The emissions present in the exhaust gas of a motor vehicle can be divided into two groups. Thus, the term “primary emission” refers to pollutant gases which form directly through the combustion process of the fuel in the engine and are already present in the untreated emission before it passes through an exhaust gas treatment system. Secondary emission refers to those pollutant gases which can form as by-products in the exhaust gas treatment system.
The exhaust gas of lean engines comprises, as well as the customary primary emissions of carbon monoxide CO, hydrocarbons HC and nitrogen oxides NOx, a relatively high oxygen content of up to 15% by volume. In the case of diesel engines, there is additional particulate emission in addition to the gaseous primary emissions, which consists predominantly of soot residues, with or without organic agglomerates, and originates from partially incomplete fuel combustion in the cylinder.
In diesel engine applications, the use of specific diesel particulate filters is unavoidable for the removal of the particulate emissions. Furthermore, complying with the emissions limits prescribed by legislation in Europe and the United States requires nitrogen oxide removal from the exhaust gas (“denitrification”). Thus, although carbon monoxide and hydrocarbon pollutant gases from the lean exhaust gas can easily be rendered harmless by oxidation over a suitable oxidation catalyst, the reduction of the nitrogen oxides to nitrogen is much more difficult owing to the high oxygen content of the exhaust gas stream.
Known methods for removing nitrogen oxides from exhaust gases are firstly methods using nitrogen oxide storage catalysts (NSCs) and secondly methods for selective catalytic reduction (SCR) by means of ammonia over a suitable catalyst, SCR catalyst for short.
The cleaning action of nitrogen oxide storage catalysts is based on the nitrogen oxides being stored in a lean operating phase of the engine by the storage material of the storage catalyst, predominantly in the form of nitrates. When the storage capacity of the NSC is exhausted, the catalyst has to be regenerated in a subsequent rich operating phase of the engine. This means that the nitrates formed beforehand are decomposed and the nitrogen oxides released again are reacted with the reducing exhaust gas components over the storage catalyst to give nitrogen, carbon dioxide and water.
Since the implementation of a rich operating phase in diesel engines is not straightforward and the establishment of the rich exhaust gas conditions required for regeneration of the NSC frequently entails auxiliary measures such as fuel post injection into the exhaust gas line, the alternative SCR method is preferably used for denitrification of diesel motor vehicle exhaust gases. In this method, according to the engine design and construction of the exhaust gas system, a distinction is made between “active” and “passive” SCR methods, “passive” SCR methods involving use of ammonia secondary emissions generated deliberately in the exhaust gas system as a reducing agent for denitrification.
For example, U.S. Pat. No. 6,345,496 B1 describes a method for cleaning engine exhaust gases, in which repeatedly alternating lean and rich air/fuel mixtures are established and the exhaust gas thus produced is passed through an exhaust gas system which comprises, on the inflow side, a catalyst which converts NOx to NH3 only under rich exhaust gas conditions, while a further catalyst arranged on the outflow side adsorbs or stores NOx in the lean exhaust gas, and releases it under rich conditions, such that it can react with NH3 generated by the inflow-side catalyst to give nitrogen. As an alternative, according to U.S. Pat. No. 6,345,496 B1, an NH3 adsorption and oxidation catalyst may be arranged on the outflow side, which stores NH3 under rich conditions, desorbs it under lean conditions and oxidizes it with oxygen to give nitrogen and water. Further disclosures of such methods are known. Like the use of the nitrogen oxide storage catalysts, however, such “passive” SCR methods have the disadvantage that one of their essential constituents is the provision of rich exhaust gas conditions, which are generally required for in situ generation of ammonia as a reducing agent.
Compared to this, in “active” SCR methods, the reducing agent is metered into the exhaust gas line from an addition tank carried in the vehicle by means of an injection nozzle. Such a reducing agent used may, apart from ammonia, also be a compound readily decomposable to ammonia, for example urea or ammonium carbamate. Ammonia has to be supplied to the exhaust gas at least in a stoichiometric ratio relative to the nitrogen oxides. Owing to the greatly varying operation conditions of the motor vehicles, the exact metered addition of the ammonia is not straightforward. This leads in some cases to considerable ammonia breakthroughs downstream of the SCR catalyst. To prevent secondary ammonia emission, an oxidation catalyst is usually arranged downstream of the SCR catalyst, which is intended to oxidize ammonia which breaks through to nitrogen. Such a catalyst is referred to hereinafter as an ammonia slip catalyst.
To remove particulate emissions from the exhaust gas of diesel motor vehicles, specific diesel particulate filters are used, which may be provided with an oxidation catalyst-containing coating to improve their properties. Such a coating serves to lower the activation energy for oxygen-based particulate burnoff (soot combustion) and hence to lower the soot ignition temperature on the filter, to improve the passive regeneration performance by oxidation of nitrogen monoxide present in the exhaust gas to nitrogen dioxide, and to suppress breakthroughs of hydrocarbon and carbon monoxide emissions.
If compliance with legal emissions standards requires both denitrification and removal of particulates from the exhaust gas of diesel motor vehicles, the described measures for removing individual pollutant gases are combined in a corresponding conventional exhaust gas system by connection in series. For example, WO 99/39809 describes an exhaust after treatment system wherein an oxidation catalyst for oxidation of NO in NOx to NO2, a particulate filter, a metering unit for a reducing agent and an SCR catalyst follow on each other. To prevent ammonia breakthroughs, an additional ammonia slip catalyst is generally required downstream of the SCR catalyst, and continues the series of catalysts on the outflow side of the SCR catalyst.
In this respect, both synthetic and natural zeolites and their use in promoting certain reactions, including the selective reduction of nitrogen oxides with ammonia in the presence of oxygen, are well known in the art. Zeolites are aluminosilicate crystalline materials having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, may range from about 3 to 10 angstroms in diameter.
EP 1 961 933 A1, for example, relates to a diesel particulate filter for treating exhaust gas comprising a filter body having provided thereon an oxidation catalyst coating, an SCR-active coating, and an ammonia storage material. Among the materials which may be used as the catalytically active component in the SCR reaction, said document mentions the use of zeolites selected from beta zeolite, Y-zeolite, faujasite, mordenite and ZSM-5 which may be exchanged with iron or copper.
EP 1 147 801 A1, on the other hand, relates to a process for reducing nitrogen oxides present in a lean exhaust gas from an internal combustion engine by SCR using ammonia, wherein the reduction catalyst preferably contains ZSM-5 zeolite exchanged with copper or iron. Said document further concerns an SCR catalyst having a honeycomb substrate and deposited thereon a coating containing ZSM-5 zeolite exchanged with iron.
EP 2 123 614 A2 for its part concerns a honeycomb structure containing zeolites and an inorganic binder. In particular, a first zeolite included in said structure is ion-exchange with a metal including Cu, Mn, Ag, and V, and a second zeolite is further included which is exchanged with a metal including Fe, Ti, and Co. Regarding the types of zeolites used for the first and second zeolite, these include zeolite beta, zeolite Y, ferrierite, ZSM-5 zeolite, mordenite, faujasite, zeolite A, and zeolite L.
Finally, U.S. Pat. No. 7,332,148 B2 describes a stabilized aluminosilicate zeolite containing copper or iron, wherein the stabilized zeolite includes ZSM-5, ZSM-8, ZSM-11, ZSM-12, zeolite X, zeolite Y, zeolite beta, mordenite, and erionite.
Accordingly, the prior art relates an awareness of the utility of metal-promoted zeolite catalysts including, among others, iron-promoted and copper-promoted zeolite catalysts, in particular for the selective catalytic reduction of nitrogen oxides with ammonia.
Presently, however, increasingly strict legislature with respect to emissions, and in particular regarding motor vehicle exhaust gas emissions, requires improved catalysts and exhaust treatment systems using such catalysts for the treatment thereof. Thus, exhaust gas emission legislation in the European Union for exhaust gas emission stage Euro 6 now requires reduction of NOx emissions for most passenger cars powered by diesel engines. For this purpose, exhaust gas emissions are tested using the New European Driving Cycle (NEDC), also referred to as the MVEG (Motor Vehicle Emissions Group) cycle, which is laid down in European Union Directive 70/220/EEC. One way of meeting this requirement includes the application of SCR catalyst technology to the exhaust gas systems of the vehicles in question.
As opposed to the old European driving cycle (ECE-15) driving cycle, a particular feature of the NEDC is that it integrates a so-called extra-urban driving cycle, such that testing may better represent the typical usage of a car in Europe, and, accordingly, the typical emission pattern linked thereto. More specifically, in the NEDC, the old European driving cycle ECE-15 is performed in the time period of 0 to 800 seconds, after which the extra-urban driving cycle is conducted in the time period up to 1200 seconds.
It would be desirable to provide an improved catalyst, in particular for use in selective catalytic reduction, wherein said catalyst is, for example, better adapted to the actual emission conditions encountered in motor vehicle use, such as for example those encountered in the NEDC.