The direct formation of H 2O 2 from H 2 and O 2 over colloidal palladium

Abstract

Additional evidence is presented for the catalytic role of aqueous colloidal palladium in the direct conversion of H 2 and O 2 to H 2O 2. Reactions typically were carried out at an O 2/H 2 ratio of 2, and 25 °C, by introducing the gases at 1 atm through a frit into the aqueous slurry. The source of palladium was either PdCl 2 or reduced palladium supported on silica gel. During the course of the reaction the palladium is distributed among PdCl 4 2−, colloidal palladium, palladium deposited on the frit, and Pd/SiO 2 when SiO 2 is present. Although the amount of colloidal palladium differs, depending on its source and the time on stream, the rate of H 2O 2 formation is proportional to the amount of colloid present at a particular time. Maximum rates were observed for colloids prepared from Pd/SiO 2; however, the largest H 2O 2 concentration of 0.7 wt% was attained when PdCl 2 was the source of colloidal palladium. The ultimate steady-state concentration of H 2O 2 is, in part, affected by the rate of H 2O 2 decomposition, which is relatively large over the Pd/SiO 2 sample that was tested. The rate of decomposition also is determined by the H + concentration. Results obtained with a mixture of 16O 2/ 18O 2 confirm that oxygen remains undissociated during the formation of H 2O 2, which may explain why palladium is uniquely suited as a catalyst for this reaction.