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

doi:10.11949/0438-1157.20190065

Abstract

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

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

CLC Number:

TQ 032.4

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.

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.

Figures/Tables 9

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）

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

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)

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)

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

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

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)

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)

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

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

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

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

费托催化剂η-Fe₂C (011)上CH₄形成及C－C耦合机理研究

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