a pore size from BET report; b cubic unit cell parameter, a0=\(\sqrt{2}\)d110; cmesopores void fraction; d diameter of the spherical cavities; e the wall thickness hw=\(\frac{\sqrt{3}a_{0}}{2}-d\).
Raman spectra of different supports are displayed in Fig. S3. For SBA-16 and AT-10 support (only modified by Al species), three peaks appearing at 491, 604, and 977 cm-1 should be assigned to pure silica.[24] An obvious peak at about 879 cm-1 appearing in the spectrum of AT-10 sample should be assigned to the non-spinel γ-Al2O3phase.[25] As reported, two peaks at 491 and 604 cm-1 should be ascribed to the tri- and tetracyclosiloxane rings generated by the condensation of surface (–OH) groups. Meanwhile, the band at 977 cm-1 is resulted from the surface Si–OH stretching mode.[26] As for the supports modified by Al and Ti species, the peaks assigned to silica and Al2O3 become too weak to be observed due to strong intensities of peaks belonging to TiO2 phase. Four characteristic peaks at 144 cm-1 399, 515 and 634 cm-1 observed in the spectra of AT-7.5, AT-5, AT-2.5 and AT-0 samples are ascribed to anatase phase.[27]
The UV–vis DRS spectra of various supports with different compositions are shown in Fig. S4A. All supports exhibit different adsorption bands with the change of addition amount of Ti sources. There are no adsorption bands appearing in AT-10 samples due to Al and Si species are transparent in the UV-vis DRS detected region. The absorption bands in the samples containing Ti species are resulted from the ligand-to-metal charge transfer from the O2- to Ti4+to form its charge-transfer excited state, (Ti3+–O-).[28]For the samples incorporated into Ti atoms, the first peaks in the adsorption range of 200-220 cm-1 is ascribed to the charge-transfer transitions of oxygen to tetrahedrally coordinated Ti4+ ions in the group of Ti(OSi)4, which can demonstrate that Ti atoms are successfully incorporated into the framework of SBA-16 materials.[29] The bands ranged from 220 to 230 nm can be observed in AT-5, AT-2.5 and AT-0 samples, which are ascribed to the group of Ti(OH)–(OSi)3 in framework of SBA-16 silica.[30] Meanwhile, obvious absorption bands in the region of 300-330 nm can be observed in samples containing Ti atoms, which can be ascribed to Ti-O-Ti bonds and the titanium sites with high coordination numbers.[31] These results confirm that the Ti species should be co-existed in the framework and external framework as TiO2 crystal phases in the Al-Ti-SBA-16 composites, which is in accordance with Raman measurements. In addition, band gap energy can be estimated from a plot of (α)1/2versus photon energy (hv). The band gap energy for TiO2phases can be obtained by extrapolating the linear part of the rising curve to zero. Higher band gap energy will indicate smaller particle size of TiO2 crystal inside the Al-Ti-SBA-16 composites. The absorption coefficient α can be calculated from the following equation of α=2303ρA/(lcm), in which ρ (TiO2) =3.9 g·cm-3, A is the absorption intensity, l is the optical path length, c is the molar concentration for TiO2 and m is molecular weight of TiO2.[32] From Fig. S4B, the band gap energies follows in the order of AT-7.5 (3.00 eV) > AT-5 (2.94 eV) > AT-2.5 (2.80 eV) > AT-0 (2.50 eV). Therefore, the particle size of TiO2 crystal in the samples follows the reverse order and increase with the additional amounts of Ti sources.
27Al NMR was also applied to obtain the existent form for Al species in the Al-Ti-SBA-16 composite. It can be generally recognized that the band appearing at about 6 ppm can be ascribed to the octahedral structural unit AlO6, which can be treated as extra-framework Al species. The band at about 33  ppm has been ascribed to the extra-framework coordination of Al3+ as pentahedral AlO5 unit. Meanwhile, the chemical shift at about 53 ppm is assigned to tetrahedral structural unit AlO4, in which aluminum is covalently connected with four Si atoms through oxygen bridges.[33, 34] The27Al NMR spectrum of AT-5 support is shown in Fig. S5. It presents three peaks belonging to AlO4, AlO5 and AlO6 structural units, indicating that the composites synthesized by this two-step method possess different Al species in framework and extra-framework.
The XPS spectra for different supports are shown in Fig. S6. Ti 2p envelop shows two characteristic peaks at about 485 and 464 eV, being ascribed to Ti 2p3/2 and Ti 2p1/2 species, which is in accordance with the reported literature.[35] It is obvious that the intensity of the above two peaks increase with the Ti contents in the supports. This result can confirm the appearance of TiO2 phase on the surface of support, which can be seen in Raman and XRD results. From Al 2p XPS envelop, it can also be seen that the intensity of the characteristic peak increase with the Al contents in the supports, which is similar with that of Ti 2p XPS spectra. As shown in O1s XPS envelop, an obvious peak at 533.0 eV can be assigned to the Si-O-Si bond. As the composition of Ti species reaches 10%, a weak peak at about 529 eV can be observed, which should be assigned to the oxygen in the Ti-O-Si bond.[35, 36] This may demonstrate that Ti atoms have been successfully incorporated into the framework of SBA-16 material. The binding energy of AT-10 sample shifting to lower values compared with SBA-16 sample may be caused by the Al incorporation of Al atoms into SBA-16 material, which can be verified from 27Al NMR result. All Si 2p XPS spectra show intensive peaks in the region ranged from 100 to 105 eV. For the spectra of supports containing Al atoms, the bands at about 101 eV, and 103 eV should be ascribed to the bonds of Si-O-Al and Si-O-Si respectively.[37] The binding energy of bands assigned to the Ti-O-Si bond is lower than that of SiO2.[38] The Si 2p XPS spectra exhibit that the binding energies of Al-Ti-SBA-16 composites shift to lower values compared with SBA-16 silica, which should be due to the incorporation of Al and Ti species into SBA-16 materials.
The SEM images of different supports are shown in Fig. S7. The SBA-16 pure silica presents regular particles with the morphology of cross-linked sphere. The AT-0 sample modified by Al species through the post-synthetic method exhibits a similar morphology with SBA-16 silica. Compared with SBA-16 and AT-10 samples, the AT-7.5, AT-5 and AT-2.5 samples exhibit different morphologies with relatively lower regularity, which should be caused by the addition of Ti sources. However, particles with the shape of inerratic polyhedron can be observed in AT-0 sample. In addition, the SEM mapping result of AT-5 sample is presented in the Fig. S8. It can be seen that the Al and Ti atoms dispersed well in the surface of AT-5 composite. The SEM EDS results of AT-0 and AT-5 samples are displayed in Fig. S9. The Al composition in AT-0 sample is detected as 11.4%, and Al and Ti compositions in the AT-5 sample are 6.4% and 5.1%, which are similar with theoretical value of Al and Ti compositions. Hence, it can further confirm that the stepwise synthetic method is effective to obtain serial Al-Ti-SBA-16 composites.
The TEM images of SBA-16 and Al-Ti-SBA-16 materials are displayed in Fig. S10. The (111) or (100) crystal faces with highly ordered degree can be clearly observed in all supports. Accompanied with small angel XRD results, the TEM measurements can also demonstrate that serial SBA-16 silica and Al-Ti-SBA-16 materials possesses highly ordered cubic body-centred Im3m symmetry structure.[22]