## 3 Results
### Reusability Tests
(**A**): Pd/TiO\({}_{2}\), (**B**): Au\({}_{1}\)Pd\({}_{3}\)/TiO\({}_{2}\) (**C**): Au\({}_{1}\)Pd\({}_{1}\)/TiO\({}_{2}\) (**D**): Au\({}_{3}\)Pd\({}_{1}\)/TiO\({}_{2}\) and (**E**): Au/TiO\({}_{2}\). shifts [60; 61; 62]. As previously presented, XPS gives information about the net electron transfer during bimetallic alloying however, caution has to be taken due to the complicated combination of initial and final state effects [63]. Observing Pauling electronegativities of gold (2.54) and palladium (2.20), it suggests that there may be a slight electron transfer from palladium to gold. Regarding previous studies [64; 65; 60], the binding energy of both Pd and Au tends to decrease when forming the alloy. Furthermore, particle size can have an effect as well on the binding energy of metals [66]. As observed in Figure 7, for Au, the peaks associated with Au4f\({}_{7/2}\) shift to lower energy (83.4 eV for Au monometallic, and between 83.1 and 83.3 eV for the alloys), in agreement to previous studies. Moreover, we can explain this shift considering the particle size variation. For pure gold, mean particle size is 4 nm, while, for the alloys, it decreases to a range between 3 and 3.4 nm. Whilst this is a small decrease in particle size, the electronic structure of the atoms can be modified to a large extent. The same explanation applies to Pd (Pd3d\({}_{3/2}\) core level). Observing the data presented in Table 3 and the binding energy shifts reported in Table S3 and Figure 8, there is a shift to lower binding energies as expected when forming the alloy (it decreases from 340.46 eV for monometallic Pd, to approximately 340.3 eV) and in agreement with previous reports [65], but in the case of Au:Pd 3:1, binding energy shifts to a higher value (340.47 eV), again explained with the decrease of mean particle size to 3 nm for this catalyst. We could conclude that the Pd3d\({}_{3/2}\) and the Au4f\({}_{7/2}\) core levels shift to lower binding energies upon alloying and results were in good agreement with the core level shifts of Au4f\({}_{7/2}\) and Pd3d\({}_{3/2}\) reported in the literature. Regarding the oxidation state of Pd, a volcano plot was observed when modifying Au content. Pd\({}^{0}\) species slightly increased until the ratio Pd/Au was 1 and decreased for higher Au content. Representative Pd\({}^{\text{II}}\)/Pd\({}^{0}\) atomic ratio calculated from XPS spectra for the catalysts is shown in Table 3. The surface area of the support (63 m\({}^{2}\)/g) decreased approximately 20% when supporting the catalyst either due to the coverage of PVA or the intrinsic error of the system. Figure 7: Au(4f) spectra for the series of Au\({}_{x}\)Pd\({}_{y}\)-TiO\({}_{2}\) catalyst. The catalytic performance of the Au\({}_{x}\)Pd\({}_{y}\)-TiO\({}_{2}\) catalysts for the formic acid decomposition at 50 \({}^{\circ}\)C is presented in Figure 9. As observed, Au presents very low activity towards the decomposition of formic acid. On the other hand, a small amount of Pd with Au (Au\({}_{x}\)Pd\({}_{1}\)-TiO\({}_{2}\)) produced a synergistic effect exhibiting a high catalytic activity with a TOF of 627 h\({}^{-1}\) at 50 \({}^{\circ}\)C after 5 min. The observed catalytic behaviour has been extensively studied before and reported [67; 68]. The narrow particle size distribution of the supported preformed colloidal Au-Pd nanoparticles could be one of the most plausible reasons for this increment in activity. Figure 8: Au(4d) + Pd(3d) spectra for the series of Au\({}_{x}\)Pd\({}_{y}\)-TiO\({}_{2}\) catalysts. Figure 9: Conversion of formic acid after 4 h as a function of the Pd: Au molar ratio.