The authors report observation of the NMR line of ' Pt atoms in the surface layer of small platinum-metal particles on which ' CO has been chemisorbed. The surface ' Pt atoms are resolved from those of ' Pt atoms deeper in the particle by spin-echo double resonance between ' Pt and ' C. The particles, supported on g-alumina, had dispersions (fraction of the atoms that are on the surface) of 26% and 76%%uo. Comparison with '9 Pt resonance in Pt carbonyls suggests that the magnitude of the Knight shift of the surface Pt is less than 0.2%. Analysis of the ' Pt spin-lattice relaxation indicates that the small surface Knight shiA results from cancellation of 6s and 5d corepolarization contributions as was found theoretically by Weinert and Freeman for clean Pt surfaces. The ' C-' Pt indirect spin coupling is found to be very similar. to those in diamagnetic platinum carbonyl molecules. The results show that CO bonds via the C atom and verify that concepts from studies of large single crystals are valid for the small particles. The key features of the ' Pt line shapes in these small platinum particles are described by a simple phenomenological model of the spatial Knight-shift variation inside these particles. The model successfully describes the major structure seen in the NMR line shapes of samples with dispersions ranging from 5% to 76go.
The detailed NMR line of i95 Pt atoms in the surface layer of small particles of Pt metal on which CO has been chemisorbed has been observed. The surface 195 Pt are resolved from the 195 Pt resonances of atoms deeper in the particle by spin-echo double resonance between 195 Pt and 13 C. The 195 Pt resonance position and 13 C-195 Pt indirect spin coupling are found to be very similar to those in diamagnetic Pt-carbonyl molecules. The results show that CO bonds via the C atom and verify that concepts deduced from studies of large single crystals are valid for the small particles.PACS numbers: 68.10.Jy, 68.20.+t, 76.70.Fz Though NMR has been a powerful technique for studying solids and liquids, its use for studying surfaces has been more limited. In their recent article, Duncan and Dybowski 1 point out that "the systems in the past have consisted primarily of physically adsorbed species. Through analysis of relaxation times and changes in isotropic chemical shifts, these early studies provided information on . . . the transport mechanisms to and from the surface and the rate of exchange between the chemisorbed monolayer and the liquid-like overlayers." The studies to date have been on nuclei of adsorbed species, typically on material such as carbon or zeolites. Several years ago, we set out to detect the NMR of the surface layer of atoms in a metal, choosing Pt because of its significance in surface science and because of the NMR properties of 195 Pt. In this paper we report the successful measurement of the position and detailed NMR line shape of the surface 195 Pt for Pt coated with CO.The first results of our group (Rhodes and coworkers) 2 " 5 showed by chemical means that the 195 Pt resonance of various coated samples was mainly in the vicinity of 195 Pt in diamagnetic compounds, but the surface NMR was not resolved from a nearby peak due to the NMR of deeper layers. Therefore the surface Pt line position was only approximately known and we could only guess at its width or shape. Using spin-echo double resonance (SEDOR) between 13 C and 195 Pt in samples of highly dispersed Pt catalysts with chemisorbed carbon monoxide ( 13 CO), we have now completely resolved the NMR of surface 195 Pt. 6 SEDOR is a pulse technique involving simultaneous excitation of the NMR of two different types of nuclei, e.g., 13 C and 195 Pt. Provided the nuclei are near enough to be coupled, the resonance of the 13 C affects the resonance of the 195 Pt, either directly through the nuclear dipolar interaction or indirectly through the electrons via the pseudoexchange and pseudodipolar interactions. 7 The strength of the coupling depends strongly on the proximity of the nuclei, so that observation of the size of the SEDOR effect can distinguish first neighbors from second neighbors, etc. Thus, for 13 CO on Pt, SEDOR gives a unique signature to the surface Pt atoms. Indeed, the coupling to deeper Pt layers is too weak for us to detect.We obtain the line shape of the surface Pt nuclei, showing quantitatively that the position of the 195 ...
The authors report measurement of ' Pt spin-lattice relaxation times Tl and spin-spin relaxation times T2 of small particles of Pt supported on alumina. Tl and T2 were measured at various static fields H p, for frequencies vp of 45, 55, and 74 MHz, and at temperatures of 4.2, 77, and 300 K. Though strong functions of H p/vp at any given vp, the relaxation times Tl and T2 at fixed Hp/vp are independent of particle size. Tl is longest at the position (Hp/vp) corresponding to the "surface peak" described in paper I (the preceding paper), indicating that conduction-electron spins are largely tied up for surface Pt atoms. The peak in Tl shifts position with change in surface coating exactly as does the peak in NMR echo amplitude, showing that the change in Hp/vp of the surface peak as a function of surface coating is most likely a chemical shift.
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