Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2002-12-03
2004-05-18
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
Reexamination Certificate
active
06737036
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally directed to catalytic compositions in which one of the components thereof is a composite of at least two metal oxides in the form of a sol having a very small particle size. The metal oxides are secured to bulk particles made up of metal oxides to provide a component of a catalyst composition containing washcoat which exhibits high activity and/or selectivity for converting various compounds to harmless byproducts. The catalytic composition may also include an optionally catalytically active metal such as a precious metal. A hydrothermal process of preparing the composite for the catalyst composition is also disclosed.
BACKGROUND OF THE INVENTION
Catalyst compositions are generally applied to a substrate to form thereon a washcoat so that the active component of the washcoat (e.g. a precious metal and/or base metal) is available for contact with compounds capable of being converted (e.g. pollutants) to a harmless compound (e.g. carbon dioxide).
Catalytic converters typically employ a monolith structure as the substrate for applying the washcoat. The monolith structure is typically comprised of a plurality of polygonal channels. The channels provide significant surface area for placement of the washcoat and therefore provide a highly active surface for the catalytic reaction.
More recently, catalyst compositions forming washcoats have been placed on existing substrates of devices which come into contact with a gas stream such as a stream of air (see for example, U.S. Ser. No. 08/682,174 filed Jul. 16, 1996). As disclosed therein, washcoats have been placed on the fins of a radiator, various components of air conditioning systems and have been proposed for other non-monolithic structures.
Catalyst compositions for forming washcoats include a catalytically reactive metal such as precious metal, base metals or combination thereof and a support for the active metal such as alumina, titania and the like. For some applications, the washcoat may also contain an oxygen storage component (OSC) such as ceria.
Current methods for forming a washcoat on a monolithic structure involve either applying a composite to the monolith before the precious metal is applied, or the precious metal may first be applied to the support material and then the composite applied thereafter. The components of the composition can be calcined before being applied to the monolithic structure and typically have a particle size in the micron range. Once the catalyst composition is applied to the monolith calcination is performed to fix the catalyst composition to the substrate.
The support material such as alumina, titania and the like is typically in the form of particles having a particle size of approximately one micron or larger. The catalytically active metal such as precious metal is deposited into the pores of the particles. However, when the catalytically active metal penetrates the micron sized particles by becoming lodged in the pores it is not readily accessible to the reactants present in the gas stream. In addition, when oxygen storage components (e.g. ceria) are used to form the washcoat, the oxygen storage component is not totally functional because of the lack of access of the gas to be treated to the active sites in the middle of the particle. Typical washcoat forming catalyst compositions are produced with micron scale particles for the support material and the OSC component which arise in part from the calcination of the particles at elevated temperatures which typically cause agglomeration.
Submicron (i.e. nanometer) sized particles are known in the art. Such particles have been produced in a particle size range of from about 1 to 30 nanometers for use in the making of catalysts, superconductors, and oxides for recording media as disclosed in Moser, U.S. Pat. Nos. 5,417,956 and 5,466,646, each of which is incorporated herein by reference.
Materials known as sols are also known in the art. According to C. Jeffrey Brinker et al.
Sol
-
Gel Science
, pp. 1-11 Academic Press (1999), a sol is defined as a colloidal suspension of solid particles in a liquid. The colloidal nature of the suspension arises when the dispersed phase is so small that gravitational forces are negligible and interactions are dominated by short-range forces, such as van der Waals attraction and surface charges. Sols are differentiated from gels which are generally characterized by molecules which reach macroscopic dimensions extending throughout the solution. Thus, a gel contains a continuous solid skeleton enclosing a continuous liquid phase.
Sols include nanometer sized particles which have been used to coat bulk particle sized oxides. Typically, the sol material (e.g. nanometer sized metal oxides) have been coated on micron sized bulk oxides and thereafter a precious metal has been applied to form a catalytic composition. The preparation of sol particles may be accomplished by hydrothermal process technology as disclosed in R. Vivekanandan et al. “Hydrothermal Preparation of Ba (Ti, Zr) O
3
Fine Powders”,
Mat. Res. Bull
, Vol. 22, pp. 99-108 (1986) and M. Avudaithai “Ultrafine Powders of SrTiO
3
From the Hydrothermal Preparation And Their Catalytic Activity In The Photolysis Of Water”
Mat. Res. Bull
, Vol. 22, pp. 641-650 (1987), each of which is incorporated herein by reference.
The prior art use of single component or physically mixed sol particles, however, has generated impervious coatings that tend to crack and are easily poisoned due to the lack of pore structure generated. While the employment of smaller particles for a catalyst composition is desirable, the use of such particles has not gained commercial acceptance because washcoats formed thereby have not provided sufficient catalytic performance because of cracking and/or poisoning as mentioned above.
It would therefore be a significant advance in the art of preparing catalyst compositions to provide a stable catalyst composition which include components having very small particle sizes wherein such components are uniformly dispersed on bulk particle supports. Such compositions provide ready access of the catalytically active metal to the gas stream being treated. It would be a further advance in the art to provide a stable catalyst composition which employs catalytically active metals more effectively and evidences higher activity and/or selectivity for said gas stream.
SUMMARY OF THE INVENTION
The present invention is generally directed to a catalyst composition including a component produced as a sol containing fine particle metal oxides secured to support particles having a larger particle size. A catalytically active metal is deposited on the fine particles or on the support or both. The catalyst composition of the present invention provides ready access of the catalytically active metal to the gas stream to be treated in the substantial absence of cracking and poisoning.
More specifically, the present invention is directed to a catalyst composition comprised of a composite of two or more small particle metal oxides which have been secured to a larger particle support. The composite has a catalytically active metal secured to the small particles and/or the larger particle support. The resulting catalyst composition is useful for the catalytic conversion of compounds such as hydrocarbons, nitrogen oxides and the like. The present invention also encompasses methods of forming the composite to produce the fine particle structure and to methods of employing the composite to produce a catalyst composition for the catalytic conversion of a wide variety of chemical compounds to harmless by-products.
In one aspect of the present invention, there is provided a catalyst composition including a composite formed of at least two metal oxides comprised of “Primary Particles” as defined herein which has been secured to a support having a larger particle size than the Primary Particles.
In particular, there is provided a composite for use in the formation of a catalyst composition comprising:
(a) at least two met
Dettling Joseph C.
Wu Joseph H-Z.
Engelhard Corporation
Negin Richard A.
Silverman Stanley S.
Strickland Jonas N.
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