Concavity Tuning of Intermetallic Pd-Pb Nanocubes for Selective Semihydrogenation Catalysis

Article abstract of DOI:10.1021/acs.chemmater.8b02337

Although considerable studies on pursuing high-performance Pd-based catalysts for the semihydrogenation of alkynes have been carried out, the creation of catalyst with high activity, selectivity and stability simultaneously toward semihydrogenation reactions remains a significant challenge. Herein, for the first time we report a facile synthetic strategy to realize the intermetallic Pd-Pb nanocubes with different concave degree by selectively utilizing small molecules. These obtained Pd-Pb nanocrystals exhibit high activity in the semihydrogenation of alkynes, where their performances are highly shape- and composition-dependent with Pd-Pb concave nanocubes showing the optimized alkene selectivity of 94.6% and activity of 179.2 h^-1, much higher than those of 10% Pd/C. Detailed X-ray photoelectron spectroscopy results show that the higher ratio of metallic Pd results in the higher activity for semihydrogenation of phenylacetylene and the higher ratio of Pb²⁺ and Pb/Pd contribute to higher styrene selectivity. The density functional theory calculations further reveal that the favorable adsorption energy of phenylacetylene and desirable desorption energy of styrene on the Pd₃Pb surface are critical for the phenylacetylene semihydrogenation with excellent activity and high selectivity. Furthermore, the Pd-Pb concave nanocube can endure at least five cycles with very limited conversion and selectivity decays, representing an efficient Pd-based catalyst for selective hydrogenation and beyond.

Full text of DOI:10.1021/acs.chemmater.8b02337

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Article
Concavity Tuning of Intermetallic Pd-Pb Nanocubes
for Selective Semihydrogenation Catalysis
Junbo Zhang, Weiwei Xu, Lai Xu, Qi Shao, and Xiaoqing Huang
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02337 • Publication Date (Web): 22 Aug 2018
Downloaded from http://pubs.acs.org on August 23, 2018
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Concavity Tuning of Intermetallic Pd-Pb Nanocubes for
Selective Semihydrogenation Catalysis
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Junbo Zhang,1+ Weiwei Xu,2+ Lai, Xu,2*Qi Shao,1 and Xiaoqing Huang1*
1College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu,
215123, China.
2Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-
Based Functional Materials & Devices, Soochow University, Jiangsu, 215123, China.
*Corresponding author. Email: xulai15@suda.edu.cn; hxq006@suda.edu.cn
ABSTRACT: Although considerable studies on pursuing high-performance Pd-based catalysts for
the semihydrogenation of alkynes have been carried out, the creation of catalyst with high activity,
selectivity and stability simultaneously towards semihydrogenation reactions remains significant
challenge. Herein, for the first time we report a facile synthetic strategy to realize the intermetallic
Pd-Pb nanocubes with different concave degree by selectively utilizing small molecules. These
obtained Pd-Pb nanocrystals exhibit high activity in the semihydrogenation of alkynes, where their
performances are highly shape- and composition-dependent with Pd-Pb concave nanocubes showing
the optimized alkene selectivity of 94.6% and activity of 179.2 h-1, much higher than those of 10%
Pd/C. Detailed X-ray photoelectron spectroscopy results show that the higher ratio of metallic Pd
results in the higher activity for semihydrogenation of phenylacetylene and the higher ratio of Pb2+
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and Pb/Pd contribute to higher styrene selectivity. The density functional theory calculations further
reveal that the favorable adsorption energy of phenylacetylene and desirable desorption energy of
styrene on the Pd3Pb surface are critical for the phenylacetylene semihydrogenation with excellent
activity and high selectivity. Furthermore, the Pd-Pb concave nanocube can endure at least five
cycles with very limited conversion and selectivity decays, representing an efficient Pd-based
catalyst for selective hydrogenation and beyond.
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INTRODUCTION
Nanomaterials composed of precious metals have drawn great attentions in recent years.1,2 In
particular, nanostructured palladium (Pd) or platinum (Pt) is an efficient heterogeneous catalyst for
fuel cell reactions, organic reactions and series of industrial production of chemicals due to their
unique physical/chemical properties.3-15 However, the monometallic Pd catalysts are not highly
selective toward the desired products when more than one reducible groups are presented in the
reaction system, such as in the acetylenic, furfuralor or nitroaromatic compounds hydrogenations
because the other reaction processes may be more likely to occur in dynamics and thermodynamics
rather than the target reaction process. To maximally overcome this limitation, introducing the
second metal is considered as an efficient strategy to enhance the selectivity of the target products
via modifying the electronic structures of Pd.16-18 Alkynes semihydrogenation is an extremely
important reaction in the synthesis of fine chemicals, the target products of this reaction are also an
important monomer for synthetic resins as well as ion exchange resins.12,16 However, to date, while
several PdM nanoparticles (NPs) have been reported with enhanced alkenes selectivity for the
alkynes semihydrogenation,20-26 the activities are still unsatisfied.21,27 The design of catalysts with
high selectivity and activity simultaneously for semihydrogenation is highly desirable.20,21
From the view of structure, the unique intermetallic nanocrystals with the specific crystal
structures and highly ordered surface atom arrangements can provide the spatial separation of active
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sites, which cannot be afforded by the conventional disordered nanocrystals.18,28-33 The intermetallic
catalyst (IMC) can provide strong interaction, charge transfer and orbital rehybridization to achieve
catalysis optimization. In addition, due to the high enthalpy of mixing, the IMCs can usually exhibit
outstanding stability in catalytic reactions.22,31-34 Nevertheless, pursuing effective structure control on
IMCs remains a great challenge,22,23,26 not to mention the systematic morphological regulation of
IMCs. To this end, the realization of systematic control of intermetallic catalysts is highly challenge
yet desirable, especially for catalysis optimization.26,35-40
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EXPERIMENTAL SECTION
Chemicals. Palladium(II) acetylacetonate (Pd(acac)2, 99%), lead(II) acetylacetonate (Pb(acac)2,
99%), lead(II) formate (Pb(HCOO)2, 99%), oleylamine (C18H37N, OAm, > 70%), benzoic acid
(C7H6O2, reagent grade, 99%), dodecyltrimethylammonium chloride (C15H34NCl, DTAC, 99%) and
benzaldehyde (C7H6O, 99%) are all purchased from Sigma-Aldrich. Lead(II) chloride (PbCl2,
99.99%), lead(II) acetate trihydrate (Pb(Ac)2·3H2O, > 99%) were obtained from Aladdin. Benzoic
acid (C7H6O2, 99%), benzyl alcohol (C7H8O, 99%), aniline (C6H7N, 99%), cyclohexane (C6H12,
analytical reagent, ≥ 99.5%) and ethanol (C2H6O, analytical reagent, ≥ 99.7%) were obtained from
Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the chemicals were used as received
without further purification. The water (18 Mꢀ/cm) used in all experiments was prepared by passing
through an ultra-pure purification system.
Synthesis of Pd3Pb NCs, Pd3Pb SCNCs and Pd3Pb CNCs. In a typical synthesis of Pd3Pb NCs,
7.6 mg Pd(acac)2, 3.2 mg Pb(Ac)2·3H2O, 6.6 mg DTAC and 5 mL OAm were added into a vial
(volume: 35 mL). After the vial was capped, the mixture was ultrasonicated for around 1 h. The
resulting homogeneous mixture was then heated from room temperature to 180 ºC in around 45 min
and maintained at 180 ºC for 5 h in an oil bath, before it was cooled to room temperature. All the
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Pd3Pb nanomaterials were collected by centrifugation and washed three times with
ethanol/cyclohexane mixture. For the synthesis of Pd3Pb SCNCs and Pd3Pb CNCs, all the conditions
are similar to those of Pd3Pb NCs but changing 6.6 mg DTAC to 40 mg benzoic acid and 0.1 mL
benzaldehyde, respectively. Before starting the catalytic reaction, the Pd3Pb intermetallic
nanomaterials were load on commercial carbon black and subjected to thermal annealing in air at
250 ºC for 1 h and then in 5% H2/N2 atmosphere at 300 ºC for 1 h.
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Characterizations. Power X-ray diffraction (PXRD) patterns were collected using an X`Pert-Pro
X-ray powder diffractometer equipped with a Cu radiation source (λ = 0.15406 nm). The
morphology and size of the nanomaterials were determined by transmission electron microscope
(TEM, Hitachi, HT7700) with an accelerating voltage of 120 kV. High-resolution TEM (HRTEM),
TEM energy dispersive X-ray spectroscopy (TEM-EDS) and high angle annular dark field scanning
TEM (HAADF-STEM) were conducted on an FEI Tecnai F20 transmission electron microscope at
an acceleration voltage of 200 kV. All the samples were prepared by dropping cyclohexane or
ethanol dispersion of samples onto carbon-coated copper grids and dried under ambient condition.
Low-resolution energy dispersive X-ray spectroscopy was performed on a scanning electron
microscope (SEM, Hitachi, S-4700). All the X-ray photoelectron spectroscopy (XPS) spectra of Pd-
Pb NCs, SCNCs and CNCs were collected by XPS (Thermo Scientific, ESCALAB 250 XI). The
concentration of each ion for each catalyst was analyzed by the inductively coupled plasma atomic
emission spectroscopy (710-ES, Varian, ICP-AES).
Hydrogenation reactions of Pd3Pb NCs, SCNCs and CNCs. For a typical hydrogenation
reaction, 3 wt% Pd of Pd3Pb/C was added into a 50 mL glass pressure vessel. After the addition of
0.1 mmol phenylacetylene, 10 mL ethanol and 5 mg catalyst into a glass pressure vessel, the vessel
was pressurized with H2 (2 atm). The reaction was conducted at 50 ºC under stirring at 400 rpm.
After the completion of the reaction, the liquid mixture was collected by centrifugation at 12000 rpm
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for 2 min. The liquid samples were detected by gas chromatography with flame ionization detector
(GC-FID). Formula for the activity of phenylacetylene hydrogenation reaction:
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nphenylacetylene converted
Semihydrogenation Activity =
nPd×h
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RESULTS AND DISCUSSION
Herein, for the first time we report a wet-chemical way to synthesize well-defined Pd3Pb
intermetallic nanomaterials with different concave degree named nanocubes (NCs), slight concave
nanocubes (SCNCs) and concave nanocubes (CNCs), which gave rise to highly active, selective and
stable semihydrogenation of alkynes. Significantly, the successful creation of the well-defined Pd3Pb
nanomaterials with same composition provides an ideal frame for exploring the effect of concave
degree on semihydrogenation. We found that the Pd3Pb SCNCs and Pd3Pb CNCs could deliver high
conversion and selectivity as well as outstanding stability compared with 10% Pd/C, in which Pd3Pb
CNCs show the highest conversion of 99.9%, styrene selectivity of 94.6% and the activity of
semihydrogenation of 179.2 h-1. The intermetallic structure also makes the Pd3Pb CNCs highly stable
with slight activity decay over five cycles. The high performance of the Pd3Pb CNCs has also been
successfully expanded to other alkynes.
We prepared Pd3Pb NCs with concave feature through a wet-chemical approach, in which
Pd(acac)2 and Pb(Ac)2•3H2O were used as Pd and Pb precursors, OAm was applied as solvent and
surfactant (see Supporting Information for details) and DTAC, benzoic acid or benzaldehyde was
chosen as morphology control agent. The synthetic parameters, such as the concentrations of the
small molecules, the precursors and the species have been explored in details (Figures S1-5), in
which the formation of Pd3Pb NCs can be attributed to the introduction of halogen ions, while the
creation of Pd3Pb CNCs and Pd3Pb SCNCs can be ascribed to the effects from aldehyde group and
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carboxyl group, respectively.
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Figure 1. (a, b) TEM images, (c) PXRD pattern, (d) TEM-EDX, (e) HRTEM image, (f) HAADF-
STEM image and elemental mappings and (g) the corresponding line-scans of the Pd3Pb CNCs. The
inset in (a) is the 3D structure model of the Pd3Pb CNCs.
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Typical transmission electron microscopy (TEM) (Figure 1a-b) images of Pd3Pb CNCs show
that the majority of the products have concave cubic profiles with the yield approaching to 100%.
The Pd3Pb CNCs are monodisperse with an average diameter of 13.6 nm, which is similar with the
size (15.2 nm) calculated by the Scherrer-equation (Figure S6a-c). As shown in Figure 1c, the
PXRD pattern of the Pd3Pb CNCs can be indexed to the standard XRD pattern of intermetallic Pd3Pb
(Joint Committee on Powder Diffraction Standards, JCPDS, No. 20-0827), where Pb atoms occupy
the eight vertexes of the cube and Pd atoms occupy the centers of six faces in one unit cell. The
composition ratio of Pd/Pb is 73.1 0.2/26.9 0.2, as revealed by the ICP-AES, which is consistent
with the TEM-EDX (73.5 0.4/26.5 0.4) (Figure 1d) and SEM-EDX (75.0 0.2/25.0 0.2)
results (Figure S6d). As shown in the HRTEM image (Figure 1e), the distinct lattice spacings of
0.198, 0.225 and 0.232 nm, corresponding to the (200), (-111) and (111) planes of the ordered Pd3Pb
intermetallic structure, are observed. Furthermore, the high-index steps were frequently observed on
the surface of Pd3Pb CNCs (Figure S7), which are beneficial for the catalysis enhancement.41,42 The
elemental distributions of Pd and Pb were confirmed by the STEM-EDX elemental mappings, where
both Pd and Pb distribute evenly in the whole CNC (Figure 1f). The line-scans verify that both Pd
and Pb distribute evenly in the whole CNC with weak element distribution in the center part (Figure
1g).
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Figure 2a and S8a show the representative TEM and STEM images of the Pd3Pb NCs in which
nanocubes are the major product. The Pd3Pb NCs have uniform edge length distribution of around 12
0 nm and the size calculated by Scherrer-equation calculation is 13.7 nm (Figure S9). As shown in
Figure S8c, the distinct PXRD diffraction peaks of the Pd3Pb NCs can be consistently indexed to the
reflections of the intermetallic Pd-Pb materials (JCPDS, No. 20-0827), similar to the XRD
diffraction of Pd3Pb CNCs. The composition ratio of Pd/Pb is 73.3 0.3/26.7 0.3, as revealed by
the ICP-AES, which is consistent with the TEM-EDX (73.6 0.5/26.4 0.5) (Figure S8e) and
SEM-EDX (73.9 0.2/26.1 0.2) results (Figure S9d). The displayed facets in the HRTEM shows
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the lattice spacing of 0.402 nm, corresponding to the (100) plane of cubic phase Pd3Pb (space group:
Pm-3m) intermetallic structure (Figure 2c). Both the line-scans and elemental mappings show the
uniform Pd and Pb distributions of the Pd3Pb NCs (Figure 2e,g). When DTAC was replaced with
benzoic acid, Pd3Pb SCNCs was obtained (Figure 2b, 2d, 2f and 2h). As shown in Figure S10c, the
Pd3Pb SCNCs have an average edge length of 12.5 nm (13.8 nm by the Scherrer-equation). The
composition ratio of Pd/Pb is 74.9 0.2/25.1 0.2, as revealed by the ICP-AES, which is consistent
with the TEM-EDX (73.8 0.6/26.2 0.6) (Figure S8f) and SEM-EDX (73.7 0.5/26.3 0.5)
results (Figure S10d). The lattice spacings of 0.400 and 0.398 nm, corresponding to the (100) plane
of cubic phase Pd3Pb intermetallic structure were observed (Figure 2d). The element distributions in
the Pd3Pb SCNCs were revealed by the HAADF-STEM-EDX elemental mapping and STEM line
scanning, where the Pd and Pb were distributed evenly through the whole SCNC (Figure 2f,h).
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Figure 2. (a) TEM image, (c) HRTEM image, (e) corresponding line-scans of the Pd3Pb NCs and (g)
HAADF-STEM image and elemental mappings of Pd3Pb NCs. (b) TEM image, (d) HRTEM image,
(f) corresponding line-scans of the Pd3Pb NCs and (h) HAADF-STEM image and elemental
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mappings of the Pd3Pb SCNCs. The inset in (a) and (b) are the 3D structure models of the Pd3Pb
NCs and Pd3Pb SCNCs.
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The hydrogenation of alkynes to alkenes is of great significance in the production of polyolefins
as well as synthetic rubbers.29 Nanostructured Pd is considered to be one of the most active
hydrogenation of alkynes catalysts,22 while it shows low selectivity for alkenes due to
overhydrogenation. To this end, the created Pd3Pb NCs, SCNCs and CNCs with modulated concave
degrees were adopted as catalysts for alkyne hydrogenations for investigating whether the Pd3Pb
intermetallic nanomaterials with concave feature can potentially result in the enhanced
semihydrogenation performance (Figure 3a).
Before starting catalytic reaction, the Pd3Pb nanomaterials were loaded on commercial carbon
(Figure S11). The calcining process was carried out to remove the organic compounds on the surface,
as well as expose the active components of the catalyst particles, where the intermetallic Pd3Pd phase
of all the Pd3Pb nanomaterials were largely maintained (Figure S12-13). Figure 3b-e shows the time
profile for the hydrogenation of phenylacetylene by using Pd3Pb nanomaterials with different
concave degree supported on C at 50 ºC under 2 atm H2. All the performances of different Pd3Pb
nanomaterials are compared with that of 10% Pd/C. As shown in Figure 3b, Pd3Pb NCs catalyst
shows a very high styrene selectivity of > 99%, whereas a low conversion of phenylacetylene (28%).
On the contrary, the Pd3Pb SCNCs and Pd3Pb CNCs achieve almost complete conversion (> 99%)
with high selectivity of 85.4% and 94.6% toward styrene, respectively (Figure 3c,d). It is notable
that Pd3Pb CNCs could greatly hinder the overhydrogenation of styrene and the 94.6% selectivity for
styrene was maintained even when prolonging the reaction time to 5 h. In addition, the optimized
Pd3Pb CNCs exhibit the favorable semihydrogenation of phenylacetylene activity with the
outstanding activity of 179.2 h-1, 5.4, 10.5 and 8.1 times higher than those of the conventional Pd3Pb
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SCNCs (32.8 h-1), Pd3Pb NCs (17.0 h-1) and 10% Pd/C (22.1 h-1), respectively (Table S1). On the
sharp contrast, for 10% Pd/C, a relative high conversion to styrene could be achieved, while the
selectivity to styrene was very limited (Figure 3e). Therefore, the excellent selectivity of the created
Pd3Pb intermetallic nanomaterials is unique and valuable because many conventional catalysts
including the Lindlar catalyst cause the rapid overhydrogenation of alkenes into alkanes at a high
conversion. The experimental results confirmed that introducing Pb to Pd was an efficient strategy
for inhibiting overhydrogenation.
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Figure 3. (a) Hydrogenation of phenylacetylene over different concave degree Pd-Pb catalysts and
benchmark catalyst: (b) Pd3Pb NCs, (c) Pd3Pb SCNCs, (d) Pd3Pb CNCs and (e) Pd/C. Reaction
conditions: phenylacetylene (0.1 mmol), ethanol (10 mL), H2 (2 atm), 50 °C, catalysts (5 mg; 10%
Pd/C, 5 mg).
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Significantly, the enhanced semihydrogenation performance of the Pd3Pb intermetallic CNCs
can also be expanded to other alkynes. As shown in Table 1, all the different alkynes can be readily
transformed into the corresponding alkenes with excellent conversions at high selectivity. The
performances of the Pd3Pb nanomaterials were further investigated by measuring the stability as a
function of the reaction cycles. In all the Pd3Pb nanomaterials, 98% conversion of the Pd3Pb CNCs
can be maintained even after five cycles (Figure 4a). In addition to the outstanding conversion and
high styrene-selectivity (94.6%) of the Pd3Pb CNCs, their styrene selectivity is also very stable with
only 0.7% decay after five cycles (Figure 4b), showing their unusual structural stability in the mild
reaction conditions. No significant loss in catalytic conversion and selectivity is also observed for all
Pd3Pb catalysts. While the structure and composition of the Pd3Pb CNCs/C can be preserved
(Figures S14), the 10% Pd/C is not very stable with severe aggregation after five cycles (Figure
S15).
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Table 1. Semihydrogenation of alkynes using intermetallic Pd3Pb CNCs/C.[a]
Entry
Substrate
Time
[min]
300
Alkyen
conv.[%]
99.9
Alkene
sel. [%]
96.3
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1-Octyne
1-Phenyl-1-propyne
Diphenylacetylene
1-Phenyl-1-pentyne
4-Ethynyltoluene
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[a]Reaction conditions: substrate (0.1 mmol) and 3.04 wt% of Pd3Pb intermetallic CNCs in 10 mL
ethanol were stirred at 50 ºC under 2 atm H2 pressure.
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Figure 4. (a) phenylacetylene conversion and (b) styrene selectivity for five rounds of successive
reactions over the as-obtained catalysts.
Generally, the Pd surface atoms cannot selectively absorb phenylacetylene and styrene, and the
Pb atoms show weak ability for the activation of H2 molecules and strong ability for poisoning the
metallic Pd.21,24 Since the catalytic reactions mainly occur on surfaces of nanomaterials, we
performed XPS experiments to investigate the surface properties of the Pd/C, Pd3Pb NCs/C, Pd3Pb
SCNCs/C and Pd3Pb CNCs/C for decoding the conversion and selectivity differences (Figure 5 and
Table S2). As shown in Figure 5a, the Pd 3d5/2 peak of Pd3Pb NCs/C (336.0 eV), Pd3Pb SCNCs/C
(336.0 eV) and Pd3Pb CNCs/C (336.0 eV) shift to higher binding energy by 1.1 eV, 1.1 eV and 1.1
eV than Pd/C (334.9 eV), respectively.26,28,43 The results show that, compared with monometallic Pd
the introduction of Pb into Pd can greatly modify the electronic structure of Pd due to the presence of
intimate interactions between Pd and Pb. Figure 5a,b and Table S2 show that Pd0 is the major form
among the Pd-Pb catalysts. It should be pointed out that the Pd0/Pd2+ ratio of Pd3Pb CNCs/C (7.7) is
the highest of all the Pd3Pb nanomaterials. Since the metallic Pd is active for semihydrogenation of
alkynes,20-26 it is believed that the higher ratio of Pd0 in the Pd3Pb nanomaterials results in higher
conversion (99.9%) for semihydrogenation of alkynes (Figure 5c).
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The catalyst surface concentration of Pb and chemical state of Pb of the intermetallic
nanomaterials were also analyzed. The fitting analysis shows the coexistence of Pb0 and Pb2+ among
Pd3Pb nanomaterials, where the peaks at about 142.2 eV and 137.4 eV correspond to Pb0 4f5/2 and
Pb0 4f7/2, and the peaks at about 144.0 eV and 139.0 eV belong to Pb2+ 4f5/2 and Pb2+ 4f7/2,
respectively (Figure 5d and Table S2). For Pd3Pb SCNCs/C and Pd3Pb CNCs/C, the higher ratio of
Pb2+ is beneficial for improving styrene selectivity since the Pb2+ modified Pd prefers to poison the
overhydrogenation of styrene (Table S2).28 In addition, it revealed that the order for Pb/Pd molar
ratio is the Pd3Pb NCs (0.364) > Pd3Pb CNCs (0.325) > Pd3Pb SCNCs (0.307), similar to the
tendency of styrene selectivity, where the higher ratio of Pb/Pd for Pd3Pb CNCs/C (0.325) results in
the higher styrene-selectivity (94.6%) (Figure 5d-f). All in all, the higher ratio of metallic state Pd
results in the higher conversion for semihydrogenation of phenylacetylene and the higher ratio of
Pb2+ and Pb/Pd contribute to higher styrene selectivity.
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Figure 5. (a) Pd 3d XPS spectra, (b) Pd0/Pd2+ molar ratios, (c) phenylacetylene conversion, (d) Pb 4f
XPS spectra, (e) Pb/Pd molar ratios and (f) styrene selectivity.
To illuminate the influence of the different surfaces on the performance of semihydrogenation of
phenylacetylene, we used density functional theory (DFT) to calculate catalytic processes on
different surfaces. Firstly, as shown in Table S3 and Figure S16, the adsorption energies of H2 on
the (100), (110) and (111) surface are -0.41 eV, -0.15 eV and -0.28 eV, respectively. The adsorption
energies of H on the (100), (110) and (111) surfaces are 0.01 eV, -0.29 eV and -0.52 eV. It can be
seen from Figure S16 that the dissociation of H2 on Pd3Pb (100) surface is endothermic by 0.43 eV.
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It indicates that the dissociation of H2 on this surface is a little energetically unfavorable, which may
not be beneficial for catalytic activity of hydrogenation reactions. In contrast, H2 dissociation is
energetically favorable on Pd3Pb (111) and the (110) surface comes second.
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Figure 6. Step-by-step hydrogenation mechanism of phenylacetylene to phenylethane on the Pd3Pb
(111) surface. Numbers in the parentheses indicate the barriers of elementary steps; Pb, grey; Pd,
blue; C, green; H, yellow.
In order to further investigate the reaction mechanisms of hydrogenation of phenylacetylene on
these different surfaces, the energy barriers on the (111) (110) (100) surfaces of Pd3Pb were
computed by transition states (TS) search. As shown in Figure 6, on the (111) surface,
phenylacetylene tends to adsorb on two adjacent Pd atoms with adsorption energy of -1.23 eV, which
promotes the hydrogenation of phenylacetylene. The kinetic barriers of the first two-step
hydrogenation are calculated to be only 0.25 eV (TS1) and 0.29 eV (TS2) on the (111) surface,
suggesting that the (111) surface of Pd3Pb is highly active for the hydrogenation of phenylacetylene
to styrene. The hydrogenation of styrene to the PhCHCH3 intermediate is found to be the rate-
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determining step in the whole processes of the hydrogenation of phenylacetylene, with a barrier of
0.58 eV. The desorption energy of styrene on the Pd3Pb (111) catalyst surface is 1.18 eV, which is
much higher than its hydrogenation barrier. This result indicates that the Pd3Pb (111) surface is very
active for the hydrogenation of styrene to phenylethane. On the other hand, on (110) surface (see
Figure S17), the energy barriers of the first two-step hydrogenation are calculated to be only 0.41 eV
(TS1) and 0.22 eV (TS2), which suggests that the (110) surface of Pd3Pb is also highly active for the
hydrogenation of phenylacetylene to styrene. However, the barrier of TS3 is 0.91 eV which is a little
larger than the desorption energy of styrene (0.86 eV). It indicates that there is a competition
between the desorption of styrene and the hydrogenation to PhCHCH3. Compared with (111) surface,
(110) surface shows a higher hydrogenation selectivity. Lastly, the reaction mechanisms on (100)
surface was shown in Figure S18. The energy barriers of TS1 and TS2 were 0.67 eV and 0.75 eV,
which are much larger than those on the (111) surface (0.25 eV, 0.29 eV) and (110) surface (0.41eV,
0.22 eV). It suggests that (100) surface shows a lower activity for the hydrogenation of
phenylacetylene to styrene, which may result from the unfavorable dissociation of H2 on (100)
surface. Besides, the desorption energy of styrene is 1.22 eV, which almost equals to the barrier of
TS3 (1.23 eV), which also shows a competition between the desorption and hydrogenation of styrene.
That is to say, the (100) surface shows a higher selectivity than (111) and (110) surface. Therefore,
the three different surfaces have different catalytic performances: the (111) surface shows the best
catalytic activity but the lowest selectivity; the (100) surface shows the highest selectivity but the
lowest catalytic activity; but the (110) surface shows an idealized activity for hydrogenation from
phenylacetylene to styrene and an optimum selectivity. This trend is consistent with the general rule
that the surface with lower catalytic activity has a higher selectivity.
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Furthermore, we found that the activity and selectivity of the catalytic hydrogenation of
phenylacetylene are closely related to the distribution of Pd and Pb atom on the surface. As shown in
Figure S19, the atomic distribution of the (100), (110), and (111) surface of Pd3Pb were displayed. It
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can be seen that on the (100) surface of Pd3Pb, there are four Pb atoms around the Pd atom. The Pd
atom on the (110) surface is surrounded with two Pd atoms and two Pb atoms. On the (111) surface,
the central Pd atom is surrounded with four Pd atoms and two Pb atoms. It is easy to find that the
atomic distribution of the surface can greatly affect the catalytic performance and selectivity; for
instance, on (100) surface the Pd was surrounded with four Pb atoms which are inert element, and
the (100) surface also shows the lowest catalytic performance but has the highest selectivity among
the three surfaces. For the (110) and (111) surface, there are more Pd atoms around the central Pd
atom in (111) surface than those of (110) surface, indicating higher activity and lower selectivity for
(111) surface. Based on this trend, we can speculate that there may be more Pd atoms distributed on
the surface of SCNCs and CNCs, than those of NCs, because the conversion of phenylacetylene is
higher on the SCNCs and CNCs surface. At the same time, compared with the surface of SCNCs, the
surface of CNCs would have higher concentration of Pb, because CNCs show a higher selectivity
than SCNCs.
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CONCLUSIONS
To summarize, we have demonstrated a wet-chemical strategy to create series of intermetallic
Pd3Pb NCs with different concave degree that can be used as efficient catalysts for
semihydrogenation of alkynes with both superior activity and excellent selectivity. The intermetallic
Pd3Pb CNCs exhibited excellent catalytic performance for phenylacetylene semihydrogenation with
99.9% conversion and 94.6% selectivity, better than other Pd3Pb nanomaterials as well as the
commercial Pd/C, making it among the most active catalysts for alkynes semihydrogenation reported
to date. Simultaneously, the Pd3Pb CNCs exhibited high stability with minor decay after five cycles.
XPS results show that the higher ratio of metallic state Pd results in the higher conversion for
phenylacetylene semihydrogenation and the higher ratios of Pb/Pd and Pb2+ contribute to higher
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styrene selectivity. The DFT calculations reveal that the excellent activity and high selectivity of the
acetylene semihydrogenation are due to the favorable adsorption energy of phenylacetylene and
desirable desorption energy of styrene on the Pd3Pb surface. We expect our study will stimulate the
rational design of efficient Pd-based nanomaterials with well-defined size, structure, composition
and phase for semihydrogenation reactions.
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ASSOCIATED CONTENT
Supporting Information. Figure S1-19&Table S1-3. This material is available free of charge via the
Internet at http://pubs.acs.org.
Corresponding Author
xulai15@suda.edu.cn; hxq006@suda.edu.cn
Author Contributions
J. Z. and W. X. contributed equally to this work.
Acknowledgements
This work was financially supported by Ministry of Science and Technology (2016YFA0204100,
2017YFA0208200), the National Natural Science Foundation of China (21571135), Young
Thousand Talented Program, Jiangsu Province Natural Science Fund for Distinguished Young
Scholars (BK20170003), Six Talent Peak Project in Jiangsu Province (Grant No. XNY-042), Natural
Science Foundation of Jiangsu Higher Education Institutions (17KJB150032), the Collaborative
Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD), and the start-up supports from
Soochow University.
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