Diffusion of adsorbates

NMR Nuclear magnetic resonance [223, 224] Chemical shift of splitting of nuclear spin states in a magnetic field H [225], C [226, 227], N [228], F [229], 2 Xe [230] Other Techniques Chemical state diffusion of adsorbed species... 

Gomer R 1990 Diffusion of adsorbates on metai surfaces Rep. Prog. Phys. 53 917... 

Eath. In other cases, attachment can be essentially permanent, but in oth cases, only detached molecules undergo transport. In contrast, the following four instances illustrate cases where diffusion of adsorbate molecules occurs in their adsorbed state within phases that are distinct from the pore fluid ... 

The model is intrinsically irreversible. It is assumed that both dissociation of the dimer and reaction between a pair of adjacent species of different type are instantaneous. The ZGB model basically retains the adsorption-desorption selectivity rules of the Langmuir-Hinshelwood mechanism, it has no energy parameters, and the only independent parameter is Fa. Obviously, these crude assumptions imply that, for example, diffusion of adsorbed species is neglected, desorption of the reactants is not considered, lateral interactions are ignored, adsorbate-induced reconstructions of the surface are not considered, etc. Efforts to overcome these shortcomings will be briefly discussed below. 

To visualize the fundamental steps of chemisorptions and reactions that occur at surfaces, in situ STM investigations typically monitor the diffusion or transformation of adsorbed molecules. A series of snapshots of preselected surface regions, compiled into a STM movie, can reveal the evolution of surface phenomena. On metal surfaces, the surface diffusion of adsorbates is usually so rapid that the... 

A mass balance for diffusion of adsorbate into a spherical pellet may be written as ... 

Surface diffusion of adsorbed species over the surface to the growth center... 

In some cases, surface diffusion - that is, the diffusion of adsorbate molecules along the interface in the pores - may contribute substantially to the mass transfer of the adsorbate, and in such cases, the effective diffusivity may become much larger than the case with pore diffusion only. 

Adsorbed molecules may diffuse laterally at the interface. Although surface diffusion is well-known in classical surface chemistry 33), data on adsorbed macromolecules is sparse. Burghardt and Axelrod3+) and Michaeli et al. 35) have both demonstrated rapid interface diffusion of adsorbed albumin. 

Figure 19. The foam stability, film thickness and surface diffusion of adsorbed /3-cas as a function of the concentration of added Tween 20 in a/w thin films. The /3-cas concentration was held constant at 0.2 mg/ml (8.33 nM). Reproduced from reference [41] with the permission of VCH Verlagsgesellschaft.

The importance of adsorbent non-isothermality during the measurement of sorption kinetics has been recognized in recent years. Several mathematical models to describe the non-isothermal sorption kinetics have been formulated [1-9]. Of particular interest are the models describing the uptake during a differential sorption test because they provide relatively simple analytical solutions for data analysis [6-9]. These models assume that mass transfer can be described by the Fickian diffusion model and heat transfer from the solid is controlled by a film resistance outside the adsorbent particle. Diffusion of adsorbed molecules inside the adsorbent and gas diffusion in the interparticle voids have been considered as the controlling mechanism for mass transfer. 

Subsequently a well-defined area at the surface is depleted from the adsorbate layer by a focused laser pulse. Since thermal equilibrium at the surface is rapidly recovered, the bare spot can be refilled only by surface diffusion of adsorbates from the surrounding areas [31]. A second laser impulse is applied to desorb the transported adsorbates after a time interval t from the first pulse. The corresponding amount of material can be quantified by mass spectrometry. For the idealized case of a circular depletion region, with a step-like coverage gradient and a concentration-independent diffusivity, the time-dependent refilling from Fick s first law is [32,33] ... 

Takeda N, Ohtani M, Torimoto T, Kuwabata S, Yoneyama H. Evaluation of diffusibility of adsorbed propionaldehyde on titanium dioxide-loaded adsorbent photocatalyst films from its photodecomposition rate. J Phys Chem B 1997 101 2644-2649. 

As shown in Figure 6.49a, the cracks grow by slip dissolution due to diffusion of active water molecules, halide ions, etc., to the crack tip, followed by a rupture of the protective oxide film by strain concentration, fretting contact between the crack faces. This is followed by dissolution of the fresh exposed surface and growth of the oxide on the bare surface. For the alternative mechanism of hydrogen embrittlement in aqueous media, the critical steps involve diffusion of water molecules or hydrogen ions to the crack tip reduction to hydrogen atoms at the crack tip surface diffusion of adsorbed atoms to preferential surface locations absorption and diffusion to critical locations in the... 

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