1. Field of the Invention
The present invention relates to after treatment of exhaust gas from an internal combustion engine in terms of removal or reduction of harmful compounds. More particularly, the invention focus on removal of particulate matter and reduction of nitrogen oxides in engine exhaust from lean burn internal combustion engines, and in particular diesel engines.
2. Description of the Related Art
Lean burn engines are known to be energy efficient, but have the disadvantage of forming particulate matter and nitrogen oxides, which must be removed or at least reduced in the engine exhaust.
To prevent environmental pollution and to fulfil several governmental requirements, modern diesel engines are provided with an exhaust gas cleaning system comprising in series an oxidation catalyst for the removal of volatile organic compounds, a particulate filter for the removal of particulate matter and a catalyst being active in the selective reduction of nitrogen oxides (NOx).
It is also known to integrate the SCR catalyst into the particulate filter.
Selective catalytic reduction of NOx in exhaust gas is usually accomplished by reaction with ammonia introduced as such or as a precursor thereof, which is injected into the exhaust gas upstream of the SCR catalyst for the selective reduction of nitrogen oxides, mainly nitrogen dioxide and nitrogen monoxide (NOx), to nitrogen.
For this purpose numerous catalyst compositions are disclosed in the literature.
Lately, zeolites promoted with copper or iron, have found great interest, particularly for use in automotive application.
Copper containing zeolite catalysts for NH3-SCR applications have shown high activity at low temperature. However, in certain applications the catalyst can be exposed to high temperature excursions in exhaust gases. Furthermore the exhaust gas contains high concentrations of water vapour from the combustion engine, which can deteriorate the zeolite catalyst performance. The hydrothermal stability is often an issue for Cu-based zeolites catalysts as one possible catalyst deactivation mechanism is the degradation of the zeolite framework due to its instability towards hydrothermal conditions, which is furthermore enhanced by the presence of copper.
Deactivation of copper containing zeolite catalysts in NH3-SCR applications is typically caused by degradation of the zeolite framework due to its instability towards hydrothermal conditions, which is furthermore enhanced by the presence of copper. However the stability is especially important for automotive applications in which the catalyst will experience high temperature excursions in an exhaust stream containing water.
Deactivation of the catalyst is in particular a problem in exhaust gas cleaning systems provided with a particulate filter, which must periodically be actively regenerated in order to prevent build up of pressure over the soot laden filter.
Active regeneration is performed by burning of captured soot. The regeneration can be initiated by injection of fuel into the exhaust gas upstream the oxidation catalyst or by electrical heating of the particulate filter.
During the active regeneration exhaust gas temperature at outlet of the filter can reach more than 850° C. and a content of water vapour more than 15% and up to 100% for periods of time between 10 and 15 minutes depending on the amount of soot captured in the filter.