DEUTERIUM NUCLEAR-MAGNETIC-RESONANCE CHARACTERIZATION OF PARTICLE-SIZE EFFECT IN SUPPORTED RHODIUM CATALYSTS

Deuterium adsorbed onto Rh particles of various sizes (1.1-9 nm) supported on Al2O3 was studied by 2H NMR spectroscopy. Near zero overpressure of D2, δD shifts downfield with increasing size of particles, approaching a limiting value ca. -120 ppm when particle size exceeds 2 nm. The dependence of δD on particle size of the odd-electron metal Rh(4d7) is opposite to the dependence observed for the even-electron metal Pt(5d8) which we have also investigated. δD also shifts downfield upon increased overpressure of D2. All these variations of chemical shifts can be satisfactorily accounted for by considering the spin density on the metal particle. The linewidth depends on the particle size but not on the overpressure of D2. Hence the major relaxation mechanism is not paramagnetic interaction which is responsible for the variations of δD. © 1992.