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.