Effects of Sodium on the Catalytic Performance of CoMn Catalysts for Fischer-Tropsch to Olefin Reactions

Article abstract of DOI:10.1021/acscatal.6b03478

The effects of a sodium (Na) promoter on the catalytic performance of cobalt-manganese (CoMn) catalysts for Fischer-Tropsch to olefin (FTO) reactions were investigated. For the sample without Na, Co⁰ was found to be the active phase for the traditional Co-based Fischer-Tropsch reaction with low CO₂ selectivity. The olefin/paraffin (O/P) ratio was found to be low with a C_2-4⁼ selectivity of only 15.4 C%. However, with the addition of Na, cobalt carbide (Co₂C) quadrangular nanoprisms with the (101) and (020) facets exposed were formed. The Co₂C nanoprisms displayed a high C_2-4⁼ selectivity (54.2 C%) as well as a low methane selectivity (5.9 C%) under mild reaction conditions. The O/P ratio for C_2-4 reached 23.9, and the product distribution deviated greatly from the classical Anderson-Schulz-Flory (ASF) distribution. Co₂C nanoprisms were considered to be an effective FTO active phase with strong facet effects. The Na promoter played a key role in the evolution of the FTO catalysts. The addition of Na, which acted as an electronic donor to cobalt, resulted in stronger CO adsorption and enhanced CO dissociation, which also benefited the formation of the Co₂C phase, leading to highly stable and active catalysts. The effects of other alkali promoters were also studied, and only the K promoter had an effect similar to that of Na on the CoMn catalysts for promoting the FTO reaction. (Graph Presented).

Full text of DOI:10.1021/acscatal.6b03478

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Article
Effects of sodium on the catalytic performance
of CoMn catalysts for Fischer-Tropsch to olefins
Zhengjia Li, Liangshu Zhong, Fei Yu, Yunlei An, Yuanyuan Dai, Yanzhang Yang,
Tie-Jun Lin, Shenggang Li, Hui Wang, Peng Gao, Yuhan Sun, and Mingyuan He
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03478 • Publication Date (Web): 14 Apr 2017
Downloaded from http://pubs.acs.org on April 14, 2017
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Effects of sodium on the catalytic performance
of CoMn catalysts for FischerꢀTropsch to
olefins
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b,c
Zhengjia Li a,b, Liangshu Zhong b,*, Fei Yu b,c, Yunlei An b,d, Yuanyuan Dai
Yanzhang Yang b,c, Tiejun Lin b, Shenggang Li b,e, Hui Wang b, Peng Gao b, Yuhan Sun
b,e,*, Mingyuan He a
,
a
School of Chemistry and Molecular Engineering, East China Normal University,
Shanghai 200062, PR China.
b
CAS Key Laboratory of Lowꢀcarbon Conversion Science & Engineering, Shanghai
Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, PR
China
c University of the Chinese Academy of Sciences, Beijing 100049, PR China
d School of Environmental and Chemical Engineering, Shanghai University, Shanghai
200444, PR China
e
School of Physical Science and Technology, ShanghaiTech University, Shanghai
200031, PR China
ABSTRACT: The effects of the sodium (Na) promoter on the catalytic performance
of CobaltꢀManganese (CoMn) catalysts for FischerꢀTropsch to olefins (FTO) reaction
were investigated. For the sample without Na, Co0 was found to be the active phase
for traditional Coꢀbased FischerꢀTropsch reaction with low CO2 selectivity. The
=
Olefin/Paraffin (O/P) ratio was found to be low with a C2ꢀ4 selectivity of only 15.4
C%. However, with the addition of Na, cobalt carbide (Co2C) quadrangular
nanoprisms with the (101) and (020) facets exposed were formed. The Co2C
nanoprisms displayed a high C2ꢀ4= selectivity (54.2 C%) as well as a low methane
selectivity (5.9 C%) under mild reaction conditions. The O/P ratio for C2ꢀ4 reached
23.9, and the product distribution deviated greatly from the classical
AndersonꢀSchulzꢀFlory (ASF) distribution. Co2C nanoprisms were considered to be
an effective FTO active phase with strong facet effect. The Na promoter played a key
role in the evolution of the FTO catalysts. The addition of Na, which acted as an
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electronic donor to cobalt, resulted in stronger CO adsorption and enhanced CO
dissociation, which also benefited for the formation of the Co2C phase, leading to
highly stable and active catalysts. The effects of other alkali promoters were also
studied, and only the K promoter had the similar effect as Na on the CoMn catalysts
for promoting the FTO reaction.
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KEYWORDS: FischerꢀTropsch to Olefins, Cobalt Carbide, Lower Olefins, Syngas,
Nanocatalysis, Sodium
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1. INTRODUCTION
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Lower olefins (C2ꢀ4=) are extensively used in the chemical industry as building
blocks to synthesize a wide range of products such as polymers, solvents, drugs,
cosmetics, and detergents1ꢀ4. They are traditionally produced by naphtha cracking or
as byproducts in the oil refining process5. With the rapid depletion of the limited
petroleum reserves, alternative production routes for these major commodity
chemicals via nonꢀoilꢀbased processes have attracted increasing attention6.
FischerꢀTropsch synthesis (FTS) is a surfaceꢀcatalyzed polymerization process that
uses the CHx monomers formed by the hydrogenation of adsorbed CO, leading to
hydrocarbons with a broad range of carbon chain length6. The direct conversion of
syngas derived from coal, biomass, and natural gas into lower olefins via the
FischerꢀTropsch to olefins (FTO) process is another attractive option due to its
simplified operation and low energy consumption7. FischerꢀTropsch catalysts can be
used for the selective formation of lower olefins by catalyst modification. Recently,
promoted ironꢀbased catalysts have been extensively studied for the FTO reaction1ꢀ2,
8ꢀ12. de Jong et al. found that Na and Sꢀpromoted iron catalysts using an inert support
with weak interaction exhibited excellent selectivity to lower olefins (61 C%)8. It was
suggested that Na reduced methane selectivity by increasing the chain growth
probability while sulfur reduced the hydrogen coverage on the catalyst resulting in an
even lower methane selectivity and higher olefin selectivity in the products9. The size
effect of iron carbide nanoparticles was also studied by de Jong et al. and it was found
that smaller iron carbide particles display higher surface specific activities mainly due
to higher methane production5, 13ꢀ14. Ma et al. developed a Zn and Naꢀmodulated Fe
catalyst, in which Zn served as the structural promoter and Na altered the electronic
structure. The modification of the electronic structure of the catalyst surface
suppressed the hydrogenation of double bonds and promoted the desorption of
products, which rendered the catalyst unexpectedly reactive toward olefins, especially
C5+ olefins15. However, ironꢀbased FTO catalysts always suffer from coke formation
and sintering due to the high reaction temperature. In recent years, CobaltꢀManganese
catalysts were also investigated for the FTO reaction16ꢀ17. However, the performance
is not good enough for potential industrial applications.
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Product selectivity can be improved by adjusting the process conditions, as well
as by optimizing the catalyst composition and structure. The use of electronic
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promoters to improve the catalytic performance is a wellꢀinvestigated topic in the
context of Coꢀbased FTS catalysts18ꢀ20. Mn has been described as a perspective
promoter to enhance both the catalytic activity and hydrocarbon selectivity21. Bell et
al. found that the Mn promoter led to higher CO surface coverage and facilitated CO
dissociation, which resulted in a heavier hydrocarbon product distribution owing to
less H availability as well as more rapid C1 monomer generation18. Bezemer et al. also
found the chain growth probability increased with the addition of Mn for Coꢀbased
catalysts supported on carbon nanofiber20. The effect of alkali metals on the FTS
reaction has been much studied for Feꢀbased catalysts, but much less for Coꢀbased
catalysts22. Previous studies of Coꢀbased catalysts have shown that the addition of
alkali elements to the supported Co catalysts resulted in a sharp decrease in the
catalytic activity and an increase in the selectivity towards heavier hydrocarbons. The
O/P ratio and the CO2 selectivity also increased upon alkali addition22ꢀ23. Kruse et al.
found that the Kꢀpromoted CoMn catalysts exhibited excellent oxygenates selectivity
up to ~50.0 % with a CO conversion of more than 40 %, and the product distribution
was tuned toward olefins by adjusting the reaction conditions, and the combined
oxygenate/olefin selectivities of ~65 % at a CO conversion up to ~20 % were
obtained for the Co4Mn1K0.1 and Co2Mn1K0.1 catalysts24. However, there were no
specific studies on the effect of Na promoter on the CoMn catalysts for syngas
conversion.
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Recently, we discovered Co2C nanostructures to possess strong facet effects and
Co2C nanoprisms with specifically exposed facets of (101) and (020) as a new FTO
active phase exhibited high C2ꢀ4= selectivity, low methane selectivity and high activity
under mild reaction conditions (250 oC and 1 bar)25. The residual Na was suggested to
play an important role in achieving such a promising catalytic performance. However,
little information was available about the effect of Na on the CoMn catalyst for the
FTO reaction. We reported here an investigation of the Naꢀpromoted CoMn FTO
catalysts, which combined detailed catalyst characterization and performance studies.
These efforts were undertaken with the aim of clarifying the chemical basis for the
observed effects of the Na promoter on the CoMn catalysts. In addition, connections
were established between catalyst structure and performance, which explained the
dependence of the activity and selectivity on the promoter. These findings strongly
favored the hypothesis that the addition of the Na promoter to the CoMn catalysts
promoted the formation and stabilization of the Co2C nanoprisms and the transition of
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the catalytic performance from traditional FTS to FTO. In addition, the effects of
other alkali promoters on the CoMn catalysts were also studied.
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2. EXPERIMENTAL SECTION
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2.1 Catalyst Preparation
The CoMn catalyst without Na was prepared by the coꢀprecipitation method.
Typically, an appropriate amount of cobalt nitrate (Co(NO3)2·6H2O, Sinopharm
Chemical Reagent Co., Ltd) and manganese nitrate (50 wt.% Mn(NO3)2, aq,
Sinopharm Chemical Reagent Co., Ltd) were dissolved in deionized water to form a 2
mol/L mixed salt solution (Co/Mn (atomic ratio) = 2/1). Meanwhile, ammonium
carbonate anhydrous ((NH4)2CO3, Sinopharm Chemical Reagent Co., Ltd) was
dissolved in deionized water to form a 2 mol/L alkali solution as the precipitant. The
aboveꢀmentioned salt and alkali solutions were simultaneously added into a beaker
with 100 ml deionized water under mechanical stirring. The pH value was maintained
o
at 8.0±0.1 and the temperature was kept at 30±1 C during the precipitation process.
After aging for 2 hours at 60 oC, the obtained suspension was centrifuged and washed
o
with deionized water for 8 times, dried at 80 C for 12 hours. Then the samples were
calcined in a furnace at 330 oC for 3 hours under static air.
Naꢀpromoted catalysts with different Na loading (0.1, 0.2, 0.4 and 0.6 wt.% Na)
were prepared by excessive impregnation of aqueous solution of sodium carbonate.
Typically, an appropriate amount of sodium carbonate (Na2CO3, Sinopharm Chemical
Reagent Co., Ltd) was dissolved in 21 ml deionized water. Then, the solution was
mixed with 10 g of CoMn catalysts by magnetic stirring under room temperature for 1
hour. The mixtures were then dried at 110 oC for 12 hours. The samples were calcined
o
in a furnace at 330 C for 3 hours under static air. Other alkali metalꢀpromoted
catalysts were prepared with a similar procedure. The obtained samples were termed
CoMnꢀxM, where x and M denoted the concentration (x=0, 0.1, 0.2, 0.4 and 0.6
wt.%) and identity of the promoter.
2.2 Catalyst Characterization
The concentration of Na was obtained with an Inductively Coupled Plasma
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Optical Emission Spectrometer (ICPꢀOES, Optima 8000, Perkin Elmer).
Xꢀray diffraction (XRD) patterns of catalysts were recorded on a Rigaku Ultima
IV Xꢀray powder diffractometer using Cu Kα radiation with a wavelength of 1.54056
Å at 40 kV and 40 mA. For phase identification, the samples were scanned from 10°
to 90° with a rate of 1°/min (1.2 s per 0.02°) in the continuous scanning mode. For
semiꢀquantitative analysis, the samples were scanned from 40° to 48° in a step
scanning mode with a rate of 0.24°/min (5 s per 0.02°). The content of the different
phases was calculated by reference intensity ratios (RIR). Generally, the integrated
intensity of the most intense peak of each phase was obtained from fitting and
deconvolution of the XRD patterns. The mass fraction of x phase (wx) was calculated
from the following equation:
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ꢂ
ꢃ
ꢀꢁ =
(1)
ꢅ
ꢈ
ꢆ
∑
ꢄꢂꢄ
ꢃ
ꢆꢉꢊ
ꢇꢅꢇ
ꢆ
where I is the integrated intensity of the most intense peak, and the RIR values are
available from the PDF cards.
TEM micrographs and particle size distribution of the catalysts were obtained
with a JEOL JEM 2000FX microscope operating at 200 kV. For the TEM
measurements, the solids were dispersed in ethanol and dropped on a copper
microgrid covered by a holey carbon film. Nanoparticle size distribution for each
sample was determined using samples of ~300 nanoparticles. The surface mean
diameter of the nanoparticles in a catalyst sample was calculated from the following
equation:
ꢈ
ꢎ
∑
ꢌ ꢍ
ꢆ
ꢆ
ꢆ
̅
ꢋ = ∑
(2)
ꢈ
ꢆ
ꢏ
ꢆ
ꢌ ꢍ
ꢆ
where ꢐꢑ is the number of particles with diameter ꢋꢑ in a nanoparticle sample of
o
size N. For the TEM characterization, the catalysts were reduced at 300 C with a
mixture of H2 and Ar (10 % H2 v/v, 12000 ml/(h·gcat)) for 5 hours at the atmospheric
pressure with a heating ramp of 1 oC/min.
Temperatureꢀprogrammed reduction (TPR) experiments were performed with
Micromeritics AutochemꢀII 2920 instrument equipped with a thermal conductivity
detector (TCD) and an MKS Cirrus 2 mass spectrometer. The samples were pretreated
o
in a Helium flow at 200 C for 120 min. Once the TCD signal was stable, the gas
stream was switched to 5 % (v/v) H2/Ar, and the temperature was then raised from 50
to 800 oC at a rate of 5 oC/min.
Nitrogen adsorption measurements were performed on a Micromeritics 2420
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instrument at ꢀ196 C. ~150 mg sample was degassed at 300 C in vacuum for 6
hours. The specific area was calculated by the BrunauerꢀEmmettꢀTeller (BET)
equation. The total pore volume and the average pore size were determined with the
Barrett–Joyner–Halenda (BJH) method.
The rate and extent of cobalt carbonization, where CO dissociates to form Co2C
and CO2, was assessed by a Micromeritics AutochemꢀII 2920 instrument equipped
with a thermal conductivity detector (TCD) and an MKS Cirrus 2 mass spectrometer.
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The catalyst was reduced in situ and exposed to CO at 250 C and 1 bar for a fixed
amount of time ranging from 30 min to 120 min. Then, the tube was flushed with He
for 30 min to remove the gas phase. The yield of Co2C was then determined by
flushing the tube with 5 % H2/Ar and monitoring the effluent for methane using an
MKS Cirrus 2 mass spectrometer.
Fourier transform infrared spectroscopy (FTIR) measurements were made using
a Thermo Scientific Nicolet 6700 FTIR spectrometer equipped with a liquid N2
cooled MCT detector. The spectra were collected in the diffuse reflectance mode from
650 to 3000 cmꢀ1 with a resolution of 4 cmꢀ1 and an accumulation of 64 scans. The
highꢀpurity carbon monoxide (>99.9 %) were used as the probe gas. Helium (>99.999
%) and hydrogen (>99.999 %) were used as the flushing gas and the reducing gas,
respectively. The catalyst (~50 mg) was placed in an infrared cell with a ZnSe
window and reduced in situ with a hydrogen flow of 40 ml/min for 5 hours under
o
atmospheric pressure and 400 C. Subsequently, the cell was flushed for 30 min with
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He with a flow rate of 40 ml/min. The cell was cooled down to 250 C and the
background spectra were recorded. CO was then fed to the cell and IR spectra were
collected repeatedly until the spectra were no longer changing over 30 min.
2.3 Catalyst Evaluation
o
The FTS reaction was performed at 1 bar and 250 C in a tubular quartz
fixedꢀbed reactor (diameter, 8 mm). A moulding process was necessary to form large
particles with sizes of 40~60 mesh (0.3~0.45 mm) prior to the reaction in order to
avoid blockage and improve diffusion. The catalyst was diluted with 0.4 g SiC
powder to remove any temperature gradient within the catalyst bed. The catalyst was
o
reduced prior to the reaction at 300 C with a mixture of H2 and Ar (10 % H2 v/v,
12000 ml/(h·gcat)) for 5 hours at atmosphere pressure, and the heating ramp was 1
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oC/min. The temperature was then dropped to 250 C in He (99.999 %) flow for 30
min to purge the residual reducing gas. Subsequently, the feed flow was switched to a
mixture of N2, H2 and CO (N2/H2/CO=3/64.6/32.3 v/v/v, 3000 ml/(h·gcat)). The
reaction was carried out for 24 hours. During the reaction, the reactor effluent was
analyzed onꢀline by an Aglient 7890A gas chromatograph equipped with two columns
and two detectors. Analysis of N2 (reference), H2, CO, CH4 and CO2 was performed
using a packed column (J&W Q&5A) and a thermal conductivity detector (TCD)
using Helium as the carrier gas. Hydrocarbons from C1 to C6 were analyzed using a
capillary column (Agilent 19095PꢀK25) and a flame ionization detector (FID). The
selectivity was calculated as the percentage of equivalent carbons present in the
hydrocarbon product (C%).
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CO conversion was calculated on a carbon atom basis according to the following
equation:
ꢚꢛ
ꢠꢚꢛ
ꢆꢜꢝꢞꢟ
ꢒꢓ ꢒꢔꢐꢕꢖꢗꢘꢙꢔꢐ =
ꢡꢢꢟꢝꢞꢟ × 100%
(3)
ꢚꢛ
ꢆꢜꢝꢞꢟ
where COinlet and COoutlet represent moles of CO at the inlet and outlet, respectively.
CO2 selectivity was calculated according to the following equation:
ꢚꢛ
ꢏ ꢡꢢꢟꢝꢞꢟ
ꢒꢓꢣ ꢤꢖꢥꢖꢦꢧꢙꢕꢙꢧꢨ = ꢚꢛ
× 100%
ꢡꢢꢟꢝꢞꢟ
(4)
ꢠꢚꢛ
ꢆꢜꢝꢞꢟ
where CO2 outlet denotes moles of CO2 at the outlet.
The selectivity of the individual product CnHm based was obtained from the
following equation:
ꢌꢚ ꢫ
ꢜ
ꢬ ꢡꢢꢟꢝꢞꢟ
ꢒꢌꢩꢪ ꢤꢖꢥꢖꢦꢧꢙꢕꢙꢧꢨ = ꢚꢛ
× 100%
(5)
ꢠꢚꢛ
ꢠꢚꢛ
ꢏ ꢡꢢꢟꢝꢞꢟ
ꢆꢜꢝꢞꢟ
ꢡꢢꢟꢝꢞꢟ
where CnHm outlet represents moles C of the product at the outlet.
The catalytic performance changed significantly at the initial stage. It was very
difficult to perform a full analysis of all the products in the short time period at 1 bar.
In this case, the method by difference was used to calculate the C5+ + Oxy. selectivity
(CO converted to products other than CO2 and the C1ꢀC4 products), where C5++Oxy.
represented hydrocarbons with 5 or more carbons and Oxy. denoted oxygenate
products.
3. RESULTS AND DISCUSSION
3.1 Composite and Textural Properties
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The concentrations of cobalt, manganese and sodium in the unpromoted and
promoted catalysts are presented in Table S1. The concentrations determined with
ICP were similar to the nominal concentrations. For all the CoMn catalysts, the
Co/Mn atomic ratios are approximately 2. For the Naꢀpromoted samples, the Na
loading are 0.006, 0.09, 0.15, 0.37 and 0.60 wt.%, respectively.
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Textural properties were acquired for the reduced and spent CoMn catalysts with
different Na concentrations and presented in Table S2. The SBET, Vtotal and Dpore
values of the various reduced catalysts range from 45.3 to 61.2 m2/g, 0.19 to 0.23
cm3/g, and 11.1 to 14.9 nm, respectively, whereas the SBET, Vtotal and Dpore values of
spent catalysts range from 37.1 to 56.5 m2/g, 0.13 to 0.18 cm3/g, and 10.1 to 12.0 nm.
The specific surface area decreased slightly for most samples after the reaction except
for CoMnꢀ0Na and CoMnꢀ0.6Na. In addition, both the pore volume and pore diameter
decreased after the reaction.
3.2 Reducibility, Morphology and Particle Size Distribution
The TPR results for all the samples are presented in Figure 1a. Two distinct
peaks centered around 250 and 378 oC for the CoMn catalyst without the Na promoter
o
and three distinct peaks centered around 168, 250 and 378 C for the CoMn catalyst
with the Na promoter were identified. The signals from the mass spectrometer
revealed that all three peaks for the Naꢀpromoted CoMn catalyst involved the
consumption of hydrogen and the formation of water (Figure S1). In order to assign
each peak in the TPR profiles, the CoMnꢀ0.4Na sample after the reduction at different
temperatures was investigated by XRD as shown in Figure 1b. Compared with the
calcined CoMnꢀ0.4Na sample, XRD patterns of the CoMnꢀ0.4Na sample reduced for
12 hours at 150 oC showed an increase in the cell volume from 535.1 Å to 541.6 Å. In
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addition, the XRD result for the sample reduced for 12 hours at 250 C indicated the
presence of diffraction peaks corresponding to the CoMn composite oxide with lower
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valency. It was suggested that the first peak around 168 C in the TPR profiles was
caused by partial reduction of the CoMn composite oxide and the second peak around
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250 C was due to the further reduction. The XRD measurement of the CoMnꢀ0.4Na
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catalyst reduced for 12 hours at 375 C revealed the presence of diffraction peaks
corresponding to metallic Co (fcc and hcp) and MnO, indicating the third peak around
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378 C originated from the reduction of the CoMn composite oxide to Co0 and MnO.
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For the CoMnꢀ0.6Na catalyst, the third peak was split into two peaks as high Na
loading would hamper the reduction of CoO to Co0 26
.
TEM images and particle size distributions for the reduced samples are shown in
Figure S2. For all the reduced catalysts, sphereꢀlike nanoparticles were observed with
size distributions ranging from 5 to 30 nm. All the samples were in the CoxMn1ꢀxO
phase, and the dꢀspacings of 2.13 Å and 2.48 Å represented the (200) and (111)
facets, respectively. The surface mean diameters increased slightly with increasing Na
concentration from 14.5 nm to 17.8 nm.
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Figure 1. (a) TPR profiles of CoMn catalysts with different Na concentrations. (b)
XRD patterns of calcined and reduced CoMnꢀ0.4Na catalysts at different
temperatures.
3.3 Effect of the Na Promoter on the Active Phase
Xꢀray powder diffraction was carried out to study the structure of the spent
CoMn catalysts. The results showed the presence of diffraction peaks corresponding
to a mixture of the CoMn composite oxide, MnO, fcc Co, and Co2C as shown in
Figure 2a. The step scanning patterns for 2θ between 43° and 48° are shown in Figure
2b. With the increasing Na concentration, the intensity of fcc Co(111) facet decreased
while the intensity of Co2C(021) facet increased. To get semiꢀquantitative information
of the Co2C weight percentage in all the cobaltꢀcontaining phases, the peaks from the
step scanning patterns of the spent catalysts with different Na concentrations after the
reaction for 24 hours were fitted as shown in Figure S3. For CoMnꢀ0Na, the peaks
belonging to Co2C were rather weak and the Co2C content was only about 2.8 wt.% in
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all the cobalt species. With increasing Na concentration, the weight percentage of
Co2C increased and reached the highest for CoMnꢀ0.4Na (33.2 wt.%). Thus, the Na
promoter appears to facilitate the formation of Co2C. However, further increasing the
Na concentration slightly decreased the amount of Co2C to about 31.1 wt.%.
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Figure 2. XRD patterns of spent CoMn catalysts with different Na concentrations. (a)
Continuous scanning with 2θ from 30° to 90°. (b) Step scanning with 2θ from 43° to
48°.
TEM images of the spent CoMn catalysts with different Na concentrations are
shown in Figure 3. In the case of the spent CoMnꢀ0Na sample, the FTS active phase
(fcc Co0) was observed with a spherical morphology without specific exposed facets.
However, the FTO active phase, Co2C nanoprisms with specific exposed facets of
(101) and (020), were not detected. Recently, we discovered Co2C nanostructures to
possess strong facet effects and Co2C nanoprisms with specifically exposed facets of
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(101) and (020) as a new FTO active phase exhibited high C2ꢀ4 selectivity, low
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methane selectivity and high activity under mild reaction conditions (250 C and 1
bar)25. With the presence of Na promoter (CoMnꢀ0.1Na and CoMnꢀ0.2Na), fcc Co0
and Co2C both in the shape of nanosphere and nanoprism were identified. For the
unpromoted and promoted CoMn catalysts with low Na concentrations (CoMnꢀ0Na,
CoMnꢀ0.1Na, and CoMnꢀ0.2Na), the transition of the active phase from fcc Co0 to
Co2C nanoprisms was observed. For the Naꢀpromoted CoMn catalysts with high Na
concentrations (CoMnꢀ0.4Na and CoMnꢀ0.6Na), Co2C nanoprisms marked by red
rectangle in Figure 3 were observed, while the FT active phase, fcc Co0, was
undetected. This trend can be correlated with the XRD results (Figure S3). For the
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spent CoMn catalysts with high Na concentration, such as CoMnꢀ0.4Na and
CoMnꢀ0.6Na, only the Co2C phase was observed, and no metallic Co0 was detected.
These findings strongly support the hypothesis that the presence of the Na promoter
could stimulate the formation of Co2C nanoprisms. Davis and coꢀworkers had studied
the FTS reaction on Co2C, and found that a fraction of the Co2C was reduced to
metallic Co0 during the FTS process27. Bulk Co2C appeared unstable under FTS
conditions, especially at high reaction temperature. However, in the present study, the
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Co2C nanoprisms were stable at the high reaction temperature of 250 C for a much
longer reaction time (>100 h) as shown in Figure S4, suggesting that the Na promoter
also benefited the stabilization of Co2C.
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Figure 3. TEM images of spent CoMn catalysts with different Na concentrations. (a)
CoMnꢀ0Na, (b) CoMnꢀ0.1Na, (c) CoMnꢀ0.2Na, (d) CoMnꢀ0.4Na, (e) CoMnꢀ0.6Na.
The nanoparticles marked by red circles and rectangles were in the shape of
nanosphere and nanoprism, respectively.
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3.4 Effects of the Na Promoter on Catalytic Performance
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The CoMn catalysts were tested in the FischerꢀTropsch reaction at 250 oC, 1 bar
and a H2/CO ratio of 2 (v/v) to determine the influence of the Na promoter on the
catalytic activity and product selectivity as shown in Figure 4. Each sample was tested
twice at the same reaction conditions in order to understand the statistical significance
of the measurement. As shown in Table S4, both CO conversion and products
selectivities were similar for each test with low relative deviation (always less than
6 %), indicating that the catalytic results were reproducible and reliable. The sample
without the Na promoter (CoMnꢀ0Na) exhibited a high catalytic activity (72.0 C%), a
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high methane selectivity (24.4 C%), a low C2ꢀ4 selectivity (14.3 C%), a low CO2
selectivity (15.7 C%), and a low O/P ratio (C2ꢀ4=/C2ꢀ40 = 0.9). In the present study, the
reaction temperature for all samples was 250 oC, while the normal reaction
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temperature for Coꢀbased FT catalyst was below 220 C with low CO2 selectivity. At
high reaction temperature, the high CO conversion for the CoMnꢀ0Na sample resulted
in high concentration of water. The high reaction temperature and high water partial
pressure improved the activity of the water gas shift (WGS) reaction and led to high
CO2 selectivity. In order to prove the correlation between the reaction temperature
and CO2 selectivity, the CoMnꢀ0Na catalyst was also tested at different reaction
temperatures as shown in Table S3. The CO conversion, CO2 selectivity, methane
selectivity and selectivities to lower olefins increased with the reaction temperature,
while the C5++Oxy. selectivity and O/P ratios decreased. Once Na was added as in the
case of CoMnꢀ0.1Na, methane selectivity decreased to 7.5 C% while C5+ selectivity
increased to 69.6 C%. Previous studies of Coꢀbased FT catalysts have also shown that
the addition of alkali promoters to the supported Co catalysts resulted in an increase in
the selectivity towards heavier hydrocarbons23. Eri et al. reported that the chain
growth probability of FTS on a cobalt/alumina catalyst was 0.75 in the absence of the
alkali promoter, and increased up to 0.87 upon the addition of the alkali promoter22.
Li et al. reported that Co2C is highly efficient for CO nondissociative adsorption,
whereas the Co metal is highly active for CO dissociative adsorption. The interface
between the cobalt metal and its carbide phase benefited for CO insertion to form
oxygenates28. According to the XRD (Figure 2 and S3) and TEM (Figure 3) results of
the spent CoMnꢀ0.1Na sample, both metallic Co0 and Co2C were identified. It is
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reasonable to speculate that the outlier of the CoMnꢀ0.1Na sample was caused by the
coexistence of metallic Co0 and Co2C. With the further increase in the Na
concentration, the CO conversion decreased significantly from 72.0 C% to 5.4 C%.
Davis et al. reported that the activity of cobalt carbide was very low, and the CO
conversion under the similar reaction conditions to those in the present study was
about 12.6 C%27. As Co2C was generated for the samples with the Na addition, it was
speculated that the decrease in the CO conversion was caused by the transition of the
active phase from Co0 to Co2C nanoprisms. Compared with the sample without the Na
promoter, the Naꢀpromoted CoMn catalysts exhibited high CO2 selectivity. Previous
studies indicated that alkali promoted Co2C catalysts exhibited high activity and
stability for the low temperature WGS reaction29. The presence of the Na promoter for
the sample CoMnꢀ0.6Na resulted in a decrease in methane selectivity from 24.4 C%
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to 9.1 C%, but an increase in C2ꢀ4 selectivity from 14.3 C% to 56.3 C% and O/P
ratios from 0.9 to 24.9. The product distributions of the CoMn catalysts with different
Na concentrations are shown in Figure 4f, and the methane selectivity deviated
gradually from the traditional ASF distribution with increasing Na concentration. The
deviation in methane selectivity was caused by a strong facet effect of the Co2C
nanoprisms with the exposed facets of (101) and (020), which were found to suppress
the formation of methane and benefit the selectivity to olefins25. The products
selectivities were also studied at comparable CO conversion (~20 C%) by changing
the GHSV. As shown in Figure S5, the trend in the product selectivities at the CO
conversion of about 20 C% was similar to that at other CO conversions (Figure 4).
However, as the contact time of the CoMnꢀ0.6Na sample increased significantly at the
lower GHSV, the product selectivities shifted towards heavier hydrocarbons. Previous
studies reported that the reinsertion of αꢀolefins into the growing chain is the main
secondary reaction, causing an increase in the growth probability with increasing
chain length and the dependency on the catalyst contact time30.
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Figure 4. Catalytic performance of the CoMn catalysts with different Na
concentrations at 250 C, 1 bar, 3000 ml/(h·gcat), H2/CO=2. (a) CO conversion, (b)
CO2 selectivity, (c) Product selectivity, (d) Lower olefin selectivity, (e) O/P ratios of
C2ꢀ4, (f) product distributions.
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For Feꢀbased FT catalysts, the Na promoter always led to a lower methane
selectivity and a significant increase in the O/P ratio and the WGS activity31ꢀ32. The
effect of the Na promoter is usually explained by a decrease in the strength of CꢀO
bond in the adsorbed CO molecule, leading to a higher coverage of the dissociated
CO on the surface9. We speculated that the addition of the Na promoter have the
similar effect on the Coꢀbased FT catalysts. Therefore, the addition of Na promoter to
the CoMn catalysts could enhance the dissociation of CO and promoted the formation
of Co2C nanoprisms with FTO activity. The Na promoter can also have an electronic
effect on the Co2C nanoprisms and altered the electronic structure of Co2C leading to
low methane selectivity and high olefin selectivity. In the case of Feꢀbased FT
catalysts, the maximum extent of iron carbonization was observed at the optimal Na
concentration. However, higher concentrations of the Na promoter inhibited iron
carbide formation and resulted in lower activity and higher rate of light hydrocarbon
formation32. In our study, the presence of the Na promoter in the CoMn catalysts
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resulted in a sharp decrease in methane selectivity and a significant increase in C2ꢀ4
selectivity and O/P ratios. The best catalytic performance was obtained with the
CoMnꢀ0.4Na catalyst, which had a high catalytic activity (20.4 C%), a low methane
selectivity (5.9 C%), a high lower olefin selectivity (54.2 C%), and a high O/P ratio
(O/P=23.9). However, further increase of Na concentration in the CoMn catalyst led
to increase of methane selectivity and decrease of O/P ratio.
3.5 Origin of the Na Promoter Effects
The extent of Co2C formation was assessed by the quantity of methanated
carbon. The CO carbonization extent was normalized by the mass of the catalyst and
the extent of CoMnꢀ0Na. The CO carbonization rate was assessed by the CO
carbonization extent as a function of the CO exposure time and normalized by the rate
of CoMnꢀ0Na exposed in CO at 30 minutes. The CO carbonization rate and extent of
the CoMn catalyst with different Na concentrations are shown in Figure 5. The
carbonization rate decreased with the CO exposure time. However, the rate and extent
of carbonization increased sharply with Na promoter concentration. The carbonization
rate of CoMnꢀ0.4Na was almost six times than that of CoMnꢀ0Na. A maximum rate
and extent of carbonization were reached for CoMnꢀ0.4Na, consistent with the
catalytic performance, XRD and TEM.
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Figure 5. Relative carbonization rate and extent of the CoMn catalysts with different
Na concentrations. (a) Relative carbonization rate with exposure time, (b) Relative
carbonization rate and extent of the CoMn catalysts with different Na concentrations
at 30 min exposure time.
In situ IR spectra revealed the presence of CO adsorbates on the catalyst surface
when the reduced catalysts were exposed to CO at the reaction temperature. Figure 6
shows the in situ IR spectra obtained after 30 minutes of CO adsorption on the
different catalysts. Close to the rotational/vibrational transitions of CO in the gas
phase around 2174 and 2114 cmꢀ1, the region between 2060 and 1990 cmꢀ1 reflected
the stretching vibrations of CO linearly adsorbed to the surface of the metallic Co0
phase32. The Na promoter induced a shift of this peak toward lower wavenumbers as
presented in Figure 6b. The red shift of the linearly adsorbed CO peak was consistent
with stronger binding of CO to the Co surface18, 33. It has been reported that the
probability of the CꢀO bond dissociation depended mainly on the strength of the
metalꢀcarbon (MꢀC) bond. Strong MꢀC bonds are favored by electronꢀrich metal
centers due to πꢀbackbonding, which at the same time weakens the CO triple bond
leading to lower carbonyl stretching frequencies18, 33. The Na promoter, which acts as
an electronic donor to cobalt, would increase the π* backꢀdonation from Co to
adsorbed CO and hence decrease the carbonyl stretching frequency34. The spectral
region, roughly between 1900 and 1700 cmꢀ1, was commonly assigned to the
bridgeꢀbonded CO on the metallic Co0 surface. The relative peak area centered at
1413 cmꢀ1 increased with Na concentration and reached a maximum for the sample
with 0.4 wt.% Na as shown in Figure 6c. The absence of the peak at 1413 cmꢀ1 for the
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unpromoted catalyst suggested that this peak was attributed to the CO adsorbate
species interacting with the Na promoter. Previous studies of the metal
oxidesꢀpromoted Coꢀbased catalysts by Bell et al. reported that cations within the
metal oxide promoters serve as Lewis acids and speculated that CO may adsorb onto
Co in a tilted configuration so as to interact with nearby metal oxides, thereby
facilitating CO dissociation35. The Na ions have empty orbital to accept electrons and
act as Lewis acids with weak Lewis acidities36. Therefore, it was suggested that the
Na ions exposed at the edge of cobalt could act as Lewis acids enhancing the
adsorption of CO. We note that this effect was sensitive to the fractional coverage of
the metal by the Na promoter and reached a maximum at a coverage of 0.4 wt.%. The
peak centered at 1587 cmꢀ1 was associated with adsorbates related to the interface of
Co and MnOx18. The Na promoter induced a shift of this peak toward higher
wavenumber as shown in Figure 6d. The blue shift of the adsorbed CO peak in the
interface of Co and MnOx ꢀ was consistent with weaker binding of CO. With the
increasing Na concentration, part of Na promoter were interacted with MnOx and
changed its electronic structure, which would suppress the promotion effect of MnOx
promoter. Thus, we proposed that the addition of the Na promoter acted as an
electronic donor to cobalt, as well as a Lewis acid exposed at the edge of cobalt, both
resulting in stronger CO adsorption and enhanced CO dissociation, which attributed to
the formation of Co2C nanoprisms with the promising FTO performance.
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Figure 6. IR study of adsorbed CO on the reduced CoMn catalysts with different Na
concentrations. (a) In situ IR spectra measured at 250 C and 1 bar. (b) Peak
wavenumber of linearly adsorbed CO changing with Na concentrations. (c) Ratio of
the area at 1413 cmꢀ1 of the CoMnꢀxNa (Ax) to that of the CoMnꢀ0.4Na (A0.4)
changing with Na concentrations. (d) Peak wavenumber of adsorbed CO on CoꢀMn
interface changing with Na concentrations.
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3.6 Effects of Other Alkali Promoters
The effects of other alkali promoters on the CoMn catalysts were also
investigated. All catalysts with other alkali promoters were prepared with the same
alkali concentration of 0.4 wt.%. As shown in Table S4, the activity decreased
dramatically with the atomic number of the alkali promoter. The CO2 selectivities of
CoMnꢀ0.4Li, CoMnꢀ0.4Na, CoMnꢀ0.4K and CoMnꢀ0.4Rb were 28.9 C%, 46.5 C%,
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39.7 C%, and 20.7 C%, respectively, whereas the C2ꢀ4 selectivities were 20.0 C%,
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54.2 C%, 57.4 C%, and 45.8 C%. In addition, the O/P ratios for these four samples
were 1.1, 23.9, 18.0, and 5.8, respectively. CoMnꢀ0.4Na was found to be the best FTO
catalyst with the lowest methane selectivity and the highest O/P ratio. The product
distributions of the CoMn catalysts with different alkali promoters are presented in
Figure S6. For CoMnꢀ0Na, CoMnꢀ0.4Li, and CoMnꢀ0.4Rb, the deviation of methane
selectivity in the ASF distribution was not observed. Methane selectivities for
CoMnꢀ0.4Na and CoMnꢀ0.4K were much lower than those predicted from the ASF
distribution. To uncover the active phase of the CoMn catalysts with different alkali
promoters, XRD and TEM studies were carried out as shown in Figures S7 and S8.
The active phases of CoMnꢀ0.4Li were mixtures of fcc Co0 and spherical Co2C. XRD
results of CoMnꢀ0.4Na, CoMnꢀ0.4K and CoMnꢀ0.4Rb revealed the existence of
Co2C. TEM images showed that the Co2C in CoMnꢀ0.4Na had the shape of
nanoprisms, whereas the obtained Co2C in CoMnꢀ0.4K and CoMnꢀ0.4Rb were
mixtures of nanospheres and nanoprisms. The spherical Co2C nanoparticles were
considered as the reason of deactivation in Coꢀbased FT reaction with low activity
and high methane selectivity. As a result, the appearance of spherical Co2C
nanoparticles for CoMnꢀ0.4K and CoMnꢀ0.4Rb was corresponding to the worse FTO
performance with higher methane selectivity compared with CoMnꢀ0.4Na.
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4. CONCLUSIONS
CoMn catalysts with different concentrations of the Na promoter were prepared
and evaluated for the FTO reaction. Characterization of the catalysts by techniques
such as ICP, XRD, N2 adsorption, H2ꢀTPR, and TEM were performed to investigate
the effect of the Na promoter on the formation of the active phase and reveal the
relationship between catalyst structure and performance. The Na promoter was found
to induce the formation of Co2C with specifically exposed facets, which was
responsible for the transition of the syngas reaction from traditional FTS to FTO. The
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maximum C2ꢀ4 selectivity (54.2 C%) with a low methane selectivity (5.9 C%) was
obtained over the CoMnꢀ0.4Na catalysts under mild reaction conditions, and the
active phase was exclusively attributed to Co2C nanoprisms. The O/P ratio for C2ꢀ4
reached a very high value of 23.9, and the product distribution deviated greatly from
the classical ASF distribution. Furthermore, the rate and extent of carbonization
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increased with increasing Na concentration. In situ IR studies revealed that the Na
promoter acted as an electronic donor to cobalt, leading to stronger CO adsorption and
enhanced CO dissociation. Therefore, the addition of the Na promoter to the CoMn
catalysts promoted the formation and stabilization of Co2C nanoprisms with
specifically exposed facets of (101) and (020), which displayed excellent FTO
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catalytic performance with low methane and high C2ꢀ4 selectivity simultaneously.
The effects of other alkali promoters were also examined to show that only the K
promoter had the similar effect on the CoMn catalysts for the FTO reaction.
In addition to the study of the Na promoter effects on the CoMn catalysts, more
efforts are still needed to reveal the role of Mn and the detailed formation mechanism
of Co2C. Obviously, the Co2Cꢀbased FTO route would become more feasible with the
further improvement in the activity, selectivity and stability. CO2 selectivity could be
decreased by hindering the WGS activity. In addition, the catalytic activity and
stability could be substantially increased by the introduction of modifiers and/or the
use of a suitable support (such as ZrO2 and Al2O3).
AUTHOR INFORMANTION
Corresponding Authors
*Eꢀmail for L. S. Zhong: zhongls@sari.ac.cn
*Eꢀmail for Y. H. Sun: sunyh@sari.ac.cn
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge.
Elemental analysis, textural properties, catalytic performance data, mass spectra,
XRD patterns, TEM images and products distributions. (PDF)
ACKNOWLEDGMENTS
This work has been supported by the Natural Science Foundation of China
(21403278, 21573271, 91545112), Shanghai Municipal Science and Technology
Commission, China (15DZ1170500, 16DZ1206900), the Ministry of Science and
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Technology of China (2016YFA0202802) and the Chinese Academy of Sciences
(QYZDBꢀSSWꢀSLH035, Youth Innovation Promotion Association).
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