One-Step Viscous-Paste Synthesis of Ni–Co Spinel Oxides: Defect Engineering, Oxygen Mobility, and Vacancy-Mediated Catalysis

Ni–Co spinel oxide was synthesized via a direct one-step thermally induced viscous-paste route, in which solid Ni and Co salts were mixed with citric acid and only 3.3 wt % deionized water to form a viscous paste, eliminating the need for homogeneous solution processing. XPS analysis confirmed the coexistence of Co²⁺, Co³⁺, Ni²⁺, and Ni³⁺ species in the spinel structure. In conjunction with XRD, EPR, H₂-TPR, and O₂-TPD characterizations, the catalyst prepared by this route was found to exhibit a higher proportion of Co²⁺ and Ni²⁺, superior oxygen mobility, abundant oxygen vacancies, and enhanced low-temperature reducibility compared with hydrothermal and coprecipitation counterparts. These properties substantially lowered the light-off temperature for toluene oxidation (T₁₀ = 63 °C, T₂₀ = 95 °C). Based on the structure–activity relationship of the catalyst, we propose that low-temperature lattice oxygen migration and a vacancy-mediated mechanism drive toluene oxidation. Furthermore, the catalyst displayed excellent durability under harsh conditions, maintaining high activity at a WHSV of 48,000 mL·g^–1·h^–1 and remaining stable even at 90% relative humidity. This straightforward and scalable viscous-paste strategy provides an effective route for constructing spinel catalysts with high active-site density and robust mass-transport characteristics, offering advanced opportunities for practical VOCs abatement.