contd..

The catalyst deactivation causes ;

• Increase in light off temperature, and
• Decrease in maximum conversion efficiency

Two types of catalyst of deactivation are encountered in practice:

• Catalyst poisoning
• Thermal deactivation

Catalyst Poisoning

The contaminants can poison the catalyst in the following manner:

• Deposition on the active catalyst sites chemically reacting with catalyst
• Accumulation of the contaminants on the outer surface of the catalyst physically restricting contact of the exhaust gases with the catalyst. This is termed as ‘blanketing effect'.

Lead as tetra ethyl lead was used for many years as antiknock additive in gasoline. Lead oxides and other lead compounds formed during combustion cause very rapid degradation of the catalyst performance. About 10 to 30 percent of the lead in the fuel gets deposited on the catalyst sites and catalyst washcoat. A typical effect of lead on the conversion efficiency of a Platinum/Rhodium 3- way catalyst is shown on Fig. 5.25. A lead deposition of about 0.5 % of catalyst weight causes 50% drop in the conversion efficiency. Now, the gasoline almost all over the world is lead free.

Sulphur naturally occurs and is present in small amounts in petroleum fuels It causes catalyst poisoning, Pd being more sensitive than Pt and Rh.. In a test, after 160,000 km vehicle operation fuel with sulphur of 575 ppm increased the catalyst light off temperature to 299º C from 277 º C with 40 ppm sulphur fuel.

Zinc and phosphorous additives used in lubricating oil get converted to oxides during combustion and form zinc pyrophosphate glaze over large areas of the catalyst surface,

which seals the passage of exhaust gas to the catalyst sites. Silicon coming from contamination of fuel clogs the protective sheath of the sensor restricting the diffusion of gases to the surface of the sensor element. It affects the response of oxygen sensor which adversely affects conversion efficiency of the closed loop controlled three-way catalysts.