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WO1994016817A1 - Catalyst of fibrous material for purification of exhaust gases from e.g. vehicles and method of making it - Google Patents

Catalyst of fibrous material for purification of exhaust gases from e.g. vehicles and method of making it Download PDF

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Publication number
WO1994016817A1
WO1994016817A1 PCT/SE1993/001035 SE9301035W WO9416817A1 WO 1994016817 A1 WO1994016817 A1 WO 1994016817A1 SE 9301035 W SE9301035 W SE 9301035W WO 9416817 A1 WO9416817 A1 WO 9416817A1
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WO
WIPO (PCT)
Prior art keywords
fibres
catalyst
particles
active material
process according
Prior art date
Application number
PCT/SE1993/001035
Other languages
French (fr)
Inventor
Jan-Erik Otterstedt
Anders TÖRNCRONA
Lars LÖWENDAHL
Per Johan Sterte
Original Assignee
Otterstedt Jan Erik
Toerncrona Anders
Loewendahl Lars
Per Johan Sterte
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Otterstedt Jan Erik, Toerncrona Anders, Loewendahl Lars, Per Johan Sterte filed Critical Otterstedt Jan Erik
Priority to AU58684/94A priority Critical patent/AU5868494A/en
Publication of WO1994016817A1 publication Critical patent/WO1994016817A1/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/50—Catalysts, in general, characterised by their form or physical properties characterised by their shape or configuration
    • B01J35/58—Fabrics or filaments
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/024—Multiple impregnation or coating
    • B01J37/0244—Coatings comprising several layers

Definitions

  • the present invention relates to a catalyst, preferably one which is used for purifying exhaust gases from vehicles, and a process for the production of such catalysts.
  • Catalysts for purifying exhaust gases from, for instance, vehicles are known earlier. Especially diesel driven vehicles need such catalysts since exhaust gases from such vehicles contain chemical compounds such as hydrocarbons, nitrogen oxides, sulphur oxides and solid particles of different types in an unacceptable amount for the environ ⁇ ment. These compounds create damage in the nature and are serious health risks for humans.
  • the nitrogen and sulphur oxides cause acidifying of soil and lakes and a part of the hydrocarbons which are found in the diesel exhaust gases are also cancer creating. Additionally, the hydrocarbons in combination with nitrogen oxides in the presence of sunlight will create ozone near the soil surface, attacking the vegetation.
  • Particles traps exist in the shape of monolithes (honeycomb structure) into which the exhaust gases are forced through the porous walls so that the particles are caught in the walls. After a period of time the filter will be clogged and it must then be demounted and regenerated. This is done by blowing hot air through the filter and thereby burning away the trapped carbon particles. To lower the ignition temperature of the carbon particles, the monolith is covered with oxidation catalysts. A manometer controls the pressure fall through the filter and indicates when it is time for regenerating.
  • the carrier structure is of a monolithic type.
  • the mono ⁇ liths have a porous cover of Al-,0, on the surface. This cover is a carrier for oxidation catalysts.
  • the filter in which most of the particles are caught are regenerated in situ during operation.
  • diesel oil is injected directly into the exhaust gases.
  • the diesel oil is immediately gasified and quickly reaches the catalytically active sites where it is oxidized.
  • the oxidation which is strongly exothermic, raises the temperature of the exhaust gases so much that the carbon particles trapped in the filter are burnt. Hereby the pressure fall through the catalyst is lowered and the diesel injection is terminated.
  • fibres are used as a direct carrier for different oxidation catalysts in the form of metal oxides.
  • Si0 2 or A1 3 0, soles are used as binders.
  • the fibres are formed into sheets and dipped into a slurry consisting of binder and catalytically active substance. The sheets are finally calcined.
  • the catalyst systems mentioned above all have some effect but they have not reached the level of limits which will exist in the future.
  • One problem is that for instance the particles of monoliths have shown to function unsatis- factory due to clogging.
  • Another problem with the known types of catalysts is that the exhaust gases which are to be purified must penetrate deeply into the porous structure to reach the catalytically active sites.
  • the porous layer has a thickness of 5000-25000 nm which is a reason that the catalytically active sites in the under parts of the porous layer are used insufficiently.
  • One object of the present invention is therefore to solve the problems with the known catalysts and to provide a catalyst in which the catalytically active materials are used completely, which has the desired effect and which can be regenerated continuously in situ in a simple and effective way.
  • a catalyst has for this purpose been provided which comprises fibres carrying cata- lytically active material, which catalyst is characterized in that the fibres have been provided with a porous surface structure which provides a support for the catalytically active material.
  • the fibres consist of quartz fibres, aluminium oxide fibres, aluminium silicate fibres, zirconium dioxide fibres or silicium carbide fibres.
  • the porous surface structure according to the invention is built up by small particles of silicium dioxide, aluminium oxide, aluminium silicate, zirconium dioxide or titanium dioxide.
  • the catalytically active material can, according to the invention, consist of noble metals such as platinum, palladium, rhodium, rhenium, indium and ruthenium or oxides of transition metals such as copper, cobalt, silver, lanthanum, cerium, iron, chrome, molybdenum and manganese which possibly can be doped with the noble metals mentioned above.
  • noble metals such as platinum, palladium, rhodium, rhenium, indium and ruthenium or oxides of transition metals such as copper, cobalt, silver, lanthanum, cerium, iron, chrome, molybdenum and manganese which possibly can be doped with the noble metals mentioned above.
  • the invention also comprises a process for the production of the catalysts above, which process is characterized in that possibly pretreated and recharged fibres are covered with particles which are less than 500 nm, suitably 5-100 nm from soles in preferably several layers whereupon they are fixed and finally covered with catalytically active material .
  • the fibres should, according to the invention, be pre ⁇ treated by washing with solvents for organic compounds or burning of possible organic substances and autoclaving at elevated temperature.
  • the fibres may be recharged by means of a diluted solution of a cationic polymer or basic aluminium chloride.
  • the particles in the sole can be ion-exchanged, suitably with a cation exchanger, for removal of sodium or potassium ions.
  • the par- tides are deposited on the surface of the fibres in alternative positive and negative layers, possibly by means of recharging.
  • the fixing occurs according to the invention by heating the covered fibres in an oven under addition of water.
  • the catalytically active material is, according to the invention, covered by either impregnating of salt solu ⁇ tions with subsequent elution of the solvent and possible oxidation or reduction of the ions or by ion exchanging.
  • the fibres which are used according to the invention consist of fire-proof inorganic materials such as Si0 2 , A1 2 0 3 , aluminium silicate, Zr0 2 and SiC.
  • the diameter of the different fibres is preferably less than 20 ⁇ m.
  • they may be treated by solvents (benzine acetone) .
  • solvents benzine acetone
  • Another method to make the fibres more receptive is first to burn off the organic impurities and thereafter autoclave the fibres in an autoclave at elevated temperature.
  • the autoclave liquid may comprise a strongly diluted sole with a small particle size. A thin layer of diluted sole particles will then be deposited on the fibre surface during autoclaving.
  • the fibre surface receptacle To make the fibre surface receptacle for charged particles, it must have a charge which is opposite to the one of the charged particles. If these charges are the same the fibres must be recharged. If negatively charged particles are to be covered on the fibre surface, the fibres can be recharged by laying them in a diluted solution of a cationic polymer.
  • the content of polymer may be 0,01% to 1%.
  • the pH value for the recharging can be chosen with respect primarily to the surface chemistry of the fibre, though also to the polymer.
  • Cationic polymers have, however, most often a wide pH interval within which they can be used.
  • the repeating units in such polymers may consist of tertiary amines having some hydroxyl group in the main chain.
  • Berocell 6100 which is produced by Berol Nobel AB, Sweden.
  • Another recharger which can be used is basic aluminium chloride. This contains positive Al ⁇ 7+ complexes at pH ⁇ 4.
  • the particles which are to be used as a part of the catalyst must have good hydrothermal stability. By this it is meant that they shall have good stability at a high temperature and high humidity. This can be obtained if the sole with the particles are ion-exchanged several times with a cation exchanger. At the ion-exchanging counter ions of such types as Na+ and K+ are removed which give the particles a low hydrothermal stability. These can be replaced by other cations, for example NH 4 +, which give a better hydrothermal stability.
  • the sole particles must be able to be recharged. This can be done by slowly adding a sole to a diluted solution of rechargers.
  • rechargers cationic polymers for basic aluminium chloride may for instance be used.
  • An alternative to recharging can also be to use two different soles having opposite charges.
  • the sole particles can be covered on the fibres by means of different methods. Such a method is to pour through diluted soles having alternatively positive and negative particles. The particles are then recharged as mentioned above.
  • the fixing can be done by heating the covered fibres in an oven to which water is added. The atmosphere in the oven will then partly consist of water vapour. This will result in that the porous structure is sintered to the fibre surface, provided that the temperature is sufficiently high in the oven.
  • Another effect of the hydrothermal treatment is that the smallest pores in the porous structure disappear. This is positive as in this way the burying of expensive catalytically active substances in the small pores is avoided.
  • the catalytically active noble metals can be covered on the porous structure by direct impregnation or ion exchanging.
  • Direct impregnation means that the pores of the porous structure are filled with a solution of a salt.
  • a salt is chlorides, nitrates and ammonium complexes of Pt, Pd, Rh, Re, In and Ru.
  • the solvent is eluated by drying.
  • the desired catalytically active noble metals are thereafter obtained by oxidation and reduction reactions respectively.
  • Depositing of catalytically active noble metals by ion exchanging means that the pH dependent surface charge of the porous structure is used. At a pH below the zero charge point of the porous structure, it will be positively charged and can adsorb negatively charged noble metal complexes. At a pH above the zero charge point of the porous structure, it will be negatively charged and can adsorb positively charged noble metal complexes. After the porous structure has been completed with positively or negatively charged noble metal complexes the material is dried. The catalytically active noble metal is thereafter formed by oxidation and reduction reactions respectively.
  • catalytically active metal oxides are to be covered on the porous structure, this is carried out by direct impreg ⁇ nation.
  • the pores in the porous structure are filled with a solution of a metal salt.
  • the solvent is eludated by drying and the catalytically active metal oxide is obtained after calcining the material.
  • the catalytically active metal oxides may be doped with noble metals to give them a higher activity in this way. This can be done by addition of a smaller amount of a noble metal solution to the metal salt solution which thereafter is directly impregnated in the porous structure. By oxidation and reduction reactions metal oxides and noble metals will be obtained in the desired shape.
  • the critical detail of the present invention is to bring about the porous surface on usually dense fibres. A typical surface manufactured via the invention was investigated with relation to the specific surface, pore area and the pore volume distribution of the fibres. These values are measured by physical adsorption-desorption of nitrogen (N 2 ) at 77°K.
  • BET Brunauer, Emmett, Teller
  • the appearance of the porous structure was investigated with TEM (Transmission Electron Microscope) .
  • TEM Transmission Electron Microscope
  • the difference in density between fibre and porous structure made it possible to separate these in spite of the fact that they can consist of the same material with relation to the elements.
  • the analyses were made with a TEM, JEOL.
  • the catalysts according to the present invention may be used to remove soot, carbon monoxide, nitrogen oxides and hydrocarbons from different kinds of exhaust gases. They are specially suitable to be used for cleaning diesel exhaust gases since they are a filter for soot particles at the same time as they catalyse the reactions, which results in that the molecular impurities are eliminated.
  • the catalysts can also advantageously be used to burn hydro- carbons in process gases from organic synthesis industry, mechanical industry and painting workshops. Exhaust gases containing large molecules of organic solvents are especi ⁇ ally adapted to be cleaned by catalysts according to the present invention as the pore structure of the catalyst allows for a quick and effective mass transport of large molecules to the active sites. Catalysts according to the present invention can also advantageously be used for purifying car exhaust gases.
  • composition of the catalysts can be chosen so that they are specially adapted for certain purposes.
  • platinum and rhodium should be chosen among the noble metals to be deposited on the porous surface of the fibre. If for the same purpose oxides of transition metals are used, these should be oxides of copper, cobalt, silver, lanthanum, cerium, iron, chrome, molybdenum and manganese. To improve the effect, these oxides can be doped by noble metals such as platinum, palladium, rhodium, rhenium, indium and ruthenium.
  • platinum and palladium should be chosen among the noble metals, or among oxides of the transition metals copper, cobalt, silver, lanthanum, cerium, iron, chrome, molybdenum and manganese. These oxides can suitably be doped with platinum, palladium, rhodium, rhenium, indium and ruthenium.
  • the autoclave was of stainless steel and was covered with Teflon on the inner side.
  • the liquid in the autoclave consisted of 3g ion exchanged Ludox ⁇ M (DuPont, d-7nm) , pH 9.
  • the ion exchanger which was used was Dowex HCRS(E) (Dow Chemical) .
  • the remainder of the liquid consisted of distilled water. The autoclaving took place at 200°C, 15-16h. The fibres were washed with 250g distilled water.
  • the fibres were thereafter put into a column and 50 ml 0,2% Si0 2 cationised ammonium stabilized 22 nm silisic acid sole was allowed to flow through at a speed of 5 cm/ in.
  • the sole was cationised at pH 7 with Berocell 6100 in an amount corresponding to 2*10 "3 g polymer/m 2 (Si0 2 ) .
  • 50 ml of distilled water rinsed away the sole excess.
  • the next sole layer consisted of 0,2% Si0 2 ammonium stabilized 22 nm silisic acid sole, anionic, at pH 7. 50 ml thereof was allowed to flow through at a speed of 5 cm/min. The sole excess was rinsed away with 50 ml distilled water.
  • the fibres were covered 4 times with 50 ml 0,2% Si0 2 sole.
  • the fibres were taken out of the column and dried at 120°C, 2h.
  • the fibres were hydrother ally treated at 750°C, 24h.
  • the specific surface before the hydrothermal treatment was measured to 40 m 2 /g and to 30 m/g thereafter.
  • the pore- size distribution for the material before and after the hydrothermal treatment appears from the diagrams of the figures 1-4.
  • Figs. 1 and 2 show the distribution of pore-volumes in the porous layer.
  • the pore-volume is indicated as a function of pore diameter measured in Angstrom.
  • Fig. 1 shows the pore- volume distribution before the hydrothermal treatment and Fig. 2 shows the same after the hydrothermal treatment.
  • Figs. 3 and 4 show the pore-area distribution. In these diagrams the pore-area is indicated as a function of the pore-diameter measured in Angstrom.
  • Fig. 3 shows the pore- area distribution before the hydrothermal treatment and Fig. 4 shows the same after the hydrothermal treatment.
  • the cationised sole had 1,2-l ⁇ 'g Al 2 0,/m 2 (Si0 2 ) .
  • the soles were added alternatively cationic/anionic through the column at a speed of 5 cm/min having a washing step in between consisting of 50 ml distilled water pH adjusted to pH 4.
  • the fibres were dried at 120°C, 2h and finally hydrothermally treated at 750°C, 24h.
  • the fibres were covered 4 times with 50 ml 0,2% Si0 2 silisic acid sole.
  • the fibres were taken out of the column and dried at 120°C, 2h.
  • Finally the fibres were hydrothermally treated at 750°C, 24h.
  • Quartz fibres were pretreated according to example 1. lg fibres was put into a polypropene receptacle containing 100 ml 2% Al , rod-shaped b ⁇ hmite at pH 4. The clean rod-shaped b ⁇ hmite had a specific surface approximately 170 m/g. The fibres were washed twice with 100 ml distilled water, pH adjusted to pH 4. The fibres were dried at 120°C for 2 h. Finally the material was hydrothermally treated at 814°C, 1,5 h, whereby the b ⁇ hmite went through a solid phase reaction to A1 2 0,. By the hydrothermal treatment the specific surface of the porous structure was lowered to approximately 120 ⁇ r/g.
  • Catalyst carrier material according to Examples 1-5 was covered with noble metals through direct impregnation.
  • the impregnation was made at pH 7 with a solution containing Pt(II) (NH,) 4 2"1 .
  • the material was dried at 110°C, lh.
  • There ⁇ after the impregnated catalyst was treated in a tube reactor.
  • a hot air stream was blown through the tube reactor at a speed of 500 ml/min, 500°C for 40 min.
  • the platinum oxide was reduced with H 2 , 450°C, 200 ml/min during 1 h.
  • the catalyst then contained approximately 1,7 mg Pt/g(kat) .
  • Catalyst carrier material according to the Examples 1-5 was covered with Ag 2 0.
  • the material was direct impregnated with a solution containing AgNO, at pH 7.
  • the material was dried at 100°C for l,5h.
  • the catalyst was thereafter heat-treated in an oven at 450°C, 2,5 h in an air atmosphere. Silver oxide was formed herethrough.
  • the catalyst contained approximately 58 mg Ag 2 0/g(kat) .
  • Catalyst carrier material according to the Examples 1-5 was covered with Ag 2 0 doped with Pt.
  • First Ag 2 0 was applied according to Example 7 and thereafter Pt according to Example 6.
  • the catalyst thereafter contained approximately 58 mg Ag 3 0/g(kat) and approximately 0,2 mg Pt/g(kat) .
  • the carrier has a very low transport resistance for reactants since the diffusion path is very short.
  • the reactants have direct access to a substantially greater number of active sites compared to mono ⁇ lithic structures since the frame here has a comparatively large specific surface.
  • the structure can simultaneously act as a filter for solid particles which can be continuously burnt off during operation.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Catalysts (AREA)
  • Exhaust Gas Treatment By Means Of Catalyst (AREA)

Abstract

The invention relates to a catalyst comprising fibres which carry catalytically active material and the invention is characterized in that the fibres have been provided with a porous structure which provides the support for the catalytic material. The invention also comprises a process for the production of the catalyst. It is characterized in that fibres are covered with small particles which are fixed and finally covered with catalytically active material.

Description

Catalyst of fibrous material for purification of exhaust gases from e.g. vehicles and method of making it .
TECHNICAL FIELD:
The present invention relates to a catalyst, preferably one which is used for purifying exhaust gases from vehicles, and a process for the production of such catalysts.
FIELD OF THE INVENTION:
Catalysts for purifying exhaust gases from, for instance, vehicles are known earlier. Especially diesel driven vehicles need such catalysts since exhaust gases from such vehicles contain chemical compounds such as hydrocarbons, nitrogen oxides, sulphur oxides and solid particles of different types in an unacceptable amount for the environ¬ ment. These compounds create damage in the nature and are serious health risks for humans. The nitrogen and sulphur oxides cause acidifying of soil and lakes and a part of the hydrocarbons which are found in the diesel exhaust gases are also cancer creating. Additionally, the hydrocarbons in combination with nitrogen oxides in the presence of sunlight will create ozone near the soil surface, attacking the vegetation.
The solid particles found in diesel exhaust gases have a tendency to adsorb poisonous chemical compounds on the surface. Since these particles are within the interval of size in which they are absorbed by the body, they are a serious health risk. Medical investigations have shown a connection between diesel exhaust gases and cancer. The authorities in western Europe, USA and Japan have in recent years started to increase demands with the aim to lower the waste from the diesel vehicles. Besides creating environmentally friendlier combustibles, cleaning equipment for the exhaust gases has also been developed which can be broadly divided into two categories, namely particle traps and continuous catalytic systems.
Particles traps exist in the shape of monolithes (honeycomb structure) into which the exhaust gases are forced through the porous walls so that the particles are caught in the walls. After a period of time the filter will be clogged and it must then be demounted and regenerated. This is done by blowing hot air through the filter and thereby burning away the trapped carbon particles. To lower the ignition temperature of the carbon particles, the monolith is covered with oxidation catalysts. A manometer controls the pressure fall through the filter and indicates when it is time for regenerating.
Another system comprises a filter part and a catalyst part. The carrier structure is of a monolithic type. The mono¬ liths have a porous cover of Al-,0, on the surface. This cover is a carrier for oxidation catalysts. The filter in which most of the particles are caught are regenerated in situ during operation. When the air pressure through the filter exceeds a certain value, diesel oil is injected directly into the exhaust gases. The diesel oil is immediately gasified and quickly reaches the catalytically active sites where it is oxidized. The oxidation, which is strongly exothermic, raises the temperature of the exhaust gases so much that the carbon particles trapped in the filter are burnt. Hereby the pressure fall through the catalyst is lowered and the diesel injection is terminated. A further system is described in European patent applica¬ tion 0 373 648. In this publication fibres are used as a direct carrier for different oxidation catalysts in the form of metal oxides. Si02 or A130, soles are used as binders. The fibres are formed into sheets and dipped into a slurry consisting of binder and catalytically active substance. The sheets are finally calcined.
A further system for purifying exhaust gases is described in US patent 4 346 557 in which a combination of fibre filter and a monolith is used. The burning of soot occurs principally in the same way as mentioned above.
TECHNICAL PROBLEM:
The catalyst systems mentioned above all have some effect but they have not reached the level of limits which will exist in the future. One problem is that for instance the particles of monoliths have shown to function unsatis- factory due to clogging. Another problem with the known types of catalysts is that the exhaust gases which are to be purified must penetrate deeply into the porous structure to reach the catalytically active sites. The porous layer has a thickness of 5000-25000 nm which is a reason that the catalytically active sites in the under parts of the porous layer are used insufficiently.
One object of the present invention is therefore to solve the problems with the known catalysts and to provide a catalyst in which the catalytically active materials are used completely, which has the desired effect and which can be regenerated continuously in situ in a simple and effective way. THE SOLUTION:
According to the present invention, a catalyst has for this purpose been provided which comprises fibres carrying cata- lytically active material, which catalyst is characterized in that the fibres have been provided with a porous surface structure which provides a support for the catalytically active material.
According to the invention it is advantageous that the fibres consist of quartz fibres, aluminium oxide fibres, aluminium silicate fibres, zirconium dioxide fibres or silicium carbide fibres.
The porous surface structure according to the invention is built up by small particles of silicium dioxide, aluminium oxide, aluminium silicate, zirconium dioxide or titanium dioxide.
The catalytically active material can, according to the invention, consist of noble metals such as platinum, palladium, rhodium, rhenium, indium and ruthenium or oxides of transition metals such as copper, cobalt, silver, lanthanum, cerium, iron, chrome, molybdenum and manganese which possibly can be doped with the noble metals mentioned above.
The invention also comprises a process for the production of the catalysts above, which process is characterized in that possibly pretreated and recharged fibres are covered with particles which are less than 500 nm, suitably 5-100 nm from soles in preferably several layers whereupon they are fixed and finally covered with catalytically active material . The fibres should, according to the invention, be pre¬ treated by washing with solvents for organic compounds or burning of possible organic substances and autoclaving at elevated temperature.
According to the process, the fibres may be recharged by means of a diluted solution of a cationic polymer or basic aluminium chloride.
The particles in the sole can be ion-exchanged, suitably with a cation exchanger, for removal of sodium or potassium ions.
According to the invention it is suitable that the par- tides are deposited on the surface of the fibres in alternative positive and negative layers, possibly by means of recharging.
The fixing occurs according to the invention by heating the covered fibres in an oven under addition of water.
Finally, the catalytically active material is, according to the invention, covered by either impregnating of salt solu¬ tions with subsequent elution of the solvent and possible oxidation or reduction of the ions or by ion exchanging.
DETAILED DESCRIPTION OF THE INVENTION:
The invention will be further described in the following in more detail.
The fibres which are used according to the invention consist of fire-proof inorganic materials such as Si02, A1203, aluminium silicate, Zr02 and SiC. The diameter of the different fibres is preferably less than 20 μm. To make the fibres more receptive for being covered by sole particles, they may be treated by solvents (benzine acetone) . Hereby the organic residues from the production are washed away from the fibre surface. Another method to make the fibres more receptive is first to burn off the organic impurities and thereafter autoclave the fibres in an autoclave at elevated temperature. The autoclave liquid may comprise a strongly diluted sole with a small particle size. A thin layer of diluted sole particles will then be deposited on the fibre surface during autoclaving.
To make the fibre surface receptacle for charged particles, it must have a charge which is opposite to the one of the charged particles. If these charges are the same the fibres must be recharged. If negatively charged particles are to be covered on the fibre surface, the fibres can be recharged by laying them in a diluted solution of a cationic polymer. The content of polymer may be 0,01% to 1%. The pH value for the recharging can be chosen with respect primarily to the surface chemistry of the fibre, though also to the polymer. Cationic polymers have, however, most often a wide pH interval within which they can be used. The repeating units in such polymers may consist of tertiary amines having some hydroxyl group in the main chain. An example of such polymers is Berocell 6100 which is produced by Berol Nobel AB, Sweden.
Another recharger which can be used is basic aluminium chloride. This contains positive Alπ 7+ complexes at pH~4.
The particles which are to be used as a part of the catalyst must have good hydrothermal stability. By this it is meant that they shall have good stability at a high temperature and high humidity. This can be obtained if the sole with the particles are ion-exchanged several times with a cation exchanger. At the ion-exchanging counter ions of such types as Na+ and K+ are removed which give the particles a low hydrothermal stability. These can be replaced by other cations, for example NH4+, which give a better hydrothermal stability.
As the particle's preferably are deposited on the fibre surface in alternative positive and negative layers, the sole particles must be able to be recharged. This can be done by slowly adding a sole to a diluted solution of rechargers. As rechargers cationic polymers for basic aluminium chloride may for instance be used. An alternative to recharging can also be to use two different soles having opposite charges.
The sole particles can be covered on the fibres by means of different methods. Such a method is to pour through diluted soles having alternatively positive and negative particles. The particles are then recharged as mentioned above.
It is important that the structure will not be worn away when the fibres are subjected to mechanical stress and therefore they must be fixed. The fixing can be done by heating the covered fibres in an oven to which water is added. The atmosphere in the oven will then partly consist of water vapour. This will result in that the porous structure is sintered to the fibre surface, provided that the temperature is sufficiently high in the oven. Another effect of the hydrothermal treatment is that the smallest pores in the porous structure disappear. This is positive as in this way the burying of expensive catalytically active substances in the small pores is avoided.
The catalytically active noble metals can be covered on the porous structure by direct impregnation or ion exchanging. Direct impregnation means that the pores of the porous structure are filled with a solution of a salt. Example of such salts are chlorides, nitrates and ammonium complexes of Pt, Pd, Rh, Re, In and Ru. The solvent is eluated by drying. The desired catalytically active noble metals are thereafter obtained by oxidation and reduction reactions respectively.
Depositing of catalytically active noble metals by ion exchanging means that the pH dependent surface charge of the porous structure is used. At a pH below the zero charge point of the porous structure, it will be positively charged and can adsorb negatively charged noble metal complexes. At a pH above the zero charge point of the porous structure, it will be negatively charged and can adsorb positively charged noble metal complexes. After the porous structure has been completed with positively or negatively charged noble metal complexes the material is dried. The catalytically active noble metal is thereafter formed by oxidation and reduction reactions respectively.
If catalytically active metal oxides are to be covered on the porous structure, this is carried out by direct impreg¬ nation. The pores in the porous structure are filled with a solution of a metal salt. The solvent is eludated by drying and the catalytically active metal oxide is obtained after calcining the material.
The catalytically active metal oxides may be doped with noble metals to give them a higher activity in this way. This can be done by addition of a smaller amount of a noble metal solution to the metal salt solution which thereafter is directly impregnated in the porous structure. By oxidation and reduction reactions metal oxides and noble metals will be obtained in the desired shape. The critical detail of the present invention is to bring about the porous surface on usually dense fibres. A typical surface manufactured via the invention was investigated with relation to the specific surface, pore area and the pore volume distribution of the fibres. These values are measured by physical adsorption-desorption of nitrogen (N2) at 77°K. To calculate the specific surface in m2/g, the so- called BET equation for the adsorption term was used. (BET = Brunauer, Emmett, Teller) . This equation is well known to one skilled in the art and will therefore not be explained further here. The pore area and the distribution of the pore volume was obtained from the desorption isotherm. The analyses were made in a Digisorb 2600 (Micromerics) .
The appearance of the porous structure was investigated with TEM (Transmission Electron Microscope) . The difference in density between fibre and porous structure made it possible to separate these in spite of the fact that they can consist of the same material with relation to the elements. The analyses were made with a TEM, JEOL.
The catalysts according to the present invention may be used to remove soot, carbon monoxide, nitrogen oxides and hydrocarbons from different kinds of exhaust gases. They are specially suitable to be used for cleaning diesel exhaust gases since they are a filter for soot particles at the same time as they catalyse the reactions, which results in that the molecular impurities are eliminated. The catalysts can also advantageously be used to burn hydro- carbons in process gases from organic synthesis industry, mechanical industry and painting workshops. Exhaust gases containing large molecules of organic solvents are especi¬ ally adapted to be cleaned by catalysts according to the present invention as the pore structure of the catalyst allows for a quick and effective mass transport of large molecules to the active sites. Catalysts according to the present invention can also advantageously be used for purifying car exhaust gases.
The composition of the catalysts can be chosen so that they are specially adapted for certain purposes.
If for example carbon monoxides, hydrocarbons, nitrogen oxides and soot particles in diesel exhaust gases are to be removed, platinum and rhodium should be chosen among the noble metals to be deposited on the porous surface of the fibre. If for the same purpose oxides of transition metals are used, these should be oxides of copper, cobalt, silver, lanthanum, cerium, iron, chrome, molybdenum and manganese. To improve the effect, these oxides can be doped by noble metals such as platinum, palladium, rhodium, rhenium, indium and ruthenium.
To burn organic solvents in air, platinum and palladium should be chosen among the noble metals, or among oxides of the transition metals copper, cobalt, silver, lanthanum, cerium, iron, chrome, molybdenum and manganese. These oxides can suitably be doped with platinum, palladium, rhodium, rhenium, indium and ruthenium.
The invention will be described further in the following with relation to the production of covering of fibres with porous layers and partly production of completed catalysts.
EXAMPLE 1:
Quartz fibres (Quartz & Silice) having d=2μm were burnt at 600°C, lh. 5g of quartz fibres were put into an autoclave containing 250g autoclave liquid. The autoclave was of stainless steel and was covered with Teflon on the inner side. The liquid in the autoclave consisted of 3g ion exchanged Ludox ΞM (DuPont, d-7nm) , pH 9. The ion exchanger which was used was Dowex HCRS(E) (Dow Chemical) . The remainder of the liquid consisted of distilled water. The autoclaving took place at 200°C, 15-16h. The fibres were washed with 250g distilled water.
lg of fibres was thereafter laid into a polypropylene receptacle containing 100 ml 0,4% cationic polymer, Berocell 6100 (Berol Nobel AB) , at pH 7. The excess of polymer solution was decanted off and the fibres washed twice with 100 ml distilled water. 100 ml of 2% Si02 ammonium-stabilized 22 nm silica sole, pH 7, original material from DuPont was added. The sole excess was decanted off and the fibres washed twice again with distilled water.
The fibres were thereafter put into a column and 50 ml 0,2% Si02 cationised ammonium stabilized 22 nm silisic acid sole was allowed to flow through at a speed of 5 cm/ in. The sole was cationised at pH 7 with Berocell 6100 in an amount corresponding to 2*10"3g polymer/m2(Si02) . 50 ml of distilled water rinsed away the sole excess. The next sole layer consisted of 0,2% Si02 ammonium stabilized 22 nm silisic acid sole, anionic, at pH 7. 50 ml thereof was allowed to flow through at a speed of 5 cm/min. The sole excess was rinsed away with 50 ml distilled water. In this example the fibres were covered 4 times with 50 ml 0,2% Si02 sole. The fibres were taken out of the column and dried at 120°C, 2h. Finally, the fibres were hydrother ally treated at 750°C, 24h. The specific surface before the hydrothermal treatment was measured to 40 m2/g and to 30 m/g thereafter. The pore- size distribution for the material before and after the hydrothermal treatment appears from the diagrams of the figures 1-4.
Figs. 1 and 2 show the distribution of pore-volumes in the porous layer. The pore-volume is indicated as a function of pore diameter measured in Angstrom. Fig. 1 shows the pore- volume distribution before the hydrothermal treatment and Fig. 2 shows the same after the hydrothermal treatment.
Figs. 3 and 4 show the pore-area distribution. In these diagrams the pore-area is indicated as a function of the pore-diameter measured in Angstrom. Fig. 3 shows the pore- area distribution before the hydrothermal treatment and Fig. 4 shows the same after the hydrothermal treatment.
EXAMPLE 2 :
lg of quartz fibres treated in an autoclave according to Example 1 was laid into 0,4% Al?(OH) SC1, pH 4, 100 ml. The excess was decanted off and the fibres washed twice with pH 4 adjusted distilled water, 100 ml. 2% Si02 ammonium stabilized 22 nm silisic acid sole, pH 4, 100 ml was added. The sole excess was thereafter washed twice with 100 ml distilled water, pH adjusted to 4. The fibres were put into a column and 50 ml 0,2% Si02 ammonium stabilized 22 nm silisic acid sole was poured through at a rate of 5 cm/min. The cationised sole had 1,2-lθ'g Al20,/m2(Si02) . The soles were added alternatively cationic/anionic through the column at a speed of 5 cm/min having a washing step in between consisting of 50 ml distilled water pH adjusted to pH 4. The fibres were dried at 120°C, 2h and finally hydrothermally treated at 750°C, 24h.
EXAMPLE 3
(S-Al , fibres (ICI, Great Britain) having a d=5μm were burnt at 600°C for lh. lg of fibres was put into a polypro- pen receptacle containing 100 ml 0,4% cationic polymer, Berocell 6100 (Berol Nobel AB, Sweden), at pH 8. The excess of polymer solution was decanted off and the fibres washed twice with 100 ml distilled water pH adjusted to pH 8. 100 ml 2% SiO? ammonium stabilized silisic acid sole, pH 8, was added. The silisic acid sole was ion exchanged with Ludox TM (DuPont, USA) where the sodium ions had been exchanged by ammonium ions. The sole excess was decanted off, the fibres washed twice with distilled water pH adjusted to pH 8. The fibres were thereafter put into a column and 50 ml 0,2% Si02 cationised 22 nm silisic acid sole was allowed to flow through at a rate of 5 cm/min. The sole was cationised at pH 8 with Berocell 6100 with an amount corresponding to 2-10"3g polymer/nr(SiO-,) .
50 ml distilled water, pH 8, rinsed away the sole excess. The next sole layer consisted of 0,2% Si02 ammonium stabi¬ lized 22 nm silisic acid sole, anionic, at pH 8. 50 ml thereof was allowed to flow through at a rate of 5 cm/min. The sole excess was rinsed off with 50 ml distilled water, pH 8.
In this example the fibres were covered 4 times with 50 ml 0,2% Si02 silisic acid sole. The fibres were taken out of the column and dried at 120°C, 2h. Finally the fibres were hydrothermally treated at 750°C, 24h.
EXAMPLE 4
Ceramic fibres (44% A120,, 54% Si02) from Karborundum, d=9 μm were burnt at 600°C for lh. lg of fibres as put into a polypropene receptacle containing 100 ml 0,4% cationic polymer, Berocell 6100, at pH 8. This example was carried out in the same way as Example 3.
EXAMPLE 5:
Quartz fibres were pretreated according to example 1. lg fibres was put into a polypropene receptacle containing 100 ml 2% Al , rod-shaped bόhmite at pH 4. The clean rod-shaped bδhmite had a specific surface approximately 170 m/g. The fibres were washed twice with 100 ml distilled water, pH adjusted to pH 4. The fibres were dried at 120°C for 2 h. Finally the material was hydrothermally treated at 814°C, 1,5 h, whereby the bδhmite went through a solid phase reaction to A120,. By the hydrothermal treatment the specific surface of the porous structure was lowered to approximately 120 πr/g.
EXAMPLE 6:
Catalyst carrier material according to Examples 1-5 was covered with noble metals through direct impregnation. The impregnation was made at pH 7 with a solution containing Pt(II) (NH,)4 2"1. The material was dried at 110°C, lh. There¬ after the impregnated catalyst was treated in a tube reactor. A hot air stream was blown through the tube reactor at a speed of 500 ml/min, 500°C for 40 min. Finally the platinum oxide was reduced with H2, 450°C, 200 ml/min during 1 h. The catalyst then contained approximately 1,7 mg Pt/g(kat) .
EXAMPLE 7:
Catalyst carrier material according to the Examples 1-5 was covered with Ag20. The material was direct impregnated with a solution containing AgNO, at pH 7. The material was dried at 100°C for l,5h. The catalyst was thereafter heat-treated in an oven at 450°C, 2,5 h in an air atmosphere. Silver oxide was formed herethrough. The catalyst contained approximately 58 mg Ag20/g(kat) .
EXAMPLE 8:
Catalyst carrier material according to the Examples 1-5 was covered with Ag20 doped with Pt. First Ag20 was applied according to Example 7 and thereafter Pt according to Example 6. The catalyst thereafter contained approximately 58 mg Ag30/g(kat) and approximately 0,2 mg Pt/g(kat) .
By means of the present invention, a catalyst which is superior to earlier known catalysts, especially those having monolithic structure, has been obtained. It has the following advantages compared to these:
1. The carrier has a very low transport resistance for reactants since the diffusion path is very short.
2. The reactants have direct access to a substantially greater number of active sites compared to mono¬ lithic structures since the frame here has a comparatively large specific surface.
3. The structure can simultaneously act as a filter for solid particles which can be continuously burnt off during operation.
The invention is not limited to the examples presented but can be modified in different ways within the scope of the claims.

Claims

CLAIMS:
1. A catalyst comprising fibres which carry catalyti¬ cally active material c h a r a c t e r i z e d i n that the fibres have been provided with a porous surface structure which provides a support for the catalytically active material.
2. Catalyst according to claim 1, c h a r a c t e r ¬ i z e d i n that the fibres consist of quartz, fibres, aluminium oxide fibres, aluminium oxide fibres, aluminium silicate fibres, zirconium dioxide fibres or silicium carbide fibres.
3. Catalyst according to either of claims 1 and 2, c h a r a c t e r i z e d i n that the porous surface structure is built up by small particles of silicium dioxide, aluminium oxide, aluminium silicate, zirconium dioxide or titanium dioxide.
4. Catalyst according to any of the claims 1 to 3, c h a r a c t e r i z e d i n that the catalytically active material consists of noble metals such as platinum, palladium, rhodium, rhenium, indium and ruthenium or oxides of transition metals such as copper, cobalt, silver, lanthanum, cerium, iron, chrome, molybdenum and manganese which, optionally, have been doped with said noble metals.
5. Process for the production of catalysts according to any of the claims l to 4, c h a r a c t e r i z e d i n that possibly pretreated and recharged fibres are covered with particles having a size less than 500 nm, suitably 5-100 nm from soles in preferably several layers, whereupon they are fixed and finally covered with catalyti¬ cally active material.
6. Process according to claim 5, c h a r a c t e r ¬ i z e d i n that the fibres are pretreated by washing with solvents for organic compounds or burning of possible organic substances and autoclaving at elevated temperature.
7. Process according to any of the claims 5 or 6, c h a r a c t e r i z e d i n that the fibres are recharged by means of a diluted solution of a cationic polymer or basic aluminium chloride.
8. Process according to claim 5, c h a r a c t e r ¬ i z e d i n that the particles in the sole are ion exchanged with cation exchangers for removing of sodium and/or potassium ions.
9. Process according to claim 5, c h a r a c t e r ¬ i z e d i n that the particles are deposited on the fibre surface in alternatively positive and negative layers, optionally by means of recharging.
10. Process according to any of the claims 5 to 9, c h a r a c t e r i z e d i n that the fixation occurs by heating of the covered fibres in an oven during water addition.
11. Process according to any of the claim 5 to 10, c h a r a c t e r i z e d i n that the catalytically active material is added through impregnation by salt solutions, followed by eludation of the solvent and possibly oxidation or reduction of the ions or by ion exchanging.
PCT/SE1993/001035 1993-01-28 1993-12-02 Catalyst of fibrous material for purification of exhaust gases from e.g. vehicles and method of making it WO1994016817A1 (en)

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EP1837076A1 (en) * 2006-03-20 2007-09-26 Ford Global Technologies, LLC Diesel particulate filter catalyst with low no2 emissions
US7390869B2 (en) 2005-06-13 2008-06-24 Eastman Chemical Company Process for removing metal species in the presence of hydrogen and a porous material and polyester polymer containing reduced amounts of metal species
US7797931B2 (en) 2006-03-20 2010-09-21 Ford Global Technologies, Llc Catalyst composition for diesel particulate filter

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Cited By (10)

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Publication number Priority date Publication date Assignee Title
WO2002049759A1 (en) * 2000-12-21 2002-06-27 Basf Aktiengesellschaft Particle-fibre-agglomerates
US7390869B2 (en) 2005-06-13 2008-06-24 Eastman Chemical Company Process for removing metal species in the presence of hydrogen and a porous material and polyester polymer containing reduced amounts of metal species
US8530609B2 (en) 2005-06-13 2013-09-10 Grupo Petrotemex, S.A. De C.V. Process for removing metal species in the presence of hydrogen and a porous material and polyester polymer containing reduced amounts of metal species
US8921509B2 (en) 2005-06-13 2014-12-30 Grupo Petrotemex, S.A. De C.V. Process for removing metal species in the presence of hydrogen and a porous material and polyester polymer containing reduced amounts of metal species
EP1837076A1 (en) * 2006-03-20 2007-09-26 Ford Global Technologies, LLC Diesel particulate filter catalyst with low no2 emissions
US7771669B2 (en) 2006-03-20 2010-08-10 Ford Global Technologies, Llc Soot oxidation catalyst and method of making
US7797931B2 (en) 2006-03-20 2010-09-21 Ford Global Technologies, Llc Catalyst composition for diesel particulate filter
US8052937B2 (en) 2006-03-20 2011-11-08 Ford Global Technologies, Llc Soot oxidation catalyst and method of making
US8137636B2 (en) 2006-03-20 2012-03-20 Ford Global Technologies, Llc Soot oxidation catalyst and method of making
US8241579B2 (en) 2006-03-20 2012-08-14 Ford Global Technologies, Llc Diesel exhaust gas treatment system

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SE470573B (en) 1994-09-19

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