Pivotal role of triethanolamine species in Rhodi-um-catalysed carbon dioxide photoreduction

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 do-nors (SEDs) remains mandatory. Despite significant progress in artificial photocatalysis, a detailed understanding of the influence of the SEDs on the reaction mechanism is still lack-ing. 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 pres-ence 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 cata-lyst, thus highlighting the effect of different SED-intermediates and underling the im-portance of an appropriate SED.