Enhanced Hydrogen Supply to Atomically Dispersed Copper Sites through Close Cooperation with Oxygen Vacancies in Black TiO2 to Promote CH4 Production in CO2 Electrolysis

CO 2 electroreduction (eCO 2 R) holds promise as an environmentally friendly approach to reducing greenhouse gas emissions. Cu is a representative catalyst with high eCO 2 R activity. However, its selectivity for CH 4 synthesis is still insufficient due to the slow eight-electron transfer to a single carbon, the predominance of C–C coupling reactions toward C 2+ products on Cu, as well as occurrence of the hydrogen evolution reaction. Here, for high CH 4 selectivity, we demonstrate a genuine hydrogen supply to atomically dispersed Cu sites (AD-Cu) via the cooperative function of oxygen vacancy (V O ) formed on defective black anatase TiO 2 ( Cu–TiO 2 –H 2 ), that is prepared by exposing Cu-doped TiO 2 (Cu–TiO 2 ) to hydrogen gas. Cu–TiO 2 –H 2 exhibited a remarkable Faradaic efficiency for CH 4 production of 63% and a partial current density of −120 mA cm –2 . The catalytic mechanism for the high CH 4 selectivity was elucidated using a variety of spectroscopies, such as electron spin resonance, reversed double-beam photoacoustic spectroscopy (RDB-PAS) and in situ Raman measurements, with the support of quantum chemical calculations. In situ Raman measurements revealed that Cu–TiO 2 –H 2 greatly accelerates proton consumption for the hydrogenation of *CO intermediates and that the surface pH on Cu–TiO 2 –H 2 is sufficiently high to stabilize *CHO intermediates, key species for CH 4 formation. DFT calculations support the stability of the intermediates during the process of forming *CHO. All our results suggest that V O contiguous to AD-Cu on Cu–TiO 2 –H 2 promotes water dissociation and smoothly supplies hydrogen to AD-Cu on Cu–TiO 2 –H 2, thus facilitating CH 4 formation in eCO 2 R.