On-Demand, Ultraselective Hydrogenation System Enabled by Precisely Modulated Pd-Cd Nanocubes

Article abstract of DOI:10.1021/jacs.9b10816

The pursuit of efficient hydrogenation nanocatalysts with a desirable selectivity toward intricate substrates is state-of-the-art research but remains a formidable challenge. Herein, we report a series of novel PdCd_x nanocubes (NCs) for ultraselective hydrogenation reactions with flexible tuning features. Obtaining a desirable conversion level of the substrates (e.g., 4-nitrophenylacetylene (NPA), 4-nitrobenzaldehyde (NBAD), and 4-nitrostyrene (NS)) and competitive selectivity for all potential hydrogenation products have been achieved one by one under optimized hydrogenation conditions. The performance of these PdCd_x NCs displays an evident dependence on both the composition and the use of Cd and a need for a distinct hydrogen source (H₂ or HCOONH₄). Additionally, for the selectivity of hydrogen to be suitably high, the morphology of the NCs has a very well-defined effect. Density functional theory calculations confirmed the variation of adsorption energy for the substrate and hydrogenation products by carefully controlled introduction of Cd, leading to a desirable level of selectivity for all potential hydrogenation products. The PdCd_x NCs also exhibit excellent reusability with negligible activity/selectivity decay and structural/composition changes after consecutive reactions. The present study provides an advanced strategy for the rational design of superior hydrogenation nanocatalysts to achieve a practical application for desirable and selective hydrogenation reaction efficiency.

Full text of DOI:10.1021/jacs.9b10816

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Article
On-demand, Ultraselective Hydrogenation System
Enabled by Precisely Modulated Pd-Cd Nanocubes
Yonggang Feng, Weiwei Xu, Bolong Huang, Qi Shao, Lai Xu, Shize Yang, and Xiaoqing Huang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b10816 • Publication Date (Web): 18 Dec 2019
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On-demand, Ultraselective Hydrogenation System Enabled by
Precisely Modulated Pd-Cd Nanocubes
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†∥
Weiwei Xu,‡∥ Bolong Huang,§∥ Qi Shao, Lai Xu, Shize Yang, Xiaoqing
†
‡*
⊥
Yonggang Feng,
Huang
†
†
*
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu, 215123, China
‡Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional
Materials and Devices, Soochow University, Jiangsu 215123, China
§Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom,
Kowloon, Hong Kong SAR
⊥
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States
ABSTRACT: The pursuit of efficient hydrogenation nanocatalysts with a desirable selectivity towards intricate substrates
is state-of-the-art research but remains a formidable challenge. Herein, we report a series of novel PdCd nanocubes
NCs) for the ultraselective hydrogenation reactions with flexible tuning feature. Obtaining a desirable conversion level of
x
(
the substrates (e.g., 4-nitrophenylacetylene (NPA), 4-nitrobenzaldehyde (NBAD) and 4-nitrostyrene (NS)) and
competitive selectivity for all potential hydrogenation products have been achieved one by one under optimized
hydrogenation conditions. The performance of these PdCd
and the use of Cd and a need for a distinct hydrogen source (H
x
NCs displays an evident dependence on both the composition
or HCOONH ). Additionally, for the selectivity of
2
4
hydrogen to be suitably high, the morphology of the NCs has a very well-defined effect. Density functional theory
calculations confirmed the variation of adsorption energy for the substrate and hydrogenation products by carefully
controlled introduction of Cd, leading to a desirable level of selectivity for all potential hydrogenation products. The
x
PdCd NCs also exhibit excellent reusability with negligible activity/selectivity decay and structural/composition changes
after consecutive reactions. The present study provides advanced strategy for the rational design of superior
hydrogenation nanocatalysts to achieve a practical application for desirable and selective hydrogenation reaction
efficiency.
INTRODUCTION
hydrogen dissociation ability of the active noble metal
and reduce the binding energy of the desired products on
nanocatalysts. However, with previously reported
nanocatalysts, it remains a difficult task to selectively
reduce the substrate with multiple unsaturated functional
groups (e.g., alkynes, alkenes, aldehydes and nitroarene)
while obtaining potential hydrogenation products with a
desired selectivity.4, 6, 17 Nevertheless, achieving selective
hydrogenation in the presence of competing unsaturated
functional groups is of a great importance to the
pharmaceutical and chemical industry because many
specific hydrogenation products are difficult to produce
by simple organic synthesis.18, 19 Therefore, it is a highly
desirable yet formidable challenge to achieve a sufficient
Selectivity is fundamental to heterogeneous
hydrogenation and is of great scientific and industrial
interest.1-3 Currently, noble metals (e.g., Pd and Pt) that
can readily activate hydrogen (H ) are the best choice for
2
the hydrogenation of unsaturated hydrocarbons.2,
Unfortunately, precise control of the hydrogenation of the
desirable product over bare noble metals is challenging,
which usually results in a low selectivity. In general, a
highly selective noble-metal nanocatalyst should
primarily hydrogenate the reactant whilst avoiding
consecutive hydrogenation of the desired product. In fact,
the hydrogenation selectivity of noble metals can be
encouraged by the introduction of a second metal or
organic/inorganic compound,7-16 since this can lower the
4-6
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level of selectivity of intricate substrates to on-demand
hydrogenation products.
analytical reagent, ≥ 99.7%), N, N-Dimethylformamide
(DMF, ≥ 99.5%), formic acid (CH O , analytical reagent,
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99.9%), acetone (C H O, ≥ 99.9%) and sodium iodide
To achieve precise hydrogenation performance, the
nature of the nanocatalyst (size, compositions and
morphology, etc.) must be carefully controlled, which
provides an ensemble control of the adsorption of
hydrogenation sources, reactants and products.20-25 As
predicted by our theoretical study, the adsorption energy
(
NaI, analytical reagent, ≥ 99.9%) were all purchased
from Sinopharm Chemical Reagent Co. Ltd. (Shanghai,
China). All the chemicals were used as received without
further purification. The water (18 MΩ cm-1) used in all
experiments was obtained by passing through an ultra-
pure purification system (Aqua Solutions).
x
of hydrogen sources on different PdCd nanocatalysts can
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Synthesis of PdCd
typical preparation of PdCd0.82 NCs, Pd(acac)
CdAc ·2H O (8.8 mg), DL-mandelic acid (150.0 mg), OAm
x
NCs and PdCd0.82 NPs. In a
be readily controlled (table S1), for which PdCd
nanocatalysts may potentially possess desirable
performance for selective hydrogenation. Here in this
work, we have introduced Cd into Pd to form PdCd NCs
x
as a potential electrocatalyst for the hydrogenation based
on flexible control of compositions. We have developed a
2
(7.6 mg),
2
2
(4 mL) and OA (1 mL) were added to a vial (volume: 30
mL). After the vial had been capped, the mixture was
ultrasonicated for approximately 0.5 h. The resulting
homogeneous mixture was heated from room
temperature to 160 °C for approximately 0.5 h and kept at
series of PdCd
x
nanocubes (NCs) (x = 0.53, 0.67, 0.82 and
selective hydrogenation of 4-
1
.13) for the
1
60 °C for 5 h in an oil bath. After cooling to room
nitrophenylacetylene (NPA) with both nitro and alkynyl
groups. Remarkably, a desirable level of selectivity with a
switchable feature for all five potential hydrogenation
products, and an excellent reusability have been achieved
temperature, the colloidal products were collected by
centrifugation and washed with a cyclohexane/ethanol
mixture. For the syntheses of PdCd0.53, PdCd0.67, and
PdCd1.13 NCs, all the conditions are similar to those of
PdCd0.82 NCs, except for changing the amount of
CdAc ·2H O to 4.4 mg, 6.6 mg, and 11.0 mg, respectively.
2 2
The preparation of PdCd0.82 NPs is similar to that of
PdCd0.82 NCs, except for replacing DL-mandelic acid
x
over these systematically tuned PdCd NCs under optimal
hydrogenation conditions. The detailed theoretical study
revealed that the tuning of the adsorption energy for the
substrate and hydrogenation products by the precise
introduction of Cd plays a key role in obtaining a
competitive selectivity. Significantly, the outstanding
(
150.0 mg) with benzoin (150.0 mg).
hydrogenation performance of PdCd
x
NCs can be readily
Synthesis of Pd NCs. In a typical preparation of Pd
NCs, Na PdCl (7.4 mg), PVP (100 mg), NaI (5 mg), and
2 4
applied to other intricate substrates (i.e., 4-
nitrobenzaldehyde (NBAD) and 4-nitrostyrene (NS)),
DMF (10 mL) were added to a vial (volume: 30 mL). After
the vial had been capped, the mixture was ultrasonicated
for approximately 10 min. The resulting homogeneous
mixture was heated from room temperature to 120 °C for
approximately 0.5 h and kept at 120 °C for 5 h in an oil
bath. After cooling to room temperature, the colloidal
products were collected by centrifugation and washed
with an ethanol/acetone mixture three times.
making it
a
practical potential hydrogenation
nanocatalyst system with a very competitive selectivity for
fine chemicals and beyond.
EXPERIMENTAL SECTION
Chemicals.
Pd(acac)
(Na PdCl
oleic acid (C18
PVP, K30) and commercial Pd/C (10 wt%, 2-5 nm Pd
nanoparticles) were all purchased from Sigma-Aldrich.
Cadmium acetate dihydrate (Cd(CH COO) ·2H O,
CdAc ·2H O > 99.99%) and ricinoleic acid (C18 , RA,
97%) were purchased from Aladdin. DL-mandelic acid
, > 99%) was purchased from Energy Chemical.
Benzoin (C14 , > 98%) was purchased from Acros. 4-
Nitrophenylacetylene (C NO NPA, 99%) was
purchased from Beijing HWARK Chem. Cyclohexane
12, analytical reagent, ≥ 99.5%), ethanol (C O,
Bis(acetylacetonate)palladium(II)
sodium tetrachloropalladate(II)
, > 98%), oleylamine (C18 37N, OAm, > 70%),
, OA, > 85%), polyvinylpyrrolidone
(
2
,
99%),
2
4
H
Characterizations. The samples were prepared for
characterization by dropping their cyclohexane or ethanol
dispersions onto carbon-coated copper grids using
pipettes. The grids were dried under ambient conditions.
Low-magnification TEM was conducted on a HITACHI
HT7700 transmission electron microscope at an
acceleration voltage of 120 kV. High-magnification TEM
and STEM were conducted on a FEI Tecnai F20
transmission electron microscope at an acceleration
voltage of 200 kV. PXRD patterns were collected on
X’Pert-Pro MPD diffractometer (Netherlands PANalytical)
with a Cu Kα X-ray source (λ = 1.540598 Å). The
34 2
H O
(
3
2
2
2
2
34 3
H O
>
8 8 3
(C H O
12 2
H O
8
H
2
2
,
>
(
6
C H
2 6
H
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to eliminate the interactions between the slabs. The
concentration of catalyst was determined by ICP-AES
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(
(
Varian 710-ES). All the X-ray photoelectron spectroscopy
XPS) measurements of these catalysts were collected
climbing image nudged elastic band (CI-NEB) method
was used to search the transition states and track
minimum-energy paths for the reactions.32, 33
with a Thermo Scientific, ESCALAB 250 XI system.
Hydrogenation measurements. The obtained PdCd
x
The optimized lattice parameters for the primitive
PdCd are 3.06, 3.06 and 3.76 Å. The PdCd [220] surface
was modeled by a 3x3 supercell. All the slabs contained
four atomic layers, and the top two layers were allowed to
relax. For the calculation of the adsorption energy of
small molecules on the slabs, we provide a definition for
the calculation of the adsorption energy (Ead) as:
NCs, PdCd0.82 NPs and Pd NCs were further loaded on
carbon black (Vulcan XC72R carbon, C) with a total Pd
content of 5 wt%, as determined by ICP-AES. All the
hydrogenation runs were performed in a glass pressure
vessel (volume: 48 mL, Chemical glass), to which solvent
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(
10 mL, DMF), PdCd
wt%) and substrate (0.68 mmol NPA/NBAD/NS) were
added. When H was the hydrogen source, the glass
pressure vessel was sealed and purged three times with H
under stirring. The glass pressure vessel was then
pressurized to 0.1 MPa using H and heated to 60 °C
under constant stirring. When HCOONH was the
hydrogen source, HCOONH (1.5 mmol) was added to the
x
(x = 0.53, 0.67, 0.82, 1.13) (10 mg, 5
E
ad = Eslab&mol - (Emol + Eslab
)
2
where Eslab&mol,
Emol, and Eslab are the energies of the slabs
2
with the adsorbed molecule, the single molecule and the
slabs, respectively.
2
4
RESULTS AND DISCUSSION
4
A series of PdCd
through a facile wet-chemical approach, where the
compositions of PdCd NCs can be readily realized by
x
NCs were successfully prepared
glass pressure vessel. The glass pressure vessel was then
sealed and heated to 60 °C under constant stirring.
Samples were taken at fixed intervals by a sample tube.
The samples were filtered and analyzed by a gas
chromatograph (GC, Shanghai Yiyou, GC-7860, column:
HP-5, 30 m × 0.32 mm × 0.25 μm) with a flame ionization
detector (FID). All hydrogenation runs of PdCd0.82 NPs,
Pd NCs and the commercial Pd/C were performed under
x
tuning the amount of Cd precursor supplied while
keeping the amount of Pd precursor constant (see
Experimental Section). To achieve better control over
the Pd-Cd nanostructure,
a
variety of synthetic
parameters were optimized. We found that the proper
amount of reducing agent and use of oleic acid (OA) are
essential for the preparation of well-defined PdCd NCs
(figures S1-3). The representative high-angle annular
dark-field scanning transmission electron microscope
x
the same conditions, changing PdCd NCs to PdCd0.82 NPs,
Pd NCs and the commercial Pd/C. The turnover
frequency numbers (TOF) based on Pd of these catalysts
was calculated using the following equation.
(
HAADF-STEM) image shows that the PdCd0.82 NCs are
fairly uniform in term of size and shape, with an average
edge length of 10.8 nm (Figure 1A). The composition of
the PdCd0.82 NCs was determined to be Pd:Cd = 55.1:44.9
by using an energy dispersive X-ray spectrophotometer
DFT models and calculations. All the calculations
(
EDS), which is consistent with results from inductively
were performed with the “Vienna ab initio simulation
package” (VASP5.4.4).26-29 The electron interactions were
represented by the projector augmented wave (PAW)
method and plane-wave basis functions with a kinetic
energy cut-off of 400 eV.30 The electronic interaction
effect in the calculations was described by the generalized
gradient approximation (GGA) with the Perdew-Burke-
Ernzerh (PBE) functional. Ground-state atomic
geometries were obtained by minimizing the energy and
force to 1.0×10-5 eV/atom and 0.05 eV/Å, respectively. To
examine the effect of van der Waals interactions on
reaction energetics, calculations were performed by using
the DFT-D3 functional.31 The vacuum spacing in the
coupled plasma atomic emission spectroscopy (ICP-AES,
Pd:Cd = 55.4:44.6) (figure S4). As Figure 1B shows, all
the peaks of the powder X-ray diffraction (PXRD) pattern
can be indexed to a body-center unit cell, where the 2θ
values of 39.1°, 42.5°, 50.4°, 61.7° 68.0°, 75.6° and 83.2° can
be indexed to the diffraction of the (111), (200), (002),
(
220), (202), (311) and (222) planes of PdCd alloy,
respectively (JCPDS file no. 06-0570). The crystalline
nature of the PdCd0.82 NCs was further confirmed by high-
resolution HAADF-STEM (Figure 1C). The corresponding
FFT pattern shows the crystal structure of PdCd0.82 NC,
where the PdCd0.82 NC exposes the (220) crystal face along
[001] zone axes (Figure 1D). Figure 1E exhibits enlarged
direction along the Z axis, with respect to the surface, was
HRTEM image from the area indicated by the white
1
5 Å between neighboring slab images, which is sufficient
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dashed rectangle in Figure 1D. As shown in Figure 1F, line-scan analysis show that Pd and Cd are uniformly
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the lattice spacing of crystal plane that parallel to the
edge of cube is measured to be 0.154 nm, assigning to the
distributed throughout the whole NC (Figures 1G and
1H).
(
220) plane of PdCd (JCPDS 06-0570). The HAADF-STEM
image and corresponding elemental mappings and the
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Figure 1. (A) HAADF-STEM image and (B) PXRD pattern of PdCd0.82 NCs. (C) High resolution HAADF-STEM image of
PdCd0.82 NC. (D) Corresponding FFT image of (C). (E) Enlarged high resolution HAADF-STEM image from the selected
area in (C). (F) Integrated pixel intensities of PdCd (taken from the white solid rectangle in (E)) along the arrow
directions, which are perpendicular to the (220) facet. (G) HAADF-STEM image and corresponding elemental mappings
and (H) line-scan analysis across the blue arrow. Inset in (A), cube model of Pd-Cd. Inset in (B), atomic model of Pd-Cd.
By simply changing the amount of Cd precursor
supplied, with other conditions kept the same as the
synthesis of the PdCd0.82 NCs, the PdCd0.53, PdCd0.67 and
PdCd1.13 NCs were readily obtained (see Experimental
Section). Typical STEM images of the PdCd
displayed in figures S5A, 5D and 5G, where all the
different PdCd NCs exhibit similar morphologies and
x
NCs are
x
sizes. Detailed characterizations were further carried out
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to reveal the crystalline nature and elemental
distributions of these PdCd NCs. The EDS results show
mappings, where Pd and Cd are distributed uniformly
throughout the NCs (figures S5B, 5C, 5E, 5F, 5H and 5I).
Precise surface control of PdCd NCs offer a potential for
tuning their catalytic properties. X-ray photoelectron
spectroscopy (XPS) was carried out to reveal the
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that the atomic ratios of Pd/Cd in PdCd0.53, PdCd0.67 and
PdCd1.13 NCs are 64.9/35.1, 60.0/40.0 and 47.2/52.8,
respectively, consistent with the ICP-AES results (figure
S4). As more Cd is introduced, the PXRD peaks are
shifted to smaller 2θ values assigned to the PdCd alloy
phase, indicating the increased d-spacing and the dilation
of the lattice-constant is due to the incorporation of Cd
into the Pd fcc lattice (Figure 2A). This observation
demonstrates that the PdCd alloy phase is maintained in
x
electronic interaction between the Pd and Cd of PdCd
NCs (Figures 2B and 2C). The 3d3/2 and 3d5/2 peaks of Pd
in PdCd NCs exhibit consecutive negative shifts with
x
x
increasing Cd, suggesting that there is a slight electron
transfer from Cd to Pd. The 3d3/2 and 3d5/2 peak positions
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of Cd in the PdCd
proportion of metallic Cd peaks are positively correlated
with Cd peaks. The d-band of PdCd NCs further
x
NCs have no obvious shift, but the
x
these PdCd NCs, while the d-spacing can be tuned by
controlling the Cd content, which ensures the potential
control of their catalytic performance by readily
optimizing Cd content. The crystalline nature of these
PdCd NCs was further characterized by HRTEM (the
x
insets of figures S5A, 5D and 5G). The lattice fringes are
x
confirmed the electronic interaction between the Pd and
Cd, where its center of gravity gradually shifted down
with the increasing amounts of Cd (Figure 2D),
indicating the variation of adsorption capability for PdCd
NCs. Therefore, the PdCd
compositions exhibit tunable surface properties.
x
all approximately 0.15 nm, which can be assigned to the
x
nanostructures with different
(
220) plane of PdCd. The spatial distributions of Pd and
Cd species in these PdCd NCs were then identified by
HAADF-STEM, line-scan analysis and elemental
x
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Figure 2. (A) PXRD patterns and enlarged PXRD patterns of PdCd
different PdCd NCs. (D) Surface valence band of PdCd NCs. All the spectra are background corrected. The white bar
indicates the center of gravity.
x
NCs. XPS spectra of (B) Pd 3d and (C) Cd 3d of
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Scheme 1. Schematic illustration showing different hydrogenation order of alkynyl and nitryl over PdCd
and HCOONH as hydrogen sources.
x 2
NCs using H
4
Pd-based nanocatalysts have been widely used in
hydrogenation reactions, and the quest for superior
hydrogenation nanocatalysts that combine high activity,
selectivity, and stability is state-of-the-art research.34-36 As
the hydrogen source under mild conditions (see Materials
and Methods). As shown in Figure 3A, PdCd0.82 NCs
exhibits a desirable conversion and selectivity to NS. The
main product of NS (97.6 %) and trace NEY (2.4 %) were
detected in a very short time period (0.5 h). The
selectivity for NS (96.0 %) was maintained even after a 5 h
reaction, indicating that PdCd0.82 is only able to reduce
alkynyl to vinyl under this reaction condition. Inspired by
a
proof-of-concept
hydrogenation of NPA was first adopted as the model
reaction to evaluate the performance of the PdCd NCs
application,
the
selective
x
with different composition ratios. In this study,
remarkable selectivity with a switchable feature for all five
potential hydrogenation products was successfully
realized, and the performance of PdCd NCs exhibits an
x
obvious composition dependence as well as a distinct
this result, the hydrogenation properties of PdCd0.53
,
PdCd0.67 and PdCd1.13 NCs were also tested under identical
conditions. For PdCd1.13 NCs, the selectivity to NS was still
obtained at a suitable level, while the hydrogenation
conversion rate was decreased, revealing that an
increased Cd content weakens the activity (figure S6A).
In contrast, the high conversion rate of NPA still
remained for PdCd0.67 and PdCd0.53 NCs, but the main
hydrogenated product was converted to NEY and EA,
respectively, indicating that an improved hydrogenation
activity can be obtained by decreasing the ratio of Cd
(Figures 3B and 3C). It is worth noting that the
hydrogen source dependence (Scheme 1).
In detail, the selective hydrogenation of NPA into NS,
4
-Nitroethylbenzene (NEY), 4-Ethylaniline (EA), 4-
Aminophenylacetylene (APA) and 4-Aminostyrene (AS)
has been readily achieved under optimal hydrogenation
conditions (Figure 3). As a starting point, the PdCd0.82
2
NCs was applied to the hydrogenation of NPA using H as
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conditions. As shown in figure S7A, the NS is hardly
intermediate products of NS and NEY were detected in
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the initial hydrogenation period over PdCd0.67 and
PdCd0.53 NCs, respectively. Therefore, we can conclude
that the product transformation path is from NPA to NEY
hydrogenated by PdCd1.13 and PdCd0.82 NCs, while a high
conversion could be observed over PdCd0.67 and PdCd0.53
NCs. As expected, for NEY, only PdCd0.53 NCs exhibit a
high conversion (figure S7B). Therefore, the satisfied
2
and eventually to EA under H , where the hydrogenation
trend of the functional groups is according to the order of
alkynyl > vinyl > nitryl under this reaction condition. To
confirm this conclusion, we further investigated the
selectivity to the specific products over PdCd
hydrogenation mainly results from the activity difference
of PdCd NCs with different ratios of Cd.
x
NCs in NPA
x
x
hydrogenation properties of these PdCd NCs by directly
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using NS or NEY as the substrate under identical reaction
Figure 3. Hydrogenation of NPA over PdCd
using H as the hydrogen source over (A) PdCd0.82 NCs, (B) PdCd0.67 NCs and (C) PdCd0.53 NCs. The conversion of NPA
and selectivity to specific products vs. time using HCOONH as the hydrogen source over (D) PdCd1.13 NCs, (E) PdCd0.82
NCs and (F) PdCd0.53 NCs. The light blue areas in (D, E, F) represent error bars of conversion.
x
NCs. The conversion of NPA and selectivity to specific products vs. time
2
4
The desired selectivity to NS, NEY and EA has thus
been realized, and a distinct composition effect and
hydrogenation trend (alkynyl > vinyl > nitryl) have been
observed. To further achieve the same level of selectivity
to all hydrogenation products, the nitryl must be initially
reduced to yield APA and AS. Considering that metal-
catalyzed reduction features various reducing agents
other than molecular hydrogen,37-39 the use of different
hydrogen sources might be an efficient approach to
change the hydrogenation trend. As a hydrogen source,
were further motivated to explore the performance of
PdCd NCs for selective NPA hydrogenation by using
HCOONH . Significantly, a complete switch of selectivity
to the products was achieved by changing the hydrogen
source from H to HCOONH . For PdCd1.13 NCs, the main
hydrogenation product changed from NS to APA under
the same hydrogenation conditions by replacing H with
HCOONH (Figures 3D and S6A). Over the course of the
x
4
2
4
2
4
hydrogenation reaction, it is clear that NPA was totally
converted after 3 h, and the selectivity of APA reached
92.0 %. AS (8.0 %) was also detected in this process
(Figure 3D). The hydrogenation properties of other
4
HCOONH is an alternative, simple, and inexpensive
reducing agent, which can offer eco-friendly reaction
conditions and even enhance efficiency.38 Therefore, we
PdCd
x
NCs were also investigated. As shown in Figure 3E
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PdCd0.82 NCs exhibit a higher hydrogenation conversion performance of Pd NCs is very similar to that of the
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rate over PdCd1.13 NCs, while the main product is
converted to AS (100 %). It is worth noting that the APA
was also detected in the initial reaction period, indicating
that the hydrogenation reaction proceeded on a different
commercial Pd/C (figure S13), demonstrating that the
hydrogenation rate is hard to control by pure Pd
nanocatalysts and leads to
a
low selectivity for
intermediate products in NPA hydrogenation. These
results show that the introduction of Cd to Pd NCs is
2 4
hydrogenation path by changing H with HCOONH as
the hydrogen source. The PdCd0.67 NCs exhibits similar
performance to PdCd0.82 NCs but are unable to maintain a
high selectivity for AS and is further converted to EA after
achieving a high conversion level (figure S6B). For the
PdCd0.53 NCs, it is obvious that all of the NPA was
converted into fully hydrogenated EA as the final
reduction product (Figure 3F). It is worth emphasizing
that the NS and NEY can be detected in short reaction
time, indicating the same hydrogenation path over
beneficial for
a
desirable selectivity for different
hydrogenation products of NPA through precisely
weakening the activity of Pd. In addition, PdCd0.82 NPs
were also chosen as references to explore the shape/facet
effect in the hydrogenation reaction. As shown in figure
S14, PdCd0.82 NPs exhibits similar hydrogenation rates but
shows a significantly reduced NS or AS selectivity
compared with PdCd0.82 NCs, whether H
the hydrogen source. In other words, the systematic
tuning of PdCd NCs is essential to achieve the desired
0
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0
2 4
or HCOONH is
PdCd0.53 NCs whether using H
2
or HCOONH
4
as the
x
hydrogen source, which demonstrates that the distinct
hydrogen source effect can only be observed for the
competitive level of hydrogenation of NPA into NS, NEY,
EA, APA and AS.
PdCd
hydrogenation path was observed in the NPA
hydrogenation by using HCOONH as a hydrogen source,
x
NCs with high Cd contents. Therefore, a distinct
To understand the mechanism of two distinct
hydrogenation paths over various PdCd
x
NCs by using
4
where the hydrogenation trend of functional groups is in
the order of nitryl > alkynyl > vinyl, in contrast with the
different hydrogen sources, series of detailed
a
experiments were further carried out. Considering that
the surface properties of nanocatalysts largely affect the
adsorption/desorption of the substrate and corresponding
hydrogenation reaction by using H
source. In addition, to make a more precise comparison,
the turnover frequency (TOF) of PdCd NCs based on Pd
2
as the hydrogen
products, the PdCd
possess different
adsorption/desorption of nitryl and alkynyl. To make a
direct comparison, experiments involving the
hydrogenation of the physical mixture of phenylacetylene
(PA) and nitrobenzene (NB) over PdCd NCs were
conducted under different hydrogen sources. When H is
x
NCs with different Cd contents can
x
near the highest selectivity of the desired product was
provided. As shown in table S2, the TOF of reactions
surface properties for the
using H
that of those using HCOONH
Meanwhile, the Cd concentration has more effect on the
TOF of reactions when using H as the hydrogen source
table S2). Moreover, control experiments of the
2
as a hydrogen source is generally higher than
4
as a hydrogen source.
x
2
2
(
adopted as the hydrogen source, the hydrogenation rates
of PA and NB decrease with the increased content of Cd,
but the hydrogenation rate of PA is always much higher
3 2
additional introduction of NH in the presence of H have
been carried out. The results reveal that the
hydrogenation rates or products are close to the cases
x
than that of NB in all the PdCd NC-catalyzed reactions
only using H
because the excessive supply of H
NH and prevents the hydrogenation trend as induced by
2
as the hydrogen source (Figure S8), mostly
(figure S15). In addition, while PdCd0.82 and PdCd1.13 NCs
maintain a superior selectivity for styrene (SY) at different
conversions, PdCd0.53 and PdCd0.67 NCs struggle to
preserve the demanded selectivity for SY as the reaction
proceeds. Significantly, a completely different catalytic
2
blocks the action of
3
HCOOONH
HCOOONH
4
. This exactly proves the specific role of
played in this reaction.
4
x 4
behavior was observed for all PdCd NCs once HCOONH
Pd NCs (figures S9 and S10) and PdCd0.82 NPs (figure
S11) of a similar size were also prepared to better
understand the roles of the composition and morphology
was applied as the hydrogen source with other reaction
conditions being the same. The hydrogenation rate of PA
by using HCOONH as the hydrogen source is slightly
2
lower than that of H , while a much higher hydrogenation
4
of PdCd
x
NCs in the highly selective hydrogenation
reaction. As shown in figure S12, Pd NCs exhibit a much
higher hydrogenation rate in the hydrogenation of NPA
rate of NB was obtained, which is even higher than the
hydrogenation rate of PA (figure S16). The contrary
results of the hydrogenation rates for PA and NB with
different hydrogen sources are consistent with the
2 4
whether H or HCOONH is the hydrogen source, where
NPA was fully hydrogenated into EA in a short time. The
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when the concentration of Cd increases and the trend
different selectivity of NPA hydrogenation by using
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different hydrogen sources.
matches well with XPS results. We further performed
adsorption energy analysis for these established models.
Since HCOONH decomposes to HCOOH and NH , the
To illuminate the influence of the surfaces of PdCd
x
4
3
NCs with different compositions on the catalytic
performance, we used density functional theory (DFT) to
investigate the catalytic processes. First, we constructed
PdCd NCs with a (220) surface that matched with the
experimental results. To simulate different compositions,
HCOOH is the main reductive part as the hydrogen
source in our simulations. As shown in tables S1 and S4,
the adsorption of all the species (including the substrate,
hydrogen sources, and possible products) decreases as the
Cd concentration increases. Strong adsorption of the
substrate and hydrogen sources implies high catalytic
efficiency. Therefore, the activity of the PdCdx NCs
decreases with higher Cd concentrations.
0
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9
0
x
we built the surface for pure Pd and PdCd NCs with
ratios of Pd to Cd as 0.5, 0.8 and 1.25, respectively, on the
top surface of the (220) plane (figure S17). As shown in
table S3, the electron transferred from Cd to Pd increases
Figure 4. The adsorption configurations of adsorbates on pure Pd and PdCd
hydrogenation of phenylacetylene on the PdCd0.5 (220) surface. (A) The different optimized configurations of NPA, NS
and NEY adsorbed on the surface of pure Pd and PdCd NCs. The numbers represent the distances between the N atom
(or C atom) and the surface (unit: Å). (B, C) Step-by-step hydrogenation mechanism of phenylacetylene to phenylethane
on the (B) PdCd0.5 (220) surface and (C) PdCd1.25 (220) surface. Numbers in parentheses indicate the barriers of elementary
steps. Cd, yellow; Pd, blue; C, gray; H, white.
x
NCs and reaction pathway for the
x
The distinct hydrogen source (H
dependence for PdCd NCs in hydrogenation reactions is
also considered in this simulation. H should be adsorbed
2
or HCOONH
4
)
stronger bindings with alkynyl and vinyl groups than
nitryl groups on the surface of Pd and PdCd NCs, so that
x
x
2
alkynyl and vinyl groups would be hydrogenated
preferentially, which is in agreement with the reductive
order (alkynyl > vinyl > nitryl) observed experimentally.
on the metal surface first and then be activated to
hydrogenate the substrates. Figure 4A shows the
optimized configurations of NPA, NS and NEY adsorbed
on the surface of pure Pd and PdCd
Cd is in the coordinated environment that displays
-
+
HCOONH
4
can be ionized into HCOO , NH
3
and H in
-
-
x
NCs. Notably, Pd or
solutions. HCOO can provide a negatively charged H
anion that is highly reductive and nucleophilic, where the
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-
H anion prefers to attack N atoms with positive charges
in nitryl groups; when the reduction reaction starts, the
proton H+ would attack the O atom with a negative
charge in a nitryl group (figure S18).40-42 The reduction of
nitryl groups occur more preferentially than the
hydrogenation of alkynyl and vinyl groups, mainly
the minimum adsorption energy of -0.98 eV (table S4);
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thus, NEY would desorb most easily and become the final
product. The case of PdCd has a similar trend to that of
PdCd0.8. Finally, NS will be the final product on the
PdCd1.25 surface due to its lowest adsorption energy of -
0.87 eV. This trend is fully in line with the experimental
-
+
because nitryl can directly react with HCOO (or H ).
4
data. When HCOONH is the hydrogen source, in which
case EA, AS, and APA are the possible products, we could
also determine the final product based on the minimum
adsorption energy rule from table S4, and the trend is
also consistent with the experimental data. Furthermore,
the differences between the adsorption energies of NPA
and NS (NS and NEY, APA and AS, AS and EA) decrease
with increasing Cd concentrations, indicating that the
adsorption energy of the reactants and the corresponding
reduction products are getting closer. Thus, the reactant
tends to be desorbed more easily, broadly agreeing with
the experimental results.
Whether H or HCOONH is used as a source of
2
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hydrogen, the selectivity has been tuned with the increase
of Cd concentration. As shown in Figures 4B and 4C, we
studied the mechanisms of hydrogenation of
phenylacetylene to phenylethane on the PdCd0.5 (220)
surface and PdCd1.25 (220) surface by the transition state
(
TS) search and the related structures can be seen in
figure S19. It is noticed that on the PdCd0.5(220) surface,
the adsorption energies of phenylacetylene and styrene
are -1.74 eV and -1.20 eV, respectively, which are much
larger than the activation barriers, thus leading to the
final product being phenylethane. However, on the
PdCd1.25(220) surface, the activated barriers of the first
and third reaction steps reach 1.18 eV and 1.90 eV, which
are much higher than the adsorption energies of
phenylacetylene and styrene. These data indicate that on
the PdCd1.25 (220) surface, it is not easy for the reaction to
occur and the catalytic activity is low. When the
concentration of Cd doping increases, on the one hand,
the adsorption of reactants (phenylacetylene, styrene) has
weakening effects, on the other hand, the activation
barriers of reactions on PdCd1.25 surfaces are higher than
that on PdCd0.5 surfaces. Both results show that PdCd1.25 is
less effective than PdCd0.5 for the reaction of the
hydrogenation of phenylacetylene to occur. This is
consistent with the phenomena observed in the
experiment: the increase of Cd concentration weakens the
catalytic effect on the hydrogenation of phenylacetylene.
Therefore, the adsorption energies of the substrates are
significantly reduced as the Cd concentration increases.
That is, the decreases of the adsorption energies can
protect intermediates from further hydrogenation and
lead to the final product since it will be more easily
desorbed from the surface. Based on this rule, we could
determine the final product in different situations. For
Encouraged by these efficient hydrogenation results,
hydrogenation reactions with other similar substrates
(NBAD and NS) over PdCd NCs were also carried out. As
x
expected, a desirable level of conversion for NBAD and a
competitive NS selectivity for all potential hydrogenation
products have been achieved under the optimized
hydrogenation conditions (table S5 and table S6), which
also exhibits an obvious composition dependence and a
distinct hydrogen source dependence. From the
comparison of catalysts performance in our work with
some benchmark catalysts in others’ works, it is apparent
that most previous studies solely focused on achieving a
high selectivity of one desired hydrogenated product. In
contrast, the realization of high selectivity for all potential
hydrogenation products under one catalysis system is the
most striking feature of our work (table S2).
A practically relevant hydrogenation catalyst should
exhibit both high performance (activity and selectivity)
and excellent stability. We have also evaluated the
stability of PdCd NCs by consecutive reactions, where
x
constant amounts of substrate were added in each run. As
shown in Figures 5 and S20, after being reused six times,
a superior conversion level was maintained for all PdCd
NCs, with no obvious deactivation of the PdCd NCs in
x
example, for PdCd0.5 with H
2
as the hydrogen source, EA
x
will be the final product because its adsorption energy of -
1.32 eV is lower than that of NS and NEY. For PdCd0.8, the
adsorption configuration shows that NEY is parallel to the
surface and does not bind to the metal surface, indicating
the physical adsorption of NEY on the PdCd0.8 surface
rather than the chemical bindings. Additionally, NEY has
the consecutive reactions. A slightly decreased selectivity
to different hydrogenation products of NPA was observed.
The detailed characterizations for the catalysts after
recycling experiments were carried out. It is clear to see
that there are no obvious morphology/composition
changes after hydrogenation in these cases (figure S21).
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is reasonable to infer that the decreased selectivity is
Uniform elemental distributions have been also
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confirmed by the elemental mappings and the line-scan
analysis (figure S22). Further XPS studies reveal that
there are no obvious differences of Pd and Cd for catalysts
mainly attributed to the subtle surface change of chemical
states under the reducing conditions and the adsorption
of reaction species.
(
figure S23). Considering the largely maintained
structure of PdCd NCs after recycles (figure S21-S23), it
x
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Figure 5. The reusability of PdCd
x
NCs. The consecutive hydrogenation reactions of NPA over (A) PdCd0.67 NCs and (B)
PdCd0.82 NCs using H as the hydrogen source. The consecutive hydrogenation reactions of NPA over (C) PdCd0.82 NCs
2
and (D) PdCd1.13 NCs using HCOONH
4
as the hydrogen source. The reaction time is 2 h for each run.
irreplaceable role of Cd and uniform nanocube
CONCLUSIONS
morphology in the remarkable selective hydrogenation
has been identified. DFT calculations have been
performed to elucidate the origin underlying the
In summary, we have presented a wet-chemical
strategy for a series of well-defined and nanocubic PdCd
x
nanocatalysts with a tunable composition for the flexible
selectivity control of NPA hydrogenation for all potential
exceptional selectivity of PdCd NCs, in which the tuning
x
of adsorption energy of hydrogenation products by
hydrogenation products. The performance of the PdCd
x
precise modulation of Cd play a key role in boosting the
selectivity. The PdCdx NCs also exhibit excellent
reusability with negligible activity/selectivity decay and
NCs exhibited strong dependence of both composition
tuning and hydrogen source selections, where the
1
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Page 12 of 14
structure/composition
reactions. The generality
hydrogenation performance of PdCd
applied to other similar substrates (e.g., NBAD and NS).
The present finding opens promising opportunities
towards the rational design of practically relevant
hydrogenation nanocatalysts with extremely high
selectivity for broad applications such as fine chemical
production and beyond.
changes
after
the
consecutive
outstanding
(5) Chen, G. X.; Xu, C. F.; Huang, X. Q.; Ye, J. Y.; Gu, L.; Li, G.;
Tang, Z. C.; Wu, B. H.; Yang, H. Y.; Zhao, Z. P.; Zhou, Z. Y.; Fu, G.;
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Zheng, N. F. Interfacial electronic effects control the reaction
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(6) Marshall, S. T.; O’Brien, M.; Oetter, B.; Corpuz, A.; Richards,
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Effect of lead acetate in the preparation of the Lindlar catalyst. J.
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x
can be readily
(
8)
Wang, X.; Liu, D.; Song, S.; Zhang, H. Pt@CeO
2
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multicore@shell self-assembled nanospheres: clean synthesis,
structure optimization, and catalytic applications. J. Am. Chem.
Soc. 2013, 135, 15864-15872.
ASSOCIATED CONTENT
(
9) Song, S.; Li, K.; Pan, J.; Wang, F.; Li, J.; Feng, J.; Yao, S.; Ge,
Supporting Information.
X.; Wang, X.; Zhang, H. Achieving the trade-off between
selectivity and activity in semihydrogenation of alkynes by
The Supporting Information is available free of charge on the
ACS Publications website site.
fabrication of (asymmetrical Pd@Ag Core)@(CeO
2
Shell)
nanocatalysts via autoredox reaction. Adv. Mater. 2017, 29,
1605332-1605338.
(10) Tian, H.; Huang, F.; Zhu, Y.; Liu, S.; Han, Y.; Jaroniec, M.;
Yang, Q.; Liu, H. The development of yolk-Shell-Structured
Pd&ZnO@carbon submicroreactors with high selectivity and
stability. Adv. Funct. Mater. 2018, 1801737.
Detailed information for characterizations, DFT
calculations and hydrogenation performances, including
Figures S1- S23 and Tables S1-S6.
AUTHOR INFORMATION
(11) Liu, P.; Qin, R.; Fu, G.; Zheng, N. Surface coordination
chemistry of metal nanomaterials. J. Am. Chem. Soc. 2017, 139,
Corresponding Author
2
(
122-2131.
12) Li, M.; Zhang, N.; Long, R.; Ye, W.; Wang, C.; Xiong, Y.
PdPt alloy nanocatalysts supported on TiO : maneuvering metal-
*xulai15@suda.edu.cn.
2
*hxq006@suda.edu.cn.
hydrogen interactions for light-driven and water-donating
selective alkyne semihydrogenation. small 2017, 13, 1604173-
1604173.
Author Contributions
∥
Y.F., W.X. and B.H. contributed equally to this work.
(
13) Chan, C. W. A.; Mahadi, A. H.; Li, M. M. J.; Corbos, E. C.;
Tang, C.; Jones, G.; Kuo, W. C. H.; Cookson, J.; Brown, C. M.;
Bishop, T. P.; Tsang, S. C. E. Interstitial modification of
palladium nanoparticles with boron atoms as a green catalyst for
selective hydrogenation. Nat. Commun. 2014, 5, 5787.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
(14) Niu, W.; Gao, Y.; Zhang, W.; Yan, N.; Lu, X. Pd-Pb alloy
nanocrystals with tailored composition for semihydrogenation:
taking advantage of catalyst poisoning. Angew. Chem. Int. Ed.
2015, 127, 8389-8392.
(15) Wei, H.; Liu, X.; Wang, A.; Zhang, L.; Qiao, B.; Yang, X.;
Huang, Y.; Miao, S.; Liu, J.; Zhang, T. FeOx-supported platinum
This work was financially supported by the Ministry of
Science
and
Technology
(2016YFA0204100,
2
017YFA0208200), the National Natural Science Foundation
of China (21571135), Young Thousand Talented Program, Six
Talent Peak Project in Jiangsu Province (Grant No. XNY-
042), the Collaborative Innovation Center of Suzhou Nano
Science & Technology, the Priority Academic Program
Development of Jiangsu Higher Education Institutions
single-atom
and
pseudo-single-atom
catalysts
for
chemoselective hydrogenation of functionalized nitroarenes. Nat.
Commun. 2014, 5, 5634.
(
16) Mitsudome, T.; Yamamoto, M.; Maeno, Z.; Mizugaki, T.;
Jitsukawa, K. One-step synthesis of core-Gold/shell-Ceria
nanomaterial and its catalysis for highly selective
semihydrogenation of alkynes. J. Am. Chem. Soc. 2015, 137, 13452-
(
PAPD), the 111 Project, and Joint International Research
Laboratory of Carbon-Based Functional Materials and
Devices, and the start-up supports from Soochow University.
Lai Xu acknowledges financial support from the National
Natural Science Foundation of China (Grant No. 91961120).
1
3455.
(17) Kahsar, K. R.; Schwartz, D. K.; Medlin, J. W. Control of
metal catalyst selectivity through specific noncovalent molecular
interactions. J. Am. Chem. Soc. 2014, 136, 520-526.
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