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Surface vacancy

Dehydrogenation is considered to occur on the corners, edges, and other crystal defect sites on the catalyst where surface vacancies aid in the formation of intermediate species capable of competing for hydrogen with ethylbenzene. The role of the potassium may be viewed as a carrier for the strongly basic hydroxide ion, which is thought to help convert highly aromatic by-products to carbon dioxide. [Pg.198]

The presence of a defect in the lattice (impurity, surface, vacancy...) breaks the symmetry and induces perturbations of the electronic structure in its vicinity. Thus it is convenient to introduce the concept of local density of states (LDOS) at site i ... [Pg.373]

The notion of point defects in an otherwise perfect crystal dates from the classical papers by Frenkel88 and by Schottky and Wagner.75 86 The perfect lattice is thermodynamically unstable with respect to a lattice in which a certain number of atoms are removed from normal lattice sites to the surface (vacancy disorder) or in which a certain number of atoms are transferred from the surface to interstitial positions inside the crystal (interstitial disorder). These forms of disorder can occur in many elemental solids and compounds. The formation of equal numbers of vacant lattice sites in both M and X sublattices of a compound M0Xft is called Schottky disorder. In compounds in which M and X occupy different sublattices in the perfect crystal there is also the possibility of antistructure disorder in which small numbers of M and X atoms are interchanged. These three sorts of disorder can be combined to give three hybrid types of disorder in crystalline compounds. The most important of these is Frenkel disorder, in which equal numbers of vacancies and interstitials of the same kind of atom are formed in a compound. The possibility of Schottky-antistructure disorder (in which a vacancy is formed by... [Pg.2]

A silicon surface, no mater how well it is prepared, is not perfectly flat at the atomic scale, but has surface defects such as surface vacancies, steps, kinks sites, and dopant atoms. The dissolution of the surface is thus not uniform but modulated at the atomic scale with higher rates at the defects and depressed sites. The micro roughness of the surface will increase with the amount of dissolution due to the sensitivity of the reactions to surface curvature associated with the micro depressed sites. These sites, due to the higher dissolution rates, will evolve into pits and eventually into pores. Depending on the condition, a certain amount of dissolution is required before the initiation of pores on all types of materials. [Pg.201]

Fig. 4.22 On the surface of a solid, there are a wide variety of atomic processes. A formation of a surface vacancy-adatom pair, or their recombination B association or dissociation of adatoms with an atomic cluster and cluster diffusion C diffusion of a surface vacancy, especially toward the lattice step D falling off a lattice step of an adatom E diffusion of a substitutional or interstitial impurity atom and its interaction with an adatom F diffusion of an adatom and its long range interactions with other adatoms G diffusion, dissociation and activation of a ledge atom H dissociation and activation of a kink atom into an adatom, a ledge atom, or an adatom on the layer above. Fig. 4.22 On the surface of a solid, there are a wide variety of atomic processes. A formation of a surface vacancy-adatom pair, or their recombination B association or dissociation of adatoms with an atomic cluster and cluster diffusion C diffusion of a surface vacancy, especially toward the lattice step D falling off a lattice step of an adatom E diffusion of a substitutional or interstitial impurity atom and its interaction with an adatom F diffusion of an adatom and its long range interactions with other adatoms G diffusion, dissociation and activation of a ledge atom H dissociation and activation of a kink atom into an adatom, a ledge atom, or an adatom on the layer above.
Adatoms, surface vacancies, and other features of surface structure are depicted in Fig. 12.1. [Pg.223]

When the surface is initially singular, the required ledges can be nucleated by the clustering of supersaturated surface vacancies in monolayer surface cavities or... [Pg.291]

Free charge carriers generated upon optical excitation either get trapped at the surface vacancies or undergo charge recombination (1) to (3). [Pg.311]

From the numbers that are mentioned above it is obvious that the direct observation of surface vacancies is a formidable, if not an impossible, experimental challenge. The very combination of a low density... [Pg.352]

Two alternative approaches exist. The first one involves significantly lowering the temperature to values where the diffusion of vacancies can be observed with a technique like STM. At lower temperatures a surface vacancy can then be artificially created by ion bombardment or direct removal of an atom by the tip. This approach has been applied successfully to several semiconductor surfaces [29-31]. For metal surfaces, although vacancy creation at a step by direct tip manipulation of the surface has been demonstrated [32], to our knowledge, no studies have been published where the diffusion of artificially created vacancies in a terrace has successfully been measured. The second approach involves the addition of small amounts of appropriate impurities that serve as tracer atoms in the first layer of the surface [20-24]. The presence and passage of a surface vacancy is indirectly revealed by the motion of these embedded atoms. If one seeks to measure both the formation energy and the diffusion barrier of surface vacancies explicitly, a combination of these two approaches is needed. [Pg.353]

Diffusion of embedded atoms through exchange with surface vacancies. [Pg.354]

Figure 4 A ball-model (top view) of a diffusion event in which the passage of a surface vacancy leads to a multi-lattice-spacing displacement of the indium atom (bright). The arrow indicates the random walk pathway of the vacancy, and the indium-vacancy exchanges are marked with crosses to show the pathway of the indium between its beginning and endpoints. Figure 4 A ball-model (top view) of a diffusion event in which the passage of a surface vacancy leads to a multi-lattice-spacing displacement of the indium atom (bright). The arrow indicates the random walk pathway of the vacancy, and the indium-vacancy exchanges are marked with crosses to show the pathway of the indium between its beginning and endpoints.
At temperatures close to RT, on most surfaces, it is well established that steps are the sole sources and sinks for adatoms [6]. Although never experimentally proven, the same can be expected for surface vacancies. In fact, this assumption was already explicit in the development of the numerical model that we presented in the previous section. In this section, we review data that directly supports this idea, again limiting ourselves to the case of In/Cu(00 1). These data justify the approach that was used in the previous section. [Pg.363]

If we assume that the steps are indeed the sources and sinks for surface vacancies and we confine ourselves to the simplest case where there is no interaction between the vacancy and the tracer atom, the recombination probability of a vacancy, prec, introduced in the previous section, will decrease with increasing distance from a step. This is schematically illustrated in Fig. 10. This decrease in prec with distance from a step allows us to experimentally verify whether the steps are indeed the sole sources and sinks for vacancies. The experimental verification consists of the following. Assume that we are tracking the motion of an embedded atom somewhere in a terrace, a given distance away from a step. Once a vacancy has formed at the step, has diffused to the embedded atom, and has had an initial exchange with the atom (i.e. in our measurements we observe the embedded atom to make a jump), the probability for it to have further encounters with the same vacancy is determined by the value 1 — prec. Since prec decreases with increasing distance to the step, the vacancy will on average have more encounters with the tracer atom the further it is away from the step. This will cause the atom... [Pg.363]

Figure 10 A schematic illustration of the effect of the presence of a step on the diffusion of a surface vacancy, (a) Schematic topography, with a step in the middle, (b) The recombination probability depends logarithmically on the distance, (c) Random walks that bring the vacancy far from the step will result on average in a much larger number of encounters with a tracer atom on the terrace than shorter random walks. Figure 10 A schematic illustration of the effect of the presence of a step on the diffusion of a surface vacancy, (a) Schematic topography, with a step in the middle, (b) The recombination probability depends logarithmically on the distance, (c) Random walks that bring the vacancy far from the step will result on average in a much larger number of encounters with a tracer atom on the terrace than shorter random walks.
Our measurements indeed show the expected logarithmic increase in mean square jump length with increasing distance away from a step, and provide confirmation that steps are indeed the sole sources and sinks for surface vacancies. Having established the role of the steps as sources and sinks for vacancies we speculate on how the vacancies are formed at the steps. One can envision several different mechanisms at steps that lead to the formation of a surface vacancy. All of them involve removing an atom from a kink, a step or (least likely) from the terrace itself. Since the attachment and detachment of atoms from kinks is often the energetically least costly way to detach an atom from a step, this appears to be the most likely initial process in the formation of a surface vacancy. Calculations on this problem have been performed in the context of the observations on Mn/Cu(00 1) that were mentioned previously [48-50]. In these calculations it was indeed found that the formation of step vacancies at kinks and the subsequent diffusion of these vacancies along a step and their release into the terrace is the most likely scenario for the creation of surface vacancies in a terrace. [Pg.364]

By definition, the rate at which the tracer atom is displaced by a surface vacancy is the product of the vacancy density at the site next to the tracer times the rate at which vacancies exchange with the tracer atom. For the case where the interaction between the tracer atom and the vacancy is negligible, the activation energy obtained from the temperature dependence of the total displacement rate equals the sum of the vacancy formation energy EF and the vacancy diffusion barrier ED. When the measurements are performed with finite temporal resolution and if there is an interaction present between the vacancy and the indium atom, this simple picture changes. [Pg.365]

Surface vacancies were shown to be responsible for the motion of embedded In and Pd atoms in the Cu(00 1) surface. The density of surface vacancies at room temperature is extremely low, but they diffuse through the surface at an extremely high rate leading to significant diffusion rates of Cu(00 1) terrace atoms. In the STM measurements the rapid diffusion of these vacancies leads to long jumps of embedded tracer atoms. Measurements of the jump length distribution show a shape of the distribution that is consistent with the model that we discussed in Section 3. In turn, this shows that the vacancy-mediated diffusion process can be accurately described with the model that is presented in Section 3, provided that the interaction between the tracer atom and the surface vacancy is properly taken into... [Pg.368]

For the calculation of the net adsorption enthalpies of transactinides on metal surfaces the partial molar enthalpies of solution and the enthalpy of displacement are required. These values can be obtained using the semi-empirical Miedema model [66-70] and the Volume-Vacancy or Surface-Vacancy model [32,70,71]. Data for these calculations are given in [34,72,73]. [Pg.231]


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