Chor Wong

Kinetics and mechanism of oxidation of basal plane on graphite

The kinetics and mechanism of oxidation of carbon atoms exposed at monolayer steps on graphite surface have been studied with the etch‐decoration and transmission electron microscopy technique and two pieces of direct evidence are shown for the importance of surface diffusion in the overall kinetics. It is found that the rate of carbon removal, or the turnover frequency, depends on the population density of these edge carbon atoms, being substantially higher on low‐density surfaces. In the argon flush experiment, it is found that surfaces with low edge carbon densities continue to ’’burn’’ for prolonged periods of time after O2 is cut off from the gas phase. Two independent mechanisms are revealed by the experimental results: (1) reaction resulted from direct collision of O2 on the edge carbon, and (2) reaction of the edge carbon with the migrated oxygen which is first chemisorbed on the basal carbon. Furthermore, from the results of the argon flush experiments, the following results can be calculated: su...

The mode of attack of oxygen atoms on the basal plane of graphite

Gold decoration of the basal lattice plane of graphite etched by atomic oxygen has been made possible by first desorbing the surface oxide layer on the basal plane. The formation of monolayer pits, as revealed by gold decoration, shows that anisotropy does exist, i.e., carbon atoms are preferentially attacked by atomic oxygen in the direction of a axes, rather than the direction of c axis. The monolayer pits are initiated from lattice vacancies which are continually formed by atomic oxygen. The monolayer pits created by atomic oxygen are, unlike the pits by O2 which are circular, hexagonal in shape, the sides of which are composed of 〈101l〉 lattice planes.

Catalysis of carbon oxidation by transition metal carbides and oxides

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 MoO3 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 Mo2C) were all hexagonal, unlike the uncatalyzed reaction in which pits were circular, and the edges of the hexagonal pits were composed of the zigzag {1010} 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 (Cr2O3, WO3, and Ta2O5), 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.

Etch decoration: scanning electron microscopy technique for measuring carbon gasification rates

Transmission electron microscopy has been used for some important discoveries underlying gas‐carbon reaction kinetics. This technique requires cleavage of the sample to about 700 A thickness. This study demonstrates that the same results by TEM can also be obtained by a much easier technique using scanning electron microscopy. Furthermore, the SEM technique is applicable to uncleavable materials and provides additional microscopic information on the surface.