1. INTRODUCTION:

Metallic nanoparticles are of interest in the fields of material science, microelectronics, pharmaceuticals and catalysis^1-8. Novel catalytic and optical properties of metal nanoparticles have been correlated with their high specific area and size effect. Chemical reduction of metal ions in the bulk yields non-homogeneous, and larger particles owing to coagulation. In some of the chemical methods, stabilizers of diverse structural characteristics have been used for preparing desired size metal nanoparticles^9,10. Water-in-oil microemulsions consist of nano-sized aqueous droplets encompassed by a monolayer of surfactant molecules dispersed in a non-polar organic medium. Chemical reduction of metal ions within the aqueous core of the water-in-oil microemulsion, do not lead to aggregation of metal atoms and thus lead to the formation of metal nanoparticles.

The drop size of microemulsion can be manipulated by carefully designing its composition, therefore, it offers the scope of synthesizing monodisperse nanoparticles. Shape and size of the nanomaterial is a function of initial concentration of metal ions as well as [water]/[surfactant] (=W) ratio in the microemulsion. The catalytic activity of metals also depends on the nature of support material used ¹¹.

For low temperature oxidation of CO to CO₂ various approaches have been adopted including those utilizing noble metal particles ¹². The work reported here was undertaken with an objective to generate metal nanoparticles which could be most effective in catalytically oxidizing CO to CO₂ at as low temperature as possible. Conversion of CO to CO₂ is one of the key reaction in three-way automotive exhaust catalyst system. In the present work, platinum nanoparticles have been synthesized by the reduction of Pt⁺⁴ ions in water-in-oil microemulsion consisting of sodium bis (2-ethylhexyl)sulphosuccinate (AOT)+cyclohexane+water. Effects of [water]/[surfactant] (=W) ratio and polymer (as stabilizer) in the microemulsion on the particle size and efficiency of the as- synthesized catalyst has been investigated.

2. MATERIAL AND METHODS:

2.1 Material:

Cyclohexane (C₆H₁₂) (Merck), sodium bis (2-ethylhexyl)sulphosuccinate (AOT) (Sigma), poly(ethylene glycol)(PEG)(Aldrich)(MW 10,000), hydrazine hydrate(NH₂.NH₂.H₂O) (Qualigens), H₂[PtCl₆].6H₂O(Sigma), Alumina(Al₂O₃) (Puralox/Condea) and tetrahydrofuran(THF)(Merck) were used as such. Double distilled water (specific conductance= 0.2x10^-5 S.cm^-1) was used in the preparation of microemulsions.

2.2 Methods:

2.2.1: Preparation of metal nanoparticles:

For preparing water-in-oil microemulsion, an aqueous solution of H₂[PtCl₆].6H₂O was slowly added to a mixture of surfactant (AOT) and cyclohexane. The mixture was homogenized by sonicating for 30 minutes. The final composition of the microemulsion was: H₂[PtCl₆].6H₂O = 0.01M; AOT: 0.12M and [water]/[AOT] (W=5,15). Reduction of metal ions was achieved by adding, drop-wise, excess of NH₂.NH₂.H₂O into the microemulsion. Microemulsion was, then broken by a slow addition of THF. The platinum particles, thus released, were directly adsorbed onto the calculated amount of alumina powder giving platinum+alumina composite containing 2% platinum (w/w). The metal+ alumina mixture was calcined at 500⁰C for two hrs and then cooled to the room temperature. The final product was stored in a moisture-free atmosphere before use.

2.2.2 TEM Analysis:

Transmission electron microscope (TEM) images of as-synthesized Pt+alumina composite were obtained using a JEOL 200 CX instrument (beam current: 20uA; accelerating voltage: 100 KV; magnification: 100K)

2.2.3 XRD Analysis:

X-ray diffraction (XRD) pattern of the as-synthesized nanomaterial was recorded using a diractometer (Philips, PW 1710 BASED; Cu.Kα radiation wave length:

1.5406A⁰ ) equipped with PC-APO diffraction software.

2.2.4. Catalytic Studies:

For studying the efficiency of the as-synthesized Pt+alumina composite catalyst oxidation of CO to CO₂ was carried out in a specially designed vertical quartz tube reactor (Fig.1.). 0.1 gm of the catalyst sample was loaded over the porous frit in the middle of the reactor. A regular flow of mixture of gases: CO(1%); O₂ (5%) and Argon (94%) was maintained through the catalytic reactor. The gas composition at the reactor outlet as a function of temperature over 50-500⁰C, was evaluated using a quadrupole mass spectrometer (Balzers Quadstar 421).

Fig.1. Vertical quartz tube reactor

3. RESULTS AND DISCUSSION:

3.1. TEM Analysis:

TEM micrographs of as-synthesized nanomaterial composite are depicted in Fig.2. It is evident that the particle size (Table-1) increases with the increase in [water]/[surfactant] (=W) ratio in water-in-oil microemulsion. It is obvious since on increasing the value of W, the aqueous core size of water-in-oil microemulsion is enlarged allowing larger number of metal atoms to aggregate therein.

Fig. 2. Transmission electron micrograph (TEM) images of platinum nanoparticles synthesized in water-in-oil microemulsion (AOT+water+cyclohexane): (a) W=5; (b) W=10; (c) W=15 and (d) freshly prepared platinum nanoparticles dispersed in microemulsion (AOT+water+cyclohexane; W=10)

3.2. XRD Analysis:

X-ray diffraction (XRD) spectra (Fig.3) of the synthesized platinum+alumina composite shows three reflections at 2θ =35.9⁰, 38.5⁰ and 46.0⁰. Whereas, reflections at 2θ = 38.5⁰and 46.0⁰ correspond to planes (111) and (200) respectively of the face centered cubic (fcc) crystal structure of platinum, the reflection at 2θ = 35.9⁰ is assigned to (002) plane of body centered tetragonal lattice. This suggests the presence of mixed phase in the synthesized platinum nanoparticles.

The particle size of the as-synthesized nanomaterial is obtained from XRD spectra using Sherer formula-

d = 0.89 λ / β.Cosθ (3.1)

Where d = particle diameter; θ = Angle of diffraction for the selected crystal plane; λ = wave length of X-ray (1.5406 A⁰); and β = half maximum line width. The values of particle diameter thus calculated for as-synthesized metal nanoparticles are presented in the parentheses in table-1.

Fig.3. X-ray diffraction (XRD) spectra of platinum nanoparticles synthesized in water-in- oil microemulsion (AOT+water+cyclohexane; W=10); adsorbed on alumina and preheated at 500 ⁰C.

Table:1 Effect of [water]/[surfactant] (=W) ratio in water-in-oil microemulsion on the size of as-synthesized platinum (2%)+alumina composite particles preheated at 500⁰C. (values in parenthesis are those calculated from Sherer Formula)

[water]/[Surfactant] (=W)	Particle diameter (nm)
5	5-6 (6.5)
10	8-10 (10.8)
15	12-15(16.0)
Preshly prepared platinum particles dispersed in microemulsion (W=10)	3-4 (3.2)

3.3. Catalytic reaction studies:

Catalytic oxidation of CO to CO₂ was carried out, over 50-500⁰C, in a vertical quatz reactor, described in section: (2.2.4). Plots of ion current (arbitrary units) for CO and CO₂, with mass numbers 24 and 44, respectively, as a function of reaction temperature from mass spectral analysis are given in Fig.4(a,b,c). The magnitude of ion current is proportional to the amount of CO or CO₂. The catalytic activity of the as-synthesized nanomaterial is judged in term of the temperature at which half of the initial amount of CO converts to CO₂. Lower the temperature required for converting CO to CO₂, higher is the catalytic activity.

In case of pure alumina (Al₂O₃) powder used as a catalyst, conversion of CO to CO₂ completes at 300⁰C (Fig. 4a; down-ramp). But when alumina + Pt(2%) composite was the catalyst (Pt nanoparticles prepared without using polymer (PEG) as stabilizer in microemulsion), such temperature comes down to 225⁰C (Fig. 4b; down ramp). The above conversion temperature is further lowered to 200⁰C, when alumina + Pt(2%) composite catalyst contained platinum nanoparticles synthesized in microemulsion stabilized by the polymer, PEG (1% w/v).

Fig.4(a). Plot of ion current (au) (in Mass spectra) versus temperature (⁰C) Catalyst =pure alumina; T_min:300⁰C (down ramp).

Fig.4(b). Plot of ion current(au) (in Mass spectra) versus Temperature(⁰C): Catalyst=Pt(2%)+Alumina (Pt nanoparticles prepared without using the polymer as stabilizer in microemulsion) ; T_min =225⁰C(down ramp).

Fig.4(c) Plot of ion current (au) (in Mass spectra) versus Temperature (⁰C): Catalyst=Pt(2%)+Alumina (Pt nanoparticles prepared using PEG(1% )(w/v) in water-in-oil microemulsion (AOT+water+cyclohexane; W=10) ; T_min =200⁰C (down ramp)

4. CONCLUSION:

Platinum nanoparticles have been synthesized using water-in-oil microemulsion by the reduction of Pt⁺⁴ ions with hydrazine hydrate (NH₂.NH₂.H₂O). The as-synthesized nanomaterial has been characterized through TEM and XRD analysis. Platinum nanoparticles are present in mixed phase comprising both face centered cubic as well as body centered tetragonal crystal lattices. Effects of [water]/[surfactant] ratio and the presence of polymer, PEG, in the microemulsion on the size of metal nanoparticles and catalytic activity for CO to CO₂ conversion has been studied.

5. ACKNOWLEDGEMENT:

The authors are grateful to the University Grant Commission (UGC), New Delhi, for providing financial help in the form of a research project for carrying out this work.

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Received on 16.03.2011 Modified on 02.04.2011

Asian J. Research Chem. 4(6): June, 2011; Page 1005-1008