Promoter-Dependent Effects of Trace CeO2 Modification on Al2O3-Supported Ni–Ru and Ni–Rh Catalysts for CO2 Methanation

The influence of trace CeO2 modification on Ni-based CO2 methanation catalysts was investigated as a function of the noble-metal promoter. Four catalysts with nominal loadings of 10 wt.% NiO and 0.3 wt.% RuO2 or Rh2O3 were supported on γ-Al2O3 and 0.2 wt.% CeO2-modified γ-Al2O3. The samples were characterized by XRD, SEM–EDS, N2 physisorption, H2-TPR, and CO2-TPD and tested over 250–700 °C at atmospheric pressure and a H2/CO2 molar ratio of 4. The XRD and N2 physisorption results showed that trace CeO2 modification did not substantially alter the γ-Al2O3 structure or mesoporous texture, while NiO remained the predominant crystalline Ni-containing phase. The effect of CeO2 on reducibility was promoter dependent. In the Ni-Rh series, CeO2 modification shifted the intermediate-temperature H2-consumption maximum from 403 to 387 °C, and the contribution of this region increased from 20.0 to 41.5%. In contrast, the principal H2-TPR maxima of the Ni–Ru catalysts remained nearly unchanged. Total quantified CO2 desorption decreased after CeO2 modification in both series, but the contribution of the 250–500 °C region increased from 8.7 to 26.5% in the Ru-containing system and decreased from 27.4 to 4.2% in the Rh-containing system. On unmodified Al2O3, Ni–Rh/Al2O3 exhibited the highest apparent concentration-based CO2 conversion throughout the 300–650 °C range, reaching 100% at 450 °C with 92.47% CH4 selectivity. At 550 °C, CeO2 modification increased the apparent conversion of the Ru-containing catalyst from 78.5 to 90.5% but decreased that of the Rh-containing catalyst from 94.0 to 83.5%. Thermodynamic calculations showed that the decrease in CH4 selectivity and increase in CO formation at elevated temperatures were consistent with the increasing contribution of the reverse water–gas shift reaction, although the experimental CH4/CO distribution did not attain complete equilibrium at 600–700 °C. Overall, trace CeO2 produced promoter-dependent changes in NiOx reducibility and the distribution of CO2 adsorption states; these effects may also have involved changes in metal–support interfacial properties rather than representing a universal synergistic effect.