Can Water Act as a Catalyst?

Water is usually regarded as a passive solvent and fire suppressant, yet aqueous microdroplets can display remarkable chemical reactivity. This article asks whether water can act as a catalyst. After defining catalysts and explaining their importance in industrial chemistry, it contrasts the relative inertness of bulk water with the unusual behavior of water at microscopic interfaces. At curved droplet surfaces, incomplete solvation and charge separation can create strong electric fields that promote the formation of highly reactive hydroxyl and hydrogen radicals. These species can drive oxidation and reduction reactions or combine to form hydrogen peroxide and molecular hydrogen. Oppositely charged droplets can also produce microlightning capable of initiating reactions in surrounding gases. Interfacial water therefore lowers effective energy barriers, opens new reaction pathways, and enables chemistry under mild conditions. However, because water-derived species may be consumed and reactivity depends on the interface, curvature, charge, and invested energy, water is not a classical catalyst. It is better described as a quasi-catalyst. Controlling aqueous interfaces may open cleaner and more energy-efficient ways of making chemical transformations.