Fischer Tropsch Synthesis using promoted cobalt-based catalysts

Article abstract of DOI:10.1016/j.cattod.2016.04.012

A series of CoMnO_x catalysts with lanthanum and phosphorus promoters were prepared by wet impregnation and investigated for syngas conversion to hydrocarbons activity via Fischer Tropsch Synthesis. The effects of the promoters on the catalyst structure were examined by ICP, XRD, TPR and XPS measurements. The results of the catalytic tests showed that the addition of promoters altered the product selectivities when compared to the unpromoted catalyst.

Full text of DOI:10.1016/j.cattod.2016.04.012

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Fischer Tropsch Synthesis using promoted cobalt-based catalysts
Sarwat Iqbala, Thomas E. Daviesa, James S. Haywarda, David J. Morgana, Khalid Karimb,
Jonathan K. Bartleya, Stuart H. Taylora, Graham J. Hutchingsa,∗
a Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK
b SABIC Technology and Innovation, P.O. Box 42503, Riyadh 11551, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of CoMnOx catalysts with lanthanum and phosphorus promoters were prepared by wet impreg-
nation and investigated for syngas conversion to hydrocarbons activity via Fischer Tropsch Synthesis.
The effects of the promoters on the catalyst structure were examined by ICP, XRD, TPR and XPS mea-
surements. The results of the catalytic tests showed that the addition of promoters altered the product
selectivities when compared to the unpromoted catalyst.
Received 6 April 2016
Accepted 7 April 2016
Available online xxx
Keywords:
Fischer Tropsch Synthesis
CoMnOx
© 2016 Elsevier B.V. All rights reserved.
Promoters
Lanthanum
Phosphorus
Syngas
1. Introduction
In addition to the promotional effect of rare earth metals, some
studies have also shown a profound promotion effect from the
Fischer Tropsch Synthesis (FTS) is an important process for
the synthesis of hydrocarbons from syngas mixtures (CO + H2).
Recently interest in FTS has increased as a promising alternative
process for the production of low-sulphur fuels from the conversion
of syngas.
addition of group 15 elements, particularly phosphorus on sup-
ported metal oxides for the FTS reaction [8–10]. Bae et al. [11]
reported an increase in FTS activity and C5+ selectivity by modi-
fication of a Co/Al2O3 catalyst by the addition of an appropriate
amount of phosphorus (1–2 wt%). Other studies have reported an
increase in the reducibility of cobalt species, which was shown to
have originated from the weak interaction between cobalt and the
phosphorus-modified surface. This weak interaction arises from
the partial transformation of the Al2O3 surface to aluminium phos-
phate [12]. An increase in catalytic activity and stability has been
reported by promotion of a Ru/Co/SiO2 catalyst with Zr and P.
Addition of an appropriate amount of Zr and P into the oxide
supported catalyst was postulated to have prevented the cobalt
particle aggregation and enhanced the stability of the catalyst
[13].
Although a significant amount of work regarding the effect of
La and/or P promoter on Co based FTS catalysts has been reported,
the main focus has been on Al2O3, SiO2, TiO2, and activated car-
bon supported catalysts. However, it appears that no study in the
open literature has been devoted to the effect of these promoters
on CoMnOx catalysts. Therefore, it would be interesting to examine
the impact of both La and P addition on a CoMnOx catalyst. In the
present study we have compared the effect of addition of La and
P dopants on the catalytic activity of CoMnOx catalysts. Further-
more the effect of sequential addition of both dopants to a CoMnOx
catalyst has also been studied.
CoMnOx catalysts have been extensively studied for the con-
version of syngas to light hydrocarbons, due to their relatively low
selectivity towards CO2 and CH4 [1–3]. Further to this, the addition
of rare-earth metals into Co-based catalysts has been reported to be
beneficial in FTS [4,5], and the promotional effect of lanthanum on
supported metal oxides in particular has been widely studied. Bar-
rault et al. [5] reported an increase in specific activity for FTS by the
addition of a La promoter to supported Co catalysts. A La/Co/Al2O3
catalyst system was studied by Vada et al. [4], and it was observed
that low loading of La (La/Co = 0.05) increased the overall catalytic
activity in the FTS reaction. The effect of La addition on the prop-
erties of precipitated metal oxides, such as Fe-based catalysts, has
also been reported [6]. Haddad et al. [7] performed a characteriza-
tion study of La-promoted Co/SiO2 systems and showed that La(III)
can regulate the strong Co-support interactions and the reducibility
of Co oxide.
∗
Corresponding author.
E-mail address: hutch@cardiff.ac.uk (G.J. Hutchings).
http://dx.doi.org/10.1016/j.cattod.2016.04.012
0920-5861/© 2016 Elsevier B.V. All rights reserved.
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Table 1
2
2. Experimental
ICP elemental analysis of La and P content in the catalysts.
2.1. Catalyst preparation
Catalysts
La content (wt%)
Theoretical
P content (wt%)
Theoretical
Measured
Measured
2.1.1. CoMnOx
La0.05
La0.1
La0.5
La1
P0.03
P0.05
P0.1
La0.1-P0.03
La0.1-P0.05
0.05
0.10
0.50
1.0
0
0
0
0.1
0.1
0.03
0.08
0.20
1.10
0.00
0.00
0.00
0.07
0.13
0
0
0
0
0.03
0.05
0.10
0.03
0.05
0.00
0.00
0.00
0.00
0.01
0.06
0.08
0.01
0.02
CoMnOx catalysts were prepared according to the procedure
used previously [8,9]. An aqueous solution was prepared containing
equimolar amounts of cobalt nitrate hexahydrate (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 dis-
tilled water (1 dm3, 80 ◦C), and dried (110 ◦C, 16 h).
at 144 W (12 mA × 12 kV). High resolution and survey scans were
performed at pass energies of 40 and 160 eV respectively. Spec-
tra were calibrated to the C (1s) signal at 284.8 eV, and quantified
using CasaXPS v2.3.17, utilizing sensitivity factors supplied by the
manufacturer.
2.1.2. CoMnOx-La and CoMnOx-P
The addition of La and P dopants was performed by the
wet impregnation method. For the synthesis of CoMnOx-La, the
required amount of lanthanum nitrate (La(NO3)3·6H2O, Sigma
Aldrich, 99.999%) was dissolved in 2 cm3 of deionized water
and stirred for approximately 15 min until a clear solution was
obtained. The dried precursor form of CoMnOx was added to the
solution and stirred at room temperature to form a paste. The paste
was subsequently dried (110 ◦C, 16 h). The resulting material was
calcined in static air (500 ◦C, 24 h, with a ramp rate from ambient
of 20 ◦C min−1).
2.3.2. Powder X-ray diffraction (XRD)
Powder X-ray diffraction (XRD) was performed on a X’Pert Pro
diffractometer with a monochromatic Cu-K␣ source (ꢀ = 0.154 nm)
operated at 40 kV and 40 mA. Data were collected over a 2ꢁ range
from 10◦ to 80◦, and phases identified by matching with the ICDD
database.
CoMnOx-P catalysts were prepared by following the same
method, except that ammonium phosphate (NH4H2PO4, sigma
Aldrich, 98%) was used as a phosphorus source instead of lan-
thanum nitrate.
2.3.3. Temperature programmed reduction (TPR)
Temperature programmed reduction/oxidation was carried
out using a TPDRO 1100 series analyser. Samples (25 mg) were
pre-treated for 1 h at 130 ◦C (20 ◦C min−1) in a flow of Argon
(20 cm3 min−1). Following this the gas flow was changed to 10%
2.1.3. CoMnOx-La-P
H2/Ar and the temperature was ramped to 800 ◦C (10 ◦C min−1
)
In order to prepare the CoMnOx-La-P catalysts, CoMnOx-La was
first prepared by wet impregnation as reported above, followed by
a drying step (110 ◦C, 16 h). In the next step the P dopant was added
by wet impregnation. The resulting slurry was dried (110 ◦C, 16 h)
and calcined in static air (500 ◦C, 24 h, and 20 ◦C min−1).
with a 5 min hold at the Tmax. H2/O2 consumption was monitored
using a TCD detector.
2.3.4. Inductively Coupled Plasma (ICP-AES)
Catalyst bulk elemental composition was determined by Induc-
tively Coupled Plasma (ICP) (ICPE-9000, Shimadzu). 25 mg of the
calcined catalyst was dissolved in 0.5 cm3 of aqua regia at room
temperature overnight. The solutions obtained were diluted with
distilled water up to 10 ml. The prepared solutions were subjected
to ICP analysis. For calibration, standard solutions of 1000 ppm of
La and 1000 ppm of P were mixed and diluted to 0.2, 0.5, 1.0, 2.5,
and 5.0 ppm.
Analysis showed that the amounts of dopant were in general
agreement with the theoretical amounts. The error in P content
was consistently within 0.03 wt%, and the error in La content was
within 0.02 wt%. One exception to this was La0.5, which had an
error of −0.3 wt%. The full data are shown in Table 1.
2.2. Catalytic activity
The catalysts were pelleted and sieved (0.65–0.85 mm), before
samples (0.5 g) were loaded into stainless steel fixed bed reactors
(internal diameter 8 mm). Catalysts were reduced in situ at 400 ◦C
for 16 h in pure hydrogen (GHSV = 600 h−1) before being allowed to
cool to room temperature. The reactor was subsequently pressur-
ized to 6 barg with syngas (CO:H2 = 1:1 molar ratio). All catalysts
were tested under the same reaction conditions, (240 ◦C, 6 barg,
GHSV = 600 h−1).
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 3800 GC. Hydrocarbons were analysed using a CP-Al2O3/KCl
column and a flame ionisation detector. Permanent gases and C1-
C4 hydrocarbons were analysed using molecular sieve 13X and
Poropak Q columns with TCD and FID detectors in series. The prod-
uct stream was cooled in a wax trap (∼25 ◦C) to retain the liquid
products. Calibrations were performed with standard samples (C1-
C5 hydrocarbon mixture diluted with nitrogen, BOC certified) for
data quantification. Nitrogen was used as an internal standard to
correct for contraction of the gas volume following reaction.
3. Results and discussion
3.1. CoMnOx-P
The catalysts doped with various loadings of P were tested
for FTS activity under identical conditions and the results are
presented in Table 2. A comparison of the CoMnOx and CoMnOx-
P catalyst performance indicates that all the P doped catalysts
showed a lower CO conversion and CO2 selectivity. The addition of
phosporus increased the selectivity to alkenes particularly propene
and butene. It was apparent that increasing the phosphorus content
decreased the selectivity to alkenes, and increased the CH4 selec-
tivity. All of these catalysts required around 90 h to stabilize and no
deactivation was observed after the stabilization was achieved dur-
ing the period of our investigation. The deactivation of the catalyst
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 operating
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Table 2
Effect of addition of P promoter on FTS activity of CoMnOx catalyst.
CoMnOx
CO conversion (%) 35.5
CoMnOx-P0.03
30
CoMnOx-P0.05
31
CoMnOx-P0.1
25.8
Product selectivity (%)
CH4
C2-C4
C5+
CO2
Alcohols
22.1
17.2
17.0
37.0
6.7
17.0
37.0
17.0
27.0
2.0
19.0
40.9
19.0
21.0
0.1
25.0
32.8
17.0
25.0
0.2
Reaction conditions: Catalyst 0.5 g, 240 ◦C, data collected at 110 h, 6 Barg, CO:H2
1:1 mol ratio, GHSV 600 h−1
.
Fig. 2. TPR profiles of P-doped CoMnOx catalysts.
the catalysts. This is likely due to the low amounts of the promoter
present, and also due to their addition on to the surface, as opposed
to incorporation into the bulk structure.
The TPR profiles and the corresponding data of the CoMnOx and
CoMnOx-P doped catalysts are displayed in Fig. 2 and Table 3. For
each catalyst two distinct reduction signals could be observed with
a small signal at lower temperature. The first signal at 240–260 ◦C
(A) is assigned to the reduction of Co(III) to Co(II); this would seem
to imply the presence of at least some Co3O4 in the material. Based
on this, it is probably that the shoulders observed in the XRD at
an angle of 36.06◦ can be assigned to discrete cobalt oxide phases.
The second TPR signal observed between 400 and 440 ◦C (B) can
be assigned to the reduction of Co(II) to Co(0) [14,15]. The third
signal (C) is attributed to the subsequent reduction of the oxides of
manganese [15,16].
Results of H2-TPR showed that even the lowest loading (P0.03) of
P promoter on CoMnOx could greatly influence the reduction tem-
perature of Co and Mn oxides. The addition of 0.03% P to CoMnOx
resulted in an increase in the reduction temperature of both Co
and Mn oxide phases. Further increase in the amount of P to 0.05%
showed further increase in the reduction of Co(II) oxide signal by
16 ◦C and also the reduction temperature of manganese oxide (C)
was increased by 22 ◦C. P0.1 showed a slight increase in the reduc-
tion temperature of Co(II) and almost no change in the Mn oxide
reduction signal. The general trend was that increasing the amount
of phosphorus dopant increased the reduction temperatures of all
the observed signals. A decline in the reducibility of both oxides
was obvious; since the activity of cobalt catalysts for FTS is linked
to the reducibility, this is thought to have contributed to a loss in
the catalytic activity.
Fig. 1. XRD pattern of P doped catalysts.
by the addition of P runs contrary to the literature on phosphorus
promotion in Co/Al2O3 [11–13]. However, it should be noted that
the promotion effect in all previous studies was concluded to stem
from the formation of a weakly-interacting aluminium phosphate
support phase. In this study, where no support is used and the phos-
phorus is directly applied to the surface of the catalyst, the P does
not act as a promoter, instead it appears to act by blocking access
to the active sites.
All catalysts doped with P were characterised by XRD and the
data set is presented in Fig. 1. These catalysts showed the diffraction
pattern for the mixed Co and Mn spinel oxide (with an interme-
diate tetragonal structure between CoMn2O4 and Mn3O4) (ICDD
018-0408). The characteristic reflections of mixed CoMn2O4 phases
remained at the same position upon impregnation of P at all levels.
No reflections associated with the P containing components were
observed. Small shoulders were visible on the lower-angle side of
the main reflection at 36.06◦. It is difficult to directly assign these
reflections, as reflections will occur in this region for CoO, Co3O4,
Mn3O4 and CoMn2O4. Whilst the overall pattern is one of the mixed
metal spinel structure, the potential overlap of reflections makes it
difficult to definitively state the full phase composition. XRD analy-
sis was unable to resolve any structural differences between any of
Hydrogen consumption data for each reduction signal is pre-
sented in Table 3. The quantity of hydrogen consumed for the
reduction of Co(II) to Co(0) was found to be similar for all the
P-doped catalysts, but the hydrogen consumption for Mn oxide
Table 3
Reduction data from TPR analysis of the P-doped CoMnOx catalysts.
Catalysts
Signal A (◦C)
Hydrogen
Signal B (◦C)
Hydrogen
Signal C (◦C)
Hydrogen
consumption (␮mol/g)
consumption (␮mol/g)
consumption (␮mol/g)
CoMnOx
P0.03
P0.05
239
239
254
267
33
33
10
21
402
418
434
439
2551
2545
2588
2538
530
577
599
597
3435
3705
3279
2865
P0.1
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Table 4
XPS derived molar concentrations for undoped and P-doped CoMnOx catalysts.
Catalysts
Concentration/at%
Co/Mn
Co + Mn/OLattice
Co (2p)
Mn (2p)
O (1s)
P (2p)
CoMnOx
P0.03
P0.05
15.34
15.05
14.81
13.48
23.2
61.46
62.03
62.05
62.74
0
0.66
0.68
0.68
0.62
0.76
0.78
0.78
0.80
22.07
21.92
21.58
0.85
1.22
2.2
P0.1
reduction increased for P0.03 and then decreased with the addition
of more P into CoMnOx catalyst.
To ascertain the concentration and oxidation states of the syn-
thesised catalysts, XPS analysis was performed and the derived
molar ratios are given in Table 4. For clarity the molar ratios
have been corrected for the adventitious carbon. For the undoped
CoMnOx catalyst the Mn(2p3/2) and Co(2p3/2) binding energies
were 642.2 eV and 780.6 eV respectively; the latter is characteris-
tic of cobalt in Co-Mn mixed metal oxides [17] and CoO [18–20]
with the satellite structure encompassing the energy range ca.
785–790 eV due to the 3d → 4s shake-up process and suggests the
presence of Co(II).
The Mn binding energies were more difficult to directly assign
due to the similarity in binding energies of Mn compounds and
potentially weak satellite structure which may aid elucidation [20],
however the multiplet splitting of the Mn(3s) signal maybe used
to further ascertain the nature of the Mn species and here was
measured at 5.4 ( 0.2) eV suggestive of Mn(III).
Fig. 3. XRD pattern of La doped catalysts.
Table 6
XPS derived molar concentrations for La-doped CoMnOx catalysts.
Catalysts
Concentration/at%
Co/Mn
Co + Mn/OLattice
Co (2p)
Mn (2p)
O (1s)
La (3d)
La0.05
La0.1
La0.5
La1.0
16.48
14.90
12.05
12.31
22.31
22.78
24.27
22.44
60.86
62.03
62.11
63.31
0.36
0.28
1.57
1.94
0.74
0.65
0.50
0.55
0.78
0.79
0.75
0.68
The surface of the undoped sample also comprised of adven-
titious carbon and small amounts of carbonate and hydroxide,
as evidenced by the asymmetry of the O(1s) signal to the high
binding energy side (not shown). Fitting of the lattice oxygen and
hydroxide/C-O species revealed a (Co + Mn)/O ratio of 0.76 in excel-
lent agreement with the bulk stoichiometry expected for tetragonal
CoMn2O4 (i.e. 0.75). However, the Co/Mn ratio is slightly higher
than that expected from the bulk stoichiometry possibly suggesting
some segregation of cobalt oxide in the near surface region.
Table 4 shows the molar concentrations of the P-doped sam-
ples. Phosphorus was characterised by a binding energy of 133.3 eV,
up to 0.5 and 1% showed a significant decline in the CO conversion
and also the selectivity to methane and carbon dioxide increased
markedly. The decrease in CO conversion upon addition of La into
the CoMnOx catalyst and a shift of the selectivity towards lower
carbon-number products is similar to the effects of potassium
doping, in which the dopant blocks CO dissociating sites [21].
Lowering the dissociative ability of the catalyst would cause the
effects observed. The addition of phosphorus appears to have a
similar effect, although it was less pronounced; it is possible that
the lanthanum is selectively binding to certain sites, whereas the
acidic phosphorus is non-selectively blocking the surface. This is
somewhat in agreement with the conclusions of Prieto et al. [22].
The XRD patterns of CoMnOx-La catalysts are shown in Fig. 3.
All of the La doped catalysts were found to be very similar to the
pure CoMnOx catalyst; no additional phases were observed and the
reflections remained at the same position. All the catalysts showed
the diffraction pattern for the mixed spinel oxide of Co and Mn
(with an intermediate tetragonal structure between CoMn2O4 and
Mn3O4) (ICDD 018-0408).
4−
characteristic of phosphate (PO43−) or pyrophosphate (P2O7
)
[20]. The Co and Mn (2p) spectra were identical in energy and shape
to that of the undoped sample whilst the combined metal to lattice
oxygen ratio was slightly higher than that of the undoped CoMnOx
(ca. 0.80 vs 0.76).
3.2. CoMnOx-La
The catalysts doped with various loadings of lanthanum were
tested for FTS activity under identical conditions and the data are
presented in Table 5. All the La-doped catalysts displayed lower
CO conversion, lower selectivity to CO2 and higher selectivity to
alkenes. The selectivity to CO2 and CH4 was decreased markedly
for a lower loading of La (0.05 and 0.1%). A further increase in La
Surface molar concentrations of the elements present for the
CoMnOx-La catalysts, derived by XPS, are presented in Table 6.
The analysis revealed the La to be present in its 3+ state with a
Table 5
Effect of addition of La promoter on FTS activity of CoMnOx catalyst.
CoMnOx
CoMnOx-La0.05
33
CoMnOx-La0.1
31
CoMnOx-La0.5
21.8
CoMnOx-La1
18.8
CO conversion (%)
36
Product selectivity (%)
CH4
C2-C4
C5+
CO2
Alcohols
22.1
17.2
17.0
37.0
6.7
17.0
27.0
31.0
24.5
0.3
15.0
37.6
28.0
19.0
0.4
21.7
31.0
15.0
31.0
1.3
26
20.5
13.5
38.5
1.9
Reaction conditions: Catalyst 0.5 g, 240 ◦C, data collected at 130 h, 6 Barg, CO:H2 1:1 mol ratio, GHSV 600 h−1
.
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Table 7
Reduction data from TPR analysis of the La-doped catalysts.
Signal A (◦C)
Hydrogen
Signal B (◦C)
Hydrogen consumption
Signal C (◦C)
Hydrogen
consumption (␮mol/g)
(␮mol/g)
consumption (␮mol/g)
CoMnOx
La0.05
La0.1
La0.5
La1
238
241
246
251
265
32
13
48
77
35
401
416
402
421
455
2557
2711
2534
2503
2628
537
582
594
643
685
3633
3994
3592
3566
3528
Table 8
Effect of a combined addition of La and P promoters on CoMnOx.
La(3d5/2) binding energy of 834.6 eV characteristic of La2O3. For
the La-doped samples the Mn region remains consistent with that
of the un-doped spinel and consistent with the XRD data which
suggests no bulk incorporation. A decrease in the amount of Co
was observed with an increase in La content.
CoMnOx-La0.1 CoMnOx-La0.1-P0.03 CoMnOx-La0.1-P0.05
CO conversion (%) 31
29.8
29.0
Product selectivity (%)
CH4
C2-C4
C5+
CO2
Alcohols
15.0
37.6
28.0
19.0
0.4
11.0
46.5
26.0
14.5
2.0
13.7
52.0
19.0
13.0
2.3
The effect of La addition on the reduction profile of La-doped
catalysts is shown in Fig. 4 and Table 7. Addition of 0.05% La into
CoMnOx increased the reduction temperature of Co(II) (B) by 15 ◦C
and a shift of 45 ◦C towards higher temperature was observed in the
reduction temperature of Mn oxide (C). Further addition of 0.1 and
0.5% La further increased the reduction temperature of Co oxide and
Mn oxide reduction signals. The catalyst doped with 1% La showed
the highest reduction temperature amongst the series of La doped
catalysts for the reduction of both Co(II) and Mn oxide reduction
signals. Hydrogen consumption data for La-doped catalysts showed
a very little change in the reduction signal of Co(II). The amount
of hydrogen consumed for Mn oxide reduction increased with an
addition of 0.05% La and decreased on further addition of La.
It has been reported that La addition into the supported Co cat-
alysts can facilitate the cobalt oxide reduction, particularly for the
Co(II) to Co(0) which provides more cobalt active sites and there-
fore an increase in the catalytic activity [23]. However, we have
observed an increase in the reduction temperature with an addition
of La promoter into CoMnOx catalyst, and hence a significant loss
of catalytic activity. The blocking of CO dissociative sites is possible
with La addition as previously reported for potassium promoters
[21,24], which can lead to a loss in catalytic activity. Increasing the
La loading on the catalysts lowered the surface concentration of
Co implying that the La is primarily positioning itself on the cobalt
surface, which is in agreement with the theory that it is selectively
blocking the CO dissociation sites on the cobalt in a manner similar
to potassium.
Reaction conditions: Catalyst 0.5 g, 240 ◦C, data collected at 110 h, 6 Barg, CO:H2
1:1 mol ratio, GHSV 600 h−1
.
3.3. CoMnOx-La-P
The catalysts doped both with La and P were tested and the cat-
alytic data is presented in Table 8. The catalyst prepared with the
sequential addition of 0.1% of La and 0.03% of P showed a significant
increase in the selectivity towards alkenes and a lower CO conver-
sion compared with the CoMnOx-La0.1 catalyst. This catalyst also
showed low selectivities to CH4 and CO2. Further increase in the
amount of P to 0.05% showed a comparable product spectrum and
the same activity as P0.03 catalyst.
The XRD patterns of CoMnOx-La-P are shown in Fig. 5. Both of
these catalysts were found to be similar to the P-doped catalysts.
The characteristic reflections of CoMn2O4 did not show any shift
in their position on addition of dopants. Again like P and La doped
catalysts, no phases associated with the P or La were observed. As
before, shoulder reflections can be observed, and are tentatively
assigned to Co3O4.
The TPR profiles and the corresponding data are shown in Fig. 6
and Table 9. As was seen with the CoMnOx-P series of catalysts,
increasing the loading of the phosphorus promoter has increased
the reduction temperature of both the Co(II) and Mn(x) reduction
Fig. 4. TPR profiles of La-doped CoMnOx catalysts.
Fig. 5. XRD pattern of La and P doped catalysts; ᭹ Co3O4.
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Table 9
Reduction data from TPR analysis of the La-P-doped catalysts.
Catalysts
Signal A (◦C)
Hydrogen
Signal B (◦C)
Hydrogen consumption
Signal C (◦C)
Hydrogen
consumption (␮mol/g)
(␮mol/g)
consumption (␮mol/g)
La0.1
La0.1-P0.03
La0.1-P0.05
246
240
239
47
24
17
402
418
425
2534
2654
2922
594
604
621
3593
4149
3669
Table 10
XPS derived molar concentrations for La and P-doped CoMnOx catalysts.
Catalysts
Concentration/at%
Co (2p)
Co/Mn
Co + Mn/OLattice
Mn (2p)
O (1s)
La (3d)
P (2p)
La0.1-P0.03
La0.1-P0.05
14.35
14.13
22.44
22.05
62.13
62.2
0.34
0.63
0.74
0.99
0.64
0.64
0.77
0.78
activity. When La and P added were added by impregnation to
the CoMnOx catalyst an increased selectivity to hydrocarbons was
observed and the selectivity to CH4 and CO2 was decreased in com-
parison to the undoped CoMnOx catalyst. In addition a considerable
decrease in CO conversion was observed. La appears to selectively
block Co sites on the surface, and promotes the formation of lower
carbon number products.
Acknowledgement
We would like to thank Sabic for financial support.
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We have investigated and contrasted the effect of La and P
dopants on cobalt manganese oxide (CoMnOx) catalyst for FTS
Please cite this article in press as: S. Iqbal, et al., Fischer Tropsch Synthesis using promoted cobalt-based catalysts, Catal. Today (2016),
http://dx.doi.org/10.1016/j.cattod.2016.04.012