Promoting effect of lanthana in the hydrogenation of carbon monoxide over supported rhodium catalysts

The catalytic activity in the CO+H 2 reaction is reported for a number of supported rhodium catalysts including Rh/La 2 O 3 , Rh/SiO 2 , Rh/Al 2 O 3 , as well as ternary phases Rh/La 2 O 3 /SiO 2 with lanthana loadings ranging from 0.1% to 8.5%. The highest conversions to C 2 -oxygenates were found to occur over the lanthana containing catalysts. In the particular case of ternary catalysts a clear promoting effect of lanthana was observed, the highest activity and selectivity to oxygenates being found at quite low lanthana loading: 0.25%. Our studies of both characterization and catalytic activity suggest that the metal particles were coated by lanthana. According to the lanthanum and rhodium contents determined in the solutions resulting from the impregnation treatment, redissolution of lanthana during this step of the preparation of the catalysts was found to occur. An interpretation of the mechanism of coating of the metal crystallites by lanthana is presented.