Al-doped TiO2 mesoporous material supported Pd with enhanced catalytic activity for complete oxidation of ethanol

Article abstract of DOI:10.1016/j.jssc.2017.01.028

Pd catalysts supported on Al-doped TiO₂ mesoporous materials were evaluated in complete oxidation of ethanol. The catalysts synthesized by wet impregnation based on evaporation-induced self-assembly were characterized by X-ray diffraction, measurement of pore structure, XPS, FT-IR, temperature programmed reduction and TEM. Characteristic results showed that the aluminium was doped into the lattice of mesoporous anatase TiO₂ to form Al-O-Ti defect structure. Catalytic results revealed that Al-doped catalysts were much more active than the pristine one, especially at low temperature (≤200?°C). This should be ascribed to the introduction of aluminium ions that suppressed the strong metal-support interaction and increased the active sites of Pd oxides, enhanced the stabilized anatase TiO₂, improved well dispersed high valence palladium species with high reducibility and enriched chemisorption oxygen.

Full text of DOI:10.1016/j.jssc.2017.01.028

Author’s Accepted Manuscript
Al-doped TiO mesoporous material supported Pd
2
with enhanced catalytic activity for complete
oxidation of ethanol
Jing Zhu, Wentao Mu, Liqing Su, Xingying Li,
Yuyu Guo, Shen Zhang, Zhe Li
www.elsevier.com/locate/yjssc
PII:
S0022-4596(17)30036-1
http://dx.doi.org/10.1016/j.jssc.2017.01.028
YJSSC19676
DOI:
Reference:
To appear in:
Journal of Solid State Chemistry
Received date: 23 November 2016
Revised date: 19 January 2017
Accepted date: 30 January 2017
Cite this article as: Jing Zhu, Wentao Mu, Liqing Su, Xingying Li, Yuyu Guo,
Shen Zhang and Zhe Li, Al-doped TiO mesoporous material supported Pd with
2
enhanced catalytic activity for complete oxidation of ethanol, Journal of Solid
State Chemistry, http://dx.doi.org/10.1016/j.jssc.2017.01.028
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Al-doped TiO2 mesoporous material supported Pd with enhanced catalytic
activity for complete oxidation of ethanol
Jing Zhu, Wentao Mu, Liqing Su, Xingying Li, Yuyu Guo, Shen Zhang, Zhe Li*
College of Chemistry and Chemical Engineering, Taiyuan University of
Technology, Taiyuan 030024, Shanxi, China
mlczjsls123@163.com
mwt15035687833@163.com
suliqing0163@163.com
lixingying0479@link.tyut.edu.cn
guoyuyu0455@link.tyut.edu.cn
zhangshen0472@link.tyut.edu.cn
lizhe@tyut.edu.cn
*Corresponding author. Tel./fax: +86 03516018975.
Abstract
Pd catalysts supported on Al-doped TiO2 mesoporous materials were evaluated in
complete oxidation of ethanol. The catalysts synthesized by wet impregnation based
on evaporation-induced self-assembly were characterized by X-ray diffraction,
measurement of pore structure, XPS, FT-IR, temperature programmed reduction and
TEM. Characteristic results showed that the aluminium was doped into the lattice of
mesoporous anatase TiO2 to form Al-O-Ti defect structure. Catalytic results revealed
that Al-doped catalysts were much more active than the pristine one, especially at low
temperature (≤ 200 ℃). This should be ascribed to the introduction of aluminium
ions that suppressed the strong metal-support interaction and increased the active sites
of Pd oxides, enhanced the stabilized anatase TiO2, improved well dispersed high
valence palladium species with high reducibility and enriched chemisorption oxygen.
graphical abstract
1

Al-doped Pd/TiO2 exhibited optimal catalytic performance for ethanol oxidation and CO2
yield by the suppression of SMSI.
Keywords: mesoporous Al-doped TiO2; Pd/TiO2; ethanol oxidation; oxide Pd
1. Introduction
Removal of volatile organic compounds (VOCs) has been extensively studied for
their formidable harm to human health and to atmospheric pollutions [1-5]. The
application of alternative fuel such as ethanol-gasoline dramatically couples reduction
of HC and CO emissions with increment of emitted ethanol and other intermediate
products [6]. It is of scientific and environmental considerations to investigate
ethanol’s elimination in detail.
One of the most effective and environmentally friendly purification technology
to ethanol-fueled vehicles exhaust is catalytic combustion, through which the
unburned ethanol, intermediate acetaldehyde and ester can be oxidized into CO2 and
H2O [7]. Many research groups have extensively studied complete oxidation of
ethanol over the past few decades [8-14]. Among them, large amount attention was
devoted to titania (TiO2), which was widely used as carriers [5]. But traditional TiO2
supported catalysts can no longer satisfy the need of catalytic performance for their
weak thermostability and low specific surface area. Attempts have been made to
improve performance of titania by the exploration of mesoporous TiO2 materials with
packed porous [15]. While in preparing mesoporous titania, the structures and
properties of the final products could be influenced by two critical issues, one was the
retardation of cooperative assembly with templates for the high reactivity of
hydrolysis and condensation of titanium precursors, which has been relieved by a
method called evaporation induced self-assembly (EISA) [16]. The other lied in the
contradiction between framework reservation and single crystal with high crystallinity
2

during calcination at high temperature to removal surfactants and to promote TiO2
crystallization [17, 18]. It has been accepted that the addition of metal to TiO2 could
obviously effect phase transformation, consolidate skeletal architecture and improve
catalytic performance. Tarek Barakat et al. have investigated T50 of total oxidation of
toluene with group V metal oxides doped hierarchically structured macro-mesoporous
TiO2, and the results revealed that doping with Nb2O5 and V2O5 can enhance the
catalytic performance of TiO2 support [19]. The introduction of aluminium to anatase
TiO2 materials has been demonstrated by many research groups [20-22]. While to the
best of our knowledge, literature on the Al doped TiO2 mesoporous material prepared
by structure directing agents is quite rarer than those of pure mesoporous materials
such as SiO2, TiO2, Al2O3 [23-27].
Palladium based catalysts have been confirmed to be highly efficient for catalytic
combustion of ethanol [14]. It has been a general consensus that ethanol oxidation
over palladium based catalysts is structure sensitive reaction, and the dispersion as
well as particle size of palladium are of the predominant factors for catalytic
performance [5, 28]. To improve catalytic properties, it is important to obtain smaller
size palladium particle. For the existence of the strong metal-support of Pd/TiO2,
researchers have found anomalous chemisorption behavior under reduction
atmosphere [29]. In the procedure of oxidation of ethanol, CO and H2 might be
produced and CO has been considered harmful to Pd/TiO2 catalyst, so it is crucial to
reduce the effect of redox species to catalysts. Additionally, the chemisorbed oxygen
in the interface could play key role to reaction, it is necessary to boost the diffusion of
gaseous phase oxygen from air to the interface of Pd-mesoporous TiO2 [30, 31].
Nevertheless, even the Pd-based catalysts have been widely used for the catalytic
oxidation of most VOCs，there remains a relatively blank field about influence of Al
on chemical state and distribution of palladium，on the interaction between Pd and
TiO2 as well as the catalytic performance of ethanol oxidation.
The aim of this work is to prepare Al-doped TiO2 mesoporous materials by a
classical EISA procedure using of triblock copolymer (P123) as structure directing
agents and to load Pd by a wet-impregnation method. The catalytic performance was
investigated for catalytic oxidation of ethanol, especially the total oxidation to CO2,
3

and then the relationship between structure and performance was discussed.
2. Experimental
2.1 Preparation of samples
Procedure to prepare Al doped-TiO2 mesoporous materials were described in this
study, and the synthesis of mesoporous titania was followed a previous EISA method
[32]. In a typical synthesis, 4.5 g P123 was dissolved in 26 mL 1-butanol at room
temperature. Next, 15 mL tetrabutyl titanate and moderate aluminum iso-propoxide,
which was fully grind in a mortar and pestle, were added in turn into the P123
solution slowly and being stirred for 2 h. The solution in another container consisting
of well mixed 2.8 mL 37 wt. % hydrochloric acid, 4 mL distilled water and 26 mL
1-butanol was added drop by drop to the above mixture under stirring for 3 h. Then
the sol was dried at 50 ℃ for 48 h and 100 ℃ for 24 h at a dish, the final gel was
heated in air in a muffle oven to 350 ℃ and 500 ℃ respectively at a ramp rate of
1 ℃/min, the range of Al element loading of (0, 2.5, 5, 7.5 and 10 wt. % Al) and
denoted as: T-350, 2.5AT-350, 5AT-350, 7.5AT-350, 10AT-350, while the sample
calcined at 500 ℃ denoted as above but change 350 to 500 just as T-500 and so on.
Palladium was loaded on the carriers calcined at 500 ℃ by a wet-impregnation
method, and the entire samples contained Pd content of 1wt. %, theoretically. The
samples prepared in different conditions were denoted as: Pd/ x AT-500 (x=0 with no
A, 2.5, 5, 7.5 and 10).
2.2 Activity test
The catalytic performance of the samples was evaluated on a fixed-bed reactor
connected with an online gas chromatograph. The compositions of ethanol and
products were analyzed on a GC-950 apparatus with two FID detectors, one of which
incorporated a methanator. 0.30 g catalyst was being tested (40-60 mesh) with the
feed gas consisted of 0.5 vol. % C2H5OH, 5 vol. % air and a balance of N2. The flow
4

rate was 100 mL/min of total gas and the gas hourly space velocity (GHSV) was
24000 h−1. Ethanol conversion (Xi) and the yields (Yj) of by-products and carbon
dioxide from ethanol combustion were calculated according to the formulas followed:
푋푖 = 퐶
× 100%
푌 = 퐶
×
× 100%
푖,푖푛−퐶푖,표푢푡
1
푗,표푢푡
푗
퐶푖,푖푛
퐶푖,푖푛
푛푟푎푡푖표
Where Ci and Cj are the concentrations of ethanol and products from ethanol
combustion, respectively, nratio is the products and reactants carbon atom ratios. And
퐶푖 = 퐴푖 × 푓
퐶 = 퐴푗 × 푓
푗 푗
푖
Where A and f are the peak areas and the relative correction factors of reactants (i)
or products (j), respectively.
2.3 Characterizations
Low-angle and wide-angle X-ray diffraction (XRD) patterns of all samples were
collected in θ-2θ mode on Rigaku D/MAX-2550 apparatus (Cu Kα1 radiation,
λ=1.5406 Å). Intensities of the diffraction peaks were recorded for 2θ range 0.5-80º
with a step size of 0.01º and the scanning speed was 8º/min.
Measurement of pore structure from the N2 adsorption-desorption isotherms
were measured on a NOVA 2000e surface area and pore size were analyzer at 77K
(liquid nitrogen temperature). Prior to measurement, all samples were degassed at
250 ℃ for 240 minutes under vacuum. The specific surface areas of the samples
were calculated by the Brunauer-Emmett-Teller (BET) method. The mesopore size
distribution and total pore volumes were determined using the Barrett-Joyner-Halenda
(BJH) method.
Fourier transformed infrared spectroscopy (FT-IR) analysis was carried out using
NICOLET360 type spectrometer. The spectra of solids were obtained using KBr
pellets. Before measurement, the catalyst and potassium bromide were mixed together
in accordance with the quality ratio 1:100. The vibrational transition frequencies are
reported in wave numbers (cm-1).
Transmission electron microscopy (TEM) and high resolution transmission
electron micrographs (HRTEM) were carried out on JEOL-2100F electron
5

microscope, operating at 200 kV.
Hydrogen temperature-programmed reduction (H2-TPR) was performed on an
AutoChem II 2920 chemisorption analyzer (Micromeritics, Norcross, GA, USA)
equipped with a TCD detector. A 100 mg catalyst sample was pretreated at 300 ℃
for 0.5 h in N2 atmosphere to remove the adsorbed matters. After cooling to room
temperature, the sample was exposed to a flow of 10 Vol. % H2/90 Vol. %Ar (50
mL/min), and the temperature was increased linearly at 10 ℃/min.
X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Kratos
Axis Ultra DLD multifunctional surface analysis photoelectron spectrometer with
X-ray source at 450 W and Al-Kα radiation (1486.6 eV). The element binding
energies were referenced to the C 1s line at 284.6 eV and measurements were
restricted an estimated error of ± 0.1 eV.
Pulse H2 chemisorption on a Micrometrics AutoChem II 2720 with TCD to
monitor H2 consumption was used to determined surface Pd atoms exposed on the
surface. Before measurement, 0.1g sample was pretreated by helium flow for 1 h at
200℃ to remove gaseous species adsorbed, and then cooling down to 60℃. The
calculation of turnover frequencies (TOF) based on ethanol conversion (TOFi),
product generation (TOFj) and the amount of Pd atoms. A H2/Pd average
stoichiometry of 1 has been assumed [33]. The turnover frequencies of Pd catalysts
were calculated as follows
푌푗퐶푖푉푖,푖푛
푇푂퐹 = 퐶 푋 푉
푠
푇푂퐹 =
푖
푖 푖,푖푛
−1
−1
[
]
[
푠
푛푟푎푡푖표푛푃푑
]
푖
푗
푛푃푑
Where Ci is the concentration of ethanol in gas mixture; Xi is the conversion of
ethanol; Vgas is the total gas flow rate; nPd is the mole number of Pd atom calculated
by pulse H2 chemisorption. And in our work, the initial concentration of ethanol in gas
mixture, total gas flow rate, reaction time and mass of catalyst are the same,
respectively.
6

3. Results and discussion
3.1 Evaluation of the catalytic activity
Fig. 1 shows the temperature dependence of total oxidation of ethanol for the Al
doped 1 wt. % Pd/ xAT-500 and Pd/T-500 catalysts from ambient temperature to
450 ℃. On different amount of Al doping TiO2 supported Pd catalysts, it can be
found the activities of ethanol oxidation over TiO2 supported Pd catalysts were greatly
improved by the introduction of aluminium. It is obviously to see that at reaction
temperature of 200 ℃, the conversion of ethanol over Pd/7.5AT-500 has reached
69.5 %, which is apparent higher than that over Pd/T-500. At the same temperature
point, CO2 yield over Pd/7.5AT-500 was 68.0 %, which was significantly higher
compared with that over others. To further gain insight into the relation between
active Pd atom and catalytic efficiency, turnover frequencies of Pd/7.5AT-500 and
Pd/T-500 were calculated on the basis of ethanol conversion and product yield and the
amount of active site measured by H2-pulse chemisorption are shown in Table.1. It
can be seen the TOFs of ethanol conversion (TOFi) and CO2 generation (TOFj) over
Pd/7.5AT-500 is significantly higher than that over Pd/T-500. Compared with
Pd/T-500, the Pd/7.5AT-500 obtained much higher TOFi and TOFj at the same
temperature, which means the higher ethanol oxidation and CO2 production from
ethanol oxidation per unit time on per active sites. Especially, at reaction temperature
of 200℃, the ratio of TOFi (3.17 s-1) and TOFj (1.62 s-1) of Pd/7.5AT-500 is 1.96
which is very close to carbon atom ratio of ethanol and CO2 which means ethanol
molecular involved in reaction largely converted to CO2, while that over Pd/T-500 is
1.62, thus the enhanced catalytic efficiency was obtained at the Pd/7.5AT-500. These
results suggest that the Pd/TiO2 with aluminium achieved much higher reactivity than
TiO2 without dopants prepared by EISA supported Pd catalysts. What worth noting is
that no CO was observed on the Pd/7.5AT-500 catalyst through the whole reaction of
ethanol (shown in Fig.S1). While as addressed before, acetaldehyde and CO were the
main by-products for the catalyst with low content of noble metal loading [34]. The
7

doping of Al could influence the reaction path of oxidation of ethanol. The yields of
other by-products showed no significant differences.
Fig. 1. Ethanol conversion and curves for Yields to CO2 Pd/T-500 and Pd/xAT-500 prepared by
aluminium doped TiO2 mesoporous material supported 1wt. % Pd
Table 1 TOFs of Pd/T-500 and Pd/7.5AT-500 for ethanol oxidation.
TOFa of Pd/T-500 (10-1,S-1)
TOFa of Pd/7.5AT-500 (10-1,S-1)
Tem.℃
Con.% TOFbi
Yield %
0
TOFcj
Con.%
14.8
18.2
30.3
33.0
50.3
69.5
91.6
94.6
96.0
96.6
TOFbi
3.45
Yield% TOFcj
75
4.7
3.5
0.30
0.23
0.23
0.15
0.58
1.66
4.13
5.08
5.64
5.93
0
0
0
100
125
150
175
200
225
250
275
300
0
0
4.25
0.6
0.26
3.5
0
0
7.05
5.3
2.46
2.3
1.4
0.17
1.14
2.69
8.16
9.57
10.47
10.60
7.68
10.8
21.7
68.5
78.3
81.1
83.6
85.8
5.02
9.1
9.0
11.70
16.16
21.31
22.01
22.32
22.44
10.11
31.65
36.42
37.69
38.89
39.90
25.8
64.4
79.2
87.9
92.6
21.1
62.8
74.7
81.7
82.7
8

350
400
450
92.6
92.6
92.6
5.935
5.93
5.93
82.7
82.7
82.7
10.60
10.60
10.60
96.6
96.6
96.6
22.44
2.244
22.44
85.8
85.8
85.8
39.90
39.90
39.90
aTOF= turnover frequency (ethanol oxidation activity per surface Pd atoms sites as measured by
H2 pulse chemisorption.
TOFbi = turnover frequency of ethanol conversion
TOFcj = turnover frequency of CO2 production
3.2 XRD analysis
To identify structure of ordered mesoporous materials, low-angel XRD were
carried out for all the materials contained different content of aluminium. As shown in
Fig. 2a. All samples exhibits a diffraction peaks at 2θ value about 1.2℃ and an
additional broad peak at range 2.0-2.5°indicate the mesoporous structure with short
range order [22, 35]. Comparatively, the peak intensity between Al-doped samples
and undoped one have a certain extent improvement, suggesting the introduction of
aluminium improves the order of the mesoporous structure. While the peak at 1.2°
corresponding to the d-spacing of 7.36 nm. And the wide-angle X-ray diffraction
patterns of different content Al-doped TiO2 mesoporous nanocomposites calcined at
350 ℃ are shown in Fig. S2. In comparison with pure TiO2, the patterns of Al doped
TiO2 samples indicate a low level crystallinity of anatase. While an increase in the
broadening peaks of the (101) anatase peak with increasing dopant of Al can be
observed for all concentrations. Different carries with improved crystallinity when
calcination temperature was risen to 500 ℃ can be seen in Fig. 2b. And evidence for
template removal of samples has been confirmed by FT-IR (Fig. S3). According to
PDF#73-1764, the major crystal phase of supports is anatase while the weak
diffraction peak at around 27.4° on pure TiO2 is assigned to rutile, indicating that
phase transformation occurred in preparing TiO2 mesoporous material with EISA
procedure under certain condition. However, no signal of rutile phase can be seen for
supports contain aluminium. Apparently, the introduction of aluminium inhibits the
9

formation of rutile from anatase phase during heat treatment at 500 ℃. What counts
on most is that anatase was confirmed a favourable factor to promote catalytic
reaction.
On the other hand, the diffraction peak observed at palladium based catalysts at
2θ position of around 33.6° can be indexed to PdO (JCPDS75-0584) [36, 37]. While
no peak related to metallic Pd can be observed, and this may due to the low content.
In the meantime, no characteristic diffraction peaks for individual aluminium species
are observed in the samples. By comparing the full width at half-maximum (FWHM)
of the strongest diffraction peak at around 25.4°, it is found that the (101) plane of
Al-doped sample exhibits 2-fold higher FWHM value than pure TiO2. It has been
certified that XRD peaks become less sharper when the grain size decreases or the
crystallinity reduced [25, 38]. According to the (101) plane of anatase phase, the
average crystallite sizes of all samples were estimated according to the Scherrer
equation (Table 2). For all the supports calcined at 500 ℃, the crystallite sizes of
anatase phase of TiO2 can be roughly summarized to be about 16 to 9 nm. The mean
size of 7.5AT-500 achieves the relatively small one of 9.06 nm, meaning the
introduction of moderate amount of aluminium can embarrass the grain growth of
anatase.
Fig. 2 Low-angle XRD patterns (a) of TiO2 and Al-doped TiO2 mesoporous materials calcined at
500 ℃ and wide-angle XRD patterns (b) of mesoporous materials and catalysts Pd/T-500 and
Pd/7.5AT-500 (a)and wide-angle (b)
a
b
10

Table 2 Phase content, FWHM and crystallite size of supports and catalysts
sample
anatase Content (wt. %)
anatase FWHM (°)
anatase Crystallite Size (nm)
10AT-500
7.5AT-500
5AT-500
100
100
100
98.37
-
0.72
0.89
0.78
0.45
0.87
0.44
11.11
9.06
10.38
16.14
9.23
T-500
Pd/7.5AT-500
Pd/T-500
-
18.17
3.3 Measurement of pore structure
N2 adsorption-desorption isotherm measurements were used to analyze the
mesoporous structure of the samples calcined at 500 ℃, and the curves are shown in
Fig. 3. The BET specific surface areas, pore volumes and pore sizes are listed in Table
2.
The N2 adsorption-desorption isotherms on samples present similar appearance
of a flat “S”-shape of type IVa curves with a clear absorption hysteresis loop located
at 0.40 < P/P0 < 1.0 of H2-type hysteresis loops according to IUPAC classification,
indicating the presence of “ink-bottle” mesopores [39]. The capillary condensation
stage on curves of samples contain 7.5 wt. % aluminum have distinct sharp adsorption
trend compared to pure TiO2, revealing the existence of narrow pore size distribution
and relatively uniform channel in agreement with BJH pore-size distribution patterns
of Fig. 3 (insert). It could be found from data of Table 3 that the BET specific surface
area changes extremely when moderate aluminium were added to support material
and catalysts, respectively. The low BET surface area and broad pore size distribution
of pure TiO2 calcined at 500 ℃ remain practically constant with literary [40]. In
contrast, pore size of 7.5AT-500 is 4.28 nm, smaller than pure TiO2 (4.87 nm), in
respect that rutile between intra anatase grains would expand primary interspace for
rutile possess larger volume, the introduction of aluminium inhibits fast crystallization
and phase transformation of anatase to rutile, as a result the relatively smaller pore
11

were preserved [41].
Consequently, the mesochannel and enhanced stabilized anatase (according to
XRD results) can be retained to supply the efficient transport pathway for reactants
and products, which is propitious to the high catalytic activity. After loading of
palladium, interestingly, samples with and without aluminium present the opposite
trends, it is noticeable that an instinct decrease of BET surface area (from 159.39
m2/g-1 to 98.67 m2/g-1) pore size (from 4.28 nm to 3.39 nm) and pore volume (from
0.14 cm3g-1 to 0.13 cm3g-1) can be observed, indicating the successful incorporation of
palladium species into the mesoporous architecture of the supports [41]. Reference
accounted that high surface area and pore volume consisted with open aperture and
short channel length can benefit absorption and mass transfer of reactant molecules,
reduce the diffusion distance of product molecules in the pores and the probability of
the occurrence of secondary reactions, and improve anti-coking ability of the catalyst
as well as the stability of charcoal [42, 43].
Fig. 3 N2 adsorption-desorption isotherms and pore size distributions (insert) of pristine
mesoporous TiO2 (T-500), 7.5 wt. % aluminium doped TiO2 mesoporous material (7.5AT-500). 1
wt. % Pd/TiO2 (Pd/T-500) and 1 wt. % Pd/7.5AT (Pd/7.5AT-500)
Table 3 Textural properties of supports and catalysts
sample
SBET (m2/g-1)
V (cm3g-1)
Dpore (nm)
12

T-500
18
0.02
0.07
0.14
0.13
4.87
6.50
4.28
3.39
Pd/T-500
7.5AT-500
Pd/7.5AT-500
40
159
98
3.4 Transmission electron microscopy (TEM)
Typical TEM and HRTEM images of 1 wt. % palladium loaded 7.5 wt. %
aluminium-doped TiO2 are shown in Fig. 4. It can be seen that the mesoporous
structure has been successfully retained [22]. The lattice fringe of the selected
particles is 0.35 nm, which belongs to the spacing of anatase. The TiO2 particles have
a size about 9 nm, which is consistent with the XRD data. Some metal agglomeration
could be observed while it is difficult to distinguish Pd/PdO clusters/particles on the
Al-doped TiO2 mesoporous material supported palladium catalyst [44].
Fig. 4 TEM and HRTEM imagers of Pd/7.5AT -500prepared by 7.5 wt. % aluminium doped TiO2
mesoporous material supported 1 wt. % Pd
a
b
c
d
13

3.5 H2-TPR analysis
H2-TPR measurements were carried out to identify Pd species and interaction
between Pd and supports. The TPR profiles of Pd catalysts supported on TiO2 and Al
doped-TiO2 are shown in Fig. 5
A negative H2 consumption peak at around 75 ℃ can be observed because
formation of β-PdH in treatment with H2/Ar mixture at room temperature and it
could be decomposed to H2 below 100 ℃ [45, 46]. The slight shift to lower
temperature of Pd/7.5AT-500 compared with Pd/T-500 could be attributed to the
weak extent of the strong metal-support interaction (SMSI) [47]. The reduction peak
at around 85 ℃ for Pd/7.5AT-500 is most likely due to reduction of Pd oxide to Pd
metal, which is significantly lower than that of Pd/T-500 meaning the Pd oxides is
more reducibility on Pd/7.5AT-500 than that of Pd/T-500. Actually, it has been
convinced that defective structures of TiO2 caused by dopants with oxidation state
lower than tetravalence could produce vacancies considered to arouse higher lattice
oxygen mobility thus enhance reduction of Pd species [48]. So the significant lower
reduction temperature and asymmetric peak at 85 ℃ for Pd/7.5AT-500 might be
related to the increased number of titania defects. Further characteristic method
should be applied to identify existence of defected TiO2. And the third peak
corresponded to the reduction of surface TiO2 particles [49, 50]. From the peak
intensity and temperature range, the amount of hydrogen consumption for TiO2
reduction on Pd/T-500 is significantly higher than that on Pd/7.5AT-500. The
different degree of SMSI over Pd catalysts was observed by the TiO2 reduction
pattern. It has been demonstrated that Pd particle on the surface could be decorated
by TiO2 overlayer and promotes TiO2 reduction more vigorously in precious study
which is consistent with observation in our Pd/T-500 [51]. The SMSI state Pd atom
of Pd/T-500 can adopt electron from TiO2 and form electron-rich metal which might
be hard to be oxidized in the catalytic cycle and left a large amount of TiOx. The
reducibility of Palladium species could be weakened. In general, alcohol oxidation
14

has close relation with reducibility of active component. The lower reduction
temperature of small palladium oxides particles have easier access to higher catalytic
performance [52]. And based on the H2-TPR results, the introduction of aluminium
could suppress the SMSI and hence the reducibility of palladium species. The more
readily reducible palladium species exhibits facile approach reduction and oxidation
cycle in the catalytic process. The results properly supported the conclusion that the
introduction of aluminium can weaken the metal-support interaction between Pd and
TiO2.
Fig. 5 H2-TPR profile of Pd/T-500and Pd/7.5AT-500 prepared by 7.5 wt. % aluminium doped TiO2
mesoporous material supported 1 wt. % Pd
3.6 XPS evaluate
To deep investigate the influence of doping element on catalyst, we analyzed changes
in oxidation state by X-ray photoemission spectroscopy (XPS) to provide information
on surface segregation of Pd based 7.5 wt. % Al species doped TiO2 and Pd/T-500.
Characteristic XPS of survey spectra and core level single spectra of the bonding
partners are presented in Fig. 6 (a-f).
Fig. 6a shows the wide survey scan of the samples, compared to curve of
Pd/T-500, Al-doped sample presents obvious peak of Al species, and binding energies
of 74.6 and 118.6 eV in the spectra correspond to the photoelectron peak of Al 2p and
Al 2s, respectively, which can be ascribed to the formation of Al3+ [53]. Furthermore,
15

high resolution XPS of Al 2p (Fig. 6b) presents that the main peak at around 74 eV is
in accordance to aluminium ions situating in the TiO2 lattice as Ti-O-Al structure
[54-56].
Fig. 6c shows the Ti 2p curves of Pd based Al-doped and undoped TiO2
mesoporous materials and details for the peak fitting parameters are presented in Fig.
S4a. For spin-orbit coupling effect, the Ti 2p core level could split into 2p3/2 and 2p1/2
with binding energies of 458.4 eV and 463.9 eV, respectively. Due to the pronounced
asymmetry of peaks, curves were decomposed into several contributions due to
Shirley background and Caussian-Lorentzian fitted method related to oxidation states
of Ti possess. For each doublet, the ratio of the area of the two peaks A(Ti 2p1/2)/A(Ti
2p3/2) was equal to 1/2 and the binding energy separation between Ti 2p1/2 and Ti 2p3/2
in the same valence conditions was kept in the region of 5.6-5.7 eV, which are in
agreement with previous literature [56]. The Ti 2p curve shows the existence of Ti3+
and Ti4+ in catalyst of Pd/T-500. The peaks at 458.6 eV (2p3/2) and 464.3 eV (2p1/2)
signify the appearance of Ti tetravalent ions, besides, the two at lower binding energy
of 456.3 eV (2p3/2) and 462 eV (2p1/2) are attributed to the existence of Ti3+ [22, 57,
58]. The peak at 457.4 eV is the satellite peak of titanium species. While it is worth
noting that only peaks related to Ti4+ species can be observed in Pd/7.5AT-500. The
additional peak at 459.3 eV is a characteristic of Ti species in oxidation state related
to Al (probably Al2TiO5) [59]. We speculate that the presence of Ti3+ might be due to
the reducibility of TiO2, especially when connected with Pd. SMSI effect for TiO2,
titanium oxides would migrate on the surface of metal, which induces charge transfer
on metal-support [60]. The binding energy of Ti 2p3/2 peak (Ti4+) shifts slightly to a
lower position (by 0.3 eV) compared Pd/7.5AT-500 with Pd/T-500, and this negative
shift suggests the existence of more serious TiO2 defect, regarding that the
chemisorption of oxygen atom can be enhanced on the vacancy of defected TiO2 [31].
According to high resolution Pd 3d XPS spectrum in Fig. 6d and details for the
peak fitting parameters in Fig. S4b, oxidation states of Pd and relative amount can be
obtained. A Pd 3d5/2 peak at 336.4 eV for both doped and undoped palladium catalysts
corresponds to Pd2+. The binding energy of 334.4 eV in Pd/T-500 can be assigned to
16

metallic Pd0, and Pd0 is not detected on Pd/7.5AT-500 but a peak at 337.6 eV
belonging to Pd4+ is presented [61-63]. In the case of Pd/T-500, the binding energy of
Pd0 is slightly shifted to lower binding energy ( around 0.5 eV) compared to date
reported before, revealing that electrons are donated from TiO2 to Pd, which can be
another indicative of SMSI as reported before [64, 65]. This SMSI effect is higher in
Pd/T-500 than in Pd/7.5AT-500, meaning the introduction of aluminium can suppress
the interaction on Pd and TiO2.
The O 1s spectrum of Fig. 6e and fitting parameters listed in Fig. 6f are used to
show the types of surface oxygen species in the catalysts. According to the
photoelectron peak positions and literatures reported before, the high binding energy
peak is assigned to surface adsorbed oxygen. It can be seen, O 1s spectrums reveal
apparent shoulder peak at 531.2 eV (named O ) on both one which should be ascribed
Ⅱ
to the adsorbed oxygen on the surface of palladium catalysts. The doped catalyst
possess higher capacity of adsorbed oxygen than the undoped one, this can be
attributed to the serious TiO2 defect (as evidenced by XPS of Ti 2p). Chemisorption
oxygen were considered presented high mobility and deep involved in the VOCs
removal [66]. In addition, the binding energy peak at about 529 eV is related to
oxygen atom linked to Ti4+ and the binding energy of 527.6 eV is ascribed to one
linked to Ti3+ which can only be detected in undoped catalyst [62, 67]. The unknown
feature at around 528.5 eV marked by sat.
(f)
17

Fig. 6 XPS spectra of Pd/T-500 and Pd/7.5AT-500 prepared by 7.5 wt. % aluminium doped TiO2
mesoporous material supported 1 wt. % Pd :(a) survey spectra; (b) Al2p spectrum (c) Ti2p
spectrum; (d) Pd3d spectrum;(e) O1s spectrum; (f) peak fitting parameter for O1s spectrum
3.8 Discussion on influence of Al doping
According to all the observations and the literature, there is a bold speculation
accounted for the correlation between Al dopant and ethanol oxidation.
As shown in Table 1. The catalytic efficiency of Pd/T-500 is significantly lower
than Pd/7.5AT-500 correlated to ethanol conversion and CO2 production based on
TOFs and shown in Table 1 at low temperature. The enhanced catalytic activities
should be attributed to aluminium dopants. It is essential to figure out why and how
aluminium influences the catalytic activity. It has been proven previously that the
combination or at least one of stabilized anatase of TiO2, activated cationic metal
/metal component and abundant adsorbed oxygen could lead to high activity [30, 31].
The results of XRD (Fig. 2a) and XPS pattern indicate that both (i) the stabilized
anatase with relative high surface area and (ii) cationic Pd and abundant adsorbed
oxygen are presented in Pd/7.5AT-500. While how this positive influence of the
introduction of aluminium on catalyst properties and catalytic activity are discussed
below. There are two possible reasons for the optimal performance of Pd/7.5AT-500.
One is the surface different atom ratio, for there is an optimum metal/titania ratio
below which would induce the charge recombination in TiO2, while on the contrary,
18

the high metal contents would increase recombination at metal particles which endure
adverse effect [68]. In addition to metal/titania ratio, oxygen/titania ratio can affect
catalytic performance, for that below stoichiometric value of 2.0 (TiO2) would
increase the formation of oxygen vacancies and decrease electron hole recombination
rate [64]. To determine the surface atomic ratio for the catalytic performance, we
calculated the surface Pd/Ti and O/Ti of the Al-doped sample and that of pure TiO2
(Table 4). For the Al-doped catalyst, the Pd/Ti ratio was found to be 0.06, just a litter
higher than that of Pd/T-500, while the O/Ti ratio was 3.29, and that of Pd/T-500 was
3.00. These variations are not convictive enough to explain such an increased catalytic
performance on the oxidation of ethanol.
Table 4 Surface atom ratio from XPS
Sample
Pd/Ti atomic %
O/Ti atomic %
Pd/T-500
0.04
0.06
3.00
3.29
Pd/7.5AT-500
The other reason lies in the strong metal-support interaction between palladium
and TiO2. SMSI between Pd and TiO2 after exposure to air at relatively high
temperature should exist for the reducibility of TiO2, which has been observed by
Ouyang [69]. To identify the effect of introduction of aluminium on SMSI, it is
necessary to distinguish whether the doped Al cation situated at Ti substitution sites or
interstitial sites [70]. If Al3+ is incorporated into interstitial void of anatase lattice, the
lattice parameters will increase and the diffraction peaks will shift to lower angle
region in the XRD patterns [53, 54]. In our paper, the peak of anatase (101) appears at
25.42° for the 7.5 wt. % Al doped TiO2, which is a little higher than that of 25.28° for
the undoped one, indicting trivalence Al has substituted Ti4+ in the anatase lattice [71].
The shift of binding energy related to Ti 2p3/2 of Ti4+ to lower position (by 0.3 eV) is
consistent with the wide-angle XRD results. Determined by electron structure of
doped ions, new energy level might be formed in pristine material. Charge transfer
from Al3+ to Ti4+ would inhibit phase transformation, and this can suppress the
reduction atmosphere and electron transfer between metal and support, which could
19

result in lower SMSI. It can be seen that the amount of the Pd2+ state is approximately
60% in both doped and undoped catalysts. And the percent of Pd0 in Pd/T-500 and
Pd4+ in Pd/7.5AT-500 possess almost unanimously, respectively. Therefore as detected
by XPS, the introduction of Al leads to the transformation of low valence Pd to higher
valence Pd species. Given that the species of active sites correlates with catalytic
performance of ethanol oxidation, high valence cationic Pd is much more active than
metallic one.
Based on the presence of Ti (Ⅳ) and cationic Pd with higher valence, it is
reasonable to propose that the introduction of Al produce an obvious negative effect
on SMSI. The catalytic system of Pd/T-500, possessing a SMSI effect higher than
Pd/7.5AT-500, is less effective in terms of ethanol oxidation and CO2 yield, in contrast
to the improvement effect of SMSI of Pd/T-500 reported recently for HCHO oxidation
[66]. This adverse result suggests that the effect of SMSI on VOCs oxidation may be
connected with the VOC species. Based on surface reaction mechanism, on the
surface of catalyst, ethanol would be dehydrogenized into ethyoxyl, and then ethyoxyl
would be further oxidized into CO2 and H2O at the presence of oxygen [14]. Catalytic
system with lower SMSI is more effective in terms of dehydrogenation from organics
[64]. The introduction of aluminium produces a boost of ethanol oxidation might be
ascribed to the allocation of Pd oxides and the higher reducibility Pd oxides exhibit
facile approach reduction and oxidation cycle in the catalytic process. And detailed
research about effects of metal oxide-support interaction on approach of ethanol
oxidation should be carried out.
4. Conclusions
In our contribution, we have reported a wet impregnation assisted EISA
procedure to prepare palladium catalysts supported on Al doped anatase TiO2
mesoporous materials. Aluminium has been incorporated into anatase TiO2 framework
by substituting Ti4+ ions and this introduction brought about anatase monocrystalline,
narrow mesoporous size dispersion and high specific surface area of Pd catalyst. The
20

obvious decrease of initiation temperature and optimal catalytic performance on
reaction temperature below 200 ℃ were observed for ethanol conversion and CO2
production on the doped Pd/AT-500 relative to undoped one, which could be
attributed to the new properties of Al-doped TiO2. We proposed that the introduction
of Al suppresses the charge transfer from TiO2 to Pd (SMSI) and facilitates Pd oxides
which has higher reducibility act as catalytically active sites, and the abundant
chemisorption oxygen generated on the defected mesoporous TiO2 is beneficial for
surface ethyoxyl to form CO2 and H2O.
Conflict of Interest:
The authors declare that they have no conflict of interest.
Acknowledgements:
The authors gratefully acknowledge the Project supported by the State Key
Program of National Natural Science of China (Grant No. 21336006) and Shanxi
Province Scientific and Technological Project (20140313002-2).
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204-208.
Highlights:


Palladium catalysts supported on Al-doped TiO2 mesoporous materials were studied
The introduction of aluminium can enhance TiO2 anatase thermostability and increase
defect TiO2.


The Pd/Al-TiO2 catalysts possess higher ethanol conversion and CO2 yield compared
Pd/TiO2.
The influence of aluminium on SMSI and on ethanol oxidation of catalytic performance
of Pd/TiO2 catalysts were evaluated by TPR and XPS.
25