Polystyrene Microspheres: Inactive Supporting Material for Recycling

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Polystyrene Microspheres: Inactive Supporting Material for Recycling and Recovering Colloidal Nanocatalysts in Solution M. A. Mahmoud, B. Snyder, and M. A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400

ABSTRACT Alumina and silica have been the most commonly used solid supports in the recovery of colloidal nanocatalysts in solution. In order to avoid possible involvement of the support in the catalytic mechanism, polystyrene microspheres are here demonstrated to be effective and nonreactive supports on which the nanocatalyst can be easily attached by using the swelling and shrinking properties of the polystyrene microspheres. The activation energy of the reduction of 4-nitrophenol with sodium borohydride on platinum nanocubes free in solution is comparable to those on polystyrene microspheres. SECTION Surfaces, Interfaces, Catalysis

during the synthesis or reactions, many reactions lead to nanoparticle aggregation. Furthermore, because of the small size of the nanoparticles, it is difficult to recover them or recycle them. For this reason, supporting material is always used such as polymers, carbon compounds, and the most dominantly used materials, silica or alumina.16 In many catalytic reactions using the latter supporting materials, they are believed to be involved in the catalysis.8 In this letter, a possible solution to the recycling problem in nanocatalysis4 is proposed based on using PS microspheres to which the nanoparticles are attached by using the swelling and shrinking properties of the PS microspheres in solution.22 In order to test for the inert nature of the PS support, the activation energy of the reduction reaction of 4-nitrophenol with borohydride on 20 nm platinum nanocubes (PtNCs) on the support is compared with that in the colloidal solution. The two activation energies are found to be comparable. Supporting that, the PS microspheres are inactive support. Twenty nanometer PtNCs capped with trimethyltetradecylammonium bromide (TTAB), prepared by the borohydride reduction of platinum salt, were found to have sharp corners and edges observed even at low transmission electron microscope (TEM) resolution.6 Figure 1A shows the TEM of those particles. Colloidal PtNCs were used to catalyze the reduction reaction of 4-nitrophenol, with borohydride at different temperatures, to 4-aminophenol. The TEM images of colloidal PtNCs after the catalysis show that the sharp corners of the cubes became slightly rounded. The morphology of the particles was not greatly affected by the temperature at which the reaction was carried out. The reaction temperatures were 25, 30, 35, and 40 °C. Figure 1B shows the TEM image of

T

he use of nanoparticles in catalysis has attracted great attention in the past few decades1,2 because of their high surface-to-volume ratio and their catalytic dependence on size and shape. Besides increasing the number of the chemically unsaturated thermodynamically active surface atoms,3 catalysis with nanoparticles is heterogeneous both in solution colloidal reactions4,5 and on supported nanoparticle gas-phase reactions.6-8 For the microcatalyst, surface chemists define the active site on the catalyst by active centers.9 The active centers are located on sharp corners and edges of a nanocatalyst. The active centers in the case of the nanocatalyst are present in greater quantity and are more active.3 Not only do the optical10 and catalytic properties of the metals change when the metals are on the nanoscale,11 but the electrochemical properties are also affected, e.g., the reduction potential value becomes more negative as the size of the metal particle approaches the nanoscale.12 Platinum is the most commonly used nanocatalyst,4,13,14 and different shapes and sizes of platinum nanoparticles have been prepared.1,15 Polystyrene (PS) microspheres have no catalytic or photocatalytic properties compared with commonly used catalyst supports, e.g., silica and alumina, that proved to have some catalytic properties.8,16 Reduction of aromatic nitrocompounds is an economically important reaction. Some metals are used as a catalyst for catalytic reduction of nitrocompounds by borohydride, e.g., silver,17 gold,18 and platinum19 nanoparticles. In some reports, bimetallic compounds, e.g., Pt-Ni, are used to catalyze this reduction reaction,20 and metallic nanoparticles supported on the surface of microgels were also used to catalyze this reduction reaction.21 One problem to solve in nanocatalysis is the difficulty in separating the nanocatalyst at the end of the reaction for recycling. Since the capping materials are usually stripped off

r 2009 American Chemical Society

Received Date: September 24, 2009 Accepted Date: October 15, 2009 Published on Web Date: November 05, 2009

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DOI: 10.1021/jz9000449 |J. Phys. Chem. Lett. 2010, 1, 28–31

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PtNCs after the reduction reaction was carried out at 40 °C. The size of the particles did not change during the reaction, but the corners were rounded. No small particles were observed in the TEM images after the reaction, only reshaping was detected. The reason for reshaping and disappearance of the sharp corners and edges were observed and explained previously4 by the valence unsaturation of the atoms located in these locations. The reshaping of the PtNCs corners was also observed by dissolving the PtNCs in water and keeping it at 80 °C for 3 h. During the catalysis, the reshaping occurs faster. Therefore, as reported in many studies,3,23,3,24 as the number of sharp edges and corners increases, the activity of the nanocatalyst increases. PtNCs supported on the surface of PS were used to catalyze the borohydride reduction reaction of 4-nitrophenol at different temperatures. It is difficult to obtain a highly magnified SEM image of PtNCs present on the surface of PS because the PS is not conductive. The shape and size of the PS microspheres did not change and the nanoparticle distribution did not show aggregation after the catalysis at different temperatures (25, 30, 35, and 40 °C). The distribution of the microsphere catalyst was found by the SEM to be unchanged after the catalysis. The reaction between borohydride and 4-nitrophenol is pseudo first order if the ratio between the concentration of

borohydride and 4-nitrophenol is greater than 10 to 1. If this ratio decreases and approaches unity; the kinetic behavior of the reaction will change. The borohydride decomposes at slightly high temperature producing hydrogen. However to maintain a high ratio of borohydride to 4-nitrophenol, and keep the reaction first order in the activation energy determination experiments, a high concentration of borohydride was used. During the kinetic study, we optically followed the concentration of the remaining 4-nitrophenol with time. The first order expression was satisfied in this reaction i.e. the relationship between the natural logarithm of the unreacted concentration of 4-nitrophenol and the reaction time was linear. From the slope of the straight line, the rate constant of the reaction is calculated. The rate constant for the reduction of 4-nitrophenol with sodium borohydride in the presence of colloidal PtNCs at different temperatures are shown in Figure 2 A. The PtNCs supported on the surface of PS microspheres were found to be stable and did not leach out into the solution after heating at 60 °C for 2 h. This was determined by SEM imaging, which showed no free PtNCs in the solution. The first-order linear plot of the reduction of 4-nitrophenol by the borohydride in the presence of PtNCs supported on the surface of PS microspheres (at different temperature) are shown in Figure 2B. As expected from the Arrhenius expression, the rate constant was found to increase as the temperature at which the reaction is taking place increases. The same catalyst concentration (corresponding to 10 μL) was used at each temperature for colloidal PtNCs. For PtNCs supported on the surface of PS, the concentration used corresponded to 150 μL. To compare the activity of the colloidal with supported PtNCs on the surface of PS, the activation energy for the reduction reaction of 4-nitrophenol by borohydride was calculated from the Arrhenius plots for the reactions catalyzed by colloidal PtNCs and PtNCs supported on the surface of PS. Figure 3 shows the Arrhenius plot for the reduction of 4-nitrophenol in the presence of

Figure 1. TEM images of (A) pure PtNCs and (B) PtNCs after the catalytic reaction was carried out at 40 °C.

Figure 2. First-order kinetic plots for the reduction of 4-nitrophenol with borohydride, at different temperatures, in the presence of colloidal PtNCs (A), and PS coated with PtNCs (B). The rate constants at each temperature are given in each panel.


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10 h. The borohydride decomposed, producing hydrogen gas, and the pressure right after adding the borohydride was very high, so an injector needle was used to diffuse the gas for 10 min. The PtNCs produced by this method required the removal of the large particles formed. The solution of PtNCs was centrifuged at 3000 rpm for 30 min. The large particles precipitated out, and the uniformly sized particles remained in the supernatant solution. To use the PtNCs in catalysis, the particles were cleaned from the excess capping material (TTAB) by centrifugation at high speed (∼13 000 rpm) for 5 min and redispersion in DI water twice. The particles were collected in 10 mL of DI water. To support PtNCs on the surface of PS, 2.1 mL of PtNCs was mixed with 800 μL of 9.7 ( 0.7 μm PS colloidal microspheres (1.0 wt. %, Duke Scientific Corp) in a glass vial. After stirring for 3 min, 100 μL of tetrahydrofuran was added to swell the PS.22 The resulting solution was further stirred for 10 more minutes and then heated in a water bath at 80 °C for 2 h. The PS-PtNCs microspheres were cleaned by centrifugation at 2000 rpm to remove the free PtNCs. PSPtNCs microspheres were then dispersed in 15 mL of DI water. This cleaning process was repeated six times before the final clean precipated PS-PtNCs microspheres were dispersed in 1 mL of DI water. The catalysis experiment was carried out in a 4 mL long neck quartz cuvette. Thirty microliters (2 mM) of aqueous solution of 4-nitrophenol was diluted with 1.67 mL of DI, then 100 μL of cleaned colloidal PtNCs or PtNCs supported on the surface of PS was added. The resulting solution was shaken gently and 2 mL of ice-cold (0.06 M) sodium borohydride was added at once. The drop in the peak corresponding to 4-nitrophenol was determined from ultraviolet-visible (UV-vis) optical measurements, using an Ocean optics HR4000Cg-UV-NIR. A Zeiss Ultra60 was used for scanning electron microscopy (SEM) measurement. A JEOL 100C TEM was used to characterize colloidal PtNCs.

Figure 3. Arrhenius plots for the reduction reaction of 4-nitrophenol with borohydride in the presence of colloidal PtNC catalyst (black) or supported on PS nanospheres (red).

colloidal PtNCs and PtNCs supported on the surface of PS. The activation energy was found to be almost the same in the case of colloidal PtNCs and those supported on PS (∼3 kcal/mol). From Figure 2, the rate constant at any temperature is approximately 7 times slower when the nanoparticles are on the PS than when they are free in solution. This could be due to (1) a difference in the surface area of the metallic nanoparticles available for catalysis by the nanoparticles under the two conditions, (2) a difference in the ability of the reactants to reach the catalytic surface of the nanoparticles, and/or (3) differences in the effective concentration of the nanoparticles. The second possibility could result from the fact that the capping material of the nanoparticle is actually anchored inside the polymer pores. This could make it sterically more difficult for the reactant molecules to reach the surface of the nanoparticles and collide with each other and react when bound to the PS microspheres. PS is an effective support for the platinum nanocatalyst. Because of its inert properties, and it does not change the activity of the nanocatalyst. The activation energy for the reduction reaction of 4-nitrophenol by sodium borohydride is almost the same in both colloidal nanocatalysts and those supported on PS. The catalyst supported on the surface of PS can be recycled from the solution at low-speed centrifugation to be recycled. PtNCs were prepared by borohydride reduction of tetrachloroplatinate, in the presence of TTAB as a capping material.1 In a 100 mL round-bottom flask, 3.3 g of TTAB was dissolved in 80 mL of deionized (DI) water. Potassium tetrachloroplatinate salt (0.0415 g) dissolved in 10 mL of DI water was added immediately to the TTAB solution, and the resulting solution became turbid. This turbidity disappeared after stirring and heating at 50 °C in an oil bath for 10 min. An icecold solution of 0.12 g of sodium borohydride dissolved in 10 mL of DI water was added and stirred, and the flask was closed tightly with a septum. The resulting solution was heated in an oil bath at 50 °C with low-speed stirring for


AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: [email protected]. ACKNOWLEDGMENT The authors would like to thank the Airforce AFOSR - Ctr of Excellence on BIONIC * under Grant No. 3306FF9.

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Polystyrene Microspheres: Inactive Supporting Material for Recycling

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