FTIR Spectroscopic Study and CO Hydrogenation on V, Nb, and Ta Oxide Promoted Rh/SiO2Catalysts

Abstract

The CO hydrogenation over Rh/SiO2catalysts promoted with oxides of Vb transition metals was studied. Two different kinds of promoter effects occurred which were established by high temperature reduction (HT reduction, Tred≥ 673 K) and HT calcination (Tcalc≥ 973 K), followed by reduction at 673 K, respectively. While HT reduction favors formation of ethanol, HT calcination, followed by reduction at 673 K enhances the selectivity toward methanol. The overall activity 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/SiO2with promoter oxide causes a slight high frequency shift for linearly bonded CO on rhodium. Reduction at 673 K leads to a decrease in CO chemisorption capacity. In this state the 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 capability 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 bridge-bonded CO occur. A high frequency component at around 1920 cm−1characteristic of bridge-bonded CO which is present after LT reduction (Tred=573 K) disappears after HT reduction.