Application to the Processes of Aluminum Deposition and Dissolution

The thoughts developed here on the link between reaction mechanism and experimental transfer coefficients have been used to assign a mechanism for A1 electrodeposition and dissolution reactions from a thf-hydride bath. An analysis of results from that work has served to clarify the confusing literature that has also given attention to the kinetics and mechanism of A1 deposition from a hydride bath.  

In the CP-rich bath type, experimental transfer coefficients were ttc= a - 0.5 0.05, while in the H -rich baths the values were ttc= 0.3 0.05 and = 0.65 0.05. The stoichiometric number for the process was calculated by coupling the charge-transfer resistance with the extrapolated exchange current density from low and high overpotential regions, respectively, and by back-reaction correction. It was determined to be 3 by both methods and for all bath compositions. It has been suggested that 

At first this would appear to be a reasonable conclusion, but upon reevaluation of how a stoichiometric number becomes incorporated into the transfer coefficients, this mechanism cannot be correct since it has been shown in this review (Section VI) that it is impossible for both non-rds electron-number coefficients to be fractional. So, although one of those could be fractional, giving, say, 1/3, the other non-rds contribution would have to be a whole number integer. With this in mind, it is evident that the experimentally derived transfer coefficients for the A1 reaction, given that they are all near to about 1/2 for both bath types, must describe the transition state of the rds. 

An interpretation of reaction mechanisms from Tafel slopes depends on the potential dependence of the surface activity or coverage, 0, of adsorbed reaction intermediates combined with that of any coupled electron-transfer step through its p. For cases where the 0,s tend to 1 and thence become potential independent, the Tafel b value can be -118 mV (i.e., P = 

It might be thought that the mechanism for A1 deposition would involve three consecutive one-electron transfer steps, which conceptually would be the most obvious pathway then cathodic as having values of ca. 0.5 (p), 1.5 (1 + P), or 2.5 (2 -i- P), would be expected with the corresponding pseudo-capacitances arising for Al(-i-I) and Al(-i-II) in-terediate species in the second or third steps, respectively. In addition, v values would be one for each of the three steps if they were individually rate determining. None of these expectations is consistent with the experimental kinetic data 

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