Catalysts for selective catalytic reduction denox technology

Abstract: Catalysts for the selective catalytic reduction of NOx by ammonia are made from porous formed supports having a certain fraction of the pores with pore diameters larger than 600 Angstrom units. Thus macropores can be introduced in a formed TiO₂ support by adding burnout materials or some inorganic oxide prior to forming the titania into the formed support. Alternatively, titania is precipitated onto a porous inorganic oxide such as SiO₂, Al₂O₃, ZrO₂, AlPO₄, Fe₂O₃ and B₂O₃ to form the support. Then conventional DeNOx catalytic metals are added. The support can be made by first impregnating the inorganic oxide particles with titania via a soluble precursor, forming titania and then shaping the coated particles into the desired shape such as a monolith. In a second embodiment, the inorganic oxide particles are first formed into the desired shape and then impregnated with the titania forming material. Barium sulfate can be added to the surface of the porous inorganic oxide to make an improved support.

Description: 1. Field of the Invention This invention relates to catalysts made of porous titania deposited on an inorganic support, and which contain additional ingredients such as oxides of vanadium and tungsten for the reduction of nitrogen oxides by ammonia. 2 Description of the Previously Published Art Nitrogen oxides have been selectively reduced in the presence of ammonia and various types of catalysts. U. S. Patent No. 4,048,112 discloses a catalyst for the selective reduction of nitrogen oxides in exhaust gases in the presence of ammonia. The catalyst is made of vanadium oxide supported on a carrier of titanium oxide in the anatase form. U. S. Patent No. 4,085,193 discloses catalysts for removing NO x . Among the materials disclosed is titania and at least an oxide of molybdenum, tungsten, iron, vanadium, nickel, cobalt, copper, chromium or uranium. U. S. Patent No. 4,010,238 discloses a process to remove NO x in the presence of ammonia and a vanadium oxide catalyst of the formula V x A y O z where v is vanadium, A is Cu, Zn, Sn, Pb, Ti, P, Cr, Fe Co and Ni, x and y are each a numeral of 0.5 to 12 and z is a numeral of 1 to 60. US. Patent No. 4,225,462 discloses using barium sulfate and vanadium (IV) oxide sulfate to make a water resistant catalyst for reducing nitrogen oxide with ammonia. U. S. Patent No. 4,176,089 discloses making high surface area, low bulk density mixed silica-titania materials useful for catalysts for the selective reduction of nitrogen oxides with ammonia. The catalysts are made by premixing the alkoxides of titanium and silicon and adding the premixed alkoxides to a hydrolysis medium so as to form a silica-titania precipitate. U. S. Patent No. 4,188,365 discloses a catalyst for the vapor phase reduction of nitrogen oxides with ammonia. The catalyst has a catalytic metal compound on a shaped carrier of TiO₂ and a clay mineral having an average particle size of 0.1 to 100 microns. The shaped carrier can also contain inorganic fibrous material and small particles of silica hydrogel and silica sol. U. S. Patent No. 4,221,768 discloses a process to remove nitrogen oxides with ammonia by using a catalyst made of an intimately mixed binary (TiO₂-SiO₂) or ternary (TiO₂-ZrO₂-SiO₂) oxide with no cladding and with no co-precipitation. These binary and ternary oxides are not mere mixtures of the individual oxides. Great Britain Patent No. 2,149,680 discloses a catalyst for the removal of nitrogen oxides made of (A) 80 to 95% by weight of an oxide obtained by thermal treatment of either binary hydrous oxide of titanium and silicon, a binary hydrous oxide of titanium and zirconium or a ternary hydrous oxide of titanium, zirconium and silicon in the presence of sulfuric acid or ammonium sulfate, (B) from 0 to 5% by weight of vanadium oxide, and (C) from 1 to 15% by weight of an oxide of tungsten, molybdenum, tin or cerium. U. S. Patent No. 4,280,926 (and its divisional 4,520,124) form a slurry of 1-20 mm long fibers which can be a silica fiber with diameter of 0.1-30 microns. Then either TiO₂ or Al₂O₃ forming materials are added and a sheet is made by papermaking means. These sheets can be formed into a honeycomb structure. The sheets are then impregnated with catalytically active agents. U. S. Patent No. 4,113,660 discloses making a titania catalyst using a metatitanic acid sol or gel. T. Shikada et al. in "Reduction of Nitric Oxide by Ammonia over Silica Supported Vanadium Oxide Catalysts (III). Effects of Additives" in Nenryo Hyokai Shi, vol. 58 page 1055 (1979) disclose treating silica gel with titanic sulfate solution, neutralizing with ammonia water, drying, adding vanadyl oxalate solution, drying and finally calcining in air at 350°C for 3 hours to produce a catalyst for the reduction of nitric oxides by ammonia. They found that the amount of TiO2 required for modifying the surface of the silica is not high. A 3% level was sufficient and that as the TiO2 content increased beyond this point the activity declined. U.S. Patent No. 4,705,770 discloses making an attrition resistant catalyst from titanyl sulfate which can be used in a fluidized bed. Small particles are made for fluidization. There appears to be no discussion of macroporosity; all the mean pore diameters are 23,2 nm (232 Angstrom units) or less. Furthermore, they do not intend to form a shaped support which has macroporosity. EP-A-0 256 359 discloses a catalyst containing pores having a diameter of 10 to less than 100 nm and .... having a diameter of 100 nm to 12000 nm and comprising titanium and at least one metal selected from Mo, " and V as metal elements of catalytically active ingredients, and the use of said catalyst for removing NO in exhaust gases. Additionally said catalyst may comprise silica. EP-A-0 268 265 discloses a denitration catalyst comprising an oxide of titanium, an oxide of tungsten nd an oxide of vanadium as catalytically active ingredients and having a pore volume of 0.1 to 0.5 cm³/g. The titania content is 50 to 99 wt% and the catalyst may further contain silica. 3. Objects of the Invention It is an object of this invention to enhance the catalyst NO x removal performance and reduce the manufacturing cost of an SCR catalyst. It is a further object of this invention to extend the catalyst life by improving the poison resistance. It is a further object of this invention to reduce the manufacturing cost by reducing the amount of titania employed in the monolith structure of an SCR catalyst. It is a further object of this invention to reduce or limit the SO₂ oxidation activity while at the same time enhancing NO x removal activity. It is a further object of this invention to reduce the operating temperatures for given NO x removal requirements. It is a further object of this invention to reduce the required reactor volume for a given NO x conversion and hence simultaneously reduce the reactor pressure drop. It is a further object of this invention to reduce the unreacted ammonia at the exit of the SCR reactor. These and further objects will become apparent as the description of the invention proceeds. The feasibility of NOx removal through selective catalytic reduction with NH₃ in the presence of O₂ has been known for sometime and proceeds as follows: 4NO + 4NH₃ + O₂→4N₂ + 6H₂O 2NO₂ + 4NH₃ + O₂→3N₂ + 6H₂O In contrast to previous NOx removal (DeNOx) catalysts which utilize a bulk titania support, we have obtained a superior catalyst consisting of titania with the appropriate catalytic metal oxide function supported on a porous inorganic oxide. Based on reaction engineering principles, we have successfully mathematically modeled the overall catalytic DeNOX reaction under various conditions. Further mathematical catalyst optimization studies have led us to uncover a route to improved DeNOX catalysts. Our model predicts that catalysts having a fraction of pores larger than 60 nm (600 Angstrom units) will have better activities than those without these large pores. As will be described below, we have successfully obtained catalysts incorporating this porosity and, as predicted by our mathematical modeling, the DeNOX activity is superior to that of commercially available catalysts which contain very few or no pores larger than 60 nm (600 Angstrom units). Furthermore, by partially replacing the expensive bulk titania by a low-cost porous metal oxide, the cost of preparing the catalyst is substantially reduced while the strength of the catalyst is increased. The titania can be precipitated onto a porous inorganic oxide such as SiO₂, Al₂O₃, ZrO₂, AlPO₄, Fe₂O₃ and B₂O₃, for example, by two procedures. In the preferred first procedure, the inorganic oxide particles are first impregnated with titania via a soluble precursor so that the internal and external surface of the particles becomes coated with titania. These coated particles, after calcining at about 500°C, are referred to as precursor powder. They may be formed, for example by extrusion, into a useful catalyst support shape such as a monolith, extrudate, bead or plate. In the second titania adding procedure, the inorganic support is extruded into the final form and then impregnated with titania forming a titania coating on the surface area of the inorganic support particles throughout the formed body. In another embodiment of the invention, barium sulfate is added to the surface of the inorganic oxide such as SiO₂ to make an improved support. This can either be done by the direct addition of barium sulfate or by adding a barium compound which will form barium sulfate in situ. The TiO₂ can be added either concurrently with or following the addition of the barium sulfate. When a catalytic metal oxide such as V₂O₅ is added to make a catalyst, the DeNOX activity is greater than a similar catalyst made without the barium sulfate addition. The preferred amount of barium sulfate to be added is from 1 to 15 wt% of the formed metal oxide support. A preferable inexpensive inorganic oxide support is silica (either gel or precipitated) such as Syloid 74 (W. R. Grace & Co.) or Zeofinn XP-113 (Zeofinn Oy, Finland). Preferably, the surface area of the support with the titania and catalyst is from a catalytically effective amount up to 350 m²/cm³, or, more preferably, one that is between 25 m²/cm³ and 200 m²/cm³. Preferably the pore diameter distribution of the formed body should cover the range from 0-10 000 nm (0 to 100,000 Angstrom units). Preferably the support should have a micropore diameter distribution between 0-60 nm (0 and 600 Angstrom units), more preferably between 5-15 nm (50 and 150 Angstrom units, and a macropore diameter distribution between 60-10000 nm (600 and 100,000 Angstrom units), more preferably between 100-2000 nm (1,000 and 20,000 Angstrom units). Although the mathematical model tends to maximize the porosity, we prefer, due to the limitation of strength, a maximum total porosity of 0.80 cm³/cm³, or more preferably, between 0.5 and 0.75 cm³/cm³. The microporosity is preferably between 0.05 and 0.50 cm³/cm³ and macroporosity preferably between 0.05 and 0.50 cm³/cm³. Brief Description of the Figure The Figure is a schematic diagram of a tubular reactor set-up used to make the support of this invention. Description of the Preferred Embodiments One embodiment involves producing the macroporous titania support by depositing the titania on a porous inorganic oxide support. One procedure to do this is to apply the titania to particles of the porous inorganic oxide. These titania containing inorganic oxide particles are referred to as precursor powder since they will be used to form the formed porous support on which the catalyst particles will be deposited. To apply the titania onto the appropriate inorganic oxide support, a soluble titania precursor is used. The soluble titania precursor can be one of, but not limited to: titanyl sulfate (e.g., UNITS 992, Kemira), titanyl chloride (e.g., UNITI 102, Kemira), titanium tetra-alkoxide, and titania sol (Nalco). The soluble precursor can be hydrolyzed by the addition of base in the presence of the supporting metal oxide. The base may be any of, but not limited to, alkali metal hydroxides or carbonates, ammonium hydroxide or carbonate, any of the alkyl amines, urea, or hexamethylenetetraamine. When titanium tetra-alkoxide is used, it can be impregnated as an alcohol solution onto the metal oxide support or it can first be dissolved in acidic solution (preferably hydrochloric acid) and then hydrolyzed in the presence of the metal oxide support. The hydrolysis may be carried out in a reaction pot wherein the base solution is added to a mixture of the titania precursor and the inorganic oxide support. Alternatively, the components may be added simultaneously to a reaction pot so as to cause precipitation of the titania precursor in the presence of the inorganic oxide support. Another alternative is to carry out the hydrolysis in a tubular reactor. Such a tubular reactor process is schematically described in the Figure. The flows of the reactants are adjusted such that the pH of the suspension exiting from the tube is sufficient to cause precipitation of the titania precursor. A pH range between 5-12 can be used although a pH of 7 is preferable. In the second titania coating procedure, the inorganic support is extruded into the desired form such as a monolith and then it is impregnated with titania forming a titania coating on the surface area of the formed inorganic support. In this case, the TiO₂ may be impregnated by dipping the extruded body in TiO₂ sol. Following deposition of the TiO₂ on the inorganic oxide support, the support is calcined for about 2 hours preferably between 300-900°C more preferably 400-650°C. The final fraction of titania on the total support is between 5 and 80 wt%, preferably between 15 and 50 wt%. Below 5 wt% the catalyst has a low activity and above about 80 wt% the catalyst becomes economically unattractive to manufacture. The formed support is characterized by porosity which is measured in units of cm³/cm³. The porosity is obtained from the pore volume of the support and the bulk density. For example, for a support having a pore volume of 0.96 cm³/g and a bulk density of 0.75 g/cm³, the porosity would be the product of these two values which is 0.72 cm³/cm³. The catalyst is made by adding to the calcined support metal oxides such as V₂O₅, MoO₃, WO₃, Fe₂O₃, CuSO₄, VOSO₄, SnO₂, Mn₂O₃, Mn₃O₄ or a combination thereof. The catalyst component or components are added to the support after the titania has been precipitated on the inorganic oxide either before or after the support shape is formed. The amount of the catalyst component is from a catalytically effective amount up to 25% by weight of the entire catalyst. The catalyst may be prepared by one of several means. Thus, for example, the TiO₂ -inorganic oxide support can first be impregnated with WO₃ before or after extrusion. For this purpose, an aqueous solution of ammonium metatungstate may be used to impregnate the support to incipient wetness. The amount of ammonium metatungstate to be used depends on the desired final percentage of WO₃ (which may preferably range from 1 to 20 wt%). The impregnated support may then be dried and calcined for example at 500-550°C. for 2 hours. The support, with or without WO₃, can then be impregnated with V₂O₅ before or after the extruded body is formed. The V₂O₅ precursor used can be an aqueous solution of vanadyl oxalate at a concentration which is appropriate to give the final desired V₂O₅ content (typically 0.5-20 wt%) in the catalyst. If the impregnation is to be carried out before the extrusion, then the support powder may be impregnated to incipient wetness with the V₂O₅ precursor solution followed by drying (100-150°C. for about 12 hours). The dried powder can then be used for extrusion. Alternatively, if the V₂O₅ is to be impregnated after the extrusion, the extruded body is first dried and calcined (for example, at 500-550°C. for 2 hours) and then dipped in the appropriate V₂O₅ precursor solution. The extrusion can be carried out in a number of ways familiar to those skillful in the art. The present catalyst is characterized by the presence of titania in the anatase phase. When silica is used as the metal oxide, the titania/silica product is not a chemical composite on the atomic scale. Instead, it is a physical mixture with anatase titania finely dispersed on bulk silica. The anatase titania particle size, as measured by the X-ray line broadening method (H.P. Klug and L.E. Alexander, "X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials") typically averages about 5 nm (50 Angstrom units). It can also be measured by EXAFS (extended X-ray absorption fine structure). It is important to point out here that for the purpose of this invention, it is undesirable, indeed disadvantageous, to have a silica-titania chemical composite such as described in U. S. Patent 4,221,768 and GB 2,149,680. The latter patents teach the preparation of a Ti-Si composite oxide wherein the final product does not contain a distinct anatase titania phase. Similarly U. S. Patent 4,176,089 also has a co-precipitation which results in an undesirable catalyst because the anatase phase would not be present. In the present case, our final titania/silica product shows the presence of titania anatase phase and it is not a chemical composite on the atomic scale. It is finely dispersed anatase titania on bulk silica. The inorganic oxide support chosen should largely determine the desired pore structure in the formed catalyst. The desired pore structure should have an average micropore diameter of less than 60 nm (600 Angstrom units), more preferably between 5-15 nm (50 and 150 Angstrom units) and a macropore diameter distribution between 60-10 000 nm (600 and 100,000 Angstrom units), more preferably between 100-2000 nm (1000 and 20,000 Angstrom units). If the inorganic oxide support does not give the desired pore structure, burn-out material may be added to give the right amount of macroporosity. In the latter case, one of several burn-out materials may be used including, but not limited to, polyethylene oxide, methylcellulose, starch or flour, polyethylene or any of the polymeric microspheres or microwaxes. In another preferred embodiment, BaSO₄ is added during precipitation of the TiO₂. This appears to add sulfate in the support which is advantageous for higher activity. The barium sulfate can be added as barium sulfate or as a soluble barium salt precursor such as barium chloride, barium nitrate, barium acetate, barium hydroxide or mixtures of these salts. Conversion to barium sulfate is achieved by adding sulfuric acid. Having described the basic aspects of our invention, the following examples are given to illustrate specific embodiments thereof. Example 1 This example illustrates the synthesis of 40% TiO₂ on precipitated silica. 3.1 kg of Zeofinn Oy Grade XP113 SiO₂ were mixed with 16.26 liters of Kemira UNITI 992 titanyl sulfate (9.3% TiO₂, 1.32 g/cm³). The resultant slurry was dripped at a rate of 6 liters per hour into 13 liters of heel water. The heel was stirred and maintained at a pH of between 2.5 and 3.0 by the addition of concentrated aqueous ammonia. After the addition of the titanyl sulfate-silica slurry was complete, the ammonia addition was continued until the pH was raised to 7.0. The mixture was stirred for an additional 30 minutes before the product was collected by filtration. After washing with distilled water, the support was dried and calcined to 550°C. A sample, formed into a round disc 2,54 cm (1") diameter, 1 cm thick, pressed at 4536 kg (10,000 lbs), was analyzed by mercury intrusion porosimetry and found to have a total porosity of 0.61 cm³/cm³ with .29 cm³/cm³ of porosity in pores greater than 60 nm (600 Angstrom units) in diameter. The surface area, measured by the BET method, was found to be 131 m²/cm³ and X-ray diffraction analysis demonstrated that the product contained anatase TiO₂. A solution of vanadyl oxalate was prepared by dissolving 0.56 g of V₂O₅ and 1.453 g of oxalic acid in 9 ml of hot water. The solution was used to impregnate 15.0 g of the TiO₂/SiO₂ powder from the above preparation, using the method of incipient wetness. The resulting dried powder was calcined at 520°C for 3 hours. Example 2 This example illustrates the synthesis of 40% TiO₂ on silica gel. The procedure described in Example 1 was followed using Syloid 74 (W. R. Grace & Co.) SiO₂ which is a silica gel. A sample of the calcined product, formed into a round disc 2,54 cm (1") diameter, 1 cm thick, pressed at 4536 kg (10,000 lbs), was analyzed by mercury intrusion porosimetry and found to have a total porosity of 0.72 cm³/cm³ with 0.20 cm³/cm³ of porosity in pores greater than 60 nm (600 Angstrom units) in diameter. The surface area, measured by the BET method, was found to be 162 m²/cm³ and X-ray diffraction analysis demonstrated that the product contained anatase TiO₂. A solution of vanadyl oxalate was prepared by dissolving 61.2 g of V₂O₅ and 157.5 g of oxalic acid in 1.80 l of hot water. The solution was used to impregnate 1.64 kg of the TiO₂/SiO₂ powder from the above preparation, using the method of incipient wetness. The resulting dried powder was calcined at 520°C for 3 hours. Example 3 This example prepares a catalyst according to the present invention where barium sulfate is also added. A solution of 2.50 g. of BaCl₂ in 50 ml of H₂O was impregnated into 25 g of SiO₂ (Syloid X6000 sold by W. R. Grace & Co.). The solids were dried and slurried with 150 ml of 6M H₂SO₄. After the solids were collected by filtration and washed with water, the white powder was dried and calcined at 300°C for 2 hours. The product was combined with 185.8 g of TiOSO₄ solution (KEMIRA UNITI 992, 120 g TiO₂/1) and 400 ml of H₂O. NH₄OH was added to neutralize the mixture which was then filtered. The solids were washed with water, dried, and calcined at 500°C for 6 hours. A solution of vanadyl oxalate was prepared by dissolving 1.887 g of V₂O₅ and 4.855 g of oxalic acid in 50 ml of hot water. The solution was used to impregnate the product using the method of incipient wetness. The resulting dried powder was calcined at 520°C for 3 hours. Example 4 A comparison catalyst was prepared from anatase TiO₂. A solution of vanadyl oxalate was prepared by dissolving 0.097 g of and NH₄VO₃ and 0.194 g of oxalic acid into 10 ml of hot water. The solution was used to impregnate 15.0 g of TiO₂ powder having a surface area of 100 m²/g using the method of incipient wetness. The resulting dried powder was calcined at 520°C for 3 hours. Example 5 This comparison example prepares a catalyst from a physical mixture of anatase TiO₂ and silica. A solution of vanadyl oxalate was prepared by dissolving 0.097 g of NH₄VO₃ and 0.194 g of oxalic acid in 28 ml of hot water. The solution was used to impregnate a mixture of 6 g of TiO₂ powder (Sakai Chemical) and 9 g of SiO₂ powder (Davison 952 W. R. Grace & Co.) using the method of incipient wetness. The powder was calcined at 520°C for 3 hours. Example 6 This comparison example prepares a catalyst where the silica and titania form a binary oxide as described in U. S. Patent No. 4,221,768. A support was prepared following the procedure of Example 1 of U.S. Patent No. 4,221,768 as follows. 50 ml of titanyl chloride solution (Kemira Uniti 102, containing 342 g TiO₂ per liter) was diluted to 200 ml with ice cold water. Then, 76.5 g of Nalco 1034A colloidal silica (34% SiO₂) was added. While the mixture was stirred at about 25°C, aqueous ammonia was slowly added until the pH reached 9.7. The mixture was aged by allowing it to stand for 2 hours. The resulting TiO₂-SiO₂ gel was isolated by filtration, washed with water, and dried at 150°C for 10 hours. The solid was ground and calcined at 500°C for 3 hours. The resulting powder was found by BET analysis to have a surface area of 253 m²/cm³. Analysis by X-ray diffraction demonstrated that the product was amorphous. A solution of vanadyl oxalate was prepared by dissolving 0.56 g of V₂O₅ and 1.44 g of oxalic acid in 15 ml of hot H₂O. The solution was used to impregnate 15.0 g of the above TiO₂-SiO₂ powder using the method of incipient wetness. The powder was calcined at 520°C for 3 hours. Example 7 The activities of the catalyst powders of Examples 1-6 were determined as follows. In a fixed bed reactor, 0.58 cm³ of powder were charged and the fixed bed reactor was heated in a sand bath to the temperatures set forth in Table 1. A gas containing 900 ppm NO, 1000 ppm NH₃, and 4% O₂ in N₂ was passed through the catalyst bed at a rate of 200 1/hr. and the amount of NO removed measured. The results are presented in Table 1. Activity of Catalyst Powders Example % Conversion 250°C 300°C 350°C 1 47.4 86.5 98.8 2 18.5 48.1 89.1 3 23.6 60.1 90.0 4 10.5 33.8 74.3 5 7.4 22.2 42.7 6 16.7 33.8 57.2 The results presented in Table 1 demonstrate the superior performance of the catalysts of Example 1-3 of this invention. The catalyst of Example 4, prepared from anatase TiO₂, is representative of the composition of current commercial SCR catalyst and is shown to have poorer performance than the catalysts of this invention. Physical mixture of silica with such an anatase TiO₂ support leads to a lowered NOx conversion as shown by the catalyst of Example 5. An atomically mixed support, demonstrated by Example 6, also is shown to have poorer performance. Example 8 This example compares a catalyst according to the present invention with a commercial catalyst. 500 g of the TiO₂/SiO₂ support prepared in Example 1 was impregnated to incipient wetness with a vanadyl oxalate solution (prepared from V₂O₅ and oxalic acid solution) to give a final V content of 1.5 wt%. This material was then extruded into a 3 cell x 3 cell monolith piece. The cell pitch is 7.4 mm where pitch is defined as the sum of the width of the cell opening plus the wall thickness. The monolith was air-dried and then calcined at 500°C for 2 hours. The activity of this monolithic catalyst piece was obtained in a side by side comparison with a similarly shaped commercial catalyst which was based on TiO₂ with near zero macroporosity. Reactor conditions used were: 6% O₂, 10% H₂O, 350°C; 400 ppm NO; 3000 ppm SO₂; 400 ppm NH₃; SV 7500 h⁻¹ (STP). The Table 2 below compares the porosity and NOx removal activity ratio of the catalyst of the present invention and the commercial catalyst. The SO₂ oxidation conversion of the new catalyst was found to be 0.2% which is comparable to that of the commercial catalyst. Table 2 shows that with the higher porosity and inclusion of macroporosity, the performance of the new catalyst is significantly improved (by some 60%) over that of the less porous commercial catalyst. Catalyst Porosity (cm³/cm³) Activity Ratio Microporosity Less than 600 AU (60 nm) Macroporosity greater than 600 AU (60 nm) New Catalyst 0.32 0.28 1.6 Commercial catalyst 0.49 0.03 1.0 Where AU is Angstrom units Example 9 In this example, the inorganic oxide support, silica (Syloid), is first extruded into cylindrical pellets 0.32 cm (1/8") by 0.32 cm (1/8") . The pellets are dipped in TiO₂ sol (11% TiO₂, Nalco) to fill up the pores. The impregnated body is dried at 150°C and then calcined at 500°C for 1 hour. The dipping, drying and calcining are repeated 3 more times so that the final TiO₂ loading on the extruded body is about 20% by weight. It is believed that this catalyst support would give a good catalyst for NO x removal after depositing a metal oxide catalytic component as in the prior examples. Example 10 This example described the preparation of a catalyst powder for extrusion. A titania catalyst powder suitable for extrusion was prepared as follows. 1690 g of titania powder was combined with 48 g of clay and the mixture dried at 200°C. A solution of 7.57 g of vanadium and 30.30 g of oxalic acid in 280 ml of water was prepared and combined with a solution pf 150.9 g of ammonium tungstate in 300 ml of water. The solution was then used to impregnate the mixture of titania and clay. The product was dried at 110°C, around, and sieved to below 80 mesh (sieve opening 177µm). Example 11 This is a comparison example which describes the extrusion of bodies without the introduction of macroporosity. A mixture of 100 g of the powder prepared in Example 10 was made with 2.5 g of Methocel 20-213 (Dow Chemical Co.) and 35 ml of water. The mixture was extruded to produce 0.32 cm (1/8") diameter cylinders which were dried and calcined at 550°C. The resultant cylinders were analyzed by mercury intrusion porosimetry and found to contain only a small fraction of macroporosity (0.05 cm³ of pores with diameters greater than 60 nm (600 Angstrom) units/cm³ of total volume). Example 12 This example describes the extrusion of bodies containing macropores. A mixture of 100 g of the powder prepared in Example 10 was made with 15 g of Methocel 20-213 (Dow Chemical Co.) and 45 ml of water. The mixture was extruded to produce 0.32 cm (1/8") diameter cylinders which were dried and calcined at 550°C. The resultant cylinders were analyzed by mercury intrusion porosimetry and found to contain a substantial fraction of macroporosity (0.17 cm³ of pores with diameters greater than 60 nm (600 Angstrom) units/cm³ of total volume). Example 13 This example describes the extrusion of bodies containing macropores and which further contain silica. A mixture of 100 g of the powder prepared in Example 10 was made with 20 g of Methocel 20-213 (Dow Chemical Co.), 11.1 g of silica (Syloid 63, Davison Chemical), and 35 ml of water. The mixture was extruded to produce 0.32 cm (1/8") diameter cylinders which were dried and calcined at 550°C. The resultant cylinders were analyzed by mercury intrusion porosimetry and found to contain a substantial fraction of macroporosity (0.21 cm³ of pores with diameters greater than 60 nm (600 Angstrom) units/cm³ of total volume). Example 14 This example measures the activity of the extrudates of Examples 11-13. The activities of the extrudates described Examples 11, 12 and 13 were determined as follows. In a fixed bed reactor, about 0.3 cm³ of extrudates were charged and the fixed bed reactor was heated in a sand bath to the temperatures set forth in Table 3 below. A gas containing 1275 ppm of NO, 1275 ppm of NH₃, and 4% O₂ in N₂ was passed through the catalyst bed at a rate of 200 1/hr and the amount of NO removed was measured. The rate of NO removal was calculated and the results are presented in Table 3. Rate of NO Removal by 0.32 cm (1/8 inch) extrudates Example Porosity 10⁷ mole NO removed/cm³-sec Total Macropores 310°C 380°C 11 0.54 0.05 6.9 11.0 12 0.60 0.17 9.2 15.4 13 0.58 0.21 8.5 14.9 The data in Table 3 demonstrate the improved performance obtained by the introduction of macroporosity. The catalyst prepared in Example 12 differs from that in Example 11 only in the presence of the additional macroporosity and it is substantially more active than the catalyst of Example 11. The catalyst of Example 13 demonstrates that the presence of such macroporosity allows further addition to the catalyst by an inexpensive porous material such as silica while maintaining a high activity.

Claims: A catalyst for the selective removal of nitrogen oxides from exhaust and waste gases in the presence of ammonia comprising (a) a formed porous support consisting of titania and at least one other inorganic oxide and (b) a metal oxide catalytic component deposited on said support before or after the support shape is formed and selected from the group consisting of V₂O₅, MoO₃, WO₃, Fe₂O₃, CuSO₄, VOSO₄, SnO₂, Mn₂O₃, Mn₃O₄ and mixtures thereof, said metal oxide being present in an amount from a catalytically effective amount to 25% by weight of the entire catalyst, said catalyst being characterized by the titania being deposited on the inorganic oxide and by the formed support having (i) a total porosity of up to 0,80 cm³/cm³ which is made up of a micropore porosity, comprising pores having a pore diameter of 60 nm (600 Å) or less, of 0,05 to 0,5 cm³/cm³ and a macroporosity, comprising pores having diameters greater than 60 nm (600 Å), of 0,05 to 0,5 cm³/cm³, (ii) said titania being present in an amount between about 5 to 80 wt% of the total support, (iii) said titania being substantially present in the anatase phase and (iv) said inorganic oxide being present in an amount of at least 20 wt% of the total support. A catalyst according to Claim 1, wherein the total porosity is between 0.5 and 0.8 cm³/cm³. A catalyst according to Claim 1 or 2, wherein the surface area of the catalyst is less than 350 m²/cm³. A catalyst according to any of Claims 1 to 3, wherein the surface area of the catalyst is between 25 m²/cm³ and 200 m²/cm³. A catalyst according to any of claims 1 to 4, wherein the titania is present in an amount of between about 15 to 50 wt% of the total support. A catalyst according to any of claims 1 to 5, wherein the pore size distribution of the formed body is in pores having a pore diameter in the range of greater than 0 - 10 µm (0 to 100,000 Å). A catalyst according to Claim 6, wherein the formed support has a micropore size distribution, comprising pores having a diameter of 60 nm (600 Å) or less, having an average pore diameter of 5 - 15 nm (50 - 150 Å) and a macropore size distribution, comprising pores with a pore diameter between greater than 60 nm and 10 µm (600 and 100 000 Å), having an average pore diameter between 100 nm and 2 µm (1000 and 20 000 Å). A catalyst according to Claim 7, wherein the average micropore diameter is between 8 and 12 nm (80 and 120 Å) and wherein the average macropore diameter is between 500 nm and 1,5 µm (5,000 and 15,000 Å). A catalyst according to any of Claims 1 to 8, further comprising barium sulfate in an amount of from 1 - 15% by weight of the formed support. A catalyst according to any of claims 1 to 9, wherein said other inorganic oxide is selected from the group consisting of SiO₂, Al₂O₃, ZrO₂, AlPO₄, Fe₂O₃, B₂O₃, and mixtures thereof. A catalyst according to Claim 10, wherein the porous inorganic oxide is silica gel or precipitated silica. A catalyst according to any of Claims 1 to 11, wherein the catalyst is shaped as a monolith, extrudate, bead, or plate. A precursor powder for making a catalyst according to Claim 1 comprising particles consisting of titania and at least one other inorganic oxide, said precursor powder being characterized by the titania being deposited on the inorganic oxide and by the particles having (i) a total porosity of up to 0,80 cm³/cm³ which is made up of a micropore porosity, comprising pores having a pore diameter of 60 nm (600 Å) or less, of 0,05 to 0,5 cm³/cm³, and a macroporosity, comprising pores having diameters greater than 60 nm (600 Å), of 0,05 to 0,5 cm³/cm³, (ii) said titania being present in an amount between about 5 to 80 wt% of the total powder, (iii) said titania being substantially present in the anatase phase and (iv) said inorganic oxide being present in an amount of at least 20 wt% of the total powder. A precursor powder according to Claim 13, further comprising a metal oxide catalytic component deposited on said powder selected from the group consisting of V₂O₅, MoO₃, WO₃, Fe₂O₃, CuSO₄, VOSO₄, SnO₂, Mn₂O₃, Mn₃O₄, and mixtures thereof, said metal oxide being present in an amount from a catalytically effective amount for the selective removal of nitrogen oxides from exhaust and waste gases in the presence of ammonia to 25% by weight of the entire powder. A precursor powder according to Claim 13 or 14, wherein the inorganic oxide is selected from the group consisting of a silica gel, precipitated silica and mixtures thereof. A process for making a catalyst according to Claim 1 comprising (a) forming a porous formed support from titania and inorganic oxide particles and (b) depositing a metal oxide catalytic component on said support before or after the support shape is formed and selected from the group consisting of V₂O₅, MoO₃, WO₃, Fe₂O₃, CuSO₄, VOSO₄, SnO₂, Mn₂O₃, Mn₃O₄ and mixtures thereof, said metal oxide being present in an amount from a catalytically effective amount to 25% by weight of the entire catalyst, said process being characterized by depositing the titania on the inorganic oxide particles before or after forming of the porous support, said porous support having (i) a total porosity of up to 0,80 cm³/cm³ which is made up of a micropore porosity, comprising pores having a pore diameter of 0 - 60 nm (0 - 600 Å), of 0,05 to 0,5 cm³/cm³ and a macroporosity, comprising pores having diameters greater than 60 nm (600 Å), of 0,05 to 0,5 cm³/cm³, (ii) said titania being present in an amount between about 5 to 80 wt% of the total support, (iii) said titania being substantially present in the anatase phase and (iv) said inorganic oxide being present in an amount of at least 20 wt% of the total support. A process according to Claim 16, wherein the inorganic particles impregnated with titania in step (a) are made by impregnating the inorganic oxide particles with a soluble titania precursor and calcining the impregnated inorganic oxide. A process according to any of Claims 16 or 17, comprising forming the support into an extruded monolith. A process according to any of Claims 16 to 18, further comprising adding to said inorganic oxide BaSO₄ or a soluble barium salt precursor which will form BaSO₄ upon reacting, said barium sulfate being present in an amount from 1 - 15% by weight of the formed support. A process according to Claim 19, wherein said BaSO₄ is added with the titania. A process according to Claim 19 or 20, wherein said BaSO₄ is added before the titania is added. A process according to any of Claims 19 to 21, wherein the barium sulfate precursor is selected from the group consisting of barium chloride, barium nitrate, barium acetate, barium hydroxide and mixtures thereof. A process for producing a precursor powder according to claim 13 comprising particles of titania on inorganic oxide particles for making a DeNOx catalyst, said process being characterized by (a) impregnating the porous inorganic oxide particles with a soluble titania precursor, and (b) calcining the impregnated inorganic oxide particles to form titania and said inorganic oxide particles, whereby said precursor particles have (i) a total porosity of up to 0,80 cm³/cm³ which is made up of a micropore porosity, comprising pores having a pore diameter of 0 - 60 nm (0 - 600 Å), of 0,05 to 0,5 cm³/cm³ and a macroporosity, comprising pores having diameters greater than 60 nm (600 Å), of 0,05 to 0,5 cm³/cm³, (ii) said titania being present in an amount between about 5 to 80 wt% of the total precursor, (iii) said titania being substantially present in the anatase phase and (iv) said inorganic oxide being present in an amount of at least 20 wt% of the total precursor. A process according to Claim 23, wherein the titania precursor is selected from the group consisting of titanyl sulfate, titanyl chloride, titanium tetra-alkoxide, titania sol and mixtures thereof. A process according to Claim 23 or 24, wherein the soluble titania precursor is impregnated on the particles by hydrolyzing the precursor by the addition of base in the presence of the inorganic oxide particles. A process according to Claim 25, wherein the base is selected from the group consisting of alkali metal hydroxides, alkali metal carbonates, ammonium hydroxide, ammonium carbonate, alkyl amine, urea, hexamethylenetetraamine or mixtures thereof. A process according to any of Claims 23 to 26, where the inorganic oxide particles are in an aqueous suspension in a tubular reactor and the soluble titania precursor is hydrolyzed by adjusting the pH of the suspension exiting from the tube to cause precipitation of the titania precursor. A process according to Claim 27, where the pH is adjusted to be in the range of 5 - 12. A process for the selective removal of nitrogen oxides from exhaust and waste gases, characterized by contacting the exhaust or waste gases with ammonia at a temperature of from about 150 °C to about 500 °C in the presence of a catalyst according to Claims 1 to 12.

Inventors: BEECKMAN JEAN WILLEM, HEGEDUS LOUIS, PAN WIE-HIN, SOLAR JEFFREY PAUL, BEECKMAN, JEAN WILLEM, HEGEDUS, LOUIS, PAN, WIE-HIN, SOLAR, JEFFREY PAUL
Applicants: GRACE W R & CO, W.R. GRACE & CO.-CONN.
Application year: 1989
Main classification: B01D53
IPC codes: B01J21, B01D53, B01J23, B01J35