DE102009033716B4 - A process for the preparation of an open-pore structure permeable by a fluid as well as a use of the structure produced by the process - Google Patents

Abstract

The invention relates to a method for producing an open-pored structure through which fluids can flow, the structure and uses thus produced. The object of the invention is to provide an open-pored structure which can be flowed through by a fluid and catalytically influenced, which can be used mechanically and thermally with long-term stability even in very small dimensions for influencing the respective fluid. The structure according to the invention is formed of at least one ceramic fiber and on the surface of which at least one catalytically active chemical element is bonded to ceramic material of the fiber (s). The preparation can be carried out by extruding powdered ceramic material in a suspension with a polymer and a solvent to form at least one fiber and then forming a green structure from it, and then, during a heat treatment, expelling the organic constituents and sintering the structure. Subsequently, the sintered structure is coated on the surface with a solution in which at least one chemical compound of a catalytically active element is coated, and thereafter a chemical reduction of the chemical compound is carried out in a further heat treatment in a reducing atmosphere, resulting in a cohesive connection of the at least one catalytically active element on the surface of the structure.

Description

The invention relates to a method for producing an open-pore structure, which can be flowed through by fluids, as well as a use of the structure produced with the ancestor

Open-pore structures, which are to be flowed through by fluids, so gases or liquids, are known in many forms to z. As a separation or other influence, such as a conversion of substances contained in the fluid to make.

Certain properties and parameters have to be fulfilled when flowing through. As a rule, the resulting pressure loss should be kept small. When used as a catalyst support, mechanical long-term stability at higher temperatures and possibly also in chemically aggressive fluids must be taken into account. In addition, a discharge of catalytically active substances or their ineffectiveness by adhering contaminants on their surface and their chemical transformation should be avoided. The degradation over the lifetime should be kept small.

In addition, a large surface overflowed by the fluid is desired.

For example, open-cell foams provide large surfaces, good distribution of fluid flow, and small diffusion paths. However, they can not or can only be produced in miniaturized form with very small dimensions and at the same time have sufficient mechanical strength.

Another problem with open-pored foams is the homogeneous application with a catalytically active substance, which is problematic especially in the interior of a foam body. A maximum catalytic effect can not be achieved over the entire surface actually available.

There are also loose beds, which are formed with individual pellets or granules, known for such applications. In these, however, a relatively small surface is available and the available diffusion paths are short. As a major disadvantage but a discharge of material can not be avoided and it is a high pressure loss to accept.

There are also ceramic honeycomb structures in use, flow through the cavities or channels fluids. They have a high mechanical strength. A discharge of catalyst material can also be largely avoided. But here too there are shortcomings in the possible miniaturization, the use of large surfaces and in some embodiments, an increased pressure loss occurs.

Out DE 10 2005 027 649 A1 An exhaust aftertreatment arrangement is known in which a layer of fibers is arranged on a support structure. The fibers can be coated with washcoat and also connected to each other.

The DE 10 2005 005 467 A1 relates to composites of ceramic hollow fibers of gas or liquid transporting ceramic material, in which a composite is formed with sintered hollow fibers.

It is therefore an object of the invention to provide an open-pore structure which flows through a fluid and can be catalytically influenced thereby, which can be used for long-term stability, even in very small dimensions, for influencing the respective fluid mechanically and thermally.

According to the invention, this object can be achieved by a method according to claim 1. Claim 7 characterizes a use of the structure produced by the method.

An open-pore structure produced according to the invention is formed from at least one ceramic fiber and, in addition, at least one catalytically active chemical element is connected in a materially bonded manner to ceramic material of the fiber (s) on its surface. In this case, a single sufficiently long fiber, which has been wound several times, form an open-pored structure. The structure can also be formed with a plurality of individual such fibers.

In this case, one or more fibers are connected to one another in a material-locking manner by means of sintered bridges at a plurality of positions.

For example, a knitted, woven, knitted, braided, nonwoven, scrim or network may have been made with the fiber (s).

Suitable ceramic materials are selected from Al ₂ O ₃ , ZrO ₂ , Si ₃ N ₄ , Y ₂ O ₃ , La ₂ O ₃ , Mn ₂ O ₃ , CeO ₂ , Cr ₂ O ₃ , Co ₂ O ₃ , SrO and CaO or a hydroxide or carbonate thereof. Accordingly, the fiber (s) may also be made, for example, from the following perovskite ceramics La _x Sr _1-x Mn _y Cr _{1 -y} O ₃ or Lax Ca _1-x Mn _y Cr _1-y O ₃ . The mentioned ceramics can also be used as a mixture of at least two of these chemical compounds or with at least one doped or offset added element or chemical compounds are used.

Suitable catalytically active elements are selected from Ni, Cu, Co, Pt, Pd, Rh, Ru, Ir and Os.

It may also be favorable that an intermediate layer of a ceramic material is formed on the surface of fibers, on the surface of which then at least one catalytically active chemical element is adhesively bonded to the ceramic material of the intermediate layer. In this case, the material of the intermediate layer may be a material having a thermal expansion coefficient that differs from the thermal expansion coefficient of the fiber material with a maximum value of 1 x 10 ^-6 m / K.

The intermediate layer can also be formed with the same material with which the fiber is made. In both cases, this is intended to make the specific surface of the intermediate layer larger than the specific surface of the uncoated fiber (s). This can be achieved by a lower sintering temperature, which is selected for the sintering of the intermediate layer to be formed on the carrier layer. The higher sintering temperature for the support structure leads to a higher strength of the support structure compared to the strength of the intermediate layer.

The fibers used for the production of structures according to the invention should have outside diameters of at least 0.01 mm to a maximum of 1 mm, preferably 0.05 mm. The structures may advantageously have a volume in the range 0.025 cm ³ to 50 cm ³ . As a result, they can be used in small training for mobile applications. An example of this is the use of fuel cells, in which particularly suitable fuel such. As hydrogen, can be obtained by means of a reforming of a hydrocarbon compound. So then electric power for the operation of small mobile consumers, eg. As notebooks are provided. The required fuel can be carried as a liquid or gaseous hydrocarbon compound having a higher calorific value in a suitable tank, possibly under elevated pressure.

In the production of structures according to the invention, the procedure is such that powdered ceramic material is extruded in a suspension with a polymer and a solvent to form at least one fiber. The greenest structure formed therefrom is then subjected to a heat treatment in which the organic components are expelled and the structure is also sintered.

It is also possible to add organic fibers. These may be cellulose fibers, for example. In a heat treatment, the organic components can be burned out, so that cavities are formed within a structure, which increase the porosity.

After extrusion, one or more of these filaments may be processed because of their then still plastic deformability to set a desired geometry, dimension and porosity of the structure.

Following this heat treatment, the sintered structure is coated on the surface with a solution in which at least one chemical compound of a catalytically active element is contained. This can be done by soaking in or spraying with the solution. At the thus coated structure, in a further heat treatment, a chemical reduction of the chemical compound is carried out in a reducing atmosphere. This leads to a cohesive connection of the at least one catalytically active element to the surface of the structure. Before this or possibly also during this heat treatment, the solvent can be evaporated.

As already mentioned, between the sintering and the further heat treatment for the chemical reduction, an intermediate layer on the ceramic surface of the sintered structure of the same ceramic material or a ceramic material having a thermal expansion coefficient of the coefficient of thermal expansion of the fiber material with a maximum value of 1 · 10 ^-6 m / K deviates, be trained.

This can also be achieved with a suspension which is formed at least with a corresponding ceramic powder and a binder. The suspension is applied to the surface of the already pre-sintered structure and expelled during a heat treatment of the binder and sintered the ceramic powder. The sintering is carried out so that the specific surface of the fiber (s) increases. This can be achieved by observing certain temperatures, heating rates and holding times. Alone or in addition, however, it is also possible to make a powder selection in which the powder for the intermediate layer has a smaller particle size and / or a different particle size distribution in comparison with the powder used for fiber production.

A bimodal particle size distribution of powder used for an intermediate layer is advantageous. Such a structure can be well flowed through by a fluid and has a high specific surface area. Thus, a powder mixture which is formed with a first particle size in the range 50 nm to 100 nm and a second particle size in the range 0.5 μm to 1 μm can be used for the interlayer formation.

The properties of a structure produced according to the invention can be influenced in a defined manner in the production. The outer diameter, the fiber (s) forming the structure, their specific surface area, the way in which the fiber (s) are formed, the volume of fibers in the structure and the number and size of sintered bridges and pores can be taken into account become.

In the invention, the fiber (s) for the structure can be prepared by the polysulfone method. It can, as in the DE 10 2008 056 721 A1 described, be proceeded. Their disclosure content is fully referenced here.

Accordingly, five process steps can be performed in this process. Thus, in a first step (i), a dispersion is prepared with a ceramic powder and a solvent for a polysulfone. The dispersion is completed in a second step (ii), in which a mixture consisting of the solvent and the polysulfone is added to the dispersion prepared in this way. In this case, a proportion of polysulfone ≤ 1% by mass and the proportion of ceramic powder of at least 85% by mass is maintained in the dispersion. However, it is also possible to use a higher proportion of polysulfone of up to 5% by mass and possibly a smaller proportion of ceramic powder than 85% by mass. With a polysulfone content above 1% by mass, it is advantageously possible to increase the porosity and thus also the specific surface area.

The polysulfone solution added in the previously prepared dispersion in the second process step (ii) should have a proportion of solvent which allows complete dissolution of the polysulfone and does not reach or at least only slightly exceeds the saturation limit of the polysulfone solvent.

The sole addition of the solvent in the first process step (i), the surface of the ceramic powder particles wetted with the solvent and thereby the surface tension ratios are favorably influenced to improve the dispersing behavior for the second process step (ii).

Thereafter, in a third step (iii), the dispersion having a pasty consistency is subjected to shaping, preferably extruded. Despite the solids content of at least 85% by mass, the dispersion is plastically deformable. In a fourth step (iv), moldings thus obtained are immersed in or wetted with a solvent for the solvent contained in the dispersion. Polysulfone is not soluble in this solvent. As a result, at least almost solvent-free molded articles can be obtained.

The green-strength moldings are then subjected to a heat treatment in a fifth step (v), as already explained in more detail above. During the heat treatment, first the organic components still contained are expelled by pyrolysis and the shaped body is sintered. The heat treatment can also take place in two stages, each of which complies with other conditions.

When sintering, however, can be largely dispensed with additional effort. It can often be sintered under normal atmospheric conditions, ie in air and at normal pressure.

In this procedure, it is particularly advantageous that in the first four process steps at low temperatures (room temperature) can be used.

Thus, in the process steps (i) to (iv), the maximum temperature of 70 ° C should not be exceeded. Preferably, the temperature should be less than 40 ° C. It can also be used at normal ambient temperature and below.

It is favorable to use long-chain polysulfone. The molecular weight should be at least 30,000 U.

In the process step in which the reduction of catalytically active chemical elements takes place, forming gas (nitrogen + 5% hydrogen), pure hydrogen, varigon (argon + 5% hydrogen), nitrogen monoxide may be contained in the reducing atmosphere. However, such a gas can also be used alone for the reducing atmosphere.

For the manufacture of the structure, a powdery ceramic material with a particle size between 0.05 microns and 10 microns can be used and a mean particle size d ₅₀ are adhered to 0.15 microns.

It is also preferable to use a salt as a chemical compound of a catalytic one to use active chemical element. This may be a chloride, for example RhCl ₃ . Salts are known to dissolve well in water until a maximum saturation limit has been reached. Thus, aqueous salt solutions, each with a particularly suitable salt content can be easily prepared. Their handling is straightforward and the required safety precautions low.

The invention will be explained by way of example below.

Example I

For a structure according to the invention fibers of ZrO ₂ were prepared by the polysulfone process. The ceramic powder used had an average particle size d ₅₀ of 0.15 microns. The suspension was formed with 70 mass% ZrO ₂ powder, 3 mass% polysulfone and 27 mass% solvent.

The suspension was extruded and the threads thus obtained with lengths between 5 mm to 10 mm then placed in the form of a support structure. In this case, a sufficiently large porosity of about 90% was maintained. After forming a nonwoven fabric with these filaments, sintering was conducted at a temperature between 1200 ° C and 1400 ° C at a heating rate of 5 K / min.

Then, the threads of the support structure were coated on the surface with a suspension which was also formed with ZrO ₂ in powder form. As a binder, polyethyleneimine was included in the suspension. The solids content of the suspension was 20 g / l. Ethanol was used as the solvent.

The thus prepared structure was then sintered at 950 ° C for a period of 2 hours. A heating rate of 3 K / min was observed. After sintering, a specific surface area of 4 m ² / g was reached.

The sintered structure was then dipped in a 1.4% by weight RhCl ₃ .3H ₂ O solution. The solvent was water. Upon heat treatment in air and then under a forming gas atmosphere at 750 ° C, the water was evaporated and the chloride was converted to Rh ₂ O ₃ and then reduced to catalytically active rhodium on the surface. It was able to form a material connection on the surface.

Mass fractions for the support structure, intermediate layer and catalytically active rhodium 100: 10: 1 were observed.

The structure thus prepared had a volume of 1.5 cm ³ . It can be used for the reforming of ethanol, as an example of a suitable hydrocarbon compound, to hydrogen.

Between 0.058 ml and 0.108 ml of ethanol and 0.108 ml and 0.213 ml of water without carrier gas were evaporated per minute and passed through the catalyst formed with the structure. The S: C ratio was 3: 1. The reforming was carried out at a constant temperature of 750 ° C. In the reformate were after the catalytically assisted reaction more than 42% hydrogen, about 40% water, about 10 vol .-% CO ₃ , 9.5 vol .-% CO ₂ , less than 0.1 vol .-% CH ₄ and no ethanol included.

The structure showed no degradation over an operating time of 6000 min.

Example II

It was a ceramic structure, prepared as in Example I.

As catalytically active elements rhodium and nickel were firmly bonded to the surface. The procedure was as follows:

The sintered structure was used in a 0.7 mass% RhCl ₃ .3H ₂ O solution containing 1.37 mass% Ni (NO ₃ ) ₂ .6H ₂ O. The solvent was water. In a heat treatment under air and then under forming gas at 750 ° C, the water was evaporated and then the chloride and nitrate converted to the respective oxide, which then settle by reduction at the surface of the structure as catalytically active rhodium and nickel and with this cohesively could be connected.

The following mass fractions were observed for the support structure, the intermediate layer, rhodium and nickel of 100: 10: 0.5: 0.5.

The structure thus prepared had a volume of 1.5 cm ³ . It can be used for the reforming of ethanol, as an example of a suitable hydrocarbon compound, to hydrogen.

Between 0.058 ml and 0.108 ml of ethanol and 0.108 ml and 0.213 ml of water without carrier gas were evaporated per minute and passed through the catalyst formed with the structure. The S: C ratio was 3: 1. The reforming was carried out at a constant temperature of 750 ° C. In the reformate were after the catalytically assisted reaction more than 42 vol .-% hydrogen, about 40 vol .-% water, about 10 vol .-% CO, 9.5 vol .-% CO ₂ , less than 0.1 Vol .-% CH ₄ and no ethanol included.

The structure showed no degradation over an operating time of 6000 min.

Claims (15)

Process for the production of an open-pored structure through which a fluid can flow, in which powdered ceramic material in a suspension with a polymer and a solvent extrudes to at least one fiber, forms a green structure therefrom and subsequently expels the organic constituents during a heat treatment and the structure is sintered, so that in the case of one fiber, several positions of the fiber or, in the case of several fibers, the fibers are bonded to one another by means of sinter bridges, and subsequently the sintered structure on the surface with a solution in the at least one chemical compound of a catalytically active element is included, coated; whereupon a chemical reduction of the chemical compound is carried out in a further heat treatment in a reducing atmosphere, which leads to a cohesive connection of the at least one catalytically active element on the surface of the structure with the ceramic material of the fiber (s) or - in a further process step, is performed between the sintering and the further heat treatment for the chemical reduction, an intermediate layer on the ceramic surface of the sintered structure of the same ceramic material or a ceramic material having a coefficient of thermal expansion, such as the thermal expansion coefficient of the fiber material with a maximum value is deviated from 1 × 10 ^-6 m / K, it is formed that the specific surface is materially connected to the catalytically active element, is increased. A method according to claim 1 or 2, characterized in that the intermediate layer is sintered at a lower temperature than the support structure. Method according to one of the preceding claims, characterized in that the fiber (s) for the structure are produced by the polysulfone method. Method according to one of the preceding claims, characterized in that the reducing atmosphere is obtained by the use of a forming gas, hydrogen, or gas containing or consisting of nitrogen monoxide gas. Method according to one of the preceding claims, characterized in that a powdered ceramic material having a particle size between 0.05 microns and 10 microns used and a mean particle size d ₅₀ is adhered to 0.15 microns. Method according to one of the preceding claims, characterized in that a salt of this element is used as the chemical compound of a catalytically active element. Use of an open-pore structure through which a fluid can flow, produced by a method according to one of the preceding claims, for reforming at least one hydrocarbon compound, characterized in that - the structure of at least one ceramic fiber is formed such that one fiber has multiple positions of the fiber or in the case of several fibers, the fibers are bonded to one another in a material-locking manner by means of sintered bridges, and - the surface of the ceramic material of the fiber (s) is adhesively bonded to at least one catalytically active chemical element, or - an intermediate layer of a material on the surface of the fiber (s) ceramic material is formed, on the surface of which at least one catalytically active chemical element is integrally bonded to the ceramic material of the intermediate layer, wherein the material of the intermediate layer is the same material as the material of the fiber (s) od the material of the intermediate layer has a thermal expansion coefficient which deviates from the coefficient of thermal expansion of the material of the fiber (s) to a maximum value of 1 × 10 ^-6 m / K and the specific surface area of the intermediate layer is greater than the specific surface area of the uncoated fiber (n ). Use of a structure according to claim 7, characterized in that the ceramic material of the fiber (s) is selected from Al ₂ O ₃ , ZrO ₂ , Si ₃ N ₄ , Y ₂ O ₃ , La ₂ O ₃ , Mn ₂ O ₃ , CeO ₂ , Cr ₂ O ₃ , Co ₂ O ₃ , SrO and CaO or a hydroxide or carbonate thereof. Use of a structure according to claim 7, characterized in that the fiber (s) is / are formed from La _x Sr _1-x Mn _y Cr _{1 -y} O ₃ or Lax Ca ₁ -x Mn _y Cr _{1 -y} O ₃ . Use of a structure according to one of claims 8 or 9, characterized in that the ceramic material is formed from a mixture of at least two chemical compounds or additionally contains at least one chemical element. Use of a structure according to any one of claims 7 to 10, characterized in that the at least one catalytically active element is selected from Ni, Cu, Co, Pt, Pd, Rh, Ru, Ir and Os. Use of a structure according to any one of claims 7 to 11, characterized in that it is in the form of a knitted fabric, woven, knitted, braided, non-woven, scrim or network. Use of a structure according to any one of claims 7 to 12, characterized in that the fiber (s) has an outer diameter of at least 0.01 mm and a maximum of 1 mm and the structure has a volume in the range of 0.025 cm ³ to 50 cm ³ / have. Use of a structure according to any one of claims 7 to 13, characterized in that the fiber (s) have / have a specific surface of at least 3 m ² / g. Use of a structure according to any one of claims 7 to 14 for reforming ethanol.