Аннотация:The increase of the cost of oil extraction makes the conversion of natural gas, coal, and
biomass into hydrocarbons, alcohols and ethers one of the main trends in natural fuel
processing industry. This process, also known as Fischer-Tropsch synthesis (FTS), is based on
the catalytic conversion of synthesis gas into more valuable hydrocarbons, alcohols and
ethers on iron or cobalt-containing catalysts. Comparing to iron catalysts, the advantages of
cobalt systems are their increased time stability and lower process pressure and
temperature. The properties of carbon nanotubes (CNTs) favour their use as a catalyst
support for FTS. The chemical inertness of CNTs, their structural rigidity, high thermal
conductivity, tuneable surface properties and localization of catalyst particles distinguish
them from oxide or other carbon-based supports [1].
In this work we investigate the FTS on cobalt catalysts supported on CNTs and
transformations of metal and support at different stages of catalyst life cycle: fabrication,
reduction and catalytic reaction. For this purpose, pristine CNTs were functionalized to the
different degrees with oxygen-containing groups. It was found that the stability of catalysts
to sintering is determined by the geometry of the support surface, while the homogeneity of
both Co distribution and its dispersion depends on the degree of functionalization of the
support [1-3]. TEM, XPS, Raman spectroscopy, and low temperature nitrogen sorption were
used to control the structure of both supports and catalysts.
The activity of Co/CNT catalysts loaded with 15 wt.% of Co linearly depended on the
metal dispersion in the dCo range of 4–25 nm, which confirms the absence of interaction of
the catalyst with the support, even in the case of CNTs containing a large number of
functionalities. Despite the selectivity to C5+ hydrocarbons was found to be lower for
samples with dCo of about 4 nm than that for the catalysts with dCo larger than 5 nm, the
highest yield of this fraction was observed in the case of smaller particles. Thus, the total
amount of surface Co plays a more significant role in the formation of condensed
hydrocarbons than the quality of the active centres. At the same time, in the case of
simultaneously low Co content and a high degree of surface functionalization, the formation
of metal particles smaller than 3 nm with the amorphous structure was observed revealing
the metal-support interaction at the stage of catalyst preparation. Further studies showed
the absence of FTS activity over this catalyst and the formation of carbon shells around
amorphous Co particles. TEM images of Co catalyst supported on oxidized CNTs are shown in
Fig.1.
To investigate the support evolution Co was washed from the catalyst after annealing,
reduction and FTS. It was found that the annealing and reduction significantly change the
structure of CNTs which results in the increase of the SBET and total pore volume. The change
in the composition and content of functional groups was thoroughly studied It was found
that about 70% of oxygen had been removed by the beginning of the FTS. The methanation
of CNTs during reduction was demonstrated by TEM (Fig.2). Nitrogen-doped CNTs were also
studied as a support for Co catalysts in the FTS.