*2.1. Structural, Morphological, Textural and Optical Properties of the Samples*

The XRD patterns of the analysed samples are shown in Figure 1. The precipitation of manganese chloride (II) with NaOH and the employed calcination temperature (600 ◦C for 2 h) allowed to obtain the Mn3O4.The signals at 2θ = 18.1◦ , 28.9◦ , 31.0◦ , 32.4◦ , 36.0◦ , 38.1◦ , 44.3◦ and 50.8◦ are, indeed, in accordance with the PDF card. No.: 00-080-0382 of pure Mn3O<sup>4</sup> (Hausmannite). Bare ZrO<sup>2</sup> was obtained with the ammonia-driven precipitation of zirconyl nitrate. The signals fitted with the PDF card No. 00-079-1771 of zirconium oxide, with the typical diffraction peaks at 2θ = 30.2◦ , 35.2◦ and 50.3◦ . Interestingly, the co-precipitation with NaOH of both metals salt precursors created substantial changes in the crystalline structure of manganese oxide. The addition of 5 wt.% of zirconium oxide

led to a mixed Mn2O3/Mn3O<sup>4</sup> phase being present the diffraction peak at 2θ = 32.9◦ ; that is, the typical fingerprint of Mn2O<sup>3</sup> (PDF card No. 00-071-0636, [12,22]), together with the signals at 2θ = 38.2◦ (overlapped with the signal of Mn3O4) and 55.1◦ that are also ascribed to manganese (III) oxide [12,22]. The increase of ZrO<sup>2</sup> content (MnOx-10%ZrO<sup>2</sup> sample) restored the main presence of Mn3O<sup>4</sup> with a trace of Mn2O3. In both the mixed oxides, the signals related to ZrO<sup>2</sup> are absent, probably due to the low amount of hosted oxide and/or to the good dispersion of zirconium oxide on manganese oxide. substantial changes in the crystalline structure of manganese oxide. The addition of 5 wt.% of zirconium oxide led to a mixed Mn2O3/Mn3O4 phase being present the diffraction peak at 2θ = 32.9°; that is, the typical fingerprint of Mn2O3 (PDF card No. 00-071-0636, [12,22]), together with the signals at 2θ = 38.2° (overlapped with the signal of Mn3O4) and 55.1° that are also ascribed to manganese (III) oxide [12,22]. The increase of ZrO2 content (MnOx-10%ZrO2 sample) restored the main presence of Mn3O4 with a trace of Mn2O3. In both the mixed oxides, the signals related to ZrO2 are absent, probably due to the low amount of hosted oxide and/or to the good dispersion of zirconium oxide on manganese oxide.

precipitation of zirconyl nitrate. The signals fitted with the PDF card No. 00-079-1771 of zirconium oxide, with the typical diffraction peaks at 2θ = 30.2°, 35.2° and 50.3°. Interestingly, the co-precipitation with NaOH of both metals salt precursors created

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 3 of 17

**Figure 1.** X-ray Diffraction (XRD) patterns of the examined samples. **Figure 1.** X-ray Diffraction (XRD) patterns of the examined samples.

The ion radius of Zr4+ (0.84 Å) is similar of Mn2+(0.83 Å), and this can favour the ionic exchange between these cations [20,23]. On the contrary, the smaller radius of Mn3+ (0.64 Å) makes the Zr4+/Mn3+ exchange more difficult. Probably, when the amount of ZrO2 is low, the Zrions partially replace the Mn2+ promoting, in this way, the main presence of Mn3+, whereas a higher amount of ZrO2 led to a preferential surface covering of the MnOx instead of a lattice incorporation of the zirconium ions in MnOx [20,23]. Thus, it can explain the major presence of Mn (III) on MnOx-5%ZrO2 sample, and the coexistence of Mn II and III in the MnOx-10%ZrO2. The main crystalline size of the samples (Table 1) was determined by applying the Scherrer formula on the principal diffraction peaks of the oxides (2θ =36.0° for Mn3O4, 30.2° for ZrO2, 32.9° for MnOx-5% ZrO2, whereas for MnOx-10% ZrO2, the value was mediated considering both the 2θ = 32.4° and 36.0° signals). The addition of zirconium oxide led to a slight increase of the crystalline size of manganese oxide, whereas the bare ZrO2 showed the lowest crystalline size (8 nm). However, this latter oxide, in accordance with the surface area values reported in the literature [18], showed the lowest surface area (Table 1, 26.2 m2/g), whereas the bare Mn3O4 exhibited the highest BET surface area (99.6 m2/g). Compared to the bare Mn3O4, the slight increase of the crystallite size of the mixed oxides determined a decrease of their surface area, which were about 85–86 m2/g for both the MnOx-ZrO2 samples (Table 1). The ion radius of Zr4+ (0.84 Å) is similar of Mn2+(0.83 Å), and this can favour the ionic exchange between these cations [20,23]. On the contrary, the smaller radius of Mn3+ (0.64 Å) makes the Zr4+/Mn3+ exchange more difficult. Probably, when the amount of ZrO<sup>2</sup> is low, the Zr ions partially replace the Mn2+ promoting, in this way, the main presence of Mn3+, whereas a higher amount of ZrO<sup>2</sup> led to a preferential surface covering of the MnO<sup>x</sup> instead of a lattice incorporation of the zirconium ions in MnO<sup>x</sup> [20,23]. Thus, it can explain the major presence of Mn (III) on MnOx-5%ZrO<sup>2</sup> sample, and the coexistence of Mn II and III in the MnOx-10%ZrO2. The main crystalline size of the samples (Table 1) was determined by applying the Scherrer formula on the principal diffraction peaks of the oxides (2θ = 36.0◦ for Mn3O4, 30.2◦ for ZrO2, 32.9◦ for MnOx-5% ZrO2, whereas for MnOx-10% ZrO2, the value was mediated considering both the 2θ = 32.4◦ and 36.0◦ signals). The addition of zirconium oxide led to a slight increase of the crystalline size of manganese oxide, whereas the bare ZrO<sup>2</sup> showed the lowest crystalline size (8 nm). However, this latter oxide, in accordance with the surface area values reported in the literature [18], showed the lowest surface area (Table 1, 26.2 m2/g), whereas the bare Mn3O<sup>4</sup> exhibited the highest BET surface area (99.6 m2/g). Compared to the bare Mn3O4, the slight increase of the crystallite size of the mixed oxides determined a decrease of their surface area, which were about 85–86 m2/g for both the MnOx-ZrO<sup>2</sup> samples (Table 1).


**Table 1.** Structural, textural and optical properties of the examined samples.

**Table 1.** Structural, textural and optical properties of the examined samples. **Sample Crystallite Size (nm) a BET Surface Area** 

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 4 of 17

<sup>a</sup> Estimated by XRD.

The SEM-EDX measurements (Figure 2) were performed to evaluate the presence of zirconium oxide on MnOx. The adopted precipitation methods led to, indifferently to the investigated samples, a non-homogenous morphology with spherical particles (Figures 2a and S1). From the EDX elemental analysis (Figure 2b,c, Table 2), it is possible to note that a little surface segregation of zirconium in the MnOx-10%ZrO<sup>2</sup> sample was detected, whose zirconium wt.% was 3.7 times higher (instead of twice as expected considering the nominal concentration) compared to the MnOx-5%ZrO2. In accordance with the XRD data, the increase of the amount of ZrO<sup>2</sup> led to an enrichment of zirconium oxide on the surface of MnOx, whereas in the MnOx-5%ZrO<sup>2</sup> mixed oxide, the ZrO<sup>2</sup> was mainly embedded in the lattice of MnOx. The SEM-EDX measurements (Figure 2) were performed to evaluate the presence of zirconium oxide on MnOx. The adopted precipitation methods led to, indifferently to the investigated samples, a non-homogenous morphology with spherical particles (Figures 2a and S1). From the EDX elemental analysis (Figure 2b,c, Table 2), it is possible to note that a little surface segregation of zirconium in the MnOx-10%ZrO2 sample was detected, whose zirconium wt.% was 3.7 times higher (instead of twice as expected considering the nominal concentration) compared to the MnOx-5%ZrO2. In accordance with the XRD data, the increase of the amount of ZrO2 led to an enrichment of zirconium oxide on the surface of MnOx, whereas in the MnOx-5%ZrO2 mixed oxide, the ZrO2 was mainly embedded in the lattice of MnOx.

**Figure 2.** (**a**) SEM image of the MnOx-5%ZrO2 as representative sample; Energy Dispersive X-ray (EDX) spectra of MnOx-10%ZrO2 (**b**) and MnOx-5%ZrO2 (**c**). The EDX spectra were mediated considering four different zones of the samples. **Figure 2.** (**a**) SEM image of the MnOx-5%ZrO<sup>2</sup> as representative sample; Energy Dispersive X-ray (EDX) spectra of MnOx-10%ZrO<sup>2</sup> (**b**) and MnOx-5%ZrO<sup>2</sup> (**c**). The EDX spectra were mediated considering four different zones of the samples.


**Table 2.** EDX elemental analysis of the examined samples. The presence of carbon was due to the carbon tape used to perform the measurements.

The surface valence state of the components of the catalysts were analysed through X-ray photoelectron spectroscopy (XPS) (Table 3). Interestingly, on the surface of MnOxbased samples, the ratio of the Mn3+/Mn2+ ions obtained considering the area of the deconvoluted spectra (see Figure S2, spectra of MnOx-5%ZrO<sup>2</sup> as representative sample) was the highest for the MnOx-5%ZrO<sup>2</sup> mixed oxide. As pointed to also by the structural properties determined by XRD, in this sample, a strong ionic interaction between the Mn2+ and the Zn4+ was particularly favoured, leading to an increase in the amount of Mn3+ ions. Moreover, in this sample, the ratio between the surface lattice oxygen (Oα) located at about 530 eV and the chemisorbed/defective oxygen (Oβ) at 532 eV was also the highest (Table 3, Figure S2), suggesting that the ionic exchange between the zirconium and the manganese ions also promoted a higher concentration of the manganese oxide surface oxygens. These, as reported, can participate in VOCs oxidation, improving the catalytic activity of the catalysts [24]. Finally, the binding energy of the Zr 3d5/2 at about 182.0 eV is the typical fingerprint of ZrO<sup>2</sup> [25]. The surface atomic percentage of Zr was 3.5 higher (2.77%) on the MnOx-10%ZrO<sup>2</sup> compared to MnOx-5%ZrO<sup>2</sup> (0.73%) confirming, as too stated by the EDX analysis, the surface covering of zirconia on manganese oxide, verified increasing the amount of ZrO2.

**Table 3.** XPS analysis and binding energy (BE in eV) of the components of the investigated samples.


The UV-DRS of the samples were reported in the Figure 3. The MnOx-based samples (Figure 3a) showed a remarkable lower reflectance compared to the bare ZrO<sup>2</sup> (Figure 3b), and thus can be highlighted considering also the colours of the as-synthesized powders (dark brown for the MnOx-based materials and white for the zirconium oxide). The optical bandgaps of the semiconductor oxides were estimated plotting the modified Kubelka– Munk function versus hν, as reported in the literature ([26], inset Figure 3b as representative sample). Interestingly, as established by XRD, the good crystallinity of ZrO<sup>2</sup> and its nanosize (8 nm, Table 1) allowed us to obtain a ZrO<sup>2</sup> with a lower bandgap (3.02 eV) compared to the other E<sup>g</sup> reported in the literature for this oxide (about 5.0 eV that, however, can be narrowed down to 2–1.5 eV on the basis of the adopted preparation method [18,27]). No substantial variations were observed comparing the unmodified Mn3O<sup>4</sup> and the MnOx-ZrO<sup>2</sup> based-oxides, with an E<sup>g</sup> of 3.26–3.29 eV (Table 1). Probably, the low amount and the good dispersion of ZrO<sup>2</sup> on MnO<sup>x</sup> did not alter the bandgap of the manganese oxide. All the manganese oxide-based samples exhibited a similar E<sup>g</sup> to TiO<sup>2</sup> (3.0–3.2 eV on the basis of the crystalline form [28]); thus, they can exploit the UV-A portion of solar irradiation.

portion of solar irradiation.

**Figure 3.** (**a**) UV-DRS (Diffuse Reflectance Spectroscopy) of the MnOx-based samples; (**b**) UV-DRS spectra of bare ZrO2. In the inset; the estimation of the optical bandgap through the modified Kubelka–Munk function. **Figure 3.** (**a**) UV-DRS (Diffuse Reflectance Spectroscopy) of the MnOx-based samples; (**b**) UV-DRS spectra of bare ZrO<sup>2</sup> . In the inset; the estimation of the optical bandgap through the modified Kubelka–Munk function.

TiO2 (3.0–3.2 eV on the basis of the crystalline form [28]); thus, they can exploit the UV-A

### *2.2. Photocatalytic, Thermocatalytic and Photothermo-Catalytic Removal of Toluene in Gas 2.2. Photocatalytic, Thermocatalytic and Photothermo-Catalytic Removal of Toluene in Gas Phase*

*Phase*  Figure 4a shows the solar photocatalytic activity in the oxidation of toluene at room temperature after 5 h of irradiation. The highest conversion value was obtained with the MnOx-5% ZrO2 (84%) followed by the MnOx-10% ZrO2 and the bare Mn3O4 (51% and 47%, respectively), whereas pure ZrO2 exhibited the lowest conversion value (33%). In accordance with the literature [3,29], in our experimental condition, the only detected by-products were carbon dioxide and water with traces of benzaldehyde (selectivity in the range 1–3%). Although a real comparison with the other reported data for this reaction is very difficult, due to the various experimental conditions adopted by the other research groups (Table 4), the performance of MnOx-5% ZrO2 mixed oxide is very promising, being similar to (considering the initial concentration of 1000 ppm of toluene) or slightly lower than the most used TiO2-based photocatalysts, or to other unconventional semiconductors (Table 4). Figure 4a shows the solar photocatalytic activity in the oxidation of toluene at room temperature after 5 h of irradiation. The highest conversion value was obtained with the MnOx-5% ZrO<sup>2</sup> (84%) followed by the MnOx-10% ZrO<sup>2</sup> and the bare Mn3O<sup>4</sup> (51% and 47%, respectively), whereas pure ZrO<sup>2</sup> exhibited the lowest conversion value (33%). In accordance with the literature [3,29], in our experimental condition, the only detected by-products were carbon dioxide and water with traces of benzaldehyde (selectivity in the range 1–3%). Although a real comparison with the other reported data for this reaction is very difficult, due to the various experimental conditions adopted by the other research groups (Table 4), the performance of MnOx-5% ZrO<sup>2</sup> mixed oxide is very promising, being similar to (considering the initial concentration of 1000 ppm of toluene) or slightly lower than the most used TiO2-based photocatalysts, or to other unconventional semiconductors (Table 4).

**Table 4.** Data comparison of the photocatalytic oxidation of toluene.


**Figure 4.** (**a**) Solar photocatalytic oxidation of toluene after 5h of irradiation; (**b**) effect of the wt.% of ZrO2 on MnOx in the solar photocatalytic toluene conversion; (**c**) thermocatalytic oxidation of toluene; (**d**) photothermo-catalytic oxidation of toluene on the investigated materials. **Figure 4.** (**a**) Solar photocatalytic oxidation of toluene after 5h of irradiation; (**b**) effect of the wt.% of ZrO<sup>2</sup> on MnO<sup>x</sup> in the solar photocatalytic toluene conversion; (**c**) thermocatalytic oxidation of toluene; (**d**) photothermo-catalytic oxidation of toluene on the investigated materials.

**Table 4.** Data comparison of the photocatalytic oxidation of toluene. **Catalysts Experimental Conditions Toluene Conversion Ref.**  MnOx-5%ZrO2 1000 ppm Toluene, 5 h irradiation solar lamp (300 W, 10.7 mW/cm2), room T, 150 mg catalyst 84% this work Brookite TiO2-5% CeO2 1000 ppm Toluene, 2 h irradiation solar lamp (300 25% [3] It is worth noting that 5 wt.% of zirconia was the best amount to obtain a synergistic positive effect on the MnOx. Indeed, the samples prepared with the same procedures reported in the par. 4.1 but adding the 3 wt.% and 15 wt.% of zirconium oxide caused a decrease of activity (66% of toluene conversion for MnOx-3% ZrO<sup>2</sup> and 48 % for MnOx-15% ZrO2, i.e., the same conversion value of the bare manganese oxide). The results pointed to a photocatalytic "volcano" trend (Figure 4b). The positive effects of the addition of ZrO<sup>2</sup> on MnO<sup>x</sup> reached the maximum with 5% of zirconia, following a progressive decrease at higher amounts. This can be reasonably due, as confirmed by XRD, SEM-EDX and XPS measurements, to a progressive surface coverage of MnOx, due to the presence of a large amount of ZrO2. This caused a decrease in the photoactivity considering also the lower photocatalytic performance of bare ZrO<sup>2</sup> compared to manganese oxide (Figure 4a). The

detection of a specific amount of the hosted oxide on the main oxide is a typical trend of the mixed oxide-based semiconductors. A large amount of the second component (hosted oxide) can cover the surface active sites of the main oxide, decreasing the overall photocatalytic activity of the photocatalyst [35,36].

Thermal catalytic combustion is the most used process to increase the removal efficiency of toluene. The thermocatalytic activity of the investigated samples is reported in the Figure 4c. Moreover, for this catalytic approach, MnOx-5% ZrO<sup>2</sup> gave the best results. The T<sup>90</sup> (the temperature at which the 90% of toluene conversion was reached) values were 216 ◦C, 231 ◦C, 240 ◦C and 383 ◦C for MnOx-5% ZrO2, MnOx-10% ZrO2, Mn3O4, and the bare ZrO<sup>2</sup> respectively, confirming the order of activity measured in the photocatalytic tests at room temperature.

To further decrease toluene T90, the solar photothermo-catalytic tests were employed on the same investigated samples (Figure 4d). The solar-assisted thermo catalytic approach allowed one to decrease the T<sup>90</sup> of 36 ◦C (180 ◦C) with respect to thermocatalytic tests on the best mixed oxide, the MnOx-5% ZrO<sup>2</sup> catalyst, and in general, a decrease of T<sup>90</sup> was verified for all the catalysts, with even the same order of activity: MnOx-5% ZrO<sup>2</sup> > MnOx-10% ZrO<sup>2</sup> (T<sup>90</sup> = 217 ◦C) > Mn3O<sup>4</sup> (T<sup>90</sup> = 226 ◦C) > ZnO<sup>2</sup> (T<sup>90</sup> = 245 ◦C). Interestingly, the highest T<sup>90</sup> decrease was verified with the bare zirconium oxide (138 ◦C lower compared to the thermocatalytic toluene T90) where the activation of the zirconia photocatalytic properties was fundamental to promote the toluene total oxidation.

The positive synergistic effect due to the addition of a small amount of zirconia on the MnO<sup>x</sup> and the solar multi-catalytic approach led to obtaining a low toluene T90, considering the absence of noble metals co-catalysts. The obtained value of T<sup>90</sup> with the MnOx-5% ZrO<sup>2</sup> sample (180 ◦C) is comparable or lower with respect to the other MnOxbased catalysts reported in the literature (in the range 200 ◦C–270 ◦C considering an initial toluene concentration of 1000 ppm [20,37]).

The influence of the gas hourly space velocity (GSHV) was reported in the Figure S3a considering the best sample (MnOx-5% ZrO2). We have chosen, for all the tests, a GSHV of 8 <sup>×</sup> <sup>10</sup><sup>4</sup> mL/gcat·h, indeed, as expected, and as reported in the literature [38], with a high flow rate; the conversion rate of toluene to CO<sup>2</sup> and water (the only by-products detected also in all the thermo and photothermo-catalytic tests) was slower, whereas a GSHV < 8 <sup>×</sup> <sup>10</sup><sup>4</sup> mL/gcat·h did not substantially modify the conversion rate.
