We investigate the effects of doping on the formation energy and concentration of oxygen vacancies at a metal-oxide surface, using MgO(100) as an example. Our approach employs density-functional theory, where the performance of the exchange-correlation functional is carefully analyzed, and the functional is chosen according to a condition on density-functional theory ionization energies. The approach is further validated by coupled-cluster calculations, including single, double, and perturbative triple substitutions, for embedded clusters. We demonstrate that the concentration of oxygen vacancies at a doped oxide surface is largely determined by the formation of a macroscopically extended space-charge region.

Magnesium oxide (MgO) is used in a variety of industrial applications due to its low cost and structural stability. In heterogeneous catalysis, MgO and Li-doped MgO have been studied as catalysts for the oxidative coupling of methane. In this work, we analyze the structure and stability of defect complexes comprising Li dopants and oxygen vacancies in MgO, combining scanning tunneling microscopy, photon-emission experiments, and density-functional theory computations. The experimental results strongly indicate that after annealing Li-doped MgO to temperatures of 600 K and higher, Li evaporates from the surface, but Li defects, such as substitutional defects, interstitials, or defect complexes comprising Li remain in the bulk. Our calculations show that bulk defect complexes containing F 2+ color centers, that have donated their two electrons to two adjacent Li defects, are the most stable configurations at realistic pressure and temperature conditions.

Surface oxidation processes are crucial for the functionality of Cu-based catalytic systems used for methanol synthesis, partial oxidation of methanol or the water-gas shift reaction. We assess the stability and population of the "8"-structure, a | 3 2 −1 2 | oxide phase, on the Cu(111) surface. This structure has been observed in x-ray photoelectron spectroscopy and low-energy electron diffraction experiments as a Cu(111) surface reconstruction that can be induced by a hyperthermal oxygen molecular beam. Using density-functional theory calculations in combination with ab initio atomistic thermodynamics and Boltzmann statistical mechanics, we find that the proposed oxide superstructure is indeed metastable and that the population of the "8"-structure is competitive with the known "29" and "44" oxide film structures on Cu(111). We show that the configuration of O and Cu atoms in the first and second layers of the "8"-structure closely resembles the arrangement of atoms in the first two layers of Cu 2 O(110), where the atoms in the "8"-structure are more constricted. Cu 2 O(110) has been suggested in the literature as the most active low index facet for reactions such as water splitting under light illumination. If the "8"-structure were to form during a catalytic process, it is therefore likely to be one of the reactive phases.Typeset by REVT E X 1

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Platinum is known as a catalyst with exceptional reactivity for many important reactions, e.g. the oxygen reduction reaction. To reduce the high cost of pure platinum catalysts, platinum on a carbon support is widely used in industrial fuel cell applications. However, these Pt/C systems suffer from poor stability. As a cost-efficient and more durable alternative, Pt single-atom catalysts on a TiN support have recently been suggested, and it has been shown that the single-atom catalysts are stable when anchored at a nitrogen vacancy site on the TiN surface in a nitrogen-lean environment. To further explore the perspective of Pt/TiN catalytic systems, we provide insights into the stability and morphology of Pt nanostructures at the TiN(100) surface, using a density-functional theory approach in combination with ab initio atomistic thermodynamics. Our results show that the formation of two-dimensional Pt nano-layers is preferred over the formation of three-dimensional Pt nano-clusters on the TiN substrate. Similar to the single-atom catalysts, nano-layers of Pt can be stabilized on the TiN(100) surface by surface nitrogen vacancies under nitrogen-lean conditions. By analyzing the electronic metal-support interaction (EMSI) between the Pt nano-layer and the TiN surface with surface defects, we demonstrate that a strong EMSI between the surrounding Ti and Pt atoms is important for stabilizing the catalyst nano-layer at the TiN surface, and that N vacancies lead to stronger Pt-Ti interaction. This work provides a rational computational platform for the design of new generation high-performance Pt-based fuel cells.