Al2O3 Thin Films for Investigation of Catalytic Solid−Liquid

J. Phys. Chem. B 2001, 105, 3187-3195

3187

Pt and Pt/Al2O3 Thin Films for Investigation of Catalytic Solid-Liquid Interfaces by ATR-IR Spectroscopy: CO Adsorption, H2-Induced Reconstruction and Surface-Enhanced Absorption Davide Ferri, Thomas Bu1 rgi,* and Alfons Baiker Laboratory of Technical Chemistry, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Zu¨ rich, Switzerland ReceiVed: June 22, 2000; In Final Form: October 16, 2000

Model platinum catalysts have been designed to study the platinum-solvent interface in situ using attenuated total reflection (ATR) infrared spectroscopy. Pt and Pt/Al2O3 thin films were evaporated on a Ge internal reflection element (IRE) and characterized by XRD, XPS, AFM, STM, and IR spectroscopy. Changes within the adsorbate layer of the Pt catalyst during cleaning with O2 and H2 were followed. After cleaning, the catalyst surface was probed by CO adsorption from CH2Cl2. For the Pt/Al2O3 film the spectrum of adsorbed CO showed a band at 2000 cm-1, which is typical for Pt/Al2O3 catalysts. The stretching vibration of linearly bonded CO exhibited a coverage-dependent frequency shift due to vibrational coupling, thus showing the existence of large clean domains on the reactive catalyst surface even in the presence of an organic solvent. CO adsorption from CH2Cl2 was slow before the cleaning process. However, subsequent admission of H2 resulted in an instantaneous and drastic increase of the CO absorption signal. The origin of this effect is a structural change of the Pt particles induced by dissolved hydrogen, which was directly monitored by ATR spectroscopy using CO as probe molecule. STM investigations showed sintering of the Pt particles upon hydrogen treatment in CH2Cl2 at room temperature, which leads to a surface-enhanced infrared absorption (SEIRA).

Introduction Vibrational spectroscopy plays a key role in fundamental studies of chemical reactions and catalysis at metal surfaces. Rich information on the structure and orientation of adsorbates as well as on their interaction with the metal surface can be obtained from vibrational spectra of the adsorbate layer. Powerful experimental techniques, such as electron energy loss spectroscopy (EELS),1 second-harmonic and sum-frequency generation (SFG),2 and infrared reflection absorption spectroscopy (IRRAS)3 have extensively been used to investigate the metal-gas interface under well-controlled conditions. On the other hand, many catalytic reactions take place at the metalliquid interface. Information gained at the metal-gas interface is not easily transferred to reaction conditions, since the behavior of a catalyst is likely to be different in the presence of a solvent. This insight motivates the development of techniques for the investigation of metal-liquid interfaces under reaction conditions. Attenuated total reflection (ATR)4 is ideally suited for vibrational spectroscopy at the solid-liquid interface. The evanescent wave selectively probes the region near the interface. In this way the contribution from the liquid can be kept reasonably small. Other advantages of the ATR technique are the absence of diffusion problems often encountered in infrared reflection absorption (IRRAS) experiments using thin layer cells and the possibility to optimize the signal-to-noise ratio by multiple reflection techniques. Work utilizing the ATR technique in spectro-electrochemistry has begun to appear in recent years.5 * Author to whom correspondence should be addressed. Fax: ++41 1 632 11 63. E-mail: [email protected].

In these studies a thin metal film deposited on the internal reflection element (IRE) is used as the electrode. The feasibility of the approach has been demonstrated for a variety of electrode materials such as Au, Pt, Pd, Fe, Ag, and Ni (see references in ref 6), although stability of some of the thin metal films in aqueous environment and under the applied potential sweeps is problematic (e.g., Cu and Pt).7,8 In catalysis research, on the other hand, the ATR technique has not received much attention yet. It is one aim of this work to show the feasibility of in situ ATR-IR spectroscopy for the study of liquid-phase heterogeneous catalysis by platinum metals. In the following we present an investigation of Pt and Pt/Al2O3 thin films evaporated on Ge internal reflection elements, serving as models for a supported Pt catalyst, and designed for in situ ATR spectroscopy. Besides ex situ characterization of the optical, electronic, and morphological properties of the films, their behavior in contact with an organic solvent and in the presence of dissolved O2 and H2 was investigated. Furthermore, CO adsorption from organic solvent was used as a probe to characterize the Pt catalyst in situ, to check the cleanliness of the Pt in contact with a commercial organic solvent, and to investigate the structural behavior of the Pt particles under reaction conditions. A few ATR-IR studies on Pt thin films have already been reported in the field of spectro-electrochemistry,9-12 although the used films were considered unstable under some conditions.7 Recently CO electro-oxidation at Pt films was studied using ATR.13 Zippel and co-workers7, 14 studied CO adsorption on Pt and Pd thin films by ATR spectroscopy from both the gas and the aqueous phase. The Pt films were unstable in contact with water, whereas Pd films were more resistant.

10.1021/jp002268i CCC: $20.00 © 2001 American Chemical Society Published on Web 03/31/2001

3188 J. Phys. Chem. B, Vol. 105, No. 16, 2001 Experimental Materials. CH2Cl2 (J. T. Baker) solvent was stored over 5A molecular sieves. Al2O3 tablets (Balzers, 99.3%) and Pt wires (Balzers, 99.99%) were used for electron beam evaporation. The gases supplied by PANGAS were N2 grade 4.5 (99.995 vol %), O2 grade 5 (99.999 vol %), CO (0.5 vol% in Ar), and H2 grade 5 (99.999 vol %). Preparation of Pt and Pt/Al2O3 Thin Films. A trapezoidal 52 × 20 × 2 mm Ge internal reflection element (IRE) (Graseby Specac) was coated at room temperature with Al2O3 and/or Pt thin films by physical vapor deposition with a Balzers BAE370 vacuum coating system. The coating material (Pt or Al2O3) was heated in a graphite crucible by means of an electron beam generated with an incandescent filament in vacuum of about 1.5 × 10-5 mbar for Pt and 5 × 10-6 mbar for Al2O3. A sputtering quartz crystal sensor was used to measure the mass thickness of the films. The deposition rate was 0.5 and 1.0 Å/s for Pt and Al2O3, respectively. The distance between the evaporation source and the IRE target was about 33 cm resulting in even films over the entire IRE. After each use the IRE was polished with 0.25 µ diamond paste and thoroughly cleaned with ethanol. Thin Film Characterization. The evaporated thin films were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning tunneling microscopy (STM), atomic force microscopy (AFM), and ellipsometry. XRD analysis was carried out with a Siemens D5000 diffractometer using Cu KR (λ ) 1.5148 Å) radiation. Al2O3 films with thickness up to 1 µm were prepared for analysis. The morphology of the films was investigated using a commercial scanning tunneling and atomic force microscope (TopoMetrix TMX2000) operating in air. Freshly cut Pt/Ir wire (80% Pt, Goodfellow) was used as the STM tip and micrographs were obtained in constant current mode. For AFM pyramidal silicon nitride tips (TopoMetrix) were used. Small fragments of a Ge IRE were used for analysis. For each sample, different areas were imaged to get representative information on the structure of the films. The structural features were, however, not sensitive to the imaged area. Thickness and refractive index of the Al2O3 film were checked with a Plasmos SD 2300 ellipsometer at an angle of incidence of 70˚ using a He-Ne laser (632.8 nm). XPS measurements were performed on a Leybold Heraeus LHS11 apparatus. X-rays were generated by a Mg source (1253.6 eV) operating at 240 W. The spectrometer energy scale was calibrated using the Au 4f7/2, Ag 3d5/2, and Cu 2p3/2 lines at 84.2, 367.9, and 932.4 eV. Spectra were recorded at constant pass energy of 31.5 eV. No charging was observed for the investigated samples. ATR-IR Spectroscopy. All ATR-IR spectra were recorded on a Bruker IFS-66 spectrometer equipped with a liquid nitrogen cooled medium band MCT detector using a commercial ATRIR unit (Wilks Scientific). Spectra were obtained by accumulating 200 scans at 4 cm-1. The angle of incidence and the active reflections were close to 45˚ and 8, respectively. The experiments were performed using unpolarized light. In situ ATR-IR experiments were carried out at room temperature using a home-built flow-through stainless steel cell. After evaporation of the thin films the coated IRE was mounted within the walls of the cell by means of two viton O-rings on the side on which the solution was admitted and a nitrile O-ring on the other side. The gap between the IRE and the cell wall was 200-300 µm.

Ferri et al. The solvent was stored in two separate reservoirs equipped with glass frits where it could be saturated with different gases. The solvent was pumped through the cell at 1.7 mL/min by means of a microdosing system (MDP-2, SWIP). A 3-way PTFE valve allowed switching between the reservoirs. Teflon tubing was used. The time required to pump the solvent from the reservoir to the cell was 2.5 min. The following protocols were used for the experiments. After assembling the cell and alignment of the optics the probe chamber was purged with dried air overnight. The solvent was saturated with N2 for about 1 h and then circulated through the cell and recycled in the reservoir. The ATR signal was allowed to stabilize (2 h), and the reference spectrum was recorded, before proceeding with the experiment. Spectra are presented in absorbance units as A ) -log(I/I0), where I and I0 are the reflected intensity of the sample and reference, respectively. When applying the cleaning procedure O2-saturated solvent from a second reservoir was pumped through the cell (30 min), followed by H2-saturated solvent (max. 30 min). Spectra were acquired at 5 min intervals. The cleaning procedure was repeated one, two, or three times either with or without recycling the solvent. CO adsorption was performed by flowing CH2Cl2 saturated with 0.5% CO in Ar over the Pt films. In the following, the gas-saturated solvent will be named according to the gas (N2, O2, H2, CO) for simplicity. In test experiments the above procedures were also applied to the bare Ge IRE and the IRE coated with a Al2O3 thin film. Results Thin-Film Characterization. Optical properties. Pt and Al2O3 films with different thickness were characterized. For this purpose layers of 1, 2, and 3 nm Pt and of 10, 50, and 100 nm Al2O3 were evaporated on the Ge IRE. The Al2O3 films showed a broad absorption below 900 cm-1 in the ATR-IR spectrum typical for Al2O3. A weak band around 950 cm-1 can be assigned to germanium oxide, which is present on the IRE before evaporation.8 The Al2O3 films did not significantly attenuate the IR light above 900 cm-1. The Pt thin films on the other hand absorbed the IR radiation over the whole mid-IR region, although no distinct absorption bands could be observed. For the 1 nm film about 15% of the IR intensity was absorbed by the platinum (for 2 and 3 nm thickness, 60 and 85%, respectively). For the experiments described below film thickness of 1 and 100 nm was chosen for Pt and Al2O3, respectively. The refractive index (n) of the freshly deposited 100 nm Al2O3 layer was 1.62 as determined by ellipsometry, indicating that the evaporated layer is amorphous. Crystalline Al2O3 is characterized by a refractive index slightly higher than 1.7.15 The obtained value for n is in good agreement with previously reported data for films prepared by MOCVD,16 ion beam evaporation,17 and rf sputtering.18 Amorphous surfaces were previously obtained by sputtering Al2O3 on ZnSe (n ) 1.67).19 In contrast, Sperline and coworkers assumed the optical properties of sputtered Al2O3 films to be identical to the ones for crystalline Al2O3.20 XRD analysis of up to 1 µm thick Al2O3 films confirmed that the evaporated metal oxide is amorphous. XPS Analysis. Figure 1 shows the Pt 4f XPS spectra of the 1 nm Pt/100 nm Al2O3 and 1 nm Pt films on Ge and demonstrates that the platinum in both thin films was in the reduced state. For the Pt film on Ge a shoulder on the high binding energy side of the Pt 4f5/2 line at about 77 eV indicates a small fraction (about 1%) of oxidized Pt. An important point emerging from the XPS analysis is that the d valence bands of both Pt films closely resemble the ones of a clean Pt(111) single crystal

Pt and Pt/Al2O3 Films for Study of Catalytic Interfaces

J. Phys. Chem. B, Vol. 105, No. 16, 2001 3189

Figure 1. XPS spectra of Pt films and Pt(111) sample as reference showing C 1s, Pt 4f, and the valence band region. (a) clean Pt(111) sample, (b) 1 nm Pt on Ge, (c) 1 nm Pt/100 nm Al2O3 on Ge.

sample with bands at 1.6 and 4.5 eV, indicating the metallic character of the Pt films (Figure 1). For the Pt/Al2O3 film the Al 2s signal was detected at 119.6 eV, in close agreement with the Al 2s signal at 119.3 eV measured for a commercial 5% Pt on Al2O3 catalyst (Engelhard 4759). From the ratio of the integrated signals Pt 4f/Al 2s, using the sensitivity factors reported by Wagner,21 a surface atomic ratio of Pt/Al was derived as 63/37. No signal of the underlying Ge IRE could be detected, showing that the 100 nm Al2O3 film is continuous. On the other hand Ge signals were detected for the 1 nm Pt film on Ge. Ge 3d lines with an intensity ratio of 2:1 were found at 29.1 and 32.4 eV, indicative of Ge and GeO2. The Pt/Ge surface atomic ratio was close to 70/30. After treating the 1 nm Pt film on Ge at 300 ˚C for 1 h in flowing H2 the XPS spectrum exhibited drastic changes which could be traced to an interfacial reaction and the formation of PtGe.22 The binding energy of the Pt 4f lines increased by about 1 eV and the lines became sharper. Furthermore, the band in the valence spectrum at around 1.6 eV vanished and the Pt/Ge surface atomic ratio decreased to 50/50. The Pt/Al2O3 film on Ge was treated in CH2Cl2 saturated first with N2, then CO, H2, and again N2 for 1 h each. Before and after this treatment the XPS spectral features were very similar showing that the thin Pt films are stable under these conditions. In particular the Pt/Al surface atomic ratio did not change significantly. The XPS analysis furthermore showed that carbon contaminants covered the Pt films as expected for samples exposed to air. About 20% of the C signal appeared around 288 eV, indicating a substantial amount of carbonate-like species (Figure 1). Morphological Characterization. On the micrometer scale the bare Ge IRE surface was characterized by scratches from the polishing procedure on the otherwise flat IRE as revealed by AFM and STM. The scratches were still visible after evaporation of a 100 nm Al2O3 thin film onto the IRE. Figure 2a shows a 100 × 100 nm STM image of such a thin film in the vicinity of a scratch. The surface is characterized by agglomerates of flat particles of about 20 nm size. Upon evaporation of 1 nm Pt onto the Al2O3 film the STM image changed drastically (Figure 2b). Smaller particles were clearly visible and the conductivity of the sample (as probed by the STM) was much higher than before evaporation of Pt. The individual particles, which form a densely packed island film, were oblate ellipsoidal shaped

Figure 2. STM images (100 × 100 nm) of (a) a 100 nm Al2O3 film, and (b) a 1 nm Pt on 100 nm Al2O3 film, evaporated on Ge.

(flat and round) with a typical height of 1 nm and a diameter of 6 nm. We assign these structures to Pt particles. For the Pt film evaporated directly on the Ge IRE a similar film structure was observed (Figure 3a). In comparison to the Pt on Al2O3 film the individual Pt particles were a little smaller and elongated, typically of 1 nm height, 3-4 nm width, and 6 nm length. Furthermore, the Pt particles were oriented on the Ge. In the top part of Figure 3a the Pt particles are preferentially oriented from bottom left to top right, whereas in the center of the image the preferential orientation is horizontally. The observed orientation of the Pt particles indicates that the Pt island growth is influenced by the underlying polycrystalline Ge. The influence of the treatment applied in the ATR experiments, as described below, on the structure of the Pt films was also investigated by STM. Figure 3b shows the STM image of the same Pt thin film as shown in Figure 3a after treatment in CH2Cl2. The treatment consisted of immersing the sample in CH2Cl2 solvent, which was subsequently saturated with N2, CO, H2, and N2 again for 1 h each. After treatment the sample was rinsed with CH2Cl2 and imaged with the same STM tip as used to record the image in Figure 3a. Comparison between Figures 3a and 3b shows that after treatment the Pt particles are larger than before with a typical diameter and height of 7 and 1.5 nm, respectively. Also the Pt particles are much rounder and less

3190 J. Phys. Chem. B, Vol. 105, No. 16, 2001

Ferri et al.

Figure 4. ATR-IR spectra showing the effect of dissolved O2 and H2 on the Pt/Al2O3 thin film. After (a) 30 min O2 flow, and (b) 10 min H2 flow. The circle indicates not compensated solvent signals (CH2 outof-plane deformation, 1264 cm-1). The asterisk indicates signals from atmospheric CO2.

Figure 3. STM images (100 × 100 nm) of a 1 nm Pt film evaporated on Ge: (a) as evaporated, and (b) after N2, CO, and H2 treatment in CH2Cl2.

oriented (if any) after treatment. The STM analysis reveals that the described treatment leads to a sintering of the Pt particles. As a further characterization of the bare evaporated Al2O3 film, pyruvic acid adsorption from CH2Cl2 was studied by ATRIR spectroscopy and compared to commercial R-Al2O3.23 Similar spectra of pyruvate species were recorded on both surfaces indicating similar base properties. In Situ ATR-IR Spectroscopy. Influence of H2 and O2 on the Pt Films. Platinum thin films sputtered on Ge have previously been found to be unstable when contacted with water resulting in a peeling of the film.7 The Pt films investigated in this work were stable in CH2Cl2, even when the solvent was saturated with CO, H2, or O2. Over several hours contact time no significant increase of the reflected intensity was observed, which would indicate a loss of Pt. When a reactive metal sample, such as platinum, is exposed to air a thin contamination layer is formed. It is known that oxidation/reduction cycles have a cleaning effect on Pt.24 The ATR experiments as described in the experimental section allow changes in the adsorbate layer at the platinum/solvent interface to be followed in situ. When the Pt thin films were contacted with O2- and H2-saturated CH2Cl2, the ATR-IR spectra exhibited dramatic changes. Figure 4 shows the ATR spectra for the Pt/ Al2O3 thin film after the first O2/H2 cycle. Spectrum (a) was

recorded after admission of O2 and spectrum (b) after subsequent admission of H2. For the reducing step (Figure 4b) strong negative peaks at ca. 2040 cm-1, 1570, and 1350 cm-1 and a weaker negative peak at 1830 cm-1 were observed. The signals at 2040 and 1830 cm-1 are associated with linearly bound and bridged CO. Sharp positive signals were observed at 1338 and 1128 cm-1. Figure 4b also shows, in contrast to Figure 4a, a sharp positive band at 1264 cm-1. This is the non-compensated CH2 out-ofplane deformation mode of CH2Cl2, which is the strongest signal in ATR spectra of the solvent. The presence of this band shows that, with respect to the reference spectrum, more solvent is probed by the evanescent wave, in agreement with removal of the contamination layer. When the reducing step with H2 lasted more than 10 min, a CO peak started to slowly grow near 2000 cm-1. When a second and third O2/H2 cycle were applied, CO and the species associated with the band at 1338 cm-1 were removed by oxygen and reappeared upon hydrogen flow. The 1570 and 1350 cm-1 bands showed opposite behavior being formed with oxygen and removed with hydrogen. In the O-H stretching region a broad signal at around 3175 cm-1 appeared with H2 and disappeared with O2 flow. A weaker broad signal at 3500 cm-1 showed opposite behavior, decreasing and increasing during H2 and O2 flow, respectively. The described spectral changes between subsequent O2 and H2 treatment became smaller with the number of cycles applied. It should be noted that the observed spectral changes upon O2 and H2 treatment were fully reproducible and that no significant spectral changes were observed when applying the same treatment to the Al2O3-coated IRE. The Pt thin film on the Ge IRE showed basically the same behavior as described above for the Pt/Al2O3 thin film on Ge. However, the relative intensities of the various bands were different, indicating another composition of the initial contamination layer. CO Adsorption. CO adsorption from CH2Cl2 on the Pt films was carried out with and without preceding O2/H2 (oxidation/ reduction) cycles as described above. CO adsorption was also attempted from the gas phase on the dry Pt and Pt/Al2O3 films at room temperature. After several hours adsorption time no CO signal could be detected in the ATR spectra to within our detection limit (approximately 2 × 10-4), showing that CO can

Pt and Pt/Al2O3 Films for Study of Catalytic Interfaces

J. Phys. Chem. B, Vol. 105, No. 16, 2001 3191 TABLE 1: Vibrational Frequencies (peak maximum) for Linearly Adsorbed CO on Pt and Pt/Al2O3 Thin Films As Obtained from ATR-IR Spectra; Comparison between Adsorption with and without Preceding O2/H2 Cleaning Cycles for Low (LC) and High Coverage (HC) without cleaning

O2/H2 cleaning

LC, before H2 HC, before H2 after H2 Pt Pt/Al2O3

Figure 5. ATR-IR spectra of CO adsorbed from CH2Cl2 on (a) Pt and (b) Pt/Al2O3 thin films after applying the O2/H2 cleaning. Spectra are offset for clarity.

Figure 6. ATR-IR spectra showing CO adsorption on the Pt thin film after applying the O2/H2 cleaning procedure. Spectra are measured at 5, 10, and 20 min intervals (overall adsorption time of 2 h) and are slightly offset for clarity. The intensity of the linearly bound CO is increasing with adsorption time.

not to a significant extent displace the contamination layer under these conditions. CO Adsorption from CH2Cl2 after O2/H2 Cycles. Figure 5 shows ATR spectra, which were obtained after one O2/H2 cycle and after CO adsorption for (a) the Pt and (b) the Pt/Al2O3 thin film on Ge. Only signals corresponding to linear (COL, 2058 cm-1) and bridged (COB, 1830 cm-1) CO could be detected. For COL the initial spectra, recorded at low coverage, displayed a signal at ca. 2030 and 2010 cm-1 on Pt and Pt/Al2O3, respectively. A blue shift of 28 and 48 cm-1 was observed with increasing CO coverage. For bridged CO the signal appeared at 1822 cm-1 for low and shifted to 1830 cm- for high coverage (∆ν ) 8 cm-1). More than 1 h was required for complete stabilization of the CO signal. The coverage-dependent blue shift of the COL signal on the Pt film is depicted in Figure 6. Table 1 summarizes the vibrational frequencies for ν(CO)L on Pt and Pt/Al2O3 measured under different conditions. Figures 5 and 6 also show that for the Pt-coated IRE COL displayed a slight negative-going peak at the high-energy side, which was not observed in the case of Pt/Al2O3. The COL signal was generally broader for the Pt film. A shoulder on the main COL signal could be detected at about 2000 cm-1 for Pt/Al2O3.

2046 2050

2058 2058

2044 2049

LC

HC

2030 2010

2058 2058

The shape of the band associated with COB was also different for the two films. In particular the COB band was broader for the Pt/Al2O3 film, as can be seen from Figure 5. No CO signals could be detected on the bare Al2O3 film showing that the signals in Figure 5 are exclusively associated with CO bound to Pt. The frequency of adsorbed CO is affected by concomitant adsorption of other molecules depending on their electron donor or acceptor character.25,26 To study the influence of O2 and H2 on the signal, CO was adsorbed first without complete saturation. For the Pt film subsequent H2 flow slowly caused a red shift (2058 to 2046 cm-1) of the COL band and a slight increase in absorbance. After 1 h of flowing H2 the COL band started to decrease rapidly and vanished almost completely. However, a small COL signal could still be observed after 3 h on stream. The COB signal also appeared constant first and shifted to about 1820 cm-1. In parallel to the decrease in COL, COB was completely removed. The same process was faster on Pt/Al2O3. The strong decrease of the COL signal started after about 30 min of flowing H2. Figure 7a shows a plot of the peak height of the COL signal as a function of time for Pt/Al2O3. Note the S-shaped curve for the COL signal as a function of time during H2 flow. For the Pt/Al2O3 film the signal at about 2000 cm-1 became prominent with flowing H2. The CO associated with this band was the most resistant towards removal by H2. With increasing number of CO/H2 cycles adsorption and removal of CO became slower (Figure 7a) and the signal at 2000 cm-1 more pronounced. Correspondingly, COB also exhibited a signal close to 1800 cm-1, whereas the signal at 1830 cm-1 disappeared upon flowing H2. The behavior of the COL signal under a stream of O2 was different. On Pt/Al2O3 COL was first blue shifted to 2063 cm-1 (∆ν ) 6 cm-1), consistent with concomitant O2 adsorption, and then back again to 2060 cm-1. The absorbance decreased about 30% in 1 h, as can be seen from Figure 7b. CO was almost completely removed by the following H2 treatment and could easily be readsorbed afterwards (Figure 7b). CO Adsorption from CH2Cl2 without Preceding O2/H2 Cycles. CO adsorption was also studied without preceding O2/H2 treatment of the thin films. Figures 8 and 9 display the CO spectra recorded (a) before and (b) after H2 admission for Pt and Pt/Al2O3, respectively. The experiments were performed as follows: N2-saturated solvent was pumped over the IRE for about 2 h. When CH2Cl2 saturated with CO was then admitted to the Pt thin film on Ge a weak and broad band at 2046 cm-1 started to grow slowly reaching 2058 cm-1 at high coverage (Figure 8a). The main ν(CO)L values are summarized in Table 1 for Pt and Pt/Al2O3. In the region below 1950 cm-1 a slight increase in the baseline was observed together with the formation of a broad signal at around 1820 cm-1. These CO bands were still growing after 4 h. When N2-saturated CH2Cl2 was pumped over the surface the signals remained constant, indicating that adsorbed CO is stable under these conditions. Upon admitting H2 after CO adsorption an instantaneous and drastic change of the CO spectrum was observed (Figure 8b).

3192 J. Phys. Chem. B, Vol. 105, No. 16, 2001

Ferri et al.

Figure 9. ATR-IR spectra showing CO adsorption on the Pt/Al2O3 thin film without preceding O2/H2 cleaning. The experimental procedure was similar to the one for Figure 7. Spectrum (a) was recorded after 30 min CO adsorption. Spectra (b) were recorded 5 (black trace) and 95 min (grey trace), respectively, after switching to H2.

Figure 7. Time-dependence of COL signal (maximum intensity) measured for the Pt/Al2O3 film. Before recording the spectra O2/H2 cleaning was applied. Treatment: (a) CO- and H2-saturated CH2Cl2 were alternately pumped over the film; (b) CO, O2, H2- and COsaturated CH2Cl2 were pumped successively over the film.

independent of the amount of adsorbed CO. A small negative signal was also observed at about 1570 cm-1. The shape of the COL band also changed: After H2 admission a negative-going peak at higher wavenumber was observed for the Pt film. The strong COL band decreased in intensity and shifted to lower frequency with time, indicating hydrogen-induced CO removal from the metal surface. After 4 h the signal was reduced to about 30% of its largest intensity. A broad band near 1820 cm-1 associated with COB was also enhanced and then removed by hydrogen. CO could be readsorbed again by flowing COsaturated CH2Cl2 over the film. Readsorption was much faster than for the first adsorption before flowing H2. For the Pt/Al2O3 thin films similar behavior was observed. Comparison between spectra (a) and (b) (black trace) in Figure 9 shows that the intensity of the COL signal increased drastically upon admission of H2. However, for COL no negative-going peak was observed after H2 admission (Figure 9b), in contrast to the Pt film. The low-energy side of the COL signal displayed a shoulder at 2000 cm-1 that was not observed for the Pt-coated IRE. This shoulder appeared clearly when flowing H2 and became prominent at very low CO coverage (Figure 9b, grey trace). Similarly, the COB signal at about 1850 cm-1 (high coverage) broadened toward lower wavenumbers at lower coverage. Together with the 2000 cm-1 signal a broad band appeared at about 1780 cm-1 concomitant to removal of the 1850 cm-1 signal. CO adsorption was not observed on the bare Ge IRE. Discussion

Figure 8. ATR-IR spectra showing CO adsorption on the Pt thin film without preceding O2/H2 cleaning. N2-saturated CH2Cl2 was first pumped over the film. After the following CO adsorption (3 h) spectrum (a) was recorded. Thereafter H2-saturated solvent was pumped over the film. Spectra (b) were recorded 5 (black trace) and 40 min (grey trace), respectively, after switching to H2.

The signal at 2058 cm-1 disappeared and a very strong absorption was detected at 2044 cm-1. Typically after admission of H2 the signal was about three to four times higher than before,

The STM results show that the evaporated Pt thin films consist of particles forming a densely packed island structure. The nanometer thin Pt films do not completely cover the underneath Ge IRE (Al2O3 film). On the other hand the 100 nm thick Al2O3 film does, as deduced from the complete absence of a Ge signal in the XPS. XPS furthermore shows that the Pt is in the reduced state and that the d-valence band exhibits the typical metallic features. From these morphological and electronic structure points of view the Pt thin films represent promising model systems for a supported Pt catalyst. In this respect the Pt/Al2O3 film is preferable over the Pt film on Ge. Ge forms alloys with Pt at relatively low temperatures as shown

Pt and Pt/Al2O3 Films for Study of Catalytic Interfaces here and elsewhere.22 Furthermore, Ge or GeO2 has the potential to migrate on the Pt surface and act as a catalyst poison. The investigated films are stable in CH2Cl2. Over several hours no indication of loss of Pt could be detected. From a spectroscopic point of view the thin nanometer thick Pt films are transparent enough to apply multiple internal reflection techniques resulting in an enhanced signal to noise ratio. When working with catalytically active metals cleanliness is a crucial point. Attempts to adsorb CO from the gas phase onto the freshly deposited dry films were unsuccessful, confirming the existence of a resistant contamination layer on the Pt, in agreement with the XPS results. On the other hand, CO adsorption from CH2Cl2, although very slow, was possible even without preceding cleaning. This shows the different behavior of the Pt in contact with gas or solvent. Adsorption was fast when CO was admitted after O2/H2 treatment. We attribute this to the cleaning effect of the O2/H2 treatment. Figure 4 shows that mostly H2 is effective. Gas phase O2/H2 pre-treatment has already been shown to have a cleaning effect on Pt/Al2O3 catalysts.24 However, in the gas phase relatively high temperatures (300 °C) are required for the cleaning, whereas our results show that in the liquid phase O2/H2 treatment is effective already at room temperature. Cleaning can be viewed as a two-step process. In a first step contaminants are reduced (oxidized) and in a second step the reduction (oxidation) products (with lower desorption energies) desorb from the surface. The fact that at the solid-liquid interface this cleaning is effective at room temperature, whereas temperatures around 300 °C are required at the solid-gas interface, indicates that the solvation energy plays an important role for the removal of species from the surface, which were generated by reduction (oxidation). The function of the hydrogen in the cleaning is therefore to reduce contaminants on the catalyst. The solvent enhances the desorption of the reduced contaminants through its much larger dissolution power compared to a gas (dissolution power is proportional to density of solvent). The cleaning effect of oxygen and hydrogen at room temperature is directly evident from the ATR spectra shown in Figure 4. Upon contact with H2 in CH2Cl2 broad negative peaks appear at 1570 and 1350 cm-1. We assign these peaks to carbonate-like species,27 which are removed from the Pt surface by H2, in agreement with the XPS results depicted in Figure 1, which show the presence of carbonates on the film. As judged from the relative intensity of the CO and carbonate bands, and the fact that adsorbed CO is a very strong absorber, more carbonates than CO are removed from the surface. The sharp positive bands at 1338 and 1128 cm-1 match well with the strong C-H deformation and C-C stretching vibrational bands of ethylidyne (C-CH3) on Pt.28 At room temperature and in the presence of hydrogen ethylidyne is the most stable C2 hydrocarbon on Pt and a ubiquitous spectator species under hydrogenation conditions.29 An assignment of the two bands to ethylidyne seems therefore likely. Note that the observed changes in the adsorbate layer occurred very fast (within minutes) upon admission of hydrogen. The spectrum hardly changed afterwards. An interesting behavior is observed for the ν(O-H) region from 2900 to 3600 cm-1. Figure 4 shows a negative going signal at around 3500 cm-1 and a more complex positive signal at ca. 3100 cm-1 on H2 admission during the O2/H2 cleaning. The observed spectral changes were reversed with subsequent admission of O2. We assign the signals to water at the metalsolvent interface, since the spectra do not correspond to bulk water. It is evident from Figure 4 that the water is in different

J. Phys. Chem. B, Vol. 105, No. 16, 2001 3193 environment in the presence and absence of the contamination layer and (not shown) the presence of either H2 or O2. In particular, when changing from O2 to H2 water which gives rise to the signal at 3500 cm-1 is converted or reoriented to water giving rise to the signal at 3100 cm-1. Hence, in the latter case the water seems to be more hydrogen-bonded, which is consistent with the larger intensity of the 3100 cm-1 band. Similar reorientation effects have been described recently at Au electrochemical interfaces,30 which supports our assignment of the signals between 2900 and 3600 cm-1 to water. In these cases the water was observed to reorient with the applied potential. Our results indicate that H2 and O2 can induce similar reorientation of the interfacial water. The water stems from residual water in CH2Cl2 and likely also from water formation from O2 and H2 on the Pt film. CO adsorption can be used to probe for the cleanliness of the Pt, since it will only adsorb on free (“clean”) sites. The much faster CO adsorption after treatment with O2/H2 than before confirms the cleaning effect also observed in the spectra (Figure 4). The most important piece of information concerning cleanliness that is obtained from the CO adsorption studies is, however, the observed frequency shift with CO coverage (Figure 6). After “cleaning” with O2/H2 the COL band is observed at ca. 2030 cm-1 at low coverage. With increasing coverage the band shifts up to 2058 cm-1. The observed frequency is in good agreement with reported values for Pt and Pt/Al2O3.31 A coverage-dependent frequency shift of linearly adsorbed CO is known to arise from vibrational coupling between CO molecules in the adsorbate layer. This phenomenon is well studied on single crystals and supported Pt particles32 and has recently also been observed on colloidal Pt particles.33 It arises when domains of adsorbed CO are formed. We suggest that the coveragedependent frequency shift observed in our experiments (Figure 6) has the same origin. The CO adsorption experiments therefore show that after O2/H2 treatment at room temperature large “clean” domains exist on the Pt surface, even in the presence of a commercial solvent. The frequency change of COL upon coadsorption of a second molecule is a typical feature observed for Pt. Changes of the electron density of the metal lead to blue or red shifts of the COL signal, depending on the electron acceptor or donor nature of the coadsorbate.25,26 For example, oxygen decreases the electron density of the metal, thus lowering the extent of back donation to the 2π anti-bonding orbital of CO. This increases the C-O bond strength and stretching frequency. Hydrogen has the opposite effect, weakening the C-O bond and lowering the CO stretching frequency as observed for commercial catalysts.34,35 We also observed frequency shifts when admitting O2 or H2. When H2 was admitted to the adsorbed CO, which was adsorbed after “cleaning”, the COL and COB stretching modes shifted to lower frequency, whereas in the presence of O2 a blue shift was observed initially. This was followed by a red shift when the CO coverage was significantly decreasing due to CO oxidation. The frequency of COL furthermore showed that the state of the Pt before and after cleaning was different. As mentioned above, CO adsorption from CH2Cl2 was observed, although very slowly, on “non-cleaned” Pt with a COL stretching frequency of 2046 cm-1 at low coverage. On the “cleaned” Pt film the frequency of the initially adsorbed COL was 2030 cm-1, which shows that the Pt was more electron-deficient before the cleaning. The observed decrease in the COL signal in the presence of hydrogen shows complex time dependence (Figure 7). When hydrogen was admitted to the Pt film after it has been saturated

3194 J. Phys. Chem. B, Vol. 105, No. 16, 2001 with CO, the COL signal stayed almost constant for the first 30 min. Then the signal decreased rapidly. We can certainly exclude diffusion as the origin of this effect in our system. It is known that the CO signal is not completely proportional to coverage due to non-linear effects.36-38 We, however, propose that the observed complex time-dependence of the COL signal at the initial stage of hydrogen admission is due to the kinetics of hydrogen-induced CO removal. A likely reason for the behavior is that the densely packed CO prevents hydrogen to coadsorb, leading to a low CO removal rate. Once part of the CO is removed, the empty sites allow more hydrogen to adsorb and CO removal proceeds faster. This would be in agreement with the known effect of H2-induced “displacement” of CO on Pt.39 The bands at 2000 and 1780 cm-1 (Figure 9) are characteristic for Pt/Al2O3 catalysts,24,34,40,41 whereas unsupported Pt does not exhibit these signals. The origin of the frequency shift of the CO stretching vibration has been attributed to the influence of the oxidic support on the adsorption properties of the metal (2000 cm-1) and to the presence of 3-fold coordinated CO species (1780 cm-1). Accordingly, the 2000 cm-1 signal is not observed for the Pt thin film on Ge, thus confirming the hypothesis of Pt sites associated with the presence of the oxidic support. The occurrence and behavior of the CO signal at 2000 cm-1 underlines the similarity of the Pt/Al2O3 thin films investigated here with supported Pt catalysts. In the latter the CO associated with the 2000 cm-1 band is the most stable in thermal desorption experiments,24,34 whereas on the Pt/Al2O3 films investigated here it is the most resistant towards displacement by hydrogen. An intriguing observation is the sudden increase of the CO absorbance upon admitting hydrogen for both the Pt and Pt/ Al2O3 thin films. This effect was only observed when CO was adsorbed without preceding O2/H2 treatment and was very fast (seconds to minutes). The sudden increase in the CO signal could have two different origins. Either, the CO coverage is increased, or the same amount of CO gives rise to a stronger signal, i.e., the absorption is enhanced. Increased CO coverage itself could have two main origins. (i) CO adsorbs from the solvent after hydrogen has “cleaned” the surface, or (ii) the hydrogen reacts with an adsorbate to produce CO. The former possibility can be discarded, because the effect was also observed when the cell and the tubing were purged with N2saturated CH2Cl2 for 1 h after CO adsorption and before H2 treatment, thus completely removing any dissolved CO. Moreover, a comparison with the CO formed during O2/H2 cycles (Figure 4) excludes that the strong CO signal arises only from hydrogen-induced decomposition of some adsorbed species. We propose that the increase in intensity is due to an enhanced absorption. Several observations support this view. The intensity ratio of the CO signal before and after admission of hydrogen is 3-4 and similar for different experiments, independent of the initial signal intensity. This indicates that the amounts of adsorbed CO before and after hydrogen treatment are proportional to each other. Surface-enhanced infrared absorption (SEIRA) has already been reported for Pt42 and other thin metal films.43 For silver films enhancement factors of up to 1000 were reported.44 The enhancement is believed to arise due to an enhanced electric field within the island film and depends on its morphology (size and shape of particles and interparticle distance).45,46 Furthermore, enhancement is restricted to molecules very near the metal film.47 Figures 2 and 3 show that the evaporated Pt and Pt/Al2O3 films exhibit island morphology, which could give rise to enhanced absorption of adsorbed molecules. More importantly, comparison of the STM

Ferri et al. pictures in Figure 3 shows that hydrogen treatment alters the morphology of the island film. Since the enhancement factor is critically dependent on film morphology, it is likely that the morphological change goes along with a change in the enhancement factor. The film shown in Figure 3b gives the larger enhancement by a factor of about 3-4 than the film in Figure 3a, indicating that larger particles favor enhancement. In light of the above discussion the adsorbed CO serves as the probe by which the morphological changes of the island film induced by hydrogen can be followed in situ. The change in absorbance (enhancement) and morphology of the film is fast, on the order of seconds to a few minutes. This indicates that the platinum is very mobile in the presence of hydrogen, carbon monoxide, and solvent, even at room temperature. The restructuring of the surface may be regarded as sintering of the particles due to admission of the reducing agent and concomitant heat generation. The contamination layer may protect the particles from sintering before hydrogen admission. One effect of hydrogen in this reconstruction process is the removal of the (protective) contamination layer as indicated by Figure 4. Support for this hypothesis comes from the similar rate for the cleaning and sintering processes. From solid-gas studies at high pressure (mbar to bar) it is known that transition metal atoms (such as Pt and Rh) become mobile in the presence of strongly adsorbing molecules (such as CO and H2) even at room temperature. This facilitates reconstruction.48-50 This high mobility can be rationalized using bonding power arguments: When a molecule is strongly bound to a surface metal atom, this atom looses some bonding power by which it is attached to the underneath lattice, since some of the bonding power is used to bind the molecule. Hence, this surface atom is looser bound to the lattice and more mobile. To our knowledge this is the first report where such a reconstruction has been evidenced in situ at the solid-liquid interface at room temperature. Figures 5 and 8 show that for the Pt thin film the COL band displays a second derivative shape with a negative-going signal at the high-energy side and an increased background level at the lower-energy side of the band. This particular band-shape was only observed after reconstruction of the surface. Furthermore, it was not observed for the Pt/Al2O3 thin film. Similar dispersive band shapes have already been observed for example for water over thin Cu films,8 for CO adsorbed from the gas phase on sputtered Pt and Pd films14 and in electrochemical CO oxidation over sputtered thin-film platinum electrodes.13 Several explanations for the peculiar lines-shape have been given including a Fano resonance51 and surface-enhanced infrared absorption (SEIRA) theory.52 We propose another reason for the line-shape. For molecules in contact with a thin metallic layer on a transparent substrate, the dispersive line-shape can naturally emerge from classical electromagnetic theory for certain properties of the metal film (thickness and optical constants) as we will show in detail elsewhere. The real part of the refractive index of the absorbing molecular layer (adsorbate layer or bulk medium) exhibits a dispersive form near an absorption line, which can result in a dispersive band shape in the ATR spectra. Conclusion Using physical deposition we have prepared and characterized thin Pt and Pt/Al2O3 films for ATR-IR spectroscopy at the Ptsolvent interface. STM investigation showed the island structure of the films. XPS revealed the metallic character of the Pt. The films were stable in organic solvent and in the presence of

Pt and Pt/Al2O3 Films for Study of Catalytic Interfaces dissolved O2, H2, and CO. Once exposed to air the freshly deposited Pt films were covered with a resistant contamination layer. ATR-IR spectroscopy revealed the cleaning effect of dissolved O2 and H2. The most prominent effect was the removal of CO and carbonate species upon contact with H2-saturated solvent. Subsequent CO adsorption was fast. The linearly bonded CO furthermore showed a characteristic frequency shift with increasing coverage due to vibrational coupling, which indicates the presence of large clean domains on the Pt, even in the presence of an organic solvent. In contrast, CO adsorption was not observed from the gas phase on the dry films and was very slow from solution before the O2/H2 cleaning. Since cleaning with O2/H2 is not effective in the gas phase at room temperature, our results seem to indicate a favorable effect of the solvent for the cleaning. The Pt/Al2O3 film (but not the Pt film) showed a band at 2000 cm-1 after CO adsorption, which is a characteristic feature of supported Pt/Al2O3 catalysts. The CO associated with this band is the most resistant towards removal by H2 and is associated with Pt-Al2O3 interaction. When H2-saturated solvent was admitted the Pt particles sintered at room temperature as shown by STM. This structural change was very fast and could be monitored by infrared spectroscopy using CO as a probe molecule. The observation that such a reconstruction occurs fast at the metal-liquid interface indicates that the solvent is favorable for this process. Furthermore, reconstruction lead to enhanced infrared absorption. Since reconstruction lead to larger Pt particles (sintering) it can be concluded that the larger particles give rise to bigger enhancement factors. In conclusion, our work indicates that the investigated films, and especially the Pt/Al2O3 film, are suitable model systems for in situ investigation of heterogeneous catalysis at the Ptsolvent interface. The demonstrated similarities with supported Pt/Al2O3, the stability of the films, the possibility to clean the Pt in situ and the good signal/noise ratio due to multiple internal reflection and enhanced infrared absorption are promising for future work in this direction. Acknowledgment. The authors thank Dr. Marek Maciejewski for XRD measurements and ETH for financial support. References and Notes (1) Ibach, H.; Mills, D. L. Electron energy loss spectroscopy and surface Vibrations; Academic Press: New York, 1982. (2) Shen, Y. R. Annu. ReV. Phys. Chem. 1989, 40, 327. (3) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (4) Harrick, N. J. Internal reflection spectroscopy; Interscience Publishers: New York, 1967. (5) Bauhofer, J. Electrochemical applications of internal reflection spectroscopy. In Internal Reflection Spectroscopy: Theory and Applications; Mirabella, F. M., Ed.; Dekker: New York, 1992; Vol. 15, p 233. (6) Johnson, B. W.; Bauhofer, J.; Doblhofer, K.; Pettinger, B. Electrochim. Acta 1992, 37, 2321. (7) Zippel, E.; Kellner, R.; Breiter, M. W. J. Electroanal. Chem. 1990, 289, 297.

J. Phys. Chem. B, Vol. 105, No. 16, 2001 3195 (8) Ishida, K. P.; Griffiths, P. R. Anal. Chem. 1994, 66, 522. (9) Pham, M. C.; Adami, F.; Lacaze, P. C.; Doucet, J. P.; Dubois, J. E. J. Electroanal. Chem. 1986, 201, 413. (10) Pham, M. C.; Aeiyach, S.; Moslih, J.; Soubiran, P.; Lacaze, P. C. J. Electroanal. Chem. 1990, 277, 327. (11) Pham, M. C.; Moslih, J.; Simon, M.; Lacaze, P. C. J. Electroanal. Chem. 1990, 282, 287. (12) Pham, M. C.; Moslih, J. J. Electroanal. Chem. 1991, 310, 255. (13) Zhu, Y.; Uchida, H.; Watanabe, M. Langmuir 1999, 15, 8757. (14) Zippel, E.; Breiter, M. W.; Kellner, R. J. Chem. Soc., Faraday Trans. 1991, 87, 637. (15) Weber, M. J. Handbook of Laser Science and TechnologysOptical Materials: Part 2; CRC Press: Boca Raton, FL, 1986; Vol. 4. (16) Haanappel, V. A. C.; Corbach, H. D. v.; Fransen, T.; Gellings, P. J. Surf. Coat. Technol. 1994, 63, 145. (17) Tan, M.; Tan, S. I.; Shen, Y. IEEE Trans. Magnetics 1995, 31, 2694. (18) Stadler, B. J. H.; Oliveira, M.; Bouthillette, L. O. J. Am. Ceram. Soc. 1995, 78, 3336. (19) Couzis, A.; Gulari, E. Langmuir 1993, 9, 3414. (20) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1992, 8, 2183. (21) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond, R. M.; Gale, L. H. Surf. Interface Anal. 1981, 3, 211. (22) Hsieh, Y. F.; Chen, L. J. J. Appl. Phys. 1988, 63, 1177. (23) Ferri, D.; Bu¨rgi, T.; Baiker, A. To be published. (24) Barth, R.; Ramachandran, A. J. Catal. 1990, 125, 467. (25) Primet, M.; Basset, J. M.; Mathieu, M. V.; Prettre, M. J. Catal. 1973, 29, 213. (26) Primet, M. J. Catal. 1984, 88, 273. (27) Iwasita, T.; Rodes, A.; Pastor, E. J. Electroanal. Chem. 1995, 383, 181. (28) Malik, I. J.; Brubaker, M. E.; Mohsin, S. B.; Trenary, M. J. Chem. Phys. 1987, 87, 5554. (29) Wieckowski, A.; Rosasco, S.; Salaita, G.; Hubbard, A.; Bent, B.; Zaera, F.; Godbey, D.; Somorjai, G. A. J. Am. Chem. Soc. 1985, 107, 5910. (30) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951. (31) Sheppard, N.; Nguyen, T. T. AdV. Infrared Raman Spectrosc. 1978, 5, 67. (32) Eischens, R. P.; Pliskin, W. A. AdV. Catal. 1958, IX, 1. (33) deCaro, D.; Bradley, J. S. Langmuir 1998, 14, 245. (34) deMe´norval, L. C.; Chaqroune, A.; Coq, B.; Figueras, F. J. Chem. Soc., Faraday Trans. 1997, 93, 3715. (35) Benvenutti, E. V.; Franken, L.; Moro, C. C. Langmuir 1999, 15, 8140. (36) Crossley, A.; King, D. A. Surf. Sci. 1977, 68, 528. (37) Moskovits, M.; Hulse, J. E. Surf. Sci. 1978, 78, 397. (38) Hollins, P. Surf. Sci. 1981, 107, 75. (39) Parker, D. H.; Fischer, D. A.; Colbert, J.; Koel, B. E.; Gland, J. L. Surf. Sci. 1990, 236, L372. (40) Haaland, D. M. Surf. Sci. 1987, 185, 1. (41) Barth, R.; Pitchai, R.; Anderson, R. L.; Verykios, X. E. J. Catal. 1989, 116, 61. (42) Nakao, Y.; Yamada, H. Surf. Sci. 1986, 176, 578. (43) Hartstein, A.; Kirtley, J. R.; Tsang, J. C. Phys. ReV. Lett. 1980, 45, 201. (44) Nishikawa, Y.; Nagasawa, T.; Fujiwara, K.; Osawa, M. Vib. Spectrosc. 1993, 6, 43. (45) Osawa, M.; Ataka, K. Surf. Sci. 1992, 262, L118. (46) Nishikawa, Y.; Fujiwara, K.; Ataka, K.; Osawa, M. Anal. Chem. 1993, 65, 556. (47) Osawa, M.; Ikeda, M. J. Phys. Chem. 1991, 95, 9914. (48) Somorjai, G. A. Catal. Lett. 1992, 12, 17. (49) Somorjai, G. A.; Su, X. C.; McCrea, K. R.; Rider, K. B. Top. Catal. 1999, 8, 23. (50) Rasko, J.; Bontovics, J. Catal. Lett. 1999, 58, 27. (51) Fano, U. Phys. ReV. B 1961, 124, 1866. (52) Osawa, M.; Ataka, K. I.; Yoshii, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497.