A review of shaped carbon nanomaterials

Materials made of carbon that can be synthesised and characterised at the nano level have become a mainstay in the nanotechnology arena. These carbon materials can have a remarkable range of morphologies. They can have structures that are either hollow or filled and can take many shapes, as evidenced by the well-documented families of fullerenes and carbon nanotubes. However, these are but two of the shapes that carbon can form at the nano level. In this review we outline the types of shaped carbons that can be produced by simple synthetic procedures, focusing on spheres, tubes or fibres, and helices. Their mechanisms of formation and uses are also described

Origin of conductivity cross over in entangled multi-walled carbon nanotube network filled by iron

A realistic transport model showing the interplay of the hopping transport between the outer shells of iron filled entangled multi-walled carbon nanotubes (MWNT) and the diffusive transport through the inner part of the tubes, as a function of the filling percentage, is developed. This model is based on low-temperature electrical resistivity and magneto-resistance (MR) measurements. The conductivity at low temperatures showed a crossover from Efros-Shklovski (E-S) variable range hopping (VRH) to Mott VRH in 3 dimensions (3D) between the neighboring tubes as the iron weight percentage is increased from 11% to 19% in the MWNTs. The MR in the hopping regime is strongly dependent on temperature as well as magnetic field and shows both positive and negative signs, which are discussed in terms of wave function shrinkage and quantum interference effects, respectively. A further increase of the iron percentage from 19% to 31% gives a conductivity crossover from Mott VRH to 3D weak localization (WL). This change is ascribed to the formation of long iron nanowires at the core of the nanotubes, which yields a long dephasing length (e.g. 30 nm) at the lowest measured temperature. Although the overall transport in this network is described by a 3D WL model, the weak temperature dependence of inelastic scattering length expressed as L_phi ~T^-0.3 suggests the possibility for the presence of one-dimensional channels in the network due to the formation of long Fe nanowires inside the tubes, which might introduce an alignment in the random structure.Comment: 29 pages,10 figures, 2 tables, submitted to Phys. Rev.

Two classes of nonlocal Evolution Equations related by a shared Traveling Wave Problem

We consider reaction-diffusion equations and Korteweg-de Vries-Burgers (KdVB) equations, i.e. scalar conservation laws with diffusive-dispersive regularization. We review the existence of traveling wave solutions for these two classes of evolution equations. For classical equations the traveling wave problem (TWP) for a local KdVB equation can be identified with the TWP for a reaction-diffusion equation. In this article we study this relationship for these two classes of evolution equations with nonlocal diffusion/dispersion. This connection is especially useful, if the TW equation is not studied directly, but the existence of a TWS is proven using one of the evolution equations instead. Finally, we present three models from fluid dynamics and discuss the TWP via its link to associated reaction-diffusion equations

Maximum principles for nonlocal parabolic Waldenfels operators

As a class of Levy type Markov generators, nonlocal Waldenfels operators appear naturally in the context of investigating stochastic dynamics under Levy fluctuations and constructing Markov processes with boundary conditions (in particular the construction with jumps). This work is devoted to prove the weak and strong maximum principles for ‘parabolic’ equations with nonlocal Waldenfels operators. Applications in stochastic differential equations with α-stable Levy processes are presented to illustrate the maximum principles

Post doped nitrogen-decorated hollow carbon spheres as a support for Co Fischer-Tropsch catalysts

In this study the outer surface of porous hollow carbon spheres (HCSs) materials were functionalized by N-doping using a post-synthesis method and they were used as a Fischer-Tropsch catalyst support. Melamine was used as the nitrogen source, and carbonization was performed at different temperatures (600 and 900 °C) to introduce variable levels of N into the HCSs, with different bonding configurations. This procedure allowed for the incorporation of up to 13% N. Our results show that post-synthesis N-doping introduced marginal defects into the carbon framework and this did not affect the thermal stability of the materials. XPS studies revealed that the surface content on these materials varied and provided evidence for temperature-tunable bonding configurations. Effects associated with post-synthesis N-doping were apparent on the Co catalyst (˜10 wt.%) properties such as the inhibited reduction caused by a metal-support interaction observed by the H2-TPR and in situ PXRD techniques. As a consequence the Fischer-Tropsch performance was influenced as both the activity and stability were improved on the catalysts supported on the N-doped materials. TEM analysis of the spent catalysts demonstrated the influence of N-doping on the sintering characteristics of Co, with particles > 30 nm measured on the N-free catalyst while N-doped samples had sizes < 15 nm

Effect of a titania covering on CNTS as support for the Ru catalysed selective CO methanation

One of the major set-backs in the selective CO methanation process, as the final clean-up step in removing residual CO from reformate gas feed, is the reverse water gas shift (RWGS) reaction. This reaction is an undesired reaction because, it runs parallel with the selective CO methanation reaction. This increases the CO outlet concentration. The catalytic performance of ruthenium supported on carbon nanotubes (CNTs), nitrogen doped carbon nanotubes (NCNTs), titania coated carbon nanotubes (NCNT-TiO2 and CNTs-TiO2) and TiO2 anatase (TiO2-A) for selective CO methanation was investigated. The feed composition relevant to reformate gas was used but in the absence of steam. The experiments were conducted within a temperature range of 100 °C and 360 °C. It was observed that carbon dioxide methanation was suppressed until CO methanation attained a maximum conversion for all the catalysts studied. The Ru/NCNT showed higher activity than Ru/CNT at all temperatures examined due to the nitrogen incorporation in the carbon domains. Both Ru/CNT and Ru/NCNT however promoted the RWGS reaction at temperatures above 250 °C. The Ru/CNT-TiO2 catalyst recorded the highest activity for both the CO and selective CO methanation followed by Ru/TiO2-A. The presence of titania on the carbon nanotubes significantly retarded the RWGS reaction from about −120% CO conversion to about 80% CO conversion, while selectivity towards methane increased in all catalysts with increasing temperature

A sinter resistant Co Fischer-Tropsch catalyst promoted with Ru and supported on titania encapsulated by mesoporous silica

One of the pathways responsible for the deactivation of Fischer-Tropsch catalysts is the loss of active metal surface area due to nanoparticle agglomeration. To combat this effect efforts have been made to increase the interaction between the metal nanoparticles and the support using materials like silica. In this study, the supported metal particles were covered with a highly porous layer of silica to stabilize the Co nanoparticles on a titania support both during reduction and under reaction conditions. Co3O4 nanoparticles (size range: 8–12 nm) supported on titania were stabilized by coating them with a thin layer of mesoporous silica ( ∼ 4 nm) to make Fischer-Tropsch catalysts that are less prone to sintering (Co/TiO2@mSiO2). To mitigate the strong metal support interactions brought about by the titania and silica a Ru promoter was loaded together with the cobalt nanoparticles onto the titania (CoRu/TiO2@mSiO2). Temperature programmed XRD studies on the evolution of the Co metal nanoparticles showed that there was no significant particle size growth under reduction conditions in the temperature range from 30 to 600 °C. Chemisorption studies following reduction under hydrogen at 350 °C and 450 °C gave results consistent with the in situ XRD data when compared to the Co/TiO2. Fischer-Tropsch synthesis on the Co/TiO2@mSiO2 and CoRu/TiO2@mSiO2 catalysts encapsulated inside the mesoporous silica shell exhibited good catalytic performance without any display of significant mass transport limitations that might arise due to a silica shell coating of the active sites. For these two catalysts the Fischer-Tropsch activity increased with reduction temperature without any significant negative changes in their selectivity due to sintering, while the activity on Co/TiO2 decreased due to Co nanoparticle sintering

From waste cooking oil to oxygen-rich onion-like nanocarbons for the removal of hexavalent chromium from aqueous solutions

Vegetable cooking oil is used in domestic and commercial kitchens owing to its ability to modify and enhance the taste of the food through the frying process. However, as the oil is used through several frying cycles, it changes colour to dark brown and acquires an unpleasant smell. At this point, the waste oil is usually discarded, thereby finding its way into freshwater streams due to poor disposal and thus becoming an environmental pollutant. To provide an alternative, ‘green’ route to waste oil disposal, herein we report on the metal-free synthesis of onion-like nanocarbons (OLNCs) made from waste cooking oil via flame pyrolysis. The OLNCs were then applied in the removal of hexavalent chromium ions from aqueous solutions. The as-synthesised OLNCs were found to have similar properties (size, quasi-spherical shape etc.) to those synthesised from pure cooking oils. The Fourier-transform infrared spectroscopy data showed that the OLNCs contained C-O-type moieties which were attributed to the oxygenation process that took place during the cooking process. The OLNCs from waste oil were applied as an adsorbent for Cr(VI) and showed optimal removal conditions at pH = 2, t = 360 min, Co = 10 mg/L and Q0max = 47.62 mg/g, superior to data obtained from OLNCs prepared from pristine cooking oil. The results showed that the OLNCs derived from the waste cooking oil were effective in the removal of hexavalent chromium. Overall, this study shows how to repurpose an environmental pollutant (waste cooking oil) as an effective adsorbent for pollutant (Cr(VI)) removal. Significance: • Waste cooking oil outperformed olive oil as a starting material for the production of OLNCs for the removal of toxic Cr(VI) from water. • The superior performance of the OLNCs from waste cooking oil was attributed to the higher oxygen content found on their surface and acquired through the cooking process. • Not only are the OLNCs produced from waste cooking oil effective in the removal of Cr(VI), but they can be used multiple times before replacement, which makes them sustainable

The "silver" Japanese quail and the MITF gene: causal mutation, associated traits and homology with the "blue" chicken plumage

<p>Abstract</p> <p>Background</p> <p>The <it>MITF </it>(<it>microphthalmia-associated transcription factor</it>) gene has been investigated in mice and various vertebrates but its variations and associated effects have not yet been explored much in birds. The present study describes the causal mutation <it>B </it>at the <it>MITF </it>gene responsible for the "silver" plumage colour in the Japanese quail (<it>Coturnix japonica</it>), and its associated effects on growth and body composition, and tests its allelism with the "blue" plumage colour mutation <it>Bl </it>in <it>Gallus gallus</it>.</p> <p>Results</p> <p>The semi dominant <it>B </it>mutation results from a premature stop codon caused by a 2 bp deletion in exon 11 of <it>MITF</it>. Homozygous "white" (<it>B/B</it>) quail which have a white plumage also show a slightly lower growth, lower body temperature, smaller heart, and lighter <it>pectoralis </it>muscles but more abdominal adipose tissue than the recessive homozygous "wild-type" (<it>+/+</it>) and heterozygous "silver" (<it>B/+</it>) quail. Similar observations on cardiac and body growth were made on mice (<it>Mus musculus</it>) homozygous for mutations at <it>MITF</it>. The production of chicken-quail hybrids with a white plumage obtained by crossing <it>Bl/+ </it>chicken heterozygous for the <it>blue </it>mutation with <it>B/B </it>white quail indicated that the mutations were allelic.</p> <p>Conclusion</p> <p>The "silver" Japanese quail is an interesting model for the comparative study of the effects of <it>MITF </it>in birds and mammals. Further investigation using a chicken family segregating for the "blue" plumage and molecular data will be needed to confirm if the "blue" plumage in chicken results from a mutation in <it>MITF</it>.</p