Active centers of redox catalysts

Abstract

The development of representations about the active site structure of solid-phase catalysts, ranging from the work of H. Taylor to a modern understanding of the complex and multi-level structure of catalytic systems, is considered. The main types of active centers of catalysts for redox processes of deep, selective, and preferential conversion are analyzed. It is shown that for each type of reaction, regardless of the chemical nature of the catalyst components, the structure of the active center is characterized by certain common features and determines the direction of conversion. Particular attention is paid to the structure of active sites formed by the type of an isolated active center ("Single Site Isolation"), which allows achieving high selectivity of catalytic processes in the direction of target products obtaining and implementation of new reactions. In particular, the reaction of methane oxidative carbonylation to acetic acid was first carried out in a gas phase using molecular oxygen as an oxidant and catalysts whose active centers were presented by isolated Rh3+ ions in the composition of rhodium selenochloride. A separate type of active center is presented by atoms located on the grain boundaries of crystallites, which arise as a result of interfacing interaction between catalyst components: support, active component, modificator, as well as grain boundaries between homogeneous nanocrystallites in agglomerated systems. It is shown that an important role in the manifestation of catalytic properties plays the availability of an active center for reagents, caused by the spatial structure of catalysts. Zeolites, organometallic compounds (MOF), mesostructural oxides in which active centers are located inside the cavity channels are examples of such catalytic systems. The main strategy of research in the field of advanced catalysts is aimed at developing methods for the synthesis of catalytic materials, which provide formation as the maximum number of active centers, so their availability for reagents and subsequent conversion to target products. Designing such systems is a complex task, based on establishing a correlation between composition, structure, and size characteristics of catalytic materials.