WO2000013772A1

WO2000013772A1 - Catalyst composition for the decomposition of ozone

Info

Publication number: WO2000013772A1
Application number: PCT/US1999/017195
Authority: WO
Inventors: Michel Deeba
Original assignee: Engelhard Corporation
Priority date: 1998-09-08
Filing date: 1999-07-29
Publication date: 2000-03-16
Also published as: AU5326099A

Abstract

A catalyst composition for decomposing ozone in which at least one first coating of a first catalytic material suitable for decomposing ozone which includes either a manganese oxide or a palladium-containing material on a refractory support is placed on a substrate and at least one second coating of a second catalytic material selected from the group consisting of platinum and rhodium-containing materials and silver oxide is placed over the first coating which may provide significantly better rates of decomposition of ozone, especially in humid and/or low temperature operating conditions.

Description

CATALYST COMPOSITION FOR THE DECOMPOSITION OF OZONE

Field of the Invention

The present invention is directed to a catalyst composition for decomposing

ozone into harmless byproducts. The catalyst composition includes at least two

coatings where at least one first coating contains an effective amount of a first

catalytic material suitable for decomposing ozone and particularly a catalytic material

containing manganese oxide or palladium on a refractory support. There is also

provided at least one second coating containing an effective amount of a second

catalytic material, different from the first catalytic material, which is specifically

selected from platinum and/or rhodium-containing materials and/or silver oxide. The

catalyst composition of the present invention provides enhanced ozone

decomposition at ambient temperature conditions, especially under humid

conditions.

Background of the Invention

Ozone treating catalyst compositions are well known in the art. Such

compositions include manganese oxides, especially manganese dioxide alone or in

combination with copper oxide and aluminum oxide. Precious metal-containing

materials are also known to decompose ozone. However, when humidity is present,

catalytic decomposition is hindered. One way to lessen the effect of humidity on ozone decomposition is to heat the catalyst. Thus, these prior art catalysts are

effective in decomposing ozone at elevated temperatures and low humidity

conditions.

Enhanced ozone conversion rates at low temperature conditions especially at

high humidity are desirable to improve the efficiency of ozone conversion and to

reduce the cost of eliminating ozone from the atmosphere. The ability to convert

ozone to harmless byproducts under low temperature, high humidity conditions

would provide a cost effective way of reducing the ozone content in the atmosphere.

It would therefore be a significant advance in the art of treating atmospheres

containing ozone to improve the efficiency of the decomposition of ozone to

harmless byproducts by providing a catalyst composition that can effectively convert

ozone at low temperature and/or high humidity conditions.

Summary of the Invention

The present invention is generally directed to a catalyst composition for

decomposing ozone into harmless byproducts. Catalyst compositions of the present

invention are especially useful for the decomposition of ozone in a humidified

atmosphere and/or under low temperature operating conditions. In a preferred form of the invention, the catalyst composition for decomposing

ozone comprises:

a) at least one first coating of an effective amount of a first catalytic

material suitable for decomposing ozone which includes a manganese oxide or

palladium on a refractory support; and

b) at least one second coating containing an effective amount of a

second catalytic material which is selected from platinum and/or rhodium containing

materials and/or silver oxide.

Enhanced rates of decomposition of ozone are obtained, especially in high

humidity atmospheres, with the composition of the present invention. The present

catalyst composition can be used on a variety of substrates including those

associated with automotive vehicles such as radiators, heat exchangers such as air

conditioners and the like, especially under low temperature conditions including

ambient temperatures.

Detailed Description of the Invention

The present invention is generally directed to a catalyst composition and

method of using the same for the treatment of an atmosphere containing ozone and

specifically to the decomposition of ozone into harmless byproducts. The

composition of the present invention is especially effective in decomposing ozone under high humidity conditions even when the decomposition is conducted at

relatively low temperatures such as ambient temperatures. The present invention

therefore provides a cost effective means of reducing the ozone content of the

atmosphere because a) the atmosphere does not have to be heated to elevated

temperatures and b) the water vapor content of the atmosphere does not have to be

reduced, each of which adds to the cost of the ozone conversion operation.

As used herein, the term "high humidity" shall mean an atmosphere whose

relative humidity is above about 50%, and even above 70%. The term "low

temperature" shall mean temperatures which are typically less than about 100°F,

and even as low as room temperature or lower.

In accordance with one aspect of the present invention, a first coating of a

first catalytic material suitable for decomposing ozone which includes manganese

oxide or palladium on a refractory support (e.g. alumina) is coated with a second

coating containing an effective amount of the second catalytic material selected from

platinum and rhodium-containing compounds and silver oxide.

As used herein the term "first coating" shall mean that one of the catalytic

materials is applied to the substrate by any one of a number of processes that place

the catalytic material on or within the substrate. The "second coating" shall mean

that the second catalytic material is likewise placed on or within the substrate. The first and second coatings may, but not necessarily, form separate and distinct layers.

Instead, the first and second coatings may constitute a single layer on or within the

substrate.

One useful and preferred class of catalytic materials for the formation of the

first coating include manganese compounds, including oxides such as Mn₂O₃ and

MnO₂ with a preferred composition comprising α-MnO₂, and cryptomelane being

most preferred. The manganese compounds (e.g. manganese oxides) may be

combined with other oxides. In this regard, other useful and preferred compositions

include a mixture of MnO₂ and CuO; and ceria; MnO₂, ceria and CuO and the like.

Specific and preferred compositions comprise hopcalite which contains the

combination of CuO and Mn0₂ and, more preferably Carulite^® which contains the

combination of MnO₂, CuO and AI₂O₃ and is sold by the Ca s Chemical Company.

More specifically, ozone treating catalyst compositions for the first coating

comprise manganese compounds including manganese dioxide, non stoichiometric

manganese dioxide (e.g., XMnO_{(1 5}.₂₀₎), and/or XMn₂O₃ wherein X is a metal ion,

preferably an alkali metal or alkaline earth metal (e.g. sodium, potassium and

barium). Variable amounts of water (H₂O, OH^") can be incorporated in the structure

as well. Preferred manganese dioxides, which are nominally referred to as MnO₂

have a chemical formula wherein the molar ratio of manganese to oxide is about

from 1.5 to 2.0. Up to 100 percent by weight of manganese dioxide Mn0₂ can be used in catalyst compositions to treat ozone. Alternative compositions which are

available comprise manganese dioxide and compounds such as copper oxide alone

or copper oxide and alumina.

Useful and preferred manganese dioxides are alpha-manganese dioxides

nominally having a molar ratio of manganese to oxygen of from 1 to 2. Useful alpha

manganese dioxides are disclosed in U.S. Patent No. 5,340,562 to O'Young, et al.;

also in O'Young, Hydrothermal Synthesis of Manganese Oxides with Tunnel

Structures presented at the Symposium on Advances in Zeolites and Pillared Clay

Structures presented before the Division of Petroleum Chemistry, Inc. American

Chemical Society New York City Meeting, August 25-30, 1991 beginning at page

342; and in McKenzie, the Synthesis of Birnessite, Cryptomelane, and Some Other

Oxides and Hydroxides of Manganese, Mineralogical Magazine, December 1971 ,

Vol. 38, pp. 493-502, each of which is incorporated herein by reference. For the

purposes of the present invention, the preferred alpha-manganese dioxide is

selected from hollandite (BaMn₈O16-xH₂O) cryptomelane (KMn₈O16-xH₂O),

manjiroite (NaMn₈O16-xH₂O)or coronadite (PbMn₈O16-xH₂O).

The manganese dioxides useful in the present invention preferably have a

surface area as high as possible. A preferred surface area is at least 100 m²/g. The composition preferably comprises a binder of the type described below

with preferred binders being polymeric binders or inorganic binders such as

zirconium oxide, silica or alumina sol.

It has been found that the use of compositions comprising the cryptomelane

form of alpha manganese oxide, which also contain a polymeric binder can result in

greater than 50%, preferably greater than 60% and typically from 75-85%

conversion of ozone in a concentration range of up to 400 parts per billion (ppb).

The preferred cryptomelane manganese dioxide has a crystalline size ranging

from 2 to 10 nm and preferably less than 5 nm. It can be calcined at a temperature

range of from 250°C to 550°C and preferably below 500°C and greater than 300°C

for at least 1.5 hours and preferably at least 2 hours up to about 6 hours.

The preferred cryptomelane can be made in accordance with methods

described and incorporated into U.S. Patent Application Serial No. 08/589,182 filed

January 19, 1996 (Attorney Docket No. 3777C), incorporated herein by reference.

The cryptomelane can be made by reacting a manganese salt including salts

selected from the group consisting MnCI₂, Mn(NO₃)₂, MnSO₄ and Mn (CH₃COO)₂

with a permanganate compound. Cryptomelane is made using potassium

permanganate; hollandite is made using barium permanganate; coronadite is made

using lead permanganate; and manjiroite is made using sodium permanganate. It is recognized that the alpha-manganese dioxide useful in the present invention can

contain one or more of hollandite, cryptomelane, manjiroite or coronadite

compounds. Even when making cryptomelane minor amounts of other metal ions

such as sodium may be present. Useful methods to form the alpha-manganese

dioxide are described in the above references which are incorporated herein by

reference.

The preferred alpha-manganese dioxide for use in accordance with the

present invention is cryptomelane. The preferred cryptomelane is "clean" or

substantially free of inorganic anions, particularly on the surface. Such anions could

include chlorides, sulfates and nitrates which are introduced during the method to

form cryptomelane. An alternate method to make the clean cryptomelane is to react

a manganese carboxylate, preferably manganese acetate, with potassium

permanganate.

It is believed that the carboxylates are burned off during the calcination

process. However, inorganic anions remain on the surface even during calcination.

The inorganic anions such as sulfates can be washed away with the aqueous

solution or a slightly acidic aqueous solution. Preferably the alpha manganese

dioxide is a "clean" alpha manganese dioxide. The cryptomelane can be washed at

from about 60°C to 100°C for about one-half hour to remove a significant amount of

sulfate anions. The nitrate anions may be removed in a similar manner. The "clean" alpha manganese dioxide is characterized as having an IR spectrum as disclosed in

U.S. Patent Application Serial No. 08/589,182 filed January 19, 1996.

A preferred method of making cryptomelane useful in the present invention

comprises mixing an aqueous acidic manganese salt solution with a potassium

permanganate solution. The acidic manganese salt solution preferably has a pH of

from 0.5 to 3.0 and can be made acidic using any common acid, preferably acetic

acid at a concentration of from 0.5 to 5.0 normal and more preferably from 1.0 to 2.0

normal. The mixture forms a slurry which is stirred at a temperature range of from

50°C to 110°C. The slurry is filtered and the filtrate is dried at a temperature range

of from 75°C to 200°C. The resulting cryptomelane crystals have a surface area of

typically in the range of at least 100 m /g.

Another catalytic material for the formation of the first coating is a palladium-

containing material dispersed on a suitable refractory oxide support, preferably

alumina.

It will be understood that the term "palladium-containing material "refers to

palladium in its elemental form and/or in the form of compounds or mixtures thereof

such as for example, salts of palladium. The composition containing palladium on the support comprises from about

1.0 to 10.0 percent by weight, and preferably from about 2.0 to 5.0 percent by

weight of palladium metal on the support, based on the weight of the palladium

metal (metal and not oxide) and the support.

The second coating contains a second catalytic material selected from

platinum and/or rhodium-containing compounds and/or silver oxide.

Suitable examples of platinum and rhodium containing materials include

platinum and rhodium themselves or in the form of salts (e.g. nitrates, sulfates and

chlorides) as well as solutions thereof.

An effective amount of the first catalytic material is generally in the range of

from about 0.5 to 3.0 g/in³, preferably from about 1.0 to 2.0 g/in³. The effective

amount of the second catalytic material forming the second coating as a coating

layer is generally from about 0.01 to 1.0 g/in³, preferably from about 0.1 to 0.3 g/in³.

When the second catalytic material is applied in a post dipping operation if platinum

and/or rhodium-containing materials are used, the amount of the metal catalyst will

be as described above. However, when silver oxide catalysts are applied by post

dipping, the amount of the second catalytic material is generally from about 0.1 to

1.0 g/in³, preferably from about 0.2 to 0.5 g/in³. The first and second coatings may be applied by a variety of methods known

to those skilled in the art including coating, spraying, dipping and the like.

Particularly preferred methods include coating and post dipping. Coating is

performed by preparing a solution of the second catalytic material and then applying

a coat of the second catalytic material on the first layer which is thereafter dried and

calcined.

In another preferred method the second catalytic material is formed into a

solution and then the substrate having thereon the first coating is dipped into the

solution for a time sufficient to enable the second catalytic material to disperse into

the first coating to form a single layer system. This post dipping procedure is

followed by the drying and calcining to produce the final catalyst composition.

The catalyst composition of the present invention can be fabricated by placing

the respective coatings on a suitable substrate. Suitable substrates include alumina,

metal, ceramic, automotive surfaces such as radiators and the like. The first coating

can be applied by dipping, spraying, or coating from a slurry containing a binder

(polymeric material or inorganic zirconium acetate salts and silica and alumina gels).

A preferred method of applying the first coating of catalytic material to the substrate

is coating from a slurry. The second coating can be applied directly over the first

coating after the first layer has dried and calcined to form a two layer system.

Generally, the second coating can be applied to the first coating in the same manner as the first coating is applied to the substrate, but preferably by post dipping followed

by drying and then calcining at > 350°C.

As previously indicated, in making the two coat system, the final product may

appear as two discrete layers or as one layer with the second coat dispersed on or

within the first coat.

Example 1

A ceramic substrate was coated with a manganese oxide slurry by placing the

ceramic substrate into a 25% solution of manganese nitrate. The coated substrate

was allowed to dry. Thereafter, the coated substrate was dipped into a silver nitrate

solution and then dried at 100°C and calcined at 450°C for two hours. The sample

(S-1 ) was tested for ozone decomposition in a gas containing 6 ppm of ozone at a

space velocity of 500,000 hr^"1 at 25°, 45° and 90°C. The degree to which ozone in

the air was decomposed was measured at steady state conditions after about one

hour on steam. A second sample (S-2) was prepared in the same manner and

treated in the same way except that the air contained 1.5% water. The results are

shown in Table 1. Table 1

The method described above was repeated for two comparative samples (C-1

and C-2) except that the second layer of catalytic material (silver oxide) was omitted.

In particular, the ceramic substrate was coated only with manganese oxide and then

subjected to the ozone containing air under the conditions shown in Table 1.

As shown in Table 1 , the samples prepared in accordance with the present

invention exhibited significantly higher rates of decomposition of ozone in humid

atmospheres as compared to catalytic materials containing only a single catalytic

layer.

Example 2

A solution of palladium nitrate was impregnated onto a low density, high

surface area (280 g/in³) macroporous alumina (VGL obtained from LaRoche, Inc.)

to give a concentration of 5% palladium on alumina. The catalyst was coated to a

level of 0.8 g/in³ dried for one hour at 100°C and calcined at 500°C. Thereafter, the calcined substrate was immersed in a platinum salt solution (platinum amine

hydroxide containing 16% platinum) to form a layer of platinum over the base layer.

The catalytic material was dried for one hour at 100°C and calcined at 500°C. The

results of sample S-3 are shown in Table 2.

A second sample (S-4) was made in the same manner as S-3. Sample S-3

was employed in connection with an air stream containing 6 ppm of ozone at

500,000 hr¹ at room temperature. Ozone conversion was measured in the same

manner as Example 1. Sample S-4 was tested in the same manner except the air

sample contained 1.5% of water vapor. The results are shown in Table 2.

Table 2

Comparative samples C-3 and C-4 were prepared in the same manner as

samples S-3 and S-4 except that the layer of platinum was omitted. The

comparative samples were tested in the same manner as S-3 and S-4 and the

results are shown in Table 2. As shown in Table 2, the samples prepared in accordance with the present

invention exhibited higher decomposition range in both dry air and significantly

added decomposition rates of ozone in wet air.

Example 3

Example 2 was repeated to prepare samples S-5 and S-6 to replace the

platinum layer with a layer of rhodium using a rhodium nitrate solution containing

about 10% rhodium and samples S-7 and S-8 to replace the platinum layer with a

layer of silver oxide using a 10% silver nitrate solution. Each of the samples S-5

through S-8 were treated and tested in the same manner as described in connection

with Example 2. The results are shown in Table 3.

Table 3

As shown in Table 3, samples S-5 and S-6 each containing a coating of

rhodium over a coating of palladium exhibited a greater percentage of

decomposition of ozone in both dry and wet air as compared with comparative

sample C-3 and C-4 (see Table 2). Similarly, higher decomposition rates were obtained with samples S-7 and S-8 employing silver oxide as a coating over the coating of palladium.

Claims

What Is Claimed:

1. A catalyst composition for decomposing ozone comprising:

a) at least one first coating of an effective amount of a first catalytic

material suitable for decomposing ozone which comprises either a manganese oxide

or a palladium-containing material on a refractory support; and

b) at least one second coating containing an effective amount of a

second catalytic material which is selected from platinum and rhodium-containing

materials, silver oxide and combinations thereof.

2. The catalyst composition of claim 1 wherein the second coating is on

top of the first coating.

3. The catalyst composition of claim 1 wherein the first and second

coatings form a single layer.

4. The catalyst composition of claim 1 wherein the refractory support is

alumina.

5. The catalyst composition of claim 1 wherein the first coating further

comprises at least one other metal oxide.

6. The catalyst composition of claim 5 wherein the at least one other

metal oxide is selected from the group consisting of copper oxide and aluminum

oxide.

7. The catalyst composition of claim 1 wherein the manganese oxide has

a surface area of at least 100 m²/g.

8. The catalyst composition of claim 1 wherein the first coating contains

the combination of a palladium-containing compound and alumina and the second

coating contains a platinum-containing material or a rhodium-containing material.

9. The catalyst composition of claim 1 wherein the first coating contains

the combination of a palladium-containing compound and alumina and the second

coating contains silver oxide.

10. The catalyst composition of claim 1 wherein the amount of the first

catalytic material is from about 0.5 to 3.0 g/in³.

11. The catalyst composition of claim 1 wherein the amount of the second

catalytic material is from about 0.01 to 1.0 g/in³.

12. The catalyst composition of claim 3 wherein at least a portion of the

second catalytic material is diffused into the first catalytic material.

13. A method of converting ozone to harmless byproducts comprising:

a) applying to a substrate at least one first coating of an effective

amount of a first catalytic material suitable for decomposing ozone which comprises

either a manganese oxide or a palladium-containing material on a refractory support;

b) applying at least one second coating of an effective amount of a

materials and silver oxide; and

c) passing an ozone-containing gas over the catalytic substrate for

a time sufficient to convert ozone to said harmless byproducts.

14. The method of claim 13 comprising passing said ozone-containing gas

over the catalytic substrate under at least one of high humidity and low temperature

conditions.

15. The method of claim 14 wherein the relative humidity of said ozone-

containing gas is at least about 50% and the temperature of said gas is no more

than about 100°F.

16. The method of claim 13 wherein the amount of the first catalytic

material is from about 0.5 to 3.0 g/in³.

17. The method of claim 13 wherein the amount of the second catalytic

material is from about 0.01 to 1.0 g/in².

18. A device for converting ozone to harmless byproducts comprising a

substrate having thereon a catalyst composition comprising:

a) at least one first coating of an effective amount of a first catalytic

or palladium-containing material on a refractory support.

b) at least one second coating containing an effective amount of a

materials and silver oxide and combinations thereof.

19. The device of claim 18 wherein the second coating is on top of the first

coating.

20. The device of claim 18 wherein the first and second coatings form a

single layer.

21. The device of claim 18 wherein the refractory support is alumina.

22. The device of claim 18 wherein the first coating further comprises at

least one other metal oxide.

23. The device of claim 22 wherein the at least one other metal oxide is

selected from the group consisting of copper oxide and aluminum oxide.

24. The device of claim 18 wherein the manganese oxide has a surface

area of at least 100 m²/g.

25. The device of claim 18 wherein the first coating contains the

combination of a palladium-containing compound and alumina and the second

26. The device of claim 18 wherein the first coating contains the

combination of a palladium-containing compound and alumina and the second

coating contains silver oxide.

27. The device of claim 18 wherein the amount of the first catalytic material

is from about 0.5 to 3.0 g/in³.

28. The device of claim 18 wherein the amount of the second catalytic

material is from about 0.01 to 1.0 g/in³.

29. The device of claim 18 wherein at least a portion of the second

catalytic material is diffused into the first catalytic material.