G. Zhang, et al.
AppliedCatalysisA,General579(2019)18–29
hence activity of the Cu species [14–16]. An efficient strategy to
overcome that is the incorporation of surface oxygen containing groups
on carbon support, which can act as anchoring sites to improve the
dispersion of Cu species and also promote the reduction of CuO to Cu2O
or Cu during the calcination process resulting in the improved catalytic
activity [12,17]. However, the agglomeration and oxidation of Cu
species occur easily during the reaction, resulting in the deactivation of
catalysts [9,12,18]. Hence, obtaining the catalyst with higher catalytic
activity and stability still remains the formidable challenge in state-of-
the-art Cu based catalyst.
of hydrothermal oxidative treatment.
2.1.2. High temperature N-doping of the supports
Typically, 1 g CNT or OCNTX sample was ground with melamine
keeping the mass ratio of melamine to CNT of 4. The mixture was an-
nealed at 700 °C for 4 h in nitrogen atmosphere at a ramping rate of
2 °C/min. Finally, the N-doped CNT sample was washed by distilled
water, filtered and dried at 100 °C for 10 h. The sample for N-doping of
pristine CNT was denoted as NCNT, while the samples for N-doping of
OCNTX were denoted as NCNTX, where X represents the temperature of
hydrothermal oxidative treatment.
In recent years, N-doped carbon materials as the support have at-
tracted much attention due to their unique properties [19–21]. The
incorporation of surface nitrogen containing groups can enhance the
hydrophilicity of carbon support, which allows the easy access of metal
salt solutions to carbon surface [22]. Moreover, the surface nitrogen
containing groups are regarded as more efficient and stable anchoring
sites for metal species than the surface oxygen containing groups [23].
The surface oxygen containing groups, especially the carboxylic groups,
could decompose during the calcination process resulting in the ag-
gregation of metal species [24,25]. Whereas, the surface nitrogen
containing groups play a crucial role in the stabilization of metal spe-
cies [26–31]. Li et al. [29] concluded that the surface nitrogen con-
taining groups inhibited the reduction of active center Au3+ to Au°
during the calcinations and acetylene hydrochlorination process, thus
improved the catalytic activity and stability of Au/AC. Chen et al. [27]
reported that the surface nitrogen containing groups on CNT improved
the resistance of metallic Co against oxidation in ambient atmosphere.
Bulusheva et al. [30] found that the surface nitrogen containing groups
on CNF prevented the aggregation of Pt during the reaction process and
thereby led to an excellent stability for hydrogen production from
formic acid. Cabrele et al. [26] revealed that the surface nitrogen
containing groups on CNT had successfully suppressed the leaching of
Cu and thus improved the catalytic stability for A3-type coupling re-
actions.
This supports the fact that N-doping on carbon support could sig-
nificantly overcome the limitations, viz., oxidation, aggregation and
leaching of Cu species during the liquid phase oxidative carbonylation
process. Generally, direct N-doping of the pristine carbon support re-
sults in low content of nitrogen containing groups due to the chemical
inertness [28,32]. In the present work, we report a facile and efficient
strategy to tune the contents of surface nitrogen containing groups on
CNT by pre-oxidation and subsequent N-doping. The catalytic activity
and stability of the obtained Cu/CNT catalysts are evaluated for liquid
phase oxidative carbonylation of methanol to DMC. The influence of
surface nitrogen containing groups on the dispersion and composition
of Cu species as well as its stabilization during the oxidative carbony-
lation reaction are disclosed.
2.2. Preparation of the catalysts
The catalysts with 8 wt.% Cu loadings (nominal) were prepared
using ultrasonic assisted impregnation. Firstly, 10 mL of Cu(NO3)2
aqueous solution was added dropwise to CNT supports. Subsequently,
the mixture was ultrasonicated and stirred for 0.5 h followed by drying
at 40 °C for 10 h and then calcined at 350 °C for 4 h in nitrogen atmo-
sphere at a ramping rate of 2 °C/min. According to the different CNT
supports, the obtained catalysts were denoted as Cu/CNT, Cu/NCNT,
Cu/OCNTX and Cu/NCNTX.
2.3. Characterization of the supports and catalysts
The N2 adsorption-desorption measurements were performed on a
Beishide 3H-2000PS2 apparatus at −196 °C after outgassing the sam-
ples under vacuum at 250 °C for 4 h. The specific surface area of the
samples were calculated by the Brunauer-Emmett-Teller (BET) method,
and the pore size distribution was measured using the Barrett-Joyner-
Halenda (BJH) model.
The X-ray photoelectron spectroscopy (XPS) data was collected on
an ESCALab220i-XL electron spectrometer (VG, UK) using 300 W AlKa
radiation. The samples were compressed into a pellet of 2 mm thickness
and then mounted on a sample holder by utilizing double-sided ad-
hesive tape for XPS analysis. The sample holder was then placed into a
fast entry air load-lock chamber without exposure to air and evacuated
under vacuum (< 10−6 Torr) over night. Finally, the sample holder
was transferred to the analysis chamber for XPS study. The base pres-
sure inside the analysis chamber was usually maintained at better than
10−10 Torr. The C1 s line (284.6 eV) was taken as a reference to correct
for electrostatic charging.
The X-ray diffraction (XRD) measurements were performed on a
Bruker D4 X-ray diffractometer with Ni filtered Cu Kα radiation (40 kV,
30 mA). The patterns were recorded in steps of 0.01° with the scanning
rate at 4°/min from 5° to 85° under atmospheric pressure.
The actual Cu loading of the catalyst was determined with an in-
ductively coupled plasma-optical emission spectrometer (ICP-OES 730,
Agilent 7700). Before the measurements, ca. 100 mg prepared sample
was dissolved in the mixture solutions with 0.5 mL hydrofluoric acid
(40 wt %) and 2 mL concentrated nitric acid (65 wt %).
2. Experimental
2.1. Surface modification of the supports
The transmission electron microscopy (TEM) photographs were
taken with a JEOL JEM-2100 electron microscope, operating at 200 kV.
The powder samples were ultrasonically dispersed in ethanol at room
temperature for 30 min and transferred onto a carbon-coated copper
grid by dipping. The mean particle diameters were determined by nano
measurer software from TEM image analysis. Generally, at least 50 Cu
nanoparticles per sample were selected to estimate their size distribu-
tion and the average particle size of Cu species.
The temperature-programmed reduction (TPR) experiments were
carried out on Micromeritics Autochem 2920 equipment. Typically,
25 mg of catalyst was loaded into a U-shape quartz autoclave, and
sample was degasified with argon (20 mL/min) at 200 °C for 2 h to re-
move physisorbed moisture. After cooling to room temperature, the gas
was switched to 10% H2 in argon flow (20 mL/min), and the tem-
perature rose from 50 to 900 °C with a heating rate of 10 °C/min.
2.1.1. Hydrothermal oxidation of the supports
The carbon nanotube (CNT, 0.5–2 μm long, ID = 5–10 nm,
OD = 20–30 nm) was purchased from Chengdu Organic Chem. Co. Ltd.
(Chinese Academy of Sciences). 1.5 g CNT was added into 75 mL nitric
acid solution (0.5 mol/L) and the mixture was transferred to stainless
steel autoclave for hydrothermal oxidative treatment. Firstly, the au-
toclave was sealed and pressurized to 0.8 Mpa with nitrogen.
Subsequently, the temperature was gradually raised to 170 °C or 200 °C
at a ramping rate of 2 °C/min and maintained for 4 h under stirring at a
speed of 300 r/min. Then, the autoclave was cooled to RT and de-
pressurized. The mixture was filtered and washed by distilled water
until pH = 7. This was followed by drying in an oven at 100 °C for 10 h.
The pristine CNT sample was denoted as CNT, while the oxidized CNT
samples were denoted as OCNTX, where X represented the temperature
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