Abstract:
Reduction of ethylene with hydrogen was studied at a Pt electrode on open circuit in H2SO4 and HClO4 solutions. The reaction rate and the open circuit potential were followed as a function of the gas composition (C2H4+ H2+ He, 1 atm) and the acid concentration. The rate, v, is independent of the acid concentration and is expressed as v=kHkEPHPE/(kHPH+kEPE), where the k values are constants and PH and PE the partial pressures of H2 and C2H4. The ratio kE/kH is 5–6, which is the same value obtained for the ratio of the diffusion rate constant of C2H4 in solution to that of H2. The rate-determining step is the mass transfer step of H2 or of C2H4 depending on the condition, PH/PE < 5–6 or > 5–6, respectively. This conclusion is supported by the dependence of the open circuit potential on the gas composition. Time variation of the catalytic activity of the activated electrode is discussed in terms of the activity change of the adsorbed hydrogen atom. Newly formed adsorbed hydrogen atoms are reactive, but after remaining on the surface, some of them become stable and have a retarding effect on the reaction.
