Synthesis of hollow trimetallic Ag/Au/Pd nanoparticles for reduction of 4-nitrophenol

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Abstract

The hollow trimetallic Ag/Au/Pd nanoparticles with controllable Ag:Au:Pd ratio were successfully prepared by using a successive reduction method. Their optical properties, morphologies, structures, chemical compositions and electronic properties were carefully characterized by UV-vis, TEM, WDS, XRD and XPS techniques. The synthesized nanoparticles were also employed as catalysts for reduction of 4-nitrophenol. The results show that the trimetallic Ag/Au/Pd nanoparticles provided better catalytic activity than those obtained from bimetallic Ag/Au nanoparticles and monometallic counterparts. This study demonstrated that both atomic ratio of constituent metal and hollow interior structure strongly affect the catalytic activities of the synthesized Ag/Au/Pd nanocatalysts.

Introduction

4-Nitrophenol (4-NP) has been wildly used in several applications such as pharmaceutical, dyeing agent, plastics, pesticides and anti-corrosion lubricant [[1], [2], [3]]. This compound has been identified as one of the most hazardous and toxic pollutants generated mainly from agricultural and industrial sources [4,5]. Therefore, several methods have been introduced to remove 4-NP from wastewater with various advantages and limitations such as adsorption [6], microbial degradation [7], electrocoagulation [8] and reduction [[9], [10], [11], [12]]. Moreover, apart from the industrial and environmental viewpoints, reduction of 4-NP is considered as a model reaction for catalytic study [13]. It is well-known that without catalyst, the reduction of 4-NP is extremely slow. Therefore, many investigations have paid attention on the development of catalysts for this reaction. For example, controllable catalytic activity as a function of temperature and enhanced stability of catalysts have been introduced by using metal/polymer nanocomposites [14,15]. In addition, the efficiency catalysts have been improved by altering the employed catalysts such as their sizes, shapes, structures, supports, and metal constituents [13,[16], [17], [18]].

In connection with this, multi-metallic nanoparticles have been recently attracted because of their unique or enhanced properties, which diff ;er from those of individual monometallic nanoparticles [[19], [20], [21]]. In the field of catalytic applications, the multi-metallic nanoparticles have shown better catalytic activity than those of bimetallic and monometallic nanoparticles, due mainly to the combination of properties between single metal and second and/or third metal. In this connection, several multi-metallic nanocatalysts have been used for reduction of 4-NP such as core-shell Al2O3@AgAu [22], PtPdBi nanowires [23], Cu-Ni-Pt dendrites [24], Ni-Co-Pd-P composite [25] and core-shell Au@Pd@Ru, [26]. Those studies reported that trimetallic nanocatalysts offer better catalytic activity than those of bimetallic and monometallic nanocatalysts for the reduction of 4-NP.

Aside from enhancing the catalytic activity of nanocatalyst by employing multi-metallic nanocatalysts, literature has also shown that enhancing of catalytic performance can be successfully achieved by using hollow structure nanocatalysts. For example, Guo et al. [27] reported that hollow porous Au nanoparticles exhibit higher catalytic activity than solid Au and Ag nanoparticles because the hollow porous nanoparticles significantly increase the number of surface area and active catalytic sites. For example, Petri and co-workers [28] synthesized Ag-Au hollow structure with controlled composition by a galvanic replacement. The catalytic study for reduction of 4-NP of these materials indicated the potential of better catalytic performance than that of Ag nanoparticles which could be attributed to two factors: (1) the presence of Au in the synthesized nanoparticles and (2) the increase in surface area of catalyst resulting from formation of porous walls and hollow interiors. Therefore, a combination of multi-metallic nanocatalysts and hollow structure is one of the very interesting approach to be explored further for the development of better catalysts.

In literatures, multi-metallic nanocatalysts consisting of Ag, Au and Pd have been used for reduction of 4-NP in the form of alloy Ag-Au-Pd nanosponges [29], AgPd nanodendrites on Au bipyramids [30], sub-monolayer Au-Pd on Ag networks [31]. However, the combination of Ag, Au and Pd nanocatalysts in the form of hollow nanoparticles is rarely investigated, except the work reported by Rodrigues et al. [32]. In this work, the AgAuPd and other trimetallic nanoparticles (AgAuPt and AgPdPt) were synthesized via sequential galvanic replacement method by using polyvinylpyrrolidone in the synthetic process. For the catalytic study, this work aimed to investigate the catalytic activity of bimetallic versus the trimetallic nanocatalyst so only one atomic ratio of each constituent in bimetallic and trimetallic nanocatalyst were chosen to investigate their relative catalytic activity. More interestingly, the results showed not only enhancing of hollow AgAuPd nanocatalyst over bimetallic counterparts but also over AgAuPt and AgPdPt. In addition, in recent year, Silva et al. [33] reported the catalytic activity of solid AuPd compared with hollow AgPd nanoflowers. It was found that Au core play important role in catalytic activity. However, when compare catalytic activity between AuPd and AgPd with comparable size and composition, the catalytic activity of AgPd is higher than that of AuPd indicating the hollow interior contribution of AgPd for improvement of catalytic activity. Therefore, applying hollow interior Ag/Au/Pd as a nanocatalyst is interesting. Aside from that, our group earlier reported successfully synthesis of core-shell AuPd nanoparticles by using simple successive reduction method without polymer [34]. We also found that the AuPd nanoparticles offer interesting catalytic activity for reduction of 4-nitrophenol. In connection with this, we here further developed the synthetic route to obtain hollow trimetallic Ag/Au/Pd nanocatalysts without using polymer in the synthetic process, then fully characterized these hollow structure nanocatalysts and also investigated the influence of their chemical compositions and morphologies on their catalytic activities.

Section snippets

Materials

Gold (III) chloride trihydrate (HAuCl4 · 3H2O, 99.999%) and Silver nitrate (AgNO3, 99.8%) were purchased from Sigma-Aldrich. Palladium (II) chloride (PdCl2, 99.99%), ascorbic acid (99%), hydrochloric acid (37%) and 4-nitrophenol (C6H5NO3, 98%) were obtained from Merck. Sodium citrate tribasic dihydrate (C6H5Na3O7 · 2H2O, 99%) was purchased from Fluka. Sodium borohydride (NaBH4, 97%) was obtained from APS Chemicals. Hydroxylamine hydrochloride (NH2OH·HCl, 96%) was obtained from Carlo Erba. All

Characterization of monometallic, bimetallic and trimetallic nanoparticles

UV-vis absorption spectra for monometallic, bimetallic and trimetallic nanoparticles are shown in Fig. 1. The UV–vis spectrum of Ag nanoparticles shows distinctive peak at around 400 nm, which is the typical surface plasmon band of Ag nanoparticles [39]. The UV–vis spectrum of AgxAu1 nanoparticles are shown in Fig. 1(a). The red-shift in all AgxAu1 nanoparticles when compared with the Ag nanoparticles indicates the formation of hollow AgxAu1 nanoparticles [38,40]. From Fig. 1 (b), absorption

Conclusions

Various compositions of hollow trimetallic Ag/Au/Pd nanoparticles were successfully synthesized by a simple successive reduction method. No strong absorption peak in the UV-vis region was found for all trimetallic Ag/Au/Pd nanoparticles. The hollow structure of the synthesized Ag/Au/Pd nanoparticles was revealed via TEM images. A combination of X-ray and electron diffraction techniques also indicated the formation of composite structure rather than alloy structure. Moreover,

Acknowledgements

We wish to thank the Center of Excellence in Materials Science and Technology, Chiang Mai University for financial support under the administration of Materials Science Research Center, Faculty of Science, Chiang Mai University.

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