Synthesis and characterization of TiO_2 loaded Cr-MCM-41 catalysts

摘要

Chromium substituted mesoporous MCM-41 material (Si/Cr = 80) was synthesized by the incorporation of chromium ions during synthesis. This was then loaded with titania via sol-gel method to explore the photoactivity in visible light. This prepared catalyst was characterized by different physico-chemical techniques such as BET surface area, nitrogen physisorption, oxygen chemisorption, XRD, XPS, TPR and Raman spectroscopy. The BET surface area, however, was lower than those commonly found in our siliceous MCM-41. This is due to the partial pore breakage, as recorded by the pore size distribution analysis. The chromium metal dispersion found in the majority of the specimens studied was quite high (in the vicinity of 50%) and decreased with the loading of TiO_2. The XRD analysis showed the patterns of Cr-MCM-41, 25%TiO_2/Cr-MCM-41 similar to those of siliceous MCM-41, however, the intensity of the d_(100) peak is decreased, while the loading of titania. Raman studies of Cr-MCM-41 and 25% TiO_2/Cr-MCM-41 indicate that the chromium is well dispersed inside the MCM-41 framework, and decreasing intensity of Raman peaks at 144 cm~(-1), 397 cm~(-1), 518 cm~(-1) and 641 cm~(-1) upon loading of titania on Cr-MCM-41 indicates that the titania was interacting with chromium, and where as the peaks are associated to anatase phase of loaded titania. TPR studies revealed a change in the reduction temperature of Cr(Ⅳ) in the titania-loaded Cr-MCM-41. The reduction temperature of Cr(Ⅵ) was found to depend on the nature of the chromium species in the MCM-41 matrix. This significantly contributes to the remarkable photocatalytic activity of TiO_2/Cr-MCM-41, and this does not happen in other transition metals incorporated MCM-41. The surface composition and binding energy of Cr 2p_(3/2) peak of the Cr-MCM-41 and 25%TiO_2/Cr-MCM-41 was analyzed by XPS and showed considerable diffusion of chromium ions to the surface upon loading of titania. The binding energy value of Cr 2p_(3/2) was decreased upon loading of TiO_2 on Cr-MCM-41, indicates that Cr is strongly interacting with TiO_2. Eventually, two separate surface electronic levels corresponding to Si-O-Cr and Ti-O-Cr regions were found by XPS analysis for TiO_2/Cr-MCM-41.