An investigation of the use of zirconia as a support for rhodium catalysts

The nature and properties of zirconia-supported rhodium catalysts after high temperature treatment in an oxidising atmosphere have been studied. No interaction between the metal and the support is seen even after calcination at 700 degrees C, easily reduced rhodium oxide being the only Rh species present. Higher dispersion of this species can be achieved by using a higher surface area alumina-doped ZnO2 although it is necessary to pre-sinter the support to prevent encapsulation of the Rh. Rh/ZrO2 catalysts calcined at 500 degrees C have remarkably high activity for methane combustion, similar to that of a Pd/Al2O3 catalyst calcined at the same temperature. Whilst the use of zirconia is preferable to alumina for combustion reactions, the activity of the zirconia-supported Rh in the selective reduction of NO with propene under lean-burn conditions is less than was found previously for Rh/Al2O3. This can be attributed to a lack of isolated Rh3+ ions in the Rh/ZrO2 catalysts. Attempts have been made to inhibit the deactivation reaction of rhodium oxide with Al2O3 by pretreatment of the Al2O3 with Zr-containing precursors. The extent to which the Zr modifies the surface properties of the support are discussed.