Steady state experiments and their interpretation were performed to explore the kinetics of CO oxidation over a supported c0@4 catalyst. The rate of reaction was found to depend on temperature and the partial pressure of oxygen but not on the partial pressure of carbon monoxide. It is shown that catalyst dynamics must be accounted for to achieve real understanding. SCOPEAmong the transition metal oxides, cobalt oxide (Co304) is one of the most active catalysts for carbon monoxide oxidation. The catalytic surface is known to play an important role, yet there is no consensus in the literature regarding the reaction mechanism. The only previous extensive study of this reaction system was by Yao (1974), who emphasized the comparison of activities of cobalt oxide catalysts prepared by various methods. She showed that the catalytic activity of C0304 for CO oxidation could even compare favorably with the noble metals, but her rate data were not sufficiently definitive to attempt any mechanistic interpretations. The broad goal of the three papers that comprise this series is to elucidate the mechanistic character of oxidation-reduction catalysis by metal oxides. This first paper deals with conventional steady state experimentation. A fixed-bed reactor was used to study the dependence of conversion (and thence reaction rate) on flow rate, temperature, CO and 0 2 partial pressures, and catalyst characteristics. The interpretation of the steady state results enhances understanding, but more importantly it establishes a basis for comparison of the results of the dynamic studies described in Parts I1 and 111.

Interpretations of a variety of transient experiments are combined with information from the literature to reveal the roles played by the Co304 catalyst surface and adsorbed species. The reaction path depends on the oxidation state of the catalyst. A rather coherent picture of oxidation-reduction catalysis by a metal oxide is obtained. DEEPAK SCOPEThe mechanism of oxidation-reduction reactions on metal oxide catalysts is not well understood. In particular, very little coherence exists in the information available on the very active catalyst for CO oxidation, C084. It is believed that the trivalent cobalt ion in the spinel structure is responsible for CO oxidation.However, the roles of lattice oxygen and various adsorbed species are not at all clear. Steady state and unsteady state kinetic studies were reported in the previous papers (Parts I and 11). The dynamic studies provided a wealth of phenomenological insights that can be used to postulate a mechanism for this catalytic reaction. The postulated mechanism can then be checked for consistency with the rate equation from the steady state studies and with information from the fragmented literature. The mechanism thus postulated and evaluated may promote practical exploitation of this catalytic reaction, provide insight into the general matter of metal oxide catalysis, and suggest productive avenues for future research. CONCLUSIONS AND SIGNIFICANCEThe phenomenological conclusions from the reactor dynamics studies (Part 11) and specific chemical information from the literature provided the basis for the postulation of a mechanism for the oxidation of CO over Co304/y-A1&3. The reaction path depends in a number of ways on the oxidation state of the catalyst. From the mechanism a rate equation can be derived for the circumstances of the steady state experiments. Excellent agreement is found between this rate equation and the one obtained in Part I. This provides partial validation of the postulated mechanism. The mechanistic insights also suggest the origins of the two kinds of catalyst deactivation contemplated in Part I. Finally, a rather coherent picture of oxidation-reduction catalysis by a metal oxide is obtained.

The transient responses of a fixed-bed reactor to step increases and decreases in CO, 0 2 , and/or CO2 feed concentrations were measured and interpreted. It is shown that dynamic methods yield vastly more phenomenological and mechanistic information than steady state measurements, with significantly less experimental effort.

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