The role of Pd–Ga bimetallic particles in the bifunctional mechanism of selective methanol synthesis via CO2 hydrogenation on a Pd/Ga2O3 catalyst

Pd–Ga bimetallic nanoparticles are formed under CO2 hydrogenation to methanol (523K, 3MPa) and under pure hydrogen reduction (523K, 0.1MPa) on a Pd (1wt.%)/Ga2O3 catalyst. The main roles of the bimetallic particles in the reaction mechanism are to hydrogenate the carbonaceous species adsorbed on the Ga2O3 support and to inhibit CO production. [Display omitted] ► Pd–Ga bimetallic nanoparticles are formed upon hydrogen reduction. ► Pd–Ga bimetallic nanoparticles enhance the selectivity to methanol. ► In situ infrared experiments show that Pd–Ga/Ga2O3 behaves as truly bifunctional. ► Encapsulation of Pd by Ga2O3 is observed by electron microscopy after air exposure. The effect of palladium–gallia interaction in Pd(1wt.%)/β-Ga2O3 during selective methanol synthesis by CO2 hydrogenation was studied. A detailed quasi-in situ transmission electron microscopy analysis of the as-prepared H2-reduced catalyst, without exposing it to air, showed that Ga–Pd bimetallic (nano)particles were formed under a reductive atmosphere at or above 523K. However, these particles were unstable; upon air exposure, a dramatic and extensive encapsulation of the metallic crystallites by Ga2O3 occurred. In addition, the function of the bimetallic particles in the mechanism of methanol synthesis was investigated by in situ infrared spectroscopy at 0.7MPa. The results confirmed those of previous studies in which the stepwise hydrogenation of (bi)carbonate to formate and then to methoxy groups on the Ga2O3 surface took place via a bifunctional pathway. In this pathway, the role of the Ga–Pd bimetallic crystallites was to provide atomic hydrogen, via spillover, to the oxidic surface and to hamper both CH3OH decomposition and CO production.