Mechanistic Features of the CeO<sub>2</sub>-Modified Ni/Al<sub>2</sub>O<sub>3</sub> Catalysts for the CO<sub>2</sub> Methanation Reaction: Experimental and Ab Initio Studies
Ayesha AlKhoori, Omer Elmutasim, Aasif A. Dabbawala, Michalis A. Vasiliades, Klito C. Petallidou, Abdul‐Hamid Emwas, Dalaver H. Anjum, Nirpendra Singh, Mark Baker, Nikolaos D. Charisiou, Maria A. Goula, Angelos M. Efstathiou, Kyriaki Polychronopoulou
Abstract
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.