Diffusion on surfaces

Gilliland, E., R. F. Baddour and G. P. Perkinson. 1974. Diffusion on surfaces. Effect of concentration on the diffusivity of physically adsorbed gases. Ind. Eng. Fundam. 13 95-100. 

For detailed examples of exchange mechanisms in atomic diffusion on surfaces, see... 

We have thus far written unimolecular surface reaction rates as r" = kCAs assuming that rates are simply first order in the reactant concentration. This is the simplest form, and we used it to introduce the complexities of external mass transfer and pore diffusion on surface reactions. In fact there are many situations where surface reactions do not obey simple rate expressions, and they frequently give rate expressions that do not obey simple power-law dependences on concentrations or simple Arrhenius temperatures dependences. 

As a test problem for comparing the various methods described above, we have chosen a heptamer island on the (111) surface of an FCC crystal. Partly, this choice is made because it is relatively easy to visualize the saddle point config-mations and partly because there is great interest in the atomic scale mechanism of island diffusion on surfaces (see for example reference 60). The interaction potential is chosen to be a simple function to make it easy for others to verify and extend om results. The atoms interact via a pairwise additive Morse potential... 

Lowell and Karp measured the effect of thermal diffusion on surface areas using the continuous flow method. Figure 15.14 illustrates a fully developed anomalous desorption signal caused by thermal diffusion. 

Synergism of catalysts via spillover ca. 800 Importance of diffusion on surface... 

From the overall perspective of the present chapter, our ambition in briefly describing the energetics of surface diffusion has been to illustrate the additional complexity that is ushered in as a result of the simultaneous action of more than one type of defect. In particular, we have shown that via a counterintuitive exchange mechanism, the activation energy for surface diffusion can be lowered by 0.4 eV relative to the intuitive motion associated with an imagined random walker on a surface. In addition, we have also shown that the presence of surface steps can have significant implications for the nature of diffusion on surfaces. 

Fig. 13. A model of the extent of molecular diffusion on surfaces, so-called diffusion circles, showing the strong temperature dependence of the number of diffusion events. Circles are shown here superimposed on a (100) lattice for surface temperatures of 400 K and 950 K at lower temperatures the diffusion circles arc much more extensive (see text).

Ala-Nissila, T., Ferrando, R., Ying, S.C. Collective and single particle diffusion on surfaces, Adv. Phys. 2002,51,949. 

Experimentally, Au(lOO) surfaces in pure HCIO4 are stable and show no adatom migration. However, even small amounts of adsorbed Cl atoms greatly enhance the surface mobUity [36, 40]. A similar facilitation of diffusion on surfaces in vacuum by adsorbates has been studied experimentally and theoretically [41 5]. In order to understand this effeet, we first investigated the effect of adsorbed Cl on the adatom migration barriers by DFT, and then used these results to simulate Ostwald ripening. 

AG Kalinichev, RJ Kirkpatrick, RT Cygan. Am Mineral 2000 85 1046-1052. AG Kalinichev, RJ Kirkpatrick, J Wang, Molecular Dynamics Simulation of Ionic Sorption and Diffusion on Surfaces and Interfaces of Layered Double Hydroxides. Proceedings of 223rd ACS National Meeting, Apr. 7-11, 2002, Orlando, FL. 

Sikavitsas, V.I., and Yang. R.T., Predicting multicomponent diffusivities for diffusion on surfaces and in molecular sieves with energy heterogeneity, Chem. Eng. Sci.. 50( 19), 3057-3066 (1995). 

Following adsorbate diffusion on surfaces, the use of fast STM [43] became possible even in real time and now allows surface scientists to draw a detailed picture of adsorbate kinetics, hopping rates, and diffusion energy barriers [44, 45] as well as to explore nucleation and growth of various homoepitaxial and heteroepitaxial systems [46]. Even the dynamics of surface reactions under high pressures [47,... 

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