Surface and Catalytic Properties of the CuO/Al_2O_3 System as Influenced by Doping with CeO_2 or ZrO_2 and by γ-Irradiation

摘要

The effects of doping the CuO/Al_2O_3 system with CeO_2 or ZrO_2, or alternatively treatment with γ-irradiation, on its surface and catalytic properties were investigated using nitrogen adsorption at-196 ℃, the decomposition of H_2O_2 at 20-40 ℃ and the oxidation of CO by O_2 at 175 ℃. The pure solids were prepared by wet impregnation with copper nitrate dissolved in the least amount of distilled water of finely powdered solid Al(OH)_3 precalcined at 400 ℃, followed by drying the resulting product and subjecting the same to calcination at 500 ℃. The doped solids were prepared by treating Al(OH)_3 precalcined at 400 ℃ with a known amount of dopant, i.e. cerium or zirconyl nitrate dissolved in the least amount of distilled water, prior to impregnation with the copper nitrate solution. The amount of copper oxide thus introduced was fixed at 13.5 wt% while the amounts of dopants were varied between 1 wt% and 10 wt% CeO_2 or ZrO_2. The results obtained indicated that ZrO_2 doping increased the degree of dispersion of the CuO phase, while CeO_2 treatment had the reverse effect. doping the CuO/Al_2O_3 system with CeO_2 or ZrO_2 led to an increase of 15.4% or 8.1%, respectively, in its BET surface area. The catalytic activity of the system towards the decomposition of H_2O_2 decreased on doping with ZrO_2 but increased when CeO_2 was used as a dopant. γ-Irradiation (at 20-160 Mrad) of CuO/Al_2O_3 solids resulted in a measurable and progressive decreases in their catalytic activity towards H_2O_2 decomposition. In CO oxidation with O_2, ZrO_2 treatment of the CuO/Al_2O_3 solids brought about a progressive increase in their catalytic activity with the maximum value (a 31% increase) being observed in the presence of 3 wt% ZrO-2 but then decreasing with further increases in the amount of dopant present until the final value attained with 10 wt% ZrO_2 was smaller than that measured for the pure CuO/Al_2O_3 catalyst sample. In contrast, the addition of the smallest amount of CeO_2 (1 wt%) led to an effective increase of 69% in the catalytic activity of the CuO/Al_2O_3 system towards the O_2 oxidation of CO, which then decreased when further amounts of CeO_2 were added to the system although still exhibiting a catalytic activity greater than that of the undoped catalyst sample. Doping or γ-irradiation of the CuO/Al_2O_3 system had no influence on the activation energy for the decomposition of H_2O_2 in the presence of the resulting solid catalysts although the concentrations of catalytically active sites present on th surfaces of the solids investigated were modified by such treatment.