THE ACTIVE SURFACE SPECIES IN ALKALI-CATALYZED CARBON GASIFICATION - PHENOLATE (C-O-M) GROUPS VS CLUSTERS (PARTICLES)

The activities and catalytic roles of different species of potassium in the gasification reactions of graphite by H2O and CO2 are investigated. TEM techniques are used to measure the rates of monolayer edge recession (uncatalyzed and COK catalyzed) and rates of monolayer channeling (catalyzed by particles) on the basal plane of graphite reacting with 21 Torr H2O at 700°C. The turnover frequencies for carbon gasification are: 0.08 s−1 (uncatalyzed), 0.15 s−1 (catalyzed by COK groups), and 7.8 s−1 (catalyzed by particles). Thus the particles have a high activity, whereas the COK groups have only a small activity. TGA and literature results using mixtures of carbon and alkali salts show a sigmoidal dependence of gasification rates on catalyst loading. This is the result of catalyst dispersion and competition between the COK groups and alkali particles. A CNDO semi-empirical molecular orbital calculation is performed on model graphite substrates with -O and -O-K groups bonded to the {101l} zigzag face and {112l} armchair face. On the zigzag face, the carbon atom bridging two COK groups gains a large negative charge (−0.486) and hence is a favorable site for binding an O atom. The surface CC bonds in this structure are substantially weakened by adding O atoms on the bridging C atoms, leading to CO release. The O atoms are supplied by the dissociation of H2O and CO2. The possible reason for the alkali particles being more active than the COM (where M = alkali) groups is that the particles can dissociate H2O and CO2 at higher rates, by providing either more active sites or higher activities. The CNDO results also predict that the C-O-K groups have an inhibiting effect on the armchair face; an inhibiting effect has indeed been observed earlier in our laboratory. © 1993 Academic Press, Inc.