Nickel migration

XPS was utilized to determine the elemental composition and oxidation states of elements in approximately the top 50 A of the surface of a fresh membrane during heating. At room temperature, surface nickel was oxidized and the intensity of the nickel peak was low. When the membrane was heated to 650°C under vacuum, the intensity of the nickel peak increased substantially and the nickel was reduced to the metallic state. The ratios of Ni/Ba, Ni/Ce and Ni/Y at the surface increased when the temperature was increased, as shown in Table 1, but they decreased again when the membrane was cooled back to room temperature. Thus, the nickel migrated or otherwise became more apparent at the surface and preferentially resided there relative to the other elements at higher temperature. 

There is only a limited amount of information on the deactivation mechanisms and rates of vanadium and nickel migration. The formation of metal silicates and/or aluminates has been proposed, as they seem to form more easily by reduction and oxidation cycles. Rajagopalan et al. [8] confirm that methods involving cyclic redox aging of metals in the presence of sulfiir are needed for screening metals-tolerant catalyst. They propose a cyclic test (the cyclic propylene steam method), which addresses the redox aging of the metal, but not the nonuniform laydown and age distribution of metals on the catalyst. 

However, since the larger particles of oxide and metal nickel have almost the same size, it may be deduced that the particles are probably reduced without any important nickel migration. The same conclusion can be drawn for the standard impregnated INi samples aggregates of nickel oxide are reduced without significant Ni migration, m agreement with previous results [2, 6]. 

LiNio.8Coo.2O2 Commercial Cobalt assists in ordering the layered structure and suppresses the nickel migration into the lithium layer. Cobalt-conttiining derivatives suffer from the drastic conductivity changes with fithiation, similar to those observed in LiCo02 materials... 

Nl Films. This section deals with nickel migration and how it compares with the migration of Ag, Cu, and Pb. Ni films were evaporated on ceramic blank substrates leaving a 15 mil gap between two halves of the film. The gap was covered with a water drop ( 1 mm diameter) and biased at various voltages ranging from 0.5 to 15 volts. Ni dendrites formed under these conditions. Dendrite growth rates were calculated from the time that each dendrite took to bridge the gap. The ionic current density was 0.1 A/cm . 

However, the generation and migration of water in the half-ceU reactions must be considered in the cell design. At the nickel electrode ... 

The tlrermodynamic activity of nickel in the nickel oxide layer varies from unity in contact with tire metal phase, to 10 in contact with the gaseous atmosphere at 950 K. The sulphur partial pressure as S2(g) is of the order of 10 ° in the gas phase, and about 10 in nickel sulphide in contact with nickel. It therefore appears that the process involves tire uphill pumping of sulphur across this potential gradient. This cannot occur by the counter-migration of oxygen and sulphur since the mobile species in tire oxide is the nickel ion, and the diffusion coefficient aird solubility of sulphur in the oxide are both vety low. 

Replacement of halides with deuterium gas in the presence of a surface catalyst is a less useful reaction, due mainly to the poor isotopic purity of the products. This reaction has been used, however, for the insertion of a deuterium atom at C-7 in various esters of 3j -hydroxy-A -steroids, since it gives less side products resulting from double bond migration. Thus, treatment of the 7a- or 7j5-bromo derivatives (206) with deuterium gas in the presence of 5% palladium-on-calcium carbonate, or Raney nickel catalyst, followed by alkaline hydrolysis, gives the corresponding 3j3-hydroxy-7( -di derivatives (207), the isotope content of which varies from 0.64 to 1.18 atoms of deuterium per mole. The isotope composition and the stereochemistry of the deuterium have not been rigorously established. 

An older method,which also retains the double bond in its original location, utilizes the fact that thioketal formation from A" -3-ketones does not induce bond migration. Subsequent desulfurization with Raney nickel gives the A" -olefin (see section XI-D). 

Vanadium also promotes dehydrogenation reactions, but less than nickel. Vanadium s contribution to hydrogen yield is 20% to 50% of nickel s contribution, but vanadium is a more severe poison. Unlike nickel, vanadium does not stay on the surface of the catalyst. Instead, it migrates to the inner (zeolite) part of the catalyst and destroys the zeolite crystal structure. Catalyst surface area and activity are permanently lost. 

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