Oxidation of H2S to S by air with Fe(III)–NTA as a catalyst: catalyst degradation

The oxidation of H2S to S by air is represented by the overall reaction:[Formula: see text]The process is used with a catalyst for the removal of H2S from natural gas. A typical catalyst would be an iron(III) chelate system such as iron(III)–nitrilotriacetate (NTA is a tribasic acid, H3L, and NTA3− represents the trinegative anion). The catalyst reactions are[Formula: see text]The main difficulty with the process is the degradation of the metal chelate catalyst. Thus the Fe(II,III)–NTA system degrades through the formation of a number of weaker chelating agents and eventually to carbon dioxide, water, and ammonia. The degradation products identified are iminodiacetic acid (IDA), glycine, and oxalate. All of these degradation products were measured quantitatively by HPLC, and a degradation scheme is presented for the catalytic process. The fact that thiosulfate greatly decreases the degradation rate, and the fact that the degradation products were observed only in the stoichiometric reactions in which the ferrous NTA was oxidized by air, led to the interpretation that the hydroxyl radical or the superoxide radical ion is the reactive species that attacks the NTA and the intermediate degradation products and is responsible for the degradation of the organic ligands present.