Unexpected Spontaneous Photochemical Formation of Hydrogen Peroxide in Water Microdroplets on Silicate Mineral...

Hydrogen peroxide (H₂O₂) is a vital reactive oxygen species with significant roles in atmospheric and environmental chemistry. While its spontaneous generation in water microdroplets has gained attention, the abiotic pathways for its formation are still not fully understood. In this study, we demonstrate the rapid and spontaneous photochemical generation of H₂O₂ in pure water microdroplets on quartz surfaces under anoxic conditions. We show that H₂O₂ is efficiently formed through multiple pathways, including direct water photooxidation at the solid-water interface and, more significantly, reactions between water and quartz surface-bound peroxy radicals (≡Si-O-O·). The observed H₂O₂ production rate reached 2.5 × 10¹¹ molecules cm^{- 2} s^{- 1} in microdroplets on quartz, exceeding that of bulk water photolysis by five orders of magnitude. This process occurs across various natural silicate minerals, suggesting that photochemical reactions in water microdroplets on silicate surfaces may represent a significant yet previously overlooked source of H₂O₂ in Earth's environment. Additionally, the solid-liquid interface reactions demonstrated in this study may offer a novel approach for the industrial-scale synthesis of H₂O₂, providing an efficient and sustainable alternative to traditional methods.