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J. Phys. Chem. C 2008, 112, 8304–8310
Surface Chemistry of Monoethanolamine on Oxygen-Precovered Cu(100) Yi-Shiue Lin, Ching-Yung Wang, Che-Ming Yang, Jian-Shiun Lin, Che-Wei Kuo, and Jong-Liang Lin* Department of Chemistry, National Cheng Kung UniVersity, 1, Ta Hsueh Road, Tainan, Taiwan, Republic of China ReceiVed: December 25, 2007; ReVised Manuscript ReceiVed: March 12, 2008
The adsorption and reactions of HOCH2CH2NH2 on oxygen-precovered Cu(100) were investigated under ultrahigh vacuum conditions. Reflection–absorption infrared spectroscopy (RAIRS) studies were performed to monitor and identify the surface intermediates from HOCH2CH2NH2 decomposition, with the assistance of density functional theory (DFT) calculations. -OCH2CH2NH- was generated from scission of both the O-H and N-H bonds of HOCH2CH2NH2 and decomposed to form surface -OCH2CH2N- and to evolve CH2O and HCN observed with temperature-programmed reaction/desorption (TPR/D). The -OCH2CH2N- species further decomposed to generate H2, HCN, and N2, together with surface carbon. The bonding structures of the two surface intermediates, -OCH2CH2NH- and -OCH2CH2N-, were also investigated by DFT calculations. Introduction Hydrogen sulfide or carbon dioxide produced in refineries, petrochemical plants, natural gas processing plants, and other industries are often removed by amine treating in which alkanolamines, such as monoethanolamine, diethanolamine, and methyldiethanolamine, are most commonly used. Monoethanolamine (HOCH2CH2NH2) investigated in the present study possesses two reactive centers of OH and NH2. Adsorbed bifunctional molecules can show versatile bonding coordination, for example, monodentate forms through ligation with either a functional group with the surface or bidentate forms bonded with both functional groups.1 The design and construction of these anchored systems relate to fundamental surface processes and provide the basis for potential applications as chemical sensors, especially in the case of monodentate species.1 It has been reported that on Cu(100) precovered with oxygen ethanol molecules decompose by dehydrogenation, forming ethoxide intermediate.2 This surface species is stable up to ∼350 K and transforms into CH3CHO and H2 at higher temperatures. In the system of dimethylamine on Cu(110) in the presence of surface atomic oxygen, (CH3)2N is suggested to be the reaction intermediate generated through N-H bond scission.3 On O/Ag(110), ethylamine is shown to dissociate by sequentially losing amino’s hydrogen to form adsorbed CH3CH2N.4 This intermediate continues to deprotonate, generating CH3CN and H2 between 300 and 400 K. Surface reactions of monoethanolamine on mordenites with a high acid-site density and on Ni(100) have been reported.5,6 Adsorption of monoethanolamine at the Brønsted acid sites produces the corresponding ammonium (HOCH2CH2NH3+), which decomposes into H2O, NH3, and C2H4 at a temperature higher than 673 K.5 Dissociation of monoethanolamine on Ni(100) generates H2, CO, and N2 between 250 and 550, 330–550, and 800–1100 K, respectively, with atomic carbon left on the surface.6 Reports of HOCH2CH2NH2 reactions on single-crystal surfaces are scarce. It is interesting to study the adsorption and reactions of * Corresponding author. E-mail:
[email protected]. Phone: 886 6 2757575 ext. 65326. Fax: 886 6 2740552.
monoethanolamine on surfaces with different structures, in terms of bonding geometry, reaction pathway, surface intermediate, and product distribution, and compare to those of monofunctional molecules with OH or NH2. In the present research, the surface chemistry of HOCH2CH2NH2 on O/Cu(100) is investigated with reflection–absorption infrared spectroscopy (RAIRS) and temperature-programmed reaction/desorption (TPR/D). Experimental Section All of the 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), an Auger electron spectrometer with cylindrical mirror analyzer, and a Fourier transformed 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 the TPR/D study was capable of detecting ions in the range of 1–300 amu and of being multiplexed to acquire up to 15 different masses simultaneously in a single desorption experiment. In the 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 of the IR spectra were taken at a temperature of about 120 K, with 800–1500 scans and a 4 cm-1 resolution. The presented spectra have been ratioed against the spectra of a clean Cu(100) surface recorded immediately before the monoethanolamine dosing. The Cu(100) single crystal (1 cm in diameter) was mounted onto a resistive heating element and could be cooled to 110 K (with liquid nitrogen) and heated to 1100 K. The surface temperature was measured by a chromelalumel thermocouple that was inserted
10.1021/jp7120643 CCC: $40.75 2008 American Chemical Society Published on Web 05/13/2008
Monoethanolamine on Oxygen-Precovered Cu(100)
J. Phys. Chem. C, Vol. 112, No. 22, 2008 8305
Figure 2. Temperature-programmed reaction/desorption spectra of a O/Cu(100) surface after dosing 0.5 L of HOCH2CH2NH2 at 120 K.
Figure 1. Temperature-programmed desorption spectra for the indicated HOCH2CH2NH2 exposures on O/Cu(100), collected for m/z ) 30 amu to represent the HOCH2CH2NH2 desorption. The trace of 0.5 L has been multiplied by a factor of 3. The inset shows the relative yield of HOCH2CH2NH2 as a function of exposure.
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. Monoethanolamine (99.9%) was purchased from TEDIA and subjected to several cycles of freeze– pump–thaw. Oxygen (99.998%) was obtained from Matheson. The purity of HOCH2CH2NH2 was checked by mass spectrometry. The gas manifold for monoethanolamine was conditioned by backfilling with saturated monoethanolamine vapor pressure overnight. Prior to monoethanolamine being added to the chamber, the gas manifold and monoethanolamine itself (in a liquid state) were pumped for a while. The oxidized Cu(100) surface was prepared by exposing a clean Cu(100) surface at 500 K to 30 L (1 L ) 1 langmuir ) 10-6 Torr · s) of O2. It was estimated that the oxygen coverage (θo) for the oxidized Cu(100) used in this study was ∼0.2 monolayer (ML). The previous study showed that a long-range order started to develop at θo ) 0.34 ML, and a (2×2)R45° structure was formed at a saturation coverage of θo ) 0.48 ML.7 In our theoretical calculations, the infrared frequencies and optimized structures of -OCH2CH2NH- and -OCH2CH2Non Cu(100) were obtained by running a Cerius2-DMol3 module based on the density functional theory (DFT). A cluster with three slabs of Cu atoms (16, 9, and 4 atoms for the first, second, and third layer, respectively) was used to represent the Cu(100) surface, fixed at their lattice positions. However, it is found that the calculated results with a three-slab cluster are very close to those obtained with a two-slab one. All calculations involved use of the local density approximation (LDA) to the exchangecorrelation functional by Perdew–Wang (PWC), and doublenumerical plus d-DNP basis (DND) were employed for Cu, C, H, O, and N atoms. The calculations were spin-unrestricted and did not include relativistic effects for the core electrons. The
Figure 3. Auger electron spectra taken after exposing 1 L of HOCH2CH2NH2 to O/Cu(100) at 120 K and flashing the surface to the temperatures indicated.
convergence criteria for geometry optimizations were generally the threshold values: 1 × 10-5 hartree, 1 × 10-3 hartree/bohr, and 1 × 10-3 bohr, 1 × 10-6 hartree for energy, gradient, and atomic displacement, and self-consistent field (SCF) tolerance, respectively. All calculations employed a method based on Pulay’s direct inversion of iterative subspace (DIIS) technique to accelerate SCF convergence. Results and Discussion Temperature-programmed desorption (TPD) results showing the HOCH2CH2NH2 desorption and reaction products are presented first, followed by RAIRS results with the aid of DFT calculations to exhibit the surface intermediates generated from HOCH2CH2NH2 thermal decomposition. HOCH2CH2NH2 Molecular Desorption. Figure 1 shows the HOCH2CH2NH2 desorption spectra for the indicated exposures
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TABLE 1: Comparison of the Infrared Frequencies (cm-1) of HOCH2CH2NH2 3 L on O/Cu(100) g′Gg′
(gas)a,b
759 (s) 796 (s) 858 (m)
approx
modec
F(CH2) ω(NH2) ν(C-C)
gGt
(liguid)a,b
854 (m) 874 (m)
approx
modec
F(CH2) ν(C-C)
g′Tgd 768 (100) 803 (26.4)
ν(C-O) ν(C-N) F(CH2)
1028 (vs) 1074 (s) 1165 (w)
ν(C-N) ν(C-O) F(CH2)
1230 (m)
tw(CH2)
1359 (m) 1375 (sh) 1385 (m)
δ(COH) ω(CH2) ω(CH2)
1242 (w) 1321 (sh) 1360 (m)
tw(CH2) tw(CH2) δ(COH)
1462 (w) 1471 (w) 1623 (m)
δ(CH2) δ(CH2) δ(NH2)
1396 (sh) 1456 (m) 1462 (sh) 1482 (sh) 1595 (m)
ω(CH2) ω(CH2) δ(CH2) δ(CH2) δ(NH2)
2861(sh) 2882 (vs) 2949 (vs)
νs(CH2) νs(CH2) νas(CH2)
2860 (s)
νs(CH2)
2921 (s)
νas(CH2)
3356 (vw) 3422 (w) 3570 (m)
νs(NH2) νas(NH2) ν(OH)
3347 (b)
120 K
200 K
ω(NH2) F(CH2) 867 (w)
967 (2.0) 1037 (s) 1083 (sh) 1100 (sh)
approx
modec
ν(C-C)
1030 (32.0) 1096 (49.8) 1110 (0.5) 1137 (0.9) 1213 (1.0) 1271 (5.2) 1316 (0.3) 1341 (32)
δ(COH) ν(C-O) + ν(C-N) ν(C-C) tw(CH2) ω(CH2) tw(CH2) ω(CH2) δ(COH)
1386 (1.1)
ω(CH2)
1447 (0.4) 1462 (1.9) 1618 (25.9) 2804 (74.3) 2868 (43.8)
δ(CH2) δ(CH2) δ(NH2) νs(CH2) νs(CH2)
2989 (13.1) 3031 (11.8) 3373 (0.3) 3468 (2.9) 3596 (5.5)
νas(CH2) νas(CH2) νs(NH2) νas(NH2) ν(OH)
1003 (w) 1045 (m) 1092 (s) 1142 (w)
1365 (w)
839 (w) 863 (vw) 938 (vw) 994 (vw) 1014 (w) 1092 (s)
1349 (vw) 1367 (w)
2863 (m)
1498 (vw) 1621 (m) 2810 (m) 2856 (m)
2925 (m)
2918 (m)
3285 (w) 3346 (w)
3207 (vw) 3288 (w) 3323 (m)
1616 (w)
a Reference 14. b s, strong; vs, very strong; m, medium; w, weak; vw, very weak; sh, shoulder; b, broad. c F, rocking; ω, wagging; νs, symmetric stretching; νas, asymmetric stretching; tw, twisting; δ, bending. d Frequencies and relative intensities (in parentheses) calculated in this work.
on O/Cu(100), represented by m/z ) 30 amu which is the most abundant ion measured in our quadrupole mass spectrometer for HOCH2CH2NH2. Two desorption states, one with a small, broad feature at ∼365 K and the other with sharp peaks at ∼209 K, are found. The low-temperature state grows with exposure and is ascribed to multilayer desorption, but the former one is due to recombinative process after HOCH2CH2NH2 dissociation as shown later. The inset showing the relative HOCH2CH2NH2 desorption yield calculated only for the 209 K state as a function of exposure indicates that decomposition of HOCH2CH2NH2 predominates at lower exposures (φ e 0.5 L). Reactions of HOCH2CH2NH2 on O/Cu(100). Figure 2 shows the TPR/D spectra of the products from HOCH2CH2NH2 reactions on O/Cu(100) at 0.5 L exposure. In addition to the ions indicated in this figure, other ions, such as m/z ) 14, 15, 31, 32, 34, 38, 39, 41, 42, 43, 44, 45, 46, 52, and 56 amu, etc., were also investigated, but without a desorption feature detected. Therefore, formation of possible reaction products containing N-C-C, C-C-O, or N-C-C-O (other than HOCH2CH2NH2) as a backbone can be ruled out. The desorption features at 394 K is attributed to H2; 526 K to N2; 290 K to H2O; 334 and 418 K to HCN; 339 K to H2CO; 365 K to HOCH2CH2NH2. The identification of H2O (m/z ) 17 and 18 amu), HCN (m/z ) 26 and 27 amu), and H2CO (m/z ) 28, 29, and 30 amu) is based on the detected fragmentation patterns that are similar to the NIST reference spectra8 or to those measured using our own spectrometer. The 28 amu desorption state (526 K) may result from N2, C2H4, or CO. Formation of ethylene is first excluded, because no fragments of 26 and 27 amu are detected at this temperature. Variation of the Auger peak intensities of HOCH2CH2NH2 with temperature has been
Figure 4. Temperature-dependent infrared spectra of HOCH2CH2NH2 on O/Cu(100) at two multilayer coverages of 3 and 8 L dosed at 120 K and followed by flashing the surface to the temperatures indicated The 120 and 200 K spectra of (b) have been multiplied by a factor of one-third. All of the spectra were measured at 120 K.
Monoethanolamine on Oxygen-Precovered Cu(100)
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Figure 5. Reflection–absorption infrared spectra taken after exposing 1 L of HOCH2CH2NH2 to O/Cu(100) at 120 K and flashing the surface to the temperatures indicated. All of the spectra were measured at 120 K.
TABLE 2: Vibrational Frequencies (cm-1) and Modes of -OCH2CH2NH- on Cu(100) -OC1H2C2H2NH-/ Cu(100) DFT calculationa 447 (8.8) 556 (5.7) 647 (16.8) 845 (6.9) 882 (28.6) 1047 (30.2) 1062 (29.7) 1113 (74.3) 1189 (25.3) 1214 (3.7) 1265 (18.1) 1317 (6.0) 1343 (3.7) 1403 (17.6) 1411 (20.3) 2863 (100) 2875 (32.9) 2914 (80.1) 2941 (0.1) 3247 (0.1) a
exptl frequency
854 1057 1092
2816 2851 2895
mode ν(Cu-N) δ(NCC), δ(OCC) δ(NH) δ(OCC) ν(C-C), ν(C-N), F(C1H2) ν(C-O), F(C2H2) ν(C-O), δ(NCC) ν(C-N), ν(C-C) δ(HNC) tw(CH2) tw(CH2) ω(CH2) ω(CH2) δ(C2H2) δ(C1H2) νs(C1H2) νs(C2H2) νas(C1H2) νas(C2H2) ν(NH)
The numbers in parentheses are relative intensities.
studied and is shown in Figure 3. It is found that the N signal decreases significantly as the temperature is increased from 450 to 600 K, in contrast to the comparable C intensity in this temperature range. Although the possibility of CO formation can not be completely ruled out, this result strongly supports that the desorption state of 28 amu at 526 K is mainly due to N2. The carbon signal is still present at 980 K after the desorption of all of the reaction products. In the previous study
Figure 6. Bonding structure of -OCH2CH2NH-, with three different viewing angles.
TABLE 3: Structural Parameters of -OCH2CH2NH- on Cu(100) -OC1H2C2 H2NH-/ Cu(100) d(C1-H1a) d(C1-H1b) d(C2-H2a) d(C2-H2b) d(C1-C2) d(C1-O) d(C2-N) d(N-H) θ(OC1C2) θ(NC2C1) θ(HNC2) θ(C2NSN) θ(C1OSN) h(Cus-O)a h(Cus-N)a
1.11 Å 1.11 Å 1.11 Å 1.11 Å 1.50 Å 1.40 Å 1.45 Å 1.04 Å 114.2° 113.8° 110.2° 142.1° 143.9° 1.46 Å 1.49 Å
a h(Cus-O) and h(Cus-N) represent the distances of the O and N atoms to the copper surface, respectively.
of NH3 reaction on preoxidized Ag(110), N2 was found to evolve at 530 K from recombination of surface atomic nitrogen.9 The reaction route from HOCH2CH2NH2 to H2CO and HCN involves dehydrogenation and C-C bond rupture. The HCN
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TABLE 4: Vibrational Frequencies (cm-1) and Modes of -OCH2CH2N- on Cu(100) -OC1H2C2H2N-/ Cu(100) DFT calculationa 349 (4.0) 378 (8.0) 385 (4.6) 447 (4.4) 566 (3.2) 843 (5.0) 863 (8.1) 1012 (2.8) 1021 (12.6) 1046 (13.5) 1158 (1.7) 1191 (2.3) 1262 (11.9) 1286 (19.5) 1382 (3.1) 1388 (7.0) 2611 (1.8) 2888 (23.2) 2914 (100) 2938 (10.8) a
exptl frequency
1015 1057
mode ν(Cu-N) ν(Cu-N), ν(Cu-O) F(C2H2) δ(OC1C2) δ(NC2C1) δ(OC1C2), δ(NC2C1) ν(C1-C2) ν(C2-N), ν(C1-C2) ν(C2-N), F(CH2) ν(C1-O), δ(C1C2N) tw(C2H2) tw(C1H2) ω(C2H2) ω(C1H2) δ(C1H2) δ(C2H2) νs(C1H2) νs(C2H2) νs(C2H2), νas(C1H2) νas(C2H2)
The numbers in parentheses are relative intensities.
desorption with two maximum rates of 334 and 418 K implies that they arise from two different surface intermediates. On Ni(100), the gaseous reaction products of HOCH2CH2NH2 are H2, CO, and N2, without forming H2CO or HCN.6 The nickel surface is more reactive toward breaking the C-N bond and dehydrogenation of HOCH2CH2NH2, as compared to Cu(100). On oxygen-precovered Cu(110) and Ag(110), the only carbon-containing product generated from CH3CH2OH reaction is acetaldehyde.10 Reaction of CH3CH2NH2 on O/Ag(110) produces CH3CN.4 These two reactions only involve loss of hydrogen, and the C-C bonds remain intact. In comparison to our case, substitution of one hydrogen at β-carbon by a reactive center indeed can alter the reaction pathways. On the surfaces of Ag and Cu with adsorbed CH3CH2O, ethanol can be regenerated in the presence of surface atomic hydrogen via a recombinative route.10 Hydrogenation is also observed for CH3CH2NH on Ag(110).4 A similar process can explain the HOCH2CH2NH2 desorption state at 365 K observed in Figure 2. Ethylene glycol (HOCH2CH2OH) reactions on O/Cu(110) and O/Ag(110) have been investigated previously.11–13 On both surfaces, the two OH groups can be oxidized to evolve glyoxal ((CHO)2). Furthermore, recombinative HOCH2CH2OH formation and C-C breakage generating CH2O and adsorbed HCOO are also reported on Ag(110).12 Infrared Absorptions at Multilayer Coverages of HOCH2CH2NH2 on O/Cu(100). Monoethanolamine is a three-rotor molecule whose conformers are characterized by the dihedral angles in the order of lpN-N-C-C, O-C-C-N, and H-O-C-C. lpN represents the lone pair on the nitrogen atom. The conformers with respect to rotation about the C-C bond are denoted by G (gauche), G′ (gauche), or T (trans), depending on the dihedral angles close to 60°, -60°, or 180°, respectively. In the other two cases of H-O-C-C and lpN-N-C-C, the lower-case letter (g, g′, or t) is used instead. The relative stability of the rotational isomers for an isolated HOCH2CH2NH2 molecule has been calculated by two different theoretical approaches at the HF-SCF ab initial level using the extended 6-31G* basis set and in the DFT framework using B3LYP/6-311++G(2d,2p).14,15 Although the predicted order in the relative
Figure 7. Bonding structure of -OCH2CH2N-, with three different viewing angles.
TABLE 5: Structural Parameters of -OCH2CH2N- on Cu(100) -OC1H2C2 H2N-/ Cu(100) d(C1-H1a) d(C1-H1b) d(C2-H2a) d(C2-H2b) d(C1-C2) d(C1-O) d(C2-N) θ(OC1C2) θ(NC2C1) θ(C2NSN) θ(C1OSN) h(Cus-O)a h(Cus-N)a
1.11 Å 1.13 Å 1.11 Å 1.11 Å 1.53 Å 1.40 Å 1.46 Å 112.2° 110.1° 161.1° 125.6° 1.56 Å 0.94 Å
a h(Cus-O) and h(Cus-N) represent the distances of the O and N atoms to the copper surface, respectively.
stability for various HOCH2CH2NH2 conformers may not be completely the same, both theoretical studies indicate that the g′Gg′ is the most stable form due to the strong intramolecular OH · · · N hydrogen bond. This conformer is the dominant species in the gas phase, and its population at room temperature is
Monoethanolamine on Oxygen-Precovered Cu(100) estimated to be 63% or 73%.14,15 The infrared absorptions of HOCH2CH2NH2 measured previously in the gas phase and their assignments are listed in Table 1. However the g′Gg′ amount is negligible in liquid phase, which mainly comprises gGt molecules. Their infrared bands are also shown in Table 1.14 The gGt conformer with an intramolecular NH · · · O hydrogen bond is thought to make the OH groups more acidic and NH2 groups more basic and therefore tends to facilitate the formation of intermolecular OH · · · N hydrogen bonding in the liquid phase.14 The dominant gGt form in the liquid state is also supported by the lowest relative energy calculated for the dimers of (gGt)2, (g′Gg′)2, and (gGt)(g′Gg′) with cyclic or open-chain structure.15 The crystal structure of HOCH2CH2NH2 examined by X-ray crystallography has been reported previously.16 In the three-dimensional network, the HOCH2CH2NH2 molecules exist in g′Tg form and are hydrogen-bonded (NH · · · O and OH · · · N) to six neighboring ones. Figure 4 shows the infrared spectra of 3 and 8 L of HOCH2CH2NH2 adsorbed on O/Cu(100) at 120 K, followed by briefly annealing the surface to 200 and 230 K. In the 120 K spectrum of Figure 4a, the infrared bands appears at 867, 1003, 1045, 1092, 1092, 1142, 1365, 1616, 2863, 2925, 3285, and 3346 cm-1. The characteristic NH2 bending mode at 1616 cm-1 and NH2 and OH stretching modes between 3200 and 3400 cm-1 manifest the presence of molecular HOCH2CH2NH2 on the surface. Heating the surface to 200 K leads to some changes in the absorption feature, such as better peak resolution in general and enhanced intensities at 1092 and 1621 cm-1. The spectral feature continues to change as the surface is further heated to 230 K, including disappearance of some of the peaks and largely reduced intensities for the others. The broad nature of the peaks in the 120 K spectrum can be ascribed to the existence of several HOCH2CH2NH2 rotational isomers on the surface. Upon heating to 200 K, the improvement of peak resolution is attributed to isomeric conversion by internal bond rotation to a relatively stable form.16 Brannon et al. have measured the infrared spectra of neat HOCH2CH2NH2 and compared to that of HN(CH2CH2OH)2, giving unambiguous assignments for the observed bands of HOCH2CH2NH2 at 3170 cm-1 (NH2 symmetric stretching), 3290 cm-1 (NH2 antisymmetric stretching), and 3370 cm-1 (OH stretching).18 The 200 K spectrum of Figure 4a shows similar peak frequencies in the range of 3200–3400 cm-1, which can be assigned to the NH2 and OH stretching vibrations in accordance with the Brannon’s work. The multilayer molecules (200 K) are likely present in the form of g′Tg or gGt, which is the major conformer in the solid or liquid state.14,16 We have calculated the infrared absorptions and their relative intensities for a g′Tg HOCH2CH2NH2 molecule (Table 1) and found its C-O stretching and NH2 bending frequencies have a better match with those observed in the 200 K spectrum. Note that the theoretical relative intensities for an isolated HOCH2CH2NH2 cannot completely reflect the experimental ones for the adsorbed molecules, due to the surface dipole selection rule of RAIRS. The difference between the 200 and 230 K spectra in Figure 4a is due to HOCH2CH2NH2 desorption from multilayer. Note that no NH2 and OH stretching bands are observed in the 230 K spectrum, indicating decomposition of these functional groups for the first-layer HOCH2CH2NH2 molecules remaining on the surface. Figure 4b shows the temperature-dependent spectra of O/Cu(100) with a higher HOCH2CH2NH2 coverage obtained with 8 L exposure. Interestingly, the 120 and 200 K (Figure 4b) spectra maintain a similarity throughout heating, indicating that the adsorption layer is stable prior to its desorption. The
J. Phys. Chem. C, Vol. 112, No. 22, 2008 8309 comparison of temperature-dependent infrared absorptions between 120 and 200 K at 3 and 8 L suggests that the transformation of HOCH2CH2NH2 rotational isomers occurring during adsorption is medium-related. The activation energy of the isomeric conversion is likely to be reduced in the HOCH2CH2NH2 medium itself that is generated by 8 L exposure. Surface Intermediates Generated from HOCH2CH2NH2 Decomposition on O/Cu(100). Figure 5 shows the temperaturedependent infrared spectra of 1 L of HOCH2CH2NH2 adsorbed on O/Cu(100) at 120 K. In the TPD study, it has been shown that at 1 L some of HOCH2CH2NH2 molecules on O/Cu(100) desorb at ∼209 K, revealing the presence of intact HOCH2CH2NH2 at lower temperatures. This result is supported by the presence of the 1609 cm-1 NH2 bending band observed in the 120 K spectrum of Figure 5, although the stretching modes of its NH2 and OH are too weak to be observed. This NH2 band is no longer found after heating the surface to a temperature higher than 200 K. The infrared spectra taken from 210 to 280 K are quite similar, and only the 280 K spectrum is shown in Figure 5. This spectral resemblance is extended to 360 K. The TPR/D result (Figure 2) in combination with the primary dissociation steps of alcohol and amine molecules on oxygencovered Ag and Cu single-crystal surfaces, discovered previously, can shed some light for the identification of the intermediate that is responsible for the set of bands at 854, 1057, 1092, 2816, 2851, and 2892 cm-1. CH3CH2NH2 has been reported to decompose on O/Ag(100) via H-abstraction by surface atomic oxygen at 110 K, forming adsorbed CH3CH2NH and OH.3 Recombination of the OH groups results in H2O evolution. CH3CH2OH shows a similar dissociation step by losing the hydroxyl’s hydrogen on O/Ag(110) and O/Cu(110) upon adsorption at 180 K.10 In the present case of HOCH2CH2NH2/ O/Cu(100), the H2O desorption trace in Figure 2 indicates the dissociation of the NH2 and/or OH groups of HOCH2CH2NH2, which may occur prior to the desorption onset temperature (∼180 K). Furthermore, the surface intermediate generated from dissociation of the functional groups still possesses an OCCN backbone; therefore, its reaction through C-C bond breakage can lead to the simultaneous formation of HCN and H2CO between 320 and 360 K. The theoretical infrared frequencies of OCH2CH2NH/Cu(100), which can arise from dehydrogenation of both the NH2 and OH groups, based on DFT calculation have been obtained and agree well with the bands observed between 280 and 360 K in Figure 5. The comparison between the experimental and calculated band frequencies and mode assignments, which are based on the animated vibrations of the corresponding bands, are shown in Table 2. Figure 6 exhibits the theoretically predicted adsorption structures of this intermediate with three different viewing angles. Both the N and O atoms are bonded approximately at bridge sites and have about the same height from the surface. The more detailed bonding geometry of -OCH2CH2NH- is shown in Table 3. In Figure 2, the HOCH2CH2NH2 desorption at ∼365 K indicates hydrogenation of -OCH2CH2NH- in the presence of surface H which originates from C-H bond scission in the formation of H2CO and HCN. Since the surface is covered with -OCH2CH2NH- between 300 and 360 K, the desorption of HCN (334 K) and H2CO (339 K) in Figure 2 is attributed to the decomposition of this surface intermediate. As observed in Figure 5, the characteristic -OCH2CH2NH- bands disappear at a temperature higher than 360 K. There are only two bands located at 1015 and 1057 cm-1 in the 380 K spectrum. Our theoretical study predicts that -OCH2CH2N- is the intermediate
8310 J. Phys. Chem. C, Vol. 112, No. 22, 2008 responsible for the two bands, based on the infrared agreement. The similarity between the calculated and experimental infrared absorptions are listed in Table 4 for comparison. As -OCH2CH2NH- dehydrogenates to form -OCH2CH2N-, the ∼1090 cm-1 C-N stretching band of -OCH2CH2NH- is shifted to ∼1015 cm-1, with the C-O stretching frequency (1057 cm-1) remaining the same. The optimized -OCH2CH2N- adsorption geometry is shown in Figure 7. The O and N atoms are bonded at bridge and fourfold hollow sites, respectively. Table 5 lists the detailed structural parameters for this intermediate. The -OCH2CH2N- species is responsible for the evolution of H2 (394 K), HCN (418 K), and N2 (526 K) observed in Figure 2. The surface carbon measured with Auger electron spectroscopy (Figure 3) after the product desorption arises from the breakage of the O-C, C-C, and/or C-N bonds and dehydrogenation of this surface intermediate. Conclusion Temperature-programmed desorption and infrared spectroscopy have been employed to investigate the adsorption and reactions of HOCH2CH2NH2 on oxygen-precovered Cu(100). The vibrational study with the aid of theoretical calculations identifies the surface intermediates of -OCH2CH2NH- and -OCH2CH2N-. The former species decomposes at ∼335 K to form HCN and H2CO. In addition, it can also abstract surface hydrogen to evolve HOCH2CH2NH2 at 365 K. The -OCH2CH2N- derived from dehydrogenation of -OCH2CH2NHfurther dissociates to produce H2 at 394 K and HCN at 418 K, leaving C and N on the surface. The nitrogen atoms recombine and form gaseous N2 at 526 K. Acknowledgment. The work was supported by the National Center for High-Performance Computing and by the National Science Council of the Republic of China (NSC 95-2113-M006-017).
Lin et al. Supporting Information Available: HOCH2CH2NH2 temperature-programmed desorption spectra of Cu(100) (Supporting Figure 1); in contrast to the case of O/Cu(100), there is no 365 K desorption state; structural parameters and infrared absorptions of -CH2CH2NH- and -CH2CH2N- that we have calculated ( Supporting Tables 1–4); comparison of the structural parameters and infrared absorptions of -OCH2CH2NH- calculated with one- (16 atoms), two- (25 atoms), and three-slab (29 atoms) copper clusters (Supporting Tables 5 and 6). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Meeker, K.; Ellis, A. B. J. Phys. Chem. B 1999, 103, 995–1001. (2) Sexton, B. A. Surf. Sci. 1979, 88, 299–318. (3) Carley, A. F.; Davies, P. R.; Edwards, D.; Jones, R. V.; Parsons, M. Top. Catal. 2005, 36, 21–32. (4) Thornburg, D. M.; Madix, R. J. Surf. Sci. 1990, 226, 61–76. (5) Pirngruber, G. D.; Eder-Mirth, G.; Lercher, J. A. J. Phys. Chem. B 1997, 101, 561–568. (6) Madix, R. J.; Yamada, T.; Johnson, S. W. Appl. Surf. Sci. 1984, 19, 43–58. (7) Wuttig, M.; Franchy, R.; Ibach, H. Surf. Sci. 1989, 213, 103. (8) Standard Reference Database 69, NIST (National Institute of Standards and Technology) Chemistry WebBook, 2005. (9) Madix, R. J.; Thornburg, D. M. Surf. Sci. 1989, 220, 268–294. (10) Wachs, I. E.; Madix, R. J. Appl. Surf. Sci. 1978, 1, 303–328. (11) Bowker, M.; Madix, R. J. Surf. Sci. 1982, 116, 549–572. (12) Capote, A. J.; Madix, R. J. Surf. Sci. 1989, 214, 276–288. (13) Armand, J. C.; Robert, J. M. J. Am. Chem. Soc. 1989, 111, 3570– 3577. (14) Silva, C. F. P.; Durate, M. T. S.; Fausto, R. J. Mol. Struct. 1999, 482–483, 591–599. (15) Vorobyov, I.; Yappert, M. C.; DuPré, D. B. J. Phys. Chem. A 2002, 106, 668–679. (16) Mootz, D.; Brodalla, D.; Wiebcke, M. Acta Crystallogr. 1989, C45, 754–757. (17) Kuo, K.-H.; Lin, Y.-S.; Shin, J.-J.; Liao, Y.-H.; Lin, J.-S.; Yang, C.-M.; Lin, J.-L. J. Phys. Chem. C 2007, 111, 7757–7764. (18) Brannon, D. G.; Morrison, R. H.; Hall, J. L.; Humphrey, G. L.; Zimmerman, D. N. J. Inorg. Nucl. Chem. 1971, 33, 981–990.
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