**3. Discussion**

The mixed oxides MnOx-ZrO<sup>2</sup> here investigated showed promising performance in the removal of VOCs in the gas phase, considering the absence of noble metals co-catalysts and an initial VOCs concentration of 1000 ppm. The amount of zirconium oxide added on manganese oxide is a key parameter to improve the catalytic and the photocatalytic performance. The as-synthesized samples showed a comparable optical bandgap, in the range 3.0–3.3 eV (Table 1), similar to the TiO2, and able to exploit the UV-A portion of the solar light. The addition of zirconia on manganese oxide led to a slight decrease of the surface area (Table 1) that, however, did not comprise the catalytic activity of the mixed oxides.

The presence of a small amount of zirconium oxide on MnO<sup>x</sup> allowed, as stated by XRD and SEM-EDX, an ionic exchange between Zr4+ and Mn2+; this favoured the formation of a synergistic effect between the two oxides, with structural changes in the bulk of MnOx. These modifications led to increasing the (photo)catalytic activity compared to the bare oxides. Indeed, when reducible oxides, i.e., which own mobile/reducible oxygens, were used for the oxidation of VOCs, the total oxidation to CO<sup>2</sup> is favoured, because these oxygens can participate in the reaction with a Mars–Van Krevelen (MvK) mechanism [41,42]. The oxygen vacancies on the surface of the oxide will be subsequently filled by the O<sup>2</sup> present in the gas phase.

This mechanism was boosted up with the photothermo-catalytic approach because the photocatalytic mechanism generated the superoxide (O<sup>2</sup> •−) and hydroxyl (OH• ) radicals [43,44], that being more reactive of the molecular O2, increased the rate of the total oxidation of VOCs (reactions a–i, Figure 6) and the re-filling of the oxygen vacancies, being the mobile oxygens of MnO<sup>x</sup> activated by the heating [13,45]. For these reasons, the conversion temperatures of both toluene and ethanol oxidation were sensibly lower compared to the thermocatalysis, especially with the MnOx-5%ZrO<sup>2</sup> sample. In this way, it was possible to exploit a double positive effect: (i) the solar irradiation effect: that allowed the formation of more reactive species, (ii) the thermal effect: that activated the redox mobility of the manganese oxide oxygens [13,20,45]. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 12 of 17

*Photothermo-catalytic mechanism: Photothermo-catalytic mechanism:*

	- (a) Charge carriers formation: MnOx-5%ZrO2 + hν(solar) → MnOx-5%ZrO2 (hVB++ e−CB) (a) Charge carriers formation: MnOx-5%ZrO<sup>2</sup> + hν(solar) → MnOx-5%ZrO<sup>2</sup> (hVB <sup>+</sup>+ e<sup>−</sup> CB)

•−

+O2 •−

•−→ CO2 + H2O

• → MnOx-

	- (d) Oxygen from the mixed oxide: MnOx-5%ZrO2 → MnOx-5%ZrO2 (Vo) + 1/2 O2(g)from oxide (d) Oxygen from the mixed oxide: MnOx-5%ZrO<sup>2</sup> → MnOx-5%ZrO<sup>2</sup> (Vo) + 1/2 O2(g)from oxide
	- heat (e) VOC oxidation: VOC + O<sup>2</sup> (g) + O2(g)from oxide heat → CO<sup>2</sup> + H2O
	- (e) VOC oxidation: VOC + O2 (g)+ O2(g)from oxide ሱሮ CO2 + H2O (f) Oxygen restoring : MnOx-5%ZrO2 (Vo) + 1/2 O2(g) → MnOx-5%ZrO2 (f) Oxygen restoring: MnOx-5%ZrO<sup>2</sup> (Vo) + 1/2 O2(g) → MnOx-5%ZrO<sup>2</sup>
	- *(iii) Solar photothermal effect* heat (g) MnOx-5%ZrO<sup>2</sup> + hν heat → MnOx-5%ZrO<sup>2</sup> (Vo) + 1/2 O2(g)from oxide + OH• + O<sup>2</sup> •−
	- (g) MnOx-5%ZrO2 + hν ሱሮ MnOx-5%ZrO2 (Vo) + 1/2 O2(g)from oxide+ OH• (h) Improved VOC oxidation: VOC + O2 (g)+ O2(g)from oxide + OH• + O2 (h) Improved VOC oxidation: VOC + O<sup>2</sup> (g) + O2(g)from oxide + OH• + O<sup>2</sup> •− → CO<sup>2</sup> + H2O
	- (i) Oxygen speeded up restoring: MnOx-5%ZrO2 (Vo) + 1/2 O2(g) + O2 5%ZrO2 (i) Oxygen speeded up restoring: MnOx-5%ZrO<sup>2</sup> (Vo) + 1/2 O2(g) + O<sup>2</sup> • → MnOx-5%ZrO<sup>2</sup>

It is worth noting that the reactions (a–c) and (d–f) are also involved in the solar photocatalysis at room temperature and in the bare thermocatalytic tests, respectively. The multi-catalytic effect (reactions g–i) allowed one to increase the performance and to favour the total oxidation of the employed VOCs to CO2. It is worth noting that the reactions (a–c) and (d–f) are also involved in the solar photocatalysis at room temperature and in the bare thermocatalytic tests, respectively. The multi-catalytic effect (reactions g–i) allowed one to increase the performance and to favour the total oxidation of the employed VOCs to CO2.

Another confirmation of the proposed MvK mechanism was reported in the Figure S3b. In the phothermo-catalytic oxidation of toluene with the MnOx-5%ZrO2 sample, the air (more interesting from a practical point of view) was replaced in the gas mixture with the pure oxygen. It is possible to note that the presence of oxygen led to a beneficial effect for the toluene conversion to CO2, being the T90 lower of 25 °C (155 °C) compared to the test with air (180 °C). This can be reasonably ascribed to the easier oxygen restoring on Another confirmation of the proposed MvK mechanism was reported in the Figure S3b. In the phothermo-catalytic oxidation of toluene with the MnOx-5%ZrO<sup>2</sup> sample, the air (more interesting from a practical point of view) was replaced in the gas mixture with the pure oxygen. It is possible to note that the presence of oxygen led to a beneficial effect for the toluene conversion to CO2, being the T<sup>90</sup> lower of 25 ◦C (155 ◦C) compared to the test with air (180 ◦C). This can be reasonably ascribed to the easier oxygen restoring on

the catalyst surface (reaction i), in an oxygen-rich environment, favouring, in this way, the

and zirconium oxide (especially at low amount of ZrO2) improved the photothermo-catalytic mechanism with the redox process on MnOx that was favoured by the ionic exchange between the zirconium and the manganese ions. On the contrary, an increased amount of zirconium oxide led to a progressive deposition of the hosted oxide on the surface of MnOx covering, in this way, the surface-active sites of manganese oxide [35,36]. For these reasons, the optimal performance was obtained with 5 wt.% of ZrO2. In this contest, the mobility of the surface oxygens of the MnOx-5%ZrO2 sample was favoured by the MnOx redox properties, and consequently, it is strictly related to its reducibility. Furthermore, the

MvK route.

the catalyst surface (reaction i), in an oxygen-rich environment, favouring, in this way, the MvK route.

As stated by the characterization data, the good interaction between the manganese and zirconium oxide (especially at low amount of ZrO2) improved the photothermocatalytic mechanism with the redox process on MnO<sup>x</sup> that was favoured by the ionic exchange between the zirconium and the manganese ions. On the contrary, an increased amount of zirconium oxide led to a progressive deposition of the hosted oxide on the surface of MnO<sup>x</sup> covering, in this way, the surface-active sites of manganese oxide [35,36]. For these reasons, the optimal performance was obtained with 5 wt.% of ZrO2. In this contest, the mobility of the surface oxygens of the MnOx-5%ZrO<sup>2</sup> sample was favoured by the MnO<sup>x</sup> redox properties, and consequently, it is strictly related to its reducibility. Furthermore, the amount of the surface oxygens on MnOx-5%ZrO<sup>2</sup> was higher compared to the other samples, as detected by XPS. To have a further confirmation of the high reducibility/mobility of the surface oxygens of MnOx-5%ZrO2, the H2-temperature-programmed reduction (TPR) measurements were carried out, and the sample profiles were reported in Figure 7. In accordance with the literature data [20,46], the TPR profiles of the MnOx-based samples were characterized to broad reduction peaks, due to the occurrence of several reduction processes of the Mn ions. As expected, the MnOx-5%ZrO<sup>2</sup> sample showed the lowest reduction feature (201 ◦C) attributed to the reduction of Mn2O<sup>3</sup> to Mn3O<sup>4</sup> [46], 111 ◦C and 117 ◦C lower compared to the same reduction feature of Mn3O<sup>4</sup> and MnOx-10%ZrO2, respectively. This reduction peak was also more intense for the MnOx-5%ZrO<sup>2</sup> compared to the other MnOx-based samples confirming, as detected by XRD and XPS, the major presence of Mn3+ ions on MnOx-5%ZrO2. The higher temperature reduction signals in the range 300–480◦ were ascribed to the further reduction of Mn3O<sup>4</sup> to MnO [46]. Moreover, in this case, the sample with 5 wt.% of ZrO<sup>2</sup> showed the highest reducibility (i.e., the lowest peak temperature). This is connected to the highest mobility/reducibility of the surface oxygens of MnOx-5%ZrO2, which favours the MvK mechanism, and therefore a better VOCs abatement. The reduction temperature of bare ZrO<sup>2</sup> started at a temperature above 500 ◦C [47], and for this reason, in our analysis (in the range 50–550◦C), its reduction peak was not complete. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 13 of 17 amount of the surface oxygens on MnOx-5%ZrO2 was higher compared to the other samples, as detected by XPS. To have a further confirmation of the high reducibility/mobility of the surface oxygens of MnOx-5%ZrO2, the H2-temperature-programmed reduction (TPR) measurements were carried out, and the sample profiles were reported in Figure 7. In accordance with the literature data [20,46], the TPR profiles of the MnOx-based samples were characterized to broad reduction peaks, due to the occurrence of several reduction processes of the Mn ions. As expected, the MnOx-5%ZrO2 sample showed the lowest reduction feature (201 °C) attributed to the reduction of Mn2O3 to Mn3O4 [46], 111 °C and 117 °C lower compared to the same reduction feature of Mn3O4 and MnOx-10%ZrO2, respectively. This reduction peak was also more intense for the MnOx-5%ZrO2 compared to the other MnOx-based samples confirming, as detected by XRD and XPS, the major presence of Mn3+ ions on MnOx-5%ZrO2. The higher temperature reduction signals in the range 300–480° were ascribed to the further reduction of Mn3O4 to MnO [46]. Moreover, in this case, the sample with 5 wt.% of ZrO2 showed the highest reducibility (i.e., the lowest peak temperature). This is connected to the highest mobility/reducibility of the surface oxygens of MnOx-5%ZrO2, which favours the MvK mechanism, and therefore a better VOCs abatement. The reduction temperature of bare ZrO2 started at a temperature above 500 °C [47], and for this reason, in our analysis (in the range 50–550°C), its reduction peak was not complete.

**Figure 7.** H2-TPR (Temperature programmed reduction) profiles of the investigated samples. **Figure 7.** H<sup>2</sup> -TPR (Temperature programmed reduction) profiles of the investigated samples.

Between the photocatalytic, the thermocatalytic and the photothermo-catalytic removal of VOCs, although the solar photocatalytic reaction has the advantages of work at room temperature and that with the MnOx-5%ZrO2, it reached a similar activity of the most used TiO2-based materials (Table 4); to have a complete VOCs removal, it is neces-Between the photocatalytic, the thermocatalytic and the photothermo-catalytic removal of VOCs, although the solar photocatalytic reaction has the advantages of work at room temperature and that with the MnOx-5%ZrO2, it reached a similar activity of the most used TiO2-based materials (Table 4); to have a complete VOCs removal, it is necessary

sary to have contextual heating. For this purpose, the solar photothermo-catalysis can be an optimal solution to obtain the good performance of the thermocatalysis, but with an

the best sample (MnOx-5%ZrO2) tested in our experimental conditions showed a decrease of 36 °C and 34 °C of the toluene and ethanol T90 conversion compared to the thermocatalytic tests favouring in both the reactions; the total oxidation to CO2 (the T90 of ethanol

Finally, the stability in the time-on steam toluene removal of MnOx-5%ZrO2 was good (Figure 8, toluene solar photothermo-oxidation) and pointed to the MnOx-ZrO2 catalyst being a promising versatile material for application in thermocatalysis, photocatalysis,

conversion to CO2 was lowered of 205 °C, Tables 5 and 6).

and photothermo-catalysis.

to have contextual heating. For this purpose, the solar photothermo-catalysis can be an optimal solution to obtain the good performance of the thermocatalysis, but with an energy saving, due to the lower temperature required for the VOCs conversion. Indeed, the best sample (MnOx-5%ZrO2) tested in our experimental conditions showed a decrease of 36 ◦C and 34 ◦C of the toluene and ethanol T<sup>90</sup> conversion compared to the thermocatalytic tests favouring in both the reactions; the total oxidation to CO<sup>2</sup> (the T<sup>90</sup> of ethanol conversion to CO<sup>2</sup> was lowered of 205 ◦C, Tables 5 and 6).

Finally, the stability in the time-on steam toluene removal of MnOx-5%ZrO<sup>2</sup> was good (Figure 8, toluene solar photothermo-oxidation) and pointed to the MnOx-ZrO<sup>2</sup> catalyst being a promising versatile material for application in thermocatalysis, photocatalysis, and photothermo-catalysis. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 14 of 17

**Figure 8.** Stability test of MnOx-5%ZrO2 catalyst in the toluene solar photothermo-catalytic oxida-**Figure 8.** Stability test of MnOx-5%ZrO<sup>2</sup> catalyst in the toluene solar photothermo-catalytic oxidation.

## tion. **4. Materials and Methods**

### **4. Materials and Methods**  *4.1. Catalysts Synthesis*

*4.1. Catalysts Synthesis*  Bare manganese oxide was prepared by chemical precipitation with NaOH (1 M) (Panreac Química SLU, Castellar del Vallès (Barcelona), Spain). In particular, a certain amount of manganese (II) chloride tetrahydrate (Sigma-Aldrich, Buchs, Switzerland) was dissolved in demineralized water and heated at 70 °C. After the NaOH was added dropwise until the pH = 10. Successively, the solution was stirred and kept at 70 °C for 2 h. Bare manganese oxide was prepared by chemical precipitation with NaOH (1 M) (Panreac Química SLU, Castellar del Vallès (Barcelona), Spain). In particular, a certain amount of manganese (II) chloride tetrahydrate (Sigma-Aldrich, Buchs, Switzerland) was dissolved in demineralized water and heated at 70 ◦C. After the NaOH was added dropwise until the pH = 10. Successively, the solution was stirred and kept at 70 ◦C for 2 h. After digestion for 24 h, the slurry was filtered and dried at 120 ◦C overnight. Finally, the resultant powders were calcined in air at 600 ◦C for 2 h.

After digestion for 24 h, the slurry was filtered and dried at 120 °C overnight. Finally, the resultant powders were calcined in air at 600 °C for 2 h. A similar procedure was followed for the bare ZrO2. In this case, the zirconyl nitrate hydrate (Fluka, Buchs, Switzerland) and ammonia (as precipitant agent, 25–28%, Sigma-A similar procedure was followed for the bare ZrO2. In this case, the zirconyl nitrate hydrate (Fluka, Buchs, Switzerland) and ammonia (as precipitant agent, 25–28%, Sigma-Aldrich, Buchs, Switzerland) were used, following the same procedures reported above, and the same thermal treatments (drying at 120 ◦C, and calcination at 600 ◦C for 2 h).

Aldrich, Buchs, Switzerland) were used, following the same procedures reported above, and the same thermal treatments (drying at 120 °C, and calcination at 600 °C for 2 h). For the MnOx-ZrO2 mixed oxides, the NaOH-driven precipitation was employed using the required stoichiometric amount of zirconyl nitrate hydrate to obtain the chosen nominal concentration in weight percentage (wt.%) of ZrO2. Moreover, in this case, the For the MnOx-ZrO<sup>2</sup> mixed oxides, the NaOH-driven precipitation was employed using the required stoichiometric amount of zirconyl nitrate hydrate to obtain the chosen nominal concentration in weight percentage (wt.%) of ZrO2. Moreover, in this case, the samples were dried at 120 ◦C and calcined in air at 600 ◦C for 2 h.

### samples were dried at 120 °C and calcined in air at 600 °C for 2 h. *4.2. Catalysts Characterization*

(FWHM) of the Kα of Mn.

*4.2. Catalysts Characterization*  The sample structures were determined through the X-ray powder diffraction (XRD) The sample structures were determined through the X-ray powder diffraction (XRD) using a Smartlab Rigaku diffractometer (Rigaku Europe SE, Hugenottenallee 167 Neu-Isenburg 63263, Germany) in Bragg–Brentano mode, equipped with a rotating anode of Cu

using a Smartlab Rigaku diffractometer (Rigaku Europe SE, Hugenottenallee 167 Neu-Isenburg 63263, Germany) in Bragg–Brentano mode, equipped with a rotating anode of

with field emission scanning electron microscopy (FE-SEM) using a ZEISS SUPRA 55 VP (Carl Zeiss QEC Gmb, Garching b. München,Germany). The composition of the powders was carried out by the energy dispersive X-ray (EDX) analysis using an INCA-Oxford (Oxford Instruments plc, Tubney Woods, Abingdon, Oxfordshire, United Kingdom) windowless detector, and a resolution of 127 eV determined using the half-height amplitude

The BET surface area values were determined by N2 adsorption–desorption measurements with a Sorptomatic 1990 instrument (Thermo Quest, Milano, Italy). Before the

measurements, the catalysts were outgassed overnight at 200 °C.

Kα radiation operating at 45 kV and 200 mA. The surface morphology was examined with field emission scanning electron microscopy (FE-SEM) using a ZEISS SUPRA 55 VP (Carl Zeiss QEC Gmb, Garching b. München, Germany). The composition of the powders was carried out by the energy dispersive X-ray (EDX) analysis using an INCA-Oxford (Oxford Instruments plc, Tubney Woods, Abingdon, Oxfordshire, United Kingdom) windowless detector, and a resolution of 127 eV determined using the half-height amplitude (FWHM) of the Kα of Mn.

The BET surface area values were determined by N<sup>2</sup> adsorption–desorption measurements with a Sorptomatic 1990 instrument (Thermo Quest, Milano, Italy). Before the measurements, the catalysts were outgassed overnight at 200 ◦C.

The UV-vis Diffuse Reflectance spectra (UV-Vis DRS, Diffuse Reflectance Spectroscopy) measurements were performed with a Jasco V- 670 spectrometer (Jasco Europe S.R.L., Cremella, Italy) provided with an integration sphere and using barium sulphate (Fluka, Buchs, Switzerland) as standard. The estimation of the optical band gap of the samples was determined using the Kubelka–Munch function [26].

The X-ray photoelectron spectroscopy (XPS) was performed with a K-alpha X-ray photoelectron instrument (Thermo Fisher Scientific, Waltham, MA, USA), employing the C 1s peak at 284.9 eV (of adventitious carbon) as reference.

The H2-TPR (Temperature programmed reduction) profiles of the samples were obtained using a home-made flow equipment (gas-mixture 5 vol.% H<sup>2</sup> in Ar) and a TCD detector, following the procedures reported in ref. [48].
