Chemistry Reference
In-Depth Information
The TWC is a flow-through device consisting of a ceramic or metal substrate
which is coated with an active catalytic layer of precious metals, such as platinum
(Pt), palladium (Pd) and rhodium (Rh). The first two oxidise CO and HC, and Rh is
used to reduce NO
x
to nitrogen. High conversion efficiencies for both reduction and
oxidation of pollutants in a TWC can be only achieved through stoichiometric
combustion (i.e. maintaining the air-to-fuel ratio at the minimum necessary for
complete combustion). A shortage of air (rich fuelling conditions) would make
oxidation impossible, whilst air excess (lean fuelling conditions) would inhibit
the reduction mechanism. Stoichiometry is maintained by means of closed-loop
control, in which the oxygen concentration in the exhaust gas is measured using
a so-called “lambda” sensor. The information from the sensor is fed to the engine
control unit, and the fuel injection system is adjusted to correct the air-to-fuel ratio.
The closed-loop system operates on the basis of fast correction algorithms which
achieve oxygen adjustment in real time. Conversion efficiencies in excess of 90%
can be achieved with a properly functioning closed-loop TWC.
3.2.4 Diesel Oxidation Catalyst
Whilst NO
x
emissions from petrol vehicles can be controlled by catalytic reduction,
this is not very effective under the oxygen-rich conditions of diesel combustion.
A diesel oxidation catalyst (DOC) is similar to a TWC in terms of structure and
configuration but is only capable of oxidation. As the exhaust gases pass through
the catalyst CO, unburnt HC and volatile PM are oxidised. The conversion effi-
ciency is a function of cell size, reactive surface, catalyst load and catalyst temper-
ature, although emissions of CO and HC are typically reduced with an efficiency of
more that 95%.
DOCs offer no NO
x
-reduction capability but can lead to a conversion of NO
to NO
2
in the tailpipe, thereby resulting in an increase in primary NO
2
emissions.
The extent of the conversion depends on the catalyst specification and the exhaust
gas temperature. Typical NO
2
/NO
x
ratios range from ~10% for diesel vehicles
without oxidation after-treatment to more than 50% for DOC- or DPF-equipped
vehicles [
17
].
3.2.5 Selective Catalytic Reduction
Selective catalytic reduction (SCR) is currently the main technology for enabling
diesel vehicles to comply with the latest NO
x
emission standards. SCR systems
became standard in Euro V heavy-duty vehicles (launched in Oct. 2010), and their
use is gradually being extended to light-duty vehicles. The method involves the
introduction of ammonia (NH
3
) into the exhaust stream to chemically reduce NO
x
to nitrogen. Typically, an aqueous solution of urea (CO(NH
2
)
2
) is used as the
reagent, with the ammonia being generated via thermolysis. The urea is fed into
the system in defined doses upstream of the SCR catalyst. An oxidation catalyst
