Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – And additional al or si containing component
Reexamination Certificate
2000-07-21
2003-09-09
Dunn, Tom (Department: 1725)
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
And additional al or si containing component
C502S064000, C502S065000, C502S073000, C502S074000
Reexamination Certificate
active
06617276
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to hydrocarbon trap/catalysts that are effective for trapping hydrocarbons and oxidizing the trapped hydrocarbons in an exhaust gas. The present invention is directed to enhancing oxidation of adsorbed hydrocarbons when the hydrocarbon trap/catalyst reaches a temperature at which the trapped (adsorbed) hydrocarbons are released.
BACKGROUND
Regulatory agencies have promulgated strict controls on the amounts of carbon monoxide, hydrocarbons and nitrogen oxides which automobiles can emit. The implementation of these controls has resulted in the use of catalytic converters to reduce the amount of pollutants emitted from automobiles.
To improve the emissions performance achievable by conversion catalyst compositions, particularly during cold-start operation, it has been proposed to use an adsorbent material to adsorb hydrocarbons during the cold-start period of engine operation. A number of patents disclose the broad concept of using an adsorbent material to minimize hydrocarbon emissions during cold-start engine operation. For example, U.S. Pat. No. 3,699,683 discloses an adsorbent bed placed after both a reducing catalyst and an oxidizing catalyst. That patent also discloses that when the exhaust gas stream is below 200° C., the gas stream is directed through the reducing catalyst then through the oxidizing catalyst and finally through the adsorbent bed, thereby adsorbing hydrocarbons on the adsorbent bed. When the temperature goes above 200° C. the gas stream which is discharged from the oxidation catalyst is divided into a major and minor portion. The major portion is discharged directly into the atmosphere. The minor portion is passed through the adsorbent bed, whereby unburned hydrocarbons are desorbed, and the resulting minor portion containing the desorbed unburned hydrocarbons is then passed into the engine where the desorbed unburned hydrocarbons are burned.
Another patent disclosing the use of both an adsorbent material and a catalyst composition to treat an automobile engine exhaust stream, especially during the cold-start period of engine operation, is U.S. Pat. No. 5,078,979. The adsorbent is a particular type of molecular sieve and the catalyst material which may be dispersed in the adsorbent may be a platinum group metal.
An international application published under the Patent Cooperation Treaty, International Publication Number WO 97/22404, discloses the use of an ion exchange reaction to alter the adsorption characteristics of a zeolite, thereby forming a basic zeolite said to be useful for adsorbing hydrocarbons from-exhaust streams. The ion exchange reaction takes place by mixing an alkaline metal or alkaline earth metal salt (sodium, calcium and magnesium are specifically disclosed) in an aqueous solution with the zeolite for a sufficient time and temperature to cause ion exchange. Typical reaction times range from 0.5 to 4.0 hours at from ambient up to 100° C. and more typically 50 to 75° C. The exchanged zeolite is then filtered and washed with water and dried. According to WO 97/22404, the basic zeolite can be formed into a slurry and then coated on to a carrier substrate.
In a publication by Mark G. Stevens and Henry C. Foley (
Alkali Metals on Nanoporous Carbon: New Solid-Base Catalysts
, Chem. Commun., 519-520 (1997)), it is disclosed that cesium may be entrapped in a carbogenic molecular sieve by vapor-phase deposition. In another publication by Stevens et al., (Mark G. Stevens, Keith M. Sellers, Shekhar Subramoney and Henry C. Foley,
Catalytic Benzene Coupling on Caesium/Nanoporous Carbon Catalysts
, Chem. Commun., 2679-2680 (1998)), such cesium entrapped carbogenic molecular sieves are said to have a high affinity for hydrogen, and for breaking of the C—H bond in benzene and thereby promoting benzene condensation to a biphenyl.
Notwithstanding the foregoing, there remains a need for an improved hydrocarbon trap/catalyst for automotive cold-start operation emissions.
SUMMARY OF THE INVENTION
The present invention pertains to a hydrocarbon trap/catalyst, i.e. a hydrocarbon adsorbing material in which hydrocarbons are adsorbed at a low exhaust gas temperature characteristic of an engine start-up condition. In accordance with the present invention, this material is impregnated with an active metal to enhance oxidation of the hydrocarbons as the hydrocarbons are desorbed from the material at an elevated temperature characteristic of normal engine exhaust conditions. The invention optionally further comprises one or more layers of a support material impregnated with one or more platinum group metal catalysts, in combination with the impregnated hydrocarbon trap catalyst of the present invention.
The present invention differs from prior hydrocarbon trap/catalysts materials by providing an active metal deposited on and in intimate contact with the hydrocarbon adsorbent material but with little or no chemical reaction between the active metal and the adsorbent. While enhancing oxidation of hydrocarbons, this active metal does not affect the adsorption characteristics of the material.
The composition of the present invention typically comprises (a), as the hydrocarbon adsorbent material, a zeolite which is effective for adsorbing hydrocarbons from an engine exhaust and (b) an active metal in intimate contact with the zeolite. The invention optionally further comprises one or more layers of (c) a three way or oxidation catalyst that includes at least one platinum group metal (PGM) and preferably includes a combination of platinum group metals. Most preferably that combination comprises platinum, palladium and rhodium in a weight ratio collectively of about 12:5:1.
The active metal useful for the above purposes is essentially any alkaline metal or alkaline earth metal, such as potassium, rubidium, cesium, beryllium, magnesium, calcium, barium and strontium. Cesium is preferred. Generally, the active metal is deposited in intimate contact with the hydrocarbon adsorbent (zeolite, for example) by pouring, dipping or spraying a soluble salt solution of the active metal onto the adsorbent, which is then heated to dryness.
Optionally, the hydrocarbon adsorbent may be first deposited (prior to impregnation with the active metal) on a catalyst substrate, such as an inert monolithic or foam structure or inert pellets or beads.
While zeolite is the preferred hydrocarbon adsorbent in the present invention, other hydrocarbon adsorbents may also be useful. Among such possibilities are amorphous silica and certain forms of carbon or activated carbon, particularly including refractory forms of carbon such as C
n
fullerenes.
DETAILED DESCRIPTION OF THE INVENTION
The present invention comprises an improved hydrocarbon trap composition including a hydrocarbon-adsorbing material, such as zeolite, which is impregnated with an active metal, such that oxidation of desorbed hydrocarbons is enhanced. Such impregnation may be effected by contacting a dry zeolite with a soluble salt solution, such as an acetate or a nitrate of an active metal, namely an alkaline metal or alkaline earth metal, particularly cesium, and drying the wet zeolite with heating to remove water, leaving the metal in intimate contact with the zeolite but avoiding ion exchange therewith. A similar effect may be produced by slurrying zeolite alone in water and depositing the slurry on a monolithic catalyst substrate, drying the slurry to leave the zeolite in intimate contact with the substrate and then dipping, pouring or spraying an active metal solution over the zeolite substrate and drying that solution, with heat, as above, to leave active metal in intimate contact with the zeolite on the substrate.
Both natural and synthetic zeolites as well as acidic, basic or neutral zeolites may be used as the hydrocarbon adsorbent. Natural zeolites include faujasites, clinoptilolites, mordenites, and chabazites. Synthetic zeolites include ZSM-5, beta, Y, ultrastable-Y, mordenite, ferrierite, and MCM-22, with ZSM-5 and beta preferred. The SiO
2
:Al
2
O
3
ratio for thes
Andersen Paul Joseph
Ballinger Todd Howard
Dunn Tom
Ildebrando Christina
Johnson Matthey Public Limited Company
RatnerPrestia
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