Simple multisite models

In many physically important cases of localized adsorption, each adatom of the compact monolayer covers effectively n > 1 adsorption sites [3.87-3.89, 3.98, 3.122, 3.191, 3.214, 3.261]. Such a multisite or 1/n adsorption can be caused by a crystallographic Me-S misfit, i.e., the adatom diameter exceeds the distance between two neighboring adsorption sites, and/or by a partial charge of adatoms (A < 1 in eq. (3.2)), i.e., a partly ionic character of the Meads-S bond. The theoretical treatment of a /n adsorption differs from the description of the 1/1 adsorption by a simple Ising model. It implies the so-called hard-core lattice gas models with different approximations [3.214, 3.262-3.266]. Generally, these theoretical approaches can only be applied far away from the critical conditions for a first order phase transition. In addition, Monte Carlo simulations are a reliable tool for obtaining valuable information on both the shape of isotherms and the critical conditions of a 1/n adsorption [3.214, 3.265-3.267]. 

However, this model does not explain why, in the comparable experiment performed under HD, no D2 forms, nor why substrates other than N2 do not promote HD formation. Also, if H2 can interact with the active site, why is a substrate of any kind needed to promote HD formation Displacement of H2 is not a necessity for binding N2, but why does HD form only when N2 is being reduced One simple answer proposed by Helleren et al. is that HD formation and N2 binding occur at different places.54 It is possible that different substrates bind to and are transformed at different parts of the large FeMoco (FeMo cofactor) site of N-ases discussed below. CO inhibits nitrogen fixation in N-ases but not H2 evolution. A single site that binds H2 and N2 equivalently should be poisoned by CO for both H2 and N2 activity, and evidence increasingly points to multisite processes in the FeMoco cluster. However, a possible model (10) for HD formation at the same site as N2 activation is discussed below. 

The much more realistic multisite surface complexation (music) model recognizes that different kinds of oxo-/hydroxo-groups are developed on the surface of the oxidic supports [18], Thus, single MO(H), double M20(H) and/or triple MjO coordinated oxo-/hydroxo-groups may be developed on the surface. Moreover, this model provides a very simple formula for estimating the surface charge of the surface oxo-/hydroxo-groups. 

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