Theoretical Modeling of Highly Dispersed Catalysts under Reactive Gas Environments

In the past decade, highly dispersed catalysts (HDCs) and more specifically single atom catalysts (SACs) have become popular in the field of heterogeneous catalysis. Although these catalysts efficiently utilize all the active metal in the catalyst, their structure and reactivity are inadequately understood. For the structure and stability of HDCs and SACs on metal oxides, the sensitivity of the very small clusters and single atoms to their surrounding gas/liquid environment results in many accessible states under reaction conditions, rendering simplistic structural models unrealistic. The unique chemical properties of the active sites by their lack of metal-metal coordination or poor mixing of electronic states with the support shift the free energy pathway of catalytic reactions, resulting in nontraditional kinetic behavior. In this dissertation, we use density functional theory (DFT), atomistic thermodynamics, and microkinetic modeling to study several HDCs and SACs to understand their structure, stability, and reactivity under realistic catalytic environments.DFT-based atomistic thermodynamics were performed to investigate the surface and structure of a highly dispersed Rh/CeO2 catalyst. We found that the interaction between Rh and the CO gas product induces a counterintuitive redispersion of Rh active sites during the steam reforming of methane, reflecting the sensitivity of the Rh single atoms to their surrounding environment. We further applied the atomistic thermodynamics approach in the simulation of the surface structure of a dual single-atom Mo1+Pd1/Co3O4 catalyst. We found that under reducing gas environments containing predominantly H2 and H2O, the Co3O4(111) surface becomes hydroxylated and Co-enriched. DFT-energetics-based microkinetic modeling of the anisole hydrodeoxygenation (HDO) reaction reveals that the hydroxylated and Co-enriched Co3O4(111) surface enables the cooperation between the Mo1 and Pd1 sites by allowing the shuttling of H through metastable H2O intermediates. Finally, we also applied DFT-energetics-based microkinetic modeling in the modeling of the hydrogenation of 1-hexyne and hydrogenation/isomerization of 1-hexene over a dilute Pd-in-Au catalyst. We found that the large barriers for H2 dissociation over Pd1 and for H exchange between individual Pd1 sites results in the high selectivity of the dilute Pd-in-Au catalyst for 1-hexene in the hydrogenation of 1-hexyne, as well as for 2-hexene in the hydrogenation/isomerization of 1-hexene