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Physisorbed, Chemisorbed, and Oxidized CO on Highly Active Cu-CeO2(111) Zongxian Yang,*,†,‡ Bingling He,† Zhansheng Lu,† and Kersti Hermansson*,§ College of Physics and Information Engineering, Henan Normal UniVersity, Xinxiang, Henan, 453007, People’s Republic of China, Henan Key Laboratory of PhotoVoltaic Materials, Xinxiang 453007, People’s Republic of China, and Materials Chemistry, The Ångstro¨m Laboratory, Uppsala UniVersity, Box 538, SE-75121 Uppsala, Sweden ReceiVed: September 23, 2009; ReVised Manuscript ReceiVed: February 4, 2010
With the use of the DFT+U method, the properties of Cu adsorbed on the stoichiometric CeO2(111) surface, Cu-doped CeO2(111) (denoted as Cu0.08Ce0.92O2) surface, and CO oxidation on the stoichiometric Cu0.08Ce0.92O2 surface are studied systematically. It is found that (i) Cu is stable both as an adsorbed atom on the surface and as dopant in the surface region. Cu adsorbed at the surface is Cu(+I) while Cu as a dopant atom is Cu(+II). (ii) The Cu dopant facilitates O-vacancy formation considerably, while Cu adsorption on the stoichiometric CeO2(111) surface may suppress oxygen vacancy formation. (iii) Physisorbed CO, physisorbed CO2, as well as chemisorbed CO (carbonate) species are observed on the Cu-doped CeO2(111) surface, in contrast, on the clean ceria(111) surface, only physisorbed CO was previously observed. C-O distances, adsorption energies, and surface-induced C-O vibrational frequency shifts were used to characterize these species. I. Introduction The copper-ceria system has been identified as a promising and cost-effective catalyst for a range of important reactions, such as the water gas shift (WGS) reaction,1 i.e., CO + H2O f CO2 + H2, selective CO oxidation at low temperature, de-SOx reactions,2 and the production of hydrogen by steam reforming of methanol.3 For the CO oxidation reactions, noble metals are known to work very well,4–6 but the Cu-Ce-O catalysts have demonstrated specific activities far superior to the conventional copper-based catalysts and even comparable with or superior to platinum catalysts.7 One example of selective CO oxidation is the PROX reaction8–11 (preferential oxidation of CO in excess H2) which is of high interest for potential fuel cell applications using proton exchange membrane fuel cells (PEMFC). The ideal fuel for these systems is pure hydrogen, but CO contained in the fuel will poison the Pt alloy anode of the PEMFC, so the search for efficient catalysts to help remove CO from the H2 production process is urgent. The state of copper in Cu-Ce-O catalysts depends much on the synthesis method and conditions, and is not very clear. Without repeating the details of the preparation methods described in the literature, we merely state here that the Cu-Ce-O catalysts for CO oxidation prepared by W. Liu et al.9 in 1995 were found to display a variety of Cu species at the catalyst surface: mainly CuO, but also “isolated” Cu2+ ions incorporated in the ceria lattice, and surface Cu+ ions, which were suggested to constitute strong active sites for CO adsorption on the ceria particles. Ten years later, Avgouropoulos et al.10 summarized the complicated state of affairs and pointed out that the precise state of copper oxide in CuO-CeO2 catalysts was still under debate in the literature since a variety of copper species had been found: highly dispersed copper oxide clusters * Authors to whom correspondence should be addressed. E-mail:
[email protected] (Z.Y.);
[email protected] (K.H.). † Henan Normal University. ‡ Henan Key Laboratory of Photovoltaic Materials. § Uppsala University.
on ceria, “Cu+ substitution at the interface of the two oxidic phases”, and the formation of a solid solution with Cu2+ incorporated into the CeO2 lattice. Avgouropoulos et al.10 also concluded that the absence of CuO peaks in X-ray powder diffraction measurements for some Cu-Ce-O catalysts in the literature is consistent with the formation of finely dispersed copper oxide on the surface of ceria, or the formation of solid solution, or a combination of those two phenomena. Also after 2005, the situation remains complex. Copper-ceria catalysts with different Cu contents were synthesized and characterized in a recent paper by Gunawardana et al.,12 who found metallic Cu to form on ceria during the WGS reaction for samples with a higher molar % of Cu than Ce, but for lower Cu contents only CuO and ceria were identified. Djinovic et al.1 reported both nanosized and bulklike CuO species on the ceria surfaces of their Cu-Ce-O catalysts, but in addition presented evidence of strong metal oxide-support interactions (SMSI) with CuO partly integrated into CeO2, forming a solid solution. However, as mentioned by Djinovic et al., others13 have claimed that since the Cu2+ and Ce4+ ionic radii are very different, the formation of a solid solution, i.e., the incorporation of Cu2+ into the CeO2 lattice, should be very limited. Gamarra et al.14 in 2007 performed systematic preparations and analyses of Cu-Ce-O catalysts prepared by two different methods, one yielding copper-doped samples, Ce1-xCuxO2, the other CuO supported on ceria, CuO/CeO2. Synchrotron-derived X-ray diffraction data for the Ce1-xCuxO2 catalyst was found to be consistent with “substitutional Cu2+ incorporation into the ceria lattice along with formation of oxygen vacancies for charge balance”. For the “CuO/CeO2” preparations with a sample composition of 5 wt % or more of Cu, large CuO nanoparticles were detected, but with a lower Cu content it was not possible to identify any completely Ce-free zones. In the present paper, we make an attempt to clarify some basic issues concerning the interaction between Cu and ceria. We use quantum-mechanical calculations to explore, among other things, the charge state of Cu and the structure around it
10.1021/jp909174u 2010 American Chemical Society Published on Web 02/18/2010
Properties of Cu Adsorbed on CeO2(111) Surface for two limiting model systems, Cu/CeO2(111) and CuxCe1-xO2(111), i.e., for isolated Cu atoms adsorbed on CeO2(111) and for Cu incorporated into the CeO2 lattice, replacing one Ce4+ ion close to the (111) surface. Moreover, the surface activity of the CuxCe1-xO2(111) system will be discussed in terms of the oxidation of a CO molecule on the surface. Two elementary steps of the oxidation process will be treated: the CO adsorption (structures, energies, and the surfaceinduced C-O vibrational frequency shifts) and the oxygen vacancy formation and whether or not it is facilitated by the presence of the Cu dopant. The quantum-mechanical method we use is periodic DFT (density functional theory) calculations of the GGA+U type, i.e., with inclusion of on-site Coulomb interactions for Ce.15,16 We are only aware of two previous theoretical studies of Cudoped ceria in the literature, and none for Cu on ceria surfaces or for CO on Cu-doped ceria surfaces. GGA calculations were thus performed for bulk Cu0.25Ce0.75O2 and Cu0.125Ce0.875O2 by Wang et al.17 in a joint experimental-theoretical study, and by Shapovalov and Metiu18 for a Cu-doped CeO2(111) surface (the latter study actually focused on the CO oxidation process on the Au-doped ceria(111) surface, via carbonate formation). Reference 17 reports an almost planar, 4-fold O coordination around the Cu ions in doped bulk ceria, while at the ceria surface in ref 18 the Cu dopant was found to induce a distorted unsymmetrical structure with three nearest-neighbor Cu-O distances in the range 2.04-2.10 Å. In the present paper, we obtain a close to planar coordination figure around Cu, similar to the one observed by Wang et al.17 for the bulk. Furthermore, in contrast to the results for the Au-doped CeO2(111) surface in ref 18 we do not only observe carbonate species on the surface but also both physisorbed CO and physisorbed CO2 molecules. It should be pointed out that there are several important and interesting aspects of Cu-ceria-catalyzed CO oxidation that we do not treat in this paper, such as the “simultaneous” CO adsorption and oxygen vacancy formation, or the step involving healing of the O vacancies to complete the catalytic cycle for CO oxidation, as explored in detail for Au-doped ceria by Shapovalov and Metiu.18 In view of the scarcity of copper-ceria calculations in the literature, we leave such calculations for the future. The computational details are presented in section II. The calculated results are presented in section III, and a brief summary is given in section IV. II. Method and Systems Computational Methodology. Spin-polarized Kohn-Sham density functional theory calculations were performed using the Vienna ab initio simulation package (VASP).19–22 The electron exchange and correlation were treated within the generalized gradient approximation (GGA), using the Perdew-BurkeErnzerhof (PBE) functional.23 Since regular DFT methods have been found unable to describe the localization of the cerium 4f states in partially reduced ceria, the Hubbard parameter, U, is introduced for the Ce 4f electrons to describe the on-site Coulomb interaction (the DFT+U method);15,16 this helps to remove the self-interaction error and improves the description of correlation effects. Several studies have shown that there is no unique optimal value of U that allows one to capture all the electronic and structural properties of ceria, as well as the energetics of reduction. There is a strong linear dependency of the reduction energy as a function of the value of the effective parameter U.24–26 Huang24 analyzed the CO adsorption energetics as a function of U providing evidence that the value of this
J. Phys. Chem. C, Vol. 114, No. 10, 2010 4487 parameter presently used in the literature (U > 4 eV for GGA) can lead to severe overestimation the binding energy of CO to ceria when surface reduction is involved, while the values U ) 2-3 eV can be more appropriate. Andersson et al.27 pointed out that in order to obtain the correct insulating ground state of CeO2-x, U must satisfy U g 5 eV (GGA). Therefore, the effect of the value of U on the redox reaction energies has not been settled. As a test, we have calculated the energies of adsorption of CO and CO2 desorption at the atop-O3 site on the Cu0.08Ce0.92O2(111) surface using different U(Ce) values (e.g., 2, 4.5, and 5 eV) (Table 1_SI, Supporting Information). It is found that the CO adsorption energies are not dramatically varying with U values. This is because the electrons transferred from the adsorbed CO to the surface mainly filled the O 2p states, instead of localizing on the Ce 4f states. The calculated CO2 desorption energies are almost not changed with the U values. As for electronic structure, the total density of states (TDOS) curves of Cu0.08Ce0.92O2(111) and Cu0.08Ce0.92O2(111) with a CO adsorbate at O3 site for the different U values showed that they are very similar to each other, especially for the states below the Fermi level (Figure 1_SI and Figure 2_SI, Supporting Information). Therefore, it would be appropriate using DFT+U method with U ) 5 eV for Ce 4f to describe the Cu-CeO2(111) systems, which is in agreement with many ceria calculations in the literature and close to the value recommended by Castleton et al.28 in their systematic exploration of the effect of the U value on electron localization and structure of reduced ceria. Also, it is known that DFT fails for the description of copper oxide which has Cu2+ with a d9 electronic configuration. Recently, Wu et al.29 studied native defects in CuO using LDA+U. Despite the description of CuO being consistent with experiment, these authors do not discuss how they arrive at their value of U (U ) 7.5 eV, J ) 0.98 eV). Nolan et al.30 used experimental data for CuO to fit a value of U, tested from 1 to 9 eV, and found the spin moment and direct and indirect band gaps increased with increasing U. The spin moment is 0.64 µB and band gap is 1.48 eV for U ) 7 eV, in good agreement with the range of experimental values in the literature. As a comparison for the Cu-doped ceria system with DFT+U method for U(Cu) ) 0 and 7 eV (with U ) 5 eV fixed for Ce 4f), the effects of the Coulomb repulsion added to Cu 3d on the O-vacancy formation energies, adsorption energies of CO, and desorption energies of CO2 at different sites are discussed in section III. The Kohn-Sham orbitals were expanded using plane waves with the well-converged cutoff energy of 408 eV. The cerium 5s, 5p, 5d, 4f, 6s, the oxygen 2s, 2p, and the copper 3d, 4s electrons were treated as valence electrons. The Brillouinzone integrations were performed using a 4 × 4 × 1 Monkhost-Pack grid and a Gaussion smearing parameter of SIGMA ) 0.2 eV. Test calculations with a much smaller SIGMA (0.05 eV) for the stoichiometric CeO2(111) and the Cu0.08Ce0.92O2(111) systems with and without an O-vacancy and Cu0.08Ce0.92O2(111) with a CO adsorbate gave almost the same energetics as those for SIGMA ) 0.2 eV; e.g., the total energetic differences are within 0.01 eV and the changes in the adsorption energy and desorption energy are 0.005 and 0.0 eV, respectively (Table 2_SI, Supporting Information). In addition, the forces on each atom were nearly converged to the same criterion. Therefore, the smearing parameter SIGMA ) 0.2 eV was used throughout the paper. Systems Studied. CeO2(111) Surface. The ceria (111) surface was chosen as the substrate in this work since it is the most stable31 among the low-index surfaces. The stoichiometric ceria surface system is represented as a slab, periodically repeated in
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Figure 1. Optimized surface structures (“side view” and “top view”) for pure and Cu-decorated ceria(111): (a,b) the clean stoichiometric CeO2(111) structure with a 2 × 2 supercell; (c,d) a Cu atom adsorbed on stoichiometric CeO2(111). For clarity, only the top three layers (top four layers in some other figures) of the substrate are shown. Here and in the following figures, yellow, red, blue, brown, and black spheres represent the Ce, O, Cu, C, and the O-vacancy atoms, respectively.
the z direction (perpendicular to the surface) and separated from its images by 15 Å vacuum gaps. For this system, the periodicity along the x and y directions is described by a 2 × 2 supercell with fixed cell axes based on the optimized lattice parameter for ceria bulk (5.48 Å) with the PBE functional. Here we chose a slab thickness of 9 atomic layers for all our calculations (three O-Ce-O stacks). The three bottom atomic layers were fixed at their bulk positions to mimic the bulk, while the x, y, and z coordinates of the remaining ions were optimized until the forces on the atoms were less than 0.02 eV. The choice of slab thickness was explored by testing the convergence of some of the surface properties with respect to the slab thickness. Thus, we computed the O-vacancy formation energy for stoichiometric CeO2(111) slabs with 9 and 12 atomic layers and the values were -2.93 and -2.90 eV, respectively. The Cu adsorption energy became 2.68 eV for a 9-layer slab, and 2.69 eV for a 12-layer slab. Some previous DFT calculations for ceria(111) in the literature18 used a 6-layer slab (two O-Ce-O stacks) based on convergence tests where the surface energy changed by (only) 0.02 eV between a 6-layer slab and a 12layer slab. The CeO2(111) crystal structure with a 2 × 2 supercell is shown in Figure 1 a,b. Cu/CeO2(111) Surface. One Cu atom was placed above the 2 × 2 supercell of the CeO2(111) slab, on one side of the slab. Different initial positions along the cell diagonal were explored, and the remaining cell was perturbed from its symmetric structure, but the optimized position is the symmetric adsorption site reported in section IIIA. Doped Cu0.08Ce0.92O2(111) Surface. Here one Ce atom in the 2 × 2 supercell of the CeO2(111) slab was removed from the outermost Ce layer on one side of the slab (i.e., the second atomic later, counting the outermost O layer as layer number 1) and replaced by a Cu atom, and the x, y and z coordinates of all atoms in the upper 6 atomic layers were reoptimized. This system, corresponding to a dopant concentration of 8%, is denoted Cu0.08Ce0.92O2(111). All other computational parameters were chosen as for the undoped CeO2(111) system. We have kept the optimized cell parameter from the stoichiometric bulk ceria structure in all our calculations in this paper. Incidentally, Wang et al.17 measured the cell parameter in Cu-doped bulk
Yang et al.
Figure 2. Optimized Cu/CeO2-x(111) structures with (a) the Cu adatom next to an O-vacancy, (b,c) the Cu adatom above the O-vacancy.
Figure 3. Optimized surface structures of Cu-doped CeO2(111) surface: (a,b) symmetric structure of Cu on Ce-vacancy; (c,d) asymmetric structure of Cu on Ce-vacancy.
ceria, and observed a change less than 0.001 Å for Cu dopant concentrations in the range between 0 and 20%. Reduced Ceria Systems: CeO2-x(111), Cu/CeO2-x(111), and Cu0.08Ce0.92O2-x(111). One surface O in the 2 × 2 supercell was removed from the surface area of the CeO2(111), Cu/ CeO2(111), and Cu0.08Ce0.92O2(111) slabs. The results for the reduced undoped surface, i.e., CeO2(111) f CeO2-x(111), were already presented in ref 32. In the case of the Cu/CeO2(111) f Cu/CeO2-x(111) formation, a surface O next to the Cu adatom was removed and the system was reoptimized (Figure 2a). Also another Cu/ CeO2-x(111) calculation was performed by removing one O atom from the optimized CeO2(111) slab and putting a Cu atom in its place and then optimizing the structure (Figure 2b,c). This test was performed just to find out if Cu can adsorb above an O-vacancy and the result is given in section III. In the case of Cu0.08Ce0.92O2-x(111), there are three nonequivalent O-atom sites at the surface (Figure 3c,d). Each of these was removed in turn, and the system was reoptimized in each case. Two surface vacancies were also explored. CO Adsorbed on Cu0.08Ce0.92O2. A CO molecule was placed at a number of different starting positions above the stoichiometric Cu0.08Ce0.92O2 surface, and the coordinates of all molec-
Properties of Cu Adsorbed on CeO2(111) Surface
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ular and slab atoms (except for those in the three bottom layers, which were fixed) were reoptimized. The resulting structures are described in section IIIC. Bulk Cu, Isolated CO, CO2, and O2 Molecules. Reference calculations were performed for these systems. Thus, the properties of bulk Cu was calculated using a primitive unit cell of a fcc lattice and the optimized lattice constant is 3.64 Å.33 The free O2, CO, and CO2 molecules were simulated using a unit cell with dimensions of 12 × 12 × 12 Å3. The calculated equilibrium bond lengths of CO and CO2 are 1.18 and 1.20 Å, which are the same as in our previous work,34 and the experimental values are 1.1335 and 1.16 Å,36 respectively. Our optimized distance for O2 is 1.29 Å, and the experimental value is 1.21 Å.37 Properties Calculated. In this study, the oxygen vacancy formation energy, Evac, is calculated from
-Evac ) E(ceriaslabvac) - E(ceriaslab) + 1/2E(O2)
(1) where E(ceriaslab) is the total energy of the (undoped or doped) ceria slab supercell without a vacancy, E(ceriaslabvac) the total energy of the (undoped or doped) ceria slab supercell with a vacancy, and E(O2) the calculated total energy for the ground state of an optimized O2 molecule in gas phase. The adsorption energy of a Cu atom, Eads, is defined as
-Eads ) E(Cu/ceriaslab) - E(ceriaslab) - E(Cu)
(2) where E(Cu/ceriaslab) and E(ceriaslab) are the total energies of the optimized slabs with and without a Cu adsorbate. The adsorption energy of a CO molecule on the (doped) ceria surface is defined in analogous way, i.e., as
-Eads ) E(CO/ceriaslab) - E(ceriaslab) - E(CO)
(3) where all three energies refer to the respective optimized system. The desorption energy of a CO2 molecule product from the doped ceria surface is defined as
-Ede ) E(ceriaslabvac) - E(CO2/ceriaslabvac) + E(CO2) (4) where E(CO2/ceriaslabvac) is the total energy of the adsorbed system with a CO2 product and E(CO2) the calculated total energy for the ground state of an optimized CO2 molecule in gas phase. With the definitions defined in eqs 1-4, we have a unified picture for the energies of O-vacancy formation, CO adsorption, and CO2 desorption: positiVe energies correspond to exothermic processes. Vibrational frequency calculations were performed for the adsorbed CO and CO2 through numerical differencing of the atomic forces to generate a Hessian matrix. Diagonalization of the mass-weighted matrix gave rise to vibrational frequencies and atomic displacement vectors for the various modes. Only the atoms in the COx species were included in the vibrational analysis. Atomic charges using the Bader scheme38 and electronic densities of states (DOS) were also calculated.
III. Results and Discussion A. Cu/CeO2 Systemswith and without an O-Vacancy. Adsorption of a single Cu atom on the stoichiometric CeO2 (111) surface was investigated for different trial adsorption sites, including atop-O, atop-Ce, above the O-hollow (the hollow site at the center of the triangle created by three surface oxygen atoms, with a subsurface O anion below it), at an O-bridge site (between two surface O atoms) and at a Ce-bridge site (between two Ce ions in the second atomic layer). One adsorption site was found above the surface O, with an adsorption energy of 1.63 eV, but the most preferred adsorption site was found above the O-hollow site (shown in Figure 1c,d). The adsorption energy is 2.68 eV with three nearest-neighbor (NN) Cu-O distances of 2.03 Å and three next-nearest-neighbor (NNN) Cu-Ce distances of 2.88 Å, respectively. It is interesting to note that for Au on ceria, Chen et al.39 also obtained a symmetric metal/ ceria(111) adsorption structure, while for earlier transition metals, like Pd, we obtained an asymmetric location of the metal atom on the ceria(111) surface.40 When Cu is adsorbed in the O-hollow position, there is an electron transfer occurring from the metal to ceria. In terms of Bader charges, this transfer is 0.66 e, so that Cu attains a charge of +0.66. The extra electrons mainly localize on three surface Ce ions. Surface Cu+ ions were mentioned in the Introduction; it appears that isolated Cu ions on the O-hollow site would qualify to be called Cu+, or at least Cu(+I), i.e., with an oxidation state of +I. We have calculated the O-vacancy formation energy, Evac, when Cu is adsorbed on the O-hollow site. When one of the three equivalent NN surface O atoms coordinating the Cu atom is removed, Evac is -3.38 eV, according to eq 1. This Evac value is smaller than for the pure CeO2 (111) surface (-2.93 eV), suggesting that Cu adsorption on the stoichiometric CeO2 (111) surface may suppress oxygen vacancy formation. The resulting optimized Cu/CeO2-x structure is shown in Figure 2a. Figure 2b,c displays another local energy minimum for the Cu/CeO2-x(111) system, obtained from the adsorption of a Cu atom above an O-vacancy on the partially reduced CeO2-x (111) surface. Starting from the relaxed stoichiometric CeO2 (111) surface, and replacing a surface O atom by a Cu atom, the system was subsequently fully relaxed (except the three bottom layers, which is the protocol we have adopted here). It turns out that the Cu atom prefers to stay asymmetrically above the center of the three surface Ce cations around the O-vacancy. The optimized structure and the Cu-O coordination are shown in Figure 2b,c. The Cu/CeO2-x adsorption energy according to eq 2 is 1.46 eV, 0.77 eV smaller than the Cu/CeO2-x adsorption energy in Figure 2a. The two excess electrons created by the O-vacancy formation give rise to a significant electron transfer from the ceria surface to the Cu atom, which obtains a net charge close to -0.5, according to the Bader charge analysis. The remaining 1.5 e are mainly given to the three Ce ions (in the second atomic layer from the surface). For the adsorption of Cu on the reduced surface, CeO2-x(111), different trial adsorption sites, including atop-O, atop-Ce, O-hollow, O-bridge, and Ce-bridge sites were considered. Actually, Figure 2a,c represents two of the representative preferential adsorption sites (O-hollow and O-vacancy) of Cu on the reduced surface (Figure 3_SI and Table 5_SI, Supporting Information). B. Doped Cu0.08Ce0.92O2 Systems. The relaxed structure for Cu-doped CeO2(111), i.e., a Cu atom replacing a Ce atom in the structure, is shown in Figure 3. The relaxed structure, maintaining 3-fold symmetry, is shown in Figure 3a,b, and with the symmetry broken, we obtain the asymmetric structure in
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TABLE 1: Formation Energy (Evac, eV) of an O-Vacancy in the Uppermost O Layer of the Cu-Doped Ceria(111) Surfacea vacancy position
Evac
VO1 VO3 VO4 V* (O2) (second vacancy after VO3)
-0.67 -0.04 -0.90 -2.17
a The labeling of the O atoms is shown in Figure 3d. The labeling of the vacancies is such that VO1 means that O1 has been removed, and so on.
Figure 3c,d. In the first case, the optimized Cu position is symmetric with respect to both the top-layer O atoms and the third-layer O atoms, as it is for Ce in the undoped structure, but the Cu atom (ion) has sunk down along the z direction, toward the subsurface O layer and lies only about 0.14 Å above the center of the three subsurface oxygen ions, with a much shorter Cu-O distance (1.95 Å) than that between the Cu dopant and the oxygen atoms in the top layer (3.07 Å). Assuming that a Ce-vacancy has already been created, the “adsorption energy”, or rather “doping energy”, gained by incorporating Cu into the structure in this position is 6.46 eV. With the symmetry broken, we obtained a more stable and asymmetric Cu0.08Ce0.92O2 structure, as shown in Figure 3c,d, which is accompanied by a stabilizing energy of 1.34 eV. In this structure, the Cu adatom has relaxed to the middle of the two subsurface oxygen ions and adopt a nearly planar structure coordinated by four first-shell O ions (one surface oxygen, O1; two subsurface oxygens, O*2, O*4; and one third-layer oxygen, Ox; cf. the definition of the labels in Figure 3d). The Cu-O bonds lie in the range 1.85-1.89 Å, in agreement with the GGA results of Wang et al.,17 who reported that Cu atom in the bulk has four close oxygen neighbors with distances from 1.92 to 1.95 Å. Viewing Cu-doped ceria in relation to the Cu/ceria and Cu/ reduced ceria structures discussed in section IIIA, we can conclude that, for CeO2(111) surfaces with stoichiometric areas, O vacancies, and Ce vacancies simultaneously present on the surface, it appears that an added Cu atom would energetically prefer to adopt the asymmetric Cu-doped structure shown in Figure 3c,d. However, it must of course be kept in mind that although the Ce-vacancy formation itself is known to occur experimentally under suitable conditions, it is a costly process (we obtain a value of -18.99 eV for the energy E(optimized stoichiometric ceriaslab) - E(optimized ceriaslab with one Cevacancy) - E(Ce atom)). The Bader charge calculated for Cu incorporated in the structure as a dopant is +1.15, even larger than the Bader charge we obtain for Cu in bulk CuO (+1.02). In both cases the oxidation state for Cu is +II. Also Wang et al.17 found in their calculations that the Cu atom is more positive as a dopant in bulk ceria than it is in CuO. O-Vacancy Formation on the Doped Surface. The surface O-vacancy formation energy was calculated for the optimized asymmetric Cu0.08Ce0.92O2 structure, as shown in Table 1. The formation energies for a single neutral oxygen vacancy at the various nonequivalent surface oxygen sites are seen to lie in the range of -0.04 to -0.90 eV, which is much larger than that for the clean stoichiometric surface (-2.93 eV); i.e., vacancy formation is very much facilitated by Cu doping. The formation energy (-0.04 eV) of an oxygen vacancy at O3 site (VO3) indicates that the vacancy formation at this site is nearly spontaneous, which agrees well with the experimental results17
Figure 4. Density of states (DOS) for (a) pure stoichiometric CeO2 (111) surface, (b) unreduced Cu0.08Ce0.92O2(111), (c) reduced Cu0.08Ce0.92O2(111) with one oxygen vacancy, and (d) reduced Cu0.08Ce0.92O2(111) with two oxygen vacancies. DOSs for both spin up (solid lines) and spin down (dashed lines) are shown. The vertical dashed line at E ) 0 eV represents the Fermi energy.
that the Cu dopants always give rise to a corresponding number of O vacancies. The formation of a second oxygen Vacancy from the O2 site on the optimized surface, with an O-vacancy at the O3 site already present (denoted as V*(O2)), also needs less energy with an Evac of -2.17 eV. Although it costs more to make the second vacancy than the first, the vacancy formation energy is still much larger than for the pure surface. The Cu dopant may serve as the seed for the formation of oxygen vacancy clusters on the ceria surfaces. The stoichiometric CeO2(111) system is known to be an insulator, and the electronic density of states (DOS) features (Figure 4a) agree well with previous DFT+U results in the literature. We find that the Cu0.08Ce0.92O2 system is a conductor with finite DOS features (Figure 4b) at the Fermi level and a small DOS peak in the O 2p-Ce 4f gap. These features are not due to any smearing used in the rendering of the DOS maps. The hole states at the Fermi level have O 2p character (as shown by the single states electron density image in Figure 5a) and the small DOS peak in the gap has Cu 3d and O 2p character (the corresponding single states electron density images is shown in Figure 5b). Judging from the DOS for the unreduced system, one may predict that the hole states at or near the Fermi level will be available to accommodate extra electrons and thereby facilitate the formation of oxygen vacancies as indicated by the small Evac values found and discussed above. Indeed, the DOS for the Cu-doped surface with one oxygen Vacancy created at
Properties of Cu Adsorbed on CeO2(111) Surface
J. Phys. Chem. C, Vol. 114, No. 10, 2010 4491 TABLE 2: Optimized Geometries and Adsorption Energies (Eads, eV) for a CO on the Cu-Doped Ceria(111) Surface adsorption geometries
Eads
physisorption long O-bridge(O2-O3) atop-Ce1 atop-Ce2 chemisorption short O-bridge(O5-O6) CO2 formation atop-O1 atop-O3 atop-O4
0.09 0.20 0.18 4.59 2.88 3.50 2.74
TABLE 3: Calculated Harmonic Stretching Frequencies of CO Physisorption and Chemisorption on Cu-Doped Ceria(111)a CO
Figure 5. Single states image for the hole states of the unreduced Cu0.08Ce0.92O2(111) in the energy range of (a) from 0.0 to 0.5 eV and (b) from 0.8 to 1.6 eV, the single states image for the gap states of the reduced Cu0.08Ce0.92O2 (111) with one vacancy (c), and two vacancies (d). The isosurface value is 0.05 e/Å3.
the O3 site (Figure 4c) confirms that these hole states (both those at the Fermi level and the small gap states peak) become occupied after the creation of a surface O-vacancy which gives back two electrons to the lattice. The single states images corresponding to gap states of Figure 4c are shown in Figure 5c, the electrons occupying the gap states are mainly distributed on the Cu and its four NN oxygen neighbors, which is in agreement with the results from the partial-DOS analysis to the gap states. When the O-vacancy is formed, the Bader charge for the Cu dopant decreases from +1.15 to +1.06. When two surface oxygen Vacancies are created on the Cudoped surface (at the O2 and O3 sites), the DOS now shows a larger gap state peak below the Fermi level (Figure 4d). Now all low-lying states involving Cu and O have become occupied and the new contributions to the gap states mainly correspond to the occupation of the Ce 4f states when the two Ce cations neighboring the second O-vacancy become reduced. Again, this is in agreement with the results from the partial-DOS analysis to the gap states. Figure 5d depicts the electron density of this gap peak and shows that two Ce cations neighboring the second vacancy host a large part of electrons occupying the gap states. When the second O-vacancy is formed, the Bader charge for the Cu dopant decreases from +1.06 to +1.00. C. CO Adsorption and Oxidation on the Cu0.08Ce0.92O2(111) Surface. Only weak physisorption of CO was observed in previous studies of the stoichiometric CeO2(111) surface by Yang et al.41 (DFT), Nolan et al. (DFT+U),42,43 and Huang et al.24 (DFT+U). In the present study, we explore whether the presence of the Cu dopant changes this state of affairs. To this end, a CO molecule was placed at various positions on the Cu0.08Ce0.92O2(111) surface, including atop-O, atop-Ce, O-hollow, O-bridge, and Ce-bridge sites, etc. When the CO molecule was oriented with the O-end toward the surface, it was found to either not bind or bind very weakly, so in the following we will only discuss adsorption with the C-end toward the surface. Out of the sites tested, the bridge site between two Ce ions, was found to be nonbinding, regardless of the orientation of the CO molecule. The adsorption energies of all optimized structures are shown in Table 2. We find three types of adsorption when the (111) surface has been doped with Cu: CO physisorption, CO chemisorption, and CO2 physisorption. The C-O stretching
species
ω
CO(g) physisorption atop-Ce1 atop-Ce2 above long O-bridge chemisorption above short O-bridge
2146
∆ω
2149 2144 2101
+3 -2 -45
1743
-403
a
The experimental gas-phase frequencies are given in the text. The frequency shifts are given with respect to the optimized gas-phase CO molecule.
TABLE 4: Calculated Vibrational Frequencies for the Symmetric Stretching Mode (ω1), the Bending Mode (ω2) and the Antisymmetric Stretching Mode (ω3) in the Case of CO2 Formation on Cu-Doped Ceria(111)a CO2 species
ω1
CO2(g) atop-O1 atop-O3 atop-O4
1339 1338 1339 1336
∆ω1
ω2
-1 0 -3
605 600 600 592
∆ω2
ω3
∆ω3
-5 -5 -13
2436 2446 2447 2448
+10 +11 +11
a The experimental gas-phase frequencies are given in the text. The frequency shifts are given with respect to the optimized gas-phase CO2 molecule.
vibrational frequencies, and shifts compared to gas-phase CO and CO2 molecules, are given in Tables 3 and 4. Selected optimized structures are shown in Figures 6 and 7. The CO and CO2 adsorption energies were defined in section II. CO Physisorption. CO physisorption is observed at the “long O-bridge site” between the O2 and O3 surface ions (Figure 6), as well as at the atop-Ce sites, and the adsorption energies are 0.2 eV or smaller. It is well-known that DFT methods are in principle not well suited to describe intermolecular electron correlation effects, which are responsible for most of the CO-surface interaction in the case of CO physisorption on ceria; nevertheless, our values for CO adsorption on the Ce ions are not far from the MP2 and CCSD(T) calculated values for CO physisorption on the stoichiometric CeO2(111) surface (∼0.30 eV).44 The C-O vibrational frequencies for the three physisorption geometries are given in Table 3, and compared with the stretching frequency of a free CO molecule in the gas phase. Our calculated harmonic frequency of a free CO is about 2146 cm-1; the experimental harmonic gas-phase frequency is 2170 cm-1.45 As a rule of thumb, the CO molecule, when adsorbed
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Figure 6. Optimized structures of CO physisorbed on Cu0.08Ce0.92O2(111) surface: (a) above long O2-O3 bridge, (b) atopCe1, and (c) atop-Ce2. The optimized CO distance in the free CO molecule is 1.179 Å.
Figure 7. Optimized structures of CO chemisorbed and CO2 formation on the Cu0.08Ce0.92O2(111) surface: (a,b) short O-bridge, (c) atop-O1, (d) atop-O3, and (e) atop-O4.
with the C-end down, is known to experience a blue shift when adsorbed on a positive ion and a red shift when adsorbed toward a negative species. However, the calculated vibrational frequency shifts, ∆ω, are seen to be a slight blue shift (+3 cm-1) for adsorption on top of the Ce1 ion, and a slight red shift (-2 cm-1) above the Ce2 ion, showing the weak interaction of CO with the substrate and the complex structures in the adsorption modes (both include positive- and negative-species neighbors as shown in Figure 6, b and c). At the long O-bridge site mode, there is a red shift of -45 cm-1. CO Chemisorption. CO chemisorption is found to occur at the “short O-bridge site” between the O5 and O6 surface ions
Yang et al. (Figure 7a,b); the O5 and O6 ions in this figure lie outside the central surface unit cell, which contains the O1, O2, O3 and O4 ions. This is clearly seen from the top view of the structure in Figure 3d). The adsorbed CO molecule binds to two surface oxygen ions and forms a carbonate-like fragment with the CO bond tilting from the surface normal by about 30°. The two surface oxygen ions involved are pulled out from the surface by 0.44 Å. The C-Osurf distances are 0.14 Å longer than the third C-O bond in the carbonate-like complex, and this third bond itself is elongated compared to the intramolecular C-O bond length of the isolated CO molecule, 1.24 Å versus 1.18 Å in a free CO. The geometry of the carbonate species we find here is similar to that found on the (110) surface of stoichiometric ceria.24,41,46 A Bader analysis38 gives a net charge of -1.4 to the carbonate-like species, and seems to be a reasonable value for a bound (CO3)2- ion. Herschend et al.46 reported a net Mulliken charge of -1.7 for their carbonate ion on the (110) surface. Also the C-O frequency shift (the last line in Table 3) are typical of a carbonate species formed on the surface: we find a strong red shift of ∼-400 cm-1 with respect to the free CO molecule, of the same order of magnitude as found in experiments for ceria powder samples with carbonate species and in the theoretical calculations for CeO2(110) by Yang et al.41 and Herschend et al.46 Why is it possible for a carbonate species to form on the Cu-doped ceria(111) surface, but not on the undoped surface? We can think of at least two reasons. First of all, the O-O surface bridge has to be of the right length to form a suitable bidentate complex. On the pure stoichiometric CeO2(111) surface the O-O bridge is 3.87 Å, but a shorter distance seems to be needed. The Cu-doped ceria system offers a range of O-O distances (3.09-4.66 Å as shown in Figure 3d), and the short O-bridge (O5-O6) forming the carbonate has an O-O distance of 3.09 Å. The second factor which we suggest promotes carbonate formation is the fact that carbonate formation involves the reduction of the ceria surface, “outside” the CO3 species (Herschend et al.;46 see also discussion in Huang et al.24). We showed clearly in above that the Cu dopant facilitates reduction of the ceria surface. CO2 Formation. We find CO2 formation on top of all surface O ions (O1, O2, O3, and O4; note that O2 and O3 are structurally equivalent). The optimized structures, with some relevant interatomic distances indicated, are shown in Figure 7c-e. The O ions underneath CO are seen to be pulled out of the surface by about 1 Å or more, resulting in a CO2 species floating on the surface with an oxygen vacancy left in the surface. The nature of the adsorbed CO2 species is similar to a free CO2 molecule with C-O bond lengths close to 1.20 Å and the O-C-O angle of almost 180°. A Bader analysis38 gives a total of 16 valence electrons, the same as for a free CO2. The adsorption energies with respect to the CO molecule, i.e., eq 3 in the Method and Systems section, lie in the range of 2.74-3.50 eV shown in Table 2. The local density of states (LDOS) for the adsorbed CO2 molecule in the strongest of these adsorption modes, at the O3 site, is shown in Figure 8, and compared with that of a free CO2: they are very similar indeed. If we view the adsorption modes at the atop-O sites as the adsorption of a CO2 molecule on the reduced Cu0.08Ce0.92O2-x(111) surface, very weak interactions between the CO2 species and the substrate are found (with the desorption energies, calculated from eq 4, of -0.26 eV, at atop-O1 site; -0.25 eV, at atop-O3 site; and -0.35 eV, at atop-O4 site,
Properties of Cu Adsorbed on CeO2(111) Surface
Figure 8. Density of states for free CO2 (solid line) and adsorbed CO2 (dashed line). The vertical dashed line at E ) 0 eV represents the Fermi energy.
respectively), indicating again that CO at atop-O sites formed a CO2 species. The C-O vibrational frequency shifts, with respect to a free CO2 molecule, as well as the absolute frequencies, are given in Table 4, for the symmetric stretching mode (ω1), the bending mode (ω2), and the antisymmetric stretching mode (ω3). The experimental vibrational spectra for the isolated CO2 molecule are made slightly complicated by the fact that a so-called Fermi resonance occurs between the symmetric mode and the first overtone of the bending mode, shifting, e.g., the symmetric mode by some 50 cm-1. This is an effect of anharmonicity, and anharmonicity is of course always present in experimental, real-life situations. However, also “harmonic vibrational frequencies” have been presented in the literature, deduced from measurements of many vibrational energy levels. The experimental harmonic values given by Dennison47 from the analysis of infrared experiments are 1351 cm-1 for the symmetric mode, 672 cm-1 for the bending mode, and 2394 cm-1 for the antisymmetric mode. Our free-molecule values are given in the first row of Table 4. The frequency shifts of the adsorbed CO2 molecule involving the O3 atom are seen to be very small, confirming that, indeed, the new adsorption species is like a weakly adsorbed CO2 molecule. We conclude that the Cu-dopant facilitates the O-vacancy formation and the oxidation of CO to CO2. D. Effects of Coulomb Repulsion Term U on Cu Dopant. For the Cu atom and Cu adsorbed on the stoichiometric CeO2(111) system, Cu shows oxidation states of 0 and +I, respectively, and the DFT+U (U(Ce) ) 5 eV and U(Cu) ) 0 eV) can described the properties correctly. When the Cu ions have a +II oxidation state, with a 3d9 electronic configuration, there might be a problem for the description of this material using DFT (U(Cu) ) 0 eV), due to the approximate nature of the exchange functional in DFT, which presents the nonzero self-interaction error. Similar material such as NiO with a 3d8 electron configuration has been reported.16 So we only gave the DFT+U (U(Ce) ) 5 eV), for U(Cu) ) 0 and 7 eV, comparison for the Cu-doped system where Cu shows +II. The O-vacancy formation energies, adsorption energies of CO, and desorption energies of CO2 at different sites for U(Cu) ) 0 and 7 eV, with U ) 5 eV fixed for Ce 4f, are calculated. The results are shown in Table 3_SI. (Supporting Information). It is found that the Coulomb repulsion added to the Cu 3d affects different reactions to a different degree. For example, the O-vacancy formation energies at different sites
J. Phys. Chem. C, Vol. 114, No. 10, 2010 4493 are increased (less endothermic) by about 0.45-0.49 eV, respectively. Similarly, the Coulomb repulsion of Cu 3d enhances the CO chemisorption and increases the adsorption energies by 0.43-0.50 eV, while it had nearly no effect on the CO physisorption and CO2 desorption energies. More importantly, the qualitative order of stabilities, e.g., the site preference for oxygen vacancy formation, CO adsorption, and CO2 desorption, etc. are the same for U(Cu) ) 0 eV and U ) 7 eV (fixed U ) 5 eV for Ce). In addition, the structures of Cu0.08Ce0.92O2(111) with and without a CO adsorbate for U(Cu) ) 0 and 7 eV (fixed U ) 5 eV for Ce) are almost the same, except that the distances between Cu and its four nearest O atoms changed within 0.02 Å. Also, the trends of electrons lost by Cu atom in different structures are similar (Table 4_SI, Supporting Information). Therefore, the reduction properties of the Cu-doped ceria, and the processes for CO adsorption and CO2 desorption on the Cu-doped ceria surface can be, to some extent, reasonably described using DFT+U method with U ) 5 eV for Ce and U ) 0 eV for Cu. IV. Conclusions The model systems for Cu-CeO2(111) (i.e., Cu adsorbed on the stoichiometric CeO2 (111) surface and Cu-doped CeO2 (111) surfaces) and the adsorption of a CO molecule on the stoichiometric Cu0.08Ce0.92O2 surface were studied systematically using the DFT+U method. It is found that Cu is stable both as an adsorbed atom, Cu(+I), on the surface and as dopant, Cu(+II), in the surface region. This is qualitatively in agreement with the experiment,9 where a variety of Cu species were found at the catalyst surface: mainly CuO, but also “isolated” Cu2+ ions incorporated in the ceria lattice, and surface Cu+ ions. Also the synchrotron-derived X-ray diffraction data for the Ce1-xCuxO2 catalyst by Gamarra et al.14 confirmed the view that “substitutional Cu2+ incorporation into the ceria lattice along with formation of oxygen vacancies for charge balance” in ref 17 and our results. Cu adsorbed on the stoichiometric CeO2 (111) surface shown an oxidation state of +I may suppress the oxygen vacancy formation; in contrast, the Cu-doped Cu0.08Ce0.92O2 system shown an oxidation state of +II is an exceptional catalyst with superior activity. The Cu dopant induces large perturbation on the geometric and electronic structure of Cu0.08Ce0.92O2 which facilitates the formation of surface oxygen vacancies and enhances the reaction of CO on the surface. Besides physisorption as on the pure CeO2(111) surface, chemisorption with carbonate-like CO3 species and direct formation of CO2 with an oxygen vacancy left on the surface were also found. This is in agreement with a recent paper in which Gamarra et al.48 confirmed CO preferential oxidation over CuO/CeO2 and found carbonate species on the Ceria surface due to the CuO/Ceria interstitial sites. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 10674042) and Innovation Scientists and Technicians Troop Construction Projects of Henan Province of China and the Swedish Research Council (V.R.). Supporting Information Available: The calculated energies for the DFT+U method with different U(Ce) values and different SIGMA values; the calculated O-vacancy formation energies, adsorption energies of CO, and desorption energies of CO2 at different sites, number of electrons lost by the Cu atom in different systems with different U(Cu) values; different trial adsorption sites
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