Method of electrolytically synthesizing nitrogen trifluoride

Abstract

The present 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: a step of dissolving, in the molten salt mixture, metal ions capable of electrolytically yielding a highly oxidized metal fluoride through reaction with fluorine radicals (F.) that are generated upon the discharge of fluoride ions which are a component of the ammonium fluoride, thereby reacting the metal ions with the fluorine radicals (F.) to yield the highly oxidized metal fluoride, and reacting the highly oxidized metal fluoride with ammonium ions on a surface of the electrode and in a solution to synthesize nitrogen trifluoride gas.

Description

TECHNICAL FIELD[0001]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.BACKGROUND ART[0002]Nitrogen trifluoride (NF3) 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...

AI Technical Summary

Benefits of technology

[0033]In the invention, for synthesizing nitrogen trifluoride (NF3) from ammonium fluoride by molten salt electrolysis, 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. In the conventional electrolytic NF3 synthesis for which such metal ions are not dissolved, NF3 is synthesized by direct electrolysis occurring on the electrode surface and near the electrode. However, when 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.

[0034]In contrast, in the invention, 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, NF3, can be synthesized at a high current efficiency.

Problems solved by technology

This (CF)n has an exceedingly low surface energy and hence has poor wettability by the electrolytic bath.

There also is a problem that the CF4 and C2F6 which have been generated according to scheme (3) reduce the purity of the target NF3 gas.

There is a problem that since CF4 and NF3 have exceedingly close boiling points (the difference in boiling point is as small as 1° C.

), separation by distillation is difficult and a highly costly purification technique must be employed in order to obtain high-purity NF3.

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 NF3.

It is therefore difficult to conduct the electrolysis at a high current density.

However, this document includes no statement concerning the use of a highly oxidized metal fluoride in a method of electrolytic NF3 synthesis through molten salt electrolysis.