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

ABSTRACT Hydrogen peroxide (H 2 O 2 ) 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 H 2 O 2 in pure water microdroplets on quartz surfaces under anoxic conditions. We show that H 2 O 2 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 H 2 O 2 production rate reached 2.5 × 10 11 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 H 2 O 2 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 H 2 O 2 , providing an efficient and sustainable alternative to traditional methods.