Development of highly selective PdZn/CeO2 and Ca-doped PdZn/CeO2 catalysts for methanol synthesis from CO2 hydrogenation

Abstract

Effect of Pd, Zn, PdZn alloy and Ca-doped PdZn on CeO2 for CO2 hydrogenation to methanol was investigated. CeO2-supported PdZn and Ca-doped PdZn nanoparticles (NPs) proved to be highly selective, fairly active and quite stable for CH3OH synthesis at reasonably low temperature conditions. In the case of Ca-doped PdZn/ CeO2, methanol selectivity of ∼100% was achieved at low temperature (T = 220 °C, P = 30 bar and GHSV = 2400 mL g−1h−1) with reasonable CO2 conversion (7.7%). CeO2-supported PdZn nanoparticles (NPs) (3-6 nm, measured from HR-TEM) were successfully prepared by the chelating method using citric acid as a chelating agent. The developed catalysts were investigated using a range of characterization techniques (BET, CO-Chemisorption, CO2-TPD, H2-TPR, XRD, XPS, STEM-EDS and HR-TEM). XPS results revealed the presence of Ce+3 ions implying the generation of oxygen-vacant sites over the surface of CeO2-supported catalysts which aided in increased CO2 dissociation resulting in higher methanol rates. An in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) study was also carried out for the best performing catalyst at actual reaction conditions to determine the intermediate species and a probable reaction mechanism. Characterization results revealed the significance of CeO2 interaction with PdZn nanoparticles for selective CH3OH formation over ceria-supported PdZn nanoparticles. Addition of Ca, to the CeO2-supported PdZn catalysts, as a promoter, slightly improved the selective conversion of CO2 to methanol by raising the amount of oxygen-vacant sites as revealed by XPS results. DRIFT studies revealed the emergence of monodentate, bidentate formates, CH2O and methoxy species and their subsequent conversion to methanol and CO, steering the reaction mechanism towards formate route for selective formation of methanol.