Pivotal role of triethanolamine species in Rhodium-catalysed carbon dioxide photoreduction

Description

Abstract

To achieve high catalytic activity of artificial photosystems in carbon dioxide reduction or water reduction/oxidation into renewable energy vectors, the use of sacrificial electron donors (SEDs) remains mandatory. Despite significant progress in artificial photocatalysis, a detailed understanding of the influence of the SEDs on the reaction mechanism is still lacking. We have explored the roles of triethanolamine as SED in the selective CO₂ to formic acid photoreduction promoted by various pentamethylcyclopentadienyl rhodium(III)-based porous materials as model catalysts, including polyoxometalate-doped systems. In the presence of anionic polyoxometalates, agglomeration of protonated triethanolamine close to the catalytically active site favours H₂ evolution over CO₂ reduction, which can be modulated through the degree of confinement imposed by the pores size. Radical trapping experiments and EPR spectroscopy show that triethanolamine-radicals are crucial for a high activity in CO₂ reduction. Triethanolamine-radicals act as electron relays in the efficient two-electron activation of the catalyst, regardless whether a homogeneous or a heterogeneous catalyst is used. In contrast, triethylamine-radicals cause deactivation of the one-electron reduced catalyst, thus highlighting the effect of different SED-intermediates and underling the importance of an appropriate SED.