Effect of Surface Structure of TiO2 Nanoparticles on CO2 Adsorption

Research Article pubs.acs.org/journal/ascecg

Effect of Surface Structure of TiO2 Nanoparticles on CO2 Adsorption and SO2 Resistance Uma Tumuluri,† Joshua D. Howe,‡ William P. Mounfield, III,‡ Meijun Li,§ Miaofang Chi,∥ Zachary D. Hood,∥,⊥ Krista S. Walton,‡ David S. Sholl,‡ Sheng Dai,†,§ and Zili Wu*,†,∥ †

Chemical Sciences Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831, United States School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332, United States § Department of Chemistry, University of Tennessee, 1420 Circle Drive, Knoxville, Tennessee 37996, United States ∥ Center for Nanophase Material Science, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831, United States ⊥ School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States ‡

S Supporting Information *

ABSTRACT: The effect of surface structure of TiO2 nanocrystals on the structure, amount, and strength of adsorbed CO2 and resistance to SO2 was investigated using in situ IR spectroscopy and mass spectrometric techniques along with first-principles density functional theory (DFT) calculations. TiO2 nanoshapes, including rods {(010) + (101) + (001)}, disks {(001) + (101)}, and truncated octahedra {(101) + (001)}, were used to represent different TiO2 structures. Upon CO2 adsorption, carboxylates and carbonates (bridged, monodentate) are formed on TiO2 rods and disks, whereas only bidentate and monodentate carbonates are formed on TiO2 truncated octahedra. In general, the order of thermal stability of the adsorbed CO2 species is carboxylates ≈ monodentate carbonates > bridged carbonates > bidentate carbonates ≈ bicarbonates. TiO2 rods and disks adsorb CO2 more strongly than TiO2 truncated octahedra, which is explained by the larger number of low coordinated surface oxygen and oxygen vacancies on the rods and disks than the truncated octahedra. Further IR studies showed that the structure and binding strength of the adsorbed CO2 species are affected by the presence of SO2. Among the three TiO2 nanoshapes, CO2 binding strength for truncated octahedra shows the most decrease due to accumulation of sulfates formed during the SO2 adsorption cycle. The fundamental understanding obtained here on the effects of the surface structure, oxygen vacancies, and SO2 on the interaction of CO2 with TiO2 may provide insights for the design of more efficient and sulfur-resistant TiO2-based catalysts involved in CO2 capture and conversion. KEYWORDS: TiO2 nanoparticles, Surface structure, Oxygen vacancies, CO2 capture and conversion, SO2 resistance, IR spectroscopy

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INTRODUCTION CO2 is a major contributor to the greenhouse effect, which is a factor in anthropogenic climate change. CO2 can be removed by conventional methods such as liquid amine scrubbing,1−3 solid adsorbents,4−6 and polymer membranes.7,8 In recent years, photocatalytic conversion of CO2 and the hydrogenation of CO2 to value-added fuels (CO, CH4, etc.) have gained research interest as an alternative approach to CO2 removal.9 TiO2-based catalysts are widely used for CO2 hydrogenation and photocatalytic conversion of CO2 because of their superior oxidation properties, charge transport properties, and high corrosion resistance.10−16 A complete understanding of the fundamental mechanisms underlying the interaction of CO2 with TiO2 is important to guide the design of efficient catalysts for photocatalysis or for application as an adsorbent in CO2 capture processes. This is © 2017 American Chemical Society

essential for the development of CO2 capture, storage, and conversion technologies that promise a sustainable energy landscape for current and future society. Many studies have been reported in past decades for the photocatalytic conversion of CO2 on polycrystalline TiO2 using reductants such as water, isopropyl alcohol, NaOH solution, and prop-2-ol solution.11,14,17−20 Among the TiO2 single-crystal morphologies, the (010) surface of anatase TiO2 is the least explored for CO2 conversion via photocatalysis or CO2 capture. Very few studies have been reported on investigation of CO2 with anatase TiO2 (010), (001), and (101) surfaces using FTIR spectroscopic techniques and theoretical calculations. The emphasis among Received: July 8, 2017 Revised: August 19, 2017 Published: August 24, 2017 9295

DOI: 10.1021/acssuschemeng.7b02295 ACS Sustainable Chem. Eng. 2017, 5, 9295−9306

Research Article

ACS Sustainable Chemistry & Engineering

Teflon-lined autoclave and heated to 180 °C for 24 h. The product was centrifuged after the reaction and washed with ethanol three times. The three TiO2 nanocrystals were calcined at 300 °C in air for 2 h and stored for further use. Material Characterization. High resolution transmission electron microscopy images (HRTEM) were acquired using FEI Titan 60/300S S/TEM under 300 kV. To minimize the beam irradiation to the TEM specimens, a low electron dose was used for imaging. Image acquisition was performed using a fast camera (Gatan OneView camera) at an area with minimized beam exposure before imaging. Raman spectra were collected using an in-house-built multiplewavelength Raman system that includes laser excitation at λ = 532 nm. X-ray photoelectron spectroscopy (XPS) measurements were performed by Thermo Scientific Kα spectrometer. Each spectrum was collected with a spot size of 400 μm and operating pressure of 3.0 × 10−7 mbar using an Al Kα micro fused monochromatized source (1486.6 eV) with a step size of 0.1 eV over 50 scans. XPS spectra were analyzed using Thermo Advantage software. Raman scattering was collected using fiber optics connected directly to the spectrograph stage of the triple Raman spectrometer (Princeton Instruments Acton Trivista 555). Brunauer−Emmett−Teller (BET) surface areas of the TiO2 nanocrystals, calcined at 300 °C, were measured via nitrogen adsorption at −196 °C using a Micrometrics Gemini 275 system. TiO2 rods, disks, and truncated octahedra have surface areas of 100, 58, and 101 m2/g, respectively. Temperature-Programmed Reduction (TPR) Studies. CO TPR studies were performed in the AMI microreactor system. The catalyst was pretreated by heating it to 300 °C for 60 min in 5% O2/ He flowing at 30 cm3/min, then cooling to 30 °C and switching to He for 30 min to purge the residual gas. The catalyst was exposed to 2% CO/2% Ar/He flow at 30 °C for 30 min before being heated to 850 °C at 10 °C/min; the temperature of the catalyst was held at 850 °C for 30 min and then cooled to 30 °C. The outlet gas stream of AMI reactor was analyzed by quadrupole mass spectrometer (Omnistar GSD-320, Pfeiffer Vacuum). A blank run was also performed for each sample to evaluate the amount of CO2 desorbed from the sample when it is heated to 850 °C in the presence of He. This evolved CO2 represents the strongly bound carbonate species from the atmosphere storage of the TiO2 samples. The normalized (on weight basis) CO2 MS profiles were subtracted from the normalized CO2 MS profiles during CO TPR (Figure S1). In Situ IR Studies. Two experimental runs were performed to study the interaction of CO2 with TiO2 nanoparticles as well as the effects of SO2 on the CO2 adsorption performance of the TiO2 nanoparticles. In the first experimental run, two CO2 adsorption/ desorption cycles were performed to evaluate the effects of the surface structure of TiO2 nanoparticles on the structure and strength of adsorbed CO2 species. In the second experimental run, alternate SO2 and CO2 adsorption/desorption cycles were performed to evaluate the effects of SO2 on the CO2 adsorption performance of the TiO2 nanoparticles. The in situ spectroscopic studies consist of the following steps: (i) pretreatment, (ii) SO2 or CO2 adsorption, (iii) He purge, and (iv) temperature-programmed desorption (TPD). In the pretreatment step, the catalyst was heated to 300 °C at the rate of 10 °C/min; the temperature of the catalyst was held at 300 °C in He flow at 25 cm3/min for 60 min. SO2 adsorption was performed by exposing the catalyst to 15 ppm of SO2 flow at 25 cm3/min for 15 min at 25 °C. CO2 adsorption was performed by exposing the catalyst to 2% CO2 flow at 25 cm3/min for 15 min at 25 °C. The residual gas in the reactor was purged by He flow at 25 cm3/min for 10 min at 25 °C, and TPD was performed by heating the catalyst to 300 °C at the rate of 10 °C/min. The temperature of the catalyst was held at 300 °C for 1 min, followed by cooling the catalyst to 25 °C at the rate of 10 °C/min. IR spectra were collected using Thermo Nicolet Nexus 670 spectrometer in diffuse reflectance mode (DRIFTS) continuously throughout the CO2 adsorption cycle. Absorbance spectra were calculated as Abs= −log(I/Io), where I is the single beam spectrum during CO2 adsorption and Io is the single beam spectrum before CO2 adsorption. The outlet gases from the DRIFTS reactor (Pike

previous studies is on the evaluation of adsorbed CO2 species and the activity of TiO2 surfaces for CO2 reduction in photocatalytic applications.15,21−27 The (001) surface of TiO2 was found to be more basic and adsorbs more CO2 compared to the (101) surface, a result of the presence of more twocoordinated surface oxygen species (O2C).12 However, the effects of the surface structure of TiO2 on the strength of CO2 interaction with the TiO2 surfaces is not well studied in the literature. An understanding of these effects would be useful in designing efficient adsorbents for CO2 capture. Corrosive gases such as SO2 are ubiquitous in energy-related applications. Stability of catalysts in the presence of these corrosive gases plays a key role in designing sustainable catalysts for cost-effective CO2 removal and conversion. Despite this, only a few studies have been reported on the investigation of SO2 interaction on anatase TiO2.25,28,29 To the best of our knowledge, the effect of SO2 on CO2 interactions has not been previously investigated. A complete understanding of the interaction mechanisms of SO2 with TiO2 and the effect of SO2 on CO2 interactions with TiO2 is important for designing an efficient catalyst for photocatalysis or for application as an adsorbent in CO2 capture processes. In this work, interaction of CO2 with different morphologies of anatase TiO2 rods {(010)+(101)+(001)}, disks {(001) + (101)}, and truncated octahedra {(101)+(001)+(010)} was investigated using Fourier transform infrared (FTIR) spectroscopy and density functional theory (DFT). Carboxylates and carbonates (bridged and monodentate) were observed to form on TiO2 rods and disks, whereas only bidentate and monodentate carbonates were observed on TiO2 truncated octahedra during CO2 adsorption. Additionally, we studied the effects of SO2 on CO2 adsorption with these systems. Our studies revealed that the structure and strength of adsorbed CO2 species are not only highly dependent on the surface structure of the TiO2 nanoparticles but also affected by the presence of adsorbed SOx species.

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EXPERIMENTAL SECTION

Material Synthesis. To form the Ti(OH)4 precursor that was used for the synthesis for both rod and truncated octrahedra morphologies, aqueous TiCl4 solution was prepared by adding 6.6 mL of TiCl4 to 50 mL of aqueous HCl solution (0.43 mol/L) drop by drop under vigorous stirring in an ice bath. TiCl4 solution was added to 5 wt % NH3·H2O aqueous solution drop by drop under vigorous stirring. After a light blue Ti(OH)4 (precursor) precipitate was formed, 10 mL of 4 wt % aqueous NH3·H2O solution was added to adjust the pH to between 6 and 7. The suspension was centrifuged after aging for 2 h at room temperature. The precipitate was washed two times with water and one time with ethanol. TiO2 rods: 0.3 g of (NH4)2SO4 was added to the mixture of water and isopropanol. Two grams of fresh Ti(OH)4 precursor was dispersed in the (NH4)2SO4 solution. A suspension was obtained after stirring and ultrasonic treatment. The suspension was transferred to a 50 mL Teflon-lined autoclave and heated to 180 °C for 24 h. The product was centrifuged after the reaction and washed with deionized water three times and ethanol one time. TiO2 truncated octahedra: 2 g of fresh Ti(OH)4 precursor was dispersed in 30 mL of 50 vol % water/isopropanal. The suspension was transferred to a 50 mL Teflon-lined autoclave and heated to 180 °C for 24 h. The product was centrifuged after the reaction and washed with deionized water three times and ethanol one time. TiO2 disks: Ti(OC4H9)4 was used as a precursor. Ti(OC4H9)4 (5 mL) was added to 20 mL drop by drop under stirring. Then, 0.5 mL of 47 wt % aqueous HF was added slowly after stirring for 15 min. The suspension was stirred for 30 min before being transferred to a 50 mL 9296


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Figure 1. TEM images of TiO2 (a) rods, (b) disks, and (c) truncated octahedra. The geometry shapes for the TiO2 particles are shown in the inset. The side view of the dominant surface structure of (010), (001), and (101) for each TiO2 nanoparticle are shown at the bottom of the TEM images. Due to the size of the slab models and computational cost, CO2 adsorption energetics were studied with fewer atomic layers with the atomic coordinates for all but the top M layers constrained to the converged geometries. In these calculations, the Brillouin zone was sampled only at the Γ-point. For 001, 010, and 101 surfaces, M values (the number of layers that were unconstrained) were 3, 4, and 4, respectively. Interionic forces were optimized to be less than 0.025 eV/ Å in CO2 adsorption calculations. CO2 adsorption on oxygen-defective slabs was studied using frozen slab models beginning from the converged, reduced model of the most energetically favorable oxygen vacancy (VO), but otherwise is studied exactly as CO2 adsorption on pristine slabs. Calculations in the presence of VO were carried out only on the 001 and 101 anatase TiO2 surfaces to investigate the importance of VO defects on CO2 adsorption motifs. Our results should be taken as a lower bound on the effects of these vacancies due to the constraints used in these calculations. The creation of VO in TiO2 results in the formation of Ti3+ species, which we initially assigned to the two lowest-coordinated Ti ions adjacent to the vacancy. The (de)localization of these states is known to be dependent upon the U value used in calculations, and our results are consistent with existing literature.38 To study CO2 adsorption, we considered physisorbed, monodentate carbonate, and bridged carbonate adsorption motifs because these are the motifs that are stoichiometrically accessible for pristine and oxygen-defective slabs. We began from ideal experimental geometries of these motifs, assuming oxygen coordination to surface Ti species and carbon coordination to surface O species, as appropriate, and using starting geometries consistent with idealized structures for these binding motifs. Binding energies were calculated as

Technologies HC-900) were analyzed using a quadrupole mass spectrometer (Omnistar GSD-301 O2, Pfeiffer Vacuum). CO2 Breakthrough Experiments. Carbon dioxide breakthrough experiments were performed on a lab-built packed-bed system. All samples were pretreated at 300 °C under vacuum prior to experiments. Approximately 30 mg of sample were pelletized, crushed, and sieved to 100 μm particle size. Pelletized samples were reactivated at 150 °C overnight in vacuum. After cooling to room temperature, the sample bed was placed in the system, the N2 flow rate was set to 85 cm3/min, the CO2 flow rate was set to 15 cm3/min, and data collection was started. Once the CO2 concentration reached steady state, CO2 flow was turned off and N2 flow was returned to 100 cm3/min. 100 μm glass beads were used to measure a calibration breakthrough curve, and the CO2 capture capacity for each material was determined by integrating the area between the calibration breakthrough curve and the sample breakthrough curve. DFT Calculations. DFT with a planewave basis set and projectoraugmented-wave pseudopotentials30,31 within the Vienna Ab initio Simulation Package (VASP)32−34 with a cutoff energy of 650 eV was used to computationally study anatase TiO2 and CO2 adsorption on TiO2 facets. Ions were optimized such that all interionic forces are less than 0.01 eV/Å. The second version of the van der Waals density functional (vdW-DF2)35 was used to perform spin-polarized calculations that account for dispersion interactions, which can play an important role in adsorption of molecules on surfaces. Hubbard U corrections were also included to account for the self-interaction errors that result in incorrect electronic structures of d-block metals.36 A U correction of 2.5 eV for the 3d orbitals of Ti, which is consistent with the range of U values use to describe Ti in anatase TiO2 in existing literature, was used.37−39 Bulk rutile and anatase TiO2 were optimized initially to establish convergence for the anatase TiO2 calculations on the basis of converging energy per formula unit. A 650 eV cutoff energy and a 6 × 6 × 6 Monkhorst−Pack40 k-point sampling of the Brillouin zone were found to provide convergence of the energy difference between bulk anatase and rutile TiO2 phases of less than 1 meV per formula unit. This basis set cutoff and k-point sampling were used as a basis for the rest of the calculations in this study, which focused on anatase TiO2. The k-point sampling mesh was adjusted to preserve a comparable level of k-point density as the Brillouin zone size changes for various slab models and supercells. 1 × 1 × N asymmetric slab models were studied for the pristine (001), (010), and (101) TiO2 surfaces, where N denotes the slab thickness. The value of N and the number of constrained layers at the bottom of the slab were varied such that the first 3 interlayers of Ti−Ti spacings were converged to within 1% to establish minimal models that provide converged surface and subsurface geometries. The supercell models (2 × 2 × N, 1 × 2 × N, and 1 × 2 × N for 001, 010, and 101 surfaces respectively) were constructed to study CO2 adsorption.

E binding = − [E(CO2 + slab) − E(slab) − E(CO2 )]

(1)

Ebinding is the zero-temperature DFT binding energy, E(CO2 + slab) is the energy of the TiO2+CO2 adduct, E(slab) is the energy of the isolated TiO2 slab, and E(CO2) is the energy of the isolated CO2 molecule, computed in a 20 Å edge-length box to minimize interaction between periodic images of the molecule. To compute vibrational frequencies of the modes of adsorbed species, a harmonic approximation was used. Each atom of the adsorbed species is displaced symmetrically by 0.015 Å along each Cartesian axis to generate a Hessian matrix of mode frequencies. These frequencies are used in comparison with experimental IR frequency data. The positions of atoms in the surface were not varied during these calculations.

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RESULTS AND DISCUSSION Material Characterization. To characterize the TiO2 nanoparticles, a combination of Raman spectra, TEM images, and comparison to previous literature was used. Raman spectra 9297


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regime represents the removal of surface oxygen,49 and the high temperature CO2 evolution regime represents the removal of bulk oxygen.50 CO reacts with the lattice oxygen to form CO2, so the temperature of maximum CO2 evolution (in the low temperature regime) provides insight into the reducibility of the surfaces. The first temperature of maximum CO2 evolution for each morphology is, in order from least to greatest, TiO2 rods (496 °C) ≈ TiO2 disks (500 °C) ≪ truncated octahedra (615 °C). TiO2 truncated octahedra are the most stable (surface energy of 0.44 J/m2)46,47 and require high temperature to remove the surface oxygen compared to TiO2 disks (0.90 J/ m2)46 and rods, so the TPR results are consistent with the known surface energies. From the TPR results, it can also be reasoned that VO defects form more readily near TiO2 (001) and TiO2 (010) surfaces than near the TiO2 (101) surface. Figures 2bi−biii shows the XPS Ti 2p spectra of TiO2 rods, disks, and truncated octahedra. The nanocrystals exhibit peaks at 459 eV for Ti4+ and 457.7 eV for Ti3+, respectively.21,51 A Gaussian fitting of Ti 2p was used to calculate the Ti3+/Ti4+ ratio. This fitting indicated that the Ti3+/Ti4+ ratio in TiO2 nanocrystals is in the order of disks (0.055) > rods (0.035) > truncated octahedra (0.008). The higher Ti3+/Ti4+ ratio indicates the presence of more Ti3+ species which accompany VO defects on TiO2 disks and rods than on truncated octahedra. There are conflicting reports in the literature about the surface VO defect formation energy of TiO2 facets. Liu et al.,21 Li et al.,52 and Cheng et al.39 reported vacancy formation energy for the TiO2 (101) surface lower than that for the TiO2 (001) surface, which indicates that VO defects more readily form on TiO2 (101) surfaces than on TiO2 (001). On the other hand, Bouzouba et al.,53 Vittadini et al.,54 and our DFT calculations indicate that VO defects are more likely to form on TiO2 (001) and TiO2 (010) surfaces than on TiO2 (101) surfaces, which agrees with our experimental results. Our DFT calculations show an ordering of VO formation energies for the various facets of (001) < (010) < (101) with energies of 3.5, 3.8, and 4.8 eV, respectively. Structure of the Adsorbed CO2. To gain insight into what CO2-derived species exist on the TiO2 nanoparticle surfaces, a combination of IR spectroscopic and DFT technique was employed. The IR spectra of TiO2 rods, disks, and truncated octahedra at room temperature after pretreatment (shown in Figure S4 in the Supporting Information) exhibit characteristic peaks of OH groups at 3735, 3675, and 3650 cm−1. The peak at the highest frequency (3735 cm−1) represents the type I (atop) hydroxyls, and the bands at 3675 and 3650 cm−1 represent type II (bridging) hydroxyls55 (shown in Scheme S1 in the Supporting Information). Figures 3a−c shows the IR spectra during CO2 adsorption for 0.25 and 15 min at room temperature on TiO2 rods, disks, and truncated octahedra. The IR spectrum in Figure 3a indicates that adsorption of CO2 on TiO2 rods (Figure 3a) results in the formation of bicarbonates (3624, 1616, and 1202 cm−1), bridged carbonates (1725 and 1291 cm−1), carboxylates (1700 and 1565 cm−1), and monodentate carbonates (1502, 1453, and 1360 cm−1).21,45,56−63 The structure of these adsorbed CO2 species are shown in Scheme 1. The negative peaks at 3716 (type I) and 3671 cm−1 (type II) indicate consumption of OH groups (Scheme S1 in the Supporting Information) during CO2 adsorption, consistent with the formation of bicarbonate species. Figure S5 shows the evolution of the IR features of the adsorbed species (carbonates and bicarbonates) on TiO2 rods as a function of CO2 adsorption time, which shows that all

of the TiO2 rods, disks, and truncated octahedra (Figure S2) exhibit characteristic peaks at 398 (B1g mode), 519 (B1g mode), and 642 cm−1 (Eg mode),41−45 indicating that the TiO2 nanoparticles exist in the anatase phase. High resolution TEM images of the three nanocrystal morphologies are shown in Figure 1 and are consistent with previous reports.21,27 From the TEM images, it was determined that the TiO2 rods are mixed with (010) + (101) + (001) terminations and the disks are terminated with (001) and (101) facets with the (101) terminations on the “side” of the disk. From the thickness (∼10 nm) and width (∼33 nm) of the disks (see Figure S3), it was estimated that there are roughly 40% of (001) and 60% (101) facets exposed. The TiO2 truncated octahedra surfaces are mainly (101) facets (∼90%) with from the remainder of surface area being (001) facets (∼10%). The (101)-dominated TiO2 truncated octahedra have lower surface energy (0.44 J/m2),4647 than the TiO2 (001) disks (0.90 J/m2).46 TiO2 (001) disks have O2C-Ti5C-O2C sites (five-coordinated Ti bonded with twocoordinated O). Whereas, TiO2 (101) truncated octahedra and TiO2 (010) rods have both O3C-Ti6C-O3C and O2C-Ti5C-O2C sites.12,21,25,48 Defects and Reducibility of TiO2 Rods, Disks and Truncated Octahedra. To gain insight into the potential of VO defects to form on various anatase TiO2 surfaces, TPR with CO and XPS experiments were performed on the various crystal morphologies. The reducibility of the lattice oxygen of the three TiO2 nanocrystals was evaluated through a TPR with CO. Figure 2 shows the normalized CO2 MS intensities during CO-TPR. The TiO2 rods, disks and truncated octahedra exhibit low temperature (600 °C) CO2 evolution regimes. The low temperature CO2 evolution

Figure 2. (a) Normalized (weight basis) CO2 MS Intensity profiles during CO TPR over TiO2 rods, disks, and octahedra. XPS Ti 2p spectra of TiO2 (bi) rods, (bii) disks, and (biii) truncated octahedra nanocrystals. The CO2 MS profile shown in the figure is obtained by subtracting the normalized CO2 MS profile during blank run (heating to 850 °C in He) from the normalized CO2 MS intensity during CO TPR. 9298


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CO2 interaction with TiO2 (010), (001), and (101) surfaces has previously been investigated using FTIR spectroscopy and DFT, although there are conflicting reports regarding what CO2 species are observed on each surface,12,21,22,26,27,61,64−66 which is likely due to the differences in surface structure (variation of surface facets) or amount oxygen vacancies in the TiO2 samples used in different research groups. Mino et al.12 report the formation of linearly adsorbed CO2 on the TiO2(101) surface and various types CO32− species on the (001) surface. Ramis et al.66 and Martra61 report the formation of the CO32− species on both (001) and (101) surfaces using FTIR spectroscopy. Chen et al.26 reported that only type-I surface carbonates (monodentate carbonates) are energetically favorable on (101) surface. In this work, we observed the formation of various CO32− species along with bicarbonates on the three TiO2 nanocrystal morphologies, consistent with results reported in the literature.22,61,66 Indrakanti et al.22 report that the IR frequencies of the CO32− species on the (001) surface are lower than those of CO32− species on the (010) surface, an observation attributed to longer C−O bond distances on the (001) surface than on the (010) surface. However, our FTIR results show similar vibrational frequencies of CO32− species on both disks (001) and rods (010) surfaces. In another study, Chen et al.27 performed in situ DRIFTS study of CO2 adsorption on the (001), (101), and (100) surfaces, observing that CO−2 species were formed only on (101) and (100) surfaces. This was attributed to the larger number of oxygen vacancies (VO) on these surfaces as compared to the (001) surface. Our FTIR results show that the CO−2 species formed only on TiO2 rods ((010)) and disks ((001)) surfaces but not on truncated octahedra ((101)), which is consistent with XPS results of fewer oxygen vacancies on the truncated octahedra’s dominant (101) surface. Using DFT, CO2 adsorption on (001), (010), and (101) surfaces of anatase TiO2 was studied. Stoichiometric CO2 adsorption motifs on pristine slab surfaces were considered to gain insight into the importance of defects and coadsorbed species on capturing CO2 adsorption behaviors. Initially, physisorbed, monodentate carbonate, and bridged carbonate adsorption motifs were considered because these are stoichiometrically accessible for CO2 on pristine TiO2 surfaces. The resulting optimized structures of adsorbed CO2 are shown in Figure S8 and are provided in the Supporting Information. The linear CO2 adsorption motif is a (mostly) linear CO2 molecule coordinating through a terminal oxygen to a Ti ion on the surface. The monodentate carbonate CO2 adsorption motif consists of a bent CO2 molecule coordinating to a surface oxygen atom through the central carbon atom. The bridged carbonate CO2 adsorption motif consists of a bent CO2 molecule much like the monodentate carbonate, but coordination is bidentate through the central carbon atom and a terminal oxygen atom to surface oxygen and Ti ions, respectively. We note that the description of the adsorption motifs is based upon their initial configurations, but in many cases, the resulting configurations do not qualitatively match with the starting structures. This is particularly true for many of the chemisorbed species, where weak coordination with surface sites results in prediction of physisorbed, linear CO2 species. Because of this, often the comparison between theory and experiment of vibrational modes also is not in good qualitative agreement, limiting comparisons that can be made between these quantities.

Figure 3. IR spectra during CO2 adsorption at 0.25 and 15 min on TiO2 (a) rods, (b) disks, and (c) truncated octahedra; temperature during adsorption = 25 °C.

Scheme 1. Proposed Structures of Adsorbed CO2 Species

the adsorbed species (carbonates and bicarbonates) evolve at similar rate. The IR spectrum in Figure 3b indicates that adsorption of CO2 on TiO2 disks results in the formation of bicarbonates (3625, 1390, and 1202 cm−1), bridged carbonates (1725 cm−1), and carboxylates (1697 and 1570 cm−1).21,45,56−62 A single negative peak at 3673 cm−1 indicates that only type II OH groups participated in CO2 adsorption on TiO2 disks. Figure S6 shows the evolution of IR features of adsorbed species (carbonates and bicarbonates) during CO2 adsorption on TiO2 disks. These spectra show that bridged (1566 cm−1) and monodentate (1359 and 1341 cm−1) carbonates form initially (0.25 min), while bicarbonates are formed with prolonged exposure. The peak at 1390 cm−1 in Figure 3b is a combination of peaks from monodentate carbonates at 1341 and 1359 cm−1 and from bicarbonates at 1402 cm−1. The IR spectrum in Figure 3c indicates that adsorption of CO2 on TiO2 truncated octahedra results in the formation of bicarbonates (3593, 1608, and 1225 cm−1), bidentate carbonates (1675 and 1244 cm −1 ), and monodentate carbonates (1502, and 1424 cm−1).21,45,56−62 Similar to TiO2 rods, both type I and type II OH groups in TiO2 truncated octahedra participate in the CO2 adsorption. Figure S7 shows the evolution of IR spectra during CO2 adsorption on TiO2 truncated octahedra, indicating that all the adsorbed species (carbonates and bicarbonates) evolve at similar rate. This is comparable to our findings for TiO2 rods. The assignment of the IR bands of adsorbed CO2 species on TiO2 rods, disks, and truncated octahedra are summarized in Table S1. 9299


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CO2. For TiO2 disks, where the (001) surface makes up most of the surface, bridged carbonates are predicted to be more prevalent than monodentate carbonates. The lack of bicarbonates in the DFT calculations is a consequence of not considering the surface hydroxylation that exists under experimental conditions. The modest binding energies predicted by the DFT calculations suggest that desorption of these species should be possible at room temperature, although we observe only about 10−70% of the CO2 desorbing at room temperature from the IR data. This discrepancy allows us to conclude that the DFT models used do not fully capture the complex behavior of CO2 species on these surfaces and that some combination of VO defects, surface hydroxyls, coadsorption behavior, and complex surface structures are necessary to fully understand our experimental observations. Strength and Amount of Adsorbed CO2. To investigate the quantity of CO2 adsorbed and the strength of its association with TiO2 nanoparticles, IR spectra were recorded during He purge of adsorbed CO2 species to study the time-dependence of absorption spectra intensities, and TPD with MS was used to investigate strongly adsorbed species’ evolution to study strength of adsorption. IR spectra of adsorbed CO2 on TiO2 nanocrystals as a function of He purge time are shown in Figures S10a−c. Generally, the characteristic features of the adsorbed CO2 species (bicarbonates, carboxylates, and bridged and monodentate carbonates) exhibit some decrease in peak intensities, indicating that some portion of the adsorbed species that are weakly bound to the surface are removed during He purge at 25 °C and 1 atm. The IR intensity profiles of prominent adsorbed CO2 species on TiO2 rods, disks, and truncated octahedra during He purge as a function of time are shown in Figures 4a−c. Figure 4a shows that the majority of the

The results of studying CO2 adsorption on pristine surfaces, unsurprisingly, did not match well with experimental observations of which species exist on each surface and the adsorption energetics of these species, so further study was done on the (001) and (101) surfaces with VO defects to study how these defects affect CO2 adsorption. The structures of CO2 adsorbed on VO-defective TiO2 surfaces are shown in Figure S9 in the Supporting Information. We did not study the (010) surface because it is only a portion of rod surfaces, and the other two facets are of higher interest as examples of the leastand most-stable anatase TiO2 surfaces relevant to this study for (001) and (101), respectively. On the pristine surfaces, the dominant adsorption mode for (010) and (101) surfaces is physisorbed CO2: no other adsorption motif produced a stable structure with an exothermic adsorption. The pristine (001) surface, in contrast, allows for exothermic adsorption of both physisorbed and monodentate carbonates. Binding energetics of each motif are included in Table 1. Note that all motifs have a binding energy Table 1. Adsorption Energies of the Adsorbed CO2 Species on TiO2 (001), (010), and (101) Surfaces Computed Using DFT binding energy of adsorbed CO2 species (kJ/mol) TiO2 surface (001), pristine (001), Odefective (010), pristine (101), pristine (101), Odefective

linear CO2 adduct

monodentate carbonate

bridged carbonate

31.9 20.4

36.0 16.9

36.3 30.5

35.1 35.6 38.1

13.1 13.7 15.8

−5.5 −18.8 31.1

for each surface but that often the predicted structure is qualitatively unlike the starting motif, and the vibrational modes (see the Supporting Information, Table S2) are often very like other motifs, typically physisorbed CO2 in cases where the CO2 moves away from the surface and associates in a weakly physisorptive way (this is the case for monodentate carbonate on the pristine (101) surface, for instance). In other cases, such as bridged carbonate for the pristine (101) surface, a metastable structure like the starting motif is predicted, but the binding energy does not indicate an exothermic adsorption event. Considering the effects of VO defects on the (001) surface, we find that surface oxygen vacancies provide sites where bridged carbonate formation is most favorable, although all adsorption motifs are less stable on the oxygen defective (001) surface: roughly 12, 19, and 6 kJ/mol less stable for physisorbed CO2, monodentate carbonates, and bridged carbonates, respectively. This is attributed to the already high reactivity of the (001) pristine surface. In contrast, the introduction of VO defects on the (101) surface acts to stabilize all adsorption motifs. Binding energies of each mode increase by roughly 3, 2, and 50 kJ/mol for physisorbed CO2, monodentate carbonates, and bridged carbonates, respectively. This very large change for bridged carbonates is because the pristine (101) surface prediction for bridged carbonates was a metastable state with an endothermic adsorption of ∼19 kJ/mol, whereas the oxygen defect site allows an exothermic adsorption of about 31 kJ/mol. Overall, the DFT calculations predict that physisorbed CO2 should be observed on all surfaces, although the physisorbed CO2 IR mode peaks would overlap with peaks of gas-phase

Figure 4. Normalized IR intensity profiles of gas phase CO2 and adsorbed species during He purge on (a) TiO2 rods, (b) disks, (c) truncated octahedra; temperature during He purge = 25 °C.

carboxylates (85%), monodentate carbonates (70%), and bridged carbonates (70%) on TiO2 rods are retained after He purge. The IR intensity profiles of the bicarbonates and carboxylates on TiO2 disks during He purge in Figure 4b show that the bicarbonates (65% retained) are removed at a rate faster compared to that of carboxylates (88% retained). The IR intensity profiles of the monodentate and bidentate carbonates on TiO2 truncated octahedra in Figure 4c show that most monodentate (30% retained) and bidentate (16% retained) carbonates are removed during the He purge. Monodentate 9300


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species is carboxylates ≈ monodentate carbonates > bridged carbonates > bidentate carbonates ≈ bicarbonates.58,67 The TPD results show that TiO2 rods and disks interact more strongly with CO2 than TiO2 truncated octahedra, possibly due to the higher basicity of the TiO2 rods and disks than truncated octahedra based on both the crystallographic structure of the three TiO2 surfaces and the quantity of oxygen vacancies (VO). In addition to having more VO defects, the TiO2 disk surface ((001)) also has a higher concentration of O2C species on the surface compared to that on the TiO2 (101) surface, which has both O3C and O2C sites.12,64 Although the TiO2 (010) surface has similar O2C sites to the (101) surface, the stronger interaction of the TiO2 rods ((010)) surface with CO2 compared to TiO2 truncated octahedra ((101)) surface is due to the presence of more VO defects on the rods. It appears that the presence of intrinsic VO defects plays a role in CO2 interaction with TiO2 surfaces, partly because VO defects increase the surface basicity of TiO2, as demonstrated in other oxides.57,68 The CO2 capture capacities of TiO2 rods, disks, and truncated octahedra determined from TPD and CO2 breakthrough curves are summarized in Table 2 and Table S3. From

carbonates are removed at a faster rate than bidentate carbonates during the first 3 min of the He purge. After 3 min, the rate of removal of bidentate carbonates increases compared to removal rate of monodentate carbonates. The IR intensity profiles of the adsorbed CO2 on TiO2 rods, disks, and truncated octahedra show that CO2 interacts much more strongly with TiO2 rods and disks than with TiO2 truncated octahedra, consistent with our previous findings. Figure 5 shows the normalized CO2 MS intensity during TPD on TiO2 rods, disks, and truncated octahedra. The CO2

Table 2. CO2 Capture Capacities of Fresh and SO2 Exposed TiO2 Rods, Disks, and Truncated Octahedra Calculated from TPD Curves

Figure 5. CO2 MS intensity profiles during TPD for TiO2 rods, disks, and truncated octahedra nanoparticles.

CO2 capture capacity (μmol/m2)

MS intensities were normalized to the surface area of the TiO2 nanoparticles. The temperature of maximum CO2 desorption (TmaxCO2) is a qualitative indicator of CO2 binding strength. TiO2 rods and disks exhibit maximum CO2 desorption at 80 and 75 °C, respectively. In contrast, TiO2 truncated octahedra exhibit maximum CO2 desorption at 59 °C, which indicates weaker interaction of CO2 with TiO2 truncated octahedra than with rods or disks. The trailing of CO2 MS intensities at high temperature (80−180 °C) indicates desorption of adsorbed CO2 species (monodentate, bidentate, and bridged carbonates and bicarbonates) at different temperatures. This trailing is conspicuous, especially on TiO2 rods and disks. The IR spectra during the TPD process on TiO2 rods, disks, and truncated octahedra are shown in Figures S11a−c in the Supporting Information. The IR spectrum obtained during cooling after pretreatment at each temperature was used as the background for subtraction. IR spectra above 150 °C are not shown because no characteristic features of adsorbed CO2 species remain. The IR spectra during TPD on TiO2 rods (Figure S10a) show that the characteristic features of carboxylates (1700 and 1565 cm−1) and monodentate carbonates (1502, 1453, and 1360 cm−1) disappear between 120 and 150 °C, whereas the characteristic features of bridged carbonates (1725 and 1290 cm−1) and bicarbonates (1616 and 1202 cm−1) disappear at 100 and 75 °C, respectively. The characteristic features of carboxylates (1700 and 1570 cm−1) on TiO2 disks (Figure S11b) disappear at 120 °C, whereas those of bridged carbonates and bicarbonates disappear at 75 °C. Approximately 70% of adsorbed CO 2 species on TiO 2 truncated octahedra were removed during He purge at room temperature (Figure 4c); the characteristic features of monodentate carbonates, bidentate carbonates, and bicarbonates that remained after He purge disappear at 100 and 75 °C. In general, the order of thermal stability of adsorbed CO2

sample TiO2 rods TiO2 disks TiO2 trocta

cycle 2

% decrease in CO2 capture capacity from C1 to C2

% decrease in CO2 capture capacity (fresh and C1)

0.264

23.08

23.53

0.219

0.185

15.28

29.45

0.033

0.016

52.02

28.26

CO2 capture capacity of fresh sorbent (μmol/m2)

cycle 1

0.448

0.343

0.310 0.046

Table S3, it can be noted that the breakthrough and TPD data indicate a much larger capture capacity for TiO2 rods and disks than for truncated octahedra. The TPD profiles quantify the strongly held carbonate species after He purge, whereas the breakthrough results represent the overall CO2 capture capacity. Therefore, it is unsurprising that the CO2 capture capacity of TiO2 rods, disks, and truncated octahedra calculated from the CO2 breakthrough curves is much larger than that calculated using TPD curves. The large difference could also be due to the higher concentration of CO2 (15 vs 2%) and longer adsorption time (60 vs 15 min) used in the breakthrough experiments. The low CO2 capture capacity of TiO2 truncated octahedra (calculated from TPD curves) is due to the removal of the adsorbed CO2 species during He purge prior to TPD, indicating that many CO2 may be quite weakly bound, consistent with our DFT results. The low CO2 capture capacity (breakthrough curves; Table S2) of TiO2 truncated octahedra (2.39 μmol/m2) compared to TiO2 rods (5.40 μmol/m2) and TiO2 disks (5.81 μmol/m2) is attributed to the low basicity of the (101) surface of the octahedra and thus weak interaction with CO2. 9301


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ACS Sustainable Chemistry & Engineering Structure of Adsorbed SO2. To investigate the adsorption of SO2 on TiO2, IR spectra both during SO2 exposure and after one and two cycles of SO2 adsorption/desorption were taken. Figures 6a−c shows the absorbance spectra after 3 and 15 min

The absorbance spectrum after one TPD cycle on TiO2 truncated octahedra (Figure S13c) exhibits IR features of isolated surface sulfates at 997, 1224, 1350, and 1405 cm−1. Disappearance of the peak at 1480 cm−1 indicates that the polynuclear surface sulfates are removed during TPD, as with disks. Increases in the intensities of the sulfate peaks after a second TPD cycle indicate the accumulation of these species. There is a limited amount of previous work on SO2 interaction with anatase TiO2 in the literature.25,28,29 Wanbayor et. al25 investigated SO2 adsorption on TiO2 (001) and (101) surfaces using DFT and reported that SO2 adsorption on the TiO2 (101) surface (ΔEads= −27.59 kcal/mol) is energetically more favorable than on the TiO2 (001) surface (ΔEads= −1.36 kcal/mol). Formation of sulfate species requires the reduction of the TiO2 surface, and thus, adsorption of SO2 is more favorable on TiO2 surfaces which can be reduced easily. Even though the TiO2 truncated octahedra are hard to reduce, as shown in the CO-TPR, the IR results show that TiO2 octahedra exhibit adsorbed SO2 species similar to those on the rods and disks, possibly indicating that the strong acidic nature of SO2 is not sensitive enough to discriminate between the three TiO2 surfaces. The amount of SO2 desorbed during TPD on TiO2 rods, disks, and truncated octahedra is summarized in Table S4. Note that this does not represent the SO2 capture capacity of the TiO2 surface because only some of the adsorbed species are retained after TPD. Effect of SO2 on Structure and Strength of Adsorbed CO2. To investigate the effects of SO2 exposure of TiO2 rods, disks, and truncated octahedra on their CO2 adsorption properties, IR spectra during CO2 exposure before SO2 exposure and after one and two cycles of SO2 adsorption are shown in Figures 7a−c. The adsorbed CO 2 species

Figure 6. Absorbance spectra during SO2 adsorption at 3 and 15 min on TiO2 (a) rods, (b) disks, (c) truncated octahedra; temperature during adsorption = 25 °C.

of SO2 adsorption on TiO2 rods, disks, and truncated octahedra. The absorbance spectra of TiO2 rods (shown in Figure 6a) show features of isolated surface sulfates at 1135 (S−O), 1295 (S = O), 1342 (S = O), and 1406 cm−1 (S = O) and polynuclear surface sulfates at 1082 (S−O) and 1488 cm−1(S = O).69−79 The evolution of IR features (Figure S12a) of the adsorbed SO2 on TiO2 rods shows that the characteristic features of the isolated and polynuclear surface sulfates evolve at the same time. The evolution of IR features of the adsorbed SO2 on TiO2 disks (shown in Figure S12b) shows that isolated surface sulfates (1160, 1350, and 1372 cm−1) are formed initially (0−1 min). At longer times (1−15 min of SO2 exposure), polynuclear surface sulfates are formed (1091, 1372, and 1420 cm−1). The absorbance spectra of TiO2 truncated octahedra (shown in Figure S12c) exhibit peaks at 1138, 1327, 1366, and, 1405 cm−1, indicating the formation of isolated surface sulfate species.69−79 The peak at 1480 cm−1 suggests that some polynuclear surface sulfates are formed at longer times (5−15 min). Figures S13a−c in the Supporting Information show the absorbance spectra after TPD on TiO2 rods, disks, and truncated octahedra for two cycles of SO2 adsorption. The absorbance spectrum after one cycle of TPD on TiO2 rods (Figure S13a) exhibits the IR features of isolated surface SO2 species at 1124, 1258, and 1390 cm−1 and polynuclear surface sulfates at 1425 and 1480 cm−1, indicating that adsorbed SO2 was retained on the catalyst after TPD. Increases in the intensities of the IR peaks at 1124, 1258, 1390, 1425, and 1480 cm−1 after a second TPD cycle indicate further accumulation of surface sulfates. The absorbance spectrum after one TPD cycle on TiO2 disks (Figure S13b) exhibits IR features of isolated surface sulfates at 993, 1141, 1258, and 1372 cm−1, which indicates that some adsorbed sulfates remain after TPD. The disappearance of the peak at 1420 cm−1 indicates the removal of polynuclear surface sulfates during TPD. Similar to the TiO2 rods, the intensities of the IR features increase after the second cycle, indicating accumulation of sulfates.

Figure 7. IR spectra of adsorbed CO2 on fresh sample and after SO2 adsorption cycles 1 and 2 on TiO2 (a) rods, (b) disks, and (c) truncated octahedra; temperature during adsorption = 25 °C.

(bicarbonates, carbonates, and carboxylates) on TiO2 rods show similar peak positions before and after exposure to SO2 (Figure 7a). However, the intensity of the peaks associated with adsorbed CO2 species decreases on TiO2 rods after SO2 exposure, indicating that SO2 exposure can decrease CO2capture performance. The decrease in CO2 adsorption after SO2 exposure is attributed to accumulation of SO2 either blocking or altering the active sites. The IR spectra of adsorbed CO2 on the SO2-exposed TiO2 rods exhibit greater intensity bands at 1202, 1290 1565, 1616, 1700, and 1725 cm−1 (due to 9302


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after a second cycle of SO2 exposure, likely due to a weaker binding strength of CO2 with SO2-exposed samples. Figures 9a−c show the normalized CO2 MS intensities during TPD for cycles 1 and 2 (performed after first and

bicarbonates, carboxylates, and bridged carbonates) as compared to the peaks at 1360, 1453, and 1502 cm−1 (due to monodentate carbonates), indicating SO2 exposure affects CO2 adsorption such that bicarbonates, carboxylates, and bridged carbonates form more readily than on pre-exposure samples. The adsorbed CO 2 species on TiO2 disks after SO 2 adsorption cycles (Figure 7b) exhibit similar features to adsorbed CO2 on fresh TiO2 disks. The drastic decrease in the IR peak intensities of the adsorbed CO2 indicates a decrease in overall CO2 adsorption capacity, likely due to accumulation of sulfates. Unlike TiO2 rods, all IR peak intensities for bicarbonates, bridged carbonates, and carboxylates show a decrease on TiO2 disks after SO2 adsorption. As with TiO2 rods and disks, TiO2 truncated octahedra exhibit a decrease in CO2 adsorption after SO2 exposure (Figure 7c). The IR spectra of SO2-exposed TiO2 truncated octahedra show that CO2 adsorbs primarily as bidentate carbonates and bicarbonates, as on the fresh sample. Figures 8a−c and d−f show the IR intensity profiles of the adsorbed CO2 species during He purge at room temperature

Figure 9. CO2 MS intensities during TPD for (1) rods, (b) disks, and (c) truncated octahedra TiO2 after SO2 adsorption cycles 1 and 2. The CO2 MS intensity was normalized to surface area.

second SO2 adsorption cycle) on TiO2 rods, disks, and truncated octahedra. The CO2 MS intensities in Figure 9 are normalized to the surface area of the TiO2 nanoparticles. TiO2 rods exhibit maximum CO2 desorption (TmaxCO2) at 73 °C, which is slightly lower than the TmaxCO2 of the fresh catalyst (75−80 °C). In contrast, TiO2 disks and truncated octahedra exhibit TmaxCO2 at temperature at 63 and 47 °C, which are 16 and 22% lower than fresh catalysts, respectively. Decreases in the TmaxCO2 for TiO2 disks and truncated octahedra indicate that the presence of accumulated sulfates/sulfites has a greater impact on the CO2 binding strength of these catalysts compared to TiO2 rods. CO2 capture capacities of TiO2 rods, disks, and truncated octahedra determined from TPD curves are summarized in Table 2. The CO2 capture capacity of the TiO2 rods for cycle 1 (performed after the first SO2 adsorption cycle) is 0.343 μmol/m2, which is 24% lower than the fresh catalyst. The CO2 capture capacity further decreases 23% from cycle 1 to cycle 2. TiO2 disks exhibit a CO2 capture capacity of 0.22 μmol/m2 after the first SO2 cycle, which is 29% lower than the fresh catalyst. There is another 15% drop in the CO2 capture capacity after the second cycle. TiO2 truncated octahedra exhibit CO2 capture capacity of 0.033 μmol/m2 for cycle 1, which is 28% lower than the fresh catalyst, and 0.016 μmol/m2 for cycle 2 (a 52% decrease from cycle 1 to cycle 2). The amount of CO2 that TiO2 rods adsorb decreases after exposure to SO2, according to the IR spectra in Figure 7. Consistent with this, adsorbed CO2 species are more weakly bound to the surface from the intensity profiles depicted in Figure 8 compared to the those of fresh samples (Figure 4). It is interesting to note that the adsorption strength of the remaining CO2 species is not greatly affected by the accumulated SO2 species, as indicated by very little change in TmaxCO2 for TiO2 rods (Figure 9). Quite differently, the amount of CO2 adsorbed decreases for TiO2 disks after SO2 exposure (Figure 7), but what CO2 species adsorb are strongly bound to the surface and are barely removed during He purge at room temperature. The TmaxCO2 for TiO2 disks after SO2 exposure decreases more than 10 °C compared to the fresh catalyst,

Figure 8. Normalized IR intensity profiles of gas phase CO2 and adsorbed species during He purge on TiO2 rods, disks, and truncated octahedra after SO2 adsorption cycle 1 (a−c) and cycle 2 (d−f); temperature during He purge = 25 °C.

after one and two cycles of SO2 exposure, respectively. The IR intensity profiles after one cycle of SO2 exposure for TiO2 rods show that a larger percentage of adsorbed CO2 species including carboxylates (78% retained), monodendate carbonates (25% retained), and bridged carbonates (40% retained) are removed during He purge for SO2-exposed samples than on the fresh samples (Figure 4a). A decrease in the retention of CO2 species after He purge suggests a decrease in the binding strength of the adsorbed CO2 species that could be caused by accumulation of sulfates on the surface. A greater intensity decrease is observed after a second SO2 exposure cycle for adsorbed CO2 species (Figure 8d) compared to the first cycle, suggesting that SO2 further decreases the binding strength of adsorbed CO2 species, an observation attributable to further accumulation of sulfates. The IR intensity profiles of adsorbed CO2 species on TiO2 disks (Figures 8b and e) and TiO2 truncated octahedra (Figure 8c and f) show trends similar to those of TiO2 rods; i.e., (i) a greater quantity of adsorbed CO2 is removed after the first SO2 cycle than from the fresh sample and (ii) retention of adsorbed CO2 species further decreases 9303


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ACS Sustainable Chemistry & Engineering indicating that the binding strength of the strongly bound species that exist on the surface decreases. TiO2 truncated octahedra exhibit a decrease in both the amount (Figures 7−9) and binding strength (Figure 9) of adsorbed CO2 species after SO2 exposure. TiO2 rods and disks, which have more VO defects, should be affected more by SO2 than the TiO2 truncated octahedra, which are mostly the highly stable (101) surface. Still, the TiO2 truncated octahedra showed more deactivation for CO2 capture after SO2 exposure when compared to TiO2 rods and disks. It is possible that this is due to the adsorption of strongly coordinating species primarily occurring on the (001) facets that make up 10% of the surface area on octahedra, consistent with the lower total amount of adsorbed CO2 species detected in the TPD measurements. Still, the reason for this deviation is not confirmed and is worthy of future investigation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone: 865-576-1080. ORCID

Miaofang Chi: 0000-0003-0764-1567 Zachary D. Hood: 0000-0002-5720-4392 Krista S. Walton: 0000-0002-0962-9644 David S. Sholl: 0000-0002-2771-9168 Sheng Dai: 0000-0002-8046-3931 Zili Wu: 0000-0002-4468-3240 Notes

■

The authors declare no competing financial interest.

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CONCLUSIONS The structure, quantity, and strength of adsorbed CO2 species are found to be dependent on the surface structure of TiO2 nanoparticles. CO2 adsorbs in the form of carboxylates, bridged carbonates, and monodentate carbonates on TiO2 rods; in the forms of carboxylates and bridged carbonates on TiO2 disks; and in the forms of bidentate and monodentate carbonates on TiO2 truncated octahedra. Bicarbonates are observed on all three nanocrystal morphologies. DFT calculations show that physisorbed linear CO2 can be expected on all surfaces and that bridged carbonate species are more likely than monodentate carbonates to form on TiO2 surfaces. The differences between DFT and the experimental results are attributed to the mixed surface facets and the presence of surface hydroxyl groups on the TiO2 nanocrystals. TPD studies reveal that CO2 interacts more strongly with TiO2 rods and disks than with TiO2 truncated octahedra, an observation attributed to the higher basicity (i.e., more low-coordinated surface oxygen) and the presence of more VO defects in TiO2 rods and disks. The effect of SO2 on the adsorption of CO2 on the TiO2 nanoshapes was further investigated. SO2 adsorbs in the form of isolated and polynuclear surface sulfates on TiO2 rods and disks and as isolated surface sulfates on truncated octahedra. IR results reveal that the sulfates are retained after TPD on all TiO2 nanocrystal morphologies. TPD studies reveal that CO2 adsorption amounts and binding strengths decrease on all TiO2 nanocrystals that were pre-exposed to SO2, an observation we attribute to the presence of the sulfates retained after the SO2 adsorption cycle. Of the TiO2 nanoshapes studied, CO2 adsorption on truncated octahedra is most affected by SO2 exposure. The results obtained in this work elucidate how surface structure of TiO2 impacts interactions with acid gases (such as CO2 and SO2) and provide insights for designing more robust TiO2-based catalysts for application in capture and conversion of acid gases, processes that eventually lead to a sustainable energy future.

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adsorption, purge, and TPD on TiO2 rods, disks, and truncated octahedra (PDF)

ACKNOWLEDGMENTS This work was supported by the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center funded by U.S. Department of Energy. Part of the work including the IR, Raman and TEM was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. The DFT work was performed using EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research. Z.D.H. gratefully acknowledges a Graduate Research Fellowship from the National Science Foundation (Grant DGE-1148903) and the Georgia Tech-ORNL Fellowship. Notice: This manuscript was authored by UT-Battelle, LLC under Contract DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy. gov/downloads/doe-public-access-plan).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02295. Additional TEM images; Raman spectra of TiO2 rods, disks, and truncated octahedra; profiles from CO TPR and blank run for TiO2 rods; and IR spectra during 9304


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Effect of Surface Structure of TiO2 Nanoparticles on CO2 Adsorption

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