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Free keywords:
ammonia synthesis; CO oxidation; dynamic catalyst; heterogeneous catalysis
Abstract:
Heterogeneous catalysis and functional surfaces are intimately connected. Surface functionalities mediate the catalytic reactions. However, the surface must fulfil several requirements: adsorption of the reactants on the surface must be energetically favoured (but reversible), the surface functionalities must enhance the dissociation of chemical bonds in the reactant and the formation of new ones which will lead to the reaction's product(s), and finally the product(s) must be able to desorb and leave the surface. It was shown that for the ammonia synthesis on ruthenium single crystalline surfaces, both the geometric and electronic functionalities of step edges are crucial to dissociate nitrogen. Another well-investigated reaction is the catalytic oxidation of carbon monoxide over platinum and oxidized ruthenium. For both metals, the surface is dynamic, which leads to a change in its functionalities during reaction. These few examples of simple reactions already show the complexity of the necessary surface functionalities for heterogeneous catalysis and the problems connected with the understanding of rather simple reaction mechanisms. The key objective in catalysis research consists in tailoring the geometric and electronic properties of catalysts depending on the target reactions. One attempt in this direction is the investigation of binary alloys and, more recently, of intermetallic compounds. Here, the adsorption properties of the reactants could be drastically changed. A completely different way of changing adsorption properties also exists for semiconductor surfaces: the direct application of electric fields leads to a change of charge carriers near the surface – the electro-adsorptive effect (EAE).
