Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2001-05-25
2003-05-27
Dunn, Tom (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S239100, C502S064000, C502S077000, C502S071000, C502S079000, C502S350000
Reexamination Certificate
active
06569394
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a catalyst body for breaking down nitrogen oxides in the presence of a reducing agent. The catalyst body has an active material that contains a zeolite and titanium dioxide. The invention also relates to a process for breaking down nitrogen oxides in a gas stream, a gas stream that contains nitrogen oxides being passed over the catalyst body. In particular, at the catalyst body the nitrogen oxides, with the aid of the reducing agent and in the presence of oxygen, are converted into molecular nitrogen and water using a selective catalytic reduction (SRC) process.
A catalyst body of the type described in the introduction is known from published British Patent Application GB 2 193 655 A. The active material of the catalyst body described in that document contains a titanium dioxide with a small specific surface area and a zeolite that is obtained by ion exchange and contains copper. The zeolite has a mean pore diameter of 10 Å or less and a molar ratio of silicon oxide to aluminum oxide of 10 or more. The catalyst body described has a high mechanical strength and a good resistance, in terms of its catalytic activity, to volatile catalyst poisons, such as arsenic, selenium or tellurium. Mordenite, ZSM-5 and ferrierite are described as preferred zeolites.
Furthermore, Published, European Patent Application EP 0 393 917 A2 discloses a catalyst body for breaking down nitrogen oxides, the active material of which contains a zeolite which, after ion exchange, contains copper and/or iron. The zeolite has a molar ratio of silicon oxide to aluminum oxide of at least 10 and a pore structure in which channels in all three spatial directions have a diameter of at least 7 Å. The catalyst body is supposed to be suitable for breaking down the nitrogen oxides in a temperature range from 250 to 600° C. USY (Ultra-Stabilized Y), Beta and ZSM-20 are described as preferred zeolites.
By contrast, conventional catalyst bodies with an active material which contains titanium dioxide and additions of vanadium oxide, tungsten oxide and/or molybdenum oxide are only suitable for breaking down nitrogen oxides up to a temperature of approximately 450° C. Since exhaust gases from a combustion installation, such as for example a fossil-fired power plant, a gas turbine or an internal-combustion engine, which contain nitrogen oxides regularly reach temperatures of up to 500° C. and above, the catalyst body described in Published, European Patent Application EP 0 393 917 A2 offers a considerable advantage.
U.S. Pat. No. 5,271,913 discloses a catalyst body for breaking down nitrogen oxides, the active material of which body contains a zeolite. The zeolite is in this case impregnated with cerium oxide or an iron oxide. The catalyst body is suitable for breaking down the nitrogen oxides using the selective catalytic reduction process in a temperature range from 500 to 700° C. Furthermore, the catalyst body described has a high stability with regard to sulfur components contained in the exhaust gas. A zeolite of the ZSM-5 type is described as a preferred zeolite, the molar ratio of silicon oxide to aluminum oxide being 20 or more.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a catalyst body and a process for breaking down nitrogen oxides that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which is still suitable for breaking down nitrogen oxides in the presence of a reducing agent even in a temperature range from 400 to 750° C. For this purpose, the catalyst body is to have both a sufficient mechanical stability and a sufficient catalytic stability. A further object of the invention is to describe a process for breaking down nitrogen oxides in a gas stream, with which it is possible to effectively lower the level of nitrogen oxides even at gas temperatures of between 400 and 750° C.
With the foregoing and other objects in view there is provided, in accordance with the invention, a catalyst body for breaking down nitrogen oxide in a presence of a reducing agent. The catalyst body contains an active material having a hydrogen-ion-exchanged, acid zeolite and an active component. The active material contains 40-60% by weight of the zeolite and 40-60% by weight of the active component. The active component contains 70-95% by weight of titanium dioxide, 2-30% by weight of tungsten trioxide, 0.1-10% by weight of aluminum oxide and 0.1-10% by weight of silicon dioxide.
The first object is achieved by a catalyst body having an active material that contains a zeolite and titanium dioxide, according to the invention, by the fact that the zeolite is a hydrogen-ion-exchanged, acid zeolite.
The term hydrogen-ion-exchanged, acid zeolite is understood as being a zeolite in which the exchangeable cations have been predominantly exchanged for hydrogen ions. This can take place, for example, by thermal conversion of ammonium (NH
4
+
) ions which are contained in synthetic zeolites, by hydrogen ion exchange or by hydrolysis of a multiply charged cation-containing zeolite during a dehydration. In this context, reference is made in particular to Kirk-Othmer, “Encyclopedia of Chemical Technology”, 3rd Edition, Volume 15, John Wiley & Sons, New York, 1981, pages 640 to 669.
Unlike in the prior art, it is not necessary, in the catalyst body according to the invention, for the zeolite of the active material to be a metal-cation-exchanged, i.e. for the exchangeable cations of the zeolite to be exchanged for metal cations, for example of copper or iron.
It should be noted that the term zeolite is understood as meaning a framework aluminosilicate in which the ratio of the oxygen atoms to the sum of the aluminum and silicon atoms is 2:1. As a result of some silicon atoms of oxidation state IV being exchanged for aluminum atoms of oxidation state III, the framework or the framework structure overall acquires a negative charge. The negative charge is compensated for by cations that are in the framework structure. The cations are what are known as exchangeable cations that can readily be replaced by other cations, in particular metal cations, by ion exchange.
A zeolite is also distinguished by the fact that the framework structure has continuous pores with a characteristic pore width.
Zeolites are classified on the basis of the molar ratio of silicon oxide to aluminum oxide or according to the characteristic framework structure resulting from the molar ratio. For classification purposes, reference is made to the article “Chemical Nomenclature and Formulation of Compositions of Synthetic and Natural Zeolites” by R. M. Barrer, Pure Appl. Chem. 51 (1979), pages 1091 to 1100.
An example of a natural zeolite is mordenite or a chabazite. Examples of synthetic zeolites are A, X and Y zeolites, which represent synthetic forms of mordenite, a ZSM-5 zeolite (ZSM-5 being a trademark for a synthetic zeolite produced by Mobil Oil Company Ltd.), an USY (Ultra-Stabilized Y) zeolite or a Beta zeolite. With regard to the structure of mordenite, of ZSM-5 zeolite and of Y zeolite, reference is also made to the specialist article titled “Acidität der Lewis-Zentren in Zeolith-Katalysatoren—NO als Sondenmolekül” [“Acidity of the Lewis Centers in Zeolite Catalysts—NO as Probe Molecule], by Frank O. Witzel, Fortschrittberichte VDI, Series 3: Verfahrenstechnik, No. 292, 1992.
Extensive tests have shown that a catalyst body with an active material which contains titanium dioxide and a hydrogen-ion-exchanged, acid zeolite is suitable for catalytic reduction of the nitrogen oxides using the SCR process up to temperatures of 750° C. This is because a catalyst body of this type on the one hand is catalytically active up to these high temperatures and on the other hand has the requisite temperature stability. In addition, the catalyst body has a high degree of stability with respect to moisture and a high resistance to sulfur-containing components in an exhaust gas that is to be treated.
T
Fischer Stefan
Pajonk Günther
Witzel Frank
Dunn Tom
Ildebrando Christina
Locher Ralph E.
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