Inhibition Effect of H₂O on the Heterogeneous Reaction between Isoprene and Fe-Substituted Cryptomelane

The transportation and transformation of biogenic isoprene are vital for the organic carbon cycle in the troposphere. As a typical mineral with high oxidation potential, Fe-substituted cryptomelane oxidizes the surface monolayer of isoprene into formic and acetic acids, and simultaneously, the Mn⁴⁺ ions in the structure are reduced to Mn³⁺ and Mn²⁺. The flow of H₂O in isoprene decreases the adsorption and oxidation of isoprene significantly, even at low relative humidity (10%). As physisorbed H₂O retains Fe-substituted cryptomelane's crystal structure and oxidation ability, the adsorption and oxidation capacity recovers when H₂O is absent in the isoprene flow. Theoretical calculations on (001) surfaces show that isoprene prefers to be adsorbed by the Fe³⁺ site and H₂O tends to form hydrogen bonds. Due to the decrease in total adsorption energy of H₂O and isoprene, Fe-substituted cryptomelane favors the adsorption of H₂O in the flow of humid isoprene. The low oxidation performance at ambient relative humidity suggests that direct oxidation by aerosols of mineral dust might not be the transformation pathway of biogenic isoprene at night.