Analysis of the Chemical State in Y-zeolite Pores by Positron Annihilation Lifetime Spectroscopy

The chemical state of the active sites in the pores of zeolites is known to greatly affect their chemical and catalytic properties, e.g. the presence of Brønsted acid sites enhances their action as polar adsorbents. Ortho -positronium diffusion in the pore network has been widely used to clarify the zeolite structure, but its interaction with acid centers under different environments remains unclear. Here, a systematic study on Y-zeolites over a wide range of Si/Al ratios in the absence and presence of water in the framework was carried out using positron annihilation lifetime spectroscopy. The acidity of the pores was found to significantly inhibit positronium formation and annihilation within the crystalline micropore network in the dehydrated condition, highlighting a strong positronium oxidizing action of the acid centers. Upon water adsorption, the interaction of the acid sites with the water molecules enabled the recovery of positronium formation in the hydrophilic low-silica samples.