Surface chemistry of hydrocarbon fragments on transition metals: towards understanding catalytic processes

The central issue of this review is the importance of selectivity in heterogeneous catalysis. The implications of our mechanistic studies on model systems to the interpretation of catalytic processes are discussed. In particular, it is our thesis that the optimization of catalytic selectivity may require a different approach than that needed to increase overall catalytic activity. This is so because a given intermediate in the reaction mechanism may be slow to form, but able to react by following more than one relatively fast pathway. In the case of hydrocarbon reforming, while activity is often controlled by alkane activation, selectivity is likely to depend on the regioselectivity of the subsequent dehydrogenation of the resulting alkyl surface intermediates. Similar arguments can be used to explain the selectivity between dehydrogenation and dehydration reactions on alcohols. Additional complications arise from the build up of carbonaceous deposits on the surface of the catalyst under reaction conditions. Surface-science studies have led to the conclusion that these deposits modify the properties of the exposed catalytic metal, opening up new channels for mild reactions but still allowing for the occurrence of the more demanding reactions involved in hydrocarbon reforming and oxidation processes. Lastly, catalyst modification can be introduced in a controlled fashion, as in the case of the bestowing of chirality to normal hydrogenation catalysts by the addition of small amounts of chiral cocatalysts to the reaction mixture. A brief survey of our recent investigation of these systems is provided.