Mechanistic modeling of the high temperature water gas shift reaction on ferrochrome
Coleman, John Slocum
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This dissertation consists of investigations on ferrochrome, copper promoted ferrochrome, and gold on ferrochrome. The first investigation involved the examination of reaction mechanisms for water-gas shift over ferrochrome catalysts under high temperature operation. Mechanisms were developed to represent the two primary mechanism types presented in literature, namely the regenerative and associative mechanisms. Both mechanism types have been suggested within the literature based on the fit of rate expressions derived from them, and experimental characterizations have provided support for both mechanisms. Mechanistic kinetic modeling was performed and comparisons to experimentally available information were made between each of the mechanisms considering both mechanistic parameters and predicted behavior under reaction conditions. A combined mechanism was additionally proposed to provide both mechanistic pathways, and comparisons to each of the individual mechanisms were made. The results of the mechanistic kinetic modeling suggest that an associative mechanism does not accurately describe water-gas shift. Both the regenerative and combined mechanisms yield quantitatively and qualitatively similar predictions. The observations made during the mechanistic kinetic modeling were then used to develop a computational chemistry model of the active site for water-gas shift over ferrochrome. Cluster models representing potential active sites of the (100), (110), and (111) surfaces of magnetite were generated and compared using the enthalpy of localization for the surface oxygen species as a selection criterion. The cluster developed from the (111) surface showed the best qualitative agreement to the regenerative and combined mechanisms, suggesting the (111) surface of magnetite may be catalytically active. The combined mechanism as formulated for the mechanistic kinetic modeling of an unpromoted ferrochrome catalyst was then fit to experimental data generated for a copper promoted ferrochrome catalyst. While copper has been shown in the past to be an effective promoter for ferrochrome water-gas shift catalysts, the method of promotion is still unclear. The mechanistic kinetic study of a copper promoted ferrochrome catalyst suggested that copper acts as a substitutional promoter, modifying key parameters within the mechanistic model. The changes in the kinetic parameters caused by copper in the combined mechanistic model mirror predictions made using a "virtual" catalyst developed from the mechanistic model for an unpromoted ferrochrome. Finally, an experimental study of the preparation and performance of gold promoted low temperature ferrochrome catalysts was underataken. The key to active gold catalysts lies in the size of gold particles supported on the surface of the catalyst. The investigation of nanoparticles preparation techniques and their eventual attachment onto a ferrochrome support suggested that while monodisperse gold can be synthesized, the attachment onto the catalyst surface generates agglomerated clusters that are catalytically inactive. Traditional deposition-precipitation techniques yielded active low temperature catalysts. These catalysts were evaluated kinetically, and a rate expression for low temperature water-gas shift was developed. They were observed to deactivate appreciably under reaction conditions, and this aspect of their performance was also quantified.