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 CeO 2 for CO 2 hydrogenation to methanol was investigated. CeO 2 -supported PdZn and Ca-doped PdZn nanoparticles (NPs) proved to be highly selective, fairly active and quite stable for CH 3 OH synthesis at reasonably low temperature conditions. In the case of Ca-doped PdZn/ CeO 2 , methanol selectivity of ∼100% was achieved at low temperature (T  = 220 °C, P  = 30 bar and GHSV = 2400 mL g −1 h −1 ) with reasonable CO 2 conversion (7.7%). CeO 2 -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, CO 2 -TPD, H 2 -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 CeO 2 -supported catalysts which aided in increased CO 2 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 CeO 2 interaction with PdZn nanoparticles for selective CH 3 OH formation over ceria-supported PdZn nanoparticles. Addition of Ca, to the CeO 2 -supported PdZn catalysts, as a promoter, slightly improved the selective conversion of CO 2 to methanol by raising the amount of oxygen-vacant sites as revealed by XPS results. DRIFT studies revealed the emergence of monodentate, bidentate formates, CH 2 O and methoxy species and their subsequent conversion to methanol and CO, steering the reaction mechanism towards formate route for selective formation of methanol.