Transition metal modified TiO2-loaded MCM-41 catalysts for visible- and UV-light driven photodegradation of aqueous organic pollutants

A number of transition metal (Cr, V, Fe, Cu, Mn, Co, Ni, Mo, and La) incorporated MCM-41 mesoporous molecular sieves with Si/Me = 80 have been synthesized by a hydrothermal method. It was demonstrated that the presence of transition metal salts in the gel during synthesis hinders the action of the template, which results in pores of MCM-41 that are not well-formed. These materials were then loaded with TiO2 via the sol-gel method, and the resulting materials were explored for the destruction of 4-chlorophenol in the presence of visible, as well as UV, light. A combination of various physicochemical techniques such as N-2 physisorption, O-2 chemisorption, X-ray diffraction (XRD), diffuse reflectance UV-vis (DR UV-vis), and temperature program reduction (TPR) were used to characterize the chemical environment of these transition metals in the prepared catalysts. The dispersion of transition metals as determined by O-2 chemisorption suggests that they are well dispersed inside the MCM-41 framework, but the dispersion values decrease with the loading of TiO2. This indicates that the loaded titania promotes the transformation of incorporated metal ions into different phases. The UV-vis spectroscopy results of TM-MCM-41 (where TM = Cr, V, Fe, Cu, Mn, Co, Ni, Mo, and La) showed enhancement of light absorption in the visible range by some composite materials. The same materials loaded with titania show higher absorption in the UV range (250-400 nm) due to the presence of titania. Among all TM-MCM-41 materials, only Cr- and Cu-MCM-41 showed strong reduction transitions at lower temperatures; all other TM-MCM-41 materials showed very broad reduction transitions. Titania-loaded Cr-MCM-41 showed an interesting reduction transition at 321 degreesC due to Cr6+-->Cr5+. This transition was obtained as a result of the strong interaction of Cr-O-Ti inside the MCM-41. The photocatalytic activity under visible and UV light for the degradation of aqueous 4-chlorophenol was tested. Among all of the catalysts, only 25% TiO2/Cr-MCM-41 exhibited significant activity in visible light. Other 25% TiO2/TM-MCM-41 samples demonstrated low or no activity to operate with visible light. In contrast, the activities of all 25% TiO2/TM-MCM-41 catalysts under UV light were found to be more comparable with each other. Their catalytic performances are correlated with the UV-vis spectrum of each synthesized catalyst to reveal the specific role played by each metal ion.