Speciation Differences in the Surface Oxidizing Species on FeS₂ and NiS₂: Reactivity in Hydrogen Atom Transfer and Oxygen Atom Transfer

As the most abundant metal sulfide on Earth's surface, pyrite (FeS2) plays important roles in the transformation of various elements and contaminants. Noteworthily, pyrite-type metal disulfides (FeS2 and NiS2) exhibit enzymatic reactivity in hydrogen atom transfer (HAT) and/or oxygen atom transfer (OAT) reactions. However, the oxidizing species and their corresponding reactivity on the surface of FeS2 and NiS2 are still not clear. In this work, both FeS2-O2 and NiS2-O2 exhibit high HAT reactivity, whereas only FeS2-O2 shows OAT reactivity under alkaline and oxic conditions. The high conversion and selectivity in the oxidation of L-cysteine and benzyl alcohol to the respective L-cystine and benzaldehyde indicate the high HAT reactivity of the oxidizing species in both FeS2-O2 and NiS2-O2 systems. On the other hand, the higher oxidation selectivity of methyl phenyl sulfide (MPS) and methyl phenyl sulfoxide (PMSO) to the corresponding PMSO and phenyl methyl sulfone (PMSO2) by FeS2-O2 than those by NiS2-O2 indicates that Fe(IV)=O oxo in the FeS2-O2 system plays important roles in OAT reactions. EPR and quenching experiments reveal that superoxo, peroxo, and oxo species, instead of the produced center dot OH and H2O2, are the predominant oxidants in HAT reactions, and the DPD and electrochemical analyses indicate that the oxidizing species are located on the surface of FeS2 and NiS2. Raman analysis further confirms the presence of-Fe(III)-OOH peroxo and-Fe(IV)=O oxo on FeS2 and-Ni(III)-OOH peroxo on NiS2. This study enriches our understanding on the reactivity of peroxo and/or oxo species on the surface of metal disulfides, providing new perspective for better understanding of the reactivity of various metal disulfides on the earth.