3.2.5. Oxygen storage
In most current technology gasoline cars the signal
of the lambda sensor 41 is used as a feedback control
signal for the fuel injection system in order to ensure
that a stoichiometric fuel-air mixture is supplied in
the cylinders. However, the system's response lag
(mainly attributed to the exhaust gas travel time and
the sensor's response delay) causes the air-to-fuel
ratio to oscillate around the stoichiometric value with
the limit cycle frequency of the control system. In
some modern fuel injection systems, the A/F ratio is
oscillated deliberately by the engine management
electronic control unit. The behavior of the 3WCC
under such dynamic conditions is of high practical
interest.
The conversions of NO, CO and hydrocarbons in a
three-way catalyst, operated with cyclical variations
in the equivalence ratio, are larger than estimates
based on summation of steady-state values during the
cycle. At least part of the improved performance is
thought to be due to the ability of the catalyst to
undergo reduction-oxidation reactions. 42'43 Such a
catalyst component is usually referred to as an oxygen
storage component. In its oxidized state it can provide
oxygen for CO and hydrocarbon oxidation in a rich
exhaust-gas environment, and in the process be
reduced. When the exhaust cycles to lean condi-
tions, this reduced component can adsorb O2 or NO
(which removes NO directly or indirectly by reducing
the 02 concentration). The oxidized component can,
in turn, provide oxygen for CO and HC oxidation in
the next rich cycle. Components such as CeO2 or
ReOE which exhibit this redox behaviour are
included in the washcoat of commercial three-way
catalysts. 44-46
The classical experimental studies of Herz 42
showed that oxygen adsorption and desorption
phenomena, under periodically varying inlet condi-
tions, are attributed to the presence of cerium and, to
a much lesser extent, other washcoat species. The
function 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:
Ce203 + 10 2 ~ 2CeO 2 (1)
Ce20 3 + NO ~ 2CeOE + 1N 2 (2)
Ce203 q- H20 ~ 2CeO2 + H2. (3)
On the other hand the CeO2 may function as an
oxidizing agent of the exhaust-gas species under net
reducing conditions according to the following