Difluoromethylene emission in the reaction between ozone and tetrafluoroethene

Abstract

C2H4 production under conditions (£ < -1.35 V) where reduction of 1 is mass-transport controlled.Control experiments demonstrate that both a Mo(III) species and an additional source of electrons (or Hi) are required for acetylene reduction. Electrolysis of C2H2 alone at £ < -1.35 V in borate buffer and in the presence of substances which strongly catalyze H2 evolution (e.g., cysteine) produces no C2H4 or C2Hé• Thus, the H2 evolved at the cathode does not reduce C2H2 directly. When a solution of 1 is prereduced, purged with C2H2, and allowed to stand with no potential applied, ethylene forms only very slowly (Table I, condition D). If potential is reapplied, ethylene production resumes at the rate indicated under condition B. This result and the potential dependence of the rate of C2H4 formation suggest that a simple redox reaction between reduced Mo catalyst and C2H2 producing C2H4 and an oxidized Mo species may not be the principal means of acetylene reduction.We believe the primary function of the Mo(III) catalyst is to bind acetylene as a substrate. Although we have been unable to isolate an intermediate or confirm its presence by uv-visible spectroscopy, some interaction between Mo(III) and C2H2 must occur because reduction of acetylene, even in the presence of H2 evolution, does not proceed unless molybdenum is present. The proposed Mom-C2H2 adduct may produce C2H4 and C2Hé through routes which do not involve a simple Mo(V) => Mo(III) redox cycle. These alternative routes include: (1) addition of electrons to bound C2H2 from an external source (electrode or chemical reductant) followed by protonation, (2) insertion of H2 or hydride into the adduct in a manner similar to known homogeneous hydrogenation reactions,16 and (3) electrocatalytic hydrogenation of the adduct by adsorbed H atoms produced on the electrode surface. Mechanisms 1 and 2 could be operative in both the chemical and electrochemical models, whereas mechanism 3 would be restricted to the electrochemical system.