1440
J. Phys. Chem. 1989, 93, 1440-1447
transition and the building up of a closed-packed 2D solid phase. To test this general conclusion, measurements are being undertaken with other adsorbents as substrates.
forming the computer calculations. Financial support of this work by Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie is gratefully acknowledged.
Acknowledgment. We thank DipLChem. G. Auer for per-
Registry No. AI2O3, 1344-28-1; Pt, 7440-06-4; benzene, 71-43-2.
Proton and Deuteron Magnetic Resonance Relaxation of Benzene Adsorbed on Alumina and on a Platinum/Alumina Catalyst B. Boddenberg* and B. Beerwerth Lehrstuhl f u r Physikalische Chemie II, Universitat Dortmund, Otto- Hahn-Strasse, 0 - 4 6 0 0 Dortmund 50, West Germany (Received: March 23, 1988; In Final Form: June 28, 1988)
The proton ('H) and deuteron (2H) magnetic resonance relaxation times T I and T2of one monolayer of benzene on g-alumina and a platinum/g-alumina catalyst were measured at two Zeeman field strengths (2.1 and 8.3 T) as function of temperature in the range 270-160 K. A model for the motions of the adsorbed benzene molecules is developed that allows an almost quantitative treatment of the experimental relaxation data. By taking into account the intramolecular contribution to the 'H relaxation rates through the results obtained from 'H relaxation, the interpretation of the proton data is made feasible. From both the IH and 2H relaxation times surface diffusion coefficients are derived which compare rather well with respect to both absolute values and temperature dependence
1. Introduction
It is well-known'-3 that the spin-lattice ( T I )and spin-spin (T2) relaxation times of protons contained in molecules adsorbed on the surfaces of solids most often are strongly influenced or even dominated by the dipolar couplings with paramagnetic impurity centers of the support unless the concentration of such species is lower than the order of 10 ppm. This circumstance prevents unambiguous information about the dynamics of adsorbed molecules to be obtained with adsorbents and catalysts of practical interest. In addition, the versatile magnetic dipolar couplings of the protons with spins other than electronic such as the protons being contained in the same and the other molecules as well as in surface O H groups tremendously increase the problem of interpreting appropriately the experimental data obtained. One way to get out of this dilemma was pointed out several years ago by Pfeifer and co-workers,' who introduced the method of relaxation analysis which systematically uses the technique of substitution and dilution of the proton spin system with deuterium. This rather tedious method which, in principle, fully exhausts the information content of the proton relaxation data requires, however, low statistical error N M R data and most accurately reproducibly prepared samples. In the present paper it is proposed to use deuteron (2H) relaxation as the main source of information about the molecular dynamics of the adsorbed species. As far as present experience goes, the interaction of the 2H nuclear quadrupole moment with the electric field gradient (EFG) at the nuclear site is practically the only source for relaxation, thus getting rid of the most complicated dipolar effects. Since the EFG tensor is predominantly intramolecular in origin and, hence, has fixed orientation with respect to the molecular framework, the 2H relaxation data purely reflect the reorientational types of molecular motion. Since, however, the translational motions along the surface of the particulate adsorbents correspond to molecular reorientations as well," the evaluation of surface diffusion Coefficientsby 2H NMR should be feasible. (1) Pfeifer, H. In NMR. Basic Principles and Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer: Berlin, 1972; Vol. 7, p 53. (2) Resing, H. A. J . Chem. Phys. 1967, 46, 4701. (3) Boddenberg, B.; Moreno, J. A Ber. Bunsen-Ges. Phys. Chem. 1983,
87, 83 (4) Grosse, R.; Boddenberg, B. Z. Phys. Chem. (Munich) 1987, 152, 1.
0022-3654/89/2093- 1440$01.50/0
Finally, with the knowledge of the reorientational dynamics of the adsorbed molecules from 2H NMR, the intramolecular contribution to 'H N M R relaxation can be calculated and taken into account, whence leaving an interpretatory problem of reduced complexity. 2. Experimental Section
The preparation and characterization of the samples used for the present investigation have been described previ~usly.~For comparison purposes two samples of g-A1203were prepared where the first (sample A) was treated as described5 and the second (B) was not reduced with H2 in the final preparation step. The 'H and 2H relaxation measurements were performed at resonance frequencies w 0 / 2 s equal to 89.1 ('H), 52.7 (2H), and 13.7 MHz (2H) by using the equipment described e l s e ~ h e r e .The ~ spin-spin relaxation times T2were determined from the heights of the spin echoes produced by Hahn and CPMG pulse sequences6 in the appropriate temperature ranges (see Figure 2) in the case of 'H resonance and from the exponentially decaying free induction decays in the case of 2H resonance. The spin-lattice relaxation times TI were obtained with the aid of saturation and inversion recovery pulse sequences6in cases where the conditions T2
