*2.3. Photocatalytic, Thermocatalytic and Photothermo-Catalytic Removal of Ethanol in Gas Phase*

The ethanol being an alcohol was more reactive than the aromatic toluene, but its oxidation can give various by-products; the most common in the gas phase oxidation was acetaldehyde [3,31,39], which is also the main by-product detected in all the investigated catalytic approaches here discussed, whereas a very low selectivity (<2%) was detected in CO, formic acid, and acetic acid. In the solar photocatalytic tests, MnOx-5% ZrO<sup>2</sup> confirmed its highest activity compared to the other samples (Figure 5) with an ethanol conversion of 98% and the highest selectivity to CO<sup>2</sup> (43%), the most important feature for the VOCs removal. The mixed oxide with the 10 wt.% of ZrO<sup>2</sup> showed a little decrease of photoactivity (ethanol conversion of 86%) and a higher selectivity to acetaldehyde (60%) with respect to CO<sup>2</sup> (36%). These data, in line with the photo-oxidation of toluene (Figure 4a), pointed to, in our experimental conditions, the 5 wt.% being the optimal amount of zirconia to have a synergistic effect with MnOx. Among the bare oxides, the manganese oxide showed a higher ethanol conversion, with a higher selectivity to CO<sup>2</sup> compared to ZrO2. This latter oxide promoted the partial oxidation to acetaldehyde (selectivity of 74%) and consequently exhibited the lowest selectivity to CO<sup>2</sup> (22%).

**Figure 5. Figure 5.**  Solar photocatalytic oxidation of ethanol after 5 h of irradiation. Solar photocatalytic oxidation of ethanol after 5 h of irradiation.

In Table 5 and Figure S4, the data of the thermocatalytic oxidation of ethanol are reported. MnOx-5%ZrO<sup>2</sup> showed, also for this VOC, the best performance, with the lowest T<sup>90</sup> (189 ◦C) and a maximum conversion to acetaldehyde of 66% at 176 ◦C. Moreover, MnOx-10%ZrO<sup>2</sup> and Mn3O<sup>4</sup> showed the same maximum conversion to acetaldehyde, but at a higher temperature (200 ◦C for MnOx-10%ZrO<sup>2</sup> and 226 ◦C for bare manganese oxide). Consequently, MnOx-5%ZrO<sup>2</sup> also exhibited the lowest T<sup>90</sup> related to the conversion to CO<sup>2</sup> (361 ◦C). It is verified also for this approach, a little negative effect of the increased amount of zirconium oxide on the MnOx, with higher T<sup>90</sup> of MnOx-10%ZrO<sup>2</sup> compared to MnOx-5%ZrO2. The highest conversion to acetaldehyde was obtained with the bare zirconia (maximum conversion of 98% at 409 ◦C) confirming, as also detected in the photocatalytic tests at room temperature, the tendency of this catalyst to promote the partial oxidation of ethanol instead of the total combustion.

the highest T90 decrease was verified with the bare zirconium oxide (138 °C lower compared to the thermocatalytic toluene T90) where the activation of the zirconia photocata-

The positive synergistic effect due to the addition of a small amount of zirconia on the MnOx 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 T90 with the MnOx-5% ZrO2 sample (180 °C) is comparable or lower with respect to the other MnOx-based catalysts reported in the literature (in the range 200 °C–270 °C considering an initial tolu-

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 8·104 mL/gcat·h, indeed, as expected, and as reported in the literature [38], with a high flow rate; the conversion rate of toluene to CO2 and water (the only by-products detected also in all the thermo and photothermo-catalytic tests) was slower, whereas a GSHV <

The ethanol being an alcohol was more reactive than the aromatic toluene, but its oxidation can give various by-products; the most common in the gas phase oxidation was acetaldehyde [3,31,39], which is also the main by-product detected in all the investigated catalytic approaches here discussed, whereas a very low selectivity (<2%) was detected in CO, formic acid, and acetic acid. In the solar photocatalytic tests, MnOx-5% ZrO2 confirmed its highest activity compared to the other samples (Figure 5) with an ethanol conversion of 98% and the highest selectivity to CO2 (43%), the most important feature for the VOCs removal. The mixed oxide with the 10 wt.% of ZrO2 showed a little decrease of photoactivity (ethanol conversion of 86%) and a higher selectivity to acetaldehyde (60%) with respect to CO2 (36%). These data, in line with the photo-oxidation of toluene (Figure 4a), pointed to, in our experimental conditions, the 5 wt.% being the optimal amount of zirconia to have a synergistic effect with MnOx. Among the bare oxides, the manganese oxide showed a higher ethanol conversion, with a higher selectivity to CO2 compared to ZrO2. This latter oxide promoted the partial oxidation to acetaldehyde (selectivity of 74%)

*2.3. Photocatalytic, Thermocatalytic and Photothermo-Catalytic Removal of Ethanol in Gas* 

lytic properties was fundamental to promote the toluene total oxidation.

8·104 mL/gcat·h did not substantially modify the conversion rate.

and consequently exhibited the lowest selectivity to CO2 (22%).

ene concentration of 1000 ppm [20,37]).

*Phase* 


**Table 5.** Data of the thermocatalytic oxidation of ethanol on the investigated samples.

Interestingly also for the removal of ethanol, the multi-catalytic reaction (i.e., the photothermo-catalysis) allowed us to improve the performance related to ethanol oxidation (Table 6, Figure S5). With MnOx-5%ZrO2, the T<sup>90</sup> of ethanol conversion was lowered to 34 ◦C (154 ◦C), a value that is comparable or lower, considering an initial ethanol concentration of 1000 ppm, with respect to the other MnOx-based materials reported in the literature (in the range 127 ◦C (initial ethanol concentration of 300 ppm) −200 ◦C (initial ethanol concentration of 600–1945 ppm) [37,40]). The total oxidation to CO<sup>2</sup> was favoured on this sample, and for this reason, the maximum conversion to acetaldehyde was low (35% at 118 ◦C), with a decrease of 205 ◦C of the T<sup>90</sup> related to the conversion to CO<sup>2</sup> compared to the thermocatalytic tests.


**Table 6.** Data of the photothermo-catalytic oxidation of ethanol on the investigated samples.

A further decrease of the maximum conversion to acetaldehyde was verified with MnOx-10%ZrO<sup>2</sup> (24%), but at a higher temperature (127 ◦C) compared to the MnOx-5%ZrO2, confirming that, with these mixed oxides, the combustion of ethanol was favoured with respect to its partial oxidation. For all the tested samples, similar to the photothermal oxidation of toluene, there was a positive effect of the solar light irradiation, with a contextual decrease of conversion temperatures compared to the thermocatalytic tests (comparison between Tables 5 and 6). The unmodified zirconia remained the less active catalyst, however, the solar-assisted reaction decreased T<sup>90</sup> of ethanol conversion of 13 ◦C compared to the tests without irradiation.
