## 3 Results  
### Optimisation of the Synthesis of Monometallic Pd Catalysts-Catalyst Characterisation  
Table S2 presents elemental chemical composition and surface area analysis of the catalysts studied during the optimisation of the colloidal preparation method. SEM-EDX analysis was performed confirming the presence of Pd and quantifying the Pd loading. Pd metal loading of the as-synthesised Pd catalysts is in good agreement to the nominal value of 1 wt %. Taking into account that the support materials used, activated charcoal and TiO\({}_{2}\) (commercial P25), presented a value of surface area of 64 and 63 m\({}^{2}\)/g respectively, we could conclude that a slight decrease in the surface area for the preformed Pd colloidal nanoparticles supported on activated charcoal was found, probably due to an obstruction of the pores (average pore size: 3.7-3.8 nm) since it is similar to the mean Pd particle size as presented in the next paragraphs. On the other hand, the Pd preformed colloidal nanoparticles supported on TiO\({}_{2}\) did not present this decrease since the pore size is approximately 28 nm and the Pd particle size distribution between 2 and 8 nm (Figure S1). Mean particle size and particle size distributions of the catalysts were analysed by TEM (Table 1) and representative bright field TEM micrographs are shown in Figure S1. Variation of the PVA amount used during the synthesis of the catalysts showed a significant influence in terms of mean Pd particle size, particle size distribution and agglomeration of Pd particles on the surface. At the lowest PVA/Pd weight ratio (PVA/Pd = 0, 0.05 and 0.1) agglomeration of Pd nanoparticles and the presence of larger Pd nanoparticles were observed. Increasing the PVA/Pd weight ratio to 0.3 a lower degree of Pd particle agglomeration was observed and the mean Pd particle size was 4.8 nm. Further increase of PVA/Pd weight ratio to 0.65 and 1.2 led to a smaller and similar mean Pd particle size (3.0 and 3.1 nm, respectively). These results indicate that by using a PVA/Pd weight ratio of 0.3, the synthesis of small Pd nanoparticles with narrow particle size distribution is achievable without the necessity to use a higher amount of PVA to further synthesise smaller Pd nanoparticles. Moreover, a lower amount of PVA used during the synthesis is expected to facilitate catalytic activity, since a higher amount of Pd free active sites will be accessible due to the lower amount of PVA that may block Pd active sites, as we have been previously shown [41]. Increasing NaBH\({}_{4}\)/Pd molar ratio from 5 to 10 we observed a narrower particle size distribution for both supports (Pd/C-h, Pd/TiO\({}_{2}\)-f, PVA/Pd = 0.65 and NaBH\({}_{4}\)/Pd = 10) in the range of 2-6 nm. XPS analysis of the as-synthesised catalysts was performed to quantify the surface atomic percentage of Pd and identify Pd electronic states in the catalysts. A representative XPS survey of Pd/C-e (PVA/Pd weight ratio = 0.3) is presented in Figure S2. XPS spectra of Pd(3d) of the as-synthesised catalysts (when varying NaBH\({}_{4}\)/Pd molar ratio) are presented in Figure 1. Pd atomic content, percentage of Pd\({}^{0}\) and mean particle size are shown in Table 1. The presence of two peaks corresponding to the Pd 3d\({}_{5/2}\) and 3d\({}_{3/2}\) transitions was observed. Each peak can be deconvoluted into two different peaks according to its oxidation number. In the following section, we will refer to the Pd 3d\({}_{5/2}\) components when discussing peak positions and assignments. The peak at 335 eV approximately was assigned to the presence of metallic Pd [42] and the peak at approximately 337 eV, to the presence of Pd\({}^{\text{II}}\) species, mainly present as PdO nanoparticles [43]. Table 1 and Figure 1 show the surface atomic percentage of Pd (exposure of Pd on the surface) for both activated charcoal and titania supported Pd catalysts. It has been shown from previous catalytic studies that the amount of metal on the surface can significantly influence catalytic performance [44; 45].