Study of H2S selective oxidation on new model catalysts: Influence of composition

Phosphate of transition elements A₃M₄(PO₄)₆ (A=Fe, Ni, Zn, Mg, Cu, Cr and M=Fe, V, Cr) are prepared by solid methods, at 1223 K. Their activity in H₂S selective oxidation are compared. In spite of their low specific area, the catalysts develop a good activity (up to 17% of H₂S conversion for surface area lower than 1 m² g^?1) and an excellent sulphur selectivity (always higher than 95%). Screening experiments show that the best systems always contain Fe as element and, in these cases, Mössbauer characterisations evidence the establishment of Fe²⁺/Fe³⁺ mixed valency during the reaction. As shown by XANES and XRD results, the ability of the element in the divalent A site to promote the redox mechanism between M²⁺/M³⁺and H₂S/S⁰ and to prevent sulphidation determines the catalytic activity.