As the name implies, catalytic converters contain catalysts. A catalyst is something that participates in the reaction, lowering the activation energy and hence increasing the reaction rate. Catalysts are not consumed in the reaction. (See the tutorial, Drug Strategies to Target HIV: Enzyme Kinetics and Enzyme Inhibitors for a review of how enzymes function as catalysts.)
In car exhaust, incomplete combustion produces carbon monoxide, CO and organic hydrocarbons (VOCs). As explained in Box 1 in the main section of the tutorial, NO is also produced from the reaction of nitrogen and oxygen gas at high temperatures. The catalyst promotes the oxidation of CO and hydrocarbons (see Equations 1 and 2, below), and the reduction of nitrogen oxides (Equation 3, below).
Each of the reactions in Equations 1 - 3 is an oxidation-reduction reaction. In Equation 1, the carbon is being oxidized and the molecular oxygen is being reduced. (Determine the oxidation numbers of the products and the reactants to convince yourself of this.) Which species are being oxidized and which are being reduced in Equations 2 and 3?
Although the reactions in Equations 1 - 3 are spontaneous, they are quite slow in the absence of a catalyst. Recall, that thermodynamics tells us whether a reaction (or process) is spontaneous under a specified set of conditions; thermodynamics does not tell us how fast (or the rate with which) a reaction will proceed. Rates of reactions are the realm of chemical kinetics and are determined experimentally. From Arrhenius Theory, the rate of a reaction is dependent on the activation energy (Ea), which is the minimum amount of energy needed for a reaction to occur (See Figure 1, Path A). Hence, even though a reaction is thermodynamically favorable, it still cannot occur unless there is enough energy available (an amount greater than or equal to the activation energy) to initiate the reaction. A catalyst allows the reaction to proceed by an alternate mechanism that has a lower activation barrier (Figure 1, Path B). When a catalyst changes the reaction pathway to lower the activation energy, the reaction rate is increased, but the thermodynamics of the reaction are not changed. That is, a catalyst cannot form products that are not allowed by thermodynamics (ΔG); however, it does increase the rate of forming the products that are allowed by thermodynamics.
Path A represents a reaction without a catalyst. Path B represents the same reaction, with a catalyst. A catalyst allows the reaction to proceed by an alternate mechanism that has a lower activation barrier. This increases the reaction rate.
Noble metals" such as platinum, palladium and rhodium function as catalysts and increase the rates of the reactions shown in Equations 1 - 3. The catalyst is usually a mixture of at least two metals because one serves as a catalyst for the oxidation reaction and the other serves as a catalyst for the reduction reaction.
Modern catalytic converters are constructed from a tough, heat-resistant ceramic material that is coated with catalysts like the noble metals mentioned above. The function of the ceramic is to provide a large surface area for the catalyst. As the emission gases pass through the catalytic converter, molecules temporarily "stick" to the metal surface and react together. The more surface area that is available, the more opportunities there are for reactions, because the catalyst keeps the molecules near each other to give them time to react. Corning is currently developing ceramics that are strong and durable enough that catalysis "cells" less than 0.02 mm thick can be constructed and packed closely together, resulting in a an enormous catalytic surface area. Research into optimal ratios of metals in catalytic converters continues to improve catalytic performance as well.