VEHICLE EXHAUST GAS CATALYSTS

Abstract: Vehicle exhaust gas catalyst compositions are disclosed comprising SnO₂, CrO₃ and CuO, in the proportions SnO₂:CrO₃ weight ratio 99:1 to 1:99, CuO 0.5 to 100%, preferably 5 to 80%, based on the combined weight of SnO₂ and CrO₃.

Description: This invention relates to catalyst compositions for use primarily as vehicle exhaust gas catalysts, to control the emissions of noxious gases, particularly carbon monoxide, unburnt hydrocarbons and/or nitrogen oxides, but also having utility in other fields where similar emission control is necessary. Catalytic converters currently in use in motor vehicles to control exhaust gas emissions generally comprise one or more platinum group metals, e.g. platinum, palladium, rhodium or rhenium, alone or in combination, and thus tend to be expensive. The most commonly used catalytic material is platinum on alumina. Much research has been carried out to find a less expensive alternative, and one system that has received considerable attention is the SnO₂-Cr₂O₃ system. For example, in a series of papers: J. Catal. 41 , 202 (1976); J. Chem. Soc. Chem. Commun. 509-510 (1974); Preparation of Catalysts, Ed. B. Delmon et al, Elsevier Scientific Publishing Company, 197-206 (1976); J. Catal. 54 , 42-51 (1978); F. Solymosi et al have reported that SnO₂ doped with a small amount (0.1-1 mole%) of Cr₂O₃ is a very active catalyst for the adsorption and reduction of nitric oxide (NO) at low temperatures (150-400°C) in the presence of a reducing gas such as CO, H₂ or C₂H₄. The catalyst is prepared by reacting HNO₃ with metallic tin to produce SnO₂ which is recovered, dried at 120°C and further heated at 350°C for 3 hours and 500°C for a further five hours. The SnO₂ is then suspended in water and Cr₂O₃ added to the suspension with continuous stirring over a long period until a homogenous dispersion is obtained. The suspension is then dried, heated at 350°C for 5 hours and finally calcined in air at 900°C for 5 hours. As a result an SnO₂ catalyst is obtained having higher valency chromium ions (average valency 3.85) stabilised in the surface layer. SnO₂-Cr₂O₃ catalysts are also reported to be effective catalysts for the catalytic oxidation of CO in gaseous mixtures containing SO₂ (SU-A-736,997); catalytic removal of SO₂ (SU-A-691,185); catalytic reduction of nitrogen oxides in the presence of NH₃ (JP-A-75-108169); and as alkylation catalysts in the alkylation of phenols (US-A-4,208,537). In SU-A-736,997 such SnO₂-Cr₂O₃ catalysts are prepared by heating SnCl₂ and CrO₃ in relative proportions to give SnO₂:Cr₂O₃ mole% ratios of from 1:99 to 99:1. In JP-A-75-108169, the SnO₂-Cr₂O₃ catalysts are prepared by coprecipitation from an aqueous solution containing concentrated HCl, CrO₃ and SnCl₂·2H₂O by addition of NH₄OH. The mixed oxide precipitated was recovered, slurried in water, coated onto an alumina support, dried and calcined. In US-A-4,208,537, the SnO₂-Cr₂O₃ containing alkylation catalysts are prepared by coprecipitation, or kneading preformed SnO₂ and Cr₂O₃-containing gels to form an oxide mixture and calcining, optionally after coating onto a suitable, e.g. alumina, support. More recently it has been found (published GB-A-2,194,174) that catalyst compositions having high levels of catalytic activity in the oxidation of CO and unburnt hydrocarbons, and in the catalytic reduction of nitrogen oxides, equivalent in many cases to those of precious metal catalysts and therefore potentially useful as vehicle exhaust gas catalysts for the control of noxious emissions, are obtained by impregnating tin(IV) oxide (SnO₂) with chromium(VI) oxide (CrO₃) and calcining the resulting product. For convenience the catalyst may be regarded as a mixed SnO₂:CrO₃ catalyst. In such catalysts the tin:chromium weight ratio (calculated as SnO₂:CrO₃) may be in the range 99:1 to 1:99, but is preferably in the range 24:1 to 2:3, most preferably 4:1 to 2:3. The SnO₂:CrO₃ catalyst composition is preferably obtained by impregnating tin(IV) oxide, preferably uniformly dispersed on a high surface area ceramic support such as cordierite, and deposited thereon by the sol-gel process described by the United Kingdom Atomic Energy Authority in their GB-A-2,134,004 B, with aqeuous chromic acid, drying and calcining the resulting product. At an SnO₂:CrO₃ weight ratio of 70:30 such a catalyst gives substantially complete CO and hydrocarbon conversions at temperatures in the range 200 to 300°C, and substantially complete NO reduction at the same temperature. In accordance with the present invention, it has been found that the performance of such SnO₂:CrO₃ catalyst compositions can be improved still further, particularly with regard to their CO oxidation activity, by incorporating CuO into the catalyst. The CuO may be incorporated into the catalyst preparation in any convenient manner. Usually this will comprise impregnating the catalyst support, before or after deposition of the tin(IV) oxide and/or chromium(VI) oxide, or directly impregnating the tin(IV) oxide if no support is used, with CuO or with a suitable precursor material convertible to CuO upon calcination. Preferably the CuO is incorporated into the support (or directly into the SnO₂ if no support is used) by impregnating the support, before or after incorporating the SnO₂ and/or CrO₃, or simultaneously therewith, with an aqueous solution of a water-soluble copper salt, such as cupric nitrate, convertible to the oxide CuO on calcination, or with aqueous cupric hydroxide, followed by drying and calcination of the impregnated support. The CuO is incorporated into the catalyst preparation in amounts ranging from 0.5 to 100% by weight or more, based on the combined weight of SnO₂ and CrO₃; preferably the amount of CuO will be in the range 5 to 80% by weight, more preferably 10 to 70%, based on the combined weight of SnO₂ and CrO₃. In accordance with one aspect of this invention, therefore, there is provided a mixed non-precious metal oxide catalyst composition, having catalytic activity in the catalytic oxidation of CO and unburnt hydrocarbons and/or the catalytic reduction of nitrogen oxides, and obtained by impregnating tin(IV) oxide (SnO₂) with chromium(VI) oxide (CrO₃) in an amount sufficient to provide a mixed oxide composition containing SnO₂ and CrO₃ in a weight ratio of from 99:1 to 1:99, incorporating into the mixed oxide composition either CuO or a water-soluble copper salt or hydroxide convertible to CuO upon calcination and sufficient to provide from 0.5 to 100% by wt. of CuO based on the combined weight of SnO₂ and CrO₃, and calcining the mixture thus obtained. Preferably, such catalyst compositions are provided in supported form, i.e. supported upon a suitable catalyst support, such as alumina, silica, titania, etc., and preferably upon a high surface area, monolithic ceramic support such as cordierite. In accordance with a further aspect of the present invention, further catalytically active metal oxides may be incorporated into the catalyst composition selected from ceric oxide CeO₂, aluminium oxide Al₂O₃, ferric oxide Fe₂O₃ and vanadium pentoxide V₂O₅. Particularly preferred additions are CeO₂ and/or Al₂O₃. In accordance with this further aspect, therefore there are provided, in accordance with this invention, mixed non-precious metal oxide catalyst compositions, having catalytic activity in the catalytic oxidation of CO and unburnt hydrocarbons and/or the catalytic reduction of nitrogen oxides, and comprising a calcined mixture of tin(IV) oxide, chromium(VI) oxide, copper oxide (CuO) and one or more of ceric oxide (CeO₂), aluminium oxide (Al₂O₃), ferric oxide (Fe₂O₃) and vanadium pentoxide (V₂O₅). The additional oxide or oxides may be incorporated into the catalyst composition in a similar manner, that is to say by impregnating the support (or the SnO₂ where no support is used) with a precursor material convertible to the desired oxide upon calcination, and preferably a water-soluble precursor which is incorporated into the support (or directly into the SnO₂) by impregnating with an aqueous solution of the precursor material followed by drying and calcining. Most desirably and wherever possible the additional oxide(s) are incorporated into the support (or directly into the SnO₂) by a sol-gel process, i.e. by impregnating the support (or the SnO₂) with a suitable oxide sol, followed by drying and calcining. In general, the oxides may e incorporated into the support in any order. As before, the preferred supports are high surface area supports such as alumina, silica, titania, etc., most preferably monolithic ceramic supports such as cordierite. The total amount of the additional oxide(s) may range from 0.5 to 50% by weight, based on the combined weights of SnO₂ and CrO₃: more probably, individual amounts of the additional oxide(s) may range from 1 to 40%, most probably in the range 10 to 30%, bases on the combined weight of SnO₂ and CrO₃. Also provided in accordance with this invention are catalytic converters for the control of vehicle gas emissions and comprising a mixed non-precious metal oxide catalyst composition as above described preferably in supported form. The preparation and activity of mixed oxide catalyst compositions in accordance with this invention, in powder form, are illustrated by the following Example. EXAMPLE 5 mls of an aqueous sol containing 282 g SnO₂ per litre was pipetted into a petri dish. To this was added with stirring 2.115 mls of a solution containing 10 g CrO₃ in 50 mls water. To this mixture was then added 2.115 mls of a solution containing 30.37 g Cu(NO₃).3H₂O in 50 mls water. This was thoroughly mixed with a glass rod and then placed in an oven at 100°C to dry. When dry the mixture was ground, placed in a silica crucible and fired at 400°C for 2 hours to produce the powdered sample. The resulting powdered mixed oxide composition had a composition corresponding to SnO₂:CrO₃:CuO in the weight ratio 62:19:19. By similar techniques and altering the amount of copper added, compositions were prepared having SnO₂:CrO₃:CuO ratios respectively of 51:33:16 and 43:43:13. The catalytic activity of the powder samples was tested for CO oxidation at 200°C and propane oxidation at 300°C, and the results compared with an unsupported SnO₂:CrO₃ catalyst powder prepared in a similar manner and having a SnO₂:CrO₃ ratio of 77:23. Tests were carried out using gaseous mixtures containing either CO (0.45 vol%) O₂ (20 vol%) balance N₂ or nC₃H₈ (0.23 vol%) O₂ (20 vol%) balance N₂ under continuous flow conditions at flow rates of 0.52 L/m and 0.13 L/m respectively. CO contents of the treated gas streams were determined by infra-red analysis, hydrocarbon content by a flame ionisation detector. a) Specific Activities of SnO ₂ :CrO ₃ :CuO Samples CO and C ₃ H ₈ oxidation at 300°C b) SnO ₂ :CrO ₃ :CuO Powders T ₁₀₀ values for CO oxidation (0.5 g samples) Using the 62:19:19 ratio composition complete CO oxidation is achieved at 175°C. Comparable figure for complete CO removal using a SnO₂:CrO₃ catalyst, weight ratio 77:23 is 340°C. Whilst these figures show a decreased, but still acceptable propane oxidation activity as compared with the SnO₂:CrO₃ catalyst, they also show a substantial improvement in CO oxidation activity brought about by the addition of CuO. Other specific ratios of SnO₂ and CrO₃ effective in catalyst compositions according to this invention are Additions of CuO to the above compositions in amounts of from 0.5 to 100% by weight, preferably 5 to 80%, based on the combined weights of SnO₂ and CrO₃ show substantial reductions in the T₁₀₀ figures, particularly for CO, and a substantial increase in the level of specific activity. As indicated by the figures given, T₁₀₀(CO) values are reduced to below 200°C, down in one case to as low as 175°C. Specific activity levels show a three-fold increase to levels in excess of 0.25 moles of CO converted/g catalyst/hr, and in one case as high as 0.43. These figures demonstrate the substantial increase in activity of the catalyst compositions of the present invention, particularly with reference to CO oxidation, obtained by adding CuO to the SnO₂/CrO₃ mixture.

Claims: A non-precious metal, mixed oxide catalyst composition for the catalytic oxidation of exhaust gas emissions emanating from internal combustion engines and comprising amounts of catalytically oxidisable carbon monoxide and unburnt hydrocarbons, characterised in that the composition comprises an oxide mixture obtained by impregnating tin(IV) oxide (SnO₂) with chromium (VI) oxide (CrO₃) in an amount sufficient to provide a mixed oxide composition containing SnO₂ and CrO₃ in a weight ratio of from 99:1 to 1:99, incorporating into the mixed oxide composition either CuO or a water-soluble copper salt convertible to CuO upon calcination and in an amount sufficient to provide from 0.5 to 100% by wt. of CuO, based on the combined weight of SnO₂ and CrO₃, and calcining the mixture thus obtained. A composition according to claim 1, wherein the SnO₂:CrO₃ weight ratio in the mixture prior to calcination is in the range 24:1 to 2:3. A composition according to claim 2, wherein the SnO₂:CrO₃ weight ratio in the mixture prior to calcination is in the range 4:1 to 2:3. A composition according to claim 1, 2 or 3, wherein the CuO content, relative to the tin and chromium oxides, is from 5 to 80%, based on the combined weights of SnO₂ and CrO₃. A composition according to any one of claims 1-4, wherein the oxide mixture prior to calcination contains one or more of CeO₂, Al₂O₃, Fe₂O₃ and V₂O₅ in a total amount of from 0.5 to 50% by wt. based on the combined weight of SnO₂ and CrO₃. A composition according to any one of the preceding claims, when supported upon a high surface area refractory catalyst support. A composition according to claim 6, wherein the catalyst support is a ceramic monolith. A catalytic converter for vehicle exhaust gas systems comprising as the catalytic component a composition as claimed in any one of claims 1-7.

Inventors: HARRIS, PETER JOHN FREDERICK, HARRISON, PHILIP GEOFFREY
Applicants: ARVIN UK LIMITED
Application year: 1989
Main classification: B01D53
IPC codes: B01J23, B01D53