Langmuir 2007, 23, 7583-7586
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Dissociative Adsorption of NO on TiO2 (110)-(1 × 2) Surface: Ti2O3 Rows as Actives Sites for the Adsorption Jose Abad,*,†,‡ Oliver Bo¨hme,† and Elisa Roma´n† Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, E-28049 Madrid, Spain, and Departamento de Fı´sica, Centro de InVestigacio´ n en O Ä ptica y Nanofı´sica-CIOyN (Campus Espinardo), UniVersidad de Murcia, E-30100 Murcia, Spain ReceiVed January 30, 2007. In Final Form: April 16, 2007 The interaction of NO with TiO2 (110)-(1 × 2) surface has been studied by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, and low-energy electron diffraction, with the aim to clarify the role of ordered defects in NO reactivity toward TiO2. The interaction was studied for exposures up to 2000 L. However, the main effects occur already in the first 2 L. The exposure of the surfaces to NO resulted in the healing of defect sites without adsorption of N and low-energy electron diffraction shows that the surface (1 × 2) symmetry is not lost after the NO dose.
1. Introduction The removal of NOx has been an important issue of concern for the last 3 decades. Recently, research in this area has been intensified1,2 since the emission of these noxious gases to the atmosphere is increasing.3 Therefore, new, more severe regulations have been imposed to control and to diminish the NOx emissions, increasing the need for new and more efficient catalysts for the control of pollution.1 Among the different materials used in catalysis, TiO2 is one of the most commonly used, as support for metal oxides or metallic particles. Besides, it is an important component in the catalytic reduction of NOx;4-9 it is employed in NO photocatalysis10,11 and in NO sensors.12,13 Nevertheless, the knowledge accumulated about the basic chemistry of NOx, NO, and NO2, on oxides surfaces, and especially on TiO2, is very low.14,15 This is because atomic scale investigations of industrial catalytic materials are very difficult due to the high complexity of these systems.16 For a better understanding of these catalytic processes, the experiments on an atomic scale are generally performed on ideal models, * Corresponding author. Tel.: +34 968 39 8551. Fax: +34 968 36 4148. E-mail:
[email protected]. † CSIC. ‡ Universidad de Murcia. (1) Armor, J. N. Ed. EnVironmental Catalysis; ACS Symposium Series 552; American Chemical Society: Washington DC, 1994. (2) Thomas, J. M., Thomas, J. W. Principles and Practice of Heterogeneous Catalysis; VCH: New York, 1997. (3) Lefhof, A. S.; Shadwick, D. S. Atmos. EnViron. 1991, 25A, 491. (4) Matsumoto, S.; Ikeda, Y.; Suzuki, H.; Ogai, M.; Miyoshi, N. Appl. Catal. B 2000, 25, 115. (5) Blanco, J.; Odenbrand, C.; Avila, P.; Knapp, C. Catal. Today 1998, 45, 103. (6) Matralis, H.; Theret, S.; Bastians, Ph.; Ruwet, M.; Grange, P. Appl. Catal. B 1995, 5, 271. (7) Kung, H. H.; Kung, M. C. Catal. Today 1996, 30, 5. (8) Ozkan, U. S.; Kumthekar, M. W.; Karakas, G. Catal. Today 1998, 40, 3. (9) Masaaki, H.; Kintaichi, Y.; Inaba, M.; Ideaki, H. Catal. Today 1998, 42, 127. (10) Yamashita, H.; Ichihashi, Y.; Zhang, S. G.; Matsumura, Y.; Souma, Y.; Tatsumi, T.; Anpo, M. Appl. Surf. Sci. 1997, 121/122, 305. (11) Rusu, C. N.; Yates, J. T., Jr. J. Phys. Chem. B 2000, 104, 1729. (12) Huusko, J.; Lantto, V.; Torvela, H. Sens. Actuators B 1993, 16, 245. (13) Boccuzzi, F.; Guglielminotti, E. Sens. Actuators B 1994, 21, 27. (14) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, England, 1994. (15) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (16) Rodrı´guez, J. A.; Jirsak, T.; Dvorak, J.; Sambasivan, S.; Fischer, D. J. Phys. Chem. B 2000, 104, 319.
such as single-crystal surfaces in ultrahigh vacuum (UHV) conditions.17 Furthermore, the interaction of NO with various substrates has received much attention, not only because the catalytic decomposition of NO is important from an applied viewpoint but also because the presence of an unpaired electron in the 2π* antibonding orbital provides interesting chemistry from a fundamental viewpoint.18 There are some fundamental studies, both theoretical and experimental, using TiO2 (110) single crystals as ideal systems to model the interaction with NO.19-22 Nevertheless, there is, to the best of our knowledge, a single study about the NO interaction with TiO2 (110) at room temperature (RT), using photoemission techniques (X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS)),23 where the interaction of NO with the different kinds of defects present on the TiO2 (110) surfaces is not examined in detail. It is known that defects play a fundamental role in the interaction of molecules with oxide surfaces since defects act as active sites for the adsorption and dissociation of molecules on the surface.14 In the past years the authors have studied the different kinds of defects present on the TiO2 (110) surfaces24-26 and their interaction with NO.24,26 The TiO2 (110)-(1 × 2) surface reconstruction is characterized by the formation of added rows of Ti2O3, with the presence of undercoordinated Ti cations27,28 (ordered defects), originating Ti3+ species.25 These species are a characteristic fingerprint of the (1 × 2) surface reconstruction, which is reflected by using different spectroscopies. Thus, in UPS these species are manifested as band gap states at 0.7 eV below the Fermi (17) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons, Inc.: New York, 1994. (18) Pirug, G.; Bonzel, H. P.; Hopster, H.; Ibach, H. J. Chem. Phys. 1979, 71, 593. (19) Boccuzzi, F.; Guglielminotti, E.; Spoto, G. Surf. Sci. 1991, 251/252, 1069. (20) Sorescu, D. C.; Rusu, C. N.; Yates, J. T., Jr. J. Phys. Chem. B 2000, 104, 4408. (21) Li, J.; Wu, L.; Zhang, Y. Chem. Phys. Lett. 2001, 342, 249. (22) Lu, G.; Linsebigler, A.; Yates, J. T., Jr. J. Phys. Chem. 1994, 98, 11733. (23) Onishi, H.; Aruga, T.; Egawa, C.; Iwasawa, Y. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2597. (24) Abad, J.; Bo¨hme, O.; Roma´n, E. Surf. Sci. 2004, 549, 134. (25) Blanco-Rey, M.; Abad, J.; Rogero, C.; Me´ndez, J.; Lo´pez, M. F.; Martı´nGago, J. A.; de Andres, P. L. Phys. ReV. Lett. 2006, 96, 055502. (26) Abad, J. Ph. D. Thesis, Autonoma University, Madrid, Spain, 2005. (27) Onishi, H.; Iwasawa, Y. Surf. Sci. 1994, 313, L783. (28) Onishi, H.; Iwasawa, Y. Phys. ReV. Lett. 1996, 76, 791.
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level,14,15,29 in ELS (electron energy-loss spectroscopy) as a loss energy peak at about 1-2 eV,14,29 and in XPS as a shoulder on the low binding energy side of the Ti 2p peak.14,15,30 In this work we present for the first time a study of the interaction of NO with a TiO2 (110)-(1 × 2) surface, at room temperature. Our results indicate the healing of defect sites present in the added rows of Ti2O3, without adsorption of N and LEED (low-energy electron diffraction) shows that the surface (1 × 2) symmetry is not lost after the NO dose. 2. Experimental Section The experiments were carried out in a UHV chamber equipped with the necessary instrumentation to perform AES (Auger electron spectroscopy), ELS, XPS, UPS, and LEED. More details are given elsewhere.24 The base pressure was 2 × 10-10 mbar. The rutile TiO2 (110) sample (PI-KEM Ltd., UK) was mounted to a Ta holder with resistive heating. The surface was cleaned by repeated cycles of 1.4 keV Ar+ ion bombardment at RT for 30 min followed by 1-2 h of heating to 530 °C, until no impurities were detected by AES and XPS. The TiO2 (110)-(1 × 2) surface reconstruction was prepared by further annealing at 880 °C for 60 min;31 after this treatment the sample exhibited a sharp (1 × 2) LEED pattern. The clean surface was exposed to NO at 6 × 10-9 and 1 to 7 × 10-8 mbar for the lower doses and 3 × 10-7 mbar for the highest dose, in sequential doses up to 2000 Langmuir (L) (1 L ) 1.33 × 10-6 mbar‚s). AES, UPS, XPS, and ELS spectra were taken using a double-pass cylindrical mirror analyzer (CMA). Non-monochromatized Mg KR (1253.6 eV) X-rays were used in XPS. Narrow-scan spectra were taken at an analyzer pass energy of 50 eV, providing a resolution of 1 eV. Before the XPS data were analyzed, the contribution of the Mg KR satellite lines were subtracted and the backgrounds were removed by a Shirley routine. The fitting of the Ti 2p peaks was performed using Gaussian doublets with a spin-orbit splitting of 5.7 eV and intensity ratio of approximately 0.5, in agreement with the literature.32-34 UPS spectra were excited with He I (21.2 eV) and He II (40.8 eV) radiation. Data were recorded at an analyzer pass energy of 15 eV, which gives a resolution of 0.3 eV for He I valence band spectra. The work function (φ) was determined from the lowenergy onset of the secondary electrons in the UPS spectra at an analyzer pass energy of 5 eV, providing a resolution of 0.1 eV. The position of the Fermi level was determined from the spectrum of a Ta foil attached to the sample. The AES data were acquired in integrate mode N(E) with a primary beam energy of 3 keV and a sample current of 33 nA. ELS spectra were recorded in the integrate mode N(E), using a primary electron beam of 100 eV. This low energy has been selected to increase surface sensitivity. The energy resolution of these measurements, given by the FWHM of the elastic back-reflected peak, was 0.6 eV.
3. Results and Discussion Recently, the geometrical disposition of atoms on the (1 × 2) surface reconstruction has been quantitatively determined by full dynamical LEED and density functional theory.25 It has been found that the (1 × 2) surface reconstruction has the added Ti2O3 unit rows proposed by Onishi and Iwasawa.27 This structure can be understood as the addition of extra rows of Ti2O3 units on top of alternate rows of five-fold-coordinated Ti atoms, where O atoms are located at bulklike positions and Ti3+ ions were positioned at vacant octahedral sites, as is shown in Figure 1. (29) Henrich, V. E.; Dresselhaus, G.; Zeiger, H. J. Phys. ReV. Lett. 1976, 36, 1335. (30) Go¨pel, W.; Anderson, J. A.; Frankel, D.; Jaehnig, M.; Phillips, K.; Scha¨fer, J. A.; Rocker, G. Surf. Sci. 1984, 139, 333. (31) Biener, J.; Wang, J.; Madix, R. J. Surf. Sci. 1999, 442, 47. (32) Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., Muilenberg, G. E., Eds. Handbook of x-ray photoelectron spectroscopy; Perkin-Elmer Corp., Physical Electronics Division: Eden Prairie, MN, 1980. (33) Pe´tigny, S.; Moste´fa-Sba, H.; Domenichini, B.; Lesniewska, E.; Steinbrunn, A.; Bourgeois, S. Surf. Sci. 1998, 410, 250. (34) Briggs, D., Seah, M. P., Eds. Practical Surface Analysis, 2nd ed., Vol. 1, Auger and X-ray Photoelectron Spectroscopy; Wiley: New York, 1990.
Figure 1. Arrangement of atoms in the TiO2 (110)-(1 × 2) surface reconstruction. In the figure are also indications of the positions of the Ti4+ and Ti3+ species. The surface unit cell of 13 × 2.96 Å2 is outlined in the figure.
Figure 2 shows O 1s and Ti 2p (left and right panels, respectively) photoemission spectra acquired before (solid line) and after (dotted line) dosing 2000 L of NO to the clean (1 × 2) surface. The O 1s core level of the clean surface is located at 529.9 eV, in agreement with the literature.35 There is no obvious change during the NO exposure in the O 1s line shape. As has been previously shown,36 the (1 × 2) surface reconstruction Ti 2p3/2 core level is characterized by two components: the main feature is located at 458.5 eV24,35,37 and corresponds to Ti4+ cations (characteristics of stoichiometric TiO2) and the other is a low binding energy shoulder located at 456.8 eV (∆E ) 1.7 eV, indicated in Figure 2 with an arrow)30,33,36,38,39 corresponding to Ti3+cations situated on the Ti2O3 rows (Figure 1). As is clearly seen in Figure 2, the intensity of the low binding energy component decreases after a NO dose of 2000 L, indicating that the Ti3+ cations are oxidized with the NO exposure. In the literature the O/Ti ratio and the extent of surface reduction δ is defined as the relative density of monovalent defects.24,40
δ ) 0.01 × [Ti3+] + 0.02 × [Ti2+] ) 4 - 0.04 × [Ti4+] 0.03 × [Ti3+] - 0.02 × [Ti2+] ) ) 4 - 2 × (O/Ti) where [Tin+] are the percentages obtained in the fitting procedure. These parameters are useful for characterizing the average oxidation (reduction) state of the surface. The TiO2 (110)-(1 × 1) stoichiometric surface is characterized by a δ between 0.3 and 0.4 and a O/Ti ratio between 1.98 and 1.99, while for the (1 × 2) surface reconstruction these values are 0.09-0.12 and 1.941.95, respectively. After a dose of 2000 L the extent of surface reduction decreases to 0.06 and the O/Ti ratio increases to 1.97. These values give a defect density very close to the stoichiometric (1 × 1) surface, indicating the oxidation of the surface after the NO dose. To check the presence of N at the surface, AES measurements were performed because AES is expected to be more sensitive to adsorbed N than XPS, partly due to its lower detection limit.41 Figure 3 shows Auger Ti LMM and Ti LMV transitions for a clean (1 × 2) surface and after 50 L of NO. The Auger electron (35) Madix, R. J.; Biener, J.; Ba¨umer, M.; Digner, A. Faraday Discuss. 1999, 114, 67. (36) Abad, J.; Rogero, C.; Me´ndez, J.; Lo´pez, M. F.; Martı´n-Gago, J. A.; Roma´n, E. Appl. Surf. Sci. 2004, 234, 497. (37) Dake, L. S.; Lad, R. J. Surf. Sci. Spectra 1998, 4, 232. (38) Wang, L. Q.; Baer, D. R.; Engelhard, M. H. Surf. Sci. 1994, 320, 295. (39) Patel, R.; Guo, Q.; Cooks, I.; Williams, E. M.; Roman, E.; de Segovia, J. L. J. Vac. Sci. Technol. A 1997, 15, 2553. (40) Idris, H.; Pierce, K. G.; Barteau, M. A. J. Am. Chem. Soc. 1994, 116, 3063. (41) Brundle, C. R. J. Vac. Sci. Technol. A 1974, 11, 212.
DissociatiVe Adsorption of NO on TiO2
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Figure 2. XPS Ti 2p and O 1s spectra for the (1 × 2) clean surface before and after a dose of 2000 L of NO. In the Ti 2p spectra the arrow indicates the binding energy position of the Ti3+ species.
Figure 3. Ti LMM and Ti LMV Auger transitions for the clean (1 × 2) surface and after 50 L of NO.
emission from N (N KVV at 379 eV42) occurs at an energy that overlaps the Ti LMM transition; this fact complicates the quantitative aspects of AES studies of N on TiO2. However, we can qualitatively estimate the presence of N on the surface by comparing the line shape of Auger spectra before and after adsorption of NO. Figure 3 shows that there are no changes in the line shape of Ti LMM transition after NO exposure in the N region. Besides, the XPS results (not shown) confirm that there is no N on the surface. These results contradict those obtained by Onishi et al.23 since they detect the presence of N on a sputtered TiO2 surface, indicating that Ti3+ cations can decompose NO to nitrides. Our group has not found evidence for the presence of nitrides neither on sputtered TiO2 surfaces24 nor in the present work. These discrepancies could be due to the different sputtering conditions used in the Onishi experiments, 3 keV for 3 min, without any information about the current density, wherefore the morphology and density of defects present in these surfaces can be quite different from our surfaces (0.8 keV, 0.8 µA/cm2, 30 min; or annealing at 880 °C). The comparison between their data and our data is difficult due to the fact that they do not display UPS spectra in a direct manner, but as difference spectra before and after the NO dose. Therefore, it is not possible determine the degree of reduction of their clean surface. Figure 4 shows the band gap region UP He I spectra collected as a function of NO exposure up to 2000 L to a clean (1 × 2) surface. The band gap state is assigned to Ti 3d orbital14 and in the (1 × 2) surface reconstruction the emission of this orbital corresponds to the two Ti cations on the Ti2O3 rows, with oxidation state Ti3+. As is clearly seen in the figure, the effect of NO on the surface defects is important already at the first dose of 0.1 (42) Davis, L. E., MacDonald, N. C., Palmberg, P. W., Riach, G. E., Weber, R. E., Eds. Handbook of Auger electron spectroscopy; Perkin-Elmer Corp., Physical Electronics Division: Eden Prairie, MN, 1976.
Figure 4. UPS He I spectra of the defect band gap states showing the effect of NO exposure until 2000 L to the clean (1 × 2) surface. The spectrum of the clean (1 × 1) surface has been included for a comparison of the defect band gap state present in the (1 × 1) surface prior to annealing form the (1 × 2) surface reconstruction.
Figure 5. Coverage-exposure relationship for NO on to TiO2 (110)(1 × 2) surface. The solid curve represents a least-squares fit of the data to n ) 2 and the dotted curve to n ) 1. The data are fitted until 2000 L; however, in the figure, only the data until 50 L is shown for clarity.
L, showing a large electron depopulation of these Ti 3d states at the first stages (1.2 L) of the NO dose. Finally, for an exposure of 2000 L the band gap defect state is completely vanished, indicating that these kinds of defects are completely healed. In light of the results shown in Figure 4, the NO uptake can be judged from the intensity decrease of the band gap states. We assume that the saturated coverage of NO at RT is the maximum amount of NO that can be chemisorbed on the Ti3+defect sites. Then, if we consider two Ti3+ defect sites per unit cell (13 × 2.96 Å,2 see Figure 1) and one molecule adsorbed per Ti3+ defect site, we obtain a saturation coverage Cs ) 1.92 × 1015 cm-2. Figure 5 shows the NO coverage (θ) as a function of NO exposure. It is noticeable that for a 2 L NO exposure half the Ti3+ defect sites are healed, and a dose of 2000 L is necessary to heal completely these defects. To learn more about the kinetics of NO adsorption on TiO2 (110)-(1 × 2), the data of Figure 5 are fitted to a Langmuir
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Figure 7. ELS spectra (E ) 100 eV) to the clean (1 × 2) surface exposed to 50 and 2000 L of NO. Figure 6. UPS He II spectra to the clean (1 × 2) surface exposed to 50 L of NO. Inset shows the work function (φ) variation with NO exposure for the clean (1 × 2) surface.
adsorption isotherm:43
θ ) (bP)1/n/1 + (bP)1/n where θ is the coverage, b is a constant, and P is the exposure in L. For n ) 1 we have a first-order rate law for adsorption and for n ) 2 we have a second-order process, which corresponds, for example, to dissociative adsorption. In Figure 5, fits for n ) 1 and n ) 2 are shown. It is clearly seen that the best fit is for the last one, indicating that the adsorption on TiO2 (110)-(1 × 2) at RT is a second-order process and the sticking probability is proportional to (1 - θ)2. Further proof in favor of the dissociative adsorption is found in the valence band spectra. It seems that, if the NO adsorption were molecular, it would be possible to expect an increase in the electronic density of the TiO2 valence band in the regions at 3.5 and 9.8 eV of binding energy, which correspond to the NO molecular orbitals 1π and 5σ (∼ 15.6 eV) and 2π (∼ 9.3 eV), respectively, that have been corrected considering the TiO2 (110)(1 × 2) work function (5.8 eV). However, no significant change in the valence band emission in the vicinity of 10 eV was observed (Figure 6); therefore, the NO molecular adsorption is excluded. Since there is an extra electron associated with each Ti3+ defect site, it seems reasonable that the interaction could occur through a charge transfer from the defect to an electronegative molecule like NO. Measurements of the work function change can provide information concerning this charge transfer. The inset of Figure 6 shows the work function variation with the NO exposure. It is observed that from the first Langmuirs the work function increases 0.4 ( 0.1 eV, reaching the value of a stoichiometric surface (φ ) 6.2 ( 0.1 eV) due to a decrease in the surface free electron concentration.44 This rapid increase in the work function at low exposure is consistent with the dissociative adsorption of NO.45 In Figure 7, the band gap region of the electron loss spectra are shown for the clean (1 × 2) surface and for doses of 50 and 2000 L. The shoulder in the elastic peak around 1.2 eV has been (43) Alberty, R. A.; Silbey, R. J. Physical Chemistry; Wiley: New York, 1991. (44) Chung, Y. W.; Lo, W. J.; Somorjai, G. A. Surf. Sci. 1977, 64, 588. (45) Kanski, J.; Rhodin, T. N. Surf. Sci. 1977, 65, 63.
attributed to a transition between the occupied and the unoccupied Ti 3d orbitals (Ti 3d-Ti 3d transitions), involving Ti3+ ions,14,29,30,44 assigned to the two Ti cations on the Ti2O3 rows; see Figure 1. Upon NO exposure Figure 7 shows a clear signal attenuation of the Ti 3d-Ti 3d transition, indicating that NO was able to remove the defect signature, Ti3+ species present on the Ti2O3 rows. LEED technique shows a light background increase of the (1 × 2) LEED pattern after 2000 L of NO, but the (1 × 2) symmetry remains; since at this dose the defects of the (1 × 2) reconstruction have vanished, it stands to reason that the defect sites have been healed with the O coming from the NO molecule. Theoretical studies about the adsorption of NO on to a TiO2 (110)-(1 × 1) defect surface21 have shown that the Ti3+ sites are more favorable than the Ti4+ sites for the NO adsorption, with the O bonded to two Ti3+ sites, producing a charge transfer from Ti to NO. When an electron is transferred to the NO, the 2π* orbital will be filled with more than one electron. As a result, the N-O bond will be weakened. The fact that the (1 × 2) symmetry remains indicate that the O coming from the NO molecule is adsorbed on the Ti2O3 rows. It seems that if the NO adsorbs on to the Ti4+ sites, keeping the (1 × 2) symmetry, UPS, XPS, and ELS techniques should not show any changes in the Ti3+ signature. All the results indicate dissociative adsorption of NO at defects sites, followed by the incorporation of O into the Ti2O3 rows, oxidation of Ti3+ cations to Ti4+, and N going in the gas phase, although from the present data we cannot determine the mechanism for N desorption.
4. Conclusions The interaction of NO with TiO2 (110)-(1 × 2) surface reconstruction have been studied by XPS, UPS, AES, ELS, and LEED at RT, until a dose of 2000 L. The results obtained show that defects present in the Ti2O3 rows (Ti3+ species) are the active sites for the dissociative adsorption of NO. No N signature was seen on the surface after defect healing by NO, indicating that this occurs through dissociation. The kinetics of NO adsorption on TiO2 (110)-(1 × 2) at RT is found to follow a second-order rate law. Acknowledgment. The authors acknowledge the Spanish Ministerio de Educacio´n, project number MAT2005-3866, and Diamant 2000 for financial support. LA700253S
