NMR of platinum catalysts. I. Line shapes

Abstract

The authors report spin-echo NMR studies of ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ 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% to 58%. The studies were at fields $H_0$ of 80 to 85 kG, frequency ${\nu }_0=74\mathrm{}$ MHz, and at a temperature of 77 K. The lines are broad (3-5 kG), requiring special methods which permit substantial signal averaging (∼ 50 000 echoes). In the low-dispersion (large-particle) samples, there is a strong absorption at the position of the ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ NMR in bulk Pt metal ($\frac{H_0}{{\nu }_0}=1.138\mathrm{}$ kG/MHz) which becomes progressively weaker as the particle size decreases. A peak which is near the ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ resonance in typical diamagnetic compounds ($\frac{H_0}{{\nu }_0}\sim 1.09$ 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 ($\frac{H_0}{{\nu }_0}$) 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${\mathrm{}\left(\mathrm{OH}\right)\mathrm{}}_6$.