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

Abstract

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.