## 2 Experimental
### Electrochemical Experiments  
#### 3.2.1 Electrochemical Characterization: Background Examination in the Alkaline Media  
The electrochemical characterization of the nanoparticles was carried out by cyclic voltammetry in a 0.1 M NaOH solution at a scan rate of 0.05 V s 1 (Figure 5a-f). The sharpness, good definition, and the symmetry of the adsorption states in all nanoparticles are clear evidence of the effective surface cleanliness [36; 37; 41; 42; 43; 44]. The observed signals in the profiles correspond to hydrogen ad(b)sorption-desorption (H\({}_{\text{ad(b)s}}\)-H\({}_{\text{des}}\)), OH adsorption (OH\({}_{\text{ads}}\)), metal oxidation, and reduction processes. The position and definition of those signals depend on the ratio of Pt:Pd in the synthesized nanoparticles. For the pure Pt nanoparticles, the pairs of peaks in the positive potential sweep (a\({}_{1}\),a\({}_{2}\)) and their symmetric pairs of peaks in the negative potential sweep (a\({}_{1}\)',a\({}_{2}\)') corresponds to the desorption and the adsorption of H on the surface of Pt nanoparticles, respectively [20; 29; 36]. On the other hand, the formation of H\({}_{\text{abs}}\) and H\({}_{\text{ads}}\) on the surface of Pd nanoparticles takes place in a single peak in the negative potential sweep (b\({}_{1}\)'), whereas the desorption of H\({}_{\text{abs}}\) and H\({}_{\text{ads}}\) split into two peaks (b\({}_{1}\),b\({}_{2}\)) in the positive scan direction (Figure 5f) [45; 46; 47; 48; 49]. The OH adsorption, oxidation of Pt-OH to Pt-O, and reduction of PtO to Pt are associated with peaks a\({}_{3}\), a\({}_{4}\), and a\({}_{4}\)', respectively, in Figure 5a. On Pd, the adsorption of OH is not well defined, and the process corresponding to the Pd oxidation/reduction processes are marked by peaks b\({}_{3}\) and b\({}_{3}\)'. The comparison between Figure 5a and f demonstrates the reduction of PtO (a\({}_{4}\)') takes place at more positive potential than that of PdO (b\({}_{3}\)') on the nanoparticles. The potential values of the Pt(Pd)O reduction peak of the synthesized nanoparticles have been compared in the 4 th column of Table 3. This observation reveals a higher tendency towards surface oxidation for Pd than for Pt nanoparticles [50]. Although the reduction of metal oxide represents a sharp peak on the surface of Pd nanoparticles (Figure 5f), the reduction of metal oxide exhibits a broader peak at more positive potentials on the surface of Pt nanoparticles (Figure 5a). This process can be also understood from the comparison between values of width at half height of Pt(Pd)O reduction peak reported in the 3 rd column of Table 3[46; 47; 50]. The peak asymmetry of the oxidation/reduction reaction of the Pt and Pd surfaces clearly indicates that these are irreversible processes. Thus, in this potential region, the actual surface state depends on the scan direction. For the bimetallic nanoparticles, as seen in Figure 5, the profile changes according to the Pt:Pd ratio. Although a\({}_{1}\) and a\({}_{1}\)' peaks are entirely separate from the peaks of a\({}_{2}\) and a\({}_{2}\)' for the synthesized nanoparticles with Pt percentage above 80%, they merge in a single peak as the Pd ratio increases [33; 51; 37]. The potential of the metal oxide formation also shifts according to the composition [20; 29; 51; 52]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Nominal Atomic** & \multicolumn{3}{c}{**Calculated with**} & \multicolumn{3}{c}{**Calculated with**} \\
**Composition Pt/Pd** & **AAS (Pd) UV-Vis (Pt)** & **Calculated with** & **EDX** & & **Pt (4f\({}_{72}\)) Pd (3d\({}_{52}\))** \\ \hline Pt\({}_{4}\)Pd & 79.6 & 0.3/20.4 & 0.3 & 82 & 0.5/18 & 0.5 & 75.3 & 0.5/24.7 & 0.5 \\ Pt\({}_{3}\)Pd\({}_{2}\) & 59.4 & 0.3/40.6 & 0.3 & 62 & 0.5/38 & 0.5 & 56.6 & 0.5/43.4 & 0.5 \\ Pt\({}_{2}\)Pd\({}_{3}\) & 40.6 & 0.3/59.4 & 0.3 & 40 & 0.5/60 & 0.5 & 38 & 0.5/62 & 0.5 \\ PtPd\({}_{4}\) & 17.8 & 0.3/82.2 & 0.3 & 20.6 & 0.5/79.4 & 0.5 & 21.4 & 0.5/78.6 & 0.5 \\ \hline \hline \end{tabular}
\end{table}