J. Phys. Chem. 1988, 92, 7117-7121
7117
Dynamics of Molecular Hydrogen Adsorbed in CoNa-A Zeolite J. M. Nicol,* University of Maryland. College Park, Maryland 20742, and National Bureau of Standards,t Gaithersburg, Maryland 20899
J. Eckert, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
and J. Howard ICI plc, Wilton Materials Research Center, Wilton, Middlesbrough, Cleveland TS6 8JE, UK (Received: March 10, 1988)
The dynamics of molecular hydrogen adsorbed in the cavities of partially cobalt exchanged type A zeolite ( C O ~ , , N ~ ~ , ~ - A ) has been investigated in the energy range 0-40 meV by incoherent inelastic neutron scattering. Both rotational and vibrational excitations are identified in the spectra. The rotational tunnel splitting of the librational ground state of the adsorbed molecular hydrogen is observed at 3.8 meV. Analysis of the data in terms of a 2-fold cosine potential with two degrees of rotational freedom best accounts for the observed spectral features among several models tested. A barrier height of about 1.4 kcal/mol is calculated. A mode at 15.3 meV is accordingly assigned as a vibration of the bound hydrogen. Evidence for the torsional mode of AI(OH)4 complexes, formed in the &cages during ion exchange, is found in the vibrational spectra at 21 meV.
Introduction Detailed knowledge of the dynamics and interaction of adsorbate molecules in the cavities of zeolites is crucial for a better understanding of the catalytic properties of these materials. A considerable amount of information has been obtained on small molecules, such as ethylene, ethene, carbon monoxide, etc.,'" adsorbed in a variety of ion-exchanged zeolites. For example, it is known from X-ray diffraction studies' of single crystals and from neutron and infrared spectroscopic studiesZ that ethene adsorbs in a side-on geometry to transition-metal cations in ionexchanged type A zeolites. This observation is in agreement with the results of quantum chemical calculations involving, however, only N a ions in the zeolite framework.6 In contrast, similar information about the interaction of molecular hydrogen with the cations in zeolites has not yielded a consistent picture. For example, it is possible in IR studies to observe sidebands to H-H stretching transitions which are induced by the electrostatic field of the cavity. These have, however, at different times' been interpreted as hindered rotations or translations of the hydrogen molecule. Detailed knowledge of the adsorption properties of hydrogen in zeolites would be of enormous interest because of the simplicity and prototypical nature of this system, as far as the catalytic activity of zeolites is concerned. The recent discovery of transition-metal complexes that bind hydrogen in the molecular form while activating the H-H bond has generated the possibility of studying the H-H bond in the activated state.* The existence of such compounds raises the question of whether similar activation of molecular hydrogen may be observable in transition-metal-exchanged zeolites. Indeed, ESR studies of Ni9*'0and Rh" exchanged type X zeolites have indicated the formation of molecular hydrogen complexes that stabilize Ni and Rh ions at sites within the large cages. However, the true identity and nature of molecular hydrogen in these complexes have not yet been established. We are carrying out a program to study molecular hydrogen adsorption in a series of transition-metal-exchanged zeolites by inelastic incoherent neutron scattering spectroscopy (IINS). The sensitivity of IINS to motions involving hydrogen atoms makes the technique a particularly useful probe of the vibrational modes of bound hydrogen. Since neutrons are able to induce transitions within the librational ground state by nuclear spin flips, which are generally not allowed in optical spectroscopies, as well as identify rotational and vibrational transitions, one would expect Address for correspondence.
to obtain from the IINS experiment detailed information on the intermolecular potentials experienced by the hydrogen molecule. In this communication we report results of our initial neutron scattering study of molecular hydrogen adsorption by partially cobalt exchanged type A zeolite (CoNa-A). Both rotational and vibrational excitations are identified in the IINS spectra. The rotational transitions are analyzed in terms of a weakly hindered rotation of Hz in a double-minimum potential. Evidence of Al(OH), complexes formed during ion exchange is found in the vibrational spectra. Crystal Structure of CoNa-A. The crystal structures of Co4Na4-A.lZand have been determined by single-crystal X-ray and neutron powder diffraction, respectively. Although some discrepancy exists between the two diffraction measurements, in both studies Co and Na ions were found to be coordinated to three framework oxygen atoms in the six-rings. While the X-ray diffraction study located the Co cations projecting slightly into the a-cages (large cavity) and the Na cations projecting into the @-cage(sodalite unit), the more recent neutron diffraction study located the Co cations projecting into the @-cages and the N a cations into the a-cages. On adsorption of carbon monoxide the neutron diffraction study found the Co cations migrated to positions in the a-cages, such that the cations were arranged tetrahedrally around the eight six-ring sites. As there is no evidence for a strong interaction of hydrogen with Na ~
~~
( 1 ) Kim, Y.; Seff, K. J . Am. Chem. SOC.1978,100, 175. Riley, P. E.; Kunz, K. B.; Seff, K. J. Am. Chem. SOC.1975,97,537. (2)Howard, J.; Nicol, J. M.; Eckert, J. In Springer Series in Surface Science 2; Van Hove, M. A., Tong, S. Y., Eds.; Springer-Verlag: Berlin, 1985; p 219. (3)See, for example: Howard, J.; Robson, K.; Waddington, T. C. Zeolites 1981, I, 175. Tam, N. T.; Cooney, R. P.; Curthoys, G . J. Chem. SOC., Faraday Trans. 1 1976,72,2577,2592. Howard, J.; Kadir, Z. A. Zeolites 1984,4,45. Howard, J.; Nicol, J. M. Zeolites 1988,8, 142. (4)Kahn, R.;Cohen de Lara, E.; Thorel, P.; Ginoux,J. L. Zeolites 1982,
2, 260. ( 5 ) Adams, J. M.; Haselden, D. A. J . Solid State Chem. 1984,55, 209. (6)Sauer, J.; Zahradnik, R. Inr. J . Quantum Chem. 1984,23, 793. (7) Forster, H.; Frede, W. Infrared Phys. 1984,24, 161. Forster, H.; Schuldt, M. J . Mol. Srrucr. 1978,47,339. (8)Kubas, G.J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. J. Am. Chem. SOC.1984,106,451. Kubas, G.J.; Unkefer, C. J.; Swanson, B. I.; Eukushima, E. J . Am. Chem. Soc. 1986,108,7000. (9) Michalik, J.; Narayana, M.; Kevan, L. J. Phys. Chem. 1984,88,5236. (10)Oliver, D.;RicKard, M.; Che, M.; Bozon-Verduraz, F.; Clarkson, R. B. J . Phys. Chem. 1980,84,420. (11)Goldfarb, D.; Kevan, L. J . Phys. Chem. 1986,90, 2137. (12) Riley, P.E.; Seff, K. Inorg. Chem. 1974,13, 1355.
0022-3654/88/2092-7117%01.50/0 0 1988 American Chemical Society
7118 The Journal of Physical Chemistry, Vol. 92, No. 25, 1988
cations,’ we assume that the hydrogen is adsorbed by the Co cations in the zeolite. Similar to the case with carbon monoxide above, we expect that migration of the Co cations to the a-cages will occur upon adsorption of hydrogen. Inside the @cages Adams and Haseldens identified by neutron diffraction a tetrahedral Al(OH)4 complex, the oxygen atoms of which point toward the six-rings not occupied by Co cations. These complexes were found to be hydrogen bonding to the framework oxygens, such that at any time two-thirds of the bonds are of the type [Al(OH)-O 0). The equilibrium orientation for the diatomic molecule in this picture is parallel to the surface and perpendicular to a line joining the two nearest-neighbor atoms of the molecule on the surface. The two low-symmetry potentials also remove all degeneracies of the rotational levels so that a large number of transitions to the higher states are possible. The main difficulty in applying this model to the present system lies in the assumption of a 2-fold barrier to in-plane rotation, whereas 6-fold symmetry would be expected as described above. We have nevertheless attempted to find a solution in terms of in-plane and out-of-plane barriers that would fit our data by interpolating the results given by MacRury and S a m ~ For . ~ an ~ in-plane barrier of 4B and an out-of-plane barrier of about 4.3B, this model predicts the observed ground-state splitting of 3.8 meV and four transitions to excited states ranging from about 23 to 33 meV. This distribution is probably somewhat broader than the experimental result shown in Figure 2. Previous attempts to employ this model to account for orthc-para hydrogen separation factors25furthermore showed that more than one set of barrier heights (A, p) could reproduce the measured values. We were, however, unable to find another set of barrier heights (A, M) that accounted as well for the higher transitions when adjusted to reproduce the ground-state splitting as the one just described. We must however regard the model of MacRury and Sams in this form as inappropriate for the H,/CoNa-A system because of its use of 2-fold symmetry for the in-plane hindering potential as discussed above and because it assumes an equilibrium orientation of the molecule parallel to the surface. This may well be appropriate for oxide surfaces, such as alumina, for which this model was developed. For the internal surfaces in the zeolite cage with its strong electrostatic however, a perpendicular orientation may well be preferred as is indicated for Nzzaand indeed suggested by the above analysis of our data. Thus, while the in-plane and out-of-plane barrier model may be the most general, it must be refined to allow for the known symmetry of the problem at hand and should be fitted to the observed data. More extensive measurements are needed, however, before such a fitting procedure can reasonably be attempted. Vibrational Excitations. All the models considered above that do fit the present data leave the peak at 15.3 meV unassigned. (23) Eckert, J.; Kubas, G. J.; Dianoux, A. J. J . Chem. Phys. 1988,88, 466. (24) MacRury, T.B.; Sams, J. R. Mol. Phys. 1970, 19, 331. (25) MacRury, T.B.; Sams, J. R. Mol. Phys. 1971, 20, 51. (26) Brigot, N.; Cohen de Lara, E.; Blain, M. J . Phys. (Les Ulis, Fr.) 1981, 42. 979.
Dynamics of H2 Adsorbed in CoNa-A Zeolite In fact, if this peak were a higher rotational transition, it would have to reflect the ground-state level spacing of 3.8 meV. The latter, when convoluted with the instrumental resolution (2 meV), yields a width (4.3 meV) larger than the observed width of 3.6 meV. On the basis of these observations, we may therefore assign this peak as a vibrational excitation. Since no other vibrational peaks are clearly evident, this may represent either an approximately isotropic harmonic oscillator or most likely the vibration along the surface normal. In the latter case the vibrations parallel to the surface may be either rather weakly hindered or strongly coupled to the rotational excitations. In both cases one may expect not to observe separate well-defined peaks for those modes. Since no other unambiguous assignment of vibrational modes of H2in zeolites are known, the validity of the present identification can only be checked against some calculations and tentative observations. Infrared studies by Forster and collaborators consistently show sidebands to the H-H stretching mode at about 5.3 meV for H2in NaCa-A which are believed7 to be vibrational sidebands. In addition, a study by S t o c k m e ~ e rinvolving ~~ rather large coverages (5-9 molecules/cage) of H2in natural chabazite shows a broad density of states ranging from about 3 to 15 meV. It seems likely that H2 in chabazite is adsorbed at various sites and that these data reflect a distribution of adsorption potentials as well as some H2-H2 interactons on account of the high coverages used, The data are analyzed in terms of an Einstein oscillator frequency of about 8 meV.27 IINS studies of H2 in ZnNa-A show two broad peaks at 9 and 19 meV, which are thought to result mainly from hindered rotations of the H2 moleculeB but may also contain vibrational components as no other strong peaks are observed. Since the presence of the Co transition-metal cation in the zeolite no doubt strengthens the adsorption of H2, one would therefore expect a higher vibrational frequency in the present case than the examples just mentioned. Our identification of the 15.3-meV peak is therefore at least consistent with the general trend just noted. In an attempt to model adsorption isotherm data of H2in Zn-A and in Na-X, Kochurikhin et al.29 arrive a t a model where the H2molecule is subject to a rotational barrier of about 7 B and has vibrational modes along the surface normal of 41 meV and parallel to the surface of about 7 meV. The former is clearly unreasonable, as this value of 41 meV is almost half of that for the same mode of chemically bound molecular hydrogen in the tungsten-dihydrogen complex* discovered recently. The conclusion therefore must be again that our value of 15.3 meV for the vibrational mode of H2 in CoNa-A is entirely consistent with observations on related systems. (27) Stockmeyer, R. Zeolites 1985, 5, 393. (28) Braid, I. J.; Howard, J.; Nicol, J. M.; Tomkinson, J. Zeolites 1987, 7, 214. (29) Kochurikhin, V. E.; Zel’venskii, Ya. D. Russ. J . Phys. Chem. (Engl. Trans/.)1971, 45, 510.
The Journal of Physical Chemistry, Vol. 92, No. 25, 1988 7121
Concluding Remarks Hydrogen adsorbed at 12 K in partially Co exchanged type A zeolite at a coverage of 0.5 molecule per supercage is found to bind weakly in molecular form presumably at the Co cation sites. Annealing of the sample at room temperature and subsequent cooling to 12 K did not reveal any noticeable changes in the IINS spectrum, which indicates that even at room temperature little if any dissociation of H2occurs. The IINS spectra of the “bare” dehydrated zeolite show evidence for a torsional mode of the Al(OH)4 complexes that were suggested to be present in the P a g e by several diffraction studies. Analysis of the data for adsorbed hydrogen suggests that the molecule is bound end-on to the Co cations and performs 180’ reorientations with a barrier of 55-68 meV (1.3-1.5 kcal/mol). A further transition at 15.3 meV may be identified as a vibrational, probably that of an isotropic oscillator. It should be possible to verify the above conclusions concerning the location and orientation of the H2 molecule by neutron powder diffraction. Such an experiment is planned, and it should also serve as a check for H-H bond activation. As we pointed out, this seems unlikely, however, on the basis of a comparison of the dynamics of H2 in CoNa-A with that of the H2tungsten complex2 where such activation is known to occur. Nevertheless, it would be of considerable interest to pursue this question of whether H-H bond activation can be studied in zeolites with different types of ion exchanges. Furthermore, it would be interesting to contrast the present case of localized adsorption of H2 at the Co cation sites with that for non-ion-exchanged zeolites. Three types of Na cations are accessible for adsorption for example in Na-A zeolite30which may mean that nonlocalized absorption is likely in these systems, particularly at higher temperatures. A previous study on methane in Na-A zeolite4 has in fact shown that methane molecules are trapped only below 40 K in front of two different N a ion sites. The resulting IINS spectra3’ indeed reflect qualitatively the resulting distribution of potentials that the molecules are subject to. This is in sharp contrast to the well-defined spectra observed in this case for H2 in the presence of Co cations. More detailed theoretical calculations to model the different aspects of H2 dynamics in the various zeolites as observed by IINS and also IR are now clearly called for as well. Acknowledgment. We thank Dr. T. J. Udovic and G. C. Greene for their help in obtaining and analyzing the TOF data. J.E. acknowledges that work at Los Alamos was performed under the auspices of the U S . Department of Energy, Division of Basic Energy Sciences. Registry No. H1,1333-74-0; Co, 7440-48-4. (30) Cohen de Lara, E.; Nguyen Tan, T. J . Phys. Chem. 1976,80, 1917. (31) Cohen de Lara, E.; Kahn, R. J. Phys. ( L a Ulis, Fr.) 1981,42, 1029.
