Nitrogen doped carbon nanotubes : synthesis, characterization and catalysis

Abstract

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.