Elucidating the promotional effect of ultra-low Zn content on Cu for CO2 hydrogenation to methanol

Abstract

CO2 hydrogenation to methanol is currently of great importance for energy and environmental objectives. Herein, the effect of ultra-low amounts of Zn on Cu/SiO2 catalysts for CO2 hydrogenation was studied, maintaining the average Cu particle size in the 2–4 nm range to minimize structure sensitivity effects. The addition of Zn amounts as low as Zn/Cu = 0.01 increases the methanol formation rate up to twelve times compared to Cu/SiO2 catalyst, highlighting the importance of isolated Zn atoms in methanol synthesis catalysis. At these very low Zn contents, CuZnX alloy sites are identified, with strong affinity for CO adsorption. When Zn content is increased, CuZn alloy disappears to give rise to mostly metallic Cu likely forming Cu-ZnOx interfacial sites. Steady state and transient Operando-DRIFTS evidence that the reaction pathways depend on the nature of the active sites with reverse water–gas shift and CO hydrogenation dominant for low Zn content catalysts and formate pathway for high Zn content catalysts.