Facile reduction of para-nitrophenols: catalytic efficiency of silver nanoferns in batch and continuous flow reactors

Alok Kumar Srivastava *a, Kunal Mondalb, Kingsuk Mukhopadhyaya, N. Eswara Prasada and Ashutosh Sharma*c
aDefence Materials and Stores R & D Establishment (DRDO), GT Road, Kanpur 208013, India. E-mail: aksrivastava@dmsrde.drdo.in
bDepartment of Chemical and Biomolecular Engineering, North Carolina State University, USA
cDepartment of Chemical Engineering, Indian Institute of Technology, Kanpur-208016, Uttar Pradesh, India. E-mail: ashutos@iitk.ac.in

Received 1st September 2016 , Accepted 27th November 2016

First published on 28th November 2016


Abstract

The catalytic efficiency of silver-nanoferns (Ag-NFs) decorated on carbon microfiber surfaces has been investigated. The Ag-NFs were grown on carbon microfibers employing electrodeposition technique using an electrolytic solution of aqueous silver nitrate and boric acid. The structure of grown Ag-NFs has nanoscaled sub-branches of sizes ≤50 nm that could be controlled by applied voltage and electrodeposition time. Using a specially designed home-made glass reactor, the catalytic efficiency of the Ag-NFs grown over carbon microfibers (cAg-NF) has been measured and tested for a model catalytic reaction of para-nitrophenol reduction mediated by sodium borohydride, both in batch and continuous flow modes of operation. The cAg-NFs have shown excellent catalytic activity with a normalized rate constant κ = 3.42 s−1 g−1. The reusability for the cAg-NFs has been observed up to seven cycles of operation without much degradation in the catalytic efficiency. The integrated c-Ag-NF catalyst system and the designed reactors are simple and can be easily incorporated for facile effluent treatment or in other applications where catalytic reduction may be required.


1. Introduction

Silver is a noble metal and in its bulk state is highly unreactive due to its completely filled d-band electronic structure. However, at the nanoscale, with high surface to volume ratio, the reactivity of silver can be greatly enhanced.1–3 Various forms of silver nanostructures for example nanowires, nanoparticles, nanoferns etc. have been studied.4,5 These nanostructures have already found extensive applications in electronics, bactericidal applications, optoelectronics, surface enhanced Raman spectroscopy and catalysis.6–11 Further, the silver nanostructures are said to possess potential to be a crucial material for future technologies.12–15 In recent years, research on catalytic applications of silver nanostructures has drawn considerable interest owing to their chemical stability, relatively easier synthesis, and high catalytic activities, particularly in redox reactions.12,16–18

On the other hand, nitrophenols are chemically and biologically one of the most stable pollutants emanating in large quantities from the effluent water of pharmaceutical and petrochemical industries.19 As such, being one of the leading organic pollutants for the agricultural and industrial wastewaters, various techniques for removal of the nitrophenols have been developed in the past.20–22 Reducing the polluting para-nitrophenols (4-NP) to environmentally benign para-aminophenols is one of the most useful approach to overcome the above problem. In addition, the reduced product para-aminophenol (4-AP) is a significant precursor material used in agriculture and pharmaceutical industries to produce various analgesic and antipyretic drugs. Therefore, the development of a catalytic system capable of converting 4-NP to 4-AP in an economical, eco-friendly and efficient manner is of considerable social and industrial interest.

Reduction of the pollutant para-nitrophenol (4-NP), mediated by sodium borohydride (NaBH4), via the catalytic activity of noble metal nanostructures is one of the well-studied model reaction systems.23–25 A number of studies have been conducted in the past using a variety of nanoparticles of Ag, Au, Pt, Fe, Pd and their core shell combinations as catalysts for the reduction of p-nitrophenol using NaBH4.17,26–28 There are several reports on the catalytic reduction of 4-NP to 4-AP in heterogeneous or homogeneous medium.29 However, in most of these studies that use free standing nanoparticles/nanostructures, there are certain restrictions and disadvantages. Firstly, the nanoparticles have inherent tendency to agglomerate due to attractive van der Waals forces between the nanosized metallic nanostructures in the solution and thus adversely influencing the catalytic reaction.30 Secondly, the expensive noble metal nanostructures are difficult to recover and recycle rendering them suitable only for single cycle usability. And lastly, the metal nanoparticles being difficult to filter and recover from the catalytically reduced effluents, themselves become an environmental hazard. The problem related to the agglomeration of nanoparticles though can be overcome by surface modification techniques such as using polymers coatings, complex ligands, or surfactants, but this results in a decrease in the catalytic activity due to steric hindrance and reduced availability of active catalyst surface. However, the problem of non-reusability and non-recovery of expensive catalyst when applied in form of free standing nanoparticles remains as usual, as said earlier.

More recently, to overcome the above said drawbacks of free standing nanoparticles, few approaches have been developed to immobilize metal nanoparticles inside a matrix of porous carbon or carbon microfibers, which act as a catalytic support material. Embedding of catalyst nanoparticles prevents the aggregation and promotes suitable dispersion at the time of chemical reaction.25,27,31 The catalyst nanoparticles being embedded in a support material can be easily recycled or reused which is always welcome as it turns the process more economical.32 The chemical inertness, easy recyclability and reusability are some of the properties of carbon fibers that make them highly suitable catalyst support material.33,34 Using the above embedding approach, one can effectively prevent metallic nanostructures from forming agglomerates even in the presence of electrolytes or strong reducing agents. However, even this useful approach has a disadvantage; the total available surface area of the catalyst particles is restricted since some of them remain embedded in the matrix.27,35 This restricted access can be easily seen as loss to the potential catalytic efficiency of the nanostructures.

In the present study, we attempt to address some of above discussed drawbacks and limitations in applying silver nanostructures for catalytic reduction applications. Rather than using Ag nanostructures as free standing particles or by embedding them in the carbon fiber substrate as described above in the past studies, an innovative approach has been employed of growing silver nanoferns, a high surface area nanostructure, over the carbon microfibers. A synthesis scheme based on electrodeposition technique has been presented, for fabricating Ag-nanofern decorated carbon microfibers (cAg-NFs) with fern branches as small as 50 nm. The choice of carbon fiber here as the substrate has advantages of not only as an extremely stable material in most of the chemical environments, but its conducting nature also allows the electrodeposition of the silver nanoferns on its surface. Due to the industrial importance and usefulness, the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of NaBH4 has been chosen as the model reaction for evaluating the efficiency of the cAg-NF composite structure. The catalytic efficiency of cAg-NFs has been evaluated in batch as well as continuous modes of reactor operation. A simple reactor design for measuring and analyzing the catalytic activity of the grown cAg-NF fiber composite material in continuous flow model is also presented. These Ag-NF decorated carbon fibers demonstrate excellent catalytic activity in NaBH4 mediated reduction of 4-NP to 4-AP in terms of high rate constant and also permit reusability for repetitive use. The Ag-NFs decorated on carbon microfiber as discussed here are quite promising for catalytic applications. To the best of the authors' knowledge, they have not been studied for catalytic application as discussed in this paper. Furthermore, the reactor design for this particular catalytic reaction as a model pollutant removal application is entirely a new approach.

2. Experimental

2.1 Materials

Silver nitrate (AgNO3), boric acid (H3BO3), para-nitrophenol (PNP), and sodium borohydride (NaBH4) were purchased from Sigma-Aldrich Co., USA. The HPLC grade water was supplied by Merck Pvt. Ltd., India and used for catalytic reactions. Polyacrylonitrile derived carbon fibers with diameter ∼5 µm were purchased from Toray, USA. All chemicals were of analytical grade and used as supplied without further purification.

2.2 Synthesis of Ag nanoferns decorated carbon fibers (cAg-NF)

A schematic representation of the setup used for electrodeposition of cAg-NF is shown in Fig. 1. Briefly, an electrolyte solution is prepared by mixing 0.1 g of AgNO3 salt and 0.1 g of boric acid in 10 ml of water with the pH of the solution maintained in the range of 4.5–5.0. Strands of carbon fibers are cut to 5 cm length, followed by sonication and washing in deionized water. Thereafter, the fibers are dried for 30 min in air at 80 °C. The electrochemical deposition has been performed by using two electrode system where 2.5 × 2.5 cm2 platinum (Pt) mesh is the anode and the above prepared carbon fibers work as cathode. The fibers are dipped upto 10 mm length inside the solution during the deposition process. Electrodeposition has been carried out at three different voltages 5 V (S1), 7.5 V (S2) and 10 V (S3) applied for time duration of 10 s for each case.
image file: c6ra21977b-f1.tif
Fig. 1 Electrodeposition of silver nanoferns (Ag-NF) on carbon fibers.

2.3 Characterization

To characterize the surface morphologies and the structure of Ag-NF-carbon fibers, Field Emission Scanning Electron Microscopy (FESEM, Quanta 200, Zeiss, Germany) has been used. The elemental composition of the silver nanostructures is determined through energy dispersive X-ray spectroscopy equipment (EDX) (Oxford Instruments) attached with FESEM.

The imaging by transmission electron microscopy (TEM) has been performed by a Tecnai G2, US microscope. To prepare the TEM samples, the cAg-NF fibers were sonicated in ethanol and a drop of ethanol suspension of the Ag nanofern is dropped on a carbon-coated copper grid and dried in vacuum for few hours. For surface area estimation, BET analysis was carried out using Autosorb iQ apparatus from Quantachrome Instruments. For BET analysis, the electrodeposited portion of fibers was carefully isolated by cutting out the plain fibers. The fiber strands having Ag-NF deposition were used to obtain the surface area by using nitrogen gas adsorption–desorption technique. The X'Pert Pro, PAN analytical, Netherlands, X-ray system with Cu Kα radiation has been used for the powder X-ray diffraction (XRD) analysis to obtain the chemical structural information and determine crystallinity of the Ag-NF and carbon fibers. The thermogravimetric analysis (TGA) has been carried out, using TA instrument HiRes TGA 2950, at 10 °C min−1 heating rate in air to investigate the efficiency of electrodeposition of silver ferns.

2.4 Measurement of catalytic efficiency and reactor design

The standard reaction mixture for all the catalytic efficiency measurements has been prepared using a ratio of 70 µl volume of 4-NP solution (1 × 10−2 M) mixed to 7.0 ml of 0.1 M NaBH4 solution. The concentration of reducing agent NaBH4 has been kept very high compared to that of the para-nitrophenol to ensure that its availability remains constant throughout the catalytic reaction. Immediately after mixing, the nitrophenol molecules get ionized and transformed to nitrophenolate anions signified by an intense yellow colour of the solution. In the presence of silver nanoferns, which act as a catalyst, the yellow color of the reaction solution slowly disappears indicating conversion of nitrophenolate ions to aminophenols (4-AP). This change which indicates the progress of the catalytic reaction has been monitored by UV spectrophotometer at a wavelength of 400 nm by using Varian Cary 50 Bio UV-Vis Spectrophotometer. Sampling for UV-Vis spectroscopy is done by taking 2 ml volume of the reaction mixture that is added back to the parent solution after UV-Vis measurement.

The catalytic efficiency has been tested in batch as well as continuous flow model reactors. The schematics of the two test modes are shown in Fig. 2a–c. In the batch mode (Fig. 2a, scheme 1), the reaction has been carried out in a beaker, by adding measured amount of c-Ag-NF to the prepared reaction mixture and stirred continuously while constantly monitoring the course of reaction. In the case of continuous flow mode, two different approaches are investigated (Fig. 2b and c, schemes 2 and 3 respectively). In the scheme 2 (Fig. 2b), the flow of reaction mixture is along the axis of catalyst fibers. The orifice of a funnel is plugged with the c-AgNF fibers and the reaction mixture is allowed to pass through them at a rate controlled by a flow controller. Whereas, in scheme 3, the reaction mixture is directed to flow in the direction perpendicular to the axis of the cAg-NF fibers arranged as shown in Fig. 2c. To implement scheme 3, for the perpendicular flow mode, a tubular reactor has been specially designed as shown in the optical image of Fig. 3. As seen in the figure, the c-AgNF fibers are inserted and sealed in small orifices created by glass blowing along the length of the tube at four equidistant locations, 10 mm to each other. To maintain uniformity in the reaction, equal amount of cAg-NF fibers have been used in both the cases (schemes 2 and 3) of continuous flow reactors. The sampling for UV-Vis measurements is done after allowing 100 ml of solution to pass through the reactor in continuous flow reactors. All the reaction processes are performed at room temperature (25 °C) and at controlled humidity level of 34%.


image file: c6ra21977b-f2.tif
Fig. 2 Schematic representation for the catalytic reduction reaction schemes (a) scheme 1, batch reactor; reaction mixture is continuously stirred (b) scheme 2, continuous flow reactor; reaction mixture flows coaxial to the catalyst cAg-NF (c) scheme 3, continuous flow reactor; reaction mixture flows perpendicular to the catalyst cAg-NF.

image file: c6ra21977b-f3.tif
Fig. 3 A photographic image of the designed continuous flow reactor. The inset shows the SEM image of the inserted and sealed cAg-NF strands along the reactor tube.

3. Results and discussion

3.1 Electrodeposition of Ag-NFs

In literature, the synthesis of silver nano dendrite structure has been extensively studied using various methods like solvothermal, sonoelectromechanical, galvanic replacement, electrochemical reduction etc.19,36–40 In the present work, silver nanoferns decorated on carbon fibers have been synthesized by electrodeposition technique using potentiostatic electrochemical reduction route with aqueous solution of AgNO3 as the electrolyte. Since the carbon fiber is conducting in nature, it can perform as one of the electrodes (cathode) for the electrodeposition of the silver nanoferns on its surface. Three electrochemical reduction conditions have been used at 5 V (S1), 7.5 V (S2) and 10 V (S3) applied for short duration of time (10 s) for the each case. The voltages applied in this work are in the range of 5–10 V and since the standard reduction potential of Ag+/Agatom pair is −1.80 V with respect to standard hydrogen electrode, the over potential conditions prevail for the electrodeposition. This overpotential results in non-equilibrium conditions leading to formation of nano dendrites or fern like structures.26 The mechanism of dendrite formation under these conditions has been explained by Matsushita et al. on the basis of diffusion limited aggregation (DLA) model wherein the deposition morphology is governed by the diffusion process with individual reduced metal atoms bonding to the growing surface.41 In the case of deposition for S3, certain fragments of the deposits have been observed to fall off the cathode i.e. from the carbon fibers towards the end of the deposition process. Since this phenomenon is not observed for S1 and S2, it may be concluded that perhaps dendrites formed in S3 become too long/bulky leading to their dissociation from the fibers.

3.2 Surface morphology

FESEM imaging has been performed to study the morphology of the synthesized Ag nanofern structures. Fig. 4 shows the FESEM and TEM micrographs of silver nanoferns deposited on carbon fiber strands. Each of these fiber strands comprises of hundreds of microfibers (diameter ∼ 5 µm) as shown in the Fig. 4a and b. A comparison of morphologies of the silver nanoferns grown over carbon microfibers at different voltages can be observed from the Fig. 4c, e and f (where Fig. 4d shows the higher magnification view of Fig. 4c). They represent the nanofern growth at 5 V, 7.5 V and 10 V respectively. As observed the density of the silver nanoferns growth increases progressively as the voltage is increased, though 7.5 V and 10 V do not show appreciable difference. In terms of morphology, the fern growth was staggered in case of 5 V while at 7.5 V fully developed ferns covered the whole fiber diameter. For 10 V electrodeposition, the growth was extremely dense. Such silver nanoferns were observed to be deposited with a nearly uniform distribution on whole of the immersed part the carbon microfibers. Fig. 4d shows high magnification FESEM micrograph of a single silver nanofern which is clearly seen to possess a dendritic morphology. The obtained Ag nanofern have been found to be very much reproducible and preserve exceptional hierarchical structure spreading to several generations with morphological self-similarity. The dendritic structures as observed, comprise of an extended central trunk with smaller side branches ornamented entirely as smaller leaves. The length of the trunk is up to tens of micrometers and the diameter is around 50–100 nm. The length of the branches can extend up to 10–15 µm with a diameter of less than 50 nm. By the fractal nature of these nanoferns, a leaf grows and becomes a branch followed again by side branches and from where the next generation of leaves grow.
image file: c6ra21977b-f4.tif
Fig. 4 (a) Low magnification FE-SEM image of silver nanoferns electrodeposited at 5 V potential on carbon fibers, (b) digital image of a silver nanofern grown carbon fibers, (c), (e) & (f) a single carbon fiber showing growth of silver nanofern islands at 5 V, 7.5 V & 10 V respectively. Increase in the density of the fibers is observed increasing progressively. (d) High magnification FE-SEM image of a single branch of silver nanoferns, (g) TEM image of the silver nanofern shows the individual crystallites of the fern.

TEM imaging provides further information of the nano/microstructure of the Ag dendrites. The TEM image in Fig. 4g shows an individual silver nanofern electrodeposited at the applied potential of 5.0 V for 10 s that presents an impeccable dendritic structure comprising of a trunk, branches and leaves. The most significant observation in TEM images is that the Ag dendritic crystals grow along a preferential direction with the angle of ∼60–65° between the branches and the trunk, as may be clearly observed in the Fig. 4g. At higher voltage (S3), it is observed that there is an abundance of nanoferns visible on trunks and branches (Fig. 4f). It also shows that dendrites of silver nanoparticles start agglomerating along the fibers surfaces. This apparently is because of the higher applied deposition potential. The rate of deposition of silver atoms is known to escalate with increased applied potential (i.e., the overpotential), which is the driving force for the electrochemical reaction.42 Since the deposits at 10 V (S3) have been noticed to fall off from the carbon fibers, it appears that limits of usable overpotential were exceeded in this case. The graph of Energy-Dispersive X-ray (EDX) analysis of Ag dendrites grown on carbon fiber as substrate is shown in Fig. 5. Carbon and silver elements were easily distinguished in FESEM and EDX performed over a selected microstructural area of the composite cAg-NF. The EDX results show that the surface of cAgNF are composed of 92.87 wt% elemental silver and 7.13 wt% carbons.


image file: c6ra21977b-f5.tif
Fig. 5 Selected area EDX spectra of cAg-NF.

BET analysis of the fibers was carried out to get an estimation of the overall surface area of the catalyst in conjunction with the fibers. The analysis results gave a BET surface area of the plain carbon fibers to be 3.756 m2 g−1 while the results for cAg-NFs indicate a BET surface area of 11.405 m2 g−1, 17.489 m2 g−1 and 20.146 m2 g−1 for 5 V, 7.5 V and 10 V respectively. The results indicate that there is an increase in the surface area of the fibers due to the deposition of silver nanoferns.

3.3 XRD analysis of carbon fibers and cAg-NF fibers

The microscopic structure and crystalline nature of the blank (as purchased) carbon fibers and of the carbon fiber with electrodeposited silver nanoferns has been examined by XRD (X-ray wavelength = 1.54060 Å) diffractographs shown in Fig. 6. The obtained XRD data (intensity of peaks at 2θ) from the figure has been compared and indexed with Pearson's Crystal Data (PCD) (file no. 1819831). A broad diffraction peak for the blank (as received) carbon fibers occurs at around 2θ = 22.5° which characterizes an amorphous carbon structure.43 The peak could be indexed to (002) Bragg reflection derived from carbon. For the Ag NF/carbon fibers, the phase composition and crystallinity of the Ag were matched by the XRD data with PCD reference (file no. 261046). The sharp peaks observed at 2θ = 38.11° and 44.31° are related to (111) and (200) lattice planes of metallic silver. The other relatively smaller peaks appeared at 57.43°, 64.45° and 77.48°, which correspond to (222), (220) and (311) lattice planes, respectively. All these XRD diffraction peaks signified the characteristic face centered cubic (fcc) structure for silver. As observed from the Fig. 6, the peak intensity increases with increase in the deposition voltage. This can be attributed to more crystallinity in the nanoferns being deposited on the carbon microfibers.
image file: c6ra21977b-f6.tif
Fig. 6 XRD plot of the plain carbon fiber and cAg-NF electrodeposited at 5 V, 7.5 V and 10 V, applied for 10 s in each case.

3.4 Thermal gravimetric analysis (TGA)

The TGA was carried out in air to estimate the amount (wt%) of silver nanoferns grown on the carbon microfibers under different experimental conditions. The results are shown in plots in Fig. 7. The TGA plots show that initially up to around 500 °C, there is very small weight loss indicating the thermal stability of cAg-NFs. However, beyond 500 °C there is a sharp decline showing substantial weight loss until 700 °C with no further loss with increasing temperatures. This is easily explained by the fact that most of the carbon content starts to oxidize at about 500 °C and once this process is complete for all the available carbon content, no further weight loss is observed. Interestingly, the pure carbon fibers took longer time to completely oxidize as compared to cAg-NFs. The reason may be attributed to good thermal conductivity of silver which creates hotspots inside the fibers leading to faster degradation of the carbon. The finally obtained constant value in the plots indicates the amount of silver loading in the fibers. The loadings of silver nanoferns as calculated after pyrolysis of the fibers are determined as 14.1%, 11.9% and 5.7% in wt% for deposition cases, respectively, of 10 V (S3), 7.5 V (S2) and 5 V (S1). As it is obvious that higher amount of silver is deposited for higher over-potential voltages. Significantly, it is also observed that the increase in the loading is not appreciable in between 7.5 V and 10 V. The reason may be attributed to the dissociation of the nanoferns from the carbon fibers during the reaction as well as the diffusion limited nature of the deposition process.
image file: c6ra21977b-f7.tif
Fig. 7 TGA analysis of the silver nanoferns/carbon fibers prepared using different deposition conditions.

It has been said earlier that for the case of specimen S3 deposited at 10 V, the Ag structures tend to fall-off or dissociate from the carbon fibers explaining the above observed not so appreciable growth over 7.5 V specimen (S2).

3.5 Catalytic degradation of para-nitrophenol by cAg-NF fibers

3.5.1 Reaction mechanism. The catalytic degradation of 4-NP to 4-AP is a six-electron transfer process and the reaction can be schematically represented as shown in Fig. 8a. The role of silver nanocatalyst in various redox reactions on the basis of electrochemical current potential is well known and can be exploited for catalytic reactions such as in the present case for 4-NP. It is believed that in the presence of the reducing agent NaBH4, the silver nanoferns act as an electron transfer medium for the redox reaction. The nucleophilic BH4 ions can donate electron to the metallic nanocatalyst while the electrophilic nitrophenols accept electrons by the virtue of electron withdrawing characteristic of nitro group. On simultaneous adsorption of reactant molecule 4-NP and the reacting species BH4 to the silver nanoferns surfaces, the electron transfer occurs between the reactant and reacting species. The heterogeneous catalysis is generally known to occur by adsorption of the reactant molecule to the catalyst surface followed by diffusion of the molecule to the active site and the formation of surface complex. Thereafter, the surface complexes transform into product molecule and desorption of the product occurs. The product formation is the slowest and the rate determining step deciding the overall order of the reaction. The observed catalytic activity of the cAg-NFs catalytic system is expected by the above mechanism because of the occurrence of metal centers whereas the carbon microfiber support plays an important task of providing the support to the nanoferns. The photographs of solutions of para-nitrophenol and the catalytically converted para-aminophenols are shown in Fig. 8b and c, respectively, that present a comparative visual view of an effective catalytic performance of cAg-NFs.
image file: c6ra21977b-f8.tif
Fig. 8 (a) Schematic representation of catalytic reduction of 4-NP to 4-AP, (b) image of 4-NP solution before catalytic conversion, and (c) after complete conversion/degradation of 4-NP solution to 4-AP.
3.5.2 The catalytic performance of cAg-NFs; efficiency and reaction kinetics. The catalytic performance of the synthesized cAg-NFs prepared under the three different electrodeposition reaction conditions, specifically at 5 V (S1), 7.5 V (S2) and 10 V (S3) were evaluated in batch reactor mode (Fig. 2a). The reaction kinetics and the rate constant for the catalytic reaction mediated by cAg-NFs have been determined with the help of UV-Vis spectroscopy. For a comparison, the catalytic activity of blank carbon fibers, without any silver nanoferns, is also evaluated. Prior to above evaluations, the reaction of the reducing agent NaBH4 on 4-NP has been noted by the UV-Vis spectroscopy by observing the shift in reaction peak resulting from transformation of 4-NP to its corresponding phenolate species, as has been said earlier in the Experimental section. Accordingly, when appropriately proportionate amounts of 4-NP (70 µl) is mixed with NaBH4 solution (7 ml), an intense yellow color appeared indicating the conversion of 4-NP into phenolate species which can also be seen by the red shift from 317 nm to 400 nm in UV spectra of 4-NP in Fig. 9a. For a catalytic reaction to occur, the phenolate species should be converted to the product 4-AP and this should be observable by UV-Vis with its characteristic absorption wavelength λmax at 300 nm. The peak at 400 nm has been taken as bench mark for determining the reaction kinetics and efficiency of catalytic performance of cAg-NF in the following.
image file: c6ra21977b-f9.tif
Fig. 9 The UV-Vis absorption spectra for (a) 4-NP and 4-NP/NaBH4 (b) reaction mixture 4-NP/NaBH4 in presence of blank carbon fiber without any silver catalyst (c) spectra showing catalytic activity using cAg-NF specimen S1 for conversion of NaBH4 mediated 4-NP solution to 4-AP (amount of Ag-NF in cAg-NF catalysts = 0.45 mg), and (d) plots of time dependent UV-visible absorption (A) for the catalytic reaction of 4-NP/NaBH4 converted to 4-AP in the presence of different specimen S1–S3 with loadings of cAg-NF (i) 0.45 mg (S1) (ii) 0.96 mg (S2) and (iii) 1.13 mg (S3) (see text). The inset shows the logarithmic plots for the absorption at 400 nm.

First, the catalytic activity of blank carbon fibers (without any silver nanofern), if any, has been observed by adding the blank carbon fibers to a continuously stirred 100 ml mixture of 4-NP/NaBH4 solution in a glass beaker (batch reactor mode). As shown in Fig. 9b, there is no noticeable change in the 400 nm peak in the UV spectra even after hours of monitoring except for a very slow and slight decrease in the intensity that can be ascribed to the known adsorption capacity of carbon materials.44 Besides, an absence of the peak at 300 nm which could be accounted for the catalytically converted product 4-AP, indicates an inactivity of blank carbon fiber towards the catalytic reaction. Thus a contributory role for the carbon fiber in the catalytic performance of cAg-NF catalytic may be ruled out.

Following the same batch reactor mode procedure as above for the blank carbon fiber case, the catalytic performance for the cAg-NFs has been evaluated for the specimen S1, S2 and S3 prepared under three different electrodeposition conditions as noted earlier at 5 V (S1), 7.5 V (S2) and 10 V (S3). On adding cAg-NF fibers to the continuously stirred 100 ml mixture of 4-NP/NaBH4 solution the yellow colour of the solution begins to vanish gradually demonstrating the progress of the catalytic reaction. The course of reaction has been monitored by sampling for UV-Vis spectroscopy conducted at different stages of reaction time by taking an aliquot of 2 ml solution which was added back to the reaction mixture after measurement. The UV-Vis spectra in Fig. 9c obtained for the specimen S1 reveals that the 400 nm absorption peak nearly vanishes with increasing reaction time simultaneous with the emergence of a new peak at 300 nm corresponding to reaction product 4-AP. The above observations imply that the nanoferns in the cAg-NF fibers has a considerable activity for the catalytic reaction. Similar observations of catalytic activity for the specimen S2 and S3 have been made in their respective UV-Vis spectra (not shown here) though now with differing reaction kinetics. The plots of time dependence of absorption peak at 400 nm that relate directly to catalytic reaction kinetics for various specimen S1–S3 are shown in Fig. 9d. A near zero absorbance (A) at 400 nm may be considered as a completed catalytic reaction. The reaction time for completed catalytic reaction is observed to be the least at 12 min for the specimen S3 with highest at 33 min for S1 as may be easily noted from Fig. 9d. It is also noted that when higher cAg-NF catalyst loadings (as the case for the specimen S3) are used there is a significant improvement of catalytic process with reduced reaction time. It may be mentioned that quantified amount of silver present in the sample specimen S3 is highest as determined earlier based on TGA analysis for S1, S2 and S3 of cAg-NF respectively as 0.45, 0.96 and 1.13 mg.

The catalytic reaction rate constant (Kt) has been determined from the absorption (A) vs. time (t) plots in Fig. 9d. The following assumptions and explanations have been advanced for determining the Kt. First, since for amount of the para-nitrophenol, the used concentration of NaBH4 has been kept very high compared to what may be stoichiometrically necessary, it can be considered to be constant throughout the catalytic reaction. An isobathic point exists between 400 nm and 300 nm absorption peaks in the time dependent UV-Vis spectra which suggests that spectroscopic peaks alone of 4-NP and 4-AP may be necessary to determine of the reaction kinetics. Also, a pseudo-first-order kinetics can be considered appropriate and justified to calculate the reaction rate constants. Finally, it is noted that the measured entity absorbance (A) by the UV-Vis spectra and concentration (C) of 4-NP in the solution are linearly proportional. Hence, the concentration ratio (Ct/C0) can be determined from the corresponding absorbance ratio At/A0 at 400 nm, where C0 and Ct are the concentrations of 4-NP, respectively, at ‘0’ and at ‘t’ time as the catalytic reaction proceeds. The slope of the plots of ln(At/A0) vs. time ‘t’ (inset in Fig. 9d) determines the rate constant (Kt) for the catalytic reaction. The observed linear nature of ln (At/A0) vs. time ‘t’ for all the specimen S1–S3 of the cAg-NF justifies the stipulation that the catalytic reaction indeed followed a pseudo-first-order kinetics. The calculated values of the rate constants (Kt) from the slope of the straight line plots in Fig. 9d have been summarized in Table 1. These results suggest that the rate constant and the amount of silver loading into cAg-NF is dependent on the electrodeposition process parameters (S1, S2, S3) and this observation here is also supported by the literature reports.25,26,45 The normalized rate constant κ (obtained by dividing Kt by the mass of catalyst) has been evaluated to enable comparing with already reported values for similar Ag nanostructures applied for catalytic performance. As seen from Table 1, the largest normalized rate constant value obtained in this work is 3.42 s−1 g−1 (for S3).

Table 1 The calculated rate constants for the catalytic conversion of 4-NP to 4-AP by cAg-NF fibers for various amounts of Ag-NF catalyst loadings
Sample Wt of Ag-NFs in fibers (mg) Kt (s−1) κ (s−1 g−1)
S1 0.45 1.3 × 10−3 2.90
S2 0.96 3.0 × 10−3 3.14
S3 1.13 3.8 × 10−3 3.42


Previously reported values of κ for coral-like dendrites, banana leaf like dendrites and spherical Ag nanostructures are 1.30, 0.41 and 0.09 s−1 g−1 respectively.46 Recently, a value of 2.81 s−1 g−1 for κ was reported for silver nanodendrites.19 Tang et al. have reported Ag nanoparticles–carbon composite catalyst having κ = 1.69 s−1 g−1.47 Clearly, the normalized rate constant value for this work (highest 3.42 s−1 g−1) is much higher than those reported for comparative Ag nanoparticle/nanostructure-carbon composite based catalytic systems. Here, it may be worth mentioning that for the free standing nanoparticles the κ value as high as 1375 s−1 g−1 has been recently reported.48 The reason may be obviously attributed to much higher surface area per unit volume exposed to reaction solution in case of free nanoparticles as compared to other morphologies. Although, as said earlier the free standing Ag particles cannot be recycled besides being not easy to remove from the reaction mixture themselves are environment pollution hazard.

3.5.3 Catalytic efficiency of continuous flow reactor. Most of the industrial processes require facile methods obviously for economic reasons. Hence, it is significant to measure the catalytic efficiency of the continuous flow reactor proposed here. The catalytic efficiency of the continuous flow reactor operating in schemes 2 and 3 has been measured by determining the conversion ratio Ct/C0 at various flow rates, from 0.5 ml min−1 to 20 ml min−1, by allowing the direction of flow for 4-NP solution either along or perpendicular to the axis of cAg-NF fibers as illustrated earlier in Fig. 2b and c. The flow rates have been varied with the help of a flow-controller. The c-AgNF fibers obtained from S2 were utilized for measuring the efficacy during continuous flow regime. The variation of % conversion (Ct/C0) with the flow rate for the flow direction both along and perpendicular with respect to the catalytic cAg-NF are presented in plots in Fig. 10. For determining Ct by UV-Vis spectra, the sampling of 3 ml of the catalytically reacted 4-NP is done only after 100 ml of reaction mixture has passed through the reactor.
image file: c6ra21977b-f10.tif
Fig. 10 % conversion of 4-NP to 4-AP at different flow rates.

It is observed that as the flow rate increases the conversion decreases with a maximum value for Ct/C0 of 87% at the lowest experimented flow rate of 0.5 ml min−1. The reason may be attributed to the fact that a low flow rate allows greater residence time to 4-NP for the catalytic reaction. It is also observed that the trend of % conversion in both the cases of direction of flow is similar for very low and very high flow rate regimes. Whereas, at all the moderate flow rates, the % conversion of 4-NP is higher for the case when the flow direction is perpendicular to the fiber axis compared to the along the catalyst fiber axis case. The reason for lower efficiency of conversion for the case cAg-NF fibers along the flow direction may be attributed to the fact that the fibers being packed inside the orifice of the funnel, there is a possibility of some of the catalyst fibers not adequately coming in contact with the reaction solution as may be desirable.

In contrast, for the perpendicular flow case, since the fibers are immersed in the reaction solution and have sufficient space around them, hence they separate out, with water and reactants completely wetting it. This allows most of the fiber strands and in turn the catalyst nanoferns to be in a liberal contact with the reaction mixture.

The reaction mixture during the perpendicular flow can easily penetrate the boundary layers of dendritic structure of the nanoferns of cAg-NF. Clearly, such higher possibility of contact for catalytic Ag-nanoferns can translate to higher conversion efficiency for this scheme 3 for the reactor design. Here, it is worth mentioning that using the Ag-nanoferns catalyst developed in the present study becomes significantly advantageous due to the fractal nature of the catalyst structure with high surface area when compared to other comparative nanostructures of Ag that may be employed usually possessing smaller surface area. Finally, for the high flow rate regime where conversion Ct/C0 is found to be very low with almost similar results in both cases of flow direction, the reason apparently is too small a residence time for the 4-NP solution.

3.5.4 Reusability of cAg-NFs. The reusability of the cAg-NF fibers has been investigated for specimen type S2 by repeating the catalysis experiment. At the end of each catalytic degradation cycle, the cAg-NF hybrid fibers are collected, cleaned with fresh water and dried and then added to a new batch of 4-NP so that the catalyst gets the same environment after each cycle of reduction. It is observed that even after seven cycles of usage, there is no significant loss in catalytic activity (Fig. 11). Even, this insignificantly small increase in reduction time may be because the surface of the catalyst becomes unrefreshed as the cycle executes. It was also observed that there was a loss of 14.2% in the weight of the original catalyst sample and the sample left after seven cycles. The reason may be attributed to the loss of fibers/detachment of the catalyst from the fibers during the process of washing and experimentation. This can be another reason for the observed decrease in the catalyst activity after seven cycles. Still, the observed sufficiently high recyclability or reusability is undoubtedly a benefit in terms of cost-effectiveness and their applications.
image file: c6ra21977b-f11.tif
Fig. 11 Complete degradation of 4-NP with sample S2 shows reusability of the cAg-NF fiber strands.

The adherence of the electrodeposited silver nanoferns onto the carbon fiber is observed adequately strong such that the silver nanoferns stay fastened to the carbon microfibers throughout the catalytic reaction conditions used in this study without any significant leaching. The FE-SEM investigations carried out on the cAg-NF fibers after the repeated/recycled catalytic reactions also supported the above observed robustness.

4. Conclusions

A simple and facile process for fabricating silver nanofern decorated carbon microfibers (cAg-NF) using electrodeposition technique has been demonstrated. A simple continuous flow reactor design has also been presented for studying the catalytic conversion of para-nitrophenol to para-aminophenol. The reactor design presented may also find its utility in practical industrial applications. The efficacy of the electrodeposited cAg-NF fiber in the conversion of p-nitrophenol to p-aminophenol has been established by testing the catalytic efficiency both for the batch as well as for continuous flow reactor models. The obtained normalized reaction rate constant κ in the range 2.9–3.4 s−1 g−1 for the cAg-NF system in this study is higher than the comparable catalytic systems reported in the past based on silver nanoparticle embedded in carbon matrix or other supported Ag-nano form catalytic systems. Another added advantage of the present catalyst system is that it can be easily recovered for reuse upto 7 cycles of catalytic performance. The reactor design for the continuous flow model can be easily scaled up for effluent treatment and other applications such as in pharmaceutical industry where catalytic efficiency of silver may be usefully employed. The Ag-NF based carbon fibers discussed here may also find functional applications in sensors, actuators and microreactors.

Acknowledgements

The authors are grateful to the support of Department of Science and Technology, New Delhi, India and Defence Research and Development Organisation, New Delhi, India for their support in this work.

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Footnote

Both the authors have equal contributions.

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