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MATHESIUP,F.: Stahl u. Eisen 34, 872 (1914). MATSUBARA, A . : Trans. Am. Inst. Mining Met. Eng. 3, 1051 (1921). MOISSAN,H . : Compt. rend. 84, 1296 (1877). OLMER,F.: J. Phys. Chem. 47, 313 (1943). J . : 2.Elektrochem. 29, 79 (1923); 30, 175 (1924). SAUERWALD, G . B., AND STARKWEATHER, H. W. J.: J. Am. Chem. SOC.52,2314 (1930). TAYLOR, G. I . : J. Phys. Chem. Russe 6, 1103 (1934); Acta Physicochim. TCHOUFFAROFF, U.R.S.S. 4, 617 (1936). (18) VALLET,P. : “Methode d’btude des systemes chimiques B temperature variable”, Thesis, Jouve, Paris, 1936. (19) WUST, F., AND RUTTEN,P. : Mitt. Kaiser-Wilhelm Inst. Eisenforsch. Diisseldorf 5, 6 (1924).
(11) (12) (13) (14) (15) (16) (17)
T H E MECHANISM OF T H E FORMATION OF KOHLSCHUTTER’S SILVER SOL. II HARRY B. WEISER
AND
MAX F. ROY
Department of Chemistry, The Rice Institute, Houston, Texas Received December 4 , 1948
“Kohlschutter’s silver sol” (1) is the term commonly applied to the hydrosol formed by conducting hydrogen into a solution of silver oxide containing an excess of the solid oxide, maintained at a temperature of 50430°C. Carbon monoxide may be substituted for hydrogen as a reducing agent, but according to Kohlschutter the resulting sol is instable. The first paper (2) on this subject reported the results of tt study of the mechanism of the sol formation process with hydrogen as the reducing agent; this paper is concerned with the formation of Bilver sol using carbon monoxide as reducing agent. In the earlier paper it was demonstrated that sol formation results by the reduction of silver oxide a t 55-58°C. only when some of the solid oxide is suspended in the solution. In fused-silica or silver vessels, ultrafiltered silver oxide solutions are not reduced by hydrogen a t 55-58’C.; in glass and platinum vessels, the reduction is confined to the surface of the vessel, giving a thin silver mirror on glass and relatively large platelets of silver on platinum. The reduction of ultrafiltered silver oxide solution in a platinum vessel results from catalytic activation of the hydrogen at the surface of the metal. No catalytic activation and no reduction take place at the surface of silver or fused silica. The reduction a t the surface of glass does not result from catalytic activation of hydrogen, followed by reduction of dissolved silver oxide, but is due to reduction of a s l m of solid silver oxide precipitated on the surface of the glass by alkali dissolved from the glass; adsorption of the oxide from solution plays a minor r81e in forming the oxide film. The ease with which solid silver oxide in suspension or on the walls of the vessel is reduced indicates that hydrogen is activated a t the surface of silver oxide or a t the interface silver-silver oxide.
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32G
HARRY B. WEISER AND MAX F. ROY
With carbon monoxide as reducing agent in the formation of silver sol, Kohlschutter claims that the reduction takes place almost entirely at the surface of the suspended silver oxide and that the resulting sols are quite instable, precipitating completely in a few days. The observations reported in this paper disclose that reduction may take place from a solution of silver oxide with the formation of highly stable silver sols. EXPERIMENTAL
Procedure and reagents The apparatus, silver oxide, temperature, and general experimental procedures used for the study of the reduction of silver oxide with carbon monoxide were identical with those described previously with hydrogen as reducing agent. Because of the catalytic nature of the reduction process with hydrogen, special precautions \yere necessary in the preparation of reagents and in the purification of the containing vessels. The same precautions were observed in the following experiments xl-ith carbon monoxide as reducing agent. I n all experiments the vessels were test tubes with gas inlet tubes of the same material. Carbon monoxide was prepared by introducing 98 per cent formic acid solutions under the surface of concentrated sulfuric acid. The gas was collected in a glass gasometer of 20-liters capacity. Before introducing the gas into the silver oxide solution, it was passed slowly over potassium hydroxide pellets, through 10 per cent potassium hydroxide solution, and finally through wash bottles containing distilled water. Cnlike the washing of hydrogen, carbon monoxide was not passed through silver oxide solution a t room temperature before being conducted into the solution at 55-58"C., since, unlike hydrogen, carbon monoxide reduces aqueous silver oxide at room temperature. Reduction of saturated silver ocide in the presence of excess solid The results obtained when carbon monoxide mas led slowly into silver oxid; solutions containing excess solid oxide are given in table 1. The three sols referred to underwent no apparent change for a week or more, but after a few weeks a reddish brown layer of solid had settled out,, leaving clear deep yelloiv sols with a greenish tinge in reflected light. The sols showed no further change on standing for G months and ivere then discarded. It should be noted that, unlike hydrogen, carbon monoxide reduced silver oxide completely t.o metallic silver under the conditions of the experiment. Reduction of silver oxide in jltered saturated solutions The experiments described in table 1 were repeated with saturated silver oxide, filtered to remove excess suspended solid. The results are given in table 2. Unlike Kohlschutter's sols formed by reduction of silver oxide with carbon monoxide and the sola referred to in table 1, all sols formed from filtered silver oxide solutions described in tablt. 2 were quite stable, undergoing no change in appearance and depositing no solid on standing. It vould appear that the particles which
MECHANISM O F FORMATION OF KOHLSCH~TTER’S SILVER SOL
327
OBSERVATIONS
CONTAINER
Soft glass., , , , . The solution showed a yellow coloration within 10 t o 15 sec. and darkened to a clear deep yellow in 1.5 t o 2 hr. On continued exposure t o carbon monoxide, t h e color of the sol became a deep brownish yellow with a greenish tinge in reflected light. A silver mirror was deposited on the vessel walls and on the carbon monoxide inlet tube. The color of the suspended solid particles changed from the characteristic brown of silver oxide t o a metallic gray. The gray particles were insoluble in concentrated ammonium hydroxide but were completely soluble in cold concentrated nitric acid, indicating that the reduction t o metallic silver was complete.
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Pyrex., , , , . . , , , The results were the same a8 in soft glass, except that there was less mirror formation on the walls.
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Fused silica., . , The results were the same as in soft glass, except that there was very little or no mirror formation on the walls.
1
CONTNNER
Soft glass
.
OBSERVATIONS
Yellow coloration started a t once and darkened in 2.5-3 hr. t o a clear deep yellow with a brownish tinge in transmitted light and a greenish tinge in reflected light. Further exposure to carbon monoxide caused no observable change in the appearance of the sol. Only a very slight
Pyrex.. . . . . . . . ,/Sameas in soft glass. Fused silica
.
/Same as in glass, except that there was no mirror formation
Reduction of silver oxide in ultrajiltered saturated solutions Saturated solutions of silver oxide were ultrafiltered in a gold-plated ultrafilter in which the membrane was supported on perforated gold foil. htembranes were prepared by impregnating No. 40 Whatman filter paper with collodion from a 4 per cent solution in glacial acetic acid. The experiments with ultrafiltered solutions were carried out in fused-silica vessels only.
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HARRY B. WEISER -4ND M.4X F. ROY
Saturated solutions: The solutions acquired a yellow coloration within a few seconds after exposure to carbon monoxide. The pale yellow color increased to a deep yellow in 1.5-2 hr. Thereafter there was no further change, since the reaction n'as complete. Xo mirror formation took place. The sols were highly stable. Carbon dioxide formed in the reduction process was rvashed out by the excess carbon monoxide bubbling through the solution. Sols formed by reduction of ultrafiltered silver oxide solutions lacked the brownish tinge which characterized the sols formed from solutions which were not ultrafiltered (cf. tables 1 and 2 ) . The brownish tinge resulted from the presence of relatively large aggregates of colloidal silver, formed by reduction of relatively large colloidal aggregates of silver oxide which were not removed by filtration through filter paper but were removed by ultrafiltration. Vnsaturated solutions: .4solution was prepared by diluting 50 ml. of saturated silver oxide with an equal amount of distilled water. When carbon monoxide was led into this solution, a light yellow coloration developed immediately and deepened to a clear bright yellow in 1.5-2 hr. A similar sol with a slightly deeper yellow color \vas obtained from a solution made by diluting GO ml. of saturated silver oxide with 40 ml. of distilled water. Both sols mere highly stable. There was no mirror formation. It will be recalled that hydrogen reduces silver oxide a t 55-58°C. only at t h e surface of silver oxide or at the interface silver-silver oxide. Accordingly, silver sols are not formed by conducting hydrogen into ultrafiltered silver oxide solutions in fused-silica vessels a t 55-58°C. On the other hand, ultrafiltered silver oxide solutions, both saturated and unsaturated, are readily reduced by carbon monoxide, giving clear stable sols. Although carbon monoxide is but slightly soluble in water at 55-58"C., it is probable that the reduction of ultrafiltered silver oxide solutions in fused-silica vessels starts in solution. After the reaction starts, catalytic activation a t the surface of colloidal silver particles may play a rble, and reaction at the silica-solution interface may take place to a certain extent. Silver sols prepared by reduction of ultrafiltered silver oxide solution with carbon monoxide are limited in concentrat>ionby the solubility of silver oxide. The sols are sufficiently stable, hoTvever, that' they can be concentratzed by removal of a portion of the water by evaporation under reduced pressure. SUMM.IRY
1 . Stable, relatively pure hydrosols of silver of uniform particle size are obtained by the reduction of ultrafiltered solutions of silver oxide with carbon
monoxide in fused-silica or Pyrex vessels. Kohlschiitter obtained only nonuniform unstable hydrosols by reduction Tvith carbon monoxide of solutions of silver oxide containing suspended silver oxide. 2. Silver hydrosols formed under the proper conditions by reduction of silver oxide solutions with carbon monoxide are purer and more uniform than the sols prepared by reduction with hydrogen, for the following reasons: ( a ) the reduction with carbon monoxide is accomplished in ultrafiltered solutions of silver
NITROGENOUS CARBONS AS CATALYSTS
329
oxide, whereas the reduction with hydrogen takes place only in the presence of solid silver oxide; and (b) the reduction of silver oxide by carbon monoxide is practically complete under the proper conditions of sol formation, whereas the reduction of silver oxide by hydrogen is incomplete, giving hydrosols in which the particles are mixtures of silver oxide and silver. REFERENCES (1) KOHLSCH~TTER: Z. Elektrochem. 14, 49 (1908). (2) WEISERAND ROY:J. Phys. Chem. 37, 1018 (1933).
THE EFFECT OF ACTIVE NITROGEN AND OF CERTAIN NITROGEN COMPOUNDS ON CATALYTIC PROPERTIES OF CARBOW PAUL F. BENTE
AND
JAMES H. WALTON
Department of Chemistry, University of Wisconsin, Madison, Wisconsin Received January 2, 1943
In a recent paper (1) the results of experiments using nitrogenous carbons as catalysts for several reactions were discussed. The catalysts were all prepared from purified materials containing nitrogen, a part of which remained present as nitrogen of constitution during carbonization and activation. Since the presence of the nitrogen seems to increase the catalytic activity of the carbons, it was of interest to determine more specifically the promoting effect of such nitrogen. The investigation of this problem is complicated by the fact that the activities of carbons prepared from various organic compounds vary markedly with the materials used. Thus it is impossible in comparing a nitrogenous carbon with a non-nitrogenous carbon to say how much of the difference in activities may arise from the effect of the different source materials and how much may be specifically due to the presence of the nitrogen. The purpose of this investigation was to obtain information concerning the promoting effect of nitrogen by preparing a nitrogenous carbon from an activated nitrogen-free sugar carbon the catalytic properties of which are known. A search of the literature pertaining to the catalytic properties of active carbons yielded only one reference concerning the enrichment of nitrogen in carbons. Honig (4)heated several commercial carbons in a nitric acid-sulfuric acid mixture, causing the nitrogen content of the carbons to increase from about 0.4 to 2.5 per cent. Resulting catalytic changes however, were not noted. In the following experiments the various methods employed by the present authors in chemically fixing nitrogen to the surface of activated carbon are described, together with the resulting changes in catalytic properties of the carbon catalysts so treated. 1 This investigation was financed by a grant from the Research Committee of the University of Wisconsin, Dean E. B. Fred, Chairman.
