Oxygen Atom Exchange between Gaseous CO - American

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Oxygen Atom Exchange between Gaseous CO2 and TiO2 Nanoclusters Svatopluk Civiš,*,† Martin Ferus,† Markéta Zukalová,† Arnošt Zukal,† Ladislav Kavan,† Kenneth D. Jordan,‡,§ and Dan C. Sorescu*,§,⊥ †

J. Heyrovský Institute of Physical Chemistry, v.v.i, Academy of Sciences of the Czech Republic, Dolejškova 318223, Prague 8, Czech Republic ‡ Department of Chemistry and Center for Molecular and Materials Simulations, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States § United States Department of Energy, National Energy Technology Laboratory, Pittsburgh, Pennsylvania 15236, United States ⊥ Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *

ABSTRACT: Isotopic exchange of oxygen atoms between gaseous C18O2 and Ti16O2 nanoparticles has been studied using high-resolution Fourier transform infrared absorption and first-principles density functional theory calculations. The rate of formation of gaseous C16O2 is found to be highly dependent on the nature of the titania sample, growing with increasing calcination temperature (i.e., with decreasing surface area) for both quasi-amorphous and crystalline samples. The unprecedented faster kinetics on high-surface-area titania made from Ti(IV) isopropoxide points at fundamental differences between this material and the usual nanocrystalline TiO2 (anatase). This is attributed to unique cluster-like structure of the noncalcined ex-isopropoxide titania. The experimental observations are rationalized by calculations of the activation barriers for oxygen exchange on a (TiO2)10 cluster. The calculations predict that titanium nanoclusters with 4-fold coordinated titanium atoms have much lower barriers for O atom exchange than previously found for the oxygen defect sites on the single crystal (101) anatase surface.

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INTRODUCTION Titanium dioxide, especially in its nanocrystalline form,1,2 is an important material for electrochemical energy storage,3−5 solar energy conversion,6 and (photo)catalysis.1,3,7 Understanding the mechanisms of chemical reactions that take place at the interface of titania8 and the changes of the energy alignment of the electronic states upon molecular adsorption9 are key requirements for efficient use of this material in catalytic applications.3,7 The interactions of solid titania and gaseous CO2 are of particular interest because carbon dioxide is the end product of the photocatalytic oxidation of organic molecules, and because carbon dioxide can be photocatalytically reduced on titania surfaces to useful molecules including CO, CH4, C2H2, or CH3OH.10−13 Such reduction reactions are the key to the production of “solar fuels” starting from CO2 feedstocks.14 Titania and its surfaces are complex systems, both in stoichiometric (TiO2) and sub-stoichiometric (TiO2−x) forms.8 Various electronic defects can exist at the surface and in the bulk crystal. In particular, oxygen vacancies and interstitial/ substitutional Ti3+ ions are introduced by mild reduction of TiO2. These defects can trap electrons and are responsible for n-doping.15,16 Mild reduction of TiO2 can be carried out by © XXXX American Chemical Society

heat treatment in vacuum, inert, or reducing atmosphere such as a hydrogen/argon mixture,17 by varying the amount of oxygen during calcination,18 or by varying the water/alkoxide ratio during a sol−gel synthesis.19 In addition, reduction can be accomplished electrochemically.20,21 Both O-vacancies and Ti3+ ions can migrate in the crystal and exchange between the bulk and surface.8,22 By removing a lattice oxygen atom, a point defect and three undercoordinated (5-fold) titanium ions are formed.15 Two extra electrons remain and fill the empty orbitals of formally Ti4+ ions. Closed-shell singlet, open-shell singlet, or open-shell triplet states can result. Another important factor that influences the reactivity of TiO2 nanoparticles is the presence of undercoordinated sites on the surface. In the case of the ideal (101) anatase surface both 5- (Ti(5c)) and 6-fold coordinated (Ti(6c)) atoms are present. The former are further reduced to 4-fold coordinated (Ti(4c)) atoms when oxygen defects are present on the surface. These uncoordinated sites have been shown to enhance the CO2 Received: December 3, 2014 Revised: January 21, 2015

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DOI: 10.1021/jp512059b J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C reduction kinetics23 and to facilitate oxygen exchange between CO2 and the surface.24 Most importantly, the density of Ti(4c) coordinated sites can be appreciable in nm size TiO2 nanoparticles.25 Here we examine the reaction of CO2 with TiO2 samples prepared so as to vary the surface area/volume ratios. The reaction is studied with the aid of 16/18O-isotope labeling. In our earlier works,26,27 we used gas-phase high resolution Fourier transform infrared spectroscopy (HR-FTIR) to study O-isotope exchange at the C16O2/Ti18O2 interface. Here we extend this work to examine how interfacial isotope exchange activity relates to the nature of titania surface. This is accomplished by using the “reverse” system C18O2/Ti16O2. This avoids Ti18O2, which is expensive and unavailable with application-tailored properties, such as crystal phase and crystal facet orientation, surface area, porosity, and particle size. The use of C18O2/ Ti16O2 allows working with essentially any titania material and consequently provides large flexibility in the investigation of isotopically traced interfacial reactions. For the first time, we demonstrate that there are qualitative differences between nanocrystalline TiO2 (anatase) and quasi-amorphous titania made by hydrolysis of Ti(IV) isopropoxide, with the latter having considerably larger activity for the oxygen-exchange reaction.

TiO2 samples were calculated from adsorption data in the range of relative pressures 0.075−0.25 using the Brunauer−Emmett− Teller (BET) method.28 High resolution Fourier transform infrared (HR-FTIR) spectra of the gas phase species were measured in a 20 cm long (2.5 cm diameter) glass optical cell equipped with CaF2 windows. The cell was interfaced to a sealable glass tube joint for the transfer of the powder material under vacuum from a side ampule in which the calcination of TiO2 took place. The optical cell was equipped with two vacuum valves (ACE glass, USA) for the gas handling and connection to the vacuum line. The evacuated optical cell containing 0.8 g of Ti16O2 was placed into the vacuum chamber of a Bruker Optics IFS 125 HR high resolution Fourier transform spectrometer. The cell was connected via cajon fittings with an external small glass vessel containing C18O2 and a Baratron pressure gage (622B, 10 Torr, MKS Instruments, Germany). The spectrometer was subsequently evacuated and operated in the fast measurement mode with 80 kHz scanning mirror speed in a regime of repeated measurements.29 The spectral acquisition was performed using a CaF2 beam splitter and an InSb detector. The kinetic measurements were triggered by adding carbon dioxide (C18O2) to the optical cell containing TiO2 during the fast scanning process.30 This setup allowed acquisition of one spectrum in several seconds with a resolution of 0.01 cm−1 in a spectral range of 1800−6000 cm−1. The measured interferograms were apodized with the Blackmann-Harris apodization function.31 In the Fourier-transform-based calculation of the final spectrum, the total number of points was generated by the zero-filling factor of 2 using the OPUS 6.0 software32 and using about ten points per fwhm for a single line in the spectrum. The concentrations of the various CO2 isotopomers were determined by independent calibration using pure gases of defined isotopic ratio (carbon dioxide, 97% 18O, MSD Isotopes, Montreal, Canada and 99.9995% natural CO2, CAS 124-38-9, Linde Gas, mixture of 0.39% C16,18O2 and 98.42% C16O2). The integrated intensities of selected individual absorption lines were calculated using the OPUS 6.0 software package,32 and the data were subsequently fit by a linear regression. Computations. The adsorption configurations of CO2 and the reaction barriers for oxygen exchange between gaseous CO2 and TiO2 were investigated using density functional theory (DFT) calculations on a Ti10O20 model nanocluster. The computations were performed using the VASP code33,34 with the PBE35 exchange-correlation functional and the projectoraugmented wave (PAW) method of Blöchl36 as adapted by Kresse and Joubert.37 The calculations included corrections for both on-site Coulomb interactions by use of the GGA+U procedure38 and long-range dispersion interactions using the Tkatchenko and Scheffler (TS) method.39 A cutoff energy of 400 eV was used for the plane-wave basis set. The Ti10O20 cluster was isolated in a large supercell allowing vacuum separations of at least 10 Å along each of the three Cartesian directions. Brillouin zone sampling was restricted to the Γpoint. The minimum energy reaction pathway for oxygen exchange was determined using the climbing-image nudged elastic band (CI-NEB) method.40 The computational details are similar to those used in our previous studies of adsorption of CO2 on rutile (110)41,42 and anatase (101) surfaces.24,43

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EXPERIMENTAL SECTION Synthesis of Materials. A low surface area crystalline TiO2(anatase) sample (designated A) was synthesized by slow addition of 15 mL of TiCl4 (99.98% Aldrich) into 5 mL H2O under stirring and cooling. The product was recovered by filtration, washed with water and dried at 200 °C for 5 h. A high-surface-area titania sample (designated NANO) was prepared by hydrolysis of 48 g titanium isopropoxide (97%, Aldrich) with 6 g H2O under stirring. The precipitate was washed with water and dried at room temperature and then at 60 °C in air. A-type samples were calcinated at 200, 250, 300, 350, 400, and 450 °C, with the resulting samples being designated A200, A250, A300, A350, A400 and A450. Similarly NANO samples were vacuum-calcinated at 60, 100, 150, 200, 250, 300, 350, and 400 °C, giving rise to the NANO60, NANO100, NANO150, NANO 200, NANO250, NANO300, NANO350 and NANO400 samples, respectively. Methods. The prepared materials were characterized by Xray diffraction (XRD) and molecular nitrogen adsorption isotherm measurements (Nitrogen, Linde 5.6). XRD spectra were measured with a Bruker D8 Advance diffractometer using Cu Kα radiation. Whereas the diffraction patterns of the initial and calcinated A samples are consistent with crystalline anatase, the diffractograms of the original NANO sample and the NANO60, NANO100, NANO150, and NANO200 samples exhibited only broad envelopes characteristic of “amorphous” material. The diffractograms of NANO250 and NANO400 samples are consistent with crystalline anatase (see Supporting Information, Figure S1). The surface areas of the prepared materials were determined from nitrogen adsorption isotherm measurements at T = −196 °C (ASAP 2010, Micromeritics) as described below. Before measurement, the TiO2 sample was degassed by ramping from ambient temperature to the appropriate calcination temperature (at 0.5 deg/min) until a residual pressure of 1 Pa was attained. Degassing continued at the final temperature under vacuum for 6 h. After degassing, adsorption isotherms were recorded using a bath of liquid N2. The surface areas of the

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RESULTS AND DISCUSSION Oxygen Exchange. Gaseous C18O2 contacting solid Ti16O2 undergoes 18O/16O isotopic exchange as was found earlier for B


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The Journal of Physical Chemistry C the C16O2/Ti18O2 system.26,27 In the present study, we examined 14 different Ti 16 O 2 samples (A200−A450, NANO60−NANO400), which are either anatase-like or amorphous, as determined using XRD measurements (see the Experimental Section). The central motivation of our study was to find the most active catalyst for the oxygen exchange process by screening of preparative conditions, and by comparing crystalline and quasi-amorphous titania. The isotopic exchange between C18O2 and Ti16O2 can be monitored using the fundamental, overtones or combination bands of CO2. Figure 1

Figure 2. Anatase A200 and A400 isotope exchange activities. Panel A reports the partial pressure of C16O2 and C18O2 isotopomers vs time for A200 Ti16O2 + C18O2. Panel B shows the corresponding results for A400 Ti16O2 + C18O2. The data are fitted by the appropriate conversion curves estimated using first-order kinetic equation models. Panel C reports surface areas vs calcination temperature for A-type samples calcinated at temperatures between 200 and 450 °C. Panel D depicts the dependence of the τ1/2 values on the calcination temperature.

Figure 1. HR-FTIR spectra of the v1+v3 and 2v2+v3 combination bands of CO2 near 3600 cm−1 following oxygen isotope exchange between A400 Ti16O2 and C18O2. (a) The spectrum of C18O2 in the absence of the surface. (b) The experimental spectrum measured after 1000 s of contact of C18O2 with A400. (c) Simulated spectrum using the Winprof program44 with the parameters of the absorption lines taken from the HITRAN database.45

reports IR spectra between 3450 and 3750 cm−1, the region of the ν1 + ν3 and 2ν2 + ν3 combination bands. Panel (a) in Figure 1 depicts the spectrum of C18O2 (at a pressure of 2 Torr) in the absence of titania. Panel (b) reports the spectrum of CO2 after 1000 s of contact with the A400 sample. Significant amounts of the C16O18O and C16O2 exchange products are evident. Panel (c) reports a simulated spectrum including all three isotopologues of CO2 taken from the HITRAN reference data45 and using the Winprof program.44 Oxygen atom exchange between C18O2 and Ti16O2 is detectable in the FTIR spectra within seconds of exposure of the surface to C18O2 as seen from Figures 2 and 3, which report data for A-samples (A200 and A400) and NANO-samples (NANO200 and NANO400), respectively. In each case, exponential decay of the C18O2 partial pressure is observed accompanied by growth of the C16O2 partial pressure (see panels A and B in Figures 2 and 3). The partial pressure of the C16O18O intermediate remains quite low and nearly constant26,27 (typical pressures of 10−2 Torr; data not shown). Kinetics of isotope exchange at the C18O2/Ti16O2 interface. As noted in the Introduction, O atom exchange can occur either at bridging O atom defect sites or at 4-fold coordinated Ti atoms on the surface. The latter are expected to be more important for the amorphous samples.46 Oxygen atom exchange between gaseous C18O2 and the Ti16O2 surface with bridging O atom defects or Ti(4c) sites can be described by a two-step reaction.26,27,47,48 This is illustrated below for the bridging O atom defect case.

Figure 3. Time variation of the partial pressures of reactant (C18O2) and product (C16O2) for the NANO200 Ti16O2 + C18O2 (panel A) and NANO 400 Ti16O2 + C18O2 (panel B) samples together with the appropriate conversion curves estimated using first-order kinetic equation models. Panel C reports surface areas vs the calcination temperature. Panel D depicts the dependence of the τ1/2 values on the calcination temperature.

The partial pressure measurements (panels A and B of Figures 2 and 3) show that the signal due to C18O2 drops to near zero within hundreds to thousands of seconds (depending on the type of titania sample. In the scheme illustrated above, the reactant Ti16O2 is in excess: 1 g of TiO2 represents ca. 12 mmol, while 1.1 Torr of CO2 in the working volume of 100 mL corresponds to ca. 6 μmol. At these conditions, the reaction rate can be fit to a first-order kinetics equation with a single effective rate constant, kI(eff) (s−1). This allows the comparison of the conversion rates of C18O2/Ti16O2 isotopic exchange at a variety of titania surfaces, i.e. for samples A and NANO calcinated at different temperatures. The global reaction C


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panel D of Figures 2 and 3. Additionally, the variation of the BET surface areas vs the calcination temperature is indicated in panel C in Figures 2 and 3 for the anatase and NANO samples, respectively. For the A samples and for the T > 150 °C NANO samples, both the reaction half-times and the BET areas decrease with increasing calcination temperature. The BET surface areas can be related to the size of the nanoparticles, if one assumes spherical particles and a density of 4 g/cm3 (Table 1). Considering that the anatase crystal is a tetragonal bipyramid and not a sphere, the above-mentioned estimate is an upper limit of the particle size. The decrease of the BET surface area (the growth of the diameter, dp) with the calcination temperature is understandable in terms of the thermally induced sintering of nanoparticles.49 However, the fact that calcination (and the associated drop in the BET surface areas) increases the titania reactivity toward the O-exchange with CO2 is surprising, and it can be ascribed to the formation of Ovacancies and other active sites created by vacuum annealing. Figure 4 confirms that there is an increase of reactivity with calcination temperature for both the A-samples and the NANO-samples.

(3)

can be described by the equation: −

dp(18) (t ) dt

= kI(eff) × p(18) (t )

(4) 18

where p(18) is the corresponding partial pressure of C O2. The solution of this equation is a function of the partial pressure p(18) of the reactant: p(18) (t ) = p(18) (0) × exp( −kI(eff) × t )

(5) 16

and a function of partial pressure of the product (C O2), p(16): p(16) (t ) = rgas[p(18) (0) − p(18) (0) × exp( −kI(eff) × t )] (6)

Equation 6 takes into account that the product is partly in the gas phase and partly adsorbed on the surface, i.e. the ratio rgas between the total partial pressure of the product p(16) formed during the reaction and the real partial pressure of the product in the gas phase p(g) is given as rgas = p(g) /p(16)

(7)

The results of the fitting procedure using eqs 5 and 6 are plotted in panels A and B of Figures 2 and 3. In both cases the solid red curve reports the fit obtained using eq 5 corresponding to the time evolution of the reactant concentrations, while the solid black curves present the time evolution of the product concentrations as determined using eq 6. The detailed results of the fits are summarized in Table 1. The half-times of the corresponding reactions, τ1/2, are indicated in panels A and B of Figure 2 for the A200 and A400 samples and in panels A and B of Figure 3 for NANO200 and NANO400 samples. The dependence of the τ1/2 values on the calcination temperature of all titania samples is shown in

Figure 4. Relation between the BET surface area and the rate constant for O atom exchange for A-samples (represented in black) and for NANO samples (represented in red).

Table 1. Rate Constants and Reaction Half-Times of the Nanomaterial (NANO60−NANO400) Samples and the Anatase A Samples (A200−A450) Calcined at Various Temperatures

NANO samples series

anatase A samples series

calcination temperature (°C)

rate constant (s−1)

60 100 150 200 250 300 350 400 calcination temperature (°C)

0.0145 0.0138 0.0140 0.0150 0.0158 0.0165 0.0178 0.0188 rate constant (s−1)

48 50 50 46 44 42 39 37 halftime (s)

200 250 300 350 400 450

0.0023 0.0025 0.0027 0.0028 0.0032 0.0060

307 277 262 251 215 116

halftime (s)

BET (m2/g)

dpa (nm)

497.3 506.5 491.7 478.7 466.8 433.4 367.4 280.9

3.0 3.0 3.0 3.1 3.2 3.7 4.8 5.3

BET (m2/g)

dpa (nm)

120.9 115.3 109.9 101.7 87.70 64.90

12.4 13.0 13.6 15.3 17.0 23.0

Besides O-vacancies in the surface, 4-fold coordinated surface Ti sites should also be considered. Comparison of the measured rate constants with the measured BET surface areas of the various samples confirms that the key factor for interfacial isotope exchange is not the absolute interfacial area, but rather the number of active sites that are created as a result of vacuum calcination. Indeed, the two effects counteract one another: Thermal treatment causes a drop of surface area due to sintering of small nanocrystals, but it also creates O-vacancies (and, perhaps also, 4-fold coordinated Ti sites). The number of O-vacancies introduced by vacuum annealing is typically several percent of all surface oxygens,8 and their creation clearly dominates over the relative loss of surface area. The results reported in Table 1 and Figure 4 confirm that the hightemperature-calcined samples (of both the A- and NANOseries) react faster with C18O2 than the low-temperature calcined samples, in spite of their smaller surface area. There is another striking difference between the NANO samples and A-samples, the former exhibiting 2.8 to 8.0 times larger rate constants for the CO2(g)/TiO2(s) oxygen-exchange reactions. Such a large difference cannot be simply assigned to surface defects in anatase, but it is rationalized in terms of basically different structure of the two materials. While the Asamples behave like ordinary anatase with some defects introduced by calcination, the NANO-samples are structurally different. We propose that the ≈3 nm sized particles of NANO-

a The particle diameter, dp, is calculated from the BET surface area by assuming a spherical particle shape and a material density of 4 g/cm3.

D


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kinks. Such nanoparticles can also have four-fold coordinated Ti sites.46 We note that a small density of four-fold Ti(4c) sites can also be created on the anatase (101) surface if oxygen defects are present on the surface. Ti(4c) sites together with the regular surface Ti(5c) and Ti(6c) sites are illustrated in Figure 5a). In this section, we extend our theoretical analysis of the oxygen exchange mechanism to nanoparticles with Ti(4c) sites. For this purpose we consider a (TiO2)10 cluster that was previously characterized by Marom et al.50 The size of our clusters is roughly comparable to the particle size of NANO samples in Table 1. Common to this and other clusters from the (TiO2)2−10 set is the prevalence of Ti(4c) coordination. As noted in the Introduction, such sites can have significant influence on the reactivity of adsorbates including for CO2 reduction.23 In considering the reaction of CO2 with the (TiO2)10 cluster, the positions of all atoms were optimized except the bottom three oxygen atoms of the cluster which were kept frozen at the positions obtained upon geometry optimization of the bare cluster. The results depicted in Figure 6 correspond to PBE-TS +U (U = 2.5 eV) calculations. Results obtained using U = 0 and U = 3.5 eV are discussed below.

samples (Table 1) behave like molecular clusters, (TiO2)n with n ≈ 10. (Note that the c-lattice constant of anatase is near 1 nm, hence these clusters have only the size of several unit cells of anatase). Below, we shall bring theoretical arguments that these clusters are, indeed, more active than anatase for the CO2(g)/TiO2(s) exchange reactions. Theoretical Study of CO2 Oxygen Exchange on a (TiO2)10 Cluster. The mechanism of oxygen exchange between molecular CO2 adsorbed at a surface oxygen vacancy (VO) on the crystalline (101) anatase surface was described in detail in our previous work using DFT calculations.24 Briefly, it was demonstrated that formation of a carbonate-like configuration (see Figure 5c) at a surface oxygen defect site is the key

Figure 5. Schematic of the mechanism of oxygen exchange between a CO2 molecule and the anatase (101) surface. Results from PBE−TS +U (U = 2.5 eV) calculations. Ti 4-fold and 5-fold sites on the surface are indicated in panel (a). The O and C atoms of the original CO2 molecule are depicted in yellow and green, respectively, while the O and Ti atoms of the surface are colored in red and gray, respectively.

intermediate responsible for the oxygen exchange at such a defect site. Specifically, upon overcoming a barrier of about 10.9 kcal/mol, a C18O2 molecule weakly physisorbed (Eads = 14 kcal/mol) in a linear configuration on (101) anatase above a surface defect site (Figure 5a) can react with a surface bridging oxygen near a VO site to form a carbonate-like species (Figure 5c). The system can further evolve to form a C18O16O molecule (see Figure 5e) which requires overcoming a subsequent barrier of 16.5 kcal/mol. During this process one of the original 18O atoms of the incoming CO2 molecule (represented in yellow in Figure 5c) remains on the surface as a bridging oxygen near the VO site. The resulting C18O16O molecule can then participate in a new oxygen exchange process upon surface diffusion (or desorption and readsorption) and reaction at another VO site leading to the exchange of the second 18O and formation of a C16O2 molecule. The calculated barriers indicated in Figure 5 are fully consistent with experimental observation of oxygen exchange on the anatase surface at ambient conditions and summarized by the two step mechanism proposed in eqs 1 and 2. As shown in the previous section an enhanced rate of oxygen exchange is observed when experiments are performed using nanosize titania samples (NANO samples) rather than with anatase powder. For the NANO samples, there is a distribution of sizes and shapes as well as of different surface steps and

Figure 6. Minimum energy pathway for oxygen exchange between a CO2 molecule and the (TiO2)10 cluster as determined from PBE−TS +U (U = 2.5 eV) calculations. The coloring scheme is the same as used for Figure 5.

Based on the pathway shown in Figure 6, O atom exchange between CO2 and the (TiO2)10 cluster is initiated upon adsorption of the CO2 molecule above a Ti(4c) site (panel #0 in Figure 6) with a binding energy of 10.5 kcal/mol. From this initial configuration and upon overcoming a barrier of 5.4 kcal/ mol, the molecule binds in a (μ1−η2) configuration (panel #8) (in the Gibson notation convention51), with simultaneous formation of two bonds, one with a Ti(4c) atom and the other with a bridge O(2c) site. From here the system can further evolve to a local minimum (image #17) corresponding to a carbonate-like species bonded in a (μ2−η2) fashion to two Ti(4c) sites. This intermediate facilitates exchange of one of the oxygen atoms of the CO2 molecule with an oxygen atom of the (TiO2)10 cluster. In this process one of the original oxygen atoms of the CO2 molecule inserts into the framework of the (TiO2)10 cluster at a location originally occupied by the bridging O(2c) atom (see image #22). The reaction then encounters a barrier of only 6.4 kcal/mol for formation of a physisorbed CO2 molecule (see image #29). We also E


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The Journal of Physical Chemistry C determined reaction profiles using U = 0 and U = 3.5 eV and find only a weak dependence of the barrier heights on the value of the U parameter. In particular, the largest barrier found was 6.1 kcal/mol for U = 0 eV, compared to 6.4 kcal/mol with U = 2.5 eV. The different binding configurations of CO2 along the pathway shown in Figure 6 are characterized by appreciably different vibrational frequencies. For example, the calculated CO stretching frequencies of physisorbed CO2 (image #0) are 2369 and 1315 cm−1. These shift to 1839 and 1120 cm−1 for the (μ1−η2) configuration (see image #8), while the carbonatelike species in image #17 is characterized by calculated vibrational frequencies of 1766, 1065, and 1037 cm−1. The barriers for the oxygen exchange on the 4-fold coordinated Ti sites of the (TiO2)10 cluster are significantly smaller than those calculated for the defective anatase (101) surface. As a result, oxygen exchange can take place more readily at Ti(4c) surface sites than at bridging O atom vacancies on the surface. This suggests that oxygen exchange may occur via a different mechanism on TiO2 nanoparticles than on the anatase surface. In general, in experimental samples, there will be a distribution of TiO2 nanoparticles of different sizes and shapes with various steps and kinks. Additional experimental and computational investigations are required to provide a detailed understanding of the mechanisms for oxygen exchange with CO2 in such samples. Nevertheless, the much faster exchange reaction at the cluster-like material is rationalized by the found energy barrier of 6.4 kcal/mol, which is considerably smaller than that for regular anatase (16.5 kcal/mol).

for a (TiO2)10 cluster. The size of our clusters is roughly comparable to the particle size of NANO samples. The overall activation energy for O atom exchange between a CO2 molecule and the TiO2 surface is calculated to be 6.4 kcal/ mol at a Ti(4c) site of a (TiO2)10 cluster as compared to 16.5 kcal/mol at a bridging O vacancy site on (101) surface of the anatase crystal. The high density of uncoordinated Ti(4c) sites and the open nature of small nm size TiO2 nanoparticles relative to large anatase particles can substantially enhance the oxygen exchange reactivity even under ambient temperature conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Comparison of XRD patterns of NANO samples and standard anatase (A400 and A200 samples). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel: +420 26605 3275. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge grants of computer time at Pittsburgh Supercomputing Center and on the NETL HPCEE supercomputer system. The work at the J. Heyrovský Institute of Physical Chemistry was supported by the Grant Agency of the Czech Republic (Contracts 13-07724S and P108/12/0814) and by the Ministry of Education Youth and Sports of the Czech Republic (COST Action CM1104, Contracts LD13060 and LD14115). The work at University of Pittsburgh was performed in support of the National Energy Technology Laboratory’s Office of Research and Development under Contract DE-FE0004000.2.661.251.001. Reference in this work to any specific commercial product is to facilitate understanding and does not necessarily imply endorsement by the United States Department of Energy.

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CONCLUSION Oxygen exchange between gaseous C18O2 and 14 different solid samples of anatase and quasi-amorphous Ti16O2 was monitored using high resolution Fourier transform infrared measurements. The three main findings are listed below. (1) Upon interaction of gaseous C18O2 with Ti16O2 the concentration of C18O2 drops to almost zero while the C16O2 concentration grows to near saturation within hundreds to thousands of seconds depending on the nature of titania sample. The partial pressure of the intermediate C16O18O species remains quite low and nearly constant. (2) The analysis of the kinetic data indicates that the halftimes for the oxygen exchange reaction are dependent on the calcination temperature and decrease for titania samples prepared at increasingly higher calcination temperatures. The BET surface areas of the titania particles drop as a result of the thermal treatment. (3) There is substantial increase in reactivity toward the Oexchange reaction for high surface area NANO samples relative to the low surface area anatase (A) samples. The rate constants of the NANO samples range from 0.0145−0.0188 s−1 and are 2.4−8.0 times larger than those of the A-samples which range from 0.0023−0.0060 s−1. These findings are attributed to the unique structure of NANO sample, which is composed from isolated (TiO2)n clusters, rather than from regular anatase crystals. This structure is characterized by 4-fold coordinated Ti sites at which the oxygen exchange can occur. A secondary effect is the thermally induced increase in the density of Ovacancies, which also accelerate the exchange reaction both in nanoclusters and ordinary anatase. The increased reactivity of oxygen exchange on (TiO2)n cluster in NANO samples has been rationalized based on first-principles density functional theory calculations for the defective (101) anatase surface and

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