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Available online at www.sciencedirect.com

Materials Chemistry and Physics 108 (2008) 369–374

Synthesis of chain-like Ru nanoparticle arrays and its catalyticactivity for hydrogenation of phenol in aqueous media

Fang Lu, Jing Liu, Jie Xu ∗State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Graduate School of the Chinese Academy of Sciences,

Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China

Received 31 July 2007; accepted 7 October 2007

bstract

Chain-like Ru nanoparticle arrays, 1–3 nm in diameter and ∼280 nm in length, have been synthesized by reducing the complex of Ru ions witholy(N-vinyl-2-pyrrolidone) (PVP) by 1 MPa H2, at 353 K, in aqueous media. The morphology of these Ru nanoparticle arrays could be variedrom long chain-like to cross-linked arrays by adjusting the reaction conditions including the average molecular weight of PVP and the mole ratio

f repeat unit of PVP to Ru. The whole formation process is supposed on the basis of their TEM and TG studies. The obtained chain-like Ruanoparticle arrays show high activity in hydrogenation of phenol with broad concentration, ranging from 10 to 10,000 mg l−1 at 353 K, underMPa H2, in aqueous media. 2007 Elsevier B.V. All rights reserved.

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eywords: Aqueous media; Catalysis; Ruthenium nanoparticle arrays; Phenol

. Introduction

It is well known that the physical and chemical propertiesf transition metal materials depend on the size and shape.herefore, many methods for the control of metal materialsorphology have been developed in the past decades [1,2].olymer-mediated methods have attracted much attention as aimple and versatile route to produce nanoparticles with dif-erent size and shape by polymer preferentially binding to therystal faces of the growing particle. Much of these works haveocused on synthesizing nanomaterials of Ag, Au, Pt, Pd, Rh,u and Co [3–6].

Ruthenium has show very unique and interesting catalyticctivities for many reactions [7,8]. For instance, it serves asajor catalysts in the hydrogenation reaction [9,10], removal of

rganic pollutants [11], synthesis of low-aromatic-content dieseluels [12] and direct-methanol fuel cells [13]. However, the gen-ration of chain-like Ru nanoparticles was seldom investigated,

nly Gendanken et al. synthesized dendritic Ru nanoparticles byVP-mediated methods in ethylene glycol solution [14]. Water

s a favored solvent in green chemistry, while the synthesis

∗ Corresponding author. Tel.: +86 411 84379245; fax: +86 411 84379245.E-mail address: xujie@dicp.ac.cn (J. Xu).

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254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2007.10.010

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f chain-like structure of ruthenium nanoparticles in aqueousolution has remained a challenge.

Phenol and its derivatives represent an important groupf refractory organic pollutant which present in a varietyf industrial and agricultural wastewaters [15]. A promisingethod for the destruction of phenol is the catalytic hydro-

enation of phenol to cyclohexanol. Because of cyclohexanolLD50 = 2060 mg kg−1, LD50 (lethal dose) is the amount of aaterial, given all at once, which causes the death of 50%

f a group of test animals) having less toxicity than phenolLD50 = 500 mg kg−1) [16], catalytic hydrogenation results in aore eco-friendly aqueous solution which can be treated further

uitably [17,18].Herein, we report the synthesis of the chain-like Ru nanopar-

icle arrays by reducing the complex of Ru ions with PVP withydrogen under mild conditions in aqueous media. The obtainedhain-like Ru nanoparticle arrays were used as high-active cat-lysts in the hydrogenation of phenol in aqueous media.

. Methods

.1. Chemicals and materials

Hydrogen (H2, 99.99%), poly(N-vinyl-2-pyrrolidone) (PVP K90, averageolecular weight (MW) = 1,250,000; K30, average MW = 40,000; K15, average

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W = 7000) and ruthenium(III)chloride hydrate (RuCl3·xH2O) were obtainedrom commercial sources without further purification.

.2. Synthesis of Ru nanoparticle arrays

Ru nanoparticle arrays were prepared by reducing the complex of Ru ionsith PVP. RuCl3 (0.1 mmol) and PVP of various average MW (K90, K30 or15) (the amount PVP in each case is 1.1, 0.22, 0.11, 0.04, 0.02 and 0.01 g)ere both dissolved in 60 ml distilled water with constant stirring, at 353 K, and

tirring was allowed to continue for 2 h. A dark-brown transparent solution wasbtained where RuIII ions were complex with PVP. After PVP–RuIII aqueousolution was treated with 4.0 MPa H2 (or 1 MPa) in a 500 ml stainless steelutoclave with a Teflon liner at 353 K for 2 h, chain-like Ru nanoparticle arraysere obtained.

.3. Characterization

TEM images were acquired using a JEM-2000EX (JEOL) electron micro-cope operating at 120 kV. HRTEM images were obtained using a Philip 20ECNAI G2 field emission gun electron microscope operating at 200 kV. Sam-les for TEM and HRTEM were fresh reduced PVP–Ru aqueous solutioneposited onto a carbon-coated Cu grid. Thermogravimetric analysis (TG) mea-urements were performed on a NETZSCH STA 409 PC thermal analyzer at aeating rate of 10 ◦C min−1 with an oxygen-flow rate of 40 ml min−1.

.4. Hydrogenation of phenol

After 100 ml aqueous solution of chain-like Ru nanoparticle arrays0.002 mmol Ru) were prepared in the autoclave, 100 ml aqueous solution of phe-ol with various concentrations (20, 200, 1000, 2000, 10,000 and 20,000 mg l−1)ere added directly. After air was flushed out of the reactor, the autoclave waseated to 353 K and then was pressurized with H2 to 1.0 MPa. Then the mix-ure was stirred at 500 rpm and the reaction was considered to start. Samplesere periodically taken from the aqueous solution after the stirring was inter-

upted. The analysis for the hydrogenation product of phenol with l0 mg l−1 waserformed by the pre-concentration process, in which ether was used as extrac-or and the extraction was repeated three times. The final organic phase wasut together and placed in water bath at 313 K to obtain concentrated sampleshich were qualitatively analyzed by GC–MS on an Agilent 6890 GC equippedith a mass selective detector. Quantitative analysis was performed on an Agilent890 GC equipped with a FID and a HP-5 (30 m × 0.53 mm × 0.6 �m) capillaryolumn by the area normalization method.

. Results and discussion

Here, the aqueous solutions of Ru nanoparticle arrays wererepared from Ru salt with PVP having various averages molec-lar weight (MW), e.g. K15 (average MW = 7000), K30 (averageW = 40,000) and K90 (average MW = 1,250,000), and the var-

ous mole ratios of the repeating units of PVP to Ru are 50/1,0/1, 5/1, 2/1, 1/1, 0.5/1, respectively. Prepared Ru nanoparti-le arrays were denoted as PVP (K15)–Ru-10 when PVP–K15as used in the preparation process with the mole ratio of the

epeating units of PVP to Ru is 10/1.

.1. Effect of the average MW of PVP

To investigate the effect of varying the average MW of PVPn Ru morphology, three types of PVP with different aver-

ge MW were used. Transmission electron microscope (TEM)mages for the three synthesized products of PVP (K90)–Ru-0, PVP (K30)–Ru-10, PVP (K15)–Ru-10 are shown in Fig. 1.ig. 1A shows the typical image of PVP (K90)–Ru-10. In this

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Physics 108 (2008) 369–374

ase, the product in the dark-brown transparent solution, whichas stable for months without noticeable precipitation, was sep-

rate chain-like nanoparticle arrays, 1–3 nm in diameter and40 nm in length. High-resolution transmission electron micro-

cope (HRTEM) images show more details of chain-like Ruanoparticle arrays, which were composed of both discrete Ruanoparticles (Fig. 1B) and continuous Ru nanowires (Fig. 1C).

The image of PVP (K30)–Ru-10 is shown in Fig. 1D. In thisase, the products became more branched, and the diameter ofanoparticles was also 1–3 nm. Fig. 1E shows the image of PVPK15)–Ru-10. The resultant products, which yield a black pre-ipitate after 24 h at room temperature, are totally cross-linkedu nanoparticle arrays with 1–3 nm in diameter. These results

uggest that it would be possible to control the morphology ofu by using PVP with different average MW. Moreover, chain-

ike Ru nanoparticle arrays with high aspect ratio were obtainedy using PVP (K 90) with high-average MW.

.2. Effect of the mole ratio of the repeating units of PVP tou

Effect of the mole ratio of the repeating units of PVP to Ru onhe Ru morphology was also studied in this work. Samples usingVP (K90) were investigated for these studies. Fig. 2 shows the

mages of the samples that were synthesized by using a processimilar to that used for PVP (K90)–Ru-10, except that the moleatio of the repeating units of PVP to Ru ranging from 0.5 to0. The images of PVP (K90)–Ru-0.5 and PVP (K90)–Ru-1 arehown in Fig. 2A and B, respectively. These samples were com-osed of cross-linked Ru nanoparticle arrays, about 1–3 nm iniameter. The images of PVP (K90)–Ru-2 and PVP (K90)–Ru-5re shown in Fig. 2C and D, respectively. There are chain-likeanoparticle arrays accompanied with some branches present inhe products, about 1–3 nm in diameter and ∼60 nm in length.

hen the mole ratio was increased to 50/1, there are relativehorter chain-like nanoparticle arrays (with ∼30 nm in length),–3 nm in diameter, as shown in Fig. 2E. These results impliedhat the final morphology of Ru nanostructures synthesized wasnfluenced by the mole ratio of PVP to Ru. When the moleatio was smaller than 10, nanoparticle arrays will accompa-ied with branches. When the mole ratio was higher than 10,VP (K90)–Ru-50 have shorter arrays than PVP (K90)–Ru-10.herefore, the mole ratio of 10 was favorable for the synthesisf chain-like Ru nanoparticle arrays with high aspect ratio.

.3. Effect of concentration of hydrogen in solution

Hydrogen is an excellent choice as a reducing agent becauset imparts no residual chemical impact on a system. In ordero investigate the effect of concentration of hydrogen in solu-ion, we decreased the pressure of hydrogen to 1 MPa. Otheronditions similar to that used for PVP (K90)–Ru-10 (4 MPa).hain-like Ru nanoparticle array typically with 1–3 nm in diam-

ter, ∼280 nm in length, is shown in Fig. 2F. The length of theanoparticle arrays was much longer than that formed underMPa H2 (∼40 nm). It is known that the concentration of hydro-en in water is decreased with decreasing the hydrogen pressure.

F. Lu et al. / Materials Chemistry and Physics 108 (2008) 369–374 371

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ig. 1. TEM and HRTEM images of chain-like Ru nanoparticle arrays that wemage PVP (K90)–Ru-10; (B) HRTEM image of continuous Ru nanowires; (C)E) TEM image of PVP (K15)–Ru-10.

hus, low concentration of hydrogen in water will benefit for theynthesis of long Ru nanoparticle arrays.

.4. The formation process of chain-like Ru nanoparticlerrays

PVP is a water soluble polymer which has hydrophobic

olyvinyl skeleton with hydrophilic pyrrolidone groups, andften used in the preparation of metal nanoparticles. It is gen-rally accepted that PVP in the preparation of nanoparticlesainly acts as the protective agent that can prevent the nanopar-

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tained when different average molecular weight of PVP were used: (A) TEMM image of discrete Ru nanoparticles; (D) TEM image of PVP (K30)–Ru-10;

icles from aggregation [19]. In this investigation, no chain-likeanoparticle arrays was observed in the absence of PVP. In otherords, PVP plays a key role in the synthesis of chain-like Ruanoparticle arrays.

The preparation of Ru nanoparticle arrays was supposed toave two steps. In the first step, coordination between RuIII andyrrolidone of PVP was produced by refluxing aqueous solution

f RuCl3 in the presence of PVP. In the second step, Ru nanopar-icle arrays (PVP–Ru) were formed by reducing the complex ofVP–RuIII with hydrogen, allowing the reduction, nucleationnd growth of ruthenium.

372 F. Lu et al. / Materials Chemistry and Physics 108 (2008) 369–374

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ig. 2. TEM images of chain-like Ru nanoparticle arrays that were obtained wh/1, (C) 2/1, (D) 5/1 and (E) 50/1. (F) is a typical image of PVP (K90)–Ru-10 (

III

The TG curves of PVP (K90), PVP (K90)–Ru and PVPK90)–Ru in oxygen atmosphere are shown in Fig. 3. For pureVP, it started to decompose at about 673 K, and the percentagef decomposed PVP was almost 100% at about 723 K. Com-

ig. 3. TG spectra of the PVP (K90), PVP (K90)–RuIII and PVP (K90)–Ru.

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fferent molar ratio of the repeating unit of PVP to Ru were used: (A) 0.5/1, (B)a).

ared with TG curve of pure PVP, PVP–RuIII had two additionalecomposed peaks at 396–587 K and 702–1000 K. The decom-osed percentage ratio of the latter peak to the former peakas about 2.8, which was close to the value of the moleculareight ratio of pyrrolidone to vinyl. This suggested that the

C bond intensity between pyrrolidone and vinyl decreasednd was broken more easily, when the ruthenium ions complexedith pyrrolidone of PVP. Therefore, compared with pure PVP,

he CH2CH2 of PVP–RuIII was easier to decomposed, at theame time, the pyrrolidone of PVP–RuIII was more difficult toecomposed due to the strong complex between ruthenium ionsnd pyrrolidone. Compared with TG curve of pure PVP, PVP–Ruad only an additional decomposed peak at 783–1099 K. Thismplied that the N C bond intensity between pyrrolidone andinyl decreased, but not enough to break the bond, whenhe Ru complexed with pyrrolidone of PVP. Therefore, afterhe PVP–RuIII was reduced into PVP–Ru, the bond intensity

etween Ru and PVP was weaker than that of PVP–RuIII, whichas consistent with the results of FT-IR, we reported before [20].After the process of the coordination between RuIII and

yrrolidone of PVP, no micelle was observed in the aqueous

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olution, because the concentration of PVP (MW = 1,250,000)s 1.5 × 10−6 M, much lower than the critical micelle concen-ration (CMC) value (10−4 M) of PVP (MW > 15,000) [21].ecause of the interaction between amide unit of PVP and Ru

ons, a string-like of Ru ions along the backbone of the PVP wasormed. After the Ru ions were reduced by hydrogen, Ru atomsan nucleate and grow to form the chain-like nanoparticle arraysue to the weaker bounding to carbonyl bond of the PVP amidenit compared with Ru ions. Relative rates of nucleation androwth determined the aspect ratio of the final products [22].eduction of Ru ions was largely diminished when hydrogenecreased to 1 MPa. Therefore, the level of the super-saturationf Ru atoms greatly reduced. It induced the slow nucleation ofu nanoparticles. As a result of nucleation-growth kinetics, itas favorable for the Ru nanoparticles aggregating to the longer

ength of nanoparticle arrays [23].In all these studies, although the morphology of Ru nanopar-

icle arrays was varied under different conditions, the diameterf the nanoparticle arrays was always similar. One possibleunction for PVP is kinetically control the growth rates of dif-erent crystalline faces by interacting with these faces throughdsorption and desorption [24]. We suspect that the PVP poly-er might coil around the nanoparticle arrays which limited the

rowth directions of the nanoparticles, when they grow alongheir longitudinal directions.

All these samples were free of Ru nanoparticles even theolar ratio of the repeating units of PVP to Ru reach to 50/1. It

s not like Ag, when the molar ratio of PVP to silver nitrateas higher than 15, only silver nanoparticles were obtained

s the product. Since Ag ions was no coordinate with amidenit of PVP previous to reduce, a heavy coverage of PVP onhe all surfaces of silver nanoparticles would result in isotropicrowth for all different faces [25]. In this work, Ru ions coor-

inate with amide unit of PVP at first. Thus, these Ru ionsould be subsequently reduced to form nanoparticle arrays alonghe backbone of the PVP, resulting in anisotropic growth foru nanoparticles.

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ig. 4. The results of catalytic hydrogenation of phenol in the aqueous solution of PVVP/Ru = 10/1, water (200 ml), H2 (1 MPa), T (353 K), agitation rate (500 rpm), initialespectively. Initial TOF is the TOF at 5 min.

nd Physics 108 (2008) 369–374 373

Therefore, the whole formation process is suggested that Ruons first coordinated with amide unit of PVP, and then a string-ike of Ru ions along the backbone of the PVP was formed. Whenhe complex of PVP–RuIII was reduced by hydrogen, bond withartial electron donation of sole PVP oxygen to vacant orbitalf ruthenium surface atoms would be weaken. When Ru atomsucleated and grew along their longitudinal directions to formhe chain-like nanoparticle arrays, PVP polymer coiled aroundhe nanoparticle arrays which limited the growth directions ofhe nanoparticles.

.5. Phenol hydrogenation in aqueous media

Hydrogenation of phenol with various concentrations inqueous solution was performed at 353 K, under 1.0 MPa H2.he results of catalytic hydrogenation of phenol were shown

n Fig. 4. In all the investigated cases, chain-like Ru nanopar-icle arrays (PVP (K90)–Ru-10 (1 MPa)) showed high activity.ast hydrogenation of phenol with the concentration from 10 to0,000 mg l−1 was achieved in the aqueous solution of chain-ike Ru nanoparticle arrays without further treatment. When theoncentration is higher than 1000 mg l−1, initial TOF (turnoverrequency, tested at 5 min) (defined as number of moles of con-umed H2 per mol of Ru per hour) reaches 21,000 h−1. Theydrogenation of phenol with the concentration of 1000, 5000nd 10,000 mg l−1, was almost complete within 40, 240 and80 min, respectively. When the phenol concentration is only0 mg l−1, initial TOF arrived at 250 h−1. The hydrogenation ofhenol was almost complete at the end of 10 min and the phe-ol concentration decreased from 10 to 0.1 mg l−1. It is worthoting that the chain-like Ru nanoparticle arrays described hereperates successfully over a much wider range of phenol con-entration compared with the traditional catalytic methods [26].

he effect of the hydrophobic microenvironment on the catalyticctivity can also be noticed by comparison of the TOF (the ini-ial concentration of phenol was 1000 mg l−1) value using PVPK90)–Ru-10 (1 MPa), PVP (K15)–Ru-10 and Ru black (∼5 nm

P (K90)–Ru-10 (1 MPa). Conditions: catalyst (0.002 mmol), repeating units ofphenol concentrations in water are 10, 100, 500, 1000, 5000 and 10,000 mg l−1,

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n diameter). The TOF values were 21,000, 9600 and 200 h−1,espectively. It can be explained that the alkyl chains of PVPorm the confined hydrophobic microenvironment around thehain-like Ru nanoparticle arrays in water. The main advantagef this hydrophobic microenvironment is an anticipated increasehe concentration of phenol around the Ru catalysts.

. Conclusions

In summary, chain-like Ru nanoparticle arrays were synthe-ized by polymer-mediated aqueous-solution method. It wasound that high-average molecular weight of PVP, suitable moleatio of repeat unit of PVP to Ru and low concentration ofydrogen would profit the synthesis of nanoparticle arrays. Thispproach can synthesize the chain-like Ru nanoparticle arraysather than randomly oriented samples. The aqueous solutionontaining chain-like Ru nanoparticle arrays without furtherreatment showed high-hydrogenation activity in the hydro-enation of phenol-containing wastewater with broad phenoloncentration.

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