Article pubs.acs.org/JPCC
Adsorption Structures and Energies of Cun Clusters on the Fe(110) and Fe3C(001) Surfaces Xinxin Tian,†,‡,§ Tao Wang,∥ Yong Yang,†,‡ Yong-Wang Li,†,‡ Jianguo Wang,† and Haijun Jiao*,†,∥ †
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China National Energy Center for Coal to Liquids, Synfuels CHINA Co., Ltd., Huairou District, Beijing 101400, China § University of Chinese Academy of Sciences, Beijing 100049, China ∥ Leibniz-Institut für Katalyse e. V., Universität Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany ‡
S Supporting Information *
ABSTRACT: Spin-polarized density functional theory computations have been carried out to investigate the adsorption configurations of Cun (n = 1−7, 13) on the most stable Fe(110) and Fe3C(001) surfaces. On both surfaces the adsorbed Cun clusters favor aggregation over dispersion, and monolayer adsorption configurations are more favored thermodynamically than the two-layer adsorbed structures because of the stronger Fe−Cu interaction over the Cu−Cu bonding. On the basis of the computed adsorption energies the Fe(110) surface has stronger Cu affinity than the Fe3C(001) surface, in agreement with the experimental results. The Fe(110) surface also has stronger Cun aggregation energies and more pronounced charge transfer from surface to adsorbed Cun clusters than the Fe3C(001) surface. Different Cun growth modes have been discussed accordingly.
1. INTRODUCTION On the basis of rapid resource depletion, the unpredictable price of crude oil, and increased demand on fuels, production of clean fuels and value-added chemicals from coal, natural gas, and biomass becomes the most important alternative to oil. For such chemical transformations, Fischer−Tropsch synthesis (FTS, CO + H2 → CxHy + H2O + CO2)1−3 is the key technology which can convert synthesis gas (CO/H2) into linear hydrocarbons and a small amount of oxygenated compounds. Due to their high activity and low price, Fe-based FTS catalysts are widely used in industry.4−6 Initial Fe-based FTS catalysts are generally hematite (α-Fe2O3), and they have to be reduced before becoming active. During the reduction, α-Fe2O3 is first reduced to magnetite (Fe3O4) by using H2, synthesis gas, or CO and then to phases consisting of metallic iron, iron oxides, and iron carbides in varying proportions depending on the operating conditions.7−9 More specifically, several iron carbide phases such as ε-Fe2C, χ-Fe5C2, θ-Fe3C, and Fe7C3 have been detected experimentally.10 Extensive investigations from transmission electron microscopy, X-ray diffraction, and Mössbauer spectroscopy revealed that the Fe3C phase is responsible for the high FTS activity.11,12 Practically, Fe-based FTS catalysts consist of not only pure iron components (metallic iron, iron oxides, and iron carbides) but also promoters such as copper, potassium, and silica or zinc oxide. Particularly, the role of copper promoter in Fe-based FTS has attracted great attention and has been extensively studied experimentally.13 Cu has been widely thought to facilitate the reduction of Fe3+ species to Fe2+ species as well as to metallic Fe,14 and to lower the reduction temperature.15 de Smit et al.16 © 2014 American Chemical Society
found that upon exposure to H2 the reduction of Cu-promoted catalysts takes place easily at low temperature and is much faster than that of the unpromoted catalysts at high temperature; however, no such significant difference could be observed upon exposure to synthesis gas. Recently, Cu promoter has been found to facilitate the consecutive conversion of magnetite to α-Fe under H2, while suppressing the formation of the χ-Fe5C2 phase under CO atmosphere.17,18 In addition, Cu can promote the carburization rate,19−21 increase the activities of FTS and water gas shift (WGS, CO + H2O → CO2 + H2) reaction,22−25 and synergistic effects were observed on Cu and K co-promoted catalyst.26 The content of Cu was found to be important in determining FTS activities. For example, Ma et al.27 found that the addition of up to 2.0 wt % Cu enhances the iron reduction significantly but lowers the activity of FTS and WGS. Pansanga et al.28 investigated the Cu modified Al2O3 as support in Fe-based FTS catalysts and found significantly higher activities of catalysts supported on 10 wt % Cu modified Al2O3 than those modified with only 1 wt % Cu. Wielers et al.29 found that Cu content does not affect the product selectivity within a broad range ( 5, indicating the inability of dispersion at high coverage. The stepwise growth energy of the aggregated mode is positive for Cu2, while negative for n ≥ 3, revealing the aggregation ability on the Fe3C(001) surface at high coverage. Bader charge analysis shows that the adsorbed Cu atoms on the Fe(110) surface are negatively charged, indicating the electron transfer from metallic iron surface to the adsorbed Cu atoms. This is indeed due to their difference in electronegativity between Fe and Cu (1.80 for Fe and 1.85 for Cu).76 On the Fe3C(001) surface, however, the electron transfer from the surface to the adsorbed Cu atoms is negligible; and this may be due to the stronger electronegativity of carbon atoms (2.54). It has been shown that charge transfer between the adsorbed clusters and the substrate can supply a driving force for structural evolution. For Au adsorption on the stoichiometric CeO2(111) surface,77 for example, the charge transfer from the cluster to the surface is sufficient to induce the preference of 3D clusters. Without substantial charge transfer from the cluster to the surface for Au adsorption on the ZnO(101̅0) surface, the 2D structures of the Au clusters are preferred.78 For Cu adsorption on the ZnO(101̅0) surface, the odd-numbered Cun clusters (n = 5, 7, 9) are positively charged as donating electrons to the ZnO conduction band. Consequently these Cun clusters have polyhedral configurations,74 and Cu can nucleate as 3D islands even at low coverage due to the stronger Cu−Cu bonding energy over the Cu−substrate interaction.78 Polyhedral configuration preference behavior is also found for the charged Cu clusters over a polar ZnO(0001) surface.79 For Cun on the Fe(110) and Fe3C(001) surfaces, however, the charge transfer is rather lower, and 2D clusters are preferred. Apart from the thermodynamic properties, kinetic effect is also important in the morphology of the adsorbed Cun clusters. For example, for the formation of Cun (n = 1−7) clusters on the WN(001) surface, Han et al.,80,81 found that Cu atoms undergo rapid agglomeration at atomic layer deposition operating temperature, forming 3D clusters, whereas the Cu thin film remains essentially stable at room temperature.
Figure 6. Diffusion pathways of a single Cu atom on the Fe3C(001) surface.
The largest energy difference is 0.66 eV. Diffusion along the [100] direction (P2; 4F1 → 4F2 → 5F → 4F1) will lead to the joint of neighbored Cun belts in different iron regions, and the largest energy difference is 0.47 eV. Therefore, both diffusion pathways are accessible because of their close and moderate hopping “barrier”. The results well-verify the preceding conclusion of Cun growth mode on the (001) surface; i.e., Cu first aggregated as triangle belts along the iron region and then spread over the surface in monolayer structure.
4. DISCUSSION Under the reduction condition of Fe-based FTS, both metallic iron and iron carbides are responsible for the activity, and Fe3C is one of the active iron carbide phases detected experimentally. For understanding the Cu promotion effect in FTS, it is essential to know the binding strengths and the surface morphology of copper adsorption on both iron and iron carbides surfaces as well as their surface diffusion ability. On the most stable Fe(110) surface, the adsorbed Cu atoms favor aggregation instead of dispersion, forming 2D structures rather than 3D clusters up to a monolayer. The Cun growth mode is linear- and triangular-based on the Fe(110) surface. On the Fe3C(001) surface, the adsorbed Cu atoms prefer to disperse separately at very low coverage and aggregate along the iron region forming a triangle belt first at high coverage. The 2D structures are also more favorable than the 3D structures due to the stronger Fe−Cu interaction than the Cu−Cu interaction. For both surfaces, the aggregation properties of Cu agree well with the previous experimental studies that Cu agglomerates on reduced catalyst surface (iron surface and carburized iron surface).32,33 The adsorption energies on the Fe(110) surface are larger than those on the Fe3C(001) surface, and this reveals the higher surface affinity of the metallic Fe surface over the iron carbide 21971
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5. CONCLUSION
ASSOCIATED CONTENT
S Supporting Information *
Plane-wave cutoff test for Cun in the gas phase (Table S1), verification of the surface models (Table S2), adsorption energies and detailed structure properties of Cu1−6 on Fe3C(001) surface (Table S3), the corresponding absolute energies of Cu n (n = 1−7, 13) on the Fe(110) and Fe3C(001) surfaces (Table S4), possible Cu2−7 gas-phase structures (Figure S1), all optimized Cu1−7 configurations on the Fe(110) surface (Figures S2−S5), and all optimized Cu1−7,13 configurations on the Fe3C(100) (Figure S6−13). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 0049-381-1281-135. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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The adsorption structures and energies of the Cun (n = 1−7, 13) clusters on the most stable Fe(110) and Fe3C(001) surfaces have been systemically computed by using spin-polarized density functional theory method. On the metallic Fe(110) surface, the adsorbed Cu atoms favor aggregation instead of dispersion, and the aggregation takes place very easily due to the considerably low diffusion barrier as well as the negative aggregation and growth energies. On the Fe3C(001) surface containing exposed iron and carbon atoms, the adsorbed Cu atoms prefer to disperse separately at very low coverage and aggregate at high coverage. Furthermore, the adsorbed Cu atoms prefer to first aggregate as belts in the iron region. Therefore, we can infer that in an appropriate range of Cu coverage the surface Cu atoms would be confined to the iron region of the surface. Monolayer adsorption configurations on both surfaces are more favored thermodynamically than the two-layer adsorbed structures because of the stronger Fe−Cu interaction over the Cu−Cu bonding. It is therefore to be expected that the adsorbed Cu atoms almost epitaxially grow on surfaces as a layer-by-layer mode at the initial stage, in agreement with the experimental observations. It is found that the Fe(110) surface can adsorb Cun more strongly than the Fe3C(001) surface, indicating the lower Cu affinity of the Fe3C(001) surface as found experimentally. The adsorbed monolayer Cun clusters on the Fe(110) surface aggregate more strongly than on the Fe3C(001) surface; stronger charge transfer from surface to the adsorbed Cun clusters has been found on the former than on the latter, especially at low coverage.
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ACKNOWLEDGMENTS
This work was supported by the National Basic Research Program of China (Grant No. 2011CB201406), the National Natural Science Foundation of China (Grant Nos. 21273262 and 21273266), and the Chinese Academy of Science and Synfuels CHINA. Co., Ltd. We also acknowledge general financial support from the BMBF and the state of Mecklenburg-Vorpommern. 21972
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