NMR of platinum catalysts: Double NMR of chemisorbed carbon monoxide and a model for the platinum NMR line shape

Abstract

The authors report observation of the NMR line of ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ atoms in the surface layer of small platinum-metal particles on which ${}_{}{}^{13}{}_{}{}^{}\mathrm{CO}$ has been chemisorbed. The surface ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ atoms are resolved from those of ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ atoms deeper in the particle by spin-echo double resonance between ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ and ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$. The particles, supported on η-alumina, had dispersions (fraction of the atoms that are on the surface) of 26% and 76%. Comparison with ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ resonance in Pt carbonyls suggests that the magnitude of the Knight shift of the surface Pt is less than 0.2%. Analysis of the ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ spin-lattice relaxation indicates that the small surface Knight shift results from cancellation of 6s and 5d core-polarization contributions as was found theoretically by Weinert and Freeman for clean Pt surfaces. The ${}_{}{}^{13}{}_{}{}^{}\mathrm{-}_{}^{195}{}_{}{}^{}$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 ${}_{}{}^{195}{}_{}{}^{}\mathrm{Pt}$ 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 76%.