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 N2 physisorption, O2 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 O2 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 °C 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.