Effects of Pressure, Thermal Treatment, and O-2 Loading in MCM41, MSU-H, and MSU-F Mesoporous Silica Systems Probed by Raman Spectroscopy

摘要

We present a Raman study of the effects induced by pressure, thermal treatments, and O-2 loading in MCM41, MSU-H, and MSU-F representative mesoporous silica. We compared the starting powders with the mechanically pressed tablets produced applying pressures of similar to 0.2 and similar to 0.45 GPa. The spectra of the three untreated tablets evidence that the main value of the Si-O-Si angle decreases and that in the MCM41 and the MSU-H Si-O-Si hydrolysis occurs, whereas such a process is absent or much less efficient in the MSU-F. Despite their different networks, the three powders tend to crystallize in cristobalite when treatments are at 1000 degrees C. The MCM41 and MSU-H tablets exhibit behavior similar to their starting powders, whereas the MSU-F tablets tend to form the tridymite crystalline phase. Such a finding could be related by the differences in the Si-O-Si hydrolysis, occurring during the tablets production. Finally, we inserted O-2 molecules within the interstices of the SiO2 wall only in MSU-F. By monitoring the O-2 content as a function of the delay time from the loading end, we provide evidence that the O-2 remains in the interstices for a sufficiently large time to modify the material properties by inserting small molecules and not only through the surface functionalization.