THE HYDRATION OF PERICLASE: ATOMISTIC INSIGHTS FROM QUANTUM-CHEMICAL LOOK

A first principles density functional theory study has been carried out to investigate the hydration kinetics of periclase (MgO) through the interaction between water and periclase (001) and (111) surfaces. A single water molecule associatively adsorbs on both surfaces. Dissociative adsorption of water molecule only occurs by the co-adsorption of water on the (001) surface. Excepting this co-adsorption on the MgO(001) surface, the adsorption of two or monolayer water molecules on both surfaces are associative in nature. The interaction of water monolayer with the MgO(111) surface leads to the formation of Mg(OH)2. The calculated energy barrier for the formation of Mg(OH)2 from MgO is 93.6 kJ/mol, which is higher than the experimental values (66-84 kJ/mol). The absence of defect in the model surface sets the upper limit to the energy barrier for the hydration reaction. The effect of surface defects is demonstrated through the adsorption of a single water molecule. A single water molecule adsorbs both associatively and dissociatively on the (111) defect surface. The energy barrier for the conversion of associative to dissociative adsorption is 36.7 kJ/mol. The important new insights from this study are: (1) only MgO(111) surface is involved in the hydration of periclase and (2) surface defects play very significant role in the experimental measurement of energy barrier for hydration reaction.