Spontaneous Oxygen Isotope Exchange between Carbon Dioxide and

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Spontaneous Oxygen Isotope Exchange between Carbon Dioxide and Oxygen Containing Minerals (Do the Minerals "breathe" CO? 2

Svatopluk Civis, Milan Bousa, Arnost Zukal, Antonín Knížek, Petr Kubelík, Petr Rojík, Jana Novakova, and Martin Ferus J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11306 • Publication Date (Web): 09 Dec 2015 Downloaded from http://pubs.acs.org on December 9, 2015

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The Journal of Physical Chemistry

Intended Submission:

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Spontaneous Oxygen Isotope Exchange between Carbon Dioxide and Oxygen Containing Minerals (Do the Minerals "breathe" CO2? Svatopluk Civiš,1,* Milan Bouša,1 Arnošt Zukal,1 Antonín Knížek,1 Petr Kubelík,1 Petr Rojík,2 Jana Nováková1 and Martin Ferus1 1

J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic Dolejškova 3, CZ18223 Prague 8, Czech Republic. 2

Sokolovská uhelná, právní nástupce, a.s. Staré náměstí 69, CZ35601 Sokolov

ABSTRACT: The spontaneous isotopic exchange of oxygen atoms between dry powdered Ti16O2-containing minerals and gaseous C18O2 was studied using the gas phase high−resolution Fourier transform infrared absorption spectroscopy (FTIR) of carbon dioxide isotopologues. The absorption rovibrational spectra of all the measured carbon dioxide isotopologues were assigned and then used for the quantification of the time−dependent isotope exchange of oxygen atoms (16O) from the surface crystalline lattice of the solid mineral samples with (18O) oxygen atoms from gaseous C18O2. Similar to our previous studies devoted to the isotopic exchange activity of titanium dioxide, we determined that rutile, montmorillonite, siderite, calcite and basaltic minerals also exhibit unexpectedly significant oxygen mobilities between solid and gas phases. The rate of formation of gaseous C16O2 is found to be highly dependent on the nature of the mineral sample. Our previous studies together with the results presented here suggest that such crystal-surface oxygen isotope mobilities can be explained by two mechanisms: the cluster−like structure of finely powdered materials or the existence of oxygen deficiency sites in the structure of the surface crystal lattice.

Corresponding author: Prof. Svatopluk Civiš, E−mail: civis@jh−inst.cas.cz, Tel.: +420 26605 3275.

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1. INTRODUCTION Several papers report on laboratory studies of carbon dioxide adsorption on oxygencontaining minerals,1 soil and carbonate minerals,2,3 different types of oxides,4,5,6 titanium dioxide, magnesium vanadate7, magnesium oxide,8 aluminum oxide,9 clays,10 nontronite, palagonite and basalts including different types of mineral substrates, which all to some extent mimic the planetary soil surface of Mars (Martian regolith simulations).11,12,13,14 Laboratory measurements of CO2 and H2O adsorption indicate that such minerals or rocks exhibit a high efficiency for storing large amounts of adsorbed gas. This capacity reflects their extremely large internal surfaces.14,15, 16 Studies of lunar soil particle sizes reveal the pulverization of surface minerals by frequent impacts.11 Similar processes can be expected in the case of Mars.17 However, the impact flux on Mars may have been up to 25 times higher than that on the Moon, and all processes involving meteorite ablation, combustion and pulverization of the Martian surface are likely to have produced a fine grained regolith with a high surface area18,19 and also influenced the planetary impact plasma chemistry.20,21,22,23,24 There are two main questions related to the Martian regolith:

1. Was there ever sufficient CO2 in the Martian atmosphere to engender a greenhouse effect of sufficient magnitude to raise the surface temperature above 0 °C? Based on laboratory measurements of CO2 adsorption on basalt and nontronite,25 Fanale and Cannon26 suggested that at least 10 times more CO2 is adsorbed onto regolith than is present in the atmosphere, which means that regolith may have played an important role in the atmospheric evolution of Mars.

2. What causes the

12

C/13C ratio to be as it is? Throughout the scientific community,

there is a high interest in the experimental constraints on the fractionation of 13C/12C and 18O/16O ratios due to the adsorption of CO2 on mineral substrates at conditions relevant to those on the surface of Mars.27These constraints are significant for two reasons: explaining subtle differences in the

13

C/12C ratio in atmospheric gases and

other materials from Earth and Mars and guiding future research into the role of CO2

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absorption on the surface and its effect on the dynamics of the atmosphere over climatic, seasonal, or diurnal time scales.

In a series of our previous papers, 28, 29, 30, 31, 32, 33, 22, 34, 30, 35 we report a wide range of experiments and theoretical studies devoted to the interaction of carbon dioxide with the anatase titania surface. In a previous publication, we discovered that the solid anatase surface spontaneously exchanges oxygen atoms (labelled

16

O,

17

O and

18

O) with gas phase

CO2. The reaction rate of this exchange strongly depends on the character and structure of the solid sample. In the case of anatase prepared from TiCl4, the annealing temperature before the experiment plays a fundamental role. Experiments that were introduced in our first studies imply that a calcination temperature lower than 200 °C is not sufficient to remove traces of hydrogen chloride from the crystalline structure of titania, and such a sample does not exhibit a CO2 exchange activity. Nevertheless, the irradiation of such a sample using UV sources (broadband UV lamp or a XeCl laser) in the presence of water leads to the reduction of carbon dioxide to hydrocarbons (CH4 and C2H2).28 The vacuum calcination of titania at temperatures of approximately 450 °C leads to the removal of hydrochloric acid from the titanium surface and the formation of several types of defects in the titanium lattice. These surface defects (oxygen vacancies) result in surface reactions between TiO2 and CO2. The oxygen−exchange activity of vacuum−annealed TiO2 towards C18O2 clearly stems from surface oxygen vacancies and the corresponding Ti4+/Ti3+ exchange.6 We discovered that titania nanoparticles exhibit a much faster isotope exchange activity, even at room temperature. However, the reactivity is influenced by another important factor. In such a case, under−coordinated sites on the surface of nano-grains play a role in the oxygen mobility between gas and solid phases. In the case of the ideal (101) anatase, both five− (Ti(5c)) and six−fold coordinated (Ti(6c)) atoms are present at the surface. The former are further reduced to four−fold coordinated (Ti(4c)) atoms when oxygen defects are present on the surface. These under-coordinated sites have been shown to enhance the CO2 reduction kinetics 36 and to facilitate the oxygen exchange between CO2 and the surface.37 Most importantly, the density of Ti(4c) sites can be appreciable in nm−sized TiO2 nanoparticles.38 In the current paper, we present the results of experimental research on the exchange between CO2 and the surface oxygen atoms of several powdered minerals using the Page 3 of 24



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previously developed technique of high-resolution infrared spectroscopy of CO2 isotopologues. We detected gaseous products from the reaction of CO2 with the surfaces of natural minerals and synthetic samples containing TiO2: Natural anatase from the region Hordaland in Norway, natural rutile from the locality of Golčův Jeníkov in the Czech Republic, synthetic anatase (prepared by the hydrolysis of TiCl4, see ref.29), synthetic rutile (Bayer 5556) and clay containing a small amount of anatase (4−6%) deposited in the Sokolov Coal Basin in the Czech Republic. Due to the high reactivity (oxygen exchange) between such minerals and CO2, we have decided to also study the surface activity of selected parent minerals, i.e., crushed basalt (Rožňava, Slovak Republic), montmorillonite (Sigma Aldrich), natural calcite, siderite and silica. Contrary to several studies that build on the idea that isotopic fractionation occurs between CO2 vapor and CO2 adsorbed on an oxygen substrate, we have designed our experiments to observe the direct interaction of gaseous C18O2 with the surface oxygen of the studied minerals.

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2. EXPERIMENTAL SECTION 2.1 Sample preparation The synthesis of anatase Ti16O2 was carried out in a closed all−glass vacuum apparatus. Titanium tetrachloride (99.98% Aldrich) was twice distilled in the vacuum before use. One gram of H216O was frozen out in high vacuum by liquid nitrogen, and the ice was introduced to 2.8 ml of TiCl4 vapor through a glass−breakable valve. After mixing both reactants, the cooling bath was removed, and the reaction mixture was left at room temperature overnight. The produced HCl was collected in a side ampoule that was cooled by liquid nitrogen. Subsequently, the solid product was heated at 100°C overnight in the still−closed vacuum apparatus, while the HCl trap remained in the liquid nitrogen cooling bath. The HCl−trap was then sealed off. Please note that the cooling of HCl must not be interrupted to prevent hazardous overpressure in the trap because the volume of gaseous HCl formed in this particular case is approximately 2 L! Finally, the apparatus was opened in a glove box under Ar, and the solid white powder was collected. The material was stored under Ar at room temperature. In the next synthetic step, a portion of the powder was heated at 450°C in vacuum (10−5 Torr) for 5 h. Crystals of natural minerals were ground into a very fine powder using a mill−stone. However, the particle size is then indefinite and unknown. Crystals of TiO2 anatase from Norway, and basalt, siderite and natural clays from Sokolov have been treated correspondingly. Prior to any measurement, the samples were degassed and dried in vacuum (10−5 Torr) at a temperature of 100°C for six hours to enhance the degassing and drying process. We would like to note that the temperature of 100°C is not sufficient to make any change in the surface structure. Selected samples were also annealed in vacuum at a temperature of 450°C. This higher temperature is sufficient to create oxygen vacancies or other defects on the surface and to sinter the grains. In such a case, a different chemical reactivity can be observed.

2.2 Methods FTIR Spectroscopy FTIR spectra were measured in a 30−cm long (2.5 cm in diameter) glass optical cell equipped with CaF2 windows. The cell was interfaced to a sealable glass−tube joint for the in vacuo

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transfer of the powder material from a side ampoule, in which the annealing and degassing also took place (In certain experiments, this in−situ annealing was avoided. In this case, the optical cell was simply loaded with the sample and evacuated). The optical cell was further equipped with a second vacuum valve (ACE glass, USA) for the gas handling and the connection to the vacuum line. The optical cell containing approximately 2 g of sample was filled with C18O2 (97 % 18O, MSD Isotopes, Montreal, Canada, CAS 124−38−9). The pressure in the measuring cell was between 1 and 2 Torr and was precisely measured with an MKS Baratron pressure gauge (0–10 Torr). The spectral measurement was performed using a Bruker IFS 125 HR spectrometer (with a KBr beam splitter and a nitrogen cooled InSb detector) in the spectral range of 1800 − 6000 cm−1. The spectra were measured with a resolution of 0.01 cm−1 using the Blackmann−Harris apodization function.39 Concentrations of the various CO2 isotopomers were determined by independent calibration using pure gases of defined isotopic ratios (carbon dioxide, 97 %

18

O, 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 select individual absorption lines were calculated using the OPUS 6.0 software package,40 and the data were subsequently fit by a linear regression. BET Surface Area Measurement The surface areas of the prepared materials were determined from nitrogen adsorption isotherms. The isotherms were recorded with an ASAP 2020 (Micromeritics, United States) volumetric instrument. To achieve the necessary accuracy for the accumulation of adsorption data, the instrument was equipped with three pressure transducers (13.3 Pa, 1.33 kPa and 133 kPa). The fresh samples were degassed by starting at the ambient temperature and heating up to 80 °C (temperature ramp of 0.5 °C/min) until a residual pressure of 1 Pa was attained. After further heating at 80 °C for 1 h, the temperature was increased up to 120 °C (temperature ramp of 1 °C/min). At this temperature, degassing continued under a turbomolecular pump vacuum for 8 h. Adsorption isotherms of nitrogen were then recorded at T = −196 °C using a bath of liquid nitrogen. The surface areas of the TiO2 samples were calculated from the nitrogen adsorption data in a range of relative pressures (0.075 − 0.25) using the BET method. The Iso-Therm thermostat (e-Lab Services, Czech Republic) was used to maintain the temperature of the sample with an accuracy of ± 0.01 °C and was used for the measurement of

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carbon dioxide adsorption at 20°C. As the adsorption isotherms of carbon dioxide were determined on the same sample immediately after the nitrogen adsorption measurement, the degassing procedure was performed at 120 °C for 8 h with a turbomolecular vacuum pump. The amount of adsorbed CO2 on each sample was expressed in terms of cm3 STP per m2 of surface area calculated using nitrogen isotherm.

Scanning Electron Microscopy (SEM), Energy−Dispersive X−Ray Spectroscopy (EDX) and X−Ray Diﬀrac7on (XRD) Characteriza7on of Natural Clay Samples SEM and EDX analyses of natural clay samples were carried out using a Hitachi field−emission−scanning electron microscope S−4800 equipped with the Noran EDX system. An acceleration voltage of 25 kV was used with a nitrogen cooled semiconductor detector equipped with a beryllium window. XRD was measured using a Bruker D8 Advance diffractometer using Cu Kα radiation. Oxygen Exchange Detection using Mass Spectrometry The isotope exchange was also measured using a Balzers QMG 420 quadrupole mass spectrometer (MS). The gases used were either 18O2 oxygen (Campro Sci, 97 at. %) or C18O2 (MSD Isotopes, 97 at. %). The solid sample (1 g) was placed into a small quartz reactor (37 cm3 + 250 cm3 of connective volume). Gas reservoirs and an MKS Baratron pressure gauge were connected to the same vacuum line. The whole apparatus was first evacuated using an auxiliary vacuum system of turbo and rotary pumps. A sapphire valve was used to leak 2.2 Torr of the gases into the MS, which was evacuated by turbo and membrane pumps. The studied sample (approximately 80 mg) was evacuated prior to interaction with the gas.

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3. RESULTS AND DISCUSSION 3.1. Synthetic and Natural Anatase In all of the experiments, the isotopic exchange between C18O2 and Ti16O2 was monitored using the fundamental, overtones or a combination of CO2 bands. Figure 1 presents an example of middle infrared spectra within the regions of the fundamental ν3 together with ν1 + ν3 and ν1 + 2ν3 combination bands. The red spectrum in Figure 1 depicts the spectrum of C18O2 (at a pressure of 1 Torr) in the absence of natural titania. The black spectrum represents CO2 after 15 000 s of contact with natural anatase calcined at a temperature of 450°C (N−A450 sample). All of the C18O2 is completely converted to C16O2 whose partial pressure again reached 1 Torr. Quantification was performed using the calibration described in the Methods. According to all of the results, both

Figure 1.. Infrared spectra of the CO2 isotopologue not in contact with titania (red) and after the contact of 18

16

C O2 with Ti O2 in phase equilibrium (black). The partial pressure of CO2 reaches 1 Torr in both cases. The spectra are assigned according to ref.

41

natural anatase and the synthetic material exhibit an exchange activity. A comparison between natural and synthetic anatase conversion curves is depicted in Figure 2. Panel A depicts a decrease in C18O2 and an increase in C16O2 isotopologue partial pressures during contact with the solid synthetic anatase sample calcined at a temperature of 100°C (referred to as A100; blue squares mark the partial pressure profile of C16O2, while blue circles profile

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the partial pressure of C18O2). The conversion of C18O2 on synthetic anatase calcined at a temperature of 450°C (referred as A450) is marked in red. Panel B shows the results of a similar experiment with natural anatase calcined at temperatures of 100°C and 450°C (referred to as N−A100 and N−A450, respectively).

18

16

Figure 2. Comparison of the C O2 oxygen exchange activities of non−calcined and calcined samples of Ti O2 is shown for synthetic anatase in panel A and for natural anatase in panel B.

The conversion curves of the C18O2 partial pressure decrease together with the C16O2 increase were fit using a pseudo−first order approximation, as described in our previous works, to obtain the pseudo−first order rate constants. Briefly, because in all of the

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experiments the C18,16O2 reaction intermediate partial pressure reaches at most 3 %, the mechanism can be simplified by the following equation: C18O2 → C16O2

(1)

This simplified mechanism can be described with the appropriate differential equation: − d p(18) (t) / dt = kI(eff) × p(18) (t)

(2)

18

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

(3)

and a function of the partial pressure of the product (C16O2), p(16) : p(16) (t) = [ p(18) (t=0) − p(18) (t=0) × exp(−kI(eff) × t)] × p(16) / p(tot)

(4)

This equation takes into account that the product is partly in the gas phase and partly adsorbed on the surface, that is, it includes the ratio, p(16)/p(tot), of the total partial pressure of the product p(tot) formed during the reaction and the real partial pressure of the product in the gas phase p(16). The results of the pseudo first-order fits are depicted in Figure 2 for synthetic anatase A100 (blue) and A450 (red) samples (Panel A), together with natural anatase N−A100 (blue) and N−A450 (red) samples (Panel B). In comparison to the previously studied synthetic material, the natural anatase from Norway also exhibits a significant isotope exchange activity. Although the timescales of isotopic exchange are longer here than in case of the synthetic material, the exchange activities of both natural and synthetic samples are similarly increased by calcination. This effect is caused by the formation of vacancies in the surface layers of the crystal. To our knowledge, we are reporting for the first time the CO2 oxygen exchange mobility between a natural anatase mineral sample and gaseous carbon dioxide. However, this activity is well known for synthetic anatase, and this effect is thoroughly studied and explained on the quantum chemical level in our previous works. We also decided to test the oxygen mobility between C18O2 and another crystalline form of Ti16O2: rutile.

3.2. Synthetic and Natural Rutile The experiments with rutile TiO2 were again focused on the exploration of prospective isotopic exchange activities and their distribution between synthetic and natural samples. We have used two samples: high surface area synthetic rutile (a non−calcined sample of

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rutile from Bayer, type 5556, dried at temperature of 100°C and labelled R100) and a natural geological sample from the locality of Golčův Jeníkov (eastern parts of Czech Republic, two samples, dried at 100°C and labelled N−R100 and calcined in a furnace at 450°C and labelled N−R450). Rutile is found there in veins of orthoamphibolite, in quartz veins of paragneisses and in the contact zone between amphibolite xenoliths and the granite from Přibyslavice. We have discovered that all the samples of the rutile form of TiO2 exhibit very fast oxygen exchange. The experimental data and the conversion curves, fitted as described previously, are supplied in Figure 3. Panel A shows the conversion of C18O2 during its contact with nano−particle synthetic rutile (Bayer, type 5556). The material exhibits very fast rate of the isotopic exchange.

Figure 3. Comparison of the C18O2 oxygen exchange activities of non−calcined nanoscale samples of Ti16O2 with synthetic rutile in panel A and natural rutile in panel B.

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Figure 4. Comparison of the CO2 adsorption capacities of natural anatase and natural and synthetic rutile. As was shown in our previous study, this capacity decreases upon calcination of the sample.

When the chamber with C18O2 gas was open, the pressure decreased, and C16O2 was immediately released with a pseudo−first order reaction halftime of 0.09 s. However, the decay of C18O2 corresponds to halftime of 252 s. This disagreement can be explained by very fast initial sorption during filling of the chamber. As shown in Figure 4, this material exhibits a significantly large surface area. During less than 0.1 s, the surface is saturated and the product is released. After the valve is closed, the rest of the reactant C18O2 is adsorbed with slower rate. The behavior can again be explained by the very fine nano−structure of this material. As described in our previous papers, in such a case, the oxygen exchange is not driven by the presence of oxygen vacancies in the surface layers of the crystal lattice, but it can be explained by the existence of large surface area nano−particles. However, the mobility of oxygen atoms between gaseous CO2 and solid TiO2 was also observed in the case of crystalline natural rutile. Here, we show that the existence of active sites similar to crystal defects in anatase can also be expected in the rutile crystal structure42,43

3.3. Montmorillonite and Clay from the Sokolov Coal Basin 3.3.1. Isotopic exchange with CO2 explored using FTIR Spectroscopy. In the exploration of natural sample isotopic exchange activities, we also focused our effort on more fundamental samples containing TiO2. Samples of clay−containing TiO2 were taken from the Družba Quarry, Sokolov, from the former pottery clay pumping station depositions. Page 12 of 24


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The geological location is described as Sokolov formation, the Habartov strata. A macroscopic inspection of this sample reveals a silty clay with volcanogenic admixtures, bioturbed (from fossil soil) and permeated with coalificated plant roots. Its color is grey. The sample was characterized by X−ray diffraction analysis. The spectra are shown in Figure 5. From a mineralogical point of view, it is composed mainly from clay (kaolinite type) with an anatase admixture, possible rutile traces and siderite. The elemental composition is shown in the EDS spectra in Figure 6 and Table 1.

Figure 5. X−ray diffraction analysis of clay from the Sokolov coal basin. Anatase together with kaolinite and siderite were identified as the main constituents of the sample.

Figure 6. EDX analysis of elements in the sample of clay from the Sokolov coal basin. Among others, it is clear that this material contains Ti.

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Table 1: Results of the EDX elemental analysis of clay from the Sokolov coal basin. Among other elements, 1.18 % Ti is also present in the sample.

18

Figure 7. Comparison of the C O2 oxygen exchange activities between non−calcined and calcined samples of the Sokolov Coal Basin clay (Sample 1) (A) and montmorillonite (Sigma Aldrich) (B).

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Figure 7, panel A shows the conversion of C18O2 to C16O2 in the experiment with the Sokolov clay sample. However, in contrast with the anatase and rutile samples, calcination at a temperature of 450°C (sample labeled S450) leads to an inverse effect. Compared with the non−calcinated sample labeled S100, the pseudo−first order reaction halftime of S450 is longer for the calcinated sample. Such a difference in activities between calcinated and non−calcinated samples can be explained by an exchange mechanism that is not based on the existence of vacancies but, again, on the activity of very fine nano−particles. This effect can be observed by comparing the Sokolov clay activity with that of the neat montmorillonite. Our results show that the material without any titanium, neat montmorillonite, exhibits exchange activity. We suppose that calcination of both clay and montmorillonite leads to the sintration of the materials. During the sintration, the particle size increases, while adsorption capacity decreases. This change in activity is demonstrated in Figure 8. This decrease in adsorption capacity was also exhibited by anatase in our previous study. However, as mentioned above, the exchange activities of anatase and rutile increase during calcination because it leads to the formation of vacancies; therefore, the activity in those samples is not dependent on the sintration.

Figure 8. Comparison of neat and calcinated, synthetic and natural clay adsorption capacities. The neat samples are marked in blue, the calcinated in red. Empty circles correspond to montmorillonite M100 and M450 respectively.

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3.32 The Isotopic exchange between oxygen molecules and the montmorillonite surface studied by mass spectrometry The method of mass spectrometry was used for testing the interaction (oxygen exchange process) between molecular oxygen and the montmorillonite surface. This method was applied because molecular oxygen cannot be directly monitored using the infrared absorption technique. Additionally, our previous infrared measurements of the CO2-oxygen exchange process have also been verified. 18

O2 Interaction

The montmorillonite sample (85.4 mg) was pretreated in vacuum at 175°C. Then, 2.2 Torr of 18

O2 was allowed to interact with the sample at temperatures ranging from room

temperature to 175°C. No isotopic exchange was found at (and below) these temperatures.

C18O2 Interaction Contrary to the exchange with 18O-labeled molecular oxygen, the exchange of oxygen from C18O2 readily proceeds even at room temperature, as is depicted in Figure 9, which shows the time dependence of the decrease in the amount of the

18

O-labeled carbon dioxide

molecular ion (C18O2) and the increase in the ion current of molecular ions C18,16O2 and C16O2. This panel represents an experiment with 81.9 mg of montmorillonite at room temperature and with 2.2 Torr of C18O2. The results represent an increase of C18,16O2 , which is considered to be an intermediate isotopologue of considered complete C16O2 to C18O2 conversion referred in our previous studies (currently e.g.33) employing FTIR detection of the products.

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Figure 9. The plot shows ion currents over a 120 min timescale when C18,18O2 was in contact with the montmorillonite sample.

When the temperature was increased to 150°C, a mild additional exchange occurred. The18O contents in the gas phase decreased by another 5%, but the equilibrium was not affected.

3.4. Basalt, Siderite, Silica and Calcium Carbonate The isotopic exchange activities of natural clays from Sokolov were compared with several selected samples of minerals: basalt (locality - Císařský Quarry, Šluknov, north of the Czech Republic), siderite (locality - Rožňava, Slovak Republic), calcium carbonate (powder, Sigma Aldrich) and silicon dioxide (powder, Sigma Aldrich). Again, all the samples were dried at a temperature of 100°C to remove humidity and subsequently placed into contact with C18O2. Whereas basalt, siderite and calcium carbonate exhibited oxygen mobility, silicon dioxide is inert and C18O2 was not converted to C16O2 over a timescale of one week. A comparison of the pseudo−first order reaction halftimes shown in Figure 10, panels A – C and the adsorption capacities of the individual minerals depicted in Figure 11 demonstrates that minerals with different chemical compositions exhibit different isotopic exchange rates, which are independent of their absorption capacities, while silicon dioxide is an inert material. This effect reveals that the isotopic exchange activity of an individual material is not only a function of the vacancy frequency and the particle size, as shown previously in the cases of anatase, rutile and clays, but it also depends on the individual chemical

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composition. For instance, in the case of calcite, the effect is evident by the fact that this material already contains (CO3)2-, which may also play the role of a reaction intermediate complex in the oxygen exchange mechanism.33 A mechanistic empirical view of oxygen mobility based on molecular dynamics and its dependence on mineral type, temperature and the existence of defect sites are discussed in the following chapter.

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Figure 10. Oxygen isotopic exchange over the surfaces of natural minerals. The conversion over basalt is depicted in panel A, siderite in panel B and calcite in panel C.

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Figure 11. Comparison of the natural mineral CO2 adsorption capacities.

4. CONCLUSIONS Table 2 shows the conclusion of the systematic screening of natural anatase TiO2, natural rutile, clays from the Sokolov coal basin and several commercial and natural materials and minerals isotopic exchange activities. All of the natural and synthetic samples, except SiO2, exhibited oxygen mobility. Additionally, to examine the effect of calcination, several samples have also been heated in a furnace under vacuum at a temperature of 450°C for several hours. Table 2: Results of all the systematic screenings of isotopic exchange activities

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In our previous studies, it was shown that the rate of isotopic exchange depends on a) the number of defect sites in surface layers of crystal lattice b) the existence of nano−sized structures in the surface of studied material We also demonstrated that the rate of isotopic exchange in an individual material can be enhanced or suppressed by calcination. When the isotopic exchange is caused by defect sites, the calcination leads to an increase in the oxygen mobility between the solid and gas phases. For instance, this effect is visible as an increase in oxygen exchange rate constants for anatase (A100 and A450) and rutile (R100 and R450) upon calcination. If the oxygen exchange is caused by the particle size, then the sintration during the calcination process leads to the formation of larger and less active aggregates and subsequently to the suppression of this activity. Such behavior was observed for clay from Sokolov (S100 and S450) and montmorillonite (M100 and M450). The inactivity of quartz can be explained by a significantly large band gap, when compared with the energies of other materials. The band-gap energies were found to be almost similar for anatase and rutile, i.e., 3.85 eV and 3.93 eV, respectively. Larger band gaps are referenced for montmorillonite (5.35 eV), and carbonate minerals siderite (4.4 eV) and calcite (6 eV), which also exhibit lower activities. However, it is clear that the dependence of the isotopic exchange rate on the band gap energy is not strict because of the existence of other effects (e.g., the number of vacancies and the number of nano−particles). Still, the band gap energy of SiO2 (8.4 eV) is far removed from those found for other minerals. The lattice potential energies of anatase and rutile are found to be 12 962 and 13 347 kJ / mol, respectively. These values are comparable with the 12 535 kJ / mol found for quartz. Because this material is inactive, it implies that the rate of the isotopic exchange is not related to the lattice potential energy. In support of this argument, the activities of natural anatase and siderite are comparable, whereas the lattice potential energy of siderite reaches 25 494 kJ / mol.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Education, Youth, and Sports of the Czech Republic (COST Action CM1104, grants No. LD14115 and LD13060) and by the Grant Agency of the Czech Republic (contract No. 14-12010S).

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Infrared spectra of the CO2 isotopologue not in contact with titania (red) and after the contact of C18O2 with Ti16O2 in phase equilibrium (black). The partial pressure of CO2 reaches 1 Torr in both cases. The spectra are assigned according to ref.36 82x36mm (300 x 300 DPI)



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Comparison of the C18O2 oxygen exchange activities of non−calcined and calcined samples of Ti16O2 is shown for synthetic anatase in panel A and for natural anatase in panel B. 80x56mm (300 x 300 DPI)


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Comparison of the C18O2 oxygen exchange activities of non−calcined and calcined samples of Ti16O2 is shown for synthetic anatase in panel A and for natural anatase in panel B. 82x58mm (300 x 300 DPI)



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Comparison of the C18O2 oxygen exchange activities of non−calcined nanoscale samples of Ti16O2 with synthetic rutile in panel A and natural rutile in panel B. 80x58mm (300 x 300 DPI)


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Comparison of the C18O2 oxygen exchange activities of non−calcined nanoscale samples of Ti16O2 with synthetic rutile in panel A and natural rutile in panel B. 80x56mm (300 x 300 DPI)



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Comparison of the CO2 adsorption capacities of natural anatase and natural and synthetic rutile. As was shown in our previous study, this capacity decreases upon calcination of the sample. 80x55mm (300 x 300 DPI)


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X−ray diffraction analysis of clay from the Sokolov coal basin. Anatase together with kaolinite and siderite were identified as the main constituents of the sample. 80x46mm (300 x 300 DPI)



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EDX analysis of elements in the sample of clay from the Sokolov coal basin. Among others, it is clear that this material contains Ti. 80x53mm (300 x 300 DPI)


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Comparison of the C18O2 oxygen exchange activities between non−calcined and calcined samples of the Sokolov Coal Basin clay (Sample 1) (A) and montmorillonite (Sigma Aldrich) (B). 80x59mm (300 x 300 DPI)



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Comparison of the C18O2 oxygen exchange activities between non−calcined and calcined samples of the Sokolov Coal Basin clay (Sample 1) (A) and montmorillonite (Sigma Aldrich) (B). 80x59mm (300 x 300 DPI)


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Comparison of neat and calcinated, synthetic and natural clay adsorption capacities. 80x52mm (300 x 300 DPI)



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The plot shows ion currents over a 120 min timescale when C18,18O2 was in contact with the montmorillonite sample. 80x55mm (300 x 300 DPI)


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Oxygen isotopic exchange over the surfaces of natural minerals. The conversion over basalt is depicted in panel A, siderite in panel B and calcite in panel C. 80x58mm (300 x 300 DPI)



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Comparison of the natural mineral CO2 adsorption capacities. 80x57mm (300 x 300 DPI)


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TOC 39x29mm (300 x 300 DPI)


Spontaneous Oxygen Isotope Exchange between Carbon Dioxide and

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