The authors report spin-echo NMR studies of ' 'Pt in small particles of Pt supported on alumina. For the samples studied, the fraction of Pt atoms on the surface (called the dispersion) ranged from 4%%uo to 58%. The studies were at fields Ho of 80 to 85 kG, frequency vo-74 MHz, and at a temperature of 77 K. The lines are broad (3-5 kG), requiring special methods which permit substantial signal averaging (-50000 echoes). In the low-dispersion (large-particle) samples, there is a strong absorption at the position of the ' Pt NMR in bulk Pt metal (Ho/vp=1. 138 kG/MHz) which becomes progressively weaker as the particle size decreases. A peak which is near the ' Pt resonance in typical diamagnetic compounds (Ho/v0-1. 09. kG/MHz) is found in samples which are coated with adsorbed molecules. It disappears when the surface is cleaned. The authors show that this peak arises from the surface layer of Pt atoms, and that its position (Ho/vo) shows that coating the Pt atoms on the surface largely ties up the electron spins of the surface Pt atoms in bonds. The exact position of this peak depends on the chemical species which is adsorbed. The authors show that when Pt is cleaned, then exposed to air for long times, the surface peak reveals that the surface has reconstructed to form Pt(OH) 6.
The authors report observation of slow beats in the envelope of spin-echo decays in the ' Pt NMR of small particles Pt supported on alumina. The beats are shown to arise from the interplay between the Knight-shift inhomogeneity present in the small particles and the pseudoexchange coupling J between neighboring nuclear spins. J/2m is found to be 4.2 kHz.
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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