sodium borosilicate nanocomposite using Euphorbia milii extract and evaluation of its catalytic activity in the reduction of chromium(VI), nitro compounds and organic dyes

Accepted Manuscript Title: Biosynthesis of the palladium/sodium borosilicate nanocomposite using Euphorbia milii extract and evaluation of its catalytic activity in the reduction of chromium(VI), nitro compounds and organic dyes Authors: Mahmoud Nasrollahzadeh, Mohaddeseh Sajjadi, Mehdi Maham, S. Mohammad Sajadi, Aziz A. Barzinjy PII: DOI: Reference:

S0025-5408(17)33715-7 https://doi.org/10.1016/j.materresbull.2018.01.032 MRB 9803

To appear in:

MRB

Received date: Revised date: Accepted date:

26-9-2017 22-1-2018 22-1-2018

Please cite this article as: Nasrollahzadeh M, Sajjadi M, Maham M, Sajadi SM, Barzinjy AA, Biosynthesis of the palladium/sodium borosilicate nanocomposite using Euphorbia milii extract and evaluation of its catalytic activity in the reduction of chromium(VI), nitro compounds and organic dyes, Materials Research Bulletin (2010), https://doi.org/10.1016/j.materresbull.2018.01.032 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.

Biosynthesis of the palladium/sodium borosilicate nanocomposite using Euphorbia milii extract and evaluation of its catalytic activity in the reduction of chromium (VI), nitro compounds and organic dyes

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Mahmoud Nasrollahzadeh,a,* Mohaddeseh Sajjadi,a Mehdi Maham,b S. Mohammad Sajadic and Aziz A. Barzinjyd Department of Chemistry, Faculty of Science, University of Qom, Qom 37185-359, Iran

Department of Chemistry, Aliabad Katoul Branch, Islamic Azad University, Aliabad Katoul, Iran.

Department of Petroleum Geoscience, Faculty of Science, Soran University, PO Box 624, Soran, Kurdistan Regional Government, Iraq d

Department of Physical education, faculty of education, Ishik University, Arbil, Iraq

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Graphical Abstract

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Corresponding author. Tel.: +98 2532850953; Fax: +98 2532103595.

E-mail address:[email protected] (M. Nasrollahzadeh).

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Highlights: Green synthesis of the Pd/sodium borosilicate nanocomposite using Euphorbia milii aqueous extract.

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Pd/sodium borosilicate nanocomposite was characterized by FT-IR, XRD, FESEM, EDS, TEM and elemental mapping.

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Pd/sodium borosilicate nanocomposite exhibited good catalytic activity in the reduction of Cr(VI), 2,4DNPH, 4-NP, CR, MO, and MB.

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The catalyst could be easily recycled and reused at least 5 times.

Abstract

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For the first time, a green method is developed to prepare palladium/sodium borosilicate nanocomposite by using

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aqueous extract of the leaves of Euphorbia milii as bioreducing and stabilizing agent. The immobilization of Pd NPs

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on the surface of the sodium borosilicate glass was established by several instrumental analysis including X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy

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dispersive X-ray spectroscopy (EDS), EDS elemental dot maps and transmission electron microscopy (TEM). The

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green synthesized nanocatalyst exhibited excellent performance in the reduction of the chromium (VI) (Cr(VI)), nitro compounds such as 2,4-dinitrophenylhydrazine (2,4-DNPH) and 4-nitrophenol (4-NP) and some organic dyes

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containing Congo red (CR), Methyl orange (MO), and Methylene blue (MB). The biosynthesized heterogeneous

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catalyst was easily recovered and reused at least in 5 consecutive reactions with no losing of its performance.

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Keywords: Biosynthesis; Pd/sodium borosilicate nanocomposite; Reduction; Nitro compounds; Organic dyes;

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Chromium (VI)

Introduction

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Water pollution due to improper discharge of urban and industrial waste, toxic pollutants and improper management of solid waste, can seriously affect human health. Hexavalent chromium (Cr(VI)) as one of the toxic heavy metals with carcinogenic and mutagenic activities originated from industrial processes such as leather tanning, metal electroplating and pigment manufacturer [1]. Colored effluent discharge of various industries of textile, paper, wood, cosmetics, agriculture, plastic and leather creates serious environmental problems. Among the mentioned industries, the textile industry produces most of wastewater with high concentration of dyes in the range of 10-200 mg L-1 [2, 3].

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Estimates indicated that 15-20% of consumer paint in this industry enter into the sewage system. Dyes are aromatic organic compounds which absorb light at a wavelength of 350-700 nm (visible light region). Discharging effluent containing color materials into the environment and aquatic ecosystems can destroy beautiful landscapes and prevent penetration of light into the depths and disrupting the process of photosynthesis and loss of aquatic plants. They also

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lead to eutrophication and interference in the ecology of effluent and have damaging effects on the growth of aquatic organisms [4,5].

Many different methods have been studied by researchers for colored wastewater treatment, including biological methods, coagulation and flocculation, chemical oxidation, adsorption using activated carbon and so on [6,7]. Using chemical methods is of less concern due to being expensive and inefficient and producing secondary pollutants. As a result, researchers are looking for appropriate and eco-friendly ways to remove contaminants without producing

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secondary pollutants. One of the ways in water treatment which has received considerable attention in recent years is

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the use of metal nanoparticles (MNPs) such as Pd, Pt, Au, Ag and Cu as catalyst due to their large surface area, high

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chemical reactivity and efficiency [8-10]. Among the various nanomaterials, palladium nanoparticles (Pd NPs) have

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been widely used as homogeneous or heterogeneous catalysts [11, 12]. However, due to the high surface energy and the tendency for agglomeration of MNPs adding suitable stabilizers such as Fe3O4 [13], reduced graphene oxide [14],

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RGO@TiO2 [15] to the reaction system to avoid reducing catalytic efficiency of these particles is inevitable. Among

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the various supporting agents, sodium borosilicate glass can be used as an effective preservative to load MNPs due to its extraordinary properties and low cost. Borosilicate glass has a low thermal expansion coefficient and high chemical

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stability, which causes high resistance to thermal shock and breaking. Also, borosilicate glass has been extensively used in modern laboratory equipment and optical devices due to their stable physical and chemical properties [16, 17].

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Nanotechnology requires the synthesis of nanomaterials from different chemical compositions and morphologies and excellent control over these features. Eco-friendly reliable processes for the synthesis of nanomaterials are among the important aspect of nanotechnology. Synthesis of the MNPs with biological methods has attracted much attention,

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due to minimize or eliminate the use of harmful substances in the environment which are principles of green chemistry. The biosynthesis methods include organisms such as bacteria, fungi, plants or extracts of plants [18-22]. Biological synthesis of MNPs is considered as a simple alternative to complex physical and chemical methods due to their economic benefits and lack of negative effects on the reaction, especially in the field of pharmaceutical and medical

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applications which needs a synthesis of MNPs on a large scale without the need to use high pressure, power, temperature or hazardous chemicals. Euphorbia milii from the family of Euphorbiaceae (Spurge family) is a widely distributed plant with many applications in folk medicine for the treatment of cancer and hepatitis. Some literatures reported that the plant

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possesses antifungal, antinociceptive and molluscicidal activities (Fig. 1). The plant is a spiny, climbing succulent shrub containing a milky white latex as emulsions of proteins, alkaloids, starches, sugars, oils, tannins, resins and gums from any cut surface of that [23,24]. Phytochemical studies of Euphorbia milii revealed the presence of cardiac glycosides, steroids, phytosterols, anthocyanin, proteins, terpenoids, flavonoids and tannins such as β-sitosterol, cycloartenol, β-amyrin acetate, lupeol, euphol, triterpenes, phenols and flavonoids as both glycoside and aglycon forms such as Quercetine, luteoline and Quercetin 3-O-(2"-O-galloyl)-a-L-arabinofuranoside. Therefore, the plant

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extract can perform as a potent antioxidant bio-media and reluctant green source to synthesize the MNPs [24,25].

Fig. 1. Image of the Euphorbia Milli plant.

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In the present study, Pd/sodium borosilicate nanocomposite was synthesized for the first time by using Euphorbia

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milii leaf extract as a reducing and stabilizing agent (Scheme 1). The performance of the prepared nanocatalyst was evaluated by reduction of chromium (VI) (Cr(VI)), nitro compounds such as 2,4-dinitrophenylhydrazine (2,4-DNPH) and 4-nitrophenol (4-NP) and some organic dyes containing Congo red (CR), Methyl orange (MO), and Methylene

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blue (MB).

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Scheme 1. Preparation of the Pd/sodium borosilicate nanocomposite using Euphorbia milii leaf extract.

2. Experimental 2.1. Instruments and reagents

High-purity chemical reagents were purchased from the Merck and Aldrich chemical companies. All materials were

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of commercial reagent grade. FT-IR spectra were recorded on a Nicolet 370 FT/IR spectrometer (Thermo Nicolet,

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USA) using pressed KBr pellets. X-ray diffraction (XRD) measurements were carried out using a Philips powder

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diffractometer type PW 1373 goniometer (Cu Kα = 1.5406 A˚). The scanning rate was 2º/min in the 2θ range from 10

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to 90˚. UV-Visible spectral analysis was recorded on a double‐beam spectrophotometer (Hitachi, U‐2900) to ensure the formation of nanoparticles. Morphology and particle dispersion was investigated by scanning electron microscopy

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(SEM) (Cam scan MV2300). The chemical composition of the prepared nanostructures was measured by EDS (Energy

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Dispersive X-ray Spectroscopy) performed in SEM. The shape and size of Ag NPs were identified by transmission electron microscope (TEM) using a Philips EM208 microscope operating at an accelerating voltage of 90 kV.

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2.2. Preparation of Euphorbia milii leaf extract 50 g of dried powdered of aerial parts of Euphorbia larica was added in 300 mL double distillated water at 80 °C for

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30 min under reflux conditions. Then the extract was centrifuged in 7000 rpm and the filtrate was kept in the refrigerator to use further.

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2.3. Preparation of Pd nanoparticles using Euphorbia milii leaf extract To prepare the PdCl2 solution, 0.02 g PdCl2 was dissolved in aqueous acidic media (as adjusted using some HCl drops) while ultrasonication. Then after absolutely dissolution of the palladium salt, 10 mL aqueous extract of the plant was added dropwise to 50 mL of the prepared aqueous solution of PdCl2 with constant stirring at 80 °C until changing the color of the mixture. Reduction of palladium ions (PdII) to palladium (Pdo) was monitored by UV-Vis absorption spectroscopy, then the colored solution of Pd NPs was centrifuged at 7000 rpm for 45 min to completely separate.

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2.4. Preparation of the sodium borosilicate glass In a 250 mL flask, 200 mL distillated water and 19 g borax (Na2B4O7·10H2O) were mixed at 35 °C. Then, 50 g sodium silicate was added to the 200 mL distillated water and precipitated silica was separated via filtration. After adding both solutions, a jelly polymer was formed under slow reaction rate and then the prepared jelly was dried to strengthen

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its polymeric fibers. At the end of the reaction, the extra ions were removed through washing the product using double distillated water.

2.5. Preparation of the Pd/sodium borosilicate nanocomposite using Euphorbia milii leaf extract

1.0 g of sodium borosilicate was added to 50 mL Euphorbia milii leaf extract under constant stirring at ambient temperature for 15 min. Afterward, 20 mL of 0.01 M PdCl2 solution was added dropwise to the prepared mixture and the reaction mixture was heated under traditional reflux conditionsfor 3 h at 80 °C. The reaction mixture was allowed

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to cool, the formed precipitate was filtered and collected over a round dish. Finally, it was washed with distilled water

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and dried at 120 °C for 5 h in an oven and then characterized.

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2.6. Reduction of 4-NP by Pd/sodium borosilicate nanocomposite

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In typical, 25 mL of 2.5 mM aqueous solution of the 4-NP and 5.0 mg of Pd/sodium borosilicate nanocomposite were mixed at room temperature under constant stirring for 2 min. Then, 25 mL of the freshly prepared NaBH 4 solution

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(0.25 M), was added to the contents of the baker and the reaction process was monitored using UV-Vis

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spectrophotometer and through recording λmax changes at 400 nm. When the color of the solution was disappeared, the catalyst was separated from the reaction mixture, washed, dried and then reused for the next run.

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2.7. Reduction of 2,4-DNPH by Pd/sodium borosilicate nanocomposite The catalytic reduction of the 2,4-DNPH was conducted at ambient temperature after adding 25 mL of the freshly

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prepared aqueous NaBH4 solution (7.91 mM) to the contents of the beaker, including 25 mL of 10.076 mM aqueous solution of 2,4-diaminophenylhydrazine (2,4-DAPH) and 5.0 mg of Pd/sodium borosilicate mixed under constant stirring for 2 min. The reaction was continued until the color of the solution disappeared. The reaction process was

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monitored using UV-Vis spectrophotometer by recording the absorbance values at 353 nm. At the end of the reaction, the catalyst was recovered for further use. 2.8. Reduction of Cr(VI) by Pd /sodium borosilicate nanocomposite Typically, 5.0 mg of Pd/sodium borosilicate nanocomposite and 25 mL of 3.4 mM aqueous solution of Cr(VI) were added to 1.0 mL formic acid solution (88%) at room temperature under constant stirring. The progress of the reaction

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was monitored by recording the variation in λmax of Cr(VI) at 350 nm. After color fading of the solution, the catalyst was removed and reused. 2.9. Reduction of CR, MO and MB by using Pd/sodium borosilicate nanocomposite Also, in order to evaluate catalytic performance of the biosynthesized nanocatalyst, similar procedures were applied

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for the catalytic degradation of CR, MO, and MB. In a typical procedure, 5.0 mg of Pd/sodium borosilicate nanocomposite was added to 25 mL of 1.44 × 10-5, 3 × 10-5, 3.1 × 10-5 M aqueous solution of CR, MO, and MB, respectively. The contents of the baker were mixed under constant stirring for 2 min. Then, 25 mL of the newly prepared NaBH4 solution (5.3 × 10-3 M) was added and monitoring the progress of the reaction was done through recording the decrease in λmax at 493, 465, and 663 nm for CR, MO, and MB, respectively. At the end of the reaction,

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the solutions were colorless and the catalyst was recovered for reuse.

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3. Results and discussion

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3.1.Biosynthesis and characterization of the Pd/sodium borosilicate nanocomposite

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Through this study, we investigate the preparation of Pd NPs using Euphorbia milli aqueous extract. The Euphorbia milli aqueous extract was characterized by UV-Vis and FT-IR analysis. Fig. 2 shows the UV bonds of the extract at

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300 nm (bond Ι) and 385 nm (bond ΙΙ) due to the cinnamoyl and benzoyl systems, respectively. In fact, they are

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concerned to the π → π* transitions of polyphenolics.

Fig. 2. UV-Vis spectrum of plant extract (1) and UV-Vis spectrum of green synthesis Pd NPs at times range 4 min to 20 days (2). The surface plasmon resonance (SPR) signal in the UV-Vis spectrum of biosynthesized Pd NPs after changing the color of the solution demonstrates the formation of NPs. Further, the stability of biosynthesized NPs monitored using

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UV-Vis spectroscopy established a very good result, even after 20 days, which is due to the effect of the phytochemicals adsorbed on the surface of nanomaterials, Fig. 2(2). The FT-IR of the Pd NPs (Fig.3) shows signals around 3450, 1725, 1582 and 1360 to 1100 cm-1 indicating the OH, carbonyl group (C=O), C=C aromatic ring and C-OH, C-C and C-H vibrations, respectively. These signals clearly

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confirm the presence of plant phytochemicals on the surface of Pd NPs and those effects on protection and stability

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of NPs.

Fig. 3. The FT-IR spectrum of biosynthesized Pd NPs.

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Fig. 4 shows the FT-IR spectrum of the Pd/sodium borosilicate nanocomposite. The absorption bands appearing at 1631 cm-1 and 3428 cm-1 are corresponds to molecular water bending due to absorption of moisture with KBr during

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sample preparation and stretching vibrations during measurements, respectively. The band appeared at 1090 cm-1 is assigned to O−Si−O or Si−O−Si [26]. The FT-IR signal centered at 952 cm-1 is corresponds to B−O band in BO4

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tetrahedral and is also related to the stretching vibration of Si−O−B [27]. Also, the absorption peaks in 802 cm-1 and 472 cm-1 are concerned to the B−O−B symmetric stretching vibration and Si−O−Si bending vibration, respectively [28].

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Fig. 4. The FT-IR spectrum of biosynthesized Pd/sodium borosilicate nanocomposite.

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The antioxidant action of flavonoids resides mainly in their ability to donate electrons or hydrogen atoms (Scheme

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2). In this work, the PdII ions were reduced to nano zero valent (NZV) metallic particles by flavonoid and phenolics present in Euphorbia milli aqueous extract according the below mechanism. Flavonoids and other phenolics present

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excellent tenacity against agglomeration.

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in the extract are not only facilitating the formation of pure Pd NPs by reduction of the PdII to Pd0 but also provide

XRD pattern of the Pd/sodium borosilicate nanocomposite is shown in Fig. 5. The peaks located at 2θ = 40.05, 46.4, and 68.2° are related to(111), (20 0) and (220) planes of the face-centered cubic Pd, respectively (JCPDS No. 89-4897) [26].

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Fig. 5. The XRD powder pattern of the Pd/sodium borosilicate nanocomposite.

Fig. 6 provides the EDX spectrum of the Pd/sodium borosilicate nanocomposite. The biosynthesized Pd/sodium borosilicate nanocomposite is composed of Pd, B, Na, C, Si and O elements. As a result, the fabrication of the Pd NPs

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was confirmed through elemental analysis.

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4000 3500

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1500

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2500

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1000 B K

AuM AuM

PdL PdL

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NaK keV

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Fig. 6.The EDS spectrum of the Pd/sodium borosilicate nanocomposite.

The morphological features of the Pd/sodium borosilicate nanocomposite were ascertained using FE-SEM analysis. Fig.7 presents the typical FE-SEM images of the Pd/sodium borosilicate nanocomposite at different magnifications. As shown in Fig.7, Pd NPs are deposited on the surface of the sodium borosilicate with nearly spherical morphology.

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Fig. 7. FE-SEM images of the Pd/sodium borosilicate nanocomposite.

The shape and size of the biosynthesized nanocomposite were investigated through TME analysis. The TEM

images of the Pd/sodium borosilicate nanocomposite at different magnifications are depicted in Fig.8. The fabrication of Pd NPs on the surface of sodium borosilicate is well established with particle size distribution in nanoscale. As shown in Fig. 8, the average size of particles is 20 nm.

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Fig. 8. TEM images and histogram of particle size distribution of the Pd/sodium borosilicate nanocomposite.

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Also, the EDS elemental dot maps were performed to further investigation of the elemental composition of the Pd/sodium borosilicate nanocomposite. Fig. 9 shows that Pd NPs are highly dispersed on the sodium borosilicate

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surface which is in agreement with previous analysis results.

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Fig. 9.The EDS elemental maps of the Pd/sodium borosilicate nanocomposite. According to the obtained results, we propose a structure for the nanocomposite in which the Pd NPs have been embedded in the glass matrix (Scheme 3). The slow reaction rate during the synthesis of sodium borosilicate makes it possible to form same regular holes which would be a good host for MNPs. The flexibility of rings of boron causes

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the B-O bonds are capable to fit with metals in various sizes. Also, an appropriate interaction can be occurred between empty orbital of boron with P-orbital electron pairs of the metal or among electron pairs of oxygen with P-empty

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orbital of the metal.

3.2. Evaluation of the catalytic activity of the Pd/sodium borosilicate nanocomposite through the reduction of 4-NP,

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2,4-DNPH, Cr(VI), CR, MO and MB The catalytic performance of the biosynthesized Pd/sodium borosilicate nanocomposite was evaluated through the reduction of 4-NP in the presence of excess amounts of NaBH4. Generally, the aqueous solution of 4-NP exhibits an intense absorption about at 317 nm in neutral or acidic media. When NaBH4 is added into the 4-NP solution, the color of the solution altered from light yellow to deep yellow, due to the formation of 4-nitrophenolate ions under strong

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basic conditions. Also, a red shift of the absorption peak happens from 317 nm to 400 nm. Without using Pd/sodium borosilicate nanocomposite, the λmax of 4-nitrophenolate remains unchanged even after 100 min (Table 1, entry 1). The λmax at 400 nm gradually fades and a new peak appears at 297 nm for 4-aminophenol (4-AP) when Pd/sodium borosilicate nanocomposite is used as a catalyst (Fig. 10). Various amounts of NaBH4 (79 and 100 equivalents) and

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catalyst (3.0 and 5.0 mg) were used in a series of experiments and the optimized result was achieved with 5.0 mg of the catalyst and 100 equivalents of the NaBH4 (Table 1, entry 4). To study the effect of support and Pd NPs on the efficiency of the reduction process, the catalytic activity of the Pd/sodium borosilicate nanocomposite in reduction of 4-NP was compared with untreated sodium borosilicate and Pd NPs, separately. No reduction reaction was observed with sodium borosilicate (Table 1, entry 5), which indicates that the main role of Pd NPs in the reduction process. Also, the importance of the support was clearly identified by longer reaction time achieved with bare Pd NPs which

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was attributed to the agglomeration of the Pd NPs during the reaction (Table 1, entry 6). The better catalytic activity

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obtained with Pd/sodium borosilicate nanocatalyst is related to synergy interaction between the Pd NPs and sodium

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borosilicate which can improve the reduction of 4-NP. As shown in Scheme 4, the mechanism of the reduction of 4-

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NP consists of two steps: (1) adsorption of NaBH4 and 4-NP onto the surface of the Pd/sodium borosilicate nanocomposite and (2) electron transfer from BH4− to 4-NP through catalyst-mediated reactions and then desorption

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of 4-AP from the catalyst surface. Table 1.

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Completion time for the reduction of the 4-NP (2.5×10-3 M) to 4-AP in the presence of different amounts of catalyst and NaBH4

Catalyst (mg)

NaBH4 (equivalents)

Time

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100

100 mina

Pd/sodium borosilicate nanocomposite (3.0)

100

5 min

Pd/sodium borosilicate nanocomposite (5.0)

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4.5 min

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Pd/sodium borosilicate nanocomposite (5.0)

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2.5 min

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Sodium Borosilicate (5.0)

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45 mina

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Pd NPs (5.0)

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7 min

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Entry

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No reaction.

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Fig. 10. The UV-Vis spectra of 4-NP aqueous solution in the presence of 100equivalents of NaBH4 and 5.0 mg of the

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Pd/sodium borosilicate nanocomposite.

Scheme 4. The proposed mechanism for the reduction of 4-NP to 4-AP in the presence of Pd/sodium borosilicate nanocomposite and NaBH4.

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The catalytic performance of the Pd/sodium borosilicate nanocomposite is compared with reported other catalysts in reduction of 4-NP in the literature and comparative results showed that the present nanocatalyst exhibits better catalytic performance than other catalysts (Table 2).

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Table 2. Comparison of the catalytic performance of the other catalysts in reduction of 4-NP.

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Entry Catalyst Time Reference 1 Pd-GA/RGO 5 min 27 2 Pd/RGO 5 min 28 3 AgNPs@MWCNTs-polymer composite 5 min 29 4 Pd/FG 12 min 30 5 HMMS-NH2-Pd 60 min 31 6 PdCu/graphene 1.5 h 32 7 PtNPs AmLig 180 min 33 8 XG/Ag NPs 24 h 34 9 Pd/sodium borosilicate 170 s This work Pd-GA/RGO: Pd-gum arabic/reduced graphene oxide; Pd/FG: Pd/functionalized grapheme; HMMS: hollow magnetic

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mesoporous spheres; Pt NPs: platinum nanoparticles; AmLig: ammonium derivatives of the lignin samples; XG:

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Xanthan gum; MWCNTs: multi-walled carbon nanotubes.

In order to further evaluate the catalytic activity of biosynthesized NPs, the Pd/sodium borosilicate nanocomposite

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was used for the reduction of the 2,4-DNPH in the presence of excess amounts of NaBH4 (Scheme 5). As it was

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predictable, without loading of Pd and using sodium borosilicate, there was no change in the color of the solution even after a long time. The UV-Vis spectra of the 2,4-DNPH solution exhibits a λmax at 353 nm, which shifts to 290 nm due

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to the formation of 2,4-DAPH, after the addition of a freshly NaBH4 solution (Fig. 11). The best performance was achieved with 7.0 mg Pd/sodium borosilicate nanocomposite and 104 equivalents of NaBH4 and under these

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conditions the color of solution disappeared quickly during 17 s.

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Fig. 11.The UV-Vis spectra of 2,4-DNPH aqueous solution in the presence of 104 equivalents of NaBH4 and 7.0 mg

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of the Pd/sodium borosilicate nanocomposite.

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The Pd/sodium borosilicate nanocomposite was also used to reduce Cr(VI) in the presence of formic acid. The progress of the reduction reaction was monitored through recording the changes in absorption peak at 350 nm

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correspond to K2Cr2O7 aqueous solution. It is well known that formic acid with potent reducing features in the presence

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of the nanocatalyst can be easily decomposed to CO2 and H2without the production of intermediate materials [35]. Reduction of Cr(VI) to Cr(III) is accomplished through hydrogen transfer (Scheme 6). As can be seen in Fig. 12, the

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solution color completely disappeared during 15 min. Optimum conditions included 5.0 mg Pd/sodium borosilicate

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nanocomposite, 1.0 mL of formic acid and 100 equivalents of NaBH4.

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Fig. 12.The UV-Vis spectra of Cr(VI) aqueous solution in the presence of 5.0 mg of the Pd/sodium borosilicate

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nanocomposite and 1.0 mL of formic acid.

Furthermore, the catalytic activities of the biosynthesized nanocatalyst were investigated in reduction of various

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organic days such as CR, MO, and MB. Fig. 13 shows the UV-Vis spectra of CR, MO, and MB obtained through

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drawing 1.0 mL of the reaction mixture and diluting it to 25.0 mL within certain times during the reaction. The intensity of absorption peaks at λmax (CR) = 493 nm, λmax (MO) = 465 nm, and λmax (MB) = 663 nm were decreased

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gradually during the reaction progress. In this context, hydrogen generated from the reaction of BH4- and

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H2Otransferred to target dyes through the unique structure of Pd/sodium borosilicate nanocomposite. The optimal amounts of catalyst and the time needed to complete dye decolorization in the presence of NaBH4 (5.3 × 10-3 M) are

Table 3.

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presented in Table 3.

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Completion time for the reduction of MB, MO and CR. Entry

Dye (M)

NaBH4 (equivalents)

Catalyst (mg)

Time

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MB (3.1 × 10-5)

5.3 × 10-3

1.0

8s

MB (3.1 × 10 )

5.3 × 10

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3.0

8s

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MB (3.1 × 10-5)

5.3 × 10-3

5.0

3s

4

MO (3.0 × 10-5)

5.3 × 10-3

5.0

10 min

-3

7.0

4.37 min

5.0

3.5 min

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2

-5

-5

8

MO (3.0 × 10 )

5.3 × 10

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CR (1.44 × 10-5)

5.3 × 10-3

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3.3. Catalyst reusability The level of recyclability is an important factor affecting the practical application of the catalyst. In order to evaluate the reusability of the biosynthesized Pd/sodium borosilicate, at the end of the reaction, the catalyst was separated from the reaction mixture and reused in the next cycle. The obtained results showed that the prepared nanocatalyst can be

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used at least in 5 consecutive cycles without significant reduction in catalytic activity. The FE-SEM analysis after the 5th recycle exhibited no obvious change in morphological characteristics of the reused Pd/sodium borosilicate nanocomposite (Fig. 14). Also, the size and shape of the biosynthesized nanocomposite not much has changed after completing the 5th consecutive reaction which was established through TEM analysis (Fig. 15). Furthermore, the EDS analysis of recycled nanocatalyst well approved the previous results obtained from the FE-SEM and TEM analyzes

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(Fig. 16).

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Fig. 14. The FE-SEM images of recycled Pd/sodium borosilicate nanocomposite.

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SC RI PT

U

Fig. 15. The TEM images of recycled Pd/sodium borosilicate nanocomposite.

SiK

N

13000 12000

A

11000 10000

M

9000 8000

6000

D

7000

OK

TE

5000 4000 3000

EP

2000 1000

BK

NaK

AuM AuM

PdL PdL

keV

0

6.688

CC

0

A

Fig. 16. The EDS spectrum of recycled Pd/sodium borosilicate nanocomposite.

4. Conclusion In this study, a rapid, biocompatible, convenient and efficient method is developed for the fabrication of the Pd/sodium borosilicate nanocomposite through reduction of PdII ions by aqueous extract of the leaves of Euphorbia milii. Using bioreducing agents causes a clean synthesis of nanocatalyst without using toxic and hazardous reagents and produces non-toxic and less dangerous by-products. Also, using sodium borosilicate glass as an inexpensive support reduces

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the cost of catalyst preparation. The green synthesized nanocatalyst exhibited excellent catalytic performance for the reduction of Cr(VI), 2,4-DNPH, 4-NP, CR, MO and MB in water under environmental conditions. The biosynthesized catalyst was stable and easily recycled and reused at least 5 times without sensible loss in its catalytic efficiency.

SC RI PT

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