Dissociation of Water at the MgO(100)−Water Interface:  Comparison of Theory with Experiment

Abstract

Dissociative chemisorption of H 2 O at the MgO(100)-water interface has been investigated both experimentally and theoretically. In particular, metastable impact electron spectroscopy (MIES) was used to image the density of occupied states on the MgO(100)/Mo(100) surface for various degrees of water exposure. After multilayer water desorption, spectral features typical of surface hydroxyls are present. To further study the possibility of dissociative chemisorption of water, a theoretical and computational method called CECILIA (combined embedded cluster at the interface with liquid approach) was used to calculate the geometry, energetics, and electronic density of states (DOS) for interfacial species. Consistent with experiment, our theoretical results predict that dissociative adsorption of H 2 O at the MgO(100)-water interface is energetically more favorable than molecular adsorption. The stabilization of charged OH -and H + interface adsorbates is due to polarization of the surrounding solvent.

Theoretical Studies of Solid−Liquid Interfaces: Molecular Interactions at the MgO(001)−Water Interface

“…We can now turn our attention to the more challenging problem regarding chemical reactivity of oxide-water interfaces. In our most recent calculations, 48 we have identified a low-energy minimum for water adsorption at the MgO(001)-water interface (about 13 kcal/mol lower than adsorbed molecular H 2 O). It involves heterolytic dissociative chemisorption of water to form surface hydroxyl groups that are stabilized by solvent polarization.…”

“…For example, classical potentials have been successfully applied to the study of both MgO crystal surfaces and liquid water. [40][41][42][43][44][45][46][47] However, results from our recent study 48 of interactions at the MgO-water interface predict that dissociated water has a lower total energy than water adsorbed in the molecular form. Thus, strictly speaking, potentials of the type Mg 2+ -H 2 O, O 2--H 2 O, and H 2 O-H 2 O, cannot sufficiently describe the interfacial chemistry.…”

Mechanisms of and Effect of Coadsorption on Water Dissociation on an Oxygen Vacancy of the MgO(100) Surface

“…[30][31][32] Therefore, the experimental detection of hydroxy groups exhibits a strong dependence on the measurement conditions as well as on the surface preparation, demonstrating the complex mechanisms of water dissociation on surface defects. [22,[31][32][33][34] The ambiguity regarding the mechanism of water dissociation on the defective MgO surfaces still persists. [35] The adsorption of a water molecule on a defective MgO surface has also been examined in a number of theoretical studies.…”

“…Consequently, this dissociation pathway can explain the presence of a stable surface hy- www.chemeurj.org droxy group on the defective MgO surface in the limit of very low coverage in many experiments. [22,29,31] Surface healing dissociation pathway: The surface healing pathway in which the dissociation of water may heal the oxygen vacancy and desorbs a hydrogen molecule is shown in Figure 2b. This pathway is quite different from that suggested by Finocchi et al [7] from previous constraint MD simulations.…”

“…These results support the fact that significant numbers of hydroxy groups from water dissociation have been detected by many experiments on the defective MgO surface. [22,29,31,61] On the other hand, the barrier for hydrogen formation from the intermediate S In of only 5.88 kcal mol À1 indicates that unless the intermediate S In is further stabilized by coadsorption of other water molecules as will be discussed below, it is possible to proceed to desorption of a hydrogen molecule and healing of the surface. Present results suggest that the dissociation of water on an oxygen vacancy on the MgOA C H T U N G T R E N N U N G (100) surface can proceed by either or both of the two pathways.…”