Strong metal-promoter oxide interactions induced by calcination in V, Nb, and Ta oxide promoted Rh/SiO2 catalysts

The influence of the calcination temperature on the structure of Rh/SiO2 catalysts promoted by Vb transition metal oxides has been studied. Calcination at T > 973 K leads to the formation of crystalline RhXO4 mixed oxides with increasing abundance in the order RhTaO4 < RhNbO4 < RhVO4. Their reduction at T = 773 K yields highly dispersed Rh degrees metal particles which seem to be embedded in a matrix of partially reduced promoter oxide, The presence of Vb metal oxides enhances the noble metal dispersion as compared to unpromoted Rh/SiO2 catalysts. Calcination at T = 773 K establishes an interaction between Rh2O3 and promoter oxide which manifests itself in a decreased CO-chemisorption capacity after low temperature reduction (LTR = 523 K), This calcination induced metal-promoter interaction decreases with increasing atomic number of the promoter element. (C) 1997 Academic Press.