Fundamentals of Adsorption and Catalysis

B. VISWANATHAN NATIONAL CENTRE FOR CATALYSIS RESEARCHINDIAN INSTITUTE OF TECHNOLOGY, MADRAS

CHENNAI, INDIA

All devices where in surface to volume ratio is high are better performing systems example is brain, leaf and may other natural systems.

The reason is that the activation at the surface is different from activation in the bulk

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

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 bulk properties 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?

(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 and monomolecular films

(1990) Nano & Mesoporous MaterialsMolecular

level

1980 Development of Surface Chemistry and Catalysis Conceptually

during the last 200 years

2000

Macroscopic level

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 Surface Mass Spectrometry; EID: Electron Induced Desorption; SDMM: Surface Desorption Molecular Microscope; CIS: Characteristic Isochromat Spectroscopy; APS: Appearance Potential Spectrosco

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.

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.

Model of a heterogeneous solid surface, depicting different surface sites.

These sites are distinguishable by their number of nearest neighbours.

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]

Hydrogen adsorption

Chemisorption Models for CO

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), 4σ, 5σ and 1π electrons are calculated and compared directly to the experimental XPS and UPS data available. The NO is tilted with respect to the surface normal axis when 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), 4σ, 5σ and 1π 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 approximation and 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

A review 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, the kinetics of adsorption–desorption processes, the participation of precursor states in the adsorption mechanism, the role of various quantum properties of the H2 and D2 molecules, 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 noble metals 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 the rotational state of the molecules, ortho–para 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

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 the surfaces 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 that constantly produce harmful molecules 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 of materials, have shown great promise in 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 wide variety of structures and electronic properties. For instance, the rare-earth oxide MgO is strongly ionic, and a high-bandgap insulator, while the transition-state metal oxide TiO2 possesses 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 like CrxMg1-x O, and one has a rich variety of materials with metal-centers of different coordinations and environments. Recent experiments already demonstrate increased DeNOx , DeSOx and HDS activity of certain mixed-metal and doped-metal oxides. However, to optimize their catalytic performance it is necessary to possess an atomic/electronic-level understanding of the interaction of the pollutant molecules with the oxide surfaces.

Dr Jose Rodriguez of Brookhaven National Laboratory and his collaborators have used Accelrys' plane-wave density functional theory (DFT) code CASTEP to carry out detailed investigations of the interaction of the above pollutant molecules with the surfaces of MgO [1-8], TiO2[9, 10], Cr2O3 [5], ZnO [1], and CeO2 [2]. Also studied were the electronic properties of mixed-metal oxides [11, 12], and pure and doped metal surfaces [13]. Much of the above work also investigated the effects of structural defects (steps, kinks, corners, O-vacancies) and doping with a second metal. The Brookhaven group has also invested a significant experimental effort in order to characterize the atomic/ionic species and the electronic density of states on the oxide surfaces, using state-of-the-art spectroscopic techniques. Some of these include: X-ray absorption near-edge spectroscopy (XANES), X-ray and Ultraviolet photoemission spectroscopy (XPS, UPS), and thermal desorption mass spectroscopy (TDS). The close coupling between theory and experiment is making possible a fundamental understanding of many phenomena associated with the chemistry of molecules on oxide surfaces. In particular, the importance of band-orbital interactions for the reactivity of oxide surfaces has become clear, and a correlation between the electronic and chemical properties of mixed and doped oxides has been established. This has opened the way for using simple models based on band-orbital mixing to provide a conceptual framework for modifying or controlling the chemical activity of pure oxides, and for better engineering of mixed-metal oxides.

Adsorbed states of CO and N2 on metal surface

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]

Challenges in Catalysis for the Conversion of Fossil 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

Dream Reactions Awaiting Catalyst Development

( according to Jens Rostrup-Nielsen)CH4 + ½ O2 ↔ CH3OHCH4 + 1/2O2 ↔ CO + 2H2 2CH4 + O2 ↔ C2H4 +2 H2O nCH4 ↔ CnH2n+2 + (2n-2) H2 Dimethyl ether ↔ C2H5OHH2 + O2 ↔ H2O22NO ↔ N2 + O22N2 + 2H2O+5 O2 ↔ 4HNO3

Model of a heterogeneous solid surface, depicting different surface sites.

These sites are distinguishable by their number of nearest neighbours.

Adsorbed states of CO and N2 on metal surface

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]