Charge State of Au25(SG)18 Nanoclusters Induced by Interaction with a Metal Organic Framework Support and Its Effect on Catalytic Performance

We investigated the charge transfer between Au-25(SG)(18) nanoclusters and metal-organic framework (MOF) supports including Mil-101-Cr, Mil-125-Ti, and ZIF-8 by an X-ray photoemission technique and discussed the influence of resulted charge states of supported Au-25(SG)(18) nanoclusters on the 4-nitrophenol reduction reaction. Charge transfer from Au-25(SG)(18) to Mil-101-Cr induces positive charge Au delta+ (0 < delta < 1) while charge transfer from ZIF-8 to Au-25(SG)(18) generates negative charge Au delta- due to different metal-support interactions. Au-25(SG)(18) on Mil-125 shows metallic Au-0, similar to unsupported Au-25(SG)(18), due to negligible charge transfer. The resulted charge state of Au delta- inhibits the formation of adsorbed hydride (H-) species because of electrostatic repulsion, while Au delta+ impairs the reductive ability of adsorbed hydride (H-) species due to strong affinity between them. In comparison, metallic Au-0 in Au-25(SG)(18)/Mil-125 and unsupported Au-25(SG)(18) presents the optimum catalytic activity. The current work provides guidelines to design effective metal nanoclusters in heterogeneous catalysis through metal-support interaction exerted by metal-oxo/nitric clusters within MOFs.