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 CO 2 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 Ni 0 -CO intermediates. In contrast, Ce-containing catalysts exhibited an additional H-assisted mechanism via the formation of -HCO 3 - and -CO 3 2- intermediates, which were further converted into formate, -CHO, and ultimately CH 4 . 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 C O 2 = 89.5 %; S C H 4 = 99.5 %, at 350 °C). CO 2 -TPD, H 2 -TPR, and SEM-TEM results revealed that this was attributed to a strong metal-support interaction between the CeO 2 and the Ni 0 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 % CH 4 /40 % CO 2 mixture to a CH 4 -enriched synthetic biomethane mixture (94.1 % CH 4 /5.9 %CO 2 ). • Ni-Ce-sepiolite catalysts were assessed for the CO 2 methanation reaction. • Ce addition increased the concentration of moderate basic sites and Ni 0 dispersion. • Moderate basic sites were accounted for -HCO 3 - and -CO 3 2- species formation. • The catalyst with optimum Ni, Ce loadings was tested in 24 h biogas upgrade tests. • Optimally a 60 % CH 4 /40 % CO 2 mixture was upgraded to biomethane (94.1 % CH 4 /5.9 %).