Effect of Added Urea on the Current Efficiency of Nitrogen Trifluoride in Electrolytic Production with Carbon and Graphite Electro d es in a Molten NH₄F-HF-KF System

Abstract

Electrolytic production of NF₃ on various anodes was carried out at 120°C in molten NH₄F-HF-(NH₂)₂CO and molten NH₄F-HF-KF-(NH₂)₂CO systems by a few kinds of electrochemical methods. Either carbon or graphite was used as an anode and Pt-rod was used as a reference electrode. Anode gas was analyzed by both gas chromatography and infrared spectroscopy.
The anodic polarization curves obtained by the galvanostatic method in the NH₄F-HF(NH₂)₂CO system showed that a favourable molar fraction of urea was less than O.008 in order to decrease direct discharge of urea. From the anodic polarization curves obtained in the NH₄F-HF-KF system, it was also found that the molar fraction of KF added in the melt should be O.233 at minimum to prevent HF from permeating through the anode. When urea was added in the molten NH₄F-HF-KF system, evolution of CO₂ gas occurred due to the reaction of urea with H₂O in the melt. In the electrolysis at the potential between 4.5 V and 5.5 V (the region of electrofluorination), anode gas was mainly composed of N₂, CF₄, NF₃, CO₂, N20. Under the electrolytic conditions of current density of 15mA⋅cm^-2 and urea-molar fraction of 0.004, NF₃ was detected in the anode gas after electrolysis for 6 h and its composition ratio of anode gas, i. e. yield, reached to the maximum value of 87% after 70 h.
These results indicate that the addition of urea in the melt is effective for increasing the current efficiency of NF₃ because of the elimination of H₂O in the melt by hydrolysis of both urea and COF₂ which is one of the anodic products.