NMR of platinum catalysts. II. Relaxation

The authors report measurement of ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ spin-lattice relaxation times $T_1$ and spin-spin relaxation times $T_2$ of small particles of Pt supported on alumina. $T_1$ and $T_2$ were measured at various static fields $H_0$, for frequencies ${\nu }_0$ of 45, 55, and 74 MHz, and at temperatures of 4.2, 77, and 300 K. Though strong functions of $\frac{H_0}{{\nu }_0}$ at any given ${\nu }_0$, the relaxation times $T_1$ and $T_2$ at fixed $\frac{H_0}{{\nu }_0}$ are independent of particle size. $T_1$ is longest at the position ($\frac{H_0}{{\nu }_0}$) 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 $T_1$ shifts position with change in surface coating exactly as does the peak in NMR echo amplitude, showing that the change in $\frac{H_0}{{\nu }_0}$ of the surface peak as a function of surface coating is most likely a chemical shift.