J. Phys. Chem. 1996, 100, 6631-6636
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CO Photooxidation on TiO2(110) Amy Linsebigler, Guangquan Lu, and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: July 19, 1995; In Final Form: October 4, 1995X
The photooxidation of chemisorbed CO on TiO2(110) was investigated. Molecular O2, chemisorbed at 105 K at anion vacancy defect sites on TiO2(110), is shown to undergo excitation leading to either photodesorption of O2 or photooxidation of coadsorbed CO to produce CO2. The yield of photoproduced CO2 and photodesorbed O2 follows the same excitation curve versus photon energy with a threshold at the TiO2 band gap, 3.1 eV. The decay time for photoproduced CO2 is essentially identical with the characteristic decay time for photodesorbed R-O2, corresponding to a photodesorption cross section ) 8 × 10-17 cm2 for 3.94 eV photons. This indicates that both O2 photodesorption and CO2 photoproduction are related to the same R-O2 species. Compared to O2 photodesorption, CO2 photoproduction is a minor process. Lattice oxygen in TiO2 is not chemically involved in CO2 formation during ultraviolet irradiation.
1. Introduction The CO photooxidation reaction with molecular oxygen is one of the simplest bimolecular surface photoreactions. Previous studies of this photoreaction on metal surfaces have aimed to elucidate the mechanism of CO photooxidation with molecular oxygen.1,2 The objective of this work on the TiO2(110) surface is 2-fold: (1) to add to the understanding of surface photooxidation reactions and (2) to understand the role of the TiO2 surface, with and without surface defects, as a photocatalyst for photooxidation reactions.3-6 When organic species are photocatalytically decomposed on the TiO2 surface, CO photooxidation to CO2 is often the last step of the reaction process. For example, CO was observed as one of the products of the CH3Cl + O2 photooxidation reaction on TiO2 powdered surfaces and on the TiO2(110) surface and undergoes further oxidation with O2 to produce CO2.7,8 The surface sites of the TiO2(110) single crystal surface have previously been characterized.9-16 The TiO2(110) surface is therefore an ideal model surface to study the CO photooxidation reaction. In this paper, we present the results of the photochemical formation of CO2 from CO and O2 on the TiO2(110) surface. The detection technique for photoinduced desorption (PID) and temperature-programmed desorption (TPD) is a line-of-sight apertured quadrupole mass spectrometer.17 Our findings have determined that the active adsorbed molecular oxygen species for the CO photooxidation reaction is present at specific oxygen vacancy sites produced from annealing the surface in vacuum in the temperature range 400-900 K. The active molecular oxygen species, whose identity has not been determined in this work, is photolytically produced from R-O2 via band gap excitation in the TiO2. Possible configurations for the excited oxygen species are O22-, O2-, O-, and O3-.18-23 2. Experimental Section The experiments reported here were carried out in a stainless steel ultrahigh-vacuum (UHV) chamber with a base pressure of 400 K to produce oxygen vacancy sites (Ti3+ sites). The fully oxidized or stoichiometric TiO2(110) surface is inert for the photooxidation of CO with molecular oxygen. The bottom curve in Figure 3 exhibits no measurable signal for 48 amu (C18O2) when the oxidized surface is exposed to UV radiation after 18O2 and C18O exposure. The top curve in Figure 3 illustrates the rapid photodesorption of C18O2 when the shutter for the light source is opened to expose the preannealed surface containing a mixture of C18O and 18O2. The photodesorption trace reached a maximum within the time resolution of our measurements (∼0.2 s). For this experiment the TiO2(110) surface was preannealed to 900 K prior to 18O2 exposure at 105 K, followed by C18O exposure at 105 K. Molecular oxygen chemisorbs at the defect sites produced from annealing the surface to 900 K to a saturation coverage of 0.12 ML.17 Therefore, the comparison in Figure 3 shows that molecular oxygen is not active for photoreaction unless adsorbed at oxygen vacancy sites. The C18O2 photoproduction signal as a function of preannealing temperature is plotted in Figure 4. As the temperature of preannealing is increased above 400 K in successive experiments, the CO2 photoproduction yield increases. The inset in Figure 4 is a plot of CO2 yield versus the preannealing temperature. The plot illustrates the dependence of CO2 production from O2 adsorbed and photoactivated at oxygen vacancy sites in our controlled annealing experiments. An experimental procedure for increasing the defect density on single crystal surfaces is to Ar+ ion bombard the surface.15,35-36 Ar+ bombardment of the TiO2(110) surface produces an oxygen deficient surface with several reduced Ti states, as witnessed by an increase in the Ti (2p) intensity in XPS.15 The surface produced in this manner is disordered and exhibits no LEED pattern. The results obtained by artificially increasing the defect
Figure 4. CO2 photoformation at 105 K as a function of the TiO2(110) preannealing temperature. The inset illustrates the yield plot of CO2 versus preannealing temperature.
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Figure 5. Comparison of the photoformation of CO2 from a preannealed TiO2(110) surface to a sputtered surface, which exhibits no CO2 photoformation.
density by Ar+ bombardment are often compared to the results obtained from the slightly defective (0.12 ML) surface created by annealing to 900 K. The effect of an increased defect density from Ar+ ion bombardment was tested for the CO photooxidation reaction. Figure 5 illustrates the complete lack of CO2 photoproduction from CO and O2 on the Ar+ ion bombarded or sputtered TiO2(110) surface. For comparison, the top spectrum is the CO2 photoproduction signal from an annealed surface. This provides direct evidence that the photooxidation reaction for CO and O2 is a site specific reaction involving the specific oxygen vacancy sites produced from annealing the TiO2(110) surface. These may be bridging oxygen vacancy sites.12-14 4.2. Molecular Species Involvement in CO2 Photoproduction. A series of isotopic labeling experiments with the two reactants, CO and O2, have demonstrated the exclusive involvement of molecular chemisorbed CO and O2 in the photooxidation process. When C18O and 18O2 are used as reactants, the 48 amu signal (C18O2) is exclusively produced. This observation excludes photooxidation reactions involving lattice oxygen which produce 44 and 46 amu signals. The measurements were made with an accuracy of 1% of the CO2 yield corrected for the isotopic impurity in the 18O2 and C18O. To study the involvement of molecular CO, C18O was exposed to the surface after 16O2. The CO2 produced was observed at 46 amu (C18O16O), and no signal for 44 amu (C16O2) was observed with an accuracy of 2% of the CO2 yield. This observation excludes CO dissociation and subsequent C and 16O2 photoreaction. Therefore, the results from the two experiments above show that the photoreaction is initiated with the molecular CO and O2 species on the preannealed TiO2(110) surface. 4.3 Photon Energy Dependence and Efficiency of CO2 Photoproduction. The photoproduction of C18O2 from adsorbed 18O2 and C18O is observed only when the photon energy is greater than the band gap of rutile TiO2, 3.1 eV. No photoreaction is observed below the band-gap energy. This observation supports a substrate-mediated excitation mechanism for O2 activation for desorption and reaction. The photon energy dependence for CO2 photoproduction follows the same yield curve as O2 photodesorption from the TiO2(110) surface as illustrated in Figure 6.17 Figure 7 illustrates a key finding of this work. The kinetics of the R-channel O2 photodesorption at hν ) 3.94 ( 0.07 eV is compared to the kinetics of CO2 photoproduction, measured
Linsebigler et al.
Figure 6. Normalized photodesorption yield of O2 and the photoformation of CO2 as a function of photon energy. The photoprocesses turn on at a photon energy greater than the band gap of TiO2 indicated in the figure at 3.1 eV.
simultaneously. It may be seen that the exponential decay rate for both the R-O2 and CO2 photodesorption signals is essentially identical. The initial exponential decay rate for O2 photodesorption pertains mainly to β-O2 photodesorption17 and is not shown in the semilogarithmic plots displayed on the right of Figure 7, which is characteristic of R-O2 behavior. 4.4. CO2 Photoformation from Specific Reactant Adsorption Configuration. A study of the most favorable adsorbate configuration for CO2 photoformation was carried out by reversing the order of reactant adsorption as shown in Figure 8. For the previous set of experiments a saturation exposure of 18O2 was employed on the annealed surface prior to CO exposure. We find that 18O2 adsorbs at the oxygen vacancy sites on the TiO2(110) prior to CO exposure. This is supported by two experimental observations: (1) the removal of the high-temperature CO thermal desorption states (attributed to CO interacting with the vacancy defect sites, see Figure 1) when O2 is exposed to the surface prior to CO and (2) the saturation of the O2 photodesorption yield at the highest defect density that may be achieved by annealing (0.12 ML).17 However, when CO is exposed to the TiO2(110) surface first, followed by O2 adsorption, the photoyield of CO2 is greatly reduced. The yield for CO exposure first, followed by O2, is a factor of 3 smaller than in the reverse experiment. This is due to the influence of the chemisorbed CO on the availability of the oxygen vacancy sites for O2 adsorption. It is believed that CO interacts with the vacancy via the O end of the CO molecule adsorbed on in-plane Ti sites33 and physically blocks the vacancy sites for O2 adsorption. Furthermore, even if the CO exposure is increased 4 times beyond the saturation CO exposure used in Figure 8 (lower curve), the yield of CO2 is unchanged. 5. Discussion 5.1. Involvement of Oxygen Vacancy Sites. The CO photooxidation reaction with molecular O2 on the TiO2(110) surface is only observed in the presence of (1) oxygen vacancy sites produced from annealing the surface to T > 400 K and (2) ultraviolet light when an energy greater than the band gap of TiO2(110), 3.1 eV. Figure 3 illustrates the existence of the CO photooxidation reaction only on the annealed surface. Figure 6 illustrates the need for band-gap excitation. A thermal
CO Photooxidation on TiO2(110)
J. Phys. Chem., Vol. 100, No. 16, 1996 6635
Figure 7. (Left) Photodesorption spectra of CO2 and O2. A line divides the O2 photodesorption spectra into the initial fast β-O2 process and the slow R-O2 process used for the time decay analysis represented in the panels on the right. The time decay constants (τ) for the two photoprocesses are very similar.
Figure 9. Schematic model of the TiO2(110) surface illustrating the oxygen vacancy defect sites produced from annealing the surface: (cross-hatched symbols) bridging O2- overlayer ions; (open symbols) in-plane O2- ions; (solid symbols) Ti4+ ions in nondefective regions or Ti3+ ions in defective regions; (shaded symbols) bulk O2- ions.
Figure 8. Comparison of CO2 photoformation on the TiO2(110) surface from two different exposure sequences of O2 and CO.
reaction between CO and O2 is not observed on the oxidized or annealed TiO2(110) surface. Annealing the TiO2(110) surface to T > 400 K creates Ti3+ oxygen vacancy sites. Figure 9 schematically depicts the TiO2(110) surface with oxygen vacancy sites. Bridging oxygen vacancy sites are more thermodynamically favorable at the temperatures used in this work.37 These Ti3+ sites were observed with several different techniques such as XPS and UPS.9,15,16,27 After adsorption of 18O2 on the annealed surface, ISS shows the existence of 18O on the surface and was employed to measure the vacancy site coverage.15 Chemisorption of oxygen-containing adsorbates such as H2O, CH2O, and NO results in the chemical reduction of the species at the oxygen vacancy sites and the subsequent thermal desorption of H2, C2H4, and N2O.28 It has already been reported that the vacancy sites play an important role in the photooxidation reaction of CH3Cl with molecular oxygen on the annealed TiO2(110) surface.7,8
In the present study, the simplicity of the CO and O2 photoreaction leads to some important observations and confirms previous conclusions obtained for the thermal and photochemistry of the TiO2(110) surface. At 105 K on the annealed TiO2(110) surface, O2 adsorbs in a molecular fashion at bridging oxygen vacancy sites. CO is believed to adsorb at in-plane Ti sites, next to the bridging oxygen rows. The adsorption of O2 on oxygen vacancy sites permits the excitation of the O2 species upon exposure to the UV light via substrate-mediated excitation. It is found that the molecular O2 exists in two different adsorption configurations. One adsorption configuration (βO2) leads to the fast photodesorption of O2 (β-channel) with a cross section of =1.5 × 10-15 cm2. The other configuration (R-O2) leads to a slow O2 photodesorption process (R-channel) with a photodesorption cross section of =8 × 10-17 cm2. The R-O2 species are converted to the β-O2 species upon annealing the O2-covered surface to temperatures in the range 105-350 K, as shown in Figure 2. The complete thermal conversion of chemisorbed O2 from R-O2 to β-O2 eliminates the CO2 photoproduction from adsorbed CO, indicating that R-O2 is responsible for the CO photooxidation reaction.34
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CO adsorption prior to O2 adsorption also confirms the adsorption and excitation of O2 at oxygen vacancy sites. CO adsorption on an annealed surface interacts with oxygen vacancy sites in the absence of O2 and reduces the ability of TiO2(110) to photooxidize CO. 5.2. Involvement of Molecular O2 and Molecular CO. Isotopic labeling of reactant species has identified the molecular reactant species involved in the photooxidation reaction. Both molecular O2 and molecular CO are the reactants. Even though isotopic labeling has determined the involvement of molecular O2 in the photooxidation reaction, the exact nature of the chemisorbed oxygen has not been determined. Several oxygen ion species have been suggested as the adsorbed species responsible for the photochemistry of oxygen on powdered TiO2.18-23 The dominant candidates on TiO2, SrTiO2, and ZnO single crystal surfaces are O2- and O22-.27,38-40 5.3. Overall Photoreaction Scheme. This work, plus earlier work,17,34 indicates the following photodesorption and photoreaction scheme for R-O2 at hν ) 3.94 eV.
R-O2(a) + hν 9 8 O2(g) -17 2
(1)
R-O2(a) + CO(a,excess) + hν f CO2(g) + O(a)
(2)
Qd = 8 × 10
cm
In addition, the thermal reaction of R-O2 has been investigated, and thermal
R-O2(a) 9 8 β-O2(a) 105-350 K
(3)
8 O2(g) β-O2(a) + hν 9 -15 2
(4)
Qd = 1.5 × 10
cm
β-O2 does not photooxidize CO.28 Two important observations have been made in this work. They are (1) the O2 photodesorption yield and the CO2 photoreaction yield versus photon energy both exhibit a threshold at the TiO2 band gap, 3.1 eV, and both follow a similar normalized yield versus photon energy curve (Figure 6) and (2) the exponential decay rate for depletion of R-O2 during photodesorption is essentially identical with the rate of exponential decay of the CO2 yield during photoreaction (Figure 7). These results indicate that the photooxidation of CO on TiO2(110) is controlled by the photoexcitation of R-O2 species adsorbed on anion vacancy defect sites. Excitation of R-O2(a) results in either O2 photodesorption or photooxidation of neighbor CO adsorbate molecules. The coverage of R-O2 is dictated by its photodesorption kinetics, and the reaction of excited R-O2(a) with CO(a) to produce CO2 is a minor kinetic channel. 6. Summary 1. CO2 photoproduction from CO and O2 only occurs when oxygen vacancy sites are present on the TiO2(110) surface produced by annealing to T > 400 K. 2. O2 adsorbs at oxygen vacancy sites on the TiO2(110) surface. 3. The CO2 photoproduction occurs at energies greater than the band-gap energy of TiO2 (3.1 eV) and follows the same photoyield curve as for O2 photodesorption. 4. CO2 photoproduction involves the interaction of chemisorbed CO with excited R-O2 species. CO2 is not produced from lattice oxygen nor CO dissociation nor thermal reaction. 5. The rate of photooxidation of CO to CO2 is controlled by the coverage of R-O2 species. Thus, the photoproduction of CO2 follows the same exponential decay rate as the photodesorption of R-O2. Photooxidation is a minor kinetic channel compared to O2 photodesorption.
6. CO exposure prior to O2 exposure competes with O2 for adsorption at oxygen anion vacancy sites, causing a reduction in the rate of photoreaction. 7. The most likely candidates corresponding to R-O2 species are O2- and O22- surface species. Acknowledgment. We acknowledge with thanks the support of the Army Research Office under the AASERT program. References and Notes (1) Mieher, W. D.; Ho, W. J. Chem. Phys. 1989, 91, 2755. (2) Mieher, W. D.; Ho, W. J. Chem. Phys. 1993, 99, 9279. (3) Fujushima, A.; Honda, K. Nature 1972, 238, 37. (4) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341, and references therein. (5) Serpone, N.; Pelizzetti, E., Eds. Photocatalysis-Fundamentals and Applications; Wiley Interscience: New York, 1989; and references therein. (6) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735. (7) Lu, G.; Linsebigler, A. L.; Yates, J. T., Jr. J. Phys. Chem. 1995, 99, 7626. (8) Wong, J. C. S.; Linsebigler, A. L.; Lu, G.; Fan, J.; Yates, J. T., Jr. J. Phys. Chem. 1995, 99, 335. (9) Go¨pel, W.; Anderson, J. A.; Frankel, D.; Jaehnig, M.; Phillips, K.; Scha¨fer, J. A.; Rocker, G. Surf. Sci. 1984, 139, 133. (10) Rocker, G.; Scha¨fer, J. A.; Go¨pel, W. Phys. ReV. B 1984, 30, 3704. (11) Eriksen, S.; Egdell, R. G. Surf. Sci. 1987, 180, 263. (12) Henrich, V. E.; Dresselhaus, G.; Zeiger, H. J. Phys. ReV. Lett. 1976, 36, 1335. (13) Henrich, V. E.; Kurtz, R. L. J. Vac. Sci. Technol. 1981, 18, 416. (14) Henrich, V. E.; Kurtz, R. L. Phys. ReV. B 1981, 23, 6280. (15) Pan, J. M.; Maschhoff, B. L.; Diebold, U.; Madey, T. E. J. Vac. Sci. Technol. A 1992, 10, 2470. (16) Kurtz, R. L.; Stockbauer, R.; Madey, T. E.; Roman, E.; de Segovia, J. L. Surf. Sci. 1989, 218, 178. (17) Lu, G.; Linsebigler, A. L.; Yates, J. T., Jr. J. Chem. Phys. 1995, 102, 4657. (18) (a) Bickley, R. I.; Stone, F. S. J. Catal. 1973, 31, 389. (b) Bickley, R. I.; Munuera, G.; Stone, F. S. J. Catal. 1973, 31, 398. (19) Munuera, G.; Rives-Arnau, V.; Saucedo, A. J. Chem. Soc., Faraday Trans. 1 1979, 75, 736. (b) Gonzalez-Elipe, A.; Munuera, G.; Soria, J. J. Chem. Soc., Faraday Trans. 1 1979, 75, 749. (20) (a) Anpo, M.; Aikawa, N.; Kubokawa, Y.; Che, M.; Louis, C.; Giamello, E. J. Phys. Chem. 1995, 89, 5017. (b) Anpo, M.; Kubokawa, Y.; Fujii, T.; Suzuki, S. J. Phys. Chem. 1994, 88, 2572. (c) Anpo, M.; Chiba, K.; Tomonari, M.; Coluccia, S.; Che, M.; Fox, M. A. Bull. Chem. Soc. Jpn. 1991, 64, 543. (21) Howe, R. F.; Gratzel, M. J. Phys. Chem. 1987, 91, 3906. (22) Courbon, H.; Formenti, M.; Pichat, P. J. Phys. Chem. 1977, 81, 550. (23) Yanagisawa, Y.; Ota, Y. Surf. Sci. Lett. 1991, 254, L433. (24) Hanley, L.; Guo, X.; Yates, J. T., Jr. J. Chem. Phys. 1989, 91, 7220. (25) Lu, G.; Linsebigler, A. L.; Yates, J. T., Jr. J. Vac. Sci. Technol. A 1994, 12, 384. (26) Muha, R. J.; Gates, S. M.; Basu, P.; Yates, J. T., Jr. ReV. Sci. Instrum. 1985, 56, 613. (27) Go¨pel, W.; Rocker, G.; Feierabend, R. Phys. ReV. B 1983, 28, 3427. (28) Lu, G.; Linsebigler, A. L.; Yates, J. T., Jr. J. Phys. Chem. 1994, 98, 11733. (29) Bozack, M. J.; Muehlhoff, L.; Russell, J. N., Jr.; Choyke, W. J.; Yates, J. T., Jr. J. Vac. Sci. Technol. A 1987, 5, 1. (30) Winkler, A.; Yates, J. T., Jr. J. Vac. Sci. Technol. A 1988, 6, 2929. (31) Campbell, C. T.; Valone, S. M. J. Vac. Sci. Technol. A 1985, 3, 408. (32) Linsebigler, A. L.; Smentkowski, V. S.; Ellison, M. D.; Yates, J. T., Jr. J. Am. Chem. Soc. 1992, 114, 465. (33) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. J. Chem. Phys. 1995, 103, 9438. (34) Lu, G.; Linsebigler, A. L.; Yates, J. T., Jr. J. Chem. Phys. 1995, 102, 3005. (35) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: New York, 1994. (36) Idriss, H.; Barteau, M. A. Catal. Lett. 1994, 26, 123. (37) Henrich, V. E. Progr. Surf. Sci. 1979, 9, 143. (38) Henrich, V. E.; Dresselhaus, G.; Zeiger, H. Phys. ReV. B 1978, 17, 4908; J. Vac. Sci. Technol. 1978, 15, 534. (39) Go¨pel, W.; Rocker, G. J. Vac. Sci. Technol. 1982, 21, 289. (40) Go¨pel, W. Surf. Sci. 1977, 62, 165.
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