Atomic equilibrium adsorbates

To resolve the problem applying methods of collimated atom beams, equilibrium vapour as well as radioactive isotopes, the Hall effect and measurement of conductivity in thin layers of semiconductor-adsorbents using adsorption of atoms of silver and sodium as an example the relationship between the number of Ag-atoms adsorbed on a film of zinc oxide and the increase in concentration of current carriers in the film caused by a partial ionization of atoms in adsorbed layer were examined. 

The chlorine electrode contains gaseous chlorine in equilibrium with atomic chlorine adsorbed on the platinum black and a solution of chloride ions. Its potential is given by the equation... 

The lattice-gas model provides effective ways on atomic-molecular level for allowing the mechanical equilibrium of all solid atoms and adsorbed particles on any (flat or structural-rough) surface if one expresses... 

The energy schemes shown in Figure 12 are based on the assumption that hydrogen atoms are adsorbed on the surface and that there is no equilibrium with the gas phase. To take the equilibrium with the gas phase into account, we would have to add to each reaction that involves... 

In contrast, the rate-determining step of the nitrogenation reaction N2 2N(dissolved) occurs on interstitial sites on the surface where N atoms are adsorbed this is in accord with the rate equation (3), where K is the equilibrium 1 kps, / [N]... 

The potential energy well for chemisorption is associated with the more familiar chemical bond, although the valence band of the solid provides many unique features, and is reviewed by Grimley [41]. The experimental distinction between physisorbed and chemisorbed states is now readily made by photoemission studies of the combined adsorbate-adsorbent system, thus (thankfully) committing the otherwise rather theological discussions of borderline cases to past history. Chemisorption heats (see ref. 42) usually lie within the range 30 < q < 600 kj mole-1 and measured adatom—adsorbent atom equilibrium distances are usually very close to those observed in solid state of molecular analogues. (Such measurements are obtained by LEED or, more accurately, by surface EXAFS.)... 

So far, it has been accepted that molecules or atoms are adsorbed on sites being in registry with the substrate lattice, and the catalysis is explained by their reaction over the surface. By the adsorption of molecules or atoms, surface metal atom undergoes so often short distance shift from their equilibrium position, which is the adsorption induced reconstruction. It should be pointed out that such short distance shift of the... 

Real-time STM observations of atomic equilibrium fluctuations in an adsorbate system O/Ru(0001). Sutf. 

Lateral density fluctuations are mostly confined to the adsorbed water layer. The lateral density distributions are conveniently characterized by scatter plots of oxygen coordinates in the surface plane. Fig. 6 shows such scatter plots of water molecules in the first (left) and second layer (right) near the Hg(l 11) surface. Here, a dot is plotted at the oxygen atom position at intervals of 0.1 ps. In the first layer, the oxygen distribution clearly shows the structure of the substrate lattice. In the second layer, the distribution is almost isotropic. In the first layer, the oxygen motion is predominantly oscillatory rather than diffusive. The self-diffusion coefficient in the adsorbate layer is strongly reduced compared to the second or third layer [127]. The data in Fig. 6 are qualitatively similar to those obtained in the group of Berkowitz and coworkers [62,128-130]. These authors compared the structure near Pt(lOO) and Pt(lll) in detail and also noted that the motion of water in the first layer is oscillatory about equilibrium positions and thus characteristic of a solid phase, while the motion in the second layer has more... 

In a recent paper [11] this approach has been generalized to deal with reactions at surfaces, notably dissociation of molecules. A lattice gas model is employed for homonuclear molecules with both atoms and molecules present on the surface, also accounting for lateral interactions between all species. In a series of model calculations equilibrium properties, such as heats of adsorption, are discussed, and the role of dissociation disequilibrium on the time evolution of an adsorbate during temperature-programmed desorption is examined. This approach is adaptable to more complicated systems, provided the individual species remain in local equilibrium, allowing of course for dissociation and reaction disequilibria. 

Eqs. (1,4,5) show that to determine the equilibrium properties of an adsorbate and also the adsorption-desorption and dissociation kinetics under quasi-equilibrium conditions we need to calculate the chemical potential as a function of coverage and temperature. We illustrate this by considering a single-component adsorbate. The case of dissociative equilibrium with both atoms and molecules present on the surface has recently been given elsewhere [11]. 

Langmuir (1916), whp put forward the fir quantitative theory of the adsorption of a gaS, assumed that a gas molecule condensing from the gas phase-would adhere to the surface fora short time before evaporating and that the condensed layer was only one atom or molecule thick. If 0 is the fraction of the surface area covered by adsorbed molecules at any time, the rate of desorption is proportional to 0 and equal to k 0 where is a constant at constant temperature. Similarly the rate of adsorption will be proportional to the area of bare surface and to the rate at which the molecules strike the surface (proportional to the gas pressurep). At equilibrium the rate of desorption equals the rate of adsorption... 

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