Competing pathways of spontaneous two-electron reduction of pyruvic acid in water microdroplets

A recent experiment demonstrated that simply spraying water microdroplets containing dissolved pyruvic acid, without additional catalysts, rapidly reduces pyruvic acid to lactic acid. Although some one-electron reduction reactions in water microdroplets have been studied previously, the underlying mechanisms of such a spontaneous two-electron reduction process remain elusive. In this study, we employ ab initio molecular dynamics (AIMD) simulations to explore possible reaction pathways. We find that pyruvic acid can readily interact directly with hydrated dielectrons in the microdroplets, but an additional slow intramolecular proton transfer process is required to complete the whole reduction reaction. Alternatively, dielectrons can be attached to a proton first. Then, with H^- formed, reduction of pyruvic acid can complete rapidly. The reduction efficiency of pyruvic acid also depends on competition with side reactions such as hydrogen gas generation. If the hydrated dielectron is in a broken-symmetry instead of a closed-shell configuration, hydrogen radicals may also be generated and escape from the water microdroplets. These results deepen our understanding of multielectron reduction in microdroplet chemistry.