Catalysis of carbon oxidation by transition metal carbides and oxides

Abstract

The technique of etch decoration/transmission electron microscopy has been employed to study the catalysis of the carbon-oxygen reaction by seven group VB and VIB metal carbides and oxides. Turnover frequencies for carbon gasification were measured at 680 °C from the monolayer pit expansion rate on the basal plane of single-crystal graphite, with and without catalyst particles deposited on the surface. Among the catalysts used in this study, only MoO 3 followed the wellestablished mode of channeling, while all others catalyzed the reaction at the edges of the etch pits which were distant from the catalyst by apparently a long-range action. The etch pits formed in the presence of carbides (WC, TaC, and Mo 2C) were all hexagonal, unlike the uncatalyzed reaction in which pits were circular, and the edges of the hexagonal pits were composed of the zigzag {101̄0} faces. These pits were similar to those created directly by atomic oxygen. The results suggested that the transition metal carbides served as dissociation centers and dissociation was followed by spillover and reaction on the edges of the pits. Deformed circular pits were formed in the presence of the oxide catalysts (Cr 2O 3, WO 3, and Ta 2O 5), and the oxide catalyst particles disintegrated and dispersed on the surface. Because the reaction temperature was near or above the Tammann temperatures, it appeared that the catalysts dispersed by emission of molecular species or clusters which migrated on the basal plane of graphite and were subsequently trapped on the edges of the etch pits. Gasification of the edge carbon atoms was thus catalyzed by the trapped catalyst through direct contacts.