GB2055303A - Catalytic conversion of compounds contained in exhaust gases from an internal combustion engine - Google Patents

Abstract

Carbon monoxide, hydrocarcarbons and nitrogen oxides contained in hot exhaust gas from an internal combustion engine are converted by passing them in contact with a catalytic composite comprising a platinum-rhodium or palladium-rhodium catalyst promoted by uranium in intimate admixture therewith and 2 dispersed on a high-surface-area refractory inorganic oxide.

Description

SPECIFICATION Conversion of compounds contained in exhaust gas from an internal combustion engine Gaseous waste products resulting from the burning or combustion of hydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbon monoxide, hydrocarbon(s) and nitrogen oxide(s) as products of combustion or incomplete combustion, and pose a serious health problem with respect to pollution of the atmosphere. While exhaust gases from other carbonaceous fuelburning sources, such as stationary engines, industrial furnaces, etc., contribute substantially to air pollution, the exhaust gases from automotive engines are a principal source of pollution.In recent years, with the ever-growing number of automobiles powered by internal combustion engines, the discharge of waste products therefrom has been of increasing concern, particularly in urban areas where the problem is more acute, and the control thereof has become exceedingly important. Of the various methods which have been proposed for converting the carbon monoxide, hydrocarbon and nitrogen oxide pollutants to innocuous products, the incorporation of a catalytic converter in the exhaust system holds the most promise of meeting the increasingly rigid standards established by the responsible governmental agencies.

In order to achieve a substantially simultaneous conversion of the carbon monoxide, hydrocarbon and nitrogen oxide pollutants, it has become the practice to employ a catalyst in conjunction with a fuel-air ratio control means which functions in response to a feed-back signal from an oxygen sensor in the engine exhaust gases. The fuel-air ratio control means is typically programmed to provide fuel and air to the engine in a ratio conducive to a near stoichiometric balance of oxidants and reductants in the hot exhaust gases at engine cruising conditions, and to a stoichiometric excess of reductants at engine idling and acceleration conditions.

The class of exhaust gas conversion catalysts herein contemplated, commonly referred to as three component control catalysts, must therefore function under variable conditions. Ideally, the catalyst should be capable of functioning under dynamic net oxidizing-net reducing conditions to catalyze the reaction of said pollutants with each other and/or any of the oxygen, hydrogen, carbon dioxide or water components which occur in hot exhaust gas fluctuating between a molar excess of oxidants and a molar excess of reductants. In particular, the catalyst should be capable of functioning during those more extended periods of fuel-rich operation, such as are encountered at engine idling and acceleration conditions, when the deficiency of oxidants in the exhaust gas becomes more acute.In other words, the catalyst should be capable of effecting the conversion of the otherwise oxidizable carbon monoxide and hydrocarbon pollutants in the absence of oxidants as typified by the water gas shift and steam reforming reactions.

Catalytic composites comprising rhodium and platinum and/or palladium as the catalytic components have heretofore been proposed for the catalytic conversion of exhaust gases from an internal combustion engine. Frequently, the catalytic composite will further comprise a base metal component, typically nickel. While certain of the base metals are known to catalyze one or more of the various reactions which constitute the exhaust gas conversion process, they are in themselves substantially less effective, and in some cases ineffective, at the dynamic net oxidizing-net reducing conditions herein contemplated. Also, certain base metals demonstrate sharply decreased performance in the presence of sulphur. Other base metal components, although catalytically inert, are included in the catalytic composite for their contribution to physical and/or thermal stability.

This invention seeks to provide a novel process for the simultaneous conversion of carbon monoxide, hydrocarbon(s) and oxide(s) of nitrogen contained in hot exhaust gas from an internal combustion engine, utilizing a catalytic composite quickly responsive to the more dynamic net oxidizing-net reducing conditions and/or affording improved conversion of the carbon monoxide and hydrocarbons at fuel-rich operating conditions.

It has now been discovered that uranium, while substantially inactive in itself, is a uniquely effective promoter for the rhodium and platinum and/or palladium catalytic combination to provide improved and continuous three-component control at the more dynamic net oxidizingnet reducing conditions, and at fuel-rich operating conditions. Also, uranium, in combination with platinum and rhodium, demonstrates superior performance to other base metal promoters at conditions of high sulfur concentration.

According to the present invention there is provided a prcess for the conversion of carbon monoxide, hydrocarbon(s) and oxide(s) of nitrogen contained in hot exhaust gas from an internal combustion engine, which process comprises contacting the hot exhaust gas with a catalytic composite comprising rhodium in combination with platinum or palladium as the catalytic elements, and further comprising a uranium promoter in intimate admixture with the catalytic elements and dispersed on a high-surface-area refractory inorganic oxide.

Advantageously, the catalytic composite comprises rhodim in combination with platinum as the catalytic elements, and further comprises a uranium promoter in intimate admixture with the catalytic elements and dispersed on a high-surface-area alumina.

Preferably, the catalytic composite comprises from 0.005 to 0.15 wt.% rhodium in combination with from 0.1 to 0.6 wt.% platinum as the catalytic elements, and further comprises from 0.05 to 3 wt.% uranium promoter in intimate admixture with the catalytic elements and dispersed on a high-surface-area alumina deposited as a uniform film on a relatively low surface area honeycomb-type ceramic support.

The catalytic composite for use in this invention comprises catalytic elements, viz. rhodium in combination with platinum or palladium, and a uranium promoter dispersed on a high-surfacearea refractory inorganic oxide. The refractory inorganic oxide may be a naturally occurring material, for example a clay or silicate such as fuiler's earth, attapulgus clay, feldspar, halloysite, montmorillonite, kaolin or diatomaceous earth (frequently referred to as siliceous earth, diatomaceous silicate, or kieselguhr), and the naturally occurring material may or may not be activated prior to use by one or more treatments, including drying, calcining, steaming and/or acid treatment.Synthetically prepared refractory inorganic oxides such as alumina, silica, zirconia, boria, thoria, magnesia, titania or chromia, or composites thereof, particularly alumina in combination with one or more other refractory inorganic oxides, for example alumina-silica, alumina-zirconia, alumina-chromia, and having a total surface area of from 25 to 600 square meters per gram, are especially suitable. In some cases, the refractory inorganic oxide may also exhibit a catalytic effect alone or in combination with the other components of the catalytic composite. Alumina is a preferred refractory inorganic oxide, and the alumina may be any of the various hydrous aluminum oxides or alumina gels such as boehmite, gibbsite or bayerite.

Activated aluminas, such as have been thermally treated at a temperature in excess of 400 C with the elimination of at least a portion of the chemically and/or physically combined water and hydroxyl groups commonly associated therewith, are particularly suitable.

In one preferred embodiment of this invention, the refractory inorganic oxide is employed deposited as a thin uniform film on a relatively low-surface-area ceramic material, for example sillimanite, petalite, cordierite, mullite, zircon, zircon mullite or spodumene. The ceramic material is typically utilized as a rigid, unitary, honeycomb-type structure, frequently referred to as a monolithic structure, providing a plurality of adjacent, parallel and unidirectional channels therethrough. However, the refractory inorganic oxide may also be satisfactorily ulilized in the form of pills, pellets, granules, rings or spheres, low density spheroidal particles having an average bulk density of from 0.3 to 0.5 grams per cubic centimeter being one preferred particulate form.Low density alumina spheres are conveniently and advantageously prepared by the well-known oil drop method. Briefly in this method, an alumina sol, such as results from digesting aluminum in hydrochloric acid under controlled conditions, is dispersed as droplets in the hot oil bath whereby gelation occurs with the formation of spheroidal gel particles. In this type of operation, the alumina is set chemically utilizing ammonia as a neutralizing or setting agent. Usually, the ammonia is furnished by an ammonia precursor, such as hexamethylenetetramine, which is included in the sol. Only a fraction of the ammonia precursor is hydrolyzed or decomposed to ammonia in the relatively short period during which the initial gelation occurs.

During the subsequent aging process, the residual ammonia precursor retained in the gelled particles continues to hydrolyze and effect further polymerization of the alumina whereby desirable pore characteristics are established. After a suitable aging period, usually from 10 to 24 hours at a temperature in the 50-105 C range, the alumina spheres are washed, dried, and calcined or activated at a temperature of from 500 to 850 C. The oil drop method is described in detail in U.S. Patent No. 2,620,314 and in U.S. Patent No. 3,600,129.

The catalytic composite for use in this invention further comprises rhodium in combination with platinum or palladium as the catalytic elements of the composite. The selected highsurface-area refractory inorganic oxide carrier material, either in the form of pills, pellets, etc. or as a thin film on a honeycomb-type ceramic support, can be impregnated with the rhodium component by conventional methods which generally entail soaking, dipping one or more times, or otherwise immersing, the carrier material in an aqueous solution of a decomposable rhodium compound, preferably an aqueous rhodium trichloride solution.The platinum or palladium component is preferably impregnated on the carrier material from a common impregnating solution with the rhodium component, for example 3 common aqueous solution of chloroplatinic or chloropalladic acid and rhodium trichloride. In any case, the resulting composite is subsequently oxidized, suitably in air at a temperature of from 370 to 650 C., and/or reduced, suitably in a hydrogen atmosphere at a temperature of from 370 to 650 C.

The uranium component of the catalytic composite can be impregnated on the carrier material from a common impregnating solution with the rhodium and the platinum or palladium components, or the uranium component can be impregnated on the carrier material prior to or subsequent to said rhodium and platinum or palladium components. It is a preferred practice to initially impregnate the carrier material with the uranium component, for example from an aqueous uranyl nitrate solution, the uranium-impregnated carrier material being oxidized, preferably in air, at a temperature of from 370 to 65O'C. prior to impregnation with the rhodium and platinum or palladium components.

The catalytic composite proposed for use in this invention is suitable for use in a converter or a reactor of through-flow, cross-flow, or radial-flow design installed in the exhaust line of an internal combustion engine. The converter or reactor may be employed in series with a subsequent oxidation converter or reactor with combustion air being injected ahead of the oxidation converter to ensure conversion of the residual carbon monoxide and hydrocarbons remaining in the exhaust gases.

The following examples illustrate the improvement in carbon monoxide, hydrocarbon and nitrogen oxides conversion to be derived through practice of the present invention.

EXAMPLE I To illustrate the increased activity of a platinum-rhodium-uranium catalyst over a platinumrhodium catalyst, samples of each were prepared.

A catalytic composite comprising a high-surface-area alumina-coated ceramic honeycomb cylinder impregnated with 250 gms. of uranium, 38 gms. of platinum and 2 gms. of rhodium per cubic meter (250, 38 and 2 gms per cubic foot, respectively) was prepared. The aluminacoated ceramic honeycomb cylinder was 22 mm (7/8") in diameter, 79 mm (3 1 /8") in length and contained approximately 36.5 parallel triangular channels per square cm (236 per square inch). In the preparation of the catalytic composite, the alumina-coated ceramic honeycomb cylinder was loaded into a vacuum flask and the flask was evacuated to about 0.97 kg/cm2 (28" Hg). A 36 ml. aqueous uranyl nitrate hydrate solution (1.1 gms. of uranium) was admitted to the evacuated flask and swirled in contact with the cylinder for about 240 seconds.The resulting uranium-impregnated honeycomb cylinder was recovered from the flask, dried at 100-300 C., and calcined in air for 2 hours at 530 C. The cylinder was then reloaded into the vacuum flask and the flask was again evacuated. A 36 ml. common aqueous solution of chloroplatinic acid (0.11 gm. of platinum) and rhodium trichloride hydrate (0.0038 gm. of rhodium) was then admitted to the flask and swirled in contact with the uranium-impregnated cylinder for about 240 seconds. The resulting impregnated cylinder was recovered from the flask, dried at 100-300 C. and calcined in air at 530 C. for 2 hours. The calcined cylinder was thereafter cut into discs 1 3 mm (+") in length.

A platinum-rhodium catalyst was then prepared in substantially the same manner as described above. A 36 ml. common aqueous solution of chloroplatinic acid (0.11 gm. of Pt) and rhodium trichloride hydrate (0.0038 gm. of rhodium) was admitted to the flask and swirled in contact with the cylinder for about 240 seconds. The resulting impregnated cylinder was recovered from the flask, dried at 100-300 C. and calcined in air at 530 C. for 2 hours. The calcined cylinder was thereafter cut into discs 13 mm (+") in length.

EXAMPLE II The two catalytic composites prepared in the above example were then tested for carbon monoxide, hydrocarbon and nitric oxide conversion activity. The catalysts were evaluated with respect to a synthetic exhaust gas mixture affording net oxidizing conditions typically encountered under feedback control, and with respect to a synthetic exhaust gas mixture affording net reducing conditions typically encountered under feedback control.The composition of the synthetic exhaust gas mixtures expressed in mole percent was as follows: Net Net Oxidizing Reducing O2 0.97 0.32 CO 0.45 1.35 H2 0.15 0.45 C3H8 0.015 0.044 NO 0.11 0.11 N2 76.67 76.09 CO2 11.64 11.64 SO2 0.001 0.001 H20 10.00 10.00 The synthetic gas mixtures were alternately charged in contact with the catalytic composite, said gaseous mixtures alternating or cycling between net oxidizing and net reducing conditions at a frequency of 0.25 hertz.The synthetic exhaust gas mixtures were preheated to 500 C. and charged in contact with the catalytic composite at a GHSV of 11 9,000. Each of the catalytic composites was further evaluated in the same manner except that the gases were preheated to only 350 C. to provide a more demanding measure of catalytic conversion. In each case, the effluent exhaust gases were sampled and analyzed after a 15-minute test period with the following results: % Conversion at 500 C % Conversion at 350 C QHs CO NO C3H8 CO NO Pt-Rh-U 89 90 46 41 94 46 Pt-Rh 87 84 32 30 77 41 The catalysts were then tested and evaluated at more severe conditions.The amount of SO2, a known poison for most catalysts, in the exhaust gas was increased from 10 ppm to 45 ppm. All other conditions, including the relative compositions of the oxidizing and reducing gases were the same as in the previous test. In this case, the effluent gas samples were analyzed with the following results: % Conversion at 50Q C % Conversion at 350 C C3H8 CO NO QH8 CO NO Pt-Rh-U 84 94 55 51 92 49 Pt-Rh 81 81 37 31 74 33 Thus, the addition of uranium to the platinum-rhodium catalyst system results in improved performance at normal and high levels of sulphur concentration.

EXAMPLE 111 The performance of the platinum-rhodium-uranium catalyst was next compared with other catalyst compositions for performance during cycling, oxidizing and reducing conditions. A catalyst comprising platinum-rhodium-nickel was prepared in the manner described in the previous examples. A 36 ml. aqueous solution containing 1.9 gms. of nickel from nickel nitrate was admitted to the flask and swirled in contact with the cylinder for about 240 seconds. The resulting impregnated cylinder was recovered, dried and calcined, and further impregnated with the platinum and rhodium components, all in accordance with the preparations of the previous examples.

A platinum-rhodium-cerium catalyst was similarly prepared. A 36 ml. aqueous solution containing 1.0 gm. of cerium from cerium nitrate hydrate was admitted to the flask and swirled in contact with the cylinder for about 240 seconds. The resulting impregnated cylinder was recovered, dried and calcined, and further impregnated with the platinum and rhodium components, all in accordance with the preparations of the previous examples.

These catalysts were tested under the conditions of Example II, with the SO, concentration of the exhaust gas at approximately 45 ppm. These catalysts compare with the Pt-Rh-U catalyst as follows: % Conversion at 500 C % Conversion at 350 C C3H8 CO NO C3H8 CO NO Pt-Rh-U 84 94 55 51 92 49 Pt-Rh-Ni 80 82 57 33 77 63 Pt-Rh-Ce 82 87 42 41 86 51 Thus, the uranium promoted platinum-rhodium catalyst performed significantly better than either the nickel or cerium promoted catalysts.

EXAMPLE IV Three component control catalysts are required to convert the carbon monoxide and hydrocarbons during periods of fuel-rich operation as well as under stoichiometric and fuel-lean conditions. Under fuel-rich conditions, there are insufficient oxidants to convert all of the carbon monoxide and hydrocarbons. The catalyst must therefore be capable of facilitating the nonoxidative means of removing these two pollutants. The water-gas shift reaction CO + H2O#CO2 + H2 is one such non-oxidative means. Accordingly, the catalysts were further evaluated with respect to a synthetic gas mixture simulating fuel-rich conditions.The composition of the synthetic gas mixture, expressed in mole %, was as follows: N2 77.3 C02 11.8 H20 10.0 CO 1.36 H2 0.45 O2 0.00 NO 0.036 SO, 0.005 The gaseous mixture was preheated at 500 C. and passed in contact with each of the catalytic composites at a GHSV of 11 9,000. The effluent gases were sampled and analyzed after a 15minute period with the following results: : Conversion at 500 C % CO %C3H8 Pt-Rh-U 23.0 0.1 Pt-Rh 3.4 1.1 Pt-Rh-Ni 4.6 0.0 Pt-Rh-Ce 11.9 0.0 The foregoing examples clearly illustrate the improvement to be derived from the catalytic composite proposed by this invention, said improvement being evident with respect to operation under dynamic net oxidizing net reducing conditions, and with respect to operation under oxidant-deficient fuel-rich conditions.

Claims (8)

1. A process for the conversion of carbon monoxide, hydrocarbon(s) and oxide(s) of nitrogen contained in hot exhaust gas from an internal combustion engine which comprises contacting the hot exhaust gas with a catalytic composite comprising rhodium in combination with platinum or palladium as the catalytic elements, and further comprising a uranium promoter in intimate admixture with the catalytic elements and dispersed on a high-surface-area refractory inorganic oxide.

2. A process as claimed in claim 1 wherein the high-surface-area refractory inorganic oxide is a high-surface-area alumina.

3. A process as claimed in claim 2 wherein the high-surface-area alumina is deposited as a uniform film on a relatively low-surface-area honeycomb-type ceramic support.

4. A process as claimed in any of claims 1 to 3 wherein the catalytic composite comprises from 0.001 to 0.5 wt. % rhodium, from 0.02 to 2 wt. % platinum and from 0.01 to 10 wt. % uranium dispersed on a high-surface-area refractory inorganic oxide.

5. A process as claimed in claim 4 wherein the catalytic composite comprises from 0.005 to 0. 15 wt. % rhodium, from 0.1 to 0.6 wt. % platinum and from 0.05 to 3 wt. % uranium dispersed on a high-surface-area alumina deposited as a uniform film on a relatively low-surfacearea honeycomb-type ceramic support.

6. A process as claimed in claim 1 carried out using a catalytic composite substantially as hereinbefore described or exemplified.

7. A catalytic converter or reactor for installation in the exhaust line of an internal combustion engine and containing a catalytic composite as defined in any of claims 1 to 5.

8. In combination with an internal combustion engine, a catalytic converter as claimed in claim 7 installed in the exhaust line from the engine.