4.5 Catalyst deterioration
In the UK, three-way catalysts must currently (1996) meet emission standards for a life of 50,000 miles; however, research efforts and legislation are set to double this requirement in the very near future to the current US standard of 100,000 miles. The catalysts do deactivate with use. Indeed the ability to withstand mild deactivation is built into the design of the catalyst, and into the entire emission control system in the vehicle. This is done by setting up vehicles at efficiencies well above the legal requirements at low mileage, so that as the catalyst slowly deactivates, it will still meet the emission standards.
However, the catalyst may be exposed to conditions that result in more severe deactivation above and beyond that which is ‘allowed for’ in its lifetime. The major causes of deterioration are thermal damage (due to exposure of the catalyst to high temperatures), and poisoning by contaminants in the exhaust (notably phosphorus, lead and sulfur). Research aimed at detecting deterioration, and trying to understand its nature, has included post-mortem examinations of used catalysts, and simulated ageing studies, in which the catalyst is exposed to high temperatures or catalyst poisons.