JOURNAL OF CATALYSIS I@, 329-346 (1988) A Mechanism for Sodium Oxide Catalyzed CO2 Gasification of Carbon JOHN M. SABER,* KEITH B. KESTER,',* JOHN L. FALCONER,*~* AND LEE F. BROWN? *Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424, and tlsotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received June 8, 1987; revised September 1, 1987 Temperature-programmed reaction was used to study sodium oxide catalyzed CO* gasification of r3C. The stoichiometry of the reaction is CO2 + a”C cf CO + (1 - u)‘~CO~ + (2~ - l)‘)CO, where the factor a is dependent on CO2 conversion. This stoichiometry is consistent with a mecha- nism composed of a reversible catalyst oxidation step and an irreversible catalyst reduction step. On the surface the catalytic surface species is oxidized by CO2 and forms CO; during the reduction reaction the oxidized species decomposes. The reversible oxidation step, which incorporates sub- strate carbon into gas-phase carbon dioxide, is at equilibrium. Carbon monoxide, via the reverse of the oxidation reaction, inhibits gasification of the substrate by decreasing the number of oxidized catalytic sites on the surface. The catalyst appears both to increase the amount of oxygen on the surface and to decrease the activation energy of the reduction reaction. Similar mechanisms de- scribe potassium- and calcium-catalyzed gasification, but NaZCO, does not interact with the carbon surface as readily as KZC03 does. A similar mechanism is also consistent with steam gasification results reported by others and shows that a separate water-gas shift reaction is not required for CO, production. 0 1988 Academic Press, Inc. INTRODUCTION This paper presents a mechanism (reac- tion steps, catalyst stoichiometries) for the sodium oxide catalyzed CO2 gasification of carbon. Although the group IA carbonates and other oxygen-bearing salts catalyze carbon gasification reactions (Z-3), the mechanisms of these reactions have yet to be understood completely. Recent investi- gations (4-20) suggest that alkali metal ox- ides catalyze carbon gasification via an oxygen transfer or oxidation-reduction mechanism. Though it appears that the pro- cess involves a cyclic oxidation and reduc- tion of the catalytic species, many funda- mental questions about the process are unanswered. Langmuir (11) first proposed an oxygen- I On leave from Department of Chemistry, Colorado College, Colorado Springs, CO. Z To whom correspondence should be addressed. transfer mechanism for the uncatalyzed carbon-carbon dioxide reaction in 1915, and it has gained general acceptance (22) in one form or another. Langmuir proposed the following: co* + c + (CO)& + co (1) (C)ads * co (2) with reaction (2) as the rate-limiting step. This mechanism implies that the CO mole- cule produced in reaction (1) contains the carbon atom from the CO2 molecule. Bon- ner and Turkevich (23) showed this to be the case by reacting 14C02 with 12C at tem- peratures below those required for reaction (2). On freezing out the reacted i4C02, they observed that the specific radioactivity (emissions per unit volume) of the remain- ing gas was the same as that of the original carbon dioxide. Thus, during uncatalyzed gasification, all the i4C that reacted pro- duced a gas-phase product containing i4C. 329 0021-9517/88 $3.00 Copyright 0 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.