OXIDATIVE REDISPERSION OF METALS IN Y ZEOLITES EFFECT OF AMMONIA ON PD/HY REJUVENATION

The general strategy for the redispersion of large metal particles in zeolites used in this paper is based on the high tendency of twofold positive cations to migrate to small zeolite cavities, where the density of the negative charge is highest. Pd/NaY or Pd/HY catalysts with large palladium particles are, therefore, first oxidized in oxygen at a temperature where PdO particles react with zeolite protons to Pd2+ ions and water; the Pd2+ ions are dispersed in sodalite cages. Subsequent gentle reduction results in the formation of small palladium particles in supercages. However, for very large palladium particles, the concentration of protons in their vicinity is insufficient for complete dispersion, even in strongly acidic HY. We have found that, in such cases, exposition to gaseous ammonia gives rise to the formation of Pd(NH3)(4)2+ ions and NH4+ ions in supercage channels. These ions are highly mobile, so that concentration gradients are lowered and, after calcination, protons are again present in the vicinity of the PdO particles.