Search

A simplified computational scheme for “deposition” of a metallic (Pd) monolayer on semi-ionic supports is considered. An arbitrary selection of the initial position of the deposited layer relative to the surface of the support cannot provide the best solution for an optimal atom–atom binding. The energy and/or geometry changes were checked relative to a slide of the mono- and bilayers in parallel to the support layer and re-optimization. Two types of possible consequences are shown at the slide length of 1–3 A for γ-Al2O3(110), monoclinic ZrO2(001) (or m-ZrO2), and rutile TiO2(001) (or TiO2 below). First consequence option: either the Pd monolayer completely collapses after the shift/optimization, or it yields a new geometry (at γ-Al2O3(110) and m-ZrO2) at different steps. Second consequence option: the shift leads to a loss of the Pd stabilization while the ordered monolayer's geometry is retained (at TiO2 and m-ZrO2). The extent of such destabilization varies depending on the strength of the interaction with the support. It is thus recommended to verify Pd stabilization energy by sliding/optimizing the monolayer along one or two monolayer directions in order to avoid its energy underestimation. The activity of such monolayers for modelling catalysis is demonstrated using H2O dissociation in the Pd monolayer/rutile system.

The importance of the “heterogeneity” of a Pd monolayer induced by interaction with a semi-ionic support in catalysis was evaluated. The geometry of the Pd monolayer was optimized on the (100) plane of γ-Al2O3 at fixed unit cell parameters defined by the oxide. Simulation of the deposition of a whole Pd monolayer in the flat Pd(100) form cut from the bulk led to the formation of a slightly distorted Pd(111) monolayer. The subsequent chemisorption or dissociation of CH4 or H2O on the Pd(111) layer resulted in a new hybrid Pd(100)/(111) structure containing alternating elements of (100) and (111) planes (the parallel bands of squares and triangles), which are similar for both CH4 and H2O reactions, and two isolated Pd mono-vacancies, respectively. The hybrid Pd(100)/(111) layer without chemisorbed species was found to be more stable than the initial distorted Pd(111) layer. The catalytic capabilities of these monolayer structures were investigated for the dissociation of methane and the water–gas shift reaction (WGSR) due to the lower predicted activation barriers for CH4, H2O, and O2 dissociation on the hybrid Pd(100)/(111) layer compared to that on the pure (bulk) Pd(100) surface. Moreover, the exothermic heats of these reactions were calculated to be moderate instead of endothermic heats on the Pd(100) or Pd(111) surfaces. The heats of H2O and NH3 adsorption on various monolayers were tested, revealing their dependence on Pd atomic charges. The relevance of the model of the heterogeneous Pd monolayer for explaining the maximum reaction rate experimentally observed at different Pd coverages was discussed. The transferability of the geometry and the extent of charge inhomogeneity of the hybrid monolayer without vacancies were also tested on the same γ-Al2O3(100) support for Pt, Rh, and Ag.

The history of developing ways of calculating atomic multipole moments is briefly described along with stages of the development and specific features of contemporary approaches in this field. A way based on the approach used by Saunders et al. is proposed for extrapolating atomic multipole moments in silica zeolites, and the behavior of an electrostatic potential (EP) and field (EF) with respect to qualitative improvement of the atomic basis is studied. It is shown that the absence of EP convergence when improving the basis in sieves can be corrected by considering the ionicity of a zeolite to attain the minimum EP deviation from the most precise value limited by the change in the EF.

New and complex results on catalytic activity, thermal stability, effect of H2O and SO2 on the Pd/La2O3-CeO2-Al2O3 model system in methane combustion have been obtained. The catalyst characterization has been made by N2- physisorption, XRD, SEM/EDX, HRTEM, XPS, TPD, TPR and FTIR methods. The observed high activity is related to the stabilization of highly dispersed Pd4+ species, strongly interacting with the support. As it was expected, La2O3 prevented the sintering of the gamma-alumina carrier, in addition it probably contributed to avoid further agglomeration of the palladium species by the formation of La2PdO4 mixed phase. It is supposed that the most probable reason for the observed decrease in activity in the presence of water vapor of thermally aged sample is the increase in PdO/Pd – ratio above its optimum. The presence of both H2O and SO2 in the gas leads to formation of surface Al2(SO4)3, thus indicating a limited possibility to regenerate completely the catalyst. The reaction pathway over Pd/La2O3-CeO2-Al2O3 catalysts proceeds most probably through Mars–van Krevelen mechanism upon adsorption of water molecules on both oxidized and reduced sites and slow desorption of reaction products. For practical evaluation of the synthesized material, a sample of Pd/La2O3-CeO2-Al2O3 was examined as supported on stainless steel (Aluchrom VDM®) in the form of single monolithic channel. Two-dimensional heterogeneous models were used to simulate the methane combustion from laboratory scale up to pilot- and full-scale adiabatic monolithic reactors.

An effect of deblocking of small size (8R, D8R) pores in zeolites due to cation drift is analyzed by using ab initio molecular dynamics (AIMD) at the PBE-D2/PAW level. The effect of carbonate and hydrocarbonate species on the carbon dioxide uptake in NaKA zeolite is demonstrated. It is shown that a hydrocarbonate or carbonate anion can form strong complexes with K+ cation and withdraw it from the 8R window, so that the probability of CO2 diffusion through 8R increases. For the first time, correlations between cationic and HCO3–/CO32– positions are demonstrated in favor of their significant interaction leading to the cationic drift from 8R windows. This phenomenon explains a nonzero CO2 adsorption in narrow pore zeolites upon high Na/K exchange. In a gas mixture, such deblocking effect reduces the separation factor because of the possible passage of both components through the plane of partly open 8R windows.

Multi-layer Pd(100)/γ-Al2O3(100) slab models were constructed over a non-defective γ-Al2O3 (100) surface in order to study the Pd oxidation steps including penetration of atomic oxygen from the surface to Pd bulk. The modeling using periodic DFT/PBE revealed that near the surface both chemisorbed O atoms or physisorbed O2 can play a promotive role regarding O diffusion starting from c(2 × 2) model of O adsorption. The calculated activation energies E# for O diffusion from Pd(100) surface approach experimental E# values between the deeper layers while underestimate the rearrangement barriers at the surface. We also calculated the activation energies of CH4 and O2 dissociation considering different Pd-Pd distances in the Pd and oxidized Pd(O*) layers on different types of oxide support shown earlier.

Two variants of the introduction of transition metals (TMs) Me = Ni, Mn, Co into the Pd(100)/(100)γ-Al2O3 catalytic system are presented to reveal a possible influence of TM on the catalytic activity of Pd for CH4 dissociation. Al/Me replacement at the (100)γ-Al2O3 surface is performed at the normal (four-coordinated) or undercoordinated (two-coordinated) Al positions, on the one hand. MeAl2O4 spinel formation is admitted at a stoichiometric Me/Al ratio, on the other hand. The addition of TM to the (100)γ-Al2O3 slab weakly changes the activation energy of CH4 dissociation. The dissociation barrier is higher for the Pd deposited over (100)MeAl2O4 spinels than the barriers over Pd(100)/(100)γ-Al2O3 catalysts. The dissociation barrier grows with a MeAl2O4 cell parameter from Ni to Mn. The minimum barrier over different supports (ZrO2, Al2O3, MeAl2O4) was found for Pd(100) deposited on the defective (100)γ-Al2O3 model. Considering another step of the whole Pd/PdO redox cycle of CH4 oxidation, we evaluated the consequences of large Pd···Pd distances over MeAl2O4 for atomic O diffusion in Pd(100). Higher squares of Pd4 windows, which limit the O jumps, relative to those calculated for transition states in Pd(100)/(100)γ-Al2O3 predict an easier O diffusion in Pd/MeAl2O4. The O2 consumption at lower temperatures in the spinels was tested experimentally using temperature-programmed oxidation (TPO) method and confirmed the easier O2 chemisorption therein compared to Pd(100)/(100)γ-Al2O3. Magnetic TM moments reveal weak changes in the course of CH4 dissociation for both routes of TM addition mainly depending on reordering of Pd geometry and its magnetic moments at both (100)γ-Al2O3 or (100)MeAl2O4.

Two series of Pd–MeOx/Al2O3 (Me = Co, La, Ce) catalysts were prepared and tested in reaction of methane combustion. The first series comprises the samples obtained by impregnation of γ-Al2O3 with the corresponding salts of La and Ce and additionally modified with Co. For the second series the LaCeAl2O3 SG and LaAl2O3 SG supports were prepared by the sol–gel method and the cobalt was not included as a modifying element. The aim was to obtain catalysts with comparable activities to that of the Co-containing samples, however the using of cobalt being subject of exclusion due to its high toxicity. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Temperature programmed reduction and Fourier transform infrared spectroscopy. The modification of alumina with lanthanum and cerium by impregnation, before the cobalt introduction, decreases interaction between the PdO and the support leading to the easy transformation of PdO into the less active Pd, when the catalyst is treated at high temperatures. The cobalt-free samples prepared by sol–gel method exhibit activity and stability comparable with those of the impregnated cobalt containing Pd + Co/LaAl2O3 catalyst. The observed high activity is explained by the formation of stable PdO particles, however the needed Pd-content to achieve the same activity as Co-containing samples is remarkably higher.

The question about possible influence of oxide support on the structure of deposited metal slabs was discussed for a series of TiO2 (rutile), tetragonal form of ZrO2, and some forms of γ-Al2O3 oxides, all of them being frequently applied for metal deposition and СН4 oxidation. The Pd(100) or Pd(110) slab models were constructed over defective and non-defective γ-Al2O3(100), non-defective ZrO2(001), and TiO2(001) supercells within a wide range of charge transfer. The latter includes the transfer from support to Pd slab or an opposite way, i.e., the total charge of all Pd atoms of the unit cells varies from − 10.456 to 2.768 e (both charge boundaries correspond to defective γ-Al2O3 oxide models with higher Al or O surface concentrations, respectively). Quick charge relaxation to nearly zero values starting already from the second Pd layer (the next one after contacting Pd layer) was observed irrespective of the presence of any defects that can charge the metallic slab both positively or negatively. The slab geometry, i.e., Pd-Pd length, of all layers depends on the oxide type and the defect type. After partial defect passivation over defective γ-Al2O3 surface with O2 or OH groups, Pd-Pd length converges to the same length as obtained with non-defective γ-Al2O3 model. The different Pd-Pd bond lengths were obtained in the top of 4-layer Pd slabs deposited over ZrO2 and TiO2 after relaxation relative to the convergent value for the Pd over γ-Al2O3 support. It illustrates the importance of the support type for growing metal nanoparticles if the contact domain with the support is sufficiently large.

Possible mechanism of oxidative carbonylation of methanol via >Cu(OCH3)2Cu< binuclear cationic oxo-clusters in the CuNaX zeolite with Cu2+ cations is analyzed theoretically within the scope of periodic boundary conditions with VASP. Such scheme was first derived by Saegusa et al. (J. Org. Chem.1970, 35, 2976) and was never tested by theoretical modeling to our best knowledge. For the CO attack we have computed the activation energy value that is close to experimental values obtained in Cu-zeolites. We suppose that this scheme can correctly describe the oxidative carbonylation at medium and high Cu loading when the copper oxo-clusters can be formed.

Our work is devoted to DFT calculations of the relative rotational and diffusional barriers for CO motions in zeolite NaY. The diffusion jump of CO adsorbed in NaY from NaII to Na'II has been confirmed as the favored way for CO re-coordination via either the C or the O atom to the Na cations instead of the CO rotation, hence explaining the mechanism which is responsible for the CO exchange between different positions and the changes in the intensities of the vibrational IR spectra. The fine structure of the vibrational C-O bands is explained by the different CO locations of adsorbed mono- and dicarbonyl species. The calculated activation energy of intra-cage CO diffusion from NaII-CO to Na'II-OC matches the respective experimental barrier observed in the NaX zeolite.

An effect of deblocking of small size (8R, D8R) pores in zeolites due to cation drift is analyzed by using ab initio molecular dynamics (AIMD) at the PBE-D2/PAW level. The effect of carbonate and hydrocarbonate species on the carbon dioxide uptake in NaKA zeolite is demonstrated. It is shown that a hydrocarbonate or carbonate anion can form strong complexes with K

A novel approach to analysis of complex gaseous mixtures is presented. The approach is based on the utilization of a compact gas chromatograph in combination with an array of highly integrated and selective metal oxide (MOX) sensors. Thanks to the implementation of a multisensory detector, the device collects multiple chromatograms in a single run. The sensors in the integrated MEMS platform are very distinct in their catalytic properties. Hence, the time separation by chromatographic column is complemented by catalytic separation by a multisensory detector. Furthermore, the device can perform the analysis in a broad range of concentrations, from ppb to hundreds of ppm. Low ppb and even sub-ppb levels of detection for some analytes were achieved. As a part of this effort, nanocomposite gas sensors were synthesized for selective detection of hydrogen sulfide, mercaptans, alcohols, ketones, and heavy hydrocarbons.

The energies of CO2 interaction with MeY zeolites containing different alkali cations (Me = Li, Na, K, Cs) at small coverage of MeY zeolites are studied using DFT modeling (PBE, PBE-D2, optPBE-vdW, optB88-vdW, and optB86b-vdW functionals). The role of CO2 interaction with HCO3− is analyzed to explain unusual decreasing order of the heats of CO2 adsorption in the sequence CsY > KY > NaY > LiY recently observed. The largest isosteric heat of adsorption in CsY is assigned to the interactions with HCO3− whose maximum concentration in CsY was confirmed via IR tools.

Microstructure factor (MF) is one of the most important properties of hydrogenated silicon (HS) used in industry. It is expressed via vibrational intensities of low stretching modes (LSM) of Si–H and high stretching modes (HSM) of SiH2/SiH3 bonds, i.e. MF = IHSM/(ILSM + IHSM). Mixed character of HSM due to Si–H contributions (together with SiH2/SiH3 ones) is confirmed by vibrational analysis of Si–H valence bonds in a 120Si:18H model of amorphous HS at the Perdew-Burke-Ernzerhof (PBE)/projector augmented wave (PAW)level. Comparison with precedent solutions for di- and poly-vacancies in microcrystalline HS shows that the redistribution of its Si–H modes between HSM and LSM bands depends on the extent of perturbation of the vacancies. This relation between the HSM/LSM repartition and HS distortion explains possible MF dependence from short or medium range ordering and its link with crystallisation kinetics.

The activation barriers for Na jumps between the cationic sites NaI, NaII, and NaIII are calculated using VASP in the presence of anions (Cl−, Br−) or H2O as well as for the empty Na12A zeolite model. The inter-cage cationic NaIII→NaII’→NaIII′ path, where NaIII′ is located in the neighbor α–cage, was modeled via an intermediate state (NaII′) corresponding to two NaII cations in one 8R window. This NaII′ point can also serve as an intermediate for the second type of intra-cage Na drift, i.e., to a NaIII site in the same cage. Reasonable agreement with experimental activation energy was achieved for NaII’→NaIII′, NaII→NaIII, and NaIII→NaIII jumps. The lower activation energy is obtained for the NaII’→NaIII' jump than for NaII→NaIII, which indicates the possible role of the doubly occupied 8R window as an intermediate state in the Na transfer with high frequency and lower activation energy related to ionic current (inter-cage transfer) in Na12A.

Two possible schemes of carbonate formation (with and without water participation) in cationic form zeolites are considered. Activation energy for the formation of hydrogen carbonate in NaX zeolite from water and carbon dioxide is calculated at the DFT level with periodic boundary conditions, while the problems of modeling the formation of symmetric carbonate in the same zeolite are discussed. The formation of copper carbonate is studied using binuclear CuOCu clusters from CO2 where the influence of water on the barrier is discussed. The questions related to DFT application to binuclear copper clusters are also considered by comparison with the data obtained at the MP2 level. The reactivity of copper carbonate is tested in the reaction with methanol. © 2015 Wiley Periodicals, Inc.

The influence of copper impurities on the nucleation of dislocations in aluminum has been investigated using the method for determining the average lifetime of the metastable state. The primary data have been obtained using the molecular dynamics method. The obtained dependences of the dislocation nucleation rate on the shear stress in pure aluminum and aluminum solid solutions with copper have been plotted in the Arrhenius form. It has been found that, with an increase in the copper concentration, the activation parameters for these dependences decrease. It has been shown that an increase in the temperature leads to an increase in the sensitivity of the dislocation nucleation rate to stresses in aluminum solid solutions with 0.1–3.0 at % Cu, in contrast to pure aluminum in which the sensitivity is almost independent of the temperature. The dependences of the dislocation nucleation rate on the copper concentration and temperature over the entire range of stresses have been discussed based on the obtained approximations.

The bulk and Young moduli and heats of hydration have been calculated at the DFT level for fully optimized models of all-siliceous and cationic zeolites with and without water, and then compared to the corresponding experimental data. Upon the addition of water, the monovalent alkali ion and divalent alkaline earth ion exchanged zeolites presented opposite trends in the elastic modulus. The main contribution to the decrease in the elastic modulus of the alkali ion exchanged zeolites appeared to be a shift of cations from the framework oxygen atoms upon water addition, with the coordination number often remaining the same. The contrasting increase in elastic modulus observed for the divalent (alkaline earth) ion exchanged zeolites was explained by cation stabilization resulting from increased coordination, which cannot be achieved within a rigid zeolite framework without water.

The alkali forms of MeRHO zeolite (Me = Li, Na, K, Cs) are studied theoretically at the DFT/PBE level using projector augmented wave method (PAW) and periodic boundary conditions (PBC). The equilibrium positions of the Me cations have been optimized at the D8R prisms where they control molecular transport in the pores of MeRHO. The effect of the displacement of alkali cations has been estimated due to coordination of both carbonate or hydrogen carbonate anions that can also induce RHO framework transformations. Admitting the CO 3 −2 and HCO 3 − formations via the reactions between H 2 O and CO 2 , the energy of respective reactions was evaluated at various Si/Al moduli and analyzed regarding possible influence on the gas adsorption.

The inter-relation between chemical and mechanical properties is demonstrated regarding Young’s modulus variation of the NaX and NaKX (83% exchange Na/K) zeolites upon carbonate formation or CO2 adsorption. The force field (FF) approach is based on the combination of known FFs for the zeolite atoms, alkali cations, CO2, and carbonate atoms. Contrary variations of the Young’s modulus are obtained in NaX upon CO2 adsorption (increase far from CO2 condensation conditions) or carbonate formation (decrease). Higher decreases of the Young’s and bulk moduli are predicted for K-exchanged form that was not yet tested experimentally.

The structures and stability of bi- and trinuclear Al clusters at the cationic sites in an 8-membered ring of mordenite are calculated within the cluster approach using the DFT method. Adsorption heats of hydrogen and water, hydrogenation and hydrolysis reactions are compared with similar values for the Ga cluster at the cationic sites of a similar Ga-exchange form of mordenite whose existence is consistent with the experiment. The effect of a different arrangement of lattice aluminum atoms in the 8-membered ring of mordenite is analyzed.

The common main factor of the Na+ influence on the extra-framework aluminum (EFAL) transformations was observed while modeling partly hydrated zeolites at both periodic and cluster computational levels. It can be characterized as the attack of Lewis acid (Na+) toward electron pairs of different O atoms. The limiting step of EFAL formation is assigned to the transformation between the AlOH2+ and Al ( OH ) 2 + species. It allows the explanation of the poisoning effect of Na+ on the cracking reactions known experimentally. The role of electrostatic factor is also discussed.

In order to estimate atomic multipole moments (AMMs) and charges in the model of amorphous SiO2 a hybrid B3LYP functional with 30% of the Hartree-Fock Hamiltonian in the exchange part with the 88-31G*(Si)/8-411G*(O) basis set and the CRYSTAL06 package are used. A 192-atomic unit cell of amorphous SiO2 is chosen as a model, the calculations with which agree well with the experimental static factor of neutron scattering. The second optimized model of amorphous SiO2 (a-SiO2) with a smaller number of defects is prepared with the use of the VASP package and full optimization of the initial a-SiO2 model. For both models the atomic charges and AMMs are calculated (up to the fourth order included) and their approximation is performed. The approximation quality is compared for these models and with a model for crystalline systems whose AMMs were previously calculated. The conclusions are drawn about the applicability of charge and AMM estimates within the approaches such as the embedded cluster.

Problem of Al–O–Al linkage in aluminosilicate materials or Al-avoidance is discussed for two zeolite structures (phillipsite and brewsterite) exchanged with Mg2+ cations. All models are fully optimized at periodic Hartree–Fock and hybrid density functional theory levels (the CRYSTAL code). Their properties are then calculated at the periodic level with the same basis sets. The reasons for the instability of the zeolite structures including the Al–O–Al moieties are interpreted on the basis of cell energy decomposition. This destabilization comes from an increase in kinetic energy if the Al–O(Mg)–Al moieties are present in zeolites. This effect is discussed in parallel with already known variational evidences in favor of dominating role of kinetic energy for stability of molecular systems.

Using different computational methods (isolated cluster, periodic models) and levels of DFT theory (hybrid B3LYP, GGA PW91 and PBE, LDA-CA), one obtained an accurate approximation function for the characterization of the band splitting (BS) of symmetric and asymmetric vibrations of the CO 3 2 - anion in cationic zeolite forms. The correlation obtained between the carbonate geometry and BS does not involve the O–C–O angles as found throughout the series of cluster or periodic models considered. The parameter values of the correlation function slightly depend on the method applied for the energy computation of the models. The “BS–geometry” function suits well for the BS values computed at both the cluster and periodic levels with exception of very few deviations presented herein. The accuracy of the evaluation of the geometry is estimated versus spectroscopic data. The relevance of carbonates for the explanation of the unusually high separation of CO2 mixtures with various gases is also discussed.

One accelerated computational route to elaborate a Si(110)/γ-Al2O3 interface model passivated via plasma-assisted atomic layer deposition is proposed. The final oxidation step corresponds to a junction of a slab with a γ-Al2O3 fragment after two O → Al or three O → Al → O atomic deposition steps over a basic Si(110) surface. The sizes of both Si(110)/γ-Al2O3 components were selected to lead to a minimal mismatch while atomic coordinates were optimized using DFT with periodic boundary conditions. The extent of Si(110) passivation after each deposition step was studied by analyzing the geometry of the atoms (to detect the under-coordinated defects) and by assigning the band structure and projected densities of the s- and p-states (to detect structural defects, with distorted bond lengths and angles). The formation of boehmite-like chains with higher (octahedral) Al coordination without Al–Si bonds in the Si(110)/SiOX/AlOY interface is particularly discussed. Examples of models with a passivated interface (using or not the junction with γ-Al2O3 fragment) were obtained together with a list of typical defects.

Typical structural defects were studied theoretically in the course of O → Al → O atomic depositions on the basic Si(110) surface. The defects were determined by analyses of the band gap states and projected densities of the s- and p-states after the deposition aimed to form a Si(110)/SiOX/AlOY/γ-Al2O3 slab. The extent of Si(110) passivation after every deposition step was studied by scanning the band structure calculated using Density Functional Theory with periodic boundary conditions. The atomic structure of the optimized Si(110) surface was compared to the one of Si(100) for which more information is available. Our modeling reproduces most features of the use of trimethylaluminium or any other organic ligand as Al precursor along O2 plasma assisted atomic layer deposition (PA ALD) when the organic ligands are completely oxidized so that their participation can be neglected in the deposition as already shown experimentally. The final oxidation step corresponds to the junction of the slab deposited over Si(110) with a γ-Al2O3 fragment, whose super cell (SC) parameters have been selected to lead to the minimum mismatch. Different examples of either non-satisfactory or accurate junction of the oxidized Si(110) slab and γ-Al2O3 fragment (under two different forms) are discussed aiming to develop a route for understanding the dominant defect types at the interface. Such theoretical work should be the first step for the elaboration of computational tools for the passivation of silicon with amorphous oxides. The latter are mainly formed at the conditions of the PA ALD depositions. The list of formed typical defects at the Si(110)/SiOX/AlOY/γ-Al2O3 boundary is presented and characterized by the projected density of states and respective band structure around the band gap.

Theoretical calculations are performed on a model zeolite A where sodium ions have successively been exchanged with potassium. Using both isolated cluster and periodic DFT calculations, we made an attempt to explain how the chemisorbed carbonate species in the material contribute to the exceptionally high CO 2 over N 2 selectivity of nearly 200 found in recent experiments [Liu et al. [1] ] with the zeolite NaKA as adsorbent to capture and separate carbon dioxide from a gas mixture containing nitrogen. We have shown that the high carbonate forming at the potassium positions in the 8R windows (KII) results in a larger 8R window diameter potentially enhancing the CO 2 uptake if adsorption is measured for individual gases.

The traditional point of view about water cleavage at divalent cations in alkaline earth (AE) mordenite (MOR) and faujasite (FAU) zeolites was not supported by energy analysis at both the isolated cluster level and the computations with periodic conditions if the cation is located in the vicinity of both Al atoms. A new hypothesis about the kinetic (and not thermodynamic) stabilization of hydroxyl groups near one noncompensated distant Al atom is verified theoretically to explain the acidic nature of the AE cationic MOR and FAU forms. This model allows the development of qualitative interpretation of the temperature variations in the IR spectra upon hydration, dehydration, and rehydration.

In our paper, we show that carbonates can be formed with almost no energetic barrier from CO2 and metal-oxide binuclear MOXM species (M = Mg, Ca, Sr, Ba, with X = 1–4, depending on the cation) in alkaline earth zeolites, mordenite (MOR) and phillipsite (PHI), on the basis of quantum mechanical density functional theory (DFT) calculations at both isolated cluster and 3D periodic levels. The participation of MOXM species (X = 1 and 3) explains the source of the third O atom in CO3 species in dehydrated zeolites, on the basis of a good agreement between the calculated and experimental positions of the asymmetric and symmetric CO3 vibration bands, of the ratio of their intensities, and of the weak dependence versus the cation and framework type. The reaction of formation of dimethylcarbonate from CaCO3Ca in the 8-membered (8R) ring of MOR and methanol has also been considered, suggesting the carbonate activity as the source of CO2 at elevated temperatures.

A new CO oxidation mechanism occurring at (MgO 2Mg) 2+ oxide clusters in different zeolites is modeled at both the periodic DFT (PDFT) level for Mg-phillipsite and with the cluster approach for zeolite Y and mordenite. Using PDFT, we obtained a value of reaction activation energy (15.3 kcal/mol) that is lower than the one obtained using the isolated cluster approach (35 kcal/mol). The main reason of the lower barrier is the strong CO chemisorption, which was not obtained with the cluster approach. The lower oxidation activity of the Mg zeolite forms is due to the smaller number of active singlet MgO XMg species compared to those in the other alkaline earth zeolites. © 2011 Published by Elsevier B.V.

The PC GAMESS package was used to obtain interionic pair potentials for lithium fluoride. A molecular dynamics simulation of the volume phase and nanosized LiF particle was performed. The temperatures of fusion and self-diffusion coefficients of the volume phase and lithium fluoride nanoparticle were found; for the volume phase, they were close to the experimental data. The temperature of fusion of a particle ∼2 A in diameter decreased by ∼600 K. The possibility of a considerable increase in ionic conductivity over the temperature range 520–1122 K was demonstrated for nanosized LiF.

The activity of Prof. Eric G. Derouane expanded over numerous directions in zeolite chemistry, from the development of pioneering physical concepts to the optimization of chemical synthesis, including catalysis. His ideas were an inspiring basis of many modern experimental and theoretical studies for many researchers at the University of Namur. In this University, where Eric has been Full Professor for over 20 years, all of us have benefited from Eric's friendship and from numerous frequent and fruitful discussions in the zeolite domain. D.P.V. is particularly indebted to Eric's long term vision regarding the multiple aspects to tackle using molecular simulations of sieves in general. With this paper, we wish to honor Eric's memory by recalling the pioneering physical concepts of confinement which have influenced more than 20 years of theoretical simulations of molecular sieves. We will also briefly mention the on-going theoretical work towards template design for the syntheses of porous materials.

A model is proposed for the LaTiO3 compound, which allows for the strong coupling of the lattice, orbital, and magnetic subsystems of this crystal. The interaction of the lattice with the electronic structure of the LaTiO3 compound is described by the vibronic Hamiltonian with the constants calculated using the ab initio Hartree-Fock-Roothaan method with the inclusion of the electron correlations. It is demonstrated that a considerable contribution to the formation of the orbital structure of the LaTiO3 compound is made by the next-nearest neighbors of the Ti3+ ion. The orbital-dependent superexchange interactions in LaTiO3 are described within the Hubbard model and agree well with experiment.

The local structure and vibrations in the region of an anion vacancy are studied using the pair interionic potentials within the shell model for crystals Me+Cl− (Me+ = Rb+, K+, Na+). The pair potentials are derived from first-principles calculations of different clusters by the Hartree-Fock-Roothaan method with the one-electron states constructed in the form of molecular orbitals as linear combinations of atomic orbitals (MO LCAO). The calculations are performed with the GAMESS program package (US). The correlation corrections are included in the calculations. The validity of the model parameters is verified by comparing the calculated with experimental structural and dynamic properties of ideal alkali halide crystals.

The parameters of the F − − F − and Me + -F - pair potentials ( Me = Rb, K, Na) are determined from ab initio calculations using the Hartree-Fock-Roothaan method in which one-electron states are described by molecular orbitals formed from linear combinations of atomic orbitals. Nonempirical potentials are used to cal- culate the structure and dynamics of perfect and imperfect alkali halide crystals. The effect of an anion vacancy on lattice dynamics is studied using the recursive method. It is shown that an adequate description of an imper- fect crystal requires consideration of a defect region with ~1000 ions.

Water molecules were removed from the X-ray structure models of zeolites with comparatively small-sized unit cells (BIK, BRE, EDI, GIS, GOO, LAU, and PHI), and cations were replaced by cations of only one type (Mg or Zn). The dehydrated zeolite models obtained were optimized with the Bush and Catlow force fields. The dependences of atomic multipole moments up to the fourth order along the so-called cumulative coordinate were calculated for these models by the density functional method with periodic boundary conditions. These dependences were used to estimate the electrostatic potential and electric field in the cross section of a zeolite that was not used to calculate them. The data obtained with atomic charges borrowed directly from calculation results were quite satisfactory.

Within the framework of strong electron-lattice interaction of Mn 3+ ions and orbital dependent exchange interaction model, the crystal and magnetic structures of charge ordered phase La 1/3 Ca 1/3 MnO 3 are studied. The calculation results are in agreement with the "Wigner crystal" model. The estimated superexchange parameters confirm the qualitative conclusions of experimental works about frustrated magnetic structure with magnetic supercell (3a x b x 2c). The values of hyperfine magnetic fields on 55 Mn nuclei in different charge states in charge ordering phase are calculated.

Electrostatic potential (EP) and electric field (EF) values are calculated in periodic models of proton-substituted brewsterite (HBRE) and chabazite (HCHA) zeolite forms using the periodic DFT (PDFT) hybrid B3LYP level with 13 different basis sets. Relative root mean square differences between the EP, or EF, values estimated between the different basis sets are evaluated in several spatial domains available for adsorbate molecules in both zeolites. In these areas, the EF increase in HCHA is evaluated relative to all-siliceous CHA. The EP is interpreted in terms of ionicity of the framework. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

A new way to evaluate the electric field (EF) based on the propagation of multipole moments distributed over atomic sites is illustrated for arbitrary aluminophosphate (ALPO) sieves of ratio Al/P = 1. The atomic multipole moments (AMMs) are evaluated for all crystallographic independent atomic types of 10 ALPO structures within the scheme developed by Saunders and colleagues, considering the electron density computed with the CRYSTAL 98 code at the periodic density functional theory (PDFT) level and different basis sets. In two steps, the method uses the calculated AMMs to calibrate several approximate dependences and permits construction of molecular charge distributions for any sieve. First, atomic charges are evaluated via fitted analytical expressions versus the atomic geometries of the framework. Second, high AMMs up to hexadecapole are approximated for all atomic positions, using a cumulative coordinate scheme. Differences between calculated and approximated EF values within the porous space available for adsorbed molecules are discussed. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

An important problem when studying the interaction between a CO probe molecule and a Na4Ca4A type zeolite is the estimation of the central repulsive coefficients versus the internuclear distance of CO. In particular, this dependence cannot be estimated in the case of the unstable linear “framework oxygen–CO molecule” pair due to the electrostatic repulsive interaction. Hence, we discuss the application of two approximate forms of this dependence either allowing or disregarding the repulsive contribution in the interval wherein the vibrational CO probability distribution cannot be neglected. The consequences of these approximations are compared through calculation of the interaction energy and band shift of CO adsorbed inside Na4Ca4A. The CO spatial parameters (semi-axes) are estimated by fitting both the band shift, corresponding to two different positions of CO relative to the zeolite, and the interaction energy values to the experimental data obtained at small coverage.

a Laboratory of Molecular Beams, Department of Chemistry, Moscow State University, Leninskie Gory, Moscow, B-234, 119899, Russia b Institute for Studies in Interface Sciences, Laboratoire de Physico-Chimie Informatique, Facultes Universitaires Notre Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium c Dipartimento di Chimica Fisica, Chimica Inorganica e Chimica dei Materiali, via P. Giuria 7, 10125 Torino, Italy