Composition-Tuned Ni–Ce Mixed Oxides with Inverse CeO ₂ /Ni Structure for Enhanced Low-Temperature CO ₂ Methanation

This work systematically investigates the effect of Ni/Ce molar ratios (0.3–2.9) on the structure–activity relationship of Ni/Ce mixed oxides for low-temperature CO2 methanation. As the Ni/Ce ratio increased, the rate of CO2 conversion gradually increased. At a GHSV of 12,000 mL·g–1h–1, all catalysts exhibited 100% CH4 selectivity. The best-performing NiCe-2.1 catalyst not only achieved 37.2% CO2 conversion at 200 °C but also maintained above 85% CO2 conversion during the stability test at 250 °C. Characterization (XRD, BET, TEM, H2-TPR/TPD, CO2-TPD, XPS) results revealed that when Ni/Ce <2.1, the catalyst exhibited a conventional Ni/CeO2 structure. The specific surface area gradually decreased with an increasing Ni content. The activity enhancement of Ni/CeO2 structure primarily resulted from the absolute increase in the number of Ni–O–Ce interface sites. When Ni/Ce = 2.1, the catalyst formed an inverse CeO2/Ni structure, which re-exposed the CeO2 surface and significantly restored the specific surface area. TOF analysis confirmed that the interfacial sites in the inverse structure had higher intrinsic activity, thereby further improving the low-temperature activity. When Ni/Ce >2.1, excessive independent NiO phases caused severe nickel agglomeration, inhibiting the formation of additional Ni–O–Ce interfaces. Finally, in situ DRIFTS results revealed that the CO2 methanation on NiCe-2.1 mainly followed the formate pathway (CO2* → HCO3* → HCOO* → CH4).