Adsorbate surface binding geometries

Until a few years back, the crystal shape of solid materials was of academic curiosity only and shape was not considered to have an effect on the chemical properties and reactivities of a material. However, recent studies clearly indicate that the shape of nanocrystals does indeed affect the chemistry. For example, it has been shown that 4 nm nanocrystalline MgO particles adsorb six molecules of S02 per nm2 at room temperature and 20 Torr pressure.30 However microcrystalline MgO adsorbs only 2 molecules of S02 per nm2 under similar conditions. Similarly, the nanocrystalline aerogel prepared, AP-MgO material adsorbs four times as much C02 as the microcrystals. There are not only differences in the amounts of gaseous molecules adsorbed on these surfaces, but also the mode of surface binding can also be different. S02 binds more predominantly as a monodentate species on the AP-MgO crystal but favors a bidentate geometry on conventionally prepared, CP-MgO microcrystals. Clearly, these results indicate that the shape and size of the crystals affect the adsorptive properties of the MgO surfaces. The high reactivities of the... 

The theoretical results obtained in the present work, strongly support the conclusions of Sheppard [46] and of Lehwald et al [67]. The results in Table 10 show that the molecule binds strongly on a Ni(lOO) surface, the geometry and the electronic structure of the molecule are highly perturbed by the adsorption process, and the di-CT adsorption mode is more stable than the 7t mode, by approximately 91 kJmoT. The calculated CC bond length for the di-a mode, 1.46 A, is identical to the experimental value determined by NEXAFS [68]. An analysis of the electronic structure shows that on both adsorption complexes the degree of the [Pg.236]

The first applications of ab initio quantum-chemical calculations to systems of electrochemical interest were concerned with the adsorption of halides at metal surfaces. The adsorption of halides is of great experimental and practical importance as many electrolyte solutions contain halide anions, which tend to adsorb specifically at the metal-water interface, especially at more positive electrode potentials. Issues of interest in halide chemisorption are the nature of chemical bond with the surface ( . e., covalent or ionic), the strength of the interaction of the different halides on different substrates, the preferred binding geometry on single-crystal surfaces, the effect of the electrode potential, and the importance of including the solvent (water) in correctly modeling the properties of the chemisorption bond. 

Fig.6 Binding energies of Cu (full lines) and Ag (broken lines) on a Si(lll) surface. The perpendicular distance between the adsorbate atoms and the plane of the surface silicon atoms is denoted by h. Hollow, top, and bridge positions of the adsorbate atoms are indicated by the labels A, B, etc. as shown in the insert, u corresponds to an unrelaxed and r to a relaxed geometry of the neighboring surface Si atoms (after Ref.47)...

Note that the dissociation proceeds with a much lower barrier on the stepped surface. As the structure diagrams show, at all stages in the dissociation the species are more strongly bound on the stepped surface, for reasons discussed in connection with Eq. (87). However, the transition state is most affected, because two N atoms are bound to four metal atoms in the transition state on a perfect surface, whereas that on the stepped surface consists of five metal atoms. As noted above, geometries in which atoms bind to different metal atoms are always more stable than when the two adsorbate atoms share one metal atom. Hence, dissociation is favored over step sites, and if a surface contains such defects they may easily dominate the kinetics. 

DFT calculations of the structure of the molecularly adsorbed NO are in reasonable agreement with experiments, but overestimate the binding energy [197,198]. A barrier of 2.1 eV to dissociation is predicted by DFT, with the NO at the transition state nearly parallel to the surface and N and atoms in bridge sites [199]. This transition state geometry is similar to that of NO dissociation on other close-packed metal surfaces [200]. There is no global DFT PES so that all theoretical dynamics is based only on empirical model PES. 

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