Mediated by H Adatoms - American Chemical Society

Letter pubs.acs.org/JPCL

Ethanol Diffusion on Rutile TiO2(110) Mediated by H Adatoms Peipei Huo, Jonas Ø. Hansen, Umberto Martinez, Estephania Lira, Regine Streber, Yinying Wei, Erik Lægsgaard, Bjørk Hammer, Stefan Wendt,* and Flemming Besenbacher Interdisciplinary Nanoscience Center (iNANO), Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark S Supporting Information *

ABSTRACT: We have studied the diffusion of ethanol on rutile TiO2(110)−(1 × 1) by high-resolution scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations. Time-lapsed STM images recorded at ∼200 K revealed the diffusion of ethanol molecules both parallel and perpendicular to the rows of surface Ti atoms. The diffusion of ethanol molecules perpendicular to the rows of surface Ti atoms was found to be mediated by H adatoms in the rows of bridge-bonded O (Obr) atoms similarly to previous results obtained for water monomers. In contrast, the diffusion of H adatoms across the Ti rows, mediated by ethanol molecules, was observed only very rarely and exclusively on fully hydrogenated TiO2(110) surfaces. Possible reasons why the diffusion of H adatoms across the Ti rows mediated by ethanol molecules occurs less frequently than the cross-row diffusion of ethanol molecules mediated by H adatoms are discussed. SECTION: Surfaces, Interfaces, Catalysis

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via O−H scission.4,10,20−22 Furthermore, room-temperature STM studies by Zhang et al. revealed a dramatic increase in Had diffusion perpendicularly to the rows of surface Ti atoms, that is, in the [11̅0] direction, after high methanol21 and 2-butanol22 exposures, respectively. Among the alcohols, ethanol (CH3CH2OH or EtOH) is of particular interest because it holds promises both as a renewable energy carrier26,27 and as feedstock for future green chemistry.28,29 In addition, the decomposition of EtOH can be used to identify active sites on metal oxide surfaces.4,10,25 Recently, we provided direct evidence that molecularly and dissociatively adsorbed EtOH species coexist on regular surface Ti sites on TiO2(110).25 On the basis of time-lapsed STM images−so-called STM movies−and DFT calculations we were able to identify EtOH molecules and two types of EtO ethoxides.25 In fact, in time-lapsed STM images, the hopping rates of diffusing species along with the observed surface reactions and the STM heights allow one to distinguish between various adsorbates, and in some cases the identity of the surface species can be unraveled with certainty.1,8,10,15,25,30 Here we report on the diffusion of EtOH on rutile TiO2(110) studied by time-lapsed STM imaging and DFT calculations. The diffusion of EtOH molecules both parallel and perpendicular to the rows of Obr and Ti atoms was observed. In contrast with the diffusion along the Ti troughs, the diffusion of EtOH perpendicular to the rows of surface Ti atoms is mediated by Had species, similarly as previously observed for

he understanding of diffusion processes on surfaces is interesting both from a fundamental point of view and with a view to applications in heterogeneous catalysis. For example, in heterogeneous catalysis, the overall reactivity is often determined by active sites such as kinks, steps, and point defects.1−3 The adsorbates and intermediate species need to diffuse to the active sites where the actual reactions occur due to the special electronic environment at these sites. Alternatively, the adsorbates may be reactive at regular surface sites but can be trapped at particular sites on the surface where different reaction routes can be initiated4 or where they are nonreactive. In either case, the overall reactivity is strongly influenced by the diffusivity of the adsorbates and intermediates,3 and thus insights into the diffusion mechanism occurring on catalyst surfaces are of great interest. A very well-studied model catalyst surface is the rutile TiO2(110)−(1 × 1) surface.5−11 Because of the potential applications of TiO2 in photocatalysis, solar cells, superhydrophilicity, and water splitting, much research has been focused on this material and the processes occurring on its surfaces.12−14 Regarding the anisotropic rutile TiO2(110) surface, high-resolution STM studies have unraveled how O2 molecules oxidize H adatoms (Had, often also denoted as OHbr species)15,16 on the surface and Ti interstitials in the bulk.11,17,18 In addition, STM studies have shown that hydrogen bonds between water molecules and Had species make the diffusion across the rows possible.8,19 Likewise, the interactions of alcohols with rutile TiO2(110) surfaces have been studied.4,7,10,20−25 Previous studies showed that Obr vacancies on rutile TiO2(110) are the most stable adsorption sites and that alcohols dissociate at Obr vacancy sites © 2012 American Chemical Society

Received: December 8, 2011 Accepted: January 6, 2012 Published: January 7, 2012 283

dx.doi.org/10.1021/jz201616z | J. Phys. Chem.Lett. 2012, 3, 283−288

The Journal of Physical Chemistry Letters

Letter

water monomers.8,19 The inverse process− the diffusion of Had species across the Ti rows mediated by EtOH molecules−was observed only very rarely. Possible reasons are discussed as to why the diffusion of Had species across the Ti rows mediated by EtOH molecules occurs less frequently than the cross-row diffusion of EtOH molecules mediated by Had species. Figure 1 displays snapshots from the STM movie (“EtOHjump”) recorded on a partially hydrogenated TiO2(110) surface

Ti trough in the [001] direction (cf. Figure 1d−f). The blue line in Figure 1f indicates the overall diffusion path of the EtOHTi molecule, as was revealed from the STM movie “EtOH-jump”. The observed diffusion of EtOHTi molecules across the rows of Obr atoms bears similarity to the diffusion of monomeric water in the [11̅0] direction.8,19 Because of this similarity and the previously observed increase in cross-row Had diffusion after high methanol21 and 2-butanol22 exposures, it can be expected that also EtOH molecules mediate the diffusion of Had species. When, instead of an EtOHTi molecule, a water monomer reached a 5f-Ti site next to an Obr atom capped by a Had species, a proton of the water molecule can be transferred to the adjacent, bare Obr row.8,19 At the same time, the OHt group (terminal OH group) left behind in the Ti trough accepts the Had species from the other Obr row, and the newly formed water monomer may continue to diffuse along the Ti trough. Accordingly, one may expect that also the OH groups of EtOHTi molecules facilitate the cross-row diffusion of Had species on TiO2(110). However, as exemplified in Figure 2, we did not observe any cross-row

Figure 1. Snapshots (32 Å × 42 Å) from the STM movie “EtOHjump” that was recorded at ∼200 K on a partially hydrogenated TiO2(110). EtOH was dosed at ∼160 K (∼1.2%ML EtOH species). The STM movie was recorded with ∼0.1 nA tunneling current and ∼+1.25 V tunneling voltage. Directions throughout the Letter are identical to those indicated in panel d. An EtOHTi molecule diffuses first along a Ti trough (a−c), before it jumps, assisted by a Had species, to the next Ti trough on the right (d−f). The mediating Had species is indicated in panel a by an arrow. The blue line in panel f indicates the overall diffusion path. Symbols indicate EtOHTi molecules (large blue open dots), Had species (white filled dots), and Obr vacancies (white open dots), respectively. A lattice grid is centered on-top of 5f-Ti sites. The appearance of the snapshots within the STM movie is indicated in the lower left corners.

Figure 2. Snapshots (40 Å × 61 Å) from the STM movie “EtOHdiffusion”. The movie was recorded at ∼200 K on a partially hydrogenated TiO2(110) that was exposed to EtOH at ∼160 K (∼1.2%ML EtOH species). An EtOHTi molecule diffuses along a Ti trough, passing four Had species without jumping to the neighbor Ti troughs. Likewise, no diffusion of Had species across the Ti troughs was observed. Symbols indicate EtOHTi molecules (large blue open dots), EtObr species (crosses), EtOTi species (filled green dots), Had species (white filled dots), and Obr vacancies (white open dots), respectively.

diffusion events of Had species on partially hydrogenated TiO2(110) surfaces. In Figure 2, consecutive snapshots from the STM movie “EtOH-diffusion” are displayed, which was recorded on the same partially hydrogenated TiO2(110) surface from which we recorded the STM movie “EtOH-jump” (cf. Figure 1). In Figure 2, an EtOHTi molecule is observed at ∼200 K to diffuse along a Ti trough, passing four Obr atoms capped by Had species, but the diffusion of Had species across the Ti troughs did not occur. Likewise, in a number of additional STM movies, we did not observe cross-row diffusion of Had species on EtOH-covered partially hydrogenated TiO2(110) surfaces. The only identified interaction between the EtOHTi molecules and Had species on such TiO2(110) surfaces was that the diffusion of the EtOHTi molecules along the Ti trough is facilitated in the vicinity of the Had species. This facilitated diffusion of the EtOHTi molecules is apparent from the STM movie “EtOH-diffusion”. From Figure 2c, it can be seen that the fast diffusion of the EtOHTi molecule in the vicinity of the Had species has led to a streaky appearance of the EtOHTi molecule in the STM image. We stress that the identity of the EtOHTi molecule in Figures 1 and 2 is definite because in a later stage within the same original STM movie (5.2 s per image of 150 Å × 150 Å size) we observed it to dissociate at an Obr vacancy site. Furthermore,

at ∼200 K that was exposed to a small amount of EtOH at ∼160 K. The bare rutile TiO2(110) surface consists of alternating rows of five-fold coordinated Ti atoms (5f-Ti) and two-fold coordinated Obr atoms. In the STM image, the geometrically protruding Obr rows appear dark and the Ti rows appear bright.5−7,10 In Figure 1, various kinds of bright protrusions on the dark Obr rows are discernible: Obr vacancies show up as faint protrusions (open dots), single Had species are imaged brighter (white filled dots), and paired Had species are seen with even brighter contrast than single Had species.5 In addition to these point defects on the dark Obr rows, a much more protruding species is seen on the bright 5f-Ti sites (large blue open dot) whose position changes along the trough (cf. Figure 1a−c). This protrusion appears with an apparent height of ∼2.6 Å and originates from an EtOHTi molecule that is hopping between 5f-Ti sites.25 In Figure 1c, the EtOHTi molecule reached a 5f-Ti site right next to a Had species, and, from a comparison of Figures 1c,d, it is evident that in Figure 1d it has jumped over the Had species to the adjacent Ti trough. Subsequently, the EtOHTi molecule diffused along this adjacent 284



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the presence of an EtOTi species (filled green dots) and two EtObr species (crosses) in the proximity of the EtOHTi molecule further strengthen the above assignments. Note that only the EtOHTi molecules are expected to diffuse at 200 K because EtOTi and EtObr species are immobile on partially hydrogenated TiO2(110) surfaces at this temperature.25 In addition to the STM studies performed on partially hydrogenated TiO2(110) surfaces (mix of Obr vacancies and Had species), we also recorded STM movies on fully hydrogenated surfaces [h-TiO2(110)], an example of which is shown in Figure 3. The h-TiO2(110) surfaces were prepared by

species along the Ti troughs is strongly promoted by the presence of Had species. Interestingly, similar promotion effects have been observed previously for the diffusion of water species30 and catechol,31 respectively. Another interesting feature in the STM movie “EtOH-hTiO2” is the observation that a few of the Had species have changed their positions in the [11̅0] direction, that is, that these Had species diffused across the Ti troughs. For example, in Figure 3a, two Had pairs are evident (indicated by hexagons), and the one in the upper left corner splits into two single Had species when an EtOHTi molecule has passed by (cf. Figures 3d,e). Likewise, we observed the cross-row diffusion of single Had species. (Four selected single Had species are indicated in Figure 3 by white filled dots.) Two examples can be found in the STM images displayed in Figure 3b,d and Figure 3e,f, respectively. The new positions of single Had species are marked by white circles, and the bent gray arrows in Figure 3b,d,e indicate the directions of the identified cross-row diffusion events. In all identified Had cross-row diffusion events in STM movie “EtOH-h-TiO2”, we observed an EtOHTi molecule next to the Had species in the STM image recorded directly before the diffusion event took place (an example of this is indicated by an open blue dot in Figure 3c), and without a coadsorbed EtOHTi molecule the cross-row diffusion of Had species was not observed. We stress that the EtOHTi-mediated cross-row diffusion of Had species was exclusively observed on h-TiO2(110) surfaces but not on partially hydrogenated surfaces. To gain further insight into the diffusion of EtOH molecules on the TiO2(110) surface, we performed a series of firstprinciples DFT calculations, the results of which are summarized in Figures 4−6. Considering that the TiO2(110) crystals used in the experiments were reduced, it appears to be

Figure 3. Snapshots (73 Å × 73 Å) from the STM movie “EtOH-hTiO2” that was recorded at ∼130 K on a h-TiO2(110) surface (∼13.4% ML Had species). EtOH was dosed at ∼135 K, leading to ∼2%ML EtOH species on the surface. The large blue open dots in panels c, e, and f and the long arrows in panels e and f indicate very mobile EtOHTi species, and filled green dots in panels a and f indicate almost immobile EtOTi species. Bent gray arrows in panels b, d, and e indicate identified cross-row diffusion of Had species. Other symbols indicate previous EtOTi adsorption sites (faint open dots), single Had species (white filled dots), paired Had species (hexagons), and newly formed single Had species (circles), respectively.

letting water dissociate at the Obr vacancies, which led to the complete absence of Obr vacancies.5,15,17 The snapshots displayed in Figure 3 (2.1 s/image; 73 Å × 73 Å) are selected from the STM movie “EtOH-h-TiO2”, which was recorded at ∼130 K on a h-TiO2(110) surface characterized by ∼13.4%ML Had species. EtOH was dosed at ∼135 K, which led to ∼2%ML EtOHTi/EtOTi species on the surface. After this preparation, the prevalent static protrusions in the Ti troughs were EtOTi species (filled green dots). The sample temperature during the recording of the STM movie “EtOH-h-TiO2” was decreased by ∼65 K compared with the two STM movies described above. Despite this, we found that the EtOTi species were not completely immobile on h-TiO2(110), which is different from the situation obtained on a partially hydrogenated TiO2(110) surfaces. (See above and ref 25.) In Figure 3, the EtOTi species are indicated by filled green dots, and previous EtOTi adsorption sites of the two identified mobile EtOTi species are indicated by faint open dots, cf. Figure 3b−f. In addition, we found evidence of very mobile EtOHTi molecules (large blue open dots in Figure 3c,e,f and the long arrows in panels e and f, respectively). The streaky appearance of the EtOHTi molecules results from their high mobility. Therefore, the STM movie “EtOH-h-TiO2” reveals that the diffusion of EtOTi and EtOHTi

Figure 4. (a−c) Selected configurations of the EtOH diffusion in the [001] direction along the Ti troughs (top and side views are shown). 5f-Ti atoms are shown in red, Obr atoms are shown in gray, C atoms are shown in black, and H atoms are shown in yellow. Other O atoms of the surface are shown in light gray and O atoms of the adsorbates are shown in pink. (d) Potential energy profiles for EtOH diffusion along the [001] direction for 0 (bold curve) and 1 (faint curve) Had coverage [θ(Had)]. 285



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First, we modeled the diffusion of an EtOHTi molecule along the Ti trough via the Ti bridging site for different coverages of coadsorbed Had species (cf. Figure 4). In the absence of Had species, the EtOHTi molecule binds with an adsorption energy of ∼0.80 eV on top of 5f-Ti sites, and the diffusion barrier was found to be ∼0.52 eV.25 For 0.33 (one Had), 0.67 (two Had), and 1.0 (three Had) Had coverage, we computed EtOH diffusion barriers of ∼0.51, ∼0.42, and ∼0.38 eV, respectively. This decrease in the diffusion barrier is mainly caused by the decreased bond strength of the EtOHTi molecule when it is adsorbed in the vicinity of Had species. For 0.33, 0.67, and 1.0 Had coverage, the EtOHTi bond strength was found to be ∼0.71, ∼0.64, and ∼0.60 eV, respectively. Considering that Had coverages as high as 0.33 are not realistic5,7,10 and that even higher Had coverages are required to decrease the energy barrier for EtOH diffusion along the Ti trough substantially, we conclude that the decreased bond strength of the EtOHTi molecule caused by coadsorbed Had species (cf. Figure 4) does not explain why the diffusion of EtOHTi molecules is much more facile on h-TiO2(110) than on partially hydrogenated TiO2(110) surfaces. Rather, we believe that the promotion of the EtOHTi diffusion along the Ti troughs by coadsorbed Had species is a local effect, similar to that observed for adsorbed water dimers30 and catechol,31 respectively. This assertion is consistent with our results summarized in Figure 2 and the STM movie “EtOH-diffusion”, respectively, and the fact that low Had coverages are sufficient to promote the EtOHTi diffusion. However, to promote the EtOHTi diffusion along the Ti troughs we do not expect that the Had species are transferred to the diffusing molecules, as has been reported for the diffusion of catechol.31 Instead, we speculate that alone the presence of Had species in the vicinity of the diffusing EtOHTi molecules (at least up to three lattice sites in the [001] direction away from the Had species) can lower the diffusion barrier. Guided by our STM results in Figure 1, we further used the DFT approach to model the cross-row diffusion of an EtOHTi molecule mediated by a Had species (cf. Figure 5). In Figure 5, the energy profile and selected configurations are shown for the case when an EtOHTi molecule is adsorbed right next to an Obr atom with a Had species (cf. Figure 5a) and when the EtOHTi molecule diffuses to the adjacent Ti trough in the [110̅ ] direction. Starting from the configuration displayed in Figure 5a, a strong hydrogen bond (bond length: ∼1.65 Å) forms between the Had species and the oxygen atom of the EtOHTi molecule (cf. Figure 5b), and the EtOHTi molecule jumps to the adjacent Ti trough (cf. Figure 5c). In agreement with the experimental observations, the diffusion to the adjacent Ti trough is a facile process; the energy barrier was computed to be only ∼0.3 eV (bold curve in Figure 5d). In contrast, in the absence of the Had species (cf. Figure 5e−g), the diffusion of an EtOHTi molecule across the Obr row is hindered by a high barrier, ∼0.67 eV (faint curve in Figure 5d), underlining the fact that the formation of the hydrogen bond between the Had species and the oxygen atom of the EtOHTi molecule is a prerequisite for the EtOH diffusion in the [11̅0] direction at 200 K. In addition, we modeled the possible cross-row diffusion of Had species mediated by EtOHTi molecules, cf. Figure 6. The starting configuration for this process (cf. Figure 6a) is identical to the one we obtained when modeling the cross-row diffusion of an EtOHTi molecule mediated by a Had species (cf. Figure

Figure 5. (a−c) Selected configurations illustrating the EtOH diffusion in the [11̅0] direction in the presence of an adjacent Had species. (d) Potential energy profiles for EtOH diffusion along the [11̅0] direction with (bold curve) and without (faint curve) an adjacent Had species. (e−g) Selected configurations illustrating the EtOH diffusion in the [11̅0] direction in the absence of a Had species. Only side views are shown. The same colors are used as in Figure 4. Replicated images of the adsorbates are dimmed for clarity.

Figure 6. (a−f) Selected configurations of the EtOHTi-mediated diffusion of a Had species in the [11̅0] direction. Only side views are shown. The same colors were used as in Figure 4. (g) Corresponding potential energy profile (bold curve) along with the energy profile computed for the water-mediated diffusion of a Had species (faint curve).

appropriate to use bulk-reduced supercells to model the TiO2(110) surfaces.25 Nevertheless, because our previous DFT calculations of EtOH adsorption properties led to identical results on reduced and stoichiometric TiO2 supercells,25 we decided to conduct the present series of DFT calculations with stoichiometric TiO2 supercells rather than with reduced ones. 286



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the mediating species. In addition, breaking and formation of O−H bonds is, in the case of a water molecule as mediating species, not associated with sharp maxima in the potential energy profile. This comparison may be taken as an additional argument as to why we did not observe any cross-row diffusion of Had species on partially hydrogenated TiO2(110) surfaces because the potential energy profile is more complex when EtOHTi is the mediating species. In conclusion, we studied the diffusion of EtOH on partially and fully hydrogenated rutile TiO2(110)−(1 × 1) by timelapsed STM imaging and DFT calculations. We found that the EtOH diffusion along the Ti troughs is strongly promoted by the presence of Had species and that Had species mediate the cross-row diffusion of EtOH molecules. In contrast, the diffusion of Had species across the Ti troughs mediated by EtOH molecules was observed only very rarely. The computed potential energy profiles of the two considered cross-row diffusion processes revealed not only almost identical diffusion barriers but also that the latter process is more complex than the former. This, and a comparison with the case where a water molecule mediates the cross-row diffusion of Had species, suggests that the complex energy profile found for the crossrow diffusion of Had species mediated by EtOH molecules is likely the key to rationalize why this process occurs much less frequently than the cross-row diffusion of EtOH molecules. From the presented results addressing the EtOH diffusion on rutile TiO2(110), it can be concluded that simple arguments considering the barrier heights alone are not always sufficient to explain the experimental observations.

5a). However, this time the EtOHTi molecule needs first to dissociate, forming an EtOTi and a Had species (cf. Figure 6c). The computed barrier for the dissociation of EtOHTi is only ∼0.3 eV (cf. Figure 6g), and, therefore, we expect that this process occurs at 200 K. Subsequently, the newly formed EtOTi ethoxide moves toward the opposite Obr atom that is capped by a Had species (cf. Figure 6d−e) before it accepts this Had species, forming a new EtOHTi molecule (cf. Figure 6f). The latter process is hindered by an energy barrier of ∼0.33 eV (cf. Figure 6g), which is only marginally higher than the ∼0.30 eV barrier calculated for EtOHTi dissociation. Comparing the computed potential energy profiles for the EtOHTi diffusion to the adjacent Ti trough mediated by Had species (cf. Figure 5d) and the cross-row diffusion of Had species mediated by EtOHTi molecules (cf. Figure 6g), the barrier heights are practically identical. Therefore, simple arguments as often used to explain diffusion and reaction rates in the framework of transition-state theory seem, in the present case, not to be applicable because on partially hydrogenated TiO2(110) surfaces, only the former process was observed. We speculate that several factors need to be considered to rationalize this apparent discrepancy, the most probable being the different shapes of the potential energy profiles. Whereas the potential energy profile for the EtOHTi diffusion to the adjacent Ti trough mediated by Had species is characterized by a single broad maximum, the potential energy profile for cross-row diffusion of Had species mediated by EtOHTi molecules shows a more complex structure with a plateau between two sharp maxima (cf. Figure 6g). It is possible that this complex structure that originates from the fact that the EtOTi species must rotate and translate considerably, whereas the O−H bonds are broken, is the main reason as to why we did not observe any cross-row diffusion of Had species on partially hydrogenated TiO2(110) surfaces. That means, when adsorbed on 5f-Ti sites, a very specific orientation of the EtOHTi molecule is required to make the cross-row diffusion of Had species feasible, but other events such as the EtOHTi diffusion to the next 5f-Ti site may be less sensitive to the EtOHTi orientation. In other words, in contrast with other processes, the cross-row diffusion of Had species is highly entropically hindered. Therefore, we expect quite different preexponential factors for the various possible diffusion events. The finding that simple arguments considering the barrier heights alone are not sufficient to explain the experimental observations is further strengthened considering that the computed barrier for EtOHTi diffusion from one 5f-Ti site to the next is ∼0.2 eV higher than the barrier for diffusion across the Obr rows at Had species. However, experimentally, diffusion events along the Ti troughs are also frequently observed. In contrast with EtOHTi molecules, isolated water molecules do mediate the cross-row diffusion of Had species very efficiently.8,19 Following very similar crystal preparations, we frequently observed the water-mediated cross-row diffusion of Had species on partially hydrogenated TiO2(110) surfaces at ∼179 K.19 Therefore, we additionally show in Figure 6g the potential energy profile for cross-row diffusion of a Had species mediated by a single water molecule (faint curve in Figure 6g). For a direct comparison to the situation when EtOHTi is the mediating species, this potential energy profiles was extracted based on new DFT calculations using the same supercell as used for the calculations addressing EtOH. As in the case of EtOHTi, the calculated energy barrier is ∼0.3 eV, but the width of the potential energy profile is smaller than when EtOHTi is

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EXPERIMENTAL AND COMPUTATIONAL METHODS The experimental setup and the procedures for the preparation of reduced and hydrogenated TiO2(110) surfaces have been previously described.5,15,17 The TiO2(110) samples used here were characterized by Obr vacancy densities between 5.5 and 8.0%ML, where 1 ML is defined as the density of the (1 × 1) units, 5.2 × 1014/cm2. The EtOH was cleaned via freeze− pump−thaw cycles and introduced to the ultrahigh vacuum chamber through a microcapillary array doser. The STM images were acquired in the constant current mode using a tunneling voltage of ∼+1.25 V and a tunneling current of ∼0.1 nA. The STM movies were recorded with a rate between 2.1 and 5.2 s per image. The DFT calculations were performed using the GPAW program,32,33 where the electrons are described by means of the projector-augmented wave (PAW) method in the frozen core approximation.34 To describe the exchange-correlation effects, the generalized gradient approximation (GGA) was used with the Perdew−Burke−Ernzerhof (PBE) functional.35 TiO2(110) surfaces were modeled using 2D-periodic slabs of four TiO2 trilayers with p(3 × 1) and p(2 × 2) unit cells, respectively. Throughout, all trilayers and the adsorbates were fully relaxed. The slabs were asymmetric with the adsorbate on one side only. The climbing nudged elastic band (NEB) method36 was used to calculate diffusion and dissociation barriers.

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ASSOCIATED CONTENT

S Supporting Information *

STM movies “EtOH-jump”, “EtOH-diffusion”, and “EtOH-hTiO2”. This material is available free of charge via the Internet http://pubs.acs.org. 287



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(18) Zhang, Z.; Lee, J.; Yates, J. T. Jr.; Bechstein, R.; Lira, E.; Hansen, J. Ø.; Wendt, S.; Besenbacher, F. Unraveling the Diffusion of Bulk Ti Interstitials in Rutile TiO2(110) by Monitoring Their Reaction with O Adatoms. J. Phys. Chem. C 2010, 114, 3059−3062. (19) Wendt, S.; Matthiesen, J.; Schaub, R.; Vestergaard, E. K.; Lægsgaard, E.; Besenbacher, F.; Hammer, B. Formation and Splitting of Paired Hydroxyl Groups on Reduced TiO2(110). Phys. Rev. Lett. 2006, 96, 066107. (20) Henderson, M. A.; Otero-Tapia, S.; Castro, M. E. The Chemistry of Methanol on the TiO2(110) Surface: The Influence of Vacancies and Coadsorbed Species. Faraday Discuss. 1999, 114, 313− 329. (21) Zhang, Z. R.; Bondarchuk, O.; White, J. M.; Kay, B. D.; Dohnálek, Z. Imaging Adsorbate O-H Bond Cleavage: Methanol on TiO2(110). J. Am. Chem. Soc. 2006, 128, 4198−4199. (22) Zhang, Z. R.; Bondarchuk, E.; Kay, B. D.; White, J. M.; Dohnálek, Z. Direct Visualization of 2-Butanol Adsorption and Dissociation on TiO2(110). J. Phys. Chem. C 2007, 111, 3021−3027. (23) Zhang, Z. R.; Rousseau, R.; Gong, J. L.; Li, S. C.; Kay, B. D.; Ge, Q. F.; Dohnálek, Z. Vacancy-Assisted Diffusion of Alkoxy Species on Rutile TiO2(110). Phys. Rev. Lett. 2008, 101, 156103. (24) Jayaweera, P. M.; Quah, E. L.; Idriss, H. Photoreaction of Ethanol on TiO2(110) Single-Crystal Surface. J. Phys. Chem. C 2007, 111, 1764−1769. (25) Hansen, J. Ø.; Huo, P.; Martinez, U.; Lira, E.; Wei, Y. Y.; Streber, R.; Lægsgaard, E.; Hammer, B.; Wendt, S.; Besenbacher, F. Direct Evidence for Ethanol Dissociation on Rutile TiO2(110). Phys. Rev. Lett. 2011, 107, 136102. (26) Navarro, R. M.; Sanchez-Sanchez, M. C.; Alvarez-Galvan, M. C.; del Valle, F.; Fierro, J. L. G. Hydrogen Production from Renewable Sources: Biomass and Photocatalytic Opportunities. Energy Environ. Sci. 2009, 2, 35−54. (27) Nadeem, M. A.; Murdoch, M.; Waterhouse, G. I. N.; Metson, J. B.; Keane, M. A.; Llorca, J.; Idriss, H. Photoreaction of Ethanol on Au/ TiO2 Anatase: Comparing the Micro to Nanoparticle Size Activities of the Support for Hydrogen Production. J. Photochem. Photobiol., A 2010, 216, 250−255. (28) Idriss, H.; Seebauer, E. G. Reactions of Ethanol over Metal Oxides. J. Mol. Catal. A: Chem. 2000, 152, 201−212. (29) Rass-Hansen, J.; Falsig, H.; Jorgensen, B.; Christensen, C. H. Bioethanol: Fuel or Feedstock? J. Chem. Technol. Biotechnol. 2007, 82, 329−333. (30) Matthiesen, J.; Hansen, J. Ø.; Wendt, S.; Lira, E.; Schaub, R.; Lægsgaard, E.; Besenbacher, F.; Hammer, B. Formation and Diffusion of Water Dimers on Rutile TiO2(110). Phys. Rev. Lett. 2009, 102, 226101. (31) Li, S. C.; Chu, L. N.; Gong, X. Q.; Diebold, U. Hydrogen Bonding Controls the Dynamics of Catechol Adsorbed on a TiO2(110) Surface. Science 2010, 328, 882−884. (32) Mortensen, J. J.; Hansen, L. B.; Jacobsen, K. W. Real-Space Grid Implementation of the Projector Augmented Wave Method. Phys. Rev. B 2005, 71, 035109. (33) Enkovaara, J.; Rostgaard, C.; Mortensen, J. J.; Chen, J.; Dulak, M.; Ferrighi, L.; Gavnholt, J.; Glinsvad, C.; Haikola, V.; Hansen, H. A.; et al. Electronic Structure Calculations with GPAW: A Real-Space Implementation of the Projector Augmented-Wave Method. J. Phys.: Condens. Matter 2010, 22, 253202. (34) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (35) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (36) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS We acknowledge with thanks the support of this work by the Danish Research Agency, the Strategic Research Council, the Villum Kahn Rasmussen Foundation, the Carlsberg Foundation, the Danish Center for Scientific Computing, and the European Research Council through an Advanced ERC grant (F.B.).

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REFERENCES

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