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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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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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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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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]