Thermal reactions of 2-iodoethanol on Cu(1 0 0)

Surface Science 600 (2006) 417–423 www.elsevier.com/locate/susc

Thermal reactions of 2-iodoethanol on Cu(1 0 0) Yung-Hsuan Liao a, Chia-Yuan Chen a, Tao-Wei Fu a, Ching-Yung Wang a, Liang-Jen Fan b, Yaw-Wen Yang b, Jong-Liang Lin a,* a

Department of Chemistry, National Cheng Kung University, 1, Ta Hsueh Road, Tainan 701, Taiwan, ROC b National Synchrotron Radiation Research Center, Hsinchu, Taiwan, ROC Received 16 June 2005; accepted for publication 24 October 2005 Available online 19 December 2005

Abstract Temperature-programmed reaction/desorption, X-ray photoelectron spectroscopy, and reflection–absorption infrared spectroscopy have been employed to investigate the reactions of ICH2CH2OH on Cu(1 0 0) under ultrahigh-vacuum conditions. ICH2CH2OH can dissociate on Cu(1 0 0) at 100 K, forming a –CH2CH2OH surface intermediate. Density functional theory calculations predict that the –CH2CH2OH is most probably adsorbed on atop site. –CH2CH2OH on Cu(1 0 0) further decomposes to yield C2H4 below 270 K. No evidence shows the formation of –CH2CH2O– intermediate in the reactions of ICH2CH2OH on Cu(1 0 0) in contrast to the decomposition of BrCH2CH2OH on Cu(1 0 0) and ICH2CH2OH on Ag(1 1 1) and Ag(1 1 0), exhibiting the effects of carbon–halogen bonds and metal surfaces.
2005 Elsevier B.V. All rights reserved. Keywords: Temperature-programmed reaction; Desorption; X-ray photoelectron spectroscopy; Reflection–absorption infrared spectroscopy; Cu(1 0 0); ICH2CH2OH; Density functional theory

1. Introduction Thermal reactions of ICH2CH2OH molecules on single crystal surfaces of Ag(1 1 1) [1,2], Ag(1 1 0) [3,4], Ru(1 1 1) [5], and Ni(1 0 0) [6] have been investigated. The interests of these studies were directed toward synthesis and characterization of the oxametallacycle (–CH2CH2O–) on the surfaces. Surface oxametallacycles have often been proposed as an intermediate in olefin epoxidation and in the decomposition of oxygenated hydrocarbons on metal surfaces. On Ru(1 1 1), no spectroscopic evidence suggests the formation of –CH2CH2O– in the reactions of ICH2CH2OH [5]. However, vibrational spectroscopy has shown that –CH2CH2O– is isolated from decomposition of ICH2CH2OH on the silver surfaces. –CH2CH2O– on Ag(1 1 1) and Ag(1 1 0) is stable up to 220 K [1–3]. The oxametallacy-

*

Corresponding author. Tel.: +886 6 2757 575; fax: +886 6 2740 552. E-mail address: [email protected] (J.-L. Lin).

0039-6028/$ - see front matter
2005 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2005.10.059

cle is also suggested to be generated from ICH2CH2OH dissociation on Ni(1 0 0) by X-ray photoelectron spectroscopy [6]. Not only on Ag surfaces, recently it has been found that –CH2CH2O– is formed by dissociating BrCH2CH2OH on Cu(1 0 0) at 190 K [7]. b-Halohydrins (XCH2CH2OH, X = halogen) have two functional groups, C–X and C– OH, which are the reactive centers and their dissociations can lead to the formation of –CH2CH2O–. The bond energies for C–I, C–Br, C–Cl, and C–F are approximately 55, 70, 90, and 110 kcal mol
1 respectively. The nature of carbon–halogen bonds and metal surfaces are anticipated to affect the reaction routes of XCH2CH2OH on metal surfaces. On Cu(1 0 0), our recent studies have shown that FCH2CH2OH and ClCH2CH2OH are mainly adsorbed reversibly, in contrast to the case of BrCH2CH2OH [7–9]. On oxygen-precovered Cu(1 0 0), ICH2CH2OH and BrCH2CH2OH have been reported to dissociate to form –CH2CH2O– at 115 K [7,10]. Therefore, it is interesting to study the ICH2CH2OH reactions on Cu(1 0 0) to show the effects of carbon–halogen bond strength and precovered

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oxygen and to make a comparison to other metal surfaces. In the present research, temperature-programmed reaction/desorption (TPR/D), X-ray photoelectron spectroscopy (XPS), and reflection–absorption infrared spectroscopy (RAIRS) are employed to investigate the adsorption and decomposition of ICH2CH2OH on Cu(1 0 0). It is found that the ICH2CH2OH dissociates at 100 K, via –CH2CH2OH intermediate, to form C2H4. Unlike the cases of ICH2CH2OH on Ag(1 1 1) and Ag(1 1 0) as well as BrCH2CH2OH on Cu(1 0 0), –CH2CH2O– is not produced in the reaction of ICH2CH2OH on Cu(1 0 0). The bonding geometry and site of –CH2CH2OH are predicted by theoretical calculations in the framework of density functional theory. 2. Experimental section All TPR/D and RAIRS experiments were performed in an ultrahigh-vacuum (UHV) apparatus equipped with an ion gun for sputtering, a differentially pumped mass spectrometer for TPR/D, four-grid spherical retarding field optics for low energy electron diffraction (LEED), a cylindrical mirror analyzer for Auger electron spectroscopy (AES), and a Fourier-transform infrared spectrometer for RAIRS. The chamber was evacuated by a turbomolecular pump and an ion pump to a base pressure of approximately 3 · 10
10 Torr. The quadrupole mass spectrometer used for TPR/D studies was capable of detecting ions in 1–300 amu range and of being multiplexed to acquire up to 15 different masses simultaneously in a single desorption experiment. In TPR/D experiments, the Cu(1 0 0) surface was positioned 1 mm from an aperture, 3 mm in diameter, leading to the mass spectrometer and a heating rate of 2 K/s was used. In the RAIRS study, the IR beam was taken from a Bomem FTIR spectrometer and focused at a grazing incidence angle of 85 through a KBr window onto the Cu(1 0 0) in the UHV chamber. The reflected beam was then passed through a second KBr window and refocused on a mercury–cadmium– telluride (MCT) detector. The entire beam path was purged with a Balston air scrubber for carbon dioxide and water removal. All the IR spectra were taken at a temperature about 115 K, with 800–1500 scans and 4 cm
1 resolution. The presented spectra have been ratioed against the spectra of a clean Cu(1 0 0) surface recorded immediately before 2-iodoethanol dosing. The Cu(1 0 0) single crystal (1 cm in diameter) was mounted on a resistive heating element and could be cooled with liquid nitrogen to 110 K and heated to 1100 K. The surface temperature was measured by a chromel–alumel thermocouple inserted into a hole on the edge of the crystal. Cleaning of the surface by cycles of Ar+ ion sputtering and annealing was done prior to each experiment until no impurities were detected by Auger electron spectroscopy. Photoemission measurements were carried out at the wide range spherical grating monochromator beamline (WR-SGM) at the National Synchrotron Radiation Research Center of ROC. Total instrumental resolution, including the beamline and energy analyzer, was estimated

to be better than 0.3 eV. The photoelectrons were collected at an angle of 50 from the surface normal. All the XPS spectra presented here were first normalized to the photon flux by dividing the recorded XPS signal with the photocurrent derived from a gold mesh situated in the beamline. The binding energy scale in all the spectra was referenced to a well-resolved spin–orbit component of the bulk Cu 2p3/2 peak at 75.10 eV. ICH2CH2OH was dosed onto Cu(1 0 0) at 100 K, followed by brief annealing and photoelectron energy analysis at 100 K to monitor the surface reactions. The size of X-ray beam used was approximately 2 · 2 mm2, and the diameter of the Cu(1 0 0) crystal was 10 mm. 370 eV photon energy was used for I 4d. The X-ray photoelectron spectra shown in supporting Fig. 3 were fitted with Gaussian–Lorentzian functions based on a nonlinear least squares algorithm after Shirely background subtraction. 2-Iodoethanol (99%) was purchased from Aldrich and subjected to several cycles of freeze– pump–thaw. The purity of ICH2CH2OH was checked by mass spectrometry. The gas manifold for 2-iodoethanol was conditioned by backfilling with saturated 2-iodoethanol vapor overnight. Prior to dosing 2-iodoethanol into the chamber, the gas manifold and 2-iodoethanol itself in liquid state were pumped for a while. In the present study, the bonding geometries of –CH2CH2OH, which is the surface intermediate of ICH2CH2OH thermal decomposition on Cu(1 0 0), are predicted by theoretical calculations. All of our calculations were performed in the framework of density functional theory using the program package Cerius2-DMol3 in which the Perdew– Wang local exchange and correlation functional and double-numerical plus d-DNP basis (DND) were employed for Cu, C, H, O, and I atoms. The size of the DND basis is comparable to Gaussian 6-31G* basis sets. However, the numerical basis set is much more accurate than a Gaussian basis set of the same size. The calculations were spinunrestricted and did not include relativistic effects for the core electrons. In the calculations for the optimized geometry of –CH2CH2OH, the convergence criteria used were 1 · 10
5 Hartree for energy, 1 · 10
3 Hartree/Bohr for gradient, and 1 · 10
3 Bohr for atomic displacement. The infrared spectra of the species was obtained by carrying out two-point calculations. The mode assignments are based on the animated vibrations of the corresponding bands. 3. Results and discussion 3.1. Desorption and reactions of ICH2CH2OH on Cu(1 0 0) Multiple-ion surveys for the masses below m/z 200 were made following ICH2CH2OH adsorption on Cu(1 0 0) at 115 K. In the TPR/D studies, carbon-containing desorption species were identified to be ICH2CH2OH and C2H4 by the same relative multiplexed TPR/D peak areas as the measured cracking patterns of ICH2CH2OH and C2H4 using the same quadrupole mass spectrometer. It was also found that the TPR/D peak temperatures for dif-

Y.-H. Liao et al. / Surface Science 600 (2006) 417–423

Fig. 1. Temperature-programmed desorption spectra for the indicated ICH2CH2OH exposures on Cu(1 0 0), collected for CH2CH2OH+ (m/z 45) to represent ICH2CH2OH desorption. The inset shows the relative yield of ICH2CH2OH as a function of exposure.

ferent runs of experiments at the same ICH2CH2OH exposure might vary by 10 K, as seen in the desorption of ICH2CH2OH at 2 L exposure (197 K in Fig. 1 and 205 K in Fig. 2). Fig. 1 shows the ICH2CH2OH desorption spectra, featured by m/z 45 (CH2CH2OH+), for different ICH2CH2OH exposures. Below 1.5 L, no ICH2CH2OH desorption is observed, indicating that decomposition of ICH2CH2OH takes place on the surface. At a 2 L exposure, multilayer ICH2CH2OH molecules desorb at 197 K. The ICH2CH2OH desorption temperature shifts to 205 K at higher exposures. The desorption yield increases linearly with exposure as shown in the inset of Fig. 1. This TPR/ D result reveals that a 2 L ICH2CH2OH exposure approximately corresponds to a monolayer (ML) coverage. In the present case, the exposure for the appearance of ICH2CH2OH multilayer desorption state and its desorption temperature are similar to those of ICH2CH2OH on Ag(1 1 1) [1]. Fig. 2 shows the TPR/D spectra, collected for the + þ þ ions m/z 15ðCHþ 3 Þ; 26ðC2 H2 Þ; 27ðC2 H3 Þ, 29(CHO ), + + + 43(CH3CO ), 127(I ), and 172(ICH2CH2OH ), of 2 L ICH2CH2OH adsorbed on Cu(1 0 0) at 115 K. The broad feature at 215 K is attributed to C2H4 desorption. Multilayer ICH2CH2OH molecular desorption is observable at 205 K in all the traces of Fig. 2. Due to the exposure uncertainty of dosing ICH2CH2OH into the chamber, the surface coverage of ICH2CH2OH obtained in the experiment for Fig. 2 seems to be higher than that of Fig. 1 at the same exposure, therefore the ICH2CH2OH desorption appears at a higher temperature in Fig. 2, compared to 197 K in

419

Fig. 2. Temperature-programmed reaction/desorption spectra of a Cu(1 0 0) after dosing 2 L ICH2CH2OH at 115 K, collected for the ions + + + + þ þ of CHþ 3 , C2 H2 , C2 H3 , CHO , C2H3O , I , and ICH2CH2OH . The traces of m/z 15, 29, 43, 127, and 172 are multiplied by a factor of 4.

Fig. 1. Besides, the desorption features observed in the traces of m/z 15, 29, and 43 of Fig. 2, which are the major fragments of acetaldehyde, show no evidence for the formation of CH3CHO in contrast to the desorption behavior found in the case of ICH2CH2OH decomposition on Ag(1 1 1) and Ag(1 0 0) [1–3]. Fig. 3 shows the TPR/D spectra of C2H4 generated from ICH2CH2OH decomposition on Cu(1 0 0) at different exposures. For exposures 51 L, there are two C2H4 desorption states. At 1.5 L and higher exposures, the C2H4 desorption becomes single and broad state centered at 220 K. In the 15 L trace of Fig. 3, the sharp peak at 210 K is due to the contribution from ICH2CH2OH multilayer desorption. In addition to the evolution of ethylene, H2 desorption between 275 and 375 K is also observed in the ICH2CH2OH decomposition on Cu(1 0 0) (supporting Fig. 1). This result suggests the formation of H on the surface [11]. The study of Auger electron spectroscopy shows the deposition of oxygen on Cu(1 0 0) after the desorption of ethylene (supporting Fig. 2). 3.2. Infrared absorptions of ICH2CH2OH on Cu(1 0 0) as a function of temperature Fig. 4a shows the temperature-dependent infrared spectra of a Cu(1 0 0) surface after dosing 15 L of ICH2CH2OH at 115 K. As shown in Fig. 1, an exposure of 15 L renders a

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Fig. 3. Temperature-programmed reaction/desorption spectra for the indicated ICH2CH2OH exposures on Cu(1 0 0), collected for C2 Hþ 3 ion.

surface coverage of 7 ML. The marked band frequencies in Fig. 4a are similar to those of ICH2CH2OH on Ag(1 1 1) and in a liquid state. This comparison is made in Table 1. From 115 to 190 K, the spectral feature in terms of peak positions and peak relative intensities does not change, namely, most of the adsorbed ICH2CH2OH molecules do not desorb or react to change their chemical identity. Upon heating to 200 K, the band intensities diminish significantly. This result is ascribed to ICH2CH2OH multilayer desorption and ICH2CH2OH decomposition based on the TPR/D studies of Figs. 1–3. No infrared signals are detected after heating the surface to 220 K or higher. At such a high ICH2CH2OH coverage (15 L), the infrared absorptions of surface intermediates generated from ICH2CH2OH decomposition are likely to overlap with those of multilayer ICH2CH2OH and cannot be resolved. Fig. 4b shows the temperature-dependent spectra of a Cu(1 0 0) surface after dosing 1.5 L of ICH2CH2OH at 115 K. The exposure of ICH2CH2OH at 1.5 L is near, but, less than a monolayer coverage. In the 115 K spectrum, the infrared bands appear at 921 1018, 1045, 1078, 1118, 1182, 1235, and 1267 cm
1. Among them, the presence of 1018, 1045, and 1078 cm
1, which are attributed to m(C–C) and m(C–O) in the case of adsorbed multilayer ICH2CH2OH (shown in Table 1), suggests that the backbone of C–C–O remains intact upon ICH2CH2OH adsorption on Cu(1 0 0) at 115 K. In addition, the C2H4 desorption observed in the TPR/D result of Fig. 3 further suggests the integrity of CH2 as ICH2CH2OH is adsorbed

Fig. 4. Reflection–absorption infrared spectra taken after exposing (a) 15 L and (b) 1.5 L of ICH2CH2OH to Cu(1 0 0) at 115 K and flashing the surface to the temperatures indicated.

Table 1 Comparison of ICH2CH2OH infrared frequencies (cm
1) 15 L of ICH2CH2OH Cu(1 0 0), 115 K (this work)

ICH2CH2OH multilayer on Ag(1 1 1) [1]

ICH2CH2OH in CCl4 solution [12]

Mode

921 978 1018 1046 1078 1142 1182 1269 1373 1419 1458 2859 2945

920 978 1018 1043 1077 1141 1177 1267 1367 1416 1462 – –

795–905 977

q(CH2) m(C–C)

1068 1147 1174 1265

m(C–O) d(C–OH)

1414 1455

x,b(CH2)

x(CH2)

q = rocking; m = stretching; d = deformation; x = wagging; b = bending.

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on 115 K Cu(1 0 0). CH2 wagging mode at 1267 cm
1 is also found in Fig. 4b. Upon heating to 160 K, a spectrum similar to the 115 K one is obtained. As the surface is further heated to 180 K, the band intensities are reduced. This result indicates decomposition of the surface species responsible for the infrared bands and is consistent with the C2H4 desorption observed in TPR/D studies. 3.3. X-ray photoelectron study of ICH2CH2OH on Cu(1 0 0) as a function of temperature In the studies of decomposition of alkyl iodides on Ag and Cu single crystal surfaces, it has been found that the C–I bond scission is facile and proceeds at low temperatures. For example, C2H5I adsorbs dissociatively on Ag(1 1 0) at 150 K, forming ethyl intermediates on the surface [13]. On Ag(1 1 1), the C–I bond breakage begins at 110 K, as demonstrated by X-ray photoelectron spectroscopy and work-function measurements [14]. Interestingly and importantly, when ICH2CH2OH is dosed onto Ag(1 1 1) at 150 K, the C–I bond dissociates to form a –CH2CH2OH surface intermediate [2]. In other words for adsorbed ICH2CH2OH with an OH bonded at the b-carbon with respect to the CH2I, the C–I bond scission still proceeds at low temperatures (<150 K) on the Ag surfaces, although the exact C–I bond breaking temperature has not been determined. Furthermore, ICH2CH2OH decomposes, via C–I scission, on Ni(1 0 0) at 150 K [6]. On Cu(1 1 1) and Cu(1 0 0) surfaces, the C–I bonds of C2H5I and the iodides with longer carbon chains dissociate below 120 K [15,16]. To understand whether the C–I bond of ICH2CH2OH on Cu(1 0 0) breaks at low temperatures, the I 4d X-ray photoemission was investigated. The binding energy of I 4d is sensitive to the iodine chemical bonding. For example, the I 4d3/2 of iodobenzene on Pd(1 1 0) appears at 52.2 eV, in contrast to the adsorbed atomic iodine at 51.4 eV [17]. Due to the interaction between neighboring iodine atoms, the 51.4 eV I 4d3/2 may shift to 50.4 eV at higher coverages [17]. Fig. 5 shows the I 4d photoelectron spectra for the Cu(1 0 0) exposed to 1 L ICH2CH2OH at 100 K, followed by brief heating of the surface to the temperatures indicated. The spectra are similar from 100 to 250 K and the I 4d3/2 and 4d5/2 are located at 51.5 and 49.8 eV, respectively. The I 4d3/2 binding energy is close to that of I on Pd(1 1 0), indicating that ICH2CH2OH dissociates by C–I bond rupture upon adsorption on Cu(1 0 0) at 100 K [17]. Supporting Fig. 3 shows the I 4d spectrum of Cu(1 0 0) exposed to 6 L ICH2CH2OH at 100 K. The I 4d3/2 of adsorbed ICH2CH2OH appears at 52.6 eV and that of atomic iodine shifts to 50.8 eV due to the iodine–iodine interaction [17]. The 115 and 160 K infrared spectra of Fig. 4b are similar to those of multilayer ICH2CH2OH in Fig. 4a, strongly suggesting the formation of –CH2CH2OH generated from C–I dissociation upon ICH2CH2OH adsorption on Cu(1 0 0). Similar observations have been reported in the cases of ICH2CH2OH on Ag(1 1 1) and Ag(1 1 0) [1–3].

Fig. 5. Variation of I 4d photoelectron spectra with annealing temperature. The Cu(1 0 0) was initially exposed to 1 L ICH2CH2OH at 100 K. These spectra were taken at different spots on Cu(1 0 0).

Table 2 compares the experimental and theoretical infrared bands and mode assignments for the –CH2CH2OH on Cu(1 0 0), Ag(1 1 1), and Ag(1 0 0). The theoretical frequencies were calculated for –CH2CH2OH adsorbed at atop site of a layer of Cu atoms using density functional calculations. In contrast to Cu(1 0 0) with preadsorbed oxygen, it has been shown that ICH2CH2OH is dissociatively adsorbed on 115 K Cu(1 0 0) to form –CH2CH2O– [10]. The effect of the nature of carbon–halogen bonds of b-halohydrins is demonstrated by comparing the decomposition routes of ICH2CH2OH and BrCH2CH2OH on Cu(1 0 0) [7]. In the study of thermal reactions of BrCH2CH2OH on Cu(1 0 0), it has been found that BrCH2CH2OH decomposes to form a –CH2CH2O– surface intermediate with two sharp and strong infrared bands at 998 and 1051 cm
1 between 200 and 250 K. However no such two bands are observed in the present case of ICH2CH2OH, as shown in Fig. 4. The C–Br bonds of C2H5Br and C3H7Br dissociate on Cu(1 0 0) at 180 K, much higher than the C–I dissociation temperature (<120 K) of the corresponding iodides [15,16]. This difference in the carbon–halogen bond dissociation temperature may, in part, explain the observed variation in decomposition pathways for ICH2CH2OH and BrCH2CH2OH on Cu(1 0 0). Because the C–I bond scission of ICH2CH2OH proceeds at lower temperatures, even at 100 K, –CH2CH2OH is generated and further reacts to form C2H4. In the case of BrCH2CH2OH, the C–Br and O–H bonds are likely to dissociate at similar temperatures,

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Table 2 Comparison of the infrared frequencies (cm
1) of –CH2CH2OH on Ag(1 1 1), Ag(1 1 0), and Cu(1 0 0) 1.5 L of ICH2CH2OH Cu(1 0 0), 115 K (this work) 921 1018 1045 1078 1118 1182 1235 1267

*

Theoretical band of –CH2CH2OH (this work)

Approximate mode

–CH2CH2OH on Ag(1 1 1) [1]

845 873 1015 1036 1068

q(*CH2) CCO bending + x(CH2) m(C–Cu) + m(C–O) + CCO bending + q(*CH2) OH bending m(C–C) + m(C–O)

1184

tw(*CH2) + tw(CH2)

1263 1335 1367 1414 2882 2907 2929 2992 3379

COH bending + x(*CH2) COH bending + x(*CH2) d(CH2) d(*CH2) ms(*CH2) ms(CH2) ma(*CH2) ma(CH2) m(OH)

–CH2CH2OH on Ag(1 1 0) [3]

800

793

975

972

1070

1076 1150 1175

1175 1250

1290

Denotes the CH2 that is bonded to OH.

therefore generating –CH2CH2O– intermediate. The nature of metal surfaces also shows clear effect on the ICH2CH2OH decomposition sequence. The Ag surfaces display an interesting contrast to ICH2CH2OH reaction on Cu(1 0 0), ICH2CH2OH decomposes sequentially to form –CH2CH2OH and –CH2CH2O– on Ag(1 1 1) and Ag(1 1 0) [1–3]. Evolution of C2H4 and CH3CHO competes, in the decomposition of –CH2CH2OH on the Ag surfaces, with the formation of –CH2CH2O– surface intermediate. In the reactions of ICH2CH2OH on Cu(1 0 0), only the –CH2CH2OH intermediate is observed. The –CH2CH2OH further reacts to generate C2H4. The ethylene shows a complexed desorption behavior, as shown in Fig. 3. The cause of this is not known but it may be due to surface defect sites [1] and steric, electronic effects related to H, O, I, and –CH2CH2OH. On Ni(1 0 0), C2H4 and CH3CHO are also the reaction products of ICH2CH2OH [6]. However, Ru(1 1 1) surface can promote the dissociation of the C–C bond of ICH2CH2OH, yielding CH4 and surface carbon [5]. 3.4. Theoretical predictions for –CH2CH2OH bonding geometry and site on Cu(1 0 0) In our calculations for the adsorption geometries of –CH2CH2OH on Cu(1 0 0) by minimizing the total energies, a slab of 9Cu atoms fixed at their lattice positions was used, but allowing the variations of all the bond lengths and angles of –CH2CH2OH. Fig. 6 shows the optimized bonding geometries of –CH2CH2OH adsorbed at atop, bridging, and hollow sites and compares their relative adsorption energies. As suggested by the lowest theoretically predicted total energy, –CH2CH2OH is most likely to be adsorbed at atop site of Cu(1 0 0). The bridging and hollow –CH2CH2OH have adsorption energies 7.5 and 18.5 kcal mol
1 higher, respectively. For the most probable geometry

Fig. 6. Comparison of bonding geometries, bonding sites, and relative energies of –CH2CH2OH on Cu(1 0 0) predicted by theoretical calculations. Two perspective views for each geometry are given. DE is the energy difference in kcal mol
1 for each structure relative to the atop case.

of –CH2CH2OH on atop site as shown in Fig. 6a, the C–C bond of –CH2CH2OH points toward the midway of the bridging and hollow sites and the C–O bond points toward the surface. The angles of C–C–Cu and C–C–O are 113.1 and 113.4 respectively, which are 4 larger than the sp3 bond angle of CH4. Based on the detailed information for the optimized geometries of –CH2CH2OH at atop, bridging, and hollow sites on Cu(1 0 0), as shown in Table 3, it is found that the bond lengths of C–O and C–C for –CH2CH2OH on the three specific sites are the same, but the bond angles of CCSN (SN = surface normal) and OCC as well as the heights of the C and O from the copper surface are different. The adsorption geometries and relative adsorption energies of –CH2CH2OH adsorbed at atop, bridging, and hollow sites on Cu(1 0 0) were also calculated using a two-layer cluster (9Cu atoms for the first layer and 4Cu atoms for the second layer). In this case, the atop sites

Y.-H. Liao et al. / Surface Science 600 (2006) 417–423

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Table 3 Theoretically predicted geometric details of optimized –CH2CH2OH on Cu(1 0 0) at different sites

of the National Science Council of the Republic of China (Grant NSC 93-2113-M-006) for this research.

Atop site

Supplementary data

Bridging site

Hollow site

d(C–O) = 1.43 d(C–O) = 1.42 d(C–O) = 1.43 d(C–C) = 1.49 d(C–C) = 1.49 d(C–C) = 1.49 h(CCSN) = 119.9 h(CCSN) = 120.3 h(CCSN) = 113.1 h(OCC) = 113.4 h(OCC) = 110.1 h(OCC) = 107.4 h(C) = 1.98 h(C) = 1.75 h(C) = 1.83 h(O) = 2.11 h(O) = 2.26 h(O) = 2.11 ˚ ); h = bond angle; SN: surface normal; h(C): distance (A ˚) d: bond length (A between the Cu surface and the C that is bonded to the surface; h(O): ˚ ) between the Cu surface and the O. distance (A

are also predicted to be the most stable surface positions for –CH2CH2OH adsorption. The atop –CH2CH2OH has adsorption energies 11.6 and 22.1 kcal mol
1 lower than the bridging and hollow ones, respectively. Supporting Fig. 4 shows the optimized bonding geometries of –CH2CH2OH adsorbed at three different sites. 4. Conclusions In brief summary, ICH2CH2OH decomposes on Cu(1 0 0) to form –CH2CH2OH, which further reacts to evolve C2H4. No surface intermediate of –CH2CH2O– is found. –CH2CH2OH is generated upon ICH2CH2OH adsorption at 100 K. Theoretical calculations based on density functional theory predict that –CH2CH2OH is most likely adsorbed at atop site of Cu(1 0 0). Acknowledgement We gratefully acknowledge the National Center for High-performance Computing and the financial support

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