STRUCTURE AND STABILITY OF RHODIUM CLUSTERS IN NAY STUDIED BY NMR ANDFTIR

Small Rh clusters in NaY zeolite (Rh-c/NaY) produced by oxidation and subsequent reduction of Rh-6(CO)(16)/NaY were studied by NMR, in situ FTIR, and volumetric adsorption of CO and H-2. Hydrogen uptake on bare Rh clusters measured via H-1 NMR was 1.0(+/-0.2) hydrogen per rhodium (H/Rh), while the amount of CO determined volumetrically was 2.56(+/- 0.1) CO/Rh. These values are consistent with very high Rh dispersion. Dosing of the Rh clusters with sufficient CO regenerated Rh-6(CO)(16). The catalyst was stable to further oxidation-reduction-CO adsorption treatment cycles. Two resonance peaks were observed in the H-1 NMR spe ctra of reduced Rh/NaY in the presence of H-2: (i) a resonance at abou t 2 ppm due to hydrogen associated with the support and (ii) a broader upfield peak assigned to hydrogen adsorbed on Rh clusters. The NMR pr operties of hydrogen adsorbed on Rh clusters differed considerably fro m those observed for hydrogen on metallic Rh particles, e.g., particle s produced by impregnation on NaY zeolite [Rh(imp)/NaY]. First, the in trinsic shift of the hydrogen adsorbed on Rh clusters was over 100 ppm further upfield than hydrogen adsorbed on catalysts with Rh particles larger than 2 nm. It is suggested that this increased shift reveals t he paramagnetic character of Rh clusters. Second, the temperature depe ndence of this intrinsic shift indicated antiferromagnetic electron sp in coupling. In contrast, the NMR shift of hydrogen adsorbed on Rh(imp )/NaY was nearly constant in the same temperature range (similar to 30 0-500 K). Finally, the second moment analysis showed that hydrogen was rigidly adsorbed on Rh clusters but was mobile on the surface of larg er Rh particles. At a hydrogen coverage of 0.65 H/Rh, two hydrogen sta tes on Rh clusters were indicated by this analysis and were proposed t o be hydrogen adsorbed at face-bridging sites and edge-bridging sites.