(169v) Density Functional Theory (DFT) Analysis of CO2 Adsorption and Dissociation on Reducible Oxides, and Integration into a Microkinetic Model for Dry Reforming of Methane (DRM)

Mitigating greenhouse gas (GHG) emissions is imperative, and dry reforming of methane (DRM) is a promising avenue for upcycling two of the most potent GHGs. CH₄ and CO₂ are converted into syngas (H₂ and CO), which can be used as the feed for Fischer-Tropsch processes. Metal-support catalysts are usually employed for this reaction. CO₂ activation is one of the rate-determining steps and the activation mechanism is largely dependent on the nature of the support. Redox-active supports, such as ceria, exhibit strong favorability to form oxygen vacancies, which serve as active sites for CO₂ activation and dissociation. Whereas irreducible oxides like alumina follow the H-assisted CO₂ activation pathway, through the formation of CHO*/COOH* intermediates. However, the understanding of CO₂ activation/dissociation mechanisms on mixed oxide supports remains relatively limited.

Our experimental collaborators have synthesized a complex Ni-CeO_x-Al₂O₃ catalyst through atomic layer deposition of thin alumina layers on ceria. The incorporation of alumina overlayer suppresses the reverse water gas shift reaction, consequently enhancing the H₂:CO ratio. Meanwhile, ceria oxidizes the surface carbon species by supplying oxygen to the metal, while also facilitating CO₂ activation. Characterization techniques suggest that ceria and alumina exist as a mixed oxide in the catalyst. This observation is supported by our DFT calculations, which have elucidated the formation of a CeAlO_x type surface oxide, with the Ce³⁺ species involved in CO₂ activation. These unique properties of the mixed-oxide support confer exceptional stability and selectivity to the catalyst. Preliminary investigation of the thermodynamic energetics of CO₂ activation and dissociation on the ceria-alumina support indicates that this mixed-oxide exhibits superior CO₂ activation capabilities compared to the respective pristine oxides.

To further understand the mechanism of CO₂ activation on the support, and influence of its kinetics on the overall catalyst performance, we have developed a dual site microkinetic model (MKM) based on elementary reaction rate constants obtained from DFT calculations. Through the MKM, we evaluate the rates of CO₂ activation/dissociation on the pure oxide supports vs the Ce-Al mixed oxide support. The reducibility of such mixed oxides determines the relative rates of H₂/CO formation, thereby influencing the catalyst activity. The degree of rate control analysis confirms the CO₂ activation on vacancies to be a rate-determining step, underscoring its role in overall reaction kinetics. Altogether, this work provides novel mechanistic understanding of the influence of CO₂ activation kinetics on the overall performance of the Ni-ceria-alumina catalyst.