Reactivity inversion in the NiO---O2 system

Abstract

The rate of redistribution of carbon-14 between CO and CO2 has been investigated over NiO catalysts in the temperature range 274 [deg] to 450 [deg]C, and at pCO2/pCO ratios 0.1 to 10. Analysis of the results shows that the sign of the relation between the reaction rate and the pCO2/pCO ratio is a function of temperature and pCO2/pCO ratio. A thermodynamic model for the description of the gas-solid defect equilibrium has been formulated to explain (a) the reactivity inversion, (b) the numerical values of the mathematical relationship between reaction rate coefficients and the pCO2/pCO ratio. On the basis of the model, suggestions are advanced for (a) the rate-controlling step of the isotopic exchange reaction, (b) the nature of surface defect sites active in the catalytic reaction and of the adsorbed oxygen intermediate, (c) the slow reaction step and the gas-phase equilibrium controlling the surface concentration of oxygen during the oxidation of CO by O2 on NiO.The reactivity reversal is discussed in the framework of the inversion in physicochemical properties that is known to take place in NiO.