## 3 Results
### Reusability Tests
The main reason for this choice was that the catalytic performance of the Pd/TiO\({}_{2}\) was similar to the analogous Pd/AC catalyst and further spectroscopic characterisation could be carried out in terms of CO-DRIFTS and FTIR when the choice of support is TiO\({}_{2}\). In Table 3 the theoretical composition and characterisation data of the as-prepared catalysts are summarised. All samples were analysed by SEM-EDX mapping to confirm the loading of Pd and provide information regarding metal dispersion. SEM-EDX mapping (Figure S6) showed that Au-Pd  
Figure 5: Reusability of Pd/AC-e. Reaction conditions: substrate/metal molar ratio: 2000:1, 50 \({}^{\circ}\)C, 0.5 M HCOOH, 750 rpm. \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **\% mol** & **\% mol** & **Loading** & **Loading Au** & **Mean** & **Surface Area** & **\%** \\  & **[Pd2*]** & **[Au3+]** & **Pd** & **(wt \%) from EDX** & **(wt \%)** & **EmX** & **(m\({}^{2}\)/g)** & **Pd2* \\ \hline Pd-TiO\({}_{2}\)+ & 1 & - & 0.96 & - & 3.6 \(\pm\) 1.9 & 51 & 54 \\ Au\({}_{x}\)Pd\({}_{y}\)TiO\({}_{2}\) & 0.75 & 0.25 & 0.80 & 0.13 & 3.2 \(\pm\) 2.2 & 51 & 56 \\ Au\({}_{x}\)Pd\({}_{y}\)TiO\({}_{2}\) & 0.50 & 0.50 & 0.70 & 0.40 & 3.4 \(\pm\) 1.1 & 45 & 58 \\ Au\({}_{y}\)Pd\({}_{y}\)TiO\({}_{2}\) & 0.25 & 0.75 & 0.30 & 0.70 & 3.0 \(\pm\) 1.0 & 50 & 56 \\ Au-TiO\({}_{2}\) & - & 1 & - & 1.00 & 4.0 \(\pm\) 0.9 & 47 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Theoretical composition and characterisation data of the monometallic-bimetallic studies. nanoparticles were well dispersed in all the as-prepared catalysts, no areas with high nanoparticle density/agglomeration were evidenced. The calculated metal loading approached the theoretical values expected within experimental error. Mean particle size and particle size distribution were calculated by TEM analysis. Figure 6 displays representative TEM images and particle size distribution of the as-synthesised catalysts. TEM analysis confirmed a good level of dispersion of the supported nanoparticles in agreement with SEM-EDX mapping. The monometallic Pd/TiO\({}_{2}\) and Au/TiO\({}_{2}\) catalysts had slightly larger mean particle size (3.6-4 nm range; Table 3) compared to the bimetallic counterparts Au\({}_{x}\)Pdy/TiO\({}_{2}\) (3-3.4 nm range), suggesting that the co-reduction of Au and Pd facilitates a smaller mean particle size and in agreement with previous reports. In previous studies, we have shown by using Scanning Transmission Electron Microscope-High Angle Annular Dark Field Imaging (STEM-HAADF) and EDX analysis of individual nanoparticles that the simultaneous chemical reduction of Au and Pd in the aforementioned Au\({}_{x}\)Pdy ratio produced random homogeneous Au-Pd alloys with a face-centred cubic (fcc) structure and with the smallest nanoparticles to be Au-rich, whereas the largest ones to be Pd-rich [54]. Moreover, in previous studies, a decrease of Pd or Au/Pd mean particle size has shown a positive catalytic influence for the effective decomposition of formic acid [55; 56; 57; 58]. Figure 6: _Cont._  
Regarding XPS analyses, Au(4f) and Au(4d) + Pd(3d) spectra are shown in Figures 7 and 8. The peaks at 83-84 eV and 87 eV associated with Au4f\({}_{7/2}\) and Au4f\({}_{5/2}\) respectively are related to Au foil. The shifts observed in the binding energy of the peaks have been previously studied [59]. Table S3 presents the binding energy shifts when forming the alloy. As demonstrated by [PERSON] et al. [47], the relative binding energy of Au4f\({}_{7/2}\) shifts at 83.4 eV, thus we concluded that the gold in the as synthesised catalysts is in the metallic state. In the bimetallic catalysts, there is an overlap of the peak Au4d\({}_{5/2}\) with the Pd3d\({}_{5/2}\) peak, therefore, only Pd3d\({}_{3/2}\) will be used to analyse the binding energy  
Figure 6: Bright field TEM micrographs and corresponding histograms of the particle size distributions of the catalysts.