J. Phys. Chem. 1994,98, 7653-7656
7653
IRAS Studies of Adsorbed Ethene (CzH4) on Clean and Oxygen-Covered Cu(ll0) Surfaces Jun Kubota, Junk0 N. Kondo, Kazunari Domen,’ and Chiaki Hirose Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori- ku, Yokohama 227, Japan Received: March 23, 1994; In Final Form: May 27, 1994’
Ethene adsorption on clean and oxygen-modified Cu( 110) surfaces was investigated by infrared reflection absorption spectroscopy (IRAS). The end-on species of adsorbed ethene, as was reported by Jenks et al., was observed on a clean Cu( 110) surface. It was, however, suggested that the adsorbed ethene structures at higher temperature were different from those obtained a t lower temperature. On oxygen-modified Cu( 1 10) surfaces, end-on ethene decreased with the increase of the amount of surface oxygen. The *-adsorption occurred only when the (2 X l)-O/Cu(llO) reconstructed surface was formed. The a-bonded ethene on the (2 X 1 ) 4 / Cu( 110) adsorbed reversibly even at 110 K, and the heat of adsorption was estimated to be 32 f 7 kJ/mol from the analysis of the adsorption isobar obtained by IRAS.
Introduction The adsorption of etheneon various metal singlecrystal surfaces has been well investigated by various techniques.’-’ Ethene is well-known to form nondissociative di-u- and *-coordination on many metal surfaces. However, Jenks et al. recently reported the new orientation of adsorbed ethene on a Cu( 110) surface as revealed by infrared reflection absorptionspectroscopy (IRAS)! For this new species, only CH2 scissors and C-C stretching modes on Cu( 110) were detected by IRAS as strong bands, while the CH2 out-of plane (wagging) mode which gives a strong band for free and *-bonded ones was not observed. Thus, it was concluded that the adsorbed ethene on the Cu( 110) surface was oriented perpendicular to the surface. The ethene adsorption on the Cu( 111) and Cu(100) surfaces was investigated by IRAS and high-resolution electron energy loss spectroscopy (HREELS), and *-coordination of adsorbed ethene was only suggested.5~6The ethene adsorbed on Cu(ll1) and Cu(100) has also been investigated by ultraviolet photoelectron spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) mea~urements.~JThe long-range structure of adsorbed ethene on Cu( 110) was investigated by low-energy electron diffraction (LEED) by Ertl, and it was indicated that the adsorbed ethene was ordered along the surface rows of Cu( 1lO).9 The oxygen adsorption on the Cu( 110) surface has been studied by LEED, Auger electron spectroscopy (AES), work function measurement, UPS, surface extended X-ray absorption fine structure (SEXAFS), and scanning tunneling microscopy (STM).’&12 It has been revealed by previous investigations that the introduced oxygen reconstructs the Cu( 110) surface into a (2 X 1) phase. The growth of the (2 X 1) reconstructed islands was clearly observed by STM.12 The interactions of ethene with oxygen-modified surfaces of Pt( 11l),l3J4 Pd( Ru(0001),16 Ir( 111),17Au( 1 and Ag(l10)19havebeenexamined. On thesesurfaces,thecoadsorbed oxygen induces *-bonding of ethene. This is interpreted as that the adsorbed oxygen withdraws the electrons of metals to inhibit the formation of u-bonding. The heat of adsorption of the adsorbed species under an ultrahigh vacuum condition has been estimated by temperature programmed desorption (TPD) and microcalorimetry.m*21The electron spectroscopies usually need the ultrahigh vacuum condition for their operation, and it is difficult to apply the measurement under adsorption equilibrium conditions. On the Abstract published in Aduunce ACS Abstracts, July 1, 1994.
0022-3654f 9412098-7653SO4.50f 0
other hand, the IRAS, especially the method of polarizationmodulated IRAS, is operative under the range from ultrahigh vacuum to atmospheric pressure. It is thus possible to measure by IRAS the heat of adsorption by equilibrium methods, as was done by Truong et al.” In the present work, the adsorption structures of ethene on clean and oxygen-modified Cu( 110) surfaces were investigated by IRAS and TPD. The influence of oxygen-induced surface reconstruction to the ethene adsorption was discussed. The heat of adsorption of *-bonded ethene on the (2 X l)-O/Cu(llO) surface was estimated by the equilibrium adsorption method.
Experhnental Section The experiments were carried out in an ultrahigh vacuum system (base pressure C 2 X 10-8 Pa) which was evacuated by an oil diffusion pump with a liquid nitrogen-cooled trap. The ultrahigh vacuum system consisted of two chambers. The first chamber for the preparation of surfaces and TPD measurements had four-grid optics for LEED and a retarding field AES, a quadrupole mass analyzer (ANELVA AQA- 100) with a liquid nitrogen-cooled jacket, an Ar+ ion gun, and a gas doser. The second chamber was set up for the IRAS measurements and was equipped with BaF2 optical windows and a gas dosing system. The sample crystal was mounted on the tip of a liquid nitrogencooled translator with two Ta wires. For IRAS measurements,a JEOL JIR-100 Fourier transform infrared (FTIR) spectrometer was used with an InSb and HgCdTe composite detector (Infrared Associates). Spectral resolution was 4 cm-*, and typically 512-1024 scans were collected. The incident angle of the infrared beam to the surface was 84O. To obtain the ratio spectra of p- and s-polarized beams, a wire grid polarizer inserted in the incident beam was revolved by a stepping motor in synchronization with the scan of the interferometer. This system eliminates from IRAS spectra the interference of atmospheric water and C02. Dried air was used for the purge of all optical paths and for the air bearing of the interferometer. The details of the system will be published elsewhere.23 The Cu(ll0) sample (Johnson Matthey Japan Ltd.) was mechanically polished by diamond paste (0.2-pm mesh). The surface was cleaned by cycles of Ar+ ion (1-kV) bombardment at 373 Kand annealingat 873 K. Nocontamination wasdetected by AES, and LEED indicated the (1 X 1) structure of the Cu(1 10) surface after this preparation. For oxygen modification of the Cu( 110) surfaces, oxygen was introduced at 320 K. The amount of preadsorbed oxygen was determined from the ratio of oxygen KLL (KE = 503 eV) and 0 1994 American Chemical Society
Kubota et al.
1654 The Journal of Physical Chemistry, Vol. 98, No. 31, 1994
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1400 1200 1000 800 wavenumber / cm Figure 1. IRAS spectra of adsorbed C2H4 on the Cu(ll0) surface at 110 K at various exposures.
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copper LMM (KE = 920 eV) peaks of AES. The ratio at the maximum coverage of 0.5 ((2 X 1)-O/Cu( 110) structure) under the ultrahigh vacuum condition was used as a reference.
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Figure 2. IRAS spectra of adsorbed CzH4 on Cu( 110) measured at 110 K under 1 X 10-6 Pa of C2H4, after treatment under 1 X 10-6 Pa of C2H4
Results and Discussion
at various temperatures.
1. Clean Cu(ll0) Surface. The IRAS spectra of adsorbed CzH4 are depicted in Figure 1. At low coverage (< 0.1 langmuir),
two peaks were observed at 1275 and 906 cm-1. These bands are assigned to the CH2 scissors and wagging (out-of plane) modes, respectively. The vibrational dipole moments of the scissors and the wagging mode are perpendicular to each other. Therefore, it is considered by the appearance of the both bands that the adsorbed C2H4 was tilted on the surface. The peaks appeared at 1522 and 1257 cm-1 and increased in intensity on increased exposure to C2H4, while the bands at 1275 and 906 cm-I disappeared with the appearance of the new bands. The peaks at 1522 and 1257 cm-1 were assigned to the C = C stretching and CH2 scissor modes, respectively. The transition dipoles of these two modes are parallel to the long axis, and it is clear that the adsorbed species giving 1522- and 1257-cm-1 bands was oriented with the long axis perpendicular to the surface. The absence of the wagging band further confirms this interpretation. The C-H stretching modes are expected to appear around 2800 cm-'; however, no absorption band which was assignable to adsorbates was detected between 1800-3000 cm-I, where the spectra were measured by using an InSb detector which had a 0.02% AR/R of noise level around 3000 cm-1.2G26 The reported values of vibrational frequencies of the adsorbed C2H4 on various Cu surfaces are summarized in Table 1 along with the literature values for the adsorbates on related surfaces. The values obtained in the present study are in excellent correspondence with those reported by Jenks et al.4 An interesting feature showed up in IRAS spectra when the Cu(ll0) sample was exposed to the CzH4 gas at higher temperatures. The Cu(ll0) surface, which was exposed to CzH4 of 1 X 10-6 Pa at various temperatures for 2 min and cooled to
110 K under the C2H4 atmosphere, was measured by IRAS, as shown in Figure 2. The 1522- and 1257-cm-1 bands broadened and decreased in intensity on elevating the exposing temperature to above 135 K. It is thus considered that another structure of adsorbed CzH4 existed when the Cu(l10) surface was exposed to CzH4 at higher temperatures. Other peaks which were assignable to adsorbed C2H4 could not be clearly detected by IRAS. The decrease of the peak area of the 1257-cm-1 band, ca. 60%, almost agreed with that observed by TPD. It is difficult to attribute the reduced amount of adsorbed C2H4 to the contamination by CO, because only a trace amount of CO was detected by TPD. We conclude that the behavior is due to a structural change of the adsorbed C2H4 molecules, which might be due to disordering by adsorption at high temperature or existence of another C2H4 species which is difficult to detect by IRAS. 2. Oxygen hecovered Cu( 110). The adsorption of oxgyen on the Cu(ll0) surface has been the subject of investigation for a long time.1G12 It is well-known that the adsorbed oxygen leads to the reconstruction of the Cu( 110) surface from the (1 X 1) to the (2 X 1)-O/Cu( 110) missing (added) row structure at high coverage,l'J-lz and the nature of the oxygen-adsorbed surface is well reflected on water adsorption on oxygen-modified Cu( 110) surfaces.27-28 At low converage (00 < O.l), the adsorbed oxygen atoms are well dispersed, as reported in refs 10-12. The surface structure observed at the coverages of over 0.1 shows the growth of the (2 X 1) island, and the (6 X 2) phase is reportedly observed at very high coverage (>1 0 000 langmuir).lOJ1 The present study was carried out under the coverage far below that for the
TABLE 1: Vibrational Frequencies (cm-1) of End-On and r-Bonded Ethene Species on Various Metal Surfaces IRAS method Cu(ll0) surfaces
>0.1 langmuir this work (ref 4)
