Hydrogen on silica-supported rhodium: Surface diffusion studies with (1)H NMR spectroscopy.

(1)H NMR spectra of rhodium particles dispersed on silica under 1 Torr of H(2) are comprised of two peaks at ~3 ppm and at ~ -104 ppm. Previous studies have interpreted these peaks as hydroxyl groups on the silica and atomic hydrogen on rhodium, respectively. We demonstrate a NMR technique to study exchange between these two species (i.e., hydrogen spillover) by selective inversion of the nuclear dipoles of one species. Our results indicate that the atomic hydrogen on the metal exchanges rapidly with a small fraction of the species that constitute the peak at ~ 3 ppm. The facile hydrogen corresponds to 1/2 the amount of hydrogen on the metal and exchanges with a time constant of ~0.1 msec at 300K. We interpret the exchanging hydrogen as either hydrogen groups which lie in an annulus around each rhodium particle or as a second form of hydrogen on the rhodium.