Nanosized inorganic metal oxides as heterogeneous catalysts for the degradation of chemical warfare agents

Article abstract of DOI:10.1016/j.cattod.2015.12.023

Nanosized inorganic metal oxides, such as TiO₂, ZnO, γ-Al₂O₃, are proposed as heterogeneous catalysts for the oxidative degradation of chemical warfare agents (CWA), particularly of organosulfur toxic agents, into oxidised products with reduced toxicity. The morphology, structural and textural properties of the catalysts were investigated. Furthermore, their catalytic properties were evaluated in the oxidative abatement of (2-chloroethyl)ethylsulfide, CEES, a simulant of sulfur mustard (blistering CWA). Their performance was also compared to a conventional decontamination powder and a commercial Nb₂O₅ sample. The metal oxides powders were then employed in the active oxidative decontamination of CEES from a cotton textile substrate, mimicking a real contamination occurrence. Remarkable results in terms of abatement and degradation into desired products were recorded, achieving good conversions and decontamination efficiency with Nb₂O₅, TiO₂ and γ-Al₂O₃, under very mild conditions, with hydrogen peroxide (as aqueous solution or as urea-hydrogen peroxide adduct), at room temperature and ambient pressure. In the aim of a real on-field use, the potential environmental impact of these solids was also evaluated by bioluminescence toxicity tests on reference bacteria (Photobacterium leiognathi Sh1), showing a negligible negative impact for TiO₂, γ-Al₂O₃, and Nb₂O₅. A major biotoxic effect was only found for ZnO.

Full text of DOI:10.1016/j.cattod.2015.12.023

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Nanosized inorganic metal oxides as heterogeneous catalysts for the
degradation of chemical warfare agents
a,b,∗
a
b
d
Chiara Bisio
, Fabio Carniato , Chiara Palumbo , Sergey L. Safronyuk ,
c
d
a
b,∗∗
Mykola F. Starodub , Andrew M. Katsev , Leonardo Marchese , Matteo Guidotti
Dipartimento di Scienze e Innovazione Tecnologica and Nano-SiSTeMI Interdisciplinary Centre, Università del Piemonte Orientale “A. Avogadro”, Viale T.
Michel 11, 15121 Alessandria, Italy
CNR-Istituto di Scienze e Tecnologie Molecolari, via C. Golgi 19, 20133 Milano, Italy
National University of Life and Environmental Sciences of Ukraine, Kyiv, Ukraine
a
b
c
d
V.I. Vernadsky Crimean Federal University, Medical Academy, Simferopol
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 August 2015
Received in revised form
Nanosized inorganic metal oxides, such as TiO , ZnO, ꢀ-Al O , are proposed as heterogeneous cata-
2
2
3
lysts for the oxidative degradation of chemical warfare agents (CWA), particularly of organosulfur toxic
agents, into oxidised products with reduced toxicity. The morphology, structural and textural properties
of the catalysts were investigated. Furthermore, their catalytic properties were evaluated in the oxida-
tive abatement of (2-chloroethyl)ethylsulfide, CEES, a simulant of sulfur mustard (blistering CWA). Their
performance was also compared to a conventional decontamination powder and a commercial Nb2O5
sample. The metal oxides powders were then employed in the active oxidative decontamination of CEES
from a cotton textile substrate, mimicking a real contamination occurrence. Remarkable results in terms
of abatement and degradation into desired products were recorded, achieving good conversions and
decontamination efficiency with Nb2O5, TiO2 and ꢀ-Al2O3, under very mild conditions, with hydrogen
peroxide (as aqueous solution or as urea-hydrogen peroxide adduct), at room temperature and ambient
pressure. In the aim of a real on-field use, the potential environmental impact of these solids was also
evaluated by bioluminescence toxicity tests on reference bacteria (Photobacterium leiognathi Sh1), show-
ing a negligible negative impact for TiO2, ꢀ-Al2O3, and Nb2O5. A major biotoxic effect was only found for
ZnO.
2
8 December 2015
Accepted 31 December 2015
Available online xxx
Keywords:
Heterogeneous catalysis
Oxidation
Chemical warfare agents
Organosulfur
Metal oxide
Bioluminescent bacteria
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
many heterogeneous catalysts have been proposed for the selective
oxidation of CWAs into partially or fully oxidized non-toxic prod-
ucts [3,4]. For instance, polyoxometalates activated with transition
metal centres (i.e. V, Fe, W, Mo) [5,6] and porous oxides, such as
zeolite and metal-containing mesoporous silica [7–11], have been
studied for these purposes. Typically, these catalysts have shown
good conversion and high selectivity values in the oxidation of
organic sulfides into sulfoxides, in particular of those compounds
that mimic the oxidative abatement of blistering agents. Further-
more, these inorganic solids are characterized by a remarkable
chemical, physical and mechanical robustness and, thanks to their
high surface area, display a good dispersion of the catalytically
active sites.
Chemical warfare agents (CWAs) have been considered among
the most deadly tools humankind has ever invented deliberately.
The oxidative abatement of pollutants and toxic chemical war-
fare agents is conventionally achieved via stoichiometric reactions
based on the use of strong oxidants (mainly, sources of active chlo-
rine, such as NaOCl, Ca(OCl)2 or dichloroisocyanurate salts) with
high environmental impact and/or via thermal degradation [1,2].
These procedures are often associated with high energy consump-
tion, large over-stoichiometric amounts of reactants and high costs.
As an alternative to these well-established conventional chem-
ical abatement strategies, and aiming to overcome their limits,
More recently, phyllosilicate clay materials have also been
tested in the abatement of CWA [12,13]. The good chemical ver-
satility and the low production costs render these solids promising
candidates for the oxidative degradation of CWAs. A novel Nb(V)-
containing saponite clay (Nb-SAP) was prepared by some of us (at
∗
Corresponding author. Fax: +39 0131 360250.
Corresponding author. Fax: +39 02 50314405.
E-mail addresses: chiara.bisio@mfn.unipmn.it (C. Bisio), m.guidotti@istm.cnr.it
∗
∗
(
M. Guidotti).
http://dx.doi.org/10.1016/j.cattod.2015.12.023
920-5861/© 2016 Elsevier B.V. All rights reserved.
0
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2
the University of Eastern Piedmont, Italy) and identified as an effi-
cient catalyst for the oxidative abatement of blistering agents [14].
The synthesis method used to obtain metal-substituted saponite
material was modified to allow the insertion of Nb(V) ions within
the inorganic framework of the clay: a bifunctional redox/acid
catalyst with strong oxidizing properties, due to the presence of
Nb(V) centres [15] and Brønsted acid character, due to aluminium
ions embedded into the tetrahedral silica sheets of the saponite
clay, was thus obtained [16]. The cooperative effect of the metal
centres and the acid sites was crucial to promote the oxidative
abatement of organosulfur blistering agents with hydrogen per-
oxide under very mild conditions and, in particular, with no use
of chlorine-containing oxidising reagents. In detail, (2-chloroethyl)
ethylsulfide (CEES), an organic compound whose structure and
reactivity is similar to sulfur mustard (the blistering agent yper-
ite, used for the first time during World War I), was selectively
oxidized to the non-toxic (2-chloroethyl) ethylsulfoxide. The Nb-
saponite catalyst was able to convert more than 98% of CEES, with
a 73% selectivity to the related sulfoxide in 8 h [14]. Notably, this
performance was significantly better than the one obtained over a
conventional commercial decontamination powder, based on alu-
mina and calcium hypochlorite.
2. Experimental details
2.1. Materials
Commercial M75CBRN (chemical, biological, radiological and
nuclear agent) decontamination powder, packed in sealed batches,
was obtained at the local Military Hospital in Milan (Italy) and used
as received, as a reference material.
Bulk hexagonal-phase Nb O5 (Aldrich, 99.99%) was used as ref-
erence for the catalytic comparison.
2
2.2. Catalyst preparation
ZnO with nanosheet morphology was prepared by ther-
mal decomposition of the Zn CO (OH) ·H O precursor obtained
4
3
6
2
by chemical bath deposition (CBD), adapting a methodology
reported in the literature [36]. 0.55 g of hydrated zinc acetate
(
(
Zn(CH COO) ·2H O, Sigma–Aldrich) were added to 3.00 g of urea
3
2
2
Sigma–Aldrich) in 50 mL of deionized water, and the pH value of
the solution was adjusted to 4.5 with diluted acetic acid. The solu-
tion was transferred in an autoclave and kept at 353 K for 12 h.
Zn CO (OH) ·H O in powder form was obtained. The sample was
4
3
6
2
Besides porous and layered materials, inorganic metal oxides,
such as Al O , TiO and MgO have been also studied for their pos-
finally heated under air at 673 K to promote the decomposition of
2
3
2
Zn CO (OH) ·H O into ZnO.
4
3
6
2
itive effects on the CWA oxidation and/or degradation reactions
1,17,18]. Often, these solids have been merely used as supports to
ꢀ
-Al O was synthesized by modifying the classical sol–gel pro-
2 3
[
cedure that is normally used to produce boehmite phase [37].
In this method, the sol–gel procedure is assisted by sonication
in order to reduce the synthesis time. Urea (0.22 g, NH CONH ,
Sigma–Aldrich) was dissolved in 30 mL of deionized water. Then,
aluminum isopropoxide (3.18 g, Al[OCH(CH ) ] , Sigma) was added
to the solution. The obtained gel was stirred at room temperature
for 1 h. After this time, the gel was dried at 363 K for 12 h, in order to
obtain boehmite precursor, and submitted to a thermal treatment,
disperse catalytically active metals, rather than as directly involved
catalysts [19–21]. The catalytic decontamination performance of
such inorganic oxides is moderate when they are in the form of
bulk aggregates or micrometric dispersions, since they are able
to remove quite efficiently by physical adsorption the hazardous
agent, but then they can degrade it only partially [22–25]. On the
contrary, when they are dispersed at nanometric level, their intrin-
sic acid/base properties and hence their hydrolytic capabilities,
which are related to both the chemical nature of the metal oxide and
to the surface properties, can be directly exploited for a successful
CWA abatement [26–30].
2
2
3
2 3
−1
under oxygen flow (100 mL min ) at 773 K for 4 h, to promote the
phase transition to ꢀ-Al O .
2
3
TiO2 was prepared as follows. 3.5 mL of titanium(IV) iso-
propoxide (Sigma–Aldrich) were mixed to 3.5 mL of 2-propanol
Finally, considering the ever-growing concern about the poten-
tial detrimental effects of nanostructured inorganic oxides on living
organisms [31,32], it is also necessary to pay a constant attention
to the potential risks connected to the use of catalytically-active
nanosized solids at large scale. Since the use of nanostructured
decontamination powders is envisaged, in the present case, not
only for the abatement of CWAs in closed and confined environ-
ments, but also in on-field total-loss situations, it is necessary to
carry out an estimation of the potential detrimental effects on both
living organisms and the environment [33–35].
(
1
Sigma–Aldrich) and the solution was submitted to sonication for
5 min. 8 mL of pure water were added drop-by-drop to the previ-
ous solution under slow stirring. The final suspension was sonicated
for other 15 min and dried at 353 K overnight. The sample, in form
of white powder, was calcined under air flow (100 mL min ) at
−1
5
73 K for 2 h.
2
.3. Catalyst characterization
X-ray powder diffraction (XRPD) of unoriented ground powders
In this respect, a series of nanostructured inorganic oxides,
namely, ZnO with layered morphology, TiO2 with anatase struc-
was collected with a Thermo ARL ‘XTRA-048 diffractometer using
Cu K␣ (ꢁ = 1.54 Å) radiation. Diffractograms were recorded at room
ture and ꢀ-Al O , were synthesized and tested in the oxidative
2
3
◦
◦
−1
temperature with a step size of 0.02 and a rate of 1 2ꢂ min . The
particle size for the different samples was estimated by using the
Debye–Scherrer equation.
Transmission electron microscopy (TEM) images were obtained
using a JEOL 3010-UHR instrument operating at 300 kV and a FEI
Tecnai F20ST operating at 200 KV. Samples were ultrasonically dis-
persed in isopropanol and a drop of the suspension was deposited
on a copper grid covered with a lacey carbon film.
degradation of the chemical warfare blistering agent simulant (2-
chloroethyl) ethyl sulfide. The first set of reactions was conducted
in a batch reactor, under controlled conditions (room temperature),
and in the presence of aqueous hydrogen peroxide as an oxidant. A
critical comparison of their catalytic properties in relation to their
physico-chemical characteristics was also carried out. The second
series of reactions was performed on a textile substrate, mimick-
ing a real contamination occurrence, at room temperature, with the
urea hydrogen peroxide (UHP) adduct as an oxidant. A difference
in reactivity of the catalysts within the two series was registered
and is herein described.
N2 physisorption measurements were carried out at 77 K in
−
6
the relative pressure range from 1 × 10
to 1 P/P0 by using a
Quantachrome Autosorb1MP/TCD instrument. Prior to analysis, the
◦
samples were outgassed at 100 C for 3 h (residual pressure lower
than 10 Torr). Apparent surface areas were determined by using
Brunauer–Emmett–Teller equation (BET), in the relative pressure
range from 0.01 to 0.1 P/P . Pore size distributions were obtained by
applying the Non Local Density Functional Theory (NLDFT) method.
In the aim of a real on-field use, the potential environmental
impact of these solids was finally evaluated by bioluminescence
tests, using reference marine bioluminescent bacteria (Photobac-
terium leiognathi Sh1) as a target.
−
6
0
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3
O
S
H O or UHP
O
O
H O or UHP
2
2
2
2
S
S
Cl
Cl
Cl
room T
cat
room T
cat
CEES
CEESO2
CEESO
+
H O
2
-
HCl
-
HCl
S
S
room T
cat
HO
room T
cat
Scheme 1.
2.4. Catalytic tests
The first series of tests of catalytic oxidative abatement of
the blistering agent simulant (2-chloroethyl) ethyl sulfide (CEES;
Aldrich; 98%; Scheme 1) was performed as follows: a 14 mM solu-
tion of CEES in n-heptane (HPLC grade; Fluka) was reacted in a
round-bottom glass batch reactor, at 298 K, in the presence of
3
2
0 wt% aqueous hydrogen peroxide (70 mM; Sigma–Aldrich) and
0 mg of solid catalyst. The total volume of the mixture was 20 mL.
Samples of the reaction mixture were withdrawn at regular inter-
vals (from 0 to 24 h). The CEES conversion was monitored by
UV–Vis spectroscopy in transmission mode (1 mL quartz cuvette)
on a PerkinElmer Lambda 900 spectrometer by monitoring the
characteristic absorption maximum at 206 nm. The distribution of
products and by-products was checked by GC–MS analysis on an
Agilent 7890 Series gas-chromatograph equipped with MS detec-
tor (Supelco-5 ms column, 30 m × 0.25 mm; n-decane as internal
standard). Blank tests were run in the absence of solid catalysts.
Standard deviations of ± 4%, ± 5% were estimated, on average, for
conversion and selectivity values, respectively. Product quantifica-
Fig. 1. XRD patterns of ZnO (a), Nb2O5 (b), ꢀ-Al2O3 (c) and TiO2 (d) samples.
1
tion and identification was also checked by H NMR (300 MHz NMR
Bruker Analytische Messtechnik GmbH), monitoring the peculiar
solution. Reaction mixture for the determination of acute toxicity
contained 850 ␮L of testing suspension in 3.0 wt% of sodium chlo-
ride solution mixed with 100 ␮L of 0.1 M phosphate buffer solution
signal of CH2 S of CEES, CH2
S O of the sulfoxide (CEESO) and
CH2 SO2 of the sulfone (CEESO ) in the range between 3.75 and
2
2
.80 ppm [38].
(
pH 7.0) and 50 ␮L of diluted bacterial culture. The changes of bio-
The second series of tests for the catalytic oxidative decontam-
◦
luminescence were recorded within 10–30 min at 25 C by using a
bioluminometer BLM 8801 (Russia). In the case of chronic biotox-
icity study, the reaction mixture, described above, additionally
contained 50 ␮L of liquid nutrition medium and bioluminescence
ination of (2-chloroethyl) ethyl sulfide was performed as follows:
2
a 100% cotton textile (9 cm ; 200 mg ± 10%) was impregnated with
CEES (0.17 mmol) and immediately covered by the catalyst/UHP
mixture (10 mg of solid mixture, containing 5 mg of catalyst and
◦
intensity was measured after 18 h of incubation at 25 C.
5
mg of urea hydrogen peroxide solid, UHP, adduct (Fluka, 15–17%
active oxygen basis)). After 5 minutes the cotton sheet was brushed
and rinsed with 20 ␮L of deionized water. The decontamination
efficiency was quantified in percentage by means of an on-field
hazardous gas and chemical detector based on Open Loop Ion
Mobility Spectrometry (CHEMPRO100i, Environics Oy). The off-
vapours from the textile support were collected 2 cm above the
contamination test area. The qualitative and quantitative analysis
of the vapours has been performed according the CWA-High Sen-
sitive (11.3.7), CWA-Sensitive (11.3.7) and First Responder (9.3.6)
internal libraries, with respect to a non-treated control taken as
3. Results and discussion
3.1. Catalysts characterization
The structural properties of the catalysts were evaluated by
powder X-ray diffraction (XRPD) technique. The XRPD profiles of
the ZnO sample shows diffraction peaks at 31.8, 34.4, 36.6, 47.6
◦
and 56.7 2ꢂ, directly ascribed to the (1 0 0), (0 0 2), (1 0 1), (1 0 2)
and (1 1 0) planes, typical of the wurtzite structure (Fig. 1a) [40].
The average particle diameter (L) of the ZnO sample was esti-
mated by applying to the (1 1 0) peak the Debye–Scherrer equation
(1), where B is the full width at half the maximum intensity (FWHM)
of the (1 1 0) peak, ꢁ is the X-ray wavelength, ꢂ represents the
diffraction angle and K is the Scherrer’s constant (K = 0.89 for spher-
ical particles).
1
00% of the instrument output. Control tests were run in the
absence of solid catalysts and/or in the absence of the chemical
agent simulant.
2.5. Biological tests
The environmental biotoxicity of the catalysts was studied by
ꢀ
ꢁ
Kꢁ
bioluminescence inhibition tests, including determination of acute
and chronic effects. Luminescent bacteria strain P. leiognathi Sh1,
isolated from Azov Sea shrimp [39], was used as a biological target.
For the measurements, the overnight bacterial culture, grown in liq-
uid nutrition medium (Nutrient Broth, Himedia, containing 3 wt% of
NaCl), was diluted 100 times with aqueous 3 wt% sodium chloride
B
2ꢂ
=
(1)
L cos ꢂ
The particle size of the ZnO sample was 13.8 nm.
The X-ray profile of Nb O5 is composed of sharp peaks, indica-
2
tive of a high crystallinity degree (Fig. 1b). The main reflections at
22.6, 28.5 and 36.9 2ꢂ, ascribed to the (0 0 1), (1 0 0) and (1 0 1)
◦
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Table 1
4
planes, respectively, are attributed to the hexagonal phase, accord-
ing to the JCPDS Card no. 28–0317 [41]. The sample was composed
of nanoparticles with size around 20 nm, as determined by the
Debye–Scherrer equation applied to the (1 0 0) reflection.
Textural features of the catalysts.
Specific surface
area (SSA)
Pores size
distribution
(PSD) (nm)
Pores volume
3
−1
)
(cm
g
2
−1
)
(
m
g
The diffraction pattern of ꢀ-Al O shows a wide band centred
2
3
◦
◦
◦
at 38 2ꢂ and two reflections at 46 and 66 2ꢂ, respectively due
to (4 0 0) and (4 4 0) reflections of a ꢀ-Al O phase (Fig. 1c), thus
ZnO
37
5.0
5.4
6.0
n.d.
0.17
0.50
0.30
n.d.
ꢀ
-Al2O3
450
162
4.3
2
3
TiO2
Nb2O5
suggesting that the heating treatment was effective to promote
the phase transformation of boehmite into ꢀ-alumina [42]. The
reflections appear broad and poorly resolved and this is likely asso-
ciated to the fact that the sample here prepared was composed
by nanoparticles, below 10 nm, as estimated by TEM analysis (see
below).
n.d.: not determined.
3
−1
(
(
estimated by NLDFT method) and pores volume of 0.17 cm g
Table 1). ꢀ-Al O3 and TiO2 nanoparticles showed similar adsorp-
2
The XRPD profile of TiO2 (Fig. 1d) is characterized by the peaks
at 25.4, 37.8, 48.0, 54.1 and 55.1 2ꢂ, related respectively to the
tion isotherms of type “IV”, with an hysteresis loop of “H3” type,
typical of the presence of disordered pores whose shape is not well
defined. This porosity is probably due to particle aggregation [44].
◦
(1 0 1), (0 0 4), (2 0 0), (1 0 5) and (2 1 1) planes of the anatase phase
(JCPDS 21–1272) [43]. The Debye–Scherrer equation applied to
(1 0 1) plane allows estimating the particles size, which resulted
2
−1
The ꢀ-Al O3 sample shows a SSA of 450 m g and large PSD cen-
2
tred at 5.4 nm, whereas the TiO material was characterized by a
2
to be 11.2 nm.
2
−1
SSA of 162 m g and PSD centred at 6.0 nm and pores volume of
0
The morphology and particles shape of the catalysts were
investigated by high-resolution transmission electron microscopy
3
−1
.30 cm g (Table 1).
Nb O5 is characterized by a type “II” isotherm without an appre-
2
(
HR-TEM). The ZnO sample is composed of crystals with layered
2
−1
ciable hysteresis loop. The specific surface area was 4.3 m g , a
very low value if compared with previous samples (Table 1).
morphology, well organized in a porous structure whose pore
dimensions are dependent on the synthetic procedure (Fig. 2A). The
layered morphology of the sample was also assessed by scanning
electron microscopy images reported in a previous manuscript by
some of us [40]. This porous structure results from the intercon-
nection of ZnO grains and is typical of solids prepared at relatively
low temperature (below 773 K) [36]. TEM analysis was also used to
estimate the size and shape of the alumina nanoparticles (Fig. 2B).
3.2. Catalytic tests
The catalytic properties of the samples were evaluated in
the oxidative abatement of the blistering agent simulant (2-
chloroethyl) ethylsulfide, CEES (Scheme 1), whose chemical
reactivity is similar to sulfur mustard (blistering warfare agent, HD,
according to the NATO code), but with a far reduced toxicity [45].
The first set of experiments was carried out in a batch reactor,
under very mild conditions, at 298 K and at ambient pressure, in
This analysis indicated that ꢀ-Al O3 sample is characterized by
2
the presence of aggregates (flake-like) that are composed by small
nanoparticles of ca. 5–10 nm with rod-like shape. Finally, TEM
images of TiO2 catalyst show aggregates composed by particles
with not well defined shape and high crystallinity with size around
the presence of 30 wt% aqueous H O2 (70 mM) in n-heptane and
2
2
0 mg of catalyst. The degradation reaction of CEES (14 mM) was
1
0–15 nm (Fig. 2C), in agreement with the XRPD results. Probably,
followed by monitoring the main UV absorption of the substrate,
at 206 nm. The test was also performed without catalyst and in the
presence of H O (blank test). In this case, CEES showed a negligible
the sonication process assisting the synthesis and the slow addition
of water, aiming at promoting the hydrolysis and condensation of
the titanium precursor, were responsible of the high crystallinity
of the obtained TiO2.
The textural features of all catalysts, in terms of specific
surface area (SSA) and pore size distribution (PSD), were also
investigated by N2 physisorption experiments. ZnO shows an
adsorption–desorption isotherm corresponding to the so-called
2
2
self-decomposition (a maximum of 4 mol% degradation after 24 h).
When the catalysts were introduced, a marked CEES abatement was
observed (Fig. 3). In particular, a progressive decrease of the CEES
concentration was recorded in reasonable times, performing the
reaction in the presence of ꢀ-Al O , ZnO, TiO and Nb O5 materials.
2
3
2
2
The degradation was remarkable during the first 2 h of reaction.
The TiO2 catalyst was responsible for 44% of CEES abatement
after 6 h and of 58% after 24 h. The moderately good performance
obtained in the presence of TiO2 can be ascribed to the pecu-
liar properties of this solid, that is capable to act via a hydrolytic
pathway (with the elimination of HCl to provide (2-hydroxyethyl)
“
type III” in the Brunauer classification. A hysteresis loop “H3”
is observed for the sample, associated to the presence of slit-
shaped pores, relative to the formation of interspaces between the
nanosheets, according to TEM analysis. The SSA, estimated by the
2
−1
BET equation, resulted to be 37 m g , with a PSD centred at 5.0 nm
Fig. 2. HR-TEM micrographs of ZnO (A), ꢀ-Al2O3 (B) and TiO2 (C) samples.
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Fig. 4. Distribution of products in the oxidative abatement of CEES with hydrogen
peroxide over Nb2O5, TiO2, ꢀ-Al2O3 and ZnO. Selectivity values to sulfoxide (CEESO;
white), to sulfone (CEESO2; grey) and to other compounds (black) were all taken at
Fig. 3. Profiles of oxidative abatement of CEES with hydrogen peroxide over Nb2O5
35–40% conversion of CEES (at different reaction times). Reaction conditions: 14 mM
(
circle), TiO2 (square), ꢀ-Al2O3 (diamond), ZnO (cross), M75 decontamination pow-
CEES; 70 mM 30% aq. H2O2; n-heptane; 20 mg catalyst; 298 K.
der (triangle, test without added H2O2) and without solid catalyst (pentagon).
Reaction conditions: 14 mM CEES; 70 mM 30% aq. H2O2; n-heptane; 20 mg catalyst;
2
98 K.
Finally, the series of inorganic oxides has been compared with
M75, a commercially available decontamination powder in use
by the Italian armed forces and considered as a reference decon-
tamination tool. Such powder is composed of a solid mixture of
bentonite (a natural occurring solid containing montmorillonite
clay as major component), alumina and calcium hypochlorite. Since
the oxidant component was already present in the mixture, no
added hydrogen peroxide was needed in the tests. In general, the
performance of the M75 powder was the poorest within the tested
catalyst set and the low conversion after 24 h (ca. 14%) is to be
attributed mainly to the modest adsorption capacity of the ben-
tonite clay.
ethylsulfide) as well as via an oxidative one (via the oxidation of the
sulfide by hydrogen peroxide to sulfoxide and sulfone) [29,46,47].
It is worth underlining that the reaction took place over an anatase
phase of TiO , which often leads to the unproductive dispropor-
tionation of H O into oxygen and water [48]. However, thanks to
the presence of an excess of oxidant (5:1 mol/mol ratio H O :CEES)
in these tests, residual amounts of H O have been always detected
by iodometric tests, thus confirming that the oxidant was not the
limiting agent in the reaction.
2
2
2
2
2
2
2
A similar behavior was observed for ZnO sample, though the
degradation of CEES over this catalyst was a bit slower (37% of
abatement after 6 h, Fig. 3). The capability of ZnO to degrade organic
sulfide CWAs via non-photochemical pathways has already been
described previously [49,50]. It was shown that the SSA of zinc
oxide is an important factor to provide a good abatement rate. In
addition, ZnO does not possess a proper oxidising ability, as it is
the case for titanium dioxide samples. The low surface area and the
negligible oxidising capacities of the present ZnO sample were thus
likely the factors limiting the activity of this solid.
In terms of selectivity, under the tested conditions, two main
products were observed: (2-chloroethyl) ethylsulfoxide (CEESO)
and (2-chloroethyl) ethylsulfone (CEESO ), both derived from the
2
oxidation of the pristine sulfide (Scheme 1). Other compounds
obtained by acid/base-catalysed degradation of CEES, mainly
vinylethylsulfide (via HCl elimination) and (2-hydroxyethyl) ethyl-
sulfide, were summarised under the entry “others” in Fig. 4.
Taking into account that CEESO is the least hazardous oxida-
tion product, ZnO was the most selective catalyst to the desired
sulfoxide, in the aim of an effective and useful CWA degradation
ꢀ
-Al O shows an intermediate catalytic initial activity (within
system. Then, ␥-Al O3 gave rise to the highest amount of non-
2
3
2
the first 2 h of reaction) with a conversion of 44% after 24 h, ascrib-
able to the medium-strong Lewis acidity of the Al(␦ ) sites exposed
oxidised products (ca. 20% at 40% conversion after 24 h), among
which vinylethylsulfide was the major component. Interestingly,
+
on the surface (Fig. 3). In addition, in the present case, the small
particles size and high specific surface area favoured a higher
interaction between the catalyst and CEES substrate (see below).
Similarly to ZnO, most of the catalytic activity of the aluminum
oxide is to be related to its capability of promoting the chemical
degradation via a hydrolysis pathway, which was also responsible
of the degradation of sulfur mustard agent into thiodiglycol [51].
the most active catalyst, Nb O5, was also very selective, although
2
towards the undesired sulfone. In fact, the high oxidising ability of
this solid resulted in a rapid oxidation of the sulfide into CEESO2,
which is a product to be considered as hazardous as the pristine
sulfide, form the toxicological point of view [45].
A second set of experiments was performed mimicking a real
2
contamination situation, on a cotton textile (9 cm ; 200 mg), in the
Interestingly, Nb O5 showed the best catalytic performances in
terms of conversion, leading to the complete abatement of CEES
>99%) within 5 h (Fig. 3). Over this solid, the remarkable acidic and
presence of CEES (0.17 mM), and using a solid mixture of catalyst
with the UHP adduct. UHP was chosen as an oxidant in this case,
because it is the best option to have a “solid equivalent” of hydrogen
peroxide for decontamination powders. The contaminated textile
support was treated with the decontamination powder mixture,
brushed, after an action time of 5 min, in order to remove the
solid and rinsed with water, thus simulating a conventional on-site
immediate decontamination protocol. The effectiveness of CEES
decontamination was monitored and quantified by means of an
on-field CWA detector.
2
(
oxidising properties of the oxide played a key role in determining
such high activity. Indeed, Nb(V)-based solid oxides have shown a
remarkable Lewis acidity together with an outstanding capability to
trigger the oxidising action of aqueous hydrogen peroxide [52,53].
In the field of CWA degradation, only Nb(V)-polyoxoniobates have
shown unique alkaline reactivity for nerve agent decontamination,
but via a completely different reaction pathway [54]. However, no
studies have taken into consideration niobia so far, as a potential
catalyst for the oxidative degradation of CWA and the results are
thus particularly promising.
Remarkably, all tested catalyst led to an extensive decon-
tamination from the sulfide in short time (Fig. 5). In terms of
decontamination efficiency of the catalyst/oxidant mixtures, the
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Fig. 5. Oxidative decontamination of CEES on textile with urea hydrogen peroxide.
Decontamination efficiency with respect to a non-treated control (taken as 100%).
Reaction conditions: 20 ␮L CEES; 10 mg catalyst/UHP mixture; 20 ␮L H2O; brushing;
2
98 K.
following order was found: Nb O5 > ꢀ-Al O > ZnO > TiO . The use
2
2
3
2
of Nb O5 and ꢀ-Al O , in the presence of UHP, led to >98% and 92%
2
2
3
CEES decontamination, respectively, after 5 min. These results are
higher than the 85% conversion observed by using the reference
M75 decontamination powder, currently in use. In particular, after
treatment with the Nb O5/UHP mixture, no residual contamina-
2
tion was detected by the on-field detector. ZnO and TiO , on the
2
contrary, displayed a lower decontamination activity, but values as
high as 82% and 73% were attained, respectively. In all cases, the oxi-
dising capability of the powder is based on the use of a chlorine-free
oxidant, UHP, instead of the environmentally unfriendly calcium
hypochlorite. At the end of the decontamination tests, no hazardous
levels of off-vapours were recorded on the textile support. That
means that in none of the tests the degradation/decomposition
by-products were revealed as toxic compounds above threaten-
ing levels by means of the on-field hazardous gas and chemical
detector. Furthermore, this second series of results, collected on
the cotton textile, showed that the abatement (oxidative degrada-
tion in a confined environment, under controlled conditions) and
the decontamination properties (on-site degradation under total-
loss conditions for the catalyst) of an oxidic material are not similar
and cannot be considered fully comparable. In fact, the change
in oxidant and reaction surroundings may exalt or diminish the
decontamination efficiency of the nanostructured solids in the very
short reaction times. For instance, the catalytic performances of
Fig. 6. Acute (a) and chronic (b) effects of decontaminating materials on P. leiognathi
Sh1 bioluminescence. Nb2O5 (᭹), TiO2 (᭿), ꢀ-Al2O3 (ꢀ), ZnO (-), M75 decontamina-
tion powder (ꢁ).
3.3. Environmental impact of the catalysts
The biological effects on the environment of the nanostructured
ꢀ
-Al O were particularly enhanced in the experiments on the tex-
2 3
oxidic catalysts have been estimated by means of bioluminescent
bacteria biotests using a P. leiognathi Sh1 strain isolated from the
Azov Sea [39]. Such methods are widely used to study the toxicity
of chemicals, drugs and environmental objects, in general [56–58].
A direct comparison of the results of nanostructured oxides in
the tests for acute and chronic toxicities did not reveal any signifi-
cant difference. Indeed, strong acute and chronic inhibition effects
on the glow of luminescent bacteria were revealed for the ZnO
sample (Fig. 6a and b), which is widely known to possess marked
antibacterial properties [59,60]. In particular, in the determination
of the chronic toxicity, a complete suppression of the bacterial lumi-
nescence was revealed. Conversely, the rest of the solids as well
as the reference conventional decontamination powder M75 did
not show evident inhibition properties and did not have significant
detrimental effects on the bacterial luminescence. However, a weak
inhibition activity of TiO2 appeared and led to a 30% diminution of
tile, probably because of the cooperative positive effect of the high
specific surface area, enabling a successful adsorption and phys-
ical removal of the contaminant, together with a good chemical
degradation capability via in-situ hydrolysis, thanks to the acid
2
character of its surface. Conversely, when a conventional Al O3
solid, with micrometric particle size (Aldrich no. 19,997–4, Al O
2
3
2
−1
activated, 155 m g
specific surface area), was used under the
same conditions, a remarkably lower decontamination capability
was found: <10% vs. 92% of CEES decontamination efficiency, for the
conventional Al O and for the nanosized one, respectively. Various
2
3
previous reports show indeed that inorganic oxides (Al O , MgO
2
3
or hydrous zirconia) with a conventional non-nanostructured are
scarcely active in adsorption plus degradation of organosulfur CWA
analogues with respect to the nanosized oxide species [23,24,55].
On these systems, the contaminant is adsorbed, but the decontam-
ination capability was ca. 20–25% only. Indeed, the nanometric size
of the active inorganic oxide, that implies high surface area and a
large number of catalytically active sites, is an important feature to
have good properties of adsorption plus degradation [1,55].
the luminescence. Analogously, the M75 powder presented a grad-
−1
ual chronic toxicity for concentrations higher than 0.2 mg mL
.
From these data, it is worth highlighting that Nb O5, ꢀ-Al O ,
2
2
3
M75 powder (notwithstanding the presence of hypochlorite salts
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ronmental impact. ZnO, conversely, displayed a marked inhibition
of the bacterial luminescence that is a clear evidence of its environ-
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2
environmentally acceptable, although selective towards undesired
by-products, catalyst in the degradation of CEES. Nevertheless, it
gave excellent results in the on-site decontamination of CEES-
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The authors thank the NATO SPS Programme Multiyear Project
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chemical warfare agents, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2015.12.023