Surfactant free synthesis of gold nanoparticles within

Powder Technology 315 (2017) 147–156

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Surfactant free synthesis of gold nanoparticles within meso-channels of non-functionalized SBA-15 for its promising catalytic activity Abu Taleb Miah a, Saitanya K. Bharadwaj b, Pranjal Saikia a,⁎ a b

Department of Applied Sciences (Chemical Science Division), Gauhati University, Guwahati 781 014, Assam, India Department of Chemistry, Pragjyotish College, Guwahati 781009, Assam, India

a r t i c l e

i n f o

Article history: Received 27 October 2016 Received in revised form 14 March 2017 Accepted 3 April 2017 Available online 05 April 2017 Keywords: Wetness impregnation Mesoporous silica Au/SBA-15 Dye reduction Gold nanoparticles (Au NPs) 4-NP reduction

a b s t r a c t This work reports surfactant free synthesis of monodisperse gold (~2.3 nm) particles confined within the pore channels of ordered hexagonal mesoporous silica SBA-15 (Au/SBA-15) by a modified wetness impregnation method. The synthesized Au/SBA-15 composite showed excellent catalytic activity towards the reduction of different wastewater organic pollutants as well as their mixtures under ambient conditions. The composite was also investigated for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). XRD, UV-DR visible spectroscopy, N2 adsorption-desorption analysis, ICP-AES, FT-IR spectroscopy, and TEM were employed to characterize the catalyst samples. The wetness impregnation method followed by a washing step with dilute aqueous NH3 was found to be an effective route to synthesize stable and highly dispersed small gold nanoparticles (Au NPs) within the mesopore channels of SBA-15. All the catalytic reactions followed pseudo-first-order kinetics and catalytic efficiency of the composite was found almost constant up to five cycles. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Although gold, in the bulk state, had long been regarded as inactive for catalytic applications due to its inert nature, it exhibits an astoundingly high activity for several reactions, both in the liquid and in the gas phase when it is well dispersed on different supports [1–4]. Haruta first discovered a remarkable activity of supported gold nanoparticles (Au NPs) in CO oxidation and since then, different preparation methods have been developed to obtain highly active gold catalysts, and several potential application fields have been explored [2,5]. In general, supported Au NPs in the ultra-fine particle size range (2–4 nm) are highly reactive, although the particles sized between 10 and 50 nm have also been reported to exhibit certain reactivity [2,6]. Nevertheless, the catalytic activity seems to depend critically on the size of Au particles (high activity for size of 2 nm) when they are attached on “inert” supports such as silica and alumina. Therefore, it is crucial to gradually reduce the particle size of gold until reaching the optimum size over the “inert” support for the possible improved activity [7]. Up to now, various methods have been employed to prepare supported Au NPs. As per literature report, impregnation (IMP) is the first method to synthesize supported gold catalysts. However, this method with the most common gold precursor HAuCl4, customarily produces large Au particles after thermal treatment and hence catalytically less active [8,9]. For this reason, IMP had been considered as the unsuitable route to obtain highly ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (P. Saikia).

http://dx.doi.org/10.1016/j.powtec.2017.04.015 0032-5910/© 2017 Elsevier B.V. All rights reserved.

active catalysts since long time. As an example, Kumar et al. recently demonstrated that IMP is practically inferior to other methods such as Homogeneous Deposition-Precipitation (HDP) and Micro-Emulsion (ME) for the formation of smaller Au NPs on SBA-15 [10]. Their study showed that conversion of benzyl alcohol to benzaldehyde increased with decrease in the size of Au particles. Smaller Au particles with higher percentage of dispersion on the support SBA-15 had a beneficial effect on the catalytic conversion. In another recent study, Wang et al. prepared Cu/SBA-15 catalysts by three different methods, namely, IMP, DP, and HDP. The Cu/SBA-15 catalyst prepared by the HDP method was found to be the most efficient for methyl acetate hydrogenation due to highly dispersed smaller (4.5 nm) Cu particles and the synergetic effect between Cu0 and Cu+ species. In contrast, the other two methods IMP and DP produced Cu particles of size 85 nm and 54 nm, respectively, and limited catalytic activity was observed [11]. Since the late eighties, several other preparation methods have been developed in order to achieve capable Au catalysts. Among them, deposition-precipitation (DP) method was demonstrated to be the most successful way for the synthesis of well dispersed, ultra-small Au particles on oxide substrates having high activity for various transformation reactions [2,4]. Noteworthy that this method is not suitable for silica supports as under alkaline conditions, deposition of Au on silica leads to the loss of silica's structural properties [2,12,13]. Moreover, the weak interaction between gold and silica support caused complexity in achieving a finest particle size range of gold. In such cases, mesoporous silica with well-defined pore size seems to be appropriate support for the confinement of Au NPs because these materials could produce

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stable metal nanoparticles (MNPs) with controllable shape and size [2,7, 14]. Among different mesoporous silicas, SBA-15 predominantly exhibited excellent properties viz., large surface areas, tunable pore diameter, ordered channel structures, thick framework walls, and high hydrothermal stability when used as support for a catalytic system. Despite having all the promising properties, bare SBA-15 does not exhibit any catalytic activity. Therefore, MNPs are incorporated into SBA-15 in order to improve its catalytic activity [14]. Currently, mesoporous silica-based metal nanocatalysts are widely used in assorted reactions because of their good catalytic activity and stability arising from the confinement effect of the mesoporous channels of silica support [15–17]. Moreover, literature reveals that decoration of Au NPs with SBA-15 silica offers a new class of functional hybrid materials which are effective in various potential applications such as in electrochemistry [18], SERS [19], sensing [18,20], enzyme mimicry [21], etc. However, as far our knowledge is concerned; Au/SBA-15 catalyst has not yet been projected for catalytic reduction of any kind of organic dye pollutants. It is well-established that MNPs are wonderfully active in reduction of dyes and their activity depends critically on size, shape, crystallinity, and the surface state [22]. Very recently, Sareen et al. investigated selective catalytic reduction of m-dinitrobenzene to m-phenylenediamine over MNP (Au, Ag and Cu) impregnated 3-aminopropyltriethoxysilane (APTES) modified SBA-15, and Au/SBA-15 was found to exhibit the best catalytic activity [14]. Unfortunately, small MNPs adhere to aggregate during the process of catalytic reaction due to their high surface energy, and leads to rapid fall of catalytic activity and stability [2,23]. Stable MNPs with controllable sizes can be widely synthesized using surfactants, polymers, dendrimers, thiols, tryptophan, nucleic acids or proteins as stabilizing agents act against agglomeration [6,24,25]. However, devised surface protected MNPs are catalytically less active due to the presence of dense and robust layer which obstructs diffusion of reactants toward the active metal sites [26]. Therefore, in recent years, MNPs are immobilized onto a variety of solid supports in order to stabilize them [23]. Mesoporous materials (MMs) are promising supports for the preparation of surfactant free catalysts since they can easily entrap the MNPs within their pore channels. Because of this fact, leaching probability of the MNPs from the mesoporous matrix is expected to be low, which alleviates the agglomeration affinity of the MNPs. Considering these advantages diverse MNP/MM hybrid systems are designed which impart high activity as well as stability in numerous reactions [12,27,28]. For instance, Song et al. synthesized Pd NPs inside the hollow mesoporous SiO2 spheres, which showed superior catalytic activity and stability in Suzuki coupling reactions [29]. Zhang et al. reported the synthesis of novel hollow mesoporous @M/CeO2 (M = Au, Pd, and Au-Pd) nanoparticles. The hollow mesoporous nanospheres showed extremely high activity and stability for catalytic reduction of 4-nitrophenol due to their hollow mesoporous structural features [30]. Dry reforming of methane (DRM) was performed over highly dispersed and stable Ni NPs in mesoporous silica [31,32]. In previous study, we examined the catalytic reduction of 4-NP by Au NP supported CeO2-TiO2 mixed oxide. The Au/CeO2-TiO2 nanocomposite exhibited tremendous catalytic activity, stability, and good reusability [33]. Most approaches in the literature used for the synthesis of small Au NP supported catalysts have some drawbacks such as complicated experimental set up, use of expensive substances such as alkoxides, polymers, surfactants, and organic solvents. In addition, toxic or dangerous reducing agents are generally used for the preparation of ultra small Au NPs and the resulting Au NPs must be stabilized by further surface modification [34]. Thus it is necessary to devise a simple, relatively non-toxic, and cheap method for the synthesis of supported Au catalysts with small size. Incorporation of ammonia treatment during the preparation is reported to be an effective strategy for mesoporous silica-based gold catalysts. For example, Zhao et al. successfully performed nitridation of mesoporous SBA-15 over ammonia to create strong interaction between the surface –NHx groups and gold precursor for facilitated immobilization of Au NPs inside the channels of SBA-15 [35]. The

recent work of Kumar et al. revealed the utilization of four different methods for the synthesis of nano Au/SBA-15 catalysts [10]. Here, the synthesized catalysts required to be reduced separately by aqueous NaBH4 solution in addition to the established methods for the purpose. However, relatively large Au NPs ranging from 7.36 to 10.38 nm were formed on the support SBA-15 [10]. Against the above background, we have exploited in-situ washing with dilute NH3 with an intention to immobilize small Au NPs within the mesoporous SBA-15 channels by the modification of conventional wetness impregnation (WI) method. The small modifications introduced to the WI method probably helped in the synthesis of sub-3 nm Au particles. Worth mentioning that no protective agents like polymers, surfactants, dendrimers etc. was used to control the growth of Au NPs. The catalytic performance of Au/SBA-15 composite was investigated by studying the reductive decolorization of various dyes/dye mixtures in the presence of NaBH4. In addition, reduction of an aromatic nitro compound 4-nitrophenol (4-NP) to 4-amionophenol (4-AP) was performed over the Au/SBA-15 catalyst. The catalytic reduction/decolorization reactions were found to be very rapid and followed pseudo-firstorder-kinetics. 2. Materials and methods Hydrogen tetrachloroaurate (HAuCl4·3H2O, 99.99%), Pluronic P123 (triblock copolymer EO20·PO70·EO20) and tetraethoxysilane (TEOS) were obtained from Sigma Aldrich. Sodium borohydride (NaBH4), NH3 and HCL were purchased from Merck. Congo red (CR), methylene blue (MB), methyl orange (MO) and 4-nitrophenol (4-NP) were purchased from Himedia. All the chemicals used in this study were of analytical grade and were used without further purification. In all experiments, double distilled water was used. 2.1. Catalyst preparation SBA-15 was synthesized following the previously reported procedures [36,37] with minor modifications. At first, 6 g of pluronic P123 were dissolved in 45 mL of double distilled water under mechanical stirring. After 20 min, 180 mL of 2 M HCl solution was added dropwise and stirred until the formation of a homogeneous solution (4 h). The temperature inside the reaction vessel was maintained at ~ 40 °C. TEOS (13.93 mL) was added slowly into the reaction vessel. The resulting mixture was then left for continuous stirring at ~40 °C. After 24 h, the whole mixture was sealed in a Teflon-lined autoclave and hydrothermally aged for 75 h under static conditions at 110 °C. After cooling the mixture at room temperature for 32 h, the white products were filtered and washed with plenty of distilled water and then dried in an oven at 40 °C for 24 h. Finally, the as-obtained product was calcined at 350 °C for 12 h in order to obtain mesoporous SBA-15 silica material. A series of Au/SBA-15 composites was synthesized by varying the amount of gold (i.e., 0.5, 1, 1.5, 2 and 5 wt%) via the modified wetness impregnation (WI) method. In a typical synthesis protocol, certain amount of the SBA-15 material was added into 3 mL aqueous solution of HAuCl4·3H2O and mixed them well by a glass rod. The paste-like product so formed was aged for 24 h at room temperature (RT). Then, it was heat treated at 80 °C for 2 h. The dried product was cooled to RT for 1 h, washed off thrice with dilute aqueous NH3 (75 mL of 0.2136 M) solution and twice with double distilled water (60 mL). Centrifugation was performed between each washing step. Finally, the washed product was calcined in an oven at 200 °C for 4 h to obtain the Au/SBA-15 composite. 2.2. Catalyst characterization The synthesized samples were analyzed by powder X-ray diffraction (PXRD) on a PANalytical X'pert Pro diffractometer using Cu Kα (λ = 0.15418 nm) radiation source and a scintillation counter detector. The


intensity data were collected over the 2θ range of 0.5–5° and 10–80 with a 0.02° step size using a counting time of 1 s per point. The average crystallite size of Au NPs was estimated with the help of Scherrer equation. The Brunauer-Emmett-Teller (BET) surface area, pore volume, and pore size distributions were obtained from the nitrogen gas sorption measurement at 77 K (Autosorb I/Quantachrome instrument, USA). The Barrett-Joyner-Halenda (BJH) method was used to calculate the pore size distribution from the desorption branch of the isotherm. Prior to measurements, the sample was degassed at 423 K for 3 h to remove any surface adsorbed residual moisture. The total pore volume was estimated from the amount adsorbed at a relative pressure (P/P0) of about 0.95, whereas the surface area was estimated from BET treatment of the isotherms in the P/P0 range of 0.05–0.30. Transmission electron microscopic (TEM) studies were made on a JEM–2100 (JEOL) instrument equipped with a slow-scan CCD camera at an accelerating voltage of 200 kV. Samples for TEM were prepared by crushing the materials in an agate mortar and dispersing them ultrasonically in ethanol. After dispersion, a droplet was deposited on a copper grid supporting a perforated carbon film and allowed to dry. Diffuse reflectance UV– visible (DRUV-vis) spectroscopic measurements were performed over the wavelength range of λ = 400–1000 nm using a UV–visible spectrophotometer [Model: U–4100 spectrophotometer (solid)]. The DR spectra for the solid samples were recorded using BaSO4 as the reference material. Fourier transform-infrared (FT-IR) spectra were recorded on a FT-IR spectrometer [Model: Spectrum Two FT-IR Spectrometer (Perkin Elmer)] at ambient conditions. Measurements were performed by pelletizing the samples with KBr in the mid-infrared region at an accelerating voltage of 200 V. Amount of actual Au present in the Au/SBA15 composite was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Model: ARCOS M/s. Spectro, Germany). The solid sample was first digested in a mixture of HF, HCl, and HNO3 in a microwave oven for 2 h and further diluted with deionised water to obtain the Au content by ICP-AES. The total organic carbon (TOC) measurement was carried out using a TOC analyzer (Model: Aurora-model-1030) by wet oxidation method. The measurement was performed to elucidate the TOC value for the catalytic reduction of congo red dye (20 ppm) with NaBH4 (0.2 M, 2 mL) and Au/SBA15 (0.133 g/L) composite. 2.3. Evaluation catalytic activity (reductive decolorization of wastewater pollutants) In order to evaluate catalytic activity, the Au/SBA-15 composite was used for the reduction of different organic dyes/dye mixtures and reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in presence of NaBH4. All experiments were performed at room temperature (~25 °C). In a typical experiment, 30 mL aqueous solution of the reactant was mixed with 2 mL (0.2 M) of freshly prepared NaBH4 and 4 mg Au/SBA-15. Immediately after the addition of Au/SBA-15, time dependent absorbance intensities of the reactant were recorded with the help of UV–visible spectrophotometer in order to quantify the evolution of the reduction process. The extent of reduction was calculated using the relation: %decolorization ¼ ½ðC0 −Ct Þ=A0 100% ¼ ½ðA0 −At Þ=A0 100% where, C0/A0 is the initial concentration/absorbance and Ct/At is the concentration/absorbance of reactant solution at different time intervals. 2.4. Results and discussion Washing step plays a vital role in the generation of small Au NPs on solid oxide supports such as Al2O3, TiO2, and SiO2 [9,38,39]. In the conventional impregnation method, Au(OH)3 is precipitated upon hydrolysis of the precursor HAuCl4·3H2O, and the chloride ligands thereby produced from the precursor promote the formation of large Au

149

particles. Washing with ammonia efficiently removes the chlorine residues after impregnation [9,38,39]. Li et al. used gaseous ammonia treatment method followed by washing with water to remove the chlorines after impregnation for obtaining small Au particles on TiO2 [40]. It is revealed that inclusion of this additional washing step in the impregnation method replaces chloride ligands from the coordination sphere of gold precursor by ammine ligands, leading the formation of an ammino-hydroxo or an amino-hydroxo-aquo gold complex [Au(NH3)2(H2O)2 − x(OH)x](3 − x)+, instead of Au(OH)3. This cationic complex interacts with the surface of the supports either electrostatically, or through grafting, and then prevents the gold leaching [9]. Based on references [9,35,38–40], we added the washing process with dilute aqueous NH3 to the wetness impregnation protocol for the synthesis of Au NP/SBA-15 composite. Initially, wetness impregnation was employed to adsorb Au3+ ions on the mesoporous matrix of SBA-15. Later, Au NPs were formed by washing the Au3+ ions/SBA-15 mixture with dilute NH3 as well as distilled water. The mixture was then calcined to fabricate Au/SBA-15 composite. The channels of the mesoporous silica (SBA-15) is hydrophilic in nature which causes infiltration of the aqueous solution of the gold precursor into the pores of the porous silica by capillary forces [41,42]. Owing to this fact, the aqueous gold solution is fully sucked inside the mesopores rather than on the external surface. The NH3 washing process generated ammine ligands on the interior surface of mesoporous channels of SBA-15 host [42]. It is widely believed that the stronger the metal-support interaction, smaller is the metal particle size formed by reduction [32]. Due to enough interaction between the ammino-hydroxo-aquo gold complex [Au(NH3)2(H2O)2 − x(OH)x](3 − x)+ ions and surface silanol groups of SBA-15 in the aqueous phase, sub 3 nm gold particles are uniformly distributed within the mesoporous channels. The facilitated immobilization of gold species into the host's channels is attributed to a neutralization reaction between the basic ammine groups and the acidic HAuCl4·3H2O precursor [43]. Thus, it can be inferred that the surface coordinated ammine groups inhibited the uncontrolled growth of the Au NPs on the outer surface of SBA-15 and produced monodisperse Au NPs only within the pores of SBA-15 [35]. Also, the amine ligands undergoing complexation with Au NPs probably caused high dispersion, stabilization, and restriction of the Au NP size. However, it is still not fully clear how the gold precursor interacts with the support surface and how the precursor–support interaction affects the structure of supported Au NPs and so further extensive study is imperative for that. During synthesis, it was seen that addition of HAuCl4·3H2O solution to SBA-15 powder caused the formation of a pale yellow mixture. However, it showed a distinct color change from pale yellow to purple brown after washing it with dilute NH3 solution followed by thermal treatment. This significant color change directly evidenced the formation of Au NPs in SBA-15. Some reported works based on the synthesis of silica based Au NP catalyst systems are summarized in Table SI 1 for showing the effectiveness of our preparation approach in synthesizing relatively smaller Au NPs. Presence of Au NPs in SBA-15 was corroborated by powder X-ray diffraction (PXRD) analysis. Small-angle PXRD patterns of the synthesized pristine SBA-15 and Au/SBA-15 composite are presented in Fig. 1(A). The pristine SBA-15 showed three well-resolved diffraction peaks at 2θ = 0.89, 1.49, and 1.72°, respectively, for (100), (110), (200) planes. These planes are characteristic of the 2D P6mm hexagonal symmetry of SBA-15 silica materials [24,30]. The Au/SBA-15 sample showed similar diffraction pattern with pristine SBA-15 which confirmed the preservation of long range ordering of mesochannels after Au impregnation. However, it can be noted that impregnation of Au caused three observable distortions in the diffraction pattern of the SBA-15 material, viz., (i) decrease in peak intensity, (ii) slight broadness of peaks, and (iii) shifting of the diffraction peaks toward lower 2θ values. To make a closer inspection into the distorting features, an enlarged PXRD pattern is presented as inset. The decrease in peak intensity is attributed to a pore-filling effect resulting in weakening the scattering contrast which

150


(A)

SBA-15 Au/SBA-15

Intensity

Intensity/a.u.

(a) (b)

0.75

1

2

1.00

1.25

3

1.50

1.75

4

5

60

(B)

SBA-15 Au/SBA-15

30

(311)

(200)

Intensity

40

(220)

(111)

50

20 10 0 20

30

40

50

60

70

80

Fig. 1. (A) Small-angle and (B) wide-angle PXRD patterns of pure SBA-15 and Au/SBA-15 composite.

is in line with previous reports [14,27]. Broadness of peaks indicates a decrease of crystallinity but not a collapse of the pore structure of SBA-15 material. Shifting of the peaks may be due to slight enlargement of frameworks of silica host during the synthesis process. Similar observations were noted in earlier reports too [14,27,44]. The wide-angle PXRD patterns of pristine SBA-15 and Au/SBA-15 composite are presented in Fig. 1(B). Both the sample showed a broad band located in between 2θ = 15° and 30°, is attributed to the amorphous nature of SBA-15 silica. The intensity of this band for Au/SBA-15 composite is lower in comparison to SBA-15. In addition to the broad band, four other peaks corresponding to the (111), (200), (220), and (311) diffraction planes of fcc Au are observed [14,19]. This result indicates successful loading of Au NPs within the mesopores of SBA-15. Size of Au NPs measured by Scherrer equation was found to be ~2.3 nm. ICPAES analysis revealed that 1.8 wt% Au was loaded on SBA-15, which is almost equal to the theoretical loading (2 wt%). This observation confirms that loss of Au during the ammonia washing treatment is limited. BET surface area, pore volume, and BJH pore size distributions were obtained from the nitrogen gas sorption measurement for the Au/SBA15 sample. It gave a BET surface area of 732.53 m2/g (compared to that of 918.64 m2/g for bare SBA-15) and monodisperse mesopores with an average pore diameter of 7.5 nm as well as pore volume of 1.171 cm3 g−1. The N2 adsorption-desorption isotherm of the sample exhibited a typical type IV physisorption isotherm with H1 hysteresis loop having sharp adsorption and desorption branches [Fig. SI 1(a)]. The sharpness of the loop indicates uniformity in the pore size distribution [10,14]. The BJH pore size distribution curve [Fig. SI 1 (b)] established the characteristic feature for mesoporous materials [10,14, 27,36,37,44].

TEM images of pristine SBA-15 and Au loaded SBA-15 are shown in Fig. 2. Pristine SBA-15 exhibited 2D hexagonal ordered mesoporous (pore diameter ~ 6–7 nm) structure with long range ordering of mesochannels [Fig. 2(b)]. The HRTEM images of Au/SBA-15 composite showed highly dispersed Au NPs in the interior of SBA-15 mesopore channels [Fig. 2(d)]. The Au NPs are sphere-shaped and their average size is ~2.3 nm, with a narrow distribution (i.e., monodispersed). Particle size distribution pattern of gold in Au/SBA-15 composite is shown in Fig. 2(e). It is assumed that the pore channels effectively restrict the size of Au NPs below the channel diameter of SBA-15 host. Fascinatingly, majority of the pores and nano-channels of SBA-15 silica remained intact even after the deposition of Au NPs. The SAED patterns clearly revealed the amorphous phase of SBA-15 silica and diffuse rings indicated nano-sized particles [Inset of Fig. 2(a) & (c)]. Diffuse reflectance UV–vis spectra of pure SBA-15 and Au/SBA-15 composite are shown in Fig. SI 2. Pure SBA-15 didn't exhibit any absorption, while the Au/SBA-15 composite showed a broad absorption band in the region 560–680 nm. This absorption band is attributed to the presence of surface plasmon resonance (SPR) effect caused by ultrasmall Au NPs. Normally, Au NPs exhibit strong absorption bands in the 500–550 nm wavelength region due to SPR effect [3]. However, position of this band is gradually shifted to longer wavelength when the Au particles are b5 nm in size [4,16,45]. Thus the broad SPR band validates the existence of extensive SPR coupling from the large number of small Au NPs within the pore channels of the mesoporous SBA-15 [45]. Fig. 3 represents the FT-IR spectra of pure SBA-15 and Au/SBA-15 composite. Both the samples showed similar spectral patterns. However, absorption intensity of the Au loaded sample was slightly lower in comparison to pure SBA-15 that could be due to the onset of crystallization and incorporation of Au NPs onto SBA-15 [46]. Moreover, the broad band around 3451 cm− 1 in pure SBA-15 was found to shift to 3416 cm−1 in Au/SBA-15 composite, which may be a result of the N– H stretching vibration of ammonia at ~ 3320 cm−1. The intense broad band at 3451˗3416 cm− 1 was assigned to the vibrational stretching mode of surface silanol (Si–OH) groups, and hydrogen-bonded water molecules adsorbed on the surface [27,28]. Deformational vibrations of this particular band cause an additional absorption around 1632 cm−1 [27]. The sharp and intense peaks at ~ 1086 cm−1, 804 cm− 1, and 468 cm− 1 was due to asymmetric and symmetric stretching vibrational modes of Si–O–Si moieties of the SBA-15 host [27,46]. The weak band located at ~960 cm−1 specified the Si–OH bending vibrations [46]. 3. Catalytic activities of Au/SBA-15 composite in the reduction of organic dyes Industrially discharged colored organic dye substances are hazardous for humans to aquatic microorganisms. Decolorization of these substances leads to the generation of non-hazardous products. Therefore, dye decolorization has gained critical interest. Dyes could not be decolorized by traditional processes due to their complex aromatic structures, hydrophilic nature and high stability against light, temperature, water, chemicals, etc. However, easy and rapid decolorization could be brought about by MNP catalyzed reduction. In this work, we choose three different organic dyes, namely, Congo red (CR), methyl orange (MO) (both are anionic azo dye) and methylene blue (MB) (a cationic heterocyclic dye) for examining the catalytic reduction/decolorization efficiency of the synthesized Au/SBA-15 composite. The reduction reactions were studied spectrophotometrically and progress of the reactions was monitored by measuring the optical absorption against time. First, the reactions were carried out in the presence of only NaBH4. The related UV–vis absorption spectra are provided in Fig. SI 3. It was seen that the reductions/decolorizations proceed very slowly and remained incomplete over a long period of time. However, addition of minute quantity of Au/SBA-15 caused quick decolorization of the dyes. It means that reduction of organic dye substances by NaBH4 is not kinetically


25

151

(e)

Frequency (%)

20 15 10 5 0

1.0

1.5

2.0 2.5 Particle Size (nm)

3.0

Fig. 2. TEM-HRTEM image of pure SBA-15 (a & b) and Au/SBA-15 composite (c & d). Insets (a & c) show the corresponding selected area diffraction (SAED) pattern. Gold particle size distribution profile of fresh Au/SBA-15 composite (e).

favorable in the absence of catalyst. The reduction progress was noticed from the continuous decolorization or decrease in absorbance with time. Fig. 4(a)-(c) exhibit corresponding UV–vis absorption patterns for the decolorization of MB, MO, and CR over Au/SBA-15 catalyst along with NaBH4. It was noted that decolorization of all dyes completed within very short period. However, the time required for complete decolorization varied according to the nature of dyes. As we can observe from Fig. 4, MB required only 45 s, while MO and CR took 150 s and 210 s, respectively, to attain the same. These observations clearly defend high catalytic activities of Au/SBA-15 composite. Based on the above findings, we can thus say that Au/SBA-15 reduces the kinetic barrier by decreasing the activation energy and makes the reduction process kinetically feasible [25,27,28]. It is surmised that Au/SBA-15 composite offered mesoporous surface for elevated adsorption of both

acceptor (dyes) and donor (BH¯4) species, which enhanced the rate of reduction by increasing electron transfer between the reacting substances. This assumption was further reinforced by studying the effect of catalyst loading on CR reduction process. Accordingly, we performed five reactions varying the amount of Au/SBA-15 composite from 0.033 to 0.167 g/L. Other reaction parameters such as concentration of CR (28.71 × 10−6 M) and NaBH4 (0.2 M, 2 mL) were kept constant. Reduction profiles of aqueous CR over different catalyst amounts were shown in Fig. SI 4. It was observed that reduction efficiency increased gradually with the increase in catalyst loading. This is attributed to higher availability of surface active sites for the adsorption of reacting species with increasing catalyst loading [47]. The reactions followed pseudofirst-order kinetics as the concentration of NaBH4 was higher in comparison to the dye molecules. Based on this, apparent rate constants (kapp)

152


2.5

80

(a) max

= 664 nm

60

Absorbance

% Transmittance

2.0

40

(a) (b)

20

SBA-15 Au/SBA-15

2000

3000

1.5 1.0 0.5

0 1000

0s 45 s

0.0

4000

-1

Wavenumber/cm

500

550

2.0

Fig. 3. FT-IR spectra of (a) pure SBA-15 and (b) Au/SBA-15 composite.

600

(b)

3.1. Catalytic activities of Au/SBA-15 composite in the reduction of mixture of organic dyes Sometimes, different dye mixtures are associated with industrially discharged wastewater. Therefore, simultaneous reduction of all dyes is indispensable and considered to be challenging. In view of this, we studied the reduction of mixture of two dyes as well as of three dyes over Au/SBA-15.

700

750

max

= 464 nm 0s 90 s 150 s

Absorbance

1.5

1.0

0.5

0.0 400

450

500

550

Wavelength/nm 1.5

(c)

max

= 498 nm 0s 60 s 210 s

1.0

Absorbance

for all the reactions were calculated from the slope of linear plots of ln(At/A0) vs. decolorization time [Fig. 5(A) & (B)] and are presented in Tables 1 & 2. The activity parameter (K) values expressed dissimilar reduction tendencies of the dyes (Table 1). Effect of Au loading amount on the catalytic performances of Au/ SBA-15 was investigated for different Au loaded samples in the reduction of CR (data not shown here). The optimization results revealed that catalytic efficiency increased gradually from 0.5 to 2 wt% of Au loading. Also, the 5 wt% Au/SBA-15 showed almost parallel activity in comparison to the 2 wt% Au/SBA-15 catalyst. Based on these results, we inferred 2 wt% of Au as the optimum loading and subsequently, we carried out the diverse reduction reactions as well as other relevant studies like catalyst loading effect, stability test with this particular catalytic system. TOC analysis was carried out for reduced congo red solution. The average value of TOC was found to be 9.727 ppm. This result indicates substantial decrease in total organic content owing to its reductive decolorization with NaBH4 and Au/SBA-15 composite.

650

Wavelength/nm

0.5

0.0

3.1.1. Reduction of mixture of two dyes Three set of mixtures were formulated by combining two different aqueous dye solutions. Fig. 6 presents Au/SBA-15 catalyzed simultaneous reduction of two dyes viz., (a) MB & MO, (b) MO & CR, and (c) MB & CR, in the presence of NaBH4. It was noted that mixing processes lead to the development of new colors. For instance, mixing of MB and MO resulted in the formation of an intense green colored solution while mixing of MO and CR, and MB and CR, respectively, furnished dark orange and faint red solution. Interestingly, the dye mixtures were fully decolorized when treated with Au/SBA-15 and NaBH4, representing their successful reduction.

yellow solution turned into colorless just in 7 min when Au/SBA-15 and NaBH4 was introduced into the solution. This result suggests that Au/ SBA-15 composite effectively catalyzes the simultaneous reduction of the three probed dyes.

3.1.2. Reduction of mixture of three dyes After realizing the excellent activity of Au/SBA-15 for mixtures of two dyes; the catalyst was subsequently tested for the reduction of mixture of three dyes viz., MB, MO, and CR. It was seen that combination of these dyes formed a yellow solution. The reduction pattern of the corresponding dye mixture is presented in Fig. 6(d). As can be observed, the

3.1.3. Catalytic activity of Au/SBA-15 composite in the reduction of 4-NP Apart from organic dyes, we investigated the catalytic activity of Au/ SBA-15 composite for the reduction of 4-nitrophenol (4-NP) to 4aminophenol (4-AP). It is worthy to mention that catalytic reduction of 4-NP by NaBH4 has become a benchmark reaction to evaluate the catalytic activity of MNPs in aqueous solution because of the reasons:

400

450

500

550

600

650

Wavelength/nm Fig. 4. Time dependent UV–vis absorption profiles of aqueous (a) MB, (b) MO and (c) CR solution in presence of Au/SBA-15 (0.133 g/L) and NaBH4 (2 mL, 0.2 M).


(A)

0

MB MO CR

ln(At/Ao)

-2

(b)

-4

(c) (a)

-6

-8 0

50

100

150

200

250

Time/sec 1

(B)

0.033 g/L 0.067 g/L 0.100 g/L 0.133 g/L 0.167 g/L

0

ln(At/Ao)

-1 -2 -3 -4 0

2

4

6

8

10

12

14

16

Time/min Fig. 5. ln(At/Ao) vs. time plot for (A) Au/SBA-15 catalyzed reduction of (a) MB, (b) MO, and (c) CR with NaBH4 and (B) catalyst loading effect on CR reduction.

(a) this reaction forms an industrially important product 4-AP, via the degradation of an environmental pollutant 4-NP, (b) the product 4-AP is desorbed from the MNP surface as soon as it is formed and hence doesn't affect kinetics, (c) kinetic analysis of this reaction enables to correlate between nanoparticle characteristics and catalytic properties in a quantitative manner and thus provide insight into mechanistic aspects involved, and (d) it fulfills the criteria for model reactions, i.e., it is well-controlled, doesn't have any side reactions or by-products, easy to analyze, and requires mild conditions only [48,49]. Therefore, it is very essential to develop facile and effective strategies to reduce 4-NP to 4-AP. It is a known fact that reduction of 4-NP by aqueous NaBH4 is thermodynamically feasible (since E0 for 4-NP/4-AP = − 0.76 V and

153

H3BO3/BH¯4 = − 1.33 V vs. NHE) but kinetically restricted due to large potential difference between donor (BH¯4) and acceptor (4-NP) species which decreases the feasibility of this reaction. MNPs with appropriate redox potential could overcome the kinetic barrier by facilitating relay of electrons from donor to the acceptor species [25,27,33]. In aqueous solution, 4-NP shows an absorption peak at 282 nm [Fig. SI 5 (a)]. This peak is shifted to 405 nm upon addition of NaBH4 to the NP solution, attributing the formation of stable 4nitrophenolate ion [Fig. SI 5 (b)]. This peak remains unchanged with time, suggesting that the reaction doesn't commence in the absence of catalyst [27,33,48]. However, intensity of this absorption peak at 405 nm gradually decreased just after the addition of Au/SBA-15 composite and completely disappeared in 4 min. Within that period, two new peaks at 305 and 235 nm were formed, indicating the formation of reduced product 4-AP (Fig. 7). An induction period t0 (sometimes several minutes) is normally observed in most of the catalytic reductions of 4-NP during which no reaction takes place. This period is regarded as the diffusion time tads required for the adsorption of 4-NP onto the catalyst's surface before initiation of the reaction [48–50]. As shown in Fig. 7(b), the tads is nearly immeasurable, implying that adsorption of the reactants was very fast. Undoubtedly, the high surface area as well as mesopores of SBA-15 facilitates the rapid diffusion of 4-nitrophenolate and BH¯4 ions from aqueous solution to the Au NP surfaces which diminished the diffusion time tads. Similar phenomena were also noted by Guo et al. and Wu et al. [27,51]. The fast adsorption process on the surface of Au/SBA-15 composite creates a high 4-nitrophenolate ion concentration in the vicinity of the Au NPs. The smaller Au particles provide large number of surface active sites which promotes easy and rapid access of the 4-nitrophenolate ions, leading to the highly efficient reduction reaction. The apparent rate constant (kapp) was measured from the slope of the linear plot presented in Fig. 7(b). The kapp is estimated to be 1.045 min−1, which is comparatively higher than that of the recently reported supported Au NP catalyzed reduction of 4-NP [23,33,51,52]. The corresponding activity (K) parameter is 261.25 min−1 g−1. Based on the above pieces of information, a schematic diagram is designed in Fig. 8, showing the catalytic reduction mechanism of dyes/4NP over Au/SBA-15 catalyst in presence of NaBH4. Metal nanoparticle catalyzed reduction of organic dyes proceeds through an electron transfer process, and the rate of such a reaction greatly depends on the swiftness of the process [53]. It is revealed that small and highly dispersed noble metal nanoparticles (NMNPs) exhibit amazing catalytic activity by promoting easy access of the target molecules to reach surface of the NMNPs serving as an electron relaying system [47,53]. In other words, smaller the particle size, more is the surface area and hence, more is the catalytic active sites accessible to the reactants for good catalytic performance [15,16]. However, this trend is not flawlessly appropriate for Au NPs acting as electron relaying system. It is reported that Au NPs, when too large or small, cannot effectively relay electrons from BH4ˉ ions to an electrophile and the particles of 2–5 nm size are suitable for the same [8,54]. In this work, the good dispersion of small sized Au NPs (2.3 nm in average) on SBA-15 support could be ascribed to the high catalytic activity of the nanocomposite in reduction of various organic pollutants. Indeed, the smaller Au NPs

Table 1 Summary of reaction time, apparent rate constant (kapp), and activity parameter (K) for Au/SBA-15 catalyzed reduction of MB, MO and CR dye in presence of NaBH4. Dye conc. (10−6 M)/30 mL

Catalyst loading (g/L)

Amount of NaBH4

Reaction time

App. rate const. (kapp, min−1)

Activity parameter (K, min−1 g−1)

Correlation co-efficient (R2)

MB (48.14)

0.133 Nil 0.133 Nil Nil 0.133

0.2 M × 2 mL

45 s 90 min 150 s 90 min 90 min 210 s

9.18 0.0037 1.662 0.0012 0.00195 1.123

2295 – 415.5 – – 280.8

1 0.98 0.99 0.99 0.99 0.99

MO (61.10), CR (28.71)

154


was similar to that of the fresh catalyst [Fig. SI 6 (a)]. HRTEM analysis also revealed the presence of well dispersed Au particles of analogous sizes [Fig. SI 6 (b)]. Gold particle size distribution profile of recycled catalyst is shown in [Fig. SI 6 (c)]. ICP-AES analysis did not reveal significant leaching of Au. The whole observations confirm that the impregnated Au NPs are fixed well on the mesoporous surface of SBA-15. These results suggest that Au/SBA-15 could be used repetitively without noteworthy loss of its catalytic efficiency which demonstrated good stability and reusability of the composite catalyst. The overall study can thus clearly establish that Au/SBA-15 composite is a very potent catalyst with some striking features such as high dispersion of Au NPs within the mesopores of SBA-15 that stopped them from agglomeration. The strong metal-matrix interaction controlled the growth of Au NPs and hence the probability of leaching of Au NPs from SBA-15 matrix is reduced. Furthermore, the protocol employed in this work for the synthesis of Au/SBA-15 composite is very simple and didn't involve any harsh reaction conditions or complicated experimental arrangements. Any extra starting materials such as polymers, surfactants, alkoxides, and organic solvents were not used, and precomplexation of Au3+ ions for stabilizing nanoparticles and surface functionalization of SBA-15 were not required for this process. The ease of the catalyst synthesis process, use of aqueous medium, good control over the Au NP size, makes it an effective way to prepare mesoporous SBA-15 based Au catalysts.

Table 2 Effect of catalyst loading on reduction of CR over Au/SBA-15 composite. Catalyst loading (g/L)

Reaction time (min)

App. rate const. (kapp, min−1)

Correlation co-efficient (R2)

0.033 0.067 0.100 0.133 0.167

16 8 5 3.5 1.25

0.2187 0.4492 0.6699 1.1232 2.587

0.99 0.98 0.99 0.99 1

possess high surface-to-volume ratio and thereby rendered more Au atoms on the host's surface, which served as the active catalytic sites for the reduction reactions [47,53]. 4. Stability and reusability of Au/SBA-15 composite Fig. 9 shows the reusability profile of Au/SBA-15 composite. Five catalytic cycles were performed in order to examine the reusability of Au/ SBA-15 composite for CR reduction. At the end of one catalytic cycle, a fresh batch of aqueous CR and NaBH4 solution was added to the preceding reaction mixture and its time-dependent absorbance spectra were recorded. Though the required time after every cycle was noticed to be increased slightly; the composite was effective enough to show 100% decolorization for all the catalytic cycles (Fig. 9). After 5th cycle, the composite was recovered from the reaction mixture by centrifugation and washed it thrice with ethanol and water and dried well. The as-obtained sample was thereafter analyzed by PXRD, TEM and ICPAES technique to examine if there is any structural change occurred after the catalytic reactions. The PXRD pattern of the recycled catalyst

5. Conclusion In conclusion, we have synthesized monodisperse and highly stable Au particles with average diameter of ~2.3 nm inside the mesoporous

1.0

(a)

465 nm

664 nm

max

1.40

(b)

max

= 475 nm 0 min 1 min 5 min 10 min 15 min

1.05

0 min 1 min 3 min 5 min

0.6 0.4

Absorbance

Absorbance

0.8

0.2

0.70

0.35

0.0

0.00 400

500

600

700

800

400

450

Wavelength/nm

0.6

(c)

0.75 max

= 482 nm

0 min

(d) max

0.5

A bsorbance

0.3 max

0.2

550

600

650

= 467 nm

0.60

0.4

A bsorbance

500

Wavelength/nm

= 675 nm 35 min

0.1 0.0

0 min 1 min 3 min 5 min 7 min

0.45

max

= 664 nm

0.30 0.15 0.00

400

500

600

Wavelength/nm

700

800

450

525

600

675

750

Wavelength/nm

Fig. 6. Time dependent UV–vis absorption patterns showing Au/SBA-15 catalyzed simultaneous reduction of mixture of (a) MB & MO, (b) MO & CR, (c) MB & CR, and (d) MB, MO & CR in presence of NaBH4.


(a)

100 0 min 1 min 3 min 4 min

1.0

1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle

80

% decolorization

Absorbance

1.5

0.5

155

60 40 20

0.0 250

300

350

400

450

500

0

Wavelength/nm

-1

0

(b)

2

3

4

5

6

7

8

Fig. 9. Decolorization percentage vs. time (min) plot showing five successive catalytic cycles for the reduction of CR over Au/SBA-15 composite.

0 -1

ln(At/Ao)

1

Time/min

1

-2 -3 -4 -5 0

1

2

3

4

Time/min Fig. 7. (a) Time dependent UV–vis absorption pattern and (b) ln(At/Ao) vs. time (min) plot for Au/SBA-15 catalyzed reduction of aqueous 4-NP with NaBH4.

channels of SBA-15 silica by modified wetness impregnation method. SBA-15 served as a solid host scaffold to control the size and dispersity of Au NPs in its stable mesoporous structure and prevented them from agglomeration and leaching. The inclusion of an extra washing step with dilute NH3 in the wetness impregnation method was believed to assist the formation of small Au NPs. The catalytic properties of the Au/SBA-15 composite towards the decolorization of various dyes/dye mixtures as well as reduction of 4-NP to 4-AP were investigated by UV–visible spectroscopy. The spectroscopic results revealed excellent catalytic activities of the Au/SBA-15 composite. The composite was found to be fairly stable and reusable up to a number of catalytic cycles. On the whole, this work provides a facile, effective and surfactant free approach for the preparation of an excellent catalyst based on SBA-15 silica stabilized small Au NPs.

Acknowledgement The authors are highly thankful to Department of Science & Technology, Govt. of India for funding research project (No: SR/FT/CS-69/2011). Thanks are also due to the Department of Instrumentation & USIC-GU, SAIF-NEHU, IIT Guwahati, and NCL pune, for providing instrumental facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.powtec.2017.04.015.

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Fig. 8. Plausible schematic representation for Au/SBA-15 catalyzed reduction of different organic pollutants.

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