Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1998-12-21
2001-05-08
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S309000
Reexamination Certificate
active
06228801
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a process for producing a catalyst with a catalytically active composition on a support body by thermal spraying and a catalyst produced according to that process.
U.S. Pat. No. 3,271,326 discloses an essentially nickel-containing catalyst in which a catalytically active surface is applied by flame spraying. In that case, in a first process step, a steel support structure which had previously been mechanically roughened has aluminum flame sprayed on to it for the purpose of further roughening. Subsequently, in a second process step, the catalytically active components are applied by flame spraying to the support structure which was pretreated in that way. A characteristic of that complicated production process is that the catalytically active components which are sprayed on are heated up to their melting point.
Furthermore, German Published, Non-Prosecuted Patent Application DE 38 13 312 A1 discloses applying a titanium dioxide modified with catalytically active components as additives to a metallic structure by thermal spraying. In that procedure, the catalytic ally active components melt briefly and upon cooling fuse together at contact points. That creates a catalytically active surface held together by adhesion forces.
However, it is known that the catalytic activity of the titanium dioxide is very strongly dependent on the proportion of the titanium dioxide which is present in the anatase modification. The anatase modification of titanium dioxide in that case has the property of being converted irreversibly by high temperatures into the less active rutile modification. Nuclei of titanium oxide in the rutile modification in the crystal microstructure grow further upon heating at the expense of the anatase modification.
On that subject, German Published, Non-Prosecuted Patent Application DE 39 16 398 A1 discloses mixing a very low-melting component into the spraying material containing a thermally sensitive component such as titanium dioxide in the anatase modification. The melting point of the very low-melting component in that case is below an inactivation temperature above which, upon influence by a chemical change, for example, the amount of the thermally sensitive component is irreversibly reduced. During thermal spraying, the spraying material is heated only to the melting point of the very low-melting component, so that inactivation of the thermally sensitive component is avoided. The adhesion of the components of the spraying material to one another occurs due to the very low-melting component flowing around other particles and fusing together during the thermal spraying. That enables a surface having a high catalytic activity to be achieved by thermal spraying, even in the presence of a thermally sensitive component.
However, a very low-melting component flowing around a catalytically active component reduces the specific surface area or BET surface area of the composition being applied. In order to achieve a high catalytic activity, the layer thickness of the catalytically active composition being applied accordingly has to be increased. The longer spraying time resulting therefrom can then easily lead to thermal distortion of the support body or the support structure. The process is therefore only suitable for support structure thicknesses of more than about 3 mm.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a process for producing a catalyst and a catalyst produced thereby, which overcome the hereinafore-mentioned disadvantages of the heretofore-known processes and products of this general type, in which the catalyst includes a catalytically active composition on a support body, in which the catalyst is produced by thermal spraying without thermally sensitive components becoming inactivated and in which the catalytically active composition has a significantly increased BET surface area as compared with the prior art.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for producing a catalyst with a catalytically active composition on a support body by thermal spraying, which comprises thermally spraying a spraying material including a titanium hydroxide reactive precursor of at least one component of the catalytically active composition onto the support body, and converting the titanium hydroxide reactive precursor to form the component.
In accordance with another mode of the invention, the thermal spraying step is carried out with a titanium metahydroxide as the titanium hydroxide.
The invention is based on the concept, that is exactly contrary to the opinion of those skilled in the art, of selecting a composition for the spraying material which is not the same as that for the catalytically active composition to be applied to the support body. Rather, for at least one component of the catalytically active composition, a reactive precursor which is selected in the spraying material is converted to form the component. In this way, for example in the case of a thermally sensitive component such as titanium dioxide, the inactivation favored by a temperature rise during the spraying process can be avoided. This is because the component is only produced from the reactive precursor during or after the spraying process and as a result cannot be inactivated by a temperature rise occurring before conversion.
It has been found that a catalytically active composition produced by the process of the invention has a high BET surface area of from 50 to 70 m
2
/g. According to comprehensive studies, it can be assumed that during the conversion of the reactive precursor linking of chemical compounds occurs which lead to the formation of interconnected microcrystallites. Such an assembly of connected microcrystallites has a high specific surface area. The high BET surface area enables the layer thickness of the catalytically active composition to be considerably reduced as compared with the prior art while maintaining an equally high catalytic activity. As a result of the shorter spraying time associated therewith, a support body having a thickness of less than 100 &mgr;m is suitable for the application of the catalytically active composition without thermal distortion occurring during application. Such a material saving allows the manufacturing costs to be reduced correspondingly. Suitable support bodies are metallic or ceramic bodies of any shape, e.g. in the form of a plate, a band, a rod or a tube. It is also conceivable to use a material other than metal or ceramic for the support body, as long as it cannot be damaged by the elevated temperature during the spraying process.
In accordance with another mode of the invention, the conversion or chemical reaction of the reactive precursor is carried out by thermal activation during spraying. The spraying material in this case is heated during the thermal spraying to above a corresponding activation temperature above which a chemical reaction of the precursor commences.
In accordance with a further mode of the invention, the thermal activation of the reactive precursor can also be carried out after spraying is completed by heat treatment of the catalytically active composition applied or the support body at above the activation temperature. Such a heat treatment can also include a calcination process.
In accordance with an added mode of the invention, a suitable reactive precursor convertible by thermal activation is, in particular, a readily thermally decomposable metal salt or a compound of a metal bearing a hydroxy group (hydroxy compound). A metal salt or an ionic compound of the corresponding metal can be ionized by appropriate heat input into a cation and an anion. Such an ionization takes place, for example, in any candle flame. If the temperature during thermal spraying, which for the purposes of the present invention includes both plasma and flame spraying, is selected so as to be appropriately high, a free metal ion can react with a gas molecule of the s
Hums Erich
Khinsky Alexander
Greenberg Laurence A.
Griffin Steven P.
Ildebrando Christina
Lerner Herbert L.
Siemens Aktiengesellschaft
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