2-Ethanolamine on TiO2 Investigated by in Situ Infrared Spectroscopy

J. Phys. Chem. C 2010, 114, 11835–11843

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2-Ethanolamine on TiO2 Investigated by in Situ Infrared Spectroscopy. Adsorption, Photochemistry, and Its Interaction with CO2 Chien-Lin Tseng, Yi-Kwan Chen, Shuai-Han Wang, Zih-Wei Peng, and Jong-Liang Lin* Department of Chemistry, National Cheng Kung UniVersity, 1, UniVersity Road, Tainan, Taiwan, Republic of China ReceiVed: December 11, 2009; ReVised Manuscript ReceiVed: May 13, 2010

In situ Fourier-transform transmission infrared spectroscopy has been employed to investigate the adsorption and photochemistry of 2-ethanolamine (HOCH2CH2NH2) on TiO2 as well as the interaction of CO2 with 2-ethanolamine-modified TiO2 surfaces. Intact HOCH2CH2NH2 and the dissociative form of OCH2CH2NH2 are found to be present on the surface with the saturation coverage of 2-ethanolamine at 35 °C, in comparison to the adsorption of CH3CH2CH2OH and CH3CH2CH2NH2 on TiO2. CO2 reacts with the -NH2 basic centers of the surface 2-ethanolamine molecules, forming carbamate (-NHCOO-) and ammonium (-NH3+). Bicarbonate (HCO3-) is also formed due to the presence of residual water. The carbamate species of OCH2CH2NHCOO- has a better thermal stability than HCO3-. In the presence of O2, as the TiO2 surface with the saturation coverage of 2-ethanolamine is photoirradiated at 325 nm, the infrared studies suggest the formation of isocyanate (NCO), absorbed CO2, OOCCH2NH2, and carbonyl-containing species of formamide (HCONH2) and formic acid (HCOOH). However, for the OCH2CH2NH2/TiO2 surface, H2O, formate (HCOO), carbonates, and a surface species carrying a CdN group are found, in addition to NCO. The photoproducts of NCO and absorbed CO2 reveal the C-C bond dissociation pathway of the surface 2-ethanolamine under illumination. Photoreaction mechanisms involving hole capture at different reaction centers and formation of organoperoxide or tetraoxide species have been proposed, on the basis of the products and intermediates identified in the 2-ethanolamine photocatalytic degradation on TiO2 in the presence of 16O2 and 18O2. 1. Introduction Carbon dioxide is one of the major greenhouse gases that cause climate change. Control of CO2 emission has become a global concern and attracted extensive attention. Limiting the amount of CO2 produced or emitted into the atmosphere can be resorted to several approaches, such as separation and removal, employment of substitutional energy, and increase for the efficiency of fossil plants. In the respect of separation and removal, CO2 capture with various adsorbents and absorbents has been reported. The materials that have been employed for this purpose include (1) activated, porous or metal-oxide-loaded carbons;1-8 (2) zeolites;9-12 (3) metal-oxides, alkali-metal-doped metal-oxides and clays;13-18 (4) porous silica modified with amino groups.19-25 To date, aqueous solutions of alkanolamines have been commonly used in the amine treating processes to remove acid gases including CO2 generated in petrochemical plants, refineries and other industries. However, it requires replacement of the alkanolamines, for example, ∼2.2 kg of HOCH2CH2NH2 per ton of CO2 captured.26 Evaporation is one of the sources of the ethanolamine loss during the operations of CO2 removal and absorbent recovery. Chemical immobilization of alkanolamines to a substrate is a promising approach to solve this problem. In terms of the reasons listed below, we have investigated the HOCH2CH2NH2/TiO2 system by in situ transmission infrared spectroscopy and the interaction of 2-ethanolamine-modified surfaces with CO2. (1) HOCH2CH2NH2 is often used in the amine treating processes. (2) TiO2 is often used as photocatalyst. The escape of alkanolamines in the aqueous amine-treating * Corresponding author. Phone: 886 6 2757575ext. 65326. E-mail: [email protected].

processes would cause environmental damage. Therefore, photochemical reactions of HOCH2CH2NH2 on TiO2 are worthy to be explored. (3) 2-Ethanolamine molecules possess two distinct functional groups, OH and NH2. Their interaction with TiO2 determines the surface chemical structure of HOCH2CH2NH2-modified surfaces. (4) This system has a potential to be employed as CO2 absorbent. Infrared spectroscopy has been shown to be useful to prove the formation of carbamates and carbamic acids, as amines are used for CO2 removal.21,25-30 However, there are some ambiguous peak assignments, as shown in Table 1 in the previous CO2 studies.21,25,28-30 For example, strong, broad infrared peaks at ∼1305 cm-1 have been observed after bubbling CO2 into CH3OH solutions containing isopropylamine or 3-(1-naphthyl)propylamine, which are ascribed to HCO3- due to the presence of H2O impurity.28,29 Similar infrared peaks near 1315 cm-1 are also found, but unassigned, in the system of aminosilanefunctionalized mesoporous silica SBA-1521 or assigned to the NCOO skeletal vibration of carbamate in the polyethylenimine/ SBA-15 case.25 Furthermore, the bands from 1628 to 1650 cm-1 in Table 1 have been attributed to different species of ammonium, carbamate, and carbamic acid. The inconsistency in peak assignment needs to be further clarified. In this report, we present the results for the adsorption and thermal stability of HOCH2CH2NH2 on TiO2, the interaction of CO2 with the ethanolamine-functionalized TiO2 surfaces, and the photochemistry of HOCH2CH2NH2 on TiO2. 2. Experimental Section TiO2 powder (Degussa P25, ∼50 m2/g, anatase 70%, rutile 30%) was dispersed in water/acetone solution to form a uniform

10.1021/jp9117166  2010 American Chemical Society Published on Web 06/18/2010

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TABLE 1: Comparison of the Major Infrared Bands (cm-1) Due to CO2 Capture polyethylenimine/ SBA-15 (ref 25) 1650

δ(NH3+) of ammonium

aminosilane/ SBA-15 (ref 21) 1628 1563

1520

1410 1320 a

δ(NH3+) of ammonium ν(CdO) of carbamate

isopropylamine in dioxane (ref 28) 1650

carbamic acid

1575

carbamate

polyallylamine in CH2Cl2 (ref 30) 1631

3-(1-naphthyl)-propylamine in CHCl3 (ref 29)

ν(CdO) of carbamate 1575

NHCOO- of carbamate

ν(CdO) of carbamate NCOO vibration of carbamate NCOO vibration of carbamate

1488

NCOO vibration of carbamate

1315

unassigned

1305a

HCO3-

1308a

HCO3-

Observed in CH3OH.

mixture, which was then sprayed onto a tungsten mesh.31 After that, the TiO2 sample was mounted inside the IR cell for in situ FTIR spectroscopy. The IR cell with two CaF2 windows for IR transmission down to ∼1000 cm-1 was connected to a gas manifold that was pumped by a turbomolecular pump with a base pressure of ∼1 × 10-7 Torr. The TiO2 sample in the cell was heated to 450 °C under vacuum for 24 h by resistive heating. The temperature of the TiO2 sample was measured by a K-type thermocouple spotwelded on the tungsten mesh. Before each run of the experiment, the TiO2 temperature was held at 450 °C in a vacuum for 2 h and at 350 °C for 0.5 h in the presence of 3.0 Torr O2. After the heating treatment, the cell was evacuated. Ten Torr of O2 was introduced into the cell as the sample was cooled to 70 °C. When the TiO2 temperature reached 35 °C, the cell was evacuated again for gas or vapor dosing and an infrared spectrum was taken as reference background. HOCH2CH2NH2 (99.9%), CH3CH2CH2OH (95%), CH3CH2CH2NH2 (99+%), CO2 (99.999%), O2 (99.998%), and 18 O2 (99 atom %) used in this study were purchased from Tedia, Dsaka, Alfa Aesar, Praxair, Matheson, and Isotec, respectively. The cell pressure was monitored with a Baratron capacitance manometer and an ion gauge. In the photochemistry study, both the UV and IR beams were 45° to the normal of the TiO2 sample. The UV light source used was a combination of a 350-W Hg arc lamp (Oriel Corp), a water filter, and a bandpass filter with a bandwidth of ∼100 nm centered at ∼325 or ∼400 nm. The photon power at the position of the TiO2 sample was ∼0.24 W/cm2 for the 325 nm light and ∼0.14 W/cm2 for the 400 nm one. HOCH2CH2NH2 absorption to the light used in the present study is negligible.32 Infrared spectra were obtained with a 4 cm-1 resolution by a Bruker FTIR spectrometer with a MCT detector. The entire optical path was purged with CO2-free dry air. The spectra presented here have been ratioed against the TiO2 background spectrum. 3. Results and Discussion 3.1. Adsorption of 2-Ethanolamine on TiO2. 2-Ethanolamine has the functional groups of OH and NH2 bonded to two neighboring methylene groups. To understand the interaction between the functional groups and the TiO2 surface, adsorption infrared spectra of HOCH2CH2NH2, CH3CH2CH2OH, and CH3CH2CH2NH2 on TiO2 have been measured and analyzed. Figure 1 shows the infrared spectra after the TiO2 surface (35 °C) is covered with 2-ethanolamine at the saturation coverage, followed by surface heating to the indicated temperatures for 1 min in a vacuum. The saturation coverage of 2-ethanolamine on TiO2 was obtained by cycles of introduction of the ethanolamine vapor into the cell with the TiO2 held at 35 °C and

Figure 1. Infrared spectra taken at the indicated temperatures in a vacuum after adsorption of 2-ethanolamine on TiO2 at 35 °C.

subsequent evacuation of the cell until no variation in infrared absorption was found. The same dosing procedure was also carried out for the adsorption of CH3CH2CH2OH and CH3CH2CH2NH2. Figure 2 shows their temperature-dependent spectra. In the 35 °C spectrum of Figure 1, the enhanced absorptions due to the ethanolamine adsorption appear at 1051, 1107, 1182, 1254, 1294, 1362, 1456, 1587, 2835, 2872, 2922, 2945, 3136, and 3244 cm-1. These frequencies are compared to the absorption bands of HOCH2CH2NH2 in liquid and gas states in Table 2. HOCH2CH2NH2 has several rotational isomers that can be characterized by the dihedral angles in the order lpN-N-C-C, O-C-C-N, and H-O-C-C.33 lpN denotes the lone pair on the nitrogen atom. The rotational isomers with respect to the rotation about the C-C bond are labeled by G (gauche), G′, or T (trans). For the other two cases of H-O-C-C and lpN-N-C-C, the lower case letter is used instead. G, G′, and T correspond to the dihedral angles close to +60°, -60° and +180°, respectively. g′Gg′ is the most abundant conformer for HOCH2CH2NH2 in the gas phase at room temperature in contrast to the most populated gGt in the liquid state.33 The negative peaks between 3600 and 3750 cm-1 in the 35 °C spectrum are due to the decrease of isolated surface OH groups after their interaction with the adsorbed HOCH2CH2NH2 via hydrogen bonding. In Figure 1, the peak intensities gradually diminish with the increase of temperature. After the surface is

2-Ethanolamine on TiO2

J. Phys. Chem. C, Vol. 114, No. 27, 2010 11837 TABLE 2: Comparison of the Infrared Frequencies (cm-1) of HOCH2CH2NH2 adsorption on TiO2

Figure 2. Infrared spectra taken at the indicated temperatures in a vacuum after adsorption of 1-propanol (a) and 1-propylamine (b) on TiO2 at 35 °C.

heated to 275 °C in a vacuum, the NH2 scissoring intensity at 1587 cm-1 has decreased to ∼82%, compared to the original one (35 °C). Meanwhile the 2945 cm-1 shoulder, which can be assigned to the CH2 antisymmetric stretching mode of HOCH2CH2NH2 molecule according to Table 2, disappears. These changes indicate thermal desorption of intact surface HOCH2CH2NH2 molecules. The peak positions observed in the 275 °C spectrum are also listed in Table 2 and are attributed to chemisorbed OCH2CH2NH2, which is bonded to the surface via the oxygen atom, in comparison to the spectral changes of CH3CH2CH2OH and CH3CH2CH2NH2 on TiO2 as a function of temperature. With respect to 1-propanol adsorption, the peaks at 1228, 1400, and 3421 cm-1 observed in the 35 °C spectrum of Figure 2a, which are assigned to the CH2 twisting, OH in-plane bending, and OH stretching modes of CH3CH2CH2OH on the surface (Table 3), respectively, disappear after the surface temperature is raised to 275 °C, indicating desorption or O-H bond breakage of propanol itself. In addition, the 1632 cm-1 peak suggests water formation due to dehydrogenation of 1-propanol on the surface. The remaining bands in the 275 °C spectrum are attributed to propanoxy (CH3CH2CH2O) groups on the surface. Previous adsorption studies for methanol and ethanol on TiO2 have shown the similar dissociative pathway via OH deprotonation.35 In the case of 1-propylamine adsorption on TiO2, the infrared bands measured at 35 and 275 °C are listed in Table 3 and compared to the infrared and Raman frequencies of liquid 1-propylamine. The previous infrared study of 1-butylamine on TiO2 at room temperature has established its molecular adsorption state with a lower NH2 deformation frequency and a large

gasa

mode

3698 3676 3660 3584 3570 3442 3356

ν(OH)

vas(NH2) vs(NH2)

2949

vas(CH2)

2882 2861

vas(CH2) vs(CH2)

1623 1471 1462

δ(NH2) δ(CH2) δ(CH2)

1385 1375 1359

ω(CH2) ω(CH2) δ(OH)

1230

tw(CH2)

1100 1083 1037

liquida

3347b

mode

35 °C

ν(OH) + v(NH2)

2921

vas(CH2)

2860

vs(CH2)

1595 1482 1462 1456 1396

δ(NH2) δ(CH2) δ(CH2) ω(CH2) ω(CH2)

F(CH2) v(C-N)

1360 1321 1242 1165

δ(OH) tw(CH2) tw(CH2) F(CH2)

1074

v(C-O)

1028

v(C-O) + F(CH2) v(C-N)

275 °C

3354 3239c 3134c

3244c 3136c 2945 2922

2924

2872 2835 1587

2873 2841 1583

1456

1456

1362

1366

1294 1254 1182

1290 1254 1184

1107

1093

1051

1045

a Reference 34. b Broad peak. c 3134-3244 cm-1: overtone of δ(NH2). 2835-2841 cm-1: vs(CH2).

increase in the NH2 wagging frequency, as compared to the vibrational absorptions of liquid 1-butylamine.37 All the observed bands in the 35 °C spectrum of Figure 2b can be attributed to molecular 1-propylamine. Increasing the temperature from 35 to 150 °C leads to the preferential decrease of the peaks at 1057, 1151, and 1595 cm-1, due to desorption of relatively weakly adsorbed 1-propylamine molecules. The overall peak intensity diminishes in response to the temperature rise to 275 °C; however, no new bands are observed. This result suggests that the surface 1-propylamine molecules continue to desorb in this temperature range, without dissociation to generate new surface species. In the 35 °C spectrum of 2-ethanolamine (Figure 1), there exists strong, broad absorptions between 1000 and 1150 cm-1, which are characteristic of alkoxy groups, for example, in the case the propanoxy groups shown in Figure 2a. This result, together with the presence of NH2 groups (∼1585 cm-1), is indicative of OCH2CH2NH2 formation at 35 °C. 3.2. CO2 Absorption by 2-Ethanolamine-Modified TiO2. Two 2-ethanolamine-modified TiO2 surfaces are employed to investigate CO2 absorption, i.e., one covered with OCH2CH2NH2 and HOCH2CH2NH2 simultaneously, which is made with the saturation coverage of 2-ethanolamine on TiO2 at 35 °C, and the other one covered with OCH2CH2NH2 only, which is made by heating the saturation coverage to ∼275 °C in a vacuum, using the same procedure as that for Figure 1. Figure 3 shows the spectra for the two 2-ethanolamine-modified surfaces at 35 °C before ((a) and (d)) and after ((b) and (e)) being in contact with 2-Torr CO2 in the cell. To clearly demonstrate the change in infrared frequency due to CO2 absorption by the surface 2-ethanolamine, the difference spectra of (b) - (a) and (e) (d) are also shown in the figure. The major enhanced bands

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TABLE 3: Comparison of the Vibrational Frequencies (cm-1) for CH3CH2CH2OH and CH3CH2CH2NH2 CH3CH2CH2OH

CH3CH2CH2NH2

adsorption on TiO2 mode

35 °C

275 °C

3333 2963 2937

v(OH) vas(CH3) vas(CH2)

2878

vs(CH2)

3421 2966 2939 2921 2879 2856

2968 2941 2923 2881 2864

1447 1467 1457 1439 1422 1383

δas(CH3) δ(CH2) δas(CH3) δ(CH2) δip(OH) δs(CH3)

1460

1460

1346 1330 1294 1271

ω(CH2) δip(OH) ω(CH2) tw(CH2)

1439 1400 1381 1367 1342

1235 1132 1100 1069

tw(CH2) F(CH2) F(CH3) v(C-C) + v(C-O) v(C-C) + v(C-O) τ(C-C) + τ(C-O)

liquida

1056 1018

c

adsorption on TiO2 liquidb

mode

35 °C

275 °C 3300 3259 3230 3136 2970 2939

3377 3321

3378 3293

vas(NH2) vs(NH2)

3327

3200

3200

2960 2933

overtone of δ(NH2) vas(CH3) overtone of δ(CH3)

3236 3145 2966 2937

1439

2908 2895

vas(CH2) overtone of δ(CH2)

1383 1367 1342

2873 2862 1610

1610

vs(CH3) vs(CH2) δ(NH2)

1465

1462

1446

1423

1390

1398 1388

1273 1248 1228

1272 1248

1105 1082

1122 1082

1047

1045

1012

1012

1353 1300 1270 1209 1117

1353 1300 1270

1095

1098

1072

1072

1039 1021

1020

1120

δas(CH3) + δ(CH2) δ(CH3) + ω(CH2) δ(CH3) δ(CH3) + ω(CH2) tw(NH2) tw(CH2) ω(CH2) tw(CH2) tw(CH2) + F(CH3) + F(CH2) F(CH3) + F(CH2) F(CH3) + v(C-C) v(C-C) v(C-N) + v(C-C)

2879 2861 1595 1583 1471 1462

2883 2865 1579 1471 1462

1383

1386

1367 1302 1271 1175 1151c

1367 1313 1273 1178 1157

1076

1074

1057 1012

1049 1005

a Reference 34. b Reference 36. The first and second columns for CH3CH2CH2NH2 show the Raman and infrared frequencies, respectively. This band can be assigned to the ω(NH2) according to ref 37.

observed in the presence of CO2 are listed in Table 4. In the case of OCH2CH2NH2/TiO2, the CO2 capture leads to the increased intensities at 1302, 1512, 1568, 1625, and 1677 cm-1 in the fingerprint region. The 1568 cm-1 peak signifies the presence of the carbamate OCH2CH2NHCOO- according to Table 1 and is due to the COO stretching mode.21,28,29,40-42 The formation of the carbamate must be accompanied by its counterpart, i.e., the ammonium OCH2CH2NH3+. In the study of 2-ethanolamine adsorption on H-mordenites with Brønsted acid sites, HOCH2CH2NH3+ is reported to be formed, with two infrared peaks at ∼1500 and 1600 cm-1 for the symmetric and antisymmetric NH3+ deformation modes, respectively.38 Therefore, the concurrent appearance of the 1512 and 1625 cm-1 bands in Figure 3c is expected to have a contribution from the NH3+-containing species, i.e., OCH2CH2NH3+. In N-monosubstituted carbamates, the CNH bending mode absorbs near 1530-1540 cm-1 in the condensed state and near 1510-1530 cm-1 in solutions.39 So, the CNH bending of OCH2CH2NHCOO- can also contribute to the absorption of 1512 cm-1. It is shown later that the 1512 cm-1 band splits into two peaks at 150 °C. The 1302 cm-1 band observed in Figure 3c is attributed to bicarbonate (HCO3-) since it is the characteristic peak of sodium bicarbonate for the O-C-O2 antisymmetric stretching.39 Meanwhile, the COO antisymmetric stretching of HCO3- appears at ∼1625 cm-1.39 Therefore, HCO3- may also contribute to the 1625 cm-1 peak in Figure 3c. The formation of HCO3- could be due to the presence of residual water in the

hydrophilic OCH2CH2NH2/TiO2 layer with basic NH2 groups. For the case of TiO2 covered with OCH2CH2NH2 and HOCH2CH2NH2 (Figure 3d-f), the major infrared bands have positions similar to those, except the 1677 cm-1 band, of OCH2CH2NH2/TiO2, but have stronger intensities in general. This result suggests that the surface HOCH2CH2NH2 can also participate in CO2 capture via the basic NH2 group and that there is a specific CO2-binding surface species present in a less crowded condition, which is responsible for the 1677 cm-1 band. The infrared spectra of CO2 adsorption on a bare TiO2 surface are shown in supporting Figure 1, which is composed of the bands from adsorbed CO2 (2364 cm-1) and carbonate species (1320, 1376, and 1583 cm-1). In the case of the 2-ethanolaminemodified TiO2 surfaces, molecular CO2 adsorption on TiO2 is not found. Although the possibility for the presence of carbonates cannot be completely ruled out, they cannot account for the major peaks observed in Figure 3c,f. In the study of 3-(1naphthyl)propylamine in DMSO or in dioxane into which CO2 is bubbled, formation of the corresponding carbamic acid, with the CdO stretching frequency at 1700 or 1725 cm-1, has been reported, in contrast to the case in 2-propanol, resulting in the generation of ammonium-carbamate.29 Since the 1677 cm-1 peak in Figure 3 is only observed in the OCH2CH2NH2/TiO2 system with the TiO2 surface covered by fewer molecules than the OCH2CH2NH2 + HOCH2CH2NH2/TiO2 one, this peak could

2-Ethanolamine on TiO2

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Figure 4. Infrared spectra of OCH2CH2NH2/TiO2 taken at the indicated temperatures in the presence of ∼2 Torr of CO2.

Figure 3. Infrared spectra of two 2-ethanolamine-modified TiO2 surfaces without ((a) and (d)) and with absorbed CO2 ((b) and (e)) in the presence of 2 Torr of CO2 at 35 °C.

TABLE 4: Major Infrared Bands (cm-1) Due to CO2 Capture by the 2-Ethanolamine-Modified TiO2 Surfaces at 35 °C OCH2CH2NH2 + OCH2CH2NH2/TiO2 HOCH2CH2NH2/TiO2 1677 1625 1568 1512 1302

1620 1571 1531 1306

mode v(CdO)a δas(NH3+)b + vas(COO)c v(COO)d δs(NH3+)b + δ(CNH)d vas(O-C-O2)c

a

CdO stretching of OCH2CH2NHCOOH or OCH2CH2NHCOO (Scheme 1). b Ammonium species. c HCO3-. d Carbamate species.

SCHEME 1

be due to the carbamate or carbamic acid shown in Scheme 1a,b. In the OCH2CH2NHCOOH case, the interaction between the carbonyl group and a surface Ti4+ ion would lead to a redshift for the CdO stretching frequency. The result of Figure 3 has shown that the amino groups participate in the CO2 capture, but not all of the surface ethanolamine molecules are transformed into ammonium and carbamate since the relatively strong NH2 deformation bands at ∼1580 cm-1 are still present in 2 Torr CO2.

3.3. Thermal Stability of CO2 Captured by OCH2CH2NH2 on TiO2. The surface species of OCH2CH2NHCOO-, HCO3- and those of Scheme 1 due to CO2 absorption by OCH2CH2NH2 on TiO2 show different thermal behaviors. Figure 4 shows the infrared spectra measured at the indicated temperatures for the TiO2 surface covered with OCH2CH2NH2 in the presence of ∼2 Torr of CO2 in the closed cell. The 35 °C spectrum is due to OCH2CH2NH2 with captured CO2. After the surface is heated to 150 °C, some of the absorbed CO2 molecules are released into the gas phase based on the reduced absorptions at 1297, 1514, and 1677 cm-1, resulting in an infrared spectrum similar to that taken before CO2 capture. As the surface is cooled from 150 to 45 °C, the basic infrared feature of the CO2-containing surface is restored. Figure 5 shows the difference spectra obtained by subtracting the spectrum of OCH2CH2NH2/TiO2 in the absence of CO2 from each spectrum in Figure 4 to further demonstrate the thermal stability for each CO2-containing surface species. From 35 to 50 °C, it is found that the overall peak intensity due to CO2 capture increases, suggesting an endothermic process for more CO2 absorption in this temperature range. CO2 capture forming carbamates presumably need more space that may take place by changing the adsorption sites, geometric shapes, or/and adsorption orientations of the surface molecules to increase the intermolecular distances. These processes may require energy, especially for OCH2CH2NH2 with a hydrogen-bonding interaction. The intensity of the 1302 cm-1 band at 75 °C becomes smaller than that at 50 °C but is comparable to the intensity at 35 °C. In contrast, the peak intensity of 1677 cm-1 at 75 °C is only ∼62% of that at 35 °C. Since the peaks at 1302 and 1677 cm-1 are due to different species of HCO3- and those shown in Scheme 1, their changing trends with temperature are not the same. The 1512 cm-1 peak is split into two peaks of 1496 and 1515 cm-1 at 150 °C, due to the absorptions by the CNH group of the carbamate and the -NH3+ of the ammonium. The 1302 cm-1 has a maximum intensity at 50 °C and then gradually

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Figure 5. Difference spectra obtained by subtracting the spectrum of OCH2CH2NH2/TiO2 without CO2 from each spectrum of Figure 4.

decreases as the temperature is further raised. As a contrast to the 1302 cm-1 band of HCO3-, the 1568 cm-1 band of OCH2CH2NHCOO- has a better thermal stability in the temperature range studied. In the ab initio calculation for the complex of dimethylamine and dimethylcarbamic acid, proton transfer to form an ammonium and a carbamate is not suggested.43 However, the ammoniumcarbamate ion pair can be stabilized with the carbamate hydrogenbonded to a dimethylamine molecule and a dimethylcarbamic acid molecule. This result indicates that ammonium-carbamate formation in CO2 absorption using amines in condensed phases involves a group of neighboring molecules. The recent study44 has shown that urea-based anion receptors can compete with n-butylammonium for the n-butylcarbamate generated from CO2 absorption by n-butylamine. Therefore, in the present case the interaction between the carbamate and unreacted OCH2CH2NH2 molecules via hydrogen bonding, in addition to the ammonium-carbamate adduct, is possible. 3.4. Photoreactions of 2-Ethanolamine-Modified TiO2 Surfaces. Figure 6a shows the infrared spectra taken before and after the indicated photoirradiation times at 325 nm for the TiO2 surface with the saturation coverage of 2-ethanolamine initially in 10 Torr 16O2 in the closed cell. After the photon exposure for 240 min, the absorptions at ∼1300, 1524, 1662, and 2198 cm-1 are enhanced, indicating that photodecomposition of the surface 2-ethanolamine occurs to form new surface species. To assist in the infrared analysis through isotope shift in frequency, the photocatalytic experiment of the 2-ethanolamine-modified surface in 18O2 has been carried out, with the outcome shown in Figure 6b. Clearer changes during the photoillumination processes can be obtained by subtracting the original spectrum from those measured after the irradiation for certain times, as displayed in Figure 7. In the 10 min difference spectrum of 16 O2, the major enhanced absorptions are located at ∼1304,

Figure 6. Infrared spectra taken before and after the indicated photoirradiation times for the TiO2 with the saturation coverage of 2-ethanolamine initially in the presence of 10 Torr 16O2 and 18O2. The traces between 2100 and 2300 cm-1 have been multiplied by a factor of 4.

1521, 1566, 1631, 1664, 1695, and 2198 cm-1. The 2198 cm-1 peak is attributed to isocyanate (NCO) species bonded to the surface via the oxygen atom, which has been identified in the photochemistry study of CH3CN on TiO2.45 In the case of 18O2, the 10 min difference spectrum has a stronger absorption at ∼1664 cm-1, but without the 1695 cm-1 band. This comparative result shows that the 1695 cm-1 peak is shifted to ∼1664 cm-1, as 16O2 is replaced by 18O2. Therefore, on the basis of the peak frequencies and the isotope shift, it is proposed that surface species containing a carbonyl group (CdO) are generated, which are likely to be formic acid (HCOOH) and formamide (HCONH2) whose carbonyl stretching frequencies on TiO2 have been reported to be near 1695 cm-1.46,47 The former one can be the precursor for the formation of CO2 and the latter for NCO.46,48 In the photoreaction of 16O2, the absorptions at ∼1304, 1521, 1566, 1631, 1664, and 2198 cm-1 continue to grow with the increase of irradiation time. Except the bands of 1664 and 2198 cm-1, the absorption feature of the other peaks is similar to the spectrum of Figure 3f in terms of the peak positions and relative peak intensities, revealing two simultaneous processes of CO2 formation and CO2 capture by the remaining adsorbed, unreacted molecules in the photodecomposition of the surface 2-ethanolamine. The observed photoproducts of CO2 and NCO suggest the reaction steps of C-C bond dissociation, dehydro-

2-Ethanolamine on TiO2

Figure 7. Difference spectra of the TiO2 with the saturation coverage of 2-ethanolamine after photoirradiation for 5, 10, 30, 60, 120, 180, and 240 min at 325 nm.

genation, and oxidation for the surface 2-ethanolamine molecules. The 1664 cm-1 band is not ascribed to CO2-related species in the system of OCH2CH2NH2 + HOCH2CH2NH2/TiO2, but instead to OOCCH2NH2 with unidentate coordination for the carboxylate group. That is, the C-OH of HOCH2CH2NH2 or C-O of OCH2CH2NH2 is photooxidized to a carboxylate responsible for the 1664 cm-1 CdO stretching peak.39 Previously, it has been reported that methoxy (CH3O) on TiO2 is transformed into HCOO under photoirradiation in O2.35 In comparison of the 240 min difference spectra of 16O2 and 18O2 (Figure 7), the -NH3+ deformation frequencies remain the same at 1521 and 1631 cm-1. The HCO3- peak at 1304 cm-1 and carbamate peak at 1566 cm-1 observed in 16O2 are red-shifted by about 4 cm-1 in 18O2. The CdO stretching peak of OOCCH2NH2 generated in 18O2 is located at 1658 in contrast to the 1664 cm-1 in 16O2. For NCO, the 2198 cm-1 peak in 16 O2 is shifted to 2189 cm-1 in 18O2. The 9 cm-1 difference is consistent with that for NC16O and NC18O on TiO2 reported previously.45 Photoirradiation for the TiO2 with the saturation 2-ethanolamine coverage at 400 min in 16O2 has also been investigated. Infrared peaks similar to those obtained at 325 nm are found (Supporting Information Figure 2), although the relative peak intensities are different. Photoirradiation for the TiO2 surface covered only with OCH2CH2NH2 at 325 nm initially in 10 Torr 16O2 or 18O2 in the closed cell has been performed as well. The preparation of the OCH2CH2NH2/TiO2 surface was similar to that for Figure 3a. The infrared spectra taken before and after the selected photoirradiation times are shown in Supporting Information Figure 3 and the difference spectra are displayed in Figure 8.

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Figure 8. Difference spectra of the TiO2 covered with OCH2CH2NH2 after photoirradiation for 5, 10, 30, 60, 120, 180, and 240 min at 325 nm.

In the 5 min difference spectra of 16O2, rapid NCO formation is evidenced by the 2198 cm-1 peak. The bands at ∼1319, 1379, and 1586 cm-1 agree with carbonate infrared absorptions due to CO2 formation and adsorption on TiO2, as shown in Supporting Information Figure 1. Absorption at ∼1510 cm-1, signaling the presence of -NH3+, is either very weak or undetectable. Therefore, the relatively strong peak at ∼1622 cm-1 is not ascribed to -NH3+ but is very likely due to H2O formation. There exists a 1674 cm-1 peak in 16O2, which appears to be a shoulder in 18O2. This result suggests that the peak is not due to a functional group containing O because of the lack of isotope shift. This band is possibly originated from CdN stretching mode.39 However, note that CO2 absorbed by OCH2CH2NH2/TiO2 has a peak at 1677 cm-1 (Figure 3a). Therefore, a partial contribution from the carbamate or the carbamic acid shown in Scheme 1a,b to the 1674 cm-1 peak cannot be completely excluded. The absorptions at ∼1674 cm-1 in 16O2 and 18O2 show different evolution behaviors with time. In the former case it grows monotonically; however, it seems to be red-shifted to 1647 cm-1 in the latter one as observed in the 240 min difference spectrum of 18O2. This can be rationally explained as follows. The surface intermediate possessing a CdN group is subjected to further photodecomposition and transformed into a carbonyl-containing species, which coincidently has a Cd16O stretching frequency close to 1674 cm-1 but is red-shifted in 18O2 due to the formation of Cd18O. In the 240 min difference spectrum of 16O2, the small 1493 cm-1 peak may be related to -NH3+ and/or carbamate species. The peaks located at 1354 and 1559 cm-1 are assigned to symmetric and antisymmetric COO stretching modes of formate (HCOO) species, which can also be produced by H2CO and HCOOH

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SCHEME 2

dissociative adsorption on TiO2.47-49 This assignment is supported by the red-shifted peaks at 1332 and 1546 cm-1 observed in 18O2. Formation of HCOO is more conspicuous in the photoirradiation of 2-ethanolamine-modified TiO2 with H2O, as shown in Supporting Information Figure 4. In this experiment, the surface was prepared by the following procedure. A bare TiO2 surface at 35 °C was first in contact with 2-ethanolamine vapor until the surface coverage was approximately 10% of the saturation, judged by the infrared intensity in the CH stretching region, followed by coadsorption of H2O with the infrared band at 1621 cm-1. The infrared spectrum of the as-prepared TiO2 surface is shown in Supporting Information Figure 4a, and the difference spectra after certain times of photoirradiation at 400 nm initially in 10 Torr 16O2 in the closed cell are shown in Supporting Information Figure 4b. The infrared peaks of HCOO clearly appear at 1359, 1381, and 1569 cm-1. For a TiO2 material, absorption of photons with energy higher than ∼3.2 eV can generate electron-hole pairs. EPR (electron paramagnetic resonance) spectroscopy has been employed to investigate paramagnetic species on TiO2 surfaces under UV irradiation.50-55 The EPR study for TiO2 powder in the air and under vacuum has detected the trapped electrons at Ti4+ sites within the bulk and trapped holes at lattice oxide ions of the surfaces.53 For the adsorbed species such as HOCH2CH2NH2 and OCH2CH2NH2, they can react with the trapped holes, initiating the oxidative decomposition processes. In the photo-

reaction of 2-ethanolamine having a saturation coverage on TiO2 (Figure 7), it is proposed that NCO, OOCCH2NH2, absorbed CO2, HCONH2, and HCOOH are formed. The mechanism proposed for the photoreaction is shown in Scheme 2a. This mechanism involves formation of the transient four-membered peroxide organic species, which decomposes into formic acid and formamide.56 These two intermediates are responsible for the 1695 cm-1 peak observed in Figure 7 and have been reported to be quickly photooxidized to CO2 and NCO on TiO2.46,48 The photooxidation may also occur at the HOCH2 moiety to form the organoperoxy radicals after O2 incorporation. On the other hand, O2 receives an electron to form O2- and then recombines with H+ to form HOO•. The peroxy can react with HOO•, producing the tetraoxide species,57 which finally decomposes into HOOCCH2NH2, followed by deprotonation to form OOCCH2NH2. In the photoirradiation of OCH2CH2NH2 on TiO2 (Scheme 2b), the organoperoxide species dissociates to form HCOO and HCONH2. The latter one further rapidly photooxidizes to NCO and the former one to carbonate species. The reaction intermediate with a CdN group is originated from ˙ HNH2 photooxidation at the CH2NH2 moiety, forming a OCH2C + radical, which is transformed into OCH2CHdNH2 after losing an electron. OCH2CHdNH2+ can recombine with another OCH2CH2NH2 surface molecule, generating OCH2CHdNCH2CH2O, which is responsible for the 1674 cm-1 band in Figure 8. This process is supported by the previous photoreaction study of 1-butylamine on TiO2, with the formation of the product

2-Ethanolamine on TiO2 N-butylidene-1-butylamine (CH3CH2CH2CH2NdCHCH2CH2CH3).58 The OCH2CHdNCH2CH2O may finally decompose, through the C-C bond scission, into HCOO and HCONHCH2CH2O. The latter species may be responsible for the ∼1180 cm-1 in Figure 8. A similar amine recombination mechanism has been reported to explain the photocatalytic formylation in the photoreactions of primary and secondary amines on TiO2 .59,60 4. Conclusion 2-Ethanolamine is found to decompose by O-H bond scission to form OCH2CH2NH2 on TiO2 (35 °C), which is coadsorbed with intact HOCH2CH2NH2 at the saturation coverage. CO2 absorption by 2-ethanolamine-modified TiO2 surfaces, through the NH2 basic centers, to form ammoniums and carbamates has been demonstrated. HCO3- is also produced due to the presence of residual water. The surface carbamate is thermally more stable than bicarbonate. In the presence of O2, photodecomposition of HOCH2CH2NH2 on TiO2 occurs and can proceed by C-C bond scission, forming HCOOH and HCONH2, which are further oxidized to NCO and absorbed CO2. The photoreaction can also occur at the HOCH2 moiety to form the carboxylate species of OOCCH2NH2/TiO2. In the case of OCH2CH2NH2/ TiO2, C-C bond photodissociation also occurs and generates HCOO, NCO, and carbonate species. Besides, the CH2NH2 moiety can participate in the photoreaction, likely generating the CdN containing species of OCH2CHdNCH2CH2O. Acknowledgment. This research was supported by the National Science Council of the Republic of China under Contract NSC-97-2113-M-006-004-MY2. Supporting Information Available: Supporting Figure 1 shows the adsorption of CO2 on bare TiO2. Supporting Figure 2 shows the difference spectra of 2-ethanolamine on TiO2 under photoirradiation at 400 nm. Supporting Figure 3 shows the infrared spectra of OCH2CH2NH2/TiO2 under photoirradiation in 16O2 and 18O2. Supporting Figure 4 shows the difference of OCH2CH2NH2 + HOCH2CH2NH2 + H2O/TiO2 under photoirradiation. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Chen, J. H.; Wong, D. S. H.; Tan, C. S.; Subramanian, R.; Lira, C. T.; Orth, M. Ind. Eng. Chem. Res. 1997, 36, 2808. (2) Burchell, T. D.; Judkins, R. R.; Rogers, M. R.; Williams, A. M. Carbon 1997, 35, 1279. (3) Song, H.-K.; Lee, K.-H. Sep. Sci. Technol. 1998, 33, 2039. (4) Heuchel, M.; Davies, G. M.; Buss, E.; Seaton, N. A. Langmuir 1999, 15, 8695. (5) Vaart, R. V. D.; Huiskes, C.; Bosch, H.; Reith, T. Adsorption 2000, 6, 311. (6) Rutherford, S. W.; Do, D. D. Carbon 2000, 38, 1339. (7) Siriwardane, R. V.; Shen, M.-S.; Fisher, E. P.; Poston, J. A. Energy Fuels 2001, 15, 279. (8) Sun, Y.; Liu, X.-W.; Su, W.; Zhou, Y.; Zhou, L. Appl. Surf. Sci. 2007, 253, 5650. (9) Hayhurst, D. T. Chem. Eng. Commun. 1980, 4, 729. (10) Choudhary, V. R.; Mayadevi, S.; Singh, A. P. J. Chem. Soc., Faraday Trans. 1995, 91, 2935. (11) Calleja, G.; Pau, J.; Calles, J. A. J. Chem. Eng. Data 1998, 43, 994. (12) Takamura, Y.; Narita, S.; Aoki, J.; Hironaka, S.; Uchida, S. Sep. Purif. Technol. 2001, 24, 519. (13) Pereira, P. R.; Pires, J.; de Carvalho, M. B. Langmuir 1998, 14, 4584. (14) Ding, Y.; Alpay, E. Chem. Eng. Sci. 2000, 55, 3461. (15) Yong, Z.; Mata, V.; Rodriques, A. E. Ind. Eng. Chem. Res. 2001, 40, 204. (16) Yong, Z.; Mata, V.; Rodriques, A. E. J. Chem. Eng. Data 2000, 45, 1093.

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