2.1.5. H2-TPR Results Analysis

The H2-TPR experiments were employed to investigate the reducibility of the catalysts. The H2-TPR cures of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 samples in the range of 100–700 ◦C are shown in Figure 5. There exist three obvious characteristic peaks of hydrogen consumption. It was reported that about 15 wt.% of MnO is soluble in ZrO<sup>2</sup> due to the solidified eutectic temperature [44]. Herein, the three characteristic peaks of catalysts were mainly attributed to the reduction of MnOx, in which Peak 1 located below 250 ◦C shows the reduction of amorphous MnOx, dispersed on the surface of Mn-Zr solid solution, and peak 2 (250–380 ◦C) should be linked to the reduction of Mn2O<sup>3</sup> to Mn3O4, while peak 3 (380–480 ◦C) corresponds to the reduction of Mn3O<sup>4</sup> to MnO, respectively [26,31,45]. Moreover, the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts behave an excellent low-temperature reduction performance compared to that of MP-Mn2Zr3. Especially for the TP-Mn2Zr3 sample, the locations of reduction peaks are separately reduced to 224, 303, and 400 ◦C, suggesting it possessed a stronger mobility of reactive oxygen species and leads to a better catalytic activity for VOCs abatement [31]. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 8 of 15 °C shows the reduction of amorphous MnOx, dispersed on the surface of Mn-Zr solid solution, and peak 2 (250–380 °C) should be linked to the reduction of Mn2O<sup>3</sup> to Mn3O4, while peak 3 (380–480 °C) corresponds to the reduction of Mn3O<sup>4</sup> to MnO, respectively [26,31,45]. Moreover, the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts behave an excellent lowtemperature reduction performance compared to that of MP-Mn2Zr3. Especially for the TP-Mn2Zr3 sample, the locations of reduction peaks are separately reduced to 224, 303, and 400 °C, suggesting it possessed a stronger mobility of reactive oxygen species and leads to a better catalytic activity for VOCs abatement [31].

**Figure 5.** H2-TPR profiles of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts. **Figure 5.** H<sup>2</sup> -TPR profiles of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts.

It was found that the reduction temperature of pure zirconia usually appears above 600 °C, and the incorporation of Mn into the framework of zirconia to form a solid solution of MnxZr1−xO<sup>2</sup> would contribute to the decreasing of reduction temperature [26,29]. In this It was found that the reduction temperature of pure zirconia usually appears above 600 ◦C, and the incorporation of Mn into the framework of zirconia to form a solid solution of MnxZr1−xO<sup>2</sup> would contribute to the decreasing of reduction temperature [26,29]. In

contribution, the reduction peak of zirconia was not observed on the MP-Mn2Zr3 catalyst,

Mn2Zr3 and CP-Mn2Zr3 catalysts, which may be the result of the synergistic interaction between the surface MnO<sup>x</sup> and ZrO2 alloyed to form Mn-Zr solid solution [31,46], which

The catalytic oxidation performance of the synthesized catalysts for toluene abatement was assessed. The functional cures between the conversion of toluene on the catalysts and reaction temperature are depicted in Figure 6. All the catalysts can achieve complete catalytic oxidation for toluene below 300 °C. As shown in Figure 6a, the CP-Mn2Zr3 catalyst exhibits better catalytic oxidation activity for toluene. The values of T<sup>50</sup> (the 50% conversion of 1000 ppm toluene) and T<sup>90</sup> (the 90% conversion of 1000 ppm toluene) of the CP-Mn2Zr3 catalyst are 270 °C and 278 °C, respectively. In addition, the order of catalytic ability of catalysts is depicted as CP-Mn2Zr3 > CP-Mn3Zr1 > CP-Mn3Zr2 > CP-Mn1Zr1 > CP-Mn1Zr3. It is evident that the catalytic activity was closely linked with the Mn/Zr ratio, which controlled the component of the Mn-Zr solid solution. Thereby, it can be inferred that the Mn-Zr solid solution is in the active phase and greatly influences the catalytic

well corresponds to the XRD results.

*2.2. Evaluation of Catalytic Activity*

activity for toluene.

this contribution, the reduction peak of zirconia was not observed on the MP-Mn2Zr3 catalyst, indicating that MnxZr1−xO<sup>2</sup> species cannot be formed by the ball milling process. In contrast, the shouldered temperature reduction peak (ca. 400 ◦C) was displayed in both TP-Mn2Zr3 and CP-Mn2Zr3 catalysts, which may be the result of the synergistic interaction between the surface MnO<sup>x</sup> and ZrO<sup>2</sup> alloyed to form Mn-Zr solid solution [31,46], which well corresponds to the XRD results.
