Assignment
During lean-burn operation, it is impossible for the three-way catalytic converter to com- pletely convert all the oxides of nitrogen (NOx) which have been generated during
combustion. In such cases namely, the oxy- gen that is needed for the oxidation of the carbon monoxide and of the hydrocarbons is not split off from the oxides of nitrogen but instead is taken from the high level of residual oxygen in the exhaust gas. The NOx
accumulator catalytic converter reduces the oxides of nitrogen in a different manner.
Design and special coating
The NOxaccumulator-type catalytic con-
verter is similar in design to the conven- tional three-way converter. In addition to the platinum and rhodium coatings, the NOxconverter is provided with special addi-
tives which are capable of accumulating ox- ides of nitrogen. Typical accumulator ma- terials are the oxides of potassium, calcium, strontium, zirconium, lanthanum, and bar- ium.
The coating for NOxaccumulation and for
the 3-way catalytic converter can be applied on a common substrate.
Operating concept
At λ = 1, due to the noble-metal coating the NOxconverter operates the same as a three-
way converter. In lean exhaust gases though it also converts the non-reduced oxides of nitrogen. This conversion is not a continu- ous process as it is with the hydrocarbons and the carbon monoxide, but instead takes place in three distinct phases:
1. NOxaccumulation (storage),
2. NOxrelease, and
3. Conversion.
NOxaccumulation (storage)
On the surface of the platinum coating, the oxides of nitrogen (NOx) are oxidized cat-
alytically to form nitrogen dioxide (NO2).
The NO2then reacts with the special oxides
on the catalyst surface and with oxygen (O2)
to form nitrates. For instance, NO2com-
bines chemically with barium oxide (BaO) to form barium nitrate (NO3)2(Fig. G,
Equation 1). This enables the NOxconverter
to accumulate the oxides of nitrogen which have been generated during engine opera- tion with excess air.
There are two methods in use to determine when the NOxconverter is full and the accu-
mulation phase has finished:
Taking the catalyst temperature into ac- count (Fig. 1, Pos. 4), the model-based method calculates the quantity of stored NOx.
An NOxsensor (6) downstream of the
NOxconverter continually measures the
NOx concentration in the exhaust gas.
NOxremoval and conversion
The more NOxthat is stored, the less the
ability to chemically bind further nitrogens of oxide. This means that regeneration must take place as soon as a given level is ex- ceeded, in other words the accumulated ox- ides of nitrogen must be released and con- verted. To this end, the engine is run briefly in the rich homogeneous mode (λ < 0.8). The processes for releasing the NOx and
converting it to nitrogen and carbon dioxide take place separately from each other. H2,
HC, and CO are used as reducing agents. Re- duction is slowest with HC and most rapid with H2. NOx release takes place as follows,
whereby the following description applies with carbon monoxide (CO) as the reducing agent: The carbon monoxide reduces the ni- trate (e.g. barium nitrate Ba(NO3)2to an ox-
ide (e.g. barium oxide BaO). This leads to the generation of carbon dioxide (CO2) and
nitrogen monoxide (NO) (Fig. G, Equa- tion 2).
Subsequently, using the carbon monoxide (CO), the rhodium coating reduces the NOx
to nitrogen and carbon dioxide (CO2)
(Fig. G, Equation 3).
There are two different methods for deter- mining the end of the NOx-release phase:
The model-based method calculates the quantity of NOxstill held by the con-
verter.
A Lambda oxygen sensor (Fig. 1, Pos. 6) downstream of the converter measures the exhaust-gas oxygen concentration and outputs a voltage jump from “lean” to “rich” when conversion has finished.
Operating temperature and installation point
The NOxconverter’s ability to accumulate/
store NOxis highly dependent upon temper-
ature. Accumulation reaches its maximum
between 300 and 400 °C, which means that the favorable operating-temperature range is much lower than that of the three-way cat- alytic converter. For catalytic emissions con- trol, therefore, two separate catalytic con- verters must be installed - a three-way pre- cat near the engine (Fig. 1, Pos. 3), and an NOxaccumulator-type main converter (5)
remote from the engine (underfloor cat).
Sulphur in the NOxaccumulator-type catalytic converter
The sulphur in gasoline presents the accu- mulator-type catalytic converter with a problem. The sulphur contained in the ex- haust gas reacts with the barium oxide (ac- cumulator material) to form barium sul- phate. The result is that, over time, the amount of accumulator material available for NOxaccumulation diminishes. Barium
sulphate is extremely resistant to high tem- peratures, and for this reason is only de- graded to a slight degree during NOxregen-
eration. When sulphurized gasoline is used therefore, desulphurization must be carried at regular intervals. Here, selective measures are applied to heat the converter to between 600 and 650 °C. For instance, the engine can be run in the “stratified-charge/cat-heating mode”. Rich (λ = 0.95) and lean (λ = 1.05)
Catalytic emissions control NOxaccumulator-type catalytic converter 81
Figure 1
1 Engine with EGR system
2 Lambda oxygen sen- sor upstream of the catalytic converter 3 Three-way catalytic
converter (pre-cat) 4 Temperature sensor 5 NOxaccumulator-
type catalytic con- verter (main cat) 6 Two-step Lambda
oxygen sensor, op- tionally available with integral NOxsensor
Reaction equations for the NOxaccumulation
phase (1), removal phase (2), and conversion phase (3) G (1) 2 BaO + 4 NO2+ O2➞ 2 Ba(NO3)2 (2) Ba(NO3)2+ 3 CO ➞ 3 CO2+ BaO + 2 NO (3) 2 NO + 2 CO ➞ N2 + 2 CO2 1 2 3 4 5 6
Exhaust-gas system with three-way catalyic converter as pre-cat, and downstream NOXaccumulator-type converter
and Lambda oxygen sensors
1
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U
exhaust gases are then passed through the cat one after the other. The barium sulphate reduces to barium oxide as a result.