Reaction Pathways of Methanol Formation in CO₂ Hydrogenation over Pd-Based Catalysts

The production of methanol (CH₃OH) from CO₂ is an attractive solution for closing the carbon cycle and thus addressing both environmental concerns and raw material changes in the chemical industry. CuZn-based catalysts are the most intensively investigated materials in this regard but suffer from CH₃OH decomposition to CO with increasing CO₂ conversion. Pd-containing materials also show promising performance, but they are less understood from a mechanistic point of view. To bridge this gap, a series of catalysts based on CeO₂, ZrO₂, Ce_0.8Zr_0.2O₂, or CeO₂–SiO₂ supports with Pd or CuZnPd as active components were prepared. Comprehensive kinetic tests revealed that the catalysts containing only Pd species convert CO₂ to CO exclusively, followed by the hydrogenation of CO to CH₃OH. Using a feed consisting of CO and H₂, 100% CH₃OH selectivity was achieved. The role of Pd is to convert CO₂ to CO and to generate surface species from H₂, which are involved in the hydrogenation of CO to CH₃OH probably on the surface of support. In situ Fourier transform infrared spectroscopy tests have identified HCOO^– species formed from gas-phase CO as surface precursors of CH₃OH. In contrast to the Pd/support catalysts, their CuZnPd/support counterparts convert CO₂ directly into CH₃OH in parallel with CO. These differences were explained by structural/electronic changes in Pd due to alloying with Cu as revealed by in situ X-ray photoelectron and X-ray absorption spectroscopy. Overall, this study enhances understanding of the mechanistic aspects of product formation in the course of CO₂ hydrogenation to CH₃OH and highlights the significance of steady-state catalytic tests at different space velocities to identify primary and secondary pathways, offering valuable insights for the tailored design of efficient catalysts for CH₃OH production from CO₂.