Effects of co-feeding with nitrogen-containing compounds on the performance of supported cobalt and iron catalysts in Fischer–Tropsch synthesis

Article abstract of DOI:10.1016/j.cattod.2015.12.015

The performance of supported cobalt and iron catalysts in low and high temperature Fischer–Tropsch synthesis was investigated in the presence of small amounts of ammonia in syngas. Ammonia was co-fed to the reactor either by in-situ hydrolysis of acetonitrile or by addition of aqueous ammonia. The addition of acetonitrile and ammonia resulted in significant irreversible deactivation of alumina supported cobalt catalysts. Lower methane and higher C₅₊ hydrocarbon selectivities were observed. Iron based catalysts did not show any noticeable deactivation in the presence of acetonitrile or ammonia. Moreover, Fischer–Tropsch reaction rate slightly increased after addition of the nitrogen-containing compounds to silica and alumina supported samples. Lower methane selectivity and higher C₂–C₄ olefin to paraffin ratio were observed in the presence of ammonia when the catalysts were reduced in hydrogen. Iron catalysts activated in carbon monoxide did not demonstrate any significant effect of ammonia on the selectivity. The catalytic data were explained by irreversible formation of inactive cobalt nitrides in cobalt catalysts and transformation of metallic iron and iron oxides in the presence of acetonitrile and ammonia into iron nitrides and iron carbides active in Fischer–Tropsch synthesis.

Full text of DOI:10.1016/j.cattod.2015.12.015

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Effects of co-feeding with nitrogen-containing compounds on the
performance of supported cobalt and iron catalysts in
Fischer–Tropsch synthesis
Vitaly V. Ordomskya, Alexandre Carvalhoa,b, Benoit Legrasa, Sébastien Paula,
Mirella Virginiea, Vitaly L. Sushkevichc, Andrei Y. Khodakova,∗
a Unité de Catalyse et de Chimie du Solide—UMR CNRS 8181, Université Lille, Ecole Centrale de Lille, 59655 Villeneuve d’Ascq Cedex, France
b Department of Chemical Engineering, Federal University of Rio Grande do Sul, Porto Alegre, RS 90040-040, Brazil
c Department of Chemistry, Lomonosov Moscow State University, 119991, Leninskye Gory 1, Moscow, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
The performance of supported cobalt and iron catalysts in low and high temperature Fischer–Tropsch
synthesis was investigated in the presence of small amounts of ammonia in syngas. Ammonia was co-fed
to the reactor either by in-situ hydrolysis of acetonitrile or by addition of aqueous ammonia.
The addition of acetonitrile and ammonia resulted in significant irreversible deactivation of alumina
supported cobalt catalysts. Lower methane and higher C5+ hydrocarbon selectivities were observed. Iron
based catalysts did not show any noticeable deactivation in the presence of acetonitrile or ammonia.
Moreover, Fischer–Tropsch reaction rate slightly increased after addition of the nitrogen-containing com-
pounds to silica and alumina supported samples. Lower methane selectivity and higher C2–C4 olefin to
paraffin ratio were observed in the presence of ammonia when the catalysts were reduced in hydrogen.
Iron catalysts activated in carbon monoxide did not demonstrate any significant effect of ammonia on
the selectivity. The catalytic data were explained by irreversible formation of inactive cobalt nitrides in
cobalt catalysts and transformation of metallic iron and iron oxides in the presence of acetonitrile and
ammonia into iron nitrides and iron carbides active in Fischer–Tropsch synthesis.
Received 12 October 2015
Received in revised form
14 December 2015
Accepted 17 December 2015
Available online xxx
Keywords:
Biosyngas
Fischer–Tropsch
Catalyst
Cobalt
Iron
Nitrides
Nitrogen impurities
Acetonitrile
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
to the presence of impurities in the reaction feed. Thus, purifica-
tion of the syngas is an important and often very expensive part of
The Fischer–Tropsch (FT) technology converts syngas produced
from coal, natural gas, shell gas and biomass into hydrocarbons
and oxygenates which can be further upgraded to fine chemicals
and fuels. Both cobalt and iron catalysts are used for FT synthe-
sis on the industrial scale. Cobalt catalysts are the catalysts of
choice for low temperature FT synthesis, which produces middle
distillates and waxes [1,2], while iron catalysts are mostly used in
high temperature FT synthesis [3] which manufactures both fuels
and petrochemicals. Catalyst deactivation is a major challenge for
both cobalt and iron based FT catalysts [4,5]. The mechanism for
catalyst deactivation in FT synthesis often includes a combina-
tion of different phenomena: poisoning, surface carbon formation,
sintering, carbidization (for cobalt catalysts), oxidation, surface
reconstruction, attrition. . . FT catalysts are particularly sensitive
the whole technology.
Continuous depletion to fossil resources and strategy for diver-
sification of energy supplies have led to continuously growing
interest in the development of new technologies such as BTL
(Biomass-to-Liquids) which aim at producing hydrocarbons and
oxygenates from renewable resources (e.g. lignocellulosic biomass
and organic waste). The biosyngas can be produced from biomass
using gasification. The presence of various impurities and poisons
specific of biosyngas and gasification process such as tar, par-
ticulates, ammonia, hydrochloric acid and sulphur gases [6] can
strongly affect the performance of cobalt and iron FT catalysts. The
particularities of FT synthesis with biosyngas have been discussed
in several reviews [6–9].
Among these impurities, sulphur compounds may cause irre-
versible deactivation of FT catalysts. A number of reports [10–17]
have recently addressed the effect of sulphur impurities on the sta-
bility of iron and cobalt FT catalysts. Much less attention has been
paid so far to the effects of nitrogen compounds in syngas on the
∗
Corresponding author. Fax: +33 3 20 43 65 61.
E-mail address: andrei.khodakov@univ-lille1.fr (A.Y. Khodakov).
http://dx.doi.org/10.1016/j.cattod.2015.12.015
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: V.V. Ordomsky, et al., Effects of co-feeding with nitrogen-containing compounds on the performance
of supported cobalt and iron catalysts in Fischer–Tropsch synthesis, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.12.015

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performance and stability of FT catalysts. The effect of ammonia
[18,19] on the catalytic performance has been shown to be differ-
ent for cobalt and iron FT catalysts. The reported results however,
have been rather contradictory. Borg et al. [15] studied the cat-
alytic performance of alumina and titania supported catalysts in the
presence of 4 ppm of NH3 in syngas. No effect of ammonia on the
catalytic performance was observed. LeVinnes et al. [20] found that
nitrogen compounds produce reversible effect on the catalytic per-
formance of cobalt catalysts. The catalyst activity was restored by
treatment in pure hydrogen. Pendyala et al. [21] studied the effect
of addition of ammonia on the performance of platinum promoted
cobalt/alumina catalysts. A significant irreversible catalyst deac-
tivation was observed at ammonia levels from 1 to 1200 ppmw.
In addition, in the presence of ammonia, the catalyst exhibited
lower methane and higher C5+ hydrocarbon selectivity which were
attributed to selective poisoning of the methanation sites. Ma et al.
[22] studied the effect of different ammonia containing compounds
on the performance of precipitated iron catalysts in a slurry reac-
tor. No deactivation was observed at low ammonia concentrations,
while important catalyst deactivation was observed at concentra-
tions of ammonia higher than 400 ppm. In the work of Sango et al.
[23] significant amounts of ammonia (up to 10 wt.%) were added
to the syngas feed over unsupported iron catalysts. The catalysts
did not shown any noticeable deactivation at the ammonia con-
centration below 2% wt. In addition to the usual FT products such
as hydrocarbons and oxygenates, the reaction yielded long chained
aliphatic amines, nitriles and amides, while the selectivities to alco-
hols, aldehydes and organic acids were much lower in the presence
of ammonia.
were refluxed for 16 h in concentrated HNO3 (65 wt.%) at 120 ◦C in
an oil bath. Then, the mixture was filtered and thoroughly washed
with distilled water until the neutral pH was reached. The washed
carbon nanotubes were then dried overnight at 100 ◦C.
The iron catalysts were prepared by incipient wetness impreg-
nation of the relevant supports with aqueous solutions of hydrous
iron nitrate (Fe(NO3)3·9H2O). The concentrations of the impreg-
nating solutions were calculated to obtain 10 wt.% iron in the final
catalysts. After impregnation the catalysts were dried overnight in
an oven at 100 ◦C. Then the catalysts supported by silica and alu-
mina were calcined in a flow of air, while the carbon supported
samples were calcined in nitrogen flow at 400 ◦C for 6 h with a
1 ◦C/min temperature ramping.
The catalysts are labelled as Mx%/Support, where x indicates
metal content in the catalyst, M stands for metal (Co or Fe) and
Support specifies the support used (Al2O3, SiO2, AC or CNT). The
platinum content is indicated in the Pt-promoted catalysts.
2.2. Characterization techniques
The BET surface area, pore volume and average pore diameter
were determined by N2 physisorption using a Micromeritics ASAP
2000 automated system. The samples were degassed under vacuum
at <10 ␮m Hg in the Micromeritics ASAP 2000 at 300 ◦C for 4 h prior
to N2 physisorption.
The ex situ X-ray powder diffraction (XRD) measurements were
conducted using a Bruker AXS D8 diffractometer using Cu(K␣) radi-
ation (ꢀ = 0.1538 nm). The XRD patterns were collected in 20–70◦
(2ꢁ) range. The identification was carried out by comparison with
JCPDF standard spectra software. The average crystallite size of
Fe3O4, Fe2O3 and Co3O4 was calculated using the diffraction lines
according to Scherrer’s equation [25].
The FTIR spectra were recorded with a Nicolet Protégé 460 FT-
IR spectrometer at 4 cm−1 optical resolution. Due to high cobalt
content, all catalysts were diluted with ␣-Al2O3 (1:1). Prior to the
measurements, 20 mg of sample was pressed in self-supporting
discs and activated in the IR cell attached to a vacuum line at 400 ◦C
for 4 h followed by reduction in hydrogen at 400 ◦C. After the reduc-
tion, water formed was evacuated at 400 ◦C for 1 h. Dose per dose
adsorption of CO was carried out in the low-temperature cell in liq-
uid nitrogen. In several experiments, the catalysts were pre-treated
in situ with NH3 at 280 ◦C. Spectra processing was performed by
OMNIC 7.3 software.
The present report addresses the impact of addition of small
amounts of acetonitrile and ammonia (1500 and 2500 ppmv) to
the syngas feed on the catalytic performance of supported iron
and cobalt catalysts. In particular, the effects of cofed acetonitrile
and ammonia on CO conversion, methane, light olefins, C5+ hydro-
carbon selectivities, catalyst stability and catalyst structure are
discussed in this work. The catalytic performance has been eval-
uated in a Flowrence high-throughput system [24] (Avantium®)
equipped with 16 parallel fixed-bed milli-reactors under typical
conditions of low and high temperature FT syntheses.
2. Experimental
2.1. Catalyst preparation
The cobalt catalysts after catalytic tests in a Flowrence high
throughput unit conducted in the presence of acetonitrile were
characterized by temperature-programmed desorption (TPD). The
samples were weighed (50 mg) and loaded in the middle of a quartz
tube. Both ends of the quartz tube were sandwiched with quartz
wool. The signals were recorded by a mass spectrometer.
Commercial ␥-alumina containing 5% of silica (Al2O3, Siralox
SASOL) was used as support for preparation of cobalt catalysts.
The catalysts were prepared by incipient wetness impregnation
with aqueous solutions of cobalt nitrate. The concentrations of the
impregnation solutions were calculated to obtain 15 wt.% cobalt
in the final catalysts. The alumina supported catalyst contain-
ing 25 wt.% Co was prepared using the two-step impregnation.
In case of Pt-promoted samples, an additional incipient wetness
impregnation of Co/␥-Al2O3 with aqueous solutions of hydrogen
hexachloroplatinate (H2PtCl6), (Sigma–Aldrich) was carried out.
The platinum content was 0.1 wt.% in the final catalysts. After
impregnation and drying the cobalt catalysts were calcined in air
flow at 400 ◦C for 6 h with a 1 ◦C/min temperature ramping.
Iron catalysts were prepared using commercial amorphous sil-
ica (SiO2, CARIACT Q-10, Fuji Silysia), alumina (Siralox, Sasol),
activated carbon and carbon nanotubes. Activated carbon (AC) was
provided by CEKA S.A., washed and then heated with 1 M solution
of nitric acid at 50 ◦C for 2 h. Multi-wall carbon nanotubes (CNT,
purity ≥95%, outer diameter 20–30 nm) were purchased from the
Chengdu Limited Company of Organic Chemistry (CCOC) in China.
They were prepared by chemical vapour deposition. The raw CNTs
The SSITKA apparatus used in this work is described in Ref.
[26]. It contains two independent feed lines. The first line is dedi-
cated to unlabeled compounds and tracer (CO, H2, He and Ne), the
second one to the isotopic compounds (13CO). The pressure trans-
ducers are used to adjust the same pressure drop for both lines.
Isotopic switches were realized using a two-position four ways
Valco-valve and monitored with QMG 432 Omnistar in the Fara-
day mode. In the first experiment, the Co25%0.1%Pt/Al2O3 catalyst
was reduced hydrogen at 400 ◦C for 3 h and then exposed to syngas
(H2/CO = 5) at atmospheric pressure at 220 ◦C. After conducting 13 h
of the reaction in the syngas, the periodic switches were performed
from 12CO/H2/He/Ne to 13CO/H2/He with simultaneous measure-
ment of the isotopic transient responses. In the second experiment,
the catalyst after reduction in hydrogen was exposed to the flow
of gaseous NH3 at 220 ◦C and atmospheric pressure for 2 h. Then
the ammonia flow was stopped and the syngas with H2/CO = 5 was
directed to the catalyst. After conducting 13 h of the FT reaction in
Please cite this article in press as: V.V. Ordomsky, et al., Effects of co-feeding with nitrogen-containing compounds on the performance
of supported cobalt and iron catalysts in Fischer–Tropsch synthesis, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.12.015

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Table 1
of the acid-treated CNT have uniform diameters of about 5 nm. Rel-
atively high surface area of these materials suggests the presence
of open caps.
Supports used for preparation of cobalt and iron catalysts.
Catalyst
N2 adsorption over supports
The crystallite size of cobalt and iron oxides in the calcined sup-
ported catalysts was evaluated from XRD patterns using Scherrer’s
equation. Co3O4 was the only cobalt phase detected by XRD in the
alumina supported catalysts. In agreement with previous reports
[27,28], the cobalt crystallite size was about 10–13 nm and was not
much affected by cobalt content. Hematite (Fe2O3) was the main
iron phase in silica iron supported catalysts with the crystallite size
of 17.5 nm. Very broad XRD patterns relevant to iron oxides were
detected in the alumina supported catalysts. This suggests the pres-
ence of very small particles of iron oxide (<5 nm) and high iron
dispersion on the alumina support.
Magnetite (Fe3O4) was the major iron phase identified in the
iron catalysts supported on carbon supports which were calcined
in nitrogen. Larger magnetite crystallites were observed in car-
bon nanotubes (dFe3O4 = 12.3 nm) compared to activated carbon
(dFe3O4 = 6.7 nm). Further information about the structure of sup-
ported cobalt and iron catalysts used in this work is available from
previous publications [29–34].
SBET, (m2/g)
Vtot, (cm3/g)
Pore size, (nm)
Al2O3
CNT
SiO2
AC
175
163
307
558
0.45
0.56
1.31
0.4
8.3
5
17.5
–
syngas at atmospheric pressure, the switches from 12CO/H2/He/Ne
to 13CO/H2/He were performed on the catalyst pre-treated with
NH3.
2.3. Catalytic tests
Carbon monoxide hydrogenation was carried out in a Flowrence
high throughput unit (Avantium®) [24]. equipped with 16 parallel
milli-fixed reactors (d = 2 mm) operating at total pressure of 20 bar,
H2/CO = 2 molar ratio and GHSV = 3000–16 000 cm3 gcat−1 h−1
.
Prior to the catalytic tests, all the samples were activated in a flow
of H2 at atmospheric pressure during 10 h at 400 ◦C. For compar-
ison, several Fe based catalysts have been also activated in CO at
350 ◦C. During the reduction, the temperature ramp was 3 ◦C/min.
After the reduction, the catalysts were cooled down to 180 ◦C and
a flow of premixed syngas was gradually introduced through the
catalysts. When pressure attained 20 bar, the temperature was
slowly increased to 220 ◦C and 300 ◦C for cobalt and iron cata-
lysts, respectively. The gaseous reaction products were analysed
by on-line gas chromatography. Analysis of permanent gases was
performed using a Molecular Sieve column and a thermal con-
ductivity detector. Carbon dioxide and C1–C4 hydrocarbons were
separated in a PPQ column and analysed by a thermoconductivity
detector. C5–C12 hydrocarbons were analysed using CP-Sil5 column
and a flame-ionization detector. High-molecular-weight products
were collected at atmospheric pressure in the vials heated at 80 ◦C.
The carbon monoxide contained 5% of helium, which was used as
an internal standard for calculating carbon monoxide conversion.
The reaction rates expressed in mol gcat−1 s−1, were defined as the
number of moles of CO converted per second per gram of catalyst.
The product selectivity (S) was reported as the percentage of CO
converted into a given product and is expressed on carbon basis.
The reaction runs with pure syngas were conducted for the first
24 h on stream. Then acetonitrile was introduced to the syngas feed.
Small amounts of acetonitrile or aqueous ammonia were added
using a HPLC pump with liquid flow rates of 5 and 10 ␮L/min to get
respectively 1500 and 2500 ppmv in the gas feed. First, 1500 ppmv
was maintained for 12 h. Then, addition was adjusted to 2500 ppmv
for 12 h and finally stopped for 24 h. The catalyst was then again
exposed to three times to 2500 ppm of CH3CN for 12 h separated
by the periods of 12 h when it was exposed to the pure gas. The
experimental procedure for acetonitrile and ammonia addition is
displayed in Fig. 1.
3.2. FT synthesis with pure syngas
The catalytic performance data of cobalt and iron catalysts in
FT synthesis measured respectively at 220 ◦C and 300 ◦C in pure
syngas are shown in Table 2. Methane, light olefins, light paraf-
fins, C5+ hydrocarbons and water are the main reaction products on
cobalt catalysts under these conditions. Only very small amounts
of carbon dioxide can be detected. The FT reaction rate on cobalt
catalysts principally depends on cobalt content in the catalysts and
platinum promotion. Platinum promotion results almost in a two
times increase in the reaction rate. In agreement with previous
reports [35–37], higher FT reaction rate on Pt promoted cobalt cata-
lysts was principally attributed to better cobalt reducibility. For the
platinum-promoted Co15%0.1%Pt/Al2O3 and Co25%0.1%Pt/Al2O3
catalysts, the reaction rate increases almost linearly with cobalt
content. The methane selectivity was between 7 and 20%, while
the selectivity to C5+ hydrocarbons was in the range from 65 to 85%.
As expected, the hydrocarbon selectivities were affected by carbon
monoxide conversion. Higher methane and lower C5+ selectivities
were observed on the cobalt catalysts at lower carbon monoxide
conversion level. In agreement with previous reports [36,38–40],
promotion with small amounts of platinum (0.1 wt.%) does not
noticeably affect the hydrocarbon selectivity.
The catalytic performance of iron catalysts in FT synthesis is
usually attributed to the presence of iron carbide species. These
iron carbide species can be produced during catalyst activation
and FT synthesis. Different procedures for the activation of iron
FT catalysts have been subject of numerous reports. Most of them
focus on the activation of bulk iron catalysts. Bukur et al. [41,42]
investigated activation procedures for the 100Fe/5Cu/4.2 K/25SiO2
catalyst in hydrogen, carbon monoxide and syngas. The catalyst
activated in hydrogen showed higher FT reaction rate but at the
same time exhibited higher methane selectivity, while the catalyst
activation in carbon monoxide and syngas led to higher selectiv-
ity to long-chain hydrocarbons. These results are also consistent
with recent work by Cano et al. [43] The effect of the composition
of activating gas mixture on the performance of precipitated iron
catalyst was also studied by Sudsakorn [44] using SSITKA. The cat-
alysts activated in hydrogen showed higher activity at steady state
conditions. Pirola et al. [45] showed that the best catalytic perfor-
mance can be obtained when the high loaded iron catalysts are
activated in syngas. These results are also consistent with the report
by Herranz [46]. Syngas pre-treatment yielded the most active cat-
alysts for both unpromoted and Mn- or Ce-promoted samples. Luo
3. Results and discussion
3.1. Catalyst characterisation
The BET surface area, total pore volume and pore diameter of
the supports used for synthesis of cobalt and iron catalysts are
shown in Table 1. The textural properties of Siralox are typical for
␥-alumina with the BET surface area of 170 m2/g and pore sizes of
about 8 nm. Large mesopores formed by volume between globules
were detected in the commercial CARIACT Q-10 silica with the sur-
face area of 300 m2/g. A relatively large surface area (557 m2/g) and
micropores were observed in the activated carbon. The nanotubes
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Fig. 1. Experimental procedures used for addition of acetonitrile and ammonia.
et al. [47,48] showed that activation of the Fe100/K1.4/Si4.6/Cu2.0
catalyst with CO produced the highest syngas conversion while
activation in hydrogen generated less active catalysts. Positive
effect of activation in carbon monoxide on the catalytic perfor-
mance of iron catalysts in slurry reactor was also observed by
O’Brien et al. [49,50], while the selectivity was not affected by the
gas used for catalyst activation. It seems that the activation proce-
dure should be adapted to specific catalysts. In the present work the
catalysts were activated either in carbon monoxide or in hydrogen.
The catalytic activity of iron catalysts measured at 300 ◦C was
strongly dependent on the support. Iron catalysts supported by car-
bon materials exhibited much higher FT reaction rate compared
with those supported by oxides (e.g. alumina and silica). Further
information about the influence of the catalytic support on the
catalytic performance of iron catalysts is available from recent pub-
lication [33]. The methane selectivity under these conditions was
between 13 and 26%, while the selectivity to C5+ hydrocarbons was
in the range from 13 to 31%. Note that iron catalysts exhibited very
significant selectivity to carbon dioxide which increases with an
increase in carbon monoxide conversion. Water is a major prod-
uct of FT synthesis and high carbon dioxide selectivity observed on
iron catalysts was attributed to the fast water gas shift reaction:
CO + H2O ↔ CO2 + H2. Table 2 shows that carbon dioxide selectiv-
ity increases with carbon monoxide conversion. FT synthesis on
iron catalysts is interplay of a large number of catalytic reactions.
Depending on the rates of these reactions, the yield of carbon
monoxide varies in a broad range. The maximum theoretically pos-
sible CO2 selectivity is 50%.
Table 2
Performance of iron and cobalt catalysts at different GHSV in pure syngas before co-feeding with nitrogen containing compounds.
Catalyst
T, ◦
C
GHSV, cm3 gcat-1 h-1
Catalytic performance after 24 h run
-1
FT rate, 10-7 molCO gcat s-1
CO conv., %
Selectivity (%)
C2-C4 olefins/ parafins
+
CH4
CO2
C5
Cobalt catalysts
220
220
220
6750
3375
13500
6750
13500
6750
1.9
2.3
5.9
10.1
30.2
22.0
6.8
20.7
11.8
19.1
9.3
7.5
7.3
0
0
0
0
0.3
1.0
64.1
77.8
65.9
81.5
85.9
85.1
3.3
2.1
2.2
1.6
0.9
0.8
Co15%/ ␥-Al2O3
16.4
10.6
36.2
54.1
78.7
Co15%0.1%Pt/ ␥-Al2O3
Co25%0.1%Pt/ ␥-Al2O3
Iron catalysts
Fe10%/CNT
300
300
16 000
6750
16 000
6750
6750
16 000
6750
27.7
25.6
14.4
8.3
11.8
30.1
21.3
39.7
91.7
20.7
29.7
42.4
43.1
76.2
21.9
13.7
26.3
13.2
15.1
19.6
15.2
33.7
38
13.1
20.5
13.7
16.9
31.7
16.7
17.6
0.6
0.7
0.8
0.7
1.0
1.1
0.4
20.4
24.4
24.6
30.8
37.1
Fe10%/SiO2
Fe10%/ ␥-Al2O3
Fe10%/AC
300
300
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3.3. FT synthesis with syngas containing acetonitrile and
ammonia
shown in Fig. 4. Introduction of acetonitrile and ammonia leads to
lower carbon monoxide conversions (at similar GHSV) and lower FT
reaction rates during and after CH3CN and NH3 addition compared
to pure syngas. Interestingly, the methane and C5+ hydrocarbon
selectivities were respectively much lower and higher at the same
conversion levels on the supported cobalt catalysts during and after
addition of the nitrogen containing compounds. Previously, sim-
ilar effects on hydrocarbon selectivities on cobalt catalysts were
observed after addition of small amounts of water [51–54] and
ammonia [21] to the syngas feed. One of the possible explana-
tions of these phenomena could be selective poisoning of cobalt
methanation sites by added ammonia and its derivatives.
The olefin to paraffin ratio is also affected by co-feeding. The
olefin to paraffin ratio at similar carbon monoxide conversions
significantly increases. The decrease in methane selectivity is
accompanied by higher olefin to paraffin ratio which could result
from the lower hydrogenation activity in the presence of ammo-
nia. Note that the effect of added acetonitrile and ammonia on the
reaction selectivity was rather irreversible on cobalt catalysts. The
return to pure syngas after switching off acetonitrile and ammonia
cofeeding does not result in full recovering the C5+ and methane
selectivities to the values observed with the fresh catalyst in pure
syngas (Fig. 4). Only the olefin/paraffin ratio almost comes to the
same values as they were before addition of N-containing com-
pounds.
The selectivity data plotted versus carbon monoxide conversion
measured over different iron catalysts in the presence of nitrogen
containing compounds are shown in Fig. 5. Exposure to different
concentrations of acetonitrile results in a irreversible decrease in
methane selectivities for all the supported iron catalysts. Addi-
tion of acetonitrile also affects the hydrogenation activity of iron
catalysts. The olefin to paraffin ratio at similar carbon monoxide
conversion increases in the presence of acetonitrile. Iron catalysts
usually exhibit higher olefin selectivities compared to cobalt coun-
terparts. In our case, the ratio of olefins to paraffins was very
low (about 1) before addition of acetonitrile or ammonia (Fig. 5).
This fact could be explained by different activation procedure in
hydrogen in comparison with traditionally used CO activation. CO
activation results in formation of iron carbide which is supposed
to be an active phase in the FT synthesis over Fe catalysts. The
reduction of the catalyst in hydrogen results in the formation of
iron oxide and small amounts of metallic Fe. High hydrogenation
activity of the catalyst activated in hydrogen can be attributed to
the presence of iron metallic sites. Indeed, the iron catalysts car-
bidized in CO show significantly lower selectivity to methane and
higher selectivity to olefins (Fig. 5) with the values similar to those
obtained after catalysis in the presence of acetonitrile and ammo-
nia. It is interesting to note that addition of acetonitrile did not
lead to significant changes in the catalytic performance for the iron
catalysts activated in carbon monoxide, while the effect of added
ammonia and acetonitrile on the reaction rate and selectivity is
more significant for the catalysts activated in hydrogen.
3.3.1. Influence on the FT reaction rate
FT catalytic data obtained in the presence of ammonia
and acetonitrile over cobalt and iron catalysts are presented
in Table 2 and Figs. 2–6 . Acetonitrile is not stable at the
reaction conditions in the presence of water. It undergoes
hydrolysis into acetic acid and ammonia with subsequent
partial decomposition of acetic acid into methane and CO2:
CH3CN + 2H2O = CH3COOH + NH3 = NH3 + CO2 + CH4. The Co based
catalysts almost do not produce carbon dioxide during FT synthe-
sis. Some slight increase in CO2 on cobalt catalysts in the presence
of added CH3CN may indicate partial decomposition of acetonitrile.
Aqueous ammonia (30 wt.%) also has been added in order to check
the main effect of acetonitrile as the source of ammonia.
The FT reaction rates measured during and after addition of
different amounts of acetonitrile and ammonia to the alumina sup-
ported cobalt catalysts are given in Fig. 2. Addition of acetonitrile
results in lower FT reaction rates on all studied cobalt catalysts.
The activity decrease depends on the amounts of co-fed acetoni-
trile and on gas space velocity. Higher acetonitrile concentrations
result in a more significant decrease in FT reaction rate which is
due to the higher acetonitrile amounts in contact with the catalyst.
Interestingly, the activity drops more rapidly during acetonitrile
addition at higher syngas space velocity. Indeed, at the same reac-
tion time and acetonitrile concentrations in the feed, the catalysts
are exposed to more significant amounts of acetonitrile when the
syngas space velocity is higher. After several exposures to different
amounts of acetonitrile separated by the periods when the catalysts
were in contact with pure syngas (Figs. 1 and 2a), all cobalt cata-
lysts reaches a non-reversible deactivation steady state behaviour.
In this quasi-steady state, the catalytic activity is not any more
affected by the presence of acetonitrile in the gas feed. FT reac-
tion rate at this state is between two and four times smaller than
the initial steady state activity of the same catalysts in pure syn-
gas. Aqueous ammonia deactivates the catalysts in the similar way,
however, not so strongly in comparison with acetonitrile.
Iron catalysts show different behaviours (Fig. 3) in the presence
of CH3CN and ammonia compared to cobalt catalysts. The FT reac-
tion rate in the presence of acetonitrile and ammonia decreases for
Fe10%/AC, sharply surges for Fe10%/CNT (in the case of high GHSV)
and slightly increases in case of Fe10%/SiO2 and Fe10%/Al2O3. The
activity loss in case of 10Fe/AC can be partially recovered when co-
feeding with CH3CN is stopped. At the quasi steady state obtained
by periodic exposure of supported iron catalysts to acetonitrile
and pure syngas (Fig. 3), the FT reaction rates is almost the same
as was measured on the fresh catalysts. Addition of ammonia
also produces relatively small effect on the activity of supported
iron catalysts, except for Fe10%/CNT which activity dramatically
increases after addition of 1500 ppm of NH3 and then drops at
higher NH3 level.
Thus, the catalytic tests suggest different effects on added ace-
tonitrile and ammonia on the FT reaction rates over cobalt and iron
catalysts. On cobalt catalysts, acetonitrile and NH3 additions result
in progressive irreversible catalyst deactivation, while on iron cata-
lysts the carbon monoxide conversion rate is affected by co-feeding
with nitrogen containing compounds to a much smaller extent.
Moreover, higher FT reaction rates were observed on CNT, silica and
alumina supported iron catalysts in the presence of acetonitrile and
ammonia.
3.4. Structure and performance of the cobalt and iron catalysts
exposed to nitrogen containing compounds
The catalytic results suggest that the performance of cobalt
and iron catalysts in FT synthesis can be affected by co-feeding
with nitrogen containing compounds. The presence of acetonitrile
results in the irreversible decrease in FT reaction rate on sup-
ported cobalt catalysts and important catalyst deactivation. The
hydrocarbon selectivity is also affected. At comparable conversion
levels, lower methane and higher C5+ hydrocarbon selectivities
were detected, while the olefin to paraffin ratio becomes higher.
In order to provide further insights into the modification of the
active sites during and after cofeeding with ammonia, the catalysts
3.3.2. Effect of added acetonitrile and ammonia on the reaction
selectivity
The hydrocarbon selectivities and C2–C4 olefin to paraffin ratios
as functions of carbon monoxide conversion on cobalt catalysts are
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Fig. 2. FT reaction on cobalt catalysts measured in pure syngas and in the syngas containing different amounts of acetonitrile (a) and ammonia (b).
Fig. 3. FT reaction on iron catalysts measured in pure syngas and in the syngas containing different amounts of acetonitrile (a) and ammonia (b).
Fig. 4. Methane (a), C5+ hydrocarbon (b) selectivities and C2–C4 olefins to paraffins ratios (c) measured on supported cobalt catalysts as functions of carbon monoxide
conversion in the presence of added acetonitrile and ammonia.
exposed to nitrogen-containing compounds were characterized by
SSITKA with isotopic switches from 12CO/H2/He/Ne to 13CO/H2/He,
TPD-MS and FTIR with adsorbed carbon monoxide. The conducted
catalytic tests showed low FT reaction rate on cobalt catalysts
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7
Fig. 5. Methane C2–C4 olefins to paraffins ratios over iron catalysts activated in hydrogen (a, b) and CO (c, d) as functions of carbon monoxide conversion in the presence of
acetonitrile and ammonia.
1.00
13CH4
Ne
13CO
0.80
without NH3 treatmnet
with NH3 treatment
25CoPt/Al2O3
0.60
0.40
0.20
0.00
15CoPt/Al2O3
15Co/Al2O3
100
200
300
400
500
600
700
Temperature, °C
0
20
40
60
80
100
Time, s
Fig. 7. TPD-MS profiles of alumina supported cobalt catalysts after the catalytic tests
Fig. 6. Normalized concentrations of Ne, 13CO and 13CH4 during switches of
12CO/H2/Ne → 13CO/H2 on the Co25%0.1%Pt/Al2O3 catalyst without and with NH3
treatment (conditions: GHSV = 14 400 cm3 h−1 g−1, p = 1 atm, T = 220 ◦C, gas compo-
sition: 5.5He:1CO:5H2:0.5Ne).
with syngas containing acetonitrile.
information about the concentration of surface intermediates and
their reactivity and thus it can be extremely helpful in elucidation
of the mechanism of catalyst deactivation.
exposed to different amounts of ammonia. The decrease in reaction
rate can be due to the decrease in the concentration of active sites or
to the decrease in the site intrinsic activity (Turnover Frequency).
Note that both phenomena can occur simultaneously during the
catalyst deactivation. Unfortunately, the steady state catalytic data
do not allow discriminating between these different deactivation
mechanisms. SSITKA [26,55,56] addresses measuring the transient
response of isotopic labels in the reactor following an abrupt change
(switch) in the isotopic composition of one of the reactants. The
switch involves only isotopic composition of the feed, while chem-
ical composition remains unchanged. SSITKA provides independent
The SSITKA transient curves obtained on Co25%0.1%Pt/Al2O3
without and with exposure to ammonia are shown in Fig. 6.
Methane was the major product of carbon monoxide hydrogena-
tion at atmospheric pressure and using syngas with H2/CO ratio of 5.
The catalyst treatment with ammonia affected transient responses
of both CO and methane. The surface coverage by molecular CO
and C1 species and their mean surface residence time calculated
from the SSITKA data are displayed in Table 3. The surface cov-
erage by CO and its residence time are only slightly affected by
ammonia treatment. At the same time, the number of C1 surface
intermediates has been noticeably reduced in the catalysts exposed
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Table 3
Results of SSITKA experiments with the Co25%0.1%Pt/Al2O3 catalyst without and with treatment with NH3.
Experiment
␶
(s)
␶
(s)
CH4
ꢁCO (␮mol/g)
ꢁC1 (␮mol/g)
CO
Co25%0.1%Pt/Al2O3 without NH3 treatment
Co(25%)Pt/Al2O3 with NH3 treatment
8.2
8.6
21.4
29.2
92.0
105
25.4
18.8
to ammonia. The NH3 treatment also notably increases the resi-
dence time of C1 surface intermediates from 21.4 to 29.2 s. This
suggests that ammonia treatment produces strong impact on the
both concentration and reactivity of C1 surface intermediates. It can
be also suggested that the most active surface sites responsible for
hydrogenation of adsorbed carbon species have been deactivated
during the ammonia treatment. This could possibly explain lower
methane selectivity on the cobalt catalysts exposed to acetonitrile
and ammonia
hydrocarbon selectivities. This suggests that the cobalt nitride for-
mation proceeds selectively on the sites favouring methanation and
block the most active cobalt hydrogenating sites.
Fig. 8 shows the IR spectra of carbon monoxide adsorbed on the
Co15%/Al2O3 and Co15%0.1%Pt/Al2O3 catalysts activated in hydro-
gen and then pre-treated with ammonia. The spectra exhibit a set of
bands of adsorbed carbon monoxide. The broad band with the fre-
quency from 2000 to 2100 cm−1 can be assigned to CO adsorption
on cobalt sites, while the bands at 2179 and 2159 cm−1 seem to be
attributed to CO adsorption on Lewis acid sites and hydroxyl groups
of the alumina support, respectively. Catalyst exposure to ammonia
results in the modification of the shape of the broad low frequency
CO band. In particular, the broad band gets narrower and contribu-
tion of the lower frequencies (␯CO = 2029 cm−1) branch is slightly
reduced. This fact suggests that ammonia pre-treatment results in
disappearance of electron enriched cobalt surface sites, associated
with low-frequency shoulder. Previous reports [60–62] attribute
low frequency bands with the frequency of 2000–2030 cm−1 to the
CO molecules in interaction with the edges or step sites of metal
nanoparticles (Pt or Pd), while the band of CO with high frequen-
cies (␯CO = 2040–2060 cm−1) corresponds to the adsorption on the
terraces. It could be speculated therefore that cobalt nitride species
could preferentially form on these highly unsaturated metal sites
(Fig. 9). The blockages of edges and steps of cobalt metal nanopar-
ticles by nitrides could result in the decrease in the FT reaction
rates and modify the reaction selectivity. Addition of platinum
to the catalyst (Fig. 8b) promotes this process and decreases the
amount of unsaturated Co sites. Initially higher amount of active
sites (bands at 2029 cm−1 in Fig. 8a and b) leads to higher activity
of Pt-promoted catalyst in comparison with unpromoted catalyst.
Differently to supported cobalt catalysts, the exposure of sup-
ported iron catalysts to acetonitrile and ammonia does not lead
Thus, ammonia co-fed to the catalyst can affect the number and
intrinsic activity of active sites for FT synthesis. These modifica-
tions can be possibly attributed to the formation of cobalt nitride
in the presence of ammonia. In order to further confirm forma-
tion of cobalt nitride, the catalysts after exposure to ammonia
during the catalytic tests were characterized by TPD-MS. Fig. 7
shows the TPD-MS profiles of the alumina supported catalysts
after the catalytic tests with detection of m/e = 28. The TPD-MS
profiles of all the spend catalysts exhibited sharp peaks at 300 ◦C
measured using both m/e = 28 and m/e = 14 signals. It can be sug-
gested that these sharp peaks observed at 300 ◦C correspond to
desorption of nitrogen. This nitrogen can be produced from cobalt
nitride decomposition. The amount of nitrogen calculated from the
area of this peak correlates with the amount of reduced cobalt in
the catalysts: the N/Co ratios were 0.46 for Co15%/Al2O3, 0.74 for
Co15%0.1%Pt/Al2O3 and 0.73 for Co25%0.1%Pt/Al2O3. Cobalt nitride
formation in the presence of ammonia has been reported in pre-
vious works [57–59]. The catalytic results obtained in this work
suggest that cobalt nitride is not active in FT synthesis. Indeed, for-
mation of cobalt nitride during the catalyst exposure to acetonitrile
leads to important catalyst deactivation. At the same time, cobalt
nitride formation coincides with the modifications of FT catalyst
selectivity patterns and leads to lower methane and higher C5+
Fig. 8. FTIR spectra of carbon monoxide adsorbed on the activated Co15%/Al2O3 (a) and Co25%0.1%Pt/Al2O3 (b) catalysts before and after exposure to NH3.
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9
Fig. 9. Evolution of cobalt and iron catalysts in the presence of ammonia.
to any irreversible catalyst deactivation. Interaction of the ammo-
nia with the iron catalysts under the reaction conditions can result
in formation of iron nitrides. The catalytic activity of iron nitrides
in FT synthesis have been investigated since the pioneering works
of Anderson [63,64]. The activity of iron nitride is comparable of
that of iron carbides, though the two phases exhibit some differ-
ences relevant to the reaction selectivity [63–65]. Shultz et al. [64]
also showed that the iron nitride could be converted rapidly into
iron carbo-nitrides if the ammonia has not been fed any more to
the reactor. Upon further treatment with syngas, the carbo-nitride
changed to Hägg carbide, which is usually considered as an active
phase for FT synthesis over iron catalysts [3,12,13]. Iron carbides
after the FT reaction tests in our spent iron catalysts were previ-
ously detected [33] using XRD and Mossbauer spectrometry. The
present work is indicative of very small effect of acetonitrile on
their catalytic performance of iron catalysts activated in CO, while
significant modifications of the hydrogenation activity in the case of
the iron catalyst activated in hydrogen were observed. High hydro-
genation activity was attributed to the presence of metallic iron
in the catalysts activated in hydrogen. Ammonia addition seems
to facilitate conversion of iron oxides and metallic iron in the cata-
lysts activated in hydrogen into iron nitride and then to iron carbide
in the presence of syngas (Fig. 9). Catalytic properties of different
iron phases present under the reaction conditions in pure syngas
(iron carbide) and in the presence of acetonitrile (iron carbide, iron
nitride or iron carbo-nitride) could probably explain a relative sta-
bility of FT reaction rate in the presence of small concentrations of
acetonitrile in syngas over iron catalysts.
Consequently lower methane and higher C5+ hydrocarbon selectiv-
ities were observed on cobalt catalysts exposed to acetonitrile and
ammonia.
No noticeable deactivation was observed on the exposure of
supported iron catalysts to acetonitrile and ammonia. The effect
of nitrogen-containing additives on iron catalysts depends on the
activation procedure. Catalyst activation in hydrogen results in
irreversible modifications of the catalyst selectivity patterns after
ammonia addition. The methane selectivity is reduced, while the
olefins to paraffins ratio is increased. Iron catalysts activated in
carbon monoxide did not demonstrate any effect of acetonitrile
addition on the selectivity.
Acknowledgments
The authors gratefully acknowledge financial support for this
work from the French National Agency for Research (Biosyngop
Project, Ref. ANR- 11-BS07-026). The authors are indebted to S.
Heyte and J. Thuriot for technical assistance with high through-
put catalytic experiments. The REALCAT platform is benefiting
from a Governmental subvention administrated by the French
National Research Agency (ANR) within the frame of the ‘Future
Investments’ program (PIA), with the contractual reference ‘ANR-
11-EQPX-0037ꢀ. The Nord-Pas-de-Calais Region and the FEDER are
acknowledged for their financial contribution to the acquisition of
the equipment of the platform. A.C. is grateful to CAPES-COFECUB
program (Brazil-France) for supporting his Ph.D. research.
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Please cite this article in press as: V.V. Ordomsky, et al., Effects of co-feeding with nitrogen-containing compounds on the performance
of supported cobalt and iron catalysts in Fischer–Tropsch synthesis, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.12.015