Bimetallic PdRu nanosponges with a tunable composition for ethylene glycol oxidation

Abstract

In this study, bimetallic PdRu nanosponges with regulated Pd/Ru ratios have been obtained through a simple mixing pathway. The synthetic step is simple, and proceeds just by mixing PdCl₂ and RuCl₃ aqueous solutions into NaBH₄ solution. The physical structure and chemical composition of the as-prepared nanosponges have been examined using different methods. Ethylene glycol electrooxidation reaction tests reveal the fine catalytic performance of the nanosponges, and the introduction of an appropriate amount of Ru can promote the catalytic ability of Pd. The developed synthetic pathway and the prepared materials in this contribution lead to a new practical advancement of electrocatalysts for ethylene glycol electrooxidation.

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Introduction

The distinct benefits of direct ethylene glycol (EG) fuel cells (DEGFCs) such as them being free of pollution, and their high energy density and wide availability lay the foundation for their latent applications as prospective power source devices.^1–4 Promoting the development of DEGFCs is highly dependent on the progress of electrocatalysts for the EG oxidation reaction (EGOR).^5–7 While on the other hand, the rapid development of nanomaterials and nanotechnology enables the successful exploration of various efficient catalysts with different nanostructures.^8–11 In these terms, many investigations have been carried out in order to prepare different nanocatalysts for the EGOR through the accurate controlling of the structural and compositional properties of the catalysts.^8,9,12,13 So far, Pt and Pd-based nanocatalysts have been identified as the most powerful materials for the EGOR.^14–17 Hence, a lot of Pt and Pd nanocatalysts for the EGOR have been fabricated, such as PtAg nanotubes,¹⁸ Pd/MnO_x/graphene nanocomposite¹⁹ and core–shell PdCuBi nanoparticles.²⁰ In particular, the Pd-based nanocatalysts are considered to be suitable alternatives for the EGOR due to their advantages in terms of lower price and similar catalytic ability compared with the Pt-based nanocatalysts.^21–24 Therefore, many Pd-based nanocatalysts have rapidly emerged for the EGOR, such as Pd nanoparticles supported on phenanthroline modified carbon²⁵ and palladium-decorated FeCo@Fe core–shell nanocatalysts.²⁶

Validated by previous investigations, researchers have found that the element Ru, which can be purchased at a much cheaper price than Pd,²⁷ can improve the catalytic performance of Pd for methanol, ethanol and formic acid electrooxidation.^28–30 These phenomena are aroused from the synergetic effects between Pd and Ru. Moreover, the cheaper Ru element can reduce the usage of Pd and hence lower the cost. However, there are still only rare studies about whether the Ru element can promote the catalytic ability of Pd for the EGOR. In another aspect, the structural properties of nanocatalysts are also essential to their final catalytic ability. With a particular interest, nanocatalysts with a nanoporous structure have been recognized as effective catalysts for alcohol molecule electrooxidation over the past few years.^31–35 Due to their excellent catalytic ability which has been displayed during previous investigations, extensive contributions have been made to fabricate nanocatalysts with a nanoporous structure. Methods such as replacement³⁶ and electrochemical etching³⁷ have been developed to construct metallic nanocatalysts with nanoporous structures. Yi et al. reported a hydrothermally assisted replacement method to prepare PdRu porous nanomaterials.²⁹ Nevertheless, there is still no report about the simple and rapid preparation of bimetallic PdRu nanostructures with tunable Pd/Ru ratios, and their catalytic ability for the EGOR also remains unknown.

In view of the above reasons, in this contribution, we carried out a simple mixing approach to prepare PdRu nanosponges (PdRu NSPs) with a porous nanostructure and adjustable Pd/Ru ratio. Electrocatalytic investigations certify the enhanced EGOR electrocatalytic properties of the PdRu NSPs. Introducing an appropriate amount of Ru can lead to the best catalytic performance. The simple mixing approach and fine catalytic performance of the PdRu NSPs pave a way for their application in the EGOR.

Experimental

Materials

RuCl₃ and PdCl₂ solid powders were purchased from Sinopharm Company (Shanghai, China). The RuCl₃ and PdCl₂ powders were dissolved in water and diluted hydrochloric acid respectively, reaching a concentration of 77.2 mmol L⁻¹ and 56.4 mmol L⁻¹, respectively. NaBH₄ was bought from Aldrich. The water used in the entire experiment is referred to as Milli-Q ultrapure water (Millipore, ≥18.2 MΩ cm).

Synthesis of PdRu NSPs

Before the synthesis, H₂PdCl₄ solution was prepared by mixing 1 g of PdCl₂ and 1 mL of concentrated HCl into water, to reach a volume of 100 mL. The PdRu NSPs were prepared by adding a mixture consisting of the above prepared RuCl₃ and H₂PdCl₄ precursor solutions into 5 mL of water, in which 10 mg of NaBH₄ had been dissolved, under stirring. The respective ratios for synthesizing different PdRu NSPs are listed as follows: 0.5 mL of H₂PdCl₄ and 0.365 mL of RuCl₃ for Pd₃₂Ru₁₉; 0.5 mL of H₂PdCl₄ and 0.183 mL of RuCl₃ for Pd₃₇Ru₁₃; 0.25 mL of H₂PdCl₄ and 0.365 mL of RuCl₃ for Pd₂₃Ru₂₈; 0.5 mL of H₂PdCl₄ for pure Pd NSPs. The reaction was continued for 30 min, and the final precipitates were centrifuged and cleaned with water.

Instruments

The structure of the PdRu NSPs was firstly analyzed using a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray analyzer (SEM mode: XL30 ESEM). The transmission electron microscopy (TEM) images were made on a HITACHI H-600 analytical TEM with an operating voltage of 100 kV. Inductively coupled plasma mass spectroscopy (ICP-MS, X Series 2, Thermo Scientific USA) was employed to determine the final exact composition of the PdRu NSPs. A D8 ADVANCE (BRUKER, Germany) diffractometer using Cu-Kα radiation with a Ni filter (λ = 0.154059 nm at 30 kV and 15 mA) was used to obtain X-ray diffraction (XRD) patterns of the PdRu NSPs. A nitrogen adsorption–desorption isotherm was recorded using a Micromeritics ASAP 2010 Analyser (USA) with nitrogen. A CHI 832B electrochemical workstation (Chenhua Instruments Corp, Shanghai, China) was utilized to perform all of the electrochemical tests. The fabrication of electrocatalytic measurement devices and modification of the working electrodes were performed according to our reported pathway.^8,9 The Pd mass loaded on the electrode was 66.6 μg cm⁻² for all of the NSPs.

Results and discussion

Structure and composition characterizations

The physical structure of the as-obtained PdRu NSPs was firstly checked using SEM. As shown in Fig. 1A–C, the generated product presents a bulk “sponge”-like nanostructure with a loose and porous feature. The TEM image (Fig. 2A) suggests that the product consists of mutually connected nanoparticles. The composition of the prepared PdRu NSPs has been primarily determined using energy dispersive X-ray spectroscopy (Fig. 2C), the result of which indicates that the Pd/Ru ratio is 37 : 13, which also fits with the ICP-MS investigation. It is also noted that the final Pd/Ru ratio is higher than the Pd/Ru ratio in the precursors, this finding may be due to the slow and incomplete reduction of the RuCl₃ precursor. The explorations of the element distribution (Fig. 1D and E) in the NSPs verify that Pd and Ru elements exist homogeneously along the whole bulk structure, suggesting an alloyed nature. XRD patterns also confirm the alloyed nature of the PdRu NSPs. As shown in Fig. 2B, the NSPs present a single phase diffraction feature and have a representative face-centered-cubic structure. The nitrogen adsorption–desorption isotherm study result (Fig. 2D) also reveals the porous feature of the NSPs. The Brunauer–Emmett–Teller (BET) surface area of the NSPs was determined to be 26.2 m² g⁻¹. Changing the Pd/Ru precursor ratio allows the generation of PdRu NSPs with different Pd/Ru ratios. In order to find out if the Ru element has made a contribution to the promotion of the catalytic performance of the PdRu NSPs, pure Pd NSPs were also prepared similarly as a control test.

Fig. 1 (A–C) SEM images of the as-prepared PdRu NSPs. (D and E) The element distribution recorded in the (C) area.

Fig. 2 TEM image (A), XRD (B), EDS (C) and nitrogen adsorption–desorption isotherm curve (D) of the PdRu NSPs.

Electrocatalytic measurements

The electrocatalytic properties of the generated PdRu NSPs were examined using a cyclic voltammogram (CV) technique. The EGOR catalytic tests were conducted in 0.5 mol L⁻¹ EG and 0.5 mol L⁻¹ potassium hydroxide. The CV curves presented in Fig. 3A were normalized using the electrode surface area. The CV curves were obtained with a potential ranging from −0.8 V to 0.4 V (relative to the Ag/AgCl electrode). For each catalyst, the applied scan rate was 50 mV s⁻¹. There are two peaks generated in the test as presented in Fig. 3A, the anodic peak is related to the direct or indirect oxidation of EG (the direct oxidation of EG will produce CO₂, while the indirect oxidation of EG will lead to the production of CO), while the cathodic peak is related to the removal of the adsorbed products produced during cathodic oxidation.⁸ Generally, there are several important indexes to evaluate the electrocatalytic properties of a catalyst. The first one is the onset oxidation potential, a lower onset potential means that the alcohol molecules can be oxidized more easily. As can be learned from Fig. 3A, the onset oxidation potential follows the trend: Pd₃₇Ru₁₃ < Pd₃₂Ru₁₉ ≈ Pd₂₃Ru₂₈ < Pd. The second index is the peak current density, a higher current density represents a higher catalytic activity. Evaluated by the anodic peak current value, these NSPs show a catalytic activity of Pd₂₃Ru₂₈ (0.02 A cm⁻²) < Pd (0.022 A cm⁻²) < Pd₃₂Ru₁₉ (0.031 A cm⁻²) < Pd₃₇Ru₁₃ (0.043 A cm⁻²). The Pd₃₇Ru₁₃ NSPs show the highest catalytic ability, producing a mass activity of 650 mA mg_Pd⁻¹, which is also better than the reported results of Pd/RGO,³⁸ Pd/WC–Mo₂C³⁹ and PdTe nanowires.⁴⁰ The catalytic activity trend suggests that the incorporation of an appropriate content of Ru into the PdRu NSPs is beneficial for the promotion of the catalytic activity, while a too high Ru content will lead to a decrease of the catalytic activity. This consequence may be derived from the inner properties of the Ru element, since pure Ru element has no obvious catalytic ability for alcohol electrooxidation, but the Ru can promote the function of Pd and Pt during the electrooxidation process, as is validated by the revealed results.^28–30 The third important index is the peak anodic current value versus the peak cathodic current (denoted as: I_a/I_c). The I_a/I_c for these NSPs displays the order: Pd (1.34) < Pd₃₇Ru₁₃ (1.44) < Pd₃₂Ru₁₉ (1.77) < Pd₂₃Ru₂₈ (2.1). A trend can be observed where the I_a/I_c value increased accordingly when the Ru content was enhanced. This result suggests that the addition of the Ru element is conducive to the removal of the adsorbed products yielded in the anodic oxidation process. Except for these three indexes, the peak oxidation potential is also important for measuring the catalytic properties of the catalyst. It can be noticed that all of the bimetallic PdRu NSPs display an evidently lower peak oxidation potential compared to the pure Pd NSPs, which further signifies the important role of Ru.

Fig. 3 The plots for the EGOR, (A) CV and (B) j–t curves recorded at −0.2 V.

Current–time (j–t) is a useful method which has been widely used to explore the catalytic stability of a catalyst. Thus, we also adopt this technique to investigate the catalytic stability of these NSPs. Fig. 3B illustrates the j–t plots for these NSPs; the applied potential was −0.2 V. The stability of these NSPs toward the EGOR presents a trend that is similar to their catalytic activity. Among these NSPs, the Pd₃₇Ru₁₃ NSPs also show the distinctly highest current density, suggesting that they have the best EGOR stability. The other two bimetallic PdRu NSPs show obviously higher current densities than the mono-component Pd NSPs, indicating the important contribution of Ru. These electrochemical studies demonstrate the fine catalytic properties of the prepared bimetallic PdRu NSPs. The incorporation of the Ru element is beneficial for the enhancement of the catalytic function of the Pd catalyst. The incorporation of an appropriate content of Ru into the PdRu NSPs leads to the best catalytic performance. The fine electrocatalytic properties of the bimetallic PdRu NSPs are probably induced by the special porous nature of the NSP structure and the synergetic effects between Pd and Ru. Taking their simple preparation pathway and fine catalytic performance into account, these prepared bimetallic PdRu NSPs may become a competitive kind of catalyst for the EGOR.

Conclusions

In conclusion, bimetallic PdRu nanosponges with a porous structure were prepared via a simple and quick mixing method. Simply tuning the RuCl₃ and PdCl₂ precursor ratio allows the generation of the desired nanosponges with a regulated Pd/Ru ratio. Ethylene glycol electrooxidation investigations demonstrated the best catalytic properties of Pd₃₇Ru₁₃ nanosponges. The study carried out here also reveals that the introduction of an appropriate amount of Ru element can promote the catalytic properties of Pd. This contribution not only provides a new type of electrocatalyst with fine catalytic ability, but also offers a useful guide for the future progress of efficient anode nano-electrocatalysts for ethylene glycol oxidation.

Acknowledgements

This work was supported by National Natural Science Foundation of China with Grant No. 21190040 and 91227114.

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