New Materials in Environmental Catalysis

Doktorsavhandling, 1995

This thesis presents new approaches to the control of emissions within the field of environmental catalysis. Three catalytic systems; bimetallic catalysts, perovskites and promoted catalysts have been prepared and evaluated for complete oxidation of volatile organic compounds and for simultaneous conversion of CO, hydrocarbons and nitrogen oxides. The activities for complete oxidation of CO and hydrocarbons over coimpregnated Pd-Pt catalysts did not vary linearly with the Pd:Pt-ratio. This non-linear variation strongly implies that bimetallic Pd-Pt clusters are formed in such catalysts. Maxima in activity for oxidation of xylene and CO were reached over Pd0.8Pt0.2/Al2O3. The activity increased with increasing total noble metal content, decreasing reactant concentration and increasing oxygen concentration. Catalysts with supports of .gamma.-Al2O3, hydrothermally treated (814 °C, 100% steam) prior to impregnation with noble metal complexes, exhibited higher activity for oxidation of xylene than those having thermally (550 °C, air) treated supports. Using a technique with repeated impregnations and subsequent calcinations at 900 °C, a barrier layer, which acted as a precursor or support for perovskite formation, was created on alumina when impregnated with La3+, Sr2+, Cu2+ and Ru3+. A perovskite phase of the nominal composition La1-xSrxAl1-2yCuyRuyO3 was formed in increasing amount with increasing content of Sr and Ru in the stock solutions used. The presence of Sr and Ru seemed to retard formation of La2CuO4 and promote formation of the perovskite phase. Cu was well dispersed over the entire support while Ru was found only in the grains of the perovskite phase. The perovskite phase seemed to take an active part in the simultaneous conversion of NO, CO and C3H6. Promoting complex metal oxides based on Cu or Co, having the overall metal composition La0.45Sr0.15Ce0.35Zr0.05M1.0 (M=Cu or Co), with small amounts of Pt-Rh, yielded high activity for the simultaneous conversion of CO, C3H6 and NO. Addition of 4.4 micromol Pt-Rh to La0.45Sr0.15Ce0.35Zr0.05Co/Al2O3 yielded a three-way activity quite similar to that obtained over a commercial three-way catalyst containing more than four times as much Pt-Rh. Pd/Al2O3 exhibited three-way activity in a narrow interval around the stoichiometric balance between oxidizing and reducing gases. Addition of La or Co to Pd broadened this interval under net reducing conditions. The conversions of CO and C3H6 started at about 100 °C lower temperatures over Pd/Co/La/Bl2O3 compared with Pd/La/Al2O3. The conversion of NO under transient conditions was markedly increased over Co-promoted Pd compared with unpromoted Pd.