A new insight into the FT S mechanisms on Fe(100) surface

1. A new insight into the FTS mechanisms on Fe(100) surface FT Synthesis hpli 08-05-15

2. Outline • Motivation • Bird’s-eye picture of syn-gas reaction mechanisms & new insight • Recently theoretical progress on Fe(100) • Model selection • Results & future work

3. 国际能源供求竞争激烈 我国能源面临的问题: • 液体燃料短缺 • 能源供给和能源安全 • 环境污染 • 农村能源 • 二氧化碳排放

4. 31767 2005年我国石油缺口 1.36亿吨 29000 25200 24780 22838 22439 21073 19818 19692 17436 16065 18150 14210 14956 17500 16960 14099 14721 16700 16100 16396 13831 16074 16300 15733 16000 15005 14608 14524 13354 12384 11486 1990 91 92 93 94 95 96 97 98 99 2000 01 02 03 04 05年 石油生产量（万吨） 石油消费量（万吨） 液体燃料短缺和保障国家能源安全

5. 如何应对我国液态能源短缺 • 加强勘探，增加生产 • 利用国际资源 • 节能 • 接替石油 煤 天然气 生物质 • 制取液体燃料 • 发展不依赖石油的化工

6. 国内天然气资源状况 化石能源储采比?/年* 常规天然气资源量 （探明储量） 470亿吨油当量 煤层气 伴生气 水合物

7. 化 学 品 间接转化 经合成气 （CO＋H2） 液体燃料 天然气 直接转化 化 学 品 液体燃料 天然气的转化途径

8. Co/Al2O3(SiO2 , Zeolites) 应该写其它金属 F-T合成nCO + (2n+1)H2CnH2n+2 + nH2O nCO +2nH2 CnH2n + nH2O 合成甲醇CO + 2H2 CH3OH 合成乙醇CO + 4H2 CH3CH2OH + H2O（H2,CO供给比？） Cu/Zn/Al Rh/Nb/Si ZSM-5 -nH2O CO + H2反应的多样性 合成气不一定是从天然气来的。

9. 解离式加氢 Experimentally 1976, Araki 认为Ni表面上甲烷化反应开始于CO在多核活性位上的解离化学吸附，这种解离化学吸附是快速、无需帮助的，甲烷化的速率决定步是表面C连续加氢过程中的某一步。随后其他一些实验室在Co和Ru催化剂上也得到了同样 的结论 对于碳链增长机理，1980年, petit的实验认为在第VIII族金属催化剂上是通过CH2偶联或CHx组合进行的。 缔合式加氢 Experimentally 1985，蔡启瑞认为Araki等的实验固然清楚地显示表面C的加氢比非解离吸附的CO加氢快，但它同时也表明Cs的加氢比Hs存在下CO的解离快。因此否认了决速步是连续加氢中的某一步的观点。 1982， Mori的实验认为在Ni催化剂上甲烷生成的速率并非取决于Cs活CHx加氢的速率，而是受CO键的断裂所控制的，不论这个键的断裂是氢助或非氢助的。 Ho和Harriott的发现，用含少量的CO的H2作为反应原料气，其在Ni催化剂上甲烷化的速率明显高于H2不存在时CO的歧化的速率。缔合式机理容易解释此现象。 此外，动力学同位素效应实验也支持在Ni和Ru催化剂上的甲烷化和FTS反应支持缔合式机理。 Kelley等则认为CO插入对于碳链的增长可能更重要。 Winslow和Bell从他们的实验当中得出结论，在Ru催化剂上CO的插入对于碳链的生长，即使不占主导的话，也可以有重要贡献。 Bird’s-eye picture of syn-gas reaction mechanisms arguments 含少量H2的CO才对，核实一下 实验上怎么做的？得出那些结论？最近的 实验进展怎样？ 有那些一致和分歧的地方？ Tsai K R et al. 2nd C-U-J symp. On Heterogeneous Catalysis. Plenary Lecture A1. Ponec V. Catalysis Vol.5, Royal Soc.Chem. 1982. 48~79 Araki M et al. J.Catal. 1976, 44, 430 Sachtler J W A. et al. J.Catal. 1979, 56, 284. Mori T, et al. J.Phys.Chem. 1982, 86, 2753. Ho S V. et al. J.Catal. 1980, 64, 272. Wonslow P. et al. J.Catal. 1985, 91, 142. Kelley R D. et al. J.Catal. 1983,84,248.

10. Theoretically 目前几乎所有的理论研究（第一性原理，UBI-QEP）都默认从解离式机理开始。 P, Hu; T, Ziegler; Z, P, Liu; …… CHx+CHx mechanisms Theoretically Until…… Inderwildi and D.A.King. O. R. Inderwildi; D.A.King. et al. J.Phys.Chem.C. Letters. 2008,

11. Bird’s-eye picture of syn-gas reaction mechanisms RDS

12. Recently theoretical progress on Fe(100) ------ Methanation H2 is readily dissociated CH is the most stable species relative to CH2,CH3 D. C. Sorescu. Phys. Rev. B. 73, 155420, 2006

13. Recently theoretical progress on Fe(100) ------ C-C couplingCHx +CHx mechanism J M. H. Lo.; T. Ziegler. J. Phys. Chem. C. 2007, 111,13149

14. Direct formation of C2 from *C is not favorable due to its high Ea Ethane is more preferred to ethylene thermodynamically in the F-T synthesis Highly unsaturated -C species are more stable because of their high coordination to Fe surface Thermodynamic stability of C2 species Lo and Ziegler, J. Phys. Chem. C111, 13149 (2007)

15. summary • H2 is readily dissociated on Fe(100) surface, which agree with experimental results. • CH is the most stable species relative to CH2, CH3. • In methanation process, dissociation of CO is RDS. • Traditional widely accepted alkyl and alkene mechanism is forbidden on Fe(100) surface. • The more possible way to couple C-C bond maybe via C+CHx mechanisms . • Ethane is more preferred to ethylene thermodynamically on Fe(100) surface.

16. RDS CO dissociation • If association? Or partial dissociation? (high pressure, CO maybe difficult to decompose directly )

17. Model selection section • Bulk modeling • Layers & fixed layers • K-POINTS convergence • CO adsorption comparison • NEB methods introduction • VTST code new optimizers

18. Bulk simulation

19. How many layers? P(2×2)

20. K-Points Convergence Unit: eV

21. INCAR other important parameters NBANDS= 160 a proper value should be set. VOSKOWN=0 default is 0, whenever used PW91, set it 1. MAGMOM=26*3 associate magnetic momentum. IMAGES = 8 SPRING=-5 Transition state searching related. LCLIMB=.TURE. EDIFF = 1e-4 electronic step convergence criterion ALGO = very_FAST electronic iteration method. RMM-DIIS ISPIN = 2 spin polarized calculation (2-yes, 1-no) ENCUT= 400 cutoff energy EDIFFG =-2e-2 ionic optimization converge criterion IBRION = 3 1: quasi-New 2: CG 3: damped methods IOPT = 1 0, Use VASP optimizers specified from IBRION (default) 1, LBFGS = Limited-memory Broyden-Fletcher-Goldfarb-Shanno 2, CG = Conjugate Gradient. 3, QM = Quick-Min. 4, SD = Steepest Descent. new optimizers provided by VTST tools. ISMEAR = 1 methfessel-paxton smearing method SIGMA = 0.1 smearing width.

22. CO adsorption sites, θ=0.25 CO adsorption on Fe(100) surface Fe(100)

23. Ehollow>Etop>Ebridge

24. Nudged Elastic Band(NEB) & Climbing image NEB (CI-NEB) & VASP Transition State Tools and codes(VTST) & compared with Berny optimizer in Gaussian program are likely to be introduced next time. Affliction ………………………… TS • RDS of my work is to search Forces Converge failure frequently, looking for powerful optimizers.

25. Results 1.CO dissociation VS CO association at different coverage. 2.Thermodynamics of various species 3.Possible reaction mechanism

26. Chemisorption of CO: Kinetics Lateral interaction: crucial factor affecting the adsorption kinetics of CO Desorption barrier decreases with  Activation barrier increases with  CO is less strongly bound at higher  Lo and Ziegler, J. Phys. Chem. C111, 11012 (2007)

27. Dissociation of CO: Coverage dependence Lateral interaction: affects the CO dissociation Eact generally increases +0.06 kcal/mol C + O becomes less stable w.r.t. CO CO dissociation is suppressed at  = 0.75 ML Lo and Ziegler, J. Phys. Chem. C111, 11012 (2007)

28. Energy Profile at  = 0.25~0.5 ML 星号表示已做频率计算 It can be conclued that formation of HCO is faster than CO dissociation kinetically speaking CH maybe not only formed by hydrogenation of C, but also derived from dis of HCO

29. Energy Profile at  = 0.5~1.0 ML 2nd CO dissociation is both thermodynamically and kinetically unfavorable. 1st and 2nd HCO formation is kinetically favorable and the dissociation of them is also Thermodynamically gained.

30. Energy Profile at  = 0.75~1.0 ML S 效应 RCO与CO解离覆盖度影响

31. Oxgenates will be not likely to form 蔡启瑞等认为在Rh催化剂制乙醇的反应中， 乙烯酮为一个重要的中间体，在Fe(100)表面上 它的稳定性却不高。 Without lateral interaction Comes from where? Energy Profile at  = 0.25~0.5 ML

32. Conclusion • CH maybe not formed by hydrogenation of C, but derived from dissociation of HCO • At θ=0.25, formation and dissociation of HCO will compete well with CO dissociation • At higher coverage, formation and dissociation of HCO becomes more favorable both thermodynamically and kinetically. • Oxygenates and ketene is not likely formed due to its unstablility. Therefore, We support the C+CHx mechanism compared with other mechanisms.