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

Abstract

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.

The tendency of supports to generate oxygen vacancies and the catalytic performance of Ni/ZrO2 and Ni/Mg(Al)O in CO2 methanation

“…This substitution gives rise to a periclase solid solution with Mg 2+ vacancies. 60,61 However, these vacancies are not observed by the Rietveld refinement. According to Wang, 61 the cationic vacancy concentration in periclase decreases after calcination.…”

Suppression of Methane Formation by Regulating the Hydrogenation Capability of Metal Oxides in Alkylation of Benzene with Syngas

“…As reported, in contrast to homolysis generating two identical H* radicals favored by metal surfaces, hydrogen was activated by heterolysis over metal oxides, forming a hydroxyl group and a metal−hydrogen bond. 33,42,43 Therefore, the 3640 cm −1 band can be ascribed to the vibration of −OH, while that at 2110 cm −1 was from the stretch of Ce−H. 33 By defining the area of peaks at 3640 cm −1 under 573 K for CeO 2 as 100%, it can be seen that single CeO 2 displays a −OH band of 100% at 573 K and this value drops slightly with elevating temperatures, possibly due to the desorption of hydrogen with continuous heating.…”

Predicting Frequency from the External Chemical Environment: OH Vibrations on Hydrated and Hydroxylated Surfaces

“…A considerable number of theoretical studies have been performed for water on ceria(111) (see the comprehensive list of references in refs − ), as well as for MgO(001) (see the list in ref and some very recent ones in refs − ) and for CaO (see list of references in ref ). As for the descriptors, we note that four of the descriptors ( R (O···O), R (H···O), the angle ∠(O–H···O), and E ∥) discussed in detail in this paper were also discussed for MgO(001) and CaO(001) in refs and .…”