Theoretical investigations on hydrogen peroxide decomposition in aquo

Abstract

Hydrogen peroxide (H₂O₂) decomposition mechanisms in the absence and presence of iron ions in aqueous solution, which contain no OH radical formation, are theoretically determined. Calculating the oxygen–oxygen bond dissociation energies of H₂O₂, we confirmed that OH radical formation requires spin-forbidden transitions. Instead, we tested an H₂O₂ dimer-based decomposition mechanism and found that this mechanism provides reasonable barrier heights of 52–62 kcal mol⁻¹, which are close to the experimental activation energy. We next calculated the oxygen–oxygen bond dissociation of H₂O₂ coordinating to the iron ion hydration complex in order to explore H₂O₂ decomposition in the presence of iron ions. Surprisingly, we found that a monovalent iron ion complex provides no reaction barrier to dissociate H₂O₂, in contrast to the ferrous (Fe²⁺) and ferric (Fe³⁺) ion complexes with accompanying very high barriers. Following this result, we determined the subsequent oxygen formation mechanism of the monovalent iron ion complex and found that this mechanism needs a hydrogen bond network around H₂O₂ to proceed at room temperature. We, therefore, conclude that H₂O₂ decomposition in the presence of iron ions is driven by electron transfer to the iron ion hydration complex and proceeds by hydrogen transfers in the hydrogen bond network around H₂O₂.