Elementary Surface Research Articles

Toward a Specific Reaction Parameter Density Functional for H2 + Ni(111): Comparison of Theory with Molecular Beam Sticking Experiments

Accurate barriers for rate controlling elementary surface reactions are key to understanding, controlling, and predicting the rate of overall heterogeneously catalyzed processes. The specific reaction parameter approach to density functional theory (SRP-DFT) in principle allows chemically accurate barrier heights to be obtained for molecules dissociating on metal surfaces, and such accurate barriers are now available for four H2–metal and three CH4–metal systems. Also, there is some evidence that SRP density functionals (SRP-DFs) may be transferable among systems in which the same molecule interacts with a low-index face of metals belonging to the same group. To extend the SRP database, here we take a first step to obtain an SRP-DF for H2 + Ni(111) by comparing sticking probabilities (S0) computed with the quasi-classical trajectory method with S0 measured in several molecular beam experiments, using potential energy surfaces computed with several density functionals. We find that the SRP-DF for H2 + Pt(111) is not transferable to H2 + Ni(111). On the other hand, the PBE-vdW2 functional describes the molecular beam experiments on H2 + Ni(111), which we deem to be most accurate with chemical accuracy and may therefore be considered a candidate SRP-DF for this system, of which the quality still needs to be confirmed through comparison with an experiment to which it was not fitted. However, the different molecular beam sticking measurements that we considered showed discrepancies with one another and with the theory for incidence energies > 0.2 eV, and it would be good if better defined and more accurate experiments would be done for these energies to resolve these differences.

Kinetic modeling of LPG catalytic cracking using Langmuir–Hinshelwood–Hougen–Watson theory

In this study, HZSM-5(1) and HZSM-5(2) zeolites were prepared with Al(NO3)3 and Al(OH)3 sources, respectively. Kinetic modeling of liquefied petroleum gas (LPG) cracking over synthesized HZSM-5 zeolites with different framework aluminum (Al) locations was investigated at 500–650 °C using Langmuir–Hinshelwood–Hougen–Watson theory. In a proposed LPG cracking mechanism, 26 elementary surface reaction steps including C–H and C–C protolytic cracking, hydrogen transfer, isomerization and absorption–desorption of olefins and also 2 lumped reactions for C5+ formation, were considered. Due to the existence of numerous local optimal points, genetic algorithm was employed. The estimation of kinetic parameters was performed by minimizing the mean of the sum of squares between theoretical and experimental data.

Automated Detection and Characterization of Surface Restructuring Events in Bimetallic Catalysts

Surface restructuring in bimetallic systems has recently been shown to play a crucial role in heterogeneous catalysis. In particular, the segregation in binary alloys can be reversed in the presence of strongly bound adsorbates. Mechanistic characterization of such restructuring phenomena at the atomic level remains scarce and challenging because of the large configurational space that must be explored. To this end, we propose an automated method to discover elementary surface restructuring processes in an unbiased fashion using Pd/Ag as an example. We employ high-temperature classical molecular dynamics to rapidly detect restructuring events, isolate them, and optimize using density functional theory. In addition to confirming the known exchange descent mechanism, our systematic approach has revealed three new predominant classes of events at step edges of close-packed surfaces that have not been considered before: (1) vacancy insertion; (2) direct exchange; and (3) interlayer exchange. The discovered events enable us to construct the complete set of mechanistic pathways by which Pd is incorporated into the Ag host in vacuum at the single-atom limit. These atomistic insights provide a step toward systematic understanding and engineering of surface segregation dynamics in bimetallic catalysts.

CO Oxidation over Ce1–xPdxO2−δ Takes Place via Vacancy Hopping

Mechanism of CO oxidation over ceria-based catalysts has been widely accepted to follow Mars–van Krevelen (MvK) mechanism involving the lattice oxygen exchange. Adsorption of CO over the metallic sites, lattice oxygen exchange, and replenishment of the support by stream oxygen are considered to be the surface processes taking place during the MvK mechanism. This study reveals an alternative MvK-type mechanism for the reaction taking place over metal ion substituted ceria-based reduced catalysts involving hopping of vacancies. The mechanism was proposed on the basis of adsorption energetics of CO and O2, probed with the help of density functional theory calculations. The computational analysis of different molecular systems (CeO2, CeO2−δ, and Ce1–xPdxO2−δ) under study showed that adsorption of O2 takes place over the reduced catalyst and nonsequential adsorption of CO and O2 takes place over Ce1–xPdxO2−δ in contrast to sequential adsorption of O2 followed by CO over CeO2−δ. Differences in the elementary surface processes in the proposed mechanism with the established MvK mechanism are highlighted with the major observation being the differences in the lattice oxygen abstraction and vacancy hopping observed with newly proposed alternative MvK mechanism.

Two-field cosmological \u03b1-attractors with Noether symmetry

We study Noether symmetries in two-field cosmological α-attractors, investigating the case when the scalar manifold is an elementary hyperbolic surface. This encompasses and generalizes the case of the Poincaré disk. We solve the conditions for the existence of a ‘separated’ Noether symmetry and find the form of the scalar potential compatible with such, for any elementary hyperbolic surface. For this class of symmetries, we find that the α-parameter must have a fixed value. Using those Noether symmetries, we also obtain many exact solutions of the equations of motion of these models, which were studied previously with numerical methods.

A new look on asteroid shape modeling

A new model of asteroid shapes is proposed for simulation of asteroid lightcurves using sparse observational data. The shape model uses logarithmic 3-foci ellipsoids and has four free parameters for describing body shapes with both convex and concave elements of global topography, as well as binary systems. We demonstrate the applicability of the model for asteroids 1580 Betulia and 8567 1996 HW1. A ray-tracing method incorporating Akimov's disk function was used to calculate the flux scattered by each elementary surface element. The simulations provide good fits to observational data, revealing the possibility for wide use of the approach.

Physical modelling of changes in the energy impact on a worker taking into account high-temperature radiation

Purpose: The purpose of this investigation is development of an experimental installation and definition the intensity of thermal irradiation at workplaces to ensure safety under condition of increased thermal radiation. This purpose is justified as follows. Human health and well-being depend to a large extent on conditions of the internal environment of the premises, which affect the heat exchange of workers with the surrounding surfaces. With this the spectral composition and intensity of irradiation are also important for assessing the effect of thermal radiation. Design/methodology/approach: The investigation was carried out using special experimental device. Uniformly lit semitransparent screen was used as a source of thermal radiation source. We used photo film with heat flow sensor ДТП 02 – ДТП 03 developed by Institute of Engineering Thermophysics under Academy of Science of Ukraine as model of the elementary surface of human body. Findings: Based on results of conducted research and optimization, we propose protective compositors that are nonflammable. This fact allows the increase in safety of facilities during operation and evacuation routes in emergency case due to high-temperature effects. Research limitations/implications: In this study, was focused on the dependence of the local angular coefficient of irradiation and maximum thermal loads, as well as the transition from indirect measurements to direct ones. This allows us to predict working conditions by the thermal factor in the workplace. Practical implications: The results of this study can be recommended to the Ministry of Health and the Ministry of Industry for the development to reduce the incidence of occupational morbidity among workers who work under conditions of exposure of thermal radiation. Originality/value: It was disclosed that it is possible to determine the distance to the source of thermal radiation from the point of measurement and the angle at which the source of thermal radiation is visible with high accuracy. Moreover, measurement point may be located at a distance safe for the researcher.

Coupling of sound spaces by plane surface harmonics and its application to sound source characterization

The coupling of two acoustical spaces via an interface surface can be done using the concept of surface impedance. The latter can be constructed by expanding the sound field across the interface into a truncated series of suitably chosen elementary surface functions. The prediction of air-borne sound emitted by a sound source into an arbitrary receiving space can then be done by defining a virtual interface surface around the source and by characterizing the source as well as the receiving space across it. With respect to this surface the source can be characterized by its blocked sound pressure and its surface impedance; in such a case the receiver is characterized by its own surface impedance. Each of these three quantities is expressed by a number of continuous spatial functions. While the interface surface can have any geometry the paper focuses on interfaces made of one or several plane rectangular surfaces. The approach is demonstrated by the cases of a multi-point source and of a vibrating box source.

Surface reaction kinetics in atomic layer deposition: An analytical model and experiments

Atomic layer deposition (ALD) surface reactions are comprised of several elementary surface interactions (such as physisorption, desorption, and chemisorption) occurring at the substrate. Since ALD processes are often far from thermodynamic equilibrium, the surface saturation behavior is controlled by the kinetics of these involved interactions. In this article, we present a first-order kinetic model for ALD reaction, to simulate the cumulative effect of precursor exposure (tA), post-precursor purge (tP1), reactant exposure (tB), post-reactant purge (tP2), and substrate temperature (Tsub) on the resulting growth per cycle (GPC) in an ABAB… pulsed ALD process. Furthermore, to simulate the effect of inadequate reactor purges (tP1, and/or tP2) and undesired non-ALD side reactions, reaction pathways to account excess GPC are also taken into consideration. From our model calculations, we simulate GPC vs Tsub trends observed in ALD growth experiments and demonstrate that the process temperature window (ΔTALD) for a constant GPC depends upon the deposition cycle parameters tA, tP1, tB, and tP2. The modeled GPC vs Tsub trends are discussed and compared with SiNx, ZrN, and ZnO PEALD growth experiments.

(Invited) Etching of Advanced Semiconductor Devices

Plasma etching or Reactive Ion Etching (RIE) is the workhorse for patterning of semiconductor devices since the early 1980-ies when it replaced wet etching in manufacturing. Today, RIE is reaching levels of performance which were unimaginable back then. At the same time, etching technologies such as Atomic Layer Etching (ALE), radical, dry vapor and Ion Beam Etching are finding their way into manufacturing for certain applications. We will present an overview of dry etching technologies used in semiconductor manufacturing. The emphasis is on the elementary surface processes and how they impact the performance on the wafer. We will start from less complex etching technologies which use just one kind of etching species, such as neutrals, radicals or ions. Then we combine these techniques into cycling processes which leads to the discussion of ALE. The highest level of complexity is reached in RIE with simultaneous species fluxes. Reactor designs for the various etching technologies and process control will be covered. Finally, and an outlook into the future of semiconductor device etching will be given.

Vibrationally excited hydrogen molecules formation on a cesiated surface

Hydrogen atoms recombination via the Eley–Rideal mechanism on a cesiated surface was studied using a semiclassical collisional model. The potential energy surface governing the reaction was evaluated by ab initio calculations in the framework of density functional theory. The recent results obtained for H interaction potential with the same surface model were used together with the first principle results obtained for H2 interaction potential with Cs and Mo surface atoms. Molecular dynamics calculations have been performed for two different typical adsorption sites identified on the surface. The probabilities for the recombination reaction and for the complete set of elementary surface processes arising from the assumed initial condition were evaluated. It appears that the recombination occurs only for one adsorption site on the surface and with low probability, while for the other site H atoms are mainly scattered from the surface with a partial negative charge. The vibrational distributions of formed H2 molecules exhibit a non-Boltzmann behaviour with peaks on medium-high lying vibrational levels. The considered surface appears promising for negative ion sources, contributing to the formation of ions by means of both usually considered mechanisms, surface production and volume production via dissociative attachment. The global and state-to-state recombination coefficients were also calculated to be used in kinetic modelling of negative ion sources.

Effective diffusion length and elementary surface processes in the concurrent growth of nanowires and 2D layers

In this work, we revisit models of the diffusion-induced growth of nanowires (NWs) accompanied by the growth of crystalline layers on the nonactivated areas of the substrate surface. In the conventional approach, the presence of the surface steps is taken into account implicitly by introducing effective diffusion length of adatoms before desorption or incorporation. In contrast, here we treat adatom incorporation as an outcome of a sequence of elementary surface processes on the terraces and step edges. The surface diffusion problems are formulated and solved for two modes of the lateral growth at the substrate surface: (i) submonolayer growth of 2D islands and (ii) permanent presence of the steps forming a closed loop around the NW base. In the limiting cases where the substrate adatoms with high probability either reach the sidewall of a NW or leave the substrate surface by desorption or attachment to 2D islands, our analysis demonstrates the correspondence between this approach and the conventional one under the relevant and physically transparent choice of the effective diffusion length. It has been shown that at relatively large NW heights the steps act as the sinks not only for the atoms impinging the substrate surface but also for the atoms impinging the NW sidewalls. This explains the failure of the simple material balance arguments to predict the NW height evolution observed in the experiments on the Au-assisted growth of InAs and GaAs NWs.

Application of work function measurements in the study of surface catalyzed reactions on Rh(1 0 0)

The present article aims to show how work function measurements (WF) can be applied in the study of elementary surface reaction steps on metallic single crystal surfaces. The work function itself can in many cases not be interpreted directly, as it lacks direct information on structural and chemical nature of the surface and adsorbates, but it can be a powerful tool when used together with other surface science techniques which provide information on the chemical nature of the adsorbed species. We here, illustrate the usefulness of work function measurements using Rh(100) as our model catalyst. The examples presented include work function measurements during adsorption, surface reaction, and desorption of a variety of molecules relevant for heterogeneous catalysis. Surface coverage of adsorbates, isosteric heat of adsorption, and kinetic parameters for desorption, desorption/decomposition temperatures of surface species, different reaction regimes were determined by WF with the aid of other surface science techniques.

Generalizedα-Attractor Models from Elementary Hyperbolic Surfaces

We consider generalizedα-attractor models whose scalar potentials are globally well-behaved and whose scalar manifolds are elementary hyperbolic surfaces. Beyond the Poincaré diskD, such surfaces include the hyperbolic punctured diskD⁎and the hyperbolic annuliA(R)of modulusμ=2log⁡R>0. For each elementary surface, we discuss its decomposition into canonical end regions and give an explicit construction of the embedding into the Kerekjarto-Stoilow compactification (which in all three cases is the unit sphere), showing how this embedding allows for a universal treatment of globally well-behaved scalar potentials upon expanding their extension in real spherical harmonics. For certain simple but natural choices of extended potentials, we compute scalar field trajectories by projecting numerical solutions of the lifted equations of motion from the Poincaré half plane through the uniformization map, thus illustrating the rich cosmological dynamics of such models.

Geographic envelope of the Moon and the identification of Moon landscapes with the use of the axiomatic method

Abstract Three consequent concepts that build up the algorithm of the identification of modern landscapes on the Moon surface are suggested. They are anaglyphonosphere axiomatic and landscape concepts obtained with the use of the axiomatic method. The first concept depicts the geographic envelope of the Moon as an anaglyphonosphere layer (relief) that is a continuum (total environment). The latter becomes the research subject for both a geomorphologist and a landscape researcher. Continuity, dynamics, range (amplitude), and erosion potential determine anaglyphonosphere. Axiomatic concept means constructing the sole scheme (mathematically determined) of the search for the elementary surface units using the geometric interpretation of surface patterns of the Moon and its landscape interpretation. The landscape concept is based on the classical principles of the landscape theory and the axiomatic principles of the previous concept. The synthesis of concepts is implemented in the models of Moon landscapes of four scales: zero, linear, two- and three-dimensional. The paper offers the last two models of Davy Catena. Proposed concepts with appropriate correction can be used in parallel studies of the natural environment: geological, geomorphological, climatic, etc. The advantages of the axiomatic method consist in the objective approach to the division of the surface into specific units (the landscapes in our case). The proposed method of identifying and displaying the landscape complexes on the lunar surface can be a significant complement for the study and mapping of terrestrial planets, satellites of planet-giants, etc.

Elementary Surface Reactions on Co(0001) under Fischer–Tropsch Synthesis Conditions

A fundamental understanding of Fischer–Tropsch synthesis (FTS) is challenging but important for catalyst design. We herein report high-resolution photoemission spectroscopic characterizations of a Co(0001) surface pretreated with syngas at different conditions without exposure to air. Elementary surface reactions were observed to sensitively depend on reaction pressures and temperatures. Both direct CO dissociation and H2-assisted CO activation for CO activation occur facilely at 300 K, while the removal of the resulting oxygen adatoms needs high temperatures. Under a typical FTS reaction condition (4 bar and 490 K), the Co(0001) surface is metallic, and direct CO dissociation is the dominant mechanism for CO activation to form irreversibly adsorbed surface intermediates including carbidic carbon, hydrocarbon fragments, atomic oxygen, and hydroxyl groups. These results significantly advance the fundamental understanding of the cobalt-catalyzed FTS process.

The most active Cu facet for low-temperature water gas shift reaction

Identification of the active site is important in developing rational design strategies for solid catalysts but is seriously blocked by their structural complexity. Here, we use uniform Cu nanocrystals synthesized by a morphology-preserved reduction of corresponding uniform Cu2O nanocrystals in order to identify the most active Cu facet for low-temperature water gas shift (WGS) reaction. Cu cubes enclosed with {100} facets are very active in catalyzing the WGS reaction up to 548 K while Cu octahedra enclosed with {111} facets are inactive. The Cu–Cu suboxide (CuxO, x ≥ 10) interface of Cu(100) surface is the active site on which all elementary surface reactions within the catalytic cycle proceed smoothly. However, the formate intermediate was found stable at the Cu–CuxO interface of Cu(111) surface with consequent accumulation and poisoning of the surface at low temperatures. Thereafter, Cu cubes-supported ZnO catalysts are successfully developed with extremely high activity in low-temperature WGS reaction.

Parametric Study and Mathematical Modeling of the Hydrogen Evolution Reaction: Application to Mildly Acidic Environments Containing Acetic Acid on a Gold Surface

The present study utilizes a computational parametric study and a continuum scale mathematical model to investigate the mechanism and the kinetics of the hydrogen evolution reaction in the mildly acidic environments in the presence of acetic acid (HAc). Such environmental condition is of great interest in the metallic corrosion field of study where a comprehensive knowledge of the mechanism and kinetics of the underlying electrochemical reactions are essential for development of accurate predictive models. The metallic corrosion in aqueous acidic solutions consists of two dominant electrochemical reactions, the metal dissolution reaction, which results in deterioration of the structure, and the hydrogen evolution reaction (HER) which provides the necessary electron sink for the former reaction to spontaneously progress. In the context of metallic corrosion, for example, the inner pipeline corrosion of oil and gas transmission lines, the hydrogen evolution reaction is not limited to the hydrogen ion (H+) reduction. In this case, the weak acids such as acetic acid are also commonly assumed to be reduced at the metal surface, although its significanec has been debated [1]. This reaction can also be categorized as a hydrogen evolving reaction, where 2HAc+2e-→H2+2Ac- . Additionally, as a weak acid the concentration of the undissociated acetic acid in the solution is defined by the chemical equilibrium, HAc↔ H++Ac-. Therefore, HAc can act as an additional source for H+ at the surface and hence, increase the rate of cathodic currents [2]. Considering this system of electrochemical and chemical reactions, the following mechanistic aspects needs to be determined: a) the mechanism of the HER from H+ based on the elementary Volmer, Tafel, Heyrovsky, and surface diffusion step. b) The significance of HAc reduction, where either a CE (HAc dissociation followed by H+reduction) or an EC (HAc reduction followed by HAc association) mechanism could be expected. C) If HAc reduction was significant, the mechanism of HAc reduction based on the elementary steps. A computational parametric study for HER reaction, based on the known elementary steps was developed similar to that introduced in an earlier publication [3]. Considering the experimentally obtained apparent Tafel slope and reaction order on a rotating disk gold electrode in perchlorate solutions in the pH range from 3 to 5 and HAc concentrations up to 1000 ppm, the mechanism of the HER from H+ was discussed. The results suggest that the rate of HER from H+ was limited by the surface diffusion step at low current densities, and by the Heyrovsky desorption step at the high current densities. A comprehensive mathematical model was used to analyze the experimental polarization curves and to obtain the kinetic rate constants of the elementary steps HER from H+. This model accounts for the mass transfer by molecular diffusion, electro migration and conceive flow as well as the chemical equilibria of water and HAc. The boundary conditions at the electrode surface account for the elementary reactions associated with the HER and also the coverage by the intermediate adsorbed hydrogen atoms. This same model was then used to analyze the experimental results in the presence of HAc. The results suggest that HAc is not significantly reduced during the cathodic polarizations, implying that a CE mechanism governs this system. This study demonstrated the application of a parametric study and mathematical modeling to investigate the mechanism and the kinetic of the HER reaction. This same theoretical framework can be used in future studies to provide valuable information on the mechanism of cathodic reactions on other surfaces and also in the solutions containing other weak acids such as carbonic acid and hydrogen sulfide.

Pyrite oxidation in air-equilibrated solutions: An electrochemical study

Oxidative dissolution of pyrite (FeS2) in air-equilibrated solutions was investigated with electrochemical techniques (polarization measurements and electrochemical impedance spectroscopy) at different solution pH (1 to 5) and temperatures (25 to 40°C). We have found that, at a given temperature, the oxidation current density (jox) varies only slightly from initial pH1 to 4, but was significantly lower at initial pH5, while oxidation potential (Eox) decreases when pH increases. At the investigated temperatures, the reaction order with respect to hydrogen ions has been found to be zero, suggesting that H+ is not involved into the rate-determining step of the oxidation rate. The jox values increase with temperature, whereas Eox potentials are hardly affected by temperature from 25 to 40°C. Activation energies (Ea) were estimated from polarization data and vary between 57±28kJ/mol (initial pH5) and 19±3kJ/mol (initial pH4). The obtained values indicate that the rate-determining step of the overall oxidative dissolution of FeS2 involves a surface reaction. The importance of surface reaction is also supported by the electrochemical impedance spectroscopy measurements which show that interfacial transfer of electrons is dominated by surface charge transfer and the elementary surface reaction which controls the mineral oxidative dissolution is the electron transfer from the conduction band of pyrite to adsorbed O2.

Automatic Generation of Microkinetic Mechanisms for Heterogeneous Catalysis

A novel approach is presented for generating microkinetic mechanisms in heterogeneous catalysis. The open-source software RMG-Cat automatically develops a detailed list of elementary surface reactions, including thermodynamic properties for the adsorbates and parametrized rate coefficients for the reactions. The software proposes numerous possible surface intermediates and reactions, but it only retains those species that have a sufficiently high rate of formation. RMG-Cat was tested on the dry reforming of methane on nickel. The software correctly found the same set of elementary reactions as in a previously compiled microkinetic mechanism, as well as a few missing reactions. These results demonstrate the potential of this approach for predicting the dominant pathways in heterogeneous catalysis.