Visible-light C-heteroatom bond cleavage and detoxification of chemical warfare agents using titania-supported gold nanoparticles as photocatalyst

Article abstract of DOI:10.1039/c0jm00345j

Gold nanoparticles supported on TiO₂ effect the detoxification of soman and VX nerve gases and yperite vesicant agent at room temperature upon visible light illumination.

Full text of DOI:10.1039/c0jm00345j

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www.rsc.org/materials | Journal of Materials Chemistry
Visible-light C–heteroatom bond cleavage and detoxification of chemical
warfare agents using titania-supported gold nanoparticles as photocatalyst†
a
a
a
Sxtefan Neaxtu, Bogdan Cojocaru, Vasile I. Parvulescu, Vasile Sxomoghi,b Mercedes Alvaroc
*
^
c
*
and Hermenegildo Garcia
Received 8th February 2010, Accepted 14th April 2010
First published as an Advance Article on the web 26th April 2010
DOI: 10.1039/c0jm00345j
reports on the use the photocatalysis to detoxify model compounds of
CWAs with UV light have demonstrated the potential of this
method.4–10 However, for obvious reasons the number of reports
concerning the photodegradation of real CWAs is considerably more
scarce and limited to the use of doped titania systems,11 encapsulated
photosensitizers12 or titanate-based nanotubes.13 The main limitation
of the photocatalysis that has to be overcome is the low efficiency of
the process with visible light irradiation, this being a consequence of
the wide band gap of titania in the most photocatalytically active
anatase phase (3.2 eV).
Gold nanoparticles supported on TiO2 effect the detoxification of
soman and VX nerve gases and yperite vesicant agent at room
temperature upon visible light illumination.
Introduction
Organophosphorous esters such as pesticides and chemical warfare
agents (CWAs) are liquids with different volatilities, depending on
their boiling points at atmospheric pressure. Destruction of these
compounds is complicated by the fact that phosphorous byproducts
derived from the original compound are also persistent and can still
exhibit high toxicity. The main difficulty in detoxification of these
poisonous compounds is to effect the breaking of the P–C bond.
Some of the toxic compounds used as chemical components of mass
destruction weapons are derived from this category of substances.
Other types also contain C–S bonds that must also be disconnected to
ensure complete decontamination.
The use of supported gold nanoparticles as heterogeneous catalysts
is a topic of much current interest in chemistry, among other reasons
because it constitutes a clear example of chemical activity derived
from the nanometre particle size. Not surprisingly considering that
TiO2 is the most widely used photocatalyst and the importance of
Au/TiO2 as a heterogeneous catalyst, the photocatalytic activity of
Au/TiO2 to effect the degradation of organic compounds, water
purification and even the photocatalytic synthesis of products of
industrial relevance has attracted increasing interest.14–17
The production, storage and use of any CWAs are strictly
controlled by the Chemical Weapons Convention.1 This convention
entered into force in 1997 and obliges all the signatory states to
destroy any chemical weapon stockpile before 2012. The current
World’s declared stockpile of CWAs is about 40,000 metric tones,
which gives an idea of the importance of the issue. In addition to the
problem of destruction of massive inventories of extremely deadly
chemicals, there is a large concern in western countries about the need
for counter measures to neutralize emergency situations and any
possible terrorist attack.2
However, most of these studies are limited to the degradation of
simple organic compounds such as formaldehyde or methanol and
cannot be used to predict the catalytic activity for the degradation of
CWA.18,19 Some reports on the use of Au/TiO2 as photocatalyst for
the degradation of more complex molecules have also described the
photoactivity of this material for the decoloration of azo-dyes like
methyl orange20 or methylene blue.21 Some of these precedents have
shown visible light photocatalytic activity of Au/TiO2 arises from
photon absorption by the gold surface plasmon band and subsequent
electron injection into the valence band of titania.19 It was thus
proposed that photoexcited semiconductor nanoparticles undergo
charge equilibration when they are in contact with metal nano-
particles. Such a charge distribution has direct influence in dictating
the energetics of the composite by shifting the Fermi level to more
negative potentials.22
The photoactivity of titania has been widely studied since the
discovery of its photocatalytic activity.3 Over the years it has been
firmly demonstrated that the photocatalysis technique can degrade to
the stage of total or partial mineralization (CO2 and H2O) a wide
range of chemical substances, and it was regarded as a most prom-
ising technology for air or aqueous pollution abatement.2 Prior
In the present article we describe an extremely efficient system,
environmentally benign that at the open atmosphere only needs
visible light to effect the detoxification of contaminated surfaces.
Other alternatives such incineration or chemical degradation are not
applicable for spillovers or they require spreading corrosive and toxic
chemicals. Thus, the use of visible light operating at ambient condi-
tions employing a non-toxic solid can be an extremely advantageous
alternative. We will show here that deposition of small gold nano-
particles (5 nm average sizes) as light harvester and catalytic sites on
anatase (Au/TiO2) renders a material that exhibits an extremely high
visible-light photocatalytic activity that is able to effect detoxification
of a wide range of CWA in a few minutes.
aDepartment of Chemical Technology and Catalysis University of
Bucharest, Faculty of Chemistry, 4-12 Regina Elisabeta Bd., 030016
Bucharest, Romania. E-mail: vasile.parvulescu@g.unibuc.ro
bResearch Centre for Defence NBC and Ecology, 225 Oltenitei Sos.,
041309 Bucharest, Romania
cInstituto Universitario de Tecnologia Quimica CSIC-UPV, Universidad
Politecnica de Valencia, Av. De los Naranjos s/n, 46022 Valencia, Spain.
E-mail: hgarcia@qim.upv.es
† Electronic Supplementary Information (ESI) available: The mechanism
of degradation under UV irradiation, degradation percentage of soman,
VX and sulfur mustard, and photodegradation products of soman, VX
and sulfur mustard. See DOI: 10.1039/c0jm00345j
4050 | J. Mater. Chem., 2010, 20, 4050–4054
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Experimental
Results and discussion
Gold supported titania photocatalysts were prepared by the deposi-
tion-precipitation method, by adding the titania support (1 g) to an
100 cm3 aqueous solution of HAuCl4 (0.2 M) previously adjusted at
pH z 8.5 with a NaOH solution (0.2 M). The slurry was maintained
at 75 ꢀC, under vigorous stirring for 5 h. After stirring, the sample was
filtered, washed with deionised water until the elimination of chloride
had occurred, and then dried under vacuum at 80 ꢀC for 48 h. The
samples have been investigated by using UV–Vis spectroscopy in the
DRS mode, XPS and TEM. Diffuse reflectance spectra (DRS) in
the range of 200–800 nm were taken on a Cary 5 spectrophotometer
from Varian. The spectra were recorded against BaSO4 as the base-
line. The computer processing of the spectra with Bio-Rad Win-IR
software consisted of conversion of wavelength (nm) to wavenumber
(cmꢁ1) and calculation of the Kubelka–Munk function F(R) from the
absorbance. Samples were granulated for DRS measurements, and
the size fraction of 60–100 mesh was loaded in a quartz cell with
a Suprasil window. The spectra of the samples were obtained under
ambient conditions. XPS spectra were recorded at room temperature
using a SSX-100 spectrometer, Model 206 from Surface Science
Instrument. The pressure in the analysis chamber during the analysis
was 1.33 mPa. Monochromatized Al–Ka radiation (hn ¼ 1486.6 eV)
was used. It was generated by bombarding the Al anode with an
electron gun operated with a beam current of 12 mA and acceleration
voltage of 10 kV. The spectrometer energy scale was calibrated using
the Au 4f7/2 peak centred at 83.98 eV. Charge correction was made
with the C 1s signal of adventitious carbon (C–C or C–H bonds)
located at 284.8 eV. An estimated error of ꢂ 0.1 eV can be assumed
for all measurements. TEM micrographs were taken using a TEC-
NAI F20 instrument operated at 200 kV. Specimens were prepared by
placing a drop of the sample material onto a copper grid with
a perforated carbon film and then allowing the solvent to evaporate.
The loading amounts of gold over TiO2 Degussa P25 determined by
ICP-AES analysis were 0.4, 0.7, and 1.5 wt%, respectively. The
photocatalytic activity was measured by using an open naturally
aerated quartz reactor. As irradiation source a 125 W high-pressure
germicidal black-bulb lamp (HQV 125 W, Osram, Germany) with
a maximum emission at 365 nm and 3.0 W UVA radiated power 315–
400 nm was used. Light intensity at the distance where the sample was
placed was 810 Lx. Light intensity measurement was performed with
an 840006 Speer Scientific luxmeter. A general photocatalytic proce-
dure was carried out as follows: 20 mg of photocatalyst to which
a 100 mL 0.77 wt% solution of chemical warfare agent in dichloro-
methane were added are placed into an open polyethylene vat. The
ensemble has been introduced in the quartz reactor, perpendicularly
to the light propagation direction. (Caution: The use of soman, VX or
sulfur mustard causes death and is regulated under the NATO agree-
ment and their production, storage and use require the corresponding
authorization.) The evolution of the reaction was followed by taking
one vat at the required reaction time, extracting the photocatalyst with
dichloromethane, concentrating and analyzing the solution with
a GC-MS equipment from Thermo Electron. The initial sample,
unexposed to UV light and the irradiated samples were subjected to
identical extraction procedure with 500 mL solvent mixture (N,N-
bis(trimethylsilyl)trifluoracetamide in methylene dichloride to
produce volatile trimethylsilyl derivatives of the degradation by-
products). The decontamination rate was calculated as the percentage
of compound consumed from the initial quantity.
To support that visible light absorption takes place at gold nano-
particles, diffuse reflectance UV–Vis spectra of the parent TiO2
support and Au/TiO2 samples were recorded (Fig. 1).
As expected, parent TiO2 possesses an absorption threshold in the
UV–Vis region at around 370 nm (3.2 eV), which is characteristic for
the transition between the valence and conduction band of TiO2
particles. After gold deposition over the titania support, the presence
in the optical spectra of an absorption band in the visible region of the
spectra at around 550 nm due to surface plasmon vibrations is
observed. This absorption band in the visible region causes the pink-
violet colour attained by three Au/TiO2 samples and is responsible for
visible light absorption of Au/TiO2 that was assumed to be the origin
of their photocatalytic activity.
For gaining understanding of the photocatalytic behaviour in the
photodegradation of the three CWA, further characterization of the
Au/TiO2 materials was carried out. Fig. 2 shows the TEM micro-
graphs of the investigated samples. Statistical analysis of the particle
size distribution indicates no difference in the average diameter of the
gold nanoparticles for the three samples that was about 5 nm. No
Fig. 1 Diffuse reflectance UV–Vis spectra of (a) 0.4% Au/TiO2, (b) 0.7%
Au/TiO2, and (c) 1.5% Au/TiO2. The inset presents an expansion of the
region 500–650 nm of the spectra.
Fig. 2 TEM images and particle size distribution of (A) 0.4% Au/TiO2,
(B) 0.7% Au/TiO2, and (C) 1.5% Au/TiO2.
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of the tested catalysts indicated a shift of the peaks assigned to the 4f7/
2 levels of gold to a higher energy (85.8 eV) suggesting an oxidation
during the photocatalytic process (Fig. 3B). Based on these data we
suggest the reaction mechanism presented in Fig. 6.
Preliminary experiments carried out under dark conditions showed
that adsorption of these CWAs on Au/TiO2 does not cause decom-
position and that the amounts adsorbed on the solid can be recovered
quantitatively by percolation with CH2Cl2. It is worth commenting
that these experiments demonstrated a major difference between the
behaviour of real CWAs and the DMMP stimulant, for which some
thermal degradation by chemisorption has been observed.24
In contrast to the dark controls, visible light irradiation led to
a very spectacular and general efficiency for CWA degradation.
Three reference Au/TiO2 materials differing in the gold loading were
used in the present study. The results obtained in the visible light
photocatalytic degradation are listed in Table 1. They show that
complete degradation of three structurally different CWA (Fig. 4) can
be achieved in short exposure times. The photocatalytic activity of
Au/TiO2 increases with the gold loading and in some cases 30 min of
exposure to visible light suffices to destroy the CWA (Table 1).
Controls show that the photocatalyst temperature is about 42 ꢀC and
that the degradation effect cannot be attributed to a thermal degra-
dation.
The photocatalytic data indicates that in general the gold loading
in the range 0.4–1.5 wt% increases the photocatalytic activity of the
material, although complete disappearance of the deadly CWA at
120 min under visible light irradiation had already been obtained at
0.7 wt% Au load. A control experiment using parent TiO2 used as
support showed that in the absence of Au nanoparticles, this material
is devoid of any visible light photocatalytic activity.
Fig. 3 XPS spectrum of the 0.7 wt% Au/TiO2 sample in the Au(4f)
region before (A) and after (B) performing the photocatalytic reaction.
The product distribution changes with the conversion indicating
that after the initial degradation of the CWA, the primary
change in the Au nanoparticles particle size distribution after the
photocatalytic experiments were observed by TEM.
XP spectra of the investigated samples showed no significant
difference in either of the Au(4f) or Ti(2p) profiles of the three Au/
TiO2 samples. Au(4f) XP spectrum of all samples exhibits peaks at
the binding energies values of 84.3 and 87.9 eV for 4f7/2 and 4f5/2
electrons, respectively (Fig. 3A).
These values match well, both in binding energies and peak width,
with the corresponding data recorded for Au+.23 Similarly, the Ti 2p3/
2 signals were observed at binding energy value of 458.9 eV, typical
for TiO2 Degussa P25 material (not shown for brevity). The analysis
Fig. 4 From left to right: chemical structures of soman (O-pinacolyl
methylphosphonofluoridate), VX (O-ethyl-S2-diisopropylamino ethyl
methyl phosphonothionate), and sulfur mustard (bis(2-chloro-
ethyl)sulfide).
Table 1 Photocatalytic performance of the investigated catalysts
Catalysts
Substrate
Soman
Irradiation source
Time of exposure/min
0.4% Au/TiO2
0.7% Au/TiO2
1.5% Au/TiO2
UV
30
120
30
120
30
120
30
120
30
15
35
40
85
13
39
45
78
11
54
21
82
14
38
42
100
22
40
71
100
28
59
32
21
42
47
100
17
45
73
100
40
67
37
Visible
UV
VX
Visible
UV
Sulfur mustard
120
30
120
Visible
100
100
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with photons of energy higher than (or equal to) its band gap energy,
charge separation occurs and electrons and positive holes are created
(Fig. 6). In a fluid reaction medium, reactants can adsorb on the
semiconductor surface and react either with electrons (acceptor
molecules such as O2) or with holes (electron donor molecules).2
The preparation of finely dispersed gold nanoparticles is believed
to be a crucial step in obtaining highly active gold catalysts for the
reactions. Highly dispersed gold is regarded as a potentially useful
material for various industrial and environmental applications.25 In
the research area of heterogeneous catalysis, Au/TiO2 is one of the
preferred catalysts exhibiting excellent activity for aerobic oxida-
tions,26–29 including the catalytic oxidative degradation of CWA
simulants.24 Thus, the oxidative degradation of dimethyl methyl-
phosphonate (DMMP, a simulant of nerve gas) may occur on Au/
TiO2 catalysts under aerobic conditions without irradiation.24 One
related precedent study performed an in situ IR study of the photo-
catalytic reaction of tabun simulant by Au/TiO2.30 However, data
with a series of real CWAs proving the photocatalytic efficiency as
presented here are missing. This point is particularly important
considering that in the literature there are contradictory data showing
that the presence of Au nanoparticles increases or decreases the
photocatalytic activity inherent to titania. For instance, it has been
found that the decrease in the surface OH titanol groups caused by
deposition of gold nanoparticles is highly detrimental in the photo-
catalytic oxidation of cyclohexane promoted by Au/TiO2 as
compared to TiO2.31 Thus, the current available data do not allow the
anticipation of the visible light photocatalytic activity of Au/TiO2
with a wide range of real CWAs and the complete detoxification by
C–heteroatom bond breaking. The present study is aimed at filling
this information gap using vesicant (sulfur mustard) and nerve gas
(soman and VX).
Fig. 5 DRIFT spectra of the 0.7% Au/TiO2 photocatalysts before (a)
and after (b) performing soman detoxification.
by-products undergo consecutive degradation. What is important is
that at final reaction times innocuous products such as phosphonates,
sulfides, disulfides, sulfoxides and sulfones are present. Moreover,
FT-IR spectrum of the used Au/TiO2 photocatalyst shows that in
addition to the expected vibration bands corresponding to the inor-
ganic metal oxide, some peaks corresponding to minor amounts of
strongly adsorbed and unrecoverable organic material is still present
at 120 min visible light irradiation (Fig. 5). Therefore, complete
detoxification of vesicant and nerve agents can be achieved under
visible light irradiation under ambient conditions using Au/TiO2
photocatalysts that are stable in the process.
We also carried out analogous photocatalytic experiments using
UV light. The results are also included in Table 1. As it can be seen
there, although the degradation products are identical to those
observed in the visible light illumination, the Au/TiO2 samples are less
active under UV irradiation compared to visible light illumination.
This is a remarkable result indicating that the light absorption by the
gold surface plasmon band is responsible for the high photocatalytic
efficiency. Otherwise, TiO2 under visible light irradiation or Au/TiO2
under UV irradiation should have exhibited identical behaviour. In
addition, no influence of the Au loading should be observed if gold
was not playing any role.
In conclusion the mechanism described in Fig. 6 accounts for an
electron transfer from gold to titania, a cooperative effect of this
electron with light from either UV irradiation or from visible light
ꢁ
irradiation to generate O2 c and OHc radicals that generate a total
degradation of the investigated CWA. Remnant water and water
formed in the photocatalytic degradation participate in the hydrolysis
steps.
Conclusions
These data demonstrate that our photocatalytic system is based on
the double aptitude of semiconductors to simultaneously adsorb
chemicals and to absorb photons. When a photocatalyst is irradiated
The data presented in this study show that we have obtained efficient
photocatalysts for the complete detoxification of structurally different
CWAs. Therefore, this work provides valuable data with real CWAs
supporting the general applicability of Au/TiO2 as an efficient visible
light photocatalyst for detoxification of vesicant and nerve agents.
Compared to incineration and chemical degradation, the photo-
catalytic detoxification of chemical weapons presented here can be
taken as a clear example of the realization of the ‘‘green’’ chemistry
principles since it only uses air, it is solventless makes an efficient use
of energy and uses a stable and recyclable gold supported titania
photocatalyst.
Acknowledgements
The authors kindly acknowledge financial support provided by the
NATO’s Scientific Affairs Division in the framework of the Science
for Peace Program Sfp 981476, and PN-II- contract number 21-048/
2007. This research has also been partially supported by the CNCSIS
Fig. 6 Photodegradation of the neurotoxic compounds on the Au/TiO2
catalysts under visible irradiation.
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