J. Phys. Chem. C 2008, 112, 12477–12485
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Sum Frequency Generation Spectroscopic Investigation of TiOx/Pt(111): Surface Active Sites and Reaction Paths Probed by Formate Jun Chen,†,‡ Jun Kubota,‡ Akihide Wada,§ Junko Nomura Kondo,§ and Kazunari Domen*,‡ Japan Science and Technology Agency 4-1-8 Hon-cho, Kawaguchi-shi, Saitama 332-0012, Japan, Department of Chemical System Engineering, The UniVersity of Tokyo 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Chemical Resources Laboratory, Tokyo Institute of Technology 4259 Nagatsuda, Midori-ku, Yokohama 226-8503, Japan ReceiVed: March 5, 2008; ReVised Manuscript ReceiVed: May 25, 2008
The catalytic behavior of ultrathin titanium oxide films deposited on platinum (111) surfaces is investigated through sum frequency generation (SFG) spectroscopy of the adsorption and decomposition of formate. TiOx films of 7 Å or less in thickness are prepared on Pt(111) substrates by evaporation of titanium under oxygen, and samples are examined as-prepared or after annealing at 873 or 973 K. Formate is adsorbed to the samples by exposure to 1000 langmuir of formic acid at 250 K. The frequency of the stretching vibration (ν(C-D)) of the deuterated formate C-D bond is found to be sensitive to the interaction between the adsorption sites and formate. SFG observations under different polarization combinations reveal that most formate is adsorbed to the surface with a bridge configuration, and that only a small amount of monodentate formate is present on the surface of TiOx/Pt(111) annealed at 973 K. Investigation of the effect of annealing temperature, time, and atmosphere on formate ν(C-D) frequency and intensity, in combination with low-energy electron diffraction (LEED) observations, reveals three kinds of surface active sites: defect sites, disordered titanium ion sites, and ordered 5-fold coordinated Ti4+ sites. These adsorption sites are responsible for ν(C-D) responses at frequencies of 2134-2184, 2195, and 2210-2226 cm-1, respectively. The defect sites are suggested to be located at the boundary between the disordered phase and the ordered phase of titanium oxide. Upon heating, formate adsorbed to defect sites undergoes decomposition at temperatures below 250 K, while formate adsorbed to ordered 5-fold coordinated Ti4+ sites decomposes via reaction paths: dissociation via monodentate formate as an intermediate, and diffusion onto defect sites before dissociation. The temperature of direct decomposition of bridge formate adsorbed to Ti4+ sites appears to be related to the formate ν(C-D) frequency, with lower frequencies corresponding to higher decomposition temperatures. 1. Introduction Solid oxides are widely employed as heterogeneous catalysts in a variety of catalytic reactions. However, the structural complexity of oxide catalyst surfaces renders it difficult to accurately characterize the mechanisms of most heterogeneous catalytic reactions that occur on oxide surfaces. Surface-scanning probe techniques have revealed the presence of high densities of defects such as oxygen vacancies on many oxide surfaces.1–5 Imperfections and modifications of the electronic structure2,6,7 have also been found to play important roles in catalytic reactions.8–12 To obtain further insights into catalytic reactions on oxide surfaces, it is therefore meaningful to monitor the behavior of reactant molecules on the submonolayer level. Titanium oxide is one of most widely used heterogeneous catalysts, and is regularly applied in photocatalysis13–18 and as a support.19–22 As a representative model of oxide catalyst surfaces, the surface science of titanium oxide has attracted great interest. A variety of surface-probe techniques combined with ultrahigh vacuum (UHV) systems, such as scanning tunneling microscopy (STM),9,23–26 atomic force microscopy (AFM),27–29 low-energy electron diffraction (LEED),26,30–32 high-resolution * To whom correspondence should be addressed. E-mail: domen@ chemsys.t.u-tokyo.ac.jp. † Japan Science and Technology Agency. ‡ Department of Chemical System Engineering, The University of Tokyo. § Chemical Resources Laboratory, Tokyo Institute of Technology.
electron energy loss spectroscopy (HREELS),33,34 Fourier transform reflection-absorption infrared spectroscopy (FTRAIRS),35 and temperature-programmed desorption (TPD),33,36 have been used to investigate the surface structures and surface chemistry of single crystalline TiO2 and titanium-oxide films. A recent atomically resolved characterization of TiOx-coated particulate platinum revealed that TiOx forms an ultrathin layer encapsulating the platinum cluster, possibly providing a physical origin for the strong metal-support interaction (SMSI) effect.37 Such observations provide some insight into the nature of reactive sites on TiO2-metal catalysts. A detailed LEED, X-ray photoemission spectroscopy (XPS), and STM investigation of the structure of ultrathin TiOx films on Pt(111)26,32 revealed that the structure of TiOx is dependent on the postannealing conditions. Overall, however, few studies have focused on the catalytic characteristics of ultrathin titanium-oxide films. The behavior of molecules on a surface can be studied by IR-visible sum frequency generation (SFG) spectroscopy, a type of vibrational spectroscopy. SFG spectroscopy involves a nonlinear optical process and as such is highly surface-specific.38,39 The resonant enhancement that occurs when the exciting IR beam is coupled with the IR and Raman active vibration of adsorbed molecules provides vibrational information.40–42 The intensity of SFG light (ISF) is expressed as a function of the IR frequency (ω) by
10.1021/jp801924h CCC: $40.75 2008 American Chemical Society Published on Web 07/22/2008
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[
(2) ISF(ω) ∝ χNR +
A
∑ ω - ωqq+ iΓq q
]
Chen et al.
2
(1)
(2) where χNR is the nonresonant term of second-order nonlinear susceptibility, q is an identifier of vibrational modes, and Aq, ωq, and Γq are the transition amplitude, resonant frequency, and damping factor of the qth mode. Aq is proportional to the transition dipole and Raman tensor of the qth mode. Using this SFG technique, the present authors have previously examined the adsorption and decomposition of formate, a key species in many catalytic reactions as a reaction intermediate on catalyst surfaces, on NiO(111). It was found that two types of formate species with different configurations are adsorbed on such surfaces: a bidentate form, and a monodentate form. By monitoring the response to laser-induced temperature jumps, the reactive intermediate involved in the decomposition process of bidentate formate was also identified. In present study, the surface active sites and reaction paths on ultrathin titanium-oxide films prepared on Pt(111) substrates under different annealing conditions are characterized by SFG spectroscopy using formate as a probe molecule. The adsorption of formate upon exposure to formic acid, and the subsequent decomposition of formate upon heating, are monitored by taking SFG spectroscopic measurements at the frequencies of the C-D stretching vibration (ν(C-D)) of deuterated formate adsorbed on titanium oxide.
2. Experimental Section The SFG setup with integrated UHV system employed in the present study is the same as that used in previous studies.43,46,47 The SFG signal is obtained from frequencytunable IR and 532 nm visible pulses produced as the secondharmonic generation (SHG) of the fundamental output from a mode-locked Nd:YAG laser with 35 ps pulse width (fwhm; full width at half-maximum) and a 10 Hz repetition rate. The frequency-tunable IR (1300-3000 cm-1) pulses are generated by differential frequency generation (DFG) from frequencytunable near-IR (NIR) pulses produced by an optical parametric generator/amplifier using two β-BaB2O4 (BBO) crystals, and from 1064 nm pulses, in an AgGaS2 (AGS) crystal. The IR pulses (30 µJ/pulse at 2000 cm-1, 2 mm spot diameter) and visible pulses (100 µJ/pulse, 3 mm) cross at the sample surface with an incident angle of ca. 70°. Unless noted otherwise, the IR and visible pulses are p-polarized at the sample surface. The SFG signal is detected by a photomultiplier tube (PMT) after passing through optical filters and a monochromator. Titanium-oxide films were prepared on Pt(111) substrates (TiOx/Pt(111)) in the same UHV chamber as employed for SFG measurements. The UHV chamber is equipped with an argonion bombardment gun for substrate cleaning and a LEEDAuger electron spectroscopy (AES) for surface characterization. The Pt(111) surface was cleaned in the UHV chamber by repeated argon-ion bombardment and annealing under vacuum at 1000 K until cleanliness and orderliness were confirmed by AES and LEED observations. Titanium was then deposited on the substrate in the UHV chamber by evaporation from a titanium wire wrapped around a resistively heated tungsten wire filament. Evaporation deposition was performed at 700 K under 1 × 10-6 Torr (1 Torr ) 133.3 Pa) of O2 to allow the formation of a titanium-oxide film. Titanium-oxide films with a range of structures were then obtained by annealing the as-prepared TiOx/ Pt(111) sample under vacuum (or O2 or H2) at elevated temperature for 1-5 min. This preparation procedure is similar to that reported by Boffa et al.26 After cooling to 250 K, samples
Figure 1. AES spectra for (a) Pt(111), (b) as prepared TiOx/Pt(111) (TiOx/Pt(111)-700), and (c) annealed TiOx/Pt(111) (TiOx/Pt(111)-8735). The baselines of AES spectra are offset.
were exposed to 1000 langmuir (1 langmuir ) 1 × 10-6 Torr s) of deuterated formic acid (DCOOD), which was dried in advance using anhydrous copper sulfate and purified by freeze-pump-thaw cycling. DCOOD was used instead of HCOOH in order to obtain clearer spectra in the frequency region of the C-D stretching modes for which the present SFG setup is optimized. TPD measurements involved heating the sample at 2.8 K s-1 while monitoring at 6 mass numbers (m/e ) 4, 20, 28, 32, 44 and 48) simultaneously. 3. Results and Discussion 3.1. Structure of TiOx/Pt(111). The composition of the asprepared TiOx/Pt(111) sample (TiOx/Pt(111)-700) was determined by AES (Figure 1). The thickness of the film was estimated to be ca. 7 Å based on the AES signal of the Pt(111) substrate and the electron inelastic mean free path (IMFP) of 5 Å for TiO2.48 Similar to the observations by Boffa et al.,26 no LEED pattern could be obtained for this sample, suggesting that the TiOx film formed on the Pt(111) substrate at 700 K under 1 × 10-6 Torr of O2 is in a disordered state. However, the sample annealed under vacuum at 873 K for 5 min (TiOx/ Pt(111)-873-5) exhibited LEED spots, indicating that annealing promotes the development of structure. According to the STM and LEED results reported by Boffa et al.,26 the structure of the TiOx film develops rectangular symmetry upon annealing, resulting in a surface similar to a reduced TiO2(110) surface consisting of ordered arrays of O defects at a spacing of 3.5 Å, which is consistent with the 3.3 Å Ti-Ti distance. The TiOx film on the TiOx/Pt(111)-873-5 sample is ca. 2 Å thinner than that on the TiOx/Pt(111)-700 sample, attributable to the diffusion of titanium into the platinum substrate upon annealing. This is supported by the detection of interfacial Ti3+ between the TiOx film and the platinum bulk by XPS.26 The completeness of TiOx coverage on the platinum surface was determined by SFG measurement of the TiOx/Pt(111)-873-5 sample after exposure to CO gas at 250 K. As CO adsorbed to platinum produces a strong SFG response (data not shown), any exposure of the platinum surface should be readily detectable by this technique.41 However, no vibrational signal due to CO adsorbed to platinum was detected, demonstrating that the TiOx coverage was complete in this sample. The TiOx/Pt(111)-973-5 (annealed at 973 K for 5 min) sample also exhibited a similar structure to that of TiOx/Pt(111)-873-5
Spectroscopic Investigation of TiOx/Pt(111)
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SCHEME 1: Adsorption Configurations of Bridge and Monodentate Formate
as observed by Boffa et al.26 However, the TiOx film was found to be thinner due to increased diffusion of titanium into the platinum bulk at elevated annealing temperature, as the fraction of Ti3+ detected by XPS increases with increasing annealing temperature.26 The influences of annealing atmosphere on the surface structure of TiOx/Pt(111) was also investigated. STM measurements confirmed that the TiOx/Pt(111) sample annealed under 3 × 10-7 Torr of O2 at 873 K for 5 min exhibited the same atomic spacing as the TiOx/Pt(111)-873-5 sample annealed under vacuum.26 3.2. Formate Adsorption on TiOx/Pt(111). Formic acid has been used in many studies to investigate the catalytic behavior of organic molecules on metal-oxide surfaces.23,45,49–52 Formic acid decomposes into formate and proton on many oxides, including TiO2, MgO, and NiO, and the configuration of formate on TiO2 have been extensively characterized by STM and HREELS.25,53,54 Bridge and monodentate forms of formate have been proposed as the two main products on TiO2(110) (see Scheme 1). The formate species generated by the decomposition of formic acid thus reflects the state of surface adsorption sites. Figure 2 shows the SFG spectra of formate on TiOx/Pt(111) samples prepared at various annealing temperatures. The prepared samples were exposed to 1000 langmuir of formic acid at 250 K prior to SFG measurement. The as-prepared titanium oxide film (TiOx/Pt(111)-700)) produces a symmetric vibrational band at 2195 cm-1 that is assignable to the ν(C-D) mode of formate. A similar spectroscopic pattern (ν(C-D) at 2160 cm-1) was reported by Bandara et al. for the decomposition of formic acid on NiO(111)/Ni(111).45 It therefore appears that only one type of adsorption site is present on this disordered TiOx film. However, the annealed sample (TiOx/Pt(111)-873-5) produces two vibrational bands: a strong band at 2226 cm-1, and a weaker band at 2180 cm-1. The 2226 cm-1 band is much narrower than the 2195 cm-1 band exhibited by TiOx/Pt(111)-700. The TiOx/Pt(111)-973-5 sample also exhibited two analogous vibrational bands, at 2210 and 2134 cm-1, with the former being much stronger than the latter, along with an addition weak intermediate band at ca. 2180 cm-1 that extends the lowfrequency tail of the high-frequency (2210 cm-1) band. Although the weak low-frequency bands may appear to be attributable to a Fermi resonance with either a 2δ(C-D) overtone or a νas(OCO) + δ(C-D) combination band,55 the possibility that these bands are due to Fermi resonance can be discounted since only one formate band was observed for the as-prepared TiOx/Pt(111) sample. The group of two or three bands is therefore considered to reflect the adsorption of formate at two or three different adsorption sites, each producing different ν(C-D) vibrations. The adsorption sites responsible for these formate vibrational bands were investigated through examination of the SFG spectra for formate adsorbed on TiOx/Pt(111) after annealing at 873 K under vacuum for increasing durations. Figure 3 shows the change in the SFG response indicative of formate adsorption among samples annealed for 1.5, 3 and 4.5 min. The band intensity is initially suppressed upon annealing (1.5 min) and
Figure 2. SFG spectra of formate adsorbed on TiOx/Pt(111)-700 (squares), TiOx/Pt(111)-873-5 (diamonds), and TiOx/Pt(111)-973-5 (triangles) at 250 K.
Figure 3. SFG spectra of formate adsorbed on TiOx/Pt(111)-700 (solid diamonds), TiOx/Pt(111)-873-1.5 (solid squares), TiOx/Pt(111)-873-3 (open squares), and TiOx/Pt(111)-873-4.5 (open diamonds) at 250 K.
is somewhat red-shifted with an extended high-frequency tail. After annealing for 3 min, however, three discernible bands: weak bands at 2184 cm-1 and 2195 cm-1, and a stronger narrow band at 2226 cm-1, are exhibited. After annealing for 4.5 min, the strong band at ca. 2224 cm-1 increases markedly in intensity, while the bands at 2195 cm-1 and ca. 2184 cm-1 remain weak and act to extend the tail of the main peak to the low-frequency side. In consistence with the LEED observations, these drastic changes in the vibrational bands of formate illustrate that the TiOx film reorganizes in structure after annealing at 873 K. As TiOx/Pt(111) annealing for approximately 5 min produced a more ordered surface structure, increasing the fraction of ordered surface sites, the strengthening of the ca. 2224 cm-1 band with annealing time suggests that this band is due to formate adsorbed on ordered surface sites of the TiOx film. Similarly, the band at 2195 cm-1 is attributed to formate adsorbed on disordered surface sites. The narrowness of the ν(C-D) band at 2224 cm-1 compared to the relatively broadband at 2195 cm-1 also supports this assignment. The active sites responsible for the vibrational band of formate at 2184 cm-1 were investigated by studying the influence of
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Figure 4. SFG spectra of formate adsorbed on TiOx/Pt(111)-873-5 (squares) and TiOx/Pt(111)-873-5 (triangles) after exposure to 200 langmuir of H2 at 823 K.
H2 treatment on the state of surface sites of TiOx/Pt(111)-8735. Figure 4 shows the SFG spectra of formate adsorbed on TiOx/ Pt(111)-873-5 before and after H2 treatment at 823 K. Hydrogen treatment clearly enhances the band at 2184 cm-1, while the intensity of the 2226 cm-1 band remains approximately constant. Despite the weak interaction between molecular hydrogen and TiO2, the densities of oxygen vacancies and Ti3+ on the TiO2 surface are increased upon annealing in an H2 atmosphere.56,57 This result demonstrates that the formate responsible for the vibrational band at 2184 cm-1 is associated with defect sites, such as oxygen vacancies and Ti3+. The TiOx/Pt(111) sample annealed at 873 K for 5 min under O2 also exhibited the formate-related band at ca. 2180 cm-1 (data not shown), indicating that the defect sites on TiOx/Pt(111) produced by annealing at high temperature under vacuum cannot be repaired by the presence of molecular oxygen. This result also shows that these defect sites on TiOx/Pt(111) are not the same as the Ti3+ defects generated by ultraviolet irradiation on crystal rutile TiO2(110) surface, those defects have been found to be repairable by exposure to O2.58 Figure 5 compares the formate SFG spectra for TiOx/Pt(111)873-5 samples exposed to 1000 and 4000 langmuir of formic acid. The band at 2184 cm-1, which was found to be suppressed upon annealing for even a short time, is substantially enhanced by greater exposure to formic acid, while the band at 2226 cm-1 is somewhat weakened. STM observations revealed the sample exposed to large amounts of formic acid had developed many TiOx islands on the film surface. It thus appears that the defect sites responsible for the vibrational band of formate at 2184 cm-1 are located at the interface between the disordered phase and the ordered phase of TiOx. As shown in Figure 2, the ν(C-D) bands attributable to formate adsorbed on TiOx/Pt(111)-973-5 are red-shifted compared to the bands observed for TiOx/Pt(111)-873-5. Although both samples exhibited similar LEED patterns, the TiOx film has been shown by XPS analysis to become thinner with increasing annealing temperature.26 Therefore, the shift in the frequency of the ν(C-D) band appears not to be related to changes in surface structure, such as Ti-Ti distance and unit cell spacing, but to the influence of the platinum substrate. In the TiOx/Pt(111)-973-5 sample, the substrate is within 5 Å away from the TiOx surface, and appears to influence the interaction
Chen et al.
Figure 5. SFG spectra of formate adsorbed on TiOx/Pt(111)-873-5 after exposure to 1000 (squares) or 4000 langmuir (triangles) of formic acid at 250 K.
Figure 6. Exposure-dependent SFG spectra of formate adsorbed on (a) TiOx/Pt(111)-700, and (b) TiOx/Pt(111)-873-5. The baselines of SFG spectra are offset.
between surface TiOx and adsorbed formate. The red-shift of the lower-frequency band, which corresponds to formate adsorbed on defect sites, is larger than for the higher-frequency band, which is assigned to formate adsorbed on ordered surface sites. As defect sites, such as oxygen vacancies and Ti3+, are closer to the substrate interface than the ordered surface sites, this observation supports the conclusion that the platinum substrate has significant influence on the chemical properties of the supported TiOx layer. On the other hand, one might think that all these different vibrational bands are due to the formate adsorbed on the TiOx film which contains domains of different thickness; however, this possibility is excluded, because a broadband in the range of 2134-2226 cm-1 instead of these relatively narrow bands will be detected in this case. The formate SFG measurements suggest that the surface structure of the as-prepared sample (TiOx/Pt(111)-700) is substantially different from that of the annealed samples. Figure 6 shows two series of SFG spectra for formate on the as-prepared (TiOx/Pt(111)-700) and annealed (TiOx/Pt(111)-8735) samples upon exposure to increasing amounts of formic acid.
Spectroscopic Investigation of TiOx/Pt(111)
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Figure 7. Polarization-dependent SFG spectra of formate adsorbed on (A) TiOx/Pt(111)-700, (B) TiOx/Pt(111)-873-5, (C) TiOx/Pt(111)-973-5, and (D) TiOx/Pt(111)-873-3 at 250 K. Polarization combination (s, p) corresponds to s-polarized visible and p-polarized IR pulses.
For TiOx/Pt(111)-700, formate is not adsorbed to saturation until exposed to approximately 1000 langmuir of formic acid. This result indicates that the sticking probability of formate to the adsorption sites on TiOx/Pt(111)-700 is very low, suggesting the presence of an activation barrier to the adsorption of formate to the sites on the as-prepared sample. In contrast, formate saturation is achieved for the TiOx/Pt(111)-873-5 sample upon exposure to less than 50 langmuir of formic acid. 3.3. Configuration of Formate Adsorbed on TiOx/Pt(111). The binding between the adsorption sites on TiOx/Pt(111) and formate was investigated through polarization-dependent SFG measurements of the formate ν(C-D) vibrations. It is worthily to note that, for the adsorbate on a metal surface, the components of the transition dipole moment that are parallel to the surface are invisible in the SFG spectrum likely due to the existence of the underlying image dipole in the metal substrate; however, in the case of the adsorbate on a thin oxide layer which is supported on a metal, the parallel components of the transition dipole moment can be evidently detected in the SFG spectrum,40,43,45 probably due to the screening of the image dipole by the oxide layer. This means that the SFG signal of formate adsorbed on TiOx/Pt(111) is generated when the electric fields of the visible and IR beams have components parallel to the C-D bond, if the C-D bond is tilted from the surface normal, the ν(C-D) signal should be detected under (s, p) combination. Figure 7 shows the SFG spectra of formate on four different TiOx/Pt(111) samples under two different polarization combinations of visible and IR beams. As can be seen in the figure, under the (s, p) combination of polarizations, all the ν(C-D) bands due to formate adsorbed on TiOx (2195, 2134, 2210, 2184, and 2226 cm-1) are absent in the spectra, indicating that these
C-D bonds of formate are oriented normal to the surface. The only tilted bond appears to that at 2227 cm-1 for TiOx/Pt(111)973-5. Under the (p, p) combination of polarizations, this band is obscured by the strong band at 2210 cm-1, and is present as a slight broadening on the high frequency side. Bridge and monodentate configurations of formate have been found on TiO2(110) through STM and HREELS investigations.25,53,54 In the bridge configuration, two oxygen atoms bind individually with two titanium atoms, with the C-D bond oriented normal to the surface. In the monodentate configuration, the formate binds with one titanium atom through one oxygen atom, and the C-D bond is tilted from the surface normal. On the rutile TiO2(111) surface, which has a characteristic large Ti-Ti distance (5.5 Å), it has been suggested that formate is adsorbed in the bidentate form, by which two oxygen atoms bind with one titanium atom and the C-D bond is normal to the surface.53 As discussed above, the structure of the TiOx films on the TiOx/Pt(111)-873-5 and TiOx/Pt(111)-973-5 samples is similar to that of the reduced TiO2(110) surface. Therefore, formate is expected to adsorb to the TiOx/Pt(111)-873-5 and TiOx/Pt(111)-973-5 surfaces in the bridge configuration. That is, the 2134, 2184, 2210, and 2226 cm-1 ν(C-D) bands can be assigned as bridge formate species. The 2210 and 2226 cm-1 ν(C-D) bands correspond to formate adsorbed on ordered surface sites, likely bound through two oxygen atoms of formate to two adjacent 5-fold coordinated Ti4+ sites in the titanium rows. In contrast, the 2134 and 2184 cm-1 ν(C-D) bands, which are associated with defect sites, are likely to be bound through one oxygen atom of formate to a 5-fold coordinated Ti4+ site, as indicated by STM observations,25 with the other formate oxygen atom inserted at an oxygen vacancy binding with a Ti3+
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Figure 8. Change in SFG spectra of formate adsorbed on TiOx/Pt(111)700 with increasing substrate temperature. The baselines of SFG spectra are offset.
defect site beneath the top-layer oxygen. The weak band at 2227 cm-1 detected under (s, p) polarization combination is assigned to the ν(C-D) vibration of monodentate formate. The surface active sites to which the monodentate formate is adsorbed will be discussed below. 3.4. Decomposition of Formate on TiOx/Pt(111). As discussed above, TiOx/Pt(111) prepared under different conditions possesses characteristic surface active sites for formate adsorption, which can be elucidated through analysis of ν(C-D) frequencies. The catalytic properties of these surface active sites in terms of the decomposition of formate were then investigated by conducting SFG measurements while varying the substrate temperature. The SFG spectra for the TiOx/Pt(111)-700 sample are shown in Figure 8. The band at 2195 cm-1 gradually weakens with increasing temperature up to approximately 370 K, above which the band weakens quickly and eventually disappears by 520 K. Only one vibrational band was observed throughout this process. These results indicate that most formate is in the bridge form, which decomposes directly at temperatures above 370 K. The TPD results (data not shown) similarly indicate the onset of CO2 desorption at ca. 340 K, with maximum desorption occurring at 390 K. For TiOx/Pt(111)-873-5 (Figure 9), the band at 2180 cm-1 weakens slowly with increasing substrate temperature, and finally disappears above 425 K. The band at 2226 cm-1 starts weakening sharply at approximately 320 K, and can no longer be detected above 425 K. Interestingly, a new band at 2241 cm-1 emerges at 350 K and remains prominent up to 425 K, indicating that a new species is produced and adsorbed on the TiOx surface at elevated temperatures. Since the vibrational frequencies of the other chemical bonds,35,59,60 ν(CdO) at ca. 1710 cm-1, νa(OCO) at ca. 1560 cm-1, and νs(OCO) at ca. 1360 cm-1, are far away from the recording range, the 2241 cm-1 band can be exclusively assigned to the ν(C-D) vibrational mode. Similar spectral behavior has also been observed in the thermal decomposition of formate on NiO(111) surface,43,45 a vibrational band at higher frequency appeared with increasing substrate temperature, and static and time-resolved SFG investigations revealed that the new band is assignable to monodentate formate, which is the intermediate form produced during the decomposition of bidentate formate. Therefore, the band ob-
Chen et al.
Figure 9. Change in SFG spectra of formate adsorbed on TiOx/Pt(111)873-5 with increasing substrate temperature. The baselines of SFG spectra are offset.
Figure 10. Change in SFG spectra of formate adsorbed on TiOx/ Pt(111)-973-5 with increasing substrate temperature. The baselines of SFG spectra are offset.
served at 2241 cm-1 for TiOx/Pt(111)-873-5 is also likely to be due to monodentate formate. Furthermore, as the 2180 cm-1 band associated with defect-adsorbed formate has weakened to extinction by 325 K, the emergence of the 2241 cm-1 band at 350 K indicates the conversion of bridge formate adsorbed to ordered Ti4+ sites (ν(C-D) at 2226 cm-1) into monodentate formate. The substrate heating experiments thus revealed three formate decomposition processes on TiOx/Pt(111)-873-5: the slow decomposition of bridge formate adsorbed to defect sites, direct decomposition of a fraction of the bridge formate adsorbed to ordered Ti4+ sites at temperatures above 320 K, and decomposition of the remainder of the bridge formate adsorbed to ordered Ti4+ sites through an intermediate, likely monodentate formate, at temperatures above 350 K. Similar spectroscopic progressions were observed for TiOx/ Pt(111)-973-5 (Figure 10). The quality of the spectra is substantially improved in this case, possibly due to a surface enhancement effect61 associated with the thinning of the TiOx film on the Pt(111) with annealing at higher temperature. Upon heating the substrate, the weak band at 2134 cm-1 immediately
Spectroscopic Investigation of TiOx/Pt(111) starts to change, while the band at 2210 cm-1 starts to weaken at about 340 K and eventually disappears at temperatures above 500 K. A band at 2229 cm-1 emerges at 350 K, reaching a maximum at 400 K and then weakening to extinction by 475 K. These progressions indicate that the three formate decomposition processes observed for TiOx/Pt(111)-873-5 also occur on TiOx/Pt(111)-973-5. The frequency of the new band (2229 cm-1) is consistent with that detected under (s, p) polarization (Figure 7C), confirming that the formate formed at high temperatures is in the monodentate configuration. As observed for the other formate bands, the ν(C-D) band of the monodentate formate on TiOx/Pt(111)-973-5 is red-shifted compared to that in TiOx/Pt(111)-873-5. This result provides further evidence that the platinum substrate has a pronounced effect on the chemical properties of the supported TiOx layer. In contrast, however, with increasing temperature in the 350-425 K range, the band at 2210 cm-1 red-shifts by about 10 cm-1 and then blue-shifts back toward the original position while weakening, accompanied by a blue-shift of the band at 2229 cm-1. This behavior is suggestive of interaction between bridge formate and adjacent monodentate formate, as observed for other coadsorbed molecules.62,63 The onset of decomposition of the bridge formate responsible for the band at 2210 cm-1 occurs at higher temperature than for the formate producing the band at 2226 cm-1 (Figure 9). This decomposition onset temperature is close to the temperature of monodentate formate formation (350 K), as evidenced by the strong 2210 cm-1 band at 350 K. This suggests that the near-complete surface coverage of formate on TiOx/Pt(111)-973-5 at approximately 375 K consists of both bridge formate and monodentate formate, and the interaction between these two types of formate would be readily detectable. It can therefore be expected that the monodentate formate, which is converted from bridge formate, binds with the same 5-fold coordinated Ti4+ sites through one oxygen atom, rather than adsorbing to an alternative active site. Simulations of the dehydration process of formic acid on rutile TiO2(110) by first-principle density functional theory (DFT)49 have demonstrated that the direct decomposition of bridge formate on 5-fold coordinated Ti4+ sites is energetically unfavorable, whereas decomposition via monodentate formate is more favorable. In the present case of an ultrathin TiOx film on Pt(111), however, both decomposition processes appear to occur together. As can be seen in Figure 9, the band at 2226 cm-1 due to bridge formate is substantially weakened upon heating to 350 K, accompanied by the emergence of a weak shoulder corresponding to monodentate formate. Although the present SFG analysis suggests that the bridge formate decomposes directly, it is likely that the bridge formate is transiently converted to the monodentate form on the decomposition pathway. The transient formation of monodentate formate cannot be resolved by static spectroscopy, but is likely to be detectable by pump-probe measurements.43–46 The application of timeresolved SFG spectroscopy to this system will be reported in a forthcoming paper. In the decomposition of formate on the present TiOx films, the temperature of the direct decomposition of bridge formate adsorbed to disordered sites or ordered 5-fold coordinated Ti4+ sites appears to be related to the ν(C-D) frequency, with the formate associated with lower-frequency bands decomposing at higher temperatures. It therefore appears that these formate decompositions are related to breakage of the C-D bond, possibly representing dehydrogenation reactions as suggested by the detection of CO2 desorption by TPD for TiOx/Pt(111)700.
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Figure 11. Change in SFG spectra of formate adsorbed on TiOx/ Pt(111)-873-3 with increasing substrate temperature. The baselines of SFG spectra are offset.
The decomposition of formate was also investigated for a TiOx/Pt(111) sample annealed for a shorter period (TiOx/Pt(111)873-3). The surface of this TiOx film has a relatively high density of defect sites coexisting with disordered and ordered sites. Defect sites on TiOx play an important role in surface catalytic reactions. For example, defect sites on TiO2(110) have been found to mediate water dissociation,9 while electron beam irradiation of TiO2(110) to produce surface defects has been shown to change the kinetics of formate decomposition.52 Figure 11 show the progression of the SFG spectrum of formate adsorbed to the defect-rich TiOx/Pt(111)-873-3 surface with increasing substrate temperature. The band at 2200 cm-1 is prominent at temperature above 310 K, and correlates well with the band at 2195 cm-1 observed for TiOx/Pt(111)-700, indicating that the corresponding formate should be adsorbed to disordered surface sites. With increasing temperature, the band at 2184 cm-1 gradually weakens, paralleled by a gradual weakening of the band at 2226 cm-1. The gradual weakening of the 2226 cm-1 band differs from the rapid weakening of the corresponding band observed for TiOx/Pt(111)-873-5. This result indicates that the bridge formate adsorbed to ordered 5-fold coordinated Ti4+ sites decomposes by a different route on TiOx/Pt(111)873-3. The thermal decomposition processes of bridge formate adsorbed on both ordered surface sites and defect sites appear to progress simultaneously. The present authors have previously investigated the motion of CO molecules on Ni(111) by laser-induced temperature jump time-resolved SFG spectroscopy, through which it was revealed that CO molecules undergo site-hopping during temperature jump.64 The diffusion of CO molecules from step to terrace sites on platinum metal surface has also been demonstrated recently under pulsed laser excitation.41 Preliminary results from timeresolved SFG investigations of formate adsorbed on TiOx/ Pt(111)-873-3 also suggest that bridge formate diffuses from the ordered 5-fold coordinated Ti4+ sites to defect sites during laser-induced temperature jumps. The results of such investigations will be reported in detail in a forthcoming publication. Figure 12 shows the estimated proportional coverages of the various formate types on each of four present TiOx/Pt(111) samples. The relative coverage of each species was calculated from the square root of the intensity values, as expressed by eq (1), where the intensity values were determined by fitting the vibrational bands in Figures 8–11 as convolution of Lorentz
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Chen et al. will directly promote the formation of oxygen defect sites. An investigation of hydroxyl species will be conducted in the future. 4. Summary
Figure 12. Estimated proportional coverage of different formate species on (A) TiOx/Pt(111)-700, (B) TiOx/Pt(111)-873-5, (C) TiOx/Pt(111)973-5, and (D) TiOx/Pt(111)-873-3 with respect to substrate temperature.
Formate was used as a molecular probe to investigate the catalytic properties of ultrathin titanium-oxide film on Pt(111) by sum frequency generation spectroscopy. Several kinds of surface active sites are indicated from the adsorption and decomposition behaviors of formate on as-prepared and annealed TiOx films. Formate adsorbed to defect sites is most easily decomposed, with a characteristic decomposition temperature of below 250 K. The temperature of direct decomposition of bridge formate adsorbed to two Ti4+ sites was found to be related to the formate ν(C-D) frequency, with lower frequencies corresponding to higher decomposition temperatures. In addition to direct dissociation, two different decomposition reaction paths were observed for bridge formate adsorbed to ordered Ti4+ sites: dissociation via monodentate formate as an intermediate (350 K), and diffusion of formate from ordered Ti4+ sites to adjacent defect sites before dissociation. The results also suggest that the platinum substrate has a relatively strong influence on the catalytic activity of the supported ultrathin titanium-oxide film, with thinner TiOx films exhibiting a shift in formate ν(C-D) frequency and a change in activation energy for formate decomposition. Acknowledgment. This work was supported by the Solution Oriented Research for Science and Technology (SORST) program of the Japan Science and Technology (JST) Agency, and by the Development in a New Interdisciplinary Field Based on Nanotechnology and Materials Science program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. J.C. greatly appreciates and acknowledges the support of a JST fellowship.
Figure 13. Schematic of formate adsorption and decomposition on TiOx/Pt(111).
shapes. As shown in Figure 12D, formate adsorbed on defect sites starts to decompose at 250 K. The activation energy for formate decomposition on defect sites is therefore much lower than that required to trigger formate decomposition on ordered 5-fold coordinated Ti4+ sites (Figure 12B). On TiOx/Pt(111)873-3, defect sites also act as the final active sites for formate decomposition. With increasing substrate temperature, the formate adsorbed on ordered 5-fold coordinated Ti4+ sites moves to defect sites before undergoing thermal decomposition, resulting in a substantial decrease in apparent activation energy. It can thus be expected that most Ti4+ sites are in close proximity to defect sites on TiOx/Pt(111)-873-3 due to the relatively high density of defects. In contrast, the defect density on TiOx/ Pt(111)-873-5 is much lower, and most of the surface is ordered. A model for the adsorption and decomposition of formate on TiOx/Pt(111) is shown in Figure 13. The formate adsorbed to TiOx/Pt(111) can be classified into three types, and conversion among the three types is possible. The apparent activities for the decomposition of these three formate species are different. The high reactivity of formate adsorbed to defective sites is clearly evident in the present results, indicating that surface diffusion plays an important role in the decomposition of formate. It should be noted that the nature of hydroxyl species on the surface was not examined in detail in the present study. Hydroxyl species are expected to promote the dissociative adsorption of formic acid, and the motion of hydroxyl species
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