Unravelling the role of oxygen species in toluene oxidation over Co3O4-base catalysts: In situ DRIFTS coupled with quasi in situ XPS

The present work elucidated the role of oxygen species in toluene oxidation over Co3O4-base catalysts. Co3O4 nanoneedles, nanoflowers, nanocubes, and nanoplates (denoted as Co3O4-n, Co3O4-f, Co3O4-c, and Co3O4-p) were employed to anchor Pt nanoparticles by an electrostatic adsorption method. The catalytic activity of the Pt/Co3O4 catalyst for toluene oxidation was significantly enhanced compared to that of Co3O4. Among these catalysts, Pt/Co3O4-p exhibited outstanding catalytic activity with the temperature for 90 % toluene conversion (T90 = 167 °C) at 82 °C lower than Co3O4-p (T90 = 249 °C). The low-temperature reducibility was obviously enhanced by the Pt nanoparticles, which weakened the Co-O bond energy, and improved the capacity of oxygen mobility and the concentration of surface adsorbed oxygen. The results of in situ DRIFTS demonstrated benzoate was identified as the dominant intermediate in toluene oxidation toward Co3O4-based catalysts. Furthermore, combined with the results of quasi in situ XPS showed that oxidizing toluene largely into benzoate primarily by the active adsorbed oxygen species at low reaction temperature (50 °C), mainly following the Langmuir-Hinshelwood mechanism, while oxidizing toluene into CO2 mostly by the lattice oxygen species at high reaction temperature (180–250 °C), largely following the Mars-van Krevelen mechanism.