THERMODYNAMICS AND KINETICS OF H2O(V) CHEMISORPTION AND SOLUBILITY IN NANOMETRIC AND SINGLE-CRYSTAL MGO PARTICLES DURING SINTERING

Equilibrium chemisorption of H2O at 613 Pa on nanometric and micrometric MgO particles was measured at 549-1053 K. Degrees of coverage are calculated on the assumptions that only {100} surfaces of MgO were exposed and that each surface O2- ion reacted with H20 to form 2OH- ions. When the thermal entropy of adsorption is assumed to be equal to the correspondent value for the reaction of H2O(v) with MgO to form Mg(OH)2, the data fit a regular solution model. The enthalpy of adsorption is calculated to be -189 kJ mol-1 at low coverage and -113 kJ mol-1 at 70% coverage. At temperature of 898 K and above water dissolved rapidly enough in the nanometric MgO particles to permit the equilibrium concentration to be measured. From the temperature dependence of solubility, the heat of solution is calculated to be -84 kJ mol-1. It is shown that if the water dissolved by reacting with O2- ions to form OH- which occupy O2- ion sites, the observed catalysis of MgO sintering by water vapor can be qualitatively explained.