Synthesis of palladium nanoparticles using continuous flow microreactor

Accepted Manuscript Title: Synthesis of Palladium Nanoparticles using Continuous Flow Microreactor Author: S. Sharada Prashant L. Suryawanshi Kumar Rajesh P Sarang P. Gumfekar T.Bala Narsaiah Shirish H. Sonawane PII: DOI: Reference:

S0927-7757(16)30218-7 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.03.068 COLSUA 20549

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

26-12-2015 23-3-2016 27-3-2016

Please cite this article as: S.Sharada, Prashant L.Suryawanshi, Rajesh P Kumar, Sarang P.Gumfekar, T.Bala Narsaiah, Shirish H.Sonawane, Synthesis of Palladium Nanoparticles using Continuous Flow Microreactor, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.03.068 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of Palladium Nanoparticles using Continuous Flow Microreactor S.Sharada1, Prashant L.Suryawanshi2, Rajesh Kumar P2, Sarang P.Gumfekar3, T.Bala Narsaiah1* [email protected], Shirish H.Sonawane2* [email protected] 1

Department of Chemical Engineering, Jawaharlal Nehru Technological University Anantapur

College of Engineering, Anantapur, AP, India 2

Department of Chemical Engineering, National Institute of Technology, Warangal, TS, India

3

Department of Chemical and Materials Engineering, University of Alberta, AB, Canada

*Corresponding author. Tel: +91-8702462626.

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Graphical Abstract

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Highlights  Continuous flow microreactor based synthesis of Pd nanoparticles. 

At optimum reaction conditions 5 nm Pd particles were obtained.



The electrochemical surface area (ECSA) of Pd nanoparticles was 2.6 cm2/mg.



Pd nanoparticles size decreases with an increase in concentration of precursor and reducing agent.



The average size of Pd nanoparticles increases with increase in flow rate.

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Abstract This study reports on the use of a continuous flow microreactor for the synthesis of palladium (Pd) nanoparticles. The size of Pd nanoparticles could be controlled in the range 5 - 200 nm by varying the flow rate of the solution containing the precursor and reducing agent. TEM analysis showed cubic morphology of the particles. The electrochemical activity and electro-reduction reaction on Pd/carbon electrode were monitored using cyclic voltammetry (CV) and linear sweep voltammetry (LSV), respectively. The surface area of Pd nanocatalyst was determined to be about 2.6 cm2/mg using CV data. The open circuit potential (OCP) was found to be 0.77 V using LSV measurement.

Keywords: Continuous flow microreactor; palladium nanoparticles; cyclic voltammetry; linear sweep voltammetry; particle size distribution; electrochemical active surface area

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1. Introduction Palladium nanoparticles are widely used in various technological applications, e.g., microelectronics, bio-sensing, catalysis and fuel cells [1-2]. The Several methods are available for synthesizing palladium nanoparticles over the past decades [2-3]. Precise control over the size of palladium nanoparticles and reproducibility are important issues [3-7]. It is important to note that the catalytic activity and electron transport properties of nanoparticles depend on their size. Investigators have explored a wide variety of synthetic techniques for the preparation of palladium nanoparticles with a narrow particle size distribution. However, robust synthetic techniques are required for specific applications [8-9].A number of methods were reported for the synthesis of Pd nanoparticles such as microwave assisted synthesis, chemical reduction method and sol-gel technique [10-13]. These conventional methods exhibit lack of precise control over mixing, nucleation and growth, which subsequently affect the particle size, particle size distribution and reproducibility [12-13]. Rapid mass transfer associated with intense mixing can significantly improve the physical and chemical properties of palladium nanoparticles. Microreactor technologies offer several advantages over conventional ‘beaker-based’ method and it is a part of process intensification used in chemical synthesis [14]. Microreactors exhibit large surface area compared to their volume, which enhances the mass transfer, which in turn improves the physical and chemical properties of the nanoparticles [15]. Microreactors offer a small volume for the reaction, which allows precise control over the synthesis of nanoparticles such as palladium (Pd), platinum (Pt) and gold (Au) etc. [16-18]. Rebrove et al.[19] have shown that a microreactor system increases the heat transfer within the reaction mixture and hence improves the reaction kinetics. The key advantage of microreactors is the ability to realize the ‘lab-on-chip’ design. Scale up is relatively easier by multiplying the number of reactors [20-21].

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Recently,Woitalka et al. [22] reported that the flow pattern in a continuous flow microreactor affects the mixing, which inturn controls the mass transfer in a reactor, the particle size and its distribution. Synthesis of Pd nanoparticles in a continuous flow microreactor is a paradigm-shift opportunity to explore the versatility of this technique for the preparation of nanoparticles [2324]. Pd nanoparticle is efficient metal catalyst for hydrogenation and electrochemical reactions in fuel cells [25-29]. The cost of Pd nanocatalyst is lower than the Pt catalyst and it is abundantly available on earth than Pt catalyst. It has more oxidation potential than Pt [2, 30]. Pd is suitable for hydrogen storage and sensing applications [31].There are several methods available for synthesizing Pd nanoparticles using precursors such as palladium chloride (PdCl2), potassium tetrachloropalladates (K2PdCl4),palladium nitrate, palladium acetate with reducing agents such as hydrogen, ethylene glycol, NaBH4, xanthan gum, hydrazine, alcohol, sodium ascorbate and sodium citrate and along with capping agents such as cetyl trimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate (SDS) [32-45]. Therefore PdCl2 was chosen as a precursor and NaBH4 as a reducing agent for the preparation of palladium nanoparticles (Pd NP). We have successfully synthesized Pd nanoparticles in a continuous flow microreactor. Further, we have demonstrated the feasibility to use the synthesized Pd nanoparticles as an electrochemical catalyst. 2. Experiment 2.1. Materials Palladium (II) chloride (PdCl2, 99%, analytical grade) was purchased from Merck specialties Pvt. Ltd., Mumbai, India and used as a precursor for the synthesis of Pd nanoparticles. Sodium borohydride (NaBH4, 99%, analytical grade)was used as a reducing agent and N-Cetyl N, N, N tri-methyl ammonium bromide (CTAB) was procured from Thomas Baker Ltd., Mumbai, India

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and used as a surfactant. Millipore deionized water was used in all experiments for the preparation of all solutions. 2.2. Synthesis of Nano-Fluids The experimental setup of continuous flow microreactor and reaction conditions for various experiments are shown in figure 1. Solutions of desired concentrations of the precursor and reducing agent were prepared in distilled water. In order to stabilize the Pd nanoparticles, approximately 1.0 mM CTAB was added in solutions [46]. Initially, both aqueous solutions were introduced through a Y-shaped junction to the continuous flow microreactor using two syringes, which were operated by peristaltic pumps at a controlled flow rate. A simple copper capillary microreactor was fabricated in house (0.8 mm inner diameter and 124 cm length of the tube).The molar ratio of precursor to reducing agent was maintained at 1:4 and 1:8. If the ratio of precursor to reducing agent is less than 1:4, there is possibility of oxidation of the metal particles. The flow rate of precursor and reducing agent was maintained at 30 and 50 mL/h for two different experimental runs. The microreactor was immersed in a temperature-controlled water bath to maintain the temperature at 25 oC. Finally, Pd colloidal suspension was formed in dark brown in color. The suspension was centrifuged at 8000 rpm for 20 min, washed with water and acetone 34 times to remove unwanted impurities and salts from product and these were then dried (at a constant temperature of 100 oC) under vacuum to recover Pd nanoparticles. 2.3. Characterization Palladium nanoparticles morphology was observed using TEM (PHILIP, CM 200 model), operating at 20-200 kV and 2.4 Ao resolution. The particle size distribution was measured by dynamic light scattering technique (Malvern Nano Series ZS).The surface functionalization of Pd nanoparticles with CTAB was confirmed by FTIR analysis. The electrochemical properties of Pd 7

nanoparticles were analysed by cyclic voltammetry and linear sweep voltammetry (Bio-Logic, SP 300 EC LAB). 2.4. Electrochemical measurements The electrocatalytic performance, kinetic characteristics of oxygen reduction reaction (ORR) and durability of the catalysts were measured by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques. For cyclic voltammetry, the electrode was prepared with Pd nanocatalysts coated on 1 cm2 carbon paper strip, which was prepared as follows: Pd catalyst was prepared by dispersing 10 mg of Pd particles in 2 mL of 50 % isopropyl alcohol (IPA) in presence of sonication for 100 minutes in a bath sonicator. Then, 10 µL of Nafion (5 wt%, Dupont) solution was added and sonicated for 10 minutes to form a homogeneous slurry. Finally, the slurry was coated on carbon strip and dried at 60 oC for 1 h. CV measurements were performed in 3.5 M KCl aqueous electrolyte solution with a sweep rate of 50 mV/s between -0.2 to 1.0 V. The electrochemical active surface area (ECSA) was determined under similar conditions. A Pt wire was used as counter electrode, which was used to estimate ECSA from the hydrogen adsorption/desorption electric charges. Linear sweep voltammetry (LSV) / Oxygen reduction reaction (ORR) was important inorder to evaluate the specific activity of electrocatalyst. LSV was used to check the resistance between electrodes and membrane for the MEA with prepared Pd catalysts. Linear sweep voltammetry experiments were carried out after purging with oxygen and keeping the assembly in saturated 3.5 M KCl aqueous solution with a scan rate of 5 mV/s. LSV was also used to evaluate the constant open circuit potential (OCP) without the load applied.

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3. Results and Discussion 3.1. Reaction mechanism The reaction scheme for the formation of palladium nanoparticles by the reduction of palladium (Pd+2) to palladium (Pd0) using sodium borohydride [47] is shown in equation (1). PdCl2 + 2NaBH4

Pd0 + H2 + B2H6 + 2NaCl …………….…

(1)

As shown in figure 2-a, the reactants (precursor and reducing agent) pass through the Y-junction of the microreactor and the mixing of reactants occurs at stage-I leading to formation of Pd nanoparticles. In stage-II, nucleation takes place. After stage II, further growth occurs, due to the diffusion of solute throughout the length of the microreactor [48]. As shown in figure 2-b, the size of the particle depends on nucleation process and residence time of the particle in the microreactor. The meta-stable zone is controlled by the reagents concentration. The concentration of palladium chloride was a limiting reactant and concentration of the reducing agent is four times higher than the palladium chloride concentration. Hence, the order of reaction is the pseudo first order based on the concentration of palladium chloride It is possible to restrict the growth of the nanoparticles and its distribution by reducing the metastable zone. The meta-stable zone is controlled by residence time of the reactor by changing the flow rate of the precursor, specifically the flow rate of palladium chloride solution. CTAB is added as a surfactant below the critical micelle concentration to avoid the agglomeration of the particles. As shown in figure 2-b, CTAB acts as the capping agent to stabilize the particles and restricts the growth of the nanoparticle. Figure 2-a shows particle formation, representing the variation of concentration and different stages of formation of particles with residence time of microreactor [49].

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3.2. Stabilizing the Pd nanoparticles using CTAB FTIR analysis was used to identify the deposition of CTAB onto palladium nanoparticles. Figure 3 shows, FTIR peaks of palladium nanoparticles in presence of CTAB as surfactant. It was found that CTAB stabilizes the Pd nanoparticles shows a bond formation between palladium chloride and sodium borohydride. The absorption band between 3000 and 3700 cm–1 represents the presence of the surfactant/capping agent. The peak observed at 3650 cm-1 represents the presence of pure CTAB and O-H stretching of the surfactant. The peak was also observed at 3337 cm–1 corresponding to N-H stretching vibration of CTAB capped onto the Pd nanoparticles [50]. The methylene C-H stretching and anti-stretching vibration frequencies of pure CTAB are observed at 2880 and 2939cm−1 [50-51], while only a single peak shift was observed at 2875cm−1in case of Pd nanoparticles, which indicates the presence of -CH2 vibration of capping agent (surfactant) on the surface of Pd nanoparticle. Small additional peaks were observed in the range between 700 and1000 cm–1 (954.8, 890, 833 and 737cm−1) thereby indicating the interaction between surfactant and Pd nanoparticles (i.e. attachments of the CTAB with surface of the Pd nanoparticles) [52–54]. 3.3. Effect of concentration of precursor and reducing agent on particle size The effect of precursor PdCl2and reducing agent NaBH4concentrations on average particles size and shape is reported in Table 1-aand Figure 4, 5 and 6 (TEM Images, selected-area electron diffraction (SAED) pattern and particle size distribution (PSD)). The molar ratio of precursor to reductant (PdCl2 to NaBH4) was kept constant in the ratio of 1:4. To find out the crystallinity of Pd nanoparticles, an experiment was carried out using 0.00025M PdCl2 & 0.001M NaBH4 at a flow rate of 30 mL/h (batch C, Table 1-a). Figure 5 shows that a selected-area electron diffraction (SAED) pattern was observed with 4.8, 8 and 9.4 nm sizes of Pd nanoparticles. The

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palladium nanoparticles show perfect crystal lattice planes corresponding to (111), (220) and (311) which in turn indicate face-centered cubic (FCC) crystals [55]. As shown in Table 1-a, as the concentration of PdCl2decreased from 0.001 to 0.00025 M, the average size of Pd nanoparticles reduces from150 nm to 5 nm (shown in figure 4-ato 4-c). For the same case, the particle size distribution (PSD) decreased from 287 nm to 105 nm (as shown in figure 6-a to 6c). The average particle size, obtained from TEM image which is less than that obtained from the PSD data, due the presence of surfactant/capping agent (CTAB) onto Pd nanoparticles in aqueous solutions [39]. 3.4. Effects of volumetric flow rates of palladium chloride and sodium borohydride The average particle size of Pd nanoparticles was affected by the flow rate of the precursor and reducing agent.When the flow rates were changed from 30 to 50 mL/h,the average particle size increases from 150 nm to 200 nm as reported in TEM images (from Figure 4-a, b, c and d) and 287 nm to 309 nm as reported in PSD data (from Figure 6-a b, c and d) respectively, details are given in Table 1-b. Similarly, for the case C & E, when the flow rate was increasedfrom 30 to 50 mL/h, the average particle size was found increased from 105 nm to 216 nm from PSD data (from Figure 6-c and e) and details were given in Table 1-b. This happens due to lower flow rates or small space-time. It was observed that the super-saturation of Pd nanoparticles was achieved within a small period of time [46]. As the volumetric flow rate increases, both the residence time of the precursor and reducing agent decrease in microreactor. As shown in the Table 1-c, the Reynolds numbers are 477 for 30 mL/h and 795 for the 50 mL/h, respectively and mixing rates were 16.97 for 30 mL/h and 28.93 for 50 mL/h, respectively. 1) The Reynolds number is calculated by ⁄

(2)

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Where U is velocity of water, ρ is density of water, D is inner diameter of microreactor and μ is viscosity of water (0.89 mPa.sec at 25oC) 2) Mixing rate is calculated by ⁄

(3)

Where U is velocity of water and D isinner diameter of microreactor. In Figure 2-a, both precursor and reducing agent were introducedat the Y-junction and mixed throughout the continuous flow microreactor. As reported in literature [48, 55], the diffusional mixing time is proportional to square of mixing length (Equation 4), therefore, over short distances the diffusion is rapid. 3) Mixing time is calculated by ⁄ Where, L ischaracteristic mixing length and

(4) is diffusivitycoefficient

Hence, the decrease in the flow rate has amarginal effect on the reduction in the particle size. 3.5. Electrochemical measurements The electrochemical properties of Pd nanocatalyst were tested using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements. Figure 7 shows the CV curve of Pd nanoparticle as a catalyst saturated with 3.5 M KCl (aqueous) at a sweep rate of 50 mA/sec. From the electrochemical test results, the electrochemical surface area (ECSA) was calculated from the CV through hydrogen adsorption/desorption electric charges and the specific activity of electrocatalysts was calculated from LSV through oxygen reduction reaction (ORR). If ECSA increases, the rate of reaction increases between the surface and the flowing fluid. The charge transfer reaction occurs at the activation sites of catalyst due to adsorption limited rate. The monolayer hydrogen adsorption/desorption reaction occurs at palladium catalyst site.

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CV curve was used to calculate the electrochemical surface area (ECSA) of catalyst. The calculated ECSA of Pd catalyst was found to be 2.6cm2/mg. As shown in LSV diagram (Figure 8), the open circuit potential (OCP) value was 0.77 V without application of load. 4. Conclusion In summary, palladium nanoparticles with anarrowsize distribution were obtained incontinuous flow microreactor by diluting precursor concentration (PdCl2), reducing agent (NaBH4) and keeping the smaller flow rates (30 mL/h).Pd catalyst shows high electrochemical surface areawith 2.6 cm2/mgand higheropen circuit potential value near to 0.77 V. Lowest particle size of palladium (5 nm) was obtained at the concentration 0.00025M of PdCl2& 0.001M of NaBH4 at flow rate of 30 mL/h. The average particle size increased with an increase in the flow rate of the reactants (precursor and reductant). The average size of Pd nanoparticles decreased with a decrease in the concentration of precursor. Different particle sizes were obtained from TEM images and from PSD data, due to the presence of surfactant/capping agent (CTAB) onto Pd nanoparticles in aqueous solution. Super-saturation of Pd nanoparticles reached within a small period of time. From the FTIR, the interaction or attachment between surfactant and Pd nanoparticles with CTAB was found to be very good. Acknowledgment The Authors gratefully acknowledge the funding agency,Department of Electronics & Information Technology (DeitY) and Ministry of Communications and Information Technology (MCIT), Government of India for all the financial assistance.

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Figure Captions

Experiment No. 1

PdCl2 (M) 0.001

NaBH4 (M) 0.004

Ratio (precursor to reductant) 1: 4 (0.25)

Flow rate (mL/h) 30

Particle size (nm) (using DLS) 287

2

0.0005

0.002

1: 4 (0.25)

30

278

3

0.00025

0.002

1: 8 (0.125)

30

105

4

0.001

0.004

1: 4 (0.25)

50

309

5

0.00025

0.002

1: 8 (0.125)

50

216

Figure 1: Experimental setup for stable palladium (Pd) nanoparticle (NP) synthesis using continuous flow microreactor.

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Figure2-a: Mechanism of palladium nanoparticle formation in microreactor

Figure 2-b: Representation of reaction scheme for the formation of metallic Pd nanoparticles in the presence of stabilizing agent CTAB

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Figure 3: FTIR spectra of formation of Pd nanoparticle using CTAB as a surfactant

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a)

b)

c) d) Figure 4:TEM images of Pd nanoparticles obtained by variation of reagents concentration at different molar ratio with different flow ratea) 0.001M PdCl2& 0.004M NaBH4with flow rate 30 mL/h (Batch A), b) 0.0005M PdCl2& 0.002M NaBH4with flow rate 30 mL/h (Batch B), c) 0.00025M PdCl2& 0.001M NaBH4with flow rate 30 mL/h (Batch C) andd) 0.001M PdCl2& 0.004M NaBH4with flow rate 50 mL/h (Batch D)

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Figure 5: Selected-area electron diffraction (SAED) CCD pattern of Pd NP at 0.00025M PdCl2& 0.001M NaBH4 with flow rate 30 mL/h

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Figure 6-a): PSD curve for palladium nanoparticles (Pd NP) at 0.001M PdCl2& 0.004M NaBH4 with flow rate 30 mL/h in microreactor for Batch A

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Figure 6-b): PSD curve for palladium nanoparticles (Pd NP) at 0.0005M PdCl2& 0.002M NaBH4 with flow rate 30 mL/h in microreactor for Batch B

Figure 6-c): PSD curve of palladium nanoparticles (Pd NP) at0.00025M PdCl2& 0.001M NaBH4 with flow rate 30 mL/h in microreactor for Batch C

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Figure 6-d): PSD curve of palladium nanoparticles (Pd NP) at0.001M PdCl2& 0.004M NaBH4 with flow rate 50 mL/h in microreactor for Batch D

Figure 6-e): PSD curve of palladium nanoparticles (Pd NP) at0.00025M PdCl2& 0.001M NaBH4 with flow rate 50 mL/h in microreactor for Batch E

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Figure 7: Cyclic voltammetry of Pd NP catalyst in 3.5 M KCl (aqueous) solution as an electrolyte at sweeprate of50 mV/s.

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Figure 8: Linear sweep voltammetry (LSV) of Pd nanoparticle saturated with 3.5 M KCl (aqueous) solution at a scan rate of 5 mV/s.

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Tables Table 1-a: Synthesis of Pd nanoparticles in microreactor using various concentrations of precursor and reducing agent Batch No.

PdCl2 (M)

NaBH4 (M)

Flow Rate (mL/h)

Average size distribution from PSD (nm)

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Average particle size from TEM(nm) 150 (± 5)

A

0.001

0.004

B

0.0005

0.002

30

20 (± 5)

278

C

0.00025

0.002

30

5 (± 2)

105

287

Table 1-b: Synthesis of Pd nanoparticles in microreactor using various flow rates of precursor and reducing agent Batch No.

PdCl2 (M)

NaBH4 (M)

Flow Rate (mL/h) 30

Average particle size from TEM(nm) 150 (± 5)

Averagesize distribution from PSD (nm) 287

A

0.001

0.004

D

0.001

0.004

50

200 (± 5)

309

C

0.00025

0.002

30

5 (± 2)

105

E

0.00025

0.002

50

Not reported

216

Table 1-c: Effects of various flow rates of precursor and reducing agent with Reynolds number and space-time. Flow rate of individual reagents (mL/h) 30

Reynolds number (Re) 477

Mixing rate U/D (Sec-1) 16.97

Spacetime (Sec.) 14.73

50

795

28.3

8.77

31