The Electrochemical Oxidation of Hydrogen Sulfide in the Tafel Region and under Mass Transport Control

Abstract

H2S can be continuously oxidized about 1.21V (RHE) anode potential range at about 93% efficiency, according to the reaction scheme H2S-~ S + 2H +-l-2e-which most probably involves the formation of an SH-chemi-sorbate on the (platinum) anode surface. At mass transfer-controlled conditions a limiting current is observed; rotating tripolar electrode cell experiments yield the value of 1.94 • 10-5 cm2/sec for the diffusion coefficient of H~S in a 0.5 mole/dm~ aqueous H2SO4 solution, at 22~ In a previous publication (1) the oxidation of hydrogen sulfide at low potentials was described in a rotating tripolar wiper-blade electrode (RTE) system via continuous anode reactivation. At potentials less than about 0.5V (RHE) the most likely oxidation mechanism involves two first-order consecutive elec-trochemical reactions with the SH-chemisorbate as the intermediate product, responsible for the relatively rapid passivation (about 30 sec on an initially fresh anode surface); the RTE system was shown (1, 2) to be an efficient anode "reactivator" in the low potential range. The extension of this study into the Tafel region and the mass transport-controlled region was stipulated by two major reasons. Firstly very little information about the nature of this reaction in these regions is available in the literature; the establishment of the most likely reaction mechanism is all the more intriguing as H2S can be oxidized continuously past the range of passivation, characteristic of low potential oxidation. Secondly, oxidation at elevated potentials yields colloidal sulfur as the anode product, which is potentially useful for medicinal purposes, etc., apart from the obvious application in pollution control. Experiments in the mass transfer control regions may also be used to estimate the apparent diffusion coefficient pertaining to the electrochemical system. The present paper summarizes the results of such an investigation and is intended to offer a better understanding of the electrochemical oxidation of hydrogen sulfide over a relatively wide range of electrode potentials .