Dissociative adsorption of methane on surface oxide structures of Pd-Pt alloys

The dissociative adsorption of methane on variously oxidized Pd, Pt and Pd-Pt surfaces is investigated using density-functional theory, as a step towards understanding the combustion of methane on these materials. For Pd-Pt alloys, models of surface oxide structures are built on the basis of known oxides on Pd and Pt. The methane adsorption energy presents large variations depending on the oxide structure and composition. Adsorption is endothermic on the bare Pd(111) metal surface as well as on stable thin layer oxide structures such as the ($\sqrt{5}\times\sqrt{5}$) surface oxide on Pd(100) and the PtO$_2$-like oxide on Pt(111). Instead, large adsorption energies are obtained for the (100) surface of bulk PdO, for metastable mixed Pd$_{1-x}$Pt$_x$O$_{4/3}$ oxide layers on Pt(100), and for Pd-Pt(111) surfaces covered with one oxygen monolayer. In the latter case, we find a net thermodynamic preference for a direct conversion of methane to methanol, which remains adsorbed on the oxidized metal substrates via weak hydrogen-bond interactions

Strukturelles Design auf der Nanometerskala

For numerous technical applications, condensed materials must be specifically produced and manipulated on the nanometre scale. This frequently means the creation of an order with characteristic lengths in the range of extension of single atoms. Besides the possibility of constructing a solid "atom by atom" with suitable procedures, specific local transformations driven by energy input can also produce modified structures which display new and technically utilisable properties.Für zahlreiche technische Anwendungen müssen kondensierte Materialien auf der Nanometerskala gezielt hergestellt und manipuliert werden. Dies bedeutet häufig die Schaffung einer Ordnung mit charakteristischen Längen in der Größenordnung der Ausdehnung einzelner Atome. Neben der Möglichkeit, den Festkörper „Atom für Atom“ mit geeigneten Verfahren aufzubauen, können auch gezielte lokale Umwandlungen durch Energieeintrag modifizierte Strukturen, welche neuartige und technisch nutzbare Eigenschaften aufweisen, hervorbringen

On the Short-Time Compositional Stability of Periodic Multilayers

The short-time stability of concentration profiles in coherent periodic multilayers consisting of two components with large miscibility gap is investigated by analysing stationary solutions of the Cahn-Hilliard diffusion equation. The limits of the existence and stability of periodic concentration profiles are discussed as a function of the average composition for given multilayer period length. The minimal average composition and the corresponding layer thickness below which artificially prepared layers dissolve at elevated temperatures are calculated as a function of the multilayer period length for a special model of the composition dependence of the Gibbs free energy. For period lengths exceeding a critical value, layered structures can exist as metastable states in a certain region of the average composition. The phase composition in very thin individual layers, comparable with the interphase boundary width, deviates from that of the corresponding bulk phase.Comment: 29 pages including 7 figures, to be published in Thin Solid Film

Podosome-Driven Defect Development in Lamellar Bone under the Conditions of Senile Osteoporosis Observed at the Nanometer Scale

The degradation mechanism of human trabecular bone harvested from the central part of the femoral head of a patient with a fragility fracture of the femoral neck under conditions of senile osteoporosis was investigated by high-resolution electron microscopy. As evidenced by light microscopy, there is a disturbance of bone metabolism leading to severe and irreparable damages to the bone structure. These defects are evoked by osteoclasts and thus podosome activity. Podosomes create typical pit marks and holes of about 300-400 nm in diameter on the bone surface. Detailed analysis of the stress field caused by the podosomes in the extracellular bone matrix was performed. The calculations yielded maximum stress in the range of few megapascals resulting in formation of microcracks around the podosomes. Disintegration of hydroxyapatite and free lying collagen fibrils were observed at the edges of the plywood structure of the bone lamella. At the ultimate state, the disintegration of the mineralized collagen fibrils to a gelatinous matrix comes along with a delamination of the apatite nanoplatelets resulting in a brittle, porous bone structure. The nanoplatelets aggregate to big hydroxyapatite plates with a size of up to 10 x 20 μm2. The enhanced plate growth can be explained by the interaction of two mechanisms in the ruffled border zone: the accumulation of delaminated hydroxyapatite nanoplatelets near clusters of podosomes and the accelerated nucleation and random growth of HAP nanoplatelets due to a nonsufficient concentration of process-directing carboxylated osteocalcin cOC. © 2021 The Authors. Published by American Chemical Society

Ab initio study of element segregation and oxygen adsorption on PtPd and CoCr binary alloy surfaces

The segregation behavior of the bimetallic alloys PtPd and CoCr in the case of bare surfaces and in the presence of an oxygen ad-layer has been studied by means of first-principles modeling based on density-functional theory (DFT). For both systems, change of the d-band filling due to charge transfer between the alloy components, resulting in a shift of the d-band center of surface atoms compared to the pure components, drives the surface segregation and governs the chemical reactivity of the bimetals. In contrast to previous findings but consistent with analogous PtNi alloy systems, enrichment of Pt atoms in the surface layer and of Pd atoms in the first subsurface layer has been found in Pt-rich PtPd alloy, despite the lower surface energy of pure Pd compared to pure Pt. Similarly, Co surface and Cr subsurface segregation occurs in Co-rich CoCr alloys. However, in the presence of adsorbed oxygen, Pd and Cr occupy preferentially surface sites due to their lower electronegativity and thus stronger oxygen affinity compared to Pt and Co, respectively. In either cases, the calculated oxygen adsorption energies on the alloy surfaces are larger than on the pure components when the more noble components are present in the subsurface layers

Toxicity of Tungsten Carbide and Cobalt-Doped Tungsten Carbide Nanoparticles in Mammalian Cells in Vitro

BACKGROUND: Tungsten carbide nanoparticles are being explored for their use in the manufacture of hard metals. To develop nanoparticles for broad applications, potential risks to human health and the environment should be evaluated and taken into consideration. OBJECTIVE: We aimed to assess the toxicity of well-characterized tungsten carbide (WC) and cobalt-doped tungsten carbide (WC-Co) nanoparticle suspensions in an array of mammalian cells. METHODS: We examined acute toxicity of WC and of WC-Co (10% weight content Co) nanoparticles in different human cell lines (lung, skin, and colon) as well as in rat neuronal and glial cells (i.e., primary neuronal and astroglial cultures and the oligodendrocyte precursor cell line OLN-93). Furthermore, using electron microscopy, we assessed whether natioparticles can be taken up by living cells. We chose these in vitro systems in order to evaluate for potential toxicity of the nanoparticles in different mammalian organs (i.e., lung, skin, intestine, and brain). RESULTS: Chemical-physical characterization confirmed that WC as well as WC-Co natioparticles with a mean particle size of 145 nm form stable suspensions in serum-containing cell culture media. WC nanoparticles were not acutely toxic to the studied cell lines. However, cytotoxicity became apparent when particles were doped with Co. The most sensitive were astrocytes and colon epithelial cells. Cytotoxicity of WC-Co nanoparticles was higher than expected based on the ionic Co content of the particles. Analysis by electron microscopy demonstrated presence of WC nanoparticles within mammalian cells. CONCLUSIONS: Our findings demonstrate that doping of WC nanoparticles with Co markedly increases their cytotoxic effect and that the presence of WC-Co in particulate form is essential to elicit this combinatorial effect

Reforming the legal and institutional framework for the enforcement of civil and commercial claims in Portugal

This paper discusses the recent major reforms in the area of civil and commercial claims enforcement undertaken by the Portuguese authorities in the context of the IMF/EU-supported adjustment program. The economic literature has long recognized that slow claims enforcement affects economic growth, foreign direct investment, credit and labor markets, and firm size. The Portuguese authorities together with IMF/EU staff deployed a novel approach that has focused on incentives tackling weaknesses in the enforcement process with the aim of increasing the efficiency and effectiveness of claims enforcement and resolving court backlogs. The paper finds impressive quantifiable changes affecting, in particular, court backlog reduction, court processing speed, and private debt recovery. The economic literature suggests that this will impact positively on the payment culture and overall growth, even if such impact cannot be determined at this stage

Strukturelles Design auf der Nanometerskala

For numerous technical applications, condensed materials must be specifically produced and manipulated on the nanometre scale. This frequently means the creation of an order with characteristic lengths in the range of extension of single atoms. Besides the possibility of constructing a solid "atom by atom" with suitable procedures, specific local transformations driven by energy input can also produce modified structures which display new and technically utilisable properties.Für zahlreiche technische Anwendungen müssen kondensierte Materialien auf der Nanometerskala gezielt hergestellt und manipuliert werden. Dies bedeutet häufig die Schaffung einer Ordnung mit charakteristischen Längen in der Größenordnung der Ausdehnung einzelner Atome. Neben der Möglichkeit, den Festkörper „Atom für Atom“ mit geeigneten Verfahren aufzubauen, können auch gezielte lokale Umwandlungen durch Energieeintrag modifizierte Strukturen, welche neuartige und technisch nutzbare Eigenschaften aufweisen, hervorbringen

Strukturelles Design auf der Nanometerskala

For numerous technical applications, condensed materials must be specifically produced and manipulated on the nanometre scale. This frequently means the creation of an order with characteristic lengths in the range of extension of single atoms. Besides the possibility of constructing a solid "atom by atom" with suitable procedures, specific local transformations driven by energy input can also produce modified structures which display new and technically utilisable properties.Für zahlreiche technische Anwendungen müssen kondensierte Materialien auf der Nanometerskala gezielt hergestellt und manipuliert werden. Dies bedeutet häufig die Schaffung einer Ordnung mit charakteristischen Längen in der Größenordnung der Ausdehnung einzelner Atome. Neben der Möglichkeit, den Festkörper „Atom für Atom“ mit geeigneten Verfahren aufzubauen, können auch gezielte lokale Umwandlungen durch Energieeintrag modifizierte Strukturen, welche neuartige und technisch nutzbare Eigenschaften aufweisen, hervorbringen