SECONDARY PORE STRUCTURE OF USY ZEOLITES STUDIED BY POSITRON ANNIHILATE TECHNIQUE

Abstract

The structures of NaY and ultrastability Y-type (USY) zeolite were studied by positron lifetime spectroscopy.Four lifetimes could be obtained in NaY zeolite,the two larger components were assigned to be o-Ps annihilation in the β cage and supercage respectively.In USY zeolite,however,an additional lifetime component τ5 was observed,which was about 92ns in vacuum with continuous distribution from 39ns to 142ns by the Laplace inversion analysis of lifetime spectrum.The second larger lifetimes τ4 also increased to 14ns,compared to that in NaY zeolite.Existence of the longer lifetimes τ4 and τ5 is due to the porous structure of USY.The positron lifetime in USY zeolite was also measured as a function of air pressure ranging from 0 (vacuum) to 0.1MPa.The most interesting was the variation of the largest components τ5 and I5.They both decreased with increasing air pressure.The results can be explained as follows.First,the oxygen contained in the air could quench the Ps and reduce its lifetime and intensity.Second,the increasing density of air could also increase the probability of o-Ps pick-off annihilation and reduce its formation.These would lead to the decrease of its lifetime and intensity.