Efficient photocatalytic hydrogen production in water using a cobalt(iii) tetraaza-macrocyclic catalyst: electrochemical generation of the low-valent Co(i) species and its reactivity toward proton reduction

Abstract

A very efficient homogeneous system for visible-light driven hydrogen production in water is reported. This comprises the [Co(CR)Cl₂]⁺ cobalt(III) tetraaza-macrocyclic complex (Cat1) as a noble metal-free catalyst, [Ru(bpy)₃]Cl₂ as a photosensitizer and ascorbate/ascorbic acid as a sacrificial electron donor and buffer. This system gives up to 1000 turnovers at pH 4.0 versus the catalyst with a relatively low photosensitizer/catalyst ratio (10/1) and a high concentration of catalyst (1 × 10⁻⁴ M), thus producing a significant amount of H₂ (12.3 mL for 5 mL of solution). It also exhibits long-term stability (more than 20 hours). The efficiency of Cat1 has been compared under the same experimental conditions to those of three other H₂-evolving catalysts, which are known to operate in water, [Co{(DO)(DOH)pn}Br₂] (Cat2), [Co(dmbpy)₃]Cl₂ (Cat3) and [Rh(dmbpy)₂Cl₂]Cl (Cat4). These comparative studies show that Cat4, although based on a noble metal, is about four times less active, while Cat2 and Cat3 produce more than one hundred times less hydrogen than Cat1. The low-valent Co^I form of Cat1 has been successfully electrogenerated in CH₃CN. Its high stability can be related to the high catalytic performance of the Cat1 system. We have also shown that in acidic aqueous solution (photocatalytic conditions) reduction at a slightly more negative potential than the Co^II/Co^I couple is needed to ensure efficient catalysis; this reduction is performed by the photogenerated [Ru^II(bpy)₂(bpy^˙−)]⁺ species.