## 2 Results and Discussion

Table 2: MPNs population of phases A, B, and C of P50 and P75 and the atomic (major) Pd/Au ratio as obtained from the data of Table 1 (see the text for the meaning of \(\rm V_{\rm MNP}\), \(\rm N_{\rm MNP}\), \(\rm P_{\rm MNP}\), \(\rm N_{\rm Au}\), and \(\rm N_{\rm Pd}\)). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**\#**} & \multicolumn{2}{c}{**Cat**} & \multicolumn{2}{c}{**P50**} & \multicolumn{2}{c}{**P75**} & \multicolumn{2}{c}{**Au**} \\ \hline  & \multicolumn{2}{c}{**Edge**} & \multicolumn{2}{c}{**Pd**} & \multicolumn{2}{c}{**Au**} & \multicolumn{2}{c}{**Pd**} & \multicolumn{2}{c}{**Au**} \\ \hline  & \multicolumn{2}{c}{**Phase**} & **A** & **B** & **C** & **A** & **B** & **C** & **A** & **B** & **C** & **A** & **B** & **C** \\ \hline
1 & V\({}_{\text{NaP}}\) (nm\({}^{3}\)) & 696.91 & 18.82 & 0.90 & 696.91 & 17.16 & 1.15  
The most remarkable difference between the surface distributions of P50 and P75 was observed in the respective phases B. Although this phase contains more gold in P50 (Pd/Au \(\approx\) 0.20) than in P75 (Pd/Au \(\approx\) 0.50), it features the highest degree of metal intermixing. The larger contribution of phase B to the metal surface area of P50 is the result of the smaller size of the MNPs (D\({}_{\rm P50}\)\(\approx\) 3.2 nm; D\({}_{\rm P75}\)\(\approx\) 5.6 nm) and of their higher population in this material (P\({}_{\rm P50}\)\(\approx\) 1.8-2 \(\times\) 10\({}^{4}\) ppm; P\({}_{\rm P75}\)\(\approx\) 3.4-3.8 \(\times\) 10\({}^{2}\) ppm). It should also be appreciated that the decrease of the relative surface area of phase B from P50 to P75 occurred with the increase of the relative surface area of phase C. The Pd/Au ratio in phase C of P75 is close to 10\({}^{3}\) as compared with ca 10 in P50. All these circumstances imply that the intermixing of the two metals is strongly enhanced on going from P75 to P50. The DS reaction has been attracting the interest of many research groups for years as its effective implementation would be a real breakthrough from the environmental point of view. In this connection,  
Figure 4: Relative surface areas respectively exposed by phases A, B (left vertical axis), and C (right vertical axis) in P50. Figure 5: Relative surface areas respectively exposed by phases A, B (left vertical axis), and C (right vertical axis) in P75. bimetallic gold-palladium catalysts are among the best ones, because of their improved performance in comparison with mono-metallic palladium catalysts. The well-known promoting effect of gold [45; 46; 47] was confirmed by our data on the catalytic productivity of P50 and P75, which was, respectively, 109 and 97 mmol\({}_{\text{H}_{2}\text{O}_{2}}\)/mol\({}_{\text{H}_{2}}\), both better than a related mono-metallic Pd catalyst (64 mmol\({}_{\text{H}_{2}\text{O}_{2}}\)/mol\({}_{\text{H}_{2}}\)). This effect was attributed to the formation of gold-palladium nano-alloyed materials. Our catalytic data show that the gold promotion was more effective in P50 than in P75. In the first approximation, this can simply be attributed to the decrease of the Pd/Au atomic (molar) ratio from 5.56 (P75) to 1.84 (P50). This lead to a relatively small increase of the population of phase A, but the features of its MNPs (large size, small population, little relative surface area, gold segregation) support the hypothesis that it was of negligible relevance, if any, to catalysis. Accordingly, only phases B and C were active in both P75 and P50. On going from the former to the latter, they were both enriched in gold, and in P50, the contribution of phase B to the metal surface area underwent a 10-fold increase. This could be the cause of the higher productivity of P50, but we cannot rule out at the moment that it could be brought about by the enrichment in gold of phase C, which in P50, still represents more than 85% of the metal surface.