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) stud y of a series of Rh/CeO2 catalysts prepared from both Rh(NO3)(3) and R hCl3 metal precursors. Our attention is focused on the influence of th e reduction/reoxidafion conditions on the metal particle size distribu tion. From the analysis of both experimental and computer simulated HR EM images, we were able to conclude that the truncated cubo-octahedron model describes well the morphology of the rhodium microcrystals grow n on ceria. Likewise, we have used the computer simulation techniques to estimate the lowest size limit for HREM detection of the rhodium pa rticles in Rh/CeO2 catalysts. In our case, the smallest metal particle s 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 par ticle (D-d) can significantly deviate from the more accurate values de termined from the so-called type C distribution curves (D-m). Reductio n treatments up to 973 K induce a progressive sintering of the metal, the effect becoming much stronger upon reduction at 1173 K. We have al so found that metal dispersion is more sensitive to the reduction trea tment in Rh(Cl)/CeO2 catalysts. Among the reoxidation temperatures inv estigated by us, rhodium redispersion could only be observed at the hi ghest temperatures: 1073 K or 1173 K. By contrast, reoxidation at 773 K causes significant metal sintering effects. (C) 1998 Elsevier Scienc e B.V.