C. Cheephat et al.
AppliedCatalysisA,General563(2018)1–8
studied the effect of adding silica to Ni/Al2O3 and found that the ad-
dition of silica to alumina supports contributed to more easily reducible
Ni catalysts. Ni/Al2O3, hexaaluminate-type BaNixAl12−xO19−α and
perovskite-type LaAl5/6Ni1/6O3 catalysts were investigated for partial
oxidation of methane by Utaka et al. [13]. XPS analysis revealed that
the hexaaluminate catalysts provided Ni-rich surfaces and highly dis-
persed Ni species obtained from hexaaluminate crystals, resulting in
greatly improved catalytic activity.
(NO3)3, Sigma-Aldrich, Germany) were dissolved in deionized water.
The metal aqueous solutions were then impregnated on γ-Al2O3 and
GDC; this was followed by drying overnight in an oven at 105 °C and
calcining at 700 °C in air for 6 h at a heating rate of 10 °C min−1. For
bimetallic Re-Ni catalysts, the amount of Re loading was varied in the
range of 1, 3 and 5 wt.%; Ni was added to balance the total weigh to
10 wt.%. The catalysts were denoted as X% Re-Y% Ni, where X and Y
represent the weight percentages of Ni and Re, respectively.
The catalytic performance can be improved not only by modifica-
tions of precursors, synthesis methods or supports, but also by doping
the supported catalysts with other added metals or promoters. Ma et al.
[14] studied the effect of rare earths and other basic promoters such as
Na, Sr, Ce and La on CPOM to syngas over monolithic Ni/γ-Al2O3 cat-
alyst. The results indicated that adding a small amount of promoters
could improve the reducibility and activity of the catalyst. Ce and La
could fully restrain the side reaction and gave 100% H2 selectivity.
Chromium added to Ni/Al2O3 catalyst was tested by González et al.
[15]. Ni-Cr alloy was formed, resulting in increasing stability in me-
thane partial oxidation. Wang et al. [16] showed the beneficial effect of
Ce, La, Ca on the catalytic performance of Ni catalysts; Ce was noted to
be the best promoter. Based on the catalyst characterization results, Ce
could improve the reducibility of the Ni catalyst and could be made
highly dispersed within the catalyst matrix. Apart from the addition of
these promoters, Re has been previously reported as good catalyst and/
or promoter for hydrocarbon conversion [17–19]. Claridge et al. [17]
studied Re/γ-Al2O3 as catalyst for partial oxidation and dry reforming
of methane. They indicated that the catalytic activity of Re/γ-Al2O3 was
strongly dependent on temperature, from which high operating tem-
perature is required. Wang et al. [18] reported the conversion of me-
thane and ethane to oxygenated molecules (formaldehyde and acet-
aldehyde) over Re2O7/SiO2. They found that rhenium oxide not only
increased methane conversion but also increased oxygen transfer to
form oxygenated compounds. Enger et al. [19] studied the CPOM over
Ni/α-Al2O3 modified by Re and claimed that the addition of Re pro-
moted ethane formation at high GHSV. Recently, we found that the
addition of Re to Cu/cerium oxide and gadolinium doped ceria sig-
nificantly improved the activity of Cu catalysts in the water-gas shift
reaction [20]. The good properties of Re as catalyst promoter has also
been reported for Co/CeO2 [21] and for Ir/TiO2 [22].
Therefore, in the present work, the effect of Re promoter doping on
Ni/γ-Al2O3 catalyst for CPOM was studied. Bimetallic Re-Ni supported
on γ-Al2O3 catalyst was tested and compared to 10% Ni and 1% Rh
monometallic catalysts. We expect that addition of Re promoter would
enhance the catalytic activity of Ni/γ-Al2O3. In addition, catalytic sta-
bility test was carried out along with the investigation of the effects of
co-fed reactants (i.e., H2O and CO2). Lastly, Re-Ni/Gd-CeO2 catalyst
was prepared and tested toward the reaction to study the effect of
catalyst support. Characterization of the prepared catalysts by BET
surface area determination, X-ray diffraction, scanning electron mi-
croscopy and energy dispersive X-ray spectroscopy, transmission elec-
tron microscopy, X-ray photoelectron spectroscopy, X-ray fluorescence,
temperature programmed oxidation and H2-temperature programmed
desorption was performed to establish relationships between the ac-
tivity and physicochemical properties of the catalysts.
2.2. Catalyst characterization
Specific surface area and pore size distribution of the catalysts were
measured via the nitrogen adsorption-desorption experiments at the
liquid N2 temperature of −196 °C using a volumetric gas adsorption
instrument (Belsorpmax, MicrotracBEL, Japan). Prior to each mea-
surement, a fresh sample was degassed at 350 °C for 4 h. The surface
area and pore size distribution were determined using the Brunauer-
Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, re-
spectively.
The crystalline phases of the fresh solid catalysts were analyzed by
an X-ray diffractometer (Philips X’Pert, Netherlands) with CuK-α ra-
diation at 40 kV and 30 mA at a scanning rate of 0.02° min−1 in the
range of 10–90°. A scanning electron microscope (JEOL, JSM-6610 LV,
Japan) connected with an energy dispersive X-ray spectrometer ana-
lyzer was used for observing the morphology and elemental composi-
tion of the bimetallic Re-Ni catalysts. To analyze the shape and size of
metals in the catalyst structure, transmission electron microscope
(JEOL, JEM 2010, Japan) was used. The chemical states of elements in
the fresh catalysts were anatomized by X-ray photoelectron spectro-
scope (AXIS Ultra DLD, UK). High-resolution X-ray photoelectron
spectroscopy was also used to determine the information on the che-
mical composition and oxidation state of the bimetallic Re-Ni catalysts.
The weight contents of the loaded metals were determined by X-ray
fluorescence analysis.
H2 chemisorption analysis was performed on Micromeritics Pulse
ChemiSorb 2700 (USA). A sample (100 mg) was first placed in a quartz
sample cell and purged with He at 400 °C to remove the moisture. Then,
the sample was reduced under H2 flow at 400 °C for 2 h. After the re-
duction, the sample was purged again with He to remove hydrogen and
cooled to 100 °C. The sample was subsequently submitted to tempera-
ture programmed desorption (TPD) with He at the heating rate of 10 °C
min−1 to 800 °C A stoichiometry of H2 to metal atom of 1:1 was as-
sumed for calculating the metal surface area and metallic dispersion. It
should also be noted that, after catalyst testing, the post-reaction tem-
perature programmed oxidation (TPO) was carried out to identify the
quantity of carbon deposited on the catalyst surface by feeding 10% O2
in He into the system while increasing the temperature from room
temperature to 1000 °C (with the rate of 10 °C min-1, after purging the
post-reaction catalyst bed with helium). The quantity of carbon was
calculated by measuring the amounts of CO and CO2 generated from the
test. The calibrations of CO and CO2 were performed by injecting
known amounts of these gases into an injection valve in the bypass line,
from which the response factors were known by dividing the number of
moles of each component with the respective areas under the peaks. It
should also be noted that the spent catalysts after TPO were further
tested with thermo gravimetric analyzer-mass spectrometer
(PerkinElmer, USA) at 1000 °C to ensure that no carbon formation re-
mained on the surface of the catalysts.
2. Experimental
2.1. Catalyst preparation
The catalysts used in the present work were prepared by the im-
pregnation technique. Commercial γ-Al2O3 (Sasol, Germany) and Gd-
CeO2 (GDC; Fuel Cell Materials, USA) were crushed and sieved to ob-
tain the particle size of 0.425 mm. To prepare the catalyst precursor,
calculated amount of the required metal of nickel (II) nitrate hexahy-
drate (Ni(NO3)2⋅6H2O, Univar, Australia), ammonium perrhenate
(H4NO4Re, Sigma-Aldrich, Germany) and rhodium (III) nitrate (Rh
2.3. Apparatus and procedures
CPOM was carried out in a tubular quartz reactor with high-purity
methane (CH4, UHP grade 99.999%), oxygen (O2, UHP grade 99.995%)
and argon (Ar, UHP grade 99.995%). In order to test the catalysts,
about 100 mg of each catalyst was filled inside the reactor and then
reduced by H2 at 500 °C for 3 h. According to the temperature
2