Surfaces reactivity

Gas-surface interactions and reactions on surfaces play a crucial role in many technologically important areas such as corrosion, adhesion, synthesis of new materials, electrochemistry and heterogeneous catalysis. This chapter aims to describe the interaction of gases with metal surfaces in terms of chemical bonding. Molecular orbital and band structure theory are the basic tools for this. We limit ourselves to metals. 

Computational chemistry has reached a level in which adsorption, dissociation and formation of new bonds can be described with reasonable accuracy. Consequently trends in reactivity patterns can be very well predicted nowadays. Such theoretical studies have had a strong impact in the field of heterogeneous catalysis, particularly because many experimental data are available for comparison from surface science studies (e.g. heats of adsorption, adsorption geometries, vibrational frequencies, activation energies of elementary reaction steps) to validate theoretical predictions. 

As explained in the previous chapters, catalysis is a cycle, which starts with the adsorption of reactants on the surface of the catalyst. Often at least one of the reactants is dissociated, and it is often in the dissociation of a strong bond that the essence of catalytic action lies. Hence we shall focus on the physics and chemistry involved when gases adsorb and dissociate on a surface, in particular on metal surfaces. 

The AIMD models of the surface region represent a necessary starting point for any large-scale MD study of the interface. The limited information available on the atomistic structure of the glass substrate complicates the assessment of the accuracy of surface models obtained by classical force fields. The AIMD models, albeit of limited size, are an essential reference to define the stability of typical surface sites, that should be at least qualitatively reflected in any larger model produced by classical MD simulations. Whereas the latter are generally not suitable to investigate surface 

If the redox mediator is a hydrophobic species, the ET at the monolayer or bilayer modified electrodes may not occur by tnnneling, but rather by diffusion through the pinholes with ET at the free sites of the electrode. This was confirmed by SECM studies of ET reactions of ferrocenemethanol at tetradecanethiol SAMs on the gold surface and at the bilayer formed by phospholipid adsorption on the first tetradecanethiol layer (106). 

The tip current is recorded as a function of the tip position in the jc, y plane. When the tip scans above electroactive sites, higher tip currents will be observed because positive feedback occurs at these sites. Thus, SECM mapping of the substrate surface should allow one to identify electroactive sites on the metal surface. The feedback mode has been used to study the heterogeneous redox activity of aluminum alloys, such as AA2024. The SECM image showed locally high redox activity, which was attributed to second phase, inter-metallic inclusions (112). 

Localized corrosion usually involves change in pH and chloride ion concentration around corrosion pits. Therefore, potentiometric SECM tips can be used to study corrosion processes. When a pH microsensor was used as the SECM tip for the in situ measurements during localized corrosion of stainless steel, it was found that pH decreases at the pit initiation stage and increases during pit growth and repassivation (113). 

Alternating current scanning electrochemical microscopy (AC-SECM) was recently used to detect precursor sites for localized corrosion on lacquered tinplates (114). AC-SECM utilizes the effect of an increasing (decreasing) solution resistance as the SECM tip approaches an insulator (conductor) for mapping domains of different conductivity/electrochemical activity on surfaces immersed in electrolytes. It was demonstrated that AC-SECM could be used to visualize microscopic cracks and holes in the coating of the lacquered tinplates. 

A probe consisting of an optical fiber coated with gold was used as the SECM tip to image simultaneously the electrochemical and photoelectrochemical activity of pitting precursor sites on an oxide film (11b, 115) by combining SECM with scanning photo-electrochemical microscopy (116). It was found that the area of high electrochemical activity is correlated with lower substrate photocurrent in the area. 

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Pisani C 1993 Embedded-cluster techniques for the quantum-mechanical study of surface reactivity J. Mol. Catal. 82 229... 

Reactive Hquid infiltration (45,68,90,93,94) is similar to the CVI process used to make RBSN. Driven by capillarity, a reactive Hquid infiltrates a porous preform and reacts on free surfaces. Reactive Hquid infiltration is used to make reaction bonded siHcon carbide (RBSC), which is used in advanced heat engines and as diffusion furnace components for semiconductor wafer processing. 

The data obtained up to the present time show that the method of catalyst preparation by the reaction of organometallic compounds with surface reactive groups may be applied to generate both isolated ions of transition metals (in various valent states) or superfine metal particles on the surface of the support. 

However, the experimental evidence collected during recent years, concerning mostly the nickel-copper alloy systems, complicated this almost currently accepted interpretation of the alloy catalytic behavior (45). Chemisorptive and subsequent catalytic phenomena appeared to require a different approach for elucidation. The surface reactivity had to be treated as a localized quality of the atoms at the interface, influenced by their neighbors in the crystal lattice (78-80). A detailed general discussion of catalysis on alloys is beyond the scope of this review. In the monograph by Anderson (81) and in the review by Moss and Whalley (82), recently published, a broad survey of the catalytic reactivity of alloys may be found. 

The only metal for which Ea > and (Prefer to the same sample is Hg, because of its liquid state. However, even in this case the situation is not settled, as discussed in Section I, since 0 values obtained several years ago are regarded with suspicion by surface physicists. Nevertheless, recent measurements46 even in ambient gases point to a reproducibility of the accepted value of 0 for Hg that cannot be simply occasional, as implied in the criticisms of detractors. Another metal in the same situation as Hg would be Ga, but in this case its surface reactivity toward oxygen and its solid state at room temperature are complications that make its 0 value less reliable, although acceptably reproducible. 

R.A. Marbrow, and R.M. Lambert, Chemisorption and surface reactivity of nitric oxide on clean and sodium-dosed Ag(110), Surf. Sci. 61, 317-328 (1976). 

Whether they are called surfaces or interfaces, when the zones between parts of a structure are "thin"— from a fraction of a micrometer (the limit of the ordinary microscope) down to molecular dimensions—the matter in them assumes a character that is somewhat different from that seen when the same matter is in bulk form. This special character of a molecular population arranged as an interfacial zone is manifested in such phenomena as surface tension, surface electronic states, surface reactivity, and the ubiquitous phenomena of surface adsorption and segregation. And the stmcturing of multiple interfaces may be so fine that no part of the resulting material has properties characteristic of any bulk material the whole is exclusively made up of transition zones of one kind or another. 

An important parameter for surface reactivity is the density of atoms in the surface. The general rule of thumb is that the more open the surface, the more reactive it is. We return to this effect in much more detail in Chapter 6. Note that (110) is the most open basal plane of an fee crystal, whereas (111) exhibits the closest packing. For bcc crystals the order is the opposite, i.e. (Ill) is the most open and (110) the most packed. 

By combining the results of the Newns-Andersons model and the considerations from the tight binding model it is now possible to explain a number of trends in surface reactivity. This has been done extensively by Norskov and coworkers and for a thorough review of this work we refer to B. Hammer and J.K. Norskov, Adv. Catal. 45 (2000) 71. We will discuss the adsorption of atoms and molecules in separate sections. 

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