Catalysts for cleaning exhaust gases

Abstract: The catalyst for cleaning exhaust gases according to the present invention comprises a first layer catalyst having as an active metal on an elemental support at least one noble metal selected from the group consisting of platinum, rhodium and palladium on which a second layer catalyst having iridium as an active metal is provided as a overlayer.

Description: Field of the invention and related art statement The present invention relates to a catalyst for cleaning exhaust gases containing nitrogen oxides (hereinafter abbreviated as NOx), carbon monoxide (CO) and hydrocarbons (HC). In the automobile exhaust gas disposal, NOx is generally cleaned within a very limited extent around logical air/fuel ratio by a catalyst while utilizing CO and HC contained in the exhaust gas. However, in cases of exhaust gas disposal of lean burn gasoline engines and diesel engines, denitration is prevented completely when using a conventional catalyst because of an excessively large amount of oxygen in the exhaust gas. Recently, crystalline silicate catalysts containing cobalt or copper are proposed as the catalysts capable of cleaning NOx even in an excessive oxygen atmosphere. However, these catalysts involve the problems of deterioration due to heat, water vapor and sulfur dioxide in an exhaust gas, although they exhibit satisfactory performance at the initial stage of the reaction. We have already found after making much effort to obtain a catalyst capable of overcoming the problems mentioned above that a catalyst on which iridium is supported is a highly active and durable catalyst. Further information regarding a catalyst on which iridium is supported is explained in Japanese patent application No. 7667/1994 published on October 25, 1994 under No. 296870/1994, Japanese patent application No. 8193/1994 published on April 4, 1995 under No. 88378/1995, Japanese patent application No. 228382/1993 published on March 28, 1995 under No. 80315/1995 and Japanese patent application No. 287986/1993 published on May 30, 1995 under No. 136463/1995. Nevertheless, the catalyst supporting iridium provides a denitration rate which is not sufficient when applied to a practical car driving because of the limited cleaning ability against CO and HC at a low exhaust gas temperature of 300°C or lower. JP-A-4074534 describes a catalyst in accordance with the preamble of claim 1. JP-A-5277368 describes a catalyst having a first layer and a second layer one of which contains iridium and the other rhodium, and a third layer composed of cerium oxide, alumina and palladium. EP-A-0 485 180 describes a first layer having at least one metal selected from Pt, Pd, Rh, Ir and Ru and a second layer formed on the first layer, the second layer containing at least one metal selected from Pt, Pd, Rh, Ir and Ru. Object and summary of the invention An objective of the present invention is to provide a catalyst for cleaning exhaust gases which exhibits denitration performance not only at a high temperature but also at a temperature as low as 300°C or lower. An aspect of the present invention is a catalyst according to claim 1. The catalyst for cleaning exhaust gases according to the present invention is a highly durable and stable catalyst, which can be utilized as an exhaust gas cleaning catalyst for a lean burn gasoline engine as well as a diesel engine. Detailed description of preferred embodiments A carrier for carrying iridium as an active metal in the second layer catalyst according to the present invention is a crystalline silicate which has a X-ray diffraction pattern shown in Table A in the specification and which is represented by the formula in the molar ratio as dehydrated: (1±0.8)R 2 O• [aM 2 • 3 •bM'O•cAl 2 O 3 ]•ySiO 2 wherein R denotes an alkaline metal ion and/or hydrogen ion, M denotes at least one elemental ion selected from the group consisting of VIII group elements, rare earth element, titanium, vanadium, chromium, niobium, antimony or gallium, M' denotes an alkaline earth metal ion of magnesium, calcium, strontium or barium, a>0, 20>b≥0, a+c=1, 3000>y>11. Lattice spacing (interstitial distance) (d value) Relative strength 11.2 ± 0.3 VS 10.0 ± 0.3 VS 6.7 ± 0.2 W 6.4 ± 0.2 M 6.0 ± 0.2 M 5.7 ± 0.2 W 5.6 ± 0.2 M 4.6 ± 0.1 W 4.25 ± 0.1 M 3.85 ± 0.1 VS 3.75 ± 0.1 S 3.65 ± 0.1 S 3.3 ± 0.1 M 3.05 ± 0.1 W 3.0 ± 0.1 M VS: Very strong S: Strong M: Medium W: Weak (X-ray source: Cu Kα) A carrier for carrying as the first layer catalyst of an active metal, which is at least one noble metal selected from the group consisting of platinum, rhodium and palladium, according to the present invention may be any of those which can be used as a carrier for a standard catalyst, and γ-alumina is generally preferred. The thickness of the coating of the first layer catalyst on the elemental support is within the range from 1 to 300 µm, preferably 5 to 100 µm, and that of the second layer catalyst is within the range from 1 to 300 µm, preferably 3 to 200 µm. At least one noble metal selected as the first layer catalyst from the group consisting of platinum, rhodium, and palladium and iridium as the second layer catalyst are supported on respective carriers preferably by means of an ion exchange method or an impregnation method, and the noble metal of the first layer catalyst is coated in an amount of 0.05 to 5 g and iridium of the second layer catalyst is coated in an amount of 0.03 to 10 g, both per 1 L of the elemental support (generally a coagulate support but any heat-resistant ceramics being acceptable). In the catalyst according to the present invention, for the purpose of achieving the activity covering a wider range of low temperature, the catalyst layer carrying at least one noble metal selected from the group consisting of platinum, rhodium and palladium is provided beneath the layer carrying iridium. Thus, the first layer catalyst having as an active metal at least one noble metal selected from the group consisting of platinum, rhodium and pallatium is provided on the elemental support, and then the second layer catalyst having iridium as an active metal is provided as a overlayer to form a multilayer catalyst. When applied to an exhaust gas at a temperature of about 250°C or higher, no denitration can be effected only by the second layer iridium-carrying catalyst because of no burning of CO or HC. However, in the case where the catalyst carrying at least one noble metal selected from the group consisting of platinum, rhodium and palladium is provided as the first layer catalyst, sufficient burning of CO and HC at 250°C is effected, resulting in the increase in the temperature of the second layer iridium-carrying catalyst up to the active temperature range, whereby exhibiting the denitration performance. In addition, the denitration performance can be obtained continuously from the exhaust gas temperature ranging from 200°C to a higher temperature since platinum rhodium and palladium can exhibit denitration activity in association with the burning of CO and HC at a temperature as low as 200 to 300°C (see Japanese patent application No. 230700/1992 published on March 15, 1994 under No. 71181/1994). On the other hand, at a high exhaust gas temperature of 400°C or higher, the performance of the second layer iridium-carrying catalyst becomes predominant, and the first layer catalyst scarcely affects the reaction, whereby keeping the high performance. The exhaust gas containing NOx, CO and HC is cleaned by a catalyst which carries iridium as the second layer catalyst in the reactions represented by the formulae shown below. In the reactions shown above, formula (1) shows the activation of HC, formula (2) shows the burning of HC, formula (3) shows the denitration, and formula (4) shows the burning of CO. In the following examples, honeycomb catalysts 16-28 are not examples of the invention but only provide technical information. Example 1 * Preparation of catalyst 5616 g of water glass #1 (SiO 2 : 30%) was dissolved in 5429 g of water to yield solution A. Separately, 718.9 g of aluminum sulfate, 110 g of ferric chloride, 47.2 g of calcium acetate, 262 g of sodium chloride and 2020 g of concentrated hydrochloric acid were dissolved together in 4175 g of water to yield solution B. Solution A and solution B were fed in a certain ratio (e.g. a ratio whereby the solutions A and B are fed so as to keep the pH as close as possible to 8; in this example solution A being fed at 368g/minute and solution B at 244g/minute) to form a precipitate, and the mixture was stirred thoroughly to yield a slurry at pH 8.0. The slurry thus obtained was charged in a 20 L autoclave, to which 500 g of tetrapropyl ammonium bromide was added and the mixture was subjected to hydrothermal synthesis at 160°C for 72 hours. After synthesis, washing with water, drying and sintering for 3 hours at 500°C, crystalline silicate 1 was obtained. Crystalline silicate 1 thus obtained is represented in a molar ratio (excluding crystal water) by the formula shown below and has the crystal structure by X-ray diffraction pattern shown in Table A. 0.5Na 2 O•0.5H 2 O•[0.8Al 2 O 3 •0.2Fe 2 O 3 •0.25CaO]•25SiO 2 Crystalline silicate 1 obtained above was subjected to NH 4 ion exchange by stirring with 4N aqueous solution of NH 4 Cl at 40°C for 3 hours. After the ion exchange, the silicate was washed and dried at 100°C for 24 hours and sintered at 400°C for 3 hours to obtain crystalline silicate 1 of type H (i.e. a silicate of the type which has protons (H + ) on its surface). 100 g of γ-alumina was immersed in aqueous solution of chloro platinic acid (H 2 PtCl 6 •6H 2 O: 2.66 g/100 cc) and kept at 120°C to evaporate to dryness, and then purged with nitrogen for 3 hours at 500°C to yield powder catalyst 1. On the other hand, 100 g of crystalline silicate 1 was immersed in chloroiridic acid (H 2 IrCl 6 : 2.1 g/100 cc) and kept at 120°C to evaporate to dryness, and then purged with nitrogen for 3 hours at 500°C to yield powder catalyst 1'. * Preparation of honeycomb catalyst Then, to 100 parts of each of powder catalysts 1 and 1' respectively, 3 parts of alumina sol and 55 parts of silica sol (20% SiO 2 ) as binders and 200 parts of water were added, and each mixture was stirred thoroughly to yield slurries for wash coats. One slurry thus contains 100 parts of powder catalyst 1 for the first layer catalyst and the other slurry contains 100 parts of powder catalyst 1' for the second layer catalyst. Then a monolith support for coagelate (400-cell lattice/inch 2 ) was immersed in the slurry of powder catalyst 1, and taken out. After blowing excessive slurry off, the support was dried at 200°C. By this procedure 100 g/L of powder catalyst 1 (first layer catalyst) was coated on the substrate. Then the support coating with powder catalyst 1 was immersed in the slurry of powder catalyst 1', and taken out. After blowing excessive slurry off, the support was dried at 200°C. By this procedure 100 g/L of powder catalyst 1' (second layer catalyst) was coated. The catalyst thus obtained was designated as honeycomb catalyst 1. Example 2 * Preparation of catalyst Except for adding cobalt chloride, ruthenium chloride, rhodium chloride, lanthanum chloride, cerium chloride, titanium chloride, vanadium chloride, chromium chloride, antimony chloride, gallium chloride and niobium chloride in the molar amount as an oxide similar to that of Fe 2 O 3 instead of ferric chloride used in the synthesis of crystalline silicate 1 in Example 1, the procedure similar to that employed for crystalline silicate 1 was conducted to obtain crystalline silicates 2 to 12. The crystal structures of these crystalline silicates as X-ray diffraction patterns were shown above in Table A, with the compositions being represented by the following formula in molar ratios of the oxides (dehydrated forms): 0.5Na 2 O•0.5H 2 O•(0.2M 2 O 3 •0.8Al 2 O 3 • 0.25CaO)•25SiO 2 , wherein M denotes Co, Ru, Rh, La, Ce, Ti, V, Cr, Sb, Ga or Nb. Except for adding magnesium acetate, strontium acetate and barium acetate each in the molar amount as an oxide similar to that of CaO instead of calcium acetate used in the synthesis of crystalline silicate 1 in Example 1, the procedure similar to that employed for crystalline silicate 1 was conducted to obtain crystalline silicates 13 to 15. The crystal structures of these crystalline silicates as X-ray diffraction patterns were shown above in Table A, with the compositions being represented by the following formula in molar ratios of the oxides (dehydrated forms): 0.5Na 2 O•0.5H 2 O•(0.2Fe 2 O 3 •0.8Al 2 O 3 •0.25MeO)•25SiO 2 , wherein Me denotes Mg, Sr or Ba. Using crystalline silicates 2 to 15 and the procedure similar to that employed in Example 1, the silicates of type H were obtained and then immersed in the aqueous solution of chloroiridic acid to obtain powder catalysts 2' to 15'. * Preparation of honeycomb catalysts Each of powder catalysts 2' to 15' was processed similarly as in Example 1 to form the slurry, which was used as the second layer catalyst and coated instead of the slurry of powder catalyst 1' at the rate of 100 g/L onto the surface of the support which had already been coated with powder catalyst 1. The catalysts thus obtained were designated as honeycomb catalysts 2 to 15. Example 3 γ-Al 2 O 3 , -Al 2 O 3 , SiO 2 , ZrO 2 , TiO 2 , Al 2 O 3 •ZrO 2 (1:1 by weight), ZrO 2 •TiO 2 (1:1 by weight), Al 2 O 3 •TiO 2 (1:1 by weight), SO 4 /ZrO 2 , zeolite of type Y, silicalite, zeolite of type A, and mordenite instead of crystalline silicate 1 used in Example 1 were immersed in and impregnated with the aqueous solution of chloroiridic acid solution similarly as in Example 1 to obtain powder catalysts 16' to 28'. Then, each of powder catalysts 16' to 28' was processed similarly as in Example 1 to form the slurry, which was used as the second layer catalyst and coated instead of the slurry of powder catalyst 1' at the rate of 100 g/L onto the surface of the support which had already been coated with powder catalyst 1. The catalysts thus obtained were designated as honeycomb catalysts 16 to 28. Example 4 Instead of the aqueous solution of chloroplatinic acid used in the preparation of powder catalyst 1 in Example 1, 100 g of γ-alumina was immersed in each of aqueous solution of rhodium chloride (RhCl 3 • 3H 2 O: 1.45 g/100 cc), aqueous solution of palladium chloride (PdCl 2 •2H 2 O: 1.10 g/100 cc), mixed aqueous solution of chloroplatinic acid•rhodium chloride (H 2 PtCl 6 • 6H 2 O : 1.33 g + RhCl 3 •3H 2 O: 0.73 g/100 cc), mixed aqueous solution of chloroplatinic acid•palladium chloride (H 2 PtCl 6 •6H 2 O: 1.33 g + PdCl 2 •2H 2 O: 0.55 g/100 cc) and mixed aqueous solution of chloroplatinic acid•rhodium chloride•palladium chloride (H 2 PtCl 6 •6H 2 O: 0.89 g + RhCl 3 •3H 2 O: 0.48 g + PdCl 2 •2H 2 O: 0.37 g/100 cc), kept at 120°C, evaporated to dryness, and then purged with nitrogen for 3 hours at 500°C to obtain powder catalysts 2 to 6 Then, each of powder catalysts 2 to 6 was processed similarly as in Example 1 to form the slurry, which was used as the first layer catalyst and coated instead of powder catalyst 1 at the rate of 100 g/L onto the surface of the support similar to that employed in Example 1, and then powder catalyst 1' was coated as an overlayer similarly as in Example 1. The catalysts thus obtained were designated as honeycomb catalysts 29 to 33. Example 5 Crystalline silicate 1 of type H in Example 1 was employed instead of γ-alumina for powder catalyst 1 in Example 1 and immersed in the aqueous solution of chloroplatinic acid similarly as in Example 1 to obtain powder catalyst 7. Powder catalyst 7 as the first layer catalyst and powder catalyst 1' as the second layer catalyst were processed similarly as in Example 1 to obtain honeycomb catalyst 34. Comparative example 1 Only powder catalyst 1 was coated onto a honeycomb elemental support at the rate of 200 g/L similarly as in Example 1 to obtain honeycomb catalyst 35. Compositions of the inventive and comparative catalysts described above are shown in Table B. Honeycomb catalysts 1 to 35 obtained in Examples 1, 2, 3, 4 and 5 and Comparative 1 were examined for the activity. The results are shown below. * Gas composition NO: 500 ppm; CO: 1000 ppm, C 2 H 4 : 1500 ppm, O 2 : 8%, CO 2 : 10%, H 2 O: 10%, Balance: N 2 , GHSV: 30000 h -1 , Catalyst dimension: 15 mm x 15 mm x 60 mm (144 cells) The denitration rates of the catalysts in the initial state at the inlet gas temperatures of 250°C and 350°C are shown in Table C. Honeycomb catalyst Inlet exhaust gas temperature: 250°C Inlet exhaust gas temperature: 350°C Catalyst layer temperature (°C) NOx decomposition rate (%) Catalyst layer temperature (°C) NOx decomposition rate (%) 1 320 68 420 54 2 315 65 418 58 3 320 68 424 61 4 315 64 423 60 5 320 65 422 63 6 315 62 423 61 7 317 61 422 60 8 320 63 425 59 9 315 65 430 58 10 320 67 419 60 11 317 62 418 59 12 315 60 416 61 13 317 61 417 59 14 314 63 415 60 15 315 65 414 61 16 310 60 415 60 17 309 61 414 58 18 312 63 413 60 19 314 59 412 58 20 315 61 415 57 21 316 63 416 60 22 319 65 418 54 23 318 64 416 56 24 320 67 417 58 25 322 68 418 54 26 328 65 420 56 27 319 65 418 54 28 318 66 420 52 29 317 51 423 54 30 314 48 430 51 31 315 49 423 54 32 320 47 420 56 33 321 49 418 54 34 315 65 413 60 35 250 1 419 54 Honeycomb catalysts 1 to 15 and 29 to 34 according to the present invention shown in Table C had a sufficient denitration activity even at a low gas temperature of approximately 250°C, indicating that they can keep a high denitration activity in a wide range of temperature. In contrast, a comparative catalyst 35 did not show an effective denitration activity at a low temperature of approximately 250°C. Honeycomb catalysts 1 to 15 and 29 to 34 according to the present invention had a stable activity in a reductive atmosphere also at a high temperature of approximately 700°C, thus being ensured to be highly durable.

Claims: A catalyst for cleaning exhaust gases comprising a support a first layer catalyst on said support having as an active metal at least one noble metal selected from the group consisting of platinum, rhodium and palladium on which first layer a second layer catalyst having iridium as an active metal is provided as a overlayer, characterised in that the carrier for carrying iridium as an active metal in the second layer catalyst is a crystalline silicate which is represented by the formula in the molar ratio as dehydrated: (1±0.8)R 2 O•[aM 2 O 3 •bM'O•cAl 2 O 3 ]•ySiO 2 wherein R denotes an alkaline metal ion and/or hydrogen ion, M denotes at least one elemental ion selected from the group consisting of VIII group elements, rare earth element, titanium, vanadium, chromium, niobium, antimony or gallium, M' denotes an alkaline earth metal ion of magnesium, calcium, strontium or barium, a>0, 20>b≥0, a+c=1, 3000>y>11 and which has a X-ray diffraction pattern shown in Table A Lattice spacing (interstitial distance) (d value) Relative strength 11.2 ± 0.3 VS 10.0 ± 0.3 VS 6.7 ± 0.2 W 6.4 ± 0.2 M 6.0 ± 0.2 M 5.7 ± 0.2 W 5.6 ± 0.2 M 4.6 ± 0.1 W 4.25 ± 0.1 M 3.85 ± 0.1 VS 3.75 ± 0.1 S 3.65 ± 0.1 S 3.3 ± 0.1 M 3.05 ± 0.1 W 3.0 ± 0.1 M VS: Very strong S: Strong M: Medium W: Weak X-ray source: Cu Kα.

Inventors: IIDA KOUZO, NOJIMA SHIGERU, IIDA, KOUZO, NOJIMA, SHIGERU, Iida, Kouzo, c/o Hiroshima R&amp;D Ctr., Mitsubishi, Nojima, Shigeru, Hiroshima R&amp;D Ctr., Mitsubishi
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1995
Main classification: B01D53
IPC codes: B01D53, B01J29, F02B1, B01J35, B01J37, B01J23, B01J27