Atomically precise silver clusters for efficient chlorocarbon degradation

Article abstract of DOI:10.1039/c2ta00254j

We describe the degradation of chlorocarbons (CCl₄, C ₆H₅CH₂Cl and CHCl₃) in solution at room temperature (27 ± 4 °C) by the monolayer-protected silver quantum cluster, Ag₉MSA₇ (MSA: mercaptosuccinic acid) in the presence of isopropyl alcohol (IPA). The main degradation products were silver chloride and amorphous carbon. Benzyl chloride was less reactive towards clusters than CCl₄ and CHCl₃. Materials used in the reactions and the reaction products were characterized using several spectroscopic and microscopic tools such as ultraviolet-visible (UV/Vis) absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR), photoluminescence spectroscopy, X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), energy dispersive analysis of X-rays (EDAX) and scanning electron microscopy (SEM). We have shown that clusters are more efficient for the degradation of halocarbons than the corresponding monolayer-protected nanoparticles (Ag@MSA, particle diameter 15 ± 5 nm) at a given time and temperature. The higher reactivity of clusters is attributed to their small size and large surface area. Clusters and nanoparticles were used for reactions in supported (on neutral alumina) and unsupported forms. A possible mechanism for the reaction has been postulated on the basis of experimental results.

Full text of DOI:10.1039/c2ta00254j

Journal of
Materials Chemistry A
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PAPER
Atomically precise silver clusters for efficient
chlorocarbon degradation†
Cite this: J. Mater. Chem. A, 2013, 1,
611
*
M. S. Bootharaju,‡ G. K. Deepesh,‡ T. Udayabhaskararao and T. Pradeep
We describe the degradation of chlorocarbons (CCl4, C6H5CH2Cl and CHCl3) in solution at room
temperature (27
ꢁC) by the monolayer-protected silver quantum cluster, Ag9MSA7 (MSA:
ꢀ
4
mercaptosuccinic acid) in the presence of isopropyl alcohol (IPA). The main degradation products were
silver chloride and amorphous carbon. Benzyl chloride was less reactive towards clusters than CCl4 and
CHCl3. Materials used in the reactions and the reaction products were characterized using several
spectroscopic and microscopic tools such as ultraviolet-visible (UV/Vis) absorption spectroscopy, Fourier
transform infrared spectroscopy (FTIR), photoluminescence spectroscopy, X-ray diffraction (XRD), Raman
spectroscopy, X-ray photoelectron spectroscopy (XPS), energy dispersive analysis of X-rays (EDAX) and
scanning electron microscopy (SEM). We have shown that clusters are more efficient for the
degradation of halocarbons than the corresponding monolayer-protected nanoparticles (Ag@MSA,
particle diameter 15 ꢀ 5 nm) at a given time and temperature. The higher reactivity of clusters is
attributed to their small size and large surface area. Clusters and nanoparticles were used for reactions
in supported (on neutral alumina) and unsupported forms. A possible mechanism for the reaction has
been postulated on the basis of experimental results.
Received 10th September 2012
Accepted 12th October 2012
DOI: 10.1039/c2ta00254j
www.rsc.org/MaterialsA
1,3,5-triazine (RDX) and 2,4,6-trinitrotoluene (TNT)), and many
others. Halocarbons are harmful trace components in water
Introduction
Research on noble metal (silver and gold) quantum clusters1,2
has become increasingly fascinating in the last two decades.
Quantum clusters, composed of a few tens to hundreds of
atoms at the core and protected with ligands, are the connecting
link between nanoparticles and molecules which exhibit
molecular optical properties like absorption,3,4 emission,5,6
chirality,7 etc. These nanosystems are very interesting because
they show several applications in various elds such as catal-
ysis,8,9 biology,10 environmental remediation,11 etc. They are
highly reactive due to their extremely small size and large
surface area. They show unusual reactivity with salts.12 Metal
clusters exhibit the property of alloying with other metals.13,14
Clusters have been used for sensing metal ions15–17 and anions18
by using their luminescence property.
and air, either in the context of biological toxicity or due to their
global warming potential or undesirable atmospheric chem-
istry.22 The main sources of these are cleaning solvents, lubri-
cants, plasticizers and refrigerants. Several of these have been
replaced by less harmful chemicals. However, many of them
continue to be used due to economic considerations or the lack
of suitable replacements. CCl4 is a widely used solvent in
industry, research laboratories, dry cleaning, etc.23 It is carci-
nogenic and it can persist in ground and surface waters. Due to
this, it is one of the complex contaminants. The maximum
allowed level of CCl4 in surface water is 5 mg Lꢂ1 24 CFCs cause
.
depletion of the ozone layer.25 Various methods such as
photodecomposition,26 incineration,27 photocatalysis28,29 and
adsorption30 are developed to eliminate halocarbons from
the environment. Chlorinated halocarbons are degraded by
reductive mechanisms.31 There are four types of reductive
mechanisms: hydrogenolysis, elimination, dehydrohalogena-
tion and hydrogenation.
The environment has been contaminated by a large number
of pollutants like organic halides19 (chlorourocarbons (CFC),
C2Cl4, C2ClF3, CCl4, etc.), heavy metals20 (Hg, Pb, Cd, Cr, etc. in
various forms), energetic materials21 (hexahydro-1,3,5-trinitro-
Nanoparticles are increasingly used for environmental
applications due to increased amount of contaminants in soil
and groundwater.32 Common nanoparticles used for halo-
carbon degradation in the literature are iron in oxide and zero-
valent forms,31 MgO33 and ZnO.34 In most of the above studies,
nanoparticles form corresponding metal halides as principal
products at high temperatures. Recently, noble metal and
oxide nanoparticles have been studied in the degradation of
DST Unit of Nanoscience (DST UNS), Department of Chemistry, Indian Institute of
Technology Madras, Chennai - 600 036, India. E-mail: pradeep@iitm.ac.in
† Electronic supplementary information (ESI) available: SEM elemental analysis
and photograph of supported silver clusters, UV/Vis absorption spectrum and
TEM image of Ag@MSA nanoparticles, XPS, XRD, Raman and EDAX data of the
products of the reaction between supported clusters and CCl4. See DOI:
10.1039/c2ta00254j
‡ These authors have contributed equally.
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halocarbons.35 Our research group has pioneered the use of supplied by SRL, India. The surface area and the mean particle
noble metal nanoparticles for the removal of pesticides and size were 900 ꢀ 50 cm2 gꢂ1 and 0.13 mm, respectively.
halocarbons from drinking water.36–40
The standard Gibbs free energy change of formation (DfG0) of
Synthesis of silver clusters
AgCl(s) and CCl4(l) are ꢂ109.8 and ꢂ65.2 kJ molꢂ1, respectively.
The Ag9MSA7 cluster was synthesized according to the reported
The net free energy change for the reaction 4Ag(s) + CCl4(l) /
protocol.42 Briey, 47 mg of AgNO3(s) and 187 mg of MSA(s)
4AgCl(s) + C (graphite) is ꢂ374.0 kJ molꢂ1. This indicates that
were ground until the mixture turned orange due to the
the above reaction is feasible at 298 K and 1 atm pressure. The
formation of Ag-thiolates. Aer that, about 50 mg of NaBH4(s)
cell electromotive force (emf), E0cell, for the reduction of CCl4 by
was added to it and grinding was continued for 2–3 minutes. To
silver (in the above reaction) is calculated to be 0.97 V using the
this, 10 mL of distilled water was added which led to the
equation DfG0 ¼ ꢂ4FEc0ell, where F is Faraday’s constant
formation of clusters. The cluster was precipitated by the
(96 500 C or J/V). The reaction may be slow but it is feasible.
addition of ethanol. The precipitate was washed several times
However, by using materials at the nanoscale, particularly silver
with pure methanol to remove excess NaBH4, MSA and thio-
at the nano/cluster length scale, the reduction potential is
lates. Finally, a reddish brown powder of the clusters was
expected to reduce signicantly as nanoscale silver is metastable
obtained aer evaporation of methanol using a rotavapor.
with respect to the bulk. Due to the reduced dimension, the
reaction is likely to be kinetically more favorable. Therefore, we
may see interesting reactivity. This motivated us to study the
Synthesis of Ag@MSA nanoparticles
reaction of CCl4 with silver quantum clusters.
Ag@MSA nanoparticles were prepared as per the previous
In the context of environmental applications, there are only a report.20 Nearly 1.7 mL of distilled water containing 85 mg of
few studies on metal clusters. A study of sensitivity of Au25 AgNO3 was added to 100 mL of methanol containing 448.9 mg
clusters, one of the most studied clusters, to various metal ions of MSA which was kept at 0–5 ꢁC. The resulting solution was
was performed by Habeeb Muhammed and Pradeep.12 Subse- stirred at 4000 rpm. 25 mL of freshly prepared 0.2 M NaBH4
quently, a number of studies have shown the effect of metal ions solution was added to the above solution drop-wise. Stirring was
on monolayer-protected as well as protein-protected gold clus- continued for 45 minutes at 0–5 ꢁC. The mixture containing the
ters.10,15,41 Sensing experiments have been performed with silver precipitate of nanoparticles was centrifuged and the residue
clusters as well.16,17 There is a large need to develop techniques was washed with methanol to remove excess MSA and NaBH4.
to use such materials as commercial products. Here, we utilized Finally, a black powder of nanoparticles was obtained by
Ag9 clusters protected with mercaptosuccinic acid (MSA) for the evaporating methanol using a rotavapor.
complete catalytic degradation of CCl4, CHCl3 and C6H5CH2Cl
at room temperature. The reaction products were AgCl,
CCl3COOH, amorphous carbon and acetone. We found that
isopropyl alcohol (IPA), used to increase the solubility of halo-
carbons in water, is very important in the reaction. Mechanistic
aspects of the reaction are discussed based on experimental
results. We propose that Clꢂ ions, which are formed due to the
cleavage of Cl3C–Cl, replace the thiolates on the surface of the
cluster. As a result, stability of the cluster is lost which causes
the interaction of silver with Clꢂ, leading to the formation of
AgCl. The detached thiolates are converted to stable sulphites/
sulphates in solution, the presence of which was conrmed by
XPS. We demonstrate the increased efficiency of clusters for
halocarbon degradation compared to analogous MSA protected
silver nanoparticles. Reactions were carried out with clusters
supported on alumina also, in which observations similar to
unsupported clusters were noted.
Preparation of supported clusters and nanoparticles on
neutral alumina
Loading of clusters was done by the addition of cluster solution
(known amount) to a calculated amount of alumina followed by
shaking with a mechanical shaker. The color of the solution
disappeared immediately, indicating adsorption of clusters on
alumina. The addition of cluster solution and shaking were
continued until the supernatant retained the color of the cluster
solution, indicating saturation loading. The supernatant at this
stage showed the absorption features of the clusters, indicating
the saturation of the alumina surface with adsorbed clusters.
The saturation limit was found to be 10 mg per gram of
alumina. Aer surface saturation, the supernatant was removed
by centrifugation. Supported clusters were washed with water
followed by methanol to remove the excess cluster. A solvent-
free material was obtained by evaporation using a rotavapor.
The same method was followed to prepare Ag@MSA nano-
particles supported on neutral alumina.
Experimental section
Materials
Reaction of halocarbons with clusters and nanoparticles
Silver nitrate (CDH, India), mercaptosuccinic acid (MSA), Nearly 5 mL of CCl4 was added to the cluster solution (25 mg in
sodium borohydride (Sigma Aldrich), methanol, ethanol, 30 mL water). To this, 10 mL of isopropyl alcohol (IPA) was
isopropyl alcohol (IPA), carbon tetrachloride (CCl4), benzyl added. Here, the importance of IPA was to increase the misci-
chloride (C6H5CH2Cl) and chloroform (CHCl3) (SRL Chemical bility of CCl4 in the reaction mixture (solubility of CCl4 ¼ 800 mg
Co. Ltd., India) were purchased from various sources and used Lꢂ1 in water at 298 K). The above mixture (orange red color) was
as such without further purication. Neutral alumina was stirred at 4000 rpm for 24 h at room temperature. The CCl4 layer
612 | J. Mater. Chem. A, 2013, 1, 611–620
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disappeared and a grey colored precipitate was formed aer 90ꢁ. The electrospray ionization mass spectrometry (ESI MS)
1.5 h. These observations indicate the occurrence of reaction measurements were done in the negative and positive
between clusters and CCl4. Reactions were carried out in meth- modes using a MDX Sciex 3200 QTRAP MS/MS instrument
anol and ethanol as well in place of IPA. In these cases, no having a mass range of m/z 50–1700, in which the spray and the
reaction was seen. A probable reason is that oxidation of IPA is extraction are orthogonal to each other. The samples were
more facile than other alcohols (methanol and ethanol). Other electrosprayed at a ow rate of 10 mL minꢂ1 and an ion spray
halocarbons, CHCl3 and C6H5CH2Cl, were used in place of CCl4 voltage of 5 kV. The spectra were averaged for 100 scans. MS/MS
under identical experimental conditions. A precipitate was spectra were collected at optimized collision energies in the
formed aer 24 h in the case of benzyl chloride, indicating less range of 25–45 eV.
reactivity. In the case of supported clusters, 500 mg of the
material was used under the above experimental conditions.
Similar quantities of Ag@MSA nanoparticles (both unsupported
and supported) were also used for the reaction of halocarbons.
Results and discussion
Starting materials: clusters, nanoparticles and supported
materials
Silver nanoclusters (Ag9MSA7) and Ag@MSA nanoparticles have
been well characterized as per other reports20,42 and we present
Instrumentation
UV/Vis absorption spectra were recorded with a PerkinElmer here only the essential data. As-synthesized silver clusters show
Lambda 25 instrument in the spectral range of 200 to 1100 nm. characteristic UV/Vis absorption features at 450, 490, 626 and
FTIR spectra were recorded with a PerkinElmer Spectrum One 886 nm (Fig. 1A). These clusters showed red emission at 726 nm
ꢁ
instrument. KBr crystals were used as the matrix for preparing when excited at 590 nm in a water–methanol mixture at 5 C
the samples. X-ray photoelectron spectroscopy (XPS) measure- (inset of Fig. 1A). These two data prove the formation of clusters.
ments were done using an Omicron ESCA Probe spectrometer The evidence for the binding of MSA to the cluster core was
with polychromatic Mg Ka X-rays (hn ¼ 1253.6 eV). Experiments obtained by FTIR analysis (Fig. 1B). Pure MSA shows a charac-
were carried out at an X-ray power of 300 W and pass energies of teristic –CO– of free carboxylate and S–H stretching peaks at
50 eV for survey scans and 20 eV for specic regions. The base 1700 and 2566 cmꢂ1, respectively (trace (a)). In the cluster
pressure of the instrument was 5.0 ꢃ 10ꢂ10 mbar. The binding sample, carboxylate stretching mode got shied to 1576 cmꢂ1
energy was calibrated with respect to the adventitious C 1s and the S–H stretching peak was absent (trace (b)). The presence
feature at 285.0 eV. Most of the spectra were deconvoluted to of characteristic peaks of MSA in clusters (trace (b)) with a
their component peaks using the soware CASA XPS. Scanning characteristic shi in peak positions in comparison to parent
electron microscopy (SEM) and energy dispersive analysis of MSA (trace (a)) conrms the binding of MSA. The absence of the
X-rays (EDAX) were performed using a FEI QUANTA-200 SEM. S–H feature (marked in trace (a)) in the cluster sample shows
For the SEM measurements, samples were spotted on an the binding of MSA with the cluster through an Ag–S linkage.42
indium tin oxide (ITO)-coated conducting glass and dried in The chemical nature of silver and sulphur was conrmed by XPS
ambience. X-ray diffraction (XRD) data were collected with a (discussed later).
˚
Shimadzu XD-D1 diffractometer using Cu Ka (l ¼ 1.54 A)
Supported clusters on alumina were characterized by SEM
radiation. The samples were scanned in the 2q range of 10 to EDAX and the data are shown in Fig. S1 of ESI.† The spectrum
Fig. 1 (A) UV/Vis absorption spectrum of as-synthesized Ag9MSA7 clusters. Inset of (A) is the photoluminescence excitation and emission spectra of the clusters. They
show emission at 726 nm when excited at 590 nm, at 5 ꢁC. (B) Comparison of the FTIR spectra of MSA and Ag9MSA7 (traces (a) and (b), respectively); the latter shows the
absence of S–H stretching in (b).
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shows the presence of elements silver and sulphur which
conrms the adsorption of clusters. Important evidence to show
the presence of clusters on alumina is the red luminescence of the
supported cluster kept under a UV lamp at liquid nitrogen
temperature (inset of Fig. S1†). Bare alumina does not show any
luminescence under UV lamp. The Ag@MSA nanoparticles were
characterized by absorption spectroscopy. Silver nanoparticles
show a surface plasmon resonance band at 392 nm which proves
the formation of nanoparticles (Fig. S2†). TEM analysis of nano-
particles shows the average size to be 15 ꢀ 5 nm (inset of Fig. S2†).
A. UNSUPPORTED CLUSTERS. The reactions were performed as
described in the Experimental section. We have products in the
solid state and products in solution. While the former are
studied by XRD, Raman, IR, etc., the latter are studied by mass
spectrometry. The grey colored precipitate and colorless
supernatant obtained aer the reaction of CCl4 and clusters
were characterized with various analytical tools. The XRD
pattern of the precipitate is shown in Fig. 2. It matches exactly
with as-prepared AgCl (a precipitate obtained aer mixing
solutions of AgNO3 and NaCl). This observation conrms the
formation of AgCl as the reaction product.
The precipitate was used for Raman analysis before and aer
washing with ammonia solution (Fig. 3A). The purpose of
washing with ammonia was to remove AgCl as a soluble
complex, [Ag(NH3)2]+Clꢂ. Aer that, the solution was centri-
fuged at 10 000 rpm. A black residue was obtained which was
analyzed by Raman spectroscopy. The unwashed precipitate
(trace (a)) shows the presence of peaks at ꢄ1408 and
ꢄ1575 cmꢂ1. These are attributed to D and G bands of carbo-
naceous species which is due to the formation of carbon from
degradation of CCl4.43 Aer washing with ammonia, the peaks
got shied to ꢄ1357 and ꢄ1590 cmꢂ1 (trace (b)). This shi may
be due to the removal of impurities like AgCl which are soluble
in ammonia. The red shi of the D band may be due to variation
in defects (due to washing with NH3 solution) of the graphitic
structure and the blue shi of the G band is due to further
amorphization of carbon due to removal of impurities.44
Comparison of FTIR data of the reaction product (Fig. 3B) and
the parent cluster (trace (a) in the inset of Fig. 3B) further
supports the formation of the carbonaceous material. The
stretching modes of the carboxylate group at 1576 and
1407 cmꢂ1 are completely absent in the reaction product (trace
(b) in the inset of Fig. 3B). This conrms the removal of a
monolayer of MSA from the cluster. The peak at 1625 cmꢂ1 in
traces (a) and (b) is due to adsorbed water. A strong peak at
1384 cmꢂ1 has emerged due to the presence of carbonaceous
Reaction with CCl4
We performed reactions with unsupported and supported
clusters (on alumina) which are presented separately.
Fig. 2 XRD of as-prepared AgCl (a) and the product (b) obtained due to the
reaction of Ag9MSA7 with CCl4. The data have not been corrected for the
background.
Fig. 3 (A) Raman spectra of the reaction product of Ag9MSA7 and CCl4 before and after washing ((a) and (b), respectively) with aqueous ammonia. (B) FTIR spectrum
of the reaction product before washing. Inset of (B) is the FTIR spectra of parent clusters (a) and the reaction product (b) in a specific region.
614 | J. Mater. Chem. A, 2013, 1, 611–620
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Fig. 4 EDAX spectrum of the product obtained after the reaction of the Ag9MSA7 cluster with CCl4. The quantification table of elements in the EDAX spectrum is also
shown. Si, In and Sn are due to the indium tin oxide substrate used. (a) and (b) are SEM images of the above sample. Elements Ag, Cl, C and S of the region shown in (b)
are mapped in (c)–(f), respectively using X-ray energies of Ag La, Cl Ka, C Ka and S Ka.
species which conrms the formation of carbon, possiblyꢂa1s a present (Fig. 5A). Aer the reaction, a new peak around 200.0 eV
graphitic product.45 The other peaks at 2924 and 2852 cm in appears due to the presence of chlorine. The C 1s regions of the
Fig. 3B are due to the C–H stretching mode in graphitic carbon. cluster and the reaction product clearly indicate the almost
The presence of chemically transformed sulphur species complete disappearance of the peak at 288.9 eV due to the
(sulphite/suphate) which are adsorbed on the reaction product carboxylate46 groups in the product (Fig. S3†). This observation
and the absence of carboxylate features were conrmed by XPS is also supported by FTIR analysis which conrms the replace-
(discussed later). From all these, it is evident that a monolayer ment of the ligand MSA on the cluster. The intensity of the peak
of the cluster has been transformed chemically. However, the at 286.5 eV (due to carbon in C–O) increased compared to the
mechanism is complex and is not understood well.
parent clusters. The Ag 3d regions of both the samples are
SEM analysis of the precipitate is given in Fig. 4. Fig. 4a is a compared in Fig. 5B. Before the reaction, Ag 3d5/2 appeared at
large area SEM image. It shows the presence of AgCl crystals. 368.1 eV whereas it is at 367.6 eV aer the reaction. This clearly
The elemental maps of the area shown in (b) are presented in reveals the oxidation of silver from Ag0 to Ag+ state.16 Note that
(c)–(f). They clearly show the presence of elements Ag, Cl and a in Ag, oxidation leads to shiing of binding energy to lower
little amount of C. The EDAX spectrum collected from a portion values.16 The S 2p region due to the monolayer of the cluster is
of the sample is shown in Fig. 4. It shows the presence of shown in Fig. 5C. Before the reaction, S 2p3/2 was at 162.1 eV
elements Ag, Cl and trace amounts of S. The quantication table assigned to sulphur bound to the cluster in the thiolate form.
of elements of the same area (in Fig. 4) shows the presence of Ag Aer the reaction, it shied to 168.5 eV. This suggests the
and Cl in 1 : 1 atomic ratio which matches with AgCl compo- chemical change of the monolayer due to the reaction of the
sition. Also it shows the atomic percent of C and S in which cluster core. This peak is assigned to sulphate/sulphite
sulphur was seen less in quantity.
species.47 The Cl 2p feature (Fig. 5D) is quite broad and it is
In Fig. 5, XPS data of the precipitate (traces (b)) are compared tted into two components corresponding to two types of Cl
with parent silver clusters (traces (a)). In the survey spectra of moieties. The Cl 2p3/2 positions are at 198.9 and 201.1 eV. The
both the samples, carbon, silver, sulphur and oxygen were former peak may be due to chemisorbed chloride48 possibly
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Fig. 5 XPS of parent Ag9MSA7 clusters (a) and the product obtained after the reaction of clusters with CCl4 (b). (A–D) survey spectra, Ag 3d, S 2p and Cl 2p regions,
respectively.
from AgCl and the latter may be due to adsorbed organic
chlorine, possibly from CCl4.39,49 While all the silver got reacted variation in pH of the solution with time (Fig. 6A). Initially, the
in this sample, there could be some CCl4 le behind. pH of the reaction mixture was 5.7. The pH aer 1.0, 2.0, 3.0 and
During the progress of the reaction, we have followed the
Fig. 6 (A) Plot of pH of the reaction product of Ag9MSA7 + CCl4 with time. The estimated error is 5%. Inset is the absorption spectra of pure acetone (a) and that of the
supernatant (b) of the above reaction, after 8.0 h. (B) Comparison of positive mode ESI MS of IPA and the reaction mixture (traces (a) and (b), respectively).
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4.0 h of the reaction was 4.7, 1.8, 1.6 and 1.5, respectively. This at m/z 196.0 and 161.0 are seen. The MSA peak at m/z 149.0 was
clearly indicates the increase in the acidity of the reaction not noticed here as well. The absence of parent MSA peaks aer
mixture. The release of H+ can be due to the oxidation of IPA to the reaction indicates the chemical transformation of detached
acetone during the reaction. The UV/Vis absorption spectra of ligands. The peak at m/z 196.0 is assigned to CCl4COOꢂ. MS/MS
the supernatant of the reaction mixture and of acetone are of m/z 196.0 gives the peak m/z 161.0 due to the loss of 35Cl
compared in the inset of Fig. 6A. Pure acetone (trace (a)) shows (Fig. S4B†). MS2 of m/z 198.0 gives 161.0 and 163.0 due to the
an absorption maximum in water at 265 nm due to n–p* tran- loss of 37Cl and 35Cl, respectively. Similar losses were seen
sition of the carbonyl group. The presence of an identical peak in MS2 of m/z 200.0 and 202.0. The species CCl4COOꢂ
at 265 nm (trace (b)) in the reaction mixture conrms the (or [CCl3COO]Clꢂ) may be formed by halogen attachment to
formation of acetone from IPA.50 Formation of acetone was CCl3COOꢂ. Above assignments were again checked by
further conrmed by comparison of ESI MS of IPA and reaction comparing the calculated and experimental mass spectra for
products in solution (Fig. 6B). IPA (trace (a)) shows an intense the above species which show an exact match (Fig. S4C and D†).
peak at m/z 45.0 which is assigned to CH3C(OH)H+. The reaction Formation of CCl3COOꢂ may be explained as follows. Oxidation
product (trace (b)) shows the disappearance of the peak at m/z of acetone may be continued to form CH3COOH by active Cl
45.0 (due to IPA) with the appearance of a new peak at m/z 43.0 species formed in the reaction (active Cl species can act as
assigned to CH3CO+. The molecular ion peak of acetone was oxidizing agents) followed by the formation of chlorinated acid.
also seen at m/z 59.0. Interestingly, ESI MS data (Fig. S4†) of the However, more studies are essential to understand the forma-
reaction product show the formation of CCl3COOꢂ which was tion mechanism of such species. A possible chemical equation
tentatively assigned using mass spectrometry/mass spectrom- for the above reaction can be written as follows. The changes
etry (MS/MS) analysis. In Fig. S4A,† trace (a) is the positive mode observed in the monolayer are not included in the reaction as
ESI MS of the reaction mixture in which no MSA peak at m/z we do not know the exact mechanism of the chemical trans-
151.0 was noticed. Trace (b) is in negative mode where the peaks formation of the ligand.
Fig. 7 (A) and (B) XPS survey spectrum and expanded Ag 3d region, respectively of the product of the reaction between Ag9MSA7 and benzyl chloride. Insets of (A) are
Cl 2p and S 2p regions in the XPS of the same sample. (C) Photographs of the reaction mixtures of (a) Ag9MSA7 + CCl4 and (b) Ag@MSA nanoparticles + CCl4 initially
(0.0 h). The CCl4 layer is seen separately at the bottom. Photographs (al) and (bl) are of corresponding mixtures after the reaction for 1.5 h. Unreacted CCl4 is marked in
(bl). An enlarged photograph of the marked area of (bl) is shown in (d).
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Table 1 Comparison of various nanomaterials used for the degradation of chlorocarbons
Nanomaterials
Halocarbons
CCl4
Process
Conditions
Ambient
Products
Remarks
Ref.
Elemental iron
and zinc
Reductive
dechlorination conditions
Free metal ions,
CHCl3 and H2
Kinetics depends on pH, 51
surface area, etc.
Pd/C, Rh/C, Ru/C
and Pt/C
TiO2 (30 nm)
suspension
Polychlorinated
biphenyls (PCBs)
CHCl3, CHBr3,
and CCl3COOꢂ
Catalytic
2-Propanol, aq.
Biphenyl and
Complete dechlorination 52
in ꢄ2–3 h
dechlorination NaOH, 82 ꢁ
C
phenylcyclohexane
Photocatalysis
pH > 11, H2O
+ CH3OH
CO, CO2 and Clꢂ
Xe arc lamp at 910 W,
2 h irradiation
53
Subcolloidal
Fe/Ag particles
(<0.1 mm size)
Charge stabilized
Ag and Au NPs
(10–150 nm)
TiO2 (40 and
80 nm)
Hexachlorobenzene Reductive
Room temperature, Tetra-, tri-, and
Complete dehalogenation 54
in 24 h
(HCB)
dehalogenation in water dichlorobenzenes
CCl4, C6H5CH2Cl,
Catalytic
2-Propanol + water, AgCl, C and acetone
Complete destruction
in 12 h
35
55
CHCl3, CH2Cl2, etc. degradation
28 ꢁ
C
CCl4
Thermal
decomposition gaseous CCl4
<550 ꢁC,
CO, CO2, COCl2,
Particles of size 40 nm
Cl2, C, TiCl4, HCl and are more reactive
titanium oxychloride than 80 nm
Monolayer-
protected Ag clusters and CHCl3
(<1 nm size)
CCl4, C6H5CH2Cl
Catalytic
degradation
2-Propanol + water, AgCl, C, acetone and Complete destruction
Present
study
28 ꢁ
C
CCl3COOH within 3 h
1 : 1 (inset of Fig. S6†). Other halocarbons like CHCl3 and
C6H5CH2Cl were also degraded by unsupported and supported
clusters giving silver chloride and amorphous carbon as the
products (data are not shown). Time taken for complete
destruction of CHCl3 is comparable to the CCl4 case, whereas
for benzyl chloride, it was more. The possible reason is its
complete immiscibility in the reaction mixture. XPS data of the
reaction products of benzyl chloride and clusters are shown in
Fig. 7A and B. A survey spectrum indicates the presence of
elements C, O, Ag, S and Cl (Fig. 7A). The peak of Ag 3d5/2 at
367.7 eV conrms the presence of silver in the +1 oxidation state
formed due to the reaction (Fig. 7B). The chemical change of the
monolayer has been conrmed by the presence of S 2p3/2 at
Ag9MSA7(aq.) + (CH3)2CHOH + CCl4 / C + AgClY +
(CH3)2CO + 2H+
B. SUPPORTED CLUSTERS. Clusters supported on neutral
alumina were also used for the study of degradation of CCl4.
Reaction products were characterized by XRD, Raman and SEM
EDAX. XRD (Fig. S5A†) conrms the formation of AgCl and
Raman analysis (Fig. S5B†) reveals the presence of the carbo-
naceous material due to the degradation of halocarbon. EDAX
analysis shows clearly the presence of Cl, Ag and S (Fig. S6†).
The quantication table of elements also supports the forma-
tion of AgCl which shows the silver to chlorine atomic ratio as
Scheme 1 Schematic representation of the degradation of halocarbon, CCl4, by silver clusters along with other chemical transformations. All the chemical species
detected are not marked.
618 | J. Mater. Chem. A, 2013, 1, 611–620
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168.5 eV (inset of Fig. 7A). The nature of Cl has been understood
from the peak of Cl 2p3/2 at 198.2 (inset of Fig. 7A). This is due to
Clꢂ from AgCl. It is important to recall that as the reaction was
complete in this case, no unreacted chlorine feature was
detected.
Summary and conclusions
In summary, we have studied the reaction of mono-
layer-protected atomically precise silver clusters and nano-
particles with halocarbons at room temperature. The presence
of reaction products, silver chloride, CCl3COOꢂ and amorphous
carbon, was conrmed by various spectroscopic and micro-
scopic tools. A possible mechanism for the reaction is proposed
accounting for the observed products. The efficiency of clusters
in degrading halocarbons is considerably higher than the
corresponding nanoparticles. This was attributed to the smaller
size and reduced nobility of silver. Control experiments and the
measurement of pH were carried out to validate the proposed
mechanism. A limitation of this material is the non-reusability.
But the reaction product AgCl can be recovered and used to
make metallic silver or silver clusters back again.
It is good to compare the efficiency of degradation of
halocarbons by quantum clusters with the corresponding
nanoparticles. For that, the same quantities (25 mg) of clusters
and nanoparticles were used for the degradation of 5 mL of
CCl4 under the same experimental conditions. Surprisingly, in
the case of clusters, the color of the mixture disappeared aer
1.5 h accompanied by the formation of a grey colored precipi-
tate (Fig. 7C). But, in the case of nanoparticles, reddish black
color turned to pale brown. No precipitate of AgCl was seen aer
1.5 h, and CCl4 remained at the bottom, as marked in Fig. 7b1.
The color change reects progress in the reaction. The color
change and the presence of CCl4 conrm that clusters are more
efficient for the degradation reaction. Degradation of chlor-
ocarbons by other relevant nanomaterials is compared in Table
1. This suggests that quantum clusters of silver are good
candidates for efficient degradation of chlorocarbons. Other
materials need light, high temperatures, etc. whereas in our
study we show the possibility of degradation at room tempera-
ture by simple mixing.
Acknowledgements
We thank the Department of Science and Technology (DST),
Government of India, for constantly supporting our research
program on nanomaterials. M.S.B. thanks the CSIR for a senior
research fellowship.
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