Active Sites on Mesoporous and Amorphous Silica Materials and Their Photocatalytic Activity:  An Investigation by FTIR, ESR, VUV−UV and Photoluminescence Spectroscopies

Active sites for photometathesis on mesoporous silica materials, FSM-16 and MCM-41, and amorphous silica, which are generated by dehydroxylation of surface isolated hydroxyl groups at high temperature above 673 K, were investigated by some spectroscopies such as FTIR, ESR, VUV−UV, and photoluminescence. FTIR study revealed that the catalytic active sites are the sites exhibiting the IR bands at 891 and 910 cm-1, which are known as a “strained siloxane bridge”. The catalytic active sites generated on three kinds of silica materials have uniform activity, and higher activity over FSM-16 than MCM-41 and amorphous silica is attributed to the larger amount of active sites generated. ESR study suggested that the IR bands would be assigned to the radical sites, ⋮Si−O· (NBOHC), which would be generated together with ·Si⋮ (E‘ center) by the dehydroxylation at high temperature, and the NBOHC photoexcited under UV light below ca. 390 nm would cause the photometathesis.