CO<sub>2</sub> Hydrogenation to Methanol over Partially Reduced Cu-SiO<sub>2P</sub> Catalysts: The Crucial Role of Hydroxyls for Methanol Selectivity
Ashok Jangam, Plaifa Hongmanorom, Ming Hui Wai, Antonius Jeffry Poerjoto, Shibo Xi, Armando Borgna, Sibudjing Kawi
Abstract
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.