Platinum(II) sulfide, PtS, is an inorganic compound, a green solid with a low melting point and high chemical resistance. It has been employed in many applications as it is moderately water and acid soluble and can be readily dispersed and deposited using sputtering targets and evaporation materials.

The platinum sulfide crystal structure is unusual in that it has square planar Pt atoms and tetrahedral sulfide ions. This leads to a very high on-off ratio and a wide variety of structural and electronic properties.

Despite these properties, platinum sulfide remains relatively understudied. This is largely due to the difficulty of obtaining a pure sample of this material. The synthesis involves complex reactions which give rise to a mixture of PtS and PtS2. While PtS2 is the more stable form it can be degraded to the less stable PtS through high temperatures. This can lead to a range of errors in XPS spectra with peak positions attributed to the different forms of the compound. This confusion also affects Raman spectra with peaks falsely assigned to PtS2 contributing to the current uncertainty in identifying this material.

This article uses a combination of hydrothermal solubility measurements in a reactor that allows fluid sampling, in situ X-ray absorption spectroscopy and molecular dynamics simulations to systematically investigate the structure, composition and stability of platinum sulfide complexes in aqueous H2S-dominated hydrothermal systems at 350 degC and 600 bar. The tetrahydrosulfide, Pt(HS)42-, is found to be the dominant platinum species in solutions saturated with PtS over a range of hydrogen sulfide concentrations. This information is crucial for understanding platinum transport and remobilization in natural hydrothermal settings and should help guide future studies.

Abstract

Platinum sulfide, PtS, is an elemental green solid that does not react with air or water at ambient temperature and pressure. It is also one of the most corrosion resistant metals and is used in a variety of applications including electroplating and oxidation catalysis. In addition to its innate properties, platinum is also the active ingredient in anti-cyanide drugs known as cisplatin and a variety of other cyanide countermeasures administered by intramuscular injection (IM) which mitigate cyanide exposures in mass casualty events. However, the cyanide scavenging efficacy of these Pt(II) complexes is dependent on prior pre-treatment with dimethyl sulfoxide (DMSO).

This study combines experimental studies of aqueous H2S-dominated hydrothermal fluids using in situ X-ray absorption spectroscopy and molecular dynamics simulations to systematically investigate platinum sulfide solubility in a range of conditions at temperatures between 200 and 300 degC and pressures up to 600 bar. This study generated PtS solubility constants that demonstrate the major form of the Pt(II) sulfide complex is tetrahydrosulfide, Pt(HS)42-, for a wide range of hydrogen sulfide concentrations at near neutral pH. This result establishes a new benchmark for future PtS solubility measurements in hydrothermal systems and can help to assess Pt transport, remobilization and concentration in nature.