## 2 Results and Discussion
### TEM Characterization  
The best catalysts for the DS tested herein was Pd/\(\mu\)S-pDVB10. It gave the highest final absolute concentration (yield) in H\({}_{2}\)O\({}_{2}\) and even more so relative to the amount of the active metal. This was the result of its higher selectivity, which was retained for a much longer time in comparison with the other catalysts, and of its stability leading to the longest useful lifetime found in this work. For this reason it was characterized by transmission electron microscopy (TEM), high angular annular dark field scanning electron microscopy (HAADF-STEM) and high resolution transmission electron microscopy (HRTEM) (Figure 5) along with the commercial Pd/C catalyst (Figure 6), which was taken as a benchmark. According to the supplier, this is an egg-shell catalyst and in fact metal nanoparticles (NPs) are clearly visible in its STEM image (Figure 6 top) close to the edge of the grain, but almost none was found close to the center. By contrast, the NP distribution is pretty homogeneous in Pd/\(\mu\)S-pDVB10 (Figure 5, top). The STEM microphotograph of Pd/\(\mu\)S-pDVB10 also highlights the very open pore system of the support, where mesopores down to a few nanometers are also clearly visible. The size distribution analysis, carried out on the TEM microphotographs, showed a broad log-normal particle size distribution for both analysed samples (Figure 7).  
Pd/C is featured by a relatively narrow size distribution centred in the 1-3 nm range, a small fraction of NP up to 6 nm and none above this value. The mean NP size of this catalyst is 2.1 nm. Pd/\(\upmu\)S-pDVB10 has a broader distribution, centered in the 3-6 nm range. The NP larger than 6 nm, topping 21 nm in diameter, are 21% of their total number. The presence of relatively large NPs in Pd/\(\upmu\)S-pDVB10 is compatible with the features of the pore system of the support, because they can also be formed in relatively large mesopores. The HRTEM characterization of the catalysts provided a better insight on the structure of the nanoparticles. Lattice fringe analysis carried out on Pd NPs for Pd/C and Pd/\(\upmu\)S-pDVB10 samples exhibit spots in the FT pattern at 2.2 A and 2.0 A that are ascribed to the spacing of {111} and {200} planes, respectively, of face centered cubic (fcc) structure of metallic  
Figure 6: Representative HAADF-STEM (**top**) and TEM (**bottom, left**) microphotographs of Pd/C and HR-TEM of a Pd NP with corresponding FFT pattern (**bottom, right**).  
Pd [47; 48; 49]. This result shows that nanoparticles are formed by zerovalent palladium, but the presence of a thin layer of PdO at the surface cannot be ruled out.  
Interestingly Pd/C was not more active than Pd/\(\upmu\)S-pDVB10, although its NPs are smaller. The respective apparent rates of H\({}_{2}\) differed from each other by about 10%, but the specific rates were much closer. This suggests that the activation of H\({}_{2}\) was not structure sensitive, hence it did not depend on the morphological features of the metal phase. For this reason, the characterization of the metal phase was limited to these two catalysts only. Moreover, TEM was the only technique applied because CO chemisorption, commonly employed for this kind of investigation, is not suitable for polymer-supported materials [22]. As recalled above, in metal catalysts supported by ion-exchange sulfonated resins the NP can be located into domains of the polymer framework which are accessible only upon swelling. Therefore, gas-solid contact in dry conditions can be prevented to different extents (sometimes even completely, [50]) according to the support physico-chemical features. Even more interestingly, Pd/\(\upmu\)S-pDVB10 was much more selective than Pd/C, which fully supports our hypothesis that the diffusion of H\({}_{2}\)O\({}_{2}\) from Pd/\(\upmu\)S-pDVB10 to the liquid phase is so fast to prevent its transformation into H\({}_{2}\)O over the metal surface.