Isolated Single-Atom Pd Sites in Intermetallic Nanostructures: High

Article pubs.acs.org/JACS

Isolated Single-Atom Pd Sites in Intermetallic Nanostructures: High Catalytic Selectivity for Semihydrogenation of Alkynes Quanchen Feng,† Shu Zhao,† Yu Wang,‡ Juncai Dong,§ Wenxing Chen,† Dongsheng He,∥ Dingsheng Wang,*,† Jun Yang,⊥ Yuanmin Zhu,# Hailiang Zhu,§ Lin Gu,∇ Zhi Li,† Yuxi Liu,⊥ Rong Yu,# Jun Li,*,† and Yadong Li*,† †

Department of Chemistry, Tsinghua University, Beijing 100084, China Shanghai Synchrontron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China § Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ∥ Materials Characterization and Preparation Center, South University of Science and Technology of China, Shenzhen 518055, China ⊥ Department of Chemistry and Chemical Engineering, Beijing University of Technology, Beijing 100124, China # Beijing National Center for Electron Microscopy, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China ∇ Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: Improving the catalytic selectivity of Pd catalysts is of key importance for various industrial processes and remains a challenge so far. Given the unique properties of single-atom catalysts, isolating contiguous Pd atoms into a single-Pd site with another metal to form intermetallic structures is an effective way to endow Pd with high catalytic selectivity and to stabilize the single site with the intermetallic structures. Based on density functional theory modeling, we demonstrate that the (110) surface of Pm3̅m PdIn with singleatom Pd sites shows high selectivity for semihydrogenation of acetylene, whereas the (111) surface of P4/mmm Pd3In with Pd trimer sites shows low selectivity. This idea has been further validated by experimental results that intermetallic PdIn nanocrystals mainly exposing the (110) surface exhibit much higher selectivity for acetylene hydrogenation than Pd3In nanocrystals mainly exposing the (111) surface (92% vs 21% ethylene selectivity at 90 °C). This work provides insight for rational design of bimetallic metal catalysts with specific catalytic properties.

1. INTRODUCTION

Recently, featured with high activity and selectivity in a variety of reactions, single-atom catalysts (SACs) have received considerable interest.21−33 Inspired by this development, we wonder whether isolating contiguous Pd sites into single-atom Pd sites would be a viable approach to improve the catalytic selectivity of Pd catalysts. The long-term stability of SACs under practical reaction conditions is yet to be improved despite the high activity and selectivity. When used as heterogeneous catalysts, intermetallic compounds (IMCs) have shown excellent long-term stability in various chemical reactions because of their unusual structural stability.13,34−48 Therefore, upon isolating Pd atoms with another metal and constructing an intermetallic structure, that is, a Pd-based IMC

While precious transition metals like palladium (Pd) and platinum (Pt) are widely used in catalytic reactions due to their outstanding chemical and physical properties, they are generally less selective toward forming desired products.1−9 More selective catalytic processes require less reactants, dispense with subsequent separation procedures, and produce fewer byproducts.1−4 So from economical and environmental perspectives, improving selectivity of industrial heterogeneous catalysts is vital. For example, highly selective removal of trace amounts of acetylene impurities in the ethylene feed is a crucial step for industrial production of polyethylene.10−16 Pd is highly active for this process, but it always exhibits low ethylene selectivity, generating undesired ethane.17−20 Hence, developing effective strategies for improving the selectivity of Pd catalysts has attracted broad interests. © 2017 American Chemical Society

Received: February 15, 2017 Published: May 14, 2017 7294

DOI: 10.1021/jacs.7b01471 J. Am. Chem. Soc. 2017, 139, 7294−7301

Article

Journal of the American Chemical Society

Octadecene (ODE) was obtained from Acros. Analytical grade poly(vinylpyrrolidone) (MW = 8000), N,N-dimethylformamide (DMF, 99.5%), ethylene glycol (EG, 99%), acetone, toluene, and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Lindlar catalyst (5 wt % Pd supported on porous calcium carbonate and treated with lead and quinolone) was purchased from Strem Chemicals. All chemicals were used as received without further purification. 2.3. Synthesis of PdIn IMNCs. Pd(acac)2 (15.2 mg, 0.05 mmol), In(acac)3 (20.6 mg, 0.05 mmol), and PVP (100 mg) were dissolved in a mixture of DMF (4 mL) and EG (4 mL), followed by 15 min vigorous stirring. The resulting homogeneous light yellow solution was transferred into a 12 mL Teflon-lined stainless-steel autoclave. Then the sealed autoclave was heated at 200 °C for 12 h in an oven before it was cooled down to room temperature. The products were collected by centrifugation and washed several times with an ethanol/acetone mixture. 2.4. Synthesis of Pd3In IMNCs. Pd(acac)2 (15.2 mg, 0.05 mmol) and anhydrous InCl3 (27.6 mg, 0.125 mmol) were dissolved in 4 mL of oleylamine to get a light yellow transparent solution. ODE (4 mL) was mixed with OAm (1 mL) in a 50 mL three neck flask and preheated up to 300 °C. Then, the as-prepared yellow transparent solution was injected into the hot solution. The hot solution turned black gradually. The reaction was kept at 300 °C for 15 min before it was cooled down to room temperature. The products were separated via centrifugation at 10000 rpm for 5 min and further washed three times with a toluene−ethanol mixture. 2.5. Synthesis of Pd NPs. Pd(acac)2 (15.2 mg, 0.05 mmol) and PVP (MW = 8000, 100 mg) were dissolved in a mixture of DMF (4 mL) and EG (4 mL), followed by 15 min vigorous stirring. The resulting homogeneous light yellow solution was transferred into a 12 mL Teflon-lined stainless-steel autoclave. Then the sealed autoclave was heated at 130 °C for 6 h in an oven before it was cooled down to room temperature. The products were collected by centrifugation and washed several times with an ethanol/acetone mixture. 2.6. Preparation of MgAl2O4-Supported Catalysts. First, MgAl2O4 spinel support was synthesized by using the procedure described in the literature.56 In order to keep Pd loading of these supported catalysts at the same value (2 wt %), we loaded different amounts of nanocrystals on support materials in ethanol by stirring at room temperature until the ethanol solution turned clear. Typically, 2 mL of 1 mg/mL Pd NP stock solution and 98 mg of MgAl2O4 were used for preparation of 2 wt % Pd/MgAl2O4. For preparation of 2 wt % Pd3In/MgAl2O4, 2.7 mL of 1 mg/mL Pd3In IMNCs stock solution (containing 2 mg of elemental Pd) and 97 mg of MgAl2O4 were used. For preparation of 2 wt % PdIn/MgAl2O4, 4.2 mL of 1 mg/mL PdIn IMNCs stock solution (containing 2 mg of elemental Pd) and 96 mg of MgAl2O4 were used. The solids were then precipitated and separated via centrifugation. Finally, the supported catalysts were dried in vacuum at room temperature overnight. The weight percentage of MgAl2O4-supported catalysts is based on metal Pd.

with single-atom Pd sites, desired catalysts with satisfactory catalytic selectivity and stability might be obtainable. To explore this concept and provide a theoretical basis for this type of single-atom alloy, we use density functional theory (DFT) to model the possible hydrogenation pathways of acetylene on the surfaces of intermetallic Pm3̅m PdIn (with Pd atoms effectively and completely isolated by indium atoms) and P4/mmm Pd3In (with Pd atoms partially separated by indium atoms). Taking into account the spatial arrangement effect and surface stability (Table S1 and detailed discussion in section 3, Results and Discussion), we choose the (110) surface of Pm3̅m PdIn with single-atom Pd sites and the (111) surface of P4/ mmm Pd3In with contiguous Pd trimer sites to study the reaction mechanism of acetylene hydrogenation on these surfaces. DFT results show that the ethylene selectivity of the PdIn(110) surface with single-atom Pd sites is much higher than that of the Pd3In(111) surface with contiguous Pd trimer sites. To further confirm this theoretical prediction, we carried out controllable synthesis of PdIn and Pd3In intermetallic nanocrystals (IMNCs) and demonstrated that PdIn nanocrystals mainly exposing (110) surface shows 92% ethylene selectivity at 90 °C for acetylene hydrogenation while Pd3In nanocrystals mainly exposing (111) surface shows only 21% ethylene selectivity under the same reaction conditions. This study provides new insight in improving catalytic performance of metal catalysts.

2. EXPERIMENTAL SECTION 2.1. DFT Modeling. All the DFT calculations were performed with the Vienna Ab-initio Simulation Package (VASP) code.49,50 The electron exchange and correlation energy was treated within the generalized gradient approximation in the Perdew−Burke−Ernzerhof scheme (GGA-PBE).51 The interaction between ions and electrons was described by the projector augmented wave (PAW) method and the spin polarization was included. To examine the effect of van der Waals interaction on reaction energetics, calculations were performed by using the DFT-D3 functional with PBE−PAW potentials.52,53 Iterative solutions of the Kohn−Sham equations were expanded in a plane-wave basis set defined by a kinetic energy cutoff of 450 eV, and the Brillouin zone was sampled using a 2 × 2 × 1 Monkhorst−Pack grid. The convergence criteria for the electronic self-consistent iteration and force were set to 10−5 eV and 0.02 eV/Å, respectively. Reaction barriers were computed using the climbing image nudged elastic band (CI-NEB) method.54 The optimized lattice constants for PdIn and Pd3In are 3.30 and 4.04 Å, which are in good agreement with the experimental values of 3.25 and 4.07 Å, respectively.55 The PdIn(110) and Pd3In(111) surfaces were modeled by p(4 × 2) and p(2 × 2) supercells, respectively. Both slabs contain four atomic layers, and the top two layers are allowed to relax. A vacuum region of 15 Å was set between the periodically repeated slabs to avoid interactions. More detailed information about the models can be found in Figures S1 and S2 and Table S1. Surface energy is determined by γ = (Eslab − NEbulk)/2A, where Eslab and Ebulk are the total energies of the slab and one bulk unit cell, respectively, N is the number of bulk units in the slab, and A is the surface area of the slab. The adsorption energy of adsorbate is defined as Eads = E(adsorbate/slab) − [E(slab) + E(adsorbate)], in which E(adsorbate/slab), E(adsorbate), and E(slab) are the calculated total energy of a surface slab with the adsorbate, a gaseous-phase molecule, and a clean surface, respectively. The reaction energy and barrier are calculated by ΔrE = E(FS) − E(IS) and Ea = E(TS) − E(IS), where E(IS), E(FS), and E(TS) are the energies of the corresponding initial state (IS), final state (FS), and transition state (TS), respectively. 2.2. Chemicals. Palladium(II) acetylacetonate (Pd(acac)2, 99%), indium(III) acetylacetonate (In(acac)3, 99%), indium(III) chloride (InCl3, anhydrous), oleylamine (OAm), phenylacetylene, 1-octyne, 3octyne, and diphenylacetylene were purchased from Alfa Aesar.

3. RESULTS AND DISCUSSION 3.1. DFT Studies of Acetylene Hydrogenation on PdIn(110) and Pd3In(111) Surfaces. At first, surface stability of low-index surfaces of PdIn and Pd3In was estimated by using Wulff construction theory.57 As shown in Figure S4, the most exposed surfaces are PdIn(110) and Pd3In(111), with surface proportion of 83% and 73%, respectively. PdIn(110) and Pd3In(111) are also found to have relative low surface energy among these surfaces (Table S1). Because these most exposed surfaces play a major role in the catalytic reactions, PdIn(110) and Pd3In(111) surfaces were selected in our DFT calculations. Figure 1 shows the energy profile and the corresponding geometric structures for acetylene hydrogenation on the PdIn(110) surface. The computed barriers and reaction energies as well as the bond distances of the intermediates are summarized in Table S2. The adsorption energies for 7295


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Figure 3. Structural analysis of PdIn IMNCs: atomic resolution STEM images of PdIn nanocrystal oriented along the (a) [111] and (d) [021] zone axes. FFT patterns from the (b) [111] and (e) [021] zone axes. Ideal crystal structure models along the (c) [111] and (f) [021] zone axes. Gray and pink spheres show Pd and In atoms, respectively. All scale bars are 1 nm.

Figure 1. Step-by-step hydrogenation mechanism of acetylene to ethane on the PdIn(110) surface. Numbers in the parentheses indicate the barriers of elementary steps; Pd, blue; In, orange; C, black; H, white.

Figure 4. Structural analysis of Pd3In IMNCs: atomic resolution STEM images of Pd3In nanocrystal oriented along the (a) [2̅3̅3] and (d) [4̅11] zone axes. FFT patterns from the (b) [2̅3̅3] and (e) [4̅11] zone axes. Ideal crystal structure models along the (c) [2̅3̅3] and (f) [4̅11] zone axes. Gray and pink spheres show Pd and In atoms, respectively. All scale bars are 1 nm. Figure 2. Step-by-step hydrogenation mechanism of acetylene to ethane on the Pd3In(111) surface. Numbers in the parentheses indicate the barriers of elementary steps; Pd, blue; In, orange; C, black; H, white.

C2H3Pd intermediate. C2H4 coordinated on one Pd atom via weak π bonding has an adsorption energy of −0.34 eV, indicating that ethylene facilely desorbs (Figure S5). The barriers of the first two hydrogenation steps are only 0.36 eV (TS1) and 0.34 eV (TS2), and the reactions are highly exothermic by 1.00 and 1.38 eV, respectively, implying that the PdIn catalyst is highly active for acetylene hydrogenation to ethylene. The further hydrogenation of C 2 H4 to C 2H 5 intermediate is the rate-determining step in the whole acetylene hydrogenation profile, with a barrier of 0.68 eV (TS3), and exothermicity of 0.54 eV. The desorption energy of C2H4 on the PdIn(110) surface is only 0.34 eV, which is lower than its hydrogenation barrier. More importantly, the transition-state energy of C2H4 hydrogenation (TS3) is above the energy of gas-phase ethylene (the dashed line in Figure 1), which suggests that ethylene prefers desorption to further hydrogenation in the following process. Hence, the PdIn(110) surface should exhibit high selectivity in the semihydrogenation of acetylene.

Table 1. Adsorption Energies (Eads) and Hydrogenation Barriers (Ea) of Ethylene on Pd3In and PdIn Surfaces Pd3In(111) Pd3In(100) PdIn(110) PdIn(111) a

Eads (eV)

Ea (eV)

ΔE (eV)

APa (%)

−1.09 −0.93 −0.34 −0.86

1.16 0.46 0.68 0.93

0.07 −0.47 0.34 0.07

73 25 83 17

Area percentage (AP, %) of each facet in the Wulff shape.

hydrocarbons and hydrogen are shown in Table S3. On the PdIn(110) surface, the single Pd atom is isolated by four nearest-neighboring In atoms. C2H2 tends to adsorb on two adjacent Pd atoms with adsorption energy of −0.58 eV, which is favored by hydrogenation to C2H4 through an ethylene-like 7296


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Figure 5. (a) The normalized XANES spectra at the Pd K-edge of Pd foil, Pd NPs, Pd3In, and PdIn IMNCs. Inset shows the enlarged picture of the corresponding rectangle section. (b) The normalized XANES spectra at the In K-edge of Pd foil, Pd NPs, Pd3In, and PdIn IMNCs. (c) The EXAFS oscillation functions at the Pd K-edge of Pd foil, Pd NPs, Pd3In, and PdIn IMNCs. (d) Fourier transforms of the experimental EXAFS spectra of Pd foil, Pd NPs, Pd3In, and PdIn IMNCs. The Fourier transforms are not corrected for phase shift.

Table 2. Structural Parameters of Pd Foil, Pd NPs, Pd3In, and PdIn IMNCs Extracted from the EXAFS Fittinga sample Pd foil Pd NPs Pd3In PdIn

scattering pair Pd−Pd Pd−Pd Pd−Pd Pd−In Pd−In

CNb g

12 10.5 6.1 4.0 5.7

± ± ± ±

1.7 2.5 1.7 1.2

Rc (Å) 2.74 2.74 2.77 2.78 2.77

± ± ± ± ±

0.02 0.02 0.03 0.03 0.03

σ2d (10−3 Å2) 5.8 6.6 5.2 11.8 8.5

± ± ± ± ±

0.3 0.3 0.9 1.8 0.5

ΔE0e (eV) 3.3 3.0 −8.8 4.3 1.0

± ± ± ± ±

0.4 0.3 5.6 1.2 0.4

R factorf 0.003 0.003 0.007 0.005

a

S02 = 0.86. bCN is the coordination number. cR is interatomic distance (the bond length between Pd central atoms and surrounding coordination atoms). dσ2 is Debye−Waller factor (a measure of thermal and static disorder in absorber−scatterer distances). eΔE0 is edge-energy shift (the difference between the zero kinetic energy value of the sample and that of the theoretical model). fR factor was applied to evaluate the goodness of the fitting. gThis value was fixed during EXAFS fitting, based on the known structure of Pd foil.

leading to the formation of undesired ethane. Furthermore, as shown in Figure S7, the hydrogenation barriers increase with the reaction energies for each hydrogenation step on PdIn(110) and Pd3In(111) surfaces, which follows the Brønsted−Evans−Polanyi (BEP) relationship.58 To compare ethylene selectivity of PdIn(110) and Pd3In(111) surfaces, we define the energy variance ΔE = Ea − |Eads| to estimate the selectivity by comparing the difference between the hydrogenation barrier and the adsorption energy of C2H4.59 Here Ea and Eads are the hydrogenation barrier and adsorption energy of C2H4, respectively. When a catalyst has a low desorption energy of ethylene and a high hydrogenation barrier, it would be highly selective. The calculation results shown in Table 1 reveal that ΔE of PdIn(110) surface (0.34 eV) is much higher than that of Pd3In(111) surface (0.07 eV), which suggests that PdIn(110) surface exhibits higher selectivity than Pd3In(111) surface for the semihydrogenation of acetylene. For comparison, we also studied the hydrogenation of acetylene on PdIn(111) and Pd3In(100) surfaces (Figures S8 and 9), even though they were exposed to a lower extent in the

For comparison, we also investigated this reaction process on the Pd3In(111) surface without isolated single Pd sites. The full potential energy profile for the hydrogenation of acetylene on Pd3In(111) surface is presented in Figure 2 (see Figure S6 and Table S4 for more details). On the Pd3In(111) surface, the most favorable adsorption site for C2H2 and C2H3 is the 3-fold coordinated hollow site on the Pd trimer sites, and C2H4 adsorbs in a di-σ configuration, which exhibits a high degree of similarity to that found on the Pd(111) surface.18,19 The hydrogenation of C2H2 to C2H3 intermediate is the ratedetermining step in the whole hydrogenation process, with a barrier of 1.23 eV (TS1) and endothermicity of 0.40 eV. The activation barrier for hydrogenation of C2H3 is 0.70 eV (TS2), and this step is exothermic by 0.45 eV. The desorption energy of C2H4 is 1.09 eV, which is a little lower than its subsequent hydrogenation barrier (TS3, 1.16 eV). It is generally agreed that the comparable values of desorption energy and hydrogenation barrier of ethylene on metal active sites contribute to relatively low ethylene selectivity.20 As the temperature increases, the barrier of C2H4 hydrogenation can be overcome, 7297


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3.2. Synthesis and Structural Analysis of PdIn and Pd3In IMNCs. To confirm our theoretical prediction, we synthesized the corresponding IMNCs. Figure 3 shows atomic resolution STEM images, as well as their corresponding fast Fourier transform (FFT) patterns, and ideal crystal structure models of PdIn IMNCs oriented along two different specific zone axes. As shown in Figure S10, the XRD pattern of PdIn IMNCs agrees well with the standard peaks of Pm3̅m PdIn (JCPDS file No. 65-4804). In an individual unit cell, In atoms occupy the eight vertexes of the cube, while the Pd atom occupies the center of the cube. STEM images in Figure 3a,d reveal that the particles are single crystalline. The lattice spacing of 0.23 and 0.16 nm can be assigned to the (110) and (200) planes of Pm3̅m PdIn, respectively. Figure 3b,e confirm the simple cubic (sc) structure by the FFT patterns of characteristic reflections of [111] and [021] zone axes, respectively. In Figure 3d, the chemical order is evident due to the slight difference in brightness between two successive planes of atoms along the [200] direction, which is directly related to the Z-contrast difference between Pd and In atoms. Figure 3c,f shows the ideal crystal structure models of PdIn along the [111] and [021] zone axes, which are well consistent with the experimental STEM images shown in Figure 3a,d, respectively. This confirms the successful synthesis of PdIn IMNCs. Atomic resolution STEM images, as well as their corresponding FFT patterns, and ideal crystal structure models of Pd3In IMNCs oriented along two different specific zone axes are shown in Figure 4. As shown in Figure S11, the XRD pattern of Pd3In IMNCs matches well with the standard peaks of P4/mmm Pd3In (ICSD No. 59476). In atoms occupy the eight vertexes of the cuboid, whereas Pd atoms occupy the centers of six faces. The lattice spacings of 0.12, 0.14, and 0.18 nm can be assigned to the (311), (022), and (220) planes of P4/mmm Pd3In, respectively. As shown in Figure 4b,e, FFT patterns of characteristic reflections of [2̅3̅3] and [4̅11] zone axes confirm the tetragonal crystal structure of Pd3In IMNCs. Furthermore, the ideal crystal structure models of Pd3In along the [2̅3̅3] and [4̅11] zone axes shown in Figure 4c,f match well with the experimental STEM images shown in Figure 4a,d, respectively, which reveals the formation of Pd3In IMNCs. To further verify the structural differences between PdIn and Pd3In IMNCs, we used X-ray absorption spectroscopy (XAS) techniques to evaluate the electronic structure and the chemical bonding of Pd−In IMNCs in their local environment.31,60−67 X-ray absorption near-edge structure (XANES) of Pd K-edge for Pd−In IMNCs with reference samples Pd foil and Pd nanoparticles (NPs) is shown in Figure 5a. The monometallic Pd NPs show spectral features that are similar to that of bulk Pd

Figure 6. Acetylene hydrogenation in the presence of excess ethylene. Acetylene conversion (a) and ethylene selectivity (b) in long-term selective hydrogenation of acetylene at 90 °C on the three as-obtained catalysts. Reaction conditions: 0.5 vol % C2H2, 5 vol % H2, and 50 vol % C2H4 and balance helium, flow rate = 120 mL min−1, space velocity (SV) = 288 000 mL g−1 h−1. Catalyst weight: 2 wt % Pd/MgAl2O4 (25 mg), 2 wt % Pd3In/MgAl2O4 (25 mg), 2 wt % PdIn/MgAl2O4 (25 mg).

theoretical crystallite shape (Figure S4). From Table 1, Pd3In(100) has the lowest selectivity (ΔE = −0.47 eV) among these four surfaces, which indicates that Pd3In is not a good catalyst for semihydrogenation of alkynes. In addition, PdIn(111) is found to have much lower selectivity compared with PdIn(110) surface (0.07 vs 0.34 eV). As the predominantly exposed surface in the Wulff shape of PdIn, the PdIn(110) surface thus plays a major role in the semihydrogenation reaction. In summary, we can conclude that PdIn catalyst exhibits higher selectivity than Pd3In for semihydrogenation of alkynes.

Table 3. Liquid-Phase Semihydrogenation of Various Alkynes on Different Catalysts entrya

catalyst

substrate

time [h]

alkyne convn [%]

alkene sel [%]

alkane sel [%]

1 2 3 4 5 6 7

Lindlar catalyst Pd/MgAl2O4 Pd3In/MgAl2O4 PdIn/MgAl2O4 PdIn/MgAl2O4 PdIn/MgAl2O4 PdIn/MgAl2O4

phenylacetylene phenylacetylene phenylacetylene phenylacetylene 1-octyne 3-octyne diphenylacetylene

3.5 3.5 3.5 3.5 5 5 5.5

100 100 100 92 97 98 99

56 49 58 97 97 98 99

44 51 42 3 3 2 1

a

Reaction conditions: alkyne (5 mmol), catalysts (Lindlar catalyst, 10 mg; 2 wt % Pd/MgAl2O4, 25 mg; 2 wt % Pd3In/MgAl2O4, 100 mg; 2 wt % PdIn/MgAl2O4, 100 mg), hexane (10 mL), H2 (1 bar), 25 °C. All hydrogenation products were determined by gas chromatography (GC) using an internal standard technique. 7298


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region for the three MgAl2O4-supported catalysts. Compared with Pd/MgAl2O4 (334.5 eV), the Pd 3d peak for Pd3In/ MgAl2O4 (334.8 eV) and PdIn/MgAl2O4 (335.3 eV) is shifted by 0.3 and 0.8 eV to higher binding energy, respectively, which is consistent with previous studies.43,70,71 Such significant modification of the Pd electronic states in Pd3In and PdIn IMNCs compared with monometallic Pd likely arises from the presence of covalent interactions between Pd and In.43,71 3.3. Gas- and Liquid-Phase Semihydrogenation of Alkynes on PdIn and Pd3In IMNCs Catalysts. After obtaining PdIn and Pd3In IMNCs, we performed catalytic tests of acetylene hydrogenation on these catalysts to verify our theoretical prediction. Acetylene hydrogenations on Pd, Pd3In, and PdIn catalysts were conducted isothermally in a continuous flow of ethylene feed at 90 °C for 20 h, and the results are shown in Figure 6. PdIn catalyst shows a very high ethylene selectivity of 92% in high acetylene conversion of 96%, whereas Pd and Pd3In catalysts have much lower ethylene selectivity of 15% and 21% in full conversion of acetylene, respectively. Liquid-phase hydrogenations of phenylacetylene are also performed on Lindlar, Pd, Pd3In, and PdIn catalysts (entries 1−4 in Table 3). When the reaction time reaches 3.5 h, PdIn catalyst achieves a phenylacetylene conversion of 92% and a high selectivity of 97% toward styrene. Under the same reaction conditions, Lindlar, Pd, and Pd3In catalysts exhibit styrene selectivity of only 56%, 49%, and 58% at the full conversion of phenylacetylene, respectively. Interestingly, the good liquidphase semihydrogenation performance of PdIn catalyst could also be further extended to other alkyne substrates, including terminal and internal alkynes (entries 5−7 in Table 3).

foil. For Pd3In and PdIn IMNCs, the energy of adsorption edge (E0) and the height of the white line (Hw) are lower than those of Pd foil and Pd NPs, which suggests the electron-richness of Pd atoms in Pd3In and PdIn IMNCs compared to those in Pd foil and Pd NPs. This result is likely due to the electron transfer from In atoms to Pd atoms, which is consistent with the electronegativities of In (1.78) and Pd (2.20), as well as the DFT results (Table S6). Figure 5b shows the normalized In Kedge XANES spectra of Pd−In IMNCs as well as the reference sample (In powder). In comparison with In powder, PdIn IMNCs exhibit both higher E0 and Hw. This result reveals the electron deficiency at the In atoms in Pd−In IMNCs, which can be confirmed from the result of Pd K-edge XANES spectra. In Figure 5c, extended X-ray absorption fine structure (EXAFS) oscillations of Pd−In IMNCs are quite different from those of Pd foil and Pd NPs, with shorter periods and smaller amplitudes. From literature,61 the shorter periods are due to the longer Pd−In(Pd) distances of Pd−In IMNCs than that of monometal Pd. Smaller amplitudes indicate the low coordinate environment of Pd atoms in Pd−In IMNCs.61,64−66 For Pd NPs, EXAFS oscillations in K space at each absorption edge are similar and closely resemble that of Pd foil, but with smaller amplitude, which can be attributed to the finite size effect of NPs.61,64−66 In Figure 5d, the slight shift of the first nearestcoordination peaks in R space of PdIn and Pd3In also indicates the mild change of atomic distance.68,69 The curve fitting results of all four samples are listed in Table 2, which demonstrates that the specific Pd−In(Pd) bond length increases from 2.74 ± 0.02 Å (Pd foil and Pd NPs) to 2.77 ± 0.03 Å (PdIn) and 2.78 ± 0.03 Å (Pd3In), and the Pd−Pd bonds are absent in PdIn IMNCs. Figures S20−23 show the experimental EXAFS spectra of these four samples (for Pd foil, Pd NPs, Pd3In, and PdIn IMNCs, respectively) together with the theoretical fitting curves obtained by the fitting procedure described in Supporting Information. From these figures, it is clear that there is a good agreement between the experimental EXAFS spectra and theoretical fitting curves. Figures S24−27 show the calculated XANES spectra of Pd−In IMNCs at both Pd and In K-edge based on their crystallographic structure. The experimental XANES curves match rather well with the calculated results, which indicates that the local coordination environment and electronic structure of Pd−In IMNCs are similar to those of crystallographic data. Inasmuch as catalytic reactions mainly occur on surfaces of the particles, we have further investigated the exposed crystal facets of as-prepared PdIn and Pd3In IMNCs. In STEM measurements, a number of individual particles were randomly selected and analyzed according to the crystal model and FFT patterns. Figures S30−35 show some examples of STEM images of individual PdIn and Pd3In particles, which reveal that PdIn and Pd3In IMNCs are mainly enclosed by (110) and (111) surfaces, respectively. These results are consistent with our aforementioned DFT results. In order to check the uniformity of composition of as-obtained samples, energy dispersive spectroscopy (EDS) experiments were performed on a series of individual particles. The line elemental EDS spectra across one single particle (Figures S36 and S37, randomly selected examples) indicate homogeneous distribution of Pd and In. The composition of each particle (Tables S8 and S9) is nearly the same as that determined by XRD characterizations. Additionally, we also performed X-ray photoelectron spectroscopy (XPS) experiments to investigate the surfaces of asprepared samples. Figure S38 shows XPS analysis of Pd 3d

4. CONCLUSIONS Through computational chemistry modeling with density functional theory, we predicted that the PdIn(110) surface with single-atom Pd sites can have high catalytic selectivity for semihydrogenation of alkynes, whereas the Pd3In(111) surface with Pd trimer sites is much less selective toward this reaction. The complete isolation of contiguous Pd atoms into single Pd sites via In atoms is a feasible way to improve the catalytic selectivity of Pd catalysts. Meanwhile, constructing intermetallic structure between Pd and In can effectively stabilize the Pd single site and endow the catalysts with long-term stability. We have then synthesized PdIn IMNCs mainly exposing (110) surface and Pd3In IMNCs mainly exposing (111) surface and examined their catalytic performance for semihydrogenation of alkynes to verify our theoretical prediction. The experimental results show that PdIn IMNCs with isolated single-atom Pd sites indeed exhibit 92% ethylene selectivity at 90 °C, which is much higher than that of Pd3In IMNCs with 21% ethylene selectivity. This finding can provide new insights for designing stable isolated single site metal catalysts.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01471. Characterization techniques, more experimental details, more characterization results, and discussions (PDF)

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ORCID

Dingsheng Wang: 0000-0003-0074-7633 Lin Gu: 0000-0002-7504-031X Jun Li: 0000-0002-8456-3980 Yadong Li: 0000-0003-1544-1127 Author Contributions

Q.F., S.Z., and Y.W. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by China Ministry of Science and Technology under Contract of 2016YFA (Grant 0202801) and the National Natural Science Foundation of China (Grants 21521091, 21390393, 21590792, 91645203, U1463202, 21471089, and 21671117).

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