WO2005075702A2

WO2005075702A2 - Method for depositing a metal oxide coating on a substrate

Info

Publication number: WO2005075702A2
Application number: PCT/FR2005/050028
Authority: WO
Inventors: Hélène Katz; Pierre Alphonse; Matthieu Courty
Original assignee: Peugeot Citroën Automobiles SA; Cnrs
Priority date: 2004-01-20
Filing date: 2005-01-18
Publication date: 2005-08-18
Also published as: FR2865219A1; EP1713954A2; WO2005075702A3; FR2865219B1

Abstract

The invention concerns a method for depositing on a substrate at least one metal oxide coating, said metal oxide coating being produced by depositing on the substrate a colloidal solution (sol) of a metal hydroxide then forming said metal oxide by pyrolysis of the gel obtained from said colloidal solution. The invention is characterized in that the colloidal solution is deposited on the substrate in thixotropic state. The invention is in particular applicable to the catalysis of reactions in liquid or gas phase on solids.

Description

METHOD FOR DEPOSITING A METAL OXIDE COATING ON A SUBSTRATE

The present invention relates to a method of depositing a metal oxide coating on a substrate. The invention finds a particularly advantageous, but not limiting, application in the field of catalysis, and more especially the field of reactions in liquid or gas phase catalyzed by solids. However, the invention also applies to other very diverse fields, in particular when it is necessary to modify certain functional properties of materials, such as hardness, thermal protection, protection against chemical agents, electrical insulation, adhesion between thin insulating layers or not, etc. The functional properties of the materials used as catalysts on solids for reactions in liquid or gas phase are obtained by depositing on the substrate material a coating consisting of an intermediate layer of very porous ceramic intended to fix the catalysts and to maximize the contact surface between the liquid or gaseous reactants and the catalyst. The intermediate layers therefore play an essential role since they must both ensure good adhesion with the substrate, a metallic support for example, have a high specific surface, be chemically neutral and have good compatibility with the chosen catalyst. The vast majority of the intermediate layers used in catalysis consist essentially of ceramics, and in particular metal oxides such as alumina, on which very small particles of a metal or another oxide can be deposited, having a catalytic action. The choice of alumina as the oxide most often used in catalysis is mainly due to the fact that, prepared from suitable precursors, it retains a high specific surface at high temperature (approximately 100 m ² / g at 1000 ° c), and that it has very interesting acid-base surface properties for certain reactions. One of these precursors is the aluminum monohydroxide γ-AIO (OH) also called boehmite. Because of its crystal structure, this compound tends to form fibers. During the dehydration process leading to the formation of alumina, these fibers coalesce to give extremely porous and stable materials up to 1000 ° C, called transition aluminas. It is only when heated to a higher temperature that these transition aluminas transform into α-alumina, also called corundum; this transformation is accompanied by an almost total disappearance of the porosity. Known from American patent 3,941,719 is a process, known as the “Yoldas process”, for the preparation of a porous alumina which can be used either in monolithic form or in the form of layers. The Yoldas process has four main stages:

- hydrolysis in a large excess of water of an aluminum alkoxide AI (OR) ₃ ,

- peptization of the hydroxide thus formed until a transparent colloidal solution (sol) is obtained,

- gel formation,

- pyrolysis to obtain alumina. To have formation of aluminum monohydroxide, boehmite, during the first stage, it is preferable to maintain the temperature above 80 ° C, otherwise there is formation of trihydroxide (bayerite) which can no longer be peptized during of the next step. Gel formation is caused either by concentrating the soil by evaporation, or by adding an organic (urea) or inorganic base

(ammonium hydroxide). To prepare layers, simply deposit the soil on the substrate by a conventional means (soaking, brush, spraying, screen printing, ...), the gel then forms during the drying of the layer. Although in US patent 3,941,719 it is stated that the transparent soil resulting from the second stage of the Yoldas process can be used to make coatings on porous or non-porous substrates, the applicant has observed that, if it is indeed possible to prepare in this way thin layers of approximately 100 nm thick on glass or ceramic substrates, the layers obtained on unpolished metallic substrates, such as stainless steel, are not homogeneous. It is the merit of the applicant to have understood that the poor quality of the layers produced on metallic substrates was mainly due to the low viscosity of the boehmite sol. However, it is difficult to increase the viscosity only by concentrating the sol because the viscosity remains practically constant until the sol-gel transition, then increases sharply, exponentially, beyond. Also, the technical problem to be solved by the object of the present invention is to propose a method of depositing on a substrate a coating comprising at least one layer of metal oxide, said layer of metal oxide being produced by deposition on said substrate of a colloidal solution (sol) of a metal hydroxide then formation of said metal oxide by pyrolysis of a gel obtained from said colloidal solution, a process which would overcome the limits to the deposition of oxide layers metallic, in particular, on metallic substrates, due to the low viscosity of the colloidal solution obtained. The solution to the technical problem posed consists, according to the present invention, in that the colloidal solution is deposited on the substrate in the thixotropic state. Indeed, the applicant has been able to establish that, near the sol-gel transition, the colloidal metal oxide solution becomes thixotropic. The transition to the thixotropic state can be achieved either by concentration of the soil, or by addition of organic or inorganic compounds, acetate or formate for example, or the combination of the two. As the thickness of the deposited layer depends on both the concentration and the viscosity, it is possible to modify the thickness over a wide range by playing on these two parameters. The fact of using a thixotropic sol has two advantages, on the one hand, of making it possible to form homogeneous adherent layers on metallic substrates, and, on the other hand, of increasing the thickness of the deposited layer. Recall that thixotropy is a property presented by certain substances, such as gels, to become liquid when they are mechanically agitated and to return to their initial viscous state at rest. This reduction in viscosity under agitation is due to the temporary destruction of the internal structure of the substance under the action of shear forces and its subsequent restructuring when the force ceases to be applied. Although it is of particular interest for metallic substrates, as has been explained above, the method according to the invention is not limited to this single type of substrate but also extends to ceramic or glass substrates. The process which is the subject of the invention has been presented above with reference to the deposition of layers of porous alumina. However, since the sol-gel process makes it possible to prepare very diverse metal oxides, it is easy to extend this process to coatings of metallic or ceramic substrates with porous oxides, other than alumina, commonly used in catalysis such as silica, zirconia, titanium oxide or cerine, and more generally any metal oxide which can be deposited to modify the surface characteristics of metallic, ceramic or glass substrates. The porosity of the layers depends on the conditions of the heat treatment (pyrolysis) carried out at the end of the deposition. But it can also be modified in a wider range by adding surfactants to the soil which will act on the organization of the particles and create a new type of porosity. Another method of modifying porosity is to incorporate into the soil compounds, such as polymer particles, which decompose at low temperatures without leaving harmful residues to the properties of the coating. During the heat treatment, the decomposition of these additives creates a porosity close to that of the diameter of their particles. It should also be emphasized that the process of the invention is not limited to porous oxides only. Indeed, by modifying the mode of synthesis of the soil it It is possible to obtain materials with moderate or not porous materials at moderate temperatures, for example α-alumina from 500 ° C. This allows to consider applying to industrial parts a coating of a functional material intended to give the substrate properties that it did not have. It is thus possible to modify the optical (anti-reflection, solar collector), mechanical (hardness), tribological (anti-friction, anti-wear), thermal (refractory layer, use of the thermoelectric effect), electrical properties. (insulating or conductive layer), chemical (resistance to oxidation, to corrosion) and, of course, catalytic. The thickness of the layers obtained by the sol-gel process is generally limited because there is a considerable reduction in volume during the passage from the sol to the oxide. This reduction commonly reaches a factor of 100, that is to say that a layer of soil of 100 μm will give an oxide layer of 1 μm. If the conditions of the heat treatment are not optimized, this shrinkage causes significant cracking of the layer. To limit this shrinkage, a means consists, according to the invention, in adding to the process a step of adding to the colloidal solution of oxide particles, which are preferably prepared during a previous step, for example from 'a soil of the same nature. It is also possible to use particles of different composition, which leads to the production of composite deposits. The sol-gel process makes it possible to obtain deposits consisting of several successive layers, possibly of variable thickness, in order to obtain thicker coatings without cracking. Generally, a moderate heat treatment is applied between each layer. The thicknesses of the layers thus produced can reach a few hundred microns (100 to 500 μm), this by adding to the thixotropic solution particles of adequate size of metal oxide. This multilayer technique can also be used to prepare a coating with a gradient in chemical composition or structure. For example, in the case of catalyst deposits on steel substrates exhibiting reactivity with respect to the gaseous medium, layers with a porosity gradient can be produced. A first layer, directly in contact with the substrate, will be a dense layer and will protect the substrate from attack gaseous reagents. The following layers will be more and more porous in order to optimize the catalytic activity. The embodiments which follow, given in a nonlimiting manner, will make it clear what the invention consists of and how it can be implemented.

Example 1 concerns the synthesis of a soil by the Yoldas process. 360 cm ³ of water (20 moles) heated to 85 ° C. are poured quickly onto 50 g of aluminum tri-sec-butoxide (about 0.2 mole) with vigorous stirring. The mixture is kept at 85 ° C. with stirring for 15 min, then 7 cm ³ of a 9.2% nitric acid solution in water (0.014 mole HNO ₃ ) are added. Maintained at 85 ° C and with stirring for 24 h to obtain a clear sol. This soil is used to make dip-coating deposits on glass or stainless steel substrates. After drying at 20 ° C and then heat treatment in an oven at 500 ° C in air, the layers are examined under a microscope. The deposits prepared on steel are not homogeneous, areas of the substrate are not covered and the adhesion is poor. On the other hand, the deposits on glass are homogeneous, their thickness being between 0.1 and 0.2 μm.

Example 2 relates to a modification of the Yoldas process according to the process according to the invention. The soil of Example 1 is gradually evaporated at 85 ° C. The initial volume of the soil was 420 cm ³ for an aluminum concentration of 0.5 mol / l. After evaporation of 280 cm ³ of solvent, the soil became thixotropic; the aluminum concentration is now 1.5 mole / l. The thixotropic sol containing 1.5 mole / l of aluminum is used to make deposits on glass and stainless steel substrates under the same conditions as in Example 1. The layers are now homogeneous and adherent. Their thickness is of the order of 2 μm. Example 3 relates to the preparation of a thixotropic sol with a low aluminum concentration. The soil of Example 1 is gradually evaporated at 85 ° C. The initial volume of the soil was 420 cm ³ for an aluminum concentration of 0.5 mol / l. After evaporation of 120 cm ³ of solvent, an identical volume of ethanol is added. The soil becomes thixotropic. Since the final volume is identical to the initial volume, the aluminum concentration is always 0.5 mol / l. This thixotropic soil is used to make deposits on glass and stainless steel substrates under the same conditions as in Example 1. The layers obtained are homogeneous and adherent. Their thickness is of the order of 1 μm.

Example 4 relates to the synthesis of a platinum-alumina catalyst deposited on steel (1% Pt / alumina) A solution of 0.28 g of hexachloroplatinic acid H ₂ PtCI ₆ in 10 is added to the soil of Example 1 cm ³ of water. After stirring, a transparent light yellow color is obtained. This soil is gradually evaporated at 85 ° C. It becomes thixotropic for an aluminum concentration of 1.4 mole / l. It is used to make deposits on stainless steel substrates under the same conditions as in Example 1. The layers obtained are homogeneous and adherent. Their thickness is of the order of 2-3 μm.

Example 5 relates to the synthesis of a platinum-alumina catalyst deposited on steel (1% Pt / alumina). A solution of 0.28 g of hexachloroplatinic acid H ₂ PtCl ₆ in 10 cm ³ of water is added to the soil of Example 1. After stirring, a transparent light yellow color is obtained. 10 cm ³ of acetic acid are then added and the mixture is stirred. After 24 hours of rest, the soil became thixotropic. The aluminum concentration is approximately 0.5 mol / l. This soil is used to make deposits on glass and stainless steel substrates under the same conditions as in Example 1. The layers obtained are homogeneous and adherent. Their thickness is of the order of 2 μm. Example 6 concerns the synthesis of cerine-alumina layers deposited on steel (10% Ce / alumina). A solution of 0.12 g of cerium nitrate dissolved in 5 cm ³ of water is added to 165 cm ³ of soil prepared according to example 1. After stirring, a transparent soil is obtained. After evaporation of 115 cm ³ of solvent, the soil became thixotropic, the aluminum concentration now being 1.65 mole / l. This soil is used to make deposits on glass and stainless steel substrates under the same conditions as in Example 1. The layers obtained are homogeneous and adherent. Their thickness on glass substrates is of the order of 2 μm.

Example 7 relates to the synthesis of a rhodium-alumina catalyst deposited on steel (1.0% Rh / alumina) 0.275 g of rhodium chloride RhCI ₃ are dissolved in 360 cm ³ of water. The solution is heated to 85 ° C. and then poured quickly onto 50 g of aluminum tri-butoxide with vigorous stirring. The mixture is kept at 85 ° C. with stirring for 15 in, then 7 cm ³ of a 9.2% nitric acid solution in water (0.014 mole HNO ₃ ) are added. Maintained at 85 ° C. and with stirring for 24 h to obtain a clear yellow-colored sol. This soil is gradually evaporated at 85 ° C. After reducing its volume by a third, the soil became thixotropic. This soil is used to make dip-coating deposits on stainless steel substrates. After drying at 20 ° C, then heat treatment in an oven at 500 ° C in air, the layer is homogeneous and adherent. Its thickness is around 0.5 μm.

Example 8 concerns the synthesis of a rhodium-alumina catalyst deposited on steel (0.1% Rh / alumina) This example illustrates the possibility of producing catalyst deposits by mixing a sol containing only alumina to a soil containing a metal salt. This method makes it possible to prepare catalysts containing variable amounts of metal from two soils mixed in proportions variables. It is well suited for the synthesis of catalysts containing small amounts of precious metal (<0.5%). We indicate the procedure for preparing a 0.1% rhodium catalyst in alumina. 180 cm ³ of a soil containing only alumina, prepared according to example 1, are mixed with 20 cm ³ of a non-concentrated soil containing 1% of rhodium and prepared according to example 7. The mixture is stirred and gradually evaporated at 85 ° C. After evaporation of about a third of the solvent, the soil became thixotropic. This soil is used to make dip-coating deposits on stainless steel substrates. After drying at 20 ° C, then heat treatment in an oven at 500 ° C in air, the layer is homogeneous and adherent. Its thickness is of the order of 0.5 μm.

Example 9 concerns the synthesis of a rhodium-alumina catalyst deposited on steel (polymer addition). This example illustrates the advantage of adding additives to the soil which modify the rheology of the soil and the porosity of the ceramic. These agents, by attaching to the surface of the colloidal particles, will modify their interactions and therefore act on the rheology of the soil, but they also create a new type of porosity during the formation of the gel. In this example we describe the addition of a polyvinyl alcohol known under the trade name of Rhodoviol. A solution containing 1.58 g of Rhodoviol 4/125 (Mw = 30,000) dissolved in 63 cm ³ of water is mixed with stirring to 120 cm ³ of soil prepared according to example 7 (containing 1% Rh in alumina ). This mixture is concentrated at 85 ° C. After evaporation of 163 cm ³ it becomes thixotropic. This soil is used to make knife deposits (tape casting) on stainless steel substrates. After drying at 20 ° C, then heat treatment in an oven at 500 ° C in air, the layer is homogeneous and adherent. Its thickness is around 6 μm. Example 10 relates to the synthesis of a rhodium-alumina catalyst deposited on steel (charged soil). This example shows the preparation of thicker layers of catalysts by adding particles to a soil. Here the particles and the soil are of the same nature. 200 cm ³ of soil prepared according to Example 7 (1% Rh / alumina) are dried in the open air and then calcined in an oven at 700 ° C in air for 2 h. A gamma alumina powder is obtained, the specific surface of which is 250 m ² / g. This powder is returned to a small amount of water at 60 ° C overnight. It is then mixed with 10 cm ³ of initial soil. The mixture is dispersed under ultrasound. A thick paste is obtained with which knife deposits are made (tape-casting) on stainless steel substrates. After drying at 20 ° C, then heat treatment in an oven at 500 ° C in air, the layer is homogeneous and adherent. Its thickness is around 30 μm.

Claims

1. A method of depositing on a substrate a coating comprising at least one layer of metal oxide, said layer of metal oxide being produced by depositing on said substrate a colloidal solution (sol) of a metal hydroxide then formation of said metal oxide by pyrolysis of a gel obtained from said colloidal solution, characterized in that the colloidal solution is deposited on the substrate in the thixotropic state.

2. Method according to claim 1, characterized in that the thixotropic state of the colloidal solution is obtained by concentration of the colloidal solution.

3. Method according to one of claims 1 or 2, characterized in that the thixotropic state of the colloidal solution is obtained by addition to the colloidal solution of at least one organic or inorganic compound.

4. Method according to claim 3, characterized in that said compound is an acetate or a formate.

5. Method according to any one of claims 1 to 4, characterized in that said substrate is chosen from the list comprising a metallic substrate, a ceramic substrate, a glass.

6. Method according to any one of claims 1 to 5, characterized in that said metal oxide is chosen from the list comprising alumina, silica, zirconia, titanium oxide, cerine.

7. Method according to any one of claims 1 to 6, characterized in that the metal oxide obtained is porous.

8. Method according to any one of claims 1 to 6, characterized in that the metal oxide obtained is non-porous.

9. Method according to any one of claims 1 to 8, characterized in that it comprises a step of adding surfactants to the colloidal solution.

10. Method according to any one of claims 1 to 9, characterized in that it comprises a step of adding to the colloidal solution of oxide particles.

11. Method according to any one of claims 1 to 10, characterized in that said coating consists of a plurality of metal oxide layers produced successively.

12. Method according to claim 11, characterized in that the plurality of layers has a gradient of chemical composition.

13. The method of claim 11, characterized in that the plurality of layers has a structural gradient.

14. Method according to claim 13, characterized in that the layers have a porosity gradient.

15. Method according to any one of claims 1 to 14, characterized in that it comprises a step of adding to the colloidal solution of polymer particles.

16. Method according to any one of claims 1 to 15, characterized in that it comprises a step of adding to the colloidal solution of metal oxide particles of variable size.