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

Abstract

To realize the evolution of C₂₊ hydrocarbons like C₂H₄ from CO₂ 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_X@p-ZnO) with CuO_X uniformly dispersed among polycrystalline ZnO was synthesized. Upon illumination, the catalyst exhibited the ability to reduce CO₂ to C₂H₄ with a 32.9% selectivity, and the evolution rate was 2.7 μmol·g^-1·h^-1 with water as a hole scavenger and as high as 22.3 μmol·g^-1·h^-1 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 C₂H₄. The C-C coupling intermediate *OC-COH was experimentally identified by in situ FT-IR studies for the first time during photocatalytic CO₂ 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 CO₂ to C₂₊ hydrocarbons in a photocatalytic system.