Elsevier

Ultrasonics Sonochemistry

Volume 58, November 2019, 104616
Ultrasonics Sonochemistry

An environmentally friendly one-pot synthesis method by the ultrasound assistance for the decoration of ultrasmall Pd-Ag NPs on graphene as high active anode catalyst towards ethanol oxidation

https://doi.org/10.1016/j.ultsonch.2019.104616Get rights and content

Highlights

  • One-pot synthesis to fabricate Pd-Ag nanoparticles.

  • Environmentally friendly method to prepare Pd-Ag nanoparticles.

  • The uniform dispersity of ultrasmall Pd-Ag nanoparticles on graphene nanosheets.

  • The Pd-Ag/G displays extraordinary electrocatalytic activity and durability toward ethanol oxidation.

Abstract

An environmentally friendly one-pot synthesis approach for the decoration of Pd-Ag nanoparticles with the ultrasmall size on graphene (Pd-Ag/G) by the assistance of ultrasound is proposed in this paper. This method offers exceptional advantages over other approaches such as environmentally friendly synthesis, being low temperature, reductant, surfactant free, simple, fast and one-pot synthesis. In this work, silver formate is added to the graphene suspension at 25 °C. Then, PdCl2 is added to the suspension under stirring to fabricate Pd-Ag/G. The uniform dispersity of nanoparticles with an average size of about 2–3 nm is well confirmed by transmission electron microscopy micrographs. The resultant catalyst is applied as anode electrocatalyst towards electrooxidation reaction of ethanol. The Pd-Ag/G catalyst displays exceptional catalytic activity and durability towards electro-oxidation of ethanol. According to the obtained results, it be concluded that the combination of Ag and Pd, ultrasmall and uniform distribution of Pd-Ag nanoparticles led to the improvement of electrocatalytic activity of the Pd-Ag/G catalyst.

Introduction

Nowadays, direct ethanol fuel cells (DEFCs) have been extensively studied as ideal alternative for power source on account of their security, high efficiency, renewable capability and low environmental pollutant emission [1], [2], [3], [4], [5], [6]. Nonetheless, the application of DEFCs in commercial level still faces crucial barriers such as low activity, insufficient long-term durability as well as high-price of catalysts. To resolve these issues, an extensive spectrum of routes has been reported with respect to the tailored design of the shape, dispersity, size, and chemical components of a catalyst.

Since now, a broad spectrum of studies has been dedicated to Pt-based catalysts while researchers believe that these materials are good alternatives for DEFCs [7], [8]. Nonetheless, high price and limited resources ultimately may hinder their utilization in a commercial application. Moreover, a serious problem of these materials is CO poisoning, which limits their usage as catalysts in the fuel cells [1], [2], [3], [4], [5], [6]. A key challenge in the commercial application of DEFCs is the tailored design of highly active, durable and inexpensive catalysts. Therefore, it is important to find a stable, active and inexpensive alternative for the replacement of Pt-based nanocatalysts. Recently, Pd-based nanomaterials, particularly Pd-transition metal alloys, have appeared as efficient alternatives to Pt-based nanomaterials owing to their high performance, higher resistance to poisoning as well as inexpensiveness. Pd-based nanocatalysts are nearly inert to oxidation of alcohol in acid media while they are more active than Pt for electrooxidation process of ethanol in basic media [9], [10], [11], [12], [13]. Moreover, Pd-based binary nanocatalysts, compared to pure Pd, demonstrate higher catalytic activity toward ethanol oxidation reaction (EOR) [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14].

It was proved that the performance of a catalyst is seriously affected by the size, synergistic effect, shape and distribution of a catalyst [15], [16]. Accordingly, a broad spectrum of routes has been developed to create high-performance Pd-based catalysts. The approaches include alloying Pd with various elements [17], [18], [19], and production of Pd nanocrystals [20], [21], [22]. Alloying Pd with various metals is considered as a good method that can not only enhance the electrocatalytic performance, but also decrease the loading of Pd metal. In recent years, more studies have used transition metals such as Au [16], Co [23], Ni [24], Ag [25], and Cu [18] combined with Pd to promote the catalytic performance of Pd-based nanocatalysts.

Until now, numerous routes have been successfully reported to create Pd-based binary catalysts which often need a multistep and complicated process. Therefore, researchers have focused on the finding the simple and one-pot synthesis approaches of Pd-based binary catalysts [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. In recent years, profuse one-pot synthesis methods have been proposed to fabricate the Pd-based binary catalysts. Unfortunately, the utilization of surfactants (e.g. PVP (poly (vinyl pyrrolidone), etc.) and reductant reagents (e.g. NaBH4, EG (ethylene glycol), etc.) are serious problems in the development of one-pot synthesis approaches for the creation of Pd-based binary catalysts. Both materials (surfactants and reducing reagents) are highly expensive and injurious compounds with regard to both environment and the human health. Moreover, coalescence of metal NPs may be hindered by employing the surfactant materials. However, application of surfactant materials leads to the blockage of active sites on the surface of nanoparticles on account of the interaction with the surface of nanoparticles which leads to the degradation of catalytic activity. Therefore, these materials are injurious for catalysis process. Besides, the well-disperse and small size of Pd NPs possess extraordinary catalytic efficiency. Nonetheless, Pd NPs created with the contractual reduction route offer poor dispersity as well as huge average sizes and lead to the coalescence of Pd NPs. Likewise, the creation of Pd-based bimetallic catalysts requires a very high temperature. The utilization of high temperature goes along with the coalescence of NPs while this phenomenon is accompanied by loss of ECSA (electrochemical surface area) as well as electrocatalytic performance of a nanocatalyst. Hence, tailored design of the experiments to provide the desirable conditions (e.g. simple, one-pot synthesis, surfactant and reductant free, low temperature and environmentally friendly way) is a vital index to synthesize and commercialization of a nanocatalyst. In addition, it is essential to select the support materials appropriately in order to prevent coalescence of NPs which reduces the surface area and electrocatalytic efficiency [36], [37]. Graphene nanosheets show a unique structure of two-dimensional (2D) sheets that are one atom thick and composed of sp2-bonded carbon atoms. They provide amazing characters such as outstanding specific surface area (≈2600 m2g−1), high thermal conductivity (5000 W mK−1), high electron mobility (2.5 × 105 cm2 V−1 s−1), and strong chemical stability. The graphene nanosheets possess an opportunity for using 2D materials as the conductive sheets to anchor electrocatalysts in polymer electrolyte membrane fuel cells (PEMFCs) [38], [39], [40], [41].

In the materials chemistry, the ultrasonic-assisted strategy is known as an efficient and powerful method for the synthesis of highly active noble metal, due to its relatively simple device and time-saving operating process. Ultrasonic irradiation provides unusual physical and chemical effects that are derived from acoustic cavitation. A mechanism for all of the sonoelectrochemical generation of nanosize metals has been proposed where metallic ions are decreased utilizing a short current pulse. In the following, NPs are also dislodged by the ultrasonic pulse. It must be noted that metallic NPs in solution have the tendency to be assembled automatically while the speed of the Ostwald ripening process is boosted which results in smaller primary NPs [42], [43], [44], [45], [46], [47], [48], [49], [50].

Herein, an environmentally friendly method was employed for the decoration of ultrasmall Pd-Ag NPs on G support (Pd-Ag/G). This way offers extraordinary advantages in comparison to other approaches such as one-pot synthesis, being surfactant and reductant free, facile and being environmentally friendly and having low temperature. The green and one-pot synthesis route was occurred based on the decomposition of silver formate at 298 K followed by a galvanic replacement reaction (GRR) Ag with Pd2+ ions at room temperature (RT). Resultant Pd-Ag/G reveals extraordinary electrocatalytic performance toward EOR.

Section snippets

Materials

Alumina (Al2O3), formic acid (HCOOH) (98–100%), sodium hydroxide (NaOH), silver nitrate (AgNO3), sodium carbonate (Na2CO3), graphite powder, palladium (II) chloride (PdCl2, 56%) and ethanol (C2H5OH) were purchased from Merck Co. Chitosan ([2-amino-2-deoxy-(1-4)-b-d-glucopyranose]), (CH) with medium molecular weight, 400,000 Da, was provided from Fluka company and utilized as received. Acetic acid was diluted to a 1% aqueous solution before use. Doubly distilled water was used throughout the

Synthesis and characterization of Pd-Ag/G

An environmentally friendly method by the assistance of ultrasound was employed to decorate the ultrasmall Pd-Ag NPs on graphene support. The Pd-Ag/G catalyst was synthesized during the decomposition of silver formate at 298 K under ultrasound irradiation followed by GRR approach. In comparison to the preceding literature, this way possesses several advantages (e.g. surfactant and reductant free, fast, environmentally friendly strategy, one-pot synthesis, low temperature, and facile way). In

Conclusion

In summary, an environmentally friendly method has been introduced to prepare ultrasmall Pd-Ag nanoparticle decorated on graphene support (Pd-Ag/G). This route offers extraordinary advantages in comparison to other approaches such as being surfactant and reductant free, one-pot synthesis, low temperature, simple and environmentally friendly. Decoration of ultrasmall Pd-Ag NPs on graphene support was well confirmed by employing the TEM, XRD, and EDS analyses. The TEM micrographs display the

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