Catalytic decomposition of H2O2 over a γ-irradiated CuO-ZnO/Al2O3 system

A CuO-ZnO/Al2O3 catalyst sample was prepared by wet impregnation methods usingAl(OH)(3), zinc and copper nitrate solutions followed by drying at 110 degrees C and calcination at 600 degrees C. The nominal molar composition of the resulting material was calculated to be 0.25CuO . 0.03ZnO/Al2O3. Samples of this solid were exposed to varying dosages of gamma-irradiation (20-160 Mrad) and the effect of such treatment on their surface characteristics and catalytic activity investigated using nitrogen adsorption studies at -196 degrees C and studies of the decomposition of H2O2 at 30-50 degrees C. The results obtained indicate that doses of gamma-rays up to 80 Mrad had no significant effect on the specific surface area, S-BET of the supported mixed oxide material although this quantity increased by 20% when the solid was exposed to gamma-irradiation doses of 160 Mrad. In contrast, such treatment brought about a progressive decrease in the catalytic activity of the treated catalyst samples. Thus, the reaction rate constant (k) of the catalyzed reaction measured at 50 degrees C diminished from 8 x 10(-2) min(-1) to 0.3 x 10(-2) min(-1) on exposure of the supported mixed oxide material to a dose of 160 Mrad. What was surprising was that the activation energy (Delta E) of the catalytic reaction decreased as a function of the dose employed whereas it should have been expected to increase in the light of the observed decrease in the catalytic activity. This apparent discrepancy was resolved by recalculating the values of Delta E taking into account any possible changes in the pre-exponential factor of the Arrhenius equation brought about by gamma-irradiation. The observed decrease in the catalytic activity due to treatment with gamma-rays was attributed, mainly, to the enhanced removal of Bronsted acid centres by the action of such irradiation.