Surface science and catalysis

Abstract

Modern surface science studies have explored a large number of metal catalyst systems. Three classes of catalytic reactions can be identified: (1) those that occur over the metal surface; (2) reactions that take place on top of a strongly adsorbed overlayer and (3) reactions that occur on coadsorbate modified surfaces. Case histories for each class are presented. The first class of reactions is structure sensitive, and rough surfaces are needed for high reactivity. Ammonia synthesis, H 2 /D 2 exchange and complex hydrocarbon reaction studies all indicate the importance of high coordination sites that are present on atomically rough, open crystal surfaces, mostly in the second layer. A model is presented that correlates increased charge fluctuations at these sites with enhanced catalytic activity. The second class of reactions includes ethylene hydrogenation. It occurs via hydrogen transfer through a tenaciously held ethylidyne overlayer to weakly adsorbed ethylene molecules on top of this layer. Most catalytic reactions occur on coadsorbate modified metal surfaces. Sulphur, potassium, carbonaceous fragments and other metal constituents are employed most frequently as surface modifiers. These additives block sites and alter the bonding of the reactants, thereby changing the reaction paths. Their effects will be demonstrated through discussions of the CO/benzene, S/thiophene and CO/K interactions.