Effect of Fe 2 O 3 and Cr 2 O 3 on anatase - rutile transformation in TiO 2

Abstract

Metal oxides supported on TiO₂ are gaining in importance as a result of their applications in catalysis. Reactions carried out over TiO₂ supported catalysts include hydrodesulphurisation (HDS), partial or selective oxidation, hydrogenation of carbon monoxide, selective catalytic reduction of NO_x, coal liquefaction, and ammonia synthesis. Catalytic activity depends on the phase of the TiO₂ skeleton (anatase, rutile, or brookite). Catalytic activity is high for anatase, but decreases when it transforms to rutile on heating at high temperature for extended periods. In the present study, the effect of doping with Fe₂O₃ or Cr₂O₃ on the anatase to rutile transformation was investigated. Samples prepared by coprecipitation and wet impregnation were heated in air at various temperatures. The calcined samples were characterised by chemical analysis, XRD, surface area measurements, SEM, and grain size measurements. It was observed that the degree of anatase transformation was higher on doping with 15% Fe₂O₃ or Cr₂O₃ than with 5%. The method of preparation plays a major role in the transformation, coprecipitated samples always giving higher rutile percentages than wet impregnated ones. The activation energy for the transformation is lower for 15% than for 5% doped samples and for coprecipitated than for wet impregnated ones.