Elementary surface reaction steps calculated activation energies

The latter concept is basic to the specific dependence of the activation energies of elementary reaction steps on different surfaces. There is not always an immediate relation with the coordinative unsaturation of the metal surface atoms. Ge and Neurock noted an exeptionally low barrier for the dissociation of NO adsorbed on the non-reconstructed Pt(lOO) surface. The corresponding transition state is shown in Fig. 3.37. The calculated activation energies for NO dissociation over the (111) and (110) surfaces are 160 and 105... 

Once the thermodynamic parameters of stable structures and TSs are determined from quantum-chemical calculations, the next step is to find theoretically the rate constants of all elementary reactions or elementary physical processes (say, diffusion) relevant to a particular overall process (film growth, deposition, etc.). Processes that proceed at a surface active site are most important for modeling various epitaxial processes. Quantum-chemical calculations show that many gas-surface reactions proceed via a surface complex (precursor) between an incident gas-phase molecule and a surface active site. Such precursors mostly have a substantial adsorption energy and play an important role in the processes of dielectric film growth. They give rise to competition among subsequent processes of desorption, stabilization, surface diffusion, and chemical transformations of the surface complex. 

For Equations (21a) and (21b) to apply, internal hydrogen atom transfer in the surface intermediates is an elementary step in the chain-growth reaction. Calculations by Ciobica et al. (90) indicate that the activation energies for C—H bond cleavage or C—H bond formation of adsorbed alkylidene or alkenyl intermediates are less than 50 kj/mol. 

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