Integrated Parametric, Kinetic, and In Situ DRIFTS Analysis of CO₂ Methanation over γ-Al₂O₃-Supported Ni–Fe and Rh Catalysts

Catalytic CO₂ methanation has emerged as an important pathway for the valorization of captured CO₂ and renewable H₂ into synthetic natural gas, thereby contributing to carbon neutrality and renewable energy storage. In the present work, a comparative investigation of γ-Al₂O₃-supported 15wt.%Ni–Fe alloy (75Ni25Fe) and 3wt.%Rh (100Rh) catalysts was conducted under identical operating conditions (P = 1atm., T = 523 to 773 K, CO₂:H₂:Ar = 1:2:7, 1:4:5, and 1:6:3) by integrating parametric optimization, power-law kinetic modeling, and in situ DRIFTS analysis. The reaction parameters were optimized, and the optimum conditions were identified at 623 K, CO₂:H₂:Ar = 1:6:3, and a GHSV of 60,000 mL h⁻¹ gcat⁻¹. The excess hydrogen supply facilitated the hydrogenation steps involved, leading to higher CO₂ conversion and CH₄ selectivity than those obtained at lower H₂/CO₂ ratios. Under these conditions, the 75Ni25Fe catalyst exhibited superior CO₂ conversion, whereas the 100Rh catalyst delivered nearly complete CH₄ selectivity (~100%). Both catalyts maintained stable performance during 12 h of operation, with no significant loss in activity observed.

Power-law kinetics revealed positive fractional apparent reaction orders with respect to both CO₂ and H₂, confirming the complex and non-linear dependence of the methanation rate on reactant partial pressures. For the 75Ni25Fe catalyst, the apparent reaction orders were α = 0.28 ± 0.09 for CO₂ and β = 0.34 ± 0.07 for H₂, whereas the 100Rh catalyst exhibited α = 0.22 ± 0.06 and β = 0.56 ± 0.05, respectively. Furthermore, the apparent activation energy of the 100Rh catalyst (96 kJ mol⁻¹) was significantly lower than that of the 75Ni25Fe catalyst (114 kJ mol⁻¹), suggesting more favorable reaction kinetics under the investigated _conditions. Mechanistic insights demonstrated that the Ni–Fe alloy catalyst predominantly follows a formate-mediated hydrogenation pathway, while the Rh catalyst preferentially proceeds via RWGS reaction coupled with subsequent CO methanation.

Overall, the Rh catalyst establishes an excellent benchmark for CH₄ selectivity and activity, whereas the Ni–Fe alloy catalyst represents a highly promising and economically viable alternative for CO₂ methanation applications.