Effect of N- and P-Type Doping on the Oxygen-Binding Energy and

Article pubs.acs.org/JPCC

Effect of N- and P‑Type Doping on the Oxygen-Binding Energy and Oxygen Spillover of Supported Palladium Clusters Arthur C. Reber and Shiv N. Khanna* Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, United States S Supporting Information *

ABSTRACT: The oxygen-binding energy is one of the primary factors determining catalytic activity in oxidation reactions. One strategy for controlling the binding of a reactant to a surface is to dope the surface to create complementary donor−acceptor pairs. As oxygen is an acceptor, we have investigated the effect of doping on the oxygen-binding energy on Pd atoms and clusters supported on a rutile TiO2(110) surface. We find that the P-type doping of the TiO2 surface dramatically reduces the O-binding energy to Pd. When extended to Pd4-supported clusters, we find that the P-type dopant decreases the energy for the oxygen to bind at spillover sites directly to the TiO2 surface. In Pd4O2, the oxygenbinding energy is reduced with P-type doping, suggesting that this strategy may be used to control the oxygen-binding energy to supported catalysts.

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INTRODUCTION One mechanism for controlling the binding energy of two species on a surface is through the use of donor−acceptor pairs.1−3 When a species that prefers to donate charge and a species that prefers to accept charge are coadsorbed on a surface, their binding energy to the surface is dramatically increased. This is caused by the surface-mediated charge transfer between the donor and acceptor pair that forms a strong ionic bond and due to the redox energy gained by the electron transfer. Because the charge transfer enhancement is a cooperative process that requires both species, the removal of either species carries a severe energy penalty. Horia Metiu and co-workers have championed this concept using the terminology of Lewis base for a donor and Lewis acid for an acceptor.1 We prefer the terminology donor and acceptor because a Lewis acid and Lewis base are typically defined as species which accept or donate an electron pair, and in many of the cases we have studied, unpaired charge density is transferred from a donor to an acceptor. Furthermore, if multiple donors or multiple acceptors are bound together on a surface, their binding energy is expected to be reduced. The surface itself may behave like a donor or acceptor,4 and one strategy for tuning the binding energy of a reactant is to dope the surface. A P-type doped surface will act as an acceptor, so other donors will have enhanced binding energies and acceptors reduced binding energies. This allows the powerful concepts used in doping semiconductors and applies them to supported catalysts. The oxygen-binding energy is one of the most important factors for determining the catalytic activity of a system. The oxygen-binding energy needs to be large enough to dissociate O2; however, if the oxygen is bound too strongly to the surface of the catalyst, the reaction will be stuck in an intermediate state and will be slow to proceed. These competing factors © 2014 American Chemical Society

produce the well-known volcano plots that indicate the optimal oxygen-binding energy for a specific reaction. It is therefore desirable to be able to tune the oxygen-binding energy to a catalyst. Of particular interest is combustion catalysis, in which the lower the oxygen-binding energy to a catalyst the more rapid the combustion.5−7 Therefore, we would like to investigate the effect of doping on the oxygen-binding energy as a strategy for optimizing supported catalysts. Our hypothesis is that the P-type doping of a surface will reduce the O-binding energy of a surface-supported cluster and that the N-type doping of a surface will enhance the O-binding energy of supported clusters. To test this hypothesis, we have investigated the effect of doping on the oxygen-binding energy of PdO and Pd4O1,2 clusters on a rutile TiO2(110) surface. Supported Pd clusters are of great interest as model catalysts.8−26 We find that the Obinding energy is larger on the N-type TiO2 surface for the PdO cluster in which we have replaced a Ti atom with V-doped and smaller for the P-type TiO2 surface in which a Ti atom is replaced with a Sc atom. This effect is caused by the cooperative binding through donor−acceptor pairs. In the case of the Pd4 clusters, we find that the situation is more complicated. First of all, the V-doped clusters increase the tendency for the oxygen atoms to adopt a spillover structure in which the O atom is bound directly to the surface, while the Scdoped reduces the tendency for the oxygen to adopt a spillover geometry. In the case of Pd4O, the oxygen-binding energy is increased for the Sc-doped and V-doped clusters, except when the oxygen atom is in a spillover site. In Pd4O2, the expected Received: May 7, 2014 Revised: August 14, 2014 Published: August 15, 2014 20306

dx.doi.org/10.1021/jp5045145 | J. Phys. Chem. C 2014, 118, 20306−20313

The Journal of Physical Chemistry C

Article

trend is followed with the O-binding energy on Sc-doped surfaces being noticeably lower than on the V-doped surface. This is due to the donor−acceptor phenomenon; however, the cluster acts as a multiple donor, so that oxygen-binding energy is difficult to control at very low oxygen coverage. As the doping allows a strategy for reducing oxygen spillover and that most catalysis is likely to occur at high oxygen coverage, we find that the use of donor−acceptor pairs as a strategy to control the catalysis may be extended to clusters.

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METHODS

The electronic structure was investigated using a GGA+U density function framework using the gradient-corrected functional proposed by Perdew et al. on a 6 × 2 slab of rutile TiO2(110).27 A value of 4.5 eV was used for the U of Ti,28,29 2.0 eV for Sc, and 2.5 eV for V to incorporate the strong correlation effects. The doping was performed by replacing a surface Ti atom with a Sc atom in the P-type surface and by replacing a Ti atom with V in the N-type surface. The calculations were performed using the VASP code, and the Kohn−Sham orbitals were expanded using a plane wave basis set.30 The projector-augmented wave method was used to treat electron−ion interactions.31,32 Convergence tests were carried out to ensure reliable total energies, and a kinetic energy cutoff of 400 eV was found to give convergent results for the plane wave basis set. Due to the large size of the surface supercell, only the Γ point was used for Brillouin zone integration. We carried out geometry optimizations using a conjugate-gradient algorithm, and the structures were not considered optimized until the forces on the atoms were minimized to 0.01 eV/Å or less. Throughout this study, we performed several Bader charge analyses to determine the electronic charge on individual atomic sites.33

Figure 1. Structure and total density of states for the TiO2(110) surface, Pd on TiO2(110), O on TiO2(110), and PdO on TiO2. The binding energies and Bader charges are labeled. Ti is gray, and O is red when incorporated in the surface and blue when adsorbed on the surface. Pd is blue-green.

−0.92 e− confirming that the O is serving as an acceptor. Pd and O coadsorption greatly enhances the binding energies, validating the importance of complementary donors and acceptors in the binding energy on surfaces. Next we investigate the effect of doping the TiO2 on the binding energy of Pd and O. We replace a Ti atom with a Sc atom with one fewer valence electron to serve as an acceptor in a P-type doped surface, and we replace a Ti atom with a V atom with one additional valence electron to serve as a donor in an N-type doped surface. Figure 2 shows the geometry and

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RESULTS To investigate the role of dopants on the oxygen-binding energy in Pd-supported clusters, we first examine the energetics and electronic structure of Pd, O, and PdO bound to TiO2. We note that Pd atoms on TiO2 are not found to be catalytically active for CO oxidation, and single atom catalysis has been observed on other supports and in the gas phase.34−39 Figure 1 shows that the pristine TiO2(110) slab has a band gap energy of 2.01 eV, which is less than the calculated 2.39 eV gap of the bulk due to the presence of surface states. The experimental band gap of the solid is 3.03 eV.40 The binding energy of the Pd atom to TiO2(110) is 1.62 eV, and the charge on Pd is +0.23 e−, indicating that the Pd atom has donated some charge to the surface. The binding of Pd reduces the band gap energy to 1.14 eV and introduces three filled states into the gap, an indicator that the Pd atom will act as a donor. The binding of O on top of a Ti atom introduces two unfilled orbitals slightly above the Fermi energy, an indicator that the O atom will act as an acceptor. The binding energy of the O atom is 1.30 eV, and the net Bader charge on the adsorbed O is −0.53, showing that it is accepting charge from the surface. The coadsorption of Pd and O to the surface causes the binding energy of Pd to increase from 1.62 to 4.96 eV and the binding energy of the O to increase from 1.30 to 4.63 eV. This increase in binding energy is a signature of enhanced binding due to complementary donor−acceptor pairs. The Bader charge on Pd has changed from +0.23 to +0.92 e−, confirming that the Pd is serving as a donor, and the Bader charge on O has increased from −0.53 to

Figure 2. Structure and density of states for the Sc-doped TiO2(110) surface, V-doped TiO2(110), and with Pd adsorbed on Sc-doped TiO2(110) and O adsorbed on V-doped TiO2. The binding energies in eV and Bader charges are labeled.

electronic structure of the Sc-doped TiO2. The geometric structure of the Sc-doped TiO2 has the Sc atom pushed slightly out of the surface, and the Sc−O bond length stretches to 2.09 Å versus the Ti−O bond distance of 1.97 Å. The Bader charge on Sc is +2.10 e− vs +2.38 e− on Ti. Replacing the Ti atom with Sc makes the surface more stable by 0.43 eV. The electronic structure of the Sc-doped surface shows that there is one 20307



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binding site suggests that the enhancement is primarily due to the redox energy gained by the electron transfer rather than a simple ionic bond. For the V-doped surface, the Pd binding energy is 1.68 and 1.64 eV, marginally larger than the 1.62 eV of the pristine surface. The Bader charges are +0.33 e− and +0.23 e−, again effectively the same as that of the pristine surface. The binding energy of the Pd atom is enhanced on the P-type Sc-doped surface because of the formation of a complementary donor−acceptor pair, and the effect on the binding energy of Pd to the N-type V-doped surface is negligible. The variation of the O atom binding energy as a function of the site is shown in Figure 3C, 3D, and 3E. The O-binding energy on top of the Sc-doped site is 1.32 eV, and the geometry is found to be a bridge site between two Ti sites, rather than on top of the Sc atom. This structure is shown in the Supporting Information (Figure S1I). The binding energy drops to 1.15 eV when the O atom is moved away from the doping site. This binding energy is essentially the same as for pristine TiO2. The O-binding energy on top of the V-doped surface is 3.31 eV and becomes 2.21 and 1.98 eV when the O atom is moved one and two sites away from the dopant. The decrease in binding energy with distance is due to the ionic character of the binding enhancement, although some of the enhancement is likely due to the redox energy. The Bader charge on the adsorbed O atom is more negative than on pristine TiO2, confirming that the enhanced O-binding energy is due to the O atom and V-doped surface acting as an acceptor−donor pair. The strong V−O bond also contributes to the enhanced binding of O on top of the V site. The oxygen-binding energy of Pd and O coadsorbed on the pristine and doped surface is shown in Figure 4. The O-binding energy of the PdO molecule is 4.63 eV, and if the O atom is moved by one and two Ti sites away from the Pd atom, the O removal energy decreases to 2.65 and 2.53 eV, respectively. The oxygen-binding energy is enhanced by the ionic bonding between the adsorbed Pd and O atoms, so the oxygen-binding energy decreases when the complementary donor−acceptor pair is separated, although at 8.48 Å much of the 2.53 eV binding enhancement is due to the redox energy. The binding energy is significantly enhanced as compared to the pristine surface. We examine the P-type, Sc-doped surface in the case where the Sc dopant is underneath the Pd atom and the case where it is two Ti sites away from the Pd atom, as shown in Figures 4A and 4D. When the Pd−O molecule is intact, the O removal energy is 3.63 and 3.64 eV, significantly lower than the 4.63 eV O removal energy for the pristine TiO2 surface. The reduction in O-binding energy is caused by the Sc acceptor stabilizing the Pd donor. The Pd binding energy is already enhanced by the presence of the Sc acceptor, so the addition of an additional acceptor, the O atom, does not enhance the binding. For the case where the Pd is on top of the Sc dopant, the binding energy drops to 1.58 and 1.50 eV as the adsorbed O atom is moved to more distant Ti sites. These binding energies are both more than 1 eV lower than for the pristine TiO2 surface. When the Sc dopant is two sites away from the Pd atom, the binding energy drops to 0.89 eV when the O atom is on top of the Sc dopant and is 1.46 eV at the more distant site from the Pd atom as seen in Figure 4D. The O binds most weakly to the Sc site. The O-binding energy for the coadsorbed Pd−O is dramatically lower in the P-type Sc-doped surface than the pristine surface. This demonstrates that doping the surface may

unoccupied orbital 0.06 eV above the Fermi level, suggesting that the surface should serve as an acceptor. The Pd atom binding energy increases from 1.62 eV in the pristine surface to 2.68 eV in the Sc-doped surface, and the Bader charge increases from +0.23 e− on the pristine surface to +0.58 e− in the Scdoped surface. Both of these results point to the Pd atom and Sc dopant serving as a complementary donor−acceptor pair. The density of states shows that the unoccupied acceptor level is no longer filled, confirming that the enhanced binding energy is due to Pd and Sc serving as a donor−acceptor pair. We next explore the effect N-type doping on the oxygenbinding energy of TiO2 by replacing a Ti atom with a V atom. In the case of V-doped TiO2, the V−O bond shrinks to 1.95 Å as compared to 1.97 Å of Ti−O, and the Bader charge on the V is +2.11 e−. Replacing the Ti atom with a V atom reduces the stability of the surface by 0.03 eV. The density of states shows an extra orbital at the Fermi level which may serve as a donor. The binding of O at an adjacent Ti site results in a binding energy of 1.98 eV which is 0.68 eV larger than the 1.30 eV binding energy of the pristine TiO2 surface. The Bader charge on O has decreased to −0.79 e−. The O atom wants to accept two electrons, while the V donates one; so, the density of states now appears as an acceptor, with one unoccupied orbital just above the Fermi energy. The enhanced binding energy and enhanced charge transfer demonstrate that the V-doped TiO2 serves as a donor, and the adsorbed O atom serves as an acceptor in a complementary donor−acceptor pair. We next examine the Pd binding energies at different sites on the Sc- and V-doped TiO2 surface. Figure 3A and 3B shows the

Figure 3. (A) and (B) Position of a Pd atom adsorbed to the TiO2 surface with the binding energy and Bader charge of the Pd atom being given in the table. (C), (D), and (E) The position of O adsorbed to the TiO2 surface, with the binding energy and Bader charge on the O atom given in Table 1.

binding energy of Pd on top of the dopant and adjacent to the dopant, and the binding energies and charges are listed in Table 1. The binding energy of Pd on the Sc-doped surface is 2.66 and 2.68 eV, both larger than the 1.62 eV binding energy on pristine TiO2. The Bader charges on both sites are +0.56 e− and +0.58 e−, both more positive than the +0.23 e− of the pristine surface. The fact that the binding energy is independent of the 20308



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Table 1. Pd- and O-Binding Energies and Bader Charges on Sc-Doped, Pristine, and V-Doped TiO2a Sc-doped Pd/O BE Pd (A) Pd (B) O (C) O (D) O (E) a

2.66 2.68 1.32 1.32 1.15

eV eV eV eV eV

TiO2

V-doped

Pd/O Chg

Pd/O BE

Pd/O Chg

+0.56 +0.58 −0.60 −0.60 −0.43

1.62 eV

+0.23

1.30 eV

−0.53

Pd/O BE 1.68 1.64 3.31 2.21 1.98

eV eV eV eV eV

Pd/O Chg +0.33 +0.23 −0.58 −0.80 −0.79

Structures are shown in Figure 3.

CO oxidation. Figure 5 shows the structure of Pd4 on pristine TiO2 to be planar square. The binding energy of Pd4 to pristine

Figure 4. O removal energy of PdO adsorbed on doped and pristine TiO2 as a function of the position of the O atom. Pd atom is relaxed at the site indicated. (A) and (D) are Sc-doped TiO2 with the dopant underneath and two sites from the Pd atom; (B) is pristine TiO2; and (C) and (E) are V-doped TiO2 with the dopant underneath and two sites from the Pd Atom.

Figure 5. Structure of Pd4 deposited on Sc-doped, pristine, and Vdoped TiO2.

result in a significant reduction in the oxygen-binding energy when coadsorbed with a donor species such as Pd. The oxygen-binding energy of Pd and O coadsorbed on the V-doped and pristine surface is shown in Figure 4C and 4E. The O removal energy from the Pd−O molecule is 4.65 and 4.63 eV, quite similar to the 4.63 eV O removal energy of the dopant-free surface. The reason there is minimal change in the O removal energy is because the Pd-binding energy is unaffected by the presence of the V dopant as seen in Figure 3. However, when the O atom is bound to sites further from the Pd atom, the O-binding energy is enhanced. The binding energy where the O atom is on top of the V dopant is 4.34 eV and at 2.98 eV when the dopant is underneath the Pd atom, versus 2.65 eV for dopant-free TiO2, and is noticeably larger than the dopant-free surface. This shows that the N-type surface has little effect on the PdO molecule; however, when spillover-like binding occurs where the O atom moves away from the Pd, the binding energy is enhanced. This indicates that the N-type dopant enhances spillover. To understand the effect of doping on the oxygen-binding energy of catalysts, we have determined the structure of the Pd4 cluster on pristine, Sc-doped, and V-doped TiO2. Although the Pd atom is useful for understanding the binding energies on the surface, it is not an effective oxidation catalyst, and experiments on size-selected Pd4 have found it to have catalytic activity for

TiO2 is 2.93 eV; the HOMO−LUMO gap is 0.61 eV; and the average net charge on the Pd atoms is +0.11 e−. On the Scdoped surface, the Pd4 cluster becomes tetrahedral when the cluster is on top of the Sc dopant, with the tetrahedral geometry being 0.13 eV more stable than the square planar structure when the cluster is on top of the Sc atom. The planar structure is more stable when the cluster is a short distance from the dopant and is 0.06 eV more stable than the tetrahedral geometry. The square planar structure of Figure 5A is most stable, although it is only 0.01 eV more stable than the tetrahedral structure atop the Sc atom. The average Pd charge is +0.18 e− and +0.19 e− for Figure 5A and 5D, respectively, as compared to +0.11 e− for Pd4 on the pristine surface showing that the Pd cluster is donating charge to the Sc acceptor. This explains why the binding energy of Pd4 to the Sc-doped surface has increased from 2.93 eV on the pristine surface to 3.69 and 3.70 eV, respectively, for the two binding sites on the Sc-doped surface. Thus, P-type doping enhances the binding energy of Pd clusters to the surface. The binding energy of Pd4 to the Vdoped surface is 3.01 and 2.98 eV, respectively, and the average charges of the Pd atoms on the surface are +0.11 e− for Figure 5C and +0.15 e− for 5E. On the V-doped surface the Pd4 is planar in both cases. The tetrahedral geometries are less stable than the planar geometries by 0.31 and 1.39 eV, for reference, the energy difference between planar and tetrahedral Pd4 on 20309



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pristine TiO2, in which the planar structure is 0.33 eV more stable. We find that the Sc-doped surface substantially increases the binding energy of the Pd4 cluster to the surface because the cluster and Sc dopant act as a complementary donor−acceptor pair. We next consider the structure of the partially oxidized cluster Pd4O2 on pristine TiO2, Sc-doped TiO2, and V-doped TiO2. On pristine TiO2, Pd4O2 has a geometry in which one of the O atoms is bound to both the Ti atom on the surface as well as a Pd atom from the cluster as seen in Figure 6B. The

Figure 7. O removal energy of Pd4O2 adsorbed on doped and pristine TiO2 as a function of the position of the O atom. The bottom cluster indicates the structure where both O atoms are adsorbed on top of the cluster, and the above indicate the O-binding energy in which one O atom is on top of the Pd4 cluster and the second O atom is spilled over onto the TiO2 surface at successive distance from the cluster.

Figure 6. Ground-state structure of Pd4O2 deposited on Sc-doped, pristine, and V-doped TiO2.

second O atom is bound to a 3-fold site on top of the Pd cluster. The lowest energy structure of Pd4O2 on V-doped TiO2 has a similar geometry, although the surface-bound O atom is bound to the V atom on the surface rather than a Ti atom on the surface. For Pd4O2 on the Sc-doped surface, however, both O atoms are bound on top of the Pd cluster, as shown in Figure 6A. These surface-bound oxygen atoms are precursors to O spillover, which renders the cluster catalytically inactive and contributes to the poisoning of catalysts, so the Sc dopant may improve the activity of the Pd4. We next compare the O-binding energy for the Pd4O2 on pristine TiO2, Sc-doped TiO2, and V-doped TiO2, as a function of the binding site. For pristine TiO2, as shown in Figure 7B, the O-binding energy is 5.39 eV for the 3-fold binding site on top of the Pd4 cluster, and the binding energy is 4.64 eV for the surface-bound site adjacent to the Pd4 cluster. As the spillover oxygen atom is moved away from the cluster and the O bound to the top site of the Pd4 cluster remains, the O-binding energy decreases to 3.78 and 3.73 eV. This spillover energy is 1.13 and 1.20 eV larger than for spillover on the Pd atom. This indicates that the larger cluster increases the tendency for spillover, probably because of the Pd4O cluster having a larger redox energy. The structure where both O atoms are bound to the top of the Pd4 cluster, as shown at the bottom in Figure 7B, is 0.36 eV less stable than the surface-bound geometry. This structure also has a much lower O-binding energy of 4.28 eV. For the Pd4O2 cluster on the Sc-doped surface, the most stable structure in both cases is the structure where both O atoms are on top of the Pd4 cluster. This means that the binding energy of the O atom on top of the cluster has decreased from 5.39 to 4.76 and 4.57 eV for the Pd4O2 cluster on top of the Sc atom and adjacent to the Sc atom, respectively. In comparing the structure with the surface-bound oxygen to each other, the binding energy on the Pd4O2 on the Sc-doped surface decreases from 5.39 to 4.93 and 4.80 eV in Figure 7A and 7D, respectively. This shows that the tendency for spillover on the Sc-doped Pd4O2 is markedly reduced as compared to the

pristine TiO2. The O-binding energy of Pd4O2 on the Sc-doped surface is significantly lower than on the pristine TiO2 surface. Next, we examine the oxygen-binding energy of the Pd4O2 on the V-doped surface. We find that the lowest energy structure of the Pd4O2 is the T2 structure, with one O atom on the 3-fold site on top of the cluster and one O atom at a surface site bound to the V dopant. The O-binding energy for the lowest energy structure is 5.64 eV on top of the cluster and 4.68 eV at the surface-bound site. The O-binding energy on top of the cluster is 0.25 eV larger than that for the pristine TiO2, and the surface-bound site is 0.04 eV larger than pristine TiO2. When the Pd4O2 cluster is on top of the V dopant, the lowest energy structure is the S1 structure with both O atoms on top of the Pd4 cluster. The O-binding energy is 4.60 eV, which is smaller than that of the pristine S1 structure, although it is also larger than the Sc-doped S1 structure. If we examine the Obinding energies for the S3, S4, T3, and T4 structures in which the O atom is at a spillover site, the O-binding energy is markedly larger on the V-doped surface than the Sc-doped surface. This is also seen in the S2 and T2 structures, in which the O-binding energy of the surface-bound atom is 4.34 and 4.68 eV on the V-doped surface, while it is 4.03 and 3.80 eV on the Sc-doped surface. The oxygen-binding energy on Pd4O2 clusters is generally larger on the V-doped surface than the Scdoped surface. This is especially true for structures in which one of the O atoms has spilled over onto the TiO2 surface. We next examine the O-binding energies of Pd4O on the Scdoped, V-doped, and pristine TiO2 surface. The structures and binding energies are shown in Figure 8. For Pd4O, we see a reversal in most of the expected O-binding energies, with the O-binding energy on the 3-fold site shown in the X1 and Y1 structures, and the binding energy is 5.12 and 5.28 eV on the Sc-doped cluster and 4.99 and 5.00 eV on the V-doped cluster. The O-binding energy is 4.69 eV on the pristine TiO2 surface. Here, the O-binding energies are larger on the Sc-doped surface 20310



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Table 2. Pd4, Pd4O, and Pd4O2 Binding Energiesa Pd4 M1 N1 Pd4O X1 Y1 Pd4O X2 Y2 Pd4O X3 Y3 Pd4O X4 Y4 Pd4O2 S1 T1 Pd4O2 S2 T2 Pd4O2 S3 T3 Pd4O2 S4 T4 a

Sc Doped

Pristine

V-Doped

3.69 eV 3.70 eV 5.18 eV 5.36 eV 4.82 ev 4.99 eV 4.29 eV 4.36 eV 2.77 eV 2.49 eV 6.00 eV 5.98 eV 5.27 eV 5.22 eV 4.01 eV 3.87 eV 3.92 eV 4.11 eV

2.93 eV

3.01 2.99 4.38 4.37 4.05 4.01 3.31 3.41 2.30 2.54 5.04 4.90 4.77 5.10 3.92 3.82 3.91 3.84

4.00 eV 3.79 eV 3.26 eV 2.23 eV 4.34 eV 4.70 eV 3.81 eV 3.78 eV

eV eV eV eV eV eV eV eV eV eV eV eV eV eV eV eV eV eV

See Figres 5, 7, and 8 for labels.

larger for the Sc-doped surface than the pristine and V-doped surface. This explains why the Sc-doped surface prefers the cluster-bound arrangement. However, when we compare the Pd4O2 binding energy in the surface-bound and spillover arrangements of S2−4 and T2−4 the cluster binding energy on the Sc-doped surface is only slightly larger than for the pristine and V-doped surfaces. The binding enhancement is much greater for Pd4O than for Pd4O2, so the net result is that the binding energies on Pd4O2 are mostly lower on the Sc-doped surface and larger on the V-doped surface. To summarize our oxygen-binding energy results, we have charted the O-binding energy for PdO, Pd4O, and Pd4O2 coadsorbed to the doped and pristine surface in Figure 9. The tunability of the O-binding energy through doping is most clear for PdO coadsorbed to the surface. The O-binding energy to the Sc-doped surface is markedly lower than the pristine surface in all of the cases, and the O-binding energy is much larger than the pristine surface when the Pd and O are separated. When the Pd−O molecule is intact, the O-binding energy is negligibly larger than the pristine surface. For Pd4O adsorbed to the surface, we find that the Sc-doped surface binds O more strongly because the acceptor dopant binds Pd4O more strongly, unless the Pd4 and O atom are separated. In the case of Pd4O2, the O is bound more weakly to the Sc-doped surface in all cases, except for the S1/T1 case where both O atoms are on top of the Pd4 cluster. This indicates that the P-type Sc dopant will reduce the tendency for the cluster to spillover oxygen onto the surface. It also reveals the complexity of interactions.

Figure 8. Oxygen binding energy for Pd4O adsorbed on doped and pristine TiO2 as a function of the position of the O atom. The bottom cluster indicates the most stable and the rest indicate alternative binding sites both on the Pd4 cluster and with the O atom at a spillover site on the TiO2 surface.

than the V-doped surface. The O-binding energy is also larger on the Sc-doped surface than the V-doped surface when O is bound to the 2-fold site in structures X2 and Y2 and at the O surface-bound structures X3 and Y3. When the O atom is moved away from the cluster at a spillover site in structures X4 and Y4, then the O-binding energy is larger for the V-doped surface than the Sc-doped surface. To understand the high O-binding energies of Pd4O on the Sc-doped surface we determine the binding energy of the Pd4, Pd4O, and Pd4O2 free clusters to the surfaces as shown in Table 2. The binding energy of Pd4 to the Sc-doped surface is 0.7− 0.75 eV larger than to the V-doped surface and pristine surface. This is expected because the Pd4 cluster is a donor, so its binding is enhanced with the Sc-doped acceptor. The binding of the Pd4O cluster to the Sc-doped surface is also enhanced because the Pd4O cluster serves as a net donor. As a result, the binding energy to the Sc-doped surface is 1.18−1.36 eV larger than to the pristine surface. This results in the O-binding energy to Pd4 on the Sc-doped surface being larger than that on the pristine surface. When the Pd4 cluster and O atom are separated, as in structures X4 and Y4, the oxygen-binding energy becomes lower on the Sc-doped surface than the Vdoped surface, a return to the expected O-binding reduction for the Sc-doped surface. This result shows the complexity in applying the donor−acceptor concept to supported clusters. A donor will stabilize acceptors and destabilize donors; however, identifying if a partially oxidized cluster is a donor or acceptor is not immediately obvious. For Pd4O, the four Pd atoms wish to donate an electron each, while the O atom wants to accept two electrons, resulting in the cluster motif acting as an acceptor. For the Pd4O2 cluster, when the O is on top of the cluster as in structures S1 and T1, the Pd4O2 binding energy is dramatically

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CONCLUSIONS We have investigated the effect of doping on the oxygenbinding energy by using PdO, Pd4O, and Pd4O2 on TiO2, with Sc as our P-type dopant and V as our N-type dopant. For PdO, the donor−acceptor hypothesis is confirmed in which the Sc acceptor dopant reduces the binding energy of the acceptor O, and the V donor enhances the O-binding energy by forming donor−acceptor pairs. For Pd4O, we find that the Pd4O cluster as whole acts as a donor, so the Sc dopant actually increases the O-binding energy and the V dopant decreases the O-binding 20311



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for increasing the binding energy of the cluster to surface, enhancing the stability of the cluster.

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ASSOCIATED CONTENT

* Supporting Information S

Additional structures of the atoms bound to the surface are available. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by an Air Force Office of Scientific Research (AFOSR) Basic Research Initiative Grant FA9550-121-0481.

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REFERENCES

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Figure 9. Oxygen binding energy comparison.

energy. This demonstrates that the situation is more complicated than our simple hypothesis. In the case of Pd4O2, the O-binding energy on the Sc-doped surface is generally reduced versus the V-doped surface. Also, on the Scdoped surface the Pd4O2 cluster prefers the structure in which both O atoms are on top of the cluster, while on pristine TiO2 and V-doped TiO2 the cluster prefers a structure in which the O atom spills over onto the surface. Hence, P-type doping is likely to reduce oxygen spillover which may make supported clusters more catalytically active. Doping also offers a strategy 20312



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Effect of N- and P-Type Doping on the Oxygen-Binding Energy and

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