Comparative study on the increased photoactivity of coupled and supported manganese-silver oxides onto a natural zeolite nano-particles

• Good catalytic activity of MnO/Ag2O-nanoclinoptilolite in photodegradation of methylene blue.
• Red shifts in band gap energies of the supported MnO-Ag2O rather the supported monocomponent one.
• MnO/Ag2O-NCP shows higher activity than Ag2O/NCP and MnO/NCP alone.
• Electron scavenger role of MnO in the MnO-Ag2O/NCP system.

Supported MnO, Ag2O and MnO-Ag2O onto clinoptilolite nano-particles (NCP) were used for the photodecolorization of methylene blue (MB) aqueous solution. The catalysts were characterized by X-ray diffraction (XRD), diffuse reflection spectroscopy (DRS), transmission electron microscope (TEM), Brunauer–Emmett–Teller (BET) and Fourier transformation infra-red (FTIR). DRS results showed red shifts in band gap energies of the supported MnO-Ag2O semiconductors with respect to the supported monocomponent one (MnO or Ag2O). Also, in the coupled semiconductors Ag2O-conduction band (CB) potential (E° = −1.3 V) is enough negative to immigrate the photogenerated electrons from this level to MnO-CB level (E° = 0.1 V). This significantly prevent from electron/hole recombination which caused to increase the photocatalytic activity. Due to more negative potential of Ag2O-CB level than oxygen potential reduction, the photogenerated electrons in Ag2O-CB can change oxygen molecules to superoxide radical and hydrogen peroxide. The produced H2O2 molecules can break to hydroxyl radicals during the irradiation process. In contrast, valence band (VB) potential of MnO (E° = 2.4 V) is sufficient for oxidation of water molecules or OH anions to hydroxyl radicals. Adding sodium chloride and potassium bromated to the system caused to decrease the photocatalytic activity because these OH scavengers produce other radicals such as OCl (E° = 1.5 V), HOBr (E° = 2.4 V) and BrO2 (E° = 1.2 V) which have less oxidizing power the powerful OH (E° = 2.8 V). Photocatalytic results showed that the hybridized MnO-Ag2O/NCP catalyst containing 10% MnO and 2.1% Ag2O was more effective than the non-hybridized MnO/NCP and Ag2O/NCP. The declororization process obeyed the first-order kinetics. Some photodecloriztion intermediates such as phenol and aniline were recognized by GC-Mass.

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