On the nature of Pd species formed upon exchange of H-ZSM5 with Pd(NH3)(4)(2+) and calcination in O-2

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 O-2 was st udied by FTIR and TPD experiments. Acidic protons were found to readily exc hange with Pd(NH3)(2)(2+) complexes and NH4+ ions arising from the partial decomposition of Pd(NH3)(4)(2+) possibly at the surface of H-ZSM-5 crystall ites. 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 O-2 at 653 K allo wed the complete removal of NH3 Ligands from Pd(NH3)(2) entities and the de composition of ammonium ions into protons. Despite the high Pd loading, no large PdO particles formed and the final oxidized Pd entities could be repr esented 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 ato ms of the zeolite framework. On the contrary, calcination in O-2 at 773 K l ed to the partial sintering of isolated Pd(II) species into large PdO parti cles. The possible formation of mobile Pd(OH)(2) moieties in O-2 at 773 K, migrating along the MFI channels, is discussed.