Catalyst where high heat resistance is required [17]

Catalyst support/ substrate

 

The catalytic substrate core is made
up of metallic foil or ceramic monolith. Metal foil monoliths are usually thin
when compared to the ceramic substrate The substrate usually consists of a
honeycomb ceramic block through which the exhaust gas flows. In most
converters, the ceramic is a single honeycomb structure with many flow
passages.

 

The passages comprise of many shapes,
including square, triangular, hexagonal and sinusoidal. The ceramic monolith
which offers various advantages. Among these advantages are smaller volumes,
lower mass and greater ease of packaging. 18

 

Metal foil monoliths are made up of ferritic
iron- chromium- aluminum alloys of high thermal durability and can be used in
the case where high heat resistance is required 17 . These foils are corrugated to have a
continuous honeycomb structure with parallel square channels of elliptical
cross section and welded together to provide good durability and resistance to
thermal shock.

 

Washcoat (WC)

 

Wash coat is typically made of oxides of Al,
Ce, Zr, etc. High wash coat surface enables a high conversion despite low
residence times.  i.e. The catalytic
materials are suspended in the washcoat before they are applied to the core.
This component produces a high porosity coat and a suitable surface for PGM
with increase in surface area available to react with the engine exhaust 19.

 

Catalyst material

 

The catalyst material used in the automobile
converters is a mixture of precious metals including platinum, palladium and
rhodium. The PGMs have the capacity to convert toxic pollutants into less toxic
ones due to their catalytic activity. Platinum is the most active catalyst, but
unwanted additional reactions and high cost of PGMs limits their wide
application. Platinum and palladium helps in the oxidation of CO, and
hydrocarbons and reduce its toxicity by means of catalytic activity,