Molecular coadsorption

Structures A and B correspond to molecular coadsorption of CO and O2 to Au2. From the two molecularly coadsorbed species, CO can readily (without barrier) bind to the end of the Au-Au axis (structure A) whereas a barrier of 0.2 eV was found for CO association from the gas phase to the Au-Au bridging site of structure B, where the Au-Au bond is significantly elongated to 3.34 A. The barrier for forming B from A via displacement of CO from the end of the complex to the Au-Au bridge is rather high (on the order of 0.9 eV). In both structures A and B, the 0-0 bond is activated to a value... 

Pd(lll)H-(3x3)-QIV 2CO QH< site LEED no no 2.6 2.2 0.0 2.2 2.2 2.2 0 0.8 7.0 0 0 2.16 0.05 0 94B4 molecular coadsorption of one flat-lying CsEIg (benzene) and 2 upright CO per cell all centered over fcc-hollow sites both with relaxed bonds (El ignored) 1st substrate layer relaxed topmost Pd-Pd interlayer spacing expanded to 2.32A (average) from bulk value of 2.25A... 

The coadsorption of oxygen as well as of other electronegative additives on metal surfaces favors in general the 7t-bonded molecular state of ethylene, as the latter exhibits, compared to the di-o bonded state, a more pronounced electron donor character and a negligible backdonation of electron density from the metal surface. 

Coadsorption and Decomposition Reactions. The final area we discuss are cases where there are coadsorbed species present, which may react, and cases where molecular adsorption converts to dissociative (or associative) products. 

Studies of coadsorption at Cu(110) and Zn(0001) where a coadsorbate, ammonia, acted as a probe of a reactive oxygen transient let to the development of the model where the kinetically hot Os transient [in the case of Cu(110)] and the molecular transient [in the case of Zn(0001)] participated in oxidation catalysis16 (see Chapters 2 and 5). At Zn(0001) dissociation of oxygen is slow and the molecular precursor forms an ammonia-dioxygen complex, the concentration of which increases with decreasing temperature and at a reaction rate which is inversely dependent on temperature. Which transient, atomic or molecular, is significant in chemical reactivity is metal dependent. 

The second stage of modeling is the introduction of solvated ionic species into the model double layer. Coadsorption of HF and water yields adsorbed HgO ions the solvation stoichiometries of ions in the first monolayer and in subsequent layers are determined. The third stage of modeling is establishment of potential control in UHV. Hydrogen coadsorption is used to deflect the effective potential of the water monolayer below the potential of zero charge. The unique ways in which UHV models can contribute to an improved molecular-scale understanding of electrochemical interfaces are discussed. 

Some implications of the results for the understanding of electrochemical processes on a molecular level are discussed. The importance of UHV work on water-ion coadsorption on metal surfaces for the understanding of electrochemical processes on a molecular level is emphasized. 

In the next example, a mixed SAM is discussed which aims to utilize photoinduced energy and electron transfer processes to create a photocurrent in an approach which is reminiscent of the natural photosynthetic process. Figure 5.33 illustrates the molecular structures of the components of interest, i.e. the molecular triad ferrocene-porphyrin-fullerene (Fc-P-C6o) and a boron dipyrrin thiol (BoDy) [67]. Mixed monolayers were generated by coadsorption onto vacuum-deposited gold... 

Self-assembly of one- or two-component alkylthiol SAMs provides a convenient tool for tailoring, at a molecular level, the chemical reactivity of a gold surface by choosing and modulating the nature and amount of surface-bound chemically reactive groups (e.g., -OH, -COOH, or -NH2) in the monolayer. Particularly, mixed alkylthiol SAMs prepared by coadsorption of alkylthiols... 

Mixed monolayers provide a useful tool for surface engineering at the molecular level. The proper spacing of chemical functionalities is a fundamental condition for an effective coupling of biomolecules,38 39 or to control surface free energy for subsequent atomic layer deposition.41 Coadsorption of alkyltrichlorosilanes with different... 

---