Fischer-Tropsch synthesis [1] is used to convert syngas (CO + H2) into hydrocarbons. It is a crucial catalytic process for transforming coal, natural gas, and biomass into high quality liquid fuels and chemical resources. Iron-based catalysts possess the following advantages: wide operating temperature range, high space time yield and alkene yield, high water-gas shift activity, and low price [2]. They are studied and applied extensively [3].
Many transmission electron microscopy, X-ray diffraction, and Mössbauer spectroscopy experiments have shown that iron carbides, including Fe5C2 and Fe3C [4, 5, 6, 7, 8, 9], are the main active phases of iron-based catalysts in FTS. Although significant progress has been made in experimental and theoretical investigations in recent years, there remain some critical scientific issues not yet resolved. For example, how does the hydrocarbon chain grow [10]? How does alkene readsorption affect the product distribution? Until now, three chain propagation mechanisms have been proposed for FTS: carbide mechanism [11], hydroxy-carbene mechanism [12, 13], and CO insertion mechanism [14]. Many experimental and theoretical results have verified the existence of CxHy species on many metal surfaces [15, 16], which give strong support for the carbide mechanism.
In the traditional carbide mechanism, CH2 is considered the monomer for chain growth. However, different opinions have been proposed in recent years. Ciobica et al. [17] found that CH was the most stable C1 species on Ru(0001), and they suggested that CH was the chain propagation monomer, which agreed with the experimental results of Wu et al. [18, 19]. Liu et al. [20, 21] carried out density functional theory (DFT) calculations to investigate C-C coupling reactions on both flat and stepped Ru(0001) and stepped Rh(111), and also used CH as the carbon chain growth monomer on stepped Ru(0001) and Rh(111). Deng et al. [22] found that the most stable CHx species on Fe3C(100) was CH, and that it was mostly derived from the hydrogenation of surface C atoms, with a small quantity supplied by hydrogen-assisted CO disassociation.
Krishna et al. [23] studied the effect of the presence of C2H4 on the product distribution by using 13CO/H2 and 12C2H4. They found that the proportion of C3+ hydrocarbons in the product was increased by C2H4 readsorption. Cao et al. [24] carried out DFT calculations and proposed that the CCH species from the dehydrogenation of CCH2 and CHCH coupled with C atoms to form CCCH on Fe5C2(001). The experimental results of Turner et al. [25, 26] and Jordan et al. [27] indicated that vinyl species (CHCH2) participated in the formation of hydrocarbons as the chain growth monomer. Using the investigation of high molecular weight hydrocarbon formation on a Ru catalyst, Mims et al. [28] concluded that vinylidene (CCH2) was the stable monomer C2 species.
In this paper, we employed spin-polarized DFT and a periodic slab model to investigate the thermodynamics and kinetics of the adsorption, dehydrogenation, and cleavage of C2H4 on Fe3C(100). The aims were to elucidate the nature of C2H4 readsorption, what are the possible monomeric forms that take part in the chain propagation in FTS, and to give guidelines for further kinetic studies.
All calculations were performed using the plane wave periodic density functional method implemented in the Vienna ab initio simulation package (VASP) [29, 30, 31]. Exchange and correlation energies were calculated using the Perdew, Burke, and Ernzerhof (PBE) functional [32] in the generalized gradient approximation (GGA). The electron-ion interaction was described by the projector augmented wave (PAW) [33], and the Kohn-Sham one-electron states up to 400 eV were expanded using a plane wave basis set. Due to its large effect on a magnetic system, spin polarization was included for super- paramagnetic cementite (Fe3C) to correct the calculations.
A 1×3×1 K-point sampling within the Brillouin zone was used in the p(1×2) and p(2×2) unit cell of the periodic slab model to investigate the adsorption, dehydrogenation, and cleavage of C2H4 on Fe3C(100). The slab (without adsorbates) vacuum was set to span 1.5 nm to exclude the interactions between the periodic slabs. As illustrated in Fig. 1, a slab consisting of eight Fe layers and four C layers (8Fe + 4C) was employed.
In all calculations, the bottom six Fe layers and three C layers (6Fe + 3C) were fixed at their bulk position, while the top two Fe layers and one C layer were allowed to relax. For evaluating energy barriers, all transition states were located using the climbing image-nudged elastic band (CI-NEB) method [34]. All stable configurations and transition states were verified by vibrational frequency analysis. The adsorption energy was defined as Eads = E(adsorbates/slab) - [E(adsorbates) + E(slab)], where E(adsorbates/slab) is the total energy of the slab with adsorbates, E(adsorbates) is the total energy of the free adsorbates, and E(slab) is the total energy of the bare slab. The reaction energy and barrier were calculated by ∆rE = E(FS) - E(IS) and Ea = E(TS) - E(IS), where E(IS), E(TS), and E(FS) are the energies of the corresponding initial state (IS), transition state (TS), and final state (FS), respectively.
All possible C2H4 adsorption sites on the Fe3C(100) surface were considered systematically, and three adsorption modes, π, di-σ, and μ-bridging, were found. The calculated stable adsorption structures, key bond parameters, and adsorption energies are given in Fig. 2.
In configurations a1, a2, and a3, C2H4 adsorbs on the exposed first Fe layers by π coordination. The C and H atoms of C2H4 are in approximately the same plane, while both the two C atoms bond with the same Fe atom in the substrate to form a σ-π compound with a three-membered C-Fe-C ring structure [35]. The adsorption energies of three configurations are -0.39, -0.52, and -0.53 eV. In comparison with the C-C bond length (0.1330 nm) of C2H4 in gas phase, the C-C bond distance is elongated to 0.1426, 0.1414, and 0.1412 nm, indicating that the C-C bond strength is weakened. Because Fe3C(100) is a rough surface, adsorbed C2H4 in configuration a2 is tilted slightly, leading one of its C atoms to interact with another surface Fe atom to give the quasi di-σ adsorption configuration a5. In this configuration, the C-C bond length (0.1439 nm) is weakened further. The adsorption energy of -0.58 eV is close to those of a2 and a3, with small differences of -0.06 and -0.05 eV. In configuration a4, both C atoms bridge a first layer and a second layer Fe atom, forming a μ-bridging adsorption mode. It is the most stable structure for C2H4 on Fe3C(100) with the adsorption energy of -0.76 eV, in which the C-C bond is highly activated with the distance of 0.1480 nm, which is between that of C2H4 (0.1330 nm) and C2H6 (0.1540 nm). The four H atoms from the C-C plane have the H-C-C angle of 114°, which is between that of C2H4 (121°) and C2H6 (111°). This showed that in the μ-bridging mode, the C atoms of C2H4 have been partially rehybridized from sp2 to sp3 to give the C atoms a quasi-tetrahedron geometry.
In summary, ethylene adsorbs weakly on Fe3C(100) surface, and the μ-bridging adsorption mode with a highly activated C-C double bond is more stable than the π and di-σ adsorption modes, which is similar to the adsorption behavior of C2H4 on metal surfaces [36, 37]. At low coverages, ethylene C2H4 mainly adsorbs in the μ-bridging mode. At high coverages, all five C2H4 adsorption configurations can coexist. The highest occupied molecular orbital (HOMO) of ethylene is the bonding π orbital. The lowest unoccupied molecular orbital (LUMO) is the antibonding π* orbital. When adsorbed on Fe atoms, ethylene transfers part of its electrons of the bonding π orbital to the 3d empty orbital of Fe atoms. At the same time, the antibonding π* orbital energy is decreased to receive the backbonding electrons of the 3d orbital of the Fe atoms. Both interactions cooperatively enhance the interactions between ethylene and the surface, and thus weaken the ethylene C=C double bond, making the bond length elongated and the bond order lower.
Dehydrogenation and C-C cleavage are competitive reactions for C2H4 on Fe3C(100). It was reported that a number of surface reactions obey the Brönsted-Evans-Polanyi (BEP) relationship [38, 39], that is, the weaker adsorption configuration has the higher reactivity. On the other hand, the stronger adsorption configuration has the higher surface concentration. We chose the a1 configuration (Fig. 2a1) in the π adsorption mode and a4 configuration in the μ-bridging adsorption mode to investigate the stepwise dehydrogenation and cleavage of ethylene on the Fe3C(100) surface. Because the a2 and a3 configurations have the same adsorption modes as the a1 configuration, and the a5 configuration can be obtained from the a2 configuration, the a2, a3, and a5 configurations would have a similar reaction behavior as the a1 configuration. The reaction energies and energy barriers of all possible elementary reactions are given in Table 1. The key structures of the intermediates and transition states in the optimized dehydrogenation and cleavage pathways are displayed in Fig. 3.
Because four H atoms of C2H4 are in different surface environments, each dehydrogenation step is selective. From the energy data in Table 1, with configuration a1 as the starting point, the dehydrogenating energies for the loss of one H atom among the Ha-d (C2H4 → C2H3 + H) are quite different. Dehydrogenating Ha is kinetically most favored. It is slightly exothermic by 0.08 eV and has a barrier of 0.34 eV. In the transition state TS(a1/b1), the bond length of C1-Ha is elongated from 0.1095 nm in the initial adsorption state to 0.1541 nm. Ha occupies an on-top site Fe atom with the Fe-Ha length of 0.1611 nm. In the final state b1, the H atom adsorbs on the 3-fold site composed of first layer and second layer Fe atoms with the Fe-Ha bond lengths of 0.1744, 0.1784, and 0.1880 nm. The a1 cleavage process (C2H4 → 2CH2) was predicted to be exothermic by 0.18 eV and has an energy barrier of 1.55 eV. In the transition state TS(a1/c1), the C-C bond length is elongated to 0.2393 nm (0.1414 nm in the initial adsorption state). In the final state c1, the C-C bond is completely broken, and CH2 adsorbs on the deep hollow site composed of one first layer Fe atom and two second layer Fe atoms. For C2H4 (a1) on the Fe3C(100) surface, dehydrogenation occurs first to form an adsorbed vinyl CHCH2 species.
As compared with the first step of C2H4 dehydrogenation and cleavage, the corresponding energy barrier of b1 dehydrogenation (C2H3 → C2H2 + H) and cleavage (C2H3 → CH2 + CH) were decreased obviously (0.06 vs 0.47 eV, 0.55 vs 1.50 eV, and 0.67 vs 1.55 eV). Dehydrogenating the Hb atom is more favored kinetically. The CH group in CHCH2(b1) tilts towards to the surface, which promotes C-H activation. In the transition state TS(b1/b2), the bond length of C1-Hb is stretched from 0.1195 nm in adsorption state b1 to 0.1559 nm, and the Hb atom adsorbs on the top of a Fe atom with the Fe-Hb distance of 0.1602 nm. In the final state b2, the Hb atom resides on the 3-fold site composed of the first and second layer Fe atoms with the Fe-Hb distances of 0.1691, 0.1710, and 0.1897 nm. This reaction is exothermic by 0.27 eV and only needs a barrier of 0.06 eV to form the vinylidene species in the η3η1(C,C) adsorption mode. In the transition state TS(b1/c2) for the b1 cleavage reaction (C2H3 → CH2 + CH), the C-C bond length is elongated to 0.2474 nm (0.1400 nm in b1), and the barrier is 0.67 eV. In the final state c2, the C-C bond is totally broken to form CH and CH2 species on 3-fold sites. This process is exothermic by 0.35 eV.
Due to the approximate symmetry for configuration b2(CCH2) relative to the vertical surface and the C-C bond, dehydrogenating the Hc and Hd atoms have close reaction energies (0.80 and 0.80 eV) and barriers (1.10 and 1.12 eV). In the transition state TS(b2/b3), the C2-Hc bond is elongated from 0.1087 nm to 0.1696 nm, and Hc is located on the top site of a Fe atom with the Fe-Hc bond of 0.1533 nm. In final state b3, the acetylidene specie (CCH) is adsorbed in the η3η1(C,C) mode, and the Hc atom occupies the 3-fold site with the Fe-Hc bond of 0.1700, 0.1823, and 0.1971 nm. The cleavage reaction (CCH2 → C + CH2) for b2 was calculated to be endothermic by 0.81 eV with a barrier of 1.69 eV. In the transition state TS(b2/c3), the C-C bond length is elongated to 0.2325 nm (0.1401 nm in b2). In the final cleavage product c3, C and CH2 adsorbs at η3 sites composed of one first layer Fe atom and two second Fe atoms or two first layer Fe atoms and one second layer Fe atom.
From the energy profile of the dehydrogenation and cleavage of configuration a1 in Fig. 4, we can see that for C2H4 on Fe3C(100), dehydrogenation easily occurs, while C-C bond cleavage is not competitive. Vinylidene (CCH2) is the most stable C2 species on the surface, with the first and second dehydrogenation steps having low energy barriers of 0.34 and 0.06 eV, respectively. However, the following dehydrogenation of vinylidene (CCH2) needs to overcome a high energy barrier of 1.10 eV and it is endothermic by 0.80 eV. This indicated that vinylidene (CCH2) dehydrogenation was difficult under FTS conditions. In contrast, acetylidene (CCH) can be hydrogenated to form the vinylidene (CCH2) species, which is exothermic by 0.80 eV with a relatively low barrier of 0.30 eV.
Taking configuration a4 as the starting point, dehydrogenating the Ha atom (C2H4 → C2H3 + Ha) is kinetically the most favored first step with a total energy barrier of 0.58 eV, as illustrated in Table 1. At the beginning, Ha atom moves gradually far away from the C1 atom, tilting towards to the surface. In the transition state TS(a4/b1′), the bond length of C1-Ha is elongated to 0.1542 nm (0.1134 nm in the initial state). Meanwhile, the Ha atom resides on the top site with the Fe-Ha length of 0.1598 nm. In the final state b1′, Ha occupies the 3-fold site consisting of first layer and second layer Fe atoms with the Fe-Ha length of 0.1741, 0.1841, and 0.1851 nm. This step is endothermic by 0.06 eV. The a4 cleavage (C2H4 → 2CH2) was predicted to be endothermic by 1.13 eV and had a barrier of 1.40 eV. In the transition state TS(a4/c1′), the C-C distance is elongated from 0.1480 nm in the initial adsorption state to 0.2381 nm. In the final state c1′, the C-C bond is completely broken to form two CH2 groups in the 3-fold site and the bridge site. This indicated that on Fe3C(100), C2H4 (a4) preferred dehydrogenation to form the vinyl CHCH2 (b1′) species.
In the following step, dehydrogenating the Hb atom to form vinylidene CCH2 (b2′) presents a lower energy barrier (0.58 vs 0.62 eV) than dehydrogenating the Hc atom to form acetylene CHCH (b2′′). In the transition state TS(b1′/b2′), the C1-Hb bond is elongated to 0.1738 nm (0.1110 nm in b1′). Hb atom adsorbs on the top site of an adjacent surface Fe atom with the Fe-Hb bond length of 0.1546 nm. In the final state b2′, Hb atom occupies the shallow hollow site with the Fe-Hb bond lengths of 0.1704, 0.1877, and 0.1987 nm. The formation of vinylidene (CCH2) is endothermic by 0.08 eV. In the transition state TS(b1′/c2′) corresponding to the cleavage of configuration b1′ (C2H3 → CH2 + CH), the C-C bond is elongated to 0.1956 nm (0.1477 nm in b1′). In the final state c2′, the C-C bond is entirely broken. CH2 resides on the bridge site composed of one first and one second layer Fe atoms, while CH occupies the 3-fold site formed by two first layer Fe atoms and one second layer Fe atom. This step is endothermic by 0.38 eV and has an energy barrier of 0.74 eV.
The dehydrogenation product b2′ of configuration a4 adsorbs on the Fe3C(100) surface in the η2η3(C,C) mode, with the Hc and Hd atoms situated in different surface environments. The energy barrier of dehydrogenating Hc (CCH2 → CCH + Hc) differs from that of Hd atom (0.74 vs 0.92 eV). In the transition state TS(b2′/b3′), the C2-Hc bond is stretched from 0.1107 to 0.1706 nm, and the Hc atom moves away to the top site of a Fe atom with the Fe-Hc distance of 0.1648 nm. In the final state b3′, acetylidene (CCH) is located on the η3η3(C,C) site, and the Hc atom adsorbs on a 3-fold site formed by two first layer Fe atoms and one second layer Fe atom with the Fe-Hc bond length of 0.1721, 0.1873, and 0.1926 nm. This step is endothermic by 0.28 eV. b2′ cleavage (CCH2 → C + CH2) has an energy barrier of 0.88 eV and is endothermic by 0.81 eV. In the transition state TS(b2′/c3′), the C-C bond length is elongated to 0.2044 nm (0.1438 nm in b2′). In the final cleavage product c3′, C resides on the 4-fold site composed of two first and two second layer Fe atoms, and CH2 occupies the bridge site formed by one first and one second layer Fe atoms.
From the energy profiles in Fig. 5 for the dehydrogenation and cleavage of configuration a4 in the μ-bridging mode, we can see that the energy barrier for C-C cleavage is much higher than that for dehydrogenation, and thus dehydrogenation would occur. The three dehydrogenation steps have close energy barriers of 0.58, 0.58, and 0.74 eV, and are endothermic by 0.06, 0.08, and 0.28 eV, respectively. Vinyl (CHCH2) is the most abundant non-molecular C2 species on Fe3C(100).
The detailed mechanism of C2H4 adsorption, dehydrogenation, and cleavage on Fe3C(100) was investigated using spin-polarized DFT and a periodic slab model. C2H4 adsorbs on surface Fe atoms by both C atoms, and the μ-bridging adsorption mode is more stable than the π and di-σ adsorption modes. The interaction of C2H4 with the Fe3C(100) surface leads to the partial rehybridization of the C atoms of C2H4 (sp2→sp3), resulting in the geometry of the C atom in C2H4 changing to be quasi-tetrahedron. The ethylene (C2H4) adsorption energy correlates with the degree of bending deformation of methylene (CH2) on Fe3C(100). The four H atoms of ethylene (C2H4) are situated in different surface chemical environments, and each dehydrogenation step is different on Fe3C(100). At high coverages, all five C2H4 adsorption configurations coexist. Dehydrogenation of the π adsorption configuration a1 to form vinyl CHCH2 (b1) and vinylidene CCH2 (b2) is the most facile pathway. The first dehydrogenation step (C2H4 →CHCH2 + H) has a lower barrier of 0.34 eV and is slightly exothermic by 0.08 eV. The second dehydrogenation step (CHCH2 → CCH2 + H) only needs a barrier of 0.06 eV and is exothermic by 0.27 eV. The following dehydrogenation of vinylidene CCH2 (b2) needs to overcome a higher barrier of 1.10 eV, and it is also unfavorable thermodynamically. At low coverages, ethylene mainly adsorbs in the μ-bridging mode. The first two dehydrogenation steps of configuration a4 have the same energy barrier of 0.58 eV and are slightly endothermic by 0.06 and 0.08 eV. The third dehydrogenation step is endothermic by 0.28 eV with a higher energy barrier of 0.74 eV. Therefore, vinyl (CHCH2) and vinylidene (CCH2) are the most abundant C2 species on Fe3C(100), and it may be the major monomer of C2H4 that participates in the chain growth in FTS.
Acknowledgments
We greatly appreciate the equipment and fund support from Synfuels China Co., Ltd.
费托合成(FTS)[1]是合成气(CO + H2)经催化反应转化为碳氢化合物的过程, 是煤、天然气和生物质等含碳资源间接转化为高品位液体燃料和化工原料的一个关键步骤. Fe基催化剂具有操作温度范围宽、时空收率和烯烃产量较高、WGS反应活性高及价格低廉等特点[2], 因而得到了广泛的研究和应用[3].
大量透射电子显微镜、X射线衍射和穆斯堡尔谱研究表明, 铁碳化合物是铁基费托催化剂主要活性相, 包括Fe5C2和Fe3C等[4, 5, 6, 7, 8, 9]. 近年来费托合成的实验和理论研究取得了很大的进展, 但是仍有不少关键科学问题亟待解决, 如碳链是如何增长的[10], 烯烃再吸附又是怎样影响产物分布的. 文献报道的链增长机理主要有三大类, 分别是碳化物机理[11]、烯醇机理[12, 13]和CO插入机理[14]. CxHy物种在许多金属表面的存在[15, 16]为由Fischer和Tropsch提出的碳化物机理提供了强有力的实验和理论依据.
在传统的碳化物机理中, CH2被认为是最可能的链增长单体. 但近年来提出了许多不同的观点. Ciobica等[17]发现在Ru(0001)面, CH是最稳定的C1物种,据此提出CH是链增长的可能单体, 与Wu等[18, 19]的实验结果吻合. Liu等[20, 21]使用密度泛函理论(DFT)研究了平坦和台阶的Ru(0001)及台阶的Rh(111)面上C-C偶合反应, 发现CH可能是链增长单体. Deng等[22]提出Fe3C(100)表面最稳定的CHx物种是CH. 大部分CH是由表面C原子加氢而来, 少量由CO氢助解离提供.
Krishna等[23]使用13CO/H2和12C2H4研究了C2H4对链增长产物分布的影响, 发现C2H4再吸附可增加C3+烃类化合物在产物中的比例. Cao等[24]利用DFT计算并提出, 在Fe5C2(001)表面来源于CCH2和CHCH脱氢的CCH与C原子偶合形成CCCH. Turner等[25, 26]和Jordan等[27]的实验表明, 乙烯基CHCH2可能作为单体参与了碳氢化合物形成. Mims等[28]研究了Ru催化剂上高分子量碳氢化合物的形成, 提出亚乙烯基(CCH2)可能是C2物种的稳定单体.
本文采用DFT和周期平板模型, 从热力学和动力学角度研究了C2H4在Fe3C(100)表面的吸附、脱氢和裂解反应, 力求阐释烯烃再吸附及其参与链增长的可能形式, 为基于详细机理的动力学研究提供理论指导.
本文计算使用基于平面波DFT的VASP (Vienna Ab initio Simulation Package)[29, 30, 31]软件. 交换相关能用广义梯度近似(GGA)方法中的PBE (Perdew, Burke and Ernzerhof)[32]泛函来计算. 电子和离子相互作用通过缀加投影波PAW (projector augmented wave)函数来描述[33]. Kohn-Sham单电子采用平面波基组展开, 截断能设为400 eV. 由于自旋极化对于磁性材料的计算影响很大, 因此具有超顺磁性的Fe3C采用自旋极化的方法来计算.
本文分别使用p(1×2)和p(2×2)的周期平板模型来研究C2H4在Fe3C(100)表面的吸附和脱氢、裂解行为. 布里渊区采用1×3×1的K点描述. 为了消除层与层之间的相互作用, 真空层高度设为1.5 nm. 该模型是由8层Fe原子和4层C原子(8Fe+4C)组成的, 如图1所示.
在所有计算中均固定下面的6层Fe原子和3层C原子(6Fe+3C), 弛豫上面的2层Fe原子和1层C原子(2Fe+1C). 在计算能垒时, 采用CI-NEB方法搜寻过渡态[34]. 所有稳定的结构和过渡态均经频率分析进一步确认. 吸附能定义为: Eads = E(adsorbates/slab) - [E(adsorbates) + E(slab)], 其中, E(adsorbates/slab)是物种吸附在表面上的总能量, E(adsorbates)是自由物种的总能量, E(slab)是洁净表面的总能量. 反应前后的反应热及反应能垒分别用∆rE和Ea来表示: ∆rE = E(FS) - E(IS), Ea = E(TS) - E(IS). 其中E(IS), E(TS)和E(FS)分别代表反应物(IS)、过渡态(TS)和产物(FS)的总能量.
本文系统考察了C2H4在Fe3C(100)表面所有可能位点的吸附, 其吸附构型有π, di-σ和μ-bridging三种模式, 计算所得的稳定吸附构型、重要键参数和吸附能见图2.
构型a1, a2和a3中C2H4以π方式吸附在暴露的第一层Fe原子上. C2H4的C和H原子仍近似在同一个平面内, 此时两个C原子与底物的同一个Fe原子成键, 形成C-Fe-C三元环结构的σ-π复合物[35]. 三种构型的吸附能分别为-0.39, -0.52和-0.53 eV. 相对于气相C2H4分子的C-C键长(0.1330nm), 吸附后的C2H4的C-C键(0.1426, 0.1414和0.1412 nm)均有所拉长, 表明C-C键强度有所弱化. 由于Fe3C(100)表面并不平坦, 构型a2中吸附的C2H4分子略倾斜, 使得其中一个C原子和与表面C相连的Fe原子发生作用, 得到类di-σ吸附构型a5. 在该构型中, C-C键(0.1439 nm)被进一步弱化. 该构型的吸附能为-0.58 eV, 与a2和a3非常接近, 分别仅相差-0.06和-0.05 eV. 在构型a4中, C2H4的两个C原子均以桥式吸附在第一层和第二层Fe原子上, 形成 μ-bridging吸附. 其吸附能为-0.76 eV, 是C2H4在Fe3C(100)表面最稳定的吸附构型. 其C-C键长为0.1480 nm, 伸长活化程度最大, 介于C2H4 (0.1330 nm)和C2H6 (0.1540 nm)之间. 四个H原子明显背离C-C平面, ∠HCC为114°, 介于C2H4 (121°)和C2H6 (111°)之间. 由此可见, 以μ-bridging位吸附的C2H4分子中C原子的杂化方式由sp2部分转化为sp3, 使C原子呈近四面体结构.
综上所述, 乙烯分子在Fe3C(100)表面的吸附作用比较弱, 且μ-bridging吸附比π和di-σ方式稳定, C-C双键拉长活化程度最大, 与C2H4在纯金属表面的吸附性质相似[36, 37]. 在低覆盖度下, C2H4主要以μ-bridging吸附形式a4存在. 在高覆盖度下, C2H4的5种吸附形式都可能存在. 乙烯分子的最高占据轨道(HOMO)是成键π轨道, 而最低空轨道(LUMO)是反键π*轨道. 乙烯吸附在Fe原子上时, 将成键π轨道上的部分电子转移给Fe原子的3d空轨道. 同时, 其反键π*轨道能量降低, 接受Fe原子3d轨道的反馈电子. 两种作用都会增强表面与乙烯的作用强度, 削弱乙烯的C=C双键, 导致键长伸长, 键级降低.
C2H4在Fe3C(100)表面的脱氢和C-C裂解是竞争反应. 文献报道, 许多表面反应符合BEP关系[38, 39], 即吸附较弱的构型具有较强的反应活性. 另一方面, 吸附强的构型具有高的浓度. 因此, 我们选取了π吸附方式的a1构型(图2a1)及μ-bridging吸附方式的a4构型(图2a4), 考察了C2H4在Fe3C(100)表面的逐步脱氢和裂解过程. 由于构型a2, a3与a1的吸附方式相同, a5可由a2变形而得, 因此, a2, a3和a5应具有与a1相似的反应行为. 表1列出了各种可能基元反应的反应能垒和反应能, 每一步最优脱氢方式和裂解反应所涉及到的中间物种及过渡态结构见图3.
由于C2H4中的4个H原子所处的表面环境不同, 使得每一步脱氢都具有选择性. 由表1可见, 以构型a1为起始点, 脱去Ha-d中一个H原子(C2H4 → C2H3 + H)的难易程度存在很大差异. 其中, 脱去Ha是动力学最有利的过程, 其能垒为0.34 eV, 反应轻微放热0.08 eV. 在对应过渡态TS(a1/b1)中, C1-Ha键长由初始吸附态的0.1095 nm拉长到0.1541 nm, Ha原子以顶位方式吸附到Fe原子上, Fe-Ha键长为0.1611 nm. 终态b1中, H原子以三配位形式吸附在第一层和第二层Fe原子上, 其Fe-Ha键长为0.1744, 0.1784和0.1880 nm. a1裂解过程(C2H4 → 2CH2)的反应能垒为1.55 eV. 在过渡态TS(a1/c1)中, C-C键由初始吸附态的0.1414 nm伸长到0.2393 nm, 在终态c1中C-C键彻底断裂, CH2吸附在由一个第一层Fe原子和两个第二层Fe原子构成的三配位洞位上, C-C键断裂伴随着放出0.18 eV的热量. 由以上分析可知, C2H4(a1)在Fe3C(100)表面首先会发生脱氢反应生成吸附态的CHCH2物种(b1).
与C2H4第一步脱氢和裂解反应相比, b1对应的脱氢(C2H3 → C2H2 + H)和裂解过程(C2H3 → CH2 + CH)能垒显著降低(0.06相比于0.47 eV, 0.55相比于1.50 eV, 0.67相比于1.55 eV), 且Hb原子的脱除在动力学上最有利. CHCH2 (b1)中CH端向表面倾斜促进了C-H键的活化, 在相应过渡态TS(b1/b2)中, C1-Hb键长由中间吸附态b1的0.1195 nm伸长到0.1559 nm, Hb原子以顶位吸附在表面Fe原子上, Fe-H键长为0.1602 nm. 终态b2时, Hb原子以三配位形式吸附在第一层和第二层Fe原子上, Fe-H键长为0.1691, 0.1710和0.1897 nm. 该步反应能垒仅为0.06 eV, 放热0.27 eV, 形成η3η1(C,C)吸附的CCH2物种. 在b1裂解反应(C2H3 → CH2 + CH)的过渡态TS(b1/c2)中, C-C键长由0.1400 nm伸长到0.2474 nm, 反应能垒为0.67 eV. 在终态c2中C-C键彻底断裂, 形成三配位吸附的CH和CH2物种, C-C键断裂伴随着放出0.35 eV的热量.
相对于过C-C键的垂直面构型b2 (CCH2)近似对称, 因此脱除Hc和Hd (CCH2 → CCH + H)的能垒(1.10和1.12 eV)和反应能(0.80和0.80 eV)非常相近. 在对应过渡态TS(b2/b3)中, C2-Hc键由0.1087 nm伸长到0.1696 nm, Hc原子以顶位方式吸附到Fe原子上, Fe-H键长为0.1533 nm. 在终态b3中, CCH物种以η3η1(C,C)吸附, Hc原子以三配位形式吸附, Fe-H键长分别为0.1700, 0.1823和0.1971 nm. b2的裂解反应(CCH2 → C + CH2)能垒是1.69 eV. 在过渡态TS(b2/c3)中, C-C键由初始吸附态的0.1401 nm伸长到0.2325 nm. 在终态的裂解产物c3中, C和CH2吸附在分别由一个第一层Fe原子与两个第二层Fe原子和由两个第一层Fe原子与一个第二层Fe原子构成的η3位上, 反应吸热0.81 eV.
从构型a1脱氢和裂解的势能面曲线(图4)可以看出, C2H4在Fe3C(100)表面易于发生脱氢反应, C-C键裂解反应不具有竞争性, CCH2是最稳定的表面C2物种. C2H4脱第一个(0.34 eV)和第二个H原子的能垒(0.06 eV)都较低, 但CCH2继续脱氢是比较困难的, 需要克服较高的能垒(1.10 eV), 且吸热0.80 eV. 这表明在通常费托反应条件下, CCH2难以进行脱氢反应; 相反, CCH物种很容易加氢(能垒仅为0.30 eV, 放热0.80 eV)生成CCH2物种.
以构型a4为起点, 第一步脱去Ha (C2H4 → C2H3 + Ha)是动力学最有利的过程, 其能垒为0.58 eV (表1). 开始脱氢时Ha渐渐远离C1原子, 倾斜到表面与Fe原子成键. 在过渡态TS(a4/b1′)中, H原子以顶位方式吸附在表面Fe原子上, Fe-Ha键长为0.1598 nm, C1-Ha键长由初始吸附态的0.1134 nm伸长至0.1542 nm. 在终态b1′中Ha原子以三配位形式吸附在第一层和第二层Fe原子上, Fe-Ha键长分别为0.1741、0.1841和0.1851 nm. 整个反应是一个吸热0.06 eV的过程. a4的裂解反应(C2H4 → 2CH2)能垒为1.40 eV, 过渡态TS(a4/c1′)时C-C键由初始吸附态的0.1480 nm伸长到0.2381 nm, 在终态c1′中C-C键彻底断裂, 形成1个三配位和1个桥式吸附的CH2物种, C-C键断裂伴随着吸收1.13 eV的热量. 因此, C2H4 (a4)在Fe3C(100)表面首先发生脱氢反应生成吸附态的CHCH2物种(b1′).
CHCH2 (b1′)继续脱去Hb生成CCH2 (b2′)比脱去Hc生成CHCH (b2′′)的能垒略低(0.58相比于0.62 eV). 在b1′脱Hb原子的过渡态TS(b1′/b2′)中, C1-Hb键长由初始吸附态的0.1110 nm伸长到0.1738 nm, Hb与表面相邻的Fe原子成顶位吸附, 其Fe-Hb键键长为0.1546 nm. 在终态b2′中, Hb原子以三配位形式吸附在第一层Fe原子上, Fe-Hb键长分别为0.1704, 0.1877和0.1987 nm. 产物CCH2的生成需要吸收0.08 eV的热量. 在b1′裂解(C2H3 → CH2 + CH)过程的过渡态TS(b1′/c2′)中, C-C键长由初始吸附态的0.1477 nm伸长到0.1956 nm, 在终态c2′中C-C键彻底断裂. CH2与1个第一层Fe原子和1个第二层Fe原子成桥式吸附, CH与两个第一层Fe原子和1个第二层Fe原子成三配位吸附. 其反应能垒为0.74 eV, 反应过程需吸收0.38 eV的能量.
构型a4脱Hb后的产物b2′以η2η3(C,C)形式吸附在Fe3C(100)表面, 其Hc和Hd原子所处的表面环境不同, 因此脱除Hc和Hd (CCH2 → CCH + H )的难易程度不同, 其能垒分别为0.74和0.92 eV. 在对应过渡态TS(b2′/b3′)中, C2-Hc键由0.1107 nm伸长到0.1706 nm, Hc原子移动到Fe原子顶位上, Fe-Hc键长为0.1648 nm. 在终态b3′中, CCH物种以η3η3(C,C)吸附, Hc原子吸附在两个第一层Fe原子和一个第二层Fe原子构成的三配位上, Fe-Hc键长分别为0.1721, 0.1873和0.1926 nm. 生成产物CCH需要吸收0.28 eV的热量. b2′的裂解反应(CCH2 → C + CH2)能垒为0.88 eV. 在过渡态TS(b2′/c3′)中, C-C键由初始吸附态的0.1438 nm伸长到0.2044 nm. 在终态的裂解产物c3′中, C和CH2分别吸附在由两个第一层Fe原子与两个第二层Fe原子构成的四配位上和由第一层Fe原子与第二层Fe原子构成的桥位上, 反应吸热0.81 eV.
从μ-bridging吸附构型a4在Fe3C(100)表面进行脱氢和裂解反应的势能面曲线(图5)可以看出, C2H4的C-C裂解反应能垒远远高于脱氢反应, 因而后者更容易发生. 前三步脱氢反应能垒较接近(0.58, 0.58和0.74 eV), 且分别吸热0.06, 0.08和0.28 eV. CHCH2是表面最丰的非分子态C2物种.
采用DFT方法和周期平板模型, 对C2H4在Fe3C(100)表面的吸附及脱氢裂解进行了详细的考察. 研究表明, C2H4两个C原子吸附在Fe3C(100)表面的Fe原子上, 且μ-bridging吸附比π, di-σ吸附更加稳定. C2H4与Fe3C(100)面作用导致C2H4的C原子部分发生重新杂化(sp2→sp3), 使C原子呈近四面体结构. C2H4在Fe3C(100)表面的吸附能与其亚甲基CH2在表面弯曲变形程度有关系. 由于C2H4分子中4个H原子所处的表面化学环境不同, 使得每一步脱氢具有一定的选择性. 在高覆盖度下, C2H4的5种吸附形式都可能存在. π吸附构型a1更容易发生脱氢反应生成CHCH2(b1)和CCH2(b2). 第一步脱氢(C2H4 → CHCH2 + H)能垒为0.34 eV, 反应轻微放热0.08 eV. 第二步脱氢(CHCH2 → CCH2 + H)能垒仅为0.06 eV, 反应放热0.27 eV. CCH2(b2)继续脱氢具有较高的能垒(1.10 eV), 且热力学上也是不利的. 在低覆盖度下, C2H4主要以μ-bridging吸附形式a4存在. 构型a4的前两步脱氢能垒为0.58 eV, 轻微吸热0.06和0.08 eV, 第三步脱氢能垒略高(0.74 eV), 吸热0.28 eV. 综合以上分析, CCH2和CHCH2是Fe3C(100)表面最丰的C2物种, 或是C2H4参与链增长的主要单体形式.
致谢 感谢中科合成油技术有限公司在设备和资金上的支持.
2014, Vol. 35 Issue (1): 28-37 
PDF (823KB) 2014, Vol. 35 Issue (1): 28-37 PDF (823KB)