contd....
The oxides of base metals such as copper, chromium, nickel, cobalt etc. have been studied. The base metal oxides are effective only at higher temperatures. In addition, they sinter and deactivate when subjected to high exhaust gas temperatures experienced at high engine loads. Their conversion efficiency is severely reduced by sulphur dioxide produced by sulphur in fuel. The noble metals platinum (Pt), palladium (Pd) and rhodium (Rh) were found to meet the above mentioned performance requirements. In practice, only the noble metals are used although these are expensive.
Mixtures of noble metals are used to provide higher reactivity and selectivity of conversion. Following are typical formulations;
- Pt : Pd in 2:1 ratio for oxidation catalysts
- (Pt + Pd): Rh in ratio of 5 :1 to 10: 1 for simultaneous oxidation and reduction such as in 3-way catalysts
Palladium has higher specific activity than Pt for oxidation of CO, olefins and methane. For the oxidation of paraffin hydrocarbons Pt is more active than Pd. Platinum has a higher thermal resistance to deactivation. Rhodium is used as a NOx reduction catalyst when simultaneous conversion of CO, HC and NOx is desired as in the 3-way catalytic converters.
The amount of noble metal used typically varies from about 0.8 to 1.8 g/l (25 to 50 g/ft3) of catalytic converter volume. For a passenger car the total amount of noble metal in the converter is typically in the range 0.8 g to 2 g.
The active metal is in a highly dispersed state when impregnated on the surface of the catalyst support. The size of the noble metal particles on the fresh converter is about 50 nm. However, when used the noble metal particles sinter and may grow to a size of around 100 nm.
Catalyst Substrate
The active catalyst material is impregnated on the surface of catalyst substrate or support. The function of catalyst substrate is to provide maximum possible contact of catalyst with reactants. Following are the main requirements of catalyst substrate:
- High surface area per unit volume to keep a small size of the converter
- Support should be compatible with coating of a suitable material (washcoat) to provide high surface area and right size of pores on its surface for good dispersion and high activity of the catalyst.
- Low thermal capacity and efficient heat transfer properties for quick heat-up to working temperatures.
- Ability to withstand high operating temperatures up to around to 1000º C.
- High resistance to thermal shocks that could be caused by sudden heat release when HC from engine misfire get oxidized in the converter.
- Low pressure drop
- Ability to withstand mechanical shocks and vibrations at the operating temperatures under road conditions for long life and durability of 160,000 km and longer
The following types of catalysts supports are used;
- Pellets
- Monolithic supports
- Ceramic monoliths
- Metal monoliths
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