Transition-Metal-Substituted Titania-Loaded MCM-41 as Photocatalysts for the Degradation of Aqueous Organics in Visible Light

Abstract

Several transition metal (V, Cr, Fe)-substituted MCM-41-based specimens (Si/Me=80) were synthesized by the incorporation of the transition metal during synthesis. They were then loaded with titania via sol–gel method and the photoactivity in visible light was explored. The XRD analysis showed the patterns of neat MCM-41 similar to those of siliceous MCM-41, while the loading of titania was in its amorphous form. The BET surface areas, however, were lower than those commonly found in siliceous MCM-41. This is due to the partial pore breakage, as recorded by the pore size distribution analysis. The transition metal dispersion found in the majority of the specimens studied was quite high (in the vicinity of 50%) and decreased with the loading of TiO2. It was found that 25 wt% TiO2 loaded on calcined Cr-Ti-MCM-41 and Cr-MCM-41 exhibits activity in visible light which is comparable with that demonstrated by commercial Degussa P25, utilizing UV light for the degradation of formic acid. The same catalyst in fully reduced form as well as titania-loaded chromium-silicon oxide of adequate composition did not show any activity. TiO2/Fe-Ti-MCM-41 catalysts exhibited marginal activity for the degradation of formic acid. Similar vanadium-substituted catalysts (reduced and nonreduced) did not show any activity in visible light. These trends were also observed for the photodegradation of phenolic compounds in visible light, wherein the rates for 25% TiO2/Cr-Ti-MCM-41 were significant. This behavior is essential for the development of photocatalysts which operate using solar light. A comparative DR UV–vis study showed the substantial absorption of visible light by all the catalysts employed. Furthermore, the coexistence of Cr3+ and Cr6+ in fully oxidized chromium substituted materials was detected (Cr3+/Cr6+ ∼0.25). Extensive TPR studies revealed a number of transitions in the titania-loaded transition-metal-substituted catalysts. In particular, the temperature of the transition Cr3+→Cr2+ was found to considerably depend on the nature of chromium species in the MCM-41 matrix. This significantly contributes to the remarkable activity of TiO2/Cr-Ti-MCM-41 and TiO2/Cr-MCM-41, and this does not take place for both catalysts in their reduced form. Moreover, this transition is not present in the spent catalyst, which may be one of the possible reasons for deactivation. The surface composition of the most active catalyst was analyzed by XPS and showed considerable diffusion of chromium ions to the surface upon loading of titania. Two separate surface electronic levels were also found for TiO2/Cr-Ti-MCM-41, corresponding to Si-O-Cr and Ti-O-Cr regions. Such an arrangement creates an opportunity for a two-step excitation of titania by two photons in the visible range. The proposed route of the catalytic activity of the above material in visible light involves the reaction of dopant level electrons with surface Cr6+, which makes available valence band holes to perform oxidation reactions.