MECHANISMS OF THE FORMATION OF PDNIX IN THE CAGES OF NAY

Three mechanisms by which Pd and Ni in the same zeolite enhance each other's reducibility and form bimetallic particles have been identified. Two series of bimetallic samples are compared with monometallic samples and their physical mixtures. In one series Pd2+ ions are initially positioned in sodalite cages, while NiO clusters occupy supercages. Upon reduction Pd atoms that leave the small cages are formed, and some become attached to NiO clusters and enhance their reduction. In the other series Ni2+ and Pd2+ ions are initially located in small cages. In this case two reduction mechanisms have been identified: below 200°C PdNi dimers are formed, which subsequently migrate to supercages; the activation energy for this migration is higher than that for Pd atoms. In a third mechanism unreduced Ni2+ ions start migrating at T > 400°C to Pd or PdNix particles in supercages; subsequent reduction to PdNix particles is fast. This process is reversible; at elevated temperature in an inert atmosphere the protons that were formed during reduction selectively reoxidize Ni atoms of PdNix particles. Rereduction at T > 400°C reproduces the TPR peaks of the above process. The "leaching" of Ni atoms out of PdNix particles is confirmed by the ability of the remaining Pd clusters to form Pd hydride. © 1991.