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, Ce0.8Zr0.2O2, 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 H-2, 100% CH3OH selectivity was achieved. The role of Pd is to convert CO2 to CO and to generate surface species from H-2, 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.