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It has long been recognized that the key to achieving stringent emission standards such as ULEV is the control of cold-start hydrocarbons. This paper describes a new approach for achieving excellent cold-start hydrocarbon control. The most important component in the system is a catalyst that is highly active at ambient temperature for the exothermic CO oxidation reaction in an exhaust stream under net lean conditions. This catalyst has positive order kinetics with respect to CO for CO oxidation. Thus, as the concentration of CO in the exhaust is increased, the rate of this reaction is increased, resulting in a faster temperature rise over the catalyst.

Two major deactivation mechanisms of automotive catalysts during road usage are: 1. thermal aging 2. poison accumulation of oil-derived poisons such as zinc and phosphorus. A dynamometer-based aging cycle, incorporating a high-temperature low-poison mode to account for thermal aging followed by a low-temperature high-poison mode to account for poison accumulation, has been developed to allow the examination of catalyst formulations after exposure to both a harsh thermal and chemical aging environment. This type of aging cycle results in dynamometer-aged catalysts that are physically much more similar to road-aged catalysts than thermally-based dynamometer cycles. Using this aging-cycle, Pd-only, Pd-Rh and Pt-Rh light-off catalysts were examined. The Pd-Rh catalyst gave the best overall performance, with equivalent HC light-off performance to the Pd-only catalyst and equivalent NOx performance to the Pt-Rh catalyst.