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 reduction can be critical to the successful choice of catalytic material. Unfortunately, such information is often lacking. Such is the case in our attempts to develop a catalytic cycle from the stoichiometric reductive carbonyl coupling reaction on reduced TiO2 surfaces. To provide 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 flow-through microreactor which provides time-resolved catalyst mass measurements to +/-1 mu g while maintaining a conventional, tubular reactor, gas-solid contacting pattern has been employed. A shift in the kinetic order with respect to H-2 with increasing temperature occurs, from one-half order at 573 K to zero order at 673 K and above. A discontinuity was also observed within this same temperature range in Arrhenius plots of the reduction rates of both anatase and rutile TiO2; apparent activation energies determined were approximately 12 kcal mol(-1) above and 29 kcal mol(-1) below 623 K. Modification of the surface of anatase TiO2 with a sufficient loading of group VIII metals removes the Arrhenius plot discontinuity, increasing the rate of reduction and decreasing the apparent activation energy at low temperatures. A change in rate-determining step is indicated by these observations, and a mechanistic scheme which combines the current and previous observations within a single framework is proposed.