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The Journal of Physical Chemishy, Vol, 82, No. 15, 1978
reported here for ND4+and N(CD3)4+. These arguments support our conclusion that the distortions of ions apparently exist in lyotropic systems and that they are responsible for the observed splittings in their NMR spectra. Note Added in Proof. A recent paper by D. M. Chen and J. D. Glickson (J. Mag. Reson., 28, 9 (1977)) investigates the problem of distortions of ammonium ions in other lyotropic phases. Results of 14N,D, and H NMR spectra are analyzed exclusively in terms of the distortion modeL7 In comparing the results given in this paper with ours we note that they observe much smaller quadrupole splittings of deuterium (particularly for ND4+). Consequently they deduced a value for (e2qQINof about 3 MHz, quite bigger than the value suggested by us.
Acknowledgment. Part of this research has been sponsored by a research grant from the United StatesIsrael Binational Science Foundation. References and Notes (1) J. W. Emsley and J. C. Lindon, "NMR Spectroscopy Using Liquid Crystal Solvents", Pergamon Press, New York, N.Y., 1975. (2) C. L. Khetrapal, A. C. Kunwar, A. S. Tracey, and P. Diehl, "Nuclear Magnetic Resonance Studies in Lyotropic Liquid Crystals" in "NMR, Basic Principles and Progress", P. Diehl, E. Fluck, and R. Kosfeld, Ed., Vol. 9, Springer-Verlag, West Berlin, 1975. (3) K. Radley and L. W. Reeves, Can J . Chem., 53, 2998 (1975). (4) K. Radley, L. Reeves, and A. S. Tracey, J. fhys. Chem., 80, 174 (1976). (5) F. Fujiwara, L. W. Reeves, and A. S. Tracey, J. Am. Chem. SOC., 96, 5250 (1974). (6) L. W. Reeves and A. S.Tracey, J. Am. Chem. Soc., 96,365 (1974). (7) D. Bailey, A. D. Buckingham, F. Fujiwara, and L. W. Reeves, J . Mag. Reson., 18, 344 (1975). (8) H. Gustavsson, G. Lindblom, B. Lindman, N-0. Persson, and H. Wennerstrom in "Liquid Crystals and Oriented Fluids", Vol. 2, J. F.
S. Furuyama and T.
(9) (10)
(11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25)
(26) (27) (28) (29) (30)
(31)
Morimoto
Johnson and R. S. Porter, Ed., Plenum Press, New York, N.Y., 1973, p 161. K. Radley and A. Saupe, Mol. Phys., 32, 1167 (1976). B. Lindman and S. Forsen, "CI, Br, I NMR, Physico-Chemicaland Biological Applications", Vol. 12 of "NMR, Basic Principles and Progress", P. Diehl, E. Fluck, and R. Kosteld, Ed., Springer-Verlag, West Berlin, 1976. L. C. Snyder and S. Meiboom, J. Chem. fhys., 44, 4057 (1966). R. Ader and A. Loewenstein, Mol. Phys., 24, 455 (1972). R. Ader and A. Loewenstein, Mol. fhys., 30, 199 (1975). A. Loewenstein, Chem. fhys. Lett., 38, 543 (1976). I. Y. Wei and C. S. Johnson Jr., J . Mag. Reson., 23, 259 (1976). A. Amanzi, P. L. Barili, P. Chidichimo, and C. A. Veracini, Chem. fhys. Lett., 44, 110 (1976). A. Frey and R. R. Ernst, Chem. Phys. Lett., 49, 75 (1977). J. Charvolln, A. Loewenstein, and J. Virlet, J. Mag. Reson., 26, 529 (1977). J. Bulthuis and C. A. de Lange, J . Mag. Reson., 14, 13 (1974). H. Wennerstrom, G. Lindblom, and B. Lindman, Chem. Scr., 6, 97 (1974). S. Engstrom, H. Wennerstrom, B. Jonsson, and C. Karlstrom, Mol. Phys., 34, 813 (1977). N-0 Persson and B. Lindman, Mol. Cryst. Liq. Cryst., 38, 327 (1977). L. W. Reeves, J. Sanches de Cara, M. Suzukl, and A. S. Tracey, Mol. fhys., 25, 1481 (1973). J. L. Kurz, J. Phys. Chem., 86, 2239 (1962). The quadrupole moments, for N in particular, are susceptible to large errors. Data on eQNcan be found in J. M. Lehn and J. P. Kintzinger, "Nitrogen NMR", M. Witanowski and G. A. Webb, Ed., Plenum Press, New York, N.Y., 1973, p 84 and 8; H. Kopfermann, "Nuclear Moments", Academic Press, New York, N.Y., 1956; W. D. White and R. S. Drago, J . Chem. Phys., 52, 4717 (1970), and Erratum to this paper. W. J. Caspary, F. Millet, M. Reichbach, and B. P. Dailey, J . Chem. fhys., 51, 623 (1969). Cf. P. Pvkko, Ann. Univ. Turku., Ser. A, 103 (19671: H. H. Mantsch, H. Saitoand I. C. P. Smith, frog. NMR Spect;osc.,'ll, 212 (1977). D. T. Edmonds, M. J. Hunt, and A. L. MacKay, J. Mag. Reson., 9, 66 (1973); M. J. Hunt, ibid., 15, 113 (1974). W. C. Bailey and H. S. Story, J. Chem. fhys., 60, 1952 (1970). J. P. Kintzinger and J. M. Lehn, Helv. Chim. Acta, 58, 905 (1975). Cf. G. F. Pedulli, C. Zannoni, and A. Alberti, Mol. Phys., 10,372(1973).
Sorption of Nitric Oxide, Carbon Monoxide, and Nitrogen by Sodium Mordenite Shoro Furuyama* and Tetsuo Morlmoto Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan (Received January 12, 1978; Revised Manuscript Received May 23, 1978) Publication costs assisted by Okayama University
The sorption of NO, CO, and Nzby sodium mordenite was measured in the temperature range -80-75 "C. The strength of the sorption affinity was found to vary in the order of N2 < NO < CO. The slopes of the isotherms (log (sorption amount) vs. log (equilibrium pressure)) were almost unity at lower coverage for all three gases. At higher coverage, the slope for NO became steep, possibly due to dimerization, while the slopes for CO and Nz decreased gradually as usual. The saturation sorption amount, V,, was 25.0 mL (STP)/g for CO and Nz, which gave a value of 0.4 nm for the effective diameter of the sorbed molecules. The isosteric heat of sorption, qst, changed with temperature and coverage. At 0 = 0.02, the values of qst for CO, Nz, and NO were 8.8,8.0, and 7.1 kcal/mol at -30 "C, and 8.1, 6.6, and 6.6 kcal/mol at 30 "C, respectively. It is concluded that the magnitude of qat is principally determined by dispersion forces and that the contributions of the quadrupole and dipole moments are less significant. The temperature dependence of the thermal entropy of sorbed molecules suggests that the sorbed molecule rotates freely above 40 "C, but librates below -25 "C. However, the degree of the rotational hindrance is independent of the magnitude of the dipole moment. At higher coverage, the sorption isotherms of CO and Nzcan be expressed in a virial form. The second virial coefficient obtained decreases gradually with increases in temperature and falls in the range 1-2 X m/molecule in the temperature region 0-45 "C.
Introduction Mordenite has a bundle of elliptical (0.7 and 0.58 nm for the maximum and minimum free diameters, respectively) straight cylindrical channels running parallel to the c axis.l On account of this unique structure, mordenite can sorb permanent gases even a t room temperature.2
According to studies by Takaishi et al., the translational mode of the sorbed molecule well be described in terms of a model for a one-dimensional g a ~ . ~The - ~rotational state of the sorbed molecule is apparently affected by the magnitude of its molecular quadrupole moment. The rotation of sorbed nitrogen is seriously hindered below 0
0022-3654/78/2082-1746$01.00/00 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 15, 1978
Sorption of Gases by Sodium Mordenite
I
-2
0
-1
log Pe
1
2
I
logPe
(Torr)
co
-1
0
IOgPe
(Torr)
Flgure 3. Sorption isotherm of N P .
Flgure 1. Sorption isotherm of NO.
-2
1749
1
2
(Torr)
Figure 2. Sorption isotherm of CO.
"C, but oxygen is freely rotating even at -80 0C.3 On the other hand, the effect of the dipole upon rotational hindrance has not been studied as yet. In the preceding paper, we measured the adsorption of NO, CO, Nz, and O2 on MgO and observed that a large dipole moment resulted in strong ads~rption.~!'This is plausible for adsorption on a plane surface having a large electrostatic field but is open t o question for sorption in the pores of a zeolite.8 In the present work, we measured the sorption of NO, CO, and N2 into the pores of sodium mordenite (NasA18Si40096-24H20) and elucidated the effects of polarizability, quadrupole moment, and dipole moment upon the thermodynamics of sorption in the mordenite pores. Experimental Section The method of purification of NO and the purities of CO and N2 gases were described in a preceding paper.6 Sodium mordenite (Zeolon supplied from Norton Co.), 0.5 g, was baked a t 450 "C for 2 h in a stream of air and then degassed at room temperature at Torr (1Torr = 133 Pa) prior to the sorption experiments. The equilibrium pressure was measured by a Datametrics Barocell pressure sensor 570D-electronic manometer 1173. The sorption temperature was determined with calibrated thermometers with an accuracy of hO.1 "C over the range of 0-75 "C and h0.2 "C over the range of -80-0 "C. Results The sorption isotherms (log Vsorbvs. log P,) of NO, CO, and N2 are shown in Figures 1-3, where Vmrhand P, denote the amount sorbed and the equilibrium pressure, respectively. The figures show that the sorption affinity increases in the order N2 < NO < CO. The slope of the
log Vsorbvs. log P, curve is almost unity for all three gases, obeying Henry's law, at lower coverages. At higher coverages, the slope for NO rises steeply at about 6 mL (STP)/g of sorption, while those for N2 and CO decrease gradually, as ~ s u a l . ~In# the ~ steep rise region, it took several days to attain equilibrium. This phenomenon may be ascribed to the dimerization of NO. The Langmuir equation was applied to the isotherms of CO and N2 obtained in the range of 0-50 "C. The saturation value for the amount sorbed was 25.0 mL (STP)/g of hydrated mordenite for both of CO and Nz. This is about two thirds of the amount obtained by Takaishi et al.3 This discrepancy will be discussed later. The Langmuir equation could not be applied to the isotherm of NO. However, the molecular dimension of NO (bond length, ro = 0.114 nm) is almost the same as that of CO and N2 (ro = 0.113 and 0.106 nm, respectively), so the saturation value for the amount sorbed for NO may be close to 25 mL (STP)/g. If we adopt the one-dimensional gas model for the sorbate in mordenite pores, the sorbates are aligned linearly. Therefore, the saturation value ex(On a two-dimensional presses linear chain capacity, surface, the saturation value for amount adsorbed gives the monolayer capacity, VmS5)The effective diameter of the sorbed molecule, deff,is given by deff = L / VI
(1)
where L denotes the total pore length and VI is expressed in number of molecules. Using crystal structure data, we have L = 2(no. of unit cell/g) X (length of c axis) = 2 X 1.72 X 1020 X 7.52 X = 25.9 X 1O1O m/g and hence d,ff = 0.4 nm. This value is slightly larger than the value (0.28 nm) obtained by Takaishi et al.3 owing to the smaller VI value used in eq 1,but is comparable with van der Waals diameter of 0.4 nm for the sorbed molecule~.~ By applying the Clausius-Clapeyron equation, the value of isosteric heat of sorption, qst, was calculated as a function of temperature and coverage, and is shown in Figure 4. The qst decreases with rise in temperature, suggesting that the sorbed state changes with temperature. For NO, qst could not be determined above 8 = 0.1, because of the occurrence of dimerization. Therefore, we can say nothing about the variation of qst for NO at higher coverages. For CO and Nz, qst decreases with increasing coverage. It can be concluded that the magnitude of qst is in the order NO IN2 < CO below 8 = 0.1 in the temperature range -30-30 "C. The values of qst for NO, CO, and Nzat d = 0.02 are summarized in Table I, along with values of qat for other
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The Journal of Physical Chemistry, Vol. 82, No. 15, 1978
S. Furuyama and T. Morlmoto
TABLE I: Values of qst in Mordenite and on MgO, and Values of Polarizability, Quadrupole Moment, and Dipole Moment
qst in mordenite, kcal/mol
NO
co N*
a
b
7.1 8.8 8.0
6.6 8.1 6.6
C
6.5 4.7 15 4.5
0, CQ,
Ar
a,i
Q, e x
10-24 cm3
10-16 Cm2
d 1.72 1.99 1.76 1.57 3.01e 1.63e
d 0.28 0.34 0.27 0.09 0.65e Oe
9st on
fi,
ED+R, EQ, kcal/mol kcal/mol
ex
cm
WQ,
kcal/mol
f
f
g
4.9 5.5
1.1 2.6 1.6 0.3 6.7 0
4.0h 3.8 2.6 2.3
d 0.16 0.11 0
4.9
0 0 0
4.4 8.3 4.5
*
a Present work a t -30 "C. Present work at 30 "C. Reference 5. References are cited in ref 6. e Reference 8. See text. g Reference 6. This value is obtained by substracting A H m (1.6 kcal/mol) from qst(N,0,/2)(5.6 kcal/mol) determined in ref 6 and 7. 0111 and a1 are (in units) 2.60 and 1.63 for CO, 2.38 and 1.45 for N,, 2.35 and 1.21 for 0, , 4.10 and 1.93 for CO, , and 1.63 and 1.63 for Ar. The values for NO are not available. "Kagaku Binran" The Chemical Society of Japan, Maruzen, 1966, p 1226. f
t 15
-
10
-
a
\
E
5
c
IT
51
'
5.2
5.4
5.6
InT
5.8 (K)
6.0
Figure 5. Stharm vs. In T plot at I9 = 0.02. 0.3
0.1
0.5
8 Figure 4. qst vs. 6 plot: (0) CO;(A)N,; (0) NO; (----) value at 30 O C ; (-) value at -30 O C .
gases and the values of dipole moment, p, quadrupole moment, Q, and polarizability, a. Table I shows that qst values strongly correlate with the values of LY and Q, but not with the values of p. This is in striking contrast to results on the MgO surface. On the MgO surface, qst increases in the order O2 < N2 < CO < NO and correlates with the magnitude of p as shown in Table I. This will be discussed later. The differential molar entropy of the sorbate, &rb, was obtained with the following equation: (2) s,,,b = Sogas -I-In (76O/Pe) - qst/T is the molar entropy of the gas in the standard where state an P, is expressed in units of Torr. .?sorb is divided into two terms, Le., the thermal entropy, $therm, and the configurational entropy, Sc,,f:
sk
%arb
=
sthem
+ Sconf
(3)
and
sconf = -R(ln (6/(1 - 6))
+ $/(I - 6))
(4)3J0
By using eq 2-4, Stherm/Rwas calculated at I9 = 0.02 and
plotted against In T in Figure 5. The slope of the curve is gentle at first (region l),then becomes steep (region 2), and finally converges to 3.5 (region 3). This behavior is completely the same as that observed in the curve obtained by Takaishi et al. for N2 s o r p t i ~ n . ~The ! ~ different temperature dependencies of Sthem mean that the sorbed state changes with temperature. The value of 3.5 for the slope in region 3 has been well explained by the model of a free rotating, one-dimensional gas, oscillating two-dimensionally in a cross-sectional plane of the pores3 The steep increase of Stberm against In Tis, therefore, ascribed to change from libration to free rotation. We have, in the free rotating region stherm/R = 3.5 In T - 4.8 for NO (5) sttherm/R= 3.5 In T - 8.0
for CO
(6)
Stherm/R= 3.5 In T - 8.3
for Nz
(7)
The expression for N2,3.5 In T - 8.3, is in good agreement with the expression, 3.5 In T = 8.1, obtained by Takaishi et aL4 Sthem can be further divided into the following five terms: + S'oac (8) stherm = Soelec + Sotrans + Sorot + where soelec etc. denote the entropies of the electronic state, translation, rotation, vibration, and oscillation against the channel wall, respectively. Here the value of So,ib can be approximated as zero for all three gases. The value of soelec
The Journal of Physical Chemistry, Vol. 82,No. 15, 1978 1751
Sorption of Gases by Sodium Mordenite
TABLE 11: Second Virid Coefficient
co
N2 Temp, " C 48.9 29.7 0 - 24.7 -44.8
10IOA, (m/molecule) (0.55) 1.01 1.16
0.92 1.25 1.49
1.55 1.74
is zero for Nz and CO but it is 0.69R for NO. The remaining three terms are calculated by the following equations:" sotrms(onedimension)/R = 0.5 In T + 0.5 + In d,ff + 0.5 In (27rmk/h2) (9) soIot(freerotation)/R = In T + 1 + In (87r2K1/h2) (10) soo,,(two dimension)/R = 2 In T + 2 + In (k2/h2vlv2) (11) where k etc. in eq 9-11 have the usual meanings. By using the above relations, we can estimate values for (ulvz) X lo-% s - ~to be 11.4, 18.2, and 1.75 for N2, CO, and NO, respectively. It is interesting to note that the order of magnitude of (v1v2) is the same as that of qst. The deviation from Henry's law in the high pressure region corresponds to deviations from ideal gas behavior. If the equation of state of the one-dimensional gas is expressed in a virial form P,L/n,RT = 1 + Al(n,/L)
+ Az(n,/L)2 + A3(n,/L)3 (12)
then the sorption isotherm is given as
P, = (n,/LK1) exp(2Aln,/L
+ 3A2n,2/2L2 + ...)
(13)
+ constant
(14)
or In Pe/ns = 2Aln,/L
where P, is the pressure of the one-dimensional gas, n, is the number of sorbed molecules per gram of sample, the A's are the virial coefficients, and K1 is c o n ~ t a n t . ~By J~ plotting In P,/n, against n,, the second virial coefficients, Al, for CO and N2 were obtained from the slopes. The result is summarized in Table 11. The value of AI decreases with rise in temperature for both gases, in contrast to the case of the normal three-dimensional or two-dimensional gases on graphite.13 This unusual trend is in harmony with predictions14 based on a theory which has taken into account the three-body effect in the interaction between ~ 0 r b a t e s . I ~
Discussion The effect of the dipole moment on the thermodynamic properties of sorbates in mordenite was quantitatively studied in the present work for the first time. It has been shown that the magnitude of qst and the degree of rotational hindrance are little affected by the presence of a dipole moment. This leads to the conclusion that the electrostatic field, F, is very weak in the pores of mordenite. This conclusion is also valid from the electrostatic pojnt of view. Since mordenite is essentially a low alumina zeolite (NaBA18Si40096), the ionic charactor of the channel wall may not be large. Table I shows that the magnitude of qst is proportional to the magnitudes of a! and Q. To determine whether it is a! or Q that contributes more effectively to qst, the following calculations were made: ED+R(M)= qst(Ar)(Wa,d (15)
EQ(M) = qst(M) - ED+R(M) (16) where ED+R(M) and EQ(M)denote the components of q, arising from the sum of the dispersion force and repulsion force, and from the quadrupole moment of species M. The values are summarized in columns 7 and 8 in Table I. Since the repulsion energy is estimated to be '/3-JI2 of the dispersion energy,16the value of the dispersion energy, E D , may be 1.3-1.5 timeslarger than the value of ED+R. Anyway, Table I clearly shows that the polarizability, i.e., the dispersion force, plays a principal role in the magnitude of qs+in the pores of mordenite. The extent of the contribution from Q, Le., the interaction between the quadrupole and the electrostatic field gradient, cannot be estimated exactly, but may be very small, since a weak field does not have a large field gradient. It is suspected that rotational hindrance may result from the anisotropy of the molecular polarizability, which determines the preferential orientation of the molecular axis to the pore axis. Usually, the anisotropy of the polarizability is large in a molecule having a large quadrupole moment (Table I). As far as the present system is concerned, the correlation between rotational hindrance and quadrupole moment may not be a primary one. It is shown in columns 3 and 9 in Table I that the value of qst for O2 in mordenite is about two times larger than that on the MgO surface. Since the magnitude of qst for O2 is mainly determined by the dispersion force, the above fact means, to a first approximation, that the number of interacting pairs (sorbed O2 molecules-atoms in solids) in mordenite is about twice as many as that on the MgO surface. This situation is very reasonable when the geometries of pore and plane surfaces are compared. Three causes are considered for the steep slope in the log Vaorbvs. log P, curve for NO at higher coverages: (a) chemical adsorption, (b) sorption into the side pocket, and (c) dimerization. Chemical adsorption may be ruled out, since the sorbate can be easily removed by outgassing at room temperature. Sorption into the side pocket may not be realistic, since this phenomenon was not observed in the sorption of CO, which has molecular dimensions and a dipole moment comparable to those of NO. Therefore, dimerization remains as the only possible cause of the steep slope. In the previous work, we measured the change of the monomer fraction of NO as functions of temperature and coverage on an MgO ~ u r f a c e .It~ is highly probable that the same dimerization takes place in the pores of mordenite. Finally, we should mention the discrepancy between the values of V, for N2 obtained in the present work and by Takaishi et alS3The sample used by Takaishi et al. was synthesized by Takahashi et al. of the University of Tokyo," and its crystal perfectness seems very higha3 On the other hand, our sample is commercial Zeolon and its crystal perfectness is unknown. If some part of the sample cannot sorb gas, the real value for L becomes smaller than the calculated one, resulting in a smaller V, value. The observed discrepancy may be attributed to the difference in the degree of crystal perfectness. Acknowledgment. This work was supported in part by a Grant-in-Aid for Research (Contract No. 054182 to S.F. and No. 011004 to T.M.) from the Ministry of Education of the Japanese Government. The authors thank Professor Tetsuo Takaishi (Rikkyo University) for his helpful discussions.
References and Notes (1) W. M. Meier, 2. Krystallogr., 115, 439 (1961). (2) R. M. Barrer and D. L. Peterson, Roc. R . SOC. London, Ser. A , 280, 466 (1964).
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The Journal of Physical Chemistry, Vol. 82,
No. 15, 1978
Turkevich et al.
(3) T. Takaishi, A. Yusa, and F. Amakasu, Trans. faraday SOC.,67,
Cornell University Press, Ithaca, N.Y., 1960,p 260. (10) At lower coverage, eq 4 is actually the same as ,S = -RIn (O/(l - 8)), which was adopted in ref 6. (11) G. J. Janz, “Thermodynamic Properties of Organic Compounds”, Academic Press, New York, N.Y., 1967,p 20. (12) R. M. Barrer and R. Papadopoulos, Proc. R . Soc. London, Ser. A , 326, 315 (1972). (13) T. Takaishi, Prog. Surface Sci., 6, 43 (1975). (14) T. Takaishi, private communication. (15) T. Takaishi, “Colloid and Interface Science”, Vol. 111, M. Kerker, Ed., Academic Press, New York, N.Y., 1976, p 185. (16)D. M. Young and A. D. Crowell, “Physical Adsorption of Gases”, Butterworths, London, 1962. (17)H. Takahashi and Y. Nishimura, J. Cby Sci. Soc. Jpn., 10,61 (1970).
3565 (1971). (4) T. Takaishi, A.Yusa, Y. Ogino, and S. Ozawa, J. Chem. Scc., Faraday Trans. 7, 70,671 (1974). (5) T. Takaishi, A. Yusa, Y.Ogino, and S. Ozawa, Jpn. J. Appl. Phys., Suppl. 2,Part 2, 279 (1974). (6) S. Furuyama, H. Fujii, M. Kawamura, and T. Morimoto, J. Phys. Chem., 82, 1028 (1978). (7) S. Furuyama, T. Morimoto, and R. Hirasawa, J. Phys. Chem., 82,
1027 (1978). (8) G. L. Kington and A. C. MacLeod, Trans. Faraday Soc., 55, 1799 (1969). (9) The van der Waals radii of N and 0 atoms are 0.15and 0.14 nm, respectively. The radius for C atom is not available but that of CH, is about 0.2 nm. L. Pauling, “The Nature of the Chemical Bond”,
Carbon-I 3 Nuclear Magnetic Resonance of Simple Molecules Adsorbed on Catalytic Surfaced Dorthy Denney,t Vyacheslav
M. Mastlkhin,$
Seitaro Narnba,t$ and John Turkevlch”tt
Chemlstry Department, Princeton Universlty, Princeton, New Jersey 08540 (Received December 12, 1977: Revised Manuscrlpt Received May 5, 1978) Publication costs assisted by the National Science Foundation
Carbon-13 nuclear magnetic resonance studies were carried out on methane, carbon monoxide, acetylene, ethylene, propylene, methylacetylene, carbonyl sulfide, cyclopropane, propane, and isobutane on Na-A, Na-X, Na-Y, H-Y, Ag-Y, decationated Y zeolites, silica, and dissolved in DCC13. When iron impurities were minimized sharp lines (about 10 Hz wide) were obtained permitting determination of the chemical shift to a precision of 0.05 ppm and the hydrogen-carbon coupling constant to an accuracy of 0.2 Hz. All chemical shifts were referred to the gaseous state and the bulk magnetic susceptibility correction was obtained by measuring the shift of nonadsorbing methane. The chemical shifts are of comparable magnitude in solution as on adsorption. The shift is interpreted to be due to strong electric fields of 0.1-0.5 V k1on the surface of the adsorbents. This field causes either a change in the separation to excited states of the adsorbed molecule, or a transfer of electrons to and from the molecule and the surface, or a redistribution of electrons within the molecule. The coupling constants permit differentiation betweeii these mechanisms and also a determination of the increase or decrease of individual carbon-hydrogen distances. The temperature dependence of the width of the lines permits determination of the energy requirement for the mobility of adsorbed species. The relaxation time measurements give further information on the mobility of the molecules and of their segments in the adsorbed phase. A new mechanism of catalysis is proposed involving a change by the electric field of the catalyst in the distance of the excited, more reactive, electronic state of the adsorbed molecule.
Introduction The study of molecules on surfaces of solids and particularly on surfaces of chemically active solids has a long history. Adsorption equilibria, kinetics, and spectroscopic and optical properties of adsorbed species have been investigated for over a century. With the development of electron and nuclear magnetic resonance studies, these techniques were also applied. Electron magnetic resonance has been found both useful and sensitive in certain systems, both to characterize the nature of the crystalline field surrounding certain transition ions on the surface and to determine electron transfer between surface atoms and adsorbed mo1ecules.l It also has been useful in studying ‘Presented a t t h e 172nd National Meeting of t h e American Chemical Society, San Francisco, Calif., Sept 2, 1976. t Chemistry Department, Rutgers, State University of New Jersey, New Brunswick, N.J. f Institute of Catalysis, Siberian Branch, USSR Academy of Sciences, Visiting Fellow (19751, at Princeton University on the US-USSR Joint Program in Chemical Catalysis. f t Supported by the Energy Research and Development Administration of the US.Government.
0022-3654/78/2082-1752$0 1 .OO/O
the adsorption of paramagnetic oxygen gas on surfacesa2 Most of the work on nuclear magnetic resonance has been carried out on proton magnetic resonance. This approach to surface studies has had limited success. The width of the resonance lines both of the protons on the surface of the solid and those in the adsorbed molecules has been too broad to identify readily the contribution of the individual protons or to determine the chemical shift. The best that can be ascertained is the proton relaxation time which yields information on the nature of the motion of the adsorbed molecule. A number of techniques have been used to overcome these difficulties. When the surface chemisorbed complex (which because of its immobility has a line too broad for observation) is in equilibrium with physically adsorbed molecules (which have sharp lines), then the chemical shift of the latter can be used to determine the chemical shift of the chemisorbed species (dilution m e t h ~ d ) .Rotation ~ of the sample a t a “magic angle”4 minimizes the dipole-dipole interaction which leads to line broadening. In recent years a variety of pulse sequences have been proposed which result in high resolution spectra for solidsS5
0 1978 American
Chemical Society
