Nickel catalysts basic principles

A favorable combination of valence forces of both components seems to be the basic principle of the nickel-molybdenum ammonia catalyst. It has been found (50) that an effective catalyst of this type requires the presence of two solid phases consisting of molybdenum and nickel on the one hand and an excess of metallic molybdenum on the other. Similar conditions prevail for molybdenum-cobalt and for molybdenum-iron catalysts their effectiveness depends on an excess of free metal, molybdenum for the molybdenum-cobalt combination and iron for the molybdenum-iron combination, beyond the amounts of the two components which combine with each other. A simple explanation for the working mechanism of such catalysts is that at the boundary lines between the two phases, an activation takes place. In the case of the nickel-molybdenum catalyst, the nickel-molybdenum phase will probably act preferentially on the hydrogen and the molybdenum phase on the nitrogen. 

The addition of to a >C=C< system is thermodynamically favorable but generally difficult to achieve. In the laboratory, chemists use catalysts such as platinum black and Rainey nickel to make the reaction proceed at a reasonable rate. The kinetic barrier for this reaction can be understood in terms of simple orbital symmetry diagrams, shown in Figure 5.2. The basic principle is that in the activated state the electrons must flow in such a way as to make and break the appropriate bonds. In the case of hydrogenation, the electron flow must break the H—H o and C—C n bonds and make two C—H bonds. The electrons must flow from an occupied orbital on one molecule to an unoccupied orbital on the other, that is, from the highest occupied molecular orbital, HOMO, on one species to the lowest unoccupied molecular orbital, LUMO, on the other. 

In principle pentadienyls can bond to transition elements in at least three basic ways, tj3, and tjs (Fig. 1). These can be further subdivided when geometrical factors are considered. If r 5 coordination could be converted to rj3 orr/1, one or two coordination sites could become available at the metal center, and perhaps coordinate substrate molecules in catalytic processes. Little is known about the ability of pentadienyl complexes to act as catalysts. Bis(pentadienyl)iron derivatives apparently show naked iron activity in the oligomerization of olefins (144), resembling that exhibited by naked nickel (13). The pentadienyl groups are displaced from acyclic ferrocenes by PF3 to give Fe(PF3)5 in a way reminiscent of the formation of Ni(PF3)4 from bis(allyl)nickel (144). 

It has been demonstrated in earlier sections that the catalytic activity of nickel oxide in the room-temperature oxidation of carbon monoxide is related to the number and the nature of the lattice defects on the surface of the catalyst and that any modification of the surface structure influences the activity of the solid. Changes of catalytic activity resulting from the incorporation of altervalent ions in the lattice of nickel oxide may, therefore, be associated not only with the electronic structure of the semiconductor (principle of controlled valency ) (78) but perhaps also with the presence of impurities in the oxide surface or a modification of the surface structure because of this incorporation. In order to determine the influence of dopants on the lattice defects in the surface of the solid and on its catalytic activity, doped nickel oxides were prepared under vacuum at a low temperature (250°). Bulk doping is not achieved and, thence, one of the basic assumptions of the electronic theory of catalysis (79) is not fulfilled. 

The basic selectivity principles introduced for the reactions of nonactivated alkenes vide supra) are also valid for polar, activated alkenes such as alkyl acrylates. Whereas monosubstituted methylenecyclopropanes usually lead to the formation of product mixtures regardless of whether nickel(0) or palladium(0) is employed as catalyst, methylenecyclopropanes with (identical) geminal substituents usually give rise to only one major product and are thus especially useful for preparative syntheses. In this section general features of these reactions of substituted methylenecyclopropanes are exemplified for selected substituents and substitution patterns. 

---