Surface Reactions of Carbon Dioxide at the Adsorbed WaterâOxide

14870

J. Phys. Chem. C 2007, 111, 14870-14880

Surface Reactions of Carbon Dioxide at the Adsorbed Water-Oxide Interface Jonas Baltrusaitis,† Jennifer D. Schuttlefield,† Elizabeth Zeitler,† Jan H. Jensen,‡ and Vicki H. Grassian*,† Department of Chemistry, UniVersity of Iowa, Iowa City, Iowa 52242, and Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5, 2100 Copenhagen, Denmark ReceiVed: June 16, 2007; In Final Form: July 31, 2007

In this study, FTIR spectroscopy is used to investigate surface reactions of carbon dioxide at the adsorbed water-oxide interface. In particular, FTIR spectra following CO2 adsorption in the presence and absence of coadsorbed water on hydroxylated nanoparticulate Fe2O3 and γ-Al2O3 at 296 K are reported. In the absence of coadsorbed water, CO2 reacts with surface O-H groups to form adsorbed bicarbonate on the surface. In the presence of coadsorbed water, this reaction is blocked as water hydrogen bonds to the reactive M-OH sites. Instead, CO2 reacts with adsorbed water to yield adsorbed carbonate and protonated surface hydroxyl groups, M-OH2+, through a proposed carbonic acid intermediate. The carbonate spectra recorded between 10 and 90% RH are nearly identical to that of carbonate adsorbed on these surfaces in the presence of the liquid water. FTIR isotope studies show that there is extensive exchange between oxygen in adsorbed water and oxygen atoms in both adsorbed carbonate and gas-phase carbon dioxide. On the basis of these experimental results along with quantum chemical calculations, a mechanism is proposed for surface reactions of carbon dioxide at the adsorbed water-oxide interface.

Introduction interfaces.1-4

Oxide surfaces are important environmental As environmental interfaces, understanding the molecular-level details of the chemistry at the adsorbed water-oxide interface will provide the necessary fundamental information needed to understand chemical processes that occur under ambient conditions. Iron oxides in particular are common reactive materials found in air, water, and soil; thus, the surface chemistry of iron oxides under ambient conditions is important in several environmental processes. For example, the surface chemistry of Fecontaining mineral dust aerosol such as iron oxides with pollutant gases has been identified as an important mechanism for increasing the amount of bioavailable iron through the reduction of Fe(III) to Fe(II).5 This process is proposed to occur in a thin aqueous layer coating the iron oxide surface. As environmental interfaces, thin film water adsorbed on iron oxide and other oxide surfaces will play an important role in the chemistry and the adsorption of molecules on these surfaces under ambient conditions of pressure and relative humidity.6 As a step toward better understanding the molecular level details of the surface chemistry of the adsorbed water-oxide interface under ambient conditions of pressure, temperature, and relative humidity, we have investigated the adsorption and surface chemistry of carbon dioxide on high surface area hydroxylated Fe2O3 and γ-Al2O3 nanoparticles at 296 K as a function of relative humidity (RH). In a previous study, the reaction of CO2 under dry conditions (RH < 1%) to form adsorbed bicarbonate was investigated using FTIR spectroscopy combined with quantum chemical calculations.7 In the study described here, the reaction of CO2 in the presence of coadsorbed water in equilibrium with water vapor corresponding to * Author to whom correspondence should be addressed. † Department of Chemistry, University of Iowa, Iowa City IA 52242 ‡ Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100

1-90% RH at 296 K is investigated. Although oxide interfaces have been very well studied, to our knowledge, this is the first report of a systematic investigation of the details of CO2 adsorption at the adsorbed water-oxide interface under ambient conditions. Experimental Section Transmission FTIR Spectroscopy. In this study, transmission FTIR spectroscopy has been used to measure the vibrational spectra of nanoparticulate Fe2O3 and γ-Al2O3 surfaces in equilibrium with gas-phase CO2, H2O, and simultaneously introduced CO2 and H2O. The infrared spectra were collected using a single beam FTIR spectrometer (Mattson Research Series), equipped with a liquid-nitrogen-cooled narrowband mercury cadmium telluride (MCT) detector. A total of 250 scans were acquired at an instrument resolution of 4 cm-1 over the spectral range from 800 to 4000 cm-1. The spectrometer was purged with a commercial air dryer (Balston 75-62) which minimized H2O and CO2 concentrations in the purge air. All IR spectra were recorded at 296 K. An infrared cell made from a stainless steel cube was placed in the sample compartment of the spectrometer. Two BaF2 windows (Janos Technology Inc.) were placed into two stainless steel holders, which were sealed by O-rings to the IR cell. Approximately 5 to 15 mg of oxide powder was pressed onto half of a tungsten grid; the other half was left uncoated and was used to measure the infrared spectrum of the gas-phase. The grid was then placed into stainless steel jaws and loaded into the infrared cell. The grid containing the oxide sample was then evacuated overnight prior to adsorption studies. The infrared cell sits on a linear translator inside the internal compartment of the spectrometer so that the position of the IR cell can be changed with respect to the infrared beam path. The half of the grid coated with the oxide powder yields spectral features associated with the oxide surface and surface adsorbates

10.1021/jp074677l CCC: $37.00 © 2007 American Chemical Society Published on Web 09/14/2007

Reactions at the Adsorbed Water-Oxide Interface as well as gas-phase spectral features, whereas the uncoated half of the grid only yields information about the gas-phase. Single beam spectra of the oxide powder in the presence of the gas-phase were all referenced to those prior to exposure to give the absorbance spectra of both surface adsorbed and gas-phase species. Information about adsorbed surface species only could then be obtained by spectral subtraction of the gas-phase spectrum under identical conditions. The infrared cell is connected to a gas-handling system that consists of a gas mixing pre-chamber and a glass manifold with four ports connected to two absolute pressure transducers (MKS instruments) that operate in two different ranges from 0.001 to 10.00 Torr and 0.1 to 1000 Torr representing low (10%) relative humidity regimes, respectively. With the pressure transducers, the RH could be measured to within (1%. The gas manifold is connected to a turbo molecular pump that can be used to pump the system down to a final pressure of around 1 × 10-6 Torr. The gas handling system is connected to the IR cell, water cylinder, and CO2 gas cylinder through a Teflon tube. In simultaneous CO2 and H2O adsorption experiments, CO2 gas was first introduced into the mixing prechamber, followed by H2O vapor introduction into the glass manifold. The CO2 and H2O mixture was made by opening a valve between the mixing prechamber and glass manifold and by allowing the gas mixture to equilibrate before introduction into the infrared cell. Before each spectrum was recorded, the gas mixture was allowed to equilibrate with the oxide sample for at least 15 min. Some experiments went for longer times up to 24 h as described. ATR-FTIR Spectroscopy. For solution-phase FTIR measurements, oxide thin films were prepared by evaporating a suspension consisting of approximately 20 mg of oxide powder in 5 mg of high purity water onto a ZnSe element inside of an attenuated total reflection (ATR) horizontal liquid cell (Pike Technology). Films were dried overnight prior to introducing solutions of sodium bicarbonate (Fisher Scientific) into the horizontal liquid cell. A total of 500 scans were averaged at a 4 cm-1 resolution for the reported solution-phase spectra. Sources of Oxide Powders, Water, Carbon Dioxide and Other Reagents. Commercially available Fe2O3 and Al2O3 powders were used in these experiments. Nanoparticulate amorphous Fe2O3 with a surface area of 229 m2g-1 was purchased from Alfa Aesar. γ-Al2O3 with a surface area of 101 m2g-1 was purchased from Degussa (AlumOxid C). All surface areas were measured using a Quantachrome Nova 1200 Multipoint BET apparatus. X-ray diffraction of the powders was performed. The crystalline phase of the γ-Al2O3 sample was confirmed. Although X-ray diffraction indicated that the sample was primarily amorphous in nature, Mossbauer spectroscopy indicated that nanoparticulate Fe2O3 contained some hematite and ferrihydrate. A Hitachi S-4000 scanning electron microscope was used to acquire images of nanoparticulate Fe2O3 and γ-Al2O3. Sample imaging was done using a working distance of 12 mm, with an emission current of 10 mA and an accelerating voltage of 15 kV. For nanoparticulate iron oxide, the SEM images show particle aggregates on the order of 35 nm in diameter that are composed of approximately 5 nm particles.8 These smaller particles have grown together to form larger interwoven aggregates. The γ-Al2O3 particles are approximately 20 nm in diameter.9 1 M sodium bicarbonate solution was prepared from NaHCO3 powder (Fisher Sci, ACS grade). Water vapor was taken from the headspace of several different samples depending on the isotope used. Distilled H2O (Optima

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14871 grade) was purchased from Fisher Scientific and was degassed prior to use. Deuterium labeled water, D2O, was purchased from Aldrich (99.9 atom % D) and 18O labeled water, H218O, was purchased from ISOTEC (minimum 95 atom % 18O). Distilled and deuterium labeled water were degassed prior to use. The 18O labeled water was transferred to the flask in the presence of nitrogen atmosphere and was used as received. Carbon dioxide was purchased from Air Products (chemically pure grade, CP), and 18O labeled carbon dioxide gas, C18O2, was purchased from ISOTEC (minimum 97 atom % 18O). Quantum Chemical Calculations. Quantum chemical calculations for mononuclear and binuclear cluster models were performed using Spartan ’06 for Windows (version 1.0.3) software package.10 One P4 2.8 GHz processor with 4 GB of memory was utilized for calculations. The results were visualized using commercially available ChemCraft software.11 Quantum chemical calculations were performed on binuclear neutral clusters with a coordinated carbonate ion of the formula [M2(OH)2(µ-OH)2(H2O)11(CO3)], where M ) Fe, Al. Calculations were also performed on a mononuclear cluster model with a coordinated carbonate ion of the formula [M(OH)3(H2O)x(CO3)]2-, where M ) Al and x ) 0-4 in the presence and absence of outer-sphere water ligands. Both energy optimization and vibrational frequency calculations were performed at the B3LYP hybrid density functional theory (DFT) level of theory with a 6-31G(d) basis set. To account for anharmonicity in the calculated vibrational frequencies, a scaling factor of 0.9632 reported by Irikura et al. for B3LYP/6-31+G(d,p) was used.12 They also showed that the scaling factors depend only weakly upon the basis set and can be used for the majority of basis sets under the same level of theory. The optimized structures were determined without symmetry constraints on any of the molecules. Stationary points were identified as true minima of the PES (potential energy surface) in all cases because no imaginary frequencies were found. Results Transmission FTIR Spectroscopy of CO2 Adsorbed on Hydroxylated Nanoparticulate Fe2O3 and γ-Al2O3 Powders Under Dry Conditions and In the Presence of Coadsorbed Water at 296 K. The transmission FTIR spectra of nanoparticulate Fe2O3 and γ-Al2O3 exposed to atmospherically relevant pressures of CO2 are shown in Figure 1. Gas-phase absorptions have been subtracted from the spectra shown in Figure 1. The spectra labeled a and b in Figure 1 are of nanoparticulate Fe2O3 and γ-Al2O3, respectively, in the presence of 0.274 Torr of CO2 only at 296 K. The frequencies of some of the most intense bands observed in the spectra are labeled and are assigned in Table 1. On the basis of recent quantum chemical calculations and extensive FTIR isotope experiments,7,13-17 the absorption bands labeled in the spectra, shown in Figure 1a,b, are assigned to bidentate adsorbed bicarbonate. The absorptions at 3619, 1622, 1410, and 1220 cm-1 are specifically assigned to the ν1(OH), ν2(OCO)a, ν3(OCO)s, and δ4(COH) vibrational modes, respectively, of adsorbed bicarbonate on nanoparticulate Fe2O3. For γ-Al2O3, the frequencies for these four modes, ν1(OH), ν2(OCO)a, ν3(OCO)s, and δ4(COH), are observed at 3630, 1650, 1435, and 1230 cm-1, respectively. This assignment was recently discussed in great detail.7 The spectra labeled c and d in Figure 1 are recorded of nanoparticulate Fe2O3 and γ-Al2O3, respectively, in the presence of 0.274 Torr of CO2 and water vapor corresponding to 40% RH at 296 K. It can be seen that at the same pressure of carbon dioxide the resulting spectra are quite different when water vapor

14872 J. Phys. Chem. C, Vol. 111, No. 40, 2007

Figure 1. Transmission spectra were recorded as follows: (a) 0.274 Torr CO2 adsorption on nanoparticulate Fe2O3, (b) 0.274 Torr CO2 adsorption on γ-Al2O3, (c) 0.274 Torr CO2 and 40% RH water vapor adsorption on nanoparticulate Fe2O3, and (d) 0.274 Torr CO2 and 40% RH water vapor adsorption on γ-Al2O3. Gas-phase absorptions have been subtracted from the spectra.

TABLE 1: Comparison of the Vibrational Frequencies (cm-1) of the CO2 Adsorption Products on Nanoparticulate Fe2O3 and γ-Al2O3 in the Absence and Presence of H2O Vapor at 40% RH

vibrational mode assignments

vibrational vibrational frequencies on frequencies on nano Fe2O3 γ-Al2O3 literature references7,13-17 (cm-1) (cm-1)

0.274 Torr CO2a bicarbonate, HCO3ν1(OH) 3619 3630 1622 1650 ν2(O-C-O)a ν3(O-C-O)s 1410 1435 δ4(COH) 1220 1230 0.274 Torr CO2 + 40% RH H2Ob carbonate, CO32ν3(O-C-O)a 1496 1520 1345 1402 ν3(O-C-O)s ν1(C-O)s 1073 n.o.c n.o. n.o.c ν2(CO2)oop

3600-3627 1615-1671 1396-1500 1220-1269

1487 (1484) 1335 (1420) 1070 (1028) n.o.c (n.o.c)

a Range taken from references 7 and 13-17. b 1 M NaHCO3 on amorphous Fe(OH)3 at pH 8.0 (1 M NaHCO3 on amorphous Al(OH)3 at pH 7.8). c n.o. ) not observed.

is present. First, absorption bands near 1640 and 3400 cm-1 due to adsorbed water bending and stretching vibrations are apparent. Second, there are no absorptions seen for bicarbonate on the surface. Instead, only absorption bands due to adsorbed carbonate are observed. In particular, the transmission FTIR spectrum of nanoparticulate iron oxide in the presence of 0.274 Torr of gas-phase CO2 and 40% RH (Figure 1c) shows absorption bands at 1496, 1345, and 1073 cm-1. These three absorption bands are assigned to ν3(OCO)a, ν3(OCO)s, and ν1(CO)s vibrational modes, respectively, of adsorbed carbonate and are included in Table 1. Similar features, albeit weaker, can be observed following the exposure of hydroxylated γ-Al2O3 surfaces to 0.274 Torr of CO2 and in the presence of 40% RH water vapor (Figure 1d). In particular, the ν3(OCO)a and ν3(OCO)s modes are observed at 1520 and 1402 cm-1, respectively. These vibrational frequencies are consistent with a solvated, adsorbed carbonate ion, as inferred from solution-phase experiments discussed below.17,18 ATR-FTIR Spectroscopy. To better understand the reaction chemistry and product formation of gas-phase CO2 at the adsorbed water-oxide interface and to interpret the data shown

Baltrusaitis et al.

Figure 2. Representative (a) ATR-FTIR spectrum of 1.0 M NaHCO3 solution, (b) ATR-FTIR spectrum of 1.0 M NaHCO3 adsorbed on nanoparticulate Fe2O3, and (c) transmission FTIR spectrum of 0.274 Torr CO2 on nanoparticulate Fe2O3 that were pre-equilibrated with 40% RH H2O. Spectrum (c) is a difference spectrum as a result of the subtraction of the spectrum of nanoparticulate Fe2O3 in the presence of H2O at 40% RH from the spectrum of nanoparticulate Fe2O3 in the presence of 0.274 Torr CO2 and H2O at 40% RH. Likewise, liquid water absorptions were subtracted from the solution-phase spectra shown in (a) and (b).

in Figure 1, we have measured the spectrum of adsorbed carbonate at the liquid water-oxide interface. The ATR-FTIR spectrum of adsorbed carbonate on nanoparticulate Fe2O3 in the presence of liquid water is shown in Figure 2. In these experiments, sodium bicarbonate solution was used as the carbonate source. The ATR-FTIR spectrum of 1.0 M NaHCO3 at pH ) 8.0 is shown in Figure 2a. Interpretation of this bicarbonate spectrum has been previously discussed by Su et al.17 Absorptions observed in the spectrum with vibrational frequencies at 1648, 1615 cm-1 and 1360, 1311 cm-1 are assigned to ν4(OCO)a and ν2(OCO)s modes of HCO3-, respectively. The remaining bands at 1011 and 843 cm-1 are assigned to ν1(C-OH) stretching mode and ν6(CO3)oop bending mode, respectively. To obtain a spectrum of adsorbed carbonate at the bulk water-oxide interface, a thin layer of nanoparticulate Fe2O3 was coated onto the ZnSe crystal. The spectrum recorded of the ZnSe crystal coated with nanoparticulate Fe2O3 in the presence of a solution of 1 M NaHCO3 at pH 8 is shown in Figure 2b. Solution-phase peaks have been subtracted from the spectrum shown in Figure 2b. The remaining peaks are due to adsorbed carbonate, as has been previously discussed in detail in the literature.17 As shown by Su and Suarez,17 adsorbed carbonate is observed as the dominant product on iron and aluminum oxide and hydroxide particles following adsorption of NaHCO3 from NaHCO3 solutions in the concentration range from 0.01 to 1.0 M near neutral pH. A facile deprotonation surface reaction mechanism has been proposed to explain the absence of adsorbed bicarbonate species in the spectra.17 In addition to these ATR-FTIR spectra, the spectrum of adsorbed carbonate at the adsorbed water-oxide interface at 40% RH is also shown for comparison in Figure 2. The spectrum shown in Figure 2c is identical to the spectrum shown in Figure 1c but for these experiments the iron oxide surface was first equilibrated with H2O prior to CO2 adsorption whereas in the experiments shown in Figure 1, H2O and CO2 were introduced into the infrared cell simultaneously. Thus, it can be seen that the spectrum recorded of the carbonate ion at the adsorbed water interface is very similar to that of the liquid water interface for nanoparticulate iron oxide suggesting similar structures and

Reactions at the Adsorbed Water-Oxide Interface

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14873

Figure 3. Transmission FTIR spectra of (a) nanoparticulate Fe2O3 following exposure to mixtures of 0.274 Torr CO2 with increasing relative humidity from 0 to 11% RH (0, 1, 3, 5, 7, 9, 11) in the 1100 to 1850 cm-1 region, (b) γ-Al2O3 following exposure to mixtures of 0.274 Torr CO2 and increasing relative humidity from 0 to 10% RH (0, 1, 2, 5, 10) in the 1100 to 1850 cm-1 region, (c) nanoparticulate Fe2O3 particles following exposure to mixtures of 0.274 Torr CO2 and increasing relative humidity from 9 to 89% RH (9, 22, 32, 41, 47, 56, 68, 81, 89% RH) in the 1000 to 4000 cm-1 region, and (d) γ-Al2O3 following exposure to mixtures of 0.274 Torr CO2 and increasing relative humidity from 10 to 80% RH (10, 20, 40, 80) in the 1000 to 4000 cm-1 region. Gas-phase absorptions were subtracted from spectra.

solvation in both cases. Furthermore, the data presented in Figures 1 and 2 show that it does not matter whether CO2 and H2O are adsorbed simultaneously or if H2O is adsorbed first followed by CO2 adsorption. Similarly for γ-Al2O3, adsorbed carbonate at the liquid water interface has been reported.17 Absorptions at 1484 and 1420 cm-1 seen in the spectrum are consistent with the data shown in Figure 1d at 40% RH. Transmission FTIR Spectroscopy of Carbon Dioxide Adsorption on Hydroxylated Nanoparticulate Fe2O3 and γ-Al2O3 Powders at 296 K from 0 to 90% RH. To further probe the chemistry of carbon dioxide at the adsorbed water interface, FTIR spectroscopy was used to monitor the adsorption of CO2 on nanoparticulate Fe2O3 and γ-Al2O3 surfaces at 296 K as a function of water vapor pressure, while keeping the CO2 pressure fixed at 0.274 Torr. These spectra are shown in Figure 3 with the gas-phase absorptions subtracted and are separated according to relative humidity ranges. In particular, the spectra recorded from approximately 0 to 11% RH are shown in the top panels for each oxide, nanoparticulate Fe2O3 and γ-Al2O3, in Figure 3a,b, respectively. These low relative humidity spectra show the spectral range extending from 1100 to 1850 cm-1. The spectra recorded from approximately 10 to 90% RH (exact % RH are given in Figure 3) are shown in the bottom panel for each oxide, nanoparticulate Fe2O3 and γ-Al2O3, in Figure 3c,d, respectively. These spectra are shown in the region extending from 1000 to 4000 cm-1. At the lower relative humidity (Figure 3a,b), there is a decrease in the intensity of the adsorbed bicarbonate bands at 1622, 1410, and 1220 cm-1 for the nanoparticulate Fe2O3 and

1650, 1435, and 1230 cm-1 for the γ-Al2O3 as the relative humidity increases. The decrease in intensity of the bicarbonate bands is concomitant with an increase of the intensity of the absorption bands near 1511 and 1333 cm-1 and 1525 and 1394 cm-1 for nanoparticulate Fe2O3 and γ-Al2O3, respectively. These absorption bands can be attributed to the adsorbed solvated surface carbonate, as discussed in the previous sections. In addition, in the low relative humidity spectra, the growth of a band due to the bending mode of adsorbed water, δ(H2O), near 1640 cm-1 can be seen. This band is observed starting at 1 to 2% RH, indicating presence of molecularly adsorbed water on the surface. For both nanoparticulate Fe2O3 and γ-Al2O3, there is an increase in the intensity of the adsorbed carbonate absorptions with an increase in relative humidity as seen in Figure 3c,d, when the relative humidity is increased to near 90% RH. This is most easily seen for nanoparticulate Fe2O3 as the intensity of the 1494 and 1347 cm-1 absorption bands, assigned to ν3 (O-C-O)a and ν3 (O-C-O)s, respectively, increase significantly with increasing relative humidity. In addition, the split in the frequency of the degenerate ν3 stretching mode, ∆ν3, which is zero for D3h symmetry but nonzero for adsorbed carbonate, decreases with increasing relative humidity from 178 to 147 cm-1 for the 9 and 89% RH experiments, respectively. The γ-Al2O3 carbonate absorptions show similar trends, but these absorptions are not nearly as intense suggesting less carbonate formation on γ-Al2O3 compared with nanoparticulate Fe2O3. In addition to the growth of carbonate bands below 1600 cm-1 and the growth of the bending mode due to adsorbed water near


Baltrusaitis et al.

Figure 4. Transmission FTIR spectra of (a) nanoparticulate Fe2O3 following exposure to increasing relative humidity from 10 to 90% RH (10, 19, 31, 39, 50, 61, 69, 80, and 90) and (b) γ-Al2O3 following exposure to increasing relative humidity from 9 to 89% RH (9, 33, 66, 85, 93, and 89). Gas-phase absorptions were subtracted from spectra.

1640 cm-1, there is also the growth in integrated absorbance of the broad O-H stretching region shown in the spectra in Figure 3c,d. For nanoparticulate Fe2O3, the O-H stretching region shows a structured band, increasing in intensity with increasing RH, with absorptions at 3515, 3266, 3105, and 3011 cm-1. For γ-Al2O3, this region of the spectrum is not as structured and does not differ very much from pure water adsorption on γ-Al2O3 as reported by Al-Abadleh and Grassian.19 For comparison purposes, transmission FTIR spectroscopy was used to investigate water vapor adsorption on these oxide powders at room temperature in the absence of CO2 (see Figure 4). The spectra were recorded as a function of increasing relative humidity in the presence of water vapor. The lack of structure in the O-H stretching region, shown in Figure 4, for both nanoparticulate Fe2O3 and γ-Al2O3 surfaces can be easily seen in the spectra. The spectra show O-H stretching absorption bands centered at 3387 and 3381 cm-1, with shoulders at 3248 and 3243 cm-1 for nanoparticulate Fe2O3 and γ-Al2O3, respectively. The absorption bands with negative intensity, located at 3674 and 3652 cm-1 and 3730 cm-1 for nanoparticulate Fe2O3 and γ-Al2O3, respectively, can be attributed to hydrogen-bonded surface hydroxyl groups.20 Absorption bands around 2150 and 2128 cm-1 for nanoparticulate Fe2O3 and γ-Al2O3, respectively, are assigned to an association band (νa), which is a combination of the bending, δ, libration, νL, and hindered translation, νT, modes.19,21 The bending mode region, δ(H2O), of adsorbed water can be found at 1640 and 1646 cm-1 for nanoparticulate Fe2O3 and γ-Al2O3, respectively. Most importantly, from a comparison perspective, there is a large difference in the water absorption bands, especially in the O-H stretching region for nanoparticulate iron oxide in the presence versus the absence of adsorbed carbonate. The smaller changes seen in the water spectra for γ-Al2O3 are consistent with less carbonate formation, as indicated by the weaker absorptions bands seen in the γ-Al2O3 spectra (Figure 3a) compared with the nanoparticulate Fe2O3 (Figure 3c). Transmission FTIR Spectroscopy of Carbon Dioxide Adsorption on Hydroxylated Nanoparticulate Fe2O3 and γ-Al2O3 Powders at 296 K and 40% RH Using Different Oxygen Isotopes in Gas-Phase Carbon Dioxide and Water Vapor. Oxygen-18 isotope studies were used to further investigate reactions of carbon dioxide at the adsorbed water-oxide interface. Several different combinations of isotope-labeling experiments were performed using C16O2, C18O2, H216O, or H218O. The spectra, shown in Figure 5, are the results of these isotope experiments. Here, gas-phase CO2 in the absence (Figure

5a,f) and presence of nanoparticulate Fe2O3 (Figure 5b-e) are shown in the spectral range between 2200 and 2450 cm-1 (left), and the adsorbed water and carbonate products that result from mixed and full isotope CO2 and H2O adsorption experiments are shown in the spectral range extending from 1000 to 1800 cm-1 (right). The isotope-labeling experiments were done at 40% RH. Once again, in all of these spectra, gas-phase water absorption bands have been subtracted from the spectra. Figure 5a shows the gas-phase spectrum of C16O2 at 2349 cm-1, for which there is no corresponding adsorbed carbonate since it is in the absence of any nanoparticulate Fe2O3. In Figure 5b, the spectrum of gas-phase C16O2 at 2349 cm-1 is shown in the presence of nanoparticulate Fe2O3 at 40% RH using H216O. The corresponding absorptions for the adsorbed carbonate on the surface of nanoparticulate Fe2O3 are identical to the absorptions seen in Figure 1c, as the bending mode of adsorbed water is at 1638 cm-1, the carbonate ν3 vibrations are observed at 1496 and 1344 cm-1, and the carbonate ν1 mode is observed at 1073 cm-1. The spectra shown in Figure 5c-e are the results of the mixed and full isotope experiments in the presence of nanoparticulate Fe2O3. A change in frequency for adsorbed carbonate is clearly seen when either or both H2O or CO2 is labeled with 18O, as shown in Figure 5c,d. In all cases, gasphase CO2 and carbonate absorption bands shift toward lower wavenumbers because of an increase in mass. Absorption bands due to gas-phase CO2 and adsorbed carbonate shift the most toward lower wavenumbers when the 18O label is present in both CO2 and H2O, as shown in Figure 5e. In the fully labeled experiment (i.e., C18O2 and H218O), gas-phase CO2 has a vibrational frequency of 2314 cm-1, which is the same value observed for C18O2 in the absence of the Fe2O3 surface, shown in Figure 5f. In Figure 5e, the bending mode of adsorbed water is seen at 1632 cm-1, and the carbonate absorptions are observed at 1478, 1321, and 1013 cm-1. Additionally, as described in an earlier publication, the surface hydroxyl groups when labeled with 18O and then exposed to C16O2 and H216O show little change in the spectrum when compared to Figure 1b.22 This lack of 18O incorporation from surface hydroxyl groups is an important observation in understanding the exchange mechanism and adsorbed carbonate formation. One of the most interesting aspects of the spectra shown in Figure 5 is the isotope mixing that is observed in the gas-phase spectra. This isotope mixing does not occur in the absence of nanoparticulate iron oxide; that is, gas-phase CO2 and water vapor in the infrared cell alone do not show any isotope scrambling when C16O2 is mixed with H218O or if C18O2 is



Figure 5. Gas-phase (left) and adsorbed phase (right) transmission spectra recorded of (a) 0.274 Torr C16O2, (b) 0.274 Torr C16O2 and 40% RH H216O adsorption on nanoparticulate Fe2O3, (c) 0.274 Torr C16O2 and 40% RH H218O adsorption on nanoparticulate Fe2O3, (d) 0.274 Torr C18O2 and 40% RH H216O adsorption on nanoparticulate Fe2O3, (e) 0.274 Torr C18O2 and 40% RH H218O adsorption on nanoparticulate Fe2O3, and (f) 0.274 Torr C18O2. Shifts in the frequencies of the absorption bands of adsorbed carbonate species are indicated. Extensive isotope exchange is seen between gas-phase CO2 and H2O in these spectra for the mixed isotope experiments (see text for further details). It is important to note that no isotope mixing is observed between gas-phase CO2 and H2O in the infrared cell in the absence of the oxides.

mixed with H216O (not shown). Thus, the observed isotope scrambling is mediated by the presence of the high surface area oxide particles. Another important aspect of the spectra shown in Figure 5 is that the amount of labeling in the carbonate product appears to be complex. First, as discussed above, labeling the surface O-H groups alone does not result in a mixed labeled adsorbed carbonate product.22 Thus, only the label from adsorbed water appears to become incorporated into the adsorbed carbonate product. Second, as to be discussed in more detail below, the adsorbed carbonate product for the C18O2 + H216O experiment appears to have one 18O labeled oxygen atom, whereas in the C16O2 + H218O experiment, two 18O labeled oxygen atoms are present in the adsorbed carbonate. This conclusion is based on the frequencies observed in the spectrum shown in Figure 5 and on quantum chemical calculations (discussed below). Similar 18O isotope experiments were done on γ-Al2O3. Since the spectra of solvated carbonate are less intense, it was difficult to monitor the 18O isotope label in the adsorbed carbonate. However, extensive isotope exchange was evident in the gasphase CO2 spectra similar to that seen for nanoparticulate Fe2O3. Quantum Chemical Calculations of Mononuclear and Binuclear Cluster Compounds. To help explain the isotopelabeling experiments and to better understand the structure of adsorbed carbonate in the presence and absence of coadsorbed water, several different quantum chemical calculations at B3LYP/6-31G(d) level of theory were applied. First, calculations were done using different gas-phase carbon dioxide isotopes. The fundamental vibrational frequency was calculated for C16O2 and for one or both oxygen atoms labeled 18O in the carbon dioxide molecule. The frequencies were each calculated at the same level of theory and scaled to 0.9632, as discussed in Experimental Section. Shifts in the carbon dioxide ν3 (O-C-O) fundamental frequency that occurred upon labeling were calculated by subtracting the labeled experiment frequencies from the nonlabeled, which were then compared to the transmission FTIR experimental values from the mixed and full isotope-labeling experiments shown in Figure 5c-e. The calculated isotope shifts were used to properly identify the gasphase carbon dioxide molecules containing different amount of 18O labeled oxygen atoms. Those are noted as C16O , C16O18O, 2

TABLE 2: ν3(O-C-O) Vibrational Frequency (cm-1) Calculated for Several Isotopes of a Carbon Dioxide Molecule at B3LYP/6-31G(d) Level of Theorya: Comparison with Experimental Values

CO2 isotope

calculated ν3(O-C-O) vibrational frequency, ω, cm-1

experimental ν3(O-C-O) vibrational frequency, ω, cm-1

C16O2 C16O18O C18O2

2347 2331 2313

2349 2332 2314

a

Scaled to 0.9632 (see text for further details).

and C18O2; the calculated ν3 (O-C-O) frequency values are shown in the Table 2. These calculations showed that for the mixed isotope-labeled experiment (i.e., experiments with H218O + C16O2 and H216O and C18O2), shown in Figure 5c,d, the predominant form of CO2 is C16O18O. Second, to better understand the bonding configuration of the carbonate ion adsorbed on the metal oxide surfaces in the presence of a coadsorbed water layer, quantum chemical modeling of two different cluster models was performed. In one set of calculations, the effect of outer-sphere water molecules on the carbonate vibrational frequencies was determined using a mononuclear Al cluster model in the presence of a carbonate molecule and 0-4 water molecules. Again, the vibrational frequencies were calculated at the same level of theory done for the gas phase and a scaling factor of 0.9632 was applied for the ν3 (O-C-O)a and ν3(O-C-O)s modes. The mononuclear cluster models are shown in Figure 6, and the calculated vibrational frequencies are plotted in Figure 7. The uncoordinated “free” carbonate ion is shown as I, and the carbonate ion coordinated to an Al(OH)3 molecule is shown in cluster model II. An increasing number of water molecules (1-4) was then added to cluster model II to simulate increasing relative humidity and are shown as cluster models III-VI. Several interesting changes are seen in the structure of the carbonate ion upon coordination and subsequent solvation (Figure 6). First, there is an elongation of the Al-O bond with each outer-sphere coordinated water molecule from 1.80 Å for cluster model II with one water to 1.84 Å for cluster model with four outer-sphere water molecules. Additionally, the O-C bond in the Al-O-C linkage decreased from 1.39 to 1.32 Å


Baltrusaitis et al.

Figure 6. Energy minimized structures for (I) free gas-phase carbonate ion and (II-VI) five model clusters [M(OH)3(H2O)x(CO3)]2- (where M ) Al and x ) 0, 1, 2, 3, or 4) optimized at B3LYP/6-31G(d) level of theory. Atoms of different elements are highlighted with different colors; aluminium, green; oxygen, red; hydrogen, blue; and carbon, yellow.

as water molecules solvate the coordinated carbonate ion. The 1.32 Å bond length is very close to the C-O distance in the uncoordinated carbonate ion shown in I. Second, the carbonate ion bond increased from 1.83 to 2.00 Å as water molecules solvated the coordinated carbonate ion. These bond length changes show that with an increase in the amount of water, that is, with increasing solvation, the carbonate ion decreases its interaction and becomes less distorted in its geometry. Third, the change in carbonate structure with increasing water molecules is reflected in the vibrational frequency of the ν3 mode which is degenerate for the free ion but splits upon coordination and loss of symmetry for the complexed carbonate ion. The calculated ν3 vibrations for the coordinated carbonate ion as a function of solvating water molecules are shown in Figure 7. As the number of water molecules increase, ∆ν3, the frequency difference between the ν3 modes decreases, which is consistent with the carbonate ion relaxing toward a “less coordinated” state. These types of changes are also seen for surface adsorbed carbonate in the relative humidity studies as ∆ν3 decreases for absorbed carbonate as relative humidity increases. The closest agreement between experimental and calculated ∆ν3 was observed for the cluster model with three water molecules. In a second set of calculations, a more complex model of the surface was then used to help interpret the FTIR spectra of

the adsorbed carbonate product in the isotope experiments. In particular, two binuclear [M2(OH)2(µ-OH)2(H2O)11(CO3)] clusters, where M ) Fe, Al, were constructed. These types of cluster models have been previously used to interpret carbonate spectra adsorbed from solutions of bicarbonate.18 Geometries, optimized at B3LYP/6-31G(d) level of theory, are shown in Figure 8a,b for both iron and aluminum clusters, respectively. Once again, the vibrational frequencies were calculated at the same level of theory and scaled to 0.9632. Calculated scaled vibrational frequencies of both iron and aluminum solvated clusters were calculated for unlabeled clusters, as well as for 18O labeled clusters with different degrees of isotope substitution. The frequencies for these different clusters are given in Table 3. Comparison for both Fe and Al unlabeled calculated frequencies, ω, was determined to be in close agreement with experimental values, ν (Table 3). Calculated frequencies for ν3 (O-C-O)a mode in C16O2 + H216O adsorption experiments on both iron and aluminum cluster models are within a 20 and 8 cm-1 error, respectively. The calculated frequencies are in very good agreement with the experimental frequencies observed for adsorbed carbonate for both the iron and the aluminum oxide surfaces, with the largest difference being 23 cm-1 for the ν3 (O-C-O)s. This agreement suggests these binuclear cluster


J. Phys. Chem. C, Vol. 111, No. 40, 2007 14877 frequency is shifted by 40 cm-1 to lower wavenumbers in the C16O2 + H218O experiment, when compared with C18O2 + H216O. These results show that the adsorbed carbonate product has a larger mass for the C16O2 + H218O mixed isotope experiment compared with the C18O2 + H216O experiment. From the calculations shown in Table 3, it can be seen that the calculated frequencies in the C16O218O2- model are in better agreement with the C18O2 + H216O experimental carbonate frequencies and the calculated frequencies from the C18O216O2model are in better agreement obtained for carbonate from the C16O2 + H218O experiment. These results then provide evidence to suggest that the dominant source of 18O label in adsorbed carbonate is from water, not carbon dioxide. Discussion

Figure 7. Plot of calculated vibrational frequencies for ν3 (O-C-O)a and ν3 (O-C-O)s modes of carbonate ion coordinated to a single aluminum atom as a function of the amount of outer sphere coordinated water molecules. Calculated vibrational frequencies were taken from the models shown in Figure 6.

models will be useful in the interpretation of the shifts observed in the isotope experiments. In the full isotope substitution experiment (C18O2 + H218O) on iron oxide, a significant shift in the adsorbed carbonate vibrational frequencies is seen. This shows that all three 18O labeled oxygen atoms are incorporated into the carbonate ion on the iron oxide surface. The error difference between calculated and experimental vibrational frequencies is 2 and 23 cm-1 for both ν3 (O-C-O)a and ν3 (O-C-O)s vibrational modes, respectively. The mixed isotope experiments are the most interesting to interpret and represent some of the most surprising results of this study. As already noted, gas-phase C16O18O is present after iron oxide is exposed to either C16O2 + H218O or C18O2 + H216O. The experimental ν3 (O-C-O)a and ν3 (O-C-O)s vibrational frequencies in C16O2 + H218O experiment are at 1478 and 1337 cm-1, and are shifted to lower wavenumbers than those in C18O2 + H216O experiment. Additionally, the ν1 vibrational

Surface Reactions of Carbon Dioxide in the Absence and Presence of Co-Adsorbed Water: Product Formation. It is clear from these experiments that coadsorbed water plays a role in carbon dioxide adsorption on oxide surfaces. Under dry conditions, carbon dioxide adsorption on hydroxylated oxide surfaces yields adsorbed bicarbonate on the surface as one of the major products. The adsorbed bicarbonate product is wellestablished, and the structure of this as a bidendate bicarbonate surface species has been recently determined by quantum chemical calculations combined with isotope experiments.7 The similarity between the spectra for CO2 adsorption at 40% RH on both iron and aluminum oxides shown in Figure 1c,d and 1 M NaHCO3 adsorptions shown in Figure 2a suggests that the nature of the adsorbed species is the same in both cases. The quantum chemical calculations by Bargar et al.18 and repeated here also point to similar structures whereby carbonate is adsorbed on the surface and solvated by nearby water molecules. Besides adsorbed carbonate, there are Fe-OH2+ groups in the high relative humdidity spectra, that is, above 10% RH. Wijnja et al., by means of diffuse reflectance IR Fourier transform (DRIFT) spectroscopy and proton coadsorption data, investigated proton uptake with the adsorption of bicarbonate from solution onto goethite particles in the pH range extending from 4.8 to 7.23 They observed an increase in intensity in their DRIFT difference spectra for adsorption bands at 3703, 3540, and 3410 cm-1 and suggested that additional protonated surface

TABLE 3: Scaled Vibrational Frequencies (cm-1) of Carbonate Ion in [M2(OH)2(µ-OH)2(H2O)11(CO3)] Cluster Model (where M ) Fe, Al), Calculated at B3LYP/6-31G(d) Level of Theory for Different Isotope Experiments: Comparison to Experimental Valuesa vibrational mode and calculated frequency, ω, cm-1 position of O-substitution

18

C16O32C16O218O2-

ν3 (O-C-O)a

C18O32-

n.a.c 1 2 3 1,2 2,3 1,3 1,2,3

1497 1495 1495 1479 1494 1479 1478 1476

C16O32-

n.a.c

1531

C18O216O2-

ν3 (O-C-O)s Iron cluster model 1364 1353 1355 1363 1345 1354 1353 1344

vibrational mode and experimental frequency, ν,b cm-1 ν1

ν3 (O-C-O)a

ν3 (O-C-O)s

ν1

1496d

1344d

1073d

1063 1050 1051 1051 1008 1010 1010 993

1488e

1344e

1073e

1478f 1478f 1478g

1337f 1337f 1321g

1033f 1033f 1013g

Aluminum cluster model 1409 n.o.h

1523

1408

n.o.h

a Scaled to 0.9632 (see text for further details). b Experimental frequencies at 40% RH (see Figure 5). c n.a. ) not applicable. d Frequencies for carbonate formed from C16O2 + H216O on Fe2O3. e Frequencies for carbonate formed from C18O2 + H216O on Fe2O3. f Frequencies for carbonate formed from C16O2 + H218O on Fe2O3. g Frequencies for carbonate formed from C18O2 + H218O on Fe2O3. h n.o. ) not observed.


Baltrusaitis et al.

Figure 8. Energy minimized structures for [M2(OH)2(µ-OH)2(H2O)11(CO3)] (where M ) Fe and Al) optimized at B3LYP/6-31G(d) level of theory. Atoms of different elements are highlighted with different colors; iron, purple; aluminum, green; oxygen, red; hydrogen, blue; and carbon, yellow.

groups coexist with adsorbed carbonate. According to Wijnja et al., the reaction proceeded via reactions 1 and 2:

Fe-OH0.5- + HCO3- a Fe-OCOO1.5- + [OH- + H+] (pH 4.8-7) (1) Fe-OH0.5- + H+ a Fe-OH20.5+ (pH 4.8-7)

(2)

The perturbation of the 3000 to 3600 cm-1 region can be seen in the spectra for adsorbed water on nanoparticulate Fe2O3 in the presence of carbonate, when compared with adsorbed water alone. Several new bands appear in this region, with the most prominent at 3515 cm-1. These absorption are interpreted as being due to protonation of the Fe-OH groups upon adsorption of CO2 + H2O. Thus, we propose that some of these bands, for example, the band at 3515 cm-1, is due to the FeOH2+ species, and the other bands at lower frequency (e.g., at 3266, 3105, and 3011 cm-1) are due to strong hydrogen bonding of water molecules to Fe-OH2+. Surface Reactions of Carbon Dioxide in the Absence and Presence of Coadsorbed Water: Reaction Mechanisms. In the absence of coadsorbed water, the reaction of carbon dioxide on hydroxylated surfaces to form bicarbonate as the major product occurs by direct reaction with surface hydroxyl groups according to reaction 3,

M2-OH + CO2 a M2-O2COH

in both solution and gas phase has been extensively studied by computational methods.24-28 The established carbon dioxide hydration activation energy in the gas phase is much higher (32.3-54.5 kcal/mol) than in solution (17.7 kcal/mol).24 However, to our knowledge, there are no estimates of the carbon dioxide hydration activation energies in the presence of surfaces. Liedl et al. investigated hydrogen-bonded complexes for two carbonic acid monomers using large basis sets and the inclusion of electron correlation.27 The formation of a carbonic acid dimer according to reaction 4 was found to be favored by ∼18 kcal/ mol, depending on the basis set used.

2H2CO3 a (H2CO3)2

(4)

Adsorbed carbonic acid may be stabilized by strong hydrogen bonding with adsorbed hydroxyl groups or oxygen atoms, and then it quickly undergoes deprotonation. The proposed reactions involved in carbonate formation are

H2O(a) + CO2(g) a H2CO3(a)

(5)

H2CO3(a) a HCO3-(a) + H+

(6)

HCO3-(a) a CO32-(a) + H+

(7)

FeOH + H+ a Fe-OH2+

(8)

(3)

where M ) Fe or Al. The mechanism to form the bidendate bicarbonate structure is more complicated than reaction 3 suggests as an intermolecular proton transfer takes place to form the final adsorbed bicarbonate structure after the initial nucleophilic attack of a surface hydroxyl group.7 Furthermore, the loss of bicarbonate absorptions in the FTIR spectra as a f(RH) in the low relative humidity regime below 10% is striking and suggests that surface adsorbed water may be blocking sites for bicarbonate formation. In the higher relative humidity spectra along with the isotope data, it is clear that, as a first step in the formation of surface carbonate, in the presence of coadsorbed water, carbon dioxide hydration has to be considered. The formation of carbonic acid

Additionally, both carbonic acid and bicarbonate can undergo reactions with surface hydroxyl groups

FeOH + H2CO3(a) a Fe-OH2+ + HCO3-(a)

(9)

FeOH + HCO3-(a) a FeOCO2-(a) + H2O(a)

(10)

The formation of a carbonic acid intermediate is supported by the fact that the main source of oxygen in the adsorbed carbonate experiments is from water molecules. The isotope substitution experiments show that two 18O atoms are incorporated into the carbonate product when C16O2 + H218O react compared with only one 18O atom incorporated when the C18O2 + H216O react. The elementary reactions involved in the isotope



substitution process during the C18O2 + H216O adsorption experiments are

H216O(a) + C18O2(g) a H2C18O216O(a)

(11)

H2C18O216O(a) a H218O(a) + C16O18O(g)

(12)

H216O(a) + C16O18O(g) a H2C16O218O(a)

(13)

As shown above, this leaves one 18O labeled oxygen atom incorporated into the adsorbed carbonic acid product in agreement with the isotope data and quantum chemical calculations, even though the initial carbon dioxide reactant was doubly labeled as C18O2. Similarly, the other mixed isotope experiment involved elementary reactions 14-16:

H218O(a) + C16O2(g) a H2C16O218O(a)

(14)

H2C16O218O(a) a H216O(a) + C16O18O(g)

(15)

H218O(a) + C16O18O(g) a H2C18O216O(a)

(16)

This scheme (14-16) yields an adsorbed carbonic acid that has two 18O labels which then reacts on the surface via reactions 6 and 7 to yield the observed carbonate isotope on the surface. Reactions 13 and 16 occur because of the presence of gas-phase water molecules adsorbed on the surface, which are in greater abundance than those generated from isotope scrambling reactions. These processes are supported by the results of the gas-phase CO2 experiments shown in Figure 5. Mixed isotope experiments with exposure times up to 24 h were performed (not shown). The peak frequencies for the adsorbed carbonate product did not change indicating that equilibria 13 and 16 dominate. From the calculations, there seems to be a preference for the 18O label to be in position 3 (as defined in Figure 8). Thus, it is not involved in isotope scrambling reactions in the C18O2 + H216O experiment and does not give rise to pure C16O32- species at long exposure times. For that reason, it is proposed that only atoms involved in bonding to the surface are responsible for isotope scrambling. However, these experiments do not probe much longer time scales, and it is possible that the oxygen isotope in the carbonate product may eventually be completely converted oxygen isotope present in the water molecules. Finally, it should be noted that several experiments from the literature lend support to the results shown in this study. In particular, Bargar et al. investigated the speciation of carbonate adsorbed onto hematite in air-equilibrated aqueous solutions by means of ATR-FTIR spectroscopy and compared the results with DFT/MO-calculated structures.18 Two major species of adsorbed carbonate were identified: a monodentate binuclear inner-sphere carbonate complex (bridged structure) and a fully or partially solvated carbonate species in the vicinity of water layers at the hematite-water interface, that was retained mainly by hydrogen bonding. If the carbonate ion is not readily available (as in our experiments), then the direct reaction between CO2 and H2O has to proceed first. Henderson et al. combined temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS), and high-resolution electron energy loss spectrometry (HREELS) to examine the reaction of CO2 and gaseous H2O on vacuum-annealed TiO2(110) surfaces to form bicarbonate.29 These experimental results were explained by a surface catalyzed conversion of a precursor-bound H2O-CO2 van der Waals complex to carbonic acid, which further reacts

at unoccupied oxygen vacancies to generate bicarbonate. Rege et al. studying H2O vapor/CO2 mixture adsorption on 13X (NaX) zeolite and γ-Al2O3 by means of FTIR spectroscopy observed CO2 adsorption enhancement at low concentrations of CO2 (

Surface Reactions of Carbon Dioxide at the Adsorbed WaterâOxide

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