Nickel crystal

The melting point selected by Hultgren et al. [73HUL/DES] and also retained by this review, = (1726 4) K, is based on the following determinations  

The value of the Curie point, T), = 631 K, is taken from the study of Connelly et al. [71CON/LOO] using a calorimetric method which permits continuous observation of T, and from the study of Vollmer et al. [66VOL/KOH] using a high temperature adiabatic calorimeter. 

Pure Ni metal is defined as the nickel reference phase. As such, its Gibbs energy of formation and enthalpy of formation are zero by definition at 298.15 K and 0.1 MPa. 

The source of data for the entropy determination of nickel has been the temperature dependence of the heat capacities. The measurements of the molar heat of 

In the present review, the selected value of the standard molar entropy of crystalline nickel, Ni(cr), is based on the above determination  

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When we deposit a small amount of iron atoms on a nickel crystal, and allow the system to equilibrate, what will happen to the iron atoms What happens with Ag on Ru, Ag on Ni, and Co on Cu ... 

In the first example (46), hydrogen adsorption on different sites of a nickel crystal was studied by considering the interaction of a hydrogen atom with a limited number of metal atoms. The nickel crystal was truncated to obtain the clusters shown in Fig. 31, containing 13,10, 9 and 8 nickel atoms, and representing models for the bulk crystal and for the (111), (100) and (110) surfaces respectively. The nearest-neighbour distance in all clusters was... 

The nickel crystal has a cubic close packing arrangement with an edge length of 352.4 nm. Using this information, calculate the density of nickel. 

Nickel crystallizes in a cubic crystal system. The first reflection in the powder pattern of nickel is the 111. What is the Bravais lattice ... 

At a slightly higher pH more zinc powder is added to the electrolyte to precipitate the nickel. This precipitate is then filtered and redissolved in sulfuric acid. This solution is sent to the nickel crystallizer where each batch then produces 1680 lbs NiS04 2H20. The residual concentrated acid is used to dissolve the next batch of incoming sludge. 

The paper deals with some new data concerning the state of the metal after reduction and the catalytic functions of zeolite catalysts containing nickel and platinum. By using the molecular sieve selectivity in the hydrogenation of mesitylene it has been proved that metal (platinum) is contained in the volume of the zeolite crystal. The temperature dependence of the formation of nickel crystals was investigated. The aluminosilicate structure and the zeolite composition influence mainly the formation of the metal surface which determines the catalytic activity. In the hydrocracking of cumene and disproportionation of toluene a bifunctional action of catalysts has been established. Hydrogen retarded the reaction. 

Figure 2. Dependence of catalytic activity of zeolites type A and X on size of nickel crystals in benzene hydrogenation = type A O = type X...

Figure 11-1 Left Schematic representation of a nickel crystal in cross section showing residual valences at the surface atoms. Right Adsorption of ethene on the surface of the nickel crystal with formation of C-Ni bonds.

The copper crystals used in these studies were made from copper of both 99.999% and 99.94% purity, the nickel crystals from metal 99.92% pure nickel plus cobalt, and the iron crystals were made from Armco iron rods of the following approximate composition iron, 99.8 carbon, 0.018 manganese, 0.027 phosphorus, 0.005 sulfur, 0.029 silicon, 0.005 copper, 0.11 %. 

Leidheiser and Gwathmey (8) found that, when a spherical nickel crystal was exposed to an atmosphere of carbon monoxide at 550°, carbon deposited, as shown in Fig. 13, in the (111) region but not in the (100) or... 

Fig. 13. Carbon deposited on a nickel crystal at 550° in carbon monoxide.

Fig. 14. Carbon deposited on a nickel crystal at 600° in an 8 1 hydrogen-carbon monoxide mixture. [According to Kehrer and Leidheiser (82). ...

These data have been verified by electron microscope photographs Fig. 11 shows an example for a catalyst reduced at 500° C. Clearly, the carrier forms a large porous structure, in which the small nickel crystals find a place. 

A particular feature of the sintering process of sample 5421 is shown by Fig. 13. From a certain point on, the particles, according to magnetic data, cease to grow, while adsorption decreases still further. This occurs with samples reduced at temperatures higher than 500° C., i.e., with those samples in which 5421 begins to show inaccessible nickel of type 2 above. It appears, therefore, that these nickel crystals are here not only becoming inaccessible to CO but also to H2. 

All of the other phenomena associated with waves can also be observed in particles. For example, in 1927 Davisson and Germer accelerated a beam of electrons to a known kinetic energy and showed that these electrons could be diffracted off a nickel crystal, just as X-rays are diffracted (see Figure 3.8). Just as with photons, interference is not always seen if the wavelength spread or the slits are large, the fringes wash out. This also explains why interference is not seen with macroscopic objects, such as buckshot—the wavelength is far too small. 

This predicted wave character was verified in 1927 by C. J. Davisson and L. H. Germer who studied the diffraction of an electron beam that was directed at a nickel crystal. Diffraction is a characteristic of waves, so it was demonstrated that moving electrons have a wave character. 

Estimate the dispersion of the nickel. Hydrogen dissociatively adsorbs on nickel whereas it does not interact with the catalyst support and is not significantly adsorbed within the nickel crystal lattice. Therefore, the amount of uptake of hydrogen by the catalyst is a measure of how well the nickel has been dispersed when deposited on the support. 

Ethylene and Acetylene. On nickel (111), both ethylene and acetylene are irreversibly chemisorbed neither can be thermally desorbed. We also find that trimethylphosphine cannot displace ethylene or acetylene from these surfaces. There have been suggestions that ethylene and acetylene are not present on the surface as molecules but as molecular fragments. Many ultra-high vacuum studies of ethylene and of acetylene chemisorption on nickel crystal planes have been reported. Most of these studies seem to implicate states in which C-H bond cleavage reactions have accompanied the basic chemisorption process (19). 

The other technique is to use a rolling and recrystaUization texture. Nickel is normally used in this method, since the heat resistance of ifickel is good and the 100 <001> texture is easily obtained. A nickel ingot is rolled with a reduction ratio of more than 90%, then the rolled nickel tape is annealed in an inert atmosphere at a temperature near 1000°C for recrystaUization. After recrystallization, nickel crystals align biaxially, and 100 <001> texture was achieved. A Ce02 buffer layer is deposited heteroepitaxially on the textured nickel in order to prevent a chemical... 

Figure 2.8 Intensity of electron scattered at a fixed angle off a nickel crystal, as a function of incident electron energy. (Adapted from C. J. Davisson, Are Electrons Waves The Journal of the Franklin Institute 206, No. 5 (May, 1928). Used by permission of the Franklin Institute, Philadelphia, PA.)...

While catalyst activity is generally inversely related to the amount of aluminum and alumina present it is not desirable to remove all of these materials from the nickel. It has been proposed that some aluminum in the nickel crystal lattice creates the defect sites responsible for catalytic activity. The alumina appears to prevent the sintering of the nickel particles. With the 20% alumina that is found in the commercial catalyst, heating to 500°C results in only a 20% reduction in surface area. When only 1% alumina is present there is a 50% reduction in surface area at this temperature but no change in surface area on heating to 250°C. 

Fig. 3. Dififraction patterns from a clean (100) face of a nickel crystal. 144 volts, or 1.02 A. The bright spots of this pattern are (711) Laue diffraction beams at their maximum intensity.

Fig, 4. Sketch of the surface atoms of the nickel crystal, the orientation being the same as that of all of the diffraction patterns reproduced here. 

A typical mechanism for carbon formation is the formation of whisker structures on the catalyst. Adsorbed hydrocarbons may react and form adsorbed carbon, which is dissolved in the nickel particle. After saturation, carbon may nucleate and grow with the nickel crystal at the tip. ... 

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