576
GEORGE-MARIA SCEWAB
mits the investigation of a variety of problems connected ivith hydrocarbon oils, as typified by the following results: Tridecanoic acid, a typical aliphaticmacid,was found to be molecularly dispersed in oil, the molecular radius being 3.5 A. If an amine and a phenol are added, electrically conducting oils are producedbthe componnds found in such a solution are moderately large molecules of 6-8 A. radius. The degree of adsorption, by graphite or asphalt, of the acid from solution was determined, as re11 as the extent of its exchange reaction n-ith copper acetate added to the oil. When an oil containing the acid vas oxidized, it ivas found that a large fraction of the acid t,akes part in the formation of polymer molecules, but, up to 200 hr., only to a small extent in the formation of the solid precipitate. REFERENCES (1) GE?,fAxT, ~ D R E W J. : Chem Phys. 14, 424 (1946). (2) GEYANT, . ~ N D R E W : J. Electrochem. Soc. 94, 160 (1948). (3) G E Y I B T , ANDREW:J. Applied Phys. 19, 1160 (1948). (4) HINES, EDWARD, A 6 D GEMAKT, ANDREW: Science 110, 19 (1949). (5) PIPER, J. D., et al.: I n d . Eng. Cheni. 31, 307 (1939); 33, 1510 (1940)
OK COIZIPACT-DISPERSED SILVER GEORGE-MARIA SCHWAB Department of Inorganzc, Physzcal, and Catal@c Chenmtry, Instztute .Vzcolaos Canellopoulos, Pzraeus, Greece Recezved J u n e 27, 1949 INTRODUCTION
Compact-dispersed substances (3) consist of discrete paiticles hound together a t single surface points and kept in fixed relative positions by this binding. Many real adsorbents and catalysts, e.g., charcoal, fritted metal powders, skeleton contacts (Raney), belong to this class. They exhibit a manifold larger and often more active surface for reactions, adsorption, and catalysis than do ordinary compact substances, and therefore they must be classified as active substances. Fricke (1) enumerates fifteen methods for the preparation of active substances, but does not include the essentially electrolytic method described bel ow. PREPARATIOh-
I n the recovery of metallic silver from halide residues by the usual method of reduction in suspension with zinc and acid we were surprised by the great velocity of this process, in view of its heterogeneous nature. We first considered an action of atomic hydrogen, diffusing from the zinc surface t o remote halide par-
COMPACT-DISPERSED SILVER
577
ticles, but the reaction proceeds equally well in neutral media where no hydrogen is evolved. Diffusion of silver ions or atoms or even of electrons through the halide particles is precluded at room temperature. A review of the scarce data (2) existing on the series of action of different metals other than zinc shows that it follow well the series of electrolytic potential. Hence, the mechanism is an electrolytic one: zinc forins the anode of local elements at the contact point’s; zinc is dissolved and silver is deposited on the already formed silver cathodes from the solution which contains, at least near the halide surface, silver ions owing to the solubility of the silver halide. A microscopic examination confirmed this view: Foils or rods, cast from fused silver chloride, were brought into contact with a piece of zinc, iron, nickel, or cadmium in solutions of hydrochloric acid, sulfuric acid, sodium chloride, or sodium sulfate. Then, it was seen that a silver front proceeded from the cont’act point gradually over the whole surface of the halide body and even into its interior. Obviously, the silver front at every moment forms the cathode of the shortcircuited element, current flowing through the solution from the zinc to the silver front. The important point is that the groivth is three-dimensional, which means that the silver body formed is porous and contains conductiye electrolyte within its pores. This is due t,o the fact, directly observed, that the silver body forms an exact pseudomorph with all the details of shape and surface of the original halide body. S o w , the molal volume of silver chloride is 25.9 ml., and the atomic volume of silver 10.3 ml. only. Consequently, the pseudomorph is bound to have an overall porosity of 6 = 0.6, Pieces of compact-dispersed silver up to a size of a few millimeters could be obtained; the larger ones sometimes show deep cracks due to internal stress. The Brinell hardness is of the order of 1 kg./mm.2 These preparations, in view of their origin, somewhat correspond to Gomberg’s “molecular silver,” as used in the preparation of free organic radicals. The compact-dispersed appearance, however, gives a better possibility for dispersoid characterization. PORE SIZE FROM CATALYTIC ACTION
I n order to determine, in addition to the porosity, the true average pore diameter, one additional measurement is needed. \Ye chose the catalytic action of the compact-dispersed silver in the dehydrogenation of formic acid vapor. This is a zero-order reaction, and its velocity is proportional to the true total accessible surface, provided the activity distribution of the active centers can be taken as constant. For compact bulk silver, in former work (E), the velocity log
2’ =
i.03 - 17,600,~4.577‘
has been measured for ii surface of 1.5 cm.? \-e compact-dispersed silver. prepared vith zinc, log
c =
measured on tiyo samples of
10.2 - 23,000 ‘4.5iT
Thus, the surface of our preparation is not more active than that of ordinary silver, but,. on the contrary, s h o w an increased activation energy. According to
578
GEORGE-hlARIA SCHN’AB
our former findings (6), this can be due to a certain zinc content of the silver, taken up during its deposition, as 1 atom per cent of zinc increases the activation energy by 1 kcal.jmole. Then, our value of 23 kcal./mole would correspond to a zinc content of 5.4atom per cent. I n fact,,an x-ray examination of the sample disclosed the corresponding diminution of the lattice spacings. S o w , a compact silver alloy of this composition, because of the general relation between activation energy and frequency factor ( F ) , n-ould give a velocity of log u = 8.2
-
23,000/4.57T
A comparison of the frequency logarithms 8.2 and 10.2 showed that our preparation has a true accessible surface, y, 100 times larger than compact particles of the same size. Measurements of the catalytic dehydrogenation of ethanol gave an identical result. From this and the porosit,y value an approximate estimate of the pore diameter is possible as follows: Let the edge length of a single, say cubic, piece of the cat,alyst, the edge length of the small cubic particles of which it is supposed to consist, V. = the apparent volume of a catalyst piece, vb = the total true volume of silver in it, p = 1 - 6 , the fractional volume occupied by silver, 0, = the geometrical surface of a catalyst piece, O b = the true total surface of the small silver cubes, y = the “roughening”, i.e., the surface increase factor, measured catalytically, and A\r = the number of small silver cubes contained in one piece of the catalyst. a b
=
=
Then
hence and hence
and from equations 2 and 4: 1 v
=
8-93
579
COMPACT-DISPERSED SILVER
or With the experimental values a = 0.1 cm., p = 0.4, and y = 100, we obtain b = 4 X cm. As the porosity is nearly 50 per cent, the pore diameter is nearly equal to the cube edge, and we obtain for the pore radius: T
x2X
em.
PORE SIZE FROM SPECIFIC PENETRATION
According to Manegold (4) for a porous diaphragm a specific flow penetration (“Durchliissigkeit,” in contradistinction to the diffusion permeability), independent of the flowing substance, can be defined by:
where V is the volume of a substance of the viscosity q passed during the time t through a sample of the length L and the cross section F under the pressure TABLE 1 Measurement of specilic penetration
3.08 2.24 9.3
~
~
’
Mean value
::: 1.2
0.8
difference A p (absolute units). By means of the Hagen-Poisseuille law, D, is related to the porosity 6 and the average pore radius T by
D, =
T26 -
8
hence
,/% 2-
r
=
Compact-dispersed silver plugs were prepared by reducing in situ silver halide columns fused in the bottom of glass tubes, and water was passed through them. Table 1 gives the results. The mean value obtained, T 1 X low4cm., coincides well with the result of the catalytic measurements, T 2 X 10-~cm., especially in view of the fact
--
580
GEORGE-MARIA SCHWAB
that the influence of the pore shape and pore distribution has not been taken into account and this effect certainly acts differently in both cases. The pore size is just at the limit of microscopic visibility, in agreement with the somewhat coarse metallic appearance of the compact-dispersed silver surface. PARTICLE SIZE BY X-RAYS
The DebyeScherrer lines of the preparations are sharp but entirely smooth; this indicates that no particles above 10-4 em. nor below IO-&cm. are present. Thus the secondary particles, indicated by the b-value, are identical with the primary crystallites. It may be mentioned here that rods of compact-dispersed silver, prepared from single crystals of silver chloride, showed no lattice orientation with respect t o the original crystal; this agrees with the finding ( 5 ) that silver prepared by the superficial photographic processing of chloride or bromide single crystals is also randomly crystallized, corresponding to the general rules concerning lattice orientation in coating films (loc. cit.). CONCLUSIOKS
The good agreement of three independent methods for the estimate of the secondary structure is of importance for the comparison of these methods, especially of the catalytic one. Once more it shows that the catalytic reaction velocity is not a casual magnitude but can be predicted in a quantitative manner. As for the compact-dispersed silver, the measurements show that it is relatively little dispersed and by no means possesses specific surface properties, say like a Raney catalyst. Thus, the “molecular silver” acts on radical halides simply by virtue of its enlarged surface and not as a result of any sort of “molecular” dispersity. REFERENCES (1) FRICKE,R . : Hedvall Festskrift, p. 189 (Goteborg, 1948). (2) Gmelin’s Handbuch der anorganischen Chemie, 7th edition. Carl Winter, Heidelberg
(1920). (3) KOHLSCH~TTER, H . W. : Kolloid-Z. 77,229 (1936) ; idem in G.-M. Schwab’s Handbuch der Katalyse, Vol. 4, p. 391, J. Springer, Vienna (1943). (4) MAKEGOLD, E . : Kolloid-Z. 81, 164 (1937); see E. ZIMENSin G.-M. Schwab’s Handbuch der Katalyse, Vol. 4, p. 243, J. Springer, Vienna (1943). ( 5 ) SCHWAB, G.-M : Trans. Faraday SOC. 43, 715, 724, (1947); also previous papers. (6) SCHWAB, G.-M.: Trans. Faraday SOC. 42, 689 (1946). SCHWAB, G.-M., AND HOLZ, G.: Z. anorg. Chem. 262, 205 (1944). (7) SCHWAB, G.-M., AND THEOPHILIDES, N.: J. Phys. Chem. 60, 427 (1946).
