A COMPARISON OF NIOBIA-PROMOTED AND VANADIA-PROMOTED RH CATALYSTS - THE BEHAVIORS OF RHMO4 (M = NB, V) ON SIO2 DURING CALCINATION AND REDUCTION TREATMENTS

The behaviors of Nb2O5 and V2O5 in Rh/SiO2 catalysts are compared by X-ray diffraction. For vanadia-promoted Rh/SiO2 catalyst, RhVO4 compound was formed exclusively by mild calcination at 773 K, and larger crystallites formed by calcination at 973 K. For niobia-promoted Rh/SiO2 catalysts, RhNbO4 compound was formed almost exclusively after the high-temperature calcination at 1173 K. The RhVO4/SiO2 and RhNbO4SiO2 catalysts exhibit significant SMS1 behaviors after high-temperature reduction, typically at 773 K; the capacity of H2 chemisorption diminished almost to zero and the catalytic activity for ethane hydrogenolysis was suppressed by ca. 3.5 orders of magnitude as compared with that after the low-temperature reduction that followed. Redispersion of rhodium was observed during the decomposition of compounds in H2. Large RhNbO4 particles (139 Å) split to a number of smaller Rh particles (48 Å) after the H2 reduction at 773 K, and niobia promoter stabilized such Rh particles which exhibited almost no agglomeration during the thermal treatment in He even at 973 K. Similar redispersion was observed for RhVO4 compound, and large particles (190 A) split to smaller Rh particles with mean size of 40 Å after the H2 reduction at 773 K. However, such Rh particles agglomerated to large ones (122 Å) by the thermal treatment at 973 K. V2O5 reacted with Rh at much lower temperature than Nb2O5, with large Rh particles being converted to RhVO4 compound even by the mild calcination at 773 K. This phenomenon may find practical application in the regeneration of sintered Rh catalyst. © 1991.