ISSN 19950780, Nanotechnologies in Russia, 2011, Vol. 6, Nos. 5–6, pp. 323–329. © Pleiades Publishing, Ltd., 2011. Original Russian Text © B.G. Ershov, A.V. Anan’ev, E.V. Abkhalimov, D.I. Kochubei, V. V. Kriventsov, L. M. Plyasova, I. Yu. Molina, N. Yu. Kozitsyna, S. E. Nefedov, M. N. Vargaftik, and I. I. Moiseev, 2011, published in Rossiiskie Nanotekhnologii, 2011, Vol. 6, Nos. 5–6.
Bimetallic Pd–M (M = Co, Ni, Zn, Ag) Nanoparticles Containing Transition Metals: Synthesis, Characterization, and Catalytic Performance B. G. Ershova, A. V. Anan’eva, E. V. Abkhalimova, D. I. Kochubeib, V. V. Kriventsovb, L. M. Plyasovab, I. Yu. Molinab, N. Yu. Kozitsynac, S. E. Nefedovc, M. N. Vargaftikc, and I. I. Moiseevc a
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia b Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, pr. Lavrent’eva 5, Novosibirsk, 63090 Russia c Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia email:
[email protected],
[email protected],
[email protected] Received November 29, 2010; accepted for publication February 3, 2011
Abstract—The reductive thermolysis of Pd(OOCMe)4M(OH2) (M = NiII, CoII, ZnII) and Pd(OOCMe)4Ag2(HOOCMe)4 molecular complexes results in the generation of bimetallic Pdbased Pd–M (M = Co, Ni, Zn, Ag) nanoparticles. The composition and morphology of nanoparticles and the electron state of metal atoms were characterized using electron microscopy, elemental ICP analysis, Xray diffraction, and XAFS (XANES/EXAFS) techniques. The catalytic performance of nanoparticles was studied using the example of reactions of catalytic hydrazine decomposition and U(VI) reduction to U(IV) by hydrazine and formic acid. The catalytic performance of Pd–Ni nanoparticles is superior to that of the standard supported Pd/SiO2 catalyst containing a similar amount of Pd atoms, while Pd–Co, Pd–Zn, and Pd–Ag nanoparticles do not catalyze the studied reactions. DOI: 10.1134/S1995078011030050
INTRODUCTION Some of the most interesting nanochemical objects are nanosized intermetallide and alloy particles. The introduction of a transition element into the coordina tion shell of a noble metal is a conventional technique allowing the directional variation of the physical and chemical (including catalytic) properties of the initial metal and its compounds. One dramatic example of such an effect is the modification of palladium and platinum catalysts by transition metals resulting in a significant enhancement of the activity, stability, and selectivity of catalysts in which the active phase is rep resented by nanoparticles of the platinum metal [1–4]. Bi and polymetallic nanoparticles are usually obtained by the independent introduction of compo nents, in general, the corresponding metal salts. These are further subjected to simultaneous or sequential reduction treatment. An alternative approach we are developing [5] is based on using the recently synthesized and structur ally characterized heterobinuclear PdM(μOOCR)4L (M = CoII, NiII, MnII, ZnII; R = Me, But; L = H2O, MeCN) complexes [6–8], in which palladium(II) atoms are bound to the transition metal atom through carboxylate bridges. It was found earlier [9, 10] that the reductive thermolysis of a heterometallic
Pd(μOOCMe)4Zn(OH2) complex under mild condi tions (250ºC, 1 atm of H2/He) leads to the formation of catalytically active bimetallic PdZn nanoparticles [11]. This work studies the reductive thermal decompo sition of heterobimetallic Pd(OOCMe)4M(OH2) (M = NiII, CoII, ZnII) and Pd(OOCMe)4Ag2(HOOCMe)4 complexes using electron microscopy, element ICP analysis, in situ Xray diffraction, and XAFS (XANES/EXAFS) spectroscopy techniques. The cat alytic activity of the generated bimetallic nanoparti cles in the reactions of hydrazine decomposition and reduction of U(VI) to U(IV) by hydrazine and formic acid is also measured. The choice of these reactions is due to the fact that techniques of nuclear fuel process ing and the purification of radioactive waste include redox reactions. In them, heterogeneous catalysts based on platinum group metals are used for their ini tiation and/or acceleration [12–16]. MATERIALS AND RESEARCH TECHNIQUES Reagents and Materials Palladium(II) acetate Pd3(OOCMe)6 was obtained by the oxidation of Pd black (synthesized by the reduc tion of chemical grade PdCl2, Voikov plant) by sodium
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borohydride) by concentrated HNO3 in glacial acetic acid according to [17], purified by prolonged boiling in glacial acetic acid with a new portion of Pd black until the full termination of NO2 evolution, and twice recrystallized from glacial acetic acid with the addition of a fresh portion of Pd black. Nickel(II) acetate Ni(OOCMe)2 · 4H2O, cobalt(II) acetate Co(OOCMe)2 · 4H2O, zinc(II) acetate Zn(OOCMe)2 · 2H2O, and silver(I) acetate AgOOCMe (all of chemi cal grade, Acros Organics, Belgium) were used with out additional purification. Solvents (acetic acid, ben zene, acetonitrile, and tetrahydrofuran) (all of chemi cal grade, Acros Organics, Belgium) were purified according to standard techniques [18]. Heterometallic complexes were obtained from palladium(II) acetate and acetates of the corresponding metals according to the technique in [6]. A solution of uranyl(VI) sulfate (UO2)SO4 was prepared by dilution in sulfuric acid of ura nium(VI) trioxide that was obtained by the calcination of ammonium uranylcarbonate (NH4)4[UO2(CO3)3] deposited from a UO2(NO3)2 · 6H2O solution (chemical grade, Labtekh, Russia) at 350°C. Reference catalysts 4% Pd/SiO2 were prepared by the impregnation of silicagel MSK (granules of 0.8– 1.0 mm) by a PdCl2 solution in 2 mol/l HCl and reduction by hydrogen (400°C, 2 h). The palladium content in the supported catalyst was determined after Pd dissolution in aqua regia using spectrophotometry on the basis of a color reaction with SnCl2. METHODS A D500 Xray diffractometer (Siemens) with a hightemperature chamber was used to study the dif fraction of powder samples in situ at a pressure of 1 atm in various gas media (H2, He, air) in the range of 20–450°C, at a controlled heating and gas supply rate [19]. The sample heating rate was 10°C/min; the gas supply rate was 5–10 l/h. After the given temperature was reached, the samples were conditioned for 30 min in the given gas medium and then the next voltammo gram was recorded. The chamber was purged by helium before and after hydrogen supply for 10–15 min. Dif fractograms were obtained using the diffracted beam Bragg–Brentano scheme (CuKαradiation, graphite monochromator, automated pointwise scanning with a 2θ step of 0.05° and an accumulation time of 3 s in each point, and a scanning range of 25–70°2θ) [20]. The JCPDS PDF2 WIN database was used for an analysis of the phase composition [21]. The average size of palladium crystallites on the surface of the supported 4% Pd/SiO2 reference cata lyst was determined using the Xray diffraction tech nique on the basis of the broadening of the diffraction line with Miller index (111). Line profiles were mea sured using an ADP10 Philips diffractometer (CuKα, graphite diffracted beam monochromator) and approximated by a Gauss curve. The instrumental line
width was determined using a nonlinear interpolation of the Si powder line width. The broadening of the (111) line was determined according to the for mula of β = (B2 – b2)1/2, where B is the observed line width and b is the instrumental line width. The crystal lite size in nanometers was determined according to the formula of d = λ/βcos(θ) [6], where β is expressed in radians in 2θ units. EXAFS and XANES spectra (Pd–K, Ni–K, Zn–K) were registered using an EXAFS spectrometer of the Siberian Center of Synchrotron Radiation (STsSI, Novosibirsk, Russia) using the techniques of fluores cence transmission and yield at the electron energy of 2 GeV and an average collector current during the measurement of 90 mA. A split single–block mono chromator Si (111) crystal was used as a monochro mator. Higher harmonics were suppressed for the Ni– K and Zn–K edges using a SiO2 mirror. Argonfilled ionization chambers and an Xray fluorescent detector were used for the Xray radiation measurement. The step in measurement of EXAFS and XANES spectra was ~1.5 and ~0.3 eV, accordingly. The sample was prepared in the form of tablets of such thicknesses that the edge absorption jump was ~0.5–0.9. The oscilla tion component χ(k) was extracted using the standard technique [22, 23]. The preedge part was extrapolated to the region of EXAFS oscillations using Viktorin polynomials. The smooth spectrum part was described using cubic splines. The chosen initial point E0 of the EXAFS spectrum was the inflection point at the absorption edge. The absolute value of Fourier transform k3χ(k) was used in the local structure studies to obtain the atom radial distribution function (ARD) in the wave num ber range of 3.0–13.0 Å–1. The extraction of the struc tural information, i.e., the determination of distances, coordination numbers, and Debye factors, was carried out by simulating spectra using the EXCURV92 soft ware [23] after Fourier filtration and including litera ture Xray data for referencing solid substances at the given Debye factor values (2σ2 ~ 0.010 Å2). Electron micrographs of bimetallic powders were obtained on a Philips EM301 electron transmission microscope by measuring and calculating the size of at least 100 crystallites. The average volume–surface crystallite size was calculated according to the formula 3 2 dvs = Σn i d i /Σn i d i , where ni is the amount of crystal lites with size di. The samples were prepared by ultra sonic powder dispersion in methanol to prevent parti cle aggregation. The content of palladium and additional metals in the obtained powders was determined using an Optima 2100 DV ICP atomic emission spectrometer (Perkin Elmer, United States). Catalytic experiments were carried out in a glass thermostated vessel with a reflux condenser to prevent the evaporation of the working solutions. The working
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Fig. 1. Successive diffractograms measured under the step wise heating of the PdII(µOOCMe)4CoIIOH2 heating: (1) 20°C/air; (2) 140°C/H2; (3) 250°C/H2; (4) 460°C/H2; and (5) 20°C/air, then 20°C /H2.
solutions were prepared immediately before the exper iment by mixing calculated amounts of the initial reagent solutions. Then a catalyst sample (4% Pd/SiO2 bimetallic powder or reference catalyst) was intro duced into the reactor. Catalyst samples were calcu lated so that the active component samples of similar masses were present in the reaction system: 1 g of 4% Pd/SiO2 corresponds to 40 mg of the bimetallic pow der. The working solution volume was 10 ml. The reac tion mixture was mixed by bubbling argon. The reac tion mixture was periodically sampled in the course of the experiment, the samples were mixed with 2.0 ml of 1.0 mol/l of H2SO4, and the catalyst was separated by centrifugation. The current concentration of the prod uct of U(IV) reduction in the solution was determined spectrophotometrically using a Shimadzu3100 spec trophotometer at a wavelength of 650 nm. STUDY RESULTS Characterization of Bimetallic Nanoparticles According to XRD data (Fig. 1), wide reflexes with coherent–scattering regions (CSR) 3–5 nm in size
(а)
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appear at temperatures above 150°C in an H2 atmo sphere that are tapered at an increase in the tempera ture, shift towards higher 2θ angles, and the CSR size grows to ~50 nm at 450°C. Herewith, the initial com plexes are destroyed and the Pd(II) and M(II) atoms are reduced, forming metallic nanoparticles aggregat ing under further heating. The alloy composition in all cases is close to the stoichiometry of Pd : M = 1 : 1. Conditions were chosen for the preparative synthe sis of bimetallic nanomaterials by means of the reduc tive thermolysis of heterobimetallic complexes (1 atm of 10% H2/He, 350°C) on the basis of data of in situ Xray diffractometry. The ratio of palladium and addi tional methods in the obtained samples according to the data of an element ICP analysis corresponds to the composition of Pd : M = 1 : 1 (M = Co, Ni, Zn). According to the data of transmission electron microscopy for the PdNi, PdCo, PdZn samples (Fig. 2), the reductive thermolysis of heterobimetallic com plexes in the hydrogen atmosphere results in the for mation of large (150–200 nm) bimetallic particles of complex shapes representing aggregates of smaller particles with their size not exceeding 50 nm. This coincides with the CSR values obtained by measuring the broadening of Xray diffraction lines. XANES spectra (Ni–K, Zn–K, and Pd–K edges) of the initial heterometallic complexes, reduced sam ples, and reference compounds are presented in Fig. 3. ARD curves—the function of atom radial distri bution for the local transition metal shells (obtained from EXAFS spectra) for heterometallic Pd(OOCMe)4M(OH2) complexes (M = Ni, Zn) and reference samples—are presented in Fig. 4a, while the data for the Pd–Ni, Pd–Zn reduced samples and ref erence samples are presented in Fig. 4b. ARD curves of the palladium shell (obtained from EXAFS spectra) for the initial and reduced Pd–Ni, Pd–Zn samples and reference compounds are presented in Figs. 5a and 5b, accordingly. The obtained XAFS data evidence that nickel is present in the initial heterometallic complexes in the form of Ni(II), while zinc is present in the form of Zn(II). Reduction results in the formation of metallic Pd–Ni nanoalloys and, possibly, of highly dispersed nickel(0) nanoparticles. Highly dispersed nanoparti
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Fig. 3. XANES spectra (Kedge of Ni, Zn, Pd) of the studied samples. (I) 1—the Pd(OOCMe)4Ni(OH2) complex; 2—reduced Pd–Ni sample; 3—Ni–foil. (II) 1—the Pd(OOCMe)4Zn(OH2) complex; 2—reduced Pd–Zn sample; 3—Zn–foil. (III) 1— the Pd(OOCMe)4Ni(OH2) complex; 2—Pd(OOCMe)4Zn(OH2) complex; 3—reduced Pd–Ni sample; 4—reduced Pd–Zn sample; and 5—Pd–foil.
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Fig. 4. (a) ARD curves for the local shell of transition metals in the studied samples: (1) (Ni–K) the Pd(OOCMe)4Ni(OH2) com plex, (2) (Zn–K) Pd(OOCMe)4Zn(OH2) complex, and (3) (Pd–K) Pd–foil (amplitude × 0.3). (b) ARD curves for the local shell of transition metals in the studied samples: (1) (Ni–K) the reduced Pd–Ni sample, (2) (Zn–K) reduced Pd–Zn sample, (3) (Pd–K) Pd–foil, and (4) (Ni–K) Ni–foil.
cles of the Pd–Zn metallic alloy are observed in the reduced samples of the Pd(OOCMe)4Zn(OH2) com plex. Herewith, no characteristic signs of formation of a separate zinc(0) phase were detected. In the initial Pd(OOCMe)4Ni(OH2) and Pd(OOCMe)4Zn(OH2) complexes, palladium is present in the form of Pd(II). Reduction under mild conditions leads to the formation of metallic Pd–Ni and Pd–Zn nanoalloys. Characteristic differences in the offedge structure are observed for these systems. They are possibly due to the significant distortion of the initial palladium FCC structure in the formation of a palladium–zinc alloy.
The obtained ARD curves feature a significant dif ference in regards to the set of present peaks (metal– metal) and position of maximums. The local shell of the transition metal and palladium for Pd–Ni and Pd–Zn samples are also different. The difference is due to the fact that metallic nickel and an alloy of changing composition are formed in the Pd–Ni sys tem, while, in the case of the Pd–Zn system, an alloy with the ratio of Pd : Zn = 1 : 1 is formed, which wholly agrees with the XRD data. Catalysis by Bimetallic Nanoparticles Reactions of catalytic hydrazine decomposition and uranium(VI) reduction by hydrazine and formic
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Fig. 5. (a) ARD curves for local shell of the palladium metal for the studied samples: (1) the Pd(OOCMe)4Ni(OH2) complex, the (2) Pd(OOCMe)4Zn(OH2) complex, and (3) Pd–foil. (b) ARD curves for the local shell of the palladium metal for the studied samples: (1) the reduced Pd–Ni sample, (2) the reduced Pd–Zn sample, and (3) Pd–foil.
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% of particles of a given size 18 16 14 12 10 8 6 4 2 0 5 7 9 11 13 15 17 19 21 23 25 Particle size, nm
Fig. 6. (a) Electron microphotograph of a grain of the 4% Pd/SiO2 catalyst; (b) size distribution of palladium crystallites for the supported 4% Pd/SiO2 catalyst (dav = 12.3 nm).
acid were studied in aqueous solutions where bimetal lic nanopowders obtained by reduction of heterome tallic Pd(OOCMe)4M(OH2) (M = Co, Ni, Zn) and Pd(OOCMe)2Ag2(OOCMe)2 complexes were intro duced as catalysts. The stability of aqueous suspen sions of these nanopowders was studied first. Catalyst samples (~10 mg) were dispersed in water (V = 10 ml) by vigorous shaking and sedimentation time was assessed at room temperature. The suspension of the Pd–Co powder is destroyed 1 min after shaking is over; Pd–Zn and Pd–Ag powders are deposited in 5– 8 min and the suspension of the Pd–N powder is stable for at least 6 h.
Experiments in the catalytic hydrazine decomposi tion showed that PdNi nanoparticles (as opposed to Pd–Zn, Pd–Ag, Pd–Co, and 4% Pd/SiO2) cause the effective decomposition of hydrazine in an aqueous solution (Fig. 7). In the case of contact between the Pd–Zn powder and a hydrazine solution, the evolu tion of gas bubbles was observed, although it was not related to hydrazine decomposition. One may assume that, in this case, the destruction of Pd–Zn nanopar ticles and zinc dissolution occurs accompanied by hydrogen evolution.
A standard heterogeneous 4% Pd/SiO2 catalyst was used as a reference catalyst. According to electron microscopy data (Figs. 6a, 6b), the supported Pd phase consists of small crystallites (d < 10 nm) and aggregates (10 < d < 30 nm) distributed uniformly
A comparison of the catalytic activity of nanopow ders in the reactions of uranium(VI) reduction by hydrazine and formic acid was carried out in 0.5 mol/l aqueous H2SO4 solution, i.e., under conditions in which metallic palladium is chemically stable.
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Fig. 7. Decomposition of N2H4 0.5 (mol/l) at 40°C in the presence of 40 mg of PdNi and 1 g of 4% Pd/SiO2.
Kinetic experiments showed that only Pd–Ni nanoparticles of all the studied samples feature cata lytic activity in the reactions of uranium(VI) reduction by hydrazine and formic acid (Figs. 8, 9). Pd–Zn, Pd–Co, and Pd–Ag powders proved to be practically inactive as regards the catalysis of these reactions. Thus, bimetallic Pd–Ni nanoparticles are charac terized by pronounced catalytic activity in the reac tions of hydrogen decomposition and uranium(VI) reduction by hydrazine and formic acid. Herewith, the catalytic activity of Pd–Ni significantly exceeds the activity of the supported Pd/SiO2 catalyst at a similar mass content of palladium in the reaction system. In the case of the hydrazine decomposition reaction, the higher catalytic activity of PdNi (when compared to that of the heterogeneous 4% Pd/SiO2 catalyst with smaller particles and, therefore, a better developed surface) is possibly due to the presence within the nan opowder of nickel promoting the reaction of catalytic hydrazine decomposition. It is known [24] that metal lic nickel, cobalt, and silver per se catalyze hydrazine decomposition. The absence of catalytic activity of the Pd–Zn, Pd–Co, and Pd–Ag samples is most likely due to the decomposition of their suspensions in an aqueous solution. CONCLUSIONS Thus, studies of the catalytic activity of bimetallic powders of palladium and zinc, cobalt, and nickel in the reactions of hydrazine decomposition and ura nium(VI) reduction by hydrazine and formic acid showed that the nature of the transition metal signifi cantly affects the catalytic activity of palladium in the studied bimetallic nanoparticles. The catalytic activity of Pd–Ni nanoparticles largely exceeds that of smaller
Fig. 8. Reduction of 0.05 mol/l U(VI) by 0.2 mol/l N2H4 in 0.5 mol/l H2SO4 at 30°C in the presence of 40 mg of bimetallic PdNi, PdCo, PdZn powders and 1 g of sup ported 4% Pd/SiO2 catalyst.
[U(IV)], mol/l 0.06 0.05 0.04 4%Pd/SiO2
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Fig. 9. Reduction of 0.05 mol/l U(VI) by 0.5 mol/l HCOOH in 0.5 mol/l H2SO4 at 30°C in the presence of 40 mg of bimetallic PdNi, PdCo, PdZn powders or 1 g of supported 4% Pd/SiO2 catalyst.
palladium particles within a heterogeneous catalyst with a better developed surface. Further studies of bimetallic nanoparticles formed from heterometallic complexes are of practical and scientific interest for understanding the mechanism of heterogeneous catalytic redox processes with their participation and for the development of new promis ing catalysts, particularly for application in the tech
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niques of processing irradiated fuel and radioactive waste. 10.
ACKNOWLEDGMENTS This work was financially supported by the Russian Foundation for Basic Research (projects nos. 0903 12114, 090300514, 090300432a, and 1003 00369a), the Foundation of the President of the Rus sian Federation (Program for the Support of Leading Scientific Schools of Russia, projects no. NSh 65264.2010.3 and NSh6692.2010.3), and the Presid ium of the Russian Academy of Sciences (the program “Directed Synthesis of Inorganic Substances and the Development of Functional Materials”).
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