SELECTIVE CATALYTIC OXIDATION OF CO IN PRESENCE OF H2

OXYDATION CATALYTIQUE SELECTIVE DE CO EN PRESENCE DE H2

English Abstract

A catalyst which comprises gold, a transition metal oxide to which the gold is
complexed and tin oxide captured on an oxide support which comprises a mixture
of cerium oxide and titanium dioxide. The tin oxide is present in an amount of
less than 0,4 % by weight of the catalyst. The catalyst has particular
application in selectively oxidising carbon monoxide in a carbon
monoxide/hydrogen mixture.

French Abstract

Catalyseur composé d'or, d'un oxyde de métal de transition avec lequel l'or forme un complexe et d'un oxyde d'étain capturé sur un support d'oxyde composé d'un mélange d'oxyde de cérium et de dioxyde de titane. L'oxyde d'étain est présent en quantité inférieure à 0,4 % en poids du catalyseur. On peut utiliser ce catalyseur, de façon spécifique, pour l'oxydation sélective d'oxyde de carbone dans un mélange d'hydrogène et d'oxyde de carbone.

Claims

-7-
CLAIMS
1. A method of selectively oxidising carbon monoxide in a carbon
monoxide/hydrogen mixture comprises contacting the mixture with a
suitable oxidant in the presence of a catalyst which comprises gold, a
transition metal oxide, to which the gold is complexed, and tin oxide
captured on an oxide support comprising a mixture of cerium oxide and
titanium dioxide, the tin oxide being present in an amount of less than
0,4% by weight of the catalyst.
2. A method according to claim 1 wherein the transition metal oxide is
cobalt oxide.
3. A method according to claim 1 or claim 2, wherein the ratio of cerium
oxide to titanium dioxide is in the range 4:1 to 5:1 on a weight basis.
4. A method according to any one of claims 1 to 3, wherein the tin oxide is
present in an amount of 0,03 to 0,3 percent by weight of the catalyst.
5. A method according to claim 1, wherein the catalyst has the
composition (all percentages being by weight):
83% CeO2
15% TiO2
0,3% SnO2
1% Au
1% Co, as an oxide.
6. A method according to any one of the preceding claims wherein the
oxidant is oxygen.

-8-
7. A method according to any one of the preceding claims wherein the
oxidation takes place at a temperature in the range 80 to 130°C.
8. A catalyst comprising gold a transition metal oxide to which the gold is
complexed, and tin oxide captured on an oxide support comprising a
mixture of cerium oxide and titanium oxide, the tin oxide being present
in an amount of less than 0,4% by weight of the catalyst.
9. A catalyst according to claim 8, wherein the transition metal oxide is
cobalt oxide.
10. A catalyst according to claim 8 or claim 9, wherein the ratio of cerium
oxide to titanium dioxide is in the range 4:1 to 5:1 on a weight basis.
11. A catalyst according to any one of claims 8 to 10, wherein the tin oxide
is present in an amount of 0,03 to 0,3 percent by weight of the catalyst.
12. A catalyst according to claim 8, which has the composition (all
percentages by weight):
83 % CeO2
15 % TiO2
0,3% SnO2
1% Au
1% Co, as an oxide.
13. A method according to claim 1, substantially as herein described.
14. A catalyst according to claim 8, substantially as herein described.

Description

CA 02368378 2001-10-O1
WO 00/59631 PCT/IB00/00390
- 1 -
SELECTIVE CATALYTIC OXIDATION OF CO IN PRESENCE OF H2
BACKGROUND OF THE INVENTION
This invention relates to catalytic oxidation.
Fuel cells are electrochemical devices for continuously converting chemical
energy into direct-current electricity. The cell consists of two electronic-
conductor electrodes separated by an ionic conducting electrolyte with
provision for the continuous flow of fuel, oxidant and reaction product into
and
out of the cell. The fuel may be gaseous or a liquid; the electrolyte liquid
or
solid; and the oxidant is gaseous. The electrodes are solid, but may be porous
and contain a catalyst. Fuel cells differ from batteries in that electricity
is
produced from chemical fuels fed to them as needed.
One of the fuels which is used in a fuel cell is hydrogen. A contaminant in
many sources of hydrogen is carbon monoxide which tends to poison the
electrodes. Carbon dioxide, on the other hand, does not poison the electrodes
of a fuel cell and it is therefore desirable to oxidise as much of the carbon
monoxide to carbon dioxide before such hydrogen is fed to the fuel cell. One
of the problems encountered in oxidising carbon monoxide to carbon dioxide is
that there is simultaneous oxidation of the hydrogen. Selective oxidation of
the
carbon monoxide is difficult to achieve.
CONFIRMATION COP~I

CA 02368378 2001-10-O1
WO 00/59631 PCT/IB00/00390
_ 7 _
SUMMARY OF THE INVENTION
According to the invention, a method of selectively oxidising carbon monoxide
in a carbon monoxide/hydrogen mixture comprises contacting the mixture with
a suitable oxidant, generally oxygen, in the presence of a catalyst which
comprises gold, a transition metal oxide, preferably cobalt oxide, to which
the
gold is complexed, and tin oxide captured on an oxide support comprising a
mixture of cerium oxide and titanium dioxide, the tin oxide being present in
an
amount of less than 0,4% by weight of the catalyst.
According to another aspect of the invention, there is provided a catalyst
which
comprises gold, a transition metal oxide, preferably cobalt oxide, to which
the
gold is complexed, and tin oxide captured on an oxide support comprising a
mixture of cerium oxide and titanium dioxide, the tin oxide being present in
an
amount of less than 0,4% by weight of the catalyst. The complexing of the
gold and the transition metal oxide includes both chemical and physical
bonding. Some such complexing of the tin oxide and the gold may also be
present.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a graph showing CO oxidation of a gas containing CO, H2 and O2;
and
Figure 2 is a graph showing HZ oxidation of a gas containing CO, H2 and O2.
DESCRIPTION OF EMBODIMENTS
It has been found that the gold-based catalyst described above is particularly
effective in the selective oxidation of carbon monoxide in a mixture of carbon
monoxide and hydrogen. "Selective" means that the oxidation of the carbon

CA 02368378 2001-10-O1
WO 00/59631 PCT/IB00/00390
_ 3 _
monoxide takes place in preference to that of the hydrogen. Some oxidation of
the hydrogen will occur.
The oxidation of the mixture will take place in the presence of an oxidant
such
as oxygen and will typically take place at a temperature in the range 80 to
130°C.
The ratio of cerium oxide to titanium dioxide in the oxide support will
generally
be in the range of 4:1 to 5:1 on a weight basis.
The catalyst of the invention is further characterised by containing tin
oxide. It
is important that this oxide is present in a relatively low amount, i.e. 0,4
percent
by weight of the catalyst or less. Preferably, such oxide is present in an
amount of 0,03 to 0,3 percent by weight of the catalyst.
The oxide support may be made by providing a particulate mixture of the
cerium oxide and titanium dioxide and, drying the mixture. The mixture will be
in finely particulate form and will typically have a surface area of
approximately
80 to 200 m2/g.
The catalyst of the invention may be made by methods described in the art.
Preferably, however, the catalyst is made by a method which includes the
steps of providing a mixture of cerium oxide and titanium oxide, impregnating
the mixture with a transition metal solution and optionally a tin solution,
drying
the impregnated oxide mixture, heating the impregnated oxide mixture at a
temperature of at least 250°C in the presence of oxygen, contacting the
thus
treated oxide mixture with a gold solution at a pH above 12, decreasing the pH
of the gold solution to a pH in the range 7 to 8,5, causing gold to
precipitate on
to the oxide mixture, optionally drying the thus treated oxide mixture and
heating the treated oxide mixture at a temperature of at least 250°C in
the
presence of oxygen.

CA 02368378 2001-10-O1
WO 00/59631 PCT/IB00/00390
- 4-
An example of a preferred catalyst of the invention has the following
composition (all percentages by weight):
83 % CeOz
15 % Ti02
0,3% Sn02
1 % Au
1 % Co, as an oxide.
The catalyst may be made as follows:
42g of Ce02 and 8g Ti02 are mixed for 10 minutes in an automatic mixer. The
mixed oxides are dried at 120°C for 30 minutes and impregnated with
22,5m1
of 0,4 molar Co(N03)z and 10,5m1 of 0,1 molar SnC~2 solutions at 50°C.
The
so formed sludge is dried at 120°C for 12 hours and impregnated with 21
ml of
0,13 molar HAuC/4. The material is washed with water and dried at 120°C
for
3 hours and calcined at 400°C for 30 minutes.
The catalyst is preferably made as follows:
Stage One
Powdered cerium oxide and titanium oxide are accurately measured into a
100m1 sample bottle and mixed in a mixer mill for 10 minutes. This mixture is
heated at 120°C for 30 minutes.
Solutions of cobalt nitrate and tin nitrate are introduced into a vessel and
heated to 80°C. The heated mixture of cerium and titanium oxides, the
support
for the catalyst, is then added under constant stirring.

CA 02368378 2001-10-O1
WO 00/59631 PCT/IB00/00390
_ j _
The pressure in the mixing vessel is reduced and water is distilled off at a
constant temperature of 65°C over a 4 hour period.
The oxide support. impregnated with cobalt and tin nitrates, is dried at
120°C
for 2 hours.
The dried oxide support is sieved through a 35 micron mesh sieve and then
heated under air at 300°C for 6 hours. This has the effect of
converting the
cobalt and tin nitrates to their oxides.
Stage Two
Hydrogen tetrachloroaurate solution is added to 500m1 potassium hydroxide
solution at pH 13. The combined solution has a pH of just below 13.
The temperature of the solution is increased to 60°C and the
preheated oxide
support, impregnated with the cobalt and tin oxides, is added to form a
slurry.
The pH of the slurry is reduced to 8 by addition of 0,1 M HN03.
Magnesium citrate solution (six times the mole equivalent of the hydrogen
tetrachloroaurate) is added to the slurry at a constant rate over a 30 minute
period. The slurry is maintained at pH 8 and 60°C over a 60 minute
period.
The slurry is filtered producing a filtered catalyst. The filtered catalyst is
re-
slurried in de-ionised water and stirred for a further 30 minutes before being
filtered and washed with de-ionised water.
The catalyst is then dried at 120°C under air for 2 hours. The dried
catalyst is
sieved to -35~m and calcined at 450°C under 5% Oz for 3 hours.
The catalyst is stored prior to use.

CA 02368378 2001-10-O1
WO 00/59631 PCT/IB00/00390
- 6-
The selective oxidation of carbon monoxide in a mixture containing 0.5 percent
CO, 2 percent H2, 0,8 percent O2, the balance being nitrogen, was evaluated
using a catalyst of the type described above, designated FC6, and a similar
catalyst, except that the tin oxide was omitted, designated FC2. The oxidation
of CO is shown in Figure 1 and the oxidation of H2 is shown in Figure 2.
It will be noted from the graphs that the carbon monoxide oxidation of the
catalyst without the tin oxide achieved excellent carbon monoxide oxidation,
but also significant hydrogen oxidation, particularly at higher temperatures.
In
contrast, at higher temperatures, the oxidation of the carbon monoxide in the
presence of the catalyst of the invention approached that of the other
catalyst
but with significantly lower hydrogen oxidation.