Superparamagnetism in nanometric-size Fe-based particles on the external surface of a steamed-treated Fe-silicalite

The insertion of trivalent heteroatoms, like Al, Ga or Fe, in the crystalline framework of MFI silicalite leads to the appearance of strong Brønsted acidity, responsible of the acidic properties and of the related catalytic activity of these materials. Migration of Al, Ga or Fe to extraframework positions induced by thermal treatment, with subsequent aggregation in form of oxidic nanoparticles, leads also to the appearance of a different type of acid properties of Lewis nature. In particular, iron species can form oxo-cations (FeiOj)n+, with i>3n, or hydroxo-cations [Fei(OH)j]n+ and so on. This yields to over-exchanged samples which have shown an interesting activity in the selective catalytic reduction of NOx. Furthermore, the presence of external Fe species is noticed. In practical application, the activation of the Fe-MFI catalyst, besides by calcination in air, is frequently carried out by steaming at high temperature. After such a treatment, a considerable part of the tetrahedrally coordinated Fe, besides being extracted and converted to octahedrally coordinated species of different nuclearity, remaining entrapped within the zeolitic pores, exits from the pore framework and forms Fe-containing agglomerates of higher nuclearity. This can be easily seen from the colour of the catalyst, that shifts from snow-white to more or less light brownish. These agglomerates could be too large to find room into the zeolitic pore network and then they could play no active part in the benzene hydroxylation reaction. To investigate the structure of these Fe species, a part of the present catalyst, calcined at 823 K, was subjected to steaming and then activated in He at 823 K. Original (E0), steamed (E2) and activated (E2A) samples were then analysed by EPR. The spectrum was composed of three groups of lines which were simulated with g  2  3 and  4.2, respectively. The whole pattern of the E0 sample broadened with increasing temperature from 110 to 295 K. With E2 sample, i.e. after steaming, the EPR features at g  4.2 was still temperature-broadened, whilst the opposite behaviour was noticed for the g  2 spectral contribution. This has been interpreted on the base of superparamagnetic particles forming in the sample.