Water coordinated on Cu(I)-based catalysts is the oxygen source in CO(2) reduction to CO

Catalytic reduction of CO(2) 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(2) to CO in the gas phase online. Our experimental observations reveal that the coordinated H(2)O on Cu(I)-based catalysts promotes CO(2) adsorption and reduction to CO, and the resulting efficiencies are two orders of magnitude higher than those without H(2)O. Isotope-labeling studies render compelling evidence that the O atom in produced CO originates from the coordinated H(2)O on catalysts, rather than CO(2) itself. Combining experimental observations and computational calculations with density functional theory, we propose a detailed reaction mechanism of CO(2) reduction to CO over Cu(I)-based catalysts with coordinated H(2)O. This study offers an effective method to reveal the vital roles of H(2)O in promoting metal catalysts to CO(2) reduction.