M. Yusuf et al.
Applied Catalysis A, General 613 (2021) 118025
collected. Therefore, the core-shell Pd@Cu nanostructure on MoS2 is
more likely the cause of the high catalytic activity compared to bare
Pd@Cu, MoS2, or a mixture of both without any preparation. Since the
light intensity (0.1 W/cm2) is used in this study, the temperature after
the reaction was measured to be ±31 ◦C. Thus, the effect of temperature
was evaluated to identify the material as thermo-catalyst, photocatalyst,
or both. To obtain the completed reaction (99 % conversion) needs high
temperature (100 ◦C) without irradiation of light (Fig. 6b) indicates that
Pd@Cu/MoS2 is a more photoactive catalyst. In addition, the action
spectrum for the photocatalytic activity Pd@Cu/MoS2 was identified by
dependency of wavelength (Fig. 6c). The optical long-pass filter cut-on
was used to block the region transmission below the chosen wave-
length (i.e., 400 nm cut-on, block transmission <400 nm). The conver-
sion was obtained to be 99 % under irradiation wavelength of 400ꢀ 800
nm. The conversion reduced to 90 %, 80 %, and 49 % under irradiation
of 455–800, 515–800, and 610–800 nm, respectively. Because the con-
version of thiol in dark condition was 33 %, the amount contribution of
400ꢀ 455 nm was 13 % calculated by ((99–90)/(99–33)× 100 %) in the
total of light-irradiated conversion. Analog to other wavelengths are 15
%, 47 %, and 24 % for 455–515, 515–610, and 610–800 nm light-
induced, respectively. These results are in good accordance with the
UV–vis DRS pattern shown in Fig. 4a.
Table 1
The photocatalytic activity of Pd@Cu/MoS2 nanostructures for oxidative
coupling of thiol reactions.
Entry
Cat. (mol %)
Solvent
Time
(h)
Conv.a
(%)
Selec.a
(%)
1
Pd@Cu/MoS2
(0.5)
CH3CN:H2O (1:1)
CH3CN:H2O (1:3)
CH3CN:H2O (3:1)
CH3CN:H2O (3:1)
3
3
3
1
3
3
3
3
3
3
3
3
69.5
40.0
99.8
47.2
54.4
65.5
44.5
38.7
96.0
100
98.5
99.6
99.6
99.2
99.0
99.0
98.4
99.5
98.7
99.6
99.2
99.4
2
Pd@Cu/MoS2
(0.5)
3
Pd@Cu/MoS2
(0.5)
4
Pd@Cu/MoS2
(0.5)
5
Pd@Cu/MoS2
(0.5)
CH3CH2OH:H2O
(3:1)
6
Pd@Cu/MoS2
(0.25)
CH3CN:H2O (3:1)
7
Pd@Cu/MoS2
(0.5)
CH3CN
8
Pd@Cu/MoS2
(0.5)
H2O
From these results, the mechanism of the oxidative coupling of thiol
by Pd@Cu/MoS2 can be proposed (Fig. 7). Based on previously reported
research [60,61], the S and H moieties of thiol are adsorbed on the
surface of the catalyst. During the photoreaction under visible light,
electron-hole pairs are generated. The holes in the valence band of the
MoS2 semiconductor move to the thiol group, generating a thiyl radical
and a hydrogen protons (H+). In the presence of air, O2 can be reduced
by accepting electrons from a combination of MoS2 conduction band and
9b
10c
11d
12e
Pd@Cu/MoS2
(0.5)
CH3CN:H2O (3:1)
CH3CN:H2O (3:1)
CH3CN:H2O (3:1)
CH3CN:H2O (3:1)
Pd@Cu/MoS2
(0.5)
Pd@Cu/MoS2
(0.5)
96.8
78.8
Pd@Cu/MoS2
(0.5)
•
electron-rich Pd@Cu to provide a superoxide (O2ꢀ ). By this synergic
Reaction conditions: 0.1 mmol 4-chlorobenzenethiol, 3 ml solvent, 0.5 mol%
photocatalyst base on Pd. The reactions were conducted in air at r.t (25 ◦C)
under Xe-lamp irradiation (400ꢀ 800 nm) with light intensity of 0.10 W/cm2.
system, the thiyl radical supposedly reacts with another thiyl radical to
produce disulfide. In addition, the superoxide transforms to hydrogen
peroxide, which decomposes into water and oxygen, and encloses the
catalytic cycle. In the absence of air, the electron-hole transfer of the
catalyst by incident of light leading the formation of thiyl radical and
hydrogen radical, then disulfide product was generated by radical
coupling of thiyl radical (Fig. S5). The addition of 2,2,6,6-tetramethylpi-
peridine-1-oxyl as a radical scavenger, the desired product was not
detected implying the radical is involved in the reactions. Here, the role
of electron-hole pairs in the photocatalytic reaction can also be probed
by disturbing the reaction with electron and hole scavengers. P-benzo-
quinone is well-known as an electron and superoxide radical scavenger,
and ethylenediaminetetraacetic acid is well-known as a hole scavenger.
The result in Fig. 6b shows a decrease in the amount of disulfide product
produced, indicating the importance of the electron-hole system in
multifunctional core-shell Pd@Cu/MoS2 photocatalyst.
a
Conversions and selectivity determined by GC–MS analysis.
b
under N2 atmosphere.
c
d
e
O2 atmosphere.
ratio Pd@Cu:MoS2 (1:1).
ratio Pd@Cu:MoS2 (1:4).
optimized photocatalytic condition was then implemented with a
diverse range of thiols into their relevant disulfanes product (Table 2).
The scope of this reaction was explored by using a variety of aromatic,
heteroaromatic, and aliphatic thiols. Aromatic thiols were transformed
to their corresponding products with high selectivity (>90 %), regard-
less of the presence of either electron-withdrawing substituents or
electron-donating substituents, such as halide, methyl, and methoxy
groups (entries 1–5). Heteroatomic thiols, such as nitrogen-containing
heteroaromatic thiols (entry 6–7) and benzylic thiol (entry 8), were
also converted under the optimized reaction conditions, with excellent
conversion and selectivity as well as the aromatic thiols. Additionally,
the variety of thiols with excellent photocatalytic activity also included
aliphatic thiols (entry 9–11). Furthermore, because asymmetric disul-
fides are also essential bioactive compounds [58,59], asymmetric
aryl-alkyl disulfides (entry 12) were produced with high selectivity by
combining 1 equivalent of an aryl thiol and 1.5 equivalent of an
aliphatic thiol.
Along with photocatalytic activity, recyclability is also a critical
factor for heterogenous catalyst in organic applications. To demonstrate
the reusability of Pd@Cu/MoS2, the photocatalyst was reused five times
with 4-chlorobenzenethiol using the optimized protocol. After each
completed reaction, the catalyst was easily recovered for reuse by
centrifugation and washing with ethanol. As shown in Fig. 8, the pho-
tocatalytic activity of Pd@Cu/MoS2 remained high over the five uses
without any significant loss. The stability of the catalyst was also
examined by identifying the morphology and structure before and after
rescue. The morphology of the catalyst was analyzed by SEM (Fig. S6),
and the structure of catalyst was analyzed by XRD (Fig. 8b), after five
sequential cycles. Despite slight changes in the morphology, the struc-
ture of the catalyst was well maintained.
The excellent catalytic activity of the oxidative homocoupling of
thiols is closely related to the light harvesting ability of Pd@Cu/MoS2
and the synergic effect of Pd, Cu and MoS2 in the multifunctional core-
shell nanostructure. To demonstrate how much the light harvesting
ability improved the catalytic activity, the comparison of results in Fig. 6
are given. Fig. 6a show that the irradiation of Xe light with a 400ꢀ 800
nm wavelength enhanced the catalytic activity only using a small
amount intensity (0.1 W/cm2). For comparison, the reactions using Pd/
MoS2, Cu/MoS2 under light irradiation and in the dark were also
There have been many reports on the oxidative coupling of thiol to
disulfide reactions catalyzed via heterogeneous conventional structures
or multifunctional structures, but most need to be performed in harsh
conditions, such as in the presence of an oxidation agent or at an
elevated temperature. The high photocatalytic activity of the Pd@Cu/
MoS2 used in this study is compared with previously reported catalyst is
6