Reactivity of the Monolayers

The preparation of multilayers based on the same or on different structural units may take advantage of the reactivity of the monolayers and particularly of the tail groups. Different strategies have been adopted as an example, dithiol molecules lead to multilayers thanks to the formation of an S-S bond between molecules on the surface [20-22]. The defects present in a single monolayer are inherited by the monomolecular layer anchored to its tail group. Hence, the degree of order of a multilayer is usually much lower with respect to the first monolayer oti the substrate. In the majority of cases, bi- or tri-layers are prepared, although few hundreds of micron thick multilayers can be also obtained. 

It is worth noting that molecules anchored at a substrate exhibit peculiar behaviors with respect to the analogous molecules in solution. One of the most 

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Subsequently, the reactivity of oxygen present on reduced iron surfaces toward hydrogen will be considered. Experiments on single crystals of iron have demonstrated that the reactivity of the monolayer of oxygen atoms toward hydrogen is strongly dependent upon the structure of the iron metal surface. The results of the experiments on single crystals can rationalize a number of the experimental observations detailed in the other chapters. 

A key limitation to the application of the monolayer as a boundary lubricant is the sensitivity to temperature. Molecules are desorbed from the surfaces as the temperature rises, and the coverage ratio would drop to zero above 200 ° C at which the monolayer becomes completely ineffective. Boundary lubrication functions at high temperature regions rely mainly on chemical reactive hlms. 

To shed hght on the origin of the enhanced ORR activity, Xu and co-workers performed extensive DFT calculations to investigate the reactivity of the Pt skin [Xu et al., 2004], in particular how oxygen interacts in vacuum with the ordered PtsCo alloy and with a monolayer of Pt formed on the alloy as a model for Pt skin. Figure 9.10 identifies the various adsorption sites for O and O2. Experiments have shown that up to four layers of Pt could sustain a 2.5% compressive strain without creating any surface... 

For the same reason, Ru(OOOl) modihcation by Pt monolayer islands results in a pronounced promotion of the CO oxidation reaction at potentials above 0.55 V, which on unmodified Ru(OOOl) electrodes proceeds only with very low reaction rates. The onset potential for the CO oxidation reaction, however, is not measurably affected by the presence of the Pt islands, indicating that they do not modify the inherent reactivity of the O/OH adlayer on the Ru sites adjacent to the Pt islands. At potentials between the onset potential and a bending point in the j-E curves, COad oxidation proceeds mainly by dissociative H2O formation/ OHad formation at the interface between the Ru(OOOl) substrate and Pt islands, and subsequent reaction between OHad and COad- The Pt islands promote homo-lytic H2O dissociation, and thus accelerate the reaction. At potentials anodic of the bending point, where the current increases steeply, H2O adsorption/OHad formation and COad oxidation are proposed to proceed on the Pt monolayer islands. The lower onset potential for CO oxidation in the presence of second-layer Pt islands compared with monolayer island-modified Ru(OOOl) is assigned to the stronger bonding of a double-layer Pt film (more facile OHad formation). 

The cyclic voltammogram for a silver electrode in 0.1M LiC104 acetonitrile solution is shown in Figure 1 (curve a). At a potential of -1.5 V, cathodic current due to the reduction of Li+ ions commences. The upd of lithium has been reported previously by Kolb et al. for positive potential sweeps after substantial lithium reduction (i) however, due to the reactivity of the metallic lithium with impurities in solution, the adsorbed layer formed on the negative potential sweep is not as stable as other upd monolayers (i). An additional cathodic wave due to the reduction of lithium is observed at approximately -2.5V, and on the return sweep the lack of an anodic wave is indicative of the reactivity of the chemisorbed atoms. 

The recent enormous progress in preparing and studying surface science qualities of relevant systems [178-180] such as vanadium oxides plus their defects has given further clear evidence that electrophilic weakly bound oxygen [83] as well as more stable defect-related oxygen [48,181-183] do exist and exhibit a reactivity much in parallel to high-performance catalysts of the monolayer type [184, 185]. 

Chemical modification of electrode surfaces by polymer films offers the advantages of inherent chemical and physical stability, incorporation of large numbers of electroactive sites, and relatively facile electron transport across the film. Since th% polymer films usually contain the equivalent of one to more than 10 monolayers of electroactive sites, the resulting electrochemical responses are generally larger and thus more easily observed than those of immobilized monomolecular layers. Also, the concentration of sites in the film can be as high as 5 mol/L and may influence the reactivity of the sites because their solvent and ionic environments differ considerably from dilute homogeneous solutions [9]. 

Uniform accessibility and reactivity of the electrode interface are the main hypotheses for developing the EHD impedance theory. However, in many cases a real interface deviates from this ideal picture due for example either to incomplete monolayer adsorption leading to the concept of partial blocking (2D adsorption) or to the formation of layers of finite thickness (3 D phenomena). 

In most of the reactions discussed above the resulting monolayers are terminated by a methyl group. While these types of monolayers are useful for passivation and chemical stabilization, the low reactivity of the terminal group makes further manipulation of the surface physical or chemical properties difficult. In order to incorporate more complex organic or bio-organic structures at the interface, new strategies for coupling these molecules to the surface are required. 

As NO dissociation produces two atoms from one molecule, the reaction can only proceed when the surface contains empty sites adjacent to the adsorbed NO molecule. In addition, the reactivity of the molecule is affected by lateral interactions with neighboring species on the surface. Figure 4.10 clearly illustrates all of these phenomena [38]. The experiment starts at low temperature (175 K) with a certain amount (expressed in fraction of a monolayer, ML) of NO on the Rh(100) surface. During temperature programming, the SIMS intensities of characteristic ions of adsorbed species are followed, along with the desorption of molecules into the gas phase, as in temperature-programmed desorption (TPD) or temperature-programmed reaction spectroscopy (TPRS) (see Chapter 2). 

Electrochemical data indicate that self-assembled monolayers of 5 and 6 catalyze the two-electron reduction of O2 to H2O2. The monolayer from 6 is a more effective electrocatalyst for the reduction of O2 than that from 5 [300]. The different reactivity results from different interfacial architecture this is confirmed by infrared, X-ray photoelectron, and visible spectroscopic measurements [300] which revealed coplanar, inclined t -7z stacking of the porphyrin ring in the monolayer of 5 and head-to-tail orientation of the porphyrin ring in the monolayer of 6. Treatment of the monolayer of 8 with Co(OAc)2 in methanol resulted in electrocatalytic activity in the reduction of O2 [300]. In contrast, a monolayer of 7 treated similarly failed to catalyze dioxygen reduction [300], although treatment of a mixed monolayer of 7 and CH3(CH2)3SH with Co(OAc)2 results in electrocatalytic activity similar to that of 6. 

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