CHARACTERIZATION OF LA3-PROMOTED CO()SIO2 CATALYSTS/

In order to assess the distribution and the promotional effects of La3 +, a characterization study has been performed on a series of La3+-pro moted 20 wt% Co/SiO2 catalysts of varying La/Co atomic ratios (0, 0.05 , 0.1, 0.2, 0.3, 0.5, and 0.75) prepared by impregnation of prereduced and passivated Co/SiO2 in order to minimize effects of promotion on C o particle size distribution. La3+ was found to be well dispersed over the Co/SiO2 catalyst. However, it did not appear to be evenly distrib uted on the Co metal surface. From temperature programmed reduction,lo w La3+ loadings (La/Co less than or equal to 0.2) were found to modera te the strong Go-support interactions resulting from aqueous impregnat ion of Co/SiO2 and to improve the reducibility of the Co oxide. [Aqueo us impregnation of prereduced Col SiO2 has been shown earlier to resul t in the formation of Co silicates (Haddad, G. J., and Goodwin, J. G., Jr., J. Catal. 157, 25 (1995))]. On the other hand, high La3+ loading s (La/Co greater than or equal to 0.5) appeared to induce La-Co compou nd and/or nonreducible Go-silicate formation, possibly due to the high er pH of the impregnating solution. The results indicate the multiple effects that promoters may have on supported metal catalysts. In addit ion to possibly affecting reaction on the sites, they may act to modif y significantly the structure of the metal catalyst, for example by mo derating metal-support interactions. Obviously, one expects that promo ted catalysts prepared directly from precursors may exhibit large diff erences in metal dispersion. However, even when promoted catalysts are prepared from a precalcined or prereduced supported metal catalyst, t he act of adding a promoter may have as much an effect in modifying th e catalyst as the final presence of the promoter. (C) 1996 Academic Pr ess, Inc.