Three-way catalyst for automotive emission control

Abstract: A three-way catalyst for automotive emission control having a mechanical carrier having a support comprised substantially of alumina, a lanthanum oxide coating decorated onto the support, a catalytic compound discontinuously supported on the decorated support having a major constituent of palladium, and a discontinuous phase of titanium oxide on the alumina, lanthana, and catalytic compound composite. The method of making such three-way catalyst for automotive emission control, comprises coating an alumina support with lanthana, impregnating the coated support with a palladium compound to form a discontinuous phase on the lanthana, and impregnating the coated support and catalytic compound with an organo-titania compound to form a discontinuous phase on the lanthana and palladium compound.

Description: This invention relates to catalysts for treating automotive emissions and a method of making such catalysts. Precious metals, including palladium, have been used as active catalyst materials in automotive emission devices. However, palladium, by itself, as the dominant or sole catalyst material, is limited in its effectiveness to promote the reduction of nitrogen oxide compounds at high temperatures and is not optimum in promoting oxidation of hydrocarbons at such temperatures. An example of such palladium usage is disclosed in U.S. patent 4,123,391. A variety of cost-increasing alternatives have been used to augment the capabilities of palladium, primarily by the use of other precious metals. This invention is based on the discovery that small amounts of titania (a nonprecious metal oxide) uniquely applied will synergistically cooperate with lanthanum oxide in the presence of Pd to enhance the total catalytic effect for oxidation and reduction, particularly at very low temperatures and at very high temperatures. Titania has not been used heretofore as a catalyst enhancing ingredient nor have titania and lanthanum oxide been used together as catalyst enhancing ingredients. Titania has been incorporated in catalyst support materials for purposes of stabilising such support materials, but prevented from entering into the catalytic process by coverings of other catalytically active ingredients (see U.S. patents 4,504,598 and 4,123,391). In some cases, titania has been layered on the support in large amounts prior to being covered with the catalytically active material (see U.S. patent 4,350,613). Straight titania has been slurried onto prevalent precious metal materials (including palladium) in a continuous film about 0.075-0.25 mm) (.003-.01 inches) thick to make an oxidation catalyst more lead tolerant (see U.S. patent 4,650,782). Such catalyst construction was not intended for enhancement of reduction processes, the titania did not function as a reducing catalyst, and there was found no special catalytic synergism between titania and palladium. Lanthanum oxide has been suggested for use as a substrate stabiliser as well as other rare earth oxides (see U.S. patents 4,624,941 and 4,283,308). In such disclosures, lanthanum oxide is dispersed as a sintered powder onto a substrate support such as alumina; the lanthanum oxide and support together are then covered with one or more noble metals. No improvement in catalytic activity was attributed to the presence of La₂O₃. U.S. patent 4,791,091 found that by dispersing lanthanum oxide onto Al₂O₃ in small particle size assisted the activity of rhodium in the temperature range of 200-460°C, but failed to appreciate how lanthanum oxide could be used to increase catalytic activity of palladium at temperatures below 200°C and above 460°C. This invention provides a three-way catalyst and catalyst construction in which titania (titanium oxide) and lanthanum oxide are deployed together in a unique manner, and methods of making them. The three-way catalyst for automotive emission control comprises a support composed substantially of alumina, discontinuous lanthanum oxide decorated on said support, a palladium catalytic compound containing at least 55% by weight of Pd and optionally up to 45% by weight of Pt and supported discontinuously on the decorated support, and a discontinuous phase of titanium oxide on said composite of alumina, lanthana and catalytic compound, said titanium oxide phase amounting to from 0.1 to 8% of the total weight of the catalyst and covering not more than 40% of the exposed surface of the palladium catalytic compound. "Support" is used herein to mean a material having a high surface area per unit volume and a good adhesion for precious metal applied thereto; "carrier" is used herein to mean a material having a low surface area per unit volume and a poor adhesion to precious metals if applied thereto; "catalyst" is used to mean the aggregation of support, lanthana, catalytic compound, and titania, but excluding the carrier, and "catalyst construction" is used to mean the said aggregate together with the carrier. Preferably, alumina particles are precipitated from an alumina slurry. Lanthana is preferably precipitated from lanthanum nitrate and should be present in the catalyst in an amount of 2-30% (by weight). Preferably, palladium is precipitated from palladium nitrate and is present in an amount of .05-5% of the total weight of the catalyst; such catalytic compound (palladium) may e.g. contain platinum in an amount up to 2.5% of the compound. The discontinuous catalytic compound should cover from 0.05 to 10% of the area of the lanthana decorated support Preferably, the discontinuous titania phase is formed from an organo-titanate and the titania constitutes up to 2% of the total weight of the catalyst and is distributed so that no greater than 40% of the surface area of each exposed particle of the catalytic compound, lanthana, or support is covered by the discontinuous phase of titanium oxide. The method of this invention comprises making the catalyst by the steps of (a) discontinuously coating an alumina support with lanthana, (b) impregnating the coated support with a palladium compound to form a discontinuous phase on the lanthana and support; and (c) impregnating the coated support and catalytic compound with an organo-titanium compound to form, upon heating, a discontinuous titania phase on the discontinuous lanthana and palladium catalytic compound phases. Discontinuity of lanthana is achieved by precipitation of a crystalline compound from an inorganic solution, which crystalline compound inherently fails to continuously spread across the alumina. Maximum dispersement on the alumina is achieved up to 15% by weight La₂O₃; lanthana will spread out as the islands of lanthana build up when the content exceeds 15%. Such island build-up leads to less surface area for interaction with Pd. As lanthana is controlled to 15% or less, the build-up is of a two-dimensional character with minimal spreading. Discontinuity of palladium is achieved for a similar reason by precipitation as an inorganic crystalline phase. Discontinuity of titania is achieved by adding titanium as an organo-titanium compound and decomposing such compound by heat to form the discontinuity. Preferably, the organo-titanium compound is titanium butoxide and is dissolved in a liquid for wetting the composite, the wetted composite being calcined at a temperature of 400-600°C for 4-12 hours. Such calcination is staged to include heating to 250-280°C for 1-1/2 to 2-1/2 hours and thence heating to 400-600°C for 5-10 hours. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a graphical illustration of hydrocarbon conversion efficiency plotted as a function of temperature for (i) a catalyst construction in accordance with this invention and (ii) for a typical prior art construction lacking the titania improvement. Figure 2 is a graphical illustration of nitrogen oxide conversion efficiency plotted as a function of temperature for (i) a catalyst construction in accordance with this invention and (ii) for comparative constructions, one lacking the lanthana addition and the other lacking the lanthana/palladium/titania combination; and Figure 3 is a schematic representation of the titania palladium, lanthana deposit relationship. As shown in Fig. 3 the tree-way catalyst construction of this invention consists essentially of a mechanical carrier having a support comprised substantially of alumina, a lanthanum oxide deposit on the support, a catalytic palladium compound dispersed thereover, and titania deposited discontinuously on the palladium/lanthana/support composite. The mechanical carrier is preferably comprised of a monolithic silicate structure (i.e., cordierite), although the configuration of the carrier is not critical to the catalyst construction of this invention. The volume of the structure is measured by its exterior dimensions. It is preferred that the micropore volume of the silicate material be relatively low and the macropore volume provide at least about 90% with pore diameters greater than 200nm (2000 angstroms). The surface area of the carrier, as opposed to the material of which the structure is made, should be maximised consistent with pressure drop limitations for the particular application of the catalytic system of this invention. It is preferred that the surface area of the monolithic structure provide 50-1000m² per litre of structure, as measured by N₂ adsorption. Cell density should be maximised consistent with pressure drop limitations and is preferably in the range of 30-125 cells/cm² of (200-800 cells in²) of cross-sectional area of the structure. Alumina must be the dominant or substantial ingredient of the support (at least 50% by weight) because it has high surface area, good adhesion, and low precious metal/support chemical interaction. It is desirable to form a support slurry by adding a solution of lanthanum nitrate in water to alumina particles--gamma or alpha type. Lanthanum oxide decorated Al₂O₃ particles are formed upon drying (12 hours at 110°C) and calcination (at about 700°C for 16 hours) from such slurry. The crystalline lanthana will not form a continuous coating, but rather pockets of separated particles or patches of lanthana. The particles after such drying and calcination will then receive the discontinuous phases of palladium and titanium oxide to form a collection of specially decorated particles which are eventually applied as a washcoating to the carrier or monolithic structure heretofore described. This is carried out by forming a diluted solution of palladium nitrate (containing about 8 grams/litre of palladium) and adding the lanthana decorated alumina particles; the mixture is dried (for about 12 hours at about 110°C) and calcined (for about 16 hours at about 650°C) to produce discontinuously decorated Pd/La₂O₃ alumina composition. This composition is then impregnated with titanium butoxide dissolved in ethanol, dried at about 100°C for about 12 hours and then stage-heated to be calcined. The specially decorated TiO₂/Pd/La₂O₃ alumina particles can be deposited on the monolithic carrier by any conventional or convenient means. Typical of such means is dipping the carrier in a slurry of such specially decorated particles to form a deposit in the form of a film about 0.0075-0.25mm (.003-.01 inch) thick). It is also preferred that such deposited particles be present on the monolithic support in an amount ranging from 15-350 g/dm³ (about 400 to about 10,000g/ft³) of carrier, where the volume is measured by the exterior dimensions of the monolithic carrier. Alternatively, lanthana and palladium may be applied directly to an alumina coated monolith structure as a first coating and the discontinuous phase of titanium oxide applied over the palladium. The catalytic compound of the present invention must comprise a major constituent of palladium which in turn is supported on the lanthana/alumina substrate. Palladium is present in an amount of at least 55% by weight of the catalytic compound. Platinum may be present in such compound in an amount up to 45% and may include other components which act as stabilisers and/or interspersants, which are well known in the art. Additionally, other components known to be beneficial to the oxidation of automotive exhaust gases may be present in minor amounts in the catalyst of the instant invention such as CaO, Zr₂O₃, NiO, and Fe₂O₃. The discontinuous palladium phase is achieved by crystal impregnation of palladium from palladium compounds in solution that have been calcined. The discontinuous phase consists of titanium oxide; titanium oxide is intended to mean herein primarily the stable oxide form of TiO₂, but other forms, such as TiO, Ti₂O₃, Ti₃O₅, and Ti₂O may be present in minor proportions of less than 2%. Titanium oxide constitutes from .1% up to 8% of the total weiqht of the catalvst (preferably .1-2.% of the total weight of the catalyst). Although the discontinuous titania phase may cover up to 40% of the exposed surface of the catalytic compound, it preferably covers only 2-20% thereof. If TiO₂ is present in an amount greater than 8% by weight, the phase starts to display continuous regions and conversion efficiency is seriously affected. If present in an amount less than .1% by weight, the ability of TiO₂ to supply oxygen to Pd is unnoticeable and again affects conversion efficiency. In carrying out the method of making the three-way catalyst of the invention, to decorate the alumina support, it is impregnated with a solution of lanthanum compounds (e.g., lanthanum nitrate), then dried and calcined to crystallise lanthana. The decorated support is impregnated with palladium by wetting the decorated support with a solution of palladium compound (e.g., palladium nitrate), and then dried and calcined to crystallise a discontinuous phase of palladium. Palladium may be enhanced in catalytic activity by (i) control of the particle size for a systemic difference, and (ii) by adding palladium in a diluted palladium nitrate solution with the amount of acid being regulated to achieve pH of 3-5 and thereafter,after drying the support, wetted with such acid, calcining the composite at a temperature in the range of 400-650°C. The organo-titanium compound is applied by dissolving such compound in a liquid for wetting the alumina/lanthanum/palladium composite, the wetted composite being then calcined at a temperature of about 400-600°C for 4-12 hours. Preferably, the calcination is staged to include heating to 250-280°C for 1-1/2 to 2-1/2 hours, and thence heating to 400-600°C for 5-10 hours. The organo-titanium compound is selected from the group consisting of organo-titanates. A preferred example of such compound is titanium n-butoxide, characterised by Ti(OC₄H₉-n)₄. Equivalents to the butoxide are titanium alkoxides. Organo-titanium compounds, particularly titanium butoxide, decompose at lower temperatures, substantially below the decomposition temperature of palladium nitrate. As shown in Figure 3, the discontinuous phases of the composite represent a striking physical difference to that of the prior art. Titania covers no greater than 40% of the surface area of the exposed and separated palladium/lanthana particles. Heating lanthanum nitrate wetted alumina produces the separated crystallites of lanthana; heating palladium nitrate wetted lanthana/alumina produces crystallites of palladium on or about the lanthana or alumina. Wetting the discontinuous crystallites of Pd and the decorated alumina/lanthana with an organo-titanium compound, followed by heating, results in growing of TiO₂ on or adjacent to the Pd particles. Titania crystallites will not grow preferentially independent of the Pd. Irregularities in the gamma alumina substrate, such as exposed crystallographic planes, contamination layers, or roughness due to geometry of nucleation sites, decrease the interfacial strength between Pd and gamma alumina and promote even greater discontinuities of the Pd particles. The amount of oragno-titanium compound introduced to the catalyst must be such as to form titanium oxide in the operational range of .1-8% by weight of the total catalyst, and preferably .1-2.0%. If the titanium compound were to be less than .1% of the total catalyst, there would be an imperceptible amount of enhancement of the catalytic activity. If the titanium oxide were the exceed 8%, the catalytic activity of palladium would be retarded by masking of a greater percentage of the palladium and thus reducing its exposure. The titanium oxide functions as a catalyst in combination with palladium and lanthana to aid in reduction as well as oxidation of the gases within the automotive emissions. The titanium oxide in its discontinuous configuration functions during oxygen-lean conditions of the emission gas to form di-titanium trioxide and additional oxygen. This may be represented by the equation: 2TiO₂ → Ti₂O₃ + 1/2 O₂. During rich oxygen spikes of the emission gas, a reducing condition occurs where titanium trioxide combines with oxygen to form titanium dioxide represented by the formula: Ti₂O₃ + 1/2 O₂ → 2TiO₂. As shown in Figure 2, nitric oxide reduction (conversion) efficiency is substantially greater for the catalyst of this invention (see square symbols with broken line) throughout the temperature range of 100-600°C as compared to the reduction capability of a palladium catalyst (see diamond symbol with solid line) without lanthana and titanium dioxide, and is substantially greater in the temperature range of 100-200°C and 400-600°C as compared to the reduction capability of a palladium catalyst with TiO₂, but not lanthana (see diamond symbol with broken line). As shown in Figure 1, the conversion efficiency of oxidising hydrocarbons is substantially greater in the temperature range of 300-500°C for the invention (triangular symbol with solid line: Al₂O₃/La₂O₃/Pd/TiO₂) than that compared for a palladium catalyst devoid of titanium dioxide (broken line: Al₂O₃/La₂O₃/Pd). Thus, the unique introduction of a discontinuous phase of titania to a discontinuous palladium phase and discontinuous lanthana phase enhances the high temperature catalysis of both reduction and oxidation of nitrous oxide and hydrocarbon, and also reduces the light-off temperature for the catalyst in an amount of 20-100°C. To show the importance of process and chemical content variations, several samples were run and data therefor is shown in Table I. Each of the samples was generated by impregnating gamma alumina by incipient wetness, with lanthanum nitrate solution to produce 15% lanthana/alumina composition (42.5 grams of 0.42-0.84 mm (20-40 mesh) gamma alumina was mixed with a solution of 8.4 grams of lanthanum nitrate in 56 ml of water). It was dried overnight at 100°C and calcined at 700°C for 16 hours. For those samples having a discontinuous phase of palladium, 6 ml of palladium nitrate solution (83.5 g/l) was diluted to 56 ml to obtain a solution with 0.5 gram of palladium; to the solution was added the lanthana decorated alumina particles. The mixture was dried and calcined similarly. For those samples employing a discontinuous TiO₂ phase, a solution of 0.56 grams of titanium butoxide was prepared in 15 ml of ethanol and 5 grams of 15% lanthana/alumina composition was added to it. This latter mixture was stirred to mix it thoroughly. It was placed in an oven at 95°C to evaporate ethanol. The titania-modified material and the starting palladium/alumina composition were heated in an oven at 280°C for one hour and thence at 600°C for six hours. The catalytic activity of these materials for propane oxidation and NO reduction was determined in a simulated flow reactor (1% H₂, 1000 ppm NO, space velocity 30 K hr⁻¹ for NO test, and 500 ppm C₃H₈, 2% O₂ at 30 K hr⁻¹ for HC test). Samples 1 and 2 compare the catalytic activity without the use of La₂O₃. Sample 3 compares the catalytic activity with and without the use of an organo-titanium compound. Sample 4 compares use of a non-alumina substrate. Sample 5 compares different mode of applying slurries. Sample 6 compares added presence of conventional oxides in place of lanthana. Samples 7 and 8 compare respectively the presence of added Pt and replacement of Pd by Pt. Sample 9 compares the catalytic activity utilizing a different organo-titanium compound. Sample 10 compares greater lanthana. Samples 11 and 12 compare greater and too lean contents of TiO₂. Sample 13, the use of rapid heating rather than stepped heating of titania.

Claims: A three-way catalyst for automotive emission control, comprising a support composed substantially of alumina, discontinuous lanthanum oxide decorated on said support, a palladium catalytic compound containing at least 55% by weight of Pd and optionally up to 45% by weight of Pt and supported discontinuously on the decorated support, and a discontinuous phase of titanium oxide on said composite of alumina, lanthana and catalytic compound, said titanium oxide phase amounting to from 0.1 to 8% of the total weight of the catalyst and covering not more than 40% of the exposed surface of the palladium catalytic compound. A catalyst as claimed in claim 1 in which the amount of lanthana does not exceed 15% by weight of the catalyst. A catalyst as claimed in claim 1 or claim 2 in which said discontinuous palladium catalytic compound covers from 0.05 to 10% of said lanthana decorated support. A catalyst as claimed in any preceding claim, in which said catalytic palladium compound forms from 0.05 to 5% of the total weight of the catalyst. A catalyst as claimed in any preceding claim, in which said palladium catalytic compound contains up to 2.5% platinum. A catalyst as claimed in any preceding claim, in which said discontinuous titanium oxide phase forms from 0.1 to 2.0% of the total weight of the catalyst. A three-way catalyst construction for automotive emission control, comprising a catalyst as claimed in any preceding claim supported on a mechanical carrier. A method of making a three-way catalyst for automotive emission control according to any one of claims 1 to 6, comprising discontinuously coating an alumina support with lanthana, impregnating the coated support with a palladium-comprising compound and treating it to form a discontinuous phase of palladium catalytic compound on the lanthana and support, impregnating the coated support and catalytic compound with an organo-titanium compound and heating to form a discontinuous titanium oxide phase on the exposed portions of said discontinuous lanthana and palladium catalytic compound phases. A method as claimed in claim 8, in which the palladium catalytic compound phase is formed by wetting the lanthana coated support with a dilute palladium nitrate solution regulated to an acidity of pH 3-5 and, after drying, calcining the composite at a temperature in the range of 450-650°C. A method as claimed in claim 8 or claim 9, in which said organo-titanium compound is dissolved in a liquid for wetting the composite of the lanthana coated support and catalytic compound and the wetted composite is calcined at a temperature of 400-600°C for 4-12 hours. A method as claimed in claim 10, in which said calcination is staged to include heating to 250-280°C for 1 1/2 to 2 1/2 hours and thence heating to 400-600°C for 5-10 hours. A method as claimed in any of claims 8 to 11, in which said organo-titanium compound is titanium butoxide. A method as claimed in any of claims 8 to 12, in which said support is a collection of aggregates prepared by (i) wetting a particulate source of alumina with dilute nitric acid to form a slurry, (ii) drying said slurry to form a cake, and (iii) crushing said cake into particle form. A method as claimed in any of claims 8 to 13, in which the alumina support is applied to the monolithic carrier of a three-way catalyst construction before performing the coating steps. A method of making a three-way catalyst construction for automotive emission control which comprises depositing the particulate product made by any of claims 8 to 13 on a monolithic carrier.

Inventors: CHATTHA MOHINDER SINGH, GANDHI HAREN SAKARLAL, WATKINS WILLIAM LEWIS HENDERSO, CHATTHA, MOHINDER SINGH, GANDHI, HAREN SAKARLAL, WATKINS, WILLIAM LEWIS HENDERSON
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S. A., FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1990
Main classification: B01D53
IPC codes: B01D53, B01J37, B01J23