费托催化剂η-Fe_2C(011)上CO吸附与活化行为
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摘要
低温Fe基费托催化剂的主要活性相是_η-Fe_2C,CO在该催化剂上的吸附与活化行为是Fe基费托合成反应的重要步骤。为了从原子尺度上研究这一过程,本文基于密度泛函理论计算,在η-Fe_2C(011)完美表面和缺陷表面上对CO的吸附和活化行为进行了系统的对比研究。计算结果表明,CO在完美表面上的最稳定吸附位为与表面Fe结合的Top位,但活化前驱体位于三齿空位。CO直接解离路径因为能垒太高在_η-Fe_2C(011)完美表面上很难发生,而以HCO为中间体的H辅助CO解离路径则更有优势。当_η-Fe_2C(011)表面产生C空缺时,其生成的四齿空位成为CO的最稳定吸附位和活性位。同时,CO的直接解离能垒大幅下降,这导致CO直接解离和以HCO为中间体的H辅助解离路径可能同时发生。
Fe_2C is discriminated as the active phase of the low-temperature Fischer-Tropsch synthesis(FTS).CO adsorption and activation on this catalyst is the key step during the Fe-based FTS.In order to gain insight to this process,spin-polarized density functional theory calculations were performed to investigate the CO adsorption and activation on both the perfect and defectiveη-Fe_2C(011)surfaces.The calculated results show that the most stable configuration of CO adsorption is the top site binding with Fe atom,while the precursor state for CO dissociation is the3Fsite.The direct CO dissociation can hardly occur due to the high CO dissociation barrier,and the H-assisted CO dissociation via HCO intermediate is proposed to be as the dominant activation pathway.Furthermore,with the formation of C-vacant site,the 4Fsite works as the most stable adsorption and activation site,with largely decreases of direct CO dissociation barriers,leading to the similar overall CO activation energy barriers for both direct and H-assisted CO dissociation via formation of HCO.Therefore,they may occur simultaneously with the C-vacant site over theη-Fe_2C(011)surface.
Fe_2C is discriminated as the active phase of the low-temperature Fischer-Tropsch synthesis(FTS).CO adsorption and activation on this catalyst is the key step during the Fe-based FTS.In order to gain insight to this process,spin-polarized density functional theory calculations were performed to investigate the CO adsorption and activation on both the perfect and defectiveη-Fe_2C(011)surfaces.The calculated results show that the most stable configuration of CO adsorption is the top site binding with Fe atom,while the precursor state for CO dissociation is the3Fsite.The direct CO dissociation can hardly occur due to the high CO dissociation barrier,and the H-assisted CO dissociation via HCO intermediate is proposed to be as the dominant activation pathway.Furthermore,with the formation of C-vacant site,the 4Fsite works as the most stable adsorption and activation site,with largely decreases of direct CO dissociation barriers,leading to the similar overall CO activation energy barriers for both direct and H-assisted CO dissociation via formation of HCO.Therefore,they may occur simultaneously with the C-vacant site over theη-Fe_2C(011)surface.
引文
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[2]温晓东,杨勇,李永旺,等.费托合成铁基催化剂的设计基础:从理论走向实践[J].中国科学,2017,47(11):1298-1311.
[3]ALIREZA A,KENNETH M B.A density functional theory based elementary reaction mechanism for early steps of Fischer-Tropsch synthesis over cobalt catalyst.2.Microkinetic modeling of liquid-phase vs.gaseousphase process[J].Molecular Catalysis,2017,436:210-217.
[4]ANDREI Y K,WEI C,FONGARLAND P.Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels[J].Chemical Reviews,2007,107:1692-1744.
[5]CHEN W,LIN T J,SUN Y H,et al.Recent advances in the investigation of nanoeffects of Fischer-Tropsch catalysts[J].Catalysis Today,2018,311:8-22.
[6]RAHIMPOUR M R,ELEKAEI H.A comparative study of combination of Fischer-Tropsch synthesis reactors with hydrogen-permselective membrane in GTLtechnology[J].Fuel Processing Technology,2009,90:747-761.
[7]JAHANGIRI H,BENNETT J,GU S,et al.A review of advanced catalyst development for Fischer-Tropsch synthesis of hydrocarbons from biomass derived syngas[J].Catalysis Science&Technology,2014,4(8):2210-2229.
[8]BAO L L,HUO C F,WANG L Y,et al.Structure and stability of the crystal Fe2C and low index surfaces[J].Journal of Fuel Chemistry and Technology,2009,37(1):104-108.
[9]ZHAO S,LIU X W,JAO H J,et al.Determining surface structure and stability ofε-Fe2C,χ-Fe5C2,θ-Fe3Cand Fe4C phases under carburization environment from combined DFT and atomistic thermodynamic studies[J].Catalysis,Structure&Reactivity,2015,1:44-59.
[10]YU X H,ZHANG X M,MENG Y,et al.CO adsorption,dissociation and coupling formation mechanisms on Fe2C(001)surface[J].Applied Surface Science,2018,434:464-472.
[11]HENDRIK J MONKHORST,JAMES D P.Special points for Brillouin-zone integrations[J].Physical Review B,1976,16(4):1748-1749.
[12]METHFESSEL M,PAXTON A T.High-precision sampling for Brillouin-zone integration in metals[J].Physical Review B,1989,40:3616-3621.
[13]KRESSE G,JOUBERT D.From ultrasoft pseudopotentials to the projector augmented-wave method[J].Physical Review B,1999,59(3):1758.
[14]JOHN P P,WANG Y.Accurate and simple density functional for the electronic exchange energy:generalized gradient approximation[J].Physical Review B,1986,33(12):8800.
[15]JOHN P P,KIERON B,MATTHIAS E.Generalized gradient approximation made simple[J].Physical Review Letters,1996,77(18):3865-3868.
[16]HENKELMAN G,JONSSON H.A dimer method for finding saddle points on high dimensional potential surfaces using only first derivatives[J].Journal of Chemical Physics,1999,111(15):7010-7022.
[17]ZHAO S,LIU X W,JIAO H J,et al.Morphology control of K2O promoter on haagg carbide(χ-Fe5C2)under Fischer-Tropsch synthesis condition[J].Catalysis Today,2016,261:93-100.
[18]SCHULZ H.Short history and present trends of Fischer-Tropsch synthesis[J].Applied Catalysis A:General,1999,186(1):3-12.
[19]BILOEN P,HELLE J N,SACHTLER W M H,et al.The role of rhenium and sulfur in platinum-based hydrocarbon-conversion catalysts[J].Journal of Catalysis,1980,63(1):112-118.
[20]CLAEYS M,STEEN E.Basic studies[J].Studies in Surface Science and Catalysis,2004,152:601-680.
[21]BURTRON H D.Fischer-Tropsch synthesis:current mechanism and futuristic needs[J].Fuel Processing Technology,2001,71(1):157-166.
[22]SHARAN S,ANTONIUS P J,RUTGER A.van Santen.Direct versus hydrogen-assisted CO dissociation[J].Journal of the American Chemical Society,2009,131(36):12874-12875.
[23]OLIVER R I,DAVID A K,STEPHEN J J.FischerTropsch synthesis of liquid fuels:learning lessons from homogeneous catalysis[J].Physical Chemistry Chemical Physics,2009,11:11110-11112.
[24]CHEN B X,DUAN X Z,CHEN D,et al.Chargetuned CO activation over aχ-Fe5C2 Fischer-Tropsch catalyst[J].ACS Catalysis,2018,8(4):2709-2714.
[25]THANTH H P,DUAN X Z,CHEN D,et al.CO activation pathways of Fischer-Tropsch synthesis onχ-Fe5C2(510):direct versus hydrogen-assisted CO dissociation[J].The Journal of Physical Chemistry C,2014,118:10170-10176.
[26]MOHAMMAD R E,MANUEL P J,HANS N.Direct versus hydrogen-assisted CO dissociation on the Fe(100)surface:a DFT study[J].ChemPhysChem,2012,13(1):89-91.
[27]HUO C F,LI Y W,JIAO H J,et al.Insights into CH4formation in iron-catalyzed Fischer-Tropsch synthesis[J].Journal of the American Chemical Society,2009,131(41):14713-14721.
[28]MELISSA A P,WERNER J R.CO dissociation at vacancy sites on Haagg iron carbide:direct versus hydrogen-assisted routes investigated with DFT[J].Topics in Catalysis,2015,58(10/11):665-674.
[29]LIU J X,ZHANG B Y,LI W X,et al.Crystallographic dependence of CO activation on cobalt catalysts:HCP versus FCC[J].Journal of the American Chemical Society,2013,135:16284-16287.