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Surface defect sites

Although surface defect sites are involved in the initiation of pores they do not determine the density and dimension of the pores in the bulk PS. The bulk morphology of PS is determined by the property of semiconductors and anodization conditions. However, under certain conditions, such as using surface patterning to generate initiation sites, the bulk PS morphology can be controlled to some extent. [Pg.202]

The adsorption site, i.e. the chemisorption position of the adatoms on (within, below) the substrate surface, thanks to the polarisation dependence of SEXAFS. Often a unique assignment can be derived from the analysis of both polarisation dependent bond lengths and relative coordination numbers. The relative, polarisation dependent, amplitudes of the EXAFS oscillations indicate without ambiguity the chemisorption position if such position is the same for all adsorbed atoms. More than one chemisorption site could be present at a time (surface defect sites or just several of the ideal surface sites). If the relative population of the chemisorption sites is of the same order of magnitude, then the analysis of the data becomes difficult, or just impossible. [Pg.98]

In considering photoactivity on metal oxide and metal chalcogenide semiconductor surfaces, we must be aware that multiple sites for adsorption are accessible. On titanium dioxide, for example, there exist acidic, basic, and surface defect sites for adsorption. Adsorption isotherms will differ at each site, so that selective activation on a particular material may indeed depend on photocatalyst preparation, since this may in turn Influence the relative fraction of each type of adsorption site. The number of basic sites can be determined by titration but the total number of acidic sites is difficult to establish because of competitive water adsorption. A rough ratio of acidic to basic binding sites on several commercially available titania samples has been shown by combined surface ir and chemical titration methods to be about 2.4, with a combined acid/base site concentration of about 0.5 mmol/g . [Pg.79]

The mechanism of C02 reduction to methane at Cu electrodes has been proposed by various groups [72-74], most of which involved the splitting of adsorbed CO followed by the hydrogenation of surface C atoms. When DeWulf et al. used X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy to study the reaction [72], they observed surface-bound carbenes (Cu CH2) as an intermediate in the system. Likewise, others used both in situ infrared (IR) reflection absorption spectroscopy and surface-enhanced Raman spectroscopy to observe the initial product of C02 reduction on Cu [74]. Typically, two different linearly bound CO species were identified and attributed to adsorption on either surface defect sites or terraces. [Pg.301]

Theory indicates that supported nanoclusters typified by Os5C on MgO are bonded more strongly at surface defect sites than at defect-free sites. [Pg.73]

Calculations suggest that C—C recombination reactions to form graphene should occur preferably on terraces, whereas CH —CzHy recombinations should be favored on the rims of step edges. Experimental observations of a working cobalt catalyst by FM-RAIRS (17) are in line with the computational results the reduction in surface coverage by CO adsorbed on surface defect sites occurs concomitantly with increasing deactivation. [Pg.175]

A small amount of copper in the mixed oxide facilitates the hydrogenation of the nickel oxide. As discussed in Chapter 12, reduction of bulk nickel oxide was facilitated by the presence not only of copper but also of silver. It would seem, then, that if silver were present in the coprecipitated nickel-aluminum oxide, it should also increase the rate of nickel oxide hydrogenation. High nickel areas were obtained when the partial pressure of water in the reducing gas was kept low.3>9 The presence of a small amount of aluminum oxide in the reduced nickel particles distorts the crystallites and produces more of the catalytically active surface defect sites.5... [Pg.269]

Diffusion at 1000-1100°C gives erratic results while causing damage to the Si surface. Random pitting results, suggesting enhanced reactions at surface defect sites. In the presence of low concentrations of O2, additional vapor phase reactions occur ... [Pg.200]

Single tetralactam macrocycles on Au(lll) surfaces apparently only build disordered closed films, and clustering at step edges and surface defect sites occurs [145],... [Pg.376]

Framework substitution of Al, Si or P by a redox metal ion leads, in general to more stable redox molecular sieves (Figure 4). So-called isomorphous substitution, in udiich the metal ion is coordinated tetrahedrally by four oxygen atoms should be possible when the cuion/ oxygn between 0.22S and 0.414 [25]. It should be noted, however, tiiat the oxidation state of the metal and, hence, structure and charge of tire frameworic, may change substantially when an as-synthesized material is calcined. For example, Cr-substituted sieves generally contain Ci in the as-synthesized material but on calcination this is transformed to Cr. Since the latter contains two extra-framework Cr == O bonds it can only be anchored to a surface defect site rather than isomorphously substituted. By the same token, as-synthesized... [Pg.157]

Calculations have shed light on the mechanism of formation of silver particles by the action of light on silver halide. We find that surface defect sites present a particularly favorable trapping level for electrons. After this trapping step, silver ions diffuse to the electron, forming a deeper trap for additional electrons. [Pg.59]

Water and Methanol The interaction of H20 with the (100) surface of MgO prepared by polishing and ion-bombarded single-crystal surfaces has been studied using X-ray photoelectron spectroscopy (XPS) both before and after annealing at 900 K (Onishi et al., 1987). It was observed that at room temperature water did not adsorb on the annealed surfaces. However, ion-bombarded surfaces did display features of OH-, presumably due to the creation of surface defect sites allowing water adsorption. At 200 K, both the polished and ion-bombarded surfaces showed evidence of OH- after exposure to H20, the ion-bombarded surface being covered with a monolayer of OH- (Peng and Barteau, 1990, 1991). [Pg.130]

Finally, for attractive potentials, the concentration effect induced by imposing the tracking constraint ( reduction of dimensionality ) on the motion of the diffusing coreactant plays a relatively more important role in influencing the kinetics if the target molecule is anchored at a defect site on the surface. For repulsive ion-ion interactions, the opposite situation pertains, that is, somewhat greater enhancements in the ratio k C, Tr W)/k C W) are found when the target molecule is localized on a site maximally removed from the surface defect sites. [Pg.373]

Scamehom CA, Harrison NM, McCarthy MI (1994) Water chemistry on surface defect sites Chemidissociation versus physisorptiononMgO(lOO). J ChemPhys 101 1547-1554. [Pg.99]


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See also in sourсe #XX -- [ Pg.453 ]




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Surface defects

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