2.1.3. Other Au-Based Bimetallic Catalysts

As discussed in the last examined work, among the other Au-based bimetallic catalysts, the Au-Pt system exhibited promising performance in VOC oxidation [96–98].

Kim et al. [97] investigated catalytic oxidation of toluene employing the Au-Pt/ZnO-Al2O<sup>3</sup> catalyst prepared by impregnation in air or H2. They found that the bimetallic sample prepared in air led to an increase of the gold particle size and a decrease of the Pt with respect to the same particles synthetized in H<sup>2</sup> stream, where an inverse correlation was verified (the gold size decreased, and the platinum size increased). Due to the crucial importance of the gold nanoparticles that facilitated the total oxidation of toluene and that increased the reduction of the surface oxygen of the mixed oxide support, the catalytic performance was higher with the bimetallic sample synthetized in H<sup>2</sup> stream and calcined at 400 ◦<sup>C</sup> (Pt and Au mean size of about 5 nm). In another study [98], the same authors correlated the catalytic activity of the same bimetallic samples, even in the total oxidation of toluene, to the molar ratio of gold and platinum, finding the following order of activity: Pt75Au25 > Pt67Au33 > Pt100Au0 > Pt50Au50 > Pt33Au67 > Pt25Au75 > Pt0Au100. The small amount of gold promotes the total oxidation of toluene due to the formation of a strong metal–metal interaction.

The good affinity of gold with noble metals was also confirmed via the catalytic performance in VOC removal of the Au-Ru system [99,100]. Sreethawong et al. [99] investigated catalytic oxidation of methanol over gold–ruthenium samples prepared through impregnation and supported on SiO2. The characterization measurements (TPR, SEM, and XRD) suggested the occurrence of an interaction between the two metals exploited with a particular composition (3.32 wt%Ru–0.61 wt% Au), which led to obtaining a good catalytic activity, notwithstanding the fact that the two metals were not miscible in their bulk phase. Interestingly, if alumina was used as support, the formation of byproducts (methyl formate, formic acid, dimethyl ether, and formaldehyde) other to CO<sup>2</sup> was detected, with an increase of methanol conversion. Catalytic oxidation of methanol was also analyzed by Calzada et al. [100] with Au-Ru/TiO<sup>2</sup> catalysts prepared by deposition–precipitation with urea. The authors highlighted that the synergistic effect between the two metals was activated at a low conversion temperature (from room temperature to T = 50 ◦C), with a dependence on the bimetallic atomic ratio (the best-performing one was Ru:Au 0.75:1). Interestingly, the DRIFT (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) CO spectra (Figure 6) illustrated as the interaction between the two metals decreased CO adsorption in the Ru surface sites in the Ru-Au 0.75:1 sample, an indication of the modification of the surface gold sites.

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**Figure 6.** DRIFT (Diffuse Reflectance Infrared Fourier Transform Spectroscopy)-CO spectra of the mono- and bimetallic Ru-Au/TiO2 samples at room temperature. Figure from [100]. Copyright 2017, **Figure 6.** DRIFT (Diffuse Reflectance Infrared Fourier Transform Spectroscopy)-CO spectra of the monoand bimetallic Ru-Au/TiO<sup>2</sup> samples at room temperature. Figure from [100]. Copyright 2017, Elsevier.

Elsevier. The modifications of the bimetallic surface sites due to interaction between the two metals strongly occurring with the Ru:Au 0.75:1 ratio were the reason behind the higher catalytic activity of this bimetallic sample compared to those of the monometallic catalysts and the Ru:Au 1:1 sample. In The modifications of the bimetallic surface sites due to interaction between the two metals strongly occurring with the Ru:Au 0.75:1 ratio were the reason behind the higher catalytic activity of this bimetallic sample compared to those of the monometallic catalysts and the Ru:Au 1:1 sample. In addition, in this case, the formation of formates as intermediates of the oxidation reaction was verified.

addition, in this case, the formation of formates as intermediates of the oxidation reaction was verified. Catalytic activity in the total oxidation of toluene of the Au-Ir bimetallic catalyst supported on TiO2 was studied by Torrente-Murciano et al. [101]. Similarly to the previous case, the synergistic interaction between the two metals allowed them to sensibly decrease the T90 that was ≈230 °C for the bimetallic sample, ≈250 °C for the monometallic gold, and ≈270 °C for Ir/TiO2. The key factors that deeply influenced catalytic activity were the strong metals–support interaction exploited with the Catalytic activity in the total oxidation of toluene of the Au-Ir bimetallic catalyst supported on TiO<sup>2</sup> was studied by Torrente-Murciano et al. [101]. Similarly to the previous case, the synergistic interaction between the two metals allowed them to sensibly decrease the T<sup>90</sup> that was <sup>≈</sup><sup>230</sup> ◦C for the bimetallic sample, <sup>≈</sup><sup>250</sup> ◦C for the monometallic gold, and <sup>≈</sup><sup>270</sup> ◦C for Ir/TiO2. The key factors that deeply influenced catalytic activity were the strong metals–support interaction exploited with the bimetallic system, which also permitted diminishing the loss of activity due to the metals sintering at high temperatures. Furthermore, the intimate contact between iridium and gold modified the bimetallic surface-active sites enhancing oxygen activation.

bimetallic system, which also permitted diminishing the loss of activity due to the metals sintering at high temperatures. Furthermore, the intimate contact between iridium and gold modified the bimetallic surface-active sites enhancing oxygen activation. In the work cited in Section 2.2, Kucherov et al. [92] demonstrated a good activity of Au-Rh In the work cited in Section 2.2, Kucherov et al. [92] demonstrated a good activity of Au-Rh supported on HZSM-5 zeolite into the oxidation of the DMDS in SO<sup>2</sup> at 290 ◦C. In this case, the zeolite support, owing to a high surface area, favored a high dispersion of the metals, a feature that is beneficial for catalytic activity.

supported on HZSM-5 zeolite into the oxidation of the DMDS in SO2 at 290 °C. In this case, the zeolite support, owing to a high surface area, favored a high dispersion of the metals, a feature that is beneficial for catalytic activity. Regarding gold–copper bimetallic systems, there are certain studies with other transition metals, Regarding gold–copper bimetallic systems, there are certain studies with other transition metals, utilized together with gold for VOC oxidation. Au-Co and Au-Fe interaction were principally investigated. In the examined works, the gold atoms interacted with the second metal present as a doping agent of the support [102–104].

utilized together with gold for VOC oxidation. Au-Co and Au-Fe interaction were principally investigated. In the examined works, the gold atoms interacted with the second metal present as a doping agent of the support [102–104]. Solsona et al. [102] synthetized gold nanoparticles anchored on cobalt containing mesoporous silica (UVM-7). The interaction between gold and cobalt permitted increasing catalytic activity in the oxidation of toluene and propane with respect to Au/UVM-7 and Co/UVM-7 catalysts. The presence of gold enhanced the reducibility of cobalt, present as Co3O4 at the Au-Co interface, thus facilitating the redox cycle of cobalt, with an MvK-like mechanism, which boosted catalytic oxidation of VOCs. In the same context, Albonetti et al. [103,104] deeply investigated the catalytic behavior of gold catalysts supported on mesoporous silica (SBA-15) via an iron oxide layer obtaining the Solsona et al. [102] synthetized gold nanoparticles anchored on cobalt containing mesoporous silica (UVM-7). The interaction between gold and cobalt permitted increasing catalytic activity in the oxidation of toluene and propane with respect to Au/UVM-7 and Co/UVM-7 catalysts. The presence of gold enhanced the reducibility of cobalt, present as Co3O<sup>4</sup> at the Au-Co interface, thus facilitating the redox cycle of cobalt, with an MvK-like mechanism, which boosted catalytic oxidation of VOCs. In the same context, Albonetti et al. [103,104] deeply investigated the catalytic behavior of gold catalysts supported on mesoporous silica (SBA-15) via an iron oxide layer obtaining the Au/FeOx/SBA-15 composite. The good dispersion of nanosized gold favored the incidence of a strong synergism between gold and iron that led to an optimal activity in the combustion of methanol (T<sup>90</sup> <sup>≈</sup> <sup>140</sup> ◦C).

Au/FeOx/SBA-15 composite. The good dispersion of nanosized gold favored the incidence of a strong synergism between gold and iron that led to an optimal activity in the combustion of methanol (T90 ≈ 140 °C). As a conclusion of this generic overview of gold-based bimetallic catalysts applied at VOC As a conclusion of this generic overview of gold-based bimetallic catalysts applied at VOC oxidation, it is possible to recognize some fundamental features of these peculiar catalysts: (a) the essential action of the nanosized gold that is able to establish a metal–metal surface interaction with a wide range of both noble and transition metals; (b) the occurrence of a synergism between the two

oxidation, it is possible to recognize some fundamental features of these peculiar catalysts: (a) the essential action of the nanosized gold that is able to establish a metal–metal surface interaction with metals that allows sensibly decreasing the light-off temperatures of VOC oxidation; (c) the synergistic effect, which is not simply the addition of the single characteristics of the corresponding monometallic samples but leads to exploring new physicochemical properties; (d) the mutual interaction between the two metals which also strongly influences metals–support interaction (in particular, if the support is a reducible oxide (CeO2, MnOx, CoOx, etc.), the gold-based bimetallic cluster increases the mobility of the surface oxygen of the support, enhancing, in this way, catalytic oxidation towards an MvK mechanism). If the support is a nonreducible or hardly reducible oxide (TiO2, SiO2, and zeolites), the high dispersion of the bimetallic alloy and the modifications of the metals surface active sites allow enhancing oxygen adsorption, improving, as a final result, the overall catalytic performance. The most employed preparation methods of the supported gold-based bimetallic catalysts are impregnation (wet or wetness), deposition–precipitation, and chemical reduction.
