Combination of Mn oxidation states improves the photocatalytic degradation of phenol with ZnAl LDH materials without a source of O2 in the reaction system

Mn-doped Zn/Al layered double hydroxides (LDH) materials were synthesized by the co-precipitation method. X-ray diffraction (XRD) analysis confirmed the polytype structure of LDH materials obtained as well as the successful incorporation of Mn into the structure. Atomic absorption spectroscopy (AAS) and energy dispersive X-ray spectroscopy (EDS) studies showed a well dispersion of Mn and a good correlation between the nominal and experimental composition. Scanning electron microscopy (SEM) revealed the formation of grains close to 1 mu m length. Energy band gap (Eg) of the solids determined by UV-vis diffuse reflectance spectroscopy (DRS) showed high photoresponse in the ultraviolet (UV) region, which was increased with Mn incorporation into the LDH structure. The LDH materials were evaluated in the photocatalytic degradation of phenol in aqueous medium, under UV radiation with and without O-2 source in the reaction system. The results showed that the O-2 is not an effective electron scavenger as usually occurs in TiO2 photocatalyst. The Mn effect in the photocatalytic activity was notably marked in anaerobic condition however in aerobic or anaerobic conditions the presence of Mn increases the photoactivity. The phenol mineralization reached in anaerobic conditions was 95% (1.5 ppm) of the initial concentration (30 ppm) and it is maintained constant after 6 h of radiation while in aerobic conditions the TOC reaches only 55% (13.5 ppm). The role of Mn as charges separator is discussed and a suitable mechanism is proposed. This study shows that the Mn doped-Zn/Al LDH materials could be a promising photocatalysts under anaerobic conditions for the removal of phenol from aqueous medium. (C) 2016 Elsevier B.V. All rights reserved.