Ortho-positronium annihilation in the Na+, Co(II) and Mn(II) A-type zeolites

The annihilation of ortho-positronium in the alpha cavities of the hydrated and dehydrated Na-A-type zeolites were analyzed. The results show that the zeolite water inhibits the formation of Ortho-positronium and also reduce its lifetime with respect to those parameters detected in the dehydrated zeolite and liquid water. On the other hand, the ortho-positronium annihilation data of the stepwise dehydrated Na-A-type zeolite suggest that the loss of water of such a zeolite may be followed by positron annihilation lifetime spectroscopy revealing more details than does a typical thermogram. This may be important while characterizing surface properties in catalysts. Furthermore, the annihilation of ortho-positronium in the a cavities of A-type zeolites containing different cations was also analyzed. For instance, in the dehydrated samples of Na+ Co(II), and Mn(II) A-type zeolites, the corresponding alpha. cavities are freed of water and ortho-positronium annihilate in them with lifetimes of 4.87, 6.69 and 7.22 us, respectively. These lifetime differences are interpreted satisfactorily by considering both the number and the atomic size of the cations which lie in the a cavities; their estimated cavity radii are within the expected values of the radii. (C) 2004 Elsevier Inc. All rights reserved.