## 2 Experimental  
### Materials and Methods  
Analytical grade of ethanol (EtOH, 99.5%), and NaOH were purchased from Merck (Darmstadt, Germany). H\({}_{2}\)PtCl\({}_{6}\), K\({}_{2}\)PdCl\({}_{4}\), and NaBH\({}_{4}\) were purchased from Sigma Aldrich (St. Louis, MO, USA). The aqueous solutions were prepared with ultrapure water. Electrochemical experiments were performed on a standard three-electrode cell system including a modified glassy carbon electrode (2 mm in diameter, GCE), Pt wire, and a saturated Ag/AgCl as working, counter and reference electrodes, respectively. Electrochemical experiments were carried out with a \(\mu\)-Autolab potentiostat/galvanostat 101 controlled by Nova software (Metrohm, Switzerland). The morphology and structure of the synthesized nanoparticles were studied by using an energy dispersive spectrometer (EDX, OXFORD, INCA model system, Oxford Instruments, Oxon, UK), transmission electron microscopy (TEM, JEOL JEM-2010 microscope working at 200 kV, JEOL, Tokyo, Japan), X-ray diffraction spectroscopy (XRD, Bruker D8(Figure 2a-f), with no significant changes in the morphology of the nanoparticles. In fact, the analysis of the structure of the nanoparticles carried out by TEM showed no visible differences due to the small difference between the lattice parameter of Pt and Pd nanoparticles, which is beyond the resolution of the images [23]. The mean particle size of the nanoparticles is reported in Table 1. This mean particle size was obtained from the analysis of more than 200 nanoparticles per sample from different parts of the grid and using ImageJ ([PERSON], public domain) and Origin software (OriginLab, Northampton, MA, USA). The average size of synthesized nanoparticles according to the analysis of the TEM images is 3.41 nm, and the biggest and smallest particle sizes were observed for the Pt and Pd nanoparticles with 4.50.7 nm and 1.60.4 nm, respectively. Figure 2: Representative transmission electron microscopy (TEM) micrographs of Pt (**a**), Pt4 Pd (**b**), Pt3 Pd2 (**c**), Pt2 Pd3 (**d**), PtPd4 (**e**), and Pd (**f**) nanoparticles. Figure 3 shows the XRD patterns of the different nanoparticles. All the diffraction peaks can be indexed with the face-centered cubic (fcc) bulk structure of Pt and Pd. The nanoparticle size was also calculated by using the (200) lattice plane of the XRD measurements, using the Scherrer equation. The calculated sizes are also reported in Table 1. The results obtained are in good agreement with the TEM analysis. In this respect, it should be noted that the estimation of the particle size with the XRD for particle sizes lower than 5 nm is always \"approximate\" and small discrepancies can be observed with those obtained by TEM since the averaging and definition of size is different. The real atomic composition of synthesized nanoparticles was calculated with AAS, UV-Vis, and EDX techniques. All of the calculated atomic compositions were in good agreement with the nominal values. The results have been represented in Table 2.  
The XPS experiments were performed to determine the electronic properties of the nanoparticles, as shown in Figure 4. In some cases, XPS can also be used to characterize the surface composition. However, it is important to note that XPS is not a pure surface method because the electrons penetrate a few nanometers into the material. For nanoparticles with an average size of about 3.4 nm, the spectra contain information from the whole nanoparticle [35]. We profit this property to determine the composition of synthesized nanoparticles by XPS [34]. The results are shown in the last column of Table 2. As can be seen, the values agree, within the error of the techniques, with the nominal values and those measured by AAS, UV-Vis, and EDX techniques. Regarding the electronic properties,