Rhodium-Enhanced Reducibility and Coke Resistance in Low-Loading Ni and Co/Al2O3 Catalysts for CO2 Methanation

Low-loading Ni (2.9 wt%) and Co (2.8 wt%) catalysts supported on γ-Al2O3 were promoted with small amounts of Rh (0.8–1.9 wt%) and evaluated for CO2 methanation under high space velocity (60,000 mL gcat−1 h−1). A monometallic Rh/Al2O3 catalyst was used as a reference. Rhodium significantly enhances the reducibility of Ni2+ and Co3+/Co2+ species, lowering reduction temperatures to ~ 240 °C (Ni) and below 570 °C (Co), achieving near-complete reduction (~ 100%) for both bimetallic systems. Despite lower dispersion in the bimetallic catalysts (~ 14–15%) compared to RhAlO (~ 55%), competitive catalytic performance is achieved at elevated temperatures. CO2-TPD reveals only weak basic sites for all catalysts, and the significantly lower CO2 uptake observed for NiRhAlO does not compromise activity, suggesting that the metallic phase primarily governs CO2 activation. Catalytic tests show that while RhAlO is more active at low temperatures (28% conversion at 250 °C), NiRhAlO and CoRhAlO reach comparable conversions (> 80%) at 450 °C, with CH4 selectivity above 90% up to 350 °C. All catalysts exhibit high stability over 30 h at 450 °C, with no clear evidence of sintering or carbon deposition, as confirmed by XRD, TGA, and Raman analyses. These results suggest that Rh promotion enables low-loading Ni and Co catalysts to achieve high activity, enhanced reducibility, and high stability under demanding methanation conditions, offering a cost-effective alternative to high-loading or noble-metal-based systems.