WO1995031675A1

WO1995031675A1 - Catalytic combustion

Info

Publication number: WO1995031675A1
Application number: PCT/GB1995/001054
Authority: WO
Inventors: Julian Peter Cox; John Ward Jenkins
Original assignee: Johnson Matthey Public Limited Company
Priority date: 1994-05-11
Filing date: 1995-05-10
Publication date: 1995-11-23
Also published as: GB9409389D0; AU2414595A

Abstract

The catalytic combustion of methane at 450-1500 °C, e.g. in a gas turbine, is conducted in the presence of a catalyst wherein stannic oxide is prime catalytically active component or support.

Description

CATALYTIC COMBUSTION

This invention relates to a method and catalyst for the catalytic combustion of methane.

Catal Rev - Sci Eng, 35 (3), 319-358 (1993) is a recent review of catalytic materials for high temperature combustion. The combustion of methane is one of the world's primary sources of energy, for instance to drive gas turbines or heat furnaces and boilers. Burning methane, however, gives rise to environmental problems. At the high temperatures reached, in excess of 1500°C, substantial nitrogen fixation can occur, giving rise to emissions of oxides of nitrogen, which are considered to be one of the worst common pollutants. Furthermore, methane is the least reactive of the hydrocarbons and the product can contain unburnt methane, a "greenhouse gas". One approach to the problems of

excessively high temperature and incomplete combustion has been to catalytically combust the methane. The heat released by the reaction on a catalyst, however, raises the temperature greatly, possibly by as much as 1700°C. Platinum and palladium are active catalytically for this reaction but they cannot be used for extended periods above 800°C since they deactivate rapidly at the higher

temperatures. Accordingly, the concept arose of using several stages in the overall

reaction. A catalyst is employed at a lower temperature to combust a proportion of the methane, and this is followed by a stage employing a higher temperature, where the partially combusted methane is further reacted, especially in the presence of a less active but more thermally stable catalyst.

A remarkably good catalytically active material has now been discovered for the catalytic combustion of methane at 450-1500°C.

Accordingly, the invention provides a method for the catalytic combustion of methane, which method comprises contacting a mixture of oxygen and uncombusted or partially combusted methane at a temperature of 450-1500°C with a catalyst comprising stannic oxide and optionally platinum group metal selected from the group consisting of one or more of palladium, platinum, rhodium and ruthenium, and where this platinum group metal is present, the atomic ratio of the tin of the stannic oxide to the platinum group metal is greater than 12:1. The invention also provides a method for the catalytic combustion

of methane, which method comprises contacting a mixture of oxygen and uncombusted or partially combusted methane at a temperature of 450-1500°C with

a catalyst comprising a prime catalytically active component on a support, wherein the support comprises stannic oxide.

The invention provides also a catalyst for use in the catalytic combustion of methane, which catalyst comprises a honeycomb monolith coated with prime catalytically active component consisting essentially of stannic oxide.

Tin oxide is known as a catalyst, see for instance US patent specification 4830844, but not for use in the present method.

J Chem Soc, Faraday Trans 1, 1983, 79, 1431-1449 reports experiments carried out on the relative reactivities of adsorbed and lattice oxygen for the oxidation of methane to carbon dioxide and water on a series of antimony- tin oxide catalysts, in the hope that the results obtained would have some bearing on the mechanism of olefin oxidation on the same catalysts.

European patent specification 542375 discloses a methane combustion process consisting of reacting the methane with oxygen in the presence of a heat-resistant catalyst of pyrochloric structure having the general formula

X_aY_bZ_cSn₂0₇ where X is a trivalent metal cation

Y is a tetravalent metal cation

Z is a bivalent metal cation

0 < a < 2

O ≤ b ≤ 1

O ≤ c < 1 and a + b + c = 2

Stannic oxide is not of pyrochloric structure and is not of course of this formula.

European patent specification 266875A discloses a method of catalytic combustion which comprises contacting a hydrocarbon gas fuel at a temperature of 800 to 1500°C in the presence of oxygen with a heat-resistant catalyst composed of at least one heat-resistant inorganic carrier selected from the group consisting of oxides, carbides and nitrides of elements belonging to Groups Ila, Ilia and IV of the periodic table, particles of at least one catalytically active component selected from the group consisting of platinum group elements dispersed and carried on said carrier, and particles of an oxide of at least one base metal selected from the group consisting of magnesium, manganese, nickel, cobalt, strontium, niobium, zinc, tin, chromium and zirconium dispersed on said particles of catalytically active component. The specification discloses that the amount of the base metal added to suppress the agglomeration of noble metal particles may

range from 0.1 to 10 gram-atom per gram-atom of the noble metal. This range contrasts with the present range of greater than 12:1. In a preferred aspect, the

present stannic oxide is employed without the platinum group metal, particularly in the higher temperature stage described below. When the stannic oxide is employed with the platinum group metal, however, the ratio of greater than 12:1 means that there is a markedly larger proportion of the stannic oxide than in the

method of the reference. This reflects the stannic oxide being not present in the present invention as an additive to suppress the agglomeration of the platinum

group metal. The stannic oxide is employed as prime catalytically active component or catalytically active support. When the stannic oxide is employed as support, it can be seen that the prime catalytically active material such as palladium would be dispersed on it, rather than the other way round as in the reference.

Stannic oxide exhibits high catalytic activity and thermal stability in the catalytic combustion of methane at 450-1500°C. The temperature at which stannic oxide converts the methane (for instance 50% of the methane) is surprisingly low. The catalytic activity of the stannic oxide lasts after cycles of using it in the present method and then allowing it to cool to ambient temperature

before using it again; it resists this thermal stress well. Stannic oxide is outstanding in these respects. Other metal oxides exhibit activity at higher temperatures than does stannic oxide or possess activity which declines significantly after thermal cycles of heating and cooling.

Stannic oxide can be used in the present invention as a catalytically active support for a prime catalytically active component or as a prime catalytically active component itself. The term "prime catalytically active component" is

employed to distinguish essential catalytically active component such as palladium from any component such as a stannic oxide support which nevertheless exhibits catalytic activity. In the present invention, the atomic ratio of the tin to any of the

present platinum group metal is usually greater than 13:1, preferably greater than

15:1. The ratio is usually lower than 14000:1, preferably lower than 8000:1.

The present catalytic combustion can be employed for instance to heat a furnace or boiler, or to remove pollutant methane, for instance in industrial off-gas. Of particular interest, however, is use in gas turbines.

The temperature at which the present catalyst is used is at least

450°C. The activity of the catalyst is generally higher at higher temperatures. Hence the temperature is usually at least 500°C, preferably at least 600°C, especially at least 800°C. The effect of higher temperatures on other factors such as the stability of other components, however, needs to be borne in mind. The temperature is at most 1500°C. Less nitrogen fixation generally occurs at lower temperatures. Hence the temperature is usually no more than 1400°C, preferably no more than 1300°C. The preferred range is 500-1400°C, especially 800-1300°C.

The gaseous reactants are usually employed at a pressure of 1-50, preferably 1-40, atmospheres. In a gas turbine, the pressure can be for instance

1.5-40, preferably 4-20, atmospheres. The rate at which the gaseous reactants are passed over the present catalyst is usually 160-500,000, preferably 800-320,000, litres per hour per gram of stannic oxide, calculated at standard temperature and pressure.

It will be appreciated that for experiments on a laboratory scale lower rates of passage of the gaseous reactants are conveniently employed, but that for industrial use, for instance in a gas turbine, markedly higher rates are usually employed. As the rate of passage over the present catalyst increases, the temperature at which a given level of methane conversion (for instance 50%) occurs generally increases. Consequently, the temperature of use in a gas turbine is generally markedly higher than that employed on a laboratory scale.

The ratio of oxygen to the uncombusted or partially combusted methane in the mixture which is contacted with the present catalyst is usually 2-200, preferably 3-100, by volume. The oxygen in the present method is normally employed as air. Normally, natural oxygen is employed, rather than the oxygen- 18 employed in the mechanistic studies reported in the J Chem Soc article mentioned above. The methane is conveniently employed as natural gas.

The surface area of the present catalyst is usually 1-10 square metres per gram.

In the aspect in which the catalyst comprises a prime catalytically

active component on a support, wherein the support comprises stannic oxide, the prime catalytically active component can be for instance one or more of Pd, Pt and

Rh, especially one or both of Pd and Pt, for instance a mixture of Pd and Pt in the

weight proportions 5-15:1. In a preferred embodiment, however, the prime catalytically active component is also stannic oxide.

The present catalyst is preferably employed in a later, higher temperature, stage of a multi-stage catalytic combustion of methane. The stannic oxide can be employed in this stage as catalytically active support for a different prime catalytically active component, but preferably as prime catalytically active component itself, and especially as both support and prime catalytically active component; it is especially preferred that the support and prime catalytically active component consist essentially of the stannic oxide. Accordingly, the mixture of oxygen and uncombusted or partially combusted methane which is contacted with the present catalyst preferably is so contacted at 800-1500°C in a later stage and has been produced in an earlier stage by contacting oxygen and uncombusted or partially combusted methane with a catalyst at a temperature lower than that in the later stage. The temperature in the earlier stage is preferably between 300 and 800°C. The references to partially combusted methane herein are to methane which has been partially combusted in an earlier stage in its catalytic combustion. The temperature in the later, higher temperature, stage is usually 800-1400°C, especially 800-1300°C. The catalyst in the earlier stage can be conventional. It usually comprises one or more of Pd, Pt and Rh, especially one or both of Pd and Pt, for instance a mixture of Pd and Pt in the weight proportions 5-15:1. The

catalyst can contain a support such as alumina. Further methane can be added to the gaseous reactants between the earlier and later stages. The later stage can be followed by a non-catalytic stage operating at high temperature, for instance 1000-

1500°C, effecting spontaneous, homogeneous, reaction. When partially combusted

methane is employed in the earlier stage, the stage is preceded by a yet earlier stage, in which methane is partially combusted, usually at a temperature yet lower than that in the earlier stage, though preferably again between 300 and 800°C, and preferably with a catalyst, especially a catalyst as discussed above for the earlier

stage.

The stannic oxide can be employed in the present method as a catalytically active support in an earlier, lower temperature, stage of a multi-stage catalytic combustion of methane. The earlier stage is preferably that discussed above. Accordingly, in a preferred aspect of the present method, the temperature is between 450 and 800°C, the stannic oxide is catalytically active support for prime catalytically active component and the gas produced is passed to a later stage where it is further reacted at a temperature higher than that in the earlier stage, and preferably at 800-1500°C. The prime catalytically active component is usually platinum group metal selected from the group consisting of one or more of Pt, Pd, Rh and Ru, preferably one or more of Pt, Pd and Rh, especially one or both of Pd and Pt, for instance a mixture of Pd and Pt in the weight proportions 5-15:1. The atomic ratio of the tin of the stannic oxide to the platinum group metal is preferably 13-800, especially 15-150, particularly 15-40. In a preferred embodiment, the support consists essentially of the stannic oxide. The later stage can be non-catalytic, effecting spontaneous, homogeneous, reaction, for instance at 1000-1500°C. Preferably, however, the later stage is catalytic, especially the later stage employing stannic oxide as discussed above. Further methane can be added

to the gaseous reactants between the earlier and later stages.

It should be noted that if the atomic ratio of the tin to any of the platinum group metal is greater than 12:1 in any stage of a multi-stage method, it need not be in another stage.

In a multi-stage method, the various stages need not be physically separated. For instance, one could employ a single monolith, optionally with separable segments, bearing different catalytic materials along its length.

The stannic oxide is extremely stable to the thermal conditions of the present method. Accordingly, the method is preferably operated without changing the present catalyst for a total period of at least 1000 hours, especially at least 8000 hours.

When the stannic oxide is prime catalytically active component, it can be employed on a support, which in turn can be carried on a carrier such as a monolith. Alternatively, the stannic oxide as prime catalytically active component can be carried directly on a carrier such as a monolith. The support and carrier can

be conventional. The weight ratio of the support to the stannic oxide is usually 0-0.95. The loading of the stannic oxide on a monolith is usually 0.01-0.6g/cm³. When the stannic oxide is catalytically active support, the support can be carried on a carrier such as a monolith. The loading of the stannic oxide on the monolith is usually 0.01-0.6g/cm³. The carrier can be conventional. The

weight ratio of the stannic oxide to prime catalytically active component is usually 16-1200. In a particular embodiment, the ratio is 10-1000 and the prime catalytically active component is platinum.

The present catalyst can contain other components. These may be other components known in themselves. The component can be for instance a catalytically active component, a promoter, a stabiliser or a support. The catalytically active component can be for instance one or more of Pd, Pt and Rh. The support can be for instance one or more of alumina, silica and zirconia. The promoter can be for instance one or more transition metal oxides such as chromium oxide. Where the present catalyst is employed in a later, higher temperature, stage, eg at 800-1500°C, of a multi-stage method, platinum group metal selected from the group consisting of one or more of Pt, Pd, Rh and Ru can be employed as a

promoter, eg in an atomic ratio of the tin to platinum group metal of between 800 and 14000, for instance between 5000 and 14000; although the platinum group metal is deactivated as a prime catalytically active component at such temperatures, it is nevertheless active as a promoter. The stabiliser can be for instance one or more oxides selected from alkaline earth metal oxides (such as barium oxide) and lanthanum group metal oxides (such as lanthana). When the stannic oxide is employed as support, the stannic oxide usually constitutes at least 50% by weight of the support, and preferably the

support consists essentially of the stannic oxide.

The present catalyst can be employed as the sole catalyst in a particular stage. Alternatively, it can be employed as one or more layers with one or more layers of other catalyst in a catalyst system in which gas passes through

the successive catalyst layers in a particular stage.

The catalyst can be in a form known in itself. It usually contains a carrier. The carrier can be a powder. Preferably, however, the carrier is a monolith, especially a honeycomb monolith, for instance of cordierite, silicon nitride, fecralloy or mullite. The monolith usually contains 7-100 cells per square cm. When the stannic oxide is employed as catalytically active support or prime catalytically active component, it can be coated onto the monolith. When employed as prime catalytically active component, it can be coated onto a support in the form of a washcoat which has previously been coated onto the monolith. Alternatively, the stannic oxide as prime catalytically active component can be coated directly onto the monolith. A catalyst which is suitable for use in the catalytic combustion of methane and which comprises a monolith coated, directly or indirectly, with prime catalytically active component consisting essentially of stannic oxide is a novel composition of matter, and is a preferred catalyst for use in the present method. The monolith is preferably a honeycomb monolith. The invention is illustrated by the following Examples. The catalyst testing was performed as follows unless otherwise stated: The catalyst was tested in a temperature programmed furnace, and the conversion of the methane was measured continuously by infra-red analysis of the reactor effluent for carbon dioxide and carbon monoxide. The weight of catalyst loaded was 0.25g and this was exposed to a flowing mixture of 1% by volume methane in air (ie 1 part of methane per 99 parts of air), at lOOml/min. The temperature was ramped at

10°C/min from ambient temperature; the Examples give the temperature when the methane conversion was 10% (T₁₀) and 50% (T₅₀). The reactor was ramped to 1000°C and then cooled to ambient temperature. The temperature cycle was repeated as a measure of the thermal stability of the catalyst; this measures the ability of the catalyst to achieve similar conversion levels in subsequent ramps.

Examples 1-6

The effect of catalyst from different sources, and the effect of prior calcination is shown below in Table 1.

Table 1

Example Catalyst First Second Third Fourth Ramp Ramp Ramp Ramp

Tio / T₅₀ Tio / T₅₀ Tio T₅₀ Ti fj / T₅₀

1 SnO₂ ex 560/647 593/673 590/672 Aldrich

2 Sn0₂ ex 521/602 536/610 527/612 BDH

3 SnO, ex 545/602 548/612 545/612 STRΪΞM

4 SnO₂ 572/654 593/686 597/691 in the form of

Extrudates ex BDH

5 #1 30 578/674 578/685 590/696 590/692 hours at 1200°C

6 #2 30 582/689 595/699 582/688 591/686 hours at 1200°C

It can be seen that the calcination of the more active samples for 30 hours at 1200°C brought them to a similar level of activity, and that though this increased the T₁₀ and T₅₀, the activity was stable after subsequent ramp cycles and was still relatively high. Examples 7-12

The effect of the mass space velocity (the flow rate of the gaseous mixture over the catalyst per unit weight of the catalyst) is shown below in Table 2. The stannic oxide was that from STREM. The weights of catalyst were varied as shown, rather than being 0.25g in each instance.

Table 2

It can be seen that a ten-fold increase in weight of the catalyst employed results in a T₅₀ about 60°C lower. Comparative Examples 13-25

The comparative effect of other metal oxides is shown below in

Table 3.

Table 3

Comparative First Second Third Example Metal Oxide Ramp Ramp Ramp

Tio / T₅₀ Tio T₅₀

13 Al₂O₃ 679/791 723/811

14 SiO₂ 832/905 807/903

15 ZnO 783/952 880/926 879/921

16 ZrO₂ 646/759 677/787 687/799

17 CeO₂ 639/841 763/918 769/926

Rhone

Poulenc

18 CeO₂ 636/777 690/831 701/843

Johnson Matthey

19 SnO 575/719 676/765 680/778

20 GeO₂ 900/932 917/947 911/938

21 V₂O₅ 886/912 802/918 872/923 Comparative First Second Third Example Metal Oxide Ramp Ramp Ramp

Tifj / τ₅₀ Tio T50 T₁₀ / T₅₀

22 WO₃ 880/914 885/907 889/912

23 MoO₃ 917/983 905/961 894/947

24 TiO₂ 895/941

25 In₂O₃ * 551/673 825/904 814/904

* Volatilised on first cycle.

By comparison with Examples 1-6, the outstanding activity and thermal stability of stannic oxide can be seen, even compared with metal oxides considered to be active oxidation catalysts for other reactions (in this category one might include CeO₂, GeO₂, V₂O₅ and MoO₃); with TiO₂, which has the same, rutile, structure as SnO₂; with TiO₂ and ZrO₂, which are generally considered to lose active oxygen and form lattice defects at high temperatures and consequently might be thought might be active catalysts for the present reaction; and with stannous oxide.

Examples 26 and 27 and Comparative Example 28

The use of stannic oxide as a catalytically active support is shown below in Table 4, which also shows for comparison the corresponding use of sihca as a support. Table 4

Example Wt of First Second Third Fourth Catalyst Ramp Ramp Ramp Ramp T.o / 50 T.o T50 T10 50 T10/ 50

26 l%Pt/Sn0₂ 0.250g 398/480 540/630 541/637

(BDH)

(ie 1 part by weight of Pt per 99 of SnO^

27 5%Pd/SnO, 0.050g 256/430 354/411 382/435 388/436 (BDH) 0.250g 340/392 346/400 316/391 2.50g - /332 291/337 290/338

Compar¬ l%Pt/Si0₂ 0.250g 350/570 550/672 573/673 ative Example 28

The catalysts were prepared as follows:

Example 26 1% Pt on SnO-,

0.85g of 5.87% by weight Pt solution of tetrammine platinum(II) hydroxide was made up to 3.0g by the addition of deionised water, and was then added to 4.95g SnO₂ (BDH). The mixture was well stirred and the resulting paste then dried overnight at 120°C in an air oven. The atomic ratio of the tin to the platinum group metal is 128:1. Example 27 5% Pd on SnO-,

3.1g of palladium(II) nitrate solution, containing 8.08% by weight

palladium as metal, was added to 4.75g Sn0₂ powder (BDH). The mixture was well stirred and the resulting paste then dried overnight at 120°C in an air oven.

The atomic ratio of the tin to the platinum group metal is 13.4:1.

Comparative Example 28 1% Pt on SnO_?

1.70g of 5.87% by weight platinum solution of tetrammine platinum(II) hydroxide was made up to 11. Og by the addition of deionised water, and the diluted platinum solution then added to 9.90g silica spheres (Shell S980B). The mixture was well stirred and the wet beads then dried overnight at 120°C in an air oven.

The excellent activity and thermal stability of the palladium on stannic oxide catalyst can be seen. It can also be seen that stannic oxide produces better results when used as a support for platinum than when silica is used. Example 26 and Comparative Example 28 also show the loss in platinum activity after ramping to 1000°C; comparison with Example 2 indicates that after subjection to this temperature, the stannic oxide is better alone. Example 29

Manufacture of tin oxide coated monolith

A slurry of SnO₂ in water was prepared so that it contained 10-30% by weight solids. The viscosity of the slurry was then adjusted by the addition of glacial acetic acid in an amount of 1% by volume of the slurry to produce a suspension suitable for application to a monolith.

The suspension was then poured through a 400 cells per square inch

(62 cells per square cm) cordierite monolith, the channels blown free with compressed air, and then dried for 1 hour at 120°C. This procedure was then repeated (usually once or twice) until the desired loading of SnO₂ on the monolith was achieved (about 2g/in³, 0.12g/cm³).

The SnO₂ was then fixed onto the monolith by firing at 500°C for 2 hours in air.

This is a general method which can be used to coat monoliths of differing cell size and composition (eg fecralloy, mullite or silicon nitride). Testing of tin oxide coated monolith

A 10mm long, 12mm diameter core of the SnO₂ coated (2.31g/in³, 0.141g/cm³) cordierite monolith was tested for methane combustion. The catalyst sample (0.60g) was placed in a plug flow reactor and tested at atmospheric pressure in a gas flow of 0.5% by volume methane in air at 2.0 litres/min. The amount of methane conversion was measured continuously using a flame ionisation detector.

The reactor was heated in a furnace until the T₁₀ was reached. This was 500°C.

Examples 30 and 31 and Comparative Example 32

Testing of powder samples

The same reactor as in the preceding Example was used for testing the activity of powder samples. In each case 0.35g of powdered catalyst was used and the T₁₀ measured. The results are shown in Table 5 below.

Example Sample Treatment

(°C)

30 SnO₂ Aldrich Fresh 510

31 SnO₂ Aldrich 1000°C for 30 500 hours

Comparative 5%Pt on α- 1000°C for 48 610 Example 32 Al₂O₃ hours It can be seen that SnO₂ even after firing at 1000°C for a long period is some 100°C more active than a platinum catalyst after undergoing a similar treatment.

Claims

1. A method for the catalytic combustion of methane, which method comprises contacting a mixture of oxygen and uncombusted or partially combusted methane at a temperature of 450-1500°C with a catalyst comprising stannic oxide and optionally platinum group metal selected from the group consisting of one or more of palladium, platinum, rhodium and ruthenium, and where this platinum group metal is present, the atomic ratio of the tin of the stannic oxide to the platinum group metal is greater than 12:1.

2. A method for the catalytic combustion of methane, which method comprises contacting a mixture of oxygen and uncombusted or partially combusted methane at a temperature of 450-1500°C with a catalyst comprising a prime catalytically active component on a support, wherein the support comprises stannic oxide.

3. A method according to claim 2 wherein the prime catalytically active component is also stannic oxide.

4. A method according to any one of claims 1-3 wherein the temperature is 500-1400°C.

5. A method according to any one of the preceding claims wherein the

contacting occurs at a pressure of 1-50 atmospheres.

6. A method according to any one of the preceding claims wherein

160-500,000 litres of the mixture are passed per hour over the catalyst per gram of

stannic oxide.

7. A method according to any one of the preceding claims wherein the mixture of oxygen and uncombusted or partially combusted methane is contacted with the catalyst at 800-1500°C in a later stage and has been produced in an earlier

stage by contacting oxygen and uncombusted or partially combusted methane with a catalyst at a temperature lower than that in the later stage.

8. A method according to claim 7 wherein the temperature in the earlier stage is between 300 and 800°C.

9. A method according to claim 7 or 8 wherein the catalyst in the earlier stage comprises one or more of palladium, platinum and rhodium.

10. A method according to any one of claims 6-8 wherein further methane has been added to the mixture produced in the earlier stage.

11. A method according to claim 1 or any one of claims 4-6 as dependent thereon wherein the temperature is between 450 and 800°C, the stannic

oxide is catalytically active support for prime catalytically active component and the gas produced is passed to a later stage where it is further reacted at a

temperature of 800-1500°C.