Plasma generation of supported metal catalysts

The present invention relates to a method of producing catalytic materials which comprises passing an aerosol comprising a mixture of metal powder and support through a plasma torch.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of producing catalytic materials 
and to catalytic materials produced by this method. 
2. Description of the Prior Art 
Among the most enduring paradigms of catalysis are the methods of supported 
metal catalyst production. For decades most of these materials have been 
made via variations on the time consuming and `dirty` incipient wetness 
technique. High surface area materials are saturated with solvent 
(generally water) containing dissolved metal salts. The solvent is then 
evaporated and the salt decomposed by heating. In general the catalyst is 
then reduced. A relatively small number of supported metal catalysts are 
made by other classic techniques, in particular precipitation and ion 
exchange. Occasionally alternative approaches are explored. For example, 
following the lead of Parkyns (1) and others (2), a great deal of effort 
has gone into generating metal particles by partially/fully decomposing 
organometallic clusters. The approach has proven to be expensive and 
largely futile. The particles produced in that manner are generally found 
to be structurally and catalytically similar to those produced using the 
far less expensive incipient wetness method (3). 
In the last two decades materials processing using plasmas has dramatically 
increased. A variety of plasma processing techniques are now employed in 
the production of virtually all integrated circuits. Plasmas are also used 
to improve `materials` processing technology, for example, in the 
production of diamond films (4,5), as an alternative to flame techniques 
for the production of high quality titanium dioxide, and even to create 
polymer films with unique characteristics (6,7). Finally, plasma 
techniques have been employed with modest success to create truly novel 
materials, such as carbo-nitride films (8,9). 
Yet, there is only a single prior example of the use of plasmas to create 
novel `supported` catalytic materials (10) and this example is clearly not 
a `model` process for general supported catalyst production. There are 
also a few examples of catalytic processes accelerated by plasmas (11), 
presumably via the (homogeneous) generation of radical species (12), as 
well as examples of plasmas `activating` catalysts by removal of poisons 
or accelerating reduction (13-15). There are also examples of the use of 
plasmas to create thin support films (model catalysts) which are later 
metal impregnated using conventional `wet` chemistry (16), and `opposite 
examples` that is, systems in which plasmas spray metals onto 
conventionally prepared oxide films (17). 
Only the single use of a plasma to create supported metal catalysts is of 
direct bearing on the present invention. The method employed in earlier 
work was significantly different from that proposed herein. Specifically, 
in the earlier work, catalysts were created by D-C discharge across a 
flowing stream of hydrocarbons. The discharge `carbonized` some of the 
hydrocarbons and resulted in volatilization of metal (nickel) from the 
electrodes. 
Particle production/treatment techniques in atmospheric plasmas can be 
broken into three categories: i) particle `treatments` which do not 
involve a change in the particle chemistry, ii) particle production in 
which the final particles do not incorporate any of the gases used to 
`fire` the plasma and iii) particle production in which the plasma gas 
phase is incorporated in the final structure. Film formation using plasmas 
operating at `low` pressures (&lt;100 Torr) are not relevant to the present 
invention. The focus of this invention is particle rather than film 
fabrication. 
Reports on particle `treatments` (category (i) above) are the most common. 
Most reports involve the use of commercial plasma torches, both DC arc and 
radio frequency, running on flowing inert gases (generally argon) to which 
metal particles are fed. The metal particles are often used to create high 
density films to coat other materials, often as some type of protective 
barrier. Some commercial processes in this category are thirty years old 
(18). Generally there is no attempt in this technology to modify the 
structure of the particles. The driving concept is simply to use the high 
energy density of plasmas to `melt` the particles such that the metal can 
adopt the form of the target surface upon impact/quenching (19,20). There 
have been instances in which the technology has been employed to create 
structures somewhat different from the original feed. For example, it was 
recently demonstrated that if two torches, each with a different material, 
are run to spray the same surface simultaneously, the result is a form of 
`laminated` film (7,21). In fact, there are several examples of the use of 
particle fed torches to create films with an intimate `atomic scale` 
mixing of the two materials (19,22,23). 
The most relevant work is metal evaporation in plasmas. Repeatedly it has 
been found that metal particles are completely atomized in atmospheric 
pressure torches. One group injected pure iron and aluminum powders in the 
micron size range into a high power (32 kW) thermal arc plasma at the rate 
of about 5 gms/min. In the plasma it was presumed that the original 
particles completely evaporated and new particles on the order of 100 nm 
in size nucleated and grew in the afterglow (24). Other groups have also 
recently shown that micron sized iron particles are atomized during 
passage through a torch (25). The particles which are captured and 
examined are presumed to form by nucleation and growth of atomic species 
in the afterglow region. 
There are a variety of methods that have been employed to make category II 
particles (26). One method is to sputter a target metal with a flowing, 
but chemically inert plasma (e.g. Argon). The particles are then collected 
downstream using filters, etc. Another method is to make solid, well 
mixed, beds consisting of two materials. These mixtures are converted to 
alloys, in particulate form, in gas (no flow) thermal plasma systems. In 
particular there have been a number of reports on the generation of 
carbides in this manner (27). Yet another example of the use of plasmas to 
make particles involves injecting molecular species into a plasma. In the 
hot zone of the plasma the original molecule is destroyed, and particles 
probably form in the afterglow during cooling. In our own laboratory, we 
created iron nanoparticles by injecting an aerosol stream containing 
liquefied ferrocene into a low pressure microwave generated (argon or 
hydrogen) plasma (28). 
The greatest number of reports in which particles are created by some 
complex chemistry in the plasma zone (category III) involve the creation 
of carbides and nitrides. Research in this area is driven by the 
perception that carbide or nitride production using plasma technology has 
solid commercial potential. 
In our own laboratory (unpublished), we have succeeded in creating aluminum 
nitride particles by injecting 1 micron aluminum particles through the 
center of a nitrogen plasma generated using our torch. Other groups did 
similar work at a much earlier date. Indeed, Vissokov and Brakalov did 
nearly identical work (29). They postulated that the original aluminum 
particles were completely atomized, and that AlN particles formed during 
rapid nucleation and growth in the rapid cooling region of the afterglow. 
This analysis is consistent with the findings (both theirs and ours) that 
the final AlN particles are orders of magnitude smaller, on a volume 
basis, than the input aluminum particles. Other workers made AlN from 
aluminum particles and ammonia (30,31). 
There are numerous examples of methods to create carbides (15,27,32,33) and 
plasma methods are said to be both significantly faster and more energy 
efficient than alternative fabrication techniques. One of the more 
relevant methods involves the injection of particles into a plasma torch 
operating at atmospheric pressure which contains hydrocarbon molecules. 
According to the inventors of this technology (34) the metal particles 
(e.g., Ti, Mg, Si) completely atomize in the hot plasma region and then in 
the cooling afterglow, nucleate new particles which incorporate carbon 
atoms created during the decomposition of the hydrocarbon molecules. 
In view of the above review of prior art, there is no evidence in prior 
literature of the use of plasma torches to create traditional supported 
metal catalysts, although there is a considerable history of the use of 
plasma torches to atomize metal particles. 
Further, there is considerable interest in the use of plasma treatments to 
sinter micron scale oxide particles together to form high density solids. 
The impetus for this interest was the finding by Bennett and co-workers 
(35) that in plasmas, alumina compacts more rapidly and at significantly 
lower temperatures than it compacts when treated thermally. Since that 
time several groups have confirmed that plasma processing accelerates the 
sintering of alumina and other oxides (36-38). 
SUMMARY OF THE INVENTION 
An object of the invention is to create supported catalysts by passing 
aerosols containing metal particles (which will atomize) and traditional 
support materials through a pressure torch preferably at atmospheric 
pressure. Thus, the present invention relates to a method of producing 
catalytic materials which comprises passing an aerosol comprising a 
mixture of at least one metal powder and at least one support through a 
plasma torch. Therefore, the present invention provides a novel, and 
radically different method for the production of supported metal catalysts 
.

DETAILED DESCRIPTION OF THE INVENTION 
A plasma torch is used to create supported metal catalysts from physical 
mixtures of any type of metal or metal mixture (e.g. at least one of 
platinum (Pt), palladium (Pd), iron (Fe), vanadium (V), chromium (Cr), 
manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), 
ruthenium (Ru), rhodum (Rh), silver (Ag), osmium (Os), iridium (Ir), gold 
(Au)) and at least traditional support material (e.g. silica, alumina, 
carbon, magnesia, titania, ceria, niobia, praseolymium oxide, lanthanum 
oxide, high surface area (&gt;5 m.sup.2 /gm) carbides and nitrides). The 
metal powder is preferably present in an amount of 0.01 to 80 wt %, more 
preferably 0.1 to 2.5 wt %, and most preferably 0.5 to 1.0 wt %. The size 
of the metal powder is preferably 0.05 to 500 microns, more preferably 0.1 
to 10 microns, and most preferably 0.5 to 5 microns. The support material 
is preferably present in an amount of 20 to 99.99 wt %, more preferably 75 
to 99.9 wt %, and most preferably 99.5 to 90 wt %. The size of the support 
material is preferably 0.01 to 1000 microns, preferably 0.1 to 200 
microns, and most preferably 1 to 10 microns. The power to the torch is 
preferably 100 to 10,000 watts, more preferably 250 to 3000 watts, and 
most preferably 300 to 1500 watts. The pressure torch is preferably 
operated at atmospheric pressure, but it may be operated at pressures in 
the range of 10 Torr to 5 atm, or even 1 Torr to 30 atom. The total flow 
rate of aerosol and plasma gas is preferably 1 to 5000 standard 
liters/min. and more preferably 1 to 20 standard liters/min. The ratio of 
aerosol gas to plasma gas is preferably 1 to 1000, more preferably 1 to 
100, and most preferably 1 to 1. The average residence time of the aerosol 
in the applicator is 0.001 to 1000 sec. 
Any of the many techniques for generating aerosols can be used. Charles 
Norman Davis, "Aerosol Science," Academic Press (1966). The preferred 
methods should preferably use a dry aerosol containing less than 0.01 to 
80 volume percent solid, more preferably 0.5 to 25 volume percent solid, 
and most preferably 0.5 to 10 volume percent solids. However, "wet" 
aerosols (that is aerosols containing some liquid phase in the form of 
small droplets) and aerosols containing higher volume fraction solids may 
be employed. 
More specifically, aerosols containing particles of metal and support 
materials in appropriate (i.e. mass) ratios (ca. 1:99 metal to support) 
are injected into the center of an atmospheric pressure plasma generated 
with a commercial direct current, radio frequency or microwave torch. Any 
type of gas that can be used to generate a plasma can be used. Thus, the 
gases used to generate the plasma can be varied. Gases commonly used in 
conjunction with this invention include oxygen, hydrogen, helium and 
nitrogen, preferably argon. Other gases which may be used include 
fluorine, chlorine, neon, krypton and xenon. Gas mixtures containing at 
least two gases (e.g. Ar/H.sub.2) may be employed as well. 
Catalytic studies (selective hydrogenation of 1-butene) indicate that 
catalysts created in this novel fashion have activities similar to 
catalysts of the same composition created using conventional methods. In 
all cases the selectivity toward isomerization rather than hydrogenation 
of plasma generated catalysts was found to be superior to those of the 
commercial catalysts for isomerization. Characterization studies suggest 
that the catalysts consist of nano-scale metal particles on highly 
modified support material. 
Accordingly, as exemplified below in the Examples, catalytic materials were 
manufactured by passing an aerosol comprising a mixture of at least one 
metal powder and at least one conventional support material, carried by 
plasma gas (e.g. argon), through a microwave powered plasma torch. The 
underlying concept behind the approach is simple. It is known that some 
small (micron or less) metal particles passed through a microwave or radio 
frequency generated atmospheric pressure plasma are totally `atomized` in 
a time of the order of 0.001 second (25,29). This is due to the very high 
temperatures, now believed to be of the order of 3000 K (39,40), found in 
the center of the plasma. Thus, in a mixture of metal particles and 
support materials, it is hypothesized that metal atoms, generated during 
the atomization of the input particles, will coat the (i.e. refractory) 
support material. In the present invention, the metal powder may partially 
atomize when passing through the plasma torch and coats the support, or 
small clusters of atoms may be formed. Particles will nucleate and grow on 
the surface of the (refractory) support as the particles flow into the 
cooler regions of the afterglow. Cooling is extremely rapid in the 
afterglow (e.g. 10.sup.5 degree/second or more), thus excessive sintering 
is unlikely. 
It is understood that the point of injection of the solid aerosol(s) into 
the plasma torch is variable. In the examples described hereinbelow the 
aerosol was injected into the hottest section of plasma and then carried 
through the afterglow and finally into the coolest zone. A variation of 
the invention is to change the injection point of the aerosol. For 
example, the aerosol can be injected into the afterglow directly, thus 
bypassing the hottest zone of the plasma. Another variation of the 
invention is to divide the aerosol into two (or more) components. For 
example, one component (or more) can be injected through the hottest 
section of the torch, and a second component (or more) can be injected 
into the afterglow. 
Another variation is in the composition of the mixtures. The final 
aerosol(s) can comprise mixtures containing several supports (e.g. alumina 
and ceria) and several metals (e.g. Rh and Pt). In fact, commercial 
catalysts frequently contain several metals and several "support" oxides 
(e.g. 3-way automobile catalysts). 
In brief, catalysts were generated using a truly novel technique and then 
characterized with a number of techniques including x-ray diffraction, 
microcalorimetry, surface area measurements (BET), scanning electron 
microscopy, and chemical analysis. Also, as catalytic materials were 
manufactured by passing an aerosol, which is a mixture of metal powder and 
conventional support materials, through a plasma torch, the influence of 
applied power and to a limited extent flow rate on catalyst 
structure/chemistry was studied. The catalytic materials so created were 
tested for their catalytic behavior for selective hydrogenation of 
1-butene. 
It is interesting at this point to reflect on cost issues as one common, 
generally inappropriate, objection to plasma synthesis is its "high cost". 
On average, it was found that 200 mg of aerosol/hr can be treated, and 
thus 1 mg of palladium/hr. Thus, the hourly cost of palladium was $0.12. 
Assuming that the power supply required 1 kW to deliver 700 W of power, 
and given the cost of electricity paid by PSU is $0.10/kW hr., it cost 
only $0.10 to process 200 mg of palladium. 
The following examples are provided for a further understanding of the 
invention, however, the invention is not to be construed as being limited 
thereto. 
EXAMPLES 
Alumina and carbon supported palladium catalysts have been created in a 
unique fashion using a microwave powered atmospheric pressure argon 
plasma. All were generated from aerosols containing 0.5 weight percent 
palladium powders (average size 1 micron, 99.95% purity from Goodfellow) 
and 99.5 weight percent of a support material. The only difference in the 
starting materials was the identity of the support material. In some 
cases, either ground alumina (Grace Chemical, 15 .mu., 99.9% purity) was 
used, in the rest ground carbon (Norit C, 20 .mu.) was employed. 
In each case the aerosol was passed through a plasma torch (Astex) operated 
at between 300 and 1000 Watts, with &lt;40 W/reflected power (FIG. 1). In 
fact, two feed streams were passed through the torch. One feed stream 
consisted of pure argon (MG Industries 99.95% purity) fed at either 1.5 
slpm or 6 slpm to the `outer shell` of the 2.5 cm ID quartz torch. The 
second stream was of 1 slpm or less and contained the aerosol (approx. 200 
mg solid/hr). This stream was fed through a 3 mm ID alumina tube to the 
center point of the microwave applicator. Great efforts were employed to 
insure the resulting jet was at the center of the torch. It was estimated 
on the basis of the volumetric flow rate and estimated temperature of the 
gas in the applicator region that the average residence time of the 
particles in the `field region` of the applicator was no more than 0.1 
seconds. It is known that cooling (18,41) in the afterglows of torches of 
the type employed is very rapid (10.sup.5 .degree.K/sec), so it is 
reasonable as a first approximation to assume that melting/atomization 
occurs in the coupler region and that cooling leading to nucleation and 
growth occurs in the afterglow region. 
The aerosol particles escaping from the torch were captured either in a 
`particle trap` at the top of a `chimney` (5 cm diameter, by 20 cm height) 
placed loosely over the top of the torch, or were captured in filter paper 
(Cole Parmer, 0.2 .mu. PTFE) placed between the end of the chimney and a 
chemical pump with a capacity of 50 slpm. 
To date six techniques have been employed in analysis of the materials 
generated with the plasma torch: Chemical composition using ICP, catalytic 
activity for selective hydrogenation of 1-butene, scanning electron 
microscopy (SEM), x-ray diffraction (XRD) microcalorimetry and total 
surface area measurements (BET). Together the inventor has found using 
these techniques that the plasma torch process generates materials which 
have both similarities and differences with supported catalysts created 
with traditional techniques. 
Example I--Alumina Supports 
Ten novel catalysts were generated by passing aerosols consisting of 
alumina (99.5 wt %) and palladium particles (0.5 wt %) through an argon 
plasma generated in a microwave torch. 
Particularly exciting are results which show a clear pattern of activity 
for alumina supported catalysts. For these catalysts, activity is a 
function of applied microwave power (FIG. 2). Selectivities were very high 
in all cases (FIG. 3) (Table I). 
TABLE I 
______________________________________ 
Temperature 
1-butene Conversion 
Catalyst (.degree. C.) 
(mol/gm of Pd/min) 
Selectivity(%) 
______________________________________ 
Pd/Al.sub.2 O.sub.3 
35 0.6 68 
(300 W, 5 slm) 
Pd/Al.sub.2 O.sub.3 
35 0.6 75 
(400 W, 5 slm) 
Pd/Al.sub.2 O.sub.3 
35 0.6 67 
(450 W, 5 slm) 
Pd/Al.sub.2 O.sub.3 
35 0.6 83 
(500 W, 5 slm) 
Pd/Al.sub.2 O.sub.3 
35 0.8 66 
(550 W, 5 slm) 
Pd/Al.sub.2 O.sub.3 
35 1.1 74 
(600 W, 5 slm) 
Pd/Al.sub.2 O.sub.3 
35 6.3 89 
(650 W, 5 slm) 
Pd/Al.sub.2 O.sub.3 
35 4.9 90 
(700 W, 5 slm) 
Pd/Al.sub.2 O.sub.3 
35 6.9 90 
(800 W, 5 slm) 
Pd/Al.sub.2 O.sub.3 
35 6.7 91 
(900 W, 5 slm) 
Pd/Al.sub.2 O.sub.3, generated 
35 74.2 65 
using Incipient 
Wetness Method 
Physical Mixture 
35 0.077 64 
of Pd and Al.sub.2 O.sub.3 
Pure Al.sub.2 O.sub.3 
35 0.00011 66 
(800 W, 5 slm) *(mol/gm of 
Al.sub.2 O.sub.3 /min) 
Pd/C 35 0.4 71 
(500 W, 5 slm) 
Pd/C 35 0.3 73 
(600 W, 5 slm) 
Pd/C 35 0.7 80 
(700 W, 5 slm) 
Pd/C 35 2.4 82 
(800 W, 5 slm) 
Pd/C 35 12.9 66 
(700 W, 1.5 slm) 
Pd/C 35 21.5 79 
(800 W, 1.5 slm) 
Pd/C 35 17.1 81 
(900 W, 1.5 slm) 
Pd/C 35 14.4 82 
(1000 W, 1.5 slm) 
Pd/C, generated 
35 0.5 58 
using Incipient 
Wetness Method 
______________________________________ 
This can be explained qualitatively with a simple model. At low power the 
palladium is not fully atomized and hence high dispersion is not achieved. 
At high power the alumina "melts" and forms spherical pellets (FIG. 4) with 
virtually no surface area (Table II). 
TABLE II 
______________________________________ 
Surface Area of Pd/Al.sub.2 O.sub.3 
Power (W) Surface Area (m.sup.2 /gm) 
______________________________________ 
300 85 
400 78 
450 79 
500 64 
550 69 
600 50 
650 49 
700 26 
800 16 
Original Pd/Al.sub.2 O.sub.3 
86 
______________________________________ 
Over a relatively narrow range of plasma operating power does the palladium 
atomize, and the alumina maintain a high surface area. Over this range of 
power both high dispersions and high activities are achieved. 
Both SEM and x-ray studies support the supposition that at operating powers 
greater than about 400 watts the alumina melts and recrystallizes. The SEM 
photos (FIG. 4a, b and c) show that the particles become spherical in 
shape and that the average particle diameter is reduced by a factor 
slightly greater than two (FIG. 4a, b and c) after passage through plasmas 
operating at 600 watts or greater. It is expected highly porous 
(.about.75% voids) .gamma.-alumina would form spheres and become denser 
following melting. XRD results (FIG. 5a, b and c) showing that the plasma 
treatment at high power changes the alumina to mixtures of corundum 
(.alpha.-alumina) and .delta.-alumina also are expected for particles 
melted in the plasma zone and then recrystallized in the cooler afterglow. 
Moreover, the particles are clearly better crystallized (sharp lines) an 
the original .gamma.-alumina. 
Another issue is metal loss. Specifically metal loss by diffusion and 
thermophoresis can be eliminated by maintaining a high ratio of plasma gas 
to aerosol gas. If this ratio is allowed to decrease, a significant 
fraction of the input metal is lost. However, under the conditions 
employed in these Examples, metal loss was found to be virtually zero. 
Changes in selectivity are potentially valuable. Often times selectivity, 
not activity, determines the value of a catalytic material. The current 
example, 1-butene hydrogenation/isomerization, is instructive in this 
regard. Commercially, C.sub.4 streams in refineries are often `selectively 
hydrogenated` in order to convert 1-butene and butadiene into 2-butene. 
This improves the lifetime of the catalytic acids used in subsequent 
alkylation, as well as enhancing the octane of the alkylate. The selective 
hydrogenation is generally performed at high pressures in order to liquefy 
the C.sub.4 stream and keep the hydrogen content in the active phase 
(liquid) low. Only at low hydrogen concentrations is isomerization rather 
than hydrogenation (butane) favored. Ideally, one would prefer catalysts 
which are selective even at higher hydrogen concentrations so that 
liquefaction is not required. Thus, it is notable (FIG. 3) that in all 
cases the isomerization selectivity was distinctly higher than that of the 
commercial catalysts. The improved isomerization selectivity at high 
hydrogen concentration of the novel plasma catalysts, relative to the 
commercial catalysts, is potentially of significant value. 
From x-ray diffraction there is evidence that the plasma treatment resulted 
in the presence of additional oxide phases in the case of alumina. For 
example, only .gamma.-alumina was initially present, but after treatment 
it is evident that several phases including .delta. and corundum are 
present as well. Moreover, it is evident that the size of both the alumina 
and silica particles is significantly smaller after the plasma treatment. 
As shown in FIGS. 9a-c and 10 the size and shape of alumina particles 
before and after passage through the torch is significantly different. 
Indeed, prior to passage through the torch the particles are irregular in 
shape, have a bimodal distribution with an average particle size (by mass) 
of about 50 microns, whereas, after passage through the torch there is a 
monomodal distribution of spherical particles, and the average size is 
about 6 microns. 
Example 2--Carbon Supported Catalysts 
Eight novel supported Pd catalysts were generated on a high surface area 
activated carbon using the plasma torch method as described above. 
An exciting result was the finding that on carbon supports catalysts with 
extremely high activities higher than on alumina could be produced. The 
data of Tables I and FIG. 7 show carbon supports that where power and 
larger residence time increase activity of the product catalyst. This 
probably results since (i) carbon surface area is not reduced by the 
plasma treatment, (ii) plasma treatment creates "active sites", allowing 
stronger metal-carbon bonds to form in the afterglow and (iii) longer 
residence times leads to greater atomization of the input metal. 
All cited patents and publications referred to in this application are 
herein incorporated by reference. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the present invention, and all such 
modifications as would be obvious to one skilled in the art are intended 
to be included within the scope of the following claims. 
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