Hypothetical desorption mechanism

Hydrogen Storage Materials (Solid) for Fuel Cell Vehicles, Fig. 16 Hypothetical desorption mechanism for LiBH4 [35]... 

Fig. 21 A Schematics of albumin-bound poly(N-isopropylacrylamide) (PNIPAM)-grafted surface via albuminated DC iniferter. B Hypothetical action of temperature-dependent switching of protein desorption by squeezing out and mechanical motion...

Gordon and Roe paid their attention mainly to chain transfer rather than chain propagation process. Thus, in a hypothetical model the termination rate constant depends on the chain length (due to control of the termination reaction by reversible desorption of the polymer macromolecule from the catalyst surface). A two parameter expression for the MWD function was derived which, with a chain transfer agent, should imply a Q decrease toward an asymptotic value of 2 On the other hand, according to Roe, no modification of MWD could be expected if catalyst non-uniformity were responsible for the broad distribution. More recent data seem to support this latter hypothesis rather than the mechanism of Gordon and Roe. 

In this mechanism. Reaction (12.1) is the chemical reaction to form the adduct, which has a reaction rate constant of 1.0 x 10 cms, as determined by the RDE measurements in this work. Reaction (12.11) is the ORR RDS on the Ti407 electrode surface, whose rate constants are given in Table 12.1. Reaction (12.III) represents the reactions for peroxide formation. After HO2 formation, HO2 can react in one of two ways further 2-electron reduction to OH through Reaction (12.1V), or chemical desorption through Reaction (12.V) to form a free peroxide ion, which then enters into the bulk solution and can be detected by the ring electrode of the RRDE. The ORR on the Ti407 electrode has a mixed 2- and 4-electron transfer pathway and gives an overall electron transfer number of <4. The relative portion of Reaction (12.IV) can be expressed as x, and the portion of Reaction (12.V) can be expressed as (1-x). When x= 1, the mechanism will follow a totally 4-electron transfer pathway, and when x = 0, the mechanism will be a totally 2-electron pathway. If the x value is >0 and < 1, the ORR will have a mixed 2- and 4-electron transfer pathway. Note that this ORR mechanism is only hypothetical, to facilitate further discussion. More evidence is needed to validate the mechanism. 

Nevertheless, one must be aware of the fact that once concordance between any reaction scheme and the experimental data has been established, it is not a foregone conclusion that all other concepts about the iron mechanism are invalidated. First of all, any reaction scheme represents a simplified model. Inter alia, one must take into consideration that the inductive behavior exhibited by the transient response of the electrode during the step-pulse polarization as well as the first inductive loop displayed in AC measurements could be the result of superposition of hydrogen anodic desorption and some other unknown effect produced by the iron reaction itself. Finally, artifacts are always possible as long as our models are based on the existence of hypothetical intermediates. 

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