A Theoretical Study On Effects Of MnO On Syngas To Olefin Over Co-based Catalyst

Olefins are vital raw materials.The technologies for gasification or reforming of methane,coal and biomass to syngas are matured,and the development of syngas to olefin technology is of great significance.Co-based catalysts have the advantages of high reactivity,good stability,resistance to carbon deposition,and low activity of water-gas shift reaction,etc.The research used to catalyze the synthesis of olefins from synthesis gas has attracted more and more attention.The research of Co-based catalysts for catalyzing syngas to olefin has attracted more and more attention.Mn-modified Co-based catalysts show higher olefin selectivity and lower CH₄selectivity,while promotion mechanism of Mn additive is deficient.The Mn additive mainly exists in the form of MnO under the conditions of syngas to olefin.In this dissertation,density functional theory（DFT）and kinetic Monte Carlo method（kMC）were applied to study the effects of MnO on CO dissociation,CH₄ formation,C₁-C₁ coupling,CH₂CH₂formation and CH₃CH₃ over Co surface aiming to reveal the promotion mechanism of MnO on syngas to olefin over Co catalyst.The Co（0001）and MnO/Co（0001）models were constructed and the electronic properties of the two models were investigated.It was found that electrons transfers from Mn atoms to neighboring Co atoms after the MnO clusters were added on Co（0001）,meanwhile,the d-band center of the Co adjacent to Mn shifts away from the Fermi level,which is conducive to weakening the bonding strength between the Co atom and the adsorbates.With DFT method,the mechanism of CO dissociation and O removal on Co（0001）and MnO/Co（0001）were investigated.It was found that CO dissociation proceeds preferentially via the H-assisted pathway via CHO intermediate on both Co（0001）and MnO/Co（0001）.And the addition of MnO accelerates the dissociation of CO on cobalt surface.The adsorbed O tends to be removed in the form of H ₂O,and OH disproportionation is the predominant path for H₂O generation.With DFT method,the hydrogenation of C or CH generated by CO dissociation to CH₄ and the process of C₁-C₁ coupling between CH_x（x=0～3）were investigated.It was found that hydrogenation of CH₃ to CH₄ possess the highest activation energy among CH_x（x=0～3）hydrogenation reactions.Four possible pathways for C₁-C₁ coupling to achieve chain growth were proposed on both Co（0001）and MnO/Co（0001）.The activation energies of CH₃ hydrogenation are increased and the activation barrier of chain growth process are decreased with the addition of MnO on Co（0001）.With DFT method,the hydrogenations of C₂H_x（x=0～5）species into ethylene or ethane on Co（0001）and MnO/Co（0001）were investigated.Five possible pathways for ethylene generation and five possible pathways for ethane generation are proposed.Adsorbed CH₂CH₂ tends to hydrogenate to form CH₃CH₃ on Co（0001）.The addition of MnO promotes the desorption of the adsorbed CH₂CH₂ and inhibits CH₂CH₂hydrogenation to CH₃CH₃ on Co（0001）.With kMC method,the process of CO hydrogenation into methane,ethylene,and ethane was investigated.The main reasons why the addition of MnO reduces the selectivity of CH₄ on Co（0001）are as follows.On the one hand,the activation energy of CH₃ hydrogenation to CH₄ on MnO/Co（0001）is higher than that on Co（0001）.On the other hand,the coverage of CH_x（x=0～3）is increased with MnO addition,which is conducive to the C₁-C₁ coupling and promotes the generation of C₂ species.Paths for ethylene and ethane formation on Co（0001）and MnO/Co（0001）were determined,and contribution of each main path to the formation of the target product were defined.Desorption of CH₂CH₂ and hydrogenation of CH₂CH₃ are key factors affecting the selectivity of CH₂CH₂ or CH₃CH₃ on both Co（0001）and MnO/Co（0001）.The addition of MnO promotes the desorption of CH₂CH₂,and suppresses hydrogenation of CH₂CH₃on Co（0001）,resulting in a higher olefin/paraffin ratio（O/P）in product.