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G. Dong et al. / Journal of Catalysis 391 (2020) 155–162
nanoparticles significantly smaller and more resistant toward sin-
tering than those seen in previous catalysts.
age pore diameters and pore size distributions were obtained by
analyzing the desorption branch of the isotherms using the
Barret–Joyner–Halenda method.
Transmission electron microscopy (TEM) images were obtained
on a JEOL JEM-2100F instrument with an operating voltage of
200 kV. The high-angle annular dark-field scanning transmission
electron microscopy (HAADF-STEM) images were acquired on a
FEI Tecnai G2 F20 S-Twin instrument with an operating voltage
of 200 kV. The actual loading of silver in the catalysts was deter-
mined by ion-coupled plasma optical emission spectroscopy (ICP-
OES) using an Agilent 5100 spectrometer. The actual content of
nitrogen in the samples was estimated by using an Elementar Vario
ELIII elemental analyzer. The powdered X-ray diffraction (XRD)
patterns of the samples were collected on a Bruker D8 Advance
X-ray diffractometer in the scanning angle (2h) range of 10° to
2. Experimental
2.1. Chemicals
Cetyltrimethylammonium chloride (CTAC), triethanolamine
(TEA), and 3-aminopropyltrimethoxysilane (APTS) were purchased
from J&K Scientific Ltd. Tetraethyl orthosilicate (TEOS), cyclohex-
ane, methylbenzene, ethanol, and silver nitrate (AgNO3) were pur-
chased from Sinopharm Chemical Reagent Co., Ltd. All the reagents
were used as received without further purification.
2.2. Preparation of the MSNS support
80° at a scanning speed of 2°ꢁminꢀ1 using nickel-filtered Cu K
a
radiation (k = 1.5418 Å). UV–visible diffuse reflectance spectra
(UV–vis DRS) were recorded in the 200–800 nm range on an Agi-
lent CARY 5000 UV–vis-NIR scanning spectrophotometer with an
integrating attachment using BaSO4 as the background standard.
X-ray photoelectron spectroscopy (XPS) was carried out on a
The MSNS was synthesized via
a biphasic stratification
approach using TEOS as the silica source and CTAC as the template
agent, according to a method reported in the literature [18]. Typi-
cally, 15 g of CTAC was first dissolved in 120 mL of deionized
water, after which 0.4 g of TEA was added. The mixture was stirred
gently at 333 K for 1 h. Subsequently, a solution of 7.5 g of TEOS in
25 g of cyclohexane was added drop-wise to the water-CTAC-TEA
mixture, which was then stirred continuously at 333 K for 12 h.
The resulting mixture was collected by centrifugation and washed
with ethanol to remove the residual reactants. After drying at
393 K for 4 h, the CTAC-templated MSNS solid (denoted as Sur-
MSNS) was obtained. Finally, the Sur-MSNS was calcined at
823 K for 4 h in static air to remove the template and obtain the
desired MSNS support.
The amino-functionalized MSNS (NH2-MSNS) was synthesized
by silanization of APTS via a post-grafting strategy [19]. Typically,
1.5 g of MSNS was dispersed in 50 g of toluene to form a homoge-
neous suspension, and then 0.5 g of APTS was added. The mixture
was refluxed at 383 K for 12 h, after which it was centrifugated and
washed with ethanol to remove the residual reactants. After drying
at 393 K for 4 h, the amino-functionalized MSNS (denoted as
NH2-MSNS) was obtained.
Thermo Fisher K-Alpha spectrometer equipped with an Al K
a X-
ray radiation source (h = 1486.6 eV) and a multichannel detector.
m
A flood gun with variable electron accelerating voltages (from 6 to
8 eV) was used for charge compensation. The binding energies
were calibrated in reference to a value of BE = 284.6 eV for the car-
bonaceous C 1s XPS signal.
The N2O chemisorption, H2 temperature-programmed reduc-
tion (H2-TPR), and H2, DMO, and CO2 temperature-programmed
desorption (H2-TPD, DMO-TPD, CO2-TPD respectively) experiments
were carried out in a Micrometrics Autochem II 2920 apparatus
with a thermal conductivity detector (TCD). For N2O chemisorp-
tion, 100 mg of the solid sample was first reduced at 623 K under
a flow of 50 mLꢁminꢀ1 of 10% H2/Ar for 3 h and then cooled down to
363 K. Subsequently, the sample was exposed to the pure N2O
(30 mLꢁminꢀ1) for 1 h to ensure that the surface metallic silver
atoms were completely oxidized to Ag2O. The sample was purged
with a flow of Ar (30 mLꢁminꢀ1) for 30 min and then cooled down
to room temperature under Ar atmosphere. Next, 10% H2/Ar
(50 mLꢁminꢀ1) was introduced, and the sample was heated up to
1073 K at rate of 10 Kꢁminꢀ1, during which the hydrogen consump-
tion was monitored using the TCD [20]. In a typical procedure for
the TPD tests, 100 mg of sample was reduced at 623 K for 3 h in
a 10% H2/Ar atmosphere, followed by purging with He for 2 h at
573 K to remove physically adsorbed impurities. After cooling
the sample down to 323 K, a flow of the probe molecule (H2 for
H2-TPD, DMO for DMO-TPD, and CO2 for CO2-TPD; for the DMO-
TPD experiments the vapor of DMO was carried by Ar) was started
until saturated adsorption was reached. The probe molecule was
then removed by purging with Ar until the baseline of the MS sig-
nal was stabilized. After cooling to room temperature, the TPD pro-
file was collected in Ar from room temperature to 1073 K at a
ramping rate of 5 Kꢁminꢀ1. The desorbed probe molecule was mon-
itored by using an online mass spectrometer (MS).
2.3. Preparation of Ag/MSNS and Ag/NH2-MSNS catalysts
The MSNS and NH2-MSNS supported Ag catalysts were pre-
pared via in situ reduction of Ag salts. Typically, 1.0 g of MSNS/
NH2-MSNS was dispersed in 90 mL of ethanol by sonication to form
a uniformly dispersed suspension. After the mixture was stirred for
30 min at 303 K, 0.049 g of AgNO3 in 24.5 mL of ethanol was added
drop-wise and stirred at the same temperature for 4 h. Then the
temperature was increased to 343 K, and the mixture was stirred
for 24 h until the color of the suspension changed into brown. After
centrifugation, the product was washed with ethanol and dried at
333 K for 12 h under a nitrogen atmosphere to obtain the corre-
sponding Ag/MSNS or Ag/NH2-MSNS catalyst.
2.4. Catalyst characterizations
In situ FTIR spectra of DMO adsorbed on the catalysts were
recorded using a Perkin-Elmer Frontier spectrometer and a trans-
mission FTIR cell. Briefly, 30 mg of dried catalyst was compressed
into a self-supporting wafer and carefully loaded into the transmis-
sion cell. The catalyst was reduced at 623 K under 5% H2/N2 for 3 h
and then evacuated by N2 for 30 min to remove the chemisorbed
hydrogen. After cooling down to 353 K, DMO was evaporated
and flowed through the cell with the aid of a vacuum pump for
1 h. That was followed by evacuation to remove any weakly-
adsorbed DMO. The FTIR spectra were then recorded at the reac-
tion temperature (i.e., 493 K) with a spectral resolution of 4 cmꢀ1
and via the accumulation of 64 scans.
Fourier-transform infrared (FTIR) spectra were recorded in the
400–4000 cmꢀ1 range on a Nicolet 6700 spectrometer equipped
with a deuterium tri-glycine sulfate (DTGS) detector. The powder
samples were mixed with KBr (2 wt%) and pressed into translucent
disks at room temperature. The N2 adsorption–desorption iso-
therms of the catalysts were measured at 77 K using a Micromet-
rics ASAP 2020 apparatus. The samples were degassed at 573 K for
3 h to remove physically-adsorbed impurities prior to the mea-
surements. The specific surface areas (SBET
according to the Brunauer–Emmett–Teller (BET) method. The aver-
) were calculated