Process of preparing metal supported catalyst having high surface area

Disclosed herewith is a process of preparing a metal supported catalyst having a high surface area comprising reducing one or more metal-containing ions in a solution employing one or more reducing agents selected from the group consisting of thiourea and thioacetamide, and supporting the reduced metal or metals onto a support. The thiourea and the thioacetamide exhibit more excellent activity and durability than those of ordinary reducing agents such as sodium borohydride and other sulfur-containing reducing agents.

BACKGROUND OF THE INVENTION 
The present invention relates to a process of preparing a metal supported 
catalyst by supporting various catalyst metals on a support, especially to 
a process of reducing an ion containing the metal. 
A catalyst comprising an inorganic oxide support such as silica and 
alumina, and an individual noble metal such as platinum, gold and 
palladium or the combination thereof supported thereon has heretofore been 
employed as that for various chemical reactions and for an electrode of a 
fuel cell. Another catalyst comprising a carbon support and the same 
catalyst noble metals supported thereon has been also employed. 
The catalyst performance of these catalysts depends on the degree of 
dispersion of the catalysts metals and the performance (specific activity) 
is promoted with the increase of the surface area of the catalyst if the 
same amount of the catalyst metals is supported thereon. 
In preparing the above catalysts, the metal elements are supported onto the 
inorganic support by reducing the ions containing the catalyst metals to 
be supported, to the metal elements by means of a reducing agent. 
Since however, such a reducing agent as lithium aluminum hydride and sodium 
borohydride ordinarily employed is too strong, the particle size of the 
metals produced by the reduction increases and the particle size 
distribution becomes broader. 
In other words, the conventional method has the drawbacks such that the 
number of the particles decreases to lower the surface area per unit 
weight of the metal so that the catalytic activity is also made to be 
lowered, and the particle size becomes considerably uneven. 
Various kinds of alloy catalysts having high catalytic activity have been 
heretofore proposed after the investigation of the combination of 
supported metals (for example, Japanese patent application No. 59-141169). 
However, even in these catalysts, the catalyst metals thereof are requested 
to have a small particle size, that is, a large specific surface area for 
elevating the activity. 
Moreover, the alloy catalysts are generally prepared by alloying an alloy 
component with a noble metal already supported on the support. It is 
important from this viewpoint to prepare the catalyst supported with a 
high surface area noble metal having narrow particle size distribution, 
that is, a uniform particle size. 
Having the uniform particle size is important to obtain an alloy catalyst 
having particles of uniform alloy composition. 
These catalysts have the drawback that they are likely to be exposed to a 
high temperature so that the activity may be lost with the lapse of time 
to shorten the catalyst life if they do not have the resistance to a 
sintering reaction. 
In order to overcome these drawbacks, such a sulfur-containing and 
relatively weak reducing agent as a thiosulfate salt and a metabisulfite 
has been proposed (U.S. Pat. No. 4,956,331 and European patent publication 
No. 0329628). Although these sulfur-containing reducing agent exhibit more 
excellent catalytic activity and resistance, another reducing agent having 
much more excellent activity and resistance is of course desirable. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a process of preparing a 
metal supported catalyst having a high surface area and high durability. 
Another object of the invention is to provide a process of preparing a 
catalyst by reducing a metal-containing ion by means of a 
sulfur-containing reducing agent other than the conventional ones. 
In accordance with one aspect of this invention, there is provided a 
process of preparing a metal supported catalyst having a high surface area 
which comprises treating a solution of a metal-containing ion or ions or 
the mixture of the said solution of the metal-containing ion or ions and a 
support with one or more reducing agents selected from thiourea and 
thioacetamide, reducing the metal-containing ions to the corresponding 
metals having a high surface area to be supported or deposited onto the 
support, and separating the support from the said solution. 
In the present invention, the ions containing the catalyst metals are 
reduced with such a sulfur-containing and relatively weak reducing agent 
as thiourea or thioacetamide when one or more of the catalyst metals 
including, for example, platinum are supported onto the carbon support or 
the inorganic oxide support. 
Thereby, the particle size of the supported metals to be deposited 
decreases and becomes uniform compared with that when such a strong 
reducing agent as sodium borohydride is employed. 
This tendency becomes much more remarkable because the metal particles grow 
around the sulfur atoms liberated during the reduction process, which 
serve as nuclei. 
It is possible to effectively utilize the catalyst metals because the 
catalyst metals formed on the inorganic support and having a large surface 
area can be sufficiently in contact with a reactant to promote the 
specific catalytic activity. 
Further the catalyst metals formed according to the present invention are 
considered to have large durability against a sintering reaction which is 
expected to lower the surface area of the catalyst metals with time 
because the thermodynamic driving force of the reaction between the 
metal-containing ion and the sulfur-containing reducing agent is smaller 
than that of the prior art, to form the fine crystal particles with less 
defects. 
When a second and a third metal is further supported onto the noble metal 
supported catalyst and alloyed therewith, the composition of the particles 
after the alloying becomes nearly uniform so that the desired composition 
can be attained for the individual particles because the particle size of 
the noble metal produced in the process of this invention is uniform.

DETAILED DESCRIPTION OF THE INVENTION 
The most characteristic feature of this invention is in that, when the ion 
or ions which contain the catalyst metals, for example, a chloroplatinic 
ion are reduced to support the catalyst metals onto a carbon support or an 
inorganic oxide support such as silica and alumina, one or more 
sulfur-containing reducing agents which are relatively weak in reducing 
ability are employed. 
As an inorganic support of this invention, a carbon support of which a main 
component is a carbon element having any form such as carbon black, 
graphite and activated carbon; and a fire resistant inorganic oxide 
support such as silica and alumina may be employed. 
Since the inorganic support is utilized for a catalyst support, it is 
preferably fine particles having a large surface area, for instance, of 30 
to 2000 m.sup.2 /g and desirably having a particle size of 100 to 5000 
.ANG.. 
As a carbon support, for example, commercially available Acetylene Black 
(trade name: Shawinigan Acetylene Black and Denka Acetylene Black), 
electroconductive carbon black (trade name: Vulcan XC72R) and graphitized 
carbon black (trade name; Denka Black or the like). 
Such a noble metal as platinum, gold and palladium can be preferably 
employed as a metal to be supported onto the inorganic support of this 
invention, and other metals may also be employed. The ions containing 
these metals include the said individual metal ions; and the complex ions 
of these metals such as a chloroplatinic ion, a chloroauric ion and a 
palladium chloride ion. 
Reduction of the metal-containing ion will be described taking the 
chloroplatinic ion as an example. 
The way of supporting the platinum metal itself can be carried out 
according to any conventional method. One of the conventional methods 
comprises mixing the carbon support or the inorganic oxide support with a 
chloroplatinic acid aqueous solution, reducing the chloroplatinic ion and 
supporting the reduced platinum metal onto the support; and another 
comprises, prior to the mixing with the support, reducing the 
chloroplatinic acid and supporting the reduced platinum onto the supports. 
However, if a strong reducing agent is employed in these reactions for 
reduction, the size of platinum particles increases so that the surface 
area of the particles per unit weight considerably decreases. 
For this reason, such a weak reducing agent as thiourea and thioacetamide 
is employed to depress the decrease of the surface area of the platinum. 
The thiourea or the thioacetamide reacts with the platinum-containing ion 
in the aqueous solution, that is, the chloroplatinic ion to form finely 
divided metal sol having a large surface area. 
With the progress of the reaction, the solution turns from yellow to 
orange, and with the further growth of the metal fine crystals for several 
hours, the solution gradually becomes darker. 
Light passing through the solution exhibits the Tyndall effect showing the 
existence of colloidal particles. 
This sol is then adsorbed onto the carbon support or the inorganic oxide 
support to provide the inorganic support supported with the platinum 
through appropriate procedures such as drying. 
In other words, when the solution becomes nearly opaque, the carbon support 
and the like are added into the solution and then the liquid phase of the 
formed slurry is forced to penetrate into the pores of the inorganic 
support by agitation employing, for example, an ultrasonic agitator. 
The thicker slurry is formed by this procedure, which remains suspended and 
seldom precipitates. 
Different from the above procedures, after the chloroplatinic acid solution 
is initially added to the support to prepare the slurry, the formed slurry 
is well agitated and dispersed, for example, with an ultrasonic agitator, 
a small amount of the thiourea and/or thioacetamide aqueous solution may 
be initially added gradually and the rest of the solution may be added at 
once to reduce and deposit the platinum-containing ion to platinum under 
the existence of the support. 
Drying of thus obtained slurry e.g. at 75.degree. to 80.degree. C. for one 
to three days for removing water provides dry powder containing the salt 
of a reaction by-product. 
The by-product may be dissolved and removed by extracting the dry powder 
several times with, for instance, 100 to 200 ml of distilled water. 
In case of the graphitized carbon black support, the said slurry 
precipitates and can be separated from the aqueous phase by discharging 
the aqueous phase. After the procedure is repeated several times, the 
catalyst is dried overnight at about 110.degree. C. 
The catalyst thus prepared has a large surface area and its particle size 
is uniform. 
In place of the above procedures, a slurry-filtration-washing process can 
be utilized. 
This can be used in the case of acetylene black or the like which does not 
easily precipitate. 
For example, according to the process, an aqueous solution of 1g-Pt/100 ml 
of chloroplatinic acid is reacted with an aqueous solution of 0.4 g/25 ml 
of thiourea or the like to produce a catalyst supported with platinum of 
which a specific surface area is 185 m2/g and of which a particle size is 
uniform. 
Since the thermodynamic driving force of the reaction between the 
chloroplatinic ion and the thiourea or the like is smaller than that of 
the prior art and fine crystal particles with less defects can be 
produced, the catalyst particles produced in this process are considered 
to have larger durability against a sintering reaction in which the 
surface area of platinum decreases with time. 
The fine particles having a uniform particle size of not more than 15 .ANG. 
can be obtained by the above mentioned reaction between the thiourea or 
the like and the chloroplatinic acid. 
Relatively weak reducing agents which can be employed in the process of the 
present invention other than the above mentioned thiourea include 
thioacetamide. The thioacetamide can reduce the metal-containing ion in a 
similar manner. 
The well dispersed catalyst particles can be obtained both in the process 
in which the platinum-containing ion is reduced prior to the impregnation 
of the platinum-containing solution into the carbon support and in the 
other process, contrary to the above process, in which the 
platinum-containing ion is reduced after the impregnation. 
In place of supporting one kind of metal such as platinum, gold and 
palladium, a solution containing two or more metal-containing ions can be 
employed so that these metals can be supported at the same time. 
In the above process, it may be possible to initially support only one 
metal which is then alloyed with another metal. 
It is possible, when the carbon support is employed as an inorganic 
support, to depress the decrease of the surface area due to movement and 
agglomeration of the supported metals, when employed at a high 
temperature, by carburizing the supported metals of the carbon support 
formed to enhance the affinity between the carbon support and the 
supported metals. 
EXAMPLES 
The present invention will be now described in detail in connection with 
the following Examples showing the preparation of the platinum alloy 
catalyst according to the present invention. However, these Examples are 
not intended to limit the scope of the present invention. 
Example 1 
Chloroplatinic acid containing 1.157 g of platinum was dissolved in 300 ml 
of water to which was added 75 ml of a solution dissolving 0.5 g of 
thiourea (H.sub.2 NCSNH.sub.2). The solution was further stirred at 
27.degree. C. 
With the lapse of time, the mixed solution became from yellow to orange, 
further to dark orange. 
After the lapse of three hours, the room was darkened and the light of an 
electric bulb was applied to the vessel, then the scatter of the light was 
observed. 
The slurry in which 10 g of Acetylene Black for the catalyst support was 
suspended in 100 ml of pure water was added to the above mixed solution. 
The slurry was stirred for two minutes with an ultrasonic agitator so that 
the mixed solution was forced to penetrate into the pores of the support. 
The slurry was kept to be suspended and did not precipitate during the 
stirring operation. 
The slurry was dried in an oven at 75.degree. to 80.degree. C. overnight 
for removing water. The dry powder thus obtained was washed three times 
with about 200 ml of distilled water so that the by-products were 
extracted and removed. 
The slurry was further dried at 70.degree. C. overnight to obtain the 
carbon support supported with the platinum. 
The average platinum size of the platinum-carbon catalyst thus obtained was 
15 .ANG., and the particle size of the platinum particles was found to be 
nearly uniform by observing the catalyst with a transmission type electron 
microscope. The specific surface area of the platinum was 185 m.sup.2 /g, 
and the supported platinum was 10% in weight according to an 
electrochemical hydrogen adsorption-desorption method. 
Comparative Example 1 
Platinum was supported by the same procedures as those of Example 1 except 
that Na.sub.2 BH.sub.4 was employed instead of the thiourea of Example 1. 
The average platinum size of the platinum-carbon catalyst thus obtained was 
48 .ANG., and the platinum particles had broad particle distribution of 20 
to 100 .ANG. according to the observation with an electron microscope. The 
specific surface area of the platinum was 56 m.sup.2 /g, and the supported 
platinum was 10% in weight. 
Comparative Example 2 
Platinum was supported by the same procedures as those of Example 1 except 
that 3 g of sodium thiosulfate pentahydrate was employed instead of the 
thiourea of Example 1. 
The average platinum size of the platinum-carbon catalyst thus obtained was 
18 .ANG., and the platinum particles had narrow particle distribution 
according to the observation with an electron microscope. The specific 
surface area of the platinum was 155 m.sup.2 /g, and the supported 
platinum was 10% in weight. 
Example 2 
Platinum was supported on the carbon support by the same procedures as 
those of Example 1 except that thioacetamide (CH.sub.3 CSNH.sub.2) was 
employed instead of the thiourea of Example 1. 
The average platinum size of the platinum-carbon catalyst thus obtained was 
15 .ANG., and the particle size of the platinum particles was found to be 
uniform by observing the catalyst with an electron microscope. The 
specific surface area of the platinum was 155 m.sup.2 /g, and the 
supported platinum was 10% in weight. 
Example 3 
Gold was supported on the carbon support by the same procedures as those of 
Example 1 except that chlorauric acid was employed instead of the 
chloroplatinic acid of Example 1. 
The average gold size of the gold-carbon catalyst thus obtained was 17 
.ANG., and the particle size of the gold particles was found to be uniform 
by observing the catalyst with an electron microscope. The specific 
surface area of the gold was 165 m.sup.2 /g, and the supported gold was 
10% in weight. 
Example 4 
Palladium was supported on the carbon support by the same procedures as 
those of Example 1 except that 300 ml of an aqueous solution which had 
been prepared by dissolving palladium chloride containing 1.157 g of 
palladium in a small amount of hydrochloric acid which was then diluted 
with pure water was employed instead of the chloroplatinic acid of Example 
1 and that 40 ml of an aqueous solution of 0.85 g of thiourea was 
employed. 
The average palladium size of the palladium-carbon catalyst thus obtained 
was 13 .ANG., and the particle size of the palladium particles was found 
to be uniform by observing the catalyst with an electron microscope. The 
specific surface area of the palladium was 308 m.sup.2 /g according to a 
carbon monoxide adsorption method, and the supported palladium was 10% in 
weight. 
Example 5 
Platinum was supported on an alumina support by the same procedures as 
those of Example 1 except that .gamma.-alimina powder for a catalyst 
support was employed in place of the Acetylene black of Example 1 and that 
the drying after the washing was performed at 120.degree. C. 
The average platinum size of the platinum-alumina catalyst thus obtained 
was 17 .ANG.. The specific surface area of the platinum was 165 m.sup.2 /g 
according to a carbon monoxide adsorption method, and the supported 
platinum was 10% in weight.