April, 1956
THECATALYTIC REDUCTION OF COBALT
magnetic y-alumina. Since the samples were diluted with ?-alumina to a common percentage of paramagnetic ion, a linear relation between relaxation time and amount absorbed is to be expected; any attempt to explain the reduction of relaxation times as due solely to paramagnetic ions is unwar ranted . Water absorbed on supported copper oxide behaves anomalously. For samples with widely differing initial concentrations of paramagnetic ion the change in relaxation time with amount absorbed is nearly constant. Further, the absorbed water has a definite bluish tinge. These two facts indicate that there may be an appreciable amount of free cupric ion on the surface of supported copper oxides. Copper in this form is capable of being taken into solution instead of being firmly incorporated into the catalyst structure as supported man-
457
ganese ions are, The result is equivalent to measuring the relaxation time of solutions of different concentrations of cupric ion absorbed on y-alumina, rather than the relaxation time for water affected by interaction with a firmly bound supported copper oxide. The break in the graphs for ethanol and methanol absorbed on supported copper oxides is associated with the more limited solubility of cupric ion in these solvents. Acknowledgment.-The courtesy of the Department of Physics of Harvard University, in particular of Professors E. M. Purcell and N. Bloembergen in providing laboratory space and facilities as well as helpful guidance, is gratefully acknowledged. This work was done under contract with the Office of Naval Research, and one of us (T.W.H.) was supported by fellowships from the Sinclair Refining Company and the National Science Foundation.
THE CATALYTIC REDUCTION OF COBALT FROM ,AMMONIACAL COBALT SULFATE SOLUTIONS’B~ BY THOMAS M.
KANEKO
AND
MILTONE. WADSWORTH
Department of Metallurgy, University of Utah, Salt Lake City, Utah Received September d o , 1966
Ammoniacal cobalt sulfate solutions were reduced under hydrogen pressure in a specially designed autoclave. The rates of reduction were measured analytically by following the depletion of cobalt from solution. Linear reduction rates were obtained under the conditions of this study covering a temperature range of 150 to 245” and hydrogen partial pressures of 150 to 800 p.s.i. Colloidal graphite was added in all runs and was found to act as a heterogeneous hydrogenation catalyst. Maximum rates of reduction were obtained for an ammonia to cobalt ratio of 2 to 1, indicating that the most easily reduced cobalt complex is the diammine. These results are consistent with the proposed mechanism which involves adsorption of hydrogen and cobalt ammine complex on the catalyst surface. An exponential hydrogen pressure dependency is explained in terms of structural variations associated with the quinonoid character of colloidal graphite.
Introduction Apparatus and Experimental Procedure A high temperature-high pressure reaction unit has been The precipitation of metals by hydrogen reducspecially designed and constructed for this investigation and tion of aqueous solutions of their salts was first already has been described elsewhere.* suggested by Beketov,a and subsequently a wide The cobalt-ammine complex solutions were prepared just variety of applications have been r e p ~ r t e d . ~In~ ~ prior to each experimental run by adding concentrated recent years, high temperature-high pressure tech- ammonium hydroxide, reagent grade, to cobalt sulfate.9 Colloidal graphite10 was used to aid in the recipitation of niques of hydrogen reduction have been applied to cobalt metal powder. In the absence of cofoidal graphite, the commercial production of the rate of reduction was extremely slow. Although general descriptions of the various After placing the sample solution in the autoclave, the processes can be found in the literature, there is an latter was evacuated, flushed with nitrogen, and then reevacuated to make certain that atmospheric oxygen had been absence of basic information on the operating removed from the system. The autoclave was then allowed variables and the reaction kinetics involved. This to come to operating temperature. After thermal equilibinvestigation, therefore, was initiated to make a rium was reached, the reaction was initiated with the ietroduction of hydrogen gas to the desired working pressure. kinetic study of the reduction of ammoniacal The vapor pressure of the solution was accounted for so that solutions of cobalt sulfate using hydrogen under the true partial pressure of hydrogen was known for the conelevated temperatures and pressures. ditions of each test. Liquid samples were withdrawn from (1) Supported by the Atomic Energy Commission under Contract
No. AT(11-1)-82. (2) This paper comprises part of a thesia to be presented b y T. M. Kaneko in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Department of Metallurgy, University of Utah. (3) N. Beketov, Compl. rend., 48, 442 (1859). (4) V. N. Ipatieff, “Catalytic Reactions at High Preaaures and Temperatures,” The Macmillan Co., New York, N. Y.,1936. (5) V. G. Tronev, S. M. Bodin, et al., Iwest. Sektora Plalinyi Drwikh Blagorod. Metal., Inst. Obehchei i Nsorh. Kim., Akad. Nauk S.S.S.R., 32, 194 (1948). (6) J. G. Barsguanth and J. B. Chalelain, Mining and Metallurgy, a#, 391 (1945). (7) F. A. Forward, Mining Eng., 6 , 577 (1953).
the autoclave a t periodic intervals. The cobalt content of these samples was determined electrolytically.ll A quick test to determine if the autoclave liquor had been depleted was carried out by adding a drop of
(8) W. H. Dreaher, T. M. Kaneko, W. M. Fassell, Jr., and M. E. Wadsworth, I n d . Eng. Chem., 47, 1681 (1955). (9) All cobalt sulfate solutions used in the experimental work were prepared b y disaolving “Analytical Reagent” (Mallinckrodt Chemical Co.) cobalt sulfate cryatals, CoSOc7Hs0, in distilled water. (10) Colloidal graphite waa prepared by diasolving concentrated “Aquadag” (Acheson Colloids Co.) in distilled water. (11) A. H.Low, A. J. Weinig and W. P. Schoder, “Technical Methods of Ore Analysis,” John Wiley and Sons, Inc., New York, N. Y., 1947, p. 109.
THOMAS M. KANEKO AND MILTONE. WADSWORTH
458
the sample to a Nitroso R salt solution, which forms a color complex sensitive to extremely small concentrations of cobalt. During the normal run, any cobalt metal which may have discharged into the sample was removed with a Teflon covered Alnico magnet prior to analysis. The rate of reduction is expressed in milligrams of cobalt precipitated per cc. of solution er hour, the wei h t being determined from the depletion oFcobalt ion from s h t i o n .
Results and Discussion Ammoniacal solutions of cobalt sulfate were found t o reduce linearly under all conditions investigated, covering the ranges of 150 to 245" in temperature and 150 to 800 p.s.i. in hydrogen partial pressure. The effect of stirring speed on the rate of reduction indicated that agitation was eliminated as a variable at the stirring speed of 650 r.p.m. All subsequent experiments ,were performed a t this limiting speed or above. The rate of reduction as a function of the concentration of graphite was found to vary linearly in the concentration range of 40 to 240 mg./l. from a series of runs a t 225" and 600 p.s.i. hydrogen partial pressure. Such a dependency is expected for a, solid catalyst, the rate being a function of the surface area. The rate diminished to an extremely low value in the absence of the catalyst. The choice of "Aquadag" (colloidal graphite) was somewhat fortuitous since the many forms of carbon vary considerably in their catalytic behavior. It was a t first thought that the colloidal graphite added served as a seed or nucleating agent. Such an assumption is not consistent with the linear rates observed. Experiments a t constant temperature (225O), hydrogen partial pressure (600 p.s.i.) and colloidal graphite concentration (120 mg./l.), but a t varying ratios of ammonia t o a constant amount of cobalt sulfate (0.425 mole/l.), indicated that a maximum rate of reduction was obtained a t the ratio of two moles of ammonia to one of cobalt (Fig. 1). This
MOLAR RATIO
OF NH40H TO COSO,.
VoI. 60
after addition of NH40H in the 2 t o 1 ratio was practically neutral and indicative of the reaction \ / ++
,,I
+ NHiOH + -Co-NHa
CO++
I
+ HzO
The diammine complex is also consistent with the reduction of Co++, which requires two electrons and forms two ammonium (NH4+) ions, resulting in the product (NH4)2S04. From the data obtained in studying the effect of the partial pressure of hydrogen on the rate of reduction of cobalt(I1)-diammine sulfate a t constant concentration of 0.425 mole per liter (25 g. Co/l.), three isotherms for the hydrogen partial pressure range of 150 t o 700 p.s.i. were plotted for the temperatures of 175, 225 and 245" (Fig. 2). The curves indicate an exponential dependency of the rate on the hydrogen partial pressure. Such an exponential dependency is not unique, however, since it has been observed in other catalyst systems12.12 in which i t was found that the rate varied exponentially with the fraction of the catalyst surface covered.
60
P% "
Fig. 1.-Effect of the molar ratio of NHaOH to COSOCon the rate of reduction of cobaltrammine complex.
Fig. 2.-Effect of hydrogen partial pressure on the rate of reduction of cobalt-ammine complex.
ratio suggests that the most active species in such a system is the cobalt(I1)-diammine sulfate
The adsorption of hydrogen on the surface of the graphite catalyst may be explained by a reversible oxidation-reduction couple and involves the presence of both phenolic and olefinic groups which have been identified14 by infrared absorption.
i I 2NH3
++
c0 4Hz0
The NE3ligand is covalently bonded to the cobalt, forming a very stable complex. This bond is sufficiently strong to make it very difficult to hydrolyze. The pH of the cobalt sulfate solution
(12) H.A. Taylor and N. J. Thon, J . Am. Chem. SOC.,74, 4169 (1952). (13) M. A. Cook and A. G . Oblad, Ind. Eng. Chem. 46, 1466 (1953). (14) E. A. Kmetko, Phys. Re%, 82, 456 (1951).
I I
THECATALYTIC REDUCTION OF COBALT
April, 1956
459
fact that the rate of reduction of the cobaltammine complex is virtually independent of the nature of the cobalt metal deposit produced. Concentration gradients with the solid catalyst itself are readily produced by the centrifuging action of the impeller, whereas the molecular adsorbates, Hz and cobalt(II)-diammine sulfate, are not affected. Since the catalyst is the source of the intermediate product from which the metallic cobalt is produced, concentration gradients of this intermediate product will occur. Therefore, metallic cobalt will precipitate in regions of high graphite (or of the intermediate product) concentration. This effect should occur primarily at the walls of the autoclave if proper baffling is not present. Also, the inertia of the solid catalyst will cause “piling up” of these particles near the impeller, particularly in front of it. It has been observed in this study that the metal deposit on the impeller blades is greatest on the forward side. J J “H” form L form It was found that the insertion of a cylindrical The “H” or quinoid form of the graphite structure baffle around the impeller, providing flow parallel results in the formation of fixed olefinic bonds and to the autoclave walls and more uniform distribuproduces surface strain which is relieved by the tion of the catalyst throughout the system, virturesonance shift associated with the adsorption of ally eliminated “plating out,” except on the hydrogen. The resulting “L” or phenolic form impeller itself. These results suggest the imporhas an underlying structure which is essentially tance of proper agitation through improved impeller aromatic. The surface strain is evident from the and baffle design or the use of simple gas bubbling fact that the C-C distance in graphite is 1.42 A. systems. whereas the C=C distance is only 1.34 L.ld The Using the 2 to 1 molar ratio of ammonia t o hydrogen should therefore split on a surface site cobalt sulfate, a series of runs were made a t various such as is shown in eq. la. For this reason, the concentrations of cobalt-ammine complex a t conadsorption process kinetically appears to involve stant temperature (175”) and hydrogen partial molecular adsorption. The presence of surface pressure (670 p.s.i.). The results are shown in Fig. strain affects both the adsorption potential of 3. The shape of this isotherm suggests an adsorphydrogen and the ease with which it may be re- tion process involving the cobalt(I1)-diammine moved in subsequent reactions. Consequently, the complex associated with the active or phenolic activation energy involving hydrogen adsorption surface sites on the graphite. The concentration should vary with surface coverage. The impor- of the phenolic sites depends in turn upon the tance of structural shifts has been proposed for sev- concentration of Hz in solution according to eq. la. eral systems including a number of c a t a l y s t ~ . ~ ~ ~Consequently, ~~-l~ the measured rate of reduction Structural shifts in catalyst surfaces have been should be a function of the hydrogen partial presdemonstrated in this Laboratory by means of sure and the concentration of the cobalt complex. differential infrared spectroscopy.20 The experimental results obtained, a t first glance, appear to indicate a homogeneous reaction. Linear rates of reduction would not be expected if 15 the rate controlling step takes place a t the surface of the growing cobalt particle. These curves 2 suggest an intermediate product whose formation represents the slow or rate-controlling step. This 9 intermediate could then complete the over-all sequence of reactions necessary to produce cobalt 6 metal a t the surface of the growing particle. Such 3 a sequence of events is further supported by the
The recent work of Garten and Weiss16 clearly points out the quinone-hydroquinone character of activated carbon and carbon black. They distinguish between the reduced form (“L” carbon) and the oxidized form ((‘H’’ carbon) and show that low temperature preparation of carbon results in the formation of “L” (phenolic) form while high temperature preparation results in the formation of “H”(quinonoid) form. The reversible reaction of the adsorption of hydrogen on graphite may be represented by the equation
11
(15) V. A. Garten and D. E. Weiss, Auatr. J . Cham., 8 , No. 1, 68 (1955).
(16) A. F. Wells, “Structural Inorganic Chemistry,” Oxford University Press, London, 1945, p. 82. (17) T. H. Milliken, Jr., G. A. Mills and A. G. Oblad, Disc. Faraday Soc., N o . 8, 279 (1950); Advances i n Catalysis, 111, 199 (1950). (18) M . A. Cook, D. H. Pack and A. G. Oblad, J. Cham. Phgs., 19, 367 (1951). (19) E. B. Cornelius, T. H. Milliken and A. G. Oblad, THISJOURNAL, 69, 810 (1955). (20) R. 0. French and M. E. Wadsworth. ”Differential Infrared
“0
01
02
03
0 5
06
07
08
MIL.
Fig. 3.-Effect of the concentration of cobalt-ammine complex on the rate of reduction a t constant temperature (175’) and hydrogen partial pressure (670 p.s.i.).
The following mechanism is consistent with the experimental results
Spectra of Surfaces: Ammonia on Cracking Catalysts,” presented before the Gordon Research Conference, Catalysis Section, June, 1955.
0 4
CONCENTRATION OF COBALT AMMINE COMPLEX IN
k Hz(gas)
Hp(solution)
(1)
THOMAS M. KANEKO AND MILTON E. WADSWORTH
460 :S(s)
+ Jr iS(p) H2
(2)
Kz
+ Co( 1 _7
: s ( P ) - Co( I S(p) - CO() +I I* +!S(q) CO(].2H (slow) Co( ).2H +Coo 2NH4+ SO4-- (fast)
IS(,)
+
+
+
(3) (4)
(5)
The symbols and represent the quinonoid and phenolic sites, respectively, on the graphite surface; Co{ } represents the surface active cobalt(II)-diammine complex, and Co( }.2H represents the intermediate product which is formed during the rate-determining step represented by eq. 4. None of these symbols are intended to represent the actual structures involved. Equations 1, 2 and 3 are here considered to be equilibrium reactions. These steps may actually represent steady-state conditions. The distinction, which lies in the determination of the temperature coefficients for the K values of each reaction, is not within the scope of the data presented in this paper. According to eq. 1, the concentration of hydrogen in solution equals kPH2, where k is the Henry constant. Letting the fraction of the surface sites covered with hydrogen, be 01, the fraction of sites covered with the adsorbed cobaltthe ammine complex, iS(,)-Co{ ], be &, the fraction of the uncovered sites, jS(s), be 1 - 81, and the bulk concentration of C o { ] be represented by J., one obtains from eq. 2 and 3
following adsorption. Figure 4 represents diagrammatically the energy barriers associated with the structural model. The adsorption potential of hydrogen on the graphite increases as the amount of hydrogen adsorbed decreases. The adsorption potential, AFl, may therefore be represented by the equation
+
(11)
A F ~= A F ~
where AF," is the adsorption potential a t zero coverage and a is a constant. The plus sign must be used since AF," is negative and a is a positive number. Similarly, the activation energy, AF*, may be represented by the equation AF* = AF~* - pel (12) where AF? is the activation energy a zero hydrogen surface coverage, and a and p are constants. The results of this study, as will be shown, indicate that the quantity, K ~ P H is , , small compared to 1. Therefore, the combination of eq. 10, 11 and 12 results in the equation
k~pHze-(A~io+a~,)/~Te-(A -a01 ~ o -BBi)/RT *
(13)
The a01 terms cancel, and a t constant temperature and cobaltrammine concentration, eq. 13 becomes R
where and
(7)
Vol. 60
=
K;P,,eB'PHa/RT
(14)
8' = @K:
(15)
In logarithmic form, eq. 14 may be written log R / P a Z = p'Paz/2.3RT f log K,'
(17)
where K1, K z and k are the equilibrium constants. ~ According to the absolute reaction rate theory21-22 whereupon, a plot of log R/PH,versus P H should result in a straight line whose slope is p'/2.3RT. Rate = K - kT riCie-AF*/RT Figure 5 is such a plot for the three isotherms of (8) h pressure versus rate of Fig. 2. The agreement is where K is the transmission coefficient (convention- very good for all three curves at 175,225 and 245". ally considered to be equal to l), k and h are the The corresponding p' values were found to be 2.28, Boltzmann and Planck constants, respectively, 2.07 and 1.98, respectively. This correlation shows TiCi represents the product of the concentrations of that the assumption K ~ P H
