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The periphery of the microdroplets exhibits unusual physical properties that are not observed in bulk solutions. This work demonstrates in water microdroplets OH− can spontaneously donate electrons for CO2 hydrogenation to formic acid (FA), which may be driven by a strong electric field at or near the interfaces of water microdroplets. Surprisingly, Cu ions in microdroplets contribute to a 4000 times increase in output of FA. Control experiments show that in water microdroplets, electrons can be deposited by valence alternation of Cu to form Cu speciation, which can promote formate conversion. The authors show for the first time, without using any additives, under ambient conditions water microdroplets can capture CO2 and reduce CO2 to formate, which implies that water microdroplets may have provided a reactor for abiotic reduction reactions in the prebiotic era, thereby synthesizing organic molecules from atmospheric CO2 with a nonenzymatic production power before the advent of biotic reducing types of machinery. [ABSTRACT FROM AUTHOR]

Reactive oxygen species (ROS) formation occurs frequently under ambient conditions in enzymatic or nanomaterial-based catalysis and is vitally important during advanced oxidation processes (AOPs). In this work, we demonstrated a spontaneous formation ROS of •OH, H 2 O 2 and •O 2 − in water microdroplets under ambient conditions without any catalysts presenting a real green source of ROS. Control experiments revealed that at the interface of water microdroplets electrons were transferred from OH− to O 2 , leading to the formation of •OH (230.25 μM/min), H 2 O 2 (152.5 μM/min) and 1O 2 (•O 2 −) (0.11 μM/min), respectively. The generation of the ROS was applied for the oxidative removal of organic pollutants, such as rhodamine B (RhB), methylene blue, nile blue, and fluorescein. A multi-nozzle spraying of water produced large amounts of ROS capable of removing 2 mL RhB solution at 100 μg/mL in 4 h. In sum, compared to the previous works, although this strategy is limited by the production of the ROS, water microdroplets also will be a significant and real green source of ROS without any additives (catalysts, nanomaterials, irradiation UV light, especially chemical reagents) at all. And we believe the limitation will be greatly improved by the designing spray array in the future. [ABSTRACT FROM AUTHOR]