Mechanistic Features of the CeO₂-Modified Ni/Al₂O₃ Catalysts for the CO₂ Methanation Reaction: Experimental and Ab Initio Studies

In this study, we investigated the Ni/CeO 2 /Al 2 O 3 catalyst system to explore the influence of different synthesis parameters on interfacial phenomena and their impact on CO 2 methanation. The focus was on the textural properties of alumina, ceria loading, and the synthesis method of supported Ni, in relation to the catalyst’s activity and CH 4 selectivity. Among the catalysts studied, Ni–20Ce/ mpAl demonstrated promising results, with an X CO2 value of 70% and S CH4 value exceeding 94% at 350 °C. We observed that medium- and high-porosity alumina facilitated better ceria dispersion, while Ni-CeO 2 cogrowth led to small Ni crystallites (∼4 nm) that increased in size after 8 h of reaction. This catalyst exhibited several advantageous features for CO 2 methanation, including a high concentration of oxygen vacancies (confirmed through Raman studies) and a significant presence of surface Ce 3+ species (validated by XPS and EPR studies). It also displayed excellent carbonyl activation capacity, high H-spillover capability, and strong SMSI phenomena. CO 2 -TPD and charge transfer Bader analysis confirmed the basic (Lewis) character of the catalyst’s surface. Specifically, Ce 3+ species, along with Ni atoms, provided suitable dual sites for CO 2 adsorption at the Ni-ceria interface, forming Ni···O–C–O···Ce 3+ entities. Furthermore, our analysis using operando SSITKA-DRIFTS revealed the active participation of both Ni and the support in the CO 2 methanation reaction, validating the ab initio studies. Notably, linear and bridged adsorbed CO species (CO L and CO B ) on the Ni surface, as well as bicarbonates (HCOOOs), were identified as active reaction intermediates involving Ce 3+ –OH and Al 3+ –OH entities. Comparing the thermal stability of carbonate-type intermediates to that of carbonyls, a CO-mediated mechanism emerged as the predominant pathway over the Ni–20Ce/ mpAl catalyst.