Fischer-Tropsch synthesis: effect of ammonia on product selectivities for a Pt promoted Co/alumina catalyst

Article abstract of DOI:10.1039/c6ra27940f

The effects of co-fed ammonia in synthesis gas on the activity and product selectivities of a typical cobalt catalyst (0.5% Pt-25% Co/Al₂O₃) were investigated during the Fischer-Tropsch synthesis using a continuously stirred tank reactor (CSTR). The product selectivities were compared at a similar CO conversion level for various concentrations (10-1000 ppmv) of ammonia, as well as clean (un-poisoned) conditions. The addition of 10-1000 ppmv ammonia (concentration of ammonia with respect to the syngas feed) significantly decreased activity; the percentage of deactivation was similar (～40%) for the various concentrations of ammonia used. At similar CO conversions, the addition of ammonia caused an increase in olefin selectivity and the corresponding paraffin and alcohol selectivities were decreased compared to the ammonia free synthesis conditions. Olefin selectivity increased with increasing concentration of ammonia, and the paraffin and alcohol selectivities were decreased with increasing ammonia concentration. At similar CO conversions, ammonia addition exhibited a positive effect on hydrocarbon selectivity (i.e., lower light gas products and higher C₅+) and also light gas product selectivities (C₁-C₄) were decreased and C₅+ selectivity increased with increasing concentration of ammonia compared to ammonia free conditions.

Full text of DOI:10.1039/c6ra27940f

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Fischer–Tropsch synthesis: effect of ammonia on
product selectivities for a Pt promoted Co/alumina
catalyst
Cite this: RSC Adv., 2017, 7, 7793
Venkat Ramana Rao Pendyala, Wilson D. Shafer, Gary Jacobs, Michela Martinelli,
*
Dennis E. Sparks and Burtron H. Davis
The effects of co-fed ammonia in synthesis gas on the activity and product selectivities of a typical cobalt
catalyst (0.5% Pt–25% Co/Al2O3) were investigated during the Fischer–Tropsch synthesis using
a continuously stirred tank reactor (CSTR). The product selectivities were compared at a similar CO
conversion level for various concentrations (10–1000 ppmv) of ammonia, as well as clean (un-poisoned)
conditions. The addition of 10–1000 ppmv ammonia (concentration of ammonia with respect to the
syngas feed) significantly decreased activity; the percentage of deactivation was similar (ꢀ40%) for the
various concentrations of ammonia used. At similar CO conversions, the addition of ammonia caused an
increase in olefin selectivity and the corresponding paraffin and alcohol selectivities were decreased
compared to the ammonia free synthesis conditions. Olefin selectivity increased with increasing
concentration of ammonia, and the paraffin and alcohol selectivities were decreased with increasing
ammonia concentration. At similar CO conversions, ammonia addition exhibited a positive effect on
hydrocarbon selectivity (i.e., lower light gas products and higher C5+) and also light gas product
selectivities (C1–C4) were decreased and C5+ selectivity increased with increasing concentration of
ammonia compared to ammonia free conditions.
Received 8th December 2016
Accepted 14th January 2017
DOI: 10.1039/c6ra27940f
www.rsc.org/advances
can contain both organic and inorganic impurities such as tars,
benzene, toluene, xylene, NH3, HCN, H2S, COS, HCl, volatile
metals, dust, and soot.7 Coal, which originates from biomass,
1. Introduction
Fischer–Tropsch Synthesis (FTS) is a promising route for the
transformation of synthesis gas (CO + H2, or syngas) into valu-
able hydrocarbons (gasoline, diesel, chemicals, etc.). It is suit-
able for the production of synthetic fuels from both fossil (e.g.,
coal, natural gas) and renewable (e.g., biomass) resources.1–3
This reaction produces a wide variety of products, from gases to
heavy waxes. FTS is catalyzed by several transition metals
including Fe, Co, and Ru, whereas only Co and Fe are commonly
employed as reasonable commercial catalysts for the process.
However, cobalt catalysts may feature high stability and activity,
along with a lower deactivation rate, compared to iron cata-
lysts,4 but require a narrower range of operating temperatures
and pressures, because any overheat shis the reaction towards
methane formation and decreases the desired liquid product
selectivity.5,6
contains many of the same inorganic impurities as found in
biomass.8 Due to the variety of feedstocks that can be processed,
signicant variations in the composition of synthesis gas can be
expected. This affects the nature of the impurities that are
present (element, speciation), as well as their relative contents.
Moreover, due to the high sensitivity of FT catalysts, severe
specications regarding syngas purity are required. Iron and
cobalt catalysts share the sensitivity toward some, but not all, of
the impurities commonly found in coal and biomass-derived
syngas.
Among these impurities, high concentrations of sulfur
compounds ($500 ppbv) may cause irreversible deactivation of
cobalt and iron-based catalysts during FT synthesis.9–12 The
effect of ammonia on FTS has been reported in a few studies
over iron and cobalt catalysts and the reported results, however,
have been rather contradictory.13–18 Claeys et al.13 reported that
co-feeding of up to 25% NH3 in syngas did not affect FT activity;
similarly Borg et al.14 reported that 4.2 ppmv of ammonia does
not have an impact for a CoRe/Al2O3 catalyst. An Exxon patent15
claims that a combined concentration of 100 ppb of NH3 and
HCN in syngas will result in a catalyst half-life of only 4 days for
a supported cobalt catalyst in a slurry reactor. However, the
patent also indicates that hydrogen treatment of the catalyst can
A gasication process is used to convert coal or biomass
feedstocks into synthesis gas (CO and H2 mixture), which
undergoes the Fischer–Tropsch reaction aer H2/CO ratio
adjustment and CO2 removal. However synthesis gas also
contains various impurities that must be removed in order to
prevent FT catalyst poisoning. Biomass-derived synthesis gas
Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive,
Lexington, KY-40511, USA. E-mail: burtron.davis@uky.edu; Tel: +1 859 257 0251
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restore the initial activity. Van Berge16 reported that cobalt
BET surface area and porosity measurements of the calcined
catalysts were rapidly but reversibly deactivated by HCN and and ammonia exposed catalysts were conducted using a Micro-
NH3. In a recent investigation, Rausch et al.17 indicated that meritics 3-Flex unit. Before performing the test, the temperature
ammonia had a signicant effect on the cobalt catalyst. was gradually ramped to 160 ꢂC and the sample was evacuated
Ordomsky et al.18 reported that the addition of NH3 and CH3CN for at least 12 h to approximately 50 mTorr.
during FT synthesis has a signicant irreversible deactivation
2.2.2 Hydrogen chemisorption and pulse reoxidation.
on supported cobalt catalysts, whereas no noticeable deactiva- Hydrogen chemisorption was conducted using temperature
tion was observed with supported iron catalysts.
programmed desorption (TPD), also measured with the Zeton
In our previous investigations,19–22 we reported that an iron Altamira AMI-200 instrument. The sample weight was typically
catalyst was quite resistant to high levels (200 ppmw) of ꢀ0.22 g. The catalyst was activated in a ow of 10 cm3 minꢁ1 of
ammonia,19 whereas a cobalt catalyst underwent signicant H2 mixed with 20 cm3 minꢁ1 of argon at 350 C for 10 h, and
ꢂ
deactivation with parts per million levels of ammonia,20–22 then cooled under owing Ar to 85 ꢂC. A gas mixture of 10 cm3
regardless of whether ammonia gas or ammonium hydroxide minꢁ1 H2 and 20 cm3 minꢁ1 Ar was owed at 85 ꢂC for 15 min to
was used. The addition of ammonia caused signicant deacti- chemisorb hydrogen to the catalyst surface. The sample was
vation for alumina, titania and silica supported cobalt catalysts, then held at 85 ꢂC under 30 cm3 minꢁ1 owing of Ar for 30 min
but the rate of deactivation was higher for the silica-supported to remove and/or prevent adsorption of weakly bound species
catalyst compared to the alumina and titania supported cata- prior to increasing the temperature slowly to 350 ꢂC, the
lysts.20 In most studies reported in the open literature, the effect reduction temperature of the catalyst. The catalyst was held
of ammonia on product selectivities over cobalt catalysts has under owing argon to desorb any remaining chemisorbed
not been reported at similar CO conversion levels. The aim of hydrogen until the TCD signal returned to baseline. Further
the present study is to compare the product selectivities at details of the procedure are provided elsewhere.24
similar CO conversion levels for various concentrations of co-
For the ammonia exposed catalyst, the catalyst was initially
fed ammonia (10–1000 ppmv) as well as under clean condi- activated in a ow of 1ꢂ0 cm3 minꢁ1 of H2 mixed with 20 cm3
minꢁ1 of argon at 350 C for 10 h, and then cooled to 220 C
under owing Ar, aer which ammonia gas (1% NH3 balanced
nitrogen, 50 cm3 minꢁ1) was owed for an hour. The sample
was then cooled to 85 ꢂC under owing of Ar. A gas mixture of 10
cm3 minꢁ1 H2 and 20 cm3 minꢁ1 Ar was owed at 85 ꢂC for
15 min to chemisorb hydrogen to the catalyst surface. The
sample was then held at 85 ꢂC under owing Ar to remove and/
or prevent adsorption of weakly bound species prior to
increasing the temperature slowly to 350 ꢂC, the reduction
temperature of the catalyst.
ꢂ
tions for a Pt-promoted Co/alumina catalyst.
2. Experimental
2.1 Catalyst preparation
Sasol/Catalox alumina (high purity g-alumina, 140 m2 gꢁ1) was
used as the support for the cobalt catalyst. The catalyst was
prepared by a slurry impregnation method, and cobalt nitrate
was the precursor. In this method, which follows a Sasol
patent,23 the ratio of the volume of solution used to the weight
of alumina was 1 : 1, such that the volume of solution was
approximately 2.5 times the pore volume of the catalyst. Two
impregnation steps were used, each to load 12.5% of Co by
weight. Aer the second impregnation/drying step, the catalyst
was calcined under air ow at 350 ꢂC. The promoter was added
by incipient wetness impregnation (IWI), and the precursor
utilized for noble metal addition was tetraammineplatinum(II)
nitrate. Aer Pt addition, the sample was dried and calcined
again using the same conditions as used previously.
2.3 Catalytic activity testing
FTS reaction experiments were conducted using a 1 L continu-
ously stirred tank reactor (CSTR) equipped with a magnetically
driven stirrer with turbine impeller, a gas-inlet line, and a vapor
outlet line with a stainless steel (SS) fritted lter (2 mm) placed
external to the reactor. A tube tted with a SS fritted lter (0.5
mm opening) extending below the liquid level of the reactor was
used to withdraw reactor wax (i.e., rewax, which is solid at room
temperature), thereby maintaining a relatively constant liquid
level in the reactor. Separate mass ow controllers were used to
control the ow rates of hydrogen and carbon monoxide.
2.2 Catalyst characterization
2.2.1 BET surface area and pore size measurements. To Carbon monoxide, prior to use, was passed through a vessel
characterize the ammonia-exposed catalysts, the end-of-run containing lead oxide on alumina to remove traces of iron
catalyst along with wax was transferred to an air-free environ- carbonyls. The gases were premixed in an equalization vessel
ment (inert gas lled chamber). The catalyst sample was diluted and fed to the CSTR below the stirrer, which was operated at
with hot ortho-xylene to remove the high molecular weight FT- 750 rpm. The reactor temperature was maintained constant
ꢂ
wax products. It was not possible to completely remove the (ꢃ1 C) using a temperature controller.
FT-wax from the catalyst particles by this method. However, the
The catalyst (ꢀ12.0 g) was ground and sieved to 63–106 mm
remaining wax acts as a protective barrier for the air-sensitive before loading into a xed-bed reactor for 12 h of ex situ
catalyst particles. The extracted catalyst was treated with 1% reduction at 350 ꢂC and atmospheric pressure using a gas
O2/N2 at 300 ꢂC for 4 h to remove the wax product formed from mixture of H2/He (60 NL per h) with a molar ratio of 1 : 3. The
FTS.
reduced catalyst (ꢀ10.0 g) was transferred to a 1 L CSTR
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containing 310 g of melted Polywax 3000 under the protection of
inert nitrogen gas. The reactor used for the reduction was
weighed before and aer reduction and aer catalyst transfer in
order to obtain an accurate amount of catalyst that was added.
The transferred catalyst was further reduced in situ at 230 ꢂC at
atmospheric pressure using pure hydrogen (20 NL per h) for
another 24 h before starting the FT reaction. In this study, the
FTS conditions used were 220 ꢂC, 1.99 MPa, H2/CO ¼ 2, and
a stirrer speed of 750 rpm.
Gas, water, oil, light wax, and heavy wax samples were
collected daily and analyzed. Heavy wax samples were collected
in a 200 ꢂC hot trap connected to the lter containing dip tube.
The vapor phase in the region above the reactor slurry passed
continuously to the warm (100 ꢂC) and then the cold (0 ꢂC) traps
located external to the reactor. The light wax and water mixture
were collected daily from the warm trap and an oil plus water
sample from the cold trap. Tail-gas from the cold trap was
analyzed with an online HP Quad Series Micro GC, providing
molar compositions of C1–C5 olens and paraffins, as well as
H2, CO and CO2. The analysis of the aqueous phase used an SRI
8610C GC with a thermal conductivity detector. The organic
products were analyzed with an Agilent 6890 GC with a ame
ionization detector and a 60 m DB-5 column. Hydrogen and
carbon monoxide conversions were calculated based on GC
analysis of the gas products, the gas feed rate, and the gas ow
that was measured at the outlet of the reactor.
Fig. 1 Effect of ammonia addition on relative CO conversion, related
to the pre-exposure conditions.
Table
1 Effect of ammonia on product selectivities over a Pt-
promoted Co/alumina catalyst. The selectivities were compared at
similar CO conversions of ammonia free synthesis conditions at three
different concentrations of ammoniaa
Ammonia concentration (ppmv)
10
0
100
0
1000
0
10
100
1000
3. Results and discussion
CO conversion
Paraffin
Alcohol
36.3
78.3
5.0
36.6
77.5
1.2
36.3
78.3
5.0
36.5
74.7
2.0
28.5
80.4
2.7
28.9
72.7
0.5
To maintain experimental control, similar activation and reac-
tion conditions (partial pressures of CO and H2) were used at
various concentrations of ammonia exposure for a Pt–Co/
alumina catalyst. Aer attaining steady state CO conversion,
ammonia gas (10 to 1000 ppmv) was added to the syngas feed.
The addition of ammonia (10–1000 ppmv) caused a signicant
initial drop in CO conversion and then the CO conversion
Olen
16.7
21.3
16.7
23.3
16.9
26.8
a
Reaction conditions: T ¼ 220 ꢂC; P ¼ 1.99 MPa; H2/CO ¼ 2; 10% N2;
S.V. ¼ 4–5.2 slph per gcatalyst
.
remained relatively constant with addition of ammonia Experiments were conducted for the un-poisoned and ammonia
following the initial drop. More importantly, CO conversion did exposed catalyst at various concentrations. The addition of 10
not recover to the expected value, even aer termination of ppmv ammonia caused an increase in olen selectivity from 16.7
ammonia addition under normal FTS conditions, indicating to 21.3%, while paraffin selectivity slightly decreased to 77.5%
that the catalyst had deactivated (i.e., NHx species are strongly from 78.3% and alcohol selectivity decreased to 1.2% from 5.0%.
adsorbed to deactivate cobalt active sites).20 The relative CO For 100 ppmv addition of ammonia, olen selectivity increased
conversions for a Pt-promoted Co/alumina catalyst at various to 23.3% from 16.7%, paraffin selectivity decreased to 74.7 from
concentrations of ammonia compared to the pre-exposure 78.3% and alcohol selectivity decreased to 2.0% from 5.0%.
conditions are shown in Fig. 1. This gure shows that the Similarly, the addition of 1000 ppmv ammonia caused an
extent of deactivation is similar (ꢃ3%) for the various concen- increase in olen selectivity to 26.8% from 16.7%, while paraffin
trations of ammonia used (10–1000 ppmv). The similar extents selectivity decreased to 72.7% from 80.4% and alcohol selectivity
of poisoning are consistent with the chemisorption of the same decreased to 0.5% from 2.7%. At similar CO conversions, the
amount of ammonia at the pressure represented by 10–1000 addition of ammonia caused an increase in olen selectivity and
ppmv of ammonia.
corresponding decreases in alcohol and paraffin selectivities
The effect of ammonia on product selectivities over a Pt- compared to the ammonia free synthesis condition. With
promoted Co/alumina catalyst at various concentrations of increasing concentration of ammonia, the extents were
ammonia is shown in Table 1. In FT synthesis, it is well known increased with similar trends (i.e., increase in olen selectivity,
that the conversion level usually inuences the selectivity, in part decreases in paraffin and alcohol selectivities) compared to the
due to increasing partial pressure of water and by increasing the ammonia free synthesis condition.
partial pressure of reactants.25,26 It is therefore important to
compare the catalysts at a similar CO conversion level. similar CO conversions were compared for the unpoisoned and
The effect of ammonia on hydrocarbon selectivities at
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ammonia poisoned catalysts, which are shown in Table 2. The
addition of 10 ppmv ammonia decreased the methane selec-
tivity to 7.6% from 8.7% and the corresponding C5+ selectivity
increased to 83.9% from 81.5%. For 100 ppmv addition of
ammonia conditions, methane selectivity dropped to 6.7% from
8.7% and the C5+ selectivity was enhanced to 85.9% from
81.5%. Similarly, the addition of 1000 ppmv ammonia caused
a signicant decrease in methane selectivity (6.5% from 9.3%)
and the corresponding C5+ selectivity increased to 86.2% from
80.2%. These results clearly indicate that addition of ammonia
signicantly decreases the lower hydrocarbons selectivity (C1–
C4) and increases the corresponding higher hydrocarbon (C5+)
selectivity. With increasing ammonia concentrations (10 to
1000 ppmv), lower hydrocarbon selectivities decreased and
higher hydrocarbons selectivity increased.
Fig. 2 The effect of ammonia on 1-olefin content in total hydrocar-
bons at similar CO conversion level (ꢀ28.5%) for a Pt-promoted Co/
alumina catalyst.
A loading of 0.5% Pt is typical of 25% Co/Al2O3 research
catalysts for facilitating the reduction of cobalt oxides that
strongly interact with the support.24,27 EXAFS experiments have
revealed that Pt is in contact with Co at the atomic level, with no
Pt–Pt coordination observed;28 moreover, comparing selectivity ethylene for chain propagation.31 Linear 1-olens are known to
at constant conversion, Pt did not signicantly alter the selec- be the main primary organic products of FTS. Once formed,
tivity of FTS compared to an un-promoted catalyst.29 That is, the they can readsorb from the catalyst surface and undergo
role of Pt is primarily to increase the extent of cobalt reduction secondary reactions: hydrogenation, isomerization, re-
to, in turn, increase the surface Co metal active site density. In insertion, hydrogenolysis and hydroformylation. The observed
our earlier study,22 the effect of ammonia over Pt promoted and product can be impacted by secondary reactions. Olen
un-promoted 20% Co/SiO2 catalysts was investigated. Similar secondary reactions strongly depend on reaction conditions
trends were observed for the un-promoted and Pt-promoted (temperature, partial pressures of H2, CO and water and resi-
silica supported cobalt catalysts, and the Pt promoted catalyst dence time), the catalyst used and the olen chain length. The
deactivated more than the un-promoted catalyst during the olen content (from C3 onwards) decreased with increasing
ammonia exposure. Regarding the conversion and selectivity carbon number for both unpoisoned and ammonia poisoned
trends, Pt promoted Co/SiO2 did not display any additional catalysts. The decrease in olen content with increasing carbon
benets or drawbacks relative to the un-promoted silica sup- number (for C3+ hydrocarbons) has been attributed to differ-
ported cobalt catalyst.
ences in reactivity, intraparticle diffusional effects, residence
The 1-olen contents in total hydrocarbons as a function of time differences due to vapor pressure differences and solu-
carbon number at similar CO conversions for the un-poisoned bility variation with molecular weight.6,32–36 Diffusivity decreases
and 1000 ppmv ammonia poisoned catalysts are shown in with increases in molecular weights of the species, and higher
Fig. 2. At similar CO conversion level, the 1-olen content molecular weight 1-olens have longer residence times within
(selectivity) of the ammonia exposed catalyst is higher than for catalyst pores, thus increasing the probability for secondary
the unpoisoned catalyst at all carbon numbers (C2–C17). reactions.32–34 Increases in solubility with increases in carbon
Ethylene is more reactive than other low molecular weight 1- number also result in increased residence times for higher
olens, and thus its selectivity is correspondingly lower. The low molecular weight hydrocarbons in the reactor as well as their
ratio of ethylene to ethane was probably due to the rapid higher liquid phase concentrations, resulting in a greater extent
readsorption30 and hydrogenation and/or incorporation of of secondary reactions.6,34–36 Due to diffusional restrictions,
larger a-olens spend longer times in a pore, which increases
the probability of their readsorption on FTS active sites before
exiting the pore. In a typical liquid product produced from
cobalt catalyst, olens will appear around up to carbon number
25, whereas paraffins will appear above about carbon number
35. Olens are not observed above carbon number ꢀ25 in the
liquid products because their long pore residence time ensures
that olens will readsorb many times; only chains that termi-
nate as unreactive paraffins exit the catalyst. Thus the higher
paraffinic content of larger hydrocarbons is not the result of
direct hydrogenation of a-olens to the corresponding paraffin
of equal size,32,33 but reects instead the enhanced readsorption
of a-olens and surface chain initiation steps that lead to their
ultimate desorption as paraffins.
Table 2 Effect of ammonia on hydrocarbon selectivities at similar CO
conversion levels of with/without ammonia addition over a Pt-
promoted Co/alumina catalysta
Ammonia concentration (ppmv)
0
10
0
100
0
1000
CO conversion
C1
36.3
8.7
36.6
7.6
36.3
8.7
36.5
6.7
28.5
9.3
28.9
6.5
C2–C4
C5+
9.8
81.5
8.5
83.9
9.8
81.5
7.4
85.9
10.5
80.2
7.3
86.2
a
Reaction conditions: T ¼ 220 ꢂC; P ¼ 1.99 MPa; H2/CO ¼ 2; 10% N2;
S.V. ¼ 4–5.2 slph per gcatalyst
.
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The decrease in methane selectivity along with higher 1- (decane). The activity and selectivity results of 1-decene under
olen content of the ammonia exposed catalysts might be due FT conditions for the unpoisoned and ammonia exposed cata-
to the suppression of secondary reactions of olens (such as lysts are shown in Table 3. The 1-decene conversion of the
hydrogenation, isomerization, re-insertion and hydro- ammonia exposed (1000 ppmv) catalyst was found to be 46.0%.
formylation). Two additional experiments were carried out to The selectivity of the hydrogenation product (decane) was found
further conrm the suppression of secondary hydrogenation to be 90.0% and the remaining 10.0% selectivity was to isom-
reaction of olens, by co-feeding 1-decene to the unpoisoned erization products (cis and trans isomers of 2-decene). For the
and ammonia poisoned catalysts during FT synthesis. Gas clean catalyst, 85.4% 1-decene conversion was observed, and
chromatographic results and 1-decene conversion of the un- the selectivities of the hydrogenation and isomerization prod-
poisoned and 1000 ppmv ammonia exposed conditions are ucts were found to be 91.0% and 9.0%, respectively. These
shown in Fig. 3. The conversion of 1-decene was calculated from results clearly reveal that addition of ammonia caused a signif-
its rate of consumption assuming that the FT product formation icant suppression of secondary reactions of olens such as
rate was not affected by 1-decene addition. The added decene is hydrogenation and isomerization for the supported cobalt
primarily converted to the corresponding linear paraffin catalysts. The 1-decene addition during FT over unpoisoned and
Fig. 3 Gas chromatographic results of 1-decene conversion at clean and 1000 ppmv ammonia addition conditions.
Table 3 Hydrogenation activity results of 1-decene under FT conditions for the clean and ammonia addition catalystsa
Conversion/selectivity (%)
Unpoisoned run
Ammonia exposed run (1000 ppmv)
1-Decene conversion
Selectivity to hydrogenated product (decane)
Selectivity to isomerization products (cis and trans 2-decene)
85.4
91.0
9.0
46.0
90
10.0
a
FT reaction conditions: T ¼ 220 ꢂC; P ¼ 1.99 MPa; H2/CO ¼ 2; 10% N2; S.V. ¼ 3–6 slph per gcatalyst, 1-decene (1 mL hꢁ1).
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ammonia exposed catalysts does not have any signicant investigated by characterizing the freshly calcined and ammonia
impact on methane selectivity. This suggests that hydro- exposed catalysts by using BET surface area and hydrogen
genolysis leading to methane from added 1-decene and the chemisorption techniques. BET surface area and pore size
olens formed during synthesis played a negligible role.
distribution results of the Pt–Co/alumina catalysts are presented
The effect of ammonia on FTS activity and selectivity of in Table 4. The BET surface area and pore size distribution results
cobalt catalysts is unclear; some researchers have reported no of the freshly calcined, FT used, ammonia exposed during FT and
effect,13,14 while others have reported that ammonia had the regenerated (aer ammonia exposure mild in situ H2 treated)
a signicant impact.15–18,20,21 In the open literature, most of the catalysts had similar values (ꢃ3%, within experimental error
studies have not reported the product selectivities at similar CO range). These values suggest that ammonia exposure during FTS
conversion level. Recently, Rausch et al.17 reported that does not cause any signicant structural changes of the catalyst
ammonia addition had a signicant impact on activity and (e.g., pore collapse) as was the case with Co/SiO2 catalysts.20 The
product selectivity as well. The activity of the catalyst was hydrogen chemisorption results of the freshly calcined catalyst
decreased with increasing the concentration of ammonia. The are shown in Table 5. Ammonia addition may decrease the
addition of ammonia resulted in an increase in olen selectivity reactivity of hydrogen. For a hydrogeꢂn activated freshly calcined
and decreases in paraffin and alcohol selectivities. Ordomsky clean catalyst the H2-TPD up to 325 C shows complete desorp-
et al.18 also reported that small amounts of acetonitrile and tion of hydrogen (139 mmol gꢁ1), while for the ammonia pre-
ammonia affected the FTS activity and selectivity of the cobalt treated catalyst, only 88% ꢂof the H2 had desorbed (122 mmol gꢁ1
)
catalysts. The deactivation of the cobalt catalyst was explained by that temperature (325 C). Both fresh and ammonia-exposed
by possible formation of cobalt nitride, which may be respon- catalysts desorbed relatively similar amounts of hydrogen aer
sible for the selective blocking of methanation sites. Lower a 30 min hold at 350 ꢂC. During FT reaction, continuous addition
methane and higher C5+ selectivity of the cobalt catalysts was of ammonia caused a 40% drop in activity, but in TPD, hydrogen
explained by a decrease in intrinsic hydrogenation activity of desorbed was lowered by only 12%. Note that we owed H2 at
cobalt sites aer the formation of cobalt nitride. The effect of 85 ꢂC for 15 min during TPD measurement, which may have
ammonia on product selectivity of Pt–Co/alumina catalyst is removed a fraction of the NHx species from the surface prior to
similar to the effect of potassium promoter to an iron-based FT the TPD, so the result may not reect the effect of constant NH3
catalysts. As we mentioned in our earlier studies,37,38 the addi- addition during FT.
tion of potassium to the iron catalysts increases the strength of
The effect of ammonia on the catalytic activity can be
CO adsorption and suppresses H2. This results in a high CO/H explained by the adsorption of NHx species on the cobalt
concentration ratio on the catalyst surface and consequently surface which may or may not form surface cobalt nitride and
leads to a lower hydrogenation activity. A lower hydrogenation which severely decreases the ability to adsorb the reactants
activity would suppress the hydrogenation of surface CHx (hydrogen and CO). This changes the selectivity due to
species and also secondary hydrogenation of olens. As a result, a coverage of NHx species on metal sites that normally adsorb
low selectivity to methane and high selectivity to higher hydrogen, creating a relatively hydrogen poor surface. Ordom-
hydrocarbons and olens was observed.
sky et al.18 proposed that the formation of cobalt nitride might
To understand the reason for changes in activity and product be responsible for the changes in activity and selectivity
selectivities following ammonia exposure, the catalysts were following ammonia exposure. There are two likely scenarios for
Table 4 BET surface area and pores size distribution results of the alumina support and various Pt–Co/alumina catalysts
BET surface
area (m2 gꢁ1
Single point pore Single point
BET SA ratio
Catalyst
)
volume (cm3 gꢁ1
)
pore diam. (nm) with fresh case
g-Al2O3 support (Catalox Sba 150)
Calcined 0.5% Pt–25% Co/Al2O3
Unpoisoned 0.5% Pt–25% Co/Al2O3
Ammonia exposed 0.5% Pt–25% Co/Al2O3
Aer ammonia exposure mild in situ H2 treated 0.5% Pt–25% Co/Al2O3
143.0
98.2
95.6
98.7
96.6
0.450
0.245
0.248
0.253
0.250
12.6
10.1
10.4
10.2
10.3
—
—
0.97
1.00
0.98
a
a
a
a
Used for Fischer–Tropsch reaction (aer extraction treated with 1% O2/N2 treated at 300 ꢂC/4 h).
Table 5 Hydrogen chemisorption results of a freshly calcined Pt–Co/alumina catalyst
H2 desorbꢁed1
O2 uptakeꢁ1
Corrected
dispersion (%)
Corrected
diameter (nm)
Reductiona
(%)
Catalyst
(mmol gcat
)
(mmol gcat
)
Freshly calcined 0.5% Pt–25% Co/Al2O3
139
1653
11.2
9.2
59.0
a
Hydrogen reduction at 350 ꢂC/10 h.
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the benecial impacts on selectivity (lower methane and higher
C5+) with ammonia addition for the alumina supported cobalt
catalyst; one is that ammonia may inhibit the methanation sites
responsible for the deviation of methane selectivity above that
produced from ASF kinetics; another possibility is that
ammonia inhibits termination of chain growth. The evidence
for the former is a slight decrease in the deviation from the ASF
distribution for methane and the evidence for the latter is an
increase in the value of a. The formation of surface cobalt
nitride or adsorbed NHx might block the methanation sites,
which results in lower methane and higher C5+ selectivity. In
earlier studies39,40 it was reported that cobalt nitride forms in
the presence of ammonia over alumina and silica supported
cobalt catalysts. In one of our very recent investigations,41 we
studied the effect of COS on the activity and selectivity of a Pt–
Co/alumina catalyst. The addition of COS caused a signicant
irreversible deactivation, and lower hydrocarbon (C1–C4) selec-
tivities increased and higher hydrocarbon (C5+) selectivity
decreased compared with unpoisoned catalyst at similar CO
conversion level. S K-edge XANES analysis of the COS poisoned
cobalt catalyst indicated that the formation of cobalt sulde
(Co–S) was responsible for the decrease in CO conversion and
change in product selectivities. Similarly, in the present work,
the addition of ammonia over cobalt catalyst might cause the
formation of cobalt nitride, which might be responsible for the
changes in activity and selectivity.
Fig. 4 Run including ammonia addition and hydrogen treatment for
the Pt–Co/alumina catalyst. Replotted from ref. 20.
In our previous investigations,20,21 we reported that the
addition of ammonia caused a signicant decrease in CO
conversion over supported cobalt catalysts and that catalyst
activity did not recover even aer termination of ammonia
addition under normal FT conditions. However, the results for
this study show that the activity of the Pt–Co/alumina catalyst
virtually recovered to the initial values aer the catalyst had
been subjected to a mild in situ hydrogen treatment (i.e., similar
FTS conditions but the carbon monoxide was switched off) for
a 24 h period (Fig. 4). The addition of ammonia caused
a signicant decrease in activity and also altered the product
selectivity. Aer termination of ammonia addition, the activity
and selectivities did not recover to the initial values but
remained at the poisoned level; this may be explained by
blocking of active sites by chemisorbed surface NHx species,
thereby restricting the adsorption of reactants (H2 and CO).
These surface NHx species are not removed aer the termina-
tion of the ammonia feeding. However, mild in situ hydrogen
treatment removed the surface NHx species and the activity and
Fig. 5 Olefin/paraffin ratio of before, during, after the addition of
ammonia (10 ppmv) and also after hydrogen treatment for the Pt–Co/
alumina catalyst. (Note: After hydrogen treatment means hydrogen
treated after ammonia addition.)
to be reversed (activity and product selectivity) with mild in situ
hydrogen treatment over a Pt–Co/alumina catalyst.
4. Conclusions
product selectivity recovered to the initial values. The olen/ The effects of ammonia on activity and product selectivities
paraffin ratio of the Pt–Co/alumina prior to, during and aer were investigated by varying the concentration of ammonia
addition of ammonia and aer hydrogen treatment conditions during Fischer–Tropsch synthesis. The addition of ammonia
are shown in Fig. 5. As we discussed earlier, the addition of caused a signicant step drop in CO conversion, and the extent
ammonia resulted in an increase in the olen/paraffin ratio or of decrease in CO conversion was similar for all concentrations
olen content relative to ammonia free synthesis conditions. (10 to 1000 ppmv) tested in this study. The product selectivities
Aer the termination of ammonia addition, the catalyst did not were compared at similar CO conversion level for unpoisoned
recover the activity and product selectivities (hydrocarbons, and and ammonia poisoned catalysts. At similar CO conversion, the
olen/paraffin ratio) as well. However, aer a mild in situ addition of ammonia caused an increase in olen selectivity
hydrogen treatment, the activity and product selectivities and decreased the corresponding paraffin and alcohol selec-
recovered almost to their initial values (Fig. 4 and 5, respec- tivities. With increasing concentration of ammonia, the
tively). These results indicate that the effect of ammonia is able formation of olens increased and the formation of paraffins
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and alcohols decreased. The hydrocarbon selectivity was also
signicantly impacted by the addition of ammonia; lower
Increased Catalyst life. International: Patent WO 98/50487,
1998.
hydrocarbon selectivities (C1–C4) decreased and the corre- 16 P. J. van Berge and E. A. Caricato, Oral Presentation at
sponding higher hydrocarbon selectivity (C5+) increased for the
ammonia exposed catalysts compared to under clean condi-
Catalysis Society of South Africa (CATSA), Kruger National
Park, South Africa, 2000.
tions at similar CO conversions. The decrease in methane 17 A. K. Rausch, L. Schubert, R. Henkel, E. van Steen, M. Claeys
selectivity accompanied by the higher olen content of the and F. Roessner, Catal. Today, 2016, 275, 94.
ammonia poisoned catalyst is due to the suppression of 18 V. V. Ordomsky, A. Carvalho, B. Legras, S. Paul, M. Virgine,
secondary reactions of olens (i.e., hydrogenation and isomer- V. L. Sushkevich and A. Y. Khodakov, Catal. Today, 2016, 275, 84.
ization). The suppression of secondary reactions with ammonia 19 W. Ma, G. Jacobs, D. E. Sparks, V. R. R. Pendyala,
addition might be due to the formation of cobalt nitride. The
S. G. Hopps, G. A. Thomas, H. H. Hamdeh, A. MacLennan,
effects of ammonia on activity and product selectivities were
Y. Hu and B. H. Davis, J. Catal., 2015, 326, 149.
able to be reversed (virtually recovered to the initial values) with 20 V. R. R. Pendyala, G. Jacobs, C. Bertaux, S. Khalid and
mild in situ hydrogen treatment over an ammonia exposed Pt–
B. H. Davis, J. Catal., 2016, 337, 80.
Co/alumina catalyst.
21 V. R. R. Pendyala, M. K. Gnanamani, G. Jacobs, W. Ma,
W. D. Shafer and B. H. Davis, Appl. Catal., A, 2013, 468, 38.
22 V. R. R. Pendyala, G. Jacobs, W. Ma and B. H. Davis, in
Fischer–Tropsch Synthesis, Catalysts, and Catalysis: Advances
and Applications, ed. B. H. Davis and M. L. Occelli, CRC
Press, Boca Raton, 2015.
Acknowledgements
This work carried out at the CAER was supported by the
Commonwealth of Kentucky and DOE grant (DE-FC26-08-
NT05988).
23 R. L. Espinoza, J. L. Visagie, P. J. van Berge and F. H. Bolder,
US Pat. 5,733,839, 1998.
24 G. Jacobs, T. K. Das, Y. Zhang, J. Li, G. Racoillet and
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25 Ø. Borg, Z. Yu, D. Chen, E. A. Blekkan, E. Rytter and
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7800 | RSC Adv., 2017, 7, 7793–7800
This journal is © The Royal Society of Chemistry 2017