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Molecular dynamics (MD) simulations employing embedded atom method potentials and ultrahigh vacuum (UHV) experiments were carried out to study the mixing process between the Ni and Pt atoms in the Ni/Pt(111) bimetallic system. The barrier for a Ni atom to diffuse from the top surface to the subsurface layer is rather high (around 1.7 eV) as calculated using the nudged elastic band (NEB) method. Analysis of the relaxation dynamics of the Ni atoms showed that they undergo diffusive motion through a mechanism of correlated hops. At 600 K, all Ni atoms remain trapped on the top surface due to large diffusion barriers. At 900 K, the majority of Ni atoms diffuse to the second layer and at 1200 K diffusion to the bulk is observed. We also find that smaller Ni coverages and the presence of Pt steps facilitate the Ni-Pt mixing. By simulated annealing simulations, we found that in the mixed state, the Ni fraction oscillates between layers, with the second layer being Ni-richer at equilibrium. The simulation results at multiple time scales are consistent with the experimental data. [ABSTRACT FROM AUTHOR]

We demonstrate that the growth of a strained film is inherently less stable on a wavy substrate than on a flat substrate. For small surface undulation, the lowest strain energy state is for the film surface to adopt the same wavelength as the substrate surface in an antiphase configuration at the early stage of growth. The critical wavelength (λc) of growth instability on a wavy substrate is half of that on a flat substrate (λ0). It increases linearly with increasing film thickness (t) as λc=λ0/2+πt. Implications for strain directed self-assembly on patterned substrate are discussed. [ABSTRACT FROM AUTHOR]

Plane-wave supercell DFT calculations within the PW91 generalized gradient approximation are used to examine the influence of oxygen coverage on the structure, energetics, and electronics of the RuO2(110) surface. Filling of Obr and Ocus sites is exothermic with respect to molecular O2 at all coverages and causes changes in local Ru electronic structure consistent with the changing metal coordination. By fitting the surface energies of a large number of surface configurations to a two-body interaction model, an O atom is calculated to be bound by 2.55 eV within a filled Obr row and by 0.98 eV along an otherwise vacant Ocus row. Lateral interactions modify these binding energies by up to 20%. Ocus–Ocus interactions are repulsive and diminish binding energy with increasing Ocus filling. Due to the favorable relief of local strain, Obr–Obr interactions are attractive and favor filling of neighbor br sites. These interaction effects are relatively modest in absolute magnitude but are large enough to influence the ability of the RuO2(110) surface to promote oxidation of relatively weak reductants, such as NO and C2H4. [ABSTRACT FROM AUTHOR]

The paper presents the first-order perturbation solutions to the two-dimensional elasticity problems of strained wires embedded in a multilayered film-substrate system with homogeneous elastic properties. The solutions are accurate to the first order of the characteristic slope of the wires; the expressions of the solutions are concise for wires containing multiple facets; and the solutions are valid for single wire, multiple wires with different shapes, and a periodic array of identical wires. The perturbation solutions provide an efficient scheme for studying the effects of embedded wires on the stresses in the multilayered system. [ABSTRACT FROM AUTHOR]

Chemicalvapor deposition (CVD) processes are often employed to produce highquality materials. In some applications, a relatively fast depositionrate is required to produce thick pieces of material, e.g., on theorder of centimeter, in an economical manner. However, in some processesthe formation of solid nuclei or powder in the gas phase can be amajor obstacle to increasing the deposition rate. The homogeneouspowder formation reaction can compete with the surface depositionreaction and consume the gas phase reactants, thus reducing the availablematerial to grow on the substrate surface. Understanding the thermodynamicsas well as kinetics of these two competing reactions helps elucidatethe reaction conditions that best reduce powder formation and increasedeposition rate. As an example where gas phase powder formation cansignificantly limit the surface deposition rate CVD growth of spinel(MgAl2O4) is investigated. In this CVD process,reverse water gas shift (rWGS) is employed to generate water, whichreacts with either chlorides or fluorides of magnesium and aluminumto produce spinel. Density functional theory (DFT) was used to studythe thermodynamics of surface reactions of spinel deposition. Thisstudy shows that the surface reaction to form spinel is thermodynamicallyfavorable. However, the spinel deposition could be limited by itssurface kinetics and/or competition for reactants with gas phase powderformation. We have employed molecular dynamics (MD) with a reactiveforce field (ReaxFF) to simulate gas phase rWGS and powder formation.These simulations show that the reactants’ residence time shouldbe minimized to reduce powder formation. A comparison of magnesiumand aluminum chlorides with their corresponding fluorides indicatesthat fluorides are better in reducing powder formation. [ABSTRACT FROM AUTHOR]

In this contribution, we review our plane wave, supercell density functional theory-based comparisons of CO and NO oxidation chemistry at a RuO2(110) surface. Base metal oxides have long been of interest as catalysts for the oxidation of small molecules such as CO and NO, and understanding of their comparative chemistries on this scientifically important model metal oxide is important for understanding the factors of the control activity. The structures and energies of CO x and NO x adsorbates on the RuO2(110) surface are contrasted, and their thermodynamic stability is quantified using a first-principles phase diagram approach. Distinctly different surface species dominate the two diagrams, reflecting the very different adsorptions and oxidation chemistries of CO and NO. Furthermore, H2O has a dramatic effect on the CO oxidation phase diagram that suggests a bicarbonate ( ) as an important and unrecognised surface poison. These thermodynamic phase diagrams are complemented with mean-field microkinetic models that quantify absolute activity, provide a different window onto surface coverage and highlight the role of various surface species in poisoning activity. Consistent with experiment and previous simulations, CO oxidation is facile under conditions at which both CO and oxygen compete effectively for surface sites. Carbonates are unstable and unimportant to catalysis; facile bicarbonate formation poisons activity in the presence of even small amounts of H2O. An intermediate CO adsorption energy and weak CO2 binding make RuO2 much more effective in oxidising CO than NO, which adsorbs strongly and self-poisons even at high temperatures. RuO2 is predicted to be effective as a reversible NO adsorber. Finally, we reviewed the kinetics of several NO surface reactions and the pathway that may contribute to the decomposition of NO to N2 and N2O over partially reduced surfaces. [ABSTRACT FROM AUTHOR]

Abstract: The surface chemistry of NO x on metal oxides is important to environmental catalysis. Here we employ plane-wave, supercell DFT calculations to characterize NO x chemistry at the RuO2(110) surface as a model of a catalytically active transition metal oxide surface. We identify a range of potential NO x intermediates, and use a thermodynamic analysis to characterize their stability as a function of gas exposure conditions. Adsorbed NO (nitrosyl) and to a lesser extent NO3 (nitrate) dominate the surface phase diagram. Computed vibrational spectra are in good agreement with observation and provide new assignments of observed surface species. NO2 is thermodynamically unstable at the surface and its desorption is never favored: in contrast to its activity towards CO oxidation, RuO2(110) is not an effective NO oxidation catalyst. Rather, it could be effective as a reversible NO adsorber. Finally, we characterize the kinetics of several NO surface reactions and identify a pathway that may contribute to the decomposition of NO to N2 and N2O over partially reduced surfaces. [Copyright &y& Elsevier]

Abstract: Metal oxides are of interest as environmental oxidation catalysts, but practical applications are often limited by poorly understood surface poisoning processes. RuO2 is active for CO oxidation under UHV conditions but is deactivated by some surface poisoning processes at ambient pressures. In this work, we report kinetic models of surface poisoning during CO oxidation over RuO2(110), based on data obtained from plane-wave, supercell DFT calculations. While a surface carbonate is stable at low O2 pressures and high CO2 exposures, it is not stable under catalytic conditions. A surface bicarbonate is more stable and deactivates the RuO2 surface over a wide range of CO and oxygen pressures in the presence of trace amounts of water. [Copyright &y& Elsevier]

The effects of B2O3 vapor on the sintering and the PTCR effect of BaTiO3-based ceramics are investigated in this study. It is revealed that B2O3 vapor can be doped into both pure and Y-doped BaTiO3 ceramics during sintering. B2O3 vapor not only obviously influences the densification and the grain growth but also increases the grain lattice parameters. The PTCR effect is improved through the doping of B2O3 vapor as the room temperature resistivity is considerably decreased and the resistance jump is greatly increased. With the doping of 1 mol% B2O3 vapor, semiconducting Y-BaTiO3 with the room temperature resistivity of 30 Ω cm is obtained when the ceramics are sintered at as low as 1150 °C; while when the ceramics are sintered at 1350 °C with the doping of 0.25 mol% B2O3 vapor, the PTCR effect is enhanced by nearly two orders of magnitude and the room temperature resistivity is decreased from 25 to 9 Ω cm. [Copyright &y& Elsevier]

Abstract: The effect of natural organic matter (NOM) on arsenic adsorption by a commercial available TiO2 (Degussa P25) in various simulated As(III)-contaminated raw waters was examined. Five types of NOM that represent different environmental origins were tested. Batch adsorption experiments were conducted under anaerobic conditions and in the absence of light. Either with or without the presence of NOM, the arsenic adsorption reached steady-state within 1h. The presence of 8mg/L NOM as C in the simulated raw water, however, significantly reduced the amount of arsenic adsorbed at the steady-state. Without NOM, the arsenic adsorption increased with increasing solution pH within the pH range of 4.0–9.4. With four of the NOMs tested, the arsenic adsorption firstly increased with increasing pH and then decreased after the adsorption reached the maximum at pH 7.4–8.7. An appreciable amount of arsenate (As(V)) was detected in the filtrate after the TiO2 adsorption in the simulated raw waters that contained NOM. The absolute amount of As(V) in the filtrate after TiO2 adsorption was pH dependent: more As(V) was presented at than that at . The arsenic adsorption in the simulated raw waters with and without NOM were modelled by both Langmuir and Frendlich adsorption equations, with Frendlich adsorption equation giving a better fit for the water without NOM and Langmuir adsorption equation giving a better fit for the waters with NOM. The modelling implies that NOM can occupy some available binding sites for arsenic adsorption on TiO2 surface. This study suggests that in an As(III)-contaminated raw water, NOM can hinder the uptake of arsenic by TiO2, but can facilitate the As(III) oxidation to As(V) at TiO2 surface under alkaline conditions and in the absence of O2 and light. TiO2 thus can be used in situ to convert As(III) to the less toxic As(V) in NOM-rich groundwaters. [Copyright &y& Elsevier]