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.

The influence of support materials and preparation methods on CO2 methanation activity was investigated using Ru nanoparticles supported on amorphous ZrO2 (am-ZrO2), crystalline ZrO2 (cr-ZrO2), and SiO2.

Hydrogen production by steam electrolysis at intermediate temperatures has potential for both the high energy conversion efficiency and the flexible operability suitable for the utilization of renewable energy resources. Employment of proton-conducting solid acid electrolytes at around 200°C is considered promising but has rarely been investigated. Here, steam electrolysis was performed at 160-220°C using a solid acid electrolysis cell (SAEC) composed of a CsH 2 PO 4 /SiP 2 O 7 composite electrolyte and Pt/C electrodes. Hydrogen production was successfully demonstrated with Faraday efficiencies around 80 %. Key factors affecting the SAEC stability were investigated in detail for the first time. It was revealed that a certain part of the electrolyte migrated into the porous anode structure during the operation. The migrated electrolyte prevented the gas diffusion and flooded the Pt/C catalyst layer. It was also found that carbonaceous materials in the anode was oxidized, leading to the decrease in the number of electrochemically active sites. Based on the findings, Pt mesh was employed as an alternative anode. The SAEC with the Pt mesh anode showed superior stability, demonstrating the importance of the anode design. The present work provides a comprehensive view of the stability issues, which is essential for the development of durable and practical SAECs.

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