WO1997009114A1

WO1997009114A1 - Use of catalysts

Info

Publication number: WO1997009114A1
Application number: PCT/SE1996/001070
Authority: WO
Inventors: Magali Boutonnet Kizling; Sven JÄRÅS; Lars J. Pettersson; Ulf Winter; Marcus F. M. Zwinkels
Original assignee: Institutionen For Kemiteknik Kemisk Teknologi Kth
Current assignee: Institutionen For Kemiteknik Kemisk Teknologi Kth
Priority date: 1995-09-01
Filing date: 1996-08-29
Publication date: 1997-03-13
Anticipated expiration: 1998-03-01
Also published as: SE9503024L; SE504912C2; SE9503024D0; AU6893996A

Abstract

The present invention relates to the use of a catalyst in a catalytic combustion, which catalyst is produced by the coating of a carrier with a suspension of noble metal particles in a microemulsion. The carrier is distributed on a monolith. The invention also relates to a catalyst comprising an active phase which by microemulsion technique has been deposited on a carrier. The active phase comprises noble metal particles. The carrier comprises a metal oxide and is distributed on a monolith. The invention also relates to a method of producing a catalyst as defined above by microemulsion technique, wherein bimetallic particles are produced by the blending of not reduced noble metal particles in microemulsion and then the reduction thereof.

Description

USE OF CATALYSTS

Field of invention

The present invention relates to the use of a parti¬ cular class of catalysts in combustions as well as to new catalysts belonging to said class. The invention also relates to a method of producing said catalysts.

Background of the invention

The interest in alternative fuels began in the early 1970's with the so called energy crisis. The increased awareness of the environment during the past decades has focussed the interest on the search for alternative fuels, which are harmless to the environment. The combus¬ tion of these fuels has to be possible to perform with a high performance as well as a good selectivity to provide as low as possible a level of poisonous or unhealthy substances in the effluents.

Catalytic combustion increases the activity of the process of combustion at the same time as the selectivity may be improved, whereby the amount of many ecologically harmful substances in the the effluents is reduced, or even eliminated.

In Sweden, it has been found that an alternative fuel for engine running, which is harmless to the en-vironment as well as of economical interest, is alco- hoi based fuels, such as ethanol. This choice depends on the access to raw material in the country as well as the resources to produce ethanol. A catalytic combustion of the products of ethanolic combustion is therefore a for the future very promising process, since it together with the appropriate catalyst should be possible to design as an extraordinarily active and selective process, which thereby is harmless to the environment.

Ethanol is already used as a fuel for buses driven on trial in Stockholm. These buses are equipped with diesel engines. It is well known that ecologically harm¬ ful substances are formed when oil and petrol are combusted. Similarily, undesirable substances, such as acetaldehyde and acetic acid, are formed when ethanol is combusted.

The effluents from petrol engines are these days taken care of by car effluent catalysts, which, inter alia , convert NO to N₂ and CO to C0₂. These catalysts con¬ tain noble metals, such as Pt and Rh, deposited on a carrier which often is composed of A1₂0₃.

The same class of catalysts are at the moment used for the purification of the effluents from diesel engines, wherein ethanol is used as a fuel. The effluents from ethanol vehicles contain non-combusted ethanol and acetaldehyde, which in turn may be converted to acetic acid, which is irritating due to its particular odour.

To avoid that these substances are let out in efflu¬ ents, efforts have been made to find suitable oxidation catalysts. Previously, mainly catalysts based on noble metals have been used. However, base metal oxides have been examined as well.

Another area where catalytic combustion is of interest is combustion of fuels in such applications as industrial boilers, gas turbine burners and heat genera- ting systems, i.e. systems that primarily aim at energy recovery. However, the requirements within this field on the properties of the material and complex chemical reaction systems make this one of the most difficult applications of catalytical combustion.

Technical problem

When ethanol is used as a fuel in diesel engines it has been shown, that the catalysts that exist today work satisfactorily during the driving. However, problems arise in situations when the engine is running idle, for instance at bus stops or when in a traffic jam. The reason is that in such situations, the temperature of the catalyst decreases, whereby the activity as well as the selectivity are deteriorated substantially. Accordingly, acetic acid and acetaldehyde, as well as some non- combusted ethanol, exist in the effluents discharged in the city environment.

Thus, a substantial need of a more specific cata¬ lyst, which inter alia has the capacity of working effi¬ ciently at low temperatures, exists, since the temp- erature of the effluents from en engine running idle is 250°C or less.

To specifically design a catalyst which in an efficient way is able to take care of the effluents formed in ethanolic combustion it is necessary to identi- fy the combustion products to work out which the products formed at low temperatures are. Thereafter, it is possible to develop the capacities of the catalyst to convert these products to effluents which are less harm¬ ful to the environment. As mentioned above, effluents from ethanol vehicles contain non-combusted ethanol and acetaldehyde and these substances, together with the acetic acid, have to be removed. Hitherto, the efforts to develop catalysts which enables this efficiently at low temperatures have failed. It has been shown that in the use of catalysts for combustion for the purpose of energy recovery a pre- catalyst, which ignites at a low temperature, is of great importance.

Description of the invention

The present invention essentially solves the above defined problem by providing the use of a new class of catalysts for catalytic combustions. Said catalysts are produced by the coating of a carrier with a suspension of noble metal particles in a microemulsion, the carrier further being distributed on a monolith. The invention further relates to said catalysts and to a method for the production thereof.

Some catalysts of this kind are known per se from EP 81900804.6. However, the present use is not suggested, and even less the extraordinarily results accomplished by the catalyst according to the invention in said use.

Detailed description of the invention

More specifically, the invention relates to a new use of a catalyst in combustions, wherein the catalyst has been produced by the coating of a carrier with a sus¬ pension of noble metal particles in a microemulsion. The carrier has been distributed on a monolith as a support and in order to maximize the contact area between the active or catalytic parts of the catalyst on the carrier and the combustion gases.

In one embodiment of the invention a carrier, which has been coated with a monolayer of the noble metal par¬ ticles, is used. Thereby the contact area relative to the effluents is increased substantially.

In another embodiment of the invention a catalyst as mentioned above is used, wherein the noble metal partic¬ les are monolithic, i.e. each particle is composed of one metal species. Also included are embodiments, where the catalyst includes several metal species, as long as each particle does not include more than one species.

In an alternative embodiment of the invention the noble metal particles are bimetallic, i.e. composed of two metals. In another embodiment of the invention a catalyst is used, wherein the carrier has been coated with noble me¬ tal particles from the Pt group, preferably Pt and/or Pd.

In a certain embodiment of the invention said noble metal particles are Pt and Pd, which have been coreduced in the microemulsion. By the simultaneous reduction of the metals, blended together, surprisingly, an activity is achieved, which is substantially higher than that achieved when the metals are first reduced each one sepa¬ rately and thereafter blended.

In one embodiment of the invention the carrier is a monolith, such as A1₂0₃, Ti0₂ or SiC, preferably A1₂0₃. In another embodiment of the invention a monolith is used, which is composed of a metal, a metal alloy or inorganic oxides. The geometrical shape thereof is such, that combustion gases, or effluents, can pass without any substantial pressure drop. A preferred monolith is produ- ced from cordierite (2MgO^#5Si0₂ ^»2Al₂0₃) . A preferred shape is a cylinder with a large amount of square channels for the reduction of said pressure drop to a minimum.

In one embodiment of the invention the catalytic combustion is the combustion of alcohol based fuels, especially ethanol. The use of the catalyst according to the invention, whose activity is surprisingly substanti¬ ally higher than the previously known catalysts, especi¬ ally at low temperatures, has been proven particularly advantageous in those diesel engines, with which the ethanol fueled city buses are equipped at present.

In another embodiment of the invention the catalytic combustion is combustion in gas turbines, industrial boi¬ lers or heat generating systems. The low flash point in the use of the catalyst according to the invention should under these conditions be especially advantageous.

Another aspect of the invention is a catalyst, which comprises an active phase, which has been deposited on a carrier by microemulsion technique. The active phase comprises noble metal particles and the carrier comprises a metal oxide. Also in this case the carrier has been distributed on a monolith. The catalyst according to the invention has proven to be especially suitable for com¬ bustion reactions, especially of the kind that has been discussed above in context with the use according to the invention, such as the combustion of alcohol based or other organic materials. In one embodiment of this aspect of the invention the carrier has been coated with a monolayer of the noble metal particles.

In another embodiment of this aspect the noble metal particles are monometallic.

In an alternative to this embodiment the noble metal particles are bimetallic.

In a particular embodiment the catalyst according to the invention is composed of noble metal particles, which for instance belong to the Pt group, preferably Pt and/or Pd. In one particular embodiment the noble metal partic¬ les are Pt and Pd, which have been coreduced in the mic¬ roemulsion, i.e. they have been admixed before the reduction thereof was performed. In another embodiment the metal oxide of the carrier is A1₂0₃, Ti0₂ or SiC, A1₂0₃ being preferred at present thanks to its advantageous ageing properties.

In a special embodiment of the catalyst according to the invention the monolith is a metal, a metal alloy or inorganic oxides.

According to a further aspect the invention relates to a method of producing such a catalyst as is described above. The catalyst is manufactured by a microemulsion technique which is known per se. However, according to the invention bimetallic particles are produced by the blending of non-reduced noble metal particles in the microemulsion and then the reduction thereof.

According to a special embodiment of the method according to the invention the noble metal particles belong to the Pt group, preferably Pt and Pd.

EXAMPLE

Production of the catalyst

Three microemulsions were produced for the experi- ments, one Pt microemulsion and one Pd microemulsion as well as one microemulsion which contained both Pt and Pd. From a monolith, with 142,4 g/dm³ of A1₂0₃ carrier, test specimens were cut out. The sizes of the test specimens were designed as to fit in an existing test plant. The monolith had a cell density of 62 cells/cm².

The metal salts were reduced and deposited on the test specimens. The volume of the monolith specimens are estimated to 11000 mm³, or llxlO^-3 dm³.

The monoliths were produced with an metal charge according to Table 1. These metal levels were chosen to enable a relevant comparison with already performed tests of catalytic activity.

The term "Pt-Pd" denotes that the metal salts were coreduced, i.e. both salts were present at the reduction. "Pt+Pd" denotes that each metal salt was reduced in its own microemulsion and that they were then blended.

Table 1

Catalyst Metal concentration Metal concentration on monolith, g/dm³ on monolith, mmol/dm³

Pt 1,8 9,5

Pd 1,0 9,5

Pt, Pt 0,9, 0,5 4,8, 4,8 (1,4 in total) (9,5 in total)

The carrier consists of γ-Al₂0₃ (Condea PX140) with a specific area of about 140 m²/g. These values are valid for catalysts produced by impregnating technique as well as catalysts produced by microemulsion technique.

Catalytic reaction The experimental design enables catalytic tests of gas mixtures, whose composition is such that it resembles the effluents produced in ethanol comustion in a diesel engine. Table 2

Substance Concentration o₂ 10 vol%

H₂0 10 vol%

C0₂ 6,5 vol%

N₂ Balance

CO 300 ppm

NO 600 ppm

C₂H₅OH 200 ppm

The catalyst is tested at an excess of air of 100% (λ=2) in a gas mixture, whose composition is given in Table 2. The experiments are performed at between 75°C and 500°C with a space velocity of lOOOOOh^""1. All of the catalysts were pretreated. The pretreatment implies that the cata- lyst is heated from room temperature to 500°C, with a heating rate of 100/minute. During the heating, the reac¬ tion mixture is passed over the catalyst in the reactor. The catalyst is then cooled with a gas mixture, the com¬ position of which is 10% of 0₂, 10% of H 0 and 80% of N₂. During cooling, the space velocity of the gas mixture is 25000 h^-1.

Measuring equipment

The concentration of the total amount of hydro- carbon, nitrogen oxide and carbon monoxide in the discharge was analyzed on line. To this end, conventional instruments for analysis were used. The total amount of hydrocarbon was analyzed with a flame ion detector (FID) . Nitrogen oxide was analyzed on a chemiluminescence detec- tor (CLD) and carbon dioxide was measured with a non- dispersive infra red instrument (NDIR) . A detailed analysis of oxygen, nitrogen, carbon dioxide, oxidizing substances and hydrocarbons was performed on line in a semicontinuous manner. This was achieved on a gas chromatograph equipped with thermic conductivity, flame ionization and far ultra violet detectors.

Results

In Table 3, the catalytic experiments are summari¬ zed. Values commonly used for the characterization of catalysts have been tabulated. The light-off temperature presented refers to the temperature when a conversion of 50% of the specific component exists.

Table 3

¹ Results from catalysts deposited on Ti0₂ and produced by impregna¬ tion technique. The following reaction steps occur in the oxidation of ethanol:

C₂H₅OH + 30₂ → 2C0₂ + 3H₂0 (1)

C₂H₅OH + 0,5 0₂ → CH3CHO + H₂0 (2)

C₂H₅OH → CH₃CHO + H₂ (3)

C₂H₅OH + 0₂ → CH₃COOH + H₂0 (4)

CH₃CHO + 0,5O₂ → CH3COOH (5)

Thus, acetic acid or acetaldehyde is formed in the oxida- tion of ethanol. It is evident that acetaldehyde is for¬ med during the course of the reaction with all catalysts. On the contrary, no detectable amount of acetic acid is formed. No acetic acid could be detected when the cata¬ lysts were produced by microemulsion technique either.

Catalytic oxidation of ethanol

The catalysts produced by microemulsion technique which are the most active ones are the catalysts con¬ taining Pt (i.e. Pt, Pt-Pd, Pt+Pd) . The light-off temperatures for these catalysts are measurably lower than for the monolith which was coated with Pd only. It is true that catalysts produced by impregnation technique exhibit the same interrelated differences depending on the metal compositions, but the activities thereof are generally lower than those of the corresponding produced by microemulsion technique.

The light-off temperatures for microemulsion produced catalysts are generally lower than for the corresponding catalysts produced by impregnation. For microemulsion produced Pt, Pd and Pt-Pd catalysts the light-off temperatures are respectively 37, 39 and 20 °C lower than for the corresponding impregnated catalysts (Table 3) .

When comparing impregnated catalysts with Ti0₂ as a carrier with the corresponding γ-Al 0₃ catalysts, it is obvious that an impregnated bimetallic Pt-Pd catalyst with a Ti0₂ carrier yields a 25°C lower light-off tempera¬ ture than a γ-Al 0₃ catalyst does. Microemulsion produced Pt catalysts on A1₂0₃ still yield a light-off temperature, which is 37°C lower than that for the impregnated Pt catalysts on Ti0₂.

From these results, it is obvious that microemulsion produced catalysts are active at lower temperatures than the corresponding impregnated catalysts, irrespective of the metal composition. Pt and Pt-Pd are the most active ones. When Al 0₃ is replaced with Ti0 in the impregnated catalysts, a light-off temperature, which is 5°C lower than with the Pt-Pd catalysts on A1₂0₃ produced by microemulsion technique, results.

Formation of acetaldehyde

It is observed that for all of the above discussed catalysts acetaldehyde is formed during the course of the reaction, unregarded which method of production of the catalyst is used. Catalysts produced in a microemulsion convert a larger amount of ethanol into acetaldehyde than impregnated catalysts do. The yield is 10-15% higher than with impregnated catalysts, unregarded the metal composi¬ tion. The Pd catalyst provides the highest yield of 55%. In comparing the temperatures at which acetaldehyde begins to decrease, it is observed that for microemulsion produced Pt catalysts, the acetaldehyde conversion dec¬ reases about 35°C earlier than the corresponding impreg¬ nated catalyst. Formation of acetic acid

No acetic acid could be detected with microemulsion produced catalysts. With impregnated catalysts acetic acid is formed with a yield of 2% with the Pt catalyst and 0,38 with Pd. The detection limit for acetic acid with the present instrumentation is about 0,5 ppm.

Yield of CO

Of the microemulsion produced catalysts the Pt-Pd catalyst had the lowest light-off temperature, also when it comes to the oxidation of CO. The light-off temperatu¬ re for this catalyst is 14°C lower than the best value presented in connection with the impregnated catalysts . On comparison of the Pd catalysts it appears that the microemulsion produced Pd catalyst has a light-off tempe¬ rature which is 14°C lower than that of the corrsponding impregnated Pd catalyst. The light-off temperatures for the Pt catalysts are approximately equivalent for the two classes.

Discussion

The studies of the etanol oxidation on different catalysts show that a number of different factors have influence on the activity of the catalyst. These factors are inter alia the method of production, the nature of the metal and the metal composition. The carrier material also has an influence on the activity of the catalyst.

As regards the nature of the metal it appears that the method of production does not influence upon the dif- ference between the Pt and Pd catalysts, i.e. that Pt is more active than Pd in ethanol oxidation. This might be explained by the fact that Pd forms PdO at a temperature lower than that at which Pt forms PtO. The respective me¬ tal oxide is assumed to be less active than Pt° and Pd°. The catalyst of Pt+Pd, which was produced by the blending together of microemulsion produced Pt and Pd particles, did not exhibit the same activity as the Pt-Pd catalyst. The Pt-Pd catalyst was produced by reduction in a microemulsion which contained metal salt of both Pt and Pd. The catalytic activity of the Pt+Pd catalyst is in the same range as the corresponding catalyst with Pd par- tides only, i.e. it has a high light-off temperature as compared to the Pt catalyst. On the contrary, the Pt-Pd catalyst has a light-off temperature which is comparable to that of the corresponding Pt catalyst. This implies that to obtain a high activity with the bimetallic cata- lysts, the metal particles have to be produced so that the metals are alloyed or blended together.

It is also recognized that the bimetallic catalysts of Pt and Pd lower the light-off temperature during CO oxidation, which may be of interest within other applications, where catalysts are used, i.e. three way catalysts.

Conclusion

The catalysts produced in a microemulsion have been shown to exhibit higher activities in ethanol oxidation than those, which have been produced by impregnation technique. In these studies, only the activities of cata¬ lysts produced by γ-Al₂0₃ as carriers have been compared. However, studies of catalysts produced by impregnation technique show that Ti0₂ may be of interest as an alternative to γ-Al₂0₃.

Claims

1. The use of a catalyst in catalytic combustion, which catalyst has been produced by the coating of a carrier with a suspension of noble metal particles in a microemulsion, the carrier being distributed on a monolith.

2. The use according to claim 1, wherein the carrier has been coated with a monolayer ot the noble metal particles.

3. The use according to any one of claims 1 and 2, wherein the noble metal particles are monometallic.

4. The use according to any one of claims 1 and 2, wherein the noble metal particles are bimetallic.

5. The use according to any one of the preceding claims, wherein the carrier has been coated with particles of noble metal belonging to the Pt group, preferably Pt and/or Pd.

6. The use according to any one of claims 1-2 and 4-5, wherein the noble metal particles are Pt and Pd, which have been coreduced in the microemulsion.

7. The use according to any one of the preceding claims, wherein the carrier is a metal oxide, preferably A1₂0₃, Ti0₂ or SiC.

8. The use according to any one of the preceding claims, wherein the monolith is from a metal, a metal alloy or inorganic oxides.

9. The use according to any one of the preceding claims, wherein the catalytic combustion is the combus¬ tion of alcol based fuels, especially ethanol.

10. The use according to any one of claims 1-8, wherein the catalytic combustion is a combustion in gas turbines, industrial boilers or heat generating systems.

11. A catalyst, especially for combustion reactions, comprising an active phase, which has been deposited on a carrier by microemulsion technique, the active phase comprising noble metal particles, the carrier comprising a metal oxide and the carrier having been distributed on a monolith.

12. A catalyst according to claim 11, wherein the carrier has been coated with a monolayer of the noble metal particles.

13. A catalyst according to any one of claims 11 and 12, wherein the noble metal particles are monometallic.

14. A catalyst according to any one of claims 11 and 12, wherein the noble metal particles are bimetallic.

15. A catalyst according to any one of the preceding claims, wherein the particles are of a noble metal belonging to the Pt group, preferably Pt and/or Pd.

16. A catalyst according to any one of claims 11-12 and 14-15, wherein the noble metal particles are Pt and Pd, which have been coreduced in the microemulsion.

17. A catalyst according to any one of the preceding claims, wherein the metal oxide in the carrier is A1₂U₃, Ti0₂ or SiC.

18. A catalyst according to any one of the preceding claims, wherein the monolith is from a metal, a metal alloy or inorganic oxides.

19. A method of producing a catalyst according to any one of the preceding claims by microemulsion technique, wherein bimetallic particles are produced by blending non-reduced noble metal particles in a micro¬ emulsion and then reducing the same.

20. A method according to claim 19, wherein the bi¬ metallic particles are of noble metals belonging to the Pt group, preferably Pt and Pd.