KINETICS AND MECHANISM OF THE ANATASE/RUTILE TRANSFORMATION, AS CATALYZE.D BY T'ERRIC OXIDE AND REDUCING CONDITIONS Eunnsox

Traces of ferric oxide have a strong catalytic effect on both nucleation and growth phases of the transfornation of TiOz from anatase to rutile. This effect is greatly enhanced in atmospheres of low/6,. Using X-ray diffraction techniques, the kinetics of the phase transformation were studied at 1 atm. and 900 to 1000'Q for anatase mixed intimately with up to 0.2/s by weight hematite (Feroa) tn COz/Hz atmospheres holding/", between 10-10 and 10-17 atm. The rate data fit the contracting reaction interface equation, dd/dt : ft(1 o;trr, where a is the extent of reaction. The rate constant & is given by: fr : %.no.z (k'fu, * k"fco * Ic"'),wherek',1c", and k"' are constants. These results are consistent with a mechanism in which the phase transformation begins at nuclei formed at, the surfaces of anatase particles coated with a thin layer of hematite. In view o{ the concerted movement of Tia+ and Oeions involved in the transformation (Shannon and Pask, 1964), the nuclei must be anion vacancies (.4. 7). The number of :{I/ formed in hematite and transferred to anatase, a.nd thus the number of reacting chains in the growth phase of the reaction (e. 9., lhe rate) are controlled by reactions of the type: 2 Or&) * Hds) + 2 OII-(D) * 2 e(h) (slow) 2 OH-(h)--+ OF(h) + AV(h) * H,O(s) (fast) It is assumed that not enough hematite is present to completely coat tho anatase particles, and in fact it turns out that at high Ws."o. the rate is independent of