Search

The effect of a surface chemical reaction involving a weak soluble surface active substance on the motion of a drop in a liquid is investigated. It is shown that as a result of the Marangoni effect the non-uniformity in the distribution of the substance along the surface associated with the proper motion of the drop and the chemical reaction has an important influence on the nature of the motion of the drop and on the force exerted by the surrounding Liquid. Under certain conditions this Leads to the development of a thrust proportional to the velocity of the drop (chemoconcentration capillary effect). The condition of occurrence of the thrust is obtained, together with its dependence on various physical parameters.

The isotopic exchange reaction between the spherical particles and ions in a solution of finite volume has been modelled mathematically. The rate equat

A mathematical model has been proposed for a heterogeneous isotopic exchange reaction which involves film diffusion, surface chemical reaction and intraparticle diffusion. The exchange equation to predict the exchange fraction as a function of time for the spherical particles immersed in a solution of finite volume has been derived. The relations between the exchange fraction and dimensionless time are plotted with e(=akf/KDe, e1(=K1a2/De) and final fractional uptake as parameters. From the values of the kinetic parameters e and e1, the relative importance of each limiting step is discussed. Experimental results of the isotopic exchange reaction of calcium ion in both system CaCO3(s)/Ca2+(aq) and system calcium type resin Dowex 50W-X8/Ca2+(aq) are coincident with the theoretical equation proposed in this study.

Single-crystalline ZnS layers have been grown on GaP and GaAs substrates, using a hydrogen transport system. The growth behavior on GaAs and GaP substrates is found to be strongly influenced by the auto-doping of gallium from the substrate. These phenomena are ascertained from the Ga-doping experiments in the ZnS growth process on GaP(111)A and B substrates. By the addition of Ga atoms to the vapor phase, the growth rate on (111)A decreases and that on (111)B increases, probably due to the change of the surface chemical reaction related to Ga atoms. These Ga-doping effects are found to show nearly the same behavior as those of In-doping for the growth of ZnS on GaP.

An approximate method is proposed for solving nonlinear problems of transient mass transfer between a wall and a motionless fluid for an arbitrary concentration dependence of the diffusion coefficient. Nonlinear problems compounded with a volume or surface chemical reaction are investigated.

The techniques of field ion microscopy and atom-probe mass spectroscopy are reviewed with emphasis on the general operational principles of the instruments and selected applications in the area of surface science. The single atom imaging capability of the field ion microscope is shown to make it uniquely suited to the study of detailed, atomic processes on clean metal surfaces. As examples, investigations of single-atom surface diffusion, cluster nucleation, and surface reconstructions are discussed. The ability of the atom-probe mass spectrometer to determine the chemical identity of selected atoms observed in the field ion microscope image is shown to be useful for the microanalysis of various materials. The different types of atom-probes currently in use and their application in the areas of surface catalyzed reactions and high-temperature ceramic superconductors are described.

We present STM and photoemission studies of the reactions of Si(100)-(2×1) and Si(111)–(7×7) with NH3. STM allows us to image the occupied and unoccupied states of the reacting systems and to obtain electronic spectra with atomic lateral resolution. Thus, for the first time, a surface chemical reaction can be probed at the atomic level. We find that both surfaces are reactive even at 100K. However, both the extent of the reaction and the reaction products at 300K are different on the two surfaces. STM also shows that while surface dangling bonds are essential for low-temperature reactivity, not all triplycoordinated Si sites are chemically equivalent. On the 7×7 surface the rest-atoms are more reactive than center-adatoms which, in turn, are more reactive than corner-adatoms.

Motivated by the suggestion that mainly geometric arguments can explain the observed structure sensitivity in the cyclotrimerization of acetylene to benzene on palladium single crystals and on supported palladium particles, we studied this reaction on size-selected Pd-n clusters (1 less than or equal to n less than or equal to 30) supported on thin MgO (100) films by thermal desorption spectroscopy. Already on small Pd-n (1 less than or equal to n less than or equal to 6), benzene is readily produced, mainly at low temperature (300 K). For larger clusters (7 less than or equal to n less than or equal to 30) benzene also desorbs at a temperature of 430 K, indicating that a critical ensemble of seven atoms is necessary for high-temperature cyclotrimerization. Using density functional theory, we attribute this high-temperature desorption of benzene to the presence of three-dimensional clusters. (C) 2000 Elsevier Science B.V. All rights reserved.

The reaction of NO with a CO adsorbate has been studied in a supersonic molecular beam experiment where gas-phase oriented NO molecules strike a CO-precovered Pt(100) surface. By seeding NO in Ar and H-2, the translational energy of the NO molecules in the supersonic beam was varied continuously between 70 and 400 meV. Two quadrupole mass analyzers mounted behind the platinum target recorded the yield of scattered and desorbed NO as well as the reaction product CO2 as a function of time. The CO, signal strongly depends on the translational energy of the incident NO molecules and on the initial NO orientation, i.e. preferential N-end or O-end collisions at normal incidence to the surface. A comparison of the observed steric effects for CO2 and NO leads to the conclusion that the CO2 reaction asymmetry induces a NO trapping asymmetry. Angle-resolved time-of-flight measurements of the CO2 reaction product have been performed to investigate the dynamics of the reaction, especially the reaction mechanism. A cosine-like angular distribution of the CO2 desorption has been found, indicating that a direct or precursor-mediated reaction mechanism is improbable. A Monte Carlo simulation of the NO + CO reaction models the course of the reaction quite well, and shows that NO dissociation dominates the course of the reaction. The observed shift of the CO2 peak for different translational energies of the NO molecules can be reproduced by an energy-dependent NO dissociation. A large steric effect in the CO2 production is found at high translational energies in the experimental data. This cannot be explained solely by an orientation-dependent trapping or sticking of the NO molecules. Consequently, another step in the reaction path, presumably dissociative NO adsorption, is orientation-dependent. (C) 1997 Elsevier Science B.V.

Deuterated methanol adsorption on Si(111)-(7 × 7) has been studied by C 1s and O 1s core-line synchrotron radiation photoemission and mass spectrometry of photodesorbed ions. Experiments were performed as a function of coverage and temperature. Two different C 1s photoemission peaks hint at two different adsorbed species, the first one related to the methoxy fragment produced after dissociative adsorption, the second one is attributed to a CDx species. Both peaks disappear after heating at 350°C, though the methoxy species is more reactive, and a third peak appears. The nature of the species producing the third peak was deduced by means of the mass spectra after heating the sample at 400°C and attributed to C dispersed on the silicon surface.

We present an STM study of methanol oxidation to formate by co-dosing of oxygen and methanol onto Cu(110) at room temperature. Formate results from the reaction of adsorbed methoxy and isolated oxygen adatoms generated during dosing. Methanol-rich gas-phase mixtures have the highest efficiency for formate production generating characteristic alternating formate c(2 × 2) and O-(2 × 1) islands in the 〈001〉 direction. An oxygen-rich gas-phase composition does not produce formate due to preferential growth of O-(2 × 1) islands. Formate production at 300 K during co-dosing experiments contrasts with the case of sequential dosing as oxygen (2 × 1)-reconstructed Cu(110) surfaces are seen to have negligible activity toward formate production when subsequently dosed with methanol at 300 K.