## 2 Results and Discussion
### Catalytic Runs: Aged Catalysts  
Ageing under different conditions was investigated for Pd/\(\upmu\)S-pDVB35 and Pd/mS-pSDVB10. Figures 8 and 9 respectively illustrate the kinetic plots of consumption (top) and of the H\({}_{2}\)O\({}_{2}\) formation (bottom) over different batches of these two catalysts. For the batch of Pd/\(\upmu\)S-pDVB35 stored under the air and in a closed desiccator, the initial rate of H\({}_{2}\) consumption was decreased by about 17% (the comparison of fresh Pd/\(\upmu\)S-pDVB35 with its aged forms will be hereafter limited to the beginning of the reaction, as it did not produce H\({}_{2}\)O\({}_{2}\) any longer after 90 min). By contrast, the performances were closely comparable for the batches stored under the strictly inert N\({}_{2}\) atmosphere of a dry-box or under the environmental conditions of the laboratory and regenerated with the same treatment employed during the reduction step of its preparation (Table 4). Figure 7: Particle size distribution for Pd/C (122 NPs) and Pd/\(\upmu\)S-pDVB10 (74 NPs). Figure 8: Overall amount of H\({}_{2}\) consumed (**top**, mmol) and of H\({}_{2}\)O\({}_{2}\) formed (**bottom**, mmol) over 100 mg of aged batches of Pd/\(\upmu\)S-pDVB35 as a function of time (lines in the bottom panel are intended to serve only as guides to the eye). In these two cases, the activity was increased by about 24% in comparison with the fresh catalyst. These data show that ageing of Pd/uS-pDVB35 under oxidative conditions should be better avoided for the sake of the preservation of the activity. However, all the aged batches of this catalyst showed a 20-25% lower rate of H\({}_{2}\)O\({}_{2}\) formation in the first 80 min. This leads to a drop of the selectivity, which was relatively moderate (from 43 to 28%) for Pd/uS-pDVB35-2A (aged under air in a closed desiccator), but much higher (from 43 to less than 10%) for Pd/uS-pDVB35-3N (aged under non-oxidative conditions) and Pd/uS-pDVB35-4 AR (regenerated upon reduction). However, Pd/uS-pDVB35-2A turned out to be more stable than fresh Pd/uS-pDVB35: in fact, the final plateau in the concentration makes of H\({}_{2}\)O\({}_{2}\) started after about 180 min (instead of 90 min) and the overall production of H\({}_{2}\)O\({}_{2}\) was doubled in comparison with the fresh catalyst and its other aged batches. This indicates that, although the exposure of the catalyst to an oxidative environment made it less active and selective, it also decreased its rate of deactivation and made its useful lifetime long enough to give a higher final yield in comparison with the fresh catalyst. This is an intriguing finding, because the oxidation state of the catalyst is not a good indicator of the overall production of H\({}_{2}\)O\({}_{2}\). Figure 9: Overall amount of H\({}_{2}\) consumed (**top**, mmol) and of H\({}_{2}\)O\({}_{2}\) formed (**bottom**, mmol) over 100 mg of aged batches of Pd/uS-pSDVB10 as a function of time (the point connected with lines refer to the fresh catalyst). palladium in the catalysts for the DS has been a vividly debated issue since long [2; 14] and our results suggest that conflicting conclusions on its effect could be obtained with the same system depending on the design of the experimental set-up. This serves as an example of how much delicate the comparison of the performances of different catalysts for the DS can be, even when the experimental conditions are apparently similar and well controlled. This is especially true when the catalysts are tested in batch as to provide only'single point' data at a given time. For instance, if the data of H\({}_{2}\)O\({}_{2}\) formation of Figure 8 were compared taking only the 30 min values of conversion and selectivity, it might have been concluded that any ageing process is generally beneficial from the point of view of the selectivity, and that there is no difference between Pd/\(\upmu\)S-pDVB35-2A and the other two aged batches (Pd/\(\upmu\)S-pDVB35-3N, Pd/\(\upmu\)S-pDVB35-4 AR).