Patent Grant
7622095
The present invention relates to a catalyst composition for use in a lean NOx trap, and more particularly, to a low cost catalyst composition for use in a lean NOx trap for use in lean-burn engines such as gasoline and diesel engines and to methods of using such a composition.
Environmental concerns and government regulations have led to efforts to improve the removal of noxious combustion by-products from vehicle engine exhausts. Typically, the combustion by-products include carbon monoxide, hydrocarbons, sulfur oxides and various nitrogen oxides (NOx). In recent years, lean-burn engines such as gasoline and diesel engines have become more widely used as they offer improved fuel economy. However, because such engines operate under fuel-lean conditions, it is difficult to achieve reduction and removal of NOx in the oxidizing atmosphere of the engine exhaust. One solution to this problem has been the use of lean NOx traps (“LNT”), which typically include a catalyst comprising one or more precious metals such as platinum (Pt), palladium (Pd), or rhodium (Rh), and an alkali earth metal such as barium, and which are provided on a support material such as alumina. The precious metal catalysts are capable of absorbing or storing nitrogen oxides during lean-burn engine operation (where there is excess oxygen) and releasing and converting them when the oxygen concentration in the exhaust gas is lowered.
However, while such traps have been effective in the removal of NOx from the exhaust of lean burn engines, they require high loadings of precious metals such as platinum, which are very expensive. Accordingly, the high cost of such traps has prevented their wide application.
Attempts have been made to develop lean NOx traps with non-precious metals in an effort to reduce costs. See, for example, U.S. Pat. No. 5,837,212, which teaches a catalyst for use in a lean NOx trap comprised of manganese and potassium. However, while such a trap has effective NO storage efficiency, the trap does not effectively convert the stored NOx to N2. It is very important for the catalyst materials to exhibit good NOx storage efficiency as well as effective NOx conversion because low conversion efficiency will result in higher NOx emissions. By “storage efficiency,” we mean the efficiency of the LNT to store NOx during the lean period. By “conversion efficiency” we mean the efficiency of the LNT to convert NOx during both the lean and rich periods.
Ruthenium has also been proposed for use as a reducing catalyst component, typically in combination with other precious metals. See, for example, U.S. Pat. No. 5,756,057. While ruthenium is also considered a precious metal, it is much lower in cost than platinum and other precious metals. However, although ruthenium has generally been regarded as an effective reducing catalyst for NOx, we have not seen ruthenium being used as a catalyst for a lean NOx trap because its effeciveness for NO oxidation, which is the first step in reactions involving a lean NOx trap, has not been demonstrated.
Accordingly, there is a need in the art for a catalyst composition for use in a lean NOx trap which is low in cost, which exhibits high NOx storage efficiency, and high (NOx to N2) conversion efficiency.
The present invention meets this need by providing a lean NOx trap composition utilizing ruthenium in place of higher cost precious metals such as platinum. The lean NOx trap composition provides high NOx storage efficiency and high NOx conversion efficiency when incorporated in a lean NOx trap.
According to one aspect of the invention, a lean NOx trap composition is provided. The lean NOx trap composition includes a support material; a ruthenium catalyst wherein the ruthenium catalyst is free of other precious metals; and a NOx absorbent material comprising an alkaline earth metal; wherein the lean NOx trap composition has a storage efficiency of at least 70% at a temperature in a range of from about 400° C. to about 500° C., and a conversion efficiency of at least 50% at a temperature in a range of from about 300° C. to about 450° C.
Another aspect of the invention involves a method of removing harmful gases from an automobile exhaust containing NOx, CO, and hydrocarbons. The method includes contacting a lean NOx trap composition with a first exhaust gas mixture, the first exhaust gas mixture comprising exhaust gases from an internal combustion engine operating in a fuel-lean condition, the lean NOx trap composition comprising: a support material; a ruthenium catalyst wherein the ruthenium catalyst is free of other precious metals; and a NOx absorbent material comprising an alkaline earth metal; wherein the lean NOx trap composition has a storage efficiency of at least 70% at a temperature in a range of from about 400° C. to about 500° C., and a conversion efficiency of at least 50% at a temperature in a range of from about 300° C. to about 450° C.; and contacting the lean NOx trap composition with a second exhaust mixture, the second exhaust gas mixture comprising exhaust gases from the internal combustion engine operating in a fuel-rich condition.
We have found that a ruthenium or ruthenium oxide catalyst can effectively oxidize NO, and demonstrates excellent NOx storage efficiency when it is incorporated into a lean NOx trap. The use of ruthenium allows lean NOx traps to be produced at much lower cost than previous traps which incorporate higher cost precious metals.
The catalyst composition can comprise a ruthenium catalyst, such as ruthenium or ruthenium oxide, in combination with an alkaline earth metal on a support material. The ruthenium catalyst component functions to oxidize NO to NO2 and to reduce NOx to N2, and is generally present in an amount of about 0.1 to about 10% by weight of washcoat, typically about 0.3 to about 5% by weight of washcoat, or about 0.5 to about 3% by weight of washcoat.
The alkaline earth metal functions to absorb NOx and is generally present in an amount of about 5 to about 50% by weight of washcoat, or about 5 to about 30% by weight of washcoat, or about 5 to about 20% by weight of washcoat. The alkaline earth metal is typically barium. Suitable barium precursors include, but are not limited to, barium oxide, barium carbonate, barium nitrate, or barium acetate.
The support material of the catalyst composition provides support to the catalyst and NOx absorbent components. Suitable support materials for use in the present invention include, but are not limited to, Al2O3, SiO2, TiO2, and zeolites. An alumina support is commonly used.
The catalyst composition may further include the use of stabilizers which provide thermal stability to the composition. Suitable stabilizers include, but are not limited to, rare earth oxides, such as lanthanum and cerium, and manganese, and zirconium. Other suitable thermal stabilizers include those disclosed in commonly assigned application Ser. No. 10/710,920, entitled Processing Methods And Formulations To Enhance Stability Of Lean NOx Trap Catalysts Based On Alkali- And Alkaline Earth Metal Compounds and commonly assigned application Ser. No. 10/710,922, entitled Thermally Stable Lean NOx Trap, the disclosures of which are hereby incorporated by reference.
The catalyst composition of the present invention can be incorporated into a lean NOx trap by any suitable process. A washcoating process will be described, although other processes could be used. The support material (for example, aluminum oxide) is first washcoated onto a monolithic substrate. Suitable substrates for use in the present invention include, but are not limited to, cordierite and metallic substrates. The substrate can be repeatedly washcoated with the support material until a washcoat weight of 20% to 45% is achieved. After each deposition, the substrate is dried at about 80° C. for about six hours and then calcined at temperatures up to about 600° C. for about six hours. When the washcoat level is achieved, a solution of the ruthenium catalyst component is prepared. The solution is deposited onto the substrate as a percentage of the washcoat. The ruthenium can be deposited two to three times to ensure a uniform distribution. After each deposition of the catalyst, the substrate is dried at about 80° C. for about six hours and then calcined under reducing conditions at temperatures up to about 600° C. for about six hours. After the catalyst component has been applied, the alkaline earth metal is applied. The alkaline earth metal is deposited in an amount based on the amount of support material deposited. After each deposition, the substrate is dried at about 80° C. followed by calcination at temperatures up to about 600° C. for about six hours.
In use, the lean NOx trap of the present invention is placed in the exhaust of a vehicle having an engine which can operate in a fuel-lean condition and which contains NOx and other harmful gases. The trap is exposed to the exhaust gases from the engine operating in a fuel-lean condition and operating in a fuel-rich condition such that at least a portion of the NOx in the gas is converted to N2, preferably at a temperature between about 125° C. and 700° C.
Generally, the lean NOx trap of the present invention has a storage efficiency of at least about 70% at a temperature in a range of from about 400° C. to about 500° C., and a conversion efficiency of at least about 50% at a temperature in a range of from about 300° C. to about 450° C. Typically, the storage efficiency is at least about 80% at a temperature in a range of from about 400° C. to about 500° C., and the conversion efficiency is at least about 60% at a temperature in a range of from about 300° C. to about 400° C. The storage efficiency can be at least about 90% at a temperature in a range of from about 400° C. to about 500° C., and the conversion efficiency can be at least about 60% at a temperature in a range of from about 300° C. to about 400° C.
One of the problems we encountered with ruthenium LNTs is that they tend to have a higher light-off temperature compared with similar platinum LNTs, i.e., the catalyst is effective in converting 50% of the NOx only at higher temperatures. This results in ineffective storage efficiency at low temperatures. A high NOx storage efficiency at low temperature is important because it broadens the applications in which the traps can be used and can increase operation flexibility.
We discovered that by eliminating the use of chlorine-containing Ru precursors and/or by adding magnesium to the catalyst composition, improved low temperature storage efficiency is obtained. The magnesium functions to lower the light-off temperature, and when used in combination with a Ru catalyst prepared from chlorine-free precursors, the light-off temperature may be reduced even further.
The magnesium is generally present in an amount of from about 1 to 20% by weight of washcoat, typically about 1 to 10% by weight of washcoat.
The lean NOx trap composition of this embodiment is generally prepared as described above, with the following changes. The ruthenium catalyst component is prepared from a chlorine-free precursor (if one is to be used), such as aqueous ruthenium nitrosyl nitrate, Ru(NO)(NO3)3. After the catalyst is applied to the substrate, the barium and magnesium (if present) are then prepared as a solution from, for example, barium acetate and magnesium nitrate. The weight ratio of Ba—Mg used in the catalyst composition is generally from about 50:50 to 95:5, and typically from about 70:30 to 90:10.
The amounts of Ba and Mg deposited may vary based on the amount of aluminum oxide deposited. The Ba/Mg solution can be washcoated onto the monolith substrate in about two depositions, each deposition followed by drying at 80° C. for six hours and calcining at 600° C. for six hours.
Another problem we found is that the oxidation activity and storage efficiency of lean NOx traps using ruthenium catalysts vary greatly, depending on how the catalyst composition is prepared.
We discovered that an optimum proximity between the catalyst component and the NOx absorbent material helps to improve for the oxidation activity and storage efficiency of the lean NOx trap.
One way of achieving optimum proximity of the catalyst component (Ru) and NOx absorbent material (Ba), is to deposit the ruthenium prior to depositing the barium compound. By optimum proximity, we mean the catalyst component and NOx absorbent material are positioned on the support material in such a way that optimum NOx storage efficiency and optimum conversion efficiency is achieved. This optimum proximity results in better conversion efficiency for a lean NOx trap utilizing a ruthenium/barium combination. The optimum proximity of the ruthenium and barium also maintains NOx storage efficiency, providing effective spill-over of the oxidized NOx (NO2) to be stored. By spill-over, we mean the spillover of oxygen atoms from the ruthenium to the absorbent material (Ba) for oxidizing NO to NO2 and further to NO3−, or the spillover of the NO2 to the absorbent material.
Referring now to
The resulting trap demonstrates a NOx storage efficiency of at least 60% at a temperature between about 350 to about 475° C., and a conversion efficiency (NO to NO2) of at least 50% at a temperature between about 375 to about 450° C.
In order that the invention may be more readily understood, reference is made to the following examples of compositions within the scope of the present invention, which examples are intended to be illustrative of the invention, but are not intended to be limiting in scope.
Examples 1 and 2 compare the use of ruthenium as a catalyst for an LNT with the use of platinum as a catalyst for an LNT.
Three catalyst compositions were prepared in accordance with the present invention. The first composition comprised 1% by weight platinum on an alumina support. The second composition comprised 1% by weight palladium on an alumina support, and the third composition comprised 0.5% by weight ruthenium on an alumina support. As shown in
A first lean NOx trap was prepared from a catalyst composition comprising 2% by weight platinum and 20% barium oxide on alumina. A second lean NOx trap was prepared from a catalyst composition in accordance with the present invention which comprised 2% by weight ruthenium and 20% barium oxide on alumina. The two traps were tested for NO storage efficiency by subjecting them to lean-rich cyclic testing in a flow reactor with a space velocity of 25,000 hr−1 with the feedgas during the lean cycle containing 500 ppm NO, 5% O2, 10% water, 10% CO2, and balance nitrogen; and during the rich cycle containing 5% CO, 500 ppm NO, 10% water, 10% CO2, 1.67% H2 and balance N2. As can be seen from
The effect on light-off temperature of the use of a chlorine-free ruthenium precursor and the use of magnesium was explored in Example 3.
A series of lean NOx traps were prepared using a wet impregnation method. In the reference sample, the LNT contained 2% by weight Ru, impregnated from RuCl3 and dried at 80° C. and calcined at 600° C. under a reducing condition for 6 hours. 20% by weight Ba (from barium acetate) was then coated on and dried and calcined under the same conditions. A lean NOx trap was prepared using a ruthenium catalyst prepared using Ru(NO)(NO3)3 as the precursor. Another LNT was prepared which included Mg with a Ba—Mg weight ratio of 80:20 (prepared from barium acetate and magnesium nitrate).
The test conditions were as follows: the monolith catalyst was crushed and sieved to 40-60 mesh. For each test, 300 mg of catalyst was used. The total flow rate was kept at 750 ml/min. This amounts to an equivalent space velocity of approximately 80 k hr−1. The feedgas contained 500 ppm NO, 10% CO2, 10% H2O, with N2 balance. In addition, during the lean condition (such as during the oxidation reaction), the feedgas contained 6% O2. During the rich period, the feedgas contained 4% CO and 1.3 H2 (zero O2). The feedgas did not contain hydrocarbons or sulfur during the reaction unless specified.
The effect of the proximity of the catalyst component (Ru) and NOx absorbent material (Ba) is illustrated in Examples 4 and 5.
Model catalyst powders were prepared and tested for NO storage efficiency and NO to NO2 oxidation activity in a micro-reactor.
For the tests, equal amounts (wt %) of Ru/Al2O3 (2% Ru) and BaO/Al2O3 (20% BaO) powders were sieved and then either mechanically mixed (by shaking the sample vial) or wet ball-milled. 100 mg of powder was then mixed with 200 mg of equal sized blank cordierite. Mechanical mixing and wet ball-milling create different proximity between the active components of Ru and Ba. Mechanical mixing resulted in infinite molecular distance between Ru and Ba, while ball milling resulted in increased proximity between Ru and Ba.
Crushed monolith model catalysts comprising (2% Ru/20% BaO/Al2O3) were prepared. In the first sample, Ba was deposited onto the alumina support followed by Ru. In the second sample, Ru was deposited first followed by Ba. Both the Ru and BaO each went through 80° C. drying overnight and 600° C. reduced calcinations for 6 hours.
A third sample was prepared by impregnating Ru and Ba onto Al2O3 together followed by drying and calcining.
The above data shows that the optimum proximity of Ru and Ba is as shown in
While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the compositions and methods disclosed herein may be made without departing from the scope of the invention.
This application claims the benefit of provisional application Ser. No. 60/601,069, entitled Catalyst Composition for Use in a Lean NOx Trap, filed Aug. 12, 2004; provisional application Ser. No. 60/601,027, entitled Catalyst Composition for Use in a Lean NOx. Trap, filed Aug. 12, 2004; provisional application Ser. No. 60/601,049, entitled Catalyst Composition for Use in a Lean NOx. Trap, filed Aug. 12, 2004; and provisional application Ser. No. 60/600,933, entitled Method of Making a Catalyst Composition for Use in a Lean NOx Trap, filed Aug. 12, 2004, all of which are incorporated herein by reference.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3819536 | Dalla Betta et al. | Jun 1974 | A |
| 3972837 | Acres et al. | Aug 1976 | A |
| 4001371 | Remeika et al. | Jan 1977 | A |
| 4049583 | Lauder | Sep 1977 | A |
| 4077913 | Acres et al. | Mar 1978 | A |
| 4089810 | Diwell et al. | May 1978 | A |
| 4127510 | Harrison et al. | Nov 1978 | A |
| 4162235 | Acres et al. | Jul 1979 | A |
| 4200554 | Lauder | Apr 1980 | A |
| 4956330 | Elliott et al. | Sep 1990 | A |
| 5064803 | Nunan | Nov 1991 | A |
| 5427753 | Miura et al. | Jun 1995 | A |
| 5435981 | Ichiki et al. | Jul 1995 | A |
| 5449504 | Kasahara et al. | Sep 1995 | A |
| 5589147 | Farnos et al. | Dec 1996 | A |
| 5597771 | Hu et al. | Jan 1997 | A |
| 5635142 | Ichiki et al. | Jun 1997 | A |
| 5747401 | Cuif | May 1998 | A |
| 5747410 | Muramatsu et al. | May 1998 | A |
| 5756057 | Tsuchitani et al. | May 1998 | A |
| 5837212 | Hepburn et al. | Nov 1998 | A |
| 5879645 | Park et al. | Mar 1999 | A |
| 5972821 | Nojima et al. | Oct 1999 | A |
| 5989507 | Sung et al. | Nov 1999 | A |
| 5992142 | Pott | Nov 1999 | A |
| 6003303 | Peter-Hoblyn et al. | Dec 1999 | A |
| 6087298 | Sung et al. | Jul 2000 | A |
| 6174835 | Naito et al. | Jan 2001 | B1 |
| 6350423 | Aoyama | Feb 2002 | B1 |
| 6391822 | Dou et al. | May 2002 | B1 |
| 6419890 | Li | Jul 2002 | B1 |
| 6477834 | Asanuma et al. | Nov 2002 | B1 |
| 6497848 | Deeba et al. | Dec 2002 | B1 |
| 6531425 | Golden | Mar 2003 | B2 |
| 6555492 | Faber et al. | Apr 2003 | B2 |
| 6620762 | Tan et al. | Sep 2003 | B2 |
| 6699448 | Wu et al. | Mar 2004 | B2 |
| 6777370 | Chen | Aug 2004 | B2 |
| 6821925 | Carruthers et al. | Nov 2004 | B2 |
| 6852666 | Bouly et al. | Feb 2005 | B1 |
| 6864212 | Tanev | Mar 2005 | B2 |
| 6923945 | Chen | Aug 2005 | B2 |
| 7213395 | Hu et al. | May 2007 | B2 |
| 20020076373 | Molinier et al. | Jun 2002 | A1 |
| 20020103078 | Hu et al. | Aug 2002 | A1 |
| 20020155040 | Sakanushi et al. | Oct 2002 | A1 |
| 20030021745 | Chen | Jan 2003 | A1 |
| 20030103886 | Dou | Jun 2003 | A1 |
| 20030106306 | Nakatani et al. | Jun 2003 | A1 |
| 20030139288 | Cai et al. | Jul 2003 | A1 |
| 20030147796 | Suga et al. | Aug 2003 | A1 |
| 20030154713 | Hiratsuka et al. | Aug 2003 | A1 |
| 20030172646 | Hiratsuka et al. | Sep 2003 | A1 |
| 20030175192 | Hu et al. | Sep 2003 | A1 |
| 20030181324 | Hotta et al. | Sep 2003 | A1 |
| 20030198582 | Golden | Oct 2003 | A1 |
| 20040043343 | Kamijo | Mar 2004 | A1 |
| 20040077492 | Yaluris et al. | Apr 2004 | A1 |
| 20040209772 | Fukunaga et al. | Oct 2004 | A1 |
| 20050148463 | Jen et al. | Jul 2005 | A1 |
| 20060034741 | Li et al. | Feb 2006 | A1 |
| 20060035782 | Jen et al. | Feb 2006 | A1 |
| Number | Date | Country |
|---|---|---|
| 0 625 633 | Nov 1994 | EP |
| 0 982 487 | Mar 2000 | EP |
| 1 241 329 | Sep 2002 | EP |
| 1 302 647 | Apr 2003 | EP |
| 1 318 282 | Jun 2003 | EP |
| 1 413 718 | Apr 2004 | EP |
| 2001-009271 | Jan 2001 | JP |
| WO 0050168 | Aug 2000 | WO |
| WO 0109271 | Feb 2001 | WO |
| WO 0209852 | Feb 2002 | WO |
| WO 02058825 | Aug 2002 | WO |
| WO 02083301 | Oct 2002 | WO |
| WO 03008095 | Jan 2003 | WO |
| WO 03040259 | May 2003 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20060034740 A1 | Feb 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60601069 | Aug 2004 | US | |
| 60601027 | Aug 2004 | US | |
| 60601049 | Aug 2004 | US | |
| 60600933 | Aug 2004 | US |
