Catalyst carrier and method for producing same

In order to provide a temperature- and acid-resistant catalyst carrier based on a gas-pervious carrier body and a porous surface layer having a high silica content, while dispensing with a washcoat, the carrier body and the surface layer are designed as a chemically and physically homogeneous structure shaped in a single operation. The SiO.sub.2 content amounts to at least 99% by weight and has a specific surface from 5 m.sup.2 /g to 50 m.sup.2 /g. In a process for producing a catalyst carrier made of synthetic, amorphous silica particles with more than 99% by weight SiO.sub.2, a paste is prepared with the particles, a liquid, and a binder. Plastifying agents are added to the paste and the paste is continuously extruded into a monolithic greenware strand. The alkali and alkaline earth content of the paste is set at no more than 200 ppm and the greenware is cut to length and sintered at a temperature in a range between 800.degree. C. and 1400.degree. C.

BACKGROUND OF THE INVENTION 
The invention relates to a catalyst carrier a gas permeable support and 
with a porous surface coating of high silica content. The invention 
furthermore relates to a method for producing a catalyst support from 
synthetic, amorphous silica particles with an SiO.sub.2 content of more 
than 99%, by weight, preferably more than 99.5%, the silica particles 
being made into a plastic dough and the dough being shaped into a 
greenware and then sintered at high temperature. 
Catalysts are used in the chemical and pharmaceutical industry, among other 
industries, in the manufacture of fine chemicals and for cleaning exhaust 
gases in industrial plants as well as in gas, gasoline and Diesel engines. 
The application determines the choice of the materials and the form of the 
catalyst support. Usually it consists of a support body of a chemically 
inert material, onto which the catalytically active substance is applied. 
In general, the greater the surface area, the better the catalyzing 
action. Therefore support bodies of great specific surface area are 
preferred. Often, however, the materials suitable as support material do 
not have the required great specific surface area. In general, the support 
body is then provided with a surface coating referred to as a "wash coat," 
which consists of a material of great specific surface area. 
A catalyst support of this kind is disclosed in U.S. Pat. No. 3,804,647. In 
the catalyst support described therein, on the surface of a monolithic, 
gas-permeable support body, which can consist of a ceramic, glass-ceramic 
or vitreous composition, a slip coat is deposited, which contains a finely 
ground, porous borosilicate glass with a content by weight of 96 percent 
SiO.sub.2. After it is applied the slip coat is dried and sintered on 
tightly at about 800.degree. C. It then serves as a wash coat to increase 
the specific surface area of the catalyst support. The sintering 
temperatures and sintering time of the slip coat are coordinated such that 
any melting of the porous borosilicate glass content will be prevented so 
as to retain its high specific surface area insofar as possible. 
The catalyst support thus prepared can be made with a honeycomb structure 
and used in conjunction with a catalytic metal coating as an exhaust gas 
catalyst up to temperatures of about 870.degree. C. However, due to the 
relatively low resistance of the borosilicate glass content to creeping, 
the specific surface area of the surface layer quickly decreases, and with 
it the catalytic effect. On account of its layered construction of 
materials of different thermal expansion, such catalyst supports have a 
relatively low strength; especially where high temperatures are used the 
surface layers spall off. It has also been found that the acid resistance 
of the support and of the surface layers of the known catalyst supports is 
insufficient in many applications, such as the cleaning of Diesel motor 
exhausts containing sulfur dioxide. 
Wash coats are widely used which are made on an Al.sub.2 O.sub.3 basis. 
These, however, have the disadvantage that the Al.sub.2 O.sub.3 undergoes 
a phase conversion at about 700.degree. C. which results in a reduction of 
the specific surface area such that the specific surface area of the 
coating tends toward zero. Thus the catalytic activity is also lost. 
Basically, for the application of the wash coat, a number of steps are 
necessary, each requiring a quality assurance step. The process for the 
manufacture of such catalyst supports is thus expensive and complicated. 
German Patent Application DE-A1 39 12 504 discloses a method for preparing 
a catalyst support in the form of compacts, in which pyrogenically made 
silicon dioxide particles are homogenized with urea, methyl cellulose, 
aluminum stearate and/or magnesium stearate as well as graphite, with the 
addition of water. The dough thus prepared is then dried at a temperature 
of 80.degree. C. to 120.degree. C. and again crushed to powder. This 
powder is then pressed to form compacts and heat treated for a period of 
0.5 to 8 hours at a temperature of 400.degree. C. to 1200.degree. C. 
By the process disclosed in DE-A1 39 12 505 catalyst supports can be made 
in the form of pill-like compacts in, for example, cylindrical, spherical 
or annular shapes, with an outside diameter of 2 mm to 15 mm. 
Pyrogenically made silicon oxides are characterized by extremely fine 
particles and an accordingly low space-filling quality. Due to this 
fineness, shaping them into the known catalyst compacts of simple 
geometrical shape presents a number of difficulties. The production of 
catalyst supports with a filigree structure is not possible by the known 
method. 
SUMMARY OF THE INVENTION 
The present invention is addressed to a gas-permeable, heat- and 
acid-resistant catalyst support in which the application of a wash coat 
can be dispensed with, as well as a simple method of manufacturing 
catalyst supports resistant to fracture and creep continuously and at low 
cost. 
According to the invention as regards the catalyst support that the support 
body and the surface layer form a chemically and physically uniform 
structure which is shaped in one common procedure, has an SiO.sub.2 
content of at least 99 wt. %, and has a specific surface area between 5 
m.sup.2 /g and 50 m.sup.2 /g. Since the support body and the surface layer 
form a chemically and physically uniform structure shaped in one common 
procedure, any flaking off of the surface layer is excluded. The catalyst 
support has a chemically uniform structure in which tensions caused, for 
example, by materials of different thermal expansion coefficients when the 
temperature changes, cannot occur. For this use of the catalyst support 
according to the invention in the cleaning of exhaust gases a 
gas-permeable structure is necessary. Particularly in the case of cleaning 
exhaust gases in the automotive field catalyst supports with a honeycomb 
structure have become popular. The production of such a structure, wherein 
the support body and the surface coating are formed in a common procedure, 
is possible by extrusion, for example. 
On account of the high SiO.sub.2 content of the surface coat and support 
body, of at least 99 wt. %, the catalyst support can be used, on the one 
hand, at high temperatures such as, for example, 1000.degree. C. and more, 
and on the other hand can be used even in an environment requiring high 
acid-resistance, without the occurrence of any marked alterations of the 
support and surface coat. 
Since the specific surface area of the catalyst support is between 5 
m.sup.2 /g and 50 m.sup.2 /g, surface areas can be obtained that are 
sufficiently great for many applications, even if the catalytic substances 
are applied directly to the surface coat on the catalyst support according 
to the invention. The specific surface area is established at a maximum of 
50 m.sup.2. It has been found that pieces with a specific surface area up 
to 50 m.sup.2 /g have sufficient mechanical strength and at the same time 
a favorable pore distribution for catalytic purposes. The formation of 
micropores contributing to a greater consumption of noble metals but not 
participating in the catalytic process is suppressed. 
A catalyst support with a specific surface area ranging between 15 m.sup.2 
/g and 30 m.sup.2 /g has proven especially advantageous. This range has 
proven desirable especially with an eye to a sufficiently great surface 
area of the catalyst support with no additional wash coat, combined with 
as low a consumption as possible of catalytically active coating material. 
Particularly for great mechanical strength and high chemical resistance to 
acids, a catalyst support has proven desirable in which the SiO.sub.2 
content is at least 99 wt. % and the alkali and alkaline earth content is 
established at a level of no more than 200 ppm, preferably no more than 50 
ppm. The low alkali and alkaline earth content permits the catalyst 
support to be sintered from powder of high silicic acid content at 
relatively high temperatures without observing any formation of 
cristobalite and the accompanying destruction or weakening of the sintered 
catalyst support. The relatively high sintering temperatures, however, 
contribute to a high mechanical strength of the catalyst support, as is 
needed in many applications, such as the cleaning of automotive exhaust 
gas. 
Catalyst supports which contain metal oxides of Groups III to VI of the 
transition metals and rare earths have proven to be especially creep 
resistant. Also, the doping on of metal oxide in the form of aluminum 
oxide has proven valuable in this regard. Such metal oxides contribute not 
only to the stabilization of the specific surface area but also to the 
fixation of the alkali traces remaining in the glass, thereby also 
improving the mechanical stability of the catalyst support. Such catalyst 
supports are used especially in the conduct of processes in which the 
preservation of the catalytic effect at high temperatures is more 
important than chemical acid-resistance. It has proven advantageous, 
however, to limit the total content of metal oxides to no more than 5000 
ppm. 
In regard to the manufacturing process, plasticizers are added to the 
composition and the composition is pressed to form a monolithic greenware 
strand, wherein the composition's combined content of alkali and alkaline 
earth is established at not more than 200 ppm, preferably not more than 50 
ppm, and the greenware is sintered at a temperature in the range between 
800.degree. C. and 1000.degree. C. 
The addition of plasticizers permits the strand of greenware to be made by 
the continuous strand pressing method known in connection with ceramic 
compositions. 
However, it is essential to the suitability of the composition for forming 
a honeycomb catalyst support appropriate for high temperatures and having 
a high resistance to fracture that the alkali and alkaline earth content 
of the composition be established at a maximum of 200 ppm, preferably at 
no more than 50 ppm, and that the greenware be sintered at a temperature 
in the range between 800.degree. C. and 1400.degree. C. With this content 
of alkali and alkaline earth, it has been found that the formation of 
cristobalite, which otherwise is to be expected in sintering at relatively 
high temperatures, and the resultant destruction or weakening of the 
sintered body against mechanical stress, is prevented and the required 
specific surface area is maintained. The relatively high sintering 
temperatures assure, on the other hand, a high mechanical strength in the 
sintered catalyst supports, combined with great specific surface area. 
Prior to sintering, the greenware strand is usually dried and, if 
necessary, cleaned. 
A process has proven to be especially advantageous in which the greenware 
is sintered at a temperature in the range between 950.degree. C. and 
1150.degree. C. A high mechanical strength is thereby achieved, as well as 
temperature stability at the correspondingly high application 
temperatures. 
The use of pyrogenically made silica particles is especially suitable for 
the production of a catalyst support by the method of the invention. These 
are characterized by extreme fineness and a correspondingly great specific 
surface area, very high purity, uniform particle shape, and the absence of 
internal porosity. Powders of pyrogenic silica particles with a specific 
surface area between 50 m.sup.2 /g and 100 m.sup.2 /g have proven to be 
especially appropriate. 
A process in which silica particles with a mean diameter in the range 
between 10 nm and 40 nm are used has proven to be especially advantageous. 
Such powders are characterized by a high sintering activity and enable the 
production of catalyst supports with great specific surface areas. 
It is expedient to use agglomerated silica particles in the process. These 
agglomerated silica particles can be calcined, preferably at temperatures 
between 500.degree.and 1200.degree. C. 
The agglomeration is performed preferably by spray drying, by which very 
uniform, fine granules can be produced. Using a pressure nozzle and a 
suspension of low solid content, virtually spherical hollow granules of 
little shell thickness can be produced. These hollow granules are 
partially destroyed in the extrusion process and provide good toothing 
(stiffness) in the extrusion composition without negatively affecting the 
extrusion. 
It can furthermore be advantageous to use a mixture of agglomerated and 
unagglomerated silica particles. The use or addition of agglomerated 
silica particles brings the advantage that the extruded composition has 
better stability of shape (greater stiffness), and after extrusion shows 
less tendency to sag (deform of its own weight). Another advantage is that 
the drying can be done faster without the risk of shrinkage cracking. The 
occurrence of shrinkage cracking is reduced overall. 
In the sintering of the greenware strand, the temperature, the holding time 
and the atmosphere are preferably selected such that the catalyst support 
has, after sintering, a specific surface area between 5 m.sup.2 /g and 50 
m.sup.2 /g, preferably between 15 m.sup.2 /g and 30 m.sup.2 /g. With such 
specific surface areas it is possible in the case of many areas of 
application to dispense with the wash coat usually applied to the surface, 
whereby a suitable specific surface area is prepared for the subsequent 
coating with the catalytically active metal. The method of the invention 
is appropriate especially for the extrusion of catalyst supports with a 
honeycomb structure, such as those used for cleaning exhaust gases of 
automobiles, especially in the case of acid exhausts of Diesel engines.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The catalyst according to the invention, and the method of its manufacture 
will be further explained below with the aid of exemplary embodiments. 
Embodiment 1 
A plastic dough is prepared from the following substances: 
4252 g of pyrogenic silica with a BET surface area of 50 m.sup.2 /g and a 
total alkali content of less than 34 ppm, 
3087 g of deionized water with a conductivity of 1 S. 
190 g of methyl cellulose with a total alkali content of less than 600 ppm, 
80 g of fatty acid with a total alkali content of less than 100 ppm, 
25 g of polyglycol with a total alkali content of less than 20 ppm. 
The dough thus prepared is mixed for 15 minutes in a Z-arm kneader and then 
extruded with a piston extruder to a monolithic greenware strand with a 
honeycomb structure. For this purpose a round nozzle with a square cell 
structure is used. The cell density is 400 cells per square inch. 
Greenware strands with a diameter of 40 mm and a length of 1000 mm are 
extruded. These are then cut into 300 mm lengths and dried in a microwave 
oven. For sintering, the lengths are heated in a sintering furnace with a 
temperature rise rate of 5.degree. C./min to a temperature of 1050.degree. 
C., and held at this temperature for 30 minutes. The air feed into the 
sintering furnace is advantageously set at about 100 1/h and kept 
constant. 
The silica honeycomb bodies thus prepared have a specific surface area of 
34 m.sup.2 /g. As it appears from the weight-parts of the above-listed 
substances, the silica content of these honeycombs after burning out the 
organic substances and eliminating the moisture is better than 99.9 wt. %. 
They can be provided directly with the active catalyst coating without 
applying the usual wash coat. 
Embodiment 2 
The same dough composition as in Embodiment 1 is prepared. As additional 
components 15 g of pyrogenic Al.sub.2 O.sub.3 with a specific surface area 
(BET) of 400 m.sup.2 /g and 6 g of cerium nitrate are added. 
Greenware strands are produced from the plastic composition by the method 
described in Embodiment 1. They are heated at a temperature increase rate 
of 5.degree. C./min to a temperature of 1150.degree. C. and held at this 
temperature for 30 minutes. The air feed into the sintering furnace is set 
at about 100 1/h and held constant. 
The specific surface area of the honeycomb body thus produced amounts to 38 
m.sup.2 /g; its SiO.sub.2 content is more than 99.5 wt. %. 
Even with this catalyst support the wash coat for increasing the specific 
surface area can be dispensed with. 
By the addition of pyrogenic Al.sub.2 O.sub.3 a sufficiently great specific 
surface area is obtained despite the higher sintering temperature. The 
higher sintering temperature also results in a greater strength in the 
SiO.sub.2 honeycomb, which is suitable also for use at especially high 
temperatures on account of its great resistance to creep. 
Embodiment 3 
A greenware strand is extruded in a continuous extruder from a dough made 
from 
4000 g of a suspension consisting of 30wt. % of pyrogenic silica with a BET 
surface area of 80 m.sup.2 /g and an alkali content of 20 ppm, and 70 wt. 
% of deionized water with a conductivity of 1 .mu.S, 
1657 g of pyrogenic silica with a BET surface area of 80 m.sup.2 /g and a 
total alkali content of 20 ppm, 
172 g of methyl cellulose with a total alkali content of less than 600 ppm, 
80 g fatty acid with a total alkali content of less than 90 ppm, and 
23 g polyglycol with a total alkali content of less than 
20 ppm 
after homogenization for 30 minutes in a Z-arm kneader. For this purpose 
the extruder is equipped with a square nozzle with a cell density of 200 
cells per square inch. Honeycomb strands with an edge length of 50 mm and 
a length of 1000 mm are drawn. These strands are cut to lengths of 200 mm 
and dried in a convection oven. 
Then the pieces thus produced are heated at a rate of temperature rise of 
about 10.degree. C./min to a temperature of 1000.degree. C. and held at 
this temperature for 60 minutes. 
The honeycomb bodies thus made have a specific surface area of 28 m.sup.2 
/g; they can also be provided directly with a catalytically active 
coating. 
Embodiment 4 
Using the following substances: 
3000 g of pyrogenic silica with a BET surface of 100 m.sup.2 /g and a total 
alkali content of less than 10 ppm 
30 g of pyrogenic Al.sub.2 O.sub.3 with a BET surface of 400 m.sup.2 /g 
400 g of spray granules of a hollow shape with an average grain size of 150 
.mu.m, prepared from pyrogenic silica with a BET surface of 100 m.sup.2 
/g, calcined at 800.degree. C. in air 
3100 g of deionized water with a conductivity of 0.2 .mu.S 
190 g of methyl cellulose with a total alkali content of less than 300 ppm 
80 g of fatty acid with a total alkali content of less than 100 ppm 
25 g of polyglycol with a total alkali content of less than 20 ppm 
a homogeneous dough is prepared in a polymer-lined intensive mixer equipped 
with knife heads, and is then extruded in a continuous extruder to a 
greenware strand. To this end the extruder is equipped with an oval nozzle 
with a cell density of 600 cells per sq. inch. Honeycomb strands 1 m long 
are made. The honeycomb strands are cut into 200 mm lengths and dried in a 
microwave oven. The pieces thus made are heated up to 460.degree. C. at a 
temperature increase rate of 4.degree. C./min and held at this temperature 
for 60 minutes; then they are heated at a temperature rise of 10.degree. 
C./min to 1300.degree. C. and held at this temperature for 30 minutes. 
The honeycomb pieces thus made have a specific surface area of 48 m.sup.2 
/g. They can be provided directly with a catalytically active coating 
without applying a wash coat. 
The honeycomb pieces thus made can be used at temperatures up to 
1000.degree. C. with additional exposure to acid, without damage.