S. He et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 248–254
249
used as catalyst supports in this work were obtained via calcining
Mo3O10(C6H8N)2·2H2O at 400 ◦C for 2 h under air flow.
2.1.2. Ir/H-MoOx, Ir/MoO3, Ir/MoO3(c) (MoO3(c): commercial
MoO3), Ir/TiO2, Ir/ZrO2, Ir/AC, and Ir/SiO2
Corresponding support was impregnated with H2IrCl6 aqueous
solution and then stirred at 80 ◦C for 4 h. The samples were dried
at 50 ◦C overnight, followed by a reduction with a stream of 5 vol%
H2/Ar at 300 ◦C for 2 h. For Ir/MoO3, a lower temperature of 200 ◦C
was adopted for the reduction, employing MoO3 nanorods as the
support.
Scheme 1. Schematic illustration for the one-pot fabrication of Ir/H-MoOx nanorods
for chemoselective hydrogenation.
CNT or SiO2 was impregnated with metal-source aqueous solu-
tion and then stirred for 4 h at 80 ◦C. The samples were then dried
at 50 ◦C overnight, followed by a reduction with a stream of 5 vol%
H2/Ar at 300 ◦C for 2 h.
increasing electronic density around Ir is desired to optimize the
hydrogenation routes, in which excessive electrons will enhance
the repulsive force with C C, and also promote the electron feed-
back to * in polar C O [18,22]. Such regulation can be achieved via
electronic metal-support interactions on designed supports [4,23].
In the regard of the large-density states around increasing Fermi
level (EF) and the appreciable delocalized electrons [10,24,25], H-
MoOx is expected to present feasible electronic interactions for
chemoselective hydrogenation. As evidenced in very recent work,
the introduction of H-MoOx into Ir/SiO2 catalysts benefited the
surface, and the electronic metal-support interactions were unfor-
tunately ignored. By contrast, in cinnamaldehyde hydrogenation,
the undesired C C hydrogenation on Pd, rather than C O, was pro-
moted by using H-MoOx as the supports [14]. These interesting, but
controversial findings indicate the importance of the varied inter-
actions associated with H-MoOx supports, which needs uncovering
and further optimization.
Regarding the feasible hydrogen doping into oxides by H2
spillover from noble metals [25], we herein develop a one-pot fab-
rication of H-MoOx nanorods supported Ir (Ir/H-MoOx) catalysts for
chemoselective hydrogenation. As shown in Scheme 1, Ir nanopar-
ticles (NPs) generate from the reduction of H2IrCl6 by H2/Ar at
mild temperature, which serve as active-sites for disassociating
H2 molecules to H atoms upon further heating [26]. The highly
active H atoms migrate to the surface of MoO3, and further dif-
fuse into the bulk, thereby leading to the in-situ hydrogenation
of MoO3. Such fabrication, simultaneously combining the genera-
tion of active metal centres and the hydrogen doping on supports,
ensures the interactions between ultrafine Ir and fresh H-MoOx
surface. The accumulated electrons around the MoO6 octahedral of
H-MoOx promote the strong electronic interactions with Ir, result-
ing in negatively charge Ir␦− species favouring the activation and
turnover of C O. As expected, Ir/H-MoOx catalysts show high selec-
tivity for the hydrogenation of ␣,-unsaturated aldehydes into
unsaturated alcohols. In cinnamaldehyde (CAL) hydrogenation, the
Ir/H-MoOx present the cinnamylalcohol (COL) selectivity as high as
93%, outperforming the Ir on other supports (e.g., MoO3, SiO2, ZrO2,
and active carbon (AC)) and even conventional metal catalysts.
Moreover, the efficacy for various substrates that possess multiple
groups (e.g., crotonaldehyde, citral, furfural and nitroarenes) fur-
ther verifies our Ir/H-MoOx to be competitive for chemoselective
hydrogenation.
A typical deposition-precipitation procedure was employed to
prepare Au/SiO2 catalyst. Briefly, SiO2 was dispersed with the aque-
ous solution of HAuCl4, and pH was adjusted to 9.0 by dropwise
addition of 0.25 M NH3·H2O (aq.). After stirring for 6 h and aging
for another 2 h, the catalysts were washed with deionized water for
five times and then dried at 50 ◦C overnight, followed by a reduction
with a stream of 5 vol% H2/Ar at 300 ◦C for 2 h.
2.2. Physical characterization
X-ray diffraction (XRD) analysis was performed on Bruker D8
diffractometer using Cu K␣ radiation ( = 1.54056 Å). Scanning
electronic microscopy (SEM) and transmission electron microscopy
(TEM) investigations were taken on a ZEISS ULTRA55 and a
JEOL JEM 2100F, respectively. Energy dispersive spectrum (EDS)
attached on TEM was carried out on a JEOL JEM 2100F. The
UV–vis diffuse reflection spectra (UV-vis DRS) were carried out
on Varian Cary 5000 at room temperature. X-ray photoelectron
spectroscopy (XPS) was processed on a Perkin-Elmer PHI X-tool,
using C 1 s (B. E. = 284.6 eV) as a reference. The metal loading
was determined by an inductively coupled plasma-atomic emis-
sion spectroscopy (ICP-AES). The Brunauer–Emmett–Teller (BET)
specific surface areas were determined by adsorption-desorption
of nitrogen at liquid nitrogen temperature, using an automatic
gas adsorption analyzer (Quantachrome Autosorb-iQ-MP). The
hydrogen temperature-programmed reduction (H2-TPR) and CO
chemisorption measurement were both conducted on a XianQuan
instrument TP 5076, and the NH3 temperature-programmed des-
orption (NH3-TPD) analysis was carried out on a Micromeritics
instrument ChemSorb 2920.
2.3. Catalytic performance measurement
CAL hydrogenation was carried out in a 100 mL stainless steel
autoclave (Parr 4848 reactor controller), in which 25 mg of cata-
lyst, 2 mmol of CAL, 20 mL of EtOH and 30 mL of H2O were loaded.
The reactor was sealed and purged with H2 to remove the air for
3 times, and then the reactor was heated to the desired tempera-
ture. Hydrogen (2 MPa) was purged into the reactor after desired
temperature was reached and the stirrer was started. The products
were analyzed by Shizumadu GC-2014C with a FID detector. The
conversion (conv.; %), selectivity to COL and hydrocinnamaldehyde
(HCAL) were calculated with the formulas.
2. Experimental
2.1. Catalyst preparation
2.1.1. MoO3 nanorods
NCAL,0 − NCAL
The precursor of Mo3O10(C6H8N)2·2H2O nanowires were fabri-
CCAL
=
× 100
(1)
NCAL,0
cated according to our previous report [27]. And the MoO3 nanorods