Reaction Pathways of Methanol Formation in CO<sub>2</sub> Hydrogenation over Pd-Based Catalysts
Denis Makhmutov, Elizaveta A. Fedorova, Anna Zanina, Christoph Kubis, Dan Zhao, Dmitry E. Doronkin, Nils Rockstroh, Stephan Bartling, Udo Armbruster, Sebastian Wohlrab, Evgenii V. Kondratenko
Abstract
High Resolution Image Download MS PowerPoint Slide The production of methanol (CH 3 OH) from CO 2 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 CH 3 OH decomposition to CO with increasing CO 2 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 CeO 2, ZrO 2, Ce 0.8 Zr 0.2 O 2, or CeO 2 –SiO 2 supports with Pd or CuZnPd as active components were prepared. Comprehensive kinetic tests revealed that the catalysts containing only Pd species convert CO 2 to CO exclusively, followed by the hydrogenation of CO to CH 3 OH. Using a feed consisting of CO and H 2, 100% CH 3 OH selectivity was achieved. The role of Pd is to convert CO 2 to CO and to generate surface species from H 2, which are involved in the hydrogenation of CO to CH 3 OH 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 CH 3 OH. In contrast to the Pd/support catalysts, their CuZnPd/support counterparts convert CO 2 directly into CH 3 OH 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 CO 2 hydrogenation to CH 3 OH 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 CH 3 OH production from CO 2 .