**2. Bimetallic Catalysts for VOC Oxidation**

Bimetallic nanoparticles (NPs) are a kind of materials formed by two different metals and characterized with peculiar features [41,42]. Specifically, they can show new properties resulting from the combination of features arising from the monometallic counterparts. Usually, the obtained physicochemical properties of bimetallic systems give a holistic result, i.e., the final properties are not the simple additive features of the monometallic analogs, but in many cases, it is possible to exploit a great improvement with new properties due to the presence of synergistic effects [43,44]. Since their application in the field of petrochemistry [45], bimetallic systems are being widely applied in heterogeneous catalysis in various reactions, such as hydrogenation [46,47], reforming [48,49], H<sup>2</sup> production and purification [50–53], and oxidation [54–56]. Recently, they have also been applied in the biomedical field [57].

American Chemical Society.

On the basis of the morphology of the bimetallic system, it is possible to classify the main arrangements into three typologies: core–shell or multishell structures, heterostructures, and random or homogeneous alloys (Figure 1) [43,44,58]. On the basis of the morphology of the bimetallic system, it is possible to classify the main arrangements into three typologies: core–shell or multishell structures, heterostructures, and random or homogeneous alloys (Figure 1) [43,44,58].

*Catalysts* **2020**, *10*, x FOR PEER REVIEW 3 of 25

**Figure 1.** Possible morphologies of bimetallic nanoparticles (one metal in red, the other in dark yellow): (**a**), (**b**) core–shell system; (**c**) multishell system; (**d**) subcluster segregated systems; and ordered (**e**) and random (**f**) homogeneous alloys. Reprinted (adapted) from [58], Copyright 2008, **Figure 1.** Possible morphologies of bimetallic nanoparticles (one metal in red, the other in dark yellow): (**a**,**b**) core–shell system; (**c**) multishell system; (**d**) subcluster segregated systems; and ordered (**e**) and random (**f**) homogeneous alloys. Reprinted (adapted) from [58], Copyright 2008, American Chemical Society.

Various factors influence the final morphology of bimetallic systems: (1) the intrinsic strength of the bonds between the two metals in comparisons to the strength of the bonds between the two monometallic constituents (in particular, if the resulting alloy bond strength is greater, reciprocal mixing is preferred, and, in the opposite case, the segregation of the two metals is favored); (2) the balance in the surface energy of the metals, where the monometallic element that owns the lower surface energy will move to the surface, establishing a shell structure, while the metal with the smaller atomic sizes will collocate to the core; (3) the mixing is favored when an electron/charge transfer between the metallic counterparts is verified; and (4) magnetic effects can influence the final Various factors influence the final morphology of bimetallic systems: (1) the intrinsic strength of the bonds between the two metals in comparisons to the strength of the bonds between the two monometallic constituents (in particular, if the resulting alloy bond strength is greater, reciprocal mixing is preferred, and, in the opposite case, the segregation of the two metals is favored); (2) the balance in the surface energy of the metals, where the monometallic element that owns the lower surface energy will move to the surface, establishing a shell structure, while the metal with the smaller atomic sizes will collocate to the core; (3) the mixing is favored when an electron/charge transfer between the metallic counterparts is verified; and (4) magnetic effects can influence the final structure between the two metals [44,58,59].

structure between the two metals [44,58,59]. The morphologies are also strictly affected by the preparation method adopted. In general, the synthesis of the bimetallic systems can be carried out with the solid-state or with the solution methods [60–62]. The latter are preferred because solid-state techniques require high temperatures and long times for annealing procedures, thus decreasing the surface area of the bimetallic catalyst and affecting catalytic performance in a crucial way. By contrast, with solution methods, it is possible to The morphologies are also strictly affected by the preparation method adopted. In general, the synthesis of the bimetallic systems can be carried out with the solid-state or with the solution methods [60–62]. The latter are preferred because solid-state techniques require high temperatures and long times for annealing procedures, thus decreasing the surface area of the bimetallic catalyst and affecting catalytic performance in a crucial way. By contrast, with solution methods, it is possible to control the nucleation and growth processes by modifying the reaction parameters.

control the nucleation and growth processes by modifying the reaction parameters. In the main catalytic reactions, the bimetallic system is supported on a specific support [63]. The commonly used techniques for the preparation of supported bimetallic catalysts are: impregnation [64,65], co-precipitation [66,67], deposition–precipitation [50,68], thermal decomposition [69], and chemical reduction [70]. The choice of a proper support is another key feature that strongly influences the catalytic activity of the bimetallic system. In particular, for VOC oxidation, and for oxidation reactions in general, the utilization of an active support (i.e., having redox properties and lattice oxygen mobility), for example, CeO2 or Fe2O3, can contribute to the formation of a synergistic effect involving both the metals and the same support [68,71], whereas other supports, with high surface areas, such as zeolites, silica, alumina, etc., favor the high dispersion of the metal species and, In the main catalytic reactions, the bimetallic system is supported on a specific support [63]. The commonly used techniques for the preparation of supported bimetallic catalysts are: impregnation [64,65], co-precipitation [66,67], deposition–precipitation [50,68], thermal decomposition [69], and chemical reduction [70]. The choice of a proper support is another key feature that strongly influences the catalytic activity of the bimetallic system. In particular, for VOC oxidation, and for oxidation reactions in general, the utilization of an active support (i.e., having redox properties and lattice oxygen mobility), for example, CeO<sup>2</sup> or Fe2O3, can contribute to the formation of a synergistic effect involving both the metals and the same support [68,71], whereas other supports, with high surface areas, such as zeolites, silica, alumina, etc., favor the high dispersion of the metal species and, therefore, the interaction between the metals and the same support [16,72].

Since the first works of Haruta and co-workers [73], gold-based catalysts have generated many investigations focused on the catalytic properties of this peculiar metal. With respect to the platinum-

therefore, the interaction between the metals and the same support [16,72].

Since the first works of Haruta and co-workers [73], gold-based catalysts have generated many investigations focused on the catalytic properties of this peculiar metal. With respect to the platinum-group catalysts, gold's properties are in some cases superior; consequently, the supported gold-based materials have found many applications in catalytic reactions dealing with environmental protection and energy production [50,72]. In the specific field of VOC catalytic oxidation, gold-based catalysts play a key role. For these reasons, bimetallic alloys with gold are firstly examined. *Catalysts* **2020**, *10*, x FOR PEER REVIEW 4 of 25 group catalysts, gold's properties are in some cases superior; consequently, the supported gold-based materials have found many applications in catalytic reactions dealing with environmental protection and energy production [50,72]. In the specific field of VOC catalytic oxidation, gold-based catalysts play a key role. For these reasons, bimetallic alloys with gold are firstly examined.

### *2.1. Gold-Based Bimetallic Catalysts 2.1. Gold-Based Bimetallic Catalysts*

The important role of gold-based catalysts in heterogeneous catalysis was disclosed through the high activity of this metal in CO oxidation and selective oxidations [74,75]. After these studies, many works dealing with this catalyst have emerged, widely expanding the use of gold catalysts [72,76]. With respect to the other commonly used noble metals, namely, Pt and Pd, gold catalysts show peculiar features such as resistance to O<sup>2</sup> poisoning and high selectivity. Another key parameter that strictly affects the catalytic activity of Au catalysts is size-dependence: To be active, gold particles must be, in general, smaller than 5 nm, which balances the proportion of the low coordination surface active sites (edges and corners) [77]. The important role of gold-based catalysts in heterogeneous catalysis was disclosed through the high activity of this metal in CO oxidation and selective oxidations [74,75]. After these studies, many works dealing with this catalyst have emerged, widely expanding the use of gold catalysts [72,76]. With respect to the other commonly used noble metals, namely, Pt and Pd, gold catalysts show peculiar features such as resistance to O2 poisoning and high selectivity. Another key parameter that strictly affects the catalytic activity of Au catalysts is size-dependence: To be active, gold particles must be, in general, smaller than 5 nm, which balances the proportion of the low coordination surface active sites (edges and corners) [77].

For other applications, as well as for VOC oxidation, the catalytic activity of gold-supported samples is affected by many factors: (a) gold–support interaction; (b) gold loading; (c) the valence state of gold; (d) the adopted synthesis and pre-treatment conditions utilized; and (e) the concentration and nature of the chosen VOC target [78,79]. Among the monometallic gold catalysts employed for VOC oxidation, the Au/CeO<sup>2</sup> sample showed a great performance in the oxidation of oxygenated molecules, i.e., aldehydes, ketones, esters, and alcohols [72]. It was reported that the high activity was related to the enhancement in the surface oxygen reducibility/mobility of the CeO<sup>2</sup> support. The oxidation mechanism followed a Mars–Van Krevelen (MvK) mechanism, where the ceria lattice oxygens were actively involved in the oxidation pathway [68,80] (Figure 2). For other applications, as well as for VOC oxidation, the catalytic activity of gold-supported samples is affected by many factors: (a) gold–support interaction; (b) gold loading; (c) the valence state of gold; (d) the adopted synthesis and pre-treatment conditions utilized; and (e) the concentration and nature of the chosen VOC target [78,79]. Among the monometallic gold catalysts employed for VOC oxidation, the Au/CeO2 sample showed a great performance in the oxidation of oxygenated molecules, i.e., aldehydes, ketones, esters, and alcohols [72]. It was reported that the high activity was related to the enhancement in the surface oxygen reducibility/mobility of the CeO2 support. The oxidation mechanism followed a Mars–Van Krevelen (MvK) mechanism, where the ceria lattice oxygens were actively involved in the oxidation pathway [68,80] (Figure 2).

**Figure 2.** Lattice ceria oxygens of Au/CeO2 catalyst involved in catalytic oxidation of formaldehyde. Reprinted (adapted) from [80], Copyright 2014, American Chemical Society. **Figure 2.** Lattice ceria oxygens of Au/CeO<sup>2</sup> catalyst involved in catalytic oxidation of formaldehyde. Reprinted (adapted) from [80], Copyright 2014, American Chemical Society.

Nevertheless, the application of gold catalysts to industrial level is limited due to certain drawbacks, such as the decrease of activity in the presence of high concentration of moisture and the Nevertheless, the application of gold catalysts to industrial level is limited due to certain drawbacks, such as the decrease of activity in the presence of high concentration of moisture and the aggregation of gold nanoparticles at high temperatures [81,82].

aggregation of gold nanoparticles at high temperatures [81,82]. Gold-based bimetallic systems are considered a useful solution to overcome the cited limitations, due to the combination of properties from the gold and the second metal. Among the various metals employed to be joined with gold, it has been found that Pd, Ag, and Cu show a high miscibility, and Gold-based bimetallic systems are considered a useful solution to overcome the cited limitations, due to the combination of properties from the gold and the second metal. Among the various metals employed to be joined with gold, it has been found that Pd, Ag, and Cu show a high miscibility, and these systems can be prepared with a wide range of methodologies [83,84], whereas Pt, Ru, Co, Fe, and

these systems can be prepared with a wide range of methodologies [83,84], whereas Pt, Ru, Co, Fe, and Ni are only partially or not at all miscible with gold [84]. Consequently, Au-Pd followed by Au-

Ag and Au-Cu are the most utilized gold-based bimetallic catalysts for VOC oxidations.

2.1.1. Au-Pd Catalysts

Ni are only partially or not at all miscible with gold [84]. Consequently, Au-Pd followed by Au-Ag and Au-Cu are the most utilized gold-based bimetallic catalysts for VOC oxidations.
