ZnO nanoparticles formation by reactions of bulk Zn with H 2O and CO 2 at sub- and supercritical conditions: I. Mechanism and kinetics of reactions

Abstract

Oxidation of solid and liquid zinc by water (temperature 335–648 °C, pressure 13.7–31.2 MPa, water density 0.045–0.420 g/cm 3), supercritical CO 2 (514–600 °C, 7.1–24.4 MPa, 0.041–0.158 g/cm 3), mixture of H 2O/CO 2 (140–600 °C, 30 MPa, water density 0.026–0.569 g/cm 3, initial ratio of [H 2O] 0/[CO 2] 0 = 0.46–2.75 mol/mol) has been studied. In pure CO 2, besides CO, carbons are formed and in mixtures H 2O/CO 2 along with H 2 and CO, paraffin and aromatic hydrocarbons, alcohols and formaldehyde are synthesized. Combustible products composition depends on the temperature and ratio of [H 2O]/[CO 2] in the mixture. In every experiment ZnO nanoparticles formation was observed. Cluster mechanism of nanostructured ZnO generation has been proposed and kinetic parameters of this process have been revealed. Temperature dependence of the rate constant of Zn reacting with H 2O is described by the equation k W( T) = 1.09 × 10 −4 exp(−53.23 (kJ/mol)/ RT). At the same temperature rate constant of Zn reacting with CO 2 is 44 times less than k W, and the rate constant of Zn reacting with H 2O/CO 2 mixture is 25.4% less than k W.