Properties of catalysts
Raman spectroscopy is an effective measurement to obtain the existent form of the crystalline and amorphous oxides.[39]The Raman spectra of oxidic NiMo/SBA-16 and NiMo/Al-Ti-SBA-16 catalysts are presented in Fig. S11. As reported, the characteristic peaks in the region of 750-1000 cm-1 can be ascribed to Mo oxide species.[40] Four peaks appearing at about 825, 898, 945 and 954 cm-1 can be observed in the Raman spectra for all catalysts. The appearance of peak at 825 cm-1 should be due to Mo-O-Mo linkage in the polymerized Mo oxide species (orthorhombic MoO3).[41] The peak at about 898 cm-1 should be ascribed to the highly dispersed Mo species, which is tetrahedral coordinated and denoted as Mo(Td).[42] The intensities of the peak at 898 cm-1 present a decreasing tendency with the Al compositions in NiMo/Al-Ti-SBA-16 catalysts. Therefore, incorporation of Al species into SBA-16 material can increase the proportion of Mo(Td) species on NiMo loading catalysts. Moreover, the couple peaks at about 945 and 954 cm-1 should be assigned to octahedrally coordinated Mo oxide species, Mo(Oh) and Mo8O264- species respectively. Noticeably, the Mo(Oh) species are considered as the active phase for HDS reaction, due to the weak interaction between Mo(Oh) species and supports and a higher reducibility and efficiency in HDS reaction.[42] For NiMo/SBA-16 catalyst, the peak at about 981 cm-1 can be attributed to symmetric stretching vibration of surface dioxo species Mo(=O)2. No peaks appearing in 990-1000 cm-1 prove that the Mo species dispersed well on the surface of serial catalysts.[43]
H2-TPR measurements were detected for investigating the influences of Al and Ti modification on the reducibility of active metals and the interactions between metals and supports. H2-TPR profiles of different oxidic NiMo catalysts are shown in Fig. S12. All profiles present a broad reduction band with two peaks ranging from 350 °C to 750 °C. The first peak at low temperature in the ranges of 430-480 °C can be ascribed to the reduction step of octahedrally coordinated Ni2+ species in contact with molybdenum.[43] The second peak at high temperature from 530°C to 580°C can be assigned to the reduction of Mo6+ to Mo4+ for octahedral Mo species, Mo (Oh) and also NiMoO4phase.[44] The peak at about 616 °C appearing in NiMo/AT-2.5 catalyst should be assigned to the reduction of bulk MoO3.[45] The two reduction peaks of NiMo/AT-10 catalyst shift to higher temperatures compared with NiMo/SBA-16 catalyst. Therefore, the interaction strength between metals and support (MSI) for NiMo/AT-10 catalyst is higher than that of NiMo/SBA-16 catalyst. It should be noteworthy that positions of the first peaks for NiMo/AT-7.5, NiMo/AT-5, NiMo/AT-2.5, NiMo/AT-0 catalysts exhibit a decreasing tendency with the Ti additional amounts, which are also lower than those of NiMo/AT-10 and NiMo/SBA-16 catalysts. Therefore, Ti modification can promote the reducibility of octahedrally coordinated Ni and Mo species. However, after Ti modification, the shift of second reduction peaks to higher temperatures may be due to larger amount of Mo (Oh) species in NiMo catalyst with the increasing Ti composition, which is also indicative of the stronger interaction strength between Mo (Oh) phase and support.
UV-vis DRS spectra of oxidic NiMo loading catalysts with different Al and Ti compositions were detected for getting more information about the coordination of oxidic Ni and Mo species. As shown in Fig. S13, the broad absorption bands in the ranges of 200−400 nm appearing in all catalysts should be ascribed to the charge transfer of O2− to Mo6+. The exact position in this characteristic band reflects the state of oxidic Mo species. The absorption band ranged from 200 to 280 nm can be ascribed to tetrahedral Mo species, Mo (TD) with highly dispersion degree. Meanwhile, the adsorption band in the region of 280-400 nm is assigned to octahedral Mo species, Mo (Oh).[46] Compared with NiMo/SBA-16 catalyst, the bands in the ranges of 280-400nm for NiMo/AT-10 catalyst shows a blue shift, indicating an increasing proportion of Mo (Td) species, which is consistent with the result of Raman measurements. Meanwhile, with the increasing amount of Ti species in the catalysts, the bands in the ranges of 280-400nm exhibit a red shift to higher wavelength, demonstrating that the incorporation of Ti atoms into SBA-16 material can increase the proportion of Mo (Oh) species.
Table 2 Acid properties for different NiMo catalysts, which is determined by the above FT-IR pyridine spectra.