**3. Discussion**

It was widely accepted that the physical and chemical properties, such as component of active phase, specific surface area, Mn valence, concentration of adsorbed oxygen/lattice oxygen and reduction ability, can command the activity of the catalyst [48]. For the component factor of the active phase, we found that the ratio of Mn/Zr and their synthesis routes play a vital role in adjusting the content of active centers, and a low molar ratio (<2/3) of Mn/Zr would lead to the formation of a solid solution of MnxZr1−xO2. As the molar ratio goes up, a second phase, such as Mn2O<sup>3</sup> and Mn3O4, will be formed, as depicted in Figure 1a. This is mainly attributed to the solubility limit of manganese in the zirconia lattice [44]. However, a more complex mixed components does not necessarily result in a better catalytic activity, as the CP-Mn2Zr3 catalyst presented a better catalytic performance among the samples prepared by the co-precipitation process (Figure 6a). Moreover, the improved co-precipitation route can further promote the exposure of the active centers and enhance the catalytic performance. Therefore, it can be inferred that the optimized catalytic activity is linked to components of the active phase and mainly hinges on the ratio of Mn/Zr and their synthesis routes. On the other hand, co-precipitation and improved co-precipitation routes are ready to prepare catalysts with a large specific surface area and enhance their catalytic activities.

In addition, the concentrations of Mn4+ and O<sup>α</sup> species of oxidizing catalyst were crucial for the oxidative degradation of VOCs [43,49]. The order of the Oβ/O<sup>α</sup> follows in descending order as Mn4+/Mn3+, which was depicted as TP-Mn2Zr3 > MP-Mn2Zr3 > CP-Mn2Zr3, indicating the concentration of Mn4+ was positively correlated with the content of oxygen vacancy [40]. In combination with the results of catalytic activity and components of active phase, the improved catalytic activity should be attributed to the promoted effects of the solid solution of MnxZr1−xO2. Moreover, the shouldered shape of the reduction peak for TP-Mn2Zr3 and CP-Mn2Zr3 catalysts can also account for the formation of MnxZr1−xO<sup>2</sup> active species, which corresponds well with the results of XRD.

It was reported that the catalytic mechanism of toluene on Mn-based catalysts involves the Mv-K and L-H mechanism. Both lattice oxygen and adsorbed oxygen species can participate in the activation-oxidation process of VOCs [42,47,50]. In this paper, the enhanced catalytic activity of Mn-Zr catalysts may be ascribed to the collective effects of manganese zirconium bimetal because the formation of the solid solution of MnxZr1−xO<sup>2</sup> would be conducive to decrease the formation energy of oxygen vacancy; the oxygen molecule was absorbed and activated to active oxygen to enhance catalytic activity. Furthermore, the oxygen molecule is easily adsorbed on the defective (111) surface of the Mn-Zr catalyst with oxygen vacancy, and the distance of O–O bond was elongated, which probably suggested that the absorbed O<sup>2</sup> is easily activated and broken, which is in favor of the quick replenishment of consumed oxygen molecular during VOCs combustion [31]. This is in good agreement with the H2-TPR results. Therefore, it is supposed that the enriched exposure of the defective (111) surface of the MnxZr1−xO<sup>2</sup> active species would lead to better catalytic activity for toluene combustion, and this can account for the outstanding performance of TP-Mn2Zr3.
