L. Lohonyai

Free volumes in zeolites as probed by positrons

Abstract Positron annihilation (PA) data are presented for various zeolites, collected by lifetime spectroscopy (LS) and by recording the full energy distributions of the annihilation radiation. Long-lived PA components were found, corresponding to positronium (Ps) formation in different types of free volume within the zeolite structure. These Ps states are influenced by various factors, such as the synthesis, the presence of adsorbates, impurities, water content, etc. Our studies were aimed therefore to correlate the PA parameters (lifetime, intensity, 2γ/3γ annihilation ratio) with the variety of “free volumes” in zeolites and examine their changes upon ion sorption and filling-up. Silicalite-1 and Na-Y are favourable matrices for high Ps formation. The filling-up of cages was investigated by LS measurements on PdFe-X, PdFe- ZSM-5 and PtFe-Y systems and a correlation of PA data with the cage-filling due to formation of metallic particles was found. Mossbauer measurements have shown that these particles are bimetallic. Water removal in Na-Y and La,Na-Y was also studied by both methods.

Preliminary results of the positron annihilation in zeolites: Peak shape and 3γ-decay

In many sorts of zeolites long-living positron components are manifested, connected presumably with Ps-formation in various kinds of free space in the zeolite structure. Literature values demonstrate, however, a broad scasttr of data obtained for assumedly on the same composition of zeolite indicating the possible influence of tehnology, adsorbates, impurities, water content, etc. In the present work spectral changes of the 511 keV annihilation peak and its vicinity are evaluated, applying for the first time a combination of theS and W parameters and the 3γ-specific left plateau region measured in zeolites. To ensure reproducible experimental conditions, evaluated samples were measured. The data show individual temperature behaviours of the annihilation parameters in the studied temperature range of 90–450 K for each kind of samples. The temperature dependence becomes reproducible after the first heating run. The results are discussed with respect to the evolution of long-living positron and positronium components.

The influence of alkali cations on the structure of zeolite precursor gels investigated by positron lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) is sensitive and proved method for material structure investigations. We used it to investigate the influence of different alkali cations on the structure of zeolite precursors in order to understand better the crystallization mechanism of zeolites. Our previous PALS investigations on Li gels show different structural features obtained for differently prepared gels, indicating the importance of the way on which cations are introduced in the gel matrix. These investigations were now extended to gels containing K, Rb and Cs cations, in correlation to their ionic radius and the way of preparation.

Long-living positron and positronium states in zeolites and microcrystalline oxides

Positron annihilation (PA) investigations were performed on zeolites (X, Y and ZSM-5) and on microcrystalline MgO, Al2O3 and SiO2, providing long lifetime components attributed to o-Ps atoms. In addition to the positron lifetime (LT) measurement, the energy distribution (ED) of the annihilation gamma radiation spectrum was recorded in the 30 keV–1.5 MeV range for different samples and was compared to reference distributions for Si and GaAs samples, where no long-living Ps states are formed. Apart from the strong correlation with the water content in the samples, the positron data collected testify much more pronounced positronium hosting features for powders of the mentioned oxides than for zeolites. Positron LT spectroscopy combined with recording of the ED of the annihilation radiation provides reliable information on the formation of long living 3γ states.