Interfacial photoreduction of supercritical CO2 by an aqueous catalyst.

Carbon dioxide (CO2) emissions currently represent a major concern. Accordingly, the electrochemical reduction of CO2 is a reaction of major relevance and also of great challenges. Considering the redox potentials required for reducing CO2, multi-electron-transfer, multiproton reactions are the most viable pathways for driving CO2 reduction. [2] Nonetheless, catalysts must be employed to overcome kinetic limitations of these processes. Molecular catalysts known to reduce CO2 are usually monomeric metal complexes, such as ruthenium and rhenium carbonyl complexes, cobalt and iron porphyrins, and nickel and cobalt cyclams, but combinations of these systems into bimetallic structures have also been proposed. More recently, the introduction of supercritical CO2 (scCO2) as a reaction medium has been investigated, as comprehensively reviewed by Fujita and co-workers. Interestingly, it has been observed that increasing the CO2 pressure in the reaction medium usually does not provide any advantage. Herein, water–scCO2 binary biphasic systems are evaluated for reducing carbon dioxide using a well-known aqueous cycle with (1,4,8,11-tetraazacyclotetradecane)nickel(II) ([Nicyclam]) as the catalytic unit, given its high affinity towards CO2 upon reduction [12] (see the Supporting Information, Figure S1a) and proven efficiency in aqueous media. As for the photosensitizer (PS), an aqueous soluble salt of tris(2,2’-bipyridyl)ruthenium(II) ([Ru(bpy)3] ) was chosen. This catalytic cycle was pioneered electrochemically by Sauvage and co-workers, photochemically by Calvin and co-workers, and by Kimura et al. Based on the reduction potentials of [Nicyclam] and PS, this cycle (see the Supporting Information, Figure S1b) involves in its initial stage the absorption of light by [Ru(bpy)3] 2+ and subsequent reductive quenching of its excited state by ascorbate (kq= 2 10m 1 s ). Subsequent reactions between the reduced photosensitizer and the catalyst in presence of CO2 were shown to yield exclusively CO and H2 in aqueous media. Over the last few years, we have shown that reactions such as hydrogen evolution or oxygen reduction could be carried out efficiently at the water–1,2-dichloroethane interface at atmospheric pressure by treating aqueous protons or oxygen with organic electron donors. Herein, we show that the interface between an aqueous solution and supercritical CO2 can be the locus for CO2 reduction, as depicted in Scheme 1.