Nickel as a Catalyst
|Numerous examples of the work of nickel as a catalyst are indicated in connection with the chemical properties of nickel. Nickel intended for use as a catalytic agent is usually prepared by reducing the green hydroxide with hydrogen at a temp, below 300°; if the metal be reduced at a higher temp, than this, it is less reactive; and if it be reduced at a bright red heat, the nickel is almost inert. The presence of halogens, arsenides, phosphides, or sulphides, poisons the catalyst, i.e. these agents reduce the activity of the catalyst. Consequently, as emphasized by P. Sabatier and J. B. Senderens, and A. W. Crossley, it is necessary to employ purified hydrogen for hydrogenation reactions with nickel as catalyst. T. Kusama and Y. Uno discussed why the catalyst prepared from nickel chloride is less active than when it is prepared from the nitrate, and they concluded that it was due to the presence of unreduced chlorides. The poisoning of the nickel catalyst was studied by B. Kubota and K. Yoshikawa, F. Wolff, K. Yoshikawa, M. C. Boswell and C. H. Bayley, and G. Roberti. |
In some cases, C. Kelber found it better to scatter, say, the carbonate, over some inert material like asbestos, kieselguhr, or animal charcoal, and then reduce the carbonate. Nickel reduced from the carbonate at 450° has no action on a mixture of hydrogen and oxygen at ordinary temp., but if the nickel be spread over an inert substance, its activity is augmented. It is still more active if reduced at 300°. W. E. Gibbs and H. Liander reduced the nickel from the carbonyl, and observed that it has but little catalytic effect on the reduction of carbon monoxide, or of ethylene by hydrogen. K. Schirmacher and co-workers associated the catalyst with silica gel; and P. Breteau obtained finely-divided nickel by reducing a hot soln. of nickel sulphate with sodium hypophosphite. L. R. Ingersoll examined the catalytic effect of films of nickel spluttered in nitrogen. According to E. Maschkilleisohn, nickel has an optimum degree of dispersion for use as a catalyst, any finer dispersion is accompanied by a reduction, and finally by a cessation of its activity. The subject was studied by A. A. Balandin, L. H. Reyerson, E. Biesalsky and coworkers, A. Brochet, C. F. Fryling, C. R. Glass and L. Kahlenberg, K. Heinze, S. Iki, T. Kusama and Y. Uno, M. Lietz, C. M. Loane, E. J. Lush, E. Maschkilleisohn, W. Normann, J. E. Nyrop, K. Omiya, A. Quartaroli, B: Kubota, M. Raney, K. Schirmacher and co-workers, A. Svizuin, R. Thomas, F. Thoren, and E. Wolfson. G. M. Schwab and L. Rudolph, and H. N. Huntzicker and L. Kahlenberg, discussed the nature of the catalyst; G. Bredig and R. Allolio, the X-ray properties of the catalyst; H. L. Waterman and M. J. van Tussenbroek, and H. Adkins and L. W. Covert, the mode of preparation of the catalyst; F. Fischer and K. Meyer, the reduction temp.; G. M. Schwab and H. Schultes, and F. H. Constable, the surface area; A. Bag, the re-activation of nickel; A. W. Gauger and H. S. Taylor, the effect of various supports for the catalyst; T. Kusama and Y. Uno, the effect of chlorine; W. W. Russell and H. S. Taylor, the effect of thoria. C. Kelber studied the effect of water; F. Wolff, the effect of ferric and nickel hydroxides; and G. F. Schoorel and co-workers, the effect of high pressures on the hydrogenation. O. Schmidt compared the hydrides of the metal.
E. Bosshard and E. Fischli, W. Norman, W. Norman and W. Pungs, W. Meigen, and W. Meigen and G. Bartels favour the opinion that it is not nickel oxide, but rather the free unoxidized metal which behaves catalytically. F. Bedford and E. Erdmann, J. B. Senderens and J. Aboulenc, and W. Siegmund and W. Suida consider that no reduction of nickel oxide to the metal occurs during hydrogenation; and they suggested that possibly the suboxide, Ni2O, described by I. Bellucci and R. M. Corelli, is formed. The hydrogen transfer takes place via an intermediate phase represented by:
It is also possible that the transfer takes place through the decomposition of water, forming nascent hydrogen: Ni2O + H2O = 2NiO + 2H, which then unites with the compound being reduced whilst the nickel monoxide is again reduced to the suboxide by the hydrogen. G. Bartel's experiments on the speeds of the reaction with nickel and with the alleged suboxide did not support this hypothesis, nor did W. N. Ipateeff's experiments on the reduction of benzene. The subject was discussed by C. Kelber, and M. C. Bos well.
Another hypothesis is that the hydrogen is activated by passage into the atomic state, by occlusion or adsorption by the nickel - vide supra, the action of hydrogen on nickel. Yet a third hypothesis assumes that the hydrogen and the compound to be reduced mutually form a kind of surface film on the nickel, and that the juxtaposition enables the molecular hydrogen to do work that it could not perform by simple contact. The subject was discussed by M. Polyakoff, and R. Foresti.
B. S. Srikantan discussed the relation between the atomic energy and the efficiency of nickel as a catalyst; C. F. Fryling, R. Kuhn, O. Schmidt, H. P. Cady and W. E. White, and A. A. Balandin, the mechanism of reductions with nickel as catalyst; and B. Kubota and K. Yoshikawa, the formation of the metal hydride in hydrogenations with nickel as catalyst. G. L. Clark and co-workers showed that differences in the activities of nickel catalysts are not due to differences in the type or dimensions of the lattice.
According to H. S. Taylor, X-radiograms show that metallic catalysts possess the definite lattice structure of the crystalline material, and there are, on the surface, groups of atoms in which the crystallization process is incomplete. The surface is to be regarded as composed of atoms in varied degrees of saturation by neighbouring metal atoms, varying from those one degree less saturated than interior atoms to those which are held to the solid surface by a single constraint only, and it is by this constraint alone that these outermost atoms differ from gaseous metal atoms. These atoms can attach to themselves or adsorb three molecules, the linking between which and the nickel atom is identical with that obtaining in nickel carbonyl. This concept introduces a mechanism whereby both constituents of a hydrogenation process may be attached to one nickel atom and obviates the necessity, inherent in the Langmuir scheme, of having both reactants adsorbed on adjacent elementary spaces. The idea of metallic atoms detached to varying extents from the normal crystal lattice is in harmony with observations on such catalytic surfaces. According to E. F. Armstrong and T. P. Hilditch, the attraction between the unsaturated organic molecule and unsaturated nickel atom is held to be strong enough to loosen the nickel atom from its adjacent atom or atoms, so that when catalytic change actually occurs the nickel atom loses all contact with its neighbours. For the moment, there may exist, actually apart from the solid surface, a combination of nickel, unsaturated compound, and probably hydrogen as well. F. P. Bowden and E. K. Rideal studied the effect of cold-work and annealing on the areas of catalytic activity.