Kinetics of the Reaction of CO 2 /CO Gas Mixtures with Iron Oxide
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INTRODUCTION
THE kinetics of oxygen exchange between gases and metal or oxide substrates is important in understanding oxidation and reduction. Even when diffusion in the solid controls the overall rate, the relative kinetics of competing reactions can have a strong influence on the local thermodynamic conditions and can control which phase will form.[1–7] As part of an overall study on the role of competition between reactions of different gases at solid surfaces, the authors investigated the kinetics of oxygen exchange on the surface of iron and iron oxides. A. Kinetics of Oxygen Exchange on Iron Oxide In previous research on oxygen exchange between CO2 and iron oxide, Grabke and Viefhaus[8] found the oxygen activity dependence of the apparent rate constant, ka, to follow Eq. [1] with a constant value of m, which is expressed as follows: ka ¼ ko am O :
½1
Grabke[9,10] proposed a mechanism for the reaction based on electron transfer to an adsorbed CO2 molecule. These workers claimed the oxygen activity dependence resulted from the electron defect structure of the oxide. However, it is now known that the defect structure of wu¨stite is much more complex than assumed by Grabke. The complexity of the defect structure is such that some
ELIZABETH J. WORRAL, Metallurgist, is with Severstal Dearborn, 14661 Rotunda Drive, Dearborn, MI 48120. Contact e-mail: [email protected] KENNETH S. COLEY, Professor, is with the Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4L7, Canada. Manuscript submitted May 1, 2006. Article published online April 15, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
workers have proposed several pseudo phases to exist within the wu¨stite phase field. The existence of pseudo phases remains controversial, but the complexity of the defect structure and the fact that it changes with oxygen potential is now well established. It should be expected that such a complex defect structure would lead to a more complex relationship than that defined by Eq. [1]. B. Pseudo Phases of Iron Oxide The existence of pseudo phases in the wu¨stite phase field has been the subject of much debate. Vallet et al.[11] were the first to suggest that the wu¨stite phase was more complex than originally thought. Since the initial work by Vallet et al., the wu¨stite phase field has been probed by many methods to determine whether the pseudo phases exist. Evidence for their existence has been provided by dilatometry,[12–14] X-ray diffraction,[12,13,15,16] thermogravimetry,[17–25] neutron diffraction,[26] electromotive force,[27,28] and electrical conductivity[29,30] measurements. However, various studies have been conducted that have provided no evidence for the existence of more than one phase in wu¨stite. These studies include those which provide thermogravimetetric,[31] electromotive force,[32–34] X-ray diffraction,[35] and work function[36] measurements. C. Effect of Defect Structure on Kinetics of Interfacial Reactions In studies using the isotope exchange te
