Nanocrystalline Co3O4 catalysts for toluene and propane oxidation: Effect of the precipitation agent

A set of cobalt oxide catalysts was prepared via precipitation using various precipitants and evaluated in the total oxidation of toluene and propane. A commercial Co3O4 catalyst was used as a reference. Various techniques were used to characterize the catalysts before and after testing. The results showed that the physicochemical properties and the catalytic performance of these cobalt oxide catalysts were greatly influenced by the nature of the precipitation agent used upon preparation. All catalysts exhibited a Co3O4 cubic spinel structure, with different crystallite sizes (27–41 nm) and specific surface areas (15–40 m2 g−1). The optimal catalyst, obtained via carbonate-precipitation and named Co-CO3, achieved 90 % toluene and propane conversion at 256 and 226 °C, respectively. This Co-CO3 catalyst performed better than the commercial catalyst in the toluene oxidation and similarly to the commercial one in the propane oxidation. All catalysts maintained constant performance upon three consecutive catalytic cycles; however, they deactivated to a different extent upon longer-term stability tests depending on the molecule to be oxidized. In toluene oxidation, Co-CO3 exhibited good stability upon cycling but a poor long-term durability. To overcome this issue, a dual-bed catalytic system was proposed. In propane oxidation, Co-CO3 showed stable performance in both cases. The effects of propane and oxygen concentrations on the catalytic performance of Co-CO3 in propane oxidation were also investigated. A positive reaction order with respect to oxygen and the results of oxygen-free catalytic test revealed the dominant role of chemisorbed oxygen species in the oxidation of propane. The excellent performance of Co-CO3 was tentatively ascribed to the highly strained nanocrystalline structure, the high concentration in Co2+ and the superior reducibility.