Investigation of Cu-Mn catalytic ozonation of toluene: Crystal phase, intermediates and mechanism

Abstract

The effect of different crystal phases, i.e. spinel phase (CuMn₂O₄) and amorphous phase (Cu_0.2MnO_x), was explored in Cu-Mn catalytic ozonation of toluene. The toluene removal efficiency followed the order of Cu_0.2MnO_x (91.2%) ˃ CuMn₂O₄ (74.5%) ˃ commercial catalyst Cu_0.3MnO_x (70.3%) in 130 min, and the higher CO₂ yield (67.6%) could be also observed using Cu_0.2MnO_x. In order to investigate the effect of phases on the toluene degradation pathway, the intermediates and byproducts were identified by DRIFTS, GC-MS, and TOF-SIMS. No obvious difference was observed in the distribution of byproducts, except for the quantities, suggesting the discrepancy of oxidation rate. On the other hand, the catalysts were characterized before and after the ozonation process by TEM, BET, XPS, XRD, EPR, TGA, and TPR. It was proposed that for amorphous catalysts, the oxygen vacancy (V_o) helped the chemisorption of toluene, and adjacent Mn reacted as the main active site for the ozonation process. While, the redox pair of Cu⁺/Mn⁴⁺ and Cu²⁺/(Mn³⁺, Mn²⁺) in the spinel phase plays an important role in the generation of oxygen vacancies for O₃ decomposition.

Keywords: Catalytic ozonation; Crystal phase; Cu-Mn bimetallic oxide; Degradation pathway; Reaction mechanism.