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Materials | 2010

Nitrogen Doped Carbon Nanotubes from Organometallic Compounds: A Review

Edward N. Nxumalo; Neil J. Coville

Nitrogen doped carbon nanotubes (N-CNTs) have become a topic of increased importance in the study of carbonaceous materials. This arises from the physical and chemical properties that are created when N is embedded in a CNT. These properties include modified chemical reactivity and modified conductivity and mechanical properties. A range of methodologies have been devised to synthesize N-CNTs. One of the procedures uses a floating catalyst in which an organometallic complex is decomposed in the gas phase in the presence of a nitrogen containing reactant to give N-CNTs. Most studies have been limited to ferrocene, ring substituted ferrocene and Fe(CO)5. This review covers the synthesis (and properties) of N-CNTs and other shaped carbon nanomaterials (SCNMs) produced using organometallic complexes. It summarizes the effects that physical parameters such as temperature, pressure, gas flow rates, type and concentration of N source etc. have on the N-CNT type, size and yields as well as the nitrogen content incorporated into the tubes that are produced from organometallic complexes. Proposed growth models for N-CNT synthesis are also reported.


Chemcatchem | 2012

Palladium‐Supported Boron‐Doped Hollow Carbon Spheres as Catalysts for the Solvent‐free Aerobic Oxidation of Alcohols

Vilas Ravat; Isaac Nongwe; Neil J. Coville

Benzaldehyde is an important intermediate in the production of perfumes, pharmaceutical compounds, dyestuffs, and agrochemicals. It is commercially produced as a by-product of the oxidation of toluene into benzoic acid. It can also be produced by the hydrolysis of benzyl chloride. However, this latter process provides benzaldehyde that contains trace contamination of chlorine, which is not acceptable in the perfume and pharmaceutical industries. Several studies have been reported on the catalytic vapor-phase chlorine-free oxidation of benzyl alcohol into benzaldehyde. However, in the vapor phase oxidation process, a significant loss of carbon in the form of carbon oxides is a major problem. Other methods involve the use of toxic and expensive stoichiometric metal oxidants, such as manganese dioxide, chromate, and permanganate, and these compounds inevitably either lead to the formation of environmentally noxious waste products or to the disposal of harmful organic solvents. The development of effective catalytic routes for the aerobic oxidation of alcohols that use environmentally benign and inexpensive oxidants, such as oxygen or air, is an important challenge. Owing to the high cost of the noble metal catalysts that are required for many catalytic reactions, it is necessary to extendand enhance their catalytic performance by depositing them homogenously on a support to optimize their active surface. For this purpose, the choice of the support is of fundamental importance in influencing the stability of the metal nanoparticles. Carbon supports are excellent supports from an industrial point of view because they are stable in both acidic and basic environments and they allow facile recovery of the precious metals by burning off the carbon after catalyst deactivation. However, a drop in the surface area of the active metal, owing to leaching and/or aggregation, often limit the use of carbon as a support. Recently, carbon nanotubes (CNTs) have attracted a lot of attention as a support for metal nanoparticles. Compared to activated carbon supports that contain many inaccessible active sites, the deposition of metal particles on CNTs or on other shaped carbon materials affords advantageous substrates because most of the metal nanoparticles are expected to be exposed and accessible to the reactant. However, owing to the inertness of pristine carbon materials, a selective metal deposition procedure requires the activation of a carbon support. Jiang et al. observed that the introduction of nitrogen groups onto the surface of CNTs to give nitrogen-doped CNTs (N-CNTs) drastically increased the dispersion and stability of the metal nanoparticles, owing to strong metal–nitrogen interactions. A metal/N-CNT system gave a more active catalyst than metal that was loaded onto pristine CNTs when tested in different reactions. Furthermore, the enhancement in palladium adsorption was found to be more significant on a borondoped CNT than on a nitrogen-doped CNT. These studies concluded that both Nand B-doped carbon supports showed greater interactions with a supported metal compared with an undoped carbon support. To further study this effect, we synthesized a series of boron-doped hollow carbon spheres (B-HCSs). These materials were made by the reaction of BCl3 solution (1. M in heptanes), which led to the deposition of carbon and boron on silica spheres as a template. Removal of the silica and the addition of Pd gave Pd/B-HCSs. Recently, molecular oxygen has been used in many highly efficient catalytic aerobic alcohol oxidation reactions by using Cu, Pd, and Ru catalysts on different supports. Subsequently, some studies have been reported in which carbon was used as a catalyst support in these reactions. Carbon nanoshells are also potential metal free catalysts for the aerobic oxidation of alcohols. To test our new Pd/B-HCS catalyst, we investigated the solvent free oxidation of alcohols into their corresponding aldehydes under an O2 atmosphere (125 8C, atmospheric pressure). The B-HCS and Pd-loaded B-HCS samples were structurally characterized by using X-ray diffraction (XRD), TEM, thermogravimetric analysis (TGA), and inductively coupled plasma-optical emission spectrometry (ICP-OES). The XRD study of Pd/B-HCS samples that were calcined in air at different temperatures was performed to establish the presence of boron in the sample, the thermal stability of the carbon spheres, and the nature of the active Pd species. The XRD patterns (Figure 1) revealed that PdO (JCPDS file no.: 00-043-1024) was observed at all temperatures whereas Pd metal (JCPDS file No-01-088-2335) was only observed at higher temperatures. This result suggests that the carbon support reduces the Pd at high temperature in an oxygen environment. The intensity of the metallic Pd peak at 2 q= 408 is larger than that of PdO at 2 q= 33.638 in the Pd/ B-HCS700 catalyst. At a calcination temperature of 550 8C, diffraction peaks that were due to B2O3 (JCPDS file no.: 00-0130570) were noted, thus indicating that any B C bonds had been converted into B O bonds and that B2O3 had formed. The diffraction peaks of B2O3 were clearly observed when the [a] Dr. V. Ravat, I. Nongwe, Prof. N. J. Coville DST/NRF Centre of Excellence in Strong Materials and Molecular Sciences Institute, School of Chemistry University of the Witwatersrand Johannesburg WITS 2050 (South Africa) Fax: (+ 27) 11717-6749 E-mail : [email protected] [b] I. Nongwe Department of Chemistry, University of Johannesburg Johannesburg, Auckland Park, 2006 (South Africa)


Chemcatchem | 2010

Catalytic Activity of Metal Nanoparticles Supported on Nitrogen-Doped Carbon Spheres

Amit Deshmukh; Rafique Ul Islam; Michael J. Witcomb; Willem A. L. van Otterlo; Neil J. Coville

A support plays a crucial role in the activity and selectivity of catalysts. Heterogeneous catalysts are typically supported on inorganic oxides to enhance the surface area of the active sites. When compared to homogeneous catalysts, the supports also provide a method of separating the catalyst from the reactant/product at the end of the reaction. Carbon has been used as a support in many reactions because of its physicochemical properties that includes high surface area, thermal and mechanical stability, electronic behavior, low density, inertness, and, most importantly, its tailored structure. Carbon supports have thus been used extensively over many decades. Those used in the past, have typically been based on activated carbon. However, the recent interest in shaped carbon materials (tubes, horns, coils, onions, etc.), has led to much interest in the synthesis and properties of new carbon morphologies. For example, metals supported in particular on carbon nanotubes are being actively studied. Another form of carbon that has been used extensively in industry is the carbon sphere, but few reports on its use as a carbon support in catalysis have been described to date. Nitrogen-doped carbon spheres (N–CSs), less than 1 mm in diameter, have been synthesized and used as a support in a few studies. For example, Jang et al. reported the use of Pd supported on nitrogen doped magnetic carbon nanoparticles (Pd on a high surface area materials containing Fe) as a catalyst for C C bond forming reactions. Herein, we report on the utilization of N–CSs as supports for Ru, Pd, and V catalysts in chemoselective hydrogenation, C C bond formation (Heck, Suzuki), and oxidation reactions respectively. The doped carbon spheres were easily prepared, low-surface-area materials and formed strong bonds to the metals, leading to high activities and selectivities. In every case, the key feature related to the size of the metal nanoparticles stabilized on the spheres. We have previously reported a synthetic procedure for the fabrication of carbon spheres (CSs) by chemical vapor deposition (CVD). In brief, CSs were prepared using acetylene as the carbon source at a carbonization temperature of 900 8C for 2 h. To prepare the nitrogen-doped CSs, the process was modified to allow the acetylene gas to bubble through an ammonium hydroxide or acetonitrile solution (preferably ammonium hydroxide; see the Supporting Information for details of the synthesis). Elemental analysis of the as-synthesized material indicated the presence of 1–1.5 wt% nitrogen. The material obtained after CVD was subjected to an acid treatment with 1:3 concentrated HNO3/H2SO4 (30 mL per 0.5 g of CSs at 70 8C for 3 h). Acid-treated CSs were then washed with water until the filtrate was pH 7 and dried at 80 8C. Metal loading (3 wt%) was then carried out by a wet impregnation method. The N–CSs and metal-loaded N–CSs were structurally characterized by transmission electron microscopy (TEM; Figure 1A– C), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray (EDX) spectroscopy (Figure 1D–F). The N–CSs had diameters ranging from 400 to 750 nm (see the Supporting Information for SEM image) and were accreted (Figure 1C, inset). The morphology of the spheres remained unaffected after acid treatment and metal loading. The average diameter of the metal particles loaded on the carbon was determined from TEM analysis. EDX analysis revealed almost 2.7wt% loading for all metals (Figure 1). TEM images for Ru/N–CSs and Pd/N–CSs showed average particle sizes of 3–5 and 2–4 nm, respectively, whereas for V/N–CSs vanadium oxide formed a film over the N–CSs. Further details of the catalyst characterization will be reported elsewhere. The new catalysts were investigated for chemoselective hydrogenation of diketones (Table 1), Heck and Suzuki carbon– carbon coupling reactions (Tables 2 and 3), and oxidation of styrene (Table 4). The catalysts showed impressive activity and recyclability in all reactions. In addition, all of the reactions were performed under mild reaction conditions. Catalytic methods, utilizing molecular hydrogen to reduce olefins, ketones, and imines, is in widespread industrial practice and is relatively straightforward. However, hydrogenation of diketones to ketols (monohydroxyketones) is a more challenging task. Ketols are of immense importance for the synthesis of fine chemicals, as they provide two different functionalities for chemical manipulation within the same molecule. Several attempts have been made to hydrogenate diketones using supported cinchonidine , 2,2’-bis(diphenylphosphino)-1,1’binaphthyl (binap), and 1,5-cyclooctadiene (cod) metal complexes. Recently, a gold catalyst supported on titanium oxide was reported to effect chemoselective transfer hydrogenation of a diketone to a ketol with conversion and selectivity [a] Dr. A. A. Deshmukh, Dr. R. Ul Islam, Prof. W. A. L. van Otterlo, Prof. N. J. Coville Molecular Science Institute, and School of Chemistry University of Witwatersrand, Private Bag No. 3 Johannesburg 2050 (South Africa) Fax: (+27)11-7176749 E-mail : [email protected] [b] Dr. A. A. Deshmukh, Prof. M. J. Witcomb, Prof. N. J. Coville DST/NRF Centre of Excellence in Strong Materials University of Witwatersrand, Private Bag No. 3 Johannesburg 2050 (South Africa) [c] Prof. M. J. Witcomb Microscopy and Microanalysis Unit, University of Witwatersrand Private Bag No. 3, Johannesburg 2050 (South Africa) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.200900224.


Journal of Materials Chemistry | 2015

UV-assisted synthesis of indium nitride nano and microstructures

Mahalieo Kao; R.M. Erasmus; Nosipho Moloto; Neil J. Coville; Sabelo D. Mhlanga

Indium nitride (InN) has been made the first time by a combined thermal/UV photo-assisted process. Indium oxide (In2O3) was reacted with ammonia using two different procedures in which either the ammonia was photolysed or both In2O3 and ammonia were photolysed. A wide range of InN structures were made by these procedures that were determined by the reaction conditions (time, temperature). The reaction of In2O3 with photolysed NH3 gave InN rod-like structures that were made of stacked cones (6 h/750 °C) or discs (6 h/800 °C) and that contained some In2O3 residue. In contrast, photolysis of both In2O3 and NH3 gave InN nanowires and pure InN nanotubes filled with In metal (>90%). The transformation of the 3D In2O3 particles to the tubular 1D InN was monitored as a function of time (1–4 h) and temperature (700–800 °C); the product formed was very sensitive to temperature. The band gap of the In filled InN nanotubes was found to be 1.89 eV.


Electrocatalysis | 2015

The Effect of Reducing Agents on the Electronic, Magnetic and Electrocatalytic Properties of Thiol-Capped Pt/Co and Pt/Ni Nanoparticles

Ntombizodwa R. Mathe; Steven S. Nkosi; D.E. Motaung; Manfred Scriba; Neil J. Coville

The electronic, magnetic and electrocatalytic properties of bimetallic thiol-capped Pt/Co and Pt/Ni nanoparticles were synthesised using two reducing agents, NaBH4 and N2H4. X-ray diffraction analysis of the nanoparticles showed Pt lattice contraction upon the addition of Co or Ni to Pt indicating the formation of an alloy structure, more apparent when N2H4 was used. XPS data analysis revealed Pt metal and Pt(II) (assigned to PtO) and a higher concentration of surface metallic Ni and Co for the NaBH4-reduced samples. Both the NaBH4- and N2H4-reduced catalysts were active for the methanol oxidation reaction (MOR), with the Pt-Co-N2H4 catalyst revealing the highest activity. The N2H4 significantly affected the magnetic properties of Pt/Co and Pt/Ni particles by controlling the morphology and crystalline structure of the nanoparticles. In general, the type of reducing agent affected the final properties of the nanoparticles.


Applied Catalysis A-general | 2005

Fischer–Tropsch synthesis over iron catalysts supported on carbon nanotubes

Munga C. Bahome; Linda L. Jewell; Diane Hildebrandt; David Glasser; Neil J. Coville


Journal of Advanced Research | 2012

The synthesis, properties and uses of carbon materials with helical morphology

Ahmed Shaikjee; Neil J. Coville


South African Journal of Science | 2011

A review of shaped carbon nanomaterials

Neil J. Coville; Sabelo D. Mhlanga; Edward N. Nxumalo; Ahmed Shaikjee


Nanoscale | 2013

Incorporation of small BN domains in graphene during CVD using methane, boric acid and nitrogen gas

George Bepete; Damien Voiry; Manish Chhowalla; Zivayi Chiguvare; Neil J. Coville


International Journal of Hydrogen Energy | 2014

Methanol oxidation reaction activity of microwave irradiated and heat-treated Pt/Co and Pt/Ni nano-electrocatalysts

Ntombizodwa R. Mathe; Manfred Scriba; Neil J. Coville

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Ahmed Shaikjee

University of the Witwatersrand

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Edward N. Nxumalo

University of South Africa

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Sabelo D. Mhlanga

University of the Witwatersrand

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George Bepete

University of the Witwatersrand

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Isaac Nongwe

University of the Witwatersrand

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Linda L. Jewell

University of South Africa

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Ntombizodwa R. Mathe

University of the Witwatersrand

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Vilas Ravat

University of the Witwatersrand

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Manfred Scriba

Council for Scientific and Industrial Research

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Amit Deshmukh

University of the Witwatersrand

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