Comparison of Pd(NH3)42+ decomposition and Pd2+ reduction in zeolites NaY, NaX and CsX

Abstract

The decomposition of Pd(NH₃)₄²⁺ and further reduction to Pd⁰ in Pd_6.4NaY and three zeolites differing in alkalimetal cation or Pd loading (Pd_2.9NaX, Pd_7.4NaX and Pd_8.2CsX) are investigated by TPO, TPR and UV–VIS–NIR spectroscopy. With regard to the temperature, the order for the decomposition of the ammine complex is PdCsX < PdNaX < PdNaY and that for the reduction of bare Pd²⁺ ions located in the same type of cage within the zeolites is PdCsX < PdNaX ≈ PdNaY. These orders depend on the stabilization of the various intermediate Pd complexes. It is shown that NaX favours the location of Pd²⁺ ions in the inner sites (i.e. sodalite and hexagonal prism). Parameters important for the various locations of Pd complexes are discussed in terms of the zeolite basicity and steric hindrance for migration of Pd(NH₃)_x²⁺ complexes or Pd²⁺ ions in the inner cavities.