Synergistic catalysis of monometallic (Ag, Au, Pd) and bimetallic (AgAu, AuPd) versus trimetallic (Ag-Au-Pd) nanostructures effloresced via analogical techniques

Journal of Molecular Liquids 287 (2019) 110975

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Synergistic catalysis of monometallic (Ag, Au, Pd) and bimetallic (Ag\\Au, Au\\Pd) versus trimetallic (Ag-Au-Pd) nanostructures effloresced via analogical techniques Hanan B. Ahmed a,⁎, Hossam E. Emam b,⁎ a

Chemistry Department, Faculty of Science, Helwan University, Ain-Helwan, Cairo 11795, Egypt Department of Pretreatment and Finishing of Cellulosic based Textiles, Textile Industries Research Division, National Research Centre, Scopus affiliation ID 60014618, 33 EL Buhouth St., Dokki, Giza 12622, Egypt. b

a r t i c l e

i n f o

Article history: Received 28 March 2019 Received in revised form 5 May 2019 Accepted 13 May 2019 Available online 15 May 2019 Keywords: Dextran Nanostructures Monometallic Bimetallic Trimetallic Catalytic activity

a b s t r a c t Monometallic (Ag, Au, Pd), bimetallic (Ag\\Au, Au\\Pd) and trimetallic (Ag-Au-Pd) nanostructures, were generated using dextran as a natural polymer. The simultaneous co-reduction of multiple metal precursors with dextran gave a fine control to produce spherical shape Ag-Au-Pd trimetallic nanostructures. Small-sized Ag monometallic nanostructures of 8.7 nm were enlarged to 15.7 nm for Ag\\Au bimetallic nanostructures. While, trimetallic nanostructures from Ag-Au-Pd was produced with much smaller size of 3.8 nm and quite narrower size distribution of 2–7 nm. The spectral analysis confirmed that the alcoholic groups of dextran were responsible for the reduction of metal ions to produce nanostructures and consequently oxidized to aldehydic/ketonic groups. The catalytic performance of the synthesized nanostructures was evaluated for the reduction of p-nitroaniline and the results demonstrated that there was a strong correlation between catalytic activity and composition of nanostructures. Half time of the reduction was diminished from 3.93 to 0.90 min. for Pd monometallic and Ag-Au-Pd trimetallic nanostructures, respectively. Using of the trimetallic nanostructure as a catalyst results in acceleration the reduction reaction 151 times. The results showed a promising approach to boost catalytic activities of the trimetallic nanostructures which were prepared via quite simple green method at ambient conditions. © 2019 Published by Elsevier B.V.

1. Introduction Manufacturing of various metal nanostructures was extensively considered due to their fascinating properties to be applicable in different purposes, such as plasmonics [1–3], chemical sensing [4,5], surfaceenhanced Raman scattering (SERS) [6–8], drug delivery [9,10], and catalysis [11]. Multi-metallic alloyed nanostructures were considerably interested because of their wide ranged structural tunability and functional diversity, which made these structures applicable in various applications including nano-devices and technologies [12–15]. Additionally, some of the interesting properties for multi-metallic alloyed nanostructures could be achieved while could not be realized with monometallic particles (NPs), such as the electronic heterogeneity, site-specific response, and combinational effect of constituent metals. Also, the alloyed nanostructures can be flexibly applicable by tuning their optical [15,16], catalytic [17–19], electronic, and magnetic

⁎ Corresponding authors. E-mail addresses: [email protected] (H.B. Ahmed), [email protected] (H.E. Emam).

https://doi.org/10.1016/j.molliq.2019.110975 0167-7322/© 2019 Published by Elsevier B.V.

[20–22] properties via controlling particle shape, size, and density [23] as well as the elemental constitution. According to the literature [17,20–22,24–26], preparation of nanoalloyed structures based on Pd resulted in significant improvement in the durability of nano-catalytic activity by the incorporation of Pd in AuNPs [24,25]. In recent years, monometallic Au, Ag, and PdNPs have been successfully applicable in many fields due to their promising plasmonic and catalytic activities. Therefore, fabrication of Ag-Au-Pd alloyed nanostructures can offer novel application as well as enhance the performance of the as-existing applications [26–28]. The systematic fabrication of ternary Ag-Au-Pd alloyed nanostructures with tunable surface morphology and elemental composition was insignificantly reported in the literature. It was reported that controlling of particle size for trimetallic nanostructures was not relatively easy compared to bimetallic NPs. There are few pieces of research on the preparation of trimetallic nanoalloyed structures with well-defined morphologies, although trimetallic nanostructures were approved to exhibit new insights into the structure-composition-property relationships with the asincorporated noble metal nanoparticles. For example, González et al., studied the synthesis of Pd-Au-Ag nano-boxes from Ag nano-cubes via

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sequential or simultaneous galvanic exchange [29]. Yamauchi and coworkers designed a methodology for the synthesis of spherical Au@ Pd@Pt triple-layered core-shell nanostructures which had better catalytic activity than bimetallic core-shell nanostructures [30,31]. Recently, Choi et al., succeeded in manufacturing of Pt-Pd-Ag ternary alloyed nano-tubes with the nano-porous framework using ZnO nano-wires as sacrificial templates [32]. Nevertheless, it is still a highly interesting target to prepare multi-metallic nanostructures with well-defined morphologies due to the formidable difficulties in adjusting the nucleation/ growth kinetics of multi-metallic nano-objects in the presence of different metal salts with different reduction potentials in the same reactor medium. As such, performed NP seeds or structure-directing templates have been commonly employed to generate nanostructures with multiple metallic constituents. Therefore, the manufacturing of an efficient and straightforward methodology for the preparation of multimetallic nanostructures with desirable particle size and morphology is quietly requested to determine their properties and investigate their valuable applications. Based on numerous approaches [33–40], it was found that synthesis of nanoparticles could be successively carried out using different polymers, where, the polymer played an important role in controlling the particle size and morphological structure especially in bimetallic systems and the shape and crystal facet in catalytic ones [41,42]. The metal nanoparticles prepared from polymer-metal ion complexes were enough stable to work as an active catalyst for the organic reaction like hydrogenation of olefin in solution [43]. Therefore, the current study aims to study the catalytic performance of monometallic (AgNPs, AuNPs, PdNPs), bimetallic (Ag\\Au, Au\\Pd), and Ag-Au-Pd trimetallic nanostructures which were prepared by a sole design as quite simple, energy saving, and cost-effective method. The fabrication was depending on the employment of dextran as one type of the biological polymers, in generation, stabilization and in improving the long-term stability of the so-synthesized nanostructures. The successive preparation of the nanostructures was confirmed through various instrumental analyses like UV–Visible spectrophotometer, transmission electron microscope (TEM), Zetasizer and X-ray diffraction (XRD). Spectral data of infrared (FTIR) and 13C NMR were represented for approving the redox reaction between dextran macromolecules and metals precursors in addition to elucidating the reaction mechanism for the production of the nanostructures. Probing of the catalytic reactivity for the synthesized monometallic, bimetallic and trimetallic nanostructures in the reduction of para-nitroaniline was widely studied. 2. Experimental 2.1. Materials and chemicals Silver nitrate (AgNO3, 99.5%, from Panreac, Barcelona – Spain), Gold chloride (AuCl3, 99%, from Sigma-Aldrich – USA), Palladium chloride (PdCl2, 99%, from Sigma-Aldrich – USA), Sodium hydroxide (99%, from Merck, Darmstadt–Germany), Dextran ((C6H10O5)n, El-Nasser

Company for Pharmaceuticals and Chemicals, Egypt), para-nitroaniline (O2NC6H4NH2, N99%, from Sigma-Aldrich – USA) and Sodium borohydride (NaBH4, ≥96%, from Sigma-Aldrich – USA), were all used without any further purification. 2.2. Nanostructures preparation procedure The preparation conditions of the nanostructures and the addition sequencing of materials are summarized in Table 1. For the preparation of monometallic nanostructures colloid, to 1 g/L of dextran solution, sodium hydroxide (4 g/L) as strong alkali was added, metal salt solution (silver, gold, palladium) was added drop-wisely under continuous stirring at room temperature (RT) for 30 min. In case of synthesizing bimetallic and trimetallic nano-structures colloid, the metal salt solution was added subsequently after prepared the first one and the reaction was left to process for 30 min after addition of the metal salt. Keeping in mind that the total concentration of metal is 100, 200 and 300 mg/L for monometallic, bimetallic and trimetallic nanostructures, respectively. After addition of metal salt, and according to the type of metal salt, the reaction medium acquired a characteristic color turned, which was darkened by time, indicating the formation of the as-required nanostructures. The progression of the reaction was monitored by UV–Visible absorption spectra; aliquots from the reaction bulk were withdrawn and detected. After synthesis of nano-structural colloids, nano-composites in the solid form were prepared by transferring the colloidal solutions to petri dish, followed by drying at 120 °C overnight. The obtained solid was used for X-ray diffraction Fourier transformation infrared and nuclear magnetic resonance spectroscopy. 2.3. Analyses and characterization 2.3.1. UV–visible spectra The prepared colloidal nanostructures were manifested by an intense absorption peak due to the Surface Plasmon Resonance (SPR). Thus the UV–visible absorption spectral analyses were used for firstly approving the successive generation of nanostructures in dispersed form. The UV–visible absorption spectra of nano-structural colloids were measured in the wavelength range of 250–750 nm using a spectrophotometer (Cary 100 UV-VIS, UV–Vis-NIR Systems, from Agilent). 2.3.2. Transmission electron microscopic (TEM) Morphological features and particle size of the so-prepared nanostructures were monitored by anticipating of a JEOL-JEM-1200 (High Resolution Transmission Electron Microscope from Japan). Nanocolloidal solutions were carefully dropped on a 400 copper grid coated by carbon film and then evaporated in air at room temperature before conducted in the microscope. The diameter and particle size of the sosynthesized nanostructures were calculated by 4 pi analysis software using TEM photos. The average diameter of the nanostructures was detected at least from 50 particles.

Table 1 Samples identifications. Sample

S1 S2 S3 S4 S5 S6

Ag+ (mg/L)

100 – – – 100 100

Au+3 (mg/L)

– 100 – 100 100 100

Pd+2 (mg/L)

– – 100 100 – 100

Concentrations of reactants; Dextran = 1 g/L and NaOH = 4 g/L. Reaction duration: 30 min. Reaction temperature: room temperature (RT).

The as-produced nanostructures

AgNPs AuNPs PdNPs Au-Pd bimetallic nanostructure Ag-Au bimetallic nanostructure Ag-Au-Pd trimetallic nanostructure

Particle size (nm) Zetasizer

TEM

10.5 45.6 58.8 63.8 68.1 23.7

8.7 n.d. n.d. n.d. 15.7 3.8

Poly-dispersity index

0.40 0.44 0.76 0.58 0.37 0.73

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2.3.3. Zetasizer analyzer The average size, size distribution and poly-dispersity index of the so-prepared nanostructures were all measured by using Zetasizer analyzer (Malvern Zetasizer Nano ZS, from Malvern Instruments Ltd – UK). The instrument was attached with a He\\Ne laser lamp (0.4 mW) at a wavelength of 633 nm. Measurements were carried out at 25 °C in an insulated chamber using dynamic light scattering technique.

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sites, to grow with continuous deposition by direct impingement. However, in case of multi-metallic nanostructures based on silver, gold, and palladium, nanoparticles were supposed to in-grow simultaneously to be fully miscible within stable nano-alloyed colloids. Hence, in the present work, dextran was used as nano-generator and protecting agent to prepare monometallic, bimetallic and trimetallic nanostructures.

3.1. UV–visible spectroscopic analyses 2.3.4. X-ray diffraction (XRD) Both of the pristine dextran polymer and nano-structural dextran composites were analyzed by powder X-ray diffraction using X'Pert PRO PANalytical diffractometer, at room temperature. Diffraction patterns were recorded in the diffraction angle (2θ) range of 5–80° using monochromatized (CuKα X-radiation at 40 kV, 50 mA and λ = 1.5406 Å) with a step size of 0.03° and scanning rate of 1 s. 2.3.5. Fourier transformation infrared (FTIR) Infrared spectra were detected for pristine dextran polymer and nano-structural dextran composites using infrared Spectrometer (Jasco FT/IR 6100, from Japan) conducted to detector of deuterated triglycine sulfate (TGS). The spectra were detected in range of 4000–500 cm−1 using transmission mode (T%), resolution of 4 cm−1 with 2 cm−1 interval scanning and scanning speed of 2 mm/s. 2.3.6. Nuclear magnetic resonance spectroscopy (NMR) The spectra of 13C NMR for the dextran and trimetallic nanostructure dextran composite were recorded on Bruker Avance 300 NMR spectrometer using Bruker UltraShield 300 MHz/54 mm bore as magnet. 2.3.7. Catalytic reactivity of the prepared nanostructures The catalytic reactivity of the so-prepared monometallic (AgNPs, AuNPs and PdNPs), bimetallic (Ag/Au and Au/Pd) and trimetallic (Ag/ Au/Pd) nanostructures; was evaluated in the reduction reaction of pnitroaniline with sodium borohydride. The experiment of monitoring the catalytic action was performed briefly as follows: A 2 mL of pnitro-aniline (2 mM) was transferred to 20 mL glass vessel contained 14.5–15.5 mL distilled water (depending on volume of nanocolloid used) and then a definite volume (0.5–1.5 mL) of nanocolloid solutions was accurately added. Taking into account, the total concentration of metal in the reaction mixture is 5 mg/L, whatever the type nanostructures (mono, bi or trimetallic). To this mixture, an exact volume of 2 mL from freshly prepared NaBH4 (150 mM) was lastly added. After the immediate addition of NaBH4, the progression of the reduction reaction was followed up by measuring of absorbance spectra. An aliquot from the reaction mixture solutions was withdrawn at a given time intervals (0–90 min) and the UV–Vis absorbance spectra were evaluated using a spectrophotometer (Cary 100 UV-VIS, UV–Vis-NIR Systems, from Agilent).

The preliminary detection of the as-prepared nanostructures was carried out by visual observation of the color changes for the reaction liquors. These changes were correlated to the excitation of Surface Plasmon Resonance (SPR) for the generated nano-objects [46]. Typically, UV–Visible absorption is used to investigate SPR for Au, Pd, and Ag nano-colloids. The detection of UV–Visible spectra could give firstly a confirmation about the successive generation of nanostructures in the reaction medium, where, the physical mixing for the solutions of different nanoparticles, is detected in UV–Visible spectra as separated SPR bands, however, when more than one nano-particulate metals interact to produce multi-metallic nanostructures, the as-referred bands are not detected and a single characteristic band is observably detected [44,47]. From the spectral data in Fig. 1, it could be clearly noted that separated absorbance peaks were individually observed at 418 nm and 523 nm which are referred to AgNPs (S1) and AuNPs (S2), respectively [34,35,48–53]. For PdNPs (S3), a weak and broad absorbance band is observed at 386 nm which is in accordance with data in the literature [54]. In the case of Ag\\Au bimetallic nanostructures sample (S5) a single broadband at 500 nm was detected, corresponding to the SPR of bimetallic Ag\\Au nanostructures [52,55,56]. However, incorporation of Pd in the bimetallic nano-colloids Au\\Pd (S4) or in the trimetallic AgAu-Pd nano-colloids (S6), resulted in full disappearance of the SPR band, which is in agreement with the previously reported studies [54,57,58]. From the data shown, it could be supposed that; i) addition of alkali before metal salts might result in dissolving dextran macromolecules to smaller molecular weighted and more reducible fragments, which in turn employed in production of high size controlled nanostructures, and ii) exploitation of the reducible functional groups of dextran polymer in synthesis and stabilization of silver nanoparticles firstly, were subsequently played the role of active surface for catalyzing the overgrowth of nanogold and nano‑palladium clusters, to give highly size and shape regulated bimetallic and trimetallic nano-objects.

3. Results and discussion In general, multi- metallic nano-colloids can be basically synthesized in two processes; i) simultaneous co-reduction of metal salts in the presence of the certain protecting agent, which is often a polymer or surfactant; or ii) successive reduction of one metal over the nuclei of another one. The former reduction method mainly resulted in the generation of alloyed nanostructures while the latter produces core-shell nano-objects. Successive reduction, which is also described as the seed-mediated growth process, was applied to prepare multi-metallic nanostructures in literature [44,45]. In accordance with previously reported research works [33–40], natural polymers could be successively employed firstly for protection of nano-object to be adsorbed on the polymeric macromolecules, for acceleration of nano-cluster growth, i.e. the simultaneous growth of nano-cluster on polymeric building chains is initiated by adsorption of small nuclei at polymeric active

Fig. 1. Absorbance spectra of mono, bi and tri-metallic nanostructures prepared by dextran.

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3.2. TEM micrographs and zetasizer data For representing the topographical features of the as-prepared nanostructures, TEM typical micrographs for AgNPs (S1, as monometallic), Ag-AuNPs (S5, as bimetallic) and Ag-Au-Pd (S6, as trimetallic) were examined as shown in Fig. 2. TEM observation indicated the all examined nanostructures were almost regularly dispersed and spherical in shape. For more confirmation, particle size and size distribution for all nanostructures were measured from TEM micrographs and from zetasizer analyzer. The size distribution measured from TEM were presented in the corresponding image, while the zetasizer data for all samples (mono, bi, and trimetallic nanostructure) were collected and presented in Fig. 2c. The plotted data affirmed that, under the same experimental conditions, dextran was successfully exploited as a superior manufacturer for all of monometallic, bimetallic and trimetallic nanostructures. The particle sizes of the selected samples exhibited prominent differentiation, where, quietly small-sized AgNPs (Fig. 2a, particle size 8.7 nm, size distribution 3–15 nm) were produced. Large size Ag\\Au bimetallic nanostructure was detected (Fig. 2b, particle size 15.7 nm, size distribution 4–27 nm). While the trimetallic nanostructure of Ag-Au-Pd exhibited much lower particle size and narrower size distribution compared to monometallic (Fig. 2c, particle size 3.8 nm, size distribution 2–7 nm). For all examined samples, the measured particle size from zetasizer was higher than that measured from TEM, attributing to difference in calculation techniques. However, all

prepared samples (mono, bi and trimetallic) were in the nano-sized dimension and their size was ranged in 10.5–68.1 nm. The same behavior in particle size was detected from zetasizer, as the monometallic nanostructure (AgNPs) exhibited quite a small size (10.5 nm), then the size was grown to 63.8 nm in case of bimetallic nanostructure (Ag-AuNPs) and lastly, the size was decreased to 23.7 nm for trimetallic nanostructure (Ag-Au-PdNPs). Thus, the as-mentioned data depicts that, Ag can be successfully exploited as templates for the synthesis of nanostructures having bimetallic or trimetallic compositions, where AgNPs acted as seeds in the simultaneous co-reduction method. The presented data also showed that, the size distribution of the so-existed Ag\\Au nano-bimetallic was extremely diminished from 15.7 nm to 3.8 nm for Ag-Au-Pd trimetallic nanostructures. Referring to recent researches [59–61], the presence of more than two metal precursors in the reaction liquor results in induction of complicated reduction kinetics due to mutual interaction as well as a complex with nano-generator. However, another reports referred the reason of size decrement in case of trimetallic nanostructures compared to bimetallic structures due to the self-organization process. Where, Ag seeds were employed as sacrificial templates for production of smaller sized and highly regulated Ag-Au-Pd trimetallic nanostructures [62,63]. Well dispersion and high stability for all of the as-prepared monometallic, bimetallic and trimetallic nanostructures can be approved via the detected values of poly-dispersity index (PdI) from zetasizer

Fig. 2. TEM images and size distribution for the prepared mono, bi and tri-metallic nanostructures; [a] Ag (mono), [b] Ag\ \Au (bi) and [c] Ag-Au-Pd (tri). [d] Particle size from zetasizer analyzer for the prepared mono, bi and tri-metallic nanostructures.

H.B. Ahmed, H.E. Emam / Journal of Molecular Liquids 287 (2019) 110975

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Fig. 2 (continued).

analyzer [48,49,64]. The as-generated nanostructures in colloidal form may have uniformity with well-regulated shape and size, as in the current approach. For all the tested samples, the synthesized nanostructures showed high stability, as the values of PdI were ranged in 0.37–0.76 (Table 1). So, it could be summarized that, at room temperature, 1 g/L of dextran is satisfactory for preparation extensively size controllable monometallic, bimetallic and as trimetallic nanostructures represented in AgNPs, Ag\\Au and Ag-Au-Pd, respectively. 3.3. XRD patterns Dextran, Ag monometallic, Ag\\Au bimetallic and Ag-Au-Pd trimetallic nanostructures-dextran composites (S1, S5 & S6, respectively) were selected to be characterized by XRD patterns and the data are shown in Fig. 3. The native dextran was observed with six intense diffraction peaks at 2θ = 6.8, 15.4°, 17.3°, 18.8°, 20.6° and 29.2°, referring to the crystalline structure of dextran as reported in literature

[65]. On the other hand, in case of all nanostructures-dextran composite, the crystalline structure of dextran was extensively destroyed and the as-mentioned diffraction bands were fully disappeared, owing to the interaction between dextran macromolecules and metal ions. Additionally, other intense diffraction bands were recorded at 2θ = 30.1°, 34.3°, 35.2°, 38.2°, 41.6°, 44.5°, 46.7°, 48.3° for Ag monometallic and Ag\\Au bimetallic nanostructures-dextran composite. According to diffraction data of international center, the diffractions at 38.2° and 44.5° are known to be typical for (111) and (200) of face centered crystalline (FCC) structures for silver and gold (JCPDS data number 04-0783 card and 4-0784 card) [44,56,64,66,67]. In case of Ag-Au-Pd trimetallic nanostructures-dextran composite, the same diffraction peaks were observed. In addition to one more diffraction were assigned at 2θ = 45.5°, which is corresponded to (200) of face centered crystalline (FCC) structures for Pd (JCPDS data number 89–4897 card) [68,69]. These diffraction data were further affirmed the above-illustrated data of absorbance spectra, zetasizer and TEM micrographs for successful

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(CO), which in turn exploited in bonding and stabilizing the generated nanostructures to give highly size regulated structures. 3.5. Reaction mechanism for the production of monometallic, bimetallic and trimetallic nanostructures

Fig. 3. XRD analysis for the prepared mono (Ag), bi (Ag\ \Au) and tri-metallic (Ag-Au-Pd) nanostructures.

preparation monometallic, bimetallic and trimetallic nanostructures by using dextran.

3.4. Spectral mapping analysis More confirmation of the redox reaction between metal ions and dextran macromolecular chains for manufacturing of monometallic, Ag\\Au bimetallic and Ag-Au-Pd trimetallic nanostructures could be realized through further spectral analyses. The detection of the change in the molecular structure of dextran backbone is detected via the data of the attenuated total reflection – Fourier transformation infrared spectroscopy (ATR-FTIR) (Fig. 4) and 13C NMR (Fig. S1, supplementary file). From Fig. 4, it could be plausible that; for pristine dextran macromolecules [65], a broad band of O\\H bond stretching is observed at 3292 cm−1, while the short band presented at 2895 cm−1 is attributed to -CH group stretching. The CH2 group bending is recorded at band of 1411 cm−1 and the bending of OH is assigned at 1005 cm−1. From the represented spectra for nanostructures, it could be noted that, an observable modification in the well-defined spectra of dextran was observed after its exploitation in synthesis of nanostructures. In case of the prepared nanostructures, the bands located at 3292 cm−1 and 1005 cm−1 characterized for OH stretching and bending, respectively, are significantly decreased. Additionally, three significant new bands are appeared for nanostructures as follows; an observable new band is appeared at 1584 cm−1, which referred to carbonyl group (C_O of aldehyde and/or ketone units) [36,52,70], where, its intensity increased from monometallic to trimetallic nanostructures, and the bands of symmetric vibration for carboxylate and the O-metal stretching are detected at 1412 cm−1 and 875 cm−1, respectively [70–73]. In 13C NMR spectra (Fig. S1, Supplementary file), for native dextran, four observable signals were detected at 98.5, 93.4, 72.4 and 61.9 ppm. These signals are typically corresponding to C1, C3, C5 and C6 in the main skeleton of dextran macromolecule [74,75]. After generation of Ag-Au-Pd trimetallic nanostructures, signal of C6 (61.9) is completely disappeared and two new peaks at 173.1 and 184.3 ppm are detected which are belonging to carbonyl carbons in carboxylate, aldehydes and/or ketones. These spectral data of FTIR & 13C NMR are both confirmed the redox reaction between dextran macromolecules and metal salts to produce the nanostructure. In this reaction, metal ions are reduced to nanometallic form and the C6 (CH2-OH) of dextran is oxidized to carbonyl

According to the previously discussed data, dextran as a type of biopolymers could be superiorly employed for fabricating well-dispersed and size regulated monometallic, bimetallic and trimetallic nanostructures under the same experimental conditions. The reaction mechanism between metal ions and dextran macromolecules under the effect of NaOH as a strong alkali was postulated and schematically diagrammed in Fig. 5a. The terminal alcoholic groups in dextran backbone as reducing groups are suggested to be exploited in reduction of metal ions to generate the as-required nanostructures, while, such groups are supposed to be oxidized to aldehydic and/or ketonic groups [49,53,76]. NaOH as strong alkali acts to increase the accessibility and reducibility of polymer macromolecules. The hydroxyl groups in dextran backbone are hypothesized to be de-protonated, where, the ionic form of polymer macromolecules tended to bond with metal ions for full reduction. According to literature, addition of strong alkali resulted in more of size regulation for the soproduced nanostructures [52,53,56,64]. Production of bimetallic and trimetallic nanostructures could be summarized in the following points: I. In case of Ag\\Au bimetallic nanostructures • As previously ascribed, Ag ions postulated to be reduced by action of dextran macromolecules. After addition of Au salt, Ag particles acted as a template for further reduction of Au ions. The ionization potential, electron affinity and electronegativity of Au are higher than that of the Ag [56,77]. Therefore, the positive potential effect of the redox reaction resulted in a spontaneous interchanging of electrons and the charge transfers easily from Ag0 to Au+3. The produced AgNPs is playing a role in the co-reduction of Au+3 ions on the surface of the formerly produced Ag nano-clusters. Thus, Ag0 particles are supposed to act as seeds or active sites for successive growth of Ag\\Au bimetallic nanostructures. • Subsequently, dextran with its reducing groups and the interdiffusion of Au and Ag, led to the nano-cluster growth of welldispersed and size regulated bimetallic nanostructures [52,78].

Fig. 4. FTIR spectra for the prepared mono (Ag), bi (Ag\ \Au) and tri-metallic (Ag-Au-Pd) nanostructures.

H.B. Ahmed, H.E. Emam / Journal of Molecular Liquids 287 (2019) 110975

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II. In case of Ag-Au-Pd trimetallic nanostructures

3.6. Investigation of the catalytic reactivity for the prepared nanostructures

• Pd+2 ions are hypothesized to be reduced for producing Pd0 small nanoparticles by the reduction action of dextran, however, the three added metal ions are supposed to compete for the active sites of dextran macromolecules. • Ag\\Au bimetallic nano-alloys can act as sacrificial templates for further growth of Pd nanoparticles to give Ag-Au-Pd trimetallic nanostructures [62,63]. • Furthermore, self-organization process for more regulated growth of trimetallic nanostructures is supposed to take a place for production of high size regulated nanostructures [62,78].

Nitro-aromatic compounds, such like 4-nitroaniline and 4-nitrophenol, are well known to be highly applicable in many industries, including the manufacturing of dyes, anilines, pharmaceuticals, explosives and agrochemicals [79,80]. Moreover, these compounds are usable as intermediates in the production of antipyretic and analgesic drugs. However, these compounds are environmentally hazardous as water contaminants and were reported as mutagens, carcinogens and teratogens [81–83], and listed in environmental legislation [84]. Therefore, reduction of such nitro-aromatic compounds to decay their deleterious effect is an important issue and is required nowadays [64,85].

Fig. 5. Suggested mechanism for [a] synthesize of mono, bi and tri-metallic nanostructures by using dextran and [b] catalytic action of nanostructures in the reduction reaction of pnitroaniline with sodium borohydride.

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Fig. 5 (continued).

The as-produced nanostructures were applied as a catalyst in the reduction reaction of p-nitroaniline to p-aminoaniline by sodium borohydride. Comparison in the catalytic reactivity between different monometallic nanostructures (Ag, Au, Pd) and bimetallic nanostructures (Ag\\Au, Au\\Pd) with trimetallic nanostructure's (Ag-Au-Pd) were presented in Figs. 6 & 7. The reduction reaction was monitored via the UV spectrophotometry, as the absorption beaks of pnitroaniline at 380 and 225 nm were vanished and the absorption beaks of p-aminoaniline at 304 and 240 nm were consequently appeared [86]. The absorbance spectra for all reduction experiments as function of time were presented in Fig. 6. The absorbance at maximum wavelength and conversion percentage of p-nitroaniline were both exported for all experiments and presented in Fig. 7. In the absence of the nanostructures, the reduction rate was quite slow and none of p-nitroaniline was converted to p-aminoaniline after 90 min. Addition of the so-prepared nanostructures was accompanied by significant acceleration of the reduction reaction and the reduction rate was found to be dependent on the type of the applied nanostructure, i.e. monometallic, bimetallic or trimetallic. From the plotted data, it could be pointed out that, i) using of monometallic nanostructures of Ag, Au and Pd resulted in reduction of p-nitroaniline after 30 min (96.5%), 60 min (92%) and 10 min (92%), respectively (Figs. 6b, c, d, 7a & c), ii) compared to monometallic, bimetallic nanostructures greatly accelerate the rate of reduction reaction, where, 92.2 and 94.3% from p-nitroaniline was reduced in 5 and 10 min by using Ag\\Au and Au\\Pd bimetallic nanostructures, respectively (Figs. 5e, f, 7b & d), while, iii) applying of Ag-Au-Pd trimetallic nanostructures led to mostly full conversion (97.2%) of p-nitroaniline to p-aminoaniline within only 3 min (Figs. 6g, 7b & d). At the same metal content in the reduction reaction, the catalytic activity of the applied nanostructures followed the order of trimetallic N bimetallic N monometallic and for the monometallic; PdNPs N AgNPs N AuNPs. Kinetics of the reduction reaction for p-nitroaniline was studied for zero-order, first-order and second-order, and the kinetic parameters (rate constant, half time and correlation coefficient) were summarized in Table 2. The data showed that, the reduction of p-nitroaniline was fitted well to first-order model. The rate of the reduction reaction was

significantly accelerated by insertion of metal nanostructures into the reaction medium. The rate constant was observably increased from 5.1 × 10−3 min−1 without catalyst to 176.6 × 10−3 min−1, 241.8 × 10−3 min−1 and 767.8 × 10−3 min−1 for using Pd monometallic, Au\\Pd bimetallic and Ag-Au-Pd trimetallic nanostructures, respectively. While, the half time (t1/2, min) of reduction was dramatically diminished from 135.91 without catalyst through 3.93 min for Pd nanostructures, to 2.87 min. For Au\\Pd bimetallic nanostructures and to 0.90 min. for Ag-Au-Pd trimetallic nanostructures. These data showed that, the reduction of p-nitroaniline was accelerated by 34.6, 47.4 and 151.0 times when the prepared monometallic, bimetallic and trimetallic nanostructures immobilized in the reaction as catalyst, respectively. Data revealed that by using nanostructures with Pd participating in nucleation as it accompanied by the highest rate constant and the lowest t1/2 values. For monometallic nanostructures, PdNPs were exhibited by the fastest catalytic activity compared to AgNPs and AuNPs, and the participation of palladium in nucleation of bimetallic nanostructures was observed with greater catalytic action, which is in agreement with literature [54,57]. While, trimetallic nanostructures were observably characterized by the highest and astonishing catalytic activity compared to the others. The catalytic mechanism for the reduction of 4-nitroaniline by NaBH4 in the presence of the as-prepared nanostructures can be clarified by Fig. 5b. Hydride ion is an electron rich, and can donate hydrogen to p-nitroaniline to convert it to p-aminoaniline. But, there is a huge difference in potential between hydride and p-nitroaniline, which minimizes the possibility of this reduction reaction [87,88]. While, nanostructures acted as bridge and increase the feasibility of the reduction reaction through accelerate the electron transfer from hydride to pnitroaniline. Firstly, the hydride ions were adsorbed onto the surface of nanostructures to form new species [89], which are responsible for the reduction of 4-nitroaniline to p-aminoaniline. This reduction occurs through hydride transferring to 4-nitroaniline (1) giving nitroso intermediate (4) via intermediates (2) and (3). Intermediate 4 was then reduced by the hydride transfer to the corresponding hydroxylamine intermediate (5), which produced the final p-aminoaniline (6) by further hydride transfer.

Fig. 6. Effect of mono, bi and tri-metallic nanostructures as catalyst on the reduction of p-nitroaniline; [a] blank (without NPs), [b] AgNPs, [c] AuNPs, [d] PdNPs, [e] Ag-AuNPs, [f] Au-PdNPs and [g] Ag-Au-PdNPs.

H.B. Ahmed, H.E. Emam / Journal of Molecular Liquids 287 (2019) 110975

[a]

2.0

[b]

5 min

9

1 min

2.0

2 min

10 min

20 min 30 min

1.2

40 min 60 min

0.8

5 min

1.6

Absorbance (a.u.)

Absorbance (a.u.)

1.6

90 min

0.4

10 min 15 min

1.2

20 min 0.8

30 min

0.4

0.0

0.0 200

300

400

500

600

700

200

300

400

Wavelength (nm) [c]

500

600

[d]

2.0

2.0

5 min

20 second 1 min

10 min 1.6 20 min

Absorbance (a.u.)

Absorbance (a.u.)

1.6

30 min 1.2 40 min 50 min

0.8

60 min

2 min

4 min 6 min

1.2

8 min 10 min

0.8

0.4

0.4

0.0

0.0 200

300

400

500

600

200

700

300

400

500

600

700

Wavelength (nm)

Wavelength (nm) [e]

700

Wavelength (nm)

[f]

2.0

2.0

20 second

20 second 1 min

1 min

1.6

Absorbance (a.u.)

2 min 5 min

1.2

0.8

0.4

2 min 4 min

1.2

0.8

0.4

0.0

0.0 200

300

400

500

600

700

200

300

400

Wavelength (nm) [g]

500

Wavelength (nm) 2.0

20 second 1 min

1.6

Absorbance (a.u.)

Absorbance (a.u.)

1.6

2 min

1.2

0.8

0.4

0.0 200

300

400

500

Wavelength (nm)

600

700

600

700

10

H.B. Ahmed, H.E. Emam / Journal of Molecular Liquids 287 (2019) 110975

[b]

[a]

1.2

Au-PdNPs

1.0

Absorbance at λmax (a.u.)

AgNPs

Absorbance at λmax (a.u.)

Ag-AuNPs

Blank

1.2

AuNPs PdNPs

0.8 0.6 0.4

1.0 Ag-Au-PdNPs 0.8 0.6 0.4 0.2

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20

40

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Contact Time (minute) [d]

Conversion Percentage (%)

120

6

8

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12

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Blank AgNPs

100

Conversion Percentage (%)

[c]

4

Contact Time (minute)

AuNPs

PdNPs

80 60 40 20

100 80 Ag-AuNPs

60

Au-PdNPs 40

Ag-Au-PdNPs 20 0

0 0

20

40

60

80

0

100

2

4

6

8

10

12

Contact Time (minute)

Contact Time (minute)

Fig. 7. [a, b] Absorbance of p-nitroaniline at maximum wavelength and [c, d] conversion percentage of p-nitro aniline as function of nanostructures catalyst. [a, c] mono-metallic and [b, d] bi- and tri-metallic.

4. Conclusion The current approach represents one pot, efficient, green technique for manufacturing of monometallic (Ag, Au, Pd), bimetallic (Ag\\Au, Au\\Pd) and trimetallic (Ag-Au-Pd) nanostructures at ambient conditions by employing of dextran as nano-generator and capping agent, via simultaneous co-reduction method. Nanostructures of monometallic Ag, bimetallic Ag\\Au and trimetallic Ag-Au-Pd were produced with quite small size of 8.7, 15.7 and 3.8 nm. The spectral data approved that, the nanostructures formed by the reduction effect of the alcoholic groups in dextran macromolecules, while, the referred groups were subsequently oxidized to aldehydic and/or ketonic groups. The performance of catalytic activity for the so-prepared nanostructures was evaluated in the reduction reaction of p-nitroaniline and there was a powerful relationship between the catalytic activity and composition in nanostructures. Participation of Pd in the clustering of nanostructures resulted in astonishing catalytic action. The values of t1/2 were 3.93 min. for Pd

nanostructure and 0.90 in case of trimetallic nanostructures. The reduction reaction of p-nitroaniline was 151 times accelerated when trimetallic nanostructure was used as a catalyst. This study provides a quite simple-green technique for synthesis of monometallic, bimetallic and trimetallic nanostructures based on Ag, Au and Pd in the colloidal form. Compared to traditional methods, the technique used here was in-expensive, simple, greener, applicable and able to produce on large scale. Moreover, the using of such prepared trimetallic nanostructures exhibited promising approach for catalysis which will open the way to be used in various applications. Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.110975.

Compliance with ethical standards The authors declare that they have no conflict of interest.

Table 2 Kinetic parameters for the reduction reaction of p-nitro aniline in the presence of the prepared mono, bi and tri-metallic nanoparticles as catalyst. Catalyst

Blank AgNPs AuNPs PdNPs Ag-AuNPs Au-PdNPs Ag-Au-PdNPs

Zero-order model

First-order model

Second-order model

K0 × 10−3 (mg/L·min)

t1/2 (min)

R2

K1 × 10−3 (min−1)

t1/2 (min)

R2

K2 × 10−3 (L/mg·min)

t1/2 (min)

R2

106.8 502.4 275.6 1047.3 756.9 1013.1 1888.1

120.00 25.51 46.50 12.24 16.93 12.65 6.79

0.95 0.76 0.96 0.87 0.95 0.76 0.96

5.1 94.5 38.5 176.6 183.3 241.8 767.8

135.91 7.34 18.00 3.93 3.78 2.87 0.90

0.94 0.97 0.99 0.98 0.91 0.94 0.97

0.2 29.3 7.2 36.3 58.4 66.7 428.8

195.2 1.33 5.42 1.08 0.67 0.59 0.09

0.92 0.95 0.89 0.98 0.89 0.99 0.85

H.B. Ahmed, H.E. Emam / Journal of Molecular Liquids 287 (2019) 110975

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