To stimulate, and to inhibit: A theoretical understanding of the sodium-catalytic mechanism of coke gasification

Abstract

For the ironmaking process, previous research on alkali-catalytic coke gasification has focused mainly on macroscopic kinetic behavior to stimulate CO production and has neglected microscopic reaction mechanisms, especially the inhibition of carbon matrix graphitization. Here, this work combined sodium-vapor-adsorption coke gasification experiments and corresponding theoretical calculations to investigate the microscopic mechanism of the sodium-catalytic coke gasification reaction. X-ray diffraction and Raman characterization showed coke gasification with graphitization of the carbon matrix and inhibition by sodium addition. Molecular dynamics simulations based on the reactive force field (ReaxFF) well reproduced the experimental results and confirmed the sodium-catalytic effect. Comprehensive reaction pathways in coke gasification with and without sodium have been summarized from ReaxFF simulation trajectories, where sodium-containing elementary reactions were verified by density functional theory. Calculations showed that the sodium atoms not only reduced the activation energy substantially in the initial carbon layer oxidation, but also adsorbed at carbon layer defects spontaneously and inhibited carbon matrix reconstruction. This behavior hindered carbon layer growth but facilitated carbon layer oxidation and accelerated CO molecule production. This mechanism, which was obtained from combined experimental and theoretical approaches, provides theoretical support for an in-depth understanding of the catalytic role of sodium in carbon material gasification.