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

Electrolysis of aqueous Mn2+ 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 (similar to 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 MnO2 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/MnO2) film took place in an aqueous Na2SO4 solution, accompanied with insertion of SO42- 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/MnO2 film effectively oxidized hydroquinone (HQ) to benzoquinone (BQ), in which the high-valent Mn sites (Mn4+) in the oxide were reduced to Mn3+ 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 Mn3+ ions resulting from HQ oxidation, keeping the charge in the oxide so that the surfactants are not necessary to be deintercalated. Namely, the MnO2 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.