MECHANISM OF ALKALI AND ALKALINE-EARTH CATALYZED GASIFICATION OF GRAPHITE BY CO2 AND H2O STUDIED BY ELECTRON-MICROSCOPY

The kinetics and mechanism of graphite gasification by CO2 and H2O catalyzed by five alkali and alkaline earth metal catalysts are investigated by studying the monolayer channeling action on the basal plane of graphite. CO2 and H2O are dissociatively chemisorbed by these catalysts, followed by diffusion of O atoms/ions to the edge carbon sites, where breakage of C&.sbnd;C bonds takes place to free CO. The CC bond breakage is the rate-limiting step. The catalytic activities as measured by the turnover frequencies on the edge carbon sites follow the order K > Ba > Ca > Na > Li. The same catalytic mechanism and rate-limiting step operate in both channeling action and bulk reactions using mixed powders of carbon and catalysts. The results on channeling also indicate that the active intermediates are catalysts in the form of particles and clusters, and that the C&.sbnd;OM (where M denotes metal) phenolate groups, although present in abundant amounts, are not the active intermediates. For very small particles, the surface free energy appears to play a role in lowering the amounts of CO2 and H2O chemisorbed, hence decreasing the channeling rates. © 1992.