The deactivation mechanism of Pd supported on silica and mesoporous silica (SBA-15) using CO2-expanded methanol as solvent was studied in the direct synthesis of H2O2 in batch and semi-continuous batch reactor tests as well as its hydrogenolysis. Fresh and used catalysts were characterized by TPR and CO chemisorption. The results evidence the presence of deactivation, which can be correlated to the loss of accessible active metal surface area due to sintering of Pd, but there is also an effect of the presence of the ordered mesoporous structure and of the reaction conditions. The higher concentration of H-2 in solution in semi-continuous batch reactor tests with respect to batch reactor tests leads to a more relevant deactivation in Pd-SiO2 with respect to Pd-SBA-15, but a higher initial activity, due to the fact that H-2 accelerates the reduction of the Pd species which are less reducible in Pd-SiO2 than in Pd-SBA-15. Pd-SBA-15 shows a higher H2O2 selectivity and productivity with respect to Pd-SiO2 in batch reactor tests, related to the presence of easier reducible Pd species. Another difference is related to the different mechanism of sintering. On the SBA-15 support, due to the presence of the ordered mesoporosity, the Pd particles migrate into the SBA-15 channels forming elongated 1D-type particles. In Pd-SiO2 catalyst, instead, the sintering of the Pd particles leads to large aggregates of Pd particles in the range of 20-25 nm. (C) 2011 Elsevier B.V. All rights reserved.