The use of [formula omitted] gas mixtures to study adsorption on chromia-promoted magnetite at water-gas shift temperatures

Abstract

The adsorption of CO and CO 2 on chromia-promoted magnetite was studied volumetrically at temperatures near 650 K using CO 2 CO gas mixtures in which magnetite is the thermodynamically stable form of iron oxide. Specifically, studies were conducted (i) at 637 K using CO 2 CO gas mixtures with partial pressure ratios, P CO 2 P CO , equal to 5.45, 2.37, and 1.56, and (ii) at 613, 637, and 663 K using a CO 2 CO gas mixture with P CO 2 P CO equal to 5.45. By measuring the changes in total pressure and gas composition when a known dose of gas mixture was exposed to the sample, the individual surface coverages by CO and CO 2 were determined. At constant temperature and P CO, the surface coverage by CO was found to decrease with increasing P CO 2 ; and, at constant temperature and P CO 2 . the surface coverage by CO 2 was found to decrease with increasing P CO. These data could be fit by either a one-site or a two-site model. In the first model, CO and CO 2 compete for a fixed number of adsorption sites; and, in the second model, CO adsorption is associated with surface oxygen vacancies while CO 2 adsorption is associated with surface oxygen species. In both models, the total number of adsorption sites comprises approximately 21% of the BET surface area. While the adsorption data alone cannot distinguish between these models, the two-site model is favored by its ability to provide a quantitative correlation of the CO and CO 2 adsorption data with independent measurements of the oxygen content of the surface.