FTIR SPECTROSCOPIC STUDY AND CO HYDROGENATION ON V, NB, AND TA OXIDE PROMOTED RH SIO2 CATALYSTS/

The CO hydrogenation over Rh/SiO2 catalysts promoted with oxides of Vb transition metals was studied. Two different kinds of promoter effect s occurred which were established by high temperature reduction (HT re duction, T-red greater than or equal to 673 K) and HT calcination (T-c alc 1973 K), followed by reduction at 673 K, respectively. While HT re duction favors formation of ethanol, HT calcination, followed by reduc tion at 673 K enhances the selectivity toward methanol. The overall ac tivity increased in the order V < Nb < Ta. The impact of promotion on the metal state was characterized by FTIR spectroscopy of adsorbed CO at 85 K. Modification of Rh/SiO2 with promoter oxide causes a slight h igh frequency shift for linearly bonded CO on rhodium. Reduction at 67 3 K leads to a decrease in CO chemisorption capacity. In this state th e rhodium metal surface is thought to be partially covered by promoter oxide and hence compares to the conventional SMSI state. This effect is enhanced if the reduction is preceded by HT calcination, The capabi lity of the promoter oxide to spread over the metal surface increases in the order Ta < Nb < V. While the total CO chemisorption capacity is lowered in the SMSI state, the relative amount of linearly to bridge- bonded CO increases and characteristic changes in the band shape of br idge-bonded CO occur. A high frequency component at around 1920 cm(-1) characteristic of bridge-bonded CO which is present after LT reductio n (T-red = 573 K) disappears after HT reduction. (C) 1997 Academic Pre ss.