## 4 Conclusions  
A series of preformed Pd colloidal nanoparticles were synthesised by varying the amount of stabiliser (PVA/Pd weight ratio) and supported on activated charcoal to study the effect of stabiliser in terms of influencing the final morphology of the supported nanoparticle (Pd particle size, particle size distribution, Pd oxidation state and surface exposure of Pd). Moreover, the influence of Pd nominal loading and the choice of support were investigated. In the case of activated charcoal, the general trend for the mean Pd particle size with the variation of PVA amount was that by increasing the PVA/Pd weight ratio a progressive decrease in the mean Pd particle size was observed and above the PVA/Pd weight ratio of 0.65 a plateau was reached. In terms of Pd loading the mean Pd particle size was in a similar range 3.9-4.8 nm and did not appreciably alter at lower Pd loading. In the case of TiO\({}_{2}\) and using the optimised experimental conditions a similar particle size was observed with a higher percentage of metallic Pd species than the analogous Pd/AC sample. The catalytic performance of the synthesised catalysts was evaluated for the formic acid decomposition as a model reaction for the production of hydrogen under mild conditions. In terms of catalytic activity, a volcano type diagram was found when Pd/PVA ratio was altered. TEM and XPS analysis showed that the observed catalytic activity could be attributed to (i) the surface Pd exposure, (ii) Pd sites accessibility (coverage of the active sites) and (iii) mean Pd particle size. The effect of the Pd loading was investigated from 0.2 to 1.2 wt %, showing the highest activity for the catalysts with 1 wt % Pd loading. The major cause for the observed trend in terms of catalysis was attributed to the atomic content of Pd on the surface. In the case of the nature of support (activated charcoal versus TiO\({}_{2}\)), activated charcoal-supported Pd nanoparticles showed higher catalytic performance. This difference was attributed to (i) the higher surface percentage of metallic Pd for the Pd nanoparticles supported on activated charcoal and (ii) to the shape of the supported Pd nanoparticles: on activated charcoal Pd nanoparticles were spherical whereas hemispherical Pd nanoparticles were observed on titania, therefore decreasing the accessible active surface area exposed and subsequently, the observed catalytic activity. Reusability tests were performed for the most active monometallic Pd catalyst (Pd/PVA = 0.3 weight ratio) showing a loss of 31% of its initial activity after five uses, probably caused by the growth of Pd nanoparticles. **Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/10/5/1752/s1](http://www.mdpi.com/2076-3417/10/5/1752/s1)]([http://www.mdpi.com/2076-3417/10/5/1752/s1](http://www.mdpi.com/2076-3417/10/5/1752/s1)), Figure S1: Bright field TEM micrographs and corresponding histograms of the particle size distributions of the catalysts. Unless stated, NaBH\({}_{4}\)/Pd = 5. (**A**): Pd/AC-a (PVA/Pd = 0), (**B**): Pd/AC-b (PVA/Pd = 0.05), (**C**): Pd/AC-c (PVA/Pd = 0.1), (**D**): Pd/AC-d (PVA/Pd = 0.2), (**E**): Pd/AC-e (PVA/Pd = 0.3), (**F**): Pd/AC-f (PVA/Pd = 0.65), (**G**): Pd/AC-g (PVA/Pd = 1.2), (**H**): Pd/AC-h (PVA/Pd = 0.65, NaBH\({}_{4}\)/Pd = 10), (**I**): Pd/TiO\({}_{2}\)-f (PVA/Pd = 0.65) and (**J**): Pd/TiO\({}_{2}\)-h (PVA/Pd = 0.65, NaBH\({}_{4}\)/Pd = 10), Figure S2: Representative XPS survey of Pd/AC-e (PVA/Pd = 0.3) and quantification of the elements present on the surface. Figure S3: Bright field TEM micrographs and corresponding histograms of the particle size distributions of the Pd/AC catalysts as a function of Pd loading. (**A**): 0.2 wt % Pd/AC-e, (**B**): 0.5 wt % Pd/AC-e, (**C**): 0.7 wt % Pd/AC-e and (**D**): 1.2 wt % Pd/AC-e, Figure S4: EDX mapping of Pd/AC catalysts as a function of Pd loading.