NUKLEONIKA 2013, 58(1):245-250

 


POSITRON ANNIHILATION STUDIES OF MESOPOROUS IRON MODIFIED MCM-41 SILICA



Marek Wiertel1, Zbigniew Surowiec1, Mieczysław Budzyński1, Wojciech Gac2

1 Maria Curie-Skłodowska University, Institute of Physics,
1 M. Curie-Skłodowskiej Sq., 20-031 Lublin, Poland
2 Maria Curie-Skłodowska University, Faculty of Chemistry,
3 M. Curie-Skłodowskiej Sq., 20-031 Lublin, Poland

MCM-41 silica materials modified by iron incorporation in the stage of its synthesis were investigated. The aim of the studies was determination of the nature of iron species and the influence of its content on the structural changes of materials and following the changes of their properties. For this purpose, the N2 sorption/desorption method and positron annihilation lifetime spectroscopy (PALS) were used. Disappearance of the longest-lived ortho-positronium (o-Ps) component (tau5) present in the PALS spectra of the initial MCM-41 material in the spectra of Fe-modified MCM-41 measured in vacuum is a result of a strong chemical o-Ps quenching and/or the Ps inhibition mechanism. Filling of pores by air or N2 at ambient pressure causes reappearance of the (tau5) component with lifetime shortened in comparison to that observed in vacuum for pure MCM-41 to the extent which can be explained by usual paramagnetic quenching in air. In contrary to the tendency observed for (tau5) lifetime which is practically independent of Fe content, the relevant intensity I5 monotonically decreases. This fact suggests that only inhibition of Ps formation occurs for the samples in air. Observed anti-quenching effect of air seems to be a result of competition of two processes: neutralization of surface active centres acting as inhibitors and considerably weaker paramagnetic quenching by O2 molecules.

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