Location, ligancy and reducibility of metal ions in zeolite cages: Co and Pd in NaY

Abstract

UV-VIS diffuse reflectance spectroscopy (DRS), temperature-programmed reduction (TPR) and temperature-programmed oxidation (TPO) have been used to study ion migration and reducibility in Co/NaY and (Co + Pd)/NaY, after ion exchange of Co(NH₃)³⁺₆ and Pd(NH₃)²⁺₄. It has been found that the behaviour of the Co(NH₃)³⁺₆ ion under calcination conditions differs markedly from that of the Co(H₂O)²⁺₆ ion, studied previously. While the latter ion loses all ligands rapidly and migrates swiftly into sodalite cages and hexagonal prisms, the Co(NH₃)³⁺₆ ion loses five of its six ammine ligands easily while being reduced, even in flowing O₂, to Co(NH₃)²⁺. A stable tetrahedral configuration is attained with three cage-wall oxygens and one strongly bonded ammine group, detectable by strong absorption bands at 16.6, 17.3 and 18.5 kK. Probing with ethylenediamine shows that this complex, most remarkably, is still located in the supercages. At 500°C, the last ammine ligand can be oxidized. Its oxidation is, however, promoted in the bimetal (Pd + Co)/NaY system by the simultaneous oxidation of the ammine ligands of Pd at 300°C. While Pd ions migrate into sodalite cages at calcination temperature T_c 300°C, the monoamminecobalt(II) ions remain in supercages. The migration of Co ions to hexagonal prisms and sodalite cages is, however, observed at higher temperature. The reducibility of Co is markedly enhanced by the presence of Pd, but the locus of the enhanced reduction depends on the history of the sample.