NICKEL-METAL, OXIDE The highly specific affinity of nickel for dimethylglyoxime and certain other oximes in alkaline citric or tartaric acid solutions may be considered a stroke of luck t h a t is rare in the atomic family. Certainly it is a particular godsend for nickel analysis, which would otherwise be seriously hampered. SOURCES OF ERROR IN G R A V I M E T R I C W O R K
.
For practical purposes, the two graviinetric methods, dimethylglyoxime and electrodeposition, are still relied on for most of the work with significant amounts of nickel. Both methods have their drawbacks and sources of error t h a t could be serious in unskilled hands. Nickel dimethylglyoxime is slightly soluble in t h e methyl or ethyl alcohol which is generally used to dissolve the reagent. As a consequence, t h e tendency is t o keep the alcohol in t h e nickel solution at a minimum, under 20%. Also, during digestion on a steam bath as carried out in some laboratories, there is a tendency to lose alcohol by fractional distillation. There is danger at this point that some of the excess dimethylglyoxime reagent will come out of solution and precipitate.with t h e nickel salt. This will produce high results if the precipitate is merely dried a t 110’ C., as is recommended in some textbooks. A temperature of 110” C. will remove alcohol and water but not invisibly occluded reagent. Most people will not take the time and trouble t o ignite this precipitate t o NiO. This source of error can be averted by drying the precipitate t o constant weight a t 150’ C., which volatilizes the excess reagent. A little mystification is also introduced when ferrous iron is present. The filtrate then exhibits a strong red color, which is often mistaken for more nickel. Naturally, this colored material will not precipitate on standing but slowly vanishes as the iron is oxidized b y air. The main source of error in the electrodeposition of nickel arises from the effect of cobalt. This element, in company with nickel as it generally is, has a tendency t o attack the platinum anode. I n such cases there is a variable loss on the anode, depending on the length of electrolysis. Some of the platinum lost from the anode is deposited on the cathode with the nickel and some of it stays in solution. That tends t o make the results for nickel a little high. On the other hand, not quite all the nickel is deposited, which tends to make the results a little
low. The final net outcome represents a compensation of errors, which often brings about a result surprisingly close to the truth. The effect of cobalt on the anode can be overcome by adding a gram or two of t h e reducing agent sodium bisulfite, but t h a t step is likely t o contaminate the cathode deposit with sulfur, which must be determined and deducted from the nickel. This is a troublesome operation and the result is t h a t analysts generally prefer t o endure the loss of platinum a t the anode. AE the platinum appears in a black spongy form, it is sometimes mistaken by analysts for a n illegitimate form of carbon and so reported. These errors are not large and are accepted as part of the facts of life for routine work in many laboratories. On the other hand, for more careful work laboratories can reduce these errors by precipitating the nickel with dimethylglyoxime prior to electrodeposition. This step has the virtue of removing cobalt. It takes longer, but it is a good way to avoid the errors due to the effect of excess reagent on the dimethylglyoxime precipitate and also t h e effect of cobalt on the anode during the subsequent electrodeposition of the nickel. Most of these points are well known among specialized analysts, but apparently they have not taken their place i n the textbooks yet. LITERATURE CITED
(1)
(2)
(3) (4)
(5)
Bergman, T. O., “Essays Physical and Chemical,” pp. 420-2 and 426-32, Edinburgh (1791). Cooper, M. D., Anal. Chem., 23, 875 (1951). Cronstedt, A. F., “Forsok til mineralogie eller mineralrikets upstiillning,” translated by Brunnich, as “System of Mineralogy,” 1770. Cronstedt, A. F., “Fortsetrung der Versuche die mit einer Erztart pus der loser Koboltgruben sind angestellt worden,” Kungliga Svenska Vetenskaps Academien, 1754; German translation in AbhandZ. NaturEehre, 16, 38-44 (1756). Cronstedt, A. F., “Svenskt Biografiskt Lexikon,” Stockholm, 1931.
(6) Cronstedt, A. F., “Versuche mit einer Erztart von den lockern
Koboltgruben,” Kungliga Svenska Vetenskaps Academien, 1751; German translation in Abhandl. Naturlehre, 13, 2937 (1755).
(7) Culbertson, J. B. and Fowler, R. M., Steel, 122, 108 (May 24, 1948). (8) Gentry, C. H. R., MetaZZurgica, 38, 108 (1948). (9) Thenard, C., Phil. Mag., 20, 63-70 (1805). RECEIVED for review October 17, 1951 ACCEPTEDJanuary 28, 1952.
NICKEL OXIDES Relation between Electrochemical Reactivity and Foreign /on Content ROBERT L. TICHENOR’ Thomas A. Edison, Inc., West Orange, T h e marked electrochemical effects of adding lithium, bismuth, and iron to the nickel electrode of alkaline storage batteries have been known for many years. Heretofore, no adequate explanation of how additions of these metals affect the electrode has been published. The theory given in this paper explains these effects using the following postulator: Electrolytic oxidation and reduction of nickel oxide are terminated by creation of insulating barrier layers of nickel oxide adjacent to the electronic conductor. “Foreign” ions such as those of lithium, bismuth, and iron enter into the crystal of nickel oxide b y substitution, replacing nickel ions. *The presence of these foreign ions affects the stability of the oxidation states of adjacent nickel ions. This change in stability retards or promotes oxidation of the nickel oxide, it also affects the growth of the insulating barrier layers. Both factors may affect the extent to which the oxide can be oxidized or reduced.
M a y 1952
N. J.
HE best known electrochemical effects of adding “foreign” ions t o nickel oxides are: (1) the increase in electrochemical capacity of nickel oxideelectrodes in potassium hydroxideelectrolyte when lithium hydroxide is added t o the electrolyte, (2) the increase in electrochemical capacity of nickel oxide electrodes when a small amount of bismuth hydroxide is added t o the nickel oxide, and (3) the decrease in electrochemical capacity of nickel oxide electrodes when a small amount of ferric hydroxide is added to the nickel oxide. These effects were discovered by Thomas A. Edison many years ago during the development of alkaline storage batteries. While the knowledge of them has been of considerable importance commercially, very few attempts t o explain their mechanism have been published. Thus Crennell and Lea (2) and Foerster ( 4 ) sug-
T
1 Present
address, E. I. du Pont de Nemours & Co., Inc., Waynesboro,
Va.
INDUSTRIAL AND ENGINEERING CHEMISTRY
973
NICKEL-METAL, OXIDE gested that lithium hydroxide helps to maintain a large surface area of nickel oxide during the life of a nickel oxide electrode. Heretofore there have been no published explanations of how bismuth increases the capacity or how iron decreases it. Jungner ( 3 ) claimed t h a t iron oxide forms a nickel-iron oxide compound which is inactive electrochemically, but apparently made no suggestion as to why it is inactive. E L E C T R O C H E M I C A L F O R M A T I O N OF C O N T A C T RESISTANCES A T SURFACE O F NICKEL O X I D E
Kuchinskil and Ershler ( 5 ) studied the electrolytic oxidation (charging) and reduction (discharging) of nickel oxide in an alkaline electrolyte. They concluded that electrolytic reduction of nickel oxide electrodes produces a high resistanpe barrier layer (contact resistance) in the nickel oxide adjacent to the metal (current-carrying) contact. They believed this layer is nickelous oxide containing little if any nickel(II1) or nickel(1V) ions. At relatively high currents this resistance is created before all the oxide
I
1
Figure 1. Nickel Ion Vacanc and Adjacent Nickel(ll1) Ions in (1 00) Plane of diickel O x i d e Crystal
is reduced. Creation of the resistance, therefore, limits the quantity of charge that can be withdrawn from the oxide. Decreasing the discharge current, interrupting it, or charging allom-s oxygen t o enter the barrier, thereby restoring its conductivity. The passage of current through nickel oxide is possible, according to these views, as long as sufficient excess oxygen is present to make the oxide a conductor, and hence nickel oxide can be oxidized and reduced electrochemically between the lower limit where the barrier layer of nickelous oxide forms and an upper limit. Tho upper limit may be complete oxidation to nickel dioxide ( K O 2 )with or without water of hydration, but it may be determined by the formation of a second barrier layer of insulating nickel dioxide. The existence of this barrier layer has not been proved experimentally. However, as explained below, the effect of bismuth in increasing the capacity of nickel oxide electrodes is evidence for its existence. Nickel dioxide is a rather unstable compound at room temperature in the presence of water. However, it is t o be expected that in a storage battery electrode adjacent to the metallic conductor or grid and under the powerful oxidizing conditions attained near the end of charging, the nickel oxide would approach very closely to the composition KOZ. The upper barrier layer is not nickelic oxide (NiZOs),for the oxide can be readily oxidized electrochemically above that level. The ease with which this can be done is evidence, according to the theory given below, t h a t nickelic oxide containing only nickel(II1) ions is not produced. Such nickelic oxide would be an insulator and hence would resist further oxidation. There are two formulas for the oxide whose composition corresponds t o Ni203, either of which might be a n electrical conductor: N i i y sNii:+Nii 2; OS and NittNi+t+'03. The theory of this paper could be presented using either formula, although the first is preferred because it ob974
viates the problem of electron transfer occurring by two electroi:~ moving together (double hole conduction). ELECTRICAL C O N D U C T I V I T Y O F O X I D E S O F M E T A L S H A V I N G VARIABLE VALENCE
It has been recognized (5, 8) that oxides used in batteries a1.e electronic conductors and that their conductivity is accompanied by an excess or a deficiency of oxygen. Thus, electrolytic lead dioxide (8)having a high electrical conductivity has a composition near PbOl.gg1 instead of PbOg which would be expected for tetravalent lead. Nickelous oxide prepared in air is said ( I ) to have a composition NiOl.ocsinstead of NiO. It is generally considered ( 7 ) that nickel oxide accommodates excess oxygen by the creation of vacancies a t some of the metal ion sites while the locations of oxygen ions are unchanged. This situation is shown schematically in Figure 1 where, follo~vingde Boer and Verwey, two nickel (111) ions are shown adjacent t o a hole and the rest of the nickel ions have a valence of 2. Verwey (IO)has concluded from his work on the relation hetween electrical conductivity of metal oxides and their crystal structure that the presence of metal ions of two valence states [such as iron(I1) and iron(II1) in magnetite or nickel(I1) and nickel( 111) in nickel oxide] in crystallographically equivalent sites leads to high electrical conductivity. I n nickel oxide the current carrier is assumed ( I O ) t,o be the hole (lack of an electron) left when an electron is removed from the nickel(I1) ion to make it nickel(II1). It is thus rather well understood why the conductivity of nickel oxide increases as it is oxidized from stoichiometric nickelous oxide [containing only nickel(I1) ions] to a material containing both nickel(I1) and nickel(II1) ions. Apparently less is known about the conductivity of highly oxidized materials such as lead dioxide and nickel dioxide. Verwey (10) has studied titanium dioxide, stannic oxide, and ferric oxide. On reduction, these compounds can form lower valent cations. Verrvey attributes the rise in conductivity which accompanies their reduction t o the presence of two valence states: iron(II1) and (11),titanium(1V) and (111), and tin(1V) and (111). Thomas (8) suggested that lead dioxide owes its conductivity to the presence of interstitial lead atoms dissociated into lead(IT7) ions and 4 electrons. These electrons are considered t,o be frce like the electrons in a metal. It is, thus, unnecessary in order to account for its electrical conductivity to assume that lead diositic .which is deficient in oxygen contains a mixture of lead(II1) or lead(I1) and lead(1V) ions. The important point for the following discussion is that the electrical conductivity of titanium dioxide, stannic oxide, ferric oxide and, presumably, nickel dioxide and lead dioxide increases as oxygen is removed from them. I n discussing nickel dioxide we shall assume that its reduction creates a mixture of nickel(II1) and nickel( IV) ions, although this assumption is not the only one that will account for the material's electrical conductivity. The assumption is useful, however, in discussing the effect of various impurities on the ability of nickel oxide to accept charge when oxidized electrolytically. EFFECT O F L I T H I U M I O N S ON ELECTRICAL C O N D U C T I V I T Y O F NICKEL O X I D E S
Verwey, Haaijman, and Romeijn (9, 10) have shown that the addition to a host crystal of small amounts of ions of valence different from those of the host creates a material of markedly different electrical conductivity when the ions of the host possess variable valence. Thus, they have shown that addition of 9.03 atomic % ' of lithium oxide to nickel oxide by heating a mixture 01 the oxides in air increases the electrical conductivity from lcss than 10-4 to 0.77 ohm-' cm.-l This rise in electrical conductivity is accompanied by an increase in available oxygen as determined by analysis (10). The absorption of oxygen is represented by Equation 1 (10).
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
NICKEL-METAL, OXIDE This summarizes the analytical result that (1 - 6)NiO
+
1/4802
+ l/&Li2O +LiaKii - a 0
(1)
there is one atom of active oxygen present for every two atoms of lithium. Values of 8 aa high as 0.1 are reported (IO). This corresponds to a mixture of LizO and NiOl.oa6. Pure nickel oxide roasted in air contains much less oxygen; its empirical formula is said to be NiOl.oQ6( I ). The theory (IO)explaining how lithium ions allow nickel oxide to be oxidized by air assumes:
positive charge, these ions would tend t o stabilize the lower valence form-Le., nickel(I1) when the oxide is in a rather low state of oxidation and nickel(II1) when it is in a high state of oxidation. This stabilization arises from the need for compensating the excess positive charge of bismuth(V) or (111) with a lower charge on the adjacent nickel ions to maintain electrical neutrality in the crystal (assuming the oxygen ions retain the same configuration in the presence of bismuth as in its absence). For any given low oxidizing potential when the oxide is close to the composition N O , the addition of bismuth( 111) would decrease the electrical conductiv-
The oxygen ions of the nickel oxide-lithium oxide material have the arrangement required for oxygen in a perfect nickelous oxide crystal. Lithium ions occupy sites which would be filled by nickel(I1) ions in a perfect nickelous oxide crystal. Gnder these conditions the substitution of lithium(1) for nickel (11) would create a local negative charge, but electrical neutrality is maintained by loss of this charge from a neighboring nickel(I1) ion, converting it t o nickel(II1). The resulting structure which would be expected in the vicinity of a lithium ion is shown in Figure 2. The lithium ion here plays a role similar to the nickel ion vacancy in the earlier theory (3). The presence of both nickel(I1) and (111) ions is assumed (IO) to give rise to good electrical conductivity in this case, just as in pure nickel oxide containing excess oxygen (Figure 1). A lithium ion is easily fitted into sites normally filled by nickel (11) ions, as its "crystal" radius is 0.60 A. and that of nickel(I1) is 0.69 A. Sodium and potassium ions with radii of 0.95 and 1.33 A,, respectively, cannot substitute so easily for nickel(I1). The size factor is probably the reason why lithium hydroxide is more strongly adsorbed ( 8 ) by nickel oxides than is potassium hydroxide. .41so since, according to Verwey, the foreign ion must enter into the host crystal to influence its conductivity, it follows that lithium ions will exert a much greater effect on the conductivity of nickel oxides than either sodium or potassium ions. The above theory of Verwey (IO) applies to nickel oxide in a rather low state of oxidation such that only nickel(I1) and (111) ions are present. Under strong oxidizing conditions the situation will be different: nickel(II1) and (IV) ions will be present and, with severe oxidation, the composition will approach NOz. If lithium ions were present in this highly oxidized material occupying sites normally filled by nickel(1V) ions, there would again be a local negative charge in the crystal, if the oxygen ions retained the same arrangement as in similarly oxidized nickel oxide which contained no lithium. Electrical neutrality could be restored by loss of electrons from some of the nickel(II1) ions adjacent to the lithium ions. Lithium ions would, therefore, tend to induce nickel(1V) ions in their vicinity much as they helped create nickel (111) ions in nickelous oxide. The theory just presented predicts that lithium ions will induce a higher valence form of nickel ion when they are added to nickel oxide which is in a n oxidizing environment. Hence, when the oxide is close to NiO in composition-Le., in a weakly oxidizing environment-addition o f lithium ion helps nickel(II1) ions to form and thus raises the electrical conductivity. When the oxide is close to NiO, in composition-Le., in a strongly oxidizing environment-addition of lithium ions helps nickel(1V) ions to form [or reduces the proportion of nickel(II1) ions] and hence lowers the electrical conductivity. EFFECT OF BISMUTH I O N S ON ELECTRlCAL,CONDUCTIVITY O F NICKEL O X I D E
The oxidation state of bismuth in a nickel oxide electrode depends on its oxidation level: Near the end of discharge bismuth (111) ions are present, while under the severe oxidizing conditions near the end of charging some bismuth(V) ions are probably present. These bismuth ions would have an effecton conductivity opposite to that observed with lithium ions. Because of their large May 1952
Figure
4.
Lithium Ion Substitution and Adjacent Nickel
(111) Ion in (1 00) Plane of Nickel Oxide Crystal
ity. The presence of bismuth(V) under a high oxidizing potential such that the oxide is near NiO, in composition would increase the electrical conductivity. REDUCTION O F ELECTRICAL C O N D U C T I V I T Y O F NICKEL O X I D E BY A D D I T I O N OF FERRIC IONS
The theory just used to explain the effect of bismuth(V) and lithium on the electrical conductivity of nickel oxides can be applied to other ions. Thus iron(II1) ions would be expected to reduce the conductivity of nickel oxide when it is in a low state of oxidation such that most of its ions are nickel(I1) and the rest are nickel( 111). Under these conditions, substitution of iron( 111) for nickel(1J) would give the crystal an excess positive charge which could be balanced by the gain of an electron by an adjacent nickel(II1) ion. (Throughout this discussion, it is assumed that the oxygen lattice is not altered by the substitution.) The effect of adding iron(II1) ions would then be to reduce the Droportion of nickel(II1) and, hence, to lower the conductivity. Similarly, under a high state of oxidation when most of the niGkel is considered to be present as nickel(1V) and the rest as nickel(III), substitution of iron(II1) for nickel(1V) would give the crystal an excess negative charge which could be balanced by the loss of an electron from an adjacent nickel(II1) ion. Under these conditions, iron(II1) ions would be expected to reduce the proportion of nickel(II1) and, hence, to lower the conductivity. The first of these predictions has some experimental support: Verwey (IO)has found that the addition to a nickelous oxidelithium oxide substance (by roasting in air) of an amount of ferric oxide equivalent to its lithium oxide content produces a material having the low conductivity of pure nickelous oxide. EFFECT O F F O R E I G N I O N S ON ELECTROCHEMICAL C A P A C I T Y NICKEL O X I D E
OF
Ignoring loss of oxygen by chemical decomposition, the electrochemical capacity of an oxide is given by Faraday's law as the difference between the oxygen conte'nt after oxidation (charging) and after reduction (discharging). In general, the capacity will be increased by the addition of foreign ions if the resulting material can be either oxidized or reduced more completely than when these ions are absent.
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NICKEL-METAL, OXlDF The electrochemical capacity of an oxide electrode depends on many factors, most of which are not considered in the following discussion. This considers only the idealized situation, in which it is assumed that the various nickel oxides (or hydroxides) are identical except for the presence or absence of certain foreign ions. It is also assumed that all portions of the oxide participate in the electrochemical oxidation and reduction. These assumptions are probably not true for any group of oxide electrodes containing different foreign ions, because the presence of a foreign ion may affect the shape of the oxide particles, alter the course and extent of their electrochemical formation, and affect the amount of the oxide utilized electrochemically. T'Vhether the factors discussed in this paper are important in determining the capacity of a given oxide electrode may depend on the particular sample of oxide in the electrode, the method of making the electrode, its geometry, its history, the type of test used to measure capacity, etc.
.
Let us first consider how lithium ions affect the discharge process. Assuming that lithium ions stabilize nickel( 111) ions, it follows from the relation between electrochemical potential and thermodynamic equilibrium that reduction of nickel(II1) ions stabilized by lithium would require a more negative potential (less positive) than the reduction of unstabilized nickel(II1) ions. In a nickel oxide containing X L i + ions and Z nickel ions, of which X Yare Ni+++,about X N i + + f i o n s would be stabilized by the X E +ions and the remaining Y N i f + + ions would be expected t o give the material about the same oxidation-reduction potential as pure nickel oxide containing Y X i + + + ions and 2 - X - Y N i + + ions. Thus, lithium ions would lower the oxidation-reduction potential of nickel oxide in a low state of oxidation [when Y is small-Le., when the number of nickel(II1) ions is about equal to the number of lithium ions] more than that of the oxide in a higher state of oxidation [when Y is largei.e., when the number of nickel(II1) ions is considerably greater than the number of lithium ions]. Toward the end of discharge, the number of nickel(II1) ions within the region adjacent to the metallic conductor (grid) in which the barrier layer forms should decrease at least until it nearly equals the number of lithium ions. The presence of lithium ions will thus lower the potential a t which the barrier layer forms.
+
To see how this decrease in the potential of formation of the barrier layer affects the discharge process, it is recalled that Kuchinskii and Ershler (6) showed that in some cases a considerable quantity of higher nickel oxide of usable potential can remain after discharge has ended because the barrier layer has destroyed the electrical connection to it. In the presence of lithium, the theory predicts that the potential a t which the barrier layer forms is lowered and hence the potential of all of the oxide a t the end of discharge is lowered (although not to the same value as the barrier layer and probably not by the same amount). Remembering that the presence of lithium ions decreases the oxidation-reduction potential of nickel oxide which is in a low state of oxidation more than that of oxide in a higher state of oxidation, it follows that the oxide outside the barrier layer will tend to be reduced to a lower nickel(II1) content in the presence of lithium ion than in its absence. In making the above statement one should also take into account the nickel(II1) ions which remain after discharge because they are stabilized by lithium ions. Whether the concentration of nickel(II1) ions in an electrode after discharge is greater or less when lithium ions are present than when they are absent depends on whether the additional reduction presumably allowed by retardation of the formation of the barrier layer is greater or less than the prevention of reduction caused by stabilization of nickel (111) ions by lithium ions. If the lithium ion concentration in the oxide were large, the stabilization effect might predominate and the reduction would then be less complete. At intermediate lithium ion concentrations it seems likely that the influence of lithium ions on the barrier layer formation would be more important in determining the extent of discharge. This would be expected to be especially true a t high rates of discharge.
976
Let us now consider the effect of lithium ions on the charging process. Here the presence of a small proportion of lithium ions would assist the initial formation of nickel(1V) ions. It would he expected that two or three nickel(1V) ions would form in the neighborhood of each lithium ion a t a l o ~ e potential r than would be required for oxidation of the rest of the nickel ions. IIowever, the remainder of the nickel ions would be largely unaffpcted by the presence of lithium ions and hence the oxidation potential for formation of nickel dioxide (the upper insulating barrier) would be about the same whether lithium is present or not. The theory does not predict with certainty whether the concentration of nickel(1V) ions in a charged electrode FTiill be larger or smaller when lithium ions are present than when they are absent. The lithium ions by stabilizing nickel(I1') ions s.111 assist in their formation (which increases their concentration), but in so doing they may hasten the formation of the barrier layer of nickel dioxide, thus preventing material outside of that layer from being completely oxidized. Which factor is more important may depend on the rate of oxidation (the current), the concentration of lithium ions in the oxide, the duration of the charge, etc. For example, with low charging rates and rather short charging times the barrier layer of nickel dioxide would not be expected to appear and under these conditions the concentration of nickel(1V) ions a t the end of charging mrould be expected to be higher when lithium is present than when it is absent. The above discussion can be summarized as follows: The theory predicts that lithium ions increase the capacity of nickel oxide electrodes in one or both of the follonring ways: (1) by retarding the formation of the barrier layer formed at the end of discharge and (2) by increasing the proportion of niclrel(1V) ions in the charged electrode. It does not, however, predict the relative importance of these two factors for any given electrode; this will have to be determined by experiment. By a similar application of the theory, it can be predicted that bismuth(V) ions will raise the potential a t which the insulating barrier of nickel dioxide is formed. This would be expected t o increase the amount of oxygen Rrhich nickel oxide can take up during electrochemical oxidation (charging), although the presence of bismuth(V) will tend to stabilize nickel(II1) ions and thus act to decrease the amount of oxygen taken up. Also, bismuth(II1) will tend to stabilize nickel( 11) ions during discharge (reduction) and hence assist somewhat the growth of the barrier layer of nickelous oxide. However, the presence of bismuth(II1) would not affect greatly the potential a t which reduction is complete (just as lithium ions would not affect by much the potential of complete oxidation to nickel dioxide). I n this case, the experimental result (6) that the rise in capacity which results from adding bismuth to nickel oxide electrodes is increased by excessive charging is evidence that bismuth(V) increases the oxygen content of the charged electrode. This result is consistent with the prediction that bismuth(V) retards the formation of the upper barrier layer of nickel dioxide. The decrease in capacity when ferric ions are present in a nickel oxide electrode helps to select the predictions of the theory which apply when ferric ions are added. The theory predicts that ferric ions \\-ill assist the growth of both barrier layers. This would reduce the capacity. Ferric ions would also be expected to stabilize nickel(1V) ions a t the end of charge and nickel(I1) ions at the end of discharge. If this happened, the capacity would be increased. Apparently the latter effects are small compared to the influence of iron(II1) on the formation of the barrier layers. A P P L I C A T I O N OF T H E O R Y TO OTHER O X I D E S
Verwey (10) has shown that addition of foreign ions under airoxidizing conditions affects the electrical properties of many oxides including those of nickel, cobalt, tin, titanium, and iron as well as of other compounds (sulfides, mixed oxides) containing manganese and tungsten. The effect on these compounds is that
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
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 AND 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
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