# Patent

## Title
Process for treating fluorine compound-containing gas

## Abstract

A gas stream containing at least one fluorine compound selected from the group consisting of compounds of carbon and fluorine, compounds of carbon, hydrogen and fluorine, compounds of sulfur and fluorine, compounds of nitrogen and fluorine and compounds of carbon, hydrogen, oxygen and fluorine is contacted with a catalyst comprising at least one of alumina, titania, zirconia and silica, preferably a catalyst comprising alumina and at least one of nickel oxide, zinc oxide and titania in the presence of steam, thereby hydrolyzing the fluorine compound at a relatively low temperature, e.g. 200°–800° C., to convert the fluorine of the fluorine compound to hydrogen fluoride.

## Background
This application is a divisional of U.S. patent application Ser. No. 09/005,006, filed Jan. 9, 1998.


BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to a process for efficient decomposition treatment of a gas containing fluorine compounds such as C2F6, CF4, C3F8, C4F8, CHF3, SF6, NF3, etc. at a low temperature.
2) Related Art
Fluorine compound gases such as CF4, C2F6, etc. are used in a large amount as a semiconductor etchant, a semiconductor cleaner, etc. However, it was found that these compounds, once discharged into the atmosphere, turn into warming substances causing global warming. Post-treatment of these compounds after their use would be subject to a strict control in the future.
Compounds having a high fluorine (F) content as a molecule constituent such as CF4, C2F6, etc. have a higher electronegativity of fluorine and thus are chemically very stable. From this nature it is very hard to decompose such fluorine compounds, and it is thus in the current situations that no appropriate processes for such decomposition treatment are not available yet.
JP-B-6-59388 (U.S. Pat. No. 5,176,897) discloses a TiO2—WO3 catalyst for hydrolysis of organic halogen compounds. The catalyst contains 0.1 to 20% by weight of W on the basis of TiO2 (i.e. 92% to 99.96% of Ti by atom and 8 to 0.04% by atom of W) and has a decomposition rate of 99% at 375° C. for a duration of 1,500 hours in treatment of CCl4 in ppm order. JP-B-6-59388 suggests that organic halogen compounds having a single carbon atom, such as CF4, CCl2F2, etc. can be decomposed, but shows no examples of decomposition results of fluorine compounds.
JP-A-7-80303 discloses another Al2O3—ZrO2—WO3 catalyst for decomposition of fluorine compound gases. The catalyst is directed to combustion-decomposition of CFCs (chlorofluorocarbons) and has a decomposition rate of 98% for a duration of 10 hours in treatment of CFC-115 (C2ClF5) by combustion-decomposition reaction at 600° C. The disclosed process needs addition of hydrocarbons such as n-butane, etc. as a combustion aid, resulting in a higher treatment cost. Among organic halogen compounds to be treated, fluorine compounds are less decomposable than chlorine compounds. Furthermore, the more the carbon atoms of organic halogen compound, the less decomposable. Decomposition of compounds consisting only of carbon and fluorine such as C2F6, etc. are much less decomposable than CFC-115, but no examples of decomposition results of such compounds are shown therein.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a process for efficient decomposition treatment of compounds of carbon and fluorine, compounds of carbon, hydrogen and fluorine, compounds of sulfur and fluorine, compounds of nitrogen and fluorine and even compounds of carbon, hydrogen, fluorine and oxygen such as C2F6, CF4, C3F8, C4F8, CHF3, SF6 and NF3.
The present invention provides a process for treating a fluorine compound-containing gas, which comprises contacting a gas stream containing at least one fluorine compound selected from the group consisting of compounds of carbon and fluorine, compounds of carbon, hydrogen and fluorine, compounds of sulfur and fluorine, compounds of nitrogen and fluorine and compounds of carbon, hydrogen, oxygen and fluorine with a catalyst containing at least one of alumina, titania, zirconia and silica in the presence of steam, thereby hydrolyzing the fluorine compound to convent fluorine of the fluorine compound to hydrogen fluoride.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a process for treating a fluorine compound-containing gas according to one embodiment of the present invention.
FIG. 2 is a graph showing performances of various catalysts for decomposing a fluorine compound.
FIG. 3 is a graph showing performances of various catalysts for decomposing a fluorine compound.
FIG. 4 is a graph showing performances of various catalysts for decomposing a fluorine compound.
FIG. 5 is a graph showing performances of various catalysts for decomposing a fluorine compound.
FIG. 6 is a graph showing performances of various catalysts for decomposing a fluorine gas.
FIG. 7 is a graph showing performance of catalysts with various composition ratios for decomposing a fluorine gas.
FIG. 8 is a graph showing performance of catalysts with various composition ratios for decomposing a fluorine gas.
FIG. 9 is a graph showing relations between reaction temperature and decomposition rate of various fluorine compounds.
FIG. 10 is a graph showing relations between reaction time and decomposition rate of a fluorine compound.
FIG. 11 is a graph showing relations between reaction temperature and decomposition rate of CHF3, CF4 and C4H8 by an Al2O3—ZnO catalyst.
FIG. 12 is a graph showing relations between reaction temperature and decomposition rate of SF6 and C3F8 by an Al2O3—NiO catalyst.
FIG. 13 is a graph showing relations between reaction temperature and decomposition rate of C4F8 by an Al2O3—NiO—ZnO catalyst.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
As a result of extensive studies on development of catalysts for decomposition of fluorine compound-containing gases, the present inventors have found that catalysts must contain a metallic component capable of forming an appropriately strong bond with fluorine as the nature of catalysts, and further have found that catalysts containing a metallic component having a higher fluoride formation enthalpy show a higher decomposition activity particularly in case of compounds consisting of carbon and fluorine, because molecules of such compounds are stable by themselves. Formation of too stable a bond will gradually lower the decomposition activity of catalysts, because fluorine compounds are less releasable from the catalyst surface, whereas too weak a bonding force will not attain a satisfactory decomposition rate. C2F6, one of gases to be treated according to the present invention, is a compound of poor reactivity because of a higher intramolecular force, and it is said that a temperature of 1,500° to 2,000° C. is required for combustion of such a gas.
As a result of tests on various catalysts, the present inventors have found that catalysts of alumina (Al2O3), titania (TiO2), zirconia (ZrO2), silica (SiO2), a mixture of titania and zirconia, a mixture of alumina and magnesia (MgO), a mixture of alumina and titania, or a mixture of alumina and silica can hydrolyze fluorine compounds, and further have found that the fluorine compounds can be decomposed at a lower temperature than 800° C. thereby.
Among these catalysts, it has been found that a catalyst based on a mixture of alumina and titania has the highest activity and particularly a catalyst comprising 75 to 98% by weight of alumina and 25 to 2% by weight of titania has a particularly high activity. It can be presumed that the alumina of the catalyst based on a mixture of alumina and titania acts to attract fluorine compounds onto the catalyst, whereas the titania acts to depart the fluorine compounds from the catalyst surface.
The present inventors further have found that catalysts based on the mixture of alumina and titania further containing at least one member selected from the group consisting of zirconia, tungsten oxide, silica, tin oxide, ceria, bismuth oxide, nickel oxide and boron oxide can hydrolyze fluorine compounds. It has been found that above all the catalyst containing zirconia has a higher decomposition activity on fluorine compounds. It has been further found that the content of at least one member selected from the group consisting of zirconia, tungsten oxide, silica, tin oxide, ceria, bismuth oxide, nickel oxide and boron oxide is preferably 0.1 to 10% by weight on the basis of sum total of alumina and titania and particularly the content of zirconia is preferably 2 to 10% by weight on the basis of sum total of alumina and titania. It seems that these additive members exist in the form of single oxides or composite oxides and contribute to an improvement of decomposition activity on fluorine compounds.
In catalyst preparation, it has been found that it is preferable to use boehmite for alumina raw material and a titanium sulfate solution for a titania raw material. It has been confirmed that there are sulfate ions, SO42−, in the catalysts prepared from the titanium sulfate solution and the decomposition activity on fluorine compounds can be improved by the presence of sulfate ions. It has been found that addition of sulfuric acid is preferable during the catalyst preparation.
The present inventors further tested catalysts containing other components besides alumina and titania, specifically catalysts containing alumina and one of zinc oxide (ZnO), nickel oxide (NiO), iron oxide, tin oxide (SnO2), platinum (Pt), cobalt oxide, zirconia (ZrO2), ceria (CeO2) and silica (SiO2). As a result, it has been found that these catalysts can hydrolyze fluorine compounds and particularly catalysts containing zinc oxide or nickel oxide have a higher activity than catalysts based on the mixture of alumina and titania. It has been further found that catalysts comprising alumina and nickel oxide, admixed with sulfuric acid during the catalyst preparation have a higher activity than the catalyst without admixing with sulfuric acid. It has not been confirmed in which forms iron oxide or cobalt oxide of the catalysts containing the iron oxide or the cobalt oxide exists. Probably it seems to exist in the form of Fe2O3 or Co3O4.
It has been found that the catalysts comprising alumina and one of Zinc oxide, nickel oxide, iron oxide, tin oxide, cobalt oxide, zirconia, ceria and silica as other components preferably contain 50 to 1% by atom of one metallic element of the other components, the balance being aluminum of the alumina, and the content of platinum is preferably 0.1 to 2% by weight on the basis (100% by weight) of alumina. It has been further found that these catalysts can further contain sulfur and the content of sulfur is preferably 0.1 to 20% by weight on the basis of the alumina catalyst.
Fluorine compounds to be treated according to the present invention include, for example, CF4, C2F6, C3F8, C4F8, C5F8, CHF3, CH2F2, CH3F, C2HF5, C2H2F4, C2H3F3, C2H4F2, C2H5F, CH2OCF2, SF6, NF3, etc., among which CF4, C2F6, C3F8, C4F8, CHF3, SF6 and NF3 are used as etchants for semiconductors and CF4, C2F6 and NF3 are used as cleaners for semiconductors.
According to the present invention, all of these fluorine compounds can be hydrolyzed. Hydrolysis temperature depends upon kinds of fluorine compounds and catalyst components, and is usually 200° to 800° C., preferably 400° to 800° C. According to the present process fluorine of fluorine compound gases can be converted to hydrogen fluoride.
Hydrolysis of fluorine compounds can proceed typically according to the following reaction equations:

CF4+2H2O→CO2+4HF  (1)

C2F6+3H2O→CO+CO2+6HF  (2)

CHF3+H2O→CO+3HF  (3)

SF6+3H2O→SO3+6HF  (4)

NF3+3/2 H2O→NO+1/2 O2+3HF  (5)

Hydrolysis according to reaction equations (2) and (3) can produce CO. The present catalysts also have an ability to oxidize CO, and thus CO can be further oxidized to CO2 in the presence of oxygen.
The present invention provides a process for hydrolyzing a fluorine compound-containing gas by a catalyst comprising at least one member selected from the group consisting of alumina, titania, zirconia, silica, a mixture of titania and zirconia, a mixture of alumina and magnesia, a mixture of alumina and titania and a mixture of alumina and silica.
Furthermore, the present invention provides a process for treating a fluorine-containing gas by a catalyst comprising alumina and titania, further containing 0.1 to 10% by weight, on the basis of alumina and titania, of one of zirconia, tungsten oxide, silica, tin oxide, ceria, bismuth oxide, nickel oxide and boron oxide.
Still furthermore, the present invention provides a process for treating a fluorine compound-containing gas by a catalyst comprising alumina and at least one member selected from the group consisting of zinc oxide, nickel oxide, iron oxide, tin oxide, cobalt oxide, zirconia, ceria, silica and platinum as other components, a ratio of aluminum of alumina to the metallic element of at least one of other components by atom is 50 to 99:50–1, and further by the catalyst further containing 0.1 to 20% by weight of sulfur on the basis of the alumina. These additive components can contribute to improvement of decomposition activity of the catalysts on fluorine compounds in the form of single oxides or composite oxides with aluminum and/or other additive components.
Still furthermore, the present invention provides a process for converting fluorine in a gas to hydrogen fluoride, which comprises contacting a gas stream containing a fluorine compound comprising C2F6 with a catalyst comprising a mixture of alumina and titania and having a weight ratio of alumina to titania being 65 to 90:35 to 10, thereby hydrolyzing the fluorine compounds. Still furthermore, the present invention provides a process for converting fluorine in a gas stream to hydrogen fluoride, which comprises contacting a gas stream comprising a fluorine compound comprising C2F6 with a catalyst comprising a mixture of alumina, titania and zirconia and having a weight ratio of alumina to titania being 65 to 90:35 to 10 and a weight ratio of zirconia to sum total of alumina and titania being 2 to 10:98 to 90, thereby hydrolyzing the fluorine compound.
Still furthermore, the present invention provides a process for converting fluorine in a gas stream to hydrogen fluoride, which comprises contacting a gas stream containing at least one fluorine compound selected from the group consisting of C2F6, CF4, C4F8 and CHF3 with a catalyst comprising a mixture of alumina and zinc oxide and having an atomic ratio of aluminum to zinc being 90 to 70:10 to 30, thereby hydrolyzing the fluorine compound.
Still furthermore, the present invention provides a process for converting fluorine in a gas stream to hydrogen fluoride, which comprises contacting a gas stream containing at least one fluorine compound selected from the group consisting of C2F6, CF4, C3F8, CHF3, NF3 and SF6 with a catalyst comprising a mixture of alumina and nickel oxide and having an atomic ratio of aluminum to nickel of 95 to 60:5 to 40, thereby hydrolyzing the fluorine compound.
Still furthermore, the present invention provides a process for converting fluorine in a gas stream to hydrogen fluoride, which comprises contacting a gas stream comprising a fluorine compound comprising C4F8 with a catalyst comprising a mixture of alumina, nickel oxide and zinc oxide, thereby hydrolyzing the fluorine compound.
Still furthermore, the present invention provides a process for converting fluorine in a gas stream to hydrogen fluoride, which comprises a hydrolysis step of contacting a gas discharged from a semiconductor-etching or cleaning step using a gas stream containing at least one fluorine compound selected from the group consisting of compounds of carbon and fluorine, compounds of carbon, hydrogen and fluorine, compounds of sulfur and fluorine, compounds of nitrogen and fluorine and compounds of carbon, hydrogen, oxygen and fluorine, after addition of air and steam to the gas, with a catalyst comprising at least one of alumina, titania, zirconia and silica, thereby hydrolyzing the fluorine compound to convert the fluorine in the gas to hydrogen fluoride, as a poststep to the semiconductor-etching or cleaning step.
Still furthermore, the present invention provide a process for treating a fluorine compound-containing gas, which further comprises an alkaline washing step of contacting the gas from the hydrolysis step with an alkaline washing solution, thereby washing the gas as a poststep to the hydrolysis step. As the alkaline washing solution, there can be used conventional ones such as a solution of NaOH, Ca(OH)2, Mg(OH)2, CaCO3, etc., a slurry of Ca(OH)2, etc.
In contacting of the gas stream containing a fluorine compound with the catalyst, the concentration of the fluorine compound in the gas stream is preferably 0.1 to 10% by volume, particularly preferably 0.1 to 3% by volume, and the space velocity is preferably 100 to 10,000 h−1, particularly preferably 100 to 3,000 h−1. Space velocity (h−1) is defined by reaction gas flow rate (ml/h)/catalyst volume (ml).
In the hydrolysis of the fluorine compound, it is desirable to add steam as a hydrogen source for hydrolysis to the gas stream so as to make the amount of hydrogen atoms (H) at least equal to the amount of fluorine atoms (F) contained in the fluorine compound, thereby making the fluorine atoms (F) of decomposition products into the hydrogen fluoride (HF) form that allows easy post-treatment. Hydrogen, hydrocarbons, etc. can be used as a hydrogen source besides the steam. In case of hydrocarbons as a hydrogen source, hydrocarbons can be combusted on the catalyst, thereby effectively reducing the heat energy to be supplied.
By adding an oxidizing gas such as oxygen, etc. to the reaction gas, oxidation reaction of CO can be carried out at the same time. When the oxidation reaction of CO is incomplete, the decomposition product gas is brought into contact with the CO oxidizing catalyst, after removal of HF from the decomposition product gas, to convert CO to CO2.
In the hydrolysis (decomposition) of fluorine compound, the reaction temperature is preferably about 200° to about 800° C. Above about 800° C., a higher decomposition rate can be obtained, but the catalyst will be rapidly deteriorated, and also the corrosion rate of apparatus structural materials will be abruptly increased, whereas below about 200° C. the decomposition rate will be lowered.
As the step of neutralizing and removing the formed HF, washing by spraying an alkaline solution is efficient and preferable because of less occurrence of clogging in pipings due to crystal deposition, etc. Bubbling of the decomposition product gas through the alkaline solution or washing with the alkaline solution through a packed column may be used for the neutralization and removal of the formed HF. Alternatively, HF can be absorbed in water, followed by treatment with an alkaline solution or slurry.
As the raw material for aluminum (Al) for preparing the present catalyst, γ-alumina and a mixture of γ-alumina and δ-alumina can be used besides boehmite. However, it is preferable to use boehmite as a raw material for Al to form an oxide through final firing.
As the raw material for titanium (Ti), titania sol, titanium slurry, etc. can be used besides titanium sulfate.
As the third metallic components for silica (Si), magnesium (Mg), zirconium (Zr), etc., their various nitrates, ammonium salts, chlorides, etc. can be used.
The present catalyst can be prepared by any of ordinary procedures for preparing catalysts, such as precipitation, impregnation, kneading, etc.
The present catalyst can be used as such or upon molding into a granular form, a honeycomb form, etc. by an desired molding procedure such as extrusion molding, tabletting, tumbling granulation, etc., or as a coating on ceramic or metallic honeycombs or plates.
Only a catalytic reactor for decomposing fluorine compounds and a facility for neutralizing and removing acid components in the decomposition product gas are required for an apparatus for carrying out the present process for treating fluorine compound-containing gas.
The present invention will be described in detail below, referring to Examples which are not limitative of the present invention.
FIG. 1 shows an example of using the present process for hydrolysis treatment of a fluorine gas in a cleaning step in a plasma CVD apparatus in the semi-conductor production process.
The plasma CVD apparatus is an apparatus for vapor depositing a SiO2 film on a semiconductor wafer surface. Since the SiO2 film tends to deposit on the entire interior surfaces of the apparatus, and thus it is necessary to remove SiO2 depositions from unwanted surfaces. To clean the unwanted surface to remove SiO2 therefrom, gases containing fluorine compounds such as C2F6, CF4, NF3, etc. are used as a cleaning gas. Cleaning gas 1 containing these fluorine compounds is led to a CVD chamber to remove SiO2 under plasma excitation. Then, the chamber is flushed with a N2 gas 2, thereby diluting the cleaning gas to a desired lower fluorine compound concentration, and the diluted cleaning gas is discharged from the chamber. The discharged gas is admixed with air 3 to further lower the fluorine compound concentration by dilution with air 3 and the air-diluted discharged gas is further admixed with steam 4 and the resulting reaction gas 5 is led to a decomposition step, where the reaction gas is brought into contact with a catalyst at a desired space velocity (h−1), which is defined by reaction gas flow rate (ml/h)/catalyst volume (ml) and at a desired temperature. In that case, the reaction gas may be heated or the catalyst may be heated by an electric oven, etc. The resulting decomposition gas 6 is led to an exhaust gas washing step, where the decomposition gas 6 is sprayed with an aqueous alkaline solution to remove acid components from the decomposition gas 6 and the resulting exhaust gas 7 freed from the acid components is discharged to the system outside.
CF4, C2F6 and NF3 can be used as etchants for semiconductors, etc., and CHF3, C3F6, SF6 and C4F8 can be also used as etchants besides the above-mentioned fluorine compounds. These etchants can be treated and decomposed in the same manner as in FIG. 1 except that the cleaning step of FIG. 1 is only replaced with an etching step.
Activities or performances of various catalysts for composing fluorine compounds were investigated, and results thereof will be described below:
EXAMPLE 1 
A C2F6 gas having a purity of 99% or more was diluted with air, and further admixed with steam to prepare a reaction gas. Steam for the admixture was prepared by feeding pure water into a reactor tube from the top at a flow rate of 0.11 ml/min. by a microtube pump and gasified. The reaction gas had a C2F6 concentration of about 0.5%. Then, the reaction gas was brought into contact with various catalysts heated to 700° C. in a reactor tube at a space velocity of 3,000 h−1. Heating of the catalyst was carried out by heating the reactor tube in an electric oven.
Reactor tube was an Inconel reactor tube having an inner diameter of 19 mm, where a catalyst bed was fixed at the center of the reactor tube and had an Inconel thermowell for a thermo couple, 3 mm in outer diameter, inside the catalyst bed. Decomposition product gas discharged from the catalyst bed was bubbled through an aqueous sodium chloride solution an then discharged as an exhaust gas. C2F6 decomposition rate was calculated by the following equation by determining concentration of C2F6 in the reaction gas at the inlet to the reactor tube and concentration of C2F6 in the decomposition gas at the outlet from the alkaline washing step by FID (flame ionization detector) gas chromatography and TCD (thermal conductivity detector) gas chromatography:




  
    Decomposition  rate
  
  =
  
    1
    -
    
      
        
          
            
              
                Concentration  of  discharged
              
            
          
          
            
              
                fluorine  compound
              
            
          
        
        
          
            
              
                Concentration  of  fed
              
            
          
          
            
              
                fluorine  compound
              
            
          
        
      
      ×
      100
      ⁢
      
        
      
      ⁢
      
        (
        %
        )
      
    
  




Catalyst 1: Al2O3 

Granular alumina (NKHD-24, trademark of a product commercially available from Sumitomo Chemical Co., Ltd., Japan) was pulverized, sieved to obtain a fraction of 0.5–1 mm grain sizes, followed by drying at 120° C. for 2 hours and firing (or calcining) at 700° C. for 2 hours.
Catalyst 2: TiO2 
Granular titania (CS-200-24, trademark of a product commercially available from Sakai Chemical Industry Co., Ltd., Japan) was pulverized, sieved to obtain a fraction of 0.5–1 mm grain sizes, followed by drying at 120° C. for 2 hours and firing at 700° C. for 2 hours.
Catalyst 3: ZrO2 
200 g of zirconyl nitrate was dried at 120° C. for 2 hours and fired at 700° C. for 2 hours. The resulting powders were placed in a mold and compression molded under a pressure of 500 kgf/cm2. The molded product was pulverized and sieved to obtain zirconia grains having grain sizes of 0.5–1 mm.
Catalyst 4: SiO2 
Granular silica (CARIACT-10, trademark of a product commercially available from Fuji Silysia Co., Ltd., Japan) was pulverized and sieved to obtain a fraction of 0.5–1 mm grain sizes, followed by drying at 120° C. for 2 hours and firing at 700° C. for 2 hours.
Catalyst 5: TiO2—ZrO2 
Granular titania (CS-200-24) was pulverized to grain sizes of 0.5 mm and under. 100 g of the resulting powders was admixed with 78.3 g of zirconyl nitrate and kneaded while adding pure water thereto. After the kneading, the kneaded mixture was dried at 120° C. for 2 hours and fired at 700° C. for 2 hours. The resulting powders were placed in a mold and compression molded under a pressure of 500 kgf/cm2. The molded product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Ti:Zr=81:19 and in a weight ratio of TiO2:ZrO2=73.5:26.5.
Catalyst 6: Al2O3—MgO
Granular alumina (NKHD-24) was pulverized to grain sizes of 0.5 mm and under. 100 g of the resulting powders were admixed with 56.4 g of magnesium nitrate and kneaded while adding pure water thereto. After the kneading, the kneaded mixture was dried at 120° C. for 2 hours and fired at 700° C. for 2 hours. The resulting powders were placed into a mold and compression molded under a pressure of 500 kgf/cm2. The molded product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Mg=90:10 and in a weight ratio of Al2O3:MgO=91.9:8.1.
Catalyst 7: Al2O3—TiO2 
Granular alumina (NKHD-24) was pulverized to grain sizes of 0.5 mm and under. 100 g of the resulting powders were admixed with 17.4 g of dried powders of a metatitanic acid slurry and kneaded while adding pure water thereto. After the kneading, the kneaded mixture was dried at 120° C. for 2 hours and fired at 700° C. for 2 hours. The resulting powders were placed in a mold and compression molded under a pressure of 500 kgf/cm2. The molded product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti=90:10 and in a weight ratio of Al2O3:TiO2=85.2:14.8.
Catalyst 8: Al2O3—SiO2 
Granular alumina (NKHD-24) was pulverized to grain sizes of 0.5 mm and under. 100 g of the resulting powders were admixed with 13.2 g of dried powders of SiO2 sol and kneaded while adding pure water thereto. After the kneading, the kneaded mixture was dried at 120° C. for 2 hours and fired at 700° C. for 2 hours. The resulting powders were placed in a mold and compression molded under a pressure of 500 mgf/cm2. The molded product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Si=90:10 and in a weight ratio of Al2O3:SiO2=88.3:11.7.
Test results of the above-mentioned catalysts 1 to 8 are shown in FIG. 2, from which it is evident that the Al2O3—TiO catalyst is preferable as a hydrolysis catalyst for a C2F6 gas.
EXAMPLE 2 
In this Example, influences of changes in composition ratios of alumina to titania in Al2O3—TiO2 catalysts upon C2F6 decomposition rate were investigated under the same test procedure and conditions as in Example 1. The results are shown in FIG. 4.
Catalyst 19: Al2O3 
Boehmite powders (PURAL SB, trademark of a product commercially available from Condea Co., Ltd.) were dried at 120° C. for 2 hours. 200 g of the resulting dried powders were fired at 300° C. for 0.5 hours and further fired at an elevated temperature of 700° C. for 2 hours. The resulting powders were placed into a mold and compression molded under a pressure of 500 kgf/cm2. The molded product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm, and tested. It was found that boehmite powders used as an alumina raw material had a higher catalytic activity than granular alumina.
Catalyst 20: Al2O3—TiO2 
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were kneaded with 248.4 g of an aqueous 30% titanium sulfate solution, while adding about 200 g of pure water thereto. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 5 hours and then fired at 700° C. for 2 hours. The resulting powders were placed into a mold and compression molded under a pressure of 500 kgf/cm2. The molded product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti=90:10 and in a weight ratio of Al2O3:TiO2=85.65:14.35.
Catalyst 21: AlO3—TiO2 
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were kneaded with about 100 g of an aqueous solution containing 78.6 g of 30% titania sol in pure water. After the kneading, the kneaded mixture was dried at 120° C. for about 2 hours and then fired at 700° C. for 2 hours. The resulting powders were placed into a mold and compression molded under a pressure of 500 kgf/cm2. The molded product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti=91:9 and in a weight ratio of Al2O3:TiO2=86.25:13.75.
It was found that the catalyst prepared from the titanium sulfate solution as a titanium raw material had the highest catalytic activity, probably because of the presence of sulfate ions SO42− in the catalyst.
EXAMPLE 3
In this Example, influences of changes in composition ratios of Al2O3 to TiO2 in Al2O3—TiO2 catalysts upon C2F6 decomposition rate were investigated under the same procedure and conditions as in Example 1.
Catalyst 22: Al2O3—TiO2 
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 100 g of the resulting dried powders were kneaded with 82.4 g of an aqueous 30% titanium sulfate solution while adding about 120 g of pure water thereto. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 5 hours and then fired at 700° C. for 2 hours. The resulting powders were placed into a mold and compression molded under a pressure of 500 kgf/cm2. The molded product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti=93:7 and in a weight ratio of Al2O3:TiO2=90.0:10.0.
Catalyst 23: Al2O3—TiO2 
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 100 g of the resulting dried powders were kneaded with 174.4 g of an aqueous 30% titanium sulfate solution while adding about 70 g of pure water thereto. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 5 hours and then fired at 700° C. for 2 hours. The resulting powders were placed into a mold and compression molded under a pressure of 500 kgf/cm2. The molded product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti=87:13 and in a weight ratio of Al2O3:TiO2=80.9:19.1.
Catalyst 24: Al2O3—TiO2 
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 100 g of the resulting dried powders were kneaded with 392 g of an aqueous 30% titanium sulfate solution while adding the latter to the former. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 5 hours and then fired at 700° C. for 2 hours. The resulting powders were placed into a mold and compression molded under a pressure of 500 kgf/cm2. The molded product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti=75:25 and in a weight ratio of Al2O3:TiO2=65.4:34.6.
Activities of catalysts 19, 20 and 22–24 are shown in FIG. 5, from which is evident that the highest C2F6 decomposition rate can be obtained at an alumina content of about 85% by weight.
EXAMPLE 4
In this Example, an influence of sulfuric acid during the preparation of the Al2O3—TiO2 catalyst upon the C2F6 decomposition rate was investigated.
Catalyst 25: Al2O3—TiO2 
Boehmite powders (PURAL SB) was dried at 120° C. for one hour. 150 g of the resulting dried powders were kneaded with 58.5 g of 30% titania sol (CS-N, trademark of a product commercially available from Ishihara Sangyo Kaisha, Ltd., Japan) and an aqueous solution prepared by diluting 44.8 g of 97% sulfuric acid with 250 ml of pure water. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 5 hours and then fired at 700° C. for 2 hours. The resulting powders were placed into a mold and compression molded under a pressure of 500 kgf/cm2. The molded product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti=91:9 and in a weight ratio of Al2O3:TiO2=86.3:13.7.
Sulfate ions were present in the catalyst. Test conditions were the same as in Example 1, except that the space velocity was changed to 1,000 h−1. The test results revealed that a C2F6 decomposition rate of 80% was obtained at a reaction temperature of 650° C.
EXAMPLE 5
In this Example, C2F6 decomposition rates were investigated by adding various components to the Al2O3—TiO2 catalysts. The catalysts were prepared as follows, but test procedure and conditions were the same as in Example 1.
Catalyst 9: Al2O3—TiO2 
Granular alumina (NKHD-24) was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm, followed by drying at 120° C. for 2 hours. Then, the dried grains were impregnated with 176 g of an aqueous 30% titanium sulfate solution. After the impregnation, the grains were dried at 250°–300° C. for about 5 hours and then fired at 700° C. for 2 hours. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti=90:10 and in a weight ratio of Al2O:TiO2=85.1:14.9. The catalyst thus prepared was designated as catalyst A.
Catalyst 10: Al2O3—TiO2—ZrO2 
50 g of Catalyst A grains were impregnated with an aqueous solution of 6.7 g of zirconyl nitrate dihydrate in 38.4 g of H2O. After the impregnation, the grains were dried at 120° C. for 2 hours and then fired at 700° C. for 2 hours. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti:Zr=90:10:0.025 and in a weight ratio of Al2O3:TiO2:ZrO2=80.2:14.0:5.8.
Catalyst 11: Al2O3—TiO2—WO3 
50 g of Catalyst A grains were impregnated with 38.4 g of an aqueous solution of 6.5 g of ammonium paratungstate in H2O. After the impregnation, the grains were dried at 120° C. for 2 hours and then fired at 700° C. for 2 hours. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti:W=90:10:0.025 and in a weight ratio of Al2O3:TiO2:WO3=76.6:13.4:10.0.
Catalyst 12: Al2O3—TiO2—SiO2 
50 g of Catalyst A grains were impregnated with 38.4 g of an aqueous solution of 7.5 g of 20 wt. % silica sol in H2O. After the impregnation, the grains were dried at 120° C. for 2 hours and then fired at 700° C. for 2 hours. The resulting grain composition for catalyst was in an atomic ratio of 90:10:0.025 and in a weight ratio of Al2O3:TiO2:SiO2=82.6:14.5:2.9.
Catalyst 13: Al2O2—TiO2—SnO2 
50 g of Catalyst A grains were impregnated with 38.4 g of an aqueous solution of 5.6 g of tin chloride dihydrate in H2O. After the impregnation, the grains were dried at 120° C. for 2 hours and then fired at 700° C. for 2 hours. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti:Sn=90:10:0.025 and in a weight ratio of Al2O3:TiO2:SnO2=79.1:13.9:7.0.
Catalyst 14: Al2O3—TiO2—CeO2 
50 g of Catalyst A grains were impregnated with 38.4 g of an aqueous solution of 10.9 g of cerium nitrate hexahydrate in H2O. After the impregnation, the grains were dried at 120° C. for 2 hours and then fired at 700° C. for 2 hours. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti:Ce=90:10:0.025 and in a weight ratio of Al2O3:TiO2:CeO2=78.4:13.7:7.9.
Catalyst 15: Al2O3—TiO2—MnO2 
50 g of Catalyst A grains were impregnated with 38.4 g of an aqueous solution of 7.2 g of manganese nitrate hexahydrate in H2O. After the impregnation, the grains were dried at 120° C. for 2 hours and then fired at 700° C. for 2 hours. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti:Mn=90:10:0.025 and in a weight ratio of Al2O3:TiO2:MnO2=81.6:14.3:4.1.
Catalyst 16: Al2O3—TiO2—Bi2O3 
50 g of Catalyst A grains were impregnated with 38.4 g of an aqueous solution of 12.13 g of bithmus nitrate hexahydrate in H2O. After the impregnation, the grains were dried at 120° C. for 2 hours and then fired at 700° C. for 2 hours. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti:Bi=90:10:0.025 and in a weight ratio of Al2O3:TiO2:Bi2O3=85.1:14.8:1.1.
Catalyst 17: Al2O3—TiO2—NiO
50 g of Catalyst A grains were impregnated with 38.4 g of an aqueous solution of 7.3 g of nickel nitrate hexahydrate in H2O. After the impregnation, the grains were dried at 120° C. for 2 hours and then fired at 700° C. for 2 hours. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti:Ni=90:10:0.025 and in a weight ratio of Al2O3:TiO2:NiO=82.0:14.4:3.6.
Catalyst 18: Al2O3—TiO2—BO4 
50 g of Catalyst A grains were impregnated with 38.4 g of an aqueous solution of 1.36 g of ammonium borate octahydrate in H2O. After the impregnation, the grains were dried at 120° C. for 2 hours and then fired at 700° C. for 2 hours. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti:B=90:10:0.005 and in a weight ratio of Al2O3:TiO2:BO4=85.65:14.827:0.008.
It was found from FIG. 3 that the Al2O3—TiO2—ZrO2 catalyst had the highest activity.
EXAMPLE 6
In this Example, various catalysts containing alumina as one member were investigated for C2F6 decomposition rates under the following conditions:
A C2F6 gas having a purity of 99% or more was diluted with air, and the diluted gas was further admixed with steam to prepare a reaction gas. Steam was prepared by feeding pure water to a reactor tube from the top at a flow rate of about 0.2 ml/min. by a microtube pump to gasify the pure water. The reaction gas had a C2F6 concentration of about 0.5%, and was brought into contact with a catalyst heated to 700° C. by external heating of the reactor tube in an electric oven at a space velocity of 2,000 h−1.
The reactor tube was an Inconel reactor tube having an inner diameter of 32 mm and had a catalyst bed fixed at the center of the reactor tube. An Inconel thermowell for a thermocouple, 3 mm in diameter, was inserted into the catalyst bed. Decomposition product gas from the catalyst bed was bubbled through an aqueous calcium fluoride solution and discharged to the system outside.
The following catalysts were prepared for the test under the foregoing conditions:
Catalyst 26
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders zinc nitrate hexahydrate and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in an atomic ratio of Al:Zn=91:9 and in a weight ratio of Al2O3:ZnO=86.4:13.6.
Catalyst 27
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with an aqueous solution of 50.99 g of nickel sulfate hexahydrate and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Ni=91:9 and in a weight ratio of Al2O3:NiO=87.3:12.7.
Catalyst 28
Boehmite powder (PURAL SB) were dried at 120° C. for one hour. 300 g of the resulting dried powders were admixed with an aqueous solution of 125.04 g of nickel nitrate hexahydrate and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Ni=91:9 and in a weight ratio of Al2O3:NiO=87.3:12.7.
Catalyst 29
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 300 g of the resulting dried powders were kneaded with 354.4 g of an aqueous 30% titanium sulfate solution while adding about 300 g of pure water thereto. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 5 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in an atomic ratio of Al:Ti=91:9 and in a weight ratio of Al2O3:TiO2=86.6:13.4.
Catalyst 30
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with an aqueous solution of 115.95 g of iron nitrate nonahydrate and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm, and tested. The resulting grain composition was in an atomic ratio of Al:Fe=91:9.
Catalyst 31
Boehmite powder (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with an aqueous solution of 95.43 g of tin chloride hydrate and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then dried at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in an atomic ratio of Al:Sn=91:9 and in a weight ratio of Al2O3:SnO2=77.4:22.6.
Catalyst 32
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with an aqueous solution prepared by diluting 22.2 g of a dinitrodiamino Pt(II) nitric acid solution (Pt concentration: 4.5 wt. %) with 200 ml of pure water, and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in a weight ratio of Al2O3:Pt=100:0.68.
Catalyst 33
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 300 g of the resulting dried powders were admixed with an aqueous solution of 125.87 g of cobalt nitrate hexahydrate, and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition was in an atomic ratio of Al:Co=91:9.
Catalyst 34
Boehmite powder (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with an aqueous solution of 76.70 g of zirconyl nitrate dihydrate, and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in an atomic ratio of Al:Zr=91:9 and in a weight ratio of Al2O3:ZrO2=80.7:19.3.
Catalyst 35
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with an aqueous solution of 124.62 g of cerium nitrate hexahydrate, and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in an atomic ratio of Al:Ce=91:9 and in a weight ratio of Al2O3:CeO2=75.0:25.0.
Catalyst 36
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 300 g of the resulting dried powders were admixed with an aqueous solution of 129.19 g of 20 wt. % silica sol, and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm and tested. The resulting grain composition for catalyst was in an atomic ratio of Al:Si=91:9 and in a weight ratio of Al2O3:SiO2=89.6:10.4.
Test results of the foregoing catalysts 19 and 26–36 at a reaction temperature of 700° C. are shown in FIG. 6, C2F6 decomposition activity is highest with the Al2O3—ZnO2 catalyst and is lowered in the order of the Al2O3—NiO catalyst, and the Al2O3—TiO2 catalyst. The highest activity of catalyst 26 seems to be due to the effect of S.
EXAMPLE 7
In this Example, changes in the activity of Al2O3—NiO catalyst 28 were investigated by changing atomic ratios of Al:Ni. Test procedure and conditions were the same as in Example 6 except that the C2F6 concentration was changed to 2% and the feed rate of pure water to 0.4 ml/min.
Catalyst 28-1
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powder were admixed with an aqueous solution of 8.52 g of nickel nitrate hexahydrate, and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Ni=99:1 and in a weight ratio of Al2O3:NiO=98.5:1.5.
Catalyst 28-2
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 300 g of the resulting powders were admixed with an aqueous solution of 66.59 g of nickel nitrate hexahydrate, and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Ni=95:5 and in a weight ratio of Al2O3:NiO=92.8:7.2.
Catalyst 28-3
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with an aqueous solution of 210.82 g of nickel nitrate hexahydrate, and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Ni=80:20 and in a weight ratio of Al2O3:NiO=73.2:26.8.
Catalyst 28-4
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with an aqueous solution of 361.16 g of nickel nitrate hexahydrate, and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Ni=70:30 and in a weight ratio of Al2O3:NiO=61.4:38.6.
Catalyst 28-5
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with 562.1 g of nickel nitrate hexahydrate, and the mixture was kneaded while adding water thereto. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Ni=60:40 and in a weight ratio of Al2O3:NiO=50.6:49.4.
C2F6 decomposition rate 6 hours after the start of test is shown in FIG. 7. It was found that the Ni content of Al2O3—NiO catalysts is in a range of 5 to 40 atom %, preferably 20 to 30 atom %.
EXAMPLE 8
In this Example, changes in the activity of Al2O3—ZnO catalyst 26 was investigated by changing atomic ratios of Al:Zn. Test procedure and conditions were the same as in Example 6 except that the C2F6 concentration was changed to 2% and the feed rate of pure water to 0.4 ml/min.
Catalyst 26-1
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with an aqueous solution of 215.68 g of zinc nitrate hexahydrate and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Zn=80:20 and in a weight ratio of Al2O3:ZnO=71.5:28.5.
Catalyst 26-2
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with 369.48 g of zinc nitrate hexahydrate and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Zn=70:30 and in a weight ratio of Al2O3:ZnO=59.4:40.6.
Catalyst 26-3
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 126.65 g of the resulting dried powders were admixed with an aqueous solution of 96.39 g of zinc nitrate hexahydrate, and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Zn=85:15 and in a weight ratio of Al2O3:ZnO=78.0:22.0.
C2F6 decomposition rate 6 hours after the start of test is shown in FIG. 8. It was found that the Zn content of Al2O3—ZnO2 catalysts is in a range of 10 to 30 atom %, preferably 10 to 15 atom %.
EXAMPLE 9
In this Example, decomposition of CF4 and CHF3 was carried out with a Al2O3—NiO catalyst 28-3 under the same test procedure and conditions as in Example 6, except that the space velocity was changed to 1,000 h−1 and the fluorine compound was diluted with nitrogen in place of air. Test results at various reaction temperatures are shown in FIG. 9. It was found that the decomposition activity of Al2O3—NiO catalyst upon CF4 gas and CHF3 gas is higher than upon C2F6 gas and the Al2O3—NiO catalyst is a preferable hydrolysis catalyst for CF4 or CHF3. Furthermore, it was found that a preferable reaction temperature is 600°–700° C. for the decomposition of CF4 and CHF3, and 650°–700° C. for the decomposition of C2F6. The higher the reaction temperature, the higher the decomposition rate. However, substantially 100% decomposition rate can be obtained at 700° C., and thus a higher reaction temperature than 700° C. will be meaningless, and a reaction temperature must be as high as 800° C.
EXAMPLE 10
In this Example, influences of steam upon C2F6 decomposition were investigated under the same test conditions as in Example 6 except that the space velocity was changed 1,000 h−1. Al2O3—NiO catalyst 28-3 was used at a reaction temperature of 700° C. while supplying steam for 2 hours from the start of test, then interrupting supply of steam for 3 hours, and then starting to supply steam again. Test results are shown in FIG. 10. It was found that during the supply of steam the C2F6 reaction rate was elevated due to the occurrence of C2F6 hydrolysis.
EXAMPLE 11 
In this Example, decomposition of SF6 was investigated with Al2O3—NiO catalyst 28-3 under the same test conditions as in Example 6 except that a SF6 gas having a purity of 99% or more was used, the space velocity was changed to 1,000 h−1 and the SF6 gas was diluted with nitrogen in of air. The reaction temperature was 700° C. Concentration of SF6 in the reaction gas at the inlet to the reactor tube and concentration of SF6 in the decomposition gas at the outlet from the alkaline washing step were determined by TCD gas chromatography and the decomposition rate was calculated by the following equation. It was found that the decomposition rate was 99% or more.




  
    Decomposition  rate
  
  =
  
    1
    -
    
      
        
          
            
              
                Concentration  of 
              
            
          
          
            
              
                
                  discharged
                
                ⁢
                
                  
                
                ⁢
                
                  SF
                  6
                
              
            
          
        
        
          
            
              
                Concentration  of 
              
            
          
          
            
              
                
                  fed
                
                ⁢
                
                  
                
                ⁢
                
                  SF
                  6
                
              
            
          
        
      
      ×
      100
      ⁢
      
        
      
      ⁢
      
        (
        %
        )
      
    
  




EXAMPLE 12
In this Example, decomposition of NF3 was investigated with Al2O—NiO catalyst 28-3 under the same test conditions as in Example 11 except that a NF3 gas having a purity of 99% or more was used. Reaction temperature was 700° C. Concentration of NF3 in the reaction gas at the inlet to the reactor tube and concentration of NF3 in the decomposition gas at the outlet from the alkaline washing step were determined by TCD gas chromatography and the decomposition rate was calculated according to the following equation. It was found that the decomposition rate was 99% or more. It was found preferable to carry out the decomposition of the NF3 gas with the Al2O3—NiO catalyst at a temperature of 600°–800° C.




  
    Decomposition  rate
  
  =
  
    
      
        
          
            
              Concentration  of 
            
          
        
        
          
            
              
                discharged
              
              ⁢
              
                
              
              ⁢
              
                NF
                3
              
            
          
        
      
      
        
          Concentration  of  fed
        
        ⁢
        
          
        
        ⁢
        
          NF
          3
        
      
    
    ×
    100
    ⁢
    
      
    
    ⁢
    
      (
      %
      )
    
  




EXAMPLE 13
In this Example, activity of Al2O3—ZnO catalyst upon hydrolysis of a CF4 gas, a C4F8 gas and a CHF3 gas was investigated. Decomposition of a CF4 gas was carried out in the following manner:
At first, a CF4 gas having a purity of 99% or more was diluted with air, and the diluted CF4 gas was further admixed with steam. Concentration of CF4 in the reaction gas was about 0.5%, and steam flow rate was adjusted to about 50 times as high as that of the fluorine compound, i.e. CF4. The reaction gas was brought into contact with the catalyst heated to a predetermined temperature in a reactor tube in an electric oven at a space velocity of 1,000 h−1. Decomposition product gas from the catalyst bed was bubbled through an aqueous sodium hydroxide solution and then discharged to the system outside. Decomposition rate of CF4 was determined by TCD gas chromatography.
The Al2O3—ZnO catalyst used for the test was prepared in the following manner:
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 126.65 g of the resulting dried powders were admixed with 96.39 g of zinc nitrate hexahydrate, and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Zn=85:15 and in a weight ratio of Al2O3:ZnO=78:22.
FIG. 11 shows decomposition rates of CF4 at various reaction temperatures and also those of CHF3 and C4F8 as fed and decomposed in the same manner as above. Decomposition rates of CHF3 and C4F8 were determined by FID gas chromatography, whereby it was found that the Al2O3—ZnO catalyst had a higher activity upon the CF4 gas, the C4F8 gas and the CHF3 gas. It was also found that a higher decomposition rate can be obtained preferably at a reaction temperature of 650° C. or higher for the hydrolysis of the C4F8 gas and even at a reactor temperature of 600° C. or lower for the hydrolysis of the CHF3 gas or the CF4 gas.
EXAMPLE 14 
In this Example, the decomposition activity of as Al2O3—NiO catalyst upon a C3F8 gas, a C4F8 gas and a SF6 gas was investigated in the same manner as in Example 13. The concentration of C4F8 in the reaction gas after decomposition of C4F8 was 0.1% by volume. The Al2O3—NiO catalyst used for the test was prepared in the following manner:
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with an aqueous solution of 210.82 g of nickel nitrate hexahydrate, and the mixture was kneaded. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours, and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in an atomic ratio of Al:Ni=80:20 and in a weight ratio of Al2O3:NiO=73.2:26.8.
FIG. 12 shows decomposition rates at various reaction temperatures, where the decomposition rate of C3F8 and C4F8 was determined by FID gas chromatography and that of SF6 by TCD gas chromatography. It was found from the test results that the Al2O3—NiO catalyst had a higher activity upon the hydrolysis of the SF6 gas, C3F8 gas and the C4F8 gas, and the reaction temperature was preferably 500° C. or higher for the hydrolysis of the SF6 gas and preferably 700° C. or higher for the hydrolysis of the C3F8 gas. In the case of C4F8 gas, the reaction temperature for the hydrolysis was preferably 650° C. or higher.
EXAMPLE 15 
In this Example, decomposition activity of an Al2O3—NiO—ZnO catalyst upon C4F8 was investigated in the same manner as in Example 13. The Al2O3—NiO—ZnO catalyst used for the test was prepared in the following manner:
Boehmite powders (PURAL SB) were dried at 120° C. for one hour. 200 g of the resulting dried powders were admixed with 210.82 g of nickel nitrate hexahydrate and 152.31 g of zinc nitrate hexahydrate, and the mixture was kneaded while adding pure water thereto. After the kneading, the kneaded mixture was dried at 250°–300° C. for about 2 hours and then fired at 700° C. for 2 hours. The fired product was pulverized and sieved to obtain grains having grain sizes of 0.5–1 mm. The resulting grain composition for catalyst was in atomic ratios of Al:Ni=80:20 and Al:Zn=85:15 and in a weight ratio of Al2O3:NiO:ZnO=60.7:22.2:17.1.
FIG. 13 shows decomposition rates at various reaction temperatures, where the decomposition rate of C4F8 was determined by FID gas chromatography.

## Claims

1. A process for treating a gas, which comprises contacting a gas stream containing at least one of a compound consisting of carbon, hydrogen, fluorine and oxygen with a catalyst at a temperature of 400 to 800° C. in the presence of steam vapor, said catalyst comprising aluminum oxide and nickel oxide, and
decomposing the compound by hydrolysis and producing a treated gas containing hydrogen fluoride.

2. A process according to claim 1, which further comprises washing the treated gas with water to remove the hydrogen fluoride.

3. A process according to claim 1, which further comprises washing the treated gas with an alkaline solution or slurry to remove the hydrogen fluoride.

4. A process according to claim 1, which further comprises washing the treated gas with water and subsequently contacting the water that has absorbed the hydrogen fluoride with an alkaline solution or slurry.

5. A process according to claim 1, wherein the catalyst further comprises at least one of zinc oxide and titanium oxide.

6. A process according to claim 1, wherein the catalyst consists essentially of alumina and nickel oxide.

7. A process according to claim 1, wherein the compound is CH2OCF2.

8. A process for treating a fluorine compound-containing gas, which comprises
contacting a gas stream containing at least one of a compound consisting of carbon, hydrogen, oxygen and fluorine with a catalyst comprising alumina as an active component and nickel oxide, said catalyst containing a composite oxide of alumina and nickel oxide,
adding steam or a reaction gas containing steam and oxygen to the gas stream, and
effecting a hydrolysis reaction between the at least one of a compound and the steam, thereby producing a treated gas containing hydrogen fluoride.

9. A process according to claim 8, which further comprises washing the treated gas with water to remove the hydrogen fluoride.

10. A process according to claim 8, which further comprises washing the treated gas with an alkaline solution or slurry to neutralize the hydrogen fluoride and other acidic compounds.

11. A process according to claim 8, which further comprises washing the treated gas with water and subsequently neutralizing the water that has absorbed the hydrogen fluoride with an alkaline solution or slurry.

12. A process according to claim 8, wherein the catalyst further comprises zinc oxide.

13. A process according to claim 8, wherein the catalyst consists essentially of alumina and nickel oxide.

14. A process according to claim 8, wherein the compound is CH2OCF2.

15. A process according to claim 1, wherein the catalyst contains 7.2 to 49.4% by weight of nickel oxide.

16. A process according to claim 8, wherein the catalyst contains 7.2 to 49.4% by weight of nickel oxide.