费托催化剂η-Fe2C (011)上CH4形成及C－C耦合机理研究

doi:10.11949/0438-1157.20190065

摘要/Abstract

摘要：

Fe₂C是低温Fe基费托催化剂的主要活性相，研究其上费托反应机理具有十分重要的意义。从原子尺度上通过密度泛函理论(DFT)计算研究了Fe₂C稳定晶相η-Fe₂C的(011)表面上甲烷形成和C—C耦合的反应机理。计算结果表明，η-Fe₂C(011)表面上甲烷形成的有效能垒为1.03 eV，其低于CH_i+CH_j耦合反应的有效能垒（1.52～2.98 eV），且最可能的C—C耦合反应路径为C+CH₃。进一步比较研究了η-Fe₂C(011)表面与其他Fe基费托催化剂表面之间的CH₄和C₂₊选择性差异，发现选择性高度敏感于Fe基催化剂的表面与体相结构，其中η-Fe₂C(011)表面具有较高的甲烷选择性。

关键词: DFT计算, 费托合成, η-Fe₂C催化剂, 甲烷形成, C—C耦合

Abstract:

Fe₂C is the main active phase of low temperature Fe-based Fischer-Tropsch catalysts. It is of great significance to study the mechanism of Fischer-Tropsch reaction. Herein, spin-polarized density functional theory(DFT) calculations are performed to investigate the mechanism of CH₄ formation and C—C coupling on the η-Fe₂C(011) surface, where this crystal phase is thermodynamically stable. The calculated results show that the effective barrier of CH₄ formation on such surface is 1.03 eV, lower than those of C—C coupling, and the C + CH₃ is the most likely C—C coupling pathways. Subsequently, the effective barrier difference between the CH₄ formation and C₁—C₁ coupling is used as a descriptor to compare the difference of the Ficher-Tropsch synthesis (FTS) selectivity between the η-Fe₂C(011) surface and other Fe-based catalysts surfaces. The FTS selectivity is found to be highly sensitive to the crystal phases and surfaces of Fe-based catalysts, and the η-Fe₂C(011) surface shows relatively high CH₄ selectivity.

Key words: DFT calculations, Fischer-Tropsch synthesis, η-Fe₂C catalyst, methane formation, C—C coupling

中图分类号:

TQ 032.4

宋楠, 潘敏建, 陈炳旭, 钱刚, 段学志, 周兴贵. 费托催化剂η-Fe₂C (011)上CH₄形成及C－C耦合机理研究[J]. 化工学报, 2019, 70(7): 2540-2547.

Nan SONG, Minjian PAN, Bingxu CHEN, Gang QIAN, Xuezhi DUAN, Xinggui ZHOU. CH₄ formation and C—C coupling mechanism on (011) surface of η-Fe₂C Fischer-Tropsch catalyst[J]. CIESC Journal, 2019, 70(7): 2540-2547.

图/表 9

图1 H（a）和CHx（b）在η-Fe2C(011)面上稳定吸附构型的俯视图和侧视图及该面上涉及甲烷化基元反应的过渡态构型的俯视图和侧视图（c）（蓝色：Fe；灰色：催化剂的表面C；绿色：涉及反应的表面C；白色：H；黄色：涉及反应的H）

Fig.1 Top and side views of favorable adsorption configurations of H (a) and CHx (b) on η-Fe2C(011) surface and corresponding top and side views of TSs of elementary steps involved in methanation (c)（blue: Fe atoms; grey: C atoms of iron carbide; green: C atoms involved in reactions; white: H atoms; yellow: H atoms involved in reactions）

表1 H在完美η-Fe2C(011)面上稳定吸附构型的吸附能和对应结构参数

Table 1 Key energetics and structural parameters of identified H adsorption on perfect η-Fe2C(011) surface

Site	r_Fe－H / nm	r_C－H / nm	E_ads /eV
2F	0.172,0.170		-2.22
3F	0.170,0.178,0.184		-2.27
4F		0.112	-2.03

表2 η-Fe2C(011) 面上涉及甲烷形成基元步骤的C—H距离（dC—H）、反应能垒（Ea）和反应能（ΔrE）（括号内数据经ZPE校正）

Table 2 C—H distances (dC—H) at TSs, reaction barriers (Ea) and reaction energies (ΔrE) involved in CH4 formation on η-Fe2C(011) surface (values including ZPE in parentheses)

Reaction	d_C—H /nm	E_a /eV	Δ_rE /eV
C+ H $→$ CH	0.142	0.49 (0.44)	0.15 (0.19)
CH + H $→$ CH₂	0.147	0.93 (0.83)	0.44 (0.50)
CH₂ + H $→$ CH₃	0.160	0.27 (0.25)	-0.41 (-0.31)
CH₃+ H $→$ CH₄(g)	0.157	0.56 (0.56)	-0.18 (0.08)

表2 η-Fe2C(011) 面上涉及甲烷形成基元步骤的C—H距离（dC—H）、反应能垒（Ea）和反应能（ΔrE）（括号内数据经ZPE校正）

Table 2 C—H distances (dC—H) at TSs, reaction barriers (Ea) and reaction energies (ΔrE) involved in CH4 formation on η-Fe2C(011) surface (values including ZPE in parentheses)

Reaction	d_C—H /nm	E_a /eV	Δ_rE /eV
C+ H $→$ CH	0.142	0.49 (0.44)	0.15 (0.19)
CH + H $→$ CH₂	0.147	0.93 (0.83)	0.44 (0.50)
CH₂ + H $→$ CH₃	0.160	0.27 (0.25)	-0.41 (-0.31)
CH₃+ H $→$ CH₄(g)	0.157	0.56 (0.56)	-0.18 (0.08)

图2 η-Fe2C(011)面甲烷形成势能图（有效能垒在图中由红色箭头标出）

Fig.2 Energy profiles of CH4 formation on η-Fe2C(011) surface (corresponding effective barriers were also presented)

图3 C—C耦合反应过渡态的构型

Fig.3 Structures of TSs of C—C coupling reactions

表3 η-Fe2C(011)面上C1+C1耦合反应的能垒（Ea）、反应能（ΔEr）、C－C的距离（dTS）以及相应的有效能垒（Eeff,CHi+CHj）（括号内数据经ZPE校正）

Table 3 C－C distances (dC－C) at TSs and reaction barriers (Ea), reaction energies (ΔrE) and effective activation energies (Eeff,CHi-CHj) of C1+C1 coupling reactions on η-Fe2C(011) surface (values including ZPE in parentheses)

Reaction	d_C－C/nm	E_a/eV	Δ_rE/eV	$E e f f, C H i - C H j$ /eV
C+C	0.222	3.00 (2.98)	1.15 (1.15)	3.00 (2.98)
C+CH	0.224	1.70 (1.74)	0.56 (0.60)	1.85 (1.93)
C+CH₂	0.201	1.17 (1.18)	0.33 (0.38)	1.76 (1.87)
C+CH₃	0.197	1.14 (1.14)	0.54 (0.58)	1.32 (1.52)
CH+CH	0.192	1.89 (1.90)	0.62 (0.68)	2.18 (2.29)
CH+CH₂	0.206	1.22 (1.19)	0.54 (0.57)	1.95 (2.08)
CH+CH₃	0.203	1.60 (1.63)	0.80 (0.84)	1.92 (2.21)
CH₂+CH₂	0.215	0.96 (0.98)	-0.02 (0.09)	2.13(2.36)
CH₂+CH₃	0.203	1.19 (1.27)	0.42 (0.57)	1.95 (2.34)

表3 η-Fe2C(011)面上C1+C1耦合反应的能垒（Ea）、反应能（ΔEr）、C－C的距离（dTS）以及相应的有效能垒（Eeff,CHi+CHj）（括号内数据经ZPE校正）

Reaction	d_C－C/nm	E_a/eV	Δ_rE/eV	$E e f f, C H i - C H j$ /eV
C+C	0.222	3.00 (2.98)	1.15 (1.15)	3.00 (2.98)
C+CH	0.224	1.70 (1.74)	0.56 (0.60)	1.85 (1.93)
C+CH₂	0.201	1.17 (1.18)	0.33 (0.38)	1.76 (1.87)
C+CH₃	0.197	1.14 (1.14)	0.54 (0.58)	1.32 (1.52)
CH+CH	0.192	1.89 (1.90)	0.62 (0.68)	2.18 (2.29)
CH+CH₂	0.206	1.22 (1.19)	0.54 (0.57)	1.95 (2.08)
CH+CH₃	0.203	1.60 (1.63)	0.80 (0.84)	1.92 (2.21)
CH₂+CH₂	0.215	0.96 (0.98)	-0.02 (0.09)	2.13(2.36)
CH₂+CH₃	0.203	1.19 (1.27)	0.42 (0.57)	1.95 (2.34)

图4 η-Fe2C (011)面上CHi+CHj偶联反应的能垒(Ea)、反应物的相对能量(Ei+Ej)以及有效能垒(Eeff)

Fig.4 Barriers, relative energy of reactants and effective barriers of CHi+CHj coupling on η-Fe2C(011) surface

表4 不同碳化铁表面上最可能的C1+C1耦合路径及其能垒

Table 4 Most possible routes and their activation barriers of C1+C1 coupling on different iron carbide surfaces

Surface		The most possible C—C coupling route	E_a/eV	Ref.
Fe	(210)	C+CH₃	1.10	[38]
	(100)	C+CH₂ C+CH₃	0.73	[31] [31]
	(100)	C+CH₂ C+CH₃	0.49	[31] [31]
χ-Fe₅C₂	(100)	C+CH₃	1.02	[26]
	(001)	C+CO	0.66	[39]
	(510)	C+CH CH+CH	1.09	[16] [16]
	(510)	C+CH CH+CH	0.96	[16] [16]
θ-Fe₃C	(031)	CH+CH CH₂+CH₂	0.76	[17] [17]
θ-Fe₃C	(031)	CH+CH CH₂+CH₂	0.50	[17] [17]
η-Fe₂C	(011)	C+CH₃ CH₂+CH₂	1.14	this work this work
η-Fe₂C	(011)	C+CH₃ CH₂+CH₂	0.98	this work this work

表5 不同碳化铁表面上甲烷形成和C1+C1耦合的有效能垒及其差值

Table 5 Effective barriers of CH4 formation and C1+C1 coupling and their barrier differences on different iron carbide surfaces

Surface	$E e f f, C H 4$ /eV	$E e f f, C 1 + C 1$ /eV	ΔE_eff/eV	Site	Ref.
Fe(210)	2.13	2.19	-0.06	step	[28]
Fe(100)	2.13	1.92	0.21	step	[24-45]
χ-Fe₅C₂(100)	1.89	1.94	-0.05	step	[26]
χ-Fe₅C₂(510)	2.39	1.66	0.73	terrace	[16]
θ-Fe₃C(031)	2.29	0.99	1.30	terrace	[17]
η-Fe₂C(011)	1.03	1.52	-0.49	step	this work

表5 不同碳化铁表面上甲烷形成和C1+C1耦合的有效能垒及其差值

Table 5 Effective barriers of CH4 formation and C1+C1 coupling and their barrier differences on different iron carbide surfaces

Surface	$E e f f, C H 4$ /eV	$E e f f, C 1 + C 1$ /eV	ΔE_eff/eV	Site	Ref.
Fe(210)	2.13	2.19	-0.06	step	[28]
Fe(100)	2.13	1.92	0.21	step	[24-45]
χ-Fe₅C₂(100)	1.89	1.94	-0.05	step	[26]
χ-Fe₅C₂(510)	2.39	1.66	0.73	terrace	[16]
θ-Fe₃C(031)	2.29	0.99	1.30	terrace	[17]
η-Fe₂C(011)	1.03	1.52	-0.49	step	this work

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费托催化剂η-Fe₂C (011)上CH₄形成及C－C耦合机理研究

CH₄ formation and C—C coupling mechanism on (011) surface of η-Fe₂C Fischer-Tropsch catalyst

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