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 TiO2 utilizing a sol−gel technique.
These materials were found (refs , ) to be photoactive for the destruction of organics with visible light.
A combination of various physicochemical techniques such as N2 physisorption, O2 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 O2
chemisorption suggests that they are well dispersed inside the MCM-41 framework, but the dispersion values
decreased with the loading of TiO2. 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% TiO2/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.
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