XAFS, XPS characterization of cerium promoted Ti-TUD-1 catalyst and it's activity for styrene oxidation reaction

Article abstract of DOI:10.1016/j.catcom.2013.11.027

Titanosilicate and many others transition metals deposited mesoporous silica materials are well recognized oxidation catalysts in different types of oxidation reactions, such as propylene oxidation, styrene oxidation, CO oxidation, etc. To increase the el

Full text of DOI:10.1016/j.catcom.2013.11.027

Catalysis Communications 46 (2014) 123–127
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
XAFS, XPS characterization of cerium promoted Ti-TUD-1 catalyst and it's
activity for styrene oxidation reaction
Sandip Mandal a, Sumbul Rahman a, Rawesh Kumar a, Kyoko K. Bando b, Biswajit Chowdhury a,
⁎
a
Department of Applied Chemistry, Indian School of Mines, Dhanbad, India
b
Nanosystem Research Unit, National Institute of Advanced Industrial Science & Technology (AIST), Tsukuba, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 August 2013
Received in revised form 23 November 2013
Accepted 27 November 2013
Available online 12 December 2013
Titanosilicate and many others transition metals deposited mesoporous silica materials are well recognized
oxidation catalysts in different types of oxidation reactions, such as propylene oxidation, styrene oxidation, CO
oxidation, etc. To increase the electrophilic character of Ti-site in TUD-1 framework cerium was incorporated
during the synthesis of Ti-TUD-1 material. The synthesized material was characterized by N2 adsorption–
desorption isotherm, XPS, XANES and EXAFS techniques. From XPS result it is found that all Ti is present in
framework position by the formation of Ti\O\Si bond in Ti-TUD-1 matrix. From EXAFS experiment it is
found that after cerium doping, the electrophilic character of the Ti site was increased resulting more styrene
oxidation activity.
Keywords:
Titanosilicate
Ti-TUD-1
Cerium
© 2013 Elsevier B.V. All rights reserved.
Styrene
Oxidation
1. Introduction
tetraethylorthosilicate (TEOS, Acros) was mixed with titanium (IV)
butoxide (Acros) under continuous stirring condition followed by the
Styrene oxidation is an important reaction on different aspects
because styrene oxide and benzaldehyde are important raw material
in pharmaceutical, agrochemical and cosmetic manufacturing industry
[1–3]. To demonstrate an atom efficient route several heterogenous
catalysts like doped silica, titanosilicates and heteropoly acids have
been used as found in the literature [4–6]. It is well evidenced in the
literature that titanium form the Ti\OOH species in the presence of O2
and H2O2 and this hydroperoxo species is the active species towards
the oxidation of small alkenes and styrene results into oxygenated
products [7–10]. Therefore, an increase in the electrophilicity of Ti site
will have a promotional effect in forming more Ti\OOH with H2O2
resulting better catalytic activity towards styrene oxidation. In the
present study it is assumed that small amount of cerium will improve
the electrophilicity of Ti center of Ti-TUD-1 material enhancing the
performance of Ti-TUD-1 catalysts. The cerium doped and undoped
Ti-TUD-1 catalysts are characterized by N2 adsorption–desorption iso-
therm, XPS, XANES and EXAFS techniques to correlate the activity of
the catalysts with the characterization results.
addition of triethanolamine (TEA, Acros) and water. Then, cerium
nitrate hexahydrate (Aldrich) in s`olid form and tetraethylammonium
hydroxide (TEAOH) (Merck Germany) were added to the mixture. The
whole mixture in the ratio of TEOS:Ti (OBu)4:TEA:H2O:Ce(NO3)3·6H2O:
TEAOH = 1:0.03:2:11:0.005:1 was stirred for 24 h at room temperature.
After stirring the prepared gel was dried at 110 °C for 24 h. Finally the
dried material was taken and calcined at the 700 °C for 10 h with a tem-
perature increasing rate 1 °C/min.
2.2. Catalyst characterization
2.2.1. N2 adsorption–desorption isotherm
The nitrogen adsorption–desorption isotherm of the sample was mea-
sured at liquid nitrogen temperature with a Quantachrome Autosorb-1C-
TPD at 77 K. Pre-treatment of the samples were done at 473 K for 3 h
under high vacuum. The surface area was determined by Brunauer–
Emmett–Teller (BET) equation.
2. Experimental
2.2.2. X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) was obtained with a JPC-
9010MC (JEOL) spectrometer with Al Kα radiation (1486.2 eV). The
energy was calibrated with a C 1s peak. The pass energy was 10 eV for
all the samples.
2.1. Preparation of cerium doped Ti-TUD-1 catalyst
Doped Ti-TUD-1 catalyst was prepared by the conventional sol–
gel procedure which is described earlier [11,12]. In this procedure
2.2.3. XAFS (X-ray absorption fine structure) spectroscopy
XAFS measurements were carried out at beam line 9C of Photon
Factory (PF), in Institute of Materials Structure Science (IMSS), High
⁎
Corresponding author. Tel.: +91 326 2235663; fax: +91 326 2296563.
E-mail address: biswajit_chem2003@yahoo.com (B. Chowdhury).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.11.027

124
S. Mandal et al. / Catalysis Communications 46 (2014) 123–127
Table 1
Energy Accelerator Research Organization (KEK) in Japan, and at beam
line 14b2 at SPring 8 in Japan. The collected data were analyzed by the
software program REX2000 (Rigaku Co.). Curve fitting analysis was
carried out with parameters obtained with FEFF8 (Univ. of Washington).
Textural properties of prepared catalysts.
Catalyst
Surface area
Pore diameter
(nm)
(m2/g)
Ti-TUD-1
507.24
563.1
6.5
6.0
Ce–Ti-TUD-1
(0.1 mol% Ce)
Ce–Ti-TUD-1
(0.5 mol% Ce)
2.3. Styrene oxidation reaction by hydrogen peroxide
460.0
5.7 & 8.6
Selective oxidation of styrene was carried out in a glass reactor fitted
with a water condenser. The reaction mixture was prepared with
10 mmol styrene (Acros), 0.2 ml dodecane (Acros, internal standard)
10 mmol 30% H2O2 (Merck India) along with 10 ml DMF solvent
(Merck India) in which 0.1 g catalyst was added. The reaction was
carried out at 80 °C temperature for 24 h with vigorous stirring. After
completion of the reaction, the reaction mixture was centrifuged and
supernatant liquid was analyzed by GC (CIC-India) using SE-30 column.
in cerium doping on Ti-TUD-1 material the area of the corresponding
peak was also increases (Table 2). As XPS is a surface sensitive technique
so it gives an idea about the surface enrichment of Ti 4+ site. This surface
enrichment of Ti 4+ would probably increase the electrophilic active
sites on catalyst surface. From the relationship between the binding
energy and Ti composition on the surface it can be concluded that tita-
nium mainly present in the tetrahedral framework position on the TUD-
1 matrix and no aggregation occurred on the surface of the catalyst [14].
Fig. 3. shows the O (1s) core level XPS spectra of Ti-TUD-1 and ceri-
um doped Ti-TUD-1 material. In this case an intense peak appeared at
534.2 eV. Indirect evidence for the high dispersion of titanium in the
silica matrix comes from the observation of a single O (1s) peak at
534.4 eV. If Ti 4+ species are not dispersed on the outer silica surface,
two distinguished oxygen components one at 532.9 eV (for Si\O\Si
bonds) and another at 530.6 eV (for Ti\O\Ti bonds) could be observed.
The appearance of one intense peak indicates that the Ti is highly dis-
persed on TUD-1 matrix and there might be the formation of Ti\O\Si
bond [13] in this case. The electro-negativity of Si is more (Pauling
electro-negativity 1.8) compare to Ti (Pauling electro-negativity
1.5). Therefore the difference of electro-negativity of Ti\O bond is
more compare to Si\O bond in Ti\O\Si bridge. As the difference
of electro-negativity of Si\O bond in Ti\O\Si bridge is low so Si\O
bond is more polarizable compare to Ti\O bond. Therefore the photo-
electrons created at the silicon–oxygen interface are more efficiently
screened compare to titanium–oxygen and silicon–oxygen interface in
Ti\O\Ti and Si\O\Si bridge [15]. Thus the shift in the position of
the O1s line toward higher binding energy.
3. Result and discussion
3.1. N2 adsorption–desorption isotherm
The N2 adsorption–desorption isotherm and pore size distribution of
Ti-TUD-1 catalyst are shown in Fig. 1. The N2 adsorption–desorption
isotherms of the Ti-TUD-1 and cerium doped Ti-TUD-1 catalysts show
type IV isotherm, typical of mesoporous materials, with a broad H1
type hysteresis loop at relative pressure P/P0 = 0.6–1.0 due to capillary
condensation in the mesopores. H1 type hysteresis loops indicate that
the material has a narrow distribution of relatively uniform pores
which is also evident in pore size distribution. The specific surface
area and pore diameter determined from sorption experiment are sum-
marized in Table 1.
3.2. XPS measurement
The spectra shown in Fig. 2. reflect the characteristic spin–orbit split-
ting of Ti (2p) core levels [Ti (2p3/2) and Ti (2p1/2)]. Here attention was
only concentrated to the Ti (2p3/2) core level electrons because it gives
the spatial arrangement of the titanium species on the TUD-1 matrix.
In this investigation one peak appeared at around 461.1 eV which is
the characteristic peak for tetrahedral arrangement of Ti species [13].
After cerium doping on Ti-TUD-1 material the peak shifted towards
the lower binding energy (Table 2) compare to Ti-TUD-1 material.
This peak shifting is associated with differential charge up property
and spatial distribution of tetrahedral titanium species. With increase
3.3. Ti K-edge & Ce K-edge XANES (X-ray absorption near edge structure)
studies
The Ti K-edge & Ce K-edge XANES spectra of the prepared catalysts
are shown in Fig. 4(a) & (b). It is well-known that Ti (IV) is in a d0
Fig. 1. N2 adsorption–desorption isotherm & pore size distribution of (a) Ti-TUD-1, (b) Ce–Ti-TUD-1 (0.1 mol% Ce) and (c) Ce–Ti-TUD-1 (0.5 mol% Ce).

S. Mandal et al. / Catalysis Communications 46 (2014) 123–127
125
Fig. 2. Ti (2p) core level XPS spectra.
configuration corresponding to A1 or A1g states depending on the
tetrahedral or octahedral symmetry with four or six oxygen anions,
respectively [15]. Therefore, tetrahedral Ti (IV) is represented by a
strong single peak in the pre-edge region (4500 eV) [15]. To confirm
the presence of cerium and its chemical state Ce K-edge XANES mea-
surements were carried out [Fig. 4. (b)]. For pure CeO2 the peak comes
at around 40,480 eV which indicates that Ce is in +4 oxidation state
[16]. In the cerium doped Ti-TUD-1 samples the peaks come around
40,460 eV which indicates that cerium is present in reduced state.
Therefore it emphasizes the interaction between cerium and titanium
change the electrophilicity of Ti center which is also proved from
FT-EXAFS of Ti species and XPS studies.
Ti (as O4Ti_O) such as K2Ti2O5 [17] and fresnoite [18] are 1.89 and
1.91 Å and for four coordinate Ti such as in Ni 2.4T0.7 Si 0.05 O4 spinel
[19] and βBa2-TiO4 [20] are 1.823 and 1.81 Å.
3.5. Styrene oxidation reaction
The catalytic activity of Ce-TUD-1, Ti-TUD-1 and Ce doped Ti-TUD-1
towards styrene oxidation reaction is presented in Table 4 having prod-
uct distribution between benzaldehyde and styrene oxide. It is widely
accepted that benzaldehyde is produced by ring opening reaction of
styrene oxide which is formed duringstyrene oxidation reaction [21,22].
In this study the catalytic activity of Ce-TUD-1 catalyst is very less towards
styrene oxidation reaction. The promotional effect of cerium on Ti-TUD-
1 catalyst towards styrene oxidation reaction is well reflected by in-
creasing conversion with some fluctuation in product selectivity be-
tween benzaldehyde and styrene oxide (60% and 40% respectively).
From XPS and Ce K-edge XANES investigation, it is found that interac-
tion between cerium and titanium change the electrophilicity of Ti
center causing the increasing formation of Ti-superoxo species in the
presence of H2O2 leading to improve conversion of styrene.
3.4. XAFS (X-ray absorption fine structure) studies
The Fourier transform EXAFS spectra of the prepared catalysts
are depicted in Fig. 5 (χ function of the different measurements are
presented in Fig. S3, Supporting Information). The Ti\O bond distance
was found at around 1.8 Å from the curve fitting results (Table 3). As
per literature report an average Ti\O bond length for five-coordinate
Table 2
XPS quantitative analysis.
Catalyst
Ti(2p3/2
)
Ti (2p1/2
)
O (1s)
Ratio of area
Position (eV)
Area
Position (eV)
Area
Position (eV)
Area
Position (eV)
Area
461 eV/534 eV
Ti-TUD-1
Ce–Ti-TUD-1 (0.1 mol% Ce)
Ce–Ti-TUD-1 (0.5 mol% Ce)
461.56
461.18
461.18
20.17
31.53
32.02
466.46
465.87
465.91
36.21
22.01
21.65
531.64
531.44
531.45
140.31
92.55
101.34
534.29
534.21
534.12
1234.4
1242.38
1240.36
0.0163
0.0253
0.0258

126
S. Mandal et al. / Catalysis Communications 46 (2014) 123–127
Fig. 3. O (1s) core level XPS spectra.
4. Conclusion
of styrene and product selectivity between benzaldehyde and styrene
oxide.
In summary, cerium doped Ti-TUD-1 material was successfully
synthesized by non hydrothermal sol–gel method. From the XPS result
it is revealed that Ti are present in a tetrahedral framework position in
the formation of Ti\O\Si bond and no aggregated species of TiO2 are
present. One important phenomenon observed from XPS and Ce K-
edge XANES investigation that cerium increases the electrophilic nature
of Ti in Ti-TUD-1 catalyst resulting increasing formation of Ti-superoxo
species with H2O2. This might be responsible for improved conversion
Acknowledgment
BC & SR would like to acknowledge UGC, Govt of India for funding
under [39-802/2010 (SR)] Scheme. BC would also like to acknowledge
to DST and JSPS for funding under India–Japan Cooperative research
program. SM and RK would like to acknowledge Indian School of Mines
for providing research fellowship. K.K.B. would like to acknowledge PF-
Fig. 4. (a) Ti K-edge XANES spectra of prepared catalysts and (b) Ce K-edge XANES spectra of prepared catalysts.

S. Mandal et al. / Catalysis Communications 46 (2014) 123–127
127
Table 4
Result of styrene epoxidation reaction by using Ce-TUD-1, Ti-TUD-1 and cerium doped
Ti-TUD-1 catalyst.
Catalyst
Conversion Selectivity (%)
(%)
Yield (%)
Benzaldehyde Styrene Benzaldehyde Styrene
oxide
oxide
Ce-TUD-1
(0.5 mol% Ce)
Ti-TUD-1
(3 mol% Ti)
Ce–Ti-TUD-1
(0.1 mol% Ce)
Ce–Ti-TUD-1
(0.5 mol% Ce)
Ce–Ti-TUD-1
(1.0 mol% Ce)
2.55
12.92
15.67
20.84
13.43
100
54.36
0
2.55
7.02
9.37
12.09
7.5
0
45.63
40.2
5.9
59.8
6.3
58.06
55.84
41.93
44.16
8.73
5.93
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Appendix A. Supplementary data
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