Effect of Growth Temperature on Bamboo-shaped Carbon–Nitrogen (C–N) Nanotubes Synthesized Using Ferrocene Acetonitrile Precursor

This investigation deals with the effect of growth temperature on the microstructure, nitrogen content, and crystallinity of C–N nanotubes. The X-ray photoelectron spectroscopic (XPS) study reveals that the atomic percentage of nitrogen content in nanotubes decreases with an increase in growth temperature. Transmission electron microscopic investigations indicate that the bamboo compartment distance increases with an increase in growth temperature. The diameter of the nanotubes also increases with increasing growth temperature. Raman modes sharpen while the normalized intensity of the defect mode decreases almost linearly with increasing growth temperature. These changes are attributed to the reduction of defect concentration due to an increase in crystal planar domain sizes in graphite sheets with increasing temperature. Both XPS and Raman spectral observations indicate that the C–N nanotubes grown at lower temperatures possess higher degree of disorder and higher N incorporation

CNx-modified Fe3O4 as Pt nanoparticle support for the oxygen reduction reaction

A novel electrocatalyst support material, nitrogendoped carbon (CNx)-modified Fe3O4 (Fe3O4-CNx), was synthesized through carbonizing a polypyrrole-Fe3O4 hybridized precursor. Subsequently, Fe3O4-CNx-supported Pt (Pt/Fe3O4-CNx) nanocomposites were prepared by reducing Pt precursor in ethylene glycol solution and evaluated for the oxygen reduction reaction (ORR). The Pt/Fe3O4-CNx catalysts were characterized by X-ray diffraction, Raman spectra, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The electrocatalytic activity and stability of the as-prepared electrocatalysts toward ORR were studied by cyclic voltammetry and steady-state polarization measurements. The results showed that Pt/ Fe3O4-CNx catalysts exhibited superior catalytic performance for ORR to the conventional Pt/C and Pt/C-CNx catalysts.Web of Scienc

Nitrogen doped carbon nanotubes : synthesis, characterization and catalysis

Nitrogen containing Carbon Nanotubes (NCNT) have altered physical- and chemical properties with respect to polarity, conductivity and reactivity as compared to conventional carbon nanotubes (CNT) and have potential for use in electronic applications or catalysis. In this thesis the incorporation of nitrogen in the graphene layers of CNT by varying the synthesis parameters, the physical/chemical consequences thereof and their potential as solid base catalyst are described. NCNT were successfully grown from acetonitrile, pyridine or N,N-dimethylformamide over supported Fe-, Co- or Ni catalysts between 823 and 1123 K. The influence of the synthesis parameters on the physical- and chemical properties of the obtained NCNT could be related to thermodynamic stability of the metal-carbide/nitride; with increasing temperature the formation of metal-carbide species is more favourable than the formation of metal-nitride species. Furthermore, the type of nitrogen formed in the NCNT appeared to be temperature dependent. At low growth temperatures, the pyridinic type nitrogen, located at edges or defects in the graphene layers, was predominant in the NCNT whereas at higher growth temperatures the formation of quaternary type nitrogen, i.e. nitrogen substituting a carbon atom in the graphene layer, was favourable. Investigation of the NCNT morphology showed that multiwalled carbon nanotubes were obtained with the Co- and Ni catalyst while bamboo structured NCNT were obtained with the Fe catalyst, regardless of the C/N precursor or growth temperature. Based on the thermodynamic stability of the metal carbides, a pulsating NCNT growth favoured by the more stable Iron carbides was proposed to explain the bamboo structure while the straight tubes were explained by a continuous growth of NCNT, favoured by the less stable Co- or Ni carbides. The number and nature of the basic sites in NCNT were investigated using acid-base titrations and XPS. The amount of nitrogen determined with titrations was about two orders of magnitude lower as obtained with XPS. This was explained by the fact that XPS probes several graphene layers while titrations only probe the accessible nitrogen species. Proton uptake curves, derived from titration data, indicated that the NCNT surface consisted of various N sites with different pKa ranges. This can be rationalized by envisioning NCNT as being constructed from organic nitrogen containing building blocks having different pKa values. Furthermore, based on the appearance of the NCNT’s titration curves three classes were distinguished, related to the type of nitrogen incorporated. All NCNT displayed catalytic activity for the base catalyzed Knoevenagel condensation of benzaldehyde with ethylcyanoacetate with initial activities comparable to those displayed by activated carbon and rehydrated hydrotalcite and which could be related to the amount of pyridinic type nitrogen in the NCNT. The reaction rate decreased with time which was, based on reaction rate modelling using Langmuir-Hinshelwood kinetic, explained by a competitive adsorption between reactant and product. Based on the results of the catalytic testing NCNT can be categorized as mild solid base comparable to other mild bases like fluoro- and hydroxyl apatites and aluminophosphate oxynitrides

On the virtue of acid–base titrations for the determination of basic sites in nitrogen doped carbon nanotubes

The basicity and nature of basic species in nitrogen containing carbon nanotubes (NCNT) prepared under different conditions were investigated by acid–base titrations. Proton uptake curves were derived from the titration data and were used to establish the basicity (pKa) ranges of nitrogen species present in NCNT. Based on the evolution of the titration curves upon acid addition the NCNT were divided into three classes: (I) in which pyridinic type N with a pKa of 7–9 were predominantly present; these pyridinic groups had some buffering capacity in a pH range of 5–7; (II) where amine type N with a pKa of 7–9 were the major species; these groups showed only minor buffering capacity at pH 8–9; (III) NCNT with low basicity, i.e. most groups possessed a pKa <7 with insignificant buffering capacity. All NCNT could be regarded as being composed of organic nitrogen containing building blocks having different pKa values

Activity of nitrogen containing carbon nanotubes in base catalyzed Knoevenagel condensation

The catalytic activity of Nitrogen containing Carbon Nanotubes (NCNT), grown from acetonitrile or pyridine over supported Fe-, Co or Ni catalysts at various conditions, were tested for the base catalyzed Knoevenagel condensation of benzaldehyde and ethylcyanoacetate. All NCNT displayed activity for the reaction with initial turn over frequencies between 9 × 10−3 and 5 × 10−2 s−1 which is comparable with to those of activated basic carbons and a rehydrated hydrotalcite. Furthermore, the initial activity per gram of catalyst of NCNT was related to the amount of pyridinic type nitrogen in the NCNT. However, the reaction rate decreased with time on stream which was explained by competitive adsorption of reactant and product. The reaction rate can be described using Langmuir–Hinshelwood type kinetics including product adsorption

Tuning nitrogen functionalities in catalytically grown nitrogen-containing carbon nanotubes

Nitrogen-containing carbon nanotubes (NCNT) were grown from acetonitrile, pyridine or N,N-dimethylformamide over a supported Fe-, Co- or Ni catalyst in the temperature range 823–1123 K. The physico-chemical properties of the obtained NCNT, such as the C/N ratio or the nitrogen type, were related to the synthesis parameters. It was found that the C/N ratio increased with increasing temperature which could be related to the thermodynamic stabilities of the metal-carbides and metal nitrides. Also the type of nitrogen present in the graphene layer changed with increasing temperature from predominantly pyridinic- to quaternary nitrogen. NCNT obtained with the Fe catalyst showed bamboo morphology regardless of the C/N source or growth temperature while straight tubes were obtained with the Co- or Ni catalyst. We propose that this difference in morphology can be explained by the thermodynamic stabilities of the different metal-carbides, leading to a ‘pulsating’ growth in the case of Fe as opposed to a more continuous growth in the case of Co or Ni