U.S. patent number RE31,719 [Application Number 06/384,966] was granted by the patent office on 1984-10-30 for supported catalyst for purifying gas and method of manufacturing the same.
This patent grant is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Kunio Kimura, Atsushi Nishino, Kazunori Sonetaka.
United States Patent |
RE31,719 |
Sonetaka , et al. |
October 30, 1984 |
Supported catalyst for purifying gas and method of manufacturing
the same
Abstract
The present invention discloses an improved supported catalyst
for gas purification which comprises a catalyst carrier composed of
a molded body including alumina cement having a CaO component in an
amount of less than 40% by weight, an Al.sub.2 O.sub.3 component in
an amount of more than 35% by weight and an iron oxide component in
an amount of less than 20% by weight. Additionally, catalytic
materials and auxiliary materials may be added, depending on
necessity. A platinum group metal is then supported on the catalyst
carrier. There is also disclosed a method for manufacturing such
catalyst. .Iadd.
Inventors: |
Sonetaka; Kazunori (Hirakata,
JP), Nishino; Atsushi (Neyagawa, JP),
Kimura; Kunio (Hirakata, JP) |
Assignee: |
Matsushita Electric Industrial Co.,
Ltd. (JP)
|
Family
ID: |
14341636 |
Appl.
No.: |
06/384,966 |
Filed: |
June 4, 1982 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
936507 |
Aug 24, 1978 |
04211672 |
Jul 8, 1980 |
|
|
Foreign Application Priority Data
|
|
|
|
|
Aug 26, 1977 [JP] |
|
|
52-102971 |
|
Current U.S.
Class: |
502/63; 502/250;
502/324; 502/328 |
Current CPC
Class: |
B01D
53/34 (20130101); B01D 53/62 (20130101); B01J
23/78 (20130101); B01J 23/40 (20130101); B01D
53/8609 (20130101) |
Current International
Class: |
B01D
53/62 (20060101); B01D 53/34 (20060101); B01D
53/86 (20060101); B01J 23/40 (20060101); B01J
23/78 (20060101); B01J 23/76 (20060101); B01J
021/12 (); B01J 021/58 (); B01J 023/78 () |
Field of
Search: |
;252/455R,455Z,466J,466PT,466B ;423/213.5,239
;502/63,250,324,328 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Shine; W. J.
Attorney, Agent or Firm: Wenderoth, Lind & Ponack
Claims
What is claimed is:
1. A catalyst for use in purification of a gas which comprises a
catalyst carrier composed of alumina cement having a CaO component
of less than 40% by weight, an Al.sub.2 O.sub.3 component of more
than 35% by weight and a Fe.sub.2 O.sub.3 component of less than
20% by weight and at least one aggregate selected from the group
consisting of silica aggregate, silica-alumina aggregate, and
alumina aggregate, and a platinum group metal supported on said
catalyst carrier in an amount of 0.001 to 0.1% by weight, said
catalyst being produced by the steps comprising:
a. molding a molded body from a mixture comprising alumina cement
and said aggregate;
b. curing said molded body for the hardening thereof,
c. subsequently .Iadd.drying and/or .Iaddend.heat treating said
hardened molded body .[.at a temperature range from 250.degree. to
700.degree. C..]. to obtain a catalyst carrier; and
d. causing a platinum group metal to be supported on said catalyst
carrier by steps comprising:
1. contacting said carrier with a solution of a salt of said
platinum group metal to impregnate said solution on the surface of
said carrier; and
2. subsequently heat treating said catalyst impregnated carrier at
a temperature .[.below.]. .Iadd.from 250.degree. to
.Iaddend.700.degree. C. to convert said platinum group metallic
salt into a platinum group metal.
2. .[.A.]. .Iadd.The .Iaddend.catalyst as claimed in claim 1,
wherein said catalyst carrier further includes at least one oxide
of manganese and copper.
3. .[.A.]. .Iadd.The .Iaddend.catalyst as claimed in claim 1
wherein said catalyst carrier includes at least one member selected
from the group consisting of ferrites and zeolites.
4. .[.A.]. .Iadd.The .Iaddend.catalyst as claimed in claim 1,
wherein said catalyst carrier further includes an auxiliary agent
composed of at least one of asbestos and glass fiber.
5. A method of manufacturing a supported catalyst as claimed in
claim 1, further employing a solvent medium for said solution of
platinum group metallic salt, which medium is at least one member
selected from the group of water and alcohol. .Iadd. 6. The
catalyst as claimed in claim 1, wherein said drying step is carried
at a temperature of 80.degree. C. .Iaddend..Iadd. 7. The catalyst
as claimed in claim 1, wherein said heat treating step c. is
carried at a temperature range from 250.degree. to 700.degree. C.
.Iaddend.
Description
This is a reissue application of U.S. Pat. No. 4,211,672 which
matured from Ser. No. 936,507, filed Aug. 24, 1978. .Iaddend.
BACKGROUND OF THE INVENTION
The present invention relates to gas purification for atmospheric
pollution prevention and more particularly, to a gas purification
catalyst especially intended for purification of exhaust gases
including lamp black or soot, odor, noxious compounds, and the
like, and generated, for example, from various kinds of household
or home use burning and cooking appliances utilizing petroleum,
gas, briquet, etc., and manufacturing method for such a
catalyst.
Compositions of the exhaust gases developed from the home use
appliances, etc., as described above are not the same, but differ
according to the appliances employed, and these compositions are,
for example, carbon monoxide due to incomplete combustion,
bydrocarbons, especially olefin group hydrocarbons in the case of
burning appliances, and mainly aliphatic or fatty acid group
hydrocarbons and kinds of aldehydes in the case of cooking
appliances.
Recently, owing to increased air tightness in buildings following
the spread of aluminum sash window frames and tendency to
multistory construction of apartment houses, mansions, etc.,
ventilation of indoor air has become very difficult. Under such
circumstances, it is strongly desired to increase safety of the
household burning appliances and also to eliminate smoke and odor
from exhaust gases generated, for example, during cooking.
Meanwhile, in the home use burning appliances, technical innovation
is under way for higher performance and lower price, and thus
development of catalysts of low cost is essential to cope with such
technical progress and state of the market in this line of
trade.
Conventionally, various kinds of catalysts have been proposed and
put into production for meeting the requirements as described in
the foregoing, the outstanding catalysts being precious metal
catalysts and metallic oxide catalysts. Among known metal
catalysts, platinum, palladium or platinum black have been regarded
as particularly suitable. However, platinum catalysts
conventionally proposed are generally high in cost, and although
the metallic oxide catalysts are cheaper than such platinum
catalysts, they are still expensive to be used for the household
burning appliances. The high cost of the platinum catalysts is
atrributable not only to expensiveness of the platinum itself, but
to the fact that the alumina molded item to be employed as a
carrier is expensive and that the manufacturing process in which
platinum is caused to be supported on the carrier is rather
complicated. The carrier for the catalysts is not limited to
alumina, but heat-resistant, chemically inactive and porous
substances such as zircon, schreit-sillimanite, magnesium silicate,
aluminosilicate, etc. may be employed as disclosed, for example, in
Japanese patent publication Tokkosho 47-50980 although those having
alumina as main component are mainly employed for practical use.
Meanwhile, catalyst employing porous metal as carrier have also
been put into production recently.
The substances to be employed as carrier may be broadly divided
into ceramic materials and metallic materials, and various methods
have conventionally been proposed for manufacturing such carriers.
However, the catalysts produced by employing such carriers have
advantages and disadvantages of their own, and are expensive, thus
requiring further improvement for application thereof to the home
use burning appliances, etc.
SUMMARY OF THE INVENTION
Accordingly, an essential object of the present invention is to
provide an improved catalyst for gas purification which enables
burning appliances and the like to be used safely and comfortably
even during abnormal conditions of use as well as normal conditions
of use of such burning appliances, with simultaneous purification
of smoke and ordor of exhaust gases through oxidation during use of
cooking appliances and the like.
Another important object of the present invention is to provide an
improved catalyst of the above described type which is stable in
functioning with high performance and superior catalytic activity
and yet, is available inexpensively.
A further object of the present invention is to provide a method of
manufacturing an improved catalyst of the above described type
through simple processes on a large scale and at low cost.
In accomplishing these and other objects, according to one
preferred embodiment of the present invention, the supported
catalyst for gas purification comprises a catalyst carrier composed
of a mold body including: alumina cement having line component of
less than 40% by weight, alumina component of more than 35% by
weight and iron oxide component of less than 20% by weight,
additives such as catalytic materials and auxiliary materials
depending on necessity, and a catalyst be supported on the catalyst
carrier. The catalyst carrier includes alumina cement of more than
15% by weight, and the aggregate of less than 85% by weight. By the
structure as described above, improved catalysts with superior gas
purification performance are advantageously presented through a
simple process at low cost.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and features of the present invention will
become apparent from the following description taken in conjunction
with the preferred embodiment thereof with reference to the
accompanying drawings, in which;
FIG. 1 is a graph showing the results of measurements of spcific
surface areas by BET method when catalyst carriers made of alumina
cement are heat-treated at various temperatures,
FIG. 2 is a graph showing the relation between CO purification
rates and iron oxide content in alumina cement for catalyst carrier
composed of alumina cement and silica sand,
FIG. 3 is a graph showing a comparison of the CO purification rate
of the catalyst carrier of the present invention and that of the
conventional catalyst carrier,
FIG. 4 is a graph showing a comparison of the CO purification rate
of the catalyst carrier of the present invention and that of the
conventional catalyst carrier, each subjected to different
temperatures for heat treatments,
FIG. 5 is a graph showing a comparison of platinum supported
amounts on catalyst carriers and CO purification rates between the
catalyst carrier of the present invention and a conventional
catalyst carrier,
FIG. 6 is a graph showing a comparison of CO purification rates of
catalysts according to the present invention and conventional
catalysts,
FIG. 7 is a schematic diagram of a petroleum heater employed for
life test of catalysts according to EXAMPLE 2 of the present
invention,
FIG. 8 is a graph showing results of life tests of catalysts
according to EXAMPLE 2 of the present invention, and
FIG. 9 is a graph showing a comparison of life tests of the
catalysts of the present invention and conventional catalysts by
the use of a petroleum fueled warm air blower.
Before the description of the present invention proceeds, it is to
be noted that like symbols represent like items throughout several
graphs of the accompanying drawings.
DETAILED DESCRIPTION OF THE INVENTION
The present invention seeks to provide an improved supported
catalyst low in cost and superior in catalytic activity through
elimination of disadvantages inherent in the conventional
catalysts. The supported catalyst of the present invention, as
described in the foregoing, comprises a molded body which contains
alumina cement having lime aluminate as main component which molded
body is employed as carrier on which the catalyst is to be
supported. For causing the catalyst to be supported on the carrier,
a metallic salt of the catalyst is caused to adhere to the surface
of the carrier through a suitable method of application and then
the metallic salt is converted into a catalytic substance through
drying, heat treatment or reduction treatment.
More specifically, main components of the alumina cement employed
herein are (1) a lime component in an amount of less than 40 weight
%, (2) an alumina component in an amount of more than 35 weight %,
and (3) an iron oxide component in an amount of less than 20 weight
%.
In the carrier composed of the alumina cement, various additives
may be included depending on necessity. Although metals of platinum
group are mainly used for the catalyst, other metals and metallic
oxides may also be used.
It should be noted here that the catalyst according to the present
invention is low in cost and superior in catalytic activity, heat
resistance, abrasion resistance, etc.
In the first place, the main component of the catalyst carrier
employed in the present invention is the alumina cement which is
differentiated from Portland cement, and the alumina cement is
generally represented by mAl.sub.2 O.sub.3.nCaO, while Portland
cement is denoted by m'SiO.sub.2.n'CaO.
Although large in demand and consequently low in cost, Portland
cement has disadvantages in that it is rather inferior in the heat
resistance and slow in the speed of hardening. On the other hand,
the alumina cement is high in the heat resistance and hardening
speed, and therefore, preferable from the viewpoint of catalyst
manufacturing. The alumina cement has the composition as described
earlier, and although mechanical strength of the carrier is
increased when the CaO component thereof exceeds 40% by weight, its
heat resistance conversely decreases, with simultaneous reaction
thereof with heavy metalic oxides at high temperatures so that
manganese oxides form CaMn.sub.2 O.sub.4, etc. for example, at
temperatures higher than approximately 650.degree. C., thus giving
rise to thermal destruction of the catalyst. Meanwhile, the heat
resistance thereof is increased, if the CaO component is small, but
its mechanical strength is conversely decreased, with prolonged
curing time during molding, thus resulting in reduction of
productivity. Additionally, the heat resistance is decreased when
the Al.sub.2 O.sub.3 component is reduced to lower than 35% by
weight, while said heat resistance is improved, as the Al.sub.2
O.sub.3 component is increased. Moreover, when the iron oxide
component exceeds 20% by weight, the mechanical strength during
heating is reduced, with simultaneous reduction of the heat
resistance. The iron oxide as described above has a catalytic
function for gas purification, for example, for purification of
carbon monoxide at temperatures higher than approximately
300.degree. C. For causing such promoting effect to be displayed,
it is preferable that the iron oxide be contained at more than 2%
by weight.
The preferable compositions of the alumina cement are CaO component
of 15 to 40 weight % and particularly of 30 to 40 weight %,
Al.sub.2 O.sub.3 component of 35 to 80 weight % and particularly of
40 to 60 weight %, and iron oxide component of 0.3 to 20 weight %
and particularly of 2 to 10 weight %.
Moreover, Portland cement described earlier can not withstand
temperatures higher than 300.degree. C., and thus, is not suitable
for the gas purification purpose of the household burning
appliances in which temperatures of the catalyst are likely to
exceed approximately 300.degree. C. Although the alumina cement can
sufficiently withstand temperatures higher than 300.degree. C., it
is preferable to employ high alumina cement for withstanding
temperatures higher than approximately 700.degree. C.
It should be noted here that since the alumina cement functions as
binding agent in the catalyst carrier employing such alumina
cement, the amount of the alumina cement should be at least 15% by
weight. If the amount of the alumina cement as described above is
small, the molded item is undesirably low in mechanical strength,
with small surface harness (abrasion resistance strength) and
decreased surface area as carrier.
The present inventors have made various investigations into the
mixing proportions as described above, and found desirable mixing
proportions according to configurations of the molded items as
tabulated in the Table below.
______________________________________ Configurations of Alumina
molded items cement Aggregate
______________________________________ Pellet type 40-95% 5-60%
Honeycomb type 15-60% 40-85%
______________________________________
In the above table, the "pellet type" for the molded item
configuration refers to those formed into rod, spherical, or square
shapes, which are small in volume as the catalyst carrier, and
therefore, they are not much affected in their spalling resistance
such as resistance against thermal destruction (cracking), etc.
without any possibility for cracking at high temperatures, even
when the aggregate employed is small in quantity. Meanwhile, the
"honeycomb type" refers to those formed into column or plate-like
shapes with many circular, elliptical and square openings or holes
provided therein, and since large in volume as catalyst carrier,
they exhibit large effects in spalling resistance such as
resistance against thermal destruction, etc., thus it is necessary
to reduce their expansion and contraction at high temperatures by
increasing the amount of the aggregate to be included therein.
As the aggregates, silica group aggregate, silica alumina group
aggregate, and alumina group aggregate may initially be mentioned
and it is preferable to employ aggregates in mineral phase such as
silicate minerals, mullite, corundum, sillimanite, .beta.-alumina
and those of magnesia, chrome, dolomite, magnesite-chrome and
chrome magnesite group. Furthermore, it is also possible to use
general aggregates at the low temperature side
(250.degree.-700.degree. C. or preferably 300.degree.-700.degree.
C.) and heat resistant aggregate at the high temperature side
(higher than 700.degree. C.) depending on working temperatures of
the catalyts.
More specifically, in the silica group aggregates, there are silica
stone and the like, and these aggregates have SiO.sub.2 as main
component. In the silica alumina group aggregates, there are
included chamotte, agalmatolite, high alumina, etc. which have
SiO.sub.2 -Al.sub.2 O.sub.3 as main component, while as the alumina
group aggregates, .alpha.-Al.sub.2 O.sub.3, .beta.-Al.sub.2
O.sub.3, .gamma.-Al.sub.2 O.sub.3, .delta.-Al.sub.2 O.sub.3, etc.
are available. It should be noted here that alumina may be replaced
by aluminum hydroxide employed as starting material for conversion
into alumina through heat treatment. Moreover, silicate minerals,
mullite, corundum, sillimanite, .beta.-alumina may be employed as a
general main mineral phase. Meanwhile, materials prepared by
roughly grinding the above described aggregate or commercially
available aggregates of conichalcite silica sand, alumina,
chamotte, etc, may be used, and for general purposes, it is
convenient to employ silica sand or chamotte available in the
market. Additionally, aggregates of magnesia, chrome, dolomite,
magnesia-chrome and chrome magnesia group may be used, but since
they are normally used for extremely high temperatures, such
aggregates are not suitable for obtaining inexpensive catalyst
cariers. In general, the aggregates have only to be superior in the
spalling resistant characteristics with respect to temperatures of
approximately 1000.degree. C. at the maximum, and thus, silica
group aggregates sufficiently meet the requirements, while for
temperatures of approximately 600.degree. C. at the maximum,
inexpensive aggregates of the silica group aggregates such as plain
sand, seashore sand, etc. may be conveniently employed.
Furthermore, in the materials usable for the aggregates, there are
fibrous organic substances which are particularly effective for
reducing deterioration of mechanical strength at high temperatures,
and for which asbestos, glass fibers, etc. may be employed. For the
above purpose, ordinary asbestos having magnesia silicate as main
component is sufficient, while it is necessary to employ materials
superior in fire resistance for use at particularly high
temperatures. On the other hand, for the glass fibers, it is
preferable to employ alkali-resistant glass fibers so as to
withstand alkali component in the alumina cement. In the above
case, however, it is necessary to determine the length, thickness,
or configurations of the glass fibers, i.e., whether they are in
the state of a mat or chopped strands.
Subsequently, particle size of the aggregate will be described
hereinbelow.
Although particles of fine size may be used without any
inconvenience for aggregates having small volume such as the pellet
type, it is necessary to employ particles of large size for those
with large volume such as the honeycomb type. Especially, in the
honeycomb type of large volume wherein the spalling resistance is
particularly important, it is possible to reduce expansion and
contraction of the catalyst carrier and simultaneously to increase
the mechanical stength of such catalyst carrier by employing
particles of large size. Above all, mixing of particles of large
size and small size is preferable. Another important role of the
aggregate is to increase the specific surface area of the catalyst
carrier, which is an essential item for the catalyst carrier.
Therefore, it is preferable to employ an aggregate particularly
having a large specific surface area, for example, .gamma.-Al.sub.2
O.sub.3, etc.
Subsequently, selection of additives will be described
hereinbelow.
There may be employed generally inexpensive and low pollution
metallic oxide catalyst of manganese, copper, iron oxides, etc.
having catalytic functions themselves.
______________________________________ Catalyst Alumina cement
Configurations material + aggregate
______________________________________ Pellet shaped 0-50 wt %
20-100 wt % carrier Honeycomb shaped 0-30 wt % 20-100 wt % carrier
______________________________________
Finally, as auxiliary agents, those increasing the specific surface
area and those having auxiliary effects on the performance,
abrasion resistance, longevity, etc. may be employed. For
increasing the specific surface area of the catalyst carrier, there
is one method in which the specific surface area is increased by
the aggregate and another method in which the surface of the
catalyst carrier is made porous through inclusion of thermally
decomposable salt during manufacturing of the catalyst carrier, and
for the latter thermally decomposable salt, organic salts are
particularly suitable. It is also possible to make the carrier
surface porous by causing alcohol, carboxymethylcellulose,
polyethylene to be included in the carrier, with subsequent heat
treatment. Moreover, as auxiliary agents, zeolite, double oxides
(ferrite), silica sol, etc. may also be included. One example of a
preferred composition is shown in the table below.
______________________________________ Auxiliary Catalyst Alumina
cement Configurations agent material + aggregate
______________________________________ Pellet shaped 0-20 wt % 0-50
wt % 20-100 wt % carrier Honeycomb shaped 0-20 wt % 0-30 wt %
20-100 wt % carrier ______________________________________
In the next step, the method of manufacturing the catalyst carrier
will be explained hereinbelow.
Alumina cement is mixed with aggregate, with further addition of
catalytic materials and auxiliary material depending on the
necessity, and after dry process mixing thereof, the carrier is
molded, with addition of water or colloidal salt necessary for the
molding. In the above case, the water or collidal salt should be
added in such an amount as to suit to the configuration and size of
the item to be molded, since the molding becomes difficult, if the
amount is excessive or too small, and after molding, the molded
carrier is subjected to perfect curing in water, when it has
hardened to a certain extent to have such a strength that it will
not collapse in the case of the perfect curing in water, and
subsequent to drying or heat treatment, the resultant catalyst
carrier is obtained.
Conventionally, most of the catalyst carrier materials are
subjected to a sintering process during molding, with active
alumina and the like having large surface area being applied on the
surface of such carrier materials for supplementing its small
surface area, but it is to be noted that the present invention is
characterized in that the carrier has the sufficient strength and
ample surface area without sintering as described above.
Subsequently, the method of supporting the catalyst is broadly
divided into three kinds, i.e., co-precipitation method,
impregnation method, and application method. Although the methods
as described above respectively have their merits and demerits, the
impregnation method is employed for the platinum catalyst of the
honeycomb shape having alumina as the carrier. The impregnation
method as described above is comparatively simple, but has such
disadvantages that there are cases where the amount of supported
catalyst is restricted or wherein the surface of the carrier is
subjected to abrasion, and reduction in the area or in the number
of pores.
In the EXAMPLES according to the present invention described later,
the impregnation and application methods are mainly employed.
The catalysts to be supported are mainly of platinum group metals
including, for example, platinum, palladium, ruthenium, rhodium,
iridium, osmium, etc., and for the salts thereof, chlorides are
preferable, representative ones of which are tetrachloroplatinate
H.sub.2 PtCl.sub.4 nH.sub.2 O, hexachloro platinate H.sub.2
PtCl.sub.6 nH.sub.2 O, platindiaminodinitrate Pt(NH.sub.3).sub.2
(NO.sub.2).sub.2 nH.sub.2 O, palladium chloride PdCl.sub.2,
ruthenium chloride RuCl.sub.3, rhodium chloride, etc. For actual
use, these metallic salts as described above are dissolved into
solvents such as water or alcohol. Although the concentration
thereof may differ depending on the amount to be applied and the
methods of supporting, optimum concentration must be determined
according to the purpose for use, configuration of the carrier,
etc., since dispersion of the catalyst particles is deteriorated,
if concentration of the solvent is excessively high. Particularly,
when the platinum metal is employed, a catalyst material superior
both in initial performance and longevity and having the supported
amount of platinum in the region of 0.001 to 0.1% by weight as
compared with conventional platinum catalyst, may be obtained. More
specifically, in the known platinum catalyst, carriers of alumina,
cordierite, etc. are employed, with the supported amount of
platinum being in the range from 0.1 to 0.5% by weight, which are
commonly accepted, since deterioration of longevity is particularly
large when the supported amount is less than 0.1% by weight. On the
contrary, when the catalyst carrier according to the present
invention is employed, high performance is expected even when the
supported amount of platinum is slight. It is to be noted that
there is a close relation between the amount of catalyst supported
and performance, and the performance is improved as the supported
amount increases, but that if the supported amount is too large,
problems such as falling off the catalyst may result. Furthermore,
it is possible to improve ranges of applications, configurations,
activity at low temperature, life, etc. of the catalyst by causing
the carrier to support more than two kinds of various metals and
metallic oxides besides the adjustment of the supported catalyst
amount.
The heating of the carrier impregnated with the foregoing metallic
salt solution at temperatures below 700.degree. C. converts the
metallic salt into a catalytic metal and catalytic metal oxide.
Referring now to the drawings, there are shown in a graph of FIG. 1
results of measurements of specific surface area by the BET method
in the case where the pellet molded member composed of alumina
cement and having diameter of 5 mm and length of approximately 3 mm
is subjected to heat treatments for one hour at various
temperatures.
As is clear from the graph of FIG. 1, the specific surface area of
the carrier is rapidly increased in the vicinity of 250.degree. C.,
due to dehydration of the bonding water in the alumina cement of
the catalyst carrier. The specific surface area of the
.alpha.-alumina carrier commercially available at present is in the
range from 5 to 15 m.sup.2 /g as measured and is smaller than that
of the conventional .gamma.-alumina carrier in the range from 100
to 300 m.sup.2 /g, but from the fact that the catalyst superior in
low temperature activity is available even with the platinum
catalyst which employs .alpha.-alumina, it is seen that the
catalyst carrier using the alumina cement sufficiently exhibits its
expected functions.
Referring to the graph of FIG. 2, there are shown purification
rates for carbon monoxide with respect to samples prepared by
forming, into the pellets as stated earlier, alumina cement of
various compositions of 50 weight % and silica sand (No. 7) of 50
weight % as shown in Table 1 below, with subsequent heat treatment
of the pellets for one hour at a temperature of 350.degree. C. In
the above case, the catalyst temperatures were at 400.degree. C.
and 600.degree. C., while the measuring conditions of the
purification rates are the same as in EXAMPLE 1 mentioned
later.
As is seen from the graph of FIG. 2, as the amount of iron oxide
component in the catalyst carrier (i.e., in the alumina cement) is
increased, the purification capacity for carbon monoxide is also
increased.
TABLE I ______________________________________ Compositions (weight
%) Al.sub.2 O.sub.3 CaO SiO.sub.2 Fe.sub.2 O.sub.3 TiO.sub.2
______________________________________ i 72.5 26.5 0.3 0.5 -- ii
53.5 28.0 4.3 2.0 2.0 iii 50.5 36.5 4.4 5.0 2.5 iv 47.1 36.0 4.8
9.5 2.4 v 40.0 38.0 4.0 16.0 -- vi 36.0 38.5 5.0 20.0 --
______________________________________
In a graph of FIG. 3, there is shown a comparison of the
purification rates for carbon monoxide between pellets A composed
of the alumina cement of 50 weight % and silica sand (No. 7) of 50
weight % having compositions shown in the item iv of the above
Table 1, and pellets B composed of the commercially available
.alpha.-alumina, with the pellets A and B being subjected to heat
treatments for one hour at a temperature of 300.degree. C. after
the molding.
As is noticed from the graph of FIG. 3, the catalyst carrier
according to the present invention has a considerably favorable
catalytic function at high temperatures, while the commercially
available catalyst carrier has almost no purification capacity even
at high temperatures.
The graph of FIG. 4 compares the purification capacity at
200.degree. C. for carbon monoxide between the pellets C composed
of the alumina cement of 100 weight % having the composition shown
in the item iv of the above Table 1 and pellets D composed of the
commercially available .gamma.-alumina, with the pellets C and D
being subjected to heat treatments for one hour at a temperature of
300.degree. C. after molding, and with the platinum supporting
amount being 0.05 weight % with further heat treatment for one hour
at temperatures from 500.degree. to 800.degree. C.
From the graph of FIG. 4, it is noticed that in the catalyst
carrier of the present invention, the performance thereof is
reduced at heat treating temperatures higher than 700.degree. C.,
while the low temperature activity of the catalyst carrier composed
of .gamma.-alumina is rapidly reduced at temperatures higher than
600.degree. C., possibly due to extreme reduction of the specific
surface area. Meanwhile, the catalyst carrier of the present
invention is small in the variation of the specific surface area as
compared with that of .gamma.-alumina, and has a large surface area
even at high temperatures, with consequent small thermal
deterioration.
Referring also to a graph of FIG. 5, there is shown a comparison of
the purification capacity at 200.degree. C. for carbon monoxide
between pellets C for the catalyst carrier of the present invention
and pellets D of .gamma.-alumina, with platinum being supported
thereon through the application method being 0.0005, 0.0001, 0.01,
0.05, 0.1 and 0.2% by weight, and with the pellets C and D being
subjected to heat treatments in an electric furnace for one hour at
a temperature of 600.degree. C. As is seen from the graph, the
catalyst carrier of the present invention is free from
deterioration in performance in the range of supported platinum
amounts of from 0.001 weight % to 0.1 weight %, and the performance
tends to be rather deteriorated when the supported amount reaches
0.2 weight %, while the catalyst carrier of the pellets D shows
improved performance at the supported amount exceeding 0.05 weight
%. The tendency as described above shows that there is a close
relation between the property (surface area) of the catalyst
carrier, amount of the supported catalyst and performance, and that
the surface area (porosity in the surface) is particularly
important. The latter may partly be attributable to the fact that
since the uniform dispersing conditions of platinum differ
depending on catalyst carriers, it is rather preferable to cause
platinum to be uniformly distributed in an amount smaller than in
the conventional practice when the alumina cement is employed, and
that if the supported amount is further increased, platinum
particles are formed into a lump to be locally present at the
porous portion on the surface, thus resulting in reduction of the
porosity (i.e. reduction of surface areas). In the catalyst carrier
of the present invention, such a trend as described above tends to
be particularly increased with the increase of additives (i.e.,
with the increase of the catalyst supported amount).
As is seen from the foregoing description, one of the features of
the present invention is that the catalyst carrier has a catalytic
function at high temperatures over 300.degree. C. It should be
noted that the present invention is characterized in that:
(1) Since the alumina cement is employed, with addition thereto of
the additives depending on the necessity, the catalyst carrier of
the present invention is low in cost and can be formed into desired
shapes without passing through a sintering process and the like by
utilizing the binding power of the alumina cement itself.
(2) The catalyst carrier itself has a purification capacity for
carbon monoxide at high temperatures (300.degree. to 500.degree.
C.) and further absorbing and removing capacity for acid gases such
as sulfur dioxide, etc., by the lime component contained in the
alumina cement.
(3) The catalyst carrier has a considerably large surface area as
well as an ample surface hardness, and thus sufficiently serves the
purpose as a carrier.
(4) Adhering efficiency (including adhering strength) of the
catalyst to the catalyst carrier is sufficiently large. In other
words, wetting phenomenon between water, alcohol, etc., and the
solvent of catalyst salts is large enough to support the catalyst
effectively in a uniformly dispersed state.
(5) As compared with the commercially available alumina carrier,
the carrier of the present invention has large abrasion strength,
with small attrition loss during use, thus being stable as carrier
for a long period of time.
(6) Since re-activation of the catalyst is possible by the catalyst
carrier, catalyst of high performance with a long life can be
obtained, with an extremely small amount of supported catalyst.
For example, when the platinum metallic catalyst is inactivated by
sulfur dioxide gas as in ##STR1##
The catalyst is regenerated by CaO.H.sub.2 O in the alumina cement
as follows.
Meanwhile, in the case of metallic oxide MO.sub.x, reactivation
takes place as follows.
(7) The advantages of employing the alumina cement as the catalyst
carrier are that the surface area is large as compared with
conventional sintered carriers, since the cement particles are fine
due to the capacity of molding at normal temperatures without
necessity of sintering the molded carrier, and that in the known
sintered carriers, although it is difficult to maintain the molding
accuracy of the molded item (i.e., carrier) uniform due to thermal
contraction of approximately 10 to 30% with respect to an original
mold, such thermal contraction can be reduced below 2% for better
molding accuracy when the alumina cement (which is not sintered) is
employed.
(8) In the conventional sintered carrier whose surface is sintered,
the surface area is small, and therefore, expected performance
cannot be obtained unless a large amount of the catalyst is
supported on the carrier through adhesion. On the contrary, since
the molded carrier of the present invention is unsintered, the
carrier surface is formed through aggregation of very fine
particles, with a large surface area, only a small amount of
supported catalyst is sufficient for the purpose, and can be
dispersed uniformly on the carrier to provide the catalyst of high
performance.
(9) Although the conventional alumina carrier of neutral nature
requires a large amount of catalyst to be supported thereon, the
alumina cement carrier according to the present invention is of
alkaline nature, and is considered to accelerate effective support
of a catalyst of precious metal salts, and provides an active
precious metal catalyst in a small amount, although the process
thereof is not sufficiently clear.
The catalyst according to the present invention as described in the
foregoing is mainly intended for the purification of exhaust gases
generated from household burning appliances and cooking appliances,
and should preferably be used at comparatively low temperatures
particularly lower than 700.degree. C., but it should be noted that
the catalyst of the present invention is not limited in its
applications to the end uses as described above, but may also be
effective for the purification and oxidation of exhaust gases
generated from various plants and the like. It should also be noted
that the catalyst of the present invention is not only effective
for purification of carbon monoxide, hydrocarbon, etc., but fully
displays its function as catalyst for absorbing and eliminating
sulfur dioxide and, for converting NO into NO.sub.2 in nitrogen
oxide removing apparatus, or as catalyst for reaction (platinum
catalyst) between NO and CO or NO and NH.sub.3.
Hereinbelow, the present invention will be described with reference
to Examples for its illustration without any intention of limiting
the scope thereof.
EXAMPLE 1
Pellets having diameter of 5 mm. were prepared from 100 parts by
weight of alumina cement having the composition shown in the item
iv of Table 1 and 100 parts by weight of silica sand (NO. 7), and
after particle size adjustment thereof to average length of 3 mm (2
to 4 mm), were subjected to heat treatment for one hour at a
temperature of 300.degree. C,. to obtain the catalyst carriers. In
the next step, the catalyst carriers thus prepared were impregnated
with water solution prepared by dissolving hexachloroplatinate at a
rate of 1 g/l to ultimately have platinum supporting amounts of
0.001, 0.01, and 0.05 weight %, with subsequent drying of the
catalyst carriers for one hour at a temperature of 80.degree. C.
and heat treatment for one hour at a temperature of 500.degree. C.
in an electric furnace. The amounts of supported catalyst of 0.001,
0.01, and 0.05 weight % respectively were designated as a, b, and
c, and the catalyst prepared by causing platinum of 0.5 weight % to
be supported on the commercially available .alpha.-alumina was
deignated as d, while the catalyst prepared by molding a mixture of
25 parts by weight of the alumina cement having the composition in
the item iv of Table 1 and 75 parts by weight of .gamma.-MnO.sub.2
was designated as e.
The supported catalysts prepared in the above described manner were
loaded in quartz tubes having inner diameter of 35 mm. by
approximately 42 cc, respectively and air containing approximately
200 ppm. of carbon monoxide CO was caused to pass the catalyst
layers at space velocity of 10,000 hr.sup.-1 to obtain the CO
purification rate through measurements of CO concentration at the
inlet and outlet sides of the quartz tubes, the results of which
are shown in the graph of FIG. 6. Although the catalysts a to c of
the present invention were slightly inferior to the commercially
available platinum catalyst, d, their activity at low temperatures
was seen to be improved as compared with the catalyst e of the
manganese oxide group.
EXAMPLE 2
The catalysts obtained with reference to Example 1 were subjected
to a life test through continuous burning of an oil heater as shown
in FIG. 7, the results of which are shown in a graph of FIG. 8.
In FIG. 7, the oil heater employed for the life test generally
includes a housing 1, a top plate 2 mounted on the top of the
housing 1, a knob 3 for raising or lowering a burning wick provided
on a front panel of the oil heater, a reflecting plate 4 and a
chimney 5 provided at the front of the reflecting plate 4.
The oil heater of FIG. 7 further includes a catalyst tank 6 which
is mounted immediately above the burning wick and which is provided
with two stages of wire nettings of 10 meshes, while the catalyst
pellets of 250 g are loaded on each of the stages to be 500 g in
total for forming upper and lower catalyst layers 7. The catalyst
tank 6 of a cylindrical shape having an internal diameter of 160
mm. is fixed for being suspended to metal fittings 8 secured to the
top plate 2. During burning, the lower catalyst layer 7 was
maintained at a temperature range of 600.degree..+-.20.degree. C.,
with space velocity of exhaust gases which pass through the
catalyst layers 7 being about 20,000 Hr.sup.-1 /1 layer.
For finding the CO purification rate, the catalyst in the lower
catalyst layer 7 was taken out for subsequent drying for one hour
at a temperature of 350.degree. C., and after being kept for one
day in a desiccator, was subjected to a similar procedure to that
in Example 1.
EXAMPLE 3
Disc-shaped catalyst carriers each composed of 50 parts by weight
of alumina cement having composition as shown in the item iv of
Table 1 and 50 parts by weight of silica sand (No. 7) and having
diameter of 200 mm. and thickness of 15 mm, with 1,200 holes (each
4 mm. in diameter) being formed in the direction of thickness, were
prepared and subjected to heat treatments for one hour at a
temperature of 300.degree. C. Water solutions of chlorides of
various platinum metals were applied to upper and lower surfaces of
each of the catalyst carriers (apparent surface area of
approximately 2,600 cm.sup.2 (about 800 g) for subsequent heat
treatment for one hour at a temperature of 500.degree. C. CO
purification rates of the catalysts thus obtained are shown in
Table 2 below.
TABLE 2 ______________________________________ Catalyst metals
Supported Composition Supported amount CO purification rate (%)
(weight ratio) (weight %) 100.degree. C. 200.degree. C.
______________________________________ Pt--Pd (1:1) 0.015 49 99
Pt--Ru (1:1) 0.015 43 97 Pt--Pd (2:1) 0.02 55 99 Pd--Ru (2:1) 0.02
39 96 Rh--Ru (1:1) 0.015 30 93 Pd 0.015 37 96 Re 0.015 25 89
______________________________________
EXAMPLE 4
After sufficiently mixing 25 parts by weight of alumina cement
having composition shown in the item ii of Table 1, 25 parts by
weight of silica sand (No. 5) and 50 parts by weight of
.gamma.-alumina powder, a solution prepared by mixing 3 parts by
weight of water and 1 part by weight of ethyl alcohol was added to
form the resultant mixture into a slurry which was then injected
into a mold of silicone rubber, and after hardening, was released
from the mold for subsequent perfect curing in warm water. After
drying, the carrier thus obtained was subjected to heat treatment
for one hour at a temperature of 300.degree. C.
The resultant carrier obtained in the above described manner having
the same configuration as that described with reference to Example
3 was caused to support various kinds of catalysts and placed in an
exhaust gas passage of a petroleum fulled hot air blower (not
shown) for measuring the purification rate for CO in the exhaust
gases. In the above measurements, the catalyst was placed in two
stages, and the exhaust gases contained 50 to 100 ppm of CO and 2
to 5 ppm of SO.sub.2, while the temperature at a central portion of
the catalyst at the lower stage was 600.degree..+-.20.degree.
C.
The results of the above measurements are given in Table 3 below,
while results of a longevity test on parts of the samples are shown
in FIG. 9.
In Table 3, the sample l had .alpha.-alumina as the carrier, while
in the sample h, platinum was caused to be supported after copper
oxide having been supported. In other composite catalysts, mixed
solutions of catalyst chlorides were used for simultaneous
supporting.
TABLE 3 ______________________________________ CO Concentration
Catalyst (ppm) in composition Supported exhaust gas CO purifica-
(weight amount inlet Outlet tion ratio ratio) (weight %) side side
(%) ______________________________________ f Pt 0.01 70 5 93 g
Pt--Pd (1:1) 0.02 100 3 97
______________________________________
EXAMPLE 5
Disc-shaped catalyst carriers each composed of 35 parts by weight
of alumina cement (the item iv of Table 1), 50 parts by weight of
electrolytic manganese dioxide, 50 parts by weight of basic copper
carbonate and 10 parts by weight of silica sand (No. 7), and having
diameter of 200 mm and thickness of 15 mm, with 1,200 holes (each 4
mm in diameter) being formed in the direction of thickness, were
prepared, and after being contacted with ethanol solutions of
chlorides of various precious metals, was subjected to heat
treatments for one hour at a temperature of 500.degree. C.
The CO purification rates of the catalyst thus obtained at
respective temperatures are tabulated in Table 4 below.
TABLE 4 ______________________________________ Catalyst metals
Supported Composition amount CO purification rate (weight ratio)
(weight %) 100.degree. C. 200.degree. C.
______________________________________ Pt Pd 1:1 0.04 40 97 Pt Ru
1:1 0.04 37 95 Pt Rh 1:1 0.04 35 95 Pt Pd 2:1 0.07 45 98 Pd Ru 2:1
0.07 33 94 Rh Ru 1:1 0.07 27 90 Pd -- 0.04 30 93 Re -- 0.04 22 85
Ru -- 0.04 21 83 ______________________________________
As is clear from the foregoing description, according to the
present invention, improved gas purification catalysts of high
performance and low cost, and manufacturing method thereof can be
advantageously presented, with substantial elimination of
disadvantages inherent in the conventional gas purification
catalysts.
Although the present invention has been fully described by way of
example with reference to the accompanying drawings, it is to be
noted that various changes and modifications are apparent to those
skilled in the art. Therefore, unless otherwise such changes and
modifications depart from the scope of the present invention, they
should be construed as included therein.
* * * * *