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The Journal of Physical Chemistry, Vol. 82, No. 17, 1978
(9) J. N. Maclean, F. J. C. Rossotti, and H. S. Rossotti, J. Inorg. Nuci. Chem., 24, 1549 (1962). (10) J. D. Holford, J . Chem. Phys., 10, 582 (1942). (11) A. S.Coolidge, J . Am. Chem. Soc., 50,2166 (1928). (12) E. Johnson and L. Nash, J . Am. Chem. Soc., 72, 547 (1950). (13) F. H. MacDougal, J . Am. Chem. Soc., 58, 2585 (1936). (14) C. T. Ewing, J. P. Stone, J. R. Spann, and R. R. Miller, J. Phys. Chem., 71, 473 (1967). (15) S. Datz, W. T. Smith, Jr., and E. H. Taylor, J. Chem. Phys., 34, 558 11961). (16) K. Hagenmark, M. Blander, and E. B. Luchsinger, J. Phys. Chem., 70, 276 (1966). (17) K. Hagenmark and D. Hengstenberg, J. Phys. Chem., 71, 3337 (1967). (18) J. Kreuzer, Z. Phys. Chem., 853, 213 (1943).
Communications to the Editor
(19) F. J. C. Rossotti and H. S. Rossotti, J . Phys. Chem., 85, 926,930, 1376 (1961). (20) E. T. Adams, Jr., Biochemistry, 4, 1655 (1965). (21) F. Y.-F. Lo, 6.M. Escott, E. J. Fendler, E. T. Adams, Jr., R. D. Larsen, and P. W. Smith, J . Phys. Chem., 79, 2609 (1975). (22) E. T. Adams, Jr., W. C. Ferguson, P. J. Wan, J. L. Sarquis, and B. M. Escott, Separation Sci., 10, 175 (1975). (23) E. T. Adams, Jr., fractions, No. 3 (1967). (24) J. L. Sarquis and E. T. Adams, Jr., Arch. Biochem. Biophys., 183, 442 (1974). (25) L.-H. Tang, D. R. Powell, B. M. Escott, and E. T. Adams, Jr., Biophys. Chem., 7, 121 (1977). (26) L. Harris and K. L. Churney, J . Chem. Phys., 47, 1703 (1967). (27) W. D. Lansing and E. 0. Kraemer, J . Am. Chem. Soc., 57, 1369 (1935).
COMMUNICATIONS TO THE EDITOR Oscillatory Gas Evolution from the System Formic Acid-Concentrated Sulfuric Acid-Concentrated Nitric Acid
Sir: In 1916,Morgan1 first observed the periodic evolution of carbon monoxide from a mixture of formic acid and concentrated sulfuric acid at temperatures between 40 and 70 "C. Recently, Showalter and Noyes2v3have made a comprehensive study of this oscillatory reaction, and have adduced evidence for the chemical nature of the observed rapid pulses of gas evolution. They have also suggested a mechanism for the reaction based on hydroxyl radical catalysis of formic acid decomposition involving iron salts which are present at the parts per million level in sulfuric acid. We have carried out a preliminary study of oscillatory gas evolution in the Morgan reaction when concentrated nitric acid is present. The three reactants in the system, and formic acid, concentrated H2S04,concentrated "Os, were mixed in the proportions of 5:l:l by volume. Standard analytical grade reagents were used without further purification. To avoid an unduly vigorous reaction, a procedure was adopted in which the formic acid was added slowly to a previously prepared mixture of concentrated sulfuric and nitric acids. The reaction vessel (a large test tube) was immersed in a thermostat maintained at 50 f 0.5 "C. Periodic foaming of the solution was observed (ashad been noted by Morgan1),the rise and fall of the froth clearly indicating the oscillatory nature of the gas evolution. A manometer connected to the reaction vessel was used in preliminary experiments to observe the stepwise increase in gas pressure in the closed system. These oscillations were made more clearly visible using a gas chromatograph thermal conductivity cell as in the experiments of Showalter and no ye^.^ The evolved gases were mixed with a carrier gas (helium) and then entered the thermal conductivity cell which was connected to a pen recorder. The bursts of gas evolution occurred every 30-40 s, and were preceded by maximum frothing of the solution. Regular oscillations persist for about 0.5 h. The evolved gases were primarily carbon monoxide, carbon dioxide, and nitrogen dioxide. The presence of these product gases was confirmed by gas-phase infrared spectrophotometry using a Perkin-Elmer 457 grating instrument. The absorption bands at 2340 (CO,) and 2140 cm-l (CO) observed at 5-min 0022-3654/78/2082-1952$0 1 .OO/O
intervals during the reaction show that initially carbon monoxide is present in a greater amount than carbon dioxide, but, as the reaction proceeds, the product gases become much richer in carbon dioxide. Further research on these product gases is currently underway in these laboratories. The period of oscillation, as well as the occurrence of oscillations was found to depend critically upon stirring conditions in the reaction vessel as noted by Showalter and no ye^.^ If stirring is too slow, gas evolution is erratic, and, under rapid stirring conditions, the gases are evolved smoothly without oscillations. While this would seem to indicate a physical basis for the oscillatory behavior in a supersaturation effect as suggested by Bowers and Rawji4 in their study of the Morgan reaction, there are nevertheless other observations which point to the chemical nature of these oscillations. We have noted that bubbles are always present in the solution even in quiescent periods which indicates that the observed bursts of gas must be due to a rapidly accelerating reaction. Furthermore, the faint blue color of the reacting solution has been observed to appear and disappear several times before persisting for the duration of the reaction. These observations of color oscillations are difficult to replicate and seem to depend very sensitively on reaction conditions. The final solution is always faintly blue in color. This color was tentatively ascribed by Morgan1 to the presence of N203,but in view of the temperature (50 "C) at which we carried out the reaction and the probable presence of NO+ as an intermediate, we feel that the color may be due to a nitroso compound formed from traces of organic impurities, Considerably more work will be necessary to establish the mechanism of the Morgan reaction including concentrated nitric acid. We feel that oxynitrogen chemistry may play a more important role than in the ShowalterNoyes mechanism for the oscillatory formic acid decomposition in sulfuric acid only. It may be speculated that the additional mechanistic steps will include: (i) the reaction of formic acid and nitric acid to yield nitrous acid and C 0 2 gas: HCOOH + "03 HNOz + COZ + HzO (ii) the formation of N203and its reaction with sulfuric acid to yield nitrosonium ions as well as its decomposition to yield NO2: 2HN02 + N20s(aq)+ H 2 0
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0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82,
Communications to the Editor
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N20,(aq) + 3H2S04 N203
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No. 17, 1978 1953
2NO+ + H30+ + 3HSO4NO
+ NO2
and also (iii) the reaction between NO+ and water: NO+ + HzO + HNOz + H+ Numerous other possibilities exist, including reactions involving hydroxyl radicals, but it seems premature to develop a mechanistic scheme based on the Oregonator model5 at this stage. Acknowledgment. Two of us (F.P. and J.F.) were supported by a National Science Foundation Undergraduate Research Participation grant during the summer of 1977 when this work was started. The authors are also indebted to Professor Richard M. Noyes for his helpful comments.
v10
+
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d20;I201 N
+
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-100-
References and Notes
1
(1) J. S. Morgan, J. Chem. SOC.London, 109, 274 (1916). (2) R. M. Noyes and R. J. Field, Acc. Chem. Res., 10, 273 (1977). (3) K. Showattar and R. M. Noyes, J. Am. Chem. Soc., 100, 1042 (1978). (4) R. G. Bowers and G. Rawji, J. Phys. Chem., 81, 1549 (1977). (5) R. J. Field and R. M. Noyes, Acc. Chem. Res., 10, 214 (1977).
5
80-
-40
21
f -30 ; 0
-20
C. J. G. Raw” J. Frlerdich F. Perrlno G. Jex
Department of Chemistry Saint Louis University St. Louis, Missouri 63103 Received May 25, 1978
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0
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120 (min)
150
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Figure 2. Hydrogen exchange of ethylene and isomerization of cis-but-Bene taking place simultaneously over the cut catalyst at 100 OC: (0)number of exchanged ethylene; (0)trans-but-2-ene.
Anisotropic Properties of Molybdenum Disulfide Single Crystal in Catalysis Publication costs assisted by Hokkaido University
Sir: Molybdenum disulfide has a sandwich-like layer structure constructed by a unit cell of a trigonal prismatic form, and is easily peeled off between the sulfur layers. Accordingly, the basal plane of the crystal is composed of a sulfur sheet, and the edge surface may expose molybdenum ions being coordinatively unsaturated. By cutting the wafers of a MoS2 single crystal, one can enlarge the edge surface area significantly without changing the basal plane area appreciably. In order to know the catalytic properties of these two surfaces, the experiments were performed by employing two forms of the MoS2 single crystal catalysts, one consists of thin wafers of the single crystal and the other is obtained by cutting the wafers into small pieces to enlarge the edge surface area, which are named “uncut” and “cut” catalysts, respectively. To minimize experimental error, the reactions were carried out in a twin reactor which was designed to furnish the same experimental conditions for the two types of catalysts. About 1g of MoSz single crystal wafers (from Climax Molybdenum Development Co. Japan) was mounted on one side of the twin reactor and the cut catalyst which was prepared by cutting nearly an equal amount of wafers was mounted on the other side of the reactor, where each cell has a volume of 50 mL. These catalysts were subjected to evacuation at 450 “C for several hours, and all experiments were performed simultaneously on both catalysts to assure identical experimental conditions. The isomerization of cis-but-2-eneto trans-but-2-ene did not occur appreciably in the absence of hydrogen on either form of catalyst a t 100 O C . If hydrogen was added, however, a remarkable promotion of the isomerization 0022-3054/78/2082-1953$0 I .OO/O
reaction was observed only on the cut catalyst while no appreciable isomerization occurred on the uncut catalyst as shown in Figure la. In previous work on MoSz p ~ w d e r , ~the J hydrogen promoting effect was explained by assuming the formation of a monohydrid site such as H
I
on which isomerization as well as the intermolecular hydrogen exchange reaction proceed via alkyl intermediates. In contrast with the isomerization of cis-but-2-ene on a single crystal catalyst, the isomerization of 2-methylbut-1-ene was brought about in the absence of hydrogen on either the cut or uncut catalysts as shown in Figure lb. This fact indicates that the isomerization of 2-methylbut-1-ene is catalyzed on the sulfur layer of the MoS2 crystal. This result strongly supports our speculation6that the isomerization of 2-methylbut-1-ene proceeds through the carbenium ion instead of the alkyl intermediates formed on the molybdenum sites. The isomerization reaction through carbenium ion intermediates is undoubtedly controlled by the proton activity or the acidity, and the results obtained on the single crystal as well as on MoS2 powder indicate that the acidity of the sulfur layer of MoS2is sufficient to make a tertiary carbenium ion from 2-methylbut-1-ene but is not so acidic to make secondary or primary carbenium ion intermediates. We can recognize an induction period for the isomerization of cis-but-2-ene in Figure la. To clarify the induction phenomena observed on the cut catalyst, a series of reactions was performed systematically, and a general rule was derived that the reactions taking place through sec-alkylintermediates always exhibit some induction time 0 1978 American Chemical Society
