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CATTOD-9841; No. of Pages5
ARTICLE IN PRESS
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S. Iqbal et al. / Catalysis Today xxx (2015) xxx–xxx
precipitate ageing time on the catalytic activity of CoMnOx cata-
lysts supported on carbon. Both of these factors have been shown
to effect catalytic activity and the product selectivity previously
[17,18].
2.3. Catalyst characterization
2.3.1. X-ray photoelectron spectroscopy (XPS)
XPS was performed using a Kratos Axis Ultra-DLD photoelec-
tron spectrometer, using monochromatic Al K␣ radiation, at 144 W.
High resolution and survey scans were performed at pass ener-
gies of 40 and 160 eV, respectively. Spectra were calibrated to the
C (1s) signal at 284.5 eV, which is typical for graphitic carbon as
measured for HOPG, and quantified using CasaXPS v2.3.15, utilizing
sensitivity factors supplied by the manufacturer.
2. Experimental
2.1. Catalyst preparation
2.1.1. Co-precipitation
CoMnOx catalysts were prepared according to the procedure
given in the patent literature [16,19]. An aqueous solution was
prepared containing equimolar amounts of cobalt nitrate hexahy-
drate (Co(NO3)2·6H2O, Sigma–Aldrich, 99.999%) and manganese
nitrate tetrahydrate (Mn(NO3)2·4H2O, Sigma–Aldrich, ≥98%). This
solution was heated to 80 ◦C and aqueous ammonia (28–30% NH3
in water, Sigma–Aldrich) was added to raise the pH from 2.9 to
8.30 0.01. The resulting precipitate was recovered by filtration,
washed with distilled water (1 dm3, 80 ◦C), dried (110 ◦C, 16 h) and
calcined in static air (500 ◦C, 24 h).
2.3.2. Powder X-ray diffraction (XRD)
XRD measurements were performed using a Bruker AXS Com-
pany, D8 Advance diffractometer. Scans were taken with a 2ꢀ step
size of 0.02◦ and a counting time of 1.0 s using Cu K␣ radiation
source generated at 40 kV and 30 mA. Specimens for XRD were
prepared by compaction into a glass-backed aluminium sample
holder. Data was collected over a 2ꢀ range from 4◦ to 80◦ and phases
identified by matching with the ICDD database.
3. Results and discussion
2.1.2. Deposition precipitation
3.1. Comparison of catalyst performance of CoMnOx and
CoMnOx/C catalysts
CoMnOx/C catalysts were prepared as according to the proce-
dure given in the patent literature [16,19]. An aqueous solution
was prepared containing equimolar amounts of cobalt nitrate
hexahydrate (Co(NO3)2·6H2O, Sigma–Aldrich, 99.999%) and man-
ganese nitrate tetrahydrate (Mn(NO3)2·4H2O, Sigma Aldrich,
≥98%). Coconut shell-derived activated carbon (GCN3070, NORIT)
was added to the mixed nitrate solution to give a final catalyst
with a composition of 20% Co, 20% Mn and 60% activated carbon.
The slurry was stirred for 10 min at 80 ◦C before aqueous ammo-
nia (28–30% NH3 in H2O, Sigma–Aldrich) was added drop wise
to the nitrate solution to raise the pH from 4 to 8.30 0.01. The
resulting precipitate was recovered by filtration, washed with dis-
tilled water (1 L, 80 ◦C), dried (110 ◦C, 16 h) and heated in flowing
He.
The unaged catalysts were tested for the FTS reaction under
identical conditions and the data are presented in Table 1. A
comparison of the CoMnOx and CoMnOx/C catalyst performance
indicates the selectivity to carbon dioxide and methane was
decreased markedly with the carbon-supported catalyst. CO con-
version and the selectivity to C5+ hydrocarbons were higher
compared with the pure CoMnOx catalyst. The effect of time-on-
line is presented in Fig. 1 and this shows that the carbon-supported
catalyst attained steady state after 45 h, whereas the unsupported
CoMnOx catalyst achieved steady state only after 90 h. Neither cat-
alyst showed deactivation over the time period studied.
To study the effect of heat treatment, one batch of catalyst was
divided into four portions which were heated in flowing He sepa-
rately at 300, 400, 500, and 600 ◦C for 5 h.
3.2. Effect of preparation variables on the performance of
CoMnOx/C catalysts
To study the effect of ageing the precipitate was left in the
mother liquor for the specific time intervals of 1, 2, and 3 h followed
by heat treatment in He at 500 ◦C for 5 h.
Pretreatment of the catalyst precursor, particularly the heat
treatment temperature is a critical parameter in catalyst prepa-
ration which can affect the activity and selectivity of the catalysts
[17]. In order to study the effect of the pretreatment, one batch
of CoMnOx/C was heated at different temperatures in He. Inter-
vals of 100 ◦C were selected and although relatively large they are
considered to be appropriate for this initial screening study. CO con-
version and product selectivity data are presented in Table 2 and it
is apparent that there was a steady increase in CO conversion with
increasing heat treatment temperatures from 300 to 500 ◦C. An
2.2. Catalytic activity
The catalysts were pelleted and sieved (0.65–0.85 mm) and 0.5 g
were loaded into stainless steel fixed bed reactors (internal diame-
ter 8 mm). Catalysts were reduced in situ at 400 ◦C for 16 h in pure
hydrogen (GHSV = 600 h−1) then cooled to room temperature and
pressurized to 6 bar with syngas (CO:H2 = 1:1 molar ratio). All cata-
lysts were tested under identical reaction conditions, 240 ◦C, 6 bar,
and GHSV = 600 h−1
.
Table 1
Comparison of the catalytic activity of CoMnOx with CoMnOx/C catalyst.
A stabilization period of ∼100 h was allowed before catalyst
data was collected and the mass balance determined. Analysis
of gas products was performed by on-line gas chromatography
using a Varian GC-3800. Hydrocarbons were analyzed using a CP-
Al2O3/KCl column and a flame ionization detector. Permanent gases
and C1–C4 hydrocarbons were analyzed using molecular sieve 13×
and Poropak Q columns with TCD and FID detectors in series. Nitro-
gen was used as an internal standard. The product stream was
cooled in a wax trap (∼25 ◦C) to retain the liquid products. Calibra-
tions were performed with standard samples (C1–C5 hydrocarbon
mixture diluted with nitrogen, BOC certified) for data quantifica-
tion.
CoMnOx
CoMnOx/C
48
CO conversion (%)
36.0
Product selectivity (%)
CH4
C2
C3
C4
C5+
22.1
4.5
11.5
1.2
17.0
37.0
6.7
7.0
4.3
16.1
7.6
43.4
20.4
2.2
CO2
Alcohols
Reaction conditions: Catalyst 0.5 g, data collected at 135 h, 240 ◦C, 6 bar, CO:H2
1:1 mol ratio, GHSV 600 h−1
.
Please cite this article in press as: S. Iqbal, et al., Fischer Tropsch synthesis using cobalt based carbon catalysts, Catal. Today (2015),