A comparison of ethanol and water as the liquid phase in the direct formation of H2O2 from H2 and O2 over a palladium catalyst

Ethanol and water have been compared as the media in which hydrogen peroxide is produced from the reaction of H-2 and O-2 over a palladium catalyst. There are significant differences between the reaction in the two media with respect to the net rate of H2O2 formation, the state of the active Pd and the mechanism of the reaction. The reactions were carried out at atmospheric pressure and at 10degreesC, with O-2 and H-2 being introduced in a 4/1 ratio through a glass frit. In ethanol, using 50 mg of 5 wt % Pd/SiO2, 1.8 wt% H2O2 was attained in 7 h; whereas, about half this amount was attained in water. In addition, the net formation rate did not remain constant in water. Both systems were 0.17 N in HCl, which facilitated the formation of colloidal Pd in water but not in ethanol. The loss of activity in water is attributed to the instability of the colloid, which has been shown previously to be the active state of Pd. By contrast, these results show that supported Pd is the active state of the metal in the ethanol system. The mechanism for the formation of the nonselective product, water, also is affected by the media in which the reaction is carried out. In ethanol, water is formed by the direct reaction of H-2 and O-2, while in the aqueous phase, water appears to be formed both by the direct pathway and by the reduction of H2O2.