Effect of Mn content on the catalytic performance of CeO₂-ZrO₂-MnO_x in the oxidation of toluene

A series of CeO₂-ZrO₂-MnO_x catalysts with different Mn contents (denoted as CZM_X, where X is the molar fraction of MnO_x) were prepared by the redox precipitation method; the effect of Mn content on the catalytic performance of CZM_X in the oxidative degradation of toluene was then investigated. The results indicate that that the CZM_0.6 catalyst exhibits the best performance in toluene oxidation; a complete conversion of toluene can be achieved at 230℃. The XRD results suggest that the crystallinity of CZM_X decreases first, steps over the valley and then increases again with the increase of Mn content. From the H₂-TPR characterization, in contrast, a volcanic tendency is observed for the interaction among Ce-Zr-Mn. In particular, the CZM_0.6 catalyst displays poorest crystallinity and strongest Ce-Zr-Mn interaction, which can enhance the surface oxygen species. Meanwhile, the Raman and O₂-TPD results prove that the abundant oxygen vacancies on the catalyst surface can promote the migration of surface active oxygen species and then enhance the catalytic performance of CZM_X in toluene oxidation. In addition, the in-situ DRIFTS results illustrate that the reaction of toluene oxidation over the CeO₂-ZrO₂-MnO_x catalysts proceeds via benzoic acid as the essential intermediate, which is rapidly converted to CO₂ and H₂O in the presence of O₂.