AMBIENT-TEMPERATURE LIGHT-OFF FOR AUTOMOBILE EMISSION CONTROL

The time taken for an exhaust emission-control catalyst to reach its o perating temperature for hydrocarbon oxidation is a major barrier to a chieving ultra-low emissions from vehicles. A new approach for achievi ng rapid catalyst light-off from a vehicle cold-start has been devised and demonstrated on an automobile. An important component in the syst em is a Pd-Pt-based catalyst which, under net lean conditions in an ex haust stream containing high levels of CO, is active at ambient temper ature for the highly exothermic CO oxidation reaction. The Pd-Pt catal yst has positive-order kinetics with respect to CO for the CO oxidatio n reaction; hence, increasing the level of CO in the feed leads to inc reasing reaction rates and a faster temperature rise for the catalyst. In practice, this means that enriching the air to fuel mixture suppli ed to the engine at cold-start (with a secondary air source to provide at least the required amount of oxygen for complete CO conversion in the exhaust) enables the catalyst to reach operating temperature withi n seconds of starting the engine. In the present work, this ambient te mperature catalyst is combined with an upstream water trap (zeolite 5 Angstrom) and hydrocarbon trap (zeolites H-ZSM-5 or H-Beta). The traps delay the exposure of the catalyst to these potentially inhibiting sp ecies until it has reached a temperature at which it can effectively c ombust hydrocarbons. When tested fresh, this system demonstrated high levels of hydrocarbon conversion throughout the start-up phase of a Fe deral Test Procedure cycle. (C) 1998 Elsevier Science B.V. All rights reserved.