Water coordinated on Cu(I)-based catalysts is the oxygen source in CO reduction to CO.

Catalytic reduction of CO over Cu-based catalysts can produce various carbon-based products such as the critical intermediate CO, yet significant challenges remain in shedding light on the underlying mechanisms. Here, we develop a modified triple-stage quadrupole mass spectrometer to monitor the reduction of CO to CO in the gas phase online. Our experimental observations reveal that the coordinated HO on Cu(I)-based catalysts promotes CO adsorption and reduction to CO, and the resulting efficiencies are two orders of magnitude higher than those without HO. Isotope-labeling studies render compelling evidence that the O atom in produced CO originates from the coordinated HO on catalysts, rather than CO itself. Combining experimental observations and computational calculations with density functional theory, we propose a detailed reaction mechanism of CO reduction to CO over Cu(I)-based catalysts with coordinated HO. This study offers an effective method to reveal the vital roles of HO in promoting metal catalysts to CO reduction.