Synthesis and characterization of novel mesocomposites Co3O4 and CuO@OMS (ordered mesoporous silica) as active catalysts for hydrocarbon oxidation

RESEARCH PAPER

Synthesis and characterization of novel mesocompositesCo3O4 and CuO@OMS (ordered mesoporous silica)as active catalysts for hydrocarbon oxidation

Cezar Comanescu

Received: 31 October 2013 / Accepted: 5 February 2014

� Springer Science+Business Media Dordrecht 2014

Abstract Novel metal nanoporous transition metal

oxides MxOy (Co3O4, CuO) have been synthesized by

thermal decomposition of inorganic salts precursors

(acetates, nitrates) impregnated into hexagonal mes-

oporous silica (OMS, ordered mesoporous silica) of

SBA-15 type (prepared in-house) at different precur-

sor loadings, the mesocomposites thus obtained being

monitored after each impregnation–calcination step

by small and wide angle powder XRD. The pore size

for the ordered silica host range from 5.08 to 7.06 nm.

Retention of the hexagonal silica framework has been

observed in spite of the temperatures up to 500 �C.

Mesoporous Co3O4 has been obtained by leaching the

silica through overnight HF dissolution, which par-

tially preserved the small-range ordering found in the

parent Co3O4@OMS composite prior to leaching.

Both Co3O4 (meso) and Co3O4@SBA-15 have been

tested in methane oxidation and were found to be

superior to the bulk Co3O4 performance, with

mesoporous Co3O4 being able to fully oxidize meth-

ane to CO2 and H2O at 350 �C, while Co3O4@OMS

exhibits a lower activity with 20 % conversion at

350 �C. CuO@OMS shows the lowest activity, with

only *13 % conversion at 500 �C.

Keywords Nanoporous � Mesocomposite �Nanocasting � Catalytic activity � SBA-15 �XRD

Abbreviations

OMS Ordered mesoporous silica

TM Transition metal

XRD X-ray diffraction

SAXS Small angle XRD

WAXRD Wide angle XRD

SEM Scanning electron microscopy

TEM Transmission electron microscopy

SAED Selected area electron diffraction

ICDD International center for diffraction data

Introduction

Nanostructured metal oxides have attracted consider-

able attraction during the last decade due to useful

properties which made them excellent candidates for

catalysis, applicative electrochemistry (Maruyama and

Arai 1996) and gas sensor devices (Li et al. 2005).

Nanosized Co3O4 particles have been synthesized

recently through several synthesis routes, like sol–gel

method, template-assisted growth, hydrothermal,

Electronic supplementary material The online version ofthis article (doi:10.1007/s11051-014-2323-4) contains supple-mentary material, which is available to authorized users.

C. Comanescu

Inorganic Chemistry Department, University Politehnica

of Bucharest, Polizu St., No. 1–7, 011061 Bucharest,

Romania

C. Comanescu (&)

National Institute of Materials Physics, Atomistilor St.,

No. 105 bis, 077125 Magurele, Romania

e-mail: [email protected]

123

J Nanopart Res (2014) 16:2323

DOI 10.1007/s11051-014-2323-4

http://dx.doi.org/10.1007/s11051-014-2323-4

precipitation, and spray pyrolysis (Kandalkar et al.

2008; Shinde et al. 2006; Armelao et al. 2001; Wang

et al. 2009). Various morphologies can be attained

ranging from particles to rods and wires, depending on

the synthesis procedure (Shu et al. 2009). Thermal and

chemical stability as well as surface area and particle

size distribution greatly affect the performance of

Co3O4 in various catalytic applications while modify-

ing the magnetic properties compared to those of the

bulk cobalt oxide (Benitez et al. 2011).

The synthesis route can modify substantially the

properties of the metal oxide material (size, shape,

stability) and greatly alter its reactivity. Among the

synthesis routes afore-mentioned, nanocasting is con-

ceptually and practically the method that can afford

high-quality metal oxides because the sacrificial hard-

template is a highly ordered material to begin with,

and the metal oxide precursor can fill the pores and

decompose in an ordered framework, while simple

decomposition of the metal salt precursor would

typically lead to bulk, amorphous metal oxide phase.

Cobalt oxide Co3O4 has been explored in the past

(reports go back to 1968) as a catalyst for methane

combustion with moderate results, due to the high

temperature (and implicitly high energy) required to

achieve complete conversion (Levy 1968). Researches

focused on Pd/Al2O3 catalysts that start converting

methane at 300 �C and reach 100 % conversion at

480 �C (Baldwin and Burch 1990). It was found that

CH4 reduces PdO formed upon reaction of Pd with O2,

and that the H and CHx (x = 1–3) fragments formed

by CH4 dissociative adsorption need small quantities

of Pd metal for enhanced combustion. However,

metallic Pd was found inactive, and the presence of

crystalline PdO was thought to be beneficial to easier

reduce than the amorphous variant. A cooperative Pd–

PdO activity was employed by the authors, with Pd

active in dissociative adsorption of methane, and

PdO—formed fast and covering the metal surface in

various thicknesses—enhances the low temperature

methane oxidation due to its easier reducibility. O2 is

adsorbed stronger than CH4 onto alumina support,

which may inhibit reaction under oxidizing conditions

(Carstens et al. 1998; Hicks et al. 1990).

The size of the Pd catalyst was also found to greatly

influence the turnover frequency, from 0.02 (small

particles) to 1.3 s-1 (large Pd particles), which

suggests that in methane oxidation the catalyst struc-

ture is important, due to different reactivity of the

adsorbed oxygen species (more reactive on smaller

particles with a larger number of active sites and

defects) (Hicks et al. 1990; Cullis and Willatt 1983).

However, even if Pd has long been employed for

successful methane combustion, the precise nature of

the catalyst site is still under debate, and it was

accepted that higher crystallinity of PdO leads to

higher activity when compared to dispersed PdO

(Hicks et al. 1990). The differences observed by

various groups reveal a complicated behavior for Pd/

Al2O3, dependent on preparative routes and reaction

conditions.

Water as well as higher concentrations of CO2 have

been reported to inhibit the oxidation, with the former

being responsible of Pd(OH)2 formation at the surface,

thus rendering the active sites inactive (catalyst poison-

ing). It seems important that water should not be present

in the catalyst at the beginning of the reaction, since the

poisoning effect should be alleviated at higher

([300 �C) temperatures (Ribeiro et al. 1994).

Other oxides have been employed for methane

oxidation, like titania or tin oxide, albeit with lower

activity than Pd/Al2O3 (Cullis and Willatt 1983). Early

reports of Co3O4 used in conjunction with Pd go back to

1999 (Li and Hoflund 1999), when Li et al. showed Pd/

Co3O4 to exhibit a higher activity in methane oxidation

than the standard Pd/Al2O3. Noble metals (Au) copre-

cipitated on transition metal oxide supports also exhib-

ited unexpectedly high activity (starting above 250 �C),

although the studies were complicated by the fact that

even the support (Co3O4, NiO, MnOx, etc.) acted as

active sites at higher temperatures. Higher oxidation

state of Au was suggested to be responsible for

increasing oxidation activity (Waters et al. 1995).

One of the main drawbacks of these oxides is their

low specific surface area which hinders their use as

catalysts (Pena and Fierro 2001). Other metal oxides

(CeO2) have been investigated due to their capacity to

store oxygen but also as combination with other oxides

as binary oxides (Pengpanich et al. 2002; Zamar et al.

1995). The role played by metal oxides could be due to

generation and participation of surface oxygen species

and vacancies (Zhu et al. 2005). The main advantages

of using metal oxides instead of noble metals are the

lower cost of raw materials used, higher thermal

stability and reducing the NOx emissions coming from

the N2 bounded to the fuel. However, among the

disadvantages one may mention the lower selectivity

and thus the higher ignition temperature.

2323 Page 2 of 21 J Nanopart Res (2014) 16:2323

123

In the present study, we present the results obtained

by employing neat Co3O4 nanoparticles as well as

Co3O4(nano)@OMS, and we compare the perfor-

mance of these catalysts in methane combustion. We

also prepared a mesoporous CuO@OMS to check the

relative activity that is achieved by another transition

metal oxide embedded in mesoporous silica at similar

concentration, reaction, and condition routes. Excel-

lent selectivity was achieved by both Co-based

catalysts, the final reactivity order being Co3O4(na-

no) [ Co3O4@OMS [ CuO@OMS. This result falls

in line with the findings of McCarthy et al., who

studied and established the following single metal-

oxide catalyst performances in CH4 combustion

to decrease in the order Co3O4 [ CuO [ NiO [Mn2O3 [ Cr2O3 (McCarthy et al. 1997). One major

drawback of using neat metal oxides is that high

temperatures required to oxidize methane lead to

sintering and finally to catalyst deactivation (Zavyal-

ova et al. 2007). A mean to bypass this shortcoming is

to use a support for stabilizing and dispersing the metal

catalyst (Milt et al. 2002).

The catalytic hydrocarbon combustion is a

‘‘greener’’ alternative to classical flame-combustion

(Saracco et al. 1996). The first representative of the

alkane homologes—CH4 (methane)—has an ignition

point (consumption of 10 % of the fuel itself) of about

400 �C, which led scientists to envision new catalysts

featuring: high activity, long-term stability, low igni-

tion temperature and cost (Yisup et al. 2005).

While dispersed noble metals (Pd, Pt) on support

materials have been investigated in the past (Burch

and Loader 1994; Yang et al. 2000; Sekizawa et al.

1996), the cost, extreme functioning regime, and

sintering susceptibility plagued their use as hydrocar-

bon oxidation catalysts. A cheaper route was recently

introduced, which employs the use of perovskite-type

mixed oxides (Co, Cr), but the low specific surface

area was a major drawback, as this translated in lower

catalytic activity (Pena and Fierro 2001).

Metal oxides are a promising alternative to noble

metal catalysts used in the past due to some specific

features: they can store oxygen, can participate in

generating surface oxygen species and vacancies and

exhibit high thermal stability, albeit at the cost of

lower selectivity and rather high ignition temperature

(Zhu et al. 2005).

The large pore diameter of mesoporous materials

(Tanev and Pinnavaia 1996; Zhang et al. 1997)

enables the catalysis of large molecules or incorpora-

tion of large active species; among these materials

SBA-15 attracts more attention owing to its excellent

hydrothermal stability, less condensed structure, and

unique micro-mesoporosity, which makes it a para-

mount candidate for supports or catalyst. However,

siliceous SBA-15 lacks active sites due to its chemical

composition; hence it is necessary to introduce other

active species in the mesopores. Obtaining mesopor-

ous materials at conditions near ambient opens new

perspectives toward an easier synthetic procedure.

Moreover, lack of thermal treatment (replaced with

solvent extraction in H2O–EtOH solution) and regen-

eration of the surfactant (from the ethanol solution)

would contribute to a new cost-effective route to

mesoporous siloxanes.

Driven by the interesting properties of mesoporous

materials as supports for active catalysts, we have

synthesized Co3O4 nanoparticles by varying the type

and quantity of the co-precursor (Co(NO3)2 or

Co(CH3COO)2) and the infiltration route of the

precursor into the mesoporous OMS support (plain

mixing vs. incipient wetness method). The new

materials have been characterized by powder XRD

(low and wide angle), N2-sorption isotherms, scanning

electron microscopy (SEM) and transmission electron

microscopy (TEM), while the actual catalyst (Co3O4

or CuO) surface loading was assessed by energy-

dispersive X-ray spectroscopy (EDAX).

While a recent report shows that nanostructured

Co3O4 can be obtained by direct soft-templating

method using P123 as anionic surfactant and precise

reaction conditions (Dahal et al. 2012), the hard-

template method might still be more attractive because

it offers the possibility to reproducibly synthesize the

OMS first and thus gives a better control over the final

structural features of the mesoporous composite

Co3O4@OMS and implicitly that of mesoporous

Co3O4 obtained after HF or KOH silica leaching.

Materials and methods

Chemicals and reagents

All reagents used: TEOS (tetraetoxysilane, 98 %

purity, Sigma-Aldrich), Pluronic P123 (Sigma-

Aldrich), CH3CH2OH (94–96 %, Alfa Aesar), NaCl

(ACS reagent,[99 %, Sigma-Aldrich), and HCl conc.

J Nanopart Res (2014) 16:2323 Page 3 of 21 2323

123

(36–38 wt%, Fluka) were analytical grade and used as

received without further purification.

Nanomaterials studied

The SBA-15 type mesoporous silica has been prepared

following an optimized route which includes NaCl

addition for improved mesophase formation (Comane-

scu and Guran 2011). In short, NaCl addition and pH

adjustments were employed for an assisted mesophase

formation at temperatures lower than those typically

required for standard SBA-15 synthesis (100–145 �C)

(Zhao et al. 2000). Over a vigorously stirring solution

of Pluronic P123 [PEO20PPO70PEO20]—nonionic

block copolymer, poly(ethylene glycol)–poly(propyl-

ene glycol)–poly(ethylene glycol), small amounts of

salt (NaCl) and concentrated (37 wt%) HCl, tetrae-

thosysilane TEOS (Si(OCH2CH3)4) was added drop-

wise. The reaction mixture containing partially

hydrolyzed siloxanic precursor was sealed in a Teflon

bomb and heated in an oven at temperatures up to

90 �C. This hydrothermal treatment was carried out

for 24 h, time after which the mixture was filtered and

washed with deionized water and ethanol (for removal

of any traces of salt). The composite containing the

P123 was dried overnight at 60 �C and calcination at

550 �C (1�/min during heating) for 6 h (complete

surfactant removal) followed by natural cooling to

room temperature. This led to the final OMS samples

used herein.

A reference sample (following literature procedure,

sample S10) was prepared by stirring the reaction

mixture at 35 �C for 20 h (TEOS prehydrolysis)

followed by hydrothermal treatment at 90 �C for

24 h. While the phase separation can be detected

macroscopically in about 1 h, using a ratio ri = 0.08

NaCl (sample S12), the precipitation occurs much

faster, after 12–15 min. The initial synthesis condi-

tions are 1TEOS:0.016P123:5.9HCl:riNaCl:194H2O.

The salting effect was employed in this synthesis, as

the TEOS condensation rate will increase with NaCl

addition, still following a similar route to nonpromot-

ed condensation. S31 was prepared using a TEOS:-

NaCl ratio 1:0.16, while for S34 a 1:0.24 ratio was

utilized (Table 1).

A number of three composites have been obtained

using inorganic salts impregnated into mesoporous

silica (SBA-15 type) and subsequent thermal treat-

ment up to 550 �C. As inorganic sources, we used

Co(NO3)2, Co(CH3COO)2, and Cu(CH3COO)2. A

comparison was made between the decomposition

products of Co(NO3)2 and Co(CH3COO)2 into the

channels of mesoporous silica based on powder XRD

pattern data. We have also compared the ‘‘just

mixing’’ impregnation procedure and the ‘‘incipient

wetness’’ method (see Supplementary Fig. S2 for

impregnation procedure details).

The solubility of Co(CH3COO)2 in EtOH was

assessed in-house and found to be 28.8 g Co(CH3-

COO)2 in 1 L EtOH (0.16 M) at ambient temperature

and pressure. 8 mL of a saturated solution containing

the Co2? metal precursor was used for two steps

(2 9 4 mL) incipient wetness impregnation in 0.5 g

silica S31.

Experiments have shown, however, that using a

concentrated (e.g., saturated) Co2? precursor solution

leads to considerable amount of amorphous cobalt

oxide forming outside of the siloxanic mesopores.

Using lower concentration of Co(NO3)2 or Co(OAc)2

in ethanol solutions can help circumvent the crystal-

lization of cobalt oxide outside the siloxanic materials

pores and thus yielding higher quality materials, with

Table 1 Highlights for synthesis of mesoporous OMS samples S10, S12, S31, S34

Sample ri ([NaCl]) Temperature of TEOS

prehydrolysis (�C)

Observations

S10 (reference

sample)

– 35 �C, 20 h, then 90 �C

for 24 h

No reaction appears after TEOS

addition

into acidic NaCl solution

Macroscopically separation of

phases occurs within 1 h

S12 0.08 35 �C, 20 h then 90 �C,

24 h

Reaction proceeds faster,

commencing with phase separation

A white precipitate formes in

less than 150

S31 0.16 35 �C, 24 h then 90 �C,

24 h

Faster phase separation White precipitate formes after

100

S34 0.24 35 �C, 20 h then 90 �C

for 24 h

Fastest visible macroscopical

separation

While precipitate in less than 100


123

better dispersion of the metal oxide inside the silica

framework and higher stability. Perhaps an aspect

easily overlooked is that the efficiency of metal salt

precursor infiltrating the mesopores is enhanced due to

interaction of the silanol groups (–Si(OH)) present in

considerable amounts in the pore walls, and the Co2?

cation. This coordination mode of cobalt (II) helps

accommodating even more metal salt precursor solu-

tion (Fig. 1).

Materials characterization

Characterization of the obtained composites has been

carried out using: N2-sorption measurements, low

angle SAXS and wide-angle WAXRD powder dif-

fraction, TEM, SEM, and energy-dispersive X-ray

spectroscopy (EDXS).

Porosity assessment (BET Brunauer–Emmett–

Teller N2 isotherms) was done at 77 K over the relative

pressure range 0.01–0.995 using a Quantachrome

NovaWin 1200e analyzer. Samples were degassed

prior to loading into the sample holder at 80 �C

overnight under vacuum (10-4 bar). Pore size distri-

bution (PSD) was computed based on DFT or BJH

model, pore volume was computed at the maximum

achieved relative pressure (typically P/P0 = 0.99),

and the surface area was computed in the relative

pressure range 0.05–0.30 following the recommenda-

tion for mesoporous materials. P/P0 range is 0.05–0.35

but, however, the liner region of the 1/[W(P0/P) - 1]

versus P/P0 tends to shift to lower relative pressure for

mesoporous materials and even more so, for micropo-

rous materials (Brunauer et al. 1938).

Small angle and wide angle X-ray powder diffrac-

tion (SAXS and WAXRD) patterns were collected on a

Bruker D8 Advance and Philips PW 1820/00 diffrac-

tometers (CuKa radiation, 40 mA, 40 kV); SAXS

measurements were collected using 2h angles between

0.7� and 3� with a scanspeed of 1 and a 0.005 increment.

Transmission electron microscopy (TEM) TEM

images were collected on a Tecnai 12 (FEI), a new

generation of transmission electron microscopes oper-

ating at an accelerating voltage of 40 kV (possible

values range 20–120 kV), with a maximum possible

magnification 1209 to 300 0009. The micrographs

cover the 5–100 nm range to show the mesocomposite

structuring. Samples were ultrasonicated in C2H5OH,

and a drop of this solution was dried on a carbon

coated microgrid prior to the measurements.

Scanning electron microscopy (SEM) The images

were recorded on a scanning electron microscope

EVO 40 Carl Zeiss, equipped with a Pentafet Link

EDXS microanalysis system. All samples were cov-

ered with a thin gold layer prior to imaging.

Energy-dispersive X-ray spectroscopy (EDXS) was

used to assess the surface elemental composition of the

as-obtained mesocomposites.

Catalytic activity. Catalytic performance was

assessed in direct CH4 oxidation on a ‘‘U’’-shaped

tubular reactor made of quartz, operating at atmospheric

pressure, loaded typically with 0.1 g of Co3O4@OMS

or Co3O4-meso, deposited as a powder on the quartz

wool, in the temperature range 250–550 �C. Before the

catalytic process, the catalyst was flushed with air

(100 mL/min) for 1 h at 550 �C to remove adsorbed

species and then was cooled to 250 �C. Evaluation of

catalytic activity was done by feeding the reactor a

mixture of 10 % CH4 in N2 and air flowing at 100 mL/

min (CH4/O2:1/5); the temperature was raised from 250

to 550 �C in 50� steps. CO2 and hydrocarbons were

checked using a Porapaq QS 80/100 and Molesieve 5A

80/100 column, mounted on a DANI GC 1000

chromatograph equipped with a TCD detector.

Results and discussion

Structural characterization of mesoporous support

OMS. Three representative mesoporous silica

Fig. 1 Co2? in Td coordination environment to four silanol

groups


123

materials (S12, S31, and S34) based on modified SBA-

15 synthesis route have been obtained and character-

ized. The schematic representation of the templating

process used in preparing mesoporous oxides (and

most often, siliceous oxide) and the subsequent hard-

templating of the mesoporous siloxanic material to the

final mesoporous TM oxide MxOy is shown in

Scheme 1.

While the discovery of the new family of porous

silicon oxides is relatively new (M41S, by Beck and

co-workers at Mobil Oil R&D) (Kresge et al. 1992),

the nanoporous systems broadened and today include

representative members with quite different morphol-

ogies: hexagonal (MCM-41, HMS), 2D hexagonal

(SBA-15), 3D hexagonal (SBA-12), cubic-cage struc-

tured (SBA-16), and others (Kresge et al. 1992; Tanev

and Pinnavaia 1995; Zhao et al. 1998).

The mechanism employed when NaCl is added to

the reaction mixture involves coordination of the P123

Pluronic surfactant chain and may alter the reaction

time (faster reaction observed upon salt addition,

Table 1) and morphology (increasing the salt concen-

tration the morphology changes from nanowires-S12

to nanospheres-S34). A number of metal cations have

been investigated and are known to form complexes

with the polymeric ethylene oxide oligomers (EO)n:

Co2?, Ni2?, Mn2?, Mg2?. Ionic strength influence has

been investigated using a wide range of alkali halide

(NaCl) concentration. Denoting the nonionic surfac-

tant P123 by N0 and tetraethoxysilane by I0 the

pathway to mesoporous silica in the presence of Na?

cations can be formulated as (N0H?Na?X-)I0. The

electrostatic forces are generated by both hydrogen

bonds and complexation of EO groups (of P123) by

the small Na? cation. The structure-directing micelles

allow polymerization of the inorganic silica source

(TEOS) around their surfaces. Electrical assembly

pathways allow a lower synthesis temperature range

(20–65 �C) (Jain et al. 1999) and long-range order in

the framework, beneficial for improved access to the

catalytic site. Hydrogen bonds are formed between the

uncomplexed EO units of the cationic complex

(N0Na?) and the protons from both hydrolyzed I0

silica source (silanol groups) and the acidic reaction

media (HCl). The halide counterion X (Cl- from NaCl

and HCl) closes the charge balance at the silica-

surfactant interface. However, the X- (anion) role is

less significant, as using Na2SO4 (X = SO42-) instead

of NaCl in corresponding [Na?], we obtained similar

materials.

Porosity assessment by N2 sorption isotherms

The N2 sorption isotherms for the as-prepared samples

highlight the structural details of the final materials.

The obvious difference was observed for this sample

Scheme 1 Typical synthetic approach (templating process) to prepare nanostructured materials. Exemplified for OMS when the

inorganic precursor is TEOS (tetraethoxysilane)


123

when compared to the salt–assisted samples. This

modified synthesis procedure affords a rather broad

pore distribution on the S10 reference sample, gener-

ally uncommon for SBA-15 materials synthesized

using the classical route. Implying salt assisted route,

we have proved the powerful effect of NaCl upon the

reactive mixture, as it only affords only one PSD

domain, and the adsorption isotherm confirms this

finding with a type IV isotherm associated to a type H1

hysteresis loop. Samples S12, S31, and S32 present a

type IV isotherm with a H1 hysteresis loop (Fig. 2),

while sample S10 prepared in similar conditions but

without the addition of NaCl shows two inflexion

points in the N2 adsorption isotherm. At intermediate

relative pressure (0.4 \ P/P0 \ 0.8), the isotherm

displays considerable adsorption and small hysteresis

loop (small, cylindrical mesopores), and at high

relative pressure (P/P0 [ 0.8) an H1 hystheresis loop,

corresponding to larger mesopores. This accounts for

the broad PSD for sample S10 with pore sizes between

7.15 and 8.11 nm, while the other OMS samples have

a sharp PSD centered at 7.19 nm.

Specific surface area, SSA, pore volume, and pore

dimensions are given in Table 2. The highest SSA is

attributed to S31 which presents very good structural

characteristics (SAXS, TEM). Detailed analysis

results based on BET, BJH (adsorption branch),

t-method (microporosity), HK, SF, and NLDFT pore

diameter computations are summarized in Fig. S1a–c.

N2 sorption isotherms were employed for porosity

assessment in the case of Co3O4@S12, CuO@S34 and

Co3O4-meso samples. An optimized catalyst would

imply better pore filling, which in turn leads to higher

content of catalytically active species, so tuning the

maximum precursor concentration proves to be a

solution to higher activity materials design. Assess-

ment of the pore filling degree was done by N2

sorption isotherms on the S12 mesoporous silica,

Co3O4@S12 mesocomposite and the Co3O4-meso

(Fig. 3).

We speculate that a higher precursor loading is

possible; based on N2 physisorption data. The pore

volume of Co3O4@S12 composite is still high, which

in turn means that it can accomodate a larger amount

of Co2? precursor. As a rough estimation, doubling the

content of Co-precursor used in impregnation would

be the upper limit with respect to siloxane uptake

capacity. Leaching silica from MxOy@OMS leads to

mesoporous metal oxide MxOy-meso (Scheme 1).

Transforming the amorphous materials into crystalline

frameworks with mesopore distribution retention has

been a challenge (Kondo and Domen 2008), because

the heat treatment typically used in this process is

accompanied by high mobility and movement of

atoms which leads to mesostructure loss. A basic idea

to circumvent the loss of mesostructural order was to

remove the siloxane from the composite matrix such

that the mesoporous oxide would be obtained and used

further on in various catalytic applications, like

hydrocarbon oxidation. This goal was pursued with

success, the silica was leached from the matrix with

either KOH or HF 10 %, and the final material was

Fig. 2 N2-sorption isotherms (left) and PSD based on BJH model (right) for the four SBA-15-based samples, S10, S12, S31, and S34


123

analyzed by X-ray diffraction to confirm the Co3O4

phase in Co3O4@OMS. The final SBET of mesoporous

Co3O4 was determined to be 105 m2/g and

Vt = 0.24 cm3/g, with d0 = 3.4 nm, and were

deduced using the BJH method applied on the

absorption branch of the isotherm.

The pore characteristics for CuO–S34 composite

were deduced from the N2 sorption isotherms (Fig. 3).

The N2 sorption isotherm presents a broader

hysteresis for CuO–S34 composite consistent with a

broader PSD (Fig. 4). We observe *50 % pore filling

upon the incipient wetness method of S34 silica with

Cu2? precursor and subsequent thermal treatment,

pore volume decreases accordingly from 1.16 to

0.51 cc/g due to partial filling of silica pores with

CuO (Table 3).

The available mesoporosity could be used to further

tune the highest CuO content for a given mesoporous

sieve as support. The PSD confirms that there is not

much contraction occurring within the mesopores

upon CuO formation inside the silica host (Fig. 4,

right) The mean pore size, however, decreases to

2.9 nm, which means that smaller mesopores account

for the decrease in the pore size (CuO formed within

silica pores). DFT confirms the presence of smaller

mesopores by the broad shoulder at *1.8 nm,

consistent with formation of CuO within the mesop-

ores (Fig. 5).

Table 2 Surface area, pore volume, and pore size summary

Sample Average pore

diameter (nm)

BJH adsorption pore

diameter [mode Dv(d)] (nm)

Vt,pores

(cc/g)

t-Method micropore

volume (cc/g)

BET

(m2/g)

BJH on adsorption

(m2/g)

S10 7.063 7.159 1.035 0.035 586 385.6

S12 5.918 7.195 0.853 0.028 577 401.5

S31 5.087 7.197 1.023 0.083 804.5 478

S34 5.759 7.172 0.864 0.001 600.4 439.9

Fig. 3 N2 physisorption isotherms at 77 K of starting OMS

(S12, represented by hollow circles), Co3O4@S12 mesocom-

posite (red circles) and Co3O4-meso (brown circles). (Colour

figure online)

Fig. 4 N2 sorption (left) of S34 siloxane (the hard template, hollow circles) and CuO–S34 mesocomposite (dark gray). PSD (DFT

model) for S34 and CuO–S34 composite samples (right)


123

SAXS and WAXRD

XRD patterns are indexed to reflexions on the strong

(100) and weaker (110) and (200) planes which

suggest a P6mm group, specific to SBA-15 materials.

This corresponds to high ordering siloxanic materials

(Fig 2). While d(100) gives ‘‘a’’ (lattice parameter),

analysis of N2 adsorption isotherms determines mean

pore diameter (Table 4). These results are correlated

by the siloxanic wall thickness, which is the difference

between cell parameter ‘‘a’’ and mean pore diameter

‘‘wd.’’

The material exhibiting the thickest walls is S31,

and this implies that such material has a higher thermal

and hydrolytic stability than the other two samples

presented (S12 and S34). However, the results present

all obtained OMS materials as having very good

physical properties.

Studies have shown that SBA-15 materials contain

intrawall micropores that can interconnect the ordered

primary mesopores, and originates from the hydro-

philic part of the triblock copolymer (EO) trapped

within the silica walls. Galarneau and Fajula showed

that the size of these intrawall pores in the SBA-15

materials is a function of the hydrothermal treatment

temperature (Galarneau et al. 2001). Large intercon-

necting mesopores were observed when SBA-15

materials were aged at 140 �C. On the other hand,

the framework pores of materials aged at 35 �C were

mainly constituted by small intrawall mesopores

(2 \ Dp \ 3 nm) and micropores (Dp \ 2 nm). The

PSDs of the framework pores also showed that a

significant decrease of the intrawall pore volume

occurred upon calcination between 250 and 550 �C.

Furthermore, as for the primary mesopores, the

decrease of this volume associated with intrawall

pores during calcination was more significant for

SBA-15 aged at lower temperatures. For materials

aged at 35 �C, the specific surface area associated with

the intrawall micropores corresponds to about 40 % of

the entire surface area, whereas the specific surface

area of the material aged at 140 �C mostly corre-

sponded to pores wider than 6 nm. Substantial lattice

shrinkage between 250 and 550 �C, may usually result

in reduced specific surface areas, while smaller lattice

contraction occurs for materials aged at higher tem-

peratures. Tuning the NaCl to TEOS initial ratio ri, one

can reduce the critical micelle concentration of the

Table 3 Textural characteristics of S34 and CuO–S34

deduced from N2 adsorption isotherms (DFT model)

Sample SBET

(m2/g)

C constant Pore volume

(cm3/g)

Mean pore

width (nm)

S34 550.6 290 1.16 4.2

CuO–S34 352.1 120 0.51 2.9

Fig. 5 SAXS (left) and WAXRD (right) data for OMS samples

showing three resolved peaks indexed to (100), (110), and (200)

reflexion planes. The wide angle X-ray diffraction (WAXRD)

confirms the amorphous siloxanic framework at larger angles

(typically around 2h = 23�)


123

surfactant, acting similar to a temperature increase.

Salt effect can also lower the thermodynamic radius of

the micelles, resulting in a lowered cell parameter and

pore size. Moreover, the presence of Mn? leads to a

long-range ordered structure of the mesoporous

material, and the final siloxanes (S12, S31, S34)

exhibit only modest microporosity. The morphology

of the obtained materials represents a consequence of

dynamic interaction between the internal force field of

a growing crystal and the perturbing influence of the

surrounding environment. Addition of salt to the

synthetic process influences the surfactant concentra-

tions, viscosity, and hydrodynamical mixing thereby

give rise to diffusion and chemical gradients inducing

curvature and complexity which will finally affect the

morphology. The presence of intrawall microporosity

is essential for the chemical stability and inertness of

the silica support, and is shown to play an essential

role in inorganic precursor decomposition within the

OMS. The t-plot method confirms the presence of

microporosity for the silica samples synthesized; for

sample S13 for instance, the V(microp-

ores) = 0.035 cc/g, accounting for 3.3 % of the total

pore volume.

Impregnation of metal precursors (acetates and

nitrates) as aqueous or alcohol solutions into the OMS

supports affords the mesocomposites Co3O4@S12 and

CuO@S34 after subsequent thermal treatment. The

progress of metal precursor decomposition inside the

silica mesopores has been monitored through XRD

analysis throughout the experiment (Fig. 6, Fig. S3A–D).

In Table S4 (SI), a summary is given of the phase

composition identified by X-ray diffraction

upon decomposition of metal acetate and nitrate

heating processes. One may observe that the

Co(NO3)2�6H2O initially present in the cobalt

acetate–silica mesocomposite (after impregnation)

transforms into Co(NO3)2(H2O)4 and Co(N-

O3)2(H2O)2 (after heating at 100 �C, partial loss

of 2–4 crystallization water molecules). Thermal

treatment to 300 �C leads to Co3O4, and further

heating at 550 �C affords the final Co3O4–silica

Table 4 Structural characteristics (framework parameters) of three silica samples (SBA-15 type) from combined N2 sorption

isotherms and SAXS patterns

Sample 2h d100 = k/2 sin h(nm)

a0 = 2d100/ffiffiffi

3p

(nm)

wd = mean mesopore

diameter (nm)

twall thickness = a0 - wd

(nm)

S12 1.02 8.65 9.99 5.918 4.072

S31 0.99 8.916 10.29 5.087 5.203

S34 1.045 8.446 9.753 5.759 3.994

Fig. 6 Small-angle X-ray diffraction (SAXS right) and wide-

angle (WAXRD) diffraction of Co3O4@12 right after the

impregnation and drying step at 100 �C (red), after 1 h of

heating at 300 �C (green color) and after 12 h heating at 550 �C

(blue color). The WAXRD depicts the sample after additional

8 h at 550 �C (mesocomposite Co3O4@S12, red color) and after

dissolution of silica with hot KOH (dark cyan color) and

additional HF treatment (black color). (Colour figure online)


123

composite Co3O4@S13 (‘‘plain mixing’’). The

decomposition pathway is similar when Co(N-

O3)2�6H2O is impregnated over six steps into silica

host (Co3O4@S12, incipient wetness impregnation

method), with the main advantage that decomposi-

tion of metal nitrate outside of the silica pores is

circumvented due to progressive filling-decomposi-

tion cycles. Incipient wetness method was chosen

as the method of choice for inorganic precursor

incorporation into silica host. Decomposition of the

Co(CH3COO)2 impregnated into S31 mesoporous

silica shows after the first heating step at 100 �C

the presence of cobalt acetate hydroxide, while

heating to 300 �C leads to Co3O4 (and Co2O3-

traces), and the heating step at 550 �C produces the

final composite Co3O4@S31, where the Co3O4 is

the main phase. However, decomposition of the

cobalt (II) acetate leads to a number of impurities

identified by powder XRD (Co(OH)2, CoO(OH)

and Co2SiO4), and so the Co(NO3)2 remains the

Co(II) precursor of choice. Decomposition of the

Cu(CH3COO)2 into S34 silica leads to CuO after

the thermal treatment at 300 �C (along with traces

of cuprite Cu2O), CuO (tenorite) being the final

phase present in the CuO@S34 mesocomposite.

SAXS measurements confirm that the mesopores

are not affected by the thermal treatment at 550 �C, as

evidenced in the case of the CuO@S34 composite

below. However, one can note an interesting shift of

the main peaks which are due to framework contrac-

tion which occurs during calcination, yet the reflection

peaks corresponding to the P6mm symmetry group

show up still strong (Fig. 7) Interestingly, the 2hcorresponding to (100) plane shifts to the right to 1.08�(for CuO@S34), consistent with a contraction of d100

to 8.18 nm and of a0 to 9.44 nm.

A thorough comparison of the starting materials

and the final composite CuO or Co3O4–siloxane has

been performed through WAXRD and N2 sorption

measurement, in order to quantify the amount of Cu or

Co-salt needed to fill the pores up to a safe upper limit

such that the pores will not get clogged and the

inorganic salt will not decompose outside of the pores.

An XRD monitoring of the decomposition process was

employed: WAXRD was recorded after the first

impregnation and solvent evaporation step (100 �C),

then after heating for 2 h at 300 �C, after 20 h at

550 �C, and similar during the second incipient

wetness impregnation step (100 �C, 300 �C then

550 �C), and in case of Co3O4– after the hot KOH

silica dissolution and separation of the mesoporous

cobalt oxide obtained by base-leaching the silica from

the mesocomposite Co3O4@OMS (Fig. S3, A–D) The

impregnation steps carried out at 100 �C revealed an

interesting aspect: Co(NO3)2�6H2O@S34 (the starting

material impregnated into OMS) loses 2–4 H2O

molecules up to 100 �C, while the decomposition

route for Co(NO3)2�6H2O comprises of four events, at

80 �C (–2H2O), 120 �C (–2H2O), 150 �C (–2H2O),

and 215 �C (actual decomposition leading to Co3O4).

This proves that nanoconfined cobalt precursor has a

lower transformation energy barrier in nanoconfined

state than it does in bulk (Scheme 2).

The chemical phases involved in these temperature

processes were identified via SAXS and WAXRD to

assess both the nitrate/acetate transformations into the

mesoporous silica host and also the eventual transfor-

mations in the silica matrix due to inorganic salt

(NaCl) loadings (SAXS). The X-ray spectra were

taken after each step of the synthesis: XRD done for

the recrystallized salt used, XRD for the composite

salt-silica after first impregnation step, XRD for the

composite after 300 �C treatment, XRD after second

impregnation at 100 �C, XRD for composite after

300 �C second step, and XRD for composite after

550 �C heat treatment and also for the mesoporous

oxide used after silica dissolution from the composite

metal oxide-silica. An interesting details of the

experimental procedure is the usage of PP (polypro-

pylene) containers for stirring the SBA-15-type OMS

Fig. 7 SAXS patterns for samples S34 (pure siloxane) and

CuO@S34 composite after thermal treatment at 550 �C. The

curves are offset vertically by 500 a.u. for clarity


123

at room temperature after having added dropwise the

Co(NO3)2 ethanol precursor solution (incipient wet-

ness method) for 5 min. The hydrophobic surface (as

opposed to that of glass) of polypropylene afforded a

reasonable loading of 15.5 % as confirmed by EDAX

measurements for the final mesocomposite.

Based on the thermal decomposition of the metal salt

precursor, one can conclude that simple mixing of the

Co(NO3)2 or Co(OAc)2 solution (in water or ethanol) in

one step followed by drying at 100 �C, heating at

300 �C for 2 h and additional 20 h at 550 �C does lead

to lower quality nanomaterials. The reason is that a fair

amount of the metal oxide would form outside of the

silica pores, and as such, will behave differently than

the oxide confined in the SBA-15 mesopores, where it

originally formed by thermal decomposition of either

the cobalt nitrate or acetate. Increasing the number of

incipient wetness steps greatly enhances the quality of

the final mesocomposites (a table summarizing the

transformations identified during metal salt decompo-

sition under the experimental conditions employed here

is given in SI, Table S4). It is noteworthy to observe that

even after calcination step at 300 �C, the metal oxide

clearly shows up in the XRD patterns, consistent with

Co3O4 and CuO XRD patterns, respectively. The

framework strengthening step at 550 �C affords the

final mesocomposites.

EDAX

The EDAX measurements (Fig. 8) for the samples

Co3O4@S12 and CuO@S34 conducted in a joint

SEM-EDAX experiment confirm the presence of the

metal oxide at the surface of the composite. This

allows us to compute the percent of metal oxide

present in the composite (Table 5).

EDAX results show a percent composition based on

the elements identified. For instance, the mesocom-

posite CuO@OMS has the composition xCuO�ySiO2.

The EDAX gives the content in Cu (%) and Si (%),

which are based on the composite formulation

(MCuO = 79.54 g mol-1, MSiO2 = 60.08 g mol-1;

ACu = 63.54 g mol-1; ASi = 28.09 g mol-1):

% Cu ¼ 63:54� x

79:54� xþ 60:08� y� 100 and

% Si ¼ 28:09� y

79:54� xþ 60:08� y� 100:

ð1Þ

The proprietary software operating the joint SEM–

EDS apparatus outputs the following data upon

acquisition (Table 5).

For CuO@OMS sample, the ratio is effectively given

as 90.78 % SiO2, 9.22 % CuO. In the case of

Co3O4@OMS, we started from the mol%: 85.54 %

SiO2, 14.46 % CoO (the real form of cobalt is, however,

Co3O4 and not CoO as assumed by software), this means

that the Co3O4 content in 100 g composite

Co3O4@OMS will be: (1/3)*(14.46/74.93)*240.79 =

15.49 g, hence 15.49 wt% (MCoO = 74.96 g mol-1,

MCo3O4 = 240.79 g mol-1). The final composition

reflecting the wt% in SiO2 and metal oxide, respectively,

is given in Table 6.

More than 15 wt% Co3O4 has been incorporated

into the silica host in case of Co3O4@S12, while a

more conservative, 9.2 % CuO was found in

CuO@S34. The metal oxide loading will show a

direct correlation with the catalytic performance of the

as-synthesized samples.

Fig. 8 EDAX measurements for samples Co3O4–S12 (left) and CuO–S34 (right). The values on the abscissa are given in keV for both

samples

Co(NO3)2.6H2O Co(NO3)2.4H2O Co(NO3)2.2H2O Co(NO3)2 1/3 Co3O4 + 1/3 N2O4 +2/3 N2O5- 2 H2O - 2 H2O - 2 H2O

80 oC 120 oC 150 oC 215 oC

confined in OMS, 100oC

- (2-4) H2O

Scheme 2 Decomposition pathway for Co(NO3)2�6H2O in bulk and confined state


123

Morphology

Transmission electron microscopy (TEM)

The TEM images for sample S31 are shown in Fig. 9.

Looking at the siloxanic SBA-15 through a direction

perpendicular to the pore axis a parallel pattern is

observed (Fig. 9a, c, d).

At 10 nm scale, the channels appear parallel (view

perpendicular to the electron beam), and the pore

diameter (*7.2 nm, Fig. 9c) confirms the PSD deduced

from N2 sorption isotherms. TEM micrographs prove a

hexagonal ordering of the silica framework, catalyzed by

the NaCl addition; during the end of the mesophase

formation, an aliquot was taken and checked under the

transmission electron microscope, confirming the hex-

agonal ordering of the framework (black arrows point to

the channels in Fig. 9f). The honeycomb structure

characteristic to the SBA-15 type of materials can be

seen clearly in Fig. 9b, e.

Examination by TEM imaging (Figs. 10, 11) of the

metal oxide-silica mesocomposites reveals that the

hexagonal arrangement of the silica is preserved

during and after pores filling with inorganic precursor,

a result confirmed by SAXS diffraction data at low

angle. The differences in electron density make the

silica support show lighter contrast, and the metal

oxide (Co3O4, CuO) appear as areas of darker contrast.

Highly dispersed cobalt oxide nanoparticles can be

observed throughout the Co3O4@OMS as patches of

different size and shape. Interestingly, the diameter of

some patches exceeds that of the OMS silica host. This

is due to the micropores in the SBA-15 type of silica

(S12) that interconnect the mesopores, and allow the

extended growth of the crystalline cobalt oxide. TEM

micrographs show both empty mesopores and cobalt

oxide patches which grew in multiple neighboring

mesopores due to their inherited microporosity. When

the channels are perpendicular to the electron beam,

they appear as parallel, highly ordered channels

(Fig. 10c). The cobalt oxide loading is visible as

darker regions apparently filling up the mesopores

(Fig. 10c). Larger patches occur within the composite

and present different morphologies, depending on the

extent of cobalt oxide crystallite growth due to cobalt

nitrate infiltration in the silica channels (meso- and

micropores) and subsequent decomposition to yield

Co3O4. These patches of cobalt oxide are well

Table 5 Computation details for CuO and Co3O4 contents in the corresponding composites

EDAX ZAF quantification (standardless)

Oxides

SEC table: default

Co3O4@OMS

Elem wt% mol% K-ratio Z A F

SiO2 85.54 88.06 0.2306 0.9930 0.5807 1.0000

CoO 14.46 11.94 0.1001 0.8737 1.0070 1.0000

Total 100.00 100.00

kV 29.00, tilt 0.00, take-off, 32.09, Tc 50.0, det

type UTW, sapphire res 134.40, Lsec 105

CuO@OMS

Elem wt% mol% K-ratio Z A F

SiO2 90.78 92.88 0.2514 0.9888 0.5990 1.0000

CuO 9.22 7.12 0.0642 0.8602 1.0139 1.0000

Total 100.00 100.00

kV 29.00, tilt 0.00, take-off, 32.09, Tc 50.0,

det type UTW, sapphire res 134.40, Lsec 79

Table 6 EDAX analysis results on mesocomposites

Co3O4@S12 and CuO@S34

Sample SiO2 wt% MxOy wt%

Co3O4–S12 84.54 15.49 wt% Co3O4

CuO–S34 90.78 9.22 wt%–CuO


123

dispersed, with widths of 15–100 nm and lengths in

the 20–200 nm range. Throughout the transforma-

tions, the silica channels are preserved (Fig. 10d). The

effect of Co3O4 forming inside the mesopores leads to

decreased surface area and pore volume, results

confirmed by N2 sorption isotherm measurements

(Figs. 3, 4). The cobalt oxide is mainly formed within

the pores and they have a rod-like morphology, since

they reside in the mesoporous channels of the silica

host (Fig. 10a). While further increase of cobalt

loading should in theory increase the catalytic activity

of the composite, the effect of pore blockage must be

considered; if the pores are filled with cobalt oxide, the

contact between the gaseous mixture (CH4, O2) and

the catalyst is reduced and this will adversely affect

the catalytic performance.

Fig. 9 TEM images for

sample S31 (804 m2/g

specific surface area)


123

The presence of CuO within the OMS host is

apparent in Fig. 11a, where extended regions of high

contrast are present, and can be attributed to the metal

oxide showing darker contrast compared to silica due

to electron density difference. The OMS mesopores

are unaffected by the CuO loading and thermal

treatment, and appear as parallel channels (Fig. 11b,

c, d) when the electron beam is perpendicular to the

pores. Moreover, there is little framework contraction

during the thermal treatment. There are, however,

some patches of CuO formed on the external area

(Fig. 11c). When the channels run parallel to the

electron beam, the honeycomb structure is visible, and

cobalt oxide seems to close the channels (black

arrows, Fig. 11b).

Scanning electron microscopy (SEM)

The obtained materials feature interesting structural

characteristics, as evidenced by the SEM images. The

mesotructures reported herein have either hexagonal

nanowire array (Co3O4–S12) or nanosphere array (CuO–

S34) appearance, depending on the topology of the host.

The hexagonal feature was inferred based on the small

angle diffraction data. The SAXS patterns exhibit an

intense low angle diffraction peak and other two, lower

intensity peaks which are assigned to the (100), (110) and

(200) planes of the 2D-hexagonal space group P6mm.

The two results were corroborated, and the conclusion

was that the Co3O4@OMS composite appears as

hexagonal nanowires. SEM images of Co3O4–S12

(Fig. 12a, b) show isolated rods 300–600 nm.

We see the influence the NaCl addition has on the

final morphology of the OMS: using a 0.08:1 molar

ratio NaCl:TEOS in the initial solution mixture leads

to nanowire formation (S12), while increasing this

ratio to 0.24:1, nanosphere arrays (S34) are obtained

instead.

This morphology control was, therefore, possible due

to the salt assisted templating process promoted by

NaCl. Nanocasting using Co2? or Cu2? precursors leads

to decomposition of the initial acetate or nitrate

Fig. 10 TEM images of

Co3O4–S12 nanocomposite


123

precursors to Co3O4 or CuO into the silica host. Lower

[NaCl] leads to nanowire formation, while higher

[NaCl] allows nanosphere arrays to form, which span

multiple hundreds of nanometers.The CuO–S34 sample

(Fig. 13a, b) shows agglomerates with the appearance of

chains (Fig. 13a, black arrows) expanding to multiple

micrometers. The nanochannels are interconnected by

metal particles that have a random distribution, inherited

from the mesoporous SBA-15 based silica which

possesses scattered micropores in the nanochannel

walls.(Scheme 1) This nanocasting procedure afforded

reproducible patterned metal oxides and mesoporous

oxides–mesoporous silica mesocomposites.

Catalytic activity of SBA-15-supported MxOy

(Co3O4@S12, CuO@S34) and Co3O4-meso

CH4 oxidation catalyzed by Co3O4(meso) and Co3O4-

SBA-15 has been investigated. The reaction rate

corresponding to the total oxidation of methane is

actually complex as the mechanism implies a couple

of temperature-dependent, competing adsorption

equilibria of CO, CO2, O2, CH4, and H2O on active

centers. For the total oxidation reaction, one may write

the following half-empirical formula:

r ¼ ks Tð Þð Þ pO1=22 þ kl Tð Þð Þ pCH4 ð2Þ

where ks(T) is the reaction constant of the surface-

chemosorbed oxygen and kl(T) represents the reaction

constant of the network oxygen, pO2 and pCH4

represent the partial pressures of oxygen and methane,

respectively. Equation (2) may be simplified applying

the assumption of fast oxygen diffusion from the

gaseous phase onto the catalyst’s surface in order to

replace the oxygen consumed from the network,

together with the negligible effects of CO2 and H2O

(dry gases, low methane concentration, temperatures

higher than 400 �C and conversions lower than 75 %).

Given these conditions, the methane oxidation can be

illustrated by:

Fig. 11 TEM images of

nanocomposite CuO–S34


123

r ¼ kapCH4 ð3Þ

where ka is the apparent reaction constant. The

reaction rate r (mol CH4 s-1 kg-1) can be computed

as:

r ¼ F � Dc CH4ð Þ=mcat ð4Þ

where F is the feed of the reactor, DcCH4is the volume

of reacted methane, and mcat is the weight of the

catalyst used. The reaction constants were computed at

different temperatures in the 250–550 �C range.

Arrhenius equation (5) was used in the logarithmic

form, and ln (k) was plotted as function of the

reciprocal absolute temperature 1/T (K-1) (Fig. 14,

right)

ln k ¼ ln A�Ea=RT ð5ÞThis approach allowed analytical determination of

Ea (the activation energy) and the pre-exponential

factor corresponding to methane oxidation on two

different catalysts, Co3O4@S12 and Co3O4-meso.

Figure 14 shows the variation with temperature of

the transformed methane percentage during the oxi-

dation reaction. One may easily observe striking

differences between the two catalysts regarding their

catalytic activity: Co3O4-meso (mesoporous Co3O4

obtained after leaching the OMS support) shows

higher efficiency than SBA-15 supported Co3O4

(Co3O4@S12), while only a low activity of CuO@S34

composite was recorded at temperatures exceeding

500 �C.

Fig. 12 SEM images for composite Co3O4–S12

Fig. 13 SEM image for sample CuO–S34


123

The temperatures necessary to obtain 25 % (T25)

and, respectively, 50 % (T50) CH4 conversions are

presented in Table 7. We observe a significant differ-

ence in the temperatures needed to convert a specific

amount of CH4 using one catalyst or the other. At

370 �C for instance, while the Co3O4@S12 afforded

only 25 % conversion of CH4, the Co3O4-meso had

already converted all the methane in the feed. The

conclusion is that among the two catalysts, although

both show good results, the Co3O4-meso is clearly the

higher performance catalyst with respect to hydrocar-

bon oxidation.

The activation energies Ea were computed using

the Arrhenius graphical representations (Fig. 14,

right). For the two catalysts investigated, plotting ln

(k) versus 1,000/T (K-1), a linear dependence of ln k00

versus 1/T was obtained. The line has the intercept:

ln A (A-frequency factor or Arrhenius constant),

and the slope: -Ea/R. For Co3O4@S12, the slope is

-3.68 which corresponds to Ea = 30.66 kJ/mol,

while for Co3O4-meso the value of the slope of

-5.67 leads to an apparent activation energy of

47.04 kJ/mol (Table 7).

The values of the activation energies summarized in

Table 7 lie between 30.66 and 47.04 kJ/mol, which

characterize metallic oxides of high (47 kJ/mol—

Co3O4-meso) and very high (30.6 kJ/mol—

Co3O4@S12) catalytic activity. For comparison, even

noble metal-doped mixed oxides (2 % Pd/CeO2–

2ZrO2) present for the methane oxidation process

values of *70 kJ/mol. One may also observe a direct

proportionality between the pre-exponential factor

and the oxygen vacancies from the catalyst (i.e.,

Co3O4-meso has more vacancies as compared to the

supported Co3O4@SBA-15). The Arrhenius constant

takes different values for the two catalysts,

0.005462 s-1 for Co3O4@S12 and 0.32605 s-1 for

Co3O4-meso. This means that, if the fraction of

molecules possessing enough kinetic energy to react

were approaching unity (exp(-Ea/RT) = 1), then the

rate constant k would be equal to the pre-exponential

factor A, and the oxidation reaction catalyzed by

mesoporous Co3O4 is 0.32605/0.00546 & 59.7 times

faster than if catalyzed by Co3O4 embedded in

mesoporous silica host (Co3O4@S12).

Let f(T) be the ratio k1(T)/k2(T), where k1(T) is the

reaction rate for the oxidation reaction catalyzed by

Co3O4@S12 and k2(T) the reaction rate for the

reaction catalyzed by Co3O4-meso.

k1 ¼ A1 � e�Ea ;1R�T ; reaction rate Co3O4@S12 ð6Þ

k2 ¼ A2 � e�Ea ;2R�T ; reaction rate Co3O4�meso ð7Þ

Fig. 14 CH4 oxidation conversion (left) on Co3O4 supported on mesoporous silica Co3O4@S12 (red squares), on Co3O4-meso (black

triangle) and CuO@S34 (black circle). Arrhenius representation for Co3O4@S12 and Co3O4 (right). (Colour figure online)

Table 7 Specific parameters (activation energy Ea, preexpo-

nential factor A) for CH4 oxidation process

Sample Temp. for % CH4

conversion (�C)

Ea apparent

(kJ/mol)

ln A

25 % 50 %

Co3O4-meso 265 305 47.04 -1.0644

Co3O4@S12 372 427 30.66 -5.2097


123

f Tð Þ ¼ k1 Tð Þk2 Tð Þ ¼

A1

A2

� e�ðEa ;2�Ea ;1Þ

R�T ¼ 0:01675� e�1;970

T

ð8Þ

f 0 Tð Þ ¼ 33:0003� e�1;970

T � T�2 [ 0; 8T [ 0 ð9Þ

0\f Tð Þ ¼ k1 Tð Þk2 Tð Þ\1; 8T [ 273 K ð10Þ

The reaction rate is actually always faster for

Co3O4(meso) regardless of the temperature. We observe

that f(T) is a continuous and increasing function for any

temperature T (Eq. 8), since the first derivative f0(T) is

always positive (Eq. 9). f(T) is also always lower than

unity for any T [ 273 K (Eq. 10). To put it in words,

this means that the reaction rate will always be higher for

Co3O4-meso than for Co3O4@S12. This result can be

interpreted also in the following way: nanoconfinement

of the mesoporous oxide into the silica host reduces the

activation energy from 47.04 kJ/mol (Co3O4-meso) to

30.66 kJ/mol (Co3O4@S12), while a reverse trend is

observed for the preexponential factor, which is higher

(0.3261 s-1) for Co3O4-meso and lower for confined

Co3O4@S12 (0.0054 s-1). This reflects the lower

probability of collision between the methane and O2

molecules over confined Co3O4@S12 (which contains

12 % cobalt oxide) than over pure mesoporous Co3O4.

The activity depends on the sample textural prop-

erties, the best performance being achieved for the

material with the highest surface area and the most

open pore system. The relatively low temperatures

required for complete oxidation of CH4 also helps

circumventing structural degradation of the catalyst. A

control experiment shows that mesoporous silica

template samples prepared here (S12, S31, S34) have

no catalytic activity at temperatures up to 500 �C,

which confirms that the nanoconfined metal oxide is

responsible for the observed catalytic effect.

Moreover, the metallic oxide reactivity greatly

depends on the coordinative unsaturation degree of the

embedded metal cations. The presence of oxygen

vacancies exposes the unsaturated metal cations effi-

ciently. The two oxidation states of cobalt in Co3O4

allow its participation in reactions involving modifica-

tion of the OS (oxidation state). It has been reported that

the redox properties of the system Co2?/Co3? can

facilitate the catalytic behavior in oxidation of cyclo-

hexanol (Taghavimoghaddam et al. 2012), while also

enhancing the oxygen mobility in Co3O4@OMS mes-

ocomposite. Due to coordinative unsaturation, the

oxygen ions from the superficial layer are generally

less bounded than those from the bulk. This explains the

superficial oxygen ions participating in oxidation reac-

tions, while the mixed valence state of cobalt in Co3O4

(which can be regarded as a 1:1 mixture of Co2O3:CoO,

so Co3?/Co2?=1/1) facilitates the oxidation, Co2?/

Co3? redox couple mediating the oxidation. Substitu-

tion of Co3? with Co2? leads to an increase in

coordinative unsaturation. Among the factors that can

influence the catalytic activity are the above-mentioned

mobility of oxygen vacancies, acid–base properties,

electron affinity of cobalt ions, structural characteristics

of the support and dispersion of active catalyst species

onto the support. The high dispersion of cobalt oxide and

copper oxide in the mesoporous silica significantly

contribute to improved catalytic performance as com-

pared to bulk metal oxides. A key role for this behavior

can be attributed to the small Co3O4 particle size as

depicted before by TEM measurements. Methane

conversion is particularly effective, the conversion

achieved at 350 �C is practically [99 %. CuO meso-

composite on the other hand only showed about 13 %

conversion of methane at 500 �C. This highlights the

mesoporous Co3O4 as a superior catalyst with respect to

alkane oxidation. Mastering the oxidation reaction

mechanism and a thorough knowledge of the inorganic

precursor behavior in mesoporous molecular sieves

(siloxanes), we could gain a new perspective on highly-

efficient catalysts that could accomplish oxidation

reactions at temperatures of about 200–250 �C for

transformations that normally required noble metals or

other expensive plant investments.

Conclusions

In summary, a range of mesocomposites containing

dispersed TM oxides (M=Cu, Co) confined in mesopor-

ous silica OMS have been prepared via the hard-

templating (nanocasting) route and characterized by N2

sorption measurements, SAXS and wide angle XRD

powder diffraction, SEM, and TEM. The mesocompos-

ites obtained exhibited homogeneity, crystallinity con-

firmed by XRD diffraction, and small crystallite sizes

(\10 nm from TEM micrographs) confirmed by the

rather narrow PSD deduced from the N2 sorption

isotherm data. The influence of the silica starting OMS

and the amount of inorganic precursor employed have led

to materials that have been tested and proven to be


123

catalytically active in hydrocarbon oxidation applica-

tions. Choosing a very stable, saturated hydrocarbon in

the alkane series (CnH2n?2, n = 1, methane), we showed

that mesoporous Co3O4 exhibits a high oxidation ability

affording full methane combustion at 350 �C, followed

by the Co3O4@OMS mesocomposite (15.5 % loading)

which converted *50 % of CH4 at 430 �C, while

CuO@OMS composite only afforded a modest 13 %

conversion of methane at 500 �C. This order in catalytic

activity of nanostructured metal oxides mirrors the

previous findings on the bulk materials, which showed

a higher activity for Co3O4 than for CuO or other TM bulk

oxides. The performance of the mesoporous Co3O4

obtained is closely related to that of the very expensive

noble metal based catalysts (Pd@Al2O3), which starts to

convert methane at 300 �C. This Pd-alumina based

catalyst is actually outperformed, since Co3O4@OMS

begins methane conversion below 250 �C, with 18 %

conversion recorded by GC at 250 �C. Further tuning of

the metal precursor quantity can afford a higher activity

for the mesocomposite Co3O4@OMS which will pre-

clude the shortcomings associated with catalyst deacti-

vation in the case of pure mesoporous Co3O4, or

mesoporous framework breakdown during successive

catalytic cycles. Performing as such an active catalyst for

alkane oxidation, we believe that oxidation of other

substrates (hydrocarbons) will proceed under much

milder conditions, which in turn is associated with lower

temperatures, energy consumption and overall ‘‘greener’’

catalysis.

Acknowledgments Support of the Romanian Ministry of

Education and Research through the project PNCDI-2 No.

72-196/2008 ‘‘New complex hydrides for hydrogen storage in

hydride tank suitable for vehicular applications’’—STOHICO

and the financial support of the POSDRU-ID5159 doctoral

fellowship are acknowledged. This work was partially

supported from the Romanian Core Programme (Contract No.

45N/2014). I am grateful to Prof. Cornelia Guran for insightful

discussions. I am in debt to senior researcher Viorica Parvulescu

for catalytic studies and insightful suggestions. I strongly

acknowledge the support received from Prof. Giovanni Principi

regarding training and usage of the research facilities at

Universita Degli Studi di Padova, Italy, where most of this

research was carried out.

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Documents

Synthesis and characterization of novel mesocomposites Co3O4 and CuO@OMS (ordered mesoporous silica) as active catalysts for hydrocarbon oxidation