ISOTHERMAL REDUCTION KINETICS OF TITANIUM DIOXIDE-BASED MATERIALS

As metal oxide reduction may be a limiting or otherwise important step in a reaction cycle, a complete description of the kinetics of the re duction can be critical to the successful choice of catalytic material . Unfortunately, such information is often lacking. Such is the case i n our attempts to develop a catalytic cycle from the stoichiometric re ductive carbonyl coupling reaction on reduced TiO2 surfaces. To provid e the necessary reduction kinetics, reaction of the anatase and rutile forms of TiO2 with H-2 has been studied from 573 to 773 K. A novel fl ow-through microreactor which provides time-resolved catalyst mass mea surements to +/-1 mu g while maintaining a conventional, tubular react or, gas-solid contacting pattern has been employed. A shift in the kin etic order with respect to H-2 with increasing temperature occurs, fro m one-half order at 573 K to zero order at 673 K and above. A disconti nuity was also observed within this same temperature range in Arrheniu s plots of the reduction rates of both anatase and rutile TiO2; appare nt activation energies determined were approximately 12 kcal mol(-1) a bove and 29 kcal mol(-1) below 623 K. Modification of the surface of a natase TiO2 with a sufficient loading of group VIII metals removes the Arrhenius plot discontinuity, increasing the rate of reduction and de creasing the apparent activation energy at low temperatures. A change in rate-determining step is indicated by these observations, and a mec hanistic scheme which combines the current and previous observations w ithin a single framework is proposed.