Surfactant-Induced Electrodeposition of Layered Manganese Oxide with Large Interlayer Space for Catalytic Oxidation of Phenol

Electrolysis of aqueous Mn 2+ ions at an anodic potential of +1.0 V (vs Ag/AgCl) in the presence of cationic surfactants led to the formation of multilayered manganese oxides intercalated with the surfactant molecules. Their interlayer spacings are much larger than that (∼0.7 nm) of conventional birnessite; i.e., 3.1 nm with cetyltrimethyl ammonium (C16) and 2.4 nm with dodecyltrimethyl ammonium (C12). According to XPS analysis, the molar ratio of C16 to MnO 2 was estimated to be 0.74, in which the C16 fraction of 0.42 is associated with Cl − anions and the other (0.32) interacts with negative charges on the Mn oxide layer. C16 surfactants accommodated between the Mn oxide layers were not ion-exchanged with Na + ions in solution but extracted with ethanol. Anodic oxidation of the C16-intercalated Mn oxide (C16/MnO 2 ) film took place in an aqueous Na 2 SO 4 solution, accompanied with insertion of SO 4 2- anions for charge compensation, and not deintercalation of the surfactants. As a result, the layered structure with large interlayer space was maintained at anodic potentials. The C16/MnO 2 film effectively oxidized hydroquinone (HQ) to benzoquinone (BQ), in which the high-valent Mn sites (Mn 4+ ) in the oxide were reduced to Mn 3+ . At open circuit potential, the low-valent Mn species were accumulated, leading to a collapse of the C16-intercalated structure within 250 min; however, applying an anodic potential allowed the structure to remain unchanged, in conjunction with the acceleration of HQ oxidation. This can be attributed to the reoxidation of the Mn 3+ ions resulting from HQ oxidation, keeping the charge in the oxide so that the surfactants are not necessary to be deintercalated. Namely, the MnO 2 layer can act as an electron-transfer mediator for the oxidation of HQ occurring in the large interlayer space. The observed catalytic current was 684 times larger than the current observed on the unmodified electrode.