DIESEL OXIDATION CATALYSTS

The diesel oxidation catalyst is also termed as DOC.  The DOC works in a similar fashion as the oxidation catalytic converter in the gasoline engines. However, the performance of DOC differs from oxidation catalysts in gasoline engines  due to;

• Low exhaust gas temperatures
• Presence of particulate matter in the diesel exhaust and
• High fuel sulphur content compared to gasoline

Typically use of DOC depending on engine design and operating conditions results into;

• 30 –80%  conversion of  the gaseous HC
• 40 to 90 %  reduction of CO
• Dry soot does not get oxidized. But, oxidation of 50 to 80% of soluble organic fraction and some PAH occurs that gives 30 to 50 % reduction in total PM emissions.
• Conversion of fuel sulphur to SO₃ and emission of sulphuric acid aerosol.

   The fuel sulphur on combustion gets converted to sulphur dioxide in the combustion chamber. The SO₂ however, gets further oxidized to SO₃ on the catalyst which combines with water vapours to form sulphuric acid aerosols which is also termed as sulphate emissions. Part of SO₃ produced is stored on the catalyst. The reactions involving fuel sulphur proceed on the catalyst as below:

Oxidation of SO₂:

Sulphate storage: