B. Ernst et al., Effects of highly dispersed ceria addition on reducibility, activity and hydrocarbon chain growth of a Co/SiO2 Fischer-Tropsch catalyst, CATAL TODAY, 50(2), 1999, pp. 413-427
A series of Co-CeO2/SiO2 catalysts at cobalt iso-content (25 wt%) with vari
ous concentrations in cerium (4.5, 8.9, 21.5 and 38.2 wt%) and a 25 wt% Co/
SiO2 system have been characterized and tested in Fischer-Tropsch synthesis
for the production of high molecular weight hydrocarbons (waxes). The effe
cts of cerium oxide addition on the cobalt particle size, on the reducibili
ty of the cobalt and on the catalytic performance (activity and selectivity
) of a Co/SiO2 catalyst have been studied. The catalysts have been prepared
with an original method combining the coprecipitation of the precursors of
cobalt and cerium by oxalic acid with the hydrolysis and condensation of t
he tetraethoxysilane. The crystallite size measurements (TEM) of Co3O4 and
CeO2, which are the only structures detected by XRD and XPS in oxidized sam
ples, indicated that the presence of cerium had a dispersing effect on the
cobalt. It was also found that the ceria was well dispersed on the silica s
upport. The reduction experiments (TPR and XPS) under hydrogen at 673 K sho
wed that the reduction rate of ceria supported on SiO2 was similar to that
of ceria with high specific surface area. The cobalt reduction degree decre
ased with the addition of cerium, even in low concentration. It was however
equivalent for the Co/CeO2, Co-Ce(38.2 wt%)/SiO2 and Co/SiO2 catalysts all
owing to conclude that the extent of cobalt reduction at 773 K was not very
much affected by the reduction state of ceria. The catalytic activities (T
OF) (P=2 MPa, T=493 K, H-2/CO=2/1) of Co-CeO2/SiO2 and Co/SiO2 catalysts we
re similar. With the addition of cerium significant changes in hydrocarbon
selectivities were observed: decrease in C2+ hydrocarbon selectivity and pr
oduction of gasoline to the detriment of the C22+ fraction (chain growth pr
obability (alpha) from 0.82 to 0.87 for Co-CeO2/SiO2 catalysts and 0.92 for
the unpromoted catalyst). This modification was attributed to a smaller co
balt particle size, to the specific adsorption/desorption properties of H-2
and CO on Co-CeO2/SiO2 catalysts and also to the possible presence of form
ate species, which could be formed on the partially reduced ceria pointed o
ut by H-2 and CO TPD studies. The hydrogenation of these species could expl
ain the additional methane production in the Fischer-Tropsch synthesis unde
r pressure. (C) 1999 Elsevier Science B.V. All rights reserved.