J. Phys. Chem. C 2007, 111, 15059-15063
15059
Adsorption States and Dissociation Processes of Oxygen Molecules on Cu(100) at Low Temperature Tetsuo Katayama,† Daiichiro Sekiba,‡ Kozo Mukai,† Yoshiyuki Yamashita,§ Fumio Komori,† and Jun Yoshinobu*,† The Institute for Solid State Physics, The UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan, The Institute of Industrial Science, The UniVersity of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505, Japan, and National Institute for Materials Science, 1-1-1 Koto, Sayo-cho, Sayo, Hyogo 679-5148, Japan ReceiVed: June 19, 2007; In Final Form: July 23, 2007
The adsorption states and dissociation process of oxygen on Cu(100) at 40 K were investigated using highresolution electron energy loss spectroscopy. The dissociation of O2 occurs on the bare Cu(100) surface via a mobile precursor at 40 K. This precursor was not observed as a stable intermediate. The chemisorption state of O2 is a single peroxo species with the O-O stretching energy at 631 cm-1. This peroxo species is stabilized by the interaction with the dissociated oxygen species, and the dissociation barrier from the chemisorbed O2 depends on the coverage of atomic oxygen.
Introduction Oxygen adsorption and dissociation on transition metal surfaces is a fundamental topic in relation to heterogeneous catalytic oxidation. The dissociation of oxygen on transition metal surfaces at low temperature is classified into two mechanisms. One is the direct dissociation with no activation barrier, and the other is the precursor-mediated dissociation with some activation barriers.1 Chemisorbed O2 species on metal surfaces have been investigated with considerable interest because of a possible precursor for dissociation. Chemisorbed O2 species are categorized into peroxo and superoxo species, judging from the charge transfer from the substrate to the oxygen 2π* orbital. The bond order of the peroxo and superoxo species is less and more than one, respectively, while the O-O stretching energy of the peroxo species is about 600-800 cm-1, which is lower than that of the superoxo species (about 8001100 cm-1). Consequently, the peroxo species interact with the substrate more strongly than the superoxo species. On Pt(111),2-6 Ag(110),6-10 Pd(111),11-13 and Cu(111),14-16 the dissociation of oxygen molecules at low temperature is mediated via molecular chemisorption as a precursor. In addition, on Pt(111)2-6 and Pd(111),11-13 the chemisorption is also mediated via physisorption as a precursor. As the temperature rises, the adsorption states of O2 change from physisorption to chemisorption until finally dissociation occurs. Oxygen adsorption and dissociation on Cu(100) have been studied experimentally and theoretically. Yokoyama et al. reported that a physisorbed molecular state was observed at temperatures below 50 K, a chemisorbed O2 existed at temperatures up to 100 K, and above 100 K, atomic species were observed.17 Yata et al. investigated the adsorption and dissociation of O2 on the reconstructed Cu(100)(2x2 × x2)R45°-O surface using a supersonic molecular * Corresponding author. Phone: 04-7136-3320. Fax: 04-7136-3474. E-mail:
[email protected]. † The Institute for Solid State Physics. ‡ The Institute of Industrial Science. § National Institute for Materials Science.
beam technique.18 They proposed that the dissociation mechanism of O2 on Cu(100) was the precursor-mediated dissociation at high coverage of atomic oxygen, and the activation barrier for the dissociation from gaseous O2 was 0.33 eV on the Cu (100)-(2x2 × x2)R45°-O surface. On the clean surface, recent theoretical studies proposed that the dissociation of O2 on Cu(100) was not mediated via molecular chemisorption and the “direct” dissociation was predicted.19-23 Daelen et al.19 reported that O2 dissociated directly over a hollow site from the gas phase when O2 approached parallel to the surface. Puisto et al.21 reported that the presence of the O atom at the follow site made the dissociation barrier at the nearest atop site higher than that of the bare Cu(100) surface. Diao et al. theoretically showed that oxygen molecules dissociated directly on Cu(100) at low coverage and that the dissociation was mediated via a precursor at high coverage.22 Thus, the dissociation mechanism of O2 on Cu(100) remains under debate, and the molecular chemisorbed state on Cu(100) has not yet been fully elucidated in detail. In this study, we investigated the dissociation process of oxygen molecules using high-resolution electron energy loss spectroscopy (HREELS). We found that the dissociation of O2 occurs on the bare Cu(100) surface via a mobile precursor at 40 K. The molecular chemisorbed state is stabilized by a dissociated oxygen species on Cu(100). Experimental Section All experiments were performed in the ultrahigh vacuum (UHV) chamber with a base pressure of ∼8 × 10-9 Pa. The Cu(100) sample was cleaned by several cycles of Ne ion sputtering and annealing to 700 K. We initially performed Auger measurements of the Cu(100) surface using the LEED optics; the detection limit of impurities was less than about 1%. Thereafter, we checked the cleanliness of the Cu(100) surface using HREELS, because HREELS had higher sensitivity to impurities such as C, S, O, CO, and so forth on Cu(100). In addition, we measured the vibrational EELS spectra of Cu(100)c(2 × 2)-CO to check the cleanliness and orderliness
10.1021/jp0747407 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/22/2007
15060 J. Phys. Chem. C, Vol. 111, No. 41, 2007
Figure 1. (i-iv): HREELS spectra for the Cu(100) surface as a function of different exposures of oxygen at 40 K. (v,vi): HREELS spectra for the Cu(100) surface after 3.6 L exposure of oxygen and subsequent 70 and 100 K annealing, respectively.
of the surface, because the CO stretching vibration was quite sensitive to the environments including steps and elemental impurities on the surface. The orderliness was also confirmed by low-energy electron diffraction (LEED). The substrate was cooled to 40 K by a continuous flow of liquid He in the manipulator, and the temperature was monitored by a chromel-alumel (K-type) thermocouple spot-welded to the Ta wire which supported the Cu(100) crystal to the holder. The Cu(100) sample was heated by the radiation, electron bombardment, or both to the rear side of the crystal from a W filament. Ta wires were inserted into two holes of the Cu crystal disk in order to fix the crystal to the sample holder; the thermocouple junction was also inserted into the hole. Thus, the measured temperature by the K-type thermocouple probed the crystal temperature accurately. Oxygen gas was introduced through a pulse gas dosing system onto the sample surface at 40 K. The pulse duration, the number of shots, and the pressure in the gas line were precisely controlled to provide a certain exposure. The exposure was converted into the Langmuir unit by comparing the HREELS spectra using a pulse gas dosing system with those using a backfilling exposure method, while the purity was checked using a quadrupole mass spectrometer (QMS, Pfeiffer Prisma QMS 200). All of the HREELS spectra were taken at 40 K. In the present experiment, the incident electron energy was 5.0 eV, and the HREELS spectra were recorded in specular at an incident angle of 60°. Intensities of about 105 counts were routinely obtained in the reflected elastic beam at 4 meV resolution (LK Technologies ELS5000). Results and Discussion Figure 1i-iv shows the HREELS spectra for the Cu(100) surface exposed to 0-3.6 L of O2 at 40 K. Figure 1v,vi shows the HREELS spectra for Cu(100) exposed to 3.6 L of O2 and subsequent annealing at 70 and 100 K, respectively. Since no significant change was observed in the HREELS spectra above
Katayama et al. 2.7 L, the surface was saturated by 2.7 L O2 at 40 K. In Figure 1ii-iv, several peaks were observed. In Figure 1ii, broad peaks were observed between 362 cm-1 and 421 cm-1. With an increase in the exposure of oxygen (Figure 1iii,iv), the peak at 421 cm-1 is shifted to 435 cm-1, and additional peaks of 170, 336, 631, 1230, and 1534 cm-1 were observed, the assignment of which is discussed below. Since the ν(O-O) mode of oxygen in gas phase is observed at 1556 cm-1, a peak at 1534 cm-1 in Figure 1iii-iv is assigned to the ν(O-O) mode of a weakly bonded physisorption.24 On Pd(111)11, the ν(O-O) mode of the physisorbed state was observed at 1585 cm-1. The small energy shift from the ν(OO) mode of gaseous O2 indicates that the physisorbed state weakly interacts with the substrate. In Figure 1v, the peak at 1534 cm-1 disappeared after annealing at 70 K. This behavior agrees with the previous TPD experiment.17According to previous studies of O on Cu(100),25,26 a peak at 362 cm-1 was assigned to ν(Cu-O) of atomic O at the fourfold hollow site, and a peak at 336 cm-1 was assigned to ν(Cu-O) of atomic O at the lower coordination site such as bridge sites. These peaks were not due to ν(Cu-CO) of CO impurities, because no ν(C-O) peak (∼2080 cm-1) was observed within the detection limit of HREELS (usually
