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1

Synthesis and characterization of iron - cobalt nanoparticles embedded in mesoporous silica MCM - 41

100%

Surowiec Z. , Wiertel M. , Budzyński M. , Gac W.

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2013

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tom Vol. 58, No. 1

87-92

EN

Fe1.xCox/MCM-41 nanocomposite materials were prepared by wet impregnation of the silica mesoporous support with aqueous solution of iron and cobalt salt mixtures. Samples, after calcination, were reduced at 573 K in order to obtain homogeneous distribution of (Fe-Co)3O4 oxide species over the MCM-41 support. Bimetallic systems were achieved through high temperature reduction, carried out at 1073 K. The temperature-programmed reduction studies indicated complex nature of the oxide species. The phase analysis of the obtained samples after low temperature reduction process carried out by means of X-ray diffraction method and using 57Fe Mossbauer spectroscopy revealed the presence of nanostructured magnetite- and maghemite-like phases. The samples with higher cobalt contents (x . 0.2) contained additionally bimetallic phases. The complete transformation of oxide phases to bimetallic Fe-Co systems occurred during reduction process in hydrogen at 1073 K. It was observed that high temperature processing caused partial transformation of highly dispersed iron oxide nanocrystallites into fayalite species embedded in the silica walls.

2

Positron annihilation studies of mesoporous iron modified MCM-41 silica

100%

Wiertel M. , Surowiec Z. , Budzyński M. , Gac W.

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2013

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tom Vol. 58, No. 1

245-250

EN

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.

3

Study on the effect of atmospheric gases adsorbed in MnFe2O4/MCM-41 nanocomposite on ortho-positronium annihilation

100%

Wiertel M. , Surowiec Z. , Budzyński M. , Gac W.

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tom Vol. 60, No. 4, part 1

783--787

EN

In this paper, results of positron annihilation lifetime spectroscopy (PALS) studies of MnFe2O4/MCM- -41 nanocomposites in N2 and O2 atmosphere have been presented. In particular, the influence of manganese ferrite loading and gas filling on pick-off ortho-positronium (o-Ps) annihilation processes in the investigated samples was a point of interest. Disappearance of the longest-lived o-Ps component with τ5 present in the PAL spectrum of initial MCM-41 mesoporous material in the PAL spectra of MnFe2O4-impregnated MCM-41 measured in vacuum is a result of either a strong chemical o-Ps quenching or the Ps inhibition effects. The intensity I4 of the medium-lived component initially increases, reaching a maximum value for the sample with minimum manganese ferrite content, and then decreases monotonically. Analogous dependence for the intensity I3 of the shortest-lived component shows a maximum at higher MnFe2O4 content. Filling of open pores present in the studied nanocomposites by N2 or O2 at ambient pressure causes partial reappearance of the τ4 and τ5 components, except a sample with maximum ferrite content. The lifetimes of these components measured in O2 are shortened in comparison to that observed in N2 because of paramagnetic quenching. Anti-inhibition and anti-quenching effects of atmospheric gases observed in the MnFe2O4/MCM-41 samples are a result of neutralization of some surface active centers acting as inhibitors and weakening of pick-off annihilation mechanism, respectively.

4

An application of Mössbauer spectroscopy in the studies of iron - molybdenum silica mesoporous materials

88%

Surowiec Z. , Wiertel M. , Marynowska A. , Gac W. , Zawadzki W.

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2013

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tom Vol. 58, No. 1

137-141

EN

Iron-molybdenum silica mesoporous materials were obtained by the application of direct hydrothermal method. Four samples with different metal contents relative to silica were investigated. Incorporation of iron and molybdenum ions in the synthesis stage led to structural changes of the MCM-41 support. With an increasing metals content, cylindrical pores of silica initially transformed into bottle-ink type pores, and then into the slit-like ones. Mössbauer spectroscopy investigations indicated that Fe ions were embedded in the silica walls and then formed nanosized crystallites in the pores. Superparamagnetic phase was observed at liquid nitrogen temperature.

5

Zastosowanie metod temperaturowo programowanych do charakteryzowania perowskitowych katalizatorów manganowo-lantanowych modyfikowanych srebrem i przeznaczonych do bezpłomieniowego spalania metanu

75%

Machocki A. , Loannides T. , Stasińska B. , Gac W. , Denis A. , Piasieczna S.

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tom T. 82, nr 3

216-220

6

Katalizatory Ni/MgO-Al2O3 w reakcji reformingu metanu z parą wodną i /lub ditlenkiem węgla

63%

Pańczyk M. , Gac W. , Denis A. , Sikorska C. , Czubryt-Idzik M. , Gołębiowski A. , Stołecki K. , Borowiecki T.

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tom T. 82, nr 8-9

748-751