Ab initiocalculations on (MgO)n, (CaO)n, and (NaCl)nclusters (n=1–6)

Impact of Local Structure in Supported CaO Catalysts for Soft-Oxidant-Assisted Methane Coupling Assessed through Ca K-Edge X-ray Absorption Spectroscopy

Soft-oxidant-assisted methane coupling has emerged as a promising pathway to upgrade methane from natural gas sources to high-value commodity chemicals, such as ethylene, at selectivities higher than those associated with oxidative (O2) methane coupling (OCM). To date, few studies have reported investigations into the electronic structure and the microscopic physical structure of catalytic active sites present in the binary metal oxide catalyst systems that are known to be effective for this reaction. Correlating the catalyst activity to specific active site structures and electronic properties is an essential aspect of catalyst design. Here, we used X-ray absorption spectroscopy at the Ca K-edge to ascertain the most probable local environment of Ca in the ZnO-supported Ca oxide catalysts. These catalysts are shown here to be active for N2O-assisted methane coupling (N2O-OCM) and have previously been reported to be active for CO2-assisted methane coupling (CO2-OCM). X-ray absorption near edge structure features at multiple Ca loadings are interpreted through simulated spectra derived from ab initio full multiple scattering calculations. These simulations included consideration of CaO structures organized in multiple spatial arrangements-linear, planar, and cubic-with separate analyses of Ca atoms in the surfaces and bulk of the three-dimensional structures. The morphology of the oxide clusters was found to influence the various regions of the X-ray absorption spectrum differently. Experiment and theory show that for low-Ca-loading catalysts (≤1 mol %), which contain sites particularly active for methane coupling, Ca primarily exists in an oxidized state that is consistent with the coordination environment of Ca ions in one- and two-dimensional clusters. In addition to their unique nanoscale structures, the spectra also indicate that these clusters have varying degrees of undercoordinated surface Ca atoms that could further influence their catalytic activities. The local Ca structure was correlated to methane coupling activity from N2O-OCM and previously reported CO2-OCM reactor studies. This study provides a unique perspective on the relationship between the catalyst physical and electronic structure and active sites for soft-oxidant-assisted methane coupling, which can be used to inform future catalyst development.

Electronic structure and physicochemical properties of the metal and semimetal oxide nanoclusters

Clusters are physical entities composed of a few to thousands of atoms with capabilities to develop novel materials, like cluster-assembled materials. In this sense, knowing the electronic structure and physicochemical properties of the isolated clusters can be useful to understand how they interact with other chemical species by intermolecular forces, as free, embedded, and saturated clusters, and by intramolecular forces, acting as support clusters. In this way, in the present work, the electronic structure and physicochemical properties of metal oxide nanoclusters (MgO, Al₂O₃, SiO₂, and TiO₂) were studied by highly correlated molecular quantum chemistry methods. Through the electronic state's characterization, a semiconductor aspect was found for the titania oxide nanocluster (T_e < 0.8 eV) while the other agglomerates showed a characteristic of insulating material (T_e > 3.3 eV). From the stability index, the following stability order can be characterized: (SiO₂)₄ > (Al₂O₃)₄ > (MgO)₄ > (TiO₂)₃. Initial information of intermolecular and intramolecular forces caused by the studied clusters was calculated through the relative electrophilicity index, which classified the (MgO)₄ and (TiO₂)₃ clusters as the more reactive ones, in which the (MgO)₄ cluster was identified as a nucleophilic species, while the (TiO₂)₃ cluster as an electrophilic molecule.

A First-Principles Study on the Hydration Behavior of (MgO)n Clusters and the Effect Mechanism of Anti-Hydration Agents

Magnesia-based refractory is widely used in high-temperature industries; its easy hydration is, however, a key concern in refractory processing. Understanding the hydration mechanism of MgO will help in solving its hydration problem. Herein, the hydration behavior of (MgO)n (n = 1–6) at the molecular level and the effect mechanisms of several anti-hydration agents on the hydration of (MgO)4 were investigated with first-principles calculations. The results indicated that the following: (1) The smaller the (MgO)n cluster size, the more favorable the hydration of MgO and the tendency to convert into Mg(OH)2 crystal; (2) Anti-hydration agents can coordinate with the unsaturated Mg atom of (MgO)4 to form a bond, increasing the coordination number of Mg, thus reducing its activity when reacting with H2O; (3) The greater the number of −COOH groups and the longer the chain length in the anti-hydration agents, the better its effect of inhibiting the hydration of MgO. These findings could enhance the understanding of the mechanism of hydration of MgO and provide theoretical guidance for the design of novel anti-hydration agents.

Structural evolution of calcia during calcium deoxidation in Fe-O-Ca melt

The crystallization process of CaO in iron melt begins with nucleation, which determines the structure and size of the CaO inclusions; thus, it is important to investigate the mechanism of inclusion modification by calcium treatment. In this study, a two-step nucleation method was used to investigate the behavior during the early stages of CaO inclusion crystallization. The first principles method was applied to calculate the structures and properties of CaO crystals and CaO clusters. Then, the nucleation mechanism of CaO in the Fe-O-Ca melt has been discussed. The numerical results show that CaO clusters with cubic structures and appropriate variations are the lowest energy structures and are more stable than other isomers. The stability of the cubic (CaO)_n clusters increases with the increase in size and gradually approaches that of the CaO crystal. CaO clusters can form spontaneously in the Fe-O-Ca melt, while the transformation reaction of the CaO clusters in the Fe-O-Ca melt deeply depends on the supersaturation ratio of [Ca] and [O]. CaO clusters may remain as suspended CaO inclusions in the iron melt for a long time, and these suspensions of CaO clusters are the source of excess oxygen in the iron melt.

Simulation Investigation on Flame Retardancy of the PVAc/ATP Nanocomposite

Molecular dynamics (MD) simulations were carried out to study the effects of some key factors on the enhancement of flame retardancy of the PVAc/ATP nanocomposite. As a result, the obvious improved flame retardancy was attributed mainly to the increased dispersion of Mg ions in the PVAc matrix due to the stronger interaction between PVAc and ATP and partially to the combustion temperature of PVAc released by the escaped H2O originating from the ATP dopant. Hence, the ATP ore as a predicted additive is viewed as a prospective candidate to be applied in future organic materials to obtain better flame-retardant properties. Moreover, in our simulations, the temperature can induce a significant impact on the interaction of the PVAc/ATP nanocomposite, in which the prominent combination between PVAc and ATP could be greatly promoted at 350 K.

Electronic and vibrational second hyperpolarizabilities of (MgO)n clusters

In this work, we report results for the static second hyperpolarizability of magnesium oxide clusters including electronic and vibrational contributions. The comparison between second-order Møller-Plesset (MP2) perturbation theory and coupled cluster results to the electronic contribution points out that MP2 is a suitable method to compute this property. When computed at the MP2 level, the electronic contribution per atom converges to approximately 5000 a.u. Vibrational corrections were computed at the MP2 level through the perturbation theoretical method of Bishop and Kirtman. Results obtained showed that the term [α²]^0,0 represents around 20% of the electronic counterpart while the term [μβ]^0,0 is comparable to it. Modes that contribute significantly to [α²]^0,0 are those in which all or part of the bond lengths simultaneously increase and decrease, leading to large polarizability derivatives. In turn, modes that provide relevant contributions to [μβ]^0,0 are those in which oxygen anions move in opposite directions to the magnesium cations yielding large derivatives of the dipole moment and first hyperpolarizability.

Electro- and opto-mutable properties of MgO nanoclusters adsorbed on mono- and double-layer graphene

Inspired by recent experiments, the trapping of molecules in 2D materials has gained increasing attention due to the unique ability of the molecules to modulate the electronic and optical properties of 2D materials, which calls for fundamental understanding and predictive design strategies. Herein, we focus on mono- and double-layer graphene encapsulating various MgO clusters and explore their diverse electronic and optical properties using a number of high-level first-principles calculations. By correlating the stability of adsorption, geometry, charge transfer, band structures, optical absorption spectrum, and the van der Waals pressure, our results decode various synergies in electro- and opto-mutable properties of MgO/graphene systems. We found that 2D-MgO flakes on graphene layers exhibit surface polarization effects - in contrast to their isolated neutral flakes - and show a significant charge transfer from graphene to n-doped flakes, breaking the symmetry of graphene layers. We obtained a van der Waals pressure of ∼0.7 (0.9) GPa on bilayer graphene encapsulating MgO nanoclusters, which matches extremely well with experiment. While there is one quantum emission in the visible light region for a single MgO flake, a wide range of visible light is accessible for MgO on mono- and double-layer graphene. Overall, these findings provide new physical insights and design strategies to modulate 2D materials with several applications in optoelectronics while significantly broadening the spectrum of strategies for fabricating new hybrid 2D heterostructures by encapsulating external molecules.

Structures and Stabilities of Alkaline Earth Metal Oxide Nanoclusters: A DFT Study

The stability orders of a number of alkaline earth oxide cluster isomers (MO)n, M = Mg, Ca, Sr, Ba and 1≤n≥6 have been determined by means of density functional theory studies using the LDA-PWC functional. Among the candidate structures, the hexagonal-ring-based isomers and the slab shapes are found to display similar stabilities. Stacks of hexagonal (MO)3 rings are found to be the slightly preferred growth strategy among the (MgO)6, isomers. In contrast, the slab structures are slightly preferred for the other alkaline metal oxide (MO)6 clusters. An explanation based on packing and aromaticity arguments has been proposed. This study may have important implications for modeling and understanding the initial growth patterns of small nanostructures of alkaline earth metals.

Stability and metastability of clusters in a reactive atmosphere: theoretical evidence for unexpected stoichiometries of MgMOx

By applying a genetic algorithm and ab initio atomistic thermodynamics, we identify the stable and metastable compositions and structures of MgMOx clusters at realistic temperatures and oxygen pressures. We find that small clusters (M≲5) are in thermodynamic equilibrium when x>M. The nonstoichiometric clusters exhibit peculiar magnetic behavior, suggesting the possibility of tuning magnetic properties by changing environmental pressure and temperature conditions. Furthermore, we show that density-functional theory with a hybrid exchange-correlation functional is needed for predicting accurate phase diagrams of metal-oxide clusters. Neither a (sophisticated) force field nor density-functional theory with (semi)local exchange-correlation functionals is sufficient for even a qualitative prediction.

Structure determination of neutral MgO clusters--hexagonal nanotubes and cages

Structural information for neutral magnesium oxide clusters has been obtained by a comparison of their experimental vibrational spectra with predictions from theory. (MgO)(n) clusters with n = 3-16 have been studied in the gas phase with a tunable IR-UV two-color ionization scheme and size-selective infrared spectra have been measured. These IR spectra are compared to the calculated spectra of the global minimum structures predicted by a hybrid ab initio genetic algorithm. The comparison shows clear evidence that clusters of the composition (MgO)(3k) (k = 1-5) form hexagonal tubes, which confirm previous theoretical predictions. For the intermediate sizes (n≠ 3k) cage-like structures containing hexagonal (MgO)(3) rings are identified. Except for the cubic (MgO)(4) no evidence for bulk like structures is found.

Accurate Equilibrium Structures Obtained from Gas-Phase Electron Diffraction Data: Sodium Chloride

A novel method has been developed to allow the accurate determination of equilibrium gas-phase structures from experimental data, thus allowing direct comparison with theory. This new method is illustrated through the example of sodium chloride vapor at 943 K. Using this approach the equilibrium structures of the monomer (NaCl) and the dimer (Na(2)Cl(2)), together with the fraction of vapor existing as dimer, have been determined by gas-phase electron diffraction supplemented with data from microwave spectroscopy and ab initio calculations. Root-mean-square amplitudes of vibration (u) and distance corrections (r(a) - r(e)) have been calculated explicitly from the ab initio potential-energy surfaces corresponding to the vibrational modes of the monomer and dimer. These u and (r(a) - r(e)) values essentially include all of the effects associated with large-amplitude modes of vibration and anharmonicity; using them we have been able to relate the ra distances from a gas-phase electron diffraction experiment directly to the re distances from ab initio calculations. Vibrational amplitudes and distance corrections are compared with those obtained by previous methods using both purely harmonic force fields and those including cubic anharmonic contributions, and the differences are discussed. The gas-phase equilibrium structural parameters are r(e)(Na-Cl)(monomer) = 236.0794(4) pm; r(e)(Na-Cl)(dimer) = 253.4(9) pm; and <(e)ClNaCl = 102.7(11) degrees. These results are found to be in good agreement with high-level ab initio calculations and are substantially more precise than those obtained in previous structural studies.

Table Salt and Other Alkali Metal Chloride Oligomers: Structure, Stability, and Bonding

We have investigated table salt and other alkali metal chloride monomers, ClM, and (distorted) cubic tetramers, (ClM)(4), with M = Li, Na, K, and Rb, using density functional theory (DFT) at the BP86/TZ2P level. Our objectives are to determine how the structure and thermochemistry (e.g., Cl-M bond lengths and strengths, oligomerization energies, etc.) of alkali metal chlorides depend on the metal atom and to understand the emerging trends in terms of quantitative Kohn-Sham molecular orbital (KS-MO) theory. The analyses confirm the high polarity of the Cl-M bond (dipole moment, VDD, and Hirshfeld atomic charges). They also reveal that bond overlap derived stabilization (approximately -26, -20, and -8 kcal/mol), although clearly larger than in the corresponding F-M bonds, contributes relatively little to the (trend in) bond strengths (-105, -90, and -94 kcal/mol) along M = Li, Na, and K. Thus, the Cl-M bonding mechanism resembles more closely that of the even more ionic F-M bond than that of the more covalent C-M or H-M bonds. Tetramerization causes the Cl-M bond to expand, and it reduces its polarity.

Infrared spectroscopy of HCN-salt complexes formed in liquid-helium nanodroplets

Rotationally resolved infrared spectra are reported for the binary complexes of HCN and LiF, LiCl, NaF, and NaCl, formed in helium nanodroplets. Stark spectroscopy is used to determine the dipole moments for these complexes. Ab initio calculations are also reported for these complexes, revealing the existence of several different isomers of these binary systems. In the frequency region examined in this experimental study we only observe one of these, corresponding to the salt binding to the nitrogen end of the HCN molecule. The experimental rotational constants, dipole moments, and vibrational frequency shifts are all compared with the results from ab initio calculations for this isomer.

Using polarity for engineering oxide nanostructures: structural phase diagram in free and supported MgO(111) ultrathin films

Using an ab initio total energy approach, we study the stability of free and Ag(111)-supported MgO(111) ultrathin films. We unravel a novel microscopic mechanism of stabilization of polar oxide orientations, based on a strong modification of the MgO structural phase diagram with respect to the bulk material. We predict that, at low thickness, films which are either unsupported or deposited on Ag(111) display a graphitelike Bk structure rather than the expected rocksalt one. Our results provide a consistent interpretation of recent experimental findings, exemplify the efficiency of this novel stabilization mechanism, and suggest new methods to engineer oxide nanostructures.

(MgO)(-)(n) (n = 1-5) clusters: multipole-bound anions and photodetachment spectroscopy

Photoelectron spectra of (MgO)(-)(n) (n = 1-5) reveal a surprising trend: The electron binding energy decreases from n = 1 to 4, and then increases from 4 to 5. Ab initio calculations suggest this pattern is related to the electrostatic interaction between the extra electron and the charge distribution of the neutral cluster. This interaction is significant in MgO (-) and (MgO)-5, for which the lowest nonvanishing multipole moment (LNM) is a dipole; it is smaller for (MgO)-2 and (MgO)-3, for which a quadrupole is the LNM; and it is the smallest for (MgO)-4, for which an octopole is the LNM. The cubic (MgO)-4 is the first octopole-bound anion yet observed experimentally and characterized theoretically.