Synthesis and self-assembly of giant porphyrin discs

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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

Adsorption of CO and H2 on Transition Metal Clusters : insights from Vibrational Spectroscopy and Density Functional Theory

Adsorption of hydrogen (H2) and carbon monoxide (CO) molecules on transition metals is of paramount importance for several (catalytic) processes. These include the purification of H2 streams and the Fischer-Tropsch reaction, in which a mixture of H2 and CO is converted to synthetic fuels. As a consequence, many studies into the binding of these two molecules on transition metals have been performed. Nevertheless, fundamental questions regarding the effects of metal particle size and electron density on adsorbate binding geometry, as well as on the effects of co-adsorption, remain. The research described in this Ph.D. thesis explores these questions, both experimentally and theoretically, by focusing on well-defined transition metal clusters in the gas-phase as model systems. The first part of the thesis deals with the adsorption H2 on a variety of transition metal clusters. It is shown that the hydrogen binding sites are highly dependent on the metal, as well as on cluster size and that they are different for small clusters compared to extended surfaces. In addition, it is found that the first H2 molecule to bind on a relatively unreactive Ni4+ cluster, can bind molecularly, while it binds exclusively dissociatively on the reactive Ni5+ and Ni6+ clusters. In the second part of the thesis, investigations into the effects associated with co-adsorption of H2 and CO are presented. These effects were studied by focusing on an early (vanadium) and a late (cobalt) transition metal as case studies. In case of vanadium it is found that co-adsorption of H2 leads to a stabilization of CO against dissociation. This is shown to be predominantly a structural effect. In contrast, co-adsorption of H2 is demonstrated to have a significant electronic effect on cobalt clusters. To explain the experimental observations, a model describing the cluster size and charge dependence of the binding of CO on transition metal clusters was extended to incorporate the co-adsorption of H2. Each adsorbed hydrogen atoms lowers the electron density of the metal particle by 0.09 - 0.25 of an electron, depending on cluster size. In the last part of the thesis, the dependence of the adsorbate binding geometry on the charge state of the metal cluster is explored. For these studies, rhodium, cobalt, and nickel carbonyls were used as model systems. In case of the rhodium and cobalt carbonyls, the removal of an electron from a neutral complex leads to a destabilization of bridge bound CO ligands. This destabilization is shown to be due to the removal of an electron from an orbital that is bonding with respect to the bridge bound carbonyl groups

Turning a Cr-based heterogeneous ethylene polymerisation catalyst into a selective ethylene trimerisation catalyst

A Phillips Cr/SiO2 polymerisation catalyst was converted into an ethylene trimerisation catalyst after assembling new active sites on the silica surface in the presence of TAC (1,3,5-triphenylhexahydro 1,3,5-triazacyclohexane) as ligand and CH2Cl2 as solvent. The reaction conditions play a role in tuning the catalytic activity of this system. It is a true trimerisation catalyst at low ethylene pressures, while, at high pressures, the polymerisation activity becomes dominant. Moreover, at high ethylene pressures, polyethylene characterised by a high crystallinity degree is obtained. Instead, a highly branched heavy oligomer of ethylene characterised by a very narrow molecular weight distribution is obtained at 1 bar. 1-Hexene, formed as the main product in the ethylene trimerisation reaction at low pressures, is partially incorporated into this growing oligomer making redundant the use of a second comonomer feedstock and an expensive catalyst