2.1.1. XRD Analysis

The XRD patterns of the as-prepared Mn-Zr catalysts with different Mn/Zr ratios are shown in Figure 1. Obviously, the intensity and position of the characteristic peak varies with the Mn/Zr ratio and synthesized routes. The intensity of characteristic peaks

of MnxZr1−xO<sup>2</sup> solid solution was increased as the Mn/Zr ratio decreased, in which the diffraction peaks located at 30.42◦ , 35.28◦ , 50.75◦ , and 60.33◦ correspond to the (111), (200), (220), and (311) crystal planes of MnxZr1−xO<sup>2</sup> solid solution (JCPDS PDF#77-2157), respectively (Figure 1a). On the contrary, the low Mn/Zr ratio resulted in the mixture phase formation, such as ZrO<sup>2</sup> (JCPDS PDF# 78-0048), or a small amount of solid solution, which is ascribed to the difference in ion radius. The ion radius of Mnn+ cations (Mn2+(0.83 Å), Mn3+(0.64 Å) and Mn4+(0.53 Å)) is smaller than Zr4+(0.84 Å). Therefore, it can be speculated that Mn cations readily partially replace Zr cations in the host lattice, whereas it is very difficult [24]. Additionally, the incorporation of Mn dramatically changes the crystal phase of zirconia (Figure 1b). Moreover, the preparation route played an important role in regulating the intensity of characteristic peaks of the MnxZr1−xO<sup>2</sup> solid solution. The two-step precipitation strategy is easier to obtain a high content of active phase solid solution as well as trace amounts of manganese oxide. Besides, it can be found that TP-Mn2Zr3 has more obvious (111) crystal plane characteristic peaks compared to CP-Mn2Zr3.; the content of (111) crystal plane of TP-Mn2Zr3 sample accounted for 51.4%, which is considered the active center [31], while the CP-Mn2Zr3 is ca. 49.3%. This is the reason why the TP-Mn2Zr3 sample has a better catalytic performance. The sample (MP-Mn2Zr3) prepared by mechanical ball mills was as a reference. The result showed that the separated mixture of ZrO<sup>2</sup> and Mn2O<sup>3</sup> oxides caused no synergistic effect between MnO<sup>x</sup> and ZrO2, and a poor catalytic activity for toluene oxidation under the ball milling conditions. (220), and (311) crystal planes of MnxZr1−xO<sup>2</sup> solid solution (JCPDS PDF#77-2157), respectively (Figure 1a). On the contrary, the low Mn/Zr ratio resulted in the mixture phase formation, such as ZrO<sup>2</sup> (JCPDS PDF# 78-0048), or a small amount of solid solution, which is ascribed to the difference in ion radius. The ion radius of Mnn+ cations (Mn2+(0.83 Å ), Mn3+(0.64 Å ) and Mn4+(0.53 Å )) is smaller than Zr4+(0.84 Å ). Therefore, it can be speculated that Mn cations readily partially replace Zr cations in the host lattice, whereas it is very difficult [24]. Additionally, the incorporation of Mn dramatically changes the crystal phase of zirconia (Figure 1b). Moreover, the preparation route played an important role in regulating the intensity of characteristic peaks of the MnxZr1−xO<sup>2</sup> solid solution. The two-step precipitation strategy is easier to obtain a high content of active phase solid solution as well as trace amounts of manganese oxide. Besides, it can be found that TP-Mn2Zr3 has more obvious (111) crystal plane characteristic peaks compared to CP-Mn2Zr3.; the content of (111) crystal plane of TP-Mn2Zr3 sample accounted for 51.4%, which is considered the active center [31], while the CP-Mn2Zr3 is ca. 49.3%. This is the reason why the TP-Mn2Zr3 sample has a better catalytic performance. The sample (MP-Mn2Zr3) prepared by mechanical ball mills was as a reference. The result showed that the separated mixture of ZrO<sup>2</sup> and Mn2O<sup>3</sup> oxides caused no synergistic effect between MnO<sup>x</sup> and ZrO2, and a poor catalytic activity for toluene oxidation under the ball milling conditions.

The XRD patterns of the as-prepared Mn-Zr catalysts with different Mn/Zr ratios are shown in Figure 1. Obviously, the intensity and position of the characteristic peak varies with the Mn/Zr ratio and synthesized routes. The intensity of characteristic peaks of MnxZr1−xO<sup>2</sup> solid solution was increased as the Mn/Zr ratio decreased, in which the diffraction peaks located at 30.42°, 35.28°, 50.75°, and 60.33°correspond to the (111), (200),

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 3 of 15

**2. Results and Discussion** *2.1. Material Characterization*

2.1.1. XRD Analysis

**Figure 1.** XRD patterns of catalysts with different molar ratios of Mn/Zr (**a**) and different preparation routes (**b**) (two-step precipitation (TP); conventional coprecipitation (CP); ball milling process (MP)).

Moreover, the lattice parameters and grain size of CP-Mn2Zr3, TP-Mn2Zr3, MP-Mn2Zr3, ZrO2, and α-Mn2O<sup>3</sup> samples were summarized in Table 1. Compared with MP-Mn2Zr3 and CP-Mn2Zr3, the TP-Mn2Zr3 catalyst has the lowest lattice parameters (5.04 Å) and the smallest grain size (8.4 nm). Indeed, Mn cations with different valence exhibit a different ionic radius. The more Mnn+ cations (Mn2+, Mn3+, and Mn4+) with smaller radii (0.83, 0.64, and 0.53 Å, respectively) incorporated into c-zirconia to replace the Zr4+ (0.84 Å) can reduce the lattice parameters [28,35,36]. Based on this, we can infer that

the improved preparation process is more beneficial to the synthesis of the MnxZr1−XO<sup>2</sup> solid solution and decreases the grain size.

**Table 1.** Data obtained from XRD analyses of two-step precipitation (TP), conventional coprecipitation (CP), ball milling process (MP)-Mn2Zr3, ZrO<sup>2</sup> , and α-Mn2O<sup>3</sup> samples.


<sup>a</sup> The average crystal size was calculated by the Scherrer equation from the XRD data.
