Analysis of complex reaction systems: Application to the chemistry of silicon nanoparticle formation and polystyrene/polypropylene degradation

Complex reaction systems consisting of thousands of species and reactions are very common in many important chemical processes. Analysis of these complex reaction systems using both experimental and theoretical tools is critical to understand the reaction pathways at the mechanistic or molecular level, and control of the conversion of chemical processes can be exerted based on the information obtained. In this thesis, two different complex reaction systems were researched. The first study focused on the degradation behavior of polymers, which was motivated by surging interest in recycling processes for plastic waste. Polystyrene (PS) and polypropylene (PP) were chosen as single component models of mixed plastic waste. Neat and binary mixture pyrolyses were performed at two different temperatures. Enhancement of the degradation rate of PP with the addition of PS was observed, which suggested that the more facile degradation of PS helped to initiate the less reactive PP.; The second system investigated was the formation chemistry of silicon nanoparticles, which are believed to be one of the leading sources of particulate contamination in chemical vapor deposition processes in the semiconductor industry and have received great attention recently due to their novel optical and electronic properties. In this thesis, automated mechanism generation was applied to unravel the chemistry in this complex reaction system. An encoding algorithm for determining species' uniqueness and a revised cycle-finding algorithm that can handle complex, polycyclic, silicon-containing molecules were developed to overcome limitations of previous algorithms. A group additivity scheme for estimating thermochemical properties of silicon hydrides was established based on quantum chemical calculations at the G3//B3LYP level of theory. Critical particle sizes were predicted under different reaction conditions, and the reaction pathways for the formation of nanoparticles were analyzed from the information obtained using automated mechanism generation. Finally, the thermochemistry and the reaction kinetics and pathways of different Si₂H₂ isomers were studied to understand the impact of molecules with multiple functionalites on the overall particle formation kinetics. The results suggested that the detailed reaction kinetics and pathways involving Si₂H ₂ isomers and other molecules with multiple functionalities merit further investigation.