Effect of crystallographic structure of MnO2 on degradation of 2-CEES

Article abstract of DOI:10.1016/j.molliq.2021.115946

In this study, four MnO₂ samples with different crystallographic structures (α-, β-, δ- and γ-MnO₂) were synthesized by the hydrothermal method. The relationship between the characteristics of the four crystalline samples and their degradation ability against 2-chloroethyl ethyl sulfide (2-CEES), a simulant of the chemical warfare agent sulfur mustard, and the relevant reaction mechanism were investigated. Characterization data indicated that the various crystal types displayed different specific surface areas, frequency of lattice defects, amounts of adsorbed/inserted water, and numbers and strength of basic sites. The combined effects of these differences caused different degradation activity in 2-CEES degradation to be observed. The reaction followed pseudo-first-order kinetics, whereby γ-MnO₂ exhibited the highest degradation activity. The mechanism of 2-CEES degradation was studied by gas chromatography and infrared spectroscopy methods, and data indicated the key role played by substrate hydrolysis and oxidation in the degradation process.

Full text of DOI:10.1016/j.molliq.2021.115946

Journal of Molecular Liquids 333 (2021) 115946
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Effect of crystallographic structure of MnO2 on degradation of 2-CEES
⇑
⇑
Yueting Guo, Lingce Kong, Meiling Lei, Yi Xin, Yanjun Zuo , Wenming Chen
State Key Laboratory of NBC Protection for Civilian, Beijing 102205, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, four MnO2 samples with different crystallographic structures (a-, b-, d- and c-MnO2) were
Received 8 October 2020
Revised 25 February 2021
Accepted 18 March 2021
Available online 23 March 2021
synthesized by the hydrothermal method. The relationship between the characteristics of the four crys-
talline samples and their degradation ability against 2-chloroethyl ethyl sulfide (2-CEES), a simulant of
the chemical warfare agent sulfur mustard, and the relevant reaction mechanism were investigated.
Characterization data indicated that the various crystal types displayed different specific surface areas,
frequency of lattice defects, amounts of adsorbed/inserted water, and numbers and strength of basic sites.
The combined effects of these differences caused different degradation activity in 2-CEES degradation to
Keywords:
Manganese oxide
Crystallographic structure
Decontamination
2-chloroethyl ethyl sulfide
be observed. The reaction followed pseudo-first-order kinetics, whereby c-MnO2 exhibited the highest
degradation activity. The mechanism of 2-CEES degradation was studied by gas chromatography and
infrared spectroscopy methods, and data indicated the key role played by substrate hydrolysis and oxi-
dation in the degradation process.
Ó 2021 Elsevier B.V. All rights reserved.
1. Introduction
According to research, manganese oxide is a kind of amphoteric
oxide with strong oxidizing property, which can be used as battery
Chemical warfare agents (CWAs) are the main chemical weap-
ons because of their high toxicity, fast action, long-lasting effects,
and low killing concentration. At the present time, well-known
lethal CWAs include the nerve agents sarin (GB, O-isopropylmethyl
phosphonofluoridate), soman (GD, 3,30-dimethylbutan-2-yl), and
VX (S-[2-(diisopropylaminoethyl)ethyl]-O-methyl-ethyl), as well
as the erosive agent sulfur mustard (HD, bis(2-chloroethyl)
sulfide). Notably, HD is also known as the ‘‘king of poisons”. The
efficient and effective decontamination of HD has always been an
important subject of concern for military chemists.
Nanometer metal oxides are reactive adsorbents characterized
by small particle size, large specific surface area, abundant surface
defects [1]. Nanometer metal oxides may not only adsorb CWAs on
the surface of nanometer metal oxides but also react with active
sites in nanometer metal oxides to produce hydrolysis, elimination,
and oxidation products, making its toxicity less than CWAs. They
are thus a kind of reactive adsorbents. Therefore, nanometer metal
oxides are the focus of growing research attention. According to
published reports, the use of synthetic CuO [2], WO3 [3], V2O5
[4,5], TiO2 [6], ZrO2 [7], ZnO [8–10], MgO [11,12], and other
nano-metal oxides [13–15] have been utilized as reactive adsor-
bents for decontamination of CWAs, which indicates that nanome-
ter metal oxides have good application prospects in this field.
electrode, industrial catalyst, and oxidant. Prasad et al. [16] and
Mahato et al. [17] synthesized different kinds of mesoporous
MnO2 for the degradation of CWAs and their simulants. The result
showed that mesoporous MnO2 had good degradation activity on
CWAs and their simulants. Previous studies have shown that crys-
tal structure affects the catalytic activity of nano-metal oxides. For
a general understanding in photocatalytic field that anatase is
more active than rutile [18–20]. The (111) plane of MgO is partic-
ularly reactive, trigonal symmetry of (111) plane that possess
proper geometry to hydrolyze bound GB or GD [21]. This indicates
that it is significant to study the effect of crystal shape on degrada-
tion performance.
Manganese oxide can occur in a variety of crystalline forms,
including
a-MnO2 (hollandite), b-MnO2 (pyrolusite), d-MnO2 (bir-
nessite), and
c-MnO2 (nsutite) as well as amorphous MnO2 [22].
Different crystalline forms of manganese oxide have been proven
to display different properties in terms of electrochemical capaci-
tance [23], oxidation [24], and catalysis [25]. Results from their
study indicated that MnO2 has great prospects in the degradation
of CWAs and their simulants. At the same time, different crystal
structures may cause differences in the degradation performance
of MnO2 on CWAs. Currently, however, data on the effectiveness
of different crystal structures of manganese oxides in CWAs decon-
tamination is still lacking in the literatures. This paper mainly dis-
cusses the difference of degradation activity between different
crystal MnO2 and 2-CEES.
⇑
Corresponding authors.
E-mail addresses: zyj1688@sina.com (Y. Zuo), fhchenwm@163.com (W. Chen).
https://doi.org/10.1016/j.molliq.2021.115946
0167-7322/Ó 2021 Elsevier B.V. All rights reserved.

Y. Guo, L. Kong, M. Lei et al.
Journal of Molecular Liquids 333 (2021) 115946
In this paper, four MnO2 species characterized by different crys-
tallographic structures were synthesized by the hydrothermal
method using potassium permanganate, manganese sulfate, and
ammonium persulfate as raw materials. The HD simulant 2-CEES
was selected as the decontamination target. The samples were
characterized by X-ray diffraction (XRD), nitrogen adsorption-
desorption isotherms, scanning electron microscopy (SEM), and
transmission electron microscopy (TEM). The effects of sample
properties on decontamination activity were studied by tempera-
ture programmed desorption of CO2 (CO2-TPD), thermogravimetric
analysis (TG) and X-ray photoelectron spectroscopy (XPS). Gas
chromatography was used to test the decontamination activity of
the sample in response to 2-CEES. Gas chromatography–mass
spectrometry (GC–MS) and Fourier-transform infrared spec-
troscopy (FTIR) were employed to analyze the decontamination
products and infer the decontamination reaction mechanism.
and whose contents were kept at 90 °C for 24 h. The precipitate
obtained as a result was filtered, washed, and dried at 80 °C for 4 h.
2.3. Characterization
The phase composition of MnO2 was determined by XRD, using
a D8 Advance diffractometer, Bruker, Germany. The samples were
irradiated at CuKa (40 kV and 40 mA) within a 5–80° 2h range. A
JCPDS card was used to determine the crystal shape.
The surface morphology of samples was determined collecting
SEM images obtained with a Gemini SEM 300, ZEISS, Germany, in
low vacuum mode imaging at 30 kV acceleration voltage.
TEM images were obtained using a JEM-2010 instrument, JEOL,
Japan, and were used to observe the micro-morphology and lattice
fringes of the samples. The instrument worked at an acceleration
voltage of 200 kV, and the magnification was 500,000 to 1 million
times.
2. Experimental
XPS analysis was performed using an AXIS SUPRA instrument,
Kratos, Japan. The samples were tested by internal calibration with
2.1. Materials
C 1S binding energy under AlKa radiation.
Potassium permanganate (KMnO4) of analytical purity was pur-
chased from Beijing chemical plant. Manganese sulfate (MnSO4ꢀH2-
O) and ammonium persulfate ((NH4)2S2O8) of analytical purity
were purchased from Shanghai Macklin Biochemical Co., Ltd. 2-
CEES (CH3CH2SCH2CH2Cl) of 98% purity was purchased from Acros
Organics.
TG experiments were performed using a TG 209 F3/DSC 200 F3
instrument, NETZSCH, Germany. The weight changes of the sam-
ples were detected from room temperature to 800 °C under air
atmosphere at a heating rate of 5 °C/min.
The nitrogen adsorption–desorption curves of the samples were
obtained using an ASAP 2020 surface area analyzer manufactured
by Micromeritics, USA. The samples were first outgassed under
dynamic vacuum for 1 h at 100 °C and then allowed to cool to room
temperature. After that, the N2 adsorption–desorption isotherms
were obtained at 105 °C (N2 temp.). The specific surface area of
the sample was measured via the Brunauer–Emmett–Teller (BET)
method, and the pore diameter distribution was measured via
the Barrett–Joyner–Halenda (BJH) method.
2.2. Synthesis of MnO2 with different crystal structures
MnO2 characterized by different crystal structures were synthe-
sized implementing the hydrothermal method as described in pub-
lished reports [23,24,26], with some modifications.
The basic sites of synthetic manganese oxide were detected by
CO2-TPD. These experiments were conducted on an Auto Chem II
2920 with a thermal conductivity detector, Micromeritics, USA.
CO2 (50 cm3/min) adsorbs for 30 min to saturation, switch to He
gas flow (50 cm3/min), keep for 180 min to remove CO2 on the
surface.
The characterization of reaction products was carried out by
GC–MS, using a combined instrument manufactured by Thermo
scientific, USA. GC–MS analyses were examined in the range of
40–280 °C at a heating rate of 20 °C/ min, keep for 3 min at 280 °C.
FTIR spectrometry, which was performed using an instrument
manufactured by Thermo Fisher, USA, was employed to character-
ize the degradation products.
2.2.1. Synthesis of a-MnO2
KMnO4 (5 g) and of MnSO4ꢀH2O (12.5 g) were dissolved in
400 ml of distilled water, and the resulting solution was stirred
at room temperature (25 °C) for about 30 min. The solution was
then transferred to a Teflon-lined stainless-steel autoclave, which
was sealed and whose contents were kept a 140 °C for 12 h. The
precipitate obtained as a result was filtered, washed with distilled
water, and dried at 80 °C for 4 h.
2.2.2. Synthesis of b-MnO2
(NH4)2S2O8 (11.4 g) and MnSO4ꢀH2O (8.45 g) were dissolved in
400 ml of deionized water, and the resulting solution was stirred
at room temperature for about 30 min. This solution was trans-
ferred to a Teflon-lined stainless-steel autoclave, which was sealed
and whose contents were kept at 140 °C for 12 h. The precipitate
obtained as a result was filtered, washed, and dried at 80 °C for 4 h.
2.4. Reaction procedure
2.2.3. Synthesis of d-MnO2
The synthesized samples were made to react with 2-CEES at
room temperature to test their decontamination abilities. Nitrile
gloves and particulate respirators should be worn during the
KMnO4 (9 g) and of MnSO4ꢀH2O (1.68 g) were dissolved in
480 ml of distilled water, and the resulting solution was stirred
at room temperature (25 °C) for about 30 min. The solution was
then transferred to a Teflon-lined stainless-steel autoclave, which
was sealed and whose contents were kept a 160 °C for 12 h. The
precipitate obtained as a result was filtered, washed with distilled
water, and dried at 80 °C for 4 h.
experiment in the fume hood. 5
lL 2-CEES was pre-diluted with
0.3 ml nonane (to obtain a ~2.5 wt% solution) in seven 5 ml test
tubes endowed with a glass plug, the test tube was then added
the MnO2 sample to be tested, that is, there was 5
lL 2-CEES in
every 0.1 g sample. The tubes were shaken and mixed then placed
in a thermostat. Take a test tube to extract the remaining 2-CEES
with 2 ml acetonitrile at pre-defined time points (0.5, 1, 2, 4, 8,
16, and 24 h). The supernatant was extracted with a microporous
filter and placed into a 7890A gas chromatograph manufactured
by Agilent Technologies, USA, for quantitative analysis aimed at
investigating the reaction kinetics.
2.2.4. Synthesis of c-MnO2
(NH4)2S2O8 (4.58 g) and MnSO4ꢀH2O (3.38 g) were dissolved in
480 ml of deionized water, and the resulting solution was stirred
at room temperature for about 30 min. This solution was trans-
ferred to a Teflon-lined stainless-steel autoclave, which was sealed
2

Y. Guo, L. Kong, M. Lei et al.
Journal of Molecular Liquids 333 (2021) 115946
ship in the tunnels present in the structure render the said tunnels
prone to collapse [32,33], importantly, the appearance of these
defects may provide more active sites for the reaction with 2-CEES.
In Fig. 2 (b, d, f, h) are reported the TEM images of the nanopar-
3. Results and discussion
3.1. Characterization of the morphologies a-, b-, d-, and c-MnO2
ticles of all four MnO2 structures. The lattice fringes of
a-MnO2
The basic structure of manganese oxide derives from the basic
[MnO6] octahedral unit resulting from the coordination of one
manganese atom by six oxygen atoms [27]. Due to its different
connection methods, the network structure leads to the formation
of different lattice MnO2. These lattices are divided into three cat-
egories: one-dimensional (1D), two-dimensional (2D), and three-
measure 1.25 nm and 1.44 nm, which match the interplanar spac-
ing of the (200) and (310) crystal planes, respectively. The lattice
fringe of 0.77 nm in b-MnO2 corresponds to the interplanar spacing
of the (110) crystal plane. The lattice fringes in d-MnO2 measure
0.91 nm, 1.22 nm, and 0.63 nm, which coincide with the interpla-
nar spacing of the (ꢂ111), (002), and (001) crystal planes, respec-
dimensional (3D) tunnel structures. The tunnel structure of
MnO2 is 1D (1 ꢁ 1) and (2 ꢁ 2) [28], b-MnO2 is 1D (1 ꢁ 1) [29],
d-MnO2 is a 2D layered structure [30], and
-MnO2 is 1D (1 ꢁ 1)
and (1 ꢁ 2) [31]. The XRD patterns of -MnO2, b-MnO2, d-MnO2,
and -MnO2 are reported in Fig. 1. They correspond to
44-0141), b- (JCPDS 24-0735), and d- (JCPDS 80-1098),
a-
tively. The lattice fringes of 3.39 nm in
the interplanar spacing of the (120) crystal plane. Notably,
and -MnO2 all display lattice distortions or ambiguities (high-
c
-MnO2 are consistent with
a-, d-,
c
c
a
lighted by blue ovals), which indicates that the structure of b-
MnO2 is characterized by higher crystallinity than those of the
other three species [34]. This result is consistent with the XRD
result.
c
a
c
- (JCPDS
- (JCPDS
14-0644) structures, respectively. Notably, the intensities of the
b-MnO2 diffraction peaks are significantly higher than those of
the diffraction peaks of the other crystal forms of manganese oxide,
which indicates that b-MnO2 had high crystallinity. The average
values for the grain sizes of the four structures, as calculated by
Scherrer’s formula are listed in Table 1.
Values for the specific surface areas and total pore volumes of
a
-, b-, d-, and
is decreased in the following order:
MnO2 > b-MnO2. The specific surface area and total pore volume
obtained for -MnO2 are greater than the other three crystallo-
graphic forms of MnO2. These differences indicate that -MnO2
c
-MnO2 are listed in Table 1. The specific surface area
c
-MnO2 - MnO2 > d-
> a
c
The SEM images of the four crystalline forms of manganese
c
oxide are reported in Fig. 2 (a, c, e, g). Evidence indicates that
a-
has more pores to adsorb and destroy more 2-CEES. This result is
consistent with the decontamination activity to 2-CEES, indicating
that the specific surface area of samples is one of the key factors
affecting the decontamination activity. The nitrogen adsorption–
desorption isotherms of the four crystal structures of manganese
MnO2 and -MnO2 consist of spherical aggregates composed of
nanorods, b-MnO2 consists of nanorods, and d-MnO2 is composed
of flower cluster nanosheets. As can be evinced from Fig. 2 (a, g),
c
unlike the smooth b-MnO2 and d-MnO2,
a-MnO2 and c-MnO2
appear rough and display large numbers of defects and pores. This
oxide are reported in Fig. 3. The said isotherms for
a-, d-, and
observation stems from the heteroatoms and symbiotic relation-
Fig. 1. XRD patterns of (a)
a-MnO2 (b) b-MnO2 (c) d-MnO2, and (d) c-MnO2.
3

Y. Guo, L. Kong, M. Lei et al.
Journal of Molecular Liquids 333 (2021) 115946
Table 1
Values for the average grain size, specific surface area, total pore volume, and average pore diameter of
a-, b-, d-, and c-MnO2 samples.
Sample
-MnO2
b-MnO2
d-MnO2
Average grain size (nm)
Specific surface area (m2/g)
Total pore volume (cm3/g)
Average pore diameter (nm)
a
20
26.2
17
46.71
10.48
18.14
83.12
0.11
0.04
0.06
0.21
9
14
13
10
c
-MnO2
6.8
Fig. 2. SEM and TEM images of (a) and (b)
a
-MnO2; (c) and (d) b-MnO2; (e) and (f) d-MnO2; (g) and (h) c-MnO2.
4

Y. Guo, L. Kong, M. Lei et al.
Journal of Molecular Liquids 333 (2021) 115946
Fig. 3. Nitrogen absorption-desorption curves of a-, b-, d-, c-MnO2 samples.
Fig. 4. Fitting kinetic experimental results.
c
-MnO2 are of the ‘‘IV” type, whereby the desorption isotherm lies
Table 2
Rate constants of the two parallel first-order reactions between 2-CEES and
and -MnO2 samples.
a
-, b-, d-,
above the adsorption isotherm. According to the type of the meso-
porous hysteresis loop, it is an H3 hysteresis loop. On the other
hand, the data for b-MnO2 are consistent with those of a type ‘‘III”
c
Sample
2-CEES
k1 (sꢂ1
)
k2 (sꢂ1
)
isotherm. a-, b-, d-, and c-MnO2 piles up to form mesopores.
a
-MnO2
3.009
5.598
4.112
3.994
0.008587
0.003439
0.0039
b-MnO2
d-MnO2
3.2. The degradation kinetics of 2-CEES with MnO2
c
-MnO2
0.01881
In order to study the 2-CEES degradation activity of the syn-
thetic manganese oxide samples,
a-, b-, d-, and
c-MnO2 were made
to react with 2-CEES. -, b-, d-, and
a
c
-MnO2 degraded 2-CEES of
the most efficient 2-CEES deactivator,
during the stable stage of the reaction.
c-MnO2 only degraded 6%
73.0%, 54.4%, 56.3%, and 80.4%, respectively within 24 h. The reac-
tion of the nano-metal oxides with 2-CEES can be considered as
consisting of two or more parallel reactions taking place at differ-
ent rates. The overall reaction between MnO2 and 2-CEES may be
viewed as the superposition of two first-order reactions character-
ized by different rates. The first stage is the fast reaction stage, 2-
CEES can quickly get adsorbed onto the MnO2 surface, 2-CEES
molecules enter the pores of the samples and then react with the
active sites present on the sample surface. Once such sites are con-
sumed, however, the reaction will enter a stable stage. 2-CEES
molecules enter the reaction sites in the internal pores through dif-
fusion and evaporation [2]. At this time, the number of available
active sites is rather low, so the reaction enters its slow stage.
The kinetics of the mentioned overall reaction are represented by
equation (1), whereby t is the reaction time and qt is the residual
amount of the HD simulant 2-CEES at time point t [35–37].
3.3. Characterization of a-, b-, d-, and c-MnO2 active sites
3.3.1. Characterization of thermal stability of
Results of the synchronous thermal analyses of
MnO2 are reported in Fig. 5. As can be seen from the TG curve in
a-, b-, d-, and
c-MnO2
a-, b-, d-, and c-
Fig. 5(a), except that the weight of b-MnO2 hardly decreased before
500 °C, the weight of
a-, d-, c-MnO2 decreased gradually as the
temperature rose from room temperature to 500 °C; this weight
loss was the result of the removal of adsorbed and intercalated
water. In the 25–105 °C temperature range, what was lost was
qt ¼ q1eðꢂk tÞ þ q2eðꢂk tÞ þ q1
ð1Þ
1
2
In equation (1), q1 and q2 are the amounts of degraded 2-CEES
in the fast reaction stage and the slow reaction stage, respectively;
k1, k2 are the rate constants of the fast and slow reactions, respec-
tively; q is the amount of 2-CEES remaining at the end of the
1
reaction. The data-fitting curves obtained using equation (1) are
reported in Fig. 4. According to the fitted kinetic curves, the rate
constants of the reactions of different samples with 2-CEES are
shown in Table 2.
Based on the kinetic curves reported in Fig. 4, it can be inferred
that, although b-MnO2 and d-MnO2 displayed higher reaction rates
in the initial stage of the reaction, the relevant curves gradually
flattened after 1 h, it will enter the stable reaction stage. According
to the rate constants of the fast reactions in Table 2, k2 is positively
related to the decontamination activity. However, the number of
reaction sites present in the inner pores is very small, so even
Fig. 5. Thermogravimetric analysis data collected on the different MnO2 species. (a)
TG patterns of a-, b-, d-, and c-MnO2, and (b) DTG patterns of a-, b-, d-, and c-MnO2.
5

Y. Guo, L. Kong, M. Lei et al.
Journal of Molecular Liquids 333 (2021) 115946
the adsorbed water generated by the samples in the air and other
ways; in the 105–500 °C range, the AOH group condenses to
release more compact water, in other words, interlayer water.
Notably, interlayer water is essential for lattice stabilization, and
its release causes the lattice to collapse. The AOH group is also
structurally very important, which replaces O2ꢂ and leaves in the
form of H2O and is called AOH water [38].
a-MnO2 lost more
weight at about 105 °C than later in the experiment, indicating that
this sample comprised more physiosorbed water than intercalated
water; in the case of d-MnO2, the opposite trend was observed,
indicating that this sample comprised more intercalated than
physiosorbed water. Fig. 5(b) differential thermogravimetric grav-
ity (DTG) shows that
a-MnO2 and d-MnO2 lose water faster at
around 100 °C. The weight of
c
-MnO2 gradually decreased from
room temperature to 400 °C, which indicates that the amount of
physiosorbed and intercalated water decreased gradually. In gen-
eral, both physiosorbed and intercalated water seems affect 2-
CEES degradation activity [2]. During the decontamination process,
the combined effect of the physiosorbed water and the intercalated
water can promote the degradation of 2-CEES and make it non-
toxic. At about 450 °C, the transformation of
MnO2 has completed, so a strong peak is visible in Fig. 5(b) just
below this temperature [39]. XRD pattern of -MnO2 after heated
at 450 °C for 20 h was shown in Fig.S1. At 500–600 °C, -MnO2,
b-MnO2, and -MnO2 are known to undergo conversion to Mn2O3
c-MnO2 into b-
c
a
c
+
[40]. Notably, in d-MnO2 is present an excess of K ions with
respect to the other oxides, which prevents the conversion of
MnO2 into Mn2O3. Therefore, at 700–800 °C, the lattice oxygen
was released from d-MnO2, which caused d-MnO2 to be converted
to Mn2O3 and Mn3O4 [41].
3.3.2. Characterization of the basic sites of a-, b-, d-, and c-MnO2
Metal oxides have basic sites, and the reason for the observed
differences in decontamination activities among the four prepared
manganese oxide species may also be related to differences in the
prevalence of basic sites. In order to verify this hypothesis, CO2-
TPD analyses were performed on the samples (see results in
Fig. 6. CO2-TPD patterns of a-, b-, d-, and c-MnO2.
3.3.3. Characterization of the chemical state and oxidizability of
d-, and -MnO2
a-, b-,
Fig. 6). The experimental CO2-TPD patterns of
a-MnO2 and d-
c
MnO2 include three different types of CO2 adsorption peaks, which
are due to weakly basic sites, moderately basic sites, and strongly
basic sites [42]. Manganese oxide reacts with 2-CEES at room tem-
perature, and atmospheric CO2 will be adsorbed, occupying active
XPS analysis was employed to determine the oxidation state Mn
(Fig. 7(a)). The Mn 2p 2/3 spectrum is divided into two valence
states: Mn4+ and Mn3+. The XPS-based estimates of the percentages
of Mn3+ and Mn4+ in
a
-, b-, d-, and
c-MnO2 are listed in Table 3. In
sites and making it poisonous. The CO2-TPD pattern of
also indicative of the presence of three different types of CO2
absorption peaks. The difference is that at 350 °C -MnO2 begins
c-MnO2 is
each sample, the proportion of Mn4+ is higher than that of Mn3+
,
but the difference in percentage between Mn4+ and Mn3+ in each
sample is not significant. In organic reactions, MnO2 can be used
as an oxidant to participate in the reaction, and the main role is
high-valent manganese ions, that is Mn4+. In the reaction with 2-
CEES, Mn4+ was continuously consumed, as the oxidation reactions
occurred.
c
to transform into b-MnO2. This conclusion is consistent with TG
analysis results. With respect to the CO2-TPD patterns, the lower
the temperature at which the peaks appear, the larger the number
of basic sites that can be used for the decontamination reaction;
notably, both the reaction temperature and the number of basic
sites affect 2-CEES decontamination activity. By contrast, evidence
suggests that b-MnO2 does not contain either weakly basic or mod-
erately basic sites, only a strongly basic site, whose presence is
made evident by an intense peak at ~500 °C. This site interacts
strongly with CO2 at high temperature to form carbonate, and it
does not contribute to the decontamination reaction. The quantity
and strength of acid–base sites in metal oxides depend mainly on
the specific metal, the oxide structure, the presence of impurities,
and the particle size [43]. Importantly, however, the degradation
activity of manganese oxide samples was not determined only by
the strength and quantity of basic sites. Although the basic sites
in MnO2 samples affected the ability of the said samples to perform
substrate degradation, they were not the main factor determining
the said ability.
The XPS spectra of O1s of a-, b-, d-, c-MnO2 are shown in Fig. 7
(b). The core energy level spectrum of O1s indicates that oxygen
atoms can be divided into three categories, according to the differ-
ent ranges of the observed values for their binding energies:
532.5–533.1, 530.8–531.3, and 529.1–529.5. These values were
assigned to lattice oxygen (Olatt, MnAOAMn), surface-adsorbed
oxygen species (Osurf, MnAOH), and oxygen present in water mole-
cules chemically adsorbed onto the oxide surface (OH2O, HAOAH),
respectively. The mentioned results are consistent with the results
previously reported by Ji et al. [44]. Notably, adsorbed water is the
main source of –OH water groups. These groups are generally dis-
tributed at the edges and corners to provide active sites for the
degradation of CWAs and their simulants. Therefore, the degrada-
tion activity of the metal oxides is related to the adsorbed water
6

Y. Guo, L. Kong, M. Lei et al.
Journal of Molecular Liquids 333 (2021) 115946
Fig. 7. XPS spectra of (a) Mn 2p and (b) O 1s for a-, b-, d-, and c-MnO2.
Table 3
O and Mn surface atomic ratios in
a
-, b-, d-, and
c
-MnO2 estimated based on XPS spectra.
Mn3+ (%)
Sample
Mn4+ (%)
Olatt (%)
Osurf (%)
OH2O (%)
a
-MnO2
62.72
61.95
62.13
63.53
38.25
38.05
34.87
36.37
66.58
81.19
76.81
75.27
25.13
12.01
16.04
15.34
8.28
6.79
7.15
9.39
b-MnO2
d-MnO2
c
-MnO2
content. The proportion of OH2O to the total oxygen content follows
the following order (Table 3): -MnO2 (9.39%) > -MnO2 (8.28%) >
d-MnO2 (7.15%) > b-MnO2 (6.79%). The adsorbed water content has
a positive correlation with the decontamination activity of the var-
ious MnO2 species.
amount of physically adsorbed water is still present on the surface
of the samples, possibly as a result of the absorption of atmo-
spheric water vapor during sample preparation or storage. In gen-
eral, adsorbed/intercalated water promotes the degradation of 2-
CEES. According to the results of the CO2–TPD experiments, the
amount and strength of basic sites also affect the decontamination
activity, these parameters are different for the different crystalline
forms of manganese oxide, and the reason cannot be simply attrib-
uted to the proportional relationship. In the XPS analysis, the oxi-
dation of MnO2 was attributed to the presence of high-priced
manganese ions (Mn4+). They oxidize 2-CEES and are continuously
consumed. In the O 1s spectrum, oxygen is divided into three cat-
egories according to the binding energy. Among them, the
adsorbed oxygen provides active sites for the reaction, which is
one of the key factors of the degradation reaction.
c
a
As can be evinced from the experimental results, a correlation
existed between decontamination activity and the structural char-
acteristics of the manganese oxide crystalline species. According to
the results of the N2-BET experiments, the decontamination rate of
2-CEES increased as the specific surface area of the MnO2 sample
increased. The increase in specific surface area may mean that
the adsorption capacity of the sample is also enhanced. 2-CEES
molecules are distributed in the pores of the oxide after adsorption,
and they are chemically destroyed when they diffuse to the reac-
tion sites. Due to the large difference in specific surface area of
the synthesized MnO2, its efficiency is particularly prominent dur-
ing the degradation process. Specific surface area is also one of the
key factors affecting the degradation of 2-CEES by MnO2. SEM evi-
3.4. Reaction mechanism of 2-CEES degradation
dence indicates that
a
-MnO2 and
c
-MnO2 have more lattice defects
GC–MS and FTIR analyses were conducted to analyze the reac-
tion mixture, so as to identify the reaction products and infer the
mechanism of 2-CEES degradation on the surface of MnO2. The
hydrolytic product of 2-CEES is polar and non-volatile, so the ana-
lyte sample was subjected to a derivatization treatment with N,O-
than b-MnO2 and d-MnO2, and these defects provide reaction sites.
Lattice defects play a positive role in the decontamination process.
Although no excess water was added during the preparation of the
MnO2 samples, according to thermogravimetry results, a small
7

Y. Guo, L. Kong, M. Lei et al.
Journal of Molecular Liquids 333 (2021) 115946
Fig. 8. Reaction scheme of 2-CEES undergoing degradation on the surface of MnO2.
bis(trimethylsilyl)trifluoroacetamide before GC–MS analysis.
a-,
CRediT authorship contribution statement
b-, d-, and -MnO2 at m/z values of 73, 89, 103, 163, 178
c
correspond to the derivatized hydrolysate, that is hydroxyethyl
ethyl sulfide (HEES). The m/z values of 63, 78, 12, and 140 corre-
spond to the oxidation product: chloro ethyl ethyl sulphoxide.
The FTIR data indicated that the bands at 700 cmꢂ1 (due to CACl)
and at 1440 cmꢂ1 and 1215 cmꢂ1 (due to CH2ACl) weakened in
intensity to then disappear; at the same time, the band at
3425 cmꢂ1 (due to AOH) gradually grew in intensity, proving the
formation of the hydrolytic product.
Based on the GC–MS and FTIR data, the mechanism of reaction
between MnO2 and 2-CEES was inferred (Fig. 8). A part of 2-CEES
reacts with the adsorbed water/ intercalated water on the surface
of MnO2 to generate hydroxy ethyl ethyl sulphide (HEES). The S
atom of the other part of 2-CEES reacts with the high-valent man-
ganese ions in MnO2 to form chloro ethyl ethyl sulphoxide. 2-CEES
reacts with surface hydroxyl groups and Lewis acidic sites of MnO2
to form surface-bound alkoxy groups.
Yueting Guo: Writing - original draft. Lingce Kong: Funding
acquisition. Meiling Lei: Formal analysis, Visualization. Yi Xin:
Formal analysis, Visualization. Yanjun Zuo: Writing - review &
editing. Wenming Chen: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This work was supported by the State Key Laboratory of NBC
Protection for Civilian Protection Fund (SKLNBC2018-08).
Appendix A. Supplementary material
4. Conclusions
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molliq.2021.115946.
In the present study we investigated the effect of the reaction
between different crystalline structure of MnO2 and 2-CEE.
d-, and -MnO2 were synthesized by the hydrothermal method,
and -MnO2 displayed the highest decontamination activity, with
a-, b-,
c
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