On the nature of Pd species formed upon exchange of H-ZSM-5 with Pd(NH₃)₄²⁺ and calcination in O₂

Abstract

The chemistry of a high Pd loading Pd-H-ZSM-5 prepared by exchanging H-ZSM-5 with an aqueous solution of Pd(NH₃)₄(NO₃)₂ and calcined in O₂ was studied by FTIR and TPD experiments. Acidic protons were found to readily exchange with Pd(NH₃)₂²⁺ complexes and NH₄⁺ ions arising from the partial decomposition of Pd(NH₃)₄²⁺ possibly at the surface of H-ZSM-5 crystallites. The interaction of the Pd(II) ammine complexes with the MFI framework induced a strong absorption band at 928 cm^-1 attributed to the vibration of distorted T–O bonds (T=Si, Al). Slow calcination in O₂ at 653 K allowed the complete removal of NH₃ ligands from Pd(NH₃)₂ entities and the decomposition of ammonium ions into protons. Despite the high Pd loading, no large PdO particles formed and the final oxidized Pd entities could be represented by the mean formula Pd(II)(OH)_1.3^0.7+, suggesting the formation of isolated oxo and/or hydroxo Pd(II) complexes. A sharp IR band at 932 cm^-1 indicated that most of these complexes were anchored to the oxygen atoms of the zeolite framework. On the contrary, calcination in O₂ at 773 K led to the partial sintering of isolated Pd(II) species into large PdO particles. The possible formation of mobile Pd(OH)₂ moieties in O₂ at 773 K, migrating along the MFI channels, is discussed.

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