Positron annihilation studies of mesoporous iron modified MCM-41 silica

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 N-2 sorption/desorption method and positron annihilation lifetime spectroscopy (PALS) were used. Disappearance of the longest-lived ortho-positronium (o-Ps) component (tau(5)) 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 N-2 at ambient pressure causes reappearance of the (tau(5)) 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 (tau(5)) lifetime which is practically independent of Fe content, the relevant intensity I-5 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 O-2 molecules.