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Contents lists available at ScienceDirect

Catalysis Today
journal homepage: www.elsevier.com/locate/cattod

Location of Co and Ni promoter atoms in multi-layer MoS2

nanocrystals for hydrotreating catalysis
Yuanyuan Zhu a , Quentin M. Ramasse b , Michael Brorson a , Poul G. Moses a ,
Lars P. Hansen a , Henrik Topsøe a , Christian F. Kisielowski c , Stig Helveg a,∗
a
Haldor Topsoe A/S, Haldor Topsøes Allé 1, DK-2800, Kgs. Lyngby, Denmark
b
SuperSTEM Laboratory, STFC Daresbury, Keckwick Lane, Daresbury WA4 4AD, United Kingdom
c
Joint Center for Artificial Photosynthesis (JCAP), Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94708, USA

a r t i c l e i n f o a b s t r a c t

Article history: The location of Co and Ni promoter atoms in industrial-style hydrotreating catalysts is examined by
Received 11 June 2015 combining aberration-corrected scanning transmission electron microscopy and electron energy loss
Received in revised form 14 August 2015 spectrum imaging. The observations unambiguously demonstrate that both Co and Ni promoter atoms
Accepted 24 August 2015
occupy sites at all low-indexed edge terminations of hexagonally shaped multi-layer MoS2 nanocrystals.
Available online xxx
In contrast, similar observations for single-layer MoS2 nanocrystals show that Co-promoter atoms pref-
erentially attach at the (−1 0 0) S-edge termination and are absent at the (1 0 0) Mo-edge termination.
Keywords:
The apparent discrepancy between single- and multi-layer MoS2 nanocrystals can be explained by the
Heterogeneous catalysis
Hydrodesulfurization
2H-MoS2 crystal structure, for which successive MoS2 layers alternatingly expose Mo- and S-edge termi-
Molybdenum disulfide nations in any of the low-indexed directions. Thus, the multi-layer Co–Mo–S and Ni–Mo–S nanocrystals,
Scanning transmission electron microscopy formed in the present type of industrial-style hydrotreating catalyst, are consistently described as a super-
Electron energy loss spectroscopy position of single-layer Co–Mo–S and Ni–Mo–S structures, and in turn, provide promoted edge sites with
different steric accessibility for the organic compounds in mineral oil distillates.
© 2015 Elsevier B.V. All rights reserved.

1. Introduction To establish a molecular-level understanding of the role of pro-

moters in hydrotreating catalysis, numerous studies have focused
In modern oil refineries, a key process is the removal of sulfur on correlating the structure of the promoted catalysts with their
and nitrogen impurities from oil distillates by catalytic hydrotreat- catalytic functionality. From this work, the general consensus
ing to alleviate the demand for clean fuels. To meet the present evolved that the hydrotreating activity is associated with the
and future requirements on the impurity levels, more active and edges of MoS2 nanoparticles, and that the promotion is associated
selective catalysts are needed. To aid the design of such improved with Co or Ni atoms located at the edges of MoS2 -like struc-
catalysts in a rational way, intense research efforts are there- tures, in the so-called “Co–Mo–S” or “Ni–Mo–S” phase [7]. In fact,
fore being devoted to develop a molecular-level understanding of it has been demonstrated that the Co–Mo–S structure forms a
hydrotreating catalysis. Type I phase, consisting mainly of single-layer MoS2 nanoparti-
The industrial hydrotreating catalysts are based on nanometer- cles, and a more HDS active Type II phase, which is associated
sized MoS2 (or WS2 ) particles dispersed on a high-surface area with multi-layer MoS2 nanoparticles [3,8]. The identification of
support. Although the MoS2 nanoparticles possess hydrotreating the Co–Mo–S phase was based on, e.g., Mössbauer, extended X-
activity, it is well known that the addition of small amounts of Co or ray absorption fine structure and infrared spectroscopy [7,9–11].
Ni promotes the catalytic functionality. By Co- or Ni-promotion, the As these spectroscopic techniques average information over vol-
hydrotreating activity is known to increase overall by more than an umes considerably larger than the individual nanostructures, the
order of magnitude and the selectivities for the hydrodesulfuriza- detailed atomic arrangement of the catalytic important edge struc-
tion (HDS), hydrodenitrogenation (HDN) and hydrogenation (HYD) tures of the Co–Mo–S and Ni–Mo–S phase remained unresolved for
reactions are radically modified [1–6]. long.
Complementary real-space information of Co–Mo–S and
Ni–Mo–S structures was subsequently obtained from scanning
∗ Corresponding author. tunneling microscopy of model catalysts, consisting of Co–Mo
E-mail address: sth@topsoe.dk (S. Helveg). or Ni–Mo sulfide nanoparticles prepared on a planar Au (1 1 1)

http://dx.doi.org/10.1016/j.cattod.2015.08.053
0920-5861/© 2015 Elsevier B.V. All rights reserved.

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substrate by physical vapor deposition under ultra-high vacuum by excess of aqueous 1.0 mol/L oxalic acid to reduce Fe impu-
conditions, and from density functional theory calculations [12,13]. rities to a residual content of ca. 350 ppm or below for the
These model studies provided unprecedented new insight into different preparations. Second, the rinsed powder was tabletized
the shape and edge structures of single-layer MoS2 nanocrystals. and granulated. The graphite granulate was then (i) impregnated
Specifically, it was demonstrated that unpromoted MoS2 nanocrys- with 0.023 mol/L aqueous Co (or Ni) acetate solution, (ii) dried
tals obtain a triangular shape, which is terminated by the (1 0 0) in ambient at 383 K, (iii) impregnated with 0.070 mol/L aque-
Mo-edge termination. Under the same sulfiding conditions, Co ous (NH4 )2 [MoS4 ] solution, (iv) transferred incipiently wet to
and Ni stabilize more hexagonally shaped MoS2 nanocrystals by a quartz boat, and (v) sulfided in a flow of 10% H2 S in H2 at
attaching preferentially at the low-indexed (−1 0 0) S-edge termi- 1073 K for 6 h followed by cooling to room temperature before
nation of MoS2 nanocrystals. The model studies also showed that flushing with inert N2 . The nominal atomic ratio of Mo:Co (Ni)
Ni attaches and stabilizes a higher-indexed MoS2 edge termination. was 3:1 and the estimated Mo loading was 0.3 wt% Co (Ni) and
Relating the information from the model studies to the technologi- 0.3 wt% Mo. The samples were stored and TEM samples were pre-
cally relevant catalysts is challenging because there is no guarantee pared in a dry and O2 -free atmosphere [22]. The TEM samples
that the nanostructures formed in the model studies are similar were prepared by crushing catalyst granulates in a mortar and
to those formed using wet impregnation, high pressure sulfida- by dispersing the dry powder on standard Cu TEM-grids covered
tion and high-surface area supports as in the preparation of the with lacey carbon. The samples were only exposed to ambient
industrial-style catalysts. Consequently, it has been a key goal to conditions for a few minutes during transfer to the electron micro-
characterize the structure and chemical composition of Co–Mo–S scope.
and Ni–Mo–S phases in the industrial-style catalysts at atomic res-
olution. 2.2. Electron microscopy
Electron microscopy has emerged as a powerful tool for
visualizing nanostructures in heterogeneous catalysts made by Electron microscopy was carried out at the SuperSTEM
industrial-style procedures. In the transmission mode, electron Laboratory, Daresbury, using a Nion UltraSTEM100 dedicated
micrographs provide a two-dimensional projected view of the aberration-corrected scanning transmission electron microscope.
three-dimensional catalyst materials. Hereby valuable informa- The instrument operates with an ultra-high base vacuum below
tion was obtained about the size, stacking and shape of MoS2 5 × 10−9 Torr and is equipped with a cold field emission gun with a
nanocrystals [14–18]. Specifically, electron micrographs of the native energy spread of 0.35 eV and the Nion quadrupole–octupole
MoS2 nanocrystals are beneficially obtained in the MoS2 0 0 1 spherical-aberration corrector, with a full correction up to six-
projection, because the shape and edge terminations are thereby fold astigmatism. In this work, the microscope was operated at a
directly revealed. However, it is only due to recent advances primary beam energy of 60 keV. The probe-forming optics were
that electron micrographs of MoS2 have become available at configured to a beam convergence semi-angle of 30 mrad, corre-
atomic-resolution and single-atom sensitivity [19–21]. By exam- sponding to a probe size of ca. 0.11 nm. An estimated electron beam
ining industrial-style MoS2 -based catalysts, prepared using thin current of ca. 50 pA was impinging on the sample.
graphite supports, it was demonstrated that aberration-corrected Aberration-corrected STEM images were acquired in the high-
high-resolution (scanning) transmission electron microscopy angle annular dark field (HAADF) mode with the detector inner
((S)TEM) can unambiguously disclose information about the stack- and outer radii being calibrated at 85 mrad and 190 mrad, respec-
ing height, shape and edge terminations of supported MoS2 tively. The STEM images were acquired with a dwell time of
nanocrystals in the 0 0 1 projection [20,21]. Moreover, com- 12–36 ␮s/pixel and pixel sizes of about 10−1 Å2 /pixel, corre-
bining the STEM with concurrent electron energy-loss (EEL) sponding to an electron dose-rate in the order of 104 e− /Å2 s
spectrum imaging made it possible to show that Co atoms and a resulting signal-to-noise ratio, S/N > 3. During part of
preferentially attach to the S-edge termination and are four- the STEM image acquisitions, electron energy loss (EEL) spec-
fold coordinated by S atoms in single-layer MoS2 nanocrystals tra were concurrently recorded pixel by pixel to form the
[22]. so-called EEL spectrum image. The EEL spectra were recorded
In contrast, the location of Co and Ni promoter atoms in using a Gatan Enfina spectrometer with a collection angle of
multi-layer Co–Mo–S and Ni–Mo–S nanocrystals has not yet been 37 mrad, an energy dispersion of 0.7 eV/channel, enabling a par-
resolved. It therefore remains an open issue as to whether the pro- allel acquisition of the S L2,3 , Mo M4,5 and Co(Ni) L2,3 ionization
moter atoms are distributed in the multi-layer nanocrystal as a edge spectra, and a dwell time of 50 ms/spectrum, result-
simple superposition of the preferred locations identified for the ing in an interpretable signal (S/N ∼ 1.5) [22]. Specifically, the
single-layer nanocrystals or are attached at different edge, bulk combined STEM-EEL spectrum imaging was conducted with a
or intercalation sites of the MoS2 structures. In the following, this typical image pixel size of around 10−1 Å2 /pixel and an elec-
issue is addressed by combining STEM and EEL spectrum imaging of tron dose-rate of the order of 106 e− /Å2 s [22]. These electron
multi-layer Co–Mo–S and Ni–Mo–S nanocrystals in industrial-style doses for STEM-EEL spectrum image recording were about 1 to
hydrotreating catalysts. 3 orders lower than those applied in an investigation of elec-
tron beam damage of single-layer MoS2 films [23], where it
was shown that encapsulation of single-layer MoS2 between
2. Experimental two sheets of graphene (or to a lesser extent, the presence of
one layer of graphene) lowered the damage-rate sufficiently that
2.1. Catalyst and TEM sample preparation two-dimensional atomic-resolution EEL spectrum imaging was
possible. While no encapsulation was applied in the present study,
The industrial-style Co- and Ni-promoted MoS2 hydrotreating the ultra-thin graphite support may played the same protective
catalysts were prepared on a graphitic support by a sequen- role.
tial incipient wetness impregnation method [22]. The graphite However, despite these electron illumination conditions,
support is generally considered as a weakly interacting support dynamic changes of the MoS2 edges could still be observed dur-
and, therefore, allows an examination of the promoter distribu- ing continued exposure to the electron beam. Such changes could
tion inherent to the MoS2 nanocrystals. First, a graphitic powder have been due to the actual threshold for electron-induced sput-
(Grade AO-2, Graphene Supermarket; 1400 ppm Fe) was rinsed tering or atom migration being lower than the theoretical value

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of 66 keV for bulk MoS2 [21]. In addition, electron and phonon

excitations could also contribute to beam-induced sample changes
[24]. As the relative contribution of the different damage mech-
anisms is poorly described in general, they were addressed by
an empirical approach. That is, STEM images of a given area
were recorded before and after the acquisition of an STEM-EEL
spectrum image and only those regions that did not show notice-
able structural changes after the final STEM image acquisition
were considered for the present analysis, as also discussed in
[22].

2.3. Data analysis

For analysis of the EEL spectrum images, all spectra were first
denoised by principal component analysis [25] and then calibrated
for any energy dispersion change or shift [22]. For generating ele-
mental maps, the spectra were integrated over a 20 eV, 40 eV and
60 eV energy window above the edge onset of the S L2,3 , Mo M4,5
and Co(Ni) L2,3 ionization edges, respectively, after subtraction the
decaying background using a power-law model [22]. The integrated
spectrum intensities are represented as normalized, linearly col-
ored values of an elemental map.

3. Results and discussion

Fig. 1a shows an aberration-corrected HAADF-STEM micrograph

of the Co-promoted MoS2 hydrotreating catalyst. The micrograph
reflects a bright hexagonally shaped nanocrystal situated on a
darker support. In the HAADF detection mode, the image inten-
sity (I) is dominated by the so-called Z-contrast, which scales with
the total projected atomic number (Z), i.e. I ∼ Z˛ (˛ ≈ 1.7) [26].
The image intensity (Fig. 1a) and its Fourier transform (Fig. 1b)
therefore indicate that the nanocrystal is MoS2 oriented with the
(0 0 1) basal plane orthogonal to the electron beam and along the
(0 0 1) plane of the graphite support. Moreover, the overall inten-
sity across the MoS2 basal plane has an abrupt change (Fig. 1a)
indicating the presence of a step separating regions with differ-
ent thickness of the MoS2 nanocrystal. The nanocrystal thickness
can be addressed by a closer inspection of the image intensity of
the two contrast patches of the basal plane (Fig. 1c and d). The
lower intensity patch of the MoS2 nanocrystal reveals a dumb-
bell structure with intensity maxima separated by 0.18 nm along
1 0 0 and an asymmetric intensity distribution (Fig. 1c and e),
which can be attributed to the pair of a 2S and 1Mo atom col-
umn of the single-layer MoS2 basal plane (Fig. 1f) [21]. Likewise,
the higher intensity patch consists of a dumbbell structure with
intensity maxima separated by 0.18 nm along 1 0 0 intensity and
a symmetric intensity distribution (Fig. 1d and e). In the 2H-MoS2 Fig. 1. (a) High-resolution STEM image of the Co-promoted MoS2 hydrotreating
unit cell, two layers of MoS2 are translated and rotated by 60◦ catalyst showing a partial multi-layer MoS2 nanocrystal oriented with its (0 0 1)
around the c axis in such a way that a 2S column in one layer basal plane along the (0 0 1) plane of the graphite support and orthogonal to the
electron beam direction. The image is unprocessed. (b) Fourier transform of the
coincides with the 1Mo column in the next layer (Fig. 1f). Hereby,
image in (a) with an assignment of the crystal lattice vectors superimposed. (c and
the atomic columns in a two-layer MoS2 nanocrystal, viewed along d) Close-up on (a) at the multi-layer (ML) and single-layer (1L) regions, indicated
0 0 1, contain the same element combinations and therefore have by white boxes in (a), and the corresponding template-average (Av.) and standard
a similar contrast. Obviously, any odd or even number of layers in deviation (Std. Dev.) from the areas marked by blue and black boxes in (a). (e) Line-
a multi-layer MoS2 nanocrystal would show up as an asymmetric profiles of image intensities along the (−1 0 0) direction in (c) and (d). The profiles
are obtained from the template-average and standard deviation images as two pixel-
or symmetric dumbbell structure in the STEM images. However,
wide (0.04 nm) averages. The red curves represent Gaussian fits to the line-profiles
the thicker the crystal, the smaller the relative intensity difference obtained with fixed background intensity, corresponding to the mean intensity from
between the atomic columns in the dumbbell becomes and such the graphite support in the vicinity of the MoS2 nanocrystal. (f) Ball models showing
differences become successively more difficult to resolve for the a top and side view of a 1L and 2L MoS2 crystal structure (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
present materials, especially as multi-layer nanoparticles also need
this article.).
to be perfectly aligned along the correct zone-axis for unambigu-
ous contrast interpretation [21]. Thus, the image intensity analysis
indicates that the nanocrystal consists of a single-layer (1L) MoS2
that is partially covered by one or more additional MoS2 layers in

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Fig. 2. (a) STEM image of a single-layer MoS2 nanocrystal obtained prior to EEL spectrum imaging. The orientation of the asymmetric 2S-1Mo dumbbell pattern indicates
the relative position of the Mo and S sub-lattices and allows the assignment of the Mo- and S-edge terminations as marked. (b) STEM image showing the framed region of
(a) used for EEL spectrum imaging. (c) The corresponding element map for the S distribution. (d) The corresponding element map for the combined Mo (blue) and Co (red)
distributions. (e) The STEM image of the region in (a) acquired after EEL spectrum imaging acquisition. (f) STEM image of a multi-layer MoS2 nanocrystal obtained prior to
EEL spectrum imaging. (g) STEM image showing the framed region of (a) used for EEL spectrum imaging. (h) The corresponding element map for the S distribution. (i) The
corresponding element map for the combined Mo (blue) and Co (red) distributions. (j) The EEL spectrum at the Co L2,3 ionization edge integrated over the framed edge region
in (i). The spectrum is obtained after processing the raw data as described in Section 2 (For interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.).

such a way that the nanocrystal is largely a multi-layer (ML) MoS2 Co attaches preferentially to the S-edge on the single-layer MoS2
nanocrystal (Fig. 1f) [20]. The low electron-dose conditions, how- nanocrystal, in agreement with [22] and with ab-initio calculations
ever, imply that the detailed stoichiometry is difficult to extract of unsupported MoS2 slabs [27]. Moreover, Ref. [22] proposed the
based on STEM imaging only. In the following, the chemical com- atomic model in Fig. 3a for the low-indexed edge terminations of
position of a single-layer and a multi-layer MoS2 nanocrystal is the single-layer Co–Mo–S nanocrystals in the present industrial-
therefore addressed by the combined STEM-EEL spectrum imaging style hydrotreating catalyst and argued that this model represents
approach. an equilibrium structure due to its insensitivity to the synthesis
First, the structure and composition of a single-layer Co–Mo–S conditions and support material used in preparation of different
nanocrystal is addressed. Fig. 2a shows a STEM image of a MoS2 model and industrial-style catalysts [12,13,22,28–31]. The prepa-
nanocrystal, which is a single-layer due to the clear asymmet- ration procedure is therefore a good starting point for an analysis
ric dumbbell structure of the basal plane. The dumbbell structure of preferences in the attachment of promoter atoms to multi-layer
reflects the relative orientation of the S and Mo sub-lattices and, MoS2 nanocrystals.
thus, enables an unambiguous assignment of the low-indexed Next, the location of Co promoters associated with multi-
(1 0 0) Mo-edge and (−1 0 0) S-edge terminations (Fig. 3) [22]. layer MoS2 nanocrystals in the present hydrotreating catalyst is
Both edge terminations are present at the corner region empha- addressed. Fig. 2f shows a STEM image of a multi-layer MoS2
sized in the close-up (Fig. 2b), from which elemental maps were nanocrystal as reflected by the approximately symmetric inten-
obtained. Fig. 2c shows a uniform distribution of S across the basal sity of the dumbbell structure in the basal plane. Faint contrast
plane with intensity maxima that coincide with the S sub-lattice variations extend over several atomic columns on the basal plane
in the STEM image of this single-layer nanocrystal. Fig. 2d shows and are likely reflecting variations in the graphite support or
the combined Mo and Co map. It demonstrates also a uniform carbon layers that partially cover the nanocrystal [22]. For com-
distribution of Mo over the basal plane. The location of the Mo parison with the elemental distribution of the single-layer MoS2
atoms is not clearly resolved due to the inelastic electron scat- nanocrystals, element maps were generated at the corner empha-
tering process and lower signal-to-noise ratio of the delayed Mo sized in the close-up in Fig. 2g. The element maps reveal that
M4,5 ionization edge in the EEL spectra [22]. Moreover, the com- both S and Mo are uniformly distributed across the nanocrys-
bined Mo and Co map shows that the Co signal has an appreciable tal (Fig. 2h and i) and that Co is located at both the adjacent
intensity confined to the S-edge and is absent in the basal plane edges (Fig. 2i). The identification of Co is based on an apprecia-
and at the Mo-edge. This finding is inherent to the single-layer ble intensity distributed over several neighboring pixels reflecting
MoS2 nanocrystals, because the STEM image obtained of the same the delocalization in the inelastic electron scattering. In con-
nanocrystal after the combined STEM-EEL spectrum image acquisi- trast, individual pixels in the basal plane or on the support with
tion confirms the integrity of the single-layer edges (Fig. 2e). Thus, an appreciable intensity are attributed to noise resulting from

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the rather low signal-to-noise ratio in the EEL spectra (Fig. 2j).
The finding that Co attaches to all edge terminations on the
multi-layer MoS2 nanocrystal differs from the marked preference
for attachment to the S-edge termination on single-layer MoS2
nanocrystals. In the multi-layer nanocrystal, the edge terminations
consist of alternating Mo- and S-edge terminations, respectively,
exposed by the successive MoS2 layers in the 2H-MoS2 stack-
ing sequence. Due to this sequence, EEL spectra obtained in the
0 0 1 projection geometry sample the composition of both Mo-
and S-edge terminations, even at adjacent edges of the multi-
layer MoS2 nanocrystal. Thus, although it cannot be excluded
that some Co atoms are trapped at less preferred sites at the
Mo-edge termination, the observed presence of Co at all edge
terminations can consistently be accounted for by the preferred
attachment to the S-edge termination alone. The present multi-
layer Co–Mo–S nanocrystals can therefore be explained as a simple
superposition of single-layer Co–Mo–S nanocrystals, as sketched in
Fig. 3b.
Finally, the Ni-promoted MoS2 hydrotreating catalyst is exam-
ined in the same way by using combined STEM and EEL spectrum
imaging. Fig. 4a shows a STEM image of two hexagonally shaped
nanocrystals with an apparent partial overlap in the projected
image. For nanocrystal I, a close-up on the basal plane shows a
dumbbell structure with an apparent symmetric intensity (Fig. 4b)
and the corresponding Fourier transform is consistent with the
MoS2 (0 0 1) structure superimposed on graphite (0 0 1) (Fig. 4c). For
nanocrystal II, a more intense image signal is observed (Fig. 4d) and
the corresponding Fourier transform reveals the MoS2 structure
viewed along the (0 0 1) direction (Fig. 4e). Thus, the image in Fig. 4a
reflects two multi-layer MoS2 nanocrystals with the nanocrystal I
thinner than II. Moreover, in the region of partial overlap of the basal
planes, a Moiré pattern is present in the STEM image (Fig. 4a) due
to different rotational orientations of the nanocrystals around the
electron beam direction. The perimeter of the Moiré pattern allows
the deduction of the shape of nanocrystal I as a truncated hexagon
and that of nanocrystal II as a regular hexagon. The overlap can be
due to a geometry in which nanocrystal II partially covers nanocrys-
tal I, in the case where the two nanocrystals are located on the same
side, but on different terraces, of the graphite support. The overlap
can also result from the two nanocrystals being situated on differ-
ent sides of the graphite support. The projection geometry makes a
clear distinction between these two configurations difficult. Across
the projected area of the two nanocrystals, the element maps (Fig. 4f
and g) show that S and Mo are uniformly distributed, and that
a higher content of S and Mo is present in nanocrystal II than I,
consistent with the thickness difference. In contrast, the element
map in Fig. 4g shows that Ni is associated with a higher intensity
near all edges of both multi-layer MoS2 nanocrystals. Moreover,
the Ni intensity is higher around the relatively thicker nanocrys-
tal II than I, reflecting a higher projected abundance of promoters.
Furthermore, the Ni promoter atoms appear to be heterogeneously
distributed around the multi-layer edges (arrowheads in Fig. 4g).
The wider patches with Ni seem to correlate with regions in which
the successive MoS2 layers tend to extend further, resulting in
a staircase edge termination (arrowheads in Fig. 4a). Thus, these
findings indicate a strong preference for Ni to attach to edges of
a multi-layer MoS2 nanocrystal in a way resembling the location
of Co atoms in multi-layer Co–Mo–S nanocrystals. The single-layer
MoS2 nanocrystals in the Ni-promoted hydrotreating catalyst were
Fig. 3. (a) Ball-models for the single-layer Co–Mo–S structure in top view (upper not successfully resolved in the present experiments. However, the
model) and side view (lower model). Adapted from [22]. (b) Ball-models for the two-
edge attachment may still be rationalized in the same way as for
layer Co–Mo–S structure in top view (upper model) and side view (lower model).
(For interpretation of the references to color in this figure legend, the reader is Co with a structural model for the multi-layer Ni–Mo–S nanocrystal
referred to the web version of this article.) as in Fig. 3b, because Ni has also been suggested to preferentially
attach to the S-edge termination and not to the Mo-edge edge
[13].

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Fig. 4. (a) STEM image of two multi-layer MoS2 nanocrystals supported by graphite. (b) Close-up (2.2 nm × 2.2 nm) on the basal plane, framed by I in (a), and (c) the
corresponding Fourier transform. (d) Close-up (2.2 nm × 2.2 nm) on the basal plane, framed by II in (a), and (e) the corresponding Fourier transform. (f) The corresponding
element map for the S distribution. (g) The corresponding element map for the combined Mo (blue) and Ni (red) distributions (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of this article.).

4. Conclusion Acknowledgements

The promoter atoms in the present industrial-style Co–Mo–S The electron microscopy was performed at the SuperSTEM Lab-
and Ni–Mo–S hydrotreating catalysts are unambiguously demon- oratory, Daresbury, and supported by the EPSRC (UK). The Danish
strated to occupy sites at all <1 0 0> edge terminations of Council for Strategic Research (grant Cat-C) and the Danish Coun-
hexagonally shaped multi-layer MoS2 nanocrystals. In contrast, for cil for Independent Research (grant HYDECAT, DFF-1335-00016)
single-layer MoS2 nanocrystals, Co-promoter atoms are found to are gratefully acknowledged for financial support. C.F.K. acknowl-
attach preferentially at the S-edge termination and to be absent edges the Joint Center for Artificial Photosynthesis, a DOE Energy
at the Mo-edge termination. This apparent discrepancy between Innovation Hub, supported through the Office of Science of the US
single- and multi-layer MoS2 nanocrystals is explained by the Department of Energy (DE-SC0004993).
2H stacking sequence of the MoS2 nanocrystals that successively
expose alternating Mo- and S-edge terminations in any of the low-
indexed directions. Thus, for the present catalysts, the multi-layer References
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Please cite this article in press as: Y. Zhu, et al., Location of Co and Ni promoter atoms in multi-layer MoS2 nanocrystals for hydrotreating
catalysis, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.053