Elsevier

Applied Surface Science

Volume 401, 15 April 2017, Pages 314-322
Applied Surface Science

Full Length Article
Hydrogenation of 4-nitrophenol to 4-aminophenol at room temperature: Boosting palladium nanocrystals efficiency by coupling with copper via liquid phase pulsed laser ablation

https://doi.org/10.1016/j.apsusc.2017.01.045Get rights and content

Highlights

Abstract

Ultra-dispersed bimetallic nanomaterials have attracted much attention in the hydrogenation of highly toxic aromatic nitro compounds to aromatic amines owing to their high stability, superior activity, reusability, and unique optical and electronic properties, as compared to monometalic nanocrystals. However, the lack of facile and economically controllable strategies of producing highly pure ultra-dispersed bimetallic nanocatalysts limits their practical industrial applications. Considering the above obstacles, we present a simple and effective strategy for the formation of bimetallic (PdCu) nanocrystals by liquid phase pulsed laser ablation using a bulk Pd metal plate submerged in CuCl2 solutions with different concentrations, in contrast to the complex and costly experimental methods used previously. The microstructural and optical properties of the synthesized nanocrystals indicate that the obtained bimetallic nanostructures are highly pure and monodispersed. Moreover, bimetallic PdCu nanostructures show a higher catalytic activity than monometallic Pd nanocrystals for the hydrogenation of 4-nitrophenol to 4-aminophenol at room temperature, also exhibiting high stability for up to four recycles. The mechanism of the enhanced catalytic activity and stability of bimetallic nanocrystals is discussed in detail. Finally, we believe that the presented design strategy and utilization of bimetallic nanocrystals for catalytic applications enables the development of novel bimetallic nanostructures by liquid phase pulsed laser ablation and their catalytic application for environmental remediation.

Introduction

Recently, aromatic nitro compounds such as 4-nitrophenol and nitrobenzene have been comprehensively employed in the industrial production of insecticides, fungicides, herbicides, dyes, and explosives [1]. However, these nitro compounds are extremely toxic contaminants that affect human health and environment, accumulating in the ecosystems and the human body [2], [3]. In contrast, 4-aminophenol, the product of 4-nitrophenol reduction, is an industrially useful material, serving as an intermediate for the manufacture of analgesic drugs, photographic developers, corrosion inhibitors, and anticorrosion lubricants [4], [5]. Therefore, the conversion of 4-nitrophenol into the less harmful and reusable 4-aminophenol is an essential and critical issue [6]. In order to resolve the environmental problems caused by 4-nitrophenol, numerous approaches have been developed for its conversion into 4-aminophenol, such as electrocoagulation [7], electro-Fenton methods [8], bio-degradation [9], catalytic hydrogenation [10], and photocatalytic degradation [11]. Among these methods, catalytic hydrogenation in aqueous solution in the presence of excess NaBH4 as a reductant and noble metal-based nanocatalysts is the most efficient and economical technique [12].

Over the last few years, diverse nanocatalysts have been developed for the reduction of 4-nitrophenol to 4-aminophenol, based on noble and transition metals such as Au, Pt, Ag, Pd, Cu, Fe, and Ni [13], [14], [15], [16]. Among the reported noble metal nanostructures, the ones based on Pd have been used for the reduction of nitro compounds more frequently than other noble metal nanostructures, due to the advantages of high stability, superior activity, reusability, and unique optical and electronic properties of Pd, furthermore the Pd is at least fifty times more abundant than Pt on earth [17], [18]. However, monometallic Pd nanocrystals are inferior for industrial applications due to their high price and scarcity. To overcome this problem, researchers have recently demonstrated that upon appropriate modification of Pd nanocrystals surface with abundant low price materials resulting in the modification of the composition ratio, and structure, Pd-based nanomaterials can become promising catalysts with low price. Therefore, it is significant to obtain well-defined Pd based functional nanostructures such as bimetallic composites (PdAu, PdCu, PdNi, and PdFe), nanocomposites (Pd-RGO, Pd-C3N4, Pd-MoS2 and CeO2@Pd), and core-shell nanostructures (Pd@C) in terms of stability, resistibility, and efficiency [18], [19], [20], [21], [22]. Among the above functional nanostructures, bimetallic nanostructures are particularly attractive for catalytic applications, because the multiple metal components in bimetallic nanocrystals serve as active centers for the reduction reaction and enable fast and convenient access of the reactant molecules to activate the catalytic process [23]. Moreover, the two metal ions present in such nanostructures provide synergistic interaction opportunities, which lead to enhanced physical, electrical, and chemical properties, resulting in excellent performance in catalytic applications [24]. For example, Zhong et al. demonstrated Au-decorated Pd bimetallic nanostructures and noticed significantly improved the ethanol electro-oxidation catalytic performance and stability than monometallic nanostructures [25]. Hu et al. demonstrated the fabrication of PdAg bimetallic nanocrystals for selective dehydrogenation of formic Acid [26]. Zhu et al. designed PdNi bimetallic nanocrystals with enhanced electrocatalytic activities [27]. In this direction, the combination of Cu has been particularly attractive for catalytic applications, because the non-precious and earth-abundant nature of copper enables large-scale production at a low price [28]. Considering this, several Cu based bimetallic nanocrystals were recently developed and applied for catalytic applications. For instance, Wu eta al. developed CuAg bimetallic nanocrystals and noticed high catalytic performance on the reduction of nitrophenol [29]. Liu et al. developed several bimetallic nanocrystals (AgCu, CuAu, CuPt and CuPd) with high catalytic activity [30]. Among the studied bimetallic nanocrystals, the PdCu combination has been particularly attractive for catalytic applications because of their interesting optical, electrical and catalytic properties. Moreover, in the reaction medium, Pd nanocrystals act as centers for hydrogen dissociation, with hydrogen atoms spilling over onto neighboring Cu sites and resulting in hydrogenation. Furthermore, the Pd/Cu combination acts as an intrinsic catalyst, boosting the catalytic activity in the reaction medium [31].

However, the lack of facile and economically controllable strategies of producing highly pure and ultra-dispersed bimetallic PdCu nanocatalysts limits their practical industrial applications. To date, most of the developed PdCu bimetallic nanostructures have been fabricated by bottom-up approaches, such as wet chemical methods. These approaches require complicated synthetic processes, high-temperature conditions, costly equipment/instruments, organic reducing agents, surfactants and/or capping agents, and produce aggregated nanoparticles, resulting in reduced catalytic activity [23]. In contrast, pulsed laser ablation in liquid (PLAL), a top-down approach, is a facile, simple, and efficient method to synthesize a variety of nanocomposites, enabling the generation of versatile sizes and shapes by controlling the laser parameters. Moreover, the nanoparticles produced by PLAL are well dispersed in the liquid medium, which enhances their catalytic activity. Furthermore, the composition of these nanostructures can be easily tuned by varying the solute concentration, solvent, and ablated metal target, rather than by changing other parameters [32], [33], [34], [35].

Considering its importance and simplicity, PLAL was used for the formation of ultrapure bimetallic nanostructures. Herein, for the first time, bimetallic PdCu nanostructures were synthesized via pulsed laser ablation of a Pd target in CuCl2 solution. The ratio of metal ions was controlled by varying the CuCl2 concentration, and the catalytic performance of the as-synthesized nanocrystals was assessed by measuring the rate of 4-nitrophenol reduction to 4-aminophenol with excess NaBH4 in aqueous solution at room temperature. The results indicate that the CuCl2 concentration is an important parameter for the catalytic reaction. The catalytic activity of the optimized bimetallic PdCu-1 nanocrystals was superior to that of the monometallic Pd nanocrystals, indicating the robustness of the bimetallic nanocrystals in the catalytic reaction. We anticipate that the presented design strategy and utilization of bimetallic nanocrystals for catalytic applications enables the development of novel bimetallic nanostructures by PLAL and their catalytic application for environmental remediation.

Section snippets

Experimental

Monometallic Pd nanocrystals were produced by pulsed laser ablation of a Pd metal plate (Sigma-Aldrich, 99.9%, 0.5 mm thick) in deionized (DI) water. The pure plate was sequentially washed with acetone and ethanol and ultrasonicated to remove impurities adsorbed on the metal surface. A 10-mL glass beaker was filled with 2 mL of DI water. The container was installed on a turn table and the laser beam focused by a convex lens entered the reaction vial. The wavelength and energy density of the

Results and discussion

The synthesis of bimetallic PdCu nanocrystals with highly monodispersed and highly exposed active sites was achieved by facile liquid phase pulsed laser ablation, as illustrated in Scheme 1, and is explained as follows. When the pulsed laser is focused on the Pd metal target submerged in a solution containing Cu2+ ions, the target absorbs the laser pulse energy contributing to surface vaporization, melting, and ionization. During the pulse duration, high-temperature and pressure plasma plume of

Conclusions

Ultra-dispersed bimetallic nanocrystals were synthesized by facile liquid phase pulsed laser ablation. These novel PdCu nanocrystals exhibit exceptionally high stability and superior 4-nitrophenol to 4-aminophenol hydrogenation ability compared to monometallic Pd nanocrystals. The kinetic rate constant for optimized PdCu nanocrystals is ∼5.91 times higher than that of monometallic Pd nanostructures. In addition, the observed kinetic rate constant is much higher than those of the several

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIP) (2014R1A4A1001690 and 2016R1E1A1A01941978).

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