Hydrogenation of CO₂ to methanol over Pd–Cu/CeO₂ catalysts

CO₂ hydrogenation to methanol receives great attention recently not only in conventional CO hydrogenation process but also in CO₂ and H₂ storage technologies. CO₂ can be converted to methanol by catalytic hydrogenation, and it is supposed that CO₂ is hydrogenated to methanol via hydrocarboxyl (COOH) intermediate (rWGSR route) rather than formate (HCOO) intermediate. It was reported that CeOx/Cu(111) surface stabilizes COOH intermediate, which consecutively results in lower activation energy for methanol formation than on Cu(111) and Cu/ZnO(0001¯) surfaces. The activity of Cu/CeO₂ catalysts can be further enhanced by Pd addition, because it can enhance reducibility of Cu site. In this study, therefore, Cu/CeO₂ and Pd–Cu/CeO₂ catalysts were applied to CO₂ hydrogenation to methanol. Pd-promoted Cu/CeO₂ catalysts achieved higher methanol productivity than Cu/CeO₂ catalyst due to the increase in CO₂ conversion. Because methanol productivity over Pd weight (mmol_MeOH/g_Pd·h) simply decreased as the amount of Pd increased, it is considered that the enhanced activity of Pd–Cu/CeO₂ catalysts mainly resulted from the increase in the reactivity of active Cu sites. The dispersion and surface concentration of Cu increased due to the interaction with highly-dispersed Pd, and Cu sites were more reduced by Pd promotion due to electron donation from Pd. Pd promotion also generated more reduced CeO₂ surface. These promoting effects of Pd was the highest in the catalyst containing 1 wt.% Pd (1Pd–10Cu/CeO₂), because further increase in the amount of loaded Pd lowered the textural properties of catalyst. As a result, methanol productivity was enhanced in Pd-promoted Cu/CeO₂ catalysts and the highest in 1Pd–10Cu/CeO₂ catalyst.