Page 28 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

IO Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

In addition to the noble metals, the alumina washcoat of a three-way catalytic converter also
contains other components like Laz03 andlor BaO, which hnction as catalytic promoters or
stabilizers against aging. La203 and BaO consist 1 to 2% of the washcoat. Ceriicm is normally
present in high quantities in the washcoat in the form of CeOz (10 to 20%) and has multiple
finctions: stabilization of the washcoat layer and improvement of thermal resistance,
enhancement of noble metal catalytic activity, promotion of the water gas shift reaction and an
oxygen storage component. Iridium has remarkable activity for NO, reduction under net
oxidizing conditions but it tends to form volatile oxides. Ruthenium and Nickel have catalytic
properties for NO, reduction, as already mentioned, and nickel is also capable of suppressing
H;?S formation ([9]).
An interesting historical overview of the development of materials for catalytic converters can
be found in [3].

a) catalytic converter with spheres or pellets like support

Insulation

b) catalytic converter with ceramic m
support

Fig. 5 (from [8], p. LXVIJ)

The substrate of a catalyst can be in the form of a metallic honeycomb (fig. 4). This usually
consists of flat and corrugated metal sheets stacked one on top of the other which are fixed
together (e.g. by welding). Other forms of the substrate are: pellets or spheres (fig. Sa) or
ceramic monoliths (fig. 5b).