Photocatalytic CO₂ reduction by Ni-substituted polyoxometalates: Structure-activity relationships and mechanistic insights

The photocatalytic activity for CO2 reduction of a series of Ni-substituted polyoxometalates (POMs) differing in nuclearity, shape and size, has been investigated under visible light irradiation, with [Ru(bpy)3]2+ (bpy = 2,2 '- bipyridine) as photosensitizer and triethanolamine as sacrificial donor. The tetrabutylammonium salt of the Ni4 tetranuclear species was found to exhibit the highest CO production and its stability under photocatalytic conditions was demonstrated. The catalytic performance was significantly lower for the alkaline salt due to the separation of the POM from its counter -ions occurring only for the tetrabutylammonium salt. Photophysical experiments evidenced a bimolecular electron transfer from the reduced photosensitizer [Ru(bpy)3]+ to the Ni4 POM, the former arising from the reductive quenching of the [Ru(bpy)3]2+ excited state by triethanolamine. This was further supported by DFT calculations, which also showed that the Ni4 POM accumulates at least two electrons and four protons to carry out the CO2 reduction catalytic process.