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 H2 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 H2 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 H2 when the catalyst pretreatment was done without water. During the pretreatment of the catalyst with H2ON2, small amounts of H2 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 H2 formation steps. The rate determining steps were the CO oxidation and H2 formation steps. Non-dissociative (CO, CO2) and dissociative (H2) adsorption were described with Langmuir isotherms.