Copper-based water reduction catalysts for efficient light-driven hydrogen generation

Abstract

Although largely neglected, convenient and commercially available copper salts constitute appropriate precursors for efficient and stable water reduction catalysts (WRC). Various copper salts have been applied in the photocatalytic proton reduction together with [Ir(ppy)2(bpy)]PF6 as photosensitizer (PS) and triethylamine (TEA) as electron donor. Among them, copper(I) iodide showed the best productivity, resulting in a TONCu of 260 and a stability of 4 days. The addition of 2,2′-bipyridine (bpy) led to a significant improvement of the catalytic system. Application of 2equiv. of bpy and an increased amount of PS improved the stability as well as the productivity (TONCu 711). Remarkably, this catalytic system was reactivated seven times with only further addition of fresh PS and TEA. Consequently, this copper-based WRC was shown to be active for more than 48 days reaching a maximum TONCu of >3900, which is in the same order of magnitude as previously known noble metal WRCs. Aberration-corrected HAADF-STEM measurements revealed a part transformation of the molecularly-defined copper precursor into Cu containing nanoparticles of 5–10nm size. Additionally, by XAS measurements the conversion of the copper(I) salts into Cu(0) compounds under light irradiation was observed. With respect to low costs and high abundance of copper these WRCs are an appropriate alternative for noble metal based catalysts.