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“Investigation into the Catalytic Activity of Porous Platinum Nanostructures” Ajit M. Kalekar, Kiran Kumar K. Sharma, Anaïs Lehoux, Fabrice Audonnet, Hynd Remita, Abhijit Saha, and Geeta K. Sharma Langmuir, Just Accepted Manuscript • DOI: 10.1021/la401302p • Publication Date (Web): 15 Aug 2013 Downloaded from http://pubs.acs.org on August 17, 2013
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Investigation into the Catalytic Activity of Porous Platinum Nanostructures Ajit M. Kalekar,† Kiran Kumar K. Sharma,‡ Anaïs Lehoux,§,+ Fabrice Audonnet,¥, X Hynd Remita,§ Abhijit Saha,± Geeta K. Sharma∗† †
National Centre for Free Radical Research (NCFRR), Department of Chemistry, University of Pune, 411007, Maharashtra, India. ‡
Department of Physical Chemistry, School of Chemical Sciences, North Maharashtra University, Jalgaon, 425001, Maharashtra, India. §
Laboratoire de Chimie Physique, CNRS-UMR 8000, Bât 349, Université Paris-Sud, 91405 Orsay Cedex, France.
¥
Clermont Université, Université Blaise Pascal, Institut Pascal, BP 10448, F-63000, ClermontFerrand, France. X
CNRS, UMR 6602, IP, F-63171 Aubière, France.
±
UGC DAE Consortium for Scientific Research LB 8/III, Salt Lake, Kolkata-700 098, India.
+ Present address: Laboratoire de Physique des Solides, CNRS-UMR 8502, Bât 510, Université Paris-Sud, 91405 Orsay Cedex, France.
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Abstract: The catalytic activity of porous platinum nanostructures, viz platinum nanonets (PtNNs) and platinum nanoballs (PtNBs), synthesized by radiolysis were studied using two model reactions i) electron transfer reaction between hexacyanoferrate (III) and sodium thiosulfate and ii) the reduction of p-nitrophenol by sodium borohydride to p-aminophenol. The kinetic investigations were carried out for the platinum nanostructure catalyzed reactions at different temperatures. The pseudo-first order rate constant for the electron transfer reaction between hexacyanoferrate (III) and sodium thiosulfate catalyzed by PtNNs and PtNBs at 293 K are (9.1 ± 0.7) × 10-3 min-1 and (16.9 ±0.6) × 10-3 min-1 respectively. For the PtNNs and PtNBs catalyzed reduction of p-nitrophenol to p-aminophenol by sodium borohydride, the pseudo-first order rate constant was (8.4 ± 0.3) × 10-2 min-1 and (12.6 ± 2.5) × 10-2 min-1 respectively. The accessible surface area of the PtNNs and PtNBs determined before the reaction are 99 m2/g and 110 m2/g respectively. These nanostructures exhibit significantly higher catalytic activity, consistent with largest accessible surface area reported so far for the solid platinum nanoparticles. The equilibrium of the reactants on the surface of the platinum nanostructures played an important role in the induction time (t0) observed in the reaction. A possible role of structural modifications of PtNBs catalyzed reaction leading to change in the accessible surface area of PtNBs is being explored to explain the non-linear behavior in the kinetic curve. The activation energy of the PtNNs and PtNBs catalyzed reduction of PNP are 26 kJ/mol and 6.4 kJ/mol respectively These observations open up new challenges in the field of material science to design and synthesize platinum nanostructures which could withstand such reaction conditions. Keywords: porous platinum nanostructures, kinetics, catalytic activity, activation energy, radiolysis
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1. Introduction The exponential growth in the field of nanoscience in recent years is due to the fact that the transition from bulk to nanosized regime leads to immense changes in the physical and chemical properties of nanostructured materials.1 The nanostructured materials display fascinating properties which have various applications in nanotechnology. Among these nanomaterials, metal nanostructures are of great interest mainly because of their high surface to volume ratio and their application in catalysis.1-4 Such studies have led to the term ‘nanocatalysis’.5 The most exciting examples of catalytic properties of nanostructured materials have been demonstrated by using gold based nanocages and nanoboxes.6 The noble-metal nanoparticles, in particular, platinum is efficiently used as catalyst in many commercially important organic reactions such as cyclopropanation, cycloisomerization, H-D exchange, Suzuki coupling, hydrosilylation, electron transfer reaction.7-13 Recently, it has been also shown that Pt nanoparticles can degrade alcohol into water.14 The catalytic activity of these platinum nanoparticles have been reported to be highly dependent on the morphology, porosity, size distribution and phase composition.15-19 It has been reported that significant enhancement in the catalytic activity could be achieved when monodispersed platinum nanoparticles are linked into platinum nanowires.20 These studies have opened an avenue for the researchers to design and synthesize different metal nanostructures and numerous reports are available for the synthesis of 1D, 2D and 3D platinum nanostructures by using templates.21-28 Hard templates, such as silica, anodic aluminum oxide and mesoporous carbon are generally used for the synthesis of semiconductor nanostructures, metal nanoparticles and nanowires with high surface to volume ratio.29-31 However, the hard template synthesis requires strong chemical treatment like hydrofluoric acid for the retrieval of the metal nanostructures.
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The platinum nanoparticles stabilized/ supported by surfactants in colloidal state have been extensively used to investigate the catalytic activity using model reactions: i) catalytic electron transfer reaction between hexacyanoferrate (III) and sodium thiosulfate, Scheme 1 and ii) catalytic reduction of p-nitrophenol (PNP) to p-aminophenol (PAP) by sodium borohydride (NaBH4), Scheme 2. 2- PtNB/PtNN 2S2O 3
32Fe(CN) 6
42Fe(CN) 6
2S4O6
Scheme 1. The platinum nanostructure (PtNBs/PtNNs) catalyzed electron transfer reaction between hexacyanoferrate (III) and sodium thiosulfate.
O
NaBH4 N
O
OH
H2N
OH
PtNB/PtNN
Scheme 2. The catalytic reduction of PNP to PAP by NaBH4 in the presence of PtNNs and PtNBs. These two reactions have been used widely for investigating the catalytic activity of metal nanostructures owing to two important properties viz the reaction does not occur without a catalyst and the catalyzed reaction can be monitored easily. The size and shape dependent catalytic properties of colloidal platinum nanoparticles in the electron transfer reaction between hexacyanoferrate (III) and sodium thiosulfate were first investigated by Spiro et al.32 The investigation proposed that the catalytic activity of platinum nanoparticles could be due to the change in the size and shape of the nanoparticles involved during the electron transfer reaction.12-13,15,32-33 Interestingly, modified platinum nanoparticles have been widely studied to search for platinum nanostructures with enhanced catalytic activity
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and to understand their mechanism in the model catalytic reactions, Scheme 1 and Scheme 2, including the induction time (t0) observed in the reaction, Scheme 2. The thermodynamic parameters like activation energy34-36 and the entropy of activation15 have been determined for these reactions using spherical platinum nanoparticles as catalyst. Recently, it has been shown that colloidal platinum nanoparticles immobilized in polyelectrolyte brushes show efficient catalytic activity during the reduction of PNP by NaBH4 and proposed a mechanism whereby platinum nanoparticles react initially with NaBH4 which then undergo electron transfer to the substrate.34 The proposed mechanism is in agreement with the surface restructuring/modification of these nanoparticles and also with the mechanism of initial activation of colloidal Ag nanoparticles proposed by Pradhan et al.37 The kinetics of these heterogeneous catalyzed reaction have also been investigated emphasizing on the pseudo-first order rate equation and taking into account the adsorption isotherm of the substrate on the surface of the catalyst.12-13,15,33-35 Such studies suggest that the catalytic activity of platinum nanoparticles depend on how the platinum nanoparticles are immobilized on the surface of the support and their accessibility to the substrate as reviewed by Hervés et al.38 Additionally, the size and shape dependent catalytic activity of the reaction in Scheme 2 have been demonstrated using gold based single nanoparticle of different shapes.39 All of these studies have been carried out using colloidal nanoparticles which consist of mixture of stabilizing agents such as polymers and surfactants. Radiolysis is a powerful method to synthesize nanoparticles and nanomaterials in solutions and in heterogeneous media.40-45 In swollen hexagonal liquid crystals (SLCs), used as soft templates, platinum nanostructures with unique morphologies, such as platinum nanowires (PtNWs), porous platinum nanoballs (PtNBs) and interconnected platinum nanonets (PtNNs) have been synthesized by radiolysis. The morphology of these synthesized platinum
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nanostructures can be easily controlled and cleanly harvested without involving any harsh treatment.46 Further, these platinum nanostructures can be easily extracted and dispersed in solvents, especially in water signifying the versatility of investigating catalytic reactions in aqueous medium. Very few reports on the reduction of PNP by using bimetallic alloy nanoparticles prepared by radiolysis are available in the literature,47 however, to the authors knowledge, reports on the reduction of aromatic nitro compounds using porous platinum nanostructures prepared by radiolysis have not been reported so far. In this study, we report the catalytic activity and kinetics of the porous platinum nanostructures synthesized by radiolysis to understand the mechanism of the two catalyzed model reactions (Scheme 1 and 2).
2. Experimental Section 2.1. Materials Tetraammineplatinum (II) chloride [Pt(NH3)4Cl2] (99% purity) was purchased from Sigma Aldrich. Cetyltrimethylammonium bromide (CTAB) (99% purity) was purchased from SD fine, 1-pentanol (99.5% purity), cyclohexane (99% purity), propan-2-ol (99.6% purity) were purchased from Qualigens. Nitrogen (N2) gas with purity 99.995% was purchased from Inox. All the materials were used as received. The solutions were prepared in deionized water from MilliQ system.
2.2. Synthesis, Irradiation and Analysis of Platinum Nanostructures The SLCs were synthesized by using cetyltrimethylammonium bromide (CTAB) doped with Pt(NH3)4Cl2 metal precursor following the protocol reported elsewere.46 (See Supporting Information for details). The stable, perfectly transparent and birefringent SLCs were exposed to radiation using the
60
Co γ- source facilities at UGC-DAE CSR, Kolkata Centre and the LCP,
Orsay. The dose rate are 5.0 kGy h-1 and 7.0 kGy h-1 respectively (determine using Fricke
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dosimetry). The irradiation dose was 40 kGy for PtNNs and 80 kGy for PtNBs. The mechanism of the radiolytic reduction of platinum ions is described in the Supporting Information. After irradiation, the SLCs were destabilized by washing with propan-2-ol. The obtained platinum nanostructures were centrifuged and the residues were washed several times with propan-2-ol and water. The harvested platinum nanostructures were air dried and stored under nitrogen prior to characterization by transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), BET surface characterization and UVVis spectroscopy which are described in detail in the Supporting Information.
2.3. The electron transfer reaction between hexacyanoferrate (III) and sodium thiosulfate A reaction mixture containing 2 mL of Milli-Q water and 0.2 mL of 1 × 10-2 M hexacyanoferrate (III) were mixed in a 3 mL quartz cuvette. The reaction was initiated by adding 0.2 mL of 1 × 10-1 M sodium thiosulfate and monitored at 420 nm by using a UV-visible spectrophotometer (Shimadzu UV1800). A stock suspension of the platinum nanostructures of 50 mg/L was prepared by dispersing in Milli-Q water. The same reaction was carried out by adding 2 mL suspension of 50 mg/L platinum nanostructures instead of water. These catalyzed reactions were followed at both 420 nm and 500 nm and the absorption coefficient of platinum nanostructures was corrected as reported earlier.48 The absorbance (At) were recorded with time at 420 nm for the catalytic reaction shown in Scheme 1 in presence of 2 mL of 50 mg/L PtNBs and PtNNs. In order to study the effect of temperature on the pseudo-first order rate constant, the reactions were also performed in the temperature range of 293 K -343 K. The temperature of the solution in the quartz cuvette was controlled by water regulated peltier assembly connected to a water bath with an accuracy of ± 0.4 °C inside the cuvette holder.
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2.4. The catalytic reduction of p-nitrophenol (PNP) In this typical heterogeneous catalysis reaction, 1.5 mL of 1 × 10-4 M PNP and 1.0 mL of 5 × 10-2 M NaBH4 were first mixed and stirred for 1−2 min in a 3 mL quartz cuvette. To this mixture 0.5 mL of 50 mg/L of platinum nanostructures were added and mixed well. The reaction was followed by observing the absorbance of p-nitro phenolate anion at 400 nm. The absorption spectra were recorded within the wavelength range of 200-600 nm. The experiments were also performed by adding accurately weighed nanostructures (0.025 mg) corresponding to 50 mg/L in the reaction mixture and measurements were carried out under two conditions, a) addition of platinum nanostructures to the 1 × 10-4 M PNP solution followed by addition of 5 × 10-2 M NaBH4 solution and b) addition of 1 × 10-4 M PNP solution to a mixture of platinum nanostructures and 5 × 10-2 M NaBH4 solution, keeping a constant platinum nanostructure concentration in suspension and by weight respectively. The latter experiments were also performed in the temperature range of 283 K - 323 K to determine the temperature effect on the pseudo-first order rate constant. For the non-catalyzed reaction, instead of platinum nanostructures, 0.5 mL of Milli Q water was added and absorption spectra were recorded at the same wavelength (400 nm).
2.5. Kinetic analysis In general, the mechanism for the heterogeneous catalysis reactions are studied using the classical Langmuir-Hinshelwood equation which is based on the reaction between the chemisorbed species on the surface of the catalyst and the same mechanism has been accepted to explain a majority of surface catalytic reactions both experimentally and theoritically.49-51 Therefore, the obtained data were treated by using a Langmuir-Hinshelwood mechanism considering that the reaction proceeds by encounters between the chemisorbed atoms on the
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surface and the substrate molecules. The Langmuir-Hinshelwood model can be used to describe the relationship between the rate of reaction in Scheme 1 and 2 in the presence of platinum nanostructures as a function of time. The rate equation for heterogeneous catalysis according to the Langmuir-Hinshelwood mechanism for a single molecule adsorbed on the surface of catalyst is given by equation (1). -
dC dt
=
kreact Kad C
(1)
1 + Kad C
where, kreact is the reaction rate constant, Kad is the equilibrium constant for the adsorption of the reactant on the platinum nanostructured surface, C is the concentration at any time t. On integrating equation (1), under conditions of pseudo-first order reaction and when Kad C
