In-SituInfrared Study of Methanol Synthesis from H2/CO2over Cu/SiO2and Cu/ZrO2/SiO2

Abstract

The interactions of CO2and H2/CO2with Cu/SiO2, ZrO2/SiO2, and Cu/ZrO2/SiO2have been investigated byin-situinfrared spectroscopy with the aim of understanding the nature of the species involved in methanol synthesis and the dynamics of the formation and consumption of these species. In the case of Cu/SiO2, the only species observed during methanol synthesis are formate groups on Cu and methoxide groups on silica. When ZrO2/SiO2or Cu/ZrO2/SiO2is exposed to H2/CO2the majority of the species observed are associated with ZrO2. CO2adsorption on either ZrO2/SiO2or Cu/ZrO2/SiO2leads to the appearance of carbonate and bicarbonate species on the surface of ZrO2. In the presence of H2these species are converted to formate and then methoxide species adsorbed on ZrO2. The presence of Cu greatly accelerates the hydrogenation of bicarbonate to formate species, and the hydrogenation of formate to methoxide species. On Cu/ZrO2/SiO2methylenebisoxy species are observed as intermediates in the latter reaction. While Cu promotes the reductive elimination of methoxide species as methanol, it is observed that hydrolytic release of methoxide species from ZrO2occurs much more rapidly than reductive elimination. Thus, the methanol synthesis over Cu/ZrO2/SiO2is envisioned to occur on ZrO2, with the primary role of Cu being the dissociative adsorption of H2. The spillover of atomic H onto ZrO2provides the source of hydrogen needed to hydrogenate the carbon-containing species. It is also proposed that the formation of CO via the reverse-water–gas-shift reaction occurs on Cu.