Chromium and titanium/chromium-containing MCM-41 mesoporous silicates as promising catalysts for the photobleaching of azo dyes in aqueous suspensions. A multitechnique investigation

Cr/MCM-41 and TiO2/Cr/MCM-41 mesoporous silica were synthesized by the wet impregnation method. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS) and Electron Spin Resonance (ESR) techniques were employed for the nanocatalysts characterization and a comparison with previously reported results of UV–Vis diffuse reflectance spectroscopy (UV–vis DR), X-ray Diffraction (XRD) and N2 adsorption/desorption analysis was carried out. It could be confirmed that the lower Cr loadings in the samples result in the increased presence of highly dispersed oxidized Cr species, which allows the desired heterojunction effect after the Ti loading and the high catalytic activity in the Acid Orange 7 (AO7) degradation. In this work, the analysis of the AO7 degradation progress by UV–vis and FT-IR spectroscopy has been made using the TiO2/Cr/MCM-41(3.5) as photocatalyst, under visible and UV–vis radiation. Short irradiation times resulted in a high degradation of the azo and naphthalene groups present in the AO7 dye. At longer irradiation times, when total discoloration of AO7 solution is obtained (>97%), the biodegradability index (BOD5/COD) reaches values greater than 0.40. This photocatalytic process could be used as a pre-treatment step before conventional biological treatment.

Figure optionsDownload as PowerPoint slideHighlights
► TiO2/Cr/MCM-41 with hexagonal arrangement of channels were successfully prepared.
► The metallic species on the sieves were evidenced by XPS and ESR analysis.
► Lower Cr loadings lead to highly dispersed Cr6+ and their heterojunction with Ti.
► Irradiation time for total AO7 degradation allows the biodegradability enhancement.
► Photocatalytic processes are suggested as pre-treatments of biological process.