ab initio insights into hydrogen UPTAKE AND EVOLUTION ON electrified solid/liquid interfaces

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selenium, selenoproteins, ADHD, depression, COVID-19, oxidative stress

In the pharmaceutical industry, more and more nutritional supplements are entering the market, the composition of which contains the trace element selenium, which has scientifically proven benefits for the human body.It is important for the proper implementation of human processes as well as for the maintenance of human systems. It has been proven that with an insufficient intake of selenium, the development of chronic degenerative diseases is possible. The trace element is known for its protection against oxidative stress in the human body, thanks to selenoproteins that break down hydrogen peroxide.Over 5.9% of the world's population suffers from Attention Deficit Hyperactivity Disorder (ADHD), which is characterized by inattention, impulsivity, and hyperactivity. Millions of people suffer from depression or have experienced it at some point in their lives. It is a mood disorder characterized by feelings of inadequacy, despondency, decreased activity, pessimism, and sadness.The COVID-19 pandemic has affected millions of people worldwide and resulted in hundreds of thousands of deaths. Currently, much research is focused on supportive nutritional therapies that can mitigate the susceptibility as well as the long-term complications of COVID-19. Selenium plays a key role in strengthening immunity, preventing viral infections, and supporting therapy in critical illnesses. In addition, its deficiency can affect the severity of the disease.The conditions listed above can be influenced by selenium, thanks to selenoproteins and their influence on oxidative stress

Construction and analysis of surface phase diagrams to describe segregation and dissolution behavior of Al and Ca in Mg alloys

Segregation and dissolution behavior of Mg alloyed with Ca and Al are studied by performing density functional theory calculations considering an extensive set of surface structures and compositions. Combining ab initio surface science approaches with cluster expansion for ordered surface structures we construct surface phase diagrams for these alloys. We utilize these diagrams to study segregation phenomena and chemical trends for surfaces in contact with a dry environment or with an aqueous electrolyte. We show that the presence of water dramatically impacts the stability and chemical composition of the considered metallic surfaces. We furthermore find that the two alloying elements behave qualitatively different: whereas Ca strongly segregates to the surface and becomes dissolved upon exposure of the surface to water, Al shows an anti-segregation behavior, i.e., it remains in Mg bulk. These findings provide an explanation for the experimentally observed increase/decrease in corrosion rates when alloying Mg with Al/Ca.Comment: 12 pages, 9 figures, submitted to Phys. Rev. Material

Chemical and structural characterization of the native oxide scale on a Mg-based alloy

In this study, the structure and composition of the native oxide forming on the basal plane (0001) of Mg-2Al-0.1Ca is investigated by a correlative approach, combining scanning transmission electron microscopy (STEM) and atom probe tomography (APT). Atom probe specimens were prepared conventionally in a Ga focused ion beam (FIB) as well as a Xe plasma FIB in a cryogenic setup and subsequently cleaned in the atom probe to remove surface contamination before oxidation. While thermal energy input from the laser and longer atmospheric exposure time increased the measured hydrogen content in the specimen's apex region, cryo preparation revealed, that the hydrogen uptake in magnesium is independent of the employment of conventional or cryogenic FIB preparation. TEM measurements demonstrated the growth of a (111) MgO oxide layer with 3-4 nm thickness on the basal (0001) plane of the Mg atom probe specimen. APT data further revealed the formation of an aluminum-rich region between bulk Mg and the native oxide. The aluminum enrichment of up to ~20 at.% at the interface is consistent with an inward growth of the oxide scale

Oxidation von Palladium-Oberflächen

Die Wechselwirkung von Sauerstoffatomen mit Übergangsmetalloberflächen spielt eine wichtige Rolle in technologisch relevanten Prozessen wie Korrosion oder Katalyse. Es erscheint daher wünschenswert, einen tieferen Einblick in das Sauerstoffverhalten auf Übergangsmetalloberflächen zu gewinnen. In der vorliegenden Arbeit werden mittels Dichtefunktional-Theorie Sauerstoff-Wechselwirkungen mit den (111) und (100) Oberflächen von Palladium untersucht. Mit Hilfe der ab-initio atomistic thermodynamics Methode wird des Weiteren die Stabilität der verschiedenen O-Phasen in Abhängigkeit des chemischen Potentials von Sauerstoff bestimmt. Das Verhalten von O auf Pd wird mit Experimenten und vorhandenen theoretischen Ergebnissen für Ru, Rh und Ag, den benachbarten 4d Übergangsmetallen im Periodensystem, verglichen. Bezüglich Pd(111) wurde gefunden, dass die Besetzung von Plätzen unterhalb der Oberfläche stets mit einer starken Verzerrung des Metallgitters verbunden ist, wodurch Sauerstoffeinbau anfangs weniger günstig als die Chemisorption auf der Oberfläche ist. Das Eindringen in den Kristall beginnt erst ab einer kritischen O Bedeckung, welche im Bereich zwischen 0.50 ML und 0.75 ML liegt. Ein Vergleich mit ähnlichen Daten für Ru, Rh und Ag zeigt, dass die kritische Bedeckung von Ru bis Ag stetig kleiner wird und zudem sehr ähnlich ist zu der kritischen Bedeckung, ab der bereits die Oxidphasen thermodynamisch stabiler als die chemisorbierte Adsorbatlage werden. Dies deutet darauf hin, dass das Eindringen des Sauerstoffs ein limitierender Schritt für die Oxidbildung auf Übergangsmetalloberflächen ist. Auf der Pd(100) Oberfläche wurde das (sqrt(5) x sqrt(5))R27° Oberflächenoxid als eine verspannte, dünne PdO(101) Schicht identifiziert, welche wesentlich durch eine starke Kopplung zu dem unterliegenden Substrat stabilisiert wird. Diese Zuordnung ist kompatibel mit Daten aus Hochaufgelöste Rumpfelektronen Spektroskopie, wie der Vergleich von berechneten mit gemesenen Rumpf-Niveau Verschiebungen zeigt. Um einen Vergleich mit entsprechenden Rastertunnelmikroskop Daten zu ermöglichen, wurde ein STM-Simulationsprogramm in das benutzte Programmpaket eingebaut. Eine Untersuchung der Stabilität von verschiedenen Sauerstoffphasen auf Pd(111) und Pd(100) in Abhängigkeit von der umgebenden Sauerstoffgasphase zeigt, dass die Oberflächenoxide auf diesen Oberflächen über einen grösseren Bereich stabil sind als das bekannte Volumenoxid PdO. Ein Vergleich mit entsprechenden experimentellen Daten im Druckbereich zwischen 10 sup 9 bis 1 bar und Temperaturen bis 1000 K identifiziete deutlich kinetische Limitierungen bei der Bildung des Oberflächen- und des Volumenoxides selbst noch bei Temperaturen bis 600 K und atmosphärischen Drücken.The interactions of oxygen atoms with transition metal surfaces play an important role for technologically relevant processes such as corrosion or heterogeneous catalysis. Gaining a deeper insight into the behaviour of oxygen on this metal surfaces appears therefore desirable. In the present work the oxygen interactions with the (111) and (100) surfaces of palladium are investigated using density-functional theory. The concept of ab-initio atomistic thermodynamics is employed to determine the stability of various O-phases subject to the chemical potential of oxygen. Furthermore, the behaviour of O-atoms on Pd is compared to experiments and existing theoretical findings for Ru, Rh and Ag, the neighbouring 4d transition metals in the periodic table. Regarding Pd(111) it was found, that sub-surface oxygen incorporation is initially always less favourable than on-surface chemisorption due to the additional lattice deformation cost in the former case. Penetration into the crystal begins after a critical O-atom coverage lying in the range between 0.50 ML and 0.75 ML, is reached. A comparison with similar data for Ru, Rh and Ag shows that the critical coverage decreases progressively from Ru to Ag. It is, furthermore, quite similar to the critical coverage at which oxide phases become thermodynamically more stable than the chemisorbed adlayers. This points towards the incorporation of sub-surface oxygen being a limiting step for the oxide formation on transition metal surfaces. On the Pd(100) surface the (sqrt(5) x sqrt(5))R27° surface oxide was identified to be a strained, but commensurable thin PdO(101) layer, which is largely stabilised through a strong coupling to the underlying substrate. A comparison of calculated and measured core-level shifts (including final-state effects) shows that this assignement is compatible with high-resolution core-level spectroscopy data. A STM-simulation program was incorporated into the program-package used for the calculations, thus making the comparison to scanning tunneling mictroscopy data possible as well. An analysis of the stability of different oxygen phases on Pd(111) and Pd(100) with regard to the surrounding oxygen gas phase shows, that the surface oxides on this surfaces represent the most stable phases over a wide range of environmental conditions far exceeding the stability range of bulk PdO. Comparison with corresponding experimental data in the pressure range between 10 sup 9 and 1 bar and temperatures up to 1000 K discerns kinetic hindrances to the formation of both the surface and the bulk oxide even at temperatures as high as 600 K and atmospheric pressures

Oxygen Overlayers on Pd(111) Studied by Density Functional Theory

By use of density-functional theory we analyze the on-surface adsorption of oxygen on Pd(111) for coverages up to 1 monolayer and compare the results with corresponding data for the other late 4d transition metals, namely, Ru, Rh, and Ag. Besides the known effect of the continued d-band filling on the oxygen-metal bond strength, we also discern trends in the adsorption geometries, work functions, and electron density of states. The repulsive lateral interactions in the overlayer give rise to a pronounced reduction of the adsorption energy at higher on-surface coverages. In fact, for oxygen coverages θ > 0.5 monolayers, the thermodynamic equilibrium phase of O/Pd(111) is known to be a surface oxide. The calculations reported in this paper show that on-surface adlayers at such higher coverages, that may exist as metastable phases, still possess qualitatively the same surface chemical bond as that which is found at low coverages. The dependence of the surface relaxation on oxygen coverage exhibits some unexpected behavior

Oxygen overlayers on Pd(111) studied by density functional theory

By use of density-functional theory we analyze the on-surface adsorption of oxygen on Pd(111) for coverages up to 1 monolayer and compare the results with corresponding data for the other late 4d transition metals, namely, Ru, Rh, and Ag. Besides the known effect of the continued d-band filling on the oxygen-metal bond strength, we also discern trends in the adsorption geometries, work functions, and electron density of states. The repulsive lateral interactions in the overlayer give rise to a pronounced reduction of the adsorption energy at higher on-surface coverages. In fact, for oxygen coverages ı> 0.5 monolayers, the thermodynamic equilibrium phase of O/Pd(111) is known to be a surface oxide. The calculations reported in this paper show that on-surface adlayers at such higher coverages, that may exist as metastable phases, still possess qualitatively the same surface chemical bond as that which is found at low coverages. The dependence of the surface relaxation on oxygen coverage exhibits some unexpected behavior. I