Environmental Engineering Reference
In-Depth Information
100 %
CO, HC
NO
50 %
0
96 %
100 %
104 %
Air/fuel ratio (% stoichiometric)
Figure 8.10
A diagram of the catalytic efficiency for CO and HC oxidation and NO reduction in a
three-way catalytic converter, as a function of the air/fuel ratio.
surface coated with a catalyst, where they can react and their products evolve into the gas stream.
Furthermore, this surface reaction will only occur quickly if the surface is hot enough and the
proper catalyst is used. Current three-way oxidation-reduction catalysts utilize such catalysts as
platinum and rhodium, and they must be heated to 250
◦
C or more to be effective.
Simultaneous oxidation of CO and HC and reduction of NO in a catalytic converter requires
very close control of the air/fuel ratio in the engine. Insufficient air will inhibit oxidation, whereas too
much will prevent reduction. The window of air/fuel ratio that will remove equally all the pollutants
is quite small, only a few percent change being allowed. This is illustrated in Figure 8.10 showing
how the catalytic efficiency (percent of pollutant removed in the converter) of the oxidation and
reduction reactions depend critically upon the air/fuel ratio. Oxygen accumulation on the catalyst
surface is built into these catalysts to widen the operating window.
For a catalytic converter to work properly, every pollutant molecule must have the chance
to stick to the catalyst surface before it flows through the reactor. This requires that there be a
large surface area coated with catalyst and that the flow passages surrounding this surface be finely
divided. Either a honeycomb structure or a packed bed of catalyst-coated pebbles satisfies this
requirement. Typically, the gas passage dimension is of the order of several millimeters, and the
converter volume is about half the engine displacement. This allows the exhaust gas only about
one engine cycle period to pass through the converter and be cleansed of most of the pollutants.
When an engine is first started at ambient temperature (called a cold start), the converter does
not work until it has been warmed by the hot engine exhaust gas to its “light-off” temperature of
