What Are the Best Active Sites for CO2 Methanation over Ni/CeO2?

We examined active sites for CO2 methanation over Ni/CeO2 catalysts prepared by a wet impregnation method. Four types of Ni/CeO2 with Ni loadings of 1, 3, 5, and 10 wt % were used in this study, assuming that the Ni sites are well dispersed in the catalysts when changing the Ni loading. According to powder X-ray diffraction and scanning transmission electron microscopy, the low-loading catalysts (1 and 3 wt %) consist mainly of Ni–Ce mixed oxides. The results of temperature-programmed reduction by H2 suggested that the Ni–Ce mixed oxides under reducing conditions contain oxygen vacancies (Ni–Vox–Ce). Note that the CO2 conversion rate was proportional to the Ni loading, which probably means that Ni–Vox–Ce sites on the Ni–Ce mixed oxides are active in CO2 conversion. In contrast, when the Ni loading was high (5 and 10 wt %), the catalysts possessed many metallic Ni nanoparticles supported on CeO2 and Ni–Ce mixed oxides. Because the turnover frequencies of CO methanation for 5 and 10 wt % Ni/CeO2 were identical, the presence of a metallic Ni surface could be essential for activation in CO methanation. We focused on the fact that the CO2 conversion rate was not related to the number of oxygen vacancies on CeO2 (Ce–Vox–Ce) but was related to the number of the Ni–Vox–Ce sites. Hence, the formation of Ni–Vox–Ce sites (CO production via the reverse water-gas shift reaction) and the exposure of metallic Ni sites (methanation of the thus-formed CO) are essential for CO2 methanation. Although it has been known that oxygen vacant sites on Ni/CeO2 catalysts are important for the catalytic activity, this study suggested anew that there are two types of the sites, Ce–Vox–Ce and Ni–Vox–Ce. Furthermore, it was clarified that the latter oxygen defect is important for CO2 methanation.