Wound foil structure comprising distinct catalysts

A preferred catalytic converter structure suitable for treating automotive exhaust gases comprises a wound corrugated metal foil and gas passages defined by facing foil surfaces of adjacent turns, wherein one surface carries platinum catalyst and the facing surface separately carries palladium catalyst for concurrently treating gases flowing therethrough. A preferred method for manufacturing said structure comprises applying platinum catalyst and palladium catalyst to distinct halves in length of both foil surfaces, which halves border along a transverse axis, and winding the foil about the axis to bring the surfaces into facing relationship.

This invention relates to an improved monolith-type automotive catalytic 
converter comprising a wound foil structure carrying distinct catalysts 
strategically located for concurrently treating exhaust gases flowing 
therethrough. More particularly, this invention relates to a wound foil 
structure wherein facing foil surfaces form gas-conveying passages and 
carry different catalyst compositions. 
One method for manufacturing a monolith-type catalytic converter comprises 
suitably winding a corrugated and coated metal foil so that the 
corrugations form gas flow passages. The preferred foil is composed of a 
high temperature corrosion-resistant ferritic stainless steel alloy 
containing aluminum. The foil is preferably oxidized to cover the surfaces 
with integrally grown, densely spaced alumina whiskers and thereafter 
coated with a gamma alumina material suitable for impregnation with a 
catalyst. 
Thorough exhaust treatment is carried out using a combination of catalysts, 
typically platinum, palladium and rhodium. However, when platinum and 
palladium are present on the same alumina surface, the metals interact in 
a manner that substantially reduces their individual effectiveness. 
Therefore, platinum and palladium are preferably applied to separate 
surfaces. For monolith-type converters, the metals have been applied 
heretofore to discrete longitudinal zones arranged in series on one or 
more structures. Thus, exhaust treatment in monolith-type converters has 
heretofore been limited to successive action by different catalyst 
compositions. 
Therefore, it is an object of this invention to provide a monolith-type 
catalytic converter comprising two distinct catalyst compositions 
separately distributed for treating continuously and concurrently 
automotive exhaust gases, and a method for manufacturing said converter. 1 
It is a further object of this invention to provide an automotive catalytic 
converter comprising a wound foil structure having gas passages defined by 
foil surfaces of successive turns, which surfaces carry different catalyst 
compositions. Mutually antagonistic catalyst compositions are separated to 
optimize their effectiveness and are disposed for concurrently treating 
gases. 
It is also an object to provide a method for manufacturing a converter 
structure comprising corrugating a metal foil, applying at least two 
different catalyst compositions to selected surfaces thereof, and suitably 
winding so that the corrugations form gas passages and the facing foil 
surfaces carry distinct catalyst compositions. 
SUMMARY OF THE INVENTION 
In a preferred embodiment, a catalytic converter structure is manufactured 
by applying a platinum catalyst to substantially one half in length and a 
palladium catalyst to the remaining half of both surfaces of an 
alumina-coated metal foil, and suitably winding the foil to form a 
generally cylindrical structure having axial gas passages defined by 
facing foil surfaces that carry the different catalysts. The foil is 
several times longer than wide and the catalysts are separated by a 
transverse boundary. The foil also comprises transverse zig-zag 
corrugations. The foil is folded and wound about the boundary so that the 
corrugations of successive turns cooperate to form the gas passages. In 
addition, the outer surface of one turn and the inner surface of the next 
turn, which surfaces define the passages, comprise different halves of the 
same foil surface and thus carry different catalysts. Therefore, platinum 
and palladium catalysts are maintained separate to prevent mutual 
interference. Also, the catalysts are strategically distributed to act 
substantially concurrently upon gases flowing through the passages 
throughout the entire length of the structure.

DETAILED DESCRIPTION OF THE INVENTION 
In a preferred embodiment, an automotive catalytic converter is formed by 
folding and winding a metal foil having an alumina coating that is 
selectively impregnated with platinum and palladium catalysts. The foil 
consists of, by weight, 15% chromium, 4% aluminum, 0.5% yttrium and the 
balance iron, and is conveniently designated Fe-Cr-Al-Y or Fecralloy. The 
foil is peeled from a rotating cylindrical billet by feeding a durable 
tungsten carbide cutting tool into the rotating surface to cut or peel 
away a continuous thin metal strip. The strip is pulled away from the 
billet surface under controlled tension to form a foil having irregular 
and severely worked surfaces. Several parameters including the billet 
rotating speed, the cutting tool feed rate and the tension pulling the 
strip are adjusted to produce a foil thickness of about 50 microns. The 
peeled foil is about 7.6 cm wide and cut to a preferred length of about 
22.9 meters for forming a corrugated foil 18.3 meters long. Any cooling 
fluid applied during peeling is suitably cleaned away. 
The peeled foil is annealed for one minute at 900.degree. C. in air and 
corrugated by passing between a pair of driven rollers carrying mating 
teeth arranged in a zig-zag pattern to form a zig-zag or herringbone 
corrugation pattern in the foil, which pattern is illustrated in FIG. 1. 
The corrugations are about 0.76 mm in height and 1.78 mm in pitch 
(peak-to-peak dimension). The segments of the zig-zag pattern are oriented 
about 10.degree. from perpendicular to the foil edges and are about 1.25 
cm long. Any lubricant is cleaned away. The foil is preferably coiled to 
facilitate handling. During subsequent operations, the foil may be either 
loosely coiled to avoid metal-to-metal contact, particularly during 
furnace heating, or uncoiled and recoiled to provide access to the foil 
surface, particularly during coating operations. Preferably, coiling is 
carried out by folding and winding the foil, as hereinafter described, 
into substantially the shape of the desired catalytic converter structure. 
The foil is heated for 8 hours at 930.degree. C. in a circulating air 
atmosphere to grow high-aspect alumina whiskers that substantially cover 
the foil surfaces and improve adhesion of a subsequent ceramic coating to 
the metal. The whiskered surface is primed by spraying an alumina gel 
formed by mixing 5.0 parts by weight colloidal alpha alumina monohydrate, 
Al.sub.2 O.sub.3.H.sub.2 O, with 95 parts deionized water and adding 
concentrated nitric acid, HNO.sub.3, to lower the pH below about 2.0. 
While still wet, the primed surface is spray-coated with a gamma alumina 
powder dispersed in a similar, but less viscous gel comprising 3.0 parts 
by weight colloidal alpha alumina monohydrate in 97 parts water and nitric 
acid-stabilized below pH 2.0. The gamma alumina powder preferably has a 
porosity greater than about 1 cc pores per gram and a surface area greater 
than about 100 square meters per gram. About 70% of the particles are 
sized less than 200 mesh and greater than 325 mesh, and the balance are 
smaller than 325 mesh. The preferred coating material is prepared by 
uniformly mixing 27 parts by weight of gamma alumina particles to about 
100 parts gel, so that the dried coating is about 90% by weight gamma 
alumina. Although the colloidal alumina loses its alpha character in the 
gel, the gamma alumina survives as discrete particles having the desired 
high surface area. The first coat is air-dried and two to five additional 
coats of the particle-containing material are spray-applied and air-dried 
to produce a total coating thickness between 40 to 50 microns. The coating 
is calcined for four hours at 550.degree. C. in air, during which noxious 
NO.sub.2 fumes are driven off. The product substantially gamma alumina 
coating is tightly adherent and suitable for impregnation with noble metal 
catalysts. 
The gamma alumina coating is impregnated with a combination of base metals 
including barium, which stabilizes gamma alumina and also a noble metal 
dispersion, and cerium which enhances oxygen storage. An aqueous solution 
containing 0.03 g/ml barium nitrate and 0.05 g/ml cerium nitrate is sponge 
wiped onto both foil surfaces evenly at a rate of about 1 milliliter per 
gram of alumina coating. The foil is calcined for four hours at 
550.degree. C. The resulting coating contains about 2 weight percent 
barium in oxide form and about 2 weight percent cerium in oxide form. 
Referring to FIGS. 1, 2 and 3, the preferred coated, corrugated foil 10 
comprises a metal substrate 11 coated with alumina layers 12. Foil 10 
features edges 13, a first surface 14 and a second surface 16. A 
transverse folding axis 18 lying perpendicular to edges 13 divides first 
surface 14 lengthwise into a first half 20, and a second half 22, and 
similarly divides second surface 16 into a first half 24 and a second half 
26, lying opposite halves 20 and 22, respectively. Axis 18 does not divide 
foil 10 exactly in half, but is shifted slightly so that first halves 20 
and 24 are 9.3 meters long and second halves 22 and 26 are 9.0 meters 
long. The difference provides about a one full turn overlap after winding 
and produces a rounder structure. 
As described hereinabove, foil 10 is corrugated in a zig-zag pattern. As 
seen in FIG. 2, corrugated first surface 14 comprises peaks 28 and troughs 
30 that lie immediately opposite troughs 32 and peaks 34, respectively, of 
the second surface 16. 
In accordance with this invention, the gamma alumina coating on foil 10 is 
impregnated with two distinct noble metal compositions. A solution of a 
first composition is prepared by volumetrically dissolving about 1.4 gram 
tetraamineplatinum(II) chloride and about 0.11 gram pentaaminerhodium(III) 
chloride in 125 ml water. These amine complex weights correspond to 0.8 
gram (0.025 troy ounce) platinum and 0.04 gram (0.00125 troy ounce) 
rhodium. A second solution is similarly prepared by volumetrically 
dissolving about 0.76 gram tetraaminepalladium(II) chloride and about 0.11 
gram pentaaminerhodium(III) chloride in 125 ml water, corresponding to 0.3 
gram (0.01 troy ounce) palladium and 0.04 gram (0.00125 troy ounce) 
rhodium. 
The catalyst solutions are applied to foil surfaces 14 and 16 using sponge 
applicators 36 and 38 adapted to wipe opposite foil surfaces, as shown in 
FIG. 2. The platinum-rhodium solution is metered from a burette 42 into a 
bulb 44 that is connected to applicator 36. Bulb 44 holds a constant 
volume of solution to continually soak applicator 36 at a controlled 
solution pressure and thereby apply solution at a predetermined rate. 
Similarly, palladium-rhodium solution is metered from a second burette 46 
into a second bulb 48 connected to applicator 38. Bulb 48 maintains 
constant solution flow, similar to bulb 44. As seen in FIG. 2, foil 10 is 
initially pulled in the direction of arrow 40 so that applicator 36 is 
sponging platinum-rhodium solution onto the first half 20 of first surface 
14 and applicator 38 is sponging palladium-rhodium solution onto first 
half 24 of second surface 16. However, upon reaching axis 18, foil 10 is 
turned over so that applicator 36 sponges platinum-rhodium solution onto 
second half 26 of second surface 16 and applicator 38 sponges 
palladium-rhodium solution onto second half 22 of first surface 14. The 
entire solution volumes are applied evenly to the selected surfaces. 
Thereafter, the coating is air dried and calcined for four hours at 
550.degree. C. in an atmosphere consisting of 4% by volume hydrogen and 
96% nitrogen. Calcining destroys the amine complex salts and reduces the 
noble metals to their elemental and catalytically active state. 
Referring to FIGS. 3 through 7, the catalyst-bearing foil 10 is folded and 
wound into a preferred catalytic converter structure 50 in FIG. 7. (To aid 
in understanding the Figures, metal substrate 11 and coating layers 12 are 
not separately illustrated.) Referring to FIG. 3, foil 10 is bent about 
axis 18 to fold the foil in half lengthwise so that first half 20 of the 
first surface 14 faces second half 22. Because of the oblique orientation 
of peaks 28 and troughs 30 in the zig-zag pattern, they cannot nest upon 
folding, but cross to form passages 52. 
FIG. 4 shows a portion of folded foil 10 along a cross-sectional plane 
wherein corrugation peaks 28 contact. Along other planes, peaks 28 lie in 
juxtaposition to troughs 30, as depicted in FIG. 5, but are prevented from 
nesting by peak-to-peak contact in planes such as in FIG. 4. Since first 
half 20 carries platinum-rhodium catalyst and the second half 22 carries 
palladium-rhodium catalyst, passages 52 feature different catalyst 
compositions on opposite surfaces. 
After folding, first half 24 of second surface 16 lies opposite second half 
26 and both face outwardly. Foil 10 is then wound about axis 18; i.e., the 
bight of the fold, so that first half surface 24 is outside the second 
half surfaces 26, as depicted in FIG. 6. During winding, first half 24 of 
second surface 16 is caused to face second half 26 and the corrugation 
peaks 34 cross to form additional passages 52 in a manner similar to 
folding. Since the first half 24 carries palladium-rhodium catalyst and 
the second half carries platinum-rhodium catalyst, passages 52 formed by 
winding also feature opposite surfaces bearing different catalysts. Foil 
10 is completely wound into converter structure 50 in FIG. 7. Structure 50 
is suitable for incorporation into an automotive exhaust system for 
treating gases flowing therein. Gases flow through passages 52 and 
concurrently contact distinct platinum catalyst and palladium catalyst 
separately distributed on opposite surfaces therealong. 
In a preferred embodiment, foil 10 is folded and wound in separate steps to 
form a preferred converter structure 50 in FIG. 7. In an alternate 
embodiment, a structure is formed without a distinct folding step. 
Referring to FIG. 8, foil 10 is looped about two posts 54 in an "S" 
-pattern with axis 18 therebetween. The posts are then revolved about axis 
18 to wind foil 10 into a converter structure. The structure is 
substantially identical to structure 50 in FIG. 7, but is more elliptical 
in cross section, depending upon the distance between posts 54. This 
winding operation brings first half 20 of first surface 14 to face second 
half 22 and first half 24 of second surface 16 to face the second half 26. 
Since half surfaces 20 and 26 carry platinum catalyst and half surfaces 22 
and 26 carry palladium catalyst, gas passage through the structure feature 
different catalysts on separate and opposite surfaces, in accordance with 
this invention. 
In the described embodiments, rhodium and the base metals are distributed 
uniformly over all foil surfaces. The catalytic efficiency of rhodium is 
believed optimized at low concentrations. However, rhodium tends to sinter 
or alloy with platinum and may advantageously be separated on opposite 
passage surfaces from platinum by this invention. Similarly, cerium tends 
to adversely react with platinum. Different base metal compositions may be 
applied to discrete foil surfaces so that cerium is present with palladium 
but separated from platinum. Thus, this invention may be adapted to 
prepare distinct foil surfaces to receive particular catalysts. Further 
advantages may be obtained using other catalysts and other base metals. 
Although zig-zag corrugations are preferred, similar passages are formed by 
winding foils having straight corrugations that are oblique to the foil 
edges. Suitable passages are also produced by alternating turns of 
corrugated foils and flat foil. In another embodiment, suitable structures 
are formed by winding together two foils rather than folding over a single 
foil. An advantage of two foils is that the catalyst compositions may be 
readily applied to a continuous strip that is subsequently cut to the 
desired length. 
Although this invention has been described in terms of certain embodiments 
throughout, it is not intended to be limited to the above description, but 
rather only to the extent set forth in the claims that follow.