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Catalytic Converters
05/22/12
The term Catalytic Converter covers the stainless steel box mounted in the
exhaust system. Inside the cover is the catalyst, a ceramic or metallic base
with an active coating incorporating alumina, ceria and other oxides and combinations
of the precious metals platinum, palladium and rhodium. The base can be protected
from vibration and shock by a resilient ceramic or metallic 'mat'
The main by-products of combustion are:
Nitrogen gas (N2): Our atmosphere is 78 percent nitrogen gas, and most of
this passes right through the car engine.
Carbon Dioxide (CO2): A harmless, odorless gas composed of carbon and oxygen.
It is also a greenhouse gas that contributes to global warming.
Water vapor (H2O): Another by-product of combustion.
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The hydrogen in the fuel bonds with the oxygen in the air.
These three emissions are mostly harmless, although carbon dioxide emissions
are believed to contribute to global warming. However since the combustion process
is never perfect, other more harmful emissions are produced in the process.
Carbon monoxide (CO): A colorless, odorless gas. It is poisonous and extremely
dangerous in confined areas, building up slowly to toxic levels without warning
if adequate ventilation is not available.
Hydrocarbons or volatile organic compounds (VOCs): Any chemical compound made
up of hydrogen and carbon.
Oxides of nitrogen (NOx): Chemical compounds of nitrogen, they combine with
hydrocarbons to produce smog.
These are the three main regulated emissions, and also the ones that catalytic
converters are designed to reduce.
Catalytic converters can either be an oxidation or three-way type. Oxidation
catalysts convert carbon monoxide (CO) and hydrocarbons (HC) to carbon dioxide
(CO2) and water, but have little effect on nitrogen oxides (NOx) and particulate
matter. Three-way catalysts operate in a closed-loop system together with a
lambda, or oxygen, sensor to regulate the air/fuel ratio on gasoline engines.
The catalyst can then at the same time oxidize CO and HC to CO2 and water while
reducing NOx to nitrogen.
Most cars today are equipped with a three-way catalytic converter. The term
Three-way refers to the three emissions it helps to reduce, carbon monoxide,
hydrocarbons or volatile organic compounds (VOCs) and NOx molecules. The converter
uses two different types of catalysts, a reduction catalyst and an oxidization
catalyst. Both types consist of a base structure coated with a catalyst such
as platinum, rhodium and/or palladium. The scheme is to create a structure that
exposes the maximum surface area of the catalyst to the exhaust flow, while
also minimizing the amount of catalyst required.
The inside of the catalytic converter is a honeycomb set of passageways or
small ceramic beads coated with catalysts. A chemical reaction takes place to
make the pollutants less harmful. There are many passages for the exhaust gases
to flow, to allow for the maximum amount of surface area for the hot gases to
pass.
In order to reduce emissions, modern car engines carefully control the amount
of fuel they burn. They try to keep the air-to-fuel ratio very close to the
stoichiometric point, which is the calculated ideal ratio of air to fuel. Theoretically,
at this ratio, all of the fuel will be burned using all of the oxygen in the
air. For gasoline engines the stoichiometric ratio is about 14.7:1. This means
that for every pound of gasoline, 14.7 pounds of air will be burned. As engine
and driving conditions change, this ratio changes as well. Sometimes it will
run richer or leaner than the ideal 14.7:1.
The Reduction Catalyst:
The reduction catalyst is the first stage of the catalytic converter. It uses
platinum and rhodium to help reduce the NOx emissions. When an NO or NO2 molecule
contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule
and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms
bond with other nitrogen atoms that are also stuck to the catalyst, forming
N2.
Oxidation Catalysts:
Palladium (Pd) and platinum (Pt) metals in very small amounts convert the
hydrocarbons of unburned gasoline and carbon monoxide to carbon dioxide and
water. This catalyst aids the reaction of the CO and hydrocarbons with the remaining
oxygen in the exhaust gas. But where did this oxygen come from?
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