Mobility of adsorbed atoms

In the Au/Al203/NiAl(100) system, hemispherical particles occur even at low coverage,7 unlike the situation with titania size distribution was narrow, and particles were stable to 600 K, implying low mobility of adsorbed atoms. Paradoxically, on alumina large particles migrate and coalesce faster than small ones, presumably because the metal-support interaction is weaker but with Au/FeO the diffusivity of atoms is higher due to a lower concentration of surface defects. 

The exchange in molecular oxygen occurs with the participation of a single atom of catalyst oxygen. This is possible if oxygen is adsorbed with the dissociation into atoms or ions. However, the desorption due to the low concentration or due to the insufficient mobility of adsorbed atoms or ions occur predominantly with their recombination into molecules with ions of oxide lattice. 

The broad outlines of the kinetic interpretation of such reactions are clear enough, but there is material for deeper investigation in many details, such as the extent to which one layer of molecules may be adsorbed on a layer of a different kind already present the relative role of atoms and molecules of gases like hydrogen and oxygen adsorbed on metals and the mobility of adsorbed atoms and molecules on the surface. It is beyond the scope of the present discussion to enter into these questions. 

Since the most active catalytic sites are usually steps, kinks, and surface defects, atomically resolved structural information including atomic distribution and surface structure at low pressure, possible surface restructuring, and the mobility of adsorbate molecules and of the atoms of the catalyst surface at high temperature and high pressure is crucial to understanding catalytic mechanisms on transition metal surfaces. The importance of studying the structural evolution ofboth adsorbates... 

The rotational mobility of adsorbed molecules is caused by its rotational degree of freedom (resulting from the fact that the molecule is tightly bound to the substrate through the only atom) and by the coupling of molecular vibrations with surface atomic vibrations. The rotational motion intensity is strongly temperature-dependent and affects spectroscopic characteristics. As a result, the rotational mobility of surface hydroxyl groups was reliably detected.200 203... 

In this section a method for the direct calorimetric determination of heats of adsorption on evaporated metal films is described and results for the heals of adsorption of hydrogen on nickel, iron, and tungsten are reported. In all cases the heats of adsorption decrease with the fraction of surface covered in a mode that can satisfactorily be explained by interaction of adsorbed atoms. A criterion for mobility of the adsorbed atoms is developed... 

As was already mentioned in the introduction, we have to assume that adatoms of hydrogen adsorbed on a metal surface with a high heat of adsorption must be mobile so that they either can find the sites of highest surface energy or can move as far apart from each other as possible if the heterogeneity of surface energy of the sites is due to interaction of adsorbed atoms on neighboring sites. A rapid redistribution of adsorbed atoms by evaporation and readsorption is out of the question... 

The components of a phase interface have a wide range of mobilities. The mobility of adsorbed particles exceeds that of the atoms in the bulk of a solid. It is usually considered that the mobility of the adsorbed particles is greater than that of the surface atoms of the solid, and when the state of the solid s atoms changes, the adsorbed particles at each instant are distributed practically in equilibrium. The same relation between the mobilities is presumed for absorbed particles and the atoms of a solid in the bulk. This is apparently not always true. In the general case, each component has its own ratio of the migration activation energies, and cases are possible when the mobilities of the surface atoms of a solid and of the adsorbed particles are commensurable. The mobilities of the atoms of a solid and of absorbed particles may also be commensurable. 

The adsorbed atoms formed in step 1 are mobile on the surface and must be considered as belonging to the metallic phase although their bonding to the metal is considerably ionic in character. Pertinent results were obtained by investigating the dependence of the equilibrium concentrations of adsorbed atoms on the equilibrium potential in the case of a silver electrode (3). It was shown that the adsorbed atoms are partially... 

The first proposal is based on observations that surface diffusivities of substrate metal atoms can be greatly enhanced over the clean surface values when oxygen or sulfur is adsorbed. This has been discussed recently by Perdereau and Rhead 375). According to their ideas, metal atoms can become disconnected from the substrate lattice and skate over patches of surface already covered by adsorbate. The weakened attraction to the substrate accounts for the high diffusivities, and can also explain the mobility of substrate atoms that is required in reconstructive adsorption. 

Coadsorbed species can interact in many different ways, when adsorbed at close distance from each other. Chapter 4 mentioned the decreased pre-exponents for rate constants of desorption of adatoms adsorbed at high coverage. The pre-exponent decreases with coverage, because of the decreased mobility of the atoms in the transition state due to site blocking by the other adatoms. When a surface is... 

A study of Table 1.1 reveals interesting features as to the mobility of the adsorbed atoms. Thus, for an argon atom on the (100) face, the easiest path from one preferred site S to the next is over the saddle point P, so that the energy barrier which must be surmounted is (1251 — 855) or 396 X 10 J/molecule. Since the mean thermal energy kT at 78 K is only 108 J/molecule, the argon molecule will have severely limited mobility at this temperature and will spend nearly all of its time in the close vicinity of site S its adsorption will be localized. On the other hand, for helium on the... 

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