Cu2O nanocubes with mixed oxidation-state facets for (photo)catalytic hydrogenation of carbon dioxide

Cuprous oxide (Cu2O), an earth-abundant, low-cost metal-oxide semiconductor, has received enormous attention for its CO2 reduction ability in aqueous media by photochemical, photoelectrochemical and electrochemical methods. An unresolved problem with all of these approaches, however, is the instability of the Cu2O caused by its tendency to undergo an irreversible redox disproportionation reaction. Here, we report a way to circumvent this troublesome behaviour of Cu2O by driving the CO2 reduction in the gas-phase via heterogeneous photocatalytic hydrogenation. To this end, Cu2O nanocubes with surfaces comprising mixed oxidation-state copper Cu(0,I,II) sites, oxygen vacancies [O] and hydroxyl OH groups were synthesized. These surfaces enable heterolysis of H2 and adsorption of CO2 under mild conditions; they facilitate the reverse water–gas shift reaction, while rendering the redox disproportionation reaction reversible. This synergism provides Cu2O nanocubes with high photocatalytic activity and stability. Cu2O is a promising photocatalyst, although its stability is compromised by a disproportionation reaction that leads to the formation of Cu and CuO. Now, a method is reported to stabilize Cu2O nanocubes, rendering them highly active and stable for the photocatalytic reverse water–gas shift reaction.