Exploring the mechanism of the CO₂ methanation reaction in Ni-CeO₂-sepiolite catalysts by time-resolved operando IR spectroscopy

Nickel-cerium-based catalysts supported on sepiolite were tested for the CO₂ methanation reaction and applied for biogas upgrading. Time-resolved operando IR results revealed that the Ce-free catalyst followed a dissociative mechanism via the formation of Ni⁰-CO intermediates. In contrast, Ce-containing catalysts exhibited an additional H-assisted mechanism via the formation of -HCO₃^- and -CO₃^2- intermediates, which were further converted into formate, -CHO, and ultimately CH₄. Different Ni and Ce loadings were screened, and the catalyst with 15 % wt. Ni and 10 % wt. Ce, 15Ni-10Ce-Sep, exhibited the optimum catalytic performance (X_CO2 = 89.5 %; S_CH4= 99.5 %, at 350 °C). CO₂-TPD, H₂-TPR, and SEM-TEM results revealed that this was attributed to a strong metal-support interaction between the CeO₂ and the Ni⁰ phases. This interaction optimized the concentration of moderately basic sites and active Ni⁰ centers, achieving an ideal Ni⁰ particle size of approximately 7 nm. These sites promoted the formation of surface carbonates and dissociated hydrogen atoms, which were identified as key intermediates in the reaction mechanism. Finally, the 15Ni-10Ce-Sep catalyst was tested for direct upgrading of biogas. Under optimum conditions, and considering the biogas constituent gases, after 24-hour long stability tests, the process upgraded a 60 % CH₄/40 % CO₂ mixture to a CH₄-enriched synthetic biomethane mixture (94.1 % CH₄/5.9 %CO₂).