Tailoring Structure–Activity Relationships in Ni and Ni₃Fe/Al₂O₃ Catalysts via Sol–Gel and Impregnation Routes for Enhanced Low-Temperature CO₂ Methanation

Catalytic CO₂ methanation has emerged as a highly promising route for carbon-neutral energy storage and power-to-gas (PtG) applications; however, the rational design of highly active low-temperature catalysts remains a major challenge in heterogeneous catalysis. In this study, mono- and bimetallic Al₂O₃-supported catalysts, Ni/Al₂O₃ and Ni₃Fe/Al₂O₃, were synthesized via Sol-Gel (SG) and Incipient Wetness Impregnation (IWI) methods to investigate the influence of synthesis route on physicochemical properties and catalytic activity toward CO₂ methanation. The catalysts were characterized using EDX, XRD, H₂-TPR, and H₂-chemisorption techniques and evaluated at 523 K and atmospheric pressure under an H₂:CO₂ ratio of 24:1. EDX analysis confirmed successful incorporation of the targeted 15 wt.% metal loading with minimal deviation from nominal composition. XRD analysis revealed the formation of metallic Ni and Ni₃Fe alloy phases after in-situ reduction at 823 K. Scherrer analysis showed significantly smaller Ni crystallite size for SG-synthesized Ni/Al₂O₃ (~8.2 nm) compared to the IWI counterpart (~12.6 nm), indicating enhanced metal dispersion. H₂-TPR studies further demonstrated superior reducibility for SG-derived Ni/Al₂O₃ with the highest reduction degree (82.98%), whereas IWI-derived Ni₃Fe/Al₂O₃ exhibited enhanced reducibility among the bimetallic catalysts due to favorable alloy formation and metal-support interaction. Catalytic studies established a strong structure–activity relationship. The SG-prepared Ni/Al₂O₃ catalyst exhibited nearly two-fold higher CH₄ formation rate than the impregnated catalyst, achieving a CO₂ conversion rate of 1.79 × 10⁻⁵ mol g⁻¹ s⁻¹ due to highly dispersed metallic Ni sites. Remarkably, the IWI-synthesized Ni₃Fe/Al₂O₃ catalyst showed the best overall performance with 12.5% CO₂ conversion, 10.7% CH₄ yield, and a CO₂ consumption rate of 3.09 × 10⁻⁵ mol g⁻¹ s⁻¹, highlighting the synergistic promotional effect of Ni–Fe alloying. These findings provide critical insights for designing highly efficient Ni-based catalysts for sustainable CO₂ valorization.