Presence Of Vacant Sites Research Articles

Atomic-scale interlayer friction of gibbsite is lower than brucite due to interactions of hydroxyls

AbstractTo investigate the role of atomic-scale structure on the frictional properties of gibbsite, a dioctahedral-type aluminum hydroxide, we calculated the atomic-scale interlayer shear properties using the first-principles method based on density functional theory. We found that the presence of vacant sites within the octahedral sheet of gibbsite enables hydroxyls to move to more stable positions and reduce the repulsive force, leading to a lower atomic-scale shear stress of gibbsite compared with brucite, a trioctahedral-type magnesium hydroxide. We also estimated the macroscopic single-crystal friction coefficient of gibbsite with the assumption that only the atomic-scale interlayer friction controls macroscopic friction. The estimated single-crystal friction coefficient for gibbsite is 0.36(6), which is clearly lower than the experimentally obtained friction coefficient of the powdered gouge of gibbsite (0.74). This difference between the interlayer friction coefficient and gouge friction coefficient suggests the presence of additional mechanisms that affect the frictional strength, such as microstructures within a fault gouge.

Numerical Monte Carlo Simulations to Evaluate the Influence That Spherical Nanoparticle Size and Arrangement Have on Interparticle Charge Transport across the Surface of Dye- and Cocatalyst-Modified Materials

Mechanistic information underlying the function of illuminated mesoporous thin films of nanomaterials that contain distinct light-absorbing and electrocatalytic units can be gleaned from discrete-time random walk Monte Carlo simulations. These simulations bridge the length and time scales between individual electron-transfer events and ensemble behavior observed from bulk thin films. Most simulations are performed using models that simplify random mesoporous networks as isolated spherical nanoparticles. However, these simplifications may not provide sufficient detail to capture macroscopic experimental observations, especially when mesoporous nanomaterials consist of various geometries. Herein, we examine the role that the structure of the mesoporous thin film plays on the ability of photogenerated charges to accumulate on surface-confined redox-active electrocatalysts that require two redox events for turnover. We observe that the structure has a dramatic influence on the expected spectroscopic absorption anisotropy signal over time. We also observe that the yield for electrocatalyst turnover, as a function of the ratio of the electron-transfer time constant for self-exchange reactions and charge recombination time constant between the semiconducting mesoporous thin film and an oxidized/reduced surface-confined dye or electrocatalyst, is influenced by the total surface coverage of redox-active species. Structures consisting of spherical nanoparticles that are barely touching or partially necked are more effective at electrocatalytic turnover in the presence of vacant sites than discrete spherical nanoparticles or those that are arranged in a rodlike structure. Moreover, we show that the yield for electrocatalyst turnover is nearly independent of whether the simulations are performed on complex three-dimensional structures or simple two-dimensional planar grids. This discovery suggests that the added complexity of three-dimensional models may not be necessary to explain differences in electrocatalytic turnover yield in photoelectrochemical constructs containing surface-confined light-absorbing and electrocatalytic units.

Dynamic Monte Carlo simulation of the [formula omitted] reaction on Pt(100): TPR spectra

The catalytic reduction of nitric oxide by hydrogen over a Pt surface is studied using a dynamic Monte Carlo (MC) method on a square lattice under low pressure conditions. Using a Langmuir–Hinshelwood reaction mechanism, a simplified model with only four adsorbed species (NO, H, O, and N) is constructed. The effect on the NO dissociation rate, the limiting step in the whole reaction, is inhibited by co-adsorbed NO and H2 molecules and is enhanced both by the presence of empty sites and adsorbed N atoms at nearest neighbors. In these simulations, several experimental parameter values are included, such as: adsorption, desorption and diffusion of the reactants. The phenomenon is studied while varying the temperature over the 300–550 K range. The model reproduces well-observed TPD and TPR experimental results. For the whole NO+H2 reaction, the phenomena of “surface explosion” is observed and can be explained as the result of the abrupt production of N2 due to both the autocatalytic NO decomposition favored by the presence of vacant sites and the development of inhomogeneous fluctuations. MA simulations also allow a visualization of the spatial development of the surface explosion as heating proceeds.

Percolation and transport in an assembly of anisotropic conductors

The percolation surface was calculated for a cubic lattice where each active site is occupied by a cube that only conducts along planes normal to one of the three Cartesian directions. The transport properties of the assembly of cubes, found using a resistor network approximation are more adversely affected by the presence of vacant sites than by the anisotropy of the different cubes. Studies of the correlation length and conductivity exponents indicate that the behaviour of the material is in general three dimensional, though systems comprised of voids and cells oriented in one direction have critical exponents corresponding to two-dimensional systems. An effective medium treatment gave simple analytical results for the percolation surface and effective conductivity that were quite accurate in some limits.

Synthesis and properties of zektzerite, LiNaZrSi6O15, and its isotypes

SynopsisZektzerite and its isotypes with Zr4+ replaced by Ti4+ or Sn4+ are readily synthesized by fusion of the constituent oxides at 1550 °C followed by recrystallization of the melt or quenched glass at 750–850 °C and I bar pressure. Indexed powder X-ray data for synthetic zektzerites are presented in miniprint Table I. The powder patterns are consistent with an A-centred orthorhombic symmetry. Refined cell dimensions are given in Table III.The Ti, Zr, and Sn zektzerites melt incongruently. Table IV records melting-point data. The coefficients of thermal expansion of Zr and Ti zektzerite have been determined by X-ray dilatometry. Both have moderately low coefficients of expansion: numerical values are given in miniprint Table II.Zektzerites contain a six-repeat unit doublechain silicate anion. Although they are chemically similar to milarites, the latter contain double-ring Si12O30 anions. Despite the difference in anion constitution both have structures in which many chemically similar atoms occupy similar positions. The effect of this similarity on the physical properties (e.g. X-ray powder pattern) is enhanced by the similar size and shape of their orthorhombic unit cells. The presence of vacant sites in zektzerite that are potentially available for cation occupancy suggests that chemically complex substitutions may occur; for example LiNa2ScSi6O15 has been synthesized and is believed to be isostructural with zektzerite and emeleusite, LiNa2FeSi6O15.