Catalyst of a coating on an alloy substrate

A catalyst suitable for treating automobile exhaust emissions comprises an Al-containing Fe-base alloy substrate with a protective coating of, e.g., Al.sub.2 O.sub.3 and Cr.sub.2 O.sub.3 and a second coating of, e.g., Al.sub.2 O.sub.3 and a Pt-group metal. The coatings may be applied using sols obtained by dispersing Al.sub.2 O.sub.3, prepared by vapor phase condensation methods, in water and having appropriate convertible salts dissolved therein, followed by firing.

This invention relates to catalysts and the preparation thereof. 
There is much interest in the use of catalysts for the treatment of motor 
vehicle exhaust gases in order to eliminate the noxious constituents 
thereof, principally carbon monoxide, oxides of nitrogen and hydrocarbons. 
Such use makes heavy demands on the catalyst, which has to withstand a 
severe environment and repeated thermal cycling throughout its working 
lifetime whilst still retaining catalytic effectiveness. 
We have now prepared a catalyst which, in certain test results, has 
exhibited ability to meet the above requirements. Further, we have found 
that such a catalyst may have application in fields other than the 
treatment of motor vehicle exhaust gases. 
The present invention provides, in a first aspect, a catalyst comprising an 
Al-containing Fe-base alloy substrate carrying a protective adherent 
coating comprising a refractory oxide in association with an oxide of an 
element of the first period of transition elements, with the exception of 
iron, wherein the transition element oxide is different from the 
refractory oxide, and a second coating comprising catalytically active 
material. 
The catalysts of the present invention may be capable of catalysing both 
oxidation and reduction reactions. Thus, they may, for eample, have 
particular application in the treatment of exhaust fumes from internal 
combustion engines and also, for example, have applications in 
hydrogenation, gas burners, isomerisation and ignition systems. Catalysts 
of the invention have shown particularly outstanding results in their 
effectiveness in oxidising carbon monoxide and hydrocarbons and in 
reducing oxides of nitrogen and in maintaining this effectiveness after 
thermal cycling. Details of tests we have carried out in this respect are 
contained herein. 
The protective adherent coatings of the catalysts are very effective in 
sealing the surface of the substrate and provide a barrier layer, thereby 
reducing the tendency of catalyst poisons, which may be present in the 
substrate, from diffusing through the coating and poisoning the catalytic 
material. 
The alloy substrate may, for example, be an Fe-base alloy containing Al, Cr 
and Y and it may also be ferritic. A particular example of alloy substrate 
is the alloy of iron, chromium, aluminium and yttrium with proportions by 
weight lying in the range of up to 20% chromium, 0.5 to 12% aluminium, up 
to 3% yttrium, for example 0.1 to 3% yttrium, and the balance iron. The 
substrate may be used either oxidised or unoxidised, though we prefer that 
it is used oxidised. When oxidised, for example, by heating in air, the 
alloy substrate has the property of forming a surface layer which becomes 
enriched in alumina, as the temperature is increased from 700.degree. C. 
This surface layer is self-healing if cracked and may provide a key for 
the protective, adherent coating. 
We prefer that the catalytically active material in the second coating is 
in association with a refractory oxide, additional to the refractory oxide 
of the protective coating, when we particularly prefer that the associated 
refractory oxide and transition oxide of the protective coating occupy 
different sites on the substrate from the associated refractory oxide and 
catalytically active material of the second coating, so that the catalyst 
comprises a substrate carrying first entities comprising a transition 
element oxide and refractory oxide in association, and second entities 
comprising catalytic material and refractory oxide in association, wherein 
the first and second entities are separate from one another in the sense 
that they occupy different sites. A specific example of our particularly 
preferred catalyst comprises the alloy substrate carrying associated 
Al.sub.2 O.sub.3 and Cr.sub.2 O.sub.3 as the protective coating and 
associated Al.sub.2 O.sub.3 and Pt as the second coating, wherein the 
associated Al.sub.2 O.sub.3 /Cr.sub.2 O.sub.3 occupy different sites from 
the associated Al.sub.2 O.sub.3 /Pt. 
We have generally found that any tendency for associated Al.sub.2 O.sub.3 
/Cr.sub.2 O.sub.3 and associated Al.sub.2 O.sub.3 /Pt to occupy common 
sites results in a deterioration in the performance of the catalyst. 
We prefer that the refractory oxide in each of the protective and second 
coatings is of an element whose atomic number does not exceed 40 and we 
particularly prefer that it is of an element whose atomic number does not 
exceed 20. Examples of suitable refractory oxides are alumina (which is 
particularly preferred and which has already been mentioned) and silica. 
Mixed oxides may also be used. 
We also prefer to include a grain growth inhibitor for the refractory oxide 
in each of the coatings. By "grain growth inhibitor" is meant a substance 
which inhibits the grain growth of the refractory oxide when the latter is 
subjected to heat treatment. This is of particular significance in the 
context of the catalysts of the invention since the inhibitor reduces 
sintering of the refractory oxide during high temperature use of the 
catalyst and thereby reduces loss of surface area of the refractory oxide 
and hence loss of catalytic activity. We have found that the proportions 
by weight of grain growth inhibitor relative to weight of refractory oxide 
is significant. Thus, best results have been achieved using around 0.1% 
(by weight) of inhibitor based on the weight of refractory oxide. Results 
have, however, been less good using larger proportions by weight of 
inhibitor (e.g. 0.5% and greater). The grain growth inhibitor is 
preferably an oxide of a Group IIIA metal or of a rare earth metal. We 
particularly prefer to use yttria. 
The catalytic material used may, for example, be a noble metal and is 
preferably a platinum group metal, namely osmium, iridium, platinum, 
palladium, rhodium or ruthenium, when it may, for example, be composed of 
more than one platinum group metal. 
Our preferred catalyst comprises the alloy substrate carrying a protective 
adherent coating of Al.sub.2 O.sub.3 as the refractory oxide and Cr.sub.2 
O.sub.3 as the transition metal oxide and a second coating of Al.sub.2 
O.sub.3 as the refractory oxide and a platinum group metal as the 
catalytically active material. In the protective coating, we prefer that 
the ratio of Cr atoms to Al.sub.2 O.sub.3 surface sites is in the range of 
1:5 to 2:1, wherein one Cr atom per Al.sub.2 O.sub.3 surface site is 
particularly preferred. 
We have discussed above the value of associated entities in the protective 
coating occupying different surface sites from entities in the second 
coating. This may be achieved by using a refractory oxide in each of the 
coatings which has been prepared by a vapour phase condensation method. 
Such methods generally give products of high surface area (referred to as 
primary-particles), which are capable of dispersion in a liquid medium to 
give colloidal particles comprising loose aggregate structures of the 
primary-particles, in which aggregates there are point to point contacts 
between the primary particles in the structure and spaces within the 
aggregate structure. Also, the number of point to point contacts is low 
because of the loose nature of the structure. The morphology of the 
colloidal particles provides particular advantages in the context of the 
present invention in that the above mentioned contacts in the aggregate 
structure present very few opportunities for sintering in the final 
catalyst, which, if it took place, would reduce surface area and hence 
catalytic activity. 
By a vapour phase condensation method is meant a preparative method which 
passes through a vapour phase intermediate. Examples of vapour phase 
condensation methods are flame hydrolysis of volatile halides or 
alkoxides, evaporation and condensation methods using electron beam, D.C. 
arc or R.F. plasma heating, or metal oxidation (e.g. of Mg) to give a 
smoke which is then condensed. A specific example of such a method is the 
production of alumina by the hydrolytic decomposition of a corresponding 
volatile halide in a flame to give a product with substantially spherical 
primary-particles. Oxides produced in this way may, for example, have a 
particle diameter in the range 4 to 50 nm and a particular example is a 
finely divided alumina having a particle diameter of .about.10 nm and a 
surface area of .about.100 m.sup.2 /g. 
The catalysts of the invention may, if desired, be provided with a third 
coating which comprises a refractory oxide in association with an oxide of 
an element of the first period of transition elements, except iron, 
wherein the transition element oxide is different from the refractory 
oxide. Thus, the third coating, if provided, has the same general 
constitution as the first coating. We have found that, in some cases, the 
third coating may assist the performance of the catalyst. 
The catalysts of the invention may be prepared as follows, which 
constitutes a second aspect of the invention: 
(i) contacting an Al-containing Fe-base alloy substrate with a first 
dispersion in a liquid medium comprising colloidal particles of a 
refractory oxide and comprising a transition element oxide or precursor 
thereof, except iron, which is different from the refractory oxide; 
(ii) drying and firing to produce a protective coating of the refractory 
oxide in association with the transition element oxide on the substrate; 
and 
(iii) providing catalytically active material on the substrate. 
Step (iii) is preferably carried out by contacting the product of step (ii) 
with a second dispersion in a liquid medium comprising colloidal particles 
of the refractory oxide, which has been prepared by a vapour phase 
condensation method and comprising material which is catalytically active 
or is convertible to catalytically active material, and by converting the 
convertible material, if present, to catalytically active material. We 
particularly prefer to use convertible material comprising a water-soluble 
salt of a platinum group metal in solution in water as the liquid medium. 
Examples of such salts are chloroplatinic acid, Pt(NH.sub.3).sub.4 
Cl.sub.2 and rhodium trichloride which are readily converted to the metal 
by, for example, chemical or thermal reduction. The conversion of 
convertible material, if present, to catalytically active material may be 
effected by firing, provided that the convertible material is decomposable 
to catalytically active material under such conditions. However, we prefer 
to effect the conversion by reduction using a soluble carbonisable 
polymer. `Soluble` means soluble in the liquid medium, and the polymer may 
be provided in solution in the second dispersion. On firing, the polymer 
is initially carbonised and then reduction of convertible material to 
catalytic material is effected by carbon and/or carbon monoxide. For such 
a mode to be effective, the convertible material must, of course, be 
capable of being reduced in this way. A preferred example of such a 
polymer is polyvinyl alcohol (often referred to as PVA). 
If a third layer is desired in the present method, the product of step 
(iii) may be contacted with a third dispersion in a liquid medium 
comprising colloidal particles of a refractory oxide and comprising a 
transition element oxide or precursor thereof, except iron, which is 
different from the refractory oxide, followed by drying and firing. 
In each dispersion the colloidal particles dispersed in the liquid medium, 
which is most conveniently water, constitute a sol. The term "refractory 
oxide," when applied to the colloidal particles, also includes precursors 
thereof which are convertible to the oxide as such during subsequent 
firing, e.g., as in step (ii). Examples of such precursors are 
nitrate-stabilised aluminium oxy-hydroxide and aluminium chlorohydrate, 
which are alumina precursors. Likewise, the term "transition element 
oxide" when applied to the first and third dispersions, also includes 
precursors thereof which are convertible to the oxide as such during 
subsequent firing, for example convertible salts which are soluble in the 
liquid medium. A specific example of such a precursor is chromium (III) 
nitrate which, when the liquid medium is water, may be provided in 
solution in the first and/or third dispersions and converted to chromium 
(III) oxide during subsequent firing. 
A grain growth inhibitor, if required, may also be provided in each of the 
dispersions, wherein it may be an inhibitor as such or a precursor thereof 
which is convertible to the inhibitor during subsequent firing. It may be 
a compound of a Group IIIA metal or of a rare earth metal, for example a 
salt thereof which is soluble in the liquid medium, and which is provided 
in solution therein. Such salts may associate in some way with the 
colloidal particles in the dispersion, possibly by electrostatic 
attraction, to give what may be termed a "mixed sol". Alternatively, the 
inhibitor, if used in any of the dispersions, may be in the form of 
colloidal particles dispersed in the liquid medium, in which case it must 
be different from the refractory oxide. 
A preferred way of preparing the first and third dispersions is to mix a 
sol of the refractory oxide (e.g. an alumina aquasol) with a solution of a 
salt of the transition element (such as chromium nitrate). 
A further preferment is that the dispersions of the colloidal particles 
have been prepared by dispersing, in the liquid medium, a small particle 
size, high surface area form of the oxide obtained by a vapour phase 
condensation method, such as flame hydrolysis of a halide corresponding to 
the oxide, and which are previously described herein. 
The viscosity of the dispersions may be controlled to facilitate carrying 
out the transfer of material in the contacting steps. In this connection, 
we have found that lowering of the pH of a dispersion to some extent 
lowers its viscosity to facilitate the above mentioned transfer. 
The alloy substrate used may be in the form of a powder, a sheet, including 
a combination of plane and corrugated sheets as in a honeycomb structure, 
fibre, or a gauze. Step (i) may, for example, be carried out simply by 
immersing the substrate in the first dispersion and then removing it 
therefrom. This procedure is particularly appropriate when the substrate 
is in the form of a sheet. The drying in step (ii) converts the sol to the 
corresponding gel form. 
The various firing steps may suitably be carried out at a temperature 
within the range from 200.degree. C to 850.degree. C, preferably 
400.degree. C. The purpose of the firing may, for example, be for 
converting the gel of the refractory oxide to the corresponding anhydrous 
form, for improving adhesion of the refractory oxide to the substrate, or 
for converting any precursors of refractory oxides, transition metal 
oxides and inhibitors to refractory oxides, transition metal oxides and 
inhibitors as such, in addition to assisting conversion of convertible 
material to catalytically active material, as already described. 
The invention will now be particularly described in Examples 1 to 3 below. 
Experiment A below is for comparative purposes only and is not an example 
of the invention. The "Fecralloy" (Registered Table Mark) alloy used in 
the examples contained up to 20% Cr, 0.5 to 12% Al, 0.1 to 3% Y, and the 
balance Fe.

EXAMPLE 1 
(a) Preparation of Catalyst Support 
Finely powdered alumina, as prepared by a vapour phase condensation method 
and having a small particle size (.about.10 nm) and high surface area 
(.about.100 m.sup.2 /g) was dispersed in water to give a 9% by weight 
Al.sub.2 O.sub.3 aquasol. Aqueous yttrium nitrate solution was added to 
the sol to give 0.1% by weight of Y.sub.2 O.sub.3 equivalent. 120 ml of a 
50% by weight aqueous solution of Cr(NO.sub.3).sub.3.9H.sub.2 O were then 
added to one litre of the sol to give a "mixed sol". 
A sample of "Fecralloy" alloy of thickness 6.25 .times. 10.sup.-3 cm, which 
has been previously heated in air for 8-12 hours at 1000.degree. C to form 
an alumina rich surface layer, was immersed in a sample of the above mixed 
sol, removed, dried and fired in air at 850.degree. c for 15 minutes. The 
firing converted the yttrium and chromium nitrates to the corresponding 
oxides and gave a catalyst support comprising "Fecralloy" alloy carrying 
an adherent coating of Cr.sub.2 O.sub.3 + Al.sub.2 O.sub.3 + Y.sub.2 
O.sub.3. 
(b) Preparation of Catalyst 
A 9% by weight Al.sub.2 O.sub.3 aquasol was prepared as above. Aqueous 
yttrium nitrate solution was added to the sol to give 0.1% by weight of 
Y.sub.2 O.sub.3 equivalent. 15 g of H.sub.2 PtCl.sub.6.6H.sub.2 O were 
then dissolved in 1 1 of the sol and 2 ml of a 20% by weight solution of 
PVA added. A sample of the catalyst support prepared above was immersed in 
this sol, removed, dried and fired in air at 850.degree. C for 15 minutes. 
The firing carbonised the PVA, and the H.sub.2 PtCl.sub.6 was reduced to 
Pt which appeared as a dark sheen. 
The catalyst so-produced was further treated by immersing in a sample of 
the mixed sol from (a) above, removing, drying and firing in air at 
850.degree. C for 15 minutes. The provision of layers in the final 
catalyst may therefore be represented as 
##STR1## 
(c) Tests on Catalyst 
The catalyst was tested under conditions designed to simulate the high 
gaseous throughputs which obtain in a vehicle exhaust environment. 
A sample of the catalyst measuring 2 .times. 2 cm and of thickness 
6.25.10.sup.-3 cm was mounted in a silica tube and subjected to 50 .mu.1 
slugs of either carbon monoxide/oxygen or propane/oxygen or nitric 
oxide/carbon monoxide, flowing at a velocity of 50 ml/min, i.e. the space 
velocity of the gases was equivalent to the volume of the catalyst being 
displaced 120,000 times per hour, which is near to the upper limit of a 
typical exhaust system. The temperature required to cause complete 
conversion of the noxious components was noted. The catalyst was then 
subjected to an acelerated ageing procedure by being heated in air at 
1,000.degree. C for 8 hours, and the above measurements repeated. The 
results appear in Table 1 below. 
EXAMPLE 2 
The procedure of Example 1 was repeated with the exception that the firing 
temperature was 400.degree. C at each stage. The test results, together 
with the test results for Example 1, are summarised in Table 1 below. 
TABLE I 
__________________________________________________________________________ 
Complete Conversion Temperatures at a Space 
Velocity of 120,000 Displacements per Hour 
Nitric Oxide 
Reduction Carbon Monoxide 
by Carbon Monoxide 
Propane Oxidation 
Oxidation 
After Heating 
After Heating 
After Heating 
New at 1000.degree. in 
New at 1000.degree. in 
New at 1000.degree. C in 
Example 
Catalyst 
air for 8 hrs 
Catalyst 
air for 8 hrs 
Catalyst 
air for 8 hrs 
__________________________________________________________________________ 
1 400.degree. C 
430.degree. C 
400.degree. C 
400.degree. C 
210.degree. C 
240.degree. C 
2 320.degree. C 
390.degree. C 
350.degree. C 
350.degree. C 
180.degree. C 
220.degree. C 
__________________________________________________________________________ 
The above results show that the catalysts tested were very effective in the 
catalysis of the three specified reactions and that this effectiveness was 
only slightly reduced (in the case of propane oxidation, not at all) after 
carrying out the specified thermal treatment. The results also show that 
carrying out the firing at 400.degree. C rather than 850.degree. C may be 
beneficial, particularly in the case of NO reduction by CO. 
EXAMPLE 3 
Preparation of Catalyst 
A "Fecralloy" alloy substrate was constructed from 6 inches wide strips, 
one strip having corrugations 0.009 inch deep, and the other having 
corrugations 0.040 inch deep superimposed on previously applied 0.009 inch 
deep corrugations. These strips were placed one on the other and rolled up 
to form a substrate, of length 6 inches and diameter 4.1 inches. The 
substrate was then oxidised in air at 1000.degree. C for 2 hours, instead 
of 8-12 hours pretreatment. A catalyst was applied to each layer as 
follows: 
(a) The substrate was immersed in the sol described in Example 1 (a), then 
removed, dried and fired at 400.degree. C in air for 15 minutes. 
The substrate was then immersed in a 13.5 % by weight Al.sub.2 O.sub.3 
aquasol, made from the same finely divided alumina as described in Example 
1, and containing aqueous yttrium nitrate solution, added to the sol to 
give 0.1% by weight Y.sub.2 O.sub.2 equivalent, then removed, dried and 
fired at 850.degree. C in air for 15 minutes. 
(b) The substrate was then immersed in a sample of a sol as used in (a) 
above but to which had been added 15 g of H.sub.2 PtCl.sub.6.6H.sub.2 O 
per litre and 2 ml of a 20% by weight solution of PVA per litre. The 
substrate was then removed, dried and fired at 850.degree. C in air for 15 
minutes. The final catalyst which may be represented as 
##STR2## 
was found to have a platinum loading of 22 g/cu ft. 
EXPERIMENT A 
Preparation of Catalyst 
A substrate as used in Example 3 was treated by 
(a) Dipping in a 13.5% by weight Al.sub.2 O.sub.3 aqusol, made from the 
same finely divided alumina as described in Example 1, and containing 
aqueous yttrium nitrate solution, added to the sol to give 0.1% by weight 
Y.sub.2 O.sub.3 equivalent. The substrate was then removed, dried, and 
fired at 850.degree. C in air for 15 minutes. 
(b) To a sample of the above sol was added 15 g H.sub.2 PtCl.sub.6.6H.sub.2 
O per litre and 2 ml of a 20% by weight solution of PVA per litre. The 
substrate was immersed in this, then removed, dried and fired at 
850.degree. C in air for 15 minutes. 
The final catalyst which may be represented as 
##STR3## 
was found to have a platinum loading of 22 g/cu ft. 
Tests on Catalysts of Example 3 and Experiment A 
It will be noted that the substrate used in Example 3 was treated in the 
same way as that used in Experiment A and had the same platinum loading, 
with the exception that the substrate in Example 3 had a prior Cr.sub.2 
O.sub.3 /Al.sub.2 O.sub.3 /Y.sub.2 O.sub.3 barrier layer. 
Each catalyst was tested as follows. The catalyst was mounted on the 
exhaust system of a 2 litre petrol engine (Triumph Dolomite), running on 
low lead (0.034 gm/U.S. gal) fuel. The engine was cycled between idle and 
70 MPH road load at 1 minute intervals for 3 hours, using a procedure 
described by Haslett, viz., "A Technique for Endurance Testing of 
Oxidation Catalytic Reactors", Automotive Engineering Congress, Detroit, 
MI 1974. 
The catalyst were then removed, mounted on the exhaust system of a 1.8 
liter Austin Marina car, and used to control the exhaust emissions over a 
standard U.S.A. Environmental Protection Agency driving cycle, the 
emissions being measured by a Constant Volume Sampling (C.V.S.) Technique. 
The results for each catalyst are summarised below, where the figures are 
in g/mile. 
______________________________________ 
EXPERIMENT A CATALYST 
EXAMPLE 3 CATALYST 
Carbon Carbon 
Hydrocarbons 
Monoxide Hydrocarbons 
Monoxide 
______________________________________ 
0.16 0.92 0.06 0.71 
EMISSION CHARACTERISTIC OF TEST VEHICLE 
(WITHOUT CATALYST) 
Carbon 
Hydrocarbons Monoxide 
______________________________________ 
0.89 12.79 
______________________________________ 
These results illustrate the improvement in durability obtained by the use 
of the chromium oxide/alumina/yttria coating of the invention. 
This benficial coating has also been applied to "Fecralloy" alloy, 
preoxidised at temperatures down to 400.degree. C for times as short as 15 
minutes.