Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/24—Halogens or compounds thereof
  • C25B1/245—Fluorine; Compounds thereof
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/042—Electrodes formed of a single material
  • C25B11/043—Carbon, e.g. diamond or graphene
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier

Definitions

• the present invention relates to an anode material which, when used in applications such as electrolysis in an electrolytic bath containing hydrofluoric acid, does not produce the anode effect even upon application of a high-current-density voltage thereto and which is free from the serious sludge generation caused by electrode dissolution, can inhibit CF 4 generation, and enables the electrolysis to continue stably without suffering electrode disintegration.
• the invention further relates to a method of electrolysis.
• An electrolytic process in which a solution prepared by dissolving an inorganic or organic compound in anhydrous hydrofluoric acid (anhydrous HF) is used as an electrolytic bath to electrolytically synthesize an inorganic fluorine compound, organic fluorine compound, or fluorine gas is in practical industrial use.
• an alkali metal fluoride such as, e.g., potassium fluoride (KF) or alkaline-earth-metal fluoride (hereinafter referred to as a conduction aid) is often added to the electrolytic bath when the system is intended to be operated at a high current density.
• KF potassium fluoride
• a conduction aid alkaline-earth-metal fluoride
• materials usable as electrolyzers and electrode materials are limited because of the marked corrosiveness of HF.
• the materials usable as anode materials are limited to nickel and carbon.
• carbon anodes include reduced susceptibility to the electrode wear occurring in nickel electrodes.
• carbon anodes often pose a problem that the phenomenon in which the electrode is passivated, i.e., the so-called anode effect, occurs to make it difficult to continue the electrolysis.
• Anode reactions occurring when a carbon anode is used include a reaction generating fluorinated graphite, besides the discharge reaction of fluoride ions which is the desired reaction.
• the generated fluorinated graphite is partly decomposed either through pyrolysis due to the Joule's heat resulting from the electrode reaction or by a disproportionation reaction.
• the fluorinated graphite which is a covalent compound, shows low wettability to the electrolytic bath. Because of this, when the rate of generating fluorinated graphite is higher than the rate of decomposing the fluorinated graphite, the electrode surface is coated with the fluorinated graphite to produce the anode effect.
• the rate of generating fluorinated graphite depends on current density and, hence, the anode effect becomes more apt to occur as current density increases.
• HF intercalates into the carbon electrode to expand the electrode and this expansion often causes the electrode to crack or disintegrate.
• techniques have been put to practical use such as, e.g., a technique in which the surface of an electrode is coated with nickel by thermal spraying or plating.
• nickel itself is problematic as will be described later.
• a technique in which the concentration of a conduction aid, e.g., KF, in an electrolytic bath is increased to thereby lower the vapor pressure of HF is also applied.
• the increased concentration of a conduction aid elevates the melting point of the electrolytic bath and hence necessitates a higher operating temperature. This technique therefore has limitations.
• Nickel is widely used as the anode in electrolysis in an electrolytic bath prepared by adding a substance to be fluorinated, such as, e.g., ammonia, an alcohol, or an amine, to anhydrous HF.
• a substance to be fluorinated such as, e.g., ammonia, an alcohol, or an amine
• nickel anodes have the advantage of being free from the anode effect occurring in carbon anodes, wear of the nickel anodes proceeds during electrolysis.
• Nickel anodes are worn in an amount corresponding to 3-5% of the quantity of electricity applied, and the cost of replacing the worn nickel anodes is almost comparable to the power cost for the electrolysis.
• nickel dissolves in the electrolytic bath to increase the viscosity of the electrolyticbath, making it difficult to control the temperature of the electrolytic bath. Therefore, periodic electrolytic-bath replacement also becomes necessary.
• anode replacement, electrolytic-bath replacement, and operation stops accompanying the replacement are indispensable to nickel anodes and are factors which inhibit the profitability of electrolytic synthesis from improvement.
• Patent document 1 discloses: an electrode comprising a silicon substrate having a surface coated with a boron-doped diamond film; and a method of electrolytic fluorination in which the electrode is used.
• Patent document 2 discloses: an electrode comprising an electrically conductive carbon-material substrate having a surface coated with electrically conductive diamond; and a method of electrolytically synthesizing a fluorine-containing substance using the electrode.
• the present inventors diligently made investigations. As a result, they found that the invention described in patent document 1 has a problem that the silicon substrate is corroded by the HF present in the electrolytic bath and, hence, it is difficult to maintain the electrode structure. Moreover, they found that the invention described in patent document 2 has a problem that when the electrolytic bath has a high HF concentration, in particular, when the molar concentration of HF in the electrolytic bath is not lower than 3 times the molar concentration of a substance to be fluorinated or of a conduction aid, then HF infiltration into the carbon substrate occurs to disintegrate the carbon substrate.
• an electrode which is free from the anode effect and disintegration occurring in carbon electrodes and is free from the wear occurring in nickel electrodes is desired as an electrode for electrolysis in electrolytic baths containing HF.
• the present invention provides an electrode for electrolysis which includes: a substrate comprising an electrically conductive material, wherein the surface of the substrate is made of glassy carbon; and an electrically conductive diamond film with which at least part of the substrate is coated.
• the invention further provides a method of electrolytically synthesizing fluorine or a fluorine-containing compound in an HF-containing electrolytic bath using the electrode.
• the present invention includes the following aspects in its broadest configurations:
• an electrically conductive substrate the surface of which is made of glassy carbon and an electrode for electrolysis obtained by coating at least part of the conductive substrate with an electrically conductive diamond film undergo neither the anode effect nor electrode wear nor electrode disintegration in electrolysis in an electrolytic bath containing HF even when the electrolytic bath has a high HF concentration and are electrodes capable of long-term continuous electrolysis.
• Glassy carbon is a carbon material having a glassy appearance and produced from cellulose, a cellulosic resin, or a thermosetting resin, e.g., a furan resin, as a precursor, and by molding the precursor and then subjecting the molded precursor to solid-phase carbonization.
• a thermosetting resin e.g., a furan resin
• Glassy carbon has a homogeneous amorphous structure with no crystal form. Although there are many voids in the structure, most of the voids are closed cells and, hence, there are almost no open cells.
• HF is less apt to intercalate into inner parts of the substrate even in an electrolytic bath having a high HF concentration. This electrode hence does not suffer electrode expansion and succeeding electrode disintegration.
• the glassy carbon is also called vitreous carbon.
• the glassy carbon used in the present invention is not particularly limited, but, for examples, GC series products manufactured by TOKAI CARBON CO., LTD., and SPI-Glas series products manufactured by SPI Supplies, can be exemplified. Especially, from the standpoint of low gas permeability, GC-10 (trade name, manufactured by TOKAI CARBON CO., LTD.) and SPI-Glas 10 (trade name, manufactured by SPI Supplies) are preferable.
• US patent 6,241,956 discloses a method for producing a glassy carbon, which is herein incorporated by reference.
• the coating of the part of the surface of the substrate with electrically conductive diamond prevents the anode effect, which is attributable to the formation of fluorinated graphite, and electrode wear.
• perfluorotrimethylamine can be efficiently synthesized using an electrolytic bath having the composition (CH 3 ) 4 NF ⁇ 5HF.
• an electrolytic bath having the composition (CH 3 ) 4 NF ⁇ 5HF.
• CsF ⁇ 2HF for preventing the passivation trouble.
• electrode wear proceeds.
• carbon is used as an anode
• the anode effect occurs and the infiltration of HF into the substrate occurs to cause electrode disintegration.
• a known electrode obtained by coating the surface of an electrically conductive carbon material substrate with electrically conductive diamond is used, HF infiltration into the substrate occurs to cause electrode disintegration.
• the invention provides an electrode which comprises a substrate comprising an electrically conductive material, wherein the surface of the substrate is made of glassy carbon, and an electrically conductive diamond film with which at least part of the surface is coated, and which is for use in, e.g., the synthesis of an inorganic fluorine compound, organic fluorine compound, and fluorine gas through the electrolysis of an electrolytic bath containing HF.
• the invention further provides a method of electrolytically synthesizing fluorine or a fluorine-containing compound using the electrode.
• the electrode and the synthesis method prevent the anode effect, electrode wear, and electrode disintegration from occurring even in an electrolytic bath having a high HF concentration, and render long-term continuous electrolysis possible.
• the productivity of inorganic fluorine compounds, organic fluorine compounds, and fluorine gas is improved.
• the electrode for electrolysis of the invention is explained in detail.
• the conductive substrate of the electrode of the invention is not particularly limited in its shape so long as the substrate has a surface made of glassy carbon. Platy, rod, pipe, or spherical shape or the like can be used.
• the glassy carbon constituting the surface has a gas permeability of preferably 10 -7 cm 2 /sec or lower, more preferably 10 -10 cm 2 /sec or lower.
• Examples of glassy carbon that satisfies the preferred gas permeability include GC-10 (trade name, manufactured by TOKAI CARBON CO., LTD.) and SPI-Glas 10 (trade name, manufactured by SPI Supplies) and SPI-Glas 20 (trade name, manufactured by SPI Supplies).
• examples of glassy carbon that satisfies the more preferred gas permeability include GC-10 and SPI-Glas 10.
• Methods for coating at least part of the surface of the conductive substrate with a conductive diamond film are not particularly limited, and any desired method can be used.
• Typical production processes include hot-filament CVD (chemical vapor deposition) process, microwave CVD process, the plasma-arc jet process, and physical vapor deposition (PVD) process.
• a suitable method can be selected from these.
• a mixed gas composed of hydrogen gas and a carbon source as a raw material for diamond is used.
• An element having a different valence (hereinafter referred to as dopant) is added to the mixed gas in a slight amount in order to impart electrical conductivity to the diamond.
• the dopant preferably is boron, phosphorus, or nitrogen.
• the content of the dopant is preferably 1-100,000 ppm, more preferably 100-10,000 ppm as ratio of dopant atom to carbon atom.
• the conductive diamond film deposited ispolycrystalline, and amorphous carbon and a graphite ingredient remain in the diamond film.
• the content of the amorphous carbon and graphite ingredient should be lower. It is preferred that in Raman spectroscopy, the ratio I(D)/I(G), wherein I(D) is the intensity for a peak appearing around 1,332 cm -1 (in the range of 1,312-1,352 cm -1 ), which is assigned to diamond, and I(G) is the intensity for a peak appearing around 1,560 cm -1 (in the range of 1,540-1,580 cm -1 ), which is assigned to the G band of graphite, shouldbe 1 or larger. Namely, it is preferred that the content of diamond should be higher than the content of graphite.
• hot-filament CVD which is a typical process for coating with a conductive diamond film.
• An organic compound serving as a carbon source such as, e.g., methane, an alcohol, or acetone, and a dopant are fed to the filament together with hydrogen gas, etc.
• the filament is heated to a temperature at which hydrogen radicals or the like generates, i.e., 1,800-2,800°C, and an electrically conductive substrate is disposed in the atmosphere so as to have a temperature in a region where diamond deposition occurs (750-950°C).
• the rate of feeding the mixed gas depends on the size of the reaction vessel. However, it is preferred to use a pressure of 15-760 Torr.
• the surface of the conductive substrate it is preferred to grind the surface of the conductive substrate because the grinding improves adhesion between the substrate and the diamond layer.
• the surface is ground so as to result in an arithmetic mean roughness Ra of 0.1-15 ⁇ m and a maximum height Rz of 1-100 ⁇ m.
• Applying a diamond powder as nuclei to the substrate surface is effective in growing an even diamond film.
• a layer of fine diamond particles having a diameter of 0.001-2 ⁇ m deposits on the substrate.
• the thickness of the diamond film can be regulated by changing deposition time, the thickness thereof preferably is 1-10 ⁇ m from the standpoint of profitability.
• fluorine or a fluorine-containing compound is synthesized by an electrolytical synthesis method.
• the method is not particularly limited, but the method comprising conducting electrolysis using the electrode for electrolysis according to the present invention in an electrolytic bath comprising hydrofluoric acid and, added thereto, a substance to be fluorinated, is preferable.
• the electrolytic bath may further comprise a fluoride of an alkali metal or a fluoride of an alkaline earth metal. These fluorides (i.e., conduction aids) may be used alone or in combination of two or more thereof.
• a soft steel, nickel alloy, fluororesin, or the like can be used from the standpoint of resistance to corrosion by HF. It is preferred to wholly or partly separate the anode side from the cathode side with a partition, diaphragm, or the like in order to prevent the F 2 or fluorine compound synthesized at the anode from mingling with the hydrogen gas generated at the cathode.
• By-products such as nitrogen, oxygen, and dinitrogen monoxide also generate in slight amounts.
• the dinitrogen monoxide can be removed by passing the product through water and sodium thiosulfate.
• the oxygen can be removed by activated carbon.
• a glassy-carbon plate (GC-10, manufactured by TOKAI CARBON CO., LTD.) was used as a conductive substrate to produce a conductive-diamond electrode under the following conditions using a hot-filament CVD apparatus.
• an abrasive material composed of diamond particles having a diameter of 1 ⁇ m was used to grind the surface of the substrate.
• the ground substrate surface had a Ra of 0.2 ⁇ m and a ten-point surface roughness Rz of 6 ⁇ m.
• diamond particles having an average particle diameter of 4 nm were applied as nuclei to the substrate surface.
• the substrate was attached to the hot-filament CVD apparatus.
• a mixed gas prepared by adding 1 vol% methane gas and 0.5 ppm trimethylboron gas to hydrogen gas was continuously passed through the apparatus at a rate of 5 L/min. While thus passing the mixed gas, the internal pressure of the apparatus was kept at 75 Torr and a voltage was applied to the filament to elevate the temperature thereof to 2,400°C. At this point of time, the substrate had a temperature of 860°C.
• the CVD operation was continued for 8 hours. After completion of the CVD operation, the substrate was analyzed. It was ascertained through Raman spectroscopy and X-ray diffractometry that diamond had deposited. The ratio of a peak intensity at 1,332 cm -1 to a peak intensity at 1,560 cm -1 in Raman spectroscopy was 1/0.4. Moreover, part of this substrate was destroyed and examined with SEM. As a result, it was found to have a thickness of about 4 ⁇ m.
• the conductive-diamond electrode produced was attached as an anode in an anhydrous HF bath kept at 0°C.
• a nickel plate and platinum were used as a cathode and a reference electrode, respectively, and the anode was examined for current-potential curve by constant-current chronopotentiometry.
• the anode potential at a current density of 5 mA/cm 2 was 0.6 V. Thereafter, the anode potential was measured while stepwise increasing the current density by 5 mA/cm 2 at a time. As a result, the anode potential at a current density of 200 mA/cm 2 was 3.2 V.
• Electrolysis was conducted under the same conditions as in Example 1, except that a graphite plate was used as an anode. This anode was thus examined for current-potential curve in the anhydrous HF bath kept at 0°C.
• the anode potential at a current density of 5 mA/cm 2 was 0.7 V. Thereafter, the anode potential was measured while stepwise increasing the current density by 5 mA/cm 2 at a time. As a result, at a current density of 70 mA/cm 2 , the anode potential increased abruptly and almost no current came to flow, making it difficult to continue the electrolysis.
• the electrolysis was stopped and the anode was taken out. As a result, the anode was found to have broken into powder in the electrolyzer.
• Electrolysis was conducted under the same conditions as in Example 1, except that a nickel plate was used as an anode. This anode was thus examined for current-potential curve in the anhydrous HF bath kept at 0°C.
• the anode potential at a current density of 5 mA/cm 2 was 0.6 V. Thereafter, the anode potential was measured while stepwise increasing the current density by 5 mA/cm 2 at a time. As a result, at the time when the current density reached 50 mA/cm 2 , the anode potential began to increase with the lapse of time. Finally, almost no current came to flow, making it difficult to continue the electrolysis.
• a conductive-diamond electrode was produced in the same manner as in Example 1, except that a silicon plate was used as a conductive substrate.
• the electrode was examined for current-potential curve in an anhydrous HF bath kept at 0°C under the same electrolysis conditions as in Example 1, except that the electrode produced was used as an anode.
• the anode potential at a current density of 5 mA/cm 2 was 0.6 V. Thereafter, the anode potential was measured while stepwise increasing the current density by 5 mA/cm 2 at a time. As a result, at a current density of 200 mA/cm 2 , the anode potential was 3.8 V.
• a conductive-diamond electrode was produced in the same manner as in Example 1, except that a graphite plate was used as a conductive substrate.
• the electrode was examined for current-potential curve in an anhydrous HF bath kept at 0°C by the same method as in Example 1, except that the electrode produced was used as an anode.
• the anode potential at a current density of 5 mA/cm 2 was 0.6 V. Thereafter, the anode potential was measured while stepwise increasing the current density by 5 mA/cm 2 at a time. As a result, at a current density of 70 mA/cm 2 , the anode potential increased abruptly and almost no current came to flow, making it difficult to continue the electrolysis.
• the electrolysis was stopped and the anode was taken out. As a result, the anode was found to have broken into powder in the electrolyzer.
• a glassy-carbon plate was used as a conductive substrate to produce a conductive-diamond electrode in the same manner as in Example 1 using the hot-filament CVD apparatus.
• This electrode was attached to a (CH 3 ) 4 NF ⁇ 5HF electrolytic bath immediately after preparation of the bath.
• a nickel plate and Cu/CuF 2 were used as a cathode and a reference electrode, respectively, and constant-current electrolysis was conducted at a current density of 100 mA/cm 2 .
• the anode potential was measured and was found to be 4.6 V.
• the anode potential was 4.8 V.
• Electrolysis was conducted in a (CH 3 ) 4 NF ⁇ 5HF electrolytic bath immediately after preparation of the bath, in the same manner as in Example 2 except that a graphite plate was used as an anode.
• the electrolysis was stopped, and the anode was taken out and examined for contact angle between a surface of the electrode and water. As a result, the contact angle was found to be 150 degrees. It was hence ascertained that the so-called anode effect had occurred.
• Electrolysis was conducted in a (CH 3 ) 4 NF ⁇ 5HF electrolytic bath immediately after preparation of the bath, in the same manner as in Example 2 except that a nickel plate was used as an anode.
• a conductive-diamond electrode was produced in the same manner as in Example 1, except that a silicon plate was used as a conductive substrate.
• Electrolysis was conducted in a (CH 3 ) 4 NF ⁇ 5HF electrolytic bath immediately after preparation of the bath, in the same manner as in Example 2 except that the electrode produced was used as an anode.
• a conductive-diamond electrode was produced in the same manner as in Example 1, except that a graphite plate was used as a conductive substrate.
• Electrolysis was conducted in a (CH 3 ) 4 NF ⁇ 5HF electrolytic bath immediately after preparation of the bath, in the same manner as in Example 2 except that the electrode produced was used as an anode.
• the electrolysis was stopped, and the anode was taken out. As a result, the anode was found to have broken into powder in the electrolyzer.

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• 2009
  • 2009-02-02 JP JP2009021157A patent/JP2010174358A/ja active Pending
• 2010
  • 2010-02-01 CN CN2012104009951A patent/CN103014756A/zh active Pending
  • 2010-02-01 US US12/697,593 patent/US20100193371A1/en not_active Abandoned
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