Low frequency sound propagation in activated carbon

Activated carbon can adsorb and desorb gas molecules onto and off its surface. Research has examined whether this sorption affects low frequency sound waves, with pressures typical of audible sound, interacting with granular activated carbon. Impedance tube measurements were undertaken examining the resonant frequencies of Helmholtz resonators with different backing materials. It was found that the addition of activated carbon increased the compliance of the backing volume. The effect was observed up to the highest frequency measured (500 Hz), but was most significant at lower frequencies (at higher frequencies another phenomenon can explain the behavior). An apparatus was constructed to measure the effective porosity of the activated carbon as well as the number of moles adsorbed at sound pressures between 104 and 118 dB and low frequencies between 20 and 55 Hz. Whilst the results were consistent with adsorption affecting sound propagation, other phenomena cannot be ruled out. Measurements of sorption isotherms showed that additional energy losses can be caused by water vapor condensing onto and then evaporating from the surface of the material. However, the excess absorption measured for low frequency sound waves is primarily caused by decreases in surface reactance rather than changes in surface resistance.

The pressure-temperature phase diagram of MgH₂ and isotopic substitution

Computational thermodynamics using density functional theory ab initio codes is a powerful tool for calculating phase diagrams. The method is usually applied at the standard pressure of p = 1 bar and where the Gibbs energy is assumed to be equal to the Helmholtz energy. In this work, we have calculated the Gibbs energy in order to study the release temperature and phase modifications of MgH(2) at high pressures up to 10 GPa (100 kbar). The isotopic substitution of hydrogen with deuterium (or tritium) does not bring about any strong effects on the phase diagram. These considerations are of extreme importance for (i) the synthesis of novel substitutional magnesium based materials at high pressure and (ii) the determination of the correct reference states for the calculation of phase diagrams at high pressure. The calculated results are compared with experimental data obtained with an in situ neutron diffraction measurement.

Identification of surface states on finely divided supported palladium catalysts by means of inelastic incoherent neutron scattering

The purpose of the present investigation was to utilize the inelastic incoherent neutron scattering (INS) technique to reveal changes at the surface of technical catalysts under the influence of hydrogen in gas/solid interactions and during chemical reactions in a liquid-phase process. The formation and the properties of supported palladium hydride and changes of the hydrogen-related surface chemistry of the corresponding activated carbon supports in 20% Pd/C catalysts after short-term and long-term hydrogen cycling at different hydrogen pressures and temperatures were studied. The spectra indicate that hydrogenation of the activated carbon support by hydrogen spillover occurs to, partly, give a material that strongly resembles a-C:H (amorphous hydrogenated carbon). Indications for different relaxation phenomena and long-range phase coherence inside of supported particles of palladium hydride compared to hydrogenated palladium black were obtained. A 5% Pd/C catalyst after use in C-C coupling reactions, the Heck reaction of bromobenzene and styrene to stilbenes, was also studied after subsequent solvent extraction. Evidence for a preferential adsorption and accumulation of cis-stilbene at the catalyst surface was obtained. INS allows identification of a certain isomer from a complex reaction mixture preferentially adsorbed at the surface of a finely divided industrial heterogeneous catalyst.

Inelastic neutron scattering measurements and Ab initio calculations of hydrogen in single-crystal palladium

Using a single-crystal sample of PdH, we have measured the inelastic form factor for exciting the proton from its ground state to various excited states as a function of crystal orientation. Because of the anharmonicity of the potential well which splits otherwise degenerate states, each inelastic form factor could be measured individually. Agreement between theory and experiment is excellent, both with regard to the transition energies and to the modulation of the intensity, giving confidence in the quantum treatment of the proton and hence in the use of the model to predict other properties of the system.

The Shape and Energy of a Revolving Liquid Mass Held Together by Surface Tension

The shape of a drop of incompressible fluid held together by the action of surface tension and made to rotate about an axis is determined, the effect of gravity being neglected. Two distinct problems are investigated. In the first an isolated drop of liquid in the form of a surface of revolution is considered. At zero angular speed the drop is spherical and with increasing angular momentum the oblateness increases until a maximum angular speed is reached (where the total mechanical potential is a minimum) beyond which a new linear series of equilibrium forms emerges. A solution in the form of a toroid is also found.

The Potential Due to Two Point Charges Each at the Centre of a Spherical Cavity and Embedded in a Dielectric Medium

A method is found for determining the electrostatic potential due to two separated spherical cavities each containing a point charge with the whole system embedded in a continuous dielectric medium. The potential is obtained as a sum involving Legendre polynomials and it is shown that the coefficients of this series can be expanded as a convergent power series in the reciprocal of the distance between the centres of the cavities. This solution is suitable for numerical calculations because there is a simple relationship between successive terms in the series and because this series is convergent even at contact separation.

The Interaction Energy of an Isolated Monovalent Ion

The interaction energy of a monovalent ion in an aqueous medium at 25�C is determined. It is also found that the water molecules in the first hydration shell of the ion have a mean dipole moment far in excess of their permanent dipole moments. Thus, for example, the increase in the dipole moment of the attached water molecules. due to the presence of an ion is about 60% for the small four-coordinated Li+ ion and about 30% for the larger four-coordinated 1- ion. Calculations are also carried out on the assumption that the ions are six coordinated.

The Stability of a Rotating Liquid Mass Held Together by Surface Tension

The stability of a drop of incompressible fluid held together by the action of surface tension and made to rotate rigidly about an axis is determined, the effect of gravity being neglected. Two distinct problems are investigated. In the first is considered an isolated drop in the form of a surface of revolution and the manner in which its stability changes with angular speed is investigated. At zero angular speed, where the drop is spherical, infinitesimal disturbances are shown to be stable and beyond a certain critical angular speed a new linear series of equilibrium forms emerges, the original series becoming unstable.