ChemInform Abstract: NMR Spectroscopy as a Probe of Surfaces of Supported Metal Catalysts

Abstract

The nuclear magnetic resonance (NMR) properties of bulk metals are usually dominated by a magnetic coupling between the nuclear spin and the spin of the conduction electrons. The coupling is proportional to the density of one-electron-states at the Fermi energy and results in NMR shifts that are large in comparison with the usual chemical shift range and also in a characteristic temperature dependence of the nuclear spin lattice relaxation rate. In systems that lack the translational invariance of the extended solid, these properties depend on the local electronic environment of the resonating nucleus (the local density of states). Examples of such systems are bulk random alloys and small metal particles, such as those in supported metal catalysts. In certain cases an adsorbate on a metal also acquires such metallic NMR properties; this often happens with carbon monoxide and probably also with hydrogen, the only adsorbates considered in this review. These metallic NMR characteristics are of interest in catalysis for several reasons. In investigations of the metal (typically 195 Pt), the local density of states on the surface is different from that in the bulk so that NMR spectroscopy can be used to determine the metal dispersion. The 1 H NMR shift of adsorbed hydrogen induced by the metal surface can be used to distinguish hydrogen on the metal from that on the support, e.g., in investigations of hydrogen spillover. The spin lattice relaxation rate of adsorbed 13 CO shows that the chemisorption bond has metallic character. On a theoretical level, the local density of electron states at the Fermi energy on metal surface sites plays a role in frontier orbital theories of chemisorption. This quantity can be measured by NMR of the metal for catalysts that have undergone various surface treatments, not necessarily under vacuum conditions, and the results can be qualitatively interpreted in a much simplified version of such a theoretical framework. This review provides a brief presentation of the relevant theory of metals and a thorough discussion of the metallic characteristics observed in the NMR spectroscopy of hydrogen, carbon monoxide, and platinum.