Catalysts for treating transient NOx emissions

Abstract

A heterogeneous catalyst article having at least one combination of a first molecular sieve having a medium pore, large pore, or meso-pore crystal structure and optionally containing a first metal, and a second molecular sieve having a small pore crystal structure and optionally containing a second metal, and a monolith substrate onto or within which said catalytic component is incorporated, wherein the combination of the first and second molecular sieves is a blend, a plurality of layers, and/or a plurality of zones.

Claims

1. A catalyst composition for treating exhaust gas comprising: (a) a first molecular sieve having BEA framework, wherein said first molecular sieve is a zeolite aluminociliate and contains about 0.5 to about 5 weight percent exchanged or free iron; and (b) a second molecular sieve having a small pore crystal structure and containing about 0.5 to about 5 weight percent of exchanged or free metal selected from Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mn, Ru, Rh, Pd, Pt, Ag, In, Sn, Re, and Ir, wherein the second molecular sieve is a zeolite aluminosilicate having a silica-to-alumina ratio of about 8 to about 150, wherein said first molecular sieve and said second molecular sieve are present in a weight ratio of about 0.1 to about 1.0.

2. The catalyst composition of claim 1, wherein said second metal is copper.

3. The catalyst composition of claim 2, wherein said second molecular sieve has a framework selected from CHA, AEI, AFX, and LEV.

4. The catalyst composition of claim 1, wherein said second molecular sieve contains about 2.0 to about 4.0 weight percent copper, and said first molecular sieve and said second molecular sieve are present in a weight ratio of about 0.2 to about 0.60.

5. A catalyst article comprising: a. a catalyst composition according to claim 1; and b. a monolith substrate onto or within which said catalytic composition is incorporated, wherein said first and second molecular sieves are present as a blend, a plurality of layers, or a plurality of zones.

6. The catalyst article of claim 5, wherein said first molecular sieve is disposed in a first zone and said second molecular sieve is disposed in a second zone, and wherein at least a portion of said first zone is upstream of said second zone relative to an intended direction of gas flow past or through said catalytic composition, a majority of said first zone does not overlap said second zone, and a majority of said second zone does not overlap said first zone.

7. The catalyst article of claim 5, wherein said first molecular sieve is disposed in a first zone and said second molecular sieve is disposed in a second zone, and wherein said second zone is disposed below said first zone relative to an intended direction of gas flow past or through said catalytic composition.

8. The catalyst article of claim 5, wherein said first and second molecular sieves are disposed on said substrate as a blend.

9. The catalyst of claim 3, wherein the second molecular sieve has a CHA framework.

10. The catalyst of claim 3, wherein the second molecular sieve has an AEI framework.

11. The catalyst of claim 3, wherein the second molecular sieve has an AEI framework.

12. The catalyst of claim 3, wherein the second molecular sieve has an AFX framework.

13. The catalyst of claim 3, wherein the second molecular sieve has a LEV framework.

14. A catalytic article comprising: a. an extruded monolith comprising a first molecular sieve having a BEA framework, wherein said first molecular sieve is an aluminosilicate or an ferrosilicate and contains about 0.5 to about 5 weight percent exchanged or free iron; and b. a washcoat comprising a second molecular sieve having a small pore crystal structure and containing about 0.5 to about 5 weight percent of exchanged or free metal selected from Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Pt, Ag, in, Sn, Re, and Ir, wherein the second molecular sieve is a zeolite, wherein the washcoat is coated on the extruded monolith.

Description

(1) In order that the invention may be more fully understood, reference is made to the accompanying drawings, in which:

(2) FIG. 1 is a schematic drawing of an exhaust system embodiment according to the invention;

(3) FIG. 2 is a schematic drawing of a further exhaust system embodiment according to the invention; and

(4) FIG. 3 is a graph showing the results of NO.sub.x conversion activity tests described in Example 3 on fresh catalysts prepared according to Examples 1, 2 and 3.

(5) FIGS. 4a-4d shows different types of combinations of a first molecular sieve and a second molecular sieve on a substrate.

(6) FIGS. 5, 6a, 6b, 7, and 8a-8c are graphs showing data associated with certain embodiments of the invention.

(7) In FIG. 1 is shown an apparatus 10 comprising a light-duty diesel engine 12 and an exhaust system 14 comprising a conduit for conveying exhaust gas emitted from the engine to atmosphere 15 disposed in which conduit is a metal substrate monolith coated with a NO.sub.x Absorber Catalyst ((NAC)) also known as a NO.sub.x trap or lean NO.sub.x trap) 16 followed in the flow direction by a wall-flow filter 18 coated with a SCR catalyst according to the invention (Cu/SSZ-13 blended with an iron-in-zeolite framework BEA also ion-exchanged with additional ion-exchanged iron). A clean-up catalyst 24 comprising a relatively low loading of Pt on alumina is disposed downstream of wall-flow filter 18.

(8) In use, the engine runs lean of stoichiometric, wherein NO.sub.x is absorbed in the NAC. Intermittently, the engine is run rich to desorb and reduce NO.sub.R. During rich running operation, some NO.sub.x is reduced to NH.sub.3 and is stored on the downstream SCR catalyst for further NO.sub.x reduction. The SCR catalyst also treats NO.sub.x during intermittent rich events. NO oxidised to NO.sub.2 on the NAC is used to combust soot trapped on the filter 18 passively. The NAC is also used to combust additional hydrocarbon during occasional forced (active) regenerations of the filter.

(9) FIG. 2 shows an alternative apparatus 11 according to the invention comprising a diesel engine 12 and an exhaust system 13 therefor. Exhaust system 13 comprises a conduit 17 linking catalytic aftertreatment components, namely a 2Au-0.5Pd/Al.sub.2O.sub.3 catalyst coated onto an inert ceramic flow-through substrate 19 disposed close to the exhaust manifold of the engine (the so-called close coupled position). Downstream of the close-coupled catalyst 19, in the so-called underfloor position, is an flow-through catalyst 22 of the extruded type comprising a mixture of an aluminosilicate CHA ion-exchanged with Cu and FeCHA, having Fe present in the molecular sieve framework structure. A source of nitrogenous reductant (urea) is provided in tank 28, which is injected into the exhaust gas conduit 17 between catalysts 19 and 22.

(10) In certain embodiments, provided is a catalyst for selectively catalysing the conversion of oxides of nitrogen using a nitrogenous reductant in a feed gas whose composition, flow rate and temperature are each changeable temporally, which catalyst comprising a combination of a first molecular sieve component and a second molecular sieve component, wherein in a direct comparison tested on the Federal Test Procedure (FTP) 75 cycle the catalyst has a higher cumulative conversion of NO.sub.x at equal or lower NH.sub.3 slip than either molecular sieve component taken alone.

(11) Preferably, the catalyst has a higher cumulative conversion of NO.sub.R, preferably to elemental nitrogen, at equal or lower NH.sub.3 slip than either molecular sieve component taken alone where the cumulative molar NO:NO.sub.2 ratio in feed gas entering said catalyst is equal to, or less than 1.

(12) In certain embodiments, the SCR catalyst has the first molecular sieve component achieves the maximum NO.sub.x conversion at a lower NH.sub.3 fill level for the conditions selected than the second molecular sieve component. Preferably, the lower NH.sub.3 fill level of the first molecular sieve component is in the range of 10-80%.

(13) In certain embodiments in the SCR catalyst, the first and second molecular sieves can be selected independently from zeolites and non-zeolite molecular sieves. Preferably, one of the molecular sieve components is a small pore molecular sieve containing a maximum ring size of eight (8) tetrahedral atoms, preferably selected from the group of Framework Type Codes consisting of: ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW, LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG and ZON, with CHA, LEV, ERI, DDR, KFI, EAB, PAU, MER, AEI, GOO, YUG, GIS and VNI being particularly preferred. Preferably, the other molecular sieve component is selected from a small pore, medium pore, large pore or meso-pore size molecular sieve. Preferred medium pore molecular sieve include MFI, MWW, AEL, AFO, FER and HEU. Preferred large pore molecular sieve include FAU, AFI, AFR, ATO, BEA, GME, MOR and OFF.

(14) In certain embodiments, one or both of the first and second molecular sieves contains a substituent framework metal selected from the group consisting of As, B, Be, Co, Fe, Ga, Ge, Li, Mg, Mn, Ti, Zn and Zr. In certain embodiments, one or both of the first molecular sieve component and the second molecular sieve component contain one or more metal selected independently from the group consisting of Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag, In, Sn, Re, Ir and Pt, preferably Cr, Ce, Mn, Fe, Co, Ni and Cu.

(15) In one aspect of the invention, provided is an exhaust system for treating a flowing exhaust gas containing oxides of nitrogen from a mobile source of such exhaust gas, which system comprising a source of nitrogenous reducing agent arranged upstream in a flow direction from a selective catalytic reduction catalyst described herein. In certain embodiments, the system further comprises an oxidation catalyst disposed upstream of the source of nitrogenous reducing agent and the SCR catalyst. In certain embodiments, the system further comprises a filter disposed between the oxidation catalyst and the source of nitrogenous reducing agent.

(16) In one aspect of the invention, provided is a lean-burn internal combustion engine, such as a compression ignition engine or a positive ignition engine, comprising an exhaust system described herein. In certain embodiments, the engine comprises a NO.sub.x absorber which functions, at least in part, as the source of a nitrogenous reducing agent.

(17) In one aspect of the invention, provided is a vehicle comprising a lean-burn internal combustion engine described herein.

(18) In one aspect of the invention, provided is a method for converting oxides of nitrogen (NO.sub.x) in an exhaust gas of a mobile source the composition, flow rate and temperature of which exhaust gas are each changeable temporally, which method comprising the step of contacting the NO.sub.x with a nitrogenous reducing agent in the presence of a selective catalytic reduction catalyst comprising a combination of a first molecular sieve component and a second molecular sieve component, wherein in a direct comparison tested on the Federal Test Procedure (FTP) 75 cycle the catalyst has a higher cumulative conversion of NO.sub.x at equal or lower NH.sub.3 slip than either molecular sieve component taken alone. In certain embodiments of the method, the catalyst has a higher cumulative conversion of NO.sub.x, preferably to dinitrogen, at equal or lower NH.sub.3 slip than either molecular sieve component taken alone where the cumulative molar NO:NO.sub.2 ratio in feed gas entering said catalyst is equal to, or less than 1. In certain embodiments of the method, the NO.sub.x is converted at a temperature of at least 100 C., preferably from about 150 C. to 750 C. In certain embodiments of the method, the gas containing the NO.sub.x contacts the SCR catalyst at a gas hourly space velocity of from 5,000 hr.sup.1 to 500,000 hr.sup.1. In certain embodiments, about 60% to about 200% of theoretical ammonia contacts the SCR catalyst calculated at 1:1 NH.sub.3/NO and 4:3 NH.sub.3/NO.sub.2. In certain embodiments of the method, the NO:NO.sub.2 ratio in gas contacting the SCR catalyst is from about 4:1 to about 1:3 by volume.

(19) The following Examples are provided by way of illustration only.

EXAMPLES

Example 1

Method of Making Fresh 3 Wt % Cu/SSZ-13 (Aluminosilicate CHA) Catalysts

(20) Commercially available SSZ-13 zeolite (CHA) was NH.sub.4.sup.+ ion exchanged in a solution of NH.sub.4NO.sub.3, then filtered. The resulting materials were added to an aqueous solution of Cu(NO.sub.3).sub.2 with stirring. The slurry was filtered, then washed and dried. The procedure can be repeated to achieve a desired metal loading. The final product was calcined. The materials prepared according to this Example are referred to herein as fresh.

Example 2

Method of Making Fresh 5 Wt % Fe/Beta Catalyst

(21) Commercially available Beta zeolite was NH.sub.4.sup.+ ion exchanged in a solution of NH.sub.4NO.sub.3, then filtered. The resulting material was added to an aqueous solution of Fe(NO.sub.3).sub.3 with stirring. The slurry was filtered, then washed and dried. The procedure can be repeated to achieve a desired metal loading. The final product was calcined. The materials prepared according to this Example are referred to herein as fresh.

Example 3

Catalyst Mixtures

(22) Separate physical blends of fresh and aged 1:3 Fe/Beta:Cu/SSZ-13 by weight were prepared by physical mixture of samples made according to Examples 1 and 2. Likewise physical blends of 1:3 BEA-Ferrosilicate:CuSSZ-13 by weight were prepared.

Example 4

NOx Conversion Activity Tests

(23) The activity of the fresh powder samples prepared according to Examples 1, 2 and 3 were tested at 250 C. in a laboratory apparatus using the following gas mixture: 125 ppm NO, 375 ppm NO.sub.2 750 ppm NH.sub.3, 14% O.sub.2, 4.5% H.sub.2O, 4.5% CO.sub.2, N.sub.2 balance at a space velocity of 60,000 hr.sup.1. The test is stopped when 20 ppm NH.sub.3 is detected downstream of the sample. The results are shown in FIG. 3.

(24) From the results it can be seen that the Fe/Beta sample has a fast transient response, but limited maximum conversion. It also slips NH.sub.3 early on in the test compared with the Cu/SSZ-13 and Fe/Beta+Cu/SSZ-13 blend. Transient response is defined as the rate at which NOx conversion increases as the level of NH.sub.3 fill on the catalyst increases. The Cu/SSZ-13 has better, higher maximum conversion but a slower transient response. The combination of Fe/Beta and Cu/SSZ-13 gives fast transient response, higher maximum conversion, but also has higher conversion than the individual components at intermediate NH.sub.3 fill levels, which is evidence of synergy. Pre-aged 1:3 Fe/Beta:Cu/SSZ-13 will provide improved results as well.

Example 5

Comparison of Blends, Layers, and Zones Combinations

(25) Three samples of a 1:3 (by weight) BEA-Ferrosilicate:CuSSZ-13 combination were prepared and separately coated on substrates as a blend, zones, and layers. The three coated substrates were exposed to a test environment similar to that described in Example 4, except that the NO:NO.sub.2 ration was about 50:50. The results are shown in FIG. 5.

(26) From the results it can be seen that zones and blends achieve higher NOx conversion compared to blends.

Example 6

N2O Formation

(27) A samples of a 1:3 (by weight) FeBEA:CuSSZ-13 combination and three samples of a 1:3 (by weight) BEA-Ferrosilicate:CuSSZ-13 combination were prepared and separately coated on substrates. The FeBEA:CuSSZ-13 combination was coated as a blend, whereas the samples of BEA-Ferrosilicate:CuSSZ-13 combination were separately coated as a blend, zones, and layers. Each of the samples were exposed to a simulated diesel gas exhaust combined with NH.sub.3 dosing (20 ppm slip). The average N.sub.2O formation during exposure was recorded and is shown in FIGS. 6a and 6b.

(28) It is clear that the FeBEA:CuSSZ-13 blend produces significant N.sub.2O resulting in an apparent reduction in maximum conversion and N.sub.2 selective transient response. This reduction in conversion also outweighs that observed for the two components evaluated independently. Surprisingly, the BEA-Ferrosilicate:CuSSZ-13 blend produces substantially less N.sub.2O than that observed for any other CuSSZ-13/zeolite blend. However, layers and zones of BEA-Ferrosilicate:CuSSZ-13 maintain the low N.sub.2O make observed for the blend, but also show improved transient response under different NO.sub.2 levels (see FIG. 7).

Example 7

Effect of NO:NO2 Ratios

(29) Four samples of BEA-Ferrosilicate:CuSSZ-13 were prepared and tested for NOx conversion capacity during exposure to simulated diesel exhaust gas combined with a NH.sub.3 reductant. Testing was performed at 250 C. and gas hourly space velocity of about 60,000/hour. The results are provided in the table below. Here, the reference (ref.) catalyst is CuSSZ-13, low fill refers to an NH.sub.3 level at less than about 0.5 g/L of exhaust gas, and high fill refers to an NH.sub.3 at greater about 1 g/L of exhaust gas.

(30) TABLE-US-00003 0% NO.sub.2 50% NO.sub.2 75% NO.sub.2 3:1 BEA- Better than Better than ref Similar at low Ferrosilicate:CuSSZ-13 ref fills, better at (BLEND) high fills 1:1 BEA- Poor at low Better than ref Similar at low Ferrosilicate:CuSSZ-13 fills, better fills, better at (BLEND) at high high fills fills than ref 1:3 BEA- Similar to Much better Better than ref Ferrosilicate:CuSSZ-13 ref than ref. Much at low fills, (ZONE) better similar at high selectivity fill 1:3 BEA-Ferrosilicate Similar to Much better Better than ref layered over CuSSZ-13 ref than ref. Much at low fills, better similar at high selectivity fill

Example 8

Multiple Combinations

(31) Samples of FeBEA, CuSSZ-13, and BEA-Ferrosilicate were prepared can combined in the indicated combinations and multiple combinations shown in FIGS. 8a-8c. In the legends, the ratios are give by weight, blends are shown in parenthesises, and zones are indicated by //, with the first named component disposed upstream with respect to gas flow past the catalyst. Each of the combinations and multiple combinations were exposed to a simulated diesel gas exhaust gas stream containing an NH.sub.3 reductant. The NO:NO.sub.2 ratio in the exhaust gas was varied from only NO, 50:50 NO:NO.sub.2 (by weight), and 75% NO.sub.2 (by weight), to test the catalyst at different conditions. Each combination or multiple combination was evaluated for NO.sub.x conversion (corrected for N.sub.2O formation) as a function of NH.sub.3 fill level.