Water as a Catalyst: Imaging Reactions of O2 with Partially and Fully

1908

J. Phys. Chem. C 2009, 113, 1908–1916

Water as a Catalyst: Imaging Reactions of O2 with Partially and Fully Hydroxylated TiO2(110) Surfaces Zhenrong Zhang,† Yingge Du,‡ Nikolay G. Petrik,† Greg A. Kimmel,*,† Igor Lyubinetsky,*,‡ and Zdenek Dohna´lek*,† Chemical and Materials Sciences DiVision, and EnVironmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed: October 10, 2008; ReVised Manuscript ReceiVed: NoVember 19, 2008

The reactions of molecular oxygen with bridging hydroxyl groups (OHb’s) formed by H2O dissociation on bridging oxygen vacancies (VO’s) of TiO2(110) are studied at low and high OHb coverages as a function of the O2 exposure, using scanning tunneling microscopy, temperature programmed desorption, and electron stimulated desorption techniques. In agreement with prior studies, oxygen adatoms (Oa), hydroperoxyls (HO2), and terminal hydroxyls (OHt) are observed as intermediates of the reactions with O2 ultimately leading to H2O as a product. Here, we show that water plays an important role in the room-temperature reactions of O2 with both partially and fully hydroxylated TiO2(110). On partially hydroxylated surfaces, water is found to be involved in the reaction cycle that leads to the consumption of Oa and VO sites thus resulting in a practically Oa- and VO-free surface. In these reactions, water is observed to participate in multiple wayssas a reactant, product, and catalyst. On fully hydroxylated TiO2(110), water is found to mediate the diffusion of surface species such as OHb that would otherwise be stationary and thus brings reactants together, catalyzing the reactions with O2. As a result, the Oa, HO2, and OHt intermediates are not observed in STM, while OHb species are available on the surface. 1. Introduction The thermal and photocatalytic reactions on TiO2 have attracted much interest owing to their wide applications, such as air purification, self-cleaning glass, water splitting, solar cells, and wastewater treatment.1,2 Both oxygen and water play crucial roles in the catalytic chemistry involved in these applications.2-8 As a result, water and O2 adsorption on model rutile TiO2(110) surfaces have been extensively investigated to facilitate a better understanding of the relevant surface processes. The (110) surface of rutile TiO2 has alternating rows of 5-fold coordinated Ti4+ ions and 2-fold coordinated bridging oxygen ions (Ob) which protrude above the plane defined by the Ti4+. For partially reduced crystals, chemically active surface sitessoxygen vacancies (VO)sare present in the bridging oxygen rows on the (110) surface. It is well-known that H2O dissociates in VO defect sites forming bridging hydroxyl (OHb) pairs9-11

H2O + VO + Ob f 2OHb

(1)

Two different mechanisms for separating the OHb pairs to form the isolated OHb’s have been identified: (1) intrinsic hydrogen diffusion along the same Ob row (i.e., in the (001) direction),10 and (2) water-assisted hydrogen diffusion from one Ob row to a neighboring one (i.e., in the [11j0] direction).9 While the waterassisted mechanism is expected to dominate in the presence of molecular water at low temperatures (400 K) where a significant H2O coverage cannot be sustained due to its high desorption rate.12 * To whom correspondence should be addressed. E-mail: [email protected];[email protected];[email protected]. † Chemical and Materials Sciences Division. ‡ Environmental Molecular Science Laboratory.

Molecular oxygen has also been shown to dissociate in VO sites with one O atom healing the VO and the other O atom bonding at a neighboring Ti4+ site as an adatom, Oa:11,13-16

O2 + VO f Ob + Oa

(2)

Recently, a second O2 dissociation channel has been reported on the Ti4+ sites not adjacent to an oxygen vacancy that results in the formation of Oa pairs:17 Ti4+

O2 98 2Oa

(3)

The reactions between O2, OHb, and H2O on TiO2(110) represent an additional step in complexity, and they are far from understood. For example, ensemble-average coadsorption studies of O2 and H2O on TiO2(110)18,19 have shown that O2 can react directly with OHb in the absence of VO’s. The authors suggested that the reaction proceeds via formation of an unidentified intermediate that decomposes and yields Ti4+-bound terminal OH, OHt. The direct reaction of O2 with OHb (to produce HO2 or H2O2) was also supported by calculations using density functional theory (DFT) and ab initio molecular dynamics.20 However, those calculations also predicted that chemisorbed O2 adsorbed next to OHb was the most stable configuration, which contradicts the experimental results. Our recent combined STM and DFT study of the initial stages of the reactions with O2 on partially hydroxylated TiO2(110) [i.e., a surface where only a fraction of VO’s are converted to OHb’s via reaction 1] has identified several reactions and two stable intermediate species (HO2 and OHt).21 Those experiments also showed that O2 readily reacts with OHb to form HO2:21

10.1021/jp809001x CCC: $40.75  2009 American Chemical Society Published on Web 01/13/2009

Imaging Reactions of O2 with Hydroxylated TiO2(110)

OHb+O2 f Ob+HO2

J. Phys. Chem. C, Vol. 113, No. 5, 2009 1909

(4)

In addition to this direct reaction of OHb with O2, OHb was shown to react indirectly with O2 via an oxygen adatom initially created by reactions 2 or 3:21

OHb + Oa f OHt + Ob

(5)

Two reactions in which OHb reacts with products created as a result of O2 exposure and which result in the formation of molecular water were also observed:21

minority reaction channel is consistent with O2 dissociation on Ti4+ sites17 followed by reactions with adsorbed water to form terminal hydroxyls. Collectively, the results presented here show that water, which is produced during the O2 exposure of both partially and fully hydroxylated TiO2(110), plays a key role in the subsequent reactions. In particular, water-mediated diffusion of species that would otherwise be stationary at room temperature (such as OHb and OHt) brings reactants together, thus catalyzing the reactions.

OHb + HO2 f Ob + Oa + H2O

(6)

2. Experimental Section

OHb + OHt f Ob + H2O

(7)

The results presented here were obtained in two ultrahigh vacuum systems. The first instrument, used to aquire the atomically resolved images, was equipped with variabletemperature STM (Omicron), Auger electron spectroscopy (AES), and a quadrupole mass spectrometer (QMS). The second instrument, used for ensemble-averaged measurements, was equipped with a low-energy electron gun, closed-cycle helium cryostat, a molecular beam for adsorbate deposition, AES, and QMS. The typical base pressure for both systems was 1 × 10-10 Torr. Rutile TiO2(110) single crystals (Princeton Scientific) were cleaned by repeated cycles of Ne+ sputtering and annealing up to 900-950 K. The initial concentration of VO’s on the clean TiO2(110) surfaces presented in the STM images is (0.06-0.08) ( 0.005 ML and 0.08 ( 0.01 ML for the ensemble-averaged measurements. In the STM system, hydroxylated surfaces were prepared by adsorption of background water for extended periods of time (hours to overnight) or by controlled H2O dosing on a clean TiO2(110) (1 × 1) surface via a retractable tube doser. We observed no differences between the hydroxylated surfaces prepared by controlled dosing or background water adsorption.12 O2 was introduced on the TiO2(110) surface using the tube doser. While dosing with the tube doser, the STM tip was withdrawn about 1 µm from the surface to reduce the shadowing effect of the tip. However, due to the shadowing and lateral variations in the O2 flux across the sample, the absolute O2 exposures for the STM experiments could not be determined directly. Instead, the product of O2 pressure behind the pinhole of the doser and the dosing time was used as a relative measure of the O2 exposure. To facilitate the comparison with the ESD and TPD results obtained in a separate UHV system (described below), we converted the relative O2 exposure for the STM experiments to absolute units by matching experimental results for the loss of OHb observed under similar conditions in both systems. Commercial tungsten tips (Custom Probe Unlimited) made by wet chemical etching, were cleaned prior to use by Ne+ sputtering and UHV annealing. The STM images were collected in the constant current mode with tunneling current of ∼ 0.1 nA and a bias voltage of about +1.5 V. The resulting images were processed using WSxM software.22 Additionally, a liquid nitrogen cooled trap was used in the STM chamber to minimize the adsorption of water from the UHV background during the measurements. All STM studies were carried out at 300 K. Details of the experimental equipment and procedures used for the ESD and TPD experiments have been described previously.23,24 An effusive molecular beam was used to dose the sample with O2 and/or D2O. For the ESD experiments, the electron beam was incident at 35° with respect to the sample normal. Typical instantaneous current densities in the electron beam were ∼3 × 1015 electrons/cm2s with a beam diameter of ∼1.5 mm. The samples were uniformly irradiated by repeatedly scanning the beam of 100 eV electrons over a rectangular area that was slightly larger than the area of the molecular beam on

In addition to these reactions involving the bridging hydroxyls, the reaction of two terminal hydroxyls to form water has been proposed:19

2OHt f Oa+H2O

(8)

From this brief discussion, it is apparent that quite a few individual reactions can take place as a result of exposure of the hydroxylated TiO2(110) to O2. Since many important details for these reactions (such as activation barriers and prefactors) are unknown, an overall understanding of how they proceed on hydroxylated TiO2(110) surfaces is lacking. For example, a recent STM study17 reported that upon O2 exposure the fully hydroxylated TiO2(110) surface [i.e., one with all VO’s converted to OHb’s via reaction 1] can be converted to a nearly perfect (i.e., VO-free and stoichiometric) state. However, the reaction mechanism of this dehydroxylation process has not been explained. Here we report a combined scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and electron stimulated desorption (ESD) study of the reactions of O2 with hydroxylated TiO2(110) surfaces at (and near) room temperature. On partially hydroxylated TiO2(110), STM reveals that with increasing O2 exposure the VO coverage slowly decreases and Oa coverage increases, until a critical point is reached beyond which the concentration of both species is rapidly reduced. The rapid loss of both VO and Oa is correlated with a significant increase in the water-assisted diffusion of the OHb remaining on the surface. Water is found to participate in the reaction cycle leading to the consumption of Oa and VO leading to a practically Oa- and VO-free surface. In these reactions, water created as a reaction product is also found to mediate the diffusion of surface species further accelerating the reaction kinetics before filling the available VO and forming new OHb. Due to the autocatalytic nature of the reaction mechanism, local fluctuations in the coverage of various surface species (e.g., VO, OHb, Oa, OHt) initially result in the local annihilation of Oa and VO within certain areas on the surface. On fully hydroxylated TiO2(110) at room temperature, the only changes visible with STM at low O2 exposures during the initial stages of the reaction are the loss of OHb’s and the onset of water-assisted OHb diffusion, suggesting that OHb’s are converted to water. Consistent with this observation, water TPD shows that after O2 exposure the OHb’s are converted to water which desorbs at ∼350 K. Using the known reactions observed on partially hydroxylated TiO2(110), the results suggest that the overall stoichiometry for the dominant reaction mechanism on the fully hydroxylated surface is 1/2O2 + 2OHb f H2O + 2Ob and that water also mediates the individual reaction steps. However, both STM and ESD show that the extended O2 exposure does not produce a perfect, stoichiometric surface. Instead, a small amount of OHt accumulates on the surface after an induction period. This

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Figure 1. STM images of the same area (10 × 10 nm2) on a partially hydroxylated TiO2(110) surface with [VO] ) 0.061 ML and [OHb] ) 0.043 ML. (a) Before and (b) after O2 exposures of 5.6 × 1015 O2/cm2 and (c) after an additional dose of 2.4 × 1015 O2/cm2 (cumulative dose of 8.0 × 1015 O2/cm2).34

the crystal. For both the ESD and TPD measurements, a quadrupole mass spectrometer (QMS) with an integrating cup25 and an electron impact ionization cell was used to measure the neutral desorbates. As described previously,23 deuterium atoms (and possibly ions) desorbed from the sample by electron irradiation were efficiently converted to HD on the walls of the integrating cup and detected in the QMS. The sample was mounted on a resistively heated tantalum base plate. The sample temperatures we report were measured via a thermocouple spotwelded to the base plate. A second thermocouple was embedded in a small amount of epoxy affixed to the TiO2. Tests indicate that during TPD (at 2 K/s) the sample temperature is ∼25 K lower than the thermocouple reading, while the sample temperature is within a few Kelvin of the thermocouple reading during isothermal experiments.26 3. Results and Discussion 3.1. Role of Water in the Reactions of O2 with Partially Hydroxylated TiO2(110). As discussed in the Introduction, our prior study focused on the initial stages of oxygen reactions with partially hydroxylated TiO2(110) where only a fraction of the VO’s is converted to OHb’s via reaction 1.21 Two stable surface intermediates, HO2 and OHt, were identified together with the elemental reaction steps (reactions 4-7) leading to their formation.21 Here, we follow the coverage of all stable surface species (VO, OHb, Oa, HO2, and OHt) on the same area as a function of cumulative O2 exposure to elucidate the overall reaction mechanism leading to water as a final product. As we will show, water is found to play an important role since it directly participates in the reactions and accelerates the overall reaction kinetics. In Figures 1(a) and 1(b), we review the assignment of the species resulting from the adsorption of H2O and O2 on clean, reduced TiO2(110) and from O2 adsorption on partially hydroxylated TiO2(110).9,10,13,14,17,21,27 Figure 1(a) shows the partially hydroxylated TiO2(110) prior to O2 exposure. Due to inverse electronic contrast, the empty-state STM images of TiO2(110) show the topographically low-lying Ti4+ rows as bright lines, and the topographically high-lying Ob rows are dark. Both VO’s (Figure 1(a), red circle) and OHb’s (Figure 1(a), black circle) appear as bright spots on the dark Ob rows. However, the OHb’s are brighter than the VO’s.9,10,27 In this representative example of the surface prior to O2 exposure (Figure 1(a)), the coverage of VO’s and OHb’s are [V0] ) 0.061 ML and [OHb] ) 0.043 ML, respectively. (Throughout the text, the coverage of any species, X, relative to the number of Ti4+ ions (5.2 × 1014 cm-2), will be denoted as [X].) When partially hydroxylated TiO2(110) is exposed to O2, three types of new features that are all centered on Ti4+ rows appear on the surface: Oa, HO2, and OHt. Figure 1(b) shows examples of all three features in the same area as in Figure 1(a) after a

total O2 exposure of 5.6 × 1015 O2/cm2. The least bright features correspond to oxygen adatoms, Oa (orange circles), created via one of two O2 dissociation channels. The emergence of a single Oa (orange circle, solid line) accompanied by the disappearance of a VO is a result of O2 dissociation in that VO (reaction 2).11,13-16 In addition, Oa pairs are observed (orange circle, dashed line) which result from a minor O2 dissociation channel on Ti4+ rows without causing the VO annihilation (reaction 3).17 Elongated features of intermediate brightness (Figure 1(b), magenta circle) correspond to hydroperoxyls, HO2, created as a result of O2 reacting with OHb via reaction 4.21 Thus the formation of an HO2 is always accompanied by the disappearance of an OHb. The brightest features centered on the Ti4+ rows are OHt species (Figure 1(b), yellow circle) that are created as a result of hydrogen transfer from OHb to Oa via reaction 5.21 While the initial O2 exposure (Figure 1(b)) leads to the formation of Oa, HO2, and OHt on the surface, an additional exposure of 2.4 × 1015 O2/cm2 leads to the disappearance of all Oa and VO species and results in a surface covered primarily by OHb’s (Figure 1(c)). To quantify this surprising result, we have measured the coverage of all the species on the larger area (30 × 30 nm2) versus cumulative O2 exposure (Figure 2(a)). For O2 exposures less than 5.6 × 1015 O2/cm2, [Oa] increases monotonically (Figure 2(a), orange triangles), while both [VO] and [OHb] decrease (Figure 2(a), red circles and black squares, respectively). Note that since some of the O2 dissociates on Ti4+ sites via reaction 3 the net increase in [Oa] is larger than the decrease in [VO]. After a total exposure of 5.6 × 1015 O2/cm2, the coverages of Oa, VO, and OHb are all about the same. In the

Figure 2. (a) Coverage of VO, OHb, Oa, and OHt + HO2 species versus cumulative O2 exposure on the partially hydroxylated TiO2(110) surface, obtained from the analysis of a 30 × 30 nm2 area. (b) The percentage of features that moved between subsequent STM images (∼2 min/ image) after various O2 exposures.



Figure 3. Time evolution of partially hydroxylated TiO2(110) after terminating O2 exposure. (a-c) show a large 30 × 30 nm2 area; (d-e) show the magnified areas (13 × 13 nm2) marked in (a-c) by squares. (a) and (d) t ) 0 min; (b) and (e) t ) 8 min; (c) and (f) t )17 min. VO’s (red circles, (d)) are converted to single OHb’s (black solid circle, (e)) and OHb pairs (black dotted circles (e)). Oa’s (orange circles) (d) are converted to OHt’s (yellow circles, (e)).

next STM image (corresponding to an O2 exposure of 8 × 1015 O2/cm2), the coverage of both VO and Oa has dropped to zero while [OHb] has continued to decrease steadily. To confirm the reproducibility of the data, we have repeated the same experiment several times. In all cases, the same sharp, simultaneous decrease of the [Oa] and [VO] has been observed. To further show that the observed changes are not induced by STM tip, we have also scanned other areas that were not repeatedly imaged. Similar final Oa- and VO-free surfaces, such as that shown in Figure 1(c), have been observed arguing that the reactions are not affected by the imaging. During the O2 exposure of the partially hydroxylated surface, the combined coverage of OHt and HO2 (Figure 2(a), green triangles) versus O2 exposure increases without any dramatic change when [VO] and [Oa] abruptly decrease. Note also that the total coverage of these species is about an order of magnitude smaller than the coverage of OHb, even after the abrupt decrease of [Oa] and [VO]. While the particular image presented in Figure 1 does not show any OHt’s and HO2’s, they are observed on the larger area scans (not shown) used to determine the concentration of the species as shown in Figure 2. The concentration of OHt’s is small because they are the product of a secondary reaction (reaction 5) in which Oa’s (from O2 dissociation) are needed. In addition, the reaction is kinetically hindered since the mobilities of OHb’s and Oa’s are both small at room temperature if no molecular water is present on the surface.10,12,21 Along with the simultaneous disappearance of Oa’s and VO’s (Figure 2(a)), another striking observation is an abrupt change in the mobility of the features detected in the STM images as shown in Figure 2(b). For O2 exposures e5.6 × 1015 O2/cm2, no movement of any species is detected upon subsequent scanning of the same area (i.e., ∼2 min later) except for an occasional intrinsic diffusion of OHb along the bridging oxygen rows. After an additional O2 exposure, 60% of the OHb’s and almost all of the HO2’s and OHt’s have changed positions in between the subsequent STM scans (Figure 2(b)). Furthermore, the OHb motion occurs primarily across the Ob rows (i.e., in the [11j0] direction). As reported previously, the intrinsic diffusion rate of OHb’s at 300 K is small and occurs along the Ob rows.10,12 Therefore, it cannot account for the large OHb

mobilities observed in Figure 2(b). HO2 and OHt have also been shown to be practically immobile at 300 K when no water is present.21 However, it is well-known that H2O-assisted crossrow diffusion of OHb’s occurs at temperatures as low as 187 K.9 Therefore, the results suggest that a finite coverage of molecular water has appeared on the surface simultaneously with the disappearance of VO’s and Oa’s as indicated by cross-row diffusion of OHb’s. It is important to note that the amount of water deposited from the background during the experiment is negligible. Thus, the total amount of hydrogen on the surface is conserved as illustrated in Figure 2(a). Therefore, the H2O has to be formed by the ongoing reactions between O2 and OHb’s as shown in reactions 6 and 7. Hereafter, we use cross-row diffusion of OHb’s as a signature of the existence of molecular H2O on the surface in the STM images. (In section 3.4, we will present TPD experiments that provide direct evidence for the creation of molecular water during the O2 exposure of hydroxylated TiO2(110).) To further investigate the mechanism underlying the simultaneous disappearance of Oa and VO, we performed an experiment in which the O2 exposure was terminated once the concentrations of Oa’s and VO’s were approximately the same (i.e., close to the “critical values” observed in Figure 2(b)). The time evolution was then followed without additional O2 impinging on the surface (Figure 3). Similar to the results in Figures 1 and 2, there is also an abrupt disappearance of both Oa and VO during this experiment. However, in this case, the disappearance of Oa and VO happened 30 min after the O2 dose was stopped and only occurred locally in the area outlined by the squares in the large area images in Figures 3(a-c) while the surrounding area remained practically unchanged. To better visualize the changes, we have magnified the highlighted areas from Figures 3(a-c) and presented them in Figures 3(d-f). At the point when both [Oa] and [VO] abruptly decrease, about 50% of the OHb, OHt, and HO2 species within the area shown in Figures 3(d-f) changed their positions from image to image (data not shown). In contrast, less than 5% of the features moved in the remaining area shown in Figures 3(a-c) during the same interval. While the STM scanning speed (2 min/frame) could not follow every step of the reactions, the


Zhang et al.

images show that the disappearance of VO’s is followed by the appearance of paired or isolated OHb’s in the same region (see STM movie, Supporting Information). Prior to the disappearance of Oa’s, their conversion to OHt is also observed. Examples of these transformations are highlighted in Figures 3(d) and (e). The observations in Figure 3 support the hypothesis that water plays an important role in the O2 reactions with hydroxylated TiO2(110) surfaces. 3.2. Water-Catalyzed Reaction Scheme for Partially Hydroxylated TiO2(110). Before presenting the experimental results for O2 reactions with a fully hydroxylated surface, we will first propose a reaction scheme to explain the observations on the partially hydroxylated surface. This reaction scheme will then aid our discussion of the results on the fully hydroxylated surface and provide the basis for our proposed reaction scheme for that surface. For the experiments in Figures 1-3, the abrupt decrease in [Oa] and [VO] occurs after [Oa] has increased to the point that it is comparable to [VO]. Furthermore, the concentrations of OHb, HO2, and OHt do not change appreciably when the Oa’s and VO’s disappear. Thus the overall stoichiometry of the reactions appears to be: Oa + VO f Ob. However, as noted above, no diffusion of the Oa’s and VO’s is detected prior to their sudden disappearance, and this observation rules out simple diffusion of the oxygen adatoms as the mechanism for filling the vacancies. While various reaction schemes leading to the filling of VO’s with Oa’s can be written using the elemental steps outlined in the Introduction (i.e., reactions 1-8), the simplest scheme that is in accord with the observations in Figures 1-3 can be written (in the order of the reactions) as follows: H2O

OHb + Oa 98 OHt + Ob H2O

(5′)

OHb + OHt 98 Ob + H2O

(7′)

H2O + VO + Ob f 2OHb

(1)

Oa’s. Thus water is more likely to be formed by reactions 6 and 7, and this water, once formed, is less likely to find VO and thus more likely to catalyze further reactions. Qualitatively, this reaction scheme could lead to the autocatalytic behavior that is experimentally observed. As shown in Figure 3, the instability threshold that leads to Oa and VO annihilation does not have to lead to the homogeneous progression of this reaction sequence over the whole surface. This further points out that a critical level of reactants has to be achieved before the positive feedback leading to the accelerated, H2O-catalyzed reaction is reached. Due to random fluctuations in the coverages of all the species over the surface, this level does not have to be reached simultaneously in all surface regions. While these ideas provide a plausible explanation for the sudden Oa + VO annihilation, they do not provide an explanation for why this process occurs when [Oa] ∼ [VO], and they will need to be tested in the future using Monte Carlo simulations. However, an autocatalytic reaction mechanism, which leads to laterally nonhomogeneous reactions across the surfaces, is not unprecedented. In the case of H2 + O2 reactions on Pt(111), water has been shown to participate in the OH formation step via reaction with chemisorbed oxygen. This subsequently leads to accelerated reaction kinetics since more OH is available for subsequent reaction of OH with hydrogen.28 A variety of different reaction schemes utilizing reactions 1-8 (or potentially other reactions that have not yet been observed) can be proposed for explaining the results in Figures 1-3. However, the fact that the concentrations of HO2 and OHt do not abruptly change with O2 exposure or time, and are an order of magnitude smaller than [Oa] and [VO] (before they abruptly decrease), constrains the possible reaction mechanisms. For example, the concentrations of OHt and HO2 are too small to allow speculative reactions such as H2O

VO + OHt f OHb or

Since the encounter of the species in reactions 5 and 7 is accelerated in the presence of H2O, we have rewritten these reactions as 5′ and 7′ indicating that H2O acts as a catalyst. In this reaction scheme, O2 exposure primarily leads to the creation of Oa’s (reactions 2 and 3). Once a sufficient coverage of oxygen adatoms is achieved, further exposure to O2 is not requiredsonly water is needed to catalyze the reactions. Intrinsic OHb diffusion, which brings an OHb and an HO2 or OHt together such that they react to form water (via reactions 6 or 7, respectively), could start the catalytic reaction cycle (i.e., reactions (5′, 7′, and 1). Alternatively, a small amount of background H2O adsorption could provide the necessary trigger. Clearly, two important questions are why the Oa + VO annihilation is so sudden and why it starts to occur within local areas on the sample. While the explanation is not obvious, the results suggest that the reactions accelerate when a water molecule can, on average, catalyze the reaction sequence leading to the creation of another water molecule before it is annihilated in VO. In other words, a water molecule must assist the recombination of a sufficient number of Oa, OHb, HO2, and OHt such that further reactions between these species produce new water molecules before the original water is captured in VO. In the early stages (small O2 exposures), the VO concentration is high and the Oa concentration is low. So, not much water is produced, and any that is produced will quickly be consumed in VO. As the reactions proceed, there are fewer VO’s and more

H2O

VO + HO2 f OHb + Oa to be primarily responsible for the loss of the VO’s. However, a complete understanding of the O2 reactions with partially hydroxylated TiO2(110) will require further experimental and theoretical investigations. 3.3. Reactions of O2 with Fully Hydroxylated TiO2(110): STM Results. The surface shown in Figure 1(c) can be in principle viewed as a fully hydroxylated surface (no VO’s) albeit with lower OHb concentration. Therefore, our studies of partially hydroxylated TiO2(110) directly lead to the investigations of O2 interactions with fully hydroxylated TiO2(110) surfaces presented in this section. The reaction scheme proposed for the O2 reactions with partially hydroxylated TiO2(110) (reactions 1, 5′, and 7′) suggests an important role for both OHb and H2O in the reactions between Oa and VO. However, based on the results in Figure 2(a), another possible explanation is that OHb is just a bystander since its concentration does not undergo any abrupt changes as a result of O2 exposure. However, if this were true, O2 would not react with a fully hydroxylated surface where all the VO’s have been converted to OHb’s via reaction 1. Below we show that O2 also reacts very efficiently with fully hydroxylated TiO2(110) surfaces, despite the absence of VO’s. In this case OHb has to play an important role in the dehydroxylation process. Figure 4 shows a representative set of STM images obtained from the same area of a fully hydroxylated TiO2(110) surface



Figure 4. STM images of the same area (15 × 15 nm2) on the fully hydroxylated TiO2(110) surface with [OHb] ) 0.126 ML. (a) Before and (b-d) after cumulative O2 exposure of 2.4 × 1015, 9.6 × 1015, and 3.4 × 1016 O2/cm2, respectively.34

before and after O2 exposure. Figure 5 shows the coverage dependence of all the species on a larger area of 30 × 30 nm2. Initially, all the bright spots observed on the (dark) Ob rows in Figure 4(a) are OHb’s. For this surface, [OHb] ) 0.126 ML, which is double the VO concentration for the clean TiO2(110) surface used in these STM studies. Before imaging, the fully hydroxylated sample was flashed to ∼350 K to remove any molecular H2O.18 As a result, the percentage of OHb’s that moved from image to image on this surface prior to O2 exposure is practically zero.12 After 2.4 × 1015 O2/cm2, almost no new features are observed on the surface (Figure 4(b)), while the concentration of OHb drops to roughly half of its initial value as shown in Figure 5(a). In the meantime, the percentage of OHb’s that moved between STM images (at ∼2 min/image) increases from 0 to 48% (Figure 5(b)) indicating the formation and presence of H2O on the surface. As the O2 exposure is increased (to a total of 9.6 × 1015 O2/cm2), the coverage of OHb’s continues to decrease to as low as 0.007 ML (Figure 4(c)) and the percentage of remaining OHb’s that have moved increases to ∼80% (Figure 5(b)). In principle, these results show that the fully hydroxylated surface can be converted to a nearly stoichiometric surface by reacting with O2. A similar observation has been made in a recent STM study; however, no explanation was proposed.17 It is worth noting that once the O2 exposure is terminated no further decrease of the OHb concentration is observed (data not shown) indicating the involvement of molecular O2 in the reactions is necessary. This is in contrast to the partially hydroxylated surface on which some reactions continue even after the O2 exposure is stopped (Figure 3). For O2 exposures >5 × 1015 cm-2, two new types of bright features centered on the Ti4+ rows begin to appear (Figure 4(c)).

Figure 5. (a) Concentration of surface species obtained from the analysis of a 30 × 30 nm2 area of the fully hydroxylated TiO2(110) surface versus O2 exposure. (b) The percentage of OHb’s that moved between subsequent STM images (∼2 min/image) versus O2 exposure.

With continued O2 dosing, the concentration of these two species increases until their combined coverage is ∼0.033 ML, which is ∼25% of the initial OHb coverage, for the largest exposure (Figures 4(d) and 5(b)). This observation is also in agreement with that of a recent STM study.17 On the basis of a comparison with the Ti4+-centered features observed on partially hydroxylated TiO2(110), we believe that the more abundant species (∼80% of the total) are likely OHt’s (Figure 4(d), yellow circles). The less abundant species (Figure 4(d), magenta circles) have approximately the same shape as HO2. However, in Figure 4 they are brighter than the OHt’s, and this is inconsistent with the appearance of HO2 relative to OHt as presented in Figure 1. As such, we do not know the chemical makeup of this second (minor) species. However, the ESD data presented below indicate that the remaining species on the surface after extended O2 exposure at 300 K should be hydrogen-containing species (such as OHt or HO2). 3.4. Reactions of O2 with Fully Hydroxylated TiO2(110): TPD and ESD Results. In addition to STM, we have also employed ensemble-averaged techniques, TPD and ESD, to investigate reactions with O2 occurring on fully hydroxylated TiO2(110). On the basis of prior TPD investigations,18,23 we expect that any water formed as a result of O2 exposure should desorb at ∼300 K. However, at slightly lower temperatures, the water should remain on the surface and thus be observable via TPD. Figure 6 (green line) shows a D2O TPD spectrum for a TiO2(110) surface that was fully hydroxylated with D2O to form ODb by exposing the crystal to D2O at 400 K. (Below, surfaces with all VO’s converted to ODb will be denoted as ODb-TiO2(110).) As expected, the D2O TPD spectrum has a peak at ∼550 K due to the recombinative desorption of ODb’s.29,30 After exposing ODb-TiO2(110) to ∼2.7 × 1016 O2/ cm2 at 300 K, very little D2O desorption is observed at any temperature (Figure 6, red line), indicating that the reactions with O2 have eliminated the ODb’s without leaving any

Figure 6. D2O TPD spectra with and without exposure to oxygen. For a surface exposed to D2O at 400 K to convert the oxygen vacancies to ODb, the D2O TPD spectrum (green line) has a peak at ∼570 K due to the recombinative desorption of 2ODb’s. No D2O is observed during the subsequent TPD (red line) after dosing O2 at 300 K. If the surface is exposed to O2 at 240 K, a D2O TPD peak at ∼350 K appears (blue line).


Figure 7. Integrated D2O TPD, D2O ESD, and D ESD yields versus O2 exposure. (a) The reduced TiO2(110) surface was exposed to D2O at 400 K, eliminating the oxygen vacancies and producing ODb’s. This hydroxylated surface was then exposed to oxygen at 240 K, and D2O TPD spectra were subsequently obtained. The integrated D2O TPD intensity from 280-420 K increases rapidly and then saturates for larger O2 exposures (red triangles). In contrast, the D2O ESD signal after O2 exposure at 240 K (blue diamonds) is small but larger than the control experiments (see text for discussion). The D2O ESD signal from 0.1 ML D2O dosed on the hydroxylated surface (purple square) is about a factor of 10 larger and easily measured. (b) The D ESD versus O2 exposure at 300 K for surfaces hydroxylated with D2O. As described in the Experimental Section, the O2 exposure-dependent decay of the D ESD signal was used to calibrate the O2 exposures in the STM experiments. To do that, the O2 exposure-dependent OHb decay in Figure 5(a) was matched with D ESD decay in Figure 7(b), and the conversion factor thus obtained was further applied to all O2 exposures in the STM experiments.

significant amount of water on the surface. However, if this fully hydroxylated surface is exposed to the same amount of O2 at 240 K, the ODb recombinative desorption peak is still suppressed, but D2O desorption at ∼350 K is observed (Figure 6, blue line).31 The D2O TPD peak at ∼350 K is consistent with molecular water desorption from the Ti4+ rows on the O2 exposed surface.19,23 An important observation is that the increase in the ∼350 K TPD peak upon O2 exposure matches (within 10%) the decrease in the ODb recombinative desorption peak. Figure 7(a) (red triangles) shows the change in the integral of the low-temperature (i.e., 280-420 K) D2O TPD peak versus O2 exposure at 240 K. The amount of low-temperature D2O desorbing from the surface increases with O2 exposure and then saturates for exposures >1.2 × 1016 O2/cm2. The data can be fit with a simple exponential rise (Figure 7(a), red line). The amount of D2O desorbing in the ODb recombinative desorption peak decreases over the same O2 exposure range where the ∼350 K peak increases (data not shown). In addition to TPD, we have also used ESD to explore the reactions of O2 with fully hydroxylated TiO2(110). We have previously shown that irradiation of D2O bound to the Ti4+ rows on TiO2(110) with low-energy electrons results in ESD of both D2O and D atoms.23 This suggests that water ESD can be used to investigate formation of water during the O2 exposure of hydroxylated surfaces. Figure 7(a) shows the D2O ESD from

Zhang et al. ODb-TiO2(110) versus O2 exposure at 240 K along with several control experiments. For these experiments, the procedure was similar to the D2O TPD experiments described above except that after O2 exposure at 240 K the D2O ESD was measured by irradiating the surface with 100 eV electrons at 100 K. Two control experiments (which are described below) were also performed after each measurement of the D2O ESD at a particular O2 exposure. The key observation is that after O2 exposure of the ODb-TiO2(110) at 240 K the D2O ESD signal was small (Figure 7(a), blue diamonds) but consistently above the control experiments (Figure 7a, circles and × ’s). For both control experiments, no D2O ESD above background was observed. In the first control experiment, the ODb-TiO2(110) was held at 240 K without O2 exposure for the time required for the corresponding O2 dose. The D2O ESD was then measured at 100 K (Figure 7(a), circles). In the second control experiment, the sample was annealed at 950 K, then cooled directly to 100 K, and the D2O ESD was measured (Figure 7(a), ×’s). The results in Figure 7(a) indicate that relatively little molecular D2O exists on the surface after O2 exposure at 240 K. In comparison, the D2O ESD signal from 0.1 ML of D2O deposited on ODb-TiO2(110) at 100 K (Figure 7(a), purple square) is about a factor of 10 larger than the signal from the O2 exposed surface and is easily measured. Therefore, the results indicate that majority of the D2O TPD signal observed after O2 exposure at 240 K (Figures 6 and 7a) is not due to molecular water that resides on the surface at 240 K. Instead, the reactions with O2 at 240 K produce other species on the surface. The D2O that desorbs at ∼350 K is then produced in chemical reactions that occur upon heating the sample. Since OHt appears to be the dominant species observed in STM after O2 exposure of a fully hydroxylated surface at room temperature (see Figure 4), it is plausible that these are also the species on the surface after O2 exposure at 240 K. Further support for this hypothesis will be provided below. As already noted, no water desorption is observed during heating after O2 exposure of a hydroxylated surface at 300 K (Figure 6, red line). Not surprisingly, no water ESD is observed in this case either (data not shown). However, D ESDswhich is measured via the HD signal in the QMS (as explained in the Experimental Section)scan be used to investigate the reactions with O2 at 300 K. Since energetic electrons efficiently desorb deuterium atoms from a variety of deuterated species (e.g., D2O, ODb, ODt, and DO2), D ESD does not allow an unambiguous identification of the chemical species responsible for the signal. On the other hand, D ESD does provide a convenient way to measure the total amount of deuterium on the surface at any time. For example, Figure 7(b) shows the integrated D ESD from ODb-TiO2(110) versus O2 exposure at 300 K. The D ESD decreases exponentially with increasing O2 exposure before saturating at a constant value for exposures >1.5 × 1016 O2/ cm2. The exponential decay for the D ESD signal (Figure 7(b)) occurs over the same exposure range as the exponential growth of the D2O TPD signal (Figure 7(a), red triangles). Interestingly, the D ESD signal decreases to ∼20% of its initial value suggesting that some deuterated species remain on the surface even for large oxygen exposures for which the bridging hydroxyls have been completely eliminated. Recall that for similar experiments STM images revealed ∼0.033 ML of Ti4+centered features on the surface or ∼25% of the initial coverage of OHb. Thus the combined STM, TPD, and ESD results suggest that the species remaining on the surface after O2 exposure at room temperature (or 300 K) are ∼80% OHt, while the rest are as yet unidentified.



3.5. Water-Catalyzed Reaction Scheme for Fully Hydroxylated TiO2(110). On the basis of previous results17,21 and the experiments on the partially hydroxylated surface discussed above, we propose a reaction scheme to explain the O2 reactions with the fully hydroxylated surface. In our discussion, we assume that the reactions occurring at 300 and 240 K are the same. The main difference between these two temperatures is that water formed in the reactions eventually desorbs at 300 K, while water produced in TPD after O2 exposure at 240 K remains on the surface. As discussed below, mobile water molecules can catalyze reactions at both temperatures. The STM and TPD results (Figures 4, 5(a), and 6) show that OHb is lost as the reaction proceeds at ∼300 K, and the D ESD results (Figure 7(b)) also support this observation. The observed mobility of the OHb (Figure 5(b)) indicates that molecular water is present during the O2 exposure. Consistent with this, molecular water desorbs at ∼350 K after O2 exposure at 240 K (Figures 6 and 7(a)).31 On the basis of these observations, the overall stoichiometry of the dominant reactions on the fully hydroxylated surface appears to be the conversion of OHb to water:

O2 + 4OHb f 4Ob + 2H2O

(9)

This stoichiometry matches the stoichiometry expected from the sum of reactions 4-7 and suggests that these individual reactions primarily account for the observations. The overall stoichiometry proposed in reaction 9 also explains one otherwise puzzling observation. Comparing the concentration of OHb versus O2 exposure on the partially and fully hydroxylated surfaces (black squares in Figures 2a and 5a, respectively), it is apparent that the OHb’s are eliminated at a significantly slower rate on the partially hydroxylated surface. As shown above in reaction 9, we expect that on fully hydroxylated surfaces four OHb species will be consumed per every reacted O2. In contrast, on partially hydroxylated surfaces, only one OHb species will be consumed in the initial stages of the reaction. This is because reaction 4, which is the initial reaction step, controls the OHb consumption. Since all the surface species are stationary at that point, subsequent reactions (reactions 5-7) are too slow, and their contribution can be neglected. Therefore, the rate of OHb consumption should be four times slower on partially hydroxylated surfaces. While the scatter of the data in Figure 2(a) and in Figure 5(a) does not allow for accurate comparison of the rates, the estimated factor of 6 difference seen experimentally is in a reasonable agreement with the factor of 4 expected based on the reaction schemes we have proposed for the partially and fully hydroxylated surfaces. Two additional observations are consistent with the proposed reaction scheme. First, there is an induction period in the early stages of the reaction (Figure 5(a)) before any other species (including OHt’s) are observed via STM. Second, the amount of water desorbing from the sample after O2 exposure at 240 K matches (within ∼10%) the amount of water that desorbs in the OHb recombination peak from the hydroxylated surface (Figure 6),32 which is consistent with reaction 9. The experimental results indicate that water plays a key role in facilitating the reactions on the fully hydroxylated surface. For example, reactions 5-7 all involve OHb, and the STM results (Figure 5(b)) show that water increases the mobility of the OHb thus bringing the reactants together.33 The lack of reaction intermediates such as HO2, Oa, and OHt in the initial stages of the reaction (Figure 5(a)) can be explained if the watercatalyzed reactions 5-7 are sufficiently fast to suppress the concentration of these species. Thus once HO2 is formed, the

rest of the reactions follow quickly, keeping the concentrations of these species low (as observed). As noted already, water also appears to play a key role in catalyzing the reactions on the partially hydroxylated surface. However there are no VO’s present on the fully hydroxylated surface to react with water, and thus the water should be quite efficient at catalyzing the reactions even in the early stages. In contrast, water only becomes an efficient catalyst on the partially hydroxylated surface once all (or most) of the VO’s have been annihilated. Since the TPD experiments show that at room temperature most of the water desorbs on the time scale of the experiments (i.e., a few minutes) (see Figure 6), they may initially appear to contradict our STM results where the presence of water is inferred based on the observed cross-row OHb diffusion (Figure 5b). However, a simple calculation of the number of diffusive hops a water molecule typically makes prior to desorbing clarifies this issue: The average lifetime of a water molecule on the surface, τdes, is inversely proportional to the desorption rate, τdes ) νa-1eEa/kT, where νa is the desorption prefactor and Ea is the desorption energy. The number of hops the water molecule takes prior to desorbing, Ndiff, is given by the diffusion rate times the desorption lifetime: Ndiff ) νdiffe-Ediff/kTτdes, where νdiff and Ediff are the prefactor and activation energy for diffusion, respectively. Assuming identical prefactors for diffusion and desorption and employing the theoretically calculated H2O desorption energy of 0.79 eV and diffusion barrier of ∼0.3 eV for water on the Ti4+ rows,9 we estimate that water molecules hop on average 3 × 108 times before they desorbs at 300 K. Therefore, within its lifetime a single water molecule can diffuse over areas much larger than those imaged in our STM experiments suggesting that the amount of water necessary to induce OHb diffusion is very small and well below the TPD detection limit (∼0.01 ML). If only reactions 4-7 occurred on the fully hydroxylated surface, then one should be able to obtain a “perfect”, stoichiometric surface after the reactions are complete. However, after O2 exposure at room temperature, a low concentration of OHt’s (and another species) is observed with STM (Figures 4 and 5). Furthermore, on an initially deuterated surface, a small amount of a deuterated species remains after O2 exposure at 300 K (Figure 7(b)). These observations suggest that another reaction channel exists for the O2 reactions with a fully hydroxylated surface in addition to reactions 4-7. If reaction 3, which has been observed on reduced17 and partially hydroxylated surfaces (Figure 1(b)), also occurs on the fully hydroxylated surface, it would lead to the formation of Oa pairs. However, Oa’s are not observed on the fully hydroxylated surface. Therefore, if reaction 3 does occur, another reaction that converts the Oa to another species (presumably OHt) must also occur. For example, Oa reacting with water

H2O + Oa f 2OHt

(10)

is a likely candidate. Reaction 10 is consistent with prior studies where isotopic scrambling of oxygen has been observed after H2O adsorption on O2-saturated surfaces.19 Taken together, reactions 8 and 10 suggest an equilibrium exists between oxygen adatoms, water, and terminal hydroxyls

Since only OHt’s are observed in STM, reaction 4 probably has the equilibrium shifted toward the OHt’s (as indicated). Further support for reaction 4 is the observation (Figure 7(a))

1916 J. Phys. Chem. C, Vol. 113, No. 5, 2009 that the amount of molecular water on the surface as measured by ESD is small after O2 exposure at 240 K. Then, upon heating, the OHt’s associatively desorb producing the TPD peak observed at ∼350 K (Figures 6 and 7(a)). For the partially hydroxylated surface, where oxygen adatoms are observed, there is an effective loss channel for waterstrapping by VO and dissociationswhich shifts the chemical equilibrium toward Oa production. 4. Conclusion In summary, we have investigated the detailed reaction mechanism of O2 with OHb’s on both partially and fully hydroxylated TiO2(110) by combining the site-specific STM and ensemble-averaging TPD/ESD studies at (and near) room temperature. On partially hydroxylated surfaces, the reactions with O2 initially lead to the formation of stationary oxygen adatom (Oa), hydroperoxyl (HO2), and terminal hydroxyl (OHt) intermediates in agreement with prior studies.11,13,14,17-19,21 Upon further exposure to O2, a critical coverage of oxygen adatoms is reached (at [Oa] ≈ [VO]), where subsequent reactions lead to the simultaneous and sudden disappearance of both VO’s and Oa’s while leaving OHb’s on the surface. The formation of H2O as a result of the O2 exposure catalyzes the delivery of Oa to VO’s sites, thus leading to their mutual annihilation. On fully hydroxylated TiO2(110) surfaces, we show that most of the OHb’s can be removed via reaction with O2 such that a fully hydroxylated TiO2 surface can be converted to a nearly stoichiometric surface. Water is found to mediate the diffusion of surface species such as OHb that would otherwise be stationary, thus bringing reactants together and catalyzing the reactions with O2. As a result, the lifetimes of the Oa, HO2, and OHt intermediates are short compared to our STM image acquisition times (∼2 min), and they are not observed in our experiments. Acknowledgment. We thank M. A. Henderson and N. A. Deskins for stimulating discussions. This work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, and performed at the W. R. Wiley Environmental Molecular Science Laboratory, a DOE User Facility sponsored by the Office of Biological and Environmental Research. Supporting Information Available: Isothermal STM movie (300 K) showing the time evolution of partially hydroxylated TiO2(110) after terminating O2 exposure. The movie illustrates the annihilation of VO and Oa species. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (2) 735. (3) (4)

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Zhang et al. (5) Anpo, M.; Aikawa, N.; Kubokawa, Y.; Che, M.; Louis, C.; Giamello, E. J. Phys. Chem. 1985, 89, 5689. (6) Thompson, T. L.; Yates, J. T. Top. Catal. 2005, 35, 197. (7) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (8) Petrik, N. G.; Kimmel, G. A. Phys. ReV. Lett. 2007, 99, 196103. (9) Wendt, S.; Matthiesen, J.; Schaub, R.; Vestergaard, E. K.; Laegsgaard, E.; Besenbacher, F.; Hammer, B. Phys. ReV. Lett. 2006, 96, 066107. (10) Zhang, Z.; Bondarchuk, O.; Kay, B. D.; White, J. M.; Dohnalek, Z. J. Phys. Chem. B 2006, 110, 21840. (11) Bikondoa, O.; Pang, C. L.; Ithnin, R.; Muryn, C. A.; Onishi, H.; Thornton, G. Nat. Mater. 2006, 5, 189. (12) Li, S. C.; Zhang, Z.; Sheppard, D.; Kay, B. D.; White, J. M.; Du, Y.; Lyubinetsky, I.; Henkelman, G.; Dohnalek, Z. J. Am. Chem. Soc. 2008, 130, 9080. (13) Wendt, S.; Schaub, R.; Matthiesen, J.; Vestergaard, E. K.; Wahlstrom, E.; Rasmussen, M. D.; Thostrup, P.; Molina, L. M.; Laegsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Surf. Sci. 2005, 598, 226. (14) Du, Y. G.; Dohnalek, Z.; Lyubinetsky, I. J. Phys. Chem. C 2008, 112, 2649. (15) Diebold, U.; Lehman, J.; Mahmoud, T.; Kuhn, M.; Leonardelli, G.; Hebenstreit, W.; Schmid, M.; Varga, P. Surf. Sci. 1998, 411, 137. (16) Henderson, M. A.; Epling, W. S.; Perkins, C. L.; Peden, C. H. F.; Diebold, U. J. Phys. Chem. B 1999, 103, 5328. (17) Wendt, S.; Sprunger, P. T.; Lira, E.; Madsen, G. K. H.; Li, Z. S.; Hansen, J. O.; Matthiesen, J.; Blekinge-Rasmussen, A.; Laegsgaard, E.; Hammer, B.; Besenbacher, F. Science 2008, 320, 1755. (18) Henderson, M. A.; Epling, W. S.; Peden, C. H. F.; Perkins, C. L. J. Phys. Chem. B 2003, 107, 534. (19) Epling, W. S.; Peden, C. H. F.; Henderson, M. A.; Diebold, U. Surf. Sci. 1998, 413, 333. (20) Tilocca, A.; Di Valentin, C.; Selloni, A. J. Phys. Chem. B 2005, 109, 20963. (21) Du, Y.; Deskins, N. A.; Zhang, Z.; Dohnalek, Z.; Dupuis, M.; Lyubinetsky, I. J. Phys. Chem. C, published on Web, DOI:10.1021/ jp807030n. (22) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. ReV. Sci. Instrum. 2007, 78, 013705. (23) Lane, C. D.; Petrik, N. G.; Orlando, T. M.; Kimmel, G. A. J. Phys. Chem. C 2007, 111, 16319. (24) Lane, C. D.; Petrik, N. G.; Orlando, T. M.; Kimmel, G. A. J. Chem. Phys. 2007, 127, 224706. (25) Yates, J. T. Experimental InnoVations in Surface Science: A Guide to Practical Laboratory Methods and Instruments.; Springer-Verlag New York, Inc.: New York, 1998. (26) Uncertainties such as these are typical in the temperature measurements for oxide surfaces. (27) Zhang, Z.; Ge, Q.; Li, S. C.; Kay, B. D.; White, J. M.; Dohnalek, Z. Phys. ReV. Lett. 2007, 99, 126105. (28) Volkening, S.; Bedurftig, K.; Jacobi, K.; Wintterlin, J.; Ertl, G. Phys. ReV. Lett. 1999, 83, 2672. (29) Henderson, M. A. Surf. Sci. 1998, 400, 203. (30) Henderson, M. A. Langmuir 1996, 12, 5093. (31) As discussed in section 2, the temperature of the sample during TPD is lower than the measured temperature, such that the true temperature of the desorption peak is ∼325 K. (32) At 240 K, the water created by the oxidation reactions does not desorb until the surface is heated to higher temperature, while at 300 K the water desorbs soon after it is formed. (33) Since the residence time of O2 on the surface at 240-300 K is expected to be short, reaction 4 is not expected to depend on the mobility of the OHb. (34) While the absolute O2 exposure has not been directly calibrated in the STM system, we use the ESD and TPD results presented later to convert the relative O2 exposures to absolute units by matching the observed O2 exposure-dependent disappearance of OHb species on fully hydroxylated TiO2(110).

JP809001X

Water as a Catalyst: Imaging Reactions of O2 with Partially and Fully

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