Reduction electrochimique du trichloro-1,1,2-trifluoroethane: Effet catalytique de l'ion ammonium

Abstract

The electrochemical reduction of 1,1,2-trichlorotrifluoroethane (TCTFE), the raw material for the manufacture of chlorotrifluoroethylene (CTFE), is studied on a rotating zinc disk. TCTFE is at saturation in solutions of NH 4Cl up to 1 M in methanol—water mixtures formed with 50–90 vol% MeOH. At low concentration of TCTFE (MeOH<70 vol%), the reaction is mass-transfer limited. For a given value of the overpotential the reduction current of TCTFE increases as the NH 4Cl concentration goes from 0.2 to 1 M. Moreover, for rich mixtures of MeOH (70–80 vol%) the transition between the charge transfer and the mass-transfer limitations is shifted towards the higher values of TCTFE concentration with rising concentration of NH 4Cl. The ammonium ions act as catalyst in the electron transfer. For a zinc cathode it is assumed that the chemical reduction between the metal and TCTFE takes place simultaneously with the electrochemical reduction mediated by NH + 4 via the NH 0 4 adsorbed intermediate. This assumption is confirmed by studies on rotating electrodes of nickel, copper and cadmium and by preparative electrolyses of CTFE on nickel and copper electrodes. For high values of the polarization, the current is limited through neutralization near the cathode of the acidic NH + 4 cation by OH − generated during co-evolution of hydrogen. The values of this limiting current increase in the same order (Ni < Cu < Zn < Cd) as the hydrogen overpotentials. The results are interpreted in terms of interactions between the cathodic metal and the assumed NH 4,ads intermediate; the weak metal—hydrogen bonding energy (Zn, Cd) makes the electron transfer to the TCTFE molecule easier. The action of NH + 4 is specific in that dechlorination of TCTFE on a nickel cathode to form CTFE is possible with a fairly good current yield only if NH 4Cl is used as electrolyte.