## 3 Results
### Reusability Tests  
Reusability of the Pd/C-e (Figure 5) was studied at 50 \({}^{\circ}\)C and 0.5 M formic acid for five subsequent catalytic cycles. The reusability test was carried out by filtrating the catalyst at ambient temperature and atmospheric pressure without further washing and using it for a new reaction under the same reaction conditions. The as-synthesised catalysts exhibited a certain loss in catalytic activity, however reaching stabilisation after the third run preserving 69% of its initial activity after the fifth use. This decrease may be attributed to (i) increment in particle size or agglomeration, (ii) decrease of Pd loading by leaching, (iii) poisoning from CO, (iv) strong adsorption and poisoning of formate species and (v) formation of the Pd-H species. Representative images of the used catalyst after five runs and particle size distribution are reported in Figure S5. As previously presented, mean Pd particle size of the fresh catalyst was \(4.8\pm 2.0\) nm, however, for the catalyst after the fifth run, it increased to \(6.9\pm 3.1\) nm and the particle size distribution was broader: 50% of the particles were in the range 6-11 nm. Sintering phenomena were visible from the snapshots. Regarding XPS analysis of the used samples, a decrease of the metal content on the surface was observed. The fresh catalyst presents a 7.8%, however, it decreased to 2.3% after the fifth use. Since XPS is surface sensitive, the drop of Pd metal content for the used catalyst could be partially explained (i) by leaching, (ii) by a migration of the Pd nanoparticles from the external surface to the pores and (iii) by an increase in Pd mean particle size as previously observed. This is a possible reason for the decrease in catalytic activity for the reused catalysts, however, there \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Loading Pd/C**} & **Pd Loading** & **Mean Particle** & **Surface** & **\% Pd\({}^{\circ}\)** & **At. \% Pd** & **TOF** \\  & **(\%) EDX** & **Size (nm)** & **Area (m\({}^{2}\)/g)** & **\% Pd\({}^{\circ}\)** & **(h \({}^{-1}\))** \\ \hline
0.2 wt \%-Pd/AC-e & 0.20 & 3.9 \(\pm\) 2.3 & 71 & 55 & 1.9 & 278 \\
0.5 wt \%-Pd/AC-e & 0.48 & 4.9 \(\pm\) 1.3 & 60 & 50 & 4.6 & 272 \\
0.7 wt \%-Pd/AC-e & 0.68 & 4.9 \(\pm\) 1.4 & 71 & 50 & 4.8 & 239 \\
1 wt \%-Pd/AC-e & 1.07 & 4.8 \(\pm\) 2.0 & 50 & 55 & 7.9 & 366 \\
1.2 wt \%-Pd/AC-e & 1.25 & 3.8 \(\pm\) 1.1 & 35 & 44 & 6.9 & 326 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Structural and chemical composition of the catalysts studied during effect of loading investigation. Figure 4: Effect of Pd sol preparation in the catalytic decomposition of formic acid as a function of the NaBH\({}_{4}\)/Pd molar ratio on AC and TiO\({}_{2}\). could be a sum of factors. For example, adsorption of formate species and formation of Pd-H could be other factors that could elucidate the observed decrease in terms of catalytic activity. For this purpose, Density functional theory (DFT) studies could shed some light. Characterisation and Catalytic Performance of Au\({}_{x}\)Pd\({}_{y}\)TiO\({}_{2}\) for Formic Acid Decomposition  
In the previous section, we showed the promising catalytic activity of monometallic Pd nanoparticles supported on activated charcoal and TiO\({}_{2}\). In previous studies, many research groups have reported that the combination of Au and Pd could enhance significantly catalytic activity in a range of catalytic reactions, including oxidation of alkanes, alcohols and polyols, hydrogenation of aldehydes and in situ hydrogen peroxide synthesis [53; 54]. Therefore, we investigated the catalytic activity of Au\({}_{x}\)Pd\({}_{y}\) ratio for TiO\({}_{2}\)-supported nanoparticles for the liquid phase decomposition of formic acid. These catalysts have been prepared using the optimised synthesis procedure reported for the supported monometallic Pd catalysts (PVA/M weight ratio = 0.3 and NaBH\({}_{4}\)/M molar ratio = 10). Titania was used as support instead of activated charcoal.