Steps on surfaces

Elementary steps on surfaces and in condensed phases are more complex because the environment for the elementary reactions can change as the composition of the reaction mixture changes, and, in the case of surface reactions, there are several types of reactive sites on solid surfaces. Therefore, the rate constants of these elementary steps are not really constant, but can vary from system to system. Despite this complexity, the approximation of a single type of reaction step is useful and often generally correct. 

Elastic interaction occurs when the displacement fields from steps substantially superpose. Atoms located in the vicinity of steps tend to relax stronger compared to those farther away. The resulting displacements or lattice distortions decay with increasing distance perpendicular to the steps. Atoms situated in between two steps experience two opposite forces and cannot fully relax to an energetically more favorable position as would be the case with quasiisolated steps. The line dipoles at steps are due to Smoluchowski smoothing [160] and interact electronically. Only dipole components perpendicular to the vicinal surface lead to repulsion whereas parallel components would lead to attractive interaction. The dipole-dipole interaction seems to be weaker than the elastic one. For instance, steps on vicinal Ag(lll) have weak dipoles as was shown in a theoretical study [161]. Entropic interaction is due to the condition that steps may not cross and leads to an effective repulsive potential, the weakest interaction type. This contribution is always present and results from the assumption that cavities under the surface are unstable. Experiments and theory investigating steps on surfaces were recently reviewed [162]. 

Steps on surfaces of polyDCHD crystals (a) SEM micrograph of 15 fim diameter crystal (b) TEM micrograph of 100 nm diameter crystal showing molecular steps. 

Jeong, H. C. and Williams, E. D. (1999), Steps on surfaces experiment and theory. Surface Science Reports 34, 171-294. 

So far we have been neglecting deviations from ideal low-index orientations. Atomic steps, however, are omnipresent on real crystal surfaces and are the smallest building blocks for all manifestations of surface morphology. Steps on surfaces are indispensable for many processes ... 

FIGURE 2.4 lUnstration of the elementary reaction steps on surfaces. See insert for color representation of the figure.)... 

Elementary steps. The reactivity of the anchored complexes has been studied in only a few cases. Evidence for a few elementary steps on surfaces has been obtained, including ligand association and dissociation reactions, oxidative addition, and insertion. Understanding of these will be a prerequisite to the fundamental understanding of the elementary steps of catalysis. At this point, these can at best be inferred by analogy to molecular chemistry in solution. Therefore, a systematic approach to the chemistry of these elementary steps on surfaces is recommended. 

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