Reactions of cation radicals of EE systems. IV. The kinetics and mechanism of the homogeneous and the electrocatalyzed reaction of the cation radical of 9,10-diphenylanthracene with hydrogen sulfide

Potential step spectroelectrochemical studies of the reaction of H₂S with electrogenerated DPA·⁺ at Pt-OTE's on which PtS has been anodically formed prior to the pulsed experiment shows unusually fast consumption of cation radical during the initial stages of the pulse. Similar experiments using electrodes on which PtS has not been formed as well as those on which PtS has been formed and then cathodically discharged do not give rise to this behavior. On the basis of these experiments, it is concluded that H₂S is adsorbed onto PtS where it reacts with electrogenerated DPA·⁺ and further, that the adsorption process is kinetically slow. Open circuit relaxation spectroelectrochemical experiments of varying pulse duration show that the consumption of DPA·⁺ is initially a zero-order process involving H₂S adsorbed onto the PtS surface and, as this adsorbed material is depleted, the consumption of DPA·⁺ becomes a second-order process. In concert with the potential step experiments, these pulse time studies show the parallel nature of the heterogeneous and homogeneous reaction pathways. The zero-order rate constant for the heterogeneous process was evaluated from pulse relaxation experiments (by extrapolation to zero pulse duration) to be 1.51(±0.05) × 10^-2 M s^-1. The bimolecular rate constant for the homogeneous process was evaluated from potential step and pulsed relaxation spectroelectrochemical measurements and stopped-flow kinetic spectrophotometry to be 6.6(±1.2) M^-1 s^-1.