Surface oxygen complexes as governors of neopentane sorption in multiwalled carbon nanotubes

Multiwalled carbon nanotube (MWCNT) samples were obtained by purification and ball-milling of a MWCNT sample synthesized by catalytic chemical vapor decomposition. These samples were oxidized with KMnO₄-H₂SO₄ solution. A heat treatment was employed to decompose the oxygenated groups created by the oxidation treatment. The samples were characterized by transmission electron microscopy (TEM), adsorption of nitrogen, pore size distribution, and by thermogravimetric analysis combined with mass spectroscopy (TG-MS). The dynamics of 2,2-dimethyl-propane (neopentane) sorption was studied by frequency-response (FR) spectroscopy. The FR spectra obtained clearly demonstrate the effect of (a) the ball-milling, (b) the oxidative treatment and (c) the heat treatment on the sorption uptake processes involved in these treated samples.     

Chemical and electrochemical characterization of porous carbon materials

Chemical and electrochemical techniques have been used in order to asses surface functionalities of porous carbon materials. An anthracite has been chemically activated using both KOH and NaOH as activating agents. As a result, activated carbons with high micropore volume (higher than 1 cm³/g) have been obtained. These samples were oxidized with HNO₃ and thermally treated in N₂ flow at different temperatures in order to obtain porous carbon materials with different amounts of surface oxygen complexes. Thermal treatment in H₂ was also carried out. The sample treated with H₂ was subsequently treated in air flow at 450 °C. Thus, materials with very similar porous texture and widely different surface chemistry have been compared. The surface chemistry of the resulting materials was systematically characterized by TPD experiments and XPS measurements. Galvanostatic and voltammetric techniques were used to deepen into the characterization of the surface oxygen complexes. The combination of both, chemical and electrochemical methods provide unique information, regarding the key role of surface chemistry in improving carbon wettability in aqueous solution and the redox processes undergone by the surface oxygen groups. Both contributions are of relevance to understand the use of porous carbons as electrochemical capacitors.     

Synthesis of Pt nanocatalyst with micelle-encapsulated multi-walled carbon nanotubes as support for proton exchange membrane fuel cells

Micelle-encapsulated multi-walled carbon nanotubes (MWCNTs) with sodium dodecyl sulfate (SDS) were used as catalyst support to deposit platinum nanoparticles. High resolution transmission electron microscopy (HRTEM) images reveal the crystalline nature of Pt nanoparticles with a diameter of ∼4 nm on the surface of MWCNTs. A single proton exchange membrane fuel cell (PEMFC) with total catalyst loading of 0.2 mg Pt cm⁻² (anode 0.1 and cathode 0.1 mg Pt cm⁻², respectively) has been evaluated at 80 °C with H₂ and O₂ gases using Nafion-212 electrolyte. Pt/MWCNTs synthesized by using modified SDS-MWCNTs with high temperature treatment (250 °C) showed a peak power density of 950 mW cm⁻². Accelerated durability evaluation was carried out by conducting 1500 potential cycles between 0.1 and 1.2 V with 50 mV s⁻¹ scan rate, H₂/N₂ at 80 °C. The membrane electrode assembly (MEA) with Pt/MWCNTs showed superior performance stability with a power density degradation of only ∼30% compared to commercial Pt/C (70%) after potential cycles.     

Doped polyaniline/multi-walled carbon nanotube composites: Preparation, characterization and properties

This study reports the synthesis of doped polyaniline in its emeraldine salt form (PANI-ES) with carboxylic acid and acylchloride groups contained multi-walled carbon nanotubes (designated as c-MWNTs and a-MWNTs) by in situ polymerization. Both Raman spectra and HRTEM images indicate that carboxylic acid and acylchloride groups formed at both ends and on the sidewalls of the MWNTs. Based on the π-π* electron interaction between aniline monomers and functionalized MWNT and hydrogen bonding interaction between the amino groups of aniline monomers and the carboxylic acid/acylchloride groups of functionalized MWNT, aniline molecules were adsorbed and polymerized on the surface of MWNTs. Structural analysis by FESEM and HRTEM showed that PANI-ES/c-MWNT and PANI-ES/a-MWNT composites are core (c-MWNT or a-MWNT)- shell (doped-PANI-ES) tubular structures with diameters of several tens to hundreds of nanometers, depending on the PANI content. The conductivities of 0.5 wt% functionalized MWNT containing PANI-ES/c-MWNT and PANI-ES/a-MWNT composites are 60-70% higher than that of PANI without MWNT.     

Rapid surface functionalization of iron-filled multi-walled carbon nanotubes

Iron-filled multi-walled carbon nanotubes (Fe@MWCNTs) were surface functionalized with various functionalities via a rapid, single-step process involving ultrasonication assisted and microwave-induced radical polymerization reactions. Both hydrophobic (e.g., polystyrenes and polymethyl methacrylate) and hydrophilic (e.g., polyacrylamide, polyacrylic acids, and polyallyl alcohols) polymer chains can be chemically grafted onto the surface of MWCNTs by the same process within 10 min. The surface grafted polymers were identified by FTIR, TGA, TEM, EELS and Raman spectra. The solubilities of the surface derivatized MWCNTs are in the range of 1200–2800 mg/l in solutions. The polyacrylic acids modified iron filled MWCNTs have a saturated magnetic dipole moment of 40 emu/g at room temperature with a coerceive field of nearly zero gauss.     

Development of carbon nanotube and carbon xerogel supported catalysts for the electro-oxidation of methanol in fuel cells

Multiwalled carbon nanotubes and high surface area mesoporous carbon xerogel were prepared and used as supports for monometallic Pt and bimetallic Pt-Ru catalysts. In order to assess the influence of the oxygen surface groups of the support, the mesoporous carbon xerogel was also oxidized with diluted oxygen before impregnation. Various reduction protocols were tested, the best results corresponding to reduction with sodium borohydride. High dispersion catalysts were obtained, which showed quite good performance in the electro-oxidation of methanol. In particular, a remarkable increase in the activity was observed when the Pt-Ru catalysts were supported on the oxidised xerogel. This effect was explained in terms of the metal oxidation state, as shown by XPS. It has been shown that the oxidised support helps to maintain the metals in the metallic state, as required for the electro-oxidation of methanol. This effect was negligible in the case of the Pt catalysts.     

Catalytic activity of carbon nanotubes in the oxidative dehydrogenation of ethylbenzene

Multi-walled carbon nanotubes (MWNT), before and after different oxidative treatments and hence possessing different oxygen surface groups, were used as catalysts for the oxidative dehydrogenation of ethylbenzene (ODE), and their performances compared to those of activated carbon and graphite samples. MWNT are active catalysts for ODE and show the highest specific activity per initial surface area amongst all the materials tested in this study. Moreover, the main advantage of using MWNT over activated carbons is their higher stability under oxidative conditions. It was shown that pre-oxidised MWNT are more active for ODE during the initial stages of the reaction, highlighting the importance of oxygenated surface groups.     

99.9% purity multi-walled carbon nanotubes by vacuum high-temperature annealing

Ultrahigh purity multi-walled carbon nanotubes (MWNTs) were obtained, which were produced by a catalytic chemical vapor deposition method and subsequently annealed at vacuum pressures between 10⁻³ and 10 Pa and temperatures between 1500 and 2150 °C. Acid treatments were made for comparison. Scanning electron microscopy, transmission electron microscopy and energy dispersive spectra were used to characterize the specimens. Purities were characterized by thermogravimetric analysis. Raman spectra were also reported. The high-temperature vacuum process efficiently removed residual metal catalysts and metal oxide carriers and enhanced the graphitization of the MWNTs. The highest MWNT purity achieved is about 99.9%. This procedure presents a nondestructive, commercially viable purification method for carbon nanotubes.     

Thermal desorption of gases and solvents from graphite and carbon nanotube surfaces

The interaction of 23 gases and solvents with the basal plane of highly oriented pyrolytic graphite (HOPG) and with single-wall carbon nanotube (SWCNT) samples is studied using thermal desorption spectroscopy. Pre-exponential frequency factors used for analysis of desorption traces are obtained from vapor pressure data. Activation energies for desorption at monolayer coverage are determined using the Redhead peak-maximum method. Binding energies of non-polar adsorbates to the HOPG surface are found to scale with the adsorbate polarizability providing clear evidence for the van der Waals character of the interaction. Low coverage desorption temperatures on SWCNT samples are found to be 50-100% higher than on HOPG. Such increase has previously been attributed to physisorption in higher coordinated sites such as grooves on the external SWCNT rope surfaces. Polar adsorbates on the other hand typically desorb at much higher temperatures from SWCNT samples which is here tentatively attributed to stronger interaction with defect sites.     

Preparation and characterization of polyaniline/multi-walled carbon nanotube composites

This study describes the synthesis of doped polyaniline in its emeraldine salt form (PANI-ES) with carboxylic groups containing multi-walled carbon nanotubes (c-MWNTs) via in situ polymerization. Both Raman and FTIR spectra indicate that carboxylic acid groups formed at both ends and on the sidewalls of the MWNTs. Based on the π-π* electron interaction between aniline monomers and MWNT and hydrogen bonding interaction between the amino groups of aniline monomers and the carboxylic acid group of c-MWNT, aniline molecules were adsorbed and polymerized on the surface of MWNTs. Structural analysis using FESEM and HRTEM showed that PANI-ES/c-MWNT composites are core (c-MWNT)-shell (doped-PANI-ES) tubular structures with diameters of several tens to hundreds of nanometers, depending on the PANI content. The conductivities of these PANI-ES/c-MWNT composites are 50-70% higher than those of PANI without MWNT.     

Role of surface chemistry on electric double layer capacitance of carbon materials

A large number of porous carbon materials with different properties in terms of porosity, surface chemistry and electrical conductivity, were prepared and systematically studied as electric double layer capacitors in aqueous medium with H₂SO₄ as electrolyte. The precursors used are an anthracite, general purpose carbon fibres and high performance carbon fibres, which were activated by KOH, NaOH, CO₂ and steam at different conditions. Among all of them, an activated anthracite with a BET surface area close to 1500 m²/g, presents the best performance, reaching a value of 320 F/g, using a three-electrode system. The results obtained for all the samples, agree with the well-known relationship between capacitance and porosity, and show that the CO-type oxygen groups have a positive contribution to the capacitance. A very good correlation between the specific capacitance and this type of oxygen groups has been found.     

Oxidation of pinane over phthalocyanine complexes supported on activated carbon: Effect of the support surface treatment

The oxidation of cis-pinane with tert-butyl hydroperoxide, at room temperature and atmospheric pressure, was carried out in the presence of iron-phthalocyanines supported on activated carbons. The carbon supports were prepared from a NORIT activated carbon, which was modified by different chemical and thermal treatments (including oxidation in the gas and liquid phases). The carbon samples were characterized by nitrogen adsorption, mass titration and temperature programmed desorption (TPD). The TPD profiles were analysed by a simple deconvolution method, allowing for the determination of the amount of oxygen containing functional groups on the carbon surface. The main reaction product is 2-pinane hydroperoxide (77% selectivity at 91% conversion). Formation of 2-pinanol, pinocampheol and verbanol and the respective ketones was also observed. The influence of the surface chemistry of the carbon supports on catalytic activity and product selectivity is studied. The catalysts prepared from supports with very high or very low oxygen content exhibit low activity, whereas for supports with intermediate oxygen contents a good correlation between the amount of phenols and lactones and catalytic activity is obtained.     

Thermal desorption of deuterium from modified carbon nanotubes and its correlation to the microstructure

The process of deuterium desorption from single-wall carbon nanotubes (SWNTs) modified by atomic (D) and molecular (D₂) deuterium treatment was investigated in an ultrahigh vacuum environment using thermal desorption mass spectroscopy (TDMS). Microstructural and chemical analyses of SWNT material, modified by this deuterium interaction, were performed by means of a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The results disclose characteristic features in the TDMS spectra of deuterium evolved from the SWNT material, which can be correlated to the microstructure of nanocarbon material modified by D-treatment. The TDMS spectra of deuterium originating from the large diameter rope type nanotube structures, resulting from a prolonged low-pressure (D + D₂) gas mixture treatment, exhibit three overlapping desorption peaks: a dominant one with a desorption activation energy (E_des) of approx. 2.86 eV and lower intensity peaks at E_des of ∼1.50 and 2.46 eV. On the other hand, the TDMS spectra of deuterium taken from the “coral reef”-like carbon nanostructures, obtained after prolonged treatment of SWNTs to a high-pressure (D + D₂) gas mixture produced at high temperature, reveal the coexistence of four superimposed desorption peaks with E_des ranging from 1.23 to 4.4 eV. A dominant desorption peak with E_des ≈ 4.4 eV, can be attributed to bulk diffusion of D trapped within this nanocapsule bulk structure.     

Synthesis and characterization of double-walled carbon nanotubes from multi-walled carbon nanotubes by hydrogen-arc discharge

Double-walled carbon nanotubes (DWNTs) were synthesized in a large scale by a hydrogen arc discharge method using graphite powders or multi-walled carbon nanotubes/carbon nanofibers (MWNTs/CNFs) as carbon feedstock. The yield of DWNTs reached about 4 g/h. We found that the DWNT product synthesized from MWNTs/CNFs has higher purity than that from graphite powders. The results from high-resolution transmission electron microscopy observations revealed that more than 80% of the carbon nanotubes were DWNTs and the rest were single-walled carbon nanotubes (SWNTs), and their outer and inner diameters ranged from 1.75 to 4.87 nm and 1.06 to 3.93 nm, respectively. It was observed that the ends of the isolated DWNTs were uncapped and it was also found that cobalt as the dominant composition of the catalyst played a vital role in the growth of DWNTs by this method. In addition, the pore structures of the DWNTs obtained were investigated by cryogenic nitrogen adsorption measurements.