(206f) Cobalt Nanoparticles Supported on Graphene for Fischer-Tropsch Synthesis Conference: AIChE Annual MeetingYear: 2018Proceeding: 2018 AIChE Annual MeetingGroup: Catalysis and Reaction Engineering DivisionSession: Alternative Fuels Time: Monday, October 29, 2018 - 5:20pm-5:42pm Authors: Mignoli, T. R., University of São Paulo Hewer, T. L. R., University of São Paulo Schmal, M., University of São Paulo Alves, R. M. B., University of São Paulo Cobalt Nanoparticles Supported on Graphene for Fischer-Tropsch Synthesis Introduction 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 sites4,7. 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 Hummers 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. 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