Kari Laasonen

Single‐Shell Carbon‐Encapsulated Iron Nanoparticles: Synthesis and High Electrocatalytic Activity for Hydrogen Evolution Reaction

Efficient hydrogen evolution reaction (HER) through effective and inexpensive electrocatalysts is a valuable approach for clean and renewable energy systems. Here, single-shell carbon-encapsulated iron nanoparticles (SCEINs) decorated on single-walled carbon nanotubes (SWNTs) are introduced as a novel highly active and durable non-noble-metal catalyst for the HER. This catalyst exhibits catalytic properties superior to previously studied nonprecious materials and comparable to those of platinum. The SCEIN/SWNT is synthesized by a novel fast and low-cost aerosol chemical vapor deposition method in a one-step synthesis. In SCEINs the single carbon layer does not prevent desired access of the reactants to the vicinity of the iron nanoparticles but protects the active metallic core from oxidation. This finding opens new avenues for utilizing active transition metals such as iron in a wide range of applications.

Hydration of Li+ ion. An ab initio molecular dynamics simulation

Ab initio Car–Parrinello molecular dynamics simulations of a Li+ ion in water have been carried out using the density-functional theory with Becke–Lee–Yang–Parr (BLYP) functional and ultrasoft Vanderbildt pseudopotentials. Both structural and dynamical properties of Li+ have been studied in detail and compared with available neutron scattering and spectroscopic data. Excellent agreement is obtained with the existing experimental data for the structure of the first hydration shell around the Li+ ion. Spectral features of bound water are compared to those of bulk water. Reasonable agreement is obtained with IR and Raman experiments. The ab initio simulation results have also been used to derive a Li+–water interaction potential. The best fit of the data gave a simple single-exponential potential function, which reproduces very well the liquid structure from the original ab initio simulations. This potential model, together with the simple point charge (SPC) water model, was applied to calculate the hydratio...

Ab initio study of gas-phase sulphuric acid hydrates containing 1 to 3 water molecules

Sulphuric acid has a tendency to form hydrates, small clusters containing a few water molecules, in the gas phase. Hydrate formation has a stabilising effect on the vapour as the pressure of sulphuric acid drops (relative to unhydrated vapor), decreasing the nucleation rate. In classical nucleation theories the hydration energies and the hydrate distribution are predicted assuming that hydrates can be described as liquid droplets having thermodynamic properties of bulk liquid. To obtain a better understanding of the structures and formation energies of the smallest clusters, we have performed ab initio density functional calculations of the mono-,di-, and trihydrates. The hydrogen bonds between the molecules are found to be strong. The more water molecules the hydrate contains, the clearer ring-like structure is formed. Comparison to classical values for the hydration enthalpies confirms that the properties of bulk liquid do not describe the properties of the smallest clusters too well. The energy barrier...

Molecular dynamics simulations of gas-liquid nucleation of Lennard-Jones fluid

Grand canonical molecular dynamics simulations have been performed to determine the sizes of critical nuclei in gas–liquid nucleation. The studied system consists of Lennard-Jones (LJ) argon atoms with a potential cutoff of 4.9σ at a reduced temperature of 0.694 and at vapor supersaturations between 3.5 and 6.2. To facilitate comparison with nucleation theories, we have also determined the equilibrium vapor pressure of LJ argon as a function of temperature. The results are compared with other simulation studies, and with predictions of the classical (CNT) and density functional (DFT) nucleation theories. We find that the semiempirical version of the DFT is in excellent agreement with the simulation results, and that the CNT underestimates the number of LJ atoms in the nuclei only very slightly.

Ab initio study of O2 precursor states on the Pd(111) surface

Interactions of O and O2 with the Pd(111) surface are studied using spin-density-functional theory with gradient corrections. The investigation of potential energy surface of O atom on Pd(111) shows the face centered cubic site adsorption to be the most favorable. The diffusion barrier to an adjacent hexagonal closed pack site is 0.56 eV. Several adsorption trajectories are calculated for O2 on Pd(111). We find that the molecule dissociation is either direct but strongly activated or precursor mediated with considerably lower barrier. Three precursor states are found and identified according to geometry, energy, and vibrational frequency. Two precursor states are energetically degenerate with only slightly different geometries. Density difference analysis reveals that the electronic structure of both the molecule and the nearest Pd atoms is modified during the adsorption. However, according to density difference analysis the molecules in the precursor states are neutral and the interaction between O2 and ...

Silicon vacancy in SiC: A high-spin state defect

We report results from spin-polarized ab initio local spin-density calculations for the silicon vacancy (VSi) in 3C– and 2H–SiC in all its possible charge states. The calculated electronic structure for SiC reveals the presence of a stable spin-aligned electron-state t2 near the midgap. The neutral and doubly negative charge states of VSi in 3C–SiC are stabilized in a high-spin configuration with S=1 giving rise to a ground state, which is a many-electron orbital singlet 3T1. For the singly negative VSi, we find a high-spin ground-state 4A2 with S=3/2. In the high-spin configuration, VSi preserves the Td symmetry. These results indicate that in neutral, singly, and doubly negative charge states a strong exchange coupling, which prefers parallel electron spins, overcomes the Jahn–Teller energy. In other charge states, the ground state of VSi has a low-spin configuration.

Structure, effective pair potential and properties of halothane

Abstract Halothane, 2-bromo-2-chloro,1,1,1-trifluoroethane, is an important inhalation general anesthetic. We performed energy minimization within the Car-Parrinello scheme of density functional theory to obtain the detailed structure of a halothane molecule, in the gas phase. Then, effective model pair potentials for a flexible halothane molecule have been developed that describe halothane in solution around room temperature. The potential parameter were fitted to reproduce the known density at 298 K. Constant pressure and temperature molecularddynamics simulations were carried out at room temperature as well as at 310 K. The results are in excellent agreement with the available experimental data.

Reliable potential for small sulfuric acid-water clusters

Abstract We have constructed a reliable potential model for clusters of sulfuric acid and water. Such model requires potentials for sulfuric acid (H 2 SO 4 ), protonated sulfuric acid (HSO 4 − ), hydronium ion (H 3 O + ) and water (H 2 O). To develop this model, we have used the available ab initio data for small clusters containing one sulfuric acid. The ab initio data are well reproduced with our model. We computed the number of waters around a sulfuric acid at atmospheric conditions (300 K and 50% relative humidity) to be ca. 1.3. This is in good agreement with experiments which predict 1.3 waters around the acid. We have also tested our model with small clusters containing two sulfuric acids and find the results to be in good agreement with ab initio calculations. Larger systems with up to 50 water molecules were studied as well. Here, the hydronium ions were found to be on the surface of the cluster and the bisulfate ions inside the cluster. The clusters are very flexible and large fluctuations were observed in the sulfur–sulfur and sulfur–hydronium distance.

Molecular dynamics using the tight-binding approximation

The authors present an extension of classical molecular dynamics (MD) to include the forces calculated from electronic degrees of freedom using the tight-binding (TB) approximation. The combined MD-TB problem is solved using simulated annealing techniques. As an example they study the structures and energetics of small silicon clusters, containing up to 10 Si atoms.

Coadsorption of CO and NO on the Pd(111) Surface

Adsorption and coadsorption of nitric oxide (NO) and carbon monoxide (CO) on the Pd(111) surface are studied by combining first principles (FP) calculations and Monte Carlo (MC) simulations. From FP adsorption energies and molecule-molecule interactions we construct an interaction model, which is used in MC. We do several simulations with different coverages and CO/NO ratios. The simulations provide 0.75 monolayer (ML) for a saturation coverage, which is in excellent agreement with experiments. The results indicate that at 0.75 ML coverage, NO molecules take over the hollow sites and push CO molecules mainly onto bridge sites.