Enhanced catalytic activity of solid and hollow platinum-cobalt nanoparticles towards reduction of 4-nitrophenol

G Model

ARTICLE IN PRESS

APSUSC-33090; No. of Pages 7

Applied Surface Science xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Enhanced catalytic activity of solid and hollow platinum-cobalt nanoparticles towards reduction of 4-nitrophenol ˛ Jan Krajczewski, Karol Kołataj, Andrzej Kudelski ∗ Faculty of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 14 January 2016 Received in revised form 11 April 2016 Accepted 13 April 2016 Available online xxx Keywords: Platinum nanoparticles Hollow platinum nanospheres Pt catalysts Reduction of 4-nitrophenol

a b s t r a c t Previous investigations of hollow platinum nanoparticles have shown that such nanostructures are more active catalysts than their solid counterparts towards the following electrochemical reactions: reduction of oxygen, evolution of hydrogen, and oxidation of borohydride, methanol and formic acid. In this work we show that synthesised using standard galvanic replacement reaction (with Co templates) hollow platinum nanoparticles exhibit enhanced catalytic activity also towards reduction of 4-nitrophenol by sodium borohydride in water. Unlike in the case of procedures involving hollow platinum catalysts employed so far to carry out this reaction it is not necessary to couple analysed platinum nanoparticles to the surface of an electrode. Simplification of the analyzed reaction may eliminate same experimental errors. We found that the enhanced catalytic activity of hollow Pt nanoparticles is not only connected with generally observed larger surface area of hollow nanostructures, but is also due to the contamination of formed hollow nanostructures with cobalt, from which sacrificial templates used in the synthesis of hollow Pt nanostrustures have been formed. Because using sacrificial templates is a typical method of synthesis of hollow metal nanostructures, formed hollow nanoparticles are probably often contaminated, which may significantly influence their catalytic activity. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Nanoparticles of noble metals are widely used as catalysts in many important fields [1–3]. High price of noble metals significantly limits, however, their practical applications. To decrease the consumption of expensive noble metals in practical commercial catalysts the following approaches have been applied: (i) alloying of noble metal with non-noble metals [4,5], (ii) formation of coreshell nanoparticles with noble metal shell and non-noble metal core [6,7], and (iii) formation of noble metal hollow nanostructures [8–12]. The last approach is especially interesting. Therefore, many groups synthesised hollow nanostructures and compared their catalytic activity with analogous solid objects [8–29]. In many works higher catalytic activity of hollow nanoparticles is explained by the fact that hollow metallic structures have lower density and hence higher surface area than analogous solid nanoparticles. In the field of catalysis, platinum has attracted particular attention. Therefore, we decided to investigate platinum hollow nanoparticles. Previous comparative studies of solid and hollow platinum nanoparticles have revealed that hollow platinum

∗ Corresponding author. E-mail address: [email protected] (A. Kudelski).

nanoparticles are catalytically more active than their solid counterparts in the following electrochemical reactions: oxidation of methanol [13–24], oxidation of formic acid [25,26], oxidation of borohydride [27], reduction of oxygen [8–12] and evolution of hydrogen [28,29]. Hollow Pt nanostructures are often synthesised by the galvanic replacement reaction using sacrificial templates from a less active metal [11–18,26–28], especially cobalt [13–15,26–28]. Therefore, hollow Pt nanoparticles may be contaminated by the material from which sacrificial template was created. This may significantly influence their catalytic activity. In this work we decided to verify whether contamination by the other metal typically used for the synthesis of the sacrificial templates may, in some cases, significantly influence the catalytic activity of platinum nanostructures. As mentioned above all works showing enhanced catalytic activity of hollow platinum nanostructures have been carried out using electrochemical setups and platinum nanoparticles were deposited on the surface of an electrode (in comparative experiments the same mass of nanospheres and solid Pt nanoclusters had to be deposited). Differences in the sticking of various platinum nanoparticles to the surface of an electrode (and hence differences in the efficiency of the electron transfer between nanoparticles and an electrode) may, however, introduce significant errors in these measurements. Therefore, it would be very useful to find a model

http://dx.doi.org/10.1016/j.apsusc.2016.04.089 0169-4332/© 2016 Elsevier B.V. All rights reserved.

Please cite this article in press as: J. Krajczewski, et al., Enhanced catalytic activity of solid and hollow platinum-cobalt nanoparticles towards reduction of 4-nitrophenol, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.04.089

G Model APSUSC-33090; No. of Pages 7

ARTICLE IN PRESS

2

J. Krajczewski et al. / Applied Surface Science xxx (2016) xxx–xxx

heterogeneous catalytic reaction carried out using standard sols of platinum nanostructures – this should allow for simpler quantitative comparison of catalytic activity of hollow and solid platinum nanoparticles. We found that as such model reaction one can use reduction of 4-nitrophenol by sodium borohydride in water. 4-Nitrophenol is a common byproduct from the production of synthetic dyes, pesticides, and herbicides [30]. 4-Nitrophenol is easily reduced by sodium borohydride in the presence of metals nanoparticles [31,32]. In particular nanoparticles of coinage metals are excellent catalysts for this reduction [33]. The catalytic properties of various metal nanoparticles strongly depend on their electronic structure and their adsorption properties, and hence, for example, some bimetallic metal nanoparticles catalyse reduction of 4-nitrophenol with rates that strongly differ from a simple linear interpolation between the rates of the two pure metals [34,35]. Moreover, the progress of the reduction of 4-nitrophenol by sodium borohydride may be easily determined from a simple spectroscopic UV–vis measurement (from the temporal disappearance of the strong adsorption band due to the nitrophenolate). Therefore, reduction of 4-nitrophenol is one of the model catalytic reactions studied by many groups. Recently Xia et al. reported that under the similar catalysis conditions hollow gold nanospheres showed a higher activity than other gold nanocatalysts towards this reduction [36]. 2. Experimental 2.1. Materials H2 PtCl6 ·6H2 O, K2 PtCl4 , PtCl2 , and CoCl2 ·6H2 O were purchased from Sigma-Aldrich. A 30% HAuCl4 solution (99.99% trace metals basis) in dilute HCl was acquired from Mennica Panstwowa (Poland). Sodium borohydride and polyvinylpyrrolidone (PVP) with the average molar mass ∼40,000 g mol−1 were purchased from Fluka Analytical. Ethylene glycol, 4-nitrophenol, sodium citrate (Na3 Cit) and NaOH were acquired from POCH S.A. (Poland). Nitrogen (≥99.999%) were purchased from Air Products. All chemicals were used without further purification or treatment. Water used for all experiments was purified by Millipore Milli-Q system and has the resistivity of ca. 18 M cm.

nanoparticles. An anaerobic environment is vital for this reaction because cobalt nanoparticles are unstable in the presence of oxygen. Solution of Co nanoparticles was kept under nitrogen influx for 1 h. In this time solutions containing 10 ml of water and various amounts (15–35 ␮l) of 0.1 M K2 PtCl4 were prepared (for example, when using 15 ␮l of 0.1 M K2 PtCl4 solution the ratio of numbers of moles of Pt and Co in the final reaction mixture was equal to 0.13:1). Finally, samples of 30 ml of prepared cobalt nanoparticles were added to the prepared K2 PtCl4 solutions and kept under air. Obtained sol of hollow platinum nanoparticles was colourless. Schematic diagram of the synthesis procedure is shown in Fig. 1. For comparison experiments Au@Pt nanostructures were also produced. These nanoparticles were synthesised using procedure developed by Nilekar et al. for the deposition of platinum layers on noble metals nanoparticles [38]. In the first step 0.2 mmol of HAuCl4 and 54 mg of PVP were dissolved in 60 ml of ethylene glycol. The solution was heated to 60 ◦ C. Then freshly prepared solution of NaBH4 in ethylene glycol (160 mg of NaBH4 was dissolved in 20 ml of ethylene glycol) was added dropwise to the HAuCl4 solution. Colour of the reaction mixture became purple. In the next step 40 ml of obtained gold colloid was injected into the PtCl2 suspension (0.068 mmol of PtCl2 was added to 20 ml of ethylene glycol). The temperature of the reaction mixture was quickly ramped to 60 ◦ C and then slowly brought to 100 ◦ C and held in this temperature for 3 h. 2.3. Decomposition of 4-nitrophenol The catalytic properties of metal nanoparticles were examined by the analysis of the kinetic of the reduction of 4-nitrophenol by sodium borohydride. Typically, 2 ml of water, 20 ␮l of 0.01 M aqueous solution of 4-nitrophenol and ca. 40 ␮l of suspension of metal nanoparticles (the actually added volume was corrected in such a way that the same mass of platinum was introduced into the cuvette) were placed into a 10 mm quartz cuvette (with nominal volume of 3.5 ml) from Starna Cells, Inc. In the next step 0.3 ml of 0.1 M freshly prepared solution of sodium borohydride were added. After this the colour of the solution suddenly changed from pale yellow to tight yellow, due to formation of 4-nitrophenolate ions. To investigate the decomposition of nitro-aromatic compounds UV–vis absorption spectra were recorded in the spectral range between 250 and 500 nm every 30 s.

2.2. Synthesis of nanoparticles 2.4. Experimental techniques Pt nanoparticles were synthesised using slightly modified procedure proposed by Baranova et al. [37]. Briefly speaking H2 PtCl6 ·6H2 O was dissolved in 20 ml of 0.15 M NaOH solution in ethylene glycol to obtain final concentration of H2 PtCl6 equal to 0.03 M. The solution was rigorously stirred for 30 min. Then the solution was heated and refluxed for 3 h at 160 ◦ C. The finally produced suspension of Pt nanoparticles was black. Solid Pt nanoparticles doped by cobalt were synthesised by the addition of CoCl2 to the solution of Pt nanoparticles (the number of moles of introduces CoCl2 was equal to the number of moles of platinum). Subsequently, the solution of sodium borohydride (nNaBH4 :nCoCl2 = 2.5) has been added to reduce cobalt cations. Then the reaction mixture was kept under air to oxidise deposited metallic cobalt layer. Platinum hollow nanoparticles were synthesised using procedure similar to those reported by Liang et al. [13]. Briefly, 100 ml of water was placed in a three-neck bottle. Then 0.1 ml of 0.4 M CoCl2 and 0.4 ml of 0.1 M Na3 Cit solutions were added. Subsequently, the flask was sealed and solution was deoxygenated by nitrogen influx for 30 min. After this time 0.1 ml of freshly prepared 1 M sodium borohydride solution was rapidly injected. The reaction mixture adopted dark brown colour which indicated formation of cobalt

UV–vis absorption spectra were collected using a Thermo Scientific Evolution 201 spectrophotometer. The transmission electron microscopy (TEM) analysis were carried out using LIBRA 120 (Zeiss, Germany) electron microscope working at an accelerating voltage of 120 kV and equipped with the In-column OMEGA filter. The samples of obtained suspensions of different platinum nanoparticles for TEM measurements were dropped onto Formvar-coated 400-mesh nickel grids (Agar Scientific) and allowed to dry. To determine the chemical composition of hollow Pt nanoparticles the samples of such nanostructures have been deposited on surface of the graphite substrate and studied with a Merlin field emission scanning electron microscope (Zeiss, Germany). The elemental analysis was performed with an energy-dispersive X-ray microanalysis (EDS) probe (Bruker). 3. Results and discussion 3.1. TEM analysis of obtained metal nanoparticles Fig. 2 shows TEM micrographs of the obtained Pt and Au@Pt nanoparticles. As can be seen from images presented in Fig. 2a

Please cite this article in press as: J. Krajczewski, et al., Enhanced catalytic activity of solid and hollow platinum-cobalt nanoparticles towards reduction of 4-nitrophenol, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.04.089

G Model APSUSC-33090; No. of Pages 7

ARTICLE IN PRESS J. Krajczewski et al. / Applied Surface Science xxx (2016) xxx–xxx

3

Fig. 1. Schematic representation of the synthetic procedure that was applied for the preparation of hollow platinum nanoparticles.

and b, the average size of synthesised Pt nanoparticles is equal to 3.0 ± 0.5 nm. Micrographs of Au@Pt nanoparticles are shown in Fig. 2c. Because Fig. 2c presents images of only 22 Au@Pt nanoparticles, Au@Pt nanoparticles depictured on other TEM micrographs were also measured, and the average size of Au@Pt nanoparticles was calculated from 200 objects as equal to 27 ± 4 nm. Hollow Pt nanoparticles were synthesised by the galvanic replacement reaction (see Eq. (1)) between Co nanoparticles and solution of K2 PtCl4 .

Co + PtCl4 2− → Pt + Co2+ + 4Cl−

(1)

The number of moles of added K2 PtCl4 was significantly smaller than the number of moles of metallic cobalt (the actually used ratios of numbers of moles of Pt and Co were between 0.13:1 and 0.3:1), therefore, PtCl4 2− could not dissolve all metallic nanocobalt present in the solution. However, after galvanic replacement the reaction mixture was exposed to air, which caused oxidation of remaining Co cores leaving hollow Pt nanoparticles. From energydispersive X-ray spectroscopy (EDS) measurements of formed hollow nanoparticles we found that synthesised nanostructures are to some extent contaminated with cobalt (formed nanostructures contained between 11% and 16% of cobalt). We also found that the outer diameter of formed hollow platinum nanoparticles does not depend on the amount of added K2 PtCl4 – in all cases this diameter was equal to ca. 16 nm. On the other hand, the thickness of formed platinum hollow nanospheres strongly depends on the initial ratio of numbers of moles of metallic Co and K2 PtCl4 – the larger this ratio, the thicker shell is formed (see Fig. 3). For example, when the ratio of numbers of moles of K2 PtCl4 and Co increases from 0.13:1 to 0.30:1, the average thickness of the Pt shells increases from ca. 3 ± 1 nm to ca. 6 ± 1 nm (see micrographs a and b in Fig. 3).

3.2. Catalytic reduction of 4-nitrophenol Aqueous solution of 4-nitrophenol with the concentration of 10−4 M is pale yellow – the UV–vis absorption spectrum of 4nitrophenol is dominated by the broad band centred at 316 nm (see Fig. 4). When solution of sodium borohydride is added to the solution of 4-nitrophenol, the formed mixture changes colour to bright yellow. This change of the colour is due to the formation of the phenolic salt (with maximum absorption at 399 nm – see Fig. 4) after introduction of the alkaline solution of sodium borohydride. pKa of 4-nitrophenol is equal to 7.16 and observed colour-changing property of 4-nitrophenol makes this compound useful as a pH indicator [39]. Relatively large acidic dissociation constant of 4-nitrophenol (pKa = 7.16) in comparison with phenol (pKa = 9.99) is due to the electron-withdrawing power of para-nitro group [39]. The mixture of solutions of 4-nitrophenol and sodium borohydride does not change colour for many hours – we have not observed any significant change of the UV–vis spectra of such mixture even after 3 days. The situation changes when some metal nanoparticles are introduced to the solution containing 4-nitrophenol and sodium borohydride. In this case bright yellow colour quickly fades as a result of the reduction of 4-nitrophenolate ions to 4-aminophenol (with the strong absorption band at 303 nm). Fig. 5 shows the temporal evolution of the UV–vis absorption spectrum of the mixture of 4-nitrophenol and sodium borohydride after addition of the sol containing hollow platinum nanoparticles (synthesised in the galvanic replacement reaction – the ratio of numbers of moles of K2 PtCl4 and Co was equal to 0.22:1). As mentioned in the Introduction the progress of the reduction of 4-nitrophenolate ions may be easily followed by the measurement of the absorption of the reaction mixture at the wavelength at which strong band due to the nitrophenolate ions appears (ca. 400 nm). Fig. 6 shows temporal evolution of the absorption at 399 nm for 4 samples containing the same amount of 4-nitrophenol

Please cite this article in press as: J. Krajczewski, et al., Enhanced catalytic activity of solid and hollow platinum-cobalt nanoparticles towards reduction of 4-nitrophenol, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.04.089

G Model APSUSC-33090; No. of Pages 7

ARTICLE IN PRESS

4

J. Krajczewski et al. / Applied Surface Science xxx (2016) xxx–xxx

Fig. 3. TEM micrographs of platinum hollow nanospheres obtained using the ratio of numbers of moles of K2 PtCl4 and Co equal to: (a) 0.13:1, and (b) 0.30:1.

Fig. 2. TEM micrographs of: (a, b) solid Pt nanoparticles, and (c) Au@Pt nanoparticles.

and sodium borohydride (so, the absorption at the starting point was the same), to which small portions of various metal nanoparticles (containing the same amount of platinum) have been added. As can be seen in Fig. 6, after addition of hollow Pt nanoparticles significantly larger amount of 4-nitrophenol has been reduced than after addition of solid nanoparticles. Since the initial concentration of the reducing agent (NaBH4 ) was 150 times higher than the concentration of 4-nitrophenol, in the first approximation we assumed for this reaction the first order reaction kinetics with regard to the concentration of 4-nitrophenol. Inset in Fig. 6 shows the relationships between ln(A(t)/A(0)) and reaction time, where A(t) and A(0) are absorbance of the reaction mixture at time t and at time 0 (it means before addition of metal nanoparticles) measured at 399 nm. As can be seen in this inset when using Pt nanoparticles as catalyst the obtained ln(A(t)/A(0)) dependence is linear, which confirms that in

this case the studied reaction is actually the first order with regard to 4-nitrophenol. The rate constant (k) of this catalytic reduction can be determined from the slope of the received linear plot. For the linear plot obtained for the reduction carried out using solid Pt nanoparticles (line a) the reaction rate constant k was determined to be 4.3 × 10−3 s−1 . In the case of using hollow Pt nanoparticles we observed some derogation from the simple first order reaction kinetics with regard to the concentration of 4-nitrophenol – at the beginning the reaction is significantly slower, which suggests existence of some incubation time. An incubation time for this reaction has been also observed by Pozun et al. [35] when alloy PdCu nanoparticles have been used as catalysts, however, Pozun et al. have not suggested any mechanism of this phenomenon. Existence of the incubation time for this catalytic reaction may be due, for example, to some surface reconstruction process of hollow Pt nanoparticles. To compare the efficiency of various Pt catalysts we estimated the reaction rate constant k also for the catalytic reduction using hollow Pt nanoparticles – we applied for this process also the simple model of the first order reaction kinetics with regard to the concentration of 4-nitrophenol and obtained the value of the

Please cite this article in press as: J. Krajczewski, et al., Enhanced catalytic activity of solid and hollow platinum-cobalt nanoparticles towards reduction of 4-nitrophenol, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.04.089

G Model

ARTICLE IN PRESS

APSUSC-33090; No. of Pages 7

J. Krajczewski et al. / Applied Surface Science xxx (2016) xxx–xxx

5

0.9

0.8

0 -0.5 ln[A(t)/A(0)]

0.8 0.7 0.6 Absorbance

Absorbance

0.6

0.4

-1

a)

-1.5

c)

-2

b)

d)

-2.5 -3 100

0

0.5

200

300

Time / s

0.4

a)

0.3 b)

b)

0.2

0.2

c)

0.1

d)

a) 0 250

300

350

400

450

500

Absorbance

Fig. 4. UV–vis absorption spectrum of: (a) 10−4 M aqueous solutions of 4nitrophenol, and (b) alkalized 10−4 M aqueous solutions of 4-nitrophenol.

250

0.2

300

350 400 450 Wavelength / nm

0 20

Wavelength / nm

500

Fig. 5. Temporal evolution of the UV–vis absorption spectrum of the reaction mixture containing 4-nitrophenol and sodium borohydride after addition of the sol containing hollow Pt nanoparticles. Hollow platinum nanoparticles were synthesised in the galvanic replacement reaction using the ratio of numbers of moles of K2 PtCl4 and Co equal to 0.22:1. Spectra are shown at 30 s intervals.

reaction rate constant k equal to 8.5 × 10−3 s−1 (it means about two times larger than in case of solid Pt nanoparticles). As one can easily calculate from the basic geometric formulas the ratio of the surface area of the sphere to the volume of the sphere is equal to 3/rs , where rs is the radius of the spherical solid nanoparticle. For hollow spherical shell the ratio of the area of the outer surface to the volume of the material of which the shell is made is equal to 3rou 2 /(rou 3 − rin 3 ), where rou is the outer diameter of the shell and rin is the inner diameter of the hollow sphere. For analysed in this work thinner Pt shells (with rou = 16 nm, rin = 13 nm) the surface area of the nanoparticles is ca. 3.8 times smaller than the surface area of used in this work spherical solid Pt nanoparti-

60

100 140 180 220 260 300 Time / s

Fig. 6. Typical temporal evolution of the absorption at 399 nm for 4 samples containing the same amount of 4-nitrophenol and sodium borohydride (see Section 2), to which small portions of various metal nanoparticles (containing the same amount of platinum) have been added. Addition of: (a) solid Pt nanoparticles, (b) Au@Pt nanoparticles, (c) hollow Pt nanoparticles with the average thickness of the Pt shells equal to ca. 3 ± 1 nm, and (d) hollow Pt nanoparticles with the average thickness of the Pt shells equal to ca. 6 ± 1 nm. Inset: the relationships between ln(A(t)/A(0)) and reaction time, where A(t) and A(0) are absorbance of the reaction mixture at time t and at time 0 (before addition of metal nanoparticles) measured at 399 nm. Straight lines have been fitting to the respective experimental date using linear least squares fitting procedure.

cles (with rs = 3 nm), whereas the surface area of the thicker shells (with rou = 16 nm, rin = 10 nm) is even ca. 10 times smaller than the surface area of used in this work solid Pt nanoparticles. On the other hand, the catalytic activity of hollow nanoparticles is significantly larger than the activity of solid nanostructures. It supports the hypothesis that higher catalytic activity of hollow Pt nanoparticles towards reduction of 4-nitrophenol is not only connected with the higher surface area generally predicted for hollow metal nanostructures, but is also probably due to the geometric and electronic differences in surfaces of various nanoparticles. We found that the obtained hollow Pt nanostructures are contaminated with cobalt (material from which the sacrificial templates were created), therefore, we decided to verify whether such contamination may influence catalytic activity of Pt nanoparticles. Fig. 7 shows temporal evolution of the absorption at 399 nm for 2 samples containing the same amount of 4-nitrophenol and sodium borohydride, to which the same amount of Co-doped and non-modified solid platinum nanoparticles has been added. As can be seen in Fig. 7, Co-doped nanostructures are significantly better catalysts because decrease of the UV band due to the 4-nitrophenolate is significantly faster after addition of these nanoparticles. It means that contamination with cobalt is probably the main reason for enhanced catalytic activity of hollow Pt nanostructures. Because using sacrificial templates is a typical method of synthesis of hollow metal nanostructures, formed hollow nanoparticles are probably often contaminated, which may significantly influence their catalytic activity. We have also investigated the catalytic activity of Au@Pt nanoparticles towards reduction of 4-nitrophenol. As can be seen on Fig. 6, the activity of these nanoparticles is also larger than the activity of Pt nanoparticles (the reaction rate constant k determined from the line b in the inset in Fig. 6 is equal to 7.0 × 10−3 s−1 ),

Please cite this article in press as: J. Krajczewski, et al., Enhanced catalytic activity of solid and hollow platinum-cobalt nanoparticles towards reduction of 4-nitrophenol, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.04.089

G Model APSUSC-33090; No. of Pages 7

ARTICLE IN PRESS

6

J. Krajczewski et al. / Applied Surface Science xxx (2016) xxx–xxx

0.8

References

0.7

Absorbance

0.6

0.5

0.4

0.3

a) b)

0.2

0.1 20 60 100 140 180 220 260 300 Time / s Fig. 7. Typical temporal evolution of the absorption at 399 nm for 2 samples containing the same amount of 4-nitrophenol and sodium borohydride to which the same amount of solid platinum nanoparticles doped with cobalt and before doping with cobalt has been added (see Section 2). Addition of: (a) solid Pt nanoparticles before doping with cobalt, (b) Pt nanoparticles doped with cobalt.

although the surface area of the platinum layer in this system is likewise significantly smaller than in the case of very small Pt nanoparticles (see Fig. 2). This supports the hypothesis that the catalytic activity of the noble metal nanoparticles towards reduction of 4-nitrophenol is not only connected with their surface area, but geometric and electronic differences in the structure of the surface layer of various nanoparticles are also very important (however, in this case, the modification of the surface properties of the formed nanostructures is not caused by doping with cobalt). 4. Conclusions The first example of high catalytic activity of hollow platinum nanoparticles towards non-electrochemical reaction is reported. Contrary to studied so far catalytic processes involving hollow platinum nanoparticles to carry out this reaction it is not necessary to couple platinum nanoparticles to the surface of an electrode. Simplification of the experimental procedure may significantly decrease the errors of measured parameters. Our results support the hypothesis that the catalytic activity of noble metal nanoparticles towards reduction of 4-nitrophenol is not only connected with the surface area of nanoparticles, but is also probably strongly dependent on the geometric and electronic structure of the surface layer of metal clusters. Such differences are probably due to the contamination of the formed hollow Pt nanostructures with cobalt, from which sacrificial templates used for formation of hollow nanostructures are synthesised. Because using sacrificial templates is a typical method of synthesis of hollow metal nanostructures, the formed hollow nanoparticles are probably often contaminated, which may significantly influence their catalytic activity. Acknowledgements The electron microscope was purchased under CePT project, which was co-financed by European Union from the European Regional Development Fund under the Operational Programme Innovative Economy 2007–2013.

[1] P. Waszczuk, J. Solla-Gullón, H.S. Kim, Y.Y. Tong, V. Montiel, A. Aldaz, A. Wieckowski, Methanol electrooxidation on platinum/ruthenium nanoparticle catalysts, J. Catal 203 (2001) 1–6. [2] K.M. Bratlie, H. Lee, K. Komvopoulos, P. Yang, G.A. Somorjai, Platinum nanoparticle shape effects on benzene hydrogenation selectivity, Nano Lett. 7 (2007) 3097–3101. [3] R. Narayanan, M.A. El-Sayed, Shape-dependent catalytic activity of platinum nanoparticles in colloidal solution, Nano Lett. 4 (2004) 1343–1348. [4] L. Yang, C. Hu, J. Wang, Z. Yang, Y. Guo, Z. Bai, K. Wang, Facile synthesis of hollow palladium/copper alloyed nanocubes for formic acid oxidation, Chem. Commun. 47 (2011) 8581–8583. [5] V.R. Stamenkovic, B.S. Mun, M. Arenz, K.J.J. Mayrhofer, C.A. Lucas, G. Wang, P.N. Ross, N.M. Markovic, Trends in electrocatalysis on extended and nanoscale Pt-bimetallic alloy surfaces, Nat. Mater. 6 (2007) 241–247. [6] S. Wojtysiak, M. Kaminski, J. Krajczewski, P. Dłuzewski, A Kudelski, Adsorption of CO on various M@Pt core–shell nanoparticles: surface-enhanced infrared absorption and DFT studies, Vib. Spectrosc. 75 (2014) 11–18. [7] R.G. Chaudhuri, S. Paria, Core/shell nanoparticles: classes, properties, synthesis mechanisms, characterization, and applications, Chem. Rev. 112 (2012) 2373–2433. [8] J.W. Hong, S.W. Kang, B.S. Choi, D. Kim, S.B. Lee, S.W. Han, Controlled synthesis of Pd–Pt alloy hollow nanostructures with enhanced catalytic activities for oxygen reduction, ACS Nano 6 (2012) 2410–2419. [9] T. Kijima, Y. Nagatomo, H. Takemoto, M. Uota, D. Fujikawa, Y. Sekiya, T. Kishishita, M. Shimoda, T. Yoshimura, H. Kawasaki, G. Sakai, Synthesis of nanohole-structured single-crystalline platinum nanosheets using surfactant-liquid-crystals and their electrochemical characterization, Adv. Funct. Mater. 19 (2009) 545–553. [10] J. Wu, Z. Peng, H. Yang, Supportless oxygen reduction electrocatalysts of CoCuPt hollow nanoparticles, Phil. Trans. R. Soc. A 368 (2010) 4261–4274. [11] H. Ming-Chen, R.S. Liu, M.Y. Lo, S. Ch Chang, L.D. Tsai, Y.M. Peng, J.F. Lee, Hollow platinum spheres with nano-channels: synthesis and enhanced catalysis for oxygen reduction, J. Phys. Chem. C 112 (2008) 7522–7526. [12] Y. Zhang, Ch. Ma, Y. Zhu, R. Si, Y. Cai, J.X. Wang, R.R. Adzic, Hollow core supported Pt monolayer catalysts for oxygen reduction, Catal. Today 202 (2013) 50–54. [13] H.P. Liang, H.M. Zhang, J.S. Hu, Y.G. Guo, L.J. Wan, Ch.L. Bai, Pt hollow nanospheres: facile synthesis and enhanced electrocatalysts, Angew. Chem. Int. Ed. 43 (2004) 1540–1543. [14] J. Zhao, W. Chen, Y. Zheng, X. Li, Novel carbon supported hollow Pt nanospheres for methanol electrooxidation, J. Power Sources 162 (2006) 168–172. [15] D.J. Guo, L. Zhao, X.P. Qiu, L.Q. Chen, W.T. Zhu, Novel hollow PtRu nanospheres supported on multi-walled carbon nanotube for methanol electrooxidation, J. Power Sources 177 (2008) 334–338. [16] Y.J. Kuang, B.H. Wu, Y. Cui, Y.M. Yu, X.H. Zhang, J.H. Chen, Preparation of hollow platinum nanospheres/carbon nanotubes nanohybrids and their improved stability for electro-oxidation of methanol, Electrochim. Acta 56 (2011) 8645–8650. [17] F. Ye, J. Yang, W. Hu, H. Liu, S. Liao, J. Zeng, J. Yang, Electrostatic interaction based hollow Pt and Ru assemblies toward methanol oxidation, RSC Adv. 2 (2012) 7479–7486. [18] R. Wu, Q. Ch. Kong, Ch. Fu, S.Q. Lai, C. Ye, J.Y. Liu, Y. Chen, J.Q. Hu, One-pot synthesis and enhanced catalytic performance of Pd and Pt nanocages via galvanic replacement reactions, RSC Adv. 3 (2013) 12577–12580. [19] X. Yu, D. Wang, Q. Peng, Y. Li, High performance electrocatalyst: pt–Cu hollow nanocrystals, Chem. Commun. 47 (2011) 8094–8096. [20] L. Wang, Y. Yamauchi, Metallic nanocages: synthesis of bimetallic Pt-Pd hollow nanoparticles with dendritic shells by selective chemical etching, J. Am. Chem. Soc. 135 (2013) 16762–16765. [21] D.J. Guo, S.K. Cui, Hollow PtCo nanospheres supported on multi-walled carbon nanotubes for methanol electrooxidation, J. Colloid Interface Sci. 340 (2009) 53–57. [22] X.W. Zhou, R.H. Zhang, Z.Y. Zhou, S.G. Sun, Preparation of PtNi hollow nanospheres for the electrocatalytic oxidation of methanol, J. Power Sources 196 (2011) 5844–5848. [23] F. Bai, Z. Sun, H. Wu, R.E. Haddad, X. Xiao, H. Fan, Templated photocatalytic synthesis of well-defined platinum hollow nanostructures with enhanced catalytic performance for methanol oxidation, Nano Lett. 11 (2011) 3759–3762. [24] A. Shan, M. Cheng, H. Fan, Z. Chen, R. Wang, C. Chen, NiPt hollow nanocatalyst: green synthesis, size control and electrocatalysis, Prog. Nat. Sci. 24 (2014) 175–178. [25] D.J. Chen, Z.Y. Zhou, Q. Wang, D.M. Xiang, N. Tian, S.G. Sun, A non-intermetallic PtPb/C catalyst of hollow structure with high activity and stability for electrooxidation of formic acid, Chem. Commun. 46 (2010) 4252–4254. [26] B. Liu, H.Y. Li, L. Die, X.H. Zhang, Z. Fan, J.H. Chen, Carbon nanotubes supported PtPd hollow nanospheres for formic acid electrooxidation, J. Power Sources 186 (2009) 62–66. [27] L. Yi, Y. Song, W. Yi, X. Wang, H. Wang, P. He, B. Hu, Carbon supported Pt hollow nanospheres as anode catalysts for direct borohydride-hydrogen peroxide fuel cells, Int. J. Hydrogen Energy 36 (2011) 11512–11518.

Please cite this article in press as: J. Krajczewski, et al., Enhanced catalytic activity of solid and hollow platinum-cobalt nanoparticles towards reduction of 4-nitrophenol, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.04.089

G Model APSUSC-33090; No. of Pages 7

ARTICLE IN PRESS J. Krajczewski et al. / Applied Surface Science xxx (2016) xxx–xxx

[28] R. Ojani, R. Valiollahi, J.B. Raoof, Comparison between graphene supported Pt hollow nanospheres and graphene supported Pt solid nanoparticles for hydrogen evolution reaction, Energy 74 (2014) 871–876. [29] C.L. Lee, Ch.M. Tseng, Ag-Pt nanoplates: galvanic displacement preparation and their applications as electrocatalysts, J. Phys. Chem. C 112 (2008) 13342–13345. [30] T. Vincent, E. Guibal, Chitosan-supported palladium catalyst. 3. Influence of experimental parameters on nitrophenol degradation, Langmuir 19 (2003) 8475–8483. [31] K. Hayakawa, T. Yoshimura, K. Esumi, Preparation of gold-dendrimer nanocomposites by laser irradiation and their catalytic reduction of 4-nitrophenol, Langmuir 19 (2003) 5517–5521. [32] K. Kuroda, T. Ishida, M. Haruta, Reduction of 4-nitrophenol to 4-aminophenol over Au nanoparticles deposited on PMMA, J. Mol. Catal. A: Chem. 298 (2009) 7–11. [33] N. Pradhan, A. Pal, T. Pal, Catalytic reduction of aromatic nitro compounds by coinage metal nanoparticles, Langmuir 17 (2001) 1800–1802. [34] S.K. Ghosh, M. Mandal, S. Kundu, S. Nath, T. Pal, Bimetallic Pt-Ni nanoparticles can catalyze reduction of aromatic nitro compounds by sodium borohydride in aqueous solution, Appl. Catal. A 268 (2004) 61–66.

7

[35] Z.D. Pozun, S.E. Rodenbusch, E. Keller, K. Tran, W. Tang, K.J. Stevenson, G. Henkelman, A systematic investigation of p-nitrophenol reduction by bimetallic dendrimer encapsulated nanoparticles, J. Phys. Chem. C 117 (2013) 7598–7604. [36] Q. Xia, D. Su, X. Yang, F. Chai, Ch. Wang, J. Jiang, One pot synthesis of gold hollow nanospheres with efficient and reusable catalysis, RSC Adv. 5 (2015) 58522–58527. [37] E.A. Baranova, C. Bock, D. Ilin, D. Wang, B. MacDougall, Infrared spectroscopy on size-controlled synthesized Pt-based nano-catalysts, Surf. Sci. 600 (2006) 3502–3511. [38] A.U. Nilekar, S. Alayoglu, B. Eichhorn, M. Mavrikakis, Preferential CO oxidation in hydrogen: reactivity of core-shell nanoparticles, J. Am. Chem. Soc. 132 (2010) 7418–7428. [39] G.C. Shields, P.G. Seybold, Computational Approaches for the Prediction of pKa Values, CRC Press Boca Raton, 2014, pp. 103.

Please cite this article in press as: J. Krajczewski, et al., Enhanced catalytic activity of solid and hollow platinum-cobalt nanoparticles towards reduction of 4-nitrophenol, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.04.089