Elementary Surface Research Articles

Elementary surface chemistry during CuO/Al nanolaminate-thermite synthesis: copper and oxygen deposition on aluminum (111) surfaces.

The surface chemistry associated with the synthesis of energetic nanolaminates controls the formation of the critical interfacial layers that dominate the performances of nanothermites. For instance, the interaction of Al with CuO films or CuO with Al films needs to be understood to optimize Al/CuO nanolaminates. To that end, the chemical mechanisms occurring during early stages of molecular CuO adsorption onto crystalline Al(111) surfaces are investigated using density functional theory (DFT) calculations, leading to the systematic determination of their reaction enthalpies and associated activation energies. We show that CuO undergoes dissociative chemisorption on Al(111) surfaces, whereby the Cu and O atoms tend to separate from each other. Both Cu and O atoms form islands with different properties. Copper islanding fosters Cu insertion (via surface site exchange mechanism) into the subsurface, while oxygen islands remain stable at the surface. Above a critical local oxygen coverage, aluminum atoms are extracted from the Al surface, leading to oxygen-aluminum intermixing and the formation of aluminum oxide (γ-alumina). For Cu and O co-deposition, copper promotes oxygen-aluminum interaction by oxygen segregation and separates the resulting oxide from the Al substrate by insertion into Al and stabilization below the oxide front, preventing full mixing of Al, Cu, and O species.

TFOx: A versatile kinetic Monte Carlo program for simulations of island growth in three dimensions

A three dimensional (3D) kinetic Monte Carlo (KMC) code has been developed that simulates the general behavior of the 3D irreversible nucleation and growth of epitaxial islands, as motivated by experimental observations of oxide nuclei formation and growth during the early stages of copper oxidation. This package was originally a versatile two dimensional (2D) KMC code [Thin Film Oxidation (TFOx)] that considered a variety of elementary steps, including deposition, adsorption, surface diffusion, aggregation, desorption, and substrate-mediated indirect interactions between static adatoms. We extended TFOx to describe 3D island growth. This new version of TFOx is composed of a C++ console program and Python graphical user interface (GUI), such that parameterized simulation, parallel execution, and 3D growth capabilities are feasible. We examined the effects of the potential gradient and the Ehrlich–Schwöbel barrier and found that the 3D island morphology is significantly influenced by the incorporation of these two factors.

Reactivity of Oxygen Adatoms on Stepped Au(997) Surface toward NO and NO2

Oxygen adatoms were prepared on stepped Au(997) surface consisting of (111) terraces and monatomic (111) steps by thermal decomposition of amorphous N2O4 multilayers, including O(a) on the (111) steps and O(a) adatoms and islands on the (111) terraces. Their structures and reactivities toward NO and NO2 were investigated by means of thermal desorption spectroscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction. Oxygen adatoms interact with seven-coordinated Au atoms on the (111) steps more strongly than with nine-coordinated Au atoms on the (111) terraces. The stability of different types of O(a) species follows the order of O(a) on the (111) steps > O(a) adatoms on the (111) terraces > O(a) islands on the (111) terraces. O(a) species on both (111) steps and (111) terraces facilely react with NO to form chemisorbed NO2(a) that desorbs from the surface at elevated temperatures. Less thermally stable O(a) species on Au surface was found to exhibit enhanced reactivity in the NO + O(a) reaction. O(a) species on Au(997) surface do not react with NO2 but they suppress their chemisorption, and strong repulsive interactions exist between coadsorbed O(a) and NO2(a). Due to the limited available sites on the (111) steps, NO2(a) on the (111) steps are more facilely affected by the coadsorbed O(a) than those on the (111) terraces. These results help to fundamentally understand elementary surface reactions in NOx-involved reactions catalyzed by Au catalysts.

Improved reversibility in lithium-oxygen battery: Understanding elementary reactions and surface charge engineering of metal alloy catalyst

Most Li-O2 batteries suffer from sluggish kinetics during oxygen evolution reactions (OERs). To overcome this drawback, we take the lesson from other catalysis researches that showed improved catalytic activities by employing metal alloy catalysts. Such research effort has led us to find Pt3Co nanoparticles as an effective OER catalyst in Li-O2 batteries. The superior catalytic activity was reflected in the substantially decreased overpotentials and improved cycling/rate performance compared to those of other catalysts. Density functional theory calculations suggested that the low OER overpotentials are associated with the reduced adsorption strength of LiO2 on the outermost Pt catalytic sites. Also, the alloy catalyst generates amorphous Li2O2 conformally coated around the catalyst and thus facilitates easier decomposition and higher reversibility. This investigation conveys an important message that understanding elementary reactions and surface charge engineering of air-catalysts are one of the most effective approaches in resolving the chronic sluggish charging kinetics in Li-O2 batteries.

The Interplay of Heat Transfer and Endothermic Chemistry Within a Ceramic Microchannel Reactor

Ceramic microchannel heat-exchanger and reactor technology is capable of achieving high performance while operating under high-temperature, corrosive, and/or oxidative environments. This work describes two computational fluid dynamics (CFD) modeling studies which examine the coupling of heat transfer and endothermic methane-steam-reforming chemistry within a ceramic microchannel reactor. These modeling tools are then applied to improve microchannel-reactor design and performance. Within the reactor, methane is converted to syngas through steam reforming; the thermal requirements for this endothermic chemistry are provided by heat transfer from hot-inert gas on adjacent layers. Fluid flow, heat transfer, and complex elementary surface chemistry are all simulated using the ANSYS FLUENT models. CFD studies reveal the substantial chemical contribution of reforming on thermal gradients across and within the reactor. Improved control of the reforming temperature is also discovered through stack-design analysis, where an odd number of inert-gas layers are found to create more-uniform reactive wall temperatures. Model results provide insight on the interplay of conjugate heat transfer and chemical kinetics in reactor design.

The Automatic Measurement of Complex Surface Based on ZOO-Type CMM

In general, when using the three-dimensional measuring machine to test a workpiece, the measuring route should to be planned firstly. Regarding workpieces which are formed by elementary curve or surface, there are specific software installed in the Coordinate Measuring Machine which can be used to generate the measuring route. However, when there are workpieces with no mathematic model, measuring process usually has to be done by manual operation. In this paper, we concentrate on the technical problem of measuring the blade with complex surface. After analyzing the geometric characteristics of blade, we did research on the self-study function of the Coordinate Measuring Machine, and finally put forward a method to rewrite the self-study measuring program according to geometric characteristics, which will successfully realize the automatic measurement of complex curved surface.

Transformations of Spherical Blocks

Abstract We further explore the correspondence between $ \mathcal{N} $ = 2 supersymmetric SU(2) gauge theory with four flavors on ϵ-deformed backgrounds and conformal field theory, with an emphasis on the ϵ-expansion of the partition function natural from a topological string theory point of view. Solving an appropriate null vector decoupling equation in the semi-classical limit allows us to express the instanton partition function as a series in quasi-modular forms of the group Γ(2), with the expected symmetry W(D 4) ⋊ S 3. In the presence of an elementary surface operator, this symmetry is enhanced to an action of $ W\left( {D_4^{(1) }} \right)\rtimes {S_4} $ on the instanton partition function, as we demonstrate via the link between the null vector decoupling equation and the quantum Painlevé VI equation.

Modelling the Geometric Structure of Concrete Work Item Processing

This article presents a method of work tool modelling for machines for surfacing concrete in the form of a disc which has variable geometric friction elements. The method, based on the effectiveness criterion of the elementary interaction surface friction describes the impact of the disc material.

Analysis of 2D and 3D circular braiding processes: Modeling the interaction between the process parameters and the pre-form architecture

This paper focuses on the modeling of a 2D and 3D circular braiding process to establish the adapted relationships between the architecture of the complex-shaped braids and the process parameters by considering the transitory and steady-state process stages. The modeling approach is based on differential geometry. We describe the trajectory of a strand of yarn as a parameterized curve on an elementary surface of the mandrel, which is described geometrically by two radii of curvature of the reference system. The developed analytical relationships allow us to take into account such transitory or steady-state process-run phenomena as the motion of the braiding front, the slippage on the mandrel and the relaxation of the yarn. The model is successfully compared to other analytical models found in the literature. Additionally, we validate the model for the case of 2D and 3D circular braiding processes run in the transitory stage.

高分解光学顕微鏡でみる氷結晶の成長素過程と表面融解

高分解光学顕微鏡でみる氷結晶の成長素過程と表面融解

Mechanism and microkinetics of the Fischer–Tropsch reaction

The increasing availability of quantum-chemical data on surface reaction intermediates invites one to revisit unresolved mechanistic issues in heterogeneous catalysis. One such issue of particular current interest is the molecular basis of the Fischer-Tropsch reaction. Here we review current molecular understanding of this reaction that converts synthesis gas into longer hydrocarbons where we especially elucidate recent progress due to the contributions of computational catalysis. This perspective highlights the theoretical approach to heterogeneous catalysis that aims for kinetic prediction from quantum-chemical first principle data. Discussion of the Fischer-Tropsch reaction from this point of view is interesting because of the several mechanistic options available for this reaction. There are many proposals on the nature of the monomeric single C atom containing intermediate that is inserted into the growing hydrocarbon chain as well as on the nature of the growing hydrocarbon chain itself. Two dominant conflicting mechanistic proposals of the Fischer-Tropsch reaction that will be especially compared are the carbide mechanism and the CO insertion mechanism, which involve cleavage of the C-O bond of CO before incorporation of a CHx species into the growing hydrocarbon chain (the carbide mechanism) or after incorporation into the growing hydrocarbon chain (the CO insertion mechanism). The choice of a particular mechanism has important kinetic consequences. Since it is based on molecular information it also affects the structure sensitivity of this particular reaction and hence influences the choice of catalyst composition. We will show how quantum-chemical information on the relative stability of relevant reaction intermediates and estimates of the rate constants of corresponding elementary surface reactions provides a firm foundation to the kinetic analysis of such reactions and allows one to discriminate between the different mechanistic options. The paper will be concluded with a short perspective section dealing with the needs for future research. Many of the current key questions on the physical chemistry as well as computational study of heterogeneous catalysis relate to particular topics for further research on the fundamental aspects of Fischer-Tropsch catalysis.

Energetics of elementary reaction steps relevant for CO oxidation: CO and O2 adsorption on model Pd nanoparticles and Pd(111)

The energetics of elementary surface processes relevant for CO oxidation, particularly CO and 02 adsorption, were investigated by a direct calorimetric method on model Pd nanoparticles and on the extended Pd(111) single crystal surface. The focus of this study lies on a detailed understanding of how a nanometer scale confinement of matter affects the binding strength of gaseous adsorbates. We report adsorption energies and sticking coefficients of CO and 02 measured as a function of the adsorbate surface coverage both on pristine and O-covered Pd surfaces. The reduced dimensions of the Pd substrate were found to affect the binding strength of the adsorbates in two principle ways: (i) via the change of the local adsorption environment that can result e.g. in stronger adsorbate bonding at the particle's low coordinated surface sites and (ii) via the contraction of the Pd lattice in small clusters and a concomitant weakening of chemisorptive interaction. Particularly for 02 adsorption, the change of the adsorption site from a three-fold hollow on Pd(111) to the edge site on Pd nanoparticles (approximately 4 nm sized on average) was found to result in a strong increase of the Pd-O bond strength. In contrast, CO adsorbs weaker on Pd nanoparticles as compared to the extended Pd(111) surface. In total, the binding energies of adsorbates on Pd and with this their surface coverages turn out to depend in a non-monotonic way on the particular structure of Pd surfaces, including the local structure of the adsorption site as well as the global properties of the small clusters arising e.g. from the lattice contraction.

Establishing the LaMnO3 Surface Phase Diagram in an Oxygen Environment: An ab Initio Kinetic Monte Carlo Simulation Study

We employ first principles methods to simulate the steady-state catalytic interaction of oxygen with the LaMnO3(001) surface under varying levels of oxidative environment. First, density-functional theory together with transition-state theory is used to obtain the energetics and activation barriers of the relevant elementary surface chemical processes. The results were subsequently fed into a kinetic Monte Carlo (kMC) simulation scheme to explicitly account for the correlations, fluctuations, and spatial distributions of the oxygen ad-atoms and ad-molecules at the surface of the catalyst under steady-state conditions. Adsorbate–adsorbate lateral interactions and influence of the local environment on the reaction kinetics were adequately taken into account to compute (p, T) surface phase diagrams under equilibrium and metastable situations. The kMC-based approach allows us to describe the kinetics-related effects such as the influence of the thermochemical history of the sample on the metastable regions of...

Kinetics of Low-Pressure, Low-Temperature Graphene Growth: Toward Single-Layer, Single-Crystalline Structure

Graphene grown on metal catalysts with low carbon solubility is a highly competitive alternative to exfoliated and other forms of graphene, yet a single-layer, single-crystal structure remains a challenge because of the large number of randomly oriented nuclei that form grain boundaries when stitched together. A kinetic model of graphene nucleation and growth is developed to elucidate the effective controls of the graphene island density and surface coverage from the onset of nucleation to the full monolayer formation in low-pressure, low-temperature CVD. The model unprecedentedly involves the complete cycle of the elementary gas-phase and surface processes and shows a precise quantitative agreement with the recent low-energy electron diffraction measurements and also explains numerous parameter trends from a host of experimental reports. These agreements are demonstrated for a broad pressure range as well as different combinations of precursor gases and supporting catalysts. The critical role of hydrogen in controlling the graphene nucleation and monolayer formation is revealed and quantified. The model is generic and can be extended to even broader ranges of catalysts and precursor gases/pressures to enable the as yet elusive effective control of the crystalline structure and number of layers of graphene using the minimum amounts of matter and energy.

ChemInform Abstract: Review of Catalyst Materials and Catalytic Steam Reforming Reactions in SOFC Anodes

AbstractReview: 38 refs.

Preliminary analysis of the connection between ocean dynamics and the noise of gravity tide observed at the Sopronbánfalva Geodynamical Observatory, Hungary

An experimental development of a computer controlled photoelectric ocular system applied for the LaCoste and Romberg G949 gravimeter made the continuous observation of time variation of gravity possible. The system was operated for half a year in the Sopronbánfalva Geodynamical Observatory to test its capabilities. The primary aim of this development was to provide an alternative and self-manageable solution instead of the standard electronic (Capacitive Position Indicator) reading of this type of gravimeter and use it for the monitoring of Earth tide. It, however, turned out that this system is sensitive enough to observe the effect of variable seismic noise (microseisms) due to the changes of ocean weather in the North Atlantic and North Sea regions at microGal level (1μGal=10−8m/s2). Up to now not much attention was paid to its influence on the quality and accuracy of gravity observations because of the large distance (>1000km) between the observation place (generally the Carpathian–Pannonian basin) and the locations (centres of storm zones of the northern hydrosphere) of triggering events. Based on an elementary harmonic surface deformation model the noise level of gravity observations was compared to the spectral characteristics of seismic time series recorded at the same time in the observatory. Although the sampling rate of gravity records was 120s the daily variation of gravity noise level showed significant correlation with the variation of spectral amplitude distribution of the analysed high pass filtered (cut-off frequency=0.005Hz) seismograms up to 10Hz. Also available daily maps of ocean weather parameters were used to support both the correlation analysis and the parameterization of the triggering events of microseisms for further statistical investigations. These maps, which were processed by standard image processing algorithms, provide numerical data about geometrical (distance and azimuth of the storm centres relative to the observation point) and physical (mass of swelling water) quantities. The information can be applied for characterizing the state of ocean weather at a given day which may help the prediction of its influence on gravity measurements in the future. Probably it is the first attempt to analyse quantitatively the effect of ocean weather on gravity observations in this specific area of the Carpathian–Pannonian region.

CFD Analysis of Elementary-Kinetic Chemical Reactions and Effects on Transport Processes in Anodes of Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is one of most promising types of fuel cells with advantages of high efficiencies, flexible fuel types. As a device directly converting the free energy of chemical reactant to electrical energy and heat, the performance of SOFC is strongly affected by the chemical and electro-chemical reactions occurring in the porous anode coupled with mass, momentum and heat transfer processes. A 3D CFD model has been developed by using in-house code to analyze the various processes coupled with elementary surface reactions occurring in SOFC’s anode, to identify the effects between transfer processes and some elementary reactions occurring in the anode. It shows that single step elementary reaction can affect the distribution of surface species, but not remarkable on global conditions such as temperature and gas distribution. On the other hand, global parameters including temperature, porous characteristics affect the surface species distribution and elementary reaction processes largely.

Constrained Regional Recovery of Continental Water Mass Time-variations from GRACE-based Geopotential Anomalies over South America

We propose a “constrained” least-squares approach to estimate regional maps of equivalent-water heights by inverting GRACE-based potential anomalies at satellite altitude. According to the energy integral method, the anomalies of difference of geopotential between the two GRACE vehicles are derived from along-track K-Band Range-Rate (KBRR) residuals that correspond mainly to the continental water storage changes, once a priori known accelerations (i.e. static field, polar movements, atmosphere and ocean masses including tides) are removed during the orbit adjustment process. Newton's first law merely enables the Difference of Potential Anomalies from accurate KBRR data and the equivalent-water heights to be recovered. Spatial constraints versus spherical distance between elementary surface tiles are introduced to stabilize the linear system to cancel the effects of the north-south striping. Unlike the “mascons” approach, no basis of orthogonal functions (e.g., spherical harmonics) is used, so that the proposed regional method does not suffer from drawbacks related to any spectrum truncation. Time series of 10-day regional maps over South America for 2006–2009 also prove to be consistent with independent data sets, namely the outputs of hydrological models, “mascons” and global GRACE solutions.

CatApp: A Web Application for Surface Chemistry and Heterogeneous Catalysis

CatApp: A Web Application for Surface Chemistry and Heterogeneous Catalysis

CFD Analysis and Parameter Study of Transport Processes with Catalytic Reactions in Anodes of Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is one of most promising types of fuel cells with advantages of high efficiencies, flexibility of usable fuel types. The performance of SOFC is strongly affected by cell overall parameters, e.g., temperature, pressure, reaction rates, etc. But it is very difficult to validate internal parameters distributions and complex physical processes by experimental method. The CFD model simulation has been proved an effective method for SOFC research. In this work, a 3D CFD model is developed to simulate the working processes in SOFC’s anode. The objective of this work is to study the effects of the parameters such as temperature, permeability, pores radius and elementary surface reaction rate on the working processes, by coupling the transport processes such as mass, momentum and thermal energy with the chemical reactions of gas- and surface-phase species based on catalyst surface coverage.