**3. Bimetallic Catalysts for the Photocatalytic Oxidation of VOCs**

The urgent request for a "greener" and sustainable industrial chemistry has driven a huge field of research towards alternative ways to treat VOCs instead of catalytic combustion. Among the various AOPs (see the Introduction section), photocatalytic oxidation is the most applied process. With respect to catalytic thermal oxidation, this technique allows working at room temperature, exploiting the chemicophysical processes activated by an appropriate light radiation interacting with the surface of a semiconductor photocatalyst [40,124–126]. Specifically, dangerous organic compounds are oxidized by hydroxyl, and super oxide radicals are generated by the interaction between the photoelectrons and photoholes of the photocatalyst with water and oxygen. These photoelectrons and photoholes are formed when an adequate wavelength (λ ≤ of the band gap, Eg, of the semiconductor) irradiates the photocatalyst [127,128]. The most used photocatalysts are metal oxides or sulfides such as TiO2, ZnO, WO3, ZrO2, CeO2, Fe2O3, ZnS, and CdS, and among them, TiO<sup>2</sup> and ZnO are the most used [129,130]. Due to its properties, such as its nontoxicity, relatively low cost, and high activity, especially under UV irradiation, titanium dioxide was deeply investigated both in academic and industrial research [124,126]. With a band gap varying from 3.0 to 3.2 eV depending on the crystalline form, TiO<sup>2</sup> is able to exploit only 5% of solar radiation, thus limiting its practical applications. Photothermocatalytic oxidation is a multicatalytic approach that accepts the contemporaneous utilization of a light source to activate the photocatalyst, and thermal heat to boost the conversion of organic molecules and to increase the yield to CO2. A proper structural and/or chemical modification of titanium dioxide together with this multicatalytic approach can be considered a suitable solution to decrease the total energy consumption, maintaining the high conversion values typical of thermocatalytic oxidation of VOCs [131–133]. Another connected strategy to increase the photocatalytic performance of titanium dioxide under solar/visible light irradiation is doping with metal or nonmetal elements [134–136], and a not yet largely explored strategy is the combination of a bimetallic alloy with TiO<sup>2</sup> [137,138]. The same metals typically employed for catalytic oxidation of VOCs, such as Au, Ag, Pt, Pd, and Cu in the nanoparticle size, if irradiated with (usually) a visible light irradiation, allow exploiting the localized surface plasmon resonance (LSPR) through collective oscillations of the electrons in the surface of the metal nanoparticles. This effect, combined with the photocatalytic properties of TiO2, is a performance solution to obtain a visible-light-driven photocatalyst [124,139,140]. In addition, for this particular application, bimetallic compounds can help to overcome some of the drawbacks of single metals. For example, the LSPR of some noble metals such as Pd, Pt, and Rh is not efficiently activated by solar irradiation, and a possible combination with the most effective plasmonic metals, such as Au, Ag or Cu, leads to takeing advantages of both the LSPR effect and of the reactive catalytic behavior of the other noble metals [137].

For the above considerations, in VOC photo-oxidation, the most investigated system was the Au-Pd bimetallic compound joined with a semiconductor photocatalyst [141–145]. In these materials, the good affinity of gold and palladium, already discussed in terms of thermocatalytic performance (Section 2.1.1.), is in this case utilized to increase the photocatalytic performances of titanium dioxide or of another semiconductor oxide.

Colmenares et al. [141] synthetized an Au-Pd/TiO<sup>2</sup> photocatalyst with the original technique of sonophotodeposition (Figure 9). The bimetallic sample exhibited high activity (83%) and good selectivity (70%) in the partial oxidation of methanol to methyl formate after 120 min of UV irradiation (125 W mercury lamp λmax = 365 nm). Although the bimetallic catalyst showed a low selectivity to CO<sup>2</sup> (≈30%), demonstrating that this approach is better suitable for selective oxidation than the total oxidation of VOCs, the reported material synthesis and adopted reaction conditions are a fascinating way to obtain results with an energy-efficient procedure and a selective photocatalyst in a short time and under mild conditions.

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**Figure 9.** Sonophotodeposition setup before (**a**) and during the deposition procedure (**b**): (1) batch photoreactor; (2) argon line; (3) switched off 6 W UV lamp; (4) ultrasonic bath; (5) switched on 6W UV lamp; (6) reflux condenser; and (7) lamp cooling system (20 °C). Figure from [141], Copyright 2015, John Wiley and Sons **Figure 9.** Sonophotodeposition setup before (**a**) and during the deposition procedure (**b**): (1) batch photoreactor; (2) argon line; (3) switched off 6 W UV lamp; (4) ultrasonic bath; (5) switched on 6W UV lamp; (6) reflux condenser; and (7) lamp cooling system (20 ◦C). Figure from [141], Copyright 2015, John Wiley and Sons.

In a further study, the same research group [142] developed a density functional methodology to analyze the reaction mechanism of the selective photo-oxidation of methanol on the bimetallic Au-Pd/TiO2 sample. The theoretical investigation showed, as with the formation of a synergistic interaction between gold and palladium, a superior photoelectron–hole separation, was verified in comparison with the monometallic samples. Furthermore, it was shown that to favor total photooxidation to CO2, the dissociation of molecular oxygen should be driven preferentially on Pd to favor the formation of PdO sites, where complete oxidation (no methyl formate formation) to carbon dioxide occurred. In a further study, the same research group [142] developed a density functional methodology to analyze the reaction mechanism of the selective photo-oxidation of methanol on the bimetallic Au-Pd/TiO<sup>2</sup> sample. The theoretical investigation showed, as with the formation of a synergistic interaction between gold and palladium, a superior photoelectron–hole separation, was verified in comparison with the monometallic samples. Furthermore, it was shown that to favor total photo-oxidation to CO2, the dissociation of molecular oxygen should be driven preferentially on Pd to favor the formation of PdO sites, where complete oxidation (no methyl formate formation) to carbon dioxide occurred.

Cybula et al. [143] investigated the performance of an Au-Pd bimetallic sample supported on rutile TiO2 synthetized with a water in oil microemulsion methodology, in the photocatalytic oxidation of toluene and phenol under visible light irradiation (25 LEDs (ߣmax = 415 nm)). In particular, the authors focused on the effect of calcination temperature on materials' preparation. The bimetallic sample calcined at 350 °C achieved 65% of toluene degradation and 22% of phenol conversion after 60 min of visible light irradiation. The performances were inferior compared to the photoactivity of monometallic palladium (79% in the toluene degradation and 24% in the phenol removal); however, the synergistic effect combined with a strong metals–support interaction was better exploited in the UV-vis tests, where the intrinsic photoactivity of rutile TiO2 also made a substantial contribution in removal efficiency. In fact, with the bimetallic sample, 100% of phenol degradation was achieved after 60 min of irradiation instead of the 56% of Pd/TiO2. Cybula et al. [143] investigated the performance of an Au-Pd bimetallic sample supported on rutile TiO<sup>2</sup> synthetized with a water in oil microemulsion methodology, in the photocatalytic oxidation of toluene and phenol under visible light irradiation (25 LEDs (λmax = 415 nm)). In particular, the authors focused on the effect of calcination temperature on materials' preparation. The bimetallic sample calcined at 350 ◦C achieved 65% of toluene degradation and 22% of phenol conversion after 60 min of visible light irradiation. The performances were inferior compared to the photoactivity of monometallic palladium (79% in the toluene degradation and 24% in the phenol removal); however, the synergistic effect combined with a strong metals–support interaction was better exploited in the UV-vis tests, where the intrinsic photoactivity of rutile TiO<sup>2</sup> also made a substantial contribution in removal efficiency. In fact, with the bimetallic sample, 100% of phenol degradation was achieved after60 min of irradiation instead of the 56% of Pd/TiO2.

The interaction of the gold–palladium compound with other semiconductors was examined by the research group of Zhang et al. [144,145]. The photocatalytic oxidation of the trichloroethylene was studied on Au-Pd/BiPO4 nanorods and on Au-Pd/MoO3 nanowires. Interestingly, with the deposition of the Au-Pd alloy on the surface of the BiPO4 nanorod, the photocatalytic degradation rate increased quickly, being about 25 times higher compared to that achieved with bare BiPO4. The authors proposed, on the basis of the characterization measurements, the reaction mechanism illustrated in Figure 10. Under visible light irradiation (solar simulator with a 440 nm cut-off filter), Au-Pd/BiPO4 were excited due to the LSPR of the Au-Pd alloy. An effective charge carrier separation was achieved due to electron transfer from the conduction band (CB) of BiPO4 to the Au-Pd surface interface, whereas the photoholes remained confined in the valence band (VB) of BiPO4. Subsequently, the same photoelectrons present in the surface of the Au-Pd alloy reacted with the oxygens in the gas-phase that were successively reduced into superoxide radicals. These radicals together with the holes in the VB of BiPO4 oxidize the trichloroethylene in water and carbon dioxide. The interaction of the gold–palladium compound with other semiconductors was examined bythe research group of Zhang et al. [144,145]. The photocatalytic oxidation of the trichloroethylene was studied on Au-Pd/BiPO<sup>4</sup> nanorods and on Au-Pd/MoO<sup>3</sup> nanowires. Interestingly, with the depositionof the Au-Pd alloy on the surface of the BiPO<sup>4</sup> nanorod, the photocatalytic degradation rate increased quickly, being about 25 times higher compared to that achieved with bare BiPO4. The authors proposed, on the basis of the characterization measurements, the reaction mechanism illustrated in Figure 10. Under visible light irradiation (solar simulator with a 440 nm cut-off filter), Au-Pd/BiPO<sup>4</sup> were excited due to the LSPR of the Au-Pd alloy. An effective charge carrier separation was achieved due to electron transfer from the conduction band (CB) of BiPO<sup>4</sup> to the Au-Pd surface interface, whereas the photoholesremained confined in the valence band (VB) of BiPO4. Subsequently, the same photoelectrons present in the surface of the Au-Pd alloy reacted with the oxygens in the gas-phase that were successivelyreduced into superoxide radicals. These radicals together with the holes in the VB of BiPO<sup>4</sup> oxidize the trichloroethylene in water and carbon dioxide.

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**Figure 10.** Reaction mechanism of the trichloroethylene oxidation with the Au-Pd/BiPO4 photocatalyst. Figure from [144]. Copyright 2017, Elsevier. **Figure 10.** Reaction mechanism of the trichloroethylene oxidation with the Au-Pd/BiPO<sup>4</sup> photocatalyst. Figure from [144]. Copyright 2017, Elsevier.

The synergistic effect between gold and palladium nanoparticles that permitted increasing charge carrier separation, enhancing in this way photocatalytic activity, was verified, also employing MoO3 nanowires as a semiconductor photocatalyst [145]. The synergistic effect between gold and palladium nanoparticles that permitted increasing charge carrier separation, enhancing in this way photocatalytic activity, was verified, also employing MoO<sup>3</sup> nanowires as a semiconductor photocatalyst [145].

The exploration of the synergistic effect with a bimetallic alloy in the photocatalytic oxidation of VOCs has not yet been largely investigated in the literature, and only fa ew examples are present considering other compounds instead of the Au-Pd system [146–149]. The possibility of favoring a selective oxidation route due to the high selectivity of gold–silver nanoparticles was examined by Han et al. [146] even in partial oxidation of methanol to methyl formate. In this case, the Au-Ag/TiO2 powders prepared via chemical reduction showed good results with a methanol conversion of 90% and a selectivity to methyl formate of about 85% under UV irradiation (500 W high pressure mercury lamp, λmax = 365 nm). Similarly to the Au-Pd systems, the bimetallic alloy enhanced the photoelectron–photohole separation with electron transfer from the conduction band of TiO2 (excited by the UV irradiation) to the gold–silver surface interface. The exploration of the synergistic effect with a bimetallic alloy in the photocatalytic oxidation of VOCs has not yet been largely investigated in the literature, and only fa ew examples are present considering other compounds instead of the Au-Pd system [146–149]. The possibility of favoring a selective oxidation route due to the high selectivity of gold–silver nanoparticles was examined by Han et al. [146] even in partial oxidation of methanol to methyl formate. In this case, the Au-Ag/TiO<sup>2</sup> powders prepared via chemical reduction showed good results with a methanol conversion of 90% and a selectivity to methyl formate of about 85% under UV irradiation (500 W high pressure mercury lamp, λmax = 365 nm). Similarly to the Au-Pd systems, the bimetallic alloy enhanced the photoelectron–photohole separation with electron transfer from the conduction band of TiO<sup>2</sup> (excited by the UV irradiation) to the gold–silver surface interface.

Another alloy with silver, i.e., Ag-Pt, was studied by Zieli'nska-Jurek et al. [147] in the photooxidation of toluene under visible light irradiation (LEDs, λmax = 415 nm). The authors found an interesting correlation regarding the order of deposition of the Ag-Pt/TiO2 photocatalysts prepared by sol–gel. In particular, the best sample (best photoactivity) was the material where the silver precursor was added before the platinum one. It was fundamental, in fact, to obtain particles with a definite size and dispersion (Ag-Pt size between 6–12 nm). In this way, it was possible to increase the toluene degradation rate with respect to the monometallic samples. By contrast, the bimetallic sample prepared with a simultaneous addition of metals precursors on TiO2 gave a lower photoactivity and different metal size and distribution. The authors concluded that platinum size had a greater influence than silver in determining overall photocatalytic activity. Recently, the same research group evaluated photocatalytic performance in both toluene and acetaldehyde degradation and of *Penicillium chrysogenum*, a dangerous fungus present in the indoor environment with Ag-Pt/TiO2 and Cu-Pt/TiO2 samples [148]. Both bimetallic samples showed a higher fungicidal activity under visible light irradiation than bare TiO2, whereas in VOC degradation, the Ag-Pt system was betterperforming compared to Cu-Pt. The peculiar activity of both bimetallic samples was ascribed to the interfacial charge transfer process between the two metals and the TiO2 confirmed by the quenching of fluorescence (i.e., intensity diminution of the TiO2 photolumiscence bands) due to the presence of Another alloy with silver, i.e., Ag-Pt, was studied by Zieli'nska-Jurek et al. [147] in the photo-oxidation of toluene under visible light irradiation (LEDs, λmax = 415 nm). The authors found an interesting correlation regarding the order of deposition of the Ag-Pt/TiO<sup>2</sup> photocatalysts prepared by sol–gel. In particular, the best sample (best photoactivity) was the material where the silver precursor was added before the platinum one. It was fundamental, in fact, to obtain particles with a definite size and dispersion (Ag-Pt size between 6–12 nm). In this way, it was possible to increase the toluene degradation rate with respect to the monometallic samples. By contrast, the bimetallic sample prepared with a simultaneous addition of metals precursors on TiO<sup>2</sup> gave a lower photoactivity and different metal size and distribution. The authors concluded that platinum size had a greater influence than silver in determining overall photocatalytic activity. Recently, the same research group evaluated photocatalytic performance in both toluene and acetaldehyde degradation and of *Penicillium chrysogenum*, a dangerous fungus present in the indoor environment with Ag-Pt/TiO<sup>2</sup> and Cu-Pt/TiO<sup>2</sup> samples [148]. Both bimetallic samples showed a higher fungicidal activity under visible light irradiation than bare TiO2, whereas in VOC degradation, the Ag-Pt system was better-performing compared to Cu-Pt. The peculiar activity of both bimetallic samples was ascribed to the interfacial charge transfer process between the two metals and the TiO<sup>2</sup> confirmed by the quenching of fluorescence (i.e., intensity diminution of the TiO<sup>2</sup> photolumiscence bands) due to the presence of the metal alloy.

the metal alloy. Wolski et al. [149] studied the mechanism of methanol photo-oxidation on bimetallic Au-Cu catalyst supported on Nb2O5 with an in operando IR methodology under both UV and visible light irradiation. Interestingly, they found that photocatalytic activity is strictly related to the light sources used and to the number of Brønsted/Lewis acid sites present on the surface of the catalysts. Wolski et al. [149] studied the mechanism of methanol photo-oxidation on bimetallic Au-Cu catalyst supported on Nb2O<sup>5</sup> with an in operando IR methodology under both UV and visible light irradiation. Interestingly, they found that photocatalytic activity is strictly related to the light sources used and to the number of Brønsted/Lewis acid sites present on the surface of the catalysts. Specifically, under visible light irradiation, the synergism between gold and copper led to an increase in the amount

Specifically, under visible light irradiation, the synergism between gold and copper led to an increase in the amount of Brønsted/Lewis acid sites on the niobia, with a consequent higher activity of of Brønsted/Lewis acid sites on the niobia, with a consequent higher activity of bimetallic samples compared to that of monometallic and pure Nb2O<sup>5</sup> samples. Furthermore, the total oxidation to CO<sup>2</sup> was favored. By contrast, with UV light irradiation, the major activation of niobia (E<sup>g</sup> ≈ 3.2 eV) favored selective oxidation into dimethoxymethane, formaldehyde, and methyl formate.

In this short chapter, the state-of-the-art of the application of bimetallic structures as chemical modifiers of conventional and unconventional semiconductor photocatalysts was examined. This approach is relatively new, and the effects of alloy synergism on the photocatalytic process are currently under investigation. The promising results, especially obtained by combining the LSPR effect of both noble and transition metals with semiconductor photoactivity, together with a possible multicatalytic strategy (i.e., a photothermo atalytic approach employing a bimetallic/semiconductor catalyst and a solar/visible light source) could in the future be a fascinating strategy to develop a greener and sustainable technology applied to the removal of volatile organic compounds.
