Emissions from diesel engines cannot be treated with the three-way catalytic converter because they operate with excess air. The use of a lambda control system to measure the amount of oxygen in the exhaust system is ruled out for technical reasons. So an oxidising catalytic converter is used to reduce hydrocarbons (HC) and carbon monoxide (CO) to carbon dioxide (CO2). It is, however, not suitable for converting nitrogen oxides.

Article source: www.volkswagen.co.uk