Characterization of iron promoter in tungstated zirconia catalysts by Mössbauer spectroscopy at very low temperatures.
A tungstated zirconia (WZ) catalyst with iron promoter used for the conversion of n-pentane into isopentane has been characterized by Mössbauer spectroscopy. The Mössbauer spectra have been recorded in zero magnetic field in the temperature range 0.05-295 K and with a magnetic field up to 7 T between 4.2 and 50 K. Both the recording of Mössbauer spectra with an applied magnetic field and at extremely low temperature allowed for the demonstration that iron is present in the catalysts as (i) hematite (alpha-Fe2O3) particles a few 10 nm in size, (ii) very small oligomeric Fe(III) species, probably in solid solution in zirconia, and (iii) Fe(III) oxide clusters showing magnetic ordering, probably embedded in the first surface layer and thus forming "rafts". These latter clusters form two ensembles with quite different sizes: one with diameters of about 3 nm, the other with diameters larger than 30 nm. These results are in agreement with those recently obtained by X-ray absorption spectroscopy and electron paramagnetic resonance.
Millet
Jean-Marc M
JM
Institut de Recherches sur la Catalyse, CNRS, conventionné avec l'Université Claude Bernard, Lyon I, 2 avenue A. Einstein, F-69626 Villeurbanne, Cedex, France. millet@catalyse.cnrs.fr
Knözinger
Helmut
H
Bonville
Pierre
P