Effects of drying on the preparation of HF Co-impregnated rhodium/Al 2O 3 catalysts

Abstract

The effects of drying rate on the internal distribution of rhodium within HF co-impregnated rhodium catalysts is reported in this work. Quantitative electron probe microanalysis (EPMA) revealed catalysts with uniform rhodium distributions, inner bands (egg-whites) of Rh and inner cores (egg-yolks) of Rh. The type of internal rhodium distribution produced was found to depend strongly upon the rate of catalyst drying. Measured drying curves showed two governing regimes of drying. At temperatures below 500°C evaporation appeared to take place primarily at the external surface of the support. Also for drying temperatures less than 500°C, EPMA distributions displayed a significant degree of rhodium segregation to the external surface of the support. This segregation occurred when characteristic times for solute back diffusion were greater than the time to reach the critical moisture content. At a temperature of 500°C, evaporation appeared to occur solely from a receding front within the support (i.e. shrinking core drying). Regardless of the regime in which drying takes place, it was shown that for the egg-white catalyst character, which exists immediately after impregnation to be preserved, the total drying time must be less than the time required for diffusion to the center of the support.