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A series of nanocrystalline CoOx catalysts were synthesized by a citric acid complexation strategy. The effects of C2–C5 diols (including ethylene glycol, 1,3-propanediol, 1,4-butanediol, and 1,5-pentadiol) on catalyst structure and propane combustion performance were investigated. The results showed that the structure of CoOx transformed from pure Co3O4 to mixed-phase CoO–Co3O4 with a larger surface area and smaller crystallite size after the introduction of diol compounds. Importantly, diol compounds effectively weaken the Co–O bond strength in nanocrystalline CoOx and enhance the activity of oxygen species, thus effectively degrading propane. Due to the weakest Co–O bond strength, excellent oxygen species activity, abundant oxygen vacancies, and the strongest redox ability, the catalyst prepared by adding 1,3-propanediol (Co-PDO) exhibited the best catalytic activity (T90 = 227 °C). Moreover, the increase in pore size, surface Co2+ and oxygen vacancy content of Co-PDO in the 40 h stability test can compensate for the loss of activity caused by the decrease in specific surface area, thereby maintaining stable catalytic activity. [ABSTRACT FROM AUTHOR]

The mixed-phase CoO-Co 3 O 4 efficiently promotes the total oxidation of propane. • CoO dispersed Co 3 O 4 nanoparticles were fabricated by citric acid complex synthesis. • Mixed-phase CoO-Co 3 O 4 facilitates propane conversion due to synergistic interaction. • Low O atoms electron cloud density and weak Co-O bond boost oxidation ability. A series of pure Co 3 O 4 and mixed-phase CoO-Co 3 O 4 nanoparticles were synthesized through a facile citric acid complex strategy. Different from pure Co 3 O 4 , the extra CoO is beneficial to the formation of small crystalline size (11.8 nm), large specific surface area (62.8 m2·g−1), and high Co2+/Co3+ and O ads /O latt ratios. Moreover, the interaction between CoO and Co 3 O 4 diminishes the electron cloud density in the outer layer of O atom, weakens the strength of the Co O bond, and improves the mobility of active oxygen species. At a GHSV of 30,000 ml·g−1·h−1, the mixed-phase CoO-Co 3 O 4 converted propane to 90% only at 235 °C. After 40 h of reaction, CoO was oxidized into smaller Co 3 O 4 particles and maintained a stable catalytic activity. Further research found that the mixed-phase CoO-Co 3 O 4 still has good activity (T 90 = 250 °C) even at high GHSV (75,000 ml·g−1·h−1). The outstanding performance of nanoparticles makes it possible to achieve deep purification of VOCs under high GHSV. [ABSTRACT FROM AUTHOR]

A series of nanocrystalline Co 3 O 4 catalysts were synthesized by low-temperature liquid phase complexation with different small molecular carboxylic acids (citric acid, oxalic acid, and tartaric acid). The effects of different carboxylic acids on the catalyst structure and surface microenvironment were discussed in detail, and the catalytic performance of propane combustion was investigated. The results showed that different carboxylic acids changed the distance between Co2+ ions in the catalyst precursor, which directly affected the crystallite size (12–16 ​nm) and specific surface area (10~31 ​m2 ​g−1) of nanocrystalline Co 3 O 4. Among them, Co 3 O 4 synthesized from the cobalt citrate precursor with the largest Co2+ ion distance has the best catalytic activity. The conversion rate of propane reached 90% at 250 ​°C and showed stable performance in 40 ​h continuous reactions. The excellent catalytic performance could be allocated to the smallest crystallite size, excellent texture properties, a high degree of lattice disorder, unique surface composition, and excellent reducibility. Different small molecular carboxylic acids changed the distance between Co2+ ions in the catalyst precursor, thus strongly affecting the structure and surface microenvironment of nanocrystalline Co 3 O 4. Among them, Co 3 O 4 synthesized from the cobalt citrate precursor with the largest Co2+ ion distance has the best catalytic activity (Co-CA: T 90 ​= ​250 ​°C). Image 1 • Nanocrystalline Co 3 O 4 were prepared with various small molecular carboxylic acids. • Different carboxylic acids changed the distance between Co2+ ions in the precursor. • Co 3 O 4 synthesized with citric acid showed the best propane conversion performance. • Abundant Co3+ and reactive oxygen species enhanced the oxidation ability. [ABSTRACT FROM AUTHOR]