Photocatalytic water oxidation with cobalt-containing tungstobismutates: tuning the metal core

Abstract

A new type of tungstobismutate water oxidation catalyst (WOC) with a disordered Co|W core, [{Co(H₂O)₃}₂{CoBi₂W₁₉O₆₆(OH)₄}]¹⁰⁻ (1) was tested for visible-light-driven performance and compared to a series of isostructural Co- and Mn-containing polyoxometalates with variable transition metal contents, ([Co_2.5(H₂O)₆{Bi₂W_19.5O₆₆(OH)₄}]⁸⁻ (2) and [Mn_1.5(H₂O)₆{Bi₂W_20.5O₆₈(OH)₂}]⁶⁻ (3)). All compounds were structurally characterized, and no indications for significant decomposition under catalytic conditions for visible-light-driven water oxidation ([Ru(bpy)₃]²⁺ as photosensitizer (PS) and S₂O₈²⁻ as electron acceptor in different buffer systems) were found. For the first time, subtle differences in the core disorder patterns of isostructural POM-WOCs were revealed to be decisive for the catalytic activity of (1) (maximum TON of 21 with 97% oxygen yield for 115 μM (1)). Performance comparison of the POM series sheds new light on the structure–activity relationships for targeted POM-WOC construction. Indeed, the Co disorder differences between (1) and (2) exclusively affect the sterically more accessible external site of the two crystallographically independent Co core positions which has a 25% higher Co occupancy in (1). This points to a stereoselective reaction pattern for the tetranuclear POM core of WOC (1) which might open up novel construction strategies for the economic redesign of sandwich-type POM-WOCs. In parallel, we demonstrate for the POM series (1)–(3) that electrochemical measurements under catalytic conditions are a promising and convenient pre-screening strategy for WOC activity. Furthermore, POM/PS complex formation of (1) with [Ru(bpy)₃]²⁺ is investigated in detail, and the different roles of Mn- and Co-centers in POM-WOC synthesis are compared.