Slab Calculations Research Articles

A DFT Study of the Adsorption and Dehydrogenation of Ethanol on a Specific Pt-Sn Catalyst

The oxidation reaction mechanism (ORM) on the anodic catalyst is very important for the development of DEFCs. In the literature are proposed several reaction mechanisms on different Pt-M based catalysts, the problem is the whole identification of all intermediates and the reaction paths involved in the complex process of the ethanol reaction. To clarify the problem of selectivity in catalysts for DEFCs, the potential energy surface (PES) of the adsorption and dehydrogenation steps of the ethanol decomposition on a Pt3Sn1 is investigated in this work, using self-consistent periodic slab calculations based on density functional theory (DFT). This research reveals that ethanol does not have a unique mode of adsorption on this catalytic surface, as well as the dehydrogenation pathway does not only proceed via the ethoxy species formation, but also via the 2-hydroxyethyl species formation. Additionally, it is showed that acetaldehyde desorbs in the process of dehydrogenation of ethanol.

Graphene-like MoS2 containing adsorbents for Hg0 capture at coal-fired power plants

This research focuses on the development of a novel graphene-like MoS2 containing adsorbent for the control of mercury emissions at coal-fired power stations in order to meet the ever-tightening environmental regulations. It was found that the adsorbent loaded with 5wt% Mo demonstrated the best performance in Hg0 capture at low temperatures under different conditions such as space velocity and gas atmosphere etc. The variation in mercury concentration in flue gas due to Hg0 re-emission in FGD unit as well as the possible change in the performance of adsorbents after regeneration were also studied. Both experimental and theoretical studies were carried out to reveal the mechanism for mercury removal. It was found that the high Hg0 removal efficiency of the adsorbent with 5wt% Mo was associated with the high sulphurization ratio of the precursors and the formation of graphene-like MoS2 nanosheets with just a few atomic layers. Moreover, a DFT-D model with periodical slab calculation was established to illustrate the interactions between the Hg0 atoms and the MoS2 (001) surface. The results showed that Hg0 atoms are chemically adsorbed on the MoS2 nanosheets and the most favourable adsorption site is the TMo position. The adsorption pathways and energy profiles, obtained through the DFT study, further elucidated that the low energy barrier for Hg atoms to be adsorbed on the MoS2 nanosheets allows low temperature adsorption process to occur, which is consistent with our experimental observation. The outstanding performance of this MoS2 containing adsorbent in Hg0 capture is attributed to the significant influence of d orbitals of the Mo atom and p orbitals of the S atom towards the Hg atom. It can be concluded that the adsorbent loaded with 5wt% Mo showed great potential to be applied in coal-fired power plants for the removal of mercury as well as the recovery of mercury as a resource.

A DFT+U investigation on methylamine decomposition catalyzed by Pt4 cluster supported on oxygen defective rutile(110) TiO2

The adsorption and decomposition mechanisms of methylamine catalyzed by Pt4 cluster supported on rutile(110) titania[namely, Pt4/TiO2-R(110)] were investigated via density functional theory slab calculations with Hubbard corrections(DFT+U). The adsorption energies under the most stable configuration of the possible species and the energy barriers of the possible elementary reactions involved in methylamine decomposition were obtained. Through systematic calculations for the reaction mechanism of methylamine decomposition on the Pt4/TiO2-R(110), the most possible decomposition path is CH3NH2→CH2NH2+H→CH2NH+2H→CHNH+3H→HCN+4H→ CN+5H, which is similar to that of methylamine dissociation catalyzed by Pt(100) surface.

A DFT Study of the Adsorption and Dehydrogenation of Ethanol on a Pt3Sn1 Catalytic Surface

The selective conversion of oxygenated hydrocarbons is important for several chemical processes as the production of propylene glycol from lactic acid, oxidation of alcohols in fuel cells, hydrogen production. Particularly searching for alternative energy converters, the selective conversion of oxygenated hydrocarbons is the great interest for the development of the Direct Ethanol Fuel Cells (DEFCs). These devices are an interesting solution for the environmental pollution and sustainable development [1-4].The oxidation reaction mechanism (ORM) on the anodic catalyst in the fuel cell is very important for the development of DEFCs. In the literature are proposed several reaction mechanisms on different catalytic surfaces based on the mixture Pt-M, the problem in these proposed reaction mechanisms is that the whole identification of all the intermediates on each specific Pt based catalyst and the reaction paths involved in the complex process of the ethanol reaction on a catalytic surface is really difficult [5-8]. Currently the main steps identified in different investigations in several Pt based catalysts are in general: the ethanol adsorption, the alcohol decomposition, carbon monoxide (CO) adsorption (catalyst poisoning), the complete oxidation of the CO toward CO2 and the formation of different intermediates (acetaldehyde, acetic acid, water and others) [9-14].To contribute to clarify the problem of selectivity in catalysts for DEFCs, the potential energy surface (PES) of the adsorption and dehydrogenation steps of the ethanol decomposition on a specific catalytic surface Pt3Sn1 is investigated in this work, using self-consistent periodic slab calculations based on density functional theory (DFT). This research reveals that ethanol does not have a unique mode of adsorption on this catalytic surface, as well as the dehydrogenation pathway does not only proceed via the ethoxy species formation, but also via the 2-hydroxyethyl species formation. Additionally it is showed that acetaldehyde desorbs in the process of dehydrogenation of ethanol. These results allow understanding in detail the first steps of the ethanol oxidation on a specific catalytic surface Pt3Sn1, which is one of the most utilized catalytic mixtures based on Pt for DEFCs.1. R. Alcalá, J. Shabaker, G. W. Huber, M. A. Sánchez-Castillo and J. Dumesic. J. Phys. Chem. B 2005, 109, 2074-2085.2. V. Pacheco, V. del Colle, R. Batista de Lima, G. Tremiliosi. Electrochim. Acta 52 (2007) 2376-2385.3. R. Alcalá, M. Mavrikakis and J. Dumesic., J. Catal. 218, (2003) 178-190.4. C. Hartnig, J. Grimminger, E. Spohr. Electrochim. Acta 52 (2007) 2236-2243.5. F. Colmati, E. Antolini y E. Gonzalez, J. Power Sources 157, (2006) 98-103.6. Y. Zhou, PH. Lv, G. CH. Wang. J. Molecular Catalysis A, 258 (2006) 203-215.7. R. Watwe, B. Spiewak, R. Cortright, J. Dumesic. J. Catalysis 180 (1998) 184-193.8. M. Janik, M. Neurock, Electrochim. Acta, 52 (2007) 5517-5528.9. S. Garcı́a-rodrı́guez, T. Herranz, and S. Rojas. “New and Future Developments in Catalysis” Elsevier, (2013) 33-67.10. A. Brouzgou, A. Podias, and P. Tsiakaras. J. App. Electrochemistry, 43 (2012) 119–136.11. C. Lamy. Chapter1 n Catalysis for Sustainable Energy Production (2009) 1–46.12. F. Vigier, C. Coutanceau, F. Hahn, E. Belgsir, and C. Lamy. J. Electroanalytical Chemistry 563 (2004) 81–89.13. M. H. Shao and R. R. Adzic. Electrochimica Acta, 50 (2005) 2415–2422.14. R. B. Kutz, B. Braunschweig, P. Mukherjee, R. L. Behrens, D. D. Dlott, and A. Wieckowski. Journal of Catalysis 278 (2011) 181–188.

(Invited) The Effect of (La,Sr)MnO3 Cathode Surface Termination on Its Electronic Structure

La1-xSrxMnO3 (LSM) was one of the first perovskites employed as SOFC cathode material and is still used in the form of porous composites with the respective electrolyte [1]. In our previous study [2], we analyzed oxygen adsorption, dissociation and migration on the (001) MnO2-terminated LM surface. In pure LaMnO3, the MnO2 (001) termination was found to be the energetically most stable, however with an increase of Sr doping, the (La,Sr)O termination is predicted to become even more favorable thermodynamically [3]. In this talk, we discuss and compare the results of first principles calculations on the elementary steps (oxygen vacancy formation, O2 molecule and O atom adsorption) of the oxygen reduction reaction on (La,Sr)MnO3 on the two (001) polar terminations – (La,Sr)O and MnO2. The LSM slab calculations were performed using the plane wave VASP computer code with GGA exchange correlation functionals. Properties of oxygen vacancies in different planes from the surface were analyzed. The vacancy formation energy on the MnO2 terminated surface is smaller than that in bulk. In contrast, the vacancy energy on the (La,Sr)O termination is larger, thus strongly reducing the surface vacancy concentration. Our approach allows for a clear separation of the two distinct effects -- different surface terminations and slab cation stoichiometry (ratio of a number of (La,Sr) ions to Mn that affects the average Mn oxidation state). It is found that oxygen vacancy formation energy in the bulk (slab center) is almost independent of the Mn oxidation state in the region +2.6 to +3.25, whereas it is monotonically decreasing on both surfaces. This surface-to-bulk discrepancy will be discussed. The oxygen atom adsorption energy on the (La,Sr)O termination considerably exceeds that on the MnO2 termination. As a result, the equilibrium oxygen adsorbate concentration was found to be about two orders of magnitude larger for the (La,Sr)O termination, but on the other hand, the surface oxygen vacancy concentration is smaller by nearly six orders of magnitude. Therefore, the oxygen reduction rate on (La,Sr)O termination is expected to be significantly lower than on the MnO2 termination. [1]. M. Kuklja, E.A. Kotomin, R. Merkle, Yu.A. Mastrikov, J. Maier, PCCP 15, 5443 (2013). [2]. Yu.A. Mastrikov, R. Merkle, E. Heifets, E. A. Kotomin, J. Maier, J. Phys. Chem. C 114, 3017 (2010). [3]. S. Piskunov, E. Heifets, T. Jacob, E. A. Kotomin, E. Spohr, Phys. Rev.B 78, 121406 (2008).

Observation of spin-polarized bands and domain-dependent Fermi arcs in polar Weyl semimetal MoTe2

We investigate the surface electronic structures of polar 1T'-MoTe2, the Weyl semimetal candidate realized through the nonpolar-polar structural phase transition, by utilizing the laser angle-resolved photoemission spectroscopy (ARPES) combined with first-principles calculations. Two kinds of domains with different surface band dispersions are observed from a single-crystalline sample. The spin-resolved measurements further reveal that the spin polarizations of the surface and the bulk-derived states show the different domain-dependences, indicating the opposite bulk polarity. For both domains, some segment-like band features resembling the Fermi arcs are clearly observed. The patterns of the arcs present the marked contrast between the two domains, respectively agreeing well with the slab calculation of (0 0 1) and (0 0 -1) surfaces. The present result strongly suggests that the Fermi arc connects the identical pair of Weyl nodes on one side of the polar crystal surface, whereas it connects between the different pairs of Weyl nodes on the other side.

Surface State Tunneling Signatures in the Two-Component Superconductor UPt_{3}.

Quasiparticle interference (QPI) imaging of Bogoliubov excitations in quasi-two-dimensional unconventional superconductors has become a powerful technique for measuring the superconducting gap and its symmetry. Here, we present the extension of this method to three-dimensional superconductors and analyze the expected QPI spectrum for the two-component heavy-fermion superconductor UPt_{3} whose gap structure is still controversial. Starting from a 3D electronic structure and the three proposed chiral gap models E_{1g,u} or E_{2u}, we perform a slab calculation that simultaneously gives extended bulk states and topologically protected in-gap dispersionless surface states. We show that the number of Weyl arcs and their hybridization with the line node provides a fingerprint that may finally determine the true nodal structure of the UPt_{3} superconductor.

Ab initio and experimental studies of polarization and polarization related fields in nitrides and nitride structures

Spontaneous and piezoelectric polarization in the nitrides is analyzed. The slab model was designed and proved to be appropriate to obtain the spontaneous polarization in AlN, GaN and InN. The spontaneous polarization and polarization related electric fields in AlN, GaN and InN were determined using DFT slab calculations. The procedure generates single value of spontaneous polarization in the nitrides. It was shown that Berry phase polarization may be applied to determination of spontaneous polarization by appropriate addition of polarization induced electric fields. The electric fields obtained from slab model are consistent with the Berry phase results of Bernardini et al. The obtained spontaneous polarization values are: 8.69*10-3 C/m2, 1.88*10-3 C/m2, and 1.96*10-3 C/m2 for AlN, GaN and InN respectively. The related Berry phase polarization values are 8.69*10-2 C/m2, 1.92*10-2 C/m2, and 2.86*10-2 C/m2, for these three compounds, respectively. The GaN/AlN multiquantum wells (MQWs) were simulated using ab intio calculations. The obtained electric fields are in good agreement with those derived from bulk polarization values. GaN/AlN MQWs structures, obtained by MBE growth were characterized by TEM and X-ray measurements. Time dependent photoluminescence measurements were used to determine optical transition energies in these structures. The PL obtained energies are in good agreement with ab initio data confirming overall agreement between theoretical and experimental data.

Reflection/transmission calculation of complex particle slabs for normal incidence through dipole approximation

The computation of the reflection/transmission coefficients from normally illuminated bianisotropic metamaterial slabs through a rigorous method is presented in this paper. The bianisotropic particles that compose finite slabs are approximated as electric and magnetic dipoles. Modeling these slabs as a succession of 2D arrays, the interaction of all dipoles is described via Green’s function series, for a given wave illumination on each array, and the excited dipole moments are obtained by the resulting linear system. Finally, the reflection/transmission coefficients are derived in terms of summing the scattering from the equivalent surfaces that comprise the slab. The new algorithm is applied to a bianisotropic and a complicated chiral particle, while all results are compared to numerically simulated ones.

Identification of the thermal properties of concrete for the temperature calculation of concrete slabs and columns subjected to a standard fire—Methodology and proposal for simplified formulations

The fire resistance of concrete members is controlled by the temperature distribution of the considered cross section. The thermal analysis can be performed with the advanced temperature dependent physical properties provided by EN 1992-1-2. But the recalculation of laboratory tests on columns from TU Braunschweig shows, that there are deviations between the calculated and measured temperatures. Therefore it can be assumed, that the mathematical formulation of these thermal properties could be improved. A sensitivity analysis is performed to identify the governing parameters of the temperature calculation and a nonlinear optimization method is used to enhance the formulation of the thermal properties. The proposed simplified properties are partly validated by the recalculation of measured temperatures of concrete columns. These first results show, that the scatter of the differences from the calculated to the measured temperatures can be reduced by the proposed simple model for the thermal analysis of concrete.

Effect of transition metal-doped Ni(211) for CO dissociation: Insights from DFT calculations

Density functional theory slab calculations were performed to investigate the adsorption and dissociation of CO over pure and M-doped Ni(211) (M=Fe, Co, Ru and Rh) with the aim to elucidate the effect of transition metal doping for CO activation. Doping the step edge of Ni(211) with Fe, Co and Ru is found to enhance the binding of CO in the initial state (IS) (in the sequence by the improvement degree: Fe>Ru>Co) as well as the co-adsorption of C and O in the final state (FS) (Ru>Fe>Co). In contrast, Rh doping is unfavorable both in the IS and in the FS. Analysis of the overall potential energy surfaces (PES) suggests CO dissociation is facilitated by Fe, Ru and Co doping both kinetically and thermodynamically, wherein Fe and Ru behave extraordinary. Interestingly, Fe substitute is slightly superior to Ru in kinetics whereas the contrary is the case in thermodynamics. Rh doping elevates the energy height from 0.97eV on Ni(211) to 1.32eV and releases 0.39eV less heat relative to Ni(211), again manifesting a negative effect. Besides the classical Brønsted–Evans–Polanyi relationship, we put forward another two neat linear relations, which can well describe the feature of CO dissociation. The differences of CO adsorption and activation in the IS over pure and doped Ni(211) surfaces are rationalized via electronic structure analysis. The findings presented herein are expected to provide theoretical guidance for catalyst design and optimization in relevant processes.

Complex band structure of topological insulator Bi2Se3

Topological insulators are very interesting from a fundamental point of view, and their unique properties may be useful for electronic and spintronic device applications. From the point of view of applications it is important to understand the decay behavior of carriers injected in the band gap of the topological insulator, which is determined by its complex band structure (CBS). Using first-principles calculations, we investigate the dispersion and symmetry of the complex bands of Bi2Se3 family of three-dimensional topological insulators. We compare the CBS of a band insulator and a topological insulator and follow the CBS evolution in both when the spin–orbit interaction is turned on. We find significant differences in the CBS linked to the topological band structure. In particular, our results demonstrate that the evanescent states in Bi2Se3 are non-trivially complex, i.e. contain both the real and imaginary contributions. This explains quantitatively the oscillatory behavior of the band gap obtained from Bi2Se3 (0 0 0 1) slab calculations.

DFT + U investigation on the adsorption and initial decomposition of methylamine by a Pt single-atom catalyst supported on rutile (110) TiO2

The adsorption and initial decomposition for methylamine catalyzed by a single Pt atom supported on rutile (110) titania (namely, Pt1/TiO2–R(110)) surface have been investigated by the density functional theory slab calculations with Hubbard corrections (DFT+U). The main purpose of the work is to better understand the role of dispersed platinum metal on the surface of rutile (110) titania in the initial decomposition of methylamine. Our calculated results show that the barriers increased with the order of CH<NH<CN, which indicates that the CH bond is the easiest to decompose and the CN bond is the most difficult to break.

Accurate radiative transfer calculations for layered media.

Simple yet accurate results for radiative transfer in layered media with discontinuous refractive index are obtained by the method of K-integrals. These are certain weighted integrals applied to the angular intensity distribution at the refracting boundaries. The radiative intensity is expressed as the sum of the asymptotic angular intensity distribution valid in the depth of the scattering medium and a transient term valid near the boundary. Integrated boundary equations are obtained, yielding simple linear equations for the intensity coefficients, enabling the angular emission intensity and the diffuse reflectance (albedo) and transmittance of the scattering layer to be calculated without solving the radiative transfer equation directly. Examples are given of half-space, slab, interface, and double-layer calculations, and extensions to multilayer systems are indicated. The K-integral method is orders of magnitude more accurate than diffusion theory and can be applied to layered scattering media with a wide range of scattering albedos, with potential applications to biomedical and ocean optics.

Magnetism, Spin Texture, and In-Gap States: Atomic Specialization at the Surface of Oxygen-Deficient SrTiO_{3}.

Motivated by recent spin- and angular-resolved photoemission (SARPES) measurements of the two-dimensional electronic states confined near the (001) surface of oxygen-deficient SrTiO_{3}, we explore their spin structure by means of abinitio density functional theory (DFT) calculations of slabs. Relativistic nonmagnetic DFT calculations display Rashba-like spin winding with a splitting of a few meV and when surface magnetism on the Ti ions is included, bands become spin-split with an energy difference ∼100 meV at the Γ point, consistent with SARPES findings. While magnetism tends to suppress the effects of the relativistic Rashba interaction, signatures of it are still clearly visible in terms of complex spin textures. Furthermore, we observe an atomic specialization phenomenon, namely, two types of electronic contributions: one is from Ti atoms neighboring the oxygen vacancies that acquire rather large magnetic moments and mostly create in-gap states; another comes from the partly polarized t_{2g} itinerant electrons of Ti atoms lying further away from the oxygen vacancy, which form the two-dimensional electron system and are responsible for the Rashba spin winding and the spin splitting at the Fermi surface.

Electronic Structure of the Dark Surface of the Weak Topological Insulator Bi14Rh3I9.

Compound Bi14Rh3I9 consists of ionic stacks of intermetallic [(Bi4Rh)3I](2+) and insulating [Bi2I8](2-) layers and has been identified to be a weak topological insulator. Scanning tunneling microscopy revealed the robust edge states at all step edges of the cationic layer as a topological fingerprint. However, these edge states are found 0.25 eV below the Fermi level, which is an obstacle for transport experiments. Here, we address this obstacle by comparing results of density functional slab calculations with scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. We show that the n-type doping of the intermetallic layer is intrinsically caused by the polar surface and is well-screened toward the bulk. In contrast, the anionic "spacer" layer shows a gap at the Fermi level, both on the surface and in the bulk; that is, it is not surface-doped due to iodine desorption. The well-screened surface dipole implies that a buried edge state, probably already below a single spacer layer, is located at the Fermi level. Consequently, a multilayer step covered by a spacer layer could provide access to the transport properties of the topological edge states. In addition, we find a lateral electronic modulation of the topologically nontrivial surface layer, which is traced back to the coupling with the underlying zigzag chain structure of the spacer layer.

Particle-in-cell δf gyrokinetic simulations of the microtearing mode

The linear stability properties of the microtearing mode are investigated in the edge and core regimes of the National Spherical Torus Experiment (NSTX) using the particle-in-cell method based gyrokinetic code GEM. The dependence of the mode on various equilibrium quantities in both regions is compared. While the microtearing mode in the core depends upon the electron-ion collisions, in the edge region, it is found to be weakly dependent on the collisions and exists even when the collision frequency is zero. The electrostatic potential is non-negligible in each of the cases. It plays opposite roles in the core and edge of NSTX. While the microtearing mode is partially stabilized by the electrostatic potential in the core, it has substantial destabilizing effect in the edge. In addition to the spherical tokamak, we also study the microtearing mode for parameters relevant to the core of a standard tokamak. The fundamental characteristics of the mode remain the same; however, the electrostatic potential in this case is destabilizing as opposed to the core of NSTX. The velocity dependence of the collision frequency, which is crucial for the mode to grow in slab calculations, is not required to destabilize the mode in toroidal devices.

PUNCHING CALCULATION OF REINFORCED CONCRETE FLOOR SLABS

Abstract. In this paper is considered the modeling of a monolithic reinforced concrete buildings structures by the spatial plate-core scheme and study of the floor slabs punching problem. When creating the finite element model of the building structure the floor slabs were created by four-node quadrilateral finite elements with 24 degrees of freedom, the columns and beams were created by spatial rods with 12 degrees of freedom. The calculation of the frame has been done by the software complex «SCAD», are got the efforts in the framework elements and are determined the concentration of stresses in the connections of slabs and columns. The program «Calculation of floor slab on punching» has been developed by the high-level language Object Pascal in Delphi environment, in accordance with the design standard which allows to research different locations of columns and stiffening diaphragms on the floor slab. The ratio of bearing capacity of the slab was calculated by means of the software «Calculation of floor slab on punching». Results are well correlated with the program «Arbat» of the software complex «SCAD». Difference of the developed program is in the possibility of considering of the columns and stiffness diaphragm location options on the floor slab (central, edge, corner). Program «Calculation of floor slab on punching» is recommended for use in the design of monolithic reinforced concrete structures.

A Study on a Two-Way Post-Tensioned Concrete Waffle Slab

Post-tensioned construction has for a long period of time occupied a significant position, especially in the construction of bridges, storage tanks, but also in buildings. In this paper are presented the aspects of a square shaped waffle slab calculation, supported punctually and having a two-way post tensioning reinforcement disposed parabolically. It is described the waffle slab system, its characteristics, preliminary design of composing elements, technological aspects regarding the manufacturing of precast panels, details regarding used materials, the reinforcement layout and the calculation of prestressing force.

Treatment of delocalized electron transfer in periodic and embedded cluster DFT calculations: The case of Cu on ZnO (10(1)0).

We assess the consequences of the interface model-embedded-cluster or periodic-slab model-on the ability of DFT calculations to describe charge transfer (CT) in a particularly challenging case where periodic-slab calculations indicate a delocalized charge-transfer state. Our example is Cu atom adsorption on ZnO(10(1)0), and in fact the periodic slab calculations indicate three types of CT depending on the adsorption site: full CT, partial CT, and no CT. Interestingly, when full CT occurs in the periodic calculations, the calculated Cu atom adsorption energy depends on the underlying ZnO substrate supercell size, since when the electron enters the ZnO it delocalizes over as many atoms as possible. In the embedded-cluster calculations, the electron transferred to the ZnO delocalizes over the entire cluster region, and as a result the calculated Cu atom adsorption energy does not agree with the value obtained using a large periodic supercell, but instead to the adsorption energy obtained for a periodic supercell of roughly the same size as the embedded cluster. Different density functionals (of GGA and hybrid types) and basis sets (local atom-centered and plane-waves) were assessed, and we show that embedded clusters can be used to model Cu adsorption on ZnO(10(1)0), as long as care is taken to account for the effects of CT.