Elsevier

Science of The Total Environment

Volume 612, 15 January 2018, Pages 103-110
Science of The Total Environment

Synthesis of cobalt-impregnated carbon composite derived from a renewable resource: Characterization and catalytic performance evaluation

https://doi.org/10.1016/j.scitotenv.2017.08.187Get rights and content

Highlights

  • Fabrication of nitrogen-doped Co-biochar derived from glucose (Co-NB).

  • Homogeneous dispersion of metallic Co nanoparticles on the Co-NB surface

  • Catalytic activity of Co-NB in converting p-nitrophenol into p-aminophenol

  • Robust reusability of the Co-NB

Abstract

A novel nitrogen-doped biochar embedded with cobalt (Co-NB) was fabricated via pyrolysis of glucose pretreated with melamine (N donor) and Co(II). The Co-NB showed high catalytic capability by converting p-nitrophenol (PNP) into p-aminophenol (PAP) by NaBH4. The analyses of FE-SEM, TEM, BET, XRD, Raman, and X-ray photoelectron spectroscopy XPS of the Co-NB showed hierarchical porous structure (BET 326.5 m2 g 1 and pore volume: 0.2403 cm3 g 1) with well-dispersed Co nanoparticles (20–60 nm) on the N-doped graphitic biochar surface. The Co-NB showed higher PNP reduction capability compared to the Co-biochar without N-doping, achieving 94.3% removal within 4 min at 0.24 g L 1 catalyst dose and initial concentration of 0.35 mM PNP. Further conversion experiments under varying environmental conditions (e.g., NaBH4 concentration (7.5–30 mM), biochar dosage (0.12–1.0 g L 1), initial PNP concentration (0.08–0.17 mM)) were conducted in batch mode. The reusability of Co-NB was validated by the repetitive conversion experiments (5 cycles). The overall results demonstrated biochar potential as catalysts for environmental applications if properly designed.

Introduction

Catalysts have been frequently utilized in various fields of chemistry, physics and biology due to their high reactivity and efficiency. However, there can be a considerable loss of catalytic activity resulting from particle aggregation and instability due to inherent high surface energy, especially in the homogenous catalyst system (Wang et al., 2017). To resolve this issue, efforts have been made to develop heterogeneous catalysts with improved recyclability and sustainability (Bennett et al., 2016, Joshi et al., 2016, Roschat et al., 2016). The use of heterogeneous catalysts to promote desired reactions for various applications such as organic synthesis, conversion to value-added products, and degradation of pollutants has received considerable attention (Hajduk et al., 2017, Liu et al., 2017; J. Liu et al., 2017, Taifan and Baltrusaitis, 2016, Wang et al., 2014).

Many studies have been reported on the catalytic conversion of target chemicals using catalysts containing noble metals (Pt, Au, Ag, Rh, Ru, Pd, etc.) (Fang et al., 2017, Louie et al., 2017, Song et al., 2017). However, despite their high performances, noble metals-based catalysts are seldom used in scaled-up applications because of high cost and limited availability of the noble metals.

In recent, the potential utility of cobalt (Co) in triggering catalytic reactions has been examined as an alternative to the noble metals due to its low cost. For example, Hasan et al. (2016) fabricated a magnetic Co‑carbon composite via calcination of cobalt-based metal organic frameworks (MOFs) and demonstrated its catalytic capability in converting p-nitrophenol (PNP) into p-aminophenol (PAP), which is a useful chemical for pharmaceuticals, dyestuffs, and agrochemicals (Guo et al., 2016). In addition, Co-loaded polymers with hierarchical porous structures were prepared as a stable catalyst for PNP conversion into PAP (Zhao et al., 2016).

Carbon-based materials have gained much attention for use as support materials for catalytic metals to provide structural integrity and other desirable properties. For instance, activated carbon supports with high surface area and hierarchical porous structure have shown excellent conversion and selectivity in catalytic applications (Mouat et al., 2016, Si et al., 2017). Carbon materials prepared with renewable resources have been recently employed in the preparation of catalytic materials (Lee et al., 2017, Liu et al., 2016, Yao et al., 2016). Our previous work has used lignin as a carbon precursor to synthesize Co-biochar catalyst composite and validated its great catalytic performance in conversion of bromate (Cho et al., 2017). Xu et al. (2015) proposed environmentally benign method to prepare 3D porous graphene by converting waste papers mixed with a cobalt(II) complex. With such efforts, utilizing biomass- or biowaste-derived carbonaceous materials (i.e., biochar) is evolving into a viable option in production of environmental catalysts. In addition, a good number of recent studies have explored the potential of biochar for the use in various catalytic/environmental applications (Dehkhoda and Ellis, 2013, Kastner et al., 2015, Mohan et al., 2014, Ren et al., 2014).

Glucose derived from the hydrolysis of cellulose is a common carbon source which has merits of low cost, easy access, and fast regeneration (Tian et al., 2016). Thus, glucose was chosen as a carbon support precursor for the synthesis of biochar-based Co catalyst. Recently, nitrogen (N) doping onto the catalysts has showed enhanced catalytic performances as compared to the catalysts without N-doping (Wang et al., 2015). Blending N-containing ligands with carbon precursor followed by pyrolysis is a facile method to provide N dopant to the catalyst, and therefore melamine that contains high amount of nitrogen was used as N donor in this work. The biochar was characterized by spectroscopic and surface analyses. The catalytic ability of the biochar was compared with that of the biochar without N doping. A series of PNP conversion experiments were performed under varying experimental parameters such as NaBH4 concentration, composite dosage, and initial PNP concentration. The reusability of the biochar was also examined to demonstrate the longevity of catalytic ability of the biochar.

Section snippets

Chemical reagents & materials

Cobalt chloride hexahydrate (CoCl2  6H2O, 99%), D-(+)-Glucose (C6H12O6) and melamine (C3H6N6, 99%) were obtained from Sigma Aldrich, USA. p-Nitrophenol (PNP, C6H5NO3, 99%) and sodium borohydride (NaBH4) were purchased from Alfa Aesar, USA and Daejung Chemical, Korea, respectively. Solutions were prepared with ultrapure deionized distilled water (DDW, 18.2  cm 1).

Synthesis of biochar catalyst

The mixture solution of Co/glucose/melamine was prepared by adding 1 g CoCl2  6H2O, 4 g glucose and 1 g melamine into 20 mL DDW. An alumina

Physicochemical properties of Co-NB

To explore the morphology and particle distribution of Co-NB, FE-SEM and TEM imaging along with elemental mapping by EDS were used. The results presented in Fig. 1 reveal that Co-NB possessed irregular layers of thin carbon support on which nanoparticles in the size range of 20–60 nm were evenly deposited (Fig. 1a, b). The elemental mapping of Co-NB in the EDS analysis exhibited 94.57 wt% C, 1.56 wt% O, and 3.87 wt% Co (Fig. 1c, d, e), suggesting carbonization (i.e., graphitization) thermally

Conclusions

In this study, N-doped Co biochar was prepared by pyrolysis of mixture of glucose, Co(II), and melamine in a single process. The Co nanoparticles (20–60 nm) evenly dispersed on the carbon surface of the biochar were found in TEM/EDS mapping. The analyses of XRD, XPS, Raman and BET revealed the metallic Co phases (CoO and Co0), N-doped graphitic carbon layers (pyridinic N and pyrrolic N), and highly hierarchical porous structure. Compared to the Co biochar without N-doping, the enhancement of PNP

Acknowledgement

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1A09000800).

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