Characterization of Titania Loaded V-, Fe-, and Cr-Incorporated MCM-41 by XRD, TPR, UV−vis, Raman, and XPS Techniques

Transition metal (Me = V, Fe, and Cr) incorporated into MCM-41 mesoporous molecular sieves (Si/Me = 80) have been synthesized by hydrothermal methods and were loaded with TiO 2 utilizing a sol−gel technique. These materials were found (refs 22, 23) to be photoactive for the destruction of organics with visible light. A combination of various physicochemical techniques such as N 2 physisorption, O 2 chemisorption, diffuse reflectance UV−vis (DR−UV−vis), X-ray diffraction (XRD), Raman, temperature program reduction (TPR), and X-ray photoelectron spectroscopy (XPS) were used to characterize the chemical environment of these transition metals in the prepared photocatalysts. 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 decreased with the loading of TiO 2 . This indicates that the loaded titania promotes the transformation of incorporated metal ions into different phases. DR−UV−vis spectra of the Me-Ti−MCM-41 materials exhibit substantial absorption of visible light in the range of 400−600 nm. However, the same materials loaded with titania show higher absorption in the UV range (250−400 nm) because of the presence of titania. XRD patterns of Me-Ti−MCM-41 are similar to that of siliceous MCM-41 and demonstrate that the transition metals are atomically dispersed in the framework. The titania loaded onto the Me-Ti−MCM-41 was of low anatase crystallinity as shown by Raman. The TPR results for Me-Ti−MCM-41 revealed a lower number of reduction transitions than the titania loaded Me-Ti−MCM-41. These reduction transitions depend on the nature of transition metal species in the MCM-41 framework. The Me/Ti and Me/Si surface atomic ratios, which are determined by XPS measurements, reveal that considerable diffusion of transition metal ions to the surface occurs upon loading of titania. The XPS line shapes, binding energies, and surface atomic ratios for Me−Ti−MCM-41 indicated one type of surface electronic level such Me−O−Si, whereas two types of surface electronic levels were found in the case of 25% TiO 2 /Me−Ti−MCM-41, which corresponds to Me−O−Si and Me−O−Ti. The Me/Si and Me/Ti surface atomic ratios show that the incorporated transition metals interact preferable with the loaded titania as in the form of Me−O−Ti heterojunction instead of staying inside the framework as in the form of Me−O−Si electronic level.