Mechanism of H2S Oxidation by Ferric Oxide and Hydroxide Surfaces

The interaction of H2S with the surface of either α-Fe2O3 or amorphous ferric hydroxide at low temperatures has been characterized using FTIR and quantified using volumetric methods. The mechanism of the interaction of H2S with either reagent is ionic and involves heterolytic dissociation of H2S and exchange of S2- and SH- anions for O2- or OH-. Subsequently water is eliminated and, in coupled redox reactions, sulfide is oxidized to elemental sulfur and Fe3+ cations are reduced to Fe2+. Initially, almost all the surface sites can interact readily with H2S, to form a layer of iron sulfides. Over longer reaction times reaction occurs with deeper sites, to form bulk sulfides. Regeneration of ferric (hydr)oxide by oxidation with oxygen converts Fe2+ to Fe3+, and S2- anions are replaced by O2- with formation of elemental sulfur. The surface species involved in each reaction have been characterized using FTIR and, for the first time, direct evidence has been obtained for the presence of SH- anions as very reactive intermediates.