Metal oxide-support interactions in Fe/ZrO2 catalysts

To reveal the metal oxide-support interactions in ZrO2-supported Fe oxide c atalysts as a function of calcination temperature, 5 wt % Fe/ZrO2 was chara cterized by means of Fe-57 Mossbauer spectroscopy, IR, XRD, and thermogravi netric analysis. The catalytic activity of Fe/ZrO2 was examined at 523 K fo r NO-CO reaction. It is shown that when Fe/ZrO2 is calcined below 573 K, sm all particles of hydrated Fe3+ oxyhydroxides (superparamagnetic at 297 K) a re formed together with Fe3+ paramagnetic species ion-exchanged with the su rface OH groups of ZrO2. In addition to the Fe3+ ion-exchanged species, Fe3 + oxide clusters and crystalline alpha-Fe2O3 particles are formed by succes sive dehydration of the oxyhydroxides on calcination at 973 K. When Fe/ZrO2 is calcined at 1073 or 1173 K, the Mossbauer spectroscopic results suggest the formation of Fe3+ cations trapped in surface vacant sites of ZrO2 at t he expense of the Fe3+ oxide clusters. On the calcination at a higher tempe rature, crystalline alpha-Fe2O3 predominates with a small portion of the Fe 3+ cations being in surface vacant sites. With 2 wt % Fe/ZrO2 prepared by a coprecipitation method and calcined at 1173-1373 K, Fe3+ cations dissolved in a ZrO2 lattice are formed in addition to a small amount of magnetically split component.