Different Reactivity of Birnessite and Cryptomelane toward Isoprene: Effect of Structure, Morphology, and Exposed Faces

This study investigates the adsorption and oxidation of isoprene, the most abundant biogenic volatile organic compound, on two common Mn(IV) (hydr)oxides in Earth's surface environment, birnessite and cryptomelane. Both minerals show high adsorption capability toward isoprene under increasing environmental temperature and low relative humidity, whereas the adsorbed isoprene on birnessite is oxidized into carboxylate species, mainly formate. For both Mn(IV) (hydr)oxides, the adsorption of isoprene not only reduces Mn4+ to Mn3+ but also gives rise to the slight distortion of the crystal structure. Compared to cryptomelane, birnessite exhibits better adsorption and oxidation capability of isoprene, which is improved by the decrease in crystallinity. This is attributed to the high density of oxygen anions on the (001) surface, owing to the participation of lattice oxygen, electrophilic adsorbed oxygen, and hydroxyl groups in the oxidation of isoprene. For cryptomelane, the particles with short and thick nanorods show higher adsorption capacity but lower oxidizability than those with long and sharp nanorods, as the (001) surface of cryptomelane prefers to adsorb isoprene and Mn4+ in long sharp nanorods and accept more electrons from isoprene. Based on the experimental results, we propose that Mn(IV) (hydr)oxide in soil dust aerosols is an important regulator of atmospheric isoprene, which enhances the bioavailability of both isoprene and Mn(IV) (hydr)oxides.