Flame Synthesized Co–CeO₂ Catalysts for CO₂ Methanation

Achieving selective conversion of CO₂ to CO, CH₄, or CH₃OH remains a key challenge in catalyst design for CO₂ hydrogenation. Site-specific activity at the metal-support interface plays a crucial role, motivating efforts to optimize metal particles and their interactions with supports. In this study, we synthesized Co-CeO₂ catalysts with varying Co contents via flame spray pyrolysis (FSP) to investigate how the location and structure of Co influence activity. All samples contain ∼8 nm CeO₂ nanoparticles with a high surface area and approximately 3.8 mol % Co²⁺ ions strongly interacting with CeO₂. Catalysts with ≥5 mol % Co feature segregated CoO and Co₃O₄ particles, which are partially reduced to metallic Co at 300 °C. The highest Co-weight-normalized activity at 200 °C (3.9 ± 0.2 mmol CO₂/mol Co/s, CH₄ selectivity 85%) was observed in 10 mol % Co-CeO₂, with ∼50% Co reduction and 4-5 nm Co nanoparticles. The 2.5 mol % Co sample exhibited only 10% reduction, forming small Co clusters and creating Co²⁺-O-Ce sites that mainly favor CO formation (79% selectivity). Low Co content facilitates CO₂ hydrogenation to CO and minor CH₃OH formation, likely on oxygen vacancies, assisted by H₂ dissociation on very small metallic Co clusters. Larger Co nanoparticles predominantly produce CH₄, with minor CO and no CH₃OH. These results demonstrate that FSP enables tuning of catalyst structures for selective CO₂ hydrogenation, leveraging the synergy between small metallic Co particles and Co²⁺-O-Ce sites.