MXPA95001698A - Catalysts to reduce the emissions of nitrogen oxides based on espine - Google Patents

Abstract

The present invention relates to: the use of catalysts for the treatment of exhaust gases of internal combustion engines with high oxygen content. These catalysts may be of the type including spinel-structured mass oxide and of formula ZnA12O4 or including normal spinel structure or reversal spinel except for ZnA12O4. They can be obtained in particular by disaggregating the precursors of the constituent elements of the spinel or from a solution or suspension of salts of the constituent elements of the spinel, then proceeding to the drying and to the calcination of the as-maintained medium. They can also be obtained by combustion of a mixture of the precursors of the constituents of the spinel and a source of carbon and nitroge

Description

CATALYSTS TO REDUCE THE EMISSIONS OF NITROGEN OXIDES BASED ON SPINELAS The present invention concerns the use of catalysts for the reduction of emissions of nitrogen oxides based on spinel for the treatment of exhaust gases. It is known that the reduction of emissions of nitrogen oxides (NOx) from the exhaust gases of automobile engines is carried out in particular by "three-way" catalysts that stoichiometrically use the reducing gases present in the mixture. Any excess of oxygen is translated by a brutal deterioration of the catalyst yields. However, some engines such as diesel engines or petrol engines that operate with lean burn are economical in terms of fuel, but emit exhaust gases that permanently contain a significant excess of oxygen of at least 5% per year. example. Therefore, a standard three-way catalyst has no effect on the NOx emissions of these engines. On the other hand, the limitation of NOx emissions becomes indispensable for the hardening of the automotive post-combustion standards that now extend to this type of engines. Therefore, there is a real need for an effective catalyst for the reduction of NOx emissions.

Therefore, the object of the invention is to find a carburettor capable of being used for the treatment of exhaust gases with a high oxygen content. For this reason and according to a first embodiment, in order to reduce the emissions of nitrogen oxides, the invention concerns the use of a catalyst comprising a spinel-structured mass oxide and of the formula Z11AI2O4 intended for the treatment of gases of nitrogen. exhaust with high oxygen content. According to a second embodiment, in order to reduce the emissions of nitrogen oxides, the invention concerns the use of a catalyst that includes an oxide of normal spinel structure or reverse spinel with the exception of ZnAl2? 4 for the treatment of exhaust gases with high oxygen content. The catalysts used in the framework of the present invention have an operating zone that is at least between 450 and 600 ° C. They also have a stability that allows them to remain active even after rises in temperatures above 600 ° C or higher than 700 ° C. Other features, details and advantages of the invention will appear more fully, once proceeded to the reading of the description given below as well as the reading of various concrete examples but not limited to its illustration. As indicated above, the catalysts used in the framework of the present invention can be presented according to two embodiments which will be described in particular in the following. With respect to the whole of the description, spinel is considered to be the compounds of general and general formula AB2O4 in which A and B are, as usual, transition ions with almost similar size; A is a bivalent ion and B is a trivalent ion. The inverse spinel structure is considered to be the compounds of general formula B (AB) 04. Lastly, everything described below regarding the spinel structure applies to the normal spinel structure as well as to the reverse spinel structure. In the case of the first mode, the catalyst comprises, with an active stage, a spinel of the formula ZnAl2 4 4. In this same case, this active stage is presented in the form of a mass oxide, that is to say that the spinel ZnAl2? 4 is present in the whole volume of the active stage in a homogeneous manner and not, for example, on the surface of this. In the case of the second embodiment, the catalyst comprises as an active stage a normal spinel or a reverse spinel with a formula different from ZnAl2Δ4. In the second case, in particular, the oxide of normal spinel structure or reverse spinel responds globally to the formula (1): AB204 in which: A is at least one element selected from the groups lia, Ib, Illb, IVb and Vb as well as the transition elements of the periodic classification; B is at least one element selected within the groups IVa, Vía, Vlla, VIII, Ib to Vb of the periodic classification. The periodic classification of the elements to which it refers is the classification published in the Supplement to the Bulletin of the Chemical Society of France No. 1 (January 1966). Transition elements are considered elements of groups IV to VIII. In particular, the oxide of normal spinel structure or reverse spinel corresponds to the formula (1) in which A is at least one element selected from the group including Mg, Ti, Mn, Fe, Co, Ni, Cu, Zn and Sn; B is at least one element selected from the group including Ti, Mn, Cr, Fe, Co, Ni, Cu, Zn, Al, Ga, In, Sn, and Sb. Spinels of formula (1) can be mentioned as examples. ) in which B is aluminum or gallium. The description that has just been made also applies to the use of compounds that come in the form of a mixture of spinels of normal or inverse structure.

Furthermore, it has been mentioned that, in the composition of the catalysts used, elements such as Zn, Cu, Sn and Sb are volatile elements. Now it is known that with such elements, taking into account the preparation temperature of the spinels and the volatilization of these elements, products with gaps are formed and can be represented by the formula A? _xBxB2-x? 4. Of course such products can be used within the framework of the invention. It can be observed that lagunal products of the same type can also be prepared using quantities of starting products with stoichiometric defect. These products can also be used within the framework of the invention. The catalysts used in the context of the present invention may comprise, in addition to the spinel-based active stage of the type described above, any type of precious metals, which are customarily used in catalysis, such as, for example, the stage, the palladium, rhodium, ruthenium, silver, gold, iridium or rhenium. Furthermore, the active stages of these catalysts can be placed on any type of support customarily used in this technical field, such as, for example, lanthanide oxides such as Ce? 2, Zr02, AI2O3, TiO2 or Si02, these supports can eventually be doped, or also zeolites, silicates, crystalline silicoaluminum phosphates, crystalline aluminum phosphates; these phosphates may comprise metal substitutes, such as, for example, titanium, iron, magnesium, zinc, manganese, cobalt, gallium, lanthanum, copper, molybdenum, chromium, germanium or boron. The catalysts used in the context of the present invention can be prepared according to different procedures.

According to a first method, the oxide of the normal spinel structure or reverse spinel constituting the active stage of the catalyst is obtained by unbundling the precursors of the elements constituting the spinel. These precursors are usually oxides. They are mixed and crushed, then, if necessary, molded under pressure, for example they take the form of a tablet. Then the mixture is calcined at a temperature and for a sufficient time to obtain the desired stage. Generally, the temperature is at least 700 ° C. The duration of the calcination is low while the temperature is high. Calcination is usually done under air and can be done under a static atmosphere or under a sweep. If necessary, several roasting cycles can be carried out with intermediate crushing and molding. According to a second method, a solution or suspension of salts of the constituent elements of the spinel is first formed. If the case arises, the support with said constituent elements can also be suspended. As salts, salts of inorganic acids such as nitrates, sulfates or chlorides can be selected; nitrates are the preferred salts. It is also possible to use the salts of inorganic acids and in particular the salts of saturated aliphatic carboxylic acids or the salts of hydroxycarboxylic acids. As examples, mention may be made of formates, acetates, propionates, oxalates or citrates. Then the solution or suspension thus formed is dried. Any known type of drying can be used but the drying is preferably carried out by atomization ie by spraying the mixture in a hot (spray-drying) atmosphere. The atomization can be carried out by means of any known sprayer, for example by means of a spraying nozzle of the sprayer pina or other type. It is also possible to use atomizers of the turbine atomizer type. Regarding the different techniques that can be carried out in the present procedure, we can refer in particular to MASTERS 'base work entitled "SPRAY-DRYING" (second edition, 1976, Editions Gerge Godwin-London). It will be noted that the atomization-drying operation can also be performed by a "flash" reactor, for example of the type made by the Applicant Company and described in particular in the French patent applications No. 2 257 326, 2 419 754 and 2 431 321. In this case, the gases used to treat (hot gases) present a movement helical and flow in a whirlpool. Then the mixture to be dried is injected according to a trajectory that coincides with the axis of symmetry of the helical trajectories of said gases, which allows to perfectly transfer the amount of movement of the gases to the mixture to be treated. In fact, the gases thus have a double function: the pulverization of the initial mixture, that is, the transformation into fine droplets on the one hand, and the drying of the droplets obtained on the other hand. Furthermore, the residence time of the particles in the reactor which is extremely low (generally less than about 1/10 of a second) has the advantage, among others, of limiting eventual risks of overheating due to too long contact with the hot gases . According to the respective performances of the gases and of the mixture to be dried, the inlet temperature of the gases is, for example, between 400 and 900 ° C, preferably between 600 and 800 ° C.; the temperature of the dried solid is between 110 and 250 ° C for example, preferably between 125 and 20. After drying, the product obtained is calcined. The calcination is carried out at a temperature sufficient to obtain the calcination of the desired stage. Usually, this temperature is at least 600 ° C. The duration of the calcination can vary from between a half hour and 10 hours for example. This duration is low while the calcination temperature is high. Calcination is usually done under air and can be done under a static atmosphere or under a sweep. A third process can be carried out for the preparation of the catalysts used in the context of the present invention. In this process, a mixture of the precursors of the constituent elements of the spinel and of a source of carbon and nitrogen is first formed; this mixture can optionally be presented in the form of a solution. The precursors used are usually salts of the elements and in particular nitrates or chlorides. The usual source of carbon and nitrogen is an organic compound, such as urea or glycine. Then, a combustion of the mixture thus formed is carried out by bringing it to a sufficient temperature, generally at least 300 ° C, for example at 500 ° C. In the case of this solution, it is boiled in the oven before being overlaid. Very high calcination temperatures can be obtained transiently in the mixture. The reaction temperature can be modulated by modifying the molar ratio between the carbon / nitrogen source and the precursor. At the end of the combustion the product is crushed if necessary. Of course, it is possible, within the framework of the present invention, to use other methods of preparing the active steps different from those described above. Mention may be made, for example, of sol-salt routes or routes by aqueous or organic precipitation. The catalysts can be used in various forms such as granules, balls, cylinders or honeycomb with variable dimensions. They can also be used in catalytic systems that include a coating (wash coat) based on active steps described above, on a substrate of the type, for example, metal monolith or ceramic. The invention is applied to the treatment of exhaust gases having an excess of oxygen. Oxygen excess is considered a content (expressed in volume) in this element of at least 5%, in particular of at least 10%; this content can for example be between 5 and 15%. In this case, one of the reactions to be catalyzed is the HC (hydrocarbons) + N0X reaction. The invention is applicable to the manufacture and use of any catalyst system of the above-mentioned type based on catalysts described above for the treatment of exhaust gases having an excess of oxygen. The hydrocarbons that can be used as a reducing agent for the elimination of NOx are, in particular, the gases or liquids of the families of saturated carbides, ethylenic carbides, acetylenic carbides, aromatic carbides and the hydrocarbons of the oil cuts, such as, for example, methane, ethane, propane, butane, pentane, hexane, ethylene, propylene, acetylene, butadiene, benzene, toluene, xylene, kerosene, diesel. The organic compounds containing oxygen can be, in particular, alcohols of the type, for example, saturated alcohols such as methanol, ethanol or propanol; the ethers such as methyl ether or ethyl ether; esters such as methyl acetate and ketones. The gases which can be treated by the present invention are, for example, those that come from gas turbines, boilers of thermal power plants or also internal combustion engines, in particular diesel engines or motors operated with a lean mixture. Now they are going to give examples. In the examples presented below, the obtained products are experienced in the following way in order to estimate their catalytic performances. 1.5 g of the powdered catalyst is charged to a quartz reactor. the reaction mixture at the reactor inlet have the following composition (by volume): - NO = 900 vpm - C3H6 = 900 vpm - 02 = 5% - C02 = 10% - H2 = 10% - N2 = csp 100% Performance global is lON / h. The WH is approximately 1000 h-1. The signals of HC (C3H6), NO and N0X (N0X = NO + N02) are recorded permanently as well as the temperature in the reactor. The HC signal is given by a BECKMAN total HC detector, based on the principle of flame ionization detection. The signals of NO and N0X are given by an N0X ECOPHYSICS analyzer, based on the principle of chemistry-luminescence: it gives the values of No, NOx and NO2; the latter is calculated by difference of the N0X and NO signals. The catalytic activity is measured from the HC, NO, NO2 and NOx signals as a function of the temperature during the rise in programmed temperature from 20 to 700 ° C at a rate of 3.75 ° C / min and from the following relationships: - The conversion rate in NO (TNO) in% that is given by: T (NO) = 100 (NO ° -N0) / N0 ° with N0 ° NO signal at time t = O corresponding to the programming principle Of temperature. - The conversion rate of HC (THC) in% given by: T (HC) = 100 (HC ° -HC) / HC ° with HC ° HC signal at time t = O corresponding to the beginning of the temperature programming. - The global conversion rate of NOx (TNOx) in% that is given by: T (NOx) = 100 (NOX ° -NOX) / NOX ° with NOx NOx signal at time t = O corresponding to the beginning of the temperature programming. Finally, it is considered surface specific to the specific surface B.E.T. determined by nitrogen adsorption according to ASTM D 3663-78 standard established from the BRUNAUER-EMMETT-TELLER method described in the newspaper "The Journal of the American Society, 60, 309 (1938)".

EXAMPLE 1 This example concerns a catalyst of formula ZnAl204.

Oxides (ZnO, AI2O3) mixed and crushed in an agate mortar are used as pressurizers, then they are given a pellet form at 1 ton / cm2. The mixture is then calcined under air in an alumina crucible. The compound ZnAl204 is obtained after two roasting cycles at 1000 ° C for 15 h, with a grinding and molding in the form of an intermediate tablet. Under these conditions, the diffraction of the X-rays shows the presence of ZnAl2? 4. The specific surface BET is less than 0.50 m2 / g. The technical qualities of the catalyst are given in table 1 below. These technical qualities show: - a good level of activity (maximum of approximately 20% global conversion from N0X to 540 ° C) taking into account the very low specific surface (<0.50 m2 / g): - an area of NOx conversion that goes from 450 to 600 ° C. TABLE 1 Temperature THC TNO TN0X 399 0.0 0. 0 0. 0 450 2.3 0. 0 2. 1 498 11.1 3. 1 11. 2 550 45.6 9. 9 19. 6 599 86.0 4. 7 3. 3 649 95.2 10. 6 1. 9 EXAMPLE 2 This example relates to a catalyst of the same type as in Example 1, but prepared differently. A mixture of nitrate at 0.2 mol / 1 in Zn and 0.4 mol / 1 in Al dries by atomization on a BUCHI. The inlet temperature of the gases is 240 ° C and the outlet temperature of the product is 115 ° C with an atomization yield of 800 ml / h. The powder thus obtained is calcined for 6 h at 800 ° C in an alumina nacelle with a programmed rise at 5 ° C / min; X-ray diffraction reveals the perfectly crystallized ZnAl2? 4 stage. In this case, the BET specific surface area is 33m2 / g. The technical qualities of the catalyst are given in table 2 below. a remarkable level of activity (maximum of approximately 40% of the overall conversion rate of N0X at 550 ° C taking into account the specific surface area of the catalyst (33m / g) - a NOx conversion zone from 350 to 600 ° C. TABLE 2 Temperature THC TNO TNO? 350 1.1 6.4 4.0 400 7 .4 7 .1 8 .1 449 25. 9 16. 7 21. 4 500 56. 9 31.9 38. 3 549 80.4 36. 1 40. 2 600 94 .8 20. 2 17 .8 649 97.7 21. 9 6. 4 EXAMPLE 3 This example relates to a catalyst of the formula SnO, 0. 25Zn0.975A12 ° 4-A mixture of nitrates at 0.195 mol / 1 in Zn and 0.4 mol / 1 in Al and 0.005 mol / 1 in Sn is spray-dried on a BUCHI.

The inlet temperature of the gases is 245 ° C and the outlet temperature of the product is 115 ° C with an atomization yield of 850 ml / h. The powder thus obtained is calcined for 6 h at 800 ° C in an alumina nacelle with a programmed rise at 5 ° C / min. The diffraction of X-rays made on this powder reveals the pure stage whose system of peaks of Braggs is similar to ZnAl2? 4 perfectly crystallized. The BET specific surface area is 34 m2 / g. The technical qualities of the catalyst are given in Table 3 below. a remarkable level of activity (maximum of approximately 40% N0X conversion at 490 ° C), taking into account the specific surface area of the catalyst. - a NOx conversion zone ranging from 400 to 600 ° C. TABLE 3 Temperature THC TNO TNOx 349 0.0 1.4 0.0 398 5.2 1.6 2.9 449 29.5 15.3 20.2 499 62.7 32.8 39.7 549 86.5 27.5 32.9 599 94.6 18.3 16.3 650 97.1 17.4 5.6 EXAMPLE 4 This example concerns a catalyst of formula Z11AI2O4. ZnO and Ga2Ü4 oxides are used as pressurizers to be mixed and crushed in an agate mortar, then given the shape of a pellet at 1 ton / cm2. The mixture is then calcined under air in an alumina crucible. The compound ZnGa2? 4 is obtained after two roasting cycles at 1000 ° C for 15 h, with a grinding and molding in the form of an intermediate tablet. Under these conditions, the diffraction of the X-rays shows the presence of ZnGa2? 4 » The specific surface BET is less than 0.50 m2 / g. The technical qualities of the catalyst are given in Table 4 below. - a good level of activity (maximum of approximately 25% conversion of N0X to 530 ° C) taking into account the very low specific surface (<0.50 m2 / g). - a NOx conversion zone ranging from 400 to 650 ° C. TABLE 4 Temperature THC TNOx 300 0.0 5.9 1.0 349 0.0 7.4 2.4 399 0.0 6.6 3.3 448 2.7 8.4 7.8 499 19.1 16.6 19.7 548 55.0 21.0 22.1 599 86.3 12.0 9.9 648 94.7 8.2 6.4 676 97.0 10.9 5.4 Regarding the following examples, the Reaction mixture at the reactor inlet has the following composition (by volume): - NO = 300 vpm - CßHg = 300 vpm - 02 = 10% - C02 = 10% - H20 = = 10% - N2 = csp 100% - Load catalytic = 300 mg The overall yield is lONl / h. The WH is approximately 20,000 h. "On the other hand, the catalysts are prepared according to the method given below: A mixture of salts, sols or also oxides in suspension is carried out under agitation by dissolving the salts or putting them in suspension. of suns and oxides, that in an adequate volume of water (concentration in salts: 0.25 to 1 M / l, suspensions 50 to 250 g / 1) This mixture is harvested with a Büchi dryer at an inlet temperature of between 220 ° and 250 ° C, outlet temperature between 100 and 150 ° C and a suspension performance of between 1.5 l / min to 15 ml / min The dust thus obtained is calcined between 500 and 900 ° C, programmed rise of between / na 5 ° / mn, level between 2h and 6h at this same temperature.

EXAMPLE 5 This example concerns a catalyst of the formula Zno.9Al20. It is prepared according to the method given above using as start products Zn (N03), 6H20 and A1 (N03) 3.9H20. The technical qualities are given in table 5. TABLE 5 Temperature THC TNO TNOx 350 3.2 0 0 400 5.5 0.1 2.8 450 11.9 3.1 5.0 500 33.4 11.7 14.6 550 74.3 40.4 30.0 600 94.8 44.2 27.6 650 99.9 33.4 18.0 700 100 23.3 11.1 EXAMPLE 6 This example concerns a catalyst of the global formula SnZnGa204 on an alumina support. The tin content is 1.6% by weight. The catalyst is prepared according to the method given above using as start products Zn (N03) 2 6H20 and Ga (N03) 3 in solution at 3.24M / 1 and an alumina put into suspension, the alumina used is a Condea alumina calcined at 800 ° C. The catalyst / support weight ratio in alumina is 20/80. The technical qualities are given in table 6.

TABLE 6 Temperature THC TNO TNOj 300 2.0 0 0 350 4.5 0 0 400 10.6 0 1.6 450 26.7 2.5 6.5 500 58.3 16.8 20.2 550 90.4 29.8 32.9 600 98.7 21.2 23.9 650 100 9.7 11.2 700 100 1.8 4.0 EXAMPLE 7 This example concerns a catalyst of formula ZnGa204 on an alumina support. The alumina used is a Condea alumina calcined for 2 h at 750 ° C. The catalyst / support weight ratio in alumina is /80. The technical qualities are given in Table 7. TABLE 7 T Teemmppeerraattuurraa T THHCC TNO TNOx 350 5.1 0 0 400 11.3 0 1.6 450 25.6 4.4 8.2 500 60.9 17.4 20.4 5 55500 9 911..11 38.5 39.9 600 97.8 42.2 38.3 650 100 17.4 16.3 700 100 3.1 3.2 EXAMPLE 8 This example concerns a catalyst of formula MgAl204. The catalyst is prepared according to the method given above using as starting products Mg (N03), 6H2? and A1 (N03) 3.9H2 ?. It is used in the same conditions as the conditions of examples 5 to 7 but in an amount of 1.5 g and a WH of lOOOOh "1. The technical qualities are given in table 8. TABLE 8 Temperature THC TNO TNOx 400 0 0 0 450 2.2 1.2 0.7 500 12.6 14.7 14.7 550 49.7 33.9 33.8 600 83.6 7.7 3.7 650 92.4 16.5 7.2

Claims (12)

1. NOVELTY OF THE INVENTION Having described the foregoing invention, the content of the following is claimed as property: CLAIMS 1.- Use of a catalyst comprising a spinel-structured mass oxide and of the formula ZnAl2 4 for the treatment of exhaust gases of internal combustion engines, with high oxygen content, in order to reduce the emissions of nitrogen oxides.
2. 2.- Use of a catalyst comprising an oxide of normal spinel structure or reverse spinel with the exception of ZnAl2? 4 for the treatment of the exhaust gases of internal combustion engines, with high oxygen content, in order to reduce the Nitrogen oxides emissions.
3. 3. Use according to claim 2, characterized in that the normal spinel or reverse spinel oxide corresponds to the formula (1): AB204 in which: A is at least one element selected from the groups lia, Ib, Illb, IVb and Vb as well as the elements of transition of the periodic classification; B is at least one element selected within the groups IVa, Vía, Vlla, VIII, Ib to Vb of the periodic classification.
4. 4. - Use according to claim 3, characterized in that the oxide of normal spinel structure or reverse spinel corresponds to the formula (1) in which A is at least one element selected from the group including Mg, Ti, Mn, Fe, Co, Ni, Cu, Zn and Sn; B is at least one element selected from the group including Ti, Mn, Cr, Fe, Co, Ni, Cu, Zn, Al, Ga, In, Sn and Sb.
5. 5. Use according to claim 4, characterized in that the Normal spinel structure oxide or reverse spinel responds to formula (1) in which B is Al or Ga.
6. 6. Use according to one of the preceding claims, characterized in that the oxide used in the normal spinel structure or inverse spinel is obtained by unbundling the precursors of the elements constituting the spinel.
7. 7. Use according to one of claims 1 to 5, characterized in that the oxide used in the normal spinel structure or inverse spinel is obtained by a process during which a solution or suspension of salts of the constituent elements of the spinel is formed. The medium thus obtained is dried and calcined.
8. 8. Use according to claim 7, characterized in that the oxide used is obtained by a process during which it is dried by atomization.
9. 9. Use according to claim 7, characterized in that the oxide used is obtained by a process during which it is calcined at a temperature of at least 600 ° C.
10. 10. Use according to one of claims 1 to 5, characterized in that the oxide used with the normal spinel structure or reverse spinel is obtained by a process during which a combustion of a mixture of precursors of the constituent elements of the spinel and from a source of carbon and nitrogen.
11. 11. Use according to claim 10, characterized in that the oxide used in normal spinel structure or inverse spinel is obtained by a process during which the source of carbon and nitrogen is urea or glycine.
12. 12. Use of an oxide as defined according to any of the preceding claims for the manufacture of a catalytic system for the treatment of exhaust gases of internal combustion engines, with high oxygen content, in order to reduce emissions of nitrogen oxides.