Metal dissolution transition time

Regarding the results discussed above, the interesting aspect of these experiments is that the front velocities took on a constant value. Some data can be seen in Fig. 59. The first three examples show activation fronts in the bistable regime of Fe, Au, and Zn dissolution, respectively the last two curves display examples of pulses in an excitable regime, again for metal dissolution reactions, hi all examples, two stationary electrodes were used to probe the local potential. The velocity of the fronts or pulses were extracted from the time difference at which the transitions were measured at the two probes. In all five examples, the readings of the two probes seem to be just time-shifted versions of each other. This indicates that the structures propagate with constant shape and velocity. 

Nevertheless, there seems to be sufficient evidence that electropolishing is an essentially transport-controlled process. Thus, the transition time, r, is observed under galvanostatic conditions, with the Sand product, being constant for a given electrolyte and reciprocal with the square root of its viscosity. Yet the Faradaic efficiency is found to be virtually 100%, and hence there can be no doubt that there is no other process leading to electropolishing except anodic dissolution of the metal. 

The analysis by chronopotentiometry of a solution of titanium salt shows the existence of a potential plateau in the potential range -2.2, -2.35 V (Fig. 5). The length of the plateau (transition time, r) depends on the current intensity and on the concentration of titanium ions. In agreement with the Sand s law, it is shown that r is proportional to the reverse of the square of the current intensity. The reactions involving metallic titanium were studied by the current reversal technique which is useful for analysing the deposition and dissolution process (Fig. 6). The two bumps at the beginning and at the end of the chronopotentiogram are due to the reaction Ti " + — Ti +. These additional plateaux occur in the same potential... 

It is known that dissolving anodes begin to passivate when the value of J exceeds a critical value (J, ) which depends on the nature of the metal, the composition of the electrolyte, and temperature. According to Savchenkov and Uvarov (11), at current densities below of the metal, all the anodic current is spent on the dissolution of the metal, while at current densities > the metal may be in the active state only a limited time, t, before the metal changes to passive state. Usually the higher the current density of the anode, the smaller the value of t, which Is referred to as the transition time. The length of the transition time depends on the ratio between the rates of formation and removal of the passive layer products. Most investigators present this relationship as ... 

When studying the growth kinetics of the intermetallic layers, after the run the crucible, together with the flux, the melt and the solid specimen, was shot into cold water to arrest the reactions at the transition metal-aluminium interface. Note that the solid specimen continued to rotate until solidification of the melt, ft is especially essential in examining the formation of the intermetallic layers under conditions of their simultaneous dissolution in the liquid phase (with undersaturated aluminium melts). The time of cooling the experimental cell from the experimental temperature down to room temperature did not exceed 2 s. 

Quite recently oxonium compounds of d-block transition metals and also closely related complexes of the lanthanides were isolated for the first time. In general, the solvent is aHF, and a strong Lewis acid, preferably AsFs, is added to the solution or suspension of an appropriate metal compound. Water may be introduced in various ways, e.g. using hydrated starting material, dissolution of metal oxides or even through the addition of H-.OAsFg. Some examples of reactions leading to new oxonium fluorometallates are collected in Table 4. 

Summary. Time-resolved, atomic-scale, in-situ STM studies of phase formation at metal electrode surfaces are described. Examples include structural phase transitions within the electrode surface layer and in anionic or metallic adsorbate layers as well as metal deposition and dissolution processes. 

We have reviewed the family of dealloyed Pt-based nanoparticle electrocatalysts for the electroreduction of oxygen at PEMFC cathodes, which were synthesized by selective dissolution of less-noble atoms from Pt alloy nanoparticle precursors. The dealloyed PtCua catalyst showed a promising improvement factor of 4-6 times on the Pt-mass ORR activity compared to a state-of-the-art Pt catalyst. The highly active dealloyed Pt catalysts can be implemented inside a realistic MEA of PEMFCs, where an in situ voltammetric dealloying procedure was used to constructed catalytically active nanoparticles. The core-shell structural character of the dealloyed nanoparticles was cmifirmed by advanced STEM and elemental line profile analysis. The lattice-contracted transition-metal-rich core resulted in a compressive lattice strain in the Pt-rich shell, which, in turn, favorably modified the chemisorption energies and resulted in improved ORR kinetics. 

Stamenkovic et al. [48] recently found that on PtsNi, the O2 reduction reaction is 90 times faster than on pure Pt. Unfortunately, dissolution of the transition metal alloyed in the PtM eatalysts is a major drawback because these transition metals are eleetroehemieally soluble at a potential range between 0.3 to 1 V vs. NHE in low pH media [47]. More effort is needed to solve this problem. 

Most recently, R alloy catalysts that have the so-called structure of a R skin, i.e., with a pure R topmost atomic layer on the surface of the alloys, were reported to be the most aetive catalysts towards the ORR [14—17]. Stamenkovic et al. [14] studied the surface properties of R-M (M = Co, Ni, Fe) polycrystalline alloys prepared by sputtering, aimealing, and leaching, respectively. They found two kinds of surface structure on the alloys, depending on the preparation procedure. The merely sputtered alloys could form R-skeleton outermost layers due to the dissolution of transition metal atoms in acid electrolyte, whereas the aimealed alloys had a R-skin topmost layer containing only R. The eatalytie activity towards the ORR on these two surfaces was much higher than that on a pure polycrystalline R surface, and the Pt-skin surface displayed the highest activity. In particular, the RsNi alloy with the (111) face was 10 times more active than the... 

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