Rate determining processes metal deposition

Recent reports about the microdroplets formation in the starting periods of atmospheric corrosion [15-18] show that the idea of a thin uniform water layers is not completely in accordance with the reality. It has been observed that when a water drop is on the metallic surface, formed in the place where a salt deposit existed before, microdroplets are formed around this central drop. The cathodic process takes place in these surrounding microdroplets, meanwhile the anodic process takes place in the central drop. This idea is not consistent with the proposal of an uniform water layer on the surface and it is very probable that this situation could be obtained under indoor conditions. It has been determined that microdrops (about 1 micron diameter) clusters are formed around a central drop. An important influence of air relative humidity is reported on microdrops formation. There is a critical value of relative humidity for the formation of microdroplets. Under this value no microdroplets are formed. This value could be considered as the critical relative humidity. This situation is very similar to the process of indoor atmospheric corrosion presence of humid air, deposition of hygroscopic contaminants in the surface, formation of microdrops. Water is necessary for corrosion reaction to occur, but the reaction rate depends on the deposition rate and nature of contaminants. 

In this equation, aua represents the product of the coefficient of electron transfer (a) by the number of electrons (na) involved in the rate-determining step, n the total number of electrons involved in the electrochemical reaction, k the heterogeneous electrochemical rate constant at the zero potential, D the coefficient of diffusion of the electroactive species, and c the concentration of the same in the bulk of the solution. The initial potential is E/ and G represents a numerical constant. This equation predicts a linear variation of the logarithm of the current. In/, on the applied potential, E, which can easily be compared with experimental current-potential curves in linear potential scan and cyclic voltammetries. This type of dependence between current and potential does not apply to electron transfer processes with coupled chemical reactions [186]. In several cases, however, linear In/ vs. E plots can be approached in the rising portion of voltammetric curves for the solid-state electron transfer processes involving species immobilized on the electrode surface [131, 187-191], reductive/oxidative dissolution of metallic deposits [79], and reductive/oxidative dissolution of insulating compounds [147,148]. Thus, linear potential scan voltammograms for surface-confined electroactive species verify [79]... 

Figure 5 shows the simulation of the reaction kinetic model for VO-TPP hydro-demetallisation at the reference temperature using a Be the network with coordination 6. The metal deposition profiles are shown as a function of pellet radius and time in case of zero concentration of the intermediates at the edge of the pellet. Computer simulations were ended when pore plugging occurred. It is observed that for the bulk diffusion coefficient of this reacting system the metal deposition maximum occurs at the centre of the catalyst pellet, indicating that the deposition process is reaction rate-determined. The reactants and intermediates can reach the centre of the pellet easily due to the absence of diffusion limitations. 

Figure 6 shows the influence of the bulk diffusion coefficient, Dhi on the metal deposition profiles. Obviously, by decreasing the diffusivity the metal deposition process becomes more diffusion rate-determined. With decreasing diffusivity the transport of reactant and intermediates is decreased resulting in a less deep penetration into the catalyst pellet. Therefore, the metal deposition maximum is shifted further to the exterior of the catalyst pellet. 

The influence of the pellet radius and initial pore radius are shown in Figures 7 and 8, respectively. Figure 7 shows the metal deposition profile in case of scale-up of the catalyst pellet to the industrial size. Figure 8 shows the metal deposition profile when a typical pore radius of a hydrotreating catalyst is applied in HDM of VO-TPP. Both cases show the presence of metal deposition maxima, indicating that the deposition process becomes diffusion rate-determined. 

Despite these successes, important process parameters, like bath agitation, bath constituents and particle type are disregarded. The constants k, 0 and B inherently account for these constants, but they have to be determined separately for every set of process parameters. Moreover, the postulated current density dependence of the particle deposition rate, that is Eq. (2), is not correct. A peak in the current density against the particle composite content curve, as often observed (Section III.3.H), can not be described. The fact that the peak is often accompanied by a kink in the polarization curve indicates that also the metal deposition behavior can not be accounted for by the Tafel equation (Eq. 4). Likewise, the (1-0 term in this equation signifies a polarization of the metal deposition reaction, whereas frequently the opposite is observed (Section 111.3,(0 It can be concluded that Guglielmi s mechanism... 

For small polarizations, therefore, the overvoltage o) is proportional to the current L Since the actual potential E is equal to the sum of E, which is a constant, and , it follows that when some stage in the discharge process is rate-determining, the cathode potential is a linear function of the current at low polarizations. Behavior of this kind has been observed in connection with the discharge of hydrogen ions at platinum cathodes, and also in the deposition of certain metals (see p. 463). 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

Electroless deposition of Au in KAu(CN)2 -I- HF can be controlled by both the kinetic process and the diffusion process. The deposition is a two-step process, with initial diffusion-limited deposition of the intermediate species, followed by surface-limited reduction of this species. For electroless deposition of Pt, it has been reported that the rate-determining step is the deposition on n-Si, whereas it is the dissolution of silicon on p-Si. Electroless copper deposition does not occur on Si02-covered silicon surface due to the lack of anodic dissolution of silicon In a non-HF solution, the deposition of copper on a bare silicon surface results in the formation of oxide aroimd the metal particles. In HF solutions, the deposition of copper proceeds very slowly in the dark on both p-Si and n-Si samples due to the lack of carriers. The... 

Beyond the double layer, there is a depleted region named the diffusion layer with a thickness of microns, much wider than the double layer, formed during deposition by the consumption of a particular species. Fig. 7A is a plot of the concentration of an ionic species as a function of the distance from the surface of the electrode, showing the diffusion layer. The consumption of ions because of metal deposition generates a concentration gradient that, in steady-state conditions, is essentially determined by the redox reaction rate. If the consumption of ions arriving at the surface by diffusion is very high, the concentration of ions at the surface Cs is effectively zero, and the deposition process is controlled by diffusion. If the consumption is low, then the ion concentration at the surface is different from zero and the deposition is controlled by kinetics, i.e., by the velocity of the reaction. 

The best choice of the alloying or co-deposited elements depends on which step of the latter mechanism is the rate-determining step (normally reaction (VIII)). Different results have been obtained with ternary and quaternary combinations of these metals with rhodium, molybdenum, and iridium [104,144-156], so there is a lot left to be explored with regard to this interesting process. In the next figure, the current vs. potential profiles of a positive going scan for the different platinum modified surfaces in acidic methanol solution are shown. 

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