Free nickel site

The chemisorption of methane on nickel involves the breakage of a C-H bond (Ceyer et al., 1987) and this requires that the molecule has sufficient energy to overcome a barrier of about 52 kJ/mol (Beebe et al., 1987 and Chorkendorff et al., 1990) and that it hits a free nickel site with free neighbor sites (Alstrup et al., 1990). The rate of adsorption is an order of magnitude higher on the open (110) nickel surface plane than on the dense (111) surface plane. The activated chemisorption of methane is the rate determining step in steam reforming (Rostrup-Nielsen,... 

The Ni-catalyzed oligomerization of olefins in ionic liquids requires a careful choice of the ionic liquid s acidity. In basic melts (Table 5.2-2, entry (a)), no dimerization activity is observed. FFere, the basic chloride ions prevent the formation of free coordination sites on the nickel catalyst. In acidic chloroaluminate melts, an oligomerization reaction takes place even in the absence of a nickel catalyst (entry (b)). FFowever, no dimers are produced, but a mixture of different oligomers is... 

The metal surface area at the inlet end of the catalyst bed in experiment HGR-12 was smaller than that at the outlet end this indicates that a decrease in nickel metal sites is part of the deactivation process. Sintering of the nickel is one possible mechanism, but carbon and carbide formation are suspected major causes. Loss of active Raney nickel sites could also conceivably result from diffusion of residual free aluminum from unleached catalyst and subsequent alloying with the free nickel to form an inactive material. 

Thin films of a composite nickel-iron (9 1 Ni/Fe ratio) and iron-free oxyhydroxides were deposited from metal nitrate solutions onto Ni foils by electroprecipitation at constant current density. A comparison of the cyclic voltammetry of such films in 1M KOH at room temperature (see Fig. 6) shows that the incorporation of iron in the lattice shifts the potentials associated formally with the Ni00H/Ni(0H)2 redox processes towards negative potentials, and decreases considerably the onset potential for oxygen evolution. The oxidation peak, as shown in the voltammo-gram, is much larger than the reduction counterpart, providing evidence that within the time scale of the cyclic voltammetry, a fraction of the nickel sites remains in the oxidized state at potentials more negative than the reduction peak. 

Transition metal complexes which react with diazoalkanes to yield carbene complexes can be catalysts for diazodecomposition (see Section 4.1). In addition to the requirements mentioned above (free coordination site, electrophi-licity), transition metal complexes can catalyze the decomposition of diazoalkanes if the corresponding carbene complexes are capable of transferring the carbene fragment to a substrate with simultaneous regeneration of the original complex. Metal carbonyls of chromium, iron, cobalt, nickel, molybdenum, and tungsten all catalyze the decomposition of diazomethane [493]. Other related catalysts are (CO)5W=C(OMe)Ph [509], [Cp(CO)2Fe(THF)][BF4] [510,511], and (CO)5Cr(COD) [52,512]. These compounds are sufficiently electrophilic to catalyze the decomposition of weakly nucleophilic, acceptor-substituted diazoalkanes. 

The selectivity of the nickel(l 1 1) surface may thus be controlled by modification of the number of free step sites, and this notion was tested experimentally by blocking the steps with small amounts of silver (84). In other STM investigations it was found that when silver was deposited on nickelfl 1 1) at room temperature, the silver preferentially nucleated and grew as islands at the step edges. When this system was post-annealed to 800 K, the silver atoms were observed to become highly mobile and decorate all the step edges of nickelfl 1 1), as shown in Fig. 6b. 

These results led the workers to suggest that catalysis actually leads to the removal of surface nickel atoms, primarily due to local heating which takes place at the reaction site. Furthermore, during the catalytic process, the nickel atom is temporarily part of a liquid- or gas-phase intermediate. Once the catalytic process is complete, the authors postulated that the free nickel atom readsorbed onto the bulk nickel, adsorbed onto the inert support, remained as nickel sol in the liquid, or continued to act as a catalyst. It was claimed that this model explained several observations, such as the differences between unsupported and supported nickel. The supported metal has a greater surface area upon which the metal can readsorb, so it tends to leave fewer atoms in the product liquid. The model also explains the observation that the reaction vessel became coated with a thin film of nickel after lengthy use. This postulated etching mechanism is similar to the recent model discussed above, whereby etching results from free-radical-surface interactions. 

The main catalyst poison in steam reforming plants is sulfur that is present in most feedstocks. Sulfur concentrations as low as 0.1 ppm form a deactivating layer on the catalyst but the activity loss of a poisoned catalyst can be offset, to some extent, by raising the reaction temperature. This helps to reconvert the inactive nickel sulfide to active nickel sites. Nickel-free catalysts have been proposed for feedstocks heavier than naphtha. These catalysts consist mostly of strontium,... 

Reacting 2 eqniv. of the silver complex of the fnnctionalised carbene with [Ni(PPh3)2Cl2] yields the bis-carbene nickel(II) complex with the two carbenes andpyridines in cis-position and thus pyridine and carbene trans to each other [37], There are no free coordination sites available and the chloride anions are nncoordinated (see Figure 3.12). 

An observation of interest in the case of methane is that conversion is insensitive to bulk nickel sulfide formation on the catalyst. It was suggested by Rostrup Nielsen and Alstnip [14,15] that the ensembles of free nickel atoms available at high coverages of sulfur are sufficient for the conversion of adsorbed methane with steam or caibon dioxide. On the basis of the present study even the bulk nickel sulfide appears to have sufficient active sites for methane decomposition. 

Nickel, palladium, and platinum d complexes preferentially add polar reagents (acids alkyl, acyl, and metal halides), whereby a Ugand must dissociate to give a free coordination site (Eq. 2-39). 

Schindler et al. carried out a kinetic analysis for the reaction of isoprene (ip) with carbon dioxide and Ni(bpy)(cod) [103]. Ni(bpy)(cod) did not react directly with CO2 (see also Sect. 5.2), but reacted reversibly with isoprene to give Ni(bpy)(ip). The kinetic results support a mechanism where, in a fast preequilibrium step, one of the bonds between Ni and cod is cleaved and, in a following step, ip coordinates to the free coordination site (Scheme 5.23). At low temperatures Ni(bpy)(ip) can further react, through a consecutive equilibrium, with another ip molecule to give Ni(bpy)(ip)2. However, in the presence of CO2, the nickel-ip complex, Ni(bpy)(ip), reacts irreversibly with the heterocumulene to form the product complex (3-5-q )-3-methyl-3-pentenylato)nickel bipyridine (Scheme 5.23). The activation parameters for the latter step were calculated to be A// = 25 7 kJ/mol and = 184 24 J/(mol K). The large negative activation entropy indicates that the carboxylation reaction proceeds in an associative way, during which CO2... 

It has been shown that a small trace of sulphur on the inlet stream can reduce coke formation on the nickel catalyst. Deactivated sulphur-nickel sites inhibit the active carbon (Ca) polymerisation/isomerisation to less active carbon (Cj3) due to the lack of free sites. In other words, SR needs three to four nickel sites while carbon formation requires six to seven nickel sites. Thus, the SR catalyst loses some of its activity, but coke formation is minimal (Trimm, 1997,1999). 

Free, ionic species of metals are at their highest concentrations at lower pH, so metals tend to be more bioavailable under these conditions.121128 At acidic pH, more protons are available to saturate metal-binding sites.99 For example, metals are less likely to form insoluble precipitates with phosphates when the pH of the system is lowered because much of the phosphate has been protonated. Under basic conditions, metal ions can replace protons to form other species, such as hydroxo-metal complexes. Some of the hydroxo-metal complexes are soluble, such as those formed with cadmium, nickel, and zinc, whereas those formed with chromium and iron are insoluble. 

Figure 1.13 Point defects in nickel oxide, NiO (schematic) Ni2+ vacancy Ni2+ interstitial Li+ on a Ni2+ site Mg2+ on a Ni2+ site Fe3+ on a Ni2+ site O2- vacancy N3- on an O2-site F on an O2- site free electron free hole.

The presence of iron in nickel oxyhydroxide electrodes has been found to reduce considerably the overpotential for oxygen evolution in alkaline media associated with the otherwise iron free material.(10) An in situ Mossbauer study of a composite Ni/Fe oxyhydroxide was undertaken in order to gain insight into the nature of the species responsible for the electrocatalytic activity.(IT) This specific system appeared particularly interesting as it offered a unique opportunity for determining whether redox reactions involving the host lattice sites can alter the structural and/or electronic characteristics of other species present in the material. 

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