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Choudhary and co-workers [12–14] reported that Au supported
at 110 ◦C for 20 h to obtain the final material. The materials were
then reduced in flowing hydrogen at 250 ◦C for 2 h to yield the final
catalysts with input Au loading of 0.5–5 wt.%.
on a range of oxides (MgO, CaO, SrO, BaO, Al2O3, Ga2O3, In2O3,
Tl2O3, TiO2, Cr2O3, MnO2, Fe2O3, CoOx, NiO, CuO, ZnO, Y2O3, ZrO2,
La2O3, Ce2O3, Nd2O3, Sm2O3, Eu2O3, Tb2O3, Er2O3, Yb2O3 and
U3O8) is active for oxidation of styrene yielding styrene oxide (SO)
with anhydrous TBHP (in benzene) as oxidant. Supports play a cru-
cial role in catalytic reactions providing anchoring sites to metal
nanoparticles and influencing the reaction rates. Yin et al. [15] stud-
ied the epoxidation of styrene over Au/mesoporous Al2O3. Zhang
et al. [16] elucidated the crystal face selective deposition of Au and
the higher activity of Au nanoparticles of 2–3 nm deposited at lat-
2.2. Characterization techniques
Elemental composition (Au0/+, Ba2+ and Na+) of the catalysts was
analyzed by Inductively Coupled Plasma-Optical Emission Spec-
troscopy (ICP-OES; Spectro Arcos). Powder X-ray diffraction (XRD)
of the catalyst samples was done on a Philips X’Pert Pro diffractome-
ter using Cu-K␣1 radiation ( = 0.15406 nm) and a proportional
counter detector. Measurements were done in the scan range (2)
of 5–80◦ at a scan speed of 4◦/min. Textural properties of the cat-
alysts [specific surface area (SBET), pore volume (Vp) and average
pore diameter (dp)] were determined from N2 physisorption stud-
ies conducted at −196 ◦C using a Quantachrome USA (Autosorb-1C)
equipment. Prior to N2 adsorption, the samples were evacuated at
200 ◦C for 2 h. It was calibrated using a reference alumina sample
(supplied by Quantachrome, USA). The samples for transmission
electron microscopy (TEM) analysis were prepared by drop casting
the catalyst material (which is already dispersed in isopropanol)
on a holey-carbon film supported by a 300 mesh copper TEM grid.
The analysis was performed using a FEI Tecnai F20 instrument
with a 200 kV field emission gun. The mean particle size of Au was
determined by inspection of several micrographs taken from var-
ious positions. The mean particle diameter (dav) was calculated
¯
eral faces {1010} of a layered double hydroxide compound (LDH)
in the oxidation of styrene by TBHP. Au25 clusters immobilized on
hydroxyapatite (HAP) showed best catalytic performance for epox-
idation of styrene in toluene medium [17]. Gold supported S- and
ionic liquid fragments-containing periodic mesoporous organic sil-
icas showed good oxidation activity with H2O2 as oxidant [18,19].
Development of easily scalable, efficient, epoxide selective and sta-
ble solid catalyst for styrene epoxidation is still a challenge. Further,
formed over Au during reactions are not fully understood.
Titanate nanotubes (TNT) are particularly attractive hosts for
catalytically active metal nanoparticles because of their well-
defined mesoporous one-dimensional structure, high specific
surface area and ion-exchange capability [20–23]. Their semicon-
ducting properties lead to strong support-metal interaction which
could influence the catalytic performance of metal nanoparticles.
We report here, for the first time, the catalytic activity of Au sup-
ported on barium titanate nanotubes (Au/BaTNT) in epoxidation of
styrene with different oxidants such as O2, H2 + O2, aqueous H2O2,
aqueous TBHP and 5.5 M TBHP in decane. The influence of reac-
tion parameters such as oxidant, solvent, Au loading (on BaTNT),
catalyst amount, substrate/oxidant mole ratio, temperature and
reaction time on the yield of styrene oxide is probed. The scope of
Au(1 wt.%)/BaTNT for oxidation of a range of substituted styrenes
and FT-Raman spectroscopies are used to investigate the reactive
oxygen species formed during the oxidation reaction. Earlier, we
reported the use of gold supported on TNT for alcohol oxidation
with molecular oxygen [24,25].
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using the formula: dav = 6 nid3/ nid2, where ni is the num-
ber of particles having a diameiter of dii. One hundred particles
were chosen to determine the mean diameter of gold particles.
A Shimadzu UV-2700 spectrophotometer equipped with an inte-
grating sphere attachment (ISR 2200) was used to acquire diffuse
reflectance UV–vis (DRUV-vis) spectra of the powder samples.
BaSO4 was used as a reference standard. The DRUV-vis spectra were
recorded for bare BaTNT and Au/BaTNT and for the samples con-
tacted with a known quantity of solvent (H2O, acetonitrile—ACN),
oxidant (H2O2, TBHP) and/or styrene. FT-Raman spectra of the cata-
lysts were recorded in the range 200–1500 cm−1 using a Horiba JY
LabRaman HR 800 Micro Raman spectrometer with 630 nm exit-
ing energy generated by a He-Ne laser operating at 20 mV. The
strength and density of basic sites in the catalysts were deter-
mined by temperature-programmed desorption (TPD) studies on
a Micromeritics Auto Chem 2910 instrument using CO2 as probe
molecule. In a typical experiment, 0.1 g of the catalyst was taken in
a U-shaped quartz sample tube. The catalyst was pre-treated in He
(30 ml/min) at 250 ◦C for 1 h, then it was cooled to 25 ◦C and a mix-
ture of CO2 in He (10 vol%) was fed to the sample (30 ml/min) for
1 h. Further, the sample was flushed with He (30 ml/min) for 1 h at
100 ◦C. Baseline was checked for stability before acquiring the data
points in the temperature range of 100–500 ◦C. The area of the des-
orption peaks gave the amount of basic sites present in the catalysts.
Acidity of the samples was estimated in a similar manner using
NH3 instead of CO2 as probe molecule. X-ray photoelectron spectra
(XPS) was recorded on a VG Microtech Multilab ESCA 3000 spec-
trometer with Al-K␣ radiation (hv = 1486.6 eV). The binding energy
scale was referenced to the C 1s line at 284.6 eV.
2. Experimental
Titanium dioxide (98%, anatase TiO2), sodium hydroxide (NaOH)
and barium nitrate were obtained from Thomas Baker Chemicals
Ltd., Chloroauric acid (HAuCl4·3H2O) was procured from HiMedia
Chemicals Ltd. All reagents were used as received.
2.1. Catalyst preparation
BaTNT was prepared by ion-exchanging preformed sodium
titanate nanotubes (NaTNT) with Ba2+ ions. In a typical prepara-
tion, 2.5 g of NaTNT (dried at 100 ◦C for 2 h) was added to 120 ml of
aqueous Ba(NO3)2 solution (0.5 M) taken a beaker. The suspension
was heated to 80 ◦C and stirred for 8 h. The solid was separated and
the same procedure was repeated two more times. Then, the iso-
lated solid was washed with deionized water and dried at 110 ◦C
overnight to obtain BaTNT. The sodium form of titanate nano-
tubes (NaTNT) were prepared by alkali treatment of anatase titania
[24–26].
In a typical preparation of Au/BaTNT, an appropriate amount
of 2 mM aqueous solution of HAuCl4·3H2O was added drop-wise
to 1 g of BaTNT suspended in 100 ml of deionized water followed
by vigorous stirring at 80 ◦C for 12 h while keeping in dark. The
solid formed was filtered, washed with deionized water and dried
2.3. Reaction procedure
Epoxidation of styrene over Au/BaTNT catalysts was carried
out at atmospheric pressure taking 0.05 g of catalyst, 10 mmol of
styrene, 10 mmol of anhydrous TBHP (5.5 M in decane) or 70% aque-
ous TBHP or 30% aqueous H2O2 in a glass reactor (25 ml) placed
in an oil bath at 80 ◦C and fitted with a water-cooled reflux con-
denser and magnetic stirrer. The reaction was conducted for 15 h.
After the completion of the reaction, the reactor was cooled to
room temperature and then the catalyst was separated by cen-