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Pd and Pd based alloy nanoparticles as
visible light photocatalysts for coupling
reactions under ambient conditions
Gallage Sunari Peiris
B.Sc. (Hons) Chemistry, University of Sri Jayewardenepura, Sri Lanka 2010
Thesis completed under the supervision of Prof. Huai-Yong Zhu and Dr. Sarina Sarina,
and submitted to Queensland University of Technology, in fulfilment of the
requirements for the degree of
Doctor of Philosophy
School of Chemistry, Physics and Mechanical Engineering
Science and Engineering Faculty
Queensland University of Technology
2017
ii
Keywords
Photocatalysis; Visible light; Localized surface plasmon resonance; Plasmonic
photocatalysts; Plasmonic metal nanoparticles; Non-plasmonic metal nanoparticles;
Alloy nanoparticles; Palladium nanoparticles; Cross-coupling reactions;
Nitrobenzene reduction; Reductive N-alkylation; Organic synthesis
iii
Abstract
Photocatalysis is a rapidly emerging research field, with great potential for a
wide range of applications, since it can utilize solar energy. Solar light has received
much attention as it is the most abundant and cleanest renewable energy source,
which produces no pollution. Therefore, synthesis of fine chemicals with solar light
at ambient temperature is of the utmost interest. Nonetheless, it is still a challenge to
devise new catalysts, which exhibit high activity under the full solar spectrum and
moderate conditions. This project aimed to develop novel metal nanoparticle
photocatalysts for several important organic reactions under visible light irradiation.
The prospect of visible light irradiation driving chemical synthesis may extend the
scope of organic synthesis via a more controlled, simplified, and greener process.
Firstly, we focused on a systematic study of palladium nanoparticle-catalysed
cross-coupling and homo-coupling reactions under visible light irradiation. These
metal nanoparticles strongly absorb the light primarily through interband electronic
transitions. The excited electrons interact with the reactant molecules adsorbed on
the metal particle surface to accelerate coupling reactions. Therefore, the rate of the
catalysed reaction depends on the concentration and energy of the excited electrons,
which can be increased by increasing the light intensity. Nevertheless, mild reaction
conditions, such as ambient temperatures and pressures in the reaction systems make
it more environmentally benign.
Secondly, we incorporated palladium metal component with silver
nanoparticles to obtain silver-palladium alloy nanoparticles (Ag-Pd alloy NPs):
which can catalyse the reductive coupling of nitroarenes reactions by light irradiation
at ambient conditions. This provided a general indication for the possibility of the
design of an alloy nanoparticle photocatalysts using silver with other transition
metals, such as nickel, cobalt. This photocatalytic process is a more efficient and
greener approach than thermal reactions for the reductive coupling of nitroarenes,
and this improving the product yield by avoiding over-reduced products. The alloy
nanoparticles strongly absorb light, energizing the conduction electrons of silver,
which migrate to palladium sites at the alloy nanoparticle surface because of charge
redistribution between the two metals. The alloying affects the charge redistribution
iv
between silver and palladium, which enhances interaction between reactant
molecules and the nanoparticles. The reduction activity is sensitive to the intensity of
the irradiation, the wavelength of the incident light, metal molar ratio and
atmosphere of the reaction. When the molar ratio of silver and palladium in alloy
nanoparticles is nearly equal, the catalysts exhibited the best performance.
Finally, supported gold-palladium alloy nanoparticles (Au-Pd alloy NPs) on
zirconium dioxide (ZrO2) can act as efficient visible light photocatalysts for
reductive N-alkylation of nitrobenzene with benzyl alcohol. Here in, we studied the
possibility of gold-palladium alloy nanoparticles usage as a photocatalyst for amine
synthesis under mild conditions. The performance of the alloy nanoparticle
photocatalyst mainly depends on the alloy composition, light intensity and reaction
temperature. These heterogeneous catalysts can be easily recycled, which is
significant in the development of practical and cost-effective catalytic processes.
This finding provides a useful guideline for green amine synthesis driven by solar
energy and further reveals the possibility of designing efficient photocatalysts for a
number of organic syntheses using transition metals alloying with gold metal.
v
List of Publications
Journal Publications
1. Sunari Peiris, John McMurtrie and Huai-Yong Zhu*. Metal nanoparticle
photocatalysts: emerging processes for green organic synthesis. Catalysis
Science & Technology, 2016, 6, 320-38. - 2016 most accessed Catalysis
Science and Technology articles
2. Sunari Peiris, Sarina Sarina*, Chenhui Han, Qi Xiao and Huai-Yong Zhu.
Silver and Palladium Alloy Nanoparticles Catalysts: Reductive coupling of
Nitrobenzene through Light Irradiation. Dalton Transactions, 2017. DOI:
10.1039/C7DT00418D
3. Sunari Peiris, Sarina Sarina*, Chenhui Han, Xiayan Wu, Qi Xiao and Huai-
Yong Zhu. Non-plasmonic Palladium nanoparticles for homo-coupling and
cross-coupling reactions under visible light irradiation. Manuscript ready to
submit to Chemistry – An Asian Journal.
4. Sunari Peiris, Sarina Sarina*, and Huai-Yong Zhu. Reductive N-alkylation of
nitrobenzene with benzyl alcohol by Au-Pd alloy nanoparticles under light
irradiation. Manuscript ready to submit to RSC Advances.
5. Sarina Sarina, Sunari Peiris and Huai-Yong Zhu*. Silver metal on different
supports as a photocatalyst for reductive coupling of Nitroaromatics.
Manuscript ready to submit.
vi
Conference Presentations
1. Sunari Peiris, Sarina Sarina, Qi Xiao and Huai-Yong Zhu*. Non-plasmonic
palladium nanoparticles for homo-coupling and cross-coupling reactions
under visible light irradiation. 9th
European Meeting on Solar Chemistry and
Photocatalysis: Environmental Applications, 13-17th
June 2016, Strasbourg,
France. - Oral presentation
2. Sunari Peiris and Huai-Yong Zhu*. Ag-Pd alloy nanoparticles catalysts:
coupling of nitroaromatics through light irradiation, Nanotechnology and
Molecular Science HDR Symposium, QUT, 12-13th
Feb 2015.- Oral
presentation
3. Sunari Peiris and Huai-Yong Zhu*. Non-Plasmonic Palladium Nanoparticles
for Homo-Coupling and Cross-Coupling Reactions under Visible Light
Irradiation. Nanotechnology and Molecular Science HDR Symposium, QUT,
16-17th
Feb 2016.- Oral presentation
QUT Verified Signature
viii
Acknowledgements
First and foremost, I would like to sincerely acknowledge my principal supervisor,
Prof. Huai-Yong Zhu, for his expert guidance and encouragement throughout my
PhD. His wide knowledge and logical approach for research work has been
extremely useful for my research.
I would also like to extend my gratitude to my associate supervisor, Dr. Sarina Sarina
for her guidance, advice and for her invaluable assistance with research. Dr. Qi Xiao
is gratefully acknowledged, for his guidance in numerous ways.
Many thanks to A/ Prof. John McMurtrie, Dr. Sarina Sarina and Dr. Qi Xiao for
collaborations and valuable suggestions particularly in the method of conducting
research.
Thanks to late Dr. Chris Carvalho, Dr. Lauren Butler, Mr. Tony Raftery, Ms. Rachel
Hancock, Dr. Natalia Danilova, Mr. Peter Hegarty, Dr. Lorraine Calwell, Dr. Llew
Rintoul and Dr. Jamie Riches for giving me training and providing me assistance
with the instruments when necessary.
Warm thanks go to my lab mates, initially helping me out to find my way around the
lab, for all the good times we had and the friendship. Many thanks to all my friends
around QUT and throughout my entire life.
Thanks to QUT for the scholarship, this made my stay in Australia possible, and
ARC for the research funding.
My deepest gratitude must indeed go to my parents, for bringing me up into such
happiness and such love and having trust in me, more than I do. Finally to my
husband Dilan, for everything we have shared from the start of our wonderful
journey in life together, no words could ever express my appreciation for all that he
has done for me.
ix
Table of Contents
Keywords ................................................................................................................................. ii
Abstract ................................................................................................................................... iii
List of Publications ...................................................................................................................v
Statement of Original Authorship .......................................................................................... vii
Acknowledgements ............................................................................................................... viii
Table of Contents .................................................................................................................... ix
List of Abbreviations ................................................................................................................x
Introductory Remarks ............................................................................................................. xi
Chapter 1: Introduction and literature review ...................................................... 1
1.1 Introductory Remarks .....................................................................................................1
1.2 Article 1 ..........................................................................................................................2
Chapter 2: Supported non-plasmonic metal nanoparticles for organic synthesis
under visible light irradiation ..................................................................................... 52
2.1 Introductory Remarks ...................................................................................................52
Article 2 ..................................................................................................................................53
Chapter 3: Supported silver based alloy nanoparticle photocatalysts for organic
synthesis under visible light irradiation ..................................................................... 77
3.1 Introductory Remarks ...................................................................................................77
3.2 Article 3 ........................................................................................................................78
Chapter 4: Supported gold based alloy nanoparticle photocatalysts for organic
synthesis under visible light irradiation ................................................................... 114
4.1 Introductory Remarks .................................................................................................114
4.2 Article 4 ......................................................................................................................115
Chapter 5: Conclusions & Future work ........................................................... 139
x
List of Abbreviations
DFT : Density Functional Theory
EDS : Electron Diffraction Pattern
GC : Gas Chromatography
GC-MS : Gas Chromatography-Mass Spectrometry
LSPR : Localised Surface Plasmon Resonance
NPs : Nanoparticles
PNPs : Plasmonic Nanoparticles
SEM : Scanning Electron Microscopy
TEM : Transmission Electron Microscopy
TOF : Turnover Frequency
TON : Turnover Number
UV/Vis : Ultraviolet-Visible Spectroscopy
VOCs : Volatile Organic Compounds
XPS : X-ray Photoelectron Spectroscopy
XRD : X-ray Diffraction
xi
Introductory Remarks
“Pd and Pd based alloy nanoparticles as visible light photocatalysts for coupling
reactions under ambient conditions” investigated non-plasmonic and alloy metal
nanoparticles and their applications in fine chemical synthesis using visible light
irradiation. The object of this thesis is to develop photocatalysts using metal
nanoparticles to utilize visible light to fine chemical synthesis under mild conditions.
This thesis investigates the photocatalytic activity of the non-plasmonic palladium
metal nanoparticles, which can drive many useful organic chemical reactions under
traditional thermal catalysis conditions. Moreover, this thesis shows that the
activities of plasmonic metal nanoparticles (such as silver, gold) are enriched by
alloying them with non-plasmonic metals and enhances the efficiency of organic
reactions under visible light irradiation. The metal nanoparticle photocatalysts
function via different reaction mechanisms. The different reaction mechanisms for
the metal nanoparticle photocatalysts are highlighted. The findings of this study
demonstrate the use of visible light or sunlight to drive chemical reactions, which is
an important aspect in the view of a sustainable and green chemistry.
This thesis is a collection of published, submitted and prepared works by the author
to various scientific journals. Thus, the general formatting follows the style of the
specific journals. Repetition and redundancy in the introductory sections of each
paper are unavoidable owing to the close relationships between the subject matter
published. The following Figure 1 is a graphical representation of the structure of the
thesis.
xii
Figure 1:.Schematic illustration of the thesis structure.
CHAPTER 1: INTRODUCTION AND LITERATURE REVIEW
A review of the literature relating to the latest developments in
direct photocatalyst using plasmonic, non-plasmonic and alloy
metal nanoparticles for organic synthesis.
CHAPTER 2: SUPPORTED NON-PLASMONIC METAL
NANOPARTICLES FOR ORGANIC SYNTHESIS UNDER
VISIBLE LIGHT IRRADIATION
A study on non- plasmonic palladium nanoparticle photocatalysts
for homo-coupling and cross-coupling reactions.
CHAPTER 3: SUPPORTED SILVER BASED ALLOY
NANOPARTICLE PHOTOCATALYSTS FOR ORGANIC
SYNTHESIS UNDER VISIBLE LIGHT IRRADIATION
A study on the Ag-Pd alloy nanoparticle photocatalysts for
reductive coupling of nitrobenzene.
CHAPTER 4: SUPPORTED GOLD BASED ALLOY
NANOPARTICLE PHOTOCATALYSTS FOR ORGANIC
SYNTHESIS UNDER VISIBLE LIGHT IRRADIATION
A study on the Au-Pd alloy nanoparticle photocatalysts for
reductive N-alkylation of nitrobenzene with benzyl alcohol.
CHAPTER 5: CONCLUSION AND FUTURE WORK
Conclusions are derived based on the scientific work presented in
this thesis with respect to each chapter and the avenues for future
work are noted.
1
Chapter 1: Introduction and literature
review
1.1 INTRODUCTORY REMARKS
This chapter includes one review article:
This article is an invited perspective by Catalysis Science & Technology (2016
most accessed Catalysis Science and Technology articles). This Perspective
summarizes the overview of recent research on direct photocatalysis of supported
metal nanoparticles (plasmonic, non-plasmonic and alloy metal nanoparticles) for
organic synthesis under light irradiation and discusses the significant reaction
mechanisms that occur through light irradiation. The progress in this new burgeoning
research area is of great interest. This perspective provides a comprehensive
backdrop of the unique features of the localized surface plasmon resonance effect in
plasmonic metals and their applications in organic transformations. Herein, we
reviewed a number of different reactions carried out using metal NP photocatalysts,
including selective oxidation, selective reduction, coupling, addition and degradation
reactions. The role of light irradiation in the catalysed reactions and the light-excited
energetic electron reaction mechanisms were highlighted and discussed each reaction
mechanism individually. Further, this provides a discussion on the outlook and future
directions of this exciting new field.
QUT Verified Signature
QUT Verified Signature
3
Metal nanoparticle photocatalysts: emerging processes for
green organic synthesis
Sunari Peiris, John McMurtrie and Huai-Yong Zhu*
ABSTRACT: Metal nanoparticle photocatalysts have attracted recent interest due to
their strong absorption of visible and ultraviolet light. The energy absorbed by the
metal conduction electrons and the intense electric fields in close proximity, created
by the localized surface plasmon resonance effect, makes the crucial contribution of
activating the molecules on the metal nanoparticles which facilitates chemical
transformation. There are now many examples of successful reactions catalysed by
supported nanoparticles of pure metals and of metal alloys driven by light at ambient
or moderate temperatures. These examples demonstrate these materials are a novel
group of efficient photocatalysts for converting solar energy to chemical energy and
that the mechanisms are distinct from those of semiconductor photocatalysts. We
present here an overview of recent research on direct photocatalysis of supported
metal nanoparticles for organic synthesis under light irradiation and discuss the
significant reaction mechanisms that occur through light irradiation.
1. INTRODUCTION
Many syntheses of organic compounds use catalysts at elevated temperatures
(thermal catalysis) to achieve higher efficiencies. Nevertheless, it will be especially
valuable to drive these reactions by light irradiation at ambient temperatures, which
will avoid unwanted by-products formed at elevated temperatures.1 Throughout the
last decade; the area of heterogeneous photocatalysis has grown rapidly with the
development of new photocatalysts, which are active in visible light and suitable for
organic synthesis. Moreover, sunlight has received much attention, as it is the
cleanest and most abundant energy source. Solar light is a combination of 5% UV
(wavelength 200–400 nm), 43% visible (wavelength 400– 800 nm), and 52%
infrared (wavelength >400 nm) radiation. Given that visible and infrared light
constitutes most of the available solar emission (approximately 95%),2 developing
novel catalysts that exhibit high activity with irradiation in the solar spectrum is a
significant challenge in photocatalysis.
4
Plasmonic-metal nanoparticles (PNPs) have been recognized as a novel class of
material that is specifically efficient in harvesting light energy for chemical synthesis
due to their intense optical absorption over a wide range of the sunlight spectrum.3–7
The characteristic feature of the PNPs is their strong interaction with resonant
incident light through excitation of localized surface plasmon resonance (LSPR). The
energy of the incident light can be gained by the conduction electrons of the metal
nanostructures. The optical properties of the PNPs strongly depend upon the size and
geometry of the nanoparticle.8 This property was utilized for visible light
photocatalysis to enhance the semiconductor photocatalytic activity on water
splitting or dye degradation.9–11
In year 2010, we reported for the first time that gold
(Au) NPs on photocatalytically inert supports could be used as photocatalysts for
chemical synthesis.12
We found that nitroarenes can be directly reduced to azo
aromatic compounds using photocatalysts made of Au NPs on ZrO2 at 40 °C under
visible light, achieving high conversion rates and product selectivity.12
The work
demonstrates the potential of direct photocatalysis of PNPs for organic synthesis.
Moreover, the role of the PNPs as light energy harvesters can be utilized in alloy NPs
of a plasmonic metal and a metal with intrinsic catalytic activity for the specific
reaction under investigation. In these novel systems no electron transfer between the
NPs and support material was observed and the metallic NPs serve as both the light
absorber and host to the catalytic sites.13
Since the metal NPs serve as both light
absorber and host the catalytic sites; many potential materials (insulating solids,
porous solids, polymers and carbon-containing materials) could potentially be used
to create better photocatalysts. It follows that coupling light harvesting and catalytic
functions greatly broadens the potential applications of metal nanoparticle
photocatalysis for fine chemical synthesis.12–14
The photocatalysts of Au NPs were initially used for environmental
remediation via oxidation of volatile organic compounds (VOCs).14
Compared to
conventional oxidation methods, which involve heating, the visible light driven
catalysis process has a significant benefit as reactions can occur at ambient
temperatures. For complete oxidation of VOCs to CO2 large oxidation power of the
catalytic process is a priority as no product selectivity is required. However,
reactions involving catalytic synthesis of fine organic chemicals often favour
partially oxidized or reduced products or a specific compound from several possible
5
products. Thus, product selectivity is at least of equal importance as a fast reaction
rate. An important feature of metal NP photocatalysis is lower reaction temperatures,
which in turn leads to formation of fewer side products and hence increased product
selectivity. Ultimately, this will be reduces equipment costs and expenditure on
energy required for the catalytic process. Since 2010, we have focused on synthesis
of fine chemicals using supported metal NP catalysts under visible light radiation.12,
15 During that time, a variety of new photocatalysts comprising supported metal NPs
have also been developed for organic synthesis by other research groups.16
This review concentrates on direct photocatalysis using supported metal NPs
for organic synthesis and the distinct mechanisms of these photocatalytic processes.
We begin with a brief discussion of semiconductor photocatalysis and associated
drawbacks followed by an overview of PNPs and the characteristic features of the
LSPR effect in plasmonic metals. This is followed by an examination of the use of
metal NP photocatalysts for organic transformations and the differences in reaction
mechanisms while under light irradiation. Throughout the review we also focus on
the critical mechanisms of direct light induced energetic electron transfer from the
metal NP surface to the adsorbed reactant molecules. Finally, we provide some
discussion about the outlook and future directions of this exciting new field.
1.1. Semiconductor photocatalysis and associated drawbacks
The discovery of water splitting on a TiO2 electrode by Fujishima and Honda in 1972
ushered in a new era for heterogeneous photocatalysis.17
Semiconductor
photocatalysts such as TiO2 are promising materials for other photochemical
applications too, for instance the degradation of VOCs, dye sensitized solar cells and
super-hydrophilic materials.18–21
Semiconductors are only able to absorb photons
with energy greater than or equal to their specific band gap energy. The valence band
electrons are excited to the conduction band by leaving holes (positively charged) in
the valence band. The separated charges migrate to the particle surface and these
charges (electrons and holes) can reduce and oxidize species absorbed on the solid
surface. Unfortunately, due to its wide band gap (3–3.2 eV); TiO2 can only utilize
ultraviolet (UV) radiation, which accounts for no more than 3–5% of the total solar
energy available on the earth's surface.22, 23
Generally, chemically stable
semiconductors have wide band gaps while materials that have narrow bandgaps are
6
unstable under most photocatalytic reaction conditions and as such both are limited
in their practical usage.23
To develop novel photocatalysts which can catalyse reactions under the full
solar spectrum, a number of approaches have been developed, such as doping TiO2
with metal ions or metal atom clusters,22, 23
integrating nitrogen24
and carbon25
into
TiO2 and using other metal oxides as catalyst materials.22, 26, 27
Nevertheless,
semiconductor photocatalysts still have disadvantages, such as the high probability of
electron–hole recombination, which decreases the quantum efficiency; energy lost
during charge transfer (charges need to move on to the surface for interaction with
the reactant molecules) and relatively low charge density on the TiO2 surface. More
importantly, semiconductor photocatalysts possess a weak affinity toward many
organic reactants and low surface concentrations of active sites for catalysing the
reactions.13
Surface reactions are the slowest step, and take much longer than
generating charges and charge migration inside the semiconductor particles. Hence,
most of the light-generated charges are quenched rather than participating in
reactions on the particle surface. Given that the limitations of light absorption and
photon efficiency are associated with the intrinsic nature of semiconductor
photocatalysts, the search is on for new materials that can work under the full solar
spectrum.
1.2. Direct photocatalysis of metal nanoparticles
Metal NPs have unique properties, which are different from those of bulk materials.
For example, NPs of gold and silver can intensely absorb visible light owing to the
localized surface plasmon resonance (LSPR) effect3, 28–30
and the light absorbance
depends on their particle size and shape. There is another mechanism by which metal
NPs absorb light: one electron absorbs one photon.
The properties of metals can be explained reasonably well by the electron-sea
model, (Fig. 1A) which defines the bonding in metals as resulting from positively
charged metal atoms in fixed positions, surrounded by delocalized conduction
electrons. The mobility of the electrons in the electron sea is used to explain the high
electrical and thermal conductivity of metals. In a solid metal, a large number of
7
electronic orbitals overlap, resulting in a large number of orbitals within continuous
bands.
Figure 1: A) Schematic illustration of the electron-sea model. B) The molecular
orbital energy spacing decrease as the number of interacting atoms increases.
According to Einstein's explanation of the photoelectric effect; when a photon
of incident light collides with an electron in a metal, if the photon has enough energy
(i.e. greater than the work function (ϕ) of the metal) then the electron is ejected from
the atom (Fig. 2). When a photon has less energy than the work function, it is unable
to eject electrons, but is able to excite a metal electron to the energy levels between
the vacuum and Fermi levels of the metal. The subsequent electron–electron collision
generates more hot electrons, but with lower energy (as the overall energy is
conserved during this relaxation process).
Figure 2: Schematic illustration of light absorption in metal NPs and the
photoelectric effect.
The collision between two electrons occurs in the order of 10 fs according to
the Fermi-liquid theory.31
The energy dissipating process in the NP by electron–
8
electron collisions continues for about 500 fs,32
and subsequent electron–phonon
interaction33
completes in several picoseconds and then the NP achieves an
equilibrium state with a slight temperature increase. This process is universal for the
NPs of all metals under all wavelengths of irradiation when the energy exchange of
the NP with the surrounding medium is ignored. We have found that light absorption
by this process is more effective in driving reactions when the wavelength is
shorter.13
1.2.1 LSPR – localized surface plasmon resonance. Localized surface plasmon
resonance is another light absorption mechanism, which is observed in the visible
light range for the NPs of a few metals such as Au, Ag and Cu. A localized surface
plasmon is an optical phenomenon that arises when light is incident on a metal NP
that is smaller than the wavelength of incident light. This produces a strong
interaction between the incident electric field and the free conduction electrons of the
metal NPs (Fig. 3).34
Figure 3: Schematic illustration of a localized surface plasmon resonance.
When the frequency of the free electron oscillation is the same as that of the
incident light, constructive interference results in the strongest possible oscillation as
well as localized field strength. Moreover, the frequency and strength of the plasmon
resonance also depends on the intrinsic dielectric properties of the metal NP, the
surrounding medium and the surface polarization, which can be influenced by the
particle size and shape. It is possible to modify the LSPR of metal NPs simply by
synthesizing the preferred NP size and shapes.35
There are many findings on the
association of the LSPR effect and NP size, and it has been reported that small Au
9
NPs with diameter <5 nm do not show any LSPR absorption.13
Nonetheless, a very
recent study shows that Au clusters of about 300 atoms (which have size <2 nm)
exhibit LSPR absorption.36
This is significant because when photocatalysis takes
place on the NP surface, smaller NPs have larger specific surface area and therefore
more sites for reaction. On the other hand, large NPs exhibit stronger light absorption
which is the driving force of photocatalytic reactions. Generally, Au NPs of 5–50 nm
size exhibit a sharp absorption peak in the 520–530 nm range.37, 38
The LSPR
absorbance varies from element to element, for example, the extinction spectra of
Au, Ag, and Cu spherical NPs with diameters of 20 nm show maxima at 530, 400,
and 580 nm, respectively (Fig. 4).39
Furthermore, metal NPs can absorb the incident
light passing in their vicinity at the LSPR wavelengths.40
The PNPs are thus more
efficient light absorbers compared to semiconductors as they can concentrate the
incident photon energy on the NPs (Fig. 5).
Figure 4: Surface plasmon absorption bands for Au, Ag and Cu nanoparticles.39
When the particles grow larger, the absorption band broadens and shifts to
longer wavelengths.41, 42
For Ag NPs, it is observed that there is a shift in the
plasmon absorption band from 400 to 670 nm when the particle shape varied from a
sphere to a cube.4, 43
As the shape and/or size of the NP changes, the intensity of the
electromagnetic field at the NP surface also changes. The local electromagnetic
fields near the rough edges of noble metal NPs can be significantly enhanced due to
these changes.28, 44
In fact, the enhanced local field strength can be over 500 times
greater than the applied field when the noble metal nanostructures have sharp tips,
10
edges and concave curves, such as in nanowires, cubes, triangular plates and NP
junctions.45
Figure 5: A) Schematic illustration of electric fields of incident light and that created
by the electron oscillations near the metal NP;40
B) the extinction cross-section and
corresponding resonant wavelength for isolated metallic NPs.
When two or more metal NPs are in close proximity, they can couple to
generate an enhanced local electromagnetic field that is larger than that produced by
one NP in isolation. The spots of the enhanced field between the NPs are called hot-
spots. Finite difference time-domain (FDTD) simulations revealed that the electric
field intensity of local plasmonic hot-spots can be reached that are up to 106 times
larger than the incident electric field.46, 47
Further, the electron–hole pair generation
rate is 1000 times greater in hot-spot areas than what it generates in incident
electromagnetic field.46
The LSPR absorption of metal NPs is correspondingly
sensitive to the neighbouring environment, including solvent and support materials.
Both light absorption and activation of the reactants takes place on the metal
NPs and it is a distinct feature of plasmonic photocatalysis systems. According to
Kale et al., 48
three routes can transfer light energy into the adsorbed reactants in
direct plasmonic-metal photocatalysis: 1) elastic radiative re-emission of photons, 2)
non-radiative Landau damping (the excitation of energetic charge-carriers in the
11
metal particle causes) and 3) the interaction of excited surface plasmons with
unpopulated adsorbate acceptor states, inducing direct electron injection into the
adsorbate, which is called chemical interface damping (CID; Fig. 6). The processes 1
and 2 are conceivably amplified due to the higher intensity of scattering light and
raised electric fields near PNPs.49, 50
Figure 6: Schematic showing the three dephasing mechanisms of oscillating surface
plasmons.48
The magnitude of the field improvement, the resonant wavelength, and the
proportion of plasmon excitations that decay through these processes depend on
nanostructure geometry, size, composition, dielectric environment and separation
distance.29, 51
The light-induced surface plasmons will ultimately decay and may
produce energetic charge-carriers in the metal NPs (processes 2 and 3 above). These
carriers can transfer the energy gained from irradiation to the surroundings52
or to
heating the NPs.53, 54
The enhanced electric field itself may also accelerate the
transfer of charge carriers from the NPs. The plasmon-assisted transfer of energetic
electrons into the adsorbate can facilitate chemical reactions. There are two possible
ways for this charge carrier driven transformation to take place: direct and indirect
(Fig. 7).55
In the case of the indirect charge transfer mechanism, the optically excited
energetic charge carriers favour transfer towards the adsorbate acceptor orbitals
(LUMO), which have energies closer to the Fermi level (due to the higher
concentration of low energy electrons). On the other hand, in the direct charge
transfer mechanism, charge transfer to adsorbate occurs via plasmon-mediated
charge scattering.56
Light absorption induced NP heating may cause desorption of the
reactant molecules, which has a negative effect on the reaction rate however the
reaction rate can be greater at higher temperatures if desorption is not severe.
12
Moreover, the interaction between a reactant molecule and a metal NP surface may
lead to perturbations in electronic structure and create a polarizability change, which
might facilitate chemical reactions.57
Figure 7: Indirect (a) and direct (b) charge transfer mechanisms.55
Principally, metals have continuous electron energy levels: the conduction
electrons gain energy from light irradiation and could re-distribute this to higher
energy states from the lower levels. The wavelength (energy) of the incident photons
regulate the maximum energetic level that the electrons can reach.58
Since there is no
requirement to overcome a band gap, as there is in semiconductors, the metal NPs
have absorption over a broad wavelength range from UV to infrared. They can
absorb the energy of light and heat simultaneously, exciting their conducting
electrons to higher energy levels. This unique property is useful when the metal NPs
are used to catalyse organic reactions. Moderate heating can further assist in
achieving excellent yields of product in many cases.
2. SELECTIVE OXIDATION REACTIONS
Selective oxidation of compounds is of great interest for both fundamental
research and commercial fine chemical production.
2.1. Alcohol oxidation
Selective oxidation of alcohols to their corresponding aldehydes/ ketones, imines and
esters is an important process for chemistry research and in industrial processes.59–61
These compounds serve as vital and versatile intermediates for fine chemical
13
production,62–64
such as in perfumes, dyes, pharmaceuticals,65–67
and
agrochemicals.64, 68
The challenge is to develop catalytic systems that can achieve
high reaction rates and product selectivity simultaneously, use green oxidation
agents, such molecular oxygen and have significant tolerance of various substituted
functional groups in the alcohols. Catalytic systems under heating can attain high
reaction rates but high temperatures often negatively affect the product selectivity by
over oxidation. Photocatalytic systems working at ambient temperature exhibit great
potential for such reactions.
2.1.1 Selective oxidation and dehydrogenation of aromatic alcohols to
aldehydes/ketones. Among aromatic aldehydes, benzaldehyde is the simplest and
most widely investigated. Usually, benzaldehyde is manufactured by hydrolysis of
benzyl chloride or by oxidation of toluene.69, 70
However, the hydrolysis of benzyl
chloride yields traces of chlorine, and the commercial oxidation of toluene is
inefficient.71
Both processes produce a substantial quantity of waste. In most studies,
long reaction times, high O2 pressures and also elevated temperatures are essential
for the oxidation of alcohols to aldehydes.72, 73
Some chlorine-free catalytic processes
using the oxidants permanganate and dichromate have been employed,74–76
but they
are expensive and/or toxic. Because many of the reactions are conducted at high
temperatures and/or high pressures they can result in significant quantities of
unwanted over-oxidized by-products.
Zhang et al. reported the use of catalysts consisting of Au NPs on zeolite
supports (A, beta, Y, silicalite-1, TS-1 and ZSM-5) for the oxidation of benzyl
alcohol to synthesis benzaldehyde.77
The reactions were carried out under visible
light irradiation at close to ambient temperature (40 °C) and under an O2 atmosphere.
Decent conversions were achieved with excellent product selectivity (99%). In
contrast, much lower benzaldehyde yield (32 %<) achieved under dark at the same
temperature. The LSPR effect of the supported Au-NPs made a vital contribution to
converting aromatic alcohols with greater selectivity. This photocatalytic process
was applicable for the selective oxidation of several aromatic alcohols with different
substituent groups (cinnamyl alcohol, 4-methoxybenzyl alcohol and 4-methylbezyl
alcohol), demonstrating its general applicability. The zeolite support exhibited no
photocatalytic activity itself. However, the zeolite supports favour adsorbing benzyl
14
alcohol. The adsorption of benzyl-type alcohols on zeolite surfaces (zeolite Y and
ZSM-5) with the interaction between the hydroxyl groups of alcohols and zeolites
(Si–O–Al, SiO−, or Na+) being through hydrogen bonds.78
The Au/Y catalyst
performed the best, while Au/silicalite-1 exhibited the lowest activity, with the
overall order of catalytic activities being:
Au/Y > Au/TS-1 > Au/A > Au/beta > Au/ZSM-5 > Au/silicalite-1.77
In 2013, we reported that the yield by selective oxidation of aromatic alcohol to
aldehyde at 45 °C under visible irradiation was excellent, 99% or above, using a
catalyst of alloy NPs containing Au and Pd on ZrO2 (Au–Pd/ZrO2) with an optimal
Au/Pd molar ratio of 1 : 1.86.79
ZrO2 support is photocatalytically inert. In the
mechanism the conduction electrons of the NPs gain the absorbed light energy,
generating energetic conduction electrons on the Pd surface sites, for which the
reactant molecules have an affinity. The charge heterogeneity of the alloy NP surface
is greater than that of Au NP or Pd NP surfaces, leading to a stronger interaction
between the alloy NPs and reactant molecules.79, 80
As the distribution of Pd sites and
charge heterogeneity at the NPs play leading roles in the catalytic reactions; the
Au/Pd molar ratio has an important influence on the catalytic performance of the
alloy NP. The conversion of benzyl alcohol to benzaldehyde with the Au–Pd/ZrO2
catalyst is 100% (in the dark it is 44%). Good performance was also observed when
the aromatic alcohol contained various additional functional groups. Enache et al.
were able to get a good selectivity (96%) for the corresponding aldehydes; yet the
reaction temperatures were comparably high.72
The introduction of Au to Pd
improves selectivity, and they argue that the Au performs the electronic promoter
role for Pd and that the active catalyst has a surface that is significantly enriched in
Pd.81
The results also support the mechanism that the Au nanostructures are the
antenna for light absorption and the catalytic reaction takes place on surface Pd sites
as the hot electrons generated by light migrate to the surface Pd sites thus
accelerating the reaction. To achieve a better benzaldehyde yield there are processes
using H2O2 as a reactant and inorganic–organic hybrid materials (metal anions–
organic esters) as catalyst, in which the reaction mixture is refluxing in the dark.82, 83
Besides, non-plasmonic transition metals are widely used as catalysts for various
catalytic reactions under heating because of their inherent affinity for organic
reactants. Sarina et al. found that the Pd/ZrO2 catalyst is a very good photocatalyst
15
for this reaction (exhibiting a benzyl alcohol conversion of 94% but 29% in the
dark), while Pt/ZrO2 exhibited the poorest activity for selective oxidation of benzyl
alcohol.58
The light absorption by the non-plasmonic metal NPs follows a different
mechanism from that of the plasmon metal NPs as mentioned in section 1. The light
absorption of alloy NP structure is usually more intense compared to that of the non-
plasmonic metal NPs, but the mechanism is complicated and depends on the
composition.
Scheme 1: Proposed mechanism of aromatic alcohol oxidation over Au–Pd alloy
NPs under visible light irradiation.79
The reaction pathway of the photocatalytic process77, 79, 80
has been proposed
based on the analyses of experimental results and information from the literature. An
example is illustrated in Scheme 1. Sarina et al. suggested that Au–Pd alloy NPs
under light irradiation drives the α-H abstraction of aromatic alcohols and
consequently the transformation from alcohol into the corresponding aldehyde is
possible to achieve in an oxidant free environment at ambient temperatures.80
Moreover, it was concluded that the α-H abstraction is the rate-determining step of
the selective oxidation and takes place via a photocatalytic process on the surface of
the supported alloy NPs. Nevertheless, photocatalytic selective oxidation can be
achieved under ambient temperatures and lower oxygen pressures.80
Under visible
light irradiation, the hydrogen abstraction from the α-C atom is facilitated by the
excitation of NP electrons to the benzyl alcohol molecules adsorbed78, 84, 85
and it is
the primary step of the pathway for selective oxidation of aromatic alcohols to the
16
corresponding aldehydes or ketones.86, 87
The reaction was conducted at ambient
temperature on the Au/Y catalyst under argon atmosphere with 24.6% conversion of
benzyl alcohol to benzaldehyde observed in twenty hours under visible light
irradiation, while slight conversion was detected in the dark under the same
conditions. The two α-H atoms on the methylene group (–CH2–) of benzyl alcohol
are more active and electrochemically polarized Au-NPs readily abstracted them to
form an Au–H bond.86, 87
After this abstraction is completed, the subsequent
abstraction of the H atom from the hydroxyl group of the transient anionic species
proceeds readily producing aldehyde as the final product, while the negative charge
of the transient anions returns to the alloy NPs.80, 84, 85
Generally, the charge is
injected into the reactant adsorbed on the metal NPs and then returns to the metal
NPs after reaction completion,84, 85
in which molecular oxygen is the oxidant in the
photocatalytic oxidation process. In contrast, molecular oxygen can scavenge the hot
electrons on polarized metal-NPs to form activated O2− species.
15 These active
species remove hydrogen from metal–H bonds and yield water as the by-product. To
validate this mechanism, the 2,2′,6,6′-tetramethylpiperidine-N-oxyl (TEMPO – a
stronger hydrogen abstractor than molecular oxygen) was used and yielded
hydroxylamine (TEMPOH) instead of water (benzyl alcohol : TEMPO = 1 : 1
molecular ratio).88
No product was detected under the controlled experiments with
the zeolite Y support as the catalyst (without Au NPs) and TEMPO under otherwise
identical conditions. Based on these results, it is believed that Au–H species are
formed89
during the photocatalytic oxidation of benzyl alcohol, and the surface
hydrogen species can be removed by activated oxygen species (or TEMPO). After
the hydrogen on the NP surface reacted with molecular oxygen, the NP is able to
react with alcohol and gain hydrogen from the alcohol forming Au–H species. In the
meantime, the electron-deficient Au NPs are reloaded with released electrons from
the adsorbed benzyl alcohol compounds which form benzyl alcohol radicals.90
Afterward, these benzyl alcohol radicals may automatically release hydrogen atoms
from the hydroxyl group (–OH) to facilitate formation of the C=O bond. Finally, the
product benzaldehyde desorbs from the support. Conversion of 23% was obtained in
48 h with high benzaldehyde (100%) selectivity.15
When Ag/zeolite Y was used as
the photocatalyst, benzyl alcohol conversion of 11% was achieved in 48 h under UV
light irradiation with the product benzaldehyde (62%).91
Au–Pd alloy NPs can
exhibit better performance under visible light irradiation than pure Au or Pd NPs.
17
This is attributed to two reasons: 1) Pd sites on the alloy NPs are more active for this
selective oxidation (or dehydrogenation), and 2) the surface electronic heterogeneity
of the alloy NPs enhances the interaction with the reactants, which facilitates the
reaction.
2.1.2 Oxidation of aliphatic alcohols to esters. Conventionally, esters are
synthesized by the reaction of activated acid derivatives with alcohols.65
However,
the multi-step reaction procedures used often produce great quantities of undesirable
by-products. The strong acidic or basic conditions also limit the use of this
methodology as not all substrates are stable under such conditions. In recent years,
considerable effort has been made to develop methods for the direct synthesis of
esters by oxidative esterification of aldehydes with alcohols. Usually alcohols are
readily available as bulk materials, more stable than the carbonyl compounds as well
as being inexpensive, less toxic, and easy to use in the laboratory.92–94
Hence, they
represent attractive starting materials for large scale production and the direct
conversion of alcohols into esters represents a significant advance towards green,
economic and sustainable processes.95–99
The selective oxidation of alcohols usually
provides the required aldehydes. However, selective oxidation of aliphatic alcohols
with molecular oxygen is rather challenging, especially with no added base and
under moderate reaction conditions,100
although it is highly desirable from both
economic and environmental points of view. In 2015, our group reported a stable and
reusable catalyst of Au–Pd alloy NPs supported on phosphate anion modified
hydrotalcite suitable for the direct oxidative esterification of aliphatic alcohols.101
This is a one-pot reaction and under visible light irradiation, where we were able to
achieve excellent conversion (94%) and good selectivity (76%) even without any
additional base being added. Compared to the direct esterification in the dark
(conversion of 62%) visible light irradiation resulted in enhanced conversion at
ambient temperatures.
The proposed mechanism for the direct oxidative esterification of alcohols is
depicted in Scheme 2. The oxidative esterification reaction may proceed through an
oxidation of alcohol to aldehyde (step IV) followed by a condensation reaction. The
alcohol molecules first adsorb on the Au–Pd alloy NP surface; as it has a strong
binding affinity. The light excited hot electrons of the metal NP facilitate the
18
cleavage of the C–H bond of the alcohol adsorbed on the NP surface79
and leads to
the formation of metal alkoxide and metal hydride species (step II). Moreover, the
basic sites on the surface of the support can bind the hydrogen atom of the alcohol
and also facilitate the O–H cleavage on the metal surface.102
The basic surface sites
should lower the activation barrier of the C–H bond of the metal alkoxide
intermediate to form the aldehyde.103
Hot electrons can also enhance hydrogen
abstraction from the α-H of the metal alkoxy species and transform into aldehyde
(steps IV–V). The condensation reaction between the aldehyde and another molecule
of alcohol results in the formation of hemiacetal intermediate (step V),104–106
which is
followed by oxidative dehydrogenation to give the corresponding ester. One can
further increase the efficiency of the reaction by adequate heating (over 55 °C).
Scheme 2: Proposed mechanism for synthesis of esters from aliphatic alcohols.101
2.2. Amine oxidation
Synthesis and applications of imines play a critical role in modern organic
synthesis.107
Oxidation of secondary amines,108, 109
self-coupling of amines110, 111
and
coupling of alcohols and amines112, 113
are the alternative approaches for imine
production. Generally, it requires high temperatures (around 100 °C), O2 atmosphere
as well as relatively extended reaction hours (>24 h) to synthesis imines through
oxidation of secondary amines.109, 114
Moreover, some approaches require
stoichiometric or excess amounts of strong oxidants such as o iodooxybenzoic acid,
19
Ph(OAc)2 and MnO2 and they produce large amounts of undesired waste.115, 116
Copper based catalysts,117
Al grafted MCM-41118
ruthenium based catalyst119
have
been developed for synthesis of imines from amines.117
However, only a limited
range of amines can be oxidized into imines and this at relatively high temperatures
(>100 °C). Aschwanden et al. reported Au(OAc)3 anchored CeO2 catalyst can
oxidize benzylamine at 108 °C at 0.98 atm O2 pressure, with a product yield (up to
89%) after 16 h.120
The elevated reaction temperatures and pressure indicate that this
reaction requires demanding conditions when using supported metal NPs as the
catalyst. In 2013, the selective photocatalytic oxidation of benzylamine to the
corresponding imine was achieved at 45 °C on the Au–Pd/ZrO2 catalyst (optimal
Au/Pd molar ratio – 1 : 1.86). The yield was 95% and selectivity was above 96%
(yield at dark – 36%).79
The catalytic activity of Pd and the charge heterogeneity of
the alloy NP enhance the efficiency of photocatalytic oxidation of amine.
Recently Sarina et al. found that the non-plasmonic transition metals NPs (Pd,
Pt, Rh, and Ir) on ZrO2 exhibit good photocatalytic activity for selective oxidation of
amines under visible or UV light irradiation. The Pt/ZrO2 catalyst performed the best
activity (71%), while Ir/ZrO2 and Rh/ZrO2 exhibited the poorest activity (34–35%).58
In these systems, the metal NPs absorb light via the mechanism in which one photon
excites one electron directly and the generated hot electrons drive the reaction. While
low benzylamine conversions (17%<) resulted in the dark.
Scheme 3: Proposed mechanism for the photo oxidation of benzylamine.79
20
The most probable pathway for oxidative coupling of benzylamine is illustrated
in Scheme 3. Initially under light irradiation, benzylamine is oxidized into
benzaldehyde by the abstraction of the α-H from the –CH2– group. Afterwards, the
unreacted amines connect by nucleophilic attack of these nascent aldehydes to yield
the corresponding imines.121, 122
2.3. Aldehyde oxidation
As mentioned earlier, esterification is one of the fundamental transformations in
organic synthesis. Therefore, the development of novel approaches for synthesis of
esters has attracted the interest of chemists owing to the extensive use of these
compounds. The synthesis of ester derivatives under mild conditions comprises the
stoichiometric activation of carboxylic acid as an acyl halide, anhydride, or activated
ester amenable to subsequent nucleophilic substitution.65
However, these one-pot
conventional approaches require the consumption of heavy-metal oxidants such as
KMnO4,123
CrO3,124
and reactive hydrogen peroxide,125, 126
or other transition-metal
catalysts.127–132
However, active graphitic carbon nitride (g-C3N4) catalyst shows
lower yield (<34%) for esterification of benzaldehyde and alcohol under visible light
radiation even with H2O2 as oxidant.126
Zhang et al. reported an exciting, important
alternative transformation method for ester derivative synthesis; which converts
aldehydes in mild conditions using supported Au NP photocatalyst under visible
light.133
A number of substituted benzaldehydes were successfully converted to the
corresponding esters in good to moderate yields. The targeted ester was isolated in
78% yield when benzaldehyde was reacted with ethanol under a visible light source
in atmospheric air at room temperature. In the dark, the reaction conversion at same
temperature was only 4.4%.133
Scheme 4: Proposed mechanisms for the esterification of benzaldehyde with
alcohol.133
21
The proposed mechanism for the esterification of benzaldehyde with alcohol is
illustrated in Scheme 4 and is consistent with the literature where an oxidant converts
a hemiacetal intermediate to the ester.134–136
The Au NP surface has a strong binding
affinity towards aromatic substances; thus the benzaldehyde molecule is readily
adsorbed on to the surface. The oxidative esterification reaction proceeds through a
condensation reaction between benzaldehyde (I) and alcohol (II), which results in the
formation of hemiacetal intermediate(III). The light excited hot electron of Au NPs
facilitates a condensation step possibly through electron transfer and recombination.
Moreover, the local electromagnetic fields enhanced near rough surfaces owing to
LSPR effect of Au NPs increases the reactivity toward nucleophilic attack by the
alcohol to give the hemiacetal. Finally, the oxidative dehydrogenations give the
corresponding ester (IV). When the reactant molecules are adsorbed on the surface of
Au NPs, it is possible to assist the oxidative dehydrogenation.
3. SELECTIVE REDUCTION REACTIONS
Reduction reactions are essential in organic synthesis and biological chemistry
and consequently are under intense study.137–139
3.1. Reduction and reductive coupling of nitroarenes
Aromatic azo compounds are widely used in the textile, food, polymer and
pharmaceutical industries.140–142
Syntheses of azo compounds are often conducted
under high pressures and at high temperatures using transition-metal reducing
agents.140, 142, 143
However, some by-products formed from the reducing agent, are
harmful to the environment.140
In 2008, Grirrane et al. reported that Au NPs/TiO2
could catalyse the production of azobenzene from nitrobenzene through a twostep
and one-pot reaction at 100 °C or above.144
First, the nitroaromatic compounds are
over reduced to corresponding amines under 8.8 atm pressure of H2. Secondly, the
amines are oxidized to aromatic azo compounds under an atmosphere of O2 at a
pressure of 4.9 atm. This is because the azo compounds are unstable under the high
temperatures and high hydrogen pressures, which are required to achieve better
reduction rate.141, 145
In 2010 Wang et al. reduced nitrobenzene mostly to aniline over
silica gel supported nickel but at higher temperatures (70–90 °C) and higher H2
22
pressures (19.7 atm) to achieve the results.146
In the same year, Zhu et al. discovered
that Au/ZrO2 can catalyse a one-step reductive coupling reaction of aromatic
nitroarenes to corresponding azo compounds at 40 °C with high yields
(approximately 100%) under visible light or UV light illumination.12
Recently, Guo
et al. reported that 3 wt% Cu NPs on graphene (Cu/graphene) exhibits excellent
photocatalytic activity for coupling of nitroarenes under visible light and sunlight.
For example, the yield of azoxybenzene from nitrobenzene is 90% at 60 °C and 96%
at 90 °C and a control experiment indicated that a negligible reduction of
nitrobenzene occurred in the dark (28%<).147
The study also demonstrates the
possibility to drive such chemical syntheses with sunlight – the most abundant
energy resource.
Scheme 5: Proposed mechanisms for the reductive coupling of nitrobebzene.12
In the reactions described above, all reactants of the photocatalytic process are
in solution phase and isopropanol performs the role of both hydrogen donor and
solvent. High concentration of the reactants on the solid catalyst surface can be
achieved under ambient pressure (Scheme 5). The presence of KOH improves the
hydrogen release from isopropyl alcohol that is transformed into acetone. The Au-
NPs are able to abstract hydrogen from isopropanol creating transient Au–H species
under visible light irradiation (step II).12, 89
The LSPR effect excited the electrons and
those energetic electrons strongly interact with the electrophilic nitro groups in the
reactant molecules, facilitating the cleavage of N=O bonds by H–Au species to yield
HO–AuNP species (step III). The significant role of the surface Au–H species in the
23
reduction pathway is confirmed by adding an efficient hydrogen-abstracting reagent,
in this case, 2,2′,6,6′-tetramethylpiperidine N-oxyl (TEMPO)—which can abstract
hydrogen from Au surface, into the reaction system while the other experimental
conditions were unchanged.148
The reduced products weren't detected. The presence
of the surface Au–H species is therefore essential for the photocatalytic reaction.
3.2. Deoxygenation of epoxides
The Deoxygenation of epoxides into alkenes is an important synthetic transformation
in organic and pharmaceutical chemistry, as it allows the use of the oxirane ring as a
protective group for C=C double bonds.149–151
However, conventional deoxygenation
requires stoichiometric reagents such as phosphines, silanes, iodides and heavy
metals and produces a large amount of undesirable waste.152–156
Even in the recent
literature, high temperatures and pressures were applied for conversion of styrene
oxide to styrene to achieve better reaction efficiency.157, 158
The proposed mechanism
for the selective reduction of epoxide is also illustrated in Scheme 6. The reduction
was conducted in isopropanol solvent (also the hydrogen donor), and the species with
the hydrogen atom bonded to the Au NP surface possible to form and react with the
epoxide molecules on the Au NPs.12, 15, 89
The active Au–H species attacks the
epoxide bond to facilitate the deoxygenation. The hydrogen atoms of the Au–H
species are able to release hydrogen to reactant molecules to form alkenes.159, 160
Scheme 6: Proposed mechanism for the selective reduction reactions.160
24
3.3. Selective reduction of ketones
The selective reduction of ketones is one possible method for the preparation of
alcohols which are important building blocks in the synthesis of pharmaceutical and
agrochemical products as well as other important materials.161–163
Most catalysts used
for hydrogenation comprise either phosphorus/ nitrogen donor ligands or a
combination of each type with high temperatures and pressures being required.164–167
Recently, Ke et al. reported the synthesis of benzyl ethanol from acetophenone with
good selectivity (>99%) under visible light and mild reaction conditions.159, 160
Under
light irradiation, the Au NPs are capable of forming transient Au–H species by
abstracting hydrogen from isopropanol (Scheme 6).12, 89
The active Au–H species
attack the C=O double bonds, leading to the hydrogenation, in which the hydrogens
of Au–H species are consumed and acetophenone is transformed into benzyl ethanol.
No reduced products were detected in the control experiment; which was done with
the use of TEMPO to confirm the key role of the Au–H species in the reduction
reactions.159, 160
The enhancement of the local electromagnetic fields near rough
surfaces of the Au NPs, due to the LSPR effect, could also assist with activation of
the double bonds of the reactant molecule.
3.4. Selective reduction of azo compounds
Bacterial strains and their enzymes are used for degradation of azo compounds via
oxidation/reduction processes.168, 169
Moreover, Nam et al. used iron metal to
decolorize azo dyes by reduction of the azo groups, which resulted in formation of
aromatic amines as products.170
However, the above methods are applied for the
purpose of environmental remediation rather than chemical synthesis. Recently, it
was reported that 40% (in dark – 10%) of azobenzene was reduced to
hydroazobenzene in 6 h with a selectivity of 78% at ambient temperatures, under
irradiation with visible light in the presence of Au NPs.160
Photocatalytic abstraction
of hydrogen from isopropanol12, 89
forms the transient Au–H species which attack the
N=N double bonds leading to the hydrogenation. None of the final reductive
products were detected under the control experiment which was done with the use of
TEMPO, once again confirming the significant role of the surface Au–H species in
the reduction reactions.159, 160, 171–173
25
3.5. Selective reduction of alkenes and alkynes
The one-step amination of alkenes or alkynes is called hydroamination and this
enables the synthesis of nitrogen containing organic compounds which are widely
used in many processes in, for example, the cosmetic, pharmaceutical and
agrochemical industries.171–173
This reaction offers an atom efficient route to various
nitrogen contain organic molecules and provides a convenient pathway for the
synthesis of numerous important fine chemicals with limited by-product
formation.172–175
The high activation barrier of hydroamination reactions demands
high temperatures (generally above 100 °C) to achieve decent yields.176, 177
Zhao et
al. were able to synthesise 1-phenethyl-2-phenylacetylene (conversion 90% and
selectivity 91%) successfully, under visible light at 40 °C in 25 h under an argon
atmosphere.178
Scheme 7: Proposed mechanism for the photocatalytic hydroamination of alkynes.178
The proposed mechanism for the photocatalytic hydroamination of alkynes on
the Au NP photocatalysts is illustrated in Scheme 7. The Au NPs absorb visible light
due to the LSPR effect and the energetic conducting electrons of the AuNPs can
migrate to the conduction band of the support (step I).179, 180
The positively charged
Au NPs interact with the nucleophilic aniline, forming a N-centred radical cation
which would lead to initiation for the electrophilic attack of the electron rich sites of
alkynes (e.g. the C atom of ≡C–Ph) (step II). The N doping could form Ti3+
sites;
which have greater coordination capability than Ti4+
sites and that makes the terminal
26
C atom more favourable for the H addition. The interaction of the electron rich C–H
bond of the terminal alkynes with lower oxidation state active Ti3+
sites might
activate the alkenes. Then the N atom links to the C atom of the phenyl ring; whereas
the H atom is added onto the terminal C atom of C≡C. The adsorption of C≡C on the
surface is weaker than C≡C and the product molecules desorb from the support
surface.181, 182
Finally, the activated sites on the photocatalyst return to the desired
initial state to start over the catalytic process.
4. CROSS-COUPLING REACTIONS
The cross-coupling reactions of mediated by organometallic compounds used
for a wide range of bond forming processes, such as C–C, C–N, C–O, C–S or C–P.183
These bond forming cross coupling reactions can produce symmetrical and
unsymmetrical biaryls and have been accepted as convenient one-step methods for
constructing complex structures which are used in the synthesis of natural
materials,184–187
bioactive products,188
agrochemicals,189
medicines190
and advanced
materials.183, 191–195
4.1. C–C coupling
4.1.1 Suzuki–Miyaura coupling. Palladium phosphine complexes are usually used
as catalysts for Suzuki reactions.196, 197
However, they cause major problems in
purification of biaryl compounds and separation of the catalyst is challenging and
leads to toxic waste.198, 199
It is challenging to recycle the catalysts used in
homogeneous catalytic processes, which is an important consideration for industrial
applications and potential impacts on the environment.200
With heterogeneous
catalysts these coupling reactions require elevated reaction temperatures and
prolonged reaction times. For example, Pd NPs supported on carbon nanotubes
resulted in 87–94% yield at 70–100 °C.201
Pd NPs on polystyrene-divinyl benzene
polymer achieved high yield (100%) at 100 °C after 12 h.202
Recently, we found that
Au–Pd alloy NPs on ZrO2 can drive the same reactions under visible light irradiation
at much lower temperatures (only 30 °C) while achieving excellent yields.79, 203
The
best conversion achieved under light illumination at 30 °C for Suzuki– Miyaura cross
coupling was 96% (Au/Pd ratio 1 : 1.86) and showed decent viability on a number of
27
substrates. The contribution of thermal effect was investigated in the dark at same
temperature and 37% conversion of 3-iodotoluene achieved. The combination of
enhanced light absorption of alloy NPs, the improved interaction between the aryl
halides and the alloy NPs, as well as the catalytic activity of the Pd being the
important factors in promoting the coupling reactions.
Scheme 8: Proposed mechanism for Suzuki reaction.79
The significant step of Suzuki–Miyaura coupling is the activation of
iodobenzene on the electron-rich Pd sites on the surface of Au–Pd alloy NPs under
light irradiation (as illustrated in Scheme 8). Generally, the palladium metal could
activate the aryl halides and facilitate the Suzuki–Miyaura coupling.200, 204
The
surface charge heterogeneity is significantly enhanced by light irradiation and
subsequently increases the interaction (absorption of reactant molecules on the metal
NP surface) between reactant molecules and Au–Pd alloy NPs (step II).
Subsequently, the aromatic borate species react with the activated aryl species on the
electron-rich Pd surface sites (step III). The initial alloy surface regenerates through
reductive elimination of the cross coupling product (R1Ph–PhR2 molecule) by
completing the photocatalytic cycle.
4.1.2 Stille coupling. The palladium-catalysed cross coupling of organostannanes
with organic halides and triflates is known as the Stille reaction.205, 206
Generally,
phosphine ligands in combination with palladium precursors provide effective
catalysts for the reaction.207, 208
However, most of the phosphine ligands are
28
expensive, toxic and air sensitive; which places significant limits on their synthetic
applications.209–211
Furthermore, to achieve higher product yields; researchers are
using microwave irradiation as well as elevated temperatures.205, 206, 212
Recently,
Xiao et al. synthesised biaryls through Stille coupling under visible light irradiation
at 45 °C using Au–Pd alloy catalyst (3-methylbiphenyl yield – 81% under light
irradiation, 33% in dark).213
4.1.3 Hiyama coupling. The Hiyama cross-coupling is an effective reaction in
organic chemistry for the synthesis of unsymmetrical biphenyl derivatives. Normally,
the organosilicon reagents are nontoxic, available at low cost, can be prepared easily
and have good stability under a variety of reaction conditions. The Hiyama coupling
reaction of trimethoxyphenylsilane with a range of aryl chlorides under microwave
irradiation conditions have been studied.214
The conversion using Hiyama coupling
with Au–Pd NPs on ZrO2 increased from 55% to 71% upon irradiation with
incandescent light in the presence of Pd NPs on ZrO2 at the same temperature (45
°C).58, 213
The temperature of the reaction mixture in the dark was kept the identical
as the reaction mixture under light by a water bath but only 7% of biphenyl
derivative yield resulted. The light irradiation on Au–Pd alloy NPs extensively
enhanced the intrinsic catalytic activity of Pd even at lower temperatures for
coupling reactions. The ability to efficiently concentrate the photon flux energy to a
very small volume (by the PNPs) and to transfer this energy to the adsorbed
molecules to induce their reaction (by catalytically active metal) are outstanding
features of alloy NPs.
4.1.4 Sonogashira coupling. Among the various cross coupling reactions, the
palladium/copper catalysed Sonogashira coupling of aryl halides with terminal
alkynes provides a convenient method to generate aryl alkynes.215–217
Typically,
Sonogashira reactions require a dry organic solvent, inert atmosphere, strong base,
prolonged reaction time and a phosphine-ligated palladium complex with a copper
co-catalyst. 215, 218
However, the copper derivatives are moisture and air sensitive and
they result unwanted terminal alkyne homocoupling through the Glaser reaction.215,
218 The Pd/SNW1 (melamine-based microporous polymer) showed decent catalytic
29
activity in copper-free Sonogashira coupling in water.219
The coupling reaction was
performed in the presence of base in water at 80 °C. The 100% conversion (98%
selectivity) of iodobenzene was observed in the presence of the Pd/SNW1 as a
catalyst.219
Generally, the phosphine-assisted method is one of the well-established
methods,220, 221
which provides excellent results in the wide range of reactions.
However, phosphine ligands are comparably expensive, toxic, and unrecoverable. In
2014, Sharma et al. synthesised catalyst (SBA-15–EDTA–Pd-11%) by anchoring a
Pd–EDTA complex over the surface of organo-functionalized SBA-15 and achieved
excellent yield (100%) at 120 °C. However, these heterogeneous catalysts used for
Sonogashira coupling reactions require elevated reaction temperatures.218, 222, 223
Xiao
et al. achieved an excellent conversion (yield – 80% under light, 10% in dark) for the
Sonogashira reaction at only 45 °C under visible irradiation, using a catalyst of NP
alloys of Au and Pd on ZrO2 (Au–Pd/ZrO2).213
4.2. C–N coupling
4.2.1 Buchwald–Hartwig coupling. The Pd catalysed Buchwald–Hartwig amination
of aryl and heteroaryl halides is a prominent method for forming C–N bonds in
modern synthetic chemistry.224–226
However, this chemistry often requires elevated
temperatures, microwave heating, refluxing and long reaction times to occur at
reasonable reaction yields.227–229
Buchwald–Hartwig coupling cannot progress in the
dark except when the temperature is over 45 °C, but it can be initiated by light
irradiation to enable 35–39% conversion at 45 °C.58
In contrast, Saikai et al. could
achieve only 54% yield under heating the reaction mixture at 120 °C for 12 hours.229
Scheme 9: Schematic diagram of the pathway for the photocatalytic cross-coupling
reactions.213
30
The proposed mechanism for the cross-coupling reactions by Xiao et al. on the
Au–Pd NP photocatalysts213
is illustrated in Scheme 9. The light irradiation
facilitates the energetic electron transfer to the adsorbed aryl iodide molecule from
the metal NP, yielding a transient negative ion species. The C–X (X = halide) bond
cleavage is the rate determining step for the coupling reactions. Loss of halide atom
affords either an adsorbed phenyl radical or an organometallic aryl palladium iodide
complex on the metal surface and later triggers the coupling reactions.213
However,
in the mechanism for thermal catalysis it is driven through a different Pd2+
intermediate.230
The aryl halide undergoes oxidative addition to the Pd0 centre and
gives unsaturated Pd2+
intermediate; which coordinates with the amine and generates
tetra coordinated Pd2+
. Then it is deprotonated with a base yielding anionic amido
complex; which subsequently gives a 3-coordinate complex. Finally, it forms the C–
N bond by coupling the aryl halide with the amine.
5. ADDITION REACTIONS
5.1. Hydrochlorination
The addition of hydrogen halides to alkenes is one of the fundamental reactions in
organic chemistry.231, 232
Nevertheless, this reaction is slightly limited in scope. For
example, addition of hydrogen halides such as HCl only occurs at satisfactory rates
in the case of strained olefins.233, 234
Gaspar et al. have studied a series of
hydrochlorination reactions using a Co catalyst at room temperature in ethanol
solution.231
The sources of hydrogen and chloride are from PhSiH3 and paratoluene
sulfonyl chloride (TsCl), respectively. The mechanism proposed by Gaspar for
hydrochlorination involves olefin hydrocobaltation and the catalytic cycle is initiated
by formation of a cobalt–hydride complex (Co2+
/Co3+
and silane).
The catalysts of noble metal nanoparticles (Pt, Pd and Au) supported on
zirconia exhibit higher activities under the irradiation of visible light for the
hydrochlorination of alkenes using hydrochloric acid as the sources of chloride and
hydrogen, which is regarded as a reaction that is difficult to occur under usual
thermal conditions.235
The light-generated hot electrons on the surface of the noble
metal NPs increase the activity of the catalyst. Under light irradiation, these hot
31
electrons can interact with protons in the solution. The hydrogen atoms on the
surface of metal NPs can readily add to the terminal carbon of the C=C group in the
molecule of 4-phenyl-1-butene according to the Markovnikov rule.236
A chloride or
hydroxide anion in the solution can be attracted by the positively charged carbon to
form the final products 4-phenylbutyl chloride or 4-phenyl-2-butenol (Scheme 10).
Remarkably higher selectivity for 4-phenylbutyl chloride was observed by all the
noble metal catalysts with Pd/ZrO2 having the best overall selectivity for the
reaction.
Scheme 10: Mechanisms of addition on supported noble metal catalysts under
irradiation of visible light.146
5.2. Acetalization
Acetalization of carbonyl compounds with alcohols to form acetals is one of the most
common methods for protecting aldehydes and ketones in organic synthesis.237–239
Conventionally, this reaction is carried out in the presence of a homogeneous acid
catalyst such as p-toluenesulfonic acid, pyridinium salts or hydrochloric acid.237–239
Although these homogeneous acids show adequate catalytic performance for
acetalization reactions, they cause problems with purification of the product, large
amounts of acidic and toxic wastes and corrosive reagents that lead to severe
environmental pollution.240–243
Acidic ionic liquids have also been used as efficient
catalysts for acetalization.238
However, a number of drawbacks of ionic liquids, such
as high cost and challenging purification of end products limit the scope of practical
industrial applications. Acetalization reactions showed decent performance with a
high selectivity on Au supported MZSM-5 (M = H+, Na
+, Ca
2+, or La
3+) under visible
32
light irradiation and at 60 °C.244
In contrast, other processes employ higher
temperatures238
and refluxing conditions.237, 239
Kawabata et al. proposed a mechanism for acetalization reactions through
hemiacetal intermediates.237
Zhang et al. suggested the plasmon-mediated catalytic
mechanism for the acetalization and discussed how the LSPR effect enhances the
activity of the catalyst.244
Plasmonic Au NPs which are loaded on ion-exchanged
ZSM-5 zeolites (Au/MZSM-5, M = H+, Na
+, Ca
2+ or La
3+) perform as “antennas”
under light irradiation by efficiently absorbing visible light. Au NPs generate active
Au(δ+,δ−)
dipole moments due to the LSPR effect under visible light irradiation, at the
same time the electric nearfield enhancement (ENFE) also induced around the
surface of Au NPs.4, 7
The ENFE of Au NPs might strengthen the polarized
electrostatic fields (PEF) of MZSM-5 hence facilitating the effective activation of the
reactant aldehyde by stretching the C=O bonds.242
The “stretched” reactants with
greater molecular polarities can be activated more efficiently by active H+ catalytic
centres improving the acetalization (Scheme 11).245–248
Scheme 11: The acetalization on Au/MZSM-5 under the irradiation of visible
light.244
33
6. DEGRADATION REACTIONS
6.1. Aldehydes and alcohols
Removal of VOCs by photocatalytic treatment has drawn extensive interest as
an environmentally friendly technique over the last few decades.249
Indoor air quality
has attracted significant attention. Formaldehyde (HCHO) is one of the major
pollutants of indoor air,249, 250
and is frequently used in room decorating, plastics,
paint, glue and refurbishing processes, as well as in furniture production. It is known
to be carcinogenic, mutagenic and teratogenic.251, 252
Catalytic oxidation is an
effective approach for removing low concentrations (at the level of parts per million)
of HCHO in indoor air.253
In general, elevated temperatures and high pressures are
desirable for complete oxidation of formaldehyde.254
Normally, titania/ titania
derivatives254, 255
and peroxone (ozone/hydrogen peroxide) 256, 257
are used under UV
irradiation for catalytic degradation. Ozone forms O3−, which can directly participate
in the reaction and hydroxyl radicals generated by hydrogen peroxide to enhance the
photocatalysis. In 2008, our group reported that Au NPs on ZrO2 support could
decrease HCHO content by 64% within 2 hours with irradiation of blue light at room
temperature and pressure.14
This is the first report on direct photocatalysis of metal
NPs which was conducted under practical reaction conditions using visible
irradiation rather than conducted in ultra-high vacuum chambers or using lasers. In
later research Au NPs on mesoporous ZrO2 nano composite (ZrO2 corporate with
LAPONITE® clay) were found to be catalytically superior for oxidation of HCHO.
During the catalytic oxidation of HCHO on Au/ZrO2- nanocomposite, the higher
oxidation state (Au3+
) of the Au is reduced to metallic Au nano crystals (Au0) and
however the both the Au3+
and Au0 states are activated the HCHO oxidation.
258
Many industrial wastewater streams contain high concentrations of organic
pollutants which are difficult to degrade biologically. Phenol is one of the most
common such pollutants and is extremely toxic to the environment and may cause
harmful effects to human health even at very low concentrations. Catalytic wet air
oxidation (CWAO) is a treatment for organic pollutants in the liquid phase,
effectively oxidising them completely to carbon dioxide and water using air or pure
oxygen.259, 260
This method requires very high temperatures and O2 pressures to
enhance the catalytic activity by formation of oxygen radicals, which can react with
34
water to form hydroxyl radicals that may then oxidize phenol.259
In 2009 Zhu et al.
reported oxidation of phenol in aqueous solution, a process which is potentially
impossible to catalyse under visible light. The Au NPs on zeolite Y, SiO2 and ZrO2
converted 21%, 28% and 45% of phenol respectively under 120 h of UV
irradiation.15
The UV light absorption results in a much greater quantity of the
electron transfer from the AuNPs to the oxygen molecule than the visible light
(LSPR absorption). Therefore, more positive charges are left in 5d band (lower
energy levels) of the Au NPs when they are exposed to UV light and those are able to
oxidize the molecules that are more challenging to oxidize such as phenol. The
photocatalytic process has a significant advantage compared to the conventional
oxidation with heating as it requires much less energy input to activate the reaction
and it is outstanding for indoor air and waste water purification at ambient
temperatures. However, the oxidation power of the metal NP photocatalysts is not as
strong as TiO2, although a much larger number of charge carriers (electrons and
holes) can be generated by light and the metal NPs have a better affinity for
adsorption of organic molecules. From the point of view of practical applications,
they cannot compete (at this stage) with TiO2 for degradation for environmental
purpose, where the concentration of reactant to be decomposed is very low.
6.2. Dyes
The synthetic fabric dyes and industrial dyestuffs constitute one of the largest groups
of chemicals manufactured around the world.261, 262
Azo dyes and fluorone dyes
constitute a significant portion in the overall category of dyestuffs and are more
destructive to the environment than many other common dyes. Moreover, some of
the azo dyes and fluorone dyes and their degradation products are extremely
carcinogenic.263
A number of these dyes are resistant to self-photo degradation,
oxidation and decay by acids and bases.264
Effective utilization of light to degrade
these synthetic dyes in the presence of aqueous titania dispersions265, 266
and metal
doped titania264, 267, 268
provide attractive approaches for minimising energy resources
required for environmental remediation. The dye sulforhodamine-B (SRB) can be
degraded effectively by both sensitized and direct photocatalysis. Both positive holes
and hydroxyl radicals are found as the oxidizing species responsible for initiating the
degradation in direct photocatalysis. 266
In the presence of water, the TiO2 particles
35
absorb UV light to produce electron–hole pairs. Mainly, the dye SRB adsorbed on
TiO2 is oxidized by a photogenerated hole, which is localized at the surface of the
irradiated TiO2. The dye cation radicals formed combine with adsorbed
molecularoxygen which results in formation of degraded fragments.266, 269
Moreover,
SRB water soluble dye molecules could be excited under light irradiation and the
excited SRB molecules (SRB*) are able to inject their electrons into the substrate.166
This “dye sensitization” effect of the excited SRB molecules on the Au-NPs
promotes the formation of O2− species. The photosensitization process under light
irradiation involves initial excitation of the SRB molecules and is helpful for
injecting dye electrons to the holes left in the 5sp band.91, 266, 270
This may combine
with the LSPR effect of the AuNPs, resulting a high rate of dye degradation. Chen et
al. found that dye content can be decreased by 74% within 3 hours under blue light
irradiation with Ag NPs on ZrO2.91
According to Zhao et al., the platinum dopant
acts as an electron sink from which molecular oxygen scavenges the electrons to
yield superoxide radical anions (O2−) first and then ˙OH radicals which are known to
cause the degradation of the SRB dye.270
In 2009, a study was reported describing
the photo degradation of sulforhodamine B using Au@TiO2/ bentonite under UV and
visible light irradiation and the researchers propose that Au was photoexcited owing
to the surface plasmon resonance effect and the photogenerated electrons were
injected into O2 adsorbed on TiO2, which increased the production of superoxide and
hydroxyl radicals.268
7. CONCLUSIONS AND OUTLOOK
The discovery of direct metal NP photocatalysis was a breakthrough for fine
organic chemical synthesis, particularly those that favour “green” synthesis strategies
(using moderate reaction conditions, fewer additives and high chemoselectivity). The
increasing numbers of papers on metal NP photocatalysis in recent years have
significantly advanced knowledge in this area. The novel developments in this
dynamic field have broadened the pathways for the efficient transformation of solar
energy into chemical energy. In direct metal NP photocatalysis, the light harvesting
and catalysing reaction can be effectively coupled on the NPs. Furthermore, these
reaction systems are highly energy efficient since the intensive light absorption is by
the metal NPs, and not by the solvent, the support material, the environment, or the
36
reaction container. The hot electrons created by light irradiation are able to induce
reactions of adsorbed reactant molecules at the surface of the metal NPs. The number
of the hot electrons and energy distribution can be manipulated by tuning the light
intensity and irradiation wavelength to optimize the reaction efficiency
Subsequently, the natural affinity of the metal NP surface to organic reactants, the
high density of conduction electrons on the NP surface and the ability of the NP to
couple the stimuli of light and heat to excite conduction electrons, creates a novel
class of photocatalysts superior to semiconductor or composite photocatalysts for
synthesis of organic compounds. In principle, this photocatalyst structure is likely to
be efficient in driving various chemical reactions with sunlight (or focused sunlight),
especially the reactions of organic molecules. Therefore, it is reasonable to assume
there will be significant focus in the near future on the field of metal NP
photocatalysts. In addition, the research on direct plasmon/non-plasmon driven
photocatalysis is still in its early stages. The development of structure (size and
shape) – function and composition function relationships of the metal photocatalysis
isn't fully revealed yet. A known characteristic feature of PNPs is their tunable LSPR
wavelength based on particle geometry, such as composition, shape and size. In
principle, it is possible to design nanostructures that can absorb the complete solar
spectrum efficiently by controlling these properties through catalyst preparation. The
dependence of direct plasmon-driven photocatalytic characteristics (e.g. efficiency,
wavelength dependence, reaction selectivity) on the structure of the PNPs is expected
to be investigated in future research. Even though some significant improvements
have been made, methods and techniques need further refinement. More exciting
discoveries can be expected in the pursuit of eco-friendly fine chemical synthesis,
and environmental remediation protocols in the very near future.
Acknowledgements
We gratefully acknowledge financial support from the Australian Research Council
(ARC DP150102110).
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52
Chapter 2: Supported non-plasmonic metal
nanoparticles for organic
synthesis under visible light
irradiation
2.1 INTRODUCTORY REMARKS
This chapter includes one article ready to submit to Chemistry – An Asian
Journal.
In this Article, we focused on a systematic investigation of the palladium
nanoparticle catalysed coupling reactions under visible light irradiation at lower
reaction temperatures. Non-plasmonic transition metals, such as palladium, are
widely used as catalysts for the synthesis of important organic compounds. Until
now, knowledge about their photocatalytic ability is limited. In this paper, we
discovered that irradiation with light can significantly enhance the catalytic
performance of non-plasmonic palladium nanoparticles at ambient temperatures for
several types of coupling reactions, including the Sonogashira, Stille, Suzuki-
Miyaura and Ullmann reactions. Palladium nanoparticles strongly absorb light
mainly through interband electronic transitions. The strong interaction facilitates the
transfer of light-excited electrons to reactant molecules adsorbed to the metal
nanoparticles, and electron transfer weakens the C–X (X- Halogen) bond of the
reactant molecules and facilitates the reactions. The rate of the catalysed reaction
depends on the concentration and energy of the excited electrons, which can be
increased by increasing the light intensity or by reducing the irradiation wavelength.
This finding provides a useful guideline for green cross-coupling reactions driven by
solar energy and reveals the possibility of designing efficient photocatalysts for a
number of organic syntheses using non-plasmonic transition metals.
QUT Verified Signature
QUT Verified Signature
54
Non-plasmonic Palladium nanoparticles for homo-coupling
and cross-coupling reactions under visible light irradiation
Sunari Peiris, Sarina Sarina,*
Chenhui Han, Xiayan Wu, Qi Xiao,
and Huai-Yong
Zhu
Abstract: The Palladium nanoparticles are extensively used as thermal catalyst for
cross-coupling reactions; nonetheless, knowledge about their photocatalytic ability is
limited. We report here that the catalytic performance of palladium nanoparticles
under light irradiation can significantly enhance the coupling reactions, such as the
Suzuki-Miyaura, Sonogashira, Stille and Ullmann reactions. The photocatalytic
activity of the palladium nanoparticles depends on the metal content, reaction
temperature, intensity and wavelength of the incident light. Higher reaction rates
were observed with increases in the incident light intensity. We believe that the
conduction electrons of palladium nanoparticles gain energy from photon absorption
and the increasing population of photoexcited electrons at the nanoparticle surface
leads to higher photocatalytic ability. The photoexcited electrons interact with the
reactant molecules on the nanoparticle’s surface and accelerate the chemical reaction.
These findings provide useful guidelines for designing efficient catalysts for a
number of organic syntheses using non-plasmonic, catalytically active transition
metal nanoparticles under UV-Visible light irradiation.
Introduction
Palladium (Pd) is unarguably the most versatile and conventional catalytic
metal in chemical synthesis field.1, 2
Pd catalysed cross-coupling is a dominant and
functional reaction in organic synthesis for the formation of carbon–carbon (C-C)
bonds in industrial and fundamental synthetic chemistry laboratories.2, 3
These
coupling reactions have been extensively applied to the fabrication of synthetic
materials, natural products, and bio-active compounds. 4-8
Owing to their wide
utilization of C–C bond formation, enormous interest continues in developing more
efficient catalysts aimed at industrial applications within environmentally benign
processes. Many of the catalytic cross-coupling reactions are driven by heat using
55
both homogeneous and heterogeneous Pd catalysts to achieve viable efficiency.9-14
However heating has negative side effects such as increasing the extent of the
formation of unwanted by products that compromises the long-term stability of
catalysts.15-19
Catalysis driven by light irradiation instead of heat is particularly
fascinating in the green chemical synthesis field as it combines the efficiency of
catalysis with the possibility for use of solar energy. Photocatalysis is an ideal
process for the future as it has lesser environmental impact and greater sustainability.
The application of direct photocatalysis to drive organic synthesis reactions largely
extends to alloying plasmonic-plasmonic metals NPs and the plasmonic- non-
plasmonic metals NPs.20-24
However, there are many other organic reactions that pure
Au, Ag and Cu NPs simply cannot catalyse. Recently, we discovered that non-
plasmonic metal NPs (Pd, Pt, Rh and Ir) can intensely absorb visible light and
efficiently improve the extent of conversion of a number of reactions at much lower
reaction temperatures. 25-27
It would be of great interest to investigate the impact of
light irradiation on the catalytic performance of non-plasmonic Pd NPs for coupling
reactions in detail. There are several homogeneous catalytic processes catalysed by
Pd complexes with various ligands that have been reported for coupling reactions.10,
11, 14 Although organometallic catalysts commonly exhibit notable activity and
selectivity, the applications in the industry remain challenging owing to their
expense, the problem of aggregation of metallic particles and difficulties of
separation.15, 28
In this context, heterogeneous catalysts are a promising option, due to
their easy separation, recycling, stability, handling and the use of a lesser catalyst
amount in the production procedure. 29
Herein, we envisioned that if the
photocatalytic cross-coupling reaction using supported Pd NPs as catalysts can be
realized, the synthesis of biaryl compounds would be a much more controlled,
simplified, and greener process.
Results and Discussion
In this study, Pd NPs with various metal contents from 1% to 5% on zirconia
(ZrO2) support were prepared via an impregnation–reduction procedure as described
in the Experimental Section. The transmission electron microscopies (TEM) image
shows that the Pd NPs are distributed evenly on the ZrO2 particle surfaces, and the
mean diameters of the Pd NPs are 16 nm (Figure 1). The corresponding Pd metal
56
percentage was obtained via Scanning Electron Microscopy (SEM) with Energy
Dispersive X-Ray Analysis (EDX). Further, energy-dispersive X-ray spectroscopy
(EDS) shows that, the catalyst consists of elemental Pd distributed uniformly on the
support (Figure S1). X-ray photoelectron spectra (XPS) of the samples (Figure 2-a)
confirmed that the metal existed in a metallic state when formed on the ZrO2 support.
Figure 2-b shows the X-ray diffraction (XRD) patterns of the catalyst Pd NPs on
ZrO2 supports. All diffraction peaks can be indexed to monoclinic ZrO2, no
reflection peaks of Pd were observed by XRD, because the metal content is low and
the metal diffraction peaks may interfere with the diffraction peaks of the supporting
ZrO2 structure.
Figure 1. The catalyst characterization. (a) TEM image of the Pd NPs/ZrO2 catalyst,
(b) TEM image of a Pd NP and Particle size distribution.
Figure 2. (a) XPS profile of Pd on the ZrO2; (b) XRD pattern of ZrO2 (black curve)
and Pd/ZrO2 catalyst with different metal content.
57
Diffuse reflectance ultraviolet−visible (DR UV−vis) spectra of Pd NPs on
ZrO2 support was collected and shown in Figure 3. ZrO2 support exhibits a negligible
light absorption at wavelengths longer than 400 nm. Thus, the ZrO2 support itself
does not contribute to photocatalytic activity.30
Generally, the conduction electrons
and the bound electrons determine the light absorptivity of the metal NPs.26, 31
However, the light absorption of a metal can have contributions from the LSPR
effect and interband transitions. These vary from metal to metal. For typical
plasmonic metal NPs, such as Au, Ag etc. light absorption through the LSPR effect
results in a collective oscillation of the free electrons, whereas non-plasmonic metal
NPs absorb light by bound electron excitation to high energy levels through
interband transitions.31, 32
This photogeneration of hot electrons increases the
intrinsic catalytic activity of Pd and makes it possible to apply this non-plasmonic
NP metal catalyst to drive various reactions at ambient temperatures under light
irradiation.
Figure 3. The normalized diffuse reflectance ultraviolet−visible (DR-UV/Vis)
extinction spectra of the Pd/ZrO2 with different metal content.
Pd has the required orbital energies to associate with a carbon–carbon double
bond.33
The characteristic ground-state electronic structure (4d10
5s0) of Pd confers
outstanding catalytic activity, because it is the only transition metal having a filled d
58
orbital with an empty frontier s orbital.2 The reported cross-coupling reactions
catalysed via homogeneous and/or heterogeneous processes need elevated
temperatures as well as high pressures to achieve higher yields.12, 34-37
However, this
study demonstrates that visible light irradiation can drive the same reactions on the
supported Pd NPs under much milder reaction conditions (< 45 °C and atmospheric
pressure), achieving great yields with negligible conversion in the dark. This reveals
the photocatalytic capability of a famed thermal catalyst. Moreover, photocatalysis
active in water can offer great advantages in terms of green chemistry, and has been
actively investigated.38, 39
In comparison to the results reported in the past literature,
this photocatalytic process without a surfactant (Suzuki coupling reaction) shows a
significant yield of biphenyl and it leads to easy product separation in the industry.
Iodobenzene was used as the aryl halide substrate for all the cross-coupling
and homo-coupling reactions. The visible light irradiation increased activity of Pd
NPs photocatalyst compared with the same reactions conducted in the dark. The
visible and UV light absorption could promote the electron interband transition, these
energetic electrons at the metal NPs surface enhance the catalytic activity of the Pd
NPs. Control experiments were carried out using the support ZrO2 as the catalyst and
nil or negligible conversion was observed with light irradiation or in the dark
condition. Generally, ZrO2 exhibits an insignificant light absorption in the visible
range since its large band gap (5 eV).30
This confirms the catalytic activity is due to
the Pd NPs. The highest activity was exhibited by the photocatalyst with a Pd content
of 3 wt% and lower or higher Pd content, 1 wt% or 5 wt% exhibit obviously poorer
performance (Figure S2). The Pd loading higher than 3 wt% resulted aggregation of
the NPs, which reduces the surface area of the Pd NPs, on where the catalytic
reaction took place.
A series of different aryl halide substitutes were used to investigate the wide
applicability of non-plasmonic Pd NPs photocatalyst on all the four coupling
reactions. The light irradiation significantly increased the yield of the desired cross-
coupling product in both electron donor or acceptor substituted halides (Table 1).
59
Table 1. The Pd NP catalysed coupling reactions with different aryl halides under
visible light irradiation and in the dark (in parentheses).
60
The yields were calculated from the product content and the aryl halide conversions
measured by GC. The products were analysed by GC and mass spectrometry.
Generally, activating C−Br or C−Cl bonds are more challenging than
activating C−I bond in heterogeneous catalysis systems due to the higher activation
barrier, and usually requires harsh reaction conditions.8 In this study, bromobenzene
and chlorobenzene were used to investigate the photocatalytic activity of Pd NPs for
activating C−Br or C−Cl bonds under light irradiation. The results (C-Br -24-28%
and C-Cl -10-16%) are shown in Table 2 and Pd NP based photocatalytic process
showing a promising approach to activate the challenging C-Cl and C-Br bond at
near ambient temperatures.
Table 2. Examples of bromobenzene and chlorobenzene as substrates for cross and
homo-coupling reactions using ZrO2 supported Pd NPs under visible light irradiation.
Substrates Product Yield (%)
1 Br + Sn(Bu)3
24 (7)a
2 Br + B(OH)2
28 (17)b
3 Br
<1b
4 Cl + Sn(Bu)3
16 (6)a
5 Cl + B(OH)2
10 (3)b
The yields were calculated from the product content and the aryl halide conversions
measured by GC. The numbers in parentheses are the data for reactions controlled
under the same conditions in the dark. Reaction temperature of (a) 55 °C, (b) 60 °C.
Light intensity is 0.9 W/cm2, and the other reaction conditions were kept the same.
The light irradiation intensity was increased from 0.34 to 0.50, 0.63, and 0.80
W/cm2 and the intensity dependent conversion of the Sonogashira, Susuki, Stille and
Ullmann reactions are depicted in Figure 4. The conversion amount of iodobenzene
on Pd NPs increases gradually with the light intensity increases, with other reaction
conditions unchanged. For example, when the light intensity was 0.34 W/cm2, the
light contributions of Ullmann homo coupling reaction was only 75% and when the
light intensity increased to 0.8 W/cm2, it improved to 92%. This demonstrates that
61
irradiation intensity is a primary factor in direct metal photocatalyst systems. The
coupling reactions are induced via a single photon absorption event and it confirms
by the linear relationship between photocatalytic enhancements with the light
intensity.40
The contribution of the thermal effect studied by conducting the reactions
in the dark, by maintaining the same reaction temperature using an oil bath.
Generally, the Pd is recognized as a good thermal catalyst for the organic synthesis.2
Nevertheless, the photo thermal heating effect is negligible on Pd NPs for all the
reaction systems in the dark. This confirms that the catalytic activity has a positive
relationship on the intensity and it is owing to the Pd NPs. The photocatalytic
reaction rate depends upon the population of excited electrons with sufficient energy
to initiate the reactant molecules and stronger light intensity is able to excite more
energetic electrons of Pd NPs.26
Moreover, the number of photoexcited electrons
with sufficient energy can be increased by tuning the irradiation wavelength and this
study assists understanding of the reaction mechanism.
(a) (b)
(c)
(d)
62
Figure 4. Dependence of the photocatalytic activity of 3% Pd NPs/ZrO2 for (a)
Sonogashira, (b) Suzuki, (c) Stille, and (d) Ullmann coupling reactions on the
intensity of light irradiation. The values with percentages demonstrate the light
irradiation contribution.
The dependence of the catalytic reaction rate on the irradiation wavelength is
illustrated by the action spectrum, which is used to determine whether the reaction
occurs through a photo induced processes or a thermocatalytic process.41, 42
The
reaction rates of the photocatalytic reactions under irradiation with different
wavelengths were determined for Pd NP/ZrO2 catalysts. LED lamps with five
different wavelengths (400 ± 5, 470 ± 5, 530 ± 5, 590 ± 5, and 620 ± 5 nm) were
used, and the rates were converted to the apparent quantum yields (AQYs). The
apparent quantum yield (AQY%) was calculated as follows: apparent quantum yield
= [(Mlight–Mdark)/Np] × 100%, where Mlight and Mdark are the molecules of products
formed under irradiation and dark conditions respectively, Np is the number of
photons involved in the reaction. The irradiance intensity and reaction temperature
were held constant for coupling reactions to ensure the total input energy gained by
the metal NPs was identical, under irradiation at different wavelengths. Moreover,
the number of product molecules formed in the dark (Mdark) was deducted to exclude
the impact of thermal heating. The action spectra of Suzuki and, Stille reactions are
shown in Figure 5 and each of them compared with the light absorption spectrum of
the Pd NPs supported with ZrO2. The highest reaction activity is discovered at
wavelengths at which the catalysts intensely absorb light. Since the ZrO2 support
doesn’t contribute to the photocatalytic activity, when Pd NPs are in the system they
perform as active sites for the coupling reactions.
63
Figure 5. The action spectra for (a) Suzuki and (b) Stille cross-coupling reactions.
The light absorption spectra (left axis) are DR UV−vis spectra of Pd NPs/ZrO2
(black). The AQY values were calculated on the basis of the median of three
experiments.
The impact of the reaction temperature on the photocatalytic activity of Pd
NPs was investigated by changing the reaction temperature from 40 °C to 80 °C
using oil baths while maintaining other experimental conditions unchanged. The
photocatalytic activity of the metal NPs increases with elevated reaction
temperature.43, 44
The reaction temperature increased the product yield of the
Sonogashira coupling at 40 °C (17%) to 80 °C (100%), and the yield of Stille
coupling at 30 °C (25%) to 70 °C (96%), and the yield of Ullmann coupling at 30 °C
(24%) to 70 °C (100%). (Figure 6) The contribution of the irradiation effect was
calculated by the yield difference between the light and the dark reaction divided by
the total yield under light irradiation. The light contribution= [(Ylight–Ydark)/Ylight]
×100%, where Ylight and Ydark are the product yields under irradiation and dark
conditions, respectively. For Sonogashira coupling reaction, the product yield
difference between the light reaction and dark reaction at 40 °C is 64% and it
accounted for 94% of the total product yield. It is noted that the contribution from the
light effect decreases as the reaction temperature was raised. At elevated
temperatures, the light excites more electrons of PdNPs to higher energy levels (these
electrons can further gain energy from the light irradiation), and transfer these
excited electron to the adsorbed reactant molecules to initiate the reaction.43, 44-46
The
relative population of excited vibrational states of the adsorbed reactant molecule
64
increases according to the Bose-Einstein distribution at higher temperatures.47
This
will reduce the energy requirement from irradiation to overcome the reaction
activation barrier of the iodobenzene. On the other hand, at lower reaction
temperatures, the light-excited electrons play a predominant role in photocatalytic
activity and the thermal effect contributes much less.46
The metal NPs can utilize
both thermal energy and photon energy simultaneously and that increases the
potential usage of the solar spectrum to facilitate chemical reactions under mild
conditions.16, 17, 19, 44
(a)
(b)
(c)
(d)
Figure 6. Dependence of photocatalytic activity on different reaction temperatures
for (a) Sonogashira, (b) Suzuki, (c) Stille, and (d) Ullmann coupling reaction. Under
a thermal heating process in the black squares and the light irradiation process
coloured (red, green, blue and black ) hollow squares.
65
Figure 7. Proposed mechanism of the photocatalytic reactions with non-plasmonic
Pd NP photocatalysts. (a) The electrons follow a Fermi–Dirac distribution at the
thermal temperature of the system; electron transfer cannot occur in the dark at low
temperatures. (b) Under higher irradiation, electrons of Pd NPs populate higher
energy levels and light-excited electrons directly injected into the antibonding
orbitals (LUMO) facilitate reaction of the adsorbed reactant molecules. (c) At
elevated reaction temperatures, more excited electrons of Pd NPs populate higher
energy levels of the Pd NPs, which can readily transfer to the LUMO of the absorbed
molecule to facilitate reactions. (d) At shorter wavelengths, electrons of Pd NPs are
excited to higher energy levels, which can also readily transfer to the LUMO of the
absorbed molecule to facilitate reactions. Figure 7 depicting the electronic energy
distribution where the y-axis is energy, E, with Fermi energy, EF. The black shading
indicates filled electronic states; red coloured HOMO & LUMO states for adsorbed
reactant.
(a)
(b) (c) (d)
66
Here, we propose a possible reaction pathway for coupling reaction that uses
the Pd/ZrO2 as photocatalyst. (Figure S3) We utilized the past literature knowledge
and focussed on possible roles light irradiation facilitates the coupling reaction at the
Pd NP surfaces.23, 26, 48-50
When the Pd NPs were irradiated with light, electrons
excited to the high-energy band and increase the energetic electron population at the
metal surface (Figure 7-b).26
The rate-determining step of coupling is the activation
of aryl halide on the Pd NPs by the transfer of electrons from the Pd atoms to the
halogen atoms and this facilitates the carbon–halogen bond cleavage (oxidative
addition).51
There are two possible pathways, which the light irradiation can enhance
the photocatalytic activity.52
The energetic electrons (hot electrons), have sufficient
energy to overcome the energy barrier and can migrate to the unoccupied LUMO of
the reactant molecules under light irradiation.26, 52
Moreover, the transient electron is
able to transfer from metal NP to the chemically adsorbed reactant molecules
inducing the reactions under light irradiation.50, 53, 54
Xiao et al. performed DFT
calculations and found that the cleavage of the C−I bond will be much easier when
one electron enters an unoccupied orbital of the reactant molecule.23
Furthermore,
higher reaction temperatures increased the excited electron population at higher
energy levels of the Pd NPs and readily transferred to the LUMO of the absorbed
molecule and facilitate the bond breaking (Figure 7-c). Following on, activation of
the coupling partner molecules facilitates transmetalation. Finally, the lower energy
electrons return to the Pd and reductive elimination of the cross-coupling product
R1Ph-PhR2 molecule completes the photocatalysis cycle.
Conclusions
In summary, it is found that visible light irradiation can efficiently drive the
cross-coupling and homo-coupling reactions using Pd NPs supported by ZrO2 at low
temperatures and at atmospheric pressure. The supported Pd NPs catalyst is almost
inactive for the coupling reactions at lower temperatures without irradiation. Pd
metal NPs serve as both a visible light harvester and a provider of catalytic sites. The
energetic electrons excited by light irradiation are driving the photocatalytic reaction
and it can be improved by tuning the incident light intensity and wavelength. These
excited electrons at the surface Pd sites interact with the reactant molecules and
initiate the photoreaction. Generally, non-plasmonic metals such as Pd, have been
67
widely used as a thermal catalyst for various industrial chemical synthesis. This
study indicated that the irradiation of metal particles leads to enhanced catalytic
activity. The findings reported here may significantly broaden the application of non-
plasmonic metals as photocatalysts and reveal the possibility of green cross coupling
reactions at mild reaction conditions. The knowledge acquired in this study may
encourage further studies in non-plasmonic metal catalysts for a wide range of
organic syntheses driven by visible light.
Experimental Section
Catalysts Preparation
Photocatalysts with 3% Pd on the ZrO2 powder were prepared: 1.0 g of ZrO2
powder (particle size less than 100 nm) was dispersed in 28 mL of a given
concentration of PdCl2 solution (dissolved in dilute ammonium hydroxide (1M)
solution) with vigorous stirring. To this suspension, 20 mL of 0.05 M NaBH4
solution was added drop wise over 30 min. The mixture was aged for overnight and
then the solid was separated, washed with water and ethanol, and dried at 60 °C. The
dried solid was used directly as a catalyst. Catalysts with other Pd loadings (1, 3 and
5 % of the overall catalyst mass, expressed in wt%) were prepared in a similar
method but using different quantities of PdCl2 aqueous solution.
Characterization of Catalysts
TEM studies were carried out on a JEOL JEM-2100 Transmission Electron
Microscope with an accelerating voltage of 200 kV. The Pd content of the prepared
catalysts was determined by energy dispersion X-ray spectrum (EDS) technology
using the attachment to a FEI Quanta 200 environmental scanning electron
microscope (SEM). Diffuse reflectance UV−visible (DR-UV-vis) spectra of the
sample powders were examined by a Varian Cary 5000 spectrometer with BaSO4 as
a reference. X-ray photoelectron spectroscopy (XPS) data were acquired using a
Kratos Axis ULTRA X-ray Photoelectron Spectrometer incorporating a 165 mm
hemispherical electron energy analyser. X-ray diffraction (XRD) patterns of the
sample powders were collected using a Philips PANalytical X’pert Pro
68
diffractometer. Cu Kα radiation (λ= 1.5418 Å) and a fixed power source (40 kV and
40 mA) were used.
Photocatalytic Reactions
General Procedure for Cross-coupling Reactions: A Pyrex round bottom flask
was used as the reaction container, and after the reactants and catalyst had been
added, the flask was sealed with a rubber septum cap and stirred with a magnetic
stirrer. The flask was irradiated using a halogen lamp (from Nelson, wavelength in
the range of 400−750 nm) as the visible light source, and the light intensity was
measured to be 0.8 W/cm2. The temperature of the reaction system was carefully
regulated with an air conditioner, which attached to the reaction chamber. The
reaction setups under dark condition were maintained at the same temperature as the
corresponding reactions under light irradiation by using oil bath placed above a
magnetic stirrer to the comparison. The reaction flask was wrapped with aluminium
foil to avoid exposure of the reaction mixture to light in the dark. At given irradiation
time intervals, the product was extracted with dichloromethane (CH2Cl2) and 2 mL
aliquots were collected, centrifuged, and then filtered through a Millipore filter (pore
size 0.45 μm) to remove the catalyst particulates. The liquid-phase products were
analysed with an Agilent 6890 gas chromatography (GC) HP-5 column to measure
the change in the concentrations of reactants and products. An Agilent HP5973 mass
spectrometer was used to identify the product. The GC conversion and selectivity
were calculated from the product content and the aryl halide conversions.
Sonogashira cross-coupling Reaction: Aryl halide (1 m mol), alkyl alkyne (1.2 m
mol), photocatalysts (50 mg), cetyltrimethylammonium bromide (CTAB) (1 m
mol), and K3PO4 (2 m mol) were added to 10 mL of H2O. The reaction
temperature was 45 ± 2 °C, under a 1 atm argon atmosphere, with a reaction
time of 24 h.
Suzuki cross-coupling Reaction: Aryl halide (1 m mol), arylboronic acid (1.5 m
mol), photocatalysts (50 mg) and K2CO3 (3 m mol) were added to 10 mL H2O.
The reaction temperature was 30 ± 2 °C, under a 1 atm argon atmosphere, with a
reaction time of 6 h.
69
Stille cross-coupling Reaction: Aryl halide (0.5 m mol), tributylphenylstannane
(0.6 m mol), photocatalysts (30 mg), cetyltrimethylammonium bromide (CTAB)
(0.5 m mol), and NaOH (1.5 m mol) were added to 4 mL of H2O. The reaction
temperature was 45 ± 2 °C, under a 1 atm argon atmosphere, with a reaction
time of 24 h.
Ullmann homo-coupling Reactions: Aryl iodide (0.5 m mol), photocatalysts (30
mg), and NaOH (1.5 m mol) were added to 4 mL of an EtOH/H2O mixture [1/1
(v/v)]. The reaction temperature was 50 ± 2 °C, with a reaction time of 24 h.
Acknowledgements
We gratefully acknowledge financial support from the Australian Research
Council (ARC DP150102110). The electron microscopy work was performed
through a user project supported by the Central Analytical Research Facility
(CARF), Queensland University of Technology.
Keywords: Metal nanoparticles• coupling reaction• photocatalysis• green synthesis•
light irradiation
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Supporting Information
Non-plasmonic Palladium nanoparticles for homo-
coupling and cross-coupling reactions under visible
light irradiation
Sunari Peiris,
a Sarina Sarina,
a* Chenhui Han,
a Xiayan Wu,
a Qi Xiao,
b and Huai-
Yong Zhua
a. School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and
Technology, Queensland University of Technology, Brisbane, QLD 4001, Australia.
b. CSIRO Manufacturing, Bayview Avenue, Clayton, VIC 3168, Australia.
*Corresponding author email: [email protected]
74
Figure S1. (a) SEM image of 3% Pd/ZrO2 sample and the corresponding mapping
of Zr, O and Pd elements; (b) EDX spectrum of of 3% Pd/ZrO2 sample.
(a)
(b)
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Figure S2: Dependence of Pd metal NP photocatalytic performance on the metal
amount.
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Figure S3. Proposed catalytic cycle for coupling reactions using the Pd/ZrO2 under
light irradiation.
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Chapter 3: Supported silver based alloy
nanoparticle photocatalysts for
organic synthesis under visible
light irradiation
3.1 INTRODUCTORY REMARKS
This chapter includes one article published (online) on Dalton Transactions,
2017. DOI:10.1039/C7DT00418D
Recently, extend studies have been done using PNPs on nitrobenzene
reductive coupling, for example, gold NPs, copper NPs, Ag-Cu NPs and Au-Cu NPs
and found that the alloying between two plasmonic metals leads to significantly
enhanced photocatalytic performance. However, the NPs of several Ag alloys have
been known to be catalytically active in thermal reaction, there has been a few
reports on the photocatalysis of Ag based alloy catalysts so far. In this article, we
found that by alloying plasmonic metal Ag to a non-plasmonic metal Pd; the
photocatalytic activity in nitrobenzene reduction is increased remarkably compared
to both Ag NPs and Pd NPs. The intrinsic catalytic activity of palladium is
significantly enhanced in the alloy NPs even at ambient temperature under light
irradiation. The Ag-Pd alloy nanoparticles absorb visible light, and the light excited
energetic electrons on the metal alloy NP surface activate the reactants. The
performance of the photocatalysts depends on the metal ratio, light intensity and
wavelength. Notably, these heterogeneous catalysts are easily recycled and can be
conveniently reused, which is an important aspect in the development of practical
and cost-effective catalytic processes. This study provides a general guiding principle
for determining the applicability of the alloy NP photocatalysts as well as a clue for
designing suitable photocatalysts made from transition metal alloyed with silver.
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79
Silver and Palladium Alloy Nanoparticles Catalysts:
Reductive coupling of Nitrobenzene through Light
Irradiation
Sunari Peiris, Sarina Sarina*, Chenhui Han, Qi Xiao, and Huai-Yong Zhu
The silver-palladium (Ag-Pd) alloy nanoparticles strongly absorb visible light
and exhibit significantly higher photocatalytic activity compared to both pure
palladium (Pd) and silver (Ag) nanoparticles. Photocatalysts of Ag-Pd alloy
nanoparticles on ZrO2 and Al2O3 supports are developed for catalyze the
nitoaromatic coupling to the corresponding azo compounds under visible light
irradiation. Ag-Pd/ZrO2 exhibited the highest photocatalytic activity for nitrobenzene
coupling to azobenzene (yield of ~80 % in 3 hours). The photocatalytic efficiency
could be optimized by altering the Ag: Pd ratio of the alloy nanoparticles, irradiation
light intensity, temperature and wavelength. The rate of the reaction depends on the
population and energy of the excited electrons, which can be improved by increasing
the light intensity or by using a shorter wavelength. The knowledge developed in this
study may inspire further studies of Ag alloy photocatalysts and organic syntheses
using Ag-Pd nanoparticles catalyst driven under visible light Irradiation.
Introduction
The aromatic azo compounds are important intermediates for a variety of
specific and fine chemicals in industries, such as dyes, food additives and
pharmaceutical products.1-6 However, conventional methods used in azo compound
synthesis involve the use of transition metal reducing agents and conditions of high
temperature and pressures.3, 6-11 The metal compounds formed from the reducing
agent are of environmental concerns.3, 8, 10 However, these routes always show low
yields and poor selectivity. Therefore, it is highly desirable to develop efficient as
well as environmentally friendly process for the coupling of nitrobenzene.
Photocatalytic reaction is driven by light irradiation, applying photon energy
instead of conventional thermal energy. Thus the photocatalytic reaction is able to be
conducted under much moderate conditions (ambient temperature and pressure),
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which make it possible to achieve a certain unstable intermediate of thermal reaction
as the final product in the photocatalytic reactions.
We reported previously that plasmonic metal nanoparticles (NPs) could
intensively absorb visible light via the localized surface plasmon resonance (LSPR)
effect, and successfully achieved azo aromatic compounds directly from the coupling
of nitroaromatic compounds at room temperature and 1 atm argon atmosphere.12 The
LSPR effect is the collective oscillation of conduction electrons in the NPs, which
resonate with the electromagnetic field of the incident light. These conduction
electrons could gain the light irradiation energy and increase the high energetic
electrons at the metal NP surface, which activates the reactant molecules for the
chemical reactions. Series of studies have been done using plasmonic NPs on
nitrobenzene reductive coupling, for example gold NPs, copper NPs, Ag-Cu NPs and
Au-Cu NPs and found that the alloying between two plasmonic metals show
significantly enhanced photocatalytic performance.11-14 However when alloying
plasmonic metal Au to a non-plasmonic metal Pd, the photocatalytic activity in
nitrobenzene reduction is reduced remarkably compared to the pure Au NP.15 This is
because supported AuNPs are an efficient photocatalyst themselves for coupling of
nitrobenzene.2, 8, 12, 16, 17 The key step of the reduction of nitrobenzene is to cleavage
the N-O bonds.11,12 This N-O bond cleavage by the hydrogen atoms bound to the
metal NP surface (E.g. H–AuNP). The abstraction of a hydrogen atom from
isopropyl alcohol (IPA) plays a critical role in catalytic activity. However, Ag shows
a weaker ability to abstract H from IPA compared to Au. Hence, alloying with some
stronger H absorbing metal such as Pd will improve utilise of Ag for the
photocatalytic chemical synthesis in economical way.18 Moreover, the NPs of several
Ag alloys have been known to be catalytically active in thermal reaction, there has
been a few reports on the photocatalysis of Ag based alloy catalysts so far.14
Therefore, in this study we investigated the possibility of the alloy NPs of Ag and a
transition metal; such as Pd, developed to an efficient photocatalysts for coupling of
Nitrobenzene.
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Experimental Section Chemicals:
Zirconium(IV) oxide (ZrO2, <100 nm particle size), silver nitrate (AgNO3,
≥99.9% trace metal basis), palladium(II) chloride (PdCl2, Reagent Plus, 99%),
sodium borohydride powder (NaBH4, ≥98.0%), nitrobenzene (99.0%), potassium
hydroxide (99.0%, KOH) and isopropanol (99.5%) were purchased from Sigma-
Aldrich (unless otherwise noted) and used as received without further purification.
The water used in all experiments was prepared by being passed through an ultra-
purification system.
Catalysts Preparation:
Ag-Pd NPs/ZrO2: Catalysts with 3 wt % of pure silver NPs on ZrO2 (3%Ag/ZrO2), 3
wt % of pure Pd NPs on ZrO2 (abbreviated 3%Pd/ZrO2) and the catalysts of Ag and
Pd alloy NPs supported by ZrO2 (3%Ag−Pd/ZrO2), with different Ag/Pd ratios were
prepared by impregnation-reduction method. For example, 1.5 wt %Ag−1.5 wt %
Pd/ZrO2 (3%Ag−Pd(1:1)/ZrO2) was prepared by the following procedure: 1.0 g ZrO2
powder (particle size less than 100 nm) was dispersed in 14 mL of 0.01 M AgNO3
aqueous solution and 14 mL of 0.01 M PdCl2 dissolved in dilute ammonium
hydroxide (1M) solution were added while magnetic stirring. To this suspension, 20
mL of 0.05 M NaBH4 solution was added drop wise in 30 min. The mixture was
aged for overnight and then the solid was separated, washed with water and ethanol
by centrifugation, and dried at 60 °C. The dried solid was used directly as a catalyst.
Catalysts with other Ag/Pd ratios were prepared in a similar method, but using
different quantities of AgNO3 aqueous solution and/or PdCl2 aqueous solution.
Ag-Pd NPs/ɣ-Al2O3: Catalysts with 3 wt % of pure silver NPs on ɣ-Al2O3 (3%Ag/ ɣ-
Al2O3), 3 wt % of pure Pd NPs on ɣ-Al2O3 (3%Pd/ ɣ-Al2O3) and the catalysts of Ag
and Pd alloy NPs supported by ɣ-Al2O3 (abbreviated 3%Ag−Pd/ ɣ-Al2O3), with
different Ag/Pd ratios were prepared by impregnation-reduction method. For
example, 1.5 wt %Ag−1.5 wt % Pd/ ɣ-Al2O3 (3%Ag−Pd(1:1)/ ɣ-Al2O3) was prepared
by the following procedure: 1.0 g of ɣ-Al2O3 powder (fiber length around 200 nm)
was dispersed 14 mL of 0.01 M AgNO3 aqueous solution and 14 mL of 0.01 M
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PdCl2 dissolved in dilute ammonium hydroxide (1M) solution were added while
magnetic stirring. To this suspension, 20 mL of 0.05 M NaBH4 solution was added
drop wise in 30 min. The mixture was aged for overnight and then the solid was
separated by centrifugation, washed with water and ethanol, and dried at 60 °C. The
dried solid was used directly as a catalyst. Catalysts with other Ag/Pd ratios were
prepared in a similar method, but using different quantities of AgNO3 aqueous
solution and/or PdCl2 aqueous solution.
ɣ-Al2O3 support: 4.7 g NaAlO2 was dissolved in 12.5 mL water in a 50 mL beaker,
and stirred for 15 min to obtain a homogeneous solution. 15 mL of 5 M acetic acid
solution was added into a 100 mL beaker. Then NaAlO2 solution was poured into the
burette (50 mL) and then added into acetic acid solution drop wise under vigorous
stirring. When finish titrating, continue adding acetic acid solution (5 M) until the pH
value is adjusted to 5.0. The white precipitate of aluminium hydrate was washed with
water and recovered by centrifuge for four times (3000rpm for 15 min). The
collected white precipitate was transferred to a blue cap bottle, to which 10 g PEO
surfactant (T15 S-7) was previously added. The above mixture was kept stirring for
1h. Then the homogenous mixture was transferred into an oven and kept at 100 °C.
Every two days, fresh aluminium hydrate was prepared and the obtained white
precipitate was added into the glass bottle and the mixture was kept stirring for 1 h.
The stirred homogenous mixture was put back into oven and kept at 100 °C. This
circle will continue until we get the desired ɣ-Al2O3 length. The precipitate was kept
in the 450 °C in the furnace for 5 hours and crushed the solid using a mortar. The
powdered solid was used directly as support.
Characterization of Catalysts
Transmission electron microscopy (TEM) images and line profile analysis (By
the energy dispersion X-ray spectrum technique) were acquired on a JEOL JEM-
2100 transmission electron microscope employing an accelerating voltage of 200 kV.
The element line scanning was conducted on a Bruker EDX scanner attached to the
TEM. The composition (Ag and Pd contents) of samples was determined by using
the energy-dispersive X-ray spectroscopy (EDS) attachment of an FEI Quanta 200
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scanning electron microscope (SEM). X-ray diffraction (XRD) patterns of the
catalysts samples were collected using a Philips PANalytical X’pert Pro
diffractometer. CuKα radiation ( =1.5418 Å) and a fixed power source (40 kV and
40 mA) were used. The diffuse reflectance UV-Visible spectra (DR−UV−vis) of the
samples were examined by a Cary 5000 spectrometer. The X-ray photoelectron
spectroscopy (XPS) data were acquired using a Kratos Axis ULTRA X-ray
Photoelectron Spectrometer.
Photocatalytic Reactions
The photocatalyst (30 mg), solvent (IPA-5mL), base (KOH- 0.2 m mol) and
the reactant (nitrobenzene – 0.15 m mol) was placed in a reaction vessel and used
500 W Halogen lamp (from Nelson, wavelength in the range 400−750 nm) as the
visible light source and usual light intensity was kept at 0.80 W/cm2 unless for
investigate the light intensity impact. The temperature of the reaction system was
vigilantly regulated with an air conditioner, which attached to the reaction chamber.
The reaction setups under dark condition were maintained at the same temperature as
the corresponding reactions under light irradiation by using oil bath placed above a
magnetic stirrer to the comparison. Catalytic reduction of nitrobenzene was
conducted under the argon atmosphere. The details of the reaction systems are given
briefly as footnotes in Table for each reaction. At given irradiation time intervals, 0.5
mL aliquots were collected and removed the catalyst particulates by filtering through
a Millipore filter (pore size 0.45 m). The filtrates were analysed by gas
chromatography (HP6890 Agilent Technologies) with a HP-5 column to measure the
concentration change of reactants and products. The products were identified using a
mass spectrometer (Agilent HP5973).
Results and Discussion
The TEM images of ZrO2 and Al2O3 supported sample showed that Ag-Pd alloy
NPs dispersed uniformly on supports and the mean diameter is about 7 nm and 8-13
nm in size respectively (Figure 1-2 and Figure S1). Figure 1-(b) is a line profile
analysis of the energy dispersion X-ray (EDX) spectrum for a Ag−Pd alloy NP,
showing that the NP consists of both Ag and Pd dispersed spherically around a
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common center, which confirms that the two metals exist as an alloy NPs in this
sample. Scanning electron microscopy (SEM) analysis confirms the appearance of
both metals, Ag and Pd, in the elemental mapping in Ag-Pd(1:1)/ZrO2 sample
(Figure S2). The X-ray diffraction (XRD) patterns of the Ag-Pd alloy/ZrO2
photocatalysts is shown in Figure 1-(d). The diffraction peaks of the sample can be
indexed to the monoclinic structure of the ZrO2 crystals. The reflection peaks of Ag-
Pd alloy could not be recognized owing to the low metal content (3 wt%).
Figure 1: Catalyst characterization of Ag-Pd (1:1) /ZrO2. (a) Transmission
electron microscopy (TEM) image; (b) The line profile analysis of EDX spectra for a
typical Ag−Pd NP indicated by the square and the information of the elemental
composition and distribution of the NP; (c) Particle size distribution; (d) The XRD
pattern of the Ag-Pd (1:1) /ZrO2 photocatalysts.
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Figure 2: Catalyst characterization of Ag-Pd (1:1) /Al2O3. (a) TEM image; (b)
Particle size distribution.
UV-visible spectra of these photocatalysts are shown in the Figure 3. ZrO2 exhibits a
weak visible light absorption (band-gap is about 5 eV); consequently, the support by
itself does not contribute to photocatalytic activity.19 However, the Al2O3 support
show light absorption in the 300-400 nm range. The absorption peak at 410 nm in the
spectrum of the samples is due to the LSPR absorption of the Ag/ZrO2 (Not showing
on the graph).20, 21 In the spectrum of the alloy samples, the characteristic Ag NP
LSPR absorption peak at 410 nm is disappeared; and we can assume that Ag NPs
may blend well with the Pd NPs.22-24 The strong light absorption from 450 nm to 600
nm is attributed to scattering caused by closely spaced metal NPs and NP
aggregates.25, 26
Figure 3: UV-Visible diffuse reflectance spectra of the supported Ag-Pd (1:1)
alloy NPs catalysts and the corresponding supports.
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The X-ray photoelectron spectroscopy (XPS) spectra of the catalysts are also
shown in Figure 4. The binding energies of Ag 3d5/2 and 3d3/2 electrons are 368.3 and
374.0 eV, respectively (Figure 4-a).27 In addition, the binding energies of Pd 3d5/2
and 3d3/2 electrons are 335.2 and 340.5 eV, respectively (Figure.4- b).28 These results
confirmed that the alloy NPs are in the metallic state.
Figure 4: X-ray photoelectron spectra (XPS) binding energy of (a) Ag 3d5/2
and Ag 3d3/2 for Ag-Pd NPs on ZrO2 and Al2O3; (b) Pd 3d5/2 and Pd 3d3/2 for Ag-Pd
NPs on ZrO2 and Al2O3.
The alloy photocatalysts with different Ag and Pd contents, pure Ag and pure
Pd catalyst supported by ZrO2/ Al2O3 were also prepared in the similar method for
reference and the corresponding Ag-Pd molar ratios were also calculated and listed
in Table S1. As can be seen in figure 5, the photocatalytic performance of Ag-Pd
alloy catalysts depend on the Ag:Pd molar ratio for the reduction coupling of
nitrobenzene. The results reveal that the highest yield of target products achieved
when the alloys NPs have the Ag:Pd molar ratio of 1:1.01. Alloy NPs with other Ag:
Pd molar ratios exhibited much lesser activity, either under light irradiation or in the
dark. The conversion of nitrobenzene with the pure Ag/ZrO2 and pure Pd/ZrO2
catalysts were below 45%. Similar trends were observed for the pure Ag and pure Pd
on Al2O3 as well. The alloying affects the surface electronic properties of the NPs.
Therefore, the catalytic activity of the alloy NPs are significantly improved compare
to pure Ag NP or Pd NP. The charge heterogeneity is a key factor in the catalytic
reactions and it depends on the Ag:Pd molar ratio. Thus, molar ratio has a significant
impact on the catalytic performance of the alloy NPs.15 The electronegativity of Pd
(2.20) is higher than that of Ag (1.9) and there will be charge heterogeneity at the
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alloy NP surface, with both negatively charged (electron rich) sites and positively
charged (electron poor) sites present. Hence, the conduction electrons will flow
electron rich to poor, until equilibrium is reached and the electron chemical potential
is equal everywhere in the alloy NP. The heterogeneous charge distribution increases
the interaction between reactant molecules and it reduces the activation energy of the
reaction, hence increases the catalytic activity.
Figure 5: Dependence of Ag−Pd metal NP photocatalytic performance on the
Ag/Pd molar ratio of the alloy NPs.
The Ag-Pd alloy NP/ZrO2 photocatalysts exhibited significantly higher activity
for reductive coupling of nitrobenzene under light irradiation at 1 atm of Ar and at
60°C. In contrast, both pure Ag/ZrO2 and Pd/ZrO2 exhibited relative low activity for
the reaction under similar reaction conditions. Table 1 shows the results of the
reduction of nitrobenzene with two catalysts. A notable feature of the photocatalyst is
its selectivity towards the nitro groups. (GCMS spectras were included in supporting
information) When the reactant contains multiple reducible groups; nitro groups are
the only one reduced under light irradiation. It is difficult to achieve this by
conventional reduction at high temperatures using pressured H2, where the reducible
groups are often reduced indiscriminately.29, 30 Moreover; the control experiment
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(under dark condition) at same temperature shows significantly poor conversions.
The blank experiments without metal NPs were also conducted and negligible
conversion was observed under light irradiation or in the dark. These observations
strongly suggest that the reductive coupling catalyzed effectively by Ag-Pd NPs and
that nitroaromatic conversion are achieved under light illumination.
Table2: Photocatalytic reductive coupling of aromatic nitro compounds
photocatalyst.
The yields were calculated from the product content and the Nitrobenzene conversions
measured by GC. The products were analysed by GC and mass spectrometry. Reaction time of 8 h, [a]
Reaction time 3 h, [b] Reaction time of 6 h- rest Azoxybenzene
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The photocatalytic reductive coupling of nitrobenzene achieved a high
conversion rate of nitrobenzene and a high selectivity to the target product,
azobenzene, when illuminated with light and intensity of 0.80 W/cm2. In 3 hours,
more than 98 % of nitrobenzene was reduced and more than 80 % of the product was
azobenzene. The incident light’s wavelength and intensity dependence for the
catalytic activity of the Ag−Pd alloy NPs was investigated. At 60 °C, when the light
intensity decreases from 0.80 to 0.63, 0.5, and 0.34 W/cm2, the conversion of
nitrobenzene also decreases from 98% to 83%, 66% and then to 56% respectively for
Ag-Pd/ZrO2 in 3 hours, while the selectivity of azobenzene remains almost
unchanged. The contribution of the light irradiation to the conversion efficiency was
calculated by difference between the conversion efficiency of the reaction in the dark
and under irradiation at the same temperature. The thermal contribution for the
conversion efficiency was observed in dark condition and both relative contributions
are shown in Figure 6. The greater contribution to the overall conversion rate
achieved under the higher the light intensities. For example (Ag-Pd/ZrO2), when the
light intensity is 0.34 W/cm2, 61% of the conversion results from light irradiation
with 39% attributed to the thermal effects at 60 °C ± 2 °C. When the light intensity is
0.8 W/cm2, 78% of the conversion is due to light irradiation. Similar changes were
observed for the Ag-Pd/Al2O3 as well. The conversion dependence on the light
intensity indicates that the coupling of nitrobenzene is an electron-driven chemical
reaction over the Ag-Pd alloy NPs. Moreover, it indicates that the reaction rate can
be controlled by the intensity of the irradiation. The controlled experiment was
carried out at 60 °C under dark and observed a lower conversion of nitrobenzene.
Moreover, negligible reaction was observed in a blank experiment, which conducted
using supporting powder (ZrO2 and Al2O3) under otherwise identical conditions.
These results further confirm that the coupling of nitrobenzene was driven by light
irradiation.
90
Figure 6: Intensity influences on the reductive coupling of nitrobenzene using
(a) Ag:Pd/ZrO2 -3 hours; (b) Ag:Pd/Al2O3 -6 hours.
Additionally, the dependence of photocatalytic activity on incident light
wavelength also investigated. The photocatalytic activity depends on the energetic
electrons excited by light absorb by alloy NPs. Therefore, the reaction rate is
expected to improve by increase the number of electrons with sufficient energy to
initiate the reaction of the reactant molecules. Tuning the irradiation wavelength and
using the higher light intensity could increase the number of energetic electrons. The
action spectrum is a one-to-one mapping between the wavelength-dependent
photocatalytic rate and the light extinction spectrum.31, 32 This can be used for
determine whether the catalytic reaction is driven by light (a photocatalytic process)
or by heat (a thermocatalytic process). In this study, we conduct the photocatalytic
coupling of nitroaromatics over Ag−Pd alloy NPs at 60 ± 2 °C under irradiation with
different wavelengths with wavelengths of 400 ± 5, 470 ± 5, 530 ± 5, 590 ± 5, and
620 ± 5 nm. The reaction rates were converted to the apparent quantum yields
(AQYs).14, 33, 34 The apparent quantum yield (AQY%) was calculated as follow:
apparent quantum yield = [(Mlight–Mdark)/Np] × 100%, where Mlight and Mdark are the
molecules of products formed under irradiation and dark conditions respectively, Np
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is the number of photons involved in the reaction. The action spectrum of coupling of
nitrobenzene by Ag-Pd/ZrO2 (Figure 7) is compared with the light absorption
spectrum of the both Ag−Pd alloy NPs and Ag NPs.
The action spectra of the coupling of nitroaromatics don't match with the
absorption spectrum of the Ag−Pd alloy NPs/ZrO2 catalyst (Figure 7). At longer
wavelengths, the absorption spectrum contains a considerable contribution from
scattering.25, 26 However, the action spectra results indicate that the scattering has
little impact on the catalytic performance. Additionally, the AQY of the
nitrobenzene coupling follows the light absorption of Ag NPs, which show the
characteristic LSPR absorption peak in the range between 380 and 410 nm.20, 21
Generally; Ag NPs absorb visible light via LSPR and excited electrons to high
energy levels. These light-excited electrons can transfer to the surface Pd sites of the
alloy NPs and enhance the catalytic performance of the alloy NPs. The results of the
action spectrum confirm that the enhancement of the catalytic performance is mostly
owing to the LSPR absorption of Ag in the alloy NPs. Therefore, we could argue that
the Ag acts as an antenna, which harvests the visible light and enhance the catalytic
activity of alloy NP.
Figure 7: Photocatalytic action spectrum for reductive coupling of
nitrobenzene using Ag:Pd/ZrO2.
The reaction temperature is also a key parameter of a photocatalytic reaction,
and the activity of alloy NPs photocatalyst can be improved by increasing the
reaction temperature.35-37 The experiment conducted under five different reaction
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temperatures, while maintaining the light intensity in constant value. The
nitrobenzene yield increases clearly with the increasing reaction temperature (from
40 °C to 80 °C) (Figure 8). Nevertheless, the product selectivity changes with the
elevated reaction temperature. Until 70 °C temperature the main product is
azobenzene for Ag−Pd alloy NPs/ZrO2. At 60-70 °C, the selectivity of azobenzene
reaches a maximum for Ag−Pd alloy NPs supported ZrO2 & Al2O3 and then declines
(Figure 8-a, b). However, at higher temperature, the product azobenzene is not as
stable as at low temperature. Under harsh reaction conditions, such as high reaction
temperatures, high gas pressure, or strong base media, azobenzene could be rapidly
reduced to aniline according to known Haber’s mechanism.8,38 It is possible to select
the desired products by controlling the reaction temperature. The number of
conduction electrons at high energy levels increases with the reaction temperature.
These thermally excited electrons still able to gain more energy through the LSPR
effect.33 The thermal and photonic energies could be coupled by electrons of alloy
NPs to drive the chemical reactions effectively.
93
Figure 8: Conversion of nitrobenzene reductive coupling and selectivity of
azobenzene and aniline (a) using Ag:Pd/ZrO2; (b) using Ag:Pd/Al2O3 at different
temperatures.
The proposed mechanism of the photocatalytic coupling of nitrobenzene on the
Ag-Pd/ZrO2 catalyst is similar to the work reported by Zhu et al.12 The key step of
the reduction of nitrobenzene is to break the N-O bonds. The Pd sites are able to
abstract hydrogen from isopropanol, which is a hydrogen donor and form the
transient Pd–H species. The Pd–H species, facilitating the cleavage of N-O bonds
and release azobenzene as the product (Figure 9). Therefore, it is reasonable to
expect that Ag alloying with Pd could enhance the photocatalytic activity than pure
Pd or Ag alone.
Figure 9: Proposed reaction pathway of coupling of nitroaromatics.
The one of the significant properties of photocatalysts is reusability.39 The
activity of the 3 wt% Ag−Pd alloy NPs/ZrO2 catalyst was checked for five successive
rounds (Figure 10). The catalyst was used under light irradiation with each run
reaction conditions were kept identical. The results illustrate that the catalyst can be
recycled without dropping activity considerably. The product yield can be
maintained and Ag–Pd alloy NPs/ZrO2 is a reusable photocatalysts for reductive
coupling of nitrobenzene.
94
Figure 10: The photocatalytic stability of 3 wt% Ag−Pd alloy NPs/ZrO2 in
five cycles at 60 °C.
Conclusions
In summary, light can efficiently drive the reductive coupling of nitrobenzene
reactions with the photocatalysts of Ag-Pd alloy NPs supported by ZrO2 at ambient
conditions and within a shorter period (3 hours). Moreover, Ag-Pd alloy NPs on
ZrO2 shows superior photocatalytic activity compared to NPs made from the pure
component metals, with the optimum activity observed for alloy NPs with a Ag:Pd
weight ratio of 1:1. The combination of the light absorbing properties of the metallic
NPs and the electronic properties of the alloys results in a superior catalytic
performance regardless the support. Nevertheless, similar trends were observed for
Ag-Pd alloy NPs on Al2O3 as well. The knowledge developed in this study may
inspire further studies in novel photocatalysts of Ag and other transition metals on
different supports for an extensive range of organic synthesis driven by sunlight, an
inexhaustible and green energy source.
Acknowledgements
We gratefully acknowledge financial support from the Australian Research
Council (ARC DP150102110). The authors are thankful to Pengfei Han for
providing ɣ-Al2O3 for the experiments. The electron microscopy work was
95
performed through a user project supported by the Central Analytical Research
Facility (CARF), Queensland University of Technology.
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Table of Contents
99
Supporting Information
Silver and Palladium Alloy Nanoparticles Catalysts:
Reductive coupling of Nitrobenzene through Light
Irradiation Sunari Peiris,a Sarina Sarina,a* Chenhui Han,a Qi Xiao,b and Huai-Yong Zhua
a. School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and Technology, Queensland University of Technology, Brisbane, QLD 4001, Australia. b. CSIRO Manufacturing, Bayview Avenue, Clayton, VIC 3168, Australia. *Corresponding author email: [email protected]
100
(a)
(b)
Figure S1. TEM image of (a) Ag-Pd(1:1)/ZrO2 catalyst; (b) Ag-Pd(1:1)/Al2O3 catalyst.
101
(a)
102
(b)
Figure S2. (a) SEM image of Ag-Pd(1:1) /ZrO2 sample and the corresponding mapping of Zr, Ag and Pd elements.; (b) EDX spectrum of of Ag-Pd(1:1) /ZrO2 sample.
103
Table S1: The calculated corresponding Au-Pd molar in photocatalysts- ZrO2 and Al2O3.
Entry Catalyst Ag (Wt%)
Pd (Wt%)
Ag:Pd ratio Calculated
Wt. Ratio Experimental
Wt. Ratio Calculated
molar Ratio
1 Ag:Pd(2:1) 2 1 2:1 1.89:1 1.97:1 2 Ag:Pd(1:1) 1.5 1.5 1:1 1:1.01 1:1.01 3 Ag:Pd(1:2) 1 2 1:2 1:1.78 1:2.03 4 Ag 3 0 1:0 1:0 1:0 5 Pd 0 3 0:1 0:1 0:1
The alloy photocatalysts with different Ag and Pd contents, pure Ag and pure Pd catalyst supported by ZrO2/ Al2O3 were also prepared in the impregnation-reduction method for reference. The corresponding Ag-Pd molar ratios were calculated and calculated Ag-Pd weight ratios were compared with an experimental weight ratio obtains via SEM- EDX spectrum.
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Characterization of products
The products were identified using an Agilent 6980 gas chromatography (GC) coupling with an Agilent HP5973 mass spectrometer equipped with a HP-5 column. Reference mass spectra from Scifinder are provided for comparison. Nevertheless spectra may reflect different instrument/ ionization methods:
a) 4-Methoxybenzenamine- m/z for C7H9NO is 123.15
Reference spectrum of 4-Methoxybenzenamine found from SciFinder:
105
b) Azobenzene, 4,4'-dimethoxy - m/z for C14H14N2O2 is 242.2
Reference spectrum of Azobenzene, 4,4'-dimethoxy found from SciFinder:
106
c) 4-Bromobenzenamine - m/z for C6H6BrN is 170.9
Reference spectrum of 4-Bromobenzenamine found from SciFinder:
107
d) 4-Methylbenzenamine - m/z for C7H9N is 107.0
Reference spectrum of 4-Methylbenzenamine found from SciFinder:
108
e) 4-Chlorobenzenamine- m/z for C6H6ClN is 127.0
Reference spectrum of 4-Chlorobenzenamine found from SciFinder:
109
f) Azobenzene, 4,4'-dichloro- m/z for C12H8Cl2N2 is 251.1
Reference spectrum of Azobenzene, 4,4'-dichloro- found from SciFinder:
110
g) 4-Iodobenzenamine - m/z for C6H6IN is 218.9
Reference spectrum of 4-Iodobenzenamine found from SciFinder:
111
h) Aniline - m/z for C6H7N is 93.0
Reference spectrum of Aniline found from SciFinder:
112
i) Azobenzene - m/z for C12H20N2 is 182.2
Reference spectrum of Azobenzene found from SciFinder:
113
j) 4-Aminobenzonitrile - m/z for C7H6N2 is 118.1
Reference spectrum of 4-Aminobenzonitrile found from SciFinder:
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Chapter 4: Supported gold based alloy
nanoparticle photocatalysts for
organic synthesis under visible
light irradiation
4.1 INTRODUCTORY REMARKS
This chapter includes one article ready to submit to RSC Advances.
In this Article, we focused on formation of carbon- nitrogen (C-N) bonds using
ZrO2 supported Au-Pd alloy nanoparticles under visible light irradiation at lower
reaction temperatures. Preparation of amines under mild and waste free conditions,
using inexpensive and readily available reactants is still a challenging goal. Herein,
we were able to synthesis N-substituted amines from nitroaromatics and alcohols. In
this article, plasmonic metal gold was alloyed with a non-plasmonic metal,
palladium; the photocatalytic activity increased remarkably compared to both pure
Au NPs and Pd NPs. The Au-Pd alloy nanoparticle absorb visible light and the light
excited energetic electrons on the metal alloy NP surface and activates the reactant
molecules. These heterogeneous catalysts can be conveniently recycled, which is an
important factor in the development of practical and cost-effective catalytic
processes. This study provides a general guideline for the applicability of the alloy
NP photocatalysts, which prepared by alloying transition metals with gold.
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Reductive N-alkylation of nitrobenzene with benzyl alcohol
by Au-Pd alloy nanoparticles under light irradiation
Sunari Peiris, Sarina Sarina*, and Huai-Yong Zhu
Abstract
Gold-palladium (Au-Pd) alloy nanoparticles on ZrO2 strongly absorb visible
light and exhibit significantly high photocatalytic activity for the formation of
carbon- nitrogen (C-N) bonds by reductive N-alkylation of nitrobenzene with benzyl
alcohol. Under optimized conditions, the catalyst with 1:1.86 molar ratios Au-
Pd/ZrO2 achieved the highest photocatalytic activity and selectivity. The
photocatalytic activity of the alloy nanoparticles depends on the reaction
temperature, intensity of the incident light and metal ratio. The finding of this study
may inspire further studies on Au alloy photocatalysts and the number of organic
syntheses using Au-Pd nanoparticles catalyst driven under visible light irradiation.
Introduction
The formation of carbon- nitrogen (C-N) bonds is one of the most significant
transformations in organic synthesis chemistry, because nitrogen containing
compounds are versatile building blocks for the synthesis of polymers, dyes,
pharmaceuticals and bio-active natural compounds.1-5
For the synthesis of C-N
bonds, the most traditional procedure is the alkylation of amines with organic
halides.6-8
However, these traditional processes often proceed under high pressure
and with the use of stoichiometric acids, which make negative consequence on
environment.7, 9-11
Therefore, the development of an easily recoverable
heterogeneous photocatalyst, which can resolve the problem of the homogeneous
systems is desirable. The direct use of readily available and inexpensive nitroarenes
and alcohols as starting materials are greatly attractive for the synthesis of secondary
amines.12-16
Recently, various nitroarenes have been used as a nitrogen source in the
formation of carbon-nitrogen(C-N) bonds through the borrowing-hydrogen process
using a large excess of benzyl alcohol derivatives, in moderate to good yields.17-22
The transfer of hydrogen from alcohols to nitro compounds allows generating
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primary amines and aldehydes. The aldehyde reacts with amine to form imine and
was reduced to obtain the final product.13, 20, 22-26
However, this method using a large
excess of alcohol to provide the hydrogen, which essential for nitrobenzene
reduction.17, 27
Here, we present the one-pot synthesis of secondary amines using an
equal molar ratio of nitrobenzene and alcohol as starting materials and the hydrogen
gas (1 atm pressure) without adding any base and within 6h. Therefore, the one-pot
synthesis of N-substituted amine from nitroaromatics and alcohols is more
economical and environmentally friendly.
Experimental Section
Catalysts Preparation:
Au-Pd/ZrO2 Catalyst: Au-Pd alloy photocatalysts with different Au/Pd ratios on
ZrO2 were prepared by impregnation-reduction method. For example, For example,
1.5 wt %Au−1.5 wt % Pd/ZrO2 (3%Au−Pd(1:1)/ ZrO2) was prepared by the
following procedure: 1.0 g ZrO2 powder was dispersed in 7.6 mL of 0.01 M HAuCl4
aqueous solution and 14 mL of 0.01 M PdCl2 aqueous solution (dissolved in 0.04M
NaCl) were added while magnetically stirring. 16 mL of 0.53 M lysine was then
added into the mixture with vigorous stirring for 30 min. To this suspension, 3 mL of
0.35 M NaBH4 solution was added drop wise over 20 min. The mixture was aged for
24 h and then the solid was separated, washed with water and ethanol, and dried at
60 °C. The dried solid was used directly as a catalyst. Catalysts with other Au/Pd
ratios and pure Au and pure Pd were prepared by a similar method, but using
different quantities of HAuCl4 aqueous solution or PdCl2 aqueous solution.
Au-Pd/LDH-P Catalyst: Au-Pd alloy photocatalysts with different Au/Pd ratios on
LDH-P were prepared by impregnation-reduction method. For example, For
example, 1.5 wt %Au−1.5 wt % Pd/ LDH-P (3%Au−Pd(1:1)/ LDH-P) was prepared
by the following procedure: 1.0 g LDH-P powder was dispersed in 7.6 mL of 0.01 M
HAuCl4 aqueous solution and 14 mL of 0.01 M PdCl2 aqueous solution (dissolved in
0.04M NaCl) were added while magnetically stirring. 16 mL of 0.53 M lysine was
then added into the mixture with vigorous stirring for 30 min. To this suspension, 3
mL of 0.35 M NaBH4 solution was added drop wise over 20 min. The mixture was
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aged for 24 h and then the solid was separated, washed with water and ethanol, and
dried at 60 °C. The dried solid was used directly as a catalyst. Catalysts with other
Au/Pd ratios and pure Au and pure Pd were prepared by a similar method, but using
different quantities of HAuCl4 aqueous solution or PdCl2 aqueous solution.
LDH-P support: The LDH with a Mg: Al molar ratio of 3:1 was produced
using a sol-gel process. 28
Mg(NO3)2•6H2O (115.4 g, 0.45 mol) and Al(NO3)3•9H2O
(56.3 g, 0.15 mol) were dissolved in 600 mL of deionized water to form an acidic
aqueous solution. The alkaline solution was made by dissolving NaOH (60.0 g, 1.5
mol) and Na2CO3 (26.5 g, 0.25 mol) in 1000 mL of deionized water. Acidic and
alkaline solutions were added drop wise simultaneously into 400 mL of deionized
water at 75 °C to obtain the precipitation. The pH value was controlled to be 10. The
suspension was aged for 3 h at 85 °C under stirring. The gel suspension was filtered
and kept at 80 °C for 16 h in an autoclave. The hydrothermally treated gel was
washed with deionised water until the washings reached a pH of 7. The resulting
precipitate was dried in oven overnight at 80 °C and grounded. The calcined (at 450
°C in a flow of 100 mL min-1
dry air for 8 h) LDH (2.0 g) was dispersed in 50 mL
Na3PO4 aqueous solution (0.02 mmol/L). The suspension was stirred at room
temperature for 12 h and finally, the solid (LDH-P) was washed and dried at 110 °C
for 10 h, the resultant solid was grounded and used as the support material.
Characterization of Catalysts
Transmission electron microscopy (TEM) images were acquired on a JEOL
JEM-2100 transmission electron microscope employing an accelerating voltage of
200 kV. The element line scanning was conducted on a Bruker EDX scanner
attached to the TEM. The composition (Au and Pd contents) of samples was
determined by using the energy-dispersive X-ray spectroscopy (EDS) attachment of
an FEI Quanta 200 scanning electron microscope (SEM). X-ray diffraction (XRD)
patterns of the catalyst samples were collected using a Philips PANalytical X’pert
Pro diffractometer. CuKα radiation (λ=1.5418 Å) and a fixed power source (40 kV
and 40 mA) were used. The diffuse reflectance UV-Visible spectra (DR−UV−vis) of
the samples were examined by a Cary 5000 spectrometer. The X-ray photoelectron
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spectroscopy (XPS) data were acquired using a Kratos Axis ULTRA X-ray
Photoelectron Spectrometer.
Photocatalytic Reactions
Reductive N-alkylation of benzyl alcohol with nitrobenzene was conducted in a
hydrogen atmosphere and solution mixture bubbled with hydrogen for 1-2 min: 0.1
m mol benzyl alcohol, 0.1 m mol nitrobenzene, 2 mL of toluene and 30 mg of the
catalyst were added in a chamber in which a 500 W Halogen lamp (from Nelson,
wavelength in the range 400−750 nm) was used as a light source and the light
intensity was usually 0.8 W/cm2
(except for the experiments investigating the impact
of the intensity). The solution mixture was stirred with a magnetic stirrer during the
reaction and illuminated with incandescent light. The temperature of the reaction
system was vigilantly regulated with an air conditioner, which attached to the
reaction chamber. The reaction setups under dark condition were maintained at the
same temperature as the corresponding reactions under light irradiation by using oil
bath placed above a magnetic stirrer to the comparison. The details of the reaction
systems are given briefly as footnotes in Table for each reaction. At given irradiation
time intervals, 1 mL aliquots were collected and then filtered through a Millipore
filter (pore size 0.45 μm) to remove the catalyst particulates. The filtrates were
analysed by gas chromatography (HP6890 Agilent Technologies) with a HP-5
column to measure the concentration change of reactants and products.
Results and Discussion
In the present study, we prepared a series of Au-Pd alloy NP catalysts
supported on zirconia (ZrO2) with various different Au: Pd ratios by the
impregnation-reduction method.29-32
The as-prepared catalysts were characterized by
several techniques to confirm the composition and morphology. TEM image (Figure
1) shows that Au-Pd alloy nanoparticles (Au-Pd NPs) are well dispersed on ZrO2
support and the mean sizes of the particles are 6 nm. The metal composition and
homogeneous distribution of the catalyst was determined by using the energy-
dispersive X-ray (EDX) spectroscopy attachment of the scanning electron
120
microscope (SEM) (Figure S1). The X-ray diffraction (XRD) characterization was
performed on catalyst samples and no reflection peaks corresponding to either
metallic Au or Pd were observed owing to the low metal content (Figure 2). This
result suggests that the detection of alloy NP signals in XRD patterns is also closely
related to the ZrO2.
Figure 1: Catalyst characterization. (a) TEM images of the Au‐Pd(1:1)/ZrO2
catalysts; (b) Particle size distribution.
Figure 2: The XRD pattern of photocatalysts.
The X-ray photoelectron spectroscopy (XPS) of the samples shown in Figure 3
confirms that gold and palladium exist in the metallic state on ZrO2 support. The
binding energies of Au 4f7/2 and Au 4f5/2 electrons are 84.1 and 87.9 eV, respectively.
Moreover, the binding energies of Pd 3d3/2 and Pd 3d5/2 electrons are 340.1 and 334.9
eV, respectively. The significant feature of the Au−Pd alloy NPs, which makes them
121
useful as a photocatalysis, is that they intensely absorb visible light mainly through
the localized surface plasmon resonance (LSPR) effect of AuNPs. UV–Vis spectra
of these samples in Figure 4 indicate that the supported Au-Pd alloy NPs strongly
absorb visible light irradiation. The absorption peak in the visible light range (at 520
nm) observed for the Au NPs on ZrO2 supports is attributed to the LSPR absorption
of Au NPs.33, 34
Evidently, the visible light absorption by Au NPs is a prerequisite for
the photocatalytic activity. In contrast, zirconia exhibits a weak absorption band from
250 to 400 nm owing to the large band gap.35
The ZrO2 enables the uniform
distribution of Au-Pd alloy NPs on the support surface, and the readily recycling of
the catalysts after reaction.
Figure 3: X‐ray photoelectron spectra (XPS) binding energy of (a) Au 4f7/2 and Ag
4f5/2 for Au‐Pd NPs on ZrO2; (b) Pd 3d5/2 and Pd 3d3/2 for Au‐Pd NPs on ZrO2.
Figure 4: UV‐Visible diffuse reflectance spectra of the supported Ag‐Pd (1:1)
alloy NPs catalysts and their comparison with pure AuNPs/ZrO2 and PdNPs/ZrO2.
122
In literature, it has been reported that amine compounds could be synthesized
from the corresponding nitroaromatic compounds and benzylalcohol through various
supported metal catalysts comprising Au, Pd, Cr and Ag at higher temperatures,
pressures, strong base and/or longer reaction time (Table 1).15, 16, 27, 36
In the present
study, Au-Pd alloy NPs on ZrO2 could be used as an efficient photocatalyst for the
formation of C-N bonds from reductive N-alkylation of nitrobenzene with benzyl
alcohol (Entry 5, Table 1)
Table 1: Comparison of the reaction conditions and catalytic activity of various
heterogeneous catalysts reported in the literatures for the reductive N-alkylation of
nitrobenzene with benzyl alcohol.
OH
NO2 HN
+
Entry Catalyst Reaction Conditions Yield (%) Ref
1 Au/Ag–Mo nano-
rods
150 ◦C, 1 atm Ar, glycerol, 24h,
K2CO3
91 27
2 Au/Fe2O3 160 ◦C, 1 atm Ar, 8h, K2CO3 87
15
3 Ag/Al2O3 155 ◦C, 2 atm H2, 24h, K2CO3 93
16
4 Cu–Cr/ɣ-Al2O3 200 ◦C, 30 atm H2, 24h, K2CO3 90
36
5 Au-Pd/ZrO2
Present Study
80 ◦C, 1 atm H2, 6h, visible light 83
We prepared a series of supported Au-Pd alloy NPs catalysts with various
metal weight ratios. The 3% Au:Pd (1:1 Wt= 1:1.86 mol) alloy NPs catalysts was
found to be the most effective for the reductive N-alkylation reaction with excellent
yield and selectivity under mild reaction conditions (Table 2). Based on the current
experimental conditions, it is clear that the 3% Au-Pd(1:1) alloy NPs on ZrO2
photocatalyst shows decent conversion as well as product yield. The possible
explanation for these observations is that the charge heterogeneity at the alloy NPs’
surface results in the improved catalytic activity of the alloy structure. The alloying
affects the surface electronic properties of the NPs and the catalytic activity of the
alloy NP is significantly improved compared to mono metals. In terms of the
selectivity, all dark reactions were found to show a very poor amine yield during the
given reaction time. For example, the reductive N-alkylation of nitrobenzene with
benzyl alcohol could be proceeded efficiently at 80 ◦C in atmospheric H2, giving
>80 % yield (Entry 4, Table 2). Compared with those reported process for the
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catalytic N-alkylation of nitrobenzene with benzyl alcohol usually conducted under
harsh reaction conditions, this exhibits apparent advantage in the viewpoint from
green chemical synthesis. Blank experiments under otherwise identical reaction
conditions, but without metal alloy NPs and without any catalyst were also
conducted and were catalytically inactive in this transformation (Entry 8 & 9, Table
2).
Table 2: Photocatalytic activity test with different Au:Pd weight ratios the reductive
N-alkylation.
OH
NO2 HN
O
NH2
(c) (d) (e)
+ + +
(a) (b)
Entry Catalyst Light Reaction (Dark)
(Weight ratio) Conversion (a) (%) Yield (%)
(c) (d) (e)
1 Au/ZrO2 26 (20) / 64 (39) 36 (61)
2 3Au-Pd/ZrO2 62 (48) 6 (3) 52 (15) 42 (82)
3 2Au-Pd/ZrO2 77 (54) 65 (0) 24 (0) 11 (100)
4 Au-Pd/ZrO2 100 (64) 83(55) 8 (8) 9 (37)
5 Au-2Pd/ZrO2 85 (55) 67 (56) 20 (4) 13 (36)
6 Au-3Pd/ZrO2 68 (46) 56 (36) 25 (11) 19 (53)
7 Pd/ZrO2 49 (42) 32 (14) 35 (15) 33 (71)
8 ZrO2 6 (0) / 100 (0) /
9 No catalyst 6 (0) / 100 (0) /
Reaction conditions: photocatalyst 30 mg, Nitrobenzene 0.1 mmol,
Benzylalcohol 0.1 mmol, solvent-toluene 2 mL, 1 atm H2, reaction temperature 80
◦C, light intensity- 0.7 W/cm
2, reaction time 6 h. The conversions and yield were
calculated from the product formed and the reactant converted based on the
benzylalcohol conversion measured by gas chromatography.
The direct synthesis of amine from benzylalcohol and nitrobenzene in the
presence of different catalyst under irradiation with light was used for the
optimization of reaction conditions (Table 3). The influence of several critical
reaction conditions, such as solvents and reaction atmosphere, have been tested with
124
Au-Pd alloy NPs on ZrO2 and HT-PO43-
(LDH-P) catalysts. The Au-Pd supported on
ZrO2 has the highest activity and selectivity towards coupling reaction than on LDH-
P. The two most commonly used solvents were examined for the reaction with other
conditions were maintained unchanged. It is obvious that the Au-Pd alloy/ZrO2
catalyst exhibited the best performance when the toluene is solvent, under light
irradiation (entry 5). We carried out the reaction in argon, oxygen, hydrogen and air
atmospheres and found that hydrogen atmosphere promotes the reaction. In the
oxygen and air atmosphere, the prominent product was benzaldehyde (Entry 3, 6, 9
& 12 ). Notably, the reaction did show excellent activity, even without base additive,
which increases the green synthesis aspects.
Table 3: Optimization of reaction conditions for amine synthesis.
OH
NO2 HN
O
NH2
(c) (d) (e)
+ + +
(a) (b)
Entry Catalyst Solvent Atmosphere Conv.
[%] (a)
Yield. [%]
Au-Pd alloy/ZrO2
(c) (d) (e)
1 BTF Ar 100 20 80 0 2 BTF H2 52 9 / 42
3 BTF O2 100 / 100 / 4 Tol Ar 92 14 78 / 5 Tol H2 100 83 8 9
6 Tol O2 100 / 100 /
7
Au-Pd alloy/LDH-P
BTF Ar 94 10 84 / 8 BTF H2 78 47 12 41
9 BTF O2 100 / 100 / 10 Tol Ar 100 13 87 / 11 Tol H2 100 51 16 33
12 Tol O2 100 / 100 /
Reaction conditions: photocatalyst 30 mg, Nitrobenzene 0.1 mmol, Benzylalcohol
0.1 mmol, solvent 2 mL (Tol-toluene, BTF-Benzotrifluoride), 1 atm atmosphere,
reaction temperature 80 ◦C, light intensity- 0.7 W/cm
2, reaction time 6 h. The
conversions and yield were calculated from the product formed and the reactant
converted based on the benzylalcohol conversion measured by gas chromatography.
The influence of the irradiation light intensity on the reductive N-alkylation
was investigated (Figure 5). We have experimentally verified that higher light
125
intensities exhibit more efficient performance. Moreover, the results clearly show a
linear dependence, which further confirms the photocatalytic activity of Au-Pd alloy
metal NPs. When the irradiance was increased from 0.34 to 0.80 W.cm−2
with other
conditions unchanged, the conversion rate of benzyl alcohol increased from 70% to
100%, respectively.
Figure 5: Intensity influences on the reductive N-alkylation of nitrobenzene
and benzyl alcohol on Au:Pd(1:1)/ZrO2 ‐6 hours
We studied the evolution of the product during the time course of the reductive
N-alkylation of nitrobenzene and benzyl alcohol using Au-Pd/ZrO2 catalyst
(Figure 6). It can be seen that the conversion of the reaction increased progressively
over time, and the amine is the main product during the reaction. The selectivity and
conversion both reached to maximum from the six hours. Then, keeping other
reaction conditions identical, the longer reaction times (24h) were investigated.
However, the product yield doesn’t increase with the reaction times as expected.
126
Figure 6: Time-conversion plot for reductive N-alkylation using Au-Pd alloy/ZrO2.
The photocatalytic activities of the Au-Pd alloy NPs/ZrO2 catalysts was
tested with different reaction temperatures and found that the activity increases with
increasing reaction temperature. The experiment conducted under seven different
reaction temperatures, while maintaining the other reaction conditions constant. As
shown in Figure 7, the catalysts exhibit excellent conversion with the increasing
temperature. The product selectivity could be controlled by the reaction temperature
and dark/light conditions. Secondary amine was obtained as the main product under
light irradiation for the all the temperatures. Further, the selectivity for the desired
amine reaches to a maximum at 80 ◦C and then decreases. Based on the current
experimental conditions, it is clear that the Au-Pd alloy NPs on ZrO2 photocatalyst
shows decent conversion as well as product yield. In terms of the selectivity, all dark
reactions were found to show a poor amine yield during the given reaction time.
127
Figure 7. Conversion of reductive N-alkylation and selectivity of secondary amine,
aniline and benzaldehyde. (a) Under light irradiation (b) Under dark condition using
3% Au:Pd(1:1)/ZrO2 at different temperatures.
We provide a tentative mechanism for amine synthesis method starting from
benzylalcohol and nitrobenzene on ZrO2 supported Au-Pd alloy NPs photocatalysts.
Figure 8 and S2 were on the basis of our experimental observation and the literature.
26, 29, 30 The Au-Pd/ZrO2 catalyst was shown efficient towards production of amine
without any addition of base. This process involves mainly three steps, which
comprises the reduction of the nitrobenzene to aniline (Figure S2 (a)), oxidation of
the alcohol to aldehyde(Figure S2 (b)), and condensation of the aldehyde and the
aniline to form the corresponding amine by reducing imine(Figure S2 (c)).22, 26
Sarina et al. found that visible light irradiation of Au-Pd alloy NPs could
enhance the catalytic activity for oxidant-free dehydrogenation of aromatic alcohols
to the corresponding aldehydes at even ambient temperatures.29, 30
Firstly, the
abstraction of α-H atoms from the alcohol molecules take place from the −CH2−
group and alloy-H species is formed. Then the dehydrogenated species, undergo a
C−H bond cleavage, yielding aldehyde as the product (Figure 8-I).37, 38
In contrast,
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the reduction of the nitrobenzene to aniline is mainly facilitated by palladium.22
Generally, Pd known as an classic hydrogen storage metal and able to store hydrogen
under mild conditions.39, 40
The most widely accepted mechanism proposes two
possible reaction pathways (Figure S2). One is the direct reduction of nitrobenzene,
which involves a sequential hydrogenation/dehydration process via nitroso and
hydroxy intermediates.41, 42
The second route involves a condensation reaction
between the nitroso and hydroxy products leading to the azoxy compound, which is
then hydrogenated/dehydrated to afford the azo compound. 43
The latter compound is
subsequently hydrogenated to the hydrazo, which finally generates the amine (Figure
8-II). Moreover, under harsh reaction conditions, such as high reaction temperatures
azobenzene rapidly reduced to aniline according to known Haber’s mechanism.41
None of the azobenzene or azoxy compounds were detected during the reaction.
Finally, benzaldehyde, condenses with the amine produced, generating an
imine and tends to follow the condensation route (Figure 8-III & S2-(c)).22
In this
sense, we speculated that the imine further reduced by abstracting H to produce
corresponding amine (Figure 8-IV) and ZrO2 supported Au-Pd alloy NPs catalyst
promotes the reductive N-alkylation of nitrobenzene and benzyl alcohol under light
irradiation.
Figure 8: Proposed reaction pathway of reductive N-alkylation of nitrobenzene
and benzyl alcohol on Au:Pd (1:1)/ZrO2.
129
For practical applications of heterogeneous catalyst, the recyclability of the
catalyst is a crucial factor.44
3% Au-Pd/ZrO2 catalyst was used for five consecutive
runs for reductive N-alkylation of nitrobenzene with benzyl alcohol to investigate the
reusability of the photocatalyst (Figure 9). The catalyst was separated by
centrifugation and washed thoroughly with ethanol three times. The dried catalyst
was directly used for subsequent reactions, while keeping the other reaction
conditions identical. The results confirmed the reusability of the Au-Pd alloy
NPs/ZrO2 catalyst without significant activity loss.
Figure 9: The photocatalytic stability of 3 wt% Au−Pd alloy NPs/ZrO2 in five cycles
at 80 °C.
Conclusions
In conclusion, we demonstrated that supported Au-Pd alloy NPs can
efficiently drive reductive N-alkylation of nitrobenzene with benzyl alcohol,
achieving excellent activity and yields under mild reaction conditions. Moreover,
decent yields of N-alkyl amines were achieved by 1:1 molar ratio of benzyl alcohol
and nitrobenzene. Further, Au-Pd alloy NPs/ ZrO2 photocatalyst can be efficiently
recycled after consecutive five reaction rounds without significantly losing activity.
The knowledge learnt in this study may inspire further studies on a wide range of
organic syntheses using supported alloy NP photocatalysts.
130
Acknowledgements
We gratefully acknowledge financial support from the Australian Research Council
(ARC DP150102110). The electron microscopy work was performed through a user
project supported by the Central Analytical Research Facility (CARF), Queensland
University of Technology.
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133
Supporting Information
Reductive N-alkylation of nitrobenzene with benzyl
alcohol by Au-Pd alloy nanoparticles under light
irradiation
Sunari Peiris,
a Sarina Sarina,
a* and Huai-Yong Zhu
a
a. School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and
Technology, Queensland University of Technology, Brisbane, QLD 4001, Australia.
*Corresponding author email: [email protected]
134
Figure S1. (a) SEM image of Au-Pd(1:1)/ZrO2 sample and the corresponding
mapping of Zr, O, Au and Pd elements.; (b) EDX spectrum of of Au-Pd(1:1) /ZrO2
sample.
135
Figure S2. Proposed reaction mechanism for reductive N-alkylation of nitrobenzene
with benzyl alcohol. a) two pathways in the reduction of nitrobenzene; b) alcohol
oxidation, and c) amine/aldehyde condensation product. 1
Reference
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136
Characterization of products
The products were identified using an Agilent 6980 gas chromatography (GC)
coupling with an Agilent HP5973 mass spectrometer equipped with a HP-5 column.
Reference mass spectra from Scifinder are provided for comparison. Nevertheless
spectra may reflect different instrument/ ionization methods:
a) Benzenemethanamine, N-phenyl- - m/z for C13H13N is 183
Reference spectrum of Benzenemethanamine, N-phenyl- - found from SciFinder:
137
b) Benzaldehyde - m/z for C7H6O is 106
Reference spectrum of Benzaldehyde- found from SciFinder:
138
c) Aniline- m/z for C6H7N is 93
Reference spectrum of Aniline- found from SciFinder:
139
Chapter 5: Conclusions & Future work
Conclusions
This work has contributed to the knowledge of novel metal NP photocatalysts
(non-plasmonic NPs and their alloy NPs) on various organic synthesis reactions
under visible light irradiation. Three types of meal NP photocatalysts have been
developed using Pd metal and used for coupling reactions under visible light
irradiation. This thesis includes the photocatalytic enhancement of the alloy NPs and
non-plasmonic Pd NPs. From the results of this study, the following conclusions can
be drawn:
In chapter 2, it was revealed that irradiation with light can significantly
enhance the intrinsic catalytic performance of non-plasmonic Pd transition metal NPs
at ambient temperatures and atmospheric pressure for several types of cross-coupling
and homo-coupling reactions. Pd metal NPs functions as together a visible light
harvester and a provider of catalytic sites. Generally, Pd metal NPs absorb the visible
light via interband electronic transitions. The energetic electrons excited by light
irradiation drive the photocatalytic reaction. The rate of the catalysed reaction
depends on the concentration and energy of the excited electrons and it can be
improved by tuning the incident light intensity and wavelength. The highest yield
was obtained (80-98%) for the 3% PdNPs on ZrO2 for all the four reactions. This
study provides insight into catalyst design for the activation of C-X bond and shown that
plasmonic excitation is not the merely mechanism involved in metal NPs under light
irradiation. This study broadens the application of non-plasmonic metals as
photocatalysts under visible light irradiation and the possibility of green approach for
the fine organic chemical synthesis.
In Chapter 3, we found an effective approach to expand the application of
AgNP as a photocatalysts by incorporating transition metals such as Pd. Generally,
palladium is well known to be catalytically active for many important organic
reactions. Therefore, the coupling of light absorption of Ag NPs (via LSPR) and
catalytic property of Pd in alloy structures can drive various chemical reactions.
Herein, the reductive coupling of nitrobenzene with Ag‐Pd alloy NPs/ZrO2
140
photocatalyst under visible light at ambient conditions was studied. Ag-Pd alloy NPs
on ZrO2 support is favourable for reduction of nitroaromatics (azobenzene yield of
~80 % in 3 hours) than Ag-Pd alloy NPs/Al2O3. Further, study reveals that the
highest yield of target products achieved when the alloys NPs have the Ag:Pd molar
ratio of 1:1.01. Alloy NPs with other Ag: Pd molar ratios exhibited much lesser
activity, either under light irradiation or in the dark. The catalytic activity of the
photocatalysts can be tuned through metal ratio, irradiation intensity & wavelength
and reaction temperature. The prospect of visible light irradiation driving chemical
synthesis has potential to deliver greener, controlled industrial processes especially
for temperature sensitive synthesis.
In Chapter 4, the photocatalytic application of Au-Pd NPs was extended to a
novel reaction. We have successfully fulfilled reductive N-alkylation of nitrobenzene
with benzyl alcohol by Au-Pd alloy nanoparticles under light irradiation at ambient
reaction conditions with Au-Pd alloy NPs/ ZrO2. These alloy NPs exhibit superior
catalytic performance when compared to pure non-plasmonic and plasmonic metal
NP photocatalysts when exposed to visible light under moderate reaction conditions.
Under optimized conditions, the catalyst with 1:1.86 molar ratios Au-Pd/ZrO2
achieved the highest photocatalytic activity and selectivity (>80 % yield) for
reductive N-alkylation of nitrobenzene with benzyl alcohol. The reaction rate
depends on the number of light-excited electrons and the number of reactant
molecules on the catalyst surface. The strong affinity of palladium towards organic
molecules increases the reactant molecules on the metal NPs and facilitates the light-
excited electron transfer. Moreover, the light absorption of Au-Pd alloy plays an
important role and by tuning light intensity and reaction temperature we can obtain
optimized the reaction activity. In addition, the reaction did show excellent activity,
even without base additive, which increases the green synthesis aspects. The
knowledge learnt in this study may inspire further studies on a wide range of organic
synthesis using supported Au-Pd alloy NP photocatalysts.
In general, work presented in this thesis will help guide successful design
photocatalysts using number of supported metal NPs for fine organic chemical synthesis.
141
Future Work
The chemical industry, which turns raw materials such as petroleum by-
products, minerals and farm products into valuable chemicals that are the ingredients
of life's essential objects, plays a vital role in our everyday life. According to US
Energy information Administration (EIA), the largest consumer of delivered energy
is the basic chemicals industry, which in 2012 accounted for about 14-19% of total
industrial energy consumption and expected rises in 2040. Therefore, it is practical to
discover a clean, renewable energy source to synthetic chemicals and sunlight stands
out as the most promising choice.
Direct metal photocatalysis (plasmonic, non-plasmonic and alloy) is a rapid
emerging research field and recently it achieved significant advancements.
Nevertheless, there is more space for further improvement on photocatalytic
performance and mechanism. Based on the outcomes of this thesis as well as
reported work, Future work can be proposed from the following aspects:
1. In chapter 2, we discussed supported non-plasmonic Pd metal NP photocatalyst
for coupling reactions under light irradiation. Based on current research, it is
beneficial to conduct the further studies on different other potential non-
plasmonic/ transition metal NPs, such as Ni, Ir, Ru and Co for fine chemical
synthesis under light irradiation. Many experiments on metal photocatalytic
reactions have examined the plasmonic properties of gold, silver and their alloy
combinations. If we could use supported transition metal NPs as photocatalysts,
such a system could attract manufacturing industries and lead to controlled,
simplified, and greener chemical synthesis. Furthermore, it is useful to study
and understand the underlying light absorption properties, mechanisms and
chemical stabilities for photocatalytic reactions. Additionally, practical
implementation of non-plasmonic/ transition metal NPs enhanced chemical
reactions will require the use of inexpensive compared to noble metals.
2. The findings obtained from chapter 3 provide useful guidelines for designing
efficient photocatalysts form Ag metal NPs. Alloying transition metals,
improves the utilisation of Ag for photocatalytic chemical synthesis in a cost-
effective way. The study of Ag-Pd alloy NPs photocatalysts could extend to
further research fields from two aspects: The first is to extend the application of
Ag-Pd alloy NPs to other chemical synthesis, such as cross-coupling reactions,
142
esterification reactions, etc. This will widen the use of Ag-Pd alloy NPs to
more applications in organic synthesis. The second is to develop new alloy NPs
photocatalysts using Ag plasmonic metal with a new photocatalyst structure for
chemical synthesis.
3. The photocatalysts made from plasmonic, non-plasmonic metals and their
alloys were illustrated a promising improvement under visible light irradiation.
There is a potential to extend the support scope of the catalyst by introducing
other materials such as conducting metal nitrides (TiN and ZrN), since they
exhibit metallic properties at visible frequencies. Furthermore, metal nitrides
stabilize oxidisable metals NPs (Eg: Cu NPs) and this widens the range of
metals that can be used in catalyst preparations. I have used ZrN as a support
for different metal NPs such as Pd and Ag-Pd alloy NPs and it exhibited decent
activity in the cross-coupling and nitrobenzene coupling reactions under visible
light irradiation. This work is underway and expected to be published in future.
4. The localised surface plasmon resonance (LSPR) wavelength of a plasmonic
nanostructure can be tuned by changing the particle geometry such as
composition, size, shape, etc. Metal NPs that can absorb the entire solar
spectrum could be designed by systematically regulating those NP’s
parameters. There have been a number of reports on the particle composition
tuning to activate various reactant molecules; however, there is limited
knowledge about the particle shape effect on the photocatalytic synthesis of
fine chemicals. Therefore, it is important to study the metal NPs shape effects
in relation to the shift of their plasmon excitation band and the ability of
activating the reactant molecules. The findings might lead to develop a
connection between reactant electronic structure and photocatalysts structure,
which allows regulating the product selectivity.