Method for the selective oxidation of carbon monoxide in...

Details Method for the selective oxidation of carbon monoxide in... Method for the selective oxidation of carbon monoxide in...

2001-05-23

2004-01-06

Silverman, Stanley S. (Department: 1754)

Chemistry of inorganic compounds

Modifying or removing component of normally gaseous mixture

Carbon monoxide component

Reexamination Certificate

active

06673327

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a method for the selective catalytic oxidation of carbon monoxide (CO) in the presence of a noble metal catalyst on an alumina carrier.
BACKGROUND OF THE INVENTION
Fuel cells are being investigated in many places as a possible energy source for driving vehicles and for stationary generation of electricity. The use of fuel cells is still highly dependent on the availability of the fuel: hydrogen (H
2
). It is not to be expected that an infrastructure for hydrogen will be set up within the foreseeable future. Especially for mobile applications, it is therefore necessary to transport an available fuel, or a fuel that becomes available, and to convert this to hydrogen as the feed for the fuel cell.
A gas mixture that consists mainly of hydrogen and carbon dioxide (CO
2
) is then produced—for example via steam reforming and/or partial oxidation—from fuels such as methane, LPG, methanol, petrol, diesel and other hydrocarbons. Said gas mixture, which is rich in hydrogen, is then fed to the fuel cell which generates electricity by an electrochemical reaction of hydrogen with oxygen.
However, a certain amount of carbon monoxide (CO) is also always liberated during the conversion of said fuels into hydrogen. For instance, a gas mixture of, for example, 75% (V/V) H
2
, 24% (V/V) CO
2
and 1% (V/V) CO is produced on steam reforming of methanol. A solid polymer fuel cell, the major candidate for transport applications, is extremely sensitive to CO, which even in low concentrations (0.01% (V/V)) has an adverse effect on the performance of the fuel cell. For a usable system it is therefore necessary to remove CO down to the said level and preferably down to a lower level (<0.005% (V/V), 50 ppm). A technically attractive option for removing CO from H
2
-containing gas streams is by means of selective oxidation of CO to CO
2
at low temperature (100° C.-200° C.). In this context it is important that the consumption of hydrogen by non-selective oxidation to water is minimised.
The power of ruthenium (Ru) to catalyse the oxidation of CO is, for example, known from the ammonia synthesis process. Thus, it is known from U.S. Pat. No. 3,216,782 (Nov. 9, 1965) that 0.5% (m/m) Ru on alumina (Al
2
O
3
) is capable of oxidising 0.055-0.6% (V/V) CO in the presence of H
2
at between 120° C. and 160° C. to a level of less than 15 ppm. In this case it is necessary that the quantity of oxygen (O
2
) added is such that the molar O
2
/CO ratio is between 1 and 2. The excess oxygen which is not needed for the oxidation of CO reacts with hydrogen to give water. It has not been investigated whether this Ru catalyst is also capable of oxidising CO from a typical reformate gas to a CO level of 15 ppm under the same conditions (temperature, O
2
/CO ratio).
In the Journal of Catalysis 142 (1993), Academic Press Inc., pages 257-259, S. H. Oh and R. M. Sinkevitch describe 0.5% (m/m) Ru/&ggr;-Al
2
O
3
as highly effective in the complete oxidation, at low temperature (100° C.), of 900 ppm CO with 800 ppm oxygen (O
2
) in a gas mixture which also contains 0.85% (V/V) H
2
, with the remainder being N
2
. Data on the stability of the Ru catalyst are not given in the article and in addition the behaviour of the catalyst in a realistic reformate gas containing H
2
, CO
2
, H
2
O and CO in much higher concentrations was not investigated.
European Patent EP 0 743 694 A1 (Nov. 20, 1996) refers to an oxidation unit for the selective oxidation of CO in H
2
-rich gas at a reaction temperature of between 80° C. and 100° C. A molar ratio of O
2
/CO of 3 is used. The final CO content is a few ppm. The excess oxygen reacts with hydrogen to give water. The catalyst consists of a 0.2% (m/m)-0.5% (m/m) Pt—Ru alloy on Al
2
O
3
. No examples which would show the stability of the catalyst are given.
U.S. Pat. No. 5,674,460 (Oct. 7, 1997) describes a structured reactor for the catalytic removal of CO from H
2
-rich gas at between 90° C. and 230° C. Depending on the temperature, the catalyst in this case consists of Pt on &ggr;-Al
2
O
3
, Pt on zeolite-Y or Ru on &ggr;-Al
2
O
2
. The invention is explained solely on the basis of 5% (m/m) Pt on &ggr;-Al
2
O
3
, by means of which the CO content can be reduced to about 40 ppm at a reaction temperature of between 80° C. and 130° C. No stability data are given in this patent either.
In the Journal of Catalysis 168 (1997), Academic Press, pages 125-127, R. M. Torres Sanchez et al. describe gold on manganese oxide as an alternative catalyst for the oxidation of CO in H
2
at low temperatures (approximately 50° C.). In particular the price, due to the high gold loading (approximately 4-10% (m/m)), makes the use of this type of, catalyst less interesting. Moreover, this type of catalyst is able to withstand carbon dioxide to only a limited extent.
It is not clear from the above whether the catalysts of the prior art are suitable for the selective oxidation of CO in H
2
-rich reformate gas mixtures where there is high activity in conjunction with good stability in the temperature range 100° C.-200° C. and where a low oxygen excess can be used to minimise the hydrogen consumption.
SUMMARY OF THE INVENTION
The invention relates to a method for the selective catalytic oxidation of carbon monoxide (CO) comprising catalytically oxidizing carbon monoxide in H
2
-rich, CO
2
- and H
2
O-containing gases in the presence of a noble metal catalyst on an &agr;-Al
2
O
3
carrier with the addition of air as oxidizing agent.
DETAILED DESCRIPTION OF THE INVENTION
One aim of the present invention is to provide a method for the selective catalytic oxidation of CO from H
2
-rich, CO
2
- and H
2
O-containing (reformate) gas mixtures, making use of as small as possible an amount of oxygen and at relatively low temperature. A further aim of the present invention is to provide a catalyst which has high chemical and thermal stability and can be produced in a cost-effective manner by means of a simple method of preparation from commercially available starting materials and a low noble metal loading.
The use of commercially available &agr;-Al
2
O
3
as carrier material in the preparation of 0.5% (m/m) Ru on Al
2
O
3
led, surprisingly, to a catalyst which in the temperature range about 120° C. about 160° C. combines high activity (>99% conversion of CO) with high stability (a CO conversion of at least 97% for a period of at least 50 hours) in the oxidation of CO with a relatively small excess of oxygen in dilute reformate gas. These results were found to be appreciably better than the results which were obtained with a commercially available 0.5% (m/m) ruthenium catalyst with &ggr;-Al
2
O
3
as the carrier (specific surface area >100 m
2
/g), which is representative of the catalysts used in the abovementioned studies and reflects the prior art.
It has also been found that the addition of Pt and the lowering of the total noble metal loading resulted in a catalyst which showed even better stability for the selective oxidation of CO in both dilute and undiluted reformate gas (a CO conversion of at least 99% for a period of at least 50 hours).
It has furthermore been found that in particular the nature and the specific surface area of the Al
2
O
3
carrier used are the factors determining the exceptional performance of the Ru and Ru—Pt catalysts according to the present invention. Preferably, alumina is used in the form of &agr;-Al
2
O
3
. A highly active and stable catalyst i formed when the specific surface area of the &agr;-Al
2
O
3
is in the range from about 3 m
2
/gram to 25 m
2
/gram.
The catalysts in the present invention can be prepared in a simple manner via a standard impregnation method from commercially available starting materials. Compared with the current state of the art, the method according to the present invention has the following advantages:
complete oxidation of CO to CO
2
in the temperature range about 120° C. about 160° C. with only a small excess of oxygen (O
2
/CO=1) compared with the stoichiometrically required quantity of oxygen (O
2
/CO

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