Materials Letters 64 (2010) 843–845

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Materials Letters

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Synthesis of platinum nanoparticles on carbon aerogel by ambient pressuredrying method

Rashmi Singh ⁎, R.K. Khardekar, D.K. Kohli, M.K. Singh, Himanshu Srivastava, P. K GuptaTarget Laboratory, Laser Materials Development and Devices Division, Raja Ramanna Centre for Advanced Technology, Indore 452 013, India

⁎ Corresponding author. Tel.: +91 0731 2442619.E-mail address: rashmi@rrcat.gov.in (R. Singh).

0167-577X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.matlet.2010.01.035

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 October 2009Accepted 12 January 2010Available online 20 January 2010

Keywords:Nano-compositeSol–gel preparationElectron microscopyX-ray techniques

Platinum nanoparticles were successfully synthesized on porous carbon aerogel with narrow pore sizedistribution by ambient pressure drying method. Platinum doped carbon aerogels were synthesized by sol–gelpolymerization of resorcinol with furfural in non aqueous medium followed by ambient pressure drying andpyrolysis of organic gel. These sampleswere characterized byScanningElectronMicroscopy (SEM), TransmissionElectronMicroscopy (TEM), X-ray Diffraction (XRD) and Flow Chemisorption. TEM and XRD results showed thatsize of platinum nanoparticle varies between 2 and 5 nm depending on platinum loading and pyrolysistemperature. Hydrogen pulsed chemisorption showed 26.5% dispersion of platinum nanoparticles in carbonaerogel.

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1. Introduction

Highly dispersed platinum nanoparticles on porous carbonsupport have many potential applications in the fields of fuel cell[1], catalyst for H2/D2 isotope exchange process [2] and hydrogenstorage material [3]. Different porous carbons including carbonaerogel with controlled texture have been investigated by manygroups as supporting material for platinum and other metalnanoparticles [4]. Well dispersed platinum nanoparticles have beensynthesized on carbon aerogel support [1,5,6] using differentmethodslike wet impregnation [7], microemulsion [8], supercritical fluid [9],polyol process [10] and sputtering methods [11].

Carbon aerogels first patented by Pekala [12] have fundamentalproperties such as high conductivity, high mesoporosity, relativelysmall degree of micro porosity, and high surface area.

These properties make carbon aerogel suitable for catalyst supports.Generally carbon aerogels are prepared by sol–gel process of formal-dehyde with a phenol derivative in aqueous medium followed by lowtemperature super critical drying. Supercritical drying is expensive andtime consuming step so alternate methods such as ambient pressuredrying [13] and freeze drying [14] are also used.

An important feature of the above mentioned sol–gel based carbonaerogel synthesis is thatmetal-doped aerogels can be easily prepared byadding solublemetal salt to the initial solution. The addedmetal salt alsoacts as catalyst for gelationprocess.Metal ions are trappedwithin the gelstructure. Further temperature treatment in inert atmosphere formsmetal nanoparticles, which are distributed throughout the porosity of

the carbon phase. The technique is very versatile and can be used forproducing different metal doped carbon aerogels which are emergentmaterial in the field of heterogeneous catalysis.

We report a new method for preparation of platinum nanoparticleson carbon aerogel matrix via ambient pressure drying technique.Organic gel was synthesized from resorcinol, furfural, isopropyl alcohol(IPA) and nonionic surfactant. Hexachloroplatinic acid solution wasadded as platinum precursor to the sol, prior to polymerization, whichassures uniform distribution of metal ions in the aerogel matrix. Themethod involves use of isopropanol (IPA) solvent and surfactant whichcontributes to low surface tension of solvent. Low surface tension ofsolvent helps in reducing collapse of the pore structure during ambientpressure drying and preserve porosity in the powder. Additionally IPAreduces platinum salt to zero valence state so platinum reduction stepwas not required. In this paper we also studied and discussed effect ofplatinum loadingandpyrolysis temperature onparticle sizeof platinum.

2. Experimental

2.1. Chemicals and materials

The starting materials were Isopropanol (CH3CHOHCH3, ≥ 99%,Merck), Resorcinol (C6H6O2, ≥ 99%, Merck), Furfuraldehyde (C5H4O2,99%, Qualigens), Tween80 (Polyoxyethylene sorbitan monooleate,Merck) and Hydrogen hexachloroplatinate(IV) hydrate (37.5% Pt,H2PtCl6·xH2O, International Laboratory).

2.2. Process

Resorcinol and furfuraldehyde were taken in 1:2 molar ratios anddissolved in IPA. Tween80 was added to the solution keeping molar

Fig. 1. SEM image of carbon aerogel with 2.5% platinum.

844 R. Singh et al. / Materials Letters 64 (2010) 843–845

ratio of resorcinol to surfactant equals 25:1. After 10 min stirring po-tassium hydroxide was added as base catalyst for initial particleformation, and solution was kept at 60 °C for 2 h in oven. The metalprecursor solution obtained by dissolving hydrogen hexachloroplati-nate in isopropanol was then added to the initial solution. Resorcinol-Furfural (RF) solution turned transparent green and immediatelyblack on adding hexachloroplatinic acid solution. The solution waskept in a sealed vial and kept at 60 °C for 65 h. Solid black gel wasformed within 30 min by polycondensation reactions between res-orcinol and furfuraldehyde. Additional 65 hour curing was done forcomplete gelation and gel strengthening. The aged alcogels wereremoved from oven and kept open for drying in ambient conditions.The organic gel was fired inmuffle furnace employing a heating rate of1.5 °C/min in argon atmospheres at different temperatures.

Fig. 2. (a–f) TEM images of platinum nanoparticles on carbon aerogel synthesized under diff500 °C and f) distribution of platinum particles for the above mentioned conditions obtaine

2.3. Characterizations

Surface morphology of the carbon aerogel was observed by SEM(Philips XL30CP). Size and distribution of platinum nanoparticles dis-persed on carbon aerogel were analyzed by TEM Philips CM200. Driedpowder was ultrasonically dispersed in water and deposited on coppergrid coated with carbon film for TEM measurements. X-ray diffraction(XRD) measurements were performed at room temperature on samplesemploying a RIGAKU (Geigerflex) diffractometer operating with Cu-Kαradiation. Metal dispersion was determined from chemisorption mea-surements carried out in TPDRO1100 (Thermo) system. Samples wereout gassed under argonflow. The adsorption stoichiometry considered tocalculate platinum dispersion was taken as 2.

3. Results and discussion

Bulk density of carbon platinum aerogel was 0.225 gm/cc,calculated by simple weight volume measurements. Volume reduc-tion of gel was nearly 10% during ambient pressure drying. Furtherreduction in volume and weight was observed after pyrolysing thepowder in inert atmosphere. Scanning electron micrograph of thecarbon aerogel powder with 2.5% platinum loading and pyrolysistemperature of 500 °C is seen in Fig. 1. Micrograph of the carbonaerogel powder shows good porosity having wide range of pores.Surface morphology of these aerogels was composed of fused micro-bead particles. Interlinking of micelles formed in the presences ofsurfactant is responsible for such microbead structure.

TEM images of the powder are shown in Fig 2(a–f). TEM mea-surements reveal that the size of platinum nanoparticle depends onpercentage of platinum loading and pyrolysis temperature. Size distri-bution of platinum particle (Fig. 2f) is estimated from the histogramsobtained by TEM images. Narrow particle size distribution having peak

erent conditions: a) 5% Pt 300 °C b) 5% Pt 400 °C c)5% Pt 500 °C d)2.5% Pt 400 °C e)5%Ptd from histograms.

Fig. 3. XRD spectra of platinum nanoparticles on carbon aerogel, synthesized underdifferent conditions.

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at 2–3 nmwas observed for 2.5% platinum loading and 400 °C pyrolysistemperature (Fig. 2d). On further increasing the pyrolysis temperatureto 500 °C, particles do not grow in size but start agglomerating (Fig. 2e).Effect of pyrolysis temperature on size of platinum nanoparticles for 5%platinum loading is shown in Fig. 2a, b andc. In this case, good dispersionwith peak at 2–3 nmwas observed at 300 °C pyrolysis temperature andagglomerates of 20–25 nm were seen at higher pyrolysis temperatures(400 °C–500 °C).

Fig.3 shows XRD diffraction spectra of the platinum nanoparticlessynthesized under different conditions. The major diffraction peaks at2θ=39.6, 46.1, 67.4, 81.2 and 85.6° corresponds to the reflectionplanes of (111), (200), (220), (311) and (222) respectively, which areconsistent with the face-centered cubic (fcc) structure of platinum(JCPDS Card No. 04-0802). The average particle size can be calculatedusing Scherrer equation on the basis of the Pt (111) peak.

D =0:9λκα

B2θcosθ:

Here , D is the average particle size of the Pt cluster, λκα the wave-length of the incident X-ray (1.5412A) , 2θ is the Bragg diffraction angle

for peak position, B2θ is the full width (in radians) at half maximum(FWHM). The typical character of crystalline platinum fcc phasedemonstrates a successful reduction of platinum precursor to themetallic form. Particle size calculated using Scherrer equation is in closeagreement with the results obtained from TEM analysis. Averageparticle size was 2.9 nm for 2.5% platinum loading and 400 °C pyrolysistemperature. Particle sizegrows to around5 nmfor 5%platinum loadingand 500 °C pyrolysis temperature. These results indicate that percent-age loading of platinum and temperature are the key factors for con-trolling the size of nanoparticle.

Platinum metal dispersion and particle diameter calculated byHydrogen gas chemisorption was 26.5% and 3.84 nm respectively forsamples synthesized at 2.5% platinum loading and 500 °C pyrolyzingtemperature.

4. Conclusion

Highly dispersed platinum nanoparticles on porous carbon aerogelsamples have been synthesized by the sol–gel method via open airdrying and carbonization. Study with TEM reveals well dispersedplatinum nanoparticles on carbon aerogel support, having size in therange of 2–3 nm diameter is obtained for 2.5% platinum loading and500 °C pyrolysis temperature. Sample with 5% platinum loading alsoshows well dispersed platinum particles but at lower pyrolysis tem-perature (300 °C) which forms large agglomerates at 500 °C pyrolysistemperature. Additionally platinum nanoparticles thus obtained are inreduced state without employing any hydrogen reduction steps. Thesynthesis method proposed in this paper is simple and suitable for massproduction and may have great potential for preparing other metalloaded carbon aerogels.

Acknowledgements

We thank Dr. Gurvinderjit Singh of Laser Materials Division, CAT,for providing XRD data on our samples. We also thank Mrs. PragyaTiwari for SEM results.

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