In Situ Infrared Study of Methanol Synthesis from H2/CO over Cu/SiO2and Cu/ZrO2/SiO2

Abstract

The interactions of CO and H2/CO with Cu/SiO2, ZrO2/SiO2, and Cu/ZrO2/SiO2have been investigated by in situ infrared 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, carbonate species adsorbed on Cu and methoxide species adsorbed on Cu and silica are observed at 523 K, H2/CO=3, and a total pressure of 0.65 MPa. When ZrO2/SiO2or Cu/ZrO2/SiO2is exposed to H2/CO, the majority of the species observed are associated with ZrO2. CO adsorption on either ZrO2/SiO2or Cu/ZrO2/SiO2results in the formation of carboxylate, bicarbonate, and formate species on zirconia. In the presence of H2, formate species are hydrogenated to methoxide species adsorbed on ZrO2. The presence of Cu greatly accelerates the rate of formate hydrogenation to methoxide species, a process in which methylenebisoxy species are observed as intermediates. Cu also significantly promotes the reductive elimination of methoxide species as methanol. Thus, 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. Spillover of absorbed CO from Cu to zirconia facilitates formate formation on zirconia at lower temperatures than in the absence of Cu. The reductive elimination of methoxide species appears to be the slow step in methanol formation by CO hydrogenation. The lower rate of methanol synthesis over Cu/ZrO2/SiO2from CO as compared to CO2hydrogenation is attributed to the lack of water formation in the former reaction, preventing facile release of methoxide by hydrolysis. The enhanced rate of methanol synthesis from CO over Cu/ZrO2/SiO2as compared to Cu/SiO2is attributed to the lower energy-barrier (formate) pathway available on Cu/ZrO2/SiO2.