Microsecond X-ray Absorption Spectroscopy Identification of CoI Intermediates in Cobaloxime-Catalyzed Hydrogen Evolution

Rational development of efficient photocatalytic systems for hydrogen production requires understanding the catalytic mechanism and detailed information about the structure of intermediates in the catalytic cycle. We demonstrate how time-resolved X-ray absorption spectroscopy in the microsecond time range can be used to identify such intermediates and to determine their local geometric structure. This method was used to obtain the solution structure of the CoI intermediate of cobaloxime, which is a non-noble metal catalyst for solar hydrogen production from water. Distances between cobalt and the nearest ligands including two solvent molecules and displacement of the cobalt atom out of plane formed by the planar ligands have been determined. Combining in situ X-ray absorption and UV/Vis data, we demonstrate how slight modification of the catalyst structure can lead to the formation of a catalytically inactive CoI state under similar conditions. Possible deactivation mechanisms are discussed. Distinct CoI states are generated under light-driven conditions depending on the structure of two cobaloxime complexes (see figure). Only one is catalytically active for H2 evolution and its structure is determined using quantitative analysis of time-resolved XANES spectra.