Elsevier

Journal of Hazardous Materials

Volume 366, 15 March 2019, Pages 338-345
Journal of Hazardous Materials

Reductive transformation of nitroaromatic compounds by Pd nanoparticles on nitrogen-doped carbon (Pd@NC) biosynthesized using Pantoea sp. IMH

https://doi.org/10.1016/j.jhazmat.2018.12.009Get rights and content

Highlights

  • We propose a novel green synthesis method for Pd NPs supported on the N-doped carbon (Pd@NC).

  • The Pd@NC exhibited a high catalytic activity for the reductive transformation of nitroaromatic compounds.

  • Integrated XANES and DFT results suggest that the d-hole counts increase due to Pd 4d-C 2p hybridization in Pd@NC.

Abstract

Reductive transformation of nitroaromatic compounds is a central step in its remediation in wastewater, and therefore has invoked extensive catalytical research with rare metals such as palladium (Pd). Herein, we report Pantoea sp. IMH assisted biosynthesis for Pd@NC as an efficient catalyst for the reduction of nitroaromatics. Multiple complementary characterization results for Pd@NC evidenced the evenly dispersed Pd NPs on an N-doped carbon support. Pd@NC exhibited the superior catalytic activity in the reduction of nitroaromatic compounds (4-nitrophenol, 2-nitroaniline, 4-nitroaniline, and 2,6-dichloro-4-nitroaniline). The origin of the catalytic activity was explained by its unique electronic structure, as explored with X-ray absorption near-edge structure (XANES) spectroscopy and density functional theory (DFT) calculations. XANES analysis revealed an increase of 25.6% in the d-hole count in Pd@NC compared with Pd°, as the result of pd hybridization. In agreement with our experimental observations, DFT calculations suggested the formation of Pd-C bonds and charge re-distribution between Pd and the carbon layer, which contributed to the superior catalytic activity of Pd@NC.

Introduction

Nitroaromatic compounds, such as 2-nitroaniline (2-NA), 4-nitroaniline (4-NA), and 2,6-dichloro-4-nitroaniline (2,6-DCNA), are widely applied as the precursor for synthesis of other organic chemicals, and also used for the production of explosives, dyes, pharmaceuticals, pesticides, perfumes and plastics [1]. Nitroaromatic compounds are usually found as contaminants in the soil and groundwater generated from industrial wastewater [2]. Nitroaromatic compounds are acutely toxic and mutagenic, and many are suspected or established carcinogens. They have been enlisted as the major priority pollutants due to their environmental persistence as well as toxicological effects to living organisms and human health even at low concentrations [3]. Though one-step complete degradation of nitroaromatic compounds is practically impossible [4], their reductive transformation to aniline is a feasible and effective method for wastewater treatment [[5], [6], [7]].

Among the commonly used techniques to reduce nitroaromatic compounds, the chemical catalytic process exhibits the highest conversion rates [8]. Thus, it is highly desirable to develop cost-effective methods to synthesize an efficient catalyst [5]. To this end, microbial-assisted green synthesis has demonstrated its unique advantages of low-cost, eco-friendliness, and mild synthetic environment [9]

Palladium is of particular importance among noble metals in the catalytic field. Pd nanoparticles (Pd NPs) are the most widely used and exploited catalyst in various heterogeneous reactions such as hydrogenation, dehydrogenation, and cross-coupling reactions [10]. Thus, Pd NPs, with the highly catalytic activity and selectivity, have been successfully applied in the industrial catalysis production. However, the aggregation of Pd NPs during the catalytic reactions restrains their stability and performance [11]. A practical approach to prevent the aggregation of NPs during the catalytic reactions is to load them on a support such as carbon-based material with high surface area [12,13]. The metal-carbon interactions may tune the electronic structure of both materials, enhancing its catalytic activity [14]. Because the metal-carbon interactions are intrinsically weak, dopants such as nitrogen are usually incorporated into carbon to strengthen the interactions [15]. Hence, the synthesis of Pd NPs@N-doped carbon (Pd@NC) material with excellent catalytic activities has attracted great attentions (Table S1) [16]. These chemical routes to synthesize Pd@N-doped carbon materials generally involve multiple steps and inevitably produce wastes that are hazardous to the environment. Biosynthesis methods have been considered as an alternative to synthesize composite materials due to the advantages of low-cost, eco-friendliness, and non-toxic synthetic environment [17].

The objective of our study was to propose a green synthesis method for Pd@NC from Pd(II) solution using Pantoea sp. IMH. Multiple complementary techniques were used to characterize the biosynthesized Pd@NC. Pd@NC was employed in the reductive transformation of 4-NP, 2-NA, 4-NA, and 2,6-DCNA. In addition, XANES spectroscopy and DFT calculations were applied to explore the electronic structure of Pd@NC. The insights obtained from our work enhance the understanding on the biosynthesis of metal@carbon materials using microorganisms and catalytic mechanism.

Section snippets

Materials

Palladium chloride (PdCl2), 4-nitrophenol, 2-nitroaniline, 4-nitroaniline, 2,6-dichloro-4-nitroaniline, and sodium borohydride (NaBH4) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). The Milli-Q water (18.2 MΩ) was used to prepare solutions in the experiments.

Preparation of bacterial cells

Pantoea sp. IMH (JX861130) was employed in the study. The bacteria cells were grown to late-exponential phase in Luria − Bertani (LB) medium at 30 °C. Then cells were harvested by centrifugation (4000 g, 5 min), and were

Materials characterization

Herein, we report a novel biosynthesis for Pd@NC using Pantoea sp. IMH (Fig. 1). When mixed with cells, Pd(II) ions were either dispersed into the cytoplasm region, or trapped on the cell wall. Then, Pd(II) ions were reduced by proteins [21,22] to form Pd NPs extracellularly and intracellularly (Figure S1). Pd NPs along with cells were then calcined at 800 °C to obtain Pd@NC.

The morphology and structure of Pd@NC were characterized using multiple complementary techniques. The FE-SEM (Figure S2)

Conclusion

In sum, Pd@NC was synthesized via Pd(II) reduction by Pantoea sp. IMH followed by calcination. Multiple complementary characterizations verified that Pd NPs, with an average size of 7.6 nm, were evenly distributed in the nitrogen-doping carbon layers. The green-synthesized Pd@NC exhibited an elevated catalytic activity for the reductive transformation of nitroaromatic compounds. Pd@NC at a 3.3 mg L−1 dose exhibited the highest kPd value of 0.19 min-1 mg-1 L for the reduction of 4-NP. The

Acknowledgements

We acknowledge the financial support of the National Basic Research Program of China (2015CB932003), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB14020201), and the National Natural Science Foundation of China (41877378, 41425016, and 21337004).

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