Diesel Catalysts: Catalyst Durability

The rugged construction of Nett^® diesel exhaust purifiers, using either metallic or ceramic supports, provides excellent mechanical durability. The catalyst substrate is designed to last for the entire life-span of the engine. Good engine maintenance, however, is necessary to sustain the catalyst activity for similar period of time. The two major causes of catalyst deactivation are (1) high temperature and (2) poisoning.

The Nett^® catalyst may suffer thermal degradation when exposed to temperatures above 650°C (1200°F) for prolonged periods of time. Diesel engines have intrinsically cool exhaust gases and thermal catalyst deterioration is not likely to take place under normal operating conditions. Catalyst overheating may occur only as a result of serious engine malfunction, such as leaking injectors, when unburned fuel is oxidized in the catalyst and excessive temperature rise is observed due to the exothermic reaction.

Several chemical elements have the ability to either selectively poison the catalyst or to mask the catalytic surface, thus, preventing the contact between exhaust gases and active catalyst sites. Substances to be avoided include phosphorus, zinc, heavy metals, lead, arsenic, vanadium, and silicone. Some of these elements (e.g., phosphorus) may be contained as additives in the engine lube oil. Low lube oil consumption and the use of low-phosphorus oils are guidelines for extended catalyst life. Leaks of lube oil into the exhaust system are very detrimental and can irreversibly deactivate the catalyst within a few hours or days.