Electrolytic Production of Nitrogen Trifluoride with Formamide as a Starting Material

Abstract

The anodic reaction of formamide (HCONH₂) on amorphous carbon was studied in a molten KH₂F₃ at 120°C using a few kinds of electrochemical methods. Pt-rod was used as the reference electrode. Anodic products were analyzed by both gas chromatography and infrared spectroscopy.
The i-E curves obtained by the cyclic voltammetry were divided into four regions with an increase in the potential. The current densities observed in both region I (below ca.2 V vs. Pt) and II (ca.2-3.5 V vs. Pt) were dependent upon the HCONH₂-concentration, and, therefore, HCONH₂ discharged mainly in these regions. Besides, the (CxF)[x>2] film was formed also on the carbon surface in the region II. In the region III (ca.3.5-6.5 V vs. Pt), HCONH₂ was fluorinated and the anode gas was composed of N₂(+O₂), CF₄, NF₃, CO₂(+COF₂), N₂O and so on. As H₂O in the melt was eliminated by formation of CO₂, the content of NF₃ in the anode gas increased with the lapse of time of electrolysis and reached the maximum value of 50.9% in the case of 4.0 mol% HCONH₂ and the current density of 5.3 mA⋅cm^-2. The anode effect occurred in the region IV (over ca.6.5 V vs. Pt). In th is system, however, the peak current density at ca.6.5 V vs. Pt on the i-E curves obtained by scanning the potential to the lower side from 9 V vs. Pt was larger than that in the molten KH₂F₃ only and it also increased with increasing the number of scan cycles. In electrolysis at the current density of 5.3-11.0 mA⋅cm^-2, the anode effect often occurred and the pulsed application of very high voltages (over 40 V) to the electrolytic cell was effective for the decomposition of (CF)n film covering the surface of the carbon anode in every occurrence of the anode effect. This procedure did not decrease the yield of NF₃. The addition of 1.0 wt% LiF to the melt was also effective for the prevention of the anode effect, but it decreased the yield of NF₃.
From these results, it is concluded that the surface of the carbon anode would be partially restored through the reduction of (CxF)n and (CF)n films with HCONH₂ and that the direct discharge of HCONH₃ as well as its electrofluorination would take place on the partially restored surface of the carbon anode. The mechanism of electrochemical fluorination of HCONH₂ is also discussed in detail.