Low temperature water-gas shift reaction over gold-ferrochrome catalysts
Vajani, Gaurav Narendra
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This work consists of the investigation of gold-ferrochrome catalysts under low temperature water-gas shift reaction conditions. The goal of this research is the development of a kinetic model and understanding the deactivation mechanism of gold-ferrochrome catalyst during water-gas shift reaction. The motivation for studying these catalysts comes from the importance and need of water-gas shift for low temperature applications requiring highly active catalysts that remain stable for long term commercial use. Gold based catalysts supported on metal oxide supports show considerable promise but no kinetic models exist and very little is known about their stability. The first part involves the development of a kinetic model for low temperature water-gas shift over gold-ferrochrome catalysts. Using experimental measurement of kinetics, a power-law rate expression with a factor accounting for catalyst deactivation has been presented. This is the first reported rate expression for gold-ferrochrome catalysts used for water-gas shift. The effect of various operating conditions on the reaction rate has been discussed. The second part investigates the cause of catalyst deactivation and a mechanism of deactivation of gold-ferrochrome catalyst has been proposed. Under various operating conditions, the catalyst loses its activity rapidly. Long term stability tests and adsorption measurements provide evidence for the loss of active sites. Two aspects of catalyst deactivation are studied: firstly, possible changes in the nature of support and gold particles and secondly, detailed investigation into catalyst deactivation by carbon deposition. Characterization of active and deactivated samples using microscopic and spectroscopic techniques provides qualitative insight into the cause of catalyst deactivation. The degree of deactivation by sintering and carbon deposition has been quantified by exposing the catalyst to various environments. A novel technique for in-situ gravimetric analysis of the catalyst is developed using a tapered element oscillating microbalance and this helps shed light on the loss in catalytic activity and its relation to carbon deposition. Thermodynamic calculations provide validation for the phase of the support and ascertain the tendency of carbon deposition under low temperature water-gas shift conditions. Carbon deposition and sintering of gold, the main factors contributing to the deactivation of gold-ferrochrome have been discussed.