Are catalytic converters good for the environment?
Sorcha McCloskey, Loreto, Coleraine: 13B Key Skill Task (Feb. 2003)

Carbon monoxide is a toxic gas, which reduces the ability of haemoglobin, the oxygen carrying molecule in the blood, to transport oxygen.  Instead of the formation of oxyhaemoglobin, the haemoglobin in red blood cells reacts favourably with carbon monoxide to produce carboxyhaemoglobin.  The inhalation of carbon monoxide is constant and the gas does have a cumulative effect.  “Mild symptoms of carbon monoxide poisoning include headaches, dizziness and tiredness.  Severe symptoms are fainting and a possible fatal coma”[1].  In most countries around the world, especially in built up or urban areas, carbon monoxide levels surpass regulations put in place by the World Health Organisation, most of this carbon monoxide is produced by vehicles.  However, catalysts can be used to limit the amount of carbon monoxide produced by car engines.  Platinum, a transition metal is responsible for the catalysation of carbon monoxide to carbon dioxide.  While carbon dioxide does contribute to the greenhouse effect, it is less dangerous to human life than carbon monoxide.  The oxidation follows this reaction:

2CO(g)         +        0₂(g)              =            2CO₂(g)

A catalytic converter can then increase the rate of this reaction, it is positioned beside the engine to ensure that it maintains a high temperature; the platinum will not achieve the activation energy (E_A) for this reaction to proceed until temperatures in excess of 240^oC are reached.  If another transition metal, rhodium is mixed with the platinum, the temperature required can be brought down to 150^oC.  “The mixture of platinum and rhodium is known as a three-way catalyst, because in addition to catalysing the oxidation of carbon dioxide, it catalyses two other reactions involving emission pollutants.”[2]

Approximately 1-2g of platinum and rhodium are used but due to the honeycomb structure of a catalytic converter (Figure 1), the surface area of the catalyst becomes vast.  The gases enter the honeycomb filter and the catalysts remove the pollutants that would otherwise be emitted by reacting with the gases.        

Figure 1 This is the honeycomb structure on which the catalyst is placed.	Figure 2 This is an artist’s impression of how an actual catalytic converter looks

If there is insufficient oxygen present in a petrol engine, unburnt hydrocarbons will be left as exhaust emissions along with the carbon monoxide.  The platinum in a converter will catalyse the oxidation of C_xH_y (any hydrocarbon), producing carbon dioxide and water, the products of any complete combustion.  This is the second reaction, which eliminates pollutants in a three-way catalytic converter:

 C_xH_y   +   (2x+y)O₂           =         xCO₂   +   y/2H₂O

The catalyst needs to be warm for this to occur and this is why it is during cold starts that the majority of hydrocarbons are emitted.  A certain amount of oxygen must also be provided to oxidise both the carbon monoxide and the unburnt hydrocarbons.  To measure effectively oxygen concentration in the engine, a sensor is fitted for this purpose, which is connected to the vehicle’s fuel injection system.

78% of air is made up of nitrogen, which due to the triple bond contained within its molecule, is considered to be unreactive.  However, when high temperatures like those in a petrol engine are achieved, enough energy can be gained to split the nitrogen bond.  The nitrogen produced will react with oxygen present to form nitrogen oxides, most commonly nitrogen monoxide; NO.  Nitrogen monoxide is colourless, but it can be further oxidised to nitrogen dioxide, which is brown.  In certain areas this can lead to a build up of brown haze, smog.

Nitrogen dioxide also contributes to the formation of acid rain, when it reacts with water to form nitrous and nitric acids.  Nitrogen dioxide is also responsible for catalysing the oxidation of sulphur dioxide in the atmosphere, which is another reason why emissions of nitrogen dioxide should be curbed.  Bronchitis, a respiratory disease can also be linked to nitrogen dioxide.

The third reaction, which occurs in a three-way catalytic converter is reducing nitrogen oxides back to nitrogen and oxygen, this is where the presence of rhodium as a catalyst is necessary:

2NO(g)                =           N₂(g)   +   O₂(g)

If too much oxygen passes from the engine to the catalytic converter, the efficiency of this reaction decreases, so oxygen levels must be closely monitored.

Catalytic converters were first introduced in America in the late 1970s and have been reducing smog for the last twenty years, but currently research is underway by the Environmental Protection Agency (E.P.A.) in the United States in response to fears that catalytic converters may actually rearrange nitrogen-oxygen compounds and form harmful compounds like laughing gas, nitrous oxide.  Nitrous oxide is especially dangerous with respect to global warming as it is “…300 times more potent than carbon dioxide…”[1].  A study by the E.P.A. has also shown that unlike most gases, the emissions of nitrous oxide are increasing rapidly each year.  “The increase in nitrous oxide…stems from the growth in the number of miles travelled by cars that have catalytic converters.”[2]  As the efficiency of catalytic converters in dealing with smog increase, the production of nitrous oxide increases.

Though nitrous oxide evidently leads to global warming, there have been no laws ratified to actually lower the emission of greenhouse gases, thus nitrous oxide is not seen as a pollutant.  No one is sure either of how much nitrous oxide is actually produced by a catalytic converter, however one study has calculated that “…a car with a fuel economy of about 19 miles a gallon would produce .27 grams of nitrous oxide per mile.”[3]  It has also been calculated that since the extinction of older cars without catalytic converters, the production of nitrous oxide has increased by nearly 50%.

At the moment, the only way to rectify the production of nitrous oxide is by adjusting catalytic converters in future models.  The gas is supposedly produced when the catalytic converter warms up and if this could be achieved quicker, then nitrous oxide emission would be reduced.

Presently, there seems little consensus as to what should happen next, we cannot continue to use catalytic converters to reduce smog, while at the same time producing another harmful gas.  If we do not resolve this problem now, then in the future it will become a major crisis and as catalytic converters will be installed on every car, so that we will have to wait for the natural wastage of the cars to occur before we can reduce the effects of catalytic converters.  As shown already, catalytic converters have definitely helped in the implementing of the 1970 Clean Air Act, but if they are to still be called “…a miracle…”[4] then we must ensure no more nitrous oxide is released and that in solving this problem, we do not create another.

“You’ve got people trying to solve one problem, and as is not uncommon, they’ve created another.”[5]
 Sorcha McCloskey Loreto, Coleraine: 13B Feb 2003.