SCR catalysts having improved low temperature performance, and methods of making and using the same

Abstract

SCR-active molecular-sieve based catalysts with improved low-temperature performance are made by heating a molecular-sieve in a non-oxidizing atmosphere with steam (hydrothermal treatment), or in a reducing atmosphere without steam (thermal treatment), at a temperature in the range of 600-900° C. for a time period from 5 minutes to two hours. The resulting SCR-active iron-containing molecular sieves exhibit a selective catalytic reduction of nitrogen oxides with NH.sub.3 or urea at 250° C. that is at least 50% greater than if the iron-containing molecular-sieve were calcined at 500° C. for two hours without performing the hydrothermal or thermal treatment.

Claims

1. An SCR-active iron-containing molecular sieve, the iron-containing molecular sieve having undergone a hydrothermal treatment in a steam-containing non-oxidizing atmosphere, or a thermal treatment in a reducing atmosphere, at a temperature in the range of 600-900° C. for a time period from 5 minutes to two hours, and wherein the iron containing molecular sieve exhibits a selective catalytic reduction of nitrogen oxides with NH.sub.3 or urea at 250° C. that is at least 50% greater than a comparable iron-containing molecular sieve that was calcined at 500° C. for two hours without performing the hydrothermal treatment in the non-oxidizing atmosphere or the thermal treatment in the reducing atmosphere, wherein the molecular sieve is a zeolite or a silicoaluminophosphate (SAPO); wherein the molecular sieve is a BEA, FER, CHA, AFX, AEI, SFW, SAPO-34, SAPO-56, SAPO-18, SAPO SAV, or SAPO STA-7.

2. The SCR-active iron-containing molecular sieve according to claim 1, wherein the molecular sieve is small-pore or medium pore.

3. The SCR-active iron-containing molecular sieve according to claim 1, wherein the selective catalytic reduction is at least two times greater than if the iron-containing molecular sieve were calcined at 500° C. for two hours without performing the hydrothermal treatment in the non-oxidizing atmosphere or the thermal treatment in the reducing atmosphere.

4. The SCR-active iron-containing molecular sieve according to claim 1, wherein the selective catalytic reduction is at least three times greater than if the iron-containing molecular sieve were calcined at 500° C. for two hours without performing the hydrothermal treatment in the non-oxidizing atmosphere or the thermal treatment in the reducing atmosphere.

5. The SCR-active iron-containing molecular sieve according to claim 1, wherein the iron containing molecular sieve exhibits a selective catalytic reduction of nitrogen oxides with NH.sub.3 or urea at 200° C. that is at least 50% greater than if the iron-containing molecular sieve were calcined at 500° C. for two hours without performing the hydrothermal treatment in the non-oxidizing atmosphere or the thermal treatment in the reducing atmosphere.

6. The SCR-active iron-containing molecular sieve according to claim 1, wherein the iron containing molecular sieve exhibits a selective catalytic reduction of nitrogen oxides with NH.sub.3 or urea at 200° C. that is at least two times greater than if the iron-containing molecular sieve were calcined at 500° C. for two hours without performing the hydrothermal treatment in the non-oxidizing atmosphere or the thermal treatment in the reducing atmosphere.

7. The SCR-active iron-containing molecular sieve according to claim 1, wherein the iron present in the molecular sieve has a higher ratio of Fe.sup.2+ to Fe.sup.3+ than if the iron-containing molecular sieve were calcined at 500° C. for two hours without performing the hydrothermal treatment in the non-oxidizing atmosphere or the thermal treatment in the reducing atmosphere.

8. The SCR-active iron-containing molecular sieve according to claim 1, wherein the molecular sieve is a zeolite and the iron present in the zeolite has a higher ratio of Fe.sup.2+ to Fe.sup.3+ than if the iron-containing zeolite were calcined at 500° C. for two hours without performing the hydrothermal treatment in the non-oxidizing atmosphere or the thermal treatment in the reducing atmosphere.

9. The SCR-active iron-containing molecular sieve according to claim 1, wherein the molecular sieve is a ferrierite and the iron present in the ferrierite has a higher ratio of Fe.sup.2+ to Fe.sup.3+ than if the iron-containing zeolite were calcined at 500° C. for two hours without performing the hydrothermal treatment in the non-oxidizing atmosphere or the thermal treatment in the reducing atmosphere.

10. An SCR-active iron-containing molecular sieve, the iron-containing molecular sieve having undergone a hydrothermal treatment in a steam-containing non-oxidizing atmosphere, or a thermal treatment in a reducing atmosphere, at a temperature in the range of 600-900° C. for a time period from 5 minutes to two hours, and wherein the iron containing molecular sieve exhibits a selective catalytic reduction of nitrogen oxides with NH.sub.3 or urea at 250° C. that is at least 50% greater than a comparable iron-containing molecular sieve that was calcined at 500° C. for two hours without performing the hydrothermal treatment in the non-oxidizing atmosphere or the thermal treatment in the reducing atmosphere, wherein the molecular sieve is a silicoaluminophosphate (SAPO).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other objects, features and advantages of the invention will become more apparent after reading the following detailed description of examples of the invention, given with reference to the accompanying drawings.

(2) FIG. 1 is a graph illustrating the NO.sub.x conversion using an SCR-active, iron-containing zeolite that had been prepared using a hydrothermal treatment according to the invention, in comparison to a conventionally-prepared iron-containing zeolite that did not receive a hydrothermal treatment, but was calcined at about 500° C. for about 2 hours.

(3) FIG. 2 is a plot obtained by applying diffuse-reflectance UV-Vis spectroscopy to powder samples of the iron-containing zeolites represented in FIG. 1.

(4) FIGS. 3a and 3b are plots obtained by performing Mossbauer spectroscopy to powder samples of the iron-containing zeolites represented in FIG. 1.

(5) FIG. 4 is a graph comparing the NOx conversion as a function of NH.sub.3 fill, for each of a series of iron-containing zeolite samples prepared under respectively different processing conditions.

(6) FIG. 5 schematically depicts an example of a rotary calcining oven that can be used to perform the hydrothermal treatment or thermal treatment followed by exposure to oxygen.

EXAMPLE 1

(7) 3 wt % iron was added to a commercially available ferrierite zeolite by spray drying the ferrierite zeolite with ammonium Fe (III) oxalate in solution so as to give the desired Fe loading. One portion of the resulting iron-containing ferrierite was dried at 105° C. overnight and was activated under a flow of 10% steam in nitrogen at 800° C. for 1 hour. This material was not subjected to calcination. Another portion of the iron-containing ferrierite was calcined at 500° C. in air for 2 hours to use as reference.

(8) In the examples that follow, powder samples of the catalysts were obtained by pelletizing the original samples, crushing the pellets and then passing the powder obtained through a 255 and 350 micron sieves to obtain a powder having particle size between 255 and 350 microns. The powder samples were loaded into a Synthetic Catalyst Activity Test (SCAT) reactor and tested using the following synthetic diesel exhaust gas mixture (at inlet) including nitrogenous reductant: 500 ppm NO, 550 ppm NH.sub.3, 12% 02, 4.5% H.sub.2O, 4.5% CO.sub.2, 200 ppm CO, balance N.sub.2 at a space velocity of 330 liters per gram of powder catalyst per hour. The samples were heated ramp-wise from 150 to 550° C. at 5° C./min and the composition of the off-gases detected and the activity of the samples to promote NOx reduction was thereby derived.

(9) As shown in FIG. 1, iron ferrierite in the form of a powder catalyst that was subjected to the hydrothermal treatment under a flow of 10% steam in nitrogen at 800° C. for 1 hour, displays markedly superior low temperature (from about 175-300° C.) conversion of NO.sub.x, as compared to the iron ferrierite that was not so treated, and instead was subjected to conventional calcination at 500° C. in air for 2 hours. The catalyst subjected to the hydrothermal treatment produced about three times the conversion of NOx from compared to the conventionally treated catalyst from about 175° C. to about 250° C., (see lines (a)-(c) which show the amounts of NOx conversion at 175, 200 and 250° C., respectively). The amount of conversion using the catalyst subjected to hydrothermal treatment at 175, 200 and 250° C. was about 15, 25 and 60%, respectively, while the amount of conversion using the conventionally treated catalyst was about 5, 8 and 18%, respectively. At 300° C., the amount of conversion using the catalyst subjected to hydrothermal treatment was about twice that from the conventionally treated catalyst (>95% versus 50%)

(10) FIG. 1 also shows that catalysts subjected to hydrothermal treatment could convert comparable amounts of NOx at significantly lower temperatures than conventionally treated catalyst. Temperatures needed for 10, 50 and 90% NOx conversion in catalysts subjected to hydrothermal treatment were about 170, 240 and 280° C. but were about 220, 300 and 350° C. for conventionally treated catalyst. The lowest temperature for maximum NOx conversion was about 310° C. for catalyst subjected to hydrothermal treatment but was about 375° C. for conventionally treated catalyst.

(11) These results demonstrate that catalyst subjected to hydrothermal treatment can produce significantly higher NOx conversion compared to a comparable conventionally treated catalyst. Catalysts subjected to hydrothermal treatment convert similar amounts of NOx at much lower temperatures compared to a comparable conventionally treated catalyst.

(12) The same powder samples were analyzed using diffuse-reflectance UV-Vis spectroscopy in a Perkin-Elmer Lambda 650S spectrometer equipped with an integrating sphere using BaSO4 as a reference. The samples were placed and packed in a holder. The scan interval was set to 1 nm from 190 to 850 nm, the response time was 0.48 sec and a 10% beam attenuator was used in the reference beam. The data was converted to Kubelka-Munk and normalised to 5 to the maximum ordinate. The resulting plots are shown in FIG. 2, in which the curves are normalized to the maximum ordinate. These plots indicate that the activation of the iron ferrierite catalyst in steam and N.sub.2, leads to significant redispersion of larger Fe species into more active Fe sites, as is shown by the reduction in reflectance from 300-400 nm, where oligonuclear species are measured and in the region above 400 nm, where small clusters of iron oxide and larger Fe.sub.2O.sub.3 species are measured.

(13) Powder samples were also analyzed using Mossbauer spectroscopy. .sup.57Fe Mossbauer spectroscopy was performed at room temperature using a Wissel constant acceleration spectrometer in transmission mode using a 57Co source in a rhodium matrix. The spectrometer was calibrated relative to α-Fe. The samples were dried and placed in a holder that was glued closed. Mossbauer data were collected over a velocity range of +/−6 mm s.sup.−1 and for different periods of time depending on the sample. A calibration run was performed on an α-Fe foil over the same velocity range. All isomer shift values were reported relative to α-Fe and spectra were analysed using the Lorentzian line-shapes facility of RECOIL software [Lagarec K and Rancourt D G, Recoil: Mossbauer spectral analysis software for Windows. http://www.isapps.ca/recoil/]. The resulting spectra is shown in FIGS. 3a and 3b. FIG. 3a is a spectrum of the conventionally calcined catalyst (without hydrothermal treatment). The spectrum has two doublets, both having parameters indicative of Fe(III) in an octahedral environment as shown by an isomer shift (CS)=0.35 mm/s and quadrupole splitting (QS)=0.65 mm/s and CS=0.34 mm/s and QS=0.99 mm/s, respectively. The spectra has two doublets of approximately equal intensity centered at about −0.05 and about 0.69 mm s.sup.−1. FIG. 3b is a spectrum of the hydrothermally activated catalyst. This spectrum displays an additional doublet (CS=1.2 mm/s and QS=2.8 mm/s) not found in the spectrum from the sample produced without hydrothermal treatment (FIG. 3a). The parameters of the additional doublet are indicative of Fe(II) in a possibly octahedral environment as indicated by the values CS=1.2 mm/s and QS=2.8 mm/s. Typical values for isomer shifts for Fe(II) are between 0.7 and 1.4 mm/s and for Fe(III) are between 0.1 and 0.6 mm/s. (Edyta Tabor, Karel Zaveta, Naveen K. Sathu, Zdenka Tvaruzkova, Zdenek Sobalík; Catalysis Today 169 (2011) 16-23) One of ordinary skill in the art would recognize that both the location of the peaks and the intensity of the peaks can vary depending on numerous factors, including, but not limited to, the age of the source, the length of time of data acquisition, the presence of water in the sample, Fe loadings, as well as the type of molecular sieve used.

(14) The spectrum of the iron ferrierite activated according to the invention shows that some Fe.sup.3+ species that are present in the conventionally calcined iron ferrierite convert to Fe.sup.2+ during activation at high temperature in H.sub.2O/N.sub.2.

EXAMPLE 2

(15) An iron ferrierite was made as described above by combining 3 wt % iron with a commercially available ferrierite zeolite by spray drying the ferrierite zeolite with ammonium Fe (II) sulphate in solution so as to give the desired Fe loading. A series of powder samples were then prepared by treating the iron ferrierite at the temperatures and atmospheric conditions as shown in Table 1. Samples were prepared for determining their catalytic activity by coating the powder onto ceramic cores.

(16) TABLE-US-00001 TABLE 1 Conditions for Preparing Modified Iron Zeolite Temperature (° C.) Time Atmosphere Treatment 800 1 h  2% H.sub.2 + N.sub.2 Thermal 800 1 h 10% H.sub.2O + N.sub.2 Hydrothermal 850 1 h 10% H.sub.2O + N.sub.2 Hydrothermal 850 2 h 10% H.sub.2O + N.sub.2 Hydrothermal 750 5 h Air Calcination (Reference)

(17) The reference sample was prepared by calcination at 750° C. because the ferrierite zeolite was treated with ammonium Fe (II) sulphate and a higher temperature was needed to remove the sulfate.

(18) The test conditions for the data shown in FIG. 4 were NO.sub.x conversion at 200° C. as a function of NH.sub.3 fill, with 75% NO.sub.2/NOx, space velocity 60 k/hr, and alpha ratio 1.5. These test conditions differ from those used to produce the data shown in FIG. 1 at least because of a difference in the composition of the gas. The gas used to generate the data in FIG. 1 contained NO as the only NOx compound, whereas the gas used to generate the data in FIG. 4 contained a mixture of NO and NO.sub.2, where NO.sub.2 accounted for about 75% of the total NOx. One of ordinary skill in the art would recognize that the rates of conversion using only NO and not NO.sub.2 are slower than the rates of conversion using both NO and NO.sub.2, and therefore rates of conversion measured using only NO are worst-case conversion rates.

(19) As shown in FIG. 4, a substantial (10-20%) increase in NOx conversion was observed with ammonia fill between about 0.2 and 0.6 g/L. The three catalysts produced using hydrothermal treatment in a mixed atmosphere comprising steam and nitrogen produced the highest differences compared to the catalyst that did not undergo hydrothermal treatment. NOx conversion using catalysts that underwent hydrothermal treatment at temperatures of about 800 to about 850° C. in a mixed atmosphere of steam and nitrogen for about one hour was about 15 to 20% greater than from a comparable catalyst that did not receive a hydrothermal treatment and had only been treated in air (calcined). The catalyst producing in the reducing atmosphere (2% H.sub.2 in N.sub.2), without steam treatment, produced conversions comparable to the sample that received hydrothermal treatment at 850° C. for 1 hour.

(20) FIG. 5 schematically depicts an example of a rotary calcination oven 10 that is well-suited to perform the hydrothermal treatment according to the invention. The oven 10 is generally cylindrical, and mounted for rotation about its axis. The oven 10 is inclined at an angle θ of about 1 to about 15°. An iron-containing molecular sieve, preferably a zeolite or a SAPO, in powder form is introduced at the higher end of oven 10, as indicated by arrow 11. As the oven is rotated, the iron-containing molecular sieve moves downwardly through the rotating oven in the direction of arrow 14. A countercurrent flow of the non-oxidizing atmosphere is provided, as indicated by the arrow 12 in FIG. 5. The activated iron containing molecular sieve is then removed from the oven 10, as indicated by the arrow 15. Steam is added with the countercurrent non-oxidative or reductive gas when a hydrothermal treatment is performed.

(21) The heating of the calcination oven is preferably controlled such that three heating zones a, b and c are maintained. In zone a, the temperature increases from 25° C. at the oven inlet to the temperature at which the hydrothermal or thermal treatment is performed. In zone b, the temperature is maintained at the temperature at which the hydrothermal or thermal treatment is performed. In zone c, the temperature decreases from the temperature at which the hydrothermal or thermal treatment is performed, to about 25° C. at the oven outlet.

(22) If an atmosphere of for example nitrogen and steam, or nitrogen and hydrogen, is maintained exclusively within the oven, the activated iron containing zeolite has a grey-black color as it emerges from the oven outlet. Following coating on a substrate and ordinary calcination in air at about 500 to about 600° C., the color of the iron containing zeolite changes from grey-black to orange-beige.

(23) It has been unexpectedly discovered that if the treatment in the oven 10 is modified so as to permit, a small amount of oxygen to enter the oven after the hydrothermal treatment, as shown by the arrow 18 in FIG. 5, the performance of the iron containing zeolite thus processed is further improved. The amount of oxygen admitted near the outlet of the oven 10 is in the range of about 1 ppm to about 200,000 ppm above the ambient level of oxygen in the oven.

(24) When the treatment concludes with a controlled inclusion of oxygen within the oven above ambient levels, it was found that the iron containing zeolite exiting the oven can have a different color than the material before the treatment, and that the conversion efficiency of the iron containing zeolite is better than when oxygen was not introduced into the oven. This is the case even after the iron-containing zeolites processed in the oxygen-free oven undergo a subsequent calcination, as described above.

(25) It will be understood that the foregoing description and specific examples shown herein are merely illustrative of the invention and the principles thereof, and that modifications and additions may be easily made by those skilled in the art without departing from the spirit and scope of the invention, which is therefore understood to be limited only by the scope of the appended claims.