## 3 Results
### Optimisation of the Synthesis of Monometallic Pd Catalysts-Catalyst Characterisation
A volcano plot illustrates how the PVA/Pd weight ratio can influence the exposure of Pd on the surface (Figure 1, Table 1). An increase of PVA/Pd weight ratio up to 0.3 was accompanied by an increase of surface Pd content, from 2.3 (for PVA/Pd = 0) to 8.2 (PVA/Pd = 0.3, (Table 1). At a higher PVA/Pd weight ratio a sequential decrease was observed. It is expected, as it has been shown from previous studies that the amount of Pd exposure on the surface could be affected by (i) particle size of Pd [46; 47; 48; 49; 50], (ii) amount and presence of PVA and (iii) nature of the support [51]. Taking into account the average pore diameter of the support is around 3.7 nm, smaller Pd nanoparticles than 3.7 nm could lay deeper in the internal surface of the activated charcoal support, explaining the trend observed when PVA/Pd ratio was above 0.3. In the case of using a PVA/Pd weight ratio lower than 0.3, the mean Pd particle size was higher than 4.8 nm and it is expected the main dominant factor for Pd exposure on the surface to be influenced by particle size effects; a large Pd particle size could lead to lower surface Pd exposure. It is difficult to quantify the amount of PVA present around the Pd nanoparticles and how it will affect the deposition of the Pd preformed metal nanoparticles. Nevertheless, it is expected that the presence of PVA could partially facilitate the increase of the number of Pd nanoparticles on the surface due to the possibility of blocking pores of the support based on the hydrodynamic radius of the stabiliser. This observation has an effect on the catalyst activity and it will be discussed in the following sections. The oxidized Pd species are presented in low content in the samples (19%-38%) and the presence of oxidized Pd species is mainly due to the facile surface oxidation of metallic Pd with atmospheric O \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Catalyst**} & **PVA/Pd** & **NaBH\({}_{d}\)/Pd** & **Atomic \% of Pd** & **\% Pd\({}^{0}\)** & **Mean Particle Size (nm)** \\ \cline{3-6}  & **Weight Ratio** & **Molar Ratio** & & **Calculated by XPS** & **Calculated by TEM** \\ \hline Pd/AC-a & 0 & 5 & 2.3 & 62 & - \\ Pd/AC-b & 0.05 & 5 & 3.1 & 64 & 8.2 \(\pm\) 3.0 \\ Pd/AC-c & 0.1 & 5 & 3.8 & 62 & 7.7 \(\pm\) 2.7 \\ Pd/AC-d & 0.2 & 5 & 5.3 & 81 & 6.9 \(\pm\) 2.2 \\ Pd/AC-e & 0.3 & 5 & 8.2 & 56 & 4.8 \(\pm\) 2.0 \\ Pd/AC-f & 0.65 & 5 & 3.9 & 57 & 3.0 \(\pm\) 0.9 \\ Pd/AC-g & 1.2 & 5 & 2.5 & 53 & 3.1 \(\pm\) 1.1 \\ Pd/AC-h & 0.65 & 10 & 4.3 & 71 & 3.2 \(\pm\) 0.9 \\ Pd/TiO\({}_{2}\)-f & 0.65 & 5 & 0.7 & 43 & 5.0 \(\pm\) 2.0 \\ Pd/TiO\({}_{2}\)-h & 0.65 & 10 & 1.0 b & 65 & 3.1 \(\pm\) 0.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Palladium atomic content at the surface, % Pd\({}^{0}\) and mean particle size. Figure 1: XPS spectra of (**A**): Pd/AC-f and Pd/AC-h and (**B**): Pd/TiO\({}_{2}\)-f and Pd/TiO\({}_{2}\)-h.  
In general, a decrease in %Pd(0) content was observed with a decrease of Pd particle size (Table 1). These results are in good agreement with what is reported in the literature, where larger Pd particles are less prone to react with O\({}_{2}\) than smaller ones [52]. On TiO\({}_{2}\), Pd tends to be immobilised inside the pores in an equal or a higher degree than on the external surface explaining the low Pd content on the surface, due to the bigger pore size. Increasing NaBH4/Pd molar ratio from 5 to 10 slightly increases the amount of metallic Pd due to the presence of a higher amount of adsorbed hydride, regardless of the support (Table 1). Catalytic Activity of the as Synthesized Monometallic Pd Catalysts for the Formic Acid Decomposition  
The catalytic performance of the Pd catalysts prepared by sol-immobilisation was evaluated in the liquid-phase decomposition of formic acid as a model reaction for hydrogen generation.