AUTOCATALYTIC REDUCTION OF HEMATITE WITH HYDROGEN UNDER CONDITIONS OFSURFACE CONTROL - A VACANCY-BASED MECHANISM

An autocatalytic mechanism is shown to explain the kinetics of the sur face reaction occurring during the low-temperature reduction of sinter ed hematite pellets with hydrogen between 300 and 500 degrees C, The r esults satisfactorily fit the predictions of a mechanism based on nond issociative chemisorption of hydrogen on the oxygen vacancies at the s urface of the hematite pellet. This chemisorption process activates th e hydrogen molecule by substantially decreasing its bond energy and in creasing its bond length, The calculated activation energy of the surf ace reaction over this temperature range was found to be much smaller than those reported earlier. The larger values are believed to have be en affected by some diffusion control within the solid phase, as a res ult of the limitations of the weight loss technique which was used in those measurements. (C) 1996 Academic Press, Inc.