The results of a development program for Pd-only and Pd/Rh automotive emission control catalysts are discussed. A review of former experiences with Pd-containing systems, especially their poor tolerance against lead and sulfur poisoning is given. Model gas experiments were conducted with improved Pd-based catalysts. Not only the activity to convert simultaneously CO, NOx and hydrocarbons (HC) was investigated, but also the formation of so-called secondary emissions such as N2O or NH3. The catalyst development was supported by XPS-measurements paying special attention to the oxidation state, spread and the dispersion of Pd. Promising Pd and Pd/Rh-catalyst candidates were tested fresh and after engine aging on engine- and vehicle dynamometers. The results are compared to Pt/Rh-catalysts. For Pd-only catalysts the ratio of catalyst volume to vehicle mass seems to be one important factor to guarantee high conversion levels after aging. The Pd/Rh-technologies reported in this study showed similar activities on different vehicles and in different test cycles as Pt/Rh catalysts. With the new developed Pd/Rh-catalyst the European legislation for model year 1996 could be achieved thus representing an economical and technical alternative to Pt/Rh-catalysts. Major potential for further improving the Pd/Rh-technologies performance must be seen in an optimization work regarding fuel quality and A/F-management.

This paper reports first results of research and development work to achieve nitrogen oxide reduction under lean diesel exhaust gas conditions. Much attention is paid to the influence of operation conditions on catalyst performance. A major part of the paper deals with the influence of the hydrocarbon component, the hydrocarbon concentration and the HC/NO ratio on the activity of a special developed platinum based catalyst. Other aspects discussed are a spectroscopic characterization and a selectivity study. A hypothesis of a “dual-site” reaction mechanism for NOx-reduction in lean diesel exhaust gas precious metal based catalyst is established. Finally, first promising results on the performance of the catalyst system in a vehicle dynamometer test are given.