Article pubs.acs.org/cm
Systematic Exploration of Synthesis Pathways to Nanoparticulate ZnPd Yuan Luo,†,‡,§ Yuhan Sun,† Ulrich Schwarz,‡ and Marc Armbrüster‡,* †
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, South Taoyuan Road 27, 030001 Taiyuan, People’s Republic of China ‡ Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Straße 40, 01187 Dresden, Germany § Graduate University of the Chinese Academy of Sciences, Yuquan Road 19A, 100049 Beijing, People’s Republic of China ABSTRACT: Synthesis of the intermetallic compound ZnPd by (i) coreduction of Pd2+ and Zn2+ by Superhydride, (ii) the use of ZnH2 as a reducing agent and simultaneously as a Zn source, as well as (iii) electroless plating of nanoparticulate Zn with Pd2+ is systematically explored. Chemical and differential thermal (DTA/TG) analysis, as well as infrared spectroscopy, confirmed the formation of ZnH2 as isolable intermediate during the reduction of Zn2+ with Superhydride. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and fractional sedimentation by disk centrifugation were applied to characterize the nanoparticulate products concerning their phase composition, crystallinity, and particle size distribution, respectively. While all routes lead to nanoparticles of ZnPd, electroless plating additionally allows one to control the crystallite size of ZnPd in a range of 2−5 nm. KEYWORDS: ZnH2, nanoparticulate Zn, intermetallic compound, PdZn, size control
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INTRODUCTION Developing a clean source of energy independent of fossil fuels is one of the main tasks to solve energy-related environmental problems. The methanol economy1 provides clean hydrogen on a sustainable basis, using methanol as a storage medium. The latter bypasses many of the safety and/or handling issues of physical high-pressure or low-temperature hydrogen storage options. Considering the high hydrogen-to-carbon ratio and the easy handling of methanol makes this small molecule a very promising hydrogen storage candidate. In addition, hydrogen can be released from methanol at relatively low temperature by steam reforming:2,3 CH3OH + H 2O → CO2 + 3H 2
role of the intermetallic particles and potential size influences are hard to determine, because of the complexity of these materials. Aiming for unsupported particles, Cable11 used Pd nanoparticles as a template to synthesize unsupported ZnPd. Bronstein12 applied reduction methods to synthesize unsupported ZnPd. A possible size dependence of the methanol steam-reforming properties over supported ZnPd has been reported earlier.10,13 Controlling the size of the intermetallic particles is a demanding task, especially for bimetallic compounds in the absence of surfactants, which would block the catalytically active surface. However, using N-vinylpyrrolidone as a surfactant, Engels14 managed to obtain various sizes of bimetallic Pd−Zn alloys via the polyol reduction method. Aiming at the unsupported synthesis of nanoparticulate ZnPd without the use of surfactants with the goal of size control, we investigated three possible pathways, i.e., (i) the coreduction of Zn2+ and Pd2+, (ii) the use of ZnH2 as a reducing agent and Zn source, and (iii) the electroless plating of nanoparticulate Zn with Pd2+. The reaction intermediate ZnH2 was characterized by chemical and combined differential thermal/thermogravimetric (DTA/TG) analysis and infrared spectroscopy. Obtained nanoparticulate products were charac-
ΔH = 49.7 kJ mol−1
The catalytic activity and high selectivity toward CO2 of the intermetallic compound ZnPd4 in this reaction was discovered by Iwasa and co-workers.5 Competing with the most effective Cu−Zn-based catalysts,6 the intermetallic compound ZnPd can overcome the drawbacks of fast deactivation, poor thermal stability, and the pyrophoric nature of the Cu-based catalysts.7 In addition, the catalytic properties of the intermetallic compound can be studied in an unsupported state, which recently revealed a strong dependence of the catalytic properties from the composition of the intermetallic compound.8 To obtain highly active materials, several authors prepared supported ZnPd.9,10 While showing high specific activities, the © 2012 American Chemical Society
Received: June 12, 2012 Revised: July 11, 2012 Published: July 12, 2012 3094
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Transmission infrared spectra (Bruker, IFS 66v/S) were measured on samples diluted with KBr (Merck KGaA, IR spectroscopy grade, dried under dynamic vacuum at 423 K overnight). Pellets were pressed with 10 t in an Ar atmosphere, comprising 50 mg KBr at the bottom, a mixture of 1 mg of sample with 50 mg of KBr in the middle, and another 50 mg of KBr on the top, to avoid air contact during the measurement. Pellets composed of 150 mg of pure KBr were used as a reference and subtracted as background from the sample measurements. The microstructure of the particles was investigated using a Tecnai 10 TEM (100 kV, LaB6 cathode) equipped with a selected-area electron diffraction (SAED) detector. Samples were prepared by dispersing the product in methanol (anhydrous, 99.8%, Sigma− Aldrich), depositing this suspension on a carbon-coated copper grid (300 mesh, Plano Supplies) and drying in air. To determine the particle size distribution, fractional sedimentation by disk centrifuge measurements (CPS, Model DC24000) was applied. The density gradient was composed of a mixture of cyclohexane (≥99.9%, Merck) and halocarbon (1.82 g mL−1, Solvadis Chemag). For the measurement, a small amount of sample was suspended in cyclohexane and expeditiously injected, using a syringe, into the rotating disk at 20 000 rpm.
terized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM), with regard to their phase composition and crystallinity, while fractional sedimentation was applied to determine the particle size distribution of the samples.
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EXPERIMENTAL SECTION
Dioctylether (99%, Aldrich), ZnCl2 (99.995%, Sigma−Aldrich), Pd(II) acetylacetonate (Pd(acac)2, 99%, Sigma−Aldrich), Superhydride (LiBEt3H, 1 M in tetrahydrofuran (THF), Sigma−Aldrich) and ethylene glycol (99.8%, Sigma−Aldrich) were used as supplied and stored in an Ar-filled glovebox with O2 and H2O concentrations below 0.1 ppm. THF (≥99.5%, Sigma−Aldrich) was distilled under Ar over CaH2 before use to remove traces of water. All synthesis procedures were carried out in a glovebox. Products were centrifuged at 6000 rpm for 3 min (EBA 20, Hettich), decanted, and subsequently washed three times with 3 mL of THF. This was followed by drying in a dynamic vacuum at 100 mbar for 20 min. Coreduction Pathway. 26.07 mg ZnCl2 (0.191 mmol) and 58.19 mg Pd(acac)2 (0.191 mmol) were dissolved in 30 mL of THF, resulting in a yellow solution. The solution was heated to reflux (339 K) before injecting 1.14 mL of Superhydride (1.14 mmol) at a rate of 149.1 mL/h. Upon the addition of Superhydride, the solution turned immediately into a black suspension, accompanied by gas evolution. The suspension was stirred at 339 K for 0.5−5 h. For experiments using ethylene glycol as a reducing agent, 26.07 mg of ZnCl2 and 58.19 mg Pd(acac)2 were mixed in 20 mL of ethylene glycol and heated to 339 and 393 K for 1 and 3 h, respectively, resulting in a black product. Reduction by ZnH2. ZnH2 was synthesized by adding 1.52 mL (1.52 mmol) Superhydride at a rate of 149.1 mL/h into a solution of 104.2 mg of ZnCl2 (0.76 mmol) in 4 mL of THF at room temperature, whereupon a white precipitate formed instantly. Two different approaches were explored for the reduction of Pd2+ by ZnH2. On the one hand, the freshly formed ZnH2 was mixed with 42.5 mg of Pd(acac)2 in 25 mL of THF. The suspension was heated to 339 K and held on for 2 h during which a black precipitate formed. After washing with THF three times and drying under dynamic vacuum for 20 min, the black precipitate was suspended in 15 mL of dioctylether and annealed at 523 K for 3 h. On the other hand, ZnH2 and Pd(acac)2 solution (42.5 mg in 5 mL of THF) were suspended together in 15 mL of dioctylether. This mixture was heated to 383 or 423 K, respectively, and held for 1 h before the black precipitate was cleaned as described above. Electroless Plating. Nanoparticulate Zn was synthesized by suspending freshly prepared ZnH2 in 15 mL dioctylether in a threenecked round-bottom flask under stirring. Heating to 383 K, the suspension changed first to gray and then to black. Subsequently, the black precipitate was annealed at temperatures between 383 to 523 K for 2−10 min. For the synthesis of ZnPd, Pd(acac)2 was dissolved in a molar ratio of 1:2 (with respect to the nanoparticulate Zn) in 5 mL of THF to form a yellow solution. The solution was directly injected into the hot suspension of nanoparticulate Zn at a rate of 149.1 mL/h. The mixture was subsequently heated in the range of 383−523 K for 2−5 h. Characterization. Powder X-ray diffraction (XRD) patterns were recorded on an image plate camera (Model G670, Huber, Cu Kα1 radiation, λ = 1.54056 Å, quartz monochromator, Guinier camera). Diffraction data were collected between 3° and 100°. Prior to the measurement, all the samples were prepared between two Kapton foils in a glovebox to avoid reaction with air or moisture. Simultaneous differential thermal analysis/thermogravimetry (DTA/TG) was measured on a Netzsch Model STA 449 C Jupiter system under Ar. Sample (18.24 mg) was loaded into Al2O3 crucibles under Ar atmosphere and heated to 773 K at a rate of 2 K/min. Elemental compositions were determined by inductively coupled plasma−optical emission spectroscopy (ICP-OES) (Model Vista RL, Varian). All the samples for chemical analysis were prepared in an Arprotected atmosphere, to avoid contact with air.
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RESULTS AND DISCUSSION While the synthesis of bulk samples of intermetallic compounds by metallurgical means is usually straightforward, synthesizing unsupported nanoparticles of these compounds is challenging − especially in the absence of surfactants. Surfactants can usually be removed from metallic nanoparticles using an oxidative treatment, a procedure which must be circumvented in the case of bimetallic particles containing a metal that is easy to oxidize (such as zinc). Otherwise, the bimetallic particles will be decomposed by preferential oxidation of this component and can usually not be fully restored by a reductive treatment. Another drawback of the oxidative treatment often applied in catalysis is the sintering of the particles due to the elevated temperature. As a result, it is difficult to obtain samples with a narrow particle size distribution to detect particle size effects in catalysis unambiguously.15 Taking these points into consideration, our synthesis of single-phase and nanoparticulate ZnPd is aiming at unsupported samplesto exclude support effects in catalysiswith a narrow size distribution without the use of surfactants. As a general synthesis method of nanoparticulate intermetallic compounds without the formation of insoluble byproduct, the coreduction of organic or inorganic salts with Superhydride has been developed for the synthesis of Cu−Pd and Ga−Pd intermetallic compounds16,17 recently. While this approach yields single-phase nanoparticles of structurally ordered intermetallic compounds, controlling the size of the particles has not been approached yet by this method. The intermetallic compound ZnPd crystallizes in a CuTitype structure.18 According to recent experimental and theoretical work, the existence of a cubic high-temperature modification could not be verified.4 In the following, three different synthesis routes to nanoparticulate ZnPd are explored concerning their suitability to lead to single-phase samples without insoluble byproduct and simultaneous size control. Starting with the experimentally simplest approach, we first investigated the possibility to synthesize ZnPd by coreduction using Superhydride as a reducing agent. During the synthesis, the formation of ZnH2 was observed, which was, in turn, used as a reducing agent and Zn source in the second pathway to ZnPd. In addition, we investigated the possibility of synthesizing ZnPd via the electroless plating of nanoparticulate zinc with 3095
dx.doi.org/10.1021/cm3018192 | Chem. Mater. 2012, 24, 3094−3100
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Figure 1. Powder X-ray diffraction (XRD) patterns of (a) nanoparticulate ZnPd prepared by coreduction with different reaction times (the calculated line positions for ZnPd are given at the bottom). (b) Using ZnH2 as a reducing agent, the characteristic pattern of Pd is obtained (calculated line positions are shown) at 339 K; annealing at 523 K or direct reaction at 383 and 423 K result in ZnPd, at the bottom of the panel.
Pd2+. In accordance with the bulk studies, all obtained ZnPd materials crystallized in the tetragonal CuTi-type structure. Coreduction Pathway. Preliminary experiments showed that Pd2+ can easily and quantitatively be reduced into nanoparticulate palladium in THF at 339 K by stoichiometric amounts of Superhydride. Applying these conditions to Zn2+ results only in a very small yield of Zn. In contrast, during optimization of the coreduction conditions, a yield of 92% isolated product was obtained with a 50% excess of Superhydride. The necessary excess of Superhydride might be explained by considering its powerful nucleophilic reduction potential, leading eventually to the unwanted reduction of the acetylacetonate ions.19 Powder XRD of the isolated product showed only the reflections of the intermetallic compound ZnPd (see Figure 1a), indicating successful synthesis, according to Zn
2+
2+
+ Pd
Figure 2. (a) Infrared spectrum of ZnH2 collected at room temperature; gray areas indicate traces of remaining tetrahydrofuran (THF). (b) Differential thermal analysis/thermogravimetry (DTA/ TG) of ZnH2 from room temperature to 773 K; the exothermic peak at 358 K indicates the decomposition temperature of ZnH2, while the endothermic peak at 686 K corresponds to the melting point of the formed Zn. (c) Powder X-ray diffraction (XRD) patterns of ZnH2 after DTA/TG measurement (pattern i) and ZnH2 isolated from THF after 34 days (pattern ii); calculated line positions for Zn and ZnO are given at the bottom.
where the hydride ion in LiBEt3H is reducing Zn2+ and Pd2+ into their elemental states with a simultaneous release of hydrogen. Subsequently, the intermetallic compound ZnPd is formed in a nanoparticulate state.
+ 4LiBEt3H
→ ZnPd + 4Li+ + 4BEt3 + 2H 2 3096
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Figure 3. Powder X-ray diffraction (XRD) patterns of different crystallite sizes of (a) Zn, (bottom: calculated line positions for Zn), and (b) ZnPd (calculated line positions of ZnPd are shown below); particle size distributions of (c) Zn and (d) ZnPd also are shown. The insert in panel (c) shows the correlation of crystallite sizes of Zn and intermetallic compound ZnPd.
the possible maximum temperature, which is determined by the boiling point of THF, eliminating the temperature as a stirring parameter. This leaves decelerating the reaction by employing a weaker reducing agent. Ethylene glycol not only can act as a reducing agent but also can act as a solvent (the so-called “polyol process”).22 Its high boiling point (470 K)23 allows one to control the reaction temperature in a wide range. Unfortunately, the coreduction of Zn2+ and Pd2+ only resulted in the formation of nanocrystalline elemental Pd, without reducing the Zn2+ up to 393 K, which was the maximum temperature applied to avoid the formation of ZnO.24 Therefore, ZnPd could not be obtained using ethylene glycol. To further explore the influence of the reducing agent, and to influence the reaction in a desired way, ZnH2 and elemental nanoparticulate Zn were used as weaker reducing agents. Reduction by ZnH2. Because of the good solubility of ZnCl2 in tetrahydrofuran (THF), we tried to synthesize nanoparticulate Zn by converting ZnCl2, using Superhydride as the reducing agent in THF at room temperature. Instead of a gray or black precipitate, a white powder was obtained. Powder XRD revealed the amorphous nature of the product. Chemical analysis of the white product revealed a composition of 5.5(1) wt % C; 2.69(6) wt % H; 4.52(2) wt % O; 86.3(2) wt % Zn; 0.34(1) wt % Li; 1673 K)4 hinders secondary growth processes. Electroless Plating. Aiming for small particles, sacrificial electroless plating of nanoparticulate Zn with Pd2+ to obtain the intermetallic compound ZnPd was explored:
Figure 4. Transmission electron microscopy (TEM) micrographs and selected area electron diffraction (SAED) patterns of 12-nm Zn nanocrystallites (panels a and b, respectively) and 22-nm Zn nanocrystallites (panels c and d, respectively); the calculated positions of Zn (black) and ZnO (gray) reflections are given as quarter circles. Panels (e) and (g) show TEM micrographs of 2- and 4-nm ZnPd nanocrystallites, respectively. The corresponding SAED patterns, together with the calculated positions of ZnPd, are given in panels (f) and (h).
2Zn + Pd2 + → ZnPd + Zn 2 +
The necessary nanoparticulate Zn was obtained by thermal decomposition of ZnH2 in dioctylether. Since the decomposition is possible from 359 K onward, the rate of Zn formation and the corresponding supersaturation can be varied up to the boiling point of dioctylether (562 K).31 Synthesis at 383 K results in elemental Zn particles with a crystallite size of 12 nm, according to the Scherrer equation. Increasing the decomposition temperature to 423, 473, and 523 K resulted in crystallite sizes of 18, 22, and 25 nm, respectively (Figure 3a). Here, the increasing size can be explained by differently advanced Ostwald ripening or sintering of the primary particles with increasing temperature. With increasing annealing temperature, very broad diffraction lines of ZnO were observed in the XRD patterns, which result from the partial oxidation of
molecules of ZnH2 containing 1 molecule of THF. The low concentration of B (
