The Journal of Physical Chemistry, Vol. 82,
Communications to the Editor
N20,(aq)
-
+ 3H2S04
-
No. 17, 1978 1953
2NO+ + H30+ + 3HSO4-
N203 NO + 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
+
- 60
d20;I201 N
+
- 50
-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
-10
0
30
60
90 Time
120 (min)
150
180
210
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-ene to 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 speculation6 that 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-alkyl intermediates always exhibit some induction time 0 1978 American Chemical Society
1954
The Journal of Physical Chemistry, Vol. 82, No. 17, 1978
Communications to the Edltor
TABLE I: M i c r o w a v e Spectroscopic Analysis of Propene-d, a n d But-1-ene-d, O b t a i n e d in t h e H y d r o g e n Exchange Reaction b e t w e e n Propene-d, a n d But-1-ene in t h e Presence of H y d r o g e n over t h e C u t MoS, Single Crystal propene-d time, min % propene-d,
45 82
26.7 43.8
but-1-ene-d
1-h,
2-h,
3-h,
1-d,
2-d,
27.4 36.5
63.7 57.0
8.9 6.5
33.6 47.9
66.4 52.1
3-d, 0 0
4-d,
0 0
1 2 3 1 2 3 4 The n u m b e r i n g of t h e c a r b o n atoms is as follows: C=C-C a n d C=C-C-C.
while the reactions which would proceed through n-alkyl intermediates do not. A typical case is shown in Figure 2, where hydrogen exchange between CzH4and CzD4and the isomerization of cis-but-2-ene were performed simultaneously a t 100 “C in the presence of Dz. It is clear that hydrogen exchange between ethylene molecules has no induction time but the isomerization of cis-but-2-ene as well as the hydrogen exchange between ethylene and cis-but-2-ene show an appreciable induction time. The intermolecular hydrogen exchange of olefins stimulated by hydrogen on MoS2 powder proved to proceed by an associative mechanism through alkyl intermediate~.~~” The contribution of the metathesis reaction12 on the cut catalyst was proved negligible by using I3C2H,. These facts lead us to a conclusion that the intermolecular hydrogen exchange of olefins on the edge surface should proceed through alkyl intermediates. Accordingly, it is quite interesting whether hydrogen exchange via sec-alkyl intermediates occurs or not during the induction period, because the isomerization through sec-alkyl intermediates does not occur within this period. In order to shed light on this subject, a hydrogen exchange reaction between propene-d6 and but-1-ene-do was performed on the cut catalyst. The microwave spectroscopic analysis of propene-d6 (propene-hl) and of but-1-ene-d, are summarized in Table I. A run of 45 min indicates exchange during the induction time, which proves the formation of propene-1-hl and propene-2-hl, HDC=CDCD3 and CD2=CHCD3, and but-l-ene-l-dl and but-1-ene-2-d,, HDC=CHC2H5 and CH2=CDC2H5. These results undoubtedly indicate that the hydrogen exchange reaction through sec-alkyl as well as n-alkyl intermediates occurs during the induction period for the isomerization reaction. If the cut catalyst was exposed to either hydrogen or olefins alone for a certain period of time, no shortening of the induction time was observed, and these phenomena were reproducible with differently purified gases for the different sample of MoS2 wafers. In conformity with a strictly controlled cis-stereo chemical hydrogen addition and elimination process on the Mo(S)4 site such peculiar phenomena seem to indicate a restricted rotation of the bulky group bonded to a tight spacing Mo(S)4 site I in a manner similar to the hindered rotation about single bonds in organic molecules caused by large steric interactions.1° The details will be discussed in a future paper. Heterogeneously catalyzed reactions may be classified into “structure sensitive” and “structure insensitive” reaction^,^ however, few of the reactions have been understood on the basis of the mechanistic necessity of
specific structures. It may be concluded that isomerizations stimulated by hydrogen on the edge surface is a structure sensitive type reaction which takes place on the sites having the two degrees of coordinative unsaturation such as expected on the edge surface of a MoS2 crystal. In contrast with this reaction, the isomerization reaction taking place on the basal plane is a structure insensitive type reaction, because it is regulated by the proton activity or the acidity of the basal plane. It should be noted here that the isomerization and/or the intermolecular hydrogen exchange of olefins through alkyl intermediates require two degrees of coordinative unsaturation for the active but more complex catalytic reactions may require different structural prerequisites including ensemble operation of several sites as Muetterties and Demitras8 showed in their metal cluster experiments. Acknowledgment. The authors thank Dr. T. Kondo of Sagami Chemical Research Center for his microwave spectroscopic analysis, and they are indebted to Climax Molybdenum Development Co. Japan for furnishing the MoSz single crystal. References and Notes (1) S. Siegel, J. Catal., 30, 139 (1973). (2) A. Takeuchi, K. Tanaka, and K. Miyahara, Chem. Lett., 171, 411 (1974); A. Takeuchi, K. Tanaka, I. Toyoshima, and K. Miyahara, J . Catal., 40,94 (1975); A. Takeuchi, K.Tanaka, and K. Miyahara, ibid., 40, 101 (1975). (3) K. Tanaka, J. Catal., 37, 558 (1975). (4) K. Tanaka, T. Okuhara, S. Sato, and K. Miyahara, J. Catal., 43,360 (1976). (5) T. Okuhara and K. Tanaka, J . Am. Chem. Soc., 98, 7884 (1976). (6) T. Okuhara, K. Tanaka, and K. Tanabe, J . Chem. SOC., Chem. Commun., 180 (1977). (7) M. Boudart, A. AMag, J. E. Benson, N. A. Dougerty, and C. G. Harkins, J . Catal., 6, 92 (1966); D. W.Biakely and G. A. Somorjai, ibid., 42, 181 (1976). (8) G. C. Demitras and E. L. Muetterties, J. Am. Chem. SOC.,99, 2796 (1977). (9) K. Tanaka and T. Okuhara, Catal. Rev., Sci. Eng., 15, 249 (1977). (10) L. M. Jackman and F. A. Cotton, Ed., “Dynamic NMR Spectroscopy”, Academic Press, New York, N.Y., 1975, p 163. (11) T. Okuhara and K. Tanaka, J . Chem. SOC.,Faraday Trans. 1 , accepted for publication. (12) One of the reviewers of this paper was wedded to metathesis type hydrogen mixing on the cut catalyst. The authors could refute confidently his claim on the basis of experimental evidence: the geometrical distribution of deuterioolefins, hydrogen promoting effect, cis-stereospecific hydrogen addition and e l i m i n a t i ~ n ,and ~ ~ ~no appreciable metathesis of ‘3C2H4.
Research Institute for Catalysis Hokkaido University Sapparo, Japan
Toshio Okuhara Ken-lchl Tanaka’
Received December 27, 1977; Revised Manuscript Received March 13. 1978
