Elsevier

Applied Catalysis B: Environmental

Volume 250, 5 August 2019, Pages 242-249
Applied Catalysis B: Environmental

Shape engineering of palladium aerogels assembled by nanosheets to achieve a high performance electrocatalyst

https://doi.org/10.1016/j.apcatb.2019.02.064Get rights and content

Highlights

  • Palladium aerogels has been synthesized by self-assembly of ultrathin Pd nanosheets in carboxylic acid solvents.

  • The alkyl group of carboxylic acids, play a crucial role in the structural properties of Pd aerogels.

  • Pd aerogel synthesized in acetic acid solvent shows highest catalytic activity and stability toward ethanol oxidation.

Abstract

Three-dimensional noble metal aerogels are unique solid materials with ultralow densities, large open pores, and high surface areas. Despite several research works on noble metal aerogels assembled by nanochains, the investigation of noble metal aerogels assembled by nanosheets has not been reported to date. In this work, palladium aerogels assembled by nanosheets have been synthesized in four various carboxylic acid (RCOOH) solvents with different alkyl groups (R= H– (formic acid, FA), CH3– (acetic acid, AA), CH3CH2– (propionic acid, PA), and CH3CH2CH2– (butyric acid, BA)). It has been found that the R groups play a crucial role in the morphology of palladium aerogels so that the Pd aerogels synthesized in acetic acid and propionic acid solvents show better morphology than that of palladium aerogels synthesized in formic acid and butyric acid solvents. The electrocatalytic activity and durability of all aerogels are evaluated using cyclic voltammetry (CV) and chronoamperometry (CA) experiments toward ethanol oxidation. Based on CV and CA data, Pd aerogels synthesized in AA and PA show high catalytic activity and durability due to their unique structures.

Introduction

Synthesis of unsupported nanocatalysts with high surface area and large porosity has aroused significant interest in nanotechnology. Recently, unsupported electrocatalysts have been promoted by Company 3 M for polymer electrolyte fuel cells [1]. In commercial catalysts such as Pd/C, the corrosion of the carbonaceous support during the operation of fuel cell leads to loss of noble metal nanoparticles; hence, as time goes by, the activity of the catalyst is dramatically reduced [2]. Therefore, a wealth of studies focused on solving this problem. It has proved that self-assembly processes are the most powerful strategies for the creation of self-supported three dimensional (3-D) materials with unique structures (e.g. large surface area, abundance active sites, and high porosity) which can be applied as supportless electrocatalysts [[3], [4], [5], [6]]. Among the 3-D nanostructures, noble metal aerogels (NMAs) are very promising un-supported electrocatalysts that have ultra-low densities, high inner surface areas and large open pores [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16]]. In recent years, a variety of methods have been reported for the fabrication of NMAs, while the development of self-assembled NMAs has significantly slowed down because of the same structures. Since in all previous studies NMAs have been assembled only by nanochains, to develop this field, it is an urgent to introduce a new structure of NMAs. After the introduction and several studies on the NMAs assembled by nanochains, now it is time to introduce and study the new generation of NMAs. Compared to the nanochains, two-dimensional (2-D) nanosheets with atomic thickness, not only provide high surface area and high flexibility but also possess outstanding electrical conductivity and atom utilization efficiency [17]. Therefore, nanosheets are promising candidates to create the new generation of NMAs, unfortunately, they are prone to stacking due to the strong interlayer interactions, which can greatly decrease the surface area. Therefore, rational engineering of 2-D nanosheets into 3-D aerogels is necessary to prevent the stacking. Moreover, it is known that the size of the nanosheets affect the density and pore size distribution of the aerogels so that the smaller size of nanosheets leads to the smaller pore size and higher density in the aerogels [18]. In this regard, our research team made extensive studies on the synthesis of Pd aerogels assembled by extensive and ultrathin nanosheets using a surfactant free route.

In the present study, a new class of the Pd aerogels with different morphology was synthesized using monocarboxylic acid (RCOOH) solvents, with different alkyl groups (R= H- (formic acid, FA), CH3– (acetic acid, AA), CH3CH2– (propionic acid, PA) and CH3CH2CH2– (butyric acid, BA). Based on the obtained results, the length of the carbon chain in monocarboxylic acid plays an important role in the thickness of Pd nanosheets and the morphology of the as-prepared Pd aerogels. The electrocatalytic activities of Pd aerogels were evaluated using cyclic voltammetry (CV) and chronoamperometry (CA) experiments toward ethanol oxidation and the results were compared with the commercial Pd/C catalyst.

Section snippets

Synthesis of Pd aerogels

10 mg of palladium acetylacetonate (Pd(acac)2) was dissolved in 10 mL of each carboxylic acid solvents in 15 mL glass vials. The mixture was bubbled with CO gas for a certain time. The bubbling time for FA is 3 min, and for other solvents, it is 10 min. After bubbling, the glass vial was immediately sealed and kept in 50 °C for 4 h without shaking. Then, the black hydrogel was immersed and washed in acetone for three times. The resulting anhydrous, acetone-containing Pd gels were transferred

Results & discussion

The Pd hydrogels synthesized in four carboxylic acid solvents including FA, AA, PA and BA in 4 h and 50 °C are shown in Fig. 1. In this study, Pd (FA), Pd (AA), Pd (PA) and Pd (BA) correspond to Pd aerogels synthesized in formic acid, acetic acid, propionic acid, and butyric acid, respectively. When Pd(acac)2 is dissolved in the selected carboxylic acid, a bright yellow solution is obtained. After bubbling of carbon monoxide into the aforementioned solution, the bright yellow color of the

Conclusion

In summary, we have described a morphology engineering in the new class of Pd aerogels assembled by Pd nanosheets. In this work, Pd aerogels were synthesized in carboxylic acid solvents with different alkyl groups. It was found that the length of the alkyl group plays a vital role in the morphology of the Pd aerogels and the thickness of Pd nanosheets. The Pd aerogels synthesized in AA and PA solvents show better morphology compared to other aerogels due to their alkyl groups acting as a mild

Acknowledgment

We gratefully acknowledge the financial support from the University of Sistan and Baluchestan.

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