Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1998-12-04
2002-01-29
Bell, Mark (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S325000, C502S334000
Reexamination Certificate
active
06342465
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to a process for preparing a catalyst which has a catalytically active coating consisting of high surface area, finely divided materials and catalytically active components on an inert carrier structure, by applying the catalytically active components to the finely divided materials, producing a coating dispersion from these materials and coating the carrier structure therewith.
This type of process provides catalysts which are used in many areas of chemical engineering. They are so-called supported catalysts in which the catalytically active components, in highly dispersed form, are applied to support materials in order to ensure high catalytic activity of the catalyst with the smallest possible amount of active components. For this purpose, support materials which have a large specific surface area for taking up the catalytically active components are used. They are generally finely divided, that is powdered, thermally stable metal oxides.
In the case of automotive vehicle exhaust catalysts, the support materials are applied in the form of a coating on catalytically inert carrier structures. Carrier structures which are suitable for automotive exhaust gas treatment are so-called honeycomb structures made of ceramic or metal which have parallel flow channels through which the exhaust gas can pass. In order to coat the honeycomb structure with the support materials, the support materials are generally dispersed in water and usually homogenized by means of a milling process. Milling adjusts the average particle size of the support materials to a value between 1 and 10 &mgr;m.
The walls of the flow channels are coated by immersing the honeycomb structure, once or several times, in this coating dispersion, followed by drying and calcining. The final coating is also called a dispersion coating.
During this procedure the catalytically active components may be applied to the specific surface area of the support materials at different times. For example, it is known that the catalytically active components are deposited only after coating the honeycomb structure with the dispersion coating by immersing the coated honeycomb structure in an aqueous solution of soluble precursors of the catalytically active components. Alternatively, there is the possibility of applying the catalytically active components to the powdered support materials in a stage which precedes producing the dispersion coating.
The present invention relates to this second possibility of depositing the catalytically active components. In order to achieve a high catalytic activity, the type of deposition must ensure that the components are deposited in as finely divided a manner as possible on the specific surface area of the support materials. In addition the type of deposition should also lead to high thermal and ageing stability of the final catalyst, that is to say the particles of catalytically active components must be firmly fixed to the surface area of the support materials in order to prevent neighboring particles agglomerating when the catalyst is subjected to high temperatures.
A variety of processes have been disclosed for depositing catalytically active components onto powdered support materials. These include, for example, impregnation with an excess of impregnating solution. In that process, an aqueous solution of the catalytically active components is added to the powdered support material, wherein the volume of the solution may be much greater than the water absorption capacity of the support material. This results in a material with a pasty consistency which is dewatered, for example in an oven at elevated temperatures of 80-150° C., and is then calcined at still higher temperatures to fix the catalytically active components. During the dewatering procedure, chromatographic effects may take place which could lead to uneven distribution of the catalytically active components on the support material.
During so-called pore volume impregnation, an amount of solvent is used to make up the solution of catalytically active components which corresponds to about 70-100% of the absorption capacity of the support material for this solvent. The solvent is generally water. This solution is distributed as uniformly as possible, for example by spraying over the support material while it is rotated in a vessel. After distributing the entire amount of solution over the support material the material is still free-flowing despite the presence of water. Finally the impregnated material is dried and then calcined at elevated temperatures to fix the catalytically active components on the support material. Chromatographic effects can largely be avoided when using pore volume impregnation. It generally provides better results than the process described above for impregnation with an excess of solvent.
The disadvantage of these known processes for impregnating support materials with catalytically active components is the fact that the catalytically active components have to be fixed to the support material by drying and calcining after the impregnation process, with the consumption of large amounts of energy, in order to prevent these components being desorbed from the support material during redispersion of the support material, which is required when producing the coating dispersion.
An object of the present invention is to achieve highly dispersed distribution of the catalytically active components on the support materials and largely avoid costly drying and calcining steps. Highly dispersed catalytically active components are considered to be those with crystallite sizes of less than 10 nm, preferably between 2 and 7 nm.
SUMMARY OF THE INVENTION
The above and other objects are achieved according to the invention by a process for preparing a catalyst which has a catalytically active coating consisting of high surface area, finely divided materials and catalytically active components on an inert carrier, by applying the catalytically active components to the finely divided materials, producing a coating dispersion from these-materials and coating the carrier structure therewith.
A feature of the present invention is a process that comprises the following process steps:
(a) impregnating a powder mixture of the designated finely divided materials with a solution of precursor compounds of the catalytically active components by the pore volume impregnation process, wherein the precursor compounds are adsorbed on at least one of the materials,
(b) producing an aqueous coating dispersion by using the impregnated powder mixture,
(c) coating the carrier structure with the dispersion obtained in this way and
(d) drying and calcining the coating on the inert carrier.
Materials with a high surface area within the context of this invention are understood to be those with specific surface areas (measured according to DIN 66132) of more than 10 m
2
/g. For treating automotive exhausts, noble metals from the platinum group of the Periodic Table of Elements are preferably used as catalytically active components. These include ruthenium, rhodium, palladium, osmium, iridium and platinum, as well as mixtures thereof.
Adsorption of the precursor compounds on the support materials depends both on the surface properties of the support materials and also on the precursor compounds and the pH of the impregnating solution. It is known, for example, that nitrates of the platinum group metals are adsorbed very strongly on aluminum oxide, but that chlorides are only weakly adsorbed at the same acidity of the impregnating solution. This difference is used during the preparation of pellet catalysts in order to have an effect on the distribution of catalytically active elements in the pellets. When using nitrates, for example, a specific outer shell profile is obtained, whereas the use of chlorides leads to almost uniform penetration of the entire pellet with the active components.
It has been shown that neither very strong nor weak adsorption leads to optimum dispersion of the catalytically acti
Domesle Rainer
Klein Harald
Kreuzer Thomas
Leyrer Jürgen
Lox Egbert
Bell Mark
DMC2 Degussa Metals
Hailey Patricia L.
Smith , Gambrell & Russell, LLP
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