Influence Of Surface Area Research Articles

Influence of surface area and additives on the thermal stability of transition alumina catalyst supports. II: Kinetic model and interpretation

The common features of the gamma, delta and theta forms of transition aluminas found in their structural and chemical properties allow them to be described by a unique formula, Al 2□O 3-v/2(OH) v < > 1-v/2 in which Al represents the aluminium ions in the trivalent sites, □ the cationic vacancies in the divalent sites, OH the hydroxyl groups substituting the oxygen in the normal positions of the anionic sublattice and < > the anionic vacancies of a spinel structure. This formula satisfies the conditions of electrical neutrality of the crystal, accounts for the required ratio of the number of sites in the structure and describes the range of stoichiometries from boehmite (v = 2) to alpha alumina (v = 0). Thus the incorporation of a foreign cation MZ +, either in an Al 3+ site or in a divalent vacancy, leads to a modification of the stoichiometry of different structural elements. The evolution of transition aluminas during thermal treatment consists of dehydroxylation by removal of water followed by an exothermic structural transformation to the alpha phase, which can be described in terms of an annihilation reaction between anionic and cationic vacancies. A theoretical model for spherical particeles based on the annihilation of vacancies as the rate-determining step leads to a kinetic law involving the initial specific surface area, the valency of the foreign cation, and a parameter which depends on its ionic radius. Kinetic data, previously reported for pure and various doped aluminas, agree satisfactorily with the theoretical law. The interest in and limitations of this model are discussed in relation to the experimental results.

Geometric influences on the spontaneous passivation of copper in alkaline hexacyanoferrate(III) solutions under conditions of free convection

The passivation of copper proceeds in the presence of an oxidizing agent without an external current. Cyclovoltammetric, chronopotentiometric and open-circuit potential measurements show, that in alkaline solution the passivation rate increases with decreasing hydroxide and increasing hexacyanoferrate(III) concentration. The rate increases also by the formation of mixed potentials with NiCrSiO x surfaces and by reduction of the surface area. These effects can be explained by an increase of the diffusion-controlled cathodic partial current in comparison to the anodic partial current. The influence of surface area on the passivation rate is important on surfaces with an extension of about or smaller than the thickness of the diffusion layer.

Über die Rückbildungsfähigkeit von Netzhaut- und Glaskörperveränderungen nach Extraktion intravitrealer Kupferpartikel*

On the basis of experiments with 42 rats, quantitative statements are made regarding the influence of surface area and dwelling time of intravitreal copper particles on the electroretinogram. The influence of these factors on the extent of recovery of function after extraction of the foreign body is demonstrated. The rate of mental intoxication (metallosis retinae) is indicated separately from the mechanical damage. Ophthalmoscopic findings are also described. The practical relevance of these results to the clinical situation is discussed in detail in connection with earlier reports.

Comparison of the catalytic properties in hydrodesulfurization reaction of unsupported MoS2 and WS2 catalysts: Influence of surface areas

Unsupported MoS2 and WS2 catalysts were prepared by decomposition of thio and oxothiosalts under sulfiding atmosphere. Surface areas between 5 and 80 m2 g−1 were obtained, varying the decomposition conditions and nature of precursors. Typical samples were studied in dibenzothiophene hydrodesulfurization in a flow microreactor working at 22.5×105 Pa. The specific activity increases linearly with surface areas for both families of samples.

Kinetics of gas-solid reactions: Influence of surface area and effective diffusivity profiles

The isothermal reaction between a gas and a porous solid was studied theoretically. The conversion-time relationship for the solid is calculated for an irreversible, first-order reaction with respect to the gaseous reactant and taking into account the diffusional resistances of the gas film and the ash layer. This calculation is performed assuming the gaseous reactant is consumed inside the porous solid, but taking into account the way in which this consumption affects the porosity and hence the available surface area and the effective diffusivity. The effectiveness factor used in such a calculation is also developed in this paper and is expressed as a correction factor to the Thiele modulus.

The relation of vibrator surface area and static application force to the vibrator-to-head coupling.

Twelve young adult men with normal hearing and no history of ear disease took part in our study of the relation of vibrator surface area and static application force to the vibrator-to-head coupling. For vibrator surface areas of 1.125, 2.25, and 4.5 cm 2 coupled to the forehead under static forces of 150, 300, and 600 gm, monaural thresholds of bone-conduction hearing were determined at frequencies 250, 500, 1000, and 2000 Hz. With surface area constant, threshold improvement was frequency dependent. It decreased with increasing frequency until at 2000 Hz it was minimal. In contrast to this, with force constant, the influence of surface area was observed to begin at 2000 Hz. Preliminary study suggests this influence would extend to 4000 Hz. In view of the respective influence of application force and surface area to bone-conduction hearing, equating vibrator-to-head coupling on the basis of pressure (force per unit area), when there are two or more vibrators, may not represent an adequate control.

Influence of Surface Area on the Density of Quartz heated at 1,300°–1,500° C

DURING an investigation of silica brick it soon became apparent that the extent of the thermal alteration of quartz was dependent on its initial particle size. A search of the literature revealed only a few fragmentary pieces of information1–5. Most of this information merely compared the extent of alteration of two fractions, a fine and a coarse, and stated that the finer fraction is altered to the greatest extent. The finer fraction was coarse compared with the quartz used here. Therefore a systematic, qualitative investigation of the influence of the initial surface area of quartz on the extent of its thermal alteration, as measured by density changes and X-ray diffraction, was undertaken. The purpose of this communication is to emphasize graphically the extent of this surface area effect.