Liquid suspension of particles of a metal belonging to the platinum group and a method for the manufacture of such a suspension

A stable suspension of particles of a metal belonging to the platinum group consists of a microemulsion in which the particles are suspended. The suspension is prepared by dissolving a metal salt in a microemulsion, and reducing the salt to form a metal.

TECHNICAL FIELD 
The invention relates to a liquid suspension of particles of a metal 
belonging to the platinum group, and a method for the manufacture of such 
a suspension. The suspension of the invention is useful, for example, for 
manufacturing catalysts by depositing the metal particles on a carrier. 
The platinum group comprises ruthenium, rhodium, palladium, osmium, 
iridium, and platinum. 
BACKGROUND ART 
It is well known to prepare a suspension of platinum particles by reducing 
a platinum salt in an aqueous solution with hydrogen, for example. The 
platinum particles thus produced usually have an average size of at least 
10 nm. The suspension, however, contains particles of various sizes, which 
is due to the fact that small particles have a tendency to unite to form 
bigger particles during the reduction. This is a disadvantage if the 
particles are to be used for catalytic purposes, because in this case 
small particles of approximately the same size are desired. It is another 
inconvenience that the suspension referred to is not quite stable. The 
particles have a tendency to settle. 
DISCLOSURE OF INVENTION 
The invention aims at eliminating said inconveniences. The suspension of 
the invention is characterized in that the liquid in which the metal 
particles are suspended consists of a microemulsion. The invention also 
relates to a method for manufacturing the suspension. The method is 
characterized by reducing a metal salt dissolved in a microemulsion. 
Microemulsions have been known for about three decades. They consist of 
transparent, thermodynamically stable solutions of water and hydrocarbons, 
stabilized by a surfactant and, if desired, a second substance having a 
carbon chain of medium length and a polar end group, for example an 
aliphatic alcohol or a fatty acid. When used in the present invention the 
surfactant should be a non-ionic compound soluble in the hydrocarbon. The 
non-ionic compound shall not react with the reducing agent, meaning that 
it shall not contain double bonds or other characteristic groups which can 
be reduced (hydrated). The non-ionic compound shall not, for example, have 
aromatic groups in the hydrocarbon chain. This demand is met by non-ionic 
surfactants having been synthesized by reacting ethyleneoxide oligomers 
with high aliphatic alcohols so that they are bonded to the hydrocarbon 
chain with an ether bond, for example pentaethyleneglycol dodecylether and 
its homologs. The non-ionic compounds shall be insoluble in the solvents 
present in the microemulsion. These demands eliminate several classes of 
non-ionic surfactants, for example those having been formed by an 
esterification process, because they are reduced by, or form a 
precipitation with, hydrazine. We prefer to use polyethyleneglycol 
alkylethers, preferably containing 12-14 carbon atoms in the carbon chain, 
and 2-8 ethyleneoxide units in the polyethyleneglycol chain, for example 
tetraethyleneglycol dodecylether. The hydrocarbon shall not react 
chemically with the other components of the solution, or with the reducing 
agents, which are very reactive. This demand eliminates, for example, 
aromatic hydrocarbons which react with the reducing agents. The boiling 
temperature of the hydrocarbon shall not be close to the temperature at 
which the metal particles are precipitated, because this would result in a 
heavy evaporation of the hydrocarbon when gaseous reaction products leave 
the microemulsion. The following types of hydrocarbons are useful, fiz: 
Aliphatic non-cyclic hydrocarbons, for example the homologous series 
hexane-hexadecane, including branched isomers; aliphatic cyclic 
hydrocarbons, for example cyclohexane, methyl-cyclohexane, 
dimethyl-cyclohexanes and other cyclohexane derivatives, 
decahydronaphtalene. If the particles are to be deposited on a carrier it 
is preferred to use readily volatile alkanes (6-10 carbon atoms). 
The metal salt shall be soluble in the microemulsion. The salt can be 
dissolved in a small quantity of water, and the aqueous solution thus 
produced may be mixed into a mixture of the surfactant and the 
hydrocarbon. Alternatively, the metal salt, which usually contains some 
crystal water, may be dissolved in the surfactant, and the solution may be 
mixed with the hydrocarbon, and additional water may finally be added, if 
desired. The metal compound may be any simple salt, or the corresponding 
acid, which is soluble in alcohols and/or water. When preparing a platinum 
suspension we prefer to use chloroplatinic acid, H.sub.2 
PtCl.sub.6.xH.sub.2 O, in which x is 6 at most. Other useful platinum 
compounds are sodiumhexachloroplatinate, Na.sub.2 PtCl.sub.6.6H.sub.2 O., 
sodiumhexabromoplatinate, Na.sub.2 PtBr.sub.6.6H.sub.2 O, and 
hexabromoplatinic acid, H.sub.2 PtBr.sub.6.6H.sub.2 O. Useful salts for 
the manufacture of suspensions of the other platinum group metals are, for 
example, palladium chloride, PdCl.sub.2, sodiumhexachloropalladinate, 
Na.sub.2 PdCl.sub.6, sodiumtetrachloropalladinate, Na.sub.2 
PdCl.sub.4.3H.sub.2 O, ruthenium chloride, RuCl.sub.3.xH.sub.2 O, irridium 
chloride, IrCl.sub.3 .xH.sub.2 O, hexachloroiridic acid, H.sub.2 
IrCl.sub.6.xH.sub.2 O, osmium tetraoxide, OsO.sub.4, rhodium chloride, 
RhCl.sub.3.H.sub.2 O.

DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE DRAWINGS 
In the ternary diagram disclosed on FIG. 1 the upper corner of the triangle 
represents 100 percent by weight n-hexane, the lower right-hand corner 
represents 20 percent by weight pentaethyleneglycol dodecylether (PEGDE), 
and the lower left-hand corner represents 20 percent by weight water. The 
area L.sub.2 represents a microemulsion, i.e. an isotropic, clear 
solution. The diagram is true for a temperature of 23.degree. C. The area 
L.sub.2 will have other shapes at other temperatures. A high percentage of 
surfactant makes the suspension difficult to handle, for example when 
using the metal particles as a catalyst. Therefore, we prefer to use only 
the upper, shaded portion of the area L.sub.2, viz. the area in which the 
percentage of hydrocarbon in the microemulsion is above approximately 80 
percent by weight and the percentage of PEGDE is below approximately 20 
percent by weight. If an aqueous solution of chloroplatinic acid is added 
to the microemulsion of FIG. 1, for making a suspension according to the 
invention, the result will be a microemulsion represented by the diagram 
of FIG. 2. In the diagram of FIG. 2 the upper corner of the triangle 
represents 100 percent by weight n-hexane, the right-hand bottom corner 
represents 100 percent by weight pentaethyleneglycol dodecylether (PEGDE), 
and the left-hand bottom corner represents 100 percent by weight 
chloroplathinic acid plus crystal water. The diagram is true for a 
temperature of 23.degree. C. The preferred portion of the area L.sub.2 is 
shaded, and is disclosed on a larger scale by FIG. 3. A practical 
application of the diagram of FIG. 3 is illustrated by Example 1. The 
diagram of FIGS. 2 and 3 discloses that the smallest required percentage 
of PEGDE is approximately 5 percent by weight, and the smallest percentage 
of water+chloroplatinic acid that can be dissolved in the microemulsion is 
approximately 5 percent by weight. The finished suspension will have a 
maximum of stability if the weight ratio PEGDE to chloroplatinic acid is 
higher than 20. Ternary diagrams of the type illustrated on the drawing 
can be produced for each combination of hydrocarbon, surfactant, water, 
and metal salt. How to determine the boundary of the area L.sub.2 has been 
disclosed by S. Friberg and I. Lapczynska: Progr. Colloid & Polymer Sci. 
56, 16-20 (1975). Said article also gives some literature references on 
microemulsions. 
The reducing agent should not produce any by-products which destroy the 
microemulsion or change the area in which it can exist. Therefore, 
substances producing insoluble solid or liquid reaction products, in 
addition to metal particles, are not suitable as reducing agents. We 
prefer to use hydrogen and hydrazine, NH.sub.2.NH.sub.2. Other useful 
reducing agents are aldehydes, for example formaldehyde HCHO. 
If the metal salt is chloroplatinic acid, hydrogen chloride will be formed 
as a by-product. Consequently, the pH of the microemulsion will sink. Too 
low a pH will reduce the stability of the finished suspension. Therefore, 
we prefer to increase, before the reduction, the pH of the microemulsion 
to 9-10 by adding an aqueous solution of sodium hydroxide. This will 
result in a pH of at least 4 and preferably from 4-6 after the reduction, 
and the stability is not impaired. The reduction can be brought about in a 
simple way by adding the microemulsion, containing a dissolved metal salt, 
to a glass flask, adding the reducing agent in liquid or gaseous form to 
said flask, and shaking the flask for a few minutes at room temperature. 
By shaking vigorously the reduction time may be brought down to 
approximately 1/2 minute, which is favorable for the stability of the 
finished suspension. Without committing ourselves to any theory on the 
formation and growth of the metal crystals during the reduction, we 
believe that each "droplet" of water in the microemulsion acts as a 
crystallization nucleus during the reduction. Each "droplet" contains a 
plurality of molecules of the metal salt. The number of crystallization 
nuclei will be comparatively high, and the nuclei are separated from each 
other by a medium in which the reduced compound is insoluble. This is 
likely to mean that the formation of crystals can start simultaneously in 
each crystallization nucleus, and that the crystal growth can continue on 
each crystal as long as the microemulsion contains any unreduced metal 
salt. Consequently, the finished metal particles, each consisting of a 
single crystal, are likely to have approximately the same size, which is 
consistent with our experience. In fact, it is a characteristic feature of 
the suspension of the invention that the particle size lies within a 
narrow range. Usually a particle diameter is obtained having a standard 
deviation from the average diameter of less than .+-.10%. For example, 
when manufacturing a suspension of platinum particles in which 70% of the 
total number of particles had a size very close to 2.5 nm we found that 
only 12% of the particles had a size between 2.2 and 2.4 nm, and that only 
18% of the particles had a size between 2.5 and 2.7 nm. The particle 
diameter was determined by means of electron microscopy. The particles 
were transferred to the diffraction grating by applying a thin layer of 
the suspension on the diffraction grating, and drying said thin layer. 
Smaller or bigger particles had not been formed at all. The electron 
microscopy disclosed tht the particles are crystalline. In general, it is 
preferred that the metal particles have a size of from 2 to 5 nm. 
The suspension of the invention can be used for making a catalyst, either 
with the metal particles still being suspended in the solution, or by 
depositing the metal particles on a solid carrier, for example pumice or 
pulverulent Al.sub.2 O.sub.3. The following depositing process is 
preferred, to prevent the metal particles from agglomerating to form 
bigger aggregates. The solid carrier is moistened with the suspension, and 
the liquid of the microemulsion is evaporated, preferably by exposing the 
moistened carrier to a reduced pressure. This process is repeated until 
the carrier possesses the desired density of the metal particles. The 
surfactant is now removed by rinsing the carrier repeatedly with ethanol. 
The carrier is finally dried at a reduced pressure. 
EXAMPLE 1 
This is an example of the manufacture of a suspension of platinum 
particles. The commercial surfactant Berol 050, a polyethyleneglycol 
alkylether, was freed of impurities by being distilled in vacuum. 0.52 
grams of the purified surfactant was dissolved in 9.45 grams n-hexane. 
H.sub.2 PtCl.sub.6.xH.sub.2 O was added in a quantity equivalent with 
approximately 4.times.10.sup.-4 grams metallic platinum per gram of 
solution. This means that the solution will contain approximately 0.3 
grams water. Sodium hydroxide was added as a solution containing 1 mol per 
dm.sup.3, in a quantity equivalent with the hydrochloric acid to be formed 
during the reduction. The platinum salt was now reduced by addition of 
hydrazine in excess while stirring. It is necessary to add the hydrazine 
in excess, because the compound is decomposed by solid platinum. The 
reduction goes fast at room temperature, and produces a stable suspension 
of platinum particles having a diameter of approximately 2.5.+-.0.2 nm. 
The pH of the finished suspension is 5-6. The quantity of added hydrazine 
can be controlled by means of the fact that said final pH shall be 
attained. 
EXAMPLE 2 
This is an example of the manufacture of a suspension of palladium 
particles. 0.96 gram Berol 05 was purified by distillation in vacuum, and 
was dissolved in 8.6 grams n-hexane. 0.31 gram aqueous solution was added, 
containing 5 percent by weight PdCl.sub.2. The palladium solution had been 
given a pH of 2-3 by the addition of 1 M hydrochloric acid. The quantity 
of palladium salt referred to corresponds to 9.4.times.10.sup.-4 gram 
palladium per gram solution. Sodium hydroxide having a concentration of 1 
mol per dm.sup.3 was added in a quantity adequate for neutralizing the 
hydrochloric acid expected to be formed during the reduction. 
The palladium salt was now reduced by the addition of hydrazine. The 
hydrazine was added in an excess, so as to compensate for the hydrazine 
being decomposed by the formed solid palladium. The reduction runs fast at 
room temperature, and creates palladium particles having a diameter of 
approximately 5.0 nm. The final pH of the suspension was 5-6. 
EXAMPLE 3 
This is an example of the manufacture of a suspension of rhodium particles. 
1.95 grams Berol 05 was purified by distillation in vacuum, and was mixed 
with 7.9 grams n-hexadecane. RhCl.sub.3.xH.sub.2 O was added in a quantity 
to produce a solution containing approximately 1.15.times.10.sup.-3 gram 
Rh per gram solution. Sodium carbonate in a quantity equivalent to the 
hydrochloric acid expected to be formed during the reduction was dissolved 
in 0.15 gram water, and the solution was added to the mixture. 
The rhodium salt was now reduced by hydrogen which was made to pass through 
the solution. The reduction was completed after approximately 2.5 hours at 
a temperature of 24.degree. C. The rhodium particles of the finished 
suspension had a diameter of approximately 3.0 nm. The suspension had a pH 
of 5-6. 
EXAMPLE 4 
This is an example of the manufacture of a suspension of platinum 
particles, in which the microemulsion is based upon a cyclic aliphatic 
hydrocarbon. 
A mixture was prepared consisting of 8.8 grams cyclohexane and 1.0 gram 
Berol 050. The Berol had been purified by vacuum distillation. To this 
mixture was added H.sub.2 PtCl.sub.6.xH.sub.2 O dissolved in 2.0 grams 
water. The quantity of the platinum salt was chosen to correspond to 
2.2.times.10.sup.-4 gram metallic platinum per gram solution. 
Sodium hydroxide was added in a quantity adequate to neutralize the 
hydrochloric acid to be formed. The sodium hydroxide was added as a 
solution containing 1 mol per dm.sup.3. 
The platinum salt was now reduced by the addition of an excess of hydrazine 
while stirring. It was necessary to add the hydrazine in an excess because 
it is decomposed by the formed solid platinum. The reaction runs fast at 
room temperature, and results in a stable suspension of platinum particles 
having a size of approximately 2.5.+-.0.2 nm. The pH of the suspension is 
5-6, and the correct quantity of added hydrazine can be controlled by said 
pH being attained.