Variations in bonding of iron in porous ferrisilicates
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Variations in bonding of iron in porous ferrisilicates Károly Lázár
© Springer Science+Business Media Dordrecht 2013
Abstract Bonding strengths of iron species in different micro- and mesoporous ferrisilicates are deduced from analysis of changes in recoilless fractions extracted from various series of in situ spectra. The recoilless fractions for extra-framework ions in microporous Fe-LTA may depend strongly on the conditions. In Fe-FER, containing both framework and extra-framework types of iron, modest changes were detected. In mesoporous Fe-MCM-41 samples variations of the bonding strengths were revealed which can apparently be correlated with the partly amorphous structure of the pore walls in these substances. Keywords Recoilless fractions · Porous ferrisilicates · Bonding strength · In situ spectra
1 Introduction Probablility of the Mösbauer effect is strongly connected to the possible vibration states of the Mössbauer nucleus inside the solid lattice. The strength of the bonding of iron in the lattice may also depend on the oxidation state, coordination etc. Thus, beside evaluation of usual basic parameters (isomer shift, quadrupole splitting) valuable additional informations can be extracted from the analysis of data related to the changes in the recoilless fractions of components. The value of the recoilless fraction (fA ) can explicitly be described e.g. in the frame of the Debye model of lattice vibrations, where a characteristic temperature (D ) may be used to estimate the
Proceedings of the 32nd International Conference on the Applications of the Mössbauer Effect (ICAME 2013) held in Opatija, Croatia, 1–6 September 2013. K. Lázár (B) Centre for Energy Research, Hungarian Academy of Sciences, Budapest, Konkoly Thege M. 29-33, 1121, Hungary e-mail: [email protected]
K. Lázár
strength of bonding of the studied species in the crystal structure [1]. D is calculated usually from the temperature dependence either of the spectral area or the isomer shift (second order Doppler shift). Relative measurements can also be used, e.g. by comparing the relative areas of a reference and the studied substances mixed within a single sample [2]. Investigation of data related to the recoilless fractions of different species present in the in situ Mössbauer spectra of metallic components of supported FeRh/SiO2 catalysts was proposed already in 1984 [3]. In particular, identification of catalytically active surface components was attempted by this way—the surface component was supposed to exhibit loose bond, i.e. stronger temperature dependence in its signal intensity. In contrast, the component originated from inside of the bulk metallic particle (thereby hardly participating in the surface catalytic process), was supposed to display less expressed temperature dependence in its intensity [3]. Existence of ionic tin species on the surface of supported bimetallic Pt-Sn catalysts was also detected in analogous studies later [4]. Application of in situ measuring technique [5] is particularly advantageous for
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