1
90
R. You et al. / Journal of Catalysis 348 (2017) 189–199
oxides supported on CeO
2
and concluded that supported vanadium
were normalized to the photon flux calibrated by measuring the
5
+
5+ 4+
mainly existed as V . During ODHP reaction, the V /V redox
cycle was found to accompany the ODHP reaction but always take
place at temperature above 500 °C. At elevated temperatures
Au4f spectra of a clean Au foil. The photon energies to measure
the Nb3d and Ce4p core levels spectra were set at 400 eV, and
the binding energies were corrected by setting the binding energy
of the adventitious carbon (C1s) to 284.8 eV. Resonant photoemis-
sion spectra (RPES) in the Ce4d–4f photon absorption region were
measured with photons of 124.8 and 121.5 eV for on-resonance
spectra and of115 eV for off-resonance spectra.
(
above 500 °C), homogeneous reactions initiated by gas-phase pro-
pyl radicals formed by the activation of propane on catalyst surfaces
were observed to occur and contribute to the ODHP process [10–
1
2,36,37]. These secondary reactions of propyl radicals in the gas
phase to propene or oxygenated species like CO have been pro-
x
2 x 2
Catalytic performance of CeO rods and NbO /CeO catalysts in
posed [10,12,37–39], but experimental observations of propyl rad-
icals in ODHP reaction are few.
the ODHP reaction was evaluated with an ambient-pressure fixed-
bed flow reactor. A 200 mg sample of catalyst (60–80 mesh) mixed
with 200 mg of silicon carbide (ꢁ60 mesh) was placed in a quartz
Niobium belongs to the same group as vanadium. We recently
prepared NbO
x
supported on various CeO
2
nanoshapes as catalysts
3 8
reactor (i.d. = 10 mm). The reaction gas, consisting of C H (flow
for ODHP reaction [40]. A preliminary observation of preferential
rate 6 mL/min), O
min), was fed at a total GHSV of 15,000 mL h
2
(flow rate 5 mL/min), and Ar (flow rate 39 mL/
4
+
ꢂ1 ꢂ1
formation of Nb species suggests NbO
tinctly different from VO –CeO interaction. In this paper, a series
of NbO /CeO rod catalysts for the ODHP reaction were character-
x
–CeO
2
interaction dis-
g . The catalyst
x
2
was heated to desired reaction temperatures at a rate of 2 °C/min
and the temperature was measured with a thermocouple fixed to
the inner surface of the catalyst bed. After the reaction reached
steady state, the composition of the effluent gas was analyzed with
an online SHIMADZU GC-2014 gas chromatograph (GC) equipped
with a Porapak Q column attached to a TCD detector for the sepa-
ration and detection of O
BOND/KCl capillary column attached to a FID detector for the sep-
aration and detection of hydrocarbons. The propane and O conver-
sion and the propene selectivity were calculated as follows:
x
2
ized by synchrotron radiation photoemission spectroscopy (SRPES),
resonant photoemission spectroscopy (RPES), and synchrotron
radiation VUV photoionization mass spectroscopy (SVUV-PIMS).
5
+
The Nb precursor was proved to interact preferentially with sur-
face oxygen vacancies on CeO rods and undergo surface redox
2
2 2
, CO, and CO , and with a SH-Rt-alumina
4
+
4+
reactions to produce Nb and Ce ; meanwhile, surface reactions
on catalysts were found to dominate the ODHP reaction via the
MvK mechanism at low temperatures, while gas-phase propyl rad-
icals were unambiguously identified at high temperatures as initi-
ating homogeneous reactions via the radical mechanism.
2
À
Á
C
3
H
8
conversion ¼ ½C
3
H
8
ꢃinlet ꢂ ½C
3
H
8
ꢃoutlet =½C
3
H
8
ꢃinlet ꢄ 100%;
C
3
H
6
selectivity ¼ ½C
3
H
6
ꢃoutlet=ð½C
3
H
8
ꢃinlet ꢂ ½C
3
H
8
ꢃoutletÞ ꢄ 100%;
2
. Experimental
O
2
conversion ¼ ð½O
Synchrotron VUV photoionization mass spectroscopy (SVUV-
2
ꢃinlet ꢂ ½O
2
ꢃoutletÞ=½O
2
ꢃinlet ꢄ 100%:
All chemical reagents of A.R. grade (Sinopharm Chemical
Reagent Co., Ltd.) and all gases (C
(>99.999%), and Ar (>99.999%), Nanjing Shangyuan Industrial
Gas Factory, China) were used as received. The ultrapure water
resistance >18 M ) was used.
NbO /CeO catalysts were prepared by a conventional wetness
incipient impregnation method using niobium (V) oxalate hydrate
Nb(HC O) as the Nb precursor and ceria rods synthesized
3 8 3 6
H (>99.99%), C H (>99.99%),
PIMS) [41–45] measurements were carried out at the combustion
end station of beamline 03U in the National Synchrotron Radiation
Laboratory (Hefei, China). A quartz catalytic reactor was connected
to the SVUV-PIMS spectrometer, where the catalyst bed (2 ꢄ 2 mm)
was placed 20–30 mm from the sampling nozzle of the spectrome-
O
2
(
X
x
2
ter. During the ODHP reaction, the reaction gas, consisting of C
3 8
H
(
2
O
)
4 5
ꢀnH
2
(
flow rate: 36 mL/min), O (flow rate: 30 mL/min), and Ar (flow
2
via a hydrothermal method and calcined at 500 °C for 4 h as the
rate: 36 mL/min), was fed to a mixing chamber, and then the gas
mixture was pumped through the catalyst bed at a total pressure
of 2.5 Torr. After the catalytic reaction reached steady state at the
desired temperature, the composition of the effluent gas was
analyzed by the online SVUV-PIMS spectrometer. A homemade
time-of-flight mass spectrometer (TOF-MS) with a mass resolution
of 2000 was used, and the m/z value was calibrated based on the
flight time of hydrogen (m/z = 2.016), water (m/z = 18.011), carbon
monoxide (m/z = 27.995), oxygen (m/z = 31.990), and carbon diox-
ide (m/z = 43.990).
support [40]. The acquired catalyst precursor was finally subjected
to calcination in ambient atmosphere at 400 °C for 4 h. The NbO
CeO catalysts are denoted as Nb/CeO , where represents the
number of Nb atoms (actual content) per nm of CeO surface.
x
/
2
a
2
a
2
2
Compositions of catalysts were analyzed by a Perkin Elmer
Optima 7300 DV inductively coupled plasma-atomic emission
spectrometer (ICP-AES). BET specific surface areas were measured
2
on a Micromeritics Tristar II 3020 M analyzer under N and the
samples were degassed at 300 °C prior to measurement. Powder
X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert
PRO SUPER diffractometer with CuK radiation (k = 0.15418 nm)
operating at 40 kV and 50 mA. H
reduction (H TPR) experiments were performed on a Micromerit-
ics Autochem 2920 apparatus. A 30 mg sample of catalyst was
placed in a quartz reactor and heated in a flow of 5% H /Ar mixture
flow rate: 30 mL/min) at a heating rate of 10 °C/min. Transmission
electron microscopy (TEM) and high-resolution transmission elec-
tron microscopy (HRTEM) images were recorded on JEOL-2010 and
JEOL-2100F high-resolution transmission electron microscopes
with an electron acceleration energy of 200 kV.
2
temperature-programmed
3. Results and discussion
2
Compositions of NbO
AES, from which the numbers of Nb atoms per nm of CeO
were calculated as 0.3, 0.6, 1.4, and 3.3. BET specific surface areas
of CeO , 0.3Nb/CeO , 0.6Nb/CeO , 1.4Nb/CeO , and 3.3Nb/CeO
were measured to be 94.8, 89.1, 84.1, 78.7, and 72.0 m /g, respec-
tively. Fig. 1 shows XRD patterns of CeO rods and NbO /CeO cat-
alysts. Only diffraction patterns arising from CeO appear in all
x 2
/CeO catalysts were determined by ICP-
2
2
2
surface
(
2
2
2
2
2
2
2
x
2
2
Synchrotron radiation photoemission spectroscopy (SRPES)
experiments were carried out at the photoemission end station
of beamline 10B in the National Synchrotron Radiation Laboratory
samples, suggesting the formation of either highly dispersed or
amorphous Nb-contained species. Indicated by the full width at
half maximum (FWHM) of the CeO
of NbO does not lessen the crystallinity of CeO
Figs. 2A and B show representative TEM and HRTEM images
of CeO rods and 3.3Nb/CeO catalyst, respectively. CeO rods
2
diffraction peaks, the loading
(
Hefei, China). Fine catalyst powders were pressed onto the con-
x
2
supports.
ducting resin for the SRPES measurements performed at 100 °C
to minimize the sample charging effect. The photoemission spectra
2
2
2
You, Rui
