Impact of K and Ba promoters on CO2 hydrogenation over Cu/Al2O3 catalysts at high pressure

CO2 hydrogenation over K and Ba promoted Cu/Al2O3 catalyst was systematically investigated to study the promoter effects in a wide range of pressure conditions. The catalysts prepared by the impregnation method were characterized by XRD, physisorption, N2O-pulse chemisorption, H-2-TPR, and CO2-TPD techniques. The catalytic performance was evaluated using a fixed-bed microreactor for a pressure and temperature range of 0.40-36 MPa and 443-553 K. The influence of promoters on the formation of surface species present during the reaction was examined by in situ DRIFTS. As expected from thermodynamics, high pressure and low temperature are the favourable conditions to achieve high selectivity to methanol over the Cu/Al2O3 catalyst. Improved reaction performance towards methanol synthesis and reverse water-gas shift (RWGS) reaction was observed for the Ba and K promoted Cu/Al2O3 catalysts, respectively. Notably, with the Ba promotion the selectivity to methanol was enhanced to 62.2% compared to 46.6% of the unpromoted Cu/Al2O3 catalyst at 10 MPa and 473 K at the expense of a lowered CO2 conversion. In contrast, the K promoted catalyst exhibited high selectivity to CO (95.8%) under the same reaction conditions. Formation of dimethyl ether, significant over the unpromoted Cu/Al2O3 catalyst at 0.4-10 MPa, was strongly suppressed at 36 MPa. Ba and K promoters effectively suppressed the formation of dimethyl ether under all examined pressure conditions by weakening the acidity of the alumina support. The strong promotional effects of K was explained by the predominant coverage of both Cu and alumina surface sites, creating specific active sites stabilizing surface intermediate species and preferring the RWGS pathway. On the contrary, the Ba promoter covers the alumina surface exclusively and renders Cu accessible and more easily reducible, promoting methanol synthesis. The effects of promoters on the catalytic performance were found to be valid at low and at elevated pressures.