ACS Catalysis
Research Article
transferred to a tube furnace and sintered at 1400 °C for 4 h
under an argon flow.
Sample Characterization. The obtained catalysts were
analyzed using an X-ray powder diffractometer (D8-AD-
VANCE, Bruker and MiniFlex, Rigaku) with a Cu Kα
radiation (λ = 0.15418 nm).
confirmed by performing gas chromatography−mass spec-
trometry (GC−MS) measurements. The TOF was calculated
from the reaction rate at a low conversion level and from the
number of exposed Pd atoms estimated to be present on the
catalyst surface via eqs 1 and 2
TOF = n C/tn
0
catalyst
(1)
(2)
XPS measurements were conducted using ESCA-3200-
(
Shimadzu) with Mg Kα radiation. The pressure of chamber
ncatalyst = mcatalystN /N
Pd A
−
6
is higher than 10 Pa, and 8 kV voltage was applied to the X-
ray source. A 4f7/2 peak (84.0 eV) of deposited Au was used as
where n is the initial number of substrate moles, C is the
0
38
the reference during the measurements.
conversion of the substrate at reaction time t, ncatalyst is the
number of moles of Pd atoms exposed on the catalyst surface,
mcatalyst is the weight of the catalyst, NPd is the amount of
exposed Pd sites per gram of catalyst obtained when applying
Nitrogen adsorption/desorption measurements were per-
formed to estimate the Brunauer−Emmett−Teller surface area
of the catalysts (BELSORP-mini II, BEL). The number of
moles of exposed Pd sites found on the surface of the material
was estimated by performing experimental measurements and
formulating theoretical predictions. CO-pulse chemisorption
the CO pulse chemisorption method, and N is Avogadro’s
A
constant. For the recycling study, the Suzuki coupling reactions
were performed with aryl halides and phenylboronic acid,
maintaining the same reaction conditions as described above,
except for the use of a recycled catalyst. After the completion
of each reaction, the catalyst powder was washed with ethanol
containing a certain amount of water several times to remove
the organic and inorganic impurities from the used catalyst.
Finally, the resultant powder was allowed to dry in vacuum at
room temperature, weighed, and reused in the next run.
Nitroarene Hydrogenation Reaction Conditions. All
reactions were conducted in a 25 mL stainless-steel autoclave
fitted with a glass mantel, a 60 bar manometer, and a magnetic
stirrer. In a typical reaction, 0.5 mmol substrates and 5 mg of
the catalyst were mixed in 5 mL of the solvent. The autoclave
was then flushed three times with H , pressurized with H (0.5
(
3
BELCAT-A, BEL) was measured at 50 °C using a He flow of
−
1
0 mL·min and pulses of 0.36 mL (9.88% CO in He). Prior
to the analysis, the catalyst was treated with an Ar flow (50
mL·min ) at 400 °C for 30 min and then a He flow (50 mL·
min ) at 400 °C for 15 min. To estimate the amount of
exposed Pd, a value of 2 for the Pd/CO ratio was employed to
infer the number of exposed Pd centers on the basis of the
results of the CO pulse chemisorption experiment.
−
1
−
1
The morphology of the catalyst samples was investigated by
HAADF-STEM (JEM-ARM-200F, JEOL). In the relevant
experiments, the samples were dispersed in hexane and
dropped on carbon paste mounted on the sample holder
made of Si or Cu. The measurements were conducted using an
accelerating voltage of 200 kV.
X-ray absorption fine structure (XAFS) measurements were
performed on the AR-NW10A beamline of the Photon Factory
Advanced Ring at the Institute of Materials Structure Science,
High Energy Accelerator Research Organization, Tsukuba,
Japan. A Si(311) double-crystal monochromator was used to
obtain a monochromic X-ray beam, and spectra were obtained
in the transmission mode. Pd(1 wt %)−ZrC and BN (dried at
2
2
MPa), and reactions were performed in the autoclave at room
temperature (25 °C). The products were analyzed using GC
with an external standard of n-hexadecane and the identity of
the products was further confirmed from GC−MS measure-
ments.
DFT Studies. All of the structural relaxation and electronic
structure calculations were performed using DFT as
implemented in the Vienna Ab initio Simulation Package
4
3,44
3
00 °C) were mixed in an argon-filled glovebox, and the
(VASP).
The generalized gradient approximation with the
45
resulting mixture was pressed with a hand press apparatus to
obtain a pellet, which was sealed in a plastic bag for the
measurement to be performed. XAFS spectra were analyzed
using Athena and Artemis software packages. The FEFF6
code was used to calculate the theoretical spectra.
Perdew−Burke−Ernzerhof (PBE) functional was adopted in
the DFT calculations, and the core electrons were described
using the projector augmented wave method.
46,47
The ZrPd3
39
(0001) surfaces, based on the optimized bulk lattice
parameters, were selected as the initial model because the
40
Pd metal was used for standards. The EXAFS data were
analysed with the coordination numbers fixed as the ideal
(0001) surface of ZrPd was calculated to be characterized by
3
−
2
the lowest surface energy (1.27 J m ) among the major faces,
values. Bulk ZrPd was also analysed to be compared with our
including the (0001), (11−20), and (1000) planes. The ZrPd
3
3
Pd−ZrC catalyst in order to discuss the properties. The crystal
(0001) surfaces were modeled using a seven-layer slab with 2
× 2 lateral unit cells. A 20 Å-thick vacuum region was set to
prevent interaction between the slabs. The central three layers
of atoms of two surface models were kept fixed to hold the
characteristics of realistic surfaces, and the rest of the unit cell
was allowed to be fully relaxed during geometry optimization.
A cut-off energy value of 500 eV and a Monkhorst−Pack K-
mesh setting of 3 × 3 × 1 were employed in the calculations
structure of ZrPd used in this study was reported by Norman
3
4
1,42
and Harris.
There are two specific sites for Pd in the
structure, and the two sites were taken into account for EXAFS
fitting procedures. In Table S1, Pd −M and Pd −M (M = Zr
1
2
and Pd) denote the two specific sites of Pd in the structure. A
Pd−C path was also considered for the Pd−ZrC sample.
Suzuki Coupling Reaction Conditions. All reactions
were conducted in a 25 mL stainless-steel autoclave fitted with
a glass mantel and a magnetic stirrer. In a typical reaction, 0.5
mmol organohalide, 0.8 mmol phenylboronic acid, 1.5 mmol
K CO , and 5 mg of Pd(1 wt %)−ZrC were mixed together in
for the ZrPd (0001) surfaces. The vacuum level was defined
3
as the energy level at which the electrostatic potential became
constant. The atoms in the two bottom layers were then
removed, and the five remaining layers were fixed and used for
further molecule adsorption calculations. The surface of the
ZrC−ZrPd composite was modeled by placing a Zr Pd
2
3
5
mL of the solvent. The autoclave was then flushed three
times with Ar, and the reactions were conducted in the
autoclave at 25−60 °C. The products were analyzed by gas
chromatography (GC), and product identification was further
3
8
24
cluster on three layers of ZrC (001) to confirm the electron
donation effect of ZrC with a low value for the work function.
1
4372
ACS Catal. 2020, 10, 14366−14374