Fuels and Society The Catalytic Converter

William Kovarik
Radford University
and
Matthew E. Hermes
Kennesaw State University

Fuels and Society C: The Catalytic Converter

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In chemistry we relate macroscopic, observable change to transformations at the molecular level that go unseen through the shorthand of symbolic representation. For the most part, we are comforted by the fact that we can at least observe the macroscopic.

The catalytic converter represents a device from which we can measure successful outcomes. Without the converter, CO and NO levels from exhaust gases are much higher than with the converter in place. Throughout the country jurisdictions require measurement of tailpipe emissions to make certain that pollutants are controlled.

The operators of the gas chromatograph devices that measure our tailpipe emissions cannot see the gases the engine emits, nor of course can they see the molecules as they stream out. But we have developed methods to make measurements on these unseen yet very real gases and represent the compositions of the gases on charts or graphs or displays.

Thus in the very real world, employees are doing chemistry. They are measuring the output of macroscopic chemical reactions and representing the using the charted representations of the reaction products to place a sticker on our cars that lets us drive away for another year!

The Role of the Catalytic Converter in Smog Reduction

To understand this section fully, you need to be familiar with the principles of chemical kinetics, chemical equilibrium, thermochemistry and thermodynamics described in your chemistry text. This section amplifies on those principles and applies them to the design, manufacture and use of fuels and engines.

Catalysis:

We discussed the production of NO in the automobile engine. If you recall, nitrogen and oxygen react to a limited extent at 2400 degrees Celsius and as the gases quicly cool, the amount of NO is fixed at that temperature. When NO is exhausted it is easily oxidized to oxides that catalytically develop ozone with its damaging properties.

But we also recall that a chemical reaction can be catalyzed so that the energy of the transition state is lower than without a catalyst, and the rate can be increased.

We recognize that most of the processes in our bodies are catalytic in nature since the foods we eat are stable in the air but are rapidly converted to usable fuels, waste products, heat and energy in our living systems by catalysts called enzymes.

Catalysis Reduces the Energy Necessary to Get to the Transition State and then to Products

The Nitrogen/Oxygen Equilibrium Constant:

We also saw that as the temperature of a nitrogen/oxygen system was raised, the equilibrium constant for the production of NO was also raised and at lower temperatures, less and less NO would be present. So that at, let us say, 1000 degrees Celsius, we would expect, at equilibrium, that in the reaction:

N₂+ O₂<--> 2NO

The equilibrium amount of NO would be reduced from that at 2400 degrees Celsius. But we made the point that only at the very high temperatures of the engine itself can we get enough energy to reach the transition state for the conversion of nitrogen and oxygen to NO - likewise we cannot reach the transition state for the reverse reaction either at lower temperatures because the transition state is the same forward and backward!

The Converter:

The catalytic converter is an almost mundane component on our automobiles. It is a stainless steel vesse through which the exhaust gases from the engine pass at about 1200 degres Celsius. The converter contains catalysts that were developed in the 1970's to lower the transition state for the nitrogen/oxygen reaction. In the .1-.4sec. that the gases are present in the converter, at least threee quarters of the NO is converted back to the elements nitrogen and oxygen.

(And more than 95% of carbon monoxide and unburned hydrocarbons in the fuel are also oxidized.)

Converter catalysts are transition metals and their oxides suspended on a matrix of ceramic material designed to offer the exhaust gases an enormous surface area as they pass through. The metal/metal oxide catalysts lower the activation energy necessary to get to the transition state and increase the rate of the reverse reaction. This small, unobtrusive piece of equipment is an essential component of the effort to reduce pollution.

College of Science and Mathematics
Kennesaw State University
1000 Chastain Rd.
Kennesaw, GA 30114
770-423-6160

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