Abstract:
A catalyst and a method for producing the catalyst, for catalytic reduction of NO x  and oxidation of hydrocarbons, the catalyst comprises a zinc, copper and aluminum spinel to which, at least one element selected from palladium, platinum, rhodium, ruthenium, osmium, iridium, rhenium and/or rare earth metals, such as lanthanum and cerium, vanadium, titanium, niobium, molybdenum, tungsten and/or salts thereof and/or oxides thereof has been added.

Description:
This application is a division of co-pending application Ser. No. 08/764,464, filed Dec. 12, 1996. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to a catalyst comprising a spinel which comprises copper (Cu), zinc (Zn) and aluminum (Al), to a method for producing the catalyst comprising a spinel which comprises copper (Cu), zinc (Zn) and aluminum (Al), as well as to the use of such a catalyst. 
     Published German Patent Application No. DE 43 01 470, discloses a CuAl 2  O 4  spinel wherein tin, lead and an oxide, a salt or an elemental form of an element of the second main or subsidiary group of the Periodic Table has been added, and subsequently calcined to form the final spinel. The spinel is used to decompose laughing gas (N 2  O). However, this patent does not teach the use of this spinel as a catalyst for the catalytic oxidation of CO to CO 2  and/or for the catalytic reduction of NOR, preferably of NO and NO 2 , especially at temperatures of several hundred degrees centigrade. 
     According to the present invention, a spinel is understood to be a material having the general chemical formula A a  B b  O 4 , which has, at least microscopically, a crystallographic or crystalline cubic lattice structure with face-centered oxygen ions and with tetrahedral or octahedral gaps. All the A atoms and up to 50% of the B atoms are disposed in the tetrahedral gaps and the remaining B atoms are disposed in the octahedral gaps. The term A atom or B atom refers here only to the crystallographic arrangement. From a material point of view, the A atoms, as well as the B atoms, can be different from one another. 
     For environmental reasons, the purification of gases, particularly of exhaust gases, which are produced by internal combustion engines for example, especially, by diesel engines and engines operating on a lean mixture, as well as the decrease in the CO content of exhaust gases and the denitrification of these gases, that is, the decomposition of nitrogen oxides (NO x ), is a pressing problem. 
     Published European Patent Application No. EP 042 471 discloses a catalyst which is used for catalytic oxidation of carbon monoxide (CO) to carbon dioxide (CO 2 ) and contains the metals copper (Cu), zinc (Zn) and aluminum (Al) as metal oxides. At least 60% of the copper present is bound to the aluminum oxide as a copper oxide/aluminum oxide spinel. Zinc oxide is disposed in the free spaces of the porous spinel in a proportion by weight of between 1% to 20%. 
     However, the catalyst disclosed in EP 042 471 is known only for its purifying action with regard to carbon monoxide, which is based on the catalytic oxidation of the carbon monoxide. Whether the catalyst is also suitable for the purification of gases containing NO x  and/or hydrocarbons, as occur in the case of the above-mentioned internal combustion engines and also in incinerator power plants and the like, is not disclosed. 
     It is an object of the present invention to develop a catalyst that has good resistance to exhaust gases components, and also has a good purifying action, particularly for NO x . Moreover, it is a further object of the present invention to develop a method for the production of such a catalyst. 
     Pursuant to the present invention, these and other objects are accomplished with a catalyst wherein at least one element selected from the group consisting of palladium, platinum, rhodium, ruthenium, osmium, iridium, rhenium, lanthanum, cerium, vanadium, titanium, niobium, molybdenum, tungsten,. salts thereof and oxides thereof is added to a copper oxide/zinc oxide/aluminum oxide spinel having the chemical formula 
     
         Cu.sub.A Zn.sub.B Al.sub.C O.sub.4 
    
     wherein A+B+C≦3 and A&gt;0, B&gt;0 and C&gt;0. 
     Furthermore, these and other objects are also accomplished by providing a method for the production of such a catalyst comprising producing a spinel comprising copper, zinc and aluminum from copper oxide, zinc oxide and aluminum oxide, and incorporating in the spinel at least one element selected from the group consisting of palladium, platinum, rhodium, ruthenium, osmium, iridium, rhenium, lanthanum, cerium, vanadium, titanium, niobium, molybdenum, tungsten, salts thereof and oxides thereof. 
     As a result of the use of a Cu A  Zn B  Al C  O 4  spinel as a catalyst, wherein A+B+C≦3, A&gt;0, B&gt;0, and C&gt;0, wherein at least one of palladium, platinum, rhodium, ruthenium, osmium, iridium, rhenium, and/or rare earth, such as lanthanum and cerium, vanadium, titanium, niobium, molybdenum, tungsten and/or salts thereof and/or oxides thereof have been added, the catalyst is largely resistant to exhaust gas components such as H 2  O, NO x , CO 2  and/or SO 2  ; and the catalyst acts in an oxidizing manner on the hydrocarbons and in a catalytically reducing manner on the NO x . The reduction of NO x , takes place in an oxygen-containing gas and in the presence of reducing agents, such as hydrocarbons. Preferably, hydrocarbons are present in a sufficient concentration in the exhaust gases of the internal combustion engine. In preferred cases, reduction rates of more than 60% can be achieved at temperatures above 300° C. 
     According to the present invention, substoichiometric compounds in which Al 2  O 3  functions as a matrix and which have characteristic spinel lines in the X-ray spectrum, are also regarded as spinels. In such substoichiometric compositions, a spinel of the composition Al 2  O 4  is present in an Al 2  O 3  matrix, so that an A.sub.(1-x) Al 2  O 4  stoichiometry results. 
     In a preferred embodiment, the catalyst is preferably a copper oxide/tin oxide/aluminum oxide spinel having the chemical formula 
     
         Cu.sub.(1-B) Zn.sub.B Al.sub.2 O.sub.4 
    
     wherein 0&lt;B&lt;1. 
     In another preferred embodiment, the catalyst is a copper oxide/tin oxide/aluminum oxide spinel having the chemical formula 
     
         Cu.sub.0.5 Zn.sub.0.5 Al.sub.2 O.sub.4. 
    
     According to the present invention, the spinel is preferably used as a molded object or a split Villz billet, particularly a honeycomb. 
     According to the present invention, the spinel preferably comprises between 0.5% and 15% by weight cerium oxide, particularly between 1 and 8% by weight. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph of NO x  (NO) reduction and CO oxidation as a function of the temperature for a mixture which contains 20% ZnO, 16% CuO and 64% Al 2  O 3 , has a spinel structure and is impregnated with 1.6% by weight of CeO 2  ; 
     FIG. 2 is a graph of NO x  (NO) reduction and CO oxidation as a function of the temperature for a mixture which contains 20% ZnO, 16% CuO and 64% Al 2  O 3 , has a spinel structure and is impregnated with 86 by weight of CeO 2  ; 
     FIG. 3 is a graph of NO x  (NO) reduction and CO oxidation as a function of the temperature for a mixture which contains 20% ZnO, 16% CuO and 64% Al 2  O 3 , has a spinel structure and is additionally mixed with WO 3 , V 2  O 5  and TiO 2  ; 
     FIG. 4 is a graph of NO x  (NO) reduction and CO oxidation as a function of the temperature for a mixture which contains 20% ZnO, 16% CuO and 64% Al 2  O 3 , has a spinel structure and additionally contains 0.1% by weight of vanadium; and 
     FIG. 5 is a graph of NO x  (NO) reduction and CO oxidation as a function of the temperature for a mixture which contains 20% ZnO, 16% CuO and 64% Al 2  O 3 , has a spinel structure and additionally contains 0.5% by weight of palladium. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     A mixture which has a spinel structure and consists of 20% ZnO, 16% CuO and 64% Al 2  O 3 , hereinafter referred to as a ZnCuAl 2  O 4  spinel, impregnated with 1.6% by weight of CeO 2 , is used as the spinel. A 10 gram sample of the spinel having a particle size from 1.6 to 2 mm is transferred to a vertically oriented quartz reactor having a diameter of 20 mm and a height of approximately 500 mm, with a gas-permeable sintered glass disk disposed in the middle to expose the sample. The height of the bed was about 15 mm. A furnace was arranged around the quartz reactor which heated the middle part of the reactor over a length of about 100 mm at temperatures of up to 550° C. 
     A gas mixture, which consisted of 1,000 ppm NO, 1,000 ppm propene and 10% oxygen, with the remainder being argon as a carrier gas, was passed through the catalyst at sa space velocity of about 10000 per hour. After the gas mixture had passed through the reactor, the NO concentration was measured with a gas detector, in which any NO 2  formed was reduced in a converter to nitric oxide (NO) before the measurement. Oxidation of hydrocarbons to carbon dioxide was simultaneously observed by measuring the carbon dioxide content with the gas detector. 
     The result of the measurement with the ZnCuAl 2  O 4  spinel of Example 1 is shown in the graph of FIG. 1. The NO x  (NO) and CO 2  concentrations are plotted in ppm as a function of the temperature, with the NO x  concentration and the CO 2  concentration being marked differently. The graph shows a clear decrease in the NO x  (NO) concentration as the temperature increases. This concentration reaches a minimum at about 430° C. and subsequently rises once again. For the ZnCuAl 2  O 4  spinel with 1.6% by weight of CeO 2 , a drastic decrease in the NO x  concentration is observed at a temperature above 150° C., and hydrocarbons are simultaneously decomposed to carbon dioxide, as indicated by the increase in the CO 2  concentration. The temperature range in which reduction of the NO x  takes place lies between 150° C. and 50° C., depending on the composition of the material. 
     Preferably, the temperature intervals coincide with the temperatures which can occur in the exhaust gas system of an internal combustion engine. 
     Since this spinel has a good response behavior at relatively low temperatures, it is particularly suitable for use as a NO x  exhaust gas catalyst for internal combustion engines and preferably, for diesel engines. 
     Further testing of this catalyst revealed a high resistance to NO x , H 2  O, and CO 2 . 
     EXAMPLE 2 
     The above ZnCuAl 2  O 4  spinel, additionally impregnated with 8% by weight of CeO 2 , was used as the spinel. This spinel was prepared by impregnating a ZnCuAl 2  O 4  spinel with 8% by weight of CeO 2 . 
     A 10 gram sample of the spinel was transferred to a vertically oriented quartz reactor having a diameter of 20 mm and a height of approximately 500 mm, in which a gas-permeable sintered glass disk was disposed in the middle to expose the sample. The height of the bed was about 15 mm. A furnace was arranged about the quartz reactor which heated the middle part of the reactor over a length of about 100 mm at temperatures of up to 550° C. 
     A gas mixture comprising 1000 ppm NO, 1000 ppm propene and 10% oxygen, with the remainder being argon as a carrier gas, was passed through the catalyst at a space velocity of approximately 10000 per hour. 
     After the gas mixture had passed through the reactor, the NO concentration was measured with a gas detector, in which any NO 2  formed was reduced in a converter to nitric oxide (NO) before the measurement. Oxidation of hydrocarbons to carbon dioxide was simultaneously observed by measuring the carbon dioxide content with the gas detector. The result of the measurements with the ZnCuAl 2  O 4  spinel of Example 2 is shown in the graph of FIG. 2. The NO x  (NO) and CO 2  concentrations are plotted in ppm as a function of the temperature, with the NO x  concentration and the CO 2  concentration being marked differently. 
     In the graphs, a clear decrease can be noted in the NO x  (NO) concentration as the temperature increases. This concentration reaches a minimum at about 300° C. and subsequently rises once again. 
     For the ZnCuAl 2  O 4  spinel with 8% by weight of CeO 2 , a drastic decrease in the NO x  concentration was observed at a temperature above 200° C., and hydrocarbons were simultaneously converted to carbon dioxide, as demonstrated by the increase in the CO 2  concentration. The temperature range in which reduction of the NO x  took place lies between 200° C. and 500° C., depending on the composition of the material. 
     Advantageously, the aforementioned temperature interval coincides approximately with the temperatures which can occur in the exhaust gas system of an internal combustion engine. 
     Since the spinel of Example 2 has a good response behavior at relatively low temperatures, it is also particularly suitable for use as an NO x  exhaust gas catalyst for internal combustion engines and, preferably, for diesel engines. 
     Further testing of this catalyst also revealed a high stability toward NO x , H 2  O, and CO 2 . 
     EXAMPLE 3 
     The above-mentioned ZnCuAl 2  O 4  spinel, mixed with oxides of tungsten, vanadium and titanium, was used as the spinel. The mixture contained up to 50% by weight of the ZnCuAl 2  O 4  spinel, the remaining 50% by weight being formed of a mixture of 5% by weight WO 3 , 36 by weight V 2  O and 42% by weight TiO 2 . A 10 gram sample of the spinel was transferred to a vertically oriented quartz reactor having a diameter of 20 mm and a height of approximately 500 mm, in which a gas-permeable sintered glass disk was disposed in the middle to expose the sample. The height of the bed was about 15 mm. A furnace was arranged around the quartz reactor which heated the middle part of the reactor over a length of about 100 mm at temperatures of up to 550° C. 
     A gas mixture, comprising 1000 ppm NO, 1000 ppm propene and 10% oxygen, with the remainder being argon as a carrier gas, was passed through the catalyst at a space velocity of approximately 10000 per hour. After the gas mixture had passed through the reactor, the NO concentration was measured with a gas detector, in which any NO 2  formed was reduced in a converter to nitric oxide (NO) before the measurement. Oxidation of hydrocarbons to carbon dioxide was simultaneously observed by measuring the carbon dioxide content with the gas detector. 
     The result of the measurements with the spinel of Example 3 is shown in the graph of FIG. 3. The NO x  (NO) and CO 2  concentrations are plotted in ppm as a function of the temperature, with the NO x  concentration and the CO 2  concentration being marked differently. In the diagram, a clear decrease in the NO x  (NO) concentration can be noted as the temperature increases. This concentration reaches a minimum at about 240° C. and subsequently rises once again. For the mixture, a drastic decrease in the NO x  concentration is observed at a temperature above 150° C., and hydrocarbons are simultaneously decomposed to carbon dioxide as demonstrated by the increase in the CO 2  concentration. The temperature range in which reduction of the NO x  takes place lies between 150° C. and 500° C. depending on the composition of the material. 
     EXAMPLE 4 
     The ZnCuAl 2  O 4  mentioned above, impregnated with 0.1% of vanadium, was used as the spinel. A 10 gram sample of the spinel was transferred to a vertically oriented quartz reactor having a diameter of 20 mm and a height of approximately 500 mm, in which a gas-permeable sintered glass disk was disposed in the middle to expose the sample. The height of the bed was about 15 mm. A furnace was arranged about the quartz reactor which heated the middle part of the reactor over a length of about 100 mm at temperatures of up to 550° C. 
     A gas mixture comprising 1000 ppm NO, 1000 ppm propene and 10% oxygen, with the remainder being argon as a carrier gas, was passed through the catalyst at a space velocity of approximately 10000 per hour. After the gas mixture had passed through the reactor, the NO concentration was measured with a gas detector, in which any NO 2  formed was reduced in a converter to nitric oxide (NO) before the measurement. Oxidation of hydrocarbons to carbon dioxide was simultaneously observed by measuring the carbon dioxide content with the gas detector. 
     The result of the measurements with the spinel of Example 4 is shown in the graph of FIG. 4. The NO x  (NO) and CO 2  concentrations are plotted in ppm as a function of the temperature, the NO x  concentration and the CO 2  concentration being marked differently. In the graph, a clear decrease in the NO x  (NO) concentration can be noted as the temperature increases. This concentration reaches a minimum at about 300° C. and subsequently rises once again. For the ZnCuAl 2  O 4  spinel with vanadium, a drastic decrease in the NO x  concentration was observed at temperatures above 170° C., and hydrocarbons were simultaneously decomposed to carbon dioxide as demonstrated by the increase in the CO 2  concentration. The temperature range in which reduction of the NO x  took place lies between 170° C. and 500° C., depending on the composition of the material. 
     EXAMPLE 5 
     The above ZnCuAl 2  O 4  spinel, impregnated with 0.5% of palladium, was used as the spinel. A 10 gram sample of the spinel was transferred to a vertically oriented quartz reactor having a diameter of 20 mm and a height of approximately 500 mm in which a gas-permeable sintered glass disk was disposed in the middle to expose the sample. The height of the bed was about 15 mm. A furnace was arranged around the quartz reactor which heated the middle part of the reactor over a length of about 100 mm at temperatures of up to 550° C. 
     A gas mixture comprising 1000 ppm NO, 1000 ppm propene and 10% oxygen, with the remainder being argon as a carrier gas, was passed through the catalyst at a space velocity of approximately 10000 per hour. After the gas mixture passed through the reactor, the NO concentration was measured with a gas detector, in which any NO 2  formed was reduced in a converter to nitric oxide (NO) before the measurement. Oxidation of hydrocarbons to carbon dioxide was simultaneously observed by measuring the carbon dioxide content with the gas detector. 
     The result of the measurements with the spinel of Example 5 are shown in the graph of FIG. 5. The NO x  (NO) and CO 2  concentrations are plotted in ppm as a function of the temperature, with the NO x  concentration and the CO 2  concentration being marked differently. In the graph, a clear decrease in the NO x  (NO) concentration can be noted as the temperature increases. This concentration reaches a minimum at about 280° C. and subsequently rises once again. For the ZnCuAl 2  O 4  spinel with 0.56 by weight of palladium, a drastic decrease in the NO x  concentration was observed at a temperature above 180° C., and hydrocarbons simultaneously were decomposed to carbon dioxide as demonstrated by the increase in the CO 2  concentration. The temperature range in which reduction of NO x  took place lies between 180° C. and 500° C., depending on the composition of the material. 
     Advantageously, this temperature interval coincides approximately with the temperatures which can occur in the exhaust gas system in an exhaust of an internal combustion engine. 
     Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example, and is not to be taken by way of limitation. The spirit and scope of the present invention are to be limited only by the terms of the appended claims.