Catalyst composition

A catalyst is disclosed wherein a particulate catalyst support has cosputtered on its surface a mixture of a catalytically active metal such as Pt, Pd, Ag, Au, Re, Rh, Ru or Ir and a cosputtered support material which preferably is the same material as the catalyst support.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to catalysts comprised of a particulate substrate 
with a composite sputtered thin film on the surface thereof the film 
comprising one or more metals having catalytic activity and a 
cosputterable support material. 
Ideally a catalyst is a substance that directly accelerates a chemical 
reaction without itself undergoing either a physical or chemical change. 
Because of their unique catalytic properties and inherent stability, the 
metals: Pt, Pd, Rh, Ag, etc., or their alloys, are frequently used as 
catalysts. Usually it is too expensive to use these metals in bulk form, 
so that a small amount of metal is supported on a substrate such as 
SiO.sub.2 or Al.sub.2 O.sub.3. Another motivation for supporting the metal 
is to disperse it, so that for a given amount of catalytic material, the 
number of available catalytic sites for the chemical reaction to take 
place is increased. 
2. Prior Art 
Commonly used techniques to deposit metal catalysts on a support are 
impregnation and ion exchange. To obtain high dispersions, the support is 
typically porous and has a high surface area. In many instances, the 
support is not inert under reaction conditions and deleterious reactions 
can occur on its surface, causing loss in catalyst selectivity. Although 
high dispersions of the catalytically active phase can be obtained during 
catalyst preparation, it is often difficult to preserve this disposition 
under conditions in which the catalyst is used. In cases where the support 
surface causes loss in catalyst selectivity and where the support cannot 
help maintain the catalyst surface area under reactor conditions, a bulk 
metal catalyst can be used. The synthesis of formaldehyde by oxidative 
dehydrogenation of methanol is an example of a process which occurs at 
high temperature (.about.650.degree. C.) and uses a bulk Ag catalyst. 
An ideal form for a catalyst would therefore be a highly dispersed metal on 
a low surface area, stable support. In G.B. No. 1,455,248 and U.S. Pat. 
No. 4,046,712 J. A. Cairns et al. disclose an attempt to make such a 
catalyst comprising a hard, substantially nonporous particulate substrate 
having a surface area not greater than about 20 sq. meters per gram and a 
sputtered deposit of catalytic material comprising one or more of 
platinum, rhodium, palladium, ruthenium, osmium and iridium. In G.B. No. 
1,537,839, J. A. Cairns et al. disclose that it is sometimes beneficial to 
provide a partial covering of the support material on each sputter-coated 
support-particle. Preferably the partial covering is in the range of 5-20% 
of the available surface area. This partial covering may be carried out by 
contacting the sputter-coated support particles with a dispersion of 
powdered support material and subsequently firing to adhere the powder to 
support-particles. 
S. K. Sharma and J. Spitz, Thin Solid Films 61, L13-15 (1979) have found 
that sputtered silver films exhibit hillock growth and agglomeration 
similar to that shown by silver and other metal evaporated thin films, 
[see, e.g., A. E. Presland et al., Prog. Surf. Sci. 3, 63 (1973)]. For 
example, quartz substrates covered with a 500 .ANG. sputtered film of Ag 
have 60% of the surface area free of Ag after 4 hours of annealing in 
oxygen at 372.degree. C. It is believed that hillock growth occurs as a 
result of the relaxation of the thermal stress developed during heating 
because of the different thermal expansion coefficient of the metal film 
and the substrate and that agglomeration results from surface diffusion 
with the kinetics determined by surface energy forces. 
Sputtered composites of metals and insulators, known as cermets, have been 
used as electrical resistors for some time. [See e.g., the review article 
by B. Abeles et al., Adv. Phys. 24, 407 (1975) which also gives typical 
procedures used for cosputtering composites.] 
SUMMARY OF THE INVENTION 
Catalysts of the present invention are formed of low surface area 
particulate substrates and a sputtered composite deposit comprising one or 
more catalytically active metals such as Pt, Pd, Ag, Au, Re, Rh, Ru and Ir 
and a sputterable support material, said metal being in concentrations of 
0.01-5.0 weight percent based on the total weight of the substrate and 
sputtered deposit, i.e., total catalyst, and the relative amount of metal 
and sputterable support material in the sputtered deposit such that the 
metal comprises 3-80% of the volume of the sputtered deposit. The 
invention finds its greatest advantage with the catalytically active 
metals listed above. Other catalytically active metals such as Ni, Co, Mo, 
W and Fe are suitable for use in the present invention. Refractory oxides, 
nitrides and carbides with surface area less than 10 m.sup.2 /g are 
preferred as the particulate substrate material. 
Surprisingly, the sputtered composite catalyst of this invention has good 
catalytic activity and selectivity, despite the fact that it uses only a 
fraction of the metallic content of a bulk metal catalyst. 
The cosputtered support component of the sputtered deposit is preferably an 
oxide, nitride or carbide and preferably the same material as the 
particulate substrate. Especially preferred as the cosputtered support 
component and the substrate material are Al.sub.2 O.sub.3 and SiO.sub.2. 
The superior performance of this catalyst over sputtered catalysts known in 
the art is believed to arise from the presence of the cosputtered support 
component in the sputtered catalyst deposit. The cosputtered support 
component acts both to disperse the catalytic metal so as to increase the 
number of catalytic sites for a given amount of metal and to bind the 
metal to the substrate thereby allowing metal concentrations capable of 
sustaining industrial reaction rates without significant loss of metal 
dispersion at operating temperatures. 
DETAILED DESCRIPTION 
The catalyst of this invention comprises a low surface area particulate 
support or substrate and a thin film composite deposit which is sputtered 
onto the substrate. 
The catalyst substrate materials have low surface area and are stable under 
the reaction use conditions. Refractory materials, e.g., oxides, nitrides 
and carbides such as SiO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2 , MgO, SiC and 
Si.sub.3 N.sub.4 are preferred. Glasses that satisfy the stability 
requirement can also be used. Especially preferred are Al.sub.2 O.sub.3 
and SiO.sub.2. Surface areas for the particulate support are preferably 
less than 10 m.sup.2 /g. Particulate size is not critical but, on the 
basis of ease of preparation, the preferred particle size is greater than 
100 .mu.m as will be discussed below. 
The sputtered thin film composite is formed of one or more catalytically 
active metals such as Pt, Pd, Ag, Au, Re, Rh, Ru and Ir and a cosputtered 
support material. The cosputtered support material is preferably an oxide, 
nitride or carbide and is preferably the same material as the particulate 
substrate material to provide better binding and adhesion to the 
substrate. Especially preferred as the cosputterable support material and 
substrate materials are the oxides Al.sub.2 O.sub.3 and SiO.sub.2. 
The composite sputtered coatings of this invention can be prepared by 
sputtering from a single composite target of the metal or metals and the 
cosputterable support material or by sputtering simultaneously or 
sequentially from separate targets of the metals or metal alloy and the 
cosputterable support material. To illustrate, if the sputtered composite 
is comprised of two metals and a cosputterable support material, one, two 
or three targets can be used. A single component target would be formed of 
all three materials; two targets would be formed of one of the metal alloy 
and one of the cosputterable support material or one of metal 
cosputterable support material and one of the other metal. Three targets 
would consist of a separate target for each metal and the cosputterable 
support material. 
It is highly desirable to have a uniform mixture of the components within 
the composite deposit. This is more easily achieved when all components 
issue from a single target. In this case, the particulate substrates can 
be mixed by vibrating or tumbling them during sputtering for example by 
vibrating the particles in a dish attached to a piezoelectric driver or 
audio speaker, or by manually agitating them by means of a manipulator arm 
attached to the dish through a vacuum feedthrough, or the sputtering can 
be interrupted and the particles mixed outside the sputtering chamber. In 
the case of simultaneous/sequential sputtering from multiple targets, 
sputtering can be interrupted several times and the particles removed from 
the chamber and mixed. With multiple targets, it is also necessary to 
rotate the table on which the substrate particles rest during the 
sputtering, again to provide a good mixture of the sputtered components. A 
table rotation speed of several revolutions per minute is sufficiently 
fast for good mixing of the composite components. In the case of 
Ag/SiO.sub.2 composites prepared with such rotations, X-ray diffraction 
and transmission electron microscopy confirm that films consist of Ag 
particles less than 100 .ANG. diameter dispersed in an amorphous SiO.sub.2 
matrix. The preferred substrate particle size is greater than 100 microns. 
Below this size, mixing and efficient coating of particles becomes 
difficult. The upper size limit is only restricted by its utility as a 
catalyst support in a given apparatus and process. 
Specifically RF sputtering has been used to produce the coatings because it 
is particularly suited to the preferred cosputterable support material 
targets. Initially the sputtering chamber is evacuated to between 
10.sup.-6 and 10.sup.-7 Torr by a conventional vacuum pumping station. A 
small amount of argon gas is let into the chamber and ionized by a 13.56 
MHz ionizing RF field applied between the target materials to be sputtered 
and the substrate table. Positively charged argon ions are then 
accelerated toward the targets, which are at negative potential. The argon 
ions dislodge atomic species of the targets which deposit on the 
substrates below. The argon pressure was usually in the range 
5.times.10.sup.-3 Torr to 20.times.10.sup.-3 Torr, but other pressures 
would be workable. Further, other inert gases such as neon, krypton or 
xenon can be used, but argon is preferred because it is less expensive, 
readily available and gives high sputtering yields with most materials. 
This preparation is not limited to RF sputtering but would also be 
expected to be accomplished by other sputtering methods, e.g., ion beam 
sputtering, which uses a separate source of accelerated ions, or by either 
RF or DC magnetron (magnetic-field enhanced) sputtering. 
An advantage of sputtering from multiple targets is that the composition of 
the cosputtered composite catalyst can be varied by changing the relative 
target voltages. For example, in the preparation of Ag/SiO.sub.2 on fused 
silica particles, the concentration of Ag was controlled at 0.038%, 
0.175%, 0.63%, 1.22% and 1.87% weight percent (based on the total weight 
of the substrate and the sputtered composite) with sputtering experiments 
of equal duration by adjusting the voltage on the Ag target at 100 V, 250 
V, 500 V, 750 V and 1000 V respectively, relative to about 1500 V on the 
SiO.sub.2 target. Over this voltage range the volume concentration of Ag 
in the sputtered coating varies from 4% (100 V) to about 70% (1000 V), the 
balance being SiO.sub.2. 
In the sputtering of multimetal catalysts, from separate metal and nonmetal 
targets, e.g., Ag/Au/SiO.sub.2 on SiO.sub.2 particles, the ratio of the 
metals can also be controlled by the target voltage. Ag/Au/SiO.sub.2 films 
were prepared with Ag/Au atomic ratios of 95/5, 88/12, 75/25, 65/35 and 
60/40. These ratios depend on sputtering voltages in a predictable way 
(see, e.g., Handbook of Thin Film Technology, Maissel and Gland) according 
to 
##EQU1## 
where R is the sputtering rate and V is the sputtering voltage. The 
relative concentration of total metal to nonmetal is also controlled by 
adjusting the sum of the metal target voltages (V.sub.Ag +V.sub.Au) 
relative to the cosputterable noncataly material target (SiO.sub.2) 
voltage. 
Ag/Au/SiO.sub.2 composite thin films were also prepared by sputtering from 
a Ag/Au alloy target and a SiO.sub.2 target. Catalysts were prepared by 
coating fused silica particles from an alloy target with atomic ratio 60 
Ag/40 Au and a SiO.sub.2 target. For alloy target voltages of 500 V, 750 V 
and 1000 V relative to about 1500 V on SiO.sub.2 the metal concentrations 
varied from 0.46-1.15-1.79 weight percent for sputtering experiments of 
equal duration. The Ag/Au atomic ratio in the films in all cases was 60 
Ag/40 Au, as confirmed by atomic absorption analysis, the same as in the 
target. 
Ag/SiO.sub.2 or Ag/Au/SiO.sub.2 coatings can also be prepared from a single 
composite target. For example, an Ag/SiO.sub.2 target was fabricated by 
mixing finely-divided Ag and SiO.sub.2 powders with a weight ratio of 
88/12, respectively in a ball-mill. The mixed powders were pressed at 
10,000 lbs into a 2 inch diameter disk and then fired for 16 hours in air 
at 640.degree. C. This produced a dense disk when used as a target from 
which to sputter composite catalysts. X-ray diffraction of these thin 
films showed finely-divided Ag of about 55 .ANG. diameter dispersed in an 
amorphous silica matrix. These results are similar to those obtained by 
sputtering from separate Ag and SiO.sub.2 targets onto substrates that are 
rotated. 
The surface area of the sputtered composite coatings increases with 
increasing coating thickness in contrast to the surface area of smooth 
evaporated or sputtered metal films which is independent of film 
thickness. This is illustrated in Table I for Ag/SiO.sub.2 composite 
coatings on fused silica particles prepared by sputtering from separate Ag 
and SiO.sub.2 targets onto fused silica particles on a rotating substrate 
table. 
TABLE I 
______________________________________ 
Voltage on 
Silver 
Target Sputtering 
Surface Area 
V.sub.Ag (volts) 
Time (hrs) 
(m.sup.2 /g) 
______________________________________ 
500 3 0.112 
500 6 0.163 
750 3 0.124 
750 6 0.245 
1000 3 0.149 
1000 6 0.313 
______________________________________ 
Since the fused silica particles themselves have negligibly small surface 
area (&lt;0.03 m.sup.2 /g), the surface area of the coated particles can be 
attributed to the coating alone. From measurements of the actual metal 
loading, the surface area of the metal alone in the samples of Table I is 
estimated to be about 20 m.sup.2 /g. 
The performance of the catalysts of this invention has been demonstrated 
for a number of specific processes. 
Catalysts comprised of Ag/SiO.sub.2 sputtered on low surface area fused 
silica particles are more active for formaldehyde synthesis from methanol 
by oxidative dehydrogenation than Ag alone sputtered on the same particle 
substrates and give yields at least equivalent to bulk Ag at high 
conversion of methanol and 1-2% better yields at lower conversions with 
only 1/100 as much Ag. High activity, selectivity and stability at 
commercially useful flow rates also obtain for Ag/MgO and Ag/Au/SiO.sub.2 
films on fused silica particles. 
Catalysts of Pd/SiO.sub.2 films sputtered on low surface area, particulate 
fused silica show high stability as hydrogenation catalysts in peroxide 
synthesis.

EXAMPLES OF THE INVENTION 
EXMPLES 1-4 
Catalysts comprised of Ag/SiO.sub.2 composites sputtered onto fused silica 
were prepared by RF sputtering simultaneously from separate targets of Ag 
and SiO.sub.2. In each example, fused silica particles (average dimension 
about 2 mm), were distributed in several pyrex dishes on a rotating 
substrate table beneath the targets and coated with the sputtered deposit. 
Approximately 15 g of fused silica was used in each of Examples 1-3 and 
approximately 14 g was used in Example 4. The sputtering voltages used for 
each target are reported in Table II. 
TABLE II 
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Voltage on Voltage on 
Example Silver Target 
Silica Target 
No. V.sub.Ag (volts) 
V.sub.SiO.sbsb.2 (volts) 
______________________________________ 
1 250 1550 
2 500 1500 
3 750 1400 
4 1000 1400 
______________________________________ 
In each example, the argon pressure was maintained at 8.times.10.sup.-3 
Torr and the particulate substrates were rotated at 5 rev/min. The 
duration of the sputtering was 6 hours with periodic mixing of the 
particles outside the sputtering chamber. 
These catalysts were then used in the synthesis of formaldehyde from 
methanol. A description of the process and the experimental techniques are 
described in U.S. Pat. No. 4,219,509 (Rao and Nielsen). For a given 
conversion of methanol to formaldehyde, the yield using the composite 
catalysts is greater than for bulk silver at low conversions, and at least 
equivalent to it at higher conversion, even though the concentrations of 
Ag are only 1/100 that of the bulk Ag catalyst. By "methanol conversion" 
is meant moles of methanol converted to other products per mole of 
methanol fed. By "selectivity to formaldehyde" is meant moles of 
formaldehyde formed per mole of methanol converted. By weight percent 
metal (e.g., weight percent Ag) is meant the weight of metal divided by 
the total weight of the substrate and the sputtered deposit times 100. The 
results are reported in Table III, as are the results for bulk silver. 
EXAMPLE 5 
A catalyst comprised of a Ag/MgO composite sputtered onto fused silica was 
prepared by RF sputtering from separate targets of Ag and MgO. 
Approximately 17 g of fused silica particles (average dimension about 2 
mm), distributed in several pyrex dishes on a rotating substrate table 
beneath the targets, were coated. The sputtering voltages were 500 volts 
on the Ag target and 1200 volts on the MgO. The argon pressure was 
maintained at 3.times.10.sup.-3 Torr and the particulate substrates were 
rotated at 5 rev/min. The duration of the sputtering was 6 hours with 
periodic mixing of the particles outside the sputtering chamber. 
This catalyst was also used in the synthesis of formaldehyde and the 
results are reported in Table III. 
EXAMPLE 6 
A Ag/SiO.sub.2 composite catalyst was prepared by RF sputtering from a 
solid composite target of Ag and SiO.sub.2 homogeneously mixed and with a 
Ag/SiO.sub.2 weight ratio of 88/12. Approximately 5 g of fused silica 
particles (average dimension about 2 mm), in a single pyrex dish, were 
coated. The particles were mixed during sputtering by mechanical agitation 
of the substrate dish attached to an arm that exited the vacuum chamber 
through a vacuum-tight feedthrough. The target voltage was 2200 volts; the 
argon pressure was 8.times.10.sup.-3 Torr; and the duration of sputtering 
was 2 hours. 
Results when used in the synthesis of formaldehyde are reported in Table 
III. 
EXAMPLE 7 
A Ag/Au/SiO.sub.2 composite catalyst containing 4.5 weight percent metal 
was prepared by RF sputtering simultaneously from a Ag/Au alloy target and 
a SiO.sub.2 target. The composition of the alloy target was 60 At. percent 
Ag and 40 At.percent Au. Approximately 10 g of fused silica particles 
(average dimension about 2 mm), distributed in several pyrex dishes on a 
rotating substrate table beneath the targets, were coated with the 
sputtered deposit. The sputtering voltages were 500 volts on the alloy 
target and 1700 volts on the SiO.sub.2. The argon pressure was maintained 
at 8.times.10.sup.-3 Torr and the particulate substrates were rotated at 5 
rev/min. The duration of the sputtering was 4 hours with periodic mixing 
of the particles outside the sputtering chamber. Example 7(a) is a control 
using 60/40 Ag/Au crystals as the catalyst. 
The catalyst was also used in the synthesis of formaldehyde. The results 
are reported in Table III. 
TABLE III 
______________________________________ 
Wt % Ag Select- 
or Ag/Au Conv. ivity 
(metal/ Mole Ratio 
Bed MeOH HCHO 
nonmetal) O.sub.2 /MeOH 
T .degree.C. 
% % 
______________________________________ 
Showing 
bulk 0.260 640 75.2 92.4 
A 0.360 700 91.0 92.7 
Example 
0.180 0.264 690 66.9 94.1 
1 (Ag/SiO.sub.2) 
0.303 700 77.8 93.6 
Example 
0.63 0.28 695 75.4 93.0 
2 (Ag/SiO.sub.2) 
0.380 710 94.3 92.0 
Example 
1.2 0.265 695 76.7 93.0 
3 (Ag/SiO.sub.2) 
0.350 700 90.1 92.8 
Example 
2.05 0.269 685 76.8 92.4 
4 (Ag/SiO.sub.2) 
0.351 700 91.0 92.3 
Example 
0.58 0.260 700 64.8 94.4 
5 (Ag/MgO) 
Example 
2.0 0.236 630 68.6 92.9 
6 (Ag/SiO.sub.2) 
Example 
4.5 0.256 622 68.5 92.8 
7 (Ag/Au/SiO.sub.2) 
Example 
bulk 0.236 586 62.4 92.5 
7 (a) (Ag/Au) 
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