Catalytic hydrogen evolution and semihydrogenation of organic compounds using silicotungstic acid as an electron-coupled-proton buffer in water-organic solvent mixtures

Article abstract of DOI:10.1016/j.jcat.2019.09.011

H₂ and O₂ were produced at separate times from water electrolysis by using silicotungstic acid as an electron-coupled-proton buffer. It was found that the redox property of silicotungstic acid in water could be regulate by the introd

Full text of DOI:10.1016/j.jcat.2019.09.011

Journal of Catalysis 378 (2019) 376–381
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Catalytic hydrogen evolution and semihydrogenation of organic
compounds using silicotungstic acid as an electron-coupled-proton
buffer in water-organic solvent mixtures
⇑
Weiming Wu, Xiao-Yuan Wu, Sa-Sa Wang, Can-Zhong Lu
CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
H2 and O2 were produced at separate times from water electrolysis by using silicotungstic acid as an
electron-coupled-proton buffer. It was found that the redox property of silicotungstic acid in water could
be regulate by the introduction of organic solvents (acetonitrile, DMF and ethanol). The redox wave of
silicotungstic acid at ꢀ0.02 V vs. RHE moved in the negative direction, which was good for the production
of hydrogen. A high yield of hydrogen (ꢀ85%) was obtained in a water-ethanol system (V/V = 1:1), which
was higher than that in water (ꢀ45%). And the introduction of ethanol didn’t affect the rate of hydrogen
evolution. Moreover, the semihydrogenation of organic compounds could be achieved in the present sys-
tem under mild conditions. The conversion of phenylacetylene and acetophenone are 97% and 80% under
optimum reaction conditions, while the selectivity of styrene and 1-phenylethanol are 80% and 82%,
respectively.
Received 18 July 2019
Revised 22 August 2019
Accepted 8 September 2019
Keywords:
Silicotungstic acid
Electron-coupled-proton buffer
Water electrolysis
Hydrogen evolution
Semihydrogenation
Ó 2019 Published by Elsevier Inc.
1. Introduction
rate H2 and O2 gases, but also let the catalysts freedom from the
limitation of the low work surfaces of electrodes.
Water electrolysis is an important technology for large-scale
hydrogen production. Due to large-scale application of hydrogen
can solve energy and environment issues at a global level, this
technology has received considerable attention in the field of
chemistry [1–3]. However, the separation of hydrogen and oxygen
is a troublesome problem in this technology, because these gases
will diffuse in electrolysis devices and can give rise to hazardous
O2/H2 mixtures [4–7]. Cronin group has introduced a concept of
electron-coupled proton buffer (ECPB) to overcome such gas cross-
over, which can act to decouple electrolytic H2 and O2 production,
producing these gases at separate times [8–10]. In the reported
systems, silicotungstic acid (H4SiW12O40) is one attractive material
as a candidate of the ECPBs for the electrolysis of water due to its
high solubility and reversible redox ability in water [10]. It can
In theory, one 2e-reduced silicotungstic acid can react with two
H+ ions to produce one H2 molecule, because the reduced silico-
tungstic acid cluster provides two electrons to take part in the
hydrogen evolution. However, only 45–60% of the hydrogen is col-
lected in the reported ECPB systems [10,11]. This can be ascribed to
the low reducing ability of the 1e-reduced silicotungstic acid (E
(H5SiW12O40/H4SiW12O40) = 0.02 V vs. RHE,
E
(H+/H2) = 0 V vs.
RHE) [10,12,13]. Therefore, improving the H2 yield via regulating
the redox property of silicotungstic acid is a significant challenge,
which will be conductive to the development of the ECPB systems
for the highly efficient hydrogen evolution from water electrolysis.
Previous work reported that the redox property of silicotungstic
acid is different in water and DMF [14]. The redox potentials of sil-
icotungstic acid in DMF are strikingly more negative than its redox
potentials in water. It is believed that introducing organic solvents
will regulate the redox property of silicotungstic acid, which can
improve the H2 yield in the ECPB systems.
Aromatic hydrocarbons and b-phenyl alcohols are important
components to many industrial chemicals. These compounds can
be efficiently obtained by the catalytic semihydrogenation of the
corresponding alkynyl and carbonyl compounds under H2 atmo-
sphere [15–20]. However, this process suffers from the limitations
of rigorous reaction conditions, high cost and tedious procedures,
accept two electrons to from a 2e-reduced species (H6SiW12O40
)
in water, while water is oxidized to O2. Then, its 2e-reduced spe-
cies can directly reduce H+ ions, which come from the oxidation
of water (2H2O = 4H+ + O2), into H2 gas in the present of catalysts
(such as Pt/C) without additional inputs. This may not only sepa-
⇑
Corresponding author.
E-mail address: czlu@fjirsm.ac.cn (C.-Z. Lu).
https://doi.org/10.1016/j.jcat.2019.09.011
0021-9517/Ó 2019 Published by Elsevier Inc.

W. Wu et al. / Journal of Catalysis 378 (2019) 376–381
377
because of the usage of the inflammable and explosive hydrogen
gas. Therefore, developing a general, economic and simple route
to preparation of these compounds under mild conditions is still
of great interest and importance. It is noted that the reduced silico-
tungstic acid is an excellent ECPB for the water electrolysis and a
candidate for the selective hydrogenation of nitroarenes [10,21].
However, it seems that the semihydrogenation of this compounds
is not feasible due to their insolubility in water. The introduction of
organic solvents can remove this obstacle, because the alkynyl and
carbonyl compounds is very soluble in organic mediums. There-
fore, the reduced silicotungstic acid may be a suitable hydrogen
source for the semihydrogenation of organic compounds in
water-organic solvent mixtures.
Herein, we used various organic solvents (acetonitrile, DMF and
ethanol) to regulate the redox property of silicotungstic acid in
water. The hydrogen evolution from water electrolysis were eval-
uated in water-organic solvent mixtures with silicotungstic acid
as an ECPB. Moreover, the semihydrogenation of organic com-
pounds (phenylacetylene and acetophenone) was investigated in
the reported system under mild conditions. Our results may allow
us to highlight the promising application of polyoxometallates in
the highly efficient hydrogen evolution from water electrolysis
and semihydrogenation of organic compounds.
dimethyl formamide or ethanol) were mixed in a Schlenk flask
(10 mL). After sealed with a rubber stopper, the reaction flask
was vacuumed with a mechanical pump and refilled by Ar gas.
Then, the deep blue H6SiW12O40 solution (2 mL) was injected
slowly into the flask with continuous stirring (600 r/min). The mix-
ture was stirred for 10 min, and the evolving H2 gas was measured
by gas chromatography headspace analysis (GC2014C, Shimazu
Co., TCD, molecular sieve 5A column, Ar carrier). The yields of
hydrogen were calculated by using the following equation (assum-
ing one mole of hydrogen occupies 24.5 L at room temperature and
pressure):
Production of H2ðmLÞ
Yield of hydrogen ð%Þ ¼
24:5ꢂMole number of H4SiW12O40ðmmolÞ
ꢂ100%
2.3. Semihydrogenation of phenylacetylene and acetophenone
For the semihydrogenation of phenylacetylene and acetophe-
none, 10 mg of the catalyst (Pt/C or Pd/C, 5 wt%, Aladdin Co.),
2 mL of ethanol and a certain amount of phenylacetylene (or ace-
tophenone, Aladdin Co.) were mixed in a Schlenk flask (10 mL).
After sealed with a rubber stopper, the reaction flask was vacu-
umed with a mechanical pump and refilled by Ar gas. Then, the
deep blue H6SiW12O40 solution (2 mL) was injected slowly into
the flask with continuous stirring (600 r/min). The mixture was
2. Experimental
2.1. Electrochemical measurement
Cyclic voltammetry curves of silicotungstic acid (0.083 mol/L,
Sinopharm chemical reagent Co. (SCRC)) in water-organic solvent
mixtures were taken on a CHI660e workstation (CH Instruments,
Inc.). The electrochemical measurements were performed in a typ-
ical three electrode cell, using glassy carbon electrode (3 mm in
diameter), Pt wire and saturated Ag/AgCl electrode as working
electrode, counter electrode and reference electrode, respectively.
In all measurements, the saturated Ag/AgCl reference electrode
was calibrated with respect to reversible hydrogen electrode
(RHE).
2.2. H2 evolution from water electrolysis
The electrochemical reduction of silicotungstic acid (H4SiW12
40) was carried out by a modified method [8–10]: 10 mmol of H4-
SiW12O40 (SCRC) and 60 mL of deionized water were added into
one compartment of two-compartment H-cell, and 0.9 mL
-
O
a
H3PO4 (13.3 mmol, SCRC) and 60 mL of deionized water were
placed into the other compartment. The compartments of the
H-cells were separated by a piece of 0.18 mm-thick Nafion N-117
membrane (Alfa Aesar Co.). All electrochemical experiments were
performed by using
a classical two-electrode configuration
(TPR6010S regulated power supply, ATTEN instruments Co.). The
H4SiW12O40 compartment was equipped with a large area carbon
felt working electrode (3 ꢁ 2 cm2), and the other compartment of
the cell was equipped with a platinum mesh working electrode
(1 ꢁ 1 cm2). To fully reduce H4SiW12O40 to H6SiW12O40 by two
electrons, a potential of 8.3 V was set between the working elec-
trodes and 1931 C of charge was passed at this potential (working
current: 0.1 A, working time: 5.4 h). The color of the solution chan-
ged from colorless to deep blue when the experiment ended. Prior
to the experiment, the H4SiW12O40 solution was bubbled with
argon (60 mL/min) for 60 min, stirred vigorously and kept under
an argon atmosphere throughout the experiment.
For investigating the effect of the introduction of organic sol-
vents on the H2 yield in the ECPB system, 10 mg of Pt/C catalyst
(5 wt%, Aladdin Co.) and 2 mL of organic solvent (acetonitrile,
Fig. 1. (a) Cyclic voltammetry curves of silicotungstic acid and (b) H2 yield in
water-organic solvent mixtures (V/V = 1:1).

378
W. Wu et al. / Journal of Catalysis 378 (2019) 376–381
stirred for 10 min, and the products were detected by GC technique
(6820, Agilent Co., FID, HP-5 capillary column, N2 carrier). Prior to
GC analysis, 4 mL of dichloromethane (A.R., SCRC) was added into
the reaction solution. After shaking for 5 min and standing for
5 min, the organic phase was moved to a sample vial (10 mL),
and was dehydrated by adding a certain amount of Na2SO4 (A.R.,
tem, corresponding a Faradaic efficiency of 85% for the hydrogen
evolution, which is higher than the yield of hydrogen in water
(3.62 mL, ꢀ 45%). Because of the equilibrium between hydrogen
gas and H4SiW12O40 [10], the yield of hydrogen is less than 100%
in the present system. Control experiments further indicate that
the production of H2 is negligible in the absence of the Pt/C catalyst
or H6SiW12O40. These results confirm the catalyst and reduced sil-
icotungstic acid are indispensable for the hydrogen evolution in
water-organic solvent systems.
SCRC). Then, 2 lL of the sample was injected for GC analysis.
3. Results and discussion
Moreover, the effect of the contents of the organic solvents on
the H2 yield has been investigated in this work. The H2 yield in
the water-ethanol system with different volume ratios are shown
in Fig. 2a. The results reveal that the H2 yield increases with the
increase of the content of ethanol in the present system. When
the content of ethanol over 50%, the H2 yield has no significant
increase, indicating the optimal content of ethanol is 50%. The
voltammetry curves of silicotungstic acid in the water-ethanol sys-
tem with different volume ratios have also been obtained (see
Fig. 2b). It is found that the redox wave of silicotungstic acid moves
in the negative direction with the increase of the content of etha-
nol. The results reveal that silicotungstic acid possess strong reduc-
ing ability at a high ethanol content. This may explain the high H2
yield in the water-ethanol system (V/V = 1:1) in the present of the
The redox property of silicotungstic acid in water-organic sol-
vent mixtures (V/V = 1:1) has been investigated by cyclic voltam-
metry. As shown in Fig. 1a, the introduction of organic solvents
(acetonitrile, DMF and ethanol) can regulate the redox property
of silicotungstic acid in water. The redox wave of silicotungstic acid
(H5SiW12O40/H4SiW12O40) at ꢀ0.02 V vs. RHE moves in the nega-
tive direction, which will be good for the reaction between the
reduced silicotungstic acid and H+ ions to produce hydrogen in
the presence of the catalyst. Fig. 1b shows the H2 yield in various
water-organic solvent systems in the present of the Pt/C catalyst
and H6SiW12O40. It is found that the introduction of organic sol-
vents can improve the H2 yield in present system. The difference
in the H2 yield for various water-organic solvent systems may be
attributed to their own dielectric constants [14]. A high yield of
hydrogen (6.95 mL, ꢀ 85%) can be obtained in a water-ethanol sys-
Pt/C catalyst and H6SiW12O40
.
Fig. 3. (a) A typical apparatus configuration for determining volumes of H2 and (b)
the production of H2 as a function of the reaction time. The inset is the color of the
solution after the reaction.
Fig. 2. (a) H2 yield and (b) cyclic voltammetry curves of silicotungstic acid in the
water-ethanol system with different volume ratios.

W. Wu et al. / Journal of Catalysis 378 (2019) 376–381
379
Note that the reaction rate, as the yield, is an important factor in
the large-scale hydrogen production from water electrolysis. In
order to measure the rate of hydrogen evolution in the water-
ethanol system, 50 mg of the Pt/C catalyst was dispersed in
10 mL of deionized water (or ethanol) in a 50 mL three-necked,
round-bottomed flask. After thoroughly flushed with argon,
10 mL of the freshly produced H6SiW12O40 was added to the sam-
ple aqueous dispersion under Ar atmosphere and stirred vigorously
via a pressure-equalizing dropping funnel. The evolving H2 gas was
captured in a 50 mL measuring cylinder filled with water (see
Fig. 3a). Fig. 3b shows the production of H2 as a function of the
found that the crystal structure of silicotungstic acid is intact after
the hydrogen evolution. Transmission electron microscope (TEM)
images indicate that the morphology of the Pt/C catalyst has no
significant change. And the Pt4+ concentrations measured by
inductively coupled plasma-atomic emission spectrometry (ICP-
AES) are 4.0 and 4.1 ppm before and after the reaction, respec-
tively. Furthermore, the yield of hydrogen remains above 80% in
the 3th cycle of testing by using recycled H4SiW12O40 and Pt/C cat-
alyst. These suggest that the ECPB (H4SiW12O40) and catalyst (Pt/C)
exhibit high stability for the hydrogen evolution in the water-
ethanol system.
As mentioned above, the semihydrogenation of organic com-
pounds is usually achieved under H2 atmosphere [15–20]. How-
ever, this process suffers from the limitation of rigorous reaction
conditions, because of the usage of the inflammable and explosive
hydrogen gas. In this work, the semihydrogenation of organic com-
pounds (phenylacetylene and acetophenone) has been evaluated
under mild conditions, which the reduced silicotungstic acid is
used as a hydrogen source. As shown in Table 1, phenylacetylene
can be efficiently hydrogenated to styrene in the presence of the
reduced silicotungstic acid and Pt/C as the hydrogen source and
catalyst, respectively. Satisfying selectivity of styrene can be
obtained at high levels of phenylacetylene. It is found that the
selectivity of styrene is ꢀ80% at a high conversion (ꢀ97%) for the
semihydrogenation of phenylacetylene (see Table 1, entry 2). Fur-
thermore, acetophenone can be reduce to 1-phenylethanol, while
the reduced silicotungstic acid and Pd/C as the hydrogen source
and catalyst, respectively. The conversion of acetophenone and
reaction time. The initial rate of the H2 evolution is ꢀ0.8 mL sꢃ1
,
giving a rate of 47 mmol of H2 per hour per mg of Pt catalyst, which
confirms the highly efficient hydrogen evolution from water elec-
trolysis in the present ECPB system. It is found that there is no
obvious change for the rate of hydrogen evolution in the water-
ethanol system. Furthermore, as shown in the inset in Fig. 3b,
the color of the solution is deep blue after the reaction in the water
system. But the color of the solution can change into light brown
by introducing ethanol, indicating more reduced silicotungstic acid
take part in the hydrogen evolution. These results reveal that the
introduction of ethanol can enhance the reduce ability of silico-
tungstic acid to improve the production of H2, but do not affect
the rate of hydrogen evolution in the present system.
Due to the low boiling point for ethanol, the Pt/C catalyst and
silicotungstic acid can be easily recycled by centrifugation and dry-
ing. Fig. 4a and b shows FT-IR spectra and X-ray diffraction (XRD)
patterns of silicotungstic acid before and after the reaction. It is
Fig. 4. (a) FT-IR spectra, (b) XRD patterns of silicotungstic acid and (c, d) TEM images of the Pt/C catalyst before and after the reaction.

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W. Wu et al. / Journal of Catalysis 378 (2019) 376–381
Table 1
Catalytic hydrogenation of phenylacetylene in water-ethanol mixturea.
Entry
Catalyst
Reactant (A, mmol)
Conversion (A, %)
Selectivity (B, %)
Selectivity (C, %)
1
2
3
4
5
6
Pt/C
Pt/C
Pt/C
Pt/C
–
0.090
0.135
0.180
0.270
0.090
0.090
100
97
81
58
0
28
82
85
87
–
72
18
15
13
–
Pt/Cb
0
–
–
a
Conditions: 2e-reduced silicotungstic acid (H6SiW12O40), 0.33 mmol; catalyst, 10 mg; water-ethanol mixture (Vwater: Vethanol = 1:1), 4 mL; reaction time, 10 min.
Water, 4 mL.
b
Table 2
Catalytic hydrogenation of acetophenone in water–ethanol mixturec.
Entry
Catalyst
Reactant (D, mmol)
Conversion (D, %)
Selectivity (E, %)
Selectivity (F, %)
1
2
3
4
5
6
7
Pd/C
Pd/C
Pd/C
Pd/C
Pt/C
–
0.085
0.170
0.255
0.340
0.170
0.170
0.170
92
80
57
42
0
54
82
90
93
–
46
18
10
7
–
–-
–
0
0
–
–
Pd/Cd
c
Conditions: 2e-reduced silicotungstic acid (H6SiW12O40), 0.33 mmol; catalyst, 10 mg; water-ethanol mixture (Vwater: Vethanol = 1:1), 4 mL; reaction time, 10 min.
Water, 4 mL.
d
selectivity of 1-phenylethanol are 80% and 82% under optimum
reaction conditions. Together the amounts of hydrogen (ꢀ2.4 mL)
evolved from these systems, the Faradaic efficiencies for these
semihydrogenation reactions are ꢀ80%. Note that the conversions
of phenylacetylene and acetophenone are negligible in water (see
Table 1, entry 6 and Table 2, entry 7), even if the reduced silico-
tungstic acid and catalyst (Pt/C or Pd/C) are used as the hydrogen
source and catalyst, respectively. These results indicate that, the
introduction of organic solvents not only can improve the H2 yield
in present system, but also can help to develop the promising
application of polyoxometallates in the semihydrogenation of
organic compounds under mild conditions.
tion. And the catalyst (Pt/C) and ECPB (H4SiW12O40) exhibit high
stability for the hydrogen evolution in the present system. More-
over, the semihydrogenation of organic compounds can be
achieved in the water-organic solvent system under mild condi-
tions in the presence of the reduced silicotungstic acid as a hydro-
gen source. The conversion of phenylacetylene and acetophenone
are 97% and 80% under optimum reaction conditions, while the
selectivity of styrene and 1-phenylethanol are 80% and 82%,
respectively. Our results may allow us to highlight the promising
application of polyoxometallates in the highly efficient hydrogen
evolution from water electrolysis and semihydrogenation of
organic compounds.
Declaration of Competing Interest
4. Conclusions
None.
The introduction of organic solvents (acetonitrile, DMF and
ethanol) can regulate the redox property of silicotungstic acid in
water. The redox wave of silicotungstic acid at ꢀ0.02 V vs. RHE
moves in the negative direction. This change is good for the reac-
tion between the reduced silicotungstic acid and H+ ions to pro-
duce hydrogen in the presence of the Pt/C catalyst. A high yield
of hydrogen (ꢀ85%) is obtained in a water-ethanol system (V/
V = 1:1), which is higher than the yield of hydrogen in water
(ꢀ45%). Further experiments reveal that the introduction of etha-
nol can enhance the reduce ability of silicotungstic acid to improve
the production of H2, but do not affect the rate of hydrogen evolu-
Acknowledgements
This work was supported by the Strategic Priority Research
Program of the Chinese Academy of Sciences (XDB20000000), the
Key Program of Frontier Science, CAS (QYZDJ-SSW-SLH033), the
National Natural Science Foundation of China (21521061,
21701169, 21701172, 21773247 and 21875252), the Natural
Science Foundation of Fujian Province (2006L2005), and the China
Postdoctoral Science Foundation (2015 M570561).

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