Electrolysis of Ammonium Hydrogen Difluoride

Ruff et al. [1] first prepared NF3 in 1928 by the electrolysis of molten anhydrous ammonium hydrogen difluoride. The electrolysis was carried out at 125°C, with a voltage of 7 to 9 V, and a current of 10 A in a cell made of copper. NF3 and other gaseous products were formed at the carbon anode and Hg was eliminated at the copper cathode. All the essential aspects of Ruff s work have been described in Fluor Erg.-Bd. 1,1959, pp. 238/40. In this section, only the work published since 1950 will be reported. Several papers dealing with improvements of Ruff s process have been published, especially in the 1960s. 

The best yields of NF3 are obtained if the electrolyte has the composition NH4F-1.1HF to NH4F-1.8HF and the electrolysis is carried out between 100 and 130°C [2 to 5]. Pierce and Pace [6] were the first to substitute the original carbon anode with a nickel anode. By using a nickel anode, the contamination of NF3 with CF4 and other fluorinated carbon compounds was avoided. A disadvantage of nickel anodes is that they degrade by forming nickel fluorides [2]. 

The electrolytic cells used are very similar to the cells used for the production of elemental fluorine [4, 5, 7] cf. Fluorine Suppl. Vol. 2,1980, pp. 4/10. An electrolytic cell suitable for NF3 production on a pilot-plant scale having a capacity of about 25 kg of electrolyte was described by Massonne [5]. The cell and the gas separation skirt were made of Monel the cell cover and the cathode were made of mild steel. The anode was composed of nickel or carbon with an efficient surface of ca. 800 cm. Thus, a current density of 0.15 A/cm was possible at a current of 120 A. On the top of the cell cover there were holes for the release of Hg, the anode gas, inlets for HF, NH3, and Ng, and for the electrodes. The cell was electrically heated and a water-cooling system was attached to remove the heat at high current densities. 

The product gas mixture was expected to contain the following substances H2, NF3, Ng, N2F2, NH2F, NHF2, N2O, O2, F2, and HF. CF4 occurred when carbon anodes were used [4, 5]. Additionally, the formation of NO [8, 9] and NO2 [4] was reported. O2 and N2O result from the 

Massonne [5], in his study of the electrolysis in a pilot plant, determined the composition of the anode gas at various temperatures, current densities, and compositions of the electrolyte. He showed that the NF3 production is nearly independent of these parameters within a relatively large range. The optimum conditions were temperature 100 to 120°C, current density 0.12 to 0.15 A/cm, and an electrolyte NH4F (1.1 to 1.5) HF. Under these conditions, and after it had passed through an adsorber with NaCl and had been washed with alkali, the anode gas had the following composition (invol%)  

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