Catalytic method

A catalytic article for use in catalytic chemical conversions, which comprises; a metal monolith catalyst support and a catalytically active surfacey layer bonded to the support by sputtering, said catalytically active layer comprising an admixture of a precious metal and a base metal oxide.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to improved catalytic reaction systems and to 
methods for the preparation of catalysts. 
2. Brief Description of Related Art 
Automotive emissions are a major environmental problem in spite of the 
advances brought about by the use of catalytic converters. One factor 
limiting the performance of catalytic converters is that pollution is not 
controlled during the thirty or so seconds required to bring the converter 
catalyst to its operating temperature. In present converters, warm-up is 
dependent on heating of the catalyst by hot engine exhaust gases. Although 
electrical heating could be utilized to preheat the catalyst prior to 
engine operation, the power and the time delay required with present 
catalyst structures, ceramic or metal, have been deemed unacceptable. 
Subsequent to catalyst light-off, surface reactions on conventional 
monolithic catalysts such as are used in catalytic converters are mass 
transfer limited. Thus, the catalyst mass required for a given conversion 
level is much higher than if no mass transfer limitation existed at the 
given operating conditions. The high catalyst mass required for the 
required conversion level results in the relatively long heat-up times 
experienced, even with electrical heating. In addition, this mass transfer 
limitation is such that the conversion level of present automotive exhaust 
catalytic converters is limited to relatively low levels, typically not 
more than about 95%, even with the relatively small catalyst channel sizes 
employed. Higher conversion levels would be advantageous. 
The need to reduce catalyst warm-up time of the conventional ceramic 
monolith automotive catalysts to reduce emissions during the warm-up 
period has led to increased interest in metal monolith catalysts. However, 
merely substituting metal for ceramic in a conventional monolith structure 
yields catalysts which still have much too high a thermal mass. Although 
metal monoliths are electrically conductive and could therefore be 
electrically preheated, fast enough heat up times have not yet been 
demonstrated as feasible. Furthermore, thermal shock damage would likely 
be a problem if a conventional metal monolith were heated as rapidly as 
needed for elimination of start-up emissions. There is a critical need for 
a catalyst system which can control hydrocarbon emissions during initial 
engine operation. 
For catalytic combustors the problem is not just emissions but the ability 
to function in certain applications. For example, an automotive catalytic 
combustor gas turbine must start in roughly the same time frame as present 
automotive engines. 
The present invention provides catalysts and systems which make possible 
much more rapid warm-up of converter catalysts without electrical heating 
and near instantaneous electrical heating of catalysts in combustors and 
catalytic converters. Moreover, catalysts of the present invention enable 
much higher conversions and improved selectivity in many chemical 
conversion processes by virtue of improved mass transfer to and from the 
catalyst surface. The process of the invention provides catalyst articles 
of improved durability, efficiency and service life. 
SUMMARY OF THE INVENTION 
Definition of Terms 
In the present invention the terms "monolith" and "monolith catalyst" refer 
not only to conventional monolithic structures and catalysts such as 
employed in conventional catalytic converters but also to any equivalent 
unitary structure such as an assembly or roll of interlocking sheets or 
the like but, as appreciated in the art, does not include particulates, 
such as powders or pellets. 
For the purposes of this invention, the terms "microlith" and "microlith 
catalyst" refer to high open area monolith catalyst elements with flow 
paths so short that reaction rate per unit length per channel is at least 
fifty percent higher than for the same diameter channel with a fully 
developed boundary layer in laminar flow, i.e. a flow path of less than 
about four mm in length, preferably less than one mm or even less than 0.5 
mm and having flow channels with a ratio of channel flow length to channel 
diameter less than about five to one, but preferably less than two to one 
and more preferably less than about 0.5 to one. Channel diameter is 
defined as the diameter of the largest circle which will fit within the 
given flow channel and is preferably less than one mm or more preferably 
less than 0.5 mm. Microlith catalysts may be in the form of woven wire 
screens, pressed metal or wire screens and have as many as 100 to 1000 or 
more flow channels per square centimeter. Flow channels may be of any 
desired shape. For wire screens, flow channel length is the wire diameter 
and thus advantageously may be shorter than 0.3 mm or even shorter than 
0.1 mm. 
The terms "carbonaceous compound" and "hydrocarbon" as used in the present 
invention refer to organic compounds and to gas streams containing fuel 
values in the form of compounds such as carbon monoxide, organic compounds 
or partial oxidation products of carbon containing compounds. 
The Invention 
It has now been found that use of the microlith catalysts of the present 
invention makes possible as much as a ten fold or more reduction in 
catalyst mass as compared to that required to achieve the same conversion 
in mass transfer limited reactions of hydrocarbons using conventional 
monoliths. It has been found that the specific mass transfer rate 
increases as the ratio of channel length to channel diameter of a monolith 
catalyst is reduced below about five to one or more preferably below about 
two to one and especially below about one to one. Mass transfer of 
reactants to the surface becomes sensitive to the inlet flow rate rather 
than being significantly limited by the diffusion rate through a thick 
laminar flow boundary layer as in conventional monolith catalysts. In such 
conventional automotive monolith catalysts, the amount of pollutants 
oxidized is essentially independent of exhaust gas flow rate and thus 
percent conversion decreases with increase in flow rate. In contrast, in 
the microlith catalysts of the present invention, the amount of reactants 
oxidized typically increases with increase in flow rate. Thus if the inlet 
flow velocity is high enough, the reaction rate can even approach the 
intrinsic kinetic reaction rate at the given catalyst temperature without 
imposing an intolerable pressure drop. This means that it is practical to 
design microlith fume abatement reactors for much higher conversion levels 
than is feasible with conventional catalytic converters. Conversion levels 
of 99.9% or even higher are achievable in a microlith automotive converter 
smaller in size than a lower conversion level conventional catalytic 
converter. Conversion levels high enough for abatement of toxic fumes are 
achievable in compact reactors. 
With the short flow paths of catalysts of the present invention, pressure 
drop is low permitting the use of much smaller channel diameters for a 
given pressure drop, further reducing catalyst mass required. It has also 
been found that channel walls as thin as 0.1 mm or even less than 0.03 mm 
are practical with small channel diameters thus permitting high open areas 
even with such small channel diameters. Thus, as many as several thousand 
flow channels per square centimeter or even more are feasible without 
reducing open area in the direction of flow below sixty percent. Open 
areas greater than 65, 70 or even 80 percent are feasible even with high 
channel density microliths. 
Inasmuch as heat transfer and mass transfer are functionally related, an 
increase in mass transfer results in a corresponding increase in heat 
transfer. Thus, not only is catalyst mass reduced by use of the microlith 
catalysts of this invention, but the rate at which an automotive exhaust 
catalyst is heated by the hot engine exhaust is correspondingly enhanced. 
The reduced catalyst mass together with the increased heat transfer rate 
enables a microlith catalyst of the invention to reach operating 
temperature much sooner than would a conventional automotive catalyst. If 
placed sufficiently close to the engine exhaust manifold, a microlith 
catalyst element can even reach operating temperature in less than five 
seconds without electrical heating. Effective operating temperature for 
automotive exhaust microlith precious metal catalysts are as low as 650 or 
even as low as 550 degrees Kelvin. However, an important feature of 
microlith catalysts of the invention is that high enough operating 
temperatures are achievable prior to or during engine cranking to permit 
effective use of base metal catalysts. It has been found that a metal 
microlith composed of a high temperature alloy containing a base metal 
catalytic element such as chromium, cobalt, copper, manganese, nickel or a 
rare earth metal is catalytically active if heated to a temperature of 
about 800 degrees Kelvin, a temperature readily achieved in less than one 
second with electrical heating. Many such alloys are commercially 
available and include Haynes alloy 25, Inconel 600, and even certain 
stainless steels. With metal microliths, alloy selection is often 
determined primarily by oxidation resistance at the maximum operating 
temperature required by the given application. 
The mass of microlith catalyst elements of the invention can be so low that 
it is feasible to electrically preheat the catalyst to an effective 
operating temperature in less than about 0.50 seconds if a thin channel 
wall electrically conductive catalyst, e.g., a metal microlith, is used. 
In catalytic combustor applications the low thermal mass of catalyst 
elements of the present invention makes it possible to bring a combustor 
catalyst up to a light-off temperature as high as 1000 or even 1500 
degrees Kelvin in less than about five seconds by electrical heating and 
even in less than about one or two seconds using the power from a 
conventional automotive battery. Such rapid heating is allowable for 
microlith catalysts of the invention because sufficiently short flow paths 
permit rapid heating without the consequent thermal expansion resulting in 
destructive stress levels. 
Typically, in automotive exhaust systems of the present invention the 
catalyst elements preferably have flow paths of less than about one 
millimeter in length and may be less than about 0.1 millimeter in length 
with as little five high channel density elements required to greatly 
exceed the start-up performance of a 150 millimeter long conventional 
monolith. The short channels exhibit a low pressure drop even with 
channels as small as 0.25 millimeters in diameter. However, if 
particulates are present channel size must be large enough to avoid 
plugging. In catalytic combustor applications, where unvaporized fuel 
droplets may be present, flow channel diameter is often large enough to 
allow unrestricted passage of the largest expected fuel droplet. Therefore 
in catalytic combustor applications flow channels may be as large as 1.0 
millimeters in diameter whereas in automotive catalytic converter 
applications, flow channel diameter often can be as small as 0.5 to 0.25 
millimeters or even smaller. If desired, one, two or three microlith 
catalyst elements of the invention may be placed in front of a 
conventional monolith catalyst element to serve as a light-off reactor for 
the monolith. This approach is useful for retrofit applications. 
Although as few as one or two catalyst elements advantageously may be used 
in a given catalytic converter application to improve the cold start 
performance of conventional monolith catalysts, the low pressure drops 
possible with catalysts of the present invention makes it possible to 
utilize a large number of small diameter elements, even as many as two 
hundred in a one inch length, such that the converter diameter is not 
significantly larger than the engine exhaust pipe. This makes it much 
easier to place the converter catalyst at the exit of or even in the 
engine exhaust manifold, resulting in even faster catalyst warm up without 
electrical heating, and allows use of screens of different composition to 
achieve both hydrocarbon and NOx control. In other fume abatement 
applications, the large number elements feasible means that it is 
practical to achieve whatever conversion levels are needed, even as high 
as 99.999 percent or better. 
Although this invention has been described primarily in terms of automotive 
emissions control, the high mass transfer rates of microlith catalysts of 
the invention offers higher conversions and improved selectivity in many 
catalytic conversion processes. In particular, microlith catalysts of the 
invention offer superior performance in highly exothermic reactions such 
as the conversion of methane and other hydrocarbons to partially oxidized 
species; for example, the conversion of methane to methanol or the 
conversion of ethane to ethylene. 
The catalyst preparation method of the present invention is especially 
useful for preparing microlith catalysts in that it enables the use of an 
unlimited variety of catalyst formulations which would be difficult or 
even impossible to produce using conventional chemical deposition 
procedures. Although direct chemical coating of microlith catalysts from 
aqueous or organic solutions can be employed to produce useful catalysts, 
the method of the present invention makes possible catalysts of improved 
durability and service life. In addition, as will be appreciated by those 
skilled in the art, it is generally disadvantageous in applications 
requiring a high open area catalyst to employ the conventionally used 
slip-coating methods to produce commercial automotive exhaust catalysts. 
Slipcoating techniques result in coating thicknesses typically on the 
order of 0.02 millimeter or more, i.e., enough to significantly reduce the 
open area of a small channel microlith. Thus it is disadvantageous to use 
a slip or gel coated substrate such as described U.S. Pat. No. 3,957,692, 
or sputter coat particulates which are then applied by slip coating (such 
as the method of U.S. Pat. No. 3,966,645). Not only are such slip coats 
relatively thick but adhesion to a substate depends on penetration of 
surface porosity. 
In contrast, coatings of almost any thickness down to as little as fifty 
angstom units or even less in thickness can be obtained by the method of 
the present invention, but more preferrably at least about 75 angstrom to 
about one or two microns in thickness. Advantageously these coatings are 
impact bonded to a metal surface, ie the initial atoms penetrate the 
surface layer, and thus even a refractory metal oxide coating resists 
delamination from a metal substrate under conditions of use. In addition, 
because nonporous layers of ten or more monolayers may be deposited, a 
refractory metal oxide layer thick enough to serve as a diffusion barrier 
between the metal substrate and a precious metal catalyst coating is 
obtained.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
The present invention is further described in connection with the drawings. 
As shown in FIG. 1, in one preferred embodiment a microlith catalyst 
element 10 comprises a plurality of square flow channels 11 with 
electrical leads 15 connected to bus bars 16. Bus bars 16 are welded at a 
forty five degree angle to metallic flow channel walls 12 to ensure even 
heating of catalyst 10. Advantageously, catalyst element 10 is in the form 
of a catalytic metal screen of at least about 400 flow channels per square 
centimeter with a wire diameter sufficiently small to yield an open area 
of at least about 70 percent. Using the power of a standard automotive 
battery the catalyst may be brought to an effective operating temperature 
in less than one second, often in significantly less than 0.50 seconds. 
Thus in automotive exhaust gas service, electrical power need not be 
applied till just after start of engine cranking thus limiting maximum 
drain on the battery. Advantageously, electrical power is applied prior to 
termination of engine cranking. Typically, an automotive microlith 
catalyst element is heated to an effective operating temperature within 
one to two seconds of start of engine cranking. This rapid heating is 
important in that no delay in engine starting is required to achieve 
emissions control. Typical reactors may have from one to ten or more such 
microliths. 
FIG. 2 shows a sectional view of a three element microlithic catalyst 
reactor 20 suitable for either automotive exhaust gas treatment or for 
catalytic combustor service. Microlith catalyst elements 21 having 400 
flow channels per square centimeter are spaced apart a distance equal to 
or greater than the length of the flow paths 22 to provide for some mixing 
of gases flowing between elements 21. Catalyst elements 21 are held in 
reactor 20 by retaining rings 26 and separated from each other by spacers 
27. A microlith catalyst reactor such as shown in FIG. 2, depending on the 
application, may contain any desired number of microlith elements. With 
fine wire microlith screens, as many as one hundred or more can readily be 
placed in a one inch long reactor. 
The microlith catalysts of the present invention are readily made using 
known catalytic agents and conventional techniques of fabrication. The 
following examples describe means of making microlith catalysts but are 
not to be construed as limiting. A microlith catalyst as per FIG. 1 is 
made by vacuum sputtering platinum onto a stainless steel screen which has 
been cleaned by heating in air to 750 K. Typically the platinum coating 
may be thinner than 100 angstroms but may be thicker for greater catalyst 
life. Advantageously, a similarly thin layer of ceria or alumina may be 
deposited prior to deposition of the platinum. Catalysts containing 
palladium, iridium, rhodium or other metals can be similarly prepared. In 
many applications, especially with electrical heating, a wire screen 
formed from stainless steel or other alloy is a sufficiently active 
catalyst without additional coating. 
In a preferred embodiment of the invention, catalyst articles of the 
invention are fabricated by sputtering admixtures of a precious metal 
catalyst and a base metal oxide on catalyst supports of metal, including 
the supports described above. Sputtering is a well known technique for 
bonding thin layers of metals to substrates. Representative of 
descriptions of sputtering are those found for example in U.S. Pat. Nos. 
3,944,504 and 4,788,082, both of which are incorporated herein by 
reference thereto. The sputtering technique described in U.S. Pat. No. 
4,046,712 (incorporated herein by reference thereto) is also applicable, 
but it should be borne in mind that the support elements described in this 
patent as coated are ceramic or carbon particulates. Metallic monolith 
catalysts pose significantly different adhesion problems than the 
inherently rough surfaced particulates. Even low porosity particulates 
present relatively large surface areas, as much as twenty square meters 
per gram. A 0.5 monoatomic layer on even a one square meter per gram 
surface represents a 0.5 square meter per gram catalyst surface, an area 
much greater than the geometric surface area of a metallic monolith. Thus 
with microlith catalysts it is important to fully utilize the available 
surface. This is not as necessary with particulate substitutes inasmuch as 
even a five atom precious metal film tends to agglomerate in use, such an 
extemely thinlayer on a microlith or even monolith catalyst would not 
provide a durable, long life catalyst article for the high temperature 
applications in which such catalysts are typically used. Much thicker 
coatings are required, typically at least about fifty or more atomic 
layers and for the highest temperature applications to stabilize the film 
by cosputtering of one or more base metal oxides into precious metal 
catalyst layer, advantageously by reactive sputtering of metal in the 
presence of oxygen. Depending on the intended use it is often advantageous 
to use a base metal oxide having catalytic properties. In addition, unlike 
ceramic and carbon substrates, metal supports require a barrier coat to 
prevent diffusion of a precious metal catalyst into the metal substrate in 
elevated temperature service. Although the inventor is not to be bound by 
any theory of operation, it is believed that the bond achieved by 
sputtering a catalyst coating on a metal support is more tenacious than 
those bonds obtained by, for example, slip coating. By sputtering, atoms 
of the metal being deposited are typically implanted below the surface of 
the metal support, instead of merely on top of the surface. In a preferred 
article of the invention, the substrate or support is first coated with a 
refractory base metal oxide by sputtering. Then the catalyst is sputtered 
directly on the interposed refractory base metal oxide, without any 
intervening slip-coat. According to the invention, a small proportion of a 
base metal oxide is admixed with the catalyst metal to be sputtered. The 
proportion of base metal oxide added may be within the range of from about 
0.0001 to 10 weight percent, preferably 0.0001 to 5 weight percent. When 
the base support is a metal oxide or is first coated with a base metal 
oxide, the catalyst surface admixture bonds with a firmer adhesion. The 
technique of deposition by sputtering can be that described for example in 
U.S. Pat. No. 4,536,482 which is incorporated by reference thereto, except 
that the substrate is a metallic support for a monolithic catalyst such as 
a microlith instead of particles or pellets of refractory material. 
The admixtures of a precious metal catalyst and a base metal oxide may be 
varied in scope. Precious metal catalysts are defined herein as gold, 
silver and the platinum group metals (metals of Group VIII of the periodic 
Table of Elements). 
Representative of base metal oxides are oxides of the rare earth metals, 
such as cerium, zirconium, hafnium, thorium and the like. Alumina is also 
a useful base metal oxide. Catalytic oxides enhance catalyst activity. 
The thickness of the sputtered layers are advantageously within the range 
of from about 5 microns to 100 mm. 
Referring now to FIG. 3, there is seen in cross-sectional view an 
embodiment article 30 of the invention showing its structure. A 
catalytically active surface layer 32 comprises in admixture a precious 
metal catalyst with a refractory base metal oxide applied by sputtering 
onto layer 34 of a refractory base metal oxide. Layer 34 is also applied 
by sputtering onto catalyst support 36. 
The following Examples describe the manner and the process for making and 
using the invention and set forth the best mode contemplated by the 
inventor for carrying out the invention. 
EXAMPLE I 
A three element catalytic microlith automotive exhaust reactor having about 
2500 flow channels per square centimeter is constructed using a five 
centimeter wide strip of 70% open area screening of platinum coated 
stainless steel wires having a diameter of 0.03 mm spaced 0.20 mm apart 
and installed in the exhaust pipe of a four cylinder automotive engine. 
During engine cranking electrical power from the battery is applied 
heating the microlith catalyst elements to a temperature of 700 degrees 
Kelvin within one second whereby hydrocarbon emissions are controlled 
during initial operation of the engine. 
EXAMPLE II 
An electrically heated ten element microlith catalytic combustor is 
constructed using a screen fabricated with 0.076 mm wires of Kanthal. 
Ambient temperature air is passed through the reactor at a flow velocity 
greater than the laminar flame velocity of the fuel to be burned. The 
catalyst is then heated electrically to a temperature of 1000 degrees 
Kelvin and an intimate admixture of fuel and air is formed by spraying jet 
fuel into the air passing into the reactor. Plug flow combustion of the 
fuel is achieved. 
EXAMPLE III 
A fume abatement reactor six centimeters in length is constructed using 300 
microlith elements of screening with about thirty 0.050 mm wires of 
platinum coated nichrome per centimeter (nominally 900 flow channels per 
square centimeter). Fumes containing 50 ppm by volume of benzene in air 
are preheated to 700 degrees Kelvin and passed through the microlith 
reactor. Better than 99.9 percent conversion of the benzene is achieved. 
EXAMPLE IV 
An aluminum substrate was coated with a layer of zirconia by sputtering 
using the general procedure of U.S. Pat. No. 4,788,082. The nonporous 
layer obtained was glassy in appearance, insulative of the support and 
continuous. Pure platinum was then coated on the zirconia layer also 
following the general procedure of U.S. Pat. No. 4,788,082. In use under 
exposure to high temperature combustion gases the platinum layer 
delaminated from the still intact zirconia layer after several hours of 
use. 
EXAMPLE V 
Repeating the procedure of Example IV supra. but replacing the pure 
platinum as used therein with a coating of equal thickness of an admixture 
of zirconia and platinum (containing about 5 weight percent zirconia) an 
article is obtained which when run under similar conditions of use, does 
not delaminate.