ZnO nanoparticles formation by reactions of bulk Zn with H2O and CO2 at sub- and supercritical conditions: I. Mechanism and kinetics of reactions

Oxidation of solid and liquid zinc by water (temperature 335-648 degrees C, pressure 13.7-31.2 MPa, water density 0.045-0.420 g/cm(3)), supercritical CO2 (514-600 degrees C, 7.1-24.4 MPa, 0.041-0.158 g/cm(3)), mixture of H2O/CO2 (140-600 degrees C, 30 MPa, water density 0.026-0.569 g/cm(3), initial ratio of [H2O](0)/[CO2](0) = 0.46-2.75 mol/mol) has been studied. In pure CO2, besides CO, carbons are formed and in mixtures H2O/CO2 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 [H2O]/[CO2] 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 H2O is described by the equation k(W)(T) = 1.09 x 10(-4) exp(-53.23 (kJ/mol)/RT). At the same temperature rate constant of Zn reacting with CO2 is 44 times less than k(W), and the rate constant of Zn reacting with H2O/CO2 mixture is 25.4% less than k(W). (C) 2008 Elsevier B.V. All rights reserved.