Catalytic washcoat and method of preparation of the same

The invention is concerned with a method of adsorbing uniformly onto preformed high surface area crystalline alumina one or more catalytically active non-noble (doping) oxide(s) and preferably alumina-stablilizing non-noble doping oxide(s), both from substantially neutral aqueous colloidal solutions thereof, drying and calcining the doped alumina to form an improved washcoating composition for receiving additionally catalytic noble metal particles.

The present invention is concerned with the preparation of so-called 
catalytic washcoating compositions or washcoats comprising a refractory 
high surface area alumina, being particularly directed to improved 
washcoats comprising other selected base metal oxides, (hereinafter 
sometimes referred to as doping oxides or dopants) functioning as 
stabilizers, promoters and/or catalysts, said dopants being applied to 
said alumina by an improved technique, and such novel washcoats being 
suitable for bonding to known catalytic supports (including ceramic 
pellets and honeycombs, metallic honeycombs and the like), and for 
receiving known catalytic noble metal particles applied thereto by known 
techniques, whereby enhanced durability and/or performance is imparted to 
such catalyzed supports, for example under the extreme temperature-cycling 
conditions encountered in automotive exhausts. 
The prior patent and technical literature is replete with descriptions of 
alumina-based catalytically active oxide-doped washcoating compositions 
bearing, in addition, catalytic noble metal particles, for oxidation of 
hydrocarbons and/or reduction of nitrogen oxides (NOx) in fume abatement, 
automotive exhaust control and the like. 
Catalytically active doping oxides, including those particularly useful 
with the present invention, or their hydroxides, are substantially 
water-insoluble. Therefore, in the prior art, it has been found necessary 
to impregnate the high surface area alumina with an aqueous solution of 
water-soluble salts of the oxides, predominantly nitrates (sometimes 
chlorides or acetates), requiring subsequent (after drying) high 
temperature decomposition of such salts to the oxides. Here the 
simultaneous release of NOx (or other contaminants) into the atmosphere 
needs to be prevented or minimized by expensive pollution control 
equipment and operation. 
A significant number of patents (but by no means all) are cited in the 
appended list of documents as indicative of the scope of the 
state-of-the-art. By way of examples, reference is made here to U.S. Pat. 
Nos. 3,894,965 (1975), 4,003,976 (1977), 4,120,821 (1978), 4,323,542 
(1982) 4,407,738 (1983), and 4,006,103 (1977); also to a publication 
entitled "Ceria-Promoted Three-Way Catalysts for Auto Exhaust Emission 
Control" by Gwan Kim (Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 267-274); 
and to German Patent No. 3,223,500 assigned to Degussa AG (1983). 
By way of background, the "738" patent proposes a single step impregnation 
of alumina pellets with acidic solutions of the noble metals. The "965" 
patent coats a honeycomb with partially hydrated amphoteric oxides 
including La.sub.2 O.sub.3, Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Mn.sub.2 
O.sub.3, TiO.sub.2, ZrO.sub.2, MnO.sub.2, SiO.sub.2, SnO.sub.2, ThO.sub.2 
and Mn.sub.3 O.sub.4 and then applies an alkaline noble metal solution 
thereto. In the "976" patent, NOx is reduced on a complex catalyst 
including components selected from nickel oxide, copper oxide, manganese 
oxide, cerium oxide, lanthanum oxide, iron oxide, cobalt oxide, yttrium 
oxide, niobium oxide as well as platinum, rhodium, and palladium 
catalysts. It should be noted that, for example, alpha-alumina is there 
impregnated, by immersion, with nitrate solutions of nickel, copper, iron, 
barium and chromic acid, and subsequently dried and heated to 600.degree. 
C.= and allowed to stand (at that temperature) for three hours in the 
presence of a stream of air (Col. 9, line 62 to Col. 10, line 6). 
Such procedure illustrates one of the principal drawbacks of the prior art: 
namely, the required impregnation/drying procedure which, beside causing 
lack of uniformity, introduces deleteriously the need for protection of 
the environment because of the release of, here, oxides of nitrogen into 
air during the high temperature decomposition of nitrates. 
The "821" patent discloses a boron oxide type dopant for alumina and the 
noble metal catalyst, whereas the "542" patent focuses on uranium oxide 
type dopant as well as mentioning others (Ni, Ce). The "103" patent shows 
impregnation of Al.sub.2 O.sub.3 with a nickel nitrate solution followed 
by drying and calcining. In the Kim article, the effect of dopants, 
primarily ceria, is described. 
Here, "cerous nitrate and ammonium metatungstate were the sources of 
additive oxides" (p. .sub.269, vol. 1, lines 11-12 under "Experimental 
Section"). The German patent emphasizes ceria and iron oxides (in addition 
to nickel oxide, zirconia) as dopants which are applied from nitrate 
solutions and later converted to oxides. Without attempting the enormous 
task of being complete, we also attach a significant further list of 
patent numbers of varying degrees of general background information 
relating to this invention. 
It has been recognized that the performance and/or endurance of the 
noble-metal-on-alumina catalysts is significantly enhanced by appropriate 
selection of these non-noble metal doping oxide or dopants--the same then 
sometimes being referred to herein, including the appended claims, as 
catalytically active doping oxides. The stabilizing of the alumina under 
high temperature conditions, i.e. its endurance enhancement, is attributed 
to, preferably titania and zirconia, which appear to slow down the thermal 
sintering process of, for example, high surface area gamma-alumina to 
lower surface area alpha-alumina. Other oxide dopants are actually 
catalytically active, among them iron oxide, chromium oxide, cerium oxide, 
lanthanum oxide and nickel oxide. 
While the use of colloidal noble metal particle solutions have been 
introduced by the assignee of this application for depositing critical 
ranges of particle size on high surface area substrates, see, for example, 
U.S. Pat. No. 4,102,819, the utility of this kind of approach to the 
non-noble metal oxide washcoating has not heretofore been evident. 
Underlying the present invention is the discovery that application of a 
similar colloidal deposit approach to doping oxides on high surface area 
alumina containing substrates not only has a significant utility and 
improvement over prior application techniques, but produces the added 
beneficial results--non-analogous to the noble metal particle application 
process--of stabilizing the alumina with certain of the adhered colloidal 
doping oxide particles by apparently slowing thermal sintering of the 
substrate in high temperature use, as above mentioned, and adding 
catalytic activity (apart from the later added noble metal catalyst) by 
the same or another doping oxide, above identified, with uniformity of 
adherence over the major portion of the alumina particles of the 
substrate. It is this class of catalytically active doping oxides 
including the above-named metal oxides (to be uniformly adsorbed by, but 
in accordance with the invention, preferably not to be reacted with the 
alumina) to which the present invention is directed. The invention is 
concerned with substantially water-insoluble, catalytically active 
metallic doping oxides of base metals capable of forming water-soluble, 
heat-decomposible salts of "potentially" air-contaminant producing acids. 
Their role (i.e., the enhancement of catalytic performance and/or 
endurance) is entirely different from that of other classes of oxides, 
such as strontium oxide (and magnesium oxide) which it has been proposed 
to combine with gamma alumina to increase the abrasion resistance of a 
granular alumina catalyst carrier, as described, for example, in Japanese 
Patent No. 51-31691 (1976) by Kumio Okamoto et al. Specifically, here 
gamma alumina is deliberately reacted either with a colloidal strontium 
oxide solution or a non-colloidal strontium nitrate solution producing a 
new abrasion resistant compound, said to result from improved particle 
bonding, the strontium oxide being catalytically inactive. 
In U.S. Pat. No. 4,539,311 (1985) mention is made of an aqueous dispersion 
of hydrated alumina and colloidal ceria (Col. 3, lines 6-7). A metal 
honeycomb support is coated with the dispersion, dried and fired, 
resulting in a mixed Al.sub.2 O.sub.3 -CeO.sub.2 washcoat containing a 
predominant amount of CeO.sub.2 throughout the entire coating (Col. 3, 
lines 35-36). Here the colloid serves the function of binding the oxides 
to the metal honeycomb. The entire washcoat so formed is merely a 
variation of the conventional washcoat base for the 4% by weight (Col. 3, 
1. 37) of "antipoisoning" barium oxide which is applied by the standard 
barium nitrate solution impregnation/drying/firing technique of the prior 
art. 
Colloidal alumina solutions have also been used for non-catalytic purposes, 
for example as binders for high surface area crystalline alumina powders 
to ceramic or metallic substrates. 
Colloidal solutions of other oxides have been used to coat low surface area 
cores with what amounts to washcoat comprising such other oxides which are 
subject to subsequent "doping." In U.S. Pat. No. 4,477,492 (1984), for 
example, glass spheres (or other macroparticles) are coated with a variety 
of oxides by means of colloidal solutions, preferably silica. "When used 
as a catalyst support . . . a platinum metal salt can be dispersed within 
the porous structure of the superficial coating . . . to yield an active 
platinum metal catalyst" (col. 5, lines 10-11, 24, 26-28). Clearly, here, 
the oxide coating of the low surface area macroparticles constitutes a 
washcoat which is functionally no different from the alumina washcoats: 
subsequent doping of these washcoats, as well, with true (rather than 
colloidal) solutions of selected oxide salts results in the detrimental 
effects above described. 
In general, as pointed out above, a primary drawback of the techniques of 
the prior art resides with limitation on the product stemming from the 
non-uniform application of these dopants on the alumina by means of 
aqueous, dopant salt-containing solutions, involving impregnation, drying 
and the contaminant-producing high temperature decomposition of the salt 
to the oxide. Such prior art deposition techniques result in a deleterious 
lack of uniformity of the dopants. While we do not wish to be held to any 
particular theory, it is plausible to attribute it to the formation of a 
wide spectrum of particle sizes non-uniformly distributed in the alumina, 
as the oxides "precipitate" from the solution during the drying process. 
The release of noxious elements by the technique of the prior art 
constitutes a detrimental environmental threat. Underlying the present 
invention is the discovery that a substantially neutral colloidal solution 
of a catalytically active oxide dopant causes the colloidal particles to 
be adsorbed uniformly by the high surface area alumina, thereby 
guaranteeing the required uniformity, reproducibility and stability for 
catalyst use as, for example, in automotive exhaust control, and obviating 
the environmental hazard. 
An object of the invention, accordingly, is to provide a new and improved 
catalytic washcoat and method of preparation thereof that significantly 
obviate the deleterious effects of non-uniform dopant deposition on the 
alumina; and does so through the adsorption by the alumina of the selected 
doping base metal oxides from substantially non-polluting colloidal 
solutions thereof, without reacting therewith. 
Other and further objects will be explained hereinafter and are more 
particularly pointed out in the appended claims. 
The novel technique underlying the invention, in summary, embraces a method 
of preparing an aqueous substantially neutral colloidal solution of one or 
more of each desired catalytically active doping oxide, adsorbing the same 
uniformly onto particles of a high surface area alumina and drying and 
calcining the doped alumina to form an improved washcoating composition. 
The term "catalytically active doping oxide", as sometimes used herein, 
shall mean a catalytic oxide and/or an oxide which enhances the endurance 
of the catalytic coating, when deposited on, but not reacted with high 
surface area alumina. 
For the purpose of this invention we select a washcoat base comprising a 
preformed high surface area crystalline alumina, regardless of whether 
such "preforming" does or does not involve a colloid. This material allows 
us to locate small amounts (relative to the amount of alumina washcoat) of 
the uniformly adsorbed catalytically active dopants on the alumina's high 
surface area for enhanced performance. While we do not wish to be held to 
any theory, it is plausible that the interaction between the alumina, the 
dopants and the noble metal catalyst(s) is significantly more effective 
with the dopants on the washcoat's surface which is exposed to the 
reacting contaminants in fumes and exhausts, than it would be with the 
dopant largely buried within the bulk of the alumina. 
Whatever the theory, the preformed alumina (in contrast, for example, to 
hydrated alumina as a precursor) allows us to limit the amount of dopants 
relative to the bulk alumina, thereby also substantially retaining, rather 
than unduly diluting the overall composition of the finished noble 
metal-bearing catalyst coating. In short, in accordance with this 
invention, the washcoat comprising the preformed high surface area 
crystalline alumina is the predominant component of the catalytic coating; 
its surface serves as the recipient of the relatively small amounts of the 
uniformly adsorbed colloidal catalytically active doping oxide(s), as well 
as the noble metal catalysts. 
We turn now to the practice of the invention which utilizes the above 
discovery of the new and uniform particle adsorption from appropriate 
colloidal solutions of the doping oxide, as distinguished from aqueous 
true solutions of a salt of the doping oxide, and without reaction with 
the alumina substrate. 
We provide generally a colloidal solution and preferably, whenever 
practicable, a substantially neutral colloidal solution, in which both the 
particle size and the particle size distribution of such doping oxides 
remains stable; that is, without significant growth, for long periods of 
time. While such stable colloidal oxide solutions are old in the art, the 
novel use of the same herein produces an improved washcoat by contacting 
the solution with the high surface area alumina, thereby causing the 
colloidal particles to be adsorbed onto the alumina without significant 
change of size of these doping oxide particles. 
Experimentally, the difference between the colloidal and noncolloidal 
solutions is easily demonstrated as follows: When a colloidal solution is 
passed, for example, through a bed of high surface alumina, the 
concentration of dopant oxides in the effluent is sharply reduced and 
often brought down to substantially zero. In contrast, prior art true 
aqueous doping-component solutions, when passed, for example, through an 
alumina column, have substantially the same concentration of solute 
entering and leaving the column, the alumina being impregnated with a 
fraction of the entering solution in its original composition. This 
difference between the colloidal solution of the present invention and the 
prior art true solutions has been found to be a key in insuring uniformity 
of dopants in the colloidal case of the present invention, as 
distinguished from decided lack of uniformity in the before-discussed true 
solution case. Moreover, by choosing the substantially neutral dopant 
oxide colloids, noxious elements such as nitrates or chlorides are almost 
entirely avoided, thereby substantially eliminating any air pollution 
hazard. The seriousness of this threat can readily be understood given the 
substantial quantities of washcoat required potentially for catalysts in 
automotive and power plant exhausts; up to 15% of the weight of the 
washcoat compositions is often representing the dopants. Clearly, 
corresponding amounts, for example, of NOx would undoubtedly necessitate 
expensive equipment for atmospheric protection. 
While stable solutions are obviously preferred, as above stated, the 
principle of uniform adsorption has also been adapted, in accordance with 
the present invention, to freshly prepared transient colloidal solutions. 
Here, for example, an acidic dopant component solution is first carefully 
neutralized typically to a pH between 3 and 8 under continuous stirring, 
and is then added promptly to the high surface area alumina; that is, 
within seconds, or at the most minutes depending on the rate of growth of 
the transient colloid. In this instance, it is believed that the high 
surface area alumina particles act as seeds to adsorb the transient 
colloidal particles, preventing them from growing to the point of separate 
coalescence and eventual precipitation. In any event, the transient 
colloid technique, while delicate to control, has been found to result in 
improved washcoats comparable to the stable colloid technique as well as 
obviating the before-mentioned environmental hazard, provided that the 
doped alumina is separated from the solution, as by filtration and 
washing, prior to drying and calcining. 
Alternately, the transient colloid may be formed in the presence of the 
alumina particles or precursor thereof, thereby adsorbing the colloidal 
dopant oxide particles. This latter technique is especially suited for 
short-lived transient colloids. 
The alumina-based washcoat serves as the catalyst support for the 
catalytically active doping oxides and for the catalytic noble metal 
particles. In general, the alumina constitutes the predominant amount of 
the washcoat with the doping oxides present (individually or in 
combination) in amount of less than about 20% by weight of the total 
washcoat. While sometimes as little as a fraction of one percent of a 
catalytic oxide is beneficial, often two or more catalytic oxides are 
adsorbed advantageously by the alumina, the oxides being present in amount 
exceeding 2% by weight of the total washcoat.

Best mode embodiments for use with alumina substrates that are to be 
employed in high temperature catalytic converter applications, such as the 
before-mentioned automotive exhausts or the like, will now be described. 
EXAMPLE 1 
A typical known colloidal solution, as prepared by ion exchange, is made as 
follows: 36.6 g of hydrated ferric nitrate salt is dissolved in 75 cc 
de-ionized water. At that point the pH is between 0 and 0.5, 32 g of a 
weakly basic (polyamine) ion exchange resin, sold under the trade name 
Amberlite IRA 69 by Rohm & Haas, in the hydroxide form is added to the 
solution with stirring. The hydroxyl ions from the resin exchange with the 
nitrate ions from the ferric nitrate, producing dissolved ferric 
hydroxynitrate. As the nitric acid continues to be neutralized by the 
resin, the pH slowly rises up to about 3.1. The solution at this point is 
a deep blood-red color. Almost all the nitrate is now retained on the 
resin and the exchanged hydrogen ions have been substantially neutralized 
to form water. With most of the nitrate removed, the iron oxide colloid 
formed is presumably protected by the residual hydrogen ion and its 
counterion (nitrate) present in trace amounts. After the removal of the 
resin, the colloidal solution is stable for many weeks, provided that it 
is not destabilized by heating or by the addition of salts. 
The stable colloidal solution of iron oxide is now applied as follows: 1000 
grams of high surface area alumina (ca. 200 m.sup.2 /g) is suspended in 
1710 ml of water. 290 ml of the colloidal solution containing 35 g/l of 
Fe.sub.2 O.sub.3 is added slowly, at ambient temperature, over a period of 
twenty minutes, under vigorous stirring, and the mixing is then continued 
for an additional two hours, resulting in an alumina slurry having 
colloidal Fe.sub.2 O.sub.3 particles substantially and uniformly adsorbed 
on the alumina. The slurry is then dried at about 180.degree. C. The 
resulting dry powder has an orange color and is converted to a typical 
washcoating composition of this invention by calcination in air at 
550.degree. C. for about three hours. The washcoat contains about 1% of 
Fe.sub.2 O.sub.3 by weight. 
The washcoat is suitable for application to, for example, ceramic 
honeycombs and for deposition thereon of typical noble catalysts, e.g. Pt, 
Rh and/or Pd, by well known techniques metal such as are disclosed, for 
example in U.S. Pat. No. 4,006,103 or by colloidal noble metal particle 
deposition as in U.S. Pat. No. 4,082,699 of Common Assignee. 
EXAMPLE 2 
A stable chromia colloidal solution is prepared as follows: 
1540 grams of hydrated chromium nitrate [Cr(NO.sub.3).sub.3 
.multidot.9H.sub.2 O] is dissolved in 3300 ml of deionized water. The 
initial pH is 0.75. 140 grams of weakly basic ion exchange resin 
(Amberlite IRA 69) is added in bulk to that solution. The hydroxyl ions of 
the resin exchange with the nitrate producing chromium hydroxy-nitrate in 
solution. After half an hour an additional amount of 346 gram resin is 
added. As the nitric acid concentration continues to be neutralized by the 
resin, the pH slowly rises up to 3.1. Most of the nitrate is now retained 
on the resin. 
At this point the slurry is filtered and the filtrate is returned into the 
beaker and mixed with additional amount of 227 grams of resin. After half 
an hour of stirring, the slurry is filtered again. The filtrate has a pH 
of 4.2. 
The filtrate is returned to the beaker and a third amount of 1465 grams of 
resin was added. After stirring and filtering, the final pH obtained is 
5.4, and the chromium oxide colloidal solution has a concentration of 25 
g/l (as Cr.sup.+3). 
The stable aqueous colloid solution of chromium oxide is now applied to the 
high surface alumina as follows: 1000 grams of high surface area gamma 
alumina is suspended in 1960 cc of water. While vigorously stirring 40 ml 
of the solution containing 25 g/l chromium oxide colloid is added slowly 
for 20 minutes. The mixing is continued for an additional two hours. The 
Cr.sub.2 O.sub.3 -on-alumina slurry is then dried at 180.degree. C. The 
resulting dry powder which has a dark blueish tinge is calcined in air at 
550.degree. C. for 3 hours. The chromia dopant contributes to catalytic 
oxidation of hydrocarbons at high temperatures (above 500.degree. C.). 
Again, the resulting washcoat is applied to the alumina substrate and 
noble metal as in Example 1. 
EXAMPLE 3 
A washcoat powder containing 6.5% by weight of CeO.sub.2 is made by 
applying to 1000 g Al.sub.2 O.sub.3 suspended in 1868 ml water, 132 ml of 
a stable colloidal solution of ceria, obtained from Rhone-Poulenc 
(prepared by peptization), containing 200 g/l of ceria, followed by drying 
and calcining all as described in the preceding examples. 
EXAMPLE 4 
A washcoat powder containing 2% by weight of zirconia (ZrO.sub.2) is made 
by applying to 1000 g Al.sub.2 O.sub.3 in 1900 ml water, 100 ml of a 
stable colloidal solution of zirconia, obtained from Nyacol Corp. of 
Ashland, Mass., (prepared by hydrolysis of basic zirconium nitrate) 
containing 200 g/l of ZrO.sub.2, all followed by drying and calcining as 
described in the preceding examples showing the sintering of the alumina 
under explosive- to high-temperature. 
EXAMPLE 5 
A washcoat powder is made by adsorbing onto Al.sub.2 O.sub.3 colloidal 
nickel oxide particles from a transient colloid thereof as follows. 
Slurry 250 g of alumina in 500 ml of water and add thereto, under stirring, 
50 ml of a solution containing 7.72 g of [Ni(NO.sub.3).sub.2 
.multidot.6H.sub.2 O] or 1.25 g of Ni.sup.++. During the addition the 
initial pH of 6.0 drops to 5.7 due to the slight acidity of the nickel 
nitrate. While continuing vigorous stirring, add slowly concentrated (27%) 
aqueous ammonia until the pH reaches 7.5, whereby transient colloidal 
nickel hydroxide-containing particles are formed and are adsorbed by the 
high surface area alumina. Continue the stirring at that pH for two hours, 
until substantial completion. After filtering, the doped alumina is dried 
and calcined as described in the preceding examples. 
Note that the filtrate is colorless, and contains less than 1 ppm of nickel 
(by atomic adsorption), attesting to the above-described adsorption of the 
colloidal nickel solution. 
As a catalytic component of a washcoated, noble metal bearing ceramic 
catalyst, the NiO dopant enhances significantly the reduction of NOx in, 
for example, automotive exhausts. 
EXAMPLE 6 
Enhanced effectiveness of the colloidal-oxide dopants is attained by the 
use of more than one oxide on the same sample of alumina (200 m.sup.2 /g 
when it is doped (in succession) with the techniques and the appropriate 
amounts of colloidal solutions, as described in Examples 1-4 herein, to 
yield a mixed colloid-doped washcoat of the following composition: 
CeO.sub.2 : 6.5% by weight 
ZrO.sub.2 : 2.0% by weight 
Fe.sub.2 O.sub.3 : 1.0% by weight 
Cr.sub.2 O.sub.3 : 0.1% by weight 
For comparison, a non-colloid oxide doped washcoat of substantially the 
same composition was prepared imprepregnating the alumina, in succession 
with nitrate solutions, followed by drying and calcining. The washcoats 
were coated onto ceramic honeycombs (400 cells per inch) and received 
noble metal Pd/Rh catalysts in the ratio of 5:1 respectively by weight in 
total amount of 25 g/ft.sup.3 of the ceramic honeycomb substrate, by the 
catalyzing procedure described in U.S. Pat. No. 4,006,103. 
The activity of the two catalyst samples was compared using a catalytic 
reactor tube in a muffle furnace equipped with temperature and flow 
controls. Gas mixtures containing air, CO, CO.sub.2, steam and propane 
were flown through flow controls into the catalytic reactor at a space 
velocity of 10,000 [l/hour]. The concentration of each component was 
determined using a gas chromatograph equipped with a molecular sieve 
column and HWD detector. Percent conversions of CO and propane in the 
effluent gas mixture were measured as a function of temperature by 
comparing the concentrations of these species at room temperature. 
Light-off temperatures defined as that temperature at which 50% conversion 
occurs are reported below. 
In one series of comparative tests of carbon monoxide conversion rates to 
CO.sub.2 were measured (carbon monoxide is usually the most readily 
oxidizable contaminant in exhausts), first on the "pristine", that is 
freshly coated catalyst, and second on the "aged" catalyst, that is after 
subjecting it to a temperature of 800.degree. C. for 8 hours, during which 
period particle growth is known to occur with loss of performance. In 
another series of comparative tests, propane conversion rates to CO.sub.2 
and water were determined, propane being a typical hydrocarbon contaminant 
which is known to require significantly higher temperatures (than carbon 
monoxide) for its catalytic oxidation. Pristine and aged light-off 
temperatures are shown in the following table. 
______________________________________ 
Light-off Temperatures 
Carbon monoxide Propane 
Contaminants 
Pristine Aged Pristine 
Aged 
______________________________________ 
non-colloidal 
158.degree. C. +/ 
195.degree. C. 
305.degree. C. +/ 
405.degree. C. +/ 
-2.degree. C. -5.degree. C. 
-5.degree. C. 
colloidal of 
153.degree. C. +/ 
185.degree. C. 
305.degree. C. +/ 
315.degree. C. +/ 
the invention 
-2.degree. C. -5.degree. C. 
-5.degree. C. 
______________________________________ 
In the case of CO, there is shown some initial pristine advantage of the 
colloidal dopant, which is doubled after aging. In the case of propane, 
the pristine samples light off at about the same temperature, but the 
retention of catalytic activity, upon aging, due to the colloid dopants, 
amounts to an order of magnitude in light-off temperature differential, a 
spectacular effect. 
It should be noted that in hydrocarbon oxidation in the presence of steam, 
ceria is the main catalytic promoter element (other than the noble 
metals). 
A "singly doped" washcoat of Al.sub.2 O.sub.3, that is one doped with 
colloidal ceria alone, and then coated with noble metals, accounts for a 
major portion of the spectacular activity retention. 
It is generally known that, for instance in the case of a 
platinum-on-alumina catalyst, the higher the surface area of the alumina, 
the better the catalytic performance (see, for example, U.S. Pat. No. 
4,082,699, Col. 7, l. 12-18 and Col. 15, table 8 of Common Assignee). 
Thus, any doping oxide capable of "stabilizing" high temperature-aged 
alumina, that is slowing its surface area loss during sintering, is 
extending the useful life of the catalyst. This is so regardless of 
whether the doping oxide(s) is not, (e.g. TiO.sub.2 or ZrO.sub.2) or is, 
(e.g. CeO.sub.2 or La.sub.2 O.sub.3) a catalyst in its own right. In yet 
another comparative test we have doped a sample of alumina with a surface 
area of 205 m.sup.2 /g (by B.E.T. measurement), with two oxides, 
contributing to the catalytic performance of the noble metals, namely with 
5% CeO.sub.2 by weight and 2.5% La.sub.2 O.sub.3 by weight, in accordance 
with the method of this invention. We have heated this doped sample and a 
non-doped sample of the same alumina in air at 1100.degree. C. for eight 
hours. In this drastic procedure the area of the non-doped alumina has 
shrunk to 10.7 m.sup.2 / g, whereas that of the doped alumina has come 
down only to 81.3 m.sup.2 / g, a spectacular improvement. 
Similar beneficial aging is effected by titania and/or zirconia, which 
oxides, however, are not contributing to the catalytic efficacy of the 
noble metals. 
To combine the improvements not heretofore anticipated, we prefer to 
select, in accordance with this invention, more than one 
environmentally-safe metal oxide dopant which stabilize(s) the alumina as 
well as enhance(s) the catalytic performance of the noble metal containing 
structure. 
If the amount of metal oxide dopants is too high (in excess of about 20% as 
before stated), the adherence of the dopant-bearing washcoat to the 
substrate, e.g. ceramic honeycomb, is reduced; and, moreover, the 
sintering and coalescence of said excessive amounts of dopants at elevated 
temperatures can reduce their effectiveness. 
While the above-named colloidal oxide dopants are preferred for use in 
exhaust conversion catalysis, other oxides such as copper oxide, manganese 
oxide, cobalt oxide and rare earth oxides are also useful therein and 
subject to colloidal oxide adsorption on the alumina. 
As stated above, the alumina (being the support for the catalytically 
active dopants) is the predominant component of washcoat composition, and 
is preferably between 85% and 98% of the total weight of the dry washcoat 
composition. 
Other variations will occur to those skilled in the art and are to be 
considered within the scope of this invention.