Cobalt deposition mechanism

Scheme 18.21 Mechanism of nickel and cobalt deposition on a supported nickel nanoparticle.

At a pH above roughly 3, cobalt is deposited through a pH-dependent mechanism by the formation of a cobalt hydroxide complex (Co(OH)2 or CoOH [9, 10]. At a constant potential, the cobalt deposition current decreases with decreasing pH, indicating an increase in the inhibition of cobalt deposit. Therefore, dendrites should be suppressed at a lower pH. At a pH below roughly 3, cobalt electrodeposition occurs through the direct reduction of Co and is practically independent of pH [10]. 

As a very important topic in contamination buildup, the question is still open to what extent the data on corrosion product solubilities in the primary coolant are of importance for the behavior of trace amounts of cobalt. It seems to be still questionable whether cobalt ferrites as a well-defined compound with properties similar to the nickel ferrites can exist under PWR primary coolant conditions, whether cobalt atoms can be incorporated into a nickel ferrite lattice or whether traces of cobalt may be deposited onto the surfaces of the reactor core by adsorption on other, already deposited oxides. Such adsorption processes may occur even on the Zircaloy oxide films in the absence of any net deposition of corrosion products. Experimental investigations of the interaction of dissolved cobalt with heated Zircaloy surfaces (Lister et al., 1983) indicated that at low crud levels in the coolant cobalt deposition on surfaces is dominated by processes involving dissolved species, with adsorption/desorption processes being the responsible mechanisms. The extent of cobalt deposition is controlled by the type of oxide present on the Zircaloy surface thin black films of zirconium oxide will pick up less cobalt from the solution than thick white oxide films, even when the differences in the available surface areas of both types of oxides are taken into account. The deposition process seems to be little affected by the heat flux in the exposed metal. According to Thornton (1992), such adsorption-desorption exchange processes provide a pathway for radioactive species to be transported around the circuit even when the net movement of corrosion products is minimized this means that under such circumstances the processes of activity transport and of corrosion product transport may be decoupled. They may provide a pathway for target nuclides such as Co to be adsorbed onto fuel rod surfaces even in a core which is virtually free of deposited corrosion product particles. 

As was mentioned above, such purely physical crud deposition mechanisms are not able to explain the relative enrichment of cobalt in the layers on the fuel rod surfaces. In Section 4.4.3.2. it was discussed that under PWR conditions dissolved... 

Observations from operating BWR plants suggest that on the surfaces wetted by high-temperature reactor water, one has to expect a deposition mechanism which is similar to that on the surfaces of a PWR primary circuit. The activation products released from the activated in-core materials, as well as from the fuel rod deposits, as dissolved ions are incorporated into the oxide layers on the austenitic out-of-core surfaces directly from the reactor water. The activated crud which is resuspended from the fuel rod surfaces is also partly deposited on the out-of-core surfaces here, colloid chemistry processes may participate in the deposition process. These corrosion products often show a higher cobalt content than the non-acti-vated corrosion products that are brought in with the feedwater. During the residence time of the particulate corrosion products on the out-of-core surfaces, this excess cobalt content is reduced therefore, the activated crud can be considered as an additional source of ionic cobalt (Alder et al., 1992). 

MOSFETT s, and silicon oxide is deposited. The source/drain positions where electrical contact is to be made to the MOSFETs are defined, using the oxide-removal mask and an etch process. For shallow trench isolation, anisotropic silicon etch, thermal oxidation, oxide fill and chemical mechanical leveling are the processes employed. For shallow source/drains formation, ion implantation techniques are still be used. For raised source/drains (as shown in the above diagram) cobalt silicide is being used instead of Ti/TLN silicides. Cobalt metal is deposited and reacted by a rapid thermal treatment to form the silicide. Capacitors were made in 1997 from various oxides and nitrides. The use of tantalmn pentoxide in 1999 has proven superior. Platinum is used as the plate material. 

Although the FTS is considered a carbon in-sensitive reaction,30 deactivation of the cobalt active phase by carbon deposition during FTS has been widely postulated.31-38 This mechanism, however, is hard to prove during realistic synthesis conditions due to the presence of heavy hydrocarbon wax product and the potential spillover and buildup of inert carbon on the catalyst support. Also, studies on supported cobalt catalysts have been conducted that suggest deactivation by pore plugging of narrow catalyst pores by the heavy (> 40) wax product.39,40 Very often, regeneration treatments that remove these carbonaceous phases from the catalyst result in reactivation of the catalyst.32 Many of the companies with experience in cobalt-based FTS research report that these catalysts are negatively influenced by carbon (Table 4.1). 

Carbon formation/deposition is a difficult deactivation mechanism to characterize on cobalt-based FTS catalysts. This is due to the low quantities of carbon that are responsible for the deactivation (<0.5 m%) coupled with the presence of wax that is produced during FTS. Furthermore, carbon is only detrimental to the FT performance if it is bound irreversibly to an active site or interacts electronically with it. Hence, not all carbon detected will be responsible for deactivation, especially if the carbon is located on the support. 

Moodley, D. J., van de Loosdrecht, J., Saib, A. M., Overett, M. J., Datye, A. K., and Niemantsverdriet, J. W. 2009. Carbon deposition as a deactivation mechanism of cobalt-based Fischer-Tropsch synthesis catalysts under reahstic conditions. Appl. Catal. A, 354 102-10. 

Bartholomew and coworkers32 described deactivation of cobalt catalysts supported on fumed silica and on silica gel. Rapid deactivation was linked with high conversions, and the activity was not recovered by oxidation and re-reduction of the catalysts, indicating that carbon deposition was not responsible for the loss of activity. Based on characterization of catalysts used in the FTS and steam-treated catalysts and supports the authors propose that the deactivation is due to support sintering in steam (loss of surface area and increased pore diameter) as well as loss of cobalt metal surface area. The mechanism of the latter is suggested to be due to the formation of cobalt silicates or encapsulation of the cobalt metal by the collapsing support. 

Alternatives to activated tungsten wire emitters are also known, but less widespread in use. Cobalt and nickel [44,47] as well as silver [48] can be electrochemi-cally deposited on wires to produce activated FD emitters. Mechanically strong and efficient emitters can be made by growing fine silicon whiskers from silane gas on gold-coated tungsten or tantalum wires of 60 pm diameter. [45] Finally, on the fracture-surface of graphite rods fine microcrystallites are exposed, the sharpness of which provides field strengths sufficient for ionization. [49]... 

Although the concept of phase is well defined thermodynamically, here phase refers to a mechanically separable homogeneous part of an otherwise heterogeneous system. The concept of phase change refers here to a change in the number present or in the nature of a phase or phases as a result of an imposed condition such as temperature or pressure. To clarify and illustrate the topic at hand, we use the specific cases of electrolessly deposited nickel and electrodeposited cobalt. 

The influence of benzylidene acetone on the electrodeposition mechanism of Zn-Co alloy was investigated [436]. A relationship between corrosion resistance, microstructure, and cobalt content in Zn-Co alloys was investigated [437] using X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy [438]. The role of vitreous carbon, copper, and nickel substrates in Zn-Co deposition from chloride bath was analyzed [439]. 

One group of NADH oxidants, which does not fit the proposed reaction scheme in Fig. 2.4 are the metal complexes. Examples of this type include nickel hexacyanoferrate deposited on porous nickel electrodes [29], gold electrodes modified with cobalt hexacyanoferrate films [30] and adsorbed l,10-phenanthroline-5,6-dione complexes of ruthenium and osmium [31]. It is unclear how these systems work and no mechanism has been proposed to date. It may be worth noting that dihydronicotinamide groups have been shown to reduce aldehydes in a non-enzymatic reaction when the reaction is catalysed by zinc, a metal ion [15]. In a reaction between 1,10-phenanthroline-2-carboxaldehyde and N-propyl-l,4-dihydronicotinamide, no reaction was seen in the absence of zinc but when added to the system, the aldehyde was reduced and the nicotinamide was oxidised. This implies that either coordination to, or close proximity of, the metal ion activates... 

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