Understanding Heterogeneous Catalysis from the Fundamentals
Artikel i vetenskaplig tidskrift, 2008
Catalysis describes the acceleration of a chemical reaction by means of a substance that is itself not consumed by the overall reaction. It is not only important for numerous human activities, but it has also always been a major spur for the development of surface science. Today there is an extensive surface-science heritage of understanding, and there are examples of catalysis phenomena that are now understood from the fundamentals. For instance, modern-day theory is able to predict the turnover frequency of an industrially relevant catalytic reaction in a semi-quantitative way. A major part of this chapter sums up such a successful surface-scientific development, pointing out descriptors for metal catalysts and identifying trends in adsorption energies and activation energies for surface reactions on transition metal surfaces by extensive computations. This is done using the density-functional theory (DFT), whose accuracy in this context is secured, and analyzed in electron-structural terms, in particular the d-band model. Via correlations determined from DFT calculations, universal relationships in heterogeneous catalysis are built up, including variations in catalytic rates, volcano relations. The optimization and design of catalysts through modeling is within reach. For instance, experimental verification for pure CO methanation, for CO2 methanation, and for simultaneous CO and CO2 methanation means that a technical methanation catalyst is discovered on the basis of computational screening. To further detail the surface-science approach to catalysis it is natural to supplement this presentation with some other examples of recent work on some catalytic reactions from the fundamentals. Oxidation of some monoxides illustrates the use of kinetic Monte Carlo simulations. The successful prediction of the outcome of the ammonia synthesis from first-principles supports the view that in the future theory will be a fully integrated tool in the search for the next generation of catalysts. The hydrogen evolution reaction on MoS2 is given as an example of successful interplay between theory and experiment. It is concluded that, thanks to the strong development in surface science, the understanding of heterogeneous catalysis from the fundamentals is approaching an advanced stage. Design of new catalyst on the basis of computational screening is today a realistic perspective. The list of issues that need further considerations includes nonadiabaticity, complex reactions, and other classes of catalyst materials than transition metals.