Abstract
Cobalt hierarchical graphitic carbon nanoparticles (Co@HGC) (1), (2), and (3) were prepared by simple pyrolysis of a cobalt phenanthroline complex in the presence of anthracene at different temperatures and heating times, under a nitrogen atmosphere. The samples were used for the catalytic hydrogenation of 2,4-dinitrophenol. Samples (1) and (3) were prepared by heating at 600 °C and 800 °C respectively, while (2) was prepared by heating at 600 °C with an additional intermediate stage at 300 °C. This work revealed that graphitization was catalyzed by cobalt nanoparticles and occurred readily at temperatures of 600 °C and above. The nanocatalysts were characterized by Scanning Electron Microscopy SEM, energy dispersive X-ray analysis EDX, Raman, Xrd, and XPS. The analysis revealed the presence of cobalt and cobalt oxide species as well as graphitized carbon, while TEM analysis indicated that the nanocatalyst contains mainly cobalt nanoparticles of 3–20 nm in size embedded in a lighter graphitic web. Some bamboo-like multiwall carbon nanotubes and graphitic onion-like nanostructures were observed in (3). The structures and chemical properties of the three catalysts were correlated with their catalytic activities. The apparent rate constants kapp (min−1) of the 2,4-dinitrophenol reductions were 0.34 for (2), 0.17 for (3), 0.04 for (1), 0.005 (no catalyst). Among the three studied catalysts, the highest rate constant was obtained for (2), while the highest conversion yield was achieved by (3). Our data show that an increase in agglomeration of the cobalt species reduces the catalytic activity, while an increase in pyrolysis temperature improves the conversion yield. The nanocatalyst enhances hydrogen generation in the presence of sodium borohydride and reduces 2,4-dinitrophenol to p-diamino phenol. The best nanocatalyst (3) was prepared at 800 °C. It consisted of uniformly distributed cobalt nanoparticles sheltered by hierarchical graphitic carbon. The nanocatalyst is easily separated and recycled from the reaction system and proved to be degradation resistant, to have robust stability, and high activity towards the reduction reaction of nitrophenols.
Highlights
We investigated the catalytic reduction of 2,4-dinitrophenol using cobalt
We have previously reported the preparation of porous cobalt–carbon nanostructures by pyrolysis of cobalt complexes and studied their applications in water treatment
Since the presence of chlorine represents only 10% of the content of cobalt in the catalysts, the catalytic activity depends more on the cobalt–carbon nanostructure, which varies from catalyst to catalyst
Summary
Carbon nanotubes (CNTs) were prepared with cobalt catalyst by arc discharge [7], pulsed laser vaporization [8], and chemical vapor deposition (CVD) [9] These methods face the challenge to produce carbon nanostructures in large amounts with specific morphologies that serve unique applications. They are sophisticated and expensive, requiring catalyst, inert gas, carbon-source gas like acetylene and reducing hydrogen gas. Cobalt oxide N-graphene/activated carbon nanomaterials Co3 O4 -NGr@C prepared by pyrolysis of cobalt pyridine derivatives acted as a selective catalyst for the hydrogenation of nitroarene derivatives under demanding conditions (at 100 ◦ C for 11–17 h) in THF solvent, using triethylamine Et3 N and formic acid as the hydrogen source [21]. Carbon nanoparticles Co@HGC prepared by pyrolysis of a cobalt phenanthroline complex under different pyrolysis temperatures in the presence of additional carbon sources
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