Complexity structures of active sites in catalysis
Four types of solid catalysts, having different complexity structures, are considered in this mini-review to show the key role of self-organizing phenomena in catalysis: i) single-site heterogeneous catalysts; ii) supported metal nanoparticles; iii) zeolites and iv) transition metal oxides operating via redox cycles. These self-organizing phenomena are an intrinsic property of any catalyst and reflect different complexity structures, as the self-repair and reorganization of active sites, the mutual interactions between neighbouring sites, the mass transfer limitations (i.e. shape selective control), the dynamic networks of “sites-joined”, etc. Consequently, the catalytic process typically results in a non-linear combination of various complexity structures and hence catalytic behaviours may appear. All these aspects are considered in the present paper to illustrate the nature of active sites and their complexity in catalysis.
Many investigations have confirmed that active sites are dynamic entities that can continuously change their physico-chemical properties depending on the reaction conditions (1,2).
It has been demonstrated that active centres may include more than one single site to form “reactive groups” (3). For instance, in the case of dissociative hydrogen adsorption on Pd(111) surface, an aggregate of three or more vacant sites is needed to form an active centre for efficient H2 dissociation, as revealed by dynamic STM analysis and DFT calculations (4, 5). Similarly, spectroscopic studies have shown that the activity of sulfonic acid resins for reactions taking place in a network of –SO3H groups (i.e. dehydration reactions of alcohols) is typically higher than that of resins with isolated –SO3H, suggesting the important role of H-bonded active networks (5).
However, with the exception of the simplest gas-phase reactions, a detailed knowledge of reaction mechanisms (i.e. kinetic parameters for the individual steps) is often very difficult to achieve (6-10). During a reaction, indeed, the catalyst surface usually interacts with a ...