Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2001-02-08
2004-04-13
Hendrickson, Stuart L. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S306000, C502S314000, C502S316000, C502S328000
Reexamination Certificate
active
06720284
ABSTRACT:
The present invention relates to Au/Fe
2
O
3
catalyst materials made from a particulate, co-catalytically active Fe
2
O
3
support material with metallic Au clusters deposited thereon which have a diameter of less than 4.5 nm, various processes for their production and their use, particularly for selective low-temperature CO oxidation in reformate hydrogen.
The CO content in reformate hydrogen from a hydrocarbon reformer is about 5,000 ppm or over 10,000 ppm to 20,000 ppm immediately downstream of a methanol reformer. When using such a reformate hydrogen as combustible gas in polymer-electrolyte-membrane (PEM) fuel cells, this CO must be reduced almost completely, that is to about 30 ppm maximum not to poison the Pt/Ru—C anodes of the PEM fuel cell conventionally used. To reduce the CO content in reformate hydrogen, there are several chemical engineering concepts, of which selective CO oxidation is currently preferred for mobile applications and small stationary plants for reasons relating to cost and selectivity, but also because of the comparatively high space-time yield.
This oxidative CO removal is traditionally carried out in a multi-stage reactor by means of known high-temperature catalysts, for example Pt/Al
2
O
3
, at 200° C. The control of such a reactor system for continuously guaranteeing a residual CO content of about 30 ppm at different load states of the fuel cell is however extremely expensive and complicated. One of the main reasons for this, which occurs particularly during transfer to weak loads with larger residence times associated therewith, is the retro-shift reaction (3) competing with the reaction equations (1) and (2) shown below, and which has to be repressed, for example by rapid increase of oxygen supply while reducing the required selectivity.
CO+½O
2
→CO
2
(1)
H
2
+½O
2
→H
2
O (2)
CO
2
+H
2
→CO+H
2
O (3)
Catalyst materials have been developed, in which the Pt has been replaced by Ru or a different Pt group metal, and which have the same activity and selectivity as the traditional Pt/Al
2
O
3
catalyst material in the temperature range from 120 to 150° C. at comparable noble metal content.
For reasons relating to kinetics and process technology, it is advantageous to allow CO coarse cleaning to proceed in the temperature range from 190 to 230° C. in a fixed bed reactor operating as isothermally as possible and filled with traditional Pt/Al
2
O
3
pellets. The second or last cleaning stage (CO fine cleaning at CO starting contents of 1,000 to 2,000 ppm) is then carried out at considerably lower temperatures, for example at 120° C., using the above-mentioned catalyst materials.
Furthermore, it has been proposed to shift the CO fine cleaning to the working region of the PEM fuel cell, that is at temperatures up to 80° C., but for which a low-temperature CO oxidation catalyst is required.
It is known that metal oxide-supported Au catalysts show high catalytic activity during low-temperature oxidation of CO even in reducing atmosphere. Hence, it can be seen from Journal of Catalysis 168 (1997) 125-127, that an Au catalyst (Au/MnO
x
catalyst) supported on manganese oxides may be used for selective oxidation of CO in hydrogen. The production of the Au/MnO
x
catalyst is effected by coprecipitation of an aqueous solution of tetrachloroauric acid and manganese nitrate with an aqueous lithium carbonate solution, drying and calcining of the coprecipitate in air at 300° C. The calcined sample thus consists mainly of metallic gold particles and MnCO
3
. After measuring the catalytic activity for CO oxidation in hydrogen for one day, decomposition of MnCO
3
occurred with formation of crystalline manganese oxides, MnO, Mn
3
O
4
and Mn
2
O
3
. In addition, there was sintering of the gold particles, wherein an average particle diameter of 2.8 nm was obtained. However, the CO conversion rate of such a catalyst material is relatively low and not satisfactory for practical application.
Applied Catalysis A: General 134 (1996) 275-283 reports on the low-temperature water gas shift reaction on Au/Fe
2
O
3
catalysts produced by coprecipitation. It can be seen from this that a higher catalytic activity results with smaller gold particle diameter. The CO conversion rate of an Au/Fe
2
O
3
catalyst material produced by coprecipitation is however likewise not satisfactory.
German Offenlegungsschrift 4 238 640 describes Au/Fe
2
O
3
catalysts for hydrogenating CO and CO
2
, which likewise are produced by mixed precipitation of a gold compound and an iron salt.
The object of the present invention is to provide an Au/Fe
2
O
3
catalyst material having increased activity and selectivity, particularly for low-temperature CO oxidation, and adequate long-term stability, and processes for its production.
This object is achieved by a catalyst material according to claims 1 and 3 and processes according to claims 7, 8 and 9. Advantageous or preferred embodiments of the inventive object are given in the sub-claims.
Accordingly, the object of the invention is an Au/Fe
2
O
3
catalyst material made from a particulate, co-catalytically active Fe
2
O
3
support material with metallic Au clusters deposited thereon which have a diameter of less than 4.5 nm, which can be obtained by
a) reacting a water-soluble Fe(III) salt in an aqueous medium with a base,
b) impregnating the still moist hydroxide gel thus formed with a solution of a water-soluble Au compound to deposit complexed Au clusters on the surface of the hydroxide gel,
c) removing water from the suspension of the reaction product thus formed, and
d) subjecting the dried reaction product to calcining at temperatures between 350 and 700° C.
According to a preferred embodiment, this catalyst material also contains at least one Fe
2
O
3
sinter inhibitor selected from Al
2
O
3
, Cr
2
O
3
and MgO.
The object of the invention is also an Au/Fe
2
O
3
catalyst material made from a particulate, co-catalytically active Fe2O
3
support material containing at least one Fe
2
O
3
sinter inhibitor selected from Al
2
O
3
, Cr
2
O
3
and MgO and with metallic Au clusters deposited thereon which have a diameter of less than 4.5 nm, which can be obtained by:
i) simultaneously reacting a water-soluble Fe(III) salt, at least one water-soluble salt of Al, Cr, Mg and a water-soluble Au compound in an aqueous medium with a base,
ii) removing water from the suspension of the reaction product thus formed, and
iii) subjecting the dried reaction product to calcining at temperatures between 350 and 700° C.
The catalyst material of the invention preferably contains 2-8 wt. % Au, since the best results are obtained with such a gold deposit.
Furthermore, it is desirable that the catalyst material of the invention has as high as possible specific surface area, preferably of at least 50 m
2
/g according to the BET method. Furthermore, the Au clusters in the catalyst material of the invention have as high as possible a degree of dispersion, so that the Au clusters preferably have a diameter of less than 4 nm, also preferably of 1-3 nm.
A high specific oxide surface area and a high degree of dispersion for the Au clusters are particularly advantageous as regards kinetic points of view, since the step determining the reaction rate during CO oxidation takes place on the gold-iron oxide boundary. The degree of dispersion of the gold is therefore very important with regard to the CO conversion rate for the same Au deposit.
Regarding the CO selectivity of the catalyst materials of the invention, it has been shown that the selectivity increases for a temperature reduction from, for example 80 to 20° C. This can be explained in that at lower temperatures CO is generally absorbed more strongly than H
2
. However, the rate of CO oxidation also drops with a reduction in temperature.
The Au/Fe
2
O
3
catalyst materials of the invention show an excellent long-term stability. For example the catalyst material of the invention shows no change on one-week long storage under real reformer gas atm
Hendrickson Stuart L.
Lish Peter J
Meyer Jerald L.
Nath Gary M.
Nath & Associates PLLC
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