Molecular Adsorption on the Doped (110) Ceria Surface - The Journal

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J. Phys. Chem. C 2009, 113, 2425–2432

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Molecular Adsorption on the Doped (110) Ceria Surface Michael Nolan* Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland ReceiVed: October 21, 2008; ReVised Manuscript ReceiVed: December 8, 2008

Doping of metal oxides can be used to modify their reactivity with respect to oxygen vacancy formation and molecular adsorption, key reactions in the applications of oxides in catalysis. In this article, we study the effects of Ti, Zr, and Hf doping on CO adsorption on the (110) surface of cerium dioxide and NO adsorption at the same surface with oxygen vacancy defects, using density functional theory, corrected for on-site Coulomb interactions (DFT+U). The dopants substitute a Ce atom in the surface layer, resulting in strong structural distortions to the surface and smaller oxygen vacancy formation energies compared to the undoped surface. On the doped surfaces, CO adsorbs much more strongly compared to the undoped surface, forming a structure similar to a carbonate unit [(CO3)2-], in which a CO2 unit points away from the surface. The presence of a carbonate is confirmed by analysis of the electronic structure and vibrational frequencies. On the defective surface with one oxygen vacancy, NO adsorption is weak, so that NO sits above a defective surface. Consistent with the literature, adsorption of two NO molecules at neighboring vacancy sites results in strong adsorption and formation of a NsN-like bond, as well as lengthening of the NsO bonds. However, the energetics of this process at the doped surfaces are not as favorable as at the undoped surface. Computed energetics for CO oxidation and NO reduction show that improving the oxidative power of the oxide makes molecular reduction less favorable. 1. Introduction Cerium oxide is used in oxidation and dehydrogenation catalysis1,2 and has recently been examined as a semiconducting metal oxide gas sensor.3,4 In the Mars-van Krevelen mechanism of redox reactions,5 the molecule to be oxidized takes oxygen from the oxide, creating oxygen vacancies on the surface of the oxide. Either an oxygen molecule from the gas phase can reoxidize the vacancies (with subsequent oxidation of CO to CO2), or a molecule such as NOx can be reduced, thus completing the catalytic cycle.6 Ceria surfaces have a relatively small oxygen vacancy formation energy,7,8 which arises from the ability of ceria to change oxidation state from (formally) 4+ to (formally) 3+. Doping of ceria with other metallic elements, such as Zr, has been shown to enhance thermal stability9,10 and to promote catalytic activity.9-17 It is believed that doping facilitates the reduction of ceria,6,9-18 by weakening the bonds to the oxide of the oxygen atoms around the dopant. A number of computational studies have demonstrated that the oxygen vacancy formation energy is lower when the surface is doped with Zr,9,10,17 Pt,19 Pd,20 and Au,6,18 among others. Replacing a Ce atom on ceria surfaces [the (111) surface for Zr, Pt, Pd, and Au and (110) and (100) for Au] enhances CO oxidation; the CesO and dopantsO bonds are weakened upon formation of the doped material, reducing the oxygen vacancy formation energy and enhancing the reactivity of the surface. Most computational studies of doped ceria have investigated oxygen vacancy formation, with little work on molecular adsorption at the doped surface.6,18 In this article, we use density functional theory corrected for on-site Coulomb interactions (DFT+U) to study the effect of doping the (110) surface of ceria with Ti, Zr, and Hf on CO adsorption and NO reduction, as well as the energetics of CO * E-mail: [email protected].

oxidation to CO2 and NO reduction to N2. We are interested in the (110) surface for a number of reasons: our calculations show that this is the most reactive surface,8 it is exposed on nanorods21,22 and nanoparticles,23-25 and these exposed surfaces show enhanced activity compared to bulk surfaces.21-24 For the choice of dopant, there has been a recent body of computational work on the impact of Zr doping on the vacancy formation processes in the bulk and at the (111) surface,9,10,17 and Ti and Hf have also been studied experimentally as dopants in ceria.26,27 These dopants have formal 4+ oxidation states, which means that there are no issues with charge compensation or oxygen hole formation processes. This work also allows comparison with previous work on CO adsorption at undoped and Au-doped ceria surfaces,6,18,24,25,28 as well as adsorption of NOx at undoped ceria surfaces,29 and provides important information on the impact of doping on the fundamental molecule-surface interactions that take place during redox reactions at ceria surfaces. 2. Methods We used a slab model to describe the ceria (110) surface and a plane-wave basis set to describe the wave functions of the valence electrons.30 The cutoff energy was 396 eV. The projector augmented wave (PAW) approach of Blo¨chl31 was used to describe the interaction between the core and the valence electrons. We used a [He] core for oxygen, carbon, and nitrogen; a [Xe] core for cerium and Hf; and a [Kr] core for Zr. The Perdew-Burke-Ernzerhof (PBE) functional32 was used to account for exchange and correlation. In common with earlier studies,16,17,21-24 we used density functional theory (DFT) corrected for on-site Coulomb interactions (DFT+U), where U ) 5 eV and was applied to the Ce 4f states. For the DFT+U calculations, we have discussed issues surrounding this approach, such as the choice of U and sensitivity of the results to the value of U.7,8,18 k-point sampling was performed using the Monkhorst-Pack scheme, with a (2 × 2 × 1) grid.

10.1021/jp809292u CCC: $40.75  2009 American Chemical Society Published on Web 01/21/2009

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Figure 1. Ceria(110) surface doped with (a) Ti, (b) Zr, and (c) Hf. The dopant is the large gray sphere, Ce is white, and O is red. The structures on the left-hand side show how the surface appears when one looks down on it, and those on the right-hand side show a side view of the surface

The (110) surface is a type II surface with neutral stoichiometric planes so that no dipole moment is present upon cleaving.7,33 The slab model was 11.5 Å (seven atomic layers) thick, with a 15-Å vacuum gap in the direction perpendicular to the surface. For all calculations, we used a surface supercell that is a (2 × 2) expansion (with x and y dimensions of 10.9425 × 7.7375 Å) with eight surface oxygen atoms, so that substitution of one surface Ce atom with the dopant gave an overall dopant concentration of 4% and formation of one oxygen vacancy gave a surface vacancy concentration of 12.5%, with an overall vacancy concentration of 1.8%. This supercell reduces defect-defect interactions and allows better treatment of an adsorbed molecule. Upon introduction of the dopant and the vacancy, all layers except the bottom two were relaxed. For ionic relaxation in a fixed lattice, the forces were relaxed until they were less than 0.02 eV/Å. CO was adsorbed and relaxed at the doped surfaces in a number of configurations to probe the dependence of adsorption energy on the adsorption site. To investigate further the nature of adsorbed species, we used a mass-weighted diagonalization of the second derivative matrix to compute the vibrational frequencies of adsorbed CO, which we compared to the available experimental data. The energies of CO, O2, NO, N2, and CO2 were computed in the same cell as the surface, with a 396 eV cutoff energy and at the Γ-point 3. Results 3.1. Structure of the Stoichiometric, Doped Surfaces. We show the relaxed surface structures for the doped (110) surface in Figure 1a-c; the dopant is the gray sphere in the outermost layer in each surface. In the (110) surface, neutral layers of CeO2 stoichiometry are present. The Ce atoms in the surface layer are coordinated to two pairs of surface oxygen atoms and a pair

Nolan of subsurface oxygen atoms. Each surface oxygen atom is coordinated to two surface atoms and one subsurface Ce atom. Upon doping, Ti moves off its initial lattice site (Figure 1a), and one of the four surface oxygen atoms nearest the dopant is notably displaced from its lattice site. This results in two short TisO distances of 1.82 and 1.93 Å and two longer TisO distances of 2.13 and 3.56 Å, compared to CesO distances of 2.35 Å in the undoped surface. The TisO distances to subsurface oxygen are 1.93 and 1.97 Å. The four short TisO distances are close to those in bulk TiO2, which suggests that the dopant is seeking to replicate its most favored coordination environment and will distort the atomic structure of the host oxide to do so. The shorter dopantsO distances in the surface lead to a longer pair of CesO distances of 2.42 and 2.84 Å, and the longer dopantsO distances lead to a pair of short CesO distances of 2.05 and 2.24 Å, all involving the surface Ce atom nearest the dopant. In the Zr- and Hf-doped surfaces, the dopant remains at its lattice site. Presumably, this arises because Zr and Hf oxides can adopt a fluorite structure, so that the coordination environment of the dopant is, in principle, relatively favorable. However, the smaller ionic radii of Zr and Hf relative to that of Ce34 still produce distortions in the atomic structure around the dopant sites. When Zr is the dopant, there are two pairs of ZrsO distances: a short pair at 2.12 Å and a long pair at 2.39/ 2.45 Å. This changes the CesO distances to a long pair at 2.53 Å and a short pair at 2.26/2.28 Å. When the dopant is Hf, the value for the short pair of HfsO distances is 2.10 Å, and those for the longer pair are 2.36 and 2.38 Å; the corresponding CesO distances are 2.53/2.55 and 2.29/2.30 Å. Distortions to the remainder of the surface are small, indicating that the effect of the dopant is localized. The very strong distortions to the surface structure in Ti-doped CeO2 are a result of the smaller ionic radius34 of Ti4+ (0.62 Å) compared to Zr4+ (0.72 Å) and Hf4+ (0.71 Å), compared to 0.96 Å for Ce4+, and the unfavorable coordination environment in which the dopant is found. The formation energy of an oxygen vacancy near the dopant (which is the most stable vacancy site, Figure 2) is given by

Evac)[E(M0.04Ce0.96O1.982) + E(1/2O2)] E(M0.04Ce0.96O2) (1) The subscript on the oxygen indicates the overall stoichiometry upon removal of one oxygen atom, and M signifies the dopant. If the resulting energy is negative (exothermic), an oxygen vacancy is thermodynamically stable, whereas a positive energy signifies an energy cost to form the vacancy. In the (110) surface, the vacancy site near the dopant involves removing one of the surface oxygen atoms closest to the dopant, and the relaxed structure is shown in Figure 2. The vacancy formation energies for the first oxygen vacancy on the doped surface are +0.31, +0.48, and +0.36 eV for Ti-, Zr-, and Hf-doped (110), respectively. For all dopants, the formation energies are substantially smaller than on the undoped surface, for which the energy of forming one vacancy is computed to be 1.99 eV.8 Thus, doping of the (110) ceria surface with Ti, Zr, and Hf will enhance the oxidative ability of this ceria surface, consistent with previous studies of Zr doping of the bulk10 and the (111) surface.35 In Figure 2, we show the structure of the surface after removing an oxygen atom next to the dopant, which is indicated with a V. The most obvious impact of vacancy formation is that a surface oxygen atom is displaced from its lattice site to

Molecular Adsorption at Doped Ceria

Figure 2. Structures for one oxygen vacancy on the doped (110) surfaces for doping with (a) Ti, (b) Zr, and (c) Hf. The color scheme is the same as in Figure 1, and the vacancy site is indicated with a V in the image. The structures on the left-hand side show how the surface appears when one looks down on it, and those on the right-hand side present a side view of the surface.

bridge the dopant and the neighboring surface Ce atom. This structure has been found in other studies of vacancies in the undoped and doped (110) ceria surface.8,36 With Ti doping, the dopantsO distances are 1.81 Å (to bridging O), 1.98 Å (to surface O), and 1.99 Å (to subsurface O); as for the vacancyfree doped surface, these distances are similar to TisO distances in bulk TiO2. The CesO distances are 2.60 Å (to bridging O) and 2.37 Å (to surface O). With Zr doping, the ZrsO distances are 1.99 Å (to bridging O), 2.09 Å (to surface O), and 2.10 Å (to subsurface O). The CesO distances are 2.36 Å (to bridging O) and 2.37 Å (to surface O) for the Ce atom nearest Zr. Finally, with Hf-doping, the HfsO distances are 1.97 Å (to bridging O), 2.07 Å (to surface O) and 2.06 Å (to subsurface O). The CesO distances are 2.39 Å (to bridging O) and 2.39 Å (to surface O) for the Ce atom nearest Hf. The major difference between the dopants is the geometry around Ti, where shorter TisO distances are found, whereas the Zr- and Hf-doped surfaces show similar geometries, consistent with the ionic radius of the dopants. Ti is by far the smallest ion of those considered, or in other words, it shows the largest deviation from the ionic radius of Ce, so that the largest distortions are for the smallest dopant. Note also that, upon oxygen vacancy formation, the dopantsO distances become shorter than in the vacancy-free surface, as the dopants attempt to maximize coordination to oxygen. To examine the electronic structure of the doped defective surface, we consider the excess spin density and the electronic density of states (EDOS). The excess spin density is the difference between the majority (up) spin and minority (down) spin densities and is a useful tool for showing where the electrons released by formation of a neutral oxygen vacancy reside. In Figure 3a, we show as an example the

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Figure 3. (a) Isosurface of excess spin density for Ti-doped CeO2 with one oxygen vacancy. The spin density isosurface contours represent 0.01 electron/Å3. (b) Ce 4f PEDOS for Ti-doped CeO2 with one oxygen vacancy. The zero of energy is the top of the valence band, and the Fermi level is indicated with a vertical line.

excess spin density for Ti-doped CeO2. The spin density resides on two surface Ce atoms. The isosurface is that of a 4f electron, so that these Ce ions have been reduced from Ce4+ (Ce 4f 0) to Ce3+ (Ce 4f 1) upon vacancy formation. The distribution of the reduced Ce3+ ions is different from that of the undoped surface.8 In the latter, the two Ce ions that are nearest neighbors to the vacancy are reduced; in the case of Ti/Zr/Hf doping, one of these cations is the dopant, which is not as easily reduced as Ce. Therefore, another Ce ion farther from the vacancy site must be reduced, leading to the distribution of Ce3+ shown in Figure 3a. For Zr- and Hf-doped CeO2, the same spin densities are found, so these results are not shown here. The Ce 4f partial EDOS (PEDOS) is shown in Figure 3b. The peaks marked I and III are the Ce 4f contribution to the valence band and the unoccupied narrow band of Ce 4f states respectively. The peak marked II in Figure 3b shows the presence of occupied Ce 4f, and from many previous studies, this peak is indicative of the presence of reduced Ce3+ ions. 3.2. CO Adsorption on the Doped Surface. In this section, we investigate CO adsorption on the doped (110) surface. We have previously studied CO adsorption at the undoped24 and Au-doped (110) and (100) surfaces.18 Unlike ref 6, we make no attempt to study the catalytic cycle for the formation of CO2 at the dopant surface. Instead, we wish to compare the details of CO adsorption at the doped and undoped surfaces, because this is the initial, fundamental interaction involved in CO oxidation at ceria. The energy change upon CO adsorption at the surface is

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Eads)E(M0.04Ce0.96O2sCO) [E(M0.04Ce0.96O2) + E(CO)]

Nolan

(2)

Eads is negative when the surface with adsorbed CO is more stable than the surface separated from CO. The adsorption energy of CO on the undoped (110) surface is -1.95 eV,24 whereas on the doped surfaces, the adsorption energies are -4.30, -4.31, and -4.10 eV for Ti, Zr, and Hf doping, respectively. Consistent with the substantial reduction in the oxygen vacancy formation energy, the energy gain upon adsorption of CO is notably larger on the doped surfaces. Huang and Fabris28 pointed out that U ) 5 eV leads to overbinding of CO at the ceria(111) surface. However, this overbinding is present in both undoped and doped surfaces and thus has no impact on our aim of comparing the reactivity of undoped and doped ceria, the key point being the enhanced reactivity of the doped over undoped surface. The enhanced reactivity to CO adsorption arises because, upon adsorption at a ceria surface, CO pulls surface oxygen atoms from their lattice site: the smaller the oxygen vacancy formation energy, the more this process will be enhanced, and this results in stabilization of the intermediate. We now address the nature of that intermediate. The relaxed structures of CO adsorbed on the doped (110) surfaces are shown in Figure 4. Similarly to the undoped and Au-doped (110) surface, CO adsorbs at these surfaces to form a carbonate-like structure in which two oxygen atoms are abstracted from the surface to bond to the carbon of the carbonyl group, giving a CO3 intermediate. In ref 24, we discussed in detail the structural and electronic features of this intermediate on the undoped (110) surface, and here, we focus on the differences between the undoped and doped surfaces. A notable difference between the doped and undoped surfaces is the orientation of the CO3 intermediate; on the undoped surface, one CsO bond points away from the surface, whereas on the doped surfaces, two CsO bonds point away from the surface. In the latter, one CsO bond is from the original CO species, and the second comes from the surface. The ease with which oxygen can be removed on these surfaces allows an oxygen atom to be pulled farther from the surface than for the undoped surface. Also, looking at the orientation of the adsorbate again, the intermediate is oriented to facilitate dissociation to CO2. This is borne out by the geometry. For each doped surface, two CsO bond lengths are 1.26 and 1.29 Å, and the third (to oxygen nearest the surface) is 1.36 Å (Ti) or 1.38 Å (Zr, Hf). For the undoped surface, there is one CsO distance of 1.23 Å and two CsO distances (to the oxygen atoms nearest the surface) of 1.37 Å. These CsO distances compare to 1.14 Å in free CO,24 1.16 Å in free CO2, and 1.28 Å in Ca(CO)3. Thus, these dopants will be effective in the formation of CO2 from CO. The barrier to dissociation of the intermediate to CO2 and a defective surface for Zr doping is estimated to be 0.37 eV,37 with similar barriers for Ti and Hf doping. Such a barrier will be surmountable under experimental conditions. Looking briefly at the surface, the dopant and neighboring Ce atom in the surface move apart, with the distortions in the surface and first subsurface layer apparent in both the plan view and side view of Figure 4. We compute CsO stretching frequencies in the range of 1734-1764 cm-1 (smallest for Ti and largest for Hf), compared to 1710 cm-1 for the undoped (110) surface25 and 1647 cm-1 for the Au-doped surface.18 These values are consistent with the measured38,39 frequency of 1728 cm-1. For reference, the

Figure 4. Relaxed adsorption structures for CO adsorbed at the (a) Ti-, (b) Zr-, and (c) Hf-doped (110) ceria surface. The left side shows the surface when one looks down on it, and the right side shows the surface from the side, highlighting the structure of the adsorbate. The color scheme is the same as in Figure 1, with the addition of blue spheres for the oxygen atoms of the adsorbate and a black sphere for the carbon of the original adsorbed CO.

computed gas-phase CO stretching frequency is 2103 cm-1, indicating that the adsorbed species cannot be an intact CO molecule. The substantial shift of CsO stretching frequency upon adsorption shows unambiguously that CO does not adsorb intact. The excess spin density and the atom and Ce PEDOS are displayed in Figure 5a,b for CO adsorbed at the Ti-doped surface. (The results for Zr and Hf doping are similar and are not shown.) The spin density shows two Ce atoms reduced to Ce3+, similarly to the undoped surface. For the undoped surface, the reduced Ce atoms are both found in the surface layer; with doping, one reduced Ce atom is found in the surface layer, and one is found in the first subsurface layer. This occurs because (i) the dopants are not as easily reduced as Ce and (ii) upon pulling two oxygen atoms out of the surface layer, the nearest Ce atoms to the sites of the removed oxygen atoms are the surface Ce and subsurface Ce that are shown to be reduced in Figure 5a. The remaining two electrons that result from removal of two surface oxygen atoms are transferred to the intermediate, similarly to the undoped surface.24 In the Ce PEDOS (Figure 5b), the valence band is peak I, and the unoccupied Ce 4f states are indicated by the narrow peak III. Peak II in the Ce PEDOS arises from the formation of the reduced Ce3+ ions, in which the Ce 4f states are occupied. The presence of the reduced Ce ions upon interaction with CO is consistent with the results for the interaction of CO with the undoped and Au-doped (110) surfaces in refs 18 and 24. The PEDOS for the CO3 adsorbate in Figure 5c shows a peak

Molecular Adsorption at Doped Ceria

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Figure 5. (a) Isosurfaces of excess spin density for CO adsorbed at the doped ceria surface. The isosurface contours represent 0.01 electron/Å3, and the color scheme is the same as in Figure 4. (b) Spin-polarized Ce and (c) adsorbed (CO)3 PEDOS for CO on the Ti-doped (110) surface. The zero of energy is the top of the valence band, and the Fermi level is indicated with a vertical line.

structure characteristic of a carbonate (see Figure 7b in ref 24) and provides further evidence of formation of a carbonate group upon adsorption of CO. The wide peak III arises from the interaction of oxygen of the adsorbate with the oxide lattice. 3.3. NO Adsorption at the Doped (110) Surface. To investigate how the doping of the oxide impacts the reactions that occur during molecular reduction (and oxide reoxidation), we studied NO adsorption at the Ti-/Zr-/Hf-doped (110) surface. For NO adsorption, we start with a surface that has one surface oxygen vacancy and adsorb NO with a plausible configuration in which oxygen sits at the vacancy site. After relaxation, the adsorption energies are +0.03, -0.29, and -0.28 eV on the Ti-, Zr-, and Hf-doped surfaces, respectively; these values compare with an adsorption energy of -0.81 eV on the undoped surface. Thus, adsorbed NO is less stable on the doped surface than on the undoped surface. The structures in Figure 6 show NO over a reduced surface, where the molecule remains as NO, consistent with the adsorption energies, which indicate a physisorbed interaction, also found in a study of ref 29 in which DFT with the U correction on Ce was used. The exact position of the molecule depends on the doped surface. From the experimental40-44 literature, NO reduction requires two NO molecules to be adsorbed at the surface. This was also noted in the computational study of Yang et al.,29 although that work was performed using a standard DFT-GGA calculation, with no U correction on Ce. To simulate this possibility, we create two oxygen vacancies in each of the doped surfaces, giving the structures are shown in Figure 7 for the undoped and doped surfaces. The energy costs of the second oxygen vacancy are +4.27, +2.09, and +2.46 eV on the Ti-, Zr-, and Hf-doped surfaces, respectively, compared to +2.30 eV on the undoped surface. Following ref 29, we position a NO molecule at each vacancy site and allow full relaxation of the adsorbates. The adsorption

energies of two NO molecules at two oxygen vacancies are -2.67, -1.85, and -1.69 eV per adsorbed NO for Ti, Zr, and Hf doping, respectively, compared to -2.25 eV per adsorbed NO on the undoped surface. These adsorption energies are consistent with the vacancy formation energies in the previous paragraph: the less favorable it is to form an oxygen vacancy, the more strongly the oxidizing molecule adsorbs at the surface, so as to reoxidize the oxide. Thus, on Zr- and Hf-doped ceria, the adsorption of NO is least favorable, and on Ti-doped ceria, NO adsorption is most favorable, with the undoped surface being intermediate. However, all surfaces show a gain in energy upon adsorption of two NO molecules For each surface, the NO molecules adsorb at the vacancy sites and tilt (Figure 8) so that the N atoms of each adsorbate come together, with NsN distances of 1.29, 1.23, and 1.25 Å for the Ti-, Zr-, and Hf-doped surfaces, respectively, compared to 1.29 Å for the undoped surface. This compares with the gasphase NsN distance of 1.10 Å for N2. Although the nitrogen atoms of the NO molecules certainly approach, there must still be a barrier to formation of N2 and breaking of the NsO bonds. The corresponding NsO distances (the two NsO distances in each pair are the same) are 1.34, 1.42, and 1.40 Å for Ti, Zr, and Hf doping, respectively, and 1.36 Å for the undoped surface, compared to 1.20 Å in free NO. The oxygen atoms of each NO do sit in the vacancy site, but the dopantsO distances of 2.05 (Ti), 2.23 (Zr), 2.16 (Hf), and 2.40 Å (undoped) are longer than the dopantsO distances in the surface. Figure 9a,b shows the spin density for two NO molecules adsorbed at the defective undoped and Ti-doped (110) surfaces. There are two reduced Ce3+ ions in the surface layer whose distribution is similar to that of the surface with one oxygen vacancy. This indicates that the surface is partially reoxidized upon adsorption of NO. Figure 9c shows the N and O 2p PEDOS of the adsorbed NO molecules. The PEDOS is

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Figure 6. Relaxed adsorption structures for NO adsorbed at the (a) Ti-, (b) Zr-, and (c) Hf-doped ceria(110) surface. The left side shows the surface when one looks down on it, and the right side shows the surface from the side, highlighting the structure of the adsorbate. The color scheme is the same as in Figure 1, with the addition of a blue sphere for the oxygen atom of the adsorbate and a black sphere for the nitrogen of the original NO.

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Figure 8. Relaxed adsorption structure for 2NO adsorbed at the (a) undoped and (b) Ti-, (c) Zr-, and (d) Hf-doped (110) ceria surface with two surface oxygen vacancies. The left side shows the surface when one looks down on it, and the right side shows the surface from the side, highlighting the structure of the adsorbate. The color scheme is the same as in Figure 1, with the addition of blue spheres for the oxygen atom of the adsorbate and a black sphere for the nitrogen of the original NO.

4. Discussion

Figure 7. Structure for two vacancies in (a) the undoped (110) surface and the (b) Ti-, (c) Zr-, and (d) Hf-doped (110) surface. This view shows how the surface appears when one looks down on it, and the positions of the vacancy sites are indicated with a V in all cases. The color scheme is the same as in Figure 1.

consistent with a closed-shell molecule, indicating that the adsorbates cannot be NO radicals and, in fact, the surface transfers two electrons, one to each adsorbate, so that the oxide is partially reoxidized.

The introduction of the dopant into the (110) surface of ceria reduces the energy required for formation of an oxygen vacancy in the surface. The vacancy formation energy is still positive, which means that we do not expect oxygen vacancies to form spontaneously, in contrast to gold doping as discussed in ref 18, for example. As mentioned in other studies, the distortion of the atomic structure around the dopant site tends to weaken the CesO bonds and makes removal of oxygen easier. The adsorption of CO onto the doped surface shows the impact of the reduction of the vacancy formation energy, in that the energy gained when CO pulls two oxygen atoms out of the surface is much larger on the doped surface than on the undoped surface, leading to a greater stabilization of the surface intermediate. On the undoped surface, the orientation of the carbonate is such that CsO points away from the surface, with one short and two long CsO distances. At the doped surfaces, the adsorbate is oriented so that a CO2-like species points away from the surface, with two short and one long CsO distances; the longer distance involves carbon and oxygen at the surface. This longer CsO distance is suggestive of formation of a carbonate group that is beginning to dissociate to CO2 and oxygen (left at the surface); we compute a barrier of 0.37 eV

Molecular Adsorption at Doped Ceria

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Figure 9. Isosurfaces of excess spin density for two NO molecules adsorbed at the (a) undoped and (b) Ti-doped (110) ceria surface. The color scheme and isosurface contours are the same as in Figure 5. (c) N 2p and O 2p PEDOS for two NO molecules adsorbed at the defective Ti-doped (110) ceria surface. The zero of energy is the top of the valence band, and the Fermi level is indicated with a vertical line.

for full dissociation to CO2 (at the Zr-doped surface), which should be surmountable under experimental conditions. This demonstrates and explains the enhanced reactivity of Ti-/Zr-/ Hf-doped ceria in CO oxidation. Consistent with experiment40 and a previous first-principles study,29 NO does not adsorb strongly at defective ceria surfaces (whether doped or undoped). The adsorption of two NO molecules at a surface with two oxygen vacancies is a more realistic model. This is because adsorption of NO to reoxidize the surface would leave a lone nitrogen atom, which would be expected to wander the surface until interacting with a surface oxygen atomsgiving NO againsor with an adsorbed NO to give N2O; the latter has not been detected in experiments,40 which plausibly rules out this mechanism. When two NO molecules adsorb at neighboring oxygen vacancies, the molecules tilt so that a bond begins to form between the N atoms of each NO molecule; the resulting NsN distances are longer than in free N2 (1.10 Å), but it is clear that a NsN bond is forming. There must be a barrier to full NsO dissociation and release of N2 from the adsorption structures found in this work. This result was also found for NO adsorption at an undoped (110) ceria surface with standard DFT, indicating that DFT and DFT+U can provide similar descriptions of aspects of the chemistry of ceria. On comparing the dopants, the shortest NsN distance is found for Zr doping, which then has the longest NsO distances (1.42 Å), as well as the longest dopantsO distance (to O of the adsorbate), and it is for Zr doping that the smallest adsorption energy is found. In section 3.1, we showed that, of the three dopants considered, Zr gives the most favorable oxygen vacancy formation energies. Thus, the correlation between oxygen vacancy formation and surface reduction and reoxidation is

clear: enhancing oxygen vacancy formation will enhance the interaction of CO with a doped ceria surface, but the tradeoff will be that the interaction of NOx with the doped surface will not be as favorable. Considering the reactions

2CO + O2 f 2CO2

(3)

2NO f N2 + O2

(4)

which will be affected by ceria doping, the impact of Ti, Zr, and Hf doping on the redox activity of ceria is therefore twofold. When doped with these elements, the oxidative power of ceria is enhanced: a lower oxygen vacancy formation energy is needed, CO adsorbs more strongly at the doped surface, and we expect CO oxidation to be enhanced. On the other hand, the reductive power of ceria could be correspondingly reduced: NO adsorption is weaker and, crucially, so is adsorption of two NO molecules at a defective surface. However, against this, the energetics for NOx adsorption still show a gain, so that this aspect of ceria catalysis can be maintained. Experimental investigation of these points is warranted. 5. Conclusions When the (110) surface of ceria is doped with Ti, Zr, and Hf, first-principles DFT+U computations show that the oxidative power of the oxide is enhanced, as indicated by a reduction in the energy required for the formation of oxygen vacancies and enhanced adsorption of CO. On the other hand, the adsorption of NO at the doped surfaces is less favorable than

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