*2.2. Other Bimetallic Catalysts*

Among the other noble metals, the most employed catalysts for catalytic oxidation of VOCs are platinum-based materials [16], and similarly to gold, platinum has shown a good affinity with palladium [105–108]. In general, as can be seen from Table 2, the use of noble-metals-based bimetallic catalysts has allowed obtaining a good performance in the removal of VOCs, whereas the utilization of transition-metals-based materials has led to shift at high temperatures in the total conversion of VOCs. However, especially in recent years, the necessity to reduce the amount of expensive noble metals has led to exploring a new synergism between noble and transition metals.



<sup>1</sup> Nominal concentration, weight percentage (wt%).

Fu et al. [105] prepared, for the hydrothermal method, a Pt-Pd bimetallic sample supported on mesoporous silica. Comparably to the catalytic behavior of gold-based samples, the synergism between the two metals allowed obtaining a superior performance in the removal of toluene with respect to the monometallic samples, with an improvement in the reducibility of palladium, involved in the redox cycle PdO→Pd<sup>0</sup> , and in oxygen adsorption capability.

The good catalytic activity of the Pt-Pd system was also confirmed by Kim et al. [106] in the degradation of benzene. Catalysts were synthetized via wetness impregnation on γ-Al2O3. Metal–metal interaction was favored by the formation of small and uniform particles and, as stated in the previous paragraphs, a specific amount ratio between the two metals (the optimum in the cited work is 0.3 wt% Pt–2%wt Pd). A higher concentration of platinum led to a remarkable decrease in activity due to the blockage of the active sites. The same authors in another paper [107] confirmed with a deep XPS (X-ray photoelectron spectroscopy) analysis the crucial role of the ratio between the metals to avoid the obstruction of the catalyst surface sites. The removal of methanol, acetone, and methylene chloride was instead studied by Sharma et al. [108], utilizing ceramic Raschig rings coated with Pt and Pd on fluorinated carbon. The authors measured a higher activity of the bimetallic catalyst with respect to monometallic ones. Furthermore, the hydrophobic nature of this particular bimetallic catalyst allowed obtaining a 90% of degradation of methanol and acetone at about 150 and 300 ◦C respectively, whereas

60% of degradation was achieved at 400 ◦C for the methylene chloride. In this case, a good correlation was established with a semi-empirical Langmuir–Hinshelwood model, which is able to predict the oxidation rate of each VOC in a gas mixture (methanol, acetone, and methylene chloride). *Catalysts* **2020**, *10*, x FOR PEER REVIEW 12 of 25 model, which is able to predict the oxidation rate of each VOC in a gas mixture (methanol, acetone,

Ethanol adsorption and oxidation were investigated by the research group of Wittayakun et al. [114] with a Pt-M (M = Co, Cu, Mn) sample supported on silica MCM-41. Among the transition metals, cobalt gave the best results, and in particular, the bimetallic 0.5 wt% Pt–15 wt% Co exhibited the best ethanol adsorption and CO<sup>2</sup> desorption. Interestingly, the authors identified two different reaction mechanisms considering the platinum monometallic sample and the bimetallic platinum–cobalt one (Figure 7). and methylene chloride). Ethanol adsorption and oxidation were investigated by the research group of Wittayakun et al. [114] with a Pt-M (M = Co, Cu, Mn) sample supported on silica MCM-41. Among the transition metals, cobalt gave the best results, and in particular, the bimetallic 0.5 wt% Pt–15 wt% Co exhibited the best ethanol adsorption and CO2 desorption. Interestingly, the authors identified two different reaction mechanisms considering the platinum monometallic sample and the bimetallic platinum– cobalt one (Figure 7).

**Figure 7.** (**a**) Ethanol oxidation mechanism on 0.5 wt% Pt/MCM-41; and (**b**) ethanol oxidation mechanism on 0.5 wt% Pt–15 wt% Co/MCM-41. Figure modified from [114]. Copyright 2012, Elsevier. **Figure 7.** (**a**) Ethanol oxidation mechanism on 0.5 wt% Pt/MCM-41; and (**b**) ethanol oxidation mechanism on 0.5 wt% Pt–15 wt% Co/MCM-41. Figure modified from [114]. Copyright 2012, Elsevier.

Specifically, in the monometallic sample after the adsorption of ethanol, a formation was verified of a parallel adsorbed acetaldehyde, further converted into monodentate acetate and at end, dissociated and desorbed as carbon dioxide, methane, and water (Figure 7a). In the bimetallic catalyst, by contrast, the ethoxy species reacted with the adsorbed oxygen to give a bidentate acetate species that was transformed into carbon dioxide (Figure 7b). The modification of the ethanol adsorption led to a higher ethanol conversion with the monometallic platinum sample in comparison with the bimetallic Pt-Co sample that, conversely, showed a higher catalytic stability. Even with the bimetallic platinum-based catalysts, Chantaravitoon et al. [115] examined the performance of a Pt-Sn/γ-Al2O3 catalyst prepared with impregnation, for the oxidation of methanol. The authors noted from the temperature-programmed desorption (TPD) measurements of methanol oxidation that on the bimetallic catalyst, methanol decomposed as H2 and CO and the desorption peaks shifted at higher temperatures, increasing the amount of Sn. In addition, in this case, the monometallic Pt catalyst exhibited a better performance compared to the bimetallic one; however, the addition of a Specifically, in the monometallic sample after the adsorption of ethanol, a formation was verified of a parallel adsorbed acetaldehyde, further converted into monodentate acetate and at end, dissociated and desorbed as carbon dioxide, methane, and water (Figure 7a). In the bimetallic catalyst, by contrast, the ethoxy species reacted with the adsorbed oxygen to give a bidentate acetate species that was transformed into carbon dioxide (Figure 7b). The modification of the ethanol adsorption led to a higher ethanol conversion with the monometallic platinum sample in comparison with the bimetallic Pt-Co sample that, conversely, showed a higher catalytic stability. Even with the bimetallic platinum-based catalysts, Chantaravitoon et al. [115] examined the performance of a Pt-Sn/γ-Al2O<sup>3</sup> catalyst prepared with impregnation, for the oxidation of methanol. The authors noted from the temperature-programmed desorption (TPD) measurements of methanol oxidation that on the bimetallic catalyst, methanol decomposed as H<sup>2</sup> and CO and the desorption peaks shifted at higher temperatures, increasing the amount of Sn. In addition, in this case, the monometallic Pt catalyst exhibited a better performance compared to the bimetallic one; however, the addition of a small amount of Sn (<5 wt%) reduced the deactivation of the catalyst in the long-time tests.

small amount of Sn (<5 wt%) reduced the deactivation of the catalyst in the long-time tests. In addition, Ru-based bimetallic compounds were discreetly studied for VOC oxidation [109,110,116]. Liu et al. [109] prepared Ru-M (M = Co, Mn, Ce, Fe, Cu) samples supported on TiO2, In addition, Ru-based bimetallic compounds were discreetly studied for VOC oxidation [109,110,116]. Liu et al. [109] prepared Ru-M (M = Co, Mn, Ce, Fe, Cu) samples supported on TiO2, evaluating catalytic performance in the degradation of benzene. Among the various metals,

evaluating catalytic performance in the degradation of benzene. Among the various metals, 1% wt

1% wt Ru–5% wt Co showed the best activity; the presence of ruthenium, in fact, increased the reducibility of Co3O4. The authors stated also that the presence of water vapor inhibited benzene oxidation at T = 210 ◦C.

The total oxidation of propene was examined on Ru-Re/γ-Al2O<sup>3</sup> by Baranowska et al. [116]. As discussed before, this nonconventional combination between these two metals also allowed increasing catalytic stability in the consecutive tests instead of the overall catalytic activity, which remained superior with the monometallic Ru sample. The addition of Re (the best composition being 5% wt Ru–3% wt Re) hampered the formation of RuO<sup>2</sup> agglomerates. In this way, the dispersion of ruthenium is favored, allowing a higher stability compared to monometallic ruthenium catalysts. Ye et al. [110] performed a catalytic test regarding chlorobenzene removal with Ru-Ce/TiO<sup>2</sup> samples prepared via impregnation. Interestingly, on the basis of the crystalline phase of titanium dioxide, the catalytic activity changed. At 280 ◦C, the bimetallic sample showed a conversion of 91% and 86% if supported on TiO<sup>2</sup> rutile and TiO<sup>2</sup> P25 (80% anatase, 20% rutile), respectively. The mixed crystalline phase of P25 was the best support for the monometallic ruthenium catalyst. By contrast, with respect to the work of Baranowska et al. [116] for this reaction and with the titanium dioxide support, dispersion was not the major parameter that affected catalytic activity; indeed, on 1%wt Ru–5%wt Ce/TiO<sup>2</sup> (rutile), the abundant RuO<sup>2</sup> clusters favored both catalytic activity and stability.

Interaction with Ce/CeO<sup>2</sup> was also investigated by Yue et al. [117] but utilizing palladium. The performance of the bimetallic catalyst Pd-Ce/ZMS-5 synthetized through impregnation was evaluated on the degradation of methyl ethyl ketone (MEK). The presence of cerium oxide considerably increased the acid sites of palladium, enhancing at the same time the re-oxidation of Pd and boosting, in the end, the overall MEK degradation rate through an MvK mechanism.

Another bimetallic catalyst with palladium was prepared by Arias et al. [111]. In this work, the synergistic effect between palladium and manganese was explored utilizing alumina as support. The authors followed the oxidation of a VOC mixture (formaldehyde/methanol), concluding also in this case that an MvK-like mechanism was the reaction pathway, with the interaction between palladium and manganese favoring the oxidation of VOCs due to the activation of the reactive lattice oxygen of PdO and MnOx.

Various bimetallic samples were tested for catalytic oxidation of VOCs, studying both the physicochemical properties and the catalytic activity of silver-containing samples [113,118–120]. In particular, Jodaei et al. [113,118] tested different Ag-M bimetallic samples supported on ZMS-5 zeolite obtained via ionic exchange. The authors investigated the catalytic combustion of ethyl acetate, finding this order of activity and stability: Fe-Ag/ZSM-5 > Co-Ag/ZSM-5 > Mn-Ag/ZSM-5 > Ag/ZSM-5. The high dispersion of silver was favored by an optimal amount of iron (1.3 wt% Fe–1.75 wt% Ag), thus activating a synergistic effect between the two metals. In the same context, Izadkhah et al. [119] made a theoretical model for the removal of ethyl acetate. In particular, considering the preparation condition, the formulation, and loading of the promoter of silver, with their algorithm, it was possible to identify the optimal catalyst for this reaction. Among the first transition metal series, the bimetallic catalyst that exhibited superior performance compared to the monometallic silver was Fe-Ag/ZSM-5, thus confirming the experimental results of Jodaei et al. [113,118], Ni-Ag/ZSM-5 and V-Ag/ZSM-5.

Complete oxidation of formaldehyde at T < 90 ◦C was obtained by Qu et al. [120] with Ag-Co/MCM-41 silica. The key feature able to sensibly increase catalytic performance with respect to the monometallic silver was electron transfer between silver and cobalt that enhanced the reducibility of cobalt oxide, increasing, at the same time, the activation of surface oxygen on the bimetallic catalyst. Furthermore, the high metal–metal support interaction (SMMI) at the optimal Ag/Co mass ratio (3:1) favored a faster adsorption–dissociation of formaldehyde on the Ag species with respect to the Co3<sup>+</sup> sites (Figure 8), thus decreasing the light-off temperature of VOC oxidation.

activity.

*Catalysts* **2020**, *10*, x FOR PEER REVIEW 14 of 25

**Figure 8.** Formaldehyde oxidation on monometallic Ag/MCM-41, Co/MCM-41 and bimetallic Ag-Co/MCM-41. Figure from [120]. Copyright 2014, Elsevier. **Figure 8.** Formaldehyde oxidation on monometallic Ag/MCM-41, Co/MCM-41 and bimetallic Ag-Co/MCM-41. Figure from [120]. Copyright 2014, Elsevier.

On the same support (MCM-41 silica), Pârvulescu et al. [121] synthetized with the hydrothermal method various Co-based bimetallic mesostructures (Co-V, Co-La, Co-Nb) characterized vy a high surface area and narrow pore size distribution. The oxidation of styrene and benzene was deeply influenced by the addition of a second metal component. Indeed, although the addition of La did not result in any synergistic effect, the addition of vanadium favored the oxidation of benzene, whereas the addition of niobium facilitated the removal of styrene, demonstrating that the presence of the second metal changed the surface-active sites of cobalt. On the same support (MCM-41 silica), Pârvulescu et al. [121] synthetized with the hydrothermal method various Co-based bimetallic mesostructures (Co-V, Co-La, Co-Nb) characterized vy a high surface area and narrow pore size distribution. The oxidation of styrene and benzene was deeply influenced by the addition of a second metal component. Indeed, although the addition of La did not result in any synergistic effect, the addition of vanadium favored the oxidation of benzene, whereas the addition of niobium facilitated the removal of styrene, demonstrating that the presence of the second metal changed the surface-active sites of cobalt.

Similarly to cobalt, copper-based bimetallic catalysts also showed a good activity in the removal of VOCs [112,122,123]. Kim et al. [112] found an optimal interaction between Mn and Cu for the total oxidation of toluene. The order of activity considering other transition metals as a second component was: 5% wt Cu-15%wt Mn/γ-Al2O3 > 5% wt Co-15% wt Mn/γ-Al2O3 > 5% wt Ni-15% wt Mn/γ-Al2O3 > 15% wt Mn/γ-Al2O3 > 5% wt Fe-15% wt Mn/γ-Al2O3. The interaction between Mn and Cu favored a high dispersion of manganese, increasing, at the same time, the mobility/reducibility of manganese oxide. The oxidation of toluene was studied recently by Djinović et al. [122], who had examined the performance of monometallic CuO and bimetallic Cu-FeOx composites supported on KIL-2 silica. The utilization of two reducible oxides allowed increasing the amount and reactivity of oxygen species, which included adsorbed (O− and O2−) and lattice (O2−) oxygens at the Cu-FeOx interface, providing a substantial decrease of T90 that was ≈350 °C for the bimetallic cluster instead of ≈450 °C of monometallic copper oxide, whereas the FeOx/KIL-2 silica reached only 30% of toluene conversion at 450 °C. Similarly to cobalt, copper-based bimetallic catalysts also showed a good activity in the removal of VOCs [112,122,123]. Kim et al. [112] found an optimal interaction between Mn and Cu for the total oxidation of toluene. The order of activity considering other transition metals as a second component was: 5% wt Cu–15% wt Mn/γ-Al2O<sup>3</sup> > 5% wt Co–15% wt Mn/γ-Al2O<sup>3</sup> > 5% wt Ni–15% wt Mn/γ-Al2O<sup>3</sup> > 15% wt Mn/γ-Al2O<sup>3</sup> > 5% wt Fe–15% wt Mn/γ-Al2O3. The interaction between Mn and Cu favored a high dispersion of manganese, increasing, at the same time, the mobility/reducibility of manganese oxide. The oxidation of toluene was studied recently by Djinovi´c et al. [122], who had examined the performance of monometallic CuO and bimetallic Cu-FeO<sup>x</sup> composites supported on KIL-2 silica. The utilization of two reducible oxides allowed increasing the amount and reactivity of oxygen species, which included adsorbed (O<sup>−</sup> and O<sup>2</sup> <sup>−</sup>) and lattice (O2−) oxygens at the Cu-FeO<sup>x</sup> interface, providing a substantial decrease of T<sup>90</sup> that was <sup>≈</sup><sup>350</sup> ◦C for the bimetallic cluster instead of <sup>≈</sup>450 ◦C of monometallic copper oxide, whereas the FeOx/KIL-2 silica reached only 30% of toluene conversion at 450 ◦C.

Abdullah et al. [123] investigated the oxidation of a Cl-VOC mixture (dichloromethane (DCM), trichloroethylene (TCE) and trichloromethane (TCM)) with Cu-Cr/ZMS-5. Interestingly, in this case, the presence of water vapor in the gas feed enhanced the total oxidation to CO2. The presence of water vapor favored the formation of reactive carbocations. Furthermore, H2O was beneficial in blocking chlorine-transfer reactions. Indeed, an important deactivation effect was found with the bimetallic catalyst at a higher Cl/H gas feed ratio, and chlorination led to a decrease in metals' reducibility that resulted in a low degradation efficiency. The reaction was driven by an MvK mechanism. Abdullah et al. [123] investigated the oxidation of a Cl-VOC mixture (dichloromethane (DCM), trichloroethylene (TCE) and trichloromethane (TCM)) with Cu-Cr/ZMS-5. Interestingly, in this case, the presence of water vapor in the gas feed enhanced the total oxidation to CO2. The presence of water vapor favored the formation of reactive carbocations. Furthermore, H2O was beneficial in blocking chlorine-transfer reactions. Indeed, an important deactivation effect was found with the bimetallic catalyst at a higher Cl/H gas feed ratio, and chlorination led to a decrease in metals' reducibility that resulted in a low degradation efficiency. The reaction was driven by an MvK mechanism.

At this point, it is possible to highlight some differences through comparison of the catalytic performance of gold-based bimetallic samples with the others reported above. For the nongoldcontaining samples category, supports with a high surface area or with a tunable pore size distribution (silica, zeolite, alumina, etc.) were preferred to favor the dispersion of the active metals. Platinum-based samples gave the best results, with a second metal that in many cases enhances catalytic stability rather than overall VOC conversion. Although noble-metals–bimetallic catalysts showed the best performance, in recent years, in order to reduce the high cost of these catalysts, the addition or replacement of at least one of the noble metals with a cheaper transition metal is an interesting approach to reduce the total material cost while maintaining an acceptable catalytic At this point, it is possible to highlight some differences through comparison of the catalytic performance of gold-based bimetallic samples with the others reported above. For the nongold-containing samples category, supports with a high surface area or with a tunable pore size distribution (silica, zeolite, alumina, etc.) were preferred to favor the dispersion of the active metals. Platinum-based samples gave the best results, with a second metal that in many cases enhances catalytic stability rather than overall VOC conversion. Although noble-metals–bimetallic catalysts showed the best performance, in recent years, in order to reduce the high cost of these catalysts, the addition or replacement of at least one of the noble metals with a cheaper transition metal is an interesting approach to reduce the total material cost while maintaining an acceptable catalytic activity.
