Theoretical evaluation of the possibility of hydrogen release during the thermal activation of hydrated magnesium oxide surfaces

Abstract

The hydrated MgO surface has been simulated by the linear atomic chain (MgOHV″ MgHOMg) 2+ and the total energy of this system has been evaluated ab initio as a function of the HH (or OH) distance. A symmetrical motion of both hydrogen nuclei with respect to the central Mg 2+ vacancy was assumed. Two potential minima are found which correspond to systems that can be formally described as (2Mg 2+, 2OH −) (I, lower energy system) and (2Mg 2+, 2O −, H 2) (II, lying 140.9 kcal above I). These theoretical calculations show that the reaction 2OH − → 2O − + H 2 will occur on MgO with a thermal activation energy of about 145.0 kcal, while the chemisorption of hydrogen as hydrexyls on O − sites has a very low activation energy (4.1 kcal).