Z. Cao, et al.
AppliedCatalysisA,General578(2019)105–115
not entirely clear due to the complexity of catalysts and reaction sys-
tems, and the catalyst with a high activity and high COL selectivity at
ambient conditions is still quite challenging.
Then the sample was purged with N2 for 0.5 h to remove the weakly
adsorbed NH3. Finally, the sample was heated in N2 from 100 to 700 °C
at a rate of 10 °C/min. For CO2-TPD, a 40 ml/min of 5% CO2/He was
introduced for 1 h to achieve the adsorption equilibrium at 50 °C, and
50–800 °C was used as the desorption temperature range. For TPR, the
samples without pre-reduction were tested under a 5% H2/Ar atmo-
sphere in a temperature range of 0–800 °C, and the interference of
water and other products derived from the reduction or decomposition
process on the TCD signal of H2 consumption was eliminated by an
isopropanol-liquid nitrogen trap. Another set of TPR experiments in a
temperature range of −50–800 °C was also carried out using the pre-
reduced and re-oxidized samples. Before TPR, the dried samples were
pre-reduced at 300 °C for 1 h in a flow of hydrogen and then re-oxidized
at 300 °C for 2 h in a flow of dry air.
Diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) data of CO adsorbed on Pt/BN catalysts were collected on a
Nicolet 6700 spectrometer equipped with a diffuse reflectance acces-
sory and a MCT/A detector. A high-temperature cell connected to a gas
flow control and vacuum system was used for the in-situ reduction of
catalysts and CO adsorption. The sample in the cell was reduced ac-
cording to preparation conditions, purged with He for 0.5 h, and then
cooled to 25 °C. After vacuum degassing for 0.5 h, the background
spectrum was recorded. Subsequently, CO was introduced and the
pressure was gradually increased from 0.25 to 50 mbar, and then the
physically adsorbed CO was evacuated by again vacuum degassing. The
CO adsorption spectra under different CO pressures and vacuum were
collected using 16 scans referenced to the background spectrum.
Hexagonal boron nitride (h-BN) has large thermal conductivity,
superior temperature stability, excellent acid-base resistance and spe-
cific coordinatively unsaturated edge sites, which can be used as cata-
lyst supports [29–32] or a catalyst [33] in various catalytic reactions.
Previous reports showed that a Pt catalyst supported on h-BN gave near
100% selectivity of the C]C hydrogenation in the temperature range of
30–100 °C for a vapor phase hydrogenation of crotonaldehyde [34], and
the addition of Sn [35] or Fe [36] could enhance the selectivity of the
C]O hydrogenation. In this work, the liquid phase selective hydro-
genation of CAL over Pt/BN catalysts was explored, where the Pt/BN
exhibited a high performance for the selective C]O hydrogenation to
COL at room temperature. The effects of supports and Pt particle sizes
were examined by N2 physisorption, XRD, TEM, HRTEM, HAADF-
STEM, NH3-TPD, CO2-TPD, H2-TPR, XPS, CO-DRIFTS, etc. The influ-
ences of solvents and operation conditions were also investigated.
2. Experimental section
2.1. Catalyst preparation
The Pt/BN catalysts with a targeted Pt loading of 1.0 wt.% were
prepared by impregnating h-BN (˜25 m2/g, Aladdin, Shanghai) with
aqueous Pt(NH3)4(NO3)2 (Sino-Platinum, Kunming) solution. The im-
pregnation mixture was dried by rotary evaporator at 50 °C for 2 h after
stirring well, and further dried in an oven at 110 °C overnight. The dried
sample, denoted as Pt/BN-Dry110, was reduced at 200, 300 and 400 °C
for 1 h in a flow of hydrogen, and the obtained catalyst was denoted as
Pt/BN-R200, Pt/BN-R300 and Pt/BN-R400, respectively. For compar-
ison, the Pt catalysts supported on Al2O3, SiO2 and graphite were pre-
pared by the same procedure as Pt/BN-R300. An extra Pt/BN catalyst,
denoted as Pt-Cl/BN-R300, was also prepared by the same procedure as
Pt/BN-R300 except that H2PtCl6 was used as metal precursors.
2.3. Catalytic test
The liquid phase selective hydrogenation of CAL was carried out in a
50 ml autoclave equipped with a magnetic stirrer. The mass transfer
limitation was eliminated when an agitation speed of 1732 rpm was
used. In a typical reaction, 5.0 mmol of CAL, 16.0 ml of isopropanol,
4.0 ml of water, 0.2 ml of n-tetradecane and 100 mg of Pt catalysts were
charged into the reactor, which was flushed with N2 for 6 times and
further pressurized with H2 to the desired pressure. Then the reaction
temperature was raised to the desired value and agitation was started.
After different reaction periods, the products were analyzed by an
Agilent 7890B gas chromatograph equipped with a flame ionizing de-
tector (FID) and a HP Innowax capillary column. The conversion of CAL
and the selectivity of products were calculated on the basis of the mass
balance of carbon using n-tetradecane as an internal standard. In the
experiments on solvent effect, a total amount of the solvent added was
kept at 20 ml. In some kinetics measurements, a constant H2 pressure
was kept. All the experiments were conducted in triplicate for re-
producibility of the data and the results were within an error of 5%.
2.2. Characterization
Nitrogen adsorption-desorption isotherms were recorded at 77 K on
a Micromeritics ASAP 2020 instrument. Before the measurements, the
samples were degassed at 250 °C for 6 h. The specific surface area was
calculated by multipoint BET model, and the pore volume was obtained
based on the adsorption at P/P0 ≈ 1.
X-ray powder diffraction (XRD) patterns were recorded on a Rigaku
Rint D/MAX-2500/PC diffractometer using Cu Kα radiation operated at
40 kV and 40 mA.
Transmission electron microscopy (TEM), high resolution trans-
mission electron microscopy (HRTEM) and high-angle annular dark
field-scanning transmission electron microscopy (HAADF-STEM)
images were taken on an FEI Tecnai G2F30 or G220 microscope using
different modes. The specimen was prepared by ultrasonically disper-
sing the sample powder in ethanol, and drops of the suspension were
deposited on a carbon-coated copper grid and dried in air.
X-ray photoelectron spectroscopy (XPS) data were collected by a
Thermo Fisher Escalab 250Xi equipped with an Al Kα monochromatic
X-ray source (1486.6 eV) under ultrahigh vacuum condition (< 10−7
Pa). The adventitious carbon 1 s peak was calibrated at 284.8 eV to
compensate for any charging effects.
3. Results and discussion
3.1. CAL hydrogenation over the Pt/BN and the effect of supports
Table 1 compares the catalytic performance of the Pt catalysts
supported on BN and other three common supports, Al2O3, SiO2 and
graphite. A CAL conversion of 95.8% with the COL selectivity of 85.2%
was achieved over the Pt/BN at room temperature. For CAL hydro-
genation, the BN supported Pt exhibited an unexpected high activity
and high selectivity of C]O hydrogenation, which was far better than
Pt/Al2O3, Pt/SiO2 and Pt/G, as shown in Table 1. The Pt/Al2O3 dis-
played a higher HCAL selectivity (49.8%) and a lower COL selectivity
(34.1%), and the same order could be observed on the Pt/SiO2, in-
dicating that the C]C hydrogenation is preferential on both of the
catalysts. Similar to the Pt/BN, the Pt/G also exhibited a considerable
COL selectivity, but its activity was too low. The BN alone had little
activity for CAL hydrogenation.
Temperature-programmed desorption of ammonia (NH3-TPD) or
carbon dioxide (CO2-TPD), and temperature-programmed reduction
(TPR) experiments were carried out on a Micromeritics Auto Chem
2920 instrument equipped with a thermal conductive detector (TCD).
For NH3-TPD, 100 mg of the catalyst was initially heated in an 80 ml/
min of N2 to 300 °C at a rate of 10 °C/min and held at this temperature
for 0.5 h. After cooling to 100 °C under the N2 flow, a 5% NH3/N2 in a
loop of 1 ml was periodically introduced until saturated adsorption.
In order to elucidate the great difference in the catalytic
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