1 Photo - Stimulated Oxidation of Magnetite , and an Application

It has been discovered that magnetite ( Fe304 ) is rapidly oxidized to hematite (o<-Fe 0 ), upon illumination ( X<.310p1 ) in an 02-bear~ni atmosphere. A laboratory investigation has been carried out to determine the kinetics and mechanism of this rapid oxidation process. It has been found that the process consists of a series of constituent reactions which are common to most oxidation mechanisms: (1) atmospheric 02 dissociates on the surface into two adsorbed atoms, upon colliding with a pair of adjacent vacant adsorption sites; (2) This step is followed by the oxidation of Fe(II) to Fe(III), simultaneous with electron attachment to the adsorbed 0 atoms, to form adsorbed 0~; (3) Step (2) is followed by the rapid attachment of a sec nd electron to the adsorbed 0~, to form chemisorbed O ~; (4) Finally, the chemisorbed 02ions are incorporated into the magnetite surface layer, forming hematite (o-Fe 2 03 ) scales. The role of adsorbed H20 in the formation of hematite nuclei, and the subsequent scale formation in the absence of H 0 by "nucleation disruption", is discussed. Although the principal oxidation product is hematite, the possibility of maghemite ( Y-Fe203 ) formation, during nucleation, is discussed. The factor which allows this oxidation process to proceed in a low-temperature, low-O partial pressure, water-free, otherwise nonoxidizing environment, is the presence of the ultraviolet light. The UV illumination gives rise to photoemission, which increases the rates of Fe(II) oxidation and the attachment of the electrons to the oxygen. The slowest step in the oxidation mechanism, and therefore the rate determining step, is the attachment of an electron to adsorbed 0.