Page 42 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

24 Methods for Monitoring and Diagnosing the Eficiency of Catalytic Converters

Oxygen storage

As shown in fig. 4, a h-sensor is used in the exhaust pipe with a three-way catalytic converter
as a feedback-control of the engine air/hel ratio, In modern engines, an oscillation cycle of the
engine aidfuel ratio around the stoichiometric value (from rich to lean and vice-versa) is
deliberately imposed by the management system of the engine. In such dynamic conditions, the
catalytic converter converts bigger quantities of NO,, CO and HC than when operating under a
steady state condition. At least part of the improved performance is thought to be due to the
ability of the catalyst to undergo reduction-oxidation reactions. Such a catalyst component is
usually referred to as an oxygen storage component.

In its oxidized state, the catalyst can provide oxygen for CO and HC oxidation in a rich
exhaust gas environment, and in the process be reduced. When the exhaust cycles to lean
conditions, this reduced component can adsorb oxygen or NO, (which removes NO, directly
or indirectly by reducing the oxygen concentration). The oxidized component can, in turn,
provide oxygen for CO and HC oxidation in the next rich cycle. Components such as CeO2 or
Re02, which exhibit this reduction-oxidation behavior are included in the washcoat of
commercial three-way catalytic converters.

Studies have shown that oxygen adsorption and desorption phenomena, under periodically
varying inlet conditions, are attributed to the presence of cerium and, to a much lesser extent,
other washcoat materials ([9]). The tinction of cerium as oxygen storage component is based
on its ability to form both 3- and 4-valent oxides. Under net oxidizing conditions the following
Ce oxide reactions may take place.

Under net reducing conditions, CeO2 fhctions as an oxidizing agent according to the
following reactions:

The interaction of Ce oxides with HC is of minor importance. The oxygen storage availability
is apparently a function of the washcoat Ce content and dispersion. Moreover, the stored
oxygen available to react under operating conditions is a function of the local temperature and
reduction-oxidation environment ([9]).