U.S. patent application number 11/728939 was filed with the patent office on 2007-09-27 for gold-containing catalyst with porous structure.
Invention is credited to Marcus Baeumer, Juergen Biener, Monika M. Biener, Alex V. Hamza, Birte Jurgens, Christian Schulz.
Application Number | 20070224099 11/728939 |
Document ID | / |
Family ID | 38276663 |
Filed Date | 2007-09-27 |
United States Patent
Application |
20070224099 |
Kind Code |
A1 |
Biener; Juergen ; et
al. |
September 27, 2007 |
Gold-containing catalyst with porous structure
Abstract
The invention relates to a gold-containing catalyst with porous
structure that is obtainable through a process that comprises the
following steps: melting together of gold and at least one less
noble metal that is selected from the group consisting of silver,
copper, rhodium, palladium, and platinum, and at least partial
removal by dissolving the at least one less noble metal out of the
starting alloy thus obtained. The catalyst has high activity and
great long-term stability, despite the fact that it does not
contain a support material or a compound that serves as a support
material. The catalyst can be used to accelerate and/or to
influence the product selectivity of oxidation and reduction
reactions. The catalyst is suitable, for example, for the
oxidization of carbon monoxide to carbon dioxide, which makes it
usable, among other things, in a fuel cell, in particular a polymer
electrolyte membrane fuel cell (PEM), for protection of the anode
catalyst against blocking by carbon monoxide.
Inventors: |
Biener; Juergen; (San
Leandro, CA) ; Hamza; Alex V.; (Livermore, CA)
; Baeumer; Marcus; (Bremen, DE) ; Schulz;
Christian; (Bremen, DE) ; Jurgens; Birte;
(Lilienthal, DE) ; Biener; Monika M.; (San
Leandro, CA) |
Correspondence
Address: |
Alan H. Thompson;Assistant Laboratory Counsel
Lawrence Livermore National Laboratory
P.O. Box 808, L-703
Livermore
CA
94551
US
|
Family ID: |
38276663 |
Appl. No.: |
11/728939 |
Filed: |
March 26, 2007 |
Current U.S.
Class: |
423/247 ;
502/330 |
Current CPC
Class: |
B01J 25/00 20130101;
H01M 4/90 20130101; H01M 2008/1095 20130101; H01M 4/98 20130101;
B01J 23/52 20130101; B01J 23/56 20130101; Y02E 60/50 20130101 |
Class at
Publication: |
423/247 ;
502/330 |
International
Class: |
B01J 23/66 20060101
B01J023/66 |
Goverment Interests
[0001] The United States Government has rights in this invention
pursuant to Contract No. W-7405-ENG-48 between the United States
Department of Energy and the University of California for the
operation of Lawrence Livermore National Laboratory.
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2006 |
DE |
10 2006 014 067.2 |
Claims
1. Gold-containing catalyst with porous structure, prepared by a
method that comprises the following steps: producing a starting
alloy by melting together of gold and at least one less noble metal
that is selected from the group consisting of silver, copper,
rhodium, palladium, and platinum; and a dealloying step comprising
at least partial removal of the less noble metal by dissolving the
at least one less noble metal out of the starting alloy.
2. The catalyst defined in claim 1, comprising a ratio of gold to
the less noble metal(s) in the starting alloy is in the range from
50 atom %: 50 atom % to 10 atom %: 90 atom % liegt.
3. The catalyst defined in claim 1 or 2, comprising an
Au--Ag-starting alloy, the ratio of gold to silver in the starting
alloy is within the range from 45 atom %: 55 atom % to 20 atom %:
80 atom %.
4. The catalyst defined in one of the preceding claims, wherein the
process further comprises a step of homogenization, in which the
starting alloy is held for an adequate time at a temperature just
below the melting point.
5. The catalyst defined in one of the preceding claims, wherein the
process also comprises at least one shaping step with which the
starting alloy is given a desired shape.
6. The catalyst defined in claim 5, comprising the shaping of the
starting alloy by at least one of the shaping methods of pressing,
stamping, rolling, bending, boring, hammering, cutting, and milling
is used.
7. The catalyst defined in one of claims 5 or 6, further
comprising, after the at least one shaping step, a step of
annealing the shaped starting alloy.
8. The catalyst defined in one of the preceding claims, comprising
the dealloying step and the creation of the porous structure occur
with the use of at least one wet-chemical and/or one
electrochemical process.
9. The catalyst defined in one of the preceding claims, comprising
after the dealloying step, the ratio of gold to the at least one
less noble metal in the porous structure is in the range from 100
atom %: 0 atom % to 95 atom %: 5 atom %, determined using AAS.
10. The catalyst defined in one of the preceding claims, further
comprising a step of activation of the material obtained after the
at least partial dealloying at a temperature of roughly +40.degree.
C. to +80.degree. C. in an oxygen-containing atmosphere, optionally
in the presence of carbon monoxide.
11. The catalyst defined in one of the preceding claims, comprising
a pore structure with diameters of roughly 30 to 100 nm determined
with scanning electron microscopy and a surface in the range of 2
to 8 m.sup.2/g determined with the BET process.
12. The catalyst defined in one of the preceding claims comprises
of a powder, pellets, or a membrane-type structure, with the
membrane-type structure having a thickness of as little as about
100 mm.
13. The catalyst defined in one of claims 1 through 12 is used in a
process to accelerate and/or influence the product selectivity of
oxidation and reduction reactions.
14. The process defined in claim 13 comprises oxidation of carbon
monoxide contained in a medium to carbon dioxide.
15. The process defined in claim 14, wherein the temperature of the
carbon monoxide-containing medium is in the range from roughly
-50.degree. C. to +150.degree. C.
16. A fuel cell that contains a catalyst defined in one of claims 1
through 12.
17. The fuel cell defined in claim 16, comprises a polymer
electrolyte membrane fuel cell, preferably a low temperature
polymer electrolyte membrane fuel cell.
18. The fuel cell defined in claim 16 or 17, wherein the catalyst
comprises a framed or frameless membrane catalyst or a catalyst
charge that is arranged upstream relative to the anode catalyst in
the gas stream.
19. The catalyst defined in claim 5, wherein said resulting
gold-containing catalyst with porous structure contains no support
structure for said gold.
20. The catalyst defined in claim 1, where said less noble metals
are selected from the group consisting of silver, copper and
palladium.
Description
FIELD OF INVENTION
[0002] The present invention relates to a gold-containing catalyst
with porous structure, the use of the catalyst according to the
invention to accelerate and/or influence the product selectivity of
oxidation and reduction reactions, as well as a fuel cell with a
catalyst according to the invention.
CROSS-REFERENCE TO RELATED APPLICATIONS
[0003] This application claims the benefit of German Patent
Application No. XXXXXXXXXX, filed Mar. 27, 2006 and titled
"GOLD-CONTAINING CATALYST WITH POROUS STRUCTURE" is incorporated
herein by this reference.
[0004] The action of catalysts is known to be based on the fact
that they open a path to chemical reactions by which starting
compounds or materials can be converted into end products by
application of a small activation energy. However, catalysts not
only accelerate a chemical reaction in this manner, but they can
frequently also influence the objective of the reaction.
Consequently, catalysts have immense significance in all fields in
which an accelerated or targeted chemical conversion of educts is
desirable or necessary.
[0005] Gold-based catalysts have been known for only a few years.
They are suitable for both oxidation and reduction reactions, with
the focus of the research and application fields certainly
concentrating on the field of oxidation reactions. In this area,
gold-containing catalysts are known, for example, for oxidation of
ethylene and acetic acid to vinyl acetate or the partial or
selective oxidation of hydrocarbons. However, the best-known
application for gold-based catalysts has to be the oxidation of
carbon monoxide to carbon dioxide. This reaction is often used not
only as a model for investigation of the activity and properties of
gold-based catalysts; such catalysts are among the few systems with
which this reaction can already occur to an extent worth mentioning
at room temperature.
[0006] In the area of reduction reactions, the use of gold-based
catalysts has been described, for example, in the hydrogenation of
carbon monoxide, carbon dioxide, and acetylene. Furthermore, gold
catalysts in the form of supported gold particles have also been
investigated with very positive results for applications in the
reduction of nitrogen oxides as well as for hydrogenation reactions
of alkenes or unsaturated aldehydes. (Masatake Haruta, CatTech,
2002, 6, 102415; Masatake Haruta, Cat, Today, 1997, 36,
153-166.)
[0007] It is further known that gold, as the most noble metal, can
have a catalytic effect only under certain conditions. One of these
conditions is, for example, that the gold must be present in very
small (nano) particles. Such very small gold particles have,
however, large surface energy and, consequently, tend to coagulate
quickly, as a result of which their catalytic activity is greatly
reduced.
[0008] For this reason, it has been proposed, to immobilize gold
(particles) on a support material, in particular a transition metal
oxide. A group of processes for the production of such
catalytically active gold-containing systems are known that are
directed at positively influencing the parameters important for
catalytic systems, such as, for instance, selectivity, the reaction
conditions (such as pressure and temperature) necessary for the
progress of catalytic reactions, reaction speeds obtainable, as
well as long-term stability, for example, through the use of new
support materials or through the use of new processes for the
production of gold and/or support material particles.
[0009] Thus, it has been proposed, for instance, in DE 4238640 A1
to produce the metal oxide with immobilized gold by means of mixed
precipitation, wherein, however, part of the gold is located
inactively in the interior of the resulting particles.
[0010] In order to circumvent this disadvantage, it has been
proposed, repeatedly, to supply the gold to already existing porous
support particles. Accordingly, WO 00/64581 teaches, for example,
to first produce a particle of titanium-silicon mixed oxide using a
sol-gel process; and then to deposit the gold on it using known
processes, such as precipitation, impregnation, spluttering,
chemical vapor deposition (CVD), or physical vapor deposition.
[0011] When gold is used in dissolved form (e.g., as AuCl.sub.3) to
deposit it, for example, by impregnation on the surface of support
particles, as is proposed among other things in WO 03/106021, it is
necessary to transform the gold cations into metallic form through
an additional reduction reaction.
[0012] With wet chemical processes, problems related to
reproducibility of the chemical/physical properties of the
catalytic systems obtained are reported. These difficulties
probably are based, for example, on the fact that, among other
things, it is difficult to control the size of the gold particles,
that the catalysts are poisoned by ions such as, for example,
chloride, that different amounts of gold "get lost" in the pores of
the support material, and that, through necessary thermal secondary
processing steps, the catalytic activity of the material is altered
in a non-reproducible manner.
[0013] For this reason, it is proposed, e.g., in WO 2005/03082 to
deposit the gold by PVD on support material particles. A suitable
vacuum apparatus is required for the PVD process, which makes the
process expensive in terms of equipment and also imposes
restrictions with regard to the size and shape of the support
material used.
[0014] WO 03/059507 describes gold-containing catalytic materials
that are produced by melting gold and a less noble metal together
and then at least partially removing the less noble metal from the
material obtained through chemical or electrochemical treatment.
The metals of the groups IIB (Zn, Cd, Hg) and IIIA (B, Al, Ga, In,
Tl) are proposed as less noble metals. It is further proposed to
additionally provide a metal from one of the groups IVB, VB, VIB,
VIII, IB, IIB, IIIA, IVA, as well as magnesium and cerium as a
promoter. Also, in WO 03/059507 it is assumed that the finished
catalyst has to have a support material based on a metal oxide
since that is the only way to be able to prevent rapid
agglomeration and sintering of the gold-containing material.
[0015] From this brief overview of known processes for the
production of gold-containing catalysts, it is clear that, in each
case, multiple complex and/or expensive steps are required before
obtaining a gold-containing catalyst usable in practice.
[0016] Consequently, one object of the present invention is to
provide a gold-containing catalyst with which the disadvantages
known from the prior art are reduced. Another object of the present
invention is to provide advantageous applications for the catalyst
according to the invention.
SUMMARY OF THE INVENTION
[0017] These objects are accomplished through a gold-containing
catalyst with porous structure, prepared by a method that includes
the following steps: producing a starting alloy by melting together
of gold and at least one less noble metal that is selected from the
group consisting of silver, copper, rhodium, palladium, and
platinum; and a dealloying step including at least partial removal
of the less noble metal by dissolving the at least one less noble
metal out of the starting alloy. It is desirable that the resulting
gold-containing catalyst with porous structure contains no support
structure for the gold.
[0018] The catalyst can be used advantageously in a process to
accelerate and/or influence the product selectivity of oxidation
and reduction reactions, and more particularly in a fuel cell.
[0019] The gold-containing catalyst with porous structure according
to the invention is characterized in that it is obtainable through
a process that comprises the following steps: melting together of
gold and at least one less noble metal that is selected from the
group consisting of silver, copper, rhodium, palladium, and
platinum, and at least partial removal by dissolving the at least
one less noble metal out of the starting alloy thus obtained. The
preferred less noble metals for use in this invention include
silver, copper and palladium, with silver being most preferred. The
step of the at least partial dissolving out of the at least one
less noble metal from the starting alloy is referred to hereinafter
as a "dealloying process."
[0020] The catalyst according to the invention is surprisingly
distinguished by high activity and great long-term stability,
despite the fact that it does not contain a support material or a
compound (e.g., a transition metal oxide) that serves as a support
material. This is all the more surprising since all prior art
assumed that such a support material is absolutely necessary in
order to obtain adequate catalytic activity and stability of a
gold-based catalyst.
DETAILED DESCRIPTION
[0021] The production of a gold-containing starting alloy is known
to the person skilled in the art and may occur, for example,
through simple mixing of the metals in a desired quantitative
proportion and subsequent melting of the metals in a furnace,
optionally in a protective gas atmosphere.
[0022] Starting alloys in which the ratio of gold to the less noble
metal(s) is in the range from 50 atom %: 50 atom % to 10 atom %: 90
atom % are suitable for the catalyst according to the
invention.
[0023] In the case of a gold-silver starting alloy (Au--Ag-starting
alloy), the composition can be within the range from 20 to 45 atom
%, i.e., at a ratio of gold to silver in the range from 45 atom %
55 atom % to 20 atom %: 80 atom %. In the case of an
Au--Ag-starting alloy, higher Au-concentrations result in the
formation of a passivation layer and lower Au-concentrations do not
yield a monolithic porous metal body. The possible concentration
limits for each alloy type for all the metals disclosed herein can
be readily determined by a person skilled in the art. For the
measurements of catalytic activity described below, an
Au--Ag-starting alloy with 30 atom % Au was used.
[0024] Within the respective possible concentration limits,
additional optimization may be undertaken. What is considered
optimal in each individual case may differ; accordingly, for
example, optimization may have as its goal the highest possible
activity, the longest possible service life, or even the least
possible cost.
[0025] After the production of the starting alloy, it is
advantageous to homogenize the starting alloy. This is achieved by
holding the starting alloy for an adequate time at a temperature
just below the melting point.
[0026] One of the particular advantages of the catalyst according
to the invention is that its production starts from a starting
alloy. The starting alloy obtained can be given almost any shape
before the dealloying process, and, thus, virtually any desired
shape can be obtained. The external shape of the starting alloy is
not altered by the dealloying process described below, such that
the shape of the later catalyst can already be predefined through
the shaping of the starting alloy.
[0027] Any suitable process can be used for the shaping of the
starting alloy, such as, pressing, stamping, rolling, bending,
boring, hammering, cutting, and/or milling. Since these methods are
usually not particularly expensive from a technical standpoint (for
example, no vacuum chamber is required), virtually any desired size
and shape of the catalyst can be produced simply and
cost-effectively.
[0028] The shaped starting alloy is preferably annealed before the
dealloying process in order to reduce mechanical stresses, for 24
hours at 850.degree. C., for example.
[0029] The starting alloy may, however, for example, also be given
a desired shape or an advantageous shape for additional processing
or shaping using a casting process. If the shaping of the catalyst
according to invention occurs exclusively by means of a casting
process, the starting alloy obtained is advantageously merely
homogenized.
[0030] The at least partial dealloying and creation of the porous
structure then occurs in a next step preferably through the use of
at least one electrochemical and/or wet-chemical process. Which
process or which combination of different processes is the most
suitable in each case depends, among other things, on the
composition of the alloy and/or the intended use of the resultant
catalyst. The most suitable process or the most suitable
combination can be determined by a person skilled in the art
through a few experiments.
[0031] With the use of an electrochemical process, the partial or
complete dealloying of the less noble metal out of the starting
alloy and the extent of the dealloying (i.e., how much of the less
noble metal is still found or remains in the starting alloy) can be
very precisely controlled by adjusting the voltage or current
density of the electrical process.
[0032] For example, the silver portion can be dissolved out of the
Au--Ag-starting alloys to the extent desired by fixing the
specimens using a gold plated clamp and placing them, for example,
in a solution with 1 M HNO.sub.3 and 0.01 M AgNO.sub.3. The
solution described is appropriate for a silver pseudo-reference
electrode, but must not be used for silver/silver chloride
reference electrodes. In the latter case, there would be a risk of
contamination of the solution with chloride ions. In order to free
the starting alloy of silver, voltage above the critical potential
is applied. The dealloying process is terminated when the
electrical current drops into the range of a few microamperes.
Then, the specimen is usually washed several times with water and
then dried in air. The residual proportion of the at least one less
noble metal can be controlled by the total current conducted or the
conditions at the end of the electrolysis.
[0033] An at least partial dealloying occurs through the use of a
wet-chemical process using a solution with a composition such that
it causes dissolution of the less noble metal out of the starting
alloy. The composition of the solution is guided by the requirement
of being able to dissolve the less noble metal(s) but without
significantly attacking the gold in the starting alloy.
[0034] One example of such a solution for the at least partial
dealloying of an Au--Ag-starting alloy with 30 atom % gold consists
in a solution of 70% nitric acid. With such a solution, it is
possible, for example, to (partially) dealloy 300 .mu.m thick
specimens within one to three days at room temperature. At least
the majority of the silver portion is selectively dissolved out of
specimens by nitric acid, and nanoporous gold foams remain. The
acid is removed by washing the specimens several times with water;
then, the specimens can be dried in air.
[0035] The proportion of the less noble metal(s) can be expediently
reduced to the desired remainder through process control during the
dealloying process, with this remainder being as low as a
proportion of 0 atom %.
[0036] The material has, after partial or complete dealloying, a
ratio of gold to the at least one less noble metal in the range
from 100 atom %: 0 atom % to roughly 95 atom %: 5 atom %. These
data are based on results of analyses using atom absorption
spectroscopy (AAS), with which the entire composition of the
material can be determined.
[0037] Analyses using XPS (XPS=X-ray photoelectron spectroscopy)
yielded ratios of gold to the at least one less noble metal in the
same materials in the range from roughly 100 atom %: 0 atom % to
roughly 80 atom %: 20 atom %. To the extent the values determined
using XPS indicated a higher proportion for the at least one less
noble metal, this is based on the fact that XPS is a highly surface
sensitive measurement method. It is well known to the individuals
skilled in the art that in alloys one or more metals may be
enriched on the surface relative to the interior.
[0038] The optimum proportion of the less noble metal that should
remain in the porous gold-containing catalyst for the respective
desired purpose may be determined simply by an individual skilled
in the art by experiment. As already explained above with regard to
the mix ratio of the metals for the starting alloy, there is also
the possibility with regard to the residual metal content of the
less noble metal that the question as to what is considered optimum
can be answered differently for each applicational case; here
again, optimization may, for example, have as its goal the highest
possible activity, the longest possible service life, an optimum
degree of porosity, or even the least possible cost.
[0039] In order for the catalyst according to the invention to
develop its full catalytic activity, it is activated advantageously
by a moderately elevated temperature (e.g., in the range from
roughly +40 to +80.degree. C.) in an oxygen-containing atmosphere,
which optionally contains a certain proportion of CO. Usually, a
one-time activation of this type suffices. A presence of carbon
monoxide is advantageous since it is possible to track the course
of activation through it (after successful activation, oxidation of
carbon monoxide to carbon dioxide is detectable or increased). To
date, no more detailed information is available concerning the
mechanism of activation.
[0040] One possible explanation for this activation may be that
water which is found in the pores of the foam is removed by the
elevated temperature. It may also be possible that organic
compounds such as hydrocarbons that block the surface are likewise
removed by the elevated temperature. It is also conceivable that a
segregation of metals to the surface takes place under the
conditions selected for activation.
[0041] As investigations have shown, advantageous embodiments of
the catalyst according to the invention have a pore structure with
diameters of roughly 30 to 100 nm (determined with scanning
electron microscopy) and a surface in the range of 2 to 8 m.sup.2/g
determined with the BET process.
[0042] The plastic properties of the starting alloy make it
possible, advantageously, that the alloy can even be shaped in a
thin or very thin film, and this thin or very thin film can then be
dealloyed at least partially by one of the above described
processes.
[0043] After the dealloying process, a porous membrane-type
structure with a thickness of as little as 100 nm is available,
through which a medium, for example, a gas or a gas mixture or a
liquid may be guided. In order to obtain the greatest possible
reactions or yields, the catalyst according to the invention may be
installed, for example, in the form of a framed or frameless
membrane-type structure (membrane catalyst) at right angles to the
direction of flow of a medium.
[0044] As the inventors discovered, the catalyst according to the
invention has, for example, an excellent capability to oxidize
carbon monoxide (CO) to carbon dioxide (CO.sub.2) and to do this at
temperatures of a carbon monoxide-containing medium, for example, a
gas, a gas mixture, or a liquid, as low as about -50.degree. C. The
examples described in the following concern tests in which this
oxidation was successfully performed in the range from roughly
-20.degree. C., through 0.degree. C. and room temperature
(+23.degree. C.) all the way up to +50.degree. C. Of course, it is
to be expected that this oxidation reaction is accelerated by even
higher temperatures, for example, up to approximately 150.degree.
C., using the catalyst according to the invention. In particular,
catalytically mediated oxidation at low temperatures seems to be
especially interesting since, with it, energy and, consequently,
cost can be saved.
[0045] The property of efficient oxidation of carbon monoxide to
carbon dioxide also makes the catalyst according to the invention
interesting for application in fuel cells. In particular, with
polymer electrolyte membrane fuel cells (PEM), carbon monoxide can
block the anode catalyst. If a catalyst according to the invention,
e.g., in the form of a framed or frameless membrane catalyst, or
even in the form of a conventional catalyst charge is arranged
upstream relative to the anode catalyst in the gas stream,
disadvantageously acting carbon monoxide can be removed for the
most part, in any case, from the gas stream by this catalyst before
it is sent on to the anode catalyst.
[0046] As already mentioned above, the catalyst according to the
invention may also be produced in the form of a particulate
catalyst, for example, in powder form, and then be used, for
instance, in the form of a catalyst charge or as a finely
distributed catalyst in a reaction mixture.
[0047] For the production of the catalyst according to the
invention in particle and powder form, the material can be crushed
after the dealloying process through the use of an appropriate
process. This is comparatively simple because the material has a
clearly higher brittleness after the at least partial dealloying
than the starting alloy. In the crushing process, care must be
taken to avoid pressure loads or to keep them as small as possible
since a high pressure load could have a disadvantageous effect
through compacting or destruction of the pores.
[0048] Alternatively, according to the invention, the starting
alloy may also be produced in small particles, e.g., by dripping a
corresponding melt into a cool or cooling medium. It is likewise
possible to produce small particles from a solid starting alloy by
mechanical processing. The small particles of the starting alloy
can then be at least partially dealloyed as described and developed
into a catalyst according to the invention.
[0049] The catalyst particles or powder thus obtained can then, of
course, be again formed, for example, into larger units, such as
pellets, for example, and used as such.
[0050] The present invention is described more precisely through
the following statements. These statements are based on tests and
results that have been performed or determined using the catalyst
according to the invention for oxidation of carbon monoxide to
carbon dioxide. The oxidation of common monoxide to carbon dioxide
represents, however, only one of the many possible areas of
application for the catalyst according to the invention and is thus
to be understood merely as one suitable model system for the
presentation of properties of the catalyst according to the
invention. The statements thus serve merely to illustrate
properties of the catalyst according to invention and must not be
understood such that they are in any way restrictive for the range
of protection of the following claims. Moreover, the data in this
document concerning the composition of the catalyst according to
the invention are to be understood such that in the gold used and
in the at least one less noble metal, the usually present
impurities may, of course, still be present.
BRIEF DESCRIPTION OF DRAWINGS
[0051] FIG. 1 Scanning electron microscope images of the porous
structure according to the invention at a) 1500.times. and b)
6000.times. enlargement;
[0052] FIG. 2 an XPS-overview spectrum of a gold foam according to
the invention;
[0053] FIG. 3 CO.sub.2 yields and temperature differences with
different CO concentrations at +23.degree. C. and a flow rate of
13.3 mL/min;
[0054] FIG. 4 CO.sub.2 yields and temperature differences with
different CO concentrations at +50.degree. C. and a flow rate of
12.8 mL/min;
[0055] FIG. 5 a comparison of the yields at +50.degree. C. and
+23.degree. C.;
[0056] FIG. 6 CO.sub.2 yields and temperature differences with
different CO concentrations at 0.degree. C.;
[0057] FIG. 7: CO.sub.2 yields and temperature differences with
different CO concentrations at -20.degree. C.;
[0058] FIG. 8: long-term stability of a catalyst according to the
invention
EXAMPLES
[0059] 1. Presentation of Exemplary Gold Foams
[0060] First, an Au--Ag-starting alloy with 30 atom % Au was
produced. The finished starting alloy was rolled to roughly 5 mm
diameter into small pieces of desired thickness. Then, the
specimens were annealed for 24 hours at 850.degree. C. For the
partial dealloying and creation of the foam structure, the specimen
underwent wet-chemical treatment. The wet chemical partial
dealloying was carried out in a solution of 70% nitric acid. The
specimens were placed on a porous glass plate in a beaker. Then,
the acid was added such that the specimen was covered. Roughly
300-.mu.m-thick specimens were dealloyed in this manner within from
one to three days. The greater part of the silver portion was
selectively dissolved out of the specimens by the nitric acid, and
nanoporous gold foams remained. The acid was removed using a
syringe and replaced with water. This was changed out several times
in order to clean remaining acid residues from the specimens. The
specimens were then air dried. The specimens have the foam
structure shown in FIG. 1.
[0061] 2. Surface Analysis Using XPS
[0062] XPS-analyses were performed in a UHV apparatus from the
company Omicron. The specimens were placed on a specimen plate and
transferred into the apparatus. The x-ray source used is a
magnesium anode from the company Omicron. Detection took place
through an energy spectrometer EA 10+ from the company
SPECS/Leybold. The energy of the photons in these measurements was
1253.6 eV. In all measurements, except those in the overview
spectrum presented in FIG. 2, the pass energy was 25 eV. For the
overview spectrum, it was 100 eV.
[0063] 3. Composition of an Exemplary Catalyst
[0064] In order to determine the composition, measurements were
performed on the one hand using atom absorption spectroscopy (AAS,
from the company Zeiss). For this, the specimens were dissolved in
aqua regia and then investigated for their silver and gold content.
On the other hand, the surface composition was determined using
x-ray photoelectron spectroscopy (XPS). This process is surface
sensitive and detects only the upper atomic layer of the material.
An exemplary XPS overview spectrum is presented in FIG. 2.
[0065] From the spectrum presented in FIG. 2, it is possible to
conclude that the gold is present in metallic, not in ionic form.
The peaks at 87.75 eV and 84.1 eV can be associated with the
4f-state of metallic gold, which according to the literature should
be at 87.45 eV and 83.8 eV [L. E. Davis, G. E. Muilenberg, C. D.
Wagner, W. M. Riggs; Handbook X-Ray Photoelectron Spectroscopy,
Perkin-Elmer Corporation 1978].
[0066] Two additional peaks can be identified in the spectrum at
373.05 eV and 367.06 eV. These two peaks can be associated with the
3d-state of metallic silver. The surface composition can be
quantitatively determined through the integral intensities of the
gold and silver peaks--taking into account the different effective
cross-sections.
[0067] Using the ratios of the integral signal intensities of
silver and gold, a silver content of 4-5 atom % was determined. It
was assumed that the silver is distributed homogeneously in the
areas near the surface detected with the method. In contrast, using
AAS, a residual silver content of approx. 0.5 atom % was
determined. This points to an enrichment of the silver on the
surface.
[0068] 4. Reactor Setup for the Measurements of Carbon Monoxide
Oxidation
[0069] The reactor consists of a glass cylinder with a fritte
(diameter 2 cm), on which the specimen (diameter 5 mm) rests. The
gas is supplied via a glass tube that surrounds the cylinder in a
spiral formation to control the temperature of the gas. (It should
be noted that with the reactor setup described, depending on flow
conditions, not all gas molecules can interact with the specimen.
The gas flow rate is regulated by two flow volume regulators from
the company Brockhorst. The regulators have a maximum flow rate of
50 mL/min synthetic air and 5.6 mL/min carbon monoxide. The
deviations of the flow volume regulators from the set flow equal
.+-.2%. The flow volume regulators are calibrated using the bubble
counter method.
[0070] Temperature control is indirect via a silicon bath in which
the reactor is located and which can be temperature controlled from
-20.degree. C. to +200.degree. C. by thermostat from the company
Haak. In the interior of the reactor, there is a
nickel/chromium-nickel and a nickel/chromium-nickel-nickel/chromium
element. The reactor temperature is measured by the
Ni/Cr--Ni-element, whereas the Ni/Cr--Ni--Ni/Cr-element is arranged
such that one node contacts the specimen and the other hangs above
it. By means of this arrangement, it is possible to measure the
temperature difference between the specimen and the reactor. The
error of such a thermal element is 1% of the temperature difference
measured.
[0071] A CO.sub.2 Uras 3G from the company Hartmann and Braun, with
which the amount of carbon dioxide is measured by volume, is
connected. The carbon dioxide concentration in volume and the
temperature difference are output as a voltage signal and plotted
by a chart recorder LS-52-2 of the company Linseis. The Uras 3G is
calibrated by setting different concentrations of carbon monoxide
in synthetic air and then detecting them using the Uras 3G. The
maximum operating range of the device is 8 volume percent carbon
dioxide with an instrument error of 0.5%.
[0072] 5. Catalysis Using an Exemplary Catalyst at +23.degree.
C.
[0073] The following statements refer by way of example to a
catalyst according to the invention in the form of a gold foam that
was produced from an Au--Ag-starting alloy (30 atom % Au) using a
wet-chemical process. This material has a pore structure with
diameters of approx. 50 nm determined using scanning electron
microscopy. The surface determined with the BET-process was approx.
4 m.sup.2/g. The residual silver content was, in the entire
material of the catalyst, as described, a few atom percent
(0.5-2%), with silver enriched on the surface (4-20 atom %).
[0074] After a one-time activation of the catalyst at +50.degree.
C. in a gas stream of 50 mL/min of synthetic air, that contained 4
volume % carbon monoxide, the reaction of the carbon monoxide could
be continued at room temperature. For this, 13 mL/min of synthetic
air flowed through the test reactor, in which the specimen lying on
a ceramic fritte was exposed to the gas stream. Carbon monoxide was
mixed at different concentrations in the gas stream. In the range
from 0.14 to 7.8 volume %, constantly increasing carbon dioxide
yields could be observed. The product gases were detected using a
carbon-dioxide-specific gas detector (Uras 3G from the company
Hartmann & Braun). In addition, it was possible to determine
the temperature difference (.DELTA.T) between the reactor and the
catalyst specimen using the thermal coupler. Since the reaction
investigated (CO+1/2O.sub.2-->CO.sub.2) is an exothermic
reaction, the reaction could be detected by an elevated temperature
of the specimen compared to the reactor. FIG. 3 depicts both the
carbon dioxide yields in volume % and the positive temperature
differences. Both results clearly indicate the oxidation of carbon
monoxide with the catalyst used. The temperature used at the time
of the tests was +23.degree. C.
[0075] 6. Catalysis Using an Exemplary Catalyst at +50.degree.
C.
[0076] When one performs the same test that was described above
under 5 at a temperature of +50.degree. C., one obtains results
that are very similar to those of the test at +23.degree. C., as
can be seen in FIG. 4. Again, the yield of carbon dioxide increases
virtually linearly with increasing carbon monoxide concentration,
and the temperature differences correlate with the yields. In FIG.
5, the yields from the measurements at +23.degree. C. and
+50.degree. C. are presented in a graph.
[0077] 7. Catalysis Using an Exemplary Catalyst at 0.degree. C.
[0078] In the case of measurements at 0.degree. C. the same
behavior of the catalyst according to the invention is observed as
with measurements at higher temperatures. As presented in FIG. 6,
both the highest yield of 4.5 volume % of carbon dioxide was
measured at the highest concentration of carbon monoxide and the
lowest yield of 0.3 volume % was measured at the lowest carbon
monoxide concentration.
[0079] 8. Catalysis Using an Exemplary Catalyst at -20.degree.
C.
[0080] The carbon dioxide yields at -20.degree. C. are lower than
all those mentioned at higher temperatures, as shown in FIG. 7.
However, the fact that such a high conversion is even obtained at
such a low temperature is unexpected and surprising and clear
evidence of the qualities of the catalyst according to the
invention.
[0081] When the yields from the measurements -20.degree. C. are
considered, they no longer reflect the typical course of the curve
that was observed with all previous measurements. In contrast to
the measurements at higher temperatures, the increase is no longer
linear. Although an increase in the yield of carbon dioxide formed
was always measured with the measurements at higher temperatures
with increasing carbon monoxide concentration, this appears to no
longer be the case above 4 volume % of carbon monoxide at
-20.degree. C.
[0082] Apparently, at a lower temperature such as -20.degree. C.
the concentration of carbon monoxide is not the deciding factor for
the reaction speed, but a different factor is acting in a limiting
manner.
[0083] Without intending to establish a specific explanation, it
might be that the dissociation of the molecular oxygen under the
conditions acts on the kinetics of the reaction to determine the
speed.
[0084] 9. Long-Term Stability
[0085] A specimen with a weight of 29.1 mg and a residual silver
content of 0.5 atom % in the interior and approx. 16 atom % on the
surface was first placed in the reactor at room temperature (ca.
+20.degree. C.) and rinsed with synthetic air. After a few minutes,
4 volume % of carbon monoxide (CO) was mixed with the gas. The
entire volume flow of the gas was 50 mL/min. Under these
conditions, the catalyst was, as anticipated, inactive. After
raising the temperature to +50.degree. C., the activity of the
specimen started immediately and even continued to increase in the
following hours. The temperature of +50.degree. C. was maintained
during the further course of the experiment. The curve of the
activity is shown in FIG. 8.
[0086] FIG. 8 shows the activity of the specimen as a function of
time. However, it must be considered that the catalyst used here
(in a reactor on laboratory scale) was a very small specimen with a
weight of only about 29 mg. Consequently, it must be assumed that a
large part of the reaction gas flowed past the catalyst surface and
could not participate in the reaction. The values presented here
must, consequently, be considered relative and not absolute
units.
[0087] The invention provides a new gold-containing catalyst with
porous structure that, with a comparatively simple method for its
production, is distinguished by an outstanding suitability for
catalytic acceleration and/or catalytic influence on the product
selectivity of oxidation and reduction reactions and also has
adequate to outstanding long-term stability despite the absence of
support material. These characteristics make the catalyst according
to the invention particularly suited, for example, for the
oxidation and, consequently, the removal of carbon monoxide from a
medium, such as a gas, a gas mixture, or a liquid.
* * * * *