NO2 abatement composition with enhanced sulfur resistance

[0002] The present invention claims the priority date of copending United States Provisional Patent Application Ser. No. 60/215,018, filed Jun. 29, 2000.

[0003] The present invention is broadly concerned with NO_xabatement, particularly with NO_xabatement in the exhaust gas of lean-burn internal combustion engines.

[0004] It is well known in the art to utilize catalyst compositions, such as those commonly referred to as three-way conversion catalysts (“TWC”) to treat the exhaust gases of internal combustion engines. Such catalysts have been found to successfully promote both the oxidation of unburned hydrocarbons (“HC”) and carbon monoxide (“CO”), and the reduction of nitrogen oxides (“NO_x”) in the exhaust gas, provided that the engine is operated at or close to an air/fuel ratio balanced for stoichiometric combustion (i.e., between about 14.4 and about 14.7; “combustion stoichiometry”). Currently, much research is underway to develop gasoline fueled engines, such as those used for passenger automobiles and the like, to operate under lean conditions, for at least part of the time, to achieve better fuel economy. Under lean conditions, the air to fuel ratio (A/F) and combustion mixture supplied to the engine are maintained above combustion stoichiometry so that the resulting exhaust gases are “lean”, i.e. the exhaust gases are relatively high in oxygen content and relatively low in reductant content, e.g. HC, CO and/or hydrogen.

[0005] Although lean burn engines provide enhanced fuel economy, they have the disadvantage that a conventional three-way conversion catalyst cannot adequately abate the NO_xcomponent of pollutants in the gas stream, as is required in order to meet increasingly stringent emission control regulations being implemented throughout the world. Efforts to increase NO_xremoval from exhaust gas emissions have resulted in the development of NO_xadsorbers which typically comprise a sorbent material to remove NO_xfrom a fuel-lean gas stream. While NO_xadsorbers have been shown to remove NO_xpollutants from exhaust gases, they suffer from the serious drawback that they are readily poisoned by sulfur also present in exhaust gases.

[0006] Without being bound to any particular theory of operation, it is believed that, under lean conditions, NO_xpresent in the gas stream is sorbed onto the NO_xadsorber in the form of stable metal nitrates, thereby removing NO_xfrom the gas stream to be discharged to the atmosphere. A catalytic component is believed to promote such sorption. During rich conditions (i.e., high reductant content) reductants in the gas stream decompose the stable metal nitrates in the NO_xadsorber, releasing NO_xfrom the adsorber. The released NO_xcontacts a NO_xabatement catalyst in the presence of reductants and, under suitable conditions (e.g., sufficiently elevated temperature), the NO_xis reduced to nitrogen or to nitrogen and N₂O.

[0007] By a mechanism similar to that for the formation of the stable metal nitrates, sulfur oxides present in the gas stream are sorbed onto the NO_xadsorber in the form of very stable metal sulfate salts. Unlike the storage nitrates which readily decompose under rich conditions, decomposition of the metal sulfates requires a combination of rich gas and elevated operating temperature. This combination of conditions may not occur with sufficient frequency under normal operating conditions for typical lean bum gasoline and diesel engines. As a result, the NO_xstorage sites are gradually poisoned by sulfur, and the NO_xconversion capability of the catalyst gradually degrades to unacceptable levels for efficient exhaust purification.

[0008] The NO_xadsorber compositions disclosed herein provide enhanced recovery from poisoning by sulfur compounds, while retaining their NO_xabatement efficiency.

[0009] For a further understanding of the nature of the present invention, as well as other features and advantages thereof, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, which are meant to be exemplary, not limiting, and wherein:

[0010] FIG. 1 is a bar graph comparing the NO_xconversion efficiency and regeneration time at different temperatures for two composition;

[0011] FIG. 2 is a graphical representation of NO_xconversion levels under lean/rich modulations of 30 seconds lean (air/fuel ratio of 21.5) to 25 seconds rich (air/fuel ratio of 12.5) after 64 hours of fuel cut engine aging at 750° C. inlet temperature and 100-150 ppm sulfur present;

[0012] FIG. 3 is a graphical representation of NO_xconversion versus temperature during lean/rich modulations (30 seconds lean (air/fuel ratio of 21.5)/25 seconds rich (air/fuel ratio of 12.5)) after 64 hours of fuel cut engine aging at 750° C. inlet temperature and 100-150 ppm sulfur present;

[0013] FIG. 4 is a graphical representation of NO_xadsorption capacity over time for two compositions after 64 hours of fuel cut engine aging at 750° C. inlet temperature and 100-150 ppm sulfur present;

[0014] FIG. 5 is a graphical representation of sulfur release over time at 700° C. (air/fuel ratio of 13) for two compositions after 64 hours of fuel cut engine aging at 750° C. inlet temperature and 100-150 ppm sulfur present; and

[0015] FIG. 6 is a graphical representation of NO_xconversion over time for two compositions after 30 hours of fuel cut engine aging at 400° C. (30 seconds lean/2 seconds rich) and 10 ppm sulfur dioxide.

[0016] The shortcomings and disadvantages of the prior art are overcome by a NO_xadsorber composition comprising a substrate; and a washcoat, wherein said washcoat comprises a catalytic metal component, a trapping material, and a porous support, wherein said porous support comprises cerium oxide and an oxygenate metal selected from the group consisting of lanthanum oxide, yttrium oxide, neodymium oxide, and combinations comprising at least one of the foregoing oxygenated metals.

[0017] Further advantages and embodiments of the present invention will be understood by those skilled in the art in light of the detailed description and figures.

[0018] The NO_xadsorber composition is characterized by a porous support material comprising a ceria-containing mixed oxide, with platinum selectively anchored thereon, resulting in improved resistance to sulfur poisoning.

[0019] Ceria-containing mixed oxides are useful in three-way conversion catalysts due to their stable oxygen storage capacity (OSC), which serves to broaden the air/fuel ratio range around the combustion stoichiometric point, where high conversion levels for CO, HC, and NO_xcan be obtained simultaneously. However, such oxygen storage capacity is undesirable with NO_xadsorber compositions because extra fuel is consumed to reduce the stored oxygen, thereby reducing the fuel efficiency. Accordingly, the current state of the art teaches that it is undesirable to incorporate OSC-containing components into NO_xadsorber compositions due to the resultant negative impact on fuel economy.

[0020] In contrast to conventional wisdom, unexpected advantages associated with the incorporation of cerium-containing mixed oxides into NO_xadsorber compositions have been discovered. While not to be limited by theory, the cerium-containing compositions disclosed herein serve as sulfur scavengers which protect the NO_xadsorber storage metals from sulfur poisoning, and subsequently facilitate sulfur release from the NO_xadsorber under rich conditions and at high temperatures.

[0021] The NO_xadsorber compositions disclosed herein comprise a substrate with a washcoat comprising a porous support, a catalytic metal component, and one or more NO_xtrapping materials (such as alkali metals, alkaline earth metals, and the like, and combinations comprising at least one of the foregoing) disposed thereon. The catalytic metal component and NO_xtrapping materials can be impregnated, loaded, imbibed, coated, or otherwise disposed on and/or within the porous support.

[0022] The porous support, which comprises at least two oxygenated metal compounds, one being cerium oxide (CeO₂), and the other being any oxygenated metal compound suitable to form a stabilized cerium-containing mixed oxide. Suitable oxygenated metal compounds include zirconium oxide (ZrO₂), lanthanum oxide (La₂O₃), yttrium oxide (Y₂O₃), and neodymium oxide (Nd₂O₃), and the like, as well as combinations comprising at least one of these oxygenated metal compounds, with ZrO₂being preferred. In the case of a mixed oxide containing ZrO₂and CeO₂, the mixed oxide may comprise ZrO₂and CeO₂in the full range of ZrO₂/CeO₂weight percent ratios. That is, the mixed oxide may comprise ZrO₂in an amount greater than the amount of CeO₂, or may comprise ZrO₂in an amount less than the amount of CeO₂.

[0023] The porous support further comprises one or more other support materials suitable for use at the high operation temperatures associated with an internal combustion engine (e.g., up to about 1,000° C.). Such materials include, but are not limited to, zeolite, alumina and the like, as well as combinations comprising at least one of the foregoing, with gamma-alumina, theta-alumina, delta-alumina, and combinations thereof being preferred. The porous support may comprise up to about 50 wt % of cerium-containing mixed oxide, the balance being other support materials; with about 3 wt % to about 25 wt % cerium-containing mixed oxide, the balance being other support materials being more preferred.

[0024] The washcoat has a catalytic metal component disposed on and/or in the porous support. The catalytic metal component comprises platinum group metals, such as, platinum (Pt), rhodium (Rh), palladium (Pd), ruthenium (Ru), iridium (Ir) and osmium (Os), as well as alloys and combinations comprising at least one of the foregoing. Preferably, the washcoat is loaded with the catalytic metal component in an amount of up to about 5% of the washcoat, with an amount of about 0.2 wt % to about 3 wt % more preferred, and an amount of about 0.5 wt % to about 2.5 wt % especially preferred.

[0025] Where the catalytic metal component is a combination of platinum with one or more other metals, the other metals, e.g., palladium, rhodium and the like, are typically present in an amount less than the platinum. More particularly, with a platinum/palladium combination, the catalytic metal component can comprise up to about 85 wt % platinum and up to about 45 wt % palladium (or other metal); with about 55 wt % to about 80 wt % platinum and about 20 wt % to about 45 wt % palladium preferred. With a platinum/rhodium combination, for example, the catalytic metal component can comprise up to about 95 wt % platinum and up to about 30 wt % rhodium; with about 70 wt % to about 85 wt % platinum and about 2.5 wt % to about 20 wt % rhodium preferred; and about 70 wt % to about 80 wt % platinum and about 5 wt % to about 10 wt % rhodium especially preferred.

[0026] In addition to the catalytic metal component, the porous support may be further loaded with one or more NO_xtrapping materials, such as alkali metals, alkaline earth metal, and mixtures comprising at least one of the foregoing. Suitable trapping materials include barium (Ba), lithium (Li), potassium (K), magnesium (Mg), sodium (Na), cesium (Cs), strontium (Sr), and combinations comprising at least one of the foregoing, with a mixture of Ba and K being preferred. Generally, the washcoat comprises up to about 50 wt % trapping material, with an amount of up to about 40 wt % preferred, and an amount of about 5 wt % to about 20 wt % being especially preferred, based upon the total weight of the washcoat. For example, in the case of a trapping material comprising a Ba and K mixture, the Ba is present in an amount up to about 30 wt % of the washcoat, and K is present in an amount up to about 10 wt %. More preferably, the trapping material comprises Ba in an amount of about 7 wt % to about 20 wt % of the washcoat, and K in the amount of about 2 wt % to about 5 wt % of the washcoat.

[0027] The washcoat can be supported on a substrate which is suitably stable in the high temperature exhaust gas environment (e.g., up to about 1,000° C.), such as cordierite, mullite, metallic substrates, and the like, as well as combinations comprising at least one of the foregoing. The substrate may be in any suitable configuration, often being employed as a monolithic honeycomb structure, spun fibers, corrugated foils or layered materials. Other materials useful as a substrate in an exhaust gas system are apparent to those skilled in the art.

[0028] Formation of the NO_xadsorber catalyst comprises application of the washcoat to the substrate as a mixture or the application of the washcoat components in sequential steps, in a manner which would be readily apparent to those skilled in the art of catalyst manufacture. For example, the porous support, catalytic metal component, and/or trapping materials can be combined to form a slurry. The slurry can then be applied to the substrate. Preferably, however, the washcoat is first applied to the substrate followed by drying and calcination. The catalytic metal component and trapping materials can then be deposited on or within the washcoat by any suitable manner, such as by impregnation techniques well known to those skilled in the art. According to such techniques, the catalytic metal component and trapping materials, individually or together, would be dissolved as soluble precursors (e.g., as a salt like potassium nitrate) in an aqueous or organic solvent which is then impregnated into the porous support. Preferably, the catalytic metal components are impregnated prior to the trapping materials. The trapping material and catalytic metal component precursors can also be dissolved together in a common solution and impregnated into the washcoat in a single step.

[0029] For example, loading of the catalytic metal component onto the washcoat may be accomplished with a catalytic metal precursor solution. Suitable precursor solutions are aqueous solutions which facilitate efficient chemisorption of the catalytic metal onto the washcoat. Suitable precursor solutions include platinum nitrate solution, platinum chloride solution, and similar materials and combinations thereof, with platinum nitrate solution being preferred.

[0030] The catalytic metal precursor solution can be applied to the cerium-containing mixed oxide prior to mixing with other washcoat components. For example, in an embodiment wherein the washcoat comprises alumina and a cerium-containing mixed oxide, alumina is added after the precursor solution is applied to the mixed oxide. This preparation method facilitates the selective deposition of catalytic metal onto the cerium-containing mixed oxide surface, rather than onto other components of the porous support (e.g., the other support materials, such as BaCO₃and Al₂O₃). Selective deposition of platinum on the cerium-containing mixed oxide enhances the NO_xadsorber's resistance to sulfur poisoning.

[0031] Not to be limited by theory, it is believed that platinum facilitates the oxidation of SO₂emissions to SO₃. The SO₃in turn readily reacts with the cerium-containing mixed oxide to form cerium sulfate (Ce₂(SO₄)₃). As demonstrated in equations (a) through (d) below, Ce₂(SO₄)₃decomposes much more readily under rich conditions than other metal sulfates which typically form in NO_xadsorber systems of the prior art, such as BaSO₄. Thus, the selective deposition of platinum onto the cerium-containing mixed oxide allows for fast and efficient NO_xadsorber recovery from sulfur poisoning.

[0032] (a) Ce₂(SO₄)₃reduction by H₂is spontaneous from about 0° C. to about 1,000° C.;

[0033] Ce₂(SO₄)₃+12H₂(g)=Ce₂O₃+3H₂S(g)+9H₂O(g); [ΔG<0 from 0° C. to 1000° C.].

[0034] (b) In contrast, Ba₂SO₄reduction by H₂is spontaneous only above 800° C.;

[0035] BaSO₄+4H₂(g)=BaO+3H₂O(g)+H₂S(g); [ΔG≦0 above about 800° C.].

[0036] (c) Ce₂(SO₄)₃can be reduced by CO above 370° C.;

[0037] Ce₂(SO₄)₃+3CO(g)=Ce₂O₃+3SO₂(g)+3CO₂(g); [ΔG≦0° C. above 370° C.].

[0038] (d) In contrast, BaSO₄cannot be reduced by CO within a reasonable temperature range;

[0039] BaSO₄+CO(g)=BaCO₃+SO₂(g); [ΔG>0 from 0° C. to 1,000° C.].

[0040] The degree of selectivity of platinum for the cerium-containing mixed oxide varies with the method of catalyst preparation. As an example, the catalyst preparation method described in Example III below results in about 100% platinum deposition on the ceria-containing mixed oxide.

[0041] Other aspects and advantages of the NO_xabatement compositions disclosed herein will be more apparent to those skilled in the art in view of the following Examples and corresponding Figures, which are meant to be exemplary and not limiting. These Examples illustrate some of the possible techniques for making the NO_xadsorber compositions.

[0042] The following example can be used to form a NO_xadsorber comprising a washcoat having, as main components, 17.1 wt % barium, 10.6 wt % zirconium oxide-cerium oxide, and 65.5 wt % aluminum oxide, based upon the total weight of the washcoat of 4.7 g/in³(grams per cubic inch).

[0043] 530.41 g of 22.94% platinum nitrate solution was sprayed onto 940.6 g of mixed oxide (25 wt % ZrO₂/75 wt % CeO₂) with constant mixing. The resulting powder was stored for 12 hrs followed by mixing into a slurry comprising 7901 g of barium-alumina powder (19.1 wt % barium on a commercial delta-alumina) made by calcination of barium acetate solution with the alumina powder. The slurry was then milled and pH adjusted to 6-7 using acetic acid, followed by coating the slurry onto a substrate of 400 cpsi (cells per square inch). Excess slurry was removed from the substrate by vacuum or pressurized air and the coated catalyst was then calcined. Rhodium was then applied using a known post impregnation method followed by another calcination.

[0044] The following example is another technique which can be used to form a NO_xadsorber comprising a washcoat having, as main components, 17.1 wt % barium, 10.6 wt % zirconium oxide-cerium oxide, and 65.5 wt % aluminum oxide, based upon the total weight of the washcoat of 4.7 g/in³.

[0045] 105.17 g of 27.84% platinum nitrate solution was diluted to 460 g total solution with distilled H₂O 230 g of mixed oxide (25 wt % ZrO₂/75 wt % CeO₂) was added to the solution, followed by about one hour of mixing. After mixing, 1932 g of barium-alumina powder (19.1 wt % barium) was added to the slurry. The slurry was then milled, coated onto a substrate, calcined, and rhodium impregnated as described above.

[0046] The following example is yet another technique which can be used to form a NO_xadsorber comprising a washcoat having, as main components, 17.1 wt % barium, 10.6 wt % zirconium oxide-cerium oxide, and 65.5 wt % aluminum oxide, based upon the total weight of the washcoat of 4.7 g/in³.

[0047] 464.0 g of 29.76% platinum nitrate solution was sprayed onto 9226 g of mixed oxide (25 wt % ZrO₂/75 wt % CeO₂) with constant mixing. The resulting powder was calcined at 250° C. for 2 hrs followed by mixing into a slurry comprising 7745 g barium-alumina powder (19.1 wt % barium). The slurry was then milled, coated onto a substrate, calcined, and rhodium impregnated as described above.

[0048] For all of the Figures, the support material of Formulation A comprises cerium-containing mixed oxides as disclosed herein. The support material of Formulation B comprises alumina. These Formulations were made in accordance with the technique set forth in Example 1. Complete loading information for each formulation is shown in Table 1.

[0049] The following Figures establish that NO_xadsorbers with washcoats comprising cerium-containing mixed oxides recover a relatively large fraction of fresh NO_xconversion capability following engine aging with sulfur-containing fuel, thus allowing their application for prolonged periods on lean burn engines operating on sulfur-containing fuels.

[0050] As shown in FIG. 1, prior to aging, each formulation provided similar levels of NO_xabatement. In each case, NO_xconversions were measured as a function of inlet temperature during a lean/rich modulation cycle (30 seconds lean/2 seconds rich). On the synthetic gas tester, the duration of the rich regeneration was controlled by an oxygen sensor and regeneration times for each test condition were measured (times are indicated on each bar). The adsorber containing cerium (formulation A) required longer regeneration times. This is the anticipated result due to the oxygen storage capacity associated with the cerium, which results in consumption of reductants such as HC and CO. Such reductants could otherwise be used to regenerate and reduce NO_xstored in an adsorber. Such consumption of reductants causes a negative impact on fuel economy, and as a result, cerium incorporation into adsorber formulations has been considered undesirable.

[0051] FIGS. 2, 3 and 4 show the results of comparative testing between Formulation A and Formulation B, following a 64 hour fuel cut engine aging at 750° C. inlet temperature, and about 100-150 ppm fuel sulfur. The performance of each formulation was measured during lean/rich modulation on an engine bench. In each case, the lean cycle was 30 seconds with an air to fuel ratio of 21.5, and the rich cycle was 1.67 seconds with an air to fuel ratio of 12.5. FIG. 2 shows Formulation A having superior NO_xconversion performance following aging. FIG. 3 shows Formulation A having superior NO_xconversion performance following aging and subsequent rich desulfation. FIG. 4 shows Formulation A having superior NO_xadsorption capacity following aging and subsequent rich desulfation.

[0052] For FIG. 5 the release of sulfur from the fuel cut engine-aged samples of Example V(a) was observed (64 hour fuel cut engine aging, 750° C. inlet temperature, and about 100-150 ppm fuel sulfur), using a sulfur sensitive detector. Under equivalent conditions of inlet temperature (700° C.) and air/fuel ratio (A/F=13) for Formulation A and Formulation B, a higher quantity of sulfur was released with Formulation A, as shown in FIG. 5.

[0053] Table II compares platinum (Pt) and barium sulfate crystallite sizes of Formulation A and Formulation B after aging (64 hour fuel cut engine aging, 750° C. inlet temperature, and about 100-150 ppm fuel sulfur). In an advantage over the prior art (Formulation B), crystallite sizes are markedly smaller with Formulation A, as shown. The smaller size of each crystallite yields a larger amount of platinum and barium sulfate surface area overall, thus leading to enhanced sulfate release with Formulation A.

[0054] FIG. 6 shows the results of comparative testing between Formulation A and Formulation B, following a 30 hour modulation aging at 400° C. inlet temperature, 10 ppm SO₂. As shown, Formulation A exhibited higher levels of NO_xconversion under conditions of lean/rich modulation (30 seconds lean/2 seconds rich), thus showing increased sulfur resistance with the NO_xadsorber comprising cerium-containing mixed oxide.

[0055] The use of a washcoat comprising a cerium-containing mixed oxide was found to enhance the recovery of the NO_xadsorber from sulfur poisoning. (See FIG. 5) This is largely due to the unstable nature of the cerium/sulfur compound formed when sulfur (as SO₂or SO₃) contacts the NO_xadsorber. Cerium sulfate Ce₂(SO₄)₃is relatively unstable and quickly decomposes, resulting in relatively fast and efficient NO_xadsorber recovery from sulfur poisoning. The efficient Ce₂(SO₄)₃decomposition is largely due to reduction by H₂and CO, as shown in equations (a) through (d) set forth above.

[0056] Moreover, in another advantage, use of cerium-containing mixed oxide in the washcoat enhances the steam reforming reaction [HC+H₂O=CO+3/2H₂] within the NO_xabatement system, which ultimately yields relatively high amounts of reductants H₂and CO. Such enhanced reductant formation results in an increased decomposition rate of metal sulfates such as Ce₂(SO₄)₃, as well as the more stable BaSO₄. The increased rate of metal sulfate reduction leads to faster recovery from sulfur poisoning.

[0057] While it is known to use CeO₂in pure, bulk form in NO_xadsorbers, the discovery of the use of cerium-containing mixed oxides provides a number of advantages, including improved thermal stability of the composition after thermal aging (e.g., about 900° C. for about 6 hours under air atmosphere). With a cerium-containing mixed oxide comprising ZrO₂, for example, the cerium-containing mixed oxide is more stable than pure CeO₂after aging at high temperatures (e.g., about 800° C. to about 1,000° C.), particularly with respect to surface area. After high temperature operation, cerium-containing mixed oxide retains significantly higher surface area than pure CeO₂(e.g., 16 m²/g for aged cerium oxide (J. Kaspar et al., Catalyst Today, Vol. 50, pp. 285-298 (1999)), and 47 m²/g for aged zirconium oxide-cerium oxide (75% cerium oxide)). Accordingly, with cerium-containing mixed oxide, there is relatively more surface area to support platinum after aging than with pure CeO₂. As a result, a smaller amount of cerium-containing mixed oxide will provide the same level of efficiency, in terms of sulfate decomposition, as compared to a larger amount of pure CeO₂. This is advantageous because smaller amounts of total ceria will lead to a smaller fuel consumption penalty, particularly when the catalyst is in the fresh state.

[0058] In another advantage, in a NO_xadsorber comprising platinum supported on a cerium-containing mixed oxide, improved dispersion and less sintering of platinum and even BaSO₄is observed after aging with an inlet temperature of 750° C. for a period of 6.5 hours of engine exhaust. As shown in Table II, with cerium-containing mixed oxide in the washcoat, platinum and BaSO₄are present as much smaller crystallites after aging with an inlet temperature of 750° C. for a period of 6.5 hours of engine exhaust. than with a composition of the prior art, indicating improved platinum and BaSO₄dispersion and comparatively less sintering. Accordingly, improved durability with respect to NO_xtrapping efficiency, sulfate decomposition, sulfur release, and NO_xconversion, after aging, is observed with a NO_xadsorber composition comprising platinum supported on a cerium-containing mixed oxide.

[0059] The NO_xabatement compositions disclosed herein have particular utility when used with lean-burn engines. Lean-bum engines are those which, at least during part of their operation, operate with air to fuel ratios greater than combustion stoichiometry. For gasoline engines, that range is usually about 19 to about 55 and for diesel the range is usually about 19 to about 40.

[0060] While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting to the claims.