Selective hydrogenation of cinnamaldehyde with PtFex/Al2O3@SBA-15 catalyst: Enhancement in activity and selectivity to unsaturated alcohol by Pt-FeOx and Pt-Al2O3@SBA-15 interaction

Article abstract of DOI:10.1016/j.jcat.2017.07.026

Selective hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL) was conducted over a series of FeO_x-doped Pt catalysts supported on a 15 wt% Al₂O₃@SBA-15 composite (15AS). It was found that the addition of FeO_x to the catalyst greatly improved its performance. With an optimal Fe/Pt molar ratio of 0.25, the PtFe_0.25/15AS catalyst reached a maximum reaction rate (defined as moles of converted CAL per gram of Pt per hour) of 13.93 mol g_Pt^?1 h^?1 with 76.9% selectivity to COL. Additionally, the PtFe_0.25/15AS catalyst afforded the highest TOF value (defined as moles of converted CAL per mole of Pt active sites per second) of 1.54 s^?1. X-ray photoelectron spectroscopy analyses and H₂ temperature-programmed reduction and CO diffuse-reflectance infrared Fourier-transformation spectroscopy studies reveal that there is strong interaction between Pt nanoparticles and Al₂O₃@SBA-15 composites and also between Pt and FeO_x, resulting in Pt species with a positive charge being dominant in PtFe_0.25/15AS catalysts. Therefore, Pt species with positive charges, together with FeO_x species, are beneficial for preferential adsorption and activation of carbonyl bonds of CAL, so that the activity and selectivity to COL were improved by PtFe_x/15AS catalysts (x represents the Fe/Pt molar ratio).

Full text of DOI:10.1016/j.jcat.2017.07.026

Journal of Catalysis 354 (2017) 24–36
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Selective hydrogenation of cinnamaldehyde with PtFe /Al O @SBA-15
x
2 3
catalyst: Enhancement in activity and selectivity to unsaturated alcohol
by Pt-FeO and Pt-Al O @SBA-15 interaction
x
2 3
Huiyan Pan a,1, Junrui Li a,1, Jiqing Lu , Guimei Wang , Wenhui Xie , Peng Wu , Xiaohong Li
b
a
c,
⇑
a
a,
⇑
a
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University,
3
663 North Zhongshan Rd., Shanghai 200062, China
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
b
c
School of Physics and Materials, East China Normal University, 3663 North Zhongshan Rd., Shanghai 200062, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL) was conducted over a series
Received 9 May 2017
Revised 22 July 2017
Accepted 26 July 2017
of FeO
addition of FeO
.25, the PtFe0.25/15AS catalyst reached a maximum reaction rate (defined as moles of converted CAL
x
-doped Pt catalysts supported on a 15 wt% Al
2 3
O @SBA-15 composite (15AS). It was found that the
x
to the catalyst greatly improved its performance. With an optimal Fe/Pt molar ratio of
0
ꢀ1
ꢀ1
per gram of Pt per hour) of 13.93 mol gPt
h
with 76.9% selectivity to COL. Additionally, the
PtFe0.25/15AS catalyst afforded the highest TOF value (defined as moles of converted CAL per mole of
Keywords:
ꢀ1
Pt active sites per second) of 1.54 s . X-ray photoelectron spectroscopy analyses and H
2
temperature-
Al
PtFe
2
O
3
@SBA-15 composites
/Al @SBA-15 catalysts
programmed reduction and CO diffuse-reflectance infrared Fourier-transformation spectroscopy studies
reveal that there is strong interaction between Pt nanoparticles and Al @SBA-15 composites and also
between Pt and FeO , resulting in Pt species with a positive charge being dominant in PtFe0.25/15AS cat-
alysts. Therefore, Pt species with positive charges, together with FeO species, are beneficial for preferen-
tial adsorption and activation of carbonyl bonds of CAL, so that the activity and selectivity to COL were
improved by PtFe /15AS catalysts (x represents the Fe/Pt molar ratio).
Ó 2017 Elsevier Inc. All rights reserved.
x
2 3
O
2 3
O
Cinnamaldehyde
Selective hydrogenation
Cinnamyl alcohol
x
x
x
1
. Introduction
often challenging to attain [5]. To improve the performance, partic-
ularly the selectivity, various approaches have been developed,
which include (1) control of the morphologies of Pt nanoparticles
by different synthesis methods [1,8–16]; (2) support-induced
modification of Pt properties via strong metal–support interaction
(SMSI) [17–24]; (3) Confinement of Pt particles by metal–organic
frameworks (MOFs) [25] or surface modification of Pt surface by
long-chain ligands [26,27] or thiols [28,29]; (4) addition of promot-
ers such as Zn, Sn, Ge, Fe, Co, or Cu to Pt catalysts [30–41]. Among
these approaches, the surface modification strategy can furnish
high selectivity to COL, but rather low activity was obtained by a
compromise in some cases [26–29]. Additionally, carbon-related
materials supported Pt–M catalysts (Pt–Fe [33,34,38,39,41], Pt–
Zn [34,39], Pt–Co [36], and Pt–Sn [23]) have shown excellent
results including activity and selectivity to COL. This means that
the support, the modifier, or the second metal in the supported
Pt catalysts would influence the selective hydrogenation of CAL
in a combinational manner. The improvement in catalytic perfor-
mance for the supported Pt catalyst by only one factor seems very
limited. Two or more strategies should be adopted to achieve bet-
ter results, including activity and selectivity to COL.
Selective hydrogenation of
a,b-unsaturated aldehyde for syn-
thesis of
efficient routes, in that
tant intermediates for synthesis of fragrances and pharmaceuticals
1]. However, this reaction is not thermodynamically favored
a,b-unsaturated alcohols is one of the most atomically
a
,b-unsaturated alcohols are very impor-
[
because the C@O bond energy is higher than that of the C@C bond.
Nevertheless, efforts have been made in the past two decades to
develop efficient catalyst systems. Hydrogenation of cinnamalde-
hyde (CAL), as a typical
reported model reaction for the selective activation of C@O or
a,b-unsaturated aldehyde, is a popularly
C@C double bonds (Scheme 1) [2–7].
The most commonly used catalysts for the selective hydrogena-
tion of CAL are noble-metal-based catalysts [3–7], among which
the Pt-based catalysts have been widely investigated. However,
satisfactory selectivity to the desired cinnamyl alcohol (COL) is
⇑
Corresponding authors.
E-mail address: xhli@chem.ecnu.edu.cn (X. Li).
These authors contributed equally to this work.
1
http://dx.doi.org/10.1016/j.jcat.2017.07.026
021-9517/Ó 2017 Elsevier Inc. All rights reserved.
0

H. Pan et al. / Journal of Catalysis 354 (2017) 24–36
25
O
+H
2
+H
2
hydrocinnamaldehyde, HCAL
OH
O
+H
2
+H
2
hydrocinnamyl alcohol, HCOL
cinnamaldehyde, CAL
OH
cinnamyl alcohol, COL
Scheme 1. Selective hydrogenation of CAL.
With regard to support materials for metal catalysts, silica is
one of the most commonly used supports for the liquid-phase
hydrogenation of CAL [11,16,30,40]. Sometimes Pt catalyst sup-
with a concentration of 14.8 mg Pt/ml was added dropwise to
0.562 g of the 15AS support. The mixture was continuously stirred
and then dried in air at ambient temperature. After that, the light
yellow solid was calcined in air at 500 °C for 3 h, followed by
ported on acidic composite oxides, such as SiO
CeO –ZrO [21], facilitate hydrogenation of C@O groups in
,b-unsaturated aldehydes. Mesoporous Al @SBA-15 compos-
2 2
–ZrO [24] and
ꢀ1
2
2
reduction under H
The Fe-doped Pt/15AS catalysts were also prepared in a manner
similar to that for the Pt/15AS catalyst, using H PtCl and
Fe(NO O as the precursors. The resulting catalysts, which
ꢁ9H
contained 5 wt% Pt and various Fe contents (0.143 to 1.43 wt%),
were denoted as PtFe /15AS, where x referred to the Fe/Pt molar
ratio. Correspondingly, the x value varied from 0.1 to 1.
2
(99.999% purity, 60 mlꢁmin ) at 500 °C for 3 h.
a
2 3
O
ites, with a thin layer of amorphous alumina coated on a SBA-15
surface as a kind of special composite oxide, not only retain the
mesoporous structure of SBA-15 silica, but also introduce a large
2
6
3
)
3
2
number of Lewis acid sites originated from Al
2
O
3
. Moreover, owing
x
to interaction of alumina and silica at the interface, a small number
of Brønsted acid sites are also formed in the corresponding
Al
Al
2
O
O
3
@SBA-15
composites.
The
unique
properties
of
2.3. Catalyst characterizations
2
3
@SBA-15 composites would be evitably beneficial for interac-
tion of metal and support and thus modulate the electronic
properties of metal nanoparticles when used as a support for them.
In our preliminary studies, Pt catalyst supported on mesoporous
X-ray diffraction (XRD) patterns of the samples were collected
using a Bruker D8 Advance instrument, using CuK
Nitrogen adsorption–desorption isotherms were measured at
196 °C (77 K) using a Quantachrome Autosorb-3B system after
a radiation.
Al
lective in related asymmetric hydrogenation of
42,43].
With these findings in hand, we are motivated to apply Pt or
Pt-Fe catalyst supported on mesoporous Al @SBA-15 composites
2
O
3
@SBA-15 composites was proved to be active and enantiose-
ꢀ
a-ketoesters
the samples were evacuated at 200 °C for 10 h. The Brunauer–Em
met–Teller (BET) specific surface area was calculated using an
adsorption branch recorded in the relative pressure range from
[
2 3
O
0
.05 to 0.35. The pore size distribution curves were calculated
for the liquid-phase selective hydrogenation of CAL. It is antici-
pated that there is strong interaction between Pt nanoparticles
via analysis of the adsorption branch of the isotherm, using the Bar
rett–Joyner–Halenda (BJH) algorithm. Scanning electron micro-
scopy (SEM) images were taken on a Hitachi S4800 electron micro-
scope with an acceleration voltage of 20 kV. Transmission electron
microscopy (TEM) images were recorded with a JEM-2100F micro-
scope with a field emissive gun, operated at 200 kV and with a
point resolution of 0.24 nm. Before measurements, the samples
were dispersed in ethanol under sonication and dropped onto a
copper grid to take TEM images.
and Al
O
2 3
@SBA-15 composites and between Pt and Fe; thus the
2 3
Pt–Fe/Al O @SBA-15 catalyst can exhibit good performance in
the liquid-phase hydrogenation of CAL via modulation of the Pt
surface electronic state by acidic support and by interaction with
the second metal component.
2
. Experimental
X-ray photoelectron spectroscopy (XPS) measurements were
performed using a Thermo Fisher Scientific ESCALAB 250Xi spec-
2.1. Chemicals
trometer with an AlKa radiation (1486.6 eV) incident beam. The
Chloroplatinic acid hexahydrate (H
2
PtCl
6
ꢁ6H
2
O) and other
sample was pretreated in situ for 2 h under high-purity hydrogen
ꢀ1
chemicals were of analytical grade and used as received. Pluronic
23 (EO20PO70EO20, MW = 5800) was purchased from Sigma-
Aldrich. CAL was purchased from Alfa Aesar and was used as
received. Other reagents including Fe(NO O, Al(NO O,
tetraethoxysilane (TEOS), hydrochloric acid (HCl, 37%), and iso-
propanol were purchased from Sinopharm Chemical Reagent Co.,
Ltd. and used as received.
(>99.999%, 30 mL min ) at 400 °C in a reactor attachment of the
XPS spectrometer. The binding energy (BE) was calibrated using
CAC binding energy at 284.4 eV in order to compare the BE with
the data from the literature. The spectra shown in the figures have
been corrected by subtraction of a Shirley background. Spectral fit-
ting and peak integration were done using XPSPEAK software.
CO chemisorption on the catalysts was performed by diffuse-
reflectance infrared Fourier-transformation spectroscopy (DRIFTS).
1
3
3
) ꢁ9H
2
3
)
3
ꢁ9H
2
2
.2. Support and catalyst preparation
The catalyst was pretreated in
4
a
H
2
flow (99.999% purity,
0 ml min ) at 400 °C for 2 h before CO adsorption at 35 °C. After
the adsorption of CO, the sample was purged with N (99.999%
purity, 40 ml min ) at 35 °C for 1 h. All of the IR spectra were col-
ꢀ1
SBA-15 was prepared according to the method reported in liter-
2
ꢀ1
2 3 2 3
ature [44]. The Al O @SBA-15 mesoporous composite with a Al O
ꢀ1
content of 15 wt% (denoted as 15AS) was prepared by an ultrasonic
impregnation method [42]. The supported Pt/15AS catalyst with a
nominal Pt content of 5 wt% was prepared by an incipient wetness
lected using 64 scans at a resolution of 4 cm
.
The H temperature-programmed reduction (TPR) technique
2
was employed to analyze the reducibility of the catalyst using
Micromeritics AutoChem II Chemisorption Analyzer. A sample of
2 6
impregnation method. Briefly, 2 ml of H PtCl aqueous solution

2
6
H. Pan et al. / Journal of Catalysis 354 (2017) 24–36
ꢀ
ꢀ
5
0 mg of the as-prepared (calcined) catalyst was placed in a quartz
ꢀ
ꢀD
H
r
ðꢀr
0
Þ
g
qbRE
ꢀ
ꢀ
A
ꢀ
ꢀ1
(interphase) heat transfer gave ꢀ
2
ꢀ < 0:15, demonstrating
reactor and pretreated in a He flow (99.999% purity, 20 ml min
at 300 °C for 60 min in order to remove the adsorbed water and
carbonates, and then the sample was cooled to 40 °C in a He flow
)
h
t
T
b
R
no heat transfer limitations. The influence of the stirring rate on
catalytic performance was also investigated to make sure that
the hydrogenation reaction was out of the diffusion control zone
(see the Supporting Information for the details).
ꢀ1
(
99.999% purity, 20 ml min ). After that, the sample was heated
ꢀ1
from 40 to 700 °C at 10 °C min under a mixture of 10% H
2
–Ar
consumption was monitored by a
gas chromatograph (GC) with a thermal conductivity detector
TCD).
ꢀ1
(
20 ml min ). The rate of H
2
3
. Results and discussion
(
3.1. Selective hydrogenation of CAL over various Pt catalysts
2.4. Catalytic test
First, we investigated Pt catalysts supported on different mate-
2 3
rials including pristine SBA-15 silica, Al O , and 15AS for the
The catalysts were tested for the liquid-phase hydrogenation
liquid-phase hydrogenation of CAL. Table 1 lists the detailed
results for the liquid-phase hydrogenation of CAL over Pt/SBA-15,
Pt/15AS, and a commercial Pt/Al O catalyst. The Pt/SBA-15 cata-
2 3
lyst gives a CAL conversion of 22.3% and a COL selectivity of
40.9%. Besides COL, the products also contain HCAL (hydrocin-
namaldehyde) and HCOL (hydrocinnamyl alcohol). The commercial
of CAL in a 100 mL autoclave. For a typical test, 50 mg of the cat-
alyst was pretreated in a specially designed quartz tube under H
2
ꢀ1
(
99.999% purity, 40 ml min ) at 400 °C for 2 h before use. Then
the pretreated catalyst was immediately transferred into the
autoclave without exposure to air and mixed with solvent (con-
taining isopropanol and water with a volume ratio of 9:1) and
CAL. The reaction began when hydrogen (2.0 MPa, 99.999% pur-
ity) was introduced with stirring at 90 °C (1200 rpm). The reac-
tion was stopped after a proper time and the products were
analyzed using a GC (GC-2014, Shimadzu) equipped with a flame
ionization detector (FID) and a capillary column (DM-WAX,
2 3
Pt/Al O catalyst gives similar CAL conversion, but the COL selec-
tivity afforded by this catalyst is considerably higher (63.7%). For
the Pt/15AS catalyst, a COL conversion of 34.9% and a COL selectiv-
ity of 64.7% are obtained. That is, the Pt nanoparticles supported on
15AS composites show better results for the liquid-phase hydro-
genation of CAL.
3
0 m ꢂ 0.32 mm ꢂ 0.25
l
m). The response factor of each compo-
To explain the enhanced performance of the Pt/15AS catalyst,
the relevant Pt catalysts were characterized using SEM, XRD, and
TEM. The morphology of the 15AS composite was characterized
using SEM (Fig. 1). Amorphous alumina is uniformly dispersed
along the mesoporous channels and outside the mesopores of the
SBA-15 host, and no obvious needlelike crystalline alumina is
observed. The mesoporous structure of 15AS was also character-
ized by the low-angle XRD patterns. As displayed in Fig. 2A, simi-
larly to the SBA-15 host, the 15AS support and the Pt/15AS
catalyst exhibit an intense diffraction peak and two weak peaks,
nent was calculated using standard samples and was used to
calculate the conversion and selectivity.
It should be pointed out that the absence of internal and exter-
nal mass transfer limitations in the reaction was verified by the
Weisz–Prater criterion and the Mears criterion, respectively. The
absence of heat transfer limitation in the reaction was verified by
a Mears criterion [45]. At the maximum conversion, CWP = 6.37 ꢂ
ꢀ
6
1
0
M
< 1 and C = 0.0498 < 0.15, which ensured the absence of a
mass transfer limitation. The Mears criterion for external
Table 1
Reaction results of CAL hydrogenation with different Pt catalysts.a
Pt particle size (nm)b
Dispersion (%)c
Conv. (%)
TOF (s )d
ꢀ1
Selectivity (%)
Entry
Catalyst
Fe loading (wt.%)
Fe/Pt molar ratio (x)
COL
HCAL
HCOL
1
2
3
4
5
6
Pt/SBA-15
0
0
0
0
0
0
6.5
3.1
5.3
1.8
2.3
2.5
2.3
2.3
2.3
2.3
n.m.j
17.4
36.8
21.3
62.8
49.1
45.2
49.1
49.1
49.1
49.1
n.m.j
22.3
21.9
34.9
61.8
60.2
61.5
77.4
44.4
77.0
76.2
60.1
31.2
15.5
27.0
25.4
46.2
0.21
0.10
0.27
0.48
0.59
0.66
1.54
0.44
1.53
1.51
—
40.9
63.7
64.7
72.3
71.4
78.9
76.9
80.9
75.7
76.2
81.0
82.2
91.2
81.0
90.9
75.1
46.3
27.7
26.8
20.9
22.9
15.4
15.2
15.1
15.6
15.4
13.8
14.2
8.9
12.8
8.5
8.5
6.7
5.7
5.7
7.8
4.0
8.6
8.3
5.2
3.6
2.8
3.0
1.4
2.9
e
Pt/Al
2
O
3
Pt/15AS
PtFe0.10/15AS
PtFe0.15/15AS
PtFe0.20/15AS
PtFe0.25/15AS
PtFe0.25/15AS
PtFe0.25/15AS
PtFe0.25/15AS
PtFe0.3/15AS
PtFe0.45/15AS
0.143
0.215
0.287
0.359
0.359
0.359
0.359
0.430
0.650
1.43
1.43
0.359
0.359
0.10
0.15
0.20
0.25
0.25
0.25
0.25
0.30
0.45
1.0
f
7
g
8
h
9
1
1
1
1
1
1
1
0i
1
2
3
4
5
6
j
j
n.m.
n.m.
—
—
—
—
PtFe
PtFe
1
/15AS
/15AS
n.m.j
n.m.j
n.m.j
n.m.j
k
1
1.0
0.25
0.25
17.0
7.7
22.0
f
j
j
PtFe0.25/SBA-15
PtFe0.25/SBA-15
n.m.
n.m.
n.m.j
n.m.j
—
a
Reaction conditions: 50 mg catalyst, 7.5 mmol CAL (for entries 1–3) and 22.5 mmol CAL (for entries 4–16), 18 mL isopropanol and 2 mL water, 2 MPa H
2
, 90 °C, 1 h,
1
200 rpm. COL represents cinnamyl alcohol, HCAL represents hydrocinnamaldehyde, and HCOL represents hydrocinnamyl alcohol.
b
Pt particle size was calculated from the TEM images.
c
Dispersion was calculated based on dispersion (%) ꢂ d (Pt particle size in nm) = 1.13.
d
TOF value was defined as the number of moles of converted CAL per mole of Pt active sites per second.
e
5
0
2
1
1
wt% Pt/Al
.5 h.
5 °C, 1 h.
2 3
O catalyst was purchased from Alfa Aesar.
f
g
h
i
MPa H
2
, 0.5 h.
bar H , 0.5 h.
2
j
Not measured.
k
2
h.

H. Pan et al. / Journal of Catalysis 354 (2017) 24–36
27
(A)
Pt/15AS
5AS
SBA-15
1
1
2
3
4
5
2
theta (degree)
Pt(111)
(B)
Pt(200)
Pt(220)
Pt/SBA-15
Pt/15AS
Pt/Al O3
2
20
30
40
50
60
70
80
2
theta (degree)
Fig. 1. SEM images of (a) SBA-15 and (b) 15AS.
Fig. 2. (A) Low-angle XRD patterns of SBA-15, 15AS and Pt/15AS catalyst and (B)
wide-angle XRD patterns of Pt/SBA-15, Pt/15AS and commercial Pt/Al catalysts.
2 3
O
attributed to the (1 0 0), (1 1 0), and (2 0 0) planes of the hexagonal
structure (P6mm), respectively [44]. The difference in peak inten-
sity might be caused by different sample thicknesses for XRD mea-
surement. The Pt crystallites of the catalysts were characterized
using wide-angle XRD as well. As can be seen in Fig. 2B, the
Pt/SBA-15 catalyst shows three sharp diffraction peaks at 2h of
surface properties of Pt nanoparticles, so that both the activity
and selectivity to COL are promoted after Pt nanoparticles are
loaded onto the composite materials. Nevertheless, improvement
is still very limited at this stage.
3
9.8°, 46.4°, and 67.6°, assigned to the Pt(1 1 1), Pt(2 0 0), and Pt
(
2 2 0) planes, respectively. After alumina was coated onto
3.2. Selective hydrogenation of CAL over FeO
catalysts
x
-promoted Pt/15AS
SBA-15 to form 15AS composites, the diffraction of Pt crystallite
for the Pt/15AS catalyst became broader. As for the commercial
Pt/Al
2
O
3
catalyst, it shows extremely weak diffraction. This indi-
catalyst has quite small Pt par-
To further improve the performance of the Pt/15AS catalyst,
Fe-doped PtFe /15AS catalysts were employed for this reaction. It
cates that the commercial Pt/Al
2
O
3
x
ticles, while the Pt/SBA-15 catalyst has relatively larger Pt
particles. The Pt catalysts were also characterized using TEM and
the Pt particle size distribution was calculated accordingly
should be noted that owing to higher activity after Fe addition to
the Pt/15AS catalyst, the hydrogen consumption was too fast to
control the reaction So, we added three times as much CAL to carry
(
Figs. S2 and S3 in the Supporting Information and Fig. 3). As listed
in Table 1 as well, the average Pt particle sizes for the Pt/SBA-15,
Pt/15AS, and Pt/Al catalysts center at 6.5, 5.3, and 3.1 nm,
x
out the reaction with the PtFe /15AS catalysts. In addition, the
reaction time was also shortened to one-half according to hydro-
gen consumption for the PtFe0.25/15AS catalyst.
2 3
O
respectively. Correspondingly, turnover frequency (TOF, the num-
ber of moles of converted CAL per mole of Pt active sites per sec-
ond) based on surface Pt atoms was also calculated and listed in
As summarized in Table 1, with addition of trace amounts of Fe
to the Pt/15AS catalyst, the catalytic performance, especially for
the conversion of CAL, is significantly enhanced. As a result, the
PtFe0.10/15AS catalyst shows 61.8% conversion of CAL and 72.3%
selectivity to COL. With Fe/Pt molar ratio x increasing from 0.10
to 0.20, the activities of the catalysts remain almost constant.
When the x value increases to 0.25, the PtFe0.25/15AS catalyst gives
a CAL conversion of 77.4% and a COL selectivity of 76.9% within a
half reaction time. However, when the Fe/Pt molar ratio x is further
increased from 0.30 to 1, the CAL conversion declines instead,
2 3
Table 1. The Pt/SBA-15, Pt/Al O , and Pt/15AS afford TOF values
ꢀ1
of 0.21, 0.10, and 0.27 s , respectively.
The enhanced performance of the Pt/15AS catalyst suggests a
positive role of the support in the reaction, which is probably
due to the presence of Lewis acidic sites in the support
[
11,21,24,43]. Additionally, we deduce that the interaction of Pt
nanoparticles with Al @SBA-15 composites can modulate the
2 3
O

2
8
H. Pan et al. / Journal of Catalysis 354 (2017) 24–36
Pt/15AS
2
1
0
0
0
1
.0-2.0 2.0-3.0 3.0-4.0 4.0-5.0 5.0-6.0 6.0-7.0 7.0-8.0 >8.0
Particle Size (nm)
Fig. 3. TEM image and particle size distribution of Pt/15AS.
although the COL selectivity increases slightly. As a result, the
PtFe1.0/15AS catalyst with the highest Fe/Pt molar ratio of 1.0 only
converts 15.5% of CAL under the same conditions, although the
selectivity to COL reached a maximum of 91.2%.
MSACOL versus Fe/Pt molar ratio x also displays a volcano-like curve
ꢀ1
ꢀ1
and the highest MSACOL of 10.71 mol gPt
PtFe0.25/15AS catalyst (see the inset of Fig. 4).
The reusability of the PtFe /15AS was also investigated. As dis-
h
is furnished by the
x
In order to make clear the changing selectivity, the hydrogena-
tion of CAL with the PtFe1.0/15AS catalyst was prolonged to 2 h. To
our disappointment, although the CAL conversion increased to
played in Fig. 5, the PtFe0.25/15AS catalyst can be recovered easily
by filtration and recycled just by washing with fresh solvent three
times. Consequently, the PtFe0.25/15AS catalyst can be reused at
least six times without obvious loss in activity or selectivity to
COL. We also detected the Pt leaching amount in the filtrate, and
to our delight, the PtFe0.25/15AS catalyst is very stable and the lea-
ched Pt amount is below the ICP-AES detection limit.
2
7.0%, the selectivity to COL decreased dramatically to 81.0%. Nev-
ertheless, the selectivity to HCOL did not increase, while the selec-
tivity to HCAL increased obviously. This is a very common
phenomenon in the selective hydrogenation of CAL [14,22,26,37].
For clarity, we correlated the mass-specific rates of CAL con-
sumption (MSACAL) (defined as the moles of converted CAL per
gram of Pt per hour) and COL formation (MSACOL) (defined as the
moles of formed COL per gram of Pt per hour) with the Fe/Pt molar
Based on these results, the catalytic performance of PtFe
was greatly enhanced over that of Pt/15AS. Furthermore, among all
of the PtFe /15AS catalysts, the PtFe0.25/15AS catalyst affords the
x
/15AS
x
highest MSACAL and MSACOL. Of particular note is that the
PtFe0.25/15AS catalyst can easily be recovered and recycled.
ratio x in PtFe
x
/15AS catalysts. As shown in Fig. 4, the MSACAL
ꢀ1
ꢀ1
greatly increased from 1.05 mol gPt
h
for the Pt/15AS catalyst
To understand the difference between PtFe
x
/15AS catalysts with
ꢀ
1
ꢀ1
to 5.56 mol gPt
h
for the PtFe0.10/15AS catalyst. The highest
different Fe/Pt molar ratios, the PtFe /15AS catalysts were charac-
x
ꢀ
1
ꢀ1
MSACAL of 13.93 mol gPt
h
was obtained with the PtFe0.25/15AS
terized using some general techniques. As already mentioned, the
low-angle XRD patterns indicate that the 15AS support and the
Pt/15AS catalyst reserve the mesostructure of the SBA-15 host.
The well-ordered mesoporous structures of the 15AS support, the
catalyst with a COL selectivity of 76.9%. However, when the Fe/Pt
molar ratio x further increased, the MSACAL declined. That is, for
Fe/Pt with an optimal molar ratio, the MSACAL is increased by
almost 13 times for the bimetallic PtFe0.25/15AS catalyst in com-
parison with the monometallic Pt/15AS catalyst. Similarly, the
Pt/15AS, and the PtFe
x
/15AS catalysts were further confirmed by
N
2
sorption (Fig. 6). All the samples show typical type IV hysteresis
PtFe /15AS
PtFe0.25/SBA-15
1
4
2
x
1
0
8
6
4
2
0
Pt/SBA-15
Pt/Al O
.
1
2
3
.
.
.
10
8
6
0
.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Fe/Pt molar ratio x
4
2
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Fe/Pt molar ratio x
x
Fig. 4. Mass-specific rate of CAL consumption (MSACAL) and COL formation (MSACOL) vs. the Fe/Pt molar ratio x in the PtFe /15AS catalysts.

H. Pan et al. / Journal of Catalysis 354 (2017) 24–36
29
1
00
loops in the relative pressure range from 0.35 to 0.80, demonstrat-
ing that the mesoporous structure of the SBA-15 host is still main-
tained even after loading of alumina or Pt nanoparticles. However,
the BET specific surface area and pore volume calculated per g of
SBA-15 decrease after the loading of alumina. As listed in Table 2,
compared with that of the SBA-15 host, the BET specific surface
Conv.
Sel.
8
6
4
2
0
0
0
0
0
2
ꢀ1
area of the 15AS decreases from 768 to 493 m g and the pore
3
ꢀ1
volume decreases to 0.82 cm g . The average pore size of the
5AS is 7.8 nm, which is slightly smaller than that of the SBA-15
1
host (8.0 nm). Moreover, the loading of Pt nanoparticles in the cat-
alyst results in a further decline in the BET specific surface area
2
ꢀ1
3
ꢀ1
(
419 m g ) and pore volume (0.66 cm g ), while the addition
of Fe with Fe/Pt molar ratio x varying from 0.10 to 1.0 in the cata-
lyst hardly changes the physicochemical properties, with the BET
2
ꢀ1
specific surface area ranging from 409 to 425 m g and the pore
3
ꢀ1
volume ranging from 0.57 to 0.69 cm g . That is, although addi-
tion of FeO to Pt/15AS catalysts causes a decrease in BET specific
surface area and pore volume, the differences in those physico-
chemical parameters of the PtFe /15AS catalyst with various Fe/
Pt ratios are negligible, while the catalytic performance of the
1
2
3
4
5
6
7
x
Running time
x
Fig. 5. The reusability of the PtFe0.25/15AS catalyst for the hydrogenation of CAL.
The reaction conditions are the same as those for entry 7, Table 1.
800
700
600
500
400
300
200
100
0
.08
SBA-15
A
1
5AS
0.06
Pt/15AS
PtFe0.1/15AS
PtFe /15AS
0
0
0
.04
.02
.00
1
-
0.02
2
4
6
8
10
12
14
Pore diameter (nm)
SBA-15
5AS
Pt/15AS
1
PtFe0.1/15AS
PtFe /15AS
1
0
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
8
7
6
5
4
3
2
1
00
6
5
4
3
2
1
0
B
PtFe0.25/15AS
PtFe0.3/15AS
PtFe0.25/SBA-15
00
00
00
00
00
00
00
0
5
10
15
20
Pore size (nm)
PtFe0.25/15AS
PtFe0.3/15AS
PtFe0.25/SBA-15
0.0
0.2
0.4
0.6
0.8 1.0
P/P0
Fig. 6. N
2 1
adsorption–desorption isotherms and pore size distributions (inset) of (A) SBA-15, 15AS, Pt/15AS, PtFe0.1/15AS, and PtFe /15AS and (B) PtFe0.25/15AS, PtFe0.3/15AS,
and PtFe0.25/SBA-15.

3
0
H. Pan et al. / Journal of Catalysis 354 (2017) 24–36
Table 2
Physicochemical parameters of SBA-15, 15AS, and the related catalysts.
dispersed while others aggregate to form very large and perfect
cubic crystallite (see Fig. S4 in the Supporting Information). These
results agree well with the XRD results. It is clear that the addition
ꢀ1
)a
(cm3
ꢀ1
)a
Entry
Sample
S
BET (m2
g
V
p
g
p
D (nm)
of a small amount of FeO
leads to smaller Pt size (1.8–2.5 nm) compared with the bare
Pt/15AS (5.3 nm), while a large amount of FeO (Fe/Pt > 0.25)
results in large Pt particles (>10 nm). In some cases, the average
Pt particle size calculated from the Scherrer equation is larger than
x
(Fe/Pt ꢄ 0.25) in the Pt/15AS catalyst
1
2
3
4
5
6
7
8
SBA-15
15AS
Pt/15AS
PtFe0.10/15AS
PtFe0.25/15AS
PtFe0.30/15AS
768
493
419
425
418
409
420
668
1.02
0.82
0.66
0.69
0.61
0.57
0.59
0.94
8.0
7.8
6.9
7.3
6.8
7.4
7.5
7.5
x
2
0 nm, indicating that the Pt nanoparticles are located on the outer
surface of 15AS. Thus, the different Pt particle sizes for the
PtFe /15AS catalysts with variable Fe/Pt molar ratio might play
an important role in determining the catalytic performance. Never-
theless, when the Fe/Pt molar ratio is lower than 0.25, the PtFe /15-
1
PtFe /15AS
PtFe0.25/SBA-15
a
x
The BET specific surface area and pore volume were calculated per g of SBA-15.
x
AS catalysts still furnish rather different results, although they
have similar Pt particle sizes. Hence, the Pt particle size is not
the only factor influencing the catalytic performance.
x
PtFe /15AS catalysts is dramatically influenced by the Fe/Pt molar
ratio. Therefore, the difference in physicochemical properties is
not the key factor that affects the catalytic performance.
Besides, on recalling the performance of Pt/SBA-15, Pt/15AS,
The Pt crystallite in the PtFe
ized using wide-angle XRD. As displayed in Fig. 7, with addition of
FeO , the Pt(1 1 1) diffraction at 2h of 39.8° for the PtFe0.10/15AS
catalyst is dramatically suppressed, indicating that addition of a
trace amount of FeO is very beneficial for the dispersion of Pt
nanoparticles. With increasing FeO amount, the PtFe0.15/15AS,
PtFe0.20/15AS, and PtFe0.25/15AS catalysts give weak diffraction
peaks as well. If the FeO amount is further increased, the PtFe /15-
x
/15AS catalysts was also character-
and PtFe
x
/15AS catalysts, we find that the performance of Pt cata-
is added to the support
lyst is significantly improved after Al
2 3
O
x
and particularly after Fe is doped. Addition of alumina to the sup-
port can certainly be beneficial to dispersion of Pt particles due to
interaction of Pt with alumina. Additionally, coating of SBA-15
with alumina forms a large amount of Lewis acid, which is helpful
for selective adsorption of C@O double bonds [21,24]. Furthermore,
doping of Fe with low content also greatly enhances the catalytic
performance, indicating that there is strong interaction between
x
x
x
x
AS catalyst (x ꢃ 0.30) shows a very strong Pt(1 1 1) diffraction
peak, demonstrating that large Pt crystallites are formed.
x
Pt and FeO . In comparison, the PtFe0.25/15AS catalyst shows supe-
The TEM images of some representative catalysts are shown in
Fig. 8, as well as the Pt particle size distributions. As already dis-
cussed in the previous section, the Pt/15AS catalyst shows a broad
Pt particle size distribution, ranging from 1.0 to 8.0 nm and center-
rior performance for the tested reaction. This could be ascribed to
two reasons. The first is that Pt nanoparticles for the PtFe0.25/15AS
catalyst can disperse very well on the support, which can be con-
firmed from the XRD pattern and TEM image (Figs. 7 and 8). The
second reason might be that the PtFe0.25/15AS catalyst has optimal
surface electronic properties, which need further characterization.
x
ing at 3.0–6.0 nm (Fig. 3). After the FeO doping, the Pt particles in
the PtFe0.10/15AS, PtFe0.20/15AS, and PtFe0.25/15AS catalysts
become smaller and more uniform than those in the Pt/15AS cata-
lyst, with Pt particle sizes ranging from 2.0 to 4.0, 2.0 to 4.0, and 1.5
to 3.0 nm, respectively. As for the PtFe0.30/15AS catalyst, the Pt
particles are not uniformly dispersed; some Pt particles are well
3.3. Further investigation and discussion
x
Before thoroughly characterizing the PtFe /15AS catalysts using
special spectroscopic and temperature-programmed techniques,
we want to compare the results in this case with the literature in
advance. Additionally, the catalytic performance of the PtFe /15AS
x
catalysts, especially the outstanding PtFe0.25/15AS catalyst, should
be further explored to the greatest extent under milder conditions.
As discussed above, our findings suggest that an optimal Fe/Pt
ratio is crucial for the catalytic performance, which is in accor-
dance with observations in the literature. For instance, Neri et al.
PtFe /15AS
x
Pt (111)
x
Pt (200)
Pt (220)
1.0
0.45
[
46] investigated zeolite P-supported Pt–Fe catalysts for the selec-
tive liquid-phase hydrogenation of CAL. They found that the high-
est COL selectivity of 85% at 90% CAL conversion was obtained at a
Pt/Fe weight ratio of 1, and the selectivity decreased with further
increase in the Fe amount. However, the authors did not discuss
the reasons. Very recently, Shi et al. [47] found that Fe/Pt molar
ratios of 0.2–0.25 were optimal for the selective hydrogenation
0
.40
.35
0
0
.30
0.25
of CAL over Pt–FeO
ment method, in good accord with our observations. Besides, it is
worth noting that, in our case, the PtFe /15AS catalysts have much
higher reaction rates (including MSACAL and MSACOL) than those
x 2
/SiO catalysts prepared by a galvanic displace-
0
.20
.15
x
0
ꢀ1
ꢀ1
0.10
reported in the literature. The highest MSACAL of 13.93 mol gPt h
ꢀ1
ꢀ1
(
with the highest MSACOL of 10.71 mol gPt
h ) obtained with the
Pt/15AS
PtFe0.25/15AS catalyst in our case is much higher than those
0
achieved with similar Pt–FeO
x
/SiO
h
2
catalysts, which furnished the
and the highest MSACOL of less
PtFe0.25/SBA-15
ꢀ1
ꢀ1
highest MSACAL of 2.24 mol gPt
ꢀ
1
ꢀ1
than 2 mol gPt
h
[47].
10
20
30
2
40
50 60 70 80
theta (degree)
In addition, the role of the Al
2 3
O @SBA-15 composite (15AS)
was also emphasized by comparing the performance of the
PtFe0.25/15AS and PtFe0.25/SBA-15 catalysts prepared by the same
method. As listed in Table 2, the PtFe0.25/SBA-15 catalyst shows
x
Fig. 7. Wide-angle XRD patterns of PtFe /15AS catalysts and PtFe0.25/SBA-15
catalyst.

H. Pan et al. / Journal of Catalysis 354 (2017) 24–36
31
PtFe0.10/15AS
30
2
0
0
0
1
1.0-2.0 2.0-3.0 3.0-4.0 4.0-5.0 5.0-6.0 6.0-7.0 7.0-8.0 >8.0
Particle Size (nm)
PtFe0.20/15AS
40
30
20
10
0
1
.0-2.0 2.0-3.0 3.0-4.0 4.0-5.0 5.0-6.0 6.0-7.0 7.0-8.0 >8.0
Particle size (nm)
PtFe0.25/15AS
5
4
3
2
1
0
0
0
0
0
0
1.0-2.0 2.0-3.0 3.0-4.0 4.0-5.0 5.0-6.0 6.0-7.0 7.0-8.0 >8.0
Particle Size (nm)
x
Fig. 8. TEM images and particle size distributions of PtFe /15AS catalysts.
inferior activity (25.4% conversion of CAL) under the same condi-
tions as those for the PtFe0.25/15AS catalyst, although the selectiv-
the catalysts containing large amounts of FeO
x
(Fe/Pt > 0.25), such
as PtFe0.3/15AS catalyst, usually have much larger Pt particles with
much lower dispersion, and perfect Pt single crystals are also
formed on the support surface (see the TEM image in Fig. S4), it is
not reasonable to compare the TOF value of a Pt nanoparticle
catalyst with that of a single-crystal catalyst. Therefore, the TOF val-
ues on these catalysts (Fe/Pt > 0.25) are not provided.
ꢀ1
ꢀ1
ity to COL of 90.9% is higher; thus the MSACOL (4.15 mol gPt
h ;
see the red dot in the inset of Fig. 9) over this catalyst is much
lower than that obtained with the PtFe0.25/15AS catalyst
ꢀ1
ꢀ1
(
10.71 mol gPt h ). Moreover, a considerably decreased COL
selectivity of 75.1% was afforded by the PtFe0.25/SBA-15 catalyst
after the reaction time was prolonged (Entry 16, Table 2). There-
x
To fully understand the PtFe /15AS catalysts, a comparison was
fore, it could be concluded that not only FeO
SBA-15 composite has significant influence on the catalytic
behaviors.
x
but also Al
2
O
3
@-
also made between the TOF values obtained with Pt–Fe catalysts
supported on different materials reported in the literature. For
instance, the bimetallic Pt–Fe catalyst supported on mesoporous
carbon prepared by the incipient wetness impregnation method
Considering the different Pt particle sizes in the catalysts, TOF
ꢀ1
values based on surface Pt atoms for the PtFe
also calculated and listed in Table 2. The addition of a low content of
FeO
(Fe/Pt ꢄ 0.25) greatly enhances the intrinsic activity of the cat-
alysts. Hence, the PtFe0.25/15AS catalyst affords a TOF of 1.54 s
which is five times higher than that on the Pt/15AS catalyst. Since
x
/15AS catalysts were
afforded 0.049 s
TOF, which was calculated according to the
results obtained with PtFe/C-SA catalyst with a dispersion of 25%
x
[39]. The Fe-doped Pt catalysts supported on zeolite P showed
ꢀ1
ꢀ2 ꢀ1
,
higher TOF values (20.7–80.7 ꢂ 10
s
) compared with the
ꢀ2
ꢀ1
monometallic Pt catalyst (0.18–0.23 ꢂ 10
s
) [46]. Nevertheless,

3
2
H. Pan et al. / Journal of Catalysis 354 (2017) 24–36
as under lower hydrogen pressure or at ambient temperature. As
listed in entry 8 of Table 1, the reaction performed at ambient tem-
perature also afforded medium CAL conversion and good COL
ꢀ1
selectivity. Accordingly, a TOF of 0.44 s was furnished by the
PtFe0.25/15AS catalyst at room temperature, which is comparable
to those reported in the literature as obtained at higher tempera-
tures [46], but much higher than those reported in Ref. [39]. As
for the MSACOL obtained with the PtFe0.25/15AS catalyst at room
temperature, it is nearly twice that reported in Ref. [47]. Addition-
ally, the similar reaction results obtained under lower hydrogen
pressure, such as 1.0 MPa and 1 bar, confirm that the selective
hydrogenation of CAL is a zero-order reaction with regard to
hydrogen pressure.
3.4. Further characterization of PtFe
x
/15AS catalysts
To deeply understand the superior ability of the PtFe
x
/15AS cat-
alyst, especially the PtFe0.25/15AS catalyst, the relevant catalysts
were further characterized using a series of techniques. First, a
close investigation of the morphology of the PtFe0.25/15AS catalyst
was conducted using HRTEM (Fig. 9). The presence of FeO is con-
x
firmed by Fourier transformation (FT) identification (Fig. 9a), with
the spots at 0.29 and 0.33 nm corresponding to the (2 0 2) and
(
0
ꢀ4 1 1) planes of
.226 nm is assigned to the Pt(1 1 1) plane, as shown in Fig. 9b.
In addition to the isolated -Fe entities, some -Fe particles
are contacting Pt particles (indicated by the white line), suggesting
interactions between Pt and -Fe
To further characterize the distribution of Pt and FeO
PtFe /15AS catalysts, the HAADF-STEM images for some catalysts
2 3
e-Fe O , respectively. Also, the lattice fringe of
e
O
2 3
e
2 3
O
e
2 3
O .
x
in the
x
were taken. As shown in Fig. 10, for both PtFe0.10/15AS and
PtFe0.25/15AS catalysts, the Pt atoms form aggregations, as evi-
denced by the bright spots in the STEM images; while the Fe atoms
are uniformly distributed without forming any aggregation. Never-
theless, the signals of Pt are spatially associated with the Fe signals,
suggesting that the Pt entities are closely related to the Fe element.
To study the electronic states of Pt and Fe, some representative
catalysts were further characterized by DRIFTS using CO as a probe
molecule. The IR spectroscopic study of CO adsorption is one of the
methods commonly used to gain insight into electronic properties.
To allow a direct comparison of these data with the catalytic
Fig. 9. HRTEM images of (a, b) PtFe0.25/15AS catalyst.
x
they are much lower than those obtained with PtFe /15AS catalysts
in this study. The bimetallic Pt–Fe catalyst supported on carbon
nanotubes prepared by a two-step microwave heating method
ꢀ1
showed TOF 0.94–2.21 s for the liquid-phase hydrogenation of
CAL under similar conditions [38], which are of the same
magnitude as those obtained in our case. Additionally, although
the authors did not report the TOF value for the very similar
Pt–FeO
method, the PtFe
MSACAL and MSACOL than the Pt–FeO
have been discussed above.
x
/SiO
2
catalysts prepared by
a
galvanic displacement
results, the PtFe
flow at 400 °C for 2 h before IR spectra were taken. Fig. 11 shows
the DRIFT spectra of CO adsorbed on PtFe /15AS catalysts after
x
/15AS catalysts were pretreated in a hydrogen
x
/15AS catalysts in this study furnish higher
x
/SiO
2
catalysts [47], which
x
purging with nitrogen at 35 °C for 1 h. For all the tested catalysts,
Furthermore, we also compared the results in this case with
those obtained under similar conditions in the literature. As
only one mode of CO adsorption, linearly adsorbed CO, is identified.
x
As displayed in Fig. 11, CO linearly adsorbed onto PtFe /15AS cata-
ꢀ1
reported in Ref. [21], 5% Pt/CeO
2
–ZrO
2
catalyst showed TOF
lysts shows a broad band centered at 2073–2081 cm . For
instance, the PtFe0.10/15AS catalyst shows a linear CO adsorption
ꢀ1
ꢀ1
1
849 h (0.51 s ) and 71.1% selectivity to COL at 100 °C under
ꢀ
1
2
0 bar hydrogen pressure. Another example is that 0.1 g of 5.4 wt
at 2073 cm
and the PtFe0.20/15AS catalyst gives a linearly
ꢀ1
%
Co–1.7 wt% Pt/SiO
2
catalyst gave a 28.6% conversion of 3 ml
adsorbed CO band centered at 2077 cm . As for the IR band of
CO linearly adsorbed on PtFe0.25/15AS catalyst, it is centered at
2081 cm , a slightly higher CO vibration frequency than for the
other two PtFe /15AS catalysts. The CO band position in this case
CAL within 2 h, furnishing a selectivity to COL of 78% at 80 °C under
a hydrogen pressure of 4.0 MPa [37]. Additionally, as discussed
ꢀ1
above, 50 mg of 0.8Pt–3.8FeO
x
/SiO
2
catalyst showed a 21% conver-
x
sion of 0.5 mL CAL within 1 h, affording 84% selectivity to COL with
is a little higher than that observed on Pt catalysts supported on
alumina or on silica [48]. This means that the electron density
ꢀ
1
ꢀ1
a MSACAL of 2.24 mol gPt
h
at 150 °C under a hydrogen pressure
of 1.0 MPa [47]. In this study, 50 mg of PtFe0.25/15AS catalyst
showed a 77.4% conversion of 22.5 mmol CAL (about 2.9 ml) within
was somewhat lower on the PtFe
words, relatively strong interaction between Pt nanoparticles and
Al @SBA-15 composites or FeO occurred during calcination in
air at 500 °C. The charge transfer took place from Pt nanoparticles
to the Lewis acids and Brønsted acids in the Al @SBA-15 com-
posites or to FeO [42].
Considering that the three PtFe
CO-DRIFTS have similar Pt particle size distribution and that the
average Pt particle size is in the range of 1.8–2.5 nm, the tiny influ-
ence of Pt particle size on the CO vibration band can be neglected.
x
/15AS catalyst [48,49]. In other
0
.5 h at 90 °C under a hydrogen pressure of 2.0 MPa, affording
2
O
3
x
ꢀ1
selectivity to COL of 76.9% with a TOF of 1.54 s and MSACAL of
1
ꢀ1
ꢀ1
3.93 mol gPt
h
. Among the results mentioned above, the selec-
2 3
O
tivity to COL is comparable. Nevertheless, the activity obtained in
this study in terms of TOF or MSACAL is the highest under similar
reaction conditions.
Moreover, we also performed the selective hydrogenation of
CAL with the PtFe0.25/15AS catalyst under milder conditions, such
x
x
/15AS catalysts characterized by

H. Pan et al. / Journal of Catalysis 354 (2017) 24–36
33
Fig. 10. HAADF-STEM images of PtFe
x
/15AS catalysts and STEM-EDS elemental mapping images for the area enclosed by the orange square.
Thus, the lower CO vibration position reflects that the catalyst has
higher electron density on Pt, because an increase in the electron
2081
0.2
2073
density on Pt in turn increases the extent of back donation to the
⁄
CO 2
p
orbitals. The increase in the electron density on Pt might
to Pt nanoparticles.
be contributed by electron transfer from FeO
x
This phenomenon is quite common for Fe-related bimetallic sys-
tems [50]. Based on the above discussion, the PtFe0.25/15AS catalyst
PtFe0.25/15AS
has the lowest electron density on Pt among the three PtFe
catalysts characterized by CO-DRIFTS. This indicates that the
Pt–FeO interaction in the PtFe /15AS catalysts is quite different
with different Fe amounts. As already mentioned above, there is
electron transfer from FeO to Pt atoms for PtFe0.10/15AS and
x
/15AS
x
x
PtFe0.20/15AS
PtFe0.10/15AS
x
PtFe0.20/15AS catalysts. However, because the PtFe0.25/15AS cata-
lyst is more electron-deficient than the PtFe0.10/15AS and
PtFe0.20/15AS catalysts, electron transfer might occur in the
PtFe0.25/15AS catalyst from Pt to FeO
The Pt–Al @SBA-15 interaction and Pt-FeO
further confirmed by the H TPR results, as shown in Fig. 12. For
x
instead [47].
2200
2150
2100
2050
2000
1950
2
O
3
x
interaction were
-1
2
Wavenumber (cm )
the monometallic Pt/15AS catalyst, there are two reduction peaks,
one centered at 94 °C and the other at 345 °C. The low-temperature
reduction peak is assigned to the reduction of either PtO or bulk
2
Fig. 11. DRIFTS spectra for CO adsorbed onto the PtFe
with N at 35 °C for 1 h.
x
/15AS catalysts after purging
2

3
4
H. Pan et al. / Journal of Catalysis 354 (2017) 24–36
Al 2p + Pt 4f
(a)
Al2p
399
δ
+
Pt
345
PtFe0.25/15AS
PtFe0.25/15AS
0
Pt
104
Al2p
Pt/15AS
0
Pt
94
Fe/15AS
Pt/15AS
1
00
200
300
400
500
600
700
80
78
76
74
72
70
68
o
Temperature ( C)
Binding Energy (eV)
Fig. 12. H
2
TPR profiles of as-prepared (calcined) 0.36 wt% Fe/15AS, 5 wt% Pt/15AS,
and PtFe0.25/15AS (5 wt% Pt–0.36 wt% Fe) catalysts.
x
PtO species interacting with the oxygen atoms of the support. The
second peak can be attributed to highly dispersed particles with
strong interaction with the support [51]. The existence of these
Pt4d
(b)
0
δ
Pt
+
315.9 314.2, Pt
2 3
species may be related to the ability of Al O @SBA-15 to stabilize
PtFe0.25/15AS
oxidized Pt particles on its surface. For the bimetallic PtFe0.25/15AS
catalyst, the reduction needs a slightly higher temperature. The
peak centered at lower temperature shifted to 104 °C and the
one centered at higher temperature shifted to 399 °C, about 44 °C
higher than that for the monometallic Pt/15AS catalyst. This indi-
x
cates that the intimate and strong Pt–FeO interaction occurs in
the PtFe0.25/15AS catalyst, in good agreement with the HRTEM
and HAADF-STEM results. For reference, a Fe/15AS sample with
the same Fe loading of 0.36 wt% as that for the PtFe0.25/15AS cata-
Pt/15AS
lyst was also prepared and characterized using H
tion of FeO occurred at relatively higher temperature and showed
a very broad but weak peak in the range of 400–600 °C [52].
Accordingly, we also calculated the H consumption of each cata-
lyst as follows: H uptake for the 0.36 wt% Fe/15AS, 5 wt% Pt/15AS,
and PtFe0.25/15AS containing 5 wt% Pt and 0.36 wt% Fe was 0.025,
2
TPR. The reduc-
x
350
340
330
320
310
300
2
Binding Energy (eV)
2
ꢀ1
0
2 2
.31, and 0.35 mmol g , respectively. The H /Pt and H /Fe ratios
were also calculated accordingly. As a result, the H
2
/Pt ratio was
Fe 2p
(c)
about 1.2, lower than the stoichiometric value of 2.0 if chloropla-
0
tinic acid was completely reduced to Pt species. This suggests that
711.7
Pt species cannot be fully reduced before the reaction starts,
mainly due to strong interaction of Pt species with 15AS and FeO
species. With regard to the H /Fe ratio, it is only 0.4 even if the
reduction is continued till 700 °C during the H TPR experiments,
x
718.3
2
725.0
PtFe0.25/15AS
731.9
2
3+
much lower than the stoichiometric value of 1.5 if Fe were fully
0
reduced to Fe . On recalling that the catalyst precursor was
reduced at 500 °C and pretreated at 400 °C in flowing hydrogen
before being submitted to the hydrogenation reaction, we can con-
clude that Fe species are in the oxidation state to a great extent.
The oxidation states of the elements in the PtFe0.25/15AS cata-
lyst probed with XPS after in situ pretreatment in a hydrogen flow
at 400 °C for 2 h are shown in Fig. 13. It can be observed that there
is an overlap between Pt 4f and Al 2p peak at about 70.0–79.0 eV
740
735
730
725
720
715
710
705
700
(Fig. 13a). However, the peaks can be easily deconvoluted to Al
Binding Energy (eV)
2
p and Pt 4f regions. The BE at 71.3 eV in the Pt4f region can be
0
assigned to 4f7/2 of the Pt species. The BE at 72.3 eV is lower than
that for Pt4f7/2 of the Pt species, but higher than that for Pt4f7/2 of
the Pt species; therefore, it can be attributed to Pt4f7/2 of partially
charged Pt species (0 < d < 2). Since the BEs of Al2p and Pt4f are
Fig. 13. XPS spectra of (a) Al2p + Pt4f, (b) Pt4d for PtFe0.25/15AS and Pt/15AS
catalysts, and (c) Fe2p for PtFe0.25/15AS catalyst.
2
+
0
d+
overlapped, the Pt4d spectrum was taken as well. The Pt4d5/2 spec-
species (0 < d < 2), because its BE is lower than that of typical Pt2+
trum clearly shows two components at binding energies of 314.2
(316.3 eV) [54].
0
d+
and 315.9 eV (Fig. 13b). The former is assigned to metallic Pt
Moreover, the Pt species are dominant on the catalyst surface.
With regard to the origin of the Pt species, they could result from
53], and the latter could be assigned to positively charged Ptd+
d+
[

H. Pan et al. / Journal of Catalysis 354 (2017) 24–36
35
the strong interaction of Pt nanoparticles with Al
electron transfer from Pt atoms to Al @SBA-15 during the calci-
nation process at 500 °C in air before reduction. Additionally, elec-
tron transfer from Pt atoms to FeO occurs in the PtFe0.25/15AS
2
O
3
@SBA-15 and
PtFe0.25/15AS catalyst with Pt nanoparticles containing predomi-
nant Pt species furnishes the best catalytic performance in terms
of CAL conversion and COL selectivity. According to the discussion,
the related hypothesis of CAL adsorption and activation with the
d+
2
O
3
x
catalyst, which has already been identified by the CO-DRIFTS
results. For comparison, the Pt/15AS catalyst was also character-
ized by XPS. As displayed in Fig. 13, the deconvoluted XP spectrum
x
PtFe /15AS catalyst was also proposed (Fig. 14). Both the positive
Pt species and the FeO species can adsorb and activate CAL by
x
d+
interaction with oxygen atoms in carbonyl groups, while hydrogen
0
0
of the Pt/15AS catalyst only showed one Pt species (Pt ). This
molecules are dissociated on Pt atoms and the then transferred to
implies that the interaction between Pt and FeO
x
is very strong
attack the activated CAL to form COL.
for the PtFe0.25/15AS catalyst. Of particular note is that the signal
intensity for PtFe0.25/15AS is weaker than that for the Pt/15AS,
maybe due to Fe species covering the Pt surface [39].
4
. Conclusions
The Fe2p XPS spectra of samples are shown in Fig. 13c. Iron spe-
cies are too complicated to assign from the deconvolution curves,
especially for BE above 715.0 eV. Due to BE of 2p3/2 for FeO,
2 3
Mesoporous Al O @SBA-15 composites (15AS)-supported
monometallic Pt or bimetallic Pt–Fe catalysts were applied for
the liquid-phase selective hydrogenation of CAL. As a result,
Fe
identify them as one specific species just from the XPS results. Nev-
ertheless, from the H TPR profile of PtFe0.25/15AS, the Fe species
3 4 2 3
O , and Fe O overlapping at around 711.0 eV [55], we cannot
x
Fe-doped PtFe /15AS catalysts are proved to be active and selective
for hydrogenation of CAL to the unsaturated alcohol COL, even
under milder conditions such as at room temperature or
under atmospheric hydrogen pressure. The reaction rate of the
PtFe /15AS catalyst varies with the Fe amount, and a volcano-like
x
curve between the reaction rate and the Fe/Pt molar fraction is
observed. When the Fe/Pt molar ratio is 0.25, the PtFe0.25/15AS
2
are reduced at 400–600 °C, while the catalyst precursor is reduced
at 500 °C in this case. Furthermore, in combination with the
HRTEM image of the PtFe0.25/15AS catalyst, we can deduce that
Fe species are
The above results reflect a strong interaction between Pt and
@SBA-15 and between Pt and FeO . This interaction changes
2 3
e-Fe O .
catalyst
3.93 molCAL
to the corresponding characterizations of PtFe
Pt particles with positive charges, that is, with Pt species as dom-
inant species on the catalyst surface, are derived from electron
reaches
a
maximum
, with a selectivity to COL of 76.9%. According
/15AS catalysts, the
mass-specific
rate
of
2
Al O
3
x
ꢀ1 ꢀ1
Pt
1
g
h
the electronic properties of Pt nanoparticles, which can certainly
explain the enhanced performance of PtFe0.25/15AS catalysts in
the selective hydrogenation of CAL. The predominant Pt species
in the PtFe0.25/15AS catalyst are electron-deficient Ptd+ species,
and iron species are also in oxidation states. Electron transfer from
x
d+
2 3 x
transfer from Pt atoms to Al O @SBA-15 and from Pt to FeO . In
addition, the Fe species also in oxidation states are in close vicinity
to the Pt species. Therefore, the Pt species with positive charges are
beneficial for the preferential adsorption and activation of terminal
C@O groups. Furthermore, the FeO species are also helpful for
x
adsorption of terminal carbonyl bonds. Meanwhile, hydrogen
2 3 x
Pt to Al O @SBA-15 and from Pt to FeO causes electron-deficient
Pt species in the PtFe0.25/15AS catalyst.
In our opinion, the electron-deficient state of Pt particles (Ptd+)
is beneficial for adsorption and activation of carbonyl bonds via
d+
interaction of oxygen atoms in carbonyl groups with Pt species
atoms can attack the activated carbonyl group of CAL via dissocia-
[
x
56]. Furthermore, the FeO species are also helpful for adsorption
tive adsorption onto Pt0 species to form the desired product COL.
of terminal carbonyl bonds. Meanwhile, hydrogen atoms can
attack the activated carbonyl group of CAL via dissociative adsorp-
Hence, the activity and selectivity to the unsaturated alcohol are
both greatly enhanced with the PtFe0.25/15AS catalyst.
0
tion onto Pt species to form the desired product, COL. Hence, the
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (Grants 21273076 and 21373089), the Open
Research Fund of the Top Key Discipline of Chemistry in Zhejiang
Provincial Colleges and the Key Laboratory of the Ministry of Edu-
cation for Catalysis Materials (Zhejiang Normal University)
(
(
ZJHX2013), and a Shanghai Leading Academic Discipline Project
B409). W. Xie also acknowledges the sponsorship of the Natural
Science Foundation of Shanghai (Grant 15ZR1411400).
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