Structure Sensitivity of Methanol Synthesis From Carbon Dioxide On Copper-Based Catalysts

Thermocatalytic conversion of carbon dioxide to methanol has recently gained considerable attention because it is regarded as an optimal solution to mitigate CO₂ emissions while generating revenues to offset carbon management costs.  Currently, commercial synthesis of methanol is accomplished by hydrogenation of CO-rich syngas over Cu/ZnO/Al₂O₃ catalysts.  However, the low activity and stability of existing formulations create major barriers towards using them in realistic CO₂-rich conditions.  Despite extensive studies of methanol synthesis over Cu-based catalysts, the nature of active sites and structure sensitivity of methanol synthesis on Cu surfaces are not fully understood.  Thus, a better understanding of active sites and structure-activity relationships is essential for the rational design of new catalysts that would be suitable for CO₂ conversion applications.

We have investigated coprecipitated Cu-based catalysts possessing different metal oxide additives, Cu crystallite sizes, and varying degrees of support crystallinity.  Using a variety of ex situ and in situ characterization techniques, we have verified that Cu⁰ species are active sites for methanol synthesis from a stoichiometric CO₂/H₂ mixture.  The possibility of Cu⁺ species acting as active sites is excluded because these species are not observed on the surface of the working catalysts.  The reaction is structure-sensitive with smaller Cu particles demonstrating higher turnover frequency.  An explanation for an activity enhancement upon promotion will be presented.  Possible reaction pathways for CO₂ hydrogenation to methanol will also be discussed.