The use of an amorphous alloy catalyst for decomposition of chlorofluorocarbons.

1. Field of the Invention

The present invention relates to materials which are particularly suitable as highly active catalysts for decomposition of chlorofluorocarbons (flons) to hydrofluoric acid, hydrochloric acid and carbon dioxide, and are characterized by easy production of catalysts and easy recovery of catalytically useful components.

2. Description of the Prior Art

Investigation for decomposition of flons had been unnecessary and hence was little performed. It has been known that they decompose by burning with fuel at high temperatures such as 800 °C in an incinerator. According to recent investigation of catalytic decomposition of flons on metal oxides, decomposition of flons occurred on zeolite at temperatures of 300-500 °C.

In general, ordinary alloys are crystalline in the solid state. However, rapid quenching of some alloys with specific compositions from the liquid state gives rise to solidification to an amorphous structure. These alloys are called amorphous alloys. The amorphous alloys are single phase alloys supersaturated with various elements and have significantly high mechanical strength in comparison with the conventional industrial alloys. Some amorphous alloys with the specific compositions have a variety of superior characteristics including extremely high corrosion resistance that cannot be obtained in ordinary crystalline alloys.

One of the present inventors and co-inventors applied Japanese Patent Application No. 123111/85 of amorphous alloy electrode materials containing Ni, Ta and platinum group elements as essential components which are suitable for the anode for oxygen production by electrolysis of acidic aqueous solutions because of high activity for oxygen evolution.

The Japanese Patent Application No. 123111/85 discloses: <ul id="ul0001" list-style="none"><li>(1) Amorphous alloy electrode materials which comprise 25 to 65 at% Ta, 0.3 to 45 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and more than 30 at% Ni.</li><li>(2) Amorphous alloy electrode materials which comprise 25 to 65 at% in the total of 20 at% or more Ta and at least one element selected from the group of Ti, Zr and Nb, 0.3 to 45 of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and more than 30 at% Ni.</li></ul>

Three of the present inventors further invented surface activated amorphous alloy electrode materials suitable for electrolysis of aqueous solutions, consisting of Ni, very small amounts of platinum group metals and at least one element selected from the group of Ti, Zr, Nb and Ta, and their activation methods, and applied Japanese Patent Application Nos. 169764/85, 169765/85 and 169767/85. They further applied Japanese Patent Application No. 169766/85 for surface-activated supersaturated solid solution alloy electrode materials for electrolysis of aqueous solutions and their activation methods.

The Japanese Patent Application No. 169764/85 discloses: <ul id="ul0002" list-style="none"><li>(1) Surface-actiated amorphous alloy electrode materials consisting of 25 to 65 at% of Nb, 0.01 to 10 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and the substantial balance of Ni, to which surface activation treatment was applied for electrolysis of aqueous solutions.</li><li>(2) Surface-activated amorphous alloy electrode materials consisting of 25 to 65 at% in the total of 10 at% or more Nb and at least one element selected from the group of Ti, Zr and less than 20 at% of Ta, and 0.01 to 10 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and the substantial balance of Ni, to which surface activation treatment was applied for electrolysis of aqueous solutions.</li><li>(3) Surface-activated amorphous alloy electrode materials consisting of 25 to 65 at% of Nb, 0.01 to 10 at% of at least one element selected from the group consisting of Ru, Rh, Pd, Ir and Pt, less than 7 at% of P and the substantial balance of Ni, to which surface activation treatment was applied for electrolysis of aqueous solutions.</li><li>(4) Surface-activated amorphous alloy electrode materials consisting of 25 to 65 at% in the total of 10 at% or more Nb and at least one element selected from the group consisting of Ti, Zr and less than 20 at% of Ta, and 0.01 to 10 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, less than 7 at% of P and the substantial balance of Ni, to which surface activation treatment was applied for electrolysis of aqueous solutions.</li><li>(5) Method for activation treatment of amorphous alloys for electrodes characterized by immersion of the above-mentioned amorphous alloy electrode materials into corrosive solutions for surface enrichment of platinum group elements as a result of preferential dissolution of Ni, Nb, Ta, Ti and Zr.</li></ul>

The Japanese Patent Application No. 169765/85 discloses: <ul id="ul0003" list-style="none"><li>(1) Surface-activated amorphous alloy electrode materials consisting of 25 to 65 at% of Ta, 0.01 to 10 at% of at least one element selected from the group consisting of Ru, Rh, Pd, Ir and Pt, less than 7 at% of P and the substantial balance of 20 at% or more Ni, to which surface activation treatment was applied for electrolysis of aqueous solutions.</li><li>(2) Surface-activated amorphous alloy electrode materials consisting of 25 to 65 at% in the total of 20 at% or more Ta and at least one element selected from the group of Ti, Zr and Nb, 0.01 to 10 at% of at least one element selected from the group of Ru, Ph, Pd, Ir and Pt, less than 7 at% of P and the substantial balance of 20 at% or more Ni, to which surface activation treatment was applied for electrolysis of aqueous solutions.</li><li>(3) Method for activation treatment of amorphous alloys for electrodes characterized by immersion of the above-mentioned amorphous alloy electrode materials into corrosive solutions for surface enrichment of platinum group elements as a result of preferential dissolution of Ni, Nb, Ta, Ti and Zr.</li></ul>

The Japanese Patent Application No. 169767/85 discloses: <ul id="ul0004" list-style="none"><li>(1) Surface-activated amorphous alloy electrode materials consisting of 25 to 65 at% in the total of 5 at% to less than 20 at% of Ta and at least one element selected from the group of Ti and Zr, and 0.01 to 10 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and the substantial balance of Ni, to which surface activation treatment was applied for electrolysis of aqueous solutions.</li><li>(2) Surface-activated amorphous alloy electrode materials consisting of 25 to 65 at% in the total of 5 at% to less than 20 at% of Ta and at least one element selected from the group of Ti and Zr, and 0.01 to 10 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, less than 7 at% of P and the substantial balance of 20 at% or more Ni, to which surface activation treatment was applied for electrolysis of aqueous solutions.</li><li>(3) Method for activation treatment of amorphous alloys for electrodes characterized by immersion of the above-mentioned amorphous alloy electrode materials into corrosive solutions for surface enrichment of platinum group elements as a result of preferential dissolution of Ni, Ta, Ti and Zr.</li></ul>

The Japanese Patent Application No. 169766/85 discloses: <ul id="ul0005" list-style="none"><li>(1) Surface-activated supersaturated solid solution alloy electrode materials consisting of 20 to less than 25 at% of at least one element selected from the group of Nb and Ta, 0.01 to 10 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and the substantial balance of Ni, to which surface activation treatment was applied for electrolysis of aqueous solutions.</li><li>(2) Surface-activated supersaturated solid solution alloy electrode materials consisting of 20 to less than 25 at% of at least one element selected from the group of Nb and Ta, 0.01 to 10 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, less than 7 at% of P, and the substantial balance of Ni, to which surface activation treatment was applied for electrolysis of aqueous solutions.</li><li>(3) Surface-activated supersaturated solid solution alloy electrode materials consisting of 20 to less than 25 at% in the total of at least one element selected from the group of Ti and Zr and 5 at% or more of at least one element selected from the group of Nb and Ta, 0.01 to 10 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and the substantial balance of Ni, to which surface activation treatment was applied for electrolysis of aqueous solutions.</li><li>(4) Surface-activated supersaturated solid solution alloy electrode materials consisting of 20 to less than 25 at% in the total of at least one element selected from the group of Ti and Zr and 5 at% or more of at least one element selected from the group of Nb and Ta, 0.01 to 10 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, less than 7 at% of P, and the substantial balance of Ni, to which surface activation treatment was applied for electrolysis of aqueous solutions.</li><li>(5) Method for activation treatment of supersaturated solid solution alloys for electrodes characterized by immersion of the above-mentioned supersaturated solid solution alloy electrode materials into corrosive solutions for surface enrichment of platinum group elements as a result of preferential dissolution of Ni, Nb, Ta, Ti and Zr.</li></ul>

Furthermore, the present inventors found surface-activated amorphous alloys for methanol fuel cell and made application of Japanese Patent Application No. 154570/86.

The Japanese Patent Application No. 154570/86 discloses: <ul id="ul0006" list-style="none"><li>(1) Surface-activated amorphous alloy for methanol fuel cell, consisting of 20 to 80 at% of at least one element selected from the group of Ti and Zr, 0.5 to 20 at% of Pt, and the substantial balance of 10 at% or more of at least one element selected from the group of Ni and Co.</li><li>(2) Surface-activated amorphous alloy for methanol fuel cell consisting of 20 to 80 at% of at least one element selected from the group of Ti and Zr, 0.5 to 20 at% of Pt, at most 10 at% (at most the same at% as Pt if Pt is at most 10 at%) of at least one element selected from the group of Ru, Rh, Pd, Ir, Tl, Si, Ge, Sn, Pb and Bi, and the substantial balance of 10 at% or more of at least one element selected from the group of Ni and Co.</li><li>(3) Surface-activated amorphous alloy for methanol fuel Cell, consisting of 20 to 70 at% of at least one element selected from the group of Nb and Ta, 0.5 to 20 at% of Pt, and the substantial balance of at least one element selected from the group of Ni and Co.</li><li>(4) Surface-activated amorphous alloy for methanol fuel cell, consisting of 20 to 70 at% of at least one element selected from the group of Nb and Ta, 0.5 to 20 at% of Pt, at most 10 at% (at most the same at% as Pt if Pt is at most 10 at%) of at least one element selected from the group of Ru, Rh, Pd, Ir, Tl, Si, Ge, Sn, Pb and Bi, and the substantial balance of 10 at% or more of at least one element selected from the group of Ni and Co.</li><li>(5) Surface-activated amorphous alloy for methanol fuel cell consisting of 20 to 80 at% in the total of at least one element selected from the group of Ti and Zr and at most 70 at% of at least one element selected from the group of Nb and Ta, 0.5 to 20 at% of Pt, and the substantial balance of 10 at% or more of at least one element selected from the group of Ni and Co.</li><li>(6) Surface-activated amorphous alloy for methanol fuel cell, consisting of 20 to 80 at% in the total of at least one element selected from the group of Ti and Zr and at most 70 at% of at least one element selected from the group of Nb and Ta, 0.5 to 20 at% of Pt, at most 10 at%(at most the same at% as Pt if Pt is at most 10 at%) of at least one element selected from the group of Ru, Rh, Pd, Ir, Tl, Si, Ge, Sn, Pb and Bi, and the substantial balance of 10 at% or more of at least one element selected from the group of Ni and Co.</li></ul>

In order to overcome high operation temperatures of catalysts consisting of platinum group elements supported on ceramics for purification of exhaust gases from plants and vehicles in addition to difficulty of recovery of platinum group elements, three of the present inventors investigated the catalysts capable of operating at low temperatures such as the beginning of combustion in addition to easy recovery, and invented catalysts for purification of exhaust gases by the reaction of carbon monoxide with nitrogen oxides in exhaust gases as Japanese Patent Application No. 262986/89.

The Japanese Patent Application No. 262986/89 discloses: <ul id="ul0007" list-style="none"><li>(1) Surface-activated catalysts for purification of exhaust gases, consisting of 20 to 70 at% of at least one element selected from the group of Nb and Ta, 0.5 to 20 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and the substantial balance of at least one element selected from the group of Ni and Co, to which surface activation treatment was applied.</li><li>(2) Surface-activated catalysts for purification of exhaust gases, consisting of 20 to 80 at% of at least one element selected from the group of Ti and Zr, 0.5 to 20 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and the substantial balance of at least 10 at% of at least one element selected from the group of Ni and Co, to which surface activation treatment was applied.</li><li>(3) Surface-activated catalysts for purification of exhaust gases, consisting of 20 to 80 at% in the total of at least one element selected from the group of Ti and Zr and at most 70 at% of at least one element selected from the group of Nb and Ta, 0.5 to 20 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and the substantial balance of at least 10 at% of at least one element selected from the group of Ni and Co, to which surface activation treatment was applied.</li></ul>

Until 2000, production and use of 5 worst flons which destruct ozonosphere and induce greenhouse effect will be prohibited. They may be substituted by other flons. Current industries are using large amounts of different flons. It is, therefore, necessary to develop the technique by which used flons can be converted to hydrofluoric acid, hydrochloric acid and carbon dioxide for re-usage without consumption of a large amount of energy.

In view of the above-foregoing, there has been a strong demand for highly active and easily recoverable catalysts for conversion of flons at low temperatures with low consumption of energy.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide highly active and easily producible and recoverable catalysts by which the reaction of flons with water to form hydrofluoric acid, hydrochloric acid and carbon dioxide at relatively low temperatures close to ambient temperature.

The objective of the invention has been achieved by finding that some amorphous alloys have high catalytic activity for conversion of flons. The present invention is composed of 3 claims in which catalytically active and easily producible and recoverable alloys consist of at least one element selected from the group of Co and Ni, valve metals to form the amorphous structure by alloying with at least one element selected from the group of Ni and Co, and a very small amount of platinum group elements effective as the catalyst for the reaction of flons with water, the alloys being activated by hydrofluoric acid treatment.

The claims of the present invention are: <ul id="ul0008" list-style="none"><li>1. Catalysts for decomposition of flons characterized by immersion in hydrofluoric acids for activation of amorphous alloys consisting of 20 to 70 at% of at least one element selected from the group of Nb and Ta, 0.5 to 20 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and the substantial balance of at least one element selected from the group of Ni and Co.</li><li>2. Catalysts for decomposition of flons characterized by immersion in hydrofluoric acids for activation of amorphous alloys consisting of 20 to 80 at% of at least one element selected from the group of Ti and Zr, 0.5 to 20 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and the substantial balance of at least one element selected from the group of Ni and Co.</li><li>3. Catalysts for decomposition of flons characterized by immersion in hydrofluoric acids for activation of amorphous alloys consisting of 20 to 80 at% in the total of at least one element selected from the group of Ti and Zr and at most 70 at% of at least one element selected from the group of Nb and Ta, 0.5 to 20 at% of at least one element selected from the group of Ru, Rh, Pd, Ir and Pt, and the substantial balance of at least one element selected from the group of Ni and Co.</li></ul>

BRIEF DESCRIPTION OF THE DRAWING

The single figure shows an apparatus for preparing amorphous alloys of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to obtain easily producible and recoverable catalysts with selectively high catalytic activity for a specific reaction, it is more convenient to use alloys containing necessary amounts of effective elements rather than usage of platinum group elements supported by alumina, titania, silica, etc. However, when conventionally processed crystalline alloys are used, additions of large amounts of various elements lead often to formation of multiple phases of different chemical properties and to poor mechanical strength, and it is difficult to obtain materials with a large specific surface area necessary for effective catalysts.

On the contrary, the amorphous alloys of the present invention are prepared by the principle to prevent localization of alloy constituents due to rapid quenching of the melt of the above-mentioned compositions, and hence they are highly homogeneous and mechanically strong and tough. When these alloys are treated by immersion in hydrofluoric acids, alloy constituents unnecessary for catalytic activity are dissolved into the hydrofluoric acids with a consequent formation of a highly active catalysts with a remarkable large surface area enriched in catalytically active platinum group elements. In this treatment hydrogen evolution occurs violently on the platinum group elements in the alloys, since the platinum group elements act as the cathode in the amorphous alloys composed of a chemically homogeneous single phase solid solution. Since the hydrogen evolution provides dissolution of alloy constituents unnecessary for catalytic activity, violent hydrogen evolution on the homogeneous alloys results in rapid and uniform dissolution of alloy constituents unnecessary for the catalytic activity. This leads to the formation of highly active catalysts with a large surface area enriched in catalytically active platinum group elements.

Consequently, the catalysts for production of hydrofluoric acid, hydrochloric acid and carbon dioxide by the reaction of flons with water can be obtained by the alloys of the present invention, to which activation treatment by immersion in hydrofluoric acids is applied.

The components and compositions of the alloys of the present invention are specified as above for the following reasons:

In the alloys set forth in Claims 1 to 3, Ni and Co are basic components which form the amorphous structure when they coexist with one or more elements selected from the group of Nb, Ta, Ti and Zr. Therefore, in order to form the amorphous structure, the alloys set forth in Claim 1 consisting of at least one element selected from the group of Ni and Co and at least one element selected from Nb and Ta should contain 20 to 70 at% of at least one element selected from the group of Nb and Ta. The amorphous structure can be obtained when the alloys set forth in Claim 2 consisting of at least one element selected from the group of Ni and Co and at least one element selected from Ti and Zr contain 20 to 80 at% of at least one element selected from the group of Ti and Zr.

Furthermore, in the alloys set forth in Claim 3 consisting of at least one element selected from the group of Ni and Co, at least one element selected from Ti and Zr, and at least one element selected from Nb and Ta, the amorphous structure can be obtained if the alloys contain 20 to 80 at% in the total of at least one element selected from the group of Ti and Zr and at most 70 at% of at least one element selected from the group of Nb and Ta.

The platinum group elements Ru, Rh, Pd, Ir and Pt are all effective for the high catalytic activity, and hence the catalytic activity requires that one or more of these platinum group elements should be 0.5 at% or more. However, the additions of large amounts of platinum group elements make the alloys quite expensive and are rather detrimental for both increasing surface area and surface enrichment of platinum group elements since the beneficial effect of platinum group elements for preferential dissolution of elements unnecessary for the catalytic activity is rather decreased by excessive additions of platinum group elements. Therefore, the contents of platinum group elements should not exceed 20 at%, and the most suitable contents are 1 to 10 at%.

The preparation of the amorphous alloys of the present invention can be carried out by any kinds of methods for preparation of amorphous alloys, such as rapid quenching from the liquid state.

One embodiment of apparatus for preparing the amorphous alloys of the present invention is shown in the accompanying drawing. This is called the rotating wheel method. The apparatus is placed in a vacuum chamber indicated by a dotted rectangle. In the figure, a quartz tube 2 has a nozzle 3 at its lower end in the vertical direction, and raw materials 4 and an inert gas for preventing oxidation of the raw materials are fed from an inlet 1. A heating furnace 5 is placed around the quartz tube 2 so as to heat the raw materials 4. A high speed wheel 7 is placed below the nozzle 3 and is rotated by a motor 6.

For the preparation of the amorphous alloys, the raw materials 4 of the prescribed compositions are placed in the quartz tube 2 and the vacuum chamber is evacuated up to about 10⁻⁵ torr. After the evacuated vacuum chamber is filled with argon gas of about 1 atm, the raw materials 4 are melted by the heating furnace 5. The molten alloy impinges under the pressure of the inert gas onto the outer surface of the wheel 7 which is rotated at a speed of 1,000 to 10,000 rpm whereby an amorphous alloy is formed as a long thin plate, which may for example have a thickness of 0.01 mm, a width of 10 mm and a length of several meters.

The amorphous alloys of the present invention will be further illustrated by certain examples which are provided only for purpose of illustration and are not intended to be limiting the present invention. Example 1

A raw alloy was prepared by argon arc melting of a mixture of commercial metals so as to form Ni-40 at% Nb-2 at% Rh. After remelting of the raw alloy under an argon atmosphere, amorphous alloy ribbons were prepared by the rotating wheel method by using the apparatus shown in the figure. The amorphous alloys thus prepared were 0.01-0.05 mm thick, 1-3 mm wide and 3-20 m long ribbons. The formation of amorphous structure was confirmed by X-ray diffraction.

The highly active metallic catalyst was obtained by the surface activation treatment of this alloy by immersion in 46.5 % HF for 300-900 sec at ambient temperature. The reactor tube was prepared by placing 0.5 g of the metallic catalysts of 5 cm length in a glass tube of 8 mm inner diameter and was placed in an electric furnace. The reactant gas mixture of CFC-12 flon and water obtained by bubbling CFC-12 flon through warm water was passed through the reactor tube. The amounts of CO₂ and remaining CFC-12 flon in the gas passed through the reactor tube were analyzed by gas chromatography. HF and HCl were solved in water and determined.

Table 1 shows the reaction temperature and conversion. <tables id="tabl0001" num="0001"><img file="EP0448976A2_D0001.tif" /></tables>

Example 2

Raw alloys, whose nominal compositions are shown in Table 2, were prepared by argon arc melting of mixtures of commercial metals. After remelting of the raw alloys under an argon atmosphere, amorphous alloys were prepared by the rotating wheel method by using the apparatus shown in the figure. The amorphous alloys thus prepared were 0.01-0.05 mm thick, 1-3 mm wide and 3-20 m long ribbons. The formation of amorphous structure was confirmed by X-ray diffraction.

The highly active metallic catalysts were obtained by the surface activation treatment of these alloys by immersion in 2-46% HF solutions for 300-900 sec at ambient temperature. The reactor tube was prepared by placing 0.5 g of the metallic catalysts of 5 cm length in a glass tube of 8 mm inner diameter and was placed in an electric furnace.

The reactant gas mixture of CFC-12 flon and water obtained by bubbling CFC-12 flon through warm water was passed through the reactor tube. The amounts of CO₂ and remaining CFC-12 flon in the gas passed through the reactor tube were analyzed by gas chromatography. HF and HCl were solved in water and determined.

Table 2 shows the reaction temperature and conversion. <tables id="tabl0002" num="0002"><img file="EP0448976A2_D0002.tif" /></tables><tables id="tabl0003" num="0003"><img file="EP0448976A2_D0003.tif" /></tables>

Consequently, the amorphous alloy catalysts of the present invention have very high activities for decomposition of flons to hydrofluoric acid and carbon dioxide by the reaction with water.