Hydrogen adsorbed on palladium during water formation studied with palladium membranes

The amount of hydrogen adsorbed on palladium during water formation has bee n studied by measurements of the rate at which hydrogen permeates a palladi um membrane. The water formation and hydrogen permeation rates were measure d simultaneously for palladium membranes exposed to a hydrogen-oxygen-argon mixture on one side and to pure argon on the other side. Investigations we re carried out for the temperatures 100, 150 and 200 degreesC at a total pr essure of 760 Torr. If the rates are plotted as a function of alpha. where alpha = PH2/(PH2 + PO2), PH2 and PO2 are the pressures of hydrogen and oxyg en at the palladium surface, respectively, it is found that, at 100 degrees C, a peak occurs in the water formation rate at alpha (max) approximate to 0.2. The palladium surface is dominated by hydrogen for alpha > alpha (max) and is hydrogen deficient for alpha < <alpha>(max). This is consistent wit h a rate limiting step for the water forming reaction where adsorbed hydrog en is one of the reactants. It is also concluded that the heat of adsorptio n for hydrogen on the palladium surface is significantly lower than previou sly derived from experiments made under ultrahigh vacuum conditions. (C) 20 01 Elsevier Science B.V. All rights reserved.