A catalytic converter can be damaged
by a strong mechanical impact. In such a case, the monolith is crushed or
crumbled and as a result, the back pressure of the exhaust gas is increased. If
the damage is not noticed in time, local overheating can cause thermal
deactivation and finally melting of the honeycomb can occur. Metallic catalysts
can endure mechanical deactivation better than ceramic ones.
There is also a chance that the ceramic material cracks because of
large temperature oscillation or thermal shocks, resulting in loss of active
substances or decreased active area42.
Two types of factors can cause high catalyst temperatures.
The engine operating conditions. The
misfiring, non-combusted fuel-air mixtures undergo catalytic combustion,
which can produce very strong exothermic reactions within the catalyst.
At about 1200°C, the metallic foil
starts to soften and probably shrink. If the catalyst temperature exceeds
1400°C, the foil will melt, after which it is destroyed. At high temperatures,
catalysts may suffer from the loss of active phase through volatilization.
Metal loss through direct volatilization is generally an insignificant route of
the catalyst deactivation.
Further Causes of ageing
Fouling & Coke formation
wash coat on an automotive catalyst has a very high effective area per volume
of material. This type of structure makes them sensitive to fouling. Fouling
covers all phenomena where the surface is covered with a deposit, e.g. with
combustion residues such as soot Substances such as Mg, Zn and P as well as
metal flakes and particles from the engine and exhaust pipe can clog pores and
block active sites, reducing the efficiency of the catalyst. Coke formation is
the most widely known form of fouling. Coke might be so large that carbon
deposits block the internal pores in the catalyst. In many cases, hydrocarbons
and aromatic materials are primarily responsible for coke formation.
Pore blocking is probably one of the most
important mechanisms. Pore blocking is often connected to coke formation, and
when the amount of coke is high on the catalyst’s surface, it may be possible
for the coke itself to block off the pore structure 31,43 .This effect can be reduced by decreasing the
amount of problematic substances in the engine oils and by using more resilient
materials in the engine and exhaust pipe.