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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 3515 Received 20th November 2013, Accepted 19th December 2013 DOI: 10.1039/c3cp54901a
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Theoretical prediction of carbon dioxide reduction to methane at coordinatively unsaturated ferric site in the presence of Cu impurities Jianping Xiao*ab and Thomas Frauenheima
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Based on first principles calculations, we have found that the reducibility of a polar FeO(111) surface can be improved by incorporating some Cu dopants. The resulting coordinatively unsaturated ferric site could effectively reduce carbon dioxide to methane in a H2 atmosphere.
From the environmental and energetic points of view, CO2 reduction to hydrocarbon as fuels is an attractive prospect.1 As is known, CO2 can be electrochemically reduced to hydrocarbons (methane and ethylene) over polycrystalline Cu electrodes.2 However, the main shortcomings are attributable to quite a high overpotential in the order of 1V in the above electrochemical processes. In contrast, the industrial syngas conversion (H2/CO/CO2) based on the Cu/ZnO/Al2O3 catalysts is more feasible and efficient, but it can uniquely produce methanol instead of methane.3 In light of the d-band model,4 as late transition metals neither Zn nor Cu has a high oxygen affinity, compared with the other late transition metals (Fe, Co, and Ni). Hence, we need to design catalysts, whose active sites have higher oxygen affinity, to dissociate the two C–O bonds in a CO2 molecule to drive the CO2 reduction to the catalytic CH4 selectively. In the work of Fu et al.,5 the authors have demonstrated that coordinatively unsaturated ferrous (CUFe) sites are highly reactive in dissociating an O2 molecule into two O atoms at low temperature, where the CUFe sites are confined on a Pt(111) substrate. In addition, the in situ observations, by means of X-ray photoelectron spectroscopy (XPS) and atomically resolved scanning tunneling microscopy (STM), also indicate that facile H2O dissociation could take place at the edge of FeO(111) supported by a Au(111) substrate,6 which could demonstrate again that the CUFe sites are effective active sites for some dissociative reactions. Unfortunately, Fu et al. did not observe the dissociation of CO molecules at the CUFe sites.5 Thus, it is quite critical to enhance the oxygen
a
¨t Bremen, Bremen Center for Computational Materials Science, Universita Am Fallturm 1, 28359 Bremen, Germany b School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany. E-mail: [email protected]
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affinity of active sites of catalysts, compared with the confined CUFe sites,5 to serve in CO2 reduction. In the work of Tamaura et al.,7 they have found that CO2 can be reduced to C and O atoms on cation-excess magnetite. These cation-excess sites are composed of CUFe and coordinatively unsaturated ferric (CUFi) sites. As the CUFi sites have a higher oxygen affinity than CUFe ones, thus, the experiment reveals that the CUFi sites can enhance the scission of C–O bonds. However, in the recent theoretical investigations of Liu et al.,8 they claimed the C–O bond scission on the Fe(100) surface is not optimal, compared with Co and Ni(100) surfaces. This is attributed to the too strong binding of the CO2 molecule on the Fe(100) surface. In other words, the reduced products of CO2 can not desorb efficiently from the Fe(100) surface. As the CUFi sites have higher oxygen affinity compared with an Fe(100) surface, one can expect that the CUFi alone are not the optimal active sites of catalysts in CO2 reduction either. In a word, on the basis of the catalysts containing the CUFe sites proposed in the work of Fu et al.,5 we can design a new catalyst with the following improvements for CO2 reduction. First, further efforts are required to reduce the use of noble metals (Pt) as the substrate in inverse catalysts. Second, the oxygen affinity of active sites of catalysts must be improved to be located between the CUFe and CUFi sites. Third, from a practical point of view, hopefully, the active sites can be spontaneously formed in reactive environment, instead of prepared carefully in vacuum in advance. Pt metal was used in the above inverse catalysts because the Pt metal has high electron negativity and low oxygen affinity. Therefore, from simple structural and functional points of view, the O-terminated FeO(111) polar surface with oxygen vacancies (VO) can be used to replace the inverse catalyst above (cf. Fig. 1), simultaneously solving the main drawbacks mentioned above. Herein, the O-terminated FeO(111) surface will be abbreviated as FeO(111)–O. The FeO(111) surface is a typical polar surface,9 resulting from the alternative stacking of O anions and Fe cations. The unneutral charge in each layer give rise to a huge electrostatic field and thus macroscopic dipole moments perpendicular to the FeO(111) surface. Therefore, the polar FeO(111) surface, in
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5–8, are corresponding to cases of VO at the topmost and first two anionic layers, respectively. The formation energies of VO defects, Ef (T, p), at a given temperature and pressure, were calculated by the following expression
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Ef (T, p) = Ed + Sn0m0(T, p)
Fig. 1 Scheme of (a) a CUFe site confined on the Pt substrate and (b) the FeO(111)–O surface containing VO defects, where the Fe@sub indicates Fe cations at the subsurface, namely, the first cationic layer. Fe, O, and Pt atoms are denoted in blue, red, and yellow, respectively.
principle, should have numerous defects and adsorbates to compensate the resulting dipole moments. In the experiment of Koike et al.,10 based on a spin-polarized secondary electron technique, they found that FeO(111) thin films were ferromagnetically ordered. More surprisingly, the ´el temperaobserved ferromagnetism was even stable above the Ne ture of FeO bulk. The tentative explanation was that the defects and reconstructions on the FeO(111) surface may significantly affect the spin ordering of Fe cations in the vicinity of the surface. However, first principles calculations show the stoichiometric FeO(111) thin films exhibit ferromagnetic ordering as well,11 while the underlying mechanism is still unknown. In this work, we have employed FeO clusters (300 atoms) to investigate the pristine FeO(111)–O polar surface by means of computing X-ray absorption near edge structure (XANES). The XANES calculations were carried out using the FEFF9.0 code based on multiple scattering schemes.12 We found that Fe cations at the FeO(111) subsurface are Fe3+, instead of Fe2+. This finding could interpret successfully the observed ferromagnetism of the FeO(111) thin films in the previous experiments.10 Moreover, we have performed first principles based atomistic thermodynamics calculations to analyze the formation of VO defects on the FeO(111) surface. We have found a possibility to improve the formation of VO defects by incorporating Cu impurities. On the basis of the most stable FeO(111)–O surface containing VO defects and Cu impurities, we have studied the pathway of CO2 reduction to CH4. Herein, the Cu/FeO(111) accounts for the FeO(111)–O surface in the presence of Cu dopants. Finally, we would propose the CO2 reduction to CH4 over the Cu/FeO(111)–O surface. First principles calculations were performed based on (2 2) FeO(111) slab models with a thickness of eight atomic layers. The upper four layers are allowed to relax and the remaining ones are fixed. All studies were considered uniquely on the polar FeO(111)–O surface. Meanwhile, the Fe-termination was passivated by a monolayer of hydrogen atoms. In addition, the concentration of substitutional Cu dopants at the subsurface varies in the range from 0 to 100%, as abbreviated to nCu (0%), sCu (25%), dCu (50%), tCu (75%), and qCu (100%). We have calculated the surface reducibility by removing oxygen atoms from the first and second anionic layers. The nomenclature of tCu + 4VO is in accordance with the formation of four VO defects at the first anionic layer of the surface (fully reduced) with 3/4 Fe sites substituted by Cu dopants. The total amount of defects, 1–4 and
3516 | Phys. Chem. Chem. Phys., 2014, 16, 3515--3519
Ep,
(1)
where Ed and Ep are the total energies of surfaces with and without defects, respectively; nO and mO are the number and chemical potential of oxygen on the surface. In the reducing atmosphere with H2 gas, we assume the surface oxygen is in equilibration with gaseous H2 and H2O. Reaction energies of hydrogenation processes, Ere(T, p), were calculated with the following expression Ere(T, p) = Ead
mclean
mCO2
nHmH,
(2)
where mclean is the chemical potential of tCu + 4VO surface, Ead is the total energies of tCu + 4VO surface with adsorbates, nH is the number of hydrogen atoms. The chemical potential (m), at a given temperature (T) and pressure (p), of CO2 and H was referred to the corresponding molecules (CO2 and H2) in their gas phases. It was calculated as follows: m(T, p) = m(T, p0) + kBT ln(p/p0),
(3)
where p0 and kB are the pressure under standard condition and Boltzmann constant, respectively. The m(T, p0) was calculated in terms of thermal contributions and zero-point energy (ZPE) corrections. A negative value of Ef (T, p) and Ere(T, p) accounts for an energetically favorable process. Kohn–Sham equations were solved based on the linear combination of atomic orbitals (LCAO) scheme, implemented in CRYSTAL09 code.13 We employed Gaussian-type basis sets to optimize structure and calculate reaction energies for Fe (86-411d41G),14 O (6-31d),15 Cu (86-4111(41D)G),16 H (5-11G*),17 and C (6-31d1G).18 The Perdew–Burke–Ernzerhof19 (PBE) functional was used for electronic exchange–correlation interactions. The irreducible Brillouin zones were sampled with (6 6 1) k-point grids. The nudged elastic band method implemented in the Vienna Ab initio Simulation Package20 with a projector augmented wave (PAW) formalism was employed to calculate the activation energies.21 In Fig. 2, it shows calculated XANES at L3 and L2 edges of Fe cations at the FeO(111)–O subsurface (Fe@sub), in which the XANES intensity reflects the unoccupied d-band states of Fe cations. The unoccupied d-band states of Fe@sub cations are similar to those of Fe3+ in Fe2O3 bulk, instead of Fe2+ in FeO. This is due to the fact the dangling surface bonds from the O anions at the first anionic layer can be saturated, accompanied by the formation of Fe3+ cations. To stabilize a typical polar surface, the conventional scenario is to reduce charge density on the first anionic layer via adsorbates from the environment. However, we found that an alternative change of Fe oxidation states from Fe2+ to Fe3+ can realize the internal suppression of surface polarity. In other words, the anionic and cationic charge stacking, O2 /Fe3+, can replace the usual stabilization mechanism, namely, O /Fe2+. In contrast, the existence of Fe2+ cations cannot directly stabilize the FeO(111)–O polar surface.
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Fig. 2 Computed XANES at (a) L3 edge and (b) L2 edge of the Fe@sub of FeO(111)–O surface, compared with those of Fe cations in FeO and Fe2O3 bulk phase.
More importantly, we could also elucidate the experimental observations in the work of Koike et al.10 Regarding the formation of Fe3+ at the FeO(111)–O subsurface, it could originate from the double exchange mechanism based on Fe2+–Fe3+ pair.22 Therefore, the FeO(111) thin films can be present in ferromagnetism above ´el temperature of bulk FeO. In addition, Gota et al.23 had also the Ne claimed the metastable FeO(111)-like phase contains ferric Fe3+ cations, rather than ferrous Fe2+ cations, in their experiments. In principle, a perfect polar FeO(111)–O surface should be sufficiently stable without polar instability problem. We have calculated the formation energies of VO on the FeO(111)–O surface. Our calculations indeed indicate VO formation is not favorable. For example, it costs 3.46 eV to create a single VO at the first anionic layer of the FeO(111)–O surface under O-rich conditions, referring the mO to the oxygen molecule. Now we turn to think about how to activate the FeO(111)–O surface for CO2 reduction. In other words, we want to reduce the FeO(111)–O surface and create some CUFi sites. The first trial is to decrease the mO in the reactive atmosphere. It should enhance the formation of VO on the FeO(111)–O polar surface. Herein, we have considered the typical industrial pressure and temperature, namely, T = 600 K, p(H2) = 40 bar, p(H2O) = 1 bar. Calculated results show the formation energy of a single VO defect at the first anionic layer can be reduced to 0.78 eV (see Fig. 3). However, it is still extremely difficult to produce an amount of VO defects since the formation of two and three VO defects requires 9.56 and 4.46 eV, respectively. In our previous work,24 we found that Cu dopants can effectively enhance the reducibility of a ZnO(0001)–O polar surface, where the Cu is present as a substitutional dopant. In addition, the oxygen affinity of CUFi sites can be reduced in the presence of Cu dopants. This may avoid the ‘‘overbinding’’ problem reported in the work of Liu et al.8 Therefore, we have calculated the formation of VO on the FeO(111)–O surface with substitutional Cu dopants. Computed formation energies are shown in Fig. 3. Thanks to quite different surface reconstructions, we did not obtain any simple linear correlations between the formation energies of VO defects and the number of VO defects. However, it is quite
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Fig. 3 Calculated formation energies of VO on the FeO(111)–O surface with Cu dopants with respect to the perfect surfaces. The chemical potential of oxygen was referred to the reactive environment, T = 600 K, p(H2) = 40 bar and p(H2O) = 1 bar.
obvious that these Cu dopants always prefer to improve the formation of VO defects. Although it is unfavorable for forming VO defects at the second anionic layer, it is quite feasible to fully reduce the first anionic layer of the FeO(111)–O surface. The optimal concentration of Cu dopants was obtained in the tCu (75%) case. The formation of VO defects essentially produces a number of CUFi sites, exhibiting stronger oxygen affinity, compared with the CUFe sites confined on the Pt metal. Reaction energies of CO2 adsorption and dissociation are calculated and shown in Fig. 4. The adsorbed CO2 can be dissociated to CO and an O atom with a released reaction energy of B1 eV and activation energy of B0.8 eV. This is acceptable in our studied reaction temperature (600 K). In addition, we have considered the subsequent CO hydrogenation processes in H2 atmosphere. The resulting CO molecule can be cleaved as C and O atoms with a reaction energy of around 0.8 eV and activation energy of B0.2 eV. This turns out that the scission of the second C–O bond is quite feasible at the CUFi sites. In Fig. 4, it shows quite nice CH4 selectivity in CO2 reduction. The C atom in the resulting CO molecule has stronger hydrogen affinity compared to the O atom, leading to formation of *CHO and *CH2 species, instead of *COH and *CHOH. In contrast, the O atom from the resulting CO molecule prefers to be adsorbed in the vicinity of the CUFi site, that
Fig. 4 Calculated reaction energies of the pathway of CO2 reduction to methane on the tCu + 4VO surface. The red points show these unfavorable intermediates in CO2 reduction.
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Acknowledgements J. Xiao would acknowledge financial support from the China Scholarship Council (CSC).
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References
Fig. 5 Scheme of CO2 reduction to CH4 in H2 atmosphere over the Cu/ FeO(111)–O surface. Cu: green; Fe: blue; O: red; C: yellow; H: cyan.
is, the VO sites on the Cu/FeO(111)–O surface. In addition, the more accurate calculations with hybrid B3LYP functional can also qualitatively reproduce the trend of CH4 selectivity. Therefore, CO2 reduction reactions should follow the pathway of methanation, instead of producing methanol. Furthermore, in order to accelerate the CO2 reduction, the CH4 desorption from the surface is also quite critical. We have found that the CH4 desorption is pretty favorable (0.02 eV). This should drive the CO2 reduction approaching the pathway of methanation too. In a word, the CUFi sites do have an intermediate reactivity in the presence of Cu dopants. After the CH4 desorption, the tCu + 4VO surface gains two leftover O atoms, which is equivalent to a tCu + 2VO surface. These leftover O atoms can be cleared by hydrogen reduction with a reaction energy of approximately 4 eV (cf. Fig. 3). In the previous work of Tamaura et al.,7 they already found that carbon dioxide can be reduced to carbon atoms using cationexcess magnetite. The cation-excess magnetite was obtained by hydrogen reduction of normal magnetite at B590 K, and is protected in an N2 atmosphere. It could rationalize our calculated results. Furthermore, Shin et al.25 investigated the CO2 decomposition on two types of MFe2O4 (M = Ni and Cu) solid solutions. First, they pointed out that FeO is the intermediate during the MFe2O4 reduction. More importantly, they also found that the redox reaction was indeed accelerated by adding substituted metal (Ni and Cu) impurities. On the basis of the previous experimental studies, we propose the CO2 reduction to CH4 using catalysts in the form of Cu–FeO solid solution in H2 atmosphere. The proposed mechanism is shown in Fig. 5. In conclusion, we have performed X-ray absorption near edge structure calculations to discover the pristine FeO(111)–O polar surface. We have found that Fe cations at the subsurface are Fe3+, instead of Fe2+, as the presence of Fe3+ at the subsurface could effectively suppress the polarity of the FeO(111)–O surface. Meanwhile, the Fe3+–Fe2+ pair could initiate a double exchange mechanism to generate the FeO(111) films in ferromagnetic ordering, which is consistent with experimental observations. Moreover, we have found the formation of oxygen vacancies can be enhanced by substitutional Cu impurities. The resulting oxygen vacancies have high oxygen affinity and low hydrogen affinity, which can serve in CO2 reduction to CH4.
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1 Y. Hori, in Modern aspects of electrochemistry, ed. C. G. Vayenas, R. E. White and M. E. Gamboa-Aldeco, Springer, New York, 2008, vol. 42, p. 89. 2 Y. Hori, H. Wakebe, T. Tsukamoto and O. Koga, Electrocatalystic process of CO selectivity in electrochemical reduction of CO2 at metal electrodes in aqueous media, Electrochim. Acta, 1994, 39, 1833. 3 J. W. Andreasen, F. B. Rasmussen, S. Helveg, A. Molenbroek, K. Ståhl, M. M. Nielsen and R. Feidenhans´l, Activation of a Cu/ZnO catalyst for methanol synthesis, J. Appl. Crystallogr., 2006, 39, 209. 4 B. Hammer and J. K. Norskov, Why Gold is the Noblest of All the metals, Science, 1995, 376, 238. 5 (a) X. Guo, Q. Fu, Y. Ning, M. Wei, M. Li, S. Zhang, Z. Jiang and X. Bao, Ferrous Centers Confined on Core-Shell Nanostructures for Low Temperature CO Oxidation, J. Am. Chem. Soc., 2012, 134, 12350; (b) Q. Fu, W. Li, Y. Yao, H. Liu, H. Su, D. Ma, X. Gu, L. Chen, Z. Wang, H. Zhang, B. Wang and X. Bao, Science, 2010, 328, 1141. 6 X. Deng, J. Lee, C. Wang, C. Matranga, F. Aksoy and Z. Liu, In situ observation of water dissociation with lattice incorporation at FeO particle edges using scanning tunneling microscopy and x-ray photoelectron spectroscopy, Langmuir, 2011, 27, 2146. 7 Y. Tamaura and M. Tabata, Complete reduction of carbon dioxide to carbon using cation-excess magnetite, Nature, 1990, 346, 255. 8 C. Liu, T. R. Cundari and A. K. Wilson, CO2 reduction on transition metal (Fe, Co, Ni, and Cu) surfaces: In comparison with homogeneous catalysis, J. Phys. Chem. C, 2012, 116, 5681. 9 C. Noguera, Polar oxide surfaces, J. Phys.: Condens. Matter, 2000, 12, R367. 10 K. Koike and T. Furukawa, Evidence for ferromagnetic order at the FeO(111) surface, Phys. Rev. Lett., 1996, 77, 3921. 11 Y. L. Li, K. L. Yao, Z. L. Liu and G. Y. Gao, First-principles study of the composition, structure, and stability of the FeO(111) surface, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 72, 155446. 12 A. L. Ankudinov, B. Ravel, J. J. Rehr and S. D. Conradson, Real-space multiple-scattering calculation and interpretation of x-ray-absorption near-edge structure, Phys. Rev. B: Condens. Matter Mater. Phys., 1998, 58, 7565. 13 R. Dovesi, V. R. Saunders, C. Roetti, R. Orlando, C. M. ZicovichWilson, F. Pascale, B. Civalleri, K. Doll, N. M. Harrison, I. J. Bush, P. h. D’Arco and M. Llunell, CRYSTAL09, User’s Manual, University of Torino, Torino, 2009. 14 G. Valerio, M. Catti, R. Dovesi and R. Orlando, Ab initio study of antiferromagnetic rutile-type FeF2, Phys. Rev. B: Condens. Matter, 1995, 52, 2422.
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15 O. M. Corno, C. Busco, B. Civalleri and P. Ugliengo, Periodic ab initio study of structural and vibrational features of hexagonal hydroxyapatite Ca10(PO4)6(OH)2, Phys. Chem. Chem. Phys., 2006, 8, 2464. 16 K. Doll and N. M. Harrison, Chlorine adsorption on the Cu(111) surface, Chem. Phys. Lett., 2000, 317, 282. 17 R. Dovesi, E. Ermondi, E. Ferrero, C. Pisani and C. Roetti, Hartree-Fock study of lithium hydride with the use of a polarizable basis set, Phys. Rev. B, 1983, 29, 3591. 18 C. Gatti, V. R. Saunders and C. Roetti, Crystal-field effects on the topological properties of the electron-density in molecularcrystals- the case of urea, J. Chem. Phys., 1994, 101, 10686. 19 J. P. Perdew, K. Burke and M. Ernzerhof, Generalized Gradient Approximation Made Simple, Phys. Rev. Lett., 1996, 77, 3865. ¨ller, Efficient Iterative Schemes for 20 G. Kresse and J. Furthmu ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set, Phys. Rev. B: Condens. Matter, 1996, 54, 11169.
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21 G. Kresse and D. Joubert, From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 59, 1758. 22 C. Zener, Interaction between the d-Shells in the transition metals. II. ferromagnetic compounds of manganese with perovskite structure, Phys. Rev. Lett., 1951, 82, 403. 23 S. Gota, E. Guiot, M. Henriot and M. Gautier-Soyer, Atomicoxygen-assisted MBE growth of a-Fe2O3 on a-Al2O3(0001): Metastable FeO(111)-like phase at subnanometer thicknesses, Phys. Rev. B: Condens. Matter, 1999, 60, 14387. 24 J. Xiao and T. Frauenheim, Activity and synergy effects on a Cu/ZnO(0001) surface studied using first-principle thermodynamics, J. Phys. Chem. Lett., 2012, 3, 2638. 25 H. C. Shin, J. H. OH, J. C. Lee, S. H. Han and S. C. Choi, The carbon dioxide decomposition reaction with (NixCu1 x)Fe2O4 solid solution, Phys. Status Solidi A, 2002, 3, 741.
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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 3515 Received 20th November 2013, Accepted 19th December 2013 DOI: 10.1039/c3cp54901a
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Theoretical prediction of carbon dioxide reduction to methane at coordinatively unsaturated ferric site in the presence of Cu impurities Jianping Xiao*ab and Thomas Frauenheima
www.rsc.org/pccp
Based on first principles calculations, we have found that the reducibility of a polar FeO(111) surface can be improved by incorporating some Cu dopants. The resulting coordinatively unsaturated ferric site could effectively reduce carbon dioxide to methane in a H2 atmosphere.
From the environmental and energetic points of view, CO2 reduction to hydrocarbon as fuels is an attractive prospect.1 As is known, CO2 can be electrochemically reduced to hydrocarbons (methane and ethylene) over polycrystalline Cu electrodes.2 However, the main shortcomings are attributable to quite a high overpotential in the order of 1V in the above electrochemical processes. In contrast, the industrial syngas conversion (H2/CO/CO2) based on the Cu/ZnO/Al2O3 catalysts is more feasible and efficient, but it can uniquely produce methanol instead of methane.3 In light of the d-band model,4 as late transition metals neither Zn nor Cu has a high oxygen affinity, compared with the other late transition metals (Fe, Co, and Ni). Hence, we need to design catalysts, whose active sites have higher oxygen affinity, to dissociate the two C–O bonds in a CO2 molecule to drive the CO2 reduction to the catalytic CH4 selectively. In the work of Fu et al.,5 the authors have demonstrated that coordinatively unsaturated ferrous (CUFe) sites are highly reactive in dissociating an O2 molecule into two O atoms at low temperature, where the CUFe sites are confined on a Pt(111) substrate. In addition, the in situ observations, by means of X-ray photoelectron spectroscopy (XPS) and atomically resolved scanning tunneling microscopy (STM), also indicate that facile H2O dissociation could take place at the edge of FeO(111) supported by a Au(111) substrate,6 which could demonstrate again that the CUFe sites are effective active sites for some dissociative reactions. Unfortunately, Fu et al. did not observe the dissociation of CO molecules at the CUFe sites.5 Thus, it is quite critical to enhance the oxygen
a
¨t Bremen, Bremen Center for Computational Materials Science, Universita Am Fallturm 1, 28359 Bremen, Germany b School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany. E-mail: [email protected]
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affinity of active sites of catalysts, compared with the confined CUFe sites,5 to serve in CO2 reduction. In the work of Tamaura et al.,7 they have found that CO2 can be reduced to C and O atoms on cation-excess magnetite. These cation-excess sites are composed of CUFe and coordinatively unsaturated ferric (CUFi) sites. As the CUFi sites have a higher oxygen affinity than CUFe ones, thus, the experiment reveals that the CUFi sites can enhance the scission of C–O bonds. However, in the recent theoretical investigations of Liu et al.,8 they claimed the C–O bond scission on the Fe(100) surface is not optimal, compared with Co and Ni(100) surfaces. This is attributed to the too strong binding of the CO2 molecule on the Fe(100) surface. In other words, the reduced products of CO2 can not desorb efficiently from the Fe(100) surface. As the CUFi sites have higher oxygen affinity compared with an Fe(100) surface, one can expect that the CUFi alone are not the optimal active sites of catalysts in CO2 reduction either. In a word, on the basis of the catalysts containing the CUFe sites proposed in the work of Fu et al.,5 we can design a new catalyst with the following improvements for CO2 reduction. First, further efforts are required to reduce the use of noble metals (Pt) as the substrate in inverse catalysts. Second, the oxygen affinity of active sites of catalysts must be improved to be located between the CUFe and CUFi sites. Third, from a practical point of view, hopefully, the active sites can be spontaneously formed in reactive environment, instead of prepared carefully in vacuum in advance. Pt metal was used in the above inverse catalysts because the Pt metal has high electron negativity and low oxygen affinity. Therefore, from simple structural and functional points of view, the O-terminated FeO(111) polar surface with oxygen vacancies (VO) can be used to replace the inverse catalyst above (cf. Fig. 1), simultaneously solving the main drawbacks mentioned above. Herein, the O-terminated FeO(111) surface will be abbreviated as FeO(111)–O. The FeO(111) surface is a typical polar surface,9 resulting from the alternative stacking of O anions and Fe cations. The unneutral charge in each layer give rise to a huge electrostatic field and thus macroscopic dipole moments perpendicular to the FeO(111) surface. Therefore, the polar FeO(111) surface, in
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5–8, are corresponding to cases of VO at the topmost and first two anionic layers, respectively. The formation energies of VO defects, Ef (T, p), at a given temperature and pressure, were calculated by the following expression
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Ef (T, p) = Ed + Sn0m0(T, p)
Fig. 1 Scheme of (a) a CUFe site confined on the Pt substrate and (b) the FeO(111)–O surface containing VO defects, where the Fe@sub indicates Fe cations at the subsurface, namely, the first cationic layer. Fe, O, and Pt atoms are denoted in blue, red, and yellow, respectively.
principle, should have numerous defects and adsorbates to compensate the resulting dipole moments. In the experiment of Koike et al.,10 based on a spin-polarized secondary electron technique, they found that FeO(111) thin films were ferromagnetically ordered. More surprisingly, the ´el temperaobserved ferromagnetism was even stable above the Ne ture of FeO bulk. The tentative explanation was that the defects and reconstructions on the FeO(111) surface may significantly affect the spin ordering of Fe cations in the vicinity of the surface. However, first principles calculations show the stoichiometric FeO(111) thin films exhibit ferromagnetic ordering as well,11 while the underlying mechanism is still unknown. In this work, we have employed FeO clusters (300 atoms) to investigate the pristine FeO(111)–O polar surface by means of computing X-ray absorption near edge structure (XANES). The XANES calculations were carried out using the FEFF9.0 code based on multiple scattering schemes.12 We found that Fe cations at the FeO(111) subsurface are Fe3+, instead of Fe2+. This finding could interpret successfully the observed ferromagnetism of the FeO(111) thin films in the previous experiments.10 Moreover, we have performed first principles based atomistic thermodynamics calculations to analyze the formation of VO defects on the FeO(111) surface. We have found a possibility to improve the formation of VO defects by incorporating Cu impurities. On the basis of the most stable FeO(111)–O surface containing VO defects and Cu impurities, we have studied the pathway of CO2 reduction to CH4. Herein, the Cu/FeO(111) accounts for the FeO(111)–O surface in the presence of Cu dopants. Finally, we would propose the CO2 reduction to CH4 over the Cu/FeO(111)–O surface. First principles calculations were performed based on (2 2) FeO(111) slab models with a thickness of eight atomic layers. The upper four layers are allowed to relax and the remaining ones are fixed. All studies were considered uniquely on the polar FeO(111)–O surface. Meanwhile, the Fe-termination was passivated by a monolayer of hydrogen atoms. In addition, the concentration of substitutional Cu dopants at the subsurface varies in the range from 0 to 100%, as abbreviated to nCu (0%), sCu (25%), dCu (50%), tCu (75%), and qCu (100%). We have calculated the surface reducibility by removing oxygen atoms from the first and second anionic layers. The nomenclature of tCu + 4VO is in accordance with the formation of four VO defects at the first anionic layer of the surface (fully reduced) with 3/4 Fe sites substituted by Cu dopants. The total amount of defects, 1–4 and
3516 | Phys. Chem. Chem. Phys., 2014, 16, 3515--3519
Ep,
(1)
where Ed and Ep are the total energies of surfaces with and without defects, respectively; nO and mO are the number and chemical potential of oxygen on the surface. In the reducing atmosphere with H2 gas, we assume the surface oxygen is in equilibration with gaseous H2 and H2O. Reaction energies of hydrogenation processes, Ere(T, p), were calculated with the following expression Ere(T, p) = Ead
mclean
mCO2
nHmH,
(2)
where mclean is the chemical potential of tCu + 4VO surface, Ead is the total energies of tCu + 4VO surface with adsorbates, nH is the number of hydrogen atoms. The chemical potential (m), at a given temperature (T) and pressure (p), of CO2 and H was referred to the corresponding molecules (CO2 and H2) in their gas phases. It was calculated as follows: m(T, p) = m(T, p0) + kBT ln(p/p0),
(3)
where p0 and kB are the pressure under standard condition and Boltzmann constant, respectively. The m(T, p0) was calculated in terms of thermal contributions and zero-point energy (ZPE) corrections. A negative value of Ef (T, p) and Ere(T, p) accounts for an energetically favorable process. Kohn–Sham equations were solved based on the linear combination of atomic orbitals (LCAO) scheme, implemented in CRYSTAL09 code.13 We employed Gaussian-type basis sets to optimize structure and calculate reaction energies for Fe (86-411d41G),14 O (6-31d),15 Cu (86-4111(41D)G),16 H (5-11G*),17 and C (6-31d1G).18 The Perdew–Burke–Ernzerhof19 (PBE) functional was used for electronic exchange–correlation interactions. The irreducible Brillouin zones were sampled with (6 6 1) k-point grids. The nudged elastic band method implemented in the Vienna Ab initio Simulation Package20 with a projector augmented wave (PAW) formalism was employed to calculate the activation energies.21 In Fig. 2, it shows calculated XANES at L3 and L2 edges of Fe cations at the FeO(111)–O subsurface (Fe@sub), in which the XANES intensity reflects the unoccupied d-band states of Fe cations. The unoccupied d-band states of Fe@sub cations are similar to those of Fe3+ in Fe2O3 bulk, instead of Fe2+ in FeO. This is due to the fact the dangling surface bonds from the O anions at the first anionic layer can be saturated, accompanied by the formation of Fe3+ cations. To stabilize a typical polar surface, the conventional scenario is to reduce charge density on the first anionic layer via adsorbates from the environment. However, we found that an alternative change of Fe oxidation states from Fe2+ to Fe3+ can realize the internal suppression of surface polarity. In other words, the anionic and cationic charge stacking, O2 /Fe3+, can replace the usual stabilization mechanism, namely, O /Fe2+. In contrast, the existence of Fe2+ cations cannot directly stabilize the FeO(111)–O polar surface.
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Fig. 2 Computed XANES at (a) L3 edge and (b) L2 edge of the Fe@sub of FeO(111)–O surface, compared with those of Fe cations in FeO and Fe2O3 bulk phase.
More importantly, we could also elucidate the experimental observations in the work of Koike et al.10 Regarding the formation of Fe3+ at the FeO(111)–O subsurface, it could originate from the double exchange mechanism based on Fe2+–Fe3+ pair.22 Therefore, the FeO(111) thin films can be present in ferromagnetism above ´el temperature of bulk FeO. In addition, Gota et al.23 had also the Ne claimed the metastable FeO(111)-like phase contains ferric Fe3+ cations, rather than ferrous Fe2+ cations, in their experiments. In principle, a perfect polar FeO(111)–O surface should be sufficiently stable without polar instability problem. We have calculated the formation energies of VO on the FeO(111)–O surface. Our calculations indeed indicate VO formation is not favorable. For example, it costs 3.46 eV to create a single VO at the first anionic layer of the FeO(111)–O surface under O-rich conditions, referring the mO to the oxygen molecule. Now we turn to think about how to activate the FeO(111)–O surface for CO2 reduction. In other words, we want to reduce the FeO(111)–O surface and create some CUFi sites. The first trial is to decrease the mO in the reactive atmosphere. It should enhance the formation of VO on the FeO(111)–O polar surface. Herein, we have considered the typical industrial pressure and temperature, namely, T = 600 K, p(H2) = 40 bar, p(H2O) = 1 bar. Calculated results show the formation energy of a single VO defect at the first anionic layer can be reduced to 0.78 eV (see Fig. 3). However, it is still extremely difficult to produce an amount of VO defects since the formation of two and three VO defects requires 9.56 and 4.46 eV, respectively. In our previous work,24 we found that Cu dopants can effectively enhance the reducibility of a ZnO(0001)–O polar surface, where the Cu is present as a substitutional dopant. In addition, the oxygen affinity of CUFi sites can be reduced in the presence of Cu dopants. This may avoid the ‘‘overbinding’’ problem reported in the work of Liu et al.8 Therefore, we have calculated the formation of VO on the FeO(111)–O surface with substitutional Cu dopants. Computed formation energies are shown in Fig. 3. Thanks to quite different surface reconstructions, we did not obtain any simple linear correlations between the formation energies of VO defects and the number of VO defects. However, it is quite
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Fig. 3 Calculated formation energies of VO on the FeO(111)–O surface with Cu dopants with respect to the perfect surfaces. The chemical potential of oxygen was referred to the reactive environment, T = 600 K, p(H2) = 40 bar and p(H2O) = 1 bar.
obvious that these Cu dopants always prefer to improve the formation of VO defects. Although it is unfavorable for forming VO defects at the second anionic layer, it is quite feasible to fully reduce the first anionic layer of the FeO(111)–O surface. The optimal concentration of Cu dopants was obtained in the tCu (75%) case. The formation of VO defects essentially produces a number of CUFi sites, exhibiting stronger oxygen affinity, compared with the CUFe sites confined on the Pt metal. Reaction energies of CO2 adsorption and dissociation are calculated and shown in Fig. 4. The adsorbed CO2 can be dissociated to CO and an O atom with a released reaction energy of B1 eV and activation energy of B0.8 eV. This is acceptable in our studied reaction temperature (600 K). In addition, we have considered the subsequent CO hydrogenation processes in H2 atmosphere. The resulting CO molecule can be cleaved as C and O atoms with a reaction energy of around 0.8 eV and activation energy of B0.2 eV. This turns out that the scission of the second C–O bond is quite feasible at the CUFi sites. In Fig. 4, it shows quite nice CH4 selectivity in CO2 reduction. The C atom in the resulting CO molecule has stronger hydrogen affinity compared to the O atom, leading to formation of *CHO and *CH2 species, instead of *COH and *CHOH. In contrast, the O atom from the resulting CO molecule prefers to be adsorbed in the vicinity of the CUFi site, that
Fig. 4 Calculated reaction energies of the pathway of CO2 reduction to methane on the tCu + 4VO surface. The red points show these unfavorable intermediates in CO2 reduction.
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Acknowledgements J. Xiao would acknowledge financial support from the China Scholarship Council (CSC).
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References
Fig. 5 Scheme of CO2 reduction to CH4 in H2 atmosphere over the Cu/ FeO(111)–O surface. Cu: green; Fe: blue; O: red; C: yellow; H: cyan.
is, the VO sites on the Cu/FeO(111)–O surface. In addition, the more accurate calculations with hybrid B3LYP functional can also qualitatively reproduce the trend of CH4 selectivity. Therefore, CO2 reduction reactions should follow the pathway of methanation, instead of producing methanol. Furthermore, in order to accelerate the CO2 reduction, the CH4 desorption from the surface is also quite critical. We have found that the CH4 desorption is pretty favorable (0.02 eV). This should drive the CO2 reduction approaching the pathway of methanation too. In a word, the CUFi sites do have an intermediate reactivity in the presence of Cu dopants. After the CH4 desorption, the tCu + 4VO surface gains two leftover O atoms, which is equivalent to a tCu + 2VO surface. These leftover O atoms can be cleared by hydrogen reduction with a reaction energy of approximately 4 eV (cf. Fig. 3). In the previous work of Tamaura et al.,7 they already found that carbon dioxide can be reduced to carbon atoms using cationexcess magnetite. The cation-excess magnetite was obtained by hydrogen reduction of normal magnetite at B590 K, and is protected in an N2 atmosphere. It could rationalize our calculated results. Furthermore, Shin et al.25 investigated the CO2 decomposition on two types of MFe2O4 (M = Ni and Cu) solid solutions. First, they pointed out that FeO is the intermediate during the MFe2O4 reduction. More importantly, they also found that the redox reaction was indeed accelerated by adding substituted metal (Ni and Cu) impurities. On the basis of the previous experimental studies, we propose the CO2 reduction to CH4 using catalysts in the form of Cu–FeO solid solution in H2 atmosphere. The proposed mechanism is shown in Fig. 5. In conclusion, we have performed X-ray absorption near edge structure calculations to discover the pristine FeO(111)–O polar surface. We have found that Fe cations at the subsurface are Fe3+, instead of Fe2+, as the presence of Fe3+ at the subsurface could effectively suppress the polarity of the FeO(111)–O surface. Meanwhile, the Fe3+–Fe2+ pair could initiate a double exchange mechanism to generate the FeO(111) films in ferromagnetic ordering, which is consistent with experimental observations. Moreover, we have found the formation of oxygen vacancies can be enhanced by substitutional Cu impurities. The resulting oxygen vacancies have high oxygen affinity and low hydrogen affinity, which can serve in CO2 reduction to CH4.
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1 Y. Hori, in Modern aspects of electrochemistry, ed. C. G. Vayenas, R. E. White and M. E. Gamboa-Aldeco, Springer, New York, 2008, vol. 42, p. 89. 2 Y. Hori, H. Wakebe, T. Tsukamoto and O. Koga, Electrocatalystic process of CO selectivity in electrochemical reduction of CO2 at metal electrodes in aqueous media, Electrochim. Acta, 1994, 39, 1833. 3 J. W. Andreasen, F. B. Rasmussen, S. Helveg, A. Molenbroek, K. Ståhl, M. M. Nielsen and R. Feidenhans´l, Activation of a Cu/ZnO catalyst for methanol synthesis, J. Appl. Crystallogr., 2006, 39, 209. 4 B. Hammer and J. K. Norskov, Why Gold is the Noblest of All the metals, Science, 1995, 376, 238. 5 (a) X. Guo, Q. Fu, Y. Ning, M. Wei, M. Li, S. Zhang, Z. Jiang and X. Bao, Ferrous Centers Confined on Core-Shell Nanostructures for Low Temperature CO Oxidation, J. Am. Chem. Soc., 2012, 134, 12350; (b) Q. Fu, W. Li, Y. Yao, H. Liu, H. Su, D. Ma, X. Gu, L. Chen, Z. Wang, H. Zhang, B. Wang and X. Bao, Science, 2010, 328, 1141. 6 X. Deng, J. Lee, C. Wang, C. Matranga, F. Aksoy and Z. Liu, In situ observation of water dissociation with lattice incorporation at FeO particle edges using scanning tunneling microscopy and x-ray photoelectron spectroscopy, Langmuir, 2011, 27, 2146. 7 Y. Tamaura and M. Tabata, Complete reduction of carbon dioxide to carbon using cation-excess magnetite, Nature, 1990, 346, 255. 8 C. Liu, T. R. Cundari and A. K. Wilson, CO2 reduction on transition metal (Fe, Co, Ni, and Cu) surfaces: In comparison with homogeneous catalysis, J. Phys. Chem. C, 2012, 116, 5681. 9 C. Noguera, Polar oxide surfaces, J. Phys.: Condens. Matter, 2000, 12, R367. 10 K. Koike and T. Furukawa, Evidence for ferromagnetic order at the FeO(111) surface, Phys. Rev. Lett., 1996, 77, 3921. 11 Y. L. Li, K. L. Yao, Z. L. Liu and G. Y. Gao, First-principles study of the composition, structure, and stability of the FeO(111) surface, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 72, 155446. 12 A. L. Ankudinov, B. Ravel, J. J. Rehr and S. D. Conradson, Real-space multiple-scattering calculation and interpretation of x-ray-absorption near-edge structure, Phys. Rev. B: Condens. Matter Mater. Phys., 1998, 58, 7565. 13 R. Dovesi, V. R. Saunders, C. Roetti, R. Orlando, C. M. ZicovichWilson, F. Pascale, B. Civalleri, K. Doll, N. M. Harrison, I. J. Bush, P. h. D’Arco and M. Llunell, CRYSTAL09, User’s Manual, University of Torino, Torino, 2009. 14 G. Valerio, M. Catti, R. Dovesi and R. Orlando, Ab initio study of antiferromagnetic rutile-type FeF2, Phys. Rev. B: Condens. Matter, 1995, 52, 2422.
This journal is © the Owner Societies 2014
View Article Online
Published on 23 December 2013. Downloaded by University of Western Ontario on 25/10/2014 21:38:00.
PCCP
15 O. M. Corno, C. Busco, B. Civalleri and P. Ugliengo, Periodic ab initio study of structural and vibrational features of hexagonal hydroxyapatite Ca10(PO4)6(OH)2, Phys. Chem. Chem. Phys., 2006, 8, 2464. 16 K. Doll and N. M. Harrison, Chlorine adsorption on the Cu(111) surface, Chem. Phys. Lett., 2000, 317, 282. 17 R. Dovesi, E. Ermondi, E. Ferrero, C. Pisani and C. Roetti, Hartree-Fock study of lithium hydride with the use of a polarizable basis set, Phys. Rev. B, 1983, 29, 3591. 18 C. Gatti, V. R. Saunders and C. Roetti, Crystal-field effects on the topological properties of the electron-density in molecularcrystals- the case of urea, J. Chem. Phys., 1994, 101, 10686. 19 J. P. Perdew, K. Burke and M. Ernzerhof, Generalized Gradient Approximation Made Simple, Phys. Rev. Lett., 1996, 77, 3865. ¨ller, Efficient Iterative Schemes for 20 G. Kresse and J. Furthmu ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set, Phys. Rev. B: Condens. Matter, 1996, 54, 11169.
This journal is © the Owner Societies 2014
Communication
21 G. Kresse and D. Joubert, From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 59, 1758. 22 C. Zener, Interaction between the d-Shells in the transition metals. II. ferromagnetic compounds of manganese with perovskite structure, Phys. Rev. Lett., 1951, 82, 403. 23 S. Gota, E. Guiot, M. Henriot and M. Gautier-Soyer, Atomicoxygen-assisted MBE growth of a-Fe2O3 on a-Al2O3(0001): Metastable FeO(111)-like phase at subnanometer thicknesses, Phys. Rev. B: Condens. Matter, 1999, 60, 14387. 24 J. Xiao and T. Frauenheim, Activity and synergy effects on a Cu/ZnO(0001) surface studied using first-principle thermodynamics, J. Phys. Chem. Lett., 2012, 3, 2638. 25 H. C. Shin, J. H. OH, J. C. Lee, S. H. Han and S. C. Choi, The carbon dioxide decomposition reaction with (NixCu1 x)Fe2O4 solid solution, Phys. Status Solidi A, 2002, 3, 741.
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