A positronium-based systematic study of the physico-chemical properties of zeolite pores

Highlights: • Good correlation between positronium lifetime and major axis of zeolite pores. • Brønsted acid sites trap positrons and inhibit positronium formation. • Water physisorption suppresses positronium diffusion in high-silica zeolites. • In low-silica zeolites positronium formation is large and enhanced by adsorbed water. A systematic study of the influence of surface acidity of pores, water content, crystal size and structural type of zeolites on positronium behavior was carried out over a wide range of Si/Al ratios using the positron annihilation lifetime spectroscopy technique. A clear decrease in the ortho-positronium pick-off annihilation lifetime, as well as the total ortho-positronium formation fraction, was detected at lower Si/Al ratios due to the positronium oxidizing effect and positron trapping of the Brønsted acid sites. Upon water adsorption, the suppression of the positronium diffusion by the water molecules adsorbed in the tubular pores and their trapping action of positrons and electrons was found to significantly reduce both the ortho-positronium lifetime and intensity. These results clarified the relation between positronium behavior and the chemical reactivity of the zeolite pores.