Exhaust gases are a complex mixture of many
compounds, but three components predominate:
Carbon monoxide from partially reacted
hydrocarbons
Volatile organic compounds ("VOCs")
from partially reacted hydrocarbons
Oxides of nitrogen ("NOx")
from reactions between atmospheric nitrogen and oxygen
Diesel exhaust in particular also contains
particulate matter, very small solid particles
Types of Catalytic
Converter
The earliest catalytic converters used commercially
on cars were two-way. They oxidised carbon monoxide (CO) and hydrocarbons
("HC") into carbon dioxide and water. This left oxides
of nitrogen ("NOx") untouched.
Three-way converters include a reduction
stage to convert NOx into nitrogen and then a second chamber to
oxidise CO and HC as in the two-way converter.
The three-way + air converter introduces
additional air between the two chambers to improve the efficiency
of the oxidation stage.
Answer
The reaction between nitrogen and oxygen occurs only under
extreme conditions (such as those found in the engine cylinder).
Conditions in the catalytic converter are too mild to oxidise the
nitrogen, even in the presence of a catalyst.
Question
Why doesn't the oxidation stage
re-oxidise nitrogen from the first stage into nitrogen oxides?
