Preparation of α-MnO 2 nanowires and its application in low temperature CO oxidation

α-MnO₂ nanowires were synthesized through a simple hydrothermal route and employed as support to obtain CuO/α-MnO₂ catalysts in low temperature CO oxidation. The prepared samples were characterized by X-ray diffraction (XRD), N₂ adsorption (BET), temperature programmed reduction (TPR), thermal gravimetric/differential thermal analysis (TGA/DTA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The results showed that the prepared samples have nanowire morphology with a size about 30–40 nm in diameter. The obtained results revealed a remarkably high activity for the prepared catalysts in low temperature CO oxidation.     

Continuous electrodeposition of polypyrrole on carbon nanotube–carbon fiber hybrids as a protective treatment against nanotube dispersion

This paper describes the electrodeposition of nanometer size polypyrrole layers on carbon fibers coated with multi-wall carbon nanotubes. The obtained carbon nanotubes–carbon fiber hybrids are characterized by electrochemistry, electron microscopy, vibrational spectroscopy and thermogravimetry. The electrical properties are measured for various polymerization times and electrolytes. The conductivity is found decrease with increasing polypyrrole thickness, but all the carbon nanotube–carbon fiber hybrids remain more conductive than pristine carbon fibers having a sizing coating. Finally it is shown that polypyrrole acts as a protecting layer against carbon nanotube dispersion when sonicated in ethanol.     

In situ formation of noble metal nanoparticles on multiwalled carbon nanotubes and its implication in metal–nanotube interactions

A simple method to decorate multiwalled carbon nanotubes (MWCNTs) with Au, Ag and Cu nanoparticles is illustrated. The method consists in directly depositing the selected metals by thermal evaporation on the carbon nanotubes. Comparative measurements carried out on samples that differ in the quantity and type of the deposited metal, reveal that isolated discrete particles form on the nanotube outer wall for all three metals. The CNT-based composites have been investigated by scanning and transmission electron microscopy to determine the size, shape and distribution of the nanoparticles. The results indicate that the quantity of evaporated metal only affects the nanoparticle size and not the average particle density. Particle composition was determined by X-ray photoelectron spectroscopy study. The results are discussed in terms of metal nanoparticle–tube interactions, an important issue for the fundamental and practical applications of similar MWCNT based composites.     

Synthesis of a graphene–carbon nanotube composite and its electrochemical sensing of hydrogen peroxide

Graphene, whose structure consists of a single layer of sp²-hybridized carbon atoms, provides an excellent platform for designing composite nanomaterials. In this study, we have demonstrated a facile process to synthesize graphene–multiwalled carbon nanotube (MWCNT) composite. The graphene–MWCNT composite material is endowed with a large electrochemical surface area and fast electron transfer properties in Fe(CN)₆^3?/4? redox species. A graphene–MWCNT composite modified electrode exhibits good performance in terms of the electrocatalytic reduction of H₂O₂; a sensor constructed from such an electrode shows a good linear dependence on H₂O₂ concentration in the range of 2 × 10^?5 to 2.1 × 10^?3 mol L^?1. The detection limit is estimated to be 9.4 × 10^?6 mol L^?1. This study provides a new kind of composite modified electrode for electrochemical sensors.     

Effect of pH and carbon nanotube content on the corrosion behavior of electrophoretically deposited chitosan–hydroxyapatite–carbon nanotube composite coatings

In the first stage, chitosan (CH)–hydroxyapatite (HA)-multiwalled carbon nanotube (MWCNT) composite coatings were synthesized by electrophoretic deposition technique (EPD) on 316L stainless steel substrates at different levels of pH and characterized by X-ray diffraction (XRD), Raman spectroscopy, FTIR and field emission scanning electron microscopy (FESEM). A smooth distribution of HA and MWCNT particles in a chitosan matrix with strong interfacial bonding was obtained. In the next stage, effects of pH and MWCNT content of the suspension on the corrosion behavior and deposition mechanism were studied. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) curves revealed that increasing pH level of the suspension increases the corrosion protection properties of the deposited composite coating in simulated body fluid (SBF). Furthermore, Nyquist plots showed that increasing MWCNT content of the suspension resulted in higher amounts of R_p, but because of the capillary properties of MWCNTs and degradability of the chitosan matrix, corrosion protection level of the coatings containing HA–CH–MWCNT was lower than those of coatings containing solely HA–CH. Amperometric curves in different pH levels of the suspension revealed that the system is diffusion controlled at elevated pH values.     

Infrared hemispherical reflectance of carbon nanotube mats and arrays in the 5–50 μm wavelength region

We present the absolute infrared (5–50 μm) hemispherical reflectance of films produced from commercially available carbon nanotubes. Spectra were obtained with the NPL directional-hemispherical reflectance measurement facility. One group of samples consisted of mats of carbon nanotubes sprayed on copper or silicon substrates. Another group consisted of vertically aligned carbon nanotubes grown on silicon. Two of the materials studied exhibited the lowest hemispherical reflectance so far observed in the infrared wavelength region.     

Robust vertically aligned carbon nanotube–carbon fiber paper hybrid as versatile electrodes for supercapacitors and capacitive deionization

We developed a facile method to transfer vertically aligned carbon nanotubes (VACNT) onto a carbon fiber paper (CFP) substrate using silicone as an adhesion layer. The resulting VACNT–CFP hybrid structure displays a very low interface resistance of 0.006 Ω as a result of the Van der Waals interaction at the VACNT–CFP interface. We demonstrated in this work that the VACNT–CFP can be used as a supercapacitor electrode with excellent rate performance and cycling stability, which could be attributed to the vertically alignment and chemical stability of CNTs in the hybrid structure. Further, we found that VACNT–CFP showed good resistance to the impact of flowing water when tested as a capacitive deionizer electrode, owing to superior mechanical robustness of the hybrid structure.     

High efficiency preparation of standard grade γ-tocopherol and its antioxidant activity

Natural standard grade γ-tocopherol is of great interest due to its good antioxidant activity and special pharmacological functions. This study separated and purified γ-tocopherol from mixed tocopherols using eco-friendly and low toxicity solvents. γ-Tocopherol (0.28 g) was obtained per performance with a purity of (98.89 ± 0.68)% and a recovery rate of (93.23 ± 0.89)%. The purification conditions were as follows: elution solvent n-hexane/ethyl acetate 94.5:5.5 (v/v), sample load 0.5 g, a column height to diameter ratio of 16:1 (length and diameter, 60 × 3 cm) and an elution rate of 2 ml/min. The purity and structure of γ-tocopherol was confirmed by high-performance liquid chromatography, gas chromatography/mass spectrometry and nuclear magnetic resonance. The adsorption isotherm of γ-tocopherol on silica gel (200–300 mesh) fitted the Langmuir isotherm model well. The antioxidant activity analysis showed that γ-tocopherol had the strongest antioxidant activity followed by δ-tocopherol and α-tocopherol. This study provides an economically attractive solution for the production of natural standard grade γ-tocopherol.     

Solvothermal fabrication of three-dimensionally sphere-stacking Sb–SnO2 electrode based on TiO2 nanotube arrays

TiO₂-NTs-based Sb–SnO₂ electrode with three-dimensionally sphere-stacking structure was successfully fabricated by the solvothermal method, followed by annealing at 500 °C for 1 h. The physico-chemical properties of electrodes were characterized through scanning electron spectroscopy (SEM), X-ray diffraction (XRD) and electrochemical measurements. SEM result showed that TiO₂-NTs/Sb–SnO₂ electrode has morphology of vertically sphere-stacking coralline. Compared with Ti/Sb–SnO₂, TiO₂-NTs/Sb–SnO₂ electrode has smaller grain size and greater specific surface area which can provide with more active sites. Compared with Ti/Sb–SnO₂ electrode, TiO₂-NTs/Sb–SnO₂ has a higher oxygen evolution potential and stronger phenol oxidation peak, indicating an improved catalytic activity for phenol degradation. The kinetic analysis of electrochemical phenol degradation showed that the first-order kinetics rate constant on TiO₂-NTs/Sb–SnO₂ electrode is 1.33 times as much as that on Ti/Sb–SnO₂, confirming that the sphere-stacking Sb–SnO₂ based on TiO₂ nanotube has a good electrocatalytic activity.     

Electrodics of methanol oxidation on Pt-f-multiwalled carbon nanotube composite, prepared by γ-radiolysis

We report the electrodics of methanol oxidation on Pt-multiwalled carbon nanotube composites (Pt-f-MWCNTs), prepared by γ-radiolysis of K₂PtCl₆ in the presence of HOOC-functionalized multiwalled carbon nanotubes. The electrocatalytic activity for the methanol oxidation was studied using cyclic and linear sweep voltammetric techniques on the stationary indium tin oxide and rotating gold disc electrodes, respectively. Higher values of oxidative (anodic) current were obtained using Pt-f-MWCNTs compared to the polycrystalline Pt electrode. This phenomenon is attributed to the synergistic effect of oxy groups on MWCNTs, which alleviate CO poisoning. The electrodics of the reaction at various temperatures was studied using linear sweep voltammetry (LSV) on a rotating disc gold electrode, modified with the composite. From the Koutecky-Levich plots, the standard rate constant (k⁰) was determined to be 7.9 ± 1.9 × 10⁻⁸ cm s⁻¹.     

Effect of mixed γ- and χ-crystalline phases in nanocrystalline Al2O3 on the dispersion of cobalt on Al2O3

This paper reports the results of a study into the effect of mixed γ and crystalline phases in Al₂O₃ on the characteristics and catalytic activities for CO hydrogenation of Co/Al₂O₃ catalysts. The catalysts were characterized by X-ray diffraction, N₂ physisorption, transmission electron microscopy, and H₂ chemisorption. Increasing Co loading from 5 to 20 wt% for the mixed phase Al₂O₃-supported Co catalysts resulted in a constant increase in both the number of cobalt metal active sites and the hydrogenation activities. However, for those supported on γ-Al₂O₃, Co dispersion increased up to 15 wt%Co and declined at 20 wt%Co loading. It is suggested that the spherical-shape like morphology of the χ-phase Al₂O₃ prevented agglomeration of Co particles, especially at high Co loadings.     

Chemical composition and electrochemical activity of some chemically synthesized γ-MnO2

A series of γ-MnO₂ with Mn⁴⁺ cation vacancy fraction 1.5 < x < 2.3 was prepared via chemical synthesis. X-ray diffraction (XRD) revealed high degree of microtwinning (T _w > 70%) and de Wolff disorder (0.32 < P _r < 0.35) in the prepared samples. Thermal analysis showed four-stage decomposition corresponding to the removal of physical water, chemisorbed water, formation of Mn₂O₃ and Mn₃O₄, respectively. The chemical composition and physical properties of the prepared samples were expressed in terms of Ruetschi’s cation vacancy model for MnO₂. The discharge behavior in 9 M KOH was governed by homogeneous solid-state reduction wherein protons and electrons were simultaneously inserted into MnO₂ lattice. Their electrochemical behavior was also studied by performing cyclic voltammetry in 9 M KOH. An attempt was made to correlate obtained energy density as a function of proton transfer rate during the discharge process.     

Effect of specific surface area on capacitance in asymmetric carbon/α-MnO2 supercapacitors

α-MnO₂ has been made using a solid state synthesis and the specific surface area then modified through milling. The formation of α-MnO₂ (specific surface area 96 m² g⁻¹) has been studied by SEM and powder XRD prior to milling. Electrode films (cast using MnO₂, graphite and PVDF) have been investigated using N₂ sorption at 77 K and show a more complex relationship than their parent oxides. Specific capacitances of 235 F g⁻¹ were observed in cyclic voltammetry studies in (NH₄)₂SO₄ (aq.) electrolyte. Good cyclability was observed in hybrid C/MnO₂ cells investigated through both galvanostatic and electrochemical impedance techniques. The specific capacitances of the cells were found to correlate with S_BET of the electrode films and not that of the parent MnO₂ powders.     

A rapid quantitative protocol for measuring carbon nanotube degree of dispersion in a waterborne epoxy–amine matrix material

The available literature makes it very clear that accurate measurements of carbon nanotube dispersion quality are very complicated and the typical characterization is neither simple nor reliable. Most methods to quantify carbon nanotube dispersion reported in the literature require investigator-chosen assumptions or software interpretations that are impractical at best and misleading at worst for facile application. Herein, we report on the use of visible light absorption-based method(s) and validate that these were quantitative for discerning dispersibility differences for MWCNTs with three distinct surface chemistry modifications and concentration levels blended with polymeric materials. Ultimately, the dispersion quality was quantified via the trendline slope of the thickness-normalized absorbance values as a function of MWCNT concentration. Extremely poor dispersions were represented by statistically insignificant slope trendlines. Our data revealed that hydroxyl surface modification increased MWCNT dispersibility by a factor of ~2.8 and ~2.6 compared to the as-received MWCNT formulations via the absorption and the blackness methods, respectively. These results support and quantifiably validate that simple optical blackness values directly measured the degree of dispersion for MWCNTs in coatings applied to substrates, and our data support that this is a simple and effective quality control metric.     

Properties of electroless and electroplated Ni–P and its application in microgalvanics

Composition, microstructure, surface morphology, mechanical properties and electrochemical behaviour of electroless (el) and electroplated (ep) Ni–P deposits are studied using XPS, SEM–EDX, AFM, nanoindentation measurements, cyclic voltammetry and capacitance measurements. Ni–P layers were compared with ep Ni films and bulk Ni. Ni–P layers prepared by both techniques contain 12–14 wt% phosphorus, present in oxidation states of P⁰ and P³⁻. El and ep Ni–P deposits are amorphous and are characterised by a relatively low average surface roughness (2 and 4 nm, respectively). The ep layers possess a rhythmic-lamellar microstructure indicating a periodic change of electrodeposition conditions. The el Ni–P layers do not show such laminated structure but exhibit small surface pores, which are absent in the ep layers. Comparable values for the hardness and the reduced elasticity modulus of el and ep coatings are determined from the nanoindentation data. The observed small differences indicate that the mechanical properties of Ni–P deposits depend not only on the phosphorus content but also on the deposit microstructure. Microelectrochemical measurements with the so-called droplet cell show that the electrochemical behaviour of both el and ep Ni–P coatings is practically identical and does not depend on the location on the sample surface. Evolution of O₂ and H₂ on Ni–P are similar to pure Ni (ep and bulk), but the corrosion resistance in acid solution is much better. The very similar properties and electrochemical behaviour of el and ep Ni–P deposits suggest that both materials are suitable for various applications in microsystem technology. For different substrates and microstructures of different size and geometry, deposition conditions have still to be optimised.     

Studies on the dispersion states of Fe2O3 on γ-Al2O3 by means of Mössbauer spectroscopy and XRD

XRD and Mössbauer spectroscopy studies show that Fe₂O₃/-Al₂O₃ made by impregnation-calcination method may have monolayer dispersed Fe₂O₃ and -Fe₂O₃ crystallites. There is a threshold monolayer dispersion capacity. If the Fe₂O₃ loading is lower than the threshold, the Fe₂O₃ will disperse on the surface of -Al₂O₃ as a monolayer, then the sample will give an XRD pattern with no crystalline Fe₂O₃ peaks and a Mössbauer spectrum of a doublet with large quadruple splitting (1.11 mm/s). When the Fe₂O₃ loading is higher than the threshold, in addition to the monolayer dispersed Fe₂O₃, crystalline -Fe₂O₃ will appear, then the sample will give an XRD pattern with crystalline -Fe₂O₃ peaks and a Mössbauer spectrum with a quadrupole splitting doublet superimposed on a magnetic splitting sextuplet. Monolayer dispersion capacity obtained by quantitative XRD phase analysis and Mössbauer spectra analysis are consistent with each other, having the same value of 0.052 g Fe₂O₃/100 m² -Al₂O₃ surface.     

Temperature-dependent charge transport in TiO2–multiwalled carbon nanotube composites

Charge transport properties of TiO₂–multiwalled carbon nanotube (MWCNT) composites were investigated. The TiO₂–MWCNT composites were fabricated by spark plasma sintering of a mixture of TiO₂ nanoparticles and MWCNTs. Temperature-dependent electrical conductivities of the composites reveal that the percolation threshold for the MWCNT network is affected by temperature, and that the activation process for electron hopping is also influenced by the percolation. Based on this interdependence, an integrated charge transport model, including both the effects of the percolation and the electron hopping, is proposed for this system.     

Dry reforming of methane over palladium–platinum on carbon nanotube catalyst

A dry reforming (DR) catalyst based on bimetallic Pd–Pt supported on carbon nanotubes is presented. The catalyst was prepared using a microwave-induced synthesis. It showed enhanced DR activity in the 773–923?K temperature range at 3 atm. Observed carbon balances between the reactant and product gases imply minimal carbon deposition. A global three-reaction (reversible) kinetic model—consisting of DR, reverse water gas shift, and CH₄ decomposition (MD)—adequately simulates the observed concentrations, product H₂/CO ratios, and reactant conversions. Analysis shows that, under the conditions of this study, the DR and MD reactions are net forward and far from equilibrium, while the RWGS is near equilibrium.     

Preparation of meso-macroporous α-alumina using carbon nanotube as the template for the mesopore and their application to the preferential oxidation of CO in H2-rich gases

Meso-macroporous α-Al₂O₃ was successfully prepared by using acid-treated carbon nanotube as mesoporous forming agent and polystyrene foam as the template for the macropore. A series of Ru/meso-macroporous α-Al₂O₃ catalysts were prepared by the incipient wetness impregnation method and applied to the preferential oxidation of CO (CO-PROX) in H₂-rich gases. SEM, N₂ adsorption–desorption, H₂-TPR techniques and TEM were employed to characterize the catalysts. The results indicate that the specific surface area was markedly elevated by introducing the mesopores, which led to the higher dispersion of ruthenium nanoparticles on the surface of α-Al₂O₃. The meso-macroporous α-Al₂O₃ supported ruthenium showed very high activity and selectivity for CO-PROX.     

The effect of temperature and hydrodynamics on carbon steel corrosion and its inhibition in oxygenated acid–salt solution

Corrosion of carbon steel in hydrochloric acid (HCl)–sodium sulphate (Na₂SO₄) solution mixture was investigated using rotating cylinder electrode (RCE) for a range of rotation velocity, 0–2000 rpm, solution temperature of 32–52 °C, and different oxygen concentrations. The corrosion rat was determined by using both weight loss method and electrochemical polarization technique. Different acid and salt concentrations were used ranged from 0.01 to 0.2 M for salt and 0.5 to 5% for acid. The conjoint effect of increased oxygen concentration and high rotational velocity was studied based on experimental measurements of O₂ concentration. The effects of operating conditions on indole and cetyl trimethyl ammonium bromides (CTAB) inhibition efficiency were also studied and discussed. The results showed that increasing the rotational velocity leads to an increase in the corrosion rate depending on the concentration of salt and acid. Increasing the temperature and acid concentrations leads to an increase in the corrosion rate while the corrosion rate exhibited unstable trend with salt concentration leads to change of corrosion. It is found that increasing O₂ concentration leads to a considerable increase in the corrosion rates especially at high rotational velocity. Indole and CTAB inhibitors exhibited very good inhibition efficiency in most conditions investigated with the former exhibited better inhibition efficiency arriving up to 87% at low rotational velocities. The inhibition efficiency of both inhibitors was found to decrease with increasing velocity. In addition, indole inhibitor reveals excellent inhibition efficiency even at high temperatures while CTAB efficiency decreased appreciably with temperature increase.