Effect of the degree of dispersion of Pt over MgAl2O4 on the catalytic hydrogenation of benzaldehyde

Article abstract of DOI:10.1016/S1872-2067(17)62815-8

One of the central tasks in the field of heterogeneous catalysis is to establish structure-function relationships for these catalysts, especially for precious metals dispersed on the sub-nanometer scale. Here, we report the preparation of MgAl₂

Full text of DOI:10.1016/S1872-2067(17)62815-8

Chinese Journal of Catalysis 38 (2017) 1613–1620
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article (Special Issue of the International Symposium on Single‐Atom Catalysis (ISSAC‐2016))
Effect of the degree of dispersion of Pt over MgAl2O4 on the catalytic
hydrogenation of benzaldehyde
Feng Yan a,b, Caixian Zhao a, Lanhua Yi a,*, Jingcai Zhang b, Binghui Ge c, Tao Zhang b, Weizhen Li b,#
a Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, School of Chemistry, Xiangtan University,
Xiangtan 411105, Hunan, China
b State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
c Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
A R T I C L E I N F O
A B S T R A C T
Article history:
One of the central tasks in the field of heterogeneous catalysis is to establish structure‐function
relationships for these catalysts, especially for precious metals dispersed on the sub‐nanometer
scale. Here, we report the preparation of MgAl2O4‐supported Pt nanoparticles, amorphous aggre‐
gates and single atoms, and evaluate their ability to catalyze the hydrogenation of benzaldehyde.
The Pt species were characterized by N2 adsorption, X‐ray diffraction (XRD), aberration‐corrected
transmission electron microscopy (ACTEM), CO chemisorption and in situ Fourier transform infra‐
red spectroscopy of the chemisorbed CO, as well as by inductively coupled plasma atomic emission
spectroscopy. They existed as isolated or neighboring single atoms on the MgAl2O4 support, and
formed amorphous Pt aggregates and then nanocrystallites with increased Pt loading. On the
MgAl2O4 support, single Pt atoms were highly active in the selective catalytic hydrogenation of ben‐
zaldehyde to benzyl alcohol. The terrace atoms of the Pt particles were more active but less selec‐
tive; this was presumably due to their ability to form bridged carbonyl adsorbates. The
MgAl2O4‐supported single‐atom Pt catalyst is a novel catalyst with a high precious atom efficiency
and excellent catalytic hydrogenation ability and selectivity.
Received 14 February 2017
Accepted 15 March 2017
Published 5 September 2017
Keywords:
Platinum
MgAl2O4 spinel
Single‐atom catalyst
Selective hydrogenation
Benzaldehyde
Benzyl alcohol
© 2017, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
ters [2]. Hydrogenation of BzH can give various products by
either partial or complete reduction of the carbonyl group
Supported noble metal catalysts are widely used in the se‐
lective hydrogenation of organic compounds to give valuable
derivatives [1]. Selective catalytic hydrogenation of benzalde‐
hyde (BzH) is a green route to the production of benzyl alcohol
(BA), which is an important feedstock for the synthesis of vita‐
mins, pharmaceuticals, pesticides, fragrances and various es‐
and/or benzene ring, and thus serves as a model reaction for
probing the catalytic hydrogenation ability of specific catalysts
[3–5]. Platinum and ruthenium catalysts tend to be more
chemoselective for reduction of the carbonyl group than reduc‐
tion of the C=C bonds, and their main drawback is further hy‐
drodeoxygenation of BA into toluene (TOL) [5,6]. Because only
* Corresponding author. Tel: +86‐731‐58293719; Fax: +86‐731‐58292477; E‐mail: yilanhua@xtu.edu.cn
# Corresponding author. Tel: +86‐411‐84379738; Fax: +86‐411‐84685940; E‐mail: weizhenli@dicp.ac.cn
This work was supported by the National Natural Science Foundation of China (21403213, 21673226, 21376236, U1462121), the “Hundred Talents
Programme” of the Chinese Academy of Sciences, and National Key R&D Program of China (2016YFA0202801), and the “Strategic Priority Research
Program” of the Chinese Academy of Sciences (XDB17020100), Department of Science and Technology of Liaoning province under contract of
2015020086‐101, and the Natural Science Foundation of Hunan Province (2016JJ2128).
DOI: 10.1016/S1872‐2067(17)62815‐8 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 38, No. 9, September 2017

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Feng Yan et al. / Chinese Journal of Catalysis 38 (2017) 1613–1620
surface metal atoms are accessible to reactants and can act as
active sites, the metal catalysts are dispersed as very small par‐
ticles on support materials to give a high specific surface area.
Unlike homogeneous catalysts, which are uniform complexes
with defined central metal ions and ligands, supported metal
particles usually contain exposed atoms in various environ‐
ments, and the local coordination environments are not ade‐
quately characterized on the atomic level [7,8]. In fact, the
structures of the surface metals in a supported catalyst are
complicated because of the various sizes of the metal particles
and their varied locations on the support material. Each site
coexisting on a metal particle provides its own activity and
selectivity profile, and thus varies the overall catalytic perfor‐
mance according to its relative size and contribution.
MgAl2O4‐supported Pt catalysts were prepared by soaking 10 g
of the MgAl2O4 support powders in 200 mL of an aqueous solu‐
tion of chloroplatinic acid hexahydrate (Pt content >37.5%,
Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd.) for
10 h under magnetic stirring. Appropriate amounts of Pt were
used to give nominal loadings of 0.2 wt%, 0.5 wt% and 1 wt%.
The powders were separated by filtration and washed three
times with deionized water. The impregnated samples were
dried at 60 °C overnight, calcined at 500 °C for 5 h and finally
reduced at 300 °C in H2 for 2 h. The catalysts are denoted
0.2Pt/MgAl2O4, 0.5Pt/MgAl2O4 and 1Pt/MgAl2O4, and the actual
Pt loadings were determined by ICP‐AES to be 0.19 wt%, 0.47
wt% and 0.92 wt%, respectively.
Particle size diversity in supported metal catalysts is a major
parameter affecting their catalytic performance. Most commer‐
cial supported metal catalysts are prepared by a conventional
impregnation method, during which the precious metal atoms
may form dissolved cations, cations adsorbed on the support,
or highly dispersed or aggregated oxide and/or metallic species
and/or particles. The metal particles and their distributions are
generally characterized by high resolution transmission elec‐
tron microscopy (HRTEM), and thus the relationship between
the catalyst structure and the catalytic performance can be
established. However, characterization of the catalysts with
spherical aberration‐corrected transmission electron micros‐
copy (ACTEM), a new technique, has shown that, besides metal
particles, single‐atom metal species are also common, irrespec‐
tive of the support material [9–13]. Such single‐atom metal
species are active structures in many reactions and, in some
cases, even more active than their nanoparticle counterparts.
They have therefore become the subject of research in the new
field of “single‐atom catalysis” [13–21]. Thus, it is necessary to
reconsider the active sites of supported metal catalysts and
re‐evaluate their relative catalytic contributions. Among the
reported Pt catalysts that contain single‐atom Pt species,
Pt/MgAl2O4, with proper modification, is particularly suitable
as a model catalyst owing to its superior stability and activity
[11,22].
2.2. Catalyst characterization
Surface areas were measured on a Micromeritics ASAP 2010
apparatus using N2 adsorption isotherms and Brunau‐
er‐Emmett‐Teller (BET) analysis methods. All samples were
degassed under vacuum at 300°C for 5 h before the adsorption
measurements. The content of Pt in the catalysts was measured
on an ICP‐AES 7300DV instrument. All samples were dissolved
in aqua regia by microwave‐assisted digestion at 900 W for 1 h.
XRD patterns were recorded on a PW3040/60 X’Pert PRO
(PANalytical) diffractometer equipped with a Cu Kα radiation
source (λ = 0.15432 nm) operating at 40 kV and 40 mA.
High‐angle annular dark field scanning transmission electron
microscopy (HAADF‐STEM) images were obtained on a JEOL
JEM‐ARM200F microscope equipped with a CEOS probe cor‐
rector, with a guaranteed resolution of 0.08 nm. TEM speci‐
mens were prepared by depositing a suspension of the pow‐
dered sample on a lacey carbon‐coated copper grid. Pt disper‐
sions were measured by pulse adsorption of CO on a Mi‐
cromeritics AutoChem II 2920 automated characterization
system at 50 °C. Approximately 200 mg of the sample was
loaded into a U‐shaped quartz tube and heated to 300 °C for 30
min under pure H2 (30 mL/min) and then cooled to 50 °C in
pure He (30 mL/min). CO pulses (10%CO‐90%He) were intro‐
duced by switching a six‐way sampling valve. Pt dispersions
were estimated by assuming the adsorption stoichiometry of
CO to Pt to be equal to 1. In situ diffuse reflectance FTIR spectra
of the adsorbed CO species were acquired with a Bruker Equi‐
nox 55 spectrometer equipped with a mercury cadmium tellu‐
ride (MCT) detector at a spectral resolution of 4 cm−1 and with
32 scans. The sample powders were loaded into the IR cell,
pretreated in pure H2 (6 mL/min) at 300 °C for 1 h at a heating
rate of 10 °C/min and then cooled to 50 °C in He (30 mL/min).
The background spectrum was recorded under this condition.
Then, CO/He (1 vol%, 30 mL/min) was introduced for 30 min
and the sample was flushed with pure He (30 mL/min) before
the spectra of chemisorbed CO were recorded.
In this work, we report the preparation of MgAl2O4 spi‐
nel‐supported single‐atom to nano‐sized Pt catalysts and their
catalytic performance in the catalytic hydrogenation of BzH. We
characterized the Pt/MgAl2O4 catalysts using BET, X‐ray dif‐
fraction (XRD), ACTEM, inductively coupled plasma atomic
emission spectroscopy (ICP‐AES), CO chemisorption and Fou‐
rier transform infrared (FTIR) spectroscopy of the adsorbed CO
species, and analyzed the relationship between the catalytic
performance and the catalyst structure.
2. Experimental
2.1. Catalyst preparation
2.3. Hydrogenation of benzaldehyde
MgAl2O4 spinel support material was prepared by hydroly‐
sis of aluminum isopropoxide with magnesium nitrate hexahy‐
drate in ethanol, followed by calcination of the precipitates in
ambient air at 800 °C for 5 h with a heating rate of 2 °C/min.
Liquid‐phase hydrogenation of BzH was conducted in a
10‐mL Teflon‐lined stainless steel autoclave reactor. In a typical
hydrogenation reaction, 20 mg of the Pt/MgAl2O4 catalyst (or

Feng Yan et al. / Chinese Journal of Catalysis 38 (2017) 1613–1620
1615
Table 1
Characteristics of the Pt/MgAl2O4 catalysts.
Pore
size
(nm)
Pore
volume
(cm3/g)
Pt
Pt
1Pt/MgAl2O4
SBET
(m2/g)
Catalyst
loading dispersion a
(wt%)
(%)
0.5Pt/MgAl2O4
0.2Pt/MgAl2O4
0.2Pt/MgAl2O4 129.8
0.5Pt/MgAl2O4 124.6
7.1
7.4
7.5
0.46
0.46
0.48
0.19
0.47
0.92
92.5
63.9
53.2
1Pt/MgAl2O4
126.7
a Determined by CO chemisorption at 50 °C with assuming CO/Pt = 1.
1mol equivalent of Pt) was suspended in 5 mL ethanol con‐
taining 100 μL BzH (>97%, AIKE Reagent, ~1 mmol). After the
air was replaced three times with H2, the autoclave was
charged with H2 (1.0 MPa) and maintained at various temper‐
atures for various time under vigorous magnetic stirring. The
absence of external diffusion limitations was verified by per‐
forming the reaction under various stirring rates in preliminary
experiments. After the reaction, the autoclave was quenched
with cold water. A reusability test was conducted with the
0.2Pt/MgAl2O4 catalyst, whereby the used catalyst was collect‐
ed and recovered by filtration and washing with ethanol. The
shortfall in the recovered catalyst was replenished with cata‐
lysts from other reactors that had undergone the same number
of runs. The products were analyzed by gas chromatography
(7890B, Agilent Technologies) using an HP‐INNOWAX capillary
column (30 m × 0.32 mm, Agilent Technologies) and a flame
ionization detector.
MgAl2O4
20
30
40
50
60
70
80
2/(o)
Fig. 1. XRD patterns for the Pt/MgAl2O4 catalysts and the MgAl2O4 sup‐
port material.
shown in Fig. 1 for the Pt/MgAl2O4 catalysts are identical to that
of the MgAl2O4 support, which appears as a pure spinel crystal
phase. The lack of detectable diffractions for Pt indicates that
the mean Pt crystallite sizes are below the detection limit of
XRD (~3 nm) in all samples. Representative HAADF‐STEM im‐
ages show that 0.2Pt/MgAl2O4 presents a large amount of sin‐
gle‐atom Pt species as isolated bright dots on the MgAl2O4 sub‐
strate, and very occasionally Pt aggregates (Fig. 2(a)). Notably,
the Pt aggregates appear to be amorphous, not crystallites with
clear edges, which suggests that they do not contain enough
atoms to form well‐defined crystalline particles. Interestingly,
0.5Pt/MgAl2O4 displays many neighboring single Pt atoms in
addition to the highly isolated Pt atoms and amorphous aggre‐
gates (Fig. 2(b)). This is reasonable because the surface density
of Pt atoms on the MgAl2O4 substrate increases with increased
Pt loading. In the 1Pt/MgAl2O4 sample, the amorphous Pt ag‐
gregates become dominant, and are accompanied by occasional
isolated single Pt atoms and rare well‐defined Pt crystallites of
less than 2 nm (Fig. 2(c)). Clearly, with increased Pt loading, the
packing of the Pt atoms on the MgAl2O4 surface becomes dens‐
er, and presents various Pt species, for example, isolated single
Pt atoms, single‐layered neighboring Pt atoms, amorphous
aggregates and well‐defined crystalline particles, depending on
the local Pt atom density. These species include almost all pos‐
sible transition forms, and thus illustrate the process for the
3. Results and discussion
3.1. Catalyst structure determination
The physico‐chemical characteristics of the three
Pt/MgAl2O4 catalysts are presented in Table 1. The surface
areas, pore sizes and pore volumes of all three catalysts were
124–130 m2/g, 7.1–7.5 nm and 0.46–0.48 cm3/g, respectively,
which indicates that the textural properties of the MgAl2O4
support materials changed little during the catalyst preparation
process. The Pt loadings were accurately determined by
ICP‐AES to be 0.19 wt%, 0.47 wt% and 0.92 wt% for
0.2Pt/MgAl2O4, 0.5Pt/MgAl2O4 and 1Pt/MgAl2O4, respectively.
The Pt dispersions were respectively estimated as 92.5%,
63.9% and 53.2% using CO chemisorption. The XRD patterns
Fig. 2. HAADF‐STEM images for the 0.2Pt/MgAl2O4 (a), 0.5Pt/MgAl2O4 (b), and 1Pt/MgAl2O4 (c) catalysts.

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Feng Yan et al. / Chinese Journal of Catalysis 38 (2017) 1613–1620
formation of metal crystallites on a support substrate in slow
motion. Similar processes are expected, and indeed evident
albeit in a much faster mode, for Pt and other noble metals
dispersed on various support materials [9,10]. The Pt/MgAl2O4
catalysts retain much more of the single‐atom and amorphous
aggregate characteristics than conventional supported metal
catalysts; this is presumably due to the markedly stronger met‐
al/support interaction in this system.
3.2. Evaluation of the catalytic performance
To explore the catalytic hydrogenation ability of the
Pt/MgAl2O4 catalysts, we conducted hydrogenation of BzH un‐
der various reaction conditions and 100% BzH conversion. As
shown in Table 2, three products were obtained: BA, TOL and
benzaldehyde diethyl acetal (BDA). As illustrated in Scheme 1,
BA is produced by the selective hydrogenation of BzH, TOL is
generated through hydrodeoxygenation of the carbon‐oxygen
bonds in BzH or BA, and BDA is formed by aldol condensation
of BzH and the ethanol solvent. The results are displayed as the
molar compositions of the products. Under mild reaction con‐
ditions, with a temperature of 60 °C, H2 pressure of 1.0 MPa and
In situ FTIR spectroscopy of CO adsorbed on the Pt/MgAl2O4
catalysts was used to probe the possible differences in the co‐
ordination environments of the accessible Pt atoms in various
Pt species. As shown in Fig. 3, 1Pt/MgAl2O4 presents four bands
centered at 1820, 2030, 2073 and 2080 cm−1. The
0.5Pt/MgAl2O4 catalyst displays two distinct bands centered at
1820 and 2073 cm−1, and a shoulder peak at approximately
2030 cm−1. However, 0.2Pt/MgAl2O4 presents only a broad
asymmetric band at 2065 cm−1 and a small band at 1820 cm−1.
According to empirical interpretations of the FTIR spectra of
CO adsorbed on metals, bands in the low frequency range
(1730–1900 cm−1) are usually assigned to bridged carbonyl
species on neighboring metal atoms, whereas those in the high
frequency range (1950–2100 cm−1) are assigned to linear car‐
bonyl species on metals in various chemical environments
[23,24]. Thus, based on this expected blue shift in the stretching
frequency with increasing coordination degree (or metallic
nature) of the Pt atom, the CO adsorption bands at 2065, 2073
and 2080 cm−1 in the spectra described above were assigned to
the collective oscillation of CO linearly adsorbed on the single
Pt atoms, amorphous aggregates and crystallites. The band at
2030 cm−1 was assigned to the collective oscillation of CO line‐
arly adsorbed on the highly under‐coordinated Pt on the edges
or corners of the Pt crystallites [23]. The CO adsorption band at
1820 cm−1 was assigned to the bridged CO species adsorbed on
the terraces of the well‐defined Pt crystallites [24]. On the basis
of the relative band areas of the linear and bridged carbonyl
species, it is clear that the number of terrace atoms markedly
increases with Pt loading, which is in line with the observed
increase in the density of Pt crystallites shown in Fig. 2.
time of
4 h, the 0.2Pt/MgAl2O4, 0.5Pt/MgAl2O4 and
1Pt/MgAl2O4 catalysts were highly active in the selective hy‐
drogenation of BzH to BA, but much less active in the hydrode‐
oxygenation to TOL. The high selectivity for BA decreased
slightly from 99.4% to 99.1% and 99.0% and the low selectivity
for TOL increased slightly from 0.4% to 0.6% and 0.7% as the
Pt loading was increased from 0.2 wt% to 0.5 wt% and 1 wt%,
respectively. BDA was detected with similar selectivities and
yields of (0.2–0.3)% for all three catalysts. A comparable yield
of BDA was also obtained for 1Pt/MgAl2O4 under similar reac‐
tion conditions when the atmosphere was changed to N2. This
suggests that the formation of BDA is a parallel side reaction
with a much slower rate. When the reaction temperature was
elevated to 100 and 150 °C, the 0.2Pt/MgAl2O4 catalyst pre‐
sented a slightly decreased selectivity for BA (98.8% and
Table 2
Catalytic results for benzaldehyde hydrogenation over the Pt/MgAl2O4
catalysts at various temperatures.
Product distribution (%)
BA
Temperature Conversion
Catalyst
(°C)
(%)
100
100
100
100
100
100
100
100
100
1.5
TOL
0.4
0.5
0.4
0.6
0.8
1.9
0.7
3.9
5.0
0
BDA
0.2
0.7
1.7
0.3
0.8
1.7
0.3
0.7
1.9
62.7
0.2Pt/MgAl2O4
60
99.4
98.8
97.9
99.1
98.4
96.4
99.0
95.4
93.1
37.3
100
150
60
0.5Pt/MgAl2O4
1Pt/MgAl2O4
100
150
60
100
150
60
#
1Pt/MgAl2O4
Reaction conditions: 20.0 mg catalyst, 100 μL BAL (~1 mmol), 5.0 mL
ethanol as solvent, 1.0 MPa of H2, 4 h.
# Reaction under similar conditions except the atmosphere is N2.
CH3
+ H2
+ H2
OH
O
BzH
TOL
BA
E
t
O
OEt
H
+ H2
OEt
Fig. 3. FT‐IR spectra of CO adsorbed on the Pt/MgAl2O4 catalysts at 50
°C. The high‐frequency vibrational mode was assigned to CO linearly
bound to Pt atoms in various chemical environments, and the
low‐frequency mode was assigned to bridged carbonyl species on
neighboring Pt atoms on crystallite.
BDA
Scheme 1. Possible reaction pathways of benzaldehyde hydrogenation
over Pt/MgAl2O4 catalysts in ethanol medium.

Feng Yan et al. / Chinese Journal of Catalysis 38 (2017) 1613–1620
1617
97.9%) and increased selectivity for BDA (0.7% and 1.7%),
whereas the selectivity for TOL was almost unchanged (0.5%
and 0.4%). The 0.5Pt/MgAl2O4 catalyst, however, showed a
notable increase in the selectivity for TOL (to 0.8% and 1.9%)
and a decrease in the selectivity for BA (to 98.4% and 96.4%) in
reactions at 100 and 150 °C. Similarly, 1Pt/MgAl2O4 also
showed increased selectivity for TOL (to 3.9% and 5.0%) and
decreased selectivity for BA (to 95.4% and 93.1%). The selec‐
tivities for BDA appear to be comparable for all temperatures
and catalysts. Through hydrogenation of BzH at 100% conver‐
sion and over a wide temperature range, the catalytic hydro‐
genation ability of the Pt/MgAl2O4 catalysts was evaluated.
Selective hydrogenation of BzH to give BA is the predominant
reaction over all three Pt/MgAl2O4 catalysts. Hydrodeoxygena‐
tion of the carbon‐oxygen bonds in BzH or BA to give TOL is
negligible for 0.2Pt/MgAl2O4, but becomes meaningful when
the Pt loading is increased to 0.5 wt% and above.
The catalytic activities of the Pt/MgAl2O4 catalysts were
further investigated at various temperatures and BzH conver‐
sions by using catalysts containing similar amounts of Pt (~1
mol). As displayed in Table 3, 0.2Pt/MgAl2O4 catalyzed the
hydrogenation of BzH at temperatures as low as 20 °C with 1.0
MPa of H2. Under these conditions, the BzH conversion was
55.3% after 15 h, with BA, TOL and BDA selectivities of 98.8%,
0 and 1.2%, respectively. When the catalytic activity was nor‐
malized to the total number of Pt atoms and surface Pt atoms
determined by CO chemisorption experiments, the catalytic
reaction rate and turnover frequency (TOF) were calculated to
be 284 mol/(mol∙h) and 307 h−1, respectively. After reaction at
40 °C for 2 h, the BzH conversion reached 30.1% and the BA,
TOL and BDA selectivities were 99.3%, 0 and 0.7%, respective‐
ly. Additionally, the catalytic reaction rate and TOF increased to
1160 mol/(mol∙h) and 1254 h−1, respectively. After reaction at
60 °C for 0.3, 1 and 2 h, the BzH conversions increased from
22.5% to 53.6% and 94.9%, and the calculated rate and TOF
decreased from 3430 to 2700 and 2388 mol/(mol∙h) and from
3708 to 2919 and 2582 h−1, respectively. For 1Pt/MgAl2O4,
after reaction at 60 °C for 0.3, 1 and 2 h, the BzH conversions
increased from 19.4% to 45.9% and 90.1%, and the calculated
reaction rate and TOF decreased from 3055 to 2386 and 2072
mol/(mol∙h) and from 5472 to 4485 and 3895 h−1, respectively.
The calculated reaction rates and TOFs are clearly related to
the reaction time, although, according to the rate equation, the
intrinsic factors affecting these parameters are the instantane‐
ous concentrations of the reactants and products [25]. Thus,
the catalytic activities that were obtained at the same temper‐
ature and with similar BzH conversions were compared among
the Pt/MgAl2O4 catalysts. At BzH conversions of approximately
20%, the reaction rates for 0.2Pt/MgAl2O4 and 1Pt/MgAl2O4
were 3430 and 3055 mol/(mol∙h), respectively, and the TOFs
were 3708 and 5742 h−1, respectively. The inverse change in
the mass specific rates and TOFs suggests that the surface Pt
atoms on the Pt nanocrystallites are highly active and can par‐
tially compensate for the loss of accessible Pt atoms inside the
particles. It is notable that the selectivity of 1Pt/MgAl2O4 for
BDA at 60°C and low BzH conversions reached (10–20)%, and
that BDA was eventually almost completely converted to BA by
catalytic hydrogenation. This supports the reaction pathways
illustrated in Scheme 1. A previously described Pt/MgAl2O4
catalyst prepared by colloidal deposition has been shown to
have a TOF of 56 h−1 at 40 °C and 0.1 MPa of H2 [5]. A
Pt/dendrimer core/shell catalyst has been reported to have a
TOF of 91 h−1 under the same conditions [26], and a Pt/TiO2
catalyst displays a TOF of 2448 h−1 at 80 °C and 0.1 MPa of H2
[27]. Clearly, all of the Pt/MgAl2O4 catalysts in this work are
considerably more active under similar reaction conditions,
irrespective of Pt loading and dispersion degree; this is pre‐
sumably due to the strong interaction between the Pt species
and the MgAl2O4 spinel support.
Because 0.2Pt/MgAl2O4 represents a novel catalyst and has
the highest Pt availability and BA selectivity, we performed
recycling experiments and evaluated the reusability of the cat‐
alyst. In these experiments, we were more interested in the
stability of the catalyst and the BA selectivity than in the kinetic
activity, so we conducted the reaction under conditions that
gave total BzH conversion. As shown in Fig. 4, the BzH conver‐
sions remained at 100% and the BA selectivity at about 99.5%
over at least six runs. Even though the 0.2Pt/MgAl2O4 catalyst
was not able to provide high kinetic activity, the excellent sta‐
bility of this catalyst in terms of BA selectivity was demon‐
strated, and indicates that the highly dispersed nature of the Pt
species was unchanged during the recycling experiments.
3.3. Relationship between catalyst structure and catalytic
function
The dispersion of Pt on an MgAl2O4 spinel support can result
in the formation of isolated or neighboring single Pt atoms,
amorphous Pt aggregates and well‐defined crystalline particles;
the species formed depend on the local Pt atom density. With a
Table 3
The catalytic selectivities and activities of the Pt/MgAl2O4 catalysts for benzaldehyde hydrogenation at low conversions.
Time
(h)
Product distribution (%)
Temperature
(°C)
Conversion
(%)
Rate
TOF
(h–1)
307
1254
3708
2919
2582
5742
4485
3895
Catalyst
(mol/(mol∙h))
BA
TOL
BDA
0.2Pt/MgAl2O4
20
40
60
60
60
60
60
60
15
2
0.3
1
2
0.3
1
55.3
30.1
22.5
53.6
94.9
19.4
45.9
90.1
98.8
99.3
97.7
98.6
99.5
79.1
89.5
87.6
0
0
1.2
0.7
1.2
0.4
0.1
19.9
9.1
10.7
284
1160
3430
2700
2388
3055
2386
2072
1.1
1.0
0.4
1.1
1.4
1.7
1Pt/MgAl2O4
2
Reaction conditions: catalyst with similar amount of Pt (~0.2 mol), 100 μL BAL (~1 mmol), 5.0 mL ethanol as solvent, 1.0 MPa of H2.

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Feng Yan et al. / Chinese Journal of Catalysis 38 (2017) 1613–1620
bands for chemisorbed CO owing to the different local coordi‐
XBzH SBA
nation environments. In principle, each type of Pt species could
be distinguished and quantified by deconvoluting the charac‐
teristic band; however, there is a high risk of over‐interpreting
the FTIR spectra if we try to deconvolute the broad IR bands
into many specific bands with slightly different shifts. Here we
consider only the relative contributions of PtL and PtB atoms,
even though both include several types of Pt species. Thus, the
average TOFs of PtL and PtB were calculated to be 3277 and
4807 h−1, respectively, without further considering the differ‐
ences in the detailed structures of the PtL and PtB atoms in the
0.2Pt/MgAl2O4 and 1Pt/MgAl2O4 catalysts. Thus, it can be con‐
cluded that the PtB atoms are considerably more active than the
PtL atoms. Considering that some of the PtL atoms exist in the Pt
crystallites, we can estimate that the activity of the crystallite
surface atoms is higher than that of the single atoms on the
MgAl2O4 support. The additional ability of the 1Pt/MgAl2O4
catalyst, which is rich in terrace Pt atoms, to hydrogenate BA to
TOL suggests that the origin of this increased activity could be
the ability to form bridged carbonyl groups. This is in line with
the observation that TOL formation usually occurs on
well‐crystallized nanocatalysts [5,28,29].
100
80
60
40
20
0
S
X
1
2
3
4
5
6
Number of run
Fig. 4. Reusability of the 0.2Pt/MgAl2O4 catalyst for the hydrogenation
of benzaldehyde in ethanol. Reaction conditions: 20.0 mg catalyst, 100
μL BAL, 5.0 mL ethanol, 1.0 MPa of H2, 60 °C, 4 h.
typical impregnation method, the coexistence of multiple Pt
species seems to be inevitable, and it is currently impossible to
quantify the relative amounts of the species in the mixture.
Fortunately, the dispersion of the surface Pt atoms, which are
potential active sites accessible to reactants, can be estimated
empirically by CO chemisorption experiments, irrespective of
whether they are present on the surface of the MgAl2O4 support
or as Pt aggregates/crystallites. Accordingly, we obtained the Pt
dispersions and calculated the TOFs using the empirical as‐
sumption that the stoichiometry of CO adsorption on Pt is equal
to 1. This is appropriate for comparison with results reported
in the literature that use a similar assumption. However, the
detection of bridged carbonyl species suggests that the CO/Pt
stoichiometry needs to be corrected by deconvolution of the
contribution of the linear and bridged carbonyls using a CO/Pt
ratio of 1 or 2, respectively. As shown in Table 4, the ratios of
surface Pt with linear (PtL) and bridged (PtB) carbonyl species
were calculated to be 90/10 and 44/56 for 0.2Pt/MgAl2O4 and
1Pt/MgAl2O4, respectively. Accordingly, the Pt dispersions
were calibrated to be approximately 100% and 73.9%, respec‐
tively. As a result, the corresponding TOFs were determined to
be 3430 and 4134 h−1 at approximately 20% BzH conversion at
60 °C. Such amendments to the overall TOFs, however, still do
not reflect the intrinsic activity of specific active sites. As we
have revealed above, different Pt species present various FTIR
4. Conclusions
We have demonstrated that Pt can be readily dispersed as
single atoms on an MgAl2O4 spinel support and forms various
dispersed Pt species, such as isolated or neighboring single Pt
atoms, amorphous aggregates and crystallites, according to the
local surface density of the Pt atoms. The single‐atom Pt species
located directly on the MgAl2O4 spinel surface are highly active
in the selective catalytic hydrogenation of BzH to BA over a
wide temperature range. The terrace atoms on the Pt crystal‐
lites are more active but less selective, and have the ability to
further hydrodeoxygenate BA to TOL. These MgAl2O4 spi‐
nel‐supported Pt single‐atom catalysts represent a novel type
of catalyst with high stability and selectivity for carbonyl hy‐
drogenation.
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Feng Yan et al. / Chinese Journal of Catalysis 38 (2017) 1613–1620
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Graphical Abstract
Chin. J. Catal., 2017, 38: 1613–1620 doi: 10.1016/S1872‐2067(17)62815‐8
Effect of the degree of dispersion of Pt over MgAl2O4 on the catalytic hydrogenation of benzaldehyde
Feng Yan, Caixian Zhao, Lanhua Yi *, Jingcai Zhang, Binghui Ge, Tao Zhang, Weizhen Li *
Xiangtan University; Dalian Institute of Chemical Physics, Chinese Academy of Science; Institute of Physics, Chinese Academy of Science
CH3
+H
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Pt在MgAl2O4载体上的分散程度对催化苯甲醛加氢反应影响
鄢 峰a,b, 赵才贤a, 易兰花a,*, 张景才b, 葛炳辉c, 张 涛b, 李为臻b,#
a湘潭大学化学系环境友好化学与应用教育部重点实验室, 湖南湘潭411105
b中国科学院大连化学物理研究所催化基础国家重点实验室, 辽宁大连116023
c中国科学院物理研究所北京凝聚态物理国家实验室, 北京100190
摘要: 构建催化剂特别是在亚纳米尺度下分散的贵金属催化剂的构效关系是多相催化研究领域中的主要任务之一. 我们
采用与金属Pt具有强相互作用的MgAl2O4尖晶石作为载体, 通过简单浸渍法制备了在纳米、亚纳米和单原子尺度上分散的
Pt催化剂. 首先利用X射线衍射和原子分辨的球差校正电镜, 确定了Pt在MgAl2O4尖晶石载体表面上随负载量增大逐渐形
成孤立的和相邻的单原子Pt, 然后逐渐形成无定形Pt聚集体和小晶粒; 然后利用电感耦合等离子体光谱和CO化学吸附测

1620
Feng Yan et al. / Chinese Journal of Catalysis 38 (2017) 1613–1620
定了催化剂中Pt的含量和分散度; 进一步通过测定CO在Pt表面吸附的红外光谱, 区分了载体表面单原子和金属颗粒表面
原子的CO吸附特征结构, 并据此对不同结构的Pt原子进行了半定量估算. 考察了具有不同Pt分散结构的Pt/MgAl2O4催化剂
的催化苯甲醛选择性加氢能力, 发现以载体表面Pt单原子物种为主的催化剂, 可在较宽的温度区间内保持较高的部分加氢
产物苯甲醇的选择性(60–150 ºC, 苯甲醇选择性99.4–97.9%, 甲苯选择性~0.4%), 而以Pt纳米颗粒为主的催化剂上苯甲醇选
择性降低显著, 同时生成较多深度加氢产物甲苯(60‒150 ºC, 苯甲醇选择性99.0–93.1%, 甲苯选择性0.7–5.0%). 此外, 我们
测定了各催化剂在不同转化率(~20–90%)时催化剂加氢反应的质量比活性和转化频率(TOF), 并在较低苯甲醛转化率
(~20%)时, 估算了不同结构Pt物种对苯甲醛加氢反应的本征活性, 发现Pt纳米颗粒表面原子比MgAl2O4载体表面Pt单原子
本征活性更高(4807 h–1 versus 3277 h–1). 综上, Pt单原子催化剂具有贵金属原子利用率高, 本征活性和加氢选择性高等优
点; Pt纳米催化剂表面原子深度加氢能力强, 加氢选择性较差, 虽本征活性更高, 但不足以补偿贵金属原子利用率降低带来
的活性损失, Pt质量比活性显著低于单原子催化剂. 此外, MgAl2O4尖晶石负载的单原子Pt催化剂也具有良好的催化反应循
环稳定性, 是一种较为理想的催化苯甲醛选择性加氢制苯甲醇催化剂.
关键词: 铂; 镁铝尖晶石; 单原子催化剂; 选择加氢; 苯甲醛; 苯甲醇
收稿日期: 2017-02-14. 接受日期: 2017-03-15. 出版日期: 2017-09-05.
*通讯联系人. 电话: (0731)58293719; 传真: (0731)58292477; 电子信箱: yilanhua@xtu.edu.cn
#通讯联系人. 电话: (0411)84379738; 传真: (0411)84685940; 电子信箱: weizhenli@dicp.ac.cn
基金来源: 国家自然科学基金(21403213, 21673226, 21376236, U1462121); 中国科学院”百人计划”; 国家重点研发计划
(2016YFA0202801); 中国科学院战略性先导科技专项(XDB17020100); 辽宁省科技厅(2015020086-101); 湖南省自然科学基金
(2016JJ2128).
本文的英文电子版由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).