Transition metals dissolution kinetics

Kassner used a rotating disc, for which the hydrodynamic conditions are well defined, to study the dissolution kinetics of Type 304 stainless steel in liquid Bi-Sn eutectic. He established a temperature and velocity dependence of the dissolution rate that was consistent with liquid diffusion control with a transition to reaction control at 860 C when the speed of the disc was increased. The rotating disc technique has also been used to investigate the corrosion stability of both alloy and stainless steels in molten iron sulphide and a copper/65% calcium melt at 1220 C . The dissolution rate of the steels tested was two orders of magnitude higher in the molten sulphide than in the metal melt. 

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. 

As for TMCs, they will have an influence on both the thermodynamics and kinetics of reaction. The thermodynamic effed is easier to imderstand. It results from the fad that spedes normally formed by the dissolution of oxides (such as [Al(H20)6p or HxSi04) may read with transition metal complexes in solution, thus dragging the reaction of oxide dissolution to the right. Such a phenomenon seems to be present in the molybdate/alumina system . ... 

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. 

Chapters 18-21 discuss core-shell and advanced Pt alloy catalysts (which also can be considered to have a core-shell structure). Chapter 18 studies the fundamentals of Pt core-shell catalysts synthesized by selective removal of transition metals from transition metal-rich Pt alloys. Chapter 19 outlines the advances of core-shell catalysts synthesized by both electrochemical and chemical methods. The performance, durability, and challenges of core-shell catalyst in fuel cell applications are also discussed. Chapter 20 reviews the recent analyses of the various aspects intrinsic to the core-shell structure including surface segregation, metal dissolution, and catalytic activity, using DFT, molecular dynamics, and kinetic Monte Carlo. Chapter 21 presents the recent understanding of activity dependences on specific sites and local strains in the surface of bulk and core-shell nanoparticle based on DFT calculation results. 

Tafel slope and a decrease in the reaction order with respect to OH have been observed and mark the start of processes specific to the transition range of the overall active range of iron dissolution among them, the formation of crystallized ferrous and ferric solid species including anions and their blocking effect on the metal dissolution superimpose and change the mechanism and the kinetics. 

Presently, it is not known whether dissolution of a surface layer serves to create or remove active catalytic sites. In earlier work (Palmer and Drummond 1986), acetic acid solutions were exposed to a well-crystallized (yellow) titanium oxide surface and significantly slower kinetics were observed although with continuous use the surface returned to the typical blue oxide sheen, and the kinetics of decarboxylation reverted to the faster rates observed originally. Acetate forms strong complexes with transition metal ions, particularly at elevated temperatures (Palmer and Drummond 1988 Giordano and Drummond 1991). The stability of these complexes is directly dependent on acetate concentration, pH, and the inverse of ionic strength. 

If the charge transfer step of a redox process (such as Equation 1.106) is rate determining, the Butler-Volmer equation is obtained as follows. A similar equation is obtained for metal dissolution where the concentrations c(Ox) and c(Red) are replaced by c(Me +) and G g. As usual in chemical kinetics, the rate constants k contain an exponential term with the ratio of the standard activation free enthalpy A,G to RT. A,Gt is the barrier that the reacting system has to overcome to get to the transition state from which the products are formed. For a simple redox reaction, this involves the change of the coordination shell of solvent molecules and other ligands, i.e., for (Fe(CN)6) " . 

Active and Active-Passive Transition Impedance and Frequency-Resolved RRDE Measurements The assoeiation of impedanee and fiequency-resolved RRDE techniques initially introduced for iron [38,117,197] has been extensively applied to the prepassive and passivation ranges of Fe-Cr alloys with and without chloride added [174,194,195], Of course, the interpretation is more intricate than for pure metals (even with multiple dissolution valences). For kinetic reasons, Cr species are not detectable on the ring. In order to draw unambiguous conclusions, reasonable assumptions had to be made concerning simultaneous alloy dissolution at steady state (see thermodynamics and rate constant approach earlier) and dissolution valences of Cr. 

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