Synthesis of Highly Active and Thermally Stable Nanostructured Pt

Jan 23, 2013 - Novel and intriguing one-pot in situ method for the preparation of nanostructured Pt–clay materials under simple conditions is report...
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Synthesis of Highly Active and Thermally Stable Nanostructured Pt/ Clay Materials by Clay-Mediated in Situ Reduction Dharmesh Varade and Kazutoshi Haraguchi* Material Chemistry Laboratory, Kawamura Institute of Chemical Research, 631 Sakado, Sakura, Chiba 285-0078, Japan S Supporting Information *

ABSTRACT: Novel and intriguing one-pot in situ method for the preparation of nanostructured Pt−clay materials under simple conditions is reported. In this synthesis, an inorganic clay mineral such as synthetic hectorite (“Laponite XLG”) or natural montmorillonite (“Kunipia F”) serves as a mild and effective reducing agent for Pt ions, which is uncommon for such a clay system, and also acts as an outstanding stabilizer for the resulting Pt nanoparticles. In aqueous solution, exfoliated colloidal clay platelets forms complex with Pt ions in the initial stage of mixing. Devoid of any organic dispersants or external reducing agents, subsequently, the Pt nanoparticles (3−6 nm) generated by clay-assisted in situ reduction of Pt ions successfully anchored onto the clay nanoplatelets. The Pt−clay material features a very high surface area (312 m2 g−1) and has excellent catalytic activity, as was kinetically evaluated via the reduction of 4-nitrophenol with NaBH4. After drying, this remarkably stable nanocomposite is completely redispersible in water and displays extreme thermal stability (up to 500 °C). On the basis of these results, this synthetic strategy is anticipated to be a very simple, economical, and green approach for the synthesis of nanostructured Pt−clay materials.

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INTRODUCTION Pt nanoparticles attract significant attention in many fields, including catalysis, electronics, and biological technologies, owing to their unique size-dependent structures and properties.1−5 Great progress has been made in the preparation and characterization of Pt nanoparticles using diverse methodologies, and a variety of significant applications have been demonstrated. The development of novel Pt-based nanocomposites for highly functional materials with advanced properties and improved performance is a continuously expanding research topic. Efforts have already been made to attach Pt nanoparticles onto 1D or 2D supporting materials such as carbon nanotubes,6,7 carbon nanofibers,8 graphene,9 silica nanoparticles,10 and inorganic clay11−13 in order to construct new hybrids. Although the fabrication of these nanocomposites was successful, most approaches involve complicated modifications using organic surfactants or polymeric stabilizers.6−24 However, the addition of organic dispersants could have negative consequences such as reduced Pt activity and/or unwanted organic residues during various applications. Clay is low-cost inorganic mineral salt with a layered structure. They are of boundless interest as functional materials in the scientific field owing to their attractive properties such as appreciable surface area, ordered structure, intercalation abilities, and high exchange capacity.25−27 The clay used, i.e., Laponite XLG, which is a synthetic layered magnesium silicate (hectorite) with a lamellar crystal structure, swells when dispersed in water and gradually cleaves into discrete disklike © 2013 American Chemical Society

nanoparticles that have a negative surface charge and small positive charge at rims.28−31 Laponite dispersions have been extensively investigated both as a model for general disklike colloidal clay suspensions32,33 and as a unit for building new types of polymer nanocomposites34,35 and nanocomposite gels36−40 that have excellent optical, mechanical, and/or swelling−deswelling properties. To date, there have been several reports on composites comprising noble metal nanoparticles with different clays or layered minerals.41−49 In one such report, Datta et al. used organically modified aminoclay [i.e., CH2CH2NH2Si8Mg6O16(OH)4] to prepare composites with various metal nanoparticles.41 Additionally, Zhang et al. synthesized clay−3-aminopropyltriethoxyilane (APTES)−gold nanocomposites, in which the APTES acts as the linkage.42 Silver nanoparticles prepared by either photoinduced,43 NaBH4 reduction,44−46 or wormlike micelles24 methods have been stabilized in a clay dispersion. Moreover, Pt and Au nanoparticles embedded in clay or layered double hydroxide have been used for catalytic applications.47−49 However, the majority of the protocols used thus far involve intricate multistep syntheses, elevated temperatures, or stabilizers such as polymers and organic ligands to inhibit particle agglomeration. To the best of our knowledge, nanostructured Pt−clay materials prepared in situ without any organic modifications at Received: November 11, 2012 Revised: December 29, 2012 Published: January 23, 2013 1977

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2200TFE, JEOL) operating at 200 kV. The samples were prepared by depositing a drop of the dilute sample solution on carbon-coated Cu grids and drying them at room temperature. Energy-dispersive X-ray spectroscopy (EDS) was performed using a scanning transmission electron microscopy (STEM) detector fitted on a JEOL JEM2200TFE instrument operating at 200 kV. XRD patterns were obtained using a Rigaku SmartLab X-ray diffractometer with monochromated Cu Kα radiation (40 kV, 100 mA). X-ray photoelectron spectra (XPS) were recorded on an ESCALab MKII X-ray photoelectron spectrometer fitted with an Mg Kα radiation excitation source. Nitrogen adsorption−desorption data were obtained using a BELSORP-mini II (BEL JAPAN Inc.) operated at 77 K. Prior to measurement, the sample was added to the measurement cell, which was placed in a drying machine and heated at 80 °C overnight. After drying, helium gas was added to the cell.

room temperature have not been reported despite their potentially intriguing and cutting-edge properties. In this paper, we propose a new, versatile, and economical route to synthesize remarkably functional Pt nanocomposites using clay minerals. Moderately small and uniformely dispersed Pt nanoparticles can be prepared in an aqueous dispersion of exfoliated clay platelets without any external reducing agents or organic modifiers. This is the first example of utilizing clay to synthesize (in situ) highly active Pt nanoparticles at room temperature. Furthermore, the potential applications of these functional nanostructured Pt−clay materials were confirmed by investigating their catalytic activity.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION The goal of the proposed synthesis is the preparation of uniform and monodispersed Pt nanoparticles that are highly functional and have high thermal and dispersion stability; this is achieved by fabricating a nanocomposite with layered inorganic clay minerals. The structure of the clay is shown in Figure 1. Laponite swells when dispersed in water and gradually cleaves into

Materials. Inorganic clay used in this study, i.e., synthetic “hectorite: Laponite XLG” ([Mg5.34Li0.66Si8O20(OH)4]Na0.66 with a cation exchange capacity of 104 mequiv/100 g), was purchased from Rockwood, Ltd., UK, and used after being washed and vacuum-dried. Natural sodium montmorillonite, i.e., “Kunipia F” (Kunimine Industry Co., Japan), was used after being purified, as described in the next section. Analytical-grade potassium tetrachloroplatinate(II) (K2PtCl4), 4-nitrophenol (C6H5NO3), platinum black, and NaBH4 were purchased from Wako Pure Chemical Industries, Japan. Amorphous silicon dioxide (Snowtexs-20, silica) was kindly provided as a 20 wt % dispersion (particle size: 20 nm) by Nissan Chemical Industries, Ltd., Tokyo, Japan. All the chemicals were used as supplied. Ultrapure water supplied by a PURIC-MX system (Organo Co., Japan) was used for all the experiments. Preparation of Pt−Clay Materials. A typical experimental procedure for preparing the nanostructured Pt−clay materials is described. Clay dispersion was prepared by swelling the clay (0.2 g) in deionized water (10 g) at 40 °C for 60 min. The resultant clay dispersion (2 wt %) was serially diluted (1 g; 0.2, 0.5, and 1 wt %) and followed by the addition of aged K2PtCl4 solution (50 μL; 5 wt %) to each test solution with continuous stirring for 2 min. In some instances, different amounts of K2PtCl4 (i.e., 20, 75, 100, or 200 μL; 5 wt %) were used at a fixed clay concentration (1 wt %). The mixed solutions containing clay and Pt species were maintained in the dark under static conditions at room temperature (25 ± 1 °C) or, in some cases, at 60 ± 1 °C. The formation of Pt nanoparticles was indicated by a color change in the reaction solution from light brown-yellow to opaque black within 24 h of mixing. On the other hand, sodium montmorillonite (Kunipia F) was dispersed in water (2 wt %) by stirring for 5 h. This dispersion, which was a turbid brown liquid, was allowed to stand without disturbance for 2 weeks. The larger particles (∼50%) settled, resulting in the supernatant being more translucent (dynamic light scattering (DLS) particle size: ∼300 nm) than the original dispersion. This supernatant was carefully separated and used for the preparation of Pt nanoparticles via the procedure described above. DLS also reveals that the Laponite XLG dispersion contains ∼30 nm particles, which is in good agreement with the results of TEM measurements for a previously reported XLG/polymer nanocomposite gel.29 Catalytic Reduction of 4-Nitrophenol. A 0.5 mL sample of sodium borohydride solution (60 mmol/L) was added to 2.5 mL of 4nitrophenol solution (0.12 mmol/L) in a quartz cuvette. Subsequently, 0.01 g of as-prepared Pt−clay material was added. Immediately after adding the composite particles, UV spectra of the sample were recorded at 1 min intervals in the range 300−500 nm at 25 °C. For comparison, similar measurements were also performed with clay alone and Pt black in place of the Pt−clay material. Characterization. UV−vis absorption spectra of the Pt nanoparticles in the clay dispersion were acquired in a 1 mm quartz cuvette at room temperature using a Hitachi U-4100 UV−vis double-beam spectrometer. For the reference blank, a solution with the same composition as the sample but without the added metal precursor was used. The morphology of the nanoparticles was examined using a highresolution field-emission transmission electron microscope (JEM-

Figure 1. Structure of inorganic clay “hectorite: Laponite XLG”.

discrete disklike particles that are ∼30 nm in diameter and 1 nm in thickness.29 The shape of the clay particles is a result of the packing of the layers, which features a central sheet of O2− and OH− ions defining octahedral sites that are occupied by Mg2+ ions. This central sheet is sandwiched between two inverted sheets of tetrahedral silicates. Thus, the outer surface of these layers comprises oxygen atoms that are involved in siloxane bonds. Hydroxyl groups are present on the edges of the particles. Depending on the pH of the suspension, the edges of the clay particle can be positively charged. In synthetic Laponite, the substitution of Li+ ions for Mg2+ ions is the source of negative charge. Dispersions of these inorganic particles were prepared by adding the powder to water with mild mixing at 40 °C for 60 min. The resulting aqueous clay dispersion, which is composed of exfoliated clay particles, is homogeneous and transparent and has a fairly high pH (10.2) and negative zeta potential (−37.1 mV). Scheme 1 illustrates the procedure for the preparation of nanostructured Pt−clay materials. First, a stable aqueous 1978

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indicate that this clay concentration is sufficient for the reduction of the Pt species but is too low to stabilize the Pt material. Accordingly, with clay concentrations higher than 0.5 wt %, the resultant Pt nanoparticles containing dispersions were completely stable for weeks under static conditions at room temperature or above (e.g., 50 °C), and the dispersions did not aggregate or precipitate. The synthesized Pt−clay material contained 0.25 wt % Pt in a typical case of 1 wt % clay and 5 wt % K2PtCl4 (50 μL). Data on the sizes and shapes of the Pt nanoparticles were obtained via TEM measurements. Figure 3a shows the typical TEM micrographs of the Pt nanoparticles obtained from a clay dispersion (1 g; 1 wt %) containing K2PtCl4 solution (50 μL; 5 wt %); it is evident that well-defined, small, spherical Pt nanoparticles were formed. The high-resolution (HR) TEM image shown in Figure 3b depicts clearly visible lattice fringes that evince the formation of crystalline Pt nanoparticles. The periodicity of the lattice is ∼0.13 nm, which coincides with the (111) d-spacing of the Pt crystal. Figure 3c displays a histogram of the particle size distribution (3−6 nm) that was derived from the TEM images by surveying more than 100 Pt particles. From the TEM micrographs of the Pt nanoparticles synthesized with different concentrations of clay and K2PtCl4, it was determined that the Pt nanoparticle sizes were almost independent of the initial concentrations of both the clay and Pt species (data not shown). To illustrate the flexibility of this approach, investigations were performed using natural montmorillonite (MMT) clay. MMT is naturally abundant and is one of the most widely used cationic clay minerals (smectite group). The TEM (Figure 4a) and HR-TEM (Figure 4b) results indicate that crystalline Pt nanoparticles were successfully prepared by analogous in situ reduction of Pt species assisted by montmorillonite clay. The periodicity of the lattice is ∼0.14 nm, which coincides with the (111) d-spacing of the Pt crystal. A histogram of the particle size distribution (2−6 nm) determined from the TEM images is shown in Figure 4c. The preparation of such small and well-dispersed Pt nanoparticles in clay without any organic moieties is a challenging goal that was successfully achieved through the protocol applied in this study. In this intriguing approach, an aqueous dispersion of clay platelets facilitates the formation of uniform Pt nanoparticles through a mild reduction process. As evidenced in the viscosity measurements (Figure S2), the clay dispersion (2 wt %) alone displays an almost constant viscosity (∼5 mPa·s), and the addition of the solution containing the Pt species results in an abrupt increase in the viscosity to almost 10 times this value (∼50 mPa·s) immediately. Despite the change in the viscosity, the transparency of the solution remained, which indicates molecular interactions between the clay platelets and Pt species. It is likely that interactions between the Pt species and silanol groups (Si−OH) on the surface of the clay instigate the reduction of the Pt species. Pt nanoparticles are then generated, and they undergo wellcontrolled growth while adhering to the clay platelets. Recently, Pd species were shown to possibly undergo reduction by silanol moieties on silica-rich substrates,50 although almost no information was provided on the role of the silanol groups. In the aqueous clay dispersion in the present study, it is feasible that the silanol groups on the surfaces of the exfoliated clay platelets induce the relatively mild in situ reduction of the Pt species. The novelty of utilizing clay materials is further supported by analogous in situ reduction of Pt species by

Scheme 1. Preparation of Nanostructured Pt−Clay Materials

dispersion consisting of exfoliated clay platelets without aggregations or sediments was prepared; then, aqueous K2PtCl4 solution was added to the dispersion with continuous stirring for 2 min. The solutions were maintained in the dark under static conditions at room temperature. The light brown-yellow solution, which was present at the inception of the reaction, eventually turned opaque black within 24 h, as shown in Figure 2a; this indicates that Pt ions are reduced to Pt in the aqueous

Figure 2. (a) Optical images showing the change in the color of the clay dispersion upon the addition of K2PtCl4 and the resulting reduction to Pt nanoparticles, assisted by the clay, to ultimately form a black dispersion. (b) UV−vis spectra for clay dispersions (1 g; 0.2, 0.5, and 2 wt %) with a fixed amount of K2PtCl4 solution (50 μL; 5 wt %) under static conditions at RT after 24 h. Inset shows optical images of the corresponding solutions.

clay dispersion. It should be noted that natural light does not affect the color change to opaque black, as shown in Figure S1 of the Supporting Information. Figure 2b displays representative UV−vis absorption spectra (clay: 0−2 wt %, K2PtCl4: 50 μL; 5 wt %); the evident evolution of the surface plasmon band between 200 and 300 nm is attributable to the formation of Pt nanoparticles.24 Visual inspection of the optical images (Figure 2b, inset) reveals that the particles have a tendency to form aggregates and settle to the bottom of the vial within 2−3 days when the clay concentration is low (0.2 wt %), even though the reduction was complete; i.e., the supernatant showed no change upon the addition of an external reducing agent. These results 1979

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Figure 3. (a) TEM micrograph of Pt nanoparticles prepared in a clay dispersion (1 g; 1 wt %) containing K2PtCl4 solution (50 μL; 5 wt %). (b) Corresponding HR-TEM image showing the lattice fringes which indicates the formation of crystalline Pt nanoparticles. (c) Particle size distribution obtained by the analysis of the TEM image shown in (a).

Figure 4. (a) TEM observations of Pt nanoparticles prepared in montmorillonite clay dispersion (0.5 wt %) containing K2PtCl4 solution (50 μL; 5 wt %) at RT. (b) Corresponding HR-TEM image, which shows the lattice fringes that indicate the formation of crystalline Pt nanoparticles. (c) Particle size distribution obtained from the analysis of the TEM image in (a).

aqueous silica nanoparticles (20 nm) with Si−OH moieties; however, in this case, the resulting Pt formed aggregates (>100 nm) (Figure S3a) and sediment in the solution (Figure S3b). These indicate that silica nanoparticles are able to reduce Pt species but are incapable of controlling the size and stabilizing the resultant Pt nanoparticles. Thus, it was revealed that exfoliated clay platelets not only interact favorably with the Pt species and subsequently promote the reduction of the Pt species but also act as a support material that suppress the aggregation and precipitation of the Pt particles, even after a prolonged time or with rigorous mechanical agitation of the Pt−clay dispersion.

When a strong reducing agent such as NaBH4 was added to the clay dispersion containing Pt ions in the initial stage of mixing, the reduction of Pt species began immediately and this reaction proceeded quickly; but the generated Pt nanoparticles were 7−18 nm in size (see TEM image in Figure S4). These observations further validate that the controlled, mild, in situ reduction of Pt species that was induced by the clay is vital to the production of stable Pt nanoparticles. In addition, the current technique offers the advantage of requiring neither vigorous experimental conditions nor the use of external reducing or stabilizing agents. Furthermore, the nanostructured Pt−clay material was successfully prepared in a one-step 1980

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reaction without the common requirements such as organic solvents, templates, or seed-mediated growth. From the XRD patterns of the Pt−clay materials (Figure 5a), it is evident that the spacing [d(100)] of the clay barely

Figure 6. (a) Optical image of the Pt−clay material obtained from Pt nanoparticles formed in a clay dispersion (1 g; 2 wt %) containing K2PtCl4 solution (50 μL; 5 wt %) that can be readily redispersed in water. (b) STEM image of the Pt−clay material shown in (a). (c) EDS mapping for the elemental analysis of the Pt−clay composite that confirms the presence of the Pt−clay hybrid material.

reaction to get completed within 3 h. However, the basic characteristics of the formed Pt nanoparticles did not change significantly, except for a slightly wider size distribution and some coalescence of nanoparticles leading to an increase in the particle size to 2−8 nm (Figure S5). The use of clay platelets as a support material for Pt nanoparticles is a promising strategy for improving the specific surface area of the Pt nanomaterials. A N2 adsorption− desorption isotherm (Figure 7) revealed BET surface areas of

Figure 5. (a) XRD patterns for the powdered (i) clay and (ii) Pt−clay. (b) XPS spectrum of Pt bonds in Pt−clay materials.

changed (d = 1.38 nm) after being loaded with the reduced Pt particles. This indicates that Pt nanoparticles were not intercalated in clay but are present at the exterior of the clay layer. Furthermore, no change in the XRD pattern was similarly observed in the case of the alumino silicate mineral and silver nanoparticles system.43 Thus, the clay sheet with Pt nanoparticles did not form regular lamellar stacks with well-defined spacing. Although the XRD pattern shows no crystalline peaks for Pt nanoparticles, probably because of small Pt,50 the XPS spectra of the nanostructured Pt−clay material (Figure 5b) provide definite evidence for the presence of reduced Pt via two principal peaks at binding energy values of 72.1 eV (4f7/2) and 76 eV (4f5/2). This suggests that the Pt species has been completely reduced to its metallic state, Pt(0).51 Despite the lack of organic component, the nanostructured Pt−clay materials demonstrated high stability in water, even after being dried to a powder (Figure 6a) and redispersed (inset optical image). Several cycles of drying and redispersion were performed to demonstrate the outstanding stability of the material, which maintained uniform particle size throughout, as confirmed from the STEM image of the dried materials shown in Figure 6b. Energy dispersive X-ray spectroscopy (EDS) elemental analysis of the dry Pt−clay material (Figure 6c) indicates the presence of Pt along with the main constituents of the clay including Mg and Si, thereby confirming the formation of the nanocomposite materials. The effects of other factors on the in situ synthesis of Pt nanoparticles were also investigated. Accordingly, increased temperature accelerates the formation of Pt nanoparticles in the clay dispersion. An increase in the temperature to 60 °C results in the much faster reduction of the Pt species that enabled the

Figure 7. N2 adsorption−desorption isotherm of the clay (gray) and Pt−clay nanocomposite (black).

the clay and Pt−clay material of 314 and 312 m2 g−1, respectively. It is evident that the high surface area of the final Pt−clay material can be attributed to the clay support; the high surface area was even retained in the Pt−clay nanostructures. Barrett−Joyner−Halenda (BJH) pore-size distribution analysis revealed that the dominant pore sizes of the clay and Pt−clay were approximately 4.4 and 4.16 nm, respectively (Figure S6). Also, the Pt nanoparticles were free from strong capping 1981

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activity of the Pt−clay nanocomposite compared to that of the same amount of commercial Pt black (data shown in Figure S7); the reaction requires a significantly longer duration to complete, when Pt black is used as a catalyst instead of the Pt− clay materials. To ensure that the catalytic effect stems from the addition of Pt−clay material, the catalytic activity of the clay dispersion alone without Pt (clay + NaBH4) was also tested. The test did not display any catalytic activity for clay when acting in isolation, as shown in the inset of Figure 8. Hence, the high catalytic activity of Pt−clay material in the present study is attributed to its large effective surface area as well as the uniformity and small size of the Pt nanoparticles. Another advantage of this nanocomposite is the ability to recycle the material at least 2−3 times without any significant loss in the catalytic activity. Thermal Stability of the Pt−Clay Nanocomposites. It was found that the Pt−clay material demonstrates ultrahigh thermal stability and can withstand calcination at 500 °C under atmospheric conditions for 2 h. The calcined materials depict minor particle aggregation with no significant change in the particle sizes and morphology. In addition, the calcined Pt−clay material can retain the original activity and catalytic properties of the noncalcined materials toward the reduction of 4nitrophenol with NaBH4. Such extraordinary thermal stability indicates the potential applicability of these materials under vigorous application conditions, for instance, as high-temperature catalyst. A more detailed study in this area is currently underway. Mechanism. Our experimental results clearly demonstrate that the silanol groups are responsible for reducing the Pt species. Although the exact mechanism of the formation of Pt nanoparticles has not yet been fully elucidated, a plausible mechanism is schematically represented in Figure 9. The inorganic clay forms nanosheets when dispersed in water by cleaving into discrete disklike particles (30 nm in diameter and 1 nm thick) with a negative surface charge (zeta potential: −37.1 mV) and small positive charge at rims. In the aqueous solution of K2PtCl4, the PtCl42− ions undergo the following solvolysis reactions:53,54

molecules that would block the Pt active sites in catalytic applications. Hence, the combination of the high surface area and nanoarchitecture should be advantageous for efficient catalytic applications. Catalytic Properties of the Nanostructured Pt−Clay Materials. Pt−clay materials with uniformly distributed Pt nanoparticles are expected to offer excellent catalytic activity. The catalytic activity of these materials was tested for the reduction of 4-nitrophenol with NaBH4 in aqueous solution; this reaction is widely used for testing the catalytic activity of Pt nanoparticles.52 The reduction of 4-nitrophenol (2.5 mL, 0.12 mM) with NaBH4 (0.5 mL, 60 mM) was performed in the presence of the Pt−clay material (0.01 g). The initially light yellow 4-nitrophenol changes to yellow-green upon the addition and mixing of the NaBH4 solution. As shown in Figure 8, UV−vis spectra of the reaction mixture were acquired

Figure 8. Temporal evolution UV−vis spectra of the reduction of 4nitrophenol by NaBH4 in the presence of Pt−clay materials. The inset shows similar measurements using the clay dispersion without Pt nanoparticles.

at 1 min intervals in the range 300−500 nm immediately after the addition of the composite particles. The reduction of 4nitrophenol to 4-aminophenol was very slow in the presence of NaBH4 alone; however, after the addition of the Pt−clay material, the strong UV absorption peak corresponding to the nitrophenolate ions at 400 nm rapidly diminished. In this experiment, because the nanostructured Pt−clay is added in very small amounts, there is little interference with the absorption spectra of 4-nitrophenol. The reaction was complete within 10 min, thereby demonstrating the enhanced catalytic

PtCl4 2 − + H 2O ⇄ PtCl3(H 2O)− + Cl−

(1)

PtCl3(H 2O)− + H 2O ⇄ PtCl 2(H 2O)2 + Cl−

(2)

Figure 9. Mechanism for the formation of nanostructured Pt−clay materials. 1982

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When a known amount of aqueous K2PtCl4 solution is added to the clay dispersion with stirring, the Pt ions interact with the silanol groups (Si−OH) and form the complex under the preparation conditions. Subsequently, reduction of the Pt species is mildly induced by the silanol groups, which likely occurs via successive proton−electron transfer processes, and the resulting Pt nanoparticles, which form within 24 h, remain adhered to the clay platelets (eq 3). 2SiOH + PtCl 2 → SiOSi + Pt + 2HCl

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Education, Science, Sports and Culture of Japan (Grant-in-Aid 23350117).

(3)

The Pt nanoparticles thus obtained are stabilized as nanostructured Pt−clay material in both dispersion and dry state. This approach is very flexible and can be applied to various clay minerals in the smectic group. From eq 3, it is evident that HCl should form upon the reduction of Pt(II) by silanol groups, which would affect the pH of the reaction system. As depicted in Figure 10, the

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Figure 10. Variation in the pH of the solution with time after mixing the clay dispersion (1 g; 1 wt %) with the K2PtCl4 solution (50 μL; 5 wt %). The pH was measured at 25 °C.

variation in pH with time was monitored immediately after the addition of the Pt species to the clay dispersion: The pH, which is initially 8.8, decreases as the reaction proceeds until it reaches a plateau around 7 after 24 h, which indicates completion of the reaction. This clearly supports our hypothesis that the silanol groups are involved in the reduction of the Pt species.

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CONCLUSIONS The current study demonstrates a green, economical, and versatile strategy for the preparation of nanostructured Pt−clay material featuring highly stable and small Pt nanoparticles (3−6 nm) that are formed in situ at room temperature on a support material (clay) without agglomeration. The integration of clay and Pt nanoparticles displays a very high BET surface area, making these materials appealing for application in catalysis. Owing to its high flexibility and simple implementation, the proposed approach can be considered as a very general and powerful method of producing nanostructured Pt−clay materials that paves the way for the further development of a wide variety of novel and functional materials using clay supports. It is anticipated that this synthetic concept will bridge the two frontline disciplines of clay systems and Pt nanomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

Additional TEM images, UV−vis, and N2 adsorption− desorption data. This material is available free of charge via the Internet at http://pubs.acs.org. 1983

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dx.doi.org/10.1021/la3044945 | Langmuir 2013, 29, 1977−1984