WO2002030546A2

WO2002030546A2 - Catalytic material and method for abatement of nitrogen oxides

Info

Publication number: WO2002030546A2
Application number: PCT/US2001/031331
Authority: WO
Inventors: Shau-Lin F. Chen; Chung-Zong Wan; Alan R. Amundsen
Original assignee: Engelhard Corporation
Priority date: 2000-10-11
Filing date: 2001-10-05
Publication date: 2002-04-18
Also published as: AU2001296672A1; JP2004523336A; EP1324816A2; WO2002030546A3; KR20030061376A

Abstract

A three-way catalytic material contains palladium-containing inorganic support particles that include lanthanum and praseodymium and is preferably substantially free of neodymium and barium. The combination of lanthanum and praseodymium together with the palladium serves to signigicantly enhance NOx reduction in gas streams that do not contain significant quantities of SO2. The catalytic material may also include other platinum grup metals, e.g., platinum, rhodium, etc. Optionally, the palladium, lanthanum and praseodymium-containing particles may be different from the particles containing the other platinum group metals.

Description

CATALYTIC MATERIAL AM) METHOD FOR ABATEMENT OF NITROGEN OXIDES

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention pertains to the catalytic abatement of nitrogen oxides and, in particular, to a catalytic material effective for the conversion of NO_x and a method for its use. One unwanted component of the exhaust gases of combustion processes, such as the combustion of gasoline in automotive engines, is NO_x or nitrogen oxides. It is known to treat gas streams that contain NO_x with catalysts that comprise palladium to convert or reduce the NO_x to gaseous nitrogen. When the gas stream also contains oxidizable pollutants such as carbon monoxide (CO) and unburned hydrocarbons (HC), it is also known to treat the gas stream with a so-called three-way catalyst ("TWC catalyst") that also converts these components to innocuous substances such as carbon dioxide and water as it substantially simultaneously converts NO_x. It is well-known that the performance of a TWC is best, realized when the exhaust gas stream results from the combustion of a stoichiometrically balanced air/fuel mixture.

Governmental regulations require that emissions of NO_x be reduced to ever lower levels so there is an on-going need for increasingly effective conversion catalysts. Meeting governmental emission requirements is especially difficult in high-temperature fuel combustion processes because NO_x is more easily formed at high temperatures than at low temperatures.

Catalysts for the abatement of NO_x, carbon monoxide and hydrocarbons typically comprise a catalytic material comprising a refractory support material on which is dispersed a catalytic metal component that comprises one or more platinum group metals. There may also be a catalytic base metal component such as a transition metal of Group Nffl of the Periodic Table of Elements, e.g., iron, nickel, manganese or cobalt on the support, although in some cases the base metal component may be admixed with the support material in bulk form. The support material preferably has a high surface area to enhance the effectiveness of the catalytic metal component dispersed thereon. The catalytic material is normally provided as a thin coating or "washcoat" adhered to the walls of a refractory carrier substrate. The latter often takes the form of a body made from a suitable material such as cordierite, mullite or the like, which is formed to have a plurality of parallel, fine gas flow passages extending therethrough. Typically, there may be from about 150 to 600 or more such gas flow passages per square inch of end face area of the substrate.

Related Art U.S. Patent 4,678,770 to Wan et al, dated July 7, 1987 and entitled "Three-Way Catalyst

For Lean Exhaust Systems", discloses a three-way catalyst for lean exhaust systems. The catalyst includes rhodium and at least one other platinum group metal dispersed on a high surface area support and a rare earth oxide, the rhodium being deposited on particles that are substantially free of the rare earth oxide. At least two different types of particles comprise the catalyst. The first type comprises the rhodium on the support that is substantially free of rare earth oxides. The second type comprises platinum and/or palladium dispersed on high surface area alumina which may optionally include rare earth oxides. Optionally, a third type of particle comprising bulk rare earth oxide that may optionally have platinum and/or palladium dispersed thereon may be included. Nowhere in the description of the invention or in the Examples does this patent disclose the combination of lanthanum and praseodymium in the catalytic material.

U.S. Patent 5,075,276 to Ozawa et al, dated December 24, 1991 and entitled "Catalyst For Purification Of Exhaust Gases", discloses a catalyst for the purification of exhaust gases. The catalyst disclosed therein comprises ceria, but not lanthana (see column 2, lines 10-20). The oxides of rare earth elements that may be used in the disclosed invention are listed at column 3, lines 32-42, and do not include lanthana. As summarized in Tables 1, 2 and 3, none of the example catalytic materials or the comparative materials comprised lanthana or a combination of lanthana and praseodymia.

PCT patent application No. PCT/US95/01849, published as International Publication WO 95/35152 on December 28, 1995, discloses a layered catalyst composite. The first layer comprises at least palladium and, optionally, other platinum group metals and may also include a stabilizer and a rare earth metal component selected from ceria, neodymia and lanthana (see page 11, lines 10-13, and page 13, lines 18-21). The second layer comprises platinum, rhodium and a second oxygen storage composition that may comprise ceria and, optionally, one or more of lanthana, neodymia, yttria or mixtures thereof (see page 11, lines 13-18, and page 12, lines 1- 12). A combination of the rare earth oxides praseodymia and lanthana is not shown or suggested. U.S. patent application No. 08/722,761, filed September 27, 1996, now U.S. Patent 5,898,014, issued April 27, 1999, and commonly assigned to the assignee of the present application, discloses a catalyst composition which, in a particular embodiment, comprises platinum group metals on a support material and an oxygen storage component that may comprise neodymia and/or praseodymia and at least one of lanthanum and neodymium.

Example 3 describes a catalyst composition containing lanthana, praseodymia and neodymia.

SUMMARY OF THE INVENTION The present invention provides a catalytic material effective at least for conversion of nitrogen oxides in gas stream. The catalytic material consists essentially of a refractory support material on which is dispersed catalytically effective amounts of a platinum group metal component, lanthanum and praseodymium.

According to one aspect of the invention, the lanthanum and praseodymium may be present in the catalytic material in atomic proportions in the range of about 1 :9 to 9: 1 , for example, in the range of 1 : 5 to 5 : 1. In a particular embodiment, the lanthanum and palladium may be present in an atomic ratio of about 1:1. Optionally, the catalytic material may be substantially free of barium and, optionally, the lanthanum may comprise at least about one percent of the catalytic material by weight. In a preferred embodiment, the platinum group metal component comprises palladium.

This invention also provides an improved catalyst member effective at least for conversion of nitrogen oxides in a gas stream. The catalyst member comprises a catalytic coating deposited onto a carrier substrate, and the improvement is that the catalytic coating comprises at least one catalytic material as described above. Optionally, the catalytic material may contain palladium at a loading of from about 30 to 500 grams per cubic foot. The praseodymium and lanthanum may be present at a combined loading in the range of from about 0.03 grams per cubic inch to 0.5 grams per cubic inch. This invention also relates to a method for treating a gas stream containing NO_x, comprising contacting the gas stream with a catalytic material as described above.

The method may comprise contacting the gas stream with the catalytic material at a temperature of at least about 200°C, preferably at high temperature, e.g., at a temperature of about 500°C. The gas stream may be substantially free of SO₂. References herein and in the claims to catalytic species identified by their elemental names without reference to a compound, alloy, etc., or as "components", are meant to encompass the catalytic species in any form in which it exists in the catalytic material, i.e., in elemental form, or as a compound such as an oxide, alloy, etc. References to particular compounds, however, indicate only the stated compounds. Thus, e.g., a reference to lanthanum in a catalytic material should be interpreted to refer to all forms of lanthanum in the catalytic material, elemental, oxide, etc., but a reference to lanthana indicates only lanthanum oxide(s).

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a catalyst member comprising a catalytic material coated onto a honeycomb-type carrier member in accordance with one embodiment of the present invention; and

Figure 1 A is a partial cross-sectional view enlarged relative to Figure 1 and taken along a plane parallel to the end faces of the carrier of Figure 1.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

The present invention arises from the discovery that combining praseodymium and lanthanum species in a platinum group metal-based catalytic material in the absence of neodymium yields unexpectedly superior NO_x-conversion activity at high temperatures relative to the performance of similar materials that contain other combinations of rare earth metal species and to the performance of praseodymium and lanthanum employed separately.

Accordingly, the invention provides a catalytic material containing a platinum group metal component dispersed on a refractory support with lanthanum and praseodymium, typically in their oxide forms, with the substantial exclusion of neodymium. As used herein and in the claims,^" me tefhi " consisting essentially of therefore signifies the substantial exclusion of neodymium from the defined catalytic material and any other component which defeats the synergy of the combination of lanthanum and praseodymium disclosed herein. This may not necessarily require the complete elimination of all traces of neodymium or other such element(s), but simply the absence of quantities of these materials that would defeat the improved catalytic performance illustrated in the Examples below. Minor quantities, e.g., not more than 0.25 percent or optionally not more than 0.05 percent of neodymium by weight of the catalytic material may be present as an impurity without departing from the advantages of the invention as described below. A catalytic material according to various optional embodiments of the present invention may also be substantially free of cerium and/or optionally substantially free of rare earth metals besides neodymium and/or substantially free of alkaline earth metals (such as barium) and/or their oxides as well. As used herein and in the claims, however, "consisting of and "consisting essentially of do not preclude the layering of particles of a catalytic material in accordance with the invention with a different material, e.g., one optionally comprising neodymium and/or barium, or the physical admixture of such materials in a single slurry or washcoat layer. Such layering or mixing of materials should be performed under conditions that do not permit significant amounts of the neodymium to dissolve into a fluid medium in which the catalytic material of this invention is disposed, as this could introduce a synergy-defeating quantity of neodymium into the claimed catalytic material. A gas stream containing NO_x , e.g., an automotive engine exhaust gas stream, is treated according to the present invention by flowing the stream into contact with the catalytic material described herein, preferably at high temperatures, i.e., temperatures in the range of 200°C to 650°C, e.g., 250°C to 600°C, although the catalytic material and method of this invention is effective over a broader range, i.e., from about 150°C to 900°C. The catalytic material may also be effective for the oxidation of hydrocarbons and carbon monoxide in the NO_x-containing gas stream, as are typically present in engine exhaust gases.

As demonstrated below, a catalytic material in accordance with the claimed invention provides synergistic performance when used to treat NO_x in a gas stream and provides performance which is better than catalytic materials with other binary rare earth metal combination for treatment of NO_x in a gas stream that is substantially free of SO₂ , e.g., that contains less than about 5 pp SO₂. The improved NO_x conversion at low sulfur levels is especially advantageous because of the current trend of regulatory agencies towards lowering the limits of permissible limits on sulfur content in fuels such as gasoline and lowering the permissible sulfur oxides and NO_x emissions from combustion process using hydrocarbon fuels, such as the emissions from internal combustion engines.

There are numerous support materials known in the art on which the platinum group metal(s), the praseodymium and lanthanum species and, optionally, other non-excluded species may be dispersed. Suitable support materials generally comprise bulk (i.e., solid) refractory inorganic metal oxides such as, for example, alumina, titania, zirconia, ceria, silica-alumina, alumino silicates, aluminum-zirconium oxide, aluminum-chromium oxide, etc. The support may optionally be in the form in which the catalytic material will be used, e.g., in the form of pellets or tablets to be rendered in a catalyst bed, or molded or extruded in the form of a flow- through monolith, e.g., a honeycomb monolith. It is generally preferable, however, to employ particulate support materials, which can easily be applied onto a carrier member such as a honeycomb monolith, either before or after the catalytic components are dispersed onto it. Most preferred are high surface area materials such as activated alumina, which primarily comprises one or more of gamma-, theta- and delta-aluminas. Many high surface area support materials are subject to surface area degradation at high temperatures, which results in a diminution of catalytic performance. The loss of catalytic performance that occurs with loss of surface area has been attributed to the occlusion of catalytically active species dispersed on the surface of the support material. It is known to stabilize high surface area support materials against thermal degradation by impregnating the high surface area material with a salt solution of a stabilizing species. The impregnated material is dried and calcined in the presence of oxygen, e.g., in air, so the stabilizing species is converted into its oxide form within the interstices of the support material. Thus, incorporated into the high surface area material, the stabilizing oxide helps to stabilize the high surface area support material against thermal surface area degradation. Various kinds of stabilizers are known in the art and can be used for purposes of this invention. For example, the stabilizer can comprise an alkaline earth metal component (subject to the substantial exclusion of barium), which may be derived from one or more alkaline earth metals such as magnesium, calcium and strontium. Typically, the alkaline earth metal component comprises one or more alkaline earth metal oxides. The alkaline earth metal component can be applied to the support material in a soluble form, e.g., as a compound dissolved in a solvent, e.g., water, that is then impregnated into the support material. Upon calcination under oxidizing conditions, the solvent is driven from the support material and the alkaline earth metal compound is converted to the oxide. For example, soluble calcium may be provided as calcium nitrate or calcium hydroxide and soluble strontium may be provided as strontium nitrate or strontium acetate, all of which become the corresponding metal oxides upon calcination. Alternatively, high surface area support materials such as gamma-alumina can be stabilized against thermal degradation by impregnating the material with a solution of a rare earth metal (other than those excluded in accordance with various embodiments of the present invention) and then calcining the impregnated material to remove the solvent and convert the rare earth metal to a rare earth metal oxide. The stabilizing species may be present in an amount of up to about, e.g., 5 percent by weight of the support material.

The platinum group metal component may be dispersed onto the support material in a conventional manner, e.g., by dissolving a soluble salt compound of each platinum group metal to be used into a suitable solvent and impregnating the support material with the solution. As used herein, the term "compound", as in "platinum group metal compound" means any compound, complex, or the like of one or more platinum group metals (the "platinum group metal component") which, upon calcination r upon use of the catalyst, decomposes or otherwise converts to a catalytically active form, which is often, but not necessarily, an oxide. Preferably, the platinum group metal component comprises palladium, optionally, to the substantial exclusion of other platinum group metals. The compounds or complexes of one or more platinum group metals may be dissolved or suspended in any liquid which will wet or impregnate the support material, which does not adversely react with other components of the catalytic material and which is capable of being removed from the catalyst by volatilization or decomposition upon heating and/or the application of a vacuum. Compounds of particular platinum group metals in the finished catalytic material are referred to herein simply as the metal. Thus, the catalytic material of the present invention preferably comprises palladium. For reasons of economy and safety of handling, aqueous solutions of impregnated species, e.g., solutions of water-soluble salts, are preferred. For example, palladium may be dispersed onto the support material by impregnating the support material with an aqueous solution of palladium chloride, palladium nitrate, etc. Optionally, other platinum group metals may be dispersed onto the support material in a similar manner, e.g., by impregnating the support material with aqueous solutions of chloroplatinic acid, potassium platinum chloride, arnine- solubilized platinum hydroxide, rhodium chloride, rhodium nitrate, etc. The compound- containing liquid is impregnated into the pores of bulk support material, and the impregnated support material is dried and preferably calcined to remove the liquid and bind the platinum group metal into the support material. In the case of a particulate support material, the impregnation may optionally be achieved using an incipient wetness method by which the catalytic compound containing liquid is slowly added to a mass of the support particles to permit the particles to substantially completely absorb the liquid, as is well known in the art. In the case of a support material in the form of a larger structure, e.g., pellets, tablets or a honeycomb monolith, impregnation may also be achieved by methods well known in the art, e.g., by immersing the support into the liquid, by spraying the liquid onto the support, etc. In some cases, the completion of removal of the liquid (which may be present as, e.g., water of crystallization) may not occur until the catalyst is placed into use and subjected to the high temperature exhaust gas. During the calcination step, or at least during the initial phase of high temperature use of the catalyst, these catalytically active species are converted into a catalytically active form on the support material. Typically, the calcination is performed in the presence of oxygen, e.g., by calcining the impregnated support material in air, to convert the catalytically active species into their oxide forms.

In addition to thermal stabilizers, a catalytic material in accordance with the present invention may optionally comprise promoter components, i.e., catalytically active species other than platinum group metal compounds to enhance the catalytic activity of the platinum group metal compounds. Such optional promoters may include non-excluded compounds of alkaline earth metals, rare earth metals, alkali metal and/or transition metals such as iron, nickel, magnesium, manganese, etc. Promoters may be incorporated into the catalytic material by wet impregnation followed by drying as described above.

The palladium and/or other platinum group metals may comprise from 1 to 15 weight percent of the catalytic material, e.g., from 2 to 10 weight percent. In a typical embodiment, a catalytic material according to the present invention may comprise about 5 percent of the platinum group metals by weight. - The praseodymium and lanthanum species may be dispersed onto the support material in the same manner as, and preferably after, the platinum group metal species are dispersed thereon. The lanthanum and praseodymium are added in quantities that provide a lanthanum to praseodymium atomic ratio of at least 1:9, for example, in the range of 1:9 to 9:1 or, typically, in the range of 1 : 5 or 5 : 1 or, preferably, about 1:1. Generally, to attain the minimum desired catalytic activity when preparing a particulate catalyst material for use as a washcoat on an inert carrier, the lanthanum is present in an amount of at least about 1 percent of the catalytic material, by weight (measured as metal oxides), typically at least about 10 percent or, in a particular embodiment, 15 percent by weight.

Particulate catalytic materials of the present invention are typically rendered in the micron-sized range, e.g., 2 to 20 micrometers in diameter, by ball milling or continuous milling so that they can be formed into a slurry and applied as a washcoat on a carrier member, as is well-known in the art. Any suitable carrier member may be employed, such as a honeycomb- type carrier of the type having a plurality of fine, parallel gas-flow passages extending therethrough from an inlet to an outlet face of the carrier. The passages are defined by walls on which a coating (sometimes referred to as a "washcoat") of the catalytic material is applied so that the gases flowing through the passages contact the catalytic material. Such structures may contain from about 60 to about 1000 or more passages ("cells") per square inch of cross section ("cpsi"), more typically 200 to 600 cpsi. Such honeycomb-type carrier may be made of any suitable refractory material, for example, it may be formed from a ceramic-like material such as cordierite, cordierite-alpha-alumina, silicon nitride, zirconium mullite, spodumene, etc. Alternatively, a honeycomb-type carrier may be made of a refractory metal such as a stainless steel or other suitable iron-based, corrosion-resistant alloys. A variety of deposition methods is known in the art for depositing a coating of catalytic material on a carrier substrate and most of these can be used with a carrier prepared according to the present invention. These include, for example, disposing the catalytic material in a liquid vehicle to form a slurry and wetting the carrier substrate with the slurry by dipping the carrier into the slurry, spraying the slurry onto the carrier, etc. The liquid medium of the slurry is then removed to leave a washcoat of the catalytic material, or a precursor thereof, on the carrier substrate. The removal procedure may entail, for example, heating the wetted carrier and/or subjecting the wetted carrier to a vacuum to remove the solvent via evaporation.

As indicated above, a catalytic material in accordance with the present invention may be used in combination with other catalytic materials. Optionally, the materials may be combined as constituents of a single washcoat slurry. For example, a quantity of particulate support material may be impregnated with the palladium and with a solution of soluble salts comprising praseodymium and lanthanum compounds to produce a first constituent catalytic material in accordance with this invention, and another quantity of particulate support material may be impregnated with catalytically active metals and any desired promoters, stabilizers, etc., to produce a second constituent catalytic material. Optionally, the second constituent catalytic material may be substantially free of neodymium and/or free of any other species that may be excluded from the first constituent catalytic material. Constituent catalytic materials may be intermixed and applied as a single washcoat of catalytic material on a carrier or they may be applied as separate layers on the carrier. Thus, the palladium-, lanthanum- and praseodymium- bearing particles of the invention which are substantially free of neodymium may be applied onto a carrier as a bottom coat and a layer of another catalytic material may be applied as a top coat on top of the bottom coat. When used together as catalytic materials, platinum and rhodium are typically present in about a 5:1 weight ratio. Such an arrangement is suitable for the system of a partial lean burn engine. Optionally, a carrier may be pre-coated with a binder coat such as a washcoat of alumina before the particulate catalytic material is deposited thereon. The binder coat may be applied to the carrier in any of the same manners useful for depositing the catalytic material onto the carrier. Alternatively, or in addition thereto, the carrier may have an anchor layer applied thereto before the catalytic material and optional binder coat are deposited on the carrier. The anchor layer may be applied to the carrier by thermally spraying a metal feedstock in the form of molten or vaporized metal onto the surface of the carrier substrate. Various techniques for applying an anchor layer onto a carrier are described in commonly assigned, co-pending application serial number 09/301,626, filed April 29, 1999, which is hereby incorporated herein by reference. The coated carrier (i.e., the catalyst member) is typically disposed in a canister configured to protect the catalyst member and to facilitate establishment of a gas flow path through the catalyst member, as is well-known in the art.

Figure 1 shows generally at 10 a refractory honeycomb monolith-type carrier member of generally cylindrical shape having a cylindrical outer surface 12, one end face 14 and an opposite end face, not visible in Figure 1, which is identical to end face 14. The junction of outer surface 12 with the opposite end face at its peripheral edge portion is indicated at 14' in Figure 1. Carrier member 10 has a plurality of fine, parallel gas flow passages 16 formed therein, better seen in Figure 1 A. Gas flow passages 16 are formed by walls 18 and extend through carrier 10 from end face 14 to the opposite end face thereof, the passages 16 being unobstructed so as to permit the flow of a fluid, e.g., a gas stream, longitudinally through carrier 10 via gas flow passages 16 thereof. A layer 20 (illustrated in exaggerated thickness), which in the art and sometimes herein is referred to as a "washcoat", is adhered to the walls 18 and, in the particular embodiment of the invention shown in Figure 1A, may be comprised of a single layer comprising the catalytic material in accordance with the present invention.

The carrier member alternatively may comprise a body of beads, pellets, tablets or particles (collectively referred to as "carrier beads") made of a suitable refractory material such as gamma-alumina, and coated with the catalytic material. A body of such carrier beads may be contained within a suitable perforated container which permits the passage of the exhaust gas therethrough, as is known in the art.

The loading of the platinum group metals and other catalytically active species in the catalytic material is chosen to provide a desired degree of conversion, taking into consideration the quantity of catalytic material to be employed, the flow rate of the gas stream to be treated, the NO_x, carbon monoxide and hydrocarbon content of the gas stream, etc., in a manner well- known to those of ordinary skill in the art. When deposited as a washcoat onto a flow-type carrier such as a bed of coated pellets or a coated honeycomb monolith or the like, the amounts of the various catalytic components of the catalytic material are often presented based on grams per volume basis, e.g., grams per cubic foot (g/ft³) for platinum group metal components and grams per cubic inch (g/in³) for catalytic materials generally, as these measures accommodate different gas-flow passage cell sizes in different honeycomb-type carrier substrates. For typical automotive exhaust gas catalytic converters, the catalyst member generally comprises from about 0.5 to about 6 g/in³, preferably from about 1 to about 5 g/in³ of catalytic material washcoat on the carrier. Typically, when a catalytic material according to the present invention is being rendered as a washcoat for a carrier substrate, the loading of palladium is in the range of about 30 g/ft³ to 500 g/ft³. The loading of praseodymium plus lanthanum should be in the range of from about 0.03 g/in³ to 0.5 g/in³, e.g., about 0.3 g/in³, such as 0.15 g/in³ of each, measured as the oxide. Each of the other optional compounds, e.g., the alkaline earth metal component, may constitute, for example, from about 0.02 to 0.4 g/in³ of the washcoat.

Example 1 Seven catalytic materials (A-G), including one comprising a ternary mixture of the rare earth metal oxides of La, Nd and Pr, three comprising binary mixtures thereof and three comprising singular rare earth metal components were prepared by preparing and modifying a common base material. The base material comprised 92.5% high porosity alumina, 2.87% zirconia impregnated into the alumina and 4.6% PdO dispersed thereon. Samples of the base material were modified by the impregnation of soluble salt solutions of La, Nd and/or Pr for a total of 15 weight percent rare earth metal oxide in each sample, in the combinations shown in TABLE I. The catalytic materials were substantially free of barium and of rare earth metals other than those indicated in TABLE I. The catalytic materials were coated onto honeycomb- type carriers, which were then dried and calcined to provide 160 grams palladium per cubic foot on each resulting catalyst member. Each catalyst member was engine-aged in two eight- chamber reactors for fifty hours at 900°C. Each aged catalyst member was then evaluated in a modal gas reactor using a gas stream comprising 0.2% CO; 0.05% H₂; 0.4% O₂; 16.3% CO₂; 235 ppm propylene; 235 ppm propane; 1400 ppm NO_x; 10% H₂O, balance N₂, flowing at a rate of 26,000/hr VHSN at a temperature of 500°C, once with 45 ppm SO₂ and once without. The test gas was composed to simulate a stoichiometric air/fuel mixture with a A/F ratio perturbation of ± 0.5 at a frequency of 0.5 hertz. (In a stoichiometric mixture there is sufficient oxygen to fully combust the hydrocarbons without leaving unreacted oxygen. This generally allows for the complete combustion of the carbonaceous components of the fuel to proceed substantially simultaneously with the reduction of ΝO_x. The conventional air/fuel ratio index variable λ is used to relate a given air/foel mixture to a stoichiometric air/fuel mixture, which has an air/fuel weight ratio of 14.65 for a fuel with H C ratio of 1.90. A combustion mixture in which the air and fuel is stoichiometrically balanced is described as having an air/fuel ratio index of λ = 1. For a lean air/fuel mixture, λ > 1 ; for a stoichiometric mixture, λ = 1 ; for a fuel- rich mixture, λ < 1.) The conversion activity of each sample for the oxidation of hydrocarbons and carbon monoxide and for the reduction of NO_x was noted and the results are set forth in TABLE I.

TABLE I

Conversion (a). 500°C Conversion ( ). 500°C

Without SO. With SO.

Sample HC CQ HQ_X HC CO NQ_X

A (La-Nd) 95.8% 82.6% 84.1% 94% 72% 71.5°

B (Nd-Pr) 95.6 83.8 82.7 95% 71.4 71.5

C (Pr-La) 96.7 83.5 88.1 93.9 72 72.3

D (Pr-La-Nd) 90.7 78.7 77.4 94.5 67.6 75.3

E (La) 94.5 82 83.4 94.5 77 77.7

F (Pr) 93.8 79.7 81.5 91.5 69.4 68.4

G (Nd) 92.3 70 69.2 93.7 80.6 80.9

The data of TABLE I show that sample C, which comprised a combination of lanthanum and praseodymium according to the present invention, provided surprisingly superior NO_x reduction in the absence of SO₂ relative to the other samples combining La or Pr with neodymium. The data also show that a synergistic effect is achieved by combining lanthanum with praseodymium (88.1 %) relative to comparable quantities of either lanthanum or praseodymium (83.4% and 81.5%).

Example 2 Six other catalytic materials (H-M) were prepared and coated onto honeycomb-type carriers in substantially the same manner as those in Example 1. The catalytic materials of this Example all included barium in addition to one or more of the rare earth oxides of lanthanum, neodymium and praseodymium. The total loading of barium plus the rare earth oxides in each catalytic material comprised about 15 percent by weight of the catalytic material. The resulting catalyst members were engine-aged and tested as described above in Example 1. The results are set forth in the following TABLE H.

TABLE π

Conversion (a). 500°C Conversion (a), 500°C

Without SO . With SO .

Sample HC CO NΩ_x HC CO NO,

H (Ba, La-Nd) 96.2% 85.8% 85.6% 95.8% 75.7% 78.3%

I (Ba, Nd-Pr) 89.5 79.2 73.6 93.8 73 70

J (Ba, Pr-La) 96.3 83.7 83.9 94.6 73.4 73.4

K (Ba, Pr-La-Nd) 91.7 80.1 80.2 94.1 75.3 76.1

L (Ba, Pr) 94.2 73.5 72.3 96.1 81.9 81

M (Ba, La) 92 72.3 71.7 96.3 80.7 85.9

The data for Sample J in TABLE II show that the inclusion of barium in the catalytic material can suppress the NO_x conversion performance of the combination of lanthanum and praseodymium in the absence of SO₂ illustrated by Sample C in Example 1, but the synergistic results derived from the combination of Pr and La (Sample J) (83.9%) is still evident by comparison to Pr (Sample L) (72.3%) or La (Sample M) (71.7%).

While the invention has been described in detail with reference to a particular embodiment thereof, it will be apparent that upon a reading and understanding of the foregoing, numerous alterations to the described embodiment will occur to those skilled in the art and it is intended to include such alterations within the scope of the appended claims.

Claims

THE CLAIMSWhat is claimed is:

1. A catalytic material effective at least for conversion of nitrogen oxides in a gas stream, consisting essentially of a refractory support material on which is dispersed catalytically effective amounts of a platinum group metal component, lanthanum and praseodymium.

2. The catalytic material of claim 1 containing lanthanum and praseodymium in atomic proportions in the range of about 1 :9 to 9: 1.

3. The catalytic material of claim 2 containing lanthanum and praseodymium in atomic proportions in the range of about 1:5 to 5:1.

4. The catalytic material of claim 3 containing lanthanum and praseodymium in an atomic ratio of about 1:1.

5. The catalytic material of claim 1 or claim 3 wherein the lanthanum constitutes at least about 1 percent of the catalytic material by weight calculated as the oxide.

6. The catalytic material of claim 1 or claim 3 wherein the catalytic material is substantially free of barium.

7. The catalytic material of claim 1 or claim 3 wherein the platinum group metal component comprises palladium.

8. In a catalyst member effective at least for conversion of nitrogen oxides in a gas stream and comprising a coating deposited onto a carrier substrate, the improvement comprising that the coating comprises at least a catalytic material which consists essentially of a refractory support material on which is dispersed catalytically effective amounts of a platinum group metal component, lanthanum and praseodymium.

9. The catalyst member of claim 8 wherein the catalytic material contains lanthanum and praseodymium in atomic proportions in the range of about 1 :9 to 9: 1.

10. The catalyst member of claim 9 wherein the catalytic material contains lanthanum and praseodymium in atomic proportions in the range of about 1:5 to 5:1.

11. The catalyst member of claim 10 wherein the catalytic material contains lanthanum and praseodymium in an atomic ratio of about 1:1.

12. The catalyst member of claim 8 or claim 10 wherein the lanthanum in the catalytic material comprises at least about 1 percent of the catalytic material by weight calculated as the oxide.

13. The catalyst member of claim 8 or claim 10 wherein the catalytic material is substantially free of barium.

14. The catalyst member of claim 8 or claim 10 wherein the catalytic material contains palladium at a loading of from about 30 to 500 grams per cubic foot.

15. The catalyst member of claim 14 wherein the catalytic material contains praseodymium and lanthanum at a combined loading in the range of from about 0.03 grams per cubic inch to 0.5 grams per cubic inch.

16. The catalyst member of claim 8 wherein the coating further comprises a second catalytic material which comprises a catalytically active metal on a support material and which is substantially free of neodymium.

17. A method for the abatement of NO_x in a gas stream comprising contacting the gas stream with a catalytic material consisting essentially of a refractory support material on which is dispersed a catalytically effective amount of a platinum group metal component, lanthanum and praseodymium.

18. The method of claim 17 wherein the catalytic material comprises lanthanum and praseodymium in an atomic ratio in the range of about 1:9 to 9:1.

19. The method of claim 18 wherein the lanthanum and praseodymium are present in atomic proportions in the range of about 1 :5 to 5 : 1.

20. The method of claim 19 comprising lanthanum and praseodymium in an atomic ratio of about 1:1.

21. The method of claim 17, claim 18 or claim 20 wherein the lanthanum constitutes at least about 1 percent of the catalytic material by weight calculated as the oxide.

22. The method of claim 17, claim 18 or claim 20 comprising contacting the gas stream with the catalytic material at a temperature of at least about 200°C.

23. The method of claim 22 comprising contacting the gas stream with the catalytic material at a temperature of about 500°C.

24. The method of claim 17, claim 18 or claim 20 wherein the catalytic material is substantially free of barium.

25. The method of claim 17, claim 18 or claim 20 wherein the catalytic material comprises palladium.

26. The method of claim 25 wherein the gas stream is substantially free of SO₂.

27. The method of claim 17, claim 18, claim 19 or claim 20 wherein the gas stream is substantially free of SO₂.