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which is extremely important for supported Pd catalysts [26]. In
this case, the atomically dispersed Pd catalysts provide an excellent
opportunity to tune the active site and optimize the selectivity of
heterogeneous catalysts, offering great potential for fundamental
studies and industrial applications.
continuous mode was used for collecting data in the 2ꢂ range from
10◦ to 80◦ at a scanning speed of 10◦ min−1. X-ray photoelectron
spectroscopy (XPS) was conducted at normal emission on a Multi-
lab 2000 spectrometer (Thermo VH Scientific) using AlK␣ radiation
(1486.6 eV). The aluminum anode was operated at an accelerating
voltage of 15 kV. The pressure in the analysis chamber was main-
tained at 5 × 10−9 bar. The binding energy of samples was calibrated
using the binding energy of C1s peak at 284.6 eV as a reference.
Fourier Transform infrared spectroscopy (FTIR) were recorded
with a BRUKER FTIR spectrometry (EQUINOX 55, Bruker Co.) at
a resolution of 4.0 cm−1 and a rate of 1 spectrum per 32 s. The
scanned wave number range was 4000–400 cm−1. Pd loadings
in the catalysts were determined by inductively coupled plasma
atomic emission spectroscopy (ICP-AES) on an IRIS intrepid II XSP
instrument (Thermo Electron Corporation). CO chemisorption on a
Micromeritics Autochem 2920 was used to determine the average
Pd particle size in 0.5Pd/␥-AlOOH.
Turnover frequency (TOF) is defined as the amount of substrate
molecules consumed per catalytic site per unit time [27]. How-
ever, it is difficult to measure the initial reaction rate and accurately
determine the number of active sites. In this paper, atomically dis-
persed Pd species supported on ␥-AlOOH nanosheets (Pd/␥-AlOOH)
were prepared with different Pd loadings by simple incipient wet-
ness impregnation combined with hydrogen post-treatment. The
Pd ions interacted with ␥-AlOOH (0 1 0) facets through surface
hydroxyls and the prepared Pd/␥-AlOOH catalysts exhibited highly
catalytic activity and selectivity for the hydrogenation of SA to GBL.
Moreover, the Pd atom activity in a cluster with different sizes was
also investigated based on the experimental results and density
functional theory (DFT) calculation.
2. Experimental
2.3. Catalytic evaluation of Pd/ꢀ-AlOOH
2.1. Preparation of atomically dispersed Pd species on ꢀ-AlOOH
nanosheets
The selective hydrogenation of SA to GBL was performed in
a batch reactor (100 ml, Parr Corp.) at 513 K and 60 bar (H2). SA
(0.4 g), reduced catalyst (0.2 g) and 1,4-dioxane (40 ml) as solvent
were charged into the autoclave. Before heating, the reactor was
purged with hydrogen four times in order to remove air in the
autoclave, and then pressurized up to 40 bar using hydrogen. After
heating the reactor to reaction temperature (513 K), hydrogen pres-
sure was raised up to 60 bar. During the reaction, reaction mixture
was stirred at 500 rpm in order to avoid mass transfer limitation.
The liquid samples taken from the autoclave at regular intervals
were analyzed by gas chromatograph and liquid chromatograph.
For the gas chromatograph, a SE-54 column was used to separate
products and a FID detector was used to identify GBL, THF and BDO.
For the liquid chromatograph, an Inertsil® ODS-SP column (5 m
4.6 × 150 mm) was used to identify SA, GBL, propionic acid (PA) and
butyric acid (BA).
All chemicals used in the experiment were of analytical grade,
and used without further purification. Gama-boehmite (␥-AlOOH)
was prepared by a sol–gel process as described in the followings:
0.25 mol aluminum secondary butoxide (Sigma–Aldrich >95%) was
added to 500 mL of de-ionized water under stirring at 353 K for
2 h. To peptize the sol, 0.0175 mol HNO3 (1.0 mol L−1) was slowly
dropped into the solution. The sol was kept boiling in an open
reactor for few hours to evaporate 80% of the solvent in order to
concentrate to the minimum gelling volume for the formation of
AlOOH gel [28] and was subsequently kept at 363 K for 24 h under
reflux conditions. After drying at 333 K for 8 h, the solid was washed
with ethanol and de-ionized water three times, followed by another
drying process. Organic impurity and water was removed by calci-
nation at 523 K for 4 h.
Supported palladium catalysts were prepared using K2PdCl4
solution as palladium source and ␥-AlOOH as support by incipient
wetness impregnation. After impregnation at room temperature
for 24 h with stirring, the samples were dried and calcined at 573 K
for 2 h with a temperature ramp of 5 K min−1. Before activity tests
and characterization, all catalysts were reduced in a tube furnace
under H2/He (7.5%, v/v) atmosphere at 513 K for 3 h and stored
under vacuum in the dark. The prepared catalysts were denoted
as mPd/␥-AlOOH with different Pd loadings, where m represents
the weight percentage of Pd based on the total catalyst weight.
2.4. DFT calculations of the Pd atom and clusters adsorbed on the
ꢀ-AlOOH (0 1 0) surface
DFT calculations were carried out using the CASTEP code
in Materials Studio package. The Perdew–Burke–Ernzerhof form
of the Generalized-Gradient Approximation was employed to
describe electron exchange and correlation. The electron-ion
potential was described by means of ab initio pseudopotential
within ultrasoft formulation. A kinetic energy cutoff of 340 eV was
used in all simulations, and the Brillouin zone was sampled by using
a Monkhorst–Pack grid. Structural relaxation was performed using
a BFGS algorithm until forced on all atoms is below 0.03 eV Å−1
and maximum energy change was 10−5 eV atom−1. In the DFT cal-
culation of ␥-AlOOH bulk model, the Brillouin zone was sampled
by 5 × 1 × 4 meshes of k-points, and all atomic positions were
fully relaxed. The optimized ␥-AlOOH bulk model was used as the
starting point for the XRD simulation and the surface model con-
struction. The ␥-AlOOH (0 1 0) surface was modeled as a periodic
slab with two stacked layers, and the successive slab were sepa-
rated by a vacuum region as thick as 12 Å in order to avoid periodic
interaction. A p (4 × 5) supercell with corresponding 1 × 1 × 1 k-
point mesh was used to study the Pd atom and clusters adsorption
on the ␥-AlOOH (0 1 0) surface.
2.2. Characterization of Pd/ꢀ-AlOOH
Aberration-corrected high-angle annual dark-filed (AC-HAADF)
images were obtained on a FEI Titan ChemiSTEM 80–200 facil-
ity using a scanning transmission electron microscopy (STEM)
at 200 kV. Through collecting transmitted electrons on an annu-
lar dark-field detector from
a high angle of 22 mrad, the
AC-HAADF-STEM images are formed. As these images taken
from AC-HAADF-STEM are sensitive to the atomic number, this
microscopy is quite useful to identify the heavier element on the
support made of lighter elements. In these images, the brightest
dots are Pd species while the gray parts of image are ␥-AlOOH
nanosheets.
X-ray diffraction (XRD) patterns were recorded on a PW3040/60
X’Pert PRO (PA Nalytical) diffractometer equipped with a CuK␣
radiation source (ꢁ = 1.54056 Å), operating at 40 kV and 40 mA. A
Please cite this article in press as: C. Zhang, et al., Atomically dispersed Pd catalysts for the selective hydrogenation of succinic acid to