Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
2000-02-02
2003-05-27
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S239100, C423S247000, C502S304000, C502S349000
Reexamination Certificate
active
06569392
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to a method for manufacturing a cost-reduced, durable three-way catalyst useful to oxidize hydrocarbons, carbon monoxide and reduce nitrogen oxides in exhaust gas generated by a gasoline internal combustion engine operated near the stoichiometric A/F ratio. More particularly, the catalyst is made by a process which comprises mixing particles of three oxide materials, ceria-zirconia particles impregnated with platinum and palladium, ceria-zirconia particles impregnated with only rhodium as the precious metal, and a precious metal free gamma-alumina particle.
BACKGROUND OF THE INVENTION
Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into nonpolluting gases including carbon dioxide, moisture (H
2
O), and nitrogen. When the gasoline powered engine is operated in a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.7 and 14.4, catalysts containing precious metals like platinum, palladium and rhodium are able to efficiently convert all three gases simultaneously. Hence, such catalysts are often called “three-way” catalysts. Typically such catalysts use a relatively high loading of precious metal to achieve the high conversion efficiency required to meet stringent emission standards of many countries. This makes the catalyst expensive. In countries where the emission standards are less stringent, a durable catalyst which would meet these less stringent standards and also be less expensive catalyst would be desirable.
We have now found a method for making a durable three-way catalyst which may use a significantly lower loading of precious metal than conventional catalysts making it less expensive, but which still obtains excellent exhaust gas conversion efficiency under close to stoichiometric conditions. This and other aspects of the invention will be discussed in detail below.
DISCLOSURE OF THE INVENTION
The invention is a method for manufacturing a durable, lower cost three-way catalyst useful for treating gasoline engine exhaust gases containing hydrocarbons, carbon monoxide, and nitrogen oxides (NOx). The catalyst has a relatively high loading of rare earth metals as compared to a low loading of precious metals which reduces its cost. The manufacturing method comprises mixing particles of three different materials together. One particle material is: (a) calcined ceria/zirconia particles, having a 20:1 to 1:1 Ce:Zr atomic ratio, impregnated with 1-20 wt. % two precious metal consisting essentially of platinum (Pt) with palladium (Pd) based on the weight of the impregnated particle, preferably this total precious metal loading is 3-8 wt. The second material is (b) calcined ceria/zirconia particle, having a 20:1 to 1:1 Ce:Zr atomic ratio, impregnated with 1-20 wt. % precious metal consisting essentially of only rhodium (Rh) based on the weight of the impregnated particle, preferably this precious metal loading is 3-8 wt. The third particle is: (c) gamma-alumina particles having a particle size, on average, of less than 5 &mgr;m. This alumina particle is not impregnated with precious metal.
These powder particles are combined in amounts so as to provide precious metal of Pt:Pd:Rh of about 3-10:3-10:1 by weight in the catalyst composition. The two particle materials impregnated with precious metal comprise 10-30 (wt) % of the catalyst material mixture, i.e., the mixture of the three kinds of particles. When this catalyst material is washcoated onto a substrate, such as a honeycomb substrate often termed a “catalyst brick”, the catalyst material mixture preferably comprise about 10-20(wt) % of the total weight of the substrate plus the catalyst materials. Preferably, the total precious metal carried on the substrate is 9-17 g/ft
3
based on the volume of the substrate, e.g., the brick.
According to another aspect of the invention, it is the catalyst made by the process disclosed above and yet another aspect is the method of treating exhaust gases generated by a stoichiometric gasoline engine with the catalyst by contacting the gas with the catalyst.
PREFERRED EMBODIMENTS OF THE INVENTION
The present invention method for manufacturing a durable, low cost three-way catalyst which may be wash coated onto a substrate, such as the honeycomb substrates commonly used in catalytic converters in the automotive industry. The catalyst is a mixture of at least three particles, two of them being ceria-zirconia supports loaded with precious metal and a third not having been loaded with precious metal. This mixture of particles may be made into a slurry and then coated on the substrate. These particles and other aspects are discussed in detail below.
The ceria/zirconia particles may be made by any technique. One preferred technique involves impregnating ceria particles with a solution containing a soluble zirconium salt, water being the preferred liquid. Conveniently, water soluble zirconium compounds like nitrates and chlorides, or their mixtures, may be used. After the impregnation, the particles are dried and calcined in air for forming the ceria/zirconia particles. After drying, e.g., at 120° C. for several hours, they would be calcined in air at an elevated temperature, e.g., around 600° C. for several hours. The precise temperature and length of time of calcining are not critical. Some specific temperatures and times for certain embodiment of the invention are demonstrated in the examples below. It is believed that during calcining in the presence of air, the zirconium that is deposited on the ceria particle is converted to zirconium oxide in combination with oxygen from the air. Hence, this material is given herein as ceria/zirconia. It may be, however, that some is maintained as zirconium in the ceria particles. Either is considered acceptable and part of the present invention and is considered to be included within the terminology “ceria/zirconia” particles as used herein.
These particles are not expected to be a mere physical mixture ceria and zirconia, but rather are considered to bichemically bonded in the lattice through the oxygen atoms and hence they could be considered to be part of the same oxide. Another way to form these particles is by co-precipitation of the metal oxides, according to techniques well known to those skilled in the art in view of the present disclosure. It is believed that the close atomic proximity of the metal atoms within the oxide lattice of these particles, and optimally a relatively uniform dispersion of the oxides, contributes to the improved HC, CO, and NOx efficiency. Neither the truth nor the understanding of this theory is necessary for practice of the invention. It is provided in an attempt to explain the unexpected superior properties of the present invention catalyst.
As seen from the disclosure above, these ceria/zirconia precious metal support particles in their preferred embodiments contain an excess of cerium relative the zirconium. That is, while the Ce/Zr atomic ratio of these particle is in its broadest embodiment 20:1 to 1:1, preferably the ratio is 10:1 to 2:1. As disclosed above, ceria/zirconia particles “a” are used to support precious metal consisting essentially of platinum together with palladium, while ceria/zirconia particles “b” are used to support precious metal consisting essentially only of rhodium. Thus, the rhodium is carried as the sole precious metal on a different particle than carries the platinum/palladium mixture. The Ce/Zr atomic ratio may be the same or different for the particles “a” and “b” as is also the case for the amount of precious metal loading on these different particles.
To provide the precious metal on the ceria/zirconia particles, any technique including the well-known wet impregnation technique from soluble precious metal precursor compounds may be used. Water soluble compounds are preferred, including, but not limited to nitrate salts and materials for platinum like chloroplatinic acid. As is known in the art, after i
Hurley Ronald G.
Li Jun (John)
Li Shiyao
Brooks & Kushman P.C.
Ford Global Technologies LLC
Langel Wayne A.
Parcari Damian
Strickland Jonas N.
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