Catalytic Ozonation of Toluene over Co ₃ O ₄ Spinel Catalysts: Influence of Surface Reduction on the Catalytic Performance and Reaction Path

Toluene and ozone are typical air pollutants found in enclosed environments such as buildings and aircrafts’ cabins. Decreasing simultaneously their concentration is a major challenge that can be achieved using a catalytic ozonation of toluene. In this study, spinel cobalt oxide catalysts were synthesized by precipitation method, and underwent different NaBH4 reduction treatments. They were fully characterized by XRD, Raman spectroscopy, XPS, TPD, TPR, and nitrogen physisorption. Their activity in toluene ozonation was recorded at temperatures from ambient to 100 °C. The cobalt oxide catalyst reduced with a 0.1 M NaBH4 solution exhibited the highest toluene ozonation activity, achieving full conversion at 90 °C. This enhancement was attributed to the surface reconstruction of Co 3 O 4 induced by NaBH4 reduction, which effectively increases the abundance of oxygen vacancies and regulates the Co 2+ /Co 3+ ratio on the catalyst surface. In situ DRIFT spectroscopy further allowed to elucidate the ozonation pathway, with various intermediates (benzyl alcohol, benzaldehyde, benzoic acid, maleic anhydride, etc.) finally yielding carbon dioxide and water.