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N. Kumar et al. / Journal of Catalysis 289 (2012) 218–226
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2 h at 300 °C in flowing H2/He mixture (50% H2). CO hydrogenation
reactions were carried out with space velocity of
2. Experimental
a
24,000 scc hꢁ1 g catꢁ1 and an H2/CO ratio of 2:1. For these experi-
ments, the syngas was diluted with helium to reduce heat effects
within the bed and to ensure that the conversion was low enough
to keep the oxygenated products in the vapor state for online GC/
FID analysis. In addition, the line from the reactor exit to the sam-
pling valve was heattraced to prevent products from condensing up-
stream of the GC/FID. The sampling valves are placed in an
isothermal (90 °C) oven. The GC/FID system (Shimadzu GC-2014)
is equipped with two thermal conductivity detectors (TCD), used
to analyze CO, CO2, and H2. Oxygenates and hydrocarbons analysis
are done using a Restek™ RT-Q Bond column (25 m) connected to
the flame ionization detector (FID). Helium was used as a carrier
gas for FID column and CO/CO2 TCD column, while N2 is used for
the H2 TCD column. The FID column oven was programed to give
the best possible separation of the products without co-elution. All
selectivities are reported in terms of carbon efficiency defined as:
2.1. Catalyst preparation
Co–Pd/SiO2-based catalysts were synthesized using a conven-
tional incipient wetness impregnation method to produce two cata-
lysts, both with 2 wt% Pd but different cobalt loadings: 2 wt% and
10 wt% (designated as 2Co–2Pd and 10Co–2Pd respectively). The
SiO2 support was obtained from Alfa Aesar (Surface Area = 300 m2/
g, Pore Volume = 1 cc/g). The precursors used for cobalt and palla-
dium were cobalt nitrate and palladium (II) 2,4-pentanedionate
[Pd(CH3COCHCOCH3)2] respectively. These salts were dissolved in
a calculated amount of ethanol before impregnating over SiO2. The
catalysts were dried overnight at 110 °C and calcined under air for
2 h at 450 °C at a temperature ramp of 1 °C per minute.
2.2. Inductively coupled plasma-optical emission spectrometry
(ICP-OES)
n ꢂ ðCnÞA
The bulk elemental composition was measured using a Perkin
Elmer 2000 DV ICP-optical emission spectrometer. A repeat sample
analysis was carried out to estimate the experimental error.
Selectivity of A ð%Þ ¼
ꢂ 100
Total CO reacted
where n is the number of carbons in A and Cn)A is mol fraction of A.
The GC/FID system was calibrated with standard certified gas
mixtures prior to the experiment. Different levels of concentration
were used for the calibration, and a curve fitting was done between
the points obtained. The calibration was checked after each com-
pleted experiment to ensure the validity of the data reported.
2.3. Temperature-programed reduction (TPR)
Temperature-programed reduction (TPR) profiles of the cal-
cined catalyst were recorded using an Altamira AMI 200-R-HP unit
equipped with a thermal conductivity detector (TCD). The catalyst
sample was first purged in a fixed-bed micro-reactor system under
flowing argon at 150 °C for 1 h to remove traces of water and then
cooled to 25 °C. TPR was performed using a 10% H2/Ar mixture at a
flow rate of 50 cm3/min while the temperature was linearly
ramped from 25 °C to 750 °C at 10 °C/min.
2.6. In situ diffuse reflectance FTIR spectroscopy (DRIFTS)
FTIR spectra were collected with a Nicolet 6700 model (Thermo
Scientific) spectrometer equipped with an MCT-A detector cooled
by liquid nitrogen. KBr beamsplitter was used to obtain spectra
in the range of 4000–650 cmꢁ1. In situ measurements were carried
out in a specially designed environmental chamber (Harrick)
equipped with a gas inlet, outlet, and a heating/cooling system. A
sample holder was used to hold ꢀ20 mg of catalyst. DRIFTS spectra
were collected by using series collection for 30 min. For each spec-
trum, 32 scans at a resolution of 4 cmꢁ1 were used.
2.4. In situ X-ray diffraction (XRD)
In situ XRD experiments were carried out at Center for Nano-
phase Materials Sciences (CNMS) at Oak Ridge National Laboratory
(ORNL). These experiments were done with PANalytical X’Pert Pro
MPD X-ray diffractometer using Cu Ka radiation (k = 1.5406 Å). The
Before each experiment, the catalyst was heated in helium at
150 °C for 30 min to remove any moisture and gases. The catalyst
was reduced by flowing a mixture of hydrogen and helium (10%
H2 in He) for 2 h at 300 °C. The cell was then flushed with helium
and brought to the desired reaction temperature (230 °C and
270 °C). Backgrounds were collected at desired temperatures after
the system was allowed to equilibrate for 15 min at that tempera-
ture. Difference spectra were obtained by subtracting the back-
ground from the subsequent spectra. Two series of experiments
were performed at each temperature: CO adsorption and CO
hydrogenation. Each series was set for 30 min and was divided into
three parts. In the first part, helium was flowed for 20 s followed by
flowing CO + He for 5 min in the second part. The third part con-
sisted of flushing with helium (for CO adsorption studies) or flow-
ing H2 + He (for CO hydrogenation studies) for the rest of the time.
5% CO/He was used for CO adsorption and 10% H2/He for CO hydro-
genation experiments. The experiments were carried out at 230 °C
and 270 °C and performed at atmospheric pressure.
sample was mounted in an Anton Paar XRK hot stage, which can go
up to 900 °C and 10 bar pressure and has the provision for gas flow.
Fresh calcined catalyst was used for the in situ XRD experiments.
The gases used for the experiments are the following:
1. 2% CO, 2% Ar, balance He.
2. 4% H2, balance He.
The catalyst was crushed to fine powder (ꢀ200 mesh) before
loading in the sample chamber. The sample chamber is provided
with gas inlet and outlet lines. Scans were taken in flowing H2/He
(50 sccm). The time taken for one scan was approximately 30 min,
and the angle was varied from 15° to 70° (step size = 0.0167113°).
The catalyst was held at the intended scan temperature during the
30 min scan.
Scans were taken at room temperature, 150 °C, 200 °C, 250 °C,
and 300 °C in flowing H2/He. Between scans, the temperature was
increased at a rate of 20 °C/min. Data analysis and peak identifica-
tion were done using the software X’Pert HighScorePlus (v3.0).
3. Results and discussion
2.5. Catalyst activity test
3.1. ICP-OES
CO hydrogenation reactions at differential conversions were car-
ried out in a 1 in. glass-lined stainless steel fixed-bed micro-reactor
system at different temperatures (230 °C and 270 °C) and total pres-
sure of 10 bar. Prior to reaction, the catalyst was reduced in situ for
The results for ICP-OES are presented in Table 1. The numbers
indicate metal wt%. The metal loadings are close to their intended
values.
4