Nickel crystallization rate

Carbon-free operation can be obtained if the steady state activity a of carbon is less than one (1) (when the concentration of carbon dissolved in the nickel crystal is smaller than that at equilibrium), aj can be expressed by balancing the rate of dissociation of the hydrocarbons into adsorbed carbon atoms with the reaction with adsorbed oxygen. In terms of the simplified mechanism (Table 2), the steady state activity of carbon, aj, can be expressed by ... 

The decomposition of ethylene on spherical nickel crystals at higher temperature was also studied, but the results cannot be correlated with hydrogenation rates. The relative reactivities of the face are also different from those found in the decomposition of carbon monoxide on nickel. The possible catalytic importance of dislocations, as indicated by the decomposition experiments, is also discussed. 

Nickel carbide is not stable at steam reforming conditions. The nucleation of the carbon whisker takes place when the concentration of carbon dissolved in the nickel crystal is higher than that at equilibrium. This is reflected by the kinetics (3). After an induction period, the carbon growth proceeds with constant rate (Fig. 1) (8,9). The rate of dissociation depends strongly on type of hydrocarbon with olefins and acetylene being the most reactive (9). 

With rapid dissociation irom olefins and acetylene, the diffusion of carbon through the nickel crystal becomes the rate determining step which is reflected by the activation energy being close to that for the diffusion of carbon through nickel (1,2). Whereas with methane, the dissociation of the molecule becomes the rate determining step (3,10). 

The nickel particle size has an impact on the nucleation of carbon. The smaller the crystals, the more difficult the initiation of carbon formation. This result was demonstrated in TGA experiments summarised in Table 5.3 with two catalysts having the same activity but different dispersions of nickel [415] [425]. The catalysts were heated at a fixed rate, and it was shown that the onset temperature was approximately 100°C higher for the catalyst with small nickel crystals (approximately 7 nm) than for that with large crystals (approximately 100 nm). 

Surface Finish. As well as influencing the rate of metal removal, electrolytes also affect the quality of surface finish obtained in ECM. Depending on the metal being machined, some electrolytes leave an etched finish. This finish results from the nonspecular reflection of light from crystal faces electrochemicaHy dissolved at different rates. Sodium chloride electrolyte tends to produce a kind of etched, matte finish when used for steels and nickel aHoys. A typical surface roughness average, Ra is about 1 ]lni. 

Figure 6.2. Stress-strain curves at a strain rate of 0.001 s T = 295 K (closed symbols), and T —11 K. (open symbols) in as-received and shock prestrained nickel-270 (polycrystalline), and in <100> and <111> nickel single crystals [11].

Ni3C decomposition is included in this class on the basis of Doremieux s conclusion [669] that the slow step is the combination of carbon atoms on reactant surfaces. The reaction (543—613 K) obeyed first-order [eqn. (15)] kinetics. The rate was not significantly different in nitrogen and, unlike the hydrides and nitrides, the mobile lattice constituent was not volatilized but deposited as amorphous carbon. The mechanism suggested is that carbon diffuses from within the structure to a surface where combination occurs. When carbon concentration within the crystal has been decreased sufficiently, nuclei of nickel metal are formed and thereafter reaction proceeds through boundary displacement. 

Baranowski [680] concluded that the decomposition of nickel hydride was rate-limited by a volume diffusion process the first-order equation [eqn. (15)] was obeyed and E = 56 kJ mole-1. Later, Pielaszek [681], using volumetric and X-ray diffraction measurements, concluded from observations of the effect of copper deposited at dislocations that transportation was not restricted to imperfect zones of the crystal but also occurred by diffusion from non-defective regions. The role of nickel hydride in catalytic processes has been reviewed [663]. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

The starting material for making these pigments is cadmium sulphate, which must be free from iron, nickel and copper impurities. Cadmium sulphide is precipitated from the sulphate solution by adding an alkaline solution of pure sodium sulphide under controlled conditions of pH, temperature and addition rate. The yellow product is in the cubic crystal form, which is converted into the required hexagonal form by calcination at 500-600 °C in the absence of air. 

Figure 10.1 A comparison of the rate of methane synthesis overtwo different nickel single-crystal catalysts and supported Ni/alumina catalysts at 120 torr total reactant pressure. (Reprinted from Goodman, D.W., J. Vac. Sci. Technol., 20, 522-526, 1982. With permission from the American Institute of Physics.)...

The single crystal results are compared in Fig. 2 with three sets of data taken from Ref. 13 for nickel supported on alumina, a high surface area catalyst. This comparison shows extraordinary similarities in kinetic data taken under nearly identical conditions. Thus, for the Hj-CO reaction over nickel, there is no significant variation in the specific reaction rates or the activation energy as the catalyst changes from small metal particles to bulk single crystals. These data provide convincing evidence that the methanation reaction rate is indeed structure insensitive on nickel catalysts. 

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