Mononuclear metal complex catalysts on supports: foundations in organometallic and surface chemistry and insights...

Catalysts that consist of isolated metal atoms bonded to solid supports have drawn wide attention by researchers, with recent work emphasizing noble metals on metal oxide and zeolite supports. Progress has been facilitated by methods for atomic-scale imaging the metals and spectroscopic characterization of the supported structures and the nature of metal-support bonding, even with catalysts in the working state. Because of the intrinsic heterogeneity of support surface sites for bonding of metals and the tendency of noble metal cations on supports to be reduced and aggregated, it is challenging to determine structures of individual metal complexes among the mixtures that may be present and to determine structures of catalytically active species and reactive intermediates. A central premise of this perspective is that synthesis of supported metal complexes that have nearly uniform structures-on supports such as dealuminated HY zeolite, chosen to have relatively uniform surfaces-is a key to fundamental understanding, facilitating progress toward determining the roles of the ligands on the metals, which include the supports and reactive intermediates in catalysis. Characterization of relatively uniform and well-defined samples nonetheless requires multiple spectroscopic, microscopic, and theory-based techniques used in concert and still leaves open many questions about the nature of reactive intermediates and catalytic reaction mechanisms.