Transition metal-oxo centers in zeolites are known to be active in the conversion of methane to methanol. Here, we study this reaction over Fe-oxo sites in the zeolite SSZ-13. By comparing calculations for the fully periodic structure and a cluster for two different methods—the standard van der Waals corrected semilocal density functional PBE-D2 and ACFDT-RPA, which is a method where correlation is calculated fully nonlocally—we find that it is actually the confining environment in the zeolite that reduces the barrier for this reaction, by more than 50%, and we find that the two applied methods lead to qualitatively different results.

The competing influences of enthalpy and entropy on the adsorption and transformation of hydrocarbon molecules in zeolites have been investigated using dispersion-corrected density-functional theory in combination with advanced statistical-mechanical techniques. At the example of propane in protonated mordenite, it is demonstrated that while enthalpy favors adsorption in the narrower side pockets (SP) due to the stronger interaction with the framework, the loss of entropy is smaller for molecules in the wider main channel (MC). At ambient and elevated temperatures, the free energy favors adsorption in the MC (in agreement with experiment) and diffusion to the SP is an activated process. On the other hand, the free energy of activation for monomolecular cracking is lower for Bronsted acid (BA) sites in the SP, if the reactant is already located there. Cracking at a BA in the MC is a simple one-step reaction but as the SP is accessible to molecules only via the MC, cracking at a BA site in the SP is possible only after overcoming the barrier for diffusion from the MC to the SP. Thus, the difference in the free energies of adsorption in the MC and the SP increases the effective free-energy of activation for the reaction in SP and the reaction in the MC is favored. This result contradicts the interpretation of recent experiments on hydrocarbon transformations catalyzed by mordenite-containing BA sites and Na+ counterions in varying proportions. This experimental interpretation is based on the assumption that Na+ counterions preferentially replace BA sites located in the SP. This assumption has been critically examined using ab-initio calculations and found to be inconsistent with our theoretical predictions. It is demonstrated that the experimentally observed decrease of the reaction rate with increasing Na+/H+ ratio arises from the strongly attractive nature of the Na+ counterions, which makes the approach of the reactant to the BA site more difficult and reduces the reaction rate. We suggest that our results on the competing influence of enthalpy and entropy arising from the confinement of the reactant in cavities of different diameters have general validity for the adsorption and acid-catalyzed reactions of hydrocarbon molecules in zeolites with a complex framework structure. (C) 2015 Elsevier Inc. All rights reserved.

•CO2/CO selectivity of methanol steam reforming process is investigated.•Using DFT catalytic reactions on NiZn, PdZn and PtZn surfaces are studied.•Atomistic scenarios for reactions determining selectivity are constructed.•Decisive factors for selectivity of the catalysts are identified.•Possibilities how to improve CO2 selectivity are presented.

Scandium trifluoride (ScF3) is a rare example of a material with a simple structure exhibiting a large negative thermal expansion over a wide range of temperatures. In the present work, the thermal behavior of this material has been studied by means of density-functional molecular dynamics in the isothermal-isobaric ensemble. Our simulations reproduce the experimentally observed trends: thermal expansion is negative at low temperatures, approaches zero at approximate to 1000 K, and is positive at higher temperatures. The simulations permit us to develop an atomistic scenario of the observed phenomena, which arises from the correlated dynamics of ScF6 octahedra. The relation between the cubic-to-rhombohedral transformation under a very modest compression and the thermal behavior of ScF3 is demonstrated.

During the last decade, it has been demonstrated that the use of intermetallic compounds composed of simple and transition metals as catalysts leads to improved activity and selectivity for a number of industrially important processes. Herein, we discuss the insight gained from density functional theory (DFT) calculations for the example of the semihydrogenation of acetylene to ethylene catalyzed by compounds of Pd with Ga or Al and of Co or Fe with Al. Investigations of the potentially active surfaces demonstrate that the chemical composition of nonstoichiometric surfaces depends strongly on the preparation conditions. Ga‐ or Al‐terminated surfaces are catalytically inactive because they cannot dissociate hydrogen, but Pd‐terminated surfaces are also unfavorable because the activation energy for the rate‐controlling step is higher than the desorption energy of acetylene. Active sites promoting a selective semihydrogenation are triangular groups consisting of two simple‐metal (Al, Ga) atoms and one transition‐metal (Pd, Co) atom. Acetylene is strongly bound to simple‐metal atoms in a di‐σ configuration, whereas ethylene is weakly π bonded on top of a transition‐metal atom. The change from strong to weak bonding is of great importance for the selectivity of the process. The novel point elucidated by the DFT calculations is the strong reactivity of the non‐transition‐metal atoms arising from the strong metallocovalent bonding in the compounds, which also contributes to their stability.

The structural and magnetic properties of mixed Pt-Co and Pt-Fe trimers and tetramers in the gas-phase and supported on a free-standing graphene layer have been calculated using density-functional theory. The influence of the strong magnetic moments of the 3d atoms on the Pt atoms and the influence of the strong spin-orbit coupling contributed by the Pt atoms on the 3d atoms have been studied in detail. All mixed trimers form isocele triangles in the gas-phase. On a graphene layer the structure is influenced by the strong binding of the 3d atoms, leading to an asymmetric configuration for Pt-rich and more symmetric structures for 3d-rich clusters. The magnetic anisotropy energy defined as the energy difference for easy and hard magnetization directions varies between 5 and 13 meV/atom for the free trimers, but is strongly reduced to values between 0.7 and 6.6 meV/atom for the graphene-supported clusters. The saddle-point energy representing the barrier against magnetization reversal is on average 3 meV/atom for free trimers, it is reduced to 2 meV/atom for the more symmetric PtCo(Fe)(2) clusters, and to only about 0.3 meV/atom for the asymmetric Pt(2)Co(Fe) cluster on graphene. For the mixed tetramers the strong magnetism stabilizes a flat geometric structure, except for Pt(3)Co which forms a distorted trigonal pyramid. The geometry of the graphene-supported tetramers is very different due to the requirement of a good match to the substrate. Large magnetic anisotropy energies are found for free Pt(3)Co where the change of the magnetization direction also induces a transition from a high- to a low-moment magnetic isomer. For all other free tetramers the magnetic anisotropy energy ranges between 3 to 5 meV/atom only, it is further reduced to 0.4 to 3.8 meV/atom for the graphene-supported tetramers. The reduction is strongest for Pt(3)Fe/graphene because of the asymmetric structure of the adsorption complex. The barriers against magnetization reversal range between only 0.3 meV/atom for Pt(3)Fe/graphene and about 3 meV/atom for PtFe(3) and Pt(3)Co. Altogether our results demonstrate a strong correlation between the geometric and magnetic degrees of freedom and the necessity to base investigations of the magnetic anisotropy of nanostructures on a simultaneous optimization of the total energy with respect to all geometric and magnetic parameters.

Recently we have demonstrated that the applicability of the Tkatchenko-Scheffler (TS) method for calculating dispersion corrections to density-functional theory can be extended to ionic systems if the Hirshfeld method for estimating effective volumes and charges of atoms in molecules or solids (AIM's) is replaced by its iterative variant [T. Bučko, S. Lebègue, J. Hafner, and J. Ángyán, J. Chem. Theory Comput. 9, 4293 (2013)]. The standard Hirshfeld method uses neutral atoms as a reference, whereas in the iterative Hirshfeld (HI) scheme the fractionally charged atomic reference states are determined self-consistently. We show that the HI method predicts more realistic AIM charges and that the TS/HI approach leads to polarizabilities and C6 dispersion coefficients in ionic or partially ionic systems which are, as expected, larger for anions than for cations (in contrast to the conventional TS method). For crystalline materials, the new algorithm predicts polarizabilities per unit cell in better agreement with the values derived from the Clausius-Mosotti equation. The applicability of the TS/HI method has been tested for a wide variety of molecular and solid-state systems. It is demonstrated that for systems dominated by covalent interactions and/or dispersion forces the TS/HI method leads to the same results as the conventional TS approach. The difference between the TS/HI and TS approaches increases with increasing ionicity. A detailed comparison is presented for isoelectronic series of octet compounds, layered crystals, complex intermetallic compounds, and hydrides, and for crystals built of molecules or containing molecular anions. It is demonstrated that only the TS/HI method leads to accurate results for systems where both electrostatic and dispersion interactions are important, as illustrated for Li-intercalated graphite and for molecular adsorption on the surfaces in ionic solids and in the cavities of zeolites.

•Semi-hydrogenation of acetylene catalyzed by 3-fold surfaces of GaPd is investigated.•Atomistic scenarios for multi-step hydrogenation processes are constructed.•Decisive factors for activity and selectivity of the catalyst are identified.•One of possible 3-fold surfaces exhibits superior catalytic activity and selectivity.•Catalytic properties originate from contributions from surfaces differing in activity and selectivity.

The structural and magnetic properties of mixed PtCo, PtFe, and IrCo dimers in the gas phase and supported on a free-standing graphene layer have been calculated using density-functional theory, both in the scalar-relativistic limit and self-consistently including spin-orbit coupling. The influence of the strong magnetic moments of the 3d atoms on the spin and orbital moments of the 5d atoms, and the influence of the strong spin-orbit coupling contributed by the 5d atom on the orbital moments of the 3d atoms have been studied in detail. The magnetic anisotropy energy is found to depend very sensitively on the nature of the eigenstates in the vicinity of the Fermi level, as determined by band filling, exchange splitting and spin-orbit coupling. The large magnetic anisotropy energy of free PtCo and IrCo dimers relative to the easy direction parallel to the dimer axis is coupled to a strong anisotropy of the orbital magnetic moments of the Co atom for both dimers, and also on the Ir atom in IrCo. In contrast the PtFe dimer shows a weak perpendicular anisotropy and only small spin and orbital anisotropies of opposite sign on the two atoms. For dimers supported on graphene, the strong binding within the dimer and the stronger interaction of the 3d atom with the substrate stabilizes an upright geometry. Spin and orbital moments on the 3d atom are strongly quenched, but due to the weaker binding within the dimer the properties of the 5d atom are more free-atom-like with increased spin and orbital moments. The changes in the magnetic moment are reflected in the structure of the electronic eigenstates near the Fermi level, for all three dimers the easy magnetic direction is now parallel to the dimer axis and perpendicular to the graphene layer. The already very large magnetic anisotropy energy (MAE) of IrCo is further enhanced by the interaction with the support, the MAE of PtFe changes sign, and that of the PtCo dimer is reduced. These changes are discussed in relation to the relativistic electronic structure of free and supported dimers and it is demonstrated that the existence of a partially occupied quasi-degenerate state at the Fermi level favors the formation of a large magnetic anisotropy.

CONSPECTUS: Complex intermetallic compounds are a class of ordered alloys consisting of quasicrystals and other ordered compounds with large unit cells; many of them are approximant phases to quasicrystals. Quasicrystals are the limiting case where the unit cell becomes infinitely large; approximants are series of periodic structures converging to the quasicrystal. While the unique properties of quasicrystals have inspired many investigations of their surfaces, relatively little attention has been devoted to the surface properties of the approximants. In general, complex intermetallic compounds display rather irregular, often strongly corrugated surfaces, making the determination of their atomic structure a very complex and challenging task. During recent years, scanning tunneling microscopy (STM) has been used to study the surfaces of several complex intermetallic compounds. If atomic resolution can be achieved, STM permits visualization of the local atomistic surface structure. However, the interpretation of the STM images is often ambiguous and sometimes even impossible without a realistic model of the structure of the surface and the distribution of the electronic density above the surface. Here we demonstrate that ab initio density functional theory (DFT) can be used to determine the energetics and the geometric and electronic structures of the stable surfaces of complex intermetallic compounds. Calculations for surfaces with different chemical compositions can be performed in the grand canonical ensemble. Simulated cleavage experiments permit us to determine the formation of the cleavage planes requiring the lowest energy. The investigation of the adsorption of molecular species permits a comparison with temperature-programmed thermal desorption experiments. Calculated surface electronic densities of state can be compared with the results of photoelectron spectroscopy. Simulations of detailed STM images can be directly confronted with the experimental results. Detailed results are presented for two intermetallic compounds that have recently attracted much attention as active and highly selective catalysts for the semihydrogenation of alkynes to alkenes, but the identification of the catalytically active surfaces was found to be very difficult. The crystal structure of B20-type GaPd can be interpreted as the lowest order approximant of icosahedral Al-Pd-Mn quasicrystals. Among the low-index surfaces, the {100} surface shows 2-fold symmetry and the {210} surface pseudo-5-fold symmetry; for both the surface stoichiometry is identical to that of the bulk. Because the structure lacks inversion symmetry, the {111} surfaces have polar character and permit terminations of widely different chemical composition. Results for all three surfaces are presented and compared with the available experiments. The crystal structure of orthorhombic Al13Co4 is built by pentagonal clusters similar to those found in decagonal Al-Co and Al-Ni-Co quasicrystals. A simulated cleavage experiment shows that the constituent clusters remain intact upon cleavage, resulting in the formation of a highly corrugated (100) surface. The calculated STM images are found to be in very good agreement with experiment and permit in addition identification of possible surface modifications by the desorption of individual atoms. Pentagonal motifs on the {210} surface of GaPd and on the (100) surface of Al13Co4 consisting of simple- and transition-metal atoms have been identified as the catalytically active centers for the semihydrogenation of acetylene to ethylene.

Periodic DFT molecular dynamics and FTIR spectroscopy were used to investigate the cationic sites of ferrierite exchanged with Co(II) and Cu(II) and their complexes with NO. Particular attention was paid to the effect of the Al siting in six-membered rings forming the cationic. sites on the structure of these sites and the corresponding binding energies of Me(II) (Me = Co and Cu). Our calculations show that both the cations upon binding to cationic sites induce a rearrangement of the local structure of the zeolite framework The rearrangement is significant for the alpha and beta-2 sites while it is minor for the beta-1 site. Comparison of the observed and theoretical NO stretching frequencies of ferrierite Co(II) and Cu(II) complexes with a NO molecule permitted the assignment of IR bands to the individual types of cationic sites. For NO-Co-ferrierite, the IR bands found at 1956, 1941, and 1935 cm(-1) can be assigned to NO-Co complexes with Co(II) located in the alpha, beta-1, and beta-2 sites, respectively. Similarly for NO-Cu-ferrierite, the frequencies of 1864, 1912, 1904, and 1892 cm(-1) belong to NO-Cu complexes having Cu(II) accommodated in the alpha, beta-1, beta-2 (conformer 1), and beta-2 (conformer 2) sites, respectively. The calculated adsorption energies are systematically higher for Co(II) than for Cu(II) for all the three sites and are in the order alpha > beta-2 > beta-1 for both the cations. Our computational results further reveal that upon binding Me(II) both the local structure of the zeolite framework as well as the binding energies of Me(II) strongly depend on the Al siting in the rings forming the cationic sites. The calculated relative binding energies of Me(II) are in the order beta-1 > beta-2 > alpha for both the cations. The general tendency of Me(II) accommodated in a cationic site to react is inversely proportional to the corresponding binding energies. We also showed that FTIR spectroscopy of complexes of NO and Me(II)-exchanged ferrierite can serve to identify the Al siting in the six-membered rings forming cationic sites.

The Tkatchenko-Scheffler method for calculating dispersion correction to standard density-functional theory, which uses fixed neutral atoms as a reference to estimate the effective volumes of atoms-in-molecule and to calibrate their polarizabilities and dispersion coefficients, fails to describe the structure and the energetics of ionic solids. Here, we propose a more appropriate partitioning, based on the iterative Hirshfeld scheme, where the fractionally charged atomic reference state is determined self-consistently. We show that our new method extends the applicability of the original method in particular to study ionic systems and adsorption phenomena on surfaces of ionic solids.

Recently, the outstanding properties of Cu-SSZ-13 (a zeolite in the chabazite structure) for the selective catalytic reduction of nitrous oxides were discovered. However, the true nature of the active site is still not answered satisfactorily. In this work, we identify the active site for the given reaction from first-principles simulations of the total energy of Cu(II) ions in various positions in combination with previously published catalytic activity as a function of the copper exchange level. This attribution is confirmed by the simulation of vibrational properties of CO adsorbed to the reduced Cu(I) species. The relation between energetic considerations, vibrational calculations, and experiment allows a clear statement about the distribution of active sites in the catalyst. We furthermore discuss the structural properties of the active site leading to the high stability under reaction conditions over a large temperature range. The insights from this work allow a more targeted catalyst design and represent a step toward an industrial application of copper-exchanged zeolites in cleaning car exhaust gases.

The adsorption of alkanes in a protonated zeolite has been investigated at different levels of theory. At the lowest level we use density-functional theory (DFT) based on semi-local (gradient-corrected) functionals which account only for the interaction of the molecule with the acid site. To describe the van der Waals (vdW) interactions between the saturated molecule and the inner wall of the zeolite we use (i) semi-empirical pair interactions, (ii) calculations using a non-local correlation functional designed to include vdW interactions, and (iii) an approach based on calculations of the dynamical response function within the random-phase approximation (RPA). The effect of finite temperature on the adsorption properties has been studied by performing molecular dynamics (MD) simulations based on forces derived from DFT plus semi-empirical vdW corrections. The simulations demonstrate that even at room temperature the binding of the molecule to the acid site is frequently broken such that only the vdW interaction between the alkane and the zeolite remains. The finite temperature adsorption energy is calculated as the ensemble average over a sufficiently long molecular dynamics run, it is significantly reduced compared to the T = 0 K limit. At a higher level of theory where MD simulations would be prohibitively expensive we propose a simple scheme based on the averaging over the adsorption energies in the acid and in the purely siliceous zeolite to account for temperature effects. With these corrections we find an excellent agreement between the RPA predictions and experiment.

We have studied Ti-based gum-metal alloys to understand their ideal strength behavior by the first-principles density functional theory. The approximant of the gum metal, G1-type Ti3Nb model structure, is determined to be the most favorable one among the possible configurations of four Nb atoms and twelve Ti atoms in a 16-atoms supercell. The ideal tensile strength and the ideal shear strength of the G1 structure are found to be 2.4 GPa and 1.45-1.65 GPa, respectively, which are much lower than those for conventional body-centered cubic simple metals. This is because the elastic softening occurs when the valence electron numbers per atom is around 4.24 featuring the gum-metal composition. The predicted shear strength is very close to the experimentally measured strength of gum-metal nanopillars, 1.7 GPa. Thus, it is confirmed that gum metal should be deformed by near ideal strength.

The method proposed by Tkatchenko and Scheffler [Phys. Rev. Lett. 102, 073005 (2009)] to correct density functional calculations for the missing van derWaals interactions is implemented in the Vienna ab initio simulation package (VASP) code and tested on a wide range of solids, including noble-gas crystals, molecular crystals (alpha-N-2, sulfur dioxide, benzene, naphthalene, cytosine), layered solids (graphite, hexagonal boron nitride, vanadium pentoxide, MoS2, NbSe2), chain-like structures (selenium, tellurium, cellulose I), ionic crystals (NaCl, KI), and metals (nickel, zinc, cadmium). In addition to the original formulation expressing the van der Waals (vdW) corrections as pairwise potentials whose strength is derived from the rescaled polarizabilities of the neutral free atoms, the self-consistently screened (TS + SCS) [Phys. Rev. Lett. 108, 236402 (2012)] variant of the method involving electrodynamic response effects has been examined. Analytical expressions for the forces acting on the atoms and for the components of the stress tensor needed for the relaxation of the volume and shape of the unit cell using the TS + SCS method are derived. While the calculated structures are reasonably close to experiment, the van der Waals corrections to the binding energies are often found to be overestimated in comparison with experimental data. The TS + SCS approach leads to significantly better results in some problematic cases, such as the binding energy of graphite. However, there is room for further improvements, in particular for strongly ionic systems. DOI: 10.1103/PhysRevB.87.064110

The ideal tensile and shear strengths of binary $\ensuremath{\beta}$-phase Ti${}_{3}$Nb alloys have been investigated using ab initio density functional calculations. The binary alloy is considered as an approximant to the multifunctional Ti-Nb-Ta-Zr-O alloy known as ``gum metal,'' which displays high strength, low elastic modulus, high yield strain, and very good ductility. This alloy has been reported to deform elastically until the stress approaches the ideal tensile strength. Our calculations have been performed for an optimized chemical decoration of the body-centered cubic (bcc) structure of the $\ensuremath{\beta}$ phase. Previous work has demonstrated that this model yields elastic constants in very good agreement with those measured for gum metal specimens and leads to a reasonably accurate description of the martensitic transformations between the bcc $\ensuremath{\beta}$, the orthorhombic ${\ensuremath{\alpha}}^{\ensuremath{'}\ensuremath{'}}$ and the hexagonal $\ensuremath{\omega}$ phases [Lazar et al., Phys. Rev. B 84, 054202 (2011)]. The simulations of the response to tensile and shear loading have been performed for large supercells which account also for the different orientations of the -Nb-Nb- chains characteristic for the $\ensuremath{\beta}$-phase structure relative to the direction of the applied load. The energy-strain and stress-strain curves are found to be very different from those reported for all bcc metals. Under uniaxial $\ensuremath{\langle}100\ensuremath{\rangle}$ loading we find an ideal tensile strength of 2.4 GPa, the upper limit to the tensile stress arising from a shear instability of the structure. Under uniaxial $\ensuremath{\langle}110\ensuremath{\rangle}$ load we calculate an ideal tensile strength of 2.2 or 2.8 GPa, depending on the orientation of the -Nb-Nb- chains relative to the loading direction. For a realistic multidomain structure the ideal strength is expected to correspond to the average of these values. An ideal strength of 2.6 GPa under $\ensuremath{\langle}110\ensuremath{\rangle}$ loading is roughly the same as under $\ensuremath{\langle}100\ensuremath{\rangle}$ load, despite a considerable anisotropy of the tensile moduli. For ${211}\ensuremath{\langle}111\ensuremath{\rangle}$ shear we calculate an ideal shear strength of 1.6 GPa, again as an average over different possible shearing directions relative to the Nb-Nb bonds. For the ${110}\ensuremath{\langle}110\ensuremath{\rangle}$ shear system we find a lower strength of 0.9 GPa. The structures reached at the stress maximum under $\ensuremath{\langle}100\ensuremath{\rangle}$ uniaxial tension and ${211}\ensuremath{\langle}111\ensuremath{\rangle}$ shear are identical, and since the maximal shear stress is much lower than the tensile stress, the alloy will fail by shear even under strictly uniaxial tension. The values of the ideal tensile and shear strengths are significantly low, even in comparison with those calculated for bcc V and Nb with very small shear moduli and approach the values reported for gum metal alloys.

The structural, energetic, and magnetic properties of Pt atoms and dimers adsorbed on a Ni-supported graphene layer have been investigated using density-functional calculations, including the influence of dispersion forces and of spin-orbit coupling. Dispersion forces are found to be essential to stabilize a chemisorbed graphene layer on the Ni(111) surface. The presence of the Ni-substrate leads not only to a stronger interaction of Pt atoms and dimers with graphene but also to a locally increased binding between graphene and the substrate and a complex reconstruction of the adlayer. The stronger binding of the dimer also stabilizes a flat adsorption geometry in contrast to the upright geometry on a free-standing graphene layer. These effects are further enhanced by dispersion corrections. Isolated Pt adatoms and flat dimers are found to be non-magnetic, while an upright Pt dimer has strongly anisotropic spin and orbital moments. For the clean C/Ni(111) system, we calculate an in-plane magnetic anisotropy, which is also conserved in the presence of isolated Pt adatoms. Surprisingly, upright Pt-dimers induce a re-orientation of the easy magnetic axis to a direction perpendicular to the surface, in analogy to Pt2 on a free-standing graphene layer and to the axial anisotropy of a gas-phase Pt2 dimer.