Spontaneous and Photoinduced Conversion of CO2 on TiO2 Anatase

Article pubs.acs.org/JPCC

Spontaneous and Photoinduced Conversion of CO2 on TiO2 Anatase (001)/(101) Surfaces Martin Ferus,†,‡ Ladislav Kavan,† Markéta Zukalová,† Arnošt Zukal,† Mariana Klementová,§ and Svatopluk Civiš*,† †

J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejškova 3, 18223 Prague 8, Czech Republic ‡ Institute of Biophysics, v.v.i., Academy of Sciences of the Czech Republic, Královopolská 135, 612 65 Brno, Czech Republic § Institute of Inorganic Chemistry of the AS CR, v.v.i. CZ−250 68 Husinec-Rez 1001, Czech Republic ABSTRACT: High-resolution FT-IR spectroscopy was used to study the kinetics of oxygen mobility between gaseous isotopically labeled C18O2 and solid phase Ti16O2 nanocrystalline anatase (001). Analysis of the isotopic composition of the gases produced has revealed that anatase (001) calcinated at temperatures under 500 °C is a very weakly reactive material and exhibits a low exchange mobility of oxygen atoms between the gas phase molecules of CO2 and the TiO2 lattice. Isotope exchange is blocked by both residual HF adsorbed onto the TiO2 surface and TiOF2 impurities. The presence of TiOF2 was shown by TEM and X-ray diffraction. However, the anatase (001) sample became slightly active after annealing at temperatures higher than 500 °C and upon UV illumination. Nevertheless, the effective rate constant is still 3.45 times lower than that observed for the spontaneous exchange between C18O2 and anatase (101) material calcinated at 500 °C.

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INTRODUCTION Morphological control and the design of crystal facets are often used as a strategy to optimize the performance of various crystalline semiconductors.1,2 The fundamental basis of this strategy is that atomic surface configuration and atomic coordination, which essentially determine heterogeneous reactivity, can be excellently tuned by controlling the crystal’s morphology.3 In general, the catalytic activity of inorganic (nano)crystals is governed not only by their surface composition but also by the physicochemical properties of their exposed surfaces, which are related to their surface atomic arrangement and coordination.4,5 A typical example is titanium dioxide (TiO2), which has promising energy and environmental applications.6 For example, (001) facets of anatase TiO2 are considered to be more reactive than (101).2,7 The predicted shape of anatase crystals under equilibrium conditions is a tetragonal bipyramid, which might be slightly truncated, exposing a majority of (101) and a minority of (001) facets.2,8 In contrast to (101) facets with only 50% five-coordinate Ti (Ti5c) atoms, (001) facets with 100% Ti5c atoms were sometimes considered to be more reactive in heterogeneous reactions.8 Yang et al.2 pioneered the preparation of anatase nanocrystals with a dramatically increased (001)-to-(101) ratio, which allowed for fundamental follow-up studies.9−12 The key for controlling the percentage of crystallographic facets of anatase crystals is to change the relative stability of each facet during crystal growth, which is intrinsically determined by the surface energies of the facets.5 Surface adsorbed fluorine atoms are known to be very effective in changing the surface energy of TiO2 facets. Previous results have shown that the percentage of © XXXX American Chemical Society

(001) facets of anatase crystals can be increased up to approximately 90%, or even close to 100%,13 depending on the different synthesis routes.2 Recently, Selloni and Selcuk14 used density functional theory calculations and first-principles molecular dynamics simulations to investigate the structure and reactivity of TiO2 (001) facets. According to their results, the (1 × 4) reconstructed surfaces exhibit weak reactivity. Such a result is consistent with our recent experimental observations.15−19 We have discovered, by investigating a set of anatase crystals with predominantly either (101) or (001) facets, that pure (001) exhibits lower reactivity than (101) in a common reaction with carbon dioxide.

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EXPERIMENTAL SECTION

Synthesis of Materials. Anatase (001) was prepared as follows: 2.4 mL of hydrofluoric acid (48%; Sigma-Aldrich) was added to 20 mL of titanium(IV) butoxide (purum, ≥97.0%, Sigma-Aldrich) with vigorous stirring. The mixture was autoclaved at 200 °C for 24 h. The sample was collected after 24 h of heating and washed with copious amounts of MilliQ water. The resulting solid was dried at 100 °C for 5 h. The as-received material contained between 6.5 and 20 wt % of F as determined by energy-dispersive X-ray spectroscopy (EDS) analysis. After calcination (500 °C, 1 h) the F content dropped Received: September 8, 2014 Revised: October 24, 2014

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dx.doi.org/10.1021/jp5090668 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

spectrum were acquired every 58 s with a resolution of 0.01 cm−1 in a spectral range of 1800−6000 cm−1. The concentrations of individual CO2 isotopoloques 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 integral intensities of selected individual absorption lines were calculated using the OPUS 6.0 software (Bruker, Germany), and the data were subsequently fitted by a linear regression in a wide range of CO2 partial pressures. The integral intensities of the defined isolated isotopologues lines have been determined for every measured experimental spectrum, and the quantities have been obtained by fitting from the calibration functions.

practically to zero. Reference material was prepared in the same way, but HF was replaced by the same amount of water. Methods. The prepared materials were characterized by Xray diffraction (XRD), scanning electron microscopy (SEM) (data not shown), transmission electron microscopy (TEM), and nitrogen adsorption measurements. XRD was measured using a Bruker D8 Advance diffractometer using CuKα radiation. SEM images were acquired on a Hitachi FE SEM S-4800 microscope. TEM analysis and energy-dispersive X-ray spectroscopy data (EDS) were carried out on a Jeol JEM-3010 microscope (300 kV, LaB6, 1.7 Å point resolution). The surface area of the prepared materials was determined from a nitrogen adsorption isotherm at −196 °C (ASAP 2010 apparatus, Micromeritics). The fresh TiO2 sample was degassed starting at ambient temperature up to the selected calcination temperature (temperature ramp 0.5°/min) until the residual pressure of 1 Pa was attained. Degassing continued at this temperature under vacuum for an additional 6 h. After degassing, the adsorption isotherms were recorded at the boiling temperature of liquid N2. The surface area of TiO2 samples was calculated using the Brunauer−Emmett−Teller (BET) method from adsorption data in the range of relative pressures p/p0 of 0.075−0.25 (p0 is the saturated vapor pressure of N2 at −196 °C). HR-FTIR spectra of the gas phase were measured in a 30 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 also took place. The optical cell was further equipped with two vacuum valves (ACE glass, USA) for the gas handling and connection to the vacuum line. The evacuated optical cell containing 1.0 g of Ti16O2 was placed into the vacuum chamber of a Bruker Optics IFS 125 HR highresolution Fourier transform spectrometer. The cell was connected using Cajon fittings through the inlets in a spectrometer chamber wall with a small glass vessel containing C18O2 (97 atom % 18O, MSD Isotopes, Montreal, Canada) and a Baratron pressure gauge located outside. The spectrometer was subsequently evacuated and operated in the fast measurement mode at a scanning mirror frequency of 80 kHz in a regime of repeated measurement. The spectral acquisition was performed using a CaF2 beam splitter and an InSb detector. The kinetic measurement was triggered by adding carbon dioxide (C18O2) into the optical cell containing TiO2 during the fast scanning process. To achieve a reasonable signal-tonoise ratio, this setup allowed the acquisition of 5 scans for a spectrum every 58 s with a resolution of 0.01 cm−1 in a spectral range of 1800−6000 cm−1. All the interferograms measured were apodized with the Blackmann−Harris apodization function.20 In the Fourier-transform-based recalculation of the final spectrum, the total number of points was generated by the zero filling procedure. We used the zero filling factor of 2 and generated about 10 points per full width at half maximum (FWHM) for a single line in the spectrum. To explore the contribution of UV excitation to the rate of the individual isotopologues formation, a broadband UV lamp with a peak wavelength at 366 nm (P-Lab, Czech Republic) was used to irradiate Ti16O2. The UV lamp was set vertically (5 cm from the window) to the sample cell, and the Ti16O2 surface was irradiated through the CaF2 window directly in the cell placed in the sample chamber of the Bruker IFS 125 spectrometer. The irradiation started immediately after the cell had been filled with 1.1 Torr of C18O2. Again, 5 scans for a

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RESULTS AND DISCUSSION The gaseous C18O2 contacting solid Ti16O2 exchanges spontaneously the 16/18O-isotopes in accord with our earlier study on the reverse system, C16O2/Ti18O2.17,18 In this work, we systematically screened nine different Ti16O2 samples of anatase (101) that were annealed in the temperature range of 50−500 °C. In general, the isotopic exchange between C18O2 and Ti16O2 can be monitored on all the fundamental IR bands of CO2, including overtones or combination bands. Figure 1 shows the selected region near 3600 cm−1, presenting the ν1 and ν3 combination bands of CO2.

Figure 1. Selected gas phase spectra of CO2 isotopologues combination bands ν1 + ν3 and 2ν2 + ν3 near 3600 cm−1. Anatase (001) Ti18O2 calcined in the temperature range of 50−500 °C was contacted with C16O2, and the measurement was performed after 24 h. Spectra acquired in experiments with the samples calcined at 150, 300, and 500 °C are shown for illustration. Exclusively anatase calcined at 500 °C exhibited spontaneous isotope exchange activity. The intensities of CO2 isotopologues combination bands ν1 + ν 3 and 2ν2 + ν3 are simulated using the HITRAN database21 and the Winprof 2000 program.22

In the experiment, we contacted 1 Torr of gaseous C18O2 with 1 g of Ti16O2 anatase (001) calcined in the temperature range of 100−500 °C. For illustration, spectra of the ν1 and ν3 combination bands of CO2 measured after 24 h are depicted in Figure 1 for experiments with anatase (001) calcined at 150, 300, and 500 °C. In the experiments with the samples calcined under 500 °C, the isotopic composition of gaseous C18O2 remained unchanged, and bands of C16,18O2 and C16O2 are not B



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Up to 200 °C, the surface area increases as the result of the HF release. At higher temperatures, the surface area decrease is very slow. Evidently, the sintering of anatase particles and the isotope exchange are still hindered due to the surface contamination. A detailed transmission electron microscopy (TEM) study of the as-synthesized and calcined samples was carried out to observe the crystal morphology of the anatase crystals and to reveal a reason for the hindered isotope exchange. The as-synthesized sample is composed of two phases, tetragonal anatase TiO2 and cubic TiOF2, as confirmed by electron diffraction and EDS analysis (Figures 2 and 3). The bulky crystals approximately 100 nm in size belong to the TiOF2 phase (Figure 2a,c), while anatase forms very thin platelets with a lateral size of approximately 100 nm and a thickness of up to 10 nm (Figure 2d). After calcination, the sole phase present in the sample is anatase in the form of slightly thicker platelets retaining their lateral size of approximately 100 nm (Figure 2b). Electron diffraction confirms the phase purity of the calcined sample (Figure 3). However, some intensity discrepancies are observed compared to the intensities calculated for a randomly oriented polycrystalline sample. These differences are due to preferred orientation of crystals (which is to be expected for platelets). Diffraction lines 004, 103, and 105 are strongly suppressed, which agrees well with the platelets flattened perpendicular to (001) faces. In addition, an extra peak was observed at 1.73 Å (k = 36.3), which belongs to the anatase 202 diffraction index (normally almost extinct).

present in the spectra. In the case of anatase (001) calcined at 500 °C, the isotope exchange started immediately after the sample was contacted with C18O2. The spectra are shown in Figure 1, and all the bands of individual isotopologues are compared with the simulation using the Winprof 2000 program22 and data taken from the HITRAN database.21 Obviously, despite extensive washing during synthesis, the surface of the anatase (001) as made or calcined at temperature below 450 °C in air or below 500 °C in vacuum is still contaminated by some HF residues adsorbed on the surface. The strong HF release is observed during the evacuation process preceding the nitrogen adsorption measurements. A nearly negligible decrease of the BET surface area with the calcination temperature (Table 1) is a consequence of its adsorption on the TiO2 surface. Table 1. Calcination Temperature Dependence of Anatase (001) BET Surface Area temperature (°C)

BET surface area (m2 g−1)

100 150 200 250 300 350

73.6 77.0 77.2 76.0 73.4 70.3

Figure 2. TEM/EDS observations: (a) bright-field TEM image of the as-synthesized sample (SAED in the inset), (b) bright-field TEM image of the calcined sample (SAED in the inset), (c) EDS analyses of areas indicated in (a) (1-TiOF2, 2-TiO2), (d) morphology of anatase crystals from the assynthesized sample. C



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Figure 3. Electron diffraction of as-synthesized and calcined samples (markers: red, anatase; blue, TiOF2). A strong peak (green marker) at 1.73 A (k = 36.3) in the calcined sample belongs to anatase 202 diffraction index and is only present due to dynamical diffraction effects.

This diffraction line is strongly enhanced by dynamical diffraction effects, which are commonly associated with electron diffraction of perfect crystals. Residual HF adsorbed on the surface of the moderately calcined anatase (001) sample together with still undecomposed TiOF2 impurities obviously almost completely blocks isotope exchange. The decrease of the reactant C18O2 partial pressure depicted in Figure 4, panels A, B, and C, indicates that its concentration drops to almost zero within the time domain of the measurement. The second reactant, Ti16O2, is in a strong excess, (1 g of TiO2 represents 12 mmol, while 1.0 Torr of CO2 in the working volume of 100 mL corresponds to ca. 6 μmol). At these conditions, the reaction rate can be fitted to the firstorder kinetics with only one effective rate constant, kI(eff) (s−1). This model allows a very clear comparison between the C18O2/ Ti16O2 isotopic exchange rates to be carried out for the studied samples, anatase (001) and anatase (101) calcined at the temperature of 500 °C. The fitting process treats the global reaction C18O2 → C16O2

This equation takes into account that the product is partly in the gas phase and partly adsorbed on the surface; i.e., the ratio p(18) 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)

The results of the fitting procedure using the eqs 3 and 4 are plotted in Figure 4. In the case of anatase (001), panel A depicts the experimentally measured C18O2 partial pressure decrease (red circles) together with results of the fitting (red solid line) using eq 3. Conversely, an increase in the C18O2 (black squares) is observed, and it is fit (black line) using eq 4. The first-order effective rate constant was fitted kI(eff) = 9.80 × 10−4 s−1. This measurement has been repeated under similar conditions upon irradiation with a 366 nm broadband UV lamp. These results are depicted in the panel B. The effective rate constant with UV treatment reaches kI(eff) = 1.67 × 10−3 s−1. It is clearly visible that UV photons accelerate the process. The experimental results of C18O2 contacted with anatase (101) are shown in the panel C. In this experiment, the firstorder rate constant of the C18O2 conversion is the highest, reaching kI(eff) = 5.77 × 10−3 s−1. Both fits together with the corresponding reaction halftimes are compared in Figure 5.

(1)

by describing it with the appropriate differential equation −

dp(18) (t ) dt

= kI(eff) × dp(18)(t )

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CONCLUSION Our explorations of the activity and material properties of anatase (001) TiO2 material prepared using titanium(IV) butoxide and hydrofluoric acid have shown that O-isotope exchange between CO2 and TiO2 is blocked in anatase (001) calcined below 500 °C. This occurs both by residual HF adsorbed on its surface and by TiOF2 impurities. The presence of TiOF2 was proven by TEM and X-ray diffraction, and to the best of our knowledge, its presence in moderately calcined anatase (001) prepared by the standard procedure (hydrothermal hydrolysis of Ti butoxide in the presence hydrofluoric

(2)

where p(18) is the corresponding partial pressure of C18O2. 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)

(3) 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 ] (4) D



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Figure 5. Comparison of conversion curves fit of photoinduced (1 W soft UV broadband source, λmax = 366 nm) and spontaneous oxygen isotope exchange on the anatase (001) sample (C16O2 concentration in black, C18O2 concentration in red). In contrast to the spontaneous exchange (solid curves), kinetic fit of the photoinduced reaction (dashed curves) exhibits a shorter halftime and 100% desorption of the product.

rate of isotopic exchange can be accelerated by UV illumination. However, the effective rate constant is still 3.45 times lower than that for spontaneous exchange between anatase (101) and C18O2. Obviously, the release of HF adsorbed on the surface of moderately calcined anatase (001) and the decomposition of TiOF2 impurity are complete at temperatures higher than 500 °C.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Grant Agency of the Czech Republic (Contract No. P108/12/0814) and by the Ministry of Education, Youth and Sports of the Czech Republic, projects nos. LD14115 and LD 13060 in the framework of the COST Action CM1104.

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Figure 4. Comparison of conversion curves for the isotope exchange on the anatase (001) surface (panels A and B) and anatase (101) surface (panel C). C16O2 concentration is represented by red dots, its fit by the red line; C18O2 concentration is represented by black squares together with its fit shown as a solid black curve. Spontaneous conversion of C18O2 on the anatase Ti16O2 (001) calcined at the temperature of 500 °C is depicted in the panel A and compared with the photoinduced process on the same material in the panel B. Panel C shows spontaneous conversion of C18O2 on the anatase Ti16O2 with the (101) surface measured in separate experiments.

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Spontaneous and Photoinduced Conversion of CO2 on TiO2 Anatase

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