| Fischer-Tropsch Synthesis (FTS) is an attractive technology for gas to liquid conversion, in which syngas, a mixture of CO and H2, is catalytically converted into a mixture of products such as olefins, paraffins, alcohols and others. Cobalt catalysts are believed to deactivate less rapidly and yield higher linear alkanes than iron counterparts due to high chain growth probability. Thus, cobalt catalysts are considered to be the best candidates for syngas to clean liquid fuel requirements. In the FTS process, the catalyst activity and selectivity are influenced by the property and structure of support, property of metal phase, metal dispersion and catalyst preparation method. The unique properties of CNT such as uniform pore size distribution, mesoporous structure, inert surface properties, and resistance to acid and base environments, hence CNTs can be commonly used as the supports for FTS.In the present study, we prepared a series of CNT-supported cobalt catalysts by the impregnation method and ZrO2 was used as a promoter. The support and catalysts were characterized with TEM, BET, XRD, XPS, H2-TPR and the activity test were carried out in a fixed-bed reactor. It was systematically investigated that the effect of the position of cobalt particles, pore sizes, the catalysts preparation method on the catalytic performance of the FTS. The main conclusions are as follows:1. The position of the cobalt active sites affact the catalytic performance of the FTS. When the cobalt particles attached inside the CNT pores, the catalysts'activity is higher than the catalyst in which the cobalt particles are outside the CNT pores, however, the products selecvities are almost the same. Both catalysts own the same cobalt particles, hence, the difference in the activity may be caused by the difference of the electronic property between inner and outer surface of the CNT. Theπ-electron density is shifted from the concave inner to the convex outer surface. As a result, the inner pore surface becomes electron-deficient while the outer pore surface becomes electron-enriched. The electron density loss of the inner surface could partially be compensated by the interaction with the encapsulated Co3O4, leading to destabilization of the oxidic nanoparticles and thus facilitates the reduction of the encapsulates which increase the activity.2. The cobalt particle size is mainly manipulated by the pore size of the CNT. The smaller pore CNT supported cobalt catalyst exhibited the highest activity among all of the catalysts. The high activity result from the high dispersion and high reduction. Further more, the smaller pore CNT supported cobalt catalyst also is the most stable catalyst because of the space restriction hinder the agglomeration of the cobalt particles during the reaction. 3. The method of addition ZrO2 also affact the catalytic performance on the FTS. For the catalysts which added cobalt prior to ZrO2 or cobalt and ZrO2 were added at the same time, the catalyst addition of ZrO2 increases the dispersion of the cobalt. The catalyst 10ZrCo-CNT(11) exhibits the lowest activity, because the impregnated Co prior to the ZrO2 partly covered the Co active site to some extend. 10CoZr-CNT(11) exhibit the highest activity because of the highest density of cobalt atoms in the catalyst surface.4. The influence of ZrO2 are different for the CNT with different pore size supported cobalt catalysts. As for the smaller pore CNT supported catalyst, the addition of ZrO2 increases the cobalt dispersion and reduction, leading to the highest activity. But for the larger pore CNT supported catalyst, the addition of ZrO2 does not change the cobalt particle size and the reduction behavior almost the same, however the CO conversion increases, due to an increase of the number of the cobalt atoms in the catalyst surface. |
