Pd-Doped ZnO Catalysts for Direct CO2 Hydrogenation to Methanol

Abstract

CO₂ emissions exacerbate environmental problems, and its hydrogenation to liquid fuels is a potential way to reduce emissions, but the development of efficient PdZn catalysts is still a challenge. In this study, a highly dispersed Pd/ZnO-25 catalyst was prepared by a simple and environmentally friendly solvent-free mechanical ball milling-pyrolysis method. It exhibited excellent methanol selectivity of 97.6% under relatively mild reaction conditions (3.0 MPa, 280 °C, Pd loading ≤0.60 wt %), achieved a methanol space-time yield of 28.1 g_MeOH g_Pd^–1 h^–1 (STY_MeOH-Pd), and demonstrated excellent stability over a period of 72 h. It is worth mentioning that the increase in gas hourly space velocity (GHSV) can lead to a maximum STY_MeOH of 51.6 g g_Pd^–1 h^–1, which is a significant advantage. Moreover, a range of characterization techniques were utilized to investigate the mechanism of the Pd^δ+-O_v active site. In situ FTIR demonstrated that the hydrogenation reaction of CO₂ over the Pd/ZnO-25 catalyst mainly followed the HCOO* pathway, and the key intermediates included HCOO* and CH₃O*. This study provides a valuable understanding of the active site and reaction mechanism involved in the hydrogenation of CO₂ to methanol, thereby improving the efficient use of Pd-based catalysts.