Reverse Water-Gas Shift (RWGS) Reaction on Pd/Al2O3 Catalyst: The Importance of Interface Studied by a Combination of First-Principles and Microkinetic Modeling

CO2 hydrogenation over oxide-supported metal catalysts proceeds through multiple reaction pathways and yields a variety of products, with product selectivity toward CO and CH4 being of particular interest. In this work, the reaction mechanisms of CO2 hydrogenation to CO and CH4 on Pd/Al2O3 were investigated using density functional theory (DFT) calculations and mean-field microkinetic modeling (MKM). Catalyst models with distinct Pd layer structures (monolayer (Pd7/Al2O3), bilayer (Pd11/Al2O3), and trilayer (Pd13/Al2O3)) were developed to investigate the impact of Pd layer configurations on CO2 adsorption. Results reveal that CO2 adsorption at the Pd–Al interface is energetically more favorable than on the Pd surface, highlighting the critical role of the metal–support interface in enhancing CO2 activation. The bilayer Pd11/Al2O3 model, which exhibited superior CO2 activation, was selected for detailed mechanistic studies. Three RWGS pathways (redox, carboxyl, and formate) were studied. CO from the redox pathway desorbs as gaseous CO, while CO formed via the carboxyl and formate pathways remains at the Pd–Al interface, undergoing further hydrogenation to CH4. MKM simulations demonstrate temperature-dependent selectivity: CH4 predominates below 640 K, while CO is favored above 640 K. Degree of selectivity control (DSC) identifies CO desorption from the Pd surface and interfacial O hydrogenation to OH as key steps governing CO selectivity, where reducing their energy barriers enhances CO selectivity. Degree of rate control (DRC) analysis further reveals that HCO dissociation to CH and interfacial O hydrogenation to OH are the rate-limiting steps for CH4 and CO formation, respectively. These insights provide a foundation for designing efficient catalysts for selective CO2 hydrogenation.