Surface rearrangements

A different approach consists of stepwise changing the adsorbent temperature and keeping it constant at each of the prefixed values Tx, Ts,. . ., Tn for a certain time interval (e.g. 10 sec), thereby yielding the so-called step desorption spectra s(81-85). The advantage of this method lies in a long interval (in terms of the flash desorption technique) for which the individual temperatures Ti are kept constant so that possible surface rearrangements can take place (81-83). Furthermore, an exact evaluation of the rate constant kd is amenable as well as a better resolution of superimposed peaks on a desorption curve (see Section VI). What is questionable is how closely an instantaneous change in the adsorbent temperature can be attained. This method has been rarely used as yet. 

Better surface stability could be achieved if the surface components are less mobile. Some success has been achieved with polyethylene and perhalogenated polymer surfaces (8,11,12,29). Crosslinking the surface should also decrease its mobility, and this is how (TD)2DPM provides added stability, Figure 3. Some surface rearrangement does appear to occur initially, but samples have retained good hydrophilicity (contact angle 45-55°) for a year. 

Surface Rearrangement of Diblock Copolymer Brushes—Stimuli Responsive Films... 

Further surface rearrangement of the silane hydrides to a mixture of [=SiH] and [=SiH2] is possible and is compatible with the observed experimental data [selected... 

There is the further possibility that the transition C/ + C(0) C(CO)b is either slow (Case 1) or fast (Case 2) in comparison with C(CO)a — CO( ) -f- C/. The rate expression to be derived is the same in either case, but the interpretation of the individual rate constant, is, in Equation (5) will be different. When Case 1 holds, ja represents the rate constant for the surface rearrangement when Case 2 holds, ja represents the rate constant for the desorption of (C0) . It is not possible, on the basis of present experimental evidence, to decide which case is operative. It is conceivable that each case will be operative but in different temperature ranges. Assuming for the moment that Case 1 holds, the general expressions given above can be simplified to... 

As in the carbon-carbon dioxide reaction, mechanisms A and B can be treated for the cases where either the surface rearrangement or desorption of the carbon-oxygen complex is the slow step. This has no effect on the discussion except that the significance of the rate constant js in Equation (10) is altered, as previously discussed. 

A heterogeneous catalytic reaction, by definition, necessitates the participation of at least one chemisorbed intermediate (54) and involves a sequence of interlinked and interdependent (55,56) steps, which include the adsorption of reactant(s), one or more surface rearrangements, and the desorption of product(s). More than one area of the solid may be active in promoting reaction the activity of such regions may vary from one crystallographic... 

Extensive theoretical work in support of both models of thermal etching was produced before 1970. The theoretical basis for the thermodynamic model was the concept of reduction in total surface energy by the preferential formation of low-energy, low-index planes. The true equilibrium shape of a crystal is the shape with the lowest surface energy, as noted by Curie (36) and Gibbs (37). The thermodynamic models provide no information regarding the process of surface rearrangement. 

The catalytic reactions which have been studied with the aid of single crystals can be divided into three types for convenience in discussion. Type I includes those reactions in which the surface rearranges to form facets, Type II those in which solid reaction products build up on the surface, and Type III those which apparently leave the surface unaffected. 

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