Adsorption and Decomposition of Monoethanolamine on Cu(100

Feb 10, 2009 - Copyright © 2009 American Chemical Society. * E-mail: [email protected]. Phone: 886-6-2757575ext. 65326. Fax: 886-6-2740552...
1 downloads 0 Views 655KB Size
Download PDF
J. Phys. Chem. C 2009, 113, 4147–4154

4147

Adsorption and Decomposition of Monoethanolamine on Cu(100) Yi-Shiue Lin, Jain-Shiun Lin, Ching-Yung Wang, Che-Wei Kuo, and Jong-Liang Lin* Department of Chemistry, National Cheng Kung UniVersity 1, Ta Hsueh Road, Tainan, Taiwan 701, Republic of China ReceiVed: October 24, 2008; ReVised Manuscript ReceiVed: December 24, 2008

Temperature-programmed reaction/desorption and reflection-absorption infrared spectroscopy have been employed to investigate the adsorption and decomposition of HOCH2CH2NH2 on Cu(100). HOCH2CH2NH2 molecules desorb from the multilayer and monolayer with a maximum rate at 203 and 255 K, respectively. The desorption state at 255 K corresponds to an adsorption energy of 66.0 kJ · mol-1 calculated for a firstorder kinetics with a preexponential factor of 1013 s-1. RAIRS suggests that the multilayer and monolayer may be composed of different HOCH2CH2NH2 rotational isomers. HOCH2CH2NH2 can also decompose on Cu(100), approximately at the same temperature range of monolayer desorption, mainly to evolve H2 and H2O at ∼400 K. An Ar+-sputtered Cu(100) surface is found to promote the dissociation of HOCH2CH2NH2. Temperature-dependent RAIRS, with the assistance of theoretical calculations based on density-functional theory, strongly suggests that HOCH2CH2NH2 on Cu(100) dissociates first by losing a hydrogen atom to form -OCH2CH2NH2, followed by transformation into HOCH2CH2Nd. Decomposition of the latter species is responsible for the desorption of H2 and H2O. Introduction Monoethanolamine possesses two reactive centers of OH and NH2. H2S or CO2 generated in refineries, petrochemical plants, and other industries are often removed by amine treating. Aqueous solutions of alkanolamines, such as HOCH2CH2NH2 (MEA) and (HOCH2CH2)2NH, can be used to absorb H2S or CO2 from gases. In the case of CO2, it can react with RNH2 and ROH in basic solutions to form carbamate (RHNCOO-) and carbonate (ROCOO-), respectively.1 The integral heat of absorption of carbon dioxide in monoethanolamine has been measured to be ca. 25-120 kJ/mol of CO2 in a concentration of less than 2 mol of CO2/mol of MEA.2 The absorbed CO2 can be driven from the carbamate or carbonate by elevating the temperature. However, the absorption with MEA has irreversible side reactions, leading to the chemical loss of the absorber.3 It requires the replacement of ∼2.2 kg of MEA per ton of CO2 captured.3 Copper salts, which are generally used as corrosion inhibitors in CO2 capture processes, have been found to catalyze degradation of monoethanolamine.4 Another source of monoethanolamine loss during the operations of CO2 removal and absorber recovery is due to evaporation. One way to suppress the amine evaporation is to chemically tether the CO2-capture molecules to a substrate surface. Our present research is a model study for tethering HOCH2CH2NH2 on Cu(100), relating to acidgas recovery, and is also a fundamental study for understanding the dissociation process of this molecule on the crystal surface. Adsorption layers of alkanolamines on CdSe have been investigated by using photoluminescence as a probe for the reactivity toward CO2 binding.5 There are only a few reports on the adsorption and reactions of HOCH2CH2NH2 on single crystal surfaces. Monoethanolamine is found to decompose on Ni(100), evolving H2 (250-550 K), CO (330-550 K), and N2 (800-1100 K) in temperature-programmed reaction studies.6 On the oxygen-precovered Cu(100) surface, monoethanolamine * E-mail: [email protected]. ext. 65326. Fax: 886-6-2740552.

Phone:

886-6-2757575

Figure 1. Temperature-programmed reaction/desorption spectra showing HOCH2CH2NH2 desorption.

undergoes dehydrogenation to generate -OCH2CH2NH- and -OCH2CH2N- as adsorbed intermediates.7 In the present research, we investigate the adsorption and decomposition of monoethanolamine on bare Cu(100) surfaces, using reflectionabsorption infrared spectroscopy (RAIRS) and temperatureprogrammed reaction/desorption (TPR/D). Density-functionaltheory calculations were performed to assist in identification of the intermediates from HOCH2CH2NH2 dissociation on Cu(100).

10.1021/jp8095117 CCC: $40.75  2009 American Chemical Society Published on Web 02/10/2009

4148 J. Phys. Chem. C, Vol. 113, No. 10, 2009

Lin et al.

Figure 2. Temperature-programmed reaction/desorption spectra of 2.5 L of HOCH2CH2NH2 on Cu(100) prepared by Ar+-sputtering and annealing (a) or by Ar+-sputtering only (b).

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 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 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 deg/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 rationed 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 chromel-alumel thermocouple that was 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. Monoethanolamine (99.9%) was purchased from TEDIA and subjected to several cycles of freeze-pump-thaw. The purity of HOCH2CH2NH2 was checked by mass spectrometer. The gas manifold for monoethanolamine was conditioned by backfilling with saturated monoethanolamine vapor overnight. Prior to monoethanolamine being added to the chamber, the gas manifold and monoethanolamine itself (in a liquid state) were pumped

for approximately 10 min. The exposure of HOCH2CH2NH2 is denoted by langmuir (1 L ) 10-6 Torr · s) In our theoretical calculations with the Cerius2-DMol3 module based on the density functional theory (DFT), a cluster with two slabs of Cu atoms (16 and 9 atoms for the first and second layer, respectively), fixed at their lattice positions, was used to represent the Cu(100) surface. 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 convergence criteria for geometry optimizations were generally the threshold values: 1 × 10-5 hartree, 1 × 10-3 hartree/bohr, 1 × 10-3 bohr, and 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 In this section, we present the TPR/D results first to show the HOCH2CH2NH2 desorption states and the evolution of reaction products, followed by RAIRS to show the surface intermediates from HOCH2CH2NH2 decomposition, with the aid of the theoretical calculations. HOCH2CH2NH2 Molecular Desorption. Figure 1 shows the molecular desorption spectra represented by m/z 30, which is the most abundant ion measured in our quadrupole mass spectrometer from HOCH2CH2NH2 fragmentation, for the indicated exposures on Cu(100). Two additional traces of m/z 31 and 61 at 1.0 L are also exhibited to provide more evidence for the desorption of HOCH2CH2NH2. The state at 203 K has been observed in the previous study of HOCH2CH2NH2 on oxygen-precovered Cu(100)7 and is attributed to desorption from the multilayer molecules. The assignment is also supported by the continuous increase in desorption yield from 1.0 L to higher exposures in Figure 1. The heat of vaporization of HOCH2CH2NH2 has been reported to be 55.3 kJ · mol-1.8 The desorption channel next to the 203 K state grows with increasing exposure, but can reach a saturation yield. Both the desorption states at 203 and 255 K have the 61 amu fragment

Decomposition of Monoethanolamine on Cu(100)

J. Phys. Chem. C, Vol. 113, No. 10, 2009 4149 TABLE 2: Comparison of the Infrared Frequencies (cm-1) of HOCH2CH2NH2 (g’Gg’) HOC1H2C2H2NH2a (g′Gg′)/Cu(100)

HOCH2CH2NH2b

ca. mode δ(OH), F(C H2), F(C H2) δ(OH), F(NH2) F(NH2), F(C1H2) ν(C-C) F(C1H2), δ(C1C2N), ω(NH2) F(C1H2), F(C2H2), ω(NH2) ω(NH2), ν(C-O) ν(C-O), ω(NH2) ν(C-N), δ(C1OH), tw(C1H2) ν(C-C), δ(C1OH) δ(C1OH), tw(C1H2), tw(NH2) tw(CH2), ω(NH2) tw(C1H2), δ(C1OH) ω(C1H2), ω(C2H2) ω(C1H2), ω(C2H2) δ(CH2) δ(CH2) δ(NH2) νs(C2H2) νs(C1H2) νas(C2H2) νas(C1H2) νs(NH2) νas(NH2) ν(OH) 2

1

Figure 3. Temperature-dependent reflection-absorption infrared spectra of 2.0 L of HOCH2CH2NH2 adsorbed on Cu(100).

411 (25.5) 464 (28.2) 572 (33.4) 837 (4.5) 875 (15.8) 971 (60.9) 1027 (100) 1068 (13.1) 1102 (41.5) 1122 (8.0) 1254 (8.0) 1278 (19.4) 1317 (12.0) 1348 (52.7) 1381 (5.4) 1417 (26.1) 1425 (10.6) 1558 (2.1) 2918 (34.3) 2943 (33.8) 2968 (34.1) 2999 (8.91) 3214 (18.2) 3299 (2.1) 3343 (70.5)

TABLE 1: Comparison of the Infrared Frequencies (cm-1) of HOCH2CH2NH2 (g′Tg)

a Theoretical frequencies and relative intensities. Cu(100).

g′Tga

ca. modeb

768 (100) 803 (26.4)

ω(NH2) F(CH2)

967 (2.0)

ν(C-C)

1030 1096 1110 1137 1213 1271 1316 1341 1386 1447 1462 1618 2804 2868

(32.0) (49.8) (0.5) (0.9) (1.0) (5.2) (0.3) (32) (1.1) (0.4) (1.9) (25.9) (74.3) (43.8)

δ(COH) ν(C-O), ν(C-N) ν(C-C) tw(CH2) ω(CH2) tw(CH2) ω(CH2) δ(COH) ω(CH2) δ(CH2) δ(CH2) δ (NH2) νs(CH2) νs(CH2)

2989 3031 3373 3468 3596

(13.1) (11.8) (0.3) (2.9) (5.5)

νas(CH2) νas(CH2) νs(NH2) νas(NH2) ν(OH)

HOCH2CH2NH2c

857 1004 1037 1093

1617 2826 2868 2924

a

Frequencies and relative intensities (in parentheses), ref 7. b ω, wagging; F, rocking; tw, twisting; δ, bending; νs, symmetric stretching; νas, antisymmetric stretching. c 2 L, 120 K Cu(100).

(HOCH2CH2NH2+) and almost the same intensity ratio of m/z 30 to 31 (Im/z 30/Im/z 31). Therefore, the 255 K channel is due to desorption of HOCH2CH2NH2, but from first layer with an adsorption energy of 66.0 kJ · mol-1 calculated by using the Redhead equation of first-order kinetics with a preexponential factor of 1013 s-1 (E/RTp2 ) (ν/β) exp(-E/RTp), where E is the adsorption energy, R is the gas constant, ν is the preexponential factor, β is the heating rate, and Tp is the peak temperature).9

869 1014

2866 2933

b

2 L, 220 K

Note that the desorption of the first layer HOCH2CH2NH2 on O/Cu(100) has not been observed, because it decomposes on the oxidized surface at temperatures lower than that of desorption. There is also a desorption state at 400 K, showing the same Im/z 30/Im/z 31 as that for the desorption states at 203 and 255 K. This high-temperature state is also attributed to HOCH2CH2NH2 desorption, although the m/z 61 signal is too small to be measured. The RAIRS study shown latter suggests that the 400 K channel is from a recombinative process instead of direct desorption from intact HOCH2CH2NH2 molecules. Decomposition of HOCH2CH2NH2 on Cu(100). In the TPR/D investigation of the decomposition of HOCH2CH2NH2, ions with m/z at 2, 14, 15, 16, 17, 18, 26, 27, 28, 29, 30, 31, 32, 41, 42, 43, 44, 45, and 46 have been monitored to detect hydrogen, water, carbon monoxide, ethylene, and other compounds containing -CN, -CHO, -NH2, or OH. In the NIST Standard Reference Database,10 HCN has major fragment ions at 26 and 27 amu; C2H4 at 26, 27, and 28 amu; CH2O at 29 and 30 amu; CH3NH2 at 28, 30, and 31 amu; CH2CO at 14 and 42 amu; CH3CN at 40 and 41 amu; CH3CHO at 43 and 44 amu; CH3CH2OH at 31 and 45 amu; and CH3CH2NH2 at 28, 30, 44, and 45 amu. Figure 2a shows the TPR/D spectra collected for the ions with the indicated masses (2, 18, 26, 27, 28, 29, 30, and 31 amu) at 2.5 L exposure of HOCH2CH2NH2. There are no desorption peaks for the other ions monitored. The desorption states at 203 and 255 K have been attributed to HOCH2CH2NH2 molecular desorption. H2 (m/z 2) is mainly produced at 255 and 403 K. At the latter temperature, H2O (m/z 18) is generated as well. Although some other products may be formed in the decomposition of HOCH2CH2NH2 on Cu(100), a definite assignment for the desorption species is difficult due to the limitation of the tiny signals measured, for example, the desorption peak of m/z 29 at 265 and 410 K, which may likely be in part due to CH2O, and the peaks at 446 K for m/z 27 and 28, which may possibly be due to C2H4.

4150 J. Phys. Chem. C, Vol. 113, No. 10, 2009

Lin et al.

Figure 5. Temperature-dependent reflection-absorption infrared spectra of 0.25 L of HOCH2CH2NH2 adsorbed on Cu(100).

Figure 4. Theoretically predicted HOCH2CH2NH2 (g′Gg′) on Cu(100).

adsorption

structure

of

The small signals shown in Figure 2a may imply that they are due to desorption from sample holder instead of HOCH2CH2NH2 reaction on Cu(100). We have carried out an additional TPR/D experiment, but not in a fashion of line-ofsight, i.e., the Cu(100) surface was not positioned in the front of the sampling aperture of QMS during the temperature ramp. It is found that, except for m/z 2 with a barely observable peak at 403 K, all of the other traces (m/z 18, 26, 27, 28, 29, 30, and 31) become featureless (Supporting Information, Figure 1). This contrasting result indicates that the desorption species shown at elevated temperatures in Figure 2a are indeed from Cu(100) surface reactions. It is also found that the yields for the products remain approximately the same as the exposure is reduced down to 0.25 L, as shown in Figure 2 in the Supporting Information with H2 and H2O as an example. This result indicates that only a small number of HOCH2CH2NH2 molecules decompose on Cu(100) and may result from a competition between decom-

position and molecular desorption. This competition is demonstrated by the similar desorption temperature for H2 and HOCH2CH2NH2 in Figure 2a. However, the possibility for the involvement of surface defect sites cannot be ruled out. This possibility can be tested by using a sputtered Cu(100) surface in HOCH2CH2NH2 TPR/D studies. Figure 2b shows the TPR/D spectra of 2.5 L of HOCH2CH2NH2 on a rough Cu(100) surface produced by ion bombardment but without further annealing. Parts a and b of Figure 2 are displayed next to each other with the same ordinate scale. On the sputtered surface, dehydrogenation becomes facile, producing more H2. The desorption yield of H2O also increases. The large desorption peak of m/z 28 at 359 K is attributed to CO. Because the number ratio of 13C /12C in nature is ∼1%, 13CO contributes to the signal observed between 300 and 400 K for m/z 29. However, the peak shapes for m/z 28 and 29 are not exactly the same in this temperature range, indicating that another product, very likely CH2O, also contributes to the m/z 29 at 376 K. The desorption state of m/z 27 at 437 K is probably due to HCN and/or C2H4. The desorption temperature and distribution of the products generated from HOCH2CH2NH2 decomposition on Cu(100) and rough Cu(100) are not the same, presumably due to the difference in the bonding nature of reaction sites on both surfaces. For Cu(100), the outmost atom layer contains (100) terraces, with defect sites including steps and kinks as well as adatoms and vacancies located at (100) terraces. On the other hand, large areas of (100) terraces are unlikely to exist on the sputtered Cu(100) and the surface exposed after ion bombardment is expected to be rough, possibly with small areas of different facets and with low coordinated copper atoms. Vibrational Investigations of HOCH2CH2NH2 Adsorption and Its Decomposition Intermediates. In this section, temperature-dependent RAIR spectra are presented to show the presence of HOCH2CH2NH2 adsorbed at multilayer and monolayer states and the temperature range of HOCH2CH2NH2 dissociation. Two different surface intermediates from HOCH2CH2NH2 decomposition on Cu(100) are detected in terms of the variation of the infrared absorptions with temperature.

Decomposition of Monoethanolamine on Cu(100)

J. Phys. Chem. C, Vol. 113, No. 10, 2009 4151

TABLE 3: Comparison of the Infrared Frequencies (cm-1) of -OCH2CH2NH2 HOCH2CH2NH2/Cu(100) -OC H2C H2NH2/Cu(100) 1

2

452 (0.5) 552 (1.3) 838 (4.7) 862 (13.4) 965 (100) 1026 (82.1) 1072 (4.2) 1121 (39.0) 1152 (4.3) 1248 (8.9) 1256 (3.8) 1322 (16.1) 1354 (4.9) 1407 (1.06) 1411 (20.6) 1561 (13.0) 2779 (8.4) 2852 (70.6) 2919 (13.4) 2940 (50.4) 3253 (12.9) 3333 (0.3)

ca. mode

2 L, 260 K

δ(OC C ), F(NH2), F(C H2) F(NH2), δ(C2C1N) F(C2H2), ω(NH2), δ(OC1C2) ν(C1-C2), δ(C2C1N) ω(NH2), F(CH2) ω(NH2), ν(C1-O) ν(C2-N), δ(OC1C2) ν(C1-O), ν(C2-N) tw(CH2), δ(NH2) tw(CH2) tw(CH2) ω(CH2) ω(CH2) δ(CH2) δ(CH2) δ(NH2) νs(CH2) νs(CH2) νas(C1H2) νas(C2H2) νs(NH2) νas(NH2) 1

2

0.25 L, 260 K

2

987 1020

987 1018

2862 2908

2852 2906

TABLE 4: Comparison of the Infrared Frequencies of HOCH2CH2Nd HOCH2CH2NH2/Cu(100) 1

2

HOC H2C H2Nd/Cu(100) 406 (49.0) 498 (100) 586 (4.8) 859 (2.4) 889 (4.0) 1015 (5.3) 1042 (14.4) 1063 (13.1) 1128 (3.1) 1208 (3.6) 1232 (7.3) 1281 (0.5) 1366 (0.4) 1401 (4.7) 1425 (3.5) 2851 (17.0) 2891 (20.4) 2943 (10.7) 2964 (33.9) 3263 (87.7)

ca. mode

2 L, 300 K

δ(OH) δ(OH) F(C1H2) F(CH2) ν(C1-C2) ν(C2-N), F(C2H2), δ(HOC1) ν(C1-O) ν(C2-N) δ(HOC1), tw(C2H2), ω(C1H2) tw(CH2), δ(HOC1) tw(CH2), δ(HOC1) ω(C2H2) ω(C1H2), δ(HOC1) δ(C2H2) δ(C1H2) νs(C1H2) νs(C2H2) νas(C2H2) νas(C1H2) ν(O-H)

Monoethanolamine is a three-rotor molecule whose conformers can be 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 by using the extended 6-31G* basis set and in the DFT framework by using B3LYP/6-311-++G(2d,2p).11,12 Both theoretical studies predict 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 estimated to be 63% or 73%.11,12 The crystal structure of HOCH2CH2NH2 (mp 283.3

0.25 L, 300K

852 999 1076

999 1055 1076

2852 2900

2852 2900

K) has been examined previously; the molecules in the threedimensional network exist in g′Tg form.13 Figure 3 shows the RAIR spectra of 2.0 L of HOCH2CH2NH2 adsorbed on Cu(100) at 120 K, followed by successively briefly annealing the surface to the indicated temperatures. All of the spectra were measured at ∼120 K. In the 120 K spectrum, the infrared bands appear at 857, 1004, 1037, 1093, 2826, 2868, and 2924 cm-1. From 120 to 190 K, the absorption feature remains the same. Increasing the surface temperature slightly higher to 200 K causes intensity decrease for some peaks, for example, 1037 and 1093 cm-1. In the 220 K spectrum, the absorption feature changes and the infrared bands are located at 869, 1014, 2866, and 2933 cm-1. This change occurring in the narrow temperature range 190-220 K corresponds to the desorption of multilayer HOCH2CH2NH2 molecules with the onset temperature at 190 K. The set of peaks observed in the 120 K spectrum is listed in Table 1 and compared to the calculated infrared absorptions of the g′Tg conformer of

4152 J. Phys. Chem. C, Vol. 113, No. 10, 2009

Figure 6. Theoretically -OCH2CH2NH2.

predicted

adsorption

Lin et al.

structure

of

HOCH2CH2NH2 together with the corresponding modes. It is found that the set of the peaks, except 857, 1004, and 2924 cm-1, which are due to first-layer adsorption molecules and discussed later, agrees with the absorptions of the g′Tg molecule. This infrared match also suggests the conformation that the multilayer HOCH2CH2NH2 molecules may adopt. On the basis of the TRD result shown in Figure 1, heating the surface covered with 2.0 L of HOCH2CH2NH2 to 220 K cannot remove the firstlayer HOCH2CH2NH2 molecules significantly. Therefore, the peaks observed in the 220 K spectrum of Figure 3 are attributed to HOCH2CH2NH2 directly bound to Cu(100). We have calculated the optimized adsorption structure of g′Gg′ HOCH2CH2NH2 (the dominant form in the gas phase) on Cu(100) and its infrared absorptions. These frequencies are listed in Table 2 and compared to the bands detected after heating the surface to 220 K. It is found that the experimental bands are consistent with the theoretically predicted infrared absorp-

Figure 7. Theoretically HOCH2CH2Nd.

predicted

adsorption

structure

of

tions of g′Gg′ on Cu(100). Accordingly, the 869 cm-1 is assigned to the F(CH2) + δ(CCN) + ω(NH2); 1014 cm-1 to ω(NH2) + ν(CO); and 2866 and 2933 cm-1 to νs(CH2). The optimized g′Gg′ adsorption structure is shown in Figure 4. Note that the theoretical infrared spectra only provide a prediction for possible HOCH2CH2NH2 rotation isomers present on the surface and that the interaction between adsorbed molecules is not taken into account in this calculation, which may lead to a change in the adsorption orientation with respect to the surface and in the relative infrared peak intensities. The absorption peaks for the first-layer HOCH2CH2NH2 molecules decrease in intensity after annealing the surface to 240 K. This result corresponds to the molecular desorption with the maximum rate at 255 K in the TPD study (Figure 1). Meanwhile, a new peak emerges at 987 cm-1. The concurrence of decrease of first-layer adsorption molecules and appearance of the new peak indicates that both desorption and decomposition of HOCH2CH2NH2 molecules on Cu(100) happen at 240

Decomposition of Monoethanolamine on Cu(100)

J. Phys. Chem. C, Vol. 113, No. 10, 2009 4153

TABLE 5: Structural Parametersa for -OCH2CH2NH2 and HOCH2CH2Nd -OC1H2C2H2NH2/Cu(100) d(C -H ) d(C1-H1b) d(C2-H2a) d(C2-H2b) d(C1-C2) d(C1-O) d(C2-N) d(N-H3a) d(N-H3b) h(Cus-O) h(Cus-N) θ(OC1C2) θ(C1C2N) θ(NH2) θ(C1H2) θ(C2H2) θ(C1OSN) θ(C2NSN) 1

a

1a

1.11Å 1.12Å 1.12Å 1.11Å 1.51Å 1.39Å 1.46Å 1.04Å 1.04Å 1.47Å 2.02Å 116.5° 113.1° 107.1° 106.8° 107.8° 153.4° 118.0°

HOC1H2C2H2Nd/Cu(100) d(C -H ) d(C1-H1b) d(C2-H2a) d(C2-H2b) d(C1-C2) d(C1-O) d(C2-N) d(O-H) 1

1a

1.11Å 1.11Å 1.11Å 1.11Å 1.50Å 1.43Å 1.45Å 1.00Å

h(Cus-O) h(Cus-N) θ(OC1C2) θ(C1C2N) θ(C1OH) θ(C1H2) θ(C2H2) θ(C1OSN) θ(C2NSN)

2.10Å 0.91Å 107.2° 110.8° 110.8° 109.9° 107.1° 117.6° 174.0°

d, bond length; θ, bond angle; h, height from the surface; SN, surface normal; Cus, copper surface.

K. In the 260 K spectrum, the infrared bands due to adsorbed HOCH2CH2NH2 are not present and a new set of peaks are located at 987, 1020, 2862, and 2907 cm-1. Formation of a surface intermediate as suggested by the new peaks in the 260 K spectrum is consistent with H2 desorption at ∼255 K in Figure 2a. The H2 production also provides a clue for the dissociation process of HOCH2CH2NH2 on Cu(100), i.e., dehydrogenation at 260 K. In the recent reaction study of HOCH2CH2NH2 on oxygen-covered Cu(100), HOCH2CH2NH2 can lose hydrogen to form two cyclic surface intermediates of -OCH2CH2NHand -OCH2CH2Nd.7 The former species has infrared absorptions at 854, 1057, 1092, 2816, 2851, and 2895 cm-1 and is stable up to 360 K. The latter species is derived from dehydrogenation of -OCH2CH2NH- at a temperature higher than 360 K and absorbs at 1015 and 1057 cm-1. Note that the major peaks located at 987 and 1020 cm-1 in the 260 K spectrum are no longer present after increasing the surface temperature to 300 K. So, -OCH2CH2NH- and -OCH2CH2Nd are unlikely to be the species responsible for the 260 K spectrum on the basis of the infrared peak frequencies and the thermal stability on the surface. To identify the possible species from dehydrogenation of HOCH2CH2NH2 on Cu(100) at 260 K, we have calculated the optimized adsorption structures of -OCH2CH2NH2, HOCH2CH2NH-, and HOCH2CH2Nd as well as their infrared absorption bands (Tables 3 and 4, as well as Table 1 in the Supporting Information). It is found that the theoretically predicted spectrum of -OCH2CH2NH2 is consistent with the measured 260 K spectrum of Figure 3. The comparison between the theoretical and experimental bands of -OCH2CH2NH2 as well as mode assignments are included in Table 3. In the 280 K spectrum of Figure 3, new peaks at 999 and 1076 cm-1 appear, being accompanied by the intensity decrease of 987 and 1020 cm-1. This result reveals the decomposition of -OCH2CH2NH2 and transformation into a new surface intermediate. As the surface is heated to 300 K, the infrared bands measured are located at 999, 1076, 2852, and 2900 cm-1. This set of peaks disappears in the 380 K spectrum. From 260 to 340 K, the absorptions of CH stretching vibrations are not largely perturbed, signifying integrity of the CH2CH2 moiety in the decomposition of -OCH2CH2NH2. It is further found that the new set of bands in the 300 K spectrum is consistent with the calculated bands of HOCH2CH2Nd (Table 4).

Figure 5 shows the RAIR spectra of 0.25 L of HOCH2CH2NH2 absorbed on Cu(100) at 120 K, followed by successively briefly annealing the surface to the indicated temperatures. At this smaller exposure, no HOCH2CH2NH2 multilayer exists based on the TPRD result. Therefore, the 120 K spectrum is attributed to HOCH2CH2NH2 directly bound to the Cu(100) surface, showing band frequencies (Table 1) similar to those observed in the 220 K spectrum of Figure 3, which is also due to first-layer HOCH2CH2NH2 but with a higher coverage. However the peak at 973 cm-1 in the 120 K spectrum of Figure 5 is relatively strong, suggesting a change in orientation for the adsorbed HOCH2CH2NH2, as compared to the 2.0 L case on average. In Figure 5, the absorption feature of 120 and 180 K spectra is the same. The peaks at 987 and 1018 cm-1 due to -OCH2CH2NH2 become discernible in the 240 K spectrum, which exists until 300 K. The peaks due to HOCH2CH2Nd start to appear at 260 K and completely disappear at 380 K. Both RAIRS studies (2.0 and 0.25 L) show similar temperature-dependent changes for HOCH2CH2NH2 decomposition on Cu(100). The HOCH2CH2Nd surface intermediate is responsible for the desorption of H2, H2O, and HOCH2CH2NH2 at ∼400 K observed in Figure 2a. Dissociation of the HO-C bond would generate OH groups on Cu(100), leading to the evolution of H2O.14 H2 is a result of dehydrogenation of the intermediate. Recombination of HOCH2CH2Nd with surface H can form HOCH2CH2NH2, which is desorbed from the surface at ∼400 K. Scavenging of surface H atoms by CH3CH2N to form ethylamine on Ag(110) at 370 K has been reported previously.15 Figures 6 and 7 show the theoretically predicted adsorption structures of the intermediates -OCH2CH2NH2 and HOCH2CH2Nd with three different viewing directions perpendicular to each other. Meanwhile, the detailed structural parameters for these two surface species are listed in Table 5. The -OCH2CH2NH2 is bonded at a bridge site with a gauche conformation with respect to the C-C bond, which is parallel to the surface. For the case of HOCH2CH2Nd, it is bonded at a hollow site, also with a gauche conformation. Surface intermediates such as -CH2CH2NH2, -CH2CH2NH-, and -CH2CH2N- possibly generated from C-O bond scission of HOCH2CH2NH2 are not expected to be responsible for the 260 and 300 K spectra in Figures 3 and 5, since there is no H2O desorption prior to 370 K. Previously, it has been shown that

4154 J. Phys. Chem. C, Vol. 113, No. 10, 2009 surface OH groups on copper can evolve H2O at ∼220 K.14 -CH2CH2O- possibly generated from C-N bond scission of HOCH2CH2NH2 cannot play a role either, in terms of the previous report regarding its infrared absorptions and thermal stability on Cu(100).16 -CH2CH2O- has vibrational bands measured at ∼1000 and ∼1060 cm-1 and is only stable up to ∼280 K. Other possible intermediates involving C-C bond scission of HOCH2CH2NH2 such as -CH2NH2, -CH2NH-, -CHdNH, -NdCH2, -CH2O-, and -CH2OH are ruled out to be the species that produce the 260 and 300 K spectra, since their theoretically predicted vibrational absorptions are inconsistent with the experimental results. Conclusion The adsorption and reactions of HOCH2CH2NH2 on Cu(100) have been studied under ultrahigh vacuum conditions. The TPR/D studies with a heating rate of 2 K/s show that the desorption energy for monolayer HOCH2CH2NH2 molecules is 66.0 kJ · mol-1. Moreover, studies of RAIRS and DFT show that the monolayer and multilayer may contain different isomers, g′Gg′ or g′Tg. HOCH2CH2NH2 can decompose on Cu(100) at ∼250 K to form -OCH2CH2NH2 intermediate, followed by further transformation into HOCH2CH2Nd. The latter species decomposes at ∼400 K, generating H2 and H2O. Acknowledgment. . This work was supported by the National Center for HighPerformance Computing and by the National Science Council of the Republic of China (NSC 97-2113M-006-004-MY2). Supporting Information Available: TPR/D spectra of 2.5 L of HOCH2CH2NH2 collected not in a line-of-sight mode

Lin et al. (Figure S1); TPR/D spectra of m/z 2 and 18 at 0.25, 1.0, 1.5, and 2.5 L (Figure S2); and theoretically predicted HOCH2CH2NH- infrared frequencies (Table S1). 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) Kim, I.; Svendsen, H. F. Ind. Eng. Chem. Res. 2007, 46, 5803– 5809. (3) Straziser, B. R.; Anderson, R. R.; White, C. M. Energy Fuels 2003, 17, 1034–1039. (4) Goff, G. S.; Rochelle, G. T. Ind. Eng. Chem. Res. 2006, 45, 2513– 2521. (5) Meeker, K.; Ellis, A. B. J. Phys. Chem. B 1999, 103, 995–1001. (6) Madix, M. Y.; Yamada, T.; Johnson, S. W. Appl. Surf. Sci. 1984, 19, 43–58. (7) Lin, Y.-S.; Wang, C.-Y.; Yang, C.-M.; Lin, J.-S.; Kuo, C.-W.; Lin, J.-L. J. Phys. Chem. C 2008, 112, 8304–8310. (8) da Silva, E. F.; Kuznetosva, T.; Kvamme, B.; Merz, M., Jr J. Phys. Chem. B 2007, 111, 3695–3703. (9) Redhead, P. A. Vacuum 1962, 2, 203–211. (10) Standard Reference Database 69, NIST (National Institute of Standards and Technology) Chemistry Webbook, 2005. (11) Silva, C. F. P.; Durate, M. T. S.; Fausto, R. J. Mol. Struct. 1999, 482-483, 591–599. (12) Vorobyov, I.; Yappert, M. C.; Dupre, D. B. J. Phys. Chem. A 2002, 106, 668–679. (13) Mootz, D.; Brodalla, D.; Wiebeke, M. Acta Crystallogr. 1989, C45, 754–757. (14) Eills, T. H.; Kruus, E. J.; Wang, H. J. Vac. Sci. Technol. A 1993, 11, 2117–2121. (15) Thornburg, D. M.; Madix, R. J. Surf. Sci. 1990, 226, 61–76. (16) Chang, P. T.; Shih, J.-J.; Kuo, K.-H.; Chen, C.-Y.; Fu, T.-W.; Shieh, D.-L.; Liao, Y.-H.; Lin, J.-L. J. Phys. Chem. B 2004, 108, 13320–13328.

JP8095117