(702b) On the Mechanism of Surface Convex and Concave Reduction Via Outward Solid-Phase Ionic Diffusion In Gas-Solid Reactions

Gas solid reaction is a typical chemical reaction, which is ubiquitous in process industry. One of the properties that characterize the gas-solid reactions is the surface structure, which determines the solid exposure to the gas reactant in the reaction process. Generally, a solid surface can be regarded as combinations of convexes and concaves. Many gas-solid reactions (e.g. M + O₂ -> MO) induce decreases of surface area and pore volume. Such surface structure change is widely attributed to sintering and molar volume increase. Sintering effect tends to agglomerate smaller particles into big ones and fuse surface pores. In most gas-solid reactions, solid reactants capture partial or entire gas component, which usually induces molar volume increase. Thus, the increased molar volume of solid particle narrows the inter-particle(grain) distance and plugs pores (concave). In this study, we found that there exists a third mechanism for surface convex and concave reduction via outward ionic diffusion. An iron oxidation reaction is used to represent all the gas-solid reactions involving outward ionic diffusion of solid species. It was found that surface concave tends to be buried and surface convex tends to flatten as the reaction proceeds. A 2-D continuum diffusion model is also established to simulate this solid-phase diffusion process, which shows consistent results as the experiments.