Mechanistic Insights in the Catalytic Hydrogenation of CO₂ over Pt Nanoparticles in UiO-67 Metal-Organic Frameworks

Metal nanoparticles (NPs) encapsulated within Zr-based UiO-67 metal-organic frameworks (MOFs) have increased selectivity toward methanol in CO₂ reduction reactions. However, the reduction mechanism in these systems remains unclear. We built upon prior work examining the synergistic interaction between Pt nanoparticles and Zr₆O₄(OH)₄ clusters in UiO-67 and developed five distinct models representing the possible active sites in the Pt ⊂ MOF system. Density functional theory (DFT) calculations were employed to elucidate the CO₂ reduction mechanism toward methanol, methane, and CO formation. Our findings support previous evidence showing that the interface between the Zr₆O₄(OH)₄ cluster and platinum nanoparticles plays a crucial role in the activation of CO₂ to CO or formate intermediates and its further reduction to methane and methanol, respectively. Furthermore, we found different CO₂ hydrogenation mechanisms for interfaces involving Pt-flat terraces and Pt-edges. On Pt terraces and interfaces near Pt terraces, the reaction goes via CO, which can be desorbed as CO(g) or be further reduced to methane. On interfaces near Pt-edges, the reaction proceeds via formate and preferably forms methanol over methane. We designed experiments to validate our computational insights involving large and small Pt nanoparticles interacting with Zr₆O₄(OH)₄ clusters. These experiments showed that only CO and methanol were formed when smaller nanoparticles were present. Notably, methane formed with CO and methanol in the presence of larger nanoparticles, highlighting the need for flat platinum surfaces at the interfaces for methane formation. We could also associate the IR signals corresponding to CO and bidentate formate with platinum nanoparticles and Zr₆O₄(OH)₄ clusters, respectively. Theoretical models and experimental data provided us with insights into the complexity of the reaction mechanism and emphasized the significance of understanding both the individual components of the catalytic system and their interactions in enhancing catalytic activity.