3
26
S. Shan et al. / Journal of Catalysis 344 (2016) 325–333
and improving the reducibility and sulfidability of W and Mo spe-
cies, such as the use of alternative supports (e.g., Al-modified
hexagonal mesoporous silica Al-HMS [14], Al–Ti–Mg mixed oxide,
and SBA-15 [6,7]), the incorporation of phosphorus [10], the
introduction of citric acid into the impregnation solution [15],
and the development of the solution-based combustion method
120 °C for 24 h; the resulting WMo-HNCs-Al
washed with H O, dried at 110 °C for 3 h, and calcined at 450 °C
for 5 h, producing an oxidic catalyst precursor WMo-HHD.
2 3
O was filtered,
2
WMo-IM was prepared by impregnating
successively with aqueous solutions of (NH
c
-Al
2
O
3
particles
4
)
6
W
7
O
24ꢁ6H O and
2
(NH
was dried at 110 °C for 3 h and calcined at 450 °C for 5 h. The
WO and MoO loadings of WMo-HHD and WMo-IM are 13.7
4
)
6
7
Mo O
2
24ꢁ4H O (AHM); after each impregnation, the sample
2 3
to prepare the WMoNi/c-Al O catalyst [16]. However, improved
W/Mo dispersion and higher W/Mo reducibility and sulfidability
cannot be simultaneously satisfied. Especially, the aforementioned
approaches based on the conventional incipient impregnation (IM)
method unavoidably lead to strong interaction between the metal
3
3
and 8.5 wt.%, respectively. It should be emphasized that the
W and Mo contents in WMo-HHD are repeatable and consistent
with the feeding amount of the source materials.
(
W/Mo) precursors and the support surface. This causes the
formation of bulk metal oxides and thereby the lowly dispersed
MS (M = W/Mo) slabs after sulfidation, unfavorable for the
intimate contact of W and Mo species and for the accommodation
of MS slabs for Ni species on their edges [11,17].
Two oxidic trimetallic WMoNi/
and WMoNi-IM, were prepared by impregnating WMo-HDD and
WMo-IM with a Ni(NO solution, drying at 100 °C for 3 h, and cal-
cining at 450 °C for 5 h in air. The WO , MoO , and NiO loadings of
WMoNi-HHD and WMoNi-IM are 13.1, 8.1, and 4.7 wt.%,
respectively. Another oxidic trimetallic WMoNi/ -Al catalyst,
WNi/MoNi-HHD, was obtained by mixing equivalent amounts of
WNi/ -Al (WNi-HHD) and MoNi/ -Al (MoNi-HHD) prepared
by the HHD method. The procedures for preparing WNi-HHD and
MoNi-HHD are the same as those used for WMoNi-HHD, except
that a suspension containing only W-HNCs or Mo-HNCs at higher
concentration was placed in the rotary autoclave containing
2 3
c-Al O catalysts, WMoNi-HHD
2
3 2
)
3
3
2
In our previous work [18,19], we synthesized Mo- and W-based
hybrid inorganic–organic nanocrystals (Mo- and W-HNCs) using
cetyltrimethylammonium bromide (CTAB) and tetraethylammo-
nium bromide (TEAB) as hybrid reagents to replace the conven-
c
2 3
O
c
2
O
3
c
2 3
O
6
ꢀ
6ꢀ
tional precursors of Mo-anions (Mo
7
O
24 ) and W-anions (W
7
O
24 ),
respectively; then we prepared supported monometallic Mo/
Al and W/ -Al catalyst precursors by the hydrothermal
deposition method; and finally we obtained supported bimetallic
NiMo/ -Al and NiW/ -Al catalysts by impregnating Mo/
Al and W/ -Al with a Ni(NO solution. The resultant
c-
2
O
3
c
2 3
O
c
-Al
26.2 and 4.7 wt.%, and the MoO
are 16.2 and 4.7 wt.%, so the WO
2
O
3
particles. The WO
3
and NiO loadings of WNi-HHD are
and NiO loadings of MoNi-HHD
, MoO and NiO loadings of
c
2
O
3
c
2
O
3
c
-
3
2
O
3
c
2
O
3
3
)
2
3
3
bimetallic catalysts exhibited superior HDS performance compared
with their counterparts prepared by the IM method. More impor-
tantly, we found that the combined use of Mo- and W-based HNCs
as precursors and the hydrothermal deposition method generated
W/Mo species with high dispersion, high reducibility, and high sul-
fidability, fully in compliance with the essential prerequisites for
achieving synergism effects in trimetallic WMoNi catalysts. Here
we further show that both W- and Mo-HNCs can be synthesized
with TEAB as the hybrid reagent and can be used as W/Mo precur-
WNi/MoNi-HHD are 13.1, 8.1, and 4.7 wt.%, respectively.
2.3. Characterization
The composition and structure determination of W(Mo)-HNCs
and the confirmation of their deposition onto
2 3
c-Al O were
obtained through the same methods as described elsewhere [18].
Transmission electron microscopy (TEM) and high-resolution
TEM (HRTEM) characterizations of the diluted suspension of
Mo-HNCs were conducted on a Philips Tecnai G2 F20 instrument
operated at 200 kV. Fourier transform infrared (FTIR) spectra were
obtained on a MAGNA-IR 560 spectrometer. The transparent wafer
was prepared with the mass ratio of the sample to KBr at 1:100.
The Raman spectra were recorded on a Bruker RFS 100/S FT-
Raman spectrometer (Germany), operated with a Nd-YAG laser at
a wavelength of 532 nm. The laser spot size was approximately
sors to prepare oxidic WMo/
c
-Al
2
O
3
, which can be further impreg-
-Al catalysts
nated with Ni(NO to yield trimetallic WMoNi/
3
)
2
c
2 3
O
exhibiting superior synergetic effects among the metal species
and thereby superior HDS activity.
2
. Experimental
1
–2
performed with an inductively coupled plasma instrument (ICPE-
000) and a Perkin–Elmer 2400 II CHN element analyzer.
The oxidic WMo/ -Al and WMoNi/ -Al catalysts were
characterized by X-ray diffraction (XRD), N physisorption, high-
lm with a power level of 100 mW. Elemental analysis was
2
.1. Synthesis of Mo- and W-HNCs
9
The synthesis procedure of Mo-HNCs is the same as that of W-
c
2
O
3
c
2 3
O
HNCs in our previous work [19], except that the sodium tungstate
was replaced with sodium molybdate.
2
angle annular dark-field scanning transmission electron micro-
scopy (HAADF-STEM), UV–vis diffuse reflectance spectroscopy
2 3
To prepare an oxidic WMo/c-Al O catalyst precursor, a suspen-
sion of W- and Mo-HNCs was synthesized. First, a certain amount
of a TEAB aqueous solution was mixed with an acidified sodium
molybdate solution; second, a solution of polyoxotungstates, i.e.,
(
DRS UV–vis), and temperature-programmed reduction in hydro-
gen (H TPR). The XRD patterns (2h = 10°–80°) were recorded with
an X-ray diffractometer (Shimadzu XRD 6000) using CuK radia-
tion (k = 1.5406 Å) and operated at a scanning rate of 4°/min. The
physisorption was performed at 77 K with a Micromeritics Tris-
2
a
2 4
an acidified Na WO solution, was added to the above mixture;
third, the pH value of the mixture was adjusted by a HCl solution
to 3 under stirring, yielding suspension A containing W- and Mo-
HNCs.
N
2
tar 3020 system. Specific surface areas were determined by the Bru
nauer–Emmett–Teller (BET) method and pore volumes were esti-
2
mated from N adsorption–desorption isotherms. Mesopore diam-
2
.2. Preparation of alumina-supported oxidic bimetallic WMo and
eters and size distributions were determined by the Barret–
Joyner–Halenda (BJH) method. HAADF-STEM was performed on
the above-mentioned Philips Tecnai G2 F20 instrument with a res-
olution of about 0.4 nm in STEM mode. The annular detector was
adjusted to collect the electrons scattered between 60 and 120
mrad. The contrast in HAADF images is considered to be propor-
trimetallic WMoNi catalyst precursors
2 3
Two oxidic WMo/c-Al O precursors, WMo-HHD and WMo-IM,
were prepared by the hydrothermal deposition method with W-
and Mo-HNCs as precursors, or HHD for short, and the IM method,
respectively. The procedure of the HHD method is as follows: sus-
pension A prepared above was transferred into a rotary autoclave
1
.7
tional to Z
(Z is the atomic number of the scattering atom).
DRS UV–vis spectra were obtained on a UV–vis spectrophotometer
(Hitachi U-4100) installed with the integration sphere diffuse
2 3
containing 5.2 g of c-Al O particles (20–40 mesh) and heated at