Table 5 compiles the conversion level and selectivity under steady state at 300°C and 10 atm for the studied catalysts. It can be seen that for low promoter content (0.1 wt%), the conversion of CO increases from approximately 26% for the Co/SiO2 to values close to 32% for the promoted samples, except that promoted by Cu which exhibit a very low conversion level, close to 12%. In the series with 0.5 wt% of promoter the drop in the activity in the CoCu catalysts is even more drastic, reaching a conversion level of 8.3% and the CoZn catalyst also displays a decrease in the conversion compared with the monometallic Co/SiO2 catalyst. Even though the cobalt particle size is smaller in the promoted samples, specially in which the promoter content is higher, and the reduction degree is higher, therefore, it should be expected for an enhancement in the catalytic activity, in those samples pro­moted with Zn and Cu, the opposite behaviour was observed. This may be

interpreted as considering that in these samples the formation of spinel type oxides as a consequence of calcinations should be expected, thus reducing the extent of active sites. The presence of these mixed oxides species has already been reported for other systems. Other possibility for this behaviour is the blocking of the Co active sites by the copper itself. However, due to the small amount of promoter they were not detected by XPS.