2.1.1. Au-Pd Catalysts

Gold is miscible with palladium in all compositions; consequently, while the formation of gold–palladium alloys is favored, the segregation of single metals was, in fact, avoided [76,83]. In Table 1 are reported some of the experimental results of the application of the Au-Pd systems in catalytic oxidation of various VOCs (T<sup>90</sup> = temperature at which the 90% of conversion was achieved).

In particular, Hosseini et al. [85] synthetized Au-Pd catalysts supported on mesoporous TiO<sup>2</sup> for the removal of toluene, propene, and a gaseous mixture of both. Interestingly, they found that catalytic activity is influenced by the morphology of the core–shell structure with the best performance shown with the Au-core/Pd-shell. By contrast, with the reverse morphology (Au-shell/Pd-core) the catalytic activity was lower, due to the lower affinity of gold for oxygen adsorption (in this case, the rate determining step of the reaction that followed a Langmuir–Hinshelwood mechanism) caused by the poor ability of gold to polarize oxygen molecules. In the same context, Barakat et al. [86] investigated the catalytic stability of bimetallic Au-Pd/doped TiO<sup>2</sup> samples under severe testing conditions (exposing the catalyst to 110 h of a gaseous toluene/air stream). The bimetallic catalyst maintained a good activity even after a long time. The interaction between the Nb-doped TiO<sup>2</sup> support and the Au-Pd system allowed obtaining a cycle-like activity of the catalyst. This oscillatory behavior was related to the existence of carbonaceous compounds adsorbed on the surface of the spent catalyst that, together with the formed OH radicals, favored the reduction of palladium. The redox process of palladium was linked to the cyclic-like activity of the bimetallic sample (Figure 3). *Catalysts* **2020**, *10*, x FOR PEER REVIEW 5 of 25 Gold is miscible with palladium in all compositions; consequently, while the formation of gold– palladium alloys is favored, the segregation of single metals was, in fact, avoided [76,83]. In Table 1 are reported some of the experimental results of the application of the Au-Pd systems in catalytic oxidation of various VOCs (T90 = temperature at which the 90% of conversion was achieved). In particular, Hosseini et al. [85] synthetized Au-Pd catalysts supported on mesoporous TiO2 for the removal of toluene, propene, and a gaseous mixture of both. Interestingly, they found that catalytic activity is influenced by the morphology of the core–shell structure with the best performance shown with the Au-core/Pd-shell. By contrast, with the reverse morphology (Aushell/Pd-core) the catalytic activity was lower, due to the lower affinity of gold for oxygen adsorption



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

**Figure 3.** Catalytic oxidation of toluene under ageing condition over Au-Pd/Nb-TiO2 catalyst. Figure from [86]. **Figure 3.** Catalytic oxidation of toluene under ageing condition over Au-Pd/Nb-TiO<sup>2</sup> catalyst. Figure from [86].

Regarding toluene degradation, Xie et al. [88] studied the catalytic performance of the bimetallic gold–palladium system supported on three-dimensionally ordered macroporous (3DOM) Co3O4. The bimetallic sample showed a much higher activity compared to its nonmetallic counterparts, with a T<sup>90</sup> of about 100 and 30 ◦C lower compared to monometallic gold and palladium, respectively. The peculiar features of the 3DOM supports (such as higher porosity and ordered pore channels) were also exploit by the same authors [91], who utilized an Au-Pd bimetallic sample prepared via chemical reduction, but employing the Mn2O<sup>3</sup> as support. The bimetallic catalysts confirmed the higher activity compared to monometallic gold and palladium in the oxidation of different VOCs, such as methane and o-xylene. To further boost catalytic activity, doping with Fe of the Au-Pd/3DOM-Mn2O<sup>3</sup> catalyst allowed modifying the structural properties of the alloy NPs. With this modification, oxygen activation and the methane adsorption ability were increased, enhancing, as final result, the overall catalytic activity.

Catalytic performance in various mixtures of VOCs (toluene/m-xylene, ethyl acetate/m-xylene acetone/m-xylene, and acetone/ethyl acetate) was examined by Xia et al. [87] on Au-Pd prepared via chemical-reduction-supported αMnO<sup>2</sup> nanotubes. In this case, the authors focused on an MvK-like mechanism with the mutual interaction of the Au-Pd nanoparticles with the α-MnO<sup>2</sup> that improved the mobility/reactivity of the surface lattice oxygen of the support. In catalytic oxidation of VOC mixture, the rate-determining step is the competitive adsorption between the various VOCs on the surface of the catalyst. With the bimetallic catalyst, the authors measured the total oxidation of a single component and of the VOC mixture at T < 300 ◦C. The same bimetallic sample also showed a high catalytic stability in the long time (50 h) on stream experiments.

Tabakova et al. [89] focused their work on the removal of benzene utilizing an Au-Pd bimetallic system synthetized by deposition–precipitation and supported on Fe-doped CeO2. The superior performance of the bimetallic catalyst with respect to the monometallic ones was highlighted through comparison of the T90, which was <sup>≈</sup><sup>95</sup> ◦C for the bimetallic sample, <sup>≈</sup><sup>180</sup> ◦C for the Pd/Fe-CeO2, and <sup>≈</sup><sup>190</sup> ◦C for the Au/Fe-CeO2. In addition, in this case, the synergistic interaction between the alloy nanoparticles and the support enhanced the mobility of the ceria surface lattice oxygen, further boosted by doping with iron. Moreover, this strong interaction facilitated the nucleation of the noble metal particles on the surface of cerium oxide.

Benzyl alcohol oxidation on Au-Pd bimetallic catalysts was extensively studied by the research group of Prati et al., and an exhaustive comparison of the performance, in this reaction, of the bimetallic Au-Pd systems present in the literature, together with a deep analysis of the various adopted preparation methods are reported in [83]. The same research group [76,83] studied the catalytic behavior of different bimetallic systems composed of Au and various other metals supported on activated carbon prepared using the sol immobilization methodology. Interestingly, they found a structural correlation depending on the second metal utilized. Specifically, an alloy structure was obtained using Pd and Pt, whereas a core–shell morphology was attained with Ru, while with Cu, a phase segregation of this metal, instead of gold, was favored. In the benzyl alcohol tests, the bimetallic synergistic effect was exploited only with copper and palladium.

The strong interaction between gold and palladium, beneficial for catalytic oxidation of benzyl alcohol, was also examined by Li et al. [90] using CeO<sup>2</sup> with different morphology as support. The different morphology of cerium oxide (rod, cube, and polyhedrons), where gold/palladium nanoparticles were deposited–precipitated (Figure 4), affected the catalytic performance. Specifically, the Au-Pd supported on ceria rod showed a higher benzyl alcohol conversion with respect to the samples supported on CeO<sup>2</sup> cubes and CeO<sup>2</sup> polyhedrons and was thus related to the smaller particle size of ceria rod compared to the other CeO<sup>2</sup> supports; moreover, this particular morphology also favored a higher concentration of ceria oxygen defects, enhancing the mobility/reducibility of the ceria surface oxygens. By contrast, the sample supported on the CeO<sup>2</sup> cube exhibited the highest selectivity in benzaldehyde.

Kucherov et al. [92] investigated the performance of mono- and bimetallic gold-based catalysts for the removal of dimethyldisulfide (DMDS), an S-VOC. They corroborate the performance of Au-Pd catalysts also for this type of VOC. The bimetallic sample supported on TiO<sup>2</sup> demonstrated a stable selectivity in benzaldehyde.

performance and assisted with the removal of DMDS at T <sup>&</sup>lt; <sup>155</sup> ◦C with the formation of SO<sup>2</sup> and elemental S. performance and assisted with the removal of DMDS at T < 155 °C with the formation of SO2 and elemental S.

Pd catalysts also for this type of VOC. The bimetallic sample supported on TiO2 demonstrated a stable

Kucherov et al. [92] investigated the performance of mono- and bimetallic gold-based catalysts

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different morphology of cerium oxide (rod, cube, and polyhedrons), where gold/palladium nanoparticles were deposited–precipitated (Figure 4), affected the catalytic performance. Specifically, the Au-Pd supported on ceria rod showed a higher benzyl alcohol conversion with respect to the samples supported on CeO2 cubes and CeO2 polyhedrons and was thus related to the smaller particle size of ceria rod compared to the other CeO2 supports; moreover, this particular morphology also favored a higher concentration of ceria oxygen defects, enhancing the mobility/reducibility of the ceria surface oxygens. By contrast, the sample supported on the CeO2 cube exhibited the highest

**Figure 4.** TEM (Transmission electron microscope) and HRTEM (High-resolution transmission electron microscope), images of: Au-Pd/CeO2-rod (**a, b**); Au-Pd/CeO2-polyhedron (**c**, **d**); and Au-Pd/CeO2-cube (**e, f**) catalysts. Figure from Ref. [90], Copyright 2020, Elsevier. **Figure 4.** TEM (Transmission electron microscope) and HRTEM (High-resolution transmission electron microscope), images of: Au-Pd/CeO<sup>2</sup> -rod (**a,b**); Au-Pd/CeO<sup>2</sup> -polyhedron (**c**,**d**); and Au-Pd/CeO<sup>2</sup> -cube (**e,f**) catalysts. Figure from Ref. [90], Copyright 2020, Elsevier.
