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

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.

Evolution of Oxygen Vacancy Sites in Ceria-Based High-Entropy Oxides and Their Role in N2 Activation

“…In the case of CaF 2 (isomorphic) for Ce 3+ ions in a coordination environment of low symmetry, g ∼ 3.67 has been reported, whereas lines with g values of 2.4 for the trigonal site of Ce 3+ ions have also been reported . As can be seen in Figure C, EPR signals at different g -values confirm the various paramagnetic centers; of high significance is the signal at g ∼2.000 which is assigned to superoxide anions (•O 2– ) attached to Ce 4+ ions; the intensity of this pulse is correlated with the concentration of oxygen vacant sites . This signal is of high intensity in the case of the Ru/HEO catalyst.…”

“…41 As can be seen in Figure 3C, EPR signals at different g-values confirm the various paramagnetic centers; of high significance is the signal at g ∼2.000 which is assigned to superoxide anions (•O 2− ) attached to Ce 4+ ions; the intensity of this pulse is correlated with the concentration of oxygen vacant sites. 42 This signal is of high intensity in the case of the Ru/HEO catalyst. Additional structural information was acquired utilizing Raman spectroscopy over the Ru-based catalysts of this study following their reduction at 650 °C for 2h in H 2 atmosphere.…”