Deactivation of three-way and oxidation catalyst dual bed emission control systems: catalyst post mortem analyses from methanol-fueled vehicles

Abstract

Dual bed platinum group metal three-way and oxidation catalysts have been in use in the USA for control of CO/HC/NO emissions from gasoline fuelled vehicles since 1978. Increasing interest in the use of alcohol as an alternative fuel to gasoline, either alone or in combination with gasoline, prompted a study by the University of Santa Clara, California, of emission and engine wear characteristics of a methanol-fueled fleet of Pinto vehicles. These vehicles were equipped with Pt/Rh three-way catalysts and Pt/Pd oxidation catalysts. Post mortem analyses have been conducted on catalysts from two Pintos which showed high emissions after service in the methanol-fueled fleet. The analyses revealed catalyst deactivation characteristics atypical of those experienced with similar gasoline-fueled vehicles and the results described indicate a specific mode of physical deactivation by oil-derived contaminants. The catalyst surface was found to be covered by a glassy layer of low melting amorphous zinc phosphate impervious to gas diffusion. This is associated with the low average running temperature of the methanol fueled vehicles but is expected to be symptomatic of gasoline fueled vehicles where catalyst running temperature is low. Heat treatment of the catalyst at higher temperature led to recovery of NO activity as the zinc phosphate layer was converted to AlPO 4 by interaction with the Al 2O 3 carrier and zinc was removed, probably via volatilization of ZnO. Lead poisoning was also observed to occur despite the fact that the methanol fuel contained zero lead. This was attributed to dissolution of lead terne plate in the fuel tank by methanol.