Defect-mediated selective hydrogenation of nitroarenes on nanostructured WS2

Article abstract of DOI:10.1039/c9sc03337h

Transition metal dichalcogenides (TMDs) are well known catalysts as both bulk and nanoscale materials. Two-dimensional (2-D) TMDs, which contain single- and few-layer nanosheets, are increasingly studied as catalytic materials because of their unique thickness-dependent properties and high surface areas. Here, colloidal 2H-WS₂ nanostructures are used as a model 2-D TMD system to understand how high catalytic activity and selectivity can be achieved for useful organic transformations. Free-standing, colloidal 2H-WS₂ nanostructures containing few-layer nanosheets are shown to catalyze the selective hydrogenation of a broad scope of substituted nitroarenes to their corresponding aniline derivatives in the presence of other reducible functional groups. Microscopic and computational studies reveal the important roles of sulfur vacancy-rich basal planes and tungsten-terminated edges, which are more abundant in nanostructured 2-D materials than in their bulk counterparts, in enabling the functional group selectivity. At tungsten-terminated edges and on regions of the basal planes having high concentrations of sulfur vacancies, vertical adsorption of the nitroarene is favored, thus facilitating hydrogen transfer exclusively to the nitro group due to geometric effects. At lower sulfur vacancy concentrations on the basal planes, parallel adsorption of the nitroarene is favored, and the nitro group is selectively hydrogenated due to a lower kinetic barrier. These mechanistic insights reveal how the various defect structures and configurations on 2-D TMD nanostructures facilitate functional group selectivity through distinct mechanisms that depend upon the adsorption geometry, which may have important implications for the design of new and enhanced 2-D catalytic materials across a potentially broad scope of reactions.

Full text of DOI:10.1039/c9sc03337h

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Defect-mediated selective hydrogenation of
nitroarenes on nanostructured WS2†
Cite this: Chem. Sci., 2019, 10, 10310
ae
a
b
ce
All publication charges for this article Yifan Sun,
have been paid for by the Royal Society
of Chemistry
‡
Albert J. Darling,‡ Yawei Li,
Kazunori Fujisawa,
Cameron F. Holder, He Liu, Michael J. Janik,* Mauricio Terrones *acde
and Raymond E. Schaak
a
a
b
ab
*
Transition metal dichalcogenides (TMDs) are well known catalysts as both bulk and nanoscale materials.
Two-dimensional (2-D) TMDs, which contain single- and few-layer nanosheets, are increasingly studied
as catalytic materials because of their unique thickness-dependent properties and high surface areas.
Here, colloidal 2H-WS
catalytic activity and selectivity can be achieved for useful organic transformations. Free-standing,
colloidal 2H-WS nanostructures containing few-layer nanosheets are shown to catalyze the selective
2
nanostructures are used as a model 2-D TMD system to understand how high
2
hydrogenation of a broad scope of substituted nitroarenes to their corresponding aniline derivatives in
the presence of other reducible functional groups. Microscopic and computational studies reveal the
important roles of sulfur vacancy-rich basal planes and tungsten-terminated edges, which are more
abundant in nanostructured 2-D materials than in their bulk counterparts, in enabling the functional
group selectivity. At tungsten-terminated edges and on regions of the basal planes having high
concentrations of sulfur vacancies, vertical adsorption of the nitroarene is favored, thus facilitating
hydrogen transfer exclusively to the nitro group due to geometric effects. At lower sulfur vacancy
concentrations on the basal planes, parallel adsorption of the nitroarene is favored, and the nitro group
is selectively hydrogenated due to a lower kinetic barrier. These mechanistic insights reveal how the
various defect structures and configurations on 2-D TMD nanostructures facilitate functional group
selectivity through distinct mechanisms that depend upon the adsorption geometry, which may have
important implications for the design of new and enhanced 2-D catalytic materials across a potentially
broad scope of reactions.
Received 5th July 2019
Accepted 15th September 2019
DOI: 10.1039/c9sc03337h
rsc.li/chemical-science
sites makes TMDs ideal candidates as active hydrogenation
catalysts. Hydrogenation catalysts that can target one reducible
Introduction
6
Two-dimensional transition metal dichalcogenide (TMD) group on a molecule when others are present and accessible are
materials are active catalysts for a broad range of chemical especially important. For example, the selective hydrogenation
1
transformations, including the hydrogen evolution reaction,
of substituted nitroarenes is important for accessing aniline
2
the electrochemical reduction of CO
treating and hydrogenation reactions.
2
,
and various hydro- compounds that are useful intermediates in pharmaceutical
The near-zero free and agrochemical production.
3
–5
7
energy of adsorption of molecular hydrogen on edge and defect
Catalysts that facilitate selective hydrogenation reactions
using H are desirable based on atom economy considerations
and the formation of water as the only reduction byproduct.
Among heterogeneous catalysts, chemically modied noble
2
8–10
a
Department of Chemistry and Materials Research Institute, The Pennsylvania State
11–13
University, University Park, PA 16802, USA. E-mail: res20@psu.edu
b
metals can achieve selective hydrogenation,
but recent
Department of Chemical Engineering, The Pennsylvania State University, University interest in new and low-cost materials has resulted in a growing
Park, PA 16802, USA. E-mail: mjj13@psu.edu
number of non-platinum group (NPG) transition metal catalysts
c
Department of Physics, The Pennsylvania State University, University Park, PA 16802,
USA. E-mail: mut11@psu.edu
14–18
for such reactions.
TMD materials are well known NPG
19
hydrogenation catalysts. The more recent ability to synthesize
two-dimensional (2-D) TMDs containing single- and few-layer
nanosheets has expanded the catalytic scope of these mate-
rials and enabled new capabilities in achieving functional group
d
Department of Materials and Science Engineering, The Pennsylvania State University,
University Park, PA 16802, USA
e
Center for 2-Dimensional and Layered Materials, The Pennsylvania State University,
University Park, PA 16802, USA
†
Electronic supplementary information (ESI) available: Additional material selectivity. For example, nanostructured MoS
catalyzes the
selective hydrogenation of substituted nitroarenes using H to
2
characterization and DFT calculation data. See DOI: 10.1039/c9sc03337h
2
‡
These authors have contributed equally to this study.
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form their corresponding anilines, but requires transition metal pattern in Fig. 1f shows further evidence for the formation of
promotors to facilitate the formation of active sites that arise crystalline 2H-WS . Polycrystallinity and strain arising from the
2
9,10,19
from a signicantly lowered metal–sulfur bond energy.
curled nanosheets and small domain sizes result in a broad, yet
Understanding how functional group selectivity emerges in 2-D characteristic and identiable diffraction pattern that is
26,27
TMDs catalysts is important, but insights remain limited. Here, consistent with 2H-WS
we show that colloidal WS nanostructures containing single- WS nanostructures are chalcogen decient (Fig. S1†). XPS,
and few-layer nanosheets, and without transition metal pro- which is sensitive to the chemical environment around each
motors, catalyze the selective hydrogenation of substituted atom and can distinguish between the two WS polymorphs,
2
.
STEM-EDS data reveal that the 2H-
28
2
2
2
nitroarenes to their corresponding aniline derivatives. conrms that the majority (ꢀ90%) of the as-synthesized sample
Complementary microscopic and computational studies contains 2H-WS , with the remainder being 1T-WS (Fig. S2 and
2
2
provide important insights into the origin of the catalytic Table S1†). The surface area of the ower-like 2H-WS nano-
2
2
ꢁ1
selectivity on the TMD nanostructures, pointing to the impor- structures is estimated by BET to be 62.7 m g , which is much
2
ꢁ1
tant roles of atomic sulfur vacancies on the basal planes and higher than that of the bulk powder sample (7.6 m
tungsten-terminated edges. These WS nanostructures provide Fig. S3†).
a well-dened platform to study how catalytic activity and In a typical catalytic experiment, the 2H-WS
selectivity can be achieved in nanostructured 2-D TMD were added to a solution mixture containing THF, deionized
g ,
2
2
nanostructures
20
materials.
water, and a substituted nitroarene. Hydrogenation was carried
out for 8 h at 120 C with 50 bar of H , which are conditions
ꢂ
2
comparable to previous studies of transition metal-based cata-
Results and discussion
15,16,29
lysts for analogous reactions.
was synthesized in solution by reacting nitrostyrene, which contains reducible nitro and vinyl groups.
, hexamethyldisilazane, and CS
in oleylamine and oleic Using the aforementioned conditions, the 2H-WS nanoowers
The model substrate was 3-
Nanostructured WS
WCl
2
6
2
2
21
acid. TEM (Fig. 1a) and HAADF-STEM (Fig. 1b) images of the facilitate >99% conversion of the nitro group to an amine and
resulting ꢀ100 nm colloidal nanostructures show that they 94% retention of the vinyl group. In contrast, commercially
22
contain nanosheets in a ower-like morphology. The crystal available Pt/C unselectively hydrogenates both nitro and vinyl
structure of 2H-WS , shown in Fig. 1c and d, consists of alter- groups under the same conditions, while no detectable prod-
2
nating stacked monolayers that contain tungsten atoms coor- ucts were obtained with bulk WS
powders (Table S2†). At lower
2
dinated by sulfur atoms in a trigonal prismatic geometry. The
H
2
pressures (20 bar), 3-nitrostyrene can also be selectively
1
Raman spectrum in Fig. 1e shows the in-plane (E2g) and out-of- transformed to 3-vinylaniline, though a longer reaction time (12
plane (A1g) vibration modes, as well as higher-order modes h) is required to achieve similar conversion (Fig. S4†).
2
3,24
[
LA(M) and 2LA(M)],
which are characteristic of 2H-WS
2
Aliquots taken during the reaction indicate that the conver-
25
ꢂ
nanostructures and indicate the presence of defects. The XRD sion of 3-nitrostyrene to 3-vinylaniline at 120 C with 50 bar of
2
H is nearly linear with time up to 8 h, reaching >99% conver-
sion and 98% selectivity (Fig. 2a). Aer 8 h, reduction of the
vinyl group results in increasing formation of the 3-ethylaniline
byproduct, and consequently a decrease in selectivity. There-
fore, reduction of the nitro group takes place prior to that of the
vinyl group, and adjusting experimental parameters can opti-
mize both activity and selectivity. As shown in Fig. 2b, the 2H-
WS
2
catalysts remain highly active and selective over ve
Fig. 2 (a) Aliquot study for the selective hydrogenation of 3-nitro-
ꢂ
styrene catalyzed by 2H-WS
2
nanostructures at 50 bar H
2
and 120 C,
showing the percentage of 3-nitrostyrene (grey), 3-vinylaniline (green),
Fig. 1 (a) TEM and (b) HAADF-STEM images of 2H-WS
2
nanoflowers. and 3-ethylaniline (pink) at different reaction times. (b) Percent
(
c) Top (basal plane) and (d) side (edge) views of the hexagonal conversion (grey) and selectivity (green) for the hydrogenation of 3-
structure for 2H-WS . (e) Raman spectrum and (f) powder XRD data for nitrostyrene to 3-vinylaniline using the same nanostructured 2H-WS
the as-prepared 2H-WS nanostructures. catalyst over five successive cycles.
2
2
2
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2
consecutive hydrogenation reactions of 3-nitrostyrene, Table 1 Conversion and selectivity of nanostructured 2H-WS cata-
lyzed hydrogenation of substituted nitroarenes to corresponding
anilines
achieving 98% conversion with >99% selectivity on the h
cycle and indicating a high degree of recyclability. TEM, XRD,
EDS, and Raman data for 2H-WS nanostructures before catal-
2
ysis and aer one and ve catalytic cycles are indistinguishable
(Fig. S5 and S6†). XPS shows no change in the 2H : 1T phase
ratio aer 5 cycles. However, the W 4f and S 2p peaks exhibit
a small shi (ꢀ0.5 eV) towards lower binding energies aer ve
cycles, suggesting that the surface of the nanostructured 2H-
a
Entry
Substrate
Conv. (%)
Selec. (%)
1
2
>99
>99
>99
94
WS catalyst becomes slightly reduced aer long-term reaction
2
with hydrogen (Fig. S7†).
A broad scope of nitroarenes with diverse reducible groups
including aldehydes, carboxylic acids, esters, amides, sulfon-
amides, nitriles, thioethers, ketones, halogens, and pyridines,
as well as combinations of groups, were also tested for conver-
sion to their corresponding anilines. As shown in Table 1, most
substrates achieved complete conversion and >99% selectivity
towards hydrogenation of the nitro group, leaving the other
functional groups unaltered. Though most halide-containing
anilines were obtained with >99% selectivity through hydroge-
nation of the corresponding nitro compounds under standard
3
4
5
6
7
>99
>99
>99
>99
>99
78
>99
>99
>99
>99
ꢂ
reaction conditions, a lower reaction temperature (100 C) was
used for 4-iodobenzene to avoid dehalogenation. Hydrogena-
tion of 4-nitrobenzaldehyde to 4-aminobenzaldehyde could only
be achieved with 78% selectivity, as hydrogenation of the
aldehyde led to the formation of 4-methylaniline. In addition,
nitrobenzene, nitrosobenzene, and N-phenylhydroxylamine
were exclusively converted to aniline under standard reaction
conditions, supporting the Haber mechanism where the nitro
groups are converted to amines with nitroso and hydroxylamine
8
9
>99
>99
>99
>99
30
intermediates.
Flower-like nanostructured WS
selective hydrogenation while bulk WS
2
is an active catalyst for
shows no hydrogena-
2
tion activity under analogous conditions (Table S2†). To 10
understand how nanostructuring of this model 2-D TMD system
enables catalysis, ADF-STEM was used to gain atomic-level
>99
>99
11
>99
>99
>99
>99
>99
>99
>99
>99
insights into the structures of the as-prepared nanosheet cata-
lysts. As shown in Fig. 3a, monolayer and bi-layer WS nano-
sheets were observed near the edges of the 2H-WS
nanoowers, 12
2
2
with visible holes on the nanosheets indicating the presence of
atomic vacancies. ADF-STEM can differentiate atoms based on
Z-contrast (Z ¼ atomic number), thus a line scan at high
magnication can be analyzed to determine which atoms are in
b
1
1
3
4
31
a particular location. Fig. 3b–d show three line scans across
the basal plane of a WS monolayer, highlighting a region with
no vacancies, a region with an individual monosulfur vacancy
), and a region with a tungsten vacancy (V ) associated with
a monosulfur vacancy (V ). While both tungsten and sulfur
vacancies are present, STEM-EDS quantication reveals that the
2
1
5
>99
>99
(V
S
W
S
a
Unless otherwise noted, hydrogenations were performed under the
(5 mg), THF (1 mL), H O (125 mL),
substrate (0.125 mmol), 8 h, 120 C. Run at 100 C.
2
H-WS nanostructures are chalcogen-decient, suggesting that following conditions: WS
2
2
2
ꢂ
b
ꢂ
sulfur vacancies dominate. Additional microscopic data also
suggest that both tungsten- and sulfur-terminated edges are
abundantly present in the ower-like nanostructures (Fig. S8†).
Although not observed, disulfur vacancies (VS2) likely also exist,
considering the sulfur vacancy concentration and previous
Surface engineering of nanostructured TMD catalysts has
been demonstrated as an effective approach to generate active
edge and vacancy sites for hydrogen activation and produc-
23
reports.
32–34
tion.
Similar insights are important for selective
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transferred to the adsorbed nitroarene molecule through active
sites (i.e. vacancies and edge sites of nanostructured WS2),
where the adsorption geometry of the nitroarene molecule plays
a key role in determining the activity and selectivity. We thus

rst investigated the adsorption of 3-nitrostyrene on the 1V
S2, and 4V surfaces of WS . Three adsorption geometries,
including two vertical modes where the molecule stands up with
the nitro group bound to the WS surface and one parallel mode
where the molecule lies at, parallel to the WS plane, were
Calculation results suggest that the
S
,
1V
S
2
2
2
36,37
investigated (Fig. S9†).
weakly adsorbed parallel conguration is preferable in the 1VS
and 1VS2 models, while the quasi-vertical conguration with
two oxygen atoms from the nitro group bound to W atoms in the
sulfur vacancy site is slightly preferred in the case of 4V
and Table S3†). The positive adsorption energies in the 1V
S2 models indicate that the vertical adsorbed states are
S
(Fig. 5
S
and
1V
unstable local minima. A high concentration of sulfur vacancies
signicantly promotes the vertical adsorption of 3-nitrostyrene
on WS . Favorable adsorption with a high concentration of
2
vacancies could also lead to higher adsorbate coverages that
motivate a vertical orientation due to surface crowding. This
vertical orientation facilitates selective hydrogenation of the
nitro group since it is positioned directly on the surface of the
Fig. 3 (a) High-resolution ADF-STEM image showing bilayer and
monolayer domains with atomic vacancies on the surface (basal plane)
WS
surface.
For the 1V and 1V models where 3-nitrostyrene preferen-
2
catalyst while the vinyl group does not interact with the
of the 2H-WS
image for a region of monolayer 1H-WS
2
nanostructures. (b) Atomically resolved ADF-STEM
. (c) Experimental ADF
2
S
S2
intensity curves and (d) structural models corresponding to the three tially adsorbs parallel to the WS surface and both the nitro and
2
line scans indicated by the green, orange, and red arrows in (b),
vinyl groups are oriented so that they could undergo catalytic
hydrogenation, we carried out additional kinetic calculations
involving stepwise hydrogenation of both the nitro and vinyl
showing the alternation of tungsten (W) and sulfur (S
2
) configurations,
) vacancies.
as well as existence of tungsten (V ) and monosulfur (V
W
S
groups. We calculated the activation barriers (E
energies (DE) for the hydrogenation of the nitro and vinyl
groups, using the 1V model as a representative parallel-
adsorption system. As summarized in Fig. S10 and S11,† the
rate-determining step for hydrogenation of the nitro group is
a
) and reaction
hydrogenation reactions on TMDs in order to understand how
selectivity emerges and therefore how it can be optimized,
manipulated, and controlled. DFT calculations were performed
to identify the optimized adsorption geometry of 3-nitrostyrene
S
2
and the hydrogen transfer process on 2-D WS with different
sulfur vacancy congurations. Monolayers of 1H-WS2 with
a monosulfur vacancy (1V ), a disulfur vacancy (1V ), and four
S
S2
evenly-distributed monosulfur vacancies (4V ) on a basal plane
S
were constructed in a 4 ꢃ 4 ꢃ 1 supercell, corresponding to 3%,
6% and 12% sulfur vacancy concentrations, respectively (Fig. 4).
2
During hydrogenation on a transition metal disulde (MS )
catalyst, molecular hydrogen is adsorbed on the surface and is
activated by either homolytic dissociation on the sulfur atoms
to form two –S–H bonds or heterolytic dissociation to yield –S–H
35
and –M–H species. The activated hydrogen atoms are then
Fig. 5 (a) Calculated adsorption energies of the three adsorption
geometries (vertical 1, vertical 2, and parallel) on the three sulfur
vacancy models (1V
energy adsorbed structures of 3-nitrostyrene absorbed on the surface
Fig. 4 Simulated structures for different types of S-vacancies on the of a 1H-WS monolayer are shown for the (b) 1V (parallel); (c) 1VS2
basal planes of 1H-WS : (a) 1V , (b) 4V , and (c) 1VS2. The vacancy sites (parallel); and (d) 4V (vertical) orientations on the different vacancy
are highlighted by red circles. models.
S S 2
, 1VS2, and 4V ) for 1H-WS basal planes. Lowest
2
S
2
S
S
S
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the transformation from R–NO
2
to R–NOOH, which has a reac- orientations, the nitro group directly bonds to the tungsten
tion barrier of 0.97 eV that is noticeably lower than that of the atoms at the edges and results in selective hydrogenation,
rst step of the vinyl hydrogenation pathway (1.07 eV). Thus, whereas the vinyl group does not directly interact with the WS2
selective hydrogenation of the nitro group is kinetically favored nanostructures.

when there is a low concentration of sulfur vacancies.
The mechanisms that result in selective hydrogenation of 3-
In addition to point defects involving sulfur vacancies on the nitrostyrene to 3-vinylaniline are different for regions of the
basal planes, the WS nanoowers also provide abundant edge nanostructured surface that have high vs. low sulfur vacancy
2
sites that could also serve as active sites for selective hydroge- concentrations and different edge terminations. However, the
nation of 3-nitrostyrene. We therefore also calculated the end result is the same – the nitro group is selectively hydroge-
adsorption energy of 3-nitrostyrene on the tungsten- and sulfur- nated relative to the vinyl group, and the selectivity arises from
terminated edges, which were constructed using 4 ꢃ 1 ꢃ 1 interactions between the substrate molecule and sulfur defects
nanoribbons. Previous DFT calculations have demonstrated on the WS surface.
2
that, at high temperatures and partial pressures of H
2
, partial
edge site coverage of sulfur and hydrogen adatoms is likely
based on the previous ab initio thermodynamics methodology.
Given the high pressures and temperatures utilized in our Colloidally synthesized 2H-WS
Conclusions
38
2
nanostructures, which have
catalytic experiments, modeling of the tungsten-terminated high surface areas and contain single- and few-layer nanosheets
edge sites included a coverage of q ¼ 0.5 and q ¼ 0.5, and with high vacancy concentrations, serve as a model 2-D TMD
S
H
the sulfur-terminated edge sites were modeled with q ¼ 1 and system for understanding the origin of selectivity during
S
q
¼ 1 (Fig. S12†). As shown in Fig. 6, S13 and S14,† the nitro hydrogenation reactions. The 2H-WS nanostructures catalyze
H
2
adsorption energies were calculated for ve distinct congura- the selective hydrogenation of substituted nitroarenes with
tions. Three adsorption geometries for the tungsten-terminated molecular hydrogen, transforming them to the corresponding
edge sites were considered, including head-on adsorption (W- anilines in the presence of a broad scope of reducible func-
1), the nitro group adsorbed to the tungsten atoms modied tionalities. Microscopic and computational studies indicate
with one sulfur atom (W-2), and the nitro group adsorbed to the that sulfur vacancies on the basal planes and tungsten-
tungsten atoms modied with one sulfur atom and one terminated edges facilitate chemoselectivity, where hydrogen
hydrogen atom (W-3). Two geometries for the sulfur-terminated preferentially transfers to the nitro group due either to the
sites were also considered: head-on adsorption (S-1) and stag- preferential vertical adsorption geometry at high basal-plane
gered head-on adsorption (S-2). Based on the optimized vacancy concentrations and tungsten-terminated edge sites, or
absorption energies, only the W-2 and W-3 congurations, to smaller kinetic barriers at lower basal-plane vacancy
involving the tungsten-terminated edge sites, revealed favorable concentrations where parallel adsorption is favored. Such
adsorption of 3-nitrostyrene via a quasi-vertical geometry, where insights are important for guiding nanostructuring efforts in 2-
the two oxygen atoms from the nitro group are separately D TMD catalyst systems, as well as other applications including
bonded to the two exposed tungsten atoms of WS (Fig. 6). For responsive gas sensors, where vacancy-rich surfaces are
2
the other three geometries, either weak or unfavorable inter- common and defect engineering could tune adsorption
39
actions were observed (Table S4†). For both the W-2 and W-3 behavior. Future efforts to synthesize more complex and
precisely-tailored TMD nanostructures, including composition-
tunable
structures,
alloys
and
noble
metal-anchored
hetero-
2
8,40–42
will provide a broader scope of systems to
enable more comprehensive studies on the nature of the active
sites, the various mechanism by which selectivity can be ach-
ieved and modied, and the role of interactions between active
species and underlying substrates.
Experimental
Materials
6
Oleylamine (technical grade, 70%), tungsten(VI) chloride (WCl ,
$
99.9%, trace metal basis), hexamethyldisilazane (HMDS,
2
reagent grade, $99%), carbon disulde (CS , $99.9%, anhy-
drous), tetrahydrofuran (THF, anhydrous, contains 250 ppm
BHT as inhibitor, $99%), azobenzene (98%), 4-nitrobenzamide
0 0
(
98%), 4-nitrobenzenesulfonamide (97%), 4 -uoro-3 -nitro-
acetophenone (97%), 3-nitrostyrene (96%), hydrazobenzene, 1-
bromo-4-nitrobenzene (99%), ethyl 4-nitrocinnamate (predom-
inately trans, 99%), 4 -aminoacetophenone (99%), 3-vinylaniline
Fig. 6 Optimized geometries of 3-nitrostyrene absorbed on the
tungsten-terminated edges of 1H-WS monolayer are shown for the
a) W-2 and (b) W-3 models from three viewing angles.
0
2
(
(contains KOH as inhibitor, 97%), 4-nitrothioanisole (96%), 1-
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chloro-4-nitrobenzene (99%), 1-iodo-4-nitrobenzene (98%), tet- reaction mixture post-reaction as an internal standard. The
radecane ($99%), nitrosobenzene ($97%), 3-ethylaniline catalyst from the reaction mixture was then removed via
(98%), p-toluidine (99%), 4-(4-nitrobenzyl)pyridine (98%), 4- centrifugation at 13 500 rpm and the resulting solution was
nitrobenzonitrile (97%), hydroxylamine hydrochloride (99%), diluted with THF and submitted for GC-MS analysis. Product
azoxybenzene, and nitrobenzene (99%) were purchased from yields were determined via calibration curves made using
Sigma Aldrich. Oleic acid (technical grade, 90%) and 4-nitro- commercially available reaction products. If no substrate signal
benzoic acid (99%) were purchased from Alfa Aesar. Methyl 4- was observed via GC-MS, it was assumed that the reaction had
nitrobenzoate ($98.0%) and 4-nitrobenzaldehyde ($98.0%) proceeded to >99% conversion. Similarly, if the desired aniline
were purchased from TCI. 10% Pt on activated carbon powder was observed as the only product, it was assumed that the
was purchased from Premetek. Solvents, including hexane and reaction had proceeded with >99% selectivity.
isopropanol, were of analytical grade. All chemicals were used
as received without further purication.
Recycling experiments
The hydrogenation of 0.5 mmol 3-nitrostyrene using 20 mg WS
2
Synthesis of 2H-WS
2
nanostructures
nanostructures as the catalyst was performed as described
above. Following the completion of the reaction, 100 mL tetra-
decane was added to the reaction mixture. The WS2 nano-
structures were separated from the reaction mixture via
centrifugation, and the resulting supernatant was diluted with
THF and submitted for GC-MS analysis to determine reaction
The colloidal synthesis of the 2H-WS nanostructures was carried
out according to the previously reported approach. 15 mL of
oleylamine was added to a 100 mL three-neck ask and degassed
for 30 min under vacuum at 120 C. HMDS (0.5 mL) was then
injected into the ask aer cooling to 100 C under argon and the
mixture was heated to 320 C. Meanwhile, WCl
mmol) was dissolved in 0.3 mL of oleic acid (0.95 mmol) and
mixed with 5 mL of oleylamine (15.2 mmol) in an argon-ushed
2
2
1
ꢂ
ꢂ
ꢂ
6
(50 mg, 0.125
2
yields, as described above. The remaining WS nanostructures
were washed and centrifuged twice with THF, and then used as
the catalyst for another hydrogenation of 3-nitrostyrene
(0.5 mmol scale). This process was repeated for a total of ve
times to evaluate the catalyst recyclability.
septum-capped vial. Upon injection, 0.24 mL of CS was intro-
2
duced to the vial, forming a homogeneous solution that became
hot due to the exothermicity of the reaction. The solution in the
vial was subsequently injected dropwise into the three-neck ask
at a rate of 10 mL h using a syringe pump. Aer 30 min, the
injection was stopped and the heating mantle was removed. Aer
cooling down to room temperature, the products were washed
three times with a 1 : 1 toluene/ethanol mixture, collected with
centrifugation, and kept as a powder under argon.
Characterization
ꢁ1
Powder X-ray diffraction (XRD) patterns were collected using
a Bruker-AXS D8 Advance diffractometer equipped with Cu Ka
radiation and a LynxEye 1-D detector. Simulated XRD patterns
of 2H-WS2 were generated using the CrystalMaker/
CrystalDiffract soware package. Transmission electron
microscopy (TEM), high-angle annular dark-eld scanning
transmission electron microscopy (HAADF-STEM) images, and
energy dispersive X-ray spectroscopy (EDS) data with element
maps were acquired using a FEI Talos F200X operating at 200
kV. High-resolution ADF-STEM images were obtained using
Hydrogenation reactions
In a typical reaction, 20 mg of the 2H-WS nanostructures in
2
powder form was added to a septum-capped glass vial along
with a Teon-coated magnetic stir bar, 4 mL of THF, 0.5 mL of
deionized water, and 0.5 mmol of substrate molecules. Soni-
cation was carried out to make sure the dispersion was fully
mixed. A 300 mL Parr stainless steel autoclave (4766-T-SS-3000-
VGR) was used for all hydrogenation reactions. The septum cap
of every sample was punctured with a disposable 18 gauge
needle to allow the partitioning of H2 into the vial. Once
prepared and sealed, the autoclave was ushed twice with H2
gas at 25 bar to remove ambient air, and then lled to the nal
3
a FEI Titan G2 60/300 TEM with a spherical aberration
corrector on both the probe- and the image-forming lens at an
accelerating voltage of 80 kV. ADF line scan was obtained using
ImageJ soware. Bruker ESPRIT 2 soware was applied for EDS
data interpretation. Micro-Raman measurements were per-
formed in a Renishaw inVia confocal microscope-based Raman
spectrometer with 514.5 nm laser. The laser spot size using the
100ꢃ objective lens was approximately 1 mm. X-ray photoelec-
pressure (50 bar H
2
). Once pressurized, the autoclave was
tron spectroscopy (XPS) experiments were performed using
a Physical Electronics VersaProbe II instrument equipped with
a monochromatic Al Ka X-ray source (hn ¼ 1486.7 eV) and
a concentric hemispherical analyzer. Charge neutralization was
performed using both low energy electrons (<5 eV) and argon
ions. The binding energy axis was calibrated using sputter
cleaned Cu foil (Cu 2p3/2 ¼ 932.7 eV, Cu 3p3/2 ¼ 75.1 eV). Peaks
ꢂ
heated to 120 C and held at that temperature for 8 h. The
2
reaction was cooled to room temperature, the H atmosphere
was discharged, and the samples were prepared for GC-MS as
described below. For the hydrogenation of substituted nitro-
arenes, a smaller scale with 5 mg of 2H-WS nanostructures in
powder form, 1 mL of THF, 0.125 mL of deionized water, and
2
0.125 mmol of substrate molecules were utilized.
were charge referenced to the CH
at 284.8 eV. Measurements were made at a takeoff angle of 45
x
peak in the carbon 1s spectra
ꢂ
GC-MS analysis
with respect to the sample surface plane. This resulted in
Typically, tetradecane (100 mL for 0.5 mmol scale reactions, 25 a typical sampling depth of 3–6 nm (95% of the signal origi-
mL for 0.125 mmol scale reactions) was injected into the nated from this depth or shallower). Quantication was carried
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out using instrumental relative sensitivity factors (RSFs) that
account for the X-ray cross section and inelastic mean free path
Acknowledgements
of the electrons. Nitrogen adsorption analysis was carried at 77 Y. S. and R. E. S. were supported by the U.S. National Science
K with an Accelerated Surface Area and Porosimetry Analyzer Foundation Grant No. DMR-1607135. A. J. D. and C. F. H. were
(ASAP 2020; Micromeritics Instrument Corp.). Prior to the supported by the ACS Petroleum Research Fund Grant No.
measurement, the sample was degassed at 333 K for 24 h under 58373ND5. Y. L. and M. J. J. recognize support from the US
4
mm Hg vacuum. Surface area was estimated using the Bru- Department of Energy, Office of Science, Basic Energy Sciences,
43
nauer, Emmett and Teller (BET) equation.
Catalysis Science Program under award # DE-SC0016529.
Computational work used the Extreme Science and Engi-
neering Discovery Environment (XSEDE), which is supported by
National Science Foundation grant number ACI-1053575. H. L.,
K. F. and M. T. acknowledge support from NSF EFRI 2-DARE
grant 1433311 and the Air Force Office of Scientic Research
Calculations
Density functional theory (DFT) calculations were performed
using Vienna ab initio simulation package (VASP) 5.4.4. The
projector-augmented-wave (PAW) approach was utilized to treat
the core electrons,
exchange–correlation functional of the generalized gradient
approximation (GGA) was used to describe electron interac-
tions. Kinetic cut-off energy for the plane-wave basis set was
set as 450 eV. Gaussian smearing method was applied with
smearing width of 0.01 eV.
44
(
AFOSR) grant 17RT0244. M. T. also acknowledges the Center
4
5,46
and the Perdew–Burke–Ernzerhof (PBE)
for 2-Dimensional and Layered Materials at the Pennsylvania
State University. Electron microscopy was performed at the
Electron Microscopy Facility at the Materials Characterization
Lab of the Penn State Materials Research Institute. We thank
Jeff Shallenberger for the help with XPS acquisition, Phillip
Smith for his assistance with collection of GC-MS data, and
Alexey Silakov for helpful discussions.
47
Monolayer WS
2
with trigonal prismatic coordination (1H-
WS ) was selected as the surface slab model for the calculation
2
of molecule adsorption and reaction pathways. Considering the
large steric hinderance of 3-nitrostyrene, we applied a 4 ꢃ 4 ꢃ 1
supercell to prevent the potential intermolecular repulsion
effect induced by coverage. Three distinct types of sulfur
vacancies were modeled: only one sulfur atom removed from
the lattice (denoted as 1V ); both top and bottom sulfur atoms
S
removed (denoted as 1VS2); and four sulfur vacancies uniformly
distributed on one side of the monolayer structure (denoted as
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Chem. Sci., 2019, 10, 10310–10317 | 10317