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Multiple Nonthermal Reaction Steps for the Photooxidation of CO to

Jan 8, 2013 - The photooxidation of CO on reduced, rutile TiO2(110) is studied on a millisecond time scale. For CO coadsorbed with a saturation covera...
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Multiple Nonthermal Reaction Steps for the Photooxidation of CO to CO2 on Reduced TiO2(110) Nikolay G. Petrik* and Greg A. Kimmel* Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, MSIN K8-88, P.O. Box 999, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: The photooxidation of CO on reduced, rutile TiO2(110) is studied on a millisecond time scale. For CO coadsorbed with a saturation coverage of chemisorbed O2 (θsat), the CO2 photon-stimulated desorption (PSD) signal is initially zero, increases to a maximum after several tens of milliseconds, and then decreases at longer times. The initial CO2 PSD signal increases ∼5 times more quickly for an oxygen coverage of 0.5θsat. The initial rate of increase of the CO2 PSD signal is proportional to the flux of UV photons. The results show that two or more nonthermal reaction steps are required to photooxidize CO adsorbed on TiO2(110). The intermediate species involved in the reactions is stable for at least 100 s at 30 K. Previous models had suggested that CO photooxidation required only one nonthermal reaction. The likely initial and final charge states of the system suggest that an electron-mediated reaction and a hole-mediated reaction are needed for the complete photooxidation reaction. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

T

surface normal along the [11̅0] azimuth.7 The noncosine distribution for the desorbing CO2 indicates that it is produced from a reaction complex on the surface that is oriented perpendicular to the bridge-bonded oxygen (Ob) rows. The experiments indicated that the reactions complex is formed from O2 adsorbed in a vacancy (VO) in an Ob row and CO adsorbed at an adjacent Ti5c site.7 The experimental results are consistent with CO2 being produced from an intermediate state that has been predicted theoretically.14,15 Recently, it has been proposed that the photooxidation of CO on TiO2(110) proceeds via electron attachment:8

iO2 is a widely used photocatalyst.1−3 Its ability to oxidize organic contaminants makes it useful, for example, in air and water purification systems and as a thin-film coating for self-cleaning surfaces.1 As a result of titanium dioxide’s practical applications and its potential use in photocatalytic water spitting, it has been the subject of a tremendous amount of research.1−3 Because it is a prototypical photocatalytic reaction, the photooxidation of CO to CO2 has received considerable attention.2 Some of the most detailed information has come from investigations of the reactions of CO and O2 on singlecrystal TiO2(110).4−11 For example, Yates and co-workers found that UV irradiation leads to either photodesorption of O2 or photooxidation of CO to produce CO2.4,5 The threshold energy for initiating the photochemical reactions corresponds to the 3 eV bandgap of TiO2, which suggests that the reactions are initiated by electrons in the conduction band and/or holes in the valence band created by the absorption of UV photons in the substrate. Previous experiments have shown that the CO2 photon-stimulated desorption (PSD) yield increased abruptly at the beginning of the UV irradiation and then monotonically decreased as the irradiation proceeds.4,5,7 Similar kinetics are also observed for the CO2 electron-stimulated desorption (ESD) yield when energetic electrons are used to create electron−hole pairs in the TiO2 instead of UV photons.8 For some experiments, the “initial” photodesorption yield has been used to characterize the various aspects of the photochemistry occurring on TiO2(110).12,13 We have previously investigated the photochemistry of CO and O2 on TiO2(110) and found that the angular distribution for the photodesorbed CO2 peaks at ∼40° with respect to the © 2013 American Chemical Society

CO + O−2 + e− → CO2 + O2b−

(1)

For this reaction, the reaction rate should be proportional to the coverage of CO and O−2 , and the flux of electrons: d[CO2 ] = ke[O−2 ][CO] dt

(2)

where ke is proportional to the flux of electrons reaching the surface and [X] is the coverage of species X. Note that the CO2 PSD signal is proportional to the reaction rate, d[CO2]/dt. Because the coverage of CO and O−2 decrease monotonically as the UV irradiation proceeds, the initial reaction rate for eq 2 should also be the maximum reaction rate, as seen experimentally.16 No reaction intermediates have been found for the CO photooxidation reaction,6,9 which is also consistent Received: December 5, 2012 Accepted: January 7, 2013 Published: January 8, 2013 344

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with a single-step reaction. Note, however, that the typical time resolutions in the earlier experiments have been ∼0.1 s,4,5,7,8 precluding investigation of the reaction kinetics at earlier times. In eq 1, O2− is proposed to be responsible for the photooxidation of CO. This species has been identified on powder samples with electron paramagnetic resonance (EPR) spectroscopy17,18 and on TiO2(110) with electron energy loss spectroscopy (EELS).19 However, the adsorption geometry and charge state of chemisorbed O2 at different oxygen coverages is not well understood.4,15,19−25 For a single O2 adsorbed in a vacancy, density functional theory (DFT) predicts that the 20,23,26 nominal charge is O2− which suggests that all the charge 2 , from the vacancy is transferred to the O2. On the other hand, experiments show that the saturation coverage of chemisorbed, θsat, corresponds to ∼2 O2 per VO.21,22 Because the charge needed to form the chemisorbed O2 primarily comes from the two electrons associated with each vacancy,27,28 the charge available per chemisorbed O2 is expected to decrease as the O2 coverage, θ(O2), increases. For θ(O2) = θsat, the vacancies are expected to have one adsorbed O2, with the remaining O2 adsorbed at Ti5c sites.7,19,23,29 Equal sharing of the charge would imply that both of these species of oxygen adsorbs as O−2 . These differences in O2 adsorption site and charge state lead to differences in the thermal and photochemistry of chemisorbed O2 for 0 < θ(O2) ≤ 0.5θsat versus 0.5θsat < θ(O2) ≤ θsat.21,22 Here we investigate the photooxidation of CO adsorbed on reduced, rutile TiO2(110) with an emphasis on the initial reaction kinetics on a millisecond time scale. For a saturation coverage of chemisorbed O2 and half that coverage, we find that the CO2 PSD signal is initially zero and gradually increases to a maximum value after several tens of milliseconds. The initial CO2 PSD signal increases more quickly for the smaller O2 coverage, but reaches approximately the same maximum value at intermediate times for both O2 coverages. The results show that the photooxidation of CO on TiO2(110) requires two or more nonthermal reaction steps and involves a metastable intermediate species. The results also suggest that a nonthermal reaction involving this intermediate species is responsible for the noncosine angular distribution of desorbing CO2 that has been previously reported.7 Figure 1a shows the 13CO2 PSD signal versus time for reduced TiO2(110) with θ(O2) = θsat (i.e., ∼2 O2/VO) and θ(13CO) = 0.9 ML. The oxygen was adsorbed at 30 K, and briefly annealed to 100 K. CO was then adsorbed, and the sample was irradiated with UV light (both at 30 K). On this time scale, the 13CO2 PSD signal appears to increase abruptly when the light is turned on, and the maximum signal occurs at or near t = 0 s. The signal then decays in a complicated, nonexponential fashion (Figure 1a, inset). The kinetics shown in Figure 1a are similar to previous reports for the photooxidation of CO on TiO2(110).4,5,7,8 However, experiments with improved time resolution show that the initial CO2 PSD signal is zero. For example, the black line in Figure 1b shows the first 350 ms of the 13CO2 PSD signal from Figure 1a where the quadrupole mass spectrometer (QMS) signal was sampled every millisecond. The signal starts at zero and gradually increases during this time, before decreasing at longer times as shown in Figure 1a. To obtain better signal-to-noise with the faster time resolution, the experiment was repeated 20 times, and the average of those individual experiments is shown.30 Because the thermal and photochemistry of chemisorbed O2 depends on the oxygen coverage,21,22 we also investigated the

Figure 1. 13CO2 and O2 PSD signals versus time on TiO2(110). The flux of UV photons was 1.7 × 1015 cm−2 s−1 and θ(13CO) = 0.9 ML. (a) 13CO2 PSD for θ(O2) = θsat. Inset: 13CO2 PSD on a semilog scale. (b) 13CO2 and O2 PSD signals at early times (note millisecond time scale). The 13CO2 PSD signals for θ(O2) = θsat (black line) and θ(O2) = 0.5θsat (red line) are initially zero and gradually increase versus time. The O2 PSD signal (dark blue line) increases with the response time of the detector (dashed green line). The shutter opening time is shown in light blue.

initial CO2 PSD kinetics for θ(O2) ∼ 0.5θsat, which corresponds to ∼1 O2 per vacancy. Figure 1b (red line) shows the CO2 PSD signal versus time for θ(O2) ∼ 0.5θsat and θ(CO) = 0.9 ML. For this lower O2 coverage, the CO2 PSD signal also starts at zero, but increases considerably faster, reaching a maximum at t ∼ 100 ms. While the CO2 PSD signals shown in Figure 1b have been normalized, the absolute signals are similar (see Figure S1, Supporting Information). As a result, the faster increase of the signal for θ(O2) ∼ 0.5θsat is also true on an absolute scale. The initial CO2 kinetics are independent of the CO coverage (see Figure S2, Supporting Information). For both θ(O2) = θsat and θ(O2) = 0.5θsat, the gradual increase in the CO2 PSD signals at early times (Figure 1b) are inconsistent with a single-step model for photooxidation, such as eq 1. As discussed in the introduction, the rate equation for a single-step model (eq 2) implies that the initial CO2 PSD signal would also be the maximum signal. By contrast, the observations show that the initial signal is zero. The gradual increase in the CO2 PSD signals versus time in Figure 1b is not due to experimental limitations such as the opening time, Δts, of the shutter for the UV light (Figure 1, light blue line). The slow increase in the CO2 yield is also not due to the time it takes desorbing molecules to reach the QMS. The green line in Figure 1b shows the QMS response time, which is faster than the observed kinetics for the CO2 PSD. (See the discussion in the Experiments section for details.) When coadsorbed O2 and CO are irradiated with UV light, O2 photodesorption is also observed.4,7 We find that the initial 345

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Figure 2. Initial CO2 PSD kinetics for different photon fluxes. For these experiments, θ(13CO) = 0.9 ML. (a) 13CO2 PSD versus time for θ(O2) = θsat and UV photon fluxes of 1.2, 4.7, and 7.7 × 1015 photons/cm2 s (blue, black and red, respectively). The shutter and detector response functions are also shown in light blue and green, respectively. (b) 13CO2 PSD signal versus time for θ(O2) = 0.5θsat and UV photon fluxes of 1.1, 3.0, and 6.9 × 1015 photons/cm2 s (blue, black, and red, respectively). (c) 13CO2 PSD signals from panel a versus photon fluence.

O2 PSD signal (Figure 1b, dark blue line) increases significantly faster than the CO2 PSD signal, reaching its maximum value in ∼20 ms. At longer times, the O2 PSD monotonically decays.4,22,31,32 In contrast to the CO2 PSD, the initial kinetics for the O2 photodesorption matches the response time of the QMS. Thus the O2 PSD is “prompt” within the time resolution of these experiments and is consistent with a one-step, holemediated desorption mechanism that has been previously proposed for the O2 photodesorption.32,33 Previous experiments on CO photooxidation have been done at higher temperatures (85−110 K),4−6,8,9 while for the experiments shown in Figure 1, the temperature during the UV irradiation was 30 K. We have also investigated CO photooxidation at higher temperatures and found that the time required for the CO2 PSD signal to increase to its maximum value decreases by about half for irradiation at 100 K for θ(O2) = θsat (see Figure S3, Supporting Information). However, the CO2 kinetics at 100 K are also incompatible with a single-step reaction. For T > 110 K, significant thermal desorption of the CO lowers the CO coverage, and O2 begins to thermally dissociate,19,21 reducing the amount of CO2 that is produced. The results in Figure 1 show that the production of CO2 from coadsorbed CO and O2 does not occur in a single nonthermal reaction. Instead, more than one nonthermal reaction step and at least one intermediate state are involved in the photooxidation of CO on TiO2(110). In that case, the initial rate of increase in the CO2 PSD signal should depend on the flux of UV photons. Figure 2a,b shows the initial CO2 PSD signals versus time for θ(O2) = θsat and θ(O2) = 0.5θsat for various photon fluxes between 1 and 8 × 1015 photons/cm2 s. As the photon flux increases, the CO2 PSD signal increases more rapidly for both O2 coverages. However, when the CO2 PSD signals are corrected for the detector response and plotted as a function of the photon fluence, the data for different fluxes collapse to a single curve (see Figure 2c). Figure 3 shows the results of experiments to test the stability of the intermediate species formed during UV irradiation. Figure 3a shows the CO2 PSD signals versus time for θ(O2) = θsat and θ(CO)=0.9 ML where the first UV irradiation was stopped after 150 ms (Figure 3a, black line). As already noted, the CO2 PSD signal gradually increases from zero during this first irradiation. When the UV shutter is closed, the CO2 signal decays to zero with the time constant of the detector response function (green dashed line). After waiting 100 s at 30 K, the sample was irradiated a second time (Figure 3a, red line). For

Figure 3. 13CO2 PSD signals versus time from reduced TiO2(110). (a) θ(O2) = θsat, or (b) θ(O2) = 0.5θsat. For both experiments, the samples were irradiated in two stages: The first UV irradiation lasted only 150 ms (black lines). After waiting 100 s, the irradiation was resumed for 60 s (second irradiation, red lines). For these experiments, the photon fluxes were ∼1.7 × 1015 photons/cm2 s and θ(13CO) = 0.9 ML.

the second irradiation, the CO2 PSD signal increases to the value it had at the end of the first irradiation with the response time of the detector (i.e., time constant ∼11 ms, see Figure S3b) showing that the photoinduced changes in the adsorbed layer are stable for at least ∼100 s at 30 K. For θ(O2) ∼0.5 θsat and θ(CO)=0.9 ML, the CO2 PSD kinetics for the second irradiation are also fast compared to the first irradiation (Figure 3b, red and black lines, respectively). Note that because very little CO or O2 desorb during the initial 150 ms irradiation, the coverage of these species is nearly unchanged at the beginning of the second irradiation. In another set of experiments, O2 and CO were briefly irradiated, and the surface was then annealed at higher temperatures prior to irradiating the film a second time at 30 K. For annealing to 100 K or less, the results were similar to those shown in Figure 3. For annealing to 120 K, the CO2 PSD 346

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Figure 4. Schematic of two-step photooxidation of CO to CO2 on TiO2(110). Oxygen, titanium, and carbon are represented by red, gray, and black spheres, respectively. (a) initial state; (b) intermediate state; (c) final state.

0.5θsat), an intermediate state corresponding to an O2−CO complex was reported.15 The barrier from the initial state to the intermediate state was 0.4 eV, and the return barrier to the initial state was 0.25 eV. From the intermediate state, the calculated barrier to the products (CO2 + O2− b ) was 0.22 eV. With these reported barriers, thermal reactions are not expected to occur at an appreciable rate at 30 K. Furthermore, if a nonthermal reaction leads to the intermediate state found in these calculations, a second nonthermal reaction would probably be required to complete the reaction. For films irradiated at 30 K and subsequently annealed to sufficiently high temperatures, thermal reactions should destroy the intermediate state. (However, because no CO2 desorption is observed during annealing, the experiments suggest that the preferred thermal reaction is back to the reactants.) The calculated intermediate state has another appealing property: The O2−CO intermediate is oriented perpendicular to the Ob rows such that the CO2 formed from a nonthermal reaction would likely be desorbed perpendicular to the Ob at an angle to the surface normal, which is consistent with the observed angleresolved CO2 desorption yields (see Figure 4 and Figure S4).7 In reaction 3, back reactions from the intermediate state to

signal promptly increases for the second irradiation but the signal intensity was reduced by ∼50%, while for annealing to 140 K or higher, no thermal desorption of CO2 is observed during annealing and the CO2 PSD signal is very small for the second UV irradiation (data not shown). These observations suggest that the reaction intermediate is a weakly bound complex of O2 and CO, consistent with the fact that previous research has failed to detect stable intermediates in the CO photooxidation reaction.2,6,9 For θ(O2) = 0.5θsat, the amount of chemisorbed O2 is approximately equal to the coverage of vacancies. Because they are the preferred adsorption sites, most of the O2 is expected to 19,20,23−26,29 adsorb in the vacancies as O2− 2 at this coverage. Experiments and theory suggest that the photooxidation reaction heals the vacancy.7,14,15 In that case, the charge originally associated with the defect ends up on the O atom that forms a normal lattice ion with a nominal charge of O2− b . Therefore, the total charge is the same for the reactants (O2− 2 + CO) and products (O2− b + CO2). Furthermore, if photogenerated holes and/or electrons initiate the nonthermal reactions, then the combined charge of these excitations should be zero. So for photooxidation involving two nonthermal reactions, it is likely that both a hole and an electron are involved: h+

e−

the reactants have not been included (e.g., (CO·O2)− →O2− 2 + CO). However, two previously reported observations suggest that back reactions may occur.7 First, CO photodesorption is significantly enhanced when O2 is coadsorbed. Second, the angular distribution of the desorbing CO is similar to that of the CO2. Together, these results suggest the CO PSD is the result of back reactions from the intermediate state. The results presented here show that increasing the oxygen coverage from 0.5θsat to θsat leads to a considerably slower rate of increase for the initial CO2 PSD signal (see, e.g., Figure 1b). However, because the geometry and charge state of the chemisorbed O2 is not well understood for θ(O2) > 0.5θsat,15,21,23,34 assessing the photooxidation reaction at high oxygen coverages is more difficult. Previous experiments suggest that O2 adsorbed in the vacancy, O2(VO), is the species involved in the photooxidation reaction independent of the O2 coverage.7 Furthermore, for both θ(O2) = 0.5θsat and θ(O2) = θsat, the angular distributions of the desorbing CO2 are similar (see Figure S4, Supporting Information), suggesting that the CO2 is produced from the same intermediate state for both low and high coverages of chemisorbed O2 . Thus O 2 chemisorbed at Ti5c sites, O2(Ti), appears to play an indirect role in the photooxidation reaction. As discussed in the Introduction, one possibility is that the O2(Ti) affects the charge state of the “active” O2(VO). Changes in the initial charge state of O2 may change the reaction rates or even the reaction mechanism.15 Another possibility is that changes in θ(O2) change the flux of electrons and holes to the surface due to band-bending effects,8,13 thus influencing the reaction rates.

e−

CO + O22 − → (CO·O2 )− → O2b− + CO2 (gas)

(3a)

or e−

h+

CO + O22 − → (CO· O2 )3 − → O2b− + CO2 (gas)

(3b)

In reaction 3a, a reaction with a photogenerated hole creates an intermediate species, and a second nonthermal reaction between the intermediate and an electron produces CO2 and heals the vacancy. In reaction 3b, the order of the hole- and electron-mediated reactions is reversed. The current results do not allow us to determine the order in which the hole- and electron-mediated reactions occur. More generally, because the distribution of charge in reduced TiO2 (with and without adsorbates) is a topic of current research for both theory and experiment,20,27,34−36 and because the experiments presented here provide no direct information about the distribution of charge on the surface during the photoreactions, the simple picture based on nominal charges should be viewed with caution. However, the results show that more than one nonthermal reaction is required to photooxidize CO independent of the model used to interpret those results. DFT has been used to investigate the reaction between CO and O2.14,15 Because these calculations were performed for systems in their electronic ground states, their connection with the nonthermal photooxidation reactions is uncertain. However, for a calculation with 1 O2 per vacancy (i.e., θ(O2) ∼ 347

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time when the photocurrent reached half its maximum value. The shutter opening time, defined as the time for the photocurrent to go from 10% to 90%, was 2.4 ms. Another source of delay in the PSD signals was due to the distribution of flight times of molecules from the TiO2(110) surface to the ionizer region of the QMS, which we call the response time of the detector. The QMS is mounted on a rotatable flange and equipped with an “integration cup”.7 Because there is no line-of-sight between the sample and the ionizer, the molecules reflect off the walls of the integration cup multiple times prior to detection, producing a spread in the arrival times of the molecules at the ionizer. The measured signals are a convolution of the PSD kinetics and this spread in arrival times. The detector response was measured by exposing an amorphous solid water (ASW) film to CO2 at 100 K. Since the CO2 does not adsorb on ASW at 100 K (i.e., has residence time that is ≪1 ms), the measured signal versus time is due to the flight time of the CO2 to the ionizer of the QMS. The CO2 signal in this experiment is shown as the green lines in Figures 1−3.

Further work is needed to develop a comprehensive understanding of these reactions for higher oxygen coverages. The results presented here show that more than one nonthermal reaction is involved in the photooxidation of CO. Note, however, that the multiple “reactions” involved in the photooxidation might not all involve making or breaking chemical bonds. Instead, some of the reactions might involve changes in the adsorption geometries of the reactants. For example, calculations suggest that O2 adsorbed in a vacancy is initially parallel to the surface, and that it tilts upward and shifts laterally to form the intermediate state (see Figure 4).14,15 In this state, the calculated distance between the O2 and CO was relatively long,15 suggesting that a new chemical species (such as carbonate 37,38 ) does not form. Thus a metastable intermediate state could still be consistent with previous results that did not observe a chemically distinct intermediate species.2,6,9 Previous investigations of the thermal dissociation of O2 in vacancies on TiO2(110) indicate that the dissociation creates an energetic oxygen atom that ends up adsorbed on nearby Ti5c sites that are not necessarily directly adjacent to the vacancy.39 Such an energetic oxygen atom could potentially be produced during photon irradiation and react with a nearby CO to produce CO2. However, this mechanism is not expected to lead to the creation of a stable reaction intermediate as indicated by the present experiments. In summary, we have investigated the initial photooxidation kinetics of CO adsorbed on TiO2(110) during UV irradiation. We find that the photon-stimulated production of CO2 is initially zero and gradually increases versus time on a millisecond time scale. The rate of increase of the initial CO2 PSD signal depends on the coverage of chemisorbed oxygen, with the signal increasing more quickly for smaller O2 coverages. The observed kinetics are not consistent with previous assumptions that CO2 is produced as the result of single nonthermal reaction between coadsorbed CO and O2. Instead, more than one nonthermal reaction step is required to produce CO2 for both high and low coverages of chemisorbed O2.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data on the CO2 PSD kinetics for different CO coverages and irradiation temperatures, and the angular distribution of the desorbing CO2 for θ(O2) = θsat and 0.5θsat are available in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: 509-371-6134 (G.A.K.). E-mail: [email protected]; phone: 509-3716151 (N.G.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. The work was performed in EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle under Contract DE-AC05-76RL01830.

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EXPERIMENTS The experiments were performed in an ultrahigh vacuum (UHV) system that has been described previously.7,40 The TiO2(110) was exposed to O2 (Air Liquid, 99.9995%) and 13 16 C O (Cambridge Isotope Laboratories, 99%) using a molecular beamline, and a QMS detected the photodesorption products. Samples were prepared by repeated cycles of sputtering (2 keV Ne+) and annealing at 950 K. The coverage of oxygen vacancies, θ(VO), was ∼0.05 − 0.08 ML as measured using water TPD.41 The O2 sticking on TiO2(110) is ∼0.8 and nearly independent of coverage at ∼30 K, and therefore θ(O2) can be accurately determined from the O2 exposure.21,22,27,42 The UV light source was a 100 W Hg lamp (Oriel #6281) coupled into the UHV chamber via a fiber optic cable. The infrared light was blocked with a water filter, while the entire UV portion was used to irradiate the sample. Photon fluxes were varied from 1 to 8 × 1015 photons/cm2s (for energies >3 eV) using neutral density filters. The UV exposure was controlled with a mechanical shutter (Uniblitz, #VS25S1T0). The shutter had a short delay prior to opening and a finite opening time (both several ms). The light blue line in Figure 1b shows the photocurrent versus time measured with a photodiode. For the experiments, t = 0 was defined as the

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REFERENCES

(1) Fujishima, A.; Zhang, X. T.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515−582. (2) Henderson, M. A. A Surface Science Perspective on TiO2 Photocatalysis. Surf. Sci. Rep. 2011, 66, 185−297. (3) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Photocatalysis on TiO2 Surfaces - Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735−758. (4) Lu, G. Q.; Linsebigler, A.; Yates, J. T. The Photochemical Identification of 2 Chemisorption States for Molecular-Oxygen on TiO2(110). J. Chem. Phys. 1995, 102, 3005−3008. (5) Linsebigler, A.; Lu, G. Q.; Yates, J. T. CO Photooxidation on TiO2(110). J. Phys. Chem. 1996, 100, 6631−6636. (6) Rohmann, C.; Wang, Y. M.; Muhler, M.; Metson, J.; Idriss, H.; Woll, C. Direct Monitoring of Photo-induced Reactions on WellDefined Metal Oxide Surfaces Using Vibrational Spectroscopy. Chem. Phys. Lett. 2008, 460, 10−12. 348

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The Journal of Physical Chemistry Letters

Letter

(7) Petrik, N. G.; Kimmel, G. A. Off-Normal CO2 Desorption from the Photooxidation of CO on Reduced TiO2(110). J. Phys. Chem. Lett. 2010, 1, 2508−2513. (8) Zhang, Z.; Yates, J. T. Electron-Mediated CO Oxidation on the TiO2(110) Surface during Electronic Excitation. J. Am. Chem. Soc. 2010, 132, 12804−12807. (9) Xu, M.; Gao, Y.; Moreno, E. M.; Kunst, M.; Muhler, M.; Wang, Y.; Idriss, H.; Woll, C. Photocatalytic Activity of Bulk TiO2 Anatase and Rutile Single Crystals Using Infrared Absorption Spectroscopy. Phys. Rev. Lett. 2011, 106, 138302. (10) Xu, M.; Noei, H.; Fink, K.; Muhler, M.; Wang, Y.; Woell, C. The Surface Science Approach for Understanding Reactions on Oxide Powders: The Importance of IR Spectroscopy. Angew. Chem., Int. Ed. 2012, 51, 4731−4734. (11) Petrik, N. G.; Kimmel, G. A. Adsorption Geometry of CO versus Coverage on TiO2(110) from s- and p-polarized Infrared Spectroscopy. J. Phys. Chem. Lett. 2012, 3, 3425−3430. (12) Yates, J. T. Photochemistry on TiO2: Mechanisms Behind the Surface Chemistry. Surf. Sci. 2009, 603, 1605−1612. (13) Zhang, Z.; Yates, J. T. Effect of Adsorbed Donor and Acceptor Molecules on Electron Stimulated Desorption: O2/TiO2(110). J. Phys. Chem. Lett. 2010, 1, 2185−2188. (14) Wu, X. Y.; Selloni, A.; Nayak, S. K. First Principles Study of CO Oxidation on TiO2(110): The Role of Surface Oxygen Vacancies. J. Chem. Phys. 2004, 120, 4512−4516. (15) Pillay, D.; Hwang, G. S. O2-Coverage-Dependent CO Oxidation on Reduced TiO2(110): A First Principles Study. J. Chem. Phys. 2006, 125, 144706. (16) For times considerably less than 1 ms, the rate constant in equation 2, ke, is not expected to be constant because it takes a finite time for electrons created within the penetration depth of the UV light to reach the surface, and other processes such as electron−hole recombination will also influence the flux of electrons to the surface. However, on a millisecond time scale, which will be relevant for the processes investigate in this paper, we expect ke to be reasonably constant. (17) Berger, T.; Sterrer, M.; Diwald, O.; Knozinger, E. Charge Trapping and Photoadsorption of O2 on Dehydroxylated TiO2 Nanocrystals - An Electron Paramagnetic Resonance Study. ChemPhysChem 2005, 6, 2104−2112. (18) Berger, T.; Sterrer, M.; Diwald, O.; Knozinger, E.; Panayotov, D.; Thompson, T. L.; Yates, J. T. Light-Induced Charge Separation in Anatase TiO2 Particles. J. Phys. Chem. B 2005, 109, 6061−6068. (19) Henderson, M. A.; Epling, W. S.; Perkins, C. L.; Peden, C. H. F.; Diebold, U. Interaction of Molecular Oxygen with the VacuumAnnealed TiO2(110) Surface: Molecular and Dissociative Channels. J. Phys. Chem. B 1999, 103, 5328−5337. (20) Chretien, S.; Metiu, H. O2 Evolution on a Clean Partially Reduced rutile TiO2(110) Surface and on the Same Surface Precovered with Au1 and Au2: The Importance of Spin Conservation. J. Chem. Phys. 2008, 129, 074705. (21) Kimmel, G. A.; Petrik, N. G. Tetraoxygen on Reduced TiO2(110): Oxygen Adsorption and Reactions with Bridging Oxygen vacancies. Phys. Rev. Lett. 2008, 100, 196102. (22) Petrik, N. G.; Kimmel, G. A. Electron- and Hole-Mediated Reactions in UV-Irradiated O2 Adsorbed on Reduced Rutile TiO2(110). J. Phys. Chem. C 2011, 115, 152. (23) Pillay, D.; Wang, Y.; Hwang, G. S. Prediction of Tetraoxygen Formation on Rutile TiO2(110). J. Am. Chem. Soc. 2006, 128, 14000− 14001. (24) Scheiber, P.; Riss, A.; Schmid, M.; Varga, P.; Diebold, U. Observation and Destruction of an Elusive Adsorbate with STM: O2/ TiO2(110). Phys. Rev. Lett. 2010, 105, 216101. (25) Tan, S. J.; Ji, Y. F.; Zhao, Y.; Zhao, A. D.; Wang, B.; Yang, J. L.; Hou, J. G. Molecular Oxygen Adsorption Behaviors on the Rutile TiO2(110)-1×1 Surface: An in Situ Study with Low-Temperature Scanning Tunneling Microscopy. J. Am. Chem. Soc. 2011, 133, 2002− 2009.

(26) Wu, X. Y.; Selloni, A.; Lazzeri, M.; Nayak, S. K. Oxygen Vacancy Mediated Adsorption and Reactions of Molecular Oxygen on the TiO2(110) Surface. Phys. Rev. B 2003, 68, 241402. (27) Petrik, N. G.; Zhang, Z. R.; Du, Y. G.; Dohnalek, Z.; Lyubinetsky, I.; Kimmel, G. A. Chemical Reactivity of Reduced TiO2(110): The Dominant Role of Surface Defects in Oxygen Chemisorption. J. Phys. Chem. C 2009, 113, 12407−12411. (28) Du, Y.; Deskins, N. A.; Zhang, Z.; Dohnalek, Z.; Dupuis, M.; Lyubinetsky, I. Formation of O Adatom Pairs and Charge Transfer upon O2 Dissociation on Reduced TiO2(110). Phys. Chem. Chem. Phys. 2010, 12, 6337−6344. (29) Wang, Z.-T.; Deskins, N. A.; Lyubinetsky, I. Direct Imaging of Site-Specific Photocatalytical Reactions of O2 on TiO2(110). J. Phys. Chem. Lett. 2012, 3, 102−106. (30) The CO2 PSD kinetics seen in Figure 1b are apparent even for a single experiment, but with a poorer signal-to-noise ratio. (31) Petrik, N. G.; Kimmel, G. A. Photoinduced Dissociation of O2 on Rutile TiO2(110). J. Phys. Chem. Lett. 2010, 1, 1758−1762. (32) Sporleder, D.; Wilson, D. P.; White, M. G. Final State Distributions of O2 Photodesorbed from TiO2(110). J. Phys. Chem. C 2009, 113, 13180−13191. (33) Thompson, T. L.; Yates, J. T. Monitoring Hole Trapping in Photoexcited TiO2(110) Using a Surface Photoreaction. J. Phys. Chem. B 2005, 109, 18230−18236. (34) Deskins, N. A.; Rousseau, R.; Dupuis, M. Defining the Role of Excess Electrons in the Surface Chemistry of TiO2. J. Phys. Chem. C 2010, 114, 5891−5897. (35) Papageorgiou, A. C.; Beglitis, N. S.; Pang, C. L.; Teobaldi, G.; Cabailh, G.; Chen, Q.; Fisher, A. J.; Hofer, W. A.; Thornton, G. Electron Traps and Their Effect on the Surface Chemistry of TiO2(110). Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 2391−2396. (36) Lira, E.; Wendt, S.; Huo, P.; Hansen, J. Ø.; Streber, R.; Porsgaard, S.; Wei, Y.; Bechstein, R.; Laegsgaard, E.; Besenbacher, F. The Importance of Bulk Ti3+ Defects in the Oxygen Chemistry on Titania Surfaces. J. Am. Chem. Soc. 2011, 133, 6529−6532. (37) Chretien, S.; Metiu, H. Density Functional Study of the CO Oxidation on a Doped Rutile TiO2(110): Effect of Ionic Au in Catalysis. Catal. Lett. 2006, 107, 143−147. (38) Kim, H. Y.; Lee, H. M.; Pala, R. G. S.; Shapovalov, V.; Metiu, H. CO Oxidation by Rutile TiO2(110) Doped with V, W, Cr, Mo, and Mn. J. Phys. Chem. C 2008, 112, 12398−12408. (39) Du, Y. G.; Dohnalek, Z.; Lyubinetsky, I. Transient Mobility of Oxygen Adatoms upon O2 Dissociation on Reduced TiO2(110). J. Phys. Chem. C 2008, 112, 2649−2653. (40) Lane, C. D.; Petrik, N. G.; Orlando, T. M.; Kimmel, G. A. Electron-Stimulated Oxidation of Thin Water Films Adsorbed on TiO2(110). J. Phys. Chem. C 2007, 111, 16319. (41) Henderson, M. A. An HREELS and TPD Study of Water on TiO2(110): The Extent of Molecular Versus Dissociative Adsorption. Surf. Sci. 1996, 355, 151−166. (42) Dohnalek, Z.; Kim, J.; Bondarchuk, O.; White, J. M.; Kay, B. D. Physisorption of N2, O2, and CO on Fully Oxidized TiO2(110). J. Phys. Chem. B 2006, 110, 6229−6235.

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dx.doi.org/10.1021/jz302012j | J. Phys. Chem. Lett. 2013, 4, 344−349