Optimization of the Pd/Cu ratio in Pd-Cu-Zn/SiC catalysts for the CO2 hydrogenation to methanol at atmospheric pressure

PdCuZn/SiC catalysts were synthesized with different Pd:Cu:Zn molar compositions and tested in the hydrogenation of carbon dioxide to methanol at atmospheric pressure. Trimetallic catalysts were compared with the corresponding bimetallic ones (PdZn/SiC, CuZn/SiC and PdCu/SiC). Catalysts were characterized by N-2 adsorption/desorption, temperature-programed reduction (TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The Pd-0 active sites were related to carbon monoxide formation via reverse water-gas-shift (RWGS), whereas the PdZn alloys catalyzed methanol synthesis. The role of copper in trimetallic catalysts was to inhibit the deposition of metallic palladium by forming a PdCu alloy that proved to be less active to CO formation. Moreover, the active sites of trimetallic catalysts were smaller and better dispersed than those of the corresponding bimetallic ones, probably due to a synergistic effect between the three metals. The catalyst with a molar composition of 37.5:12.5:50 Pd:Cu:Zn (mol.%) was selected as the most active for the methanol synthesis, as this sample showed the highest activity and selectivity to methanol. The role of copper was also shown to be crucial in trimetallic catalyst by comparing the best example with an equivalent bimetallic PdZn/SiC with a Pd: Zn molar ratio of 37.5:62.5.