Effect of the ionizing radiation on the kinetics of the reduction by hydrogen of NiO-Fe2O3 mixed oxides of various genesis

Abstract

The kinetics of the reduction by hydrogen of two series of NiO-Fe2O3 mixed oxides of different composition was studied in the temperature range 320 - 410 °C using thermogravimetry. Both series were prepared from different precursors that were thermally decomposed at different temperatures. The oxidizing ability of the surface ( the content of chemisorbed oxygen) is the only parameter from all physico-chemical properties studied that changed significantly during the pre-irridation of the system by 60Co gamma radiation (dose 505 kGy), by fast neutrons from the 252Cf source (dose Dn = 110 Gy) and by accelerated electrons (3 MeV, dose 500 kGy). A positive radiation effect was observed in the first case while the irritation by neutrons and electrons resulted in a negative effect. The magnitude of the effect depends on the composition and genesis of the system and it decreases in the sequence γ, e-, n. The radiation induced changes in the reduction kinetics of mixed oxides change their magnitude and sign in dependence on the composition and genesis. Not only the increase by an order of magnitude of the rate of the initial phase of the reduction which was ascribed to the partial reduction of hematite to magnetite but also the acceleration of the second step (reduction of Fe3O4 to Fe) was observed after the exposure to photons or fast neutrons. The radiation changes induced by electrons are limited only to the surface of the irradiated solid and they affect only the first step of the reduction. The maximum positive effect was observed in the region of high excess of hematite and its magnitude decreases with the increasing content of nickel oxide. The radiation effects decrease with the increasing temperature of the reduction and at about 400 °C the effect of irradiation is completely annealed.