New Materials in Environmental Catalysis

Abstract: 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 Pd_0.8Pt_0.2/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 La_1-xSr_xAl_1-2yCu_yRu_yO₃ 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 La_0.45Sr_0.15Ce_0.35Zr_0.05M_1.0 (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 La_0.45Sr_0.15Ce_0.35Zr_0.05Co/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.