The Preparation, Characterization And Propane Oxidative Dehydrogenation Performances Of Oxidized And Nitrogen Doped Carbon Nanotubes

Carbon materials have drawn a great deal of attention in catalysis field for the applicationas catalyst supports and metal-free catalysts. As highly active catalyst, carbon nanotubes（CNT） become one of the hottest topics in current carbon catalysis research with thesignificant progress in activation of alkanes in recent years. Researchs indicated that thesurface functionalities plays a significant role in improving catalytic properties of carbonmaterials. As most reported, many liquid-phase oxidizing acids were employed to generatefunctionalities attached carbon surface at the condition of refluxing. However, oxidation ofcarbon materials using acid is an intermission operating process with sequent separation anddrying, which is not only inefficiency but also difficult to scale up the manufacturing process.At the same time, the massive overdose of concentrate acid waste much more alkali agents toneutralize and cause a heavy potential pollution. Gas-phase oxidation method has numerousbenefits to eliminate the shortcomings above in solution method and are available in a widetemperature range with many oxidizing gases. Nitrogen dioxide （NO₂） is more reactive thanthe other oxidizing species at low temperature. Based on the C-NO₂reactivity, gas-phaseoxidation of carbon nanotubes in diluted NO₂was systemically studied in this research. Onthe other side, heteroatom doping makes it possible to control the electronic properties andconsequential catalytic performance of nanocarbons. For instance, the electron-rich nitrogenatoms in nitrogen doped CNT （NCNT） will provide additional electron for graphitic latticeand give rise to surface basicity. The incorporated N atoms can introduce electron-deficientneighbor C atoms and reduce the energy gap between the highest-occupied molecular orbitalsand the lowest-unoccupied molecular orbitals, which would benefit the binding O₂on NCNTand make it easy the electron transfer from NCNT to the adsorbed oxygen speciesconsequently promotes the catalytic activity in oxygen-involving reactions. Nitrogen dopinginto carbon nanotubes become the most promising direction in carbon catalysis. In this thesis,a series of CNT with different doping level was controllably synthesized by chemical vapordeposition method and carefully characterized their structure and composition by microscopyand spectroscopy. The correlation between structural parameters of NCNT and catalyticperformances and kinetics of oxidative dehydrogenation of propane （ODP） revealed themodulation effect of nitrogen doping on carbon catalysts for ODP. The main contents are asfollows:（1） A novel method for NO₂gas-phase oxidation of carbon nanotubes was developed. Thesurface oxygen groups modified CNT has been systematically characterized and used as a catalyst for ODP. This study revealed that NO₂oxidation method facilitated generating moresurface active carbonyl groups and increasing the specific surface area of CNT, being about1.5and1.3times for carbonyl ratio and BET area, respectively，than that produced byconventional HNO3refluxing at the same5wt%weight loss of CNT. The NO₂modified CNTshows superior catalytic performances than the other treated by O₂, HNO3vapor and HNO3solution with respect to C3H6formation rate. Kinetic measurements at differential conditiondemonstrated that the NO₂treated CNT performed with bigger pre-exponential factors thanthat oxygen or concentrated HNO3treated, due to more stable C=O active sites generated onCNT surface by NO₂oxidation. The more C=O ratio is profitable for the adsorption of α–H inpropane molecular on NO₂-treated CNT to form more intermediate state of RC···H···O=CNT,and consequently a higher dissociation probability of α–H leading to a rapid C3H8conversionto C3H6finally. This research presents an applicable and efficient method for oxidativemodification of CNT as well as other carbon materials.（2） Using toluene and aniline as carbon and nitrogen resources, a series of NCNT withadjustable nitrogen content of0-1.89atom%was synthesized by a catalytic chemical vapordeposition method at temperature of800oC and purified by concentrated HNO3refluxing forcatalytic application. The surface chemistry and structural properties were investigated byusing various characterization methods. The characterization results revealed that nitrogenelements were homogenously distributed throughout the carbon framework and formed asmany as five nitrogen-containing species. The graphitic nitrogen component in every samplewas around40%of the total nitrogen content. As the doping level raising, the structuralparameters of carbon nanotubes changed with the increasing of lattice spacing and curvatureof graphitic layers to form a typical bamboo-like structure. However, the sp²-hybridizedfeature of carbon atoms was not obviously changed as the sp²ratio kept in99.7%. The dopedCNT samples were more active in reacting with oxygen than commercial and homemadenon-doped CNT while slightly weaker in oxidation resistance, but still could be used inoxygen-enriched environments at400oC.（3） The catalytic performaces of NCNT were investigated by using ODP as ahigh-temperature probe reaction. As revealed in integral reaction measurements, NCNTshows a better catalytic activity than the non-doped CNT and a superior stability in ODPreaction. Both propane conversion and propene selectivity were increased with the increasingN content in carbon catalysts. The increment of catalytic activity was attributed to themodulation effect of N doping on the electronic structure of NCNT and dissociativeadsorption of oxygen moleculars. Chemical kinetics was first applied to study the N doping effect on ODP reaction. The overall activation energy and oxygen reaction order were foundto be lower on NCNT catalyst than that on non-doped CNT catalysts. The increment ofgraphitic nitrogen content （NG） can significantly reduce the overall activation energy asdescribed by an empirical equation that E_a=117.2-32.7NG. The careful analysis ofstructure-performance correlation indicated that CNT doped with N facilitates the electronictransport and dissociative adsorption of O₂, as well the regeneration of surface NCNT-O-Hspecies to C=O active sites, thus promotes the ODP reaction. In addition, the doped N atomsmay improve propene desorption from the surface due to the increasing repulsion betweenNCNT and propene with electron-rich C=C bond to avoid a total oxidation and maintain asuperior propene selectivity.