H/D Exchange of Cyclopentane on Pt/Mordenites: Probing for Monoatomic Pt Sites

H/D exchange of cyclopentane on Pi/Mordenite catalysts has been studied. With 0.5% Pt/HMor reduced at or below 200°C, an isotope distribution pattern characteristic of prevailing stepwise exchange (i.e., one atom exchanged per residence of an adsorbed molecule) is observed. It is attributed to monoatomic Pt1 sites for which Pt atoms in side-pockets of the main channels are likely candidates. In contrast, Pd4 clusters in NaY display prevailing multiple exchange with the characteristic double-U-shaped distribution pattern that is also observed on other supported and unsupported Pt and Pd catalysts. Physisorbed cyclopentane plugs the narrow one-dimensional pores; molecules inside these pores become largely isolated from the gas phase, though single-file diffusion permits some leakage. Molecules which have been trapped inside the channels for some time are found in isotopic equilibrium with the zeolite protons.