Oxygen Vacancy Formation on Rutile TiO2(110) and Its

Oct 20, 2007 - Density functional calculations at the B3LYP/6-31G* level were performed to mimic the oxygen vacancy formation on rutile TiO2(110) surf...
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J. Phys. Chem. C 2007, 111, 16941-16945

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Oxygen Vacancy Formation on Rutile TiO2(110) and Its Interaction with Molecular Oxygen: A Theoretical Density Functional Theory Study Nurbosyn U. Zhanpeisov* and Hiroshi Fukumura Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-8578, Japan ReceiVed: June 22, 2007; In Final Form: September 4, 2007

Density functional calculations at the B3LYP/6-31G* level were performed to mimic the oxygen vacancy formation on rutile TiO2(110) surface as well as for its interaction with molecular oxygen using extended cluster models. It was shown that the experimental temperature-programmed desorption observation on triply exceeding the concentration of O2 per vacancy site at a low-temperature saturation coverage on TiO2(110) can be well explained without making an assumption on the formation of the tetraoxygen (O4) fragment at the vacancy site, provided that the mechanism of formation of precursor defect sites on rutile TiO2(110) is properly addressed. The results obtained show that there are two kinds of dioxygen molecules adsorbed in a ratio of 1:2. The first kind of dioxygen is directly located at the vacancy site in a nearly perpendicular fashion, while the other involves two dioxygen molecules on next-nearest 5-fold coordinated titanium (Ti5C) sites with additional stabilization through a H-bonding pattern with the two hydroxyl groups nearby from both sides of the vacancy site. The latter adsorption form is expected to undergo a fast desorption, while the former one can be involved in the photooxidation of coadsorbed CO molecules.

Introduction Titanium dioxide, either as rutile or anatase, has attracted much interest during the past years, owing to its wide applications in chemisorption, catalysis, and photocatalysis, in chemistry and physics, as well as in carrying out many fundamental reactions.1 Among these reactions, the interaction of TiO2 with molecular oxygen is highly important because many selective and/or partial oxidations proceed through direct or indirect interactions with molecular oxygen on these materials.2 However, defect-free TiO2 is known to be relatively inert for the activation of dioxygen molecules, and thus the adsorption of O2 is mainly mediated by the presence of surface oxygen vacancies.2,3 Recent temperature-programmed desorption (TPD) experiments show that the full O2 coverage on TiO2(110) at a low temperature (∼120 K) would be approximately 3 times the surface vacancy population, and adsorbed O2 molecules may exist either in weakly or strongly bound states below 200 K and above 400 K, respectively.4 In addition, the experiments indicate the existence of two adsorption modes for dioxygen on reduced TiO2(110) with a ratio of 1:2, where one-third of the adsorbed molecules can photooxidize coadsorbed carbon monoxide, while the other molecules simply desorb through fast photodesorption.5 Hwang and co-workers recently proposed a new adsorption model to shed light on these different forms of adsorbed molecular oxygen on reduced TiO2(110) involving an oxygen vacancy.3 On the basis of their extensive periodic slab density functional theory (DFT) calculations on the interaction of molecular oxygen with a neutral vacancy formed by the removal of an oxygen atom from the precursor protruded O site of rutile TiO2(110), they concluded the following: (i) At the initial stage of low O2 adsorption, the first O2 molecule would strongly interact with an oxygen vacancy site via the formation of a complex nearly parallel to the (110) plane of rutile, as shown * E-mail: [email protected].

schematically in Figure 1; (ii) The second O2 molecule forms a tetraoxygen (O4) fragment located on both the vacancy and the next-nearest-neighbor 5-fold coordinated Ti5C sites (Figure 1c). The predicted tetraoxygen formation that is anchored at the oxygen vacancy site is the key intermediate that in turn allows the adsorption of the three O2 molecules per vacancy at saturation. The tetraoxygen adsorbed is found to be substantially more stable than the two separately adsorbed O2 molecules; (iii) According to these authors, the third O2 molecule would preferentially be adsorbed on one of the nearest Ti5C Lewis acid sites (Figure 1d), even though the number of such sites presented nearby leads to a similar probability to accommodate other O2 molecules. Evidently, these findings can hardly be applied to describe the mentioned above experimental TPD data4,5 that strictly distinguishes two kinds of molecular oxygen adsorbed in the ratio of 1:2 where only two-thirds of the adsorbed O2 molecules are likely to be photodesorbed off the surface. Moreover, the nearly bent and open structure found for the O4 complex3 conflicts with the more optimal cyclic ring structure (either square or rectangular) predicted for the isolated O4 particle by other theoretical investigations.6 While an oxygen vacancy is shown to be responsible for the adsorption of up to three O2 molecules on TiO2(110),3 additional data is definitely needed to properly describe the earlier experimental findings. In this paper we report on the theoretical results of DFT B3LYP/6-31G* level calculations obtained using the wellknown cluster approach for the formation of an oxygen vacancy site and its interaction with molecular oxygen. In line with our previous studies,7 here we have used molecular clusters with varied size that are formed through cutting the substructures from rutile while properly taking into account their boundary terminations, electroneutrality, and stoichiometry.8 Although our calculations satisfactorily reproduce the known adsorption models of water7,8a as well as the difficulty in binding molecular oxygen on defect-free sites of TiO2(110),2,4,5 they did not support the results and conclusions of the previous theoretical study.3

10.1021/jp074869g CCC: $37.00 © 2007 American Chemical Society Published on Web 10/20/2007

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Figure 1. Schematic representation of the formation of a neutral oxygen vacancy on rutile TiO2(110) and its consecutive interactions with the three dioxygen molecules: non-defect TiO2 (a), vacancy site at the precursor protruded O atom position (b), parallel fashion orientation for the first dioxygen molecule (c), formation of a tetraoxygen fragment for the two dioxygen molecules (d), and the adsorption form for the three dioxygen molecules (e). Grey and red balls stand for Ti and O, respectively. To distinguish the selected precursor bridging O center on which the vacancy would be formed as well as adsorbed dioxygen molecules from the rest other centers, they have denoted as blue balls.

According to our calculations, the experimental TPD observation on triply exceeding the concentration of O2 per vacancy site2,4 can be well explained without making the assumption of the formation of an O4 fragment at the vacancy site3 if one properly addresses the formation mechanism of precursor defect sites on rutile TiO2(110). The cluster approach applied here can be as effective as periodic slab calculations to describe the nature of interactions and catalysis on oxide surfaces because these chemical interactions considered here are highly local in nature.8 Method and Models DFT calculations were performed using the Gaussian 03 program packages.9 Geometry optimizations were carried out with the use of Becke’s three-parameter hybrid method with the Lee, Yang, and Parr (B3LYP) gradient-corrected correlation functional10 and the 6-31G* standard basis sets. The defectfree rutile were modeled by the extended clusters of Ti10O32H24 (model I) and Ti13O43H34 (model II) shown in Figure 2, which contains all kinds of active sites of the rutile TiO2(110) surface. The geometry optimizations were carried out taking into account the symmetry restrictions of the minerals. Results and Discussion First of all, let us consider the formation of different oxygen vacancy sites on rutile TiO2(110) through the analysis of their formation energetics. This question is important because it may shed light on our understanding of the molecular dioxygen interaction with the oxygen vacancy sites. Evidently, the oxygen vacancy site is most likely to be formed via the removal of one of the bridging oxygen in clusters I or II (Figure 2), either as O, O-, or O2-. However, as can be expected, the vacancy formation even through the removal of the neutral oxygen atom requires a lot of energy because it requires breaking two Ti-O bonds. The energy necessary for the formation of a neutral oxygen vacancy site for clusters II and I has been estimated to be 6.6 and 6.7 eV at the applied B3LYP/6-31G* level,

Figure 2. The cluster models of Ti10O32H24 (A) and Ti13O43H34 (B) used to mimic rutile TiO2(110) as well as to estimate the formation energy of different oxygen vacancy sites. The first model A is a completely symmetrical one (with a relatively small number of structural parameters for the geometry optimization), while the second model B is a slightly extended form involving additional bulk as well as Ti5C centers. The dark blue and grey balls stand for the selected precursor bridging O center on which the vacancy would be formed and the Ti5C center, respectively.

respectively. Note that, in the recent paper of Hwang and coworkers,3 this neutral vacancy is adopted for further interactions with O2 molecules without any analysis of its formation energetics. This methodology was based on previous theoretical studies11,12 where periodic slab DFT calculations were used to mimic the interaction of metal atom(s), such as Au or its dimer as well as Cu or Ag, with either the stoichiometric or neutral oxygen vacancy-containing site on rutile TiO2(110). At the same time, the removal of oxygen as a monoanion (dianion) needs a

Oxygen Vacancy Formation on Rutile TiO2(110)

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Figure 3. Molecular (a) and dissociative adsorption forms (b) for a single water molecule, dissociative adsorption of the two water molecules (c), and the formation of the oxygen vacancy site (d) on a model II cluster. The O atoms of the adsorbed water fragments are represented by blue balls in panels a, b, and c. An arrow points to the vacancy site.

much larger energy of 10.3 (30.3) and 11.2 (31.8) eV from the same clusters II and I, respectively. Here we should note that the above energies necessary for the creation of neutral (or monoanion, or dianion) vacancy sites is estimated in a conventional way, that is, as the energy difference between the initial defect-free cluster and the sum of the defect site-containing cluster and the isolated O (or O-, or O2-), respectively. The latter isolated O, O-, and O2- are in their optimal spin states, that is, as triplet, doublet, and singlet spin states, respectively. Conversely, the defect site containing clusters are treated as singlet (neutral), doublet (monocation), and singlet (dication) spin states, respectively. For all oxygen vacancy-containing clusters, there are evident alterations on charge redistributions that strongly affect not only next-nearestneighbor sites but also all centers involved as compared to the defect-free clusters. Evidently, the extra positive charge dissipation depends on the size of the clusters applied being slightly larger for the cluster II with its larger size (for either singly or doubly positively charged clusters with vacancy sites). These results show that, if these types of oxygen vacancies (i.e., neutral or charged) formed, they would be highly reactive with any impurities present nearby. Thus, one must look into other possible channels of the formation of vacancy sites by oxygen. One such possibility for the formation of vacancy sites by oxygen comes from looking into the origin of an increase in the chemical activity of both MgO catalysts and TiO2(110) by generous conventional hydrothermal pretreatments. As shown for MgO,13 the hydrothermal pretreatment can lead to the formation of a new Mg(OH)2 phase and to an increase in the number of low-coordinated active Mg2+LC and O2-LC sites by complex transformations from the precursor MgO. One of the important factors explaining an overall increase in its chemical activity is the dissociative adsorption of a pair of water molecules on most active acid-base centers of the precursor MgO.13 Taking these findings into account, one may consider the similar interactions of water with defect-free TiO2(110). In line with our previous estimations using a minimal-size Ti3O11H10 cluster model,7c,8a the molecular adsorption of water (Figure 3a) can naturally take place on the Ti5C site of TiO2(110), where water acts as a donor of a pair of electrons to the oxide surface. Its adsorption energy amounts to 1.64 eV, being slightly smaller than that of dissociative adsorption forms. For example, the dissociative adsorption of water (Figure 3b) involving the same Lewis acid and the next-nearest-neighbor basic bridging O sites are energetically much more profitable, and release 2.63 eV.

(Note that the method applied here (B3LYP) usually overestimates the binding energy of adsorbates; however, qualitative trends are well described).7,8 If the next water molecule is also dissociatively adsorbed on the same basic site as well as on another next-nearest-neighbor Ti5C acid site, then a new water molecule will form on the precursor bridging O site (Figure 3c). Removal of the newly formed water molecule creates a neutral vacancy on the bridging O site (Figure 3d). The waterassisted vacancy formation energy is estimated to be 2.11 eV, much less than the energy for non-water-assisted neutral vacancy formation (e.g., 6.6 eV for cluster II). Note, however, that this neutral vacancy is not completely free but is accompanied by two hydroxyl groups located on the two Ti5C sites located on either side of the vacancy site. These two OH-groups compensate for the missing precursor bridging O site, so they have strong bonds with the Ti5C acid sites: the removal of one of these OH-groups as an OH radical or OH- anion needs 4.0 and 8.7 eV more energy, respectively. Thus their mobility to fill the vacancy site would be strictly limited. Note also that the presence of hydroxyl groups on oxide surfaces is well documented in the literature (as measured by low-temperature CO adsorption),14 and rutile (TiO2) is not an exemption where at least four kinds of OH-groups are found to be present.14 Overall, the formation of this kind of a neutral oxygen vacancy accompanied by two OH-groups becomes energetically profitable and releases up to 1.52 eV as compared to the isolated two water molecules and initial TiO2(110). The above proposed mechanism clearly shows that the waterassisted oxygen vacancy formation is energetically much more profitable than the non-water-assisted neutral vacancy formation. In this case, some of the bridging O centers have been considered the active sites to stabilize the dissociated fragments of the two water molecules simultaneously. However, one should note that not all water molecules would interact in a similar manner (by involvement of the same bridging O site) because of the presence of numerous bridging O as well as Ti5C sites on the rutile TiO2(110) surface. In such a sense, the mechanism considered above is the simplified one. On the other hand, if water dissociation involved only a new bridging O as well as Ti5C sites, then no water-assisted vacancy formation would be evidently observed. The same holds true if the second water directly interacts with the coordinated O-H groups formed: it would lead to the formation of the energetically least profitable H-bonded complex.7a,8a Thus, they fail to explain the

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Figure 4. Molecular adsorption of dioxygen on non-defect rutile TiO2(110) (a), nearly perpendicular (b) and parallel (c) adsorption forms of a single dioxygen molecule at the vacancy site, and the two (d) and three (e) dioxygen molecule adsorption at the vacancy site and on the nextnearest-neighbor T5C acid site(s). Note an additional stabilization for the second and third water molecules comes through establishing an H-bonding pattern with the hydroxyl groups nearby (shown by arrows), respectively. Blue balls stand for the dioxygen molecules adsorbed.

role of hydrothermal pretreatments (at least) commonly used in experiments, in contrast to the mechanism proposed above. Let us move to consider the interaction of dioxygen molecule(s) with non-defect as well as oxygen vacancy-containing TiO2(110). Our calculations have shown that the adsorption energy for the most active singlet dioxygen on TiO2(110) is slightly endothermic (0.58 eV) as compared to the isolated noninteracting TiO2(110) and dioxygen in its triplet electronic ground state (see Figure 4a). This is in complete agreement with the above experimental observations on the inertness of non-defect TiO2(110) to dioxygen adsorption.2,3 However, for the oxygen vacancy site with the two OH-groups (Figure 3d), the activation of the dioxygen molecule might easily take place. According to our calculations, the first dioxygen molecule can be adsorbed directly on the vacancy site by making the tilt structure shown in Figure 4b. The tilt angle amounts to 39.2° because of additional stabilization through the interaction of its free O endatom with the nearest O atom of the hydroxyl group located at the Ti5C site (the O‚‚‚O distance is 2.731 Å), while the O atom of the dioxygen closest to the surface is shifted by 0.454 Å as compared to the precursor lattice bridging O atom position. This orientation is 0.87 eV more favorable compared to the case when the dioxygen molecule is located at the vacancy site in a parallel fashion (Figure 4c). In the latter case, the center of mass of the dioxygen molecule is 0.49 Å above the lattice position of the precursor bridging oxygen atom, while the O-O bond distance is lengthened by 0.075 Å. These results contradict the findings of the recent paper of Hwang and co-workers3 where the first O2 was found to be adsorbed on a pure oxygen defect site of TiO2(110) in a parallel fashion in regard to the selected rutile plane. Thus a proper understanding of the nature and the formation mechanism of the initial vacancy site on oxide surface are highly important and should be properly taken into account. The experimental TPD observation on triply exceeding the concentration of O2 per vacancy site2,4 can be then explained without making an assumption of the formation of the O4 cluster at the vacancy site on rutile TiO2(110). In this case, the second and the third dioxygen molecules adsorb on naturally existed

next-nearest-neighbor Ti5C Lewis acid sites by making an additional stabilization through a weak H-bonding pattern with the two pre-existed hydroxyl groups nearby, respectively (Figure 4d,e). This explains why, at the full O2 saturation coverage on TiO2(110) at low temperature the adsorbed O2 molecules would approximately be 3 times the surface vacancy population.2-4 These results show also that there are two kinds of dioxygen molecules adsorbed, where one is directly located at the vacancy site in a nearly perpendicular fashion, while the other kind resides on next-nearest Ti5C sites with additional stabilization through H-bonding pattern with the two hydroxyl groups nearby on both sides of the vacancy site. The latter adsorbed dioxygen expected to undergo a fast desorption, while the former one may be involved in the photooxidation of coadsorbed CO molecules.3,5 Summary and Conclusions We have shown that the cluster approach is well suited to explain the known experimental facts of the adsorption of dioxygen molecules on reduced rutile TiO2(110) if the formation of active site structures are properly addressed. The experimental TPD observation on triply exceeding the concentration of O2 per vacancy site2,4 can be well explained without making an assumption of the formation of the O4 fragment3 at the vacancy site on rutile TiO2(110). Instead, the first dioxygen molecule can adsorb directly on the vacancy site by making a tilt structure, while the second and third dioxygen molecules adsorb on naturally existing next-nearest-neighbor Ti5C Lewis acid sites through an additional stabilization by making a weak H-bonding pattern with the two pre-existing hydroxyl groups nearby, respectively. The latter adsorbed O2 is evidently expected to undergo a fast desorption, while the former one may be involved in the photooxidation of coadsorbed CO molecules.3,5 This also explains why, at the full O2 saturation coverage on TiO2(110) at low temperature, the adsorbed O2 molecules would be approximately 3 times the surface vacancy population.2-4 Although recent experimental scanning tunneling microscopy (STM) measurements demonstrate the presence of oxygen vacancy sites as isolated ones,15 they also clearly pointed out

Oxygen Vacancy Formation on Rutile TiO2(110) the specific limitations in performing such kinds of investigations where the STM tip itself can be strongly affected (or even oxidized) by the interaction with molecular oxygen. That is why tip changes occur often, may lead to images that are even hard to interpret, and are probably the reason for conflicting interpretations of TiO2 surface features in the STM literature.15 At the same time, these experiments showed that hydroxyl groups from spurious water in the oxygen gas stream are observed to be adsorbed dissociatively at step edges and on the in-plane Ti rows. Acknowledgment. We gratefully acknowledge the supercomputing resources provided by the Information Synergy Center of Tohoku University. The present work was supported by a Grant-in-Aid from the Ministry of Education, Science, and Culture of Japan (16072203). References and Notes (1) (a) Anpo, M., Ed. Photofunctional Zeolites: Synthesis, Characterization, Photocatalytic Reactions, Light HarVesting. Nova Scientific Publishing: Huntington, NY, 2000. (b) Zhanpeisov, N. U.; Anpo, M. Theor. Chem. Acc. 2005, 114, 235-241. (2) (a) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341-357. (b) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735-758. (c) Campbell, C. T.; Parker, S. C.; Starr, D. E. Science 2002, 298, 811814. (3) Pillay, D.; Wang, Y.; Hwang, G. S. J. Am. Chem. Soc. 2006, 128, 14000-14001. (4) Henderson, M. A.; Epling, W. S.; Perkins, C. L.; Peden, C. H. F.; Diebold, U. J. Phys. Chem. B 1999, 103, 5328-5337. (5) Lu, G. Q.; Linsebigler, A.; Yates, J. T. J. Chem. Phys. 1995, 102, 3005-3008. (6) (a) Rasmussen, M. D.; Molina, L. M.; Hammer, B. J. Chem. Phys. 2004, 120, 988-997. (b) de Lara-Castells, M. P.; Krause, J. L. Chem. Phys. Lett. 2002, 354, 483-490.

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