Silver catalyst for production of ethylene oxide and method for manufacture thereof

A silver catalyst having fine silver particles dispersed and deposited fast on the outer surface of a porous inorganic refractory carrier and on the inner wall surface of pores in said carrier and used in the production of ethylene oxide by the catalytic gas-phase oxidation of ethylene with molecular oxygen, which silver catalyst is characterized by containing a compound of at least one metal ion selected from the group consisting of cesium, rubidium, potassium, and thallium (monovalent) as dispersed and deposited fast in an amount in the range of 1.times.10.sup.-6 to 5.times.10.sup.-6 gram equivalent per the unit surface area, m.sup.2, of said silver on the surface of said silver.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
This invention relates to a silver catalyst to be used for the production 
of ethylene oxide by the catalytic gas-phase oxidation of ethylene with 
molecular oxygen and to a method for the manufacture of the silver 
catalyst. 
2. Description of the Prior Art: 
The silver catalyst which is used in the commercial production of ethylene 
oxide by the catalytic gas-phase oxidation of ethylene with molecular 
oxygen is required, for satisfactory performance of the function thereof, 
to exhibit high selectivity and high activity and enjoy a long catalyst 
life as. 
Various studies have been made to date for the purpose of improving the 
performance of the silver catalyst and consequently fulfilling the 
requirement and efforts have been made to improve carriers, reaction 
promoters, silver compounds and the like. Numerous reports covering 
carriers have been published. The specifications of U.S. Pat. Nos. 
3,207,700, 4,368,144, 2,766,261, 3,172,893 and 3,664,970, and the 
specifications of Japanese patent publication SHO No. 43(1968)-13,137, SHO 
No. 45(1970)-22,419 and SHO No. 45(1970)-11,217 are their examples. Most 
of them, however, concern pore distributions and specific surface areas of 
carriers. 
In the specification of U.S. Pat. No. 2,125,333, there is a description to 
the effect that an alkali metal salt containing sodium or potassium and a 
metal salt thereof is used as an additive for the silver catalyst in the 
manufacture of ethylene oxide. 
In the specification of U.S. Pat. No. 2,238,474, there is a description to 
the effect that sodium hydroxide improves the activity of the silver 
catalyst for the production of ethylene oxide and potassium hydroxide has 
an adverse effect upon the activity of the silver catalyst. 
In the specification of U.S. Pat. No. 2,765,283, there is a description to 
the effect that the silver catalyst is improved by adding 1 to 2,000 ppm 
by weight of an inorganic chlorinated substance such as sodium chloride to 
the catalyst carrier before silver is deposited on the carrier. 
In the specification of U.S. Pat. No. 2,799,687, there is a description to 
the effect that a halide such as sodium chloride or potassium chloride, 
used in an amount of 20 to 16,000 ppm, functions as an inhibitor and 
induces degradation of the activity of the silver catalyst. 
In the specification of U.S. Pat. No. 4,007,135, there is disclosed a 
catalyst for the production of alkylene oxide, which catalyst contains 
copper, gold, zinc, cadmium, mercury, niobium, tantalum, molybdenum, 
tungsten, vanadium, or desirably chromium, calcium, magnesium, strontium, 
and/or more desirably barium, and preferably further an alkali metal, in 
an amount preceding the amount naturally present as an impurity or cement 
in the carrier and sufficient to manifest the action of a promoter. 
In the specification of U.S. Pat. No. 4,168,247, there is disclosed a 
catalyst for the production of alkylene oxide, which catalyst contains 
silver deposited on a porous heat-resistant carrier possessing a specific 
surface area in the range of 0.05 to 10 m.sup.2 /g and further contains 
sodium and at least one other alkali metal selected from the group 
consisting of potassium, rubidium, and cesium in a promoting amount in 
excess of the amount naturally present as an impurity or a binding agent 
in the carrier. 
In the specification of U.S. Pat. No. 4,278,562, there is disclosure to the 
effect that a catalyst for the production of an alkylene oxide is obtained 
by depositing silver and optionally sodium or lithium in the form of 
corresponding salts on a carrier, heating the carrier and in the 
subsequent treatment, depositing thereon the salts of such alkali metals 
as potassium, rubidium, and cesium in conjunction with an amine and/or 
ammonia. 
In Japanese Patent Laid-Open No. SHO 55(1980)-145,677, there is disclosed a 
silver catalyst which, as a catalyst for the reaction of oxidation, has 
silver and, when necessary, further an alkali metal component or an 
alkaline earth metal component deposited on a non-acidic carrier 
containing alumina, silica, and titania in a total amunt of not less than 
99% by weight, containing metal of the Groups Va, VIa, VIIa, VIII, Ib, and 
IIb of the Periodic Table of Elements in a total amount of less than 0.1% 
by weight, and assuming no acid color on exposure to methyl red having a 
pKa value of +4.8. 
In Japanese Patent Laid-Open No. SHO 56(1981)-105,750, there is disclosed a 
silver catalyst for the production of ethylene oxide, which silver 
catalyst is prepared by impregnating a carrier using .alpha.-alumina as a 
principal component thereof and having a sodium content of not more than 
0.07% by weight and a specific surface area in the range of 1 to 5 m.sup.2 
/g with an impregnation having 0.001 to 0.05 gram equivalent, per kg of 
complete catalyst, of a complex of an alkali metal with boron, a complex 
of an alkali metal with molybdenum, and/or a complex of an alkali metal 
with tungsten contained in a decomposable silver solution formulated to 
give a deposition ratio of 5 to 25% by weight based on the complete 
catalyst, and then heating and reducing or thermally decomposing the 
product of impregnation. 
In Japanese Patent Laid-Open No. SHO 57(1982)-107,241, there is disclosed a 
silver catalyst for the production of ethylene oxide, which catalyst 
incorporates therein, besides silver, sodium (Na) as a cationic component 
and chlorine (C1) as an anionic component in amounts such that the atomic 
ratio of C1/Na will be less than 1. 
In the specification of U.S. Pat. No. 4,415,476, there is disclosed a 
silver catalyst for the production of ethylene oxide, which silver 
catalyst contains, besides silver, at least sodium and cesium as cationic 
components and chlorine as an anionic component. 
In Japanese Patent Laid-Open No. SHO 57(1982)-171,435, there is disclosed a 
silver catalyst for the production of ethylene oxide, which silver 
catalyst contains metallic silver particles deposited in a ratio of 5 to 
25% by weight based on complete catalyst on an .alpha.-alumina carrier 
having a sodium content of not more than 0.07% by weight and a specific 
surface area in the range of 0.5 to 5 m.sup.2 /g and 0.001 to 0.05 gram 
equivalent of at least one alkali metal or alkali metal compound per kg of 
the complete catalyst and in excess of the amount naturally present in the 
carrier. 
In the specification of U.S. Pat. No. 4,248,740, there is disclosed a 
method for the manufacture of a silver catalyst for the production of 
ethylene oxide, which method is characterized by impregnating a porous 
inorganic refractory carrier with a silver compound containing a reducing 
substance, thermally reducing the resulting product of impregnation 
thereby causing fine silver particles to be dispersed and deposited on the 
outer surface of the carrier and on the inner walls of the pores in the 
carrier, subsequently washing the composite with water and/or a lower 
alcohol, drying the wet composite, further impregnating the composite with 
a solution of a reaction promoting substance, and evaporation the 
impregnated composite to dryness. 
In the specification of EP No. EP-85237, there is disclosed a catalyst for 
the production of an alkylene oxide, which catalyst comprises silver on a 
porous inorganic refractory carrier containing at least 0.003 gram 
equivalent, per kg of complete catalyst, of cesium and/or rubidium 
chemically absorbed on the surface of the carrier and a catalyst wherein 
the amount of the chemically absorbed cesium and/or rubidium falls in the 
range of 400 to 3,000 ppm based on the complete catalyst per unit surface 
area, m.sup.2 /g, of the carrier. 
In the specification of No. GB-2117263, there is disclosed a catalyst which 
comprises a granular carrier made of alumina, silica, silica-alumina, or a 
combination thereof, possessing a surface area approximately in the range 
of 0.05 to 1.5 m.sup.2 /g, and having a characteristic ability to absorb 
selectively an alkali metal from a solution of the alkali metal, 5 to 20% 
by weight, based on complete catalyst, of a silver dispersion deposited on 
the granular carrier from a solution of an organic silver salt and 
activated in the presence of molecular oxygen at a maximum temperature not 
exceeding 500.degree. C. for a time long enough to produce an active fresh 
catalyst and consequently allowed to exist in the form of particles of an 
average particle diameter approximately in the range of 0.2 to 1.0 micron, 
and at least one alkali metal selected from among cesium, potassium, and 
rubidium deposited in an amount approximately in the range of 10 to 1,000 
ppm by weight based on the complete catalyst on the dispersed active 
silver particles from a solution composed of water and an alkanol of 1 to 
3 carbon atoms. 
Numerous reports on silver catalysts have been published as described 
above. Most of them, however, are directed to improving the performance of 
silver catalyst by addition to the catalyst of an alkali metal from a 
specific range. All these catalysts, however, have still many problems in 
terms of performance and service life as a catalyst. 
The effects manifested upon silver catalysts for the production of ethylene 
oxide by the addition thereto of reaction promoters represented by alkali 
metals have found recognition widely. They have been disclosed in numerous 
patented inventions. Most of these inventions, however, are nothing more 
than empirically unveiled effects. Virtually none of them has gone the 
length of elucidating an actual chemical action which is responsible for 
the effect involved. It is well known by persons skilled in the art that, 
owing to such true state of affairs as described above, in no few patented 
inventions, the inventors have disclosed contradicting technical concepts. 
Even in general technical literature, there are found reports such as the 
report written by Margolis and titled "Catalytic Oxidation of 
Hydrocarbons" which purport in effect that the addition of alkali metals 
results in degradation of the selectivity for ethylene oxide, suggesting 
that test results heavily hinge on methods of test adopted by individual 
researchers. This situation may well be regarded as a confusion arising 
solely from incomprehension of the true nature of chemical actions of 
reaction promoters. We, as the result of a diligent study, have succeeded 
in elucidating the chemical actions of reaction promoters and, based on 
the knowledge consequently acquired, perfected a literally ideal catalyst. 
Various inventions have been proposed concerning kinds of reaction 
promoters to be used, amounts of such reaction promoters to be added for 
effective use, and methods of addition of such reaction promoters. The 
conditions in which such reaction promoters are distributed in catalysts 
and the actions of the reaction promoters manifested in the catalysts, 
however, have not been elucidated. Exceptionally, in the specifications of 
No. EP-85237 and No. GB-2117263, there are found statements to the effect 
that chemical absorption or adsorption of alkali metals on carriers is 
effective. These statements are interpreted as implying that the 
adsorption poisoning of an alkali metal done to acid sites on a carrier 
brings about an effect of suppressing the reaction of isomerization of 
ethylene oxide into acetaldehyde which is a secondary reaction in the 
reaction for the formation of ethylene oxide. The inventors' study has 
also yielded results which support these statements. It should be 
especially noted here, however, that in the conclusion drawn from the 
inventors' study, the effect of the addition of an alkali metal to the 
silver catalyst is manifested predominantly on the performance of silver 
and only secondarily on the adsorption poisoning done to the acid sites on 
the surface of the carrier. If the adsorption poisoning to the acid sites 
on the surface of the carrier is ideally realized, it does not necessarily 
follow that this achievement will consequently bring about a marked 
improvement of the performance of the catalyst (selectivity for ethylene 
oxide). We have confirmed that the catalyst's acquisition of an ideal 
performance is not realized unless the compound such as an alkali metal is 
deposited in an optimum amount as dispersed on the monomolecular level 
(with the ions of alkali metal distributed one by one independently) on 
the surface of silver. 
In the specification of No. GB-2117263, there is found statement purporting 
in effect that the act of intentionally using a carrier possessing 
numerous acid sites for the purpose of increasing the amount of a metal to 
be chemically adsorbed on the carrier is effective. We are of an opinion 
that use of a carrier having numerous acid sites is not beneficial. 
Our conclusion has drawn regarding the chemical action of a metal additive 
manifested on the surface of silver is that the steric hindrance effect on 
the surface of silver is greater than the electronic effect advocated by 
Margolis et al.. Margolis et al. predict that the addition of an electron 
donating alkali metal tends to lower the selectivity for ethylene oxide. 
This theory evidently contradicts the effectiveness of an alkali metal 
which has found recognition widely. The primary ground on which the 
inventors adhere to the steric hindrance effect is the fact that the 
adsorption such as of an alkali metal on the monomolecular level on silver 
contributes greatly to the improvement of selectivity. The second ground 
which supports their conclusion is the fact that, in addition to such 
heavy alkali metals as cesium, rubidium, and potassium which have already 
been generally accepted as effective promoters, thallium has been 
demonstrated by a research group including the inventors to be an equally 
effective promoter and all these metals possess the large cation radii in 
common. The four metal ions of cesium, rubidium, potassium, and thallium 
(monovalent) possess the four largest cation radii in all the elements 
excepting instable radioactive elements. In such factors as 
electronegativity, ionization potential, and work function which have 
bearing on the electron effect, these metal ions having nothing to share 
in common. The steric hindrance effect on the surface of silver is 
considered to be manifested conspicuously in the suppression of 
dissociative adsorption of molecular oxygen and in the suppression of 
readsorption of produced ethylene oxide and both the forms of suppression 
are believed to contribute directly to the enhancement of selectivity. 
Methods which define ranges of amounts of metals to be added per kg of a 
catalyst and ranges of amounts of metals to be added per unit surface 
area, m.sup.2, of a carrier which are frequently found in the patented 
inventions published to date are extremely superficial and far from 
essential truths. The truth is that the performance of a catalyst is 
largely swayed by the condition in which a given metal additive is present 
in the catalyst. This is the very cause for the confusion which has 
brought about a wide variance among the test results obtained by different 
researchers. The catalysts prepared in accordance with such conventional 
techniques as described above are not perfectly satisfactory in 
performance, particularly in terms of selectivity. 
An object of this invention, therefore, is to provide a novel silver 
catalyst for the production of ethylene oxide and a method for the 
manufacture of the silver catalyst. 
Another object of this invention is to provide a catalyst which is enabled 
to acquire heretofore unattainable high selectivity and retain this 
quality for a long time by causing a reaction promoter of a fixed amount 
relative to the surface area of the silver in the catalyst to be dispersed 
and deposited fast on the monomolecular level on the surface of the silver 
and a method for the manufacture of the catalyst. 
SUMMARY OF THE INVENTION 
The objects described above are accomplished by a silver catalyst having 
fine silver particles dispersed and deposited fast on the outer surface of 
a porous inorganic refractory carrier and on the inner wall surfaces of 
pores in the carrier and used in the production of ethylene oxide by the 
catalytic gas-phase oxidation of ethylene with molecular oxygen, which 
silver catalyst is characterized by containing a compound of at least one 
metal ion selected from the group consisting of cesium, rubidium, 
potassium, and thallium (monovalent) as dispersed and deposited fast in an 
amount in the range of 1.times.10.sup.-6 to 5.times.10.sup.-6 gram 
equivalent per unit surface area, m.sup.2, of silver on the surface of the 
silver. 
These objects are accomplished by a method for the manufacture of a silver 
catalyst having fine silver particles dispersed and deposited fast on the 
outer surface of a porous inorganic refractory carrier and on the inner 
wall surfaces of pores in the carrier and used in the production of 
ethylene oxide by the catalytic gas-phase oxidation of ethylene with 
molecular oxygen, which method comprises impregnating the porous inorganic 
refractory carrier with a solution of a silver compound containing a 
reducing compound, thermally reducing the product of impregnation thereby 
enabling fine silver particles to be dispersed and deposited fast on the 
outer surface of the porous inorganic refractory carrier and on the inner 
wall surface of pores in the carrier, subsequently washing the resulting 
composite with at least one member selected from the group consisting of 
water and a lower alcohol, drying the wet composite, and subsequently 
causing a compound of at least one metal ion selected from the group 
consisting of cesium, rubidium, potassium, and thallium (monovalent) to be 
dispersed and deposited fast in an amount in the range of 
1.times.10.sup.-6 to 5.times.10.sup.-6 gram equivalent per the unit area, 
m.sup.2, of the silver on the surface of the silver, wherein said 
dispersion and deposition are effected by adsorption from an impregnant 
containing the compound of at least one metal selected from the group 
consisting of cesium, rubidium, potassium, and thallium (monovalent) and 
the impregnation and the expulsion of the solvent by drying subsequent to 
the step of adsorption and deposition are carried out at temperatures not 
exceeding 50.degree. C. 
The aforementioned objects are also accomplished by a method for the 
manufacture of a silver catalyst having fine silver particles dispersed 
and deposited fast on the outer surface of a porous inorganic refractory 
carrier and on the inner wall surface of pores in the carrier and used in 
the production of ethylene oxide by the catalytic gas-phase oxidation of 
ethylene with molecular oxygen, which method comprises impregnating the 
porous inorganic refractory carrier with a solution of a silver compound 
containing a reducing compound, thermally reducing the product of 
impregnation thereby enabling fine silver particles to be dispersed and 
deposited fast on the outer surface of the porous inorganic refractory 
carrier and on the inner wall surface of pores in the carrier, then 
heating the resulting composite in a current of gas at a temperature 
exceeding 200.degree. C. thereby decomposing and expelling the residual 
organic substance, and subsequently causing at least one metal ion 
selected from the group consisting of cesium, rubidium, potassium, and 
thallium (monovalent) to be dispersed and deposited fast in an amount in 
the range of 1.times.10.sup.-6 to 5.times.10.sup.-6 gram equivalent per 
the unit area, m.sup.2, of the silver on the surface of the silver, 
wherein said dispersion and deposition are effected by adsorption from an 
impregnant containing the compound of at least one metal selected from the 
group consisting of cesium, rubidium, potassium, and thallium (monovalent) 
and the impregnation and the expulsion of the solvent by drying subsequent 
to the step of adsorption and deposition are carried out at temperatures 
not exceeding 50.degree. C. 
DESCRIPTION OF PREFERRED EMBODIMENT 
We have found that for notably improvment of the selectivity of a silver 
catalyst, the cesium compound, for example, must be dispersed and 
deposited in an optimum amount on the monomolecular level on the surface 
of silver. With the conventional technique, however, it is extremely 
difficult to realize the deposition of the cesium compound in the 
particular manner just mentioned. Any catalyst answering this description 
has never existed in fact. To effect the dispersion and deposition of the 
cesium compound in the manner described above, use of the action of 
adsorption is advantageous. Unlike the conventional technique which 
effects adsorption by making use of acid sites on a carrier, fast 
retention of the cesium compound, for example, in a dispersed and 
deposited state on a completed catalyst requires fixation of special 
conditions because strong adsorption sites necessary for chemical 
adsorption do not exist on silver. 
When a silver catalyst, no matter whether it may be used as deposited on a 
carrier or not, is kept immersed for a long time in a solution containing 
a cesium compound, for example, the solution is observed to vary its 
concentration and reach an equilibrium concentration in a fixed length of 
time (mostly in 3 to 4 hours) and, as the result, a solute exceeding the 
concentration of the solution is seen to be deposited on silver. It may be 
safely concluded that this phenomenon constitutes one form of adsorption. 
This adsorption, however, is very weak so that the deposited solute will 
readily separate from the silver when it is treated with a solvent of a 
high dissolving power. Further, the amount of the solute adsorbed on the 
silver is notably decreased when the temperature of the impregnant is 
elevated. 
We have experimentally confirmed that the relation between the amount of 
the monovalent ion such as Cs.sup.+ ion adsorbed and the equilibrium 
concentration of the solution can be regulated by Langmuir Formula of 
Adsorption, indicating that the adsorption is a monomolecular layer 
adsorption and possesses the nature of chemical adsorption. On the other 
hand, the amount of saturated adsorption found by the test equals the 
amount required for covering the surface of silver substantially 
completely. This fact indicates that the adsorption sites involved in this 
case are not limited to any special cites on the surface of silver. From 
these test results, we have concluded the phenomenon of adsorption under 
discussion to be an action of electrostatic adsorption occurring between 
the adsorbed oxygen 0.sup.- normally present on the surface of silver and 
the monovalent metal ion such as Cs.sup.+ ion. When this weak adsorption 
is utilized for the dispersion and deposition of the monovalent metal ion, 
the process for the preparation of a silver catalyst necessitates fixation 
of special conditions. 
The conditions to be observed for the process are as follows. 
(1) While the impregnate containing a monovalent metal ion such as cesium 
ion requires to contain the solute in a prescribed amount, it should be 
prepared using a solvent in which the solute has as low solubility as 
possible. In water, for example, cesium and other similar monovalent 
compounds have excessively high degrees of solubility. It is, therefore, 
not desirable to use water along as the solvent. Cesium and other similar 
monovalent compounds can be used in the form of oxalates, carbonates, 
acetates, and other salts, oxides, and hydroxides. Desirably as the 
solvent, a lower alcohol of not more than 3 carbon atoms or a mixed 
solvent thereof is used. 
(2) The immersion in the solution of the monovalent metal ion such as 
cesium ion or other similar monovalent metal ion should be carried out at 
a low temperature of less than 50.degree. C. This temperature is desired 
to fall in the range of 0.degree. to 40.degree. C., preferably 0.degree. 
to 25.degree. C. When the immersion is made at higher temperatures, the 
amount of adsorption is notably decreased and the performance of the 
produced catalyst is degraded. 
(3) The expulsion of the solvent by drying should be carried out at a low 
temperature of less than 50.degree. C. It is desired to be made in a 
current of gas at a temperature not higher than the immersion temperature. 
When the immersion is made at a low temperature as described above but, in 
the subsequent step, the drying treatment is carried out at a high 
temperature, the adsorbed ions are separated from the silver during the 
course of the drying treatment, with the result that the amount of 
adsorption will be notably decreased and the performance of the produced 
catalyst will be degraded. 
When these conditions are selected, there is obtained a silver catalyst 
containing a cesium compound, for example, dispersed and deposited fast on 
the monomolecular level on the surface of silver. For the produced silver 
catalyst to acquire the optimum performance, the amount of cesium ion or 
other similar monovalent ion to be dispersed and deposited on the silver 
should be limited to a level falling in the range of 1.times.10.sup.-6 to 
5.times.10.sup.-6 gram equivalent, preferably 1.5.times.10.sup.-6 to 
4.times.10.sup.-6 gram equivalent, per the unit area, m.sup.2, of the 
surface of silver. The concentration of the impregnant which is required 
in fixing the amount of deposition of the cesium compound, for example, 
within the range can be easily found from the linear formula derivable 
from the Langmuir's adsorption isotherm to be obtained with respect to the 
adsorption on the surface of silver. If the cesium compound is dispersed 
and deposited in any amount exceeding the aforementioned range on the 
surface of silver, the produced catalyst possesses notably low activity. 
If this amount falls short of the lower limit of the range, then the 
produced catalyst possesses notably inferior selectivity. 
When the method of this invention is followed, the monovalent metal 
compound such as cesium compound, for example, is deposited also on the 
exposed surface of the carrier. The aforementioned range, however, has 
absolutely nothing to do with the amount of the cesium compound to be 
deposited on the exposed surface of the carrier. This range is applied 
exclusively to the amount of the monovalent metal compound such as cesium 
compound, for example, which is deposited on the surface of silver. The 
amount of the monovalent metal compound which has been dispersed and 
deposited on the monomolecular level on the surface of silver by virtue of 
adsorption is determined as follows. 
First, the amount, A (gram equivalent), of the monovalent metal compound 
such as the cesium compound adsorbed on the entire surface of silver is 
calculated as follows. 
##EQU1## 
The amount, A includes the amount of the impregnant adsorbed on the 
exposed surface of the carrier besides the amount of the impregnant 
adsorbed on the surface of silver. Then, the carrier from the same lot in 
the same amount as used in the preparation of the catalyst is subjected to 
the same procedure as used in the preparation of the catalyst, excepting 
the deposition of silver is omitted. The carrier is then immersed in a 
solution containing the cesium ion, for example, in a varying 
concentration, to obtain data on the relation between the equilibrium 
concentration for adsorption and the amount of adsorption. The data so 
obtained can be regulated by Langmuir's adsorption formula. The amount of 
adsorption on the surface of the carrier obtained in the same equilibrium 
concentration for adsorption as the equilibrium concentration for 
adsorption with the catalyst is calculated similarly to the formula (1). 
The amount thus calculated is reported as B (gram equivalent). 
Let S.sub.A (m.sup.2 /g of catalyst) stand for the specific surface area of 
the catalyst found by the BET (Brunauer-Emmett-Teller) method, S.sub.B 
(m.sup.2 /g of carrier) for the specific surface area of the carrier to be 
used, and a (wt%) [=(weight of catalyst-weight of carrier)/weight of 
catalyst.times.100] for the silver content of the catalyst, then the 
surface area, S.sub.A ' (m.sup.2 /g of catalyst), or silver in the 
catalyst on the assumption that silver particles have a hemispheric shape 
will be found as follows: 
EQU S.sub.A '=2{S.sub.A -S.sub.B .times.(100-a)/100} 
The surface area of the exposed carrier, S.sub.B ' (m.sup.2 /g of 
catalyst), in the catalyst is found as follows: 
EQU S.sub.B '=S.sub.A -S.sub.A ' 
Consequently, the amount of the cesium compound, A' (gram equivalent), 
adsorbed on the surface of silver is found as follows: 
EQU A'=A-B.times.(S.sub.B '.times.weight of catalyst) (S.sub.B .times.weight of 
carrier) 
Then, the amount of the cesium compound adsorbed per unit area, m.sup.2, of 
the surface of silver, C (gram equivalent/m.sup.2 of Ag), is found as 
follows: 
EQU C=A'(S.sub.A '.times.weight of catalyst) 
Since the relation between the amount of adsorption, A', on the surface of 
silver and the equilibrium concentration of the impregnant for adsorption 
to be found by the method of calculation indicated above can also be 
regulated by Langmuir type adsorption formula, the adsorption on the 
surface of silver is a monomolecular layer adsorption. The amount of 
saturated adsorption to be found from Langmuir type formula substantially 
agrees with the amount to be found geometrically from the ion radius of 
cesium or other similar monovalent metal. This fact proves that the 
adsorption under discussion is an adsorption on the molecular level. 
The effect of the addition of cesium or other similar monovalent metal on 
the carrier is considered to reside in poisoning the acid sites on the 
surface of the carrier thereby suppressing the activity of isomerization 
of ethylene oxide. This conclusion can be proved by the following 
experiment, for example. When ethylene is oxidized in a reaction tube 
packed with the catalyst of this invention and the outlet gas containing 
the produced ethylene oxide is passed through another reaction tube packed 
solely with a carrier of the same amount as the catalyst in the first 
reaction tube and held at the same temperature as the first reaction tube, 
the ratio of isomerization of ethylene oxide calculated from the change of 
gas composition at the inlet and the outlet of the second reaction tube 
shows loss of 1 to 4% of inlet ethylene oxide, depending on the kind of 
the carrier used. When the same test is performed using a carrier having a 
proper amount of cesium compound, for example, dispersed and deposited 
therein, the loss of inlet ethylene oxide is only less than 1%. The 
results clearly show the effect of the addition of the cesium compound, 
for example, to the carrier. In due consideration of the fact that the 
exposed surface of the carrier relative to the surface of the catalyst is 
considerably smaller than the surface of the carrier used alone, the 
effect of the addition to the carrier is believed to be not more than 
about 2% in terms of the selectivity for ethylene oxide. The inventors, in 
this respect, wish to emphasize strongly the fact that then the cesium 
compound is dispersed and deposited in a suitable amount on the surface of 
silver in accordance with the present invention, there can be realized a 
plus effect of more than 10% in terms of selectivity for ethylene oxide. 
When the adsorption of the cesium compound is carried out from a solution, 
the solute in the solution remaining within the pores adheres to the inner 
wall of the pores. The portion of the monovalent metal compound such as 
cesium compound thus deposited inside the pores is not included in the 
monovalent metal compound dispersed and deposited as defined herein. The 
expression "cesium compound, for example, dispersed and deposited" as used 
herein refers to the cesium compound deposited on the monomolecular level 
(with the cesium ions distributed independently one by one). The solute 
which has settled inside the pores are not dispersed or deposited but 
allowed to remain in the form of clusters of certain size. Since these 
clusters bring about no beneficial effect and, when occurring excessively, 
go to impairing the performance of the catalyst, it is desirable to reduce 
the amount of clusters to the fullest possible extent. For successful 
control of the clusters, it is necessary to observe faithfully the three 
conditions mentioned above, lower the concentration of the solution to the 
irreducible minimum, and keep down the equilibrium concentration for 
adsorption as much as possible. When these conditions are fulfilled, the 
amount of the monovalent metal compound entrapped inside the pores can be 
suppressed to the order of about 20% of the amount deposited by 
adsorption, so that the clusters of monovalent metal compound cannot have 
any noticeable adverse effect upon the performance of the catalyst. If 
these conditions are not fulfilled, the proportion accounted for by the 
amount of the monovalent metal compound entrapped increases possibly to 
the extent of impairing the performance of the catalyst. Another method 
conceivable for lowering the effect of the monovalent metal compound 
clusters entrapped in the pores comprises immersing the catalyst as a 
finished product in a solvent thereby allowing the entrapped clusters of 
monovalent metal compound to be preferentially dissolved out. This method, 
however, cannot be called desirable because it is not easy to determine 
and control the adsorbed amount of monovalent metal compound. 
For more effective manifestation of the effect of this invention, the 
silver deposited on the carrier is desired to be in a highly dispersed 
state. By covering the surface of the carrier with silver particles and 
consequently decreasing the exposed surface of the carrier, the effect of 
the active sites on the surface of the carrier can be decreased and the 
possible dilution of the effect of this invention can be avoided. 
The catalyst contemplated by the present invention is prepared as follows. 
As the solution of a silver compound containing a reducing compound and 
used for the present invention, any of all the known solutions answering 
the description can be adopted. Among other solutions available at all, 
those which permit high dispersion of silver advantageously are solutions 
containing alkanolamine as a reducing compound and having various silver 
compounds dissolved in alkanolamine or other amine, an aqueous silver 
nitrate solution containing formaline as a reducing component, and 
monoethylene glycol solutions of silver nitrate containing lower acid 
amides as reducing components. 
As typical examples of alkanolamine or other amine to be used as the 
reducing compound, there can be cited mono-, di-, and triethanolamines, 
mono-, di-, and tri-n-propanolamines, mono-, di- and 
tri-isopropanolamines, n-butanolamines, and isobutanolamines. As typical 
examples of the lower acid amide, there can be cited foramide, acetamide, 
propionic acid amide, glycolic acid amide, and dimethylformamide. 
As the silver salt to be used as a starting material, any of the inorganic 
silver salts and organic silver salts which are capable of reacting the 
alkanolamine and consequently forming a complex salt can be adopted. 
Typical examples of the silver salt include silver nitrate, silver 
carbonate, silver sulfate, silver acetate, silver oxalate, silver lactate, 
silver succinate, and silver glycolate. 
As the solvent to be used in this invention, water proves desirable. A 
lower aliphatic compound of 2 to 6 carbon atoms containing 1 to 3 
alcoholic hydroxyl groups in the molecular unit thereof is advantageously 
used particularly when a lower acid amine is used as a reducing compound. 
Examples of the lower aliphatic compound include monoethylene glycol, 
diethylene glycol, triethylene glycols, trimethylene glycol, monopropylene 
glycol, methyl cellosolve, ethyl cellosolve, methyl carbitol, ethyl 
carbitol, and glycerol. 
The solution of a silver compound selected from among those described above 
is used to impregnate a porous inorganic carrier. 
As the porous inorganic carrier to be used for this invention, any of the 
porous inorganic carriers heretofore known to the art can be adopted. 
Among other carriers available at all, a carrier made of alumina and/or 
silica proves particularly desirable. Especially, a carrier made of 
.alpha.- alumina gives favorable results. This carrier is desired to have 
an apparent porosity in the range of 40 to 70%, preferably 50 to 65%, and 
a BET specific surface area in the range of 0.1 to 10 m.sup.2 /g, 
preferably 0.2 to 5 m.sup.2 /g. 
The silver compound containing the reducing compound, at a temperature in 
the range of room temperature to 200.degree. C. is reduced to metallic 
silver and deposited in the form of fine particles on the inner and outer 
surfaces of the carrier. In this case, the heating temperature is desired 
to be kept down to the irreducible minimum. Better results of the heating 
are obtained when the heating is started at a low temperature and then 
continued at gradually elevated temperatures. 
After the active silver has been dispersed and adhered fast on the outer 
surface of the carrier and on the inner surface of pores in the carrier, 
the resulting composite is washed with water and/or a lower alcohol 
preferably in a boiling condition. Examples of the lower alcohol are 
aliphatic alcohols of 1 to 3 carbon atoms, such as methanol, ethanol, 
isopropanol, and n-propanol. This washing treatment is effective in 
removing alkanolamine and other organic substances from the catalyst and, 
at the same time, cleaning the surface of the produced active silver and 
enhancing the activity of the silver. The amount of silver to be deposited 
is desired to fall in the range of 2 to 25% by weight, preferably 5 to 20% 
by weight, based on the complete catalyst. The washed composite is then 
dried by being heated to a temperature in the range of 50.degree. to 
150.degree. C. The catalyst obtained consequently has deposited on the 
carrier fine and uniform silver particles having an average diameter not 
exceeding 1,000 Angstroms. 
Instead of washing and drying the composite described above after silver 
compound has been reduced to metallic silver, the remained organic 
compound may be removed by heating this composite in a current of a gas, 
suitably an inert gas such as nitrogen at a temperature exceeding 
200.degree. C., desirably falling in the range of 200.degree. to 
300.degree. C. to activate the catalyst. The heating effected in an 
atmosphere containing oxygen at a high temperature exceeding 300.degree. 
C. is undesirable because it entails heavy sintering of silver particles. 
It is desirable to adopt conditions such that the average diameter of the 
silver particles will fall not more than 2,000 Angstroms, preferably 1,000 
Angstroms. Again in this case, the amount of silver to be deposited is 
desired to fall in the range of 2 to 25% by weight, preferably 5 to 20% by 
weight, based on the complete catalyst. 
Further, the catalyst consequently produced is immersed in a solution of a 
compound of at least one metal selected from the group consisting of 
cesium, rubidium, potassium, and thallium (monovalent) in a lower alcohol 
such as methanol or ethanol, for example, so that the compound of at least 
one metal selected from the group consisting of cesium, rubidium, 
potassium, and thallium (monovalent) will be dispersed and deposited fast 
in an amount in the range of 1.times.10.sup.-6 to 5.times.10.sup.-6 gram 
equivalent per the unit area, m.sup.2 of the silver on the surface of 
silver. In this case, the immersion is made at a temperature of less than 
50.degree. C., desirably in the range of 0.degree. to 40.degree. C., and 
more desirably 0.degree. to 25.degree. C. The expulsion of the solvent by 
drying after the deposition by adsorption is carried out at a temperature 
of less than 50.degree. C., desirably in the range of 0.degree. to 
40.degree. C., and more desirably 0.degree. to 25.degree. C. This 
expulsion of the solvent is desirably carried out in a current of a gas. 
The compounds of cesium, rubidium, potassium, and thallium (monovalent) are 
used in the form of nitrates, sulfates, carbonates oxalates, hydroxides, 
oxides, and acetates. 
Examples of the lower alcohol to be used as a solvent include methanol, 
ethanol, isopropanol, n-propanol, and mixtures thereof. 
As the conditions to be used in the method for manufacture of ethylene 
oxide by the catalytic gas-phase oxidation of ethylene with molecular 
oxygen in the presence of a silver catalyst obtained by the present 
invention, all the conditions heretofore known in the art can be adopted. 
The conditions generally adopted for the commercial production of ethylene 
oxide, i.e. the reaction temperature in the range of 150.degree. to 
300.degree. C., preferably 180.degree. to 280.degree. C., the reaction 
pressure in the range of 2 to 40 kg/cm.sup.2 G, preferably 10 to 30 
kg/cm.sup.2 G, and the space velocity in the range of 1,000 to 30,000 
hr.sup.-1 (STP), preferably 3,000 to 8,000 hr.sup.-1 is desirably composed 
of 0.5 to 30% by volume of ethylene, 5 to 30% by volume of carbon dioxide 
gas, and the balance to make up 100% by volume of an inert gas such as 
nitrogen, argon, or steam, and a low hydrocarbon such as methane or ethane 
preferably plus 0.1 to 10 ppm (volume) of a halide such as ethylene 
dichloride, diphenyl chloride, vinyl chloride, monochlorobenzene or 
dichlorobenzene which is intended as a reaction inhibitor. 
Examples of the source of molecular oxygen to be used in the present 
invention, there can be cited air, oxygen, and oxygen enriched air. 
The chemical action of dispersion and deposition of the cesium ion on the 
surface of silver is believed to produce a notable steric hindrance effect 
upon various adsorbates on the surface of silver during the oxidation of 
+ethylene. One phase of this effect is manifested on adsorbates of oxygen 
species by effectively coating adjacent silver atoms and thereby curbing 
dissociative adsorption of oxygen and suppressing complete oxidation. 
Another phase of the effect is manifested in curbing and readsorption of 
the produced ethylene oxide on silver and suppressing the isomerization of 
ethylene oxide into acetaldehyde. In both the phases, the effect is 
believed to contribute directly to enhancing the selectivity for ethylene 
oxide.

Now, the present invention will be described more specifically below with 
reference to working examples and controls. It should be noted that the 
examples are purely illustrative of, and not limitative in any sense of, 
the present invention. 
The numerical values of conversion and selectivity to be indicated in the 
working examples and the controls are the results of calculation based on 
the following formulas. 
##EQU2## 
EXAMPLE 1 
A silver impregnant was prepared by dissolving 470 g of silver nitrate in 
300 g of water, keeping the solution cooled in a water bath, adding 360 g 
of ethanolamine to the solution, and thoroughly stirring the resulting 
mixture. In this impregnant was immersed 2.2 liters of .alpha.- alumina 
carrier having an apparent porosity of 57% and a BET specific surface area 
of 0.78 m.sup.2 /g. The impregnation mixture was gradually heated to 
90.degree. C., stirred at this temperature for 3 hours, heated further to 
120.degree. C., and stirred for 2 hours so as to have the reduced silver 
dispersed and deposited on the carrier. The silver-deposited catalyst thus 
obtained was washed five times with 3 liters of boiling water and then 
dried by heating in a current of nitrogen at 110.degree. to 120.degree. C. 
for 4 hours. 
Then, the dried catalyst was kept immersed in a solution of 1.60 g of 
cesium carbonate in 1,615 ml of reagent grade ethanol at 20.degree. C. for 
3 hours. Subsequently, the catalyst was deprived of excess impregnant and 
further swept with dry nitrogen flowing at a rate of 50 liters/minute for 
5 hours for thorough evaporation and expulsion of the solvent remaining in 
the pores of the carrier. In this while, the temperature of the catalyst 
was prevented from exceeding 20.degree. C. 
The catalyst obtained at this point was found to have 13.5% by weight of 
silver deposited thereon. The surface area of this silver was 1.03 m.sup.2 
/g of catalyst, the exposed surface area of the carrier was 0.14 m.sup.2 
/g of catalyst, and the amount of cesium ion deposited by adsorption on 
the surface of silver was 2.3.times.10.sup.-6 gram equivalent per the unit 
area, m.sup.2, of the surface of the silver. 
An externally heating type double-pipe stainless steel reactor 25 mm in 
inside diameter and 6,000 mm in length was packed with the catalyst. A 
mixed feed gas consisting of 20% by volume of ethylene, 8% by volume of 
oxygen, 7% by volume of carbon dioxide gas, and the balance to make up 
100% by volume of methane, nitrogen, argon, and ethane and further 
containing 2 ppm of ethylene dichloride was introduced into the catalyst 
bed and left reacting under a reaction pressure of 15 kg/cm.sup.2 G at a 
space velocity of 6,500 hr.sup.-1. The results obtained after 30 days 
reaction are shown in Table 1. Even after 6 months' continued reaction, 
this catalyst retained the performance intact. 
EXAMPLE 2 
A catalyst was prepared following the procedure of Example 1, except that a 
solution of 1.25 g of rubidium carbonate in 1,615 ml of reagent grade 
methanol was used in place of the solution of 1.60 g of cesium carbonate 
in 1,615 ml of reagent grade ethanol. The catalyst thus obtained was found 
to have 13.5% by weight of silver deposited thereon. The surface area of 
the silver was 1.03 m.sup.2 /g of catalyst, the exposed surface area of 
the carrier was 0.14 m.sup.2 /g of catalyst, and the amount of rubidium 
ion deposited by adsorption on the silver was 2.6.times.10.sup.-6 gram 
equivalent per the unit area, m.sup.2, of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results after 30 days of reaction were as 
shown in Table 1. Even after 6 months' continued reaction, this catalyst 
retained the performance thereof intact. 
EXAMPLE 3 
A catalyst was prepared following the procedure of Example 1, except that a 
solution of 1.30 g of potassium nitrate in 1,615 ml of reagent grade 
methanol was used in the place of the solution of 1.60 g of cesium 
carbonate in 1,615 ml of reagent grade ethanol. The catalyst thus obtained 
was found to have 13.5% by weight of silver deposited thereon. The surface 
area of the silver was 1.03 m.sup.2 /g, the exposed surface area of the 
carrier was 0.14 m.sup.2 /g, and the amount of potassium ion deposited by 
adsorption on the silver was 2.8.times.10.sup.-6 gram equivalent per the 
unit area, m.sup.2, of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 1. Even after 6 months' continued reaction, this catalyst 
retained the performance thereof intact. 
EXAMPLE 4 
A catalyst prepared following the procedure of Example 1, except that 2.45 
g of thallium acetate was used in place of 1.60 g of cesium carbonate. The 
catalyst thus obtained was found to have 13.5% by weight of silver 
deposited thereon. The surface area of the silver was 1.03 m.sup.2 /g of 
catalyst, the exposed surface area of the carrier was 0.14 m.sup.2 /g of 
catalyst, and the amount of thallium (monovalent) ion deposited by 
adsorption on the silver was 2.7.times.10.sup.-6 gram equivalent per the 
unit area, m.sup.2, of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 1. Even after 6 months' continued reaction, this catalyst 
retained the performance thereof intact. 
EXAMPLE 5 
A catalyst was prepared following the procedure of Example 1, except that 
an .alpha.-alumina carrier having an apparent porosity of 54% and a BET 
specific surface area of 1.12 m.sup.2 /g was used instead and a solution 
of 2.40 g of cesium carbonate in 1,600 ml of reagent grade ethanol was 
used in place of the solution of 1.60 g of cesium carbonate in 1,615 ml of 
reagent grade thanol. The catalyst thus obtained was found to have 13.5% 
by weight of silver deposited thereon. The surface area of the silver was 
1.25 m.sup.2 /g of catalyst, the exposed surface area of the carrier was 
0.36 m.sup.2 /g of catalyst, and the amount of cesium ion deposited by 
adsorption on the silver was 2.9.times.10.sup.-6 gram equivalent per the 
unit area, m.sup.2, of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 1. Even after 6 months' continued reaction retained the 
performance thereof intact. 
EXAMPLE 6 
A silver-containing impregnant was prepared by dissolving 520 g of silver 
nitrate in 300 g of water, keeping this solution cooled in a water bath, 
adding 400 g of ethanolamine to the cooled solution, and thoroughly 
stirring the resulting mixture. In this impregnant was immersed 2.2 liters 
of .alpha.-alumina carrier having an apparent porosity of 60% and a BET 
specific surface area of 2.80 m.sup.2 /g. The impregnation mixture was 
gradually heated to 90.degree. C., stirred at this temperature for 3 
hours, further heated to 120.degree. C., and stirred for 2 hours to have 
the reduced silver dispersed and deposited on the carrier. The 
silver-deposited catalyst consequently obtained was washed five times with 
3 liters of boiling water and was then dried by heating in a current of 
nitrogen at 110.degree. to 120.degree. C. for 4 hours. 
Then, the dried catalyst was kept immersed in a solution of 4.65 g of 
cesium carbonate in 1,650 ml of reagent grade ethanol at 20.degree. C. for 
3 hours. Subsequently, the catalyst was deprived of excess impregnant and 
further swept with dry nitrogen flowing at a flow rate of 50 liters/minute 
for 5 hours thorough evaporation and expulsion of the solvent remaining 
within the pores of the carrier. In this while, the temperature of the 
catalyst was prevented from exceeding 20.degree. C. 
The catalyst thus obtained was found to have 14.7% by weight of silver 
deposited thereon. The surface area of the silver was 2.42 m.sup.2 /g of 
catalyst, the exposed surface area of the carrier was 1.18 m.sup.2 /g of 
catalyst, and the amount of cesium ion deposited by adsorption on the 
silver was 2.1.times.10.sup.-6 gram equivalent per the unit area, m.sup.2, 
of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 1. Even after 6 months' continued reaction, the catalyst 
retained the performance thereof intact. 
EXAMPLE 7 
A catalyst was prepared following the procedure of Example 6, excepting 320 
g of water was used in place of 300 g of water, an .alpha.-alumina carrier 
having an apparent porosity of 62% and a BET specific surface area of 3.53 
m.sup.2 /g was used instead, and a solution of 5.10 g of cesium carbonate 
in 1,680 ml of reagent grade ethanol was used in place of the solution of 
4.65 g of cesium carbonate in 1,650 ml of reagent grade ethanol. The 
catalyst thus obtained was found to have 14.8% by weight of silver 
deposited thereon. The surface area of the silver was 2.86 m.sup.2 /g of 
catalyst, the exposed surface area of the carrier was 1.58 m.sup.2 /g of 
catalyst, and the amount of cesium ion deposited by adsorption on the 
silver was 2.0.times.10.sup.-6 gram equivalent per the unit area, m.sup.2, 
of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 1. Even after 6 months' continued reaction, this catalyst 
retained the performance thereof intact. 
EXAMPLE 8 
A silver impregnant was prepared by dissolving 470 g of silver nitrate in 
300 g of water, keeping the solution cooled in a water bath, adding 360 g 
of ethanolamine to the cooled solution, and thoroughly stirring the 
resulting mixture. In the impregnant was immersed 2.2 liters of .alpha.- 
alumina carrier having an apparent porosity of 57% and a BET specific 
surface area of 0.78 m.sup.2 /g. This impregnation mixture was gradually 
heated to 90.degree. C., stirred at this temperature for 3 hours, then 
heated further to 120.degree. C., and stirred for 2 hours so as to have 
the reduced silver dispersed and deposited on the carrier. The 
silver-deposited catalyst consequently obtained was washed 5 times with 3 
liters of boiling water and then heated in a current of nitrogen at 
110.degree. to 120.degree. C. for 4 hours. 
Subsequently, the dried catalyst was kept immersed in a solution of 1.60 g 
of cesium carbonate in 1,615 ml of reagent grade ethanol of 0.degree. C. 
for 3 hours. Then, the catalyst was deprived of excess impregnant and 
swept with dry nitrogen flowing at a rate of 50 liters/minute for 8 hours 
thorough evaporation and expulsion of the solvent remaining in the pores 
of the carrier. In this while, the temperature of the catalyst was 
prevented from exceeding 0.degree. C. 
The catalyst consequently obtained was found to have 13.5% by weight of 
silver deposited thereon. The surface area of this silver was 1.03 m.sup.2 
/g of catalyst, the exposed surface area of the carrier was 0.14 m.sup.2 
/g of catalyst, and the amount of cesium ion deposited by adsorption on 
the silver was 2.4.times.10.sup.-6 gram equivalent per the unit area, 
m.sup.2, of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 1. Even after 6 months' continued reaction, this catalyst 
retained the performance thereof intact. 
EXAMPLE 9 
A silver impregnant was prepared by dissolving 470 g of silver nitrate in 
700 g of monoethylene glycol, adding 190 g of formaldehyde to the 
solution, and thoroughly stirring the resulting mixture. In the impregnant 
was immersed 2.2 liters of .alpha.-alumina carrier having an apparent 
porosity of 57% and a BET specific surface area of 0.78 m.sup.2 /g. This 
impregnation mixture was stirred and, at the same time, heated to 
130.degree. C., stirred at this temperature for 2 hours, further heated to 
160.degree. C., and stirred for 2 hours so as to have the reduced silver 
dispersed and deposited on the carrier. The silver-deposited catalyst 
consequently obtained was washed 8 times with boiling water and then dried 
by heating in a current of nitrogen at 110.degree. to 120.degree. C. for 4 
hours. 
The dried catalyst was kept immersed in a solution of 1.45 g of cesium 
carbonate in 1,615 ml of reagent grade ethanol at 20.degree. C. for 3 
hours. Then, the catalyst was deprived of excess impregnant and swept with 
dry nitrogen flowing at a rate of 50 liters/minute for 5 hours for 
thorough evaporation and expulsion of the solvent remaining inside the 
pores of the carrier. In this while, the temperature of the catalyst was 
prevented from exceeding 20.degree. C. 
The catalyst consequently obtained was found to have 13.5% by weight of 
silver deposited thereon. The surface area of this silver was 0.91 m.sup.2 
/g of catalyst, the exposed surface area of the carrier was 0.22 m.sup.2 
catalyst, and the amount of cesium ion deposited by adsorption on the 
silver was 2.3.times.10.sup.-6 gram equivalent per the unit area, m.sup.2, 
of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 1. Even after 6 months' continued reaction, this catalyst 
retained the performance thereof intact. 
CONTROL 1 
A silver impregnant was prepared by dissolving 470 g of silver nitrate in 
300 g of water, keeping the solution cooled in a water bath, adding 360 g 
of ethanolamine to the cooled water, and thoroughly stirring the resulting 
mixture. In the impregnant was immersed 2.2 liters of .alpha.-alumina 
carrier having an apparent porosity of 57% and a BET specific surface area 
of 0.78 m.sup.2 /g. This impregnation mixture was gradually heated to 
90.degree. C., stirred at this temperature for 3 hours, then heated 
further to 120.degree. C., and stirred for 2 hours so as to have the 
reduced silver dispersed and deposited on the carrier. The 
silver-deposited catalyst consequently obtained was washed 5 times with 3 
liters of boiling water and then dried by heating in a current of nitrogen 
at 110.degree. to 120.degree. C. for 4 hours. 
Then, the dried catalyst was kept immersed in a solution of 5.05 of cesium 
carbonate in 1615 ml of reagent grade ethanol at 20.degree. for 3 hours. 
Subsequently, the catalyst was deprived of excess impregnant and further 
swept with dry nitrogen flowing at a flow rate of 50 liters/minute for 5 
hours for thorough evaporation and expulsion of the solvent remaining 
within the pores of the carrier. In this while, the temperature of the 
catalyst was prevented from exceeding 20.degree. C. 
The catalyst thus obtained was found to have 13.5% by weight of silver 
deposited thereon. The surface area of the silver was 1.03 m.sup.2 /g of 
catalyst, the exposed surface area of the carrier was 0.14 m.sup.2 /g of 
catalyst, and the amount of cesium ion deposited by adsorption on the 
silver was 5.5.times.10.sup.-6 gram equivalent per the unit area, m.sup.2, 
of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the result of 30 days' reaction were as shown 
in Table 1. 
CONTROL 2 
A catalyst was prepared following the procedure of Control 1, except that 
0.24 g of cesium carbonate was used in place of 5.05 g of cesium 
carbonate. 
The catalyst thus obtained was found to have 13.5% by weight of silver 
deposited thereon. The surface area of this silver was 1.03 m.sup.2 /g of 
catalyst, the exposed surface area of the carrier was 0.14 m.sup.2 /g of 
catalyst, and the amount of cesium ion deposited by adsorption on the 
silver was 0.4.times.10.sup.-6 gram equivalent per the unit area, m.sup.2, 
of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 1. 
CONTROL 3 
A catalyst was obtained by following the procedure of Control 1, except 
that 1.45 g of cesium carbonate was used in place of 5.05 g of cesium 
carbonate, the carrier was kept immersed in the ethanol solution of cesium 
carbonate at 70.degree. C. for 3 hours, and the outer wall temperature of 
the catalyst bed during the evaporation and expulsion of the solvent 
retained in the pores of the carrier was kept at 70.degree. C. 
The catalyst thus obtained was found to have 13.5% by weight of silver 
deposited thereon. The surface area of the silver was 1.03 m.sup.2 /g of 
catalyst, the exposed surface area of the carrier was 0.14 m.sup.2 /g of 
catalyst, and the amount of cesium ion deposited by adsorption on the 
silver was 0.8.times.10.sup.-6 gram equivalent per the unit area, m.sup.2, 
of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 1. After six months' continued reaction using this catalyst, the 
reaction temperature increased 2.degree. C. and the selectivity decreased 
to 73.2%. 
TABLE 1 
__________________________________________________________________________ 
Example 
1 2 3 4 5 6 
__________________________________________________________________________ 
Specific surface area 
0.78 0.78 0.78 0.78 1.12 2.80 
of carrier (m.sup.2 /g) 
Apparent porosity of 
57 57 57 57 54 60 
carrier (%) 
Impregnant 
(Solute) 
cesium 
rubidium 
potassium 
thallium 
cesium 
cesium 
containing carbon- 
carbon- 
nitrate 
acetate 
carbon- 
carbon- 
metal ate ate ate ate 
compound 
(Solvent) 
ethanol 
methanol 
methanol 
ethanol 
ethanol 
ethanol 
Immersion temperature in 
20 20 20 20 20 20 
impregnate containing metal 
compound (.degree.C.) 
Drying temperature after 
20 20 20 20 20 20 
deposition of metal 
compound by adsorption (.degree.C.) 
Amount of silver 
13.5 13.5 13.5 13.5 13.6 14.7 
deposited (% by weight) 
Surface area of silver 
1.03 1.03 1.03 1.03 1.25 2.42 
(m.sup.2 /g of catalyst) 
Exposed surface area of 
0.14 0.14 0.14 0.14 0.36 1.18 
catalyst (m.sup.2 /g of catalyst) 
Adsorbed ion cesium 
rubidium 
potassium 
thallium 
cesium 
cesium 
Amount (gram equivalent) of 
2.3 .times. 10.sup.-6 
2.6 .times. 10.sup.-6 
2.8 .times. 10.sup.-6 
2.7 .times. 10.sup.-6 
2.9 .times. 10.sup.-6 
2.1 .times. 10.sup.-6 
adsorbed ion per m.sup.2 of 
surface area of silver 
Reaction temperature (.degree.C.) 
230 227 224 226 232 224 
Conversion (%) 10.0 10.0 10.0 10.0 10.0 10.0 
Selectivity (%) 
83.1 82.3 81.2 82.4 82.9 82.3 
__________________________________________________________________________ 
Example Control 
7 8 9 1 2 3 
__________________________________________________________________________ 
Specific surface area 
3.53 0.78 0.78 0.78 0.78 0.78 
of carrier (m.sup.2 /g) 
Apparent porosity of 
62 57 57 57 57 57 
carrier (%) 
Impregnant 
(Solute) 
cesium 
cesium 
cesium 
cesium 
cesium 
cesium 
containing carbon- 
carbon- 
carbon- 
carbon- 
carbon- 
carbon- 
metal ate ate ate ate ate ate 
compound 
(Solvent) 
ethanol 
ethanol 
ethanol 
ethanol 
ethanol 
ethanol 
Immersion temperature in 
20 0 20 20 20 70 
impregnate containing metal 
compound (.degree.C.) 
Drying temperature after 
20 0 20 20 20 70 
deposition of metal 
compound by adsorption (.degree.C.) 
Amount of silver 
14.8 13.5 13.5 13.5 13.5 13.5 
deposited (% by weight) 
Surface area of silver 
2.86 1.03 0.91 1.03 1.03 1.03 
(m.sup.2 /g of catalyst) 
Exposed surface area of 
1.58 0.14 0.22 0.14 0.14 0.14 
catalyst (m.sup.2 /g of catalyst) 
Adsorbed ion cesium 
cesium 
cesium 
cesium 
cesium 
cesium 
Amount (gram equivalent) of 
2.0 .times. 10.sup.-6 
2.4 .times. 10.sup.-6 
2.3 .times. 10.sup.-6 
5.5 .times. 10.sup.-6 
0.4 .times. 10.sup.-6 
0.8 .times. 10.sup.-6 
adsorbed ion per m.sup.2 of 
surface area of silver 
Reaction temperature (.degree.C.) 
223 231 232 260 211 216 
Conversion (%) 10.0 10.0 10.0 5.0 10.0 10.0 
Selectivity (%) 
82.2 83.2 82.9 73.5 72.0 76.0 
__________________________________________________________________________ 
EXAMPLE 10 
A silver impregnant was prepared by mixing 420 g of silver oxalate with 200 
g of water to produce a slurry, keeping the slurry cooled in a water bath, 
adding 360 g of ethanolamine to the cooled slurry, and thoroughly stirring 
the resulting mixture. In the impregnant was immersed 2.2 liters of 
.alpha.-alumina carrier having an apparent porosity of 55% and a BET 
specific surface area of 0.70 m.sup.2 /g. The impregnation mixture was 
stirred and heated to 90.degree. C. for 1 hour, then heated further to 
120.degree. C., and stirred for 1 hour so as to have the reduced silver 
dispersed and deposited on the carrier. The silver-deposited catalyst was 
heated in a current of air at 260.degree. C. for 24 hours. 
Then, this catalyst was kept immersed in a solution of 1.16 g of cesium 
carbonate in 1,580 ml of reagent grade ethanol at 20.degree. C. for 3 
hours. Subsequently, the catalyst was deprived of excess impregnant and 
further swept with dry nitrogen flowing at a rate of 50 liters/minute for 
5 hours for thorough evaporation and expulsion of the solvent remaining 
within the pores of the carrier. In this while, the temperature of the 
catalyst was prevented from exceeding 20.degree. C. 
The catalyst consequently obtained was found to have 13.2% by weight of 
silver deposited thereon. The surface area of this silver was 0.50 m.sup.2 
/g of catalyst, the exposed surface area of the carrier was 0.36 m.sup.2 
/g of catalyst, and the amount of cesium ion deposited by adsorption on 
the silver was 2.0.times.10.sup.-6 gram equivalent per the unit area, 
m.sup.2, of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 2. Even after 6 months' continued reaction, this catalyst 
retained the performance thereof intact. 
EXAMPLE 11 
A catalyst was prepared following the procedure of Example 10, except that 
a solution of 0.95 g of rubidium carbonate in 1,580 ml of reagent grade 
methanol was used in place of the solution of 1.16 g of cesium carbonate 
in 1580 ml of reagent grade ethanol. The catalyst thus obtained was found 
to have 13.2% by weight of silver deposited thereon. The surface area of 
the silver was 0.50 m.sup.2 /g of catalyst, the exposed surface area of 
the carrier was 0.36 m.sup.2 /g of catalyst, and the amount of rubidium 
ion deposited by adsorption on the silver was 2.3.times.10.sup.-6 gram 
equivalent per the unit area, m.sup.2, of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 2. Even after 6 months' continued reaction, this catalyst 
retained the performance thereof intact. 
EXAMPLE 12 
A catalyst was prepared following the procedure of Example 10, except that 
a solution of 0.95 g of potassium nitrate in 1580 ml of reagent grade 
methanol was used in place of the solution 1.16 g of cesium carbonate in 
1580 ml of regent grade ethanol. The catalyst thus obtained was found to 
have 13.2% by weight of silver deposited thereon. The surface area of the 
silver was 0.50 m.sup.2 /g of catalyst, the exposed surface area of the 
carrier was 0.36 m.sup.2 /g of catalyst, and the amount of potassium ion 
deposited by adsorption on the silver was 2.4.times.10.sup.-6 gram 
equivalent per the unit area, m.sup.2, of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 2. Even after 6 months' continued reaction, this catalyst 
retained the performance thereof intact. 
EXAMPLE 13 
A catalyst was prepared following the procedure of Example 10, except that 
2.00 g of thallium acetate was used in place of 1.16 g of cesium 
carbonate. The catalyst thus obtained was found to have 13.2% by weight of 
silver deposited thereon. The surface area of the silver was 0.50 m.sup.2 
/g of catalyst, the exposed surface area of the carrier was 0.36 m.sup.2 
/g of catalyst, and the amount of thallium (monovalent) ion deposited by 
adsorption on the silver was 2.5.times.10.sup.-6 gram equivalent per the 
unit area, m.sup.2, of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 2. Even after 6 months' continued reaction, this catalyst 
retained the performance thereof intact. 
EXAMPLE 14 
A catalyst was prepared following the procedure of Example 10, except that 
an .alpha.-alumina carrier having an apparent porosity of 53% and a BET 
specific surface area of 1.05 m.sup.2 /g was used instead and a solution 
of 2.0 g of cesium carbonate in 1,560 ml of reagent grade ethanol was used 
in place of the solution of 1.16 g of cesium carbonate in 1,580 ml of 
reagent grade ethanol. 
The catalyst thus obtained was found to have 13.63 % by weight of silver 
deposited thereon. The surface area of the silver was 0.60 m.sup.2 /g of 
catalyst, the exposed surface area of the carrier was 0.60 m.sup.2 /g of 
catalyst, and the amount of cesium ion deposited by adsorption on the 
silver was 2.6.times.10.sup.-6 gram equivalent per the unit area, m.sup.2, 
of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 2. Even after 6 months' continued reaction, this catalyst 
retained the performance thereof intact. 
EXAMPLE 15 
A catalyst was prepared following the procedure of Example 10, except that 
the impregnation temperature of the ethanol solution of cesium carbonate 
and upper limit temperature of the catalyst bed during drying was changed 
from 20.degree. C. to 0.degree. C. and flowing time of nitrogen during 
drying was changed from 5 hours to 8 hours. 
The catalyst thus obtained was found to have 13.2% by weight of silver 
deposited thereon. The surface area of the silver was 0.50 m.sup.2 /g of 
catalyst, the exposed surface area of the carrier was 0.36 m.sup.2 /g of 
catalyst, and the amount of cesium ion deposited by adsorption on the 
silver was 2.1.times.10.sup.-6 gram equivalent per the unit area, m.sup.2, 
of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 2. Even after 6 months' continued reaction, this catalyst 
retained the performance thereof intact. 
CONTROL 4 
A silver-deposited catalyst obtained following the procedure of Example 10 
was kept immersed in a solution of 4.50 g of cesium carbonate in 1,580 ml 
of reagent grade ethanol at 20.degree. C. for 3 hours. Subsequently, the 
catalyst was deprived of excess impregnant and further swept with dry 
nitrogen flowing at a rate of 50 liters/ minute for 5 hours for thorough 
evaporation and expulsion of the solvent remaining within the pores of the 
carrier. In this while, the temperature of the catalyst was prevented from 
exceeding 20.degree. C. 
The catalyst consequently obtained was found to have 13.2% by weight of 
silver deposited thereon. The surface area of the silver was 0.50 m.sup.2 
exposed surface area of the carrier was 0.36 m.sup.2 /g of catalyst, and 
the amount of cesium ion deposited by adsorption on the silver was 
6.1.times.10.sup.-6 gram equivalent per the unit area, m.sup.2, of the 
surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 2. 
CONTROL 5 
A silver-deposited catalyst prepared following the procedure of Example 10 
was kept immersed in a solution of 0.22 g of cesium carbonate in 1,580 ml 
of reagent grade ethanol at 20.degree. C. for 3 hours. Then, the catalyst 
was deprived of excess impregnant and further swept with dry nitrogen 
flowing at a rate of 50 liters/minute for 5 hours for thorough evaporation 
and expulsion of the solvent remaining within the pores of the carrier. In 
this while, the temperature of the catalyst was prevented from exceeding 
20.degree. C. 
The catalyst consequently obtained was found to have 13.2% by weight of 
silver deposited thereon. The surface area of the silver was 0.50 m.sup.2 
/g of catalyst, the exposed surface area of the carrier was 0.36m.sup.2 /g 
of catalyst, and the amount of cesium ion deposited by adsorption on the 
silver was 0.4.times.10.sup.-6 gram equivalent per the unit area, m.sup.2, 
of the surface of the silver. 
When oxidation of ethylene was carried out by using this catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 2. 
CONTROL 6 
A silver catalyst prepared following the procedure of Example 10 was kept 
immersed in a solution of 1.16 g of cesium carbonate in 1,580 ml of 
reagent grade ethanol at 70.degree. C. for 3 hours. Then, the catalyst was 
deprived of excess impregnant and further swept with dry nitrogen flowing 
at a rate of 50 liters/minute for 3 hours for thorough evaporation and 
expulsion of the solvent remaining within the pores of the carrier. In 
this while, the temperature of the catalyst was kept at 70.degree. C. 
The catalyst consequently obtained was found to have 13.2% by weight of 
silver deposited thereon. The surface area of the silver was 0.50 m.sup.2 
/g of catalyst, the exposed surface area of the carrier was 0.36 m.sup.2 
/g of catalyst, and the amount of cesium ion deposited by adsorption on 
the silver was 0.8.times.10.sup.-6 gram equivalent per the unit area, 
m.sup.2, of the surface of the silver. 
When oxidation of ethylene was carried out by using the catalyst following 
the procedure of Example 1, the results of 30 days' reaction were as shown 
in Table 2. After six months' continued reaction using this catalyst, the 
reaction temperature increased 3.degree. C. and the selectivity decreased 
to 73.0%. 
TABLE 2 
__________________________________________________________________________ 
Example Control 
10 11 12 13 14 15 4 5 6 
__________________________________________________________________________ 
Specific surface area 
0.70 0.70 0.70 0.70 1.05 0.70 0.70 0.70 0.70 
of carrier (m.sup.2 /g) 
Apparent porosity of 
55 55 55 55 53 55 55 55 55 
carrier (%) 
Impregnant 
(Solute) 
cesium 
rubidium 
potassium 
thallium 
cesium 
cesium 
cesium 
cesium 
cesium 
containing carbon- 
carbon- 
nitrate 
acetate 
carbon- 
carbon- 
carbon- 
carbon- 
carbon- 
metal ate ate ate ate ate ate ate 
compound 
(Solvent) 
ethanol 
methanol 
methanol 
ethanol 
ethanol 
ethanol 
ethanol 
ethanol 
ethanol 
Immersion temperature in 
20 20 20 20 20 0 20 20 70 
impregnate containing metal 
compound (.degree.C.) 
Drying temperature after 
20 20 20 20 20 0 20 20 70 
deposition of metal 
compound by adsorption (.degree.C.) 
Amount of silver 
13.2 13.2 13.2 13.2 13.6 13.2 13.2 13.2 13.2 
deposited (% by weight) 
Surface area of silver 
0.50 0.50 0.50 0.50 0.60 0.50 0.50 0.50 0.50 
(m.sup.2 /g of catalyst) 
Exposed surface area of 
0.36 0.36 0.36 0.36 0.60 0.36 0.36 0.36 0.36 
catalyst (m.sup.2 /g of catalyst) 
Adsorbed ion cesium 
rubidium 
potassium 
thallium 
cesium 
cesium 
cesium 
cesium 
cesium 
Amount (gram equivalent) of 
2.0 .times. 
2.3 .times. 
2.4 .times. 
2.5 .times. 
2.6 .times. 
2.1 .times. 
6.1 .times. 
0.4 
0.8 .times. 
adsorbed ion per m.sup.2 of 
10.sup.-6 
10.sup.-6 
10.sup.-6 
10.sup.-6 
10.sup.-6 
10.sup.-6 
10.sup.-6 
10.sup.-6 
10.sup.-6 
surface area of silver 
Reaction temperature (.degree.C.) 
231 229 226 228 234 232 260 212 219 
Conversion (%) 7.5 7.5 7.5 7.5 7.5 7.5 3.9 7.5 7.5 
Selectivity (%) 
83.1 82.2 80.9 82.2 82.8 83.3 70.8 72.5 76.8 
__________________________________________________________________________ 
As already described in detail above, the conventional method which obtains 
a catalyst by the addition of a reaction promoter pays absolutely no 
consideration to the dispersion and deposition of at least one metal ion 
selected from the group consisting of cesium, rubidium, potassium, and 
thallium (monovalent) on the surface of silver and, with respect to the 
range of amount considered effective, only specifies a superificial amount 
departing far from the substantial truth. Thus, the catalyst produced by 
the conventional method fails to acquire satisfactory performance or 
sufficient catalyst life. The catalyst of the present invention realizes 
heretofore unattainable high selectivity and long catalyst life and, 
therefore, enjoys great economic advantage.