Photo-assisted oxidation of thiols to disulfides using cobalt ‘‘Nanorust’’ under visible light

This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015 New J. Chem., 2015, 39, 6193--6200 | 6193
Cite this: New J. Chem., 2015,
39, 6193
Photo-assisted oxidation of thiols to disulfides
using cobalt ‘‘Nanorust’’ under visible light†
Deepak Chauhan,a
Pawan Kumar,b
Chetan Joshi,b
Nitin Labhsetwar,*c
Sudip K. Gangulya
and Suman L. Jain*b
Heterogeneous ‘‘Nanorust’’ containing cobalt oxide has been developed for the visible light assisted
oxidation of thiols to disulfides using molecular oxygen as an oxidant under alkaline free conditions and
therefore more environmentally friendly. Pyrolysis of heterogenized tetrasulfonated cobalt(II) phthalocyanine
(CoPcS) supported on mesoporous ceria (CeO2) transforms it into a novel heterogeneous ‘‘Nanorust’’
containing CoOx-C,N@CeO2 which exhibited higher catalytic activity than the homogeneous CoPcS as well
as the ceria immobilized CoPcS catalyst. Importantly, these catalysts could easily be recovered and recycled
for several runs, which makes the process greener and cost-effective.
Introduction
Thiols are widely distributed in petroleum products causing
foul odor, corrosiveness, and environmental pollution.1
The
oxidation of thiols to disulfides constitutes an elegant approach
not only to remove thiols from petroleum products but also
extracted thiols can be used in synthetic industries for various
purposes.2
Numerous methods reported for the oxidation of
thiols to disulfides use stoichiometric oxidants, such as dichro-
mates, permanganates and metal peroxides.3
However, these
processes involve tedious work-up procedures and generate
large amounts of undesirable, toxic, waste by-products, which
cause environmental pollution.4
To eliminate such problems,
the catalytic oxidation of thiols using molecular oxygen as a
terminal oxidant has been intensively studied in recent years.5
Among the various known metal catalysts for the aerobic oxidation
of thiols into disulfides, cobalt phthalocyanine complexes have
been widely used.6
However, the homogeneous nature of cobalt
phthalocyanine complexes and requirement of strongly alkaline
conditions for the oxidation reaction are the major drawbacks
associated with these systems. Many studies have been directed to
transform these homogeneous cobalt phthalocyanine systems to
heterogeneous forms by immobilizing these phthalocyanines on
solid supports, particularly, solid basic materials, such as magne-
sium containing oxides to avoid the addition of strong soluble
bases into the systems and to overcome environmental problems.7
Recently, Beller et al. reported novel nano-metal oxides (e.g.
Fe2O3/NGr@C; ‘‘Nanorust’’ and Co3O4/NGr@C) synthesized by
pyrolysis of in situ-generated nitrogen-ligated metal complexes
on various supports to be efficient heterogeneous catalysts for
organic transformations.8
Inspired by these reports, we aimed
to develop non-precious metal oxides ‘‘Nanorust’’ catalysts for
realizing cost effective and environmentally benign methodol-
ogies for organic transformations.
Recently, visible light photoredox catalysis using solar energy
and molecular oxygen emerged to be ideal for sustainable organic
synthesis as both sunlight and oxygen are readily available and
practically inexhaustible, and no harmful oxidant-derived pro-
ducts are formed.9
Also, we have recently reported an efficient
graphene oxide immobilized iron–phthalocyanine catalyst for the
photo-induced oxidation of thiols using molecular oxygen as the
oxidant.10
Here we describe for the first time, the use of cobalt
oxide based ‘‘Nanorust’’ as an excellent visible light active catalyst
for the oxidation of thiols using molecular oxygen under alkaline
free conditions. The intended ‘‘Nanorust’’ (CoOx-C,N@CeO2)
catalyst was prepared by pyrolysis of meso-ceria supported cobalt
phthalocyanine at 600 1C for 6 h under an argon atmosphere
(Scheme 1).
Result and discussion
Synthesis and characterization of the catalyst
Firstly meso-CeO2 was synthesized following our previously
reported procedure using chitosan as the template.11
Subsequently
meso-CeO2 was used as a support matrix for immobilizing the
a
Refining Technology Division, CSIR-Indian Institute of Petroleum, Dehradun,
248005, India
b
Chemical Sciences Division, CSIR-Indian Institute of Petroleum, Dehradun,
248005, India. E-mail: suman@iip.res.in; Fax: +91-135-2660202;
Tel: +91-135-2525788
c
Environmental Materials Division, CSIR-National Environmental Engineering
Research Institute (CSIR-NEERI), Nagpur, India.
E-mail: nk_labhsetwar@neeri.res.in
† Electronic supplementary information (ESI) available: N2 adsorption–desorption
isotherm, pore size distribution and SEM image of CeO2, CoPc@CeO2, and CoOx-
C,N@CeO2. See DOI: 10.1039/c5nj00792e
Received (in Porto Alegre, Brazil)
30th March 2015,
Accepted 7th June 2015
DOI: 10.1039/c5nj00792e
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tetrasulfonated cobalt phthalocyanine complex (CoPcS) by taking
the advantages of the ionic interaction between –SO3H of phthalo-
cyanine and –OH of CeO2 to give CoPc@CeO2. The obtained
CoPc@CeO2 was pyrolysed at 600 1C in an argon atmosphere for
6 h to give ‘‘Nanorust’’ CoOx-C,N@CeO2 (Scheme 1).
The surface properties like BET surface area (SBET), mean pore
diameter (rp), total pore volume (Vp) etc. were determined by N2
adsorption–desorption isotherms at 77 K. The type (IV) loop was
observed for meso-CeO2, CoPc@CeO2 and CoOx-C,N@CeO2 and
according to IUPAC recommendation suggests a mesoporous
nature of these materials (Fig. S1a–c, ESI†).12
The calculated
surface area for meso-CeO2 was found to be 75.92 m2
gÀ1
which
is much higher than commercial grade CeO2 (23.2 m2
gÀ1
).
While for CoPc@CeO2 and CoOx-C,N@CeO2 the surface area
was found to be 49.93 m2
gÀ1
and 72.19 m2
gÀ1
respectively.
The significant decrease in surface area in CoPc@CeO2 con-
firmed the successful loading of CoPc units on the surface.
The restoration of surface area and pore size after the pyro-
lysis step in ‘‘Nanorust’’ CoOx-C,N@CeO2 was assumed due to
the breaking of the organic moieties with reopening of the
pores as well as the presence of cobalt oxides in the pores of
meso-CeO2.
SEM images of mesoporous CeO2, CoPc@CeO2 and ‘‘Nanorust’’
are shown in Fig. S2 (ESI†). The erupted ridge type structures in
the SEM pattern of meso-CeO2 was obtained, which may be due
to the increase in localized temperature during the combustion
process (Fig. S2a, ESI†). After the attachment of CoPc to meso-
CeO2, the elevated and crinkled structures were significantly
reduced which is most likely due to the presence of CoPc
moieties on the surface (Fig. S2b, ESI†). The SEM image of
CoOx-C,N@CeO2 clearly indicated the decomposition of complex
moieties and provided CeO2 type structures having rough surface
morphology (Fig. S2c, ESI†). Furthermore, the presence of Co,
C and N in EDX results of ‘‘Nanorust’’ confirmed the presence of
these elements in the synthesized material (Fig. S2e, ESI†).
In Fig. 1, the TEM images display that the synthesized
materials having the particle size in the range of 5–10 nm. The
honeycomb like pattern observed in TEM of CeO2 (Fig. 1a) is
probably due to the template structure used during the synthesis.
The almost similar morphology of ‘‘Nanorust’’ to the CeO2
support indicated the decomposition of CoPc units during
calcinations (Fig. 1b and c). Characteristic rings due to CeO2
diffraction in the SAED pattern of CoOx-C,N@CeO2 revealed the
good crystallinity of the synthesized material (Fig. 1d).
The X-ray diffraction patterns of meso-CeO2, CoPc@CeO2
and CoOx-C,N@CeO2 are shown in Fig. 2. The XRD diffracto-
gram of CeO2 consists of characteristic peaks at 2y values of 28.61
(111), 33.31 (200), 47.481 (220), 56.51 (311), 59.21 (220), 69.41 (400)
and 76.61 (331), which can be indexed to the fcc cubic space
group Fm3m (225) structure and were in good agreement with
JCPDS card no. 34-0394 (Fig. 2a).13
After the attachment of cobalt
phthalocyanine to meso-CeO2 the XRD pattern remained almost
unchanged due to the lower loading of CoPc units (Fig. 2b).
Furthermore after pyrolysis the diffraction pattern of CoOx-
C,N@CeO2 was found to be almost similar to that of meso-CeO2,
indicating that the calcination step did not change the crystalline
state of CeO2 (Fig. 2c).
The UV-Vis absorption spectra of tetrasulfonated cobalt phthalo-
cyanine (CoPcS) in dimethylformamide (DMF) show two char-
acteristics absorption bands at 305 nm (Soret band) and 660 nm
Scheme 1 Synthesis of ‘‘Nanorust’’ CoOx-C,N@CeO2 catalyst.
Fig. 1 TEM image of (a) mesoporous CeO2 (b) CoPc@CeO2 (c) CoOx-
C,N@CeO2 and (d) SAED pattern of CoOx-C,N@CeO2.
Fig. 2 XRD of (a) mesoporous CeO2 (b) CoPc@CeO2 and (c) CoOx-
C,N@CeO2.
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(Q band), respectively, due to p - p* macrocycle ring transition
(Fig. 3a).14
For the mesoporous CeO2 a strong absorption band
in the UV region was observed due to O-2p to Ce-4f transition
(Fig. 3b).15
The appearance of the Q band of CoPc in UV-Vis
spectra of CoPc@CeO2 was a clear indication of successful attach-
ment of CoPc units to meso-CeO2 (Fig. 3c). After the pyrolysis of
CoPc@CeO2, the Q band of CoPc disappeared due to thermal
degradation of the phthalocyanine ring structure. The obtained
UV-Vis spectrum for ‘‘Nanorust’’ was almost similar to meso-CeO2
but the absorption profile in the visible region was found to be
increased that was assumed due to the surface doping of CeO2
with Co, C and N elements (Fig. 3d).
The characteristic peaks in the FT-IR spectrum of CoPcS (Fig. 4a)
at 574 cmÀ1
, 640 cmÀ1
, 750 cmÀ1
correspond to the phthalocyanine
ring vibrations. The peaks at 925 cmÀ1
, 1105 cmÀ1
, and 1150 cmÀ1
were due to the aromatic ring vibrations, C–H bending vibrations,
CQN vibration and pyrrole ring vibration of the phthalocyanine
ring structure.16
The appearance of peaks at 1029 cmÀ1
, 1317 cmÀ1
,
1727 cmÀ1
confirmed the –SO3H entity in the molecule. Further-
more a broad band at 3460 cmÀ1
was due to –OH stretching
vibration of the –SO3H group.17
The FT-IR spectra of CeO2
showed its characteristics peak at 1369 cmÀ1
and 1542 cmÀ1
(Fig. 4b).18
The other peak at 1631 cmÀ1
was assumed due to
the adsorbed water on the surface of CeO2.19
The appearance
of peaks characteristic to CoPcS in the FT-IR of CoPc@CeO2
confirmed the successful attachment of CoPcS to CeO2 (Fig. 4c).
The FT-IR spectrum of ‘‘Nanorust’’ CoOx-C,N@CeO2 exhibited
peaks at 1114 cmÀ1
and 1631 cmÀ1
, probably due to the degrada-
tion of complex units and restoration of the CeO2 structure (Fig. 4d).
The thermal degradation pattern of samples was determined
using a TGA thermogram as shown in Fig. 5. The TGA curve of
CoPcS showed a very small weight loss around 100–150 1C due
to the loss of moisture and adsorbed water molecules in the
sample (Fig. 5a).20
Another major weight loss at around 375 1C
was attributed to the degradation of the phthalocyanine ring
structure. For CeO2, the weight loss observed at 100 1C was due
to the loss of moisture, which was followed by a linear weight
loss (Fig. 5b).21
After the attachment of CoPcS to meso-CeO2 the
catalyst (CoPc@CeO2) exhibited a small weight loss at 100 1C
due to evaporation of water and another weight loss in the range
of 400–450 1C which was due to the slow degradation of inter-
calated CoPc units (Fig. 5c). For the ‘‘Nanorust’’ CoOx-C,N@CeO2
the TGA pattern was found to be almost similar to that of meso-
porous CeO2 that confirmed the degradation of the complex
moieties during the pyrolysis step (Fig. 5d).
Catalytic activity
The catalytic activity of the developed materials i.e. CeO2,
CoPc@CeO2 and ‘‘Nanorust’’ CoOx-C,N@CeO2 was tested for
the photocatalytic oxidation of thiols to disulfides in aqueous
medium under visible light irradiation. n-Dodecane thiol was
chosen as a model substrate and 20 watt LED was used for visible
light illumination. The oxidized product was extracted with
diethyl ether and analyzed by GC. The results of these experi-
ments are summarized in Table 1. As shown, the photo-oxidation
of n-dodecane thiol did not occur in the absence of catalyst under
visible light irradiation (Table 1, entry 1). Homogeneous cobalt
Fig. 3 UV-Visible spectra of (a) CoPcS (b) CeO2 (c) CoPc@CeO2 and
(d) CoOx-C,N@CeO2.
Fig. 4 FT-IR of (a) CoPcS (b) meso-CeO2 (c) CoPc@CeO2 (d) CoOx-
C,N@CeO2.
Fig. 5 TGA thermogram of: (a) CoPcS (b) meso-CeO2 (c) CoPc@CeO2
(d) CoOxC,N@CeO2.
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phthalocyanine (CoPcS) showed moderate activity for this
transformation and gave 81% isolated yield of disulfide after
5 h under visible light irradiation (Table 1, entry 2). There was
negligible yield of corresponding disulfide obtained when meso-
CeO2 was used as the catalyst under identical experimental
conditions (Table 1, entry 3). The CoPc@CeO2 afforded 68%
isolated yield of the disulfides in 5.5 h (Table 2 entry 4). The
synthesized ‘‘Nanorust’’ CoOx-C,N@CeO2 was found to be most
reactive among all the tested catalysts and gave 87% isolated
yield of the disulfide in 5 h (Table 1, entry 5). To evaluate the
effect of visible light irradiation, we performed the oxidation of
n-dodecane thiol (C12) in the dark using the ‘‘Nanorust’’ catalyst
under similar experimental conditions. The reaction was found
to be very slow and gave only 18.5% yield of the corresponding
disulfide (Table 1, entry 5). Similarly the reaction did not occur in
the absence of molecular oxygen with visible light under identical
conditions (Table 1, entry 5). To establish the superiority of
‘‘Nanorust’’ we also synthesized an ordinary CoOx@CeO2 catalyst
by the wet impregnation method and used for the oxidation
of n-dodecane thiol under described reaction conditions. The
synthesized catalyst was found to be less reactive than the
‘‘Nanorust’’ and gave only 44% yield of the oxidized product
(Table 1, entry 6).
Thermal reactions were also carried out in order to explore
the thermal activity of the CoOx-C,N@CeO2 catalyst. The reac-
tion was carried out at three different temperatures 50 1C, 60 1C
and 70 1C under identical conditions by using n-dodecane thiol
as a model substrate. The isolated yield of the corresponding
disulfide after 5 h at 50 1C, 60 1C and 70 1C was found to be 54,
78 and 84% respectively. These results confirmed that the
developed nanorust CoOx-C,N@CeO2 catalyst could efficiently
work for the thermal oxidation of thiols and at higher tempera-
ture i.e. 70 1C a comparable yield of the product was obtained.
Furthermore, we investigated the effect of ‘‘Nanorust’’ catalyst
concentration on the photo-oxidation of n-dodecane thiol by
varying the catalyst amount from 0.05 to 0.4 g under described
experimental conditions (Fig. 6). As shown in Fig. 6 0.2 g catalyst
was found to be optimum and gave maximum as 88% conver-
sion to disulfide in 5 h. A further increase in catalyst amount
showed no significant increase in the rate of the reaction (Fig. 6).
Furthermore, the oxidation of various thiols was carried out
using ‘‘Nanorust’’ as the catalyst under optimized reaction condi-
tions (Scheme 2). The results of these experiments are summar-
ized in Table 2. All the thiols were converted into corresponding
Table 1 Effect of various reaction parameters on the photo catalytic
oxidation of n-dodecane thiola
Entry Catalyst Dark Light
Time
(h)
Conv.b
(%)
Yieldc
(%)
1 Blank reaction Â O 12 — —
2 CoPcS O Â 12 6 4
Â O 5.0 84 81
3 meso-CeO2 O Â 8 — —
Â O 12 8 6
4 CoPc@CeO2 O Â 12 4 3
Â O 5.5 72 68
5 ‘‘Nanorust’’
CoOx-C,N@CeO2
O Â 12 20 18.5
Â O 5 88 87
Â O — (—)d
—
6 CoOx@CeO2 O Â 8 — —
Â O 6 48 44
a
Reaction conditions: n-dodecane thiol (2 mmol), catalyst (1 mol% 0.2 g), in
the presence of molecular oxygen at room temperature (35 1C). b
Determined
by GC. c
Isolated yield. d
In the absence of molecular oxygen.
Table 2 Nanorust catalyzed photo-assisted oxidation of thiolsa
Entry Thiol Disulfide Time (h) Conv.b
(%) Yieldc
(%) TOF (hÀ1
)
1 3.5 98 95 27.1
2 3.5 96 94 26.5
3 3.5 95 94 26.5
4 4.0 92 90 22.5
5 4.0 90 87 21.7
6 4.5 90 85 18.8
7 5.0 88 87 17.4
8 5.0 81 78 15.6
9 5.5 87 83 15.0
10 5.5 85 81 14.7
11 6.0 84 82 13.6
12 6.0 82 80 13.3
a
Reaction conditions: thiol (2 mmol), catalyst (1 mol%, 0.2 g), in the presence of molecular oxygen under visible light; temperature (35 1C).
b
Determined by GC. c
Isolated yield.
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disulfides in quantitative yield with no trace of possible side
products. In general aliphatic thiols (Table 2, entries 1–6) were found
to be more reactive than aromatic ones (Table 2, entries 7–10).
However, the reaction was found to be sluggish upon increasing
the chain length of alkyl thiols and therefore required longer
reaction time (Table 2, entries 4–6).
In order to explore the practical applicability of the ‘‘Nanorust’’
CoOx-C,N@CeO2 we have performed the photo-oxidation of
kerosene blended with less reactive thiol i.e. tert-dodecane thiol
(300 ppm) and the results are shown in Fig. 7. As shown in
Fig. 7, the concentration of tert-dodecane thiol in the kerosene
sample was found to be decreased from 300 ppm to 264 ppm,
85 ppm, 44 ppm and 25.8 ppm for CeO2, CoPc@CeO2, CoPcS
and ‘‘Nanorust’’ CoOx-C,N@CeO2 respectively within 5 h.
Furthermore, the recyclability of the ‘‘Nanorust’’ catalyst was
evaluated to establish the heterogeneous nature and stability
of the catalyst. The recycling experiments were performed by
choosing the oxidation of n-dodecane thiol as a representative
example and the results are summarized in Fig. 8. As shown,
the recovered catalyst exhibited almost similar catalytic activity
at least for six recycling experiments (Fig. 8). After the six recycle
experiments, the recovered catalyst was analyzed by ICP-AES
analysis to determine the cobalt content in the recovered
catalyst. For the recycled catalyst the cobalt content was found
to be 0.22 wt% that was nearly similar to the freshly prepared
catalyst with 0.24 wt% (0.02 wt% loss after six recycling).
Furthermore, the cobalt content in supernatant solutions, after
removal of the catalyst, was determined by ICP-AES analysis to
confirm the heterogeneous nature of the catalyst. The cobalt
content in the supernatant after the first recycling experiment
was found to be 1.5 ppm and in subsequent recycling experi-
ments (for six runs), no cobalt was detected in the supernatant
solutions. These results indicated that the catalyst exhibited
marginal leaching during the first recycling experiment and
after that the catalyst showed excellent stability without any
detectable leaching.
Although the exact mechanism of the reaction is not known at
this stage; however, based on the existing report, we proposed a
plausible mechanism for the photocatalytic oxidation of thiols to
disulfides. As reported in the literature mesoporous ceria due to
its higher band gap absorbs energy specifically in the UV range.22
However, after doping with cobalt, carbon and nitrogen elements
during the pyrolysis process, the band gap was reduced signifi-
cantly which makes the synthesized nanorust visible light active.
After the absorption of visible light, electrons of the valance band
get excited into the conduction band of CeO2. These electrons are
trapped by doped cobalt and therefore cobalt works like an
electron trapper and slows down electron hole pair recombination
rate.23
Upon transfer of these electrons, the molecular oxygen gets
excited24
and reacted with thiol to produce thiolate radicals.25
These thiolate radicals subsequently dimerized to give corre-
sponding disulfide along with water as the by-product.25
Conclusion
We have demonstrated the first successful use of ‘‘Nanorust’’
synthesized by pyrolysis of cobalt phthalocyanine supported
meso-CeO2 as a highly eﬃcient heterogeneous catalyst for photo-
catalytic oxidation of thiols using molecular oxygen as the
Fig. 6 Eﬀect of ‘‘Nanorust’’ catalyst concentration on the rate of the
reaction.
Scheme 2 ‘‘Nanorust’’catalyzed photo-oxidation of thiols to disulfides.
Fig. 7 Photo-oxidation of tert-dodecane thiol in kerosene.
Fig. 8 Results of recycling experiments.
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oxidant under visible light irradiation in an alkali free environ-
ment. The developed nanorust was found to be highly photoactive
and exhibited superior activity as compared to the homogeneous
CoPc and supported CoPc@CeO2 catalyst under similar reaction
conditions. The developed nanorust was easily recovered and
could be reused for several runs with consistent activity. We
believe that our findings will be helpful in developing a light
induced novel greener process for photocatalytic transformations
of organic substrates in a sustainable way.
Experimental
Materials
The template Chitosan (85% deacylated) was purchased from Alfa
Aesar. Cobalt phthalocyanine (97%), cerium nitrate (99.99%) and
chlorosulfonic acid (99%) was purchased from Sigma Aldrich.
All other chemicals were purchased from Merck and were of
analytical grade. Cobalt phthalocyanine tetrasulfonic acid was
synthesized by treating CoPc with chlorosulfonic acid following
the literature procedure.26
All reagents were used without further
purification. Deionized water was used in all experiments.
Techniques used
The surface morphology of the material was determined by
scanning electron microscopy using Jeol Model JSM-6340F.
The micro fine structure of materials was determined by High
Resolution Transmission Electron Microscopy using FEI-TecnaiG2
Twin TEM working at an acceleration voltage of 200 kV. For TEM
analysis well dispersed aqueous samples were deposited on a
carbon coated copper grid. Electron diffraction patterns were
evaluated using the Process-Diffraction software package. The
diffraction pattern and phase structure of materials was deter-
mined by XRD using a Bruker D8 Advance diffractometer at 40 kV
and 40 mA with Cu Ka radiation (l = 1.5418 nm). Diffraction peaks
were compared with the standard database reported in the Joint
Committee on Powder Diffraction Standards (JCPDS). The vibra-
tional spectra of samples were recorded on a Perkin-Elmer spec-
trum RX-1 IR spectrophotometer from 450 cmÀ1
to 4000 cmÀ1
.
The electronic transition under UV-visible light of solid materials
was performed on a Perkin Elmer lambda-19 UV-VIS-NIR spectro-
photometer by using BaSO4 as a reference material while UV-Vis
spectra of cobalt phthalocyanine tetrasulfonate was collected in
DMF. Thermo-gravimetric analysis was carried out for determining
the thermal stability of the samples using a thermal analyzer
TA-SDT Q-600 in the temperature range 40 to 900 1C with a heating
rate of 10 1C minÀ1
under nitrogen flow. Surface properties like
BET surface area, pore size distribution etc. were estimated by the
Brauner–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH)
method using a Micromeritics ASAP2010 instrument, using liquid
nitrogen at 77 K. XPS measurements were obtained on a KRATOS-
AXIS 165 instrument equipped with dual aluminum–magnesium
anodes by using MgKa radiation (hn = 1253.6 eV) operated at 5 kV
and 15 mA with pass energy 80 eV and an increment of 0.1 eV. To
overcome the charging problem, a charge neutralizer of 2 eV was
applied and the binding energy of the C 1s core level (BEffi84.6 eV)
of adventitious hydrocarbon was used as a standard. The XPS
spectra were fitted by using a nonlinear square method with the
convolution of Lorentzian and Gaussian functions, after a poly-
nomial background was subtracted from the raw spectra. The
metal content of developed catalyst was determined by Inductively
Coupled Plasma-Atomic Emission spectroscopy using an induc-
tively coupled plasma atomic emission spectrometer (ICP-AES,
DRE, PS-3000UV, Leeman Labs Inc., USA). The samples for ICP-
AES were prepared by digesting 0.05 g catalyst with conc. HNO3 to
oxidize all organic materials and leaching out the metals in the
oxidized form. The obtained solution was heated at 70 1C for
30 min and the volume was made up to 10 mL by adding
de-ionized water. The yield of disulfides was determined by
GC-MS. The thiol contents of blended kerosene were determined
using a Mettler Toledo DL50 Rondolino potentiometer using a
platinum counter electrode. 1
H-NMR and 13
C-NMR spectra of
disulfides were collected on the Bruker Advance-II 500 MHz
instrument working at 500 MHz frequency. The thiol contents of
blended kerosene were determined using a Mettler Toledo DL50
Rondolino potentiometer using a platinum counter electrode.
Synthesis of mesoporous-ceria (CeO2)11
The mesoporous CeO2 was prepared by following a modified
template method as previously reported by us. In briefly, chitosan
powder (3 g) was dissolved in 100 mL of 5% of acetic acid with
stirring for about 1 h. To this chitosan solution, 1.5 g cerium
nitrate aqueous solution was added with stirring for 2 hours. The
precursor thus obtained was then subjected to precipitation in
50% aqueous ammonia solution. The precipitate obtained was
dried at 60 1C and followed by calcination at 550 1C for 5 h.
Synthesis of meso-CeO2 supported cobalt phthalocyanine
(CoPc@CeO2)27
In a typical synthesis, meso-CeO2 (2 g) and CoPcS (0.25 g) were
added to a round bottom flask containing 100 mL ethanol–water
(1/1) mixture and stirred for 24 hours at 80 1C. The synthesized
heterogeneous material was thoroughly washed with ethanol,
water and then dried at 60 1C for 24 h.
Synthesis of ‘‘Nanorust’’ (CoOx-C,N@CeO2)
The ‘‘Nanorust’’ CoOx-C,N@CeO2 was synthesized by the pyrolysis
of meso-CeO2 supported CoPc (CoPc@CeO2) at 600 1C in the
presence of argon for 6 h. The cobalt content of the developed
catalyst was found to be 0.24 wt% (1.36 Â 10À2
mmol Co3O4 per g
catalyst). For comparison, ordinary Co3O4@CeO2 was synthesized
by wet impregnation of CeO2 with cobalt acetate solution followed
by calcinations. For comparison we have also synthesized
CoOx@CeO2 by a wet impregnation method. Briefly, 20 mg
CoCl2Á6H2O was dissolved in 25 mL water and then 1 g of CeO2
was added to this solution and stirred and then the solvent was
removed under reduced pressure and the material was calcined
at 600 1C for 8 h.
General experimental procedure for photo-oxidation of thiols
For the photocatalytic thiol oxidation experiments, 10 mL water,
catalyst (1 mol%) and thiol (2 mmol) were added in a 25 mL
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round bottomed flask. The reaction vessel was sealed with septum
and illuminated using 20 Watt LED light (Model no.-HP-FL-20W-
F-Hope LED Opto-Electric Co. Ltd l 4 400 nm) under an oxygen
atmosphere at room temperature. The intensity on the surface of
vessel was 75 W mÀ2
as measured using an intensity meter. The
progress of the reaction was monitored by TLC and the samples
were collected every half an hour with the help of a needle. The
disulfides were extracted with diethyl ether and analyzed by GC.
After completion of the reaction, the catalyst was separated by
filtration. The recovered catalyst was washed with methanol and
dried for recycling runs. The filtrate so obtained was extracted
with diethyl ether and the combined organic layer was dried over
anhydrous MgSO4 and concentrated under reduced pressure. The
crude product was purified by column chromatography using
ethyl acetate:hexane (1:9) as the eluent. Blank reactions were
carried for confirming that catalyst, oxygen and visible light were
essential components for the oxidation of thiols. For comparison
thermal reactions were also carried out at diﬀerent temperatures
by using the CoOx-C,N@CeO2 nanorust.
Acknowledgements
Authors are thankful to Director IIP for granting permission
to publish these results. P.K. is thankful to CSIR New Delhi for
providing fellowship under Emeritus Scientist Scheme. Further-
more, CSIR, New Delhi is kindly acknowledged for funding in
CSC-0117 12th Five Year Plan Project. Dr S. Bojja, CSIR-IICT, is
kindly acknowledged for providing TEM analysis of the samples.
The analytical department of the Institute is kindly acknowledged
for providing support in the analysis of samples.
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