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.