VII- Theoretical Appendix A catalytic reaction consists of a series of elementary steps: the adsorption of the reactant on the catalyst surface, diffusion or migration on the surface for recombination, the breaking of some bonds, and the creation of some new ones to form the product molecule. The last step of every heterogeneous catalytic reaction is the desorption of the product molecule from the catalyst surface, which regenerates the catalyst site for the next sequence of the reaction (Figure S5.8). In heterogeneous catalysis, the energy barrier for each of these elementary steps is greatly determined by the electronic interaction of the relevant molecule or molecules (reactants, intermediates, or products) with the catalyst surface. These electronic interactions, at their most fundamental levels, consist of a series of complicated quantum phenomena: the dynamic interaction of orbitals with the electronic states of the metal surface, whose full understanding is a demanding task and requires a high-level quantum chemical approach. For more details see the monographs from Hoffmann and Van Santen111-114. The fundamental question of why some metal catalysts accelerate a specific type of reaction while others do not, can only be answered by decoding these complex electronic effects. Scientists in the field of heterogeneous catalysis have been quite successful in developing models that can relate the trends of catalytic reactions Figure S5.8. Schematic illustration of the various elementary steps for a catalytic reaction occurring at a metal surface. The overall rate of the reaction and selectivity towards a specific product is greatly dependent on which of these steps is ratedetermining. The figure is adopted from2.
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