Catalytic hydrogenation of CO 2 to methane using renewable H 2 is central to power-to-gas concepts. Here, NiO–CeO 2 composites obtained by flame spray pyrolysis (FSP) were used to elucidate how Ni speciation and particle size govern low-temperature CO 2 methanation. The as-prepared materials comprise ~8–10 nm CeO 2 nanocrystals with highly dispersed Ni²⁺ and, at Ni loadings ≥10 mol% Ni, segregated NiO that reduces to Ni 0 nanoparticles at 300 °C. In situ synchrotron XRD, quasi-in situ XPS, and operando IR spectroscopy reveal partial CeO 2 reduction and the coexistence of Ni 2+ –O–Ce interfacial sites and metallic Ni 0 . At 200 °C, catalysts containing 1 mol% Ni favor CO, whereas catalysts with ≥10 mol% Ni exhibit high CH 4 selectivity due to the presence of Ni 0 nanoparticles, reaching 98–99% CH 4 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 CH 4 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–CeO 2 are attributed to the synergy between small Ni 0 nanoparticles and Ni 2+ –O–Ce sites associated with oxygen vacancies, which together govern CO 2 activation and CH 4 formation. • Flame synthesis yields Ni–CeO 2 with tunable Ni speciation and particle size • Catalytic performance in CO 2 hydrogenation evaluated • Operando IR and XRD link Ni size to rWGS vs. methanation selectivity • Ni 0 nanoparticles (>=10 mol% Ni) deliver 98–99% CH 4 at 300 °C • Small Ni clusters favor CO and deactivate via soft coke at 300 °C
Evtushkova et al. (Wed,) studied this question.