WO2000072965A1

WO2000072965A1 - Zeolite catalysts for selective catalytic reduction of nitric oxide by ammonia and method of making

Info

Publication number: WO2000072965A1
Application number: PCT/US2000/014830
Authority: WO
Inventors: Ruiqiang Long; Ralph T. Yang; Kent D. Zammit
Original assignee: The Regents Of The University Of Michigan
Priority date: 1999-05-27
Filing date: 2000-05-26
Publication date: 2000-12-07
Also published as: AU5449400A

Abstract

A catalyst for the selective catalytic reduction of nitrogen oxide compounds with ammonia comprising iron exchanged zeolites. The catalyst may further include promoters ion exchanged, or doped with the zeolites. Preferably the promoter ions are rare earth metals such as cerium.

Description

ZEOLITE CATALYSTS FOR SELECTIVE CATALYTIC REDUCTION OF NITRIC OXIDE BY AMMONIA AND METHOD OF MAKING

RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application,

Serial No. 60/136,626 filed May 27, 1999, the entire disclosure of which is hereby incorporated by reference.

FIELD OF THE INVENTION This invention relates in general to the field of controlling of nitrogen oxide

(NO) emissions from fossil-fuel power plants and engines. More particularly, the invention relates to a catalyst composition for controlling nitrogen oxide emissions from fossil-fuel power plants by selective catalytic reduction (SCR) with ammonia using zeolite catalysts such as Fe-ZSM-5 and Fe-MOR catalysts, and optionally Ce- Fe-ZSM-5 and Ce-Fe-MOR catalysts. The invention also relates to a method of making the same.

BACKGROUND OF THE INVENTION Nitrogen oxides (NO, NO₂, and N₂O, collectively referred to as "NO_x") present in the exhaust gases from combustion of fossil fuels, continues to be a major source for air pollution. Nitrogen oxides contribute to photochemical smog, acid rain, ozone depletion and greenhouse effects. As a major cause of acid rain, nitrogen oxides additionally contribute to the pollution of water and soil. The current technology for reducing nitrogen oxides emissions from power plants is by selective catalytic reduction (SCR) of NO_x (where x is 1 to 2) with ammonia in the presence of oxygen, to form nitrogen and water. The overall reaction is as follows:

4NH₃ + 4NO + O₂ → 4N₂ + 6H₂O Many catalysts have been reported to be active for this reaction, such as vanadia and other transition metal oxides (e.g., V₂O5, CuO, Fe₂O₃, Cr₂O₃, Fe₂O₃- Cr₂O₃, Nb₂O₅, and the like), pillared clays and zeolite-type catalysts. For example, in the SCR reaction, a vanadia catalyst V₂O₅ + WO₃ (or MoO₃) supported on TiO₂ is commonly used as commercial catalysts. The mechanism of the reaction on the vanadia catalysts has been studied extensively and is reasonably understood although several different mechanisms have been proposed. Although the SCR technology based on vanadia catalysts has been commercialized, problems with this approach still remain. For example, the vanadia catalyst promotes high activity for the oxidation of SO₂ to SO₃, promotes formation of N₂O at high temperatures, and is toxic. Hence there are continuing efforts in developing new catalysts. H-zeolites and ion-exchanged molecular sieves have received much attention for selective catalytic reduction of NO_x by both hydrocarbon and ammonia in recent years. H-mordenite (MOR) was found active for SCR with ammonia by Pence and Thomas in "Proceedings of the AEC Pollution Control Conference", CONF-721030, 1972, page 15. Since that time, H-form (proton- form), Cu-exchanged, and Fe- exchanged zeolites (examples of such are: mordenite, Y, and ZSM-5), pillared clays,

MCM-41 (Mobil molecular sieve) and HMS (a derivative of MCM-41) have been investigated by many groups. The inventors first reported that pillared clays were active for the SCR reaction. Many transition metal ions ( such as Cr³⁺ , Cu²⁺, Fe³⁺, Co²⁺, Ni²⁺, and the like) have been exchanged with pillared clays. Among them, the Fe-exchanged pillared clays were found to be the most active catalysts. Moreover, the SCR activity of the Fe-exchanged pillared clays was improved by the presence of H₂O and SO₂, which was attributed to an increase in surface acidity due to sulfation of the catalysts. Fe-exchanged zeolites also showed high activities for the SCR reaction. Komatsu et al. in "Zeolites: A Refined Tool for Designating Catalysis Sites",

Elsevier, 1995, page 437, studied the selective catalytic reduction of NO with NH₃ on a Fe-ZSM-5 catalyst, but the activity was only moderate. The inventors believe that its moderate activity could be related to the low iron content in the Fe-ZSM-5 catalyst because the Fe-ZSM-5 was prepared by exchanging Fe³⁺ with ZSM-5 so that only a low exchange level could be reached. More recently, Ma and Grunert, Chem.

Commun., 1999, vol. 71, reported the preparation of an over-exchanged Fe-ZSM-5 catalyst by sublimation of FeCl₃ vapor into H-ZSM-5, similar to that reported earlier by Chen and Sachtler. The catalyst showed a high SCR activity and was stable in the presence of H₂O and SO₂. However, the maximum NO conversion was still lower than that of over-exchanged Cu-ZSM-5 under the same reaction conditions. The over-exchanged Fe-ZSM-5 also showed a high activity for SCR of NO by iso-butane, reported by Chen and Sachtler, Catal. Today, 1998, Vol. 42, Page 73, earlier. Thus, there is a need in the industry to continue development of catalysts that exhibit improved catalytic activity while minimizing the aforementioned limitations.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an improved catalyst composition for reducing nitrogen oxides emissions.

It is another object of the present invention to provide a method of making an improved catalyst composition for reducing nitrogen oxide emissions. In summary, the present invention provides a catalyst for selective catalytic reduction of nitrogen oxide compounds with ammonia comprising a composition of iron exchanged zeolites, and mixtures thereof. The composition may further include promoter ions exchanged , or doped, with the zeolites. Preferably the promoter ions are rare earth metals such as cerium (Ce). One embodiment of the present invention described herein is a catalyst composition comprising iron (Fe) exchanged zeolite catalysts, in particular Fe-ZSM- 5, Ce-Fe-ZSM-5, Fe-MOR, Ce-Fe-MOR catalysts and mixtures thereof, for the selective catalyst reduction (SCR) of nitric oxide by ammonia. The Fe exchanged zeolite catalyst compositions of the present invention show remarkable activity for the SCR reaction. Of significant advantage, the Fe-ZSM-5, Fe-MOR, Ce-Fe-ZSM-5 and Ce-Fe-MOR catalysts of the present invention show an activity that is much greater, for example up to about 16 times more active, than conventional commercial catalysts. The catalysts of the present invention function over a broader temperature range, and produces only N₂, as opposed to undesirable N₂O, and H₂O. The catalyst composition of the present invention also show substantially lower activity for oxidation of SO₂ to SO₃, a desirable property. Further, the catalyst of the present invention is resistant to SO₂ and H₂O, and is durable.

In another aspect of the present invention, a method of making an iron exchanged zeolite catalyst is provided. In one embodiment of the method of the present invention comprises the steps of providing a zeolite starting material of ZSM-5, MOR and other zeolites and exchanging the zeolite starting material with an iron containing material, such as FeCl₂ and other ferrous salts, and carrying out the exchanging step in air at room temperature. In another embodiment, a rare earth metal may be used as a promoter and in this instance the exchanging step further includes exchanging the zeolite starting material with a rare earth promoter, such as a Ce(NO₃)₃ solution.

BRIEF DESCRIPTION OF THE DRAWINGS Additional objects and advantages of the invention will become apparent in reading the detailed description of the invention and the claims and with reference to the figures, in which: Fig. 1 is a graph of NO_x conversion as a function of temperature showing the catalytic activities for NO_x reduction at certain reaction conditions for various Fe- ZSM-5 catalyst compositions in accordance with the present invention, and compared to a conventional vanadia catalyst.

Fig. 2 is a FT-IR spectra showing ammonia adsorbed on a Fe(58)-ZSM-5 catalyst in accordance with one embodiment of the present invention at various temperatures.

Fig. 3 is a FT-IR spectra showing NO+O₂/He adsorbed on a Fe(58)-ZSM-5 catalyst in accordance with another embodiment of the present invention at various temperatures and then subsequent processing, (a) 200, (b) 300, (c) 300 and (d) 400°C.

Fig. 4 is a FT-IR spectra showing the consumption of NH₄ ⁺ on a Fe(58)- ZSM-5 catalyst at 300 °C in a flow of He containing 1000 ppm NO and 2% O₂ in accordance with another embodiment of the present invention.

Fig. 5 is a graph of NO_x conversion as a function of temperature showing the catalytic activities for selective catalytic reduction of NO_x at certain reaction conditions for various catalyst compositions in accordance with the present invention, and compared to a conventional vanadia catalyst.

Fig. 6 is a graph of NO_x conversion as a function of temperature demonstrating the effect of Si/A ratio on the NO_x conversion of catalyst compositions in accordance with the present invention.

Fig. 7 is a graph of NO_x conversion as a function of temperature illustrating the catalytic performance for selective catalytic reduction of NO by ammonia on various Fe-exchanged molecular sieve catalysts in accordance with another embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT The catalyst composition of the present invention is comprised of iron (Fe)- exchanged zeolite catalysts.

In another embodiment of the present invention, the Fe exchanged zeolite catalysts are further exchanged, or doped, with a rare earth metal, preferably cerium (Ce) and oxides tehreof such as CeO₂. Other suitable rare earth metals are La, Pr and oxides thereof such as La^ and PrOx.

Fe exchanged, and optionally additionally Ce-exchanged, zeolites finding use in the present invention include: ZSM-5, Mordenite (MOR), SAPO, Clinoptilolite, chabazite, ZK-4 and ZK-5, which are all known commercial zeolites. More specifically, the above zeolites having a silicon to aluminum ratio (Si/Al) of about 1 to 100 are included. For purposes of this description, the catalyst compositions of the present invention employ the following catalyst designation: the number in parentheses following the metal atom indicates the metal-exchange level (in percent of ion exchanged) and the number in parenthesis at the end indicates the Si to Al ratio (Si/Al) in the zeolite. Thus, a catalyst designation of Fe(58)-ZSM- 5(10) represents a Fe-exchange level of 58% and a Si/Al of 10. The metal exchange level (or percent of ion exchanges) were determined by neutron activation analysis . Of particular advantage the present invention provides a zeolite catalyst composition of Fe-exchanged ZSM-5 catalyst with a Si/Al ratio of 10, which has shown the highest activity among all known catalysts for SCR of NO by ammonia. The catalyst of the present invention are much more active than the commercial vanadia catalysts and do not exhibit the deficiencies associated with the vanadia catalysts. Preferably, the Fe exchanged and optionally additionally Ce doped, zeolites will have a Si to Al ratio in the range of approximately 1 to 100 , and more preferably in the range of approximately 5 to 40.. Preferably, the catalyst of the present invention will exhibit a metal exchange level (in percent of ion exchanges) in the range of approximately

1% to 300% , more preferably in the range of approximately 20% to 150%, with a metal exchange level of approximately 60% being most preferred. Of particular advantage, the present invention also provides an improved method of making the zeolite catalysts. In one embodiment of the method of the present invention comprises the steps of providing a zeolite starting material of NH₄-ZSM-5 or MOR, and exchanging the zeolite starting material with an iron containing material, such as a FeCl₂ solution and other ferrous salts, and carrying out the exchanging step in air at room temperature. In another embodiment, a rare earth metal may be used as a promoter and in this instance the exchanging step further includes exchanging the zeolite starting material with a rare earth promoter, such as a Ce(NO₃)₃ solution. Other suitable iron containing materials include FeCl₃, FeBr₂, Fe(CH₃COO)₂,

Fe, and the like. Other suitable rare earth metal promoters include Y, La, Pr, Nd, Sm and Tb and oxides thereof, including Y₂O₃, Nd₂O₃, Sm₂O₃, Gd₂O₃ and TbO_x. The iron containing material will preferably have an Fe concentration in the range of about 0.1 to 15 wt. %. In the case where a FeCl₂ solution is used, the Fe concentration is preferably about 1.6 wt. %. When using NH₄-ZSM-5 as the starting material, the Si/Al is preferably about 10.

When employing a promoter such as Ce, the concentration of Fe in the iron containing material may be reduced. In one example, when using a Ce(NO₃)₃ solution as a promoter exchanged with the starting material, and a FeCl₂ solution, the Fe concentration is preferably about 1.14 wt. %.

Also, when employing the promoter, it is preferred that the promoter be exchanged with the starting material first, and then exchange the zeolite starting material with the Fe containing material. The exchanging step is generally carried out for 0.1 to 72 hours. In another embodiment of the present invention, the starting material is H-ZSM-5, preferably having a Si/Al of about 10. In this embodiment, the Fe containing material is a Fe and HC1 solution and the exchanging step is carried out at about 50°C in a flow of helium (He). Preferably the Fe concentration in the Fe containing material is about 2.21 wt. %. Alternatively, H-ZSM-5 may be exchanged with solid FeCl₃ at about 320°C in He. In this instance the Fe concentration in the FeCl₃ solid is about 8.6 wt. %. Additionally, the Ce promoter may also be employed. In this embodiment, the exchanging step is generally carried out for 0.1 to 24 hours. The Fe exchanged zeolite catalysts were prepared by using the above methods (methods 1 and 3 are conventional methods, method 2 (Fe+Hcl) is the method of the present invention) and the catalytic activity was evaluated. The catalytic activity experiments were performed at 1 arm with a conventional fixed- bed, flow reactor connected to a chemiluminescent NO_x analyzer. The reaction products were simultaneously analyzed by gas chromatography (GC). Figure 1 shows the percent of NO_x conversion with different catalysts of the present invention, and compared directly with conventional catalyst (i.e. 4.4% V₂O₅- 8.2% WO₃/TiO₂). These results were obtained under the following reaction conditions: 50mg catalyst, 1 OOppm NO, 1 OOOppm NH₃, 2%O₂, balance helium (He), and at a gas hourly space velocity (GHSV) of 4.6 x 10⁵ 1/h at ambient conditions. Surprisingly high NO_x conversions were obtained based on the Fe-ZSM-5 catalysts of the present invention, over a broad temperature window (375-600°C) and under a high GHSV of 4.6xl0⁵ 1/h. At 400-500°C, NO_x conversions reached nearly 100%. Since the maximum NO_x conversion on conventional H-ZSM-5 was only 64% obtained at 600°C under the same reaction conditions, Fe³⁺ in the zeolite framework played an important role in this reaction. It appears that the activities were little influenced by the Fe content between 1.14% by weight to 3.57% by weight in ZSM- 5. According to an alternative embodiment of the present invention, it was found that the addition of a small amount (for example 0.054% by wt.) of cerium further increased the activities as shown in Figure 1. In comparison, the conventional catalyst 4.4%V₂O₅+8.2%WO₃/TiO₂ showed substantially lower activities in NO_x reduction under the same conditions, and the NO_x conversions decreased sharply when the temperature was above 450°C (Figure 1) due to oxidation of ammonia by oxygen. However, when the amount of the conventional catalyst was increased from

50 mg to 800 mg while keeping all other conditions identical, the maximum NO_x conversion was increased from 64.0% at 400°C to 99.8% at 375°C. This indicates that the Fe-ZSM-5 catalysts of the present invention were more than 16 times as active as the commercial catalyst as shown in Table 1 below: Table 1. Results of No_x reduction by NH₃ over Fe-ZSM-5 and 4.4% V₂O₅-

8.2%WO₃/TiO₂ catalyst. Catalyst Temperature NO_x Conversion Selectivity

(°C) (%) N₂ N₂O

Fe(130)-ZSM-5 375 96.4 100 0

(50mg) 400 99.1 100 0

450-500 >99.9 100 0

4.4%V₂O₅-8.2%WO₃TiO₂ 350 99.7 97.3 2.7

(800mg) 375 99.8 95.1 4.9

400 96.0 91.5 8.5

Reaction conditions in Table 1 are: [NO] = [NH₃] = lOOOppm, [O₂] = 2%, He = balance and total flow rate= 500ml/min

The same ratio of 16 can be obtained by comparing the overall, first-order rate constants. In addition, on the Fe-ZSM-5 catalysts, no N₂O was detected in the entire temperature range of 300-600°C, with only N₂ and H₂O as the reaction products. With the conventional V₂O₅+WO₃/TiO₂ catalyst, a significant amount of undesirable N₂O was formed in the reaction. N₂O is known to be the most stable of the greenhouse gases, and it consumes the ozone layer in the atmosphere, and thus its suppression, achieved by the present invention, is a significant advance in the art. Because combustion gases contain water vapor and sulfur dioxide (along with NO_x, O₂ and other gases), resistance to H₂O and SO₂ is an important factor for SCR catalysts. Accordingly, the effects of H₂O and SO₂ on the catalytic performance of the catalyst of the present invention, and in particular Ce-Fe(42)- ZSM-5 was evaluated. It was found that the effects of H₂O and SO₂ increased the activity and also widened the reaction temperature window. The Ce-Fe-ZSM5 catalyst is also stable in the SCR reaction. During a run of 60 h on stream at 375°C, under the conditions of 100 mg sample, 1000 ppm NO, 1000 ppm NH₃, 2% O₂, 5%

H₂O, 500 ppm SO₂ and GHSV = 2.3x10⁵ 1/h, NO_x conversion remained at ca. 99- 100%. Activities for SO₂ oxidation were measured by collecting SO₃ from products as BaSO₄ precipitates. The Ce-Fe-ZSM-5 catalyst of the present invention showed a much lower activity for SO₂ oxidation to SO₃ (0.7% vs. 3.8% for 4.4%V₂O₃+8.2%WO₃/TiO₂) at 375°C under the same reaction conditions.

The mechanism of NO_x reduction by NH₃ on the catalyst of the present invention was studied by in situ FT-IR. A Fe-ZSM5 sample (20 mg) was pressed as a self-supporting wafer and loaded in an infrared cell with Si windows. The sample was pretreated at 500°C for 15 minutes in a flow of He and then cooled to 100°C for adsorption of NH₃. After the sample adsorbed NH₃ (1000 ppm NH₃ in He) for 30 minutes followed by purging with He, NH₄ ⁺ coordination in ZSM-5 was formed as shown in Figure 2, at bands at 3351-2730, 2154, 1944 and 1466 cm^"1. The negative peak at 3608 cm^'1 was due to consumption of OH groups because the background spectrum was from the He-pretreated FE-ZSM5. An increase in temperature resulted in desorption of NH₄ ⁺ from the sample. However, some NH₄ ⁺ ions still remained in the Fe-ZSM-5 catalyst at 450°C (Figure 2). This suggests that strong

Brønsted acid sites existed on the sample. In a separate experiment, when the Fe- ZSM-5 was exposed to 1000 ppm NO and 2% O₂ at 200°C, three bands at 1880, 1624 and 1580 cm^"1 were detected as illustrated in Figure 3. This can be assigned to weakly adsorbed NO, NO₂ and NO₃ ^" species, respectively. This result suggests that NO was oxidized to NO₂ by O₂ on this surface. After purging with He for 5 min at

400°C, all of the bands vanished. In another experiment, NH₃ was first adsorbed on the Fe-ZSM-5 to form NH₄, and the sample was subsequently exposed to NO/O₂/He at 300°C. The NH₄ ⁺ bands vanished gradually in 5 minutes, as illustrated in Figure 4. This indicates that NH₄ ⁺ reacted with NO/NO₂ to form N₂, as identified by gas chromatography. When the Fe-ZSM-5 was exposed to the reactant mixture

(NO+NH₃+O₂/He) at 250-450°C, only adsorbed NH₄ ⁺ ions were detected. The above results taken together indicate the following scheme for NO reduction by NH₃ on the Fe-ZSM-5 catalyst of the present invention. NH₃ is first activated to NH₄ ⁺ on Brønsted acid sites of the zeolite, then weakly adsorbed and gaseous NO and NO₂ (resulting from NO oxidation by O₂) and NO₃ ^" species react with NH₄ ⁺ to form N₂ and H₂O. The reaction between NO₂ (NO₃.) and NH₄ ⁺ is much faster than that between NO and O₂.

The high activity of the inventive catalyst could be attributed to its strong Brønsted acidity (thus a high concentration of NH₄ ⁺ ions) and a high activity for oxidation of NO to NO₂. It is well known that both Brønsted acidity and the formation of NO₂ are important for the SCR reaction on zeolite-type catalysts. In an alternative embodiment of the present invention, a series of Fe- exchanged zeolite were studied as catalysts for the selective catalytic reduction (SCR) of NO with ammonia. It was found that Fe-ZSM-5 and Fe-mordenite catalysts were highly active for the SCR reaction. Nearly 100% NO conversions were obtained at 400-550°C under conditions with a high space velocity (GHSV=4.6x 10⁵ 1/h). However, Fe-Y and Fe-MCM-41 with larger pore sizes showed lower activities for this reaction. For Fe-ZSM-5 catalysts, the SCR activity decreased with increasing Si/Al ratio in the zeolites. As Fe-exchange level in the Fe-ZSM-5 catalysts was increased from 58 to 252%, NO conversion increased at lower temperatures (e.g., 300°C), but decreased at higher temperatures (e.g., 600°C). The addition of cerium to Fe-ZSM-5 further increased the SCR activity and durability in the presence of H₂O and SO₂. Compared with the commercial vanadia catalyst, the Fe-ZSM-5 catalyst was 16 times more active; it also functioned in a broader temperature window, produced only N₂ rather than N₂O and H₂O, and showed a substantially lower activity for oxidation of SO₂ to SO₃. A number of experiments were performed to demonstrate the present invention. The experiments are provided herein for illustration purposes only and are in no way intended to limit the scope of the invention.

EXPERIMENTAL Preparation of Catalyst. The starting materials used for preparation of the catalysts are as follows. NH₄-ZSM-5 (Si/Al = 10) was obtained from Air Products and Chemicals Inc. The other NH₄-form zeolites were prepared from Na-form zeolites by exchanging with 0.5 M NH₄C1 solution (three times) at room temperature. ZSM-5 (Si/Al = 40) and ZSM-5 (Si/Al = 100) were kindly supplied by Exxon. Mordenite (MOR, Si/Al = 6.4) and Y zeolite (Si/Al = 2.4) were obtained from Zeolyst International Company and Strem Chemicals, respectively. Al-MCM- 41 (Si/Al = 10) was prepared according to the procedure described elsewhere. FeCl₂-4H₂O (99%), FeCl₃ (99%) and Fe(NO₃)₃-9H₂O (98.5%) were obtained from Aldrich. Iron powder and HC1 solution (2.00 M) were supplied by Fisher. The Fe-exchanged zeolite catalysts were prepared using the method of the present invention. In each experiment, 2 g NH₄-ZSM-5 (Si/Al = 10, 40 and 100), NH₄-MOR, or NH₄-Y was added to 200 ml of 0.05 M FeCl₂ solution with constant stirring. After 24 hours, the mixture was filtered and washed five times with deionized water. Ce(0.8)-Fe(42)-ZSM-5 was obtained from 3g NH₄-ZSM-5 exchanged with 200 ml 0.05 M Ce(NO₃)₃ solution, followed by exchanging with 200 ml 0.05 M FeCl₂ solution. The exchange processes were performed at room temperature for 1 day. Fe(93)-ZSM-5(10) was prepared by exchanging NH₄-ZSM-5 (Si/Al = 10) with FeCl₂ solution three times. Table 2 sets forth the preparation conditions and iron content of the various Fe-exchanged zeolite catalysts according to one embodiment of the present invention, as well as conventional catalysts: Table 2. Preparation conditions and iron contents of Fe-exchanged zeolite catalyst.

Sample Si/Al Fe Content ion-exchange Preparation (wt.%) extent (%) Condition

Fe(58)-ZSM-5 (10) 10/1 1.59 57.7 Exchanging NH₄- ZSM-5 with FeCl₂ solution for 24 hours at room temperature (once)

Ce-Fe(42)-ZSM-5 (10) 10/1 1.17 42.3 Exchanging NH₄- ZSM-5 with Ce(NO₃)₃, then FeCl₂ solution for 24 hours at room temperature (once)

Fe(93)-ZSM-5 (10) 10/1 2.56 93.0 Exchanging NH₄- ZSM-5 with FeCl₂ solution for 24 hours at room temperature (three times)

Fe(130)-ZSM-5 (10) 10/1 3.58 130 Exchanging H- ZSM-5 with Fe+HCl solution at 50°C for lO days in He

Fe(252)-ZSM-5 (10) 10/1 6.94 252 Exchanging H- ZSM-5 with FeCl₃ vapor at 320°C Fe(57)-ZSM-5 (40) 40/1 0.43 57.3 Exchanging NH₄- ZSM-5 with FeCl₂ solution for 24 hours at room temperature (once)

Fe(65)-ZSM-5 (100) 100/1 0.20 65.3 Exchanging NH₄- ZSM-5 with FeCl₂ solution for 24 hours at room temperature (once)

Fe(60)-MOR (6.4) 6.4/1 2.41 59.6 Exchanging NH₄- mordenite with FeCl₂ solution for 24 hours at room temperature (once)

Fe(67)-Y (2.4) 2.4/1 5.59 66.9 Exchanging NH₄- Y with FeCl₂ solution for 24 hours at room temperature (once)

Fe(85)-MCM-41 (10) 10/1 2.33 84.7 Exchanging H- Al-MCM-41 with Fe+HCl solution at 25°C for 3 days in a flow of He

In order to prepare over-exchanged Fe-ZSM-5 (i.e., where Fe-exchange level is above 100%), two other methods were used. Fe(130)-ZSM-5(10) was obtained from exchanging 2 g H-ZSM-5 with a mixed solution that contained 200 ml 0.1 M HCl and 0.73 g iron powder. The ion-exchange was performed at 50°C for 10 days in a flow of He, in order to prevent oxidation of the ferrous ion to the ferric form. Fe(252)-ZSM-5(10) was prepared by a sublimation of FeCl₃ vapor into H-ZSM-5 at 320°C for 2 hours, followed by hydrolytic removal of chlorine, by following the procedure given by Chen and Sachtler. H-ZSM-5 was prepared by calcining NH₄- ZSM-5 at 500°C for 3 h. Fe(85)-MCM-41(10) was prepared by exchanging H-Al- MCM-41 (Si/Al = 10) with a solution containing 200 ml 0.1M HCl and 0.73g Fe at room temperature for 3 days in a flow of He. The obtained catalysts were first dried at 120°C in air for 12 h, then calcined at 600°C for 6 h. Fe²⁺ in the catalysts was oxidized to Fe³⁺. Finally, the obtained samples were ground to 60-100 mesh. The Fe and Al contents in the samples were measured by neutron activation analysis.

The iron exchange extent was calculated by 3 x (number of iron ions) / (number aluminum ions). The preparation of the catalysts and the resulting Fe contents are summarized in Table 2.

In addition to the Fe-exchanged catalysts of the present invention, 2.27 wt.%Fe₂O₃/H-ZSM-5 and V₂O₅+WO₃/TiO₂ catalyst (4.4 wt.%V₂O₅+8.2 wt.%WO₃/TiO₂) were also used for comparison. The catalysts were prepared by incipient wetness impregnation. The V₂O₅+WO₃/TiO₂ catalyst had nearly identical SCR activity and behavior as that of the commercial SCR catalyst supplied by a major catalyst manufacturer. Catalytic Activity Measurement. The SCR activity measurement was carried out in a fixed-bed quartz reactor, using 50 mg (0.065 ml) of catalyst. The flue gas was simulated by blending different gaseous reactants to represent the various emission gases. Two sets of flowmeters were used to control the flow rates of the individual reactants. Helium, NH₃/He (premixed), and No/He (premixed) gases were controlled by rotameters, whereas SO₂/He (premixed) and O₂ were controlled by mass flowmeters (FM 4575 Linde Division). The typical reactant gas composition was as follows: 1000 ppm NO, 1000 ppm NH₃, 2% O₂, 500 ppm SO₂ (when used), 5% water vapor (when used), and balance He. The total flow rate was 500 ml/min (ambient conditions) and thus a very high GHSV was obtained, i.e. 4.6 x 10⁵ 1/h. The premixed gases (1.01% NO in He, 1.00% NH₃ in He, and 0.99% SO₂ in He) were supplied by Matheson. Water vapor was generated by passing He through a heated gas-wash bottle containing deionized water. The tubings of the reactor system were wrapped with heating tapes to prevent formation and deposition of ammonium sulfate/bisulfate and ammonium nitrate. The NO and NO₂ concentrations were continually monitored by a chemiluminescent NO/NO_x analyzer

(Thermo Electron Corporation, Model 10). To avoid errors caused by the oxidation of ammonia in the converter of the NO/NO_x analyzer, an ammonia trap containing a phosphoric acid solution was installed before the sample inlet to the chemiluminescent analyzer. The products were also analyzed by a gas chromatograph (Shimadzu, 14A) at 50°C with 5 A molecular sieve column for N₂ and Porapak Q column for N₂O. Hence the nitrogen balance and the product selectivities for N₂ and N₂O could be obtained. During the reaction, the temperature was increased from room temperature to 600°C step by step. At each temperature point, in 50°C intervals (i.e., 250, 300, 350, 400, 450, 500, 550 and 600°C), data was obtained when the SCR reaction reached steady state after five minutes.

S0₃ Analysis. To measure the amount of SO₃ from SO₂ oxidation during the SCR reaction, a conventional wet analysis method was adopted. With the same reactor used for measuring the catalyst SCR activity, the effluent was bubbled through a solution containing BaCl₂ and HCl. After 12-24 hour runs at 375°C under the conditions of 1000 ppm NO, 1000 ppm NH₃, 2% O₂, 5% H₂O, 500 ppm SO₂ and GHSV = 4.6 x 10⁵ 1/h, the SO₃ was quantitatively captured and precipitated as BaSO₄. Considering that a small amount of ammonium sulfates (resulting from the reaction of SO₃, H₂O and NH₃) might stay in the tubing between the SCR reactor and BaCl₂ solution after the reaction. The BaSO₄ precipitate was collected on an ashless filter paper which was burned along with the precipitate in a crucible, so the amount of the precipitate was accurately measured. Catalytic performance of Fe-ZSM-5. The SCR activities of H-ZSM-5,

Fe₂O₃/H-ZSM-5, Fe-ZSM-5, and V₂O₅+WO₃/TiO₂ catalysts are shown in Figure 5. Under the conditions with a high GHSV (i.e. 4.6 x 10⁵ 1/h), the H-ZSM-5(10) catalyst showed moderate activity. NO conversion increased from zero to 65% when the reaction temperature was increased from 250 to 600°C. This is in good agreement with the previous result that H-ZSM-5 was active for the SCR reaction at high temperatures. After 2.27 wt.% Fe₂O₃ was doped on the H-ZSM-5, NO conversions were improved at lower temperatures, but the highest NO conversion was only 70% at 550°C. However, when the same amount of iron ions were exchanged to ZSM-5 in accordance with the present invention, surprisingly high NO conversions were obtained on the Fe(58)-ZSM-5(10) catalyst, which also gave a broad temperature window (Figure 5). Nearly 100% NO conversion was obtained at 400-550°C on the Fe-ZSM-5 catalyst of the present invention. The above results indicate clearly that Fe³⁺ in the catalyst played an important role for the SCR reaction. By comparison, the commercial V₂O₅+WO₃/TiO₂ catalyst showed substantially lower NO conversions under the same conditions, and the NO conversions decreased sharply when the temperature was above 450°C (Figure 5) due to oxidation of ammonia by oxygen. However, when the amount of the commercial catalyst was increased from 50 mg to 800 mg while keeping all other conditions identical, the maximum NO conversion was increased from 57.0% at 400°C to 99.8% at 375°C. This indicates that Fe-ZSM-5 catalysts were more than 16 times as active as the commercial catalyst. The SCR activity can also be represented quantitatively by the first-order rate constant (k), since the reaction is known to be first order with respect to NO under stoichiometric NH₃ conditions on a variety of catalysts. By assuming a plug flow reactor (in a fixed bed of catalyst) and free of diffusion limitation, the rate constant can be calculated from the NO conversion (X) by

where F₀ is the molar NO feed rate, [NO]₀ is the molar NO concentration at the inlet

(at the reaction temperature), and W is the catalyst amount (g). From the NO conversions and reaction conditions, first-order rate constants could be calculated. The k values of the inventive Fe(58)-ZSM-5(10) catalyst were 2078 and 2793 cm³/g/s at 400 and 450°C, respectively. By comparison, the k values for the commercial vanadia catalyst were only 417 cm³/g/s at 400°C and 386 cm³/g/s at

450°C. In addition, on the inventive Fe(58)ZSM-5(10) catalyst, no N₂O was detected in the entire temperature range of 250-600°C, with only N₂ and H₂O as the reaction products. With the conventional V₂O₅+WO₃/TiO₂ catalyst, a significant amount of N₂O was formed at high temperatures (e.g., 5% of N₂O yield was obtained at 375°C). The nitrogen balance in this work was above 95%. The maximum SCR activity on the Fe-ZSM-5 catalyst was also superior to any other catalysts reported in the literature.

Effect ofFe content on SCR activity over Fe-ZSM-5 (10) catalysts. The effect of the Fe content on the SCR activity is summarized in Table 3.

Table 3. Catalytic performance of Fe-ZSM-5(Si/Al=10) catalyst at different ion exchange percentages in the absence of H₂O and SO₂.^a

Catalyst Temperature NO Conversion to N₂ k_b TO (%) (cm³/g/s)

Fe(58)-ZSM-5 (10) 3 30000 2 277..00 101

350 72.5 450

400 99.6 2080

450 >99.9 2790

500 >99.9 2990

550 99.2 2220

600 92.0 1230

Fe(93)-ZSM-5 (10) 3 30000 3 333..00 128

350 82.0 597

400 99.0 1730

450 >99.9 2790

500 >99.9 2990

550 95.6 1260

600 91.0 1180

Fe(130)-ZSM-5 (10) 3 30000 3 355..00 138

350 85.4 670

400 99.1 1770

450 >99.9 2790

500 >99.9 2990

550 98.6 1970

600 91.5 1200

Fe(252)-ZSM-5 (10) 3 30000 5 577..00 270

350 97.0 1220

400 99.7 2.19

450 95.0 1210

500 90.0 995

550 86.0 905

600 67.0 541

Ce(0.8)-Fe(42)-ZSM-5 (10) 3 30000 3 355..55 131

350 74.5 476

400 >99.9 2600

450 >99.9 2790

500 >99.9 2990

550 99.0 2119

600 92.5 1265 ^a Reaction conditions: 0.05g catalyst, [NO] = [NH₃] = 1000 ppm, [O₂] = 2%, He = balance, total flow rate = 500 ml/minute and GHSV = 4.6 X 10⁵ 1/h. ^b First-order rate constant, as defined in the text, calculated by Equation 1. At low temperatures (e.g., 300°C), both NO conversion and the first-order rate constant (k) increased with increasing Fe content on the Fe-ZSM-5(10) catalysts. But at higher temperatures (> 550°C), they dropped more rapidly with an increase in Fe content. This may be related to oxidation of NH₃ by O₂ at high temperatures because a high iron content on the catalyst may be beneficial to oxidation of NH₃. At 400-500°C, NO conversion reached nearly 100% on the Fe- ZSM-5(10) catalysts with 58-130% of Fe-exchange level. Also, no N₂O was detected by GC during the reaction for these catalysts.

Effect of Si/Al ratio on SCR activity over Fe-ZSM-5 catalysts. It is known that the Si/Al ratio in zeolites affects their acidity-basicity and cation exchange capacity. The effect of Si/Al ratio on SCR activity over the catalysts of the present invention was studied. As shown in Figure 6, the ratio of Si/Al had a strong effect on NO conversion for the Fe-ZSM-5 catalysts. With an increase in Si/Al ratio, NO conversions decreased rapidly at the temperature range of 250-600°C (Figure 6). High NO conversions were obtained over Fe(58)-ZSM-5 (Si/Al = 10), but very low

NO conversions were obtained on Fe(65)-ZSM-5 (Si/Al = 100). These results indicate that the Fe-ZSM-5 catalysts with low Si/Al ratio in the range of about 1 to 40 is favorable to a high NO conversion.

SCR activities on different Fe-exchanged molecular sieves. The SCR activity on other different Fe-exchanged molecular sieves (Fe-MOR, Fe-Y, and Fe-

MCM-41) were also studied by the inventors. The results are shown in Figure 7. Like the Fe(58)-ZSM(10) catalyst, the Fe(60)-MOR(6.4) catalyst also showed high activities and a broad temperature window. Also, no N₂O was detected in the entire temperature range. In comparison, Fe(67)-Y(2.4) was less active than the above two catalysts. The lowest SCR activity was found on Fe(85)-MCM-41 (10) catalyst. The maximum NO conversion on the Fe-MCM-41 catalyst was only 25.5% at 550°C under the condition with 4.6 x 10⁵ 1/h. The above results suggest that the structure of molecular sieves has a strong effect on the SCR activity.

Effect ofCe Promoter. In accordance with an alternative embodiment of the present invention, a Ce promoter is added to the catalyst composition. Specifically,

Ce and Fe are exchanged with the zeolite. Table 3 shows the catalytic activity for a Ce(0.8)-Fe(42)-ZSM-5(10) catalyst in accordance with this alternative embodiment of the invention. As shown the Ce exchanged catalyst was also very active in the SCR reaction. NO conversions reached near 100% at 400-500°C. As compared with Fe(58)-ZSM-5, the addition of cerium increased SCR activity at lower temperatures (>400°C). Effect ofH₂0 + S0₂ on SCR activity. Because resistance to H₂O and SO₂ is an important factor for SCR catalysts, we studied the effect of H₂O and SO₂ on the catalytic performance over the Fe-ZSM-5 catalysts (Table 4). The results are shown in Table 4.

Table 4. Catalytic performance of Fe-ZSM-5(Si/Al=10) catalyst in the presence of H₂O and SO₂.^a

Catalyst Temperature NO Conversion to N₂ k, 10³ TO (%) (cm³/g/s)

Fe(58)-ZSM-5 (10) 3 30000 2 200..00 72

350 70.0 419

400 >99.9 2600

450 >99.9 2790

500 >99.9 2990

550 >99.9 3180

600 94.0 1370

Fe(130)-ZSM-5 (10) 3 30000 3 300..00 114

350 81.8 594

400 >99.9 2600

450 >99.9 2790

500 >99.9 2990

550 >99.9 3180

600 95.0 1460

Ce(0.8)-Fe(42)-ZSM-5 (10) 3 30000 2 233..55 85.8

350 74.5 476

400 >99.9 2600

450 >99.9 2790

500 >99.9 2990

550 >99.9 3180

600 97.7 1842 ^a Reaction conditions: 0.05g catalyst, [NO] = [NH₃] = 1000 ppm, [O₂] = 2%, [H₂O]

= 5%, [SO₂] = 500 ppm, He = balance, total flow rate = 500 ml/minute and GHSV = 4.6 X 10⁵ l h. ^b First-order rate constant, as defined in the text, calculated by Equation 1.

At low temperatures (e.g., 300°C), NO conversions and first-order rate constants (k) decreased in the presence of H₂O and SO₂ on Fe(58)-ZSM-5(10), Fe(130)-ZSM-5(10) and Ce(0.8)-Fe(42)-ZSM-5(10) catalysts. But at high temperatures, it was found that H₂O and SO₂ increased SCR activities and also widened the temperature windows for the two catalysts (Tables 3 and 4). However, after a run of 60 hours on stream at 375°C, under the conditions of 100 mg sample, 1000 ppm NO, 1000 ppm NH₃, 2% O₂, 5% H₂O and 500 ppm SO₂, the activity on

Fe(58)-ZSM-5(10) decreased from 99.5 to 80.0%. By comparison, NO conversion on the inventive Ce(0.8)-Fe(42)-ZSM-5(10) was only changed from 99.9 to 99.0%. This indicates that the Ce-Fe-ZSM-5 was stable in the SCR reaction and cerium plays a stabilization role for Fe-ZSM-5. SO₂ Oxidation Activity. Using the wet chemical method described in the foregoing to quantitatively measure the amounts of SΟ₃ generated in the reaction effluents, SO₂ conversions at 375°C for Fe(58)-ZSM-5(10), Ce(0.8)-Fe(42)-ZSM- 5(10) and 4.4%V₂O₅+WO₃/TiO₂ were obtained. The conversion for SO₂ to SO₃ was about 0.7% for Fe(58)-ZSM-5(10) and Ce(0.8)-Fe(42)-ZSM-5(10), which was much lower than that (3.8%) on the vanadia catalyst under the same reaction conditions.

The foregoing demonstrates that the Fe exchanged zeolite, and optionally additionally exchanged with Ce, catalysts of the present invention have higher activities than all known catalysts for the SCR reaction of NO with ammonia. In particular, for Fe-ZSM-5(10) catalysts, when the Fe-exchange level was increased from 58 to 252%, the NO conversion increased at lower temperatures (e.g., 300°C) but decreased at high temperatures (>500°C, Table 2). This is in good agreement with our previous result on Fe-TiO₂-PILC catalysts that NO conversion at high temperatures decreased with an increase in iron content, but is contrary to the result reported by Ma and Grunert that the SCR activity on Fe-ZSM-5 with an exchange degree of < 100% was inferior to that on the over-exchanged Fe(300)-ZSM-5(14).

Over-exchanged Fe-ZSM-5 catalysts have attracted much attention for the SCR of NO with hydrocarbon because they were highly active and resistant to H₂O and SO₂ as compared with the Cu-ZSM-5 catalysts. Feng and Hall first reported that Fe(183)-ZSM-5(19) prepared by an ion-exchange from oxalate salt was an active and durable SCR catalyst with iso-butane as the reductant. The SCR activity of

Fe(183)-ZSM-5(19) was much higher than that of under-exchanged Fe-ZSM-5 catalysts (i.e., ion-exchange level < 100%). Chen and Sachtler studied the same reaction on Fe(300)-ZSM-5 catalysts prepared by using anaerobic sublimation of volatile FeCl₃ to H-ZSM-5. They also reported high activity and durability of the over-exchanged Fe-ZSM-5 catalysts for SCR with hydrocarbons. Using the procedure of Chen and Sachtler, Ma and Grunert prepared an over-exchanged Fe(300)-ZSM-5(14) catalyst and investigated its catalytic performance for the SCR of NO with ammonia. They also concluded that the SCR activity of the over- exchanged catalyst was superior to that of under-exchanged Fe-ZSM-5 catalysts obtained by conventional aqueous exchange. But the maximum NO conversion on the Fe(300)-ZSM-5(14) in their work was below 85% under the condition with a lower space velocity of 3.04 10⁵ 1/h, which was significantly lower than that (near

100%) on the Fe(58)-ZSM-5(10) catalyst of the present invention obtained at a higher space velocity (4.6 x 10⁵ 1/h) as tested herein. It is noted that, although over- exchange is important for Fe-ZSM-5 catalysts to obtain a high activity for SCR with hydrocarbons, it may not be necessary for SCR with ammonia. The mechanisms for these two SCR reactions are very different. The SCR reaction with hydrocarbons generally involves the adsorption/oxidation of nitrogen oxides and hydrocarbons. The surface acidity of catalyst does not play an important role and no direct relationship between catalytic activity and acidity has been found for this reaction of SCR with hydrocarbon. Conversely, for the SCR reaction of NO with ammonia, the subject of the present invention, the reaction path involves the adsorption of ammonia on the Brønsted or Lewis acid sites and reaction between the ammonia adspecies and nitrogen oxides. In this reaction, surface acidity is important because strong acidity is beneficial to NH₃ adsorption. Work has demonstrated that there is a direct correlation between SCR activity and surface acidity for many catalysts (e.g., V₂O₅+WO₃/TiO₂, pillared clay and zeolite catalysts), and an increase in surface acidity improves the catalytic activity. The results obtained by the inventors, indicated that a higher Fe-exchange level did not result in a higher acidity. However, a higher iron content may increase the activity for oxidation of ammonia by oxygen at high temperatures. This will result in a decrease in NO conversion at high temperatures over the Fe-ZSM-5(10) catalysts with higher iron contents.

Komatsu et al. reported that the oxidation activity of ammonia by oxygen increased with copper content on Cu-ZSM-5 and the catalysts with higher copper contents also showed lower NO conversions at high temperatures for the SCR reaction of NO with ammonia.

The above results also indicated that the ratio of Si/Al in ZSM-5 had a strong effect on NO conversion for the Fe-ZSM-5 catalysts. Although the ion-exchange level for Fe(58)-ZSM-5(10), Fe-(57)-ZSM-5(40) and Fe(65)-ZSM-5(100) were close, NO conversions decreased sharply with increasing Si/Al ratio (Figure 6). It is known that ZSM-5 with a lower Si/Al ratio has a higher Brønsted acidity so that a high SCR activity will be achieved on the Fe-ZSM-5 catalyst. For different Fe- exchanged molecular sieves, NO conversions were found to decrease in an order of Fe(58)-ZSM-5(10) > Fe(60)-MOR(6.4) > Fe(67)-Y(2.4) » Fe(85)-MCM-41(10)

(Figure 7). This indicates that the structure of molecular sieve played an important role for the SCR reaction with ammonia. ZSM-5 has a unique pore structure that consists of two intersecting channel systems: one straight and the other sinusoidal and perpendicular to the former. Both channel systems have ten-membered-ring elliptical openings (0.52-0.57 ran in diameter). The mordenite pore structure consists of elliptical and noninterconnected channels parallel to the c-axis of the orthorhombic structure. Their openings are limited by twelve-membered rings (0.6- 0.7 nm). Y-zeolite is a faujasite-type zeolite with 1.5-3.0 of Si/Al ratio, whose large cavities of 1.3 nm in diameter (supercages) are connected to each other through apertures of 1.0 nm. By comparison, MCM-41, a new member of molecular sieve family, shows a regular hexagonal array of uniform pore openings with pore dimensions between 1.5 and 10 nm. The average pore diameter of the Al-MCM- 41(10) was 4.3 nm. Hence, the pore sizes of the above molecular sieves increase according to a sequence of ZSM-5 < MOR < Y < MCM-41, but the SCR activities on the above Fe-exchanged zeolites decreased in this order. However, large pore diameter is clearly beneficial to diffusion rates for the SCR reaction. This contradiction suggests that the SCR reaction could be free of diffusion limitation and the SCR reaction was achieved at pore openings near the crystallite surfaces. For the H-ZSM-5 catalyst, Eng and Bartholomew proposed a scheme for reducing NO to N₂ by ammonia. One pair of HN₄ ⁺ ions (i.e., two NH₃ molecules adsorbed onto neighboring Brønsted acid sites) reacts with one NO₂ molecule (resulting from oxidation of NO by O₂) to form a complex, which subsequently reacts with another NO molecule to produce N₂ and H₂O. They suggested that the oxidation of NO to NO₂ was the rate-limiting step for the reaction. According to the above catalytic performance results, it seems that Fe-exchanged zeolites also follow this reaction pathway. Fe³⁺ ions on the catalysts increased the oxidation rate of NO to NO₂ and thus increased the SCR activity significantly as compared with H-ZSM-5 (Figure 5).

A small pore diameter and a low Si/Al ratio in molecular sieves would promote the formation of NH₄ ⁺ pairs, so that Fe-ZSM-5(10) and Fe-MOR(6.4) catalysts showed high activities. On the contrary, pairs of NH₄ ⁺ would be difficult to form on Fe- exchanged molecular sieves with large pore sizes and high Si/Al ratios (due to a lack of adjacent acid sites) thus very low SCR activities were obtained on Fe(85)-MCM-

41(10) and Fe(65)-ZSM-5(100) catalysts.

In summary, the present invention of Fe exchanged zeolite catalysts (which may be optionally exchanged with Ce) are shown to be highly reactive for the SCR reaction of NO with ammonia. Nearly 100% NO_x conversion was obtained at 400- 550°C under conditions with a high space velocity (GHSV = 4.6 x 10⁵ 1/h). But conventional catalyst of Fe-Y(Si/Al = 2.4) and Fe-MCM-41 (Si/Al = 10) with larger pore diameters showed lower activities for this reaction. For Fe-ZSM-5 catalysts of the present invention, the SCR activity also decreased with increasing Si/Al ratio in the zeolites. The addition of cerium to Fe-ZSM-5 further increases the SCR activity and durability in the presence of H₂O and SO₂. As compared with the commercial vanadia catalyst, the Fe-ZSM-5(10) catalysts were much more active; they functioned in a broader temperature window and produced only N₂ (rather than N₂) and H₂O; they also showed a substantially lower activity for oxidation of SO₂ to SO₃.

Claims

CLAIMSWhat is claimed is:

1. A catalyst for selective catalytic reduction of nitrogen oxide compounds with ammonia, comprising: a composition of iron exchanged zeolites, and mixtures thereof.

2. The catalyst of Claim 1 wherein said composition further comprises promoter ions exchanged with said zeolites.

3. The catalyst of Claim 1 wherein said zeolite is selected from the group of chabazite, clinoptilolite, ZK-4, ZK-5, ZSM-5 and mordenite (MDR).

4. The catalyst of Claim 2 wherein said promoter ions are selected from the group of Ce, La, Pr, Y, Nd, Sm, Gd, Tb and oxides thereof.

5. The catalyst of Claim 1 wherein said zeolite is ZSM-5.

6. The catalyst of Claim 1 wherein said zeolite is mordenite (MOR).

7. The catalyst of Claim 1 wherein said catalyst is comprised of Fe-

ZSM-5 or Ce-Fe-ZSM-5.

8. The catalyst of Claim 1 wherein said catalyst is comprised of Fe- ZSM-5 and has a Si/Al ratio in the range of approximately 1 to 100.

9. The catalyst of Claim 1 wherein said catalyst is comprised of Fe- ZSM-5 and has a Si/Al ratio of approximately 10.

10. The catalyst of Claim 1 wherein said catalyst is comprised of Ce-Fe- ZSM-5 and has a Si/Al ratio in the range of approximately 1 to 100.

11. The catalyst of Claim 1 wherein said catalyst is comprised of Ce-Fe- ZSM-5 and has a Si/Al ratio of approximately 10.

12. The catalyst of Claim 1 wherein the percent of iron ions exchanged is in the range of approximately 1% to 300%.

13. The catalyst of Claim 1 wherein the percent of cerium ions exchanged is in the range of approximately 1% to 300%.

14. The catalyst of Claim 2 wherein the promoter ions are provided by rare earth metal compounds selected from the group of Y₂O₃, Nd₂O₃, Sm₂O₃, Gd₂O₃, TbO_x, CeO₂, La_jO_j, or PrO_x.

15. The catalyst of Claim 1 wherein said catalyst is selected from the group of:

Fe(58)-ZSM-5 (10); Ce-Fe(42)-ZSM-5 (10); Fe(93)-ZSM-5 (10); Fe(130)-ZSM-5 (10); Fe(252)-ZSM-5 (10); Fe(57)-ZSM-5 (40); Fe(65)-ZSM-5 (100); and Fe(60)- MOR(6.4).

16. The catalyst of Claim 4 wherein said rare earth ions are present in a concentration in the range of about 0.1 to 15 wt. %.

17. A method of making an iron exchanged zeolite catalyst for the selective catalytic reduction of nitrogen oxide compounds with ammonia, comprising the steps of: providing a zeolite starting material; exchanging said zeolite starting material with an iron containing material; and carrying out said exchanging step in air or inert gas at room temperature.

18. The method of Claim 17 wherein said exchanging step further includes exchanging the zeolite starting material with a promoter ion.

19. The method of Claim 17 wherein said iron containing material is selected from the group of FeCl₂, Fe and other ferrous salts.

20. The method of Claim 18 wherein said promoter ions are rare earth metal ions selected from the group of Ce, La, Pr, Y, Nd, Sm, Gd and Tb.