Effect of alkali and alkaline earth metal ions on benzyl alcohol oxidation activity of titanate nanotube-supported Au catalysts

Article abstract of DOI:10.1039/c5ra06496a

Sodium titanate nanotubes (NaTNTs) were prepared by alkali treatment of anatase titania. They were then ion-exchanged with alkali and alkaline earth metal ions to get ATNTs (A = Li⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺

Full text of DOI:10.1039/c5ra06496a

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Effect of alkali and alkaline earth metal ions on
benzyl alcohol oxidation activity of titanate
nanotube-supported Au catalysts†
ab
Cite this: RSC Adv., 2015, 5, 47740
Devadutta Nepakab and Darbha Srinivas
*
Sodium titanate nanotubes (NaTNTs) were prepared by alkali treatment of anatase titania. They were then
ion-exchanged with alkali and alkaline earth metal ions to get ATNTs (A ¼ Li+, K+, Cs+, Mg2+, Ca2+, Sr2+and
Ba2+). Gold (1–5 wt%) was supported on these nanotubes by a deposition–precipitation method and
investigated as a catalyst for the selective oxidation of benzyl alcohol with air/molecular oxygen (1 atm)
under solvent- and alkali-free conditions. Detailed characterization by X-ray powder diffraction, high
resolution transmission electron microscopy, N2-physisorption, diffuse reflectance UV-visible
spectroscopy, X-ray photoelectron spectroscopy and CO2-temperature-programmed desorption
techniques revealed that the basicity of the catalyst influences the uptake, mean particle size, electronic
properties and oxidation activity of the supported gold. Benzaldehyde formed with a selectivity of about
99%. The catalytic activity (turnover frequency) was found to have a direct relationship with the basicity
and an inverse relationship with the Au particle size. Among the catalysts investigated, Au/BaTNTs,
having higher basicity, smaller Au particles and higher metal dispersion, showed enhanced catalytic
activity than the other Au/ATNT catalysts. Pd addition to Au leading to Au–Pd/BaTNTs increased the
activity (TOF) but lowered the selectivity for benzaldehyde (80 wt%). Titanate nanotubes donate electron
density to Au particles, yielding electron rich Au ions, which are responsible for activating molecular
oxygen and oxidizing benzyl alcohol. Au/BaTNTs, having higher basicity and lower size Au nanoparticles
than the other Au/ATNT, activates molecular oxygen more easily and thereby enhances the catalytic activity.
Received 11th April 2015
Accepted 11th May 2015
DOI: 10.1039/c5ra06496a
www.rsc.org/advances
The catalytic performance of supported gold nanoparticles
(Au NPs) can be altered by controlling the particle size of Au, by
Introduction
choosing the right kind of support and by building strong
contact of gold with the support (through the metal–support
peripheral interface or via charge transfer from the support to
gold).5 The effect of strong metal–support interactions (SMSI)
was rst observed in TiO2-supported noble metal systems
reported by Tauster et al.5 in 1978. One of the early hypotheses
for this effect was that the alteration of the charge state of metal
by electron transfer from or to a support leads to an improve-
ment in the ability to activate reactants and thereby inuencing
the catalytic properties of the metal.6 This hypothesis was
conrmed experimentally by several researchers. Bruix et al.7
successfully interpreted the enhanced activity of platinum
particles supported on ceria in a water–gas shi reaction as the
“electronic metal–support interaction” (EMSI), a term that was
recently coined by Campbell.8 However, intrinsic metal effects,
such as electronic quantum size effect9 and structure–sensitivity
geometrical effect10 should be ruled out for a complete under-
standing of the EMSI effect. For supported metal particle/
cluster catalysts, complete elimination of intrinsic metal
effects is particularly difficult or even impossible. Milone et al.11
reported that the catalytic properties of supported gold catalysts
strongly depend on the type and structure of the support. From
Heterogeneous catalysis by gold is now a very fascinating
research topic. Previously it was ignored for a long time as bulk
gold is chemically inert. The eld of catalysis by nano-gold was
provoked by two contemporaneous discoveries made by
Hutchings1 and Haruta2 for acetylene hydrochlorination and
low-temperature CO oxidation, respectively. This led to several
other explorations and exploitations of nano-gold and gold
containing catalysts in oxidation, epoxidation, direct synthesis
of hydrogen peroxide, hydrogenation, coupling and water–gas
shi reactions through green chemistry practices.3 In partic-
ular, the selective liquid-phase oxidation of alcohols to the
corresponding carbonyl compounds using gold catalysts under
moderate conditions is highly desirable from economic and
environmental perspectives.3,4
aCatalysis Division, CSIR-National Chemical Laboratory, Pune 411 008, India. E-mail:
d.srinivas@ncl.res.in; Fax: +91 20 2590 2633; Tel: +91 20 2590 2018
bAcademy of Scientic and Innovative Research (AcSIR), New Delhi 110 001, India
† Electronic supplementary information (ESI) available: XRD, CO2-TPD proles,
DRUV-vis spectra and XPS proles of Au/ATNT, N2 physisorption of supports,
correlation proles of basicity versus uptake and particle size of Au and B.E.
values versus TOF and catalytic activity data. See DOI: 10.1039/c5ra06496a
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the comparative study of Fe2O3-, ZnO-, CaO-, and Al2O3-sup-
In the preparation of 1, 3 and 5 wt% Au supported on ATNTs,
ported gold catalysts, they found that the basicity and lattice an appropriate amount of
oxygen of supports have signicant effects on the catalytic HAuCl4$3H2O was added drop-wise to 1 g of ATNTs suspended
performance. in 100 mL of deionized water. The suspension was stirred
2 mM aqueous solution of
Recently, 1D nanoscale alkali titanates have attracted much vigorously for 12 h while maintaining the temperature at 80 ꢀC.
interest in heterogeneous catalysis as catalyst supports. In All of this operation was done in the dark or by covering the
particular, aer the simple alkaline hydrothermal synthesis, contents with aluminium foil. The solid obtained was ltered,
sodium titanate nanotubes (NaTNTs) reported for the rst time washed with deionized water and dried at 110 ꢀC for 20 h to
by Kasuga et al.12 have emerged as a promising functional obtain the gold loaded ATNTs. The materials were then reduced
material. The NaTNTs have well-dened mesopores with a wall in owing hydrogen (20 mL minꢁ1) at 250 ꢀC for 2 h to yield the
thickness of a few nanometers. They expose a large proportion nal catalysts.
of the Na+ counterions on the inner and outer surfaces of the
Au–Pd (2 wt%, 1 : 1)/BaTNTs were prepared in the same
nanotubes. These Na+ ions are ion-exchangeable.13 Ion- manner as described above by simultaneously adding the
exchange has been established as an effective method for required amounts of aqueous HAuCl4$3H2O (2 mM) and
preparing highly dispersed supported metal catalysts with a Pd(OAc)2 (2 mM) solutions to a BaTNT suspension.
narrow metal particle size distribution for use in heterogeneous
catalysis.14 Moreover, the semiconducting properties of titanate
nanotubes may lead to a strong electronic interaction between
the support and metal, which could improve the catalytic
Catalyst characterization techniques
performance in redox reactions. Specically, titanate nanotubes The metal loadings were determined by inductively coupled
have been used as a catalyst support for platinum catalyzed plasma-optical emission spectroscopy (ICP-OES; Spectro Arcos).
cyclohexene hydrogenation–dehydrogenation,15 iridium and Au/ATNTs were digested in aqua-regia. Standard solutions of
cobalt catalyzed water splitting for hydrogen production,16 gold known concentration were used for calibration. X-ray powder
catalyzed water–gas shi reaction17 and CO oxidation18,19 and diffraction (XRD) patterns of the catalyst samples were recorded
bimetallic gold–palladium catalyzed direct synthesis of immediately aer reduction in a H2 ow (250 ꢀC, 2 h) on a
hydrogen peroxide.20 Recently, we have reported the application Philips X’Pert Pro diffractometer using Cu-Ka radiation (l ¼
of Au and Au–Pd supported on NaTNTs for the oxidation of 0.15406 nm) and a proportional counter detector with a scan-
primary and secondary alcohols.21 In continuation of this work, ning angle (2q) of 5–80ꢀ at a scan speed of 4ꢀ minꢁ1. High
we report here a study of Au supported on various alkali and resolution transmission electron microscopy (HRTEM) images
alkaline earth metal ion-exchanged titanate nanotubes for were taken on a FEI Technai-F30 instrument with a 300 kV eld
benzyl alcohol oxidation. The inuence of the basicity of the emission gun. The specic surface area, pore volume and pore
support on the physicochemical characteristics and catalytic diameter of the catalysts were determined from N2 adsorption–
activity of gold is investigated.
desorption isotherms measured at ꢁ196 ꢀC using Quantach-
rome, USA (Autosorb-1C) equipment. Prior to N2 adsorption, the
ꢀ
samples were evacuated at 200 C for 2 h. A reference alumina
sample (supplied by Quantachrome, USA) was used to calibrate
the instrument. Diffuse reectance UV-visible (DRUV-vis) spec-
troscopic measurements of the powder samples were performed
Experimental
Catalyst preparation
Titanium dioxide (98% anatase TiO2), sodium hydroxide on a Shimadzu UV-2550 spectrophotometer equipped with an
(NaOH), and alkali and alkaline earth metal nitrates were integrating sphere attachment (ISR 2200); BaSO4 was used as
obtained from Thomas Baker Chemicals Ltd. Chloroauric acid the standard. Temperature-programmed desorption (TPD)
(HAuCl4$3H2O) was purchased from HiMedia Chemicals Ltd. studies were carried out on a Micromeritics Auto Chem 2910
and palladium acetate (Pd(OAc)2) was procured from Aldrich instrument using CO2 as the probe molecule to quantify the
Co. All reagents were used as received and without any further amount of basic sites. In a typical experiment, 0.1 g of the
purication.
catalyst was taken in a U-shaped, ow-through, quartz sample
NaTNTs were prepared as reported by us earlier.21 To tube. Prior to measurements, the catalyst was pre-treated in He
synthesize ion-exchanged titanate nanotubes, ATNTs (A ¼ Li+, (30 mL minꢁ1) at 250 ꢀC for 1 h. It was then cooled to 25 ꢀC and a
K+, Cs+, Mg2+, Ca2+, Sr2+ and Ba2+), 2.5 g of dried NaTNTs was mixture of CO2 in He (10 vol%) was fed to the sample (30 mL
suspended in 120 mL of 1.0 M aqueous solution of alkali and minꢁ1) for 1 h. Then, the sample was ushed with He (30 mL
alkaline earth metal nitrates. The suspension was stirred for 8 h minꢁ1) for 1 h at 100 ꢀC. Before starting the desorption analysis,
while maintaining the temperature at 80 ꢀC. The solid was the baseline was checked for stability. The CO2-TPD measure-
separated and the ion-exchange procedure was repeated ments were carried out in the temperature range of 100–500 ꢀC.
another two times. The solid isolated was washed with deion- The area of the desorption peaks gave the amount of basic sites
ized water, ltered and dried at 110 ꢀC overnight to obtain present in the catalyst. X-ray photoelectron spectra (XPS) of the
ATNTs. In the case of Cs+, Sr2+ and Ba2+ exchanged materials, samples were acquired on a VG Microtech Multilab ESCA 3000
the concentration of the nitrate solution used was 0.5 M instead with Al Ka radiation (hn ¼ 1486.6 eV). The peak corresponding
of 1.0 M, as solubility is an issue with these salts.
to carbon 1s (at 285 eV) was taken as the reference in estimating
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binding energy (BE) values of Au in the catalyst. The peak
position was determined with a precision of ꢂ0.2 eV.
Reaction procedure
Known quantities of catalyst (0.05 g) and benzyl alcohol (25
mmol) were charged into a triple necked, glass, round-bottom
ask (25 mL) and placed in a temperature controlled oil bath.
The reactor was connected with
a water-cooled reux
condenser, a molecular oxygen/air-lled rubber balloon and a
magnetic stirrer. The temperature of the reactor was raised to
80–120 ꢀC and the reaction was conducted for a specic time. It
was then cooled to 25 ꢀC and the catalyst was separated by
centrifugation. For identication and quantication of the
products GC-MS (Varian CP-3800; CP-Sil8CB—30 m ꢃ 0.25 mm
ꢃ 0.25 mm capillary column) and gas chromatography (Varian
3800; CP-8907—15 m ꢃ 0.25 mm ꢃ 0.25 mm column and a
ame ionization detector) techniques were employed.
Fig. 1 XRD patterns of BaTNTs and Au/BaTNTs.
Results and discussion
Structural and textural characterization
where Vm ¼ volume occupied by a Au atom (1.69 ꢃ 10ꢁ23 cc),
NaTNTs showed XRD peaks at 2q ¼ 10.1, 24.3, 28.4 and 48.5ꢀ
corresponding to reections from the (200), (110), (211) and
(020) planes, respectively (JCPDS les: 31-1329 and 41-0192).
The ion-exchanged and Au loaded samples depicted similar
XRD patterns indicating that they all have the same X-ray crystal
structure as the NaTNTs. The diffraction peak for the (200)
plane had, however, marginally shied to a lower 2q value (by
0.3ꢀ) when the cation in the ATNTs changed from Li+ to Ba2+.
This shi in XRD peak position is due to an increase in the
interplanar spacing with the increasing size of the counterion.
But the positions of the other peaks remained nearly the same.
Only representative XRD patterns of BaTNTs and Au (1–5 wt%)/
BaTNTs are shown in Fig. 1, while the patterns for all the other
catalysts are deposited in the ESI.† No additional peaks due to
Au were detected in 1 wt% Au deposited samples (except for Au/
CaTNTs) indicating that the size of the Au particles in all of
those catalysts is below the detection limit of X-rays. However, at
higher loadings (ca., 3 and 5 wt%), additional peaks at 38.2ꢀ and
44.6ꢀ corresponding to Au (111) and Au (200) were observed
(Fig. 1) and decrease in the intensity of (200) [relative to (020)]
was noted. This is due to the blockage of inter-planar spacing
with Au nanoparticles or structural deformation as a conse-
quence of strong interactions of Au with the support titanate
nanotubes.
am ¼ area occupied by a Au atom (0.869 ꢃ 10ꢁ15 cm2), dav
¼
mean particle size of Au (HRTEM), Vi ¼ volume of the ith particle
(HRTEM) and Ai ¼ surface area of the ith particle (HRTEM). The
mean particle size and percentage dispersion of Au estimated
from the HRTEM images are listed in Table 1. Wherever the
characteristic peaks due to Au were observed in the XRD
proles, the average crystallite size of Au was determined using
the Debye–Scherrer formula and reported in Table 1. The crys-
tallite (XRD) and particle sizes (HRTEM) of Au (1 wt%)/CaTNTs
are comparable. The value of dav for different Au (1 wt%)/ATNTs
is in the range 5.2–11.6 nm and the Au dispersion is between 10
and 20%.
The chemical composition (Au, alkali/alkaline earth metal
ion and Na+ content) of the Au/ATNT catalysts was determined
by ICP-OES (Table 1). The Na/Ti mole ratio of “bare” NaTNTs
was 0.65, which is nearly equal to the theoretical value of 0.67
conrming the molecular formula of NaTNTs as Na2Ti3O7. Ion-
exchange with other alkali/alkaline earth metal ions occurs
along the length of the nanotube. Na+ ions in the interlayer
region were difficult to exchange and hence, remnants of non-
exchangeable Na+ may be noted in the compositions of Au/
ATNTs (Table 1).
Representative N2 adsorption–desorption isotherms along
with pore size distribution curves of BaTNTs and Au (1 wt%)/
BaTNTs are shown in Fig. 3. These materials showed N2 phys-
isorption isotherms typical of type IV with a H2-hysteresis loop.
The isotherms represent mesoporosity with an open-ended pore
structure. The samples showed broad pore size distribution
curves. The size of the alkali/alkaline earth metal ion has an
inuence on the textural properties. The SBET of Au/ATNTs was
in the range 122–182 m2 gꢁ1 and the average pore diameter was
between 4.1 and 5.6 nm (Table 1). Bare supports have a SBET
between 132 and 196 m2 gꢁ1 and an average pore diameter
between 4.5 and 6.6 nm (ESI†). A marginal decrease in the
specic surface area and average pore diameter is thus noted
HRTEM images and Au particle size distribution curves of Au
(1 wt%)/ATNTs are shown in Fig. 2. These images conrmed the
presence of a hollow tubular multi-walled morphology of the
support titanate nanotubes and the Au nanoparticles on the
interior and outer surfaces of the nanotubes. Metal dispersions
(D) were determined using eqn (1) and (2),
6Vm
D ¼
(1)
amdav
P
6
niVi
P
dav
¼
(2)
niAi
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Fig. 2 HRTEM images and particle size distribution histograms of Au (1 wt%)/ATNT catalysts.
upon loading Au. A decrease in the total pore volume of Au/ showed an additional weak overlapping peak at 250–300 ꢀC
ATNTs compared to the supports was also observed.
which is missing in the other ATNT samples.
Carbon dioxide was used as a probe molecule to determine
Among alkali ion exchanged Au/ATNT catalysts the uptake of
the basicity of the Au/ATNT catalysts. The TPD proles (ESI†) Au (Au content in the catalyst) decreased with increasing
contained two desorption peaks, in the temperature regions basicity of the catalyst. A similar observation was found also for
ꢀ
ꢀ
100–300 C and 300–500 C corresponding to weak and strong alkaline earth metal ion exchanged Au/ATNT catalysts. In
basic sites, respectively. The desorption temperature and general, the percentage Au uptake (with respect to the input
amount of CO2 desorbed refer to the strength and density of the value) is higher for alkaline earth metal ion than alkali ion
basic sites. The basicity of the catalysts is reported in Table 1. In exchanged ATNTs. Also a decrease in the mean size (dav) of Au
general, the alkaline earth metal ion exchanged titanates have with increasing basicity of the catalyst was observed (ESI†) with
higher basicity than the alkali metal ion exchanged catalysts. It the values of Au supported on Cs+, Mg2+ and Ca2+-exchanged
appears that the amount of ions exchanged also determines the NaTNTs deviating from this relationship. The reason for this
basicity value. The basicity of the catalysts inuenced the abnormal behaviour is not clear at this point of time. However,
uptake and particle size of Au (ESI†). Au (1 wt%)/NaTNTs it appears that the strength of basicity and relative concentra-
tion of strong basic sites may inuence the metal particle size.
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Table 1 Chemical composition, textural properties and basicity of Au/ATNT catalysts
Chemical compositiona
(wt%)
Textural propertiesb
Pore
Basicityd (mmol mꢁ2
)
Pore
Au particle/
Au
Alkaline
ion
SBET
volume diameter crystallite
dispersionc
S. no. Catalyst
Au
Na+ (m2 gꢁ1
)
(cc gꢁ1
)
(nm)
sizec (dav, nm) (%)
Weak Strong Total
1
2
3
4
5
6
7
8
Au (1 wt%)/LiTNTs
0.62
0.65
0.55
1.25
3.33
0.52
2.21
—
0.92 178
0.56
0.50
0.45
0.41
0.38
0.32
0.41
0.50
0.44
0.39
0.36
0.37
0.36
0.35
0.37
5.1
5.6
5.5
5.3
5.1
4.1
5.6
5.4
5.5
5.1
5.0
5.4
5.3
5.1
5.2
11.6
7.2
10
16
18
—
—
11
12
11
19
—
—
20
—
—
25
0.46
0.43
0.43
0.40
0.37
0.45
0.21
0.28
0.25
0.36
0.41
0.21
0.32
0.42
0.66
0.80
0.87
1.05
1.13
0.98
1.72
2.12
2.84
2.82
1.9
1.78
2.79
2.07
1.72
1.79
1.26
1.29
1.70
1.53
1.35
2.17
2.19
3.12
2.75
2.26
2.19
3.00
2.39
2.06
2.45
Au (1 wt%)/NaTNTs
Au (1 wt%)/KTNTs
Au (3 wt%)/KTNTs
Au (5 wt%)/KTNTs
Au (1 wt%)/CsTNTs
7.2
182
9.90
9.32
8.80
16.42
2.50
4.22
9.21
6.80
5.35
13.81
12.00
9.26
0.61 153
0.36 135
0.22 122
6.3
7.3e
15.4e
10.0
9.9
2.6
126
Au (1 wt%)/MgTNTs 0.90
0.62 165
0.61 175
0.30 173
0.26 166
0.27 152
0.42 155
0.36 146
0.28 140
0.30 149
Au (1 wt%)/CaTNTs
Au (1 wt%)/SrTNTs
Au (3 wt%)/SrTNTs
Au (5 wt%)/SrTNTs
Au (1 wt%)/BaTNTs
Au (3 wt%)/BaTNTs
Au (5 wt%)/BaTNTs
0.91
0.72
2.40
3.30
0.68
2.80
4.10
10.5 (10.0e)
5.4
9
10
11
12
13
14
15
5.8e
8.7e
5.2
5.5e
6.4e
4.2
Au–Pd (1 : 1, 2 wt%)/ 0.70 (0.82f) 11.82
BaTNTs
a
ICP-OES. Alkaline ion means A+ ion in ATNTs. b N2-physisorption. c HRTEM. d CO2-TPD; weak (100–300 ꢀC) and strong (300–500 ꢀC). e Crystallite
f
size from XRD. Value in parentheses is the Pd content.
> SrTNTs > KTNTs. Deconvolution of the XPS lines in the Au
binding energy region (ESI†) revealed the presence of another
set of signals at higher BE values but with lower intensity
assignable to Aud+ ions. Pusztai et al.23 reported that, on titanate
nanowires, gold atoms can occupy ion exchange positions at a
lower loading. They observed a higher portion of Au+/Au0 on the
catalyst aer reduction with H2 compared to reduction with
NaBH4. This may be a reason for the presence of a small
quantity of Aud+ species in ATNT samples. There are reports,26,27
which argue that hydride transfer is involved in C–H bond
activation, and the cationic gold is catalytically active. Abad
et al.27 have investigated the active sites on Au/CeO2. Recently,
Zhao et al.28 have also reported the active sites on Au/Ni2O3
involved in the oxidation of alcohol. They showed a direct
correlation between the concentration of Au+ or Au0 species and
catalyst supports. The Au+/Au0 ratio in the present catalysts was
determined (Table 2). The lower the BE value, the higher the
catalytic activity was over the Au/ATNT catalysts (ESI†). For Au–
Pd (2 wt%, 1 : 1)/BaTNTs, for the core level XPS spectrum the Au
4f7/2 line appeared at 82.7 eV (ESI†). The weak intensity of these
Au 4f lines is indicative of the location of Au inside the titanate
nanotubes and masking of the Au surface by Pd particles.
Smaller particle sizes can also lead to lowering in BE values. The
spectral lines for Pd0 appeared at 334.8 eV (3d5/2) and 340.0 eV
(3d3/2).
Spectral characterization
DRUV-vis spectroscopy provided evidence for the presence of Au
nanoparticles by showing a typical localized surface plasmon
resonance (LSPR) band at 480–580 nm (ESI†). The peak position
of the LSPR band (lmax) is related to the Au mean particle size. It
is related to the shape of the Au particle and dielectric function
of the support material. As seen from Fig. 4, lmax of the LSPR
band increased with the increasing particle size of Au. We have
already noted that the particle size of Au has an inverse relation
with the basicity of the catalyst. In other words, the spectral/
electronic properties of Au are greatly inuenced by the
nature of the support and Au particle size. Similar observations
were made also by other researchers.22
Au (1 wt%) supported on KTNTs, SrTNTs and BaTNTs
showed a 4f7/2 line in the XPS spectra at 83.2, 83.0 and 82.8 eV,
respectively, metallic gold in gold foil shows this line at 84.0
eV.23 This suggests that Au on ATNTs is rich in electron density.
The formation of such electron rich Au species was attributed to
occur through transfer of the electron density from the
support.23,24 Arrii et al.25 have reported the dependence of Au 4f7/
binding energy on the nature of the support. On TiO2 and
2
Al2O3, the BE of Au 4f7/2 showed a negative shi by ꢁ0.6 and
ꢁ0.9 eV, respectively. A large shi up to ꢁ1.1 eV was observed
for the spent Au/TiO2 catalyst. When the cations were changed
from K+ to Sr2+ and Ba2+ in the ATNTs, the BE of the Au signals
shied systematically downward (Table 2). This observation
indicates that as the basicity of the support increased, the Fermi
level of the metal particles shied to a lower binding energy.
Thus, the XPS spectra point out electron transfer from ATNTs to
Au nanoparticles, the extent of which follows the order: BaTNTs
Catalytic activity
The catalytic activity of Au supported on ATNTs for benzyl
alcohol oxidation with molecular oxygen as the oxidant was
studied at 120 ꢀC and under solvent- and alkali-free conditions
(Table 3). Benzaldehyde was the selective product (selectivity ¼
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Table 2 XPS data (binding energy values in eV) of Au/ATNTs catalysts
þ
Au0
Aud+
4f7/2
Aud
Au0
Catalyst
4f7/2
4f5/2
4f5/2
Au (1 wt%)/KTNTs
Au (1 wt%)/SrTNTs
Au (1 wt%)/BaTNTs
Au–Pd (1 : 1, 2 wt%)/BaTNTs
0.28
0.27
0.43
—
83.2
83.0
82.8
82.7
86.9
86.7
86.5
—
85.2
85.2
85.3
85.4
88.9
88.9
89.0
—
3). Au (1 wt%) supported on Ba2+ ion-exchanged NaTNTs [Au (1
wt%)/BaTNTs] showed the highest catalytic activity [benzyl
alcohol conversion ¼ 43 wt% and TOF ¼ 622 hꢁ1 (based on the
total metal content) and 2237 hꢁ1 (based on the exposed surface
metal atoms)]. This reaction occurred also with air instead of
molecular oxygen over Au (1 wt%)/BaTNTs but then the
conversion under the same reaction conditions was 24.6 wt%
with a benzaldehyde selectivity of 96.5 wt%. The support
(BaTNTs) alone could not catalyze this reaction to a notable
extent (benzyl alcohol conversion ¼ 2.8 wt%). The gold nano-
particles were smaller in size (dav ¼ 5.2 nm) and highly
dispersed (metal dispersion ¼ 20%) on BaTNTs than on other
supports (Table 1). The smaller the particle size, the higher the
number of exposed surface active sites and interaction with the
support and the higher the catalytic activity. The catalytic
activity of Au (1 wt%) supported on KTNTs, SrTNTs and BaTNTs
with almost similar Au particle sizes followed the order: Au (1
wt%)/BaTNTs > Au (1 wt%)/SrTNTs > Au (1 wt%)/KTNTs (Table
3, run no. 3, 9 and 12a). This trend in catalytic activity parallels
the variation in electron transfer from the support to the Au
particles. With a view to see the inuence of the addition of Pd
on the catalytic activity of Au, we have prepared a Au–Pd (2 wt%,
1 : 1)/BaTNT composition. Addition of Pd decreased the mean
particle size of Au from 5.2 to 4.2 nm and the metal dispersion
increased from 20 to 25%. Au–Pd/BaTNTs showed a remarkably
higher catalytic activity than the catalyst without Pd (benzyl
alcohol conversion ¼ 92.3 wt% as against 43 wt%). But the
benzaldehyde selectivity dropped from 99 to 80.5 wt%. Note
that a conversion of 92.3% was achieved on Au–Pd in just 5 h
instead of 10 h over the Au catalysts. Hence, Au supported on
NaTNTs exchanged with larger cations exhibited higher cata-
lytic activity due to the higher amount of eꢁ density at Au in
those catalysts, which could easily activate molecular oxygen
and enable the oxidation of benzyl alcohol.29 Hsu et al.14
observed such electronic effects on Pt catalysts with similar
supports. Fig. 5 shows an interesting correlation between the
basicity and mean particle size/catalytic activity (TOF). The
particle size of Au decreased with increasing basicity (basic sites
per unit surface area) of the catalyst while the TOF showed an
increasing trend. In other words, the basicity of the support,
mean particle size of Au and catalytic activity are inter-related to
each other. The points related to Au (1 wt%) on CaTNTs and
MgTNTs have fallen away from this relationship because there
could be other factors like the strength of basicity that would
inuence the values of particle size and TOF. Fig. 6 depicts the
variation of the oxidation activity of Au as a function of its
Fig. 3 N2-physisorption isotherms (top) and pore size distribution
curves (bottom) of BaTNTs and Au (1 wt%)/BaTNTs.
Fig. 4 Variation of LSPR band position (lmax) with the mean particle
size (dav) of supported Au.
99 wt%). While the support had little effect on benzaldehyde
selectivity, a remarkable inuence of it on the benzyl alcohol
conversion and turnover frequency (TOF) was observed (Table
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particle size. Catalytic activity increased with decreasing Au
particle size. It is interesting to note that all the catalysts have
followed this trend unlike that observed in Fig. 5. This claries
that other than the density, the strength of basicity plays a
crucial role. With increasing Au content from 1 to 5 wt% on
BaTNTs, the conversion of benzyl alcohol increased from 43 to
69.7 wt% but the selectivity for benzaldehyde decreased from 99
to 95.2 wt% (Table 3). An increase in activity with increasing Au
content was observed also with KTNT and SrTNT supports.
Effect of the reaction conditions
The catalytic activity increased with increasing reaction time
(ESI†). Only a marginal drop in benzaldehyde selectivity was
noted at higher conversions. Also an increase in activity with the
amount of catalyst was noted (ESI†). When the reaction
temperature was raised from 80 to 100 and then to 120 ꢀC,
benzyl alcohol conversion on Au (1 wt%)/BaTNTs increased
from 30.2 to 36.3 and then to 43 wt%. This increase was from 8.4
to 15.2 and then to 17 wt% over Au (1 wt%)/NaTNTs. The
selectivity for benzaldehyde was 99 wt% at all of those condi-
tions (ESI†).
Fig. 5 Variation of the mean particle size and TOF with the basicity for
Au (1 wt%)/ATNTs. Reaction conditions: catalyst ¼ 0.05 g, benzyl
alcohol ¼ 25 mmol, p (O2) ¼ 1 atm, reaction temperature ¼ 120 ꢀC, and
reaction time ¼ 10 h.
Catalyst reusability
detected. To conrm the heterogeneity of the oxidation reaction
over Au (1 wt%)/BaTNTs, the reaction at 120 ꢀC was stopped
aer 2 h (when the conversion was 17.2 wt%), the catalyst was
taken out and the reaction was continued without the catalyst
for another 8 h and the reaction mixture was analyzed by GC. No
detectable change in benzyl alcohol conversion was observed
(ESI†) suggesting that the reaction was heterogeneous in
nature.
Fig. 7 displays the reusability of Au (1 wt%)/BaTNTs in benzyl
alcohol oxidation. Aer the rst run, the catalyst was separated
from the reaction mixture by ltration, washed with water, dried
at 110 ꢀC for 4 h and then, reused in the next recycle conducted
at the same conditions. Such recycles were done ve times. The
catalyst was recyclable. The XRD pattern of the spent catalyst is
nearly the same as that of the fresh one (Fig. 1). ICP-OES anal-
ysis of the spent catalyst showed no loss of Au content but a
little loss of Ba2+ ions (0.6 wt%) at the end of the 5th recycle was
Table 3 Catalytic activity data for the oxidation of benzyl alcohol over Au/ATNT catalystsa
Run no.
Catalyst
Conversion (wt%)
TOF (hꢁ1
)
Aldehyde selectivityb (wt%)
1
2
3
4
5
6
7
8
Au (1 wt%)/LiTNTs
Au (1 wt%)/NaTNTs
Au (1 wt%)/KTNTs
Au (3 wt%)/KTNTs
Au (5 wt%)/KTNTs
Au (1 wt%)/CsTNTs
Au (1 wt%)/MgTNTs
Au (1 wt%)/CaTNTs
Au (1 wt%)/SrTNTs
Au (3 wt%)/SrTNTs
Au (5 wt%)/SrTNTs
Au (1 wt%)/BaTNTs
Au (1 wt%)/BaTNTs
Au (3 wt%)/BaTNTs
Au (5 wt%)/BaTNTs
Au–Pd (1 : 1, 2 wt%)/BaTNTs
7.2
17.0
21.9
34.1
45.3
8.8
11.6
13.4
37.8
46.3
54.1
43.0
24.6
62.5
69.7
92.3
114 (852)
99.0
99.0
99.0
98.2
93.6
99.0
99.0
99.0
99.0
96.2
94.1
99.0
96.5
97.0
95.2
80.5
257 (1153)
392 (1583)
268
134
166 (1068)
127 (805)
147 (976)
516 (1791)
190
9
10
11
12a
12b
13
14
15
161
622 (2237)
356 (1280)
220
167
819 (4653)
a
Reaction conditions: catalyst ¼ 0.05 g, benzyl alcohol ¼ 25 mmol, p (O2) ¼ 1 atm, reaction temperature ¼ 120 ꢀC, and reaction time ¼ 10 h,
turnover frequency (TOF) ¼ moles of benzyl alcohol converted per mole of Au in the catalyst (ICP-OES) per hour. TOF values in parentheses are
those calculated based on moles of benzyl alcohol converted per mole of surface metal atoms (estimated from HRTEM) per hour. Run no. 12a
is performed with molecular oxygen. Run no. 12b was conducted with air. For run no. 15, reaction time ¼ 5 h and the products include
benzaldehyde (80.5%), benzoic acid (1.0%), benzyl benzoate (10.2%), benzene (6.2%) and toluene (2.1%). b Balance selectivity is benzyl benzoate.
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Fig. 8 Tentative mechanism for the oxidation of benzyl alcohol over
Au/ATNTs.
Fig. 6 Correlation between the turnover frequency (TOF) and mean
particle size of Au. Reaction conditions are the same as in Fig. 5.
reaction mechanism is shown in Fig. 8. The particle sizes of Au
decreased with increasing basicity of the support while the
catalytic activity had increased (Fig. 5). The higher the basicity,
the higher the activation of benzyl alcohol and the higher the
conversion. At the same time, the lower the particle size of Au,
the higher the available surface area and support–Au interac-
tion and the higher the activation of molecular oxygen and
thereby, the higher the catalytic activity. Thus, basicity has a
direct (by way of activating benzyl alcohol) and indirect (by way
of support–metal interaction) inuence on the catalytic activity.
The mean particle size of Au on different supports decreased in
the order: Au/BaTNTs < Au/SrTNTs < Au/KTNTs. The smaller the
particle size the higher the SMSI effect, and there would be
more peripheral Au that are in the d+ state and hence, the
higher the hydride ion abstraction and thereby, the higher the
catalytic activity. This behavior is clearly evident from the XPS
and catalytic activity data.
Fig. 7 Recyclability of Au (1 wt%)/BaTNTs in the oxidation of benzyl
alcohol with molecular oxygen. Reaction conditions are the same as in
Fig. 5.
Conclusions
Gold particles supported on alkali and alkaline earth metal ion-
exchanged titanate nanotubes (ATNTs) were synthesized, char-
acterized and their catalytic activities for benzyl alcohol oxida-
tion with molecular oxygen/air were investigated. Gold particles
on ATNTs have a mean diameter in the range of 5.2–11.6 nm
and a Au dispersion in the range of 10–20%. The oxidation of
benzyl alcohol formed benzaldehyde with a selectivity of 99
wt%. Alkali and alkaline metal ion exchange had a marked
effect on the catalytic activity of titanate nanotubes. Among the
catalysts investigated Au supported on BaTNTs showed the
highest catalytic activity. The basicity of the support has an
inuence on the mean particle size and catalytic activity of Au.
With increasing basicity, the particle size of Au had decreased
while the activity of the catalyst had increased. This study
teaches that the catalytic activity of Au can be enhanced by
altering the titanate support through exchange with Ba2+ ions.
Since Au draws electron density from the basic support, it can
activate molecular oxygen more easily forming a superoxide like
The electrooxidation of ethanol over Au supported on glassy
carbon requires a partial coverage of the Au surface with its
oxide species. In CO oxidation, it was found that the oxidation
activity increases with an increase in the concentration of the
Au+ species; but the presence of metallic Au (electron rich) was
essential. According to these reports, the active sites consist of
an ensemble of metallic Au atoms and a cationic Au+
species.30–32 In the present case, benzyl alcohol is activated on
the support surface (as a consequence of its basic nature). Then
the abstraction of a hydride ion is favoured at the metal–
support interface (at coordinatively unsaturated sites/Au+ ions).
The electron rich Au nanoparticles activate molecular oxygen to
produce activated oxygen species, which help in removing H
from AuH species to produce water as the by-product. Mean-
while, the benzyloxy cation releases a proton and forms the nal
product benzaldehyde.33,34 A schematic representation of the
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47748 | RSC Adv., 2015, 5, 47740–47748
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