CO₂ Hydrogenation to Methanol over Partially Reduced Cu-SiO_2P Catalysts: The Crucial Role of Hydroxyls for Methanol Selectivity

The involvement of surface hydroxyl species in controlling methanol selectivity for a CO2 hydrogenation reaction was investigated over Cu-phyllosilicate (Cu-SiO2P) catalysts prepared by a urea-assisted hydrothermal synthesis method. The role of hydroxyls involvement was investigated by treating the Cu-SiO2P catalyst between 225 and 350 °C in H2 gas. The presence of Cu-containing phyllosilicate structures in Cu-SiO2P catalysts was confirmed through TEM and XPS analyses. As evidenced by in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy, Cu+ centers are dominant surface species for Cu in Cu-SiO2P catalysts during the hydrogenation reaction. Also, bidentate formate species are the prominent intermediates to direct methanol formation via methoxy intermediate species. The hydroxyl intervention in CO2 hydrogenation for a hydroxyl-abundant Cu-SiO2P catalyst reduced at 225 °C is confirmed by the appearance of a new band at 2944 cm–1 (−C–H) by high-pressure in situ DRIFTS CO2 hydrogenation experiments. H2/CO-TPR revealed the uniformity and presence of surface hydroxyls in Cu-SiO2P catalysts. The best catalyst Cu-SiO2P reduced at 225 °C gave a stable CO2 conversion of 3.5% and a methanol yield of 140 mg MeOH/g-cat.h (a methanol selectivity of 77%) at 225 °C and 3 MPa pressure for a 24 h reaction time. The presence of hydroxyls generated due to partial reduction of the Cu-SiO2P catalyst and surface enriched with Cu+ species could be the reasons for its superior catalytic performance.