CATALYTIC ACTIVITY OF NICKEL

NICKEL-CATALYSTS predicted by his theory. I n view of its wide applicability it is believed that the theory of this paper can be extended to explain the effects of impurities on the electrochemical capacity of oxides other than nickel, such as manganese dioxide and lead dioxide. ACKNOWLEDGMENT

The author wishes to thank Sol S. Jaffe, Wm. Meirowitz, and Henry J. Wittrock for many helpful discussions and criticisms of this paper. Thanks are due to Thomas A , Edison, Inc., for permission to publish it. LITERATURE CITED (1)

Boswell, M. C., and Iler, R. K., J. Am. Chem. Soc., 58, 924 (1936).

(2) Crennell, J. T., and Lea, F. M., “Alkaline Accumulators,” pp. 69, 103, 106, London, Longmans, Green and Co., 1928. (3) De Boer, J . W., and Verwey, E. J. W., Proc. Phys. Soc. London, 49. Extra Part No. 274. 59 (19373. (4) Foerater, F., “Elektrochemie Wasseriger Losungen,” 4th ed., p. 267, Leipzig, J. A. Barth, 1923. (5) KuchinskiY, E. M., and Ershler, B. V., J . Phys. Chem. U.S.S.R., 20, 5 3 9 4 6 (1946). (6) Moulton, J. D., Thomas A. Edison, Inc.. private communication. (7) Seitz, F., “Modern Theory of Solids,” p. 468, New York, McGraw-Hill Book Co., 1940. (8) Thomas, U. B., J . Electrochem. Soc., 94, 42 (1948). (9) Verwcy, E. J. W., I-Iaaijman, P. IT., and Romcijn, F. C., Chem. Weekblad, 44, 705-8 (1948). (10) Verwey, E. J. W., Haaijman, P. W., Romeijn, F. C.. and Osterhout, G. W., Philips Research Repts., 5, 173 (1950). RECEIVED for review October 17, 1951. ACCEPTEDJanuary 15, 1952.

C a t a Iys t s

CATALYTIC ACTIVITY OF NICKEL Theoretical Aspects D. A. DOWDEN Research Department, Billingham Division, Imperial Chemical Industries, Ltd., Co. Durham, England Chemisorption of substrates b y metals depends upon the “residual valencies” of the giant-molecule crystallites. These valencies are discussed qualitatively using band theory and Pauling’s resonatingvalence-bond theory. Vacant d orbitals appear necessary for rapid, general chemisorption and therefore also for catalytic activity of certain types. The binding patential of the valencies varies inversely as the Fermi energy of the metal electrons and inversely as the ionization potential of the relevant, substrate valence electrons. These concepts are employed in a rationalization of the properties (specific area and specific activity) of nickel and its alloys as catalysts in industry and research.

T I S conventional and useful to consider the total activity of a catalyst, in appropriate units per &nit mass of catalyst, as arising from two factors. The first of these is the specific area (area per unit mass of solid) which, correctly estimated, measures the area of solid surface accessible t o fluid substrates or effective in solid reactions, and the second is the specific activity (activity per unit area), indicating the efficacy of the surface units. Provided that the solid particles are not too small-i.e., approaching atomic dimensions-specific area is independent of specific activity for a given solid. Generally speaking, specific area is much less sensitive to small variations in catalyst content and condition than is specific activity, which latter is consequently the vital factor in all catalyses by solids. Of old, for both academic and industrial purposes, the total activity of simple substances was broadly related t o the periodic table despite occasional confusion from area variables. I n greater detail two influences on specific activity were recognized, the first originating in electronic configuration (61), and the second being structural or geometric ($1); neither was in conflict with the active center theory (67). More recently the stable, solid combinations of the elements have been shown (11) to catalyze the reactions according to different mechanisms depending upon whether the solid is a n electronic conductor (metal, semimetal, or semiconductor) or a n insulator. None of these classifications is rigid and all are related t o the position of the solid constituents in the periodic table. The new quantum theory shows that the major part of the total energy (extranuclear) of any system of nuclei and electrons a t

I

May 1952

0 ” K. can be calculated by assigning the electrons two to each allowed electron energy level, and by summing the energy over all occupied levels. At 0 ” K., and at other temperatures for processes of similar entropy change, the changes in total energy control both equilibrium and rate. The pattern of electron levels in energy-Le., the electronic structure or configuration-decides the total energy and it is in the light of this relationship that one refers to the effects of electronic structure on change in general and to heterogeneous catalysis in particular. On this basis a geometric factor is not a fundamental requirement, but the idea is retained because complexity obscures the detail of the dependence of geometry upon electronic structure. The new emphasis upon the electron structure of solids in the study of heterogeneous catalysis is thus revealed as a closer investigation of the correlation with the periodic table. It is now a truism that chemisorption of at least one reactant is a requisite of catalysis and a great step forward has been made when its equilibrium and rate can be related t o the properties of the solid adsorbent. EQUILIBRIUM A N D RATE

As in other complicated reactions, simplification ensues for homogeneous surfaces if the over-all change is split into a number of “irreducible” steps-i.e., irreducible in the sense of its not being possible or useful t o subdivide further on the basis of existing data. Obviously there are sorptive processes (adsorption and desorption) and reactions on the surface (simple or chain) in which, by stoichiometry and analogy, the participants are formally the common species or their derivatives (atoms, radicals, and i6ns). Chemisorption is the factor common t o all the interchanges and its equilibrium and rate must be examined for each substrate. Accumulating studies of the kinetics of homogeneous processes suggest a simplification, at least of qualitative value, which should be applicable t o reactions with a solid surface. Thus in a group of reactions where the energy surfaces are geometrically similar it appears that the free energy of activation, AF*, is proportional to the reciprocal of the free energy change, AF (43). Extended t o the Lennard-Jones (Si) representation of activated adsorption (Figure l), this means t h a t the activation energy for adsorption of

INDUSTRIAL AND ENGINEERING CHEMISTRY

977

NICKEL-CATA LYSTS a given substrate varies inversely as the adsorption energy over a set of similar surfaces-e.g., metallic surfaces. Analogous curvea connecting two or more adsorbed states can be drawn to facilitate the discussion of changes within the chemisorbed layer.

I

I

1

1

VAN DER W M L S ADS

-/’ CUCMISDRPTION

DtSTANCE FROM SURFACE

Figure 1.

____L

Activated Adsorption

Everything, then, hinges upon the free energy of chemisorption viewed as the reaction between a giant molecule ( a crystallite) and some simpler species. The bonds formed in adsorption must arise from the same kind of forces which yield ionic, covalent, or physical bonds in the simpler molecules, but whereas in the latter the valence concepts are straightforward, this is far from true for the solid surface. The valence properties of metallic surfaces will be considered before proceeding t o the strength of the chemisorptive bonds. ELECTION CONFIGURATION AND VALENCY OF METALS

The characteristics of electrovalency and covalency are electron transfer and electron sharing, respectively, each without violating the Pauli principle, and accompanied by a decrease in the free energy of the system. These were of old related to electron configuration by means of the Lewis-Kossel octet theory, which has become transformed on the one hand into the method of molecular orbitals and on the other into that of the directed valence bond. Modern physics of the solid state provides twol almost complementary, models of metals, the one based upon band theory (40, 64) related to the molecular orbital method, and the other upon Pauling’s resonating-valence-bond theory (40, 46, W), derived from the valence-bond method, and these must be examined to find the metal analogs of the electrovalency and covalency of simpler species. Band Theory. The metal electrons are confined to a potential box, so that the discrete atomic levels have broadened gt the observed interatomic distance into overlapping bands. Each band is related to an energy level of the free atoms and only the electrons (valence electrons) in excess of the inert-gas configuration need be considered; thus in the long periods one can conveniently distinguish 3d, 48,and 4p bands (titanium t o selenium), 4d, 58, and 5 p bands (zirconium to tellurium), or 5d1 63, and 6 p bands (hafnium to polonium). At 0 ” K. the outermost electrons of the atoms occupy the levels in pairs up t o the Fermi energy (Emm.) and the properties of the solid are fixed by the energy-density of levels, n ( E ) a3 a function of energy, E, measured 978

upward from the base of the band, and by the extent to which the levels are filled. I n Figure 2 these parameters are shown diagrammatically (for nickel) in the conventional potential energy box; the energy zero, corresponding t o a free electron a t rest outside the metal, is the broken horizontal line a t the top, while the bottom of the band is inaicated by the full line below the metal region. The metal work function, ‘p, is also shown and the potential well of a substrate species with a filled level, a t a diatance proportional to its ionization potential below zero. Electron level density is represented by t,he curve (full and broken) drawn so that its distance from the left-hand boundary of the metal is proportional to n(E). The, following observations are relevant: 1. At 2’ > 0 ” K. the only unpaired electrons are those with energy close to Emax,. 2. The rate of increase of the t,otal energy of the system per added electron is inversely proportional to n ( E ) a t E,,,. for a given number of metal atoms. 3. The value of n ( B ) a t E,,,. is much greater for levels of d character than those of s p type. Consider the transitional elements of t,he first, long period as members of integral electron concentration (ne,valence electrons per atom) in a continuous series of solid solutions formed between adjacent metals from titanium to selenium. Then semiempirical theory and experiment suggest that with increasing ne from 4(Ti) t o 10.6 (4%: GCu) a relatively narrow and dense band of levels, principally of 3d type, is being filled. If the energy of the bottom of the band is assumed t o depend largely upon atomic volume (as in the t,heory of Hume-Rothery alloys, 29), the Fermi energy and chemical potential of the electrons increase continuously with n. but, a t a rate depending upon % ( E ) . Emax, increases much faster as soon as the d band has been filled; it also depends upon the lattice geometry. The unfilled d levels (“holes in t h e d band”) close t o the top of the d band, as evidenced by st,rong paramagnetism or ferromagnetism, contain unpaired electrons; these may be paired with the s and p electrons of hydrogen, copper, zinc, aluminum, etc. The unpaired electrons must therefore be counted among those available for binding substrates a t the crystal surface and can be called residual-valence electrons. These electrons do not contribute significantly t o the metal cohesion, but there may exist surface states containing paired electrons which arise from the unsaturation of the cohesive elect,rons a t the surface (see below). Complete filling of the holes in the d band, like completion of an octet, is tantamount t o saturation of the residual valency, as the work required t o put an additional electron into the system then becomes very large. Elements and alloys of ne > 10.6 can pos688s d band holes in small concentration due t o thermal activation of electrons from the d band t o the s band with an activation energy N 1 e.v.

Resonating-Valence-Bond Theory. Pauling’s conception of the bonds in metals is probably more congenial to the chemist. He ascribes t o the transitional metals an electronic configuration obtained by hybridizing the five d orbitals, one s orbital, and three p orbitals t o yield nine strong, binding d s p orbitals which on the basis of magnetic and geometric data are divided into the following groups : “Atomic” orbitals, principally of d character, not effective in cohesion but responsible for the magnetic properties and therefore the analog of the d band holes. “Bonding” orbitals ( d s p ) , producing cohesion, of varying d content and containing no unpaired electrons (in the interior of the solid). The number of electrons per atom in these orbitals is called the metallic valency. ‘LMetallic”orbitals, largely of s p type; on the average at least a fraction of one of these is empty to permit unsynchronixed resonance of the valence electrons. According to the simplest picture i t is possible for a vacant metallic orbital of d character t o appear but this seems unlikely. Comparison of this model with the physical properties of metals and alloys shows that unfilled atomic orbitals are the seat of residual valency in the bulk metal. The two pictures of the solid state are contrasted in Figure 3 for metals adjacent t o nickel. Pauling’s structures cannot be adequately represented because they are mixtures of configurations and only one of these is shown for each element. The full line across each section is the Fermi energy; the d and s bands have been displaced with respect

INDUSTRIAL AND ENGINEERING CHEMISTRY

Vol. 44, No. 5

NICKELCATALYSTS to this energy to show how the d bands are flled and ferromagnetism disappears at copper. In the same way the orbitals of the valence bond model are arranged to show the occupancy of the bonding and the atomic orbitals. The egure does not represent the relative energies in detail. Similar configurational changes occur in the other long periods.

METAL

Figure

2.

SUBSTRACTE

Electron Levels in M e t a l and Substrate

At the solid surface bonding orbitals (which are fully occupied in the metal bulk by valence electrons) become, a t least in part, available for binding substrates and must be included in the residual valencies of the metal surface. Zener (7i) has recently set up a group of postulates, based upon empirical reasoning, which purport to describe the magnetic and structural properties of the elements; i t supplements the concepts already given, in the region where they are weakesti.e., among the metals of strong, constant paramagnetism. Residual AfBnity. Both theories can be used to describe the trends in the binding potential of the residual valencies of the surface. The bend theory does this via the Fermi energy of the metal electrons, which, providing the ratio of metal valence electrons t o substrate valence electrons is very large-Le., the metal crystallite is not too small-controls the maximum kinetic energy of the electrons in the metal-substrate complex. On Pauling’s theory the binding potential should be related to the strength of the residual valence orbitah and connected in some way with the cohesive energy of the metal lattice. Since the chemical potential of the metal electrons increases with ne, the residual affinity must diminish with increasing ne and the rate of change is best shown by the variations in electronlevel density with Emx.. ADSORPTION AND ABSORPTION

Approaching the solid surface the substrate molecule passes, perhaps fleetingly, through the states of van der Waals’ adsorption and chemisorption before entering the lattice into the absorbed state. The binding in each of these states, being dependent in part upon a residual valency of the lattice, depends upon the electron configuration of the metal. It also depends upon the electron configuration of the substrate. Physical Adsorption. This term usually implies a condensation with an exothermic heat < 5 kg.-cal. mole-I, but recent work upon chemisorption and catalysis (63) suggests that the former

May 1952

can occur with a small heat and emphasizes the principal connotation “Ictck of electron transfer or exchange.” The theoretical studies of Prosen and Sachs (49) indicate that the attractive potential in van der Waals’ adsorption depends upon the electron density. On the other hand, the polarizability of d shells should be rather less than that of s p electrons. It would be fortuitous if these opposing effects cancelled, and the heat of adsorption should undergo some change when the electron-level density alters sharply, as when a d band is just filling. The polarizability of simple substrates is roughly measured by their ionization potentials (32),which in turn can be estimated by the methods of Mulliken from the electronic structure of the molecule. Thus the heats depend intimately upon the structure of the reactants, but the variations from these sources must be 6 on the same scale, this suggests t h a t adsorption at a clean metal surface must always produce a dipole layer with its negative side outward. Such a conclusion is premature, because ignorance of the detailed electronic configuration of the adsorbed complex prevents the proper choice of the correct valence states (41). Absorption. Within the metal there should be the same kind of dependence of heat of solution (“occlusion”) upon the Fermi energy as is shown by the heat of adsorption, and in fact there exists a parallel variation in these two properties for hydrogen, both becoming increasingly exothermic with decreasing n, (10, 86, 70). This section has sketched the energy pattern of sorption on bare surfaces of metals of the long periods in its relation to electronic configuration of solid and substrate. Completion of the picture requires a closer investigation of the individual bonds at the surface, for which the Pauling model is best suited. METAL-SUBSTRATE COMPLEX

The properties of the palladium-containing binary and ternary alloys of palladium, hydrogen, and the group IB metals, together with a comparison of the extent and rate of chemisorption of hydrogen by the metals of groups 8 and lB, show that vacant d orbitals are a requisite for strong, general chemisorption (9, 10). This implies that the residual metal valencies resident at the surface in the unsaturated electrons of bonding orbitals are not alone sufficient for strong chemisorption; unfilled atomic orbitals are also necessary. The process of chemisorption must then be somewhat a8 follows, in which the two stages are represented separately:

[

M - H

1% IL,

=

0

=

0

h metal center, M , having, for descriptive purposes only, one hole, h,, in the d band and two vacant bonding orbitals combines with a hydrogen atom. In the first stage the hydrogen atom donatesl its s electron t o the metal to fill the hole and the subsequent bond can be ionic (as shown) or covalent. I n the first case the unused surface bonding orbital electrons can then be

980

extended to overlap the proton and the final bonding represents more or less saturated surface valencies. The polarity of the bond depends upon the electronegativities of the metal crystal and the hydrogen atom. More simply this can be written M % H f , in which the adsorbed species increases the ligancy of the surface metal atoms, and t o some extent there restores the resonating bond systein of the metal. More complex reactants such as ethylene are bound similarly, but the number of scission products and consequent combinations is greatly increased. The possibility t h a t the most loosely held pair of T electrons enter the holes in the d band, while those of lower level combine with the bonding electrons, seems t o be contraindicated by the magnetic properties of the alloys of nickel with polyvalent metals such as antimony (50), where 3 to 4 valence electrons occupy d band holes in nickel. The first step in the above scheme must be critical, because chemisorption is rcduced in heat and rate in its absence; in the absence of d band holes, this trigger reaction does not occur. Preservation of the dependence of heat of adsorption upon the Fermi energy necessitates that the heat contributed by the second step should vary directly with that of the first step, and the direction of the electron transfers in each part makes this likely Various forniulaticns have been proposed for the content of the d s p hybrid bond, which must in some ways be like the metal valence bonds, and suggest a relationship between heats of sublimation and chemisorption similar to that of Eucken (6, 16). Eley ( 1 4 ) has used the metal cohesive energies and formal bond energies of some simple substrates, together with Pauling’s postulates (44) on the additivity of normal, covalent bond strengths, to calculate heats of chemisorption which are in moderate agreement with experiment. However, the inadequacy of this simple approach can be realized from Beeck’s results (!?-4) for hydrogen and ethylene on chromium and tungsten, .ir-hich show t h a t each gas has the same adsorption heat on both metals, despite the fact that their sublimation heats have a ratio >2. I n covalent bonding in molecules, the phenomenon of saturation is found; moreover, as in the study of inorganic complexes, it is usual to find that with increasing number of coordinated groups the bond strength decreases. The corresponding aspects of the residual valencies of the metallic, giant molecule are intimately connected in d metals, with the hole equilibrium between bulk and surface, and between surface atoms. If it is first assumed that no work is done in bringing a hole from the interior to the surface of a clean metal, then the sole factor influencing the concentration of the holes in the surface is the free energy of the chemisorption process which forms “bound” holes. As long as the free energy of adsorption is large enough to make chemisorption irreversible, holes will always migrate to the surface t o be converted into “chemisorbed” holes. The same applies t o coupled holes. Thus nickel possesses 0.6 hole per atom in the 3d band, or a t any instant 0.6 is the fraction of bulk atoms carrying one hole. In the covered surface the concentration of bound holes can be as high as one per atom; in the bare surface it remains at 0.6. Possibly, if some sorbed species were sufficiently active, coupled holes, two or more in a group, could be induced a t the surface, but the data on massive nickel do not support such an outcome. Single holes or coupled holes must repel each other, and unless chemisorption a t each center results in complete “neutralization,’, holes will tend to remain away from occupied sites, EO that an adsorption act requiring a local aggregation of holes (polycentric adsorption) will then be greatly slowed. Consider the nickel-copper continuous series of solid solutione in which, a t a sufficiently low temperature, the ,3d band of nickel is just full a t the alloy containing 0.6 atomic fraction of copper (40, 64). Then from pure nickel up t o this composition the initial heat of adsorption of hydrogen fa1k slowly as E,,,. Increases, while at the same time the fractlon of surface covered with adatoms remains constant and close t o unity. At low temperatures there will be, presumably, no chemisorption on the more copper-rich alloys. As the temperature is raised, electrons are raised from the d band t o the top of the s band,

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

Vol. 44, No. 5

NICKELCATALYSTS requiring an amount (Wh) of work increasing from zero at 0.6 copper t o ca. 50 kg.-cal. mole-’ at pure copper, according t o simple band theory (39). These thermally generated holes become bound in chemisorption. Thus at moderate temperatures t h e initial heat of adsorption on t h e copper-rich alloys is decreased from 28 kg.-cal. mole-’ by Wh, and the activation energ for adsorption is increased by an amount 0: Wa. As the initiafheat declines, the normal fall of heat with surface coverage mbre rapidly reaches the value at which chemisor tion is impossible-i.e., t h e fraction of surface covered by atatoms rapidly diminishes m the copper content is increased beyond 0.6. I n real catalysts t h e activity of t h e copper-rich alloys will be extremely sensitive t o impurities, heterogeneities, mechanical faults (dislocations, etc.), and concentration fluctuations (19). On a large crystal of homogeneous surface, the heat of adsorption remains steady with increasing coverage until dipole interaction begins, but the heat begins to fall rapidly only when the filled orbitals of the metal-substrate complex begin t o interact. The rate of attainment of the stationary state must depend upon the concentration of holes, as does the heat of adsorption (via Emax.), but the extent of surface covered by irreversibly adsorbed gas depends primarily upon the substrate (via the effective coverage per adsorbed molecule) and not upon the hole concentration, provided this is not too small. I n the region of reversible chemisorption the surface covered by chemisorbed gas is proportional t o some function of the hole concentration (according to the number of holes required per site). Iron is an example of a metal containing coupled holes, where the atoms carry, presumably randomly, either two or thrqe holes in the ratio of 4 to 1 and the possibilities are enormously increased. I n the ferromagnetic alloys of nickel with copper, zinc, etc., i t is not known whether the holes appear with equal frequency at each kind of atom. It is possible, then, t o visualize repulsive geometric effects due t o ,the size of the substrate and the properties of the holes, and saturation effects due t o a n insufficiency of d band holes; thus on some surfaces the transition from adsorbed ethylene as (2Ci 4H.) through 2(: CHz) to (CH2 - CH2)with a falling heat might occur with increasing coverage. I n princ*ipleit should be possible t o realize chemisorption with a heat of the order usually associated with physical adsorption.

+’

RATE AND KINETICS

The association of extensive chemisorption with the presence of unfilled d orbitals, and the increasing heats of chemisorption with decline in atomic number of the d metals (metals with d band holes), leads immediately t o the conclusion that high general activity will be found only with d metals and t h a t activity will fall in each period as one proceeds t o the left from group 8. The incompleteness of information on reaction mechanisms, even such simple processes as the ortho-parahydrogen conversion and the hydrogen isotope exchange, prevents the derivation of a more precise relation between rate and electronic configuration. Nevertheless, these reactions of hydrogen show clearly the natdre of the correlation. Hydrogen Reactions. Theory and experiment (17) agree that t h e homogeneous reaction H pHz oHz H is bimolecular and has an activation energy ca. 7 kg.-cal. mole-’. Translated t o the sorbed layer on a metal, as a reaction between chemisorbed atoms and physically adsorbed molecules, it becomes the Rideal mechanism. RIDEALMECHANISM (5.9). Since the adsorption heats of the molecular species are almost equal and the activation energies for chemisorption almost zero, the activation energy of the aurface exchange must also be -7 kg.-cal. mole-’. Semitheoretical estimates (14) of the heat of chemisorption of hydrogen show that adatoms are t o be expected in large concentration at normal pressures on d metals; when the surface concentration, e, of atoms is independent of pressure, this leads to first-order kinetics. I n the stationary state the true activation energy

+

M a y 1952

-

+

must be almost independent of the metal, in this approximation, whereas the rate = e ; the surface is then always saturated, the maximum value of e being fixed by the number of d band holes per atom, and filling these holes reduces the concentration of adatoms while increasing the apparent activation energy by the energy required t o transfer an electron from the d band to the s band. Experiment had, of course, established in advance that the heterogeneous reaction is of the first order with a n activation energy -7 kg.-cal. mole-’, but it now appears that the agreement F a y be fortuitous. Couper and Eley (9) (palladium-gold alloys) and Rienlicker (59) (copper-palladium alloys) have demonstrated that on filling the d band the apparent activation energy actually shows a big increase. The Rideal mechanism seems to be the only probable one on those surfaces where the heats of chemisorption are large. I n general, for mobile chemisorbed films, it implies a regime where adatoms coexist with molecules in the first layer and therefore where heats are low and a dissociative mechanism is feasible. BONHOEFFER AND FARKAS MECHANISM ( 7 ) . Reaction could proceed equally well between adatoms with desorption as molecular hydrogen the rate-controlling step, and a true activation energy equal t o the heat of desorption. This process can operate only a t e N 1 (for d metals) where the adsorption heats are not too large and chemisorption is reversible-that is, where theory has yet little t o say about the variation of heat with e. The data of Couper and Eley (9) show that the apparent activation energy remains unchanged over the palladium-gold alloys from pure palladium u p to the filling of the d band and then rises sharply. If the apparent activation energy were also the true one, this observation would mean t h a t the heat of desorption remains constant until thed band is full and then increasesrapidly, a not unlikely variation. However, Trapnell and Rideal have shown that the apparent activation energy may really be controlled by the manner in which heats of chemisorption decrease with e, in the region where chemisorption is reversible. On the palladium-gold alloys this requires that the dependence of the heats on 8, near surface saturation by adatoms, should be the same on the metals from palladium up t o the equiatomic composition. Eley (14)has suggested a compromise in which the Rideal mechanism operates over the palladium-rich alloys and the Bonhoeffer-Farkas process over the gold-rich ones. Complex Reactions. The ideas inherent in t h e mechanisms proposed for the hydrogen reactions can b e traced in the far more complicated situations visualized for the hydrogenation of unsaturated hydrocarbons and exemplified by Twigg’s analysis (68) of hydrogenation and exchange with ethylene:

(1) C2Hd I CHz-CHp

Tsi

el

Psi

‘-’

Psi

es

N‘i

e2

This scheme leads to expressions for the rate of .hydrogenation ( a t low temperatures = k3paz el), the rate of exchange (higher temperatures = k3pH2el), and the rate of double bond migration ( = [ksksk6/kc k , ] 1 ” p ~ ~ 8in 1 )which 01, the surface concentration of adsoibed ethylene, and the individual rate constants can depend upon the electronic structure of the metal. T h e ionization potential of the outermost T electrons of ethylene is smaller than

+

INDUSTRIAL AND ENGINEERING CHEMISTRY

981

NICKEL-CATALYSTS that of the hydrogen-binding electrons. Ethylene is therefore expected to have the higher heat of chemisorption and to be preferentially adsorbed up to the surface coverage a t which the d orbitals in the surface are almost saturated; this gives first-order kinetics for hydrogenation on all d metals. As before O1 decreases as the d band holes are occupied and Wh may appear in the apparent activation energy. Beeck (2-4) has studied the hydrogenation of ethylene, a t 0’ C. over a number of metallic elements, finding that the kinetics are first order Kith respect t o hydrogen and zero order with respect to ethylene in all cases. The apparent activationsenergy is constant a t about 11 kg.-cal. mole-’ for all those metals of group 8 which were examined, despite large variations in the rate constant. The original interpretation given by Beeck is that the over-all rate of hydrogenation is proportional t o the fraction of surface not covered by adsorbed ethylene. The fast process is the reaction of adsorbed hydrogen with ethylene, while the slow and ratedetermining process is the removal of adsorbed ethylene from the surface by hydrogenation with adsorbed hydrogen. On this basis one might expect that the reaction rate would decrease as the heat of adsorption of ethylene increases; the rate therefore decreases as the Fermi energy decreases, as the chemical potential decreases, or on Pauling’s model as the valency v , the percentage d character, d, and v X d decrease. Schuit (62) has given an astonishing (for an apparently complex process) correlation of Beeck’s data by using the relation loglo Ica (v x d). This means that the apparent free energy of activation decreases as ( v X d ) increases, but data are too sparse to allow the development of a satisfactory relationphip between (v X d ) and heats of chemisorption. General Conclusions. It appears then that reactions proceed via radical intermediates, the heat of chemisorption of which controls the surface concentrations and the rates of the steps in the reaction sequence. The heats of chemisorption are controlled by the electron configuration as discussed above. I n any long period of the periodic table, chemisorption of niolecules by odd electrons and lone pairs, or through splitting into atoms, radicals, biradicals, etc., is possible only in large surface concentration when the metal possesses holes in a d band, Consequently high activity a t low temperatures is also associated with such an electron configuration. Reactions proceeding on surfaces where chemisorption occurs but with a small heat of adsorption are expected to be faster when the initial heat is lorn. and where the whole surface chemisorbs-Le., as the chemical potential of the d metal electrons increases. Since the Fermi energy is sensitive to lattice type, abrupt changes in activity can be expected a t phase boundaries-e.g., among the HumeRothery alloys. Where surface processes utilize a substrate ion as intermediate, the over-all rate may depend upon the rate of formation of the ion or upon its concentration, both of which are controlled by the electronic work function of the metal in an obvious way (11). In each long period the work function of the pure metals appears to increase from group 4 to a flat maximum in group 8 and then t o decrease again in groups 1 and 2. This must then be the pattern of activity for a reaction controlled by positive ion formation and the reverse is true when negative ion formation is essential. I n the next sections these concepts are applied to metallic catalysts containing nickel. GENERAL ACTIVITY OF METALLIC NICKEL AND IT§ A L L O Y §

Metallic nickel catalysts include all those containing pure nickel or nickel alloys as separate phases, regardless of form or support. Only the metals of groups 8 and lB, together with cadmium, tin, and lead, possess oxides which are reducible in the pure state at moderate temperatures and are also of,interest catalytically. A comparison of the free energies of formation of nickel oxide (NiO) and manganous oxide (MnO) shows how the reducibility

982

of nickel oxide depends upon its electronic configuration. The major difference lies in the heats of formation, and the calculation of these heats, using Born-Haber cycles, reveals that the difference arises largely from a big increment- in the second ionization potential on going from manganese to nickel and to the greater cohesive energy of nickel. At a given temperature, metaluc phases exist only below a certain partial pressure of oxygen and sulfur (or their compounds), which can easily be estimated from thermodynamic data. Pure Nickel. The magnetic properties of nickel and its alloys are the basis of the simple theories of the solid state which describe nickel as having 0.6 hole per atom in the 3d band and set these filled in alloys with ne = 10.6. The low temperature, electronic specific heat data of Keesom and Kurrelmeyer (SO) in the alloy systems iron-nickel and nickel-copper confirm this picture, in essentials, and show that n(Bmx.)in the quaitet iron, cobalt, nickel, and copper reaches a flat maximum between n, = 10 and n, = 10.7and falls to a low value a t copper. Thus nickel possesses the largest chemical potential of any elemental d metal in the first long period; from the last section its general activity is expected t o be the greatest. The Feimi energy of ferromagnetic nickel is less than that of paramagnetic nickel (18); consequently changes in reaction characteristics must be expected as reaction temperatures pass through the Curie point; this is the magnetochemical effect. The correlations s u g gest that for processes of the kind discussed above, the intrinsic activity should increase with the dkappearance of the fer1omagnetic state. Alloys with Subgroup B Elements. All these elements possess loosely held valence electrons which can enter the 3d band of nickel (50) and decrease it8 activity. Moreover, even those having irreducible oxides, such as zinc, must be considered in reducing . systems because of the possibility of reactions like Ni ZnO or-KiZn alloy -j- 1/202. In this section interest is focused upon the B metals forming solid solutions or intermediate compounds with nickel-e.g., copper, gold, zinc, cadmium, aluminum, carbon, silicon, tin, phosphorus, arsenic, antimony, tellurium, etc.among which aluminum and silicon possess oxides that are too irreducible to be effective in ordinary practice. Addition of these elements to nickel as homogeneous alloys, or casual admixture of the elements or their progenitors u i t h catalyst or reactants fills the d-band holes and reduces activity; their effectiveness in this respect varies directly with valency. -4t higher temperatures the much smaller activity of the alloys and compounds with filled d bands should also vary inversely with the valency of the modifying element, a t a given atomic concentration. Alloys with Subgroup A Metals. Most of these possess irreducible oxides (except perhaps molybdenum and tungsten a t moderate temperatures) and their practical interest lies only in the marginal effects noted above. Addition of these metals t o nickel presumably reduces the chemical potential of the electrons of the metal, causing a large increase in heats of chemisorption an’d a decrease in activity, probably along the lines of the Schuit correlation, If Pauling’s valencies are taken for the metals, then one notes a coincidence of variable valence (at chromium and manganese) with abnormal structures, and the change from endothermic to exothermic hydrogen occluders (10) Then nickel alloys with n, < 6 should begin to absorb hydrogen extensively and hydrogen deactivation should occur. Alloys with Group 8 Metals. All metals of this group have reducible oxides and the common ones form continuous series of solid solutions with nickel. I n the same period, cobalt and iron must reduce the general activity of nickel, while in different periods palladium and platinum with similar band forms should increase the activity. The palladium-rich alloys, showing “anomalous” hydrogen occlusion, will be hydrogen “deactivated” (in which state d band holes are occupied, in the bulk, by electrons from hydrogen) and under certain conditions will have irreproducible activities (9, 11).

INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY

+

-

Vol. 44, No. 5

NICKELL ATA LYSTS Poisons, Clearly all elements, other than palladium, platinum, and perhaps a few of the rarer, group 8 elements, must deactivate nickel, even when the interaction is only one of chemisorption. The term "poisoning" is reserved for a catastrophic decrease in activity associated with small amounts of additives. The rules devised above lead straight t o the group B elements and their compounds, especially those of high valency, as the most potent. The large valency (normal, not Pauling's) requires a large Fermi energy and therefore a small sublimation energy, so that this group of poisons is relatively volatile and produces a mobile alloy surface lattice; such poisons are thus easily distributed. Strongly electropositive and electronegative elements such a s the alkali metals and the halogens, respectively, are chemisorbed as tightly bound cations and anions with resulting geometric blockage of the surface. The surface dipole is also altered, and this should produce a change in the polarity of the bonds holding a chemisorbed substrate and should probably modify the relationship between heat of adsorption and surface coverage. Oxygen and sulfur produce effects as electronegative species and perhaps, especially in the case of sulfur, as group B elements; sulfur possesses six electrons which might in special circumstances occupy the d band of nickel. All reactants possessing electrons of relatively low ionization potential can function as poisons (38); the reversibility of the poisoning depends in 'an obvious way upon the heat of chemisorption and upon the thermodynamics of possible bulk phasea. SPECIFIC A R E A S

OF NICKEL CATALYSTS

A t each stage in the genesis of a catalyst the specific area is dependent upon the previous history through a complex set of chemical and diffusional reactions, often involving solids which are not metals; being activated processes (collectively causing sintering a t high temperatures), the paramount variable controlling their rates is the temperature. Empirically the onset of a rapid decrease in the specific area of a normal crystalline solid can be associated with a temperature somewhere between the surface Tammann temperature (0.3 X melting point, K.) and the Tammann temperature of the bulk (0.5 X melting point, O

K.) (6). Pure Nickel Catalysts. Usually the preparation of catalytic nickel involves the presence of nickel oxide a t a preliminary stage. Nickel oxide (NiO) melts at 2090" C., so that lattice mobility must be expected a t 500" C., whereas with metallic nickel it is appreciable a t a.250" C. in industrial contacts. Thus a t temperatures above 250" C. the specific area of a pure nickel catalyst will rapidly approach the geometric area regardless of the mode of preparation-Le., the formation and treatment of nickel oxide-unless special steps are taken. Pure nickel is commonly employed a t temperatures below 250' C., so that preparative variables can have large effects upon the specific area in the quasi-stationary state of the catalyst. The relevant topochemistry is too extensive to be discussed here, but it is notable that temperatures are kept as low as possible in each reaction stage, thus producing defective lattices, which in turn increase the rates and lower the temperatures required a t succeeding steps. Addition of foreign ions can produce similar defects. Clearly both specific area and specific activity are sensitive to the two mutually dependent variables, temperature and extent of reduction of the nickel oxide. In practice, optimum conditions are approximated by utilizing easily decomposed hydroxides and neutral or basic salts of nickel, such as the formate and oxalate, or the carbonates. Catalysts prepared by the extraction of suitable alloys of nickel with aqueous alkalies or acids are made under almost ideal conditions, since the metallic nickel is produced a t temperatures as low as 0" C. without auxiliary reduction. Raney nickel, in powder or granule form, is such a catalyst, and appears to contain extensively

May 1952

defective crystals (66); however, these catalyab can hardly ever be called "pure nickel." In view of the low melting point of nickel sulfide, sulfur must be counted among the most dangerous of poisons to nickel catalysts. .Nickel Alloy Catalysts. Large electron concentration implies small cohesive energies, low melting points, and consequently greater lattice mobility a t a given temperature. One therefore expects big changes in the specific area of metallic catalysts as the d band is filled (61). It is known that the activation energy of diffusion in metals increases with the absolute melting point (68), and the diffusion constants must show a similar trend The data of Matano (37; 63, p. 60) for the systems nickel-gold, palladium-gold, and platinum-gold show rapid increases in the diffusion constants (at ca. 1000 " C.) a t about the composition where magnetic properties indicate a just filled d band. Near the same composition, the melting points in the nickel-gold solid solutions show a minimum, those in the palladium-gold series begin to fall rapidly, while in the platinum-gold system the liquidus shows similar behavior. On the other hand, in the nickel-copper series of solid solutions no comparable variations in melting characteristics are found either a t the L'critical" composition or elsewhere, despite a rapid increase in diffusion constants (98). Thus with present information it seems safe to associate both melting (cf. Jones, 99) and diffusion phenomena with electronic configuration (Fermi energy) and to await a more complete correlation. In cases where alloys of nickel with subgroup B elements are used at moderate temperatures, as when pure nickel is too active, i t is clear that loss of specific area will be pronounced unless the catalyst is supported. Metals with irreducible oxides will always produce nickel alloys supported upon the irreducible oxide, Nickel can always be made more refractory by addition t o i t of metals to its left in the first long period, but only with accompanying loss of activity. Supports. The function of a support is to induce and maintain a high specific area in metallic catalysts without affecting their specific activity. All of these requirements are seldom met; metals and the more readily formed excesR semiconductors cannot be used in this service unless simultaneous modification of the nickel activity is also required. Carbon and the refraotory oxides remain, the latter of high lattice energy, being found among the oxides of the metals forming small cations of large electrovalence-e.g., magnesia, alumina, silica, and their congeners. The most important of these are the network formers which yield amorphous oxides of large specific area, The mode of operation of the supporting oxide can vary between two extremes; on the one hand i t is simply the retention of small agglomerates of parent oxide and then metallic nickel in isolated pores-e.g., kieselguhr (@)-and on the other hand the formation of stable oxide compounds-e.g., NiAl2O4-which are reduced only slowly and permit the formation of large numbers of reduction nuclei (probably forming first a t regions away from the mobile cation vacancies in NiAlzOc/~-AlzOa solid solutions). Although supports shift the incidence of sintering t o higher temperature ranges, the specific area of supported alloys must still show the same dependence upon the occupation of the d bands. In this section it has been shown that specific area as well a8 specific activity is a function of electronic configuration; next the ubiquity of this theme will be demonstrated for a wide range of reaction types. SPECIFIC ACTIVITY OF NICKEL CATALYSTS

A considerable amount of direct experimental evidence now exists which demonstrates the correctnesa of the activity pattern outlined by the theory. Only a part of this collection concerns

INDUSTRIAL AND ENGINEERING CHEMISTRY

983

catalysts containing nickel; nevertheless, because of the similarities in band structure, cognate data on systems based upon palladium and platinum will also be noted. Reducing Atmospheres. Theory and experiment are expected to be in best agreement for reactions carried out in reduchg atmospheres over binary alloys of metals having reducible oxides. ORTHO-PARAHYDROGEN CONVERSION. The work of Couper and Eley (9) upon the palladium-gold series of solid solutions provides P most elegant illustration of the coincident drop in activity and increase in apparent activation energy as the 4d band fills. Results by Rienacker and Sarry (59) on the palladium-copper alloys, when disentangled from order-disorder effects, are qualitatively similar; there is a rapid fall in activity a t about the composition where the 5d band becomes full (ne = 10.5), but the rise in apparent activation energy is delayed until n, = 10.8 to 10.9. In both systems the frequency factor declinw as the d band fills. HYDROGENATION O F UNSATURaTED COIIPOUXDS. Hydrogenation of multiple bonds has been the object of systematic work by Rienacker et al. using ethylene [nickel-copper (66), palladiumcopper (58), platinum-copper (58)] and cinnamic acid [nickelcopper (56)], by Reynolds (51) and Dowden and Reynolds ( I S ) (nickel-copper, nickel-iron) with styrene and benzene, by Long, Frazer, and O t t (39)and Emmett and Skau (15) with benzene (nickel-copper, palladium-gold, cobalt-iron). In all cases the catalyst activity drops off to a low value in the region where the d band is full, although the way in vhich this occurs varies from system to system; the apparent activation energy always increases in the copper and silver-rich alloys, but again the observed step-up does not invariably appear in the close neighborhood of the critical composition. Addition to nickel of metals (iron) that lower the Fermi energy also decreases its activity ( I S ) . Comparable activity patterns appear whether the reactants are liquid or gaseous, whether the metals are unsupported or supported, and whether the catalysts are p r q ~ a r e dby evaporation, with alloying by fusion, by reduction of the oxides, or by alkali extraction of ternary alloys with aluminum. Industrial practice is concerned onlv v ith catalysts of optimum activity, so that examples conforming to the entire pattern are not easy to find. The hydrogenolysis and saturation of natural fats and fatty acids to give long-chain alcohols (48) provide one of the best cases from the patent literature; methane production from carbon monoxide and hydrogen during methanol synthesis over metallic catalysts is another. DEHYDRATION. One of the most studied reactions is the decomposition of formic acid vapor. Rienacker et al. are again foremost [nickel-copper (64), palladium-copper (60),platinumcopper ( 6 7 ) ] followed by Dowden and Reynolds ( I S ) who also examined the decomposition of methanol (nickel-copper). Results are in general conformity with the theory; the same kinds of variation noted above are present. Oxidizing Atmospheres, Formation of oxide films confuses the issue in reactions over nickel alloys; Tammann (65) found t h a t platinum and palladium are most active in the series of alloys, platinum-silver, platinum-gold, palladium-silver, palladium-gold, and that activity falls off as the unpaired electrons of the group 8 metals are paired off. I n the oxidation of sulfur dioxide platinum is much more active than gold (8, @), while in ammonia oxidation over palladium-silver alloys the rate falls off with increasing silver content (I). By way of contrast the rate of decomposition of aqueous hydrogen peroxide over the nickel-copper series ( I S ) shows a reverse dependence upon electronic configuration, being most rapid over the copper-rich alloys and falling t o zero as the 3d band emptiee; this effect is additional to t h a t arising from corrosion (see below). Corrosion Reactions.. I n their own way these processes are even more complex than those encountered in heterogeneous catal984

ysis, yet the importance of electronic configuration is easily observed. Uhlig (69) has developed such a theory of corrosion and has demonstrated the rapid attack by sea water upon the d metals of the nickel-copper series. The same pattern has been found bj. Hudson ( 8 6 ) for atmospheric attack, in the presence of sulfur compounds (nickel-copper) and by Irmann ( 9 7 ) for corrosion b y sulfuric acid (nickel-copper). Preferential attack on the alloys without d band holes is observed in high temperature oxidation ( 4 6 ) and in corrosion by aqueous hydrogen peroxide (22). Some of these effects may be equilibrium rather than rate phenomena. CONCLUSIONS

The foregoing discussion provides a preliminary theoretical basis for the study of the electronic factor in heterogeneous catalysis and shows how it can be used to explain the behavior of metallic nickel and its alloys. The theme can be developed i n many directions in its applications to induEtria1 catalysis (12). Moreover, useful parallels can be drawn for homogeneous catalyais in liquid media by transitional metal ions, electrochemical reactions, etc. Apart from the need for more experimental work to elucidate the nature of the activity pattern over wider ranges of electron configuration for dissimilar substrates, the ideas are closely dependent upon the current theories of thc metallic state. The time approaches when advances in both fields wiil be miitually advantageous. NOMENCLATURE

energy, measured upward from a band bottom maximum kinetic energy of metal electrons (Fermi energy) = electron-volt = change of free energy = free energy of activation = number of “holes” per atom in a d band = ionization potential = rate constant = electron concentration, valence electrons per atom counting from bottom of d band = energy-density of electron levels = number of “bonding” orbitals per atom (Pauling) = work t o raise an electron from d band t o s band = surface concentration, fraction of surface covered = electron-exit work-function = thermodynamic potential of metal electrons =

=

LITERATURE CITED

(1) Adadurov, I. E.,

Deich, Ya. M . , and Prozorovskii, N. 9., .I. Applzed Chem. (U.S.S.R.), 9, 807 (1936). (2) Beeck, O., “Advances in Catalysis and Related Subjects,” Vol. 2, p. 151, New York, Academic Press, 1950; Dkcusaiotis Faraday SOC.,8 , 118 (1950). (3) Beeck, O., Cole, W. A , , and Wheeler, A., Ibid., 8, 314 (1950). (4) Beeck, O., and Ritchie, A. W , Ibid., 8, 159 (1950). and Anderson, J. S., J . Chom (5) Bevan, D. J. M., Shelton, J. P., Soc., 1948, 1729. (6) Boer, J. H. de, “Electron Emission and Adsorption Phenomena,’’ Cambridge, Clarendon Press, 1935. (7) Bonhoeffer, K. F., and Farkas, A., Z. physik. Chem., B12, 231 (1931). ( 8 ) Boreskov, G. K., J . Phys. Chem. (U.S.S.R.), 19, 535 (1945). (9) Couper, A . , and Eley, D. D., Discussions Famday Soc., 8 , 172 (1950). (10) Dowden, D. A., Division of Colloid Chemistry, 120th Meeting AM. CHEM.SOC., New York, N. Y., September 1951. (11) Dowden, D. A., J . Chem. Soc., 1950, 242. (12) Dowden, D. A., “Modern Industrial Catalysts,” unpublished lectures to National Association of Spanish Chemists, Santander. 1950. (13) Dowden, D. A., and Reynolds, I?. W., Discussiom Faraday SOC.,8 , 184 (1950). (14) Eley, D. D., Ibid., 8, 34 (1950). (15) Emmett, P. H., and Skau, N., J . Am. Chem. SOC.,6 5 , 1029, (1943). (16) Eucken, A., 2.Elektrochem., 28, 6 (1922). (17) Farkas, A., “Orthohydrogen, Parahydrogen and Heavy Hydrogen,” Cambridge, Cambridge University Press, 1935.

INDUSTRIAL AND ENGINEERING CHEMISTRY

Vol. 44, No. 5;

N ICKEL-CATALYSTS (18) Fletcher, G. C., and Wohlfarth, E. P., Phil. Mag., 42, 106

(1951). (19) Goldman, J. E., Phys. Rev., 82, 339 (1951). (20) Greene, J. B., and Manning, M. F., Phys. Rev., 63, 203 (1943). (21) Griffith, R. H., “Advances in Catalysis and Related Subjects,” Vol. 1, p. 91, New York, Academic Press, 1948. (22) Grube, G., and Jedele, A,, 2. Elektrochem., 38, 799 (1932). (23) Gurney, R. W., Phys. Rev., 47, 479 (1935). (24) Herring, C., and Nichols, M. H., Rev. Modern Phys., 21, 185 (1949). (25) Himmler, W., 2. physilc. Chem., 195, 244, 253 (1950). (26) Hudson, J. C., Trans. Faraday SOC.,25, 177 (1929). (27) Irmann, R., Abhandl. Inst. Metall., Aachen, 1, 39 (1916). (28) Johnson, W. A,, Metals Technol., Tech. Pub. 2007 (1946). (29) Jones, H., Proc. Phys. SOC.(London), 49, 243, 250 (1937). (30) Keesom, W. H., and Kurrelmeyer, B., Physica, 7, 1003 (1940). (31) Lennard-Jones, J. E., Trans. Faradag SOC.,28, 333 (1932). (32) London, F., 2. Physik, 63, 245 (1930). (33) Long, J. H., Fraaer, J. C. W., and Ott, E., J. A m . Chem. SOC., 56, 1101 (1934). (34) McCartney, J . T., Seligman, B., Hall, W. K., and Anderson, R. B., J. Phys. and Colloid Chem., 54, 505 (1950). (35) Manning, M. F., Phys. Rev., 63, 190 (1943). (36) Manning, M. F., and Chodorow, M. I., Ibid., 56, 787 (1939). (37) Matano, Proc. Math. SOC.Japan, 15, 405 (1938). (38) Maxted, E. B., J. SOC.Chem. Ind. (London),67, 93 (1948). (39) Mott, N. F., Phil. Mag., 22, 287 (1936). (40) Mott, N. F., and Jones, H., ”Theory of the Properties of Metals and Alloys,” London, Oxford University Press, 1936. (41) Mulliken, R. S., Phys. REV.,46, 549 (1934); J. Chem. Phys., 2, 782 (1934); 3, 573 (1935). (42) Neumann, B., 2. Elektrochem., 35, 42 (1929). (43) Ogg, R. A., and Polsnyi, M., Trans. Faraday Soc., 31, 1375 (1935). (44) Pauling, L., “Nature of the Chemical Bond,” 2nd ed., Ithaca, N. Y., Cornell University Press, 1940. (45) Pauling, L., Proc. Roy. SOC.(London), A196, 343 (1949).

’

(46) Pilling, N. B., and Bedworth, R. E., IND.ENG.CHEM.,17, 372 (1925). (47) Price, W. C., Chem. Revs., 41, 257 (1947). (48) Procter and Gamble Co., Brit. Patents 562,609, 562,610 (1944); 570,957 (1945). (49) Prosen, E. J. R., and Sachs, R . G., Phys. REV.,61, 65 (1942). (50) Rado. G. T.. and Kaufmann, A. R., Ibid., 60, 336 (1941). (51) Reynolds, P. W.. J . Chem. SOC.,1950, 265. (52) Rideal, E. K., Proc. Cambridge Phil. Soc., 35, 130 (1939). (53) Rideal, E. K., and Trapnell, B. M. W., Discussions Faraday SOC.,8, 114 (1950). (54) Rienlicker, G., and Bade, H., 2. anorg. Chem., 248, 45 (1941). (55) Rienacker, G., and Bomnier, E. A,, Ibid., 242, 302 (1939). (56) Rieniicker, G., and Burmann, R., J . p a k t . Chmn., 158, 95 (1941). (57) Rienacker, G., and Hildebrandt, H., 2. anorg. Chem., 248, 52 (1941). (58) RienLcker, G., Muller, E., and Burmann, B., Ibid., 251, 55 (1943). (59) Rienacker, G., and Sarry, B., Ibid., 257, 41 (1948). (60) Rienacker, G., Wessing, G., and Trautmann, G., Ibid., 236, 252 (1936). (61) Russell, A. S., Nature, 117, 47 (1926). (62) Schuit, G. C. A , , D i s c u s s i o n s Faraday SOC.,8, 204 (1950). (63) Seith, W., “Diffusion in Metallen,” p. 50, Berlin, Julius Springer, 1939. (64) Seitz, F., “Modern Theory of Solids,” New York, McGrawHill Book Co., 1940. (65) Tammann, G., 2. anorg. Chem., 111, 92 (1920). (66) Taylor, A,, and Weiss, J., Nature, 141, 1055 (1938) (67) Taylor, H. S., Proc. Roy. SOC. (London), A108, 105 (1925): J. Phys. Chem., 30, 145 (1926). (68) Twigs, G. H., Discussions Faraday SOC.,8, 152 (1950). (69) Uhlig, H. H., Trans. Electrochem. SOC.,85, 307 (1944). (70) Wagner, C., J . Chem. Phys., 19, 626 (1951). (71) Zener, C., Phys. Rev., 81, 440 (1951). RECEIVED for review Ootober 17, 1951. ACCEPTEDFebruary 5 , 1952.

NICKEL COMPOUNDS AS CATALYST RAW MATERIALS JOHN G. DEAN* The International Nickel Co, Inc., New York 5, Several million pounds ,of nickel compounds are used each year in the United States in the production of catalysts, largely for hydrogenation reactions. The inorganic salts serve as reagents for preparing nickel hydroxide and carbonate, which yield on reduction so-called precipitated catalysts. Nickel nitrate is particularly suited for the preparation of impregnated catalysts, while nickel formate is used in making liquid-process Catalysts. Raney nickel, usually derived from nickel-aluminum alloys, is unique in that intermetallic compounds are involved. Costs vary widely with the particular nickel derivatives used, but performance is the main consideration because of the small quantities of catalyst required on a percentage basis in most hydrogenation reactions.

ICKEL catalysts were unknown until just before the start of the present century, when the classical work of Sabatier

N

(86)called attention to their potentialities and opened up vastly important applications. Today, they are possibly the most widely studied of all catalytic materials, as evidenced by the hundreds of scientific articles indexed under this broad topic each year by abstract journals. They conform t o the classical definition of a catalyst in that they affect the rate of chemical reactions without being chemically changed at the end of the process. They do not alter the free energy or change the equilibria involved, but Present addresi‘ Dean Researoh Servicea, Tuckahoe, N.

May 1952

Y.

N. Y.

when properly applied can be markedly effective in lowering the energy of activation in specific cases and thus exerting powerful influences on the rates of certain reactions and the actual course of complex chemical processes. The most familiar and b y far the largest application of nickel catalysts is in the hydrogenation of organic materials containing unsaturated carbon-to-carbon bonds (7‘). In this function they belong to the so-called contact type of catalyst as used in heterogeneous systems-i.e., the nickel is present as part of a solid phase, with the catalyzed reaction taking place in liquid and gas phases in contact with the surface of the nickel. The process of fat hardening provides an outstanding example of this type of catalysis. Hydrogen can be bubbled indefinitely without effect into cottonseed oil being heated and agitated in an appropriate vessel, but once a fraction of a per cent of h e l y divided active nickel catalyst is added, a vigorous reaction starts and in a matter of minutes th’e normally liquid oil can be converted t o a stable solid, making such products as margarine practical possibilities

(W. The basic problem in the production of nickel catalysts centers around the enlargement and maintenance of the catalyst surface. Nickel compounds play a major role in this operation, serving primarily as intermediates in the conversion of massive nickel of no measurable catalytic activity under most conditions, to higharea, active forms capable of forming transition complexes with

INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y

985