Abstract: The pristine transition metals (TMs) offer limited choices for high-performance heterogeneous catalysts because they are usually restricted in less optimal regions on the reactivity volcano map. Us...

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Topics: Oxophilicity (56%)

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7 results found

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01 Jan 2011-

Abstract: Catalytic selectivity in the epoxidation of ethylene to form ethylene oxide on alumina-supported silver catalysts is dependent on the geometric structure of catalytically active Ag particles and reaction conditions. Shape and size controlled synthesis of Ag nanoparticles is used to show that silver nanocubes exhibit higher selectivity than nanowires and nanospheres. For a given shape, larger particles offer improved selectivity. The enhanced selectivity toward ethylene oxide is attributed to the nature of the exposed Ag surface facets; Ag nanocubes and nanowires are dominated by (100) surface facet and Ag nanospheres are dominated by (111). Furthermore, the concentration of undercoordinated surface sites is related to diminished selectivity to ethylene oxide. We demonstrate that a simple model can account for the impact of chemical and physical factors on the reaction selectivity. These observations have also been used to design a selective catalyst for the ethylene epoxidation reaction.

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Topics: Ethylene oxide (56%), Selectivity (52%), Ethylene (51%) ... read more

145 Citations

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Abstract: The strength of interaction between a metal and oxygen and/or carbon is a crucial factor for catalytic performance, materials stability, and other important applications. While these are fundamental properties in materials science, there is no general understanding of what makes a metal oxophilic or carbophilic, especially for main group metals. In this work, we elucidate the factors that control how oxophilic or carbophilic a metal is by creating a predictive model and applying it to a variety of data sets for transition metals and main group metals, including DFT-calculated adsorption energies and experimental formation energies. Our model is easily interpretable and accurately describes oxophilic and carbophilic trends across different regions of the periodic table. This model captures the ionic contribution to bonding, the adsorbate-sp contribution to bonding, and the adsorbate-d contribution to bonding by using the reduction potential, matrix coupling elements, band centers, and band filling. For transition metals, the adsorbate–surface d coupling is the major factor that determines oxophilicity relative to carbophilicity. For metals that do not contain d electrons either in their core or valence shell (Li, Be, Na, Mg, Al, K, and Ca), the reduction potential and the adsorbate–surface s coupling are the major factors. As a simple application, we demonstrate the utility of oxophilicity and carbophilicity in rapidly screening metal dopants for improved selectivity for ethylene epoxidation on silver-based catalysts. Using our model, we establish a direct relationship between the electronic properties of the metal dopants and their calculated selectivity for ethylene epoxidation. The results suggest that transition metals with high adsorbate–surface d coupling and s block metals with low adsorbate–surface s coupling are good silver-dopant candidates for this reaction. Overall, the improved linkage between a metal's electronic structure and its interaction with carbon or oxygen will be broadly useful in design of functional materials for a variety of applications.

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Topics: Oxophilicity (63%), Valence electron (53%), Block (periodic table) (53%) ... read more

2 Citations

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Abstract: The effects of Re oxospecies, Cl, and alkali metal (AM = Li, Na, K, Rb, Cs) promoters for silver catalysts of ethylene epoxidation are considered using the DFT approach. The formation of Ag-bound AMReOx surface complexes that affect the catalyst performance is demonstrated. The interplays between silver, alkali metals, and anionic promoters are discussed. The Li-based systems mostly affect the catalyst activity while other alkali metals additionally affect the selectivity towards ethylene oxide in a varying extent. The Rb dopant shows potential as a Cs substitute or co-promoter. The facile composition–performance descriptor comprising the correlation of the substrate binding energy and charge over oxygen atom is proposed. The results obtained can be used in the applied designing of Ag-based epoxidation catalysts.

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Topics: Ethylene oxide (54%), Catalysis (53%), Alkali metal (51%) ... read more

2 Citations

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Abstract: We present ab initio calculations for atomic oxygen adsorption on Ir(111) for a wide range of oxygen coverages, $\ensuremath{\Theta}$, namely from 0.11 to 2.0 monolayers (ML), including subsurface adsorption and thin surface-oxide-like structures. For on-surface adsorption, oxygen prefers the fcc-hollow site for all coverages considered. Similarly to oxygen adsorption on other transition metal surfaces, as $\ensuremath{\Theta}$ increases from 0.25 ML to 1.0 ML, the binding energy decreases, indicating a repulsive interaction between the adsorbates. For the coverage range of 0.11 to 0.25 ML, there is an attractive interaction, suggesting the possible formation of a local $(2\ifmmode\times\else\texttimes\fi{}2)$ periodicity with a local coverage of $\ensuremath{\Theta}=0.25$ ML. Pure subsurface oxygen adsorption is found to be metastable and endothermic with respect to the free ${\text{O}}_{2}$ molecule. For structures with coverage beyond one full ML, we find the incorporation of oxygen under the first Ir layer to be exothermic. As the subsurface O coverage increases in these structures from 0.5 to 1.0 ML, the energy becomes slightly more favorable, indicating an attractive interaction between the O atoms. The structure with the strongest average O binding energy is however a reconstructed trilayer-like structure that can be described as a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ oxide-like layer in $p(2\ifmmode\times\else\texttimes\fi{}2)$ surface unit cell, with coverage 1.5 ML. Through calculation of the surface Gibbs free energy of adsorption, taking into account the pressure and temperature dependence through the oxygen atom chemical potential, the calculations predict only three thermodynamically stable regions, namely, the clean surface, the $p(2\ifmmode\times\else\texttimes\fi{}2)\text{-O}$ phase, and bulk ${\text{IrO}}_{2}$. Thin trilayer surface oxide structures are predicted only to form when kinetic hindering occurs, in agreement with recent experimental work.

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1 Citations

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Abstract: Ethene epoxidation on Ag-based catalysts, an important heterogeneous catalytic reaction with large-scale global wide production, has generated continuous debate on the active site of epoxidation over the past 60 years. The controversy is not only on the roles of the phase transition from Ag metal to oxidation but also on the necessity of the minority crystal facets of the Ag metal, i.e., Ag(100). Herein, we identify, via a machine-learning reaction exploration, that ethene oxidation on the Ag metal surfaces has three low-energy pathways and the most important one, the dehydrogenation of oxometallacycle intermediate (OMC-DH), is entirely overlooked previously. By computing the free energy profile and performing microkinetics simulation, we show that irrespective of the reaction conditions the dehydrogenation path is always dominant for ethene oxidation on both Ag(100) and Ag(111) metal surfaces (>90%), which rationalizes the low selectivity to combustion products (CO2 and H2O) in low oxygen pressure experiments and rules out the chance of Ag metal phases being the active site of ethene epoxidation under industrial conditions (high O2 pressures). The universal presence of the OMC-DH pathway and the general low selectivity on metal sites are then confirmed by evaluating this mechanism on different catalysts. Our results highlight the power of machine-learning-based reaction exploration for resolving the complex reaction network and also point the direction to reveal the true active site of Ag-based catalyst in ethene oxidation.

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Topics: Sampling (statistics) (58%)

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79 results found

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Abstract: Generalized gradient approximations (GGA’s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. [S0031-9007(96)01479-2] PACS numbers: 71.15.Mb, 71.45.Gm Kohn-Sham density functional theory [1,2] is widely used for self-consistent-field electronic structure calculations of the ground-state properties of atoms, molecules, and solids. In this theory, only the exchange-correlation energy EXC › EX 1 EC as a functional of the electron spin densities n"srd and n#srd must be approximated. The most popular functionals have a form appropriate for slowly varying densities: the local spin density (LSD) approximation Z d 3 rn e unif

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Topics: Local-density approximation (60%), Orbital-free density functional theory (57%), Density functional theory (56%) ... read more

117,932 Citations

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Abstract: We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order ${\mathit{N}}_{\mathrm{atoms}}^{3}$ operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ``metric'' and a special ``preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order ${\mathit{N}}_{\mathrm{atoms}}^{2}$ scaling is found for systems containing up to 1000 electrons. If we take into account that the number of k points can be decreased linearly with the system size, the overall scaling can approach ${\mathit{N}}_{\mathrm{atoms}}$. We have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable. \textcopyright{} 1996 The American Physical Society.

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Topics: DIIS (51%)

64,484 Citations

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Abstract: We present a detailed description and comparison of algorithms for performing ab-initio quantum-mechanical calculations using pseudopotentials and a plane-wave basis set. We will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temperature density-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order Natoms2 scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge density including a new special ‘preconditioning’ optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. We have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio molecular-dynamics package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.

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Topics: Projector augmented wave method (55%), Conjugate gradient method (55%), Iterative method (54%) ... read more

40,008 Citations

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Abstract: The method of dispersion correction as an add-on to standard Kohn-Sham density functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coefficients and cutoff radii that are both computed from first principles. The coefficients for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination numbers (CN). They are used to interpolate between dispersion coefficients of atoms in different chemical environments. The method only requires adjustment of two global parameters for each density functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of atomic forces. Three-body nonadditivity terms are considered. The method has been assessed on standard benchmark sets for inter- and intramolecular noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean absolute deviations for the S22 benchmark set of noncovalent interactions for 11 standard density functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C(6) coefficients also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in molecules and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems.

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Topics: Density functional theory (56%), London dispersion force (53%), Dispersion (optics) (51%)

22,557 Citations

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Abstract: A modification of the nudged elastic band method for finding minimum energy paths is presented. One of the images is made to climb up along the elastic band to converge rigorously on the highest saddle point. Also, variable spring constants are used to increase the density of images near the top of the energy barrier to get an improved estimate of the reaction coordinate near the saddle point. Applications to CH4 dissociative adsorption on Ir~111! and H2 on Si~100! using plane wave based density functional theory are presented. © 2000 American Institute of Physics. @S0021-9606~00!71246-3#

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Topics: Saddle point (60%)

10,941 Citations