Fischer-Tropsch synthesis on Co/AlO catalyst: Effects of carbidization and reduction conditions
Academic Article
Overview
Research
Identity
Other
View All
Overview
abstract
Alumina supported cobalt (Co) catalysts are the preferred choice for Fischer-Tropsch synthesis (FTS) when using natural gas as raw material, which is the case of Qatar. It is thus of economical interest to find ways of increasing activity and/or selectivity of these catalysts. One way of modifying catalytic activity, consists of transformation of the active Co phase crystallites. A shift from the abundance of the face-centered cubic (fcc) Co to hexagonal-close packed (hcp) Co phase has been found to increase catalyst activity [1]. This can be accomplished through the carbidization-reduction step with Co carbide as an intermediate. The catalyst was prepared by incipient wetness impregnation of alumina support (15% Co/AlO). Its fcc form was obtained by reducing calcined catalyst in pure hydrogen at 375C for 10 h. The hcp form was subsequently prepared by carbidization with pure CO for 14 h at 220C, followed by reduction in pure hydrogen at 220 or 250C for 9 h. The catalyst was tested in a fixed-bed reactor at 220C, 20 bar, H/CO ratio of 2 and a space velocity of 3.5 NL/g-cat/h. The figure shows thermogravimetric results during the first reduction step which decreases weight of the sample due to the Co(II,III) Co transformation and weight gain during the carbidization step. After 14 h of carbidization, the stoichiometric CoC amount is obtained. Temperature programmed hydrogenation and oxidation of this sample showed evidence for the presence of CoC and absence of free carbon. CO conversion during FTS decreased from 59% (H reduction) to 55% or 48% after carbidization-reduction pretreatment (at 220 and 250C respectively); suggesting the need for future studies on alternative reduction/carbidization conditions. Methane selectivity increased from 8% for H reduced catalyst to 11% after both carbidization-reduction pretreatments, probably due to presence of surface and/or bulk carbides. [1] Karaca H. et al. Journal of Catalysis 277 (2011) 14.
