Image simulation and experimental HREM study of the metal dispersion in Rh/CeO2 catalysts.: Influence of the reduction/reoxidation conditions

This work reports on a high resolution electron microscopy (HREM) study of a series of Rh/CeO2 catalysts prepared from both Rh(NO3)(3) and RhCl3 metal precursors. Our attention is focused on the influence of the reduction/reoxidafion conditions on the metal particle size distribution. From the analysis of both experimental and computer simulated HREM images, we were able to conclude that the truncated cubo-octahedron model describes well the morphology of the rhodium microcrystals grown on ceria. Likewise, we have used the computer simulation techniques to estimate the lowest size limit for HREM detection of the rhodium particles in Rh/CeO2 catalysts. In our case, the smallest metal particles are well above the detection limit, and therefore, the reported size distributions account well for the actual ones. Three different (A-C) size distribution curves have been built up. From their analysis, we conclude that metal dispersions estimated from the mean size metal particle (D-d) can significantly deviate from the more accurate values determined from the so-called type C distribution curves (D-m). Reduction treatments up to 973 K induce a progressive sintering of the metal, the effect becoming much stronger upon reduction at 1173 K. We have also found that metal dispersion is more sensitive to the reduction treatment in Rh(Cl)/CeO2 catalysts. Among the reoxidation temperatures investigated by us, rhodium redispersion could only be observed at the highest temperatures: 1073 K or 1173 K. By contrast, reoxidation at 773 K causes significant metal sintering effects. (C) 1998 Elsevier Science B.V.