EP1843844A1

EP1843844A1 - Catalytically active composition for the selective methanation of carbon monoxide and method for producing said composition

Info

Publication number: EP1843844A1
Application number: EP06707761A
Authority: EP
Inventors: Christian Kuhrs; Markus HÖLZLE; Till Gerlach; Michael Hesse
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2005-01-24
Filing date: 2006-01-19
Publication date: 2007-10-17
Also published as: US7560496B2; WO2006077236A1; CA2595466A1; DE102005003311A1; US20080139676A1; JP2008528250A

Abstract

The invention relates to a catalytically active composition for the selective methanation of carbon monoxide. Said composition is characterised in that it contains as the active component at least one element selected from the group consisting of ruthenium, rhodium, nickel and cobalt and a carbon-based support. The invention also relates to the use of said catalytically active composition for the selective methanation of carbon monoxide and to the use of the composition for generating hydrogen in fuel-cell applications.

Description

Catalytically active composition for the selective methanation of carbon monoxide and process for its preparation

description

The invention relates to a catalytic composition and a process for the selective methanation of carbon monoxide, in particular for use in fuel cell systems.

Low-temperature fuel cells can only be operated with hydrogen or hydrogen-rich gases of a defined quality. The CO concentration depends on the energy source used and the reforming process used. The removal of higher CO concentrations is possible with the shift process with further formation of hydrogen. However, depending on the method design, a residual concentration of CO remains, generally in the range from 0.5 to 1.5% by volume. When using Cu catalysts, for example, CO removal can be made possible down to 3,000 ppm. The CO content in the hydrogen-rich gas must be further reduced as much as possible in order to avoid poisoning of the anode catalyst.

The reduction of CO contained in the gas stream to below the required limits is usually carried out in a fine cleaning stage. Today, selective oxidation is the common CO removal method. The selective oxidation has a high level of development, but in addition to the disadvantage of only moderate selectivity has the need for a precisely dosed air supply, resulting in a high measurement and control engineering effort. Added to this is the addition of the oxidant oxygen to the gas a safety problem. The removal of CO by reaction with H ₂ (methanation) has considerable advantages over selective CO oxidation due to its unproblematic implementation.

The CO methanation (hydrogenation of carbon monoxide to methane) is carried out according to the reaction equation:

CO + 3H ₂ → CH ₄ + H ₂ O ΔH = -206.2 kJ / mol

As a competing reaction, the conversion of CO ₂ to methane takes place:

CO ₂ + 4H ₂ → CH ₄ + 2H ₂ O ΔH = -164.9 kJ / mol

The particular challenge for selective CO methanation is that preferably CO should be hydrogenated and not CO ₂ , as this would consume more hydrogen. According to thermodynamics, the CO methanation is opposite the CO ₂ methanation preferred. It is known that below a limit of 200 to 300 ppm CO concentration in the fuel gas does not use the CO ₂ methanation. The CO concentration in the fuel gas is about 10,000 ppm, which is 50 times higher than the specified limit. The CO ₂ content is about 15 to 25 vol .-% an order of magnitude above the CO content. Accordingly, a CO-selective catalyst is indispensable.

The selective methanization of CO has long been known. First, CO was methanized on the Ni catalyst, but CO ₂ had to be previously washed out. In 1968, a ruthenium catalyst for the selective CO methanation of Baker et al. (US Pat. No. 3,615,164), where a ruthenium or rhodium catalyst is used on an alumina support material. Also, in Chemical Abstracts, Volume 74, 1971, no. 35106u, the selective methanation of CO in a gas mixture containing hydrogen, carbon dioxide and carbon monoxide, at temperatures in the range 125-300 ⁰ C using ruthenium catalysts. In US-A-3663162 of 1972, a Raney nickel catalyst is claimed for this reaction.

EP-A-1174486 combines a methanation step with a selective oxidation unit for the purpose of lower oxygen consumption and lower CO ₂ methanation rate.

In EP-A-0946406 two methanation stages of different temperature levels are interconnected. The advantage here is that no or less CO _{2 is} methanized at the high-temperature stage, but even a large part of the carbon monoxide is degraded. In the subsequent low-temperature methanation, the residual removal of CO.

WO 97/43207 describes the combination of a first stage for selective oxidation with a subsequent methanation stage. With this combination, both processes can be operated under optimal conditions.

Other recent applications, such as EP-A-1246286, in which a methanation reactor of a selective oxidation unit is preferred as the final process step of gas purification for ease of construction and ease of operation, also include optimized process steps, but also use conventional ones Catalysts predominantly based on ruthenium or nickel.

JP-A-2004097859 describes catalysts for the removal of CO in hydrogen-containing gas streams by reaction with H ₂ . Catalysts mentioned are inorganic see support on which one or more metals selected from the group Ru, Ni and Co, are applied. Support materials are TiO ₂ , ZrO ₂ , Al ₂ O ₃ and zeolites.

JP-A-2002068707 relates to a process for removing CO from hydrogen-containing gas by selective methanation of CO using a catalyst having a Ru component and an alkali metal and / or alkaline earth metal on a refractory inorganic oxide support.

The use of coal as a catalyst carrier has not previously been described for the methanation of carbon monoxide.

The processes of the prior art do not allow to ensure a sufficient reduction in the CO content while sparing the CO ₂ content. The proposed catalysts either do not work selectively or act only in a narrow temperature range.

The exothermic reaction leads to hot spots. For this reason, a wide temperature window must be mobile. Another problem is the adiabatic temperature increase in monoliths, when these are used as shaped catalyst bodies, which is the case in practice.

In particular, for fuel cell applications, the required maximum CO content in the fed hydrogen-rich gas and the necessary high selectivity (methanation of CO, but not of CO ₂ ) over a broad temperature window are still a great development potential for suitable deactivation-resistant catalysts.

The object of the invention was therefore to provide a catalyst for the selective CO methanation, which receives its selectivity and activity in a wide temperature range.

The object has been achieved according to the invention in that, for the selective methanation of carbon monoxide, a catalytically active composition is used which contains as the active component ruthenium, rhodium, nickel or cobalt and a support material based on coal and is optionally doped.

The invention thus provides a catalytically active composition for the selective methanation of carbon monoxide, which is characterized in that it contains as active component at least one element selected from the group consisting of ruthenium, rhodium, nickel and cobalt, and a support material based on coal. Objects of the invention are also the use of this catalytically active composition for the selective methanation of carbon monoxide and for use in fuel cell applications.

It has surprisingly been found that a Ru, Rh, Ni or Co-containing catalyst on a carbon support, which is optionally doped with Fe in particular, the methanation of CO in a wide temperature range of about 100 to 300 ° C in a nearly ensures constant selectivity over a long period of time. Conventional catalysts show a significant drop in selectivity with increasing temperature. By applying the catalyst according to the invention a significantly lower control effort is required because the temperature window in the methanation of the CO must be kept less accurate. In addition, a well working catalyst even at high temperatures directly downstream of the pre-purification stage (TTK - low temperature conversion), which is operated at about 220 to 280 ⁰ C, followed.

The catalytically active composition contains as active component at least one element selected from the group consisting of Ru, Rh, Ni and Co, preferably Ru.

Coal, such as activated carbon acid activated activated carbon, graphite or pyrolytic carbon is used as support material according to the invention, preferably activated carbon moldings are used.

The loading of the carrier material with the active component is preferably 0.1 to 20 wt .-%, particularly preferably 1 to 10 wt .-%.

To increase their activity and / or selectivity, the active component and / or the carrier material can be doped. In particular iron, niobium, manganese, molybdenum and zirconium are suitable as doping elements. Preference is given to doping with iron.

The doping elements are used in an amount of preferably 0.1 to 20 wt .-%, particularly preferably 1 to 10 wt .-%.

The preparation of the catalyst according to the invention is carried out in a customary manner, for example by bringing the active components, preferably in the form of their salts / hydrates, into solution and then applying them to the carbon carrier in a suitable manner, for example by impregnation. Thereafter, the catalyst is dried, possibly calcined, optionally reduced and passivated if necessary.

The result is a catalytically active composition which is outstandingly suitable for the selective methanation of carbon monoxide. Depending on the particular Reaction conditions while the desired significant depletion of CO in the gas mixture is achieved.

Advantageously, therefore the CO carried in a temperature range of preferably 100 to 300 ⁰ C, the selective methanation.

The catalytically active composition is therefore particularly suitable for use in hydrogen production for fuel cell applications.

Further embodiments of the present invention can be taken from the claims, the description and the examples. It goes without saying that the features mentioned above and those still to be explained below of the article according to the invention can be used not only in the respectively indicated combination but also in other combinations without departing from the scope of the invention.

The invention will be explained in more detail with reference to the following embodiments, but without thereby making a corresponding limitation.

Examples

To evaluate the results of the examples, the variables selectivity and conversion were used. The selectivity is the quotient of the amount of CO converted and the amount of methane produced (in% by volume). The indication "ve" means that CO _{2 is} fully preserved, and sales are based on CO.

example 1

Preparation of a C-based catalyst with 5 wt.% Ru and 1 wt.% Fe, 3 mm

strands

4.4 g of ruthenium (III) chloride hydrate were dissolved in 15.0 ml of deionized water and 2.4 g of iron (III) chloride hydrate in 10.0 ml of deionised water. The solutions were combined and diluted with deionized water to 90% of the water uptake of the activated charcoal carrier, which in this case was 0.95 cπrVg (total volume 41.0 ml).

Activated carbon strands with a diameter of 3 mm and a length of about 2 to 5 mm were introduced and soaked in drops of the prepared solution. Carrier and impregnation solution were well mixed throughout the impregnation process.

Subsequently, the catalyst was dried in a rotary kiln at 90 ° C with 150 l / h of nitrogen for six hours. Immediately after the drying process the catalyst was reduced in the rotary kiln with a flow of 15 l / h of hydrogen and 60 l / h of nitrogen. The oven was heated to 500 ° C within two hours and then held at 500 ⁰ C for three hours. Thereafter, the catalyst was cooled to room temperature under nitrogen. Gradually more and less air and less nitrogen were fed in over 2 hours, whereby the catalyst was passivated. The temperature of the catalyst was not more than 15 ° C above room temperature. For the activity test described under 2 a), the catalyst was comminuted to 1-2 mm chippings.

Examples 2 a) and b)

Selective methanation

2 a) An electrically heated tubular reactor with a volume of 50 ml and a diameter of 14 mm was used for the experiment.

At the bottom, 4 ml of steatite spheres with a diameter of 1.8 to 2.2 mm were installed, to which the catalyst mixture was then added. The catalyst mixture consisted of 10 g of catalyst (as 1 to 2 mm grit), which in the case of the catalyst according to Example 1 corresponds to a volume of about 21 ml, with about 10 ml steatite spheres with a diameter of

1, 8 to 2.2 mm was well mixed. The feed used was 14 ml of steatite spheres with a diameter of 1.8 to 2.2 mm, which filled the residual volume of the reactor.

The catalyst was initially at 90 l / h of nitrogen and 10 l / h of hydrogen at

230 ° C for one hour reduced. The gas composition chosen for the experiment is typical of the exit of the cryogenic post stage after reforming of methane: 33 vol.% H ₂ ; 28% by volume of N ₂ ; 25% by volume H ₂ O; 13% by volume of CO ₂ ; 0.5% by volume of CO; 0.5% by volume of CH ₄ . A load of 5 lg _Ka t ^"1 h ^{" 1 was} chosen.

After all gases had been adjusted and the reactor (after the reduction at 230 ^° C.) had cooled to 150 ^° C., the test was started. Every three hours the temperature was raised within 10 minutes to 25 ° C, the Maximaltempera- structure was 300 ⁰ C.

2 b) The experiment described under 2 a) was repeated, but using a conventional catalyst based on Al ₂ O ₃ with 5 wt .-% Ru and 1 wt .-% Fe (as 1 to 2 mm grit) was used ,

The following results were achieved: selectivity (see also diagram 1) Temperature ⁰ C 5% Ru + 1% Fe / C 5% Ru + 1% Fe / Al ₂ O ₃

240 71% 9%

260 62% 7%

280 44% 7%

300 61% 6%

Sales (see also diagram 2)

Temperature ⁰ C 5% Ru + 1% Fe / C 5% Ru + 1% Fe / Al ₂ O ₃

240 95% 97%

260 97% 98%

280 89% 99%

300 90% 99%

From Diagram 2 it is clear that the conversion of both catalysts is comparable (although slightly higher for the conventional catalyst based on Al ₂ O ₃ ). However, diagram 1 shows that a significantly higher selectivity is achieved with the catalyst according to the invention. In addition, it can be clearly seen that, especially at low temperature, the catalyst according to the invention offers very good selectivities.

Example 3 a)

70 g of 3 mm extrudates of Supersorbon SX 30 (Fa Lurgi) were initially introduced and activated at 80 ° C. with 150 ml of HNO ₃ (conc.) For five hours. The activated carbon was then washed and dried at 120 ⁰ C.

7.3 g of ruthenium (III) chloride were dissolved in water and mixed with a solution containing 2.4 g of iron (III) chloride, diluted to 41 ml of water and slowly added to the activated carbon. The catalyst was dried at 90 ⁰ C under nitrogen, then reduced in a nitrogen stream of hydrogen at 500 ° C. After cooling, the material was passivated at room temperature.

Example 3 b)

The catalyst according to Example 3 a) was first activated in the reactor with a hydrogen / nitrogen gas mixture and then at a load of 2.5 l gκ _at ^"1 h ^{" 1} in a gas stream with 33 vol .-% H ₂ ; 25% by volume H ₂ O; 28.25 vol.% N ₂ ; 13% by volume of CO ₂ ; 0.25% by volume of CO; 0.5 vol .-% CH ₄ operated. The temperature was varied between 120 and 220 ⁰ C in 10 K increments. The measurement results for selectivity, conversion and final CO concentration are shown in the following table.

In this example, the very wide temperature window in which the catalyst can be operated becomes clear.

Examples 4

The catalyst of the invention according to Example 1 was at the load of 2.5 lg _cat ^"1 h ^{" 1} and the following gas composition (33 vol .-% H ₂ ; 25 vol .-% H ₂ O; 28.25 Vol.% N ₂ , 13% by volume of CO ₂ , 0.25% by volume of CO, 0.5% by volume of CH ₄ ) were operated for a period of 1000 hours at a temperature of 175 ° C. Over the runtime, a CO concentration of <50 ppm was realized. Over time, CO _{2 was} unaffected by the reaction. The concentration of 50 ppm CO is the limit for the operation of fuel cells based on polymer electrolyte membranes.

The evolution of the CO concentration as a function of time is shown in Diagram 3.

Following the experiment, the temperature of the reaction was varied. The results are shown in the following table:

The example emphasizes the long-term stability of the catalyst. Example 5

The inventive catalyst according to Example 1 was operated in series with a commercially usable catalyst for the low-temperature conversion. The catalyst for the selective methanization underwent a load of 2.5 l gκ _at ^"1 h ^{" 1} .

The input and output values for both reaction stages are shown in the table below. Example 5 a) shows the values for the operation of a TTK catalyst at 210 ^° C., 5 b) for the operation of a TTK catalyst at 220 ^° C.

CO ₂ CO ₂ H ₂ N ₂ H ₂ O CH ₄

output

Methani-

20 ppm 15.9 VoI. % 43.7 VoI% 40.3 VoI. % dry 0, 14% vol

175 ° C

output

Methani-

44 ppm 15.8 VoI. % 43.7 VoI% 40.3 VoI. % dry 0, 2 vol.% sation

190 ⁰ C

output

Methani-

46 ppm 15.8 VoI. % 43.4% VoI% 40.6 VoI. % dry 0, 3 vol.% sation

200 ⁰ C

output

Methani-

16C ppm 15.4 VoI. % 42.4 VoI% 41, 6 VoI. % dry 0, 6 vol.% sation

210 ° C

Example 6

The inventive catalyst according to Example 1 was subjected to a series of atmospheric changes under operating conditions. In this case, at a constant reactor temperature of 175 ° C., a gas composition 1 (2.5 l gκ _at ^"1 h ^{" 1} , 33% by volume H ₂ , 25% by volume H ₂ O, 28.25% by volume N ₂ , 13% by volume of CO ₂ , 0.25% by volume of CO, 0.5% by volume of CH ₄ ) were converted to air after brief purging with nitrogen. After renewed nitrogen purge was switched back to the original gas composition 1.

This procedure tests the load capacity of the catalytic converter during typical startup and shutdown processes in a PEM fuel cell. In the table below, the conversion and selectivity values and the resulting CO concentration are plotted according to the individual atmospheric changes:

From the example, it is clear that the catalyst is stable despite the change in atmosphere significantly undercuts the limit concentration of CO of less than 50 ppm.

Claims

claims

1. Catalytically active composition for the selective methanation of carbon monoxide, characterized in that it contains as active component at least one element selected from the group consisting of ruthenium, rhodium, nickel and cobalt, and a support material based on coal.

2. Catalytically active composition according to claim 1, characterized in that the active component and / or the carrier material with at least one EIe- ment, selected from the group consisting of iron, niobium, manganese, molybdenum and zirconium doped.

3. Catalytically active composition according to claim 1 or 2, characterized in that the active component is ruthenium.

4. Catalytically active composition according to one of claims 1 to 3, characterized in that it is doped with iron.

5. Catalytically active composition according to one of claims 1 to 4, characterized in that the total loading of the carrier material with the

Active component 0.1 to 20 wt .-% is.

6. Use of a catalytically active composition according to any one of claims 1 to 5 for the selective methanation of carbon monoxide.

7. A process for the selective methanation of carbon monoxide, characterized in that a catalytically active composition is used, which contains as active component at least one element selected from the group consisting of ruthenium, rhodium, nickel and cobalt, and a support material on Kohleba- sis.

8. The method according to claim 7, characterized in that the methanation takes place in a temperature range of 100 to 300 ° C.

9. Use of a catalytically active composition according to any one of claims 1 to 5 in hydrogen production for fuel cell applications.

3 Fig.