Adsorbate-substrate rearrangement

Figure 3. Slow adsorbate-substrate rearrangement phenomena after adsorption of incomplete monolayer (peaks A1 + A2) and subsequent extended polarization at constant potential between peaks A2 and A3. STM-images recorded in 0.01 M HCIO4 + 0.005M Pb or Tl. (a) STM image recorded in the system Pb 7Ag(l 11) after 600 s extended polarization. Window size 12 nm grayscale range 0.07 nm. The voltammograms represent the voltammetric behavior before and after 600 s polarization, (b) STM image of the transformed coverage in the system n" /Ag(l 11) after 3000 s extended polarization. Window size...

The induction period could result from a more complex behavior than reduction, which can in principle be done ex situ. Appearance of the induction period in the case of nitrophenol hydrogenation over Au nanoparticles (Fig. 9.38) was ascribed to the surface restructuring induced by the adsorbed substrate. A rearrangement of the surface atoms seems to be necessary to create catalyticaUy active sites as, for example, comers or edges on the surface. 

An example of the fomiation of a new reconstmction is given by certain fee (110) metal surfaces. The clean surfaces have (1x1) synunetry, but become (2x1) upon adsorption of oxygen [16, 38]. The (2x1) synuiietry is not just due to oxygen being adsorbed into a (2 x 1) surface unit cell, but also because the substrate atoms rearrange themselves... 

It has been proposed recently [28] that static friction may result from the molecules of a third medium, such as adsorbed monolayers or liquid lubricant confined between the surfaces. The confined molecules can easily adjust or rearrange themselves to form localized structures that are conformal to both adjacent surfaces, so that they stay at the energy minimum. A finite lateral force is required to initiate motion because the energy barrier created by the substrate-medium system has to be overcome, which gives rise to a static friction depending on the interfacial substances. The model is consistent with the results of computer simulations [29], meanwhile it successfully explains the sensitivity of friction to surface film or contamination. 

In contrast, the pentavalent Sb-119 ions at the interfaces are weakly bonded to the oxide ion layer of the hematite surfaces in neutral and slightly acidic region, while in the acidic region most of the adsorbed Sb-119 ions are in the zeroth or first metal ion layers of the substrate forming Sb-O-Fe bonds. The pentavalent Sb-119 ions having once been incorporated into the surface metal ion sites retain their chemical form, even when the pH of the aqueous phase is raised above 7. Heating of suspensions at 98°C results in chemical rearrangement of the hematite surfaces to yield pentavalent Sb-119 ions in the second or deeper metal ion layers. 

When an atom or molecule is adsorbed on a surface new electronic states are formed due to the bonding to the surface. The nature of the surface chemical bond will determine the properties and reactivity of the adsorbed molecule. In the case of physisorption, the bond is rather weak, of the order of 0.3 eV. The overlap of the wave functions of the molecule and the substrate is rather small and no major change in the electronic structure is usually observed. On the contrary, when the interaction energy is substantially higher, there are rearrangements of the valence levels of the molecule, a process often denoted chemisorption. The discrete molecular orbitals interact with the substrate to produce a new set of electronic levels, which are usually broadened and shifted with respect to the gas phase species. In some cases completely new electronic levels emerge which have no resemblance to the original orbitals of the free molecule. 

This combination of techniques allows us to determine the structure of the adsorbed species while on the metal surface and after desorption into the gas phase. Furthermore, molecular rearrangements in the adsorbed overlayer as a function of both the substrate temperature and background pressure can be studied. 

LEED has been used to determine the structure of a wide variety of surfaces, including clean and reconstructed surfaces of metals and semiconductors, and atomic and molecular physisorption and chemisorption on many different substrates (see part 5). As the theoretical and experimental tools of LEED have improved, the structure of systems with larger and more complex unit cells have been determined. Successful LEED structure determinations have been carried out for systems with several molecules adsorbed in unit cells up to 16 times larger than the substrate unit cell,/1 / and for reconstructed surfaces where the structural rearrangement involves several surface layers./2/... 

In the monocyclic series, the 2,4-cyclohexadienones rearrange to photoproducts of potential synthetic value. However, for efficient 1,2-acyl migration, only a relatively small number of substrates are suitable. These must be highly substituted, for example (48). Whereas on direct excitation in methanol, cleavage to the isomeric ketenes (49) (4> 2 0.42) predominates, the remarkably stereoselective 1,2-acyl shift to the bicyclohexenone (50) is found either in trifluoroethanol or when the dienone is adsorbed on silica gel. The conversion to (50) is followed by a reversible phototransformation to the cross-conjugated dienone (51) and accompanied by aromatization to (52) to a minor extent. Such reactivity has also been verified for tetra- and penta-methylated 2,4-cyclohexadienones. The only photoreaction of the hexamethylated homolog, on the other hand, is ketene formation. ° ... 

The observation that these solute molecules rearrange on the surface agrees with the report of a rearrangement of AFP molecules on the ice surface." An early stage of adsorption can be pictured as isolated adsorbed molecules, conformationally disordered and randomly distributed on the surface of the substrate. The final stage involves close-packed adsorbents with relatively uniform molecular orientation and conformation. We speculate that initially, not all wfAFP molecules would contact the silica surface with their... 

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