Photocatalytic C-C Coupling from Carbon Dioxide Reduction on Copper Oxide with Mixed-Valence Copper(I)/Copper(II).

To realize the evolution of C hydrocarbons like CH from CO reduction in photocatalytic systems remains a great challenge, owing to the gap between the relatively lower efficiency of multielectron transfer in photocatalysis and the sluggish kinetics of C-C coupling. Herein, with Cu-doped zeolitic imidazolate framework-8 (ZIF-8) as a precursor, a hybrid photocatalyst (CuO@p-ZnO) with CuO uniformly dispersed among polycrystalline ZnO was synthesized. Upon illumination, the catalyst exhibited the ability to reduce CO to CH with a 32.9% selectivity, and the evolution rate was 2.7 μmol·g·h with water as a hole scavenger and as high as 22.3 μmol·g·h in the presence of triethylamine as a sacrificial agent, all of which have rarely been achieved in photocatalytic systems. The X-ray absorption fine structure spectra coupled with in situ FT-IR studies reveal that, in the original catalyst, Cu mainly existed in the form of CuO, while a unique Cu surface layer upon the CuO matrix was formed during the photocatalytic reaction, and this surface Cu site is the active site to anchor the in situ generated CO and further perform C-C coupling to form CH. The C-C coupling intermediate *OC-COH was experimentally identified by in situ FT-IR studies for the first time during photocatalytic CO reduction. Moreover, theoretical calculations further showed the critical role of such Cu sites in strengthening the binding of *CO and stabilizing the C-C coupling intermediate. This work uncovers a new paradigm to achieve the reduction of CO to C hydrocarbons in a photocatalytic system.