Fundamentals of Adsorption and Catalysis

8/7/2019 Fundamentals of Adsorption and Catalysis

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Fundamentals of Adsorption and

Catalysis

B. VISWANATHAN

NATIONAL CENTRE FOR CATALYSIS RESEARCH

INDIAN INSTITUTE OF TECHNOLOGY, MADRAS

CHENNAI, INDIA


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All devices where in surface to

volume ratio is high are betterperforming systems example is

brain, leaf and may other natural

systems.

The reason is that the activationat the surface is different from

activation in the bulk


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Multi-functionality

Surface site is differently active compared to the

sites in the bulk of the material

Multi-functionality is easily possible

B A

CH3 CH CH CH2

H

H

OH

basic acidic

0

20

40

60

80

100

120

140

7.9%

11%

81.1%

Solid acid

Solid acid-base

Solid base

Solid base

catalysts (10)

Solid acid-base

bifunctional

catalysts (14)

Solid Acid

catalysts

(103)

Number

Total

(127)


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Some basic questions that we seek answers for

How do interfaces behave?Do they behave as an algebraic sum of the behaviour of the two phases?

Do the phases at the interface retain their identity?

If the phases are changed in configurations and structure, what is the driving force for such

changes?

To how many layers in each of these phases, these configurational changes are felt?

From what depth or number of layers deep down from the surface or interface the bulkproperties of these phases are manifested?

If the surfaces and interfaces are a dynamic one, why do we need the study of the surfaces in

static mode?

Is it for the qualitative and quantitative elemental composition?

Is it to assess whether there is any accumulation or depletion of species from other phases?

Is there any accumulation of the species from one phase thus leading to binding at the surface.?What is the nature of this adsorption?

What is the adsorption strength?

What is the structure of the adsorbed state as compared to the free molecules in its own state?

How do the properties of these molecules in the adsorbed state differ from that they exhibit in

their free state?


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(1850) Catalysis

(1875) Electrochemistry, Surface, TD and Instrumentation

( 1850) Tribology

(1925)Adsorption Science and Electron Emission

(1955) Surface Analytical Techniques

(1960) Microporous Solids

(1975) Clusters andmonomolecular films

(1990) Nano &

Mesoporous

Materials

Molecular

level

1980

Development of Surface Chemistry and Catalysis

Conceptually

during the last 200 years

2000

Macroscopic

level


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Fig. 2.2. Representation of the techniques based on Electrons in electron, ion,

neutral and photon out LEED: Low Energy Electron Diffraction; HEED: High

Energy Electron diffraction; RHHED: Reflected High Energy Electron Diffraction;ILEED: Ineleastic Low Energy Electron Diffraction; AES:Auger Electron

Spectroscopy; EELS: Electron Energy Loss Spectroscopy; EIID: Electron Induced

Ion Desorption; SEPSMS: Electron Probe SurfaceMass Spectrometry; EID:

Electron Induced Desorption; SDMM: Surface Desorption MolecularMicroscope;

CIS: Characteristic Isochromat Spectroscopy; APS:Appearance Potential

Spectrosco


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Fig. 2.3. Schematic representation of the techniques that can be generated from

Photon- in photon, neutral, electron or ion-out methodology. XPS: X ray

Photoelectron Spectrroscopy; ESCA: Electrons Spectroscopy for Chemical

Analysis.


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Fig. 2.4. Schematic representation of the techniques that can be generated from

Ions-in ion-, neutral-, electron- or photon-out methodology. ISS: Ion Scattering

Spectroscopy, SIMS: Secondary Ion Mass Spectrometry, INS: Ion Neutralization

Spectroscopy, PIX: Proton Induced X ray emission.


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Model of a heterogeneous solid surface, depicting different

surface sites.These sites are distinguishable by their number of nearest neighbours.


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Clusters of atoms with single cubic packing having 8, 27, 64, 125 and 216

atoms.[In an eight-atom cluster, all of the atoms are on the surface. However, the dispersion D, defined

as the number of surface atoms divided by the total number of atoms in the cluster, declines

rapidly with increasing cluster size]


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Hydrogen adsorption


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ChemisorptionModels for CO


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DFT Studies on Clusters

The interaction of NO with Pd clusters has been studied by means of the LCGTO-

DF method. Metal cluster models (up to 13 atoms) with different size and geometry

have been used to describe the atop, bridge and three-fold sites. The use of

different model core potentials to increase the size of the cluster model treated and

to save computational time has been discussed. The binding energies of N(1s), 4,

5 and 1 electrons are calculated and compared directly to the experimental XPS

and UPS data available. The NO is tilted with respect to the surface normal axiswhen adsorbed on top and bridge sites by about 52.6 and 46.7 degrees,

respectively. On the two types of three-fold sites (hcp and fcc) the NO remains

upright. The bending angle is very sensitive to the cluster size and affects the

binding energies of N(1s), 4, 5 and 1 orbitals. The NO adsorption energies on

the different adsorption sites have been estimated using different cluster models.

The vibrational frequencies have been calculated in the harmonic approximationand they are in reasonable agreement with the available experimental values. The

cluster model approach is discussed in terms of its reliability to determine the

adsorption energies and the favored site of adsorption


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Areview of the published results on the adsorption of some simple gases on metal

surfaces at low substrate temperatures (Ts

30 K, down to liquid helium

temperatures) is given. The methods of investigating low-temperature adsorption of

gases are briefly discussed. Attention is focused primarily on the adsorption of

hydrogen on transition metals and noble metals. The results of experimental studies

on transition metals include information about the state of the adsorbed particles

(atoms or molecules), the spectra of the adsorption states, thekinetics of

adsorptiondesorption processes, the participation of precursor states in the

adsorption mechanism, the role of various quantum properties of the H2 and D2molecules, the influence of two-dimensional phase transitions, the structure of the

adsorbed layer (adlayer), and electron-stimulated processes. Experimental studies of

the adsorption of hydrogen on noblemetals in conjunction with theoretical

calculations provide information about the fine details of the quantum sticking

mechanism, in particular, the trapping of molecules into quasi-bound states and the

influence of diffraction by the lattice of surface atoms. Data on the role of therotational state of the molecules, orthopara conversion, and direct photodesorption

are examined. A review of the relatively few papers on the adsorption of oxygen,

carbon monoxide, and nitrogen is also given


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Stringent federal and environmental regulations have placed a high priority on

developing catalysts to prevent N-, S-, and C-containing pollutants from entering the

earth's atmosphere. Accelrys' quantum physics code CASTEP has been used to

carry out a detailed study of the interaction of various pollutant molecules on thesurfaces of rare-earth, transition-metal, and mixed-metal oxides, and to investigate

how these interactions change as a function of surface defects and doping with

different metals. The insight gained from these studies, augmented with

sophisticated spectroscopy techniques is providing invaluable guidance in the

design of new metal-oxide-based catalysts.

Chemisorption of NO 2 on a Cr-doped MgO(100) surface. Electrons in Cr 3d levels

above the MgO valence band lead to strong pollutant binding and facilitate N-O

bond dissociation

Numerous industrial processes involve combustion or oxidation of chemicals and fuels


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that constantly produceharmfulmolecules like NO, NO2 , N2O , SO2, H2S, CO etc.

Besides being hazardous to human health through environmental pollution, these

molecules cause millions of dollars worth of damage annually in the form of acid rain

and building corrosion. One cannot overstate the importance of designing better

catalysts to prevent these molecules from entering the earth's atmosphere.Metal-oxides, as a general class ofmaterials, have shown great promisein such

applications. In fact, the surface chemistry of oxides is relevant to many technological

applications: catalysis, photo-electrolysis, electron-device fabrication, corrosion

prevention, and sensor development, to name a few. They possess a widevariety of

structures andelectronic properties. Forinstance, the rare-earth oxide MgO is strongly

ionic, and a high-bandgap insulator, while the transition-statemetal oxide TiO2possesses half the bandgap as MgO, and can best be described as an iono-covalent

material. Add to this scenario mixed-metal oxides like MgMoO4 , FeMoO4 or NiMoO4 ,

and doped oxides likeCrxMg1-xO, and one has a rich variety of materials with metal-

centers of different coordinations and environments. Recent experiments already

demonstrate increased DeNOx, DeSOxand HDS activity of certain mixed-metal and

doped-metal oxides. However, to optimize their catalytic performance it is necessary topossess an atomic/electronic-level understanding of the interaction of the pollutant

molecules with the oxide surfaces.

D J R d i f B kh N ti l L b t d hi ll b t h d


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Dr Jose Rodriguezof Brookhaven National Laboratory andhis collaborators have used

Accelrys' plane-wave density functional theory (DFT) codeCASTEP to carry out detailed

investigations of theinteraction of the above pollutantmolecules with the surfaces of

MgO [1-8], TiO2[9, 10], Cr2O3 [5], ZnO [1], andCeO2 [2]. Also studied were the

electronic properties ofmixed-metal oxides [11, 12], and pure and dopedmetal surfaces[13]. Much of the above workalso investigated theeffects of structural defects (steps,

kinks, corners, O-vacancies) and doping with a secondmetal.

The Brookhaven group has also invested a significantexperimentaleffortin order to

characterize the atomic/ionic species and theelectronic density of states on the oxide

surfaces, using state-of-the-art spectroscopic techniques. Some of theseinclude: X-ray

absorption near-edge spectroscopy (XANES), X-ray and Ultraviolet photoemissionspectroscopy (XPS, UPS), and thermal desorption mass spectroscopy (TDS).

The close coupling between theory andexperimentis making possible a fundamental

understanding ofmany phenomena associated with the chemistry ofmolecules on oxide

surfaces. In particular, theimportance of band-orbitalinteractions for the reactivity of

oxide surfaces has become clear, and a correlation between theelectronic and chemical

properties ofmixed and doped oxides has been established. This has opened the wayfor using simplemodels based on band-orbitalmixing to provide a conceptual framework

formodifying or controlling the chemical activity of pure oxides, and for better

engineering ofmixed-metal oxides.

Ad b d f CO d N2


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Adsorbed states of CO and N2 on

metal surface

Cl f i h i l bi ki h i 8 27 64 125 d 216


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Challenges in Catalysis for the Conversion ofFossil Fuels

Fossil fuel Function Challenges in catalysis Basic Science challenges

coal Utilization Gasification C-C bond activation

Clean up CO2, NOx reduction, S and

particulates

CO2, NOx reduction

chemistry

Oil Utilization Catalytic combustion -

Natural gas

Clean up

Utilization

Clean up

CO2 reduction

FT, other Gas to liquid

processes, H2 production

CO2, NOx, reduction

CO2, NOX reduction

chemistry

C-H bond activation

CO2, NOx reduction

chemistry

D R i A i i C l


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Dream Reactions Awaiting Catalyst

Development

( according to Jens Rostrup-Nielsen)

CH4 + O2 CH3OH

CH4 + 1/2O2 CO + 2H2

2CH4 + O2 C2H4 +2 H2OnCH4 CnH2n+2 + (2n-2) H2

Dimethyl ether C2H5OH

H2 + O2 H2O2

2NO N2 + O2

2N2 + 2H2O+5 O2 4HNO3

M d l f h t lid f d i ti diff t


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Model of a heterogeneous solid surface, depicting different

surface sites.These sites are distinguishable by their number of nearest neighbours.

Ad b d t t f CO d N2


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Adsorbed states of CO and N2 on

metal surface

Cl sters of atoms ith single c bic packing ha ing 8 27 64 125 and 216


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Fundamentals of Adsorption and Catalysis