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

Abstract

A detailed high resolution electron microscopy investigation of different Rh/lanthana catalysts has been carried out. Samples prepared by wet impregnation using either aqueous or acetone solutions of rhodium nitrate and metal deposition by evaporation under high vacuum have been studied. The nanostructural evolution of the support and the metal phase through the different preparation steps has been tracked. Thus, the effects of reduction treatments in a wide range of temperatures (473–973 K) have been taken into consideration. Scanning electron microscopy images evidence an intense dissolution of the lanthanide sesquioxide crystallites during the impregnation step with the rhodium nitrate aqueous solution. Such an effect could not be detected in the 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. Increasing the reduction temperature leads to a significant sintering of rhodium present both on the surface and the bulk of the support. Decorated rhodium particles are clearly visible in the three catalysts in the whole range of reduction temperatures. The drastic structural rearrangements which take place in the support during the reduction treatment and support dragging during sintering, are most likely the driving force for the decoration of the metal particle surfaces.