Oxidation behaviour of sperrylite and platarsite (100) surfaces: A DFT study

Abstract

The density functional theory (DFT) was employed to investigate the oxidation mechanism and electronic structures of sperrylite (PtAs2) and platarsite (PtAsS) (100) surface. The cluster expansion (CE) and virtual crystal approximation (VCA) methods were used to construct the PtAsS models. The PtAsS constructed model by CE possessed symmetry of P213, while the VCA maintained the Pa-3 symmetry. The computed surface energies and morphologies for all surface models, depicted the (100) plane the preferred cleavage. The oxidation mechanisms of the (100) surfaces of sperrylite and platarsite favoured the mono atomic oxygen bonding, which resulted from the dissociation of the O2 molecule on the surfaces. The adsorption energies were more exothermic for PtAs2 (100) surface oxidation (–315.82 kJ.mol–1), suggesting that sperrylite highly oxidises than the platarsite mineral. The PtAsS (CE) (100) surface oxidises stronger than the PtAsS (VCA) (100) surface, which is owed to the less (100) surface stability of the PtAsS (CE) than for PtAsS (VCA). The partial density of states (PDOS) and Bader charges indicated that the adsorbed As, S and As/S atoms donate electrons to the O2 atoms. These finding clearly illustrated that the arsenide PGMs primarily oxidises through the As sites for sperrylite and As and S sites for platarsite. Therefore the study has demonstrated the oxidation behaviour of the sperrylite and platarsite platinum group minerals (PGMs) and unravelled the surface hindrance for collector interaction during floatation which results in poor floatability of arsenide PGMs.