2.1.2. Au-Ag and Au-Cu Catalysts

The establishment of a strong interaction between gold and silver with the formation of an alloy or of bimetallic clusters was investigated by our research group both in VOC oxidation and in H<sup>2</sup> purification towards the preferential oxidation of CO (PROX reaction) [50,53,68]. In particular, we evaluated the catalytic activity of Au-Ag and Au-Cu bimetallic samples supported on CeO<sup>2</sup> toward the degradation of 2-propanol and ethanol. A higher activity was found of the gold–silver sample with respect to Au-Cu and the monometallic counterparts. The higher activity of the gold–silver system was correlated to a higher mobility/reactivity of ceria surface oxygens, due to a strong synergistic interaction between the gold–silver nanoparticles and the cerium oxide. A linear correlation was stated

2.1.2. Au-Ag and Au-Cu Catalysts

from [68], Copyright 2015, Springer Nature.

considering the T<sup>50</sup> of alcohol oxidation and the TPR (temperature-programmed reduction) initial temperature, i.e., the temperature at which the reduction of ceria surface oxygens started considering the analyzed samples (Figure 5). The Au-Ag/CeO<sup>2</sup> catalyst displayed the lowest reduction temperature and T50. correlation was stated considering the T50 of alcohol oxidation and the TPR (temperatureprogrammed reduction) initial temperature, i.e., the temperature at which the reduction of ceria surface oxygens started considering the analyzed samples (Figure 5). The Au-Ag/CeO2 catalyst displayed the lowest reduction temperature and T50.

synergistic interaction between the gold–silver nanoparticles and the cerium oxide. A linear

*Catalysts* **2020**, *10*, x FOR PEER REVIEW 8 of 25

The establishment of a strong interaction between gold and silver with the formation of an alloy or of bimetallic clusters was investigated by our research group both in VOC oxidation and in H2 purification towards the preferential oxidation of CO (PROX reaction) [50,53,68]. In particular, we evaluated the catalytic activity of Au-Ag and Au-Cu bimetallic samples supported on CeO2 toward the degradation of 2-propanol and ethanol. A higher activity was found of the gold–silver sample

**Figure 5.** Temperature at which the 50% of conversion of ethanol and 2-propanol was achieved (T50) versus TPR (temperature-programmed reduction) initial temperature: (filled diamond) Au-Ag/CeO2; (filled circle) Au-Cu/CeO2; (filled square) Au/CeO2; (filled triangle) Ag/CeO2; (times) Cu/CeO2. Figure **Figure 5.** Temperature at which the 50% of conversion of ethanol and 2-propanol was achieved (T50) versus TPR (temperature-programmed reduction) initial temperature: (filled diamond) Au-Ag/CeO<sup>2</sup> ; (filled circle) Au-Cu/CeO<sup>2</sup> ; (filled square) Au/CeO<sup>2</sup> ; (filled triangle) Ag/CeO<sup>2</sup> ; (times) Cu/CeO<sup>2</sup> . Figure from [68], Copyright 2015, Springer Nature.

Nagy et al. [93] studied the performance of Au-Ag nanoparticles supported on SiO2 synthetized from the adsorption of bimetallic colloids in the oxidation of benzyl alcohol. The authors focused their research on the crucial importance of the molar ratio between the two metals. In particular, a synergistic effect was verified that reflects a higher activity at a low Ag/Au molar ratio (best result Ag/Au = 23/77). For the authors, the synergy is activated by the optimal concentration of the two metals, which increased the activation of both oxygens from gas-phase and from the support. In the same context, a correlation between catalytic activity and the concentration of gold and silver was measured by our research group in the PROX reaction [50] with a higher concentration of gold or silver with respect to the second metal that was detrimental for the overall catalytic performance, whereas the best results were obtained with an approximately equal concentration of gold and silver Nagy et al. [93] studied the performance of Au-Ag nanoparticles supported on SiO<sup>2</sup> synthetized from the adsorption of bimetallic colloids in the oxidation of benzyl alcohol. The authors focused their research on the crucial importance of the molar ratio between the two metals. In particular, a synergistic effect was verified that reflects a higher activity at a low Ag/Au molar ratio (best result Ag/Au = 23/77). For the authors, the synergy is activated by the optimal concentration of the two metals, which increased the activation of both oxygens from gas-phase and from the support. In the same context, a correlation between catalytic activity and the concentration of gold and silver was measured by our research group in the PROX reaction [50] with a higher concentration of gold or silver with respect to the second metal that was detrimental for the overall catalytic performance, whereas the best results were obtained with an approximately equal concentration of gold and silver (1% wt–1% wt).

(1% wt-1% wt). The crucial importance of the molar ratio between gold and the other metal was also stated in the review of Bracey et al. [94], focused on the Au-Cu system. Specifically, in one of the analyzed works, the following order of reaction in catalytic oxidation of propene is reported: AuCu (1:3 molar ratio)/TiO2 > AuCu (1:1 molar ratio)/TiO2 > AuCu/TiO2 (3:1 molar ratio) > Au/TiO2. The content of copper, in fact, strongly influenced the dispersion of the metal nanoparticles, with a high amount of copper in the alloy that caused a decrease in the size of the metal particles, thus contributing to enhancint the activity and selectivity into propene oxide [95]. In the same review, it was illustrated that, when investigating another reaction, such as selective oxidation of benzyl alcohol to benzaldehyde, the more active bimetallic catalyst was the sample with the higher concentration of gold (the catalyst AuCu/SiO2 with a molar ratio of 4:1). Similarly to the previous examples, the The crucial importance of the molar ratio between gold and the other metal was also stated in the review of Bracey et al. [94], focused on the Au-Cu system. Specifically, in one of the analyzed works, the following order of reaction in catalytic oxidation of propene is reported: AuCu (1:3 molar ratio)/TiO<sup>2</sup> > AuCu (1:1 molar ratio)/TiO<sup>2</sup> > AuCu/TiO<sup>2</sup> (3:1 molar ratio) > Au/TiO2. The content of copper, in fact, strongly influenced the dispersion of the metal nanoparticles, with a high amount of copper in the alloy that caused a decrease in the size of the metal particles, thus contributing to enhancint the activity and selectivity into propene oxide [95]. In the same review, it was illustrated that, when investigating another reaction, such as selective oxidation of benzyl alcohol to benzaldehyde, the more active bimetallic catalyst was the sample with the higher concentration of gold (the catalyst AuCu/SiO<sup>2</sup> with a molar ratio of 4:1). Similarly to the previous examples, the bimetallic catalyst was prepared by impregnation, but in this case, a higher concentration of gold is fundamental to achieve a high selectivity (98%) to benzaldehyde.

The above-discussed literature data on AuCu bimetallic catalysts were mainly focused on the selective oxidation of VOCs, whereas the work of Nevanperä et al. [96] dealt with catalytic oxidation of DMDS with bimetallic gold-based catalysts (Au-Cu and Au-Pt) supported on γ-Al2O3, CeO2, and CeO2-Al2O<sup>3</sup> prepared by surface redox reduction. Among the examined supports, the alumina gave the best results, whereas the addition of gold enhanced the catalytic activity of both monometallic copper and platinum samples, Au-Cu catalysts being the most active system. Interestingly, the authors noted that the same Au-Cu catalyst led to the formation of dangerous byproducts, such as carbon monoxide and formaldehyde. This was attributed to the high concentration of reactive surface oxygens favored by the presence of copper oxide and to the dissociation of the oxygen that started at a lower temperature with respect to the monometallic samples, with the consequent modification of the surface acid and basic sites of the bimetallic catalyst. By contrast, selectivity towards CO<sup>2</sup> and H2O was higher in the Au-Pt sample.
