Density functional study of O2 adsorption on (100) surface of γ‐uranium

Chemisorption of the oxygen molecule on the (100) surface of γ‐uranium was investigated using the generalized gradient approximation to Density Functional Theory. Dissociative adsorptions of O₂ are found to be significantly favored compared to molecular adsorptions. Interstitial adsorptions of molecular oxygen are less probable, as no bound states are found in this case. Only after dissociation of O₂ is atomic oxygen diffusion through the surface possible. The O 2p orbitals are found to hybridize with U 5f bands, and some of the U 5f electrons become more localized. A significant charge transfer from the first layer of the uranium surface to the oxygen atoms is found to occur, making the bonding partly ionic. For the most favored site, the dissociative chemisorption energy is ～9.5 eV, which indicates a strong reaction of uranium surface with oxygen. Spin polarization does not have a significant effect on the chemisorption process. For most of the sites and approaches, chemisorption configurations are almost same for both spin‐polarized and non‐spin‐polarized cases. For the most favored chemisorption sites of oxygen on uranium, paramagnetic adsorption is slightly stronger, by 0.304 eV, compared to magnetic adsorption. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Structure Sensitivity of CO Oxidation on Co3O4: A DFT Study

The reaction mechanism of CO oxidation on the Co₃O₄ (110) and Co₃O₄ (111) surfaces is investigated by means of spin‐polarized density functional theory (DFT) within the GGA+U framework. Adsorption situation and complete reaction cycles for CO oxidation are clarified. The results indicate that 1) the U value can affect the calculated energetic result significantly, not only the absolute adsorption energy but also the trend in adsorption energy; 2) CO can directly react with surface lattice oxygen atoms (O_2f/O_3f) to form CO₂ via the Mars–van Krevelen reaction mechanism on both (110)‐B and (111)‐B; 3) pre‐adsorbed molecular O₂ can enhance CO oxidation through the channel in which it directly reacts with molecular CO to form CO₂ [O₂(a)+CO(g)→CO₂(g)+O(a)] on (110)‐A/(111)‐A; 4) CO oxidation is a structure‐sensitive reaction, and the activation energy of CO oxidation follows the order of Co₃O₄ (111)‐A(0.78 eV)>Co₃O₄ (111)‐B (0.68 eV)>Co₃O₄ (110)‐A (0.51 eV)>Co₃O₄ (110)‐B (0.41 eV), that is, the (110) surface shows higher reactivity for CO oxidation than the (111) surface; 5) in addition to the O_2f, it was also found that Co³⁺ is more active than Co²⁺, so both O_2f and Co³⁺ control the catalytic activity of CO oxidation on Co₃O₄, as opposed to a previous DFT study which concluded that either Co³⁺ or O_2f is the active site.     

A theoretical study of atomic oxygen chemisorption on the Ni(100) and Ni(111) surfaces

Atomic oxygen chemisorption has been studied for the fourfold hollow site of the Ni(100) surface and for the threefold hollow site of the Ni(111) surface. To model the Ni(100) surface, 10 different clusters in the range Ni₅ to Ni₄₁ were used, and for the Ni(111) surface, 11 different clusters in the range Ni₁₃ to Ni₄₃ were used. A detailed analysis of the orbital occupations of the cluster with and without oxygen for the different clusters shows that there are three different possible bonding mechanisms. In two of these, the basic feature is that a₁ electrons of the cluster are kicked out by the oxygen 2p_z orbital and moved to holes in the 2p_{x, y} orbitals. A picture where the oxygen electrons fit into the electronic structure of the cluster is emphasized. The third mechanism, which is applicable for only one cluster, can be described as the formation of two covalent bonds of E symmetry. The final best estimate of the oxygen chemisorption energy for the Ni(100) surface is about 130 kcal/mol, and for the Ni(111) surface, about 115 kcal/mol. In particular for the Ni(111) surface, an excited oxygen state with radical character is identified, which might be a catalytically important species. The excitation energy to reach this state should be on the order of 10 kcal/mol for the Ni(111) surface.     

Angle and spin resolved Auger and photoelectron spectroscopy on Rb-layers by means of circularly polarized VUV radiation

Normal incidence circularly polarized VUV radiation with energies around 23 eV creates spin polarized photoelectrons from thick layers of Rb on Pt(111) and thus excites oriented 4p hole states. The preferential spin direction of the Auger electrons and its dependence upon the emission angle has been measured and is compared with the corresponding angular dependence of the primary photoelectron spin polarization also measured. Since the CVV Auger decay relates to as ² pair of valence electrons, the cross comparison of results for photoelectrons and Auger electrons studies the questions on whether photoemission and Auger decay occur in sequence, assuming an independent two step model, and whether the valences-electrons couple to a singlet state configuration.     

The structural,electronic, and magnetic properties of the stoichiometric (001) surface of double perovskite Sr2FeMoO6

The structural, electronic, and magnetic properties of the stoichiometric (001) surface of double perovskite Sr₂FeMoO₆ have been studied by using a 10‐layer FeMoO₄ and SrO terminated (001)‐oriented slab model and the first‐principles projector augmented wave potential within the generalized gradient approximation as well as taking into account the on‐site Coulomb repulsive (U = 2.0 eV for Fe and 1.0 eV for Mo). An outwards relaxation is observed for several layers near surface, and the accompanying layer rumpling has a decrease tend from surface layer to inner layer. Along Fe–O–Mo–O–Fe or Mo–O–Fe–O–Mo chains, the oxygen atom is closer to the adjacent Mo atom than to the adjacent Fe atom. In FeO₆ or MoO₆ octahedra, the two axial TM?O bonds are not equal, and especially, the surface dangling bond makes the remaining one axial TM?O bond slightly shorter than four equally equatorial TM?O bonds. The half‐metallic nature and a complete (100%) spin polarization character ensure the FeMoO₄ and SrO terminated (001)‐oriented slab of double perovskite Sr₂FeMoO₆ a potential application in spintronics devices. The Fe⁺³ and Mo⁺⁵ ions are still in the (3d⁵, S = 5/2) and (4d¹, S = 1/2) states with positive and negative magnetic moments respectively and thus antiferromagnetic coupling via oxygen between them. There is no direct interaction between two nearest Fe–Fe or Mo–Mo pairs, whereas the hybridizations between Fe 3d and 4s, O 2s and 2p, as well as Mo 4d, 5s and 5p orbitals are fairly significant. Copyright © 2016 John Wiley & Sons, Ltd.     

O2, CO2, and H2O Chemisorption on UN(001) Surface: Density Functional Theory Study

We performed density functional theory calculations of O₂, CO₂, and H₂O chemisorption on the UN(001) surface using the generalized gradient approximation and PW91 exchange-correlation functional at non-spin polarized level with the periodic slab model. Chemisorp-tion energies vs. molecular distance from UN(001) surface were optimized for four sym-metrical chemisorption sites. The results showed that the bridge parallel, hollow parallel and bridge hydrogen-up adsorption sites were the most stable site for O₂, CO₂, and H₂O molecular with chemisorption energies of 14.48, 4.492, and 5.85 kJ/mol, respectively. From the point of adsorbent (the UN(001) surface), interaction of O₂ with the UN(001) surface was of the maximum magnitude, then CO₂ and H₂O, indicating that these interactions were associated with structures of the adsorbate. O2 chemisorption caused N atoms on the surface to migrate into the bulk, however CO₂ and H₂O had a moderate and negligible effect on the surface, respectively. Calculated electronic density of states demonstrated the electronic charge transfer between s, p orbital in chemisorption molecular and U6d, U5f orbital.     

Massively parallel direct SCF calculations on large metal clusters: Ni5-Ni481

Summary The convergence of the cluster model with respect to excitation energies, ionization potentials and hydrogen chemisorption energy in the four-fold hollow site of the Ni(100) surface is studied for a sequence of cluster models from Ni₅ up to Ni₁₈₁. For the largest, Ni₄₈₁, cluster studied, only the structure of the occupied levels for one state is obtained. The concept of bond-preparation is found to be essential for the evaluation of chemisorption energies also for clusters with more than 100 atoms. The cluster excitation energies show a slow decrease such that even for Ni₁₈₁ the step between the lower excited states is still 0.1–0.2 eV. The effect ofp-functions on surrounding cluster atoms is found to be 3–4 kcal/mol independent of cluster-size. The direct SCF program DISCO was parallelized using the TCGMSG toolkit in order to perform the calculations. The easy strategy utilized is analysed and exhaustive timings on the Alliant Campus/800 MPP system with 200 CPU's are presented.     

Resonant emission of UO<Subscript>2</Subscript>, U<Subscript>3</Subscript>O<Subscript>8</Subscript>, and UO<Subscript>2+<Emphasis Type="Italic">x</Emphasis></Subscript> valence electrons under SR excitation near the O<Subscript>4,5</Subscript>(U) absorption edge

The structure of the resonant electron emission (REE) spectra of UO₂ (REE appears under the excitation with synchrotron radiation near the O_4,5(U) absorption edge at ∼100 eV and ∼110 eV) is studied with regard to the X-ray O_4,5(U) absorption spectrum of UO₂ and a quantitative scheme of molecular orbitals based on the X-ray electron spectroscopy data and the results of a relativistic calculation of the electronic structure of UO₂. The structure of the REE spectra of U₃O₈ and UO_2+x is studied for comparison, and the effect of the uranium chemical environment in oxides on it is found. The appearance of such a structure reflects the processes of excitation and decay involving the U5d and electrons of the outer valence MOs (OVMOs, from 0 to ∼13 eV) and inner valence MOs (IVMOs, from ∼13 eV to ∼35 eV) of the studied oxides. It is noted that REE spectra show the partial density of states of U6p and U5f electrons. Based on the structure of REE spectra, it is revealed that U5f electrons directly participate (without losing the f nature) in the chemical bonding of uranium oxides and are delocalized within CMOs (in the middle of the band), which results in the enhancement of the intensity of the REE spectra of CMO electrons during resonances. The U6d electrons are found to be localized near the bottom of the outer valence band and are observed in the REE spectra of the studied oxides as a characteristic maximum at 10.8 eV. It is confirmed that U6p electrons are effectively involved in the formation of IVMOs, which leads to the appearance of the structure in the region of IVMO electron energies during resonances. This structure depends on the chemical environment of uranium in the considered oxides.     

Effect of spin polarization for hydrogen adsorbed on Si(111)(1×1) surface: First‐principles calculations

The role of spin polarization on adsorption of atomic and molecular hydrogen on Si(111)(1×1) surface is examined by comparing the results of the local spin density approximation (LSD) and those of the local density approximation (LDA). A large improvement of the adsorption energies (around 0.8 eV/H) was found for the H atom adsorbed on Si(111)(1×1) surface. The inclusion of spin polarization reduces the overbinding between the H atom and the silicon surface and its effect is much more pronounced when the H atom is far away from the surface. Despite of the large changes in the adsorption energies, the main character of the potential energy surface of the H atom on Si(111)(1×1) surface is retained. An opposite effect is found in the charge‐density‐transfer map of LSD results as compared to LDA results for the H atom approaching the surface through the H3 path, in which the H atom loses electrons rather than gains electrons from the surface. The fact that the H atom tends to lose electrons in the silicon bulk has already been reported by the experimental studies for the behavior of the H atom in the p‐type silicon. For the molecular hydrogen on Si(111)(1×1) surface, the effect of the spin polarization is so small that it can be neglected. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 79: 47–55, 2000     

Oxygen Atom Adsorption and Diffusion on Pd Low-index Surfaces and （311） Stepped Surface

Introduction Atom adsorption on transition metal surfaces has attracted special attention as a base for understanding the fundamental processes of oxidative catalysis. Particularly interesting is the adsorption and diffusion of oxygen on well-defined metal surfaces. An oxygen covered palladium surface, for example, plays a central role in several important reactions such as oxidation of carbon monoxide and ammonia. In particular, the (100), (111), (110) surfaces and the interactions with oxyge…     

Dynamics of O2 Chemisorption on a Flat Platinum Surface Probed by an Alignment‐Controlled O2 Beam

O₂ adsorption on Pt surfaces is of great technological importance owing to its relevance to reactions for the purification of car exhaust gas and the oxygen reduction on fuel‐cell electrodes. Although the O₂/Pt(111) system has been investigated intensively, questions still remain concerning the origin of the low O₂ sticking probability and its unusual energy dependence. We herein clarify the alignment dependence of the initial sticking probability (S ₀) using the single spin‐rotational state‐selected [(J ,M )=(2,2)] O₂ beam. The results indicate that, at low translational energy (E ₀) conditions, direct activated chemisorption occurs only when the O₂ axis is nearly parallel to the surface. At high energy conditions (E ₀>0.5 eV), however, S ₀ for the parallel O₂ decreases with increasing E ₀ while that of the perpendicular O₂ increases, accounting for the nearly energy‐independent O₂ sticking probability determined previously by a non‐state‐resolved experiment.     

Magnetic order and valence fluctuation in a Pu?Ga intermetallic compound studied via a first principles calculation

In order to reveal electronic properties of a plutonium-gallium intermetallic compound (Pu₃Ga), and its potential implication for microscopic mechanisms for effects of Ga doping on the electronic and structural properties, as well as the phase stability of delta-phase Pu Ga alloy, a first principles calculation on the magnetic properties of this system is implemented by using density functional theory (DFT) plus on-site Coulomb repulsion U with nonmagnetic, ferromagnetic, and antiferromagnetic (AFM) orders, while the intermediate correlation effect, which is beyond the scope of pure itinerant and localized electronic model, is investigated by using a many-body technique combining DFT and dynamical mean-field theory considering the dynamical correlation effect due to the incompletely filled Pu 5f orbitals and the relativistic effect by inclusion of spin-orbit coupling (SOC). Our findings show that Pu₃Ga is a bad metal with AFM order, which is in good agreement with the experimental magnetic measurement. SOC further splitting Pu 5f states into j = 5/2 and j = 7/2 manifolds, the former exhibits metallic character, while the latter insulating feature. Occupation analysis establishes that an average occupancy of Pu 5f electrons in Pu₃Ga is n_f = 4.9598, this result together with the spectrum function indicates that 5f electrons in this system might be a localized state with strong valence fluctuation. Additionally, optimization of lattice parameter, density of state, and momentum-resolved electronic spectrum function are also presented.     

Geometrical,spectroscopic, and magnetic properties of an oxygen atom adsorbed on the Ni(100) surface

Spin-polarized linear combinations of Gaussian-type orbital–model core potential–local spin density (LCGTO –MCP –LSD ) computations have been performed for oxygen chemisorption on a Ni(100) surface simulated by four different clusters. Results show that the oxygen atom chemisorbs preferentially on the fourfold hollow site with an equilibrium distance of 1.931 Å and a vertical vibrational frequency of 401 cm^?1. The corresponding experimental values are 1.960 Å and 423 or 430 cm^?1. A satisfactory agreement with experiment is also found for the adsorption energy (6.7 vs. 5.6 eV). The bridge position lies at only 0.4 eV above the fourfold hollow one. It is found that oxygen adsorption leaves the bare cluster total spin magnetic moment unchanged, but induces appreciable reductions of the local atomic moment on the surface nickel atoms. © 1993 John Wiley & Sons, Inc.     

Interaction of CO and NO with the spinel CuCr2O4 (100) surface: A DFT study

The characteristics of CO and NO molecules at Cu²⁺ and Cr³⁺ ion sites on the CuCr₂O₄ (100) surface have been studied by first principles calculations based on spin‐polarized density functional theory (DFT). The calculated results show that adsorption energies for X‐down(C, N) adsorption vary in the order: Cu²⁺‐CO>Cr³⁺‐NO≈Cr³⁺‐CO>Cu²⁺‐NO. CO molecules are preferentially adsorbed at Cu sites, whereas NO molecules adsorb favorably at Cu²⁺ and Cr³⁺ ion sites. The C‐O and N‐O stretching frequencies are red‐shifted upon adsorption. Combining the analysis of frontier molecular orbitals and Mulliken charge, for CO and NO X‐down adsorption systems, the 5σ orbitals donate electrons and the 2π^* orbitals obtain back‐donated electrons. Although for NO with O‐down adsorption systems, the NO‐2π^* orbitals obtain back‐donated electrons from substrates without 5σ‐donation. Coadsorption calculations show the CO/NO mixture adsorb selectively at the Cu²⁺ion site but simultaneously at the Cr³⁺ ion site, respectively. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Chiral Selective Chemistry Induced by Natural Selection of Spin‐Polarized Electrons

The search to understand the origin of homochirality in nature has been ongoing since the time of Pasteur. Previous work has shown that DNA can act as a spin filter for low‐energy electrons and that spin‐polarized secondary electrons produced by X‐ray irradiation of a magnetic substrate can induce chiral selective chemistry. In the present work it is demonstrated that secondary electrons from a substrate that are transmitted through a chiral overlayer cause enantiomeric selective chemistry in an adsorbed adlayer. We determine the quantum yields (QYs) for dissociation of (R)‐ or (S)‐epichlorohydrin adsorbed on a chiral self‐assembled layer of DNA on gold and on bare gold (for control). The results show that there is a significant difference in the QYs between the two enantiomers when adsorbed on DNA, but none when they are adsorbed on bare Au. We propose that the effect results from natural spin filtering effects cause by the chiral monolayer.     

Origin of Thermal and Hyperthermal CO2 from CO Oxidation on Pt Surfaces: The Role of Post‐Transition‐State Dynamics,Active Sites,and Chemisorbed CO2

The post‐transition‐state dynamics in CO oxidation on Pt surfaces are investigated using DFT‐based ab initio molecular dynamics simulations. While the initial CO₂ formed on a terrace site on Pt(111) desorbs directly, it is temporarily trapped in a chemisorption well on a Pt(332) step site. These two reaction channels thus produce CO₂ with hyperthermal and thermal velocities with drastically different angular distributions, in agreement with recent experiments (Nature, 2018 , 558, 280–283). The chemisorbed CO₂ is formed by electron transfer from the metal to the adsorbate, resulting in a bent geometry. While chemisorbed CO₂ on Pt(111) is unstable, it is stable by 0.2 eV on a Pt(332) step site. This helps explain why newly formed CO₂ produced at step sites desorbs with far lower translational energies than those formed at terraces. This work shows that steps and other defects could be potentially important in finding optimal conditions for the chemical activation and dissociation of CO₂.     

Origin of Thermal and Hyperthermal CO2 from CO Oxidation on Pt Surfaces: The Role of Post‐Transition‐State Dynamics,Active Sites,and Chemisorbed CO2

The post‐transition‐state dynamics in CO oxidation on Pt surfaces are investigated using DFT‐based ab initio molecular dynamics simulations. While the initial CO₂ formed on a terrace site on Pt(111) desorbs directly, it is temporarily trapped in a chemisorption well on a Pt(332) step site. These two reaction channels thus produce CO₂ with hyperthermal and thermal velocities with drastically different angular distributions, in agreement with recent experiments (Nature, 2018 , 558, 280–283). The chemisorbed CO₂ is formed by electron transfer from the metal to the adsorbate, resulting in a bent geometry. While chemisorbed CO₂ on Pt(111) is unstable, it is stable by 0.2 eV on a Pt(332) step site. This helps explain why newly formed CO₂ produced at step sites desorbs with far lower translational energies than those formed at terraces. This work shows that steps and other defects could be potentially important in finding optimal conditions for the chemical activation and dissociation of CO₂.     

Identifying the O2 diffusion and reduction mechanisms on CeO2 electrolyte in solid oxide fuel cells: A DFT + U study

The interactions and reduction mechanisms of O₂ molecule on the fully oxidized and reduced CeO₂ surface were studied using periodic density functional theory calculations implementing on‐site Coulomb interactions (DFT + U) consideration. The adsorbed O₂ species on the oxidized CeO₂ surface were characterized by physisorption. Their adsorption energies and vibrational frequencies are within ?0.05 to 0.02 eV and 1530–1552 cm^?1, respectively. For the reduced CeO₂ surface, the adsorption of O₂ on Ce⁴⁺, one‐electron defects (Ce³⁺ on the CeO₂ surface) and two‐electron defects (neutral oxygen vacancy) can alter geometrical parameters and results in the formation of surface physisorbed O₂, O₂^a? (0 < a < 1), superoxide (O₂^?), and peroxide (O₂^2?) species. Their corresponding adsorption energies are ?0.01 to ?0.09, ?0.20 to ?0.37, ?1.34 and ?1.86 eV, respectively. The predicted vibrational frequencies of the peroxide, superoxide, O₂^a? (0 < a < 1) and physisorbed species are 897, 1234, 1323–1389, and 1462–1545 cm^?1, respectively, which are in good agreement with experimental data. Potential energy profiles for the O₂ reduction on the oxidized and reduced CeO₂ (111) surface were constructed using the nudged elastic band method. Our calculations show that the reduced surface is energetically more favorable than the unreduced surface for oxygen reduction. In addition, we have studied the oxygen ion diffusion process on the surface and in bulk ceria. The small barrier for the oxygen ion diffusion through the subsurface and bulk implies that ceria‐based oxides are high ionic conductivity at relatively low temperatures which can be suitable for IT‐SOFC electrolyte materials. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Determination of uranium and plutonium in plutonium based fuels by sequential amperometric titration

A method is described for the sequential determination of uranium and plutonium in plutonium bearing fuel materials. Uranium and plutonium are reduced to U(IV) and Pu(III) with titanous chloride and then titrated with dichromate to two end points which are detected amperometrically using two polarized platinum electrodes. Uranium-plutonium solutions of known concentrations containing plutonium in the proportions of 4, 30, 50, and 70% were analyzed with precisions better than 0.3%, maintaining the amounts of plutonium per aliquot in the range of 2–10 mg. No significant bias could be detected. Several samples of (U, Pu)O₂ and (U, Pu)C were analyzed by this procedure. The effects of iron, fluoride, oxalic acid and mellitic acid on the method were also studied.     

Computational Design of One‐Dimensional Ferromagnetic Semiconductors in Transition Metal Embedded Stannaspherene Nanowires

Developing low dimensional semiconductors with moderate band gaps, intrinsic ferromagnetism and large magnetic anisotropy energies (MAEs) is very desirable for high‐speed nano‐spintronic devices, which, however, still remains a big challenge. Here, via first principles calculations, a potential route to realize such materials is proposed based on a new class of one‐dimensional transition metal (TM) embedded stannaspherene (Sn₁₂^2–) nanowires [TM₂(Sn₁₂)]_∞ (TM = Ti‐Ni). Three semiconductors with robust ferromagnetism are achieved with TM = V, Cr and Fe, which all exhibit direct or quasi‐direct band gaps around 1.0 eV, rendering their great potentials for visible light optoelectronic applications. Interestingly, [Cr₂(Sn₁₂)]_∞ and [Fe₂(Sn₁₂)]_∞ are both identified as bipolar magnetic semiconductors (BMS) with valence and conduction band edges spin polarized in the opposite directions, which are promising for realizing switch of carriers’ spin orientation by electrical gating, while [V₂(Sn₁₂)]_∞ exhibits a half semiconductor (HSC) property with valence and conduction band edges spin polarized in the same direction and can be used for spin‐polarized carriers generation. Moreover, sizable MAEs are discovered in these nanowires, which are at least two orders of magnitude larger than those of Fe, Co and Ni bulks and also significantly larger than current ferromagnetic semiconductors.