Surface population fraction

An interesting approach recently applied to doped nanocrystals is the heterocrystalline core-shell method commonly applied to pure nanocrystals. In a series of papers (102, 103), Mn2+ CdS nanocrystals were synthesized in inverted micelles under conditions very similar to those described above and in Figs. 8, 9, and 13. The poor luminescent properties of the resulting Mn2+ CdS nanocrystals were attributed to nonradiative recombination at unpassivated CdS surface states. From the discussion in Section I and II.C, however, it is likely that a large fraction if not all of the Mn2+ ions resided on the surfaces of these as-prepared nanocrystals as observed for Co2+ (Fig. 9). This interpretation is supported by studies in other laboratories that showed large Mn2+ surface populations in Mn2+ CdS nanocrystals grown by the same inverted micelle approach (63). Nevertheless, growth of a ZnS shell around these Mn2+ CdS nanocrystals led to an approximately ninefold increase in Mn2+ 4T 1 > 6A i... 

Dealuminification. Recent investigations have shown the importance of the dealloying of S-phase (Al2CuMg) particles on the corrosion of aluminum aircraft alloys, specifically aluminum alloy 2024-T3. In 2024-T3, the S-phase particles represent approximately 60% of the particle population. These particles are of the order of 1 pm diameter, with a separation of the order of 5 pm representing an surface area fraction of 3%.56 The selective removal of aluminum and magnesium from these particles leaves behind a porous copper particle that becomes the preferential site for oxygen reduction.57, (Corcoran)5... 

After studying Chapter 5, it will be more obvious that in order to estimate the reaction rate between CO and Oj, one needed to know the fraction of the catalyst surface populated by each of these reactants. Once a catalyst surface with its own chemistry enters the picture, some unexpected phenomena may take place. For example, attractive forces (such as van der Waals) necessary to initiate vapor-liquid transitions may enter the picture when the adsorbed molecules are in close proximity of one another. As such, a surface condensation may take place at temperatures much higher than the saturation temperatures at the given pressure. When such surface condensations occur, the surface of the catalyst is no longer a randomly populated matrix of a substrate with the adsorbates. On the contrary, the surface looks like a very ordered system (Figure III). 

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. 

Example 11.17 Uniformly sized spheres are fed to a CSTR where they undergo a reaction that consumes the surface at a constant rate of k", in meters per second. What fraction of the initial population will survive the reactor and what wiU be the average size upon exiting the reactor ... 

Fig. 8. Scattering the transition state from the surface. Measured vibrational distribution of NO resulting from scattering of laser-prepared NO(v = 15) from Au (111) at incidence = 5 kJ mol-1. Only a small fraction of the laser-prepared population of v = 15 remains in the initial vibrational state. The most probable scattered vibrational level is more than 150 kJ mol-1 lower in energy than the initial state. Vibrational states below v = 5 could not be detected due to background problems. These experiments provide direct evidence that the remarkable coupling of vibrational motion to metallic electrons postulated by Luntz et al. can in fact occur. (See Refs. 44 and 59.)...

However, in contrast to microbiological experiments and near-surface studies, modelling of sulfate reduction in pore water profiles with in the ODP program has demonstrated that natural populations are able to fractionate S-isotopes by up to more than 70%c (Wortmann et al. 2001 Rudnicki et al. 2001). Brunner et al. (2005) suggested that S isotope fractionations of around -70%c might occur under hyper-sulfidic, substrate limited, but nonlimited supply of sulfate, conditions without the need of alternate pathways involving the oxidative sulfur cycle. 

Photoemission experiments with flat surfaces revealed that atoms of lower coordination may have a different population of d-orbitals and a different local density of states (138-140). These effects have been also predicted and analyzed theoretically (94-97, 136, 137), and should be always considered. The only question is whether they manifest themselves in the chemisorption and catalytic behavior. In any case, the impression is that by making metal particles small in size, one can cause the electronic structure of a certain fraction of the metal atoms to vary more than by making a bulk solution alloy. 

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