Heterogeneous catalysts modify the rate of a chemical reaction by changing the activation barrier for at least one critical elementary step. This kinetic operation requires catalysts to be reactive themselves. The main difference to other reactive materials is their cyclic operation returning them after having performed a turnover of reaction in their initial state of geometric and electronic structure. This property is referred to as “dynamical”.

Solid surfaces can behave dynamically most easily when they are not in a highly ordered state in order to minimize collective activations of the geometric structure. Collective excitation of the electronic structure would lead easily to over-reaction and hence preclude selective reactions (concept of site isolation). These requirements call for a hierarchical structure of a working catalyst in which a stable matrix contains adaptive sites as clusters which change their properties according to the chemical potential of the gas phase to accommodate for adsorption of reactants and desorption of products.

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