Balance of activation and ring-breaking for toluene oxidation over CuO-MnO x bimetallic oxides

Highlights: • The reaction rates of Cu₂Mn₁ is approximately 4 times that of CuO and Mn₃O₄. • The superior reducibility of Cu₂Mn₁ is not main reason for the increased activity. • The rate of ring-break is matched with the rate of toluene activation over Cu₂Mn₁. • Mn–O is activation site, whereas Cu–O is adsorption site in O–Cu–O–Mn–O. • The matching of adsorption and activation sites is effective to design catalysts. CuMn oxides have been studied for many years to catalytic degradation of toluene, but there are still many divergences on the essence of their great catalytic activity and reaction mechanism. A series of CuMn bimetallic oxides were synthesized for the catalytic oxidation of toluene in this study. Cu₂Mn₁ exhibited the highest toluene oxidation rate per specific surface area, which was approximately 4 times that of monometallic CuO and Mn₃O₄. Benzoic acid was the only intermediates which could be observed during toluene oxidation. Between monometallic CuO and Mn₃O₄, toluene was more difficult to be activated by Mn₃O₄ to generate benzoic acid (toluene activation), whereas benzoic acid was oxidized (ring-breaking) by CuO with more difficulty. As for CuMn, the superior reducibility combined with the balance between ring-breaking of benzoic acid and activation of toluene-to-benzoic acid determined the high toluene oxidation rate. DFT simulations exhibited that in O–Cu–O–Mn–O structure, the Mn–O site was a more effective activation site for toluene-to-benzoic acid oxidation, whereas Cu–O mainly performed as an adsorption site for toluene. This work identifies the different roles of Cu and Mn entities in toluene oxidation and provides the novel design strategy for toluene removal catalysts.