UBC Theses and Dissertations
CH₄ decompostion kinetics on supported Co and Ni catalysts Zhang, Yi
Methane activation is important in a number of reactions that aim to convert natural gas to more valuable products using supported metal catalysts. As a potential alternative to steam reforming and partial oxidation, catalytic decomposition of CH4 may provide H2 without CO contamination for use with PEM fuel cells. However, the mechanism of carbon deposition and catalyst deactivation during CH4 decomposition is complex and not fully understood. The present work is aimed at clarifying some aspects of catalyst deactivation during the decomposition of CH4 at moderate temperatures on low loading Co and Ni catalysts. The experimental observations presented in the present work suggest that catalyst deactivation was a consequence of the competition between the rate of encapsulating carbon formation and the rate of carbon diffusion. Stable activity or catalyst deactivation during CH4 decomposition was observed, depending on which of these two rates was greater. The experimental observations also show that the gas phase composition KM , and catalyst properties such as metal particle size and metal-support interaction have a critical effect on catalyst deactivation: catalyst deactivation was reduced with increasing KM and with increasing metal particle size; catalyst deactivation was increased by a strong metal-support interaction. A general kinetic model of CH4 decomposition on supported metal catalysts has been developed based on experimental observations and the deactivation mechanism described above. The initial rate increase was described by including the rate of carbon nucleation at the tailing face of the metal particle using two methods: Cluster nucleation (Kinetic Model I) and Boltzmann nucleation (Kinetic Model II). The fit of literature data to Kinetic Model I and Kinetic Model II confirmed the presence of carbon nucleation at the tailing face. The observed CH4 decomposition activity profiles on supported Co catalysts with either stable activity or declining activity were well described by the kinetic model. The site density profile along the metal particle was obtained and the effect of metal particle size on the CH4 decomposition activity has been quantified by fitting the observed CH4 decomposition activity profiles to the developed kinetic model.
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