The role of the structural features (Cu–Cu distance) of Cu2O in C–C coupling in CO2RRs with H2O under solar irradiation. A DFT study for elucidating the reaction mechanism

In photochemical (and photoelectrochemical) coprocessing of CO2 and H2O, the control of the selectivity plays a fundamental role for maximising the light (and Faradaic) efficiency towards a product. In fact, a selective process would avoid post-reaction separation operations, saving energy and resources. Recently, we have shown that different frustules (the siliceous skeleton of microalgae diatoms) induce the formation of distinct facets ({111} octahedral or {110} rhombohedral) of Cu2O, which are characterized by a different Cu–Cu distance (see text). Notably, each facet promotes the formation of different CO2RPs. In particular, {111} affords methanol while {110} produces C2+ species. These findings suggested that the Cu–Cu distance in the Cu2O facets may play a key role in C–C coupling, additionally opening the question about single-site or multi-site catalysis in CO2RRs. To contribute to clarifying the role of the intermetallic distance in the Cu2O-driven photocatalysis, we have modelled by DFT calculations the CO2 reduction under PCET (Proton Coupled to Electron Transfer) conditions compatible with the mild reaction conditions (0.1 MPa pressure and maximum temperature of 45 ◦C) used in our photochemical experiments. The results of the present study shed light on three key points: i. the {110} rhombohedral facet of Cu2O shows the most suited Cu–Cu distance (ca. 260 pm) for C–C coupling with respect to {111} octahedral or {100} cubic facets (Cu–Cu distance of 370 and 417 pm, respectively), ii. the multi-site mechanism is energetically preferred to the single-site catalysis for C–C coupling; iii. engineering the surface structure of the photo(electro)catalyst is essential for a high selectivity towards a given CO2RP, either a C1 or a Cn species, determining the process energy efficiency.