Search

A new grid-based algorithm developed at Molecular Simulations, and molecular dynamics method have been applied to modeling the locations of non-framework species in zeolites. This new method locates the energy minima for various frameworks and populates these sites with the non-framework species. The cation locations were predicted in dehydrated zeolite adsorbents and catalysts, such as Na88X, Ca48X, mixed cation zeolite 3A (K60Na36A), and Cu-mordenite (Si/Al=5.0) using only well-known framework structure models. Furthermore, the locations of benzene molecules in a supercage of zeolite Ca48X were correctly predicted via the application of the Monte Carlo docking and molecular dynamics methods. These examples demonstrate that when the framework type is known, the new technique can provide a realistic initial structure input for the challenging task of solving crystal structures of zeolites containing non-framework species.

The embedded atom method (EAM) was used to simulate the dynamics of deposition of supported metal clusters on fcc transition metals and the dynamic behavior of surfaces. The formation of metal clusters was observed before deposition and the geometries, stabilities and dynamics of a series of metal clusters were discussed as obtained by EAM and compared with first-principle calculations. The calculations indicate that the structure of a cluster in the vapor phase is governed by its zero kelvin stability. We find that if the cluster forms in the vapor phase, it is difficult to get epitaxial growth since the metal cohesive energy is large and the chester can not be broken by thermal motions. The effect of impurities on the deposition is also discussed. The anharmonicity, roughening and reconstruction of surface were also investigated. Three-body interactions were found to he important to understand these phenomena

The dissocn. of NO and CO has been studied on cluster models representing the copper (100) and -(111) single-crystal faces using d. functional quantum calcns. For each surface, several possible reaction paths are proposed, for which we fully optimized the reactant, product, and transition states at the local d. level (LDA). Nonlocal d. functional calcns. (NLDA) were then performed on these optimized geometries. The clusters we used, varying in size between 13 and 31 atoms, produced dissocn. barriers and energies that were reasonably well converged with cluster size. Classical transition-state theory was used to calc. the rates of dissocn. and recombination on the basis of computed frequencies of the predicted transition state and the reactant and product states. The transition states for NO and CO dissocn. on all surfaces can be described as \"tight\" transition states corresponding to preexponentials for dissocn. in the range 1010-1013 s-1. The dissocn. barrier for NO is significantly lower than that for CO. In addn., the more open Cu(100) surface is more reactive toward dissocn. than the close-packed Cu(111) surface. Nonlocal corrections to the LDA functional were found to have a small effect on dissocn. barrier height, but the effect was found to be more profound on the recombination barrier and overall dissocn. energies

The 105 zero-wavevector vibrational modes of an infinite sodalite framework with atomic positions corresponding to the sodalites M 8 [Al 6 Si 6 O 24 ]Cl 2 (M=Li, Na, K) and silica sodalite are calculated by the Wilson GF matrix method. Not all of the modes which are formally infrared or Raman active have significant spectral intensity, and point-charge or bond-polarizability models are used to generate synthetic infrared or Raman spectra in order to identify the modes which give rise to the observed bands. The synthetic spectra reproduce most of the features of the experimental spectra very satisfactorily, and for the aluminosilicate sodalites the wavenumbers of the bands are simulated to within 30 cm -1

The structure of stoichiometric (s) silver beta alumina, Ag1-xAl11O17+x2/; x=0.0, prepared via hydrogen reduction of a non-stoichiometric (NS) material with x=0.45 has been determined at 25 degrees C, 300 degrees C and 500 degrees C by Rietveld analyses of powder neutron diffraction data. In contrast to the single Ag site observed previously at 4.2 K, the silver ion distribution becomes increasingly diffuse with increasing temperature. At 25 degrees C the mirror plane structure is similar to that seen at 4.2 K, but apparently with some anti-Beevers-Ross site occupancy, and still with an increased c-axis constant compared with the NS parent. At 500 degrees C no silver cation site is well defined, consistent with a two-dimensional quasi-liquid state suggested by earlier diffuse X-ray scattering experiments.

Combinatorial chemistry [1] and high-throughput screening are now reasonably well-validated for drug discovery; we have the ‘proofs of concept’ in terms of accelerated discovery and activity optimization, with compounds, for example from Pharmacopeia’s own programs, now at pre-clinical stages. Similar principles clearly also apply to the discovery and optimization of new catalysts, as evidenced by recent overviews [2–7] and by the collective contributions in this volume [8].

We consider the application of high-throughput experimentation (HTE), including combinatorial methods, to catalyst discovery and early-phase optimization. While combinatorial- and parallel-testing methods have an already substantial history in catalysis, recent work by several groups promises significant efficiency gains. In molecular catalysis, progress is noted in library design, library synthesis by pooled, parallel, and discrete formats, catalyst testing and reaction optimization; the prime constraint for organometallic catalysts is the limited scope of synthesis procedures for non-peptide-based ligand libraries. Routes described for the synthesis of heterogeneous catalysts include hydrothermal synthesis, arraying of solution precursors, automated impregnation and precipitation, and arraying of solid precursors. The key challenge in applying HTE to heterogeneous catalysis is testing; we distinguish here between Stage 1, or "discovery" testing and Stage 2, optimization testing, and describe techniques with potential in each case. Recent examples from the literature and our own work are used to illustrate these principles and the prospects for HTE applied to catalysis.

Density functional theory provides a first-principles approach for computing the geometric and electronic structures, and a wealth of corresponding properties, of a wide range of materials types and compositions, including bulk solids, surfaces, defects and clusters of molecules. Parallel advances in hardware performance, implementation strategies and algorithms have all contributed to a rapid growth in the number of important applications. Recent developments under each of these themes are outlined and the breadth of current applications is illustrated by typical examples. Issues associated with the implementation and performance of density functional methods on parallel computer architectures are discussed.

Zeolites are crystalline and, therefore, diffraction methods are central to attempts at atomiclevel structural characterization. This overview outlines key issues of diffraction from a materials perspective and describes the various problems that zeolite materials present to the diffractionist. As a result of these challenging problems, zeolite systems often provide the incentive for diffraction technique developments, or rank high amongst the materials to which developing methods are first applied. Recent applications of diffraction techniques, particularly X-ray and neutron diffraction, to zeolite structural problems are summarized. Principal advances stem from further use of powder neutron diffraction (PND) and, particularly, exploitation of the properties of synchrotron X-radiation in both powder and single crystal diffraction experiments. In parallel, there have been continuous improvements in data analysis methods, notably in programs for Rietveld analyses of powder diffraction data. Computer simulations are also playing an increasing role in structural characterization, exemplified by the use of simulated annealing as a new approach to initial structure solutions.

We study the structure and growth sequence of small palladium clusters Pdn using the Many-Body Alloy (MBA) potential and simulated annealing techniques. Our results show the preference of compact polyhedral structures. These equilibrium structures are compared with the bulk Pd crystal in terms of cohesive energies and nearest neighbor distances. Both the cohesive energy and the nearest neighbor distance show a slow convergence to bulk behaviors. By analyzing the detailed structures and cohesive energies, we find that Pd4, Pd7 and Pd13 are magic number structures, which are the consequence of their high symmetry and large coordination number.

We show that the frequencies of certain optically active lattice vibrational modes in a large number of zeolites can be used as direct structural probes. Mode frequencies are found to depend on the cosine of the T-O-T angle and the Si/Al ratio of the framework. The relationship between mode frequency and local framework structure is discussed in terms of a complete lattice dynamics calculation for specific zeolite structures as well as a lattice dynamical model of amorphous glasses.

The non-framework cation positions for Li+ in dehydrated zeolite Li-A(BW) and for Na+ in zeolite 4A are simulated by a straightforward combination of Monte Carlo and structure optimization techniques, using only a framework structure model and appropriate interatomic potentials.

The complete structures of dehydrated potassium zeolite L, K{sub 9}Al{sub 9}Si{sub 27}O{sub 72}, at 298 and 78 K and of the same zeolite at 78 K containing, on average, 1 molecule of perdeuteriobenzene/unit cell have been determined by powder neutron diffraction. Comparison with earlier literature data indicates that little change in structure accompanies dehydration, benzene sorption, or temperature change 298 K {ge} T {ge} 78 K. The use of powder neutron diffraction has enabled a direct measure of the aluminum partitioning between the two inequivalent T-sites in the structure that, consistent with the complementary measure from bond length arguments, demonstrates a distinct aluminum preference for the 12-ring, Sil (T4) sites. The nonframework K{sup +} configuration shows only subtle changes over the range of conditions studied. Perdeuteriobenzene is observed at 78 K in capping positions above the channel wall (type D) K{sup +} cations. Simple atom-atom potential modeling of the benzene site based on the structural results yields only a small activation barrier to benzene molecule reorientation.

The low-temperature structure of stoichiometric silver beta alumina, AgAl11O17, is described for the first time. A powder neutron diffraction study at 4.2K reveals that there is a single three-fold silver location of unit occupancy in each mirror plane. The silver site lies between the mid-oxygen and Beevers-Ross positions where the Ag-O contact distances can be minimised. The Ag-O interactions, in particular the cooperative movement of the spacer oxygens, are responsible for the superlattice which has been described previously. The Beevers-Ross, anti-Beefers-Ross and interstitial aluminium sites are vacant and the c axis is expanded relative to the non-stoichiometric parent.

Synchrotron X-radiation provides unique opportunities for diffraction experiments and, therefore, for extending our understanding of the structure - property interplay in catalyst systems. The present status of opportunities and applications of synchrotron X-ray diffraction techniques in the structural chemistry and catalysis science areas is overviewed, and illustrated by selected recent results.

Linear relationships between the magnitudes of the /sup 29/Si isotropic chemical shifts and the number of aluminum atoms in FAU and LTA framework materials indicate second-neighbor effects and provide an alternative method of determining framework compositions from /sup 29/Si NMR spectra. Mean values for the change in chemical shift per added aluminum atom are 6.28 and 0.70 ppm for first- and second-neighbor sites, respectively. 35 references, 2 figures, 2 tables.

The special features of synchrotron X-radiation and the types of instrumentation required for a range of synchrotron X-ray diffraction experiments are outlined. A diverse range of applications to inorganic and heterogeneous catalyst systems are discussed. Grazing incidence X-ray diffraction experiments have revealed a number of surface structures and followed surface phase transformations; similar studies of boundary and interfacial structures are cited. Experiments on catalytic metals dispersed on supports have exploited anomalous scattering effects. High-resolution powder diffraction has assisted phase identifications, provided quantitative data on peak broadening (such as arise from particle size effects or stacking disorder), enabled studies of subtle superstructures and a growing number of ab initio structure solutions, and permitted Rietveld refinements of a range of inorganic materials. The potential for performing measurements on individual crystallites in the micrometre size regime has b...

Caracterisation par spectrometrie RMN du solide, et par diffraction, des neutrons de la zeolite A modifiee par echange d'ions d'aluminium et de gallium

In this paper we demonstrate the usefulness of a nearest neighbor force model for studying the structural dependence of the lattice dynamics of zeolites. Interpreting Raman scattering in the context of this model confirms earlier empirical vibrational assignments in zeolites and underlines the importance of short range forces in determining the frequencies of certain zeolite vibrational modes. The model describes fully the low frequency bond bending mode of A1 symmetry, although long range forces are shown to be required to model complete zeolite vibrational spectra.

Powder neutron diffraction has been used to measure directly the silicon–aluminium partitioning between the two tetrahedral sites in zeolite L; the similar environments of the two sites prevent simple measurement of this partitioning by 29Si n.m.r. spectroscopy, but enable its use as a probe of framework composition.

In the first complete structure determination of a hydrated polycrystalline zeolite, powder neutron diffraction and molecular modelling have been used to determine proton positions in a hydrated lithium gallosilicate with the ABW framework.