Flame Synthesized Ni – CeO₂ Catalysts for low-temperature CO₂ methanation: Size-dependent selectivity revealed by operando spectroscopy

Catalytic hydrogenation of CO₂ to methane using renewable H₂ is central to power-to-gas concepts. Here, NiO–CeO₂ composites obtained by flame spray pyrolysis (FSP) were used to elucidate how Ni speciation and particle size govern low-temperature CO₂ methanation. The as-prepared materials comprise ∼8–10 nm CeO₂ nanocrystals with highly dispersed Ni²⁺ and, at Ni loadings ≥ 10 mol% Ni, segregated NiO that reduces to Ni⁰ nanoparticles at 300 °C. In situ synchrotron XRD, quasi-in situ XPS, and operando IR spectroscopy reveal partial CeO₂ reduction and the coexistence of Ni²⁺–O–Ce interfacial sites and metallic Ni⁰. At 200 °C, catalysts containing 1 mol% Ni favor CO, whereas catalysts with ≥ 10 mol% Ni exhibit high CH₄ selectivity due to the presence of Ni⁰ nanoparticles, reaching 98–99% CH₄ at 300 °C. Across the full Ni content range (1–30 mol% Ni), Ni-normalized rates at 200 °C are comparable, while product selectivity shifts markedly with Ni particle size. Long-term testing at 300 °C demonstrates stable CH₄ production for the 10 mol% Ni catalyst, whereas the small-cluster-rich 1 mol% Ni sample deactivates due to the accumulation of soft coke. The high activity and stability of FSP-made Ni–CeO₂ are attributed to the synergy between small Ni⁰ nanoparticles and Ni²⁺–O–Ce sites associated with oxygen vacancies, which together govern CO₂ activation and CH₄ formation.