Segregation of platinum from mordenite channels on calcination and reduction pretreatments

Samples of Pt/NaMOR (Pt/NaM) and Pt/HMOR (Pt/HM) were prepared through ion exchange of MOR with [Pt(NH3)4](NO3)2 and subsequently treated with calcination at 723 K in air and reduction at 673 K in H2. The segregation of Pt in the prepared samples during the treatment was monitored with different characterization techniques. Transmission electron microscopy results showed that platinum in freshly exchanged and in calcined samples stayed dominantly in channels of MOR. However, a significant portion of platinum in channels segregated to and sintered at the external surface during the reduction. A high NH/NPt ratio of 0.80 in 0.5Pt/NaM compared to that of 0.55 in 0.5Pt/HM in hydrogen chemisorption suggested that the segregation of platinum was partially retarded by Na+ ions in NaM. Xenon tended to physisorb on reduced Pt particles in the channels of Pt/NaM and caused a downfield shift to 129Xe NMR peak. The shift revealed that a fraction of segregated platinum (or a fraction of Pt retained in the channels) remained constant for Pt/NaM samples with different Pt loadings. Pt particles retained in channels of highly loaded 5Pt/NaM blocked pores (d∼0.70 nm) and thereby substantially blocked the diffusion of gas molecules. Consequently, a low uptake of both hydrogen and xenon in 5Pt/NaM was observed in isotherm measurements.