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Cite This: ACS Appl. Nano Mater. 2019, 2, 3307−3314
Surface Modification of PdZn Nanoparticles via Galvanic Replacement for the Selective Hydrogenation of Terminal Alkynes Masayoshi Miyazaki,† Shinya Furukawa,*,‡,§ Tomoaki Takayama,† Seiji Yamazoe,§,∥ and Takayuki Komatsu*,†
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Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-E1-10 Ookayama, Meguro-ku, Tokyo 152-8551, Japan ‡ Institute for Catalysis, Hokkaido University, N10 W21, Kita-ku, Sapporo 001-0021, Japan § Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan ∥ Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, 1-1 minami-Osawa, Hachioji-shi, Tokyo 192-0397, Japan S Supporting Information *
ABSTRACT: In this study, a novel intermetallic compound-derived trimetallic surface site in nanoscale via the galvanic replacement reaction (GRR) is described. A PdZn/SiO2 catalyst, which exhibited high activity for the hydrogenation of phenylacetylene, was prepared. The GRR between the metallic Zn of the SiO2supported PdZn intermetallic nanoparticles and the third metal precursor (e.g., Pb, Bi, Sn, Au, Ag, and Ga) was carried out, affording well-modified surface Pd sites. Among a series of catalysts modified by the third metals, the Pb-replaced catalyst exhibited an excellent yield of styrene, with the minimum overhydrogenation rate for alkane. The Pb-replaced catalysts were characterized by X-ray diffraction, scanning transmission electron microscopy−energy-dispersive X-ray spectroscopy, CO Fourier transform infrared, and X-ray absorption fine structure measurements. The combination of these characterization methods revealed that (1) surface Zn atoms are successfully replaced by Pb during the GRR and (2) the prepared catalysts exhibit a bulk PdZn intermetallic structure, with their surface modified by Pb. The as-prepared catalysts were used for the selective hydrogenation of phenylacetylene to styrene. A control experiment using a Pd−Zn−Pb trimetallic solid-solution alloy led to a low catalytic activity, highlighting the validity and specificity of the Pb-modified PdZn surface structure. KEYWORDS: galvanic replacement, intermetallic compound, alkyne hydrogenation, surface modification, PdZn nanoparticles
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INTRODUCTION Alloying of noble metals with other metals is one of the most widely used methods for improving their catalytic performance, including activity, selectivity, and stability.1−6 Typically, such an improvement is related to the modification of the active sites appropriate for the target reaction. The effect of this modification can be generally classified as an ensemble effect,7,8 caused by the dilution of active sites, and a ligand effect,9−11 caused by electronic perturbation related to the second metal. With respect to controlling the nature of active sites, intermetallic compounds are attractive candidates for drastically changing the electronic and geometric states of active metals, typically rendering a considerably enhanced catalytic performance compared with those of pure metals in various reactions, including the selective hydrogenation of alkynes,12−15 nitroarenes,16 and α,β-unsaturated aldehydes.17 These unique catalytic properties of intermetallic compounds can be attributed to their specific crystal and electronic structures, that is, well-structured bimetallic surface atomic arrangements and electronically modified active sites. The © 2019 American Chemical Society
crystal structures of intermetallic compounds are thermodynamically determined by the combination of constituent elements and their composition. If the composition is varied to modify the electronic state, the crystal structure will also be changed in a majority of cases. Therefore, it is difficult to independently control the geometric structure and electronic state. For solid-solution alloys, the addition of the third metal has been employed to modify catalytic properties.18−20 However, for intermetallic compounds, the addition of the third metal typically leads to a drastic change in the crystal structure. In this context, it is imperative to develop a flexible catalyst design in which the geometric and electronic structures of the active sites can be independently tuned. The galvanic replacement reaction (GRR) between zerovalent metal and cationic species (eq 1) demonstrates promise to overcome this challenge.21,22 Received: April 24, 2019 Accepted: May 7, 2019 Published: May 7, 2019 3307
DOI: 10.1021/acsanm.9b00761 ACS Appl. Nano Mater. 2019, 2, 3307−3314
Article
ACS Applied Nano Materials n M + m Nn + → n Mm + + m N
PdZn + x M
(1)
The application of the GRR to Pt, Pd, and Au nanoparticles affords core−shell and hollow structures via exchange.23−25 With the application of the GRR to a binary metal, only one element can be selectively replaced on the basis of the differences in the reduction potential. For example, metallic Fe atoms constituting binary Pt−Fe nanoparticles have been selectively replaced by metallic Ru atoms by the GRR.26,27 Moreover, the GRR is allowed only on the metal nanoparticle surface and not in the bulk. Therefore, the use of the GRR for intermetallic compounds would provide a well-modified surface structure, in which one of the two component metals is selectively replaced by the third metal without changing the geometric arrangement. In this study, a series of Pd-based intermetallic nanoparticles with their surfaces modified in nanoscale by the third metals (M = Cu, Rh, Sn, Au, Pb, or Bi) were prepared by the GRR. In addition, the performance of the prepared catalysts for the selective hydrogenation of alkynes was investigated to understand the role of the surface structure in catalysis. In summary, a novel catalyst design based on intermetallic compounds by the GRR was reported, followed by its application to the highly selective semihydrogenation of alkynes (Scheme 1).
where M represents the third metal species used for the GRR and x represents the atomic ratio of the added M with respect to Pd taken to be 1.0. Catalytic Reaction. The hydrogenation of phenylacetylene was carried out in a 50 mL three-neck round-bottom flask equipped with a silicone rubber septum and gas storage balloon (2 L). Prior to the reaction, the catalyst (10 mg) was reduced under flowing H2 (45 mL min−1) at 403 K for 45 min in the flask, followed by cooling to room temperature. Second, a THF (5 mL) solution of phenylacetylene (0.5 mmol) was added to the flask to initiate the reaction under 1 atm of H2 at room temperature. Third, the products were analyzed by gas chromatography−flame ionization detection (GC−FID) (Shimadzu, GC-2025), via an instrument equipped with a capillary column (Shimadzu GLC, SH-Rtx-1701, 30 m × 0.25 mm × 0.25 um). Characterization. Powder X-ray diffraction (XRD) patterns of the prepared catalyst were recorded on a Bruker D8 ADVANCE diffractometer using a Cu Kα X-ray source to examine the crystal structure. To flatten the baseline, the XRD pattern of the silica support was subtracted from those of the supported catalysts. High-angle annular dark-field scanning TEM microscopy (HAADF-STEM) images were recorded on a JEOL JEMARM200M microscope equipped with an energy-dispersive X-ray (EDX) analyzer (EX24221M1G5T). X-ray photoelectron spectroscopy (XPS) analysis was conducted using an ULVAC PHI 500 VersaProbe spectrometer. The catalyst was pressed into a pellet and placed on a sample holder for the spectrometer in a glovebox filled with N2. The sample holder was transferred into the spectrometer using a transfer vessel without being exposed to air. The obtained spectra were calibrated with Si 1s and C 1s as references for binding energy. Pd and Zn K-edge X-ray absorption fine structure (XAFS) measurements of the catalysts were carried out at BL01B1 in the SPring-8 operated at 8 GeV using Si(311) and Si(111) single-crystal monochromators, respectively. First, the catalyst was pressed into a pellet (diameter of 13 mm) and reduced in a flow of H2 at 403 or 673 K for 1 h and then sealed in polyethylene packs under an inert atmosphere. XAFS spectra were recorded at a liquid He temperature using a cryostat in fluorescence mode. To analyze the XAFS data, the k3-weighted χ spectra were Fourier transformed into the r space, using the Fourier transformation range of 2.5−16 Å−1, and the curve-fitting range was 2.0−2.9 Å. The back-scattering amplitude and phase shift functions were calculated by FEFF8. The composition of the catalysts after GRR was estimated by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Shimadzu ICPE-9000 apparatus. CO pulse chemisorption was performed using BELCAT II (Microtrac BEL) to estimate the Pd dispersion of the prepared catalysts. Prior to chemisorption, the catalyst was pretreated under a 5% H2/Ar flow (40 mL min−1) at 130 °C for 0.5 h. After the reduction pretreatment, He was introduced at the same temperature for 10 min to remove the chemisorbed hydrogen, followed by cooling to room temperature. A 10% CO/He pulse was introduced into the reactor, and the supplied CO flow was quantified downstream by a thermal conductivity detector. Computational Details. Periodic density functional theory (DFT) calculations were performed using the CASTEP code28 with Vnderbilt-type ultrasoft pseudopotentials29 and the Perdew−Burke− Ernzerhof exchange-correlation functional30 based on the generalized gradient approximation. The plane-wave basis set was truncated at a kinetic energy of 400 eV. A Fermi smearing of 0.1 eV was utilized. The reciprocal space was sampled using a k-point mesh with a spacing of typically 0.04 Å−1, as generated by the Monkhorst−Pack scheme.31 Geometry optimizations were performed on supercell structures using periodic boundary conditions. The surface was modeled using a metallic slab with a PdZn(101)−(2 × 4) structure, a thickness of six atomic layers, and 15 Å of vacuum spacing. Some or all of the Zn atoms at the surface were replaced with Pb atoms to reproduce the surface-modified structures. The convergence criteria for structure
Scheme 1. Illustration of the Surface Modification of PdBased Intermetallic Nanoparticles with the Replacement of the Second Metal by a Third Metal Using the GRR Method and Catalytic Application for Selective Semihydrogenation of Phenylacetylene
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METHODS
Catalyst Preparation. A series of intermetallic catalysts were prepared by (pore filling) co-impregnation using silica as the support. First, a mixture of an aqueous solution of Pd(NO3)3 and each of the precursors of the second metal salts, i.e., Cu(NO3)3·9H2O (Wako, 99%), Zn(NO3)2·6H2O (Kanto, 99%), Ga(NO3)3·nH2O (Wako, 99.9%), In(NO3)3·9H2O (Kanto, 99.9%), SnCl2 (Kanto, 97%), SbCl3 (Kanto, 98%), Pb(NO3)2·3H2O (Kanto, 99.5%), and Bi(NO3)3· 5H2O (Kanto, 99.5%), was added to dried silica gel (CARiACT G-6, Fuji Silysia; SBET = 470 m2 g−1). Pd loading was adjusted to 3 wt %. Second, the mixtures were sealed overnight at room temperature, followed by reduction under flowing H2 at 1073 K for 1 h and drying over a hot plate. Surface Modification of PdZn + M Trimetallic Catalysts via the Galvanic Replacement Reaction. In a standard procedure, 100 mg of PdZn/SiO2 was first allowed to settle in a three-neck round-bottom flask, followed by pre-reduction under flowing H2 at 673 K for 45 min. Second, after the flask had been cooled and the H2 replaced with Ar, aqueous solutions of the third metal precursors dissolved in 10 mL of degassed H2O were added to the reactor. Third, the flask was placed in an oil bath and heated at 90 °C for 18 h. Finally, the suspensions were subjected to centrifugation and washed with H2O, followed by drying using a rotary evaporator. Hereafter, the prepared catalysts are expressed as 3308
DOI: 10.1021/acsanm.9b00761 ACS Appl. Nano Mater. 2019, 2, 3307−3314
Article
ACS Applied Nano Materials optimization and energy calculation were set to (a) an SCF tolerance of 2.0 × 10−6 eV per atom, (b) an energy tolerance of 1.0 × 10−5 eV per atom, (c) a maximum force tolerance of 0.05 eV Å−1, and (d) a maximum displacement tolerance of 1.0 × 10−3 Å.
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RESULTS AND DISCUSSION First, a series of Pd-based intermetallic compounds (PdmMn/ SiO2; M = Bi, Cu, Ga, In, Pb, Sb, Sn, or Zn) were prepared, and their catalytic performance for the hydrogenation of phenylacetylene was examined to determine the appropriate base material for the GRR. XRD confirmed the formation of each desired intermetallic phase (Figure S1). For quantitative evaluation, the initial hydrogenation rate of phenylacetylene (R1) at a low-conversion region (
