Spontaneous H2O2 Accumulation under O2 Driven by Sacrificial Oxidation of NiS2 without External Electrical/Photonic Input

Although spontaneous H₂O₂ accumulation by sacrificial oxidation of metal disulfides (FeS₂, NiS₂, etc.) under oxic conditions without external electrical/photonic input has been observed, the mechanistic understanding of HCO₃^– on the production of H₂O₂ is still not clear. In this study, HCO₃^– apparently accelerates the production of H₂O₂ on NiS₂, and the produced H₂O₂ increases from 53.85 to 90.12 μM with increasing concentrations of HCO₃^– from 0 to 0.5 M at pH 9.0. Electrochemical analyses indicate that HCO₃^– effectively mediates the 2e reduction of O₂ to H₂O₂ on the surface of NiS₂. EPR and Raman analyses rule out the primary contributions of soluble ^•OH and O₂^•– to the production of H₂O₂ and reveal the presence of both ≡Ni–OO^• superoxo and ≡Ni–OOH peroxo on NiS₂, which are important precursors for H₂O₂. The presence of CO₃^•– also indicates that HCO₃^– is an important H-donor during the reduction of O₂ to H₂O₂. DFT calculations further reveal that it is thermodynamically more favorable for ≡Ni–OO^• and ≡Ni–OOH to abstract H from HCO₃^– than from H₂O to produce H₂O₂, confirming that HCO₃^– is a better H-donor than H₂O to produce H₂O₂ via the H-abstraction pathway. This study provides insight into the importance of HCO₃^– on spontaneous H₂O₂ accumulation by sacrificial oxidation of NiS₂ under oxic conditions without external electrical/photonic input.