SUMMARY OF THE INVENTION
[0013] The above and other objects of the present invention can be achieved by a structured catalyst for selective reduction of nitrogen oxides contained in the lean exhaust gas of a combustion engine by means of ammonia using a compound that can be hydrolyzed to ammonia and that is injected into the exhaust gas stream in the form of an aerosol, where the structured catalyst is in the form of a monolithic catalyst body through which a large number of flow paths for the exhaust gas run from an inlet face to an outlet face. It is a feature of the invention that the catalyst contains a reduction catalyst for selective reduction of nitrogen oxides by ammonia and a hydrolysis catalyst for the hydrolysis of the compound that can be hydrolyzed to ammonia, where the hydrolysis catalyst is applied to the reduction catalyst in the form of a coating.
[0014] A separate hydrolysis catalyst can be omitted through the arrangement of hydrolysis catalyst and SCR reduction catalyst in direct contact with each other in accordance with the invention. A considerably reduced structure length for the overall catalyst system results from this. The advantages of the invention are, however, not limited just to this geometric factor. Rather, synergistic effects that lead to a better catalytic performance compared to separate catalysts result from the close contact between the hydrolysis catalyst and the reduction catalyst.
[0015] In the catalyst in accordance with the invention the reduction catalyst forms the catalyst support for the hydrolysis catalyst. Because of this, a separate catalyst for the hydrolysis catalyst, with its very large thermal mass, is omitted. Thus, after a cold start the catalyst in accordance with the invention reaches its operating temperature, and thus its fall catalytic performance, very much sooner than a separate catalyst.
[0016] For the operation of the catalyst in an exhaust gas treatment system the compound that can be hydrolyzed to ammonia, usually urea, has to be supplied to it in an amount sufficient for reduction of the nitrogen oxides. A sufficient amount of urea is present when the ammonia that is formed from it by hydrolysis is in a mol ratio to the nitrogen oxides contained in the exhaust gas from about 0.8:1 to 1.2:1 (see EP 9 385 164 B1). The urea can be injected into the exhaust gas in upstream from the catalyst in the form of an aqueous urea solution, as a solid or in molten form. Methods for dispensing urea into the exhaust gas stream and control of the amount of urea are known from the prior art and are not an object of this invention.
[0017] Through the known techniques for adding urea to the exhaust gas stream of a vehicle the urea reaches the catalyst in the form of an aerosol. It is absorbed by the hydrolysis layer and hydrolyzed. The resulting ammonia is formed in direct proximity to the reduction catalyst with locally very high partial pressure and is partially consumed on the spot by the catalyst for the reduction of the nitrogen oxides. The unconsumed part passes into the gas phase of the exhaust gas stream and provides the downstream parts of the catalyst with reducing agent. All in all, the catalyst in accordance with the invention enables a considerably better use of the ammonia. Because a large part of the ammonia does not have to pass into the gas phase of the exhaust gas stream in order to develop its reducing effect, the potential ammonia escape is also reduced and the oxidation catalyst that is optionally arranged downstream of the catalyst is thus omitted.
[0018] In many applications the desired effect can be achieved just by coating the face of the body of the reduction catalyst with the hydrolysis catalyst so that the volume of the reduction catalyst is only slightly reduced by the hydrolysis catalyst.
[0019] The overall better utilization of the urea with the catalyst in accordance with the invention also reduces the problem of the possible formation of nitrous oxide (N2O) by nonselective oxidation of ammonia on the reduction catalyst itself or by the connected oxidation catalyst, especially at high exhaust gas temperatures, which comes up with the conventional SCR process.
[0020] Another advantage of the structured catalyst is that hydrolysis and reduction take place at approximately the same temperature, since there is only a minimal temperature difference between the hydrolysis catalyst and the underlying reduction catalyst. Because of this, the catalyst still provides good reduction results even in a case when with separated catalyst systems the temperature at the reduction catalyst would be below the window of activity of the reduction catalyst because of the temperature difference between the hydrolysis catalyst and the reduction catalyst as a consequence of the selected operating point of the engine. In addition, the hydrolysis catalyst also acts as a protective layer for the underlying reduction catalyst, which is sensitive to the deposition of particulate matter and other contaminants from the exhaust gas and can be impaired, while experience shows that such contaminants have hardly any effect on the activity of the hydrolysis catalyst.
[0021] In one particular embodiment of the invention the reduction catalyst can be designed as a so-called complete catalyst, i.e., it consists entirely of catalytically active mass and is obtained, for example, by tabletting or extruding the catalytically active mass or a precursor of it. Preferably, the reduction catalyst is extruded in the form of a honeycomb with parallel channels that form the flow paths for the exhaust gas and are regularly arranged over the cross section of the catalyst body. The flow channels are bounded by channel walls. The hydrolysis catalyst is applied in the form of a coating to the channel walls of the flow channels.
[0022] For coating the catalyst body the components of the hydrolysis catalyst are processed to a coating dispersion with which the flow channels are coated by known techniques. The cell density (number of flow channels per square centimeter) of these complete catalysts is approximately between 20 and 100 cm−2.
[0023] Since the urea is chiefly deposited onto the channel walls in the front part of the honeycomb as the material flows through the honeycomb, it is advantageous to apply the hydrolysis catalyst only in the front part of the reduction catalyst for optimum utilization of it. It is sufficient when the hydrolysis catalyst is applied onto the reduction catalyst only over 10 to 70% of the length of the honeycomb starting from the inlet face.
[0024] In a preferred embodiment of the invention the reduction catalyst and hydrolysis catalyst are applied to an inert support in the form of coatings one on top of the other, where the hydrolysis layer lies on the reduction layer and directly contacts the exhaust gas. All of the known forms of catalyst supports are suitable as inert supports, i.e., honeycombs of metal or ceramic with cell densities with 20 to 200 cm−2. Also advantageous are mixing structures as catalyst supports such as, for example, the Sulzer mixer, which simultaneously serve for optimum distribution of the urea aerosol, or the released ammonia, over the catalyst cross section.
[0025] If honeycombs are used as the support, the measures discussed above for matching the active components to the changing concentrations of urea aerosol, ammonia and nitrogen oxides along the honeycomb can be correspondingly used.
[0026] The compositions based on titanium dioxide that are noted in EP 0 385 164 B1 are suitable as SCR catalysts. In particular, the reduction catalyst can contain the following components:
[0027] A) titanium dioxide,
[0028] B) at least one oxide of tungsten, niobium, molybdenum, silicon, boron, aluminum, phosphorus, zirconium, barium, yttrium, lanthanum or cerium or mixed oxides thereof or mixed oxides with titanium dioxide and
[0029] C) at least one oxide of vanadium, iron or copper.
[0030] Preferably the titanium dioxide is 30 to 100 wt % in the form of the anatase modification with a specific surface of greater than 40 m2/g. For stabilization against high temperature stresses it can be stabilized with 1 to 20 wt % tungsten oxide, molybdenum oxide, silicon dioxide or lanthanum oxide.
[0031] Alternatively to the described composition of the reduction catalyst, it can also contain one or more zeolites that are charged or exchanged with copper and/or iron and optionally also with cerium or molybdenum. A mixture of the titanium dioxide-containing composition with the zeolite-containing composition is also possible.
[0032] The compositions known from the prior art are suitable for the hydrolysis and reduction catalysts that are used in the catalysts in accordance with the invention. In particular, titanium dioxide, aluminum oxide, silicon dioxide, zirconium dioxide or their mixed phases and compounds with each other are used as hydrolysis catalysts. Oxides of pentavalent and hexavalent elements like SO3, WO3, Nb2O5and MoO3can be employed as stabilizers and to increase the activity.
[0033] With the reducing activity of the SCR catalyst there is always a potential for the formation of nitrous oxide by oxidation gas by oxidation of the ammonia. This formation is essentially catalyzed by the additional components of the reduction catalyst, especially by vanadium pentoxide. The formation of nitrous oxide increases with increasing exhaust gas temperature and will be the greater, the higher the partial pressure of the ammonia. To reduce the formation of nitrous oxide, one particular embodiment of the catalyst called for the active component vanadium pentoxide to be applied with increasing concentration along the reduction catalyst. In the simplest case a stepwise distribution of concentration is sufficient since this can be produced, for example, by the appropriate impregnation of the reduction catalyst with vanadium pentoxide precursors. Accordingly, a low concentration of vanadium pentoxide is thus called for in the inlet side portion of the catalyst, since here the partial pressure of ammonia and the exhaust gas temperature as well are still high. With increasing distance from the inlet phase the partial pressure of the ammonia decreases because of the reaction with the nitrogen oxides. For this reason the concentration of vanadium pentoxide here can be increased so as to achieve good conversion rates still even with the already reduced partial pressures of the ammonia and nitrogen oxides. The increased concentration of vanadium oxide is preferably applied over a length of 10 to 90% of the honeycomb starting from the outlet face. Advantageously, the two said measures are combined with each other, i.e., the hydrolysis catalyst is applied only in the front region, while the concentration of vanadium pentoxide is increased in the complimentary rear region of the reduction catalyst.