GB2046118A

GB2046118A - Auto Exhaust Catalysts

Info

Publication number: GB2046118A
Application number: GB8002237A
Authority: GB
Original assignee: American Cyanamid Co
Current assignee: Wyeth Holdings LLC
Priority date: 1979-03-22
Filing date: 1980-01-23
Publication date: 1980-11-12
Also published as: FR2451773A1; IT1143068B; DE3010800A1; NL8000554A; JPS55129135A; IT8047688A0

Abstract

Extruded support particles for noble metal catalysts for treatment of exhaust emissions are found to improve efficiency of the catalyst in use when the particles are shaped as concave polylobal solid particles having a concavity index greater than 1, a void fraction of 0.25-0.60 and a ratio of geometric surface area to geometric volume of 1000-24 inch<-1> (approximately 390-10 cm<-1>).

Description

SPECIFICATION Auto Exhaust Catalysts The invention relates to improvements of catalysts for use in catalytic conversion of internal combustion engine exhausts. Most automobiles recently manufactured in the United States have been equipped with catalytic converters to reduce the emission of carbon monoxide and hydrocarbons by catalytic treatment of the engine exhaust gas. The most widely used catalytic converters for this use employ noble metal catalysts on refractory supports of alumina or the like. The present invention is an improvement of that type of exhaust conversion catalysts and particularly those which have small particulate support particles of extruded alumina or other suitable refractory support material.In a most preferred embodiment of the invention the catalyst comprises one or several noble metals that are active for catalytic oxidation of hydrocarbons and carbon monoxide and further comprises another noble metal that is active for the catalytic reduction of NOx components. A combination of platinum or platinum and palladium with rhodium and with added improvers on extruded alumina support particles is especially suitable for this purpose.

The particulate catalysts of the invention when in use are packed in a catalytic conversion reactor. Exhaust gas emissions from an engine are fed through the packed reactor in which the gas contacts catalytic noble metals at the particle surface.

A high ratio of the external surface area of the particle to the reactor volume containing the catalyst particles is especially important in catalytic exhaust conversion reactors, in which most of the effective contact of gas to catalyst occurs at the outer surface of the particles. Only a very small proportion, if any, of the catalytic contact occurs by diffusion of gas to catalytic surface areas within the particles. Greater external particle surface area in the given volume of a reactor can be obtained by reducing the catalyst particle size but the reduction of particle size increases pressure drop across the reactor.

Engine operating requirements limit the permissible pressure drop that can be tolerated for an exhaust conversion reactor, hence limit the minimum particle size that can be used in a reactor for that use. An object of the invention is to provide catalyst particles, for use in catalytic conversion reactors, that will provide greater external particle surface area and hence provide greater surface area available for catalytic contact in a reactor of given size with a given pressure drop.

Using no more catalytic metal than was used on particulate catalysts of the conventional cylindrical extrudate shape, significant improvement of catalyst performance is obtained by the use of a more efficient particle shape, in accordance with the present invention.

In the prior art, the conventional extrudate shape for emission control catalyst supports has been an extruded cylinder of small diameter, e.g.

one-eighth inch, and of small length e.g. threesixteenths inch. In accordance with the invention we provide emission control catalysts having extruded support particles with concave polylobal cross sectional shapes. Examples of concave polylobal cross sectional shapes are dumb-bell shapes, trilobal and quadrilobal shapes and other concave polylobal extrudate shapes having cross sections of the kind described in detail in U.S.

Patent No. 3,990,964 patented November 9, 1976 and in U.S. Patent No. 3,966,694 patented June 29, 1976, and in U.S. Patent No. 3,857,780 patented December 31, 1 974. The specifications and drawings of all of those patents are incorporated herein by reference for the descriptions therein and particularly for the descriptions of concave polylobal particle shapes in those patents.

A geometric solid is concave whenever there are at least two separate points lying within or on the surface of the solid which can be connected by a straight line which is not completely contained within said solid or on its surface. The concavity index of any concave solid will be greater than 1.0 as explained in U.S. Patent No.

3,990,964.

Concave catalyst particles in the size range suitable for use in the present range have a characteristic ratio of geometric surface area to geometric volume in the range from about 1000 inch-' to 24 inch~ and preferably between 200 inch and 40 inch~1.

Void fraction is a value describing the closeness of particle packing characteristic of a given particle shape and size. It is the ratio of the volume of void space between particles to the total container volume in a container packed full with said particles. Particles suitable for use in the present invention will have a characteristic void fraction value in the range from 0.25 to 0.60 and preferably between 0.35 and 0.50.

Furthermore, catalyst particles suitable for use in the present invention are extrudates having concave polylobal cross sectional shape. We use the word polylobal to define the outer shape defined by two or more outer surfaces of intersecting lobes, such as cylindrical segments or the like joined to the unitary particle structure.

These lobes in cross-section will appear as segments of intersecting circles, or as circular segments joined to other members in a manner to make a polylobal unitary structure. In our most preferred embodiments the polylobal shape will appear in cross section as a solid area enclosed by two to five intersection circles. We prefer the symmetrical extrudate solid shapes having three or four outer lobes formed by segments of three or four intersecting cylinders (intersecting circles in cross section) of equal diameter on centers defined by the corners of an equilateral triangle in the case of three lobes, or of equilateral squares in the case of four lobes, extended along the axis of the extrudate solid.However, the invention comprises other polylobal concave shapes which may vary from the symmetric shapes we prefer, provided such asymmetric polylobal concave shapes have the characteristic ratios within the ranges defined above.

We use the term trilobal shaped to define the extrudate shape that is defined by the outer segments of three intersecting cylinders and the term quadrilobal shape to define the extrudate shape that is defined by the outer segments of four intersecting or tangent cylinders. All of the extrudate shapes useful for this invention are filled solids, i.e. solid shapes having no internal cavities (by this definition we do not mean to exclude the inherent porous structure of any alumina materials which may be selected to constitute the shaped extrudate particle).

Extruded support particles having the concave polylobal shape characteristics defined above will provide more outer particle surface area for a given unit of support weight of volume than will extrudate cylinders of the same weight or volume of the same support material. Furthermore, the concave cross section particles are found to present less flow resistance in closely packed reactors than conventional cylinders, and so the concave extrudate particles can be used in a size that is smaller than a given cylinder particle size to provide an equivalent pressure drop across a reactor. Both the concave shape of the extrudate particles as defined above, and the smaller usable particle diameter contribute to the increase of available geometric surface area on the catalyst particles in a reactor of given volume packed with these particles.

The concave extrudate shape that we most prefer is a concave quadrilobal shape having a cross section defined by the outer segments of four tangent or overlapping circles or equal diameter centered on the four corners of a square having sides in the range from about 3/8 to the full length of the circle diameters. This cross section projected along the axis of extrusion provides solid particles having a concave outer surface defined by four intersecting or tangent cylinder sections. The length of a side of the smallest square that can envelop the quadrilobe cross section is the diameter we refer to in describing the diameter of the quadrilobe particle.

We have found that such quadrilobe extrudate particles, having diameter of .094 inch, with the four small cylinders of diameter .047 centered on corners of a .047 inch square, when packed in a catalytic conversion reactor of given volume will cause about the same pressure drop across the reactor in operation as the pressure drop caused by cylindrical extrudate particles of one-eighth inch diameter in the same reactor. Accordingly, we compare shaped particles with cylindrical particles which have different diameters because both will cause approximately the same pressure drop across the same reactor.

A most preferred material for the support particles of the invention is a low density base alumina made with rehydratable alumina and preferably also with a combustible filler, such as microcrystalline cellulose. A typical support material of this kind is one that is made by the process described in U.S. Patent No. 4,119,474 patented October 1 0, 1 978 and which meets the specifications described in that patent. The same material is used for making the extruded polylobal shapes as well as the extruded cylinders which are used for comparison in the examples below.

Any other suitable alumina support material that can be formed in the concave shapes described, as by extrusion, and that is suitable for use as a refractory catalyst support for noble metal catalysts in an exhaust gas conversion reactor may be used as the support material in a catalyst embodying the invention.

The invention is described in more detail below using a three-way conversion catalyst which comprises platinum as the noble metal catalyst for conversion by oxidation of hydrocarbons and carbon monoxide and which comprises rhodium as the noble metal catalyst for conversion by reduction of NOx components in the exhaust emissions. The invention is useful as well with other combinations of noble metal catalysts or with only one selected noble metal catalyst, which may be useful for oxidation only or for reduction only or for both.

In the preferred embodiments described in more detail below, iron and cerium improvers were added to improve the properties of emission control catalysts (see U.S. Patent No. 3,224,831) but the invention may be used in catalysts having no added improvers or in those having other selected promoters or improvers for the catalyst in addition to or instead of those described in the #amples.

Example 1 Catalyst support particles of low density alumina are prepared from a mixture of rehydratable alumina and microcrystalline cellulose, extruded and finished by calcining as described in U.S. Patent No. 4,119,474, except the mixture of alumina and microcrystalline cellulose prepared for extrusion is extruded through an extrusion die shaped to form the quadrilobe extrudates described above having diameter of 0.094 inch. The average length of the finished support particles is about threesixteenths of an inch and the geometric surface to volume ratio is 71 inch~1. Supports for the control catalyst are made the same except the extrusion mixture is extruded through a circular die to form cylindrical extrudates having diameter of one eighth inch, average length about three- sixteenths inch and geometric surface to volume ratio of 43 inch~1. A ten-liter measure of finished extrudates is impregnated with catalytic and promoter metals, dried, calcined and reduced by conventional techniques to make finished catalyst particles containing metals in the following amounts:: 8 gm/liter Fe 8 gm/liter Ce .0725 gm/liter Rh .580 gm/liter Pt .0795 gm/liter P A ten-liter measure of the cylindrical support particles is impregnated and finished by the same procedures, using the same quantities of the metal compounds to make the same volume of comparison catalyst. The catalytic and promoter metal weight contents per unit of volume containing the packed catalyst will be the same for the comparison catalyst as for the test catalyst, regardless of any difference in the support weights. Thus, a reactor of given volume filled with either the quadrilobe test catalyst or the cylindrical comparison catalyst particles will contain the same amounts of catalytic and promoter metals.

The test and comparison catalysts prepared as described above were packed in respective catalyic conversion reactors of the same volume and each was tested on an engine test stand under identical conditions for one thousand hours.

The engines were operated to simulate driving conditions at average speed of 50 miles per hour.

The test parameters were periodically measured during the test run to determine Hydrocarbon Efficiency %, CO/NOx crossover efficiency, and time in seconds to reach 50% efficiency from cold start. At the outset of the tests (0 miles) the Hydrocarbon Efficiency and CO/NOx crossover efficiency did not vary significantly for the test and comparison catalysts. All of those values were between 95 and 100 percent efficiency.

Vehicle EPA efficiency values measured on the new catalyst were only slightly higher for the test catalyst than for the control. However, at zero miles the time to reach 50% conversion time from cold start was significantly better with the quadrilobe catalysts than with the cylinders, as measured for all three impurities, hydrocarbon, carbon monoxide and NOx. On continued running, the hydrocarbon efficiency and CO/NOx crossover efficiencies gradually fell off for both the test and control catalysts, but the rate of decline was noticeably more gradual for the test catalyst.

The Hydrocarbon (HC) Efficiency for the quadrilobe shaped catalyst fell below 90% for the first time after running time equivalent to 25,000 miles. The HC efficiency of the cylindrical catalyst had already declined to values below 90% at 17,500 miles and remained below 90% in all subsequent tests. HC efficiency for the quadrilobes did not fall below 80% efficiency until the test at 45,000 miles, but the cylinders first measured below 80% HC efficiency at 32,500 miles and remained below 80% in all subsequent tests.

The CO/NOx Crossover Efficiency of the quadrilobe catalyst first fell below 90 percent at 15,000 miles; the cylindrical catalyst fell below 90% at 7,500 miles and in all subsequent tests.

After 25,000 miles, all of the CO/NOx crossover efficiency measurements to the end of the test remained in a range about 70% + 3% for the quadrilobe catalyst, while all those measurements for the cylindrical catalyst after 25,000 miles ranged lower, erratically, between 55 and 70%.

and mostly near 60% crossover efficiency.

In the time-toreach 50% conversion tests, the times (seconds after cold start) for cylindrical catalyst to attain 50% conversion were substantially longer, on every point of comparison with the quadrilobe catalysts, in every measurement made from zero miles to 50,000 miles. In every test for time-to-reach 50% conversion, at mileages from zero to 50,000 miles the time-to-reach 50% conversion for each of the HC, CO and NOx conversions, was less for the quadrilobe catalyst, by differences that ranged from 15 seconds to over 100 seconds and these differences generally become more pronounced as the two catalysts were used for longer times.

The test results clearly demonstrated that the concave polylobal extrudates were superior to cylindrical extrudates for use as supports for noble metal catalysts in catalytic emission control converters, providing significantly better conversion efficiency and significantly faster startup performance throughout the life of the catalyst, without the need for increased amounts of catalyst metals or promoter metals.

Claims

1. Catalyst for treatment of exhaust emissions from internal combustion engines comprising extruded refractory support particles shaped as concave polylobal solids having concavity index greater than one, void fraction in the range from about 0.25 to about 0.60 and ratio of geometric surface area to geometric volume in the range from about 1,000 to about 24 inch~1, said particles further comprising catalytic noble metal on said support particles for catalytic oxidation of incompletely oxidized components of said exhaust emissions in a catalytic conversion reactor packed with said particles.

2. Catalyst defined by Claim 1, wherein said void fraction is in the range from about 0.35 to about 0.50, and said ratio of geometric surface area to geometric volume is in the range from about 200 to about 40 inch~1.

3. Catalyst as defined by Claim 1 or Claim 2, wherein the polylobal solids have a cross-section defined by from two to five intersecting or tangent circles.

4. Catalyst as defined by Claim 4, wherein the polylobal solids are quadrilobal shapes.

5. Catalyst defined by Claim 4, wherein the quadrilobal shape is defined by segments of four overlapping or tangent cylinders of equal diameter centered on respective corners of a square.

6. Catalyst defined by Claim 5, wherein said square has sides whose length is from 3/8 up to equal said diameter of said cylinders.

7. Catalyst defined by any preceding claim, wherein said catalytic noble metal is selected from platinum, palladium and mixtures thereof.

8. Catalyst defined by any preceding claim, wherein said noble metal catalyst further comprises noble metal catalyst on said support particles for catalytic reduction of NOx components in said emissions.

9. Catalyst defined by Claim 8, wherein the catalytic noble metal further comprises rhodium.

10. Catalyst defined by Claims 7 and 9, wherein the catalytic noble metals consist of platinum and rhodium.

11. Catalyst defined by any preceding claim, wherein said extruded refractory support consists of or comprises alumina.

12. Catalyst defined by Claim 11, wherein the alumina support particles are calcined extrudates made from rehydratable alumina, optionally with a combustible filler.

13. Catalyst defined by Claim 12, wherein the combustible filler is microcrystalline cellulose.

14. Catalyst defined by Claim 1 and substantially as hereinbefore described.