research article synthesis by microwaves of bimetallic...

Hindawi Publishing CorporationJournal of NanotechnologyVolume 2013 Article ID 578684 9 pageshttpdxdoiorg1011552013578684

Research ArticleSynthesis by Microwaves of BimetallicNano-Rhodium-Palladium

M Ugalde12 E Chavira2 M T Ochoa-Lara1 I A Figueroa2 C Quintanar3 and A Tejeda2

1 Laboratorio Nacional de Nanotecnologıa Centro de Investigacion en Materiales Avanzados SCAvenida Miguel de Cervantes 120 31109 Chihuahua CHIH Mexico

2 Instituto de Investigaciones en Materiales Universidad Nacional Autonoma de Mexico Apartado Postal 70-36004510 Mexico DF Mexico

3 Facultad de Ciencias Ciudad Universitaria Universidad 3000 Circuito Exterior SN 04510 Mexico DF Mexico

Correspondence should be addressed to M Ugalde magaliugaldecimavedumx

Received 29 May 2013 Revised 31 August 2013 Accepted 12 September 2013

Academic Editor Jorge Seminario

Copyright copy 2013 M Ugalde et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

An improved acrylamide sol-gel technique using a microwave oven in order to synthesize bimetallic Rh-Pd particles is reportedand discussed The synthesis of Pd and Rh nanoparticles was carried out separately The polymerization to form the gel of bothRh and Pd was carried out at 80∘C under constant agitations The method chosen to prepare the Rh and Pd xerogels involved thedecomposition of both gels The process begins by steadily increasing the temperature of the gel inside a microwave oven (from80∘C to 170∘C) In order to eliminate the by-products generated during the sol-gel reaction a heat treatment at a temperature of1000∘C for 2 h in inert atmosphere was carried out After the heat treatment the particle size increased from 50 nm to 200 nmproducing the bimetallic Rh-Pd clusters It can be concluded that the reported microwave-assisted sol-gel method was able toobtain nano-bimetallic Rh-Pd particles with an average size of 75 nm

1 Introduction

Bimetallic alloy (solid solutions or intermetallics com-pounds) nanostructures synthesized from two single com-ponents have been of interest because of their superior prop-erties in comparison with their respective single-componentspecies that is Ag Pd Rh and so forth [1] The synthesisof noble metals and alloys in nanometric scale has beenextensively investigated in nanotechnology because of theiroptical and electronic properties as well as for their usefulapplications in many fields such as medicine [2] catalysisand sensors [3ndash6] Hardness highmelting and boiling pointsand high thermal and electrical conductivity are some of theexisting properties of these noble metals

Rhodium and palladium (Rh and Pd) crystallize in a facecentered cubic (fcc) unit cell Both metals add s electronsto the collective d band of palladium [7] and increase thelattice parameter of the palladium host lattice [8 9] It wasfound that rhodium behaves as an absorber of hydrogen at

high pressure of gaseous hydrogen when situated within thepalladium lattice [10 11]

Recently Pd nanocrystals have been prepared in aqueoussolutions giving rise to a great variety of shapes includingtruncated octahedron cube octahedron and thin plate [12ndash14] making them ideal candidates as seeds for growingbimetallic nanostructures The control of crystal size andits dispersion are among the main goals of nanocrystalpreparation This is due to the physicochemical properties ofa bimetallic nanocrystal that can be tailored by controllingtheir particle size shape and elemental composition aswell as their internal and surface structures Recently thecomplexity of nanomaterials can be further enhanced bythe formation of multimetallic nanostructures (eg core-shell and dumbbell) Synthesis parameters such as cappingagent metal ion and reaction temperature also play animportant role in the overgrowth process [15] Among theavailable methods to obtain nanosized bimetallic particles ofrhodium-palladium are the coimpregnation method [16 17]

2 Journal of Nanotechnology

nanocrystals stabilized in micelles [18 19] gas evaporation[20 21] polyol process [22] and the sol-gel method [23]

Different methods have been developed to produce metalnanoparticles but these differ significantly depending onthe application required Osseo-Asare and Arriagada [18]stabilized Rh particles using the ionic (poly (N-vinyl-2-pyrrolidone)-co-(1-vinyl-3-alkylimidazolium halide)) copol-ymer in ionic solvents They found a distribution of particlesof sim3 plusmn 06 nm These nanoparticles were used as catalystsfor arene hydrogenation Similarly Pileni [19] produced Rhnanoparticles through chemical reduction This synthesismethod is widely used to produce nanoparticles which willbe used as catalysts on an industrial scale They reportedthat the Rh nanoparticles are formed by reducing Rh ions inethanol-water solvent with the polymers of polyvinylpyrroli-done (PVP) [20 21] The PVP encapsulates the nanoparticlesin the solution and prevents their accumulation The growthof the nanoparticles may be limited during the reduction bythe space restrictions imposed by the three-dimensional PVPnetwork The particles generated from this method showedan average size of 32 plusmn 07 nm However it was noted thatwithin the network of the Rh-PVP therewere Cl atomsThesespecies came from the reagent used (RhCl

3(3H2O)) During

the synthesis the Cl atoms were entrapped as an impurity inthe interlattice planes of the material The particles producedcan only be used for catalysis since the aforementionedreagent stabilizes them Thus the impurities anchored in thestructure may detrimentally affect their catalytic propertiesTherefore it is imperative to produce impurity-free Rh Pdor Rh-Pd nanoparticles in order to widen their possibleapplications [24ndash42]

Based on the above the objective of this work is tosynthesize an impurities-free bimetallic alloy of palladium-rhodium through amodified polyacrylamide sol-gel methodusing microwave radiation The motivation of this study isbased on the observation that a possible synergism may existbetween both metallic atoms as they are located in closeproximity The possible union effect can be reflected in thechemical performances of both metals which differ fromthe chemical behavior of each of the metallic componentsalone Finally it is worth mentioning that sol-gel method waschosen because nanoparticle size control can be achieved bymeans of heat treatments or pH variations

2 Experimental Procedure

21 Materials and Synthesis Procedure The reagents usedin this work were Rhodium metal (999 Sigma-Aldrich)Palladium acetate (Pd(OAc)

299 Aldrich) sulfuric acid

(H2SO4 J T Baker) nitric acid (HNO

3 70 J T

Baker) ammonium hydroxide (NH4OH 28ndash30 J T

Baker) ethylenediaminetetraacetic acid (EDTA 99 Fluka)NN1015840-methylenebisacrylamide (995 Fluka) 221015840-azobis(2-methylpropionamidine) dihydrochloride (98 Fluka) acry-lamide monomer (99 Sigma-Aldrich) and distilled water

The synthesis of Pd and Rh nanoparticles was carriedout separately using an acrylamide sol-gel technique[43] The synthesis of Rh nanoparticles started dissolving

439mmoL of Rh metal in H2SO4

and distilled water(80mL) at 70∘C forming a yellowish solution Later thesolution was cooled down to ambient temperature priorto adjusting the pH ca to 7 by adding NH

4OH following

the method outlined in [44] As soon as a pH ca 7 wasreached the solution was heated up to 80∘C and 516mmoLof EDTA was added to complex the rhodium ions avoidingthe formation of agglomerates After this 211mmoL ofacrylamide monomer and 972mmoL of cross-linking agentNN1015840 methylenebisacrylamide were added to the initialsolution Finally to produce the gel 55mmoL of initiator221015840-azobis(2-methylpropionamidine)dihydrochloride wasadded to reach the polymerization [43] The pH of thesolution was constantly monitored by means of a pH meter(Thermo Scientific Orion star A221) The polymerizationof Rh was carried out at 80∘C under constant agitation Itis worth noting that all the processes were done under airatmosphere In neutral or basic conditions the time requiredto form the gel decreases considerably [26]

The synthesis of Pd nanoparticles was carried out bydissolving Pd acetate in 300mL of distilled water at 80∘CAfter this 12mL of HNO

3and H

2O2were added Later the

solution was cooled down to ambient temperature prior toadjusting the pH ca to 7 by adding NH

4OH similar to the

method used for Rh As soon as a pH ca 7 was reached thesolution was heated up to 80∘C and 102mmoL of EDTAwasaddedThis EDTA reagent is used in this process with the aimof trapping the Pdmetal ions and preventing the formation ofagglomerates until the reaction starts By doing this the Pdions remain stable during the decomposition of EDTA [43]

In order to start the polymerization 5627mmoL ofacrylamide monomer and 1297mmoL of the cross-linkerNndashN1015840 methylenebisacrylamide were added Immediately368mmoL of a chemical initiator 120572ndash1205721015840 azodiisobutyrami-dine dihydrochloride was incorporated to the solution inorder to increase interconnection velocity [43 44] Thepolymerization occurred at 80∘C in air under continuousmagnetic stirring

The method chosen to prepare the Rh and Pd xerogelsinvolves the decomposition of both gels The process beginsby heating up both gels inside a microwave oven that isfrom 80∘C to 170∘C The microwave is equipped with aninfrared temperature sensor installed in such fashion thatit can measure the temperature of the material in analysiswithin the reaction area In this process the oven increasesthe temperature steadily Once a temperature of 170∘C hasbeen reached (under an argon flux) the process to generatethe xerogel of Rh and Pd lasts 30min and 3min respectivelyThe produced Rh and Pd xerogels were pulverized usingan agate electric mortar (Retsch GmbH-Mortar Grinder RM100) and heated at 1000 plusmn 4∘C for 2 h under an argon flux

22 Characterization The crystalline structure of the mate-rials produced was determined by XRD using a BrukerAXS D8-advance diffractometer with Cu K120572 radiation (120582 =15406 A) 40KV and 30mA equipped with a graphitediffracted beammonochromatorThe diffractogram patternswere collected at room temperature over the 2120579 range of 25∘

Journal of Nanotechnology 3

to 70∘ with a step size of 002∘ and time per step of 06 sA thermal analyzer (STD Q600 TA-Instruments) using astandard alumina pan with a heating rate of 10∘Cminminus1 in airatmosphere over a range of temperatures of 30∘C to 1000∘Cwas used to study decomposition temperatures

Themicrostructure andmorphologywere observed usinga scanning electron microscope (SEM JSM5800L JEOL)and a high-resolution transmission electron microscope(HRTEM JEM-2200FS JEOL) The samples were preparedby dispersing ca 2mg of Rh-Pd nanocrystals in 4mL of 1-propanol and were then sonicated for 15min Finally a dropof dispersion was deposited on a lacey carbon supportedcopper grid (200mesh) A series of SEM micrographs at10 kV and HR-TEM at 200 kV were taken The compositionof the material was determined with an Energy Dispersive X-Ray (EDX) spectroscope with an Oxford Incax-sight 7688detector The size distribution was determined from themicrographs taken with the HRTEM The size distributionhistograms were calculated using ldquoImage Jrdquo software (ImageProcessing and Analysis in Java) In this work the diametersof at least 100 particles were measured and mean values weredetermined The statistical error was also included

3 Results and Discussion

31 X-Ray Diffraction Patterns The combination of the XRDand TGA analysis proved to be useful when observing thenumber of phases that changed during the applied heattreatments to the gel and xerogel Figure 1(a) shows thegel and xerogel diffractograms of Rh Both diffractogramsshowed the presence of two distinctive crystalline phasesthat is ammonium sulfate (NH

4)2SO4(PDF 01-78-0579) and

edetic acidC10H16N208(PDF00-033-1672)The former could

be attributed to the addition of NH4OH which was used to

neutralize pH of the solution in the reactionThe latter couldbe associated with the reaction between the polymer andthe EDTA The xerogel diffractogram clearly shows six lowintensity peaks that correspond to edetic acid this evidencesthat the edetic acid is being removed during this process

ThePddiffraction patterns of the xerogel and gel productsare displayed in Figure 1(b) The Pd gel showed a typicalhalo for amorphous materials It is thought that this materialhas been produced by the remained amount of acrylamidepolymer although a small Bragg peak was observed atsim2120579 = 40∘ This low intensity peak could be attributed toPd (PDF 03-065-6174) On the other hand the Pd xerogeldiffractogram not only showed the amorphous contributionand the peak that corresponds to Pd but also displayed anumber of peaks that were identified as palladium oxide(PdO) (PDF 01-075-0584) Pd (NO

3)2(H2O)2(PDF 01-085-

2483) and NH4NO3(PDF 00-08-0452) It is thought that

these compounds are the product of the transformation of thegel to xerogel

After the heat treatment at 1000∘C for 2 h all by-productsshown in Figures 1(a) and 1(b) disappearedThis can be clearlyobserved in Figure 1(c) where only the peaks of Pd (PDF03-065-6174) and Rh (PDF 01-087-0714) metallic elements

are displayed Therefore the formation of the bimetalliccompound Rh-Pd is supported by these results

32 Thermogravimetric Analysis (TGA) The TGA curvesare important to determine the temperatures to which theformation of organic matter and subproducts take placeduring the sol-gel synthesis The results obtained from thistechnique did help to control the process parameters toproduce the Rh-Pd bimetallic material

The TGA results obtained from the rhodium and palla-dium gel and xerogel were analyzed separately and are shownin Figure 2 The TGA gel and xerogel traces for rhodium(Figure 2(a)) showed a weight loss at sim230∘C and 241∘Crespectively indicating that there was a process of polymerdegradationThe removal of (NH

4)2SO4occurred at 328∘C in

the gel and at 378∘C in xerogel It was also observed that theformation of Rh

2O3occurred at 597∘C and 620∘C for the gel

and xerogel respectively When increasing the temperatureup to 1000∘C particles of Rh were obtained confirming theelimination of all by-products generated before and duringheat treatment

Figure 2(b) exhibits the thermograms for the Pd gel andxerogel For the Pd gel an initial weight loss at 148∘C wasevident This was attributed to the evaporation of H

2O

while at 260∘C the weight loss corresponded to the removalof NH

4NO3 Similarly at 400∘C the elimination of Pd

(NO3)2(H2O)2compound is detected and finally at 505∘C

the decomposition of PdOwas observedThe TGA for the Pdxerogel curve displays aweight loss at 293∘C as a consequenceof the degradation of both the acrylamide and NH

4NO3

compounds Some (Pd (NO3)2(H2O)2) traces were detected

around 400∘C and then the observed drop of the TGA curveat 557∘C indicates that after this temperature the formationof Pd particles can be favored

From these results and considering the decompositionpoints of Rh

2O3and PdO both compounds were heat treated

at 1000∘C for 2 h under an Ar flux After this it was expectedthat the growth of those Pd-Rh particles might occur Table 1shows the average of weight loss obtained by means ofthermal analysis

33 Scanning Electron Microscopy (SEM) Figure 3(a) showsthe bimetallic material before being subjected to the heattreatment at 1000∘C This image clearly shows a morphol-ogy mainly composed by elongated granules ranging from500 nm to 1 120583m Figure 3(b) shows the morphology of thebimetallic Rh-Pd grains produced after applying the heattreatment it was observed that the material started to growforming sphero-hexagonal shapes between 1 and 7 120583m TheEDS elemental analysis of the Rh-Pd particles is shown inFigure 4 The average composition percentage for the Rhand Pd was 3453 and 6547 respectively Figure 5 showsthe elemental SEM mapping performed to the producedRh-Pd particles This analysis confirmed the homogeneousdistributions of the Rh and Pd elements in the material

34 High Resolution Transmission Electron Microscopy(HRTEM) In order to observe the Rh-Pd particles by


10 20 30 40 50

In

tens

ity (a

u)

Gel

Xerogel

Ammonium sulfate (NH4)2SO4Edetic acid (C10H16N2O8)

2Θ-scale

(a)

Inte

nsity

(au

)

Gel

Xerogel

Palladium PdPalladium oxide PdO

10 20 30 40 50 60 70

Palladium nitrate hydrate Pd (NO3)2(H2O)2Ammonium nitrate NH4NO3

2Θ-scale

(b)

30 40 50 60 70 80 90

Inte

nsity

(au

)

Palladium (Pd) Rhodium (Rh)

larr

larrlarrlarr

larr

larr

2Θ-scale

(c)

Figure 1 XRD patterns of (a) Rh (b) Pd and (c) bimetallic Rh-Pd

HRTEM these were dispersed by ultrasonication TheHRTEM micrographs showed the formation of clusterswith a polyhedron shape and distribution of the grainsrsquo sizeranging from 50 to 200 nm (Figure 6(a)) Figure 6(c) showsthe micrograph of one Rh-Pd particle with a size of sim100 nmIt is thought that the change in size and geometry of thoseparticles could be attributed to the sonication process which

fractured and broke the sphero-hexagonal shape particlesproducing nano-hexagonal shapes particles with an averagesize of 75 nm as shown in Figure 6(b)

The results presented in this work for the synthe-sis of bimetallic Rh-Pd by sol-gel with acrylamide usingmicrowaves were in good agreement with those reported in[45 46] They reported the synthesis of Pd-Rh bimetallic


200 400 600 8000

20

40

60

80

100 Acrylamide

Wei

ght (

)

GelXerogel

RhT = 230 241∘C

T = 378 328∘C

T = 597 620∘C

(NH4)SO4

Rh2O3

Temperature (∘C)

(a)

0

20

40

60

80

100

Wei

ght (

)

Temperature (∘C)0 200 400 600 800

PdO

1000

GelXerogel

PdT = 293∘C

T = 400∘C

T = 148∘C

T = 260∘C

T = 505∘CT = 557∘C

NH4NO3

NH4NO3

Pd(NO3)2(H2O)2

(b)

Figure 2 TGA curves from gel and xerogel for (a) rhodium and (b) palladium

Table 1 Study of thermal transitions for the Rh and Pd gel and xerogel respectively

Rhodium thermal transition Gel119879 (∘C)

Xerogel119879 (∘C)

Gelweight loss ()

Xerogelweight loss ()

Removal of the polymer (acrylamide) 241 230 13 9Elimination of (NH4)2SO4 compound 328 378 76 82Rh2O3 formation 620 597 99 99

Palladium thermal transition Gel119879 (∘C)


Gelweight loss ()


Removal of the polymer (acrylamide) 148 148 96 96Elimination of (NH4)2SO4 compound 273 300 562 92Pd (NO3)2(H2O)2 formation 400 mdash 45 mdashPdO decomposition 505 557 031 061

(a) (b)

Figure 3 SEM analysis for the (a) material before the heat treatment and (b) Rh-Pd alloy powders after applying the heat treatment at 1000∘Cfor 2 h


10120583m Electron image 1

(a)

O

Rh

Pd

Spectrum 1

0 2 4 6 8 10 12 14 16 18 20(keV)

Full scale 702 cts cursor 19974 (0 cts)

(b)

Figure 4 EDS analysis for the Rh-Pd alloy particles after applying the heat treatment at 1000∘C for 2 h


Pd La1 Rh La1 O Ka1

Figure 5 SEM elemental mapping for the bimetallic Rh-Pd

nanodendrites These particles consisted of Rh branchesanchored to a Pd nanocrystal core The palladium nanocrys-tals synthesized by different methods exhibit distinct formsthat is truncated octahedron cube and thin plate Accordingto Kobayashi Pd has been successfully employed as seed togrow Rh branchesThe bimetallic particles are around 20 nmHowever as mentioned above the synthesized method usedby Kobayashi is based on reducing Na

3RhCl6from L-

ascorbic acid in an aqueous solutionThismethod has severalshortcomings one of the most important is that varying theconcentration of Na

3RhCl6during the synthesis could only

control the degree of Rh branching A high concentration ofRh in the solution causes that only part of Rh can be anchoredaround the Pd and the excess of the produced nano-Rh thatfails to be anchored is dispersed in the formed solution thatis it fails to complete its reactionswith Pd to form the bimetalOn the other hand the varieties of morphologies found byKobayashi for Pd grains used as inoculants to grow the Rh

branches were comparable to those reported in this workfor the synthesis of bimetallic Rh-Pd by the modified sol-gel technique This could be attributed to the heat treatmentapplied in which the sintering process of the bimetallicmaterial begins causing the grain growth forming sphero-hexagonal shaped-like morphology particles Besides thismethod did help the formation of a bimetallic compoundwith a reasonably well homogeneous distribution as it wasclearly observed in the SEMmapping

4 Conclusion

The reported microwave-assisted sol-gel method was ableto obtain nano-bimetallic Rh-Pd particles with an averagesize of 75 nm Thermogravimetric analysis helped determinenot only the decomposition temperatures but also the heattreatment conditions needed to obtain nano-Rh-Pd-particlesAfter the heat treatment at 1000∘C2 h the subproducts that


(a)

50 60 70 80 90 100 1100

2

4

6

8

10

12

14

Dist

ribut

ion

Particle size (nm)

Average particle size = 75 plusmn 05nm

(b)

(c)

Figure 6 Micrographs of HRTEM of the nano-bimetallic Rh-Pd (a) Sphero-hexagonal shapes (b) average-size distribution of particles(sim75 nm) and (c) grain of the bimetallic Rh-Pd of sim50 nm

were produced during the sol-gel reaction were removedThesynthesis of the nano-bimetallic Rh-Pd was confirmed byXRD and EDXTheHRTEMmicrographs showed the forma-tion of clusters with a polyhedron shape and distribution ofthe grainsrsquo size ranging from 50 to 200 nmThe clusters weresuccessfully dispersed with an ultrasonic cleanerThe sphero-hexagonal shaped-like morphology particles reported in thiswork could be attributed to the heat treatment processFinally it is worth noting that our Rh-Pd particles are notrestricted to a single application as the resulting Rh-Pdnanoparticles were found to be free of impurities since thestabilizer used (EDTA)was completely removedwith the heattreatment

Acknowledgments

M Ugalde acknowledges the financial support from CONA-CyT through the scholarship 207277 I A Figueroa acknowl-edges the financial support from PAPIIT UNAM throughproject IB100712RR180712 The authors also express their

acknowledgment for the technical support of LaboratorioNacional de Nanotecnologıa Centro Nacional de Nanotec-nologıa and CIMAV and especially to Karla Campos Vene-gas Carlos Ornelas Omar Novelo and Caın Gonzalez fortheir interesting useful discussions

References

[1] R Ferrando J Jellinek and R L Johnston ldquoNanoalloys fromtheory to applications of alloy clusters and nanoparticlesrdquoChemical Reviews vol 108 no 3 pp 845ndash910 2008

[2] T Ragheb and L A Geddes ldquoElectrical properties of metallicelectrodesrdquoMedical and Biological Engineering and Computingvol 28 no 2 pp 182ndash186 1990

[3] Q Yuan and X Wang ldquoAqueous-based route toward noblemetal nanocrystals morphology-controlled synthesis and theirapplicationsrdquo Nanoscale vol 2 no 11 pp 2328ndash2335 2010

[4] H-C Kim S-M Park W D Hinsberg and I R DivisionldquoBlock copolymer based nanostructures materials processesand applications to electronicsrdquo Chemical Reviews vol 110 no1 pp 146ndash177 2010


[5] M-C Daniel and D Astruc ldquoGold nanoparticles assemblysupramolecular chemistry quantum-size-related propertiesand applications toward biology catalysis and nanotechnol-ogyrdquo Chemical Reviews vol 104 no 1 pp 293ndash346 2004

[6] D V Talapin J-S Lee M V Kovalenko and E V ShevchenkoldquoProspects of colloidal nanocrystals for electronic and optoelec-tronic applicationsrdquo Chemical Reviews vol 110 no 1 pp 389ndash458 2010

[7] N F Mott and H Jones Theory of the Properties of Metals andAlloys Oxford University Press London UK 1936

[8] R Coles ldquoLattice spacings in Ni-Cu and Pd-Ag alloysrdquo Journalof the Institute of Metals vol 84 p 346 1956

[9] A Maeland and T B Flanagan ldquoLattice spacings of gold-palladium alloysrdquo Canadian Journal of Physics vol 42 no 11pp 2364ndash2366 1964

[10] T B Flanagan B Baranowski and S Majchrzak ldquoRemarkableinterstitial hydrogen contents observed in rhodium-palladiumalloys at high pressuresrdquo Journal of Physical Chemistry vol 74no 24 pp 4299ndash4300 1970

[11] B Baranowski S Majchrzak and T B Flanagari ldquoDiffusion ofhydrogen in rhodium-palladium alloysrdquoThe Journal of PhysicalChemistry vol 77 no 23 pp 2804ndash2807 1973

[12] B Lim Y Xiong Y Xia et al Journal of Materials Research vol3 p 1367 1988 Angewandte Chemie International Edition vol46 p9279 2007

[13] B Lim M Jiang J Tao P H C Camargo Y Zhu and YXia ldquoShape-controlled synthesis of Pd nanocrystals in aqueoussolutionsrdquoAdvanced FunctionalMaterials vol 19 no 2 pp 189ndash200 2009

[14] B Lim H Kobayashi P H C Camargo L F Allard J Liu andY Xia ldquoNew insights into the growth mechanism and surfacestructure of palladium nanocrystalsrdquo Nano Research vol 3 no3 pp 180ndash188 2010

[15] F-R Fan D-Y Liu Y-F Wu et al ldquoEpitaxial growth of hetero-geneous metal nanocrystals from gold nano-octahedra to pal-ladium and silver nanocubesrdquo Journal of the American ChemicalSociety vol 130 no 22 pp 6949ndash6951 2008


[17] MRassoul F Gaillard EGarbowski andM Primet ldquoSynthesisand characterisation of bimetallic Pd-Rhalumina combustioncatalystsrdquo Journal of Catalysis vol 203 no 1 pp 232ndash241 2001

[18] K Osseo-Asare and F J Arriagada ldquoSynthesis of nanosizeparticles in reverse microemulsionsrdquoCeramic Transactions vol12 pp 3ndash16 1990

[19] M P Pileni ldquoReverse micelles as microreactorsrdquo Journal ofPhysical Chemistry vol 97 no 27 pp 6961ndash6973 1993

[20] I P Beletskaya A N Kashin A E Litvinov V S Tyurin PM Valetsky and G Van Koten ldquoPalladium colloid stabilized byblock copolymer micelles as an efficient catalyst for reactions ofC-C and C-heteroatom bond formationrdquo Organometallics vol25 no 1 pp 154ndash158 2006

[21] R W Siegel S Ramasamy H Hahn Z Li T Lu and R Gron-sky ldquoSynthesis characterization and properties of nanophaseTiO2

rdquo Journal of Materials Research vol 3 no 6 pp 1367ndash13721988

[22] D-H Kim and J Kim ldquoSynthesis of LiFePO4

nanoparticles inpolyol medium and their electrochemical propertiesrdquo Electro-chemical and Solid-State Letters vol 9 no 9 pp A439ndashA4422006

[23] R Uyeda ldquoThe morphology of fine metal crystallitesrdquo Journalof Crystal Growth vol 24-25 pp 69ndash75 1974

[24] F Robert G Oehme I Grassert and D Sinou ldquoInfluenceof amphiphile concentration on the enantioselectivity in therhodium-catalyzed reduction of unsaturated substrates inwaterrdquo Journal ofMolecular Catalysis A vol 156 no 1-2 pp 127ndash132 2000

[25] T Mizuno Y Matsumura T Nakajima and S Mishima ldquoEffectof support on catalytic properties of Rh catalysts for steamreforming of 2-propanolrdquo International Journal of HydrogenEnergy vol 28 no 12 pp 1393ndash1399 2003

[26] A Gniewek A M Trzeciak J J Ziołkowski L Kepinski JWrzyszcz and W Tylus ldquoPd-PVP colloid as catalyst for Heckand carbonylation reactions TEM and XPS studiesrdquo Journal ofCatalysis vol 229 no 2 pp 332ndash343 2005

[27] R Narayanan and M A El-Sayed ldquoEffect of colloidal catalysison the nanoparticle size distribution dendrimer-Pd vs PVP-Pdnanoparticles catalyzing the Suzuki coupling reactionrdquo Journalof Physical Chemistry B vol 108 no 25 pp 8572ndash8580 2004

[28] VV Shumyantseva S Carrara V Bavastrello et al ldquoDirect elec-tron transfer between cytochrome P450scc and gold nanoparti-cles on screen-printed rhodium-graphite electrodesrdquoBiosensorsand Bioelectronics vol 21 no 1 pp 217ndash222 2005

[29] I Willner R Baron and B Willner ldquoIntegrated nanoparticle-biomolecule systems for biosensing and bioelectronicsrdquo Biosen-sors and Bioelectronics vol 22 no 9-10 pp 1841ndash1852 2007

[30] J M D Zapiter B M Tissue and K J Brewer ldquoRutheniumand rhodium complexes anchored to europium oxide nanopar-ticlesrdquo Inorganic Chemistry Communications vol 11 no 1 pp51ndash56 2008

[31] H S Nalwa Handbook of Nanostructured Materials and Nan-otechnology vol 1 Academic Press San Diego Calif USA2000

[32] M Harada D Abe and Y Kimura ldquoSynthesis of colloidaldispersions of rhodium nanoparticles under high temperaturesand high pressuresrdquo Journal of Colloid and Interface Science vol292 no 1 pp 113ndash121 2005

[33] J Jimenez-Mier G Herrera E Chavira L Banos J Guzmanand C Flores ldquoSynthesis and structural characterization ofYVO3

prepared by sol-gel acrylamide polymerization andsolid state reaction methodsrdquo Journal of Sol-Gel Science andTechnology vol 46 no 1 pp 1ndash10 2008

[34] X-D Mu J-Q Meng Z-C Li and Y Kou ldquoRhodiumnanoparticles stabilized by ionic copolymers in ionic liquidslong lifetime nanocluster catalysts for benzene hydrogenationrdquoJournal of the American Chemical Society vol 127 no 27 pp9694ndash9695 2005

[35] T Ashida KMiura T Nomoto et al ldquoSynthesis and characteri-zation of Rh(PVP) nanoparticles studied by XPS andNEXAFSrdquoSurface Science vol 601 no 18 pp 3898ndash3901 2007

[36] Y Huang J Chen H Chen et al ldquoEnantioselective hydro-genation of ethyl pyruvate catalyzed by PVP-stabilized rhodiumnanoclustersrdquo Journal of Molecular Catalysis A vol 170 no 1-2pp 143ndash146 2001

[37] J-L Pellegatta C Blandy V Colliere et al ldquoCatalytic investiga-tion of rhodiumnanoparticles in hydrogenation of benzene andphenylacetylenerdquo Journal of Molecular Catalysis A vol 178 no1-2 pp 55ndash61 2002

[38] S Morneta S Vasseura F Grassteb et al ldquoMagnetic nanopar-ticle design for medical applicationsrdquo Progress in Solid StateChemistry vol 34 pp 237ndash247 2006


[39] A P Alivisatos ldquoSemiconductor clusters nanocrystals andquantum dotsrdquo Science vol 271 no 5251 pp 933ndash937 1996

[40] U Kreibig and M Vollmer Optical Properties of Small MetalClusters Springer Berlin Germany 1995

[41] B Fegley Jr P White and H K Bowen ldquoProcessing and char-acterization of ZrO

2

and Y-Doped ZrO2

powdersrdquo AmericanCeramic Society Bulletin vol 64 no 8 pp 1115ndash1120 1985

[42] M Ugalde E Chavira M T Ochoa-Lara and C QuintanarldquoNew synthesis method to obtain Pd nano-crystalsrdquo Journal ofNano Research vol 14 pp 93ndash103 2011

[43] A Sin and P Odier ldquoGelation by Acrylamide a quasi-universalmedium for the synthesis of fine oxide powders for electroce-ramic applicationsrdquo Advanced Materials vol 12 no 9 pp 649ndash652 2000

[44] H Wang L Gao W Li and Q Li ldquoPreparation of nanoscale 120572-Al2

O3

powder by the polyacrylamide gel methodrdquo Nanostruc-tured Materials vol 11 no 8 pp 1263ndash1267 1999

[45] HKobayashi B Lim JWang et al ldquoSeed-mediated synthesis ofPd-Rh bimetallic nanodendritesrdquo Chemical Physics Letters vol494 no 4ndash6 pp 249ndash254 2010

[46] S Kishore J ANelson J H Adair and P C Eklund ldquoHydrogenstorage in spherical and platelet palladium nanoparticlesrdquoJournal of Alloys and Compounds vol 389 no 1-2 pp 234ndash2422005

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ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Journal ofNanomaterials


nanocrystals stabilized in micelles [18 19] gas evaporation[20 21] polyol process [22] and the sol-gel method [23]

Different methods have been developed to produce metalnanoparticles but these differ significantly depending onthe application required Osseo-Asare and Arriagada [18]stabilized Rh particles using the ionic (poly (N-vinyl-2-pyrrolidone)-co-(1-vinyl-3-alkylimidazolium halide)) copol-ymer in ionic solvents They found a distribution of particlesof sim3 plusmn 06 nm These nanoparticles were used as catalystsfor arene hydrogenation Similarly Pileni [19] produced Rhnanoparticles through chemical reduction This synthesismethod is widely used to produce nanoparticles which willbe used as catalysts on an industrial scale They reportedthat the Rh nanoparticles are formed by reducing Rh ions inethanol-water solvent with the polymers of polyvinylpyrroli-done (PVP) [20 21] The PVP encapsulates the nanoparticlesin the solution and prevents their accumulation The growthof the nanoparticles may be limited during the reduction bythe space restrictions imposed by the three-dimensional PVPnetwork The particles generated from this method showedan average size of 32 plusmn 07 nm However it was noted thatwithin the network of the Rh-PVP therewere Cl atomsThesespecies came from the reagent used (RhCl

3(3H2O)) During

the synthesis the Cl atoms were entrapped as an impurity inthe interlattice planes of the material The particles producedcan only be used for catalysis since the aforementionedreagent stabilizes them Thus the impurities anchored in thestructure may detrimentally affect their catalytic propertiesTherefore it is imperative to produce impurity-free Rh Pdor Rh-Pd nanoparticles in order to widen their possibleapplications [24ndash42]

Based on the above the objective of this work is tosynthesize an impurities-free bimetallic alloy of palladium-rhodium through amodified polyacrylamide sol-gel methodusing microwave radiation The motivation of this study isbased on the observation that a possible synergism may existbetween both metallic atoms as they are located in closeproximity The possible union effect can be reflected in thechemical performances of both metals which differ fromthe chemical behavior of each of the metallic componentsalone Finally it is worth mentioning that sol-gel method waschosen because nanoparticle size control can be achieved bymeans of heat treatments or pH variations

2 Experimental Procedure

21 Materials and Synthesis Procedure The reagents usedin this work were Rhodium metal (999 Sigma-Aldrich)Palladium acetate (Pd(OAc)

299 Aldrich) sulfuric acid

(H2SO4 J T Baker) nitric acid (HNO

3 70 J T

Baker) ammonium hydroxide (NH4OH 28ndash30 J T

Baker) ethylenediaminetetraacetic acid (EDTA 99 Fluka)NN1015840-methylenebisacrylamide (995 Fluka) 221015840-azobis(2-methylpropionamidine) dihydrochloride (98 Fluka) acry-lamide monomer (99 Sigma-Aldrich) and distilled water

The synthesis of Pd and Rh nanoparticles was carriedout separately using an acrylamide sol-gel technique[43] The synthesis of Rh nanoparticles started dissolving

439mmoL of Rh metal in H2SO4

and distilled water(80mL) at 70∘C forming a yellowish solution Later thesolution was cooled down to ambient temperature priorto adjusting the pH ca to 7 by adding NH

4OH following

the method outlined in [44] As soon as a pH ca 7 wasreached the solution was heated up to 80∘C and 516mmoLof EDTA was added to complex the rhodium ions avoidingthe formation of agglomerates After this 211mmoL ofacrylamide monomer and 972mmoL of cross-linking agentNN1015840 methylenebisacrylamide were added to the initialsolution Finally to produce the gel 55mmoL of initiator221015840-azobis(2-methylpropionamidine)dihydrochloride wasadded to reach the polymerization [43] The pH of thesolution was constantly monitored by means of a pH meter(Thermo Scientific Orion star A221) The polymerizationof Rh was carried out at 80∘C under constant agitation Itis worth noting that all the processes were done under airatmosphere In neutral or basic conditions the time requiredto form the gel decreases considerably [26]

The synthesis of Pd nanoparticles was carried out bydissolving Pd acetate in 300mL of distilled water at 80∘CAfter this 12mL of HNO

3and H

2O2were added Later the

solution was cooled down to ambient temperature prior toadjusting the pH ca to 7 by adding NH

4OH similar to the

method used for Rh As soon as a pH ca 7 was reached thesolution was heated up to 80∘C and 102mmoL of EDTAwasaddedThis EDTA reagent is used in this process with the aimof trapping the Pdmetal ions and preventing the formation ofagglomerates until the reaction starts By doing this the Pdions remain stable during the decomposition of EDTA [43]

In order to start the polymerization 5627mmoL ofacrylamide monomer and 1297mmoL of the cross-linkerNndashN1015840 methylenebisacrylamide were added Immediately368mmoL of a chemical initiator 120572ndash1205721015840 azodiisobutyrami-dine dihydrochloride was incorporated to the solution inorder to increase interconnection velocity [43 44] Thepolymerization occurred at 80∘C in air under continuousmagnetic stirring

The method chosen to prepare the Rh and Pd xerogelsinvolves the decomposition of both gels The process beginsby heating up both gels inside a microwave oven that isfrom 80∘C to 170∘C The microwave is equipped with aninfrared temperature sensor installed in such fashion thatit can measure the temperature of the material in analysiswithin the reaction area In this process the oven increasesthe temperature steadily Once a temperature of 170∘C hasbeen reached (under an argon flux) the process to generatethe xerogel of Rh and Pd lasts 30min and 3min respectivelyThe produced Rh and Pd xerogels were pulverized usingan agate electric mortar (Retsch GmbH-Mortar Grinder RM100) and heated at 1000 plusmn 4∘C for 2 h under an argon flux

22 Characterization The crystalline structure of the mate-rials produced was determined by XRD using a BrukerAXS D8-advance diffractometer with Cu K120572 radiation (120582 =15406 A) 40KV and 30mA equipped with a graphitediffracted beammonochromatorThe diffractogram patternswere collected at room temperature over the 2120579 range of 25∘






4)2SO4(PDF 01-78-0579) and





3)2(H2O)2(PDF 01-085-












2O





4NO3









10 20 30 40 50

In

tens

ity (a

u)

Gel

Xerogel


2Θ-scale

(a)

Inte

nsity

(au

)

Gel

Xerogel


10 20 30 40 50 60 70


2Θ-scale

(b)

30 40 50 60 70 80 90

Inte

nsity

(au

)


larr

larrlarrlarr

larr

larr

2Θ-scale

(c)






200 400 600 8000

20

40

60

80

100 Acrylamide

Wei

ght (

)

GelXerogel

RhT = 230 241∘C

T = 378 328∘C

T = 597 620∘C

(NH4)SO4

Rh2O3

Temperature (∘C)

(a)

0

20

40

60

80

100

Wei

ght (

)

Temperature (∘C)0 200 400 600 800

PdO

1000

GelXerogel

PdT = 293∘C

T = 400∘C

T = 148∘C

T = 260∘C

T = 505∘CT = 557∘C

NH4NO3

NH4NO3

Pd(NO3)2(H2O)2

(b)





Gelweight loss ()





Gelweight loss ()



(a) (b)




(a)

O

Rh

Pd

Spectrum 1

0 2 4 6 8 10 12 14 16 18 20(keV)


(b)



Pd La1 Rh La1 O Ka1



3RhCl6from L-





4 Conclusion



(a)

50 60 70 80 90 100 1100

2

4

6

8

10

12

14

Dist

ribut

ion

Particle size (nm)


(b)

(c)



Acknowledgments



References















































2

and Y-Doped ZrO2





O3











CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials








4)2SO4(PDF 01-78-0579) and





3)2(H2O)2(PDF 01-085-












2O





4NO3









10 20 30 40 50

In

tens

ity (a

u)

Gel

Xerogel


2Θ-scale

(a)

Inte

nsity

(au

)

Gel

Xerogel


10 20 30 40 50 60 70


2Θ-scale

(b)

30 40 50 60 70 80 90

Inte

nsity

(au

)


larr

larrlarrlarr

larr

larr

2Θ-scale

(c)






200 400 600 8000

20

40

60

80

100 Acrylamide

Wei

ght (

)

GelXerogel

RhT = 230 241∘C

T = 378 328∘C

T = 597 620∘C

(NH4)SO4

Rh2O3

Temperature (∘C)

(a)

0

20

40

60

80

100

Wei

ght (

)

Temperature (∘C)0 200 400 600 800

PdO

1000

GelXerogel

PdT = 293∘C

T = 400∘C

T = 148∘C

T = 260∘C

T = 505∘CT = 557∘C

NH4NO3

NH4NO3

Pd(NO3)2(H2O)2

(b)





Gelweight loss ()





Gelweight loss ()



(a) (b)




(a)

O

Rh

Pd

Spectrum 1

0 2 4 6 8 10 12 14 16 18 20(keV)


(b)



Pd La1 Rh La1 O Ka1



3RhCl6from L-





4 Conclusion



(a)

50 60 70 80 90 100 1100

2

4

6

8

10

12

14

Dist

ribut

ion

Particle size (nm)


(b)

(c)



Acknowledgments



References















































2

and Y-Doped ZrO2





O3











CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




10 20 30 40 50

In

tens

ity (a

u)

Gel

Xerogel


2Θ-scale

(a)

Inte

nsity

(au

)

Gel

Xerogel


10 20 30 40 50 60 70


2Θ-scale

(b)

30 40 50 60 70 80 90

Inte

nsity

(au

)


larr

larrlarrlarr

larr

larr

2Θ-scale

(c)






200 400 600 8000

20

40

60

80

100 Acrylamide

Wei

ght (

)

GelXerogel

RhT = 230 241∘C

T = 378 328∘C

T = 597 620∘C

(NH4)SO4

Rh2O3

Temperature (∘C)

(a)

0

20

40

60

80

100

Wei

ght (

)

Temperature (∘C)0 200 400 600 800

PdO

1000

GelXerogel

PdT = 293∘C

T = 400∘C

T = 148∘C

T = 260∘C

T = 505∘CT = 557∘C

NH4NO3

NH4NO3

Pd(NO3)2(H2O)2

(b)





Gelweight loss ()





Gelweight loss ()



(a) (b)




(a)

O

Rh

Pd

Spectrum 1

0 2 4 6 8 10 12 14 16 18 20(keV)


(b)



Pd La1 Rh La1 O Ka1



3RhCl6from L-





4 Conclusion



(a)

50 60 70 80 90 100 1100

2

4

6

8

10

12

14

Dist

ribut

ion

Particle size (nm)


(b)

(c)



Acknowledgments



References















































2

and Y-Doped ZrO2





O3











CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




200 400 600 8000

20

40

60

80

100 Acrylamide

Wei

ght (

)

GelXerogel

RhT = 230 241∘C

T = 378 328∘C

T = 597 620∘C

(NH4)SO4

Rh2O3

Temperature (∘C)

(a)

0

20

40

60

80

100

Wei

ght (

)

Temperature (∘C)0 200 400 600 800

PdO

1000

GelXerogel

PdT = 293∘C

T = 400∘C

T = 148∘C

T = 260∘C

T = 505∘CT = 557∘C

NH4NO3

NH4NO3

Pd(NO3)2(H2O)2

(b)





Gelweight loss ()





Gelweight loss ()



(a) (b)




(a)

O

Rh

Pd

Spectrum 1

0 2 4 6 8 10 12 14 16 18 20(keV)


(b)



Pd La1 Rh La1 O Ka1



3RhCl6from L-





4 Conclusion



(a)

50 60 70 80 90 100 1100

2

4

6

8

10

12

14

Dist

ribut

ion

Particle size (nm)


(b)

(c)



Acknowledgments



References















































2

and Y-Doped ZrO2





O3











CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





(a)

O

Rh

Pd

Spectrum 1

0 2 4 6 8 10 12 14 16 18 20(keV)


(b)



Pd La1 Rh La1 O Ka1



3RhCl6from L-





4 Conclusion



(a)

50 60 70 80 90 100 1100

2

4

6

8

10

12

14

Dist

ribut

ion

Particle size (nm)


(b)

(c)



Acknowledgments



References















































2

and Y-Doped ZrO2





O3











CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a)

50 60 70 80 90 100 1100

2

4

6

8

10

12

14

Dist

ribut

ion

Particle size (nm)


(b)

(c)



Acknowledgments



References















































2

and Y-Doped ZrO2





O3











CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials













































2

and Y-Doped ZrO2





O3











CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



research article synthesis by microwaves of bimetallic...

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