Psi-k - Ab initio (from electronic structure) calculation of complex processes in materials

Working Group 6 - Surface Science, Catalysis and Corrosion

Juergen Hafner, University of Vienna, Vienna, Austria

Surfaces are the cutting edge of material sciences, i.e., a surface is the place where molecules from the gas phase or a liquid come into contact with a material, and where chemical bonds of these approaching molecules may be cut and new bonds formed. If we look around, everything we see is surfaces, and to understand the properties of materials, to understand how materials can be produced, how they can be grown, to understand why and how materials corrode or rust, to understand how to protect materials, for example how to make the surface hard, and to understand how catalysis works and how it may be improved, -- for all this, one has to understand surfaces.

Numerous experiments in ultra-high vacuum as well as theoretical studies on surfaces at zero temperature and pressure have been performed over the last decades in order to gain a better understanding of the mechanisms, which, for example, underlie the phenomena of catalysis and corrosion. Often the results achieved this way cannot be extrapolated directly to the technologically relevant situation of finite-temperature and high-pressure. Accordingly, modern surface science has realized that bridging the so-called pressure-gap (getting out of the vacuum) is the inevitable way to go. Of similar importance are studies in which the temperature is changed systematically (warming up and cooling down). Both aspects are being taken into account in recent experiments and ab initio calculations.

It is evident that there is still much to learn about the intricate molecular and atomic processes that occur at surfaces, although significant advances in this exciting field have been made over the years. We also note that the impact of ab initio simulations has been particularly profound in this area. Scientists of the European groups, that take part in this working group, are at the forefront of theoretical research on catalysis and corrosion.

The stumbling-stone on the way to further progress is mainly the difference in the length and time scales involved in computer- and laboratory experiments. So far surface science has concentrated on the identification and characterization of stable geometries and transition states. In the future, a comprehensive description of all competing processes will be required. To achieve this, the interplay of many different individual reactions during a catalytic process will be modelled using statistical-mechanical approaches (kinetic Monte Carlo (MC) or "smart" MC), the parameters (energy barriers and prefactors) being derived from modern density-functional calculations on large supercells. Recent results indicate that it will be possible not only to model known experimental facts, but to develop a theory of reactions kinetics that is PREDICTIVE and whose parameters are calculated from first principles and carry true microscopic meaning.

A further important step just being realized in catalysis research is the need to consider realistic environment (high pressure and high temperature). Recent theoretical work, followed by experiments has shown that catalysts change significantly under actual working conditions. For example, for oxidation catalysis it occurs that the surface of a metallic catalyst undergoes significant changes, e.g. an oxide layer (or a sub oxide) is formed, and thus adsorption, dissociation, and reactions are very different from those studied previously for pristine metal surfaces. Similarly, the active sites in sulfidic catalysts used in environmentally important hydrodesulfurization reactions change apppreciably as a function of the chemical potential of all reactants. Here it is important to emphasize that the microscopic characterization of the active sites of a catalyst under working conditions is probably more accessible to a computer- than to a laboratory experiment.

A very important class of catalysts are zeolites. These are mesoporous alumino-silicates with very complex crystal structures - complex enough to be out of reach for ab-initio calculations until recently. The importance of zeolites is in the combination of their acidic function with the molecular-sieving properties arising from their pore-structure. Their functionality may be further enhanced by introducing nanoclusters of metals, leading to bi-functional catalysts. Ab-initio simulations of important catalytic reactions in industrially used zeolites will be feasible in a near future. Furthermore, investigations of catalyst--support interactions and bifunctional catalysts will be within the reach of ab-initio simulations, planned to address the modeling of bifunctional catalysis.

Our discussion so far has concentrated on certain aspects of heterogeneous catalysis, but the same ab-initio techniques will enable us to realize important progress in other fields: corrosion, passivation, formation of protective overlayers, formation of nanostructured films for electronic and magnetic devices. At this point the interests of the working group on "Surface science, catalysis and corrosion" overlaps with the research themes of the working groups on magnetism, interfaces and semiconductors for recording purposes.

MEMBERS OF THE WORKING GROUP [14 members]