Patent Application
20080095682
Refer now to the figures, which are exemplary embodiments, and wherein the like elements are numbered alike.
Unmodified manganese catalysts have a narrow window for ammonia SCR because at moderate temperatures (e.g., 300° C.); these catalyst begin to oxidize NH3 unselectively to NOx. Titania was a logical element to try because is not volatile at high temperatures, but it doesn't work well with manganese. With respect to tungsten, it seemed unlikely to work since it tends to be volatile at high temperature. For example, V—W—Ti—O catalysts where W(VI) is supported on the surface of anatase titania suffers from the fact that WO3 can volatilize at temperatures of about 600° C. In addition, WO3 is volatile when supported on alumina. So, Mn—W—Al—O catalysts are not expected to be viable after high temperature aging.
Although, to our knowledge, there are no reports of tungsten in oxide systems with a coordination number greater than 6, a tungsten fluorite structure was sought. In fluorite, if all atoms are at normal crystallographic positions, metal atoms are coordinated by 8 oxygen atoms, with the metal atom at the center of a cube of 8 oxygen atoms.
Some embodiments of the Ce—Zr—R-A-M-O compositions (e.g., the Ce—Zr—Mn—W—O catalyst, and the Ce—Zr—W—O catalyst) seemed to solve one or more of the above problems. The temperature window expanded, and no evidence of tungsten volatility at 850° C. and 950° C. was found. As oppose to merely an intimate mixture of WO3 with another oxide, it is believed that our composition is a new compound. The fact that tungsten appears to go into the fluorite structure in the present compositions is unprecedented. Further, the simultaneous presence of Ce—Zr—Mn-A-O (wherein A is W, Nb, Ta, and/or Mo) in a fluorite is also unprecedented, as is the simultaneous presence of Ce—Zr—W-A-O (wherein A, which is optional, is Nb, Ta, and/or Mo) in a fluorite. Not to be bound by theory, for tungsten, the fluorite structure is believed to be significant because, unlike alumina (either spinel γ-Al2O3 or corundum α-Al2O3) or titania (either anatase or rutile) or silica (whether as a zeolite, or amorphous silica, or quartz) the fluorite structure accommodates tungsten atoms in lattice positions. In the other oxides, the tungsten is merely mixed with the oxide and not part of the lattice.
The Ce—Zr—R-A-M-O catalyst, e.g., a mesoporous Ce—Zr—Mn-A-M-O fluorite catalyst (optionally comprising both fluorite and MnWO4 phases), is a NOx reduction catalyst, that can attain NOx reduction light-off temperatures of about 150° C., with NOx conversion at about 150° C. to about 300° C., and thermal stability up to and exceeding about 850° C. As used herein, a material is “mesoporous” if greater than or equal to 50% of its pore volume is in pores having a size (as measured along a major axis) of 2 nanometers (nm) to 50 nm. This catalyst is particularly useful for NOx reduction and is capable of reducing the NOx without the need for a precious metal catalyst (e.g., without the need to convert some of the NO to NO2). Two common mechanisms for catalytic reduction of NOx, known as the “Standard Reaction” and the “Fast Reaction,” are:
4NO+4NH3+O2→4N2+6H2O Standard Reaction
2NO+2NO2+4NH3→4N2+6H2O Fast Reaction
When the “Standard Reaction” attains better results, a catalyst for forming NO2 is not needed; e.g., no precious metal (e.g., platinum) catalyst is needed with or upstream of the NOx catalyst. The catalyst can be employed as a downstream SCR device, e.g., and underfloor catalyst or disposed after a particulate filter, or can be combined with the particulate filter to reduce NOx and also eliminate soot.
If the SCR device is downstream of the Ce—Zr—R-A-M-O catalyst that functions as a particulate filter, the SCR catalyst may be any of various materials known to reduce NOx with ammonia, such as zeolites containing copper or iron, or vanadia (optionally with tungsta or molybdena) supported on anatase titania. In another embodiment the Ce—Zr—R-A-M-O catalyst can be disposed in the SCR, downstream of a particulate filter.
The catalyst formula is Cea—Zrb—Rc-Ad-Me-Ox. The subscripts a, b, c, d, e, and x, represent atomic fractions, subject to the proviso that (a+b+c+d+e=1). In addition, the value of “a” can be about 0.1 to about 0.6, or, more specifically, about 0.25 to about 0.5, and even more specifically, about 0.3 to about 0.4. “b” can be about 0.25 to about 0.7, or, more specifically, about 0.3 to about 0.6, and even more specifically, about 0.35 to about 0.5. “c” can be about 0.02 to about 0.5, or, more specifically, about 0.1 to about 0.4, and even more specifically, about 0.2 to about 0.35, or, in some embodiments 0.05 to about 0.2. When “R” is Mn, “d” can be about 0.04 to about 0.2, or, more specifically, about 0.05 to about 0.15, and even more specifically, about 0.07 to about 0.12. When “R” is W, “d” can be less than or equal to about 0.2, or, more specifically, about 0.04 to about 0.2, yet more specifically, about 0.05 to about 0.15, and even more specifically, about 0.07 to about 0.12. “e” can be less than or equal to about 0.15, or, more specifically, about 0.03 to about 0.1. The precise value for “x” (and therefore the precise value for δ) depends on the type of metal component, its charge, atomic fraction, and the requirement that the metal oxide have a neutral overall charge. Examples of the catalyst include, for example, Ce0.35—Zr0.35—Mn0.25—W0.05—O; Ce0.30—Zr0.30—Mn0.30—W0.1—O; and Ce0.45—Zr0.45W0.1—O2+δ.
“R” is Mn or W. If “R” is Mn, “A” comprises W, Mo, Ta, and Nb, as well as combinations comprising at least one of the foregoing. If R is W, A, which is optional, comprises Mo, Ta, and Nb, as well as combinations comprising at least one of the foregoing. “M”, which is optional, comprises trivalent rare earth ion(s) (e.g., dysprosium (Dy), erbium (Er), ytterbium (Yb), holmium (Ho), erbium (Er), thulium (Tm), lanthanum (La), yttrium (Y), lutetium (Lu), samarium (Sm), and gadolinium (Gd), as well as combinations comprising at least one of the foregoing. In some embodiments, the catalyst is free of La and Y. As defined herein, “free” means that La and Y were not added to the catalyst, but may exist as an impurity in a reagent used to make the catalyst.
The Cea—Zrb—Rc-Ad-Me-Ox catalyst can be formed in various fashions. For example, water soluble salts of cerium, zirconium, “M”, and “R” (if “R” is Mn), can be prepared in a sufficiently acidic solution to dissolve the salts (e.g., a slightly acidic solutions, such as a pH of less than or equal to about 2.0, or, more specifically, a pH of about 0.5 to about 2.0). For W, however, (when “R” is W), a water soluble salt of the tungsten, preferably a sodium tungstate or ammonium tungstate is prepared, e.g., by dissolving the salt in water to give a solution with a pH of about 7.5 to about 10.0, or, more specifically, about 7.5 to about 9.0. A salt (e.g., a sodium and/or ammonium salt) of “A” can be dissolved into the basic solution (e.g., having a pH of about 7.5 to about 10.0). For example, for the cerium, zirconium, “R” (when “R” is Mn), and “M”, the salts can be nitrates, acetates, and/or chlorides (preferably nitrates and/or acetates), with nitric acid or acetic acid being suitable acids for acidifying this solution. For “A” and “R” (when “R” is W), the salt can be alkali or ammonium salts (preferably sodium salts), with sodium hydroxide, potassium hydroxide, or tetraalkylammonium hydroxide optionally present for maintaining a suitably basic solution.
The salts can separately be dissolved in the appropriate solutions and then combined. Alternatively, the acidic solutions can be formed as one solution and the basic solutions can be formed as one solution. In one example, cerium (III) nitrate hydrate can be dissolved in water without additional acid, e.g., to yield a solution with a pH of about 1.2 to about 1.4. Zirconyl nitrate hydrate can be dissolved in water with a bit of nitric acid, e.g., to yield a solution with a pH of about 0.7, while manganese nitrate can be dissolved without extra acid, e.g., to yield a solution with a pH of about 1.4. These solutions can, optionally, be initially combined for form a single acidic solution for combination with the basic solution(s), or each can be combined directly with the basic solution(s) in a single step. With respect to the basic solution, Na2WO4, for example, can be dissolved without extra base, e.g., to yield a solution with a pH of about 8.5, and/or [NH4]10W12O41 can be dissolved without extra base, e.g., to yield a solution with a pH of about 7.8-8.0. Sodium or ammonium molybdates, tantalates, or niobates can be similarly dissolved. As with the acidic solution(s), the basic solutions can be initially combined to form a single basic solution, or can be individually added directly to the acidic solution(s) to form the precipitate.
These solutions can be mixed, e.g., by adding the basic solution(s) to the acidic solution(s), to form an initial precipitate. Optional heating, e.g., to about 50° C. to about 90° C., of one or more of the solutions, prior to mixing, can be employed. Precipitation is completed by adding NaOH or KOH to raise the pH, e.g., to about 8.8 to about 9.2. The solution is then optionally heated to a temperature of about 50° C. to about 90° C., or, more specifically, about 60° C. to about 80° C. (e.g., about 70° C.), and stirred for less than or equal to about 6 hours or so (e.g., about 1 to about 6 hours, or, more specifically, about 1.5 to about 2.5 hour (e.g., about 2 hours). The precipitate can then be washed to remove soluble salts, dried (e.g., actively and/or passively), and calcined. Calcination can be at a temperature of less than or equal to about 700° C., or, more specifically, about 600° C. to about 700° C., or, yet more specifically, about 650° C. to about 700° C.
It is possible to make and use precursor solutions with concentrations of XO2 of 100 grams per liter (g/liter) or more (wherein “X” represents Ce, Zr, “R” (when “R” is Mn), and “M”). In this nomenclature of XO2, the amount of CeO2, ZrO2, and MnO2 equivalent to the Ce(III) salt (e.g., CE(III) nitrate), zirconyl salt (e.g., zirconyl nitrate), and Mn(II) salt (e.g., Mn(II) nitrate) is calculated and put into the precursor solution (e.g., nitrate solution). When “R” is W, an initial solution containing a tungsten concentration equivalent to up to 20 g/liter WO3 is prepared. For example, the concentration of tungsten, in its original basic solution prior to addition to the XO2 acidic solution(s), can be equivalent to about 15 g/liter to about 25 g/liter WO3 (e.g., about 20 g/liter WO3), while the concentration of Ce, Zr, Mn, and “M” can each be about 50 g XO2/liter to about 100 g XO2/liter.
Also, it is noted that while precursor nitrates were used in preparing the above samples, it is possible to use other water-soluble salts of cerium, zirconium, and manganese including, for example, acetate and/or chloride salts. If chloride salts are used, the precipitate will be washed to ensure essentially complete removal of sodium chloride.
It is noted that not all materials provide desirable catalysts. As can be seen in
Similarly, employment of titanium dioxide (TiO2), e.g., the use of various amounts of TiO2 in the composition, did not improve the performance as is illustrated in
The catalyst structure can affect the performance, e.g., NOx and/or NH3 conversion. Both (NH4)10W12O41 and Na2(WO4) have proven useful in the synthesis of the Ce—Zr—R-A-M-O catalyst. It is noted that in some of the synthesized catalysts, the catalyst contains both fluorite and MnWO4 phases. Next, we can make a rough estimate of the ratio of fluorite to MnWO4 in Ce0.3Zr0.3Mn0.3W0.1O2 that has been calcined at 700° C. based on our X-ray diffraction results. We examine the 200 fluorite reflection and the 011 MnWO4 reflection. The most intense feature in the X-ray diffraction patterns occurs at about 30° 2-theta in our diffraction pattern; this diffraction intensity results from contributions of fluorite (100) and MnWO4 (1-bar 1 1). So, we base our analysis on reflections due solely to fluorite, namely the (200) reflection occurring at about 34° 2-theta) and (011) for MnWO4, occurring at about 26.3° 2-theta. We determine maximum intensity of these two reflections and weight them according to the relative intensity of these reflections, as reported on JCPDS cards 13-0434 (for MnWO4) and 43-1002 (for CeO2). JCPDS is an acronym for the Join Committee on Powder Diffraction Standards, now associated with the International Centre for Diffraction Data. The MnWO4/fluorite ratio can, roughly, be described as:
An exemplary synthesis method comprises dissolving an appropriate amount of Na2WO4 in an aqueous alkali hydroxide solution (e.g., NaOH and/or KOH solution). Separately, nitrates of cerium, zirconium, and manganese, (e.g., Ce(NO3)3 hydrate, ZrO(NO3)2 hydrate, and Mn(NO3)2) can be dissolved in an aqueous acidic acid that optionally contains a bit of nitric acid to promote dissolution. These solutions are slowly combined and the Ce—Zr—Mn—W—O material is precipitated. The precipitate is washed and calcined.
When the Ce—Zr—R-A-M-O catalyst is used in the SCR device, e.g., in an underfloor location, the SCR device comprises a porous support with the Ce—Zr—R-A-M-O catalyst located therein and/or thereon. Examples of porous support materials include cordierite, metallic supports, silicon carbide (SiC), alumina (e.g., zirconium toughened alumina), and the like, as well as combinations comprising at least one of the foregoing materials. The porous support can be in the form of a monolith, sponge, foil, and so forth, and may also comprise a protective coating, e.g., of phosphate or metal phosphate.
The Ce—Zr—R-A-M-O catalyst can, alternatively or in addition, be located in the particulate filter. The particulate filter can comprise any filter and design capable of removing particulate matter from the exhaust stream and preventing the emission of such particulate matter into the atmosphere. In addition, when the Ce—Zr—R-A-M-O catalyst is also disposed in the filter, the filter is capable of reducing NOx. For example, after calcination at 600° C., some of the Ce—Zr—R-A-M-O catalysts are capable of NOx conversion down to less than 50 parts per million by volume (ppm) (i.e., 85% conversion) can be attained over a temperature of about 200° C. to about 415° C.
As noted above, the Ce—Zr—R-A-M-O catalyst can be used in a particulate filter. A particulate filter generally comprises a shell and a filter element with a retention material disposed therebetween. The filter element removes particulate matter from the exhaust stream. The filter element can comprise any material designed for use in the environment and which can remove particulate matter from a gaseous stream. Some possible materials include ceramics (e.g., extruded ceramics), metals (e.g., extruded, sintered metals), and silicon carbide; and so forth, as well as combinations comprising at least one of the foregoing materials, such as cordierite, aluminum oxide, aluminum phosphate, sintered steel (such as sintered stainless steel). For example, the filter element can comprise a gas permeable ceramic material having a honeycomb structure consisting of a plurality of channels, e.g., parallel channels. The channels can be divided into alternating inlet channels and outlet channels. The inlet channels are open at an inlet end of the filter element and closed at the opposite end of the filter element, while outlet channels are closed at the inlet end and open at the outlet end. The inlet and outlet channels are separated by porous longitudinal sidewalls that permit the exhaust gases to pass from the inlet channels to the outlet channels along their length.
Optionally, a composition appropriate for soot oxidation can be disposed in one or more of the inlet channel(s), while the Ce—Zr—R-A-M-O catalyst can be disposed in one or more of the outlet channel(s). These materials may be wash coated, imbibed, impregnated, physisorbed, chemisorbed, precipitated, sprayed, or otherwise applied to the filter element. Depending upon the location of the Ce—Zr—R-A-M-O catalyst in the particulate filter, the composition for soot oxidation may also be chosen for its reactivity with other exhaust components. For example, a soot oxidation composition may be chosen that does not unselectively oxidize ammonia (or urea) without coreaction of NOx, within the temperature range of interest (e.g., 150° C. to about 500° C. or so).
Soot oxidation catalysts are any material capable of oxidizing soot, with an ignition temperature below about 550° C., that does not have activity to oxidize NH3 at temperatures below about 600° C. Possible examples of such catalysts are those of the catalysts discussed in U.S. Patent Publication No. 2005-0282698 that lack ammonia oxidation activity. Many base metal catalyst compositions will provide this sort of activity. In general, soot oxidation catalysts containing platinum (Pt) and/or palladium (Pd) will not be suitable, however.
If an oxidation catalyst is employed, as with the SCR device, the oxidation catalyst comprises a substrate, catalytic material, and shell, with a retention material disposed between the substrate and the shell. The catalyst material(s) for the oxidation catalyst can be a catalyst capable of oxidizing at least one of HC and CO to water and CO2, respectively, and preferably that does not oxidize NO to NO2. The substrate can comprise materials employed in the particulate filter and/or the SCR device. Although the substrate can have any size or geometry, the size and geometry are preferably chosen to optimize surface area in the given exhaust emission control device. Typically, the substrate has a honeycomb geometry, with the combs through-channel having any multi-sided or rounded shape, with substantially square, triangular, pentagonal, hexagonal, heptagonal, or octagonal, or similar geometries preferred due to ease of manufacturing and increased surface area.
As mentioned above, seemingly due to the mechanism of operation of the present catalyst, the catalyst has improved NOx reduction and ammonia conversion when operated under the “Standard Reaction” conditions. As a result exhaust from a diesel engine can optionally be passed from the manifold directly through the particulate filter and from the particulate filter, directly through an optional SCR and then directly vented to the environment (i.e., without the use of additional or other exhaust treatment devices). In this embodiment, the Ce—Zr—R-A-M-O catalyst can be disposed in the particulate filter and/or in the SCR. Regardless of the catalysts location, ammonia (and/or urea) is also introduced to the catalyst, by introduction directly to the device comprising the catalyst, or introduction to the exhaust stream upstream thereof. Alternately, ammonia could be produced on-board through thermal desorption of ammonia from an ammoniacal salt, such as Mg(NH3)6Cl2, contained in a dedicated ammonia delivery system. Once the catalyst has attained a temperature of greater than or equal to about 150° C., it attains light-off, converting the NH3 and NOx to nitrogen and water.
If the catalyst is located in the particulate filter, e.g., in the outlet channels of the filter, the exhaust stream flows into the inlet channels and passes through the walls of the channels, thereby filtering out soot and particulate matter from the stream. As the stream passes through the outlet channels, the NOx and ammonia are converted to nitrogen and water. After the stream exits the filter, it can optionally be treated to oxidize HC and CO in the stream prior to venting to the environment.
When an oxidation catalyst is employed, it is preferable to employ an oxidation catalyst that does not convert the NO to NO2, but oxidizes the HC and CO. From the oxidation catalyst, the stream can pass through a particulate filter to remove the soot and/or other particulate matter. If the Ce—Zr—R-A-M-O catalyst is not located in the filter, the NOx is reduced downstream in a SCR comprising the Ce—Zr—R-A-M-O catalyst.
The following examples are provided merely to further illustrate the present Ce—Zr—R-A-M-O catalyst and not to limit the scope hereof.
168.3 milliliter (ml) solution of ammonium para-tungstate, (NH4)10W12O41 (concentration 12.5 grams (g) WO3 per liter (1)), was poured to a solution obtained by mixing of 220.5 ml Ce(NO3)3, 158 ml ZrO(NO3)2, and 79.5 ml Mn(NO3)2 with concentrations 50 g MO2/l. The pouring of ammonium para-tungstate was performed at room temperature (20-23° C.) at the rate of 10 milliliters per minute (ml/min) under intensive stirring. After ammonium para-tungstate was added to the mixed nitrate solution, the resultant suspension was heated to 70° C., and then, 2N KOH was added until attaining pH 9±0.1. The suspension was stirred under these conditions for 2 hours. The suspension was filtered, and the precipitate was washed with distilled water until there was an absence of nitrates in the filtrate. Finally, the precipitate was washed with ethanol. The precipitate was dried, first in air and then in a drying furnace at 110° C. to 120° C. for 12 to 14 hours, and, after that, was calcined at 600° C. for 4 hours.
336.5 ml solution of ammonium para-tungstate, (NH4)10W12O41 (concentration 12.5 g WO3/l), was poured to a solution obtained by mixing of 187 ml Ce(NO3)3, 134 ml ZrO(NO3)2 and 95 ml Mn(NO3)2 with concentrations 50 g MO2/l. The pouring of ammonium para-tungstate was performed at room temperature (20-23° C.) at the rate of 10 ml/min under intensive stirring. After ammonium para-tungstate was added to the mixed nitrate solution, a suspension obtained was heated to 70° C., and then, 2N KOH was added until attaining pH 9±0.1. The suspension was stirred under these conditions for 2 hours. The suspension was filtered, and the precipitate was washed with distilled water till the absence of nitrates in the filtrate. Finally, the precipitate was washed with ethanol. The precipitate was dried first in air and then in a drying furnace at 110° C. to 120° C. for 12 to 14 hours, and, after that, was calcined at 600° C. for 4 hours.
When calcined at low temperatures (e.g., less than or equal to 700° C.), both materials (Samples 1 and 2) comprise a fluorite structure. Additionally, MnWO4 was detected in Sample 2 after calcination at 700° C., but not after calcination at 600° C., while MnWO4 is not detected after calcination at 600° C. or 700° C. in Sample 1.
Fresh and/or aged Samples 1 and 2, as well as reference catalysts (as is explained in each figure), were tested. The aging conditions were 16 hours (hr) at the aging temperature in an environment comprising 28 parts per million by volume (ppm) SO2, 4.5 volume percent (vol %) H2O, 14 vol % O2, balance N2 and Ar. The evaluation conditions that were then employed to test the catalysts comprised: 350 ppm NO, 350 ppm NH3, 4.5 vol % H2O, 14 vol % O2, balance He.
In one embodiment, a NOx reduction catalyst consists essentially of Cea—Zrb—Rc-Ad-Me-Ox fluorite, wherein only elements that do not reduce NOx conversion can additionally be incorporated into the catalyst. “a” can be about 0.1 to about 0.6, or, more specifically, about 0.25 to about 0.5, and even more specifically, about 0.3 to about 0.4. “b” can be about 0.25 to about 0.7, or, more specifically, about 0.3 to about 0.6, and even more specifically, about 0.35 to about 0.5. “c” can be about 0.02 to about 0.5, or, more specifically, about 0.1 to about 0.4, and even more specifically, about 0.2 to about 0.35, or, in some embodiments 0.05 to about 0.2. When “R” is Mn, “d” can be about 0.04 to about 0.2, or, more specifically, about 0.05 to about 0.15, and even more specifically, about 0.07 to about 0.12. When “R” is W, “d” can be less than or equal to about 0.2, or, more specifically, about 0.04 to about 0.2, yet more specifically, about 0.05 to about 0.15, and even more specifically, about 0.07 to about 0.12. “e” can be less than or equal to about 0.15, or, more specifically, about 0.03 to about 0.1. The NOx reduction catalyst is capable of reducing NOx (e.g., promoting a chemical reaction between NOx and reduced nitrogen species (such as ammonia) to make, predominantly, nitrogen (N2)).
The catalyst disclosed herein has low light-off temperatures of less than or equal to about 200° C., even less than or equal to about 175° C., e.g., about 140° C. to about 160° C. Catalyst also has a wider operating range of greater than or equal to about 200° C., or, more specifically, greater than or equal to about 300° C., and even greater than or equal to about 400° C.
Additionally, the Ce—Zr—R-A-M-O catalyst disclosed herein is unexpectedly highly thermally stable (e.g., stable at temperatures of greater than or equal to 700° C.), and retains NOx reduction capabilities even after 16 hours of aging under hydrothermal conditions at a temperature of 850° C. Since this catalyst exhibits greater NOx reduction capabilities without the presence of NO2, an oxidation catalyst upstream of this NOx catalyst can be eliminated while high NOx conversion is attained. It is further noted that the present catalysts show good tolerance to sulfur (e.g., SO2 and SO3) and superior oxygen atom transport, e.g., compared to neat ceria. Hence, it is believed that the Ce—Zr—R-A-M-O catalysts will function as effective soot oxidation catalysts as well as a NOx reduction catalyst. Therefore, a single material can be used to perform the two more difficult functions of diesel exhaust gas detoxification: removal of NOx and removal of carbonaceous particulate matter.
For example, diesel emissions control systems that do not employ the Ce—Zr—R-A-M-O catalyst disclosed herein, and that utilize SCR and particulate traps, will often have, as a first catalyst in the emissions control system, an oxidation catalyst. This catalyst (typically Pt based) serves two or three functions: oxidizing CO and HC coming from the engine, oxidizing NO to NO2 (e.g., for the FAST NOx reaction), and serving as a heat source to heat up the particulate trap to temperatures sufficient to cause its regeneration by oxidizing carbon stored in the trap. The heating is accomplished by injecting sufficient (relatively large, compared to normal engine operation) amounts of hydrocarbons upstream of the engine as a late pulse during the combustion cycle (so that the hydrocarbons emerge from the cylinder largely uncombusted) and/or in the exhaust pipe, upstream of the oxidation catalyst. The hydrocarbons, when oxidized over the oxidation catalyst, increase the temperature of the exhaust gas sufficient to heat the particulate trap to soot regeneration temperatures.
Downstream of this oxidation catalyst is generally a particulate trap followed by an SCR catalyst (since the SCR catalyst generally fails at temperatures below particulate trap regeneration temperatures); e.g., zeolite-based SCR catalysts and vanadia-titania-based catalysts fail at a temperature below about 750° C. However, since the Ce—Zr—Mn-A-M-O catalyst disclosed herein are more temperature tolerant, they are better able to withstand the middle position (between the oxidation catalyst and the particulate trap).
Not to be bound by theory, the Ce—Zr—R-A-M-O catalysts (Ce—Zr—Mn-A-M-O catalysts) appear to reduce NO more effectively than NO2, thereby allowing the use of Pd oxidation catalysts (at a substantial cost savings to the use of Pt catalysts) to heat exhaust to temperatures needed to regenerate a particulate trap. Pd catalysts are less effective than Pt catalysts at oxidizing NO to NO2 which appears desirable for the new catalysts disclosed herein. In other words, unlike iron zeolite catalysts and V—W—TiO2 catalysts used for mobile applications, the Ce—Zr—R-A-M-O catalyst does not require the use of Pt oxidation catalysts to convert NO to NO2 because it is active for the “Standard Reaction”. This substantially reduces cost.
The Ce—Zr—R-A-M-O catalysts (e.g., Ce—Zr—Mn-A-M-O catalysts) could be coated onto a particulate trap, allowing the combination of the SCR and particulate matter (PM) control with a single coated substrate, permitting helpful reductions in system complexity and cost. After the SCR and particulate trap catalysts, an additional oxidation catalyst can be used to oxidize CO slipping from the particulate trap during high temperature regeneration and to oxidize any ammonia that slips from the SCR catalyst.
Not to be bound by theory, the present composition enables a process for reducing NOx, where the NOx is mostly NO (e.g., greater than or equal to about 60 vol % NO, based on the total vol % of the NOx), using ammonia such that at temperatures below about 300° C., the characteristics of NOx reduction improve as the NO/NO2 ratio increases. The NOx reduction characteristics of the catalyst at temperatures below 300° C. are better when the NOx is about 95 vol % NO, 5 vol % NO2 compared to when the NOx is 50 vol % NO, 50 vol % NO2. Hence, for many embodiments of the Ce—Zr—R-A-M-O catalyst, the NOx reduction characteristics at temperatures below about 300° C. are better when NOx is greater than or equal to about 75 vol % NO (e.g., less than or equal to 25 vol % NO2) than when NOx is less than or equal to 50 vol % NO (e.g., 50 vol % NO2), based on the total volume of the NOx. It is noted that NOx in normal engine is exhaust is typically greater than 75 vol % NO and usually greater than 90 vol % NO. As a result, the present catalyst benefits from the normal exhaust conditions, able to use the exhaust NOx as it, without converting the NO/NO2 ratio. The present catalyst can reduce greater than or equal to about 50% of NOx in a gas stream, or more specifically, greater than or equal to about 80 vol % of NOx in a gas stream, and, even more specifically, greater than or equal to about 90 vol % of NOx in a gas stream (based upon a total volume of NOx in the gas stream), in at a temperature of about 150° C. to about 550° C., or, more specifically, the temperature is about 150° C. to about 400° C. Additionally, the present catalyst can have a NOx reduction temperature window of about greater than or equal to about 250° C., even greater than or equal to about 300° C., and even greater than or equal to about 350° C., where greater than or equal to about 50%, and even greater than or equal to about 70% NOx conversion can be attained.
Ranges disclosed herein are inclusive and combinable (e.g., ranges of “up to about 25 wt %, or, more specifically, about 5 wt % to about 20 wt %”, is inclusive of the endpoints and all intermediate values of the ranges of “about 5 wt % to about 25 wt %,” etc). As used herein, “combination” is inclusive of blends, mixtures, alloys, oxides, copolymers, reaction products, and the like, as applicable. Furthermore, the terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another, and the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context, (e.g., includes the degree of error associated with measurement of the particular quantity). The suffix “(s)” as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., the colorant(s) includes one or more colorants).
All cited patents, patent applications, and other references are incorporated herein by reference in their entirety. However, if a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference.
While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
