Communicationis to the Editor
The Journal of Physical Chemistry, Vol. 83, No. 3, 7979 427
An alternative interpretation may be given to the deviations from Henry's law observed for the very dilute solutions of benzene in water. Following McMillan and Mayer,') one may show that the coefficient of the quadratic B P X B (and ~ hence term in the fugacity expression fB = ~ X + the dimerization constant) is directly proportional to the virial coefficient for pairwise interaction of solute molecules. The virial coefficient can be expressed in terms of cluster integrals involving the intermolecular potential energy functi.on for pairs of molecules. Thus, the dimerization constant may in principle be predicted provided that sufficiently accurate information about the attractive and repulsive forces between dissolved benzene molecules is available, This calculational method is entirely analogous to that used to relate equilibrium constants for the formation of molecular aggregates in the vapor phase to virial coefficients and ultimately to intermolecular potential energy function^.^^^ Although attempts have been made to predict properties of dissolved hydrocarbon aggregates from assumed potential energy functions, the lack of accurate th.ermodynamic data for dilute aqueous solutions of the hydrocarbons has in our opinion hampered the development of theories of hydrophobic interaction. Currently we are obtaining vapor pressure data for the water-benzene system at a number of temperatures in the range 20-45 O C . Our preliminary results clearly show that AHo for the di.merization of benzene is a relatively large positive quantity, as might have been anticipated for an aggregation stabilized in large part by hydrophobic interactions. The final derived values of AGO, AH", and ASo for the association of benzene in water should be of considerable interest in relation to our understanding of hydrophobic effects. We are also investigating the dependence of the limiting Henry's law constant, KH", on temperature in order to infer thermodynamic constants for the transfe.r reaction benzene (ideal gas) = benzene (ideal dilute solution in water)
that "diphenyl is not to be considered a good model for the actual benzene dimer".
Our initial results indicate that AHo for this solution process is about -6.9 kcal/mol at 35 " C and that, ACpo is on the order of 70 cal/deg, in fair agreement with results inferred from earlier calorimetric and solubility studies.1°
Acknowledgnzent. This study was supported by Grant No. CHE77-03668 from the National Science Foundation. We thank Professor H. S. Frank for sending us a manuscript copy of ref 5 in advance of publication.
References and Notes (1) W. Kauzmann, Adv. Protein Chem., 14, 1 (1959); F. Franks, Water, Compr. Treat., 4, 1-94 (1973). (2) A. Ben-Naim, J. Wilf, and M. Yaacobi, J. Phys. Chem., 77, 95 (1973). (3) J. H. Saylor, J., M. Stuckey, and P. M. Gross, J . Am. Chem. Soc., 60, 373 (1938). (4) A. A. Taha, R. D.Grigsby, J. R. Johnson, S.D. Christian, and H. E. Affsprung, J. Chem. Educ., 43, 432 (1966). (5) W. J. Green, and H. S. Frank, J . Solution Chem., in press. (6)A. E. Korvezen, Red. Trav. Chim., Pay-Bas, 70, 697 (1951); 72, 483 (1953). (7) See, for example, P. 0. P. Ts'o in "Basic Principles of Nucleic Acid Chemistry", Vcl. 1, Academic Press, New York, 1974, pp 537-562. (8) W. McMillan and J. Mayer, J . Chem. Phys., 13, 176 (1945). (9) N. Davidson, "Statistical Mechanics", McGraw-Hili, New York, 1962, pp 337-341. (10) S. J. Gill, N. F. Nichols, and I. Wadso, J. Chem. Thermodyn.,8, 445 (1976), and references therein. (1 1) A reviewer has objected to our making a direct comparison between the thermodynamic constants of hydrophobic Interaction obtained by Ben-Naim for 2(benzene) = diphenyl and the present dimerization data for benzene in water. Ben-Naim uses model compounds to deduce the indiirect part of the free energy of association for a particular configuration of the "dimer", and this value should not neCeSSarih/amroximate the total free enerav of dimerization reoorted here. Irt ihe'words of another reviewer, 6 6 present results confirm
0022-3654/79/2083-0427$01 .OO/O
Department of Chemistry The University of Oklahoma Norman, Oklahoma 730 19
Edwin E. Tucker" Sherrll D. Christian
Received July 20, 1978
Identification of Catalytically Active Sites on Reduced Molybclena-Alumina Catalysts Publication costs assisted by the National Science Foundation
Sir: Kokes and Dent1 were able to identify active intermediates in the hydrogenation of olefins over ZnO using IR spectroscopy. 'This approach is not generally applicable, however, because either the steady state concentration of the intermediate is too low, or else its lifetime is too short. We report herein an alternative approach in which information concerning the nature of the active sites may be deduced from IR studies of "poison" molecules which are selectively andl very strongly adsorbed on the catalytic centers. NO has been found2 to be a selective poison for propylene metathesis over molybdenum carbonyl supported on silica or alumina (after activation at elevated temperatures). Recently we have found it to selectively poison olefin hydrogenation, but not metathesis or isomerization, over reduced molybdena-alumina catalystsQ3IR spectra from these poison molecules for the latter system are presented in Figure 1. These spectra are characteristic of a dinitrosyl surface complex similar to those formed in homogeneous sys,tems as described by Cotton and J o h n ~ o nsimilar ;~ epectra have been reported for reduced chromia-silica catalyst^.^" Kemball and Howe2 also reported similar spectra but assumed the poisoning was due to a single NO molecule per site. With our system both bands (Figure 1)grew at the same rate when small doses of NO were introduced. When a 51 mixture of H2:C3H6was allowed to react over an activated (at 200 "C) molybdenum carbonyl on yalumina catalyst (prepared by vacuum sublimation) ,7 the product distribution was similar to that obtained for our reduced (to about I. e/Mo) molybdena-alumina catalyst, but the overall rate of reaction (propylene disappearance) was over 15 times greater. Both catalysts were equally poisoned (99% reduction in rate) by about 50 bmol of NO, Le., by about 2 NO for 40 to 50 Mo. The metathesis rate was also repressed with the carbonyl catalyst, but not with the reduced molybdena-alumina. Isotopic 15N0was used to confirm that species formed on molybdena-alumina were dimeric. Table I shows the frequencies observed for 14N0,15N0,and mixtures of these molecules. With 15N0,the two bands were shifted to lower =: frequencies by the expected amount [U('~NO)/U(~~NNO) 1.0151,as observed by Kugler and Grydera5r6A 1:1mixture of 14N0 and I5MO produced the expected spectrum consisting of two sets of triplet bands. The relative intensities of each of the triplet sets was about as expected, Le., 14N0-14NO:14NO-15NO:15NO-15N0 N 1:2:1. The complex was quite stable. Half saturation with '"0 produced the two bands for 15NO-J5N0. After evacuation and adding I4NO,the two bands for 14NO-14N0 were found along with those for 15NO-15N0. No bands for 14NO-15N0 were detected even after standing for 1 h at about 40 " C in 5 torr of 14N0. However, because these bands could not be resolved to the base line, it is not possible to say that @ 1979 American Chemical Society
428
The Journal of Physical Chemistry, Vol. 83, No. 1, 1979
Communications to the Editor
TABLE I: Spectra of NO Adsorbed on Molybdena-Alumina Catalystsu frequency, t 2 cm-' catalyst reduced MOO,/ Y-A~Z 0, reduced MOO,/ Y-Als0 3 reduced MOO, ?'*A12 0 3 activated Mo(CO),/ y-Al,O,C dinitrosyl Mo complexd
,'
isotope W 14N0 1820
,
W ,
1710
I5NO
1795
I4NO t ''NOb 14N0
1820,1805, 1710,1695, 1795 1685 1815 1705
14N0
1820
1685
3.710
Catalyst reduced 1.3 < electrons/Mo < 1.8; it was the same 8% Mo on y-alumina studied p r e v i o ~ s l y . ~ In the case of w , the high and low frequency bands were shoulders which could be detected at various dosing !evels. With w all three bands could be readily determined. Unpublished data of R. H. Howe and A. Kazusaka. Reference 4.
the carbonyl catalyst is about M O ~ ' ,while ~ that for the reduced molybdena-alumina is closer to Mo4+,and the rate accelerates rapidly and approaches that for the carbonyl catalyst as the extent of reduction is increased above 1 ~ / M o What, .~ then, is the active site for activation of Hz? In view of the very small fraction of molybdenum sites which must be poisoned3 to eliminate hydrogenation, and the fact that these sites hold two molecules of NO, it is clear that they must be multiply coordinatively unsaturated, It is therefore tempting to suggest that Mo3+centers may be involved.
1900
I a00
CM -'
Figure 1. Infrared spectra of NO on molybdena-alumina reduced with H,. The spectra were obtained by saturating the reduced catalyst pellet with NO at room temperature. This was followed by 1-h evacuation at the temperatures indicated at the right of the spectra. The procedure is nearly identical with that used in the saturation poisoning studies reported in ref 3.
14NO-15N0 was completely absent. On evacuation a t elevated temperature (Figure l),NO could be removed with difficulty, but N 2 0 was formed in the process. The sites which are active for hydrogenation over the two catalysts studied are presumably similar; this notion is strengthened by their response to NO. This leads to an interesting question, viz., the average oxidation state for
Acknowledgment. This work was supported by grants from the National Science Foundation (No. CHE77-07772) which we gratefully acknowledge. We are indebted to Professor Russell Howe for supplying us with the data for the molybdenum carbonyl catalyst shown in the table. References and Notes (1) R. J. Kokes and A. L. Dent, J . A m . Chem. Soc., 92, 6709 (1970). (2) C. Kemball and R. F. Howe, J . Chem. Soc., Faraday Trans. 7, 70,
(3) (4) (5) (6) (7)
1153 (1974). E. A. Lombard0 and W. K. Hall, to be published. F. A. Coiton and 8.F. G. Johnson, Inorg. Chem., 3, 1609 (1964). E. L. Kugler and J. W. Gryder, J . Catal., 36, 152 (1975). E. L. Kugler, J. W. Gryder and R. J. Kokes, J. Catal,, 36,142 (1975). R. L. Burwell, Jr., and A. Brenner, J . Catal., 52, 353 (1978).
Laboratory for Surface Studies Department of Chemistry University of Wisconsin Milwaukee, Wisconsin 5320 I Received July 18, 1978
W. S. Milkman W. Keith Hall'
