Ever since Sir Hugh Stott Taylor proposed that a catalyzed chemical reaction is not catalyzed uniformly over the entire solid surface of the catalyst but only at certain 'active sites' or centers it is understood that for supported metal catalysts the catalytic action is connected with exposed atoms on the corners, edges, and various types of surface facets/defects of metal crystals. The presence of heterogenous sites is a hallmark of supported metal catalysts of various sizes and shapes. Even with shape-controlled metal nanocrystals, there exists a diversity of exposed metal “sites” that possess different coordination environments, leading to different catalytic activity/selectivity. Furthermore, metal-support interactions, which are required to stabilize metal particles/clusters during a catalytic reaction, produce new sites that could render different catalytic properties. Because of the complexities, accurate determination of active sites and therefore the understanding of their catalytic behaviors of supported metal catalysts are elusive for most of important catalytic reactions. The recent development of isolated single metal atoms as practical catalysts provides a platform for investigating the fundamental nature of heterogeneous catalysts. A single-atom catalyst (SAC) usually does not manifest as catalytically single site because of the heterogenous nature of supports and the strong interactions between the isolated metal atoms and the various types of anchoring sites on the support surfaces. The active site in a SAC, however, has to involve the isolated single metal atom. The electronic properties of the supported single metal atoms, however, can be strongly modulated by its nearest neighbors. For certain catalytic reactions, metal dimers (including hetero-atom dimers), trimers, and tetramers may provide unique advantages. The phase space of these cluster catalysts can be tremendous, leading to vast opportunities in developing new catalysts for targeted chemical reactions. The complexity in identifying active centers, especially the structural transformation and evolution of active metal sites during a targeted catalytic reaction, poses a formidable challenge. Stabilization strategies for confining metal atoms and subnano metal species on solid supports have been developed for practical industrial applications.

Probing catalytically active species: From single metal atoms to subnano clusters

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.