Electrons and Hydroxyl Radicals Synergistically Boost the Catalytic Hydrogen Evolution from Ammonia Borane over Single Nickel Phosphides under Visible Light Irradiation

Article abstract of DOI:10.1002/open.201900335

From the perspective of tailoring the reaction pathways of photogenerated charge carriers and intermediates to remarkably enhance the solar-to-hydrogen energy conversion efficiency, we synthesized the three low-cost semiconducting nickel phosphides Ni

Full text of DOI:10.1002/open.201900335

DOI: 10.1002/open.201900335
Full Papers
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Electrons and Hydroxyl Radicals Synergistically Boost the
Catalytic Hydrogen Evolution from Ammonia Borane over
Single Nickel Phosphides under Visible Light Irradiation
[a, b]
[a]
[a]
Jin Song,
Xiaojun Gu,* and Hao Zhang
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À 1
From the perspective of tailoring the reaction pathways of
photogenerated charge carriers and intermediates to remark-
ably enhance the solar-to-hydrogen energy conversion effi-
ciency, we synthesized the three low-cost semiconducting
nickel phosphides Ni P, Ni P and Ni P, which singly catalyzed
turnover frequency (TOF) value of 82.7 min , which exceeded
the values of reported metal phosphides at 298 K. The
enhanced activities of nickel phosphides were attributed to the
visible-light-driven synergistic effect of photogenerated elec-
trons (e ) and hydroxyl radicals ( OH), which came from the
oxidation of hydroxide anions by photogenerated holes. This
was verified by the fluorescent spectra and the capture experi-
ments of photogenerated electrons and holes as well as
hydroxyl radicals in the catalytic hydrogen evolution process.
À
*
2
12
5
3
the hydrogen evolution from ammonia borane (NH BH ) in the
3
3
alkaline aqueous solution under visible light irradiation at 298 K.
The systematic investigations showed that all the catalysts had
higher activities under visible light irradiation than in the dark
and Ni P had the highest photocatalytic activity with the initial
2
[
28–30]
1. Introduction
chemical energy conversion.
Importantly, the photogener-
ated charge carriers in semiconductors can promote many
In consideration of the global environment problems, hydrogen
energy has become a promising alternative to fossil fuels.
redox reactions including hydrogen evolution from small
[1,2]
[31,32]
molecules such as water.
For photocatalytic splitting of
The efficient storage and release of hydrogen are main barriers
during developing the hydrogen economy based on fuel
NH BH , the key is to explore low-cost catalytically active
3 3
semiconductors. It has been shown that non-precious metal
phosphides composed of P and earth abundant metals such as
[3,4]
cells. Ammonia borane (NH BH ) with low toxicity and high
3
3
hydrogen content (19.6 wt%) is an excellent chemical hydrogen
Ni have been used in water splitting as cocatalysts in photo-
[5–7]
[33,34]
storage material.
Up to now, a variety of catalysts based on
catalysis and catalysts in electrocatalysis.
However, no
non-noble metals such as Ni and Co have been explored for the
attention has been paid to using these metal phosphides as
photocatalysts without photosensitizers or photogenerated
electron acceptors such as metal nanoparticles probably due to
hydrogen evolution from NH BH3 (NH BH +2H O!NH BO +
3
3
3
2
4
2
[
8–18]
3
H ), which is a thermodynamics-controlled process.
How-
2
ever, their activities still remain low compared with the noble
that they have narrow band gaps and thus have no enough
[19–27]
[35]
metal catalysts.
Thus, it is highly desirable to explore new
redox potential to directly split water.
Different from the
catalytic systems using low-cost and efficient catalysts for
hydrogen evolution.
Compared with the thermocatalysis, the visible-light-driven
photocatalysis is considered as a friendly route for solar-to-
water splitting with non-spontaneous characteristics, NH BH ,
3
3
À 1
À 1
which has weak BÀ N (~117 kJ·mol ) and BÀ H (~430 kJ·mol )
[25,36]
bonds,
is relatively easy to split though the catalytic
reaction. In addition, photogenerated charge carriers such as
electrons and highly active groups such as hydroxyl radicals,
which are often used to promote chemical reactions, can
benefit the cleavage of BÀ H and BÀ N bonds in the photo-
catalytic splitting of NH BH . Bearing these aspects in mind, we
[
a] J. Song, Prof. X. Gu, H. Zhang
Inner Mongolia Key Laboratory of Coal Chemistry
School of Chemistry and Chemical Engineering
Inner Mongolia University
3
3
considered that if semiconducting metal phosphides such as
Ni P are directly used as catalysts under visible light irradiation,
Hohhot 010021, Inner Mongolia, China.
E-mail: xiaojun.gu@yahoo.com
b] J. Song
x
y
[
the efficiency of hydrogen evolution from NH BH3 will be
3
Academician Expert Workstation of Ecological Governance and Green
Development of Bayan Nur
Department of Ecology and Resource Engineering
College of Hetao
Bayan Nur 015000, Inner Mongolia, China.
enhanced through the synergistic effect of photogenerated
electrons and hydroxyl radicals.
Herein, we reported a series of low-cost metal phosphides
Ni P, Ni P and Ni P with different structures, which were for
2
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5
3
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.201900335
the first time used as single catalysts for hydrogen evolution
from NH BH in the alkaline aqueous solution under visible light
©
2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This
3
3
is an open access article under the terms of the Creative Commons Attri-
bution Non-Commercial NoDerivs License, which permits use and distribu-
tion in any medium, provided the original work is properly cited, the use is
non-commercial and no modifications or adaptations are made.
irradiation at 298 K. Compared with the activities of all the
catalysts in the dark, their visible-light-driven activities were
enhanced and Ni P had the highest activity. Moreover, the
2
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photocatalytic hydrogen evolution mechanism was also dis-
cussed on the basis of the related experiments.
2.2. Structural Characterization
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The crystalline phases of as-synthesized nickel phosphides were
characterized using PXRD. The results showed that the pure
phases of Ni P, Ni P and Ni P were synthesized (Figure S1–S3).
2. Results and Discussion
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The surface areas of Ni P, Ni P and Ni P were 33.2, 11.8 and
2
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3
2
2
.1. Synthesis Strategy
20.9 m /g (Figure S4–S6). The TEM images showed that the
particles of Ni P, Ni P and Ni P were in the nanoscale (Figure 2
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Visible-light-responsive materials with low cost were selected to
and S7). The lattice fringes with spacing of 0.221, 0.193 and
0.180 nm corresponded to the (111) plane of Ni P, (240) plane
catalyze hydrogen evolution from NH BH3 since they can
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harvest visible light and then produce separated charge carriers
including holes and electrons, which can promote the catalytic
of Ni P and (222) plane of Ni P. The XPS investigations showed
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that the Ni 2p3/2 edges in the three nickel phosphides were
deconvolved into three peaks (Figure 3). The peaks at 853.0,
852.7 and 852.4 eV were assigned to the positive charge Ni
[37]
reaction.
Among the catalytically active species, nickel
δ+
phosphides were selected as photocatalysts on the basis of
three considerations. Firstly, nickel phosphides are active
components for catalytic hydrogen evolution from NH BH , but
[
22,41,42]
for Ni P, Ni P and Ni P, respectively.
The value of δ was
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δ(Ni P)>δ(Ni P )>δ(Ni P). The peaks at 855.9, 856.0 and
3
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2
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5
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2
+
[43]
their activities are still low. Secondly, the photogenerated
charge carriers induced by the visible light might benefit the
enhanced catalytic reaction rate of nickel phosphides. Thirdly,
three nickel phosphides Ni P, Ni P and Ni P with different
855.9 eV were consistent to the Ni for Ni P, Ni P and Ni P,
2 12 5 3
respectively, due to the surface oxidation. The peaks at 861.3,
860.9 and 860.5 eV were assigned to the satellites of Ni 2p3/2
.
The spectra of Ni 2p1/2 were similar to those of Ni 2p3/2. In the P
2p3/2 spectra (Figure S8), the peaks at 128.9, 129.1 and 129.3 eV
2
12
5
3
structures, where there are different electron transfer character-
[38,39]
δÀ
[44]
istics from Ni to P,
provided us a chance to systematically
confirmed the presence of P for Ni P, Ni P and Ni P,
2
12
5
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study the photocatalytic chemistry of metal phosphides. Since
respectively, indicating that there were different characteristics
of charge transfer from Ni to P in the three nickel phosphides
with different structures.
À
OH ions in the aqueous solution are easily oxidized by
photogenerated holes from semiconductors to form highly
[40]
reactive hydroxyl radicals.
The inorganic base NaOH was
In the UV-vis spectra, Ni P, Ni P and Ni P had the visible
2
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selected as the resource of hydroxyl radicals, which were
light absorption (Figure S9). The steep absorption edge sug-
gested the electron transfer from valence band to conduction
band. In order to verify the electron transfer of the nickel
phosphides, we studied their transient photocurrent density
under visible light irradiation (λ�420 nm). The results showed
that the transient photocurrent appeared in the nickel
beneficial for the cleavage of BÀ N bonds in NH BH . Once the
3
3
À
holes were consumed by OH ions, the remaining photo-
generated electrons could efficiently participate in the splitting
of NH BH . In the light of the above contents, the visible-light-
3
3
driven synergistic effect of photogenerated electrons and
hydroxyl radicals could lead to the efficient catalytic hydrogen
evolution from NH BH in the alkaline aqueous solution over
phosphides and Ni P had the highest photocurrent density
2
(Figure 4a). These results also confirmed that the three nickel
3
3
the nickel phosphides with different structures (Figure 1).
Figure 1. Schematic illustration of the catalytic hydrogen evolution proce-
dure over single semiconducting nickel phosphides with different structures
under visible light irradiation.
2 5
Figure 2. TEM and HRTEM mages of (a, c) Ni P and (b,d) Ni12P .
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Figure 4. (a) Time versus transient photocurrent density and (b) Nyquist
plots of the three nickel phosphides.
Figure 3. XPS patterns of Ni 2p for (a) Ni
2
P, (b) Ni12
P
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and (c) Ni
3
P.
À 1
phosphides had semiconducting characteristics and Ni P had
82.7 min , which was the highest in the values of reported
2
higher catalytic reduction ability. It should be noted that the
photocurrent response of Ni P and Ni P were positive while the
metal phosphide catalysts (Table 1). The 87.5, 78.7 and 88.2% of
activity enhancement of Ni P, Ni P and Ni P was caused by the
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photocurrent response of Ni P was negative, indicating that
contribution of visible light irradiation comparable to the dark
reaction, respectively. The different activity enhancement could
be attributed to the different light absorption ability and the
different charge separation efficiency with increasing the Ni/P
ratio from 2 to 3 in these nickel phosphides, which might lead
to the different usage efficiency of photogenerated charge
carriers and hydroxyl radicals. In addition, the characteristic of
electron transfer from Ni to P in the three nickel phosphides
with different Ni/P ratios was different (Figure 3), which led to
their different photocatalytic hydrogen evolution activities. It
1
2 5
Ni P and Ni P were n-type semiconductors and Ni P was p-
2
3
12 5
[45]
type semiconductor. These different semiconducting charac-
teristics might lead to generating different charge carriers in
the conductive process, which might further influence the
photocatalytic properties of corresponding catalysts. In addi-
tion, the EIS experiments were carried out to get further
insights into the hydrogen evolution ability of nickel phos-
phides. It was found that Ni P and Ni P had the smallest and
2
3
biggest charge transfer resistance, respectively (Figure 4b),
which was consistent with the corresponding transient photo-
current density.
3 3
Table 1. Activities of catalysts for hydrogen evolution from NH BH .
À 1
Catalyst
TOF (min
)
Reference
2
.3. Catalytic Performance and Mechanism
Ni
Ni0.8
Ni
2
P
W
82.7
25.0
19.6
79.5
46
72.2
58.4
246.8
70.0
42.3
93.8
This work
6
0.2
14
15
16
17
18
20
24
25
37
In order to investigate the effect of visible light irradiation on
the catalytic performance, the hydrogen evolution from NH BH
in the alkaline aqueous solution over Ni P, Ni P and Ni P was
measured. As shown in Figure 5, the activities of three catalysts
were enhanced under visible light irradiation compared with
their activities in the dark. Specially, Ni P exhibited the highest
Co/(CeOx)0.91/NGH
Cu0.72Co0.18Mo0.1
CoP
Ni0.7Co1.3
Pd Ni /MCN
Cu
Co/CTF
Co/C
3
3
2
12
5
3
P
74
26
x
Co1À xOÀ GO
2
N
4
-580
activity with the initial turnover frequency (TOF) value of
3
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Figure 6. Fluorescent spectra of Ni
alkaline aqueous solution and TA (5 mM) under visible light irradiation.
2
P in (a) 10.0, (b) 30.0 and (c) 50.0 mM of
Figure 5. (a) Time versus volume of hydrogen evolution from the alkaline
aqueous NH BH over the three nickel phosphides under different conditions
and (b) the initial TOF values.
3
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Figure 7. Time versus volume of hydrogen evolution from NH
aqueous solution with different alkaline concentrations over Ni
visible light irradiation.
3
BH
3
in the
should be noted that the crystalline phases of three nickel
phosphides were maintained after the catalytic hydrogen
evolution reaction (Figure S1–S3), indicating that these photo-
catalysts were stable in the present alkaline environment.
The formation of radical intermediates under the light
irradiation of photoactive materials in aqueous environment
2
P under
on the surface of catalyst. Besides, the base NaOH also acted as
a catalyst promoter for the hydrolytic dehydrogenation of
[46,47]
plays an important role in the photocatalytic reactions.
It is
[16–18]
known that hydroxyl radicals can react with TA and generate
TAOH, which emits fluorescence at around 426 nm. To inves-
NH BH .
This suggested that the formation of hydroxyl
3
3
radicals benefited the catalytic activity enhancement under
visible light irradiation. To further make sure the existence of
hydroxyl radicals, we tested the photocatalytic activity of Ni P,
where 2-propanol was chosen as the scavenger of hydroxyl
radicals. The results showed the activity decreased due to the
decrease of hydroxyl radicals (Figure 8), indicating that the
formation of hydroxyl radicals was a positive factor for photo-
catalytic hydrogen evolution from NH BH .
tigate the effect of hydroxyl radicals, which generated in a
À
process of oxidation of OH or H O by photogenerated holes,
2
2
on the catalytic activity, we selected TA as fluorescence probe
to trace the hydroxyl radicals. The results showed that the
À
concentration of OH affected the amount of produced
hydroxyl radicals and there was different intensity of
fluorescence peaks, which appeared at about 426 nm (Figure 6).
More importantly, the photocatalytic hydrogen evolution rate
increased with increasing the base concentration (Figure 7). In
3
3
In the present catalytic system, the hydroxyl radicals
À
generated through the oxidation of OH ions by photogen-
[48,49]
detail, Ni P had the highest activity in 0.5 M of NaOH. However,
erated holes.
In order to confirm this judgment, we
2
its activity decreased when the alkaline concentration increased
to 0.7 M. The high-concentration base induced the formation of
more hydroxyl radicals, which participated in the cleavage of
chemical bonds in NH BH , while the excessive hydroxyl radicals
measured the photocatalytic activity of Ni P, where KI was used
2
as capture agent of holes. The results showed that the catalytic
activity decreased due to the decrease of holes (Figure 8),
suggesting that the formation of holes had a positive effect on
the catalytic reaction rate in the alkaline aqueous solution.
3
3
impeded the adsorption of catalytic substrates NH BH and H O
3
3
2
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the BÀ N bond and the photogenerated electrons attacked the
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BÀ H bond to generate H with the assistance of water.
2
To further understand the role of visible light irradiation in
the activity enhancement of catalysts, the catalytic performance
of Ni P under different light intensity was investigated. It was
2
found that increasing the light intensity resulted in an almost
linear enhancement of catalytic activity (Figure 10). These
phenomena could be attributed to that more electrons and
hydroxyl radicals generated from Ni P in the alkaline environ-
2
ment under high-intensity visible light irradiation, leading to
that more photogenerated electrons and hydroxyl radicals were
used to attack and break BÀ H and BÀ N bonds in NH BH
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In the present reaction, water molecules participate in the
[5,51]
hydrogen evolution.
In order to confirm this judgment, we
used D O instead of H O to investigate the kinetic isotope effect
2
2
Figure 8. Time versus volume of hydrogen evolution from the alkaline
(KIE) in the catalytic reaction over Ni P, Ni P and Ni P. The
2 12 5 3
aqueous NH
3
BH
3
over Ni
2
P without scavenger or in the presence of 2-
results showed that the hydrolysis of NH BH in D O had a low
3
3
2
propanol, K Cr
2
2
O
7
and KI under visible light irradiation.
reaction rate compared to that in H O under light irradiation
2
(Figure 11 and S10). The KIE constants were 2.9, 2.8 and 2.4
calculated according to the hydrogen evolution rate. This
indicated that water molecules were involved in the hydrogen
evolution from the aqueous NH BH , which was similar to the
Besides the photogenerated holes, the photogenerated elec-
[50]
trons also played an important role in the catalytic reaction.
To check this, K Cr O was selected as scavenger of electrons in
3
3
[52]
hydrogen evolution through the hydrolysis of NaBH4.
2
2
7
the catalytic process. The results showed that the photocatalytic
activity decreased after the introduction of K Cr O (Figure 8),
2
2
7
indicating that the photocatalytic hydrogen evolution was
remarkably affected by the photogenerated electrons.
In view of the above results regarding hydroxyl radicals and
photogenerated charge carriers, a possible mechanism for
photocatalytic hydrogen evolution from NH BH in the alkaline
3
3
aqueous solution was proposed (Figure 9). After the absorption
of visible light by nickel phosphides, the photogenerated
electrons and holes formed, separated and migrated to the
catalyst surface. In the meantime, the hydroxyl radicals
generated through the reaction between photogenerated holes
À
and OH ions. For the decomposition of NH BH for hydrogen
3
3
evolution, the cleavage of BÀ N bonds, which form through
enjoying lone pair electrons between NH and BH3 groups,
3
should first happen. On the basis of that the hydroxyl radicals
benefited the enhancement of photocatalytic hydrogen evolu-
tion from NH BH3 in the experiment, it could be rationally
3
deduced that the hydroxyl radicals attacked NH BH to break
3
3
Figure 10. (a) Time versus volume of hydrogen evolution from the alkaline
aqueous NH BH over Ni P under visible light irradiation with different
Figure 9. Possible mechanism for the hydrogen evolution from NH
3
BH
3
in
3
3
2
the alkaline aqueous solution over nickel phosphides under visible light
irradiation.
intensities and (b) the dependence of initial TOF values on the light
intensity.
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Figure 11. Time versus volume of hydrogen evolution from NH BH in H O
or D O over Ni P and Ni P under visible light irradiation.
2 2 3
3
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2
It is known that the hydrogen evolution from aqueous
[53]
NH BH is a thermodynamics-controlled process. In order to
3
3
evaluate the effect of reaction temperature on the photo-
catalysis, we measured the photocatalytic performance of Ni2P
under the photothermal condition. The results showed that
compared with the photocatalytic activity of Ni P at 298 K, its
2
activity was significantly enhanced under photothermal con-
À 1
dition and its initial TOF value was 138.3 min (Figure S11),
À 1
which was much higher than the value (82.7 min ) at room
temperature.
The practical application of catalysts requires their good
recycle durability. So the long-time catalytic durability test of
Ni P under visible light irradiation was carried out at 298 K. The
2
results showed that the catalyst had good durability and 100%
of H selectivity even after 20 cycles of catalysis (Figure 12a).
2
After the catalytic cycles, the crystalline phase of Ni P was
2
unchanged (Figure 12b). With increasing the number of cata-
lytic cycles, the rate of hydrogen evolution decreased, which
might be due to the agglomeration of catalyst particles after
the long-time photocatalytic reaction (Figure 12c). In addition,
the increase of boron species in the catalytic system might also
[5]
lead to the gradual decrease of hydrogen evolution rate.
3. Conclusions
Figure 12. Durability test for the hydrogen evolution from the alkaline
aqueous NH BH over Ni P, (b) the PXRD pattern and (c) the TEM image of
catalyst after 20 cycles of catalysis.
3
3
2
In summary, three semiconducting nickel phosphides Ni P,
2
Ni P and Ni P with different structures were synthesized. The
12
5
3
compounds were used as single catalysts for hydrogen
evolution from NH BH under visible light irradiation. Among all
3
3
the catalysts, Ni P had the highest room-temperature activity. In
related hydrogen evolution performance of the catalysts. Based
on the obtained results, a new mechanism for the photo-
catalytic hydrogen evolution from NH BH in the alkaline
2
the catalytic process, the synergistic effect of photogenerated
electrons and hydroxyl radicals in the alkaline aqueous
condition enhanced the activities of nickel phosphides under
visible light irradiation. This effect has been experimentally
testified by the fluorescent spectra and the capture of photo-
generated charge carriers and hydroxyl radicals as well as the
3
3
aqueous solution has been presented. This work sheds light on
exploring low-cost and visible-light-responsive semiconductors
with well-defined structures, which is beneficial for rationally
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Full Papers
designing new high-performance photocatalytic systems to-
ward a greener world using clean energy.
Calculation Method
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
The initial TOF value was calculated at the NH
BH conversion of
3
3
72%. The calculation was shown as follow:
Experimental Section
TOF ¼ 3
nNH3BH3
ꢀ72%
catalystt
n
Chemicals
In the equation, nNH3BH3 is the molar amount of NH BH3 at its
conversion of 72%, ncatalyst is the total molar amount of nickel
phosphide and t is the time at the NH BH conversion of 72%.
3
Nickel chloride hexahydrate (NiCl
·6H O, Sinopharm Chemical
2
2
Reagent Co. Ltd, >99%), sodium hypophosphite (NaH PO , Aladdin,
2
2
3
3
9
9%), trisodium citrate dihydrate (Na C H O ·2H O, Sinopharm
3 6 5 7 2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
Chemical Reagent Co. Ltd, >99%), phosphorus red (P, Aladdin,
9%), sodium acetate (CH COONa, Sinopharm Chemical Reagent
9
3
Co. Ltd, >99%), nickel acetate tetrahydrate (Ni(CH COO) ·4H O,
3
2
2
Acknowledgements
Sinopharm Chemical Reagent Co. Ltd, >99%), potassium dichro-
mate (K Cr O , Sinopharm Chemical Reagent Co. Ltd, 99.8%),
2
2
7
The authors gratefully acknowledge the financial support from the
National Natural Science Foundation of China (21761025), the
Program for New Century Excellent Talents in University of the
Ministry of Education of China (NCET-13-0846) and the Pilot
Support Project for Ecological Conservation and Restoration of
Landscape, Forest, Lake and Grassland in Lake Ulansuhai
potassium hydroxide (KOH, Sinopharm Chemical Reagent Co. Ltd,
85%), potassium iodide (KI, J&K Chemical, >99%), sodium
>
hydroxide (NaOH, Sinopharm Chemical Reagent Co. Ltd, >96%),
ammonia borane (NH BH , Aldrich, 97%), terephtalic acid (C H O ,
3
3
8
6
4
TA, J&K Chemical, 99%), 2-propanol ((CH ) CHOH, J&K Chemical,
3
2
99.9%) and deuterium oxide (D O, J&K Chemical, 99.8% atom D)
2
were obtained without purification.
(2019HYYSZX).
Synthesis and Catalytic Study
[
22]
Conflict of Interest
For the synthesis of Ni P,
the mixture of NiCl ·6H O (1.00 g),
2 2
2
Na C H O ·2H O (0.25 g) and NaOH (2.80 g) in water (45 mL) was
3
6
5
7
2
stirred for 1 h to give a green solid. Then, the green solid (0.25 g)
The authors declare no conflict of interest.
and NaH PO2 (1.25 g) were heated at 573 K for 2 h in an Ar
2
[
39]
atmosphere to give Ni P. For the synthesis of Ni P , the mixture
2
12 5
of Ni(CH
COO)
·4H
O (0.95 g) and phosphorus red (0.70 g) was
Keywords: synergistic effect · ammonia borane · hydrogen
evolution · catalysts
3
2
2
dissolved in water (30 mL) under stirring for 20 min. Then, the
mixture was hydrothermally treated at 473 K for 12 h to give Ni P .
1
2 5
[
54]
For the synthesis of Ni P,
the mixture of NiCl ·6H O (5.0 g),
3
2 2
NaH PO2 (24.4 g) and CH COONa (2.9 g) was dissolved in water
2
3
[
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Manuscript received: November 12, 2019
Revised manuscript received: February 2, 2020
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