The role of water in the oxidation of lead and bismuth by high-density water-oxygen fluid

The paper presents the research results on oxidation of liquid lead (Pb)^l and bismuth (Bi)^l by high density water-oxygen fluid (ρ_H2O = 0.91–4.69 and ρ_O2 = 0.75–2.93 mol/dm³). Time dependences of the pressure were obtained by the method of H₂O/O₂ fluid injection into the reactor at different temperatures (623–873 K), from which the dependences of the amount of O₂ in the reaction mixture were calculated and the oxidation kinetics was described. The results revealed a non-monotonic dependence of the (Pb)^l oxidation rate and mechanisms on temperature and H₂O/O₂ molar ratio, caused by the transition from kinetic to diffusion modes. The oxidation rate of (Bi)^l was observed to increase with increasing temperature and is governed by diffusion. The oxidation of (Pb)^l at 653–723 K and molar ratio H₂O/O₂ = 1.45–1.70 was found to lead to the formation of a porous oxide layer consisting of polyhedral Pb₃O₄ crystals. The oxidation of (Bi)^l gives rise to the synthesis of Bi(OH)₃ and Bi₂O₃ in the form of plates, ribbons, and rods exhibiting diverse crystal structures. It is shown that H₂O molecules can participate in oxidation of (Pb)^l as a catalyst, whereas in the oxidation of (Bi)^l they act as the reagent. The results obtained may be useful in developing new approaches to the synthesis of oxide particles using supercritical water fluids, evaluating the stability of catalysts based on variable valence metal oxides, and ensuring the safety of power units.