Theory of Low Coordination Number Active Sites on Surfaces

Theory of Low Coordination Number Active Sites on Surfaces 

While low coordination number sites, steps, and kinks, are the active sites for bond breaking in platinum, the atomic terrace sites with larger coordination numbers may also become active sites with unique chemistry for other elements. It will perhaps become possible to identify the bond-breaking ability of various coordination number sites of a given metal in breaking H—H, C—H, C —C, 0=0, N=N, etc., chemical bonds. By varying the atomic surface structure, which would change the relative concentrations of the different coordination number surface sites, the product distribution in surface chemical reactions may be markedly varied. 

Tsang and Falicov (49) have calculated the charge density distribution at corner sites in ionic and rare gas crystal surfaces. For ionic solids, low coordination number surface sites should have large charge density variations 

Perhaps one of the important conclusions of these studies that points to the unique chemistry of surface irregularities, steps, and kinks, which appear to be active sites, is the controlling influence of the local atomic structure, local surface composition, and local bonding between adsorbates and surface sites. The microstructure of the metal surface controls bond scission and thus the rate and path of chemical reactions. Calculations taking into account this local bonding picture should help to unravel the elementary bond-breaking steps in catalytic surface reactions. 

Studies of the sticking probability of oxygen on silicon surfaces revealed a change from 10 5 to 1 with increasing density of surface steps (52a). Crystals with a large density of steps may be prepared by cleavage, and their chemistry can be readily studied. Ibach has associated the increased activity of disso-ciatively chemisorbed oxygen to the presence of electron orbitals that became available on silicon atoms at low coordination number step sites on surfaces. 

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X. Theory of Low Coordination Number Active Sites on Surfaces. 60... 

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