Functionalized Rutile TiO2(110) as a Sorbent To Capture CO2 through

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

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Functionalized Rutile TiO (110) as a Sorbent to Capture CO through Noncovalent Interactions: A Computational Investigation Akshinthala Parameswari, Soujanya Yarasi, and G. Narahari Sastry J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09311 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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The Journal of Physical Chemistry

Functionalized Rutile TiO2 (110) as a Sorbent to Capture CO2 through Noncovalent Interactions: A Computational Investigation Akshinthala Parameswari†§, Yarasi Soujanya*†§, and G. Narahari Sastry*†§ †Centre

for Molecular Modeling, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500007, Telangana, India §Academy of Scientific and Innovation Research (AcSIR), New Delhi 201002, India Corresponding author: Mail id: [email protected]

ABSTRACT: The present study evaluates the CO2 adsorption capacity of functionalized rutile TiO2 (110), using the density functional theory and ab initio molecular dynamics simulations. The defect-free TiO2 surface is functionalized (f-TiO2) with alkanolamines (AKAs) namely, monoethanolamine (MEA), 3-aminopropanol (3AP); and amino acids (AAs) glycine (GLY), -alanine (-ALA). These functionalized adsorbents attain stability through bifunctional/bidentate binding of weakly acidic OH/COOH groups to TiO2 surface. While the AKAs bind parallel to the surface, where the AAs bind perpendicular to the TiO2 surface, exhibiting several binding sites favourable for multiple, cooperative noncovalent interactions with CO2. The nature and the strength of these interactions are evaluated in terms of binding energies, vibrational frequencies, Quantum Theory of Atoms in Molecules (QTAIM) approach, and charge transfer analysis. The subtle yet substantial interactions of CO2 with f-TiO2 will lead to physisorption and diffusion through pores/channels of f-TiO2 based solid adsorbents, which consequently reduce the energy required for the regeneration process. Among all the configurations of CO2 binding with f-TiO2, GLY-TiO2---CO2 display highest binding energy of -46 kJ/mol and the trend follows as, GLY > 3AP > MEA > β-ALA. Finally, this study examines the possibility of f-TiO2 based solid adsorbents as promising materials for CO2 capture at reduced cost in view of their preference towards physisorption of CO2 gas molecules.

INTRODUCTION The concentration of atmospheric CO2 has increased from ~100 ppm in the year 1700 to ~400 ppm in 2017 and if the current trend continues, it is likely to reach 550 ppm by 2100.1,2 This increase in the CO2 level after the dawn of industrial revolution has its catastrophic effect on environment, vegetation and marine eco systems. Therefore, developing an efficient, stable and selective capture of CO2 materials, i.e. cost-effective is an important area of research. Amine scrubbing is one of the conventional technologies used to separate CO2, in which aqueous MEA is popularly used.3,4,5,6 Due to the aqueous nature of amines, large amount of energy is required to regenerate the solvent and also has drawbacks like equipment corrosion and degradation in the presence of other flue gases.7,8,9 CO2 capture by using solid adsorbents (SAs) is considered to be a promising technology as it is less corrosive, less toxic and exhibits high efficiency in CO2 capture compared to conventional liquid amines. Further, SAs are relatively easy to handle and can be regenerated by pressure, temperature and vacuum swing processes,10 thereby reducing the parasitic energy along 1

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with the capital and operating costs for CO2 capture. To address the problem of low adsorption capacity of pure SAs, there is a growing interest in adopting the adsorption processes by using functionalized solid adsorbents (f-SA).11 These f-SAs are obtained by either chemically grafting or physically immobilizing basic functional groups on the supports such as silica,12 carbon nanotubes,13 MOFs,14,15 activated carbon,16, zeolites,17 polymers and metal oxides

18etc.

which

tremendously enhance the CO2 adsorption capacity. SAs with chemically grafted amines have shown improved performance for multiple cycles compared to SAs with physically immobilized amines.19 Extensive studies on the f-SAs have focused mainly on improving its working capacity. However, the more reasonable metric for assessing the f-SA is the energy required to adsorb and desorb the CO2. Therefore, the design of f-SAs should not only target enhancing the CO2 working capacity but also to minimize the overall heat required for the adsorbent regeneration.20 Hence, one of the criteria used in this study is to develop sorbent models in which strong binding interactions are avoided and thereby the formation of carbamate, carbamic acid and carbonate by-products is prevented.21 The formation of such strongly bound species increases the energy penalty in terms of regeneration. Therefore, in this study, f-SAs with the adsorption sites favourable for CO2 binding through noncovalent interactions have been preferred. Typically, the magnitude of physisorption energy is ∼20 to 50 kJ/mol22 with major contribution from electrostatic, induction and the dispersion interactions. Our group has been working on the fundamental understanding of long-range interactions, especially on their nature, occurrence and causative effect in various chemical and biological systems.23 It is well established that the nature and strength of physisorption is governed by several noncovalent interactions like cation- interactions, - interactions, hydrogen bond, halogen bond, metal-ion lone pair interactions etc., which cooperatively influence each other.24,25 Hence it is highly important to understand how these long-range dispersion interactions regulate CO2 binding with the adsorption sites of f-TiO2. TiO2 is one of the most studied metal oxides owing to number of its applications including heterogeneous catalysis, photo-electrolytic water splitting, solar cells, gas sensors etc.26,27 The high surface area, good thermal and mechanical stability, low-cost and good resistance in acidic and oxidative conditions makes TiO2 as a potential candidate to study in the domain of surface science.28,29 In view of the biological applications of TiO2 based implants, amino acids especially glycine binding to TiO2 surface has been reported.30,31,32 Recently, Kapica-Kozar et al. have shown the importance of TiO2 modification with various amines for CO2 capture in achieving higher adsorption capacity.33 Apart from TiO234,35,36 studies reporting AKAs binding to hydroxylated Cr2O3,37 Al(100)38 Cu(100),39 Al2O340 are available in the literature, however their performance towards CO2 capture is not evaluated. Importantly the majority of studies reported on reduced or oxidized TiO2 (rutile,41,42 anatase,43 and brookite,44) surface reveal that CO2 adsorption prefers take place in chemisorption mode.45,46 In our earlier study we have demonstrated the efficacy of amino acids as effective sorbents to capture CO2.47 In continuation of our studies on developing novel sorbents for CO2 capture,47,48,49 in the present study, we adopted an approach to cut down the regeneration cost of sorbents by tuning the CO2 binding strength to f-TiO2, preferably through non bonded interactions. In order to compare the effect of functionalization of TiO2 on CO2 binding, in the current study we first discussed the results obtained on isolated molecules of AKAs (MEA, 3-AP) and AAs (GLY and -ALA) in Sec 3.1. Then, the stable orientations of AKAs and AAs that are chemically tethered to TiO2 surface are described in Sec 3.2. And in the following sections Sec 3.2.1 and 3.2.2, stable orientations of CO2 binding sites which are labelled as C(1-2)M, C(1-2)N, C(1-2)O and C(1-2)P are discussed. Finally, molecular dynamics (MD) simulations have been performed on the most stable CO2 configuration as described in Sec 3.3. Coadsorption of CO2 with 2


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preadsorbed H2O·f-TiO2 systems is discussed in Sec 3.4. Overview of all the systems and corresponding labels considered in this is shown in Scheme1.

2. COMPUTATIONAL DETAILS All the electronic structure calculations are performed using periodic density functional theory (DFT) based methods as implemented in DMol3 module of Material Studio.50 a generalized gradient approximation (GGA) with Perdew-BerkErnzerhof (PBE)51 functional is used to describe the energy of exchange and correlations terms. All-electron core treatment with double numerical plus polarization (DNP basis set) was taken.52 DMol3 provides high accuracy of localized atomic orbitals and low basis set super position error as clearly examined by Yasuji et al.53 For optimizations, the convergence parameters are set to be 1.0×10-5 Hatree/atom for energy, 0.002 Hatree/Å for atomic forces, 0.005 Å for maximum displacement respectively. In addition, the numerical integration performed on real space grid with a fine global orbital cut-off 5.2 Å. A vacuum slab of 15 Å was used in z-direction to prevent interaction between slabs.54 The Brillouin zone was sampled at the Gamma point for isolated molecules , and 2×2×1 for slab calculations via the Monkhorst-Pack k-point mesh.55 To include non-local correlation effects, Grimme (D2)56,57 and Tkatchenko and Scheffler (TS)58 correction potentials are used. Both the schemes use the same damping function to determine the longrange behaviour of dispersion correction, but the later one accounts for the polarizability and volume. And the atomic volumes are derived from the Hirshfeld partitioning of the electron density.59 The binding energies (BEs) of different complexes were calculated according to the following equations, BECO2 = E(moiety·CO2) ― (Emoiety +ECO2)

(1)

BEmoiety = E(𝑓 ― TiO2) ― (ETiO2 + Emoiety)

(2)

BECO2 = E(𝑓 ― TiO2·CO2) ―(E(𝑓 ― TiO2) + ECO2)

(3) (4)

BE𝑛CO2 = E(𝑓 ― TiO2·𝑛CO2) ―(E(𝑓 ― TiO2) + E𝑛CO2) BEH2O = E(𝑓 ― TiO2·H2O) ―(E(𝑓 ― TiO2) + EH2O) BECO2 coads = E(𝑓 ― TiO2·H2O·CO2) ―(E(𝑓 ― TiO2·H2O) + ECO2)

(5) (6)

Where, the in Eq. 1, E(moiety·CO2)represents the total energy of moiety·CO2 complex, andEmoiety, ECO2are the energies of individual moiety and free CO2molecule respectively. Similarly, in equations 2-6 are the BEs of moiety, CO2 with f-TiO2, multiple CO2, H2O with f-TiO2 and CO2 coadsorption energy in presence of H2O with f-TiO2 respectively. Where as in Eq. 4 E(𝑓 ― TiO2·𝑛CO2) represents the total energy of f-TiO2.nCO2 complex, and E(𝑓 ― TiO2) is energy of f-TiO2 complex and EnCO2 is the energy of the CO2 molecule, n represents the number of CO2 molecules. The negative BEs correspond to stable configurations and TS dispersion corrected BEs have been considered throughout this work for the reliable description of the long-range dispersion interactions between adsorbate and adsorbent. A supercell (4×2) with four-layer thickness (a=11.836, b=12.9938 and c=24.282) was generated from the unit cell of rutile TiO2 (lattice constants of a=4.5918, b=4.534 and c=2.9673) by cleaving at (110) plane and with the vacuum of 15 Å in z-direction to prevent interaction between slabs. Figure 1. shows the pictorial view of rutile TiO2 surface, which consist of six–fold-coordinated Ti atom (6f), five-fold -coordinated Ti atom (5f) and bridging (BR) in plane oxygen atoms

3



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(IP), where Ti4+ ions act as Lewis acidic sites, and O-2 ions act as Lewis basic sites. We have allowed to relax all the layers of (4×2) TiO2 surface in all electronic structure calculations. To evaluate the nature and strength of interaction between adsorbate and adsorbent, the topological analysis of electron density and its derivatives were estimated using AIM-UC code with electron density grid files.60 These grid files are being generated by doing singe point calculations with the input parameters taken to be the same as in the geometry optimization. The theory of AIM-UC provides an elegant approach to unravel both inter and intra molecular interactions with the help of electron density (ρ) and Laplacian (∇²ρ) values. The physical significance of ρ(r) is the gradient of the electron density, whereas the Laplacian ∇²ρ (r) represents areas of local charge concentration and depletion. The strength of a bond is defined by the magnitude of the charge density at bond critical point (BCP). If charge density at BCP is ∇²ρ BCP (r) < 0, the density is locally concentrated resulting in shared interactions, while in the case ∇²ρ BCP (r) > 0 the electron density is depleted representing closed-shell interactions. The topological properties of ρ and ∇²ρ for four f-TiO2 systems as well as most stable binding configurations of CO2 on f-TiO2 are tabulated in Table 2 and Table 3. We performed ab initio molecular dynamics simulations (AIMD) simulations in VASP 5.4. 61,62 using DFT within vdWDF1 approximation.63 Most stable binding configurations of CO2 on f-TiO2 surface from the static calculations are taken as starting geometries to carry out the AIMD simulations. The projector augmented wave (PAW)64 method with titanium 4s 3d and oxygen 2s 2p levels were expanded into plane waves with the energy cut-off of 300 eV. The top two layers were allowed to relax and the bottom two layers were kept frozen during the simulation to reduce the computational time. The Brillouin zone integration was sampled at gamma point via Monkhorst-Pack method. The dynamical motion of CO2 molecules is explored by solving the Newtonian equation of motion with a time step of 0.5 fs for the total simulation time of 1 ps using blocked Davidson and residual minimization (RMM-DIIS) algorithm. MD simulations utilized a thermodynamical ensemble of constant number of particles, volume and temperature (NVT) with Nose-Hoover thermostat65 at 300 K. After completion of simulations, the resulting trajectories were taken into VMD software to visualize and calculate the radial distribution function (RDFs).66 Data acquired from the statistical analysis of probability of interaction of atoms in RDFs is well correlated with the static calculations, and also quantifies the strength of binding. Vibrational frequencies of four f-TiO2 systems as well as most stable binding configurations of CO2 on f-TiO2 are tabulated in Table 4.

3. RESULTS AND DISCUSSION 3.1 CO2 Binding with isolated molecules Initially, the geometry optimizations of CO2 with isolated molecules (CO2·MEA, CO2·3AP, CO2·GLY, CO2·β-ALA) are carried out. The obtained BEs are in similar range except that of CO2·MEA (~-7 kJ/mol), due to intramolecular H-bond (N---H) between nitrogen lone pair and H(OH) group. However, this N---H interaction is not possible after chemical tethering of MEA to TiO2 in MEA-TiO2 system (Figure.3). As evident from the Figure 2. The gauche conformation of MEA is considered for the CO2·MEA complex, as it is stable than the corresponding trans conformation by 0.07 eV.34 Henceforth all further calculations are carried out with gauche conformation of MEA. In all the CO2·MEA, CO2·3AP, CO2·GLY, CO2·β-ALA complexes, the electrophilic carbon atom of CO2 interacts with nucleophilic nitrogen atom, which is a type of Lewis acid-base interaction. Apart from C···N dipole interaction, a weak H-bond is present in C-H---O 4


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interaction is observed in CO2·3AP, CO2·β-ALA complexes. Although the bond lengths of CO2 are not much influenced on its binding, the bond angle however is deflected by ~5° in all isolated systems except CO2·MEA due to the weak C--N interaction. Hydrogen bonding and acid−base interactions are the driving force for AKAs and AAs to interact with CO2. The binding energies obtained with PBE, PBE+D2 and PBE+TS for each investigated CO2 complex are given in Table S1. To quantify the local donor/acceptor character of CO2 on its binding with MEA, 3AP, GLY, β-ALA complexes, The CT is calculated between CO2 and isolated molecules by summing up the Hirshfeld charges 67 of the individual molecules are tabulated in Table 1, negative CT values implies charge flow from amino group to CO2. Albeit the magnitude of CT among CO2·MEA, CO2·3AP, CO2·GLY, CO2·β-ALA complexes is more or less similar (~-0.04), it is negligible in CO2·MEA due to intramolecular H-bond (N---H) decreases the density of nitrogen lone pair.

3.2 Functionalization of TiO2 with AKAs and AAs The geometry optimization of defect-free-TiO2 functionalized with AKAs (MEA, 3-AP), AAs (GLY and -ALA) in the absence of CO2 has been carried out to identify the stable configuration. From the studies reported on the binding of amines and amino acids on TiO2, it’s clear that, AKAs & AAs can bind either in molecular and dissociated form. In the current study, we have chosen the most stable dissociative form. While the AKAs preferentially bind parallel to the TiO2 surface, the AAs bind perpendicular to the TiO2 surface as shown in Figure 3 (MEA-TiO2, 3AP-TiO2, GLY-TiO2, -ALATiO2) and involves strong binding from oxygen atom OH/COOH with titanium atoms of TiO2. The BEs are in the range of chemisorption, which (~-150 to -200 kJ/mol ) are well agreement with experimental studies of AKAs and AAs on TiO2(refer Table S2).34,68,30, 70 Evident from the Figure 3, the terminal nitrogen and oxygen atoms of MEA align optimally (-O-CH2-CH2-NH2 (3.113 Å) with two adjacent Ti (5f) sites at distance of 2.984 Å resulting higher BE of -226 kJ/mol. This is rationalized by strong interaction of nitrogen atom of amine with deficient titanium atom of TiO2 at distance of 2.337 Å (with BCP2 ∇2ρ, 7.5357). This alignment aids amino and methylene protons to form H-bonds with proximal bridging oxygen O(BR) atoms with distance of 2.225 Å and 2.655 Å respectively. The magnitude of these interactions were measured in terms of electron density ρ and its Laplacian ∇2ρ values at bond critical points (BCP3-5) and ring critical points (RCP1) are listed in Table 2 and as same is shown in the Figure S1 respectively. Then this MEA-TiO2 system is further used to generate the possible binding configurations of CO2. Similarly, 3AP binds to TiO2 surface as shown in the Figure 3. However, there exists a mismatch of the length of its structure (4.982 Å) with the length equivalent to three adjacent titanium atoms in a row (5.885 Å). This discrepancy is manifested in lower BE of 3AP (-193 kJ/mol), lower ρ, ∇2ρ values of BCP1 and 2 and wider bond angle of CNTi (~5°) than that of MEA-TiO2 model. Even though the interaction between amino nitrogen and titanium atom is weaker in 3AP-TiO2 than that of MEA-TiO2, it is however compensated to some extent by amino and methylene protons by participating in H-bonding with proximal bridging oxygen O(BR) atoms, with distance of 2.263 Å and 2.621 Å respectively. The BCP3 for (H2C)H---O(BR) 3AP-TiO2 is almost twice that of BCP3 in MEA-TiO2, as evident from the Table 2. The lone pair of the amino nitrogen interacts weakly with electron deficient titanium atom at N-Ti distance of 2.621 Å for 3AP-TiO2 compared to 2.337 Å for MEA-TiO2. Although the structures of MEA and 3AP belong to AKAs, the presence of additional methylene group in 3AP molecule increases N-Ti binding distance which results in lower BE of 5



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3AP-TiO2 (-193 kJ/mol) compared to MEA-TiO2 (-226 kJ/mol). However, this feature leads to increased CO2 adsorption capacity of 3AP than in MEA. In agreement with earlier experimental studies 69 both GLY and -ALA orient perpendicularly to the surface, leaving amino protons free to bind with incoming CO2 as shown in the Figure 3. In GLY-TiO2 structure, as evident from the Figure 3, amino group is oriented in such a way that one of its protons is in close contact with O(COO-) group (2.406 Å) resulting the nitrogen atom more basic to bind efficiently with C(CO2). -ALA-TiO2 has slightly higher BE (-174 kJ/mol) than GLY-TiO2 (-169 kJ/mol). Because of H-bonding between surface bound proton and oxygen of carboxyl group (HO(BR)---O(COO)) at distance of 2.283 Å in -ALA-TiO2 compared to 2.359 Å in GLY-TiO2. The magnitude of this interaction is reflected at their BCP3 in Table 2 and the corresponding geometry is shown in the Figure S1 respectively. Both amino acids (GLY and -ALA) are stabilized through dissociative bidentate bridging mode with carboxylic oxygen atoms binding with two adjacent titanium atoms and carboxyl proton is transferred to surface oxygen atom.

3.2.1 CO2 binding on MEA-TiO2 and 3AP-TiO2 Optimization of f-TiO2 with CO2 led to various configurations, C(1-4) M, C(1-4)N, and optimal ones are shown in Figure 4 and the remaining configurations are given in Figure S2, and BEs are given in Table S5. C(1)M configuration exhibits BE of -34 kJ/mol, stabilizes through H-bonding and acid-base interactions (Figure 4a). The H-bonding between oxygen atom of CO2 with amino proton of MEA (CO2)O---H(NH2) at a distance 2.378 Å. The C---O(BR) acid-base interaction between carbon atom of CO2 with bridging oxygen atom of the surface (CO2)C---O(BR) 3.177 Å further reflected in their ρ(0.0156) and ∇2ρ(0.1523) values of RCP2 shown in Table 3 and Figure S4. The geometrical parameters of possible close contacts in C(1)M remain same as in MEA-TiO2. As shown in the Figure 4, the bond length (O=C) of CO2 that interacts with the amino proton is slightly elongated (1.179 Å) than the other C=O bond length (1.172 Å). In C(2)M configuration (in Figure 4b), CO2 is in the vicinity of methylene protons showing weak interaction with the BE of -8 kJ/mol. It is important to identify from the orientations of CO2 on MEA-TiO2 surface, that these two configurations C(1)M and C(2)M can still exist even in the case of multiple CO2(vide infra)(see Figure 6) on MEA-TiO2 surface because of adequate gap between two adjacent five coordinated Ti (5f) rows (6 Å). Further, one of the amino protons can still interact with the adjacent hydroxyl group of MEA as reported.34 For 3AP-TiO2, configurations of CO2 C(1-2)N are shown in the Figure 4. It is intriguing to observe that the C(1)N exhibits BE of -40 kJ/mol, -6 kJ/mol more than in C(1)M, in spite of mismatch of its structure to the surface titanium atoms. In C(1)N, (Figure 4c) the oxygen atom of CO2 binds with one amino proton (CO2)O---H(NH2) at a distance, 2.543 Å and carbon atom of CO2 with the O(BR) atom at a distance of 3.076 Å and other geometrical parameters of possible close contacts remain same as in 3AP-TiO2. Slightly higher BE value of C(1)N is reasoned to the presence of additional RCP3 and higher ρ and ∇2ρ values of RCPs (1 & 2). Another important observation one can make in this configuration is that the binding of amino nitrogen of 3AP with titanium atom gets stronger in presence of CO2 compared to MEA-TiO2. As evident from Table 3, significant increase in ρ(0.1558 from 0.1370) and ∇2ρ(3.6634 from 2.8924) values of BCP2 in C(1)N. These observations are further corroborated with AIMD simulations (vide infra). In C(2)N (Figure 4d) configuration, as expected a weaker interaction with only methylene protons gives BE of -14 kJ/mol, as shown in the Figure 4d. Comparison of binding energies of CO2 interaction with f-TiO2 configurations, C(1-4) M, C(1-4)N, obtained with PBE, PBE+D2 and PBE+TS are given in Table S3 along with C(1-3) O, C(1-3)P. 6


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Tenney et al. have attributed the inability of MEA-TiO2 to capture CO2 as due to difficulty in accessing the amino nitrogen atom to form the carbamate species.68 Though their result is interpreted as misfortune for CO2 capture, it is indeed a favourable feature to reduce the cost of regeneration. Because, the formation of by-products (carbonate and carbamate) comparatively adds more energy to regenerate the sorbent. CO2 capture through physisorption mode as shown above for MEA-TiO2 and 3AP-TiO2 is therefore an important cost-effective strategy. Moreover, for both the models, multiple noncovalent interactions keep the amino nitrogen in close contact with the titanium atom leaving no space for the formation of carbamate. It is only the magnitude of this interaction is different in both the moieties. As shown in the Table 2, high values Laplacian (∇²ρ) of electron density at bond critical point 2, reflects the strong interaction of MEATiO2 as compared to 3AP-TiO2 (7.5357 in MEA comparted to 2.8924 in 3AP). It may be possible in earlier study that scanning tunnelling microscope (STM) images based on which the results were analysed show the geometries that are only stable at measurement conditions and often detailed atomic configurations of corresponding images are unclear due to high mobility of CO2.68 Further authors assumed that the mechanism of formation carbonate and carbamate species in their functionalized TiO2 systems is similar to the reaction of CO2 with aqueous amines. Among the mechanisms proposed for the reaction between CO2 and MEA, the zwitterion mechanism is the most commonly accepted mechanism, which is known to get influenced by the solvent water and other interactions. Besides, the presence of two amino and water molecules are decisive for the formation of carbamate and carbonate species. If carbonate and carbamate species formation had taken place, it might be due to the presence of water molecules required for the hydrolysis. It is well established that water had major influence in hydrolysis of TiO2 surface and in the creation of oxygen vacancy sites.

70

It is worth mentioning in this context that it is difficult for CO2 to bind in

chemisorption mode to the pristine TiO2 surface without the existence of hydroxyl groups or defects. Because CO2 is an inert molecule with the reduction potential of CO2/CO2- is about 1.9 eV, which is much higher than the TiO2 conduction band, preventing electron transfer from TiO2 to CO2.71

3.2.2 CO2 binding on GLY-TiO2 and -ALA-TiO2 Possible configurations of CO2 with GLY-TiO2 C(1-3)O and -ALA-TiO2 C(1-3)P surface, optimal ones are shown in Figure 5 and the remaining configurations are in Figure S3 and BEs are given in Table S5. In C(1)O configuration, carbon of CO2 simultaneously binds with the amino nitrogen (3.010 Å) and with O(BR) (3.149 Å). In addition, O(CO2) participates in strong H-bonding with the surface bound proton H-O(BR) at a distance of 2.083 Å as shown in the Figure 5a. All these cooperative noncovalent interactions result in the formation of RCP2 and RCP2 and C(1)O exhibits a high BE of -46 kJ/mol. However, this kind of interactions are not possible for -ALA-TiO2 due to the presence of additional methylene group in its structure. This is manifested in significantly higher BEs -46 kJ/mol in C(1)O than -23 kJ/mol in C(1)P. In another C(2)O configuration, the BE is significantly lower (-22 kJ/mol) than that of C(1)O as CO2 is away from surface bound proton. CO2 is participating a weak H-bonding with the amino proton at a distance of 3.063 Å as shown in the Figure 5. In another C(2)P configuration, the BE is significantly lower (-12 kJ/mol) than that of C(1)P due to the orientation of CO2. In order to evaluate the strength of CO2 binding with the f-TiO2 surface, calculated vibrational frequencies for all the models shown in Figure 4 and Figure 5 are summarised in Table 4. Among all the vibrational modes, frequencies corresponding to asymmetrical and symmetrical stretching of CO2 and NH2 group can be considered as indicators for key geometric and energetic parameters that are important for CO2 adsorption. The values of these bands are found to 7



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be in consistent with the reported physisorbed near-linear CO2 asymmetric and symmetric bands, ~2350 and 1300 cm-1 respectively.42 It is clear from the Table 4 that a red shift varied from 14 to 20 cm-1 is observed in asymmetrical stretching mode of CO2---f-TiO2 compared to CO2 on pristine TiO2 surface. It is very intriguing to observe a blue shift of about 40 cm-1 in asymmetric stretching band of CO2 adsorbed to reduced TiO2 (2350 cm-1) when compared to free CO2 (2310 cm-1) indicating geometry distortions of CO2 along with the bond length elongation, which actually promote the chemisorption. Both asymmetrical and symmetrical stretching modes of amino group in AKAs are significantly red shifted by about ~100 cm-1 compared to AAs, due to participation of amino protons in H-bonding with O(BR) atoms, as described earlier sections. Apart from the effect of the distortions in the geometry of the molecule, Ti (5f) metal centre may also play a direct role in the frequency shift, by attracting the nearby oxygen atom in the CO2 molecule during the vibration. These results again corroborate that functionalized TiO2 systems capture CO2 majorly through physisorption mode. The calculated CT between CO2 and both isolated molecules as well as most stable binding configurations of CO2 on fTiO2 have been calculated by summing up the Hirshfeld atomic charges are tabulated in Table 1. Contrary to CO2 binding with isolated molecules, positive CT for all the configurations except for C(1)P, indicates flow of charge from CO2 to the f-TiO2 surface. This might be due to change in pKa of amino protons and methylene protons as result of chemically grafting of moieties on to TiO2 surface. The CT values further prove that after chemical bonding of moieties to TiO2 surface, the interaction is predominantly between amino protons and oxygen atoms of CO2.

3.2.3 Adsorption of multiple CO2 on MEA-TiO2 To understand the binding of multiple CO2 molecules with f-TiO2 surface, DFT calculations have been performed on MEA-TiO2 surface as an example. Starting from the geometry of C(1)M (having one CO2), three configurations namely, 2C(1)M, 3C(1)M and 3C’(1)M have been generated. 2C(1)M (-64 kJ/mol) is obtained by adding of one CO2 to C(1)M configuration and further addition of CO2 to 2C(1)M generates 3C(1)M and 3C’(1)M with binding energies of -79 and 84 kJ/mol respectively. These configurations are preferred in the current study for two reasons; first is to integrate the two most stable C(1)M and C(2)M configurations and to know the ability of multiple CO2 to bind in between the titanium rows and also above the f-TiO2 surface. The optimized geometries are shown in Figure 6. 2C(1)M with already bound CO2 intact, addition of another CO2 finds its space to bind to second amino proton. In this process, the (CO2)O---H(NH2 interaction gets attenuated . Interestingly, addition of second CO2 to C(1)M (-34 kJ/mol) further enhances the binding of 2C(1)M (-64 kJ/mol). It is observed that each O(CO2) of two CO2 molecules form bifurcated hydrogen bonds with two amino protons stabilizing the structure significantly as evident from shorter distances shown in the Figure 6. The 3C(1)M is obtained with sequential inclusion of third CO2 to 2C(1)M. In this configuration, CO2 bind with methylene protons similar to C(2)M geometry through weaker interactions giving rise to -15 kJ/mol more to 2C(1)M the structure. Another possibility of second CO2 binding is identified as the structure 3C’(1)M with -5 kJ/mol more stabilization, shown in the Figure 6. The configuration of each CO2 molecule is governed by competition between attractive quadrupole-quadrupole interaction and steric repulsion between CO2 molecules. Therefore, it can be inferred that a (4x2) model of MEA-TiO2 surface adsorb maximum of three CO2 molecules, making this as a potential sorbent for CO2 capture. These results also infer that CO2 can bind both in the gap between two adjacent five coordinated Ti (5f) rows and in the pore cavity formed by f-TiO2.

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3.3 CO2-dynamics on stable configurations After understanding the CO2 binding mechanism with f-TiO2 from the electronic structure calculations, we continued further to evaluate the time dependent dynamics of CO2 with f-TiO2 surface by using AIMD calculations. For this, we have chosen only one configuration (C(1)M, C(1)N, C(1)O and C(1)P) as representative example for each of f-TiO2 models. The CO2 binding with f-TiO2 surface can be described in terms of the interatomic C-N, H-O, O-Ti, N-Ti bonding interactions, which are expressed in the form of radial distribution functions (RDFs). AIMD calculations have been performed for the most stable geometry (C(1)M, C(1)N, C(1)O and C(1)P of each f-TiO2 model for 1 ps simulations time at 298 K. The snapshots of most stable geometry obtained during the 1 ps simulation time is depicted in the Figure S5a and Figure S5b for all the four models. And the calculated RDFs between the atoms of CO2 and f-TiO2 that are actively engaged in the binding are illustrated in the Figure 7. Analysis of the trajectory of entire simulation time for all the four models reveals that the CO2 molecule move only in the vicinity of functionalised moiety and participate in long-range van der Waal interactions including amino, methylene protons and with O(BR) atoms represented in dotted lines in the Figure S5. In addition, CO2 molecule rotate around to facilitate each of its oxygen atoms to bind with possible binding sites of TiO2 in all the systems. As shown in the Figure 7, a pronounced peak in g(r) between CO2 and amine protons is observed at r = 2.0−3.5 Å in C(1)M, C(1)N, where much stronger interactions are identified. For C(1)M, as shown in the Figure 7, two peaks corresponding to H(NH2)---O(CO2) interaction appear at 2.5 Å which is within the range of Hbonding. Similar to the geometry of C(1)M, AIMD study also reveals that the structure of MEA-TiO2 is stabilized through bifurcated H-bonding interactions between one of the oxygen atoms of CO2 and two amino protons (~2.61 Å) along with the other interactions depicted in the Figure 7. In agreement with data obtained from static calculations, engaged amino nitrogen with titanium in C(1)M brings its protons close to the surface, thereby enabling the binding with the O(BR) atoms. Whereas, in C(1)N the free amino protons interact with O(CO2) and also display more and stronger such type of non-bonded interactions than in C(1)M, confirming the explanation for its slightly higher BE. Unlike in MEA, during the simulation time, the geometry of 3AP is bound to the TiO2 surface only through O-Ti bond (1.75 to 1.89 Å), leaving the remaining fragment away from the TiO2 surface (Figure S5). Whereas in C(1)M the maximum elongation of N(NH2)---Ti bond is only about ~0.2 Å from the stable geometry. The peak in g(r) representing interaction between N(NH2)---C(CO2) which is not seen in C(1)M and C(1)N is conspicuous in C(1)O and C(1)P, albeit less intense than other interactions. As shown in the Figure 7, due to longer length of its structure, one can notice different mode of longrange interactions in C(1)O and C(1)P. For example, in C(1)O, O(CO2) binds through strong H-bond to surface bound proton of TiO2 surface; the same oxygen chooses to bind with H(CH2) in C(1)P. The interaction between N(NH2)--C(CO2) is found in both the models and is varied between 3.0-4.0 Å. Importantly in C(1)O , there is one more interaction between C(CO2) and O(BR) as shown in the Figure 7. Though the corresponding intensity is low, the binding is found to be strong with a distance ranging from 1.9 to 3.1Å. Similar to the structure obtained from static calculations, a delicate balance of long-range interactions between CO2 with GLY-TiO2 is C(1)O contribute to highest BE of -46 kJ/mol. All these results suggest that CO2 bind strongly with the acidic amine protons, while the carbon interaction with nitrogen atom (C-N) is relatively weak. Thus, the computational study clearly reveals that AAs functionalized on TiO2 leads to stronger adsorption sites for the capture of CO2, especially at low pressures.72

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3.4 Coadsorption of CO2 with H2O.f-TiO2 systems Water plays a detrimental role in reducing the CO2 adsorption capacity for most of the solid adsorbents by binding competitively to the surface atoms. Therefore, in this study, we particularly examined the coadsorption of CO2 and H2O on pristine TiO2 surface and on all the four f-TiO2 models. Optimized geometries of CO2 on preadsorbed H2O·TiO2 are presented in the Figure 8. The obtained results of adsorption of CO2 and H2O on pristine TiO2 (-43 kJ/mol) are in agreement with the reported values by Sorescu et al73 (-47 kJ/mol) (see Table S4). For the four f-TiO2 models, (WM, WN, WO, WP where W represents water) we first carried out the calculations to locate the water binding sites. The binding configurations of single H2O molecule on each of f-TiO2 as presented in the Figure 9. On these optimized geometries, we located the CO2 binding sites, i.e CO2 interactions on pre-adsorbed H2O·f-TiO2 surface, labeled as C(1)WM , C(1)WN , C(1)WO, C(1)WP represented in the Figure 10. Both H2O and the CO2 molecules interact with various sites of functionalized surface mostly through H-binding. It’s very interesting to observe that the BE (-66, -73, -116 and -50 kJ/mol) of water molecule on four f-TiO2 is almost twice than that of CO2 with pre-adsorbed H2O·f-TiO2 (-35, -33, 37 and -22 kJ/mol). As shown in Figure 10, the pre-adsorbed H2O molecule is found to competitively bind with favorable binding sites of CO2 through H-bonding. However the availability of additional binding sites due to methylene protons facilitate CO2 to bind with f-TiO2 albeit with slightly reduced BEs, +1, -7, -9, -1 kJ/mol for C(1)WM , C(1)WN , C(1)WO, C(1)WP configurations respectively.(see Table S6) To summarize due to competitive interaction of water, CO2 binding energy is slightly decreased by +1, -7, -9, -1 kJ/mol for C(1)WM , C(1)WN , C(1)WO, C(1)WP configurations respectively. This shows that the adsorption properties of CO2 are not significantly affected even if H2O molecule occupy favorable binding sites of CO2. Therefore, these materials are considered to be efficient sorbents to capture CO2.

4. CONCLUSIONS f-SAs are attractive materials for CO2 capture in view of their several promising features to achieve good adsorption capacity. Functionalization of TiO2 surface with various organic moieties is a reasonable approach to steer the incoming CO2 molecules to bind through physisorption, consequently reducing the regeneration energy. The current study investigates the possibility of developing TiO2-supported AKAs and AAs as promising adsorbents to capture CO2. The results from electronic structure calculations and ab initio molecular dynamics simulations demonstrated that the functionalization of TiO2 surface results in potential CO2 binding sites primarily through multiple, cooperative noncovalent interactions. The nature and strength of these interactions were evaluated by using BEs, vibrational frequencies and QTAIM analysis. Both classes of moieties are found to competitively interact with CO2. BEs of ~-40 kJ/mol is obtained for GLY-TiO2---CO2 and 3AP-TiO2---CO2, strikingly due to its molecular orientation and easily accessible amino group protons. Since the long-range dispersion interactions are primarily responsible for CO2 binding to f-TiO2, the dispersion corrected BEs are nearly twice to the BEs from pure functional. BEs obtained with PBE+TS are found to be in agreement with the previous studies. Furthermore, the CO2 adsorption properties of are not significantly affected by H2O, indicating the robust ness of these hybrid f-TiO2 models. The study opens up the possibility of developing similar hybrid sorbents for the economical and efficient capture of CO2.

ASSOCIATED CONTENT Supporting Information 10


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Binding energies and some of the important geometrical parameters of each system considered in this are given in tables (S1 to Table S6). Representation of bond critical points and ring critical points obtained from QTAIM analysis (FigureS1 and S4), other binding configurations of CO2 on f-TiO2 (Figure S2 and S3), Snapshots of MD simulations (Figure S5a and S5b), RDF graphs of f-TiO2 (Figure S6). This material is available free of charge via Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: 04027191251 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS A.P. thanks CSIR, New Delhi for the senior research fellowship. GNS thanks DST for the grant of J.C Bose fellowship.

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Figure 1. a) Side and b) top views of (4×2) rutile TiO2 (110) surface. Ti(6f), Ti(5f), denotes the six, five coordinated titanium atoms and O(BR), O(IP) bridging, in plane oxygen atoms respectively. Red, O; grey, Ti.

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Figure 2. Optimized complexes of a) CO2·MEA, b) CO2·3AP, c) CO2·GLY and d) CO2·β-ALA at PBE/DNP level. Bond distances are in Å, bond angles are in degrees, and binding energies are in kJ/mol. Color code: red, O; blue, N; dark grey, C; white, H.

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Figure 3. Binding configurations of TiO2 functionalized with a) MEA, b) 3AP, c) GLY and d) β-ALA molecules, optimized at PBE/DNP level. Bond distances are in Å and binding energies are in kJ/mol. The ‘O(BR)’ label refers to bridging oxygen atom.

Figure 4. Binding configurations of CO2 on MEA-TiO2 and 3AP-TiO2 systems, a) C(1)M, b) C(2)M, c) C(1)N and d) C(2)N, optimized at PBE/DNP level. Bond distances are in Å, bond angles are in degrees and binding energies are in kJ/mol. The C(1)M and C(2)M label refers to number of binding configurations of CO2 on MEA-TiO2 system, where C(1)N and C(2)N refers to number of binding configurations of CO2 on 3AP-TiO2 system.

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Figure 5. Binding configurations of CO2 on GLY-TiO2 and β-ALA-TiO2 systems, a) C(1)O, b) C(2)O, c) C(1)P, d) C(2)P, optimized at PBE/DNP level. Bond distances are in Å, bond angles are in degrees and binding energies are in kJ/mol. The C(1)O and C(2)O label refers to number of binding configurations of CO2 on GLY-TiO2 system, where C(1)P and C(2)P refers to number of binding configurations of CO2 on β-ALA-TiO2 system.

Figure 6. Binding configurations of multiple CO2 on MEA-TiO2 system, 2C(1)M, 3C(1)M 1, and 3’C(1)M, optimized at PBE/DNP level. The 2C(1)M, 3C(1)M and 3’C(1)M label refers to two , three CO2 molecules respectively. Bond distances are in Å and binding energies are in kJ/mol. The above three binding configurations are generated with sequential addition of CO2 molecules to C(1)M. The binding energies corresponds to nCO2 molecules.

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Figure 7. Plot of radial distribution function g(r)from AIMD simulations of a) C(1)M, b) C(1)N, c) C(1)O and C(1)P . The indicated distances are depicted as figures in the inset.

Figure 8. Binding configurations of a) CO2 on TiO2, b) H2O on TiO2, c) CO2 and H2O on TiO2, optimized at PBE/DNP level. Bond distances are in Å, bond angles are in degrees and binding energies are in kJ/mol.

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Figure 9. Binding configurations of H2O on four f-TiO2 systems, a) WM, b) WN, c) WO and d) WP optimized at PBE/DNP level. Bond distances are in Å, bond angles are in degrees and binding energies are in kJ/mol. The W label refers to water molecule, M, N, O, P label refers to four f-TiO2 systems.

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Figure 10. Coadsorption configurations of both CO2 and H2O on four f-TiO2 systems, (a) C(1)WM, b) C(1)WN, c) C(1)WO and d) C(1)WP), optimized at PBE/DNP level. Bond distances are in Å, bond angles are in degrees and binding energies are in kJ/mol. The C(1)WM, C(1)WN, C(1)WO and C(1)WP label refers to binding configurations of both CO2 and H2O on f-TiO2 system.

Table 1. The CT is calculated for CO2 interacting with isolated molecules and CO2 on four f-TiO2 configurations by summing up the Hirshfeld atomic charges of CO2 molecule, at PBE/DNP level. (Charges of free CO2 molecule, C = 0.2906, O = -0.1452, O = -0.1452). CO2 with isolated molecules C

CO2 on four f-TiO2

CO2·MEA

CO2·3AP

CO2·GLY

CO2·β-ALA

C(1)M

C(1)N

C(1)O

C(1)P

0.2861

0.2759

0.2742

0.2783

0.3003

0.2975

0.2922

0.2849

0.1296

0.1292

0.1429

O

-0.1467

-0.1568

-0.1635

-0.1579

0.1305

O

-0.1377

-0.1565

-0.1571

-0.1511

0.1264

0.1294

0.1251

0.1425

CT

0.0016

-0.0374

-0.0464

-0.0307

0.0434

0.0385

0.0379

0.0005

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Table 2. Electron density (ρ) and Laplacian (∇²ρ) of electron density values (a.u) at Bond Critical Points (BCPs), Ring Critical Points (RCPs) of four f-TiO2 systems.

MEA-TiO2

3AP- TiO2

ρ

∇²ρ

ρ

∇²ρ

BCP1

0.8597

22.9453

0.9146

20.2597

BCP2

0.2748

7.5357

0.1370

2.8924

BCP3

0.0413

0.7308

0.0733

1.3047

BCP4

0.0796

1.4937

0.0767

1.4401

BCP5

0.0380

0.7059

-

-

RCP1

0.0703

0.5927

0.0899

0.6002

RCP2

-

-

-

-

RCP3

-

-

0.0701

0.5064

GLY-TiO2

β-ALA-TiO2

BCP1

0.3938

11.9561

0.3953

11.8544

BCP2

0.4009

12.1545

0.4223

12.7001

BCP3

-

-

0.0717

1.3857

RCP1

0.1360

0.9011

0.1358

0.9155

Pictorial view of indicated ρ and ∇²ρ values are presented in Figure S1

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Table 3. Electron density (ρ) and Laplacian (∇²ρ) of electron density values (a.u) at Bond Critical Points (BCPs), Ring Critical Points (RCPs), Cage Critical Points (CCPs) of CO2 with four f-TiO2 systems. C(1)M

C(1)N

ρ

∇²ρ

ρ

∇²ρ

BCP1

0.8825

19.4908

0.9089

19.9079

BCP2

0.2647

7.2507

0.1558

3.6632

BCP3

0.0412

0.7319

0.0744

1.3289

BCP4

0.0870

1.6599

0.0769

1.4386

BCP5

0.0333

0.6125

-

-

BCP6

0.0478

1.0639

0.0386

0.7942

BCP7

0.0312

0.5601

0.0381

0.6795

RCP1

0.0698

0.5859

0.0917

0.6352

RCP2

0.0156

0.1523

0.0170

0.2015

RCP3

-

-

0.0728

0.5354

CCP1

0.0866

0.5160

-

-

C(1)O

C(1)P

ρ

∇²ρ

ρ

∇²ρ

BCP1

0.4395

13.0860

0.4073

12.0599

BCP2

0.4089

12.3402

0.4333

12.7389

BCP3

0.0602

0.9428

0.0512

0.7663

BCP4

0.0901

1.9162

-

-

BCP5

0.0328

0.5951

-

-

BCP6

-

-

0.0405

0.7408

RCP1

0.1381

0.9286

0.1373

0.9108

RCP2

0.0180

0.1739

0.0323

0.3508

RCP3

0.0278

0.3915

-

-

Pictorial view of indicated ρ and ∇²ρ values are presented in Figure S4.

Table 4. Vibrational frequencies (cm-1) of NH2 group of four f-TiO2 systems and the frequencies of CO2 and NH2 group of four f-TiO2 systems. (νasy, νsym values of 2350 and 1300 cm-1 for CO2 on pristine TiO2) IR Mode

MEA-TiO2

3AP-TiO2

GLY-TiO2

β-ALA-TiO2

νasy(NH2) νsym(NH2)

3380

3383

3463

3457

3266

3289

3370

3361

C(1)M

C(1)N

C(1)O

C(1)P

3379

3381

3444

3436

3272

3287

3358

3352

2330

2330

2328

2324

1296

1299

1286

1298

νasy (NH2) νsym (NH2) νasy (CO2) νsym (CO2)

The label νasy, νsym refers to asymmetrical and symmetrical modes.

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Page 22 of 22

TOC GRAPHIC

Scheme 1. Schematic representation of models considered in the current study and corresponding nomenclature used to represent each model.

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Functionalized Rutile TiO2(110) as a Sorbent To Capture CO2 through

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