Tuning the Nature of the Anion in Hydrated Layered Double

Subscriber access provided by University of South Dakota

Article

Tuning the Nature of the Anion in Hydrated Layered Double Hydroxides for H2 Production under Ionizing Radiation Maxime LAINE, Yuan Yuan LIAO, Fanny VARENNE, Pierre Picot, Laurent J. Michot, Elodie Barruet, Valérie Geertsen, Antoine Thill, Manuel Pelletier, Jean-Blaise Brubach, Pascale Roy, and Sophie Le Caer ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01240 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 9, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Nano Materials

Tuning the Nature of the Anion in Hydrated Layered Double Hydroxides for H2 Production under Ionizing Radiation Maxime Lainé,1 Yuanyuan Liao,1 Fanny Varenne,1 Pierre Picot,1 Laurent J. Michot,2 Elodie Barruet,1 Valérie Geertsen,1 Antoine Thill,1 Manuel Pelletier3, Jean-Blaise Brubach,4 Pascale Roy,4 and Sophie Le Caër1* 1 LIONS, NIMBE, UMR 3685, CEA, CNRS, Université Paris-Saclay, CEA Saclay, F-91191 Gif-sur-Yvette Cedex, France 2 Laboratoire Phenix Sorbonne Universités, UPMC Université, Paris 06, CNRS, UMR 8234, Paris F-75005, France 3 LIEC, UMR 7360 CNRS, Université de Lorraine, Vandoeuvre-les-Nancy F-54500, France 4 Synchrotron SOLEIL, AILES Beamline, L’Orme des Merisier, Saint-Aubin, BP 48, F-91192 Gif-sur-Yvette Cedex, France *Corresponding author: [email protected] Abstract Layered double hydroxides (LDHs) are adsorbent materials able to store significant amounts of anionic radionuclides under various levels of hydrations. In view of this storage application, it is crucial to understand their evolution under ionizing radiation, particularly as a function of their hydration state. We thus synthesized four LDHs, with Mg/Al molar ratio around 3.5 and four different anions: NO3-, CO32-, ClO4- and Cl- chosen for their contrasted behavior towards ionizing radiation. For all these materials, the initial interlamellar distance increases slightly within the first step of relative humidity (RH), 0-3%, and remains constant in the 3-74% RH range. Infrared spectroscopy reveals a lowering of the anion symmetry when it is confined in LDHs, caused by the interaction with surface OH groups and water molecules together with a decrease of degrees of freedom upon confinement. When RH increases, the environment of the anions becomes more and more symmetric. Under ionizing radiation, the nature of the anion in the interlamellar space and the hydration state control the H2 production. The dry LDH containing nitrate anions leads to very low H2 yields as nitrate anions scavenge precursors of dihydrogen. Materials containing perchlorate (inert towards ionizing radiation) and carbonate (hydroxyl scavenger) anions lead to very similar H2 yields which increase in the 0-3% RH range, and then remain stable in the 3-74% range, with a maximum value approximately half of that obtained for bulk water. This shows that the material itself contains hydroxyl scavengers. The sample containing chloride anions produces the highest H2 amounts, with H2 yields twice higher than the yield measured in bulk water. This peculiar behavior can be linked to specific reactions involving confined chloride anions. This work shows that the anion of the LDH sample can be selected depending on the applications. Keywords: layered double hydroxides; radiolysis; reaction mechanisms; hydration; infrared spectroscopy; storage applications 1 ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. Introduction Nuclear energy currently provides about 11% of the world electricity production, in a relatively clean and cost-effective manner. However, nuclear power plants generate various nuclear wastes, whose management and storage are a major issue. Among the numerous radionuclides produced, radioactive iodine deserves particular attention as it is highly volatile and known for its harmful effects on human health.1 Problems associated with 129I are particularly acute due to the long half time of this isotope (about 107 years). In contrast, 131I is short-lived, with a half time of about 8 days, but is a significant component of hospital radioactive wastes.2 Both radionuclides have then to be appropriately stored.3 In that regard, the behavior of adsorbed iodine is particularly relevant. Numerous materials have been proposed up to now as adsorbents. One may quote zeolite-based materials,4 zerovalent iron,5 illite minerals6 or functionalized clay minerals.7 Developing alternate iodine sorbent materials, able to store higher amounts of radionuclides at low cost, remains however an open research question. Among such materials, layered double hydroxides (LDHs) appear as promising candidates as they are cheap and efficient anion-exchanging materials with high adsorption capacity.8-13 Figure 1 displays a general structural scheme of LDHs. The chemical formula of these layered materials can be written as , where and stand for

metals at +II and +III oxidation state, respectively. is a compensating anion located in the interlamellar space and the number of water molecules in the interlamellar space. The surface of the materials, sometimes referred to as anionic clays, is covered with hydroxyl groups whose hydrogen atoms interact with anions and water molecules. The charge density of the sheets as well as the chemical and geometrical properties of the anions control the interlamellar distance as well as the hydration and swelling ability of these materials.14-17

Figure 1. Layered double hydroxide structure scheme. In the present work, the MII or MIII cations are MgII or AlIII, respectively. The water molecules are not represented on the figure.

2 ACS Paragon Plus Environment

Page 2 of 37

Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Being positively charged, LDHs are efficient adsorbents for anionic species as illustrated in different studies that investigated the removal of anionic azo dyes from aqueous streams by LDHs,18 a key issue for treating waste waters from the textile industries.19-20 Along similar lines, the efficient capture of iodine by LDHs intercalated with polysulfides was also demonstrated recently.21 In the context of the nuclear waste management, adsorption is not the sole issue that must be considered. A crucial one in such applications is related to the behavior of the adsorbent when submitted to ionizing radiation. Whereas this issue is addressed in the literature for the case of cationic clay minerals,22-24 to our knowledge, such information is currently not available for LDH materials. In contrast with cationic clay minerals for which interlamellar cations (Na+, Li+, Ca2+….) are not affected by ionizing radiation, interlamellar anions in LDHs can be reactive towards ionizing radiation. Radiolysis in LDHs is therefore expected to be more complex than their cationic counterparts. In order to evaluate the potential of the materials for the storage of radionuclides (129I for instance), the ideal method would be to insert such nuclides in the various LDHs and to evaluate their evolution over periods of time corresponding to the decay time of the radionuclides. However, such experiment requires special safety conditions that can hardly be met in most laboratories and could involve too long durations. In contrast, using accelerated electrons (beta particles) is a fast and valid alternative approach for investigating the impact of ionizing radiation on the materials. Indeed, radioactive 129I and 131I are respectively generating beta and gamma emissions and beta emissions mostly, which in turn are at the origin of further modifications. Moreover, during this storage of radionuclides for extended periods of time, the water content of the sample can drastically change. It is therefore crucial to apprehend how the relative humidity can affect the behavior of the materials under ionizing radiation. Bearing this in mind, the aim of the present paper is i) to synthesize LDHs with various anions and to study their behavior for increasing relative humidity (RH). For that purpose, a detailed infrared study was performed prior to irradiation. ii) To quantify H2 production, chosen as a probe of the chemical modifications induced by irradiation (see R1) in these materials taking into account both RH and anion nature. Water radiolysis can be written as follows:25-26

, ● , ● , , , ! ,

(R1)

We focused on 4 anions whose behavior under ionizing radiation are significantly different: • ClO4-, which is inert towards ionizing radiation; • NO3-, which is in particular a solvated electron scavenger, the solvated electron being a precursor of H2; • CO32- and Cl-, which are both scavengers of the HO● radical known to react with H2 thanks to the Allen chain (R2-R3):25

• + → + • (R2) 3 ACS Paragon Plus Environment


• + → • + (R3) Therefore, if the Allen chain plays a role in LDHs, then carbonate and chloride anions are expected to lead to an increase of the H2 production, due to their ability to scavenge the hydroxyl radical. In the present work, we will show that LDH-NO3- leads to the lowest H2 yields, and LDH-Clto the largest, while LDH-ClO4- and LDH-CO32- lead to intermediate yields. Therefore, we will mainly focus on LDH-NO3- and LDH-Cl- in the infrared part investigating their properties as a function of hydration.

2. Materials and methods 2.1. Synthesis and measurement of the Mg/Al ratio The synthesis of LDHs was carried out using magnesium and aluminum precursors containing the chosen compensating anions NO3-, Cl- and ClO4- according to the constant pH coprecipitation method widely described in the literature.14, 16, 27-29 Briefly, freshly de-ionized water was used. The various solutions of metal salts were prepared with a total cationic concentration of 1 M and an Al/(Al + Mg) molar ratio of 1/5, using the adequate reagents (Table 1). For LDH containing the NO3-, Cl- and ClO4- anions, the appropriate metal solution was added dropwise thanks to a pressure-equalizing dropping funnel. A 2 M NaOH solution was added simultaneously with another isobaric dropping funnel in order to maintain the pH at a constant value of 10. For the system containing carbonate anions, the same set-up was used, but the pH was maintained at 10 thanks to the continuous addition of an aqueous mixture of 0.2 M Na2CO3 and of 2 M NaOH. Syntheses were carried out at room temperature under argon flow. Table 1. Metallic reagents used in each synthesis together with the corresponding theoretical formula of the obtained LDH. The experimental Mg/Al ratio, measured by ICP-MS, is displayed in the right column. LDH

Magnesium reagent

Aluminum reagent

NO3ClClO4CO32-

Mg(NO3)2.6H2O MgCl2.6H2O Mg(ClO4)2 Mg(NO3)2.6H2O

Al(NO3)3.9H2O Al(Cl3).6H2O Al(ClO4)3.9H2O Al(NO3)3.9H2O

Measured Mg/Al ratio Mg4Al1(OH)10(NO3).yH2O 3.46 Mg4Al1(OH)10Cl.yH2O 3.64 Mg4Al1(OH)10(ClO4).yH2O 3.91 Mg4Al1(OH)10(CO3)0,5.yH2O 3.49 Theoretical formula

Once the addition of the metal salt solution was complete, the suspension was kept at pH 10 at room temperature for one hour. For the systems containing NO3-, Cl- and ClO4- anions, the precipitate was washed several times with deionized water in order to remove the remaining dissolved salts. For the compound containing the carbonate ions, the solid was washed and resuspended in a 0.1 M Na2CO3 solution. It was then again thoroughly washed with deionized water. All samples were then dried overnight at 353 K and finally placed in desiccators at 4 4 ACS Paragon Plus Environment

Page 4 of 37

Page 5 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


different RHs: 3% (using silica gel), 11% (LiCl saturated solution), 43% (K2CO3 saturated solution) and 74% (NaNO3 saturated solution). Higher RHs were not investigated because the samples then became gels. The 0% RH was prepared just before the experiments by pumping the samples heated at 383 K for 48 hours under primary vacuum. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) measurements were conducted to determine the exact chemical composition of the synthesized samples. Experiments were performed on a quadrupole ICP-MS (X series, Thermo Elemental), equipped with a slurry nebulizer and an impact ball nebulization chamber. Argon was used as nebulizing gas and helium was the collision gas. The plasma flux was set at 10 L·min-1. The samples were dissolved beforehand in 2% nitric acid. Using this method, the Mg/Al ratio was thus measured for the four samples (Table 1, right column). All samples exhibit a Mg/Al molar ratio ≈ 3.5, except for LDH-ClO4-, for which it is about 3.9. All samples were then characterized by using several techniques as described hereafter. Moreover, the characterization of the samples by AFM, the measurement of the amounts of metallic impurities and the characterization by nitrogen adsorption/desorption at 77 K are provided in Supporting Information (Figures S1 to S3 and Tables S1 to S3). 2.2. X ray diffraction measurements (XRD) Powder X-ray diffraction patterns were collected thanks to a Bruker D8 Advance diffractometer. The device is equipped with a Cu emitter (λCuKα = 1.541 Å, 40 kV/40 mA), a grazing parabolic Göbel mirror and a position sensitive detector (Vantek) allowing collection of the diffracted pattern. Since this device is not equipped with a RH controller, samples at 0% RH were further analyzed using a Small Angle X-rays Scattering (SAXS) XENOCS XEUSS 2.0 device which combines the copper Kα line with a DEXTRIS 1M detector. The samples were placed between two Kapton sheets on a sample holder and successive acquisitions of 10 minutes each were carried out during pumping. Standard procedures were then applied to the diffraction images collected using the pySAXS software in order to obtain scattered intensities calibrated as a function of the scattering vector. 2.3. Thermogravimetric Analysis (TGA) Thermogravimetric Analysis (TGA) was carried out using a Mettler-Toledo TGA/DSC 1 analyzer. Approximately 20 mg of each sample was placed in a 70 µL alumina crucible and heated from 298 to 773 K at a heat flow of 5 K·min-1 and then from 773 to 1073 K at a heat flow of 10 K.min-1 under a dinitrogen flux of 50 mL·min-1. Data were analyzed using the STARe software. 2.4. Infrared (IR) spectroscopy to characterize the hydration of the samples The evolution of the samples with RH was analyzed by IR spectroscopy at the AILES beamline of synchrotron SOLEIL (Saint Aubin, France).



The hydration controlled set-up used has been described elsewhere.30-31 In short, samples, deposited on a thin diamond disc under argon atmosphere, were fixed on a sample holder at precise normal incidence relative to the incident beam. A copper holder allows introducing the disc into a humidity-controlled copper cell of 1 cm3 volume. The cell is vacuum (UHV)compatible (leak rate < 1 × 10-9 mbar·L·s-1) thanks to the use of all metal elements and is evacuated using a turbomolecular pump. The cell body is connected to the cold tip of a closed cycle cryostat through a copper braid in order to maintain the cell at a fixed temperature. During measurements, the cell temperature is monitored with a precision of ± 0.1 K by means of a Si-diode and a resistive heater. The set-up allows performing in situ infrared measurements under controlled vapor pressure and at stable temperature. For the hydration cycle, the disc with the deposited sample was first heated at 330 K for a few hours under vacuum (10-6 mbar) to remove adsorbed water molecules. The sample was then cooled down to 298 K. This corresponds to the driest state presented in this study. Subsequently, IR spectra of LDHs were recorded at controlled water vapor pressures (P) at 296.0 ± 0.1 K for adsorption and desorption processes. P was converted to RH, using the pressure dependence: %RH = (⁄() × 100%, with () the equilibrium vapor pressure of water (() = 28.0 mbar at 296 K). It is worth noting that the quantity of water adsorbed during hydration depends on the initial state of the sample. The O-H stretching band measured for the various samples suggests that LDH-NO3- and LDH-ClO4- allowed for a fully dried state as opposed to the other two samples where significant quantity of water remained in the initial (or driest) state. Absorbance spectra were recorded at equilibrium for a given value of RH in the mid-infrared (MIR) range, from 500 to 4000 cm−1 with a 4 cm−1 resolution using a KBr beamsplitter and an HgCdTe detector. In order to remove contributions from water vapor and diamond window, absorbance spectra with the empty diamond disc were recorded for the same RH values and the resulting absorbance spectrum was subtracted. All spectra result from averaging 400 scans measured with the mobile mirror speed of 2 cm·s-1. 2.5. Irradiation and H2 production measurement by gas chromatography

The conditioning procedure for irradiation experiments and gas chromatography analysis has been described previously.23-24 Briefly, samples placed in Pyrex glass ampoules under Ar were irradiated using a Titan Beta, Inc. linear accelerator (LINAC)32 with 10 MeV electron pulses of 10 ns duration at a repetition rate of 2 Hz to avoid macroscopic heating. The dose per pulse ((30 ± 2) Gy·pulse-1, with 1 Gy = 1 J·kg-1) was determined by the Fricke dosimetry33 before each experiment. We considered that the measured dose determined by the Fricke dosimeter was the same in the porous systems under study. Indeed, as stated by Fano’s theorem34, significant modifications of the dose whatever the microscopic variations of density are not expected. This fact was already checked using Monte-Carlo simulations for gammarays in the case of porous silica.35 All samples were exposed to a total 200 kGy irradiation dose, performed by 4 steps of 50 kGy. The radiolytic G-values were then calculated by considering that the dose is absorbed by the whole sample (i.e., material and water). We can estimate that a dose of a few hundred of MGy would be of practical relevance. This dose is out of reach within our experimental conditions. 6 ACS Paragon Plus Environment

Page 6 of 37

Page 7 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nevertheless, the present study provides insight into the behavior of LDHs under irradiation. H2 formed under irradiation was measured by gas chromatography (µGCR3000 SRA Instrument) using ultra-high purity argon as carrier gas.23-24 3. Results 3.1. Evolution of interlamellar distance with hydration by XRD The evolution of the d003 distance (Figure 1) determined using X-Ray diffraction is displayed in Figure 2 as a function of RH for the 4 samples. The overall variation of the d003 distance with RH displays similar features for the four samples. It gains around 0.4 Å in the 0-3% RH range and then remains constant in the 3-74% RH range.

Figure 2. d003 distance evolution as a function of RH for the four studied samples. The dotted lines guide the eyes. Since the thickness of a sheet is about 4.77 Å,29, 36 the interlamellar distances range from 2.92 to 3.33 Å for LDH-CO32-; 2.97 to 3.38 Å for LDH-Cl-; 2.88 to 3.33 Å for LDH-NO3- and 4.20 to 4.58 Å for LDH-ClO4- in the 0-74% RH range. Note that this interlamellar distance is very similar for all the samples, except for LDH-ClO4- for which it is higher by more than one Angstrom. This can be assigned to the larger size of the perchlorate anion as compared to the other anions. Diffraction patterns at a selected RH (43%) are given for all samples in S4 in Supporting Information. 3.2.Determination of relative water amount by TGA The thermogravimetric analysis curves of the LDHs equilibrated at different RHs are displayed in Figure 3. Low temperature mass losses are characteristics of water adsorbed in the samples.14, 17, 37-41 Different types of water can be identified: i) interparticular water that corresponds to water in the pores between LDH crystallites, and ii) surface water, i.e. water in the interlamellar space, on the outer surfaces of the sheets and in the micropores resulting from imperfect stacking of sheets. Interparticular water is removed between 323 and 378 K 7 ACS Paragon Plus Environment


whereas surface water is desorbed at higher temperatures, between 378 and 473 K. In the case of LDHs, no clear features can be observed in the second range, which prevents distinguishing water in the interlamellar space, referred to as intrinsic water, from water in the micropores, referred to as extrinsic water.42 Above 473 K, dehydroxylation occurs together with the release of various compounds resulting from decomposition of interlayer anions. In the case of LDH-CO32-, dehydroxylation and carbon dioxide release are observed from 493 to 673 K,38 and these two phenomena overlap. Additional mass loss at higher temperature can be assigned to the release of nitrogen monoxide arising from nitrate impurities used in the synthesis.38 For LDH-Cl-, dehydroxylation occurs from 553 to 673 K followed by dichlorine release from 673 to 873 K.38 For LDH-NO3-, dehydroxylation takes place between 553 K and 693 K, while nitrogen monoxide, arising from nitrate decomposition, is launched between 553 K and 923 K. Lastly, in LDH-ClO4-, dehydroxylation is found in the 553-723 K range, while dioxygen release occurs in the 623-723 K range.38 This analysis suggests that complete dehydration occurs at higher temperature for LDH-CO32and LDH-Cl- (slightly below 500 K) as compared to LDH-NO3- and LDH-ClO4- (around 425 K).

Figure 3. TGA curves measured for five RHs for the various LDHs: (a) LDH-CO32-, (b) LDH-Cl-, (c) LDH-NO3- and (d) LDH-ClO4-. The nature of the gases released during heating is given on the figure. Dehydroxylation leads to the formation of H2O. On the basis of TGA curves (Figure 3), it is possible to determine the proportions of interparticular and surface water in the different samples as a function of RH.42 It should be 8 ACS Paragon Plus Environment

Page 8 of 37

Page 9 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


noted here that a special protocol was implemented for samples at 0% RH in order to avoid water re-adsorption upon introduction of the samples in the TGA set-up. Samples, shortly after having been fully dried (as described in the experimental section) were placed into the TGA device. They were then heated at 383 K for one hour and cooled down to ambient temperature under dinitrogen flow before analysis.

Figure 4. Relative mass fractions of interparticular water (a) and surface water (b) at the five studied RHs. Figure 4 reveals various features regarding the uptake of water and the evolution of its status with increasing RH. As expected, no interparticular water is measured at 0% RH. Its amount increases markedly in the 0-3% RH range and then slightly in the 3-43% RH range. It increases more between 43 and 74% RH. This latter evolution is the more pronounced in the case of LDH-NO3-. For the four samples, the amount of surface water (intrinsic and extrinsic water) increases from 0 to 3% RH and then remains almost constant. LDH-CO32- appears to hydrate more than the other samples at low RH. These results are consistent with the d003 values obtained from XRD measurements (Figure 2). Indeed, the amount of surface water being almost constant, except under 3% RH , the quantity of water in the interlamellar space and, hence, the d003 distance are constant in the 3-74% RH range. This behavior is clearly different from that observed in cationic swelling clays, for which the interlayer distance increases stepwise.43 3.3. Evolution of interactions within LDH through hydration by infrared spectroscopy To further describe the hydration processes in the various samples using infrared spectroscopy, we will first focus on the evolution of the O-H stretching band in the 2800-4000 cm-1 range as a function of RH (from 0 to 80%) for the various samples (Figure 5). The whole spectra are given in Supporting Information (Figures S5 to S8). For all the studied samples, the O-H stretching band is very broad. In addition, as stated in the experimental section, the driest state of both LDH-CO32- and LDH-Cl- still contains significant quantity of water as 9 ACS Paragon Plus Environment


attested by the modest evolution of the integrated area of the O-H stretching band. In these cases, the changes are only representative of the reorganization of the water network.

Figure 5. Evolution of the O-H stretching band during water adsorption for the four samples. The small sharp features observed below 3000 cm-1 are due to carbon pollution. In the case of LDH-CO32-, at the lowest RH values, a sharp component is observed at 3700 cm-1. It is due to the O-H stretching mode of weakly H-bonded –OH groups. It quickly disappears when RH increases due to the formation of stronger H-bonds. Note that in this case only, a component is detected at very low wavenumbers, around 2880 cm-1. This can be assigned to water molecules bridged with the carbonate anion.44 In the case of LDH-NO3-, components around 3575 and 3650 cm-1, corresponding to the O-H stretching mode of hydroxide groups or water molecules engaged in few hydrogen bonds, are always observed, whatever the RH value. Components at low wavenumber are present and correspond to strong hydrogen bonds that are due to the presence of anions. The main component around 3400 cm-1 corresponds to water involved in hydrogen interactions. It increases significantly with RH. Another component around 3150 cm-1 and attributed to the O-H stretching mode of hydroxyl groups or water molecules for which the hydrogen atom is in interaction with the oxygen atoms of nitrate anions, also increases significantly with RH.44 In the case of LDH-ClO4-, components at high wavenumbers, ranging from 3650 to 3700 cm-1, are observed whatever RH. A sharp component around 3600 cm-1, corresponding to rather weak H-bonds, increases in intensity with RH. With increasing RH, features around 3250 cm-1 and 3450 cm-1 become more and more prominent. The first one corresponds to strong H-bonds between water molecules or hydroxyl groups with the anion, and the latter one to moderate H-bonds between water molecules and hydroxyl groups. The behavior of LDH-Cl- is strikingly different from the behavior of the other samples. Apart from the component around 3700 cm-1, corresponding to very weak hydrogen bonds, and that disappear very quickly due to the formation of stronger H-bonds, starting from RH ~ 2%, the global spectra suddenly change and shift towards lower wavenumbers. The maximum of the O-H stretching band is around 3470 cm-1 at low RH and then decreases to 3370 cm-1 at high RH values. This indicates that, when the interlamellar space is filled, water changes from “liquid-like” to “ice-like”. 10 ACS Paragon Plus Environment

Page 10 of 37

Page 11 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The next section will analyze in details the IR results obtained for LDH-NO3- and LDH-Clsamples that, as we will see in the irradiation section, lead to very different H2 production yields, i.e. the lowest one in the first case and the highest in the latter. The two other samples are discussed in Supporting Information (Table S4 and Figure S9). In the whole spectra (Figures S5-S8), the observed vibrational bands are due to the vibrations of water molecules and of OH groups (O-H stretching band in the 3000-4000 cm-1 range, and water bending mode at 1630 cm-1), of anions and also to lattice (Figure S5). Contrary to the chloride anion, nitrate anions possess vibrational modes whose analysis gives insight into phenomena at stake during hydration. We will first investigate the vibrational modes corresponding to nitrate anions. In LDHs, nitrate anions are confined and it is then relevant to compare the positions of their vibrational modes with those reported for the free anion (Table 2) that has D3h symmetry. In LDH, the symmetry of the anion lowers, due to interactions with water molecules and OH groups. Therefore, some bands, normally inactive in infrared spectroscopy, such as the ν1 mode, either become active or split. As band positions evolve with RH (Figure 6), we first analyze the band positions at 0% RH (Table 2). In this table, we also report some literature data on LDHs.14, 45 However, as in most studies RH values are not specified, direct comparison between data is not straightforward. Table 2. Summary of vibrational modes of nitrate anion (non-degenerate N-O symmetric stretching mode ν1, non-degenerate N out-of-plane bending mode ν2, double-degenerate N-O asymmetric stretching mode ν3 and bending inside the plane ν4) in various environments: free, in double lamellar hydroxides14, 45 and in the present work. Values for the present study are those obtained at 0% RH.

ν1 ν2 ν3 ν4

Free NO3- 45 Raman IR/cm-1 /cm-1 1049 830 1350 1355 680 690

In hydrotalcites14, 45 Raman IR/cm-1 /cm-1 14 1050 1044 827 1360 1355 667 712

Present work (0% RH) IR/cm-1 1032 816/833 1353/1486 not identified

For the free ion, the ν1 mode is not observed by IR spectroscopy. In dry LDH, this mode is active in LDH-NO3- and observed through the presence of a weak band at 1032 cm-1. This can be assigned to a distortion of the D3h symmetry, resulting from the local chemical environment of the nitrate anion and from H-bonding.41 The presence of this vibration was already reported by Miyata,14 who observed it at 1050 cm-1. Whereas a single ν2 mode is observed in the free ion at 830 cm-1, this mode is split in dry LDH-NO3- where two signals are observed at 816 and 833 cm-1, i.e. close to the values reported by Miyata at 820 and 840 cm-1.14 In a recent work,41 the band at 834 cm-1 was assigned to a planar nitrate arrangement, whereas the band at 825 cm-1 was attributed to tilted nitrate anions. The data obtained in the present study with two band positions may then suggest that both planar and tilted nitrate anions could be present in the interlamellar space of dry LDH-NO3-. The question then arises 11 ACS Paragon Plus Environment


of whether these two structural arrangements exist in the same interlamellar space, or in different ones. Considering the work of Wang et al.29 who showed that nitrate orientation in Mg/Al LDHs strongly depends on the hydroxide layer charge, it may be tentatively proposed that layer charge heterogeneity may explain the coexistence of two structural arrangements in a macroscopic sample. The ν3 band corresponds to a single band position at 1350 cm-1 for the free ion. In contrast, the situation in dry LDH-NO3- is much more complex with the presence of two main components located at 1353 and 1486 cm-1. Such a splitting was not observed in reference 42, but was mentioned in Miyata’s work.14 The ν4 mode was reported at 667 cm-1 in hydrotalcite.45 However, its position cannot be exploited in LDH samples, as it is located in the spectral region corresponding to lattice vibrations. It is of particular interest to follow the variation of these various modes upon increasing RH (Figure 6). As far as the ν1 mode is concerned, its position shifts towards 1049 cm-1. In the case of the ν2 mode, splitting vanishes with increasing RH and the final value obtained at high RH (828 cm-1) is very similar to the IR ν2 mode of the free anion. The splitting of the ν3 mode also evolves with increasing RH, and at high RH (80%), the massif corresponding to ν3 exhibits a main band at 1337 cm-1 and a shoulder at 1394 cm-1 (Figure 6). Interestingly, most changes appear to occur for RH ≤ 40%, as both the ν1 and ν2 modes exhibit changes before 40% RH. In the dry system, a competition exists between the repulsion between anions and the formation of H-bonds from –OH groups towards anions. Therefore, in this state, anions are located near the hydroxide layer, with two possible structural arrangements. Water enters in the interlamellar space, which is first accompanied by an increase in d003 distance (Figure 2), and then, for RH above 3%, a more complete H-bond network develops. The environment of the nitrate anion therefore becomes more comparable to that of the free anion. It can be assumed that it is located in the midplane of the interlamellar space. Note that at increasing RH, water will also enter gradually in the different types of porosity in which nitrate anions can also be found.

Figure 3. Left: zoom on each vibrational mode band of the nitrate anion according to RH. Right: evolution of the position of the band maximum according to RH for ν1 (in black) and ν2 (in red). For ν2, it is possible to visualize the loss of splitting. The ν2 value of free nitrate anions, obtained in IR spectroscopy, is given as a dotted line. The ν3 band is not plotted. 12 ACS Paragon Plus Environment

Page 12 of 37

Page 13 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In the case of LDH-Cl-, no vibrational modes due to the presence of the chlorine ion are expected. Figure 7a evidences however two bands below 3% RH decreasing in intensity and almost disappearing with increasing RH: the position of the first one varies from 760 to 735 cm-1, while the second one shifts from 880 to 845 cm-1 with increasing RH. Meanwhile, the lattice modes shift from 590 to 560 cm-1. This may indicate that the environment of the anion becomes more and more symmetric when water is adsorbed. In this latter case, the chloride anion is in a strong H-bond network with 6 OH groups and 4 water molecules, as suggested by simulations.46 The particular strength of the corresponding H-bond network is evidenced by the O-H stretching band (Figure 5) that becomes ice-like starting from 2% RH. Figure 7b displays a zoom on the water bending mode. The band in this spectral region is dominated by a contribution whose position shifts from 1607 to 1637 cm-1 when RH increases from 0 to 95%. The water bending mode is usually found in the 1640-1660 cm-1 wavenumber range in bulk water.47 In the present case, the frequency determined is all the more red-shifted than the RH value is low, suggesting that a non-isotropic environment has a significant effect on the water bending mode. Another contribution is also observed around 1550-1560 cm-1. Such band can also be attributed to a water bending mode.48 This band could be due to crystalline water in strong interaction with the anion.48 Note that the same type of contribution was observed for all samples (Figures S5 to S8). It is however attributed to the formation of a monodentate carbonate-metal complex when the sample is very dry in LDH-CO32-. This contribution then disappears when RH increases (Figure S6 and corresponding discussion). In contrast to LDH-Cl- and LDH-ClO4- samples, this crystalline water in strong interaction with the anion component strongly increases with RH in LDH-NO3- (Figure S5). This suggests that nitrate anions are present in all types of porosity.

Figure 7. Evolution of the IR signals of LDH-Cl- during water adsorption: (a) in the 4001000 cm-1 region; (b) in the water bending region. To conclude, in all cases, the environment of the anion becomes more symmetric when RH increases. Moreover, we evidence here that the LDH-Cl- sample exhibits a special behavior as compared to the other ones, with a very sudden stiffening of the H-bond network as soon as RH is greater than 2%. Having well characterized and analyzed how the samples evolve with 13 ACS Paragon Plus Environment


RH, we can now understand the evolution of their H2 production yields under irradiation as a function of the nature of the anion and of the water amount. 3.4. H2 production yields under irradiation Cumulated H2 production was measured as a function of the cumulated dose, for the four samples at various RHs (see an example in the case of LDH-NO3- in S10). H2 production evolves linearly with the dose (S10). The slopes of the lines allow determining the H2 radiolytic yields, the evolution of which is displayed in Figure 8. As the amount of metallic impurities in all samples is lower than a few ppm (Table S1 in Supporting Information), radiolytic yields can be safely assigned for LDH samples. It must also be pointed out that the irradiation conditions used in our study do not affect the structure, mineralogy and water content of the materials, as FT-IR, TGA and XRD experiments of irradiated samples do not reveal any significant changes compared to non-irradiated ones (Figure S11 in Supporting Information). Indeed, considering the total dose used here (200 kGy) and the radiolytic yields, only a minor percentage of water molecules and OH groups are affected by ionizing radiation. Moreover, low linear energy transfer particles such as accelerated electrons are not expected to change the mineralogy of the samples under our experimental conditions.

Figure 8. Dihydrogen radiolytic yields for different LDHs: LDH-CO32- (black squares), LDHCl- (red dots), LDH-NO3- (blue up triangles) and LDH-ClO4- (dark cyan down triangles) as a function of: (a) RH and (b) the total water content. The value obtained in bulk water (4.5 x 10-8 mol.J-1) is given for comparison. Lines are plotted to guide the eyes. Figure 8a reveals that H2 yields vary with both anion nature and RH. As far as RH is concerned, the H2 yield of all samples, except LDH-NO3-, increases in the 0-3% RH range and remains constant for higher RH, which is consistent with the evolution of the water amount and of the d003 distance. The behavior of LDH-NO3- is markedly different as, for this sample, the yields increase monotonously with RH. Figure 8b clearly reveals that anion nature is the main factor controlling dihydrogen production, and depending on the anion, 3 types of behavior are observed: • For LDH-NO3-, the value of the H2 yield measured at 0% RH (3x10-11 mol·J-1) is close to the detection limit of the gas chromatography. H2 yields are very low, largely inferior to the yield obtained in bulk water and they increase slightly with RH. Such behavior appears to be 14 ACS Paragon Plus Environment

Page 14 of 37

Page 15 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


consistent with the fact that NO3- is a scavenger of both the solvated electron49 and the solvated electron precursor, the so-called “dry” electron,50 that are both dihydrogen precursors.25, 50 Still, such an assumption should be confirmed by identifying decomposition products of the nitrate anion. For this reason, the nature of the gases produced under irradiation was investigated by mass spectrometry (data not shown). Except H2, the only other gas measured was O2. As dioxygen production in a porous medium is rather unusual,22, 26, 51 and considering that the radiolysis of solid nitrates is known to induce O2 formation,25 this O2 formation can be linked to the (pre)-solvated electron scavenging effect of nitrate anions. Surprisingly, nitrite species that are formed upon radiolysis of nitrate25 were not detected. This might be due to the trapping of the nitrite species in the structure of LDH-NO3-. The slight increase in H2 yields with increasing RH can be attributed to the higher amount of interparticular water when RH increases (Figure 4). As the H2 yield remains very low even at high RH, it can be inferred that nitrate anions are also present in interparticular water, which is consistent with the infrared spectroscopy results described above.

• LDH-CO32- and LDH-ClO4- display similar behavior (Figure 8). Very low radiolytic yields, close to the detection limits, are obtained at 0% RH. H2 yields then reach values around 2 × 10-8 mol·J-1 for RH equal to 3% and remain constant thereafter. This similarity in behavior may appear surprising as ClO4- is known to be inert under ionizing radiation whereas CO32- is known to be a hydroxyl radical scavenger. This suggests that hydroxyl radical scavengers are present in the LDH material itself, since adding such scavengers does not induce any modification in H2 radiolytic yields, as also already reported in the case of nanometric silica.26 • Finally, LDH-Cl- exhibits significantly higher H2 yields than the other samples. This is true at 0% RH where the obtained value, 3.3 × 10-8 mol·J-1, is close to that of bulk water. It remains true for higher RH where yields around 1.1 × 10-7 mol·J-1, i.e. more than 2 times that of bulk water, are obtained. The high yields measured in the LDH-Cl- sample are rather unexpected, as similarly to the carbonate anion, the chloride anion is known to be a hydroxyl radical scavenger. In aqueous solution, it is even more efficient than the carbonate anion (rate constants of 4.3 × 109 L·mol-1·s-1 and 3 × 108 L·mol-1·s-1 for chloride52 and carbonate53, respectively) for scavenging hydroxyl radicals. The origin of the high radiolytic yields obtained for LDH-Cl- then deserves further investigation. 4. Discussion: H2 reaction mechanisms in LDHs under ionizing radiation All the experimental evidence gathered in the study shows that H2 radiolytic yields are mainly controlled by the nature of the compensating anion, and for a given anion, by the amount of water present in the sample. Consequently, reaction mechanisms must take into account both anion reactivity and water content to explain dihydrogen production. Moreover, the electron dose rate is also expected to impact the radiolytic yields26 . For the present study, the use of accelerated electrons clearly provides higher doses per unit of time than radioisotopes decay would. In contrast, the time of irradiation during storage may clearly be significantly longer. One may consider, nevertheless, the reaction mechanisms proposed below as general trends. Compared to previous studies dealing with cationic clay minerals in which the cation is inert under ionizing 15 ACS Paragon Plus Environment


Page 16 of 37

radiation,23-24 the presence of anions adds complexity to the process. Therefore, for the sake of clarity, we will start by describing reactions in the sample containing perchlorate anions which are known to be inert towards ionizing radiation, i.e. we will not consider anions. By analogy with cationic clay minerals,22-24, 54 we can propose the following reaction scheme. The first step consists in the production of an electron-hole pair by interaction of radiation with the LDH structure according to (R4):

./ + ℎ

(R4)

Electrons can then be either trapped in the LDH structure (R5) or migrate to the surface of the sheets. In the latter case, electrons can react on hydroxyl groups present on the surface by dissociative attachment (R6). They can also be solvated by water molecules, for instance those present in the interlamellar space (R7): 123

445

(R5)

+ 6, 7 → 6, 7 + ● 85

(R6)

(R7)

The hole can also migrate towards surface hydroxyl groups (R8) and, if transferred at interfaces, react with water molecules (R9):

ℎ + 6, 7 → 6, 7● +

ℎ + → + ●

(R8)

(R9)

The formed species can then lead to dihydrogen production, where is by convention the electron with one water molecule:

+ → + 2

(R10)

+ → ● +

(R11)

+ ● → +

(R12)

● + ● →

(R13)

It must be stressed that water confinement in LDHs is equivalent or more intense than that measured in swelling clay minerals.23-24 Indeed, the interlamellar distance is about 3 Å for LDH-CO32-, LDH-Cl- and LDH-NO3-. It is about 4.6 Å for LDH-ClO4- (Figure 2). These values are similar, or even smaller, than the interlayer distance in synthetic montmorillonite or saponite containing one water layer (5 Å). In these latter two cases, H2 radiolytic yields were always higher than that measured in bulk water, except for samples equilibrated at 0% RH, but remained smaller than the yields measured for LDH-Cl- in the present work. Figure 8 clearly shows at a fixed RH, the amount of water is not the key factor of the H2 production. IR 16 ACS Paragon Plus Environment

Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spectroscopy has revealed that the organization of the H-bond network is different in LDH-Clthan in the other samples (Figure 5), and that the H-bond network becomes more “ice-like’’, starting from RH = 2% than in the other cases. The radiolysis of ice, however, leads to smaller H2 yields than the radiolysis of liquid water. Indeed, under gamma irradiation, the H2 production is measured to be 4.5 × 10-8 mol·J-1 at room temperature in bulk water, whereas it is 2.6 × 10-8 mol·J-1 at 258 K in ice (see 55 and references therein). Therefore, the change in the organization of water molecules in the LDH-Cl- sample cannot account for the high H2 yields measured. An additional explanation, as suggested by the modifications observed in the H-bond network (Figure 5) in this sample would be that the migration properties of the intermediate species would be different in LDH-Cl- as compared to the other ones. This would affect the implied reactions, resulting in the excess production of hydrogen atoms as compared to its production in bulk water and in the other samples. In LDH-CO32-, LDH-Cl- and LDH-NO3-, the preceding reaction mechanisms remain, but additional reactions due to the behavior of anions take place. As a first step for dealing with anions, reactions known in the literature for aqueous solutions can be written.25 For LDH-NO3-, the nitrate anion clearly acts as an electron scavenger since the radiolytic yields of dihydrogen remains remarkably low, whatever RH (Figure 8). By analogy with previous works carried out in aqueous phase,49, 56 the following reactions can then be written:

:! + → :! ●

(R14)

:! ● + → : ● + 2

(R15)

As stated in the introduction, ;! and ;7 anions favor H2 production by removing the Allen chain (R2-R3). Thus, in solution, the H2 production was shown to increase with the chloride concentration.56-57 In the case of carbonate53, 58 and chloride52, 59-60 anions, reactions traditionally found in the literature for aqueous solutions are:

;! + ● → ;! ● + ;! ● + ● → ;! +

;7 + ● → ;7 ●

(R16) (R17)

(R18)

As already mentioned, the rate constant of (R16)53 is roughly ten times lower than (R18)52. Furthermore, in LDH the number of chloride anions is double that of carbonate anions (Table 1). Therefore, the scavenging capacity of chloride should be approximately 20 times higher than that of carbonate anions. However, as the similarity between H2 yields observed for LDH-CO32- and LDH-ClO4- must be linked to the presence of scavengers in the LDH structure, the high H2 yields measured in LDH-Cl- must be explained by additional reactions. We suggest here, based on preliminary NAP-XPS experiments,61 that reactions such as (R19) and (R20) would lead to increased dihydrogen production:

2;7 ● → 2;7 +

(R19)



;7 ● + ● → ;7 +

(R20)

5. Conclusion To assess the behavior of LDHs materials in the context of the storage of radionuclides and in particular the effect of β- radiation, four samples with Mg/Al ratio around 3.5 and containing different anions (NO3-, CO32-, ClO4- and Cl-) were synthesized. Their behavior with regard to water uptake was characterized using various techniques (ATG, XRD, infrared spectroscopy…). All these materials exhibit a limited swelling at very low RH (under ~3%) but no further swelling was ever observed at higher RH. Infrared spectroscopy revealed that, at low RH, the symmetry of the anion is lowered in this confined medium, due to interaction with various -OH groups and water molecules. In all studied cases, the environment of the anion then becomes more and more symmetric when RH increases. For all systems, the O-H stretching band is very broad, but the LDH-Cl- sample exhibits a specific behavior, as a “liquid water-ice”-like transition is observed as soon as RH is larger than 2%. The behavior of these materials under ionizing radiation was then studied. At similar total water content, the samples exhibit contrasted H2 yields, evidencing that the nature of the anion mostly controls the dihydrogen production. Moreover, the organization of the H-bond network was shown not to play a significant role in the H2 production under irradiation. Radiation chemistry experiments evidence three types of behavior depending on anion nature: i)

ii)

iii)

LDH-NO3- leads to very small H2 yields (< 10-9 mol·J-1 in the whole RH range studied). This is due to the fact that nitrate anions are scavengers of pre-solvated and solvated electrons that are precursors of dihydrogen. In this case, the H2 yields increase with the total water amount; LDH-ClO4- and LDH-CO32- leads to very similar dihydrogen yields, which are at most half the value measured in bulk water (~ 2 × 10-8 mol·J-1). The corresponding anions are inert towards ionizing radiation and hydroxyl scavengers, respectively. This shows that the material itself contains hydroxyl scavengers. LDH-Cl- produces significant amounts of dihydrogen upon irradiation, as the H2 yields can be at least twice the value obtained in bulk water (~ 1.1 × 10-7 mol·J-1). These yields are even higher than in synthetic cationic swelling clay minerals, evidencing that Cl- contributes to dihydrogen production, according to reactions we have proposed.

In the two latter cases, the evolution of the H2 yields with RH follows that of the interlamellar space: it increases in the 0-3% RH range and then remains stable. Consequently, the present study suggests that when these LDH-ClO4-, LDH-Cl- and LDH-CO32- are used for radionuclides storage within geological repositories, RH changes are not expected to affect the H2 production. In contrast, in LDH-NO3-, the H2 yield will increase with increasing RH although remaining at a much lower level than in the other LDHs. 18 ACS Paragon Plus Environment

Page 18 of 37

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The behavior of LDHs under ionizing radiation is mainly controlled by the nature of the anion. The proper adsorbent can then be chosen depending on the desired application. For instance, H2 (and gases) production is minimized when nitrate anions are present in the interlamellar space, making these materials promising ones for the storage of radionuclides, when a limited impact on the material itself is required for safety reasons. On the contrary, LDH-Cl- materials can be used in the context of nuclear energy in order to efficiently produce significant amounts of dihydrogen that can then be used as another energy source. Associated Content: Supporting Information The Supporting Information is available free of charge. Characterization of the samples by AFM, ICP-MS, dinitrogen adsorption-desorption at 77 K; XRD diffraction patterns at one selected RH; evolution of the whole IR spectra as a function of RH; H2 cumulated production as a function of the cumulated dose; characterization of the samples before and after irradiation. Acknowledgments This work was supported by a grant from Région Ile-de-France in the framework of DIM Oxymore. This work was also supported by a public grant from the “Laboratoire d’Excellence Physics Atom Light Matter” (LabEx PALM) overseen by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” program (reference: ANR-10LABX-0039). Synchrotron SOLEIL is acknowledged for providing beam time and technological support. References (1) (2)

(3)

(4)

(5)

(6) (7) (8)

Ten Hoeve, J. E.; Jacobson, M. Z. Worldwide Health Effects of the Fukushima Daiichi Nuclear Accident. Energie Environ. Sci. 2012, 5, 8743-8757. Ravichandran, R.; Binukumar, J. P.; Sreeram, R.; Arunkumar, L. S. An Overview of Radioactive Waste Disposal Procedures of a Nuclear Medicine Department. J. Med. Phys. 2011, 36, 95-99. Riley, B. J.; Vienna, J. D.; Strachan, D. M.; McCloy, J. S.; Jerden Jr., J. L. Materials and Processes for the Effective Capture and Immobilization of Radioiodine: A Review. J. Nucl. Mat. 2016, 470, 307-326. Chapman, K. W.; Chupas, P. J.; Nenoff, T. M. Radioactive Iodine Capture in SilverContaining Mordenites through Nanoscale Silver Iodide Formation. J. Am. Chem. Soc. 2010, 132, 8897-8899. Lee, J. W.; Cha, D. K.; Oh, Y. K.; Ko, K. B.; Song, J. S. Zero-Valent Iron Pretreatment for Detoxifying Iodine in Lliquid Crystal Display (LCD) Manufacturing Wastewater. J. Hazard. Mater. 2009, 164, 67-72. Kaplan, D. I.; Serne, R. J.; Parker, K. E.; Kutnyakov, I. V. Iodide Sorption to Subsurface Sediments and Illitic Minerals. Environ. Sci. Technol. 2000, 34, 399-405. Riebe, B.; Dultz, S.; Bunnenberg, C. Temperature Effects on Iodine Adsorption on Organo-Clay Minerals. App. Clay Sci. 2005, 28, 9-16. Khan, A. I.; O'Hare, D. Intercalation Chemistry of Layered Double Hydroxides: Recent Developments and Applications. J. Mat. Chem. 2002, 12, 3191-3198.



(9) (10)

(11)

(12)

(13)

(14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

(22)

(23)

Wang, Q.; O'Hare, D. Recent Advances in the Synthesis and Applications of Layered Double Hydroxide (LDH) Nanosheets. Chem. Rev. 2012, 112, 4124-4155. Lu, H.; Zhu, Z.; Zhang, H.; Zhu, J.; Qiu, Y.; Zhu, L.; Kuppers, S. Fenton-Like Catalysis and Oxidation/Adsorption Performances of Acetaminophen and Arsenic Pollutants in Water on a Multimetal Cu-Zn Fe-LDH. ACS Appl. Mater. Interfaces 2016, 8, 25343-25352. Jawad, A.; Liao, Z.; Zhou, Z.; Khan, A.; Wang, T.; Ifthikar, J.; Shahzad, A.; Chen, Z.; Chen, Z. Fe-MoS4: An Effective and Stable LDH-Based Adsorbent for Selective Removal of Heavy Metals. ACS Appl. Mater. Interfaces 2017, 9, 28451-28463. Santos, R. M. M.; Tronto, J.; Briois, V.; Santili, C. V. Thermal Decomposition and Recovery Properties of ZnAl-CO3 Layered Double Hydroxide for Anionic Dye Adsorption: Insight into the Aggregative Nucleation and Growth Mechanism of the LDH Memory Effect. J. Mater. Chem. A 2017, 5, 9998-10009. Varadwaj, G. B. B.; Oyetade, O. A.; Rana, S.; Martincigh, B. S.; Jonnalagadda, S. B.; Nyamori, V. O. Facile Synthesis of Three-Dimensional Mg-Al Layered Double Hydroxide/Partially Reduced Graphene Oxide Nanocomposites for the Effective Removal of Pb2+ from Aqueous Solution. ACS Appl. Mater. Interfaces 2017, 9, 1729117306. Miyata, S. The Syntheses of Hydrotalcite-Like Compounds and Their Structures and Physico-Chemical Properties I: The Systems Mg2+-Al3+-NO3−, Mg2+-Al3+-Cl−, Mg2+Al3+-ClO4−, Ni2+-Al3+-Cl− and Zn2+-Al3+-Cl−. Clay. Clay Miner. 1975, 23, 369-375. Hou, X.; Bish, D. L.; Wang, S.-L.; Johnston, C. T.; Kirkpatrick, R. J. Hydration, Expansion, Structure, and Dynamics of Layered Double Hydroxides. Am. Mineral. 2003, 88, 167-179. Iyi, N.; Fujii, K.; Okamoto, K.; Sasaki, T. Factors Influencing the Hydration of Layered Double Hydroxides (LDHs) and the Appearance of an Intermediate Second Staging Phase. Appl. Clay Sci. 2007, 35, 218-227. Marappa, S.; Vishnu Kamath, P. Water Molecules in Hydrotalcite like Layered Double Hydroxides: Interplay between the Hydration of the Anions and the Metal Hydroxide Layer. Z. Anorg. Allg. Chem. 2015, 641, 927-934. Sobhana, L.; Sarakha, M.; Prevot, V.; Fardim, P. Layered Double Hydroxides Decorated with Au-Pd Nanoparticles to Photodegradate Orange II from Water. App. Clay Sci. 2016, 134, 120-127. Darmograi, G.; Prelot, B.; Layrac, G.; Tichit, D.; Martin-Gassin, G.; Salles, F.; Zajac, J. Study of Adsorption and Intercalation of Orange-Type Dyes into Mg–Al Layered Double Hydroxide. J. Phys. Chem. C 2015, 119, 23388-23397. Darmograi, G.; Prelot, B.; Geneste, A.; Martin-Gassin, G.; Salles, F.; Zajac, J. How Does Competition between Anionic Pollutants Affect Adsorption onto Mg–Al Layered Double Hydroxide? Three Competition Schemes. J. Phys. Chem. C 2016, 120, 10410-10418. Ma, S.; Islam, S. M.; Shim, Y.; Gu, Q.; Wang, P.; Li, H.; Sun, G.; Yang, X.; Kanatzidis, M. G. Highly Efficient Iodine Capture by Layered Double Hydroxides Intercalated with Polysulfides. Chem. Mater. 2014, 26, 7114-7123. Fourdrin, C.; Aarrachi, H.; Latrille, C.; Esnouf, S.; Bergaya, F.; Le Caër, S. Water Radiolysis in Exchanged-Montmorillonites: the H2 Production Mechanisms. Environ. Sci. Technol. 2013, 47, 9530-9537. Lainé, M.; Balan, E. A., Th. ; Martin, F.; von Bardeleben, H.-J.; Robert, J.-L.; Le Caër, S. Reaction Mechanisms in Talc under Ionizing Radiation: Evidence of a High Stability of H● Atoms. J. Phys. Chem. C 2016, 120, 2087-2095.


Page 20 of 37

Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24)

(25) (26) (27)

(28)

(29)

(30)

(31)

(32) (33) (34) (35)

(36) (37)

(38)

(39)

(40)

Lainé, M.; Balan, E.; Allard, T.; Paineau, E.; Jeunesse, P. M., M.; Robert, J.-L.; Le Caër, S. Reaction Mechanisms in Swelling Clays under Ionizing Radiation: Influence of the Water Amount and of the Nature of the Clay Mineral. RSC Adv. 2017, 526, 526534. Spinks, J. W. T.; Woods, R. J., An Introduction to Radiation Chemistry, 3rd ed., Wiley-Interscience Publication, New York, USA, 1990. Le Caër, S. Water Radiolysis: Influence of Oxide Surfaces on H2 Production under Ionizing Radiation. Water 2011, 3, 235-253. Constantino, V. R.; Pinnavaia, T. J. Structure-Reactivity Relationships for Basic Catalysts Derived from a Mg2+/Al3+/CO3−Layered Double Hydroxide. Catal. Lett. 1994, 23, 361-367. Constantino, V. R.; Pinnavaia, T. J. Basic Properties of Mg1-X(2+) Alx(3+) Layered Double Hydroxides Intercalated by Carbonate, Hydroxide Chloride and Sulfate Anions. Inorg. Chem. 1995, 34, 883-892. Wang, S.-L.; Wang, P.-C. In Situ XRD and ATR-FTIR Study on the Molecular Orientation of Interlayer Nitrate in Mg/Al-Layered Double Hydroxides in Water. Colloids Surf., A 2007, 292, 131-138. Vita, N.; Brubach, J.-B.; Hienerwadel, R.; Bremond, N.; Berthomieu, D.; Roy, P.; Berthomieu, C. Electrochemically Induced Far-Infrared Difference Spectroscopy on Metalloproteins Using Advanced Synchrotron Technology. Anal. Chem. 2013, 85, 2891-2898. Dalla Bernardina, S.; Alabarse, F.; Kalinko, A.; Roy, P.; Chapuis, M.; Vita, N.; Hienerwadel, R.; Berthomieu, C.; Judeinstein, P.; Zanotti, J.-M. New Experimental Set-Ups for Studying Nanoconfined Water on the AILES Beamline at SOLEIL. Vib. Spectrosc 2014, 75, 154-161. Mialocq, J. C.; Hickel, B.; Baldacchino, G.; Juillard, M. The Radiolysis Project of CEA. J. Chim. Phys. 1999, 96, 35-43. Fricke, H.; Hart, E. J. in Radiation Dosimetry, Vol. 2, Second Edition ed. (Eds.: Attix, F. H. Roesch, W. C.), Academic press, New York and London, 1966, pp. 167-232. Fano, U. Note on the Bragg-Gray Cavity Principle for Measuring Energy Dissipation. Radiat. Res. 1954, 1, 237-240. Rotureau, P.; Renault, J. P.; Lebeau, B.; Patarin, J.; Mialocq, J. C. Radiolysis of Confined Water: Molecular Hydrogen Formation. ChemPhysChem 2005, 6, 13161323. Zigan, F.; Rothbauer, R. Neutron Diffraction Measurement on Brucite. Neues Jahrb. Mineral. Monatsh. 1967, 4, 137-143. Kanezaki, E. Direct Observation of a Metastable Solid Phase of Mg/Al/CO3- Layered Double Hydroxide by Means of High Temperature in Situ Powder XRD and DTA/TG. Inorg. Chem. 1998, 37, 2588-2590. Kloprogge, J. T.; Kristóf, J.; Frost, R. L., in 2001, a Clay Odyssey: Proceedings of the 12th International Clay Conference, Bahía Blanca, Argentina, July 22-28, 2001, Vol. 1, Elsevier, 2001, p. 451. Zhang, J.; Xu, Y. F.; Qian, G.; Xu, Z. P.; Chen, C.; Liu, Q. Reinvestigation of Dehydration and Dehydroxylation of Hydrotalcite-Like Compounds Through Combined TG-DTA-MS Analyses. J. Phys. Chem. C 2010, 114, 10768-10774. Conterosito, E.; Palin, L.; Antonioli, D.; Viterbo, D.; Mugnaioli, E.; Kolb, U.; Perioli, L.; Milanesio, M.; Gianotti, V. Structural Characterisation of Complex Layered Double Hydroxides and TGA-GC-MS Study on Thermal Response and Carbonate Contamination in Nitrate and Organic Exchanged Hydrotalcites. Chem. Eur. J. 2015, 21, 14975-14986. 21 ACS Paragon Plus Environment


(41)

(42) (43)

(44)

(45)

(46)

(47) (48)

(49)

(50)

(51)

(52)

(53)

(54) (55)

(56)

Sjåstad, A. O.; Andersen, N. H.; Vajeeston, P.; Karthikeyan, J.; Arstad, B.; Karlsson, A.; Fjellvåg, H. On the Thermal Stability and Structures of Layered Double Hydroxides Mg1–xAlx(OH)2(NO3)x.mH2O (0.18≤ x≤ 0.38). Eur. J. Inorg. Chem. 2015, 2015, 1775-1788. Yun, S. K.; Pinnavaia, T. J. Water Content and Particle Texture of Synthetic Hydrotalcite-Like Layered Double Hydroxides. Chem. Mater. 1995, 7, 348-354. Ferrage, E.; Lanson, B.; Sakharov, B. A.; Drits, V. A. Investigation of Smectite Hydration Properties by Modeling Experimental X-ray Diffraction Patterns: Part I. Montmorillonite Hydration Properties. Am. Mineral. 2005, 90, 1358-1374. Kloprogge, J. T.; Frost, R. L. Fourier Transform Infrared and Raman Spectroscopic Study of the Local Structure of Mg-, Ni-, and Co-Hydrotalcites. J. Solid State Chem 1999, 146, 506-515. Kloprogge, J. T.; Wharton, D.; Hickey, L.; Frost, R. L. Infrared and Raman Study of Interlayer Anions CO32–, NO3–, SO42– and ClO4– in Mg/Al-Hydrotalcite. Am. Mineral. 2002, 87, 623-629. Kirkpatrick, R. J.; Kalinichev, A. G.; Bowers, G. M.; Yazaydin, A. Ö.; Krishnan, M.; Saharay, M.; Morrow, C. P. NMR and Computational Molecular Modeling Studies of Mineral Surfaces and Interlayer Galleries: A Review. American Mineral. 2015, 100, 1341-1354. Brubach, J.-B.; Mermet, A.; Filabozzi, A.; Gerschel, A.; Roy, P. Signatures of the Hydrogen Bonding in the Infrared Bands of Water. J. Chem. Phys 2005, 122, 184509. Pejov, L.; Jovanovski, G. Low Bending Vibrations of Crystalline Water Molecules: an Ongoing Quest or a Final Word. Topical Review: a Tribute to Academician Bojan Soptrajanov. Contributions, Sec. Nat. Math. Biotech. Sci., MASA 2017, 38, 69-82. Balcerzyk, A.; El Omar, A. K.; Schmidhammer, U.; Pernot, P.; Mostafavi, M. Picosecond Pulse Radiolysis Study of Highly Concentrated Nitric Acid Solutions: Formation Mechanism of NO3• Radical. J. Phys. Chem. A 2012, 116, 7302-7307. Pastina, B.; LaVerne, J. A.; Pimblott, S. M. Dependence of Molecular Hydrogen Formation in Water on Scavengers of the Precursor to the Hydrated Electron. J. Phys. Chem. A 1999, 103, 5841-5846. Le Caër, S.; Rotureau, P.; Brunet, F.; Charpentier, T.; Blain, G.; Renault, J. P.; Mialocq, J.-C. Radiolysis of Confined Water: Hydrogen Production at a High Dose Rate. ChemPhysChem 2005, 6, 2585-2596. Balcerzyk, A.; Schmidhammer, U.; El Omar, A. K.; Jeunesse, P.; Larbre, J.-P.; Mostafavi, M. Picosecond Pulse Radiolysis of Direct and Indirect Radiolytic Effects in Highly Concentrated Halide Aqueous Solutions. J. Phys. Chem. A 2011, 115, 91519159. Ghalei, M.; Ma, J.; Schmidhammer, U.; Vandenborre, J.; Fattahi, M.; Mostafavi, M. Picosecond Pulse Radiolysis of Highly Concentrated Carbonate Solutions. J. Phys. Chem. B 2016, 120, 2434-2439. Thomas, J. K. Physical Aspects of Radiation-Induced Processes on SiO2, GammaAl2O3, Zeolites and Clays. Chem. Rev. 2005, 105, 1683-1734. Frances, L.; Grivet, M.; Renault, J. P.; Groetz, J.-E.; Ducret, D. Hydrogen Radiolytic Release from Zeolite 4A/Water Systems under γ Irradiations. Rad. Phys. Chem. 2015, 110, 6-11. Kelm, M.; Bohnert, E. Radiation Chemical Effects in the Near Field of a Final Disposal Site–II: Simulation of the Radiolytic Processes in Concentrated NaCl Solutions. Nucl. Technol. 2000, 129, 123-130.


Page 22 of 37

Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(57)

(58)

(59)

(60) (61)

Kelm, M.; Bohnert, E. Radiation Chemical Effects in the Near Field of a Final Disposal Site–I: Radiolytic Products Formed in Concentrated NaCl Solutions. Nucl. Technol. 2000, 129, 119-122. Cai, Z.; Li, X.; Katsumura, Y.; Urabe, O. Radiolysis of Bicarbonate and Carbonate Aqueous Solutions: Product Analysis and Simulation of Radiolytic Processes. Nucl. Technol. 2001, 136, 231-240. Jayson, G. G.; Parsons, B. J.; Swallow, A. J. Some Simple, Highly Reactive, Inorganic Chlorine Derivatives in Aqueous Solution. Their Formation using Pulses of Radiation and their Role in the Mechanism of the Fricke Dosimeter. J. Chem. Soc., Faraday Trans. 1 1973, 69, 1597-1607. Woods, R. J.; Lesigne, B.; Gilles, L.; Ferradini, C.; Pucheault, J. Pulse Radiolysis of Aqueous Lithium Chloride Solutions. J. Phys. Chem. 1975, 79, 2700-2704. Rochet, F., personnal communication, 2017.



Graphical abstract. 70x29mm (600 x 600 DPI)

ACS Paragon Plus Environment

Page 24 of 37

Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Layered double hydroxide structure scheme. In the present work, the MII or MIII cations are MgII or AlIII, respectively. The water molecules are not represented on the figure. 209x109mm (96 x 96 DPI)



Figure 2. d003 distance evolution as a function of RH for the four studied samples. The dotted lines guide the eyes. 78x62mm (600 x 600 DPI)


Page 26 of 37

Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. TGA curves measured for five RHs for the various LDHs: (a) LDH-CO32-, (b) LDH-Cl-, (c) LDHNO3- and (d) LDH-ClO4-. The nature of the gases released during heating is given on the figure. Dehydroxylation leads to the formation of H2O. 78x56mm (600 x 600 DPI)





Page 28 of 37

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60







Page 30 of 37

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Relative mass fractions of interparticular water (a) and surface water (b) at the five studied RHs. 78x62mm (600 x 600 DPI)



Figure 4. Relative mass fractions of interparticular water (a) and surface water (b) at the five studied RHs. 78x62mm (600 x 600 DPI)


Page 32 of 37

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Evolution of the O-H stretching band during water adsorption for the four samples. The small sharp features observed below 3000 cm-1 are due to carbon pollution. 78x28mm (600 x 600 DPI)



Figure 6. Left: zoom on each vibrational mode band of the nitrate anion according to RH. Right: evolution of the position of the band maximum according to RH for ν1 (in black) and ν2 (in red). For ν2, it is possible to visualize the loss of splitting. The ν2 value of free nitrate anions, obtained in IR spectroscopy, is given as a dotted line. The ν3 band is not plotted. 78x28mm (600 x 600 DPI)


Page 34 of 37

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Evolution of the IR signals of LDH-Cl- during water adsorption: (a) in the 400-1000 cm-1 region; (b) in the water bending region. 72x30mm (600 x 600 DPI)



Figure 8. Dihydrogen radiolytic yields for different LDHs: LDH-CO32- (black squares), LDH-Cl- (red dots), LDH-NO3- (blue up triangles) and LDH-ClO4- (dark cyan down triangles) as a function of: (a) RH and (b) the total water content. The value obtained in bulk water (4.5 x 10-8 mol.J-1) is given for comparison. Lines are plotted to guide the eyes. 78x62mm (600 x 600 DPI)


Page 36 of 37

Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Dihydrogen radiolytic yields for different LDHs: LDH-CO32- (black squares), LDH-Cl- (red dots), LDH-NO3- (blue up triangles) and LDH-ClO4- (dark cyan down triangles) as a function of: (a) RH and (b) the total water content. The value obtained in bulk water (4.5 x 10-8 mol.J-1) is given for comparison. Lines are plotted to guide the eyes. 78x62mm (600 x 600 DPI)


Tuning the Nature of the Anion in Hydrated Layered Double

Recommend Documents