Nanocomposite of montmorillonite and silver nanoparticles: Characterization and application in catalytic reduction of 4-nitrophenol

Nanocomposite of montmorillonite and silver nanoparticles: Characterization and application in catalytic reduction of 4-nitrophenol

Materials Chemistry and Physics 140 (2013) 493e498 Contents lists available at SciVerse ScienceDirect Materials Chemistry and Physics journal homepa...

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Materials Chemistry and Physics 140 (2013) 493e498

Contents lists available at SciVerse ScienceDirect

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Nanocomposite of montmorillonite and silver nanoparticles: Characterization and application in catalytic reduction of 4-nitrophenol Petr Praus a, *, Martina Turicová a, Martina Karlíková b, Libor Kvítek b, Richard Dvorský c Department of Analytical Chemistry and Material Testing, V SB-Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava-Poruba, Czech Republic b Department of Physical Chemistry, Palacký University, 17. listopadu 12, 771 46 Olomouc, Czech Republic c Institute of Physics, V SB-Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava-Poruba, Czech Republic a

h i g h l i g h t s  Ag nanoparticles with an average size of 6.9 nm were reduced on montmorillonite.  Ag nanoparticles were fixed in montmorillonite pores forming a stable nanocomposite.  Ag in the nanocomposite showed catalytic activity for reduction of 4-nitrophenol.  Reaction kinetics was explained by the LangmuireHinshelwood model.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 December 2012 Received in revised form 7 March 2013 Accepted 19 March 2013

Silver ions previously intercalated into a montmorillonite (MMT) interlayer were reduced by sodium borohydride forming a nanocomposite of MMT and silver nanoparticles (AgeMMT) with no other stabilizing additives. Within 360 min no coagulation of an aqueous AgeMMT dispersion was observed. However, after 24 h the coagulation was indicated by a red shift of absorption maximum from 408 nm to 434 nm and by broadening of the absorbance band. The nanocomposite was characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and measurements of specific surface area (SSA). It contained 4.94 wt. % of silver. Ag nanoparticles with an average size of 6.9 nm were located on the external MMT surface, mostly in its pores. AgeMMT was used as a catalyst for reduction of 4-nitrophenol with sodium borohydride forming 4aminophenol. After 30 s the reaction kinetics changed from zero order to first order, which was explained by means of the LangmuireHinshelwood model. The whole reduction was completed after 290 s. During this time min. 95 wt. % of Ag nanoparticles stayed fixed on the MMT support. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: A. Microporous materials A. Nanostructures A. Metals A. Composite materials D. Surface properties D. Optical properties

1. Introduction According to contemporary definitions nanomaterials are considered natural, fabricated or incidentally formed materials having 50% or more particles with one or more external dimensions in the range of 1e100 nm [1]. Their outstanding properties have been described in many books, comprehensive reviews and research papers [2]. Clay minerals, layered clays in particular, present an important group of natural nanomaterials. Their crystallites are characterized by a layered structure with one dimension in the nanorange. Existence of several types of surfaces is also typical of * Corresponding author. Tel.: þ420 59 732 1527. E-mail address: [email protected] (P. Praus). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.

them: external surfaces including edges and internal surfaces of interlayer spaces. A clay particle is an assembly of layered crystallites and a clay aggregate is an assembly of clay particles. Therefore, we may distinguish between interlayer, interparticle, and interaggregate pores of different sizes and shapes [3]. In general, nanoparticles tend to agglomerate and must be stabilized by steric or electrostatic effects mostly by using various organic polymers or surfactants. Another effective way is their stabilization on porous supports. Some layered clays were used for deposition of nanoparticles forming nanocomposites. Nanoparticles of ZnO, ZrO2 and TiO2 were immobilized on montmorillonite and ones of palladium were supported on kaolinite [4,5]. Silver nanoparticles were supported on silicates, such as montmorillonite [5,6], halloysite [7], kaolinite [5,8], chabazite [9],


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laponite [7], mica [10], colloidal silica [11] etc. Nanosized oxides of various transitional metals, including iron, chromium, cobalt, manganese and cerium, were immobilized on laponite clay and montmorillonite forming mesoporous solids with high surface areas [12]. Pillared clays [13,14] can be also assigned to nanocomposites because different oxide particles are incorporated into clay interlayers. Nanoparticles of ZnS and CdS were used for the fabrication of a nanocomposite with montmorillonite that was applied in photocatalytic reduction of carbon dioxide [15,16]. A nanocomposite of platinum nanoparticles and anionic clay e double-layer zinc hydroxide was also fabricated [17]. Nanoparticles of noble metals, silver in particular, are intensively studied due to their unique electrical, optical, catalytic and biologic properties, which are different from properties of bulk materials. They can be used in different fields of chemistry as effective catalysts [18,19], excellent antibacterial materials [20,21], in medicine [22], and also in construction of new sensors for analytical applications [23,24]. The aim of this paper was to stabilize silver nanoparticles on montmorillonite with no other additives. The resultant nanocomposite was characterized and used as a catalyst for reduction of 4-nitrophenol. This reduction was monitored by UVeVIS spectrometry and reaction kinetics was evaluated. The 4-nitrophenol is not only one of the most dangerous waste in chemical industry but it is also the starting molecule in manufacturing of many analgesic and antipyretic drugs, such as paracetamol, phenacetin and acetanilide. It can be also applied as a corrosion inhibitor, photographic developer and also as a hairdyeing agent [25]. For these facts and due to a simple spectrophotometric determination method the reduction of 4-nitrophenol to 4-aminophenol by borohydride is often used as a model reaction for investigation of catalytic activity of noble metal (Ag, Au, Pt, Pd) nanoparticles [26e28]. 2. Experimental 2.1. Reagents All chemicals were of analytical reagent grade: silver nitrate (Safina, Czech Republic), sodium borohydride and 4-nitrophenol (all from SigmaeAldrich). Water used for the preparation of all solutions was deionised by reverse osmosis. Naþ-rich montmorillonite SWy 2 (Crook County, Wyoming) had a cation exchange capacity of 1.2 meq g1, which was determined by saturation with NH4 þ and analysis of released metal ions. The <5 mm fraction separated by sedimentation was used for the following experiments.

TEM micrographs were recorded on a CCD camera with a resolution of 1024  1024 pixels. Size of nanoparticles was evaluated using the Pixel-Fox software. Powder samples were dispersed in ethanol and the dispersions were exposed to ultrasound for 10 min. A drop of the diluted dispersions was placed on a holey-carbon-coated copper grid and allowed to dry at the laboratory temperature. Selected-area electron diffraction (SAED) patterns were evaluated using the Process Diffraction software package. 2.4. Specific surface area measurements Specific surface areas of powder samples were measured with Sorptomatic 1990 (Thermo Electron Corporation, USA) using nitrogen and calculated by the Advance Data Processing software according to the BET isotherm at p/p0 ratio of up to 0.3 and 77.3 K. Before BET measurements, the samples were outgassed for 3 h at the laboratory temperature. Monolayer surface areas were calculated by fitting 10 points of each isotherm. The specific surface areas of micro- and mesopores were calculated by means of the Horvath Kawazoe [30] and BJH [31] models. 2.5. X-ray powder diffraction An X-ray powder diffraction study was performed using a powder diffractometer INEL CPS 120 (INEL, France) equipped with a curved position-sensitive detector PSD 120 MB/11 (reflection mode, Ge-monochromatized, CuKa1 radiation), which allows simultaneous data collection over 0 e120 with steps of 0.031 in 2-theta. Diffraction patterns were acquired in ambient atmosphere under constant conditions (2000 s, 35 kV, 20 mA). 2.6. Molecular UVeVIS and atomic absorption spectrometry UVeVIS absorption spectra were measured by a double-beam spectrometer Lambda 25 (Perkin Elmer, USA). All spectra were recorded using 1 cm quartz cuvettes within the range of 200 nme 800 nm. The cuvettes were filled with the aqueous dispersions of AgeMMT and their absorption spectra were scanned. An atomic absorption spectrometer Spectra AA30 (Varian Inc., USA) was used for the determination of silver in AgeMMT using a standardised method [32]. Aireacetylene flame was employed as an atomisation technique. The AgeMMT composite was dissolved in a mixture of HF and HNO3. The Ag content was 4.94 wt. % calculated as an average of two parallel analyses (5.00 wt. % and 4.88 wt. %).

2.2. Reduction procedure 2.7. Catalytic activity of AgeMMT Silver nanoparticles were prepared by reduction of silver ions with sodium borohydride. For this purpose, 10 mg of MMT intercalated with Agþ ions [29] was placed into a test tube and 5 mL of 0.010 mol L1 of NaBH4 solution was added and shaken. This reaction was vigorous and took only several minutes under laboratory temperature (24  2  C). Presence of the Ag nanoparticles was confirmed by UVeVIS spectrometry when typical absorption bands with their maxima around 410 nm were observed. After the reduction, AgeMMT was filtered out by 0.60 mm membrane filters and washed several times with water to remove residual Agþ ions. 2.3. Transmission electron microscopy Transmission electron microscopy was carried out on a JEOL JEM 3010 microscope operated at 300 kV (LaB6 cathode, point resolution 0.17 nm) with an Energy Dispersive X-ray detector attached.

Catalytic activity of AgeMMT was investigated by means of reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride in aqueous solutions. Typically, 300 mL of the 1  104 mol L1 4-nitrophenol solution was mixed with 0.010 g of AgeMMT. Thereafter, 5 mL of the sodium borohydride solution of 1  102 mmol L1 was added to the dispersion and the reaction was monitored by UVeVIS spectrometry. Absorption spectra of resulting 4-nitrophenolate in a range of 250e600 nm were recorded every 30 s. UVeVIS spectra were recorded by a spectrophotometer Specol S600 (Analytic Jena AG, Germany) by monitoring the decrease of absorbances at 415 nm. Study of the catalytic activity of AgeMMT was conducted at the laboratory temperature. The kinetic experiment was repeated three times and the standard deviation of determined kinetic parameters was about 12%.

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3. Results and discussion 3.1. Characterization of AgeMMT The AgeMMT nanocomposite was prepared by the procedure described in the Experimental section. Montmorillonite was firstly intercalated by Agþ cations and then borohydride was added to reduce them to metallic silver [29]. Silver atoms were nucleated on the MMT surface forming Ag nanoparticles. The resultant nanocomposite was characterized by TEM, XRD and specific surface area measurements. Fig. 1 shows TEM micrographs of Ag nanoparticles located on montmorillonite. Fig. 1 (right) also provides a detailed view of Ag nanoparticles grown on the MMT surface. SAED patterns revealed their cubic and hexagonal structures. Sizes of Ag nanoparticles estimated from the TEM micrograph varied from 3 nm to 14 nm and were characterized by a log-normal distribution. The mean size of Ag nanoparticles was 6.9 nm, which is several times lower than sizes of Ag nanoparticles prepared by homogenous nucleation and stabilization in solutions (d  25 nm) [33,34]. The presence of such small Ag nanoparticles on the MMT surface agrees with the results of molecular modelling published by Tokarský et al. [35]. According to this study, Ag nanoparticles are preferentially nucleated on MMT (001) planes rather than on MMT (100) edges and adhesion of these nanoparticles decreases with their increasing size and thickness. This implies that only the small Ag nanoparticles remained on the MMT surface and the larger ones were released to the solution as demonstrated in Fig. 1.


In addition, heterogeneous nucleation is characterized by lower critical free energy (DG*) than homogenous nucleation: DG*het ¼ DG*homo f ðqÞ, where f(q) < 1 is a function of the contact angle q defined as f(q) ¼ (2  3cos q þ cos3 q)/4. Therefore, nucleation rate J proportional to exp(DG*/kT), where k is the Boltzmann constant and T is the thermodynamic temperature, is higher at heterogeneous nucleation and consequently, a higher number of smaller Ag nanoparticles should be formed in comparison with homogenous nucleation as mentioned above. The specific surface area of MMT and AgeMMT was measured by the BET method. A pore size distribution was calculated by the HorvatheKawazoe model of micropores and the BJH model of mesopores. It was found that the SSA of micropores decreased from 49.5 m2 g1 of MMT to 18.3 m2 g1 of AgeMMT. In case of mesopores, the SSA increased from 17.1 m2 g1e19.4 m2 g1. It is also possible to mention here that the MMT interlayer space is not accessible to nitrogen molecules and thus interlayer galleries cannot be considered micropores characterized by the BET method. Error of the SSA measurements expressed as a relative standard deviation was estimated at 0.4% from repeated measurements of a standard sample of alumina. Therefore, these SSA changes were likely caused by processes connected with the deposition of Ag nanoparticles. Taking into account that the treatment of MMT with sodium borohydride solutions leads to the SSA enlargement [6] we can deduce the MMT micropores were likely plugged by Ag nanoparticles and nanoclusters and new mesopores among Ag nanoparticles were created as well.

Fig. 1. TEM micrograph of AgeMMT nanocomposite. Overall view of AgeMMT (top left), detailed view of Ag nanoparticles attached to MMT surface (top right), detailed view of Ag nanoparticles composed of smaller nanocrystals (bottom left), large Ag nanoparticles in the solution (bottom right).


P. Praus et al. / Materials Chemistry and Physics 140 (2013) 493e498

In order to recognize whether the nanoparticles were also intercalated in the MMT interlayer, XRD patterns of MMT and Age MMT were recorded (Fig. 2). A shift of diffraction peaks corresponding to the 001 diffraction of MMT and AgeMMT was minimal. It means that the d001 interlayer distance was not increased and no intercalation of the Ag nanoparticles occurred. An explanation of this phenomenon inheres in i) release of intercalated silver ions from the MMT interlayer space by their ion exchange with sodium ions coming from NaBH4 and ii) their consequent reduction to metallic silver on the montmorillonite external surface. An average Ag crystallite size L111 ¼ 9.8 nm was calculated for broadening of the 111 diffraction according to the Scherrer equation L111 ¼ K l/b cos Q, where K ¼ 0.94 is the constant, b ¼ 0.01481 is the X-ray diffraction broadening at half of the maximum intensity in radians, l ¼ 0.15406 nm and Q ¼ 18.7 are the radiation wavelength and Bragg angle, respectively. Comparing the value of L111 with the range of nanoparticles size found by TEM (3e14 nm) it indicates that some Ag nanoparticles were probably composed of smaller Ag nanocrystals as also visible in Fig. 1 (bottom left). By annealing at 370  C for 2 h the Ag(111) diffraction peak was shifted from 37.5 to 38.5 (2 theta) as a result of recrystallization of defective cubic nanostructure, which follows from silver diffraction intensities reported in a powder diffraction database of JCPDS (ICCD 4-783). After the annealing L111 increased to 11.9 nm.

Fig. 3. UVeVIS spectra of AgeMMT and MMT dispersions.

silver was immobilized in AgeMMT. It well agrees with an idea of filling MMT pores with Ag nanoparticles forming a stable nanocomposite.

3.2. Stability of AgeMMT dispersion Stability of the AgeMMT dispersion was verified by recording a series of UVeVIS spectra at different times. The dispersion of 0.01 mg of AgeMMT in 25 mL of water was placed into a 1 cm quartz cuvette in the UVeVIS spectrometer and several spectra at different times from 1.5 min to 360 min and after 24 h were recorded. Within 360 min, the spectra with an absorption maximum at 408 nm caused by Ag surface plasmon absorption were identical. After 24 h, the absorption maximum was shifted to 434 nm and the absorption band became broader (Fig. 3). It indicates the coagulation of AgeMMT particles. The AgeMMT dispersions were shaken for 0.5, 1.0, 1.5, 3.0, 5.0 and 10.0 min, filtrated through 0.60 mm membrane filters and the first 10 mL of the filtrates were collected, acidified by HNO3 and analysed by AAS to determine the content of silver. This filtration took about 1 min. It was found that 0.04e0.21 wt. % of Ag was released from AgeMMT, which is 0.81e4.25 wt. % of the total content of Ag in AgeMMT. It means that more than 95.75 wt. % of

Fig. 2. XRD patterns of MMT and AgeMMT.

3.3. Reduction of 4-niprophenol by AgeMMT The AgeMMT nanocomposite was used as a catalyst for the reduction of 4-nitrophenol by sodium borohydride, which was added in a 100 times higher excess. The reduction occurred by transfer of electrons from BH4  (donor) to 4-nitrophenol (acceptor) after their adsorption on Ag nanoparticles surface. The reaction mixture was monitored by measurements of UVeVIS absorption spectra shown in Fig. 4. In Fig. 4 spectra of the reaction mixture of 4-nitrophenol, NaBH4 and AgeMMT are presented. The highest spectrum belongs to the beginning of the reduction immediately after addition of NaBH4 to the solution of 4-nitrophenol. All the next spectra were measured in an interval of 30 s (see Experimental) and the reaction course was monitored by decrease of the absorption peak of 4-nitrophenol at 400 nm (Fig. 5). During the reduction a new peak of 4-

Fig. 4. UVeVIS absorption spectra at different times of 4-nitrophenol (400 nm) and 4aminophenol (290 nm) during reduction catalysed by AgeMMT.

P. Praus et al. / Materials Chemistry and Physics 140 (2013) 493e498

r ¼ kapp KNP cNP



It is also necessary to mention that no catalytic activity of montmorillonite itself was observed. As given above, most of the Ag nanoparticles were fixed in the MMT pores, however, they were accessible by 4-nitrophenol molecules. The silver nanoparticles are stable in the reduction environment of solution of NaBH4, which was proved in the study performed by N. Pradhan et al. [27] and was confirmed many times in many studies of catalytic activity of silver nanoparticles and composites of silver nanoparticles [7,26,28,38,39]. 4. Conclusion

Fig. 5. Dependence of 4-nitrophenol absorbance on time of reduction catalysed by AgeMMT.

aminophenol emerged at 290 nm [26e28]. No additional peaks were observed in this kinetic experiment because at low concentrations of silver nanoparticles used in the reduction the absorption peak of surface plasmon absorption of silver nanoparticles (at 434 nm) was very low (absorbance is lower than 0.1). Two isosbestic points at 282 nm and 315 nm indicated that 4-aminophenol was a unique product of the reduction. The maximal absorbances at 400 nm were taken to construct a kinetic curve demonstrated in Fig. 5. Fig. 5 shows two distinct parts on the kinetic curve. The first part corresponding to a steep decrease of absorbances within the time interval of 0e30 s was well described by an equation of the zeroorder reaction kinetics: A ¼ 0.0339 (0.0067) t þ 1.823 (0.122) (r ¼ 0.9855), where A and t are absorbance and reaction time, respectively. This means that the reduction rate was constant at the reaction beginning. The second part of the curve between 35 s and 295 s well matched the first-order reaction kinetics equation indicating that the reaction rate decreased with the decreasing concentration of 4-nitrophenol: ln A ¼ 0.00664 (0.00026) t  0.0519 (0.0144) (r ¼ 0.9914), where the symbols were explained above. It well agrees with the fact that the concentration of borohydride was much higher than that of 4nitrophenol and thus it can be considered constant (pseudo-firstorder reaction) [36,37]. This change of the reaction kinetics can be explained using the LangmuireHinshelwood model of heterogeneous catalysis describing the 4-nitrophenol reduction rate r as follows

KNP cNP r ¼ kapp 1 þ KNP cNP


where kapp is the apparent kinetic parameter depending on experimental conditions, KNP and cNP are the adsorption constant and concentration of 4-nitrophenol, respectively. For high concentrations of 4-nitrophenol, that is at the beginning of the reaction, KNPcNP >> 1and KNPcNP þ 1 z KNPcNP. Then Eq. (1) reduces into

r ¼ kapp


The reaction rate is constant and of zero order. For low concentrations of 4-nitrophenol it holds KNPcNP << 1, then KNPcNP þ 1 z 1 and Eq. (1) can be arranged in an equation describing the first-order reaction rate

In this study, the AgeMMT nanocomposite was prepared by reduction of Agþ ions, previously intercalated in the MMT interlayer, in borohydride solution without any stabilizing additives. The nanocomposite contained 4.94 wt. % of silver. Ag nanoparticles of the average size of 6.9 nm were fixed on MMT, especially in its pores. The existence of such small nanoparticles was ascribed to fast heterogeneous nucleation on the MMT external surface. Within 360 min no coagulation of the aqueous AgeMMT dispersions was observed. However, after 24 h the coagulation was indicated by a red shift and broadening of the Ag surface plasmon absorbance band. The catalytic activity of AgeMMT was tested in case of reduction of 4-nitrophenol with borohydride forming 4-aminophenol. Reaction kinetics of this reduction changed from zero order to firstorder, which was explained in terms of the LangmuireHinshelwood model of heterogeneous catalysis. During the reduction time (5 min) the AgeMMT dispersion was stable and Ag nanoparticles were mostly (min. 95 wt. %) immobilized on the MMT support. Montmorillonite played an important role in the formation of small Ag nanoparticles without the necessity of stabilizing additives. In future research, montmorillonite and other clay minerals will be tested for the preparation of different nanocomposites for various catalytic and photocatalytic applications. Acknowledgement The financial support of the Regional Material Technology Research Centre (CZ.1.05/2.1.00/01.0040) was gratefully acknowledged. The authors would like to thank Dr. M. Valásková, D.Sc. for  recording the XRD patterns, S. Studentová for measurements of  specific surface areas (both from VSB-Technical University of Ostrava) and Dr. M. Klementová (Institute of Inorganic Chemistry of   the ASCR, Husinec-Re z) for obtaining the TEM images of AgeMMT. References [1] European Commission, Commission recommendation of 18 october 2011 on the definition of nanomaterial, Off. J. Eur. Union 275 (2011) 38e40. [2] C.N.R. Rao, A. Müller, A.K. Cheetham, Nanomaterials Chemistry, Wiley-VCH, Weinheim, 2007. [3] F. Bergaya, G. Lagaly, in: F. Bergaya, B.K.G. Theng, G. Lagaly (Eds.), Handbook of Clay Science, Elsevier Ltd., 2006, pp. 1e18. [4] R. Kun, K. Mogyorósi, J. Németh, L. Körösi, Sy. Papp, I. Dékány, J. Therm. Anal. Calorim. 79 (2005) 595e604. [5] S. Papp, R. Patakfalvi, I. Dékány, Colloid Polym. Sci. 286 (2008) 3e14.  [6] P. Praus, M. Turicová, V. Machovi c, S. Studentová, M. Klementová, Appl. Clay Sci. 49 (2010) 341e345. [7] P. Liu, M. Zhao, Appl. Surf. Sci. 255 (2009) 3989e3993. [8] R. Patakfalvi, A. Oszkó, I. Dékány, Colloids Surf. A 220 (2003) 45e54. [9] S.M. Kuznicki, C.H. Lin, L. Wu, H. Yin, M. Danaie, D. Mitlin, Clays Clay Miner. 56 (2008) 655e659. [10] C.W. Chiu, P.D. Hong, J.J. Lin, Langmuir 27 (2011) 11690e11696. [11] Z.J. Jiang, C.Y. Liu, Y. Liu, Appl. Surf. Sci. 233 (2004) 135e140. [12] J.J. Li, Y. Mu, X.Z. Xu, H. Tian, M.D. Duan, L.L.D. Li, Y.P. Hao, S.Y. Qiao, G.Q. Lu, Microporous Mesoporous Mater. 114 (2008) 214e221. [13] K. Ohtsuka, Chem. Mater. 9 (1997) 2039e2050.


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