Catalysis Communications

Catalysis Communications 10 (2009) 1305 1309 Contents lists available at ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/locate/catcom Change in the catalytic reactivity of Pt nanocubes in the presence of different surface-capping agents Cheonghee Kim, Hyunjoo Lee * Department of Chemical and Biomolecular Engineering, Specialized Graduate School of Hydrogen and Fuel Cell, Yonsei University, Seoul 120-749, South Korea article info abstract Article history: Received 1 November 2008 Received in revised form 29 January 2009 Accepted 16 February 2009 Available online 26 February 2009 Keywords: Pt nanocubes Shape control Surface-capping agent Reactivity Polyvinylpyrrolidone (PVP; MW 55,000)-capped and tetradecyltrimethylammonium bromide (TTAB; MW 336)-capped Pt nanoparticles were synthesized with the same cubic shape and similar particle size (8.1 vs. 8.6 nm, respectively). These Pt nanocubes were used as catalysts for reactions of H +,C 2 H 4,C 6 H 6, and O 2 NC 6 H 4 OH on the Pt surface. With increasing molecular size of the reactants (H + <C 2 H 4 <C 6 H 6 ), the ratio of the catalytic activity of TTAB- to PVP-capped nanocubes shows a dramatic increase because TTAB with short alkyl chains provides a larger number of clean Pt atom ensembles for larger reactants. However, similar activity between the two types of nanocubes was measured for a solution-phase reaction (O 2 NC 6 H 4 OH) where the surface-capping agents are able to spread out into solvent media, exposing bare Pt atoms. In this case, the influence of the alkyl length of the surface-capping agent was minimized. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Colloidal metallic nanoparticles have been used as catalysts for various reactions, including the selective oxidation of ethylene glycol to glycolate by Au colloids [1], conversion of cellulose to C6- alcohols by Ru nanoclusters [2], oxidation of ethylene to ethylene oxide by Ag colloids [3], electron transfer, Suzuki-coupling reaction or selective hydrogenation by Pt nanoparticles [4 8]. It is well-known that the size and shape of such colloidal nanoparticles have a strong influence on their catalytic properties. In particular, the surface-capping agent, which is usually added during nanoparticle synthesis to stabilize the high surface energy of the nanoparticles, has a strong effect on catalytic activity and selectivity. Miyazaki et al. synthesized Pt nanoparticles using polyvinylpyrrolidone, poly(n-isopropylacrylamide), and sodium polyacrylate as capping agents, revealing that the rate of NO reduction by CH 4 varies among the different agents [9]. The authors proposed that the various capping agents resulted in different morphology of nanoparticle (e.g., hexagonal, square, triangular), resulting in turn in contrasting catalytic properties. As demonstrated in this earlier work, the choice of surface-capping agent commonly influences the size or shape of the nanoparticle; consequently, the effect of the capping agent on catalytic properties is blurred by size or shape effects. To isolate the effect of the surface-capping agent, two different capping agents, PVP and TTAB, were used to synthesize the same * Corresponding author. Tel.: +82 2 2123 5759; fax: +82 2 312 6401. E-mail address: azhyun@yonsei.ac.kr (H. Lee). shape of Pt nanocubes with a similar size. PVP, which is the most widely used capping agent in the synthesis of metallic nanoparticles such as Au, Ag, Pt, and Pd [10 14], is an amphiphilic polymer with very long chain length (MW 55,000). In contrast, TTAB has a short alkyl chain (MW 336) with an ammonium head group. Alkyltrimethylammonium bromide has recently been used in the synthesis of gold nanorods [15,16] and various shapes of Pt or Pd nanocrystals [17,18]. While the C@O bond located on a PVP monomer unit is known to have a pseudo covalent bonding with a metallic surface [19], TTAB has no functional group having a strong interaction with a metallic surface. In this study, we used PVP- and TTAB-capped Pt nanocubes as catalysts to test the reactions of electrocatalytic H + adsorption/ desorption, C 2 H 4 hydrogenation in the gas phase, C 6 H 6 hydrogenation in the gas phase, and O 2 NC 6 H 4 OH hydrogenation in the solution phase. The surface-capping agents lie on the Pt surface for electrocatalytic or gas-phase reaction, but spread out freely through the solution media for solution-phase reaction. The effect of the molecular size of the reactants (H + <C 2 H 4 <C 6 H 6 <O 2 NC 6 H 4 OH) was evaluated for PVP- and TTAB-capped Pt nanocubes. 2. Experimental PVP- and TTAB-capped Pt nanocubes were synthesized as reported previously [17,20]. TEM images were taken using a JEOL 2100 at 200 kv. The nanocubes were washed and concentrated, and the solution was then dropped on silicon wafer and dried at room temperature overnight for X-ray photoelectron spectroscopy (XPS) using a SIGMA probe (ThermoVG) equipped with a 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.02.013

1306 C. Kim, H. Lee / Catalysis Communications 10 (2009) 1305 1309 monochromatic Al Ka X-ray source (15 kv, 100 W). The binding energy was calculated with reference to the maximum intensity of the C 1s signal at 285 ev. The obtained spectra were analyzed using XPS PEAK 41 software. Thermogravimetric analysis (TGA) was performed using a TA instruments Q5000 at a heating rate of 10 C/min under N 2 flow. The concentrated nanocube solution was dropped onto a gold working electrode (5 mm in diameter) and dried overnight at room temperature. The amount of Pt nanocubes in the solution was evaluated by ICP-MS (Perkin Elmer OPTIMA 3000DV). The cyclic voltammogram was measured in 0.5 M H 2 SO 4 solution at a scan rate of 50 mv/s. A three-electrode cell was used with a standard calomel electrode (SCE) as the reference electrode, and a platinum wire inside a glass frit as a counter electrode. The charge (measured in H desorption) was used to evaluate the active sites of Pt nanocubes. Reactor setup for ethylene hydrogenation and benzene hydrogenation was as reported previously [17,21]. Briefly, the nanoparticles were assembled and deposited on a silicon wafer by the Langmuir Blodgett technique. The silicon wafer was then held in compression against a rubber O-ring to a reactor cell. Ethylene or benzene hydrogenation was performed in the reaction cell under continuous gas flow. The flow reactor was periodically sampled by Gas Chromatography (HP 6890) to measure the conversion. Hydrogenation of p-nitrophenol (99%, Aldrich) was performed in a solution phase. A total of 0.2 ml of concentrated colloidal solution with 89 lg of Pt nanoparticle was added to 1 ml of 0.8 M NaBH 4 aqueous solution. The mixture was stirred for 15 min at room temperature. Seven milliliters of 3.4 mm p-nitrophenol was added to the mixture and stirred until the reaction was complete. Reaction progress was measured with UV vis (Shimadzu UV-2401PC) at 5 min intervals. 3. Results and discussion Fig. 1 shows TEM images of PVP- and TTAB-capped Pt nanocubes. PVP-capped nanocubes have an average size of 8.1 ± 0.8 nm, comprising 73% cubes, 21% tetrahedra, and 6% irregular shapes. TTAB-capped nanocubes have an average size of 8.6 ± 1.1 nm, with 85% cubes, 10% tetrahedral, and 5% irregular shapes. The size and shape distributions are similar for the two sets of nanocubes. Analysis by X-ray photoelectron spectroscopy was performed for both sets of nanocubes after dropping the nanocubes solution onto a silicon wafer and drying. Fig. 2a and c shows Pt peaks for two different chemical states of Pt 0 and Pt 2+. Pt 2+ possibly resulted from PtO formed on the Pt surface or from unreacted platinum salt entangled in the capping layer. Pt 0 4f 7/2 and Pt 2+ 4f 7/2 peaks are located on 70.2 and 71.2 ev for PVP-capped nanocubes, and on 71.5 and 72.5 ev for TTAB-capped nanocubes. The binding energy on PVP-capped nanocubes is significantly lower than the typical values for Pt 4f peaks [22]. Borodko et al. reported the formation of strong >C@O Pt bonds on the surface of PVP-capped Pt nanoparticles [19]. This bond with the electron-donating effect of PVP to Pt possibly makes the binding energy of Pt on the PVP-capped nanocubes much lower than that of TTAB-capped nanocubes. A single N 1s peak appears on 399.8 ev for PVP-capped nanocubes, while two different N 1s peaks are observed for TTAB-capped nanocubes on 398.3 and 397.8 ev as shown in Fig. 2b and d. As noted by Nikoobakht and El-Sayed [16], two different TTAB layers (one having a strong interaction with the metal surface and the other having a weak interaction with the first layer) may exist on the Pt surface. The peak area ratio of N/Pt is 0.55 for PVP-capped nanocubes and 0.04 for TTAB-capped nanocubes. These findings indicate that the platinum surface is covered by nitrogen to a much greater degree on PVP-capped nanocubes than on TTABcapped nanocubes. Fig. 3 shows TGA data for PVP only, PVP-capped nanocubes, TTAB only, and TTAB-capped nanocubes. PVP only shows a sharp decrease in mass at 425 C. The mass of PVP-capped nanocubes starts to decrease at a lower temperature, but mass reduction was observed even between temperatures of 450 C and 620 C. TTAB only was decomposed at a much lower temperature of 255 C. In the case of TTAB-capped nanocubes, two steps of decomposition occurred at 210 C and 560 C. El-Sayed and Nikoobakht showed that when cetyltrimethylammonium bromide present on a gold surface is heated, the alkyl chain decomposes first, then the ammonium head group at a much higher temperature [16]. Likewise, free TTAB or alkyl chains of TTAB present on a platinum nanocubes surface are decomposed at a relatively low temperature, whereas ammonium head groups are decomposed at a relatively high temperature. Organic capping agents on a platinum surface appear to be decomposed via a variety of decomposition modes, showing mass reduction over a broad temperature range. Turnover frequency (TOF) was compared for various catalytic reactions when PVP- or TTAB-capped nanocubes were used as catalysts. H adsorption/desorption was performed using Pt nanocubes as electrocatalysts. A known amount of nanocubes was deposited on an Au electrode, and a cyclovoltammogram (CV) was run in a 0.5 M H 2 SO 4 solution at a scan rate of 50 mv/s for H adsorption/ desorption in order to estimate the active sites. Table 1 lists the percentage of platinum atoms active for the access of H +. The Fig. 1. TEM images of (a) PVP-capped Pt nanocubes with a size of 8.1 ± 0.8 nm (73% cube, 21% tetrahedra, 6% irregular shape) and (b) TTAB-capped Pt nanocubes with a size of 8.6 ± 1.1 nm (85% cube, 10% tetrahedra, 5% irregular shape).

C. Kim, H. Lee / Catalysis Communications 10 (2009) 1305 1309 1307 (a) Pt 4f (b) N 1s 78 76 74 72 70 68 404 402 400 398 396 (c) Pt 4f (d) N 1s 78 76 74 72 70 68 404 402 400 398 396 Fig. 2. XPS spectra of (a), (b) PVP-capped Pt nanocubes and (c), (d) TTAB-capped Pt nanocubes. C 1s peak resulted from hydrocarbon contamination was calibrated as 285.0 ev and the position of other peaks was determined accordingly. Weight % (Free PV P) 100 (a) 100 Weight % (PVP-Pt) B) Weight % (Free TTA 100 (b) 100 95 95 60 90 60 90 40 85 40 85 20 20 0 0 75 75 0 100 200 300 400 500 600 700 0 0 100 200 300 400 500 600 700 0 Temp( o C) Temp( o C) Fig. 3. TGA data for (a) PVP-capped Pt nanocubes and (b) TTAB-capped Pt nanocubes under N 2 flow. Weight % (TTAB-Pt ) Table 1 Turnover frequency (TOF) for various catalytic reactions when PVP or TTAB-capped Pt nanocubes were used as catalysts. Fourth column shows the ratio of each activity. PVP-capped Pt nanocubes TTAB-capped Pt nanocubes TTAB/PVP H desorption (% active site) a 13.3 50.0 3.8 C 2 H 4 hydrogenation (TOF, s 1 ) at 373 K b 2.8 20.0 7.1 Benzene hydrogenation (TOF, s 1 ) at 3 K c 0.07 0.94 13.4 p-nitrophenol hydrogenation(tof, s 1 ) at RT 0.59 0.58 0.98 a b c The number of Pt atoms on clean surface was calculated based on the size of Pt nanocubes. Cited from Ref. [18] for PVP-capped nanocubes and TTAB-capped nanocubes. Cited from Ref. [21] for TTAB-capped nanocubes. The data for PVP-capped nanocubes were obtained in the same reaction condition as a TTAB case.

1308 C. Kim, H. Lee / Catalysis Communications 10 (2009) 1305 1309 Absorbanceo 2.0 1.5 1.0 0.5 (a) 0 min 5 min 10 min 20 min 30 min Absorbance 2.0 1.5 1.0 0.5 (b) 0 min 5 min 10 min 20 min 30 min 0.0 0.0 300 350 400 450 500 550 300 350 400 450 500 550 Wavelength (nm) Wavelength (nm) Fig. 4. UV vis spectral change over time for hydrogenation of p-nitrophenol when using (a) PVP- or (b) TTAB-capped Pt nanocubes as metallic catalysts. number of Pt atoms on an ideally clean surface was calculated based on the size of Pt nanocubes. The number of Pt atoms that H + can actually access for adsorption/desorption was estimated from the H desorption curve. For the PVP-capped nanocubes, only 13.3% of Pt atoms on the surface of the nanocubes were used for H adsorption/desorption. In contrast, 50.0% of the surface Pt atoms were catalytically active for H adsorption/desorption in the case of TTAB-capped nanocubes. Catalytic activity for ethylene hydrogenation has been reported previously for these Pt nanoparticles [17]. At 373 K, TOF was observed to be 2.8 s 1 for PVP-capped nanocubes and 20.0 s 1 for TTAB-capped nanocubes. Catalytic activity for benzene hydrogenation for TTAB-capped nanocubes has been reported to be 0.94 s 1 at 3 K [21]; at the same reaction condition, TOF for PVP-capped nanocubes is 0.07 s 1. All three catalytic reactions described above were performed in the case that the organic capping agents lie on the Pt surface. An ensemble of clean Pt atoms is necessary to ensure that the reactants adsorb onto the clean Pt atoms and then undergo catalytic reactions. When the reactant molecule was H +, the activity of TTAB-capped nanocubes was 3.8 times higher than that of PVPcapped nanocubes. With increasing size of the reactant molecule to ethylene and benzene, the activity ratio of TTAB- to PVP-capped nanocubes also increases (to 7.1 and 13.4, respectively). In the case of large reactant molecules, TTAB-capped nanocubes show much higher activity than PVP-capped nanocubes. TTAB with a short alkyl chain would hinder the Pt surface to a lesser degree than PVP with a polymeric alkyl chain would, thereby blocking fewer catalytically active sites. In addition, more ensembles of clean Pt atoms on TTAB-capped nanocubes show the significant impact on catalytic activity, resulting in a higher activity ratio (TTAB-/PVP-capped nanocubes) in the case of larger reactant molecules. Catalytic activity was also tested for the hydrogenation of p-nitrophenol. The PVP- or TTAB-capped nanocubes were dispersed in p-nitrophenol aqueous solution and the intensity of the UV vis spectrum peak at 400 nm was monitored to evaluate catalytic activity. Fig. 4 shows the reduction of p-nitrophenol over time, as observed by UV vis. Initial TOF was 0.59 s -1 for PVP-capped nanocubes and 0.58 s 1 for TTAB-capped nanocubes. Although p-nitrophenol is larger even than benzene, catalytic activity was similar for PVP- and TTAB-capped nanocubes. When the nanocubes were dispersed in aqueous solution as colloids, the organic agents spread out instead of lying on the Pt surface. In this case, the length of alkyl chain would have had minimal effect; instead, the density of pseudo covalent bonding between organic agent and the Pt surface would have been of greater importance. The bonding density appears to be similar between the two types of nanocubes, indicated by similar reactivity. The shape of the cubic nanoparticles was preserved after gas-phase or solution-phase catalytic reactions. 4. Conclusions Pt nanocubes of similar shapes and size distributions were synthesized with two different organic capping agents (PVP and TTAB). Compared with PVP-capped nanocubes, TTAB-capped nanocubes showed 3.8 times higher activity for H + adsorption/ desorption, 7.1 times higher activity for C 2 H 4 hydrogenation, and 13.4 times higher activity for C 6 H 6 hydrogenation. TTABcapped nanocubes have more ensembles of clean Pt atoms and these ensembles make dramatic difference to the catalytic activity with the increasing molecular size of the reactants. TTAB- and PVP-capped nanocubes showed similar activity for p-nitrophenol hydrogenation in a solution phase. In contrast to the above reactions, in this case the capping agents could spread out freely in the solution, minimizing the effect on reactivity of alkyl chain length of the capping agent. This work clearly demonstrates that the surface-capping agents have a marked effect on the catalytic properties, depending on the characteristics of the catalytic reactions. 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