Reaction Pathways of 2-Iodoacetic Acid on Cu(100): Coverage

Mar 31, 2010 - the reaction paths of ICH2COOH on Cu(100) at a coverage e1 monolayer. ... to the photon flux by dividing the recorded XPS signal with t...
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Reaction Pathways of 2-Iodoacetic Acid on Cu(100): Coverage-Dependent Competition between C-I Bond Scission and COOH Deprotonation and Identification of Surface Intermediates Yi-Shiue Lin,† Jain-Shiun Lin,† Yung-Hsuan Liao,† Che-Ming Yang,† Che-Wei Kuo,† Hong-Ping Lin,† Liang-Jen Fan,‡ Yaw-Wen Yang,‡ and Jong-Liang Lin*,† †

Department of Chemistry, National Cheng Kung University 1, Ta Hsueh Road, Tainan, Taiwan, ROC, and ‡ National Synchrotron Radiation Research Center 101, Hsin-Ann Road, Hsinchu, Taiwan, ROC Received December 4, 2009. Revised Manuscript Received February 6, 2010

The chemistry of 2-iodoacetic acid on Cu(100) has been studied by a combination of reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reaction/desorption (TPR/D), and theoretical calculations based on density functional theory for the optimized intermediate structures. In the thermal decomposition of ICH2COOH on Cu(100) with a coverage less than a half monolayer, three surface intermediates, CH2COO, CH3COO, and CCOH, are generated and characterized spectroscopically. Based on their different thermal stabilities, the reaction pathways of ICH2COOH on Cu(100) at temperatures higher than 230 K are established to be ICH2COOH f CH2COO þ H þ I, CH2COO þ H f CH3COO, and CH3COO f CCOH. Theoretical calculations suggest that the surface CH2COO has the skeletal plane, with delocalized π electrons, approximately parallel to the surface. The calculated Mulliken charges agree with the detected binding energies for the two carbon atoms in CH2COO on Cu(100). The CCOH derived from CH3COO decomposition has a CC stretching frequency at 2025 cm-1, reflecting its triple-bond character which is consistent with the calculated CCOH structure on Cu(100). Theoretically, CCOH at the bridge and hollow sites has a similar stability and is adsorbed with the molecular axis approximately perpendicular to the surface. The TPR/D study has shown the evolution of the products of H2, CH4, H2O, CO, CO2, CH2CO, and CH3COOH from CH3COO decomposition between 500 and 600 K and the formation of H2 and CO from CCOH between 600 and 700 K. However, at a coverage near one monolayer, the major species formed at 230 and 320 K are proposed to be ICH2COO and CH3COO. CH3COO becomes the only species present on the surface at 400 K. That is, there are two reaction pathways of ICH2COOH f ICH2COO þ H and ICH2COO þ H f CH3COO þ I (possibly via CH2COO), which are different from those observed at lower coverages. Because the C-I bond dissociation of iodoethane on copper single crystal surfaces occurs at ∼120 K and that the deprotonation of CH3COOH on Cu(100) occurs at ∼220 K, the preferential COOH dehydrogenation of monolayer ICH2COOH is an interesting result, possibly due to electronic and/or steric effects.

1. Introduction Rearrangement of acetolactone by ring-opening can generate different CH2COO isomers, such as þCH2-COO-, •CH2COO•, and CH2dC•-O-O•.1 •CH2COO- anion radicals can be formed by negative ion chemical ionization of appropriate precursors, for example, CH3COOH and (CH3)3SiCH2COOSi(CH3)3, using nitrous oxide or nitrogen trifluoride.1 Electron detachment of •CH2COO- leads to a mixture of acetolactone and • CH2COO•.1 The CH2COO group also appears in the arsenobetaine of (CH3)3AsþCH2COO-, which can be found in biological samples and participates in transport of arsenic in environmental systems.2 In the cesium salt of tetra(acetato)diplatinate(III), Cs3[Pt2(CH2COO)2(CH3COO)2Cl2]Cl 3 3H2O, the CH2COO ligands bridge between two Pt ions.3 Although the relevant concepts between the fields of organometallic chemistry and surface science have long been recognized, there are no reports for the adsorption *Corresponding author: e-mail [email protected]; Ph 886-62757575 ext 65326; Fax 886-6-2740552. (1) Schr€oder, D.; Goldberg, N.; Zummack, W.; Schwarz, H.; Poutsma, J. C.; Squires, R. R. Int. J. Mass Spectrom. Ion Processes 1997, 165/166, 71. (2) Cannon, J. R.; Edmonds, J. S.; Francesconi, K. A.; Raston, C. L.; Saunders, J. B.; Skelton, B. W.; White, A. H. Aust. J. Chem. 1981, 34, 787. (3) Yamaguchi, T.; Sasaki, Y.; Ito, T. J. Am. Chem. Soc. 1990, 112, 4038. (4) Albert, M. R.; Yates, J. T., Jr. The Surface Scientist’s Guide to Organometallic Chemistry; American Chemical Society: Washington, DC, 1987.

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and reaction of CH2COO on metal single crystals to date.4 Besides, this species may be involved in heterogeneous oxidation of hydrocarbons or decomposition of oxygenated hydrocarbons. To explore the possibility of forming CH2COO on a metal single crystal and to understand its adsorption structure and surface reactivity, the ICH2COOH/Cu(100) system is investigated. 2-Iodoacetic acid is naturally a proper precursor for CH2COO isolation on Cu(100) in terms of the known primary dissociation steps of the C-I bonds in iodoalkanes5 and COOH groups in carboxylic acids on copper surfaces.6-10 The C-I bond dissociation of iodoethane on Cu(111) to form ethyl groups occurs at ∼120 K, and deprotonation of acetic acid on copper surfaces to generate acetate happens below 300 K. Furthermore, in the present case of ICH2COOH, the two functional groups of ICH2 and COOH are actually bonded together; the effect of their mutual interaction on the functional group dissociation, possibly producing various reaction intermediates, needs to be considered and is worthy to be investigated. (5) Lin, J.-L.; Bent, B. E. J. Phys. Chem. 1992, 96, 8529. (6) Sexton, B. A. J. Vac. Sci. Technol. 1980, 17, 141. (7) Bowker, M.; Madix, R. J. Appl. Surf. Sci. 1981, 8, 299. (8) York, S. M.; Hag, S.; Kilway, K. V.; Phillips, J. M.; Leibsle, F. M. Surf. Sci. 2003, 522, 34. (9) Sexton, B. A. Chem. Phys. Lett. 1979, 65, 469. (10) Surman, M.; Lackey, D.; King, D. A. J. Electron Spectrosc. Relat. Phenom. 1986, 39, 245.

Published on Web 03/31/2010

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This article presents XPS, RAIRS, and TPR/D results to reveal the reaction paths of ICH2COOH on Cu(100) at a coverage e1 monolayer. Competition between the C-I scission and COOH dehydrogenation of ICH2COOH/Cu(100) is found to be coverage-dependent, therefore leading to different surface intermediates, such as CH2COO and ICH2COO observed below 300 K. Two other intermediates, CH3COO and CCOH, are also observed at higher temperatures. In addition, the optimized bonding structures of CH2COO and CCOH are provided by DFT calculations.

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, 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 ∼210-10 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. The identification of products from ICH2COOH reaction on Cu(100) was based on the detected fragmentation patterns, with the aid of the NIST reference spectra.11 In TPR/D experiments, the Cu(100) 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(100) 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 1000 scans and 4 cm-1 resolution. The presented spectra have been ratioed against the spectra of a clean Cu(100) surface recorded immediately before 2-iodoacetic acid dosing. The Cu(100) 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 Arþ 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 (WRSGM) 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 wellresolved spin-orbit component of the bulk Cu 2p3/2 peak at 75.10 eV. The size of X-ray beam used was approximately 2  2 mm2. 620 eV photon energy was used for I 4d, C 1s, and O 1s. The X-ray photoelectron spectra obtained were fitted with GaussianLorentzian functions based on a nonlinear least-squares algorithm after Shirley background subtraction. ICH2COOH (99%, Showa), CH3COOH (99.7%, Showa), and CD3COOD (99.99 atom % D, Aldrich) were subjected to several cycles of freezepump-thaw. The ICH2COOH sample we used contained water impurity which showed thermal desorption up to ∼200 K; therefore, in the study of ICH2COOH on Cu(100) the ICH2COOH was (11) Standard Reference Database 69, NIST (National Institute of Standards and Technology) Chemistry WebBook , 2005.

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Figure 1. Reflection-absorption infrared spectra taken after dosing 0.5 langmuir of ICH2COOH on Cu(100), followed by flashing the surface to the indicated temperatures. either dosed at ∼200 K or dosed at a lower temperature and then heated to ∼200 K or higher temperatures for RAIRS and XPS analysis. Computational Method. In our theoretical cluster-model calculations for the bonding geometries and infrared bands of CH2COO and CCOH, two slabs of total 25 Cu atoms fixed at their lattice positions were used. All of the calculations were performed in the framework of density functional theory using the program package Cerius2-DMol3, in which generalized gradient approximation proposed by Perdew and Wang (GGAPW91) was employed for the exchange-correlation functional. Double-numeric quality basis set with polarization functional (DNP) were used for the all electron calculations including relativistic effect for the core electrons. The geometry optimization convergence threshold for energy change, maximum force, and maximum displacement between optimization cycles was 0.00001 Ha, 0.001 Ha/A˚, and 0.0005 A˚, respectively. A preliminary transition state for the surface reaction CH2COO þ H f CH3COO was obtained using the integrated linear synchronous transit/quadratic synchronous transit method, followed by TS optimization. The confirmation of the pathway connecting the reactant, transition state, and product was achieved using intrinsic reaction path calculation, based on the nudged elastic band algorithm.

3. Results and Discussion We first studied the ICH2COOH molecular desorption as a function of exposure. The temperature-programmed desorption spectra of ICH2COOHþ ion (186 amu) are shown in Supporting Figure 1, with the inset of desorption yield vs exposure. As displayed, the desorption begins at ∼1 langmuir, corresponding to a saturated first layer coverage, and the yield increases with the exposure due to desorption of multilayer ICH2COOH molecules. 3.1. ICH2COOH/Cu(100) Study at a Coverage Less than or Close to 0.5 Monolayer. 3.1.1. Temperature-Dependent Infrared Spectroscopy: Formation of CH3COO and CCOH. Figure 1 shows the temperature-dependent infrared spectra for 0.5 langmuir of ICH2COOH adsorbed on Cu(100). ICH2COOH desorption is only found in TPR/D study at an exposure larger than 1.0 langmuir, so a 0.5 langmuir renders a DOI: 10.1021/la904576z

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Table 1. Comparison of the CH3COO Infrared Frequencies (cm-1) 0.5 langmuir of ICH2COOH/ Cu(100),a 400 K

CH3COO/ Cu(110)b

modeb

1334 1387

1335 1391

νs(CH3) νas(CH3)

1428 This work. bReference 9.

1434

νs(OCO)

a

CH3COO/ Cu(100)a 1332 1388 1400 1423

submonolayer coverage. In the 210 K spectrum, there is a broad, unresolved band at 1429 cm-1. Heating the surface to 240 K causes a decrease in the peak width but a slight increase in intensity at 1429 cm-1. The changes of the peak shape and intensity with temperature suggest the variation of surface species and/or their relative concentrations. More detailed structural information in this temperature range will be given later in the X-ray photoelectron emission analysis. The infrared absorption feature develops into other stages at higher temperatures. At 300 K, a set of distinguishable peaks at 1334, 1387, and 1428 cm-1 appears and persists up to 500 K. In the temperature range of 500-550 K, a dramatic absorption change occurs, generating a new 2023 cm-1 peak at the sacrifice of the three peaks. This result must be originated from chemical transformation of adsorption species. The new species responsible for the sharp band at 2023 cm-1 no longer exists at 660 K. It is noticed that the set of peaks at 1334, 1387, and 1428 cm-1 is similar to that of CH3COO on Cu(110) reported previously.8 This comparison is listed in Table 1. To further support the CH3COO formation from ICH2COOH decomposition on Cu(100), we have measured the infrared absorptions of adsorbed CH3COO by dissociative adsorption of CH3COOH on the same copper facet and shown the result in Figure 2. The absorption characteristics from 300 to 650 K in Figure 2 are similar to those of Figure 1 in terms of the peak positions, relative peak intensities, and their variation with temperature. Note that the previous studies have shown that CH3COO is the only species present on Cu(100) and Cu(110) surfaces at ∼400 K after CH3COOH deprotonation.8-10 Although in Table 1 the bands at ∼1335 and 1390 cm-1 have been assigned previously to CH3 deformation modes, they may also be in part due to OCO stretching as suggested in the CD3COOD study shown later. The antisymmetric OCO stretching frequency of CH3COO on Cu(100) is expected to be near 1600 cm-1; however, this fundamental transition is not allowed if the acetate is adsorbed with upright configuration according to the surface dipole selection rule of RAIRS.9 In Figure 2, the absorption bands in the 190 K spectrum give no indication for the formation of CH3COO. They are attributed to adsorbed CH3COOH with the CdO stretching mode at 1655 and 1692 cm-1, CH3 antisymmetric deformation mode at 1435 cm-1, and C-O stretching mode at 1290 cm-1.12 The two CdO stretching frequencies are red-shifted as compared to the gas value of 1788 cm-1, indicating that there are two acetic acid adsorption states and the CdO groups strongly interact with the surface. Increasing the temperature to 210 K leads to the disappearance of the 1290 and 1692 cm-1 peaks, which is attributed to desorption of weakly adsorbed acetic acid molecules. A separate TPR/D experiment also shows acetic acid desorption at 200 K. The characteristic CH3COO infrared bands between 1300 and 1500 cm-1 appear in the 240 K spectrum. It is concluded that deprotonation of CH3COOH on Cu(100) occurs between 210 and 240 K. According to the infrared results shown in Figures 1 and 2, the CH3COO must decompose between 500 and 550 K because of the disappearance of its infrared bands in this temperature range. In (12) Burneau, A.; Genin, F.; Quiles, F. Phys. Chem. Chem. Phys. 2000, 2, 5020.

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Figure 2. Reflection-absorption infrared spectra taken after dosing 0.5 langmuir of CH3COOH on Cu(100), followed by flashing the surface to the indicated temperatures.

addition to the formation of the new surface species responsible for the peak at 2023 or 2025 cm-1, the TPR/D experiments also detect evolution of gas products between 500 and 700 K. Figure 3 shows the TPR/D spectra for the ions of 2, 14, 15, 16, 18, 28, 42, 44, and 60 amu after adsorption of 0.3 langmuir of CH3COOH on Cu(100). Thermal reaction of CH3COOH on Cu(110) has been reported by Bowker et al. previously.7 Deprotonation of CH3COOH on Cu(110) occurs as the first dissociation step, forming adsorbed CH3COO and H. H2 from recombination of the surface H is evolved at ∼310 K. The CH3COO on Cu(110) is stable up to ∼590 K and then decomposes to yield CH4(m/z 15 and 16), H2O(m/z 18), CO(m/z 28), CO2(m/z 28 and 44), CH2CO(ketene, m/z 14 and 42), CH3COOH(m/z 60), and carbon fragment. A mechanism involving (CH3CO)2O (acetic anhydride) intermediate has been invoked to explain the simultaneous formation of CH3COOH and CH2CO. In the present Cu(100) study, H2 from CH3COOH deprotonation desorbs at 329 K, and the same products as those from CH3COO decomposition on Cu(110) are observed, but at a slightly lower temperature of ∼550 K, as shown in Figure 3. However, the chemistry of CH3COO on Cu(100) and Cu(110) is not completely the same. Another reaction channel forming H2 and CO occurs at 640 K on Cu(100). Note the temperature match for the H2 and CO evolution (Figure 3) and the significant reduction for the peak near 2025 cm-1 (Figures 1 and 2), indicating that the surface species responsible for the 2025 cm-1 is the origin for the H2 and CO formation at 640 K. There are several surface candidates, such as adsorbed CO and acetylene and those shown in Scheme 1, which may be responsible for the peak near 2025 cm-1. First of all, carbon monooxide and acetylene are ruled out since their desorption from Cu(100) is reported to occur below 300 K.13,14 The species of Scheme 1a (13) Borguet, E.; Dai, H. L. J. Chem. Phys. 1994, 101, 9080. (14) Dvorak, J.; Hrbek, J. J. Phys. Chem. B 1998, 102, 9443.

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Figure 3. Temperature-programmed reaction/desorption spectra of 0.3 langmuir of CH3COOH on Cu(100), representing the formation of H2, CH4, H2O, CO, CO2, CH2CO, and CH3COOH. Scheme 1

(ketenylidene) has been suggested to be the surface intermediate formed in the acetone decomposition on preoxidized Ag(111).15 Ketenylidene on Ag(111) has a strong absorption band near 2020 cm-1; however, it only persists to ∼360 K. It seems unlikely that this species can be stable up to 600 K on Cu(100). The structures of Scheme 1b,c, possessing a carbon-carbon triple bond, are the most probable species that can account for the 2025 cm-1 peak and the concomitant H2 and CO formation upon its dissociation. Previously, Hochstrasser et al. have investigated the infrared absorptions of HCtCOH (ethynol) generated from photoisomerization of ketene in argon matrix and have provided, in particular, the shift in frequency for ethynol isotopmers.16 The CC stretching frequency of ethynol is affected by H, D isotope exchange and shows different changes in wavenumber for the acetylenic H and hydroxy H. The fundamental transition for the CC stretching of HCtCOH is located at 2202.1 cm-1, which is shifted to 2099.1 cm-1 for DCtCOH and to 2199.4 cm-1 for HCtCOD.16 Based on Hochstrasser’s result, a comparison of the infrared bands between CH3COOH and CD3COOD after their decomposition on Cu(100) should be able to identify the surface structure that exhibits the 2025 cm-1 peak in Figure 2. The CC stretching frequencies of OCtCH and OCtCD (Scheme 1b) may have a (15) Sim, W. S.; King, D. A. J. Phys. Chem. 1996, 100, 14794. (16) Hochstrasser, R.; Wirz, J. Angew. Chem., Int. Ed. Engl. 1990, 29, 411.

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Figure 4. Reflection-absorption infrared spectra taken after dosing 0.5 langmuir of CD3COOD on Cu(100), followed by flashing the surface to the indicated temperatures.

difference of ∼100 cm-1, in contrast to a few wavenumbers for CtCOH and CtCOD (Scheme 1c). Since Scheme 1a does not contain H, the absorption frequency due to the cumulated double bond group would not be changed for both the acetic acids used. Figure 4 shows the temperature-dependent infrared spectra for 0.5 langmuir of CD3COOD on Cu(100). CD3COO on Cu(100) has three major infrared bands located at 1363, 1386, and 1421 cm-1 in the 400 K spectrum, which can be assigned to the symmetric OCO stretching of acetate at multiple bonding sites and/or with multiple bonding configurations as reported previously for HCOO on Ag(111).17 Note that the CD3 deformation modes should be shifted to ∼1000 cm-1 and cannot account for the peaks from 1363 to 1421 cm-1. Decomposition of acetate on Cu(100) shows isotope effect as the H is replaced by D, as indicated by the inconsistent disappearance temperature of the acetate peaks in Figures 2 and 4. Reaction of CD3COO on Cu(100) also results in a sharp, strong absorption band in the spectra from 550 to 650 K. Most importantly, its peak frequency is 4 cm-1 lower than that produced by CH3COO decomposition. This result strongly supports that CtCOH is the surface intermediate from CH3COO decomposition on Cu(100) and is responsible for the peak near 2025 cm-1. 3.1.2. Temperature-Dependent X-ray Photoelectron Emission: Formation of CH2COO and CH3COO. Although the RAIRS results can successfully identify the intermediates of CH3COO and CtCOH in the decomposition of ICH2COOH on Cu(100) at higher temperatures, the information it offers at lower temperatures is little. Therefore, XPS has also been employed to monitor the reaction process of ICH2COOH on Cu(100). The temperature-dependent C 1s, O 1s, and I 4d spectra for 0.2 langmuir of ICH2COOH on Cu(100) are shown in Figure 5. Since it has been established that CH3COO is the species present on Cu(100) from ICH2COOH decomposition at 400 K, we begin our analysis from the 400 K spectrum in Figure 5. The C 1s (284.8 and 287.7 eV) and O 1s (531.0 eV) main emission (17) Sim, W. S.; Gardner, P.; King, D. A. J. Phys. Chem. 1996, 100, 12509.

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Figure 6. Theoretically predicted adsorption structures of CCOH at bridge site (a) and hollow site (b). Table 2. Calculated Infrared Bands (cm-1) for the Optimized CtCOH and CH2COO on Cu(100)a CtC-OH CtC-OH bridge site hollow site

mode

CH2COO

mode

450 F(CH2) þ δ(OCC) 613 δ(OCO) 667 tw(CH2) 696 δ(CCO) 827 ω(CH2) 926 νs(OCO) 990 tw(CH2) 1230 ν(C-O) þ ν(C-C) 1324 ν(C-O) þ δ(CH2) 1428 δ(CH2) 3069 νs(CH2) 3168 νas(CH2) a δ: bending; F: rocking; tw: twisting; ω: wagging; ν: stretching.

324, 449 1086 1216 2075 3548

356 1021 1215 1924 3513

δ(CCO) νs(CtC-O) δ(COH) νas(CtC-O) ν(OH)

Figure 5. X-ray photoelectron emission of C 1s, O 1s, and I 4d after dosing 0.2 langmuir of ICH2COOH on Cu(100), followed by flashing the surface to the indicated temperatures.

peaks observed in the 400 K spectrum are indeed similar to those for acetate on Cu(110).7 CH3COO on Cu(110), prepared by dissociative adsorption of acetic acid at 370 K, exhibits two C 1s peaks at ∼285.0 and 288.0 eV due to the CH3 and COO, respectively. On the other hand, the O 1s binding energy for the carboxylate group appears at ∼531.5 eV. In the 400 K spectrum of Figure 5, the I 4d signals of 49.7 eV (I 4d5/2) and 51.5 eV (I 4d3/2) are due to adsorbed I.18 In the 230 K spectrum, the photoelectrons of O 1s and I 4d are peaked at almost the same binding energies as those observed at 400 K, indicating the presence of carboxylate groups and I atoms on the surface. However, the C 1s signal becomes more complex but can be deconvoluted into the acetate C 1s peaks with smaller areas than those found at 400 K and two other peaks at 283.0 and 287.0 eV. In other words, vanishing of the species responsible for the 283.0 and 287.0 eV peaks leads to the growth of acetate from 230 to 400 K. The C 1s peaks at 283.0 and 287.0 eV are ascribed to the surface intermediate of CH2COO on the basis of following reasons. First, it contains no COOH or C-I group that is consistent with the XPS result at 230 K.7 Second, no thermal desorption product is found between (18) Liao, Y.-H.; Chen, C.-Y.; Fu, T.-W.; Wang, C.-Y.; Fan, L.-J.; Yang, Y.-W.; Lin, J.-L. Surf. Sci. 2006, 600, 417.

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Figure 7. Theoretically predicted adsorption structure of CH2COO on Cu(100).

230 and 400 K in the ICH2COOH TPR/D study (Supporting Figure 2), suggesting that the structure of CH2COO moiety remains intact. C-C bond breakage of CH2COO would generate CH2 and CO2 on Cu(100) and finally turn into C2H4 and CO2 desorption below 400 K,19 but this is not the case. Similarly, C-H scission of CH2COO can result in H2 desorption at ∼320 K; however, this reaction channel is not observed either. The surface CH2COO can abstract adsorbed H from ICH2COOH deprotonation to form CH3COO. We have calculated the most probable CH2COO adsorption structure on Cu(100) and its Mulliken charges on each atom. The charges on the carbon atoms of the CH2 and COO are calculated to be -0.32 and þ0.56 in electron units, respectively. Since C 1s binding energy is generally located (19) Lin, J.-L.; Bent, B. E. J. Am. Chem. Soc. 1993, 115, 6943.

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Figure 8. Theoretical reaction pathway of CH2COO þ H f CH3COO on Cu(100).

at ∼285.0 eV for neutral carbons in hydrocarbons, the C 1s peak positions for the CH2 (283.0 eV) and COO (287.0 eV) of the CH2COO agree with the charge effect on core electron binding energy.20 We have also measured the temperature-dependent X-ray photoelectron spectra for 0.45 langmuir of ICH2COOH (Supporting Figure 3), which can render a coverage close to a half monolayer. The evolution of the chemical species with temperature at this exposure is similar to that found in the 0.2 langmuir case. 3.1.3. Theoretical Predictions for the Adsorption Structures and Infrared Bands of CCOH and CH2COO on Cu(100) and for the Reaction Pathway of CH2COO þ H f CH3COO. Density functional theory calculations have been employed to predict the bonding sites, optimized structures, and infrared bands of CCOH and CH2COO on Cu(100). The cluster calculation results show that CCOH adsorbed on bridge site or hollow site has almost the same total energy (only a difference of 0.3 kcal/mol) but is lower than that of CCOH on atop site by ∼8.0 kcal/mol. Figure 6 shows the adsorption geometries of CCOH on bridge (Figure 6a) and hollow (Figure 6b) sites, with the molecular CCO axis basically perpendicular to the surface. Detailed structural parameters for the CCOH are listed in the tables close to the optimized clusters. It is found that the adsorption sites have some effects on the CCOH internal structure, especially for the d(CC) and θ(OCC). The CC bond lengths of CCOH on bridge site and on hollow site are calculated to be 1.248 and 1.270 A˚, indicative of CtC triple-bond character.21 The calculated band frequencies (Table 2) relating to the CCO stretching of CtCOH also reflect the change in the CC bond length. Furthermore, the theoretical result predicts that the CtCOH is more likely to be adsorbed at bridge site in terms of the better match for the experimental CtC stretching at ∼2025 cm-1. In the case of CH2COO on Cu(100), the optimized adsorption structure is shown in Figure 7. The four (20) Vickerman, J. C. Surface Analysis; John Wiley & Sons: New York, 1997. (21) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Principles of Structure and Reactivity, 4th ed.; Baker & Taylor Books: Bridgewater, NJ, 1997.

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atoms constituting the skeleton of CH2COO are roughly located in the same plane parallel to the surface. The calculated CC bond distance (1.44 A˚) of the CH2COO is located between 1.34 and 1.54 A˚, which are the typical lengths for C-C single bond and CdC double bond, respectively.21 Although the two C-O bonds in the CH2COO have not exactly the same length (1.291 and 1.330 A˚), they are less than the typical C-O bond length (∼1.43 A˚) but larger than the CdO one (∼1.20 A˚).21 Therefore, the CH2COO on Cu(100) is proposed to have delocalized π electrons over the CCOO skeleton. The predicted infrared bands for the optimized CH2COO are shown in Table 2. The infrared bands involving the C-O stretching of the CH2COO appear at 1230 and 1324 cm-1, which are much lower than the COO symmetric (∼1430 cm-1) and antisymmetric stretching (∼1600 cm-1) of CH3COO on Cu(100).9 Figure 8 shows the path calculated for the chemical process of CH2COO þ H f CH3COO on Cu(100). In the calculation, the distance between the two reactants in the initial state is set large enough to avoid chemical bond formation. In the initial state, the H atom is bonded at a hollow site, which is its most stable surface position. The activation energy for the reaction path of CH2COO þ H f CH3COO is predicted to be 18 kcal/mol, revealing that this hydrogenation process of CH2COO is feasible. The detailed structural parameters for the transition state are also included in Figure 8. In the transition state, the C-C bond of CH2COO is tilted away from the surface to accommodate the approaching H atom. 3.2. Study for ICH2COOH/Cu(100) near One Monolayer Coverage. 3.2.1. Temperature-Dependent X-ray Photoelectron Emission: Formation of ICH2COO and CH3COO. Figure 9 shows the temperature-dependent X-ray photoelectron spectra of C 1s, O 1s, and I 4d for 1.0 langmuir of ICH2COOH on Cu(100). In the 230 K spectrum, the I 4d emission signal between 48.0 and 54.0 eV is fitted with four peaks at 49.9 and 51.7 eV for the adsorbed I and at 50.7 and 52.5 eV for another surface species containing the ICH2 group. The area ratio of the latter species to DOI: 10.1021/la904576z

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the adsorbed I is 1.4. This result is unlike the cases of 0.2 and 0.45 langmuir of ICH2COOH, which shows complete C-I bond scission for all of the adsorbed molecules at 230 K. In the respect of O 1s, the main signal is peaked at 531.2 eV, manifesting predominantly the presence of carboxylate species, with a small shoulder at ∼533.0 eV due to COOH group.7 In the 230 K C 1s spectrum, the main peaks are located at 285.1 and 287.8 eV showing two types of carbon atoms with different bonding environments. The 287.8 eV peak agrees with the C 1s binding energy of carboxylate groups (COO). Therefore, it can be concluded that the major species present on Cu(100) at 230 K for 1.0 langmuir of ICH2COOH are carboxylates. From the results of 0.2 and 0.45 langmuir of ICH2COOH, CH2COO and CH3COO are two possible species that should be considered in the analysis of the 1.0 langmuir XPS data. The small peak of 283.3 eV in the 230 K C 1s spectrum of Figure 9 indeed shows the formation CH2COO, but it is a minor surface species. Although formation of CH3COO is possible, it is not the only carboxylate species present at 230 K. If this is the case, the iodine signal would be completely from adsorbed I, which is not supported by the presence of ICH2 groups observed in the 230 K I 4d spectrum of Figure 9. Therefore, ICH2COO is proposed to be formed at the higher ICH2COOH coverage in addition to CH3COO and CH2COO, which can account for the iodine signal from ICH2 and oxygen signal from COO. Based on the area ratio of ICH2 to adsorbed I, ICH2COO is the major species present on the surface at 230 K. As the temperature is raised to 320 K to induce further C-I bond rupture, the iodine area ratio decreases. This C-I dissociation process has come to an end at 400 K, and the peaks of C 1s and O 1s indicate the formation of CH3COO. 3.3. Comparison of the Dissociation of the C-I and COOH of ICH2COOH on Cu(100). Scheme 2 summarizes the reaction pathways of ICH2COOH on Cu(100). It has been known that the C-I bond scission occurs upon adsorption of iodoethane on Cu(111) at ∼120 K,22 and dehydrogenation of acetic acid to form acetate on Cu(100) occurs at ∼220 K (Figure 2). At a coverage less than a half monolayer for ICH2COOH on Cu(100), the reaction intermediate CH2COO can be isolated on the surface at 230 K, which recombines with H to generate CH3COO. However, near a monolayer coverage, the amount of ICH2 functional groups on Cu(100) is substantial at 320 K. This may be due to electronic and/or steric effects. Probably, the activation energies for dissociation of the two functional groups (ICH2 and COOH) are largely changed at a crowded condition leading to the preferred ICH2COO formation at 230 K. (22) Lin, J.-L.; Bent, B. E. J. Phys. Chem. 1992, 96, 8529.

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Figure 9. X-ray photoelectron emission of C 1s, O 1s, and I 4d after dosing 1.0 langmuir of ICH2COOH on Cu(100), followed by flashing the surface to the indicated temperatures.

The depressed C-I bond scission may also be due to a particular ICH2COOH adsorption configuration near a monolayer coverage; i.e., the OH group is attached to the surface, and the ICH2 moiety is pushed away from it. In the crowded environment which restrains the C-C bond bending, the C-I dissociation is therefore retarded because the surface sites can not be accessed. Langmuir 2010, 26(11), 8218–8225

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4. Conclusion At a coverage smaller than 0.5 monolayer, ICH2COOH decomposes on Cu(100) at 230 K to form adsorbed CH2COO and CH3COO. DFT theoretical calculation predicts that the skeletal CCOO plane of CH2COO is approximately parallel to the surface, suggesting a strong interaction between the surface and the whole molecule. CH2COO can abstract surface H, generating CH3COO which is stable up to ∼500 K. Between 500 and 600 K, CH3COO dissociates to evolve gaseous products of H2, CH4, H2O, CO, CO2, CH2CO, and CH3COOH. Decomposition of CH3COO also produces CtCOH, which is predicted to be adsorbed at a bridge site of Cu(100) with the CCO molecular axis approximately perpendicular to the surface. This surface species with an infrared absorption at ∼2025 cm-1 is the origin for the formation of CO and H2 in the temperature range of 600-700 K.

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At a monolayer coverage, ICH2COO is the major surface species formed at 230 K, which further reacts at 400 K to generate CH3COO on the surface. Acknowledgment. This research was supported by the National Science Council of the Republic of China (Grant NSC 972113-M-006-004-MY2). Supporting Information Available: TPD spectra of ICH2COOH on Cu(100) (Supporting Figure 1); TPR/D spectra of 0.3 langmuir of ICH2COOH on Cu(100) (Supporting Figure 2); XP spectra of C1s, O1s, and I4d of 0.45 langmuir of ICH2COOH on Cu(100) (Supporting Figure 3). This material is available free of charge via the Internet at http://pubs.acs.org.

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