Disk-Shaped Cobalt Nanocrystals as Fischer–Tropsch Synthesis Catalysts Under Industrially Relevant Conditions

Abstract

Colloidal synthesis of metal nanocrystals (NC) offers control over size, crystal structure and shape of nanoparticles, making it a promising method to synthesize model catalysts to investigate structure-performance relationships. Here, we investigated the synthesis of disk-shaped Co-NC, their deposition on a support and performance in the Fischer–Tropsch (FT) synthesis under industrially relevant conditions. From the NC synthesis, either spheres only or a mixture of disk-shaped and spherical Co-NC was obtained. The disks had an average diameter of 15 nm, a thickness of 4 nm and consisted of hcp Co exposing (0001) on the base planes. The spheres were 11 nm on average and consisted of ε-Co. After mild oxidation, the CoO-NC were deposited on SiO2 with numerically 66% of the NC being disk-shaped. After reduction, the catalyst with spherical plus disk-shaped Co-NC had 50% lower intrinsic activity for FT synthesis (20 bar, 220 °C, H2/CO = 2 v/v) than the catalyst with spherical NC only, while C5+-selectivity was similar. Surprisingly, the Co-NC morphology was unchanged after catalysis. Using XPS it was established that nitrogen-containing ligands were largely removed and in situ XRD revealed that both catalysts consisted of 65% hcp Co and 21 or 32% fcc Co during FT. Furthermore, 3–5 nm polycrystalline domains were observed. Through exclusion of several phenomena, we tentatively conclude that the high fraction of (0001) facets in disk-shaped Co-NC decrease FT activity and, although very challenging to pursue, that metal nanoparticle shape effects can be studied at industrially relevant conditions.

Full text

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Topics in Catalysis (2020) 63:1398–1411
https://doi.org/10.1007/s11244-020-01270-7
1 3
ORIGINAL PAPER
Disk‑Shaped Cobalt Nanocrystals asFischer–Tropsch Synthesis
Catalysts Under Industrially Relevant Conditions
T.W.vanDeelen1· J.M.Harmel1· J.J.Nijhuis1· H.Su2,3· H.Yoshida1,4· R.Oord1· J.Zečević1· B.M.Weckhuysen1·
K.P.deJong1
Published online: 6 May 2020
© The Author(s) 2020
Abstract
Colloidal synthesis of metal nanocrystals (NC) offers control over size, crystal structure and shape of nanoparticles, making
it a promising method to synthesize model catalysts to investigate structure-performance relationships. Here, we investigated
the synthesis of disk-shaped Co-NC, their deposition on a support and performance in the Fischer–Tropsch (FT) synthesis
under industrially relevant conditions. From the NC synthesis, either spheres only or a mixture of disk-shaped and spherical
Co-NC was obtained. The disks had an average diameter of 15nm, a thickness of 4nm and consisted of hcp Co exposing
(0001) on the base planes. The spheres were 11nm on average and consisted of ε-Co. After mild oxidation, the CoO-NC
were deposited on SiO2 with numerically 66% of the NC being disk-shaped. After reduction, the catalyst with spherical plus
disk-shaped Co-NC had 50% lower intrinsic activity for FT synthesis (20bar, 220°C, H2/CO = 2v/v) than the catalyst with
spherical NC only, while C5+-selectivity was similar. Surprisingly, the Co-NC morphology was unchanged after catalysis.
Using XPS it was established that nitrogen-containing ligands were largely removed and insitu XRD revealed that both
catalysts consisted of 65% hcp Co and 21 or 32% fcc Co during FT. Furthermore, 3–5nm polycrystalline domains were
observed. Through exclusion of several phenomena, we tentatively conclude that the high fraction of (0001) facets in disk-
shaped Co-NC decrease FT activity and, although very challenging to pursue, that metal nanoparticle shape effects can be
studied at industrially relevant conditions.
Keywords Cobalt· Nanocrystals· Anisotropy· Disk· Model catalyst· Fischer–Tropsch synthesis
1 Introduction
Controlled synthesis of supported metal nanoparticles is
one of the main challenges in designing heterogenous cata-
lysts for major industrial processes as well as fundamental
research [1–3]. Traditional catalyst synthesis methods have
in common that nanoparticle formation takes place on the
support [4]. Recent advances in catalyst synthesis include
the use of colloidal techniques to synthesize nanocrystals
(NC) in solution followed by deposition on a support [4, 5].
By separating the NC synthesis from deposition onto the
support, the properties of the NC can be manipulated with
greater precision, leading for example to NC of particular
sizes, crystal structures and shapes [6]. Among these proper-
ties, nanoparticle shape is arguably the hardest to tune using
conventional techniques, so here colloidal methods could
offer a clear advantage over existing catalyst synthesis tech-
niques, provided that challenges regarding NC deposition
onto the support and ligand removal are solved [7].
Electronic supplementary material The online version of this
article (https ://doi.org/10.1007/s1124 4-020-01270 -7) contains
supplementary material, which is available to authorized users.
* K. P. de Jong
k.p.dejong@uu.nl
1 Inorganic Chemistry andCatalysis, Debye Institute
forNanomaterials Science, Utrecht University,
Universiteitsweg 99, 3584CGUtrecht, TheNetherlands
2 Laboratory ofMaterials andInterface Chemistry &
Center forMultiscale Electron Microscopy, Department
ofChemical Engineering andChemistry, Eindhoven
University ofTechnology, PO box513, 5600MBEindhoven,
TheNetherlands
3 Institute ofComplex Molecular Systems, Eindhoven
University ofTechnology, PO box513, 5600MBEindhoven,
TheNetherlands
4 The Institute ofScientific andIndustrial Research, Osaka
University, 8-1 Mihogaoka, Ibaraki, Osaka567-0047, Japan
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1399Topics in Catalysis (2020) 63:1398–1411
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Anisotropic NC are often metastable compared to spheri-
cal NC, which are usually the equilibrium shape in particular
in the case of metals [8, 9]. By using ligands that bind pref-
erentially to specific surface facets, it is possible to inhibit
growth in certain directions and to drive the process towards
a kinetically controlled product [10, 11]. Distinct crystal fac-
ets are exposed at the surface of these particles, which may
lead to distinct catalytic performance. Additionally, different
shapes also imply different relative amounts of corner, edge
and terrace sites, the reactivity of which can vary depend-
ing on the type of reaction [12]. The effect of NC shape on
catalytic performance has already been investigated mainly
for noble metal [13–15] or for metal oxide NC [16], yet few
accounts of shape variation for supported base metal NC
catalysts exist.
The shape of NC plays an important role if the catalyzed
reaction is structure sensitive. One pronounced example of
such a reaction is the Fischer–Tropsch (FT) synthesis [12].
In FT, a mixture of H2 and CO called synthesis gas is con-
verted into hydrocarbon fuels and chemicals over a cata-
lyst, typically based on Fe or Co [17]. The cobalt-catalyzed
FT reaction is sensitive to various structural aspects of the
Co particles [18]. For example, a size dependency of the
catalytic performance of cobalt is generally reported, with
the surface-specific activity and C5+-selectivity decreasing
sharply below a particle size of 6–8nm [19–21]. Addition-
ally, several studies indicated that the crystal structure of
Co is relevant, with hcp being more active than fcc [22–26],
although this topic remains controversial [27]. Given the
potential influence of the crystal structure, the shape of Co
particles might be important as well, as shape partially gov-
erns the surface structure of nanoparticles.
For the potential effect of the NC shape on catalytic FT
performance we should resort mainly to theoretical and sur-
face science studies, as little experimental data is available
on complex model catalysts under high-pressure conditions.
Theoretical studies predict that the B5 surface ensemble is
essential for CO-dissociation, which is considered an impor-
tant step for activity and selectivity. The occurrence of coor-
dinatively unsaturated sites, including B5 sites, varies with
particle size [28, 29] and crystal structure [29] and offers
one explanation for the observed structure sensitivity. Fur-
thermore, a computational comparison of the FT activity of
various Co crystal planes indicated that higher index planes
of hcp Co are orders of magnitude more active than the close
packed (0001) [25]. Surface science experiments on single
crystal planes of hcp Co showed that relatively more CH4 is
produced on hcp (0001) than on higher index planes, leading
to a lower selectivity towards C5+-products [30, 31]. How-
ever, a recent experimental study indicated that hcp (0001)
displayed intermediate activity and C5+-selectivity, with
both increasing in the order hcp {11
20
} < {0001} < {10
11
}
[32]. All of these aspects of surface composition could in
principle be affected by the shape of the Co-NC, although it
is also possible that surface defects [33], hydrocarbon cov-
erage [34] or CO-induced surface reconstruction [35–41]
might in practice annihilate the effects of Co-NC shape.
Surface science cannot always be directly translated
to more complex catalysts and high-pressure systems [5].
To bridge the gap between surface science and supported
catalysts under industrially relevant reaction conditions,
3D model catalysts containing anisotropic Co particles are
required. Examples of anisotropic cobalt nanoparticles in
FT include unsupported particles, such as CoMnOx nano-
prisms [42], Co nanocubes [43], bimetallic nanorods [44],
and Co3O4 nanorods [45], plates or cubes [32]. Furthermore,
hcp nanorods supported either on SiO2–Al2O3 [46] or on Ni
or Cu foams [47] have also been employed. Clearly, syn-
thetic approaches are still required to produce anisotropic
model systems that closely resemble conventional supported
cobalt catalyst, with stable Co-NC shapes yet without addi-
tives, such as a second metal, that might influence the FT
performance.
Here, we report our efforts to advance the synthesis,
characterization and testing of shape-controlled Co-NC
dispersed on a support as relevant model catalysts for the
Fischer–Tropsch synthesis. We consistently obtained a mix-
ture of disk-shaped and spherical Co-NC. The disks were
15nm in diameter with a thickness of 4nm and consisted of
hcp Co with (0001) terminating the base planes, while the
spherical particles consisted of ε-Co. After low-temperature
oxidation of the Co-NC, the CoO-NC were deposited on
SiO2 (50m2 g−1). Numerically 66% of the NC were disks,
leading to a higher occurrence of hcp (0001) on the cobalt
surface compared to purely spherical Co-NC. The FT activ-
ity of the disks/spheres mixture was half that of spheres
only, while the C5+-selectivity was identical. Remarkably,
the shape endured the FT conditions, as 62% of the NC after
catalysis were disks of the original dimensions. Nitrogen-
containing ligands were largely removed, and both catalysts
featured the same crystalline composition, leaving the pos-
sibility for enhanced hcp (0001) exposure in disks. Based
on these experiments, it was concluded that the disk shape
of Co-NC had a strong influence on the FT performance,
most probably caused by the introduction of a high number
inactive low-index Co planes.
2 Experimental Methods
2.1 Synthesis ofDisk‑Shaped NC
A mixture of disk-shaped and spherical Co-NC was synthe-
sized based on an adapted procedure originally developed
by Puntes etal. [10, 48]. The ligands, 50mg oleic acid (90%
technical grade, Sigma-Aldrich) and 600mg octadecylamine
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1400 Topics in Catalysis (2020) 63:1398–1411
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(> 99.0% (GC), Sigma-Aldrich), were transferred to a three-
necked 100mL flask with two septa and a reflux condenser
connected to a Schlenk line. The mixture was heated to
80°C under vacuum using a heating mantle while magneti-
cally stirring at 600rpm. After 30min, the setup was flushed
three times with N2 before switching to N2 for the remainder
of the synthesis. Then, 7.5mL 1,2-dichlorobenzene (99%
Anhydrous, Sigma-Aldrich) was added to the mixture with-
out air-exposure, a thermocouple was inserted and the mix-
ture was heated to reflux temperature (188°C). Meanwhile,
270mg dicobalt octacarbonyl (95%, stabilized with hexane,
Acros Organics) was transferred to a vial in an N2-glovebox
and the vial was left open inside the glovebox for 40min to
age. After ageing, the Co2(CO)8 was dissolved in 1.5mL
1,2-dichlorobenzene and the vial was closed using a cap with
a septum and removed from the glovebox. Once the liquid
was at reflux temperature, the cobalt precursor was rapidly
injected into the center of the flask using a syringe with a
Ø1.2 × 40mm needle under 600rpm stirring. The synthesis
was quenched after 5min using a water bath. The water bath
was removed after ~ 3min when the temperature of the reac-
tion mixture was below 30°C. Simultaneously, the stirring
was stopped and the stirring plate placed 5cm below the
flask to magnetically separate the reaction product. The top
7mL contained the highest concentration of disk-shaped
Co-NC and was transferred to a glass centrifuge tube after
5min and exposed to air from this point on. Consequently,
the NC were oxidized to CoO to facilitate their uniform dis-
tribution over a support at a later stage [7]. The centrifuge
tube was filled to 20mL with 2-propanol (≥ 99.5% (GC),
Merck) and centrifuged at 2500G for 10min. The purple/
brown supernatant was decanted and the residue redispersed
in 0.5mL n-hexane (99%, Acros Organics) using sonication.
Another precipitation-redispersion cycle was performed by
filling the tube to 20mL with 2-propanol, centrifuging at
2500G for 40min, decanting the supernatant and finally
redispersing the residue in 10mL n-hexane using sonication.
2.2 Deposition ofDisk‑Shaped NC
The disk-shaped CoO-NC were deposited on SiO2 (Aerosil
OX 50, Evonik) within 4h after synthesis. To obtain enough
material with sufficient metal loading, two NC synthesis
batches were deposited consecutively on the same batch
of support. A glass round bottom vessel with an overhead
mechanical stirrer was placed in a sonication bath. 300mg
SiO2 and 20mL n-hexane were added and the top of the
vessel was loosely covered with aluminum foil. Stirring and
sonication were applied to suspend the SiO2 for 20min and
then the first CoO-NC batch in 10mL n-hexane was added
dropwise in 15min under continued stirring and sonication.
The mixture was left for 45min after the addition and then
the n-hexane was slowly evaporated in 15min by a applying
a flow of N2 through the vessel, all under sonication and stir-
ring. This entire procedure was repeated a second time by
suspending the dried uniformly grey powder again in 20mL
n-hexane and adding the second batch of CoO-NC. After-
wards, the dried powder was suspended in 10mL n-hexane
and transferred to a glass centrifuge tube. The solid was
precipitated by centrifuging at 2500G for 1min and the
supernatant was decanted. The residue was 5 times resus-
pended in 5mL n-hexane and precipitated by centrifuging
at 2500G for 1min and finally dried at ambient temperature
under vacuum for 16h. The dry powder was pressed and
sieved to a grain size of 75–150µm (100mg was obtained).
2.3 Synthesis ofSpherical NC
Exclusively spherical Co-NC were synthesized according to
a comparable hot-injection method, however, only oleic acid
was used as ligand according to a procedure described before
[49]. The synthesis was performed in N2 atmosphere using a
Schlenk line. 65mg oleic acid was degassed under vacuum
at 100°C for 30min under 650rpm magnetic stirring in a
3-necked 100mL flask with a condenser and two septa. The
flask was flushed with N2 and the synthesis was continued
under N2. Then, 7.5mL 1,2-dichlorobenzene was added
under N2 atmosphere and the solution was heated to 165°C.
Meanwhile, 270mg dicobalt octacarbonyl was dissolved in
1.5mL 1,2-dichlorobenze and sealed under N2 atmosphere
inside a glovebox. This cobalt precursor was then rapidly
injected (needle: Ø0.9 × 70mm) into the pre-heated solution
at 750rpm stirring and reacted vigorously. After 20min, the
reaction was quenched and cooled to at ambient temperature
using a water bath. One septum was then removed to oxidize
the Co-NC to CoO-NC by air-exposure at low temperature
for 1h (650rpm stirring). The suspension was divided after
over two glass centrifuge tubes, which were filled further to
20mL with 2-propanol and centrifuged at 2200G for 30min
to precipitate the NC. The supernatant was decanted, the
precipitate redispersed in 0.5mL n-hexane by sonication
and the tubes were filled again to 20mL with 2-propanol
and centrifuged at 2200G for 40min. This precipitation-
redispersion cycle was repeated two more times and after-
wards, the residue was redispersed and combined in 2mL
n-hexane in total.
2.4 Deposition ofSpherical NC
The spherical CoO-NC were deposited on the same low
surface area SiO2 as the disk-shaped CoO-NC, according
to a method described before [49]. The SiO2 had already
been pressed and sieved to a fraction of 75–150µm. The
support (500mg) was transferred to a 100mL 3-necked
flask and a mixture of the Co-NC in n-hexane and 6.5mL
1-octadecene (90%, technical grade, Sigma-Aldrich) was
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1401Topics in Catalysis (2020) 63:1398–1411
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added under 400rpm magnetic stirring. The flask was then
equipped with a condenser, glass plug and septum and con-
nected to a Schlenk line. The suspension was degassed under
vacuum at 100°C for 15min and as a result, the n-hexane in
the suspension evaporated. Then, the flask was flushed with
N2 and the deposition was continued under N2. The suspen-
sion was heated to 200°C for 30min, after which the heating
mantle was removed, causing the solution to cool to room
temperature within 30min. The sample was washed by first
centrifuging at 1500G for 5min and decanting the superna-
tant. Subsequently, the precipitate was resuspended in 2mL
n-hexane through sonication, 6mL acetone was added and
the sample was isolated by centrifuging at 1500G for 5min
and decanting the supernatant. This precipitation-resuspen-
sion cycle was repeated five times, followed by a last cycle in
which the n-hexane/acetone mixture was replaced by 20mL
acetone. Finally, the sample was dried in three consecutive
steps, i.e. 60°C for 1h in static air, 120°C for 3h in static
air and 80°C for 3h under vacuum, and pressed and sieved
to a grain size of 75–150µm.
2.5 Characterization
Transmission electron microscopy (TEM) was performed
on a Tecnai 20 (FEI) operated at 200kV. Samples of col-
loidal suspension were prepared by drop casting a diluted
sample directly on a carbon-coated TEM grid. In the case
of SiO2-supported CoO-NC, the powder was first suspended
in 2-propanol by sonication and subsequently drop casted
on a carbon-coated TEM grid. The TEM images of sup-
ported CoO-NC were manually analyzed using ImageJ. In
the case of the colloidal suspensions, the images of CoO-NC
were automatically classified as either disks or spheres and
manually reviewed before analysis. All reported NC sizes
are number averaged and recalculated to the equivalent Co0
size to compensate for lattice expansion due to a 3nm thick
CoO passivation layer. Because of the limited height of the
disks, these NC were assumed to oxidize completely to CoO.
The metallic surface area was calculated using the surface
area weighted mean diameter, again after correcting for the
3nm CoO passivation layer. A cross-sectional area of 0.0662
nm2 per cobalt atom was assumed in order to calculate the
metallic surface area [50].
Vitrification of freshly synthesized colloidal suspensions
for cryogenic transmission electron microscopy (cryo-TEM)
was performed using a Vitrobot Mark IV (FEI) vitrifica-
tion robot. The Co-NC were vitrified in the original reaction
suspension within 30min from magnetically separating the
synthesis product. For the vitrification, 3µL of Co-NC sus-
pension was applied to a 200 mesh, lacey carbon-coated Cu
grid at 25°C in a chamber saturated with toluene vapor. Sub-
sequently, the grid was blotted for 3s with filter paper and
plunge-frozen in liquid N2 (− 196°C). The samples were
kept in liquid N2 for several hours and loaded into the Titan
Krios (FEI) equipped with a field emission gun operating
at 300kV and energy filter. Images were recorded using a
2k × 2k Gatan charge coupled device (CCD) camera. High-
resolution transmission electron microscopy (HR-TEM) was
performed in areas where no solvent appeared present. Low
dose selective area diffraction (SAED) was used for structure
analysis and the diffraction patterns were processed accord-
ing to the method described by Leijten etal. [51].
Electron tomography was carried out using an FEI Talos
F200X operated at 200kV in bright field TEM mode. The
CoO-NC/SiO2 sample was suspended in ethanol by ultra-
sonication. A few droplets of the suspension were deposited
onto a Quantifoil R2/1 carbon film supported parallel-bar Cu
grid which already contained Au nanoparticles of 5nm in
diameter. TEM images were recorded with a Ceta camera
(4096 × 4096pixels) over a tilt range of − 6° to + 76° with
tilt increments of 2° at a nominal magnification of 74,000
times. The tilt series was aligned using IMOD software [52]
using the Au nanoparticles as fiducial markers. The aligned
tilt series were binned by 4 and reconstructed using the back-
projection algorithm. The resulting reconstructed volumes
had a final voxel size of (0.572nm)3. Segmentation of disk-
shaped CoO-NC, spherical CoO-NC and the SiO2 support
in the reconstructed volumes was performed in FIJI [53].
Adequate threshold values were applied to median-filtered
reconstructed volumes. Disk-shaped NC, spherical NC and
the SiO2 support were defined in 3D by manually delimiting
their boundary. Volume rendering of the segmented volumes
was carried out in FIJI.
Inductively coupled plasma-optical emission spectros-
copy (ICP-OES) was performed on a SPECTRO ARCOS
after digestion of the sample in aqua regia.
X-ray photoelectron spectroscopy (XPS) was measured
on a Kratos Axis Ultra DLD with an X-ray source operat-
ing at 225W and 15keV, and equipped with an Al anode
(EKα = 1486.6eV). The samples were fixed using Ag-tape at
ambient conditions and loaded in the setup. High-resolution
spectra of Co2p, Si2p, O1s and C1s were measured at 40eV
pass energy, while N1s and the survey scan were recorded
at 160eV pass energy. Kratos Axis sensitivity factors were
used for quantification. XPS measurements were performed
on three distinct locations on all samples and the results
were averaged. In case of the N1s scan, multiple measure-
ments of the same location were accumulated to improve the
signal-to-noise ratio. The binding energies were calibrated
by setting the C–C/C–H bond peak to 284.8eV.
In situ X-ray diffraction (XRD) was performed on a
Bruker D8 Discover with a Mo (Kα10.709Å) source in
Debye–Scherrer transmission (capillary) geometry. The
X-ray beam was focused on a quartz capillary with 1000µm
OD and a wall thickness of 10µm using a Göbel-mirror. The
setup was equipped with an energy dispersive LynxEye XE
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1402 Topics in Catalysis (2020) 63:1398–1411
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Position Sensitive Detector (PSD). Details on the complete
setup can be found in recent publications [54, 55]. The pat-
terns were measured between 5 and 39° 2θ with an incre-
ment of 0.07° and time per step of 20.5s. Multiple scans
were accumulated to improve the signal-to-noise ratio only
if the patterns were identical. Rietveld Quantitative Phase
Analysis (Rietveld QPA) was performed on the measured
diffractograms using Bruker TOPAS v5 software. Details
of the Rietveld refinement procedure are given in Support-
ing Methods.
Co-S/SiO2 was diluted with 2 mass equivalents SiO2
(Aerosil OX 50) to obtain a similar overall cobalt loading
as Co-D/SiO2. Typically, 7mg sample in the sieve frac-
tion 75–150μm was loaded in the capillary and fixed with
quartz wool at either end, resulting in a catalyst bed length
of ~ 20mm. The capillary was placed in the setup and the
catalysts were reduced insitu at 350°C for 8h (1°C min−1)
in a 1.2mL min−1 flow of 25 vol% H2 in He at 1bar. After
reduction, the temperature was lowered to 180°C with
3°C min−1 and at this temperature, a 3.0mL min−1 flow
of synthesis gas H2/CO = 2(v/v) was introduced. After
10min stabilization, the back-pressure regulator was set to
10.6bar. Only after the set pressure was reached and stable,
the temperature was increased to 220°C with 1°C min−1,
which was defined as TOS = 0h. Insitu XRD patterns were
recorded both during reduction and FT.
2.6 Fischer–Tropsch Synthesis
The Fischer–Tropsch synthesis performance of the catalysts
was investigated using a Flowrence (Avantium) 16 paral-
lel reactor setup. 80mg Co-D/SiO2 and 12mg Co-S/SiO2
were first diluted with 100mg SiC (212–425µm) and then
loaded into stainless steel plug-flow reactors of 2.6 and
2.0mm (ID), respectively. The samples were dried for 2h
in an He flow and reduced insitu in a 25vol% H2 in He flow
at 350°C for 8h after a heating ramp of 1°C min−1. After
reduction, the temperature was lowered to 180°C with 3°C
min−1 and the reactors were pressurized to 20bar in an H2
flow. Then, the synthesis gas feed was introduced with H2/
CO = 2v/v and 5vol% He as internal standard and after 1h
stabilization, the reactors were heated to 220°C with 1°C
min−1, which was defined as the starting point for FT. After
the run, the catalysts were treated in H2 at 200°C for 10h
to remove carbonaceous products from the catalyst surface
and cooled down further under He.
The reaction products were analyzed online using an
Agilent 7890A GC with two separate channels. In the first
channel, the hydrocarbon products were analyzed using an
Agilent J&W PoraBOND Q column in combination with
an FID. In the second channel, the permanent gasses were
separated on a ShinCarbon-ST column and analyzed using
a TCD. The catalytic activity was given as CO conversion
(XCO) cobalt-time yield (CTY) and turnover frequency
(TOF). The TOF was based on the CTY and the average
metallic surface area of the spent catalyst, as determined by
TEM. The selectivity (in %C) to C1-C4 hydrocarbons was
determined as SC1-C4 = 100 FCn n (FCO,in XCO)−1 with n rep-
resenting the product’s carbon number and F the flow of the
hydrocarbon or CO. The selectivity to C5+ hydrocarbons was
calculated as SC5+ = 100%C-SC1-4.
3 Results andDiscussion
Cobalt nanocrystals (NC) were synthesized by a hot-injec-
tion synthesis using oleic acid and octadecylamine as ligands
with the aim to synthesize disk-shaped NC and the results
were analyzed by TEM (Fig.1). Both disk- and spherical-
shaped CoO-NC were obtained from a single synthesis with
the ratio between the two varying strongly with the reaction
conditions, which made separation necessary, in line with
literature [10]. Enrichment of the disk-shaped over spherical
NC was realized by a magnetic field, using the difference
in the magnetic properties between the different NC. In our
case, the large magnetic moment of the spheres resulted in
faster precipitation from the solution compared to the disks
under the weak magnetic field of the stirring plate (Fig.S1).
This led to enhanced concentrations of disks in the top layer
of the reaction mixture (Fig.1a–e). After magnetic separa-
tion, the work-up procedure also affected the disk-to-sphere
ratio, as the disk yield decreased with more precipitation-
redispersion cycles (Fig.1a–c). This indicates that spheres
were preferentially precipitated during centrifugation or
alternatively, that the fraction of disks already varied in the
original reaction mixture of different batches.
The elongated, rod-like particles (mainly visible in
Fig.1a) were identified as stacks of disk-shaped particles
orientated side to side. From these particles, the average
dimensions of the disks were determined (Fig.S2). The
average diameter of the disks was 15.5 ± 3.5nm and the
thickness 4.2 ± 0.6nm, leading to an aspect ratio (height/
diameter) of 0.27. As a comparison, all-spherical CoO-NC
of 8.3 ± 1.1nm were synthesized using only oleic acid as
ligand, as described before [49] (Fig.1f).
A batch containing disk-shaped NC after three precip-
itation-redispersion cycles was analyzed in more detail
(Fig.1c). The NC in the TEM image were semi-automati-
cally divided into two groups (Fig.1e) based on their con-
trast, which is a measure for their thickness, in relation to
their size and morphology. The large, low-contrast NC were
(truncated) hexagonally-shaped and the smaller, high-con-
trast NC were spherical. To examine the 3D shape of these
NC, the stage was tilted 50° inside the TEM (Fig.1d, S3 and
supporting movie S1). Analysis of 100 NC from both groups
(Fig. S4) revealed that the size of the particles was 15.4nm
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1403Topics in Catalysis (2020) 63:1398–1411
1 3
at 0° and 11.9nm at 50° in the case of the low-contrast
particles, proving that these NC were indeed smaller in the
z direction as expected for disk-shaped NC. When consid-
ering an aspect ratio of 0.27, the 2D projection length at
50° should be 13.1nm (Fig.S5), which matches the exper-
imental value quite well. On the other hand, the smaller,
high-contrast particles were 10.7nm at 0° and 10.6nm at
50°, showing that their diameter was the same in all direc-
tions and that these NC were indeed spherical. Because the
shapes of the CoO-NC could be accurately distinguished,
the numerical disk yield could be determined and was 53%
for the batch after three precipitation–redispersion cycles.
Disk-shaped Co-NC are a metastable phase [8, 56], mak-
ing the synthesis results highly susceptible to small varia-
tions in the procedure. Therefore, optimizing the reaction
conditions was essential to obtain a high fraction of ani-
sotropic particles. Important synthesis parameters included
the speed of magnetic stirring and the speed of precursor
injection, as faster stirring or slower injection through a thin-
ner needle led to enhanced formation of 10–30nm spheri-
cal NC and a lower yield of disk-shaped NC. Furthermore,
higher disk yields were obtained with batches of Co2(CO)8
that had been aged 40min in the glovebox prior to use in
synthesis. These phenomena have not been reported before
for this synthesis method and illustrate the delicate nature
of this approach.
The crystal structure of the as-synthesized Co-NC
before oxidation was analyzed using a combined cryo-
and high-resolution-TEM approach (Fig.2). A Co-NC
suspension was vitrified shortly after NC synthesis and
transported and loaded in liquid N2 to avoid oxidation of
the Co-NC. The vitrification protected the sample from
oxidation and HR-TEM was possible because there was
little to no solvent present in the TEM sample. Electron
Fig. 1 TEM images of several
batches of mixed spheres/
disks CoO-NC after different
washing procedures followed
by dispersion and drying on the
carbon foil (a–e). The wash-
ing procedures consisted of a
one precipitation-redispersion
cycle, b two precipitation-
redispersion cycles and c–e
three precipitation-redispersion
cycles. c–e Images of the same
location with d showing the
sample after tilting by 50° and e
with an overlay highlighting the
particles that were identified as
disks (red) and spheres (aqua). f
Spherical CoO-NC synthesized
using only oleic acid as ligand
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1404 Topics in Catalysis (2020) 63:1398–1411
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diffraction of the area in Fig.2a resulted in the diffraction
pattern in the inset of Fig.2a and was radially averaged
to give Fig.2b. The main peak at d = 2.06Å could not be
unambiguously assigned to one crystal structure, as this
lattice spacing is present in all three metallic cobalt phases
(Fig. S6). The diffraction peak at d = 2.19Å, however, only
matched hcp cobalt. Furthermore, the peaks at d-values of
1.26 and 2.19Å could be ascribed to {11
20
} and {10
10
}
of hcp cobalt, respectively, which were more intense than
expected. Considering that {11
20
} and {10
10
} have perio-
dicity in the x,y direction and not in the z direction, this
proved that the hcp cobalt was present in anisotropic, disk-
shaped NC and that the short z axis is along the [0001]
direction, in line with literature [10]. Based on the inten-
sity ratios of the hcp Co phase, it is likely that the peak at
d = 2.06Å has another dominant contribution either from
fcc or ε-cobalt.
Further information on the crystal structure of the NC
before oxidation was obtained by investigating an individual
NC in more detail with HR-TEM on the same cryogenic
sample. The Co-NC in the framed area in Fig.2c was iden-
tified as a disk-shaped NC because of its low contrast and
diameter of 15nm. Lattice fringes of this particle were
imaged and the Fourier transform revealed hexagonal sym-
metry of the crystal lattice, indicative of hcp cobalt. The
determined lattice spacings were 2.28 and 1.29Å, which
were again matched to {11
20
} and {10
10
} of hcp Co. Addi-
tionally, the other prominent particle of 11nm with higher
contrast was identified as a spherical NC and did not show
hexagonal symmetry. Its lattice spacings matched ε-cobalt
best (Fig.S7), indicating that the other crystal structure
observed by electron diffraction was most likely ε-cobalt,
which is consistent with literature [10]. It should be empha-
sized that this concerned the crystal structure of the Co-NC
before oxidation and that it is uncertain what crystal struc-
ture would evolve after low-temperature oxidation, insitu
reduction and catalysis.
The CoO-NC after two precipitation-redispersion cycles
were deposited on low surface area SiO2 (50m2 g−1) using
a method that avoids the use of high temperatures to affect
the NC shape as little as possible. This particular type of
SiO2 was selected, because it had yielded very stable Co-NC
based catalysts under FT conditions before [49]. The sample
containing mainly disk-shaped NC was designated as Co-D/
SiO2. Two washing steps after NC synthesis were the opti-
mal compromise between uniform attachment of the CoO-
NC with lower ligand concentration on one hand (Fig.S8)
and progressively lower NC yields on the other. Based on
ICP-OES, a cobalt loading of 1.4wt% was obtained using
this deposition method. The Co-D/SiO2 sample was ana-
lyzed by TEM (Fig.3a–c). The NC were largely uniformly
distributed over the support, although some areas with clus-
tered disk-shaped and spherical NC were encountered as
well (Fig.3c). Despite this occasional clustering of NC, NC
were largely well dispersed on the support, making the cata-
lyst useful for further investigations.
It was challenging to distinguish the disk-shaped NC from
the SiO2 support because of the low contrast of the CoO-
NC associated with their limited thickness when orientated
perpendicular to the electron beam. However, based on a
combination of contrast, size and morphology, it was pos-
sible to identify the CoO-NC and discriminate between disks
and spheres (Fig.3a, b). After analysis of 203 NC, 66% of
the NC were classified as disks and 34% as spheres. In addi-
tion, a reference catalyst containing exclusively spherical
CoO-NC was prepared by depositing the 8.3nm CoO-NC on
the same low surface area SiO2 (Fig.3d). This catalyst was
designated as Co-S/SiO2. The obtained cobalt loading was
9.6wt% based on ICP-OES. Furthermore, the CoO-NC were
Fig. 2 High-resolution TEM under cryogenic conditions of as-syn-
thesized disk-shaped and spherical Co-NC with little to no solvent
present. a Overview of several clustered Co-NC with the selected
area electron diffraction pattern in the inset. b Intensity profile
obtained by radially averaging and baseline correcting the SAED
pattern. c Several Co-NC at higher magnification. The framed area
contained a 15nm, low contrast NC that was classified as a disk; the
inset gives the Fourier transform of the framed area
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1405Topics in Catalysis (2020) 63:1398–1411
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uniformly distributed over the support and no areas contain-
ing clustered NC were observed. This catalyst was described
in more detail in an earlier publication [49].
Electron tomography of Co-D/SiO2 was performed to
investigate the morphology of the NC on the support in
more detail (Fig.4, complete tilt series from − 66 to + 76°
included as supporting movie S2). The disks could be clearly
distinguished from the spheres using the tilt series, because
only the projected aspect ratio of the disks varied with the
tilting angle. The accuracy of the method applied before
to classify the NC as disks or spheres based on size, mor-
phology and contrast was confirmed by tomography and the
obtained results for this limited sample size (only 26 NC in
field of view) matched the earlier observation of ~ 65% of
the NC present as disks. Furthermore, a 3D representation
of a selected part of the sample was obtained through recon-
struction and segmentation of the tilt series (Fig.4c, recon-
struction and 3D-rendered volume as supporting movies S3,
S4). From this representation, the morphology of NC can be
clearly inferred as well as their interaction with the support.
It showed that the disk-shaped NC typically attached to SiO2
with one base plane. Future work on metal-support interac-
tions for the differently shaped NC could involve, e.g., XPS
experiments to explore electronic interactions of the Co-NC
with the support.
The FT performance of the catalysts was evaluated over
100h on stream at similar CO conversion levels (Fig.5,
Table1). The cobalt-weight-based activity (cobalt-time
yield, CTY) against TOS is shown in Fig.5b. Co-S/SiO2
was twice as active per unit weight of cobalt as Co-D/SiO2.
Because of the different particle sizes and shapes, the activ-
ity was compared as cobalt-surface-based activity (turnover
frequency, TOF). The metallic surface area necessary for the
TOF was calculated from the NC sizes in the spent catalyst,
which will be discussed later (Table1). The TOF of Co-D/
SiO2 was 49 × 10–3s−1 and the TOF of Co-S/SiO2 was 86 ×
10–3s−1, the former value being in line with other work on
Co/SiO2 using conventional catalyst preparation techniques
such as impregnation [57]. However, the TOF of Co-S/SiO2
was remarkably high, possibly pointing to a higher fraction
of more active hcp Co [22–26] resulting from the NC-based
synthesis compared to conventional synthesis methods.
The selectivities towards C5+, as well as C1- and C2–C4,
products were similar (Table1) and overall ~ 5% lower than
on conventionally prepared Co/SiO2 catalysts [57]. Con-
sequently, the ASF distributions and derived chain growth
probabilities (α) were comparable as well. Plots of the
C5+-selectivity as a function of TOS and the ASF distribu-
tions are included as Fig.S9. The effect of crystal structure
on selectivity is still under debate, as some reports indi-
cated no difference between hcp and fcc Co [22, 27], while
others observed higher C5+-selectivity on hcp Co [24, 58].
However, in our case, similar C5+-selectivity of both cata-
lysts is in line with expectations when considering that hcp
(0001) surfaces are virtually inactive compared to higher
index surfaces. Therefore, if (0001) concentrations for Co-D/
Fig. 3 TEM images of pristine
samples of CoO-NC deposited
on SiO2. a An area of Co-D/
SiO2 with an uniform distribu-
tion of CoO-NC and b the same
location with an overlay high-
lighting the NC that were identi-
fied as disks (red) and spheres
(aqua). c An area of Co-D/
SiO2 where the CoO-NC were
clustered. d Co-S/SiO2 with an
uniform CoO-NC distribution
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1406 Topics in Catalysis (2020) 63:1398–1411
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SiO2 were to be enhanced, these additional surfaces would
contribute only marginally to the overall product spectrum
and as a result, the selectivity would hardly change. Thus,
changing the NC shape from spheres to disks did not influ-
ence the selectivity of the catalyst.
Overall, the main difference in performance was the fac-
tor 2 lower intrinsic activity of the disk-containing Co-D/
SiO2 sample, which could have several reasons. First, the
activity could be lower as a result of the shape and thus
the exposed facets, which would match the theoretical and
experimental finding that hcp (0001) is less active than
higher-index hcp or fcc facets [25, 32]. Second, residual
ligands could still be present in the case of Co-D/SiO2 and
block part of the cobalt surface. Third, the surface area of
Co-D/SiO2 could be lower than expected if the degree of
reduction is low or if there is substantial NC growth, for
example in the areas where clustering of NC was observed
(Fig.3c). These potential explanations were investigated in
more detail and are described below.
The structure of the spent catalysts was analyzed after
FT and subsequent passivation using TEM (Fig.6). The
sizes and shapes of 160 cobalt nanocrystals in Co-D/SiO2
were quantified (Fig.6a, b), showing numerically 62%
disk-shaped NC with d = 14.5nm and 38% spherical NC
with d = 10.2nm. The aspect ratio of the disks could not
be reliably determined for this sample and was assumed to
have remained unchanged. These results were very similar
to those of the pristine catalyst and the disk shape was thus
remarkably stable under FT conditions. Furthermore, a more
clustered part of the catalyst was also observed where some
particle growth had occurred (Fig.6c). This area probably
originated from an area of clustered CoO-NC in the pristine
catalyst. The size of the NC in Co-S/SiO2 was unchanged as
well with spherical particles of 8.7nm on average (Fig.6d).
The stability of the Co-NC in terms of both size and shape
under reaction conditions meant that catalytic performance
might be related to the disk shape.
Two types of ligands were used to synthesize disk-
shaped Co-NC (oleic acid and octadecylamine) while
spherical Co-NC were synthesized using only oleic acid.
For oleic acid, efficient ligand removal has been shown [7],
but could be different for linear amines. We assessed the
Fig. 4 Electron tomography results of the pristine Co-D/SiO2 sam-
ple, consisting of a mixture of disk-shaped and spherical CoO-NC
deposited on SiO2. a TEM image of Co-D/SiO2 at 0° tilt angle. The
small (~ 5 nm), high-contrast particles are gold fudicial markers on
the TEM grid. b The same TEM image with an overlay highlighting
the NC that were identified as disks (red) and spheres (aqua). This
location was reconstructed and the framed area was segmented. c
Volume-rendered 3D reconstruction of the framed area in b, shown
at two different angles obtained by rotation along the z axis (dashed
line). The z axis also indicates the direction of the projection in the
original TEM image a, b. The SiO2 support is depicted in grey, disk-
shaped CoO-NC in red and spherical CoO-NC in aqua. An animation
showing a 360° rotation of this segmented sample together with the
tilt series and the full reconstruction file are included as supporting
movies S2-4
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1407Topics in Catalysis (2020) 63:1398–1411
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influence of the octadecylamine ligand in FT by investi-
gating the amount of surface nitrogen species using XPS
on the catalysts as-prepared and after 100h FT followed
by passivation. Low concentrations of N were detected
on both as-prepared catalysts and on bare SiO2 (Fig.7a,
complete survey scans in Fig.S10) The N-to-Co atom ratio
was 4 times higher on Co-D/SiO2 (0.07) than on Co-S/
SiO2 (0.02). After FT and passivation, Co-D/SiO2 was
partially covered in carbon and only an indication of N
was observed. Spent Co-S/SiO2 was too covered in car-
bon to yield useful data on Co and N contents. The N/Co
atom ratio of spent Co-D/SiO2 seemed similar to that of
as-prepared Co-S/SiO2, so excess N-species in Co-D/SiO2
from the NC synthesis procedure were removed during
reduction/FT. Harmel etal. [46] quantified that the content
of the ligand hexadecylamine decreased from 8wt% to
3wt% during reduction under H2 at 270°C, which is sub-
stantially lower than the reduction temperature of 350°C
applied here. Therefore, it seemed that our reduction pro-
cedure was sufficient to activate the catalyst and minimize
the effect of remaining N-species.
To get more insight into the structure of the catalysts
under FT conditions, insitu XRD was performed on both
catalysts at 10bar, 220°C and an H2/CO ratio of 2v/v
(Fig.7b, Fig.S11). To obtain satisfying Rietveld refine-
ment results, it was necessary to allow preferred orien-
tations of hcp Co crystallites, similar to what has been
reported before [55]. We ascribe this to polymorphism
of Co in the Co-NC/SiO2 samples, since XRD analysis of
metallic cobalt is complicated by the propensity of cobalt
to include stacking faults [22, 59]. Our analysis method
did not quantify this intergrowth structure, but accommo-
dated much of it in the hcp contribution by increasing the
intensity of the (0002) peak. The fraction of hcp Co could
therefore be overestimated. However, both samples were
refined using the same method and given the similarity of
the results for both samples, it appears that polymorphism,
if present, took place to the same extent in both samples.
The crystalline cobalt phase in both catalysts consisted
of approximately 65% hcp Co. The fcc Co and CoO com-
positions were 21% fcc Co and 12% CoO for Co-D/SiO2
and 32% fcc Co and 3% CoO for Co-S/SiO2. These values
are in line with results obtained by Andreev etal. [60]
using NMR on Co/SiC at elevated temperatures under
static Ar atmosphere and in reasonable agreement with
Cats etal. [54] who used operando XRD on Co/TiO2
under FT conditions. In general, however, direct compari-
son of these results to literature is cumbersome, since the
obtained Co phase strongly depends on particle size, metal
precursor, support and activation procedure [61, 62]. Fur-
thermore, the average crystallite sizes of metallic cobalt
Fig. 5 Fischer–Tropsch synthesis results of Co-D/SiO2 and Co-S/
SiO2 at 20bar, 220°C and H2/CO = 2v/v. The GHSV was 1560 h−1
for Co-D/SiO2 and 39,900h−1 for Co-S/SiO2. a CO conversion and b
cobalt-time yield as a function of time-on-stream
Table 1 Fischer–Tropsch synthesis results for Co-D/SiO2 and Co-S/SiO2 reported at similar CO conversions and averaged between 90 and 100h
on stream at 20bar, 220°C and H2/CO = 2v/v
a In 10–5 molCO gCo−1 s−1
b Based on the CTY and the metallic Co surface area of the spent catalysts as derived by TEM
Sample GHSV (h−1) XCO (%) CT Y
aTOFapp
(10–3s−1)bC5+-sel. (%C) C2–C4-sel.
(%C)C1-sel. (%C) α Co SA spent
catalysts (m2
gCo−1)
Co-D/SiO21560 8.9 6.9 49 75 8.6 16 0.78 56
Co-S/SiO239,900 8.1 15 86 78 9 13 0.76 71
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1408 Topics in Catalysis (2020) 63:1398–1411
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was 3–5nm, significantly smaller than the particle dimen-
sions obtained by TEM. This probably indicated that the
Co-NC were polycrystalline, which could be related to the
occurrence of stacking faults.
The degree of reduction of the crystalline phase was
high with 88% for Co-D/SiO2 and 97% for Co-S/SiO2 and
therefore excluded as a reason for substantial activity differ-
ences. Once in the reduced state, the XRD profiles remained
unchanged for the duration of the experiment. The crystal-
lites thus remained stable without sintering or restructur-
ing during reduction and the first 18h FT, in line with the
TEM analyses. This also indicated that a significant loss
of surface area in Co-D/SiO2 due to sintering of NC was
unlikely. Furthermore, the hcp Co content in Co-D/SiO2 was
similar to that in Co-S/SiO2, but nevertheless substantial
in both samples. Finally, the average crystallite dimensions
of 3–5nm prevented any conclusions on preferred orien-
tations and surface termination in disk-shaped NC with a
diameter of 16nm and 4nm thickness. Overall, activity dif-
ferences due to enhanced hcp Co content was excluded, but
preferred crystallite orientations within the polycrystalline
disk-shaped NC and with that enhanced hcp (0001) termina-
tions were still possible.
Combining the obtained data on the ligands and NC size,
shape and crystal structure of the catalysts under reaction
conditions, the cobalt specific surface area and the frac-
tion of the surface terminated by hcp (0001) were calcu-
lated (Fig.8). This served as an estimation of the structural
difference between the Co-NC disk-containing catalyst and
the catalyst containing exclusively spherical NC. To do so,
we assumed that the base planes of the disks were termi-
nated by hcp (0001) and that the disks were attached to SiO2
with one base plane (Fig.4, supporting movie S4), thereby
rendering that plane inaccessible for reactants. This is a con-
servative estimate, as the curvature of the SiO2 might in
practice make these base planes partially accessible, which
would increase the relative occurrence of hcp (0001). Fur-
thermore, the dimensions of the as-synthesized NC were
used in surface area calculations, i.e. disks with d = 15.5nm
and h = 4.2nm and spheres with d = 10.7nm.
Besides the surface of the disks, also the surface of the
spheres has to be considered. According to Wulff reconstruc-
tions of spherical hcp cobalt particles by Liu etal. [25], 18%
of the surface of a spherical hcp cobalt particle is terminated
by (0001). Therefore, the maximal hcp (0001) occurrence in
Co-S/SiO2 was 18%. Compared to an estimated hcp(0001)
occurrence of 39% in Co-D/SiO2, this means that 21% of the
metallic cobalt surface area of Co-S/SiO2 is in a higher index
and probably more active form than for the disk-contain-
ing Co-D/SiO2. This holds when all NC consist of hcp Co,
while a difference in hcp (0001) exposure of 33% would be
obtained if only disks were hcp Co. Considering the uncer-
tainty and error margin in this analysis, we conclude that
the difference in hcp (0001) exposure could be of the right
magnitude to account for much of the observed activity dif-
ference. Therefore, we identified this as a plausible cause for
Fig. 6 TEM images of Co/
SiO2 samples after > 100h of
Fischer–Tropsch synthesis and
passivation. a An area of Co-D/
SiO2 with an uniform distribu-
tion of NC and b the same loca-
tion with an overlay highlight-
ing the NC that were identified
as disks (red) and spheres
(aqua). c An area of Co-D/SiO2
where the NC were clustered. d
Co-S/SiO2 with a homogeneous
NC distribution
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1409Topics in Catalysis (2020) 63:1398–1411
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the differences in catalytic performance of the disk-shaped
and spherical NC, also because of the exclusion of other fac-
tors that might lead to such a substantial loss of activity, e.g.
surface coverage by ligands, or NC restructuring or growth,
as discussed before.
This is, to the best of our knowledge, the first time that a
supported model catalyst has been synthesized to investigate
the effect of Co-NC shape in FT without complications of a
secondary metal or interference of ligands and it illustrates
the potential of colloidal techniques for model catalyst syn-
thesis. This work shows that nanoparticle shape can be an
important design parameter for supported cobalt FT catalysts
when operated under industrially relevant conditions. Fur-
thermore, based on these results, we tentatively propose that
the extended low-index surface facets of the disks are the
main contributor to the lower TOF.
4 Conclusions
We investigated the synthesis of disk-shaped cobalt
nanocrystals (Co-NC), their deposition on a support and the
subsequent effect of their anisotropy on the Fischer–Tropsch
synthesis. Disk-shaped hcp Co-NC were synthesized mixed
with spherical ε Co-NC. After oxidation to CoO, these NC
were deposited on low surface area silica. Electron micros-
copy analysis revealed that numerically 66% of the deposited
CoO-NC were disks and 34% were spheres. After in situ
reduction of the CoO-NC to Co, the Fischer–Tropsch activ-
ity of disk-containing catalyst was only half that of a cata-
lyst containing only spherical NC. Interestingly, the catalysts
were stable and the size and shape of the NC were retained,
even for the disk-shaped Co-NC. XPS showed that most
of the amine ligands were removed during activation/FT
and insitu XRD revealed that the crystal structure of both
catalysts was similar, with NC with 3–5nm polycrystalline
domains and approximately 65% hcp Co and 21 to 32% fcc
Co. Based on these experiments, we tentatively propose that
changing the Co-NC shape from spheres to disks reduced FT
activity because of hcp (0001) facets prevailing. This work
clearly demonstrates the potential of colloidal methods to
synthesize new, well-defined model catalysts to investigate
structure-performance relationships in catalysis even under
high-pressure conditions.
Fig. 7 Nitrogen species on the catalysts before and after 100h FT fol-
lowed by passivation and cobalt crystal structure during FT. a N1s
XPS signal around 400eV of the catalysts in the pristine and spent
state and a bare SiO2 reference. The data was offset with 1500CPS
increment at 380eV binding energy, except for Co-S/SiO2 spent (off-
set -8000CPS). b cobalt phase composition during Fischer–Tropsch
synthesis as determined using insitu XRD and Rietveld refinement.
The average crystallite sizes per metallic Co phase are indicated next
to the corresponding columns. The amount of CoO was too low to
reliably determine a crystallite size. The diffractograms were obtained
over the first 18h on stream at 10bar, 220°C and H2/CO = 2v/v by
accumulating six scans of 3h. No change in the diffraction patterns
was observed over the course of the reaction
Fig. 8 Calculated accessible Co specific surface area (open bars) of
Co-D/SiO2 and Co-S/SiO2 under FT conditions. The hcp (0001) sur-
face area is indicated and broken down for the disk-shaped (red) and
spherical (aqua) Co-NC
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1410 Topics in Catalysis (2020) 63:1398–1411
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Acknowledgements T.W.v.D., H.S., B.M.W. and K.P.d.J. acknowledge
Shell Global Solutions and the Netherlands Association for Scientific
Research (NWO) for funding through the CHIPP framework. H.Y.,
J.Z. and K.P.d.J. acknowledge the European Research Council, EU FP7
ERC Advanced Grant No. 338846. H.Y. acknowledges the Program for
Advancing Strategic International Networks to Accelerate the Circula-
tion of Talented Researchers by JSPS. We thank Nico Sommerdijk,
Hans Meeldijk and Paul Bomans for their involvement with the (cryo-)
TEM measurements, and Peter Munnik, Peter van den Brink, Sander
van Bavel and Marco de Ridder from Shell Technology Center Amster-
dam for XPS measurements and useful discussions.
Author Contributions TWD synthesized and characterized the cata-
lysts, evaluated their catalytic performance and drafted the manuscript.
JH assisted with sample preparation and JJN synthesized the reference
catalyst. HS performed cryo-TEM and interpretation, HY and JZ were
involved with tomography and reconstruction, and RO and BMW per-
formed insitu XRD and assisted with data analysis. KPJ contributed
to experimental design, data analysis and writing of the manuscript.
All authors have given approval to the final version of the manuscript.
Funding This work was funded by Shell Global Solutions and the
Netherlands Association for Scientific Research (NWO) for funding
through the CHIPP framework. Further financial support was obtained
from the European Research Council, EU FP7 ERC Advanced Grant
No. 338846 and the Program for Advancing Strategic International Net-
works to Accelerate the Circulation of Talented Researchers by JSPS.
Data Availability Data is available upon reasonable request.
Compliance with Ethical Standards
Conflict of interest There are no conflicts of interest to declare.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
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