2.1.2. BET Analysis

It was accepted that the catalytic activity of nanocatalysts is closely related to their surface texture, such as specific surface area, average pore size, and pore volume. Figure 2 shows the nitrogen adsorption-desorption curves of as-obtained catalysts (TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3), and it can be observed that these catalysts have similar adsorption isotherms, which can be classified as typical type IV adsorption isotherms based on the IUPAC classification [37]. Hysteresis loops demonstrated relative pressure in the range of 0.4–1 in all three samples, and there was no adsorption saturation platform in the range of higher relative pressure, suggesting the catalysts have an irregular pore structure that may be caused by the slit-shaped pores. The type IV isotherm is usually the basis for judging whether there are mesopores in the catalyst materials [8,38]. Considering the presence of type IV adsorption isotherm and H3 hysteresis loop in the catalyst, it can be determined that irregular mesopores were formed in all the catalysts [39]. Figure 2b shows the pore size distribution of the catalyst, which further confirmed the existence of the mesoporous structure in the catalysts. The pore size mainly distributed in the range of 2–40 nm and centered around 10 nm. Specifically, the value of the specific surface area, average pore diameter, and pore volume of the catalyst were listed in Table 2. It was observed that the catalyst prepared by mechanical stirring showed the lowest specific surface area, only 23.7 m2/g, whereas the catalyst doped with Zr obviously demonstrated an improved specific surface area. The specific surface areas of TP-Mn2Zr3 and CP-Mn2Zr3 catalysts were 99.7 and 139.5 m2/g, respectively, which may promote the exposure of active sites on the surface of the catalysts [31] and contribute to the improved catalytic performance. Among these oxides, the average pore sizes of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts are 10.8, 5.9, and 13.5 nm, respectively. It was found that the TP-Mn2Zr3 catalyst has the largest pore volume of 0.27 cm3/g, followed by CP-Mn2Zr3 (0.20 cm3/g) and MP-Mn2Zr3 (0.08 cm3/g) catalysts. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 5 of 15 **Table 2.** The surface area, average pore diameter, and pore volume of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts. **Samples SBET (m<sup>2</sup> /g) Average Pore Diameter (nm) <sup>a</sup> Pore Volume (cm<sup>3</sup> /g) <sup>b</sup>** TP-Mn/Zr = 2/3 99.7 10.8 0.27 CP-Mn/Zr = 2/3 139.5 5.9 0.20 MP-Mn/Zr = 2/3 23.7 13.5 0.08 <sup>a</sup> Bases on the total adsorption average pore width (4V/A by BET, A = SBET). <sup>b</sup> Based on the BJH Adsorption cumulative volume.

**Figure 2.** Nitrogen adsorption-desorption isotherms (**a**) and BJH pore-size distributions (**b**) of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts. **Figure 2.** Nitrogen adsorption-desorption isotherms (**a**) and BJH pore-size distributions (**b**) of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts.

oclinic zirconium dioxide into cubic zirconium dioxide [26].

In order to further study the morphology of the catalysts, TEM experiments were carried out on the CP-Mn2Zr3 and TP-Mn2Zr3 catalysts, as shown in Figure 3. The results suggested that both CP-Mn2Zr3 and TP-Mn2Zr3 exhibit morphology of nanoparticles (Figure 3a,c) and the lattice spacings of ca. 2.54 and 2.94 Å , well attributed to the (111) crystal plane and (200) crystal plane of the MnxZr1−xO<sup>2</sup> solid solution, respectively (Figure 3b,d). This is in line with the XRD results. Therefore, it can be inferred that Mn ions are readily embedded in the lattice of ZrO<sup>2</sup> and form Mn-Zr solid solution via co-precipitation or optimized two-step precipitation route and lead to the phase transformation from mon-

2.1.3. HRTEM Analysis

3g-1, STP )

 TP**–**Mn2Zr3 CP**–**Mn2Zr3 MP**–**Mn2Zr3

**(a)**

and MP-Mn2Zr3 catalysts.


**Table 2.** The surface area, average pore diameter, and pore volume of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts.

**Table 2.** The surface area, average pore diameter, and pore volume of TP-Mn2Zr3, CP-Mn2Zr3, and

TP-Mn/Zr = 2/3 99.7 10.8 0.27 CP-Mn/Zr = 2/3 139.5 5.9 0.20 MP-Mn/Zr = 2/3 23.7 13.5 0.08 <sup>a</sup> Bases on the total adsorption average pore width (4V/A by BET, A = SBET). <sup>b</sup> Based on the BJH

**(b)**

**/g) Average Pore Diameter (nm) <sup>a</sup> Pore Volume (cm<sup>3</sup>**

**/g) <sup>b</sup>**

 TP**–**Mn2Zr3 CP**–**Mn2Zr3 MP**–**Mn2Zr3

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 5 of 15

dV/dW (cm

3g-1 nm-1

)

<sup>a</sup> Bases on the total adsorption average pore width (4V/A by BET, A = SBET). <sup>b</sup> Based on the BJH Adsorption cumulative volume.

### 2.1.3. HRTEM Analysis 2.1.3. HRTEM Analysis In order to further study the morphology of the catalysts, TEM experiments were

MP-Mn2Zr3 catalysts.

**Samples SBET (m<sup>2</sup>**

Adsorption cumulative volume.

In order to further study the morphology of the catalysts, TEM experiments were carried out on the CP-Mn2Zr3 and TP-Mn2Zr3 catalysts, as shown in Figure 3. The results suggested that both CP-Mn2Zr3 and TP-Mn2Zr3 exhibit morphology of nanoparticles (Figure 3a,c) and the lattice spacings of ca. 2.54 and 2.94 Å, well attributed to the (111) crystal plane and (200) crystal plane of the MnxZr1−xO<sup>2</sup> solid solution, respectively (Figure 3b,d). This is in line with the XRD results. Therefore, it can be inferred that Mn ions are readily embedded in the lattice of ZrO<sup>2</sup> and form Mn-Zr solid solution via coprecipitation or optimized two-step precipitation route and lead to the phase transformation from monoclinic zirconium dioxide into cubic zirconium dioxide [26]. carried out on the CP-Mn2Zr3 and TP-Mn2Zr3 catalysts, as shown in Figure 3. The results suggested that both CP-Mn2Zr3 and TP-Mn2Zr3 exhibit morphology of nanoparticles (Figure 3a,c) and the lattice spacings of ca. 2.54 and 2.94 Å , well attributed to the (111) crystal plane and (200) crystal plane of the MnxZr1−xO<sup>2</sup> solid solution, respectively (Figure 3b,d). This is in line with the XRD results. Therefore, it can be inferred that Mn ions are readily embedded in the lattice of ZrO<sup>2</sup> and form Mn-Zr solid solution via co-precipitation or optimized two-step precipitation route and lead to the phase transformation from monoclinic zirconium dioxide into cubic zirconium dioxide [26].

**Figure 3.** High resolution transmission electron microscope (HRTEM) images of CP-Mn2Zr3 (**a**,**b**) and TP-Mn2Zr3 (**c**,**d**). **Figure 3.** High resolution transmission electron microscope (HRTEM) images of CP-Mn2Zr3 (**a**,**b**) and TP-Mn2Zr3 (**c**,**d**).

### 2.1.4. XPS Analysis 2.1.4. XPS Analysis

**(a)**

Mn2Zr3.

Intensity(a.u.)

MP-Mn2Zr3

CP-Mn2Zr3

TP-Mn2Zr3

Intensity(a.u.)

XPS experiments were performed to analyze the components and valence states of elements, as shown in Figure 4 and Table 3. It was observed that the surface elements of the catalyst are mainly composed of O (ca. 530 eV), Zr (ca. 183), and Mn (ca. 642 eV) (Figure 4a) [30,40]. Figure 4b shows the XPS spectra of Mn 2p; it was integrated into two char-XPS experiments were performed to analyze the components and valence states of elements, as shown in Figure 4 and Table 3. It was observed that the surface elements of the catalyst are mainly composed of O (ca. 530 eV), Zr (ca. 183 eV), and Mn (ca. 642 eV) (Figure 4a) [30,40]. Figure 4b shows the XPS spectra of Mn 2p; it was integrated into

acteristic peaks by deconvolution. The characteristic peaks located at 641.7 eV and 643.4

**(b)**

**(d)**

TP-Mn2Zr3

CP-Mn2Zr3

MP-Mn2Zr3

**Figure 4.** XPS spectra of (**a**) full spectra, (**b**) Mn 2p, (**c**) O 1s, (**d**) Zr 3d over TP, CP, and MP-

Intensity(a.u.)

Binding energy(ev) <sup>190</sup> <sup>188</sup> <sup>186</sup> <sup>184</sup> <sup>182</sup> <sup>180</sup> <sup>178</sup> <sup>176</sup>

Zr3s

1000 800 600 400 200 0

(529.7)

Binding energy(ev)

**(c)** O1s <sup>O</sup><sup>α</sup>

O<sup>β</sup> (531.0)

540 538 536 534 532 530 528 526

Zr3pC1s Zr3d O1s Mn2p O KLL

Mn Aguer peak

eV can be attributed to Mn3+ and Mn4+ species, respectively [1,17,41].

 TP-Mn2Zr3 CP-Mn2Zr3 MP-Mn2Zr3

Mn3s

Mn3p

Intensity(a.u.)

645 640

Zr4+(184.3)

Mn4+(643.3) Mn3+(641.7)

Binding Energy(ev)

Binding energy(ev)

Mn 2p

TP-Mn2Zr3

CP-Mn2Zr3

MP-Mn2Zr3

Zr4+(181.9) Zr 3d

two characteristic peaks by deconvolution. The characteristic peaks located at 641.7 eV and 643.4 eV can be attributed to Mn3+ and Mn4+ species, respectively [1,17,41]. acteristic peaks by deconvolution. The characteristic peaks located at 641.7 eV and 643.4 eV can be attributed to Mn3+ and Mn4+ species, respectively [1,17,41].

XPS experiments were performed to analyze the components and valence states of elements, as shown in Figure 4 and Table 3. It was observed that the surface elements of the catalyst are mainly composed of O (ca. 530 eV), Zr (ca. 183), and Mn (ca. 642 eV) (Figure 4a) [30,40]. Figure 4b shows the XPS spectra of Mn 2p; it was integrated into two char-

**Figure 3.** High resolution transmission electron microscope (HRTEM) images of CP-Mn2Zr3 (**a**,**b**)

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 6 of 15

and TP-Mn2Zr3 (**c**,**d**).

2.1.4. XPS Analysis

**Figure 4.** XPS spectra of (**a**) full spectra, (**b**) Mn 2p, (**c**) O 1s, (**d**) Zr 3d over TP, CP, and MP-**Figure 4.** XPS spectra of (**a**) full spectra, (**b**) Mn 2p, (**c**) O 1s, (**d**) Zr 3d over TP, CP, and MP-Mn2Zr3.


Mn2Zr3. **Table 3.** Analysis of the surface composition and content of samples based on XPS results.

Furthermore, the ratio of Mn4+/Mn3+ of the catalyst calculated based on the relative area was closely related to the preparation routes. The TP-Mn2Zr3 catalyst prepared by an improved precipitation method had a higher Mn4+/Mn3+ value, which reached 0.68, which followed the MP-Mn2Zr3 (0.53) and CP-Mn2Zr3 catalysts (0.50), respectively. The strong interaction between Mn and Zr in Mn-Zr solid solution can promote the occurrence of the charge transfer process and produce more Mn4+ [30], which plays a key role in the catalytic degradation of toluene. In other words, the stronger interaction between Mn and Zr in the catalyst of TP-Mn2Zr3 can be conducive to higher catalytic oxidation of toluene. Meanwhile, it is widely accepted that the oxygen mobility of manganese-based oxides is closely related to the transformation ability of manganese species between different valence states, and the increase of the concentration of high valence metal cations promotes the chemical potential and mobility of oxygen [42].

O 1s XPS spectra of the three catalysts were exhibited in Figure 4c, which can be split into two peaks by deconvolution, the bind-energy located at 529.0−530.0 eV and 531.0−532.0 eV can be ascribed to the surface lattice oxygen (Oα) and chemisorbed oxygen and/or defect oxides (O2−, O<sup>2</sup> <sup>2</sup>−, or O−) (Oβ) [28], respectively. It was noted that the content of O<sup>α</sup> is more than 60% for the three samples, suggesting the O<sup>α</sup> is dominated. Furthermore, the decreasing trend of

Oβ/O<sup>α</sup> value is as follows: TP-Mn2Zr3(0.65) > MP-Mn2Zr3(0.63) > CP-Mn2Zr3 (0.59), which is positively correlated with the value of Mn4+/Mn3+(TP-Mn2Zr3 (0.68) > MP-Mn2Zr3(0.53) > CP-Mn2Zr3 (0.50)). It was reported that both lattice oxygen and chemisorbed oxygen and/or defect oxides are active oxygen species that can participate in the oxidation of toluene, which improves the oxidation performance of the catalyst [19,35]. Herein, the optimized coprecipitation catalyst displays better catalytic activity due to the contribution of lattice oxygen and absorbed oxygen.

The spin-splitting peaks of the Zr 3d orbit can be observed in Figure 4d, centered at 181.9 eV and 184.3 eV and belong to 3d5/2 and 3d3/2 orbit, respectively, suggested that Zr cations exist in the catalyst in the form of tetravalent [30]. Meanwhile, it was found that there was hardly a change in the position of the Zr 3D orbital splitting peak in TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 samples. It was worth noting that the XPS signal peak of Zr in TP-Mn2Zr3 and CP-Mn2Zr3 samples originated from the Mn-Zr solid solution, while the signal peak of Zr in the MP-Mn2Zr3 sample was derived from pure zirconia. This indicates that the Zr4+ ion is very stable [43], though the Mnn+ ions were entered the framework of ZrO2. Moreover, the proportion of Mn, Zr, and O in the catalyst is related to the preparation routes (Table 3). The order of the proportion of Mn elements follows: MP-Mn2Zr3 > CP-Mn2Zr3 > TP-Mn2Zr3, while that of Zr and O is the opposite; this suggested that the concentration of Mn-Zr solid solution various with the content of Mn ions entered the framework of ZrO2.
