IR spectroscopy of adsorbed NO as a useful tool for the characterisation of low concentrated Fe-silicalite catalysts

Abstract

FT-IR spectroscopy of adsorbed NO was used to investigate the electronic and geometric structure of the extraframework iron species formed upon thermal activation of four Fe-silicalite samples with Si/Fe ratios ranging from 25 to 150. An heterogeneous family of grafted Fe 2+ species, classified in terms of their first coordination shell, i.e. as L 2N n Fe 2+ (where L are the framework SiO − groups chemically linked to the Fe species and N are the framework oxygen atoms of the vicinal SiOSi bridges interacting with Fe centres via electrostatic forces) has been detected as the primary product of the Fe migration. Depending upon the number of N ligands (3, 2 or 1) the Fe 2+ species can adsorb one, two, and three NO molecules, respectively, with formation of well-defined nitrosylic complexes. L 2N n Fe 2+ ( n=1 and 2) are supposed to be isolated species, while L 2N 3Fe 2+ are probably dimeric or polymeric clusters. Fe 3+ extraframework species can be reduced to Fe 2+ by prolonged outgassing in vacuo. The formation of large Fe 2O 3 clusters is inferred for the most concentrated sample. O 2 dissociates with formation of adsorbed oxygen on all extraframework Fe species at 573 K. At room temperature only dimeric or polymeric L 2N 3Fe 2+ species are able to dissociate O 2.