Nickel adsorbed species

The temperature dependence of the extent of adsorption was not interpreted, except that the results were considered to be consistent with the magnetic measurements of Selwood (see Section II,C) which indicate that the number of carbon-metal bonds between adsorbed species and the surface increases threefold between 120°and 200°C due to extensive dissociative chemisorption. The authors proposed that two forms of chemisorbed benzene exist at the nickel surface, (i) an associatively adsorbed form which can be displaced by further benzene, and which may be w- or hexa-dissociatively adsorbed form that requires the presence of hydrogen to bring about its removal from the surface. 

Sachtler [195] proposed a dual-site mechanism in which the hydrogen is dissociated on the Ni surface and then migrates to the substrate that is coordinated to the adsorbed dimeric nickel tartrate species. In their model, adsorption of modifier and reactants takes place on different surface atoms in contrast to Klabunovskii s proposal. Adsorbed modifier and reactant are presumed to interact through hydrogen bonding (Scheme 14.5). The unique orientation of adsorbed modifier molecules leads to a sterically favored adsorbed reactant configuration to achieve this bonding. 

Nowack et al. [17] determined adsorbed iron III and nickel-EDTA species in soil by reverse phase ion-pair high-performance liquid chromatography. Iron III EDTA was found to be the main species present occuring at 30-... 

Mouvet, Ch., and A. C. M. Bourg (1983), "Speciation (Including Adsorbed Species) of Copper, Lead, Nickel and Zinc in the Meuse River", Water Res. 17/6, 641 -49. 

The autocatalytic process observed on Ni(l 10) was very similar to results reported previously on nickel powder (82). In that work the rate of decomposition was measured as a function of the amount of formic acid adsorbed and the formic acid pressure. At 60°C and pressures from 0.4 to 1.4 Torr the rate of decomposition decreased as the formate coverage increased. Such behavior can be accounted for if the attractive interactions between the adsorbed species observed for the formic acid decomposition on Ni(llO) or... 

Nickel is strongly adsorbed by soil, although to a lesser degree than lead, copper, and zinc (Rai and Zachara 1984). There are many adsorbing species in soil, and many factors affect the extent to which nickel is adsorbed, so the adsorption of nickel by soil is site specific. Soil properties such as texture, bulk density, pH, organic matter, the type and amount of clay minerals, and certain hydroxides influence the retention and release of metals by soil (Richter and Theis 1980). 

Mouvet C, Bourg ACM. 1983. Speciation (including adsorbed species) of copper, lead, nickel, and zinc in the Meuse River. Water Research 17 641-649. 

The high selectivity of nickel for demethylation is explained by its high affinity for the adsorption of primary carbon atoms and its high ability to form 1,2-di-adsorbed species.272 On cobalt, where excessive methane formation is characteristic, consecutive C—C bond ruptures are believed to occur via species with multiple attachment to the metal.252... 

The adsorbed species discussed above are simplified representations of the actual surface complexes since the number of participating surface metal atoms and the number of bonds between the molecule and the surface are not known. Hydrogenolysis reactions, however, are believed to take place on multiple surface sites, specifically, on a certain ensemble of atoms. For example, on the basis of kinetic data, Martin concluded273 that at least 12 neighboring nickel atoms free from adsorbed hydrogen are required in the complete cleavage of ethane ... 

From the changes in magnetic susceptibility of nickel—silica catalysts during ethylene adsorption at room temperature, Selwood [55] has concluded that ethylene exists both as an associatively and a dissociatively adsorbed species. On increasing the temperature, the dissociative adsorption becomes more important. Thus at 100° C, the susceptibility changes are consistent with the formation of six bonds to the surface for each adsorbed ethylene molecule, suggesting the following process... 

Fig. 6. Adsorption isotherms for 14C-ethylene and the effect of hydrogen on the adsorbed species for (a) nickel—alumina and (b) palladium—alumina at 20°C [63],...

Adsorption of hex-l-ene, a mixture of cis- and frans-hex-2-ene, and c/s-hex-3-ene on nickel—silica results in identical infrared spectra [83]. Addition of hydrogen results in an intensification of the spectrum suggesting that the initial spectrum results from dissociatively adsorbed species, a conclusion substantiated by the observation that the gas in equilibrium with the surface during the initial adsorption contains isomerised hexenes. Evacuation of the hydrogen causes a decrease in intensity and the reappearance of the initial spectrum. 

Khulbe and Mann [155] have obtained infrared spectra of allene adsorbed on silica-supported cobalt, nickel, palladium, platinum and rhodium. The spectra were similar for all the metals, although variations in band intensity from metal to metal were observed. Addition of hydrogen to the allene-precovered surface resulted in similar spectra to those found for chemisorbed and hydrogenated propene in which the surface species was thought to be an adsorbed prop-1-yl group. The authors concluded that the initial allene spectrum was consistent with the adsorbed species being a 1 2-di-o-bonded allene (structure K)... 

Very recently, Bailey and Richards (23) have shown that a high degree of sensitivity for adsorbed species can be achieved by measuring the absorption of infrared radiation on a thin sample cooled to liquid helium temperature. The optical arrangement used in these studies is shown in Figure 10. The modulated beam produced by the interferometer is introduced into the UHV sample chamber and reflected off a thin slice of monocrystalline alumina covered on one side by a 1000 k film of nickel or copper. Radiation absorbed by the sample is detected by a doped germanium resistance thermometer. The minimum absorbed power detected by this device when operated at liquid helium temperature is 5 x 10 14 W for a 1 Hz band width. With this sensitivity absorbtivities of 10"4 could be measured. 

The most recent results for hydrogen chemisorbed on Raney nickel by Kelly et aj[ (30) are shown in Fig. 8. The inelastic neutron TOF spectra are taken at 77 K in the energy-loss mode with the background spectrum of the reduced Raney Ni sample after desorption of adsorbed species (Fig. 8(a)) subtracted. Figure 8(b) is the pectrum for a hydrogen monolayer. Features at -120 meV (960 cm-i)and -140 meV (1120 cm-1)are in excellent agreement with previous results of Renouprez et al (28,29). In addition, a peak at 78 meV (624 cm"1) is clearly resolved. 

Inelastic neutron scattering from adsorbed species on Raney nickel. 

The reaction sequence outlined in Scheme 4.2 is more complete, showing the primary metalalkyl, 10b, as well as the adsorbed Ti-allyl, 13. With catalysts other than palladium or nickel this species is not present so the boxed portion of this reaction sequence would not take place. 

Pulse electrolysis, which is often used to improve the quality of electrodeposits (see below), was found to have a strong effect on the structure of nickel deposits by modifying the interfacial sorption properties [6.62]. Molecular species such as H2 or Ni(OH)2 are easily desorbed during the relaxation time, improving the electrocrystallization process. Conversely, specifically adsorbed species such as Hads or anions inhibit the electrodeposition process of nickel. 

Organic inhibitors in the nickel bath also influence the texture of nickel deposits. The inhibition effects are related to their molecular structure [6.69]. In the presence of brightners with unsaturated ethylenic or acetylenic compounds, the [110] texture is preferentially formed. With aryl-sulfonic compounds used as leveling agents, the [100] or [211] textures are favored. The modification of the crystal growth has been interpreted by an adsorption-hydrogenation-desorption model. The nature and the strength of a bond between a metallic surface and an adsorbed species depend on the... 

Optical spectroscopy has been applied with a good deal of success to the identification of chemisorbed species and of the nature of the surface bond. Infrared spectra have been most useful in studies of simple molecules, such as carbon monoxide adsorbed on platinum or nickel, and ultraviolet spectra for the characterisation of more complex interipediates, such as carbonium ions and ion radicals. The frequency of the adsorption band (or bands) often serves to identify the adsorbed species by comparison with spectra of known compounds. Quantitative information may then in principle be obtained by measuring the area under the adsorp-... 

Pure nickel oxides, which are catalytically active, are usually prepared by decomposition at a moderate temperature of nickel salts or the hydroxide (19). However, if the pyrolysis is performed in air, the oxide surface is not free from adsorbed species, especially adsorbed oxygen (20). Moreover, oxygen excess may penetrate the bulk of the... 

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