Nickel-Containing Keggin-Type Polyoxometalates as Hydrogen Evolution Catalysts: Photochemical Structure-Activity Relationships

Abstract

In search of structure-activity relationships for polyoxometalate (POM)-based water reduction catalysts, nickel-monosubstituted Keggin-type POMs ([Ni(H₂ O)XW₁₁ O₃₉ ]^n- ; XP, Si, Ge) were compared with respect to their activity in photochemical hydrogen evolution. The title compound series was characterized by single-crystal X-ray diffraction methods and a wide range of spectroscopic and electrochemical techniques. Nickel substitution was identified as a crucial feature for catalytic activity through comparison with nickel-free reference POMs. Furthermore, turnover number (TON) and turnover frequency strongly depended on the heteroatom X, and the highest TON among the series was recorded for [Ni(H₂ O)GeW₁₁ O₃₉ ]^6- . Photochemical hydrogen evolution activity was compared with redox and onset potentials obtained from electrochemical analyses. Furthermore, activity trends were correlated with electronic structure properties derived from density functional theory calculations.

Keywords: electrochemistry; nickel; photochemistry; polyoxometalates; structure-activity relationships.