Elsevier

Composites Part B: Engineering

Volume 172, 1 September 2019, Pages 309-315
Composites Part B: Engineering

Controlled organization of building blocks to prepare three-dimensional architecture of Pd–Ag aerogel as a high active electrocatalyst toward formic acid oxidation

https://doi.org/10.1016/j.compositesb.2019.05.021Get rights and content

Highlights

  • Fast strategy for the creation of Pd–Ag aerogel assembled by nanochaines.

  • Pd–Ag aerogel shows high catalytic activity and durability toward formic acid oxidation.

  • The porous 3D network of Pd–Ag aerogel is formed during the self-assembly process.

Abstract

Among self-assembled architectures, noble metal aerogels have emerged as state-of-the-art catalysts, and these unique architectures play a vital role in widespread applications. Until now, profuse ways have been introduced for the development and synthesis of noble metal aerogels, which suffer from long gelation time as well as complexity. In this work, a facile and efficient method is presented to create the Pd–Ag aerogel. This route offers advantages such as a surfactant-free route, short gelation time and a simple strategy. The Pd–Ag aerogel was prepared under controlled condition by using a galvanic displacement of sacrificial Ag nanoparticles with H2PdCl4 followed by supercritical CO2 drying. Porous three-dimensional architecture of Pd–Ag aerogel is confirmed by the TEM, FESEM, XRD, ICP-AES and N2 isotherm. The resultant aerogel displays exceptional catalytic activity and durability toward electro-oxidation of formic acid on account of two vital reasons: 1) the unique architecture of resultant aerogel reflects exceptional advantages such as meso-and macroporous properties, and 2) the combination of Ag with Pd, leading to the acceleration in the oxidation of formic acid (synergistic effects).

Introduction

Nowadays, direct formic acid fuel cells (DFAFCs) have aroused profuse attention on account of widespread advantages (e.g. significant energy density, security, low toxicity, low pollutant emission, etc.) [[1], [2], [3], [4], [5]]. However, the commercialization of DFAFCs still faces crucial drawbacks such as insufficient activity, scanty long-term durability and high-price of catalysts. Hence, a wealth of methods has been reported to overcome these issues with respect to the rational design of the dispersity, shape, bulk compounds, and size of a catalyst [[6], [7], [8], [9], [10]].

The advent of revolutionary technologies in nanoscience, such as self-assembly, has attracted a lot of interests for the creation of unique architectures. In recent years, numerous studies on three-dimensional (3D) porous noble metal nanostructures have stimulated extensive interests toward different assembled architectures by fine-tune of the nanoparticles (NPs) as building units. These porous noble metal nanostructures have received a lot of attention in various industries such as photonic, optoelectronic, catalytic and sensing technologies on account of their great surface area, large active sites, and high porosity [10,11]. The tailored design of nanomaterials with unique structures (e.g. nanosheets, nanowires, etc.) has created a lot of opportunities to promote catalytic activity and durability [12,13]. In this context, noble metal aerogels (NMAs) with extended metal backbone networks have emerged as state-of-the-art catalysts.

The NMAs are a new class of 3D nanostructures with unique characters (e.g. ultra-low density, extraordinary porosity and huge surface area) [[14], [15], [16], [17], [18], [19], [20], [21]]. These exceptional 3D networks are created by utilizing self-assembly of building units. Interestingly, in this process (self-assembly), building units (metal nanoparticles) are linked together, leading to the creation of a 3D network with exceptional characters. On account of the exceptional physicochemical characters, which are related to the macro-scale, the NMAs have attracted extensive interest in nanoscience, and are a long-desired goal in recent studies. The NMAs, not only reveal the intrinsic properties of aerogels (e.g. extensive surface area, and nanoscale particle sizes), but also provide the exceptional characters of metal nanoparticles (such as extraordinary catalytic activity and conductivity). Likewise, the self-supporting characteristic of NMAs leads to the enhancement of durability and lack of corrosion of metal nanoparticles by support [[14], [15], [16], [17], [18], [19], [20], [21]]. Hence, the NMAs can serve as suitable candidates for widespread applications (e.g. fuel cell, optic, etc.). In recent years, numerous studies have been dedicated to develop the aerogels. The creation of self-supporting nanoporous titanium aerogel from the TiO2 aerogel template utilizing a magnesiothermic reduction route has been reported by Xu and co-workers [22]. These products maintained some unique characteristics (e.g. monolithic shape, large surface area, and nanoscale porous framework). Du and colleagues reported a simple method for the fabrication of multifunctional silica nanotube aerogels (SNTAs) via chemical vapor deposition (CVD) of silica onto the sacrificial carbon nano-skeleton of a carbon aerogel [23]. These SNTAs are not only porous, nanotubular, transparent, and ultralow density but also hydrophobic, thermal resistant, mechanically robust and machinable. Recently, Sun and co-workers have devoted themselves to the investigation of the interaction between subwavelength structures and electromagnetic waves. They have demonstrated that the hierarchical porous carbon aerogels that contain randomly abundant subwavelength structures can dramatically change their optical reflection [24]. Furthermore, Sun and co-workers have proved that the density, framework and micropores of carbon aerogels play a crucial role in electromagnetic properties [25]. Moreover, it is proved that the photocatalysis efficiency of titanium oxide (TiO2) sheet increases under the shading of super black carbon aerogel/TiO2 composite [26]. As mentioned above, the NMAs offer exceptional characteristics of metal nanoparticles such as high conductivity. Hence, they are promising candidates especially for the super black or hot-electron-induced materials.

For the first time, the synthesis of NMAs was reported by Alexander Eychmüller and co-workers. They are realized the controlled destabilization of citrate-stabilized NPs in aqueous solution to produce the NMAs [14]. Since then, a broad spectrum of approaches has been reported for the fabrication of NMAs. The crucial step to create the NMAs is the gelation process. Until now, a wealth of studies has focused on the facilitation of gelation process. Nonetheless, the gelation processes suffer from complexity as well as long gelation time. Likewise, one of the vital problems in the creation of NMAs is the utilization of surfactants. The surfactant compounds lead to the degradation of catalytic performance on account of their strong interaction to the surface of NMAs and the blockage of active sites.

In this paper, a facile and useful strategy is introduced for the creation of Pd–Ag bimetallic aerogel. This route offers advantages including a facile route, surfactant free method and short gelation time. The Pd–Ag aerogel was prepared by using a galvanic replacement of Ag nanoparticles with H2PdCl4 as metal precursor followed by supercritical CO2 drying. The Ag nanoparticles were fabricated by reducing AgNO3 in the presence of glyoxylic acid monohydrate (GA) and sodium carbonate (SC). Next, Pd–Ag aerogel was formed using galvanic replacement of Ag nanoparticles by H2PdCl4 as metal precursor in controlled condition. The supercritical CO2 drying was applied to transform hydrogel to aerogel. Resultant Pd–Ag aerogel served as a supportless catalyst toward formic acid oxidation (FAO), and represented exceptional electrocatalytic activity and durability. Fig. 1 shows the schematic illustration of the fabrication of Pd–Ag aerogel, and its application toward FAO.

Section snippets

Materials

Hydrochloric acid (HCl), silver nitrate (AgNO3), Glyoxylic acid monohydrate (C2O3·H2O), sodium carbonate (Na2CO3), and PdCl2 were purchased from Merck Co. The PdCl2 was dissolved in hydrochloric acid to make H2PdCl4 precursor solution.

Characterization

To elucidate the morphology of resultant aerogel, field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were acquired on MIRA3 TESCAN and Zeiss-EM10C–100 KV, respectively. A Bruker, D8 ADVANCE equipped with a Cu Kα

Synthesis of Pd–Ag hydrogel/aerogel

In previous studies, it was proved that the combination of Ag and Pd metals will help to enhance the catalytic performance owing to the synergetic effects. Therefore, this research focuses on the creation of bimetallic aerogel. The Pd–Ag aerogel was prepared by using a galvanic replacement reaction (GRR) method followed by supercritical CO2 drying. The sacrificial Ag nanoparticles were created by the reducing AgNO3 in the presence of SC utilizing GA as the reducing agent followed by adding H2

Conclusions

In summary, a facile and useful strategy is introduced to produce the Pd–Ag aerogel. This route offers several advantages over other approaches such as facile strategy as well as shorter gelation time. The Pd–Ag aerogel was fabricated by utilizing a galvanic displacement of sacrificial Ag NPs with H2PdCl4 under controlled condition followed by supercritical drying. Assembly of 3D unique of Pd–Ag aerogel is confirmed by the TEM and FESEM. The Pd–Ag aerogel reflects exceptional catalytic

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