Nanostructural evolution of high loading Rh/lanthana catalysts through thepreparation and reduction steps

A detailed high resolution electron microscopy investigation of different R h/lanthana catalysts has been carried out. Samples prepared by wet impregna tion using either aqueous or acetone solutions of rhodium nitrate and metal deposition by evaporation under high vacuum have been studied. The nanostr uctural evolution of the support and the metal phase through the different preparation steps has been tracked. Thus, the effects of reduction treatmen ts in a wide range of temperatures (473-973 K) have been taken into conside ration. Scanning electron microscopy images evidence an intense dissolution of the lanthanide sesquioxide crystallites during the impregnation step with the r hodium nitrate aqueous solution. Such an effect could not be detected in th e catalyst prepared using an acetone solution of the same metal precursor. On its hand, HREM indicates the presence of a highly dispersed metal phase in the catalyst prepared in water after reduction at low temperatures. Incr easing the reduction temperature leads to a significant sintering of rhodiu m present both on the surface and the bulk of the support. Decorated rhodiu m particles are clearly visible in the three catalysts in the whole range o f reduction temperatures. The drastic structural rearrangements which take place in the support during the reduction treatment and support dragging du ring sintering, are most likely the driving force for the decoration of the metal particle surfaces. (C) 1999 Elsevier Science B.V. All rights reserve d.