Chemical Engineering Journal 215-216 (2013) 616–625

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Chemical Engineering Journal

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Easily scalable synthesis of Ni nanosols suitable for the hydrogenationof 4-nitrophenol to p-aminophenol under mild condition

M. Blosi a,⇑, S. Albonetti b, A.L. Costa a, N. Sangiorgi a, A. Sanson a

a ISTEC-CNR, Institute of Science and Technology for Ceramics, National Research Council, Via Granarolo 64, 48018 Faenza, Italyb Department of Industrial Chemistry and Materials, Bologna University, Viale del Risorgimento 4, 40136 Bologna, Italy

h i g h l i g h t s

" Nickel sol is synthesized by using a microwave assisted route." Synthesis versatility and high sol concentration make the process easily scalable." Synthesized nickel nanoparticles show very good catalytic activity." Prepared nanoparticles are promising material as catalysts for SOFC anodic layer." Nickel formation goes through Ni-glycolate which turns completely into metal.

a r t i c l e i n f o

Article history:Received 2 July 2012Received in revised form 22 October 2012Accepted 2 November 2012Available online 21 November 2012

Keywords:Nickel nanoparticlesStable nanosolsPolyol synthesisMicrowaveCatalysis4-NitrophenolAECSA

1385-8947/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cej.2012.11.013

⇑ Corresponding author. Tel.: +39 0546699718; faxE-mail address: magda.blosi@istec.cnr.it (M. Blosi)

a b s t r a c t

Ni nanoparticles with a dimension ranging between 80 and 200 nm were synthesized by a microwave-assisted polyol route. The reaction was carried out at 200 �C in diethylenglycol (DEG) in the presenceof a binary chelating agent system (polyvinylpyrrolidone PVP and dodecylamine DDA), in order to obtainstable and highly concentrated nanosols whose ratio was carefully optimized. Stable suspensions wereproduced with the future prospect of improving the performance of the anodic layer in solid oxide fuelcells (SOFCs), in which the metal is employed as catalyst. Studying the influence of different reactiontimes on the final product, it was possible to identify the reaction path that goes through the formation,after 10 min, of crystalline Ni-glycolate that turns completely into crystalline Ni metal after 20 min. Theas obtained Ni nanoparticles were characterized by XRD, TG–DTA, FT-IR, TEM. Preliminary tests on theircatalytic and electrochemical activities were carried out by studying the hydrogenation of 4-nitrophenol(4-NP) to p-aminophenol (4-AP) in the presence of NaBH4 as probe reaction and by calculating the elec-trochemically active surface area (AECSA) with cyclic voltammetry.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, the synthesis of metal nanoparticles withcontrolled size and shape has been a general goal addressed byseveral material scientists [1]. Most papers, however, deal withnanopowders preparations, involving solid precipitation and lowsolid suspension concentration [2–4]. Only few reports have beenfocused on the production of stable nanosols [5,6] with increasedsolid concentration [7]. The one-step synthesis approach is partic-ularly important for obtaining suspensions nucleating the particledirectly into the fluid, therefore avoiding separation and re-dispersion steps. This approach also ensures a better control overaggregation phenomena and particle size growth. On the otherhand the use of sols represents an essential goal both from a safety

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point of view and for the subsequent steps of colloidal processingof ceramics.

Ni nanoparticles have been synthesized in literature throughmany methods [8–10] and with various reducing agents [4,9,11,12],we focussed on polyol synthesis because it is an highly versatileroute, allowing the preparation of nanosized metal particles witha good control on particle size and morphology [13].

‘‘Polyol’’ stands for polyalcohols with an high boiling tempera-ture and able to solve inorganic salts. Its mild reducing propertiesand chelating effect, are fundamental for the production of welldispersed magnetic metal nanoparticles such as Ni, Co, Fe, thateasily aggregate due to magnetic dipole interactions and the highelectron affinity [14].

Many papers were published on Ni NPs preparation, but in com-parison to other metals [15–20] some of them were related to thepolyol route [21–26] and none of them concerned the preparationof stable colloidal sols at high solid contents.

Ni(CH3COO)2 Chelatin agents

MW Heating (200°C)

Cation reduction

Colloidalsuspension

g

EG or DEG

Scheme 1. Synthesis flow sheet.

M. Blosi et al. / Chemical Engineering Journal 215-216 (2013) 616–625 617

Nickel nanoparticles are still a worth investigation for theirimportant electric, magnetic and catalytic properties [27–29], forexample in hydrogenation [30–32] and oxidation [33–35] reactions.Ni has also a key role in solid oxide fuel cell (SOFC) as catalyst of theH2 oxidation reaction. Stable Ni nanosuspension, for example, wouldrepresent an important starting point for the production of typicalanodic layers. The latter is generally a cermet (ceramic– metal) com-posed by the Ni as metallic part and the electrolytic material as theceramic one. In order to ensure good cell performance, the nickelparticles should form a percolating phase within the network ofthe ceramic phase. In the most common process, the anode is pro-duced using as starting powder a mixture of NiO and the electrolyte.The former is then reduced in situ to metallic nickel [36,37].

Ni-nanosols could be effectively used for the production of highlyactive anodes via infiltration process, which has been recentlyshown to be an effective way to improve the electrochemical perfor-mance of the electrodes of a SOFC [38]. With this method, a stableand highly concentrated sol can be exploited for the infiltration ofthe metal inside the ceramic layer previously produced. Moreover,starting from particles already in a metallic state, instead of NiOpowders, can reduce the health risks linked to the oxide handling.

In this paper we reported the study and optimization of themicrowave-assisted polyol method for the synthesis of Ni suspen-sion with a high solid loading. The proposed microwave assistedsynthesis, usable also in large scale continuous production, enablesprocess intensification. Moreover several features makes the devel-oped processes a potential up scalable one. Firstly the reductionoccurs only by the solvent action so avoiding the use of hazardousreducing agents commonly used for the synthesis of metals[4,9,11,12] and not easy to handle in large quantities. Moreoverthe stability is a useful feature for the production in a continu-ous-flow system to avoid the powder separation and recoverysteps. Finally the high versatility of the synthesis together withthe high solid loading are key characteristics for developing advan-tageous large scale production.

Although the solid concentration obtained after synthesis wasimproved with respect to the literature [38], a post synthesis con-centration by ultrafiltration, here not described, was performed inorder to reach the very high solid contents (5–6 wt.%) useful forSOFC devices. An increased solid loading reduces the number ofapplication steps needed to deposit the right amount of catalyston the ceramic matrix. The formation of stable and concentratedNi metal nanosols follows a typical sol–gel synthesis mechanism:hydrolysis, condensation and reduction of Ni precursor salt bythe glycolic solvent (‘‘polyol’’) at 200 �C and further stabilizationof nanometric phase by optimizing the molar ratio between twodifferent chelating agents: polyvinylpirrolidone (PVP) and dodecyl-amine (DDA). The reaction path was studied by stopping the syn-thesis at different reaction times and following the evolution ofmetal-glycolate complexes as intermediate product.

Finally, some preliminary tests on the catalytic and electro-chemical properties of the prepared nanoparticles were carriedout. The catalytic performances were evaluated by studying: thehydrogenation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP)in the presence of NaBH4, a probe reaction often used in literatureto test the catalytic activity of metal nanoparticles [29,39,40]. Theelectrochemical properties, on the other hand, were assessed bycalculating the electrochemically active surface area (AECSA) ofthe synthesized nanoparticles supported on carbon black.

2. Experimental procedure

2.1. Nanoparticles synthesis

All the chemical reagents used in this experiment were analyt-ical grade (Sigma–Aldrich). The typical synthesis procedure is re-

ported in Scheme 1. For the synthesis of a stable suspension withconcentration of 0.5 wt.%, 3.9 g of polyvinylpirrolidone (PVP Mwa15.000) and 9.6 g of dodecylamine (DDA), used as chelating agents,were completely dissolved in 200 ml of glycol (ethylenglycol, EG ordiethylenglycol, DEG), finally 5 g of Ni(ac)2 were added. The solu-tion was heated to the synthesis temperature, under magnetic stir-ring in a commercial microwave oven specially designed forchemical synthesis and equipped with a reflux system (MicrosynthPlus, Milestone). Since the reduction power of polyols (EG or DEG)is enhanced at high temperature the reactions were performed at190 �C for EG, which matches with its boiling point and at 200 �Cfor DEG.

The reaction chamber of the system is provided with magneticstirring, reflux system and an optical fiber temperature controller.The microwave power is generated by 2 � 800 W magnetrons,with frequency 2.45 GHz. In order to respect the scheduled heatingramps, the power is continuously supplied and automatically mod-ulated by the instrument software. The power is automaticallychanged by the instrument to follow the temperature profile.

2.2. Chemical physical characterization

Particle size distribution was evaluated by Dynamic Light Scat-tering (DLS) using a nanometric particle size analyzer, (ZetasizerNano S, Malvern Instruments, UK); scattering data were recordedat 25 ± 1 �C in backscattering modus at a scattering angle of2h = 173�. Before measurement, samples were properly dilutedwith the glycolic solvent. Each result corresponds to the averageof five measurements and each measurement is the average of15 analyses. The instrument measures the hydrodynamic diameterthat is a diameter that includes the coordination sphere and thespecies adsorbed on the particle surface such as stabilizers, surfac-tants and so forth. DLS analysis provides also a polydispersion in-dex (PDI), which is a number ranging from 0 to 1 useful toquantify the colloidal dispersion degree: samples with PDI lowerthan 0.2 may be considered monodispersed.

UV–VIS extinction spectra were collected with a Lambda 35spectrophotometer (Perkin–Elmer, UK), working in a double beammode. Analyses were performed using a quartz cuvette as sample-holder. Samples for UV–VIS spectroscopy were prepared by dilut-ing the as prepared colloidal suspension with diethylenglycol.

Diffraction patterns were collected on the dried powders of thesample with the higher metal concentration. Each suspension wascentrifuged at 6100 rpm, washed several times with absolute eth-anol and dried for 24 h at room temperature in air. Analyses wereperformed by a diffractometer, (Bruker D8, Advance, Germany)operating in h/2h configuration, with a XCeleretor detector (20–80� 2h range, 0.02 stepsize, 0.5 s time-per-step).

618 M. Blosi et al. / Chemical Engineering Journal 215-216 (2013) 616–625

To detect the presence of adsorbed organic species on Ni nano-particles, FT-IR spectra were recorded in the 400–4000 cm�1 rangeon samples synthesized at different reaction times. Spectra wererecorded using a Fourier transform infrared spectrometer (1700Perkin–Elmer, Waltham, Massachusettes, USA).

Dried metal nanoparticles were dispersed in absolute ethanoland after 10 min of sonication they were dropped and dried on a sil-icon wafer as holder for scanning electron microscopy analyses(SEM, Leica Cambridge Stereoscan 360) and onto holey carbon foilssupported on conventional copper microgrids for transmission elec-tron microscopy analysis (TEM). A Philips CM 100 operating at 80 kVwas used. Average particle size and standard deviation were calcu-lated on more than 150 particles, by analyzing different images.

2.3. Catalytic characterization

Catalytic characterization was performed after drying the pow-ders at ambient temperature for 24 h. The activity of the as pre-pared powders was compared with a submicrometric commercialpowder of the metal (Nickel, Aldrich), with an average particle sizeof about 600 nm, which was tested in the same reaction. Thereduction of 4-nitrophenol by NaBH4 was studied at 25 �C. 0.6 gof freshly prepared NaBH4 were added to 15 ml of a 4-NP solution(5.9 � 10�3 M), obtaining a reducer concentration of 1.06 M. Aftermixing the solution, 1.8 mg of dried Ni nanopowder were addedto the alcoholic solution, to keep the following molar ratios be-tween the reagents: nNaBH4/nNi = 517.5; n(4-NP)/nNi = 2.9. In or-der to prevent surface oxidation of nanometal during the test allthe reagents were previously dissolved in absolute ethanol (99%).

The reaction was followed studying the absorption spectra ofthe solutions using a spectrophotometer (Lambda 35, Perkin–Elmer, USA) in the range between 250 and 500 nm.

The reaction is of second order (1.1), but as the concentration ofNaBH4 is in large excess with respect to the one of the reagent(4-NP), the reduction rate can be regarded independent fromNaBH4, changing the system in a pseudo first order reaction(1.2). Therefore the rate constants of the reaction were determinedby measuring the change in absorbance at 400 nm, wavelengthtypical of 4-NP in alkaline condition, as a function of time.

V ¼ k � ½4-NP� � ½NaBH4� second order ð1:1Þ

V ¼ K � ½NP� pseudo first order ð1:2Þ

2.4. Electrochemical characterization

The electrochemical cell was configurated as follows: a glassycarbon electrode modified with nickel nanoparticles served asworking electrode (WE), a platinum gauze (Sigma–Aldrich) as coun-ter electrode (CE) and a saturated calomel electrode (SCE, Amel) wasused as reference one. A 0.5 M aqueous KOH (Sigma Aldrich) solu-tion was used as electrolyte solutions for the electrochemical anal-ysis. All solutions were prepared using deionized water (Millipore)with a resistivity value of 18.2 MX cm. Before each analysis the solu-tion was deaerated by bubbling N2 gas for 20 min. The working elec-

trode was modified with a nickel nanopowder supported on carbonblack: the dried nickel nanoparticles were dispersed in absolute eth-anol and impregnated on the carbon powder (Carbon Black VulcanXC-72), in order to have an amount of metallic phase of 30 wt.%.To prepare the working electrode, 5 mg of supported catalyst weredispersed in 1 ml of diluted Nafion solution (Sigma–Aldrich,0.05 wt.% in ethanol) and 10 ll of the prepared suspension wasdropped onto the glassy carbon electrode (d = 3 mm) by a microsy-ringe. The glassy carbon substrate was polished with an aluminasuspension before using. This method of preparation allows thedeposition of the nanoparticles on the surface of the electrode andtherefore their electrochemical characterization.

Cyclic voltammetry was carried out using Autolab potentiostat(lAutolab Type III-FRA2, Eco Chemie, the Nederland). The Nova 1.6(Eco Chemie, the Nederland) software was used to control the anal-yses and to record the curves in the range of potentials between�0.2 V and +0.6 V at scan rate of 100 mV/s.

3. Results and discussion

From preliminary tests, microwave system was preferred withrespect to traditional method, because it provided suspension withlower polydispersion. In fact, it ensures heating homogeneity, soallowing a fine control on particles nucleation and growth. Theconsequent formation of stable and highly concentrated sols, al-lows to address SOFC anodic layer design requirements.

The glycolic solvents (EG and DEG) used, as a consequence oftheir bidentate (EG) and tridentate (DEG) structures, helped to pre-vent particles agglomeration due to their chelating action [41].Nevertheless, in order to avoid nanoparticles aggregation, the addi-tion of two different protecting agents, polyvinilpyrrolidone (PVP)and dodecylamine (DDA), was considered.

As described in the literature [42], the bond between PVP and Niis too weak to prevent particles aggregation, so the addition ofDDA, with a stronger interaction with the metal [14] could be use-ful in order to achieve stable nanosuspensions. PVP in fact, due toits steric hindrance establishes a weak interaction with metalsalthough the PVP – Ni (ion or metal) interaction is characterizedby to the donation of a pair of electrons from the carbonyl oxygento the nickel or to the complex formation of the nitrogen containedon the five-membered heterocycles with the metal [42] (Fig. 1a).As well DDA provides a pair of electrons from the nitrogen at theend of a 12-carbon chain and therefore a stronger interaction withthe Ni-surface (Fig. 1a).

In the developed syntheses, during the heating, the solutionturned green as consequence of the complete dissolution of thenickel salt. At the reaction temperature, 180 �C for ethylenglycol(EG) and 200 �C for diethylenglycol (DEG), the Ni2+ reduction startsand the solution turns firstly to dark green and then to black, thetypical color of metallic nickel, so confirming the reduction of theprecursor salt. For both solvents (EG, DEG) black color was ob-served after 10 min at the reaction temperature. The features ofthe synthesized samples are reported in Table 1. The ratio betweenthe two chelating agents (DDA/PVP) was optimized by performingexperiments in EG with a metal concentration of 0.01 M. After that,the most promising ratio in terms of suspension stability was usedalso for the syntheses performed in DEG.

3.1. Synthesis optimization

Sample Ni-EG01, was synthesized in ethylenglycol with a molarratio PVP/DDA = 1. Once the system reached 190 �C a blacknanoparticles layer suddenly appeared on the solvent surface. Atthe end of the reaction all the particles floated on the liquidsurface. An uniform dispersion was achieved only by shaking the

Fig. 1. (a) Chemical structure of used chelating agents: PVP, DDA. Scheme of interaction between: (b) DDA and Ni-nanoparticles, DDA surround the particles as a typicalsurfactant; (c) PVP and Ni-nanoparticle; and (d) scheme of interaction between a PVP-DDA blending and Ni nanoparticles.

Table 1Characteristics of synthesized samples.

Sample [Ni2+] (M) nDDA/nNi nPVP/nNi nDDA/nPVP Time (min)

Ni-EG01 0.01 13 13 1 45Ni-EG02 0.01 13 0 – 45Ni-EG03 0.01 0 13 – 45Ni-EG04 0.01 5 21 0.25 15Ni-DEG01 0.01 5 21 0.25 15Ni-DEG02 0.1 1.1 4.2 0.25 30Ni-DEG03 0.1 1.1 4.2 0.25 20Ni-DEG04 0.1 1.1 4.2 0.25 10

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suspension, however after 12 h the particles moved back to thesurface. In order to better understand this behavior, not previouslyreported in literature, and to evaluate the different effect of eachchelating agent, the Ni-EG02 and Ni-EG03 syntheses were per-formed by adding DDA and PVP separately. The use of DDA inNi-EG02, seemed to delay the reduction: the shift toward black col-or, in fact, appeared after 15 min at 190 �C. The higher reactivity inthe presence of PVP, can be explained considering that PVP can actas a weak reducing agent [43,44] promoting metal formation.Moreover, the particles floating at the end of the synthesis detectedin NiEG02, let us conclude that DDA is the responsible of this effect.As shown in Fig. 1b, DDA probably adsorbs on the surface of the asformed nuclei and for its strong surfactant nature induces the par-ticles floating at the liquid surface. As further confirmation of thesurfactant action of DDA, during the heating, it was detected theformation of conspicuous foam, not observed in the presence ofPVP. Both foaming formation and particles floating should beavoided for having a more uniform system and a better scalableprocess.

Ni-EG03 was synthesized adding PVP as unique stabilizingagent. In this case the solution turned black after about 5 min at190 �C, followed by a rapid precipitation of the formed particles.SEM analyses performed on the so-prepared sample showed astrong particles aggregation (Fig. 2) with the formation of asym-metric nanostructures along the longitudinal direction and a broadparticle size distribution ranging from 100 to 250 nm. The elon-gated direction of aggregation is the result of the metal-glycolcomplexes formation, typically characterized by a long-chainshape [45] PVP interaction with nanoparticles is schematized inFig. 1c. PVP is a cross-linked polymer and probably wound thenickel particles forming a nickel core-PVP shell architecture that

do not prevent the condensation mechanism driven by the me-tal-glycol interaction. On the basis of this previous results it canbe concluded that both ligands affect positively the Ni particlessynthesis mechanism as PVP promotes the reduction of Ni precur-sor and DDA enabling particles floating on the liquid surface, has akey role in their stabilization.

In order to balance the effect of the two chelating agents, a newratio between DDA/PVP was tested (Ni-EG04). Being the surfactantaction of DDA very strong, its amount was reduced with respect toPVP and the new DDA/PVP ratio was set at 0.25. At these condi-tions a synergic balance between the chelating agents was attained(Fig. 1d) and a stable and well dispersed metal suspension was pre-pared. Foaming and floating were prevented and the reduction oc-curred at 190 �C. The suspension was stable 24 h. After that time, asmall amount of Ni precipitate starts to appear. The stability wasimproved up to 4–5 days changing the solvent from EG to DEG(Ni-01DEG). With respect to EG in fact, DEG, due to its tridentatestructure, offers a better chelating action. Moreover, thanks to itshigher boiling point the reaction can be performed far from its boil-ing temperature, ensuring a better homogeneity during the parti-cles nucleation. Bubbles evolution in fact, induces turbulence inthe solution which promotes phenomena of nucleation next tothe wall and subsequent aggregation.

UV–VIS spectroscopy carried out on sample Ni-01DEG (Fig. 3)showed a peak at 340 nm corresponding to the surface plasmonresonance (SPR) of nickel colloidal nanoparticles [46,47]. As de-scribed by Creighton and Eadon [46], Ni colloids, as Sc, Ti, V, Cr,Ni, Y and Pd, give broad or at least partly resolved absorption bandsin the near-UV, for this reason it was very difficult to compare thespectra of different samples in order to get information about sizeor interaction with the surrounding media; anyway the presence ofthe typical SPR band of nickel clearly showed the formation ofnanometric particles.

In order to make the suspensions more suitable for the futureapplication on SOFC device, the solid content was increased to0.49 wt.%, keeping at 0.25 the molar ratio between DDA and PVP.To study the reaction path, samples Ni-02DEG, Ni-03DEG and Ni-04DEG were produced at different reaction times: 10, 20 and30 min. To the best of our knowledge, this is the first report con-cerning Ni nanosols at this high concentration.

Excepting for the sample synthesized at 30 min, the suspen-sions, in spite of the high concentration, showed a time stabilitysimilar to Ni-01DEG, proving that the right balance between the

Fig. 2. SEM analysis of sample Ni-EG03-MW: (a) low magnification and (b) high magnification.

Fig. 3. UV–VIS absorbance spectra of sample Ni-DEG01. The arrow indicates theplasmon resonance peak typical of nickel nanoparticles (kabs = 340 nm).

Table 2Mean particle diameter of samples synthesized at decreasing reaction times andmeasured by different techniques.

Sample Time(min)

£XRD(nm)

£DLS(nm)

PDI £SEM/TEM(nm)

St dev(nm)

Ni-DEG02 30 12 490 0.4 193 53Ni-DEG03 20 10 450 0.5 – –Ni-DEG04 10 8 285 0.4 83 32

Fig. 4. DLS size distribution of samples obtained after 30 min of reaction (Ni-02DEG) and after 10 min (Ni-04DEG).

620 M. Blosi et al. / Chemical Engineering Journal 215-216 (2013) 616–625

chelating agents enhances the stability even for a high metalcontent.

Table 2 shows the values of mean diameter measured by DLSand based on hydrodynamic diameter for samples prepared inDEG at different reaction times. The aggregates formed on samplestreated at 20 or at 30 min strongly influenced the final mean diam-eter measured by DLS. As expected, longer reaction time caused anincrease in dimensions. For all samples PDI values indicated a highpolydispersion degree, due to the presence of particles with adiameter lower than 100 nm together with aggregates with dimen-sions higher than 300 nm (Fig. 4).

Since the DLS technique refers to the hydrodynamic diameter,that is the diameter comprehensive of the species adsorbed onthe particle surface, the values are much higher than the diame-ters observed by SEM and TEM analyses (Figs. 5 and 6). TEM andSEM measurements confirmed the particle coarsening of sampleNi-DEG02 synthesized for the longest reaction time. The meandiameter moved from 83 nm, for the sample Ni-DEG04 synthe-sized in 10 min, to a value of about 180–190 nm for the sampleobtained in 30 min. As known by nucleation and growth theory[48], once the nuclei form they start to grow during the reactiontime; in this case the dwelling at 200 �C for 30 min induced amassive particles increase. For both samples a high polydisper-sion degree was observed, confirming the DLS data. Probably,the high polydispersion degree is due to the slow reduction rate,which is a consequence of the weak reducing power of DEG. Infact, this route avoids the use of hazardous reducing agents likesodium boron hydride, formaldehyde and hydrazine commonlyused for the synthesis of metals.

3.2. Structural characterization

XRD analyses (Fig. 7) confirmed the structures of samples Ni-02DEG and Ni-03DEG as cubic with the three different peaks, cor-responding to (111), (200) and (220) fcc crystalline planes of Ni(JPCD 03-1051). XRD analyses performed on sample Ni-04DEG,are showed in Fig. 8. The observed profile was identified as typicalfor metal-glycolate complexes, also observed by other researchgroups [45]. The pattern was characterized by a strong peak atlow angles (5–10�) and by large and asymmetric peaks. The me-tal-glycolate complexes are intermediate compounds generatedby the reaction of DEG with metal ions to form the Ni-DEG coordi-nation complex with a chainlike structure. They further coagulateeach other to form a rodlike complex through Van der Waals

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Fig. 5. Sample Ni-DEG04 (10 min reaction): (a) SEM and (b) TEM images; (c) particle size distribution calculated by TEM analysis (average diameter: 83 nm).

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interactions [26]. As expected, crystallite size (Table 2), calculatedusing the Scherrer formula and corresponding to the primary par-ticles size, evidenced a clear dependence with the reaction times:the shorter is the holding time at the synthesis temperature thesmaller are the particles diameters. On the other hands the dwell-ing at the synthesis temperature causes both the growth of theformed nuclei and the phase evolution from metal-glycolate com-plex to pure-metallic phase. The observed behavior for the primaryparticles is coherent with the data from DLS and TEM.

The Fourier transform IR (FT-IR) spectra (Fig. 9) shows the com-parison between two samples synthesized at different reactiontimes (10 and 30 min). In the sample treated for 10 min (Ni-04DEG) the typical bands of the metal-glycolate complex wereclearly detected. In fact at 1060, 2854 and at 2922 cm�1 the C–OH vibration and the symmetric and asymmetric vibration bandsof CH2 confirm the presence of the glycolic compound, while at460 and at 620 cm�1 the Ni–O stretching vibration and the Ni–OH deformational mode support the presence of a metal-glycolatebond [27,49,50].

Instead, in the sample treated for longer time (Ni-02DEG) thebands corresponding to the diethylenglycol have much lowerintensity and were ascribed to the residual solvent adsorbed onthe particle surface after the washing step, while, the metal-glycolatebond bands were not observed.

3.3. Catalytic characterization

To test the activity of the as-obtained Ni-nanoparticles, the directhydrogenation of 4-nitrophenol was considered as sample reaction.Direct hydrogenation catalyzed by nickel is considered to be analternative green process for the production of 4-aminophenol

[33,51]. However, the catalytic performance of nickel nanoparticlesin this reaction has been rarely investigated and very few papersreport on the hydrogenation of 4-NP at room temperature. More-over, the commercially available Raney nickel catalyst cannot beapplied on this reaction due to the fact that this material also cat-alyzes the hydrogenation of aromatic ring beyond the nitro group[52,53]. Here, the hydrogenation of 4-nitrophenol was performedat room temperature in the presence of NaBH4 and the preparednanoparticles showed an interesting catalytic activity.

Preliminary experiments on the hydrogenation of 4-NPindicated that no reaction occurs without the catalyst in thestudied conditions, confirming the absence, during our tests, ofnon-catalytic reactions.

Dried powders from samples Ni-02DEG and Ni-04DEG andcommercial Nickel (Aldrich) were tested as catalysts in the reduc-tion of 4-NP to 4-AP in the presence of NaBH4 as reducing agent.The reaction was performed in absolute ethanol in order to avoidparticles surface oxidation. The ethanol solution of 4-NP has a lightyellow color that turns to bright yellow in the presence of NaBH4,due to the formation of phenolate ions that show a strong absor-bance peak at 400 nm.

Adding Ni nanoparticles to reaction mixture caused a gradualdecoloration of the solution and ultimately a bleaching of the yel-low color, due to the formation of 4-AP, that presents no peaks inthe visible region, but two peaks at 300 nm and at 230 nm.

The catalytic reaction was followed by measuring the change inabsorbance at 400 nm, corresponding to 4-NP. Table 3 showed asummary of the obtained results, in terms of kinetic constant,conversion and turn over frequency (TOF, defined as the numberof moles of reduced 4-NP per mole of catalyst per second).The synthesized nanoparticles exhibited very good catalytic

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Fig. 6. Sample Ni-DEG02-(30 min reaction): (a) SEM and (b–c) TEM images; (d) Histogram of particle size distribution calculated by SEM analysis (averagediameter = 193 nm).

Fig. 7. XRD spectra of samples Ni-DEG02 (treated for 30 min) and Ni-DEG03(treated for 20 min).

622 M. Blosi et al. / Chemical Engineering Journal 215-216 (2013) 616–625

performance, allowing the total conversion of 4-NP at room tem-perature in few minutes. Fig. 10a shows the series of absorbancecurves referred to Ni-02DEG: after 6 min the 4-NP was totally con-verted to 4-AP, while for Ni-04DEG the same conversion wasachieved in 3 min. The rate constants were assessed consideringthe absorbance decreasing of the 4-NP peak and correspond tothe slope of ln(At/A0) as a function of time (Fig. 10b). As can be ob-served by the curves and as showed in Table 3, Ni-DEG04 synthe-sized for 10 min and characterized by the presence of metal-glycolate complex, showed a better catalytic activity than the sam-ple heated for 30 min, probably due to its lower particle diameter.

On the contrary, commercial submicrometric nickel, with an aver-age mean diameter of about 600 nm, did not show any activity andthe spectra of 4-NP remained unchanged even after 5 h. For bothhome prepared samples the hydrogenation started immediately,when the metal was put in contact with the solution and no induc-tion time was observed. Since, in this reaction, the induction timecould often be ascribed to the presence of an oxide layer on particlesurface [54], we hypothesized the absence of a compact superficialoxide film on the prepared powders.

TOF values evidenced that the prepared nanoparticles have anenhanced catalytic behavior with respect to literature data. Syn-thesized nickel showed higher TOF than nanoparticles proved inthe same catalytic reaction both at room temperature [55] and inautoclave at 100 �C in presence of H2 [53]. Furthermore our nano-particles exhibited an increased TOF with respect to Ni Raney too,tested at 100 �C in an autoclave system [53]. Even if the reactionreported by Wang was performed in a total different system withrespect to our tests (high temperature, hydrogen insufflation),TOF values indicated that our catalysts were more active.

3.4. Electrochemical characterization

To examine the electrochemical properties of the Nickel nano-particles the AECSA was calculated. This area describes the interfacebetween electrolyte and metal at which the electron transfer oc-curs and it is a clue parameter to assess the electrochemical behav-ior of nickel nanoparticles. It is fundamental to distinguishbetween the morphological surface area and the electrochemicallyactive one because the latter, responsible of the catalytic response

Fig. 8. XRD analysis performed on sample Ni-04DEG. (a) Total spectra and (b) enlargement of spectra portion evidencing the peaks typical of glycolate complex.

350135023503350ν

Fig. 9. FT-IR analysis performed on sample Ni-DEG02 (30 min reaction) and onsample Ni-DEG04 (10 min reaction).

Table 3Kinetic constant, conversion and TOF values calculated for samples Ni-DEG02 and Ni-DEG04. TOF of synthesized samples are compared with TOF of Ni nanoparticles usedin the same reaction and reported in the literature.

Sample k (s�1) Conversion (%) TOF (s�1)

Ni-DEG04 1.44 97 1.9 � 10�2

Ni-DEG02 0.56 97 0.8 � 10�2

Ni [55]a – – 0.002 � 10�2

Ni [53]b – 99 0.048 � 10�2b

Ni Raney [54]a – – 0.012 � 10�2

a Literature data.b TOF calculated at 8 h of reaction with a conversion of 99%, reaction performed

at 100 �C in autoclave and insufflating H2.

A

λλ (nm)

0s

30 s

90 s

240 s

330 s

360 s

(a)

Fig. 10. (a) UV–VIS spectra of 4-NP reduction for sample Ni-DEG02 and (b)

Fig. 11. Cyclic voltammetry analysis for samples Ni-DEG02 and Ni-DEG04 in 0.5 Maqueous KOH solution at scan rate of 100 mV/s.

M. Blosi et al. / Chemical Engineering Journal 215-216 (2013) 616–625 623

of the material, is often smaller than the total surface area avail-able. There are several methods to measure the AECSA [56], amongthem cyclic voltammetry represents one of the most versatile. Inparticular, the AECSA was measured by using the charge of forma-tion of a-Ni(OH)2 and considering that the charge density corre-sponding to the formation of a a-Ni(OH)2 monolayer is equal to514 lC/cm2 [57]. The AECSA is evaluated by dividing the average be-tween anodic and cathodic charges for the charge formation of onea-Ni(OH)2 monolayer.

Fig. 11 showed the cyclic voltammetry curves observed for thesamples Ni-DEG02 and Ni-DEG04. Two peaks, corresponding to the

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0 100 200 300 400

Ln

(A/A

0)

Time (s)

(b)

plot of ln(At/A0) vs. time, the rate constants correspond to the slopes.

624 M. Blosi et al. / Chemical Engineering Journal 215-216 (2013) 616–625

anodic (oxidation) and cathodic (reduction) reactions at the nano-particles surface were detected. The peaks were referred to theoxidation and reduction of Nickel to a-Ni(OH)2 and vice versa. Pass-ing from Ni-DEG02 to Ni-DEG04 a current decrease, correspondingto a reduction of AECSA, was detected. This correspond to calculatedvalues of electrochemically active surface area of 0.94 cm2 for Ni-DEG02 and 0.53 cm2 for Ni-DEG04. These preliminary resultsshowed electrochemical activity for both samples. The most cata-lytic active sample, however, (Ni-DEG04) showed the lower AECSA.Since the calculated AECSA is directly linked to the metallic phaseamount, it has been supposed that the presence of a metal-glycolatecomplex in sample Ni-DEG04, decreasing the metallic surface area,induced a lowering of the calculated parameter.

4. Conclusions

A microwave assisted polyol ruote was successfully optimizedto synthesize nickel metal nanoparticles with a good catalyticactivity tested in the reaction of 4-NP hydrogenation. The synthesisparameters were optimized in terms of chelating agents to metalmolar ratios and reaction time, allowing the achievement of a nick-el nanosols at high concentration (0.49 wt.%), higher than the val-ues usually reported in the literature and a time stability of severaldays. The optimized synthesis exploits the chelating and the reduc-ing power of both the solvent and of PVP, providing the completereduction of metal without adding any hazardous reducing agentsas sodium boron hydride, formaldehyde or hydrazine. The reactionpath was studied and a metal-glycolate compound was detected byXRD as intermediate of reaction, proving the chelating action of theglycolic solvent.

The catalytic tests performed on synthesized nanoparticles inthe reduction of 4-NP, chosen as probe reaction, showed promisingcatalytic performance, comparable and more active than literaturedata. Particularly, the sample Ni-DEG04, synthesized for 10 minand containing a metal-glycolate complex exhibited the best per-formance, maybe due to its smaller particle size. On the otherhands the electrochemically active surface area assessed by cyclicvoltammetry showed that the prepared samples had an electro-chemical activity, but the sample Ni-DEG04, in spite of its excellentcatalytic activity, evidenced the lowest AECSA, due to the presenceof the metal-glycolate complex which reduced the pure metal sur-face. Although the metal glycolate-complex affects negatively theAECSA, it promotes efficiently the hydrogenation reaction.

Concluding the synthesis here developed seems to be a perfectcandidate to prepare stable nickel nanosols with an electrochemi-cal activity and excellent catalytic properties, usable as catalysts inthe typical anodic layer of solid oxide fuel cells.

Acknowledgments

This work was funded by the Economic Development Ministry(MISE) of Italy, under the frame of the Project: ‘‘Celle a combusti-bile per applicazioni stazionarie cogenerative’’.

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