Reconstruction and catalytic activity of hybrid Pd(100)/(111) monolayer on γ-Al2O3(100) in CH4, H2O, and O2 dissociation.

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.

Effective numbers of modes applied to analysis of internal dynamics of weakly bound clusters.

The dependence of the volume of the chaotic component in the internal dynamics of triatomic van der Waals clusters on the angular momentum is calculated using the Monte Carlo and molecular dynamics methods. It has been found that this dependence is nonmonotonic and that its functional form varies for different values of the total energy. The effective number of rotational modes was used to clarify why a change in the volume of chaotic component of the phase space happens for certain values of the angular momentum. We conclude that a large fraction of regular trajectories in relation to all trajectories appears only when there is a possibility for the regular motion to perform a rotation different from that for a chaotic motion. When such difference is small, the regular motion disappears. The effective number of rotational modes can be used to estimate the difference in the type of rotation and is a convenient parameter which controls changes in the dynamics of the system.

Ion-exchanged binuclear Ca2OX clusters, X = 1-4, as active sites of selective oxidation over MOR and FAU zeolites.

A new series of calcium oxide clusters Ca(2)O(X) (X = 1-4) at cationic positions of mordenite (MOR) and faujasite (FAU) is studied via the isolated cluster approach. Active oxide framework fragments are represented via 8-membered window (8R) in MOR, and two 6R and 4R windows (6R+4R) possessing one common Si-O-Si moiety in FAU. Structural similarities between the Ca(2)O(X)(8R) and Ca(2)O(X)(6R+4R) moieties are considered up to X = 4. High oxidation possibilities of the Ca(2)O(2)(nR) and Ca(2)O(3)(nR) systems are demonstrated relative to CO, whose oxidation over the Ca-exchanged zeolite forms is well studied experimentally. Relevance of the oxide cluster models with respect to trapping and desorption of singlet dioxygen is discussed.

Convergence of electric field and electric field gradient versus atomic basis sets in all-siliceous and Mg substituted phillipsites.

Electrostatic potential (EP), electric field (EF), and electric field gradient (EFG) values are calculated in periodic models of magnesium substituted phillipsite (MgPHI) zeolite forms using periodic DFT (PDFT) hybrid B3LYP level with fourteen different basis sets. Relative root mean square differences between the EP, EF, or EFG values estimated between different basis sets are evaluated in several spatial domains available for adsorbate molecules in the zeolite. In these areas, the EF increase in MgPHI is evaluated relative to all-siliceous PHI types. The EP is interpreted in terms of framework ionicity for MgPHI and all-siliceous PHI models. Angular Si-O-Si dependence of the EFG asymmetry at (17)O atoms in all-siliceous zeolites is discussed.

Electric field convergence versus atomic basis sets in all-siliceous zeolites.

The electrostatic potential (EP) and electric field (EF) are calculated in the TON and CHA zeolites using periodic hybrid B3LYP, PBE, and PW91 functionals considering eight basis sets. Relative root mean square differences between the EP or EF values estimated between the different basis sets are evaluated in several domains available for adsorbate molecules in both zeolites. The EP is interpreted in terms of ionicity of the framework.