Terrace diffusion

Mobility of this second kind is illustrated in Fig. XVIII-14, which shows NO molecules diffusing around on terraces with intervals of being trapped at steps. Surface diffusion can be seen in field emission microscopy (FEM) and can be measured by observing the growth rate of patches or fluctuations in emission from a small area [136,138] (see Section V111-2C), field ion microscopy [138], Auger and work function measurements, and laser-induced desorption... 

Fig. XVIII-14. Schematic illustration of the movement of NO molecules on a Pt(lll) surface. Molecules diffuse around on terraces, get trapped at steps, escape, and repeat the process many times before eventually desorbing. [Reprinted with permission from M. Cardillo, Langmuir, 1, 4 (1985) (Ref. 140). Copyright 1985, American Chemical Society.]...

If the terrace diffusion with the diffusion constant is rate limiting, the width relaxation is intermediate as [67]... 

In this case, however, if the step separation i is small, the consecutive steps are coupled via the terrace diffusion held, and the width cannot increase as fast as but increases slowly, like the edge-diffusion-limited growth, as [68]... 

Iodine and bromine adsorb onto Au(l 11) from sodium iodide or sodium bromide solutions under an applied surface potential with the surface structure formed being dependent on the applied potential [166]. The iodine adsorbate can also affect gold step edge mobility and diffusion of the Au surface. Upon deposition of a layer of disordered surface iodine atoms, the movement of gold atoms (assisted by the 2-dimensional iodine gas on the terrace) from step edges out onto terraces occurs. However, this diffusion occurs only at the step edge when an ordered adlayer is formed [167]. 

In order to assess the role of the platinum surface structure and of CO surface mobility on the oxidation kinetics of adsorbed CO, we carried out chronoamperometry experiments on a series of stepped platinum electrodes of [n(l 11) x (110)] orientation [Lebedeva et al., 2002c]. If the (110) steps act as active sites for CO oxidation because they adsorb OH at a lower potential than the (111) terrace sites, one would expect that for sufficiently wide terraces and sufficiently slow CO diffusion, the chronoamperometric transient would display a CottreU-hke tailing for longer times owing to slow diffusion of CO from the terrace to the active step site. The mathematical treatment supporting this conclusion was given in Koper et al. [2002]. 

The hydrogenation of para-substituted anilines over rhodium catalysts has been investigated. An antipathetic metal crystallite size effect was observed for the hydrogenation of /Moluidinc suggesting that terrace sites favour the reaction. Limited evidence was found for catalyst deactivation by the product amines. Catalysts with pore diameters less than 13.2 nm showed evidence of diffusion control on the rate of reaction but not the cis trans ratio of the product. 

Since the evaporation of a solid would occur at the kink sites because the bonding is weaker, atoms would diffuse also to these sites before evaporation. A demonstration of this is to be found on the morphologies of single crystals after a period of heating in vacuum to cause substantial evaporation. The resultant surface shows an increase in the number of ledges and kinks relative to the area of the terraces. It is also to be expected that dislocations emerging at the surface of catalysts, either as edge or screw dislocations, would play a... 

Fig. 2. Kinetics scheme for NO/Pt(l 11) including terrace diffusion (1), trap-to-terrace jumps (II), and thermal desorption T/D. (Adapted from Ref 16.)...

Figure 6.15. Ion transfer to the terrace site, surface diffusion, and incorporation at kink site.

Terrace Ion-Transfer Mechanism, In the terrace siteion-transfer mechanism a metal ion is transferred from the solution (OHP) to the flat face of the terrace region (Fig. 6.15). At this position the metal ion is in the adion (adsorbed-like) state, having most of its water of hydration. It is weakly bound to the crystal lattice. From this position it diffuses on the surface, seeking a position of lower energy. The final position is a kink site. 

Figure 1. Three limiting mechanisms for atomic processes which mediate step fluctuations, a) Step-edge diffusion b) Evaporation-recondensation c) Terrace diffusion with diffusion kernel P(). By appropriate choice of P(J), this case can reduce to cases a) and b) (see text).

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