Exhaust gas treatment unit for the selective catalytic reduction of nitrogen oxides under lean exhaust gas conditions and a process for the treatment of exhaust gases
An exhaust gas treatment unit for the selective catalytic reduction of nitrogen oxides under lean exhaust gas conditions which contains at least one catalyst with catalytically active components for selective catalytic reduction (SCR components). The exhaust gas treatment unit is characterised in that the catalyst also contains, in addition to SCR components, at least one storage component for nitrogen oxides (NOx components).
1. An exhaust gas treatment unit for the selective catalytic reduction of nitrogen oxides under lean exhaust gas conditions comprising an oxidation catalyst for oxidizing nitrogen monoxide contained in the exhaust gas to nitrogen dioxide and downstream thereof one catalyst in the form of a honeycomb structure with a catalytically active component for selective catalytic reduction of nitrogen oxide and at least one storage component for nitrogen oxides, said storage component comprising at least one compound of an element selected from the group consisting of an alkali metal, an alkaline earth metal, and cerium which are able to store nitrogen dioxide by forming nitrates, and wherein said storage component for nitrogen oxides comprises no catalytically active platinum group metals.
2. The exhaust gas treatment unit according to claim 1 , wherein said catalytically active component for selective catalytic reduction of nitrogen oxides is a solid acid system of titanium dioxide and vanadia.
3. The exhaust gas treatment unit according to claim 2 , wherein the solid acid system also contains at least one component selected from the group consisting of tungsten oxide, molybdenum oxide, silicon dioxide, sulfate and zeolites, wherein the zeolites can be present in the acid H form or can be exchanged with metal ions.
4. The exhaust gas treatment unit according to. claim 1 , wherein said catalytically active component for selective catalytic reduction of nitrogen oxides contains at least one zeolite, wherein the zeolite can be present in the acid H form or can be exchanged with metal ions.
5. The exhaust gas treatment unit according to claim 1 , wherein the honeycomb structure of the catalyst is obtained by preparing an extrudable composition comprising the catalytically active component for selective catalytic reduction of nitrogen oxides and the at least one storage component for nitrogen oxides and extruding the extrudable composition to produce the honeycomb structure.
6. The exhaust gas treatment unit according to claim 1 , wherein the honeycomb structure of the catalyst is a full extrudate from said catalytically active component for selective catalytic reduction of nitrogen oxides onto which are applied the nitrogen oxide storage components in the form of a coating.
7. The exhaust gas treatment unit according to claim 1 , wherein the honeycomb structure of the catalyst is obtained by applying said catalytically active component for selective catalytic reduction of nitrogen oxides and the nitrogen oxide storage component in the form of a coating onto an inert carrier structure in the form of a honeycomb monolith.
8. The exhaust gas treatment unit according to claim 7 , wherein said catalytically active component for selective catalytic reduction of nitrogen oxides and the nitrogen oxide storage component are present in two separate layers on the inert carrier structure.
9. The exhaust gas treatment unit according to claim 8 , the layer with the nitrogen oxide storage component is applied directly to the support structure and the layer with the catalytically active component for selective catalytic reduction of nitrogen oxides is on top of the layer with the nitrogen oxide storage component and is in direct contact with the exhaust gas.
10. The exhaust gas treatment unit according to claim 1 , further comprising a hydrolysis catalyst located in the exhaust gas treatment unit, between the oxidation catalyst and the catalyst for selective catalytic reduction.
11. The exhaust gas treatment unit according to claim 1 , further comprising an ammonia barrier catalyst is located in the exhaust gas treatment unit, downstream of the catalyst for selective catalytic reduction.
12. An exhaust gas treatment unit for the selective catalytic reduction of nitrogen oxides under lean exhaust gas conditions, comprising:
(a) an oxidation catalyst for oxidizing nitrogen monoxide in exhaust gas to nitrogen dioxide; and,
(b) downstream of the oxidation catalyst a single catalyst member comprising a honeycomb support and a first layer and a second layer, wherein
(i) the first layer contacts the honeycomb support and comprises one or more storage components for storing nitrogen oxides as nitrates, wherein the one or more storage components consist essentially of one or more compounds of an element selected from the group consisting of an alkali metal, an alkaline earth metal, and cerium; and
(ii) wherein the second layer is in contact with exhaust gas and comprises a catalytically active component for selective catalytic reduction of nitrogen oxide.