Stationary and transient kinetics of the high temperature water-gas shift reaction

The high temperature water-gas shift reaction over an industrial Fe3O4-Cr2O3 catalyst was investigated by stationary and transient experiments in isothermal conditions and at elevated pressures, A new modular computer controlled catalyst evaluation unit which can be operated either as a plug flow tubular reactor (PFTR) or a gradientless reactor was used. The plug flow mode was used to produce kinetic data for power-law kinetic models and the gradientless reactor to generate kinetic data for classical kinetic models. Separate chemisorption of CO, CO2, and H-2 were done at 293, 373, 473 and 623 or 673 K to study the importance of these components as surface intermediates in the shift reaction. In PFTR the kinetic experiments were performed at 3-5 bar and 573-633 K in two separate series during the slow decay of the catalyst activity. The age of the catalyst in these experimental series was 200-280 and 725-763 h, respectively. The transient experiments were performed in the gradientless reactor at 573-623 K and 5 bar the age of the catalyst being 200-870 h. According to the stationary studies, the reaction rate is strongly dependent on the CO concentration, weakly dependent on the H2O concentration and practically independent on the CO2 and H-2 concentrations. The reaction orders with respect to CO and H2O were around 1 and 0.5. In transient experiments CO2 was always liberated faster than H-2 when the catalyst pretreatment was done without water. During the pretreatment of the catalyst with H2O/N-2, small amounts of H-2 were formed. The H2O pretreatment retarded the CO2 response. Based on these results a reaction mechanism was proposed which consisted of CO adsorption and oxidation steps as well as of H2O adsorption, decomposition and H-2 formation steps. The rate determining steps were the CO oxidation and H-2 formation steps. Non-dissociative (CO, CO2) and dissociative (H-2) adsorption were described with Langmuir isotherms.