Proton Migration on Perfect, Vacant, and Doped MgO(001) Surfaces: Role of Dissociation Residual Groups

Although migration processes on low-coordination surface sites of a magnesium oxide (MgO) surface are important in technological applications including catalysis in gas interfaces, they are not well understood. In particular, hydrated MgO(001) surfaces present charge transfer and ionic transport phenomena that sometimes do not work properly in technological devices. On the other hand, low-coordination surface sites are chemically attractive. Given that defects and adsorbed species can be characterized by the kinetics of surface processes, in this work, we investigated proton mobility pathways produced from the dissociative adsorption of water molecules on different MgO(001) surfaces: a perfect terrace, with an anionic vacancy, and with an Al-doped + cationic vacancy. For that purpose, we employed density functional theory with periodic boundary conditions combined with the climbing imaged nudged elastic band method to compute energy barriers. The dissociative adsorption of water molecules on the perfect-terrace surface depends on their state of aggregation; the anionic vacancy was filled with hydroxyl groups, giving rise to co-adsorbed protons, and the cationic vacancy favored the formation of hydroxylated centers, in good agreement with experiment. The doping and vacancies increase the surface dissociation in the absence of co-adsorbed water molecules by decreasing the barriers of proton and hydroxyl formation. For the different surfaces, considering perfect-terrace surface or surface with defects, the water, hydroxyl, and proton species, are the main sources for changing the proton migration barriers. More free surface sites of basic residual groups follow favorable kinetic mobility of protons on MgO and increase the stability of the migration products, as seen in the surface with an anionic vacancy. Therefore, we showed that defects on MgO(001) surfaces may be important in surface proton transport due to the existence of favorable dissociative adsorption mechanisms of water molecules producing specific residual groups needed to lead to most stable migration pathways.