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2009, 113, 19389–19392 Published on Web 10/19/2009
Adsorption of CO2 on the Rutile (110) Surface in Ionic Liquid. A Molecular Dynamics Simulation Tianying Yan,*,†,‡ Shu Wang,† Yuan Zhou,†,§ Zhen Cao,† and Guoran Li† Institute of New Energy Material Chemistry, Department of Material Chemistry, Nankai UniVersity, Tianjin 300071, China, and Institute of Scientific Computing, Nankai UniVersity, Tianjin 300071, China ReceiVed: September 10, 2009; ReVised Manuscript ReceiVed: October 9, 2009
The adsorption of CO2 on TiO2 rutile (110) surface in room-temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM+/PF6-), is studied by molecular dynamics simulation. Due to the strong electrostatic interactions between the O atom of CO2 with the unsaturated Ti atom of the rutile (110) surface, CO2 is adsorbed on the rutile (110) rather than dispersing in the IL bulk. At the interface, CO2 arranges itself in a highly ordered manner, with its D∞h symmetric axis parallel to the rutile (110) surface normal, and the CdO bond is elongated for the coordinating O atom. The interfacial packing pattern is CO2, PF6-, and BMIM+, in sequence, starting from the rutile (110) surface. Thus, the adsorbed CO2 molecules are confined in the narrow neighborhood adjacent to the rutile (110) surface. Room-temperature ionic liquids (ILs) are generally considered as green solvents for their environmentally benign properties, such as low volatility, nonflammability, and reusability. As promising replacements for conventional solvents, ILs have generated a wide range of applications involving gas dissolving and separating.1-4 Currently, the supercritical CO2-based separation process is receiving wide concerns, since it can be used to extract plenty of solutes from IL mixtures with negligible pollution.5,6 The most appealing advantage of this process is that the ILs would not add any contamination to the CO2 phase and the final recovered product. This is mainly due to the fact that IL is not soluble in the CO2 phase at all.1 On the contrary, research shows that the solubility and the selectivity of CO2 in ILs are “tunable” via altering the cations or the anions.7-9 Accordingly, the inner structure of the ILs after the dissolution of CO2 is important.6,10 As a complement to experiments, computer simulations have been extensively exploited in studies involving ILs.11 In a combined experimental and simulation work, Brennecke, Maginn, and co-workers indicated that the solubility of CO2 in imidazolium (im)-based ionic liquids is rather governed by the nature of the anions than the cations. They concluded that the overall organization of the ILs is identical after the addition of CO2 in the pure ILs, due to the strong Coulombic interactions which facilitate the formation of a network with CO2 filling in the interstices in the ILs. Besides, the results showed that the CO2 molecule takes on a tangential orientation to the anion in order to maximize the interactions.10 This anion-CO2 interaction was also explored by Balasubramanian and Bhargava, who believed that the optimized structures of anion-CO2 complexes are * To whom correspondence should be addressed. Phone: (86)22-23505382. Fax: (86)22-2350-2604. E-mail:
[email protected]. † Institute of New Energy Material Chemistry, Department of Material Chemistry. ‡ Institute of Scientific Computing. § Current address: College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China.
10.1021/jp908758u CCC: $40.75
Figure 1. Structure of ionic liquid BMIM+/PF6- and CO2.
dominated by Lewis acid-base interactions, which lead to the distortion of the linear CO2 configuration.12 According to the simulation of Berne and co-workers, only small angular rearrangements of the anions occur in the IL, in which the liquid structure mostly remains unchanged despite the large mole fractions of CO2.6 However, the maximum concentration would not increase as the external pressure increases,1 because the structure of ILs would be significantly disturbed to accommodate more CO2.6 Though an abundance of investigations have been applied on the bulk properties of the CO2/IL systems4 as well as IL interface properties,13 there exists no literature about the behavior of the mixture at a surface to our best knowledge. Our previous simulation on the BMIM+/NO3-/rutile (110) system shows that the adsorbed NO3- anions at the interface organized themselves into a highly ordered manner,14 while changing the anion to PF6- does not present such an ordered interfacial structure.15 On the other hand, Grimes and co-workers have conducted experiments to study the high rate photocatalytic conversion of CO2 to hydrocarbon production on the high surface area TiO2 nanotube arrays. They observed that, with a nanotube wall thickness low enough, the rate of the conversion under outdoor sunlight amounts to more than 20 times higher than that under UV illumination.16 In this study, we investigate the interfacial properties of the CO2/BMIM+/PF6- (1-butyl-3-methyl-imidazolium hexafluorophosphate, cf. Figure 1) mixture in contact with the rutile (110) surface via molecular dynamics simulation. 2009 American Chemical Society
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BMIM+/PF6- is adopted in this study because the solubility of CO2 in this IL has been well studied experimentally2,7 and computationally.6,10,12 The study indicates a strong coordinating tendency between CO2 and the unsaturated Ti atoms on the interface, and the TiO2 surface is almost exclusively occupied by CO2. The performance described here may cast new light on the research of CO2 adsorption and conversion and enrich the microscopic understanding of the general properties of ILs. To simulate the CO2/BMIM+/PF6- mixture and TiO2 rutile (110) interface, a slab of rutile crystal, with the nonhydroxylated surface cleaved on the (110) plane, as well as the CO2/BMIM+/ PF6- mixture were put into the MD simulation box with periodic boundary conditions applied on all directions. The rutile (110) slab was prepared as in our previous study.14 Briefly, we repeated the rutile unit cell along the [110], [1j10], and [001] crystallographic directions by 8, 8, and 17 times, respectively, resulting in slab dimensions of 51.975 Å × 51.975 Å × 50.303 Å. Afterward, we cleaved the (110) surfaces, in which the oxygen atoms are bis-coordinated and the titanium atoms are penta-coordinated. The two rutile (110) surfaces are then separated to 72 Å, between which 540 BMIM+/PF6- pairs and 60 CO2 molecules were put. The density of the IL is 1.354 g/cm3, which is close to the experimental density (1.36 g/cm3).17 The number of CO2 in BMIM+/PF6- is also close to the experimental solubility at 1 atm.10 The equilibrium run was proceeded with simulated annealing for the temperature of the system gradually reduced from 1000 to 300 K in 10 ns. After that, the MD production run was performed with the NVT ensemble coupled to a Nose´-Hoover thermostat18 at 300 K for 5 ns, with integration time steps of 1 fs using the velocity Verlet algorithm.19 The van der Waals and real space electrostatic interaction cutoff distance was 12 Å, and the reciprocal space electrostatic interactions were handled with conventional 3-D Ewald summation.19 The simulation was performed with the DL_POLY package.20 The force field parameters of CO2 and IL were taken from Maginn’s work,10 and the rutile (110) surface was frozen with van der Waals and electrostatic interactions with CO2 and IL. The van der Waals interactions between BMIM+ and the rutile (110) surface, as well as the partial charges on TiO2, were summarized in our previous work,14 and those between PF6- and TiO2, as well as between CO2 and TiO2, are summarized in the Supporting Information. Figure 1 shows the schematic structures and atomic notations of BMIM+/PF6and CO2. Figure 2a shows the number density profiles of the center of mass of CO2 molecules, the BMIM+, and the PF6- ions. The most striking feature is the existence of the remarkably high peaks of CO2 near the rutile (110) surface. Owing to the different amounts of adsorption at each side of the rutile (110) surface, the two CO2 peaks near the rutile slab are slightly different in height. The peak fades significantly toward the IL bulk. Unlike the previous investigations,1,8 CO2 molecules are almost totally adsorbed on the rutile (110) surface rather than dissolved in the IL bulk, leading to a bulk composed mainly of IL. It was found in a previous study that CO2 fills the interstices of the IL.6 As the interstices are created at the IL/TiO2 interface, it is reasonable that CO2 molecules fill in these interstices. Besides, the CO2 peak positions are closer to the interface than those of both of the ions, attributed to the strong interactions between CO2 and the rutile (110) surface. Immediately next to the CO2 layer is the PF6- layer, due to the strong interactions between PF6- and CO2, as shown in previous studies.6,10,12 It is also noticeable that the enhanced layers of the ions emerge immediately next to the high peaks of CO2. As a consequence of
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Figure 2. (a) Number density profiles of the center of mass of BMIM+ (black), PF6- (red), and CO2 (blue) along the radial distance of the rutile (110) surface normal direction, z. (b) Orientational ordering of the imidazolium (im) ring normal of BMIM+ (black), N-N vector on the im ring of BMIM+ (red), and D∞h symmetric axis of CO2 (blue) along the radial distance of the z-direction.
the surface segregation of the CO2 and PF6-, the bulky BMIM+ also shows an enhanced layer at the interfacial region, and the region around 3.4 Å to the rutile slab is almost exclusively occupied by the BMIM+ cations. Thus, the surface layer shows a well ordered pattern of CO2, PF6-, and BMIM+, in sequence, starting from the rutile (110) surface. Such an enhanced IL layer also confines CO2 on the surface. Comparing with our previous studies on the BMIM+/NO3-/rutile (110) interface14 and BMIM+/ PF6-/rutile (110) interface,15 in which the surface segregation of the NO3- anion is more than the PF6- anion, it seems that a hydrophobic IL is needed in order to have CO2 to be exclusively adsorbed on the surface. Otherwise, there would be competition between CO2 and the anion on the surface adsorption. The orientational ordering of CO2 and BMIM+ along the rutile (110) surface normal direction, z, is evaluated by the second Legendre polynomial, P2(θ;z) ) 〈[3 cos2 θ(z) - 1]/2〉, in which θ(z) denotes the angle of a specific direction vector with respect to the radial distance along the z-direction. Figure 2b shows P2(θ;z)’s of the im ring normal of BMIM+, the N-N vector of BMIM+, and the D∞h symmetric axis of CO2. The positive P2(θ;z) of 0.6 for the im ring normal, accompanied by the negative P2(θ;z) of -0.5 for the N-N vector immediately next to the rutile (110) surface, signifies the parallel alignment of the imidazolium ring on the surface plane. However, such closely contacted BMIM+ is extremely rare according to Figure 2a. For the enhanced BMIM+ layer, The sign of the above two P2(θ;z)’s switches, i.e., -0.5 for the im ring normal and 0.6 for the N-N vector, indicating a tilt alignment of the im ring with respect to the rutile (110) surface. Thus, in the enhanced density layer, the BMIM+ cations slant on the rutile (110) surface with a tilt alignment of the im ring with respect to the rutile (110) surface. The orientational tendency is much more evident for CO2. The P2(θ;z) of the surface segregated CO2 is close to 1.0, demonstrating that CO2 takes a vertical alignment on the rutile (110) surface, with its D∞h symmetric axis parallel to the surface normal. A snapshot, taken at the end of the simulation, is presented in Figure 3, which clearly shows the adsorption
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Figure 3. Snapshot taken at the end of the MD simulation, which shows the structure of the TiO2 (110) interfacial region (green, N atom; yellow, H atom; gray, C atom on imidazolium ring; tan, P atom; orange, F atom; blue, C atom on CO2; red, O atom on CO2; cyan, Ti; pink, O atom on TiO2).
structure at the interfacial region. CO2 stands with the linear configuration in an upright position with respect to the interface with one oxygen atom coordinating with the unsaturated Ti atom on the rutile (110) surface and the other oxygen atom pointing toward the bulk region. This particular arrangement also facilitates the interactions between CO2 and the interfacial titanium atoms. As the surface 5-fold Ti atoms at the interface are unsaturated by oxygen atoms, the oxygen atoms on CO2 tend to approach the interface to donate one O atom to coordinate with the unsaturated Ti atom. During the 5 ns MD simulation, the adsorbed CO2 molecules are very stable on the rutile (110) surface. To obtain detailed structural information for the CO2 adsorption on the rutile (110) surface, we calculated the distribution of distance between the unsaturated Ti atoms and O atoms, denoted as O1 atoms, of CO2 that are coordinated with the Ti atoms on the interface, as depicted in Figure 4a. The peak occurs around 2.11 Å, with the average distance of ca. 2.18 Å. The regular Ti-O bond length is 1.60 Å. Therefore, CO2 shows a strong tendency to coordinate with the unsaturated Ti atom. Meanwhile, the CdO bond length distributions are also presented in Figure 4b. It is revealed that the length for the CdO bond that points away from the interface nearly retains its equilibrium bond length of 1.16 Å, whereas the bond length for the one approaching the rutile (110) slab is elongated to 1.19 Å. Thus, the CdO bond energy is weakened due to the strong interaction between the O atom on CO2 with the unsaturated Ti atom on the rutile (110) surface. Due to the limitation of the classical force field, which neglects charge transfer interactions at the surface and the possibility of bond forming/ breaking, we were unable to estimate the CdO bond dissociation energy in this study. In summary, we have performed a molecular dynamics simulation on the CO2/BMIM+/PF6-/rutile (110) system. The results show that CO2 is adsorbed on the rutile (110) rather than dispersing in the IL bulk. This mainly results from the strong interactions between the CO2 molecules and the interfacial
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Figure 4. (a) Distribution of the coordinating bond length between the O1 atom (the nearer ones to the interface) of CO2 and the interfacial unsaturated Ti atom. (b) Distribution of the CdO bond length (the bold line denotes the CdO1 bond length, and the dashed line denotes the CdO2 bond length, in which the O2 atom denotes the O atom of CO2 pointing toward the IL bulk).
unsaturated Ti atoms. At the interface, CO2 arranges itself in a highly ordered manner, with its D∞h symmetric axis parallel to the rutile (110) surface normal, and the CdO bond is elongated for the coordinating O atom. Such orientational inclination is favorable for CO2 to donate one of the O atoms to coordinate with the unsaturated Ti atom on the rutile (110) surface. The interfacial packing pattern is CO2, PF6-, and BMIM+, in sequence, starting from the rutile slab. Thus, the adsorbed CO2 molecules are confined in the narrow neighborhood adjacent to the rutile (110) surface. We hope this study can stimulate further processing of CO2 in ILs, and the related experimental study is welcome. Acknowledgment. This research is supported by the NSFC (No. 20873068, 20503013) and the 973 Program (2009CB220100) of China. We are grateful to Institute of Scientific Computing (NKstars HPC program) of Nankai University for the computing time. T.Y. thanks Prof. Xueping Gao for the helpful discussion. Supporting Information Available: The van der Waals interactions between PF6- and TiO2, and those between CO2 and TiO2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Nature 1999, 399, 28. (2) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2002, 106, 7315. (3) Hanioka, S.; Maruyama, T.; Sotani, T.; Teramoto, M.; Matsuyama, H.; Nakashima, K.; Hanaki, M.; Kubota, F.; Goto, M. J. Membr. Sci. 2008, 314, 1. Kazarian, S. G.; Briscoe, B. J.; Welton, T. Chem. Commun. 2000, 2047. (4) Bara, J. E.; Carlisle, T. K.; Gabriel, C. J.; Camper, D.; Finotello, A.; Gin, D. L.; Noble, R. D. Ind. Eng. Chem. Res. 2009, 48, 2739. (5) Blanchard, L. A.; Brennecke, J. F. Ind. Eng. Chem. Res. 2001, 40, 287. Brown, R. A.; Pollet, P.; McKoon, E.; Eckert, C. A.; Liotta, C. L.;
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