Adsorption and removal mechanism of atomic oxygen on different facets of θ-Fe3C
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摘要: 碳化铁θ-Fe3C作为费托反应活性相之一，其氧化造成了催化剂的严重失活。探究氧原子在碳化铁晶面的吸附及移除机理有助于理解氧化过程，为提高催化剂的稳定性提供参考。本工作通过理论计算研究了低覆盖度下氧原子在θ-Fe3C不同晶面的吸附，其在（110）晶面吸附最强，（001）晶面次之，（011）晶面吸附最弱，即（110）容易氧化。原子热力学研究表明，增大H2O分压或降低温度有利于氧原子吸附，容易造成表面氧化。此外，在典型费托反应条件下（110）晶面氧原子的覆盖度最高，进一步证明该晶面易氧化，与低覆盖度吸附结果一致。对移除路径进行计算得出，（011）晶面吸附氧原子直接与CO反应以CO2方式移除能垒较低（0.84 eV）；（001）与（110）晶面吸附氧原子主要通过OH歧化以H2O方式移除，但后者形成O-H键需要克服的能垒更高（1.72 vs 1.47 eV）。Abstract: The Fisher-Tropsch Synthesis (FTS) is one of the most intensively studied reactions in heterogeneous catalysis, which could convert the syngas (CO and H2) from coal, natural gas, shale gas, and biomass into gasoline, diesel, and a series of important chemical products. The process provides an effective strategy for reducing environmental pollution caused by direct coal combustion, at the meanwhile, alleviating dependence on imported petroleum. During the reaction, iron-based catalysts have attracted the attention of numerous researchers due to their unique advantages, for example, low cost, flexibility in product distribution, suitability for low H2/CO ratio syngas and large operating space. And the iron-based catalysts in FTS have achieved applications successfully in industry. Iron carbide is recognized as active phases in FTS catalyzed by iron-based catalysts, among which θ-Fe3C is one of the dominant phases. Additionally, θ-Fe3C is widely applied in fields such as biomedicine and electrochemistry. However, θ-Fe3C could be oxidized under realistic conditions, affecting the performance as magnetic materials and catalysts seriously. Considering the case above, the investigation of oxidation of iron carbide θ-Fe3C is of great significance. And exploring the adsorption, as well as removal mechanism of atomic oxygen over the iron carbide facet is helpful for understanding the oxidation process, and providing a reference for improving the stability of catalysts. In this work, we have explored the adsorption of oxygen atoms from low to high coverage on three θ-Fe3C surfaces with density functional theory. Ab initio atomistic thermodynamics was utilized to investigate the effect of experimental conditions like temperature and partial pressure of H2O. According to the calculation, it was found that the adsorption at low coverage on (110) was the strongest, followed by (001), and the adsorption on (011) was the weakest, meaning that (110) can be oxidized easily. As the number of oxygen atoms adsorbed on the surface increases, the stepwise adsorption energy increases, which is a manifestation of the repulsive effect between adsorbed oxygen atoms. The average adsorption energy for each surface increases with the increase of adsorbed oxygens, and the magnitude of increase varies due to the different local structures of each facet. The average adsorption energy on (011) has a relatively small increase, indicating that the repulsion between oxygen atoms adsorbed on this crystal plane is weak; whereas opposite on (001) and (110) facets. Atomistic thermodynamic studies showed that increasing the partial pressure of H2O or decreasing the temperature will stabilize the O adsorption, leading to surface oxidation. In addition, the highest O coverage on (110) under typical FTS conditions further proved that the facet is easily oxidized, which is consistent with the adsorption results at low coverage. Finally, the removal path of adsorbed oxygen on different facets was calculated, and the results showed that adsorbed O on (011) prefer to react with CO with energy barrier of 0.84 eV. On (001) and (110), removal in the form of H2O via OH disproportionation is more favored, but the energy barrier to form O−H bond is higher for the latter facet (1.72 vs 1.47 eV).
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Key words:
- θ-Fe3C /
- oxidation /
- adsorption of atomic O /
- removal mechanism
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图 1 碳化铁θ-Fe3C各表面模型的俯视图与侧视图
Figure 1 Schematic top and side views of the θ-Fe3C surface modelsBlue balls for Fe atoms and grey balls for C atoms.
(a): (001); (b): (011); (c): (110).
图 2 低覆盖度下氧原子在各晶面吸附的最稳定构型图
Figure 2 Stable adsorption configurations of the single oxygen atom on three surfacesblue for Fe; black for C; and red for O.
(a): (001); (b): (011); (c): (110).
图 3 平均吸附能随氧原子数目的变化关系
Figure 3 Average adsorption energy (Eads-ave, eV) as a function of O number (On)
图 4 饱和覆盖度下氧原子在各晶面吸附的最稳定构型
Figure 4 Stable adsorption configurations of atomic oxygen in the saturated state on three surfacesblue for Fe; black for C; and red for O.
(a): (001); (b): (011); (c): (110).
图 5 Fe3C（001）晶面氧原子吸附吉布斯自由能随（a）温度，（b）H2O与H2分压比的变化
Figure 5 Relationship between Gibbs free energy change and (a) temperature, (b) partial pressure ratio of H2O/H2 in($p_{{\mathrm{H_2O}}}/p_{{\mathrm{H}}_2} $) at different deposited O atoms (nO) on (001)
图 6 Fe3C各晶面氧原子吸附的热力学平衡相图
Figure 6 Phase diagrams for the oxygen adsorption state (θ, nO/nm2, n in parentheses)
(a): (001); (b): (011); (c): (110).
图 7 Fe3C（001）晶面吸附氧原子移除的反应势能面图与过渡态结构的正、侧视图
Figure 7 Energy profiles for O removal and front and side views of transition states on Fe3C(001) surface blue for Fe; black for C; red for O; and white for H.
图 8 Fe3C（011）晶面吸附氧原子移除的反应势能面图与过渡态结构的正、侧视图
Figure 8 Energy profiles for O removal and front and side views of transition states on Fe3C(011) surface blue for Fe; black for C; red for O; and white for H.
图 9 Fe3C（110）晶面吸附氧原子移除的反应势能面图与过渡态结构的正、侧视图
Figure 9 Energy profiles for O removal and front and side views of transition states on Fe3C(110) surface blue for Fe; black for C; red for O; and white for H.
表 1 氧原子在各晶面吸附的相关信息，包括吸附个数（On）、吸附位点 （site）及逐步吸附能 （∆Eads, eV）
Table 1 Information of O adsorption on three surfaces, including the number of adsorbed O (On), adsorption site and stepwise adsorption energy (∆Eads)
Surface On Site ∆Eads (001) 1 3F −0.94 2 3F −0.93 3 3F −0.89 4 3F −0.88 5 3F −0.39 6 3F −0.46 7 3F −0.26 8 3F −0.26 9 CTa 0.16 (011) 1 B −0.50 2 B −0.53 3 B −0.52 4 B −0.56 5 B −0.44 6 B −0.46 7 B −0.46 8 B −0.48 9 3F 0.06 (110) 1 3F −1.30 2 3F −1.08 3 3F −0.90 4 3F −0.35 5 B −0.52 6 B −0.39 7 3F 0.00 CT site indicates that oxygen atoms are adsorbed on the carbon atom on the surface. 
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表 2 Fe3C各晶面氧原子移除相关基元反应的能垒和反应能
Table 2 The activation energy (Ea, eV) and reaction energy(Er, eV) of elementary reactions involved in the removal route
Reactions (001) (011) (110) O+H→OH Ea 1.47 1.59 1.72 Er 0.56 −0.06 0.78 OH+H→H2O Ea 1.75 1.54 2.09 Er 1.00 0.88 1.26 2OH→H2O+O Ea 0.89 1.77 1.54 Er 0.43 0.93 0.48 CO+O→CO2 Ea 2.18 0.84 2.67 Er 0.85 0.46 1.81 CO+OH→COOH Ea 1.70 1.70 2.22 Er 1.12 1.25 1.42 COOH→CO2+H Ea 0.63 1.23 1.04 Er −0.83 −0.73 −0.39
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