Nitrogen oxides storage catalysts containing cobalt

Nitrogen oxides (NOx) storage catalysts comprising cobalt and barium with a lean NOx storage ratio of 1.3 or greater. The NOx storage catalysts can be used to reduce NOx emissions from diesel or gas combustion engines by contacting the catalysts with the exhaust gas from the engines. The NOx storage catalysts can be one of the active components of a catalytic converter, which is used to treat exhaust gas from such engines.

FIELD OF THE INVENTION

The invention is related to nitrogen oxides (NOx) storage catalysts, and the use of such catalysts to treat exhaust gas containing nitrogen oxides.

BACKGROUND OF THE INVENTION

Elimination of NOxfrom the exhaust of a lean-burn engine is a challenging problem because it can only be removed via reduction. For example, a diesel engine operating under lean conditions with excess oxygen provides excellent fuel economy, however the reduction of NOxin these oxidizing conditions is very difficult. For a gasoline engine operating under stoichiometric conditions three-way catalyst technology can be used to reduce HC (hydrocarbon), CO and NOxemissions. However, as with the diesel engine, the three-way catalyst is not able to reduce NOxunder lean conditions. Therefore, with respect to both engine types there remains a need for the development of new catalyst systems to remove NOxfrom the exhaust gas under lean-burn conditions.

The conversion of NOxto stable nitrates, and storing the nitrates under lean conditions, followed by reducing the nitrates or “stored NOx” to dinitrogen under rich conditions has been investigated with some success. Some of the early NOxstorage catalysts fitted to the exhaust systems of vehicles contained barium compounds. These catalyst systems stored NOxwhen the engines operated under lean conditions, and reduced the stored NOxwhen the gas was made rich. See, B. J. Cooper, et al. inCatalysis and Automotive Pollution Control, eds. A. Curcq and A. Frennet (Elsevier, Amsterdam, 1987) p. 117. Unfortunately, the present levels of sulfur compounds in fuel and the resulting SO2in the exhaust gas has prevented the commercial introduction of these catalysts, which are relatively susceptible to sulfur poisoning.

A new generation of NOxstorage catalysts tested against simulated exhaust gas from a lean-burn gasoline engine has provided an average NOxconversion of more than 90% over a test cycle in which the air/fuel ratio was cycled between lean and rich. See, W. Bögner, et al.,Applied CatalysisB7 (1995) 153. The feed gas was alternated between oxidizing (lean) and a short reducing (rich) period every two minutes or so resulting in NOxstorage during the lean period and conversion to dinitrogen during the rich period.

Over the past decade significant efforts have been made toward the development of NOxstorage catalysts containing platinum as well as other noble metals as an oxidative and reductive catalyst. Barium has been used as the primary NOxstorage material though other NOxstorage components have also been investigated. The use of transition metals to improve resistance to sulfur poisoning has also been investigated. See, K. Yamazaki, et al.,Applied Catalysis, B 30 (2001) 459.

Lean burn gasoline and diesel engines not only improve the fuel efficiency of automobiles, but also lead to a reduction in the emission of greenhouse gases. Impeding the widespread implementation of lean burn engines is the inability of current three-way catalytic converters to reduce nitrogen oxides under oxidizing lean conditions. Extensive research has been performed in search of alternative catalysts that will reduce NOxin oxygen rich environments under steady state conditions, but an acceptable catalyst has not yet been discovered.

SUMMARY OF THE INVENTION

Nitrogen oxides (NOx) storage catalysts comprising cobalt and barium with a lean NOxstorage ratio of 1.3 or greater. The NOxstorage catalysts can be used to reduce NOxemissions from diesel or gas combustion engines by contacting the catalysts with the exhaust gas from the engines. The NOxstorage catalysts can be one of the active components of a catalytic converter, which is used to treat exhaust gas from such engines.

DETAILED DESCRIPTION OF THE INVENTION

The addition of cobalt to traditional platinum/barium NOxstorage catalysts has led to a significant increase in NOxstorage. Also, by using cobalt as the oxidizing metal in NOxstorage catalysts the amount of platinum in such catalysts can be reduced. In fact, the cobalt can completely replace the platinum in such catalyst systems. The substitution of cobalt for platinum, the later being about fifty times more expensive than cobalt, provides a significant cost reduction without a sacrifice in catalytic performance.

The NOxstorage catalysts described and claimed in this application can be used to reduce NOxemissions from diesel or gas combustion engines by contacting the catalysts with the exhaust gas from the engines. In such a case, the catalysts are typically supported on a refractory inorganic oxide. The supported catalysts are used to treat the exhaust gas from gasoline and diesel engines thereby reducing NOxemissions. In one application, the supported catalysts can form part of a catalytic converter for a vehicle.

The NOxstorage catalysts comprise cobalt and barium, and have a lean NOxstorage ratio of 1.3 or greater. Other NOxstorage catalysts of the invention can have a lean NOxstorage ratio of 1.5 or greater. Still other NOxstorage catalysts of the invention can have a lean NOxstorage ratio of 2.0 or greater.

In one embodiment, the catalyst can further contain platinum. The platinum is typically present in smaller amounts than NOxstorage traditional catalysts based solely on barium and platinum.

The presence of the cobalt in the catalyst provides the necessary oxidizing regions necessary to reduce NOxemissions. In one embodiment, the NOxstorage catalysts are used in combination with a reduction catalyst containing rhodium. The two catalytic components of the catalyst system work together to reduce NOxemissions from exhaust gas.

In another embodiment, the NOxstorage catalysts consist essentially of cobalt, platinum and barium supported on an inorganic oxide. Again, the catalyst has a lean NOxstorage ratio of 1.3 or greater. Typically, the cobalt is predominantly in the form of Co3O4and the barium in the form of BaCO3.

The term “lean NOxstorage ratio” is a catalytic performance ratio defined by the following relationship:
[xPtyBazCo]/{[xPtyBa]+[zCo]}
wherein [xPt yBa zCo] is the lean NOxstorage value for a catalyst of the invention containing cobalt, barium and platinum; [xPt yBa] is the lean NOxstorage value for a platinum-barium catalyst without cobalt; and [zCo] is the lean NOxstorage value for a cobalt catalyst. The lean NOxstorage value for each of the NOxstorage catalysts is determined by measuring the integrated area between the inlet NOxconcentration and outlet NOxconcentration in the fuel lean state from time zero to the time the outlet NOxconcentration is 300 ppm for each of the NOxstorage catalysts as represented byFIG. 1. The lean NOxstorage measurements are collected after the NOxstorage data is reproducible over several rich-lean fuel cycles. See, Example 1.

The lean NOxstorage values are determined using the actual weight loadings for each of the prepared catalysts. The term “actual weight loading” defines a catalyst by the metal's actual weight percent loading, e.g., as measured by atomic absorption or another elemental analysis technique.

One of ordinary skill in the art understands that wet impregnation techniques used to prepare supported metal catalysts typically do not provide actual weight percent loadings based on the nominal weight loading. The term “nominal weight loading” is calculated in-part by the amount of metal precursor used to prepare the catalyst and the weight percentage of metal in the metal precursor. As shown in Example 2, Table 2, the actual weight loadings of a respective metal can be very similar to or very different from the nominal weight loadings. For example, in the case of platinum, the actual weight loading is less than the nominal weight loading in all of the catalysts presented in Table 2. In contrast, there is much less of a difference between actual weight loading and nominal weight loading for cobalt.

FIG. 2is a graphical representation of lean NOxstorage values for each of the stated catalysts. The lean NOxstorage values are calculated by the following mathematical expression and data representative ofFIG. 1.

NOx⁢⁢storage=∫t=0t=to⁢(inlet⁢⁢NOx-outlet⁢⁢NOx)⁢ⅆt
where t0is the time when the outlet NOxconcentration reaches 300 ppm.

FIG. 2shows the lean NOxstorage values for the catalysts [1 Pt], [5Co], [15Ba] and [1Pt/5Co]. An exemplary calculation to determine the lean NOxstorage ratio using the data ofFIG. 1for a given catalyst of the invention is as follows. The lean NOxstorage values for [1Pt/15Ba] and [5Co] are 31 μmol/g and 6 μmol/g, respectively. The lean NOxstorage value for [1Pt/15Ba/5Co] is 67 μmol/g. Given these values the “lean NOxstorage ratio” is 67/{31+6} or 1.86.

To calculate lean NOxstorage values the actual weight loading for cobalt, barium and platinum in the inventive catalyst must be within 50% of the actual weight loadings for each of these metals in the non-inventive (comparative catalyst). In the example above, the non-inventive catalysts [1Pt/15Ba] and [Co] have actual weight loadings of 0.58%, 11.75% and 5.32% for platinum, barium and cobalt, respectively. The actual weight loadings of the inventive catalyst are 0.37%, 12.88% and 6.32% for platinum, barium and cobalt, respectively. Therefore, the actual weight loading of the metals: Pt, 0.58-0.37; Ba, 11.75-12.88; and Co, 5.32-6.32 are all within 50% for each respective metal. For example, in the case of platinum (0.58-0.37)/0.58=0.36 (36%).

The promotional effect provided by the addition of cobalt to a platinum/barium catalyst is shown by the near doubling of the NOxstorage capacity of the catalyst 1Pt/5Co/15Ba compared to the traditional 1 Pt/15Ba catalyst. As shown inFIG. 1, the time prior to a detectable outlet NOxconcentration for the 1Pt/5Co/15Ba catalyst was nearly twice as long as the time for the other two catalysts. There is also a higher steady state lean NOxconcentration associated with the 5Co/15Ba catalyst than the Pt containing catalysts. The NOxconcentration at steady state is in-part governed by the reducing efficiency of the catalyst, thus the NOxreducing efficiency of cobalt is believed to be lower than that of platinum.

FIG. 3depicts the X-ray diffraction pattern of the catalyst 1Pt/15Ba/5Co following baseline correction and smoothing using a 2ndorder polynomial, 11 point Savitzky-Golay algorithm. The peak at approximately 36.8 2θ is attributed to cubic Co3O4, a material known for its high oxidizing capacity, which can explain the relatively high promotional effect of cobalt over both manganese and iron. The peak at approximately 59.9 2θ is also attributed to Co3O4. The relatively strong peak at approximately 46.0 2θ is attributed to BaCO3.

It is believed that the Co3O4sites adjacent to barium storage sites increase the interface contact area between the oxidizing and storage component of the catalysts. This is important for increasing the NO2spillover to barium storage sites, another key step in NOxstorage. Therefore, by increasing the NO oxidation and NO2spillover to barium storage sites in the catalysts an increase in the NOxstorage is observed.

The invention is also directed to NOxstorage catalysts that are platinum-free (Pt-free) catalysts. A Pt-free catalyst contains less than 0.03 wt % platinum. The Pt-free catalysts comprise cobalt and barium. Again, these catalysts are typically supported on an inorganic oxide. The applicants were surprised to find that the NOxstorage performance of the catalyst 5Co/15Ba was essentially equivalent to that of 1 Pt/15Ba suggesting that 5 wt % Co was comparable of 1 wt % Pt with respect to NOxstorage.

FIG. 5depicts the X-ray diffraction pattern of the catalyst 5Co/15Ba following baseline correction and smoothing using a 2 order polynomial, 11 point Savitzky-Golay algorithm. The peaks at approximately 37.3 2θ and approximately 59.0 2θ is attributed to cubic Co3O4, a material known for its high oxidizing capacity, which can explain the relatively high promotional effect of cobalt over both manganese and iron. The relatively strong peak at approximately 46.0 2θ is attributed to BaCO3. The peak at approximately 23.8 2θ is also attributed to Co3O4.

The Pt-free catalysts will have a lean NOxstorage ratio of 1.3 or greater. Other Pt-free catalysts can have a lean NOxstorage ratio of 1.5 or greater. Still other Pt-free catalysts can have a lean NOxstorage ratio of 2.0 or greater.

The lean NOxstorage ratio for a Pt-free catalyst is defined by the following relationship:
[yBazCo]/{[yBa]+[zCo]}
wherein [yBa zCo] is the lean NOxstorage value for a Pt-free catalyst of the invention containing cobalt and barium; [yBa] is the lean NOxstorage value for a barium catalyst; and [zCo] is the lean NOxstorage value for a cobalt. As already described above and in Example 1, the lean NOxstorage values are determined by measuring the integrated area between the inlet NOxconcentration and outlet NOxconcentration in the fuel lean state from time zero to the time the outlet NOxconcentration is 300 ppm for each of the nitrogen storage catalysts.

Again, if one uses the catalytic data provided inFIG. 2for each of the tested catalysts, that is, a lean NOxstorage value of 5 μmol/g for [15Ba], 6 μmol/g for [5Co] and 31 μmol/g for [5Co/15Ba] one would obtain a lean NOxstorage ratio of 31/11 or 2.82.

FIG. 4is a graphical representation of catalysts with different transition metals. In this case, a traditional 0.5Pt/7.5Ba NOxstorage catalyst was modified by incorporating 2.5Fe, 2.5Mn or 2.5Co. The addition of either iron or manganese shows an increase in the NOxstorage by 25-30%, whereas Co has a considerably higher promotional effect increasing lean NOxstorage by more than 100%.

The catalyst can be supported on many types of inorganic oxides selected from the group consisting of alumina, silica, titania, zirconia aluminosilicates, and mixtures thereof with alumina being preferred. For example, high surface area alumina materials, also referred to as gamma alumina typically exhibit a BET (Brunauer, Emmett, and Teller) surface area in excess of 60 square meters per gram (m2/g), and often up to about 200 m2/g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina. Metal oxides other than activated alumina can also be used as a catalyst support. For example, bulk ceria, zirconia, alpha alumina and other materials are known for such use. Although many of these materials have a lower BET surface area than activated alumina, that disadvantage tends to be offset by the greater durability of the resulting catalyst.

The inorganic oxides can also include zeolites, in particular, zeolites in admixture with one or more of the support oxide materials listed in the above paragraph. Zeolites coated with the cobalt, barium, and optionally platinum, are of particular importance for achieving as low a light-off temperature as possible for the hydrocarbons in diesel exhaust gas. Owing to their acid surface properties, zeolites have a high cracking activity for the long-chained hydrocarbons of the exhaust gas. The long-chained molecules are therefore broken down into smaller fragments by contact with the zeolites, which smaller fragments can then more readily be oxidized by the cobalt and/or platinum in the NOxstorage catalysts. The weight ratios of the support oxides and the zeolites is from 10:1 to 2:1, and in most instances the weight ratios is from 8:1 to 3:1. Particularly advantageous and ageing-resistant oxidizing activities for carbon monoxide and hydrocarbons can be achieved with a weight ratio of 7:1 to 5:1.

Of the large number of zeolites available, the following are especially suitable beta-zeolites, zeolites of the faujasite type, such as, for example, Y-zeolites, especially de-aluminized Y-zeolites, mordenites and zeolites, having a high silicon dioxide content, of the pentasil type, especially ZSM-5. Each of the above zeolites can be used alone or in an admixture of one or more zeolites. The zeolites are preferably used in their acid H+form. De-aluminized Y-zeolites and ZSM-5 zeolites, each having a modulus of more than 30, typically more than 40, can have unique advantages. The modulus of a zeolite denotes its molar ratio of silicon dioxide to aluminum oxide.

The inorganic oxide can also be treated with a titanium-zirconium (Ti—Zr) composite oxide. The Ti—Zr composite oxide will contain a molar ratio of titanium to zirconium from 1/9 to 9/1. Preparation of a refractory inorganic support containing the Ti—Zr composite is described in U.S. Pat. No. 5,804,152, the entire disclosure of which is incorporated herein by reference.

The oxide support can be in any configuration, shape, or size which exposes the metal components of the catalyst to the exhaust gas to be treated. The choice of configuration, shape and size of the support will depend in-part on the specific use of the catalysts. Convenient shapes that can be used include solid particulate forms such as pills, pellets, granules, rings, spheres, etc. The particulate form can be advantageous if large volume of catalyst is required, or if periodic replacement of the catalyst is required.

The catalysts of the invention are typically prepared by impregnating the inorganic oxide with one or more solutions containing cobalt, platinum and barium. The process of impregnating metals on and in inorganic supports, e.g., alumina, is well known to those of ordinary skill in the art.

It is to be understood by one of ordinary skill in the art that metal containing solutions of any metal concentration can be used, particularly, if a multiple loading impregnating procedure is used. For example, if a 5 w/w % cobalt nominal weight loading is desired, one or more impregnating steps can be used until the desired loading of cobalt is achieved.

The NOxstorage catalysts can be used in combination with one or more reduction catalysts, in particular reduction catalysts containing rhodium. The reduction catalyst is needed to reduce the stored NOxunder fuel rich conditions. For example a nitrogen storage and reduction catalyst can contain both platinum and cobalt as the oxidizing metals, barium as the NOxstorage material, and rhodium as the reduction catalyst supported on a refractory inorganic oxide.

Alternatively, the NOxstorage catalysts of the invention are “Pt-free” catalysts that contain cobalt as an oxidizing metal, barium as the NOxstorage material, and rhodium as the reduction catalyst supported on an inorganic oxide.

High Throughput Experimental Setup and Testing Procedure

All catalytic tests were performed using a 16-channel parallel reactor. Details concerning the reactor has been described in J. Lauterbach et al., inApplied Catalysis, A 254, (2003) 357, of which the description and operation of the 16-channel parallel reactor is incorporated herein by reference. The reaction products from all 16 channels were analyzed simultaneously using Fourier transform infrared (FTIR) imaging. The optical setup consists of a Bruker Equinox 55 FTIR spectrometer interfaced with a 64×64 pixel mercury cadmium telluride FPA detector (Santa Barbara Focalplane, Goleta, Calif., USA) capable of collecting IR spectra of the effluents from all 16 reactors in less than 2 seconds. Details of the optical setup and analytical methods can be found in previous publications.

A typical data set collected during a switch from fuel rich to fuel lean conditions for three different catalysts is shown inFIG. 1. The outlet NOxconcentration profiles are shown as functions of time, with the dotted lines representing the inlet NOxconcentration and 300 ppm NOx. The performance of each catalyst was based on the NOxstorage, defined as the integrated area between the inlet NOxconcentration and outlet NOxconcentration, in the fuel lean state, from time zero to the time when the outlet NOxconcentration reached 300 ppm. Catalytic performance was evaluated when the NOxstorage behavior became reproducible over several cycles. The feed gas consisted of 0.14% v/v NOx6% v/v O2, 0.9% v/v CO, 0.15% v/v C2H4in helium for the fuel lean phase at a space velocity of 42,000 mL/h/gm catalyst. The fuel rich phase was simulated by replacing the oxygen with an equal volume of helium while maintaining the other flow rates constant at the fuel lean conditions. The catalysts were cycled multiple times between 15 minutes of fuel rich phase and 30 minutes of fuel lean phase. For all results reported, 150 mg of catalyst was loaded into each reactor, and all reactions were performed at a temperature of 648 K.

Fuel rich and fuel lean conditions refer to the molar ratio of oxidizing to reducing molecules as defined by the stoichiometric ratio SR:
SR={[NO]+2[O2]}/{[CO]+6[C2H4]}
A SR value of greater than 1 refers to fuel lean conditions, and a SR value of less than one refers to fuel rich conditions.

The 0.5Pt/7.5Ba/2.5Co catalyst was tested under 32 different reaction conditions with varying temperature and feed gas compositions provided in Table 1. This catalyst consistently showed an increase of three fold in the lean NOxstorage as compared to the 0.5Pt/7.5Ba for nearly all of the reaction conditions. These experimental results further verify the promotional effect of cobalt on NOxstorage over a wide range of reaction conditions. The space velocity for all of the testing in Table 1 was 42,500 (mL/hr/g catalyst)

The catalysts were synthesized via incipient wetness on gamma-Al2O3(Catalox® Sba-200 m2/g). A list of Pt/Co catalysts tested in this study is shown in Table 1. The naming convention for each catalyst throughout the paper is based on the nominal weight loading. Thus, a catalyst with a nominal weight loading of 1% w/w Pt and 15% w/w Ba is referred to as 1Pt/15Ba. Chloroplatinic acid hexahydrate, barium nitrate, cobalt nitrate, iron (III) nitrate nonahydrate, and magnesium nitrate precursors (Strem Chemicals) were dissolved in distilled water prior to impregnation.

Details concerning the impregnation procedure is described in J. Lauterbach et al., inCatalysis Today,98(3) (2004) 375, of which the impregnation procedure is incorporated herein by reference. The use of impregnating inorganic oxide supports with the desired weight loading of different metals is well known in the art. A catalyst with the desired weight loadings was prepared by dissolving the necessary amount of metal precursor in distilled water. The solutions were added to the dried oxide support until incipient wetness was obtained. The impregnated oxide supports were dried overnight in a vacuum oven at a temperature of about 393 K and then crushed before the next impregnation step. This process was repeated until the entire precursor solution had been added to the support. In particular, because of the low solubility of barium nitrate in water, it was necessary to utilize multiple impregnation steps to achieve the desired weight loadings.

After completion of the final impregnation step, the powders were crushed and calcined in a furnace. All catalysts were calcined by heating to 473 K over two hours, holding the temperature at 473 K for one hour, further heating to 823 K over three hours, holding at 823 K for two hours, and then cooling to 298 K. In addition, all catalysts were reduced in the high-throughput reactor for 1 hour in 10% v/v H2/He at 773 K before performing the reaction studies.

The preparation of 1.5 g of the NOxstorage catalyst, 1Pt/15Ba/5Co, is provided below. Similar procedures were used to prepare the other catalysts listed in Table 2. An aqueous solution of chloroplatinic acid hexahydrate (0.0398 g) was used to obtain a nominal weight percent loading of 1% Pt on 1.5 g of γ-alumina. For example, the weight fraction of platinum in chloroplatinic acid hexahydrate is 0.37. As a result, the amount of chloroplatinic acid hexahydrate required is (0.015/0.37) or 0.0398 g. Similarly, one can calculate the amount of barium nitrate (0.4282 g) and cobalt nitrate (0.3704 g) required for a nominal weight loading of Ba 15% and Co 5%, respectively.

The metal precursors were dissolved and mixed together with an appropriate amount of water based on their respective solubility. The mixture (solution) was then added to dried γ-alumina support in multiple steps based on the incipient wetness of the support material. The impregnated oxide support was dried overnight in a vacuum oven at a temperature of about 393 K and then crushed before the next impregnation step. This process was repeated until the entire precursor solution had been added to the support. In particular, because of the low solubility of barium nitrate in water, it was necessary to utilize multiple impregnation steps to achieve the desired weight loadings. After completion of the final impregnation step, the powder was crushed and then calcined as described above.

The actual weight loadings of all catalysts were verified by atomic absorption spectroscopy (S series atomic absorption spectrometer, Thermo Electron), and are listed in Table 2. The actual platinum weight loadings of the catalysts were significantly less than the nominal loadings, implying that the oxidizing capacity demonstrated by the Pt/Co catalysts was mainly associated with Co. For example, if we attempted to prepare a catalysts with 1% Pt, which we call nominal loading, the actual amount of platinum actually present in the catalyst (as measured by atomic absorption spectroscopy) was significantly less (See, Table 2).

X-ray diffraction patterns were acquired under atmospheric conditions with a Philips X'Pert diffractometer using Cu Kα radiation at 40 kV and 40 mA.

We have used a simulated diesel exhaust gas with the component compositions indicated in Table 3. The data ofFIG. 6shows the results (in terms of NOxstorage) from testing of a variety of Co containing NSR catalysts for simulated diesel exhaust conditions.FIG. 6also includes the results from two different noble metal containing catalysts (Pt and Rh). Three different reducing agents and 5% CO2in the feed stream were used for testing. He was used as the carrier gas.

All catalysts were tested at 375° C. The error bars are calculated from the repeats of the same catalyst tested at a later time. It can be inferred from these results that a catalyst composition of 5% Co and 15% Ba catalysts actually performs better than all of the tested Pt/Ba and Rh/Ba catalyst compositions. The addition of 5% Co to catalysts containing 1Pt/15Ba effectively doubles the storage capacity. A similar increase has been observed with the 1Rh/15Ba catalyst composition.

The catalyst containing both platinum and rhodium did not perform as well as the inventive catalysts. Similar, results for Pt/Rh combinations have been reported in the literature by Fridell et. al., but for different reaction conditions.