VEHICLE EXHAUST GAS SYSTEMS

Abstract: 2-way and 3-way catalyst compositions comprising tin(IV) oxide (SnO₂) and chromium(VI) oxide (CrO₃) are disclosed for use in catalytic converters for vehicle exhaust gas systems, such catalyst compositions being effective in the catalytic oxidation of carbon monoxide and unburnt hydrocarbons, and in the catalytic reduction of nitrogen oxides contained in the vehicle exhaust.

Description: This invention relates to catalyst compositions for use in the treatment of vehicle exhaust gas emissions to reduce the content of carbon monoxide and unburnt hydrocarbons, and thus to reduce pollution problems associated with vehicle exhausts. Increasingly severe environmental regulations governing the emission of noxious gases into the atmosphere, and in particular, vehicle exhaust emissions, have stimulated a demand for catalysts active in the removal of such noxious components from the gases prior to disposal or venting to the atmosphere. In particular demand are 2-way and 3-way catalysts effective in removing carbon monoxide, unburnt hydrocarbons and/or nitrogen oxides from vehicle exhaust emissions, and this has led to the development of catalytic converters for attachment to vehicle exhaust gas emission systems to control the emission of carbon monoxide, unburnt hydrocarbons and/or nitrogen oxides to the atmosphere. Preferably such catalysts are active in the removal of all three, i.e. the so-called 3-way catalysts, but 2-way catalysts, or even 1-way catalysts do have some utility, not necessarily in the field of exhaust gas emissions, but in others. Current commercial catalytic converters generally comprise one or more platinum group metals e.g. platinum, palladium, rhodium and rhenium, alone or in combination. Such converters tend therefore to be expensive. Attempts have been made either to reduce or eliminate altogether the dependence on platinum group metals in such systems. For example, in GB-A-2,070,958 an exhaust gas catalyst is disclosed comprising the combination of a platinum group metal and a base metal oxide selected from SnO₂, TiO₂ and NiMoO₄ deposited on a monolithic ceramic support coated with a refractory metal oxide, e.g. alumina. Numerous other patent applications and scientific papers have been published on the subject, with activity largely being centered on tin(IV) oxide, i.e. SnO₂, as an essential catalyst component, alone or in combination either with platinum group metals, as already mentioned, or other base metal oxides, but the fact remains that the present commercially available or acceptable converters all employ a platinum group metal as a principal component. A non-exhaustive list of other relevant art relating to the use of tin(IV) oxide in the purification of exhaust or other waste gases is as follows: Journal of Catalysis, 39 (1975), 412-418, Catalytic reduction of nitrous oxide by carbon monoxide over tin(IV) oxide; Nature, 269 (1977), 585-586, Water-promoted oxidation of carbon monoxide over tin(IV) oxide-supported palladium; Chemistry and Industry, (1976), 787-788, Catalysis of the NO-CO reaction: a further example of synergism in the Pd-SnO₂ system; Journal of Catalysis, 42 (1976), 418-424, Catalytic reduction of nitric oxide by carbon monoxide over SnO₂-CuO gels; see also J.C.S. Chem. Comm. (1973) and J.C.S. Chem. Comm. (1974) 56-57; JP-A-75-108169, Catalytic treatment of waste gases, including vehicle exhaust emissions, with ammonia in the presence of an SnO₂/Cr₂O₃ catalyst to reduce nitrogen oxides; SU-A-736,997, Carbon monoxide oxidation catalyst comprising SnO₂ and Cr₂O₃; SU-A-691,185, Sulphur dioxide removal catalyst comprising SnO₂ and Cr₂O₃; Prep. Catal. Proc. Int. Symp. 1975 (Published 1976) 197-206, Preparation and activation of tin(IV) oxide-chromium(III) oxide catalysts for nitric oxide reduction; see also J. Catal., 54 (1978), 42-51; Proc. Mech. Hydrocarbon React. Symp. (1975) 311-320, Catalytic oxidation of ethylene, ethylene oxide and propylene using tin(IV) oxide-chromium(III) oxide catalysts. Other suggested combinations particularly for the catalytic reduction of nitrogen oxides include SnO₂-V₂O₅, JP-A-74-126561; mixed oxides of Ti and one or more of Nb, Fe, W, V, Ni, Co, Cu, Cr, U, Sn, DE-A-2458888; and SnO₂-CeO₂, JP-A-74-77881. In said JP-A-75-108169 and SU-A-736,997 a method is disclosed for preparing the SnO₂-Cr₂O₃ catalyst which comprises oxidising SnCl₂ in situ with CrO₃, but so far as is known no studies have been carried out specifically on catalyst systems comprising tin(IV) oxide and chromium(VI) oxide, i.e. the system: SnO₂-CrO₃. Finally, in GB-A-1,476,347 a catalyst composition is disclosed for the catalytic oxidation and removal of ammonia from ammonia-containing gases, the catalyst comprising a complex oxide mixture of the formula Cr x A y O z where A, inter alia , can be Sn, x is 4 to 12, y is 0.2 to 8 and z is 6.2 to 42, and which composition is prepared by reacting a chromium containing compound, inter alia, chromic anhydride with a tin compound, e.g. SnCl₄, followed by calcination to convert the reaction product into oxide form. One such calcined oxide is given as having the formula Cr₉Sn₃O 19.5 . In accordance with the present invention, a catalyst composition has been discovered that has 2-way catalytic activity, i.e. in the catalytic oxidation of carbon monoxide and in the catalytic oxidation of hydrocarbons, which is comparable with, and in some cases substantially equals, the catalytic activity of the commercially available platinum based systems. The catalyst also shows some activity in the catalytic reduction of nitrogen oxides, and is therefore potentially useful as a 3-way catalyst in the treatment of vehicle exhaust gas emissions to remove nitrogen oxides as well as carbon monoxide and/or unburnt hydrocarbons. The catalyst compositions used in accordance with this invention are essentially mixed oxide compositions obtained by impregnating tin(IV) oxide (SnO₂) in solid particulate form, and preferably as an aqueous sol, with aqueous dichromic acid and calcining the resultant product at a temperature in the range 200°C to 500°C, preferably 300°C to 400°C. Usually the catalyst composition will be used in dispersed form i.e.dispersed on a high surface area catalyst support material, such as alumina, although a wide variety of other high surface area catalyst support materials are available e.g. zeolites and other molecular sieves, and may be used as catalyst support materials in this invention. Other suitable support materials include silica, magnesia and titania. Particularly preferred support materials are those comprising an alumina, or other refractory metal oxide coating on a monolithic ceramic or metal base. Such catalyst support materials are known and are commercially available, e.g. under the Trade name Cordierite. Various techniques are available for dispersing the tin(IV) oxide and chromium(VI) oxide on the support. A convenient and currently preferred technique is to impregnate the support with a suspension of SnO₂ in an aqueous or organic medium followed by drying the impregnated support to deposit the tin(IV) oxide thereon. The tin(IV) oxide-containing support is then impregnated with aqueous dichromic acid and the reimpregnated support redried prior to calcination. In an alternative procedure, the tin(IV) oxide is formed in situ by impregnating the support with SnCl₄ and then washing the impregnated support first with water to form an unstable hydrated tin(IV) oxide precipitate, SnO₂.H₂O.Cl⁻ and subsequently with aqueous ammonia to form a stable, hydrated oxide precipitate SnO₂.H₂O. The support is then washed with water to remove chloride ion, and thereafter impregnated with the aqueous dichromic acid. Alternatively the catalyst composition may simply comprise granules or pellets of tin oxide which have been impregnated with aqueous dichromic acid, dried and calcined. The relative amounts of tin(IV) oxide and chromium(VI) oxide present in the oxide mixture prior to calcination are not narrowly critical and may range from 1 part tin (calculated as tin(IV) oxide) to 99 parts chromium (calculated as chromium(VI) oxide), all parts being by weight, to 99 parts tin to 1 part chromium, calculated on the same basis. However, the preferred range is from 40 to 90 parts tin, preferably 60 to 75, to correspondingly 60 to 10 parts chromium, preferably 40 to 25, calculated on the same basis. Alternatively expressed the Sn:Cr weight ration (calculated as SnO₂:CrO₃) is preferably in the range 2:3 to 9:1, most preferably 3:2 to 3:1. The chromium concentration can be readily and easily controlled by adjusting the molar strength of the aqueous dichromic acid used to impregnate the tin(IV) oxide. Where the catalyst composition is deposited on a high surface area support, i.e. catalyst supports with a total surface area in the range 1.5 m²/dm³ to 2.5 m²/dm³, typically around 2 m²/dm³, catalyst loadings, i.e. calculated as total tin(IV) oxide/chromium(VI) oxide, may be anything from 1 to 500 g/m², depending on the activity of the particular catalytic composition, but more usually loadings are expected to be in the range 50 to 150 g/m², preferably around 100 g/m². The catalyst compositions of this invention show optimum catalytic activity for the oxidation of carbon monoxide and hydrocarbons and the catalytic reduction of nitrogen oxides at temperatures above about 250°C, e.g. up to 1100°C, and are generally operable with satisfactory results at temperatures in the range 300 to 900°C. Although the presently preferred catalyst compositions of this invention consist essentially of a calcined mixture of tin(IV) oxide and chromium(VI) oxide, the addition to or presence of other metal oxides, particularly transition metal oxides in the calcined mixture, is not to be excluded. Catalyst compositions for use according to this invention, their preparation and activity are further described with reference to the following Examples and with reference to the accompanying drawings in which: Figure 1 compares the carbon monoxide conversion of a catalyst composition according to this invention comprising granular tin(IV) oxide impregnated with chromium(VI) oxide in a weight ratio CrO₃:SnO₂ or 30:70 with that obtained with a commercial platinum-alumina catalyst; Figure 2 compares the hydrocarbon (propane) conversion of the same two catalysts; Figure 3 shows the conversion of nitrogen oxides obtained with the same CrO₃:SnO₂ catalyst. Figures 4, 5 and 6 compare the carbon monoxide, hydrocarbon (propane) and NO x conversion rates of a catalyst composition according to the invention comprising a commercial monolithic ceramic (Cordierite) catalyst support wash coated with alumina and impregnated with the CrO₃/SnO₂ catalyst with that obtained using a commercial exhaust gas catalyst comprising the same support material impregnated with platinum and rhodium. Figures 7 and 8 compare the carbon monoxide and hydrocarbon conversion rates obtained with a catalyst composition according to this invention (powdered SnO₂ impregnated with CrO₃, weight ratio CrO₃:SnO₂, 30:70) with those of a Pt-alumina catalyst using an exhaust gas from a 4-stroke internal combustion engine using unleaded fuel. Figure 9 compares the effect on CO oxidation of ageing a catalyst composition according to this invention (CrO₃ impregnated SnO₂ powder, weight ratio (CrO₃:SnO₂, 6:94) in air at 750°C for 5 hours with the effect on a Pt-alumina catalyst. Figure 10 shows the effect on hydrocarbon conversion of ageing a supported catalyst according to this invention in air at 750°C for 5 hours. As will be seen the conversion rates obtained: CO oxidation, propane oxidation and nitrogen oxide reduction, using catalyst compositions according to this invention are comparable with, and in many cases superior to, those obtained using commercial platinum-alumina and platinum-rhodium-alumina catalysts. The preparation of catalyst compositions according to the invention is illustrated by the following Examples. EXAMPLE 1 CrO₃ impregnated SnO₂ powder Method (1) . An aqueous tin(IV) oxide gel was first obtained by hydrolyzing distilled SnCl₄ with water to obtain a hydrated tin(IV) oxide (stannic acid) which was treated with ammonia to obtain a stabilised hydrated tin(IV) oxide SnO₂·H₂O. The stabilised product was washed thoroughly with water to remove all traces of chloride ion and then dried at 100°C for 2 hours. The dried product was then impregnated with chromium(VI) oxide by refluxing the tin(IV) oxide powder (1 g) in 16 ml aqueous dichromic acid (24 g.p.l.:0.24M) at 70°C for 16 hours. The CrO₃ impregnated powder was dried at 100°C for 2 hours and calcined in air for 1 to 2 hours at 300°C to 400°C. The final product had a chromium/tin content corresponding to a CrO₃:SnO₂ weight ratio of 30:70. Method (2) . Method 1 was repeated but using a stabilised aqueous tin(IV) oxide sol obtained from The Harwell Laboratory and prepared by a sol-gel transformation process. (See for example United Kingdom Atomic Energy Authority, GB-A-2,155,915). 16 ml aqueous dichromic acid (98 g.p.l.:0.98M) was added to 10 ml SnO₂ sol (368 g.p.l. SnO₂), which caused some gelation, and necessitated stirring in order to homogenise the mixture. Following the dichromic acid addition the product was dried initially in an oven at 60°C followed by calcination in air at 300°C to 400°C for 1 to 2 hours. Final CrO₃:SnO₂ ratio 30:70. Method (3) . 1 g anhydrous tin(IV) oxide (BDH Limited) was added to 16 ml aqueous dichromic acid (26.7 g.p.l.:0.267M) and left to stand at room temperature for 24 hours with occasional stirring. The powder was filtered off, dried at 60°C for 2 to 3 hours and calcined in air at 300°C to 400°C for a further 1 to 2 hours. Final CrO₃:SnO₂ ratio 30:70. EXAMPLE 2 Alumina supported SnO₂/CrO₃ catalyst Method (1) . Following the procedure of Example 1, Method 1, a stabilised tin(IV) oxide product was obtained, and washed with water to remove all traces of chloride ion, following which the tin(IV) oxide was resuspended in water and washed over a commercial monolithic catalyst support (Cordierite: an alumina wash coated ceramic block) and dried at 100°C for 2 hours. The monolith was then washed with 0.2M dichromic acid, dried at 60°C and then calcined in air at 300°C to 400°C for 1 to 2 hours. Catalyst loadings on the support were calculated to be of the order of 100 g/m² of total surface area. Method (2) . The monolithic support (Cordierite) is immersed in a stabilised tin(IV) oxide sol (The Harwell Laboratory) dried in an oven at 100°C, and then washed with 2M dichromic acid. The coated monolith is then oven dried at 60°C, and calcined in air at 300°C to 400°C for 1 to 2 hours. TEST PROCEDURES Catalyst compositions prepared as above were tested using the following gaseous compositions: CO oxidation Vol.% CO 4 - 8 O₂ 4 - 8 N₂ Balance Propane oxidation Vol.% nC₃H₈ 0.1 - 0.5 O₂ 10 - 20 N₂ Balance NO x reduction Vol.% NO 0.05 CO 5 - 10 N₂ Balance All tests reported herein were carried out either using a catalyst powder prepared as in Example 1, Method 2, or a monolithic supported catalyst block obtained as in Example 2, Method 2. Comparisons were made with a commercial powdered 1% by wt. Pt-alumina catalyst (Ventron Alfa Products), or a commercial monolithic catalyst block (platinum and rhodium impregnated Cordierite, Pt:Rh ratio 5:1, loading 2 g/dm³). The tests carried out on an actual exhaust gas composition were carried out on the exhaust gas from a Honda internal combustion engine running on unleaded fuel. Figures 1 and 2 show that a catalyst powder according to this invention comprising a CrO₃ impregnated SnO₂ at a CrO₃/SnO₂ weight ratio of 30:70 gives substantially complete CO and hydrocarbon conversions at temperatures in the range 200°C to 300°C, i.e. well within the normal temperature range of a vehicle exhaust, and very comparable with the conversions obtained using a commercial Pt-alumina powder. Figure 3 shows complete NO reduction at the same temperature, i.e. indicating the 3-way catalyst utility of the compositions of this invention. Figures 4 to 6 show the comparable results obtainable using the supported catalyst materials. Figures 7 and 8 show the 2-way catalytic activity (CO oxidation and hydrocarbon oxidation) of the powdered catalyst compositions on an actual exhaust gas, and show complete CO removal and hydrocarbon removal at 350°C, quite comparable with the results obtained using a noble metal catalyst, platinum or alumina. Figure 9 shows the effect on catalyst activity (CO oxidation) of prolonged heating of the catalyst powder. The graphs for Pt-alumina and the CrO₃/SnO₂ catalysts are, in fact, not strictly comparable since the CrO₃/SnO₂ catalyst used in this test was an unsupported CrO₃/SnO₂ powder. Figure 10 shows the effect on hydrocarbon conversion of ageing the supported catalyst (Example 2) in air at 750°C for 5 hours. All tests were carried out at a gas flow rate of 0.1 to 0.5 L/min. CO, hydrocarbon and nitrogen oxide contents of the gas streams before and after contact with the catalyst were measured by methods standard in the automotive industry, namely CO content by infra-red analysis, hydrocarbon content by FID (flame ionisation detector), and nitrogen oxides by chemiluminescence.

Claims: The use, in a catalytic converter for vehicle exhaust gas systems, of a catalyst composition effective in the catalytic oxidation of carbon monoxide and unburnt hydrocarbons contained in the exhaust, said catalyst composition comprising a mixture of tin and chromium oxides obtained by impregnating tin(IV) oxide (SnO₂) in solid particle form with aqueous dichromic acid to obtain, after drying, a mixed oxide composition consisting essentially of tin(IV) oxide (SnO₂) and chromium (VI) oxide (CrO₃) in a weight ratio of from 1:99 to 99:1, and calcining the impregnated product at a temperature in the range 200 to 500°C. The use according to claim 1 of a composition, in which said SnO₂:CrO₃ weight ratio is in the range 2:3 to 9:1. The use according to claim 1 of a composition, in which said SnO₂:CrO₃ weight ratio is in the range 3:2 to 3:1. The use according to any one of claims 1 to 3 of a composition, in which said impregnated product is obtained by treating an aqueous tin(IV) oxide sol with said aqueous dichromic acid, and drying the thus treated sol prior to calcination. The use according to any one of the preceding claims of a composition, wherein the mixed oxide catalyst composition is dispersed on a high surface catalyst support. The use according to claim 5 of a composition, wherein said support is or comprises alumina. The use according to claim 6 of a composition, wherein said support comprises alumina coated on a monolithic ceramic or metal base. A catalytic converter for vehicle exhaust systems comprising, as the catalyst, a composition as defined in any one of the preceding claims.

Inventors: HARRIS, PETER JOHN FREDERICK, HARRISON, PHILIP GEOFFREY
Applicants: ARVIN UK LIMITED
Application year: 1987
Main classification: B01D53
IPC codes: B01J23, B01D53