Chemical Heat Storage Mechanism in Alkaline Earth Metal Oxide: Ab Initio Modeling of the Initial Hydration Reaction on MgO(001) Surface

Effective management of reusable waste heat is one of
the most
important technologies for reducing energy consumption and promoting
energy conservation. In particular, chemical heat storage is a practical
solution to the spatial and temporal mismatches of energy use. Among
various chemical reactions, the reversible hydration process of periclase
(MgO) to brucite [Mg(OH)2] is a practical candidate for
the future applications in various social systems. In this article,
in order to clarify the atomistic reaction mechanism of thermochemical
heat storage in the MgO hydration reaction, we calculated potential
energy profiles to understand the complex thermochemical reaction
on the solid surface. By assuming a series of elementary reaction
processes in the early stage of the surface reaction, we have practically
estimated the reaction energetics for the hydration reaction in both
the MgO(001) terrace and step geometries. First, we calculate two
important thermodynamic parameters that characterize the thermochemical
reaction (the reaction heat and the transition temperature). Our computational
results show good agreement with the available thermodynamic data.
Then, to clarify the nature of the thermochemical heat storage process,
we introduce the 2D potential energy diagram, which is determined
by the dislocation distance of the Mg atoms and the coordination (solvation)
number of the Mg atoms. This 2D potential energy diagram clearly shows
that the activation energy is highly dependent on the surface geometry
of MgO, while an atomistic process in this rate-determining step (an
extraction of Mg atoms from the inner layer to the surface aqueous
layer) is exactly the same in both the terrace and step surface. These
computational results lead to a simple guideline for the design of
thermochemical heat storage systems.