Experimental and Theoretical Studies of Isolated-Site Catalysts: Lessons for Nanocatalysis

While supported particles of metals and oxides exhibit changes in specific activity with particle size and shape, the identification of what is meant by an active site on such catalysts is very difficult because of the diversity of possible active site. As a result, it is usually difficult to identify the exact cause for changes in catalyst activity and selectivity when changes are made in catalyst composition and structure. By contrast, single-site catalysts are exceptionally good models systems for exploring the consequences of site composition and structure on the mechanism and kinetics of catalyzed reactions because such catalysts can be prepared in such a way that almost all sites are identical, or nearly identical. Example of single sites include metal cations present in either framework or extra-framework positions in zeolites, metal oxo species dispersed onto oxide supports, and supported metal atoms and small clusters. This talk will illustrate the characterization of such structures by means of XANES and EXAFS, aided by simulations of such data based on theoretical models. Methods for studying the progress of elementary processes involved in catalyzed reactions will be illustrated for the oxidation of methanol to formaldehyde on supported vanadate species and the oxidative carbonylation of methanol to dimethyl carbonate on Cu-exchanged zeolites. These studies will also show that the hypotheses of reaction mechanism and the influence of site and support composition can be understood from first principles through the application theoretical analysis.