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Y. Xin et al. / Journal of Catalysis 375 (2019) 202–212
Ru/Nb2O5 (NP) catalysts were prepared through adsorption of
reached, as monitored by a thermal conductivity detector (TCD).
The Ru dispersion was calculated assuming a stoichiometry of
1:1 for CO adsorbed on Ru.
colloidal Ru NPs on the Nb2O5 supports. Colloidal Ru NPs with dif-
ferent sizes were prepared by the reduction of RuCl3 with ascorbic
acid in the aqueous phase at 80 °C for 1 min, 5 min and 12 h,
respectively, according to previous report [35]. Then the Nb2O5
support was added into the Ru NPs colloidal solution and stirred
for 24 h. After that, the sample was centrifuged, washed with
deionized water, dried at 50 °C in a vacuum oven, and calcined at
300 °C for 4 h under N2 atmosphere.
2.4. Catalysts test and products analysis
Typically, catalyst (0.04 g) and 4-methylphenol (0.2 g) were
loaded into a 50 mL stainless autoclave reactor (Anhui Kemi
Machinery Technology Co., Ltd) with water (15 mL) as the solvent.
After the reactor was purged with H2 for three times, 0.5 MPa H2
was charged. Then the reaction was conducted at 250 °C with the
magnetic stirring (600 r.p.m) and kept for a certain reaction time.
The stirring speed of 600 r/min was sufficient for mass transfer
(Fig. S3). After reaction, the reactor was quenched to ambient tem-
perature in a water bath. The reaction mixture was extracted with
ethyl acetate (10 g), followed by centrifugation to separate the
solid catalyst. The organic phase was qualitatively analyzed using
a GC–MS (Agilent 7890A–5975C) equipped with an HP-5 column
and quantitatively analyzed by GC-FID (Agilent 7890A). Tridecane
was taken as the internal standard. Both of the experiments and
catalyst preparations were repeated three times. The number of
conversion and yield were obtained by taking the average of three
measurements.
2.3. Characterization
The powder XRD patterns were recorded with a Rigaku D/max-
2550VB/PC diffractometer by using Cu K
a (L = 0.15406 nm) radia-
tion that was operated at 40 kV and 40 mA.
The N2 adsorption-desorption isotherms were measured at
À196 °C using a Micromeritics ASAP 2020 M sorption analyzer.
The BET method was used to calculate the specific surface area.
The transmission electron microscopy (TEM) was performed on
a JEOL 2100 electron microscope that was operated at 200 kV.
The actual Ru loading in the sample was detected by inductively
coupled plasma-atomic emission spectroscopy (ICP-AES) on a
Perkin-Elmer Optima 2100 DV spectrometer.
Raman spectra were recorded on a Renishaw Raman spectrom-
eter under ambient conditions, and the 514 nm line of the Spectra
Physics Ar+ laser was used as the excitation wavelength. The laser
3. Results and discussions
beam intensity and spectrum slit width was 2mW and 3.5 cmÀ1
,
respectively. X-ray absorption near edge structure (XANES) mea-
surement was conducted at the Canadian Light Source using the
Soft X-ray Microcharacterization Beamline with a selected energy
3.1. Characterization of the catalysts
Fig. 1 shows the XRD patterns of various Ru/Nb2O5 (WI) cata-
lysts. For sample L3-Nb2O5, only two diffraction peaks at 22.7°
and 46.2° were observed obviously, indicating its layered structure
and low degree of crystallinity [34]. M-Nb2O5 sample displayed the
structural characteristics of hexagonal Nb2O5 (TT phase). The TT
phase showed a low degree of crystallinity, which can be regarded
as a modification of the orthorhombic T phase. For the samples of
H-Nb2O5 and F-Nb2O5, the peaks at 28.4° and 36.7° were split into
two peaks, respectively, indicating the transformation from the
pseudohexagonal TT phase to the orthorhombic T phase [36]. The
sharp peaks for both of H-Nb2O5 and F-Nb2O5 samples indicate that
they are highly crystallized. Additionally, no diffraction peaks cor-
responded to Ru were detected on M-Nb2O5 and L3-Nb2O5 cata-
lysts and only a very broad peak at 42.7° corresponded to Ru is
observed in the XRD patterns of H-Nb2O5 and F-Nb2O5 (Fig. 1B).
These indicate the different particle sizes of Ru on different
Nb2O5, but all are highly dispersed.
The TEM images of four Ru/Nb2O5(WI) catalysts are presented
in Fig. 2. The Ru nanoparticles are anchored on F-Nb2O5 (Fig. 2A)
and H-Nb2O5 (Fig. 2B) hosts and have a mean distribution of
7.6 nm and 1.9 nm, respectively, while it cannot be seen Ru
nanoparticles on M-Nb2O5 (Fig. 2C) and L3-Nb2O5 (Fig. 2D). EDS-
mapping analysis of Ru/M-Nb2O5 and Ru/L3-Nb2O5 show the pres-
ence of Ru on Nb-based supports homogeneously. It is consistent
with the results from XRD and TEM, i.e. small Ru particles highly
dispersed on four Ru/Nb2O5 (WI).
window (e.g. Nb La for Nb L3-edge). A four-element Si drift solid
state detector was equipped for the fluorescence yield
measurement.
H2 temperature-programmed reduction (H2-TPR) measure-
ments of the catalysts were conducted using an apparatus
(PX200, Tianjin Golden Eagle Technology Co. Ltd). 50 mg sample
was directly heated from room temperature to 400 °C at a rate of
10 °C/min in a flow of 10 vol% H2/Ar (40 mL/min). The hydrogen
consumption was monitored using a thermal conductivity detector
(TCD).
Infrared (IR) spectra of pyridine adsorption were recorded on
NICOLET iS50 FT-IR spectrometer, with 32 scans at an effective res-
olution of 4 cmÀ1. The samples were pressed into self-supporting
disks and placed in an IR cell. The disk was dehydrated by heating
at 400 °C for 1 h under vacuum. After the cell was cooled to room
temperature, the IR spectrum was recorded as background. Pyri-
dine vapor was then introduced into the cell at room-
temperature until equilibrium was reached, and then a second
spectrum was recorded. Then evacuation was performed at
100 °C for 10 min followed by spectral acquisitions.
Diffuse Reflectance Infrared Fourier Transform Spectroscopy
(DRIFTS) was collected with a NICOLET iS50 FT-IR spectrometer
equipped with an MCT/A detector. Firstly, the catalysts (0.05 g)
were in situ reduced in the cell in H2 at 400 °C for 30 min and
cooled to 35 °C in N2, and a background was recorded. The reduc-
tion time decreases to 30 min due to the small amount of catalyst.
Then 4-methylphenol with N2 was bubbled into the cell for 30 min,
blowing with N2 for 60 min. Finally, the temperature was increased
from 35 to 150 and 200 °C, respectively and the adsorption spectra
of 4-methylphenol were recorded at different time.
The Ru dispersion was measured by CO chemisorption using the
dynamic adsorption method with an automatic chemisorbent
(ChemiSorb 2720). Before adsorption, the samples (0.1 g) were
reduced under pure H2 (60 mL/min) at 473 K for 30 min (10 K/
min), cooled to room temperature, and flushed in He for 30 min.
Then, pulses of 5% CO in He were injected until saturation was
Nitrogen sorption was carried out to measure the surface area
of all Nb2O5 supports and Ru/Nb2O5 catalysts, the data were sum-
marized in Table 1. It is found that after loading of Ru, all surface
areas decreased, but in less extent. L3-Nb2O5 and Ru/L3-Nb2O5
had the highest surface area (197/181 m2ÁgÀ1), followed with M-
Nb2O5/Ru/M-Nb2O5 (50/44 m2ÁgÀ1
)
and H-Nb2O5/Ru/H-Nb2O5
(42/36 m2ÁgÀ1). The surface areas of F-Nb2O5 and Ru/F-Nb2O5 are
least, 16 and 15 m2ÁgÀ1, respectively. These four Nb2O5 supports
were prepared by adding different organic agents. Hexadecyl tri-
methyl ammonium bromide (CTAB) was used for M-Nb2O5, pheno-
lic resin for H-Nb2O5, ammonium oxalate for L-Nb2O5 and no