Performances of Pd Nanoparticles on Different Supports in the Direct Synthesis of H₂O₂ in CO₂-Expanded Methanol

Catalysts based on Pd supported on SiO₂, SBA-15, γ-Al₂O₃ and N-CNT were investigated for the direct H₂O₂ synthesis and decomposition/hydrogenolysis in CO₂-expanded methanol in batch and semi-batch reactors working at room temperature and a pressure of 6.5 bar. The order of reactivity for the synthesis of H₂O₂ per g of Pd or for the rate of H₂O₂ decomposition, or the selectivity to H₂O₂ does not correlate with the acidity of the support. A relation was observed with the Pd particle sizes, i.e. smaller particles lead to a higher specific activity, but the sample supported on carbon nanotube shows an order of magnitude higher specific activity per g of Pd, indicating the presence of an additional factor attributed to the interaction with the carbon support which determines an intrinsic higher reactivity. However, this higher reactivity corresponds also to a higher activity in the consecutive conversion of H₂O₂, probably by hydrogenolysis reaction. A rough relation between activity in H₂O₂ synthesis and in H₂O₂ conversion was observed in the samples with the different supports, indicating the similar nature of the active sites responsible for the two reactions. Finally, the analysis of the recyclability of the catalysts and transmission electron microscopy (TEM) characterization data after the catalytic tests indicate that the use of CO₂-expanded methanol allows improving the catalytic performances in H₂O₂ direct synthesis due to the enhanced solubility of H₂ and O₂, but induces a fast catalyst deactivation due to an enhanced mobility and sintering of the supported Pd particles. The effect is present in all the four type of support used.