MXPA06011841A - Process for preparing platinum-tin-alumina catalysts useful as automotive catalytic converters - Google Patents

Abstract

Described is an easy and cost effective method for the preparation of automotive post combustion catalysts based on alumina-supported platinum and tin, which are air- and hydrogen-activated and sulfur oxide- and oxygen flow-sulfated. The inventive catalysts show a high activity regarding the oxidation of non-burnt hydrocarbons, carbon monoxide, and the reduction of nitrogen oxides present in the vehicle exhaust gas. Said catalysts are highly resistant to deactivation by sulfur oxide-poisoning present in the reacting gas.

Description

PROCEDURE FOR THE PREPARATION OF CATALYSTS OF P-LATIN O-TIN-ALUMINA FOR USE AS CATALYTIC CONVERTERS IN AUTOMOTIVE BACKGROUND OF THE INVENTION The method and the formulations described in the present invention lead to the obtaining of heterogeneous catalysts with catalytic activity in the oxidation of carbon monoxide, oxidation of hydrocarbons and simultaneous reduction of nitrogen oxides, reactions carried out in the so-called Catalytic Converter. The catalytic oxidation of carbon monoxide, unburned hydrocarbons and the reduction of nitrogen oxides, products of combustion in gasoline and diesel cars is a reaction that is carried out to significantly reduce the polluting gases of the environment and requires of the use of poly functional catalysts as described in the following documents: 1. MS Brogan, D. Swallow, R.J. Brisley, SAE Technical Paper Series 2000-01-2913. 2. B. Krutzsch, G. Wenninger, M. Weibel, P. Stapf, A.Funk, SAE Technical Paper Series, 982592 (1998). 3. B. Krutzsch, G. Wenninger, M. Weibel, P. Stapf, A.Funk, SAE. In particular, these catalysts are designed to carry out oxidation and simultaneous reduction reactions under conditions in which one does not predominate over the other. Reactions occur under conditions in which the temperature of the converter can reach up to 700 ° C. This inevitably produces secondary reactions such as the sintering of the support and the metallic phase, as well as possible deactivation of the catalyst by deposit of coke. The catalyst must also prevent its loss of activity due to the presence of gaseous compounds from the composition of gasoline or diesel and which are fixed either in the metallic phase or in the support or in both, producing an accelerated deactivation. Among the compounds that alter the lifetime of the catalysts used in the catalytic converter are the compounds of sulfur oxide produced by the combustion of sulfur compounds contained in fossil fuel [4-12]. The reactions carried out in the catalytic converter are complex and require the action of three simultaneous metallic properties: the oxidation capacity to carry out the transformation of carbon monoxide into carbon dioxide: the total combustion reaction of hydrocarbons not burned due to bad combustion in the engine and finally the reduction reaction of nitrogen oxides to nitrogen. The latter is carried out by reducing the CO in the nitrogen oxides or by the action of unburned hydrocarbons which act as reducers of nitrogen oxides. When the reactions are not completely completed then a partial reduction of the nitrogen oxides occurs, for example, the NO and N02 are reduced to N20 but not totally to nitrogen. It is clear that the selectivity, that is, the orientation to the total combustion reaction product C02 and the product of total reduction of nitrogen oxides or elemental nitrogen is one of the most important characteristics in the design of the catalytic converter, as has been demonstrated in the following works: 4. AF Diwell, C. Hallet, J.R. Taylor, T.J. Truex, Cat. And Automotive Pollution Control III. Studies in Surface Science, Vol. 71 (1991) 417. Elsevier, Amsterdam. 5. A.V. Deo, I.G. Dalla Lana, H.W. Habgood, J. Catal., 21 (1971) 270. 6. A. Datta, R.G. Cavell, R.M. Tower, Z.M. George, J. Phys. Chem., 89 (1985) 443. 7. H.S. Gandhi, M. Shelef, Appl. Catal., 77 (1997) 175. 8. E.S. Lox, B.H. Engler, E. Koberstein, SAE, 890 795 (1989). 9. H.P. Bonzel, R. Ku, J. Chem. Phys., 59 (1973) 1641. 10. S.T. Astergger, E. Bechtold, Surf. Sci., 122 (1982) 491 11. D.D. Beck, M.H. Krueger, D.R. Monroe, SAE, 910 844 (1991). 12. H.S. Gandhi, H.C. Yao, H.K. Stepien, M. Shelef, SAE, 780-606 (1978). Among the metals that have oxidizing and reducing properties in the catalytic converter is the platinum supported in alumina, however, its selectivity to the desired products (total combustion and nitrogen reduction of nitrogen oxides) is not total and the catalyst formulation for the catalytic converter based on platinum have been modified by the addition of a second noble metal such as rhodium or palladium and in some cases its formulation is supplemented with the addition of lanthanum oxides to the alumina support in which they are deposited as mentioned in the following documents: 13. DD Beck, J.W. Sommers, C.L Di aggio, Appl. Catal. B: 3 (1994) 205. 5 14. A. Gervasini, P. Carniti, V. Ragaini, Appl. Catal. B: 22 (1999) 201. 15. H. Schaper, E.B.M. Doesburg, L.L Van Reijev, Appl. Catal., 7 (1983) 211. 16. 16. J.S. C urch, N.W. Cant, Appl.Catal., 101 (1993) 105. In the present invention a method of preparation of platinum-based catalysts supported in alumina is reported which are modified by the addition of a second non-noble metal such as tin and to which they are given a pre-treatment with sulfur dioxide. The objectives of the addition of the second metal are to decrease the sintering of platinum, modify its selectivity to obtain a higher yield in the desired products such as carbon dioxide and elemental nitrogen. Furthermore, by virtue of the pre-treatment with S02, the passivation of the support (alumina) and the metal (platinum) is achieved, achieving greater resistance to the effect of the sulfur compounds contained in gasoline or diesel. The document: 17. JP59203640, "Preparation of Platinum-Tin Catalyst" (Taño Kazuo) describes a method for the preparation of catalysts for the hydroformulation reaction. This method proposes the use of coordination compounds containing an element of the Group VB of the Periodic Table, a tin compound and a hydrogen halide for the preparation of the catalyst. Likewise, the documents: EP040711, "High activity, high yield tin-modified platinum-iridium catalysts, and reforming process using same", (Baird Jr. W. C, Swan G. A. III, Boyle J. Ph.). 25 19. WO02087757, "A Multimetallic Reforming Catalyst Comprising Platinum and Tin, the Preparation and the Application, (to A., Pan J, Yang S.) 20. EP0407116," Novel Platinum - ??? coritaining Reforming Catalysts and Reforming Process ", (Baird Jr. W.C., Riley K. L .; Swan G.A. III). propose methods for the preparation of catalysts containing platinum and tin for the reformulation of naphthas, but in none of the aforementioned documents, sulfation of the catalyst is proposed as is done in the present invention.

DETAILED DESCRIPTION OF THE INVENTION One of the most specific processes of the present invention comprises the following steps: a. The impregnation of alumina (gamma) with a platinum precursor (hexachloroplatinic acid hexahydrate in aqueous medium having a pH equal to 2. b) The addition of a tin precursor (tin tetrachloride pentahydrate) c) The solutions containing the precursors of platinum and tin are kept in contact with the alumina support at room temperature and left for eight hours in agitation: The alumina support impregnated with the metal precursors is dried at 110 ° C until total evaporation of the aqueous medium; Calcination of the solid containing the two precursor metals is carried out by a heat treatment in dry air flow at a temperature of 500 ° C for six hours: f The catalyst is reduced in hydrogen flow at a temperature of 500 ° C during eight hours; g.The passivation is carried out by a treatment in oxygen flow and sulfur dioxide for 10 hours at a temperature of 500 ° C.

THE MODALITIES OF THE PROCESS The specific modalities of the previous process of preparation of the platinum and tin catalysts supported in alumina and passivated with sulfur dioxide is the following: 1. The support can be gamma alumina, eta alumina or mixture of both with surface areas included between 50 and 300 m2 / g, preferentially gamma alumina. 2. The platinum compound may be hexachloroplatinic acid, or platinum chloride contained in an aqueous solution, preferably hexachloroplatinic acid. 3. The aqueous solution containing the platinum precursor can have a pH comprised between two and five, which is obtained by the addition of an aqueous solution containing hydrochloric acid, preferably pH two. 4. The tin compound can be tin tetrachloride or tin acetate contained in an aqueous solution, preferably tin tetrachloride. 5. Metallic precursors can be added in any order, first the solution containing the platinum and then the solution containing the tin or vice versa, or both at the same time. 6. The drying temperature of the alumina impregnated with the platinum and tin precursors may be between 70 and 130 ° C, preferably 110 ° C. 7. The calcination temperature with air may be between 450 and 550 ° C, preferably 500 ° C. 10 8. The reduction temperature with hydrogen flow can be between 450 and 550 ° C preferentially 500 ° C. 9. The passivation of the reduced catalyst containing the platinum and the tin supported in alumina can be with sulfur dioxide or sulfur trioxide or the mixture of both, preferably with sulfur dioxide. 15 10. The passivation temperature with sulfur oxide can be between 300 and 600 ° C, preferably 500 ° C.

EXAMPLE 1 Preparation of: 1. Platinum catalyst / alumina (1% Pt). Catalyst A. In 100 parts of the support (alumina) is added a solution of hexachloroplatinic acid containing 1.0 parts of platinum and 3 parts of hydrochloric acid. The solution is left in contact for 2 hours and the excess water is evaporated for 12 hours. It is treated in dry air stream at 500 ° C for 8 hours and then in hydrogen flow for 8 hours at 500 ° C. The chemisorption results show that the platinum dispersion is 0.35 and the 10 scanning electron microscopy data show an elemental content of platinum of 1.02% and chlorine of 1.2% by dry weight.

EXAMPLE 2 Platinum-tin catalyst / alumina. (1.0% Pt-1% Sn). Catalyst B In 100 parts of the support (alumina) is added a solution of hexachloroplatinic acid (containing 1.0 parts of platinum), stannic chloride (containing 1.0 parts of tin) and 3 parts of hydrochloric acid The solution is left in contact for 2 hours and The excess water is evaporated for 12 hours. It is treated in dry air stream at 500 ° C for 8 hours and then in hydrogen flow for 8 hours at 20 500 ° C. The chemisorption results show dispersions of 0.35 for platinum and 0.11 for tin. The scanning electron microscopy data show elemental contents of 1.02% for platinum, 1.2% for tin and 1.2% for chlorine in dry weight. EXAMPLE 3 Platinum-tin catalyst / alumina (1.0% Pt-2% Sn). Catalyst C. In 100 parts of the support (alumina) a solution of hexachloroplatinic acid (containing 1.0 parts of platinum), stannic chloride (2.0 parts of tin) and 3 parts of hydrochloric acid is added. The solution is left in contact for 2 hours and The excess water is evaporated for 12 hours. It is treated in synthetic air stream at 500 ° C for 8 hours and then in hydrogen flow for 8 hours at 500 ° C. The chemisorption results show dispersions of 0.22 for platinum and 0.11 for tin. The scanning electron microscopy data show elementary contents of 1.0% for platinum, 2.1% for tin and 1.3% for chlorine. EXAMPLE 4 Platinum-tin catalyst / alumina (1.0% Pt-1% Sn). Catalyst D. Which comes from preparation B, passivated with S03. Catalyst B is sulfated in a flow of sulfur dioxide and oxygen for 10 hours at 500 ° C. The results of chemisorption and scanning electron microscopy show that there was no change in the dispersion or in the content of the metals. This catalyst exhibits an activity of 40% at 250 ° C in the reduction of nitrogen oxides by unburned hydrocarbons with a catalyst mass of 200 milligrams in a reactant flow (6.0 L / h) of compound per: 800 ppm of NO, 2.5% propane, 10% oxygen and helium as a 100% balance.

EXAMPLE 5 Platinum-tin alumina catalyst (1% Pt-2.0% Sn). Catalyst E. That comes from preparation C, passivated with S02. The catalyst C is sulfated in a flow of sulfur dioxide and oxygen for 10 hours at 500 ° C. The results of chemisorption and scanning electron microscopy show that there was no change in the dispersion or in the content of the metals. At 250 ° C this catalyst produced 45% conversion of the NOx to nitrogen with a catalyst mass of 200 milligrams in a reactant flow (6.0 L / h) composed of 800 ppm of NO, 4% propane, 10% oxygen and helium as a 100% balance.

EXPLANATION OF THE FIGURE Figure 1 represents the spectra determined by the photoelectron spectroscopy (XPS) of the electronic layer Pt 4d52 in the catalyst 1% Pt-2% Sn / sulfated AI203 (Catalyst E). Intensity (Accounts per second) depending on the electronic link energy (BE). This figure shows that the presence of tin on the surface of the catalyst causes a decrease in the electronic density of the surface platinum.

Claims (3)

R E I V I N D I C A C I O N S
1. The description of the present invention is considered as a novelty and therefore what is contained in the following clauses is considered as property: 1. Procedure for the preparation of sulphated platinum-tin-alumina catalysts, for the oxidation of hydrocarbons, carbon monoxide and reduction of the nitrogen oxides in the exhaust gas of internal combustion engines, characterized in that it comprises the steps of: i) mixing an amount of alumina with a solution containing the salts of platinum and tin in acid medium; ii) activate the mixture by calcination in air and reduction in hydrogen flow; iii) passivate the solid obtained by sulfation in a flow of oxides of sulfur and oxygen.
2. 2. Process for the preparation of platinum-tin-alumina catalysts for the oxidation of hydrocarbons, carbon monoxide and reduction of nitrogen oxides according to claim 1, characterized in that the alumina used according to step i) is selected from the group consisting of gamma alumina and / or eta alumina.
3. 3. Process for the preparation of platinum-tin-alumina catalysts for the oxidation of hydrocarbons, carbon monoxide and reduction of nitrogen oxides according to claim 1, characterized in that the pH of the solution containing the salts of platinum and tin in accordance with with step i) two to five are selected, preferably pH two. . Process for the preparation of platinum-tin-alumina catalysts for the oxidation of hydrocarbons, carbon monoxide and reduction of nitrogen oxides according to claim 1, characterized in that the contents of tin in the final catalyst in accordance with step i ) are between 51.0% and 5.0% dry weight. . Process for the preparation of platinum-tin-alumina catalysts for the oxidation of hydrocarbons, carbon monoxide and reduction of nitrogen oxides according to claims 1 and 3 characterized in that the platinum compound according to step i) is selected between hexachloroplatinic acid hexahydrate and / or platinum chloride. Process for the preparation of platinum-tin-alumina catalysts for the oxidation of hydrocarbons, carbon monoxide and reduction of nitrogen oxides according to claims 1 and 2 characterized in that the tin compound according to step i) is selected between tin tetrachloride and / or tin acetate. Process for the preparation of platinum-tin-alumina catalysts for the oxidation of hydrocarbons, carbon monoxide and reduction of nitrogen oxides according to claim 1, characterized in that the platinum-tin-alumina catalyst according to step ii) , it is thermally treated in dry air flow at temperatures between 450-550 ° C. Process for the preparation of platinum-tin-alumina catalysts for the oxidation of hydrocarbons, carbon monoxide and reduction of nitrogen oxides according to claim 1, characterized in that the platinum-tin-alumina catalyst according to step ii) , is reduced in hydrogen flow at temperatures between 300-600 ° C. Process for the preparation of platinum-tin-alumina catalysts for the oxidation of hydrocarbons, carbon monoxide and reduction of nitrogen oxides according to claim 1, characterized in that the platinum-tin-alumina catalyst according to step iii) , is sulfated in sulfur dioxide or sulfur trioxide flow or the mixture of both and oxygen at temperatures between 300-600 ° C.