Hydrogen autocatalytic activation

Electroless reactions must be autocatalytic. Some metals are autocatalytic, such as iron, in electroless nickel. The initial deposition site on other surfaces serves as a catalyst, usually palladium on noncatalytic metals or a palladium—tin mixture on dielectrics, which is a good hydrogenation catalyst (20,21). The catalyst is quickly covered by a monolayer of electroless metal film which as a fresh, continuously renewed clean metal surface continues to function as a dehydrogenation catalyst. Silver is a borderline material, being so weakly catalytic that only very thin films form unless the surface is repeatedly cataly2ed newly developed baths are truly autocatalytic (22). In contrast, electroless copper is relatively easy to maintain in an active state commercial film thicknesses vary from <0.25 to 35 p.m or more. 

Several benzyl derivatives exhibit potentially hazardous properties arising from the activation by the adjacent phenyl group, either of the substituent or of a hydrogen atom. Halides, in particular, are prone to autocatalytic Friedel Crafts polymerisation if the aromatic nucleus is not deactivated by electron withdrawing substituents. 

In the early nineteen-sixties Halpem, James and co-workers studied the hydrogenation of water-soluble substrates in aqueous solutions catalyzed by rathenium salts [6]. RuCh in 3 M HCl catalyzed the hydrogenation of Fe(III) to Fe(II) at 80 °C and 0.6 bar H2. Similarly, Ru(IV) was autocatalytically reduced to Ru(III) which, however, did not react further. An extensive study of the effect of HCl concentration on the rate of such hydrogenations revealed, that the hydrolysis product, [RuCln(OH)(H20)5. ] " was a catalyst of lower activity. It was also established, that the mechanism involved a heterolytic splitting of H2. In accordance with this suggestion, in the absence of reducible substrates, such as Fe(in) there was an extensive isotope exchange between the solvent H2O and D2 in the gas phase. 

Cupric and Cuprous Salts in Inert Solvents. The reduction of cupric heptanoate by hydrogen to the cuprous salt proceeds homogeneously in a variety of nonpolar solvents. In heptanoic acid solution, both the cupric and cuprous salt contribute to the activation of hydrogen, the latter being more active (Chalk and Halpern, 28). The reaction is thus autocatalytic (Fig. 4), the rate-law being of the form... 

Further cautions should be discussed. Whereas transport of hydrogen may occur at temperatures well below 400°C, the induction of catalytic activity on the support by spiltover hydrogen is an activated process and requires considerable time (up to 12 h of treatment at 430°C in hydrogen). Comparison of catalytically active surfaces must be done with similar temperatures and times of spillover pretreatment. To further complicate the analysis, there is evidence that an activated support (e.g., Al2Oa) may be able to dissociate hydrogen. The process may, therefore, be autocatalytic that is, the support first activated by spillover may be able to adsorb, dissociate, spill over, and consequently activate more support surface (137). 

The abundant chemistry of Ni(CO>4 under reductive reaction conditions leading to the formation of dinuclear nickel complexes or even to nickel clusters suggests the involvement of higher aggregates, however. An overview of the reactivity of nickel complexes, and of Ni(CO)4 in particular, is given in a series of excellent reviews by Jolly [13]. There seems to be evidence of an autocatalytic cycle for the formation of the active catalyst [14]. Parallel to this, the water-gas shift reaction (eq. (5)) occurs, resulting in the formation of carbon dioxide and hydrogen, which is known to form metal hydrides in the presence of metal carbonyls [15]. 

The Cu product is a more effective hydrogenation catalyst than the Cu species and the rate data exhibits autocatalytic behavior. The initial reaction rate was attributed to activation of H2 by Cu . The first-order dependence on Cu fell to nearly half-order with increasing concentration of the Cu salt. This was attributed to dimerization of the Cu ... 

---