CATALYTIC BEHAVIOR OF TRANSITION-METAL OXIDE IN GRAPHITE GASIFICATION BY OXYGEN, WATER, AND CARBON-DIOXIDE

Vanadium oxide is used as a model transition metal-oxide catalyst for the gasification of graphite by O2 H2O, and CO2. The following catalyst actions on the graphite basal plane are common in all three reactions: deep layer channeling, monolayer channeling, edge recession, and widening monolayer channeling. The steady-state oxidation states of vanadium in the reactions with CO2 and H2O are, respectively, V3O5 and V2O3 under the reaction conditions. The rate-limiting step in all channeling actions (for all three gasification reactions) is the oxidation on the surface of the metal oxide to increase the oxygen activity. Oxygen then diffuses through the nonstoichiometric metal-oxide particle to reach the carbon active sites where gasification takes place. (However, based on the channeling and TGA rate data, the catalyst surface oxidation is the rate-limiting step; the subsequent steps of diffusion of oxygen anion and CC bond breakage steps are not rate-limiting.) This mechanism can explain the fact that transition metal oxides are the most active catalysts for the C + O2 reaction but are only weakly active for the C + H2O and C + CO2 reactions. The origin of the phenomenon of the widening monolayer channels is attributed to the anisotropy of the catalyzed reactivities of two adjacent zigzag edges of graphite. © 1991.