H-1-NMR STUDY OF HYDROGEN ADSORPTION ON PD-NAY - INFLUENCE OF METAL-PARTICLE SIZE

Small palladium particles on NaY zeolite have been studied by H-1 NMR using hydrogen as a probe. Several species may be identified: hydrides (alpha and beta) and chemisorbed and physisorbed hydrogen. The ranges where the two hydrides exist are demonstrated by this method. NMR spe ctra show two signals depending on the hydrogen concentration. The fir st, detected at low concentration, is attributable to the alpha-hydrid e and the chemisorbed hydrogen. Its resonance shift varies from -100 t o 0 ppm (vs TMS). The variation is linear up to a coverage theta almos t-equal-to 1. Beyond this value the second signal appears and increase s at the expense of the previous one. It corresponds to the beta-hydri de; its chemical shift is positive (+26 ppm) and it is almost insensit ive to the hydrogen concentration and the metal dispersion. These resu lts show also that the Pd-H(alpha) concentration depends on the covera ge of the surface and that it is possible to produce a monolayer of hy drogen on the metal just before formation of the beta-hydride.