Position dependent phenomena during deactivation of three-way catalytic converters on vehicles

Abstract

Mapping procedures have been developed to characterise some of the position dependent phenomena occurring in three-way catalytic converters. The activity of small samples for the removal of CO, NO, propene and propane from a simulated mixture under slightly lean conditions was measured in a flow system and correlated with surface area. Contamination was determined by particle-induced X-ray emission (PIXE) with XRD used to follow structural changes in the washcoat. The procedures have been used to investigate a substantial set of converters which had seen extensive use on vehicles. Three of these converters, taken from vehicles which had failed a standard emission test, are discussed here. In one case, loss of surface area and CO/hydrocarbon/NO activity was greater at the front and is associated with phosphorus deposition. XRD measurements showed that operating temperatures were sufficiently high to result in the formation of cerium orthophosphate at the front and substantial growth in ceria particle size throughout which also contributed to activity loss. A second converter showed substantial loss of NO activity alone which was traced to high levels of lead, concentrated towards the front but significant throughout. A third converter of the same type had undergone a lesser loss of activity for NO removal at the front due to lead but the deterioration in CO and propene was greater towards the rear of the converter. This was associated with a loss of surface area caused by a period of overheating under net reducing conditions with XRD measurements showing the formation of cerium aluminate and a cerium barium magnesium hexaaluminate which are characteristic of such conditions.