ChemInform Abstract: A Comparison of Niobia- and Vanadia-Promoted Rh Catalysts: The Behaviors of RhMO4 (M: Nb, V) on SiO2 During Calcination and Reduction Treatments.

Abstract

Abstract The behaviors of Nb 2 O 5 and V 2 O 5 in Rh/SiO 2 catalysts are compared by X-ray diffraction. For vanadia-promoted Rh/SiO 2 catalyst, RhVO 4 compound was formed exclusively by mild calcination at 773 K, and larger crystallites formed by calcination at 973 K. For niobia-promoted Rh/SiO 2 catalysts, RhNbO 4 compound was formed almost exclusively after the high-temperature calcination at 1173 K. The RhVO 4 /SiO 2 and RhNbO 4 SiO 2 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 RhNbO 4 particles (139 A) split to a number of smaller Rh particles (48 A) 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 RhVO 4 compound, and large particles (190 A) split to smaller Rh particles with mean size of 40 A after the H 2 reduction at 773 K. However, such Rh particles agglomerated to large ones (122 A) by the thermal treatment at 973 K. V 2 O 5 reacted with Rh at much lower temperature than Nb 2 O 5 , with large Rh particles being converted to RhVO 4 compound even by the mild calcination at 773 K. This phenomenon may find practical application in the regeneration of sintered Rh catalyst.