What Is a Catalytic Converter? Cleaning Exhaust With a Coated Honeycomb

A petrol engine produces three pollutants the device addresses. Carbon monoxide is oxidised to carbon dioxide. Unburnt hydrocarbons are oxidised to carbon dioxide and water. Oxides of nitrogen are reduced back to nitrogen and oxygen. The difficulty is that the first two require oxygen and the third requires its absence, so a single device handling all three must operate where the exhaust contains almost exactly the oxygen needed to burn the fuel and no more, which is a narrow window. That requirement is why the device is called a three-way converter and why it forced a change in how engines are run, since achieving that composition requires measuring the oxygen in the exhaust continuously and adjusting the fuel supply in response, which is what the sensor in the exhaust stream does.

The construction is designed around getting gas into contact with very little metal:

• •A ceramic or metal honeycomb with thousands of narrow parallel channels, giving enormous surface area in a small volume
• •A rough coating on those channels, which increases the effective surface area by a further large factor
• •Platinum, palladium and rhodium dispersed across that coating as tiny particles, using a few grams in total
• •A housing that holds the fragile substrate and survives vibration and thermal cycling
• •An oxygen sensor upstream and generally a second downstream to check the device is working
• •Placement close to the engine, since the reactions require high temperature and the device must heat quickly

The device is durable and has specific vulnerabilities. It does essentially nothing until it reaches operating temperature, generally around three hundred degrees, which means a substantial proportion of the pollution from a journey is emitted in the first minutes, and short journeys are therefore disproportionately polluting. Lead poisons the catalyst permanently, which is why leaded petrol had to be eliminated before these devices could be fitted, and that connection is the reason the two changes happened together. Phosphorus and sulphur from oil and fuel degrade it more slowly. Overheating from engine misfire melts the substrate. Physical damage from impact or from thermal shock cracks it. And the precious metals make used units valuable, which has produced widespread theft from parked vehicles and a substantial recycling industry.

The underlying principle deserves stating, since the device is the most widely encountered example of it. A catalyst provides an alternative route for a reaction that requires less energy to get started, which means the reaction proceeds far faster at a given temperature without the catalyst being consumed, so a small quantity processes an unlimited amount of material. It does not change what the reaction produces at equilibrium or make an impossible reaction possible, since it only affects the rate. On a solid catalyst the reaction happens at the surface, with reactant molecules binding, rearranging and leaving, which is why surface area rather than mass determines the capacity and why the metal is dispersed as tiny particles. The same principle underlies most industrial chemistry, including fertiliser production and petroleum refining, and every enzyme in every living thing.

The device addresses three specific pollutants and the qualifications matter. It does not reduce carbon dioxide, since converting the other gases produces more of it, so the climate effect of burning fuel is untouched. Diesel engines run with excess oxygen and cannot use a three-way device, requiring separate arrangements including particulate filters and a system injecting a urea solution to handle nitrogen oxides, and the difficulty of meeting standards with those systems is the technical background to the emissions testing scandal. Particulates from tyres and brakes are unaffected by anything in the exhaust and are now a substantial share of vehicle-related particle pollution. And the real-world performance of these systems has repeatedly proved worse than test results, which drove the shift towards testing on the road rather than only in laboratories.

Two of the three reactions need oxygen and the third needs its absence, so the exhaust must contain almost exactly the oxygen required to burn the fuel, which is why engines measure it continuously and adjust. A honeycomb coated with a few grams of precious metal supplies the surface. It does nothing cold, so short journeys pollute disproportionately, and lead poisons it permanently.