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
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B21/00—Nitrogen; Compounds thereof
  • C01B21/082—Compounds containing nitrogen and non-metals and optionally metals
  • C01B21/083—Compounds containing nitrogen and non-metals and optionally metals containing one or more halogen atoms
• • 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

Definitions

• the present invention relates to a method of electrolytically synthesizing nitrogen trifluoride gas from a mixture of molten salts including ammonium fluoride. More particularly, the invention relates to an electrolytic synthesis method in which even when the cell is operated at a high current density, anode effect is inhibited from occurring and sludge generation by electrode wear is avoided and which enables the synthesis of nitrogen trifluoride gas to be continued at a high efficiency.
• Nitrogen trifluoride is the fluoride of nitrogen which was first synthesized in 1928 by Ruff et al. through molten salt electrolysis. The trifluoride was consumed in large quantities as a fuel oxidizer in planetary probe rocketries projected and practiced by NASA, the United States, and has come to have much interest. At present, nitrogen trifluoride is being used in large quantities as a gas for dry etching in a semiconductor device production step and as a cleaning gas for cleaning CVD chambers in semiconductor device or liquid-crystal display production steps.
• NF 3 there are a chemical process and a molten salt electrolysis process.
• KF ⁇ HF molten salts are electrolyzed to thereby obtain F 2 and this F 2 is reacted with ammonia in molten acid ammonium fluoride or reacted with a metal fluoride/ammonium complex to thereby obtain NF 3 .
• molten salt electrolysis process either ammonium fluoride (NH 4 F)—KF—HF molten salts or NH 4 F—HF molten salts are electrolyzed to directly obtain NF 3 .
• nickel is usually employed as an anode. This process has an advantage that NF 3 not contaminated with CF 4 can be synthesized. However, nickel dissolves during electrolysis and accumulates as a nickel fluoride sludge on the bottom of the electrolytic cell. Because of this, periodic electrolytic-bath replacement or periodic electrode replacement is indispensable, and this makes it difficult to continuously produce NF 3 . Furthermore, nickel dissolution occurs in an amount corresponding to 3-5% of the quantity of electricity applied, and increasing a current density considerably enhances nickel dissolution. It is therefore difficult to conduct the electrolysis at a high current density.
• Diamond electrodes are recently attracting attention as a carbonaceous electrode which causes no anode effect or as a nickel-substitute electrode for NF 3 production (non-patent document 1 and non-patent document 2).
• Conductive-diamond electrodes are a thermally and chemically stable electrode material, and various electrolytic processes utilizing a conductive-diamond electrode have been proposed.
• Patent document 1 proposes a treatment method in which an organic matter in a waste liquid is oxidatively decomposed with a conductive-diamond electrode employing a conductive diamond as an electrode catalyst.
• Patent document 2 proposes a method in which a conductive-diamond electrode is used as each of an anode and a cathode to electrochemically treat an organic matter.
• Patent document 3 proposes a method of ozone synthesis in which a conductive-diamond electrode is used as an anode.
• Patent document 4 proposes the synthesis of peroxosulfuric acid in which a conductive-diamond electrode is used as an anode. Furthermore, patent document 5 proposes a method of microorganism destruction in which a conductive-diamond electrode is used as an anode.
• Patent document 6 discloses that a conductive diamond is applicable to the electrolysis of molten salts including hydrogen fluoride. It was found that when a conductive diamond is used as an anode to conduct the electrolysis of NH 4 F—HF molten salts, then the wettability of the electrode by the electrolyte does not decrease and anode effect does not occur. It was further found that in this electrolysis, sludge generation by electrode wear is inhibited and CF 4 generation is exceedingly slight. Although the cause of these has not been elucidated, the following is presumed. In the conductive-diamond electrode, an outermost surface of the diamond layer comes to have fluorine ends through electrolysis. Except this, the chemically stable diamond structure does not change.
• Patent document 7 discloses a process for fluorine gas production in which a highly oxidized metal fluoride is heated in a gas stream.
• this document includes no statement concerning the use of a highly oxidized metal fluoride in a method of electrolytic NF 3 synthesis through molten salt electrolysis.
• Non-Patent Document 1 Electrochemistry, 75, 934 (2007)
• Non-Patent Document 2 Journal of Fluorine Chemistry, 296, 128 (2007)
• Patent Document 1 JP-A-7-299467
• Patent Document 2 JP-A-2000-226682
• Patent Document 3 JP-A-11-269685
• Patent Document 4 JP-A-2001-192874
• Patent Document 5 JP-A-2004-195346
• Patent Document 6 JP-A-2006-249557
• Patent Document 7 JP-A-2007-176768
• An object of the invention is to provide a method of electrolysis which can stably synthesize NF 3 while diminishing the occurrence of anode effect, generation of CF 4 , and generation of a sludge even when a carbonaceous electrode is used or a diamond electrode is used at a high current density.
• the invention provides an electrolytic synthesis method of nitrogen trifluoride, comprising electrolytically synthesizing nitrogen trifluoride gas from ammonium fluoride in an ammonium fluoride-containing molten salt mixture using a carbonaceous electrode as an anode, wherein the method comprises:
• ammonium ions can be reacted with the highly oxidized metal fluoride in the solution not to mention on the surface of the electrode and near the electrode to thereby synthesize nitrogen trifluoride gas.
• the carbonaceous electrode is one selected from an amorphous-carbon electrode, a graphite electrode, a carbon electrode containing a metal fluoride, and a conductive-diamond electrode.
• the electrode preferably comprises a base made of graphite or amorphous carbon.
• the molten salt mixture preferably comprises NH 4 F—KF—HF or NH 4 F—HF.
• the metal ions preferably comprise ions of at least one metal selected from the group consisting of Ni, Mn, Co, Li, and Cs.
• the anions serving as the counter ions preferably are fluoride ions. Although the counter ions may be ions of an oxide or another halide, such ions undergo, upon addition to the electrolytic bath, anion exchange and become a fluoride.
• the current density of the anode at the time of the electrolysis is desirably 0.01-10 A/cm 2 .
• the concentration of the metal ions in the electrolyte is desirably from 0.01 M to a saturation concentration.
• use may be made of a method in which metal ions are added in an amount not smaller than that corresponding to a saturation concentration to cause the metal ions to exist in the electrolysis system in an amount corresponding to supersaturation. In this case, however, the excess amount precipitates as a sludge or salt and the solution part has a saturation concentration. Consequently, the term saturation as used in the invention includes an embodiment in which metal ions are added in an amount corresponding to supersaturation.
• the target anode reaction is represented by the following scheme.
• reaction products include N 2 F 2 , N 2 F 4 , O 2 , and N 2 O.
• a nickel dissolution reaction represented by the following scheme proceeds to generate a nickel fluoride sludge.
• the dissolution of proper metal ions as in the invention produces the following effect.
• the current which is permitted to flow in an amount exceeding that corresponding to the rate of ammonium ion diffusion yields excess fluorine radicals, and these fluorine radicals react with the dissolved metal ions to yield a highly oxidized metal fluoride.
• This metal fluoride undergoes a disproportion reaction near the electrode and thereby becomes highly oxidized metal fluoride ions, which dissolve in the solution.
• reaction with the ammonium ions present in the solution proceeds rapidly, whereby the target compound, NF 3 , is yielded also in the solution.
• nickel ions for example, it is presumed that the reaction represented by
• the overall current efficiency (sum of fluorination near the electrode and fluorination in the anolyte) can be increased.
• NF 3 nitrogen trifluoride
• metal ions are dissolved which are capable of electrolytically yielding a highly oxidized metal fluoride through reaction with fluorine radicals (F.) generated upon the discharge of fluoride ions which are a component of the ammonium fluoride.
• F. fluorine radicals
• NF 3 is synthesized by direct electrolysis occurring on the electrode surface and near the electrode.
• current is permitted to flow in an amount exceeding that corresponding to the rate of ammonium ion diffusion, then the reaction between excess fluorine radicals and ammonium ions becomes unable to proceed on the electrode surface, resulting in a reduced current efficiency.
• the dissolved metal ions react with excess fluorine radicals to become a highly oxidized metal fluoride, and this metal fluoride is converted to highly oxidized metal fluoride ions through a disproportion reaction near the electrode. Consequently, even when current is permitted to flow in an amount exceeding that necessary for the direct electrolysis occurring on the electrode surface and near the electrode, the current is used without being wasted and the target compound, NF 3 , can be synthesized at a high current efficiency.
• the anode to be used in the invention is a carbonaceous electrode, which is reduced in sludge generation.
• an amorphous-carbon electrode, a graphite electrode, or a carbon electrode containing a metal fluoride is usable, it is especially desirable to use a diamond electrode because of its chemical stability.
• the metal fluoride in the metal-fluoride-containing carbon electrode are LiF, LiF—CaF 2 , MgF 2 , CaF 2 , and the like.
• the conductive-diamond electrode is produced by fixing a conductive diamond to an electrode base.
• the material and shape of the electrode base are not particularly limited so long as the material is electrically conductive.
• Use can be made of nonmetallic materials such as silicon, silicon carbide, graphite, and amorphous carbon and metallic materials such as titanium, niobium, zirconium, tantalum, molybdenum, tungsten, and nickel.
• a carbonaceous material such as graphite or amorphous carbon is preferred from the standpoint of chemical stability in electrolytic baths which contain hydrogen fluoride.
• the shape of the electrode use may be made of a platy, meshy, rod, or pipe shape, a spherical shape such as beads, a porous platy shape, or the like.
• Methods for fixing diamond to the electrode base also are not particularly limited, and any desired one may be used.
• Typical processes for diamond production include a hot-filament CVD (chemical vapor deposition) process, microwave plasma CVD process, plasma arc jet process, and physical vapor deposition (PVD) process.
• a gaseous mixture of hydrogen gas and a carbon source is used as a raw material for diamond in each process, an element having a different valence (hereinafter referred to as dopant) is added in a slight amount in order to impart electrical conductivity to the diamond.
• the dopant preferably is boron, phosphorus, or nitrogen.
• the content thereof is preferably 1-100,000 ppm, more preferably 100-10,000 ppm.
• amorphous carbon and a graphite ingredient remain in the conductive-diamond layer synthesized. From the standpoint of the stability of the diamond layer, it is preferred that the amount of the amorphous carbon and graphite ingredient should be smaller. It is especially preferred to use a conductive diamond in which in Raman spectroscopy, the ratio of the peak intensity at 1,332 cm ⁇ 1 assigned to diamond to the peak intensity at 1,560 cm ⁇ 1 assigned to graphite G-band is in the range of from 1:0.3 to 1:0.8.
• the hot-filament CVD process is explained below as a representative process for synthesizing a conductive-diamond electrode.
• An organic compound serving as a carbon source such as methane, an alcohol, or acetone, and a dopant are fed to a filament together with, e.g., hydrogen gas.
• the filament is heated to a temperature at which hydrogen radicals or other radicals are generated, i.e., 1,800-2,800° C., and a conductive base is disposed in this atmosphere so as to be held in a range of temperatures at which diamond deposition occurs (750-950° C.).
• the rate of feeding the gaseous mixture depends on the size of the reaction vessel. However, the pressure is preferably 15-760 Torr.
• the base preferably has an average surface roughness (Ra) of 0.1-15 ⁇ m and a ten-point surface roughness (Rz) of 1-100 ⁇ m. Furthermore, to fix a diamond powder as nuclei to the base surface is effective in growing an even diamond layer. A layer of fine diamond particles having a particle diameter of usually 0.001-2 ⁇ m deposits on the base.
• the thickness of the diamond layer can be regulated by changing deposition period. The thickness thereof is preferably regulated to 1-10 ⁇ m from the standpoint of profitability.
• any of an amorphous-carbon electrode, metal-fluoride-containing carbon electrode, and graphite electrode or the conductive-diamond-coated electrode or the like is used as an anode, and nickel, stainless steel, or the like is used as a cathode.
• electrolysis is conducted in NH 4 F—HF molten salts or NH 4 F—KF—HF molten salts at a current density of 0.01-10 A/cm 2 , whereby NF 3 can be obtained from the anode.
• the dissolution of metal ions may be accomplished by adding and dissolving a metal salt such as a fluoride (the salt preferably is NiF 2 , NH 4 NiF 3 , or the like in the case of, e.g., nickel).
• a metal salt such as a fluoride
• the concentration in which the metal ions are to be dissolved should be regulated to a value in the range of from 0.01 M to saturation.
• the metal can be used any metal which forms highly oxidized metal fluoride ions. Preferred are Ni, Mn, Co, and the like. These metals are presumed to form highly oxidized metal fluoride ions (complex ions) each including six fluoride ions coordinated.
• Ni, Mn, Co, Li, and Cs are simultaneously used as the metal to be added or as ions thereof in the invention, then not only the metal ions in a high-order oxidation state described above but also a complex salt in a high-order oxidation state, such as, e.g., Li 2 NiF 6 in the case of Li and Ni or CsNi 2 F 6 in the case of Cs and Ni, deposits on the surface of the carbonaceous electrode.
• the complex salt deposited in a high-order oxidation state is presumed to accelerate the target fluorination reaction of ammonium ions and heighten current efficiency in NF 3 production.
• the material of the electrolytic cell use can be made of a mild steel, nickel alloy, fluororesin, or the like from the standpoint of resistance to corrosion by high-temperature hydrogen fluoride. It is preferred that the anode side and the cathode side should be wholly or partly separated from each other by a partition wall, diaphragm, or the like in order to prevent the NF 3 synthesized at the anode from mingling with the hydrogen gas generated at the cathode.
• the NH 4 F—HF molten salts as an electrolytic bath may be prepared, for example, by bubbling anhydrous hydrogen fluoride gas into ammonium monohydrogen difluoride (acid ammonium fluoride) and/or ammonium fluoride.
• the NH 4 F—KF—HF molten salts as another electrolytic bath may be prepared, for example, by bubbling anhydrous hydrogen fluoride gas into a salt mixture of acid potassium fluoride and acid ammonium fluoride.
• the electrolytic bath immediately after preparation thereof contains water in an amount of about several hundreds ppm. Because of this, in electrolytic cells employing a conventional carbon electrode as an anode, it has been necessary to remove the water, for example, by conducting dehydrating electrolysis at a low current density of 0.01 A/cm 2 or lower for the purpose of inhibiting anode effect. However, the electrolytic cell employing a conductive-diamond electrode does not necessitate any special operation, such as, e.g., dehydrating electrolysis, and NF 3 can be electrolytically synthesized.
• a slight amount of HF accompanies the NF 3 generated at the anode.
• This HF can be removed by passing the NF 3 through a column packed with granular sodium fluoride.
• a small amount of nitrogen and a slight amount of oxygen and dinitrogen monoxide (nitrous oxide) are generated as by-products in the NF 3 synthesis.
• the dinitrogen monoxide (nitrous oxide) can be removed by passing the NF 3 through water and sodium thiosulfate or through a zeolite.
• the oxygen can be removed with activated carbon.
• a graphite plate was used as a conductive base to produce a conductive-diamond electrode with a hot-filament CVD apparatus under the following conditions.
• a surface of the base was polished with an abrasive material composed of diamond particles having a particle diameter of 1 ⁇ m.
• the base surface had an average surface roughness (Ra) of 0.2 ⁇ m and a ten-point surface roughness (Rz) of 6 ⁇ m.
• Ra average surface roughness
• Rz ten-point surface roughness
• diamond particles having a particle diameter of 4 nm were fixed as nuclei to the base surface, and this base was attached to the hot-filament CVD apparatus.
• a gaseous mixture obtained by mixing hydrogen gas with 1 vol % methane gas and 0.5 ppm trimethylboron gas was fed to the apparatus.
• the conductive-diamond electrode produced was attached as an anode in NH 4 F-1.7HF molten salts immediately after preparation of the molten salts.
• a nickel plate was used as a cathode.
• NiF 2 was added to the cell so as to dissolve nickel ions in the molten salts in a saturation concentration.
• constant-current electrolysis was conducted at a current density of 0.13 A/cm 2 .
• the gases generated at the anode were analyzed. As a result, the current efficiency concerning NF 3 production was found to be 50%. Almost no sludge generation occurred.
• Electrolysis was conducted under the same conditions as in Example 1, except that nickel ions were not dissolved. As a result, analysis of the gases generated at the anode at 24 hours after initiation of the electrolysis revealed that the current efficiency concerning NF 3 production was 36%.
• Electrolysis was conducted under the same conditions as in Example 1, except that a nickel plate was used as an anode. As a result, analysis of the gases generated at the anode at 24 hours after initiation of the electrolysis revealed that the current efficiency concerning NF 3 production was 58%. However, the cell which had been used for the electrolysis contained a large amount of a sludge.
• Example 2 The same electrolysis as in Example 1 was conducted, except that the current density was changed to 0.4 A/cm 2 . As a result, the current efficiency concerning NF 3 production was found to be 50%.
• Electrolysis was conducted under the same conditions as in Example 2, except that nickel ions were not dissolved. As a result, analysis of the gases generated at the anode at 24 hours after initiation of the electrolysis revealed that the current efficiency concerning NF 3 production was 12%.
• Lithium ions and nickel ions were dissolved in NH 4 F-2HF molten salts each up to a saturation concentration, and constant-current electrolysis was conducted with the apparatus used in Example 1 under the conditions of a temperature of 120° C. and a current density of 0.1 A/cm 2 . Analysis of the gases generated at the anode at 100 hours after initiation of the electrolysis revealed that the current efficiency concerning NF 3 production was 65%.
• Electrolysis was conducted under the same conditions as in Example 3, except that the metal ions were not dissolved. As a result, analysis of the gases generated at the anode at 100 hours after initiation of the electrolysis revealed that the current efficiency concerning NF 3 production was 45%.

Landscapes

• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=40946109

Family Applications (1)

Country Status (6)

Cited By (4)

Families Citing this family (4)

Citations (2)

Family Cites Families (8)

• 2008
  • 2008-07-10 JP JP2008180616A patent/JP2010018849A/ja active Pending
• 2009
  • 2009-07-09 EP EP09165061.4A patent/EP2143826B1/en active Active
  • 2009-07-09 US US12/500,213 patent/US20100006449A1/en not_active Abandoned
  • 2009-07-09 KR KR1020090062500A patent/KR101185817B1/ko active IP Right Grant
  • 2009-07-10 CN CN2009101520252A patent/CN101624708B/zh active Active
  • 2009-07-10 TW TW098123386A patent/TWI415973B/zh active

Patent Citations (2)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events