NMR of platinum catalysts. II. Relaxation

The authors report measurement of ¹⁹⁵Pt spin-lattice relaxation times T₁ and spin-spin relaxation times T₂ of small particles of Pt supported on alumina. T₁ and T₂ were measured at various static fields H₀, for frequencies ν₀ of 45, 55, and 74 MHz, and at temperatures of 4.2, 77, and 300 K. Though strong functions of H₀ν₀ at any given ν₀, the relaxation times T₁ and T₂ at fixed H₀ν₀ are independent of particle size. T₁ is longest at the position (H₀ν₀) 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 T₁ shifts position with change in surface coating exactly as does the peak in NMR echo amplitude, showing that the change in H₀ν₀ of the surface peak as a function of surface coating is most likely a chemical shift.