WO2000061265A1

WO2000061265A1 - Guard bed for emission control catalyst

Info

Publication number: WO2000061265A1
Application number: PCT/US2000/009027
Authority: WO
Inventors: Gordon James Johnston Bartley
Original assignee: Southwest Research Institute
Priority date: 1999-04-08
Filing date: 2000-04-05
Publication date: 2000-10-19

Abstract

A catalytic guard bed (10) is disposed upstream of the primary catalytic converter component (14) of an emission control system (16). The guard bed (10) is formed of a porous metallic foam (12) having a thermally stable phosphateable support layer disposed thereon. Additionally, a catalytic material is disposed on the phosphateable support layer.

Description

Title: GUARD BED FOR EMISSION CONTROL CATALYST

1. FIELD OF THE INVENTION

This invention relates generally to a guard bed for protecting a catalytic converter in an emission control system, and more particularly to such a guard bed that has a porous metallic foam element disposed upstream of the catalytic converter component of the emission control system.

2. DESCRIPTION OF THE RELATED ART

Emissions control catalysts are commonly used to convert noxious emissions into less harmful products. For example, noxious emissions control catalysts are widely used to control emissions from internal combustion engines, such as automobile engines. In recent years, the acceptable emission levels from various sources have become successively lower through new regulations imposed by the Environmental Protection Agency

(EPA) and the California Air Resources Board (CARB) in the

United States, and by appropriately designated government bodies in Europe and other countries . Recent CARB standards such as the Low Emission Vehicle (LEV) and the Ultra Low Emission Vehicle (ULEV) qualifying regulations require that emission catalysts must operate extremely effective levels of performance over a considerable length of time. Significant advances in developing thermally durable catalyst formations have allowed catalysts to be placed closer to the engine, thereby decreasing the catalyst light-off time, and lowering cold-start emissions. However, catalysts are still susceptible to poisoning by deposits formed on their surfaces from fuel and lubricating oil contaminants and additives . Such contaminants may be sulfur and lead in the fuel, while additives may be phosphorus, zinc, magnesium, calcium, and other compounds in lubricating oils . Additives are needed to reduce engine wear and improve oil performance, but can have a detrimental effect on the catalyst performance due to build-up, over time, on the catalyst itself. As the acceptable emission levels decrease, catalyst poisoning becomes increasingly more significant. Limitations on additives can only go so far in reducing catalyst poisoning without reducing the effectiveness of lubricating oils to protect the engine.

One attempt to protect the catalytic elements in an exhaust emission control system from such poisoning is described in U.S. Patent No. 5,857,326, issued January 12, 1999, to Scott Blanchet, and entitled EXHAUST POISON TRAP. The

Blanchet trap includes a tubular housing with an internal peripheral wall whereby exhaust gases passing through the tubular housing are accelerated outwardly by centrifugal force and particulate matter carried in the gas is trapped in a porous lining provided in the tubular housing. Thus, particulate matter carried in the portion of the exhaust gas stream that contacts the porous lining is removed from that portion of the gas stream prior to passage through a catalytic converter. Another system for removing soot from diesel engine exhaust gas is described in U.S. Patent No. 5,067,319, issued November 26, 1991, to Franz Moser and titled SYSTEM FOR

PURIFYING THE EXHAUST GASES OF DIESEL ENGINES. The Moser system includes an apparatus for branching the exhaust gas stream into two branch lines which are adapted to be shut off in alternation by a change-over valve. One of the branch lines includes a soot filter, whereas the other branch line constitutes a by-pass line. During operation of the engine under full load, or under partial load in excess of a predetermined value, the change-over valve is switched to direct the exhaust gas through the soot filter.

Neither of the above-described catalytic converter protection systems operate to treat all of the exhaust gas discharged from the engine 100 percent of the time. Furthermore, both of the trap systems are permanent elements of the emission control system and are not easily replaced when they become saturated or otherwise lose their effectiveness .

The present invention is directed to overcoming the problems set forth above. It is desirable to have a guard bed for protecting a catalytic converter in an exhaust gas system that is disposed normal to the gas flow, requiring that all of the exhaust gases passing to the catalytic converter are first treated to remove compounds that would otherwise poison or adversely affect the catalytic converter. Moreover, it is desirable to have a cost-effective, replaceable, catalytically active catalyst guard bed to protect the primary catalytic converter of an emission control system from the adverse effects of poison deposition. Also, it is desirable to have such a catalyst guard bed that is not only effective at removing poisons from the exhaust gases, but also avoids creating new problems in engine and emissions system performance .

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a guard bed for protecting a catalytic converter in an emission control system is disposed at a position whereat the gases containing products of combustion discharged from internal combustion engine pass through the guard bed prior to passage through the catalytic converter. The guard bed includes a porous metallic foam element disposed normal to the path of the exhaust gases and provides a substrate structure on which a thermally stable phosphateable support layer is disposed. The thermally stable phosphateable support layer includes at least one oxide of a metal selected from the group consisting of aluminum, cerium, lanthanum, praseodymium and neodymium. A catalytic material comprising at least one Group VIII metal is disposed on the phosphateable support layer.

Other features of the catalytic converter guard bed embodying the present invention include the oxides being pre- mixed and deposited as a slurry on the substrate, and then dried and fired to form the phosphateable support layer. Other features -include the support layer being aluminum oxide having at least one oxide selected from the group consisting of cerium oxide, lanthanum oxide, praseodymium oxide and neodymium oxide dispersed within the aluminum oxide. Still other features include the dispersed oxides having a combined surface area from about 10m²/g to about 200m²/ -

Still other features of the catalytic converter guard bed embodying the present invention include the support layer having a phosphation promoter comprising at least one oxide of a metal selected from the group consisting of zirconium, magnesium, calcium, strontium and barium. Yet additional features include the catalytic material disposed on the phosphateable support layer comprising from about 0.02 weight percent to about 2.00 weight percent of the support layer.

Yet another feature of the catalytic converter guard bed embodying the present invention includes the porous metallic foam element being a replaceable disposable component of the emission control system.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the structure and operation of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein: Fig. 1 is a schematic diagram of an emission control system having a protective guard bed embodying the present invention incorporated therein;

Fig. 2 is a partial cross-section of a porous metallic foam element of the guard bed embodying the present invention; and

Fig. 3 is an enlarged section of the porous metallic foam element of the guard bed embodying the present invention.

DETAILED DESCRIPTION OF A

PRESENTLY PREFERRED EXEMPLARY EMBODIMENT In the preferred embodiment of the present invention, as shown schematically in Fig. 1, a catalyst guard bed, indicated generally by the reference numeral 10, comprises a relatively thin, for example on the order of approximately one-half inch thickness, disk of a porous metallic foam 12 positioned upstream of a first catalytic converter 14 of an emission control system, indicated generally by the reference numeral 16. Desirably, the porous metallic foam element or unit 12 is positioned immediately adjacent to the catalytic converter 14, and maintained in that position by a removable housing 18, whereby the metallic foam unit 12 can be readily removed and replaced when its performance deteriorates after a period of use.

Porous metallic foam is cast metal with finely divided, evenly distributed gas bubbles that form a plurality of interconnecting passages throughout the body of the metal, as illustrated in Figs. 2 and 3. The metallic foam element 12 of the present invention is preferably formed of a FeCr alloy, a ferritic stainless alloy, Inconel, or silicon carbide, having from about five to about fifty pores per square inch of surface area, and desirably from about five to about twenty pores per square inch of surface area. Metallic foams formed of the above-described materials and pore densities have excellent strength properties and induce turbulent flow to exhaust gases passing through the unit. Turbulent flow improves exhaust gas interaction with the catalytic surface materials disposed on the metallic foam substrate. In addition, the metallic foam materials listed above have high-temperature strength characteristics whereby the integrity of the guard bed 10 is maintained under high engine exhaust gas conditions with minimal foam thickness.

The guard bed 10 is catalyzed to maintain effective reduction of cold start emissions and also to promote the formation of phosphates on its surface. Platinum Group Metals (PGM) , and in particular palladium, platinum and rhodium, are useful in effecting a catalytic reduction of hydrocarbons, carbon monoxide, and oxides of nitrogen. A second catalytic material may be used to promote formation of phosphates on the guard bed surface. The catalytically active materials are supported in or on a high surface area oxide support layer 20, as illustrated in Fig. 3, which is both resistant to thermal loss of surface area and easily phosphated.

Desirably, the high surface area support layer 20 comprises gamma, theta, or delta alumina, although other high surface area alumina phases may also be used. Desirably, the support layer 20 has a surface area from about 10m²/g to about 200m²/g and preferably from about 80m²/g to about 160m²/g. Phosphateable materials include the aforementioned alumina and, if desired, rare earth oxides such as oxides of cerium, lanthanum, praseodymium, and neodymium oxide, or mixtures thereof. The rare earth oxides may be dispersed on the alumina support from solution using standard impregnation techniques, or added as a physical mixture. If added as a physical mixture, the rare earth oxides desirably have a surface area in the range from about 10m²/g to about 200m²/g. Rare earth oxide loadings are desirably from about one to about 90 weight percent of the total wash coat, and preferably in the range from about five to about 45 weight percent of the total wash coat .

Aluminum and rare earth oxides doped with phosphation promoters such as zirconium, or alkaline earth metals such as magnesium, calcium, strontium and barium, may also be used in the formation of the support layer 20. The phosphation process is also desirably promoted by the addition of catalytic additives such as Group VIII metals, in particular platinum, palladium, rhodium, and ruthenium. When the Group VIII catalytic additives are deposited on the support layer 20, the loadings should be from about 0.02 to about 2.00 weight percent of the support layer 20. The Group VIII catalytic materials also desirably affect the conversion of hydrocarbon, carbon monoxide, and oxides of nitrogen. Thus, the catalytic guard bed 10 desirably enhances the effectiveness of the emission control system, without significantly affecting the operation of the engine. However, the primary purpose of the catalytic guard bed 10 is to effectively remove most of the primary catalyst poisons before the exhaust gases reach the downstream catalytic converter 14.

Thus, the porous metallic foam 12 provides a high surface- area substrate structure for the support layer 20. The support layer 20 disposed on the substrate structure is thermally stable and phosphateable and comprises at least one oxide of a metal selected from the group consisting of aluminum, cerium, lanthanum, praseodymium, and neodymium. A catalytic material comprising a Group VIII metal is disposed on or within the phosphateable support layer 20. Also, as described above, the oxide of at least one metal selected from the group consisting of aluminum, cerium, lanthanum, praseodymium, and neodymium, may be pre-mixed and deposited by impregnation techniques or as a physical mixture on the substrate 12 and then dried and fired to complete formation of the phosphateable support layer 20. Desirably, the oxides dispersed in the support layer have a combined surface area of from about 10m²/g to about 200m²/g. In addition, the support layer 20 may desirably include a phosphation promoter comprising at least one oxide of a metal selected from a group consisted of zirconium, magnesium, calcium, strontium, and barium.

An additional important feature of the guard bed 10 embodying the present invention includes the replaceability of the metallic foam element 12 which may be removed by opening the housing 18, removing the spent unit and replacing it with a new metallic foam element 12. This enables the guard bed 10 to remain effective, even after prolonged use under adverse conditions which would otherwise adversely affect the catalytic converter 14. The metallic foam element 12 should be removed and replaced with a new unit when vehicle emissions exceed acceptable limits. A simple replacement of the metallic foam element 12 is significantly cheaper than replacing the catalytic converter 14.

Although the present invention is described in terms of a preferred exemplary embodiment, with specific illustrative phosphateable and catalytic materials, those skilled in the art will recognize that changes in those illustrative materials may be made without departing from the spirit of the invention. Also, by way of example, the metallic foam element 12 may have a different thickness than that stated in the illustrative embodiment and may have a shape other than that of the illustrative disk shape. Such changes are intended to fall within the scope of the following claims. Other aspects, features, and advantages of the present invention may be obtained from a study of this disclosure and the drawings, along with the appended claims .

Claims

What we claim is : 1. A guard bed for protecting a catalytic converter in an emission control system, said guard bed being disposed in said emission control system at a position whereat gases containing products of combustion discharged from an internal combustion engine pass through said guard bed prior to passage through said catalytic converter, said guard bed comprising: a porous metallic foam element disposed normal to a flow path of said gases and providing a substrate structure, a thermally stable phosphateable support layer disposed on said substrate structure and comprising at least one oxide of a metal selected from the group consisting of aluminum, cerium, lanthanum, praseodymium and neodymium, and a catalytic material comprising at least one Group VIII metal disposed on said phosphateable support layer.

2. The guard bed, as set forth in Claim 1, wherein said oxides are premixed and deposited as a slurry on said substrate, and then dried and fired to form said phosphateable support layer.

3. The guard bed, as set forth in Claim 1, wherein said support layer is aluminum oxide having oxides selected from the group consisting of cerium oxide, lanthanum oxide, praseodymium oxide and neodymium oxide dispersed within said aluminum oxide, said dispersed oxides having a combined surface area of from about 10 m²/g to about 200 m²/g.

4. The guard bed, as set forth in Claim 1, wherein said support layer is aluminum oxide having a metal oxide selected from the group consisting of cerium oxide, lanthanum oxide, praseodymium oxide and neodymium oxide dispersed within said aluminum oxide .

5. The guard bed, as set forth in Claim 1, wherein said phosphateable support layer has a surface area of from about 10 m²/g to about 200 m²/g.

6. The guard bed, as set forth in Claim 1, wherein said support layer includes a phosphation promoter comprising at least one oxide of a metal selected from the group consisting of zirconium, magnesium, calcium, strontium and barium.

7. The guard bed, as set forth in Claim l, wherein said catalytic material comprises from about 0.02 weight percent to about 2.00 weight percent of said support layer.

8. The guard bed, as set forth in Claim 1, wherein said porous metallic foam element is a replaceable disposable element of the emission control system.