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Oxidation Catalytic Converters

The oxidation catalyst converts CO and HC to CO₂and H₂O at substantially low temperatures and at higher conversion efficiency tan the thermal reactors. The required oxygen for oxidation reactions is made available either by operating engine lean or by injecting secondary air ahead of the catalytic converter when engine is operated rich. Vehicles employing the oxidation catalysts were generally tuned rich for better NO_xcontrol and secondary air injection was employed

The conversion efficiency of a catalytic converter is defined as:

Light-Off Tempersature

The catalytic conversion efficiency depends upon the exhaust temperature and its relationship with temperature is shown on Fig 5. 13. The temperature at which 50% CO or HC conversion is obtained is defined as the light off temperature. The catalyst light off temperature is desired to be as low as possible for good emission reduction during actual vehicle operation. . The light off temperature for a new catalyst varies from about 220° C for CO to 260 - 270° C for HC. With ageing, the catalyst light off temperature increases. For the first generation catalytic converters, t he temperature of operation was in the range of 250 – 600 ºC and the typical gas space velocity during vehicle operation varied from 3 to 30 s -1 .