Thermal Deactivation

During normal city driving, exhaust temperatures are normally below 600° C. However, overheating of catalyst may occur due to engine malfunction such as ignition failure, misfire or excessively rich operation. The very high concentration of unburned hydrocarbons in exhaust when oxidized in converter leads to excessively high catalyst bed temperatures. When the catalyst is exposed to temperatures above 900-1000 °C, loss in catalyst surface area and loss in dispersion of catalyst particles due to sintering are obtained. Ignition failure for about 20 seconds may completely destroy the catalyst.

Sintering caused by high temperatures results in;

• Under high temperatures, the catalyst particles migrate, coalesce or atoms in vapour phase get transported from smaller particles to larger particles. These process form large particles by combination of several small catalyst particles. Thermal sintering of the catalyst particles reduces catalyst activity.
• As the temperature increases to 1200° C, -alumina changes to -alumina resulting in washcoat shrinkage, loss of micro-pores and reduction in catalyst surface area by a factor of 10. The catalyst particles are also trapped inside the collapsed pores which then are unable to come into contact with the gases.
• Thermal deactivation increases light off temperature. After ageing at 730° C, the catalyst had a surface area of 21.5 m²/g of washcoat that reduced to 11.4m²/g after ageing at 1000° C. The corresponding light off temperatures were about 250 and 320° C, respectively.