Technological Control of Emissions Catalytic Converters

Over the last decades, automobile manufacturers have employed several strategies to decrease VOC and NOx emissions from their vehicles and thereby meet governmental standards. One early technique that had some success for NOx control was to lower the temperature of the combustion flame and thereby decrease the rate of creation of thermal nitric. The temperature lowering was achieved by recirculating a fraction of the engine emissions back through the flame, which presumably lowered the flame temperature and hence the production of thermal NO by lowering the concentration of oxygen in combustion.

In recent decades, more complete control of NOx emissions from gasoline-powered cars and trucks has been attempted using catalytic converters placed just ahead of the mufflers in the vehicle's exhaust system. The original two-way converters controlled only carbon-containing gases, including carbon monoxide, CO, by completing their combustion to carbon dioxide. However, by use of a surface impregnated with a platinum-rhodium catalyst, the modern three-way converter changes nitrogen oxides back to elemental nitrogen and oxygen using unburned hydrocarbons and the combustion intermediates CO and Hz as reducing agents:


Write and balance reactions in which NO is converted to N2 by (a) CO and (b) C6H14. [Hint: The other reaction product is C02, plus H20 in the latter case.]

The carbon-containing gases in the exhaust are catalytically oxidized almost completely to C02 and water by the oxygen that is present:

The catalyst is dispersed as very tiny crystallites, initially less than 10 nm in size. An oxygen sensor in the vehicle's exhaust system is monitored by a computer chip that controls the intake air/fuel ratio of the engine to the stoichiometric amount required by the fuel in order to ensure a high level of conversion of the pollutants. The whole process is illustrated in Figure 3-7a. If the air/fuel mix is not very close to the stoichiometric ratio, the warmed catalyst will not be effective for reduction (if there is too much air), causing nitrogen oxides to be emitted into the air, or for oxidation (if there is too little air), causing CO and hydrocarbons to be emitted, as illustrated in Figure .3-7b.


100 90

_ 80



1 70


J. 60

1 50


1 40



g 30









(excess fuel) (excess air)

Air/fuel ratio

(excess fuel) (excess air)

Air/fuel ratio

FIGURE 3-7 (a) Modern catalytic converter for automobiles, with its position in the exhaust system indicated. [Source: L. A. Bloomfield, "Catalytic Converter," Scientific American (February 2000): 108.1 (b) Efficiency in conversion of catalytic converter versus air/fuel ratio. [Source: B. Harrison, "Emission Control/' Education in Chemistry 37 (2000): 127.|

n, reduction

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