*2.2. Evaluation of Catalytic Activity*

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 activity for toluene. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 9 of 15

**Figure 6.** Activity test of catalysts with different molar ratios of Mn/Zr (**a**) and prepared by different strategies (**b**). **Figure 6.** Activity test of catalysts with different molar ratios of Mn/Zr (**a**) and prepared by different strategies (**b**).

Moreover, the catalytic activity assessing experiments were also carried out on CP-Mn2Zr3, TP-Mn2Zr3, and MP-Mn2Zr3 catalysts, and the results are depicted in Figure 6b. Obviously, the TP-Mn2Zr3 catalyst exhibits a better catalytic performance with T<sup>50</sup> and T<sup>90</sup> values of 263 °C and 269 °C, respectively, followed by CP-Mn2Zr3. The performance of the catalyst with MP-Mn2Zr3 is the lowest, with T<sup>50</sup> and T<sup>90</sup> values of 275 °C and 287 °C, which is much lower than the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts. The results clearly show that the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts with the existence of the Mn-Zr solid solution exhibit higher catalytic activity for toluene abatement than that of the MP-Mn2Zr3 catalyst without the formation of the Mn-Zr solution. The outstanding catalytic performance of the TP-Mn2Zr3 catalyst for toluene should be attributed to the more ex-Moreover, the catalytic activity assessing experiments were also carried out on CP-Mn2Zr3, TP-Mn2Zr3, and MP-Mn2Zr3 catalysts, and the results are depicted in Figure 6b. Obviously, the TP-Mn2Zr3 catalyst exhibits a better catalytic performance with T<sup>50</sup> and T<sup>90</sup> values of 263 ◦C and 269 ◦C, respectively, followed by CP-Mn2Zr3. The performance of the catalyst with MP-Mn2Zr3 is the lowest, with T<sup>50</sup> and T<sup>90</sup> values of 275 ◦C and 287 ◦C, which is much lower than the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts. The results clearly show that the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts with the existence of the Mn-Zr solid solution exhibit higher catalytic activity for toluene abatement than that of the MP-Mn2Zr3 catalyst without the formation of the Mn-Zr solution. The outstanding catalytic performance of the TP-Mn2Zr3 catalyst for toluene should be attributed to the more exposed defect (111) crystal plane of MnxZr1−xO<sup>2</sup> and the improved capacity of mobility of active oxygen.

posed defect (111) crystal plane of MnxZr1−xO<sup>2</sup> and the improved capacity of mobility of active oxygen. The stability of the texture for the Mn-Zr catalysts will determine their potential application to some extent. In this work, five consecutive catalytic light-off cycle tests were performed on the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts, and the results are shown in Figure 7. The value of the T<sup>90</sup> value of the cycle 2 experiment for the TP-Mn2Zr3 catalyst is 259 °C, which is lower than that of the cycle 1 experiment (269 °C), and runs 3, 4, and 5 on the TP-Mn2Zr3 catalyst maintain a similar value of T<sup>90</sup> (259 °C ± 1). On the contrary, the The stability of the texture for the Mn-Zr catalysts will determine their potential application to some extent. In this work, five consecutive catalytic light-off cycle tests were performed on the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts, and the results are shown in Figure 7. The value of the T<sup>90</sup> value of the cycle 2 experiment for the TP-Mn2Zr3 catalyst is 259 ◦C, which is lower than that of the cycle 1 experiment (269 ◦C), and runs 3, 4, and 5 on the TP-Mn2Zr3 catalyst maintain a similar value of T<sup>90</sup> (259 ◦C ± 1). On the contrary, the CP-Mn2Zr3 catalyst performed inferior cyclic stability, in which the value of T<sup>90</sup> for cycles 1, 2, 3, 4, and 5 was 278, 269, 264, 264, and 269 ◦C, respectively. The enhanced

CP-Mn2Zr3 catalyst performed inferior cyclic stability, in which the value of T<sup>90</sup> for cycles

the activation effect at 300 °C [8]. To view the cyclic stability directly, the function curves between cycle times and toluene conversion at 265 °C are depicted as the inset of Figure 7a,b, which imply that the TP-Mn2Zr3 catalyst exhibited better cycle stability. Conversely, the order of cyclic stability of CP-Mn2Zr3 obeys the volcanic model, which implies the

inferior stability of CP-Mn2Zr3.

100

**(a)**

oxidation performance for both catalysts after running the first experiment should be attributed to the activation effect at 300 ◦C [8]. To view the cyclic stability directly, the function curves between cycle times and toluene conversion at 265 ◦C are depicted as the inset of Figure 7a,b, which imply that the TP-Mn2Zr3 catalyst exhibited better cycle stability. Conversely, the order of cyclic stability of CP-Mn2Zr3 obeys the volcanic model, which implies the inferior stability of CP-Mn2Zr3. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 10 of 15 *Catalysts* **2021**, *11*, x FOR PEER REVIEW 10 of 15 100 99.1885 99.42363 **(b)**

**Figure 7.** Activity cycle test of the TP-Mn2Zr3 catalyst (**a**) and the CP-Mn2Zr3 catalyst (**b**). **Figure 7.** Activity cycle test of the TP-Mn2Zr3 catalyst (**a**) and the CP-Mn2Zr3 catalyst (**b**). Based on studies reports, the bimetal oxide catalysts involve two reaction mechanisms in the oxidation for toluene, namely the Langmuir–Hinshelwood (L-H) and Mars–

Based on studies reports, the bimetal oxide catalysts involve two reaction mechanisms in the oxidation for toluene, namely the Langmuir–Hinshelwood (L-H) and Mars– van Krevelen (Mv-K) mechanisms. Increasing the temperature, the L-H mechanism is gradually weakened, and the Mv-K mechanism gradually occupies a dominant position. The adsorbed oxygen directly oxidizes the adsorbed organic molecules, which is accorded to the L-H mechanism; on the other hand, lattice oxygen is activated at a higher temperature, and the consumed lattice oxygen can be readily supplemented from gaseous oxygen, Based on studies reports, the bimetal oxide catalysts involve two reaction mechanisms in the oxidation for toluene, namely the Langmuir–Hinshelwood (L-H) and Mars–van Krevelen (Mv-K) mechanisms. Increasing the temperature, the L-H mechanism is gradually weakened, and the Mv-K mechanism gradually occupies a dominant position. The adsorbed oxygen directly oxidizes the adsorbed organic molecules, which is accorded to the L-H mechanism; on the other hand, lattice oxygen is activated at a higher temperature, and the consumed lattice oxygen can be readily supplemented from gaseous oxygen, which obeys the Mv-K mechanism [42,47]. van Krevelen (Mv-K) mechanisms. Increasing the temperature, the L-H mechanism is gradually weakened, and the Mv-K mechanism gradually occupies a dominant position. The adsorbed oxygen directly oxidizes the adsorbed organic molecules, which is accorded to the L-H mechanism; on the other hand, lattice oxygen is activated at a higher temperature, and the consumed lattice oxygen can be readily supplemented from gaseous oxygen, which obeys the Mv-K mechanism [42,47]. Figure 8a reveals the effect of different WHSVs on the activity of the TP-Mn2Zr3 catalyst. Obviously, the value of T<sup>90</sup> is positively correlated with WHSV; in other words, a

which obeys the Mv-K mechanism [42,47]. Figure 8a reveals the effect of different WHSVs on the activity of the TP-Mn2Zr3 catalyst. Obviously, the value of T<sup>90</sup> is positively correlated with WHSV; in other words, a longer contact time is beneficial to improve the catalytic performance. The longevity experiments for 3000 min were performed on TP-Mn2Zr3 and CP-Mn2Zr3 catalysts, as shown in Figure 8b. It was worth noting that the TP-Mn2Zr3 catalysts maintained a splendid catalytic activity (>90% toluene conversion). While the CP-Mn2Zr3 catalyst declined a lot after 1000 min and the value of toluene degradation was sharply decreased to about Figure 8a reveals the effect of different WHSVs on the activity of the TP-Mn2Zr3 catalyst. Obviously, the value of T<sup>90</sup> is positively correlated with WHSV; in other words, a longer contact time is beneficial to improve the catalytic performance. The longevity experiments for 3000 min were performed on TP-Mn2Zr3 and CP-Mn2Zr3 catalysts, as shown in Figure 8b. It was worth noting that the TP-Mn2Zr3 catalysts maintained a splendid catalytic activity (>90% toluene conversion). While the CP-Mn2Zr3 catalyst declined a lot after 1000 min and the value of toluene degradation was sharply decreased to about 50%, the conversion of toluene was only kept at ca. 33% after the 3000 min combustion reaction. longer contact time is beneficial to improve the catalytic performance. The longevity experiments for 3000 min were performed on TP-Mn2Zr3 and CP-Mn2Zr3 catalysts, as shown in Figure 8b. It was worth noting that the TP-Mn2Zr3 catalysts maintained a splendid catalytic activity (>90% toluene conversion). While the CP-Mn2Zr3 catalyst declined a lot after 1000 min and the value of toluene degradation was sharply decreased to about 50%, the conversion of toluene was only kept at ca. 33% after the 3000 min combustion reaction.

160 180 200 220 240 260 280 Temperature(℃) 0 1000 2000 3000 0 Reaction(min) **Figure 8.** The effect of different WHSVs on the activity of the TP-Mn2Zr3 catalyst (**a**); Longevity test over TP-Mn2Zr3 and CP-Mn2Zr3 in 1000 ppm toluene (**b**). **Figure 8.** The effect of different WHSVs on the activity of the TP-Mn2Zr3 catalyst (**a**); Longevity test over TP-Mn2Zr3 and CP-Mn2Zr3 in 1000 ppm toluene (**b**).

CP-Mn2Zr3 in 1000 ppm toluene (**b**).

0
