Studies On Chemistry Of Some Well-defined Flow Systems

In this dissertation, three subjects on energy, the formation of polycyclic aromatic hydrocarbon (PAH₎, catalytic ignition and oxidation, silicon deposition in low-pressure chemical vapor deposition (C_VD), were studied using simple but well-defined flow systems with advanced diagnosis methods, numerical simulations and modeling.C_hapter 1, under the background of energy deficiency,more and more concerns and strict legislation on the environment, the importance and necessity of studies on the formation mechanism of PAH_, the catalytic ignition and oxidation and the weakly rarefied effect on the silicon deposition in low pressure C_VD are described. The research methods used in this thesis are briefly outlined.C_hapter 2, the pyrolysis of two typical aromatics (benzene and toluene), well-known precursor of PAH_s, were studied at temperature range of 1200-1900 K and low pressure of～25 torr by synchrotron VUV single photoionization mass spectrometry combined with molecular-beam sampling technology. The intermediates of pyrolysis process were identified and their mole fractions vs temperature were acquired. A G3B3 method was used to calculate the reaction pathways related to many important intermediates. A detailed kinetic model including 150 species and 560 element reactions was applied to simulate the pyrolysis process. Satisfactory agreement has been achieved between experimental results and computed predictions for most species. The conclusions are as follows:1. Benzene mainly decomposes through the reaction sequence of C₆H₆→C₆H₅→o-C₆H₄→C₄H₂ + C₂H₂;while toluene decomposes through the channel C₆H₅C_H3→C₆H₅C_H2→c-C₅H₅→C₃H₃.2. The main PAH formation pathway is C₆H₆→C₉H₈→C₉H₇→C₁2H₈→C₁4H₈ in benzene pyrolysis; besides the vital role of C₂H₂ in H_AC_A mechanism leading to PAH formation, other small molecule products (C₃, C₄ and C₆-species) also contribute a lot. The PAH formation in toluene pyrolysis process has a feature: combination of aromatics/aromatic radical followed by H_/H₂ elimination to form PAH_s.3. Benzene and phenyl radical play a vital role on PAH formation in benzene pyrolysis, while benzyl radical play the most important role on PAH formation in toluene pyrolysis. 4. The initial formation temperature of species produced in pyrolysis process is qualitatively accorded with the energy surface calculated by the G3B3 mothod.5. The important reactions contributing to the formation and consumption of all species are abstracted from the detailed mechanism to preliminarily get a simple mechanism with 167 reactions by production-rate analysis of all species in benzene and toluene pyrolysis process.6. The difference between the modeling result and experimental data is analyzed by production-rate analysis of all species in benzene and toluene pyrolysis process, which gives lots of insight into improving the current modeling.Chapter 3, Catalytic oxidation of light hydrocarbons/air mixtures over Pd-based catalytic surface were studied by using wire microcalorimetry which determines the catalytic heat release rate as a function of the wire temperature. The experiments were conducted with thorough pre-treatment and extensive surface characterization using focused ion beam, backscattered electron (BSE), energy dispersive x-ray (EDX), and x-ray photoelectron spectrometry (XPS), which render the catalyst surface well controlled and characterized. Consequently the catalytic ignition temperature was successfully determined. It is also noted that the geometry of the experiment is so simple that it was readily simulated to validate chemical mechanisms by software FLUENT.1. The heat release rates of 1-4% methane at a temperature range of 400-800 K were acquired.2. When the concentration of methane increases from 1% to 4%, the ignition temperature decreases from 642 to 580 K; while pressure increases from 0.5 to 4 atm, ignition temperature decreases from 630 to 556 K. The global activation energy of methane over PdO is 21.5±0.9 kcal/mol, the reaction order of methane is 0.9±0.1 at the temperature range of 630-770 K. The reaction order increases as catalyst size decreases.3. Comparison of experimental data with modeling result indicates the importance of surface morphology. The sensitivity analysis of heat release rate indicates three important reactions: the dissociative adsorption of methane and oxygen, and the desorption of oxygen.Chapter 4, we introduced the boundary slip phenomena into the SPIN program for the simulation of rotating-disk LPCVD, in which the weakly rarefied flow effects must be included to accurately compute the deposition rate. A model reaction mechanism for silicon deposition was used to elucidate the rarefied flow effect of the reduced deposition rate with decreasing system pressure. This trend is further augmented by considering the boundary _slip phenomena, especially the temperature _slip. Furthermore, the _slip temperature T_slip and the temperature of the molecules striking the surface Tm are recognized to be two important factors associated with the temperature _slip phenomena. Their distinctive roles in affecting the deposition rate are identified in that, at relatively low disk temperatures (e.g. 800 K). The total deposition rate mainly attributes to the SiH4 deposition, which is substantially affected by the Tm-sensitive sticking coefficient. However, at higher disk temperatures (e.g. 1200 K), the total deposition rate is mainly due to the SiH2 deposition, which is substantially affected by T_slip. Considering that the model reaction mechanism contains the representative gas-phase and surface reactions, the present results are believed to be readily extendable to detailed reaction mechanisms and other similar LPCVD system.