(206f) Cobalt Nanoparticles Supported on Graphene for Fischer-Tropsch Synthesis

Fischer-Tropsch synthesis consists in the
catalytic conversion of synthesis gas (H₂ and CO) and it is a
promising alternative to produce high quality hydrocarbon fuels. Although
several studies around the Fischer-Tropsch synthesis (FTS), making the process
viable economically is still a challenge. Aspects such as activity,
selectivity, especially for heavier hydrocarbons, and the cost of the catalysts
currently employed should be further studied in order to facilitate the process
on a profitable industrial scale. Since early works, it is widely accepted that
Group VIII metals are very active in CO hydrogenation, mainly Co and Fe ^1,2. Co-based catalysts are preferred,  since it is more active than Fe-based
ones and require lower reaction temperature ¹. Some inorganic supports with
high surface area such as silica, alumina and niobium oxide have been used to
increase cobalt dispersion. FT synthesis over cobalt on alumina catalysts
allows the production of long chain alkenes even at atmospheric pressure.
Furthermore, Co/Al₂O₃ catalysts are well known to produce
linear hydrocarbons, with high selectivity for heavy hydrocarbons, and have low
activity for water gas shift reaction. However, the formation of irreversible
cobalt-aluminates during pretreatment and under reaction conditions decreases
the catalytic activity due to the loss of active cobalt metal for catalyzing
the reaction³. The carbon nanomaterials have
gained prominence in the last years due to its notable properties, such as high
mechanical resistance, high superficial area, thermal stability and high
electronic conductibility, making these materials promising supports for
heterogeneous catalysts⁴. Recent studies also indicate
that in catalysis metal nanoparticles supported on carbon nanomaterials used in
FTS showed high activities for C₅₊, as well as low selectivity for
the formation of methane and CO₂ ⁴. Promoting effects due to the
presence of a second metal over reducibility, activity and stability of cobalt
catalyst have been reported in numerous studies ⁵. Rare earth promoters such as
La, Ce, Pr and Sm are also investigated, since they may improve Co-based catalysts
performance, decreasing methane production, carbon dioxide and C2-C4 products
and increasing the selectivity for C5+ and catalyst stability⁶. In this context, graphene has
attracted the attention of the scientific community due to its unique properties,
such as high mechanical and thermal resistance, high electron mobility, high
surface area and availability of sites^4,7. High surface graphene as a
support for cobalt allow a better nanoparticles dispersion and its surface
defects may be sites for adsorption of active species for catalysis process.
Given this background, this work is focused in designing a cobalt-based
catalyst promoted with lanthanum and supported on graphene for Fischer-Tropsch
Synthesis. The materials were characterized by different techniques such as
surface area measurement, XRD, Raman spectroscopy, HRTEM for structure and
properties verification. The promotion effect was investigated by Diffuse
Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Methods

Graphene synthesis process was an
adaptation of improved Hummer’s method. Graphite flakes were chemically
oxidized with sulfuric acid, phosphoric acid and potassium permanganate for 8
h. The graphite oxide (GO) obtained was purified and subsequently exfoliated
and thermally reduced. The final support was denoted as reduced graphene oxide
(rGO). Graphene-supported cobalt catalyst was prepared with a cobalt loading of
10 wt%. and was synthesized by deposition  precipitation  method, using Co(NO₃)₂.6H₂O
diluted in ethanol as precursor solution and ammonium hydroxide as
precipitating agent. The catalyst was dried and calcined at 450 °C for 3 hours
under Ar flow.. The materials were characterized by different techniques such
as surface area measurement, XRD, Raman spectroscopy, HRTEM for structure and
properties verification. The catalysts was investigated by Diffuse Reflectance
Infrared Fourier Transform Spectroscopy (DRIFTS). Results

The products of the graphene synthesis were
characterized by Raman spectroscopy, the results are showed in Figure 1. It may be noticed the presence of
D-band (ca. 1350 cm^-1) and G-band (ca. 1580 cm^-1)
in rGOs and GOs. The occurrence of D-band is correlated with significant number
of defects and sp³ hybridization domains whereas G-band is
associated to graphitic structure and sp² hybridization domains ^4,8,9. After oxidation process,
D-band appears in GO profile, indicating the presence of defects caused by
oxygenated functions between graphene oxide layers. D/G intensity ratio is an
important parameter for understanding the structure changes in oxidation and
reduction processes. A raise in D/G ratio after reduction is observed in all
rGO if compared with GO profile. This suggests a reduction of sp² domains due
to reduction of GO and an increase of disorder degree in rGOs ⁹.

Figure 1: Raman spectra for reduced graphene oxide
(RGO), graphite oxide and graphite.

The nitrogen isotherms obtained for the
material are shown in Figure 2.
The specific surface of the materials obtained during the graphene synthesis increase
according the sequence graphite<graphite oxide<graphene oxide<reduced
graphene oxide, the data were 6, 59, 352 and 494 m²/g, respectively.
The graphite flakes adsorption isotherm is classified as Type VI accord with
the IUPAC (International Union of Pure and Applied Chemistry)¹⁰. This kind of isotherm
represents stepwise multilayer adsorption on a uniform non-porous surface,
characteristic of graphite material. After the oxidation step, the shape
isotherm of the graphite oxide change for Type IV with hysteresis loop, which
is associated with capillary condensation taking place in mesopores. The
graphite oxide has Type H4 hysteresis loop, that is generally observed with
particles with narrow slit-like pores and the mean pore diameter was 4 nm, the
pore volume was 0,024 cm³g^-1.
Thus, when the oxygen atoms are introduced on the layers of the graphite, the
interlayer distance increase, so is becomes possible the adsorption of the
nitrogen molecules between the graphite sheets.

Figure 2: Nitrogen adsorption-desorption isotherm of
the materials obtained in different steps of the reduced graphene oxide
synthesis by the thermal method. A: Graphite flakes, B: graphite oxide, C:
graphene oxide, D: reduced graphene oxide by thermal method (rGOt).

Graphene oxide and reduced graphene oxide
produced by thermal methods shown similar nitrogen adsorption/desorption
isotherms. The adsorption isotherms are classified as Type IV, typical for
mesoporous materials and the hysteresis loop, Type H3, are characteristic of
plate-like particles giving rise to slit-shaped pore. These types of adsorption
isotherms and hysteresis loop are in agreement with the characteristic expected
for graphene oxide and rGOt prepared by thermal method. The pore size of the graphene
oxide and reduced graphene oxide still similar to the value observed for
graphite oxide, around 4 nm. But, after the thermal expansion, exfoliation, and
thermal reduction the pore volume increase from 1.4 to 1.6 cm³g^-1,
which is an indication that the leaves were separated. The X-ray diffraction
pattern of the 10% Co/rGOt is in Figure 3A. All the indentified peaks for 10% Co/rGOt nanocomposite were
assigned to cobalt monoxide with cubic crystal struture (cF8) with space group
Fmm .  The CoO was the the only detected in the sample
with any contribution of Co3O4 phase.

The morphology of 10% Co/rGO nanocomposite
was examine using transmission electron microscopy. A typical micrograph is
presented in Figure 3B. The
figure shows a thin graphene sheet embedded with Co nanoparticles (higher
electron density portions on the graphene leaves). The weak contrast that the
rGO sheet in very thin. The shape of the Co nanoparticles are spherical and
highly disperse on the graphene surface. The cobalt oxide nanoparticles shows
the diameter sizes around 17 nm.

Figure 3: . A-) XRD patterns of rGO and 10%
Co/rGO compared to CoO profile. B-) TEM image of the a10% Co/rGO sample.

The nitrogen adsorption/desorption
isotherms confirmed the mesoporous structure of the synthesized  rGO and 10%
Co/rGO materials and the surface area of the materials were around 400 m²g^-1. Conclusion

                The development of a high performance catalyst for FTS, mainly with
high activity and selectivity for high-chained hydrocarbons, is essential for
economic feasibility of this technology compared to tradition fuel production.
The characterization results  shows that the rGO and 10% Co/rGO was
successfully prepared by the proposed synthesis method. References

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