High-index surfaces

The existence of active sites on surfaces has long been postulated, but confidence in the geometric models of kink and step sites has only been attained in recent years by work on high index surfaces. However, even a lattice structure that is unreconstructed will show a number of random defects, such as vacancies and isolated adatoms, purely as a result of statistical considerations. What has been revealed by the modern techniques described in chapter 2 is the extraordinary mobility of surfaces, particularly at the liquid-solid interface. If the metal atoms can be stabilised by coordination, very remarkable atom mobilities across the terraces are found, with reconstruction on Au(100), for example, taking only minutes to complete at room temperature in chloride-containing electrolytes. It is now clear that the... 

Notzel, R., Ledentsov, N. N., Daweeritz, L., and Ploog, K., Semiconductor quantum-wire structures directly grown on high-index surfaces. Phys. Rev. B 45, 3507 (1992). 

A high-indexed surface zone with a high density of steps or growth sites (kink positions) would follow the same growth law as liquid metals, i.e., the Butler-Volmer relation. Vicinal faces, characterized by low-index surface zones separated by uniformly distributed monatomic steps show an intermediate behavior. 

In this subsection we shall add further aspects to this transition, emphasizing in particular that the step free energy k Ts T) vanishes at 7r (Weeks, 1980 van Beijeren and Nolden, 1987). This fact has particular implications for vicinal (i.C., high index-) surfaces, e.g. Cu(llf), with i odd at low temperatures such a surface can be viewed as a dense regular array of steps relative to a (100) surface. 

Formation of hexane was catalyzed by Pt (110) octahedra at temperatures below 240 °C [251]. Catalysts containing Pt-truncated octahedra defined by (100) and (111) surfaces, and Pt spheres containing high-index surfaces produced 2-methylpentane. Finally, Pt (100) cubes led to the formation of cracking products (i.e., Cj-C ) under the same conditions. Interestingly, Pt spheres and truncated octahedra exhibited higher TOFs compared to octahedral (8 times at 240 °C) and cubic (50 times at 240 °C) nanoparticles. 

Tian N, Zhou ZY, Sun SG (2008) Platinum metal catalysts of high-index surfaces fiom singlecrystal planes to electrochemically shape-cruitrolled nanoparticles. J Phys Chem C 112(50) 19801-19817... 

The reactivity of a coordinatively unsaturated surface atom tends to increase with decreasing coordination number. Therefore, high-index surfaces tend to be more reactive than low-index surfaces. Steps and kinks are often the preferred sites for dissociation to occur. If dissociation leads to fragments that preferentially require a specific site or sites... 

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