Size-dependent selectivity

Analyzing the products formed on small size-selected Pd (1 < n < 30) clusters deposited on MgO(lOO) thin films indicates that the surface intermediate C4H4 is produced efficiently on all cluster sizes. Thus at least two acetylene molecules are adsorbed in a ti-bonded configuration at the initial stage of the reaction [71]. The observed size-dependent selectivity may then be understood by regarding the influence of the cluster size to steer the reaction either towards the cyclotrimerization to form or towards a direct hydrogen transfer from adsorbed C2H2 to the... 

Another important factor in catalysis is the selectivity of a catalytic reaction. So far, however, information on the atom-by-atom evolution of this astonishing catalytic selectivity is still lacking. In this example, we illustrate such a size-dependent selectivity with the polymerization of acetylene on palladium nanocatalysts [46]. This reaction over supported Pd particles reveals a direct correspondence between reactivities observed on model systems and the behavior of industrial catalysts under working conditions [66]. In ultra-high vacuum (UHV) [67] as well as under high pressure, large palladium particles of typically thousands of atoms show an increased selectivity for the formation of benzene with increasing particle size [66]. In contrast, small palladium particles of typically hundreds of atoms are less selective for the cyclotrimerization, and catalyze butadiene and butene as additional products [66]. 

Association constants were determined by Cram extraction methods, which revealed that the macrocycle 50 formed a strong complex with chloride (12 800 1300 M ). Proton NMR titration experiments conducted in CD2CI2 also revealed the size dependence selectivity of the caUx[n]pyrroles where bromide was foimd to bind approximately seven times stronger to the larger caUx[6]pyrrole compared to calix[4]pyrrole (710 25M for 50 against 10 for 47) [68]. 

Candolin, U., and Voigt, H. R., 2003, Size-dependent selection on arrival times in sticklebacks why small males arrive first. Evolution 57 862-871. 

In the resting state the cell surface water exists as polarized multilayers (possibly more strongly polarized than the bulk-phase water in the cell). This polarized water then provides size-dependent selective permeability to solutes and ions by the saltatory route. The c value of the surface fixed anionic site is such that is preferred. As a result, these anionic sites offer additional routes for facile entry of ions like K by the adsorption-desorption route than, say, by Na. Nevertheless both the saltatory and the adsorption-desorption routes are open to as well as Na", only their quantitative aspects differ. In other words there are no specific routes (or gates) or Na routes (or gates). 

Many interesting applications of CD-RTP have concentrated on the determination of PAHs and nitrogen-containing heterocycles. Successful applications of CD-RTP based on host size-dependent selectivity (molecular recognition) can be envisaged. For example, in P-CD solutions, intense RTP is observed for the smaller naphthalene (which fits neatly into the jS-CD cavity) and no phosphorescence is detectable from a larger molecule such as 1-phenylnaphthalene. 

Reaction and Transport Interactions. The importance of the various design and operating variables largely depends on relative rates of reaction and transport of reactants to the reaction sites. If transport rates to and from reaction sites are substantially greater than the specific reaction rate at meso-scale reactant concentrations, the overall reaction rate is uncoupled from the transport rates and increasing reactor size has no effect on the apparent reaction rate, the macro-scale reaction rate. When these rates are comparable, they are coupled, that is they affect each other. In these situations, increasing reactor size alters mass- and heat-transport rates and changes the apparent reaction rate. Conversions are underestimated in small reactors and selectivity is affected. Selectivity does not exhibit such consistent impacts and any effects of size on selectivity must be deterrnined experimentally. 

Hollow carbon nanotubes (CNTs) can be used to generate nearly onedimensional nanostrutures by filling the inner cavity with selected materials. Capillarity forces can be used to introduce liquids into the nanometric systems. Here, we describe experimental studies of capillarity filling in CNTs using metal salts and oxides. The filling process involves, first a CNT-opening steps by oxidation secondly the tubes are immersed into different molten substance. The capillarity-introduced materials are subsequently transformed into metals or oxides by a thermal treatment. In particular, we have observed a size dependence of capillarity forces in CNTs. The described experiments show the present capacities and potentialities of filled CNTs for fabrication of novel nanostructured materials. 

Our approach is similar to that employed in research of free cluster ions in the gas phase, where various measurements are conducted on the cluster which is mass selected out of the size-distributed clusters generated by laser sputtering. Based on the chemical compositions of the isolated MFCs, we discuss the determining factors of core size in connection with the formation processes. Some core-size dependent properties of the MFCs are also presented. 

Possibly, the most difficult aspect of determining particle size is selecting initially the correct or relevant definition and then calculating a mean or average diameter to represent the complete bulk solid. To some extent, this will depend on the following. 

In the ideal collision free environment of a molecular beam, the properties of a metal cluster can be considered to be truely isolated from cluster-substrate effects. Therefore, spectroscopic methods that can selectively extract information from metal cluster beams hold great promise for illuminating diverse size dependent properties of aggregates of metal atoms in their equilibrium configuration (23). 

Turnbull and Cech [58] analyzed the solidification of small metal droplets in sizes ranging from 10 to 300 xm and concluded that in a wide selection of metals the minimum isothermal crystallization temperature was only a function of supercooling and not of droplet size. Later, it was found that the frequency of droplet nucleation was indeed a function of not only crystallization temperature but also of droplet size, since the probability of nucleation increases with the dimension of the droplet [76]. However, for low molecular weight substances the size dependence of the homogeneous nucleation temperature is very weak [77-80]. 

An interesting application in porous silica glass employs deuterium spin-lattice relaxation rates in selectively deuterated pyridine (50). The pore-size dependence shown in Fig. 20 is linear in the reciprocal of the pore radius for both the para and ortho positions, but the significant difference in the relaxation at these two positions precludes analysis by an isotropic... 

This chapter deals with the selective preparation, TEM/EXAFS/XPS characterization and catalysis of mono- and bimetallic nanowires and nanoparticles highly ordered in silica FSM-16, organosilica HMM-1 and mesoporous silica thin films. The mechanism of nanowire formation is discussed with the specific surface-mediated reactions of metal precursors in the restraint of nanoscale void space of mesoporous silica templates. The unique catalytic performances of nanowires and particles occluded in mesoporous cavities are also reviewed in terms of their shape and size dependency in catalysis as well as their unique electronic and magnetic properties for the device application. 

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