Carbon Nitride Composite Supported Cobalt Catalysts In Fischer-Tropsch Synthesis

The catalytic performance of catalysts for Fischer-Tropsch synthesis?FTS?was mainly influenced by active phases,promoters and supports,investigation on the relationship between catalytic performance and catalyst structure is beneficial for the selection of good FTS catalysts.In this paper,three-dimensional nitrogen-doped graphene aerogel?NGA?and carbon nitride-titania composite were synthesized and used as supports for cobalt-based catalysts,their structure and performance in Fischer-Tropsch synthesis were systematically investigated.The major results are as follows:1)Co/NGA catalysts with different cobalt loadings were one-step hydrothermal synthesized during the construction of 3D N-doped graphene aerogel?NGA?.Characterization and reaction results showed that higher cobalt dispersion was found on the catalyst with higher cobalt loading,and majority of cobalt species in the freeze-dried samples was fcc phase Co⁰.Excessive cross linking originated by high Co²⁺concentration during the construction of 3D NGA led to the restraint of mass transfer of reactants to cobalt active sites,and therefore,lower FTS activity was observed on the catalysts with higher cobalt loading.Cobalt loading after the construction of 3D graphene-based hydrogel could lead to the dispersion of cobalt particles on the surface of NGA,and hence obtained higher FTS activity.The fcc phase Co⁰ in fresh Co/NGA catalysts was transformed in to hcp phase Co⁰ during FTS reaction,and as a result,catalytic activity was increased with increasing time on stream.2)Urea was used as the precursor of carbon nitrate,and the carbon nitride-titania composite was synthesized by hydrothermal-thermal decomposition method.Supported cobalt catalysts were prepared by a typical incipient wetness impregnation?IWI?method.Characterization results showed that cobalt dispersion was increased but cobalt reducibility was reduced,and hence the FTS activity of corresponding catalyst was decreased with increasing C₃N₄ cotents.Owing to the existence of C₃N₄ layer,the formation of TiO₂-Co strong interaction during reaction was restrained,and thus good reaction stability was obtained on Co/TiO₂@xC₃N₄ catalysts.Plasma treatment could significantly improve cobalt dispersion,and meanwhile enhace the cobalt reducibility,as a result,Co/TiO₂@xC₃N₄-p catalysts had more number of active sites,and hence presented higher FTS activity and good stability.