Adsorption kinetic trapping

Another contribution to variations of intrinsic activity is the different number of defects and amount of disorder in the metallic Cu phase. This disorder can manifest itself in the form of lattice strain detectable, for example, by line profile analysis of X-ray diffraction (XRD) peaks [73], 63Cu nuclear magnetic resonance lines [74], or as an increased disorder parameter (Debye-Waller factor) derived from extended X-ray absorption fine structure spectroscopy [75], Strained copper has been shown theoretically [76] and experimentally [77] to have different adsorptive properties compared to unstrained surfaces. Strain (i.e. local variation in the lattice parameter) is known to shift the center of the d-band and alter the interactions of metal surface and absorbate [78]. The origin of strain and defects in Cu/ZnO is probably related to the crystallization of kinetically trapped nonideal Cu in close interfacial contact to the oxide during catalyst activation at mild conditions. A correlation of the concentration of planar defects in the Cu particles with the catalytic activity in methanol synthesis was observed in a series of industrial Cu/Zn0/Al203 catalysts by Kasatkin et al. [57]. Planar defects like stacking faults and twin boundaries can also be observed by HRTEM and are marked with arrows in Figure 5.3.8C [58],... 

The concurrent effects of adsorption, solvent evaporation, and capillary forces can, however, lead to kinetically trapped conformations (see below). The question arises whether and under what conditions an equilibrium 2D conformation can be achieved. 

Finally, strong irreversible hysteresis has been observed in H2 adsorption/ desorption measurements on Ni2(4,4 -bipy)(N03)4 by the groups of Thomas and Rosseinsky," even to very low desorption pressures. This results from kinetic trapping of H2 molecules which can then only be released by raising the temperature to IlOK and above. 

The adsorption kinetics are found to be completely different for the two molecules, AHS and DHS. The rate of dissolution of molecules into solution from fully formed monolayers at room temperature is negligible, so equilibrium carmot be established by desorption and readsorption of monolayer components in the complete mono-layer. Equilibration could proceed through the physisorbed thiol. Rapid equilibration between the physisorbed molecule and the molecules in solution would be followed by relatively slow conversion of the physisorbed thiols to chemisorbed thiolates. If the rate constant for conversion of thiol to surface thiolate is independent of the structure of the thiol, which is likely, a chemisorbed layer would be kinetically trapped. The adsorption rate constant would then be determined by the equilibration between the physisorbed thiol and the thiols in solution, which might explain the observed large differences between AHS and DHS. A similar argument has been used by Bain et al. to explain the observed composition of mixed monolayers. This would explain the major role of the solvent in the adsorption kinetics and possibly also in the resulting film structure. A further consideration is the expected hy-... 

The Kratky-Porod formula may not be applicable for long molecules with L Ip, for which excluded volume interactions should be taken into account. Intramolecular excluded volume effects result from repulsion between segments within the same molecule, which result in an increase of the end-to-end distance. These effects are particularly strong for 2D systems, which demonstrate an increased density of segments and do not permit chain crossings. Both methods require complete visualization of a statistical ensemble of single molecules in order to determine L, 0, and (R ). In addition, they assume the observation of molecules in their natural state, in which molecules are not constrained and freely fluctuate around their equilibrium conformation. The concurrent effects of adsorption, solvent evaporation, and capillary forces can, however, lead to kinetically trapped conformations. The question arises whether and under what conditions an equilibrium 2D conformation can be achieved. 

Most rare gas separations will commence with total adsorption of all the gases of interest in an adsorbent trap. In this manner one may minimize problems arising from non-uniform introduction of sample, temperature variations I and adsorption kinetics by using lower temperatures and more adsorbent than should really be necessary. By treating all adsorption steps in this manlier one tends to reduce losses,. since in subsequent desorption steps slow processes may affect the history of the separation in detail but will not result in actual loss of sample. 

Schneider, H.M., Granick, S. Kinetic traps in polymer adsorption—exchange of polystyrene between the adsorbed state and a good solvent. Macromolecules 25, 5054 (1992)... 

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