1P181
SFG 分光法による Pt(111)基板上の TiO2 膜に吸着したフォルメートの観察(東大工*, 東工大資源研**, JST†) ○Jun Chen*†, 久保田純**, 和田昭英**, 堂免一成*†[Introduction]
Titanium oxide is widely applied as catalyst support, photocatalyst, gas sensors, coatings,and etc. These applications are essentially associated with its surface properties, therefore,
which have received considerable studies by FT-IR, TPD, electron spectroscopies and STM
techniques in recent years. In present study, titanium oxide film was prepared on Pt(111) inthe ultra high vacuum (UHV) chamber. Formic acid is used as a molecular probe tocharacterize the surface sites on the titanium oxide film. At the same time, sum frequency
generation (SFG) spectroscopy, which is a highly surface-specific technique, is applied toinvestigate the configurations of surface species. It is observed that bidentate or bridge
formate is formed by the decomposition of formic acid on the titanium oxide film.
Furthermore, we are going to apply the time-resolved SFG technique to this system.
[Experimental]
The SFG setup and the UHV instruments are the same as previous reports [1]. A cleanPt(111) surface was obtained by repeated Ar ion bombardment and annealing at 1000 K until
the cleanliness and orderliness were confirmed by AES and LEED observations. The titaniumoxide film (10-20 Å thickness) was deposited on Pt(111) at 700 K by vacuum evaporation ofTi while 1 × 10-6 Torr O2 flow was presented [2]. After cooled to 250 K, the titanium oxide
film was exposed to 1000 L (1 L = 1 × 10-6 Torr s) of DCOOD. For the SFG measurement,
frequency-tunable IR and 532 nm visible pulse (pulse width of 35 ps) were used. Thoseenergies at sample surface were 50 µJ/pulse (at 2000 cm-1) and 100 µJ/pulse, respectively. For
the TPD record, a 2.8 K/s heating rate was used, and six mass numbers were monitored
simultaneously.

m/e
44 (CO2)MS Intensity (a.u.)[Results and discussion]
The temperature programmed desorption (TPD)
spectra for saturation exposure of formic acid on
titanium oxide film at 250 K is shown in figure 1. One
desorption region clearly appear on the spectra between
340 and 450 K. CO2 is main product, which desorptionpeak locates at 390 K. On the other hand, no DCOOD
signal was detected. This indicates that these desorption
products are not from QMS cracking of formic acid but
from the decomposition of surface species formed after
exposure of formic acid on the titanium oxide film.
Compared with our previous TPD results of formic acidon NiO(111) surface [3], the mass spectrum signals aremuch weak even that of CO2 in this case. This suggeststhat the amount of the species produced on the surfaceof the titanium oxide is small.
Figure 2 shows the SFG spectrum for saturation
exposure of formic acid on the titanium oxide film at
250 K, and the thermal stability of surface species wasinvestigated. Both visible pulse and IR pulse werep-polarization. At 250 K, it exhibits only one

28 (CO)48 (DCOOD)
32 (DCDO)
20 (D2O)
4 (D2)
300

400 500 600 700
Temperature (K)800Fig. 1. TPD spectra for saturation
exposure of DCOOD on titaniumoxide film at 250 K

SFG Intensity (a.u.)

SFG Intensity (a.u.)

vibrational resonance band at 2170 cm-1, which is2170
assigned to the v(C-D) vibrational mode of bidentate
or bridge formate, demonstrating that formate isproduced by the dissociation of formic acid on thesurface of the titanium oxide film. After the formate
450 K
was formed on the titanium oxide film at 250 K, thesubstrate was heated step by step. It is observed that
400 K
the above band is reduced at 400 K and disappears on
further heating to 450 K. Compared with the TPD
350 K
results, this illustrates that formate adsorbed on thetitanium oxide film decomposes at around 400 K and300 K
the main product is CO2. Exposure dependent SFG
spectra were also carried out. It gives that the250 K
vibrational band at 2170 cm-1 achieves strongest
2000 2100 2200 2300 2400 2500
-1
Wavenumber (cm )intensity around 1000 L. This indicates that theadsorption of formate on titanium oxide is very slow
and the apparent sticking probability was Fig. 2. SFG spectra for saturation
exposure of DCOOD on titanium oxideapproximately 0.001.
film at different temperatures
On the other hand, as shown in figure 2, there isno vibrational signal of monodentate formate, which
frequency locates at around 2200 cm-1. Considering(Vis, IR)the band at 2170 cm-1 is broad, we also carried out theSFG measurement with different polarization
combinations. Shown in figure 3, after the change in(s, p)polarization of the visible pulse from p-polarization to-1
s-polarization, the SFG signal at 2170 cm disappears.
This means that the C-D bond, corresponding to thisvibrational band, is vertical to the surface of titaniumoxide film, further confirming this band at 2170 cm-1
belongs to the bidentate or bridge formate because theC-D bond of monodentate is tilted from the surface(p, p)
normal. This behavior of formate adsorbed on thetitanium oxide is different from that on NiO(111), on
which surface both types of formate are formed by the2000 2100 2200 2300 2400
-1
Wavenumber (cm )decomposition of formic acid. This suggests that thedistance between two coordinatively unsaturated
surface Ti cations, which act as lewis acid sites Fig. 3. SFG spectra of formate adsorbed
bonded with formate, is suitable to the formation of on titanium oxide film at 250 K fordifferent polarization combinations
bridge formate.
In summary, formate by the dissociation offormic acid on the titanium oxide film is sensitively detected by SFG spectroscopy. From theorientation of C-D bond, it indicates that formate exists as bidentate or bridge structure.
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