Ti-MCM-41 catalysts with variable Si/Ti ratios were synthesized by two different methods of Ti incorporation. The samples were characterized with XRD, $N_2$ adsorption, TEM, FT-IR and ESR. The photocatalytic degradation of 4-chlorophenol was carried out to investigate the photocatalytic activity of Ti-MCM-41 catalysts.
Ti-MCM-41 (pre) catalysts, which were prepared by introducing the Ti precursor (titanium tetra-iso-propoxide, TTIP) during hydrothermal synthesis of MCM-41, showed a relatively low photocatalytic activity due to the isolated $Ti^{4+}$ in the silicate framework. On the other hand, Ti-MCM-41 (post) catalysts, which were prepared by incorporating the TTIP after the MCM-41 synthesis, showed a relatively high photocatalytic activity compared to Ti-MCM-41 (pre) catalysts. It seems that $TiO_2$ in Ti-MCM-41 (post) was located at an extra-framework position and/or on the wall of MCM-41.
Among the Ti-MCM-41 (post) catalysts, the Ti-MCM-41 (post1) catalyst was prepared with calcined MCM-41 and the Ti-MCM-41 (post2) catalyst with dried MCM-41, respectively. It was observed that the Ti-MCM-41 (post2) catalysts were more active than the Ti-MCM-41 (post1) catalysts most probably due to different amount of exposed $TiO_2$.
For the Ti-MCM-41 (post) catalyst with Si/Ti=2.5, the rutile phase of $TiO_2$ appeared only when the calcination temperature was high enough (∼850℃). Only the anatase phase was observed when the calcination temperature was below 750℃. The phase transformation from anatase to rutile, which generally occurs between 400℃ and 500℃ for $TiO_2$, must be suppressed due to the presence of MCM-41.
It was found that the photocatalytic activity for the decomposition of 4-chlorophenol depended mainly on the surface area of catalysts. The poor activities of the Ti-MCM-41 catalysts calcined at 850℃ seemed to be not only due to the presence of rutile phase but also due to the low surface area.
