(628e) A Combined DFT and Microkinetic Modeling Analysis of Methane Dry Reforming on Complex Ni-CeOx-Al2O3 Catalysts

Mitigating greenhouse gas (GHG) emissions is imperative, and dry reforming of methane (DRM) is a promising avenue for upcycling CH­₄ and CO₂. They are converted to syngas, which is used as the feed for Fischer-Tropsch processes. Ni-alumina is a commonly employed catalyst due to its high initial activity and thermal stability. However, it cokes during DRM due to insufficient oxygen supply from CO₂ to adsorbed carbon species. The use of a more redox active oxide support (Ceria) can perform dual roles – providing oxygen to Ni for C oxidation and activating CO₂ at the resulting oxygen vacancies. Nevertheless, ceria's propensity towards reverse water gas shift (rWGS) reaction poses a challenge, as H-spillover from Ni, onto ceria, lowers the H₂:CO ratio.

We hypothesize that combining ceria and alumina can mitigate H-spillover while enhancing O-transport to the metal, thereby improving the catalyst’s activity and selectivity. Experimental efforts in this collaborative project observe greater stability and less coking by deposition of thin alumina layers over ceria, by atomic layer deposition. Our preliminary DFT works suggest that ceria and alumina form a CeAlO_x type overlayer, with Ce³⁺ anticipated to be actively involved in CO₂ activation. To elucidate the reaction pathway on such multicomponent oxide catalyst surfaces and gauge the relative rates of interfacial transport processes, we have developed a dual site microkinetic model (MKM).

Analysis of our MKM indicates that higher support/metal ratio results in higher H-spillover rates, thereby lowering the H₂:CO ratio. Co-feeding limited amount of H₂O to the system results in negative H-spillover rates and very high O-transport rates to Ni, thus increasing the H₂:CO above 1. Further, the MKM evaluates the role of oxygen vacancies in CO₂ activation, and their influence on overall selectivity. Altogether, this work provides novel mechanistic insights on the elementary steps involved in DRM on Ni-ceria-alumina catalysts.