Theoretical Studies On The Effect Of Crystal Structure Of Co Catalyst On Surface C Reaction Mechanism And Carbon Chain Growth Mechanism In F-T Synthesis

Cobalt-based catalysts are widely used in Fischer-Tropch synthesis?FTS?due to its high FTS activity.However,the diffusion and accumulation of surface C in FTS on Co-based catalysts blocked active sites,meanwhile,the penetration of surface C from surface of Co catalysts to the bulk phase leads to cobalt carbides,these issues lead to the deactivation of Co-based catalysts.Meanwhile,in the hydrogen-enriched FTS atmosphere,surface C could be eliminated through its hydrogenation to CH_x,the deactivation of Co catalysts could be inhibited;the formation of CH_x were favorable for carbon chain growth in FTS.On the basis of the complexities of FTS reactions and the structure diversity of Co catalyst crystal surface,the intrinsic microcosmic mechanism of the surface C reaction on Co catalyst and the effect of the crystal structure on carbon chain growth mechanism could not be determined.Therefore,the reaction mechanism of surface C?diffusion,accumulation,pernetration and hydrogenation?and the carbon chain growth mechanism on different crystal surfaces of HCP and FCC Co catalysts as well as the promoter-modified Co catalysts were systematically investigated using the quantum chemical density functional theory calculations.The effects of crystal phase and the crystal surface structure of Co catalysts on the surface C reaction mechanism and carbon chain growth mechanism as well as the effects of promoter-modified Co catalysts on surface C reaction mechanism were clarified.The main conclusions in this paper are obtained as follows:1.The microcosmic reasons for the deactivation of differen crystal phase Co catalysts were clarified:the pernetration of surface C was the main reason for the deactivation of HCP Co catalysts;the accumulation of surface C was the main reason for the deactivation of FCC Co catalysts.The type of crystal structures of resistance toward the deactivation of Co catalysts were screened:the main exposed crystal surfaces with the step B₅ active unit of HCP Co catalysts?Co?10-10?,?10-11?and?10-12?with the 75%exposure area of HCP Co catalysts?and?Co?311?,?110?and?100?with the 30%exposure area of FCC Co catalysts?of FCC Co catalysts could eliminate surface C through its hydrogenation reaction,and inhibit surface C accumulation and penetration,as a result,the ability of resistance toward the deactivation for these crystal surface structures of Co catalysts was stronger;while the Co?111?surface with the 70%exposure area of FCC Co catalysts and the Co?0001?surface with the 18%exposure area of HCP Co catalysts neither eliminated surface C,nor inhibited surface C accumulation and penetration,thus,the ability of resistance toward the deactivation for the two crystal structures of Co catalysts was poor;based on the proportion of these crystal structure in Co crystal phases,the ability of resistance toward the deactivation of HCP crystal phase Co catalysts was strong,while the FCC phase Co catalyst had poor resistance ability toward the deactivation.Important parameters judging the ability of resistance toward the deactivation of Co catalysts was obtained:surface C adsorption properties or Bader charge of surface C and the surface d-band center were the important indicators to describe the ability of resistance toward the deactivation of Co catalysts,the ability of resistance toward the deactivation of Co catalysts can be improved by increasing the adsorption ability of surface C and reducing the d-band center of crystal surface strctures of Co catalysts.2.The effect of the promoters Pt,Ru or B-modified Co catalysts on the surface C adsorption and surface C reaction mechanism?diffusion,accumulation,penetration and hydrogenation?were clarified:the adsorption ability of surface C was enhanced on Pt and Ru-modified FCC Co catalysts,the existence of Pt and Ru promoted surface C hydrogenation,and inhibited surface C diffusion,accumulation and penetration;B-modified FCC Co catalysts weakened surface C adsorption and promoted surface C hydrogenation,but surface C diffusion,accumulation and penetration could not be inhibited.Thus,the noble metal Pt and Ru promoters could inhibit the deactivation of FCC Co catalysts,while the non-metal B could inhibit the deactivation of FCC Co catalysts by blocking the active sites of surface C aggregation physically instead of inhibition the deactivation of Co catalysts by impacting surface C reaction.The ability of resistance toward the deactivation of FCC Co catalysts can be well regulated by noble metals Pt and Ru promoters.3.The carbon chain growth mechanism on HCP Co catalysts was clarified,among them,the carbon chain growth mechanism on Co?10-12?surface was obtained:RCH₂CH₂ was the main monomer,RCH₂CH₂ coupling with CH₂ via carbide mechanism to form R'CH₂CH₂?R'=H or RCH₂?can achieved the carbon chain growth,R'CH₂CH₂ hydrogenation into alkanes realized carbon chain termination.The effect of crystal surface structures of HCP and FCC Co catalysts on the key CH_x intermediate,carbon chain growth mechanism as well as the effect of methane,methanol and high carbon alcohols on the products were clarified:the crystal surface structures of Co catalysts affected the formation pathway and the main existence form of CH_x,the formation of methane affected the formation of C₂₊hydrocarbons,while the effect of methanol and high alcohols on C₂₊hydrocarbons formation could be neglected.The relationship between the crystal surface structures of Co catalyst and the carbon chain growth mechanism was obtained:the crystal surface structures?Co?10-10?,?10-11?and?10-12??with the step B₅ active unit can realized the carbon chain growth via carbide mechanism,the crystal surface structures?HCP-Co?0001?and FCC-Co?111??with the flat 4-fold active unit can achieved the carbon chain growth through CHO insertion mechanism,however,the instability of CHO limited its interaction with the main C_nH_x intermediate,which was not in favor of carbon chain growth.4.The active unit representing surface structure characteristic were proposed to adjust whether the surface structure can inhibite the deactivation and promote carbon chain growth or not:the crystal surface structures with the step B₅ active unit consisting of five Co atoms could inhibit the deactivation of Co catalyst and realize the carbon chain growth via the carbide mechanism,the crystal surface structures with the flat 4-fold active unit consisting of four Co atoms could not inhibit the deactivation of Co catalyst,the ability of resistance toward the deactivation could be enhanced by introducing Pt and Ru promoters,and achieved the carbon chain growth via the CHO insertion mechanism,which is not in favor of carbon chain growth.The microcosmic reasons is clarified that the surface C reaction mechanism?diffusion,accumulation,penetration and hydrogenation?and the carbon chain growth mechanism in FTS reaction on Co catalysts is sensitive to the cystal surface structure.These results could be used to modulate the crystal surface structures of Co catalysts,and provided the reliable theoretical clues for regulating the ability of resistance toward the deactivation of Co catalysts and improving the FTS activity for Co-based catalysts.