CO2 hydrogenation to CH4 over hydrothermal prepared ceria-nickel catalysts: Performance and mechanism study

• Ce-Ni catalyst prepared with modified hydrothermal method by adjusting the proportion of solvents. • The adsorbed CO is a side product. • Formate and methoxy species are important intermediates. • The oxygen vacancies, nickel dispersion are critical to the improvement of catalytic performance. A series of nickel-ceria catalysts were prepared with the hydrothermal method and the composition of solvents was adjusted. The performance of the catalysts on CO 2 methanation varied greatly with the volume ratio of ethylene glycol/water in the hydrothermal process. Among them, the catalyst denoted as CN-70-70EG exhibited the highest catalytic activity with a CO 2 conversion of 67.8 % at 375 °C. Catalysts were characterized by XRD, BET, H 2 -TPR, CO 2 -TPD, XPS, Raman, UV–vis DRS, SEM mapping and in-situ DRIFTS. It was revealed that the catalytic performance was dependent on the nickel dispersion, the number of surface oxygen vacancies and the ability of the catalysts to be activated at low temperatures. In-situ DRIFTS experiments showed that CO 2 was initially adsorbed in the forms of carboxylate, carbonate and bicarbonate on the ceria sites. These adsorbed species reacted with the dissociated H to produce the main intermediate of formate species. The adsorbed CO was only a side product, rather than intermediate species. Furthermore, DRIFTS experiments on hydrogenation of methanol and formic acid demonstrated that these two species could be converted into methane with the presence of the catalysts. In both situations, methoxy species bonded to surface Lewis acid sites were found to be intermediates in this reaction.