Descriptor of catalytic activity of metal sulfides for oxygen reduction reaction: a potential indicator for mineral flotation

Abstract

Froth flotation has been widely used to separate sulfide minerals from sulfide ores, where the oxygen reduction reaction (ORR) plays a significant role as the cathodic reaction. However, the intrinsic ORR mechanism over sulfide minerals and its effect on the flotation performance remain unclear. Herein, we explored the intrinsic ORR mechanism over metal sulfides through density functional theory (DFT) calculations along with the computational hydrogen electrode (CHE) model. Our results revealed that pyrite (FeS₂) and chalcopyrite (CuFeS₂) exhibited similar ORR behavior to that over platinum (Pt) but not galena (PbS) and sphalerite (ZnS). The relative computational overpotentials for the ORR were completely consistent with the experimental trend in the order of Pt < FeS₂ < CuFeS₂ < PbS < ZnS and linearly correlate with the oxygen binding energies. We thus conclude that the distinct ORR behaviors originate from the different oxygen binding strengths which are determined by the underlying electronic structure. More importantly, we demonstrated that the DFT calculated bulk centroids of the occupied S 3p band for metal sulfides strongly correlate with the experimentally measured rest potential of the counter sulfide minerals in xanthate solution, and consequently we established the descriptor–activity relationship. Using this relationship, we present that the mechanism underlying the mixed potential model regarding the xanthate–sulfide mineral interaction is essentially governed by the relative dominating role of the compositional metallic cation to the anionic sulfur in terms of the electronic structure around the Fermi level, which favors the formation of dixanthogen and metal-xanthate, respectively.