ARTICLE pubs.acs.org/JPCC
Trapping Nitric Oxide by Surface Hydroxyls on Rutile TiO2(110) Shao-Chun Li,*,†,‡ Peter Jacobson,†,§ Shu-Lei Zhao,|| Xue-Qing Gong,|| and Ulrike Diebold*,†,§ †
Department of Physics and Engineering Physics, Tulane University, New Orleans, Louisiana 70118, United States National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, P. R. China § Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10/134, 1040 Vienna, Austria State Key Laboratory of Chemical Engineering, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China
)
‡
bS Supporting Information ABSTRACT: Hydroxyls are omnipresent on oxide surfaces under ambient conditions. While they unambiguously play an important role in many catalytic processes, it is not well-understood how these species influence surface chemistry at atomic scale. We investigated the adsorption of nitric oxide (NO) on a hydroxylated rutile TiO2(110) surface with scanning tunneling microscopy (STM), X-ray/ultraviolet photoemission spectroscopy (XPS/ UPS), and density functional theory (DFT) calculations. At room temperature adsorption of NO is only possible in the vicinity of a surface hydroxyl, and leads to a change of the local electronic structure. DFT calculations confirm that the surface hydroxyl-induced excess charge is transferred to the NO adsorbate, which results in an electrostatic stabilization of the adsorbate and, consequently, a significantly stronger bonding.
1. INTRODUCTION The surface physics and chemistry of metal oxides is heavily influenced, and often even dominated, by surface defects.1 In recent years, the rutile TiO2(110) surface has evolved as a key model system to study the fundamentals of defect-related adsorption processes at the molecular level.2 4 Much attention has been paid to intrinsic defects, in particular to surface O vacancies (VO’s) as well as, more recently, subsurface Ti interstitials and how these affect the adsorption of key molecules such as O2 or CO.5 15 An important type of defect in technical oxide materials is a hydroxyl; in fact, it is believed that surface OH’s are a key ingredient for many catalytic processes on metal oxide materials. At a fundamental level it was shown that OH induces a gap state in TiO2,16 which in many respects resembles the one introduced by intrinsic defects.5,17 20 It is thus interesting to find out how the H-induced change in electronic structure affects adsorption processes at a molecular level. Density functional theory (DFT) calculations on hydroxylated surfaces have been reported;21,22 here we report direct experimental observation of the influence of hydroxyls on the adsorptive properties of TiO2. In this report we probe the influence of surface hydroxyls for the case of nitric oxide (NO) adsorption. NO has an unpaired electron in its 2π* orbital and can undergo complex reactions. The molecule is harmful to the environment, and its interaction with surfaces is central in pollution control via trapping and catalytic destruction.23,24 Previous studies25 have found that NO adsorbs rather weakly (Eads ∼ 0.4 eV) on stoichiometric TiO2(110); in ultrahigh vacuum (UHV), it desorbs around ∼127 K from surfaces with and without VO’s.25,26 VO’s were found to r 2011 American Chemical Society
facilitate the reaction of two NO molecules into N2O at higher coverages.27 This weak interaction is in contrast to the use of TiO2-based material for selective NO trapping28 in technical processes. The catalytic NO reduction of TiO2 based catalysts strongly depends on the material’s pretreatment conditions,27 which are believed to directly influence the defect type and concentration, although the exact nature of these defects has not been established. Here we report that NO interacts mainly with surface hydroxyls on TiO2(110), and is bound strongly enough to be stable at room temperature. With scanning tunneling microscopy (STM), we show that each individual NO adsorption/desorption event occurs in the proximity of a surface hydroxyl group. Core level and valence band X-ray and ultraviolet photoemission spectra (XPS, UPS) establish electron transfer to the adsorbate. DFT calculations confirm that the excess charge that is localized in the vicinity of a surface hydroxyl is donated to the NO molecule; this charge transfer results in an electrostatic stabilization of the adsorbate at the surface.
2. MATERIALS AND METHODS 2.1. Experimental Procedures. The STM measurements (constant current mode) were carried out at room temperature in UHV (base pressure: 1 10 10 mbar). XPS was performed Received: September 26, 2011 Revised: December 13, 2011 Published: December 16, 2011 1887
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Figure 2. Adsorption/desorption of NO in vicinity of surface hydroxyls on TiO2(110). (a,b) Two consecutive STM images (6 nm 6 nm, V = +1.6 V and I = 0.2 nA) of a TiO2(110) surface with ∼0.05 ML of NO taken at the same position. The images were collected in an NO background pressure of ∼2 10 9 mbar. In the area marked with the lower left (white) circle, an NO-related feature disappears, and surface hydroxyl appears at a neighboring O site. The upper right circle (black) marks the reverse process. See Supporting Information for tracking of molecules on a larger area. (c) Fluctuations of coverage of adsorbed OH and NO features versus time in the same NO background pressure. (d) Density of surface OH and NO-related adsorbates versus NO exposure, normalized to the initial density of hydroxyls. The lines are plotted to guide the eye.
Figure 1. STM images (Vsample = +1.6 V and Itunnel = 0.2 nA) of a TiO2(110) surface. (a) (10 nm 10 nm) Clean surface with ∼0.15 ML of surface O atoms hydroxylated (bright spots on dark rows, one is marked with a green arrow). (b) (10 nm 10 nm) After exposure to NO at room temperature, features with a typical “bright-dark-bright” configuration appear (blue arrow); the white arrow marks an area with more than one nearest neighbors adsorbed.(c) (3 nm 3 nm) High-resolution image of the surface in panel b; the grid’s corners mark the position of surface Ti5c cations. (d) (3.5 nm 3.5 nm) High-resolution STM image with special tip condition, showing surface hydroxyls (bright spots) and ring-shaped NO-related features.
calculations and found that the calculated energetics are nearly unchanged.
using Mg Kα radiation. UPS measurements were conducted on the 3 m TGM beamline at the Center for Advanced Microstructures and Devices (CAMD) in Baton Rouge, LA. A photon energy of 47 eV was used to enhance the Ti3+ defect state in UPS.2 The TiO2(110) sample was cleaned by sputtering and annealing. High-purity NO (99.999%) was dosed via backfilling. Exposures are quoted in Langmuirs (L) where 1 L = 1.33 10 6 mbar 3 s, and adsorbate coverages are given in monolayers (MLs), where 1 ML equals the number of Ti5c sites on the surface (∼5.2 1014 cm 2). 2.2. Computational Procedures. The plane-wave DFT calculations were performed using the Vienna Ab-initio Simulation Package (VASP)29,30 with the projector-augmented wave method (PAW).31 The exchange-correlation function was treated with the generalized gradient approximation (GGA) of the Perdew Wang (PW91) functional32 with a cutoff energy of 350 eV. The rutile TiO2(110) surface was modeled with a 4 2 surface cell; the slab consisted of 4 O Ti O trilayers with 11 Å of vacuum between slabs. This surface model is sufficiently large to exclude interactions between neighboring cells. To model the hydroxylated surface, one H atom was placed on top of a 2-fold coordinated bridging oxygen atom (O2c) in each cell. NO was adsorbed onto the slab in a range of configurations. The Brillouin zone was sampled with a (111) Monkhorst Park mesh. In the calculations, all atoms were allowed to relax up to a force threshold of 0.05 eV/Å except for those at the bottom layer of the slab. In order to calculate the electronic structure with reasonable accuracy, we corrected for on-site Coulomb interactions (DFT+U) where U = 4.2 eV was applied to the Ti 3d state. This value of U gives a gap energy of ∼3.0 eV, which is the experimental value. It needs to be mentioned that we also conducted standard DFT
3. RESULTS AND DISCUSSION The rutile TiO2(110) surface consists of alternating rows of 2-fold coordinated O2c and 5-fold coordinated Ti4+ (Ti5c) atoms. The contrast in STM is dominated by electronic effects, and O2c rows are imaged as dark rows and Ti5c as bright rows, respectively33 (see Figure 1a). Standard surface treatment in UHV removes O2c and creates VO’s at the surface. Water dissociates at these VO’s and forms two hydroxyls, which appear as bright protrusions on the dark O2c rows34 38 (see Figure 1a). In this experiment, all the VO’s were hydroxylated by water prior to NO dosage, therefore the round protrusions along the dark rows in Figure 1a are assigned to surface hydroxyls, with a coverage of ∼0.15 ML. Figure 1b shows an STM image after dosing 2 L NO at 300 K. The surface shows a new feature with a characteristic “brightdark-bright” (BDB) contrast along the Ti5c rows with a density that increases with increasing NO exposure. This new feature is assigned as NO-related. Figure 1c shows an atomically resolved STM image of the BDB feature, with the substrate lattice superimposed. The dark center and the two bright ends each occupy one neighboring Ti5c site. The surface hydroxyls, imaged as protrusions at O2c rows in Figure 1a, become nearly invisible after NO adsorption. The STM image in Figure 1d was taken with a special tip condition (possibly with an adsorbate at the end). Here the hydroxyl is imaged as a normal round protrusion as reported in the literature,34 36,38 whereas the NO-related adsorbate has a ring-like rather than linear appearance. The BDB features are mobile at room temperature. They hop as a unit preferentially along, and occasionally across, the Ti5c rows (Supporting Information Figure S1). The diffusion of NOfeatures and hydroxyls was tracked through comparison of two 1888
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Figure 3. Photoemission spectra upon NO adsorption on hydroxylated TiO2(110). (a) Valence band photoemission in normal emission mode, taken with a photon energy of 47 eV. The spectra shown from bottom to top were taken on the clean TiO2(110) surface (black), hydroxylated surface (red), and after incremental NO dosages up to 200 L. A difference spectrum (200 L; hydroxylated) is shown at the bottom. (b,c) Zoom-in regions of the OH-3σ peak and the Ti3+-related gap state of the same spectra as in panel a. (d) Integrated peak intensity of the Ti3+-related gap state in panel c as a function of NO exposure. The colors of the curves and circles are the same as in panels a and c. (e) Ti 2p XPS of a hydroxylated TiO2(110) surface (black) and the surface after dosing NO (red). The blue scatter plot was obtained by subtracting the clean from the NO-exposed spectrum after aligning the peaks.
consecutive STM frames collected at the same area and a low NO background pressure (∼2.0 10 9 mbar). Such a method has been widely applied for other adsorbates to study the diffusion and de/adsorption at the TiO2(110) surface.34,35,39 44 A quantitative evaluation of adsorbate diffustion on TiO2(110) via consecutive STM images has been discussed elsewehere.39 41 Supporting Information Figure S2 (see also Movie S1) shows an example of how each single adsorbate was tracked; two such successive STM images are shown in Figure 2a,b. Interestingly, when a BDB feature disappears, a hydroxyl appears on a neighboring O2c site (Figure 2, white circle). Conversely, once a BDB appears, a neighboring OH disappears (black circle) in the STM image. Through carefully tracking adsorbates over large areas, we see clear evidence that the simultaneous dis/appearance of OH/NO (Figure 2a,b) are correlated events and not caused by random diffusion of either OH or NO. Supporting Information Figure S3 exemplifies a few cases of adsorption/desorption events. The temporal evolution of the BDB features and OH coverage extracted from STM time-lapse images is plotted in Figure 2c. Their number densities show antiphase fluctuations, whereas the total coverage of OH + NO is approximately constant. The density of NO and OH-related features as a function of NO exposure is shown in Figure 2d; as the density of adsorbed NO increases, the OH coverage decreases. Each of these observations points toward a mechanism where a surface hydroxyl is involved in the capture and formation of an adsorbed NO. However, the H is not consumed in the adsorption reaction; when a NO desorbs from the surface, an OH is left behind. The photoemission results presented in Figure 3 support such a mechanism. UPS from the clean surface (black curves in Figure 3a,c) shows a Ti3+-derived band gap state at ∼0.9 eV that is typical for a slightly reduced surface.2 After water exposure
and dissociation at VO’s, an OH-related 3σ peak appears at ∼10.9 eV binding energy (red curves, Figure 3a,b), while the gap state at 0.9 eV shows a slight increase indicative of excess charge trapped at the hydroxyl.16 This 3σ peak decreases with NO dosage (Figure 3b), indicating that OH groups are affected by the NO adsorption. Furthermore, NO-related states appear in the valence band (difference spectra in Figure 3a and Supplementary Figure S4). The intensity of the Ti3+ derived band gap state decreases upon exposure to a few Langmuir of NO (Figure 3c,d) and levels out at ∼200 L. The residual Ti3+ state might originate from subsurface Ti interstitials or unreacted surface hydroxyls. Previously the decrease of Ti3+-related photoemission features was taken as evidence that NO adsorbs at reduced parts of the TiO2 surface at room temperature;45 this is not confirmed by our STM measurements (Supporting Information Figure S5). To further probe the effect of NO on the Ti3+ states, the Ti 2p core-level was measured with XPS (Figure 3e). Upon exposure to 10 L of NO, all XPS peaks are rigidly shifted to a lower binding energy by ∼0.1 eV, indicating adsorption-induced upward bandbending.46 The line shape of the Ti 2p3/2 core level also changes: the low binding energy shoulder of the main peak, indicative of a Ti3+ oxidation state, decreases, and the main Ti4+ state increases by 4%. This is seen more clearly in a difference spectrum (blue curve, Figure 3e), obtained after aligning the clean and NO-dosed spectra. Both the intensity and line shape of the O 1s peak is unchanged (Supporting Information Figure S6). The low XPS cross-section for nitrogen and the small NO saturation coverage prevented detection of the N 1s peak. Because the total coverage in our experiment is rather small, we cannot confirm the adsorbate’s configuration with an independent technique such as vibrational spectroscopy.47 To understand how NO interacts with the hydroxylated TiO2(110) 1889
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Figure 4. DFT optimized adsorption structure. (a) Calculated structure (side view) of a single NO at the hydroxylated TiO2(110) surface. Ti atoms are in gray, O in red, N in blue, and H in white. (b) Charge difference diagram obtained by subtracting the charges of the separated gasphase NO + hydroxylated surface from those of the adsorption system; plotted in a plane perpendicular to the surface and across the Ti5c row.
surface, DFT calculations were performed. It is known that the NO radical easily dimerizes,25,48 so we considered both the adsorption of single molecules as well as dimers in various configurations. In the low coverage regime considered here, NO-NO encounters that lead to dimerization are unlikely. In addition, the calculated STM images of the various NO dimers did not agree with the experimental ones, while recent STM measurements of isolated NO on Cu(110)49 resemble the lobe-like features observed in our images (Figure 1). We also probed whether it is possible to assume that an NO reacts with the surface hydroxyl. The resulting three-atom intermediate, HNO, was calculated unstable at the Ti5c site, however, and desorbed into the gas phase during optimization. According to prior DFT calculations,25 a single NO physisorbs at Ti5c sites in a tilted, N-end down configuration. It is reasonable to assume that the NO lands onto, and diffuses along, a Ti5c row at room temperature until it encounters a surface OH. The most stable adsorption structure of a single NO at the hydroxylated surface is shown in Figure 4a. The NO sits at the Ti5c in an upright configuration by forming a Ti5c N bond (∼2.014 Å), and the corresponding adsorption energy was estimated to be 0.64 eV, which is significantly higher than that of a clean surface (∼0.4 eV). It needs to be mentioned that adsorption of NO in a tilted configuration that shortens the NO HO distance does not affect the calculated adsorption energies, suggesting that the effect of H-bonds is trivial. The calculated adsorption energy of 0.64 eV is in line with the experimental observations that (i) NO adsorption can be determined at room temperature, and (ii) it still exhibits moderate mobility during the measurement. This increase in binding energy (0.64 vs 0.40 eV) is caused by a transfer of charge from the surface to the molecule. From calculated Bader charges, we determined that the excess charge introduced by hydroxylation is largely localized at a subsurface Ti6c, turning it into a Ti3+. However, once NO adsorbs, this subsurface Ti6c then has the same of charge as the other Ti6c cations, while the NO now has a net negative charge of 0.33 e, which is mainly located at the N (see the charge difference diagram in Figure 4b). The excess charge introduced by the H is thus transferred to the NO, as is also seen experimentally with photoemission (Figure 3). The resulting negatively charged species sticks to the surface Ti5c mainly through electrostatic interaction (see Figure 4b).
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As one can see from Figure 4b, the negative charge difference area around this Ti5c has a clear d orbital-like shape. Our calculations help to understand the role of hydroxyls in adsorbing NO on TiO2, and clarify the apparent discrepancy between the weak physisorption observed in previous TiO2(110) adsorption studies25,26 and the use of TiO2 as an NO trapping material. The interaction between NO and TiO2 is altered through the interaction with coadsorbed hydrogen. The H changes the local electronic structure, and this adsorbate-induced electronic defect helps to arrest NO on an otherwise relatively unreactive surface. We also considered the influence of surface VO’s and modeled NO adsorption in the vicinity of a VO. However, although a VO also introduces localized excess electrons, turning two surface Ti4+ into Ti3+, the NO adsorption energy on this defective surface was estimated to be 0.43 eV, nearly identical to the value obtained at the stoichiometric surface. This suggests that the drastic structural relaxation caused by a VO may hinder the charge transfer to the surface NO. In fact, in a recent DFT+U study, it has been shown that the localization of excess electrons at reduced metal oxide surface with missing lattice O is exclusively determined by the structural relaxation it induces.50 Since NO adsorption at Ti5c does not affect such relaxation at the vacancy site, the electronic structures with respect to the excess electrons would therefore tend to keep intact. By contrast, at the hydoxylated surface, the H not only brings one excess electron, but it also induces local structural deformation to the compact surface. Accordingly, the charge transfer to the NO can favor both its adsorption and the recovery the electronic as well as the structural configuration of the surface around the hydroxyl. It should also be noted that the adsorbed entity is necessarily a transient species: if the proton diffuses away, the NO is no longer bound and will desorb (Figure 2c). Isolated surface hydroxyls have a relatively high activation energy for diffusion, which implies the diffusion rate at room temperature is rather low.40 The hydroxyl diffusion can be facilitated by transiently adsorbed molecular water, however.35 If such an event occurs and H hops a few lattice sites, then the NO is no longer bound to the surface and will desorb. The H finds itself at a new location and represents a fresh adsorption site for a new NO molecule.
4. CONCLUSION These results could have interesting technical consequences, for example, in the selective catalytic reduction (SCR) of NO in H2-burning internal combustion engines. In our idealized model study the surface hydroxyls were formed through the dissociation of water at oxygen vacancies. In technical catalysts, H could be provided by a metal that splits hydrogen gas, such as Pd, and by spillover onto the oxide substrate. Generally, metal oxides are at least partially hydroxylated under ambient conditions. Our findings, i.e.; that surface hydroxyls collect charge around them, and that the transfer of this charge to an adsorbed entity are key to a strong binding of NO to TiO2, should be more universally applicable to adsorbate/oxide systems. ’ ASSOCIATED CONTENT
bS
Supporting Information. Diffusion of the NO-related adsorbates, examples of simultaneous adsorption/desorption of NO-related features in the vicinity of surface hydroxyls, difference UPS spectrum, and reduced area of TiO2 after NO
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’ AUTHOR INFORMATION Corresponding Author
*Phone: (504) 862-3179; fax: (504) 862-8702; e-mail: shaochun.
[email protected] (S.-C.L.). Phone: (+43 1) 58801-13425; fax: (+43 1) 58801-13499; e-mail:
[email protected] (U.D.).
’ ACKNOWLEDGMENT This work was supported by the Department of Energy (contract DE-FG02-05ER15702). The authors thank Yaroslav Losovyj and Erie Morales for help with the UPS measurements. X.Q.G. acknowledges financial support from NSFC(21073060). S.C.L. acknowledges financial support from NKPBRC (No. 2010CB923404). ’ REFERENCES (1) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge/New York, 1994. (2) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (3) Pang, C. L.; Lindsay, R.; Thornton, G. Chem. Soc. Rev. 2008, 37, 2328. (4) Dohnalek, Z.; Lyubinetsky, I.; Rousseau, R. Prog. Surf. Sci. 2010, 85, 161. (5) Wendt, S.; Sprunger, P. T.; Lira, E.; Madsen, G. K. H.; Li, Z.; Hansen, J. Ø.; Matthiesen, J.; Blekinge-Rasmussen, A.; Lagsgaard, E.; Hammer, B.; Besenbacher, F. Science 2008, 1159846. (6) Henderson, M. A.; Epling, W. S.; Perkins, C. L.; Peden, C. H. F.; Diebold, U. J. Phys. Chem. B 1999, 103, 5328. (7) Lira, E.; Hansen, J. Ø.; Huo, P.; Bechstein, R.; Galliker, P.; Lægsgaard, E.; Hammer, B.; Wendt, S.; Besenbacher, F. Surf. Sci. 2010, 604, 1945. (8) Scheiber, P.; Riss, A.; Schmid, M.; Varga, P.; Diebold, U. Phys. Rev. Lett. 2010, 105, 216101. (9) Wang, Z.-T.; Du, Y.; Dohnalek, Z.; Lyubinetsky, I. J. Phys. Chem. Lett. 2010, 1, 3524. (10) Tan, S.; Ji, Y.; Zhao, Y.; Zhao, A.; Wang, B.; Yang, J.; Hou, J. G. J. Am. Chem. Soc. 2011, 133, 2002. (11) Zhao, Y.; Wang, Z.; Cui, X.; Huang, T.; Wang, B.; Luo, Y.; Yang, J.; Hou, J. J. Am. Chem. Soc. 2009, 131, 7958. (12) Petrik, N. G.; Zhang, Z.; Du, Y.; DohnaI lek, Z.; Lyubinetsky, I.; Kimmel, G. A. J. Phys. Chem. C 2009, 113, 12407. ~ .; (13) Zhang, Z.; Lee, J.; Yates, J. T.; Bechstein, R.; Lira, E.; Hansen, J. A Wendt, S.; Besenbacher, F. J. Phys. Chem. C 2010, 114, 3059. (14) Papageorgiou, A. C.; Beglitis, N. S.; Pang, C. L.; Teobaldi, G.; Cabailh, G.; Chen, Q.; Fisher, A. J.; Hofer, W. A.; Thornton, G. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 2391. (15) Lira, E.; Wendt, S.; Huo, P.; Hansen, J. Ø.; Streber, R.; Porsgaard, S.; Wei, Y.; Bechstein, R.; Lægsgaard, E.; Besenbacher, F. J. Am. Chem. Soc. 2011, 133, 6529. (16) Di Valentin, C.; Tilocca, A.; Selloni, A.; Beck, T. J.; Klust, A.; Batzill, M.; Losovyj, Y.; Diebold, U. J. Am. Chem. Soc. 2005, 127, 9895. (17) Henderson, M. A.; Epling, W. S.; Peden, C. H. F.; Perkins, C. L. J. Phys. Chem. B 2002, 107, 534. (18) Yim, C. M.; Pang, C. L.; Thornton, G. Phys. Rev. Lett. 2010, 104, 036806. (19) Di Valentin, C.; Pacchioni, G.; Selloni, A. Phys. Rev. Lett. 2006, 97, 166803. (20) Minato, T.; Sainoo, Y.; Kim, Y.; Kato, H. S.; Aika, K.-i.; Kawai, M.; Zhao, J.; Petek, H.; Huang, T.; He, W.; Wang, B.; Wang, Z.; Zhao, Y.; Yang, J.; Hou, J. G. J. Chem. Phys. 2009, 130, 124502. (21) Deskins, N. A.; Rousseau, R.; Dupuis, M. J. Phys. Chem. C 2010, 114, 5891.
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