Transition metal adsorption surfaces

Transition metal adsorption results in one of two types of behavior alloy formation and the initial growth of epitaxial structures. Mn on Pd(100), Au and Pd on Cu(100) and Sn on Pt(lll) can result in substitutional structures in which the adatom replaces an host atom in the top layer of the substrate. This results in the formation of an ordered alloy which is confined to the first atomic layer of the surface. Because of the size difference between the adatom and host atom, this substitution can result in a buckling of the alloy monolayer (see table 11). 

If the assumption that results obtained from flash filament experiments can be correlated to the properties of the original chemisorbed layer is accepted, this type of study has shown a and states to be characteristic of the CO transition-metal adsorption system. However, there is still no conclusive evidence to support any one view of the mechanism causing these two states, with such widely different heats of adsorption. Explanation of these two phases in terms of adsorption on different crystal planes is not fully acceptable. For example, in certain single crystal studies on tungsten surfaces, the a and j8 phases have been shown to coexist after CO is adsorbed at room temperature, so... 

Figure 5.3. A top view of the adsorption sites for O2 preciusors on transition metal (111) surface. (Reprinted with permission from Ye Xu and Manos Mavrikakis, J Chem Phys 116 (2002) 10846. 2002 American Institute of Physics.)...

An extension of the relative simple formulation used in SCMs for surfaces with permanent eharges (see Section II.B.l) has been published recently [84]. A fictitious surface species (X ) was defined and hypothetical complexation reactions on site X were written, and thus cation-exchange reactions of permanent negative layer charges were easily incorporated into such model. The model showed not only to fit satisfactorily all of the experimental data of transition metal adsorption on montmorillonite but also to explain specific features of adsorption on clays compared to oxides. 

Consider oxygen adsorption on transition metal oxide surfaces. The role of the narrow bands of metal fi -states in the transition metals is now played by localized surface resonances and surface states, typically in the gap of semiconducting or insulating transition metal oxides. 

FIGURE 11.9 Scaling relations between adsorption energies of OH and OOH versus the O adsorption energy, (a) data for close-packed metal surfaces. Results where the solvation by HjO is included are compared to results with no solvation, (b) and (c) data of transition metal oxide surfaces. Adapted from Karlberg and Wahnstrbm (2005) and Rossmeisl et al. (2007). 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

Fig. 5(a) contains the oxygen and hydrogen density profiles it demonstrates clearly the major differences between the water structure next to a metal surface and near a free or nonpolar surface (compare to Fig. 3). Due to the significant adsorption energy of water on transition metal surfaces (typically of the order of 20-50kJmoP see, e.g., [136]), strong density oscillations are observed next to the metal. Between three and four water layers have also been identified in most simulations near uncharged metal surfaces, depending on the model and on statistical accuracy. Beyond about...

The performance of VASP for alloys and compounds has been illustrated at three examples The calculation of the properties of cobalt dislicide demonstrates that even for a transition-metal compound perfect agreement with all-electron calculations may be achieved at much lower computational effort, and that elastic and dynamic properties may be predicted accurately even for metallic systems with rather long-range interactions. Applications to surface-problems have been described at the example of the. 3C-SiC(100) surface. Surface physics and catalysis will be a. particularly important field for the application of VASP, recent work extends to processes as complex as the adsorption of thiopene molecules on the surface of transition-metal sulfides[55]. Finally, the efficiciency of VASP for studying complex melts has been illustrate for crystalline and molten Zintl-phases of alkali-group V alloys. 

Most studies of the effect of alkalis on the adsorption of gases on catalyst surfaces refer to CO, NO, C02, 02, H2 and N2, due to the importance of these adsorbates for numerous industrial catalytic processes (e.g. N2 adsorption in NH3 synthesis, NO reduction by CO). Thus emphasis will be given on the interaction of these molecules with alkali-modified surfaces, especially transition metal surfaces, aiming to the identification of common characteristics and general trends. 

This backdonation of electron density from the metal surface also results in an unusually low N-N streching frequency in the a-N2 state compared to the one in the y-N2 state, i.e. 1415 cm 1 and 2100 cm"1, respectively, for Fe(l 11)68. Thus the propensity for dissociation of the a-N2 state is comparatively higher and this state is considered as a precursor for dissociation. Because of the weak adsorption of the y-state both the corresponding adsorption rate and saturation coverage for molecular nitrogen are strongly dependent on the adsorption temperature. At room temperature on most transition metals the initial sticking coefficient does not exceed 10 3. 

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

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