Cu-Bound Formates are Main Reaction Intermediates during CO₂ Hydrogenation to Methanol over Cu/ZrO₂

Cu/ZrO₂ is a promising catalyst for the hydrogenation of CO₂ to methanol. Reaction pathways involving formates or hydroxycarbonyls have been proposed. We show here that three different types of formates can be observed under reaction conditions at 220 °C and 3 bar, one being located on metallic Cu and two others being bound to ZrO₂ . The surface concentrations of formates were determined through calibration curves and their reactivity measured during chemical transient experiments. The Cu-bound formate represented only about 7 % of surface formates, but exhibited a higher reactivity and was found to be the only formate that could account for all the production of methanol. Copper is thus not there only to activate H₂ , but also bears other crucial intermediates. This work reemphasizes that fully quantitative IR analyses and transient methods are required to unravel the role of surface species.