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Radiolytic Events in Nanostructured Aluminium Hydroxides Josiane Albert Kaddissy, Stéphane Esnouf, Delphine Durand, Dimitri Saffre, Eddy Foy, and Jean-Philippe Renault J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b13104 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on March 10, 2017

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The Journal of Physical Chemistry

Radiolytic Events in Nanostructured Aluminium Hydroxides. Josiane A. Kaddissy a, Stephane Esnouf b, Delphine Durand b, Dimitri Saffre c, Eddy Foy d, JeanPhilippe Renault *a a

LIONS, NIMBE, CEA, CNRS, Université Paris Saclay, F-91191 Gif-sur-Yvette Cedex, France

b

DEN-Service d’Etude du Comportement des Radionucléides (SECR), CEA, Université Paris-Saclay, F-

91191, Gif-sur-Yvette, France c d

AREVA TNI – 1 rue des Hérons, 78180 Montigny Le Bretonneux, France LAPA, NIMBE, IRAMAT, CEA, CNRS, Université Paris Saclay- F-91191 Gif-sur-Yvette Cedex,

France

ABSTRACT: The radiolysis of nanosized Al(OH)3 and -AlOOH has been examined. Irradiations were performed using electron beams. H2 radiolytic yields were determined with respect to structure and particle size. The measured yields have an order of magnitude of 10-9 mol.J-1, which decreases when particle size is lowered. Surface and structure analysis techniques were used to detect any change before and after irradiation and no differences were seen. Annealing experiments showed that a significant amount of molecular hydrogen was trapped inside AlOOH and Al(OH)3. H radicals, F centers,

and

centers were identified by electron paramagnetic resonance (EPR) spectroscopy. A significant part of the absorbed energy remains stored into the nanomaterials, and induces a secondary H2 production upon heating. 1 ACS Paragon Plus Environment


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1. Introduction During the storage or transportation of radioactive materials, several gases can be generated. Hydrogen is the dominant flammable gas of concern and its potential threat has drawn more and more attention. This hazardous gas might originate from three sources: metallic corrosion, radiolytic and chemical oxidation of organic compounds in wastes and radiolysis of liquid/vapor water. 1-2 However, a fourth source has been comparatively discounted: solid water. Indeed, very significant amounts of water can be trapped in the form of hydrates or hydroxides in various materials used in the nuclear industry. From a fundamental point of view, the role of structural water in processes of radiolysis of solids needs to be investigated. Usually in order to consider the behavior of water inside a material, many authors have referred to the irradiation of crystalline D2O and H2O ices. 3-5 It has been shown that irradiation alters the structure of ice and produces H, H2, O2, H2O2 and HO2. Though, their diffusion to the surface depends on mechanical treatments, structure, particle size etc. and the related mechanisms are not well defined. 6-7 So far, limited data exists on the behavior of hydroxides under irradiation in general and of the prototypal AlOOH and Al(OH)3 in particular.

8-10

AlOOH (Boehmite) and Al(OH)3 (Bayerite)

formed as corrosion products on fuel cladding, were selected to serve as model compounds for other hydroxides and oxyhydroxide encountered in the storage and disposal of nuclear wastes. Recently, hydrogen production under γ irradiation of similar materials was studied, 11 suggesting that Al(OH)3 was not radiolysable despite its numerous hydroxyl groups. Paradoxical results found in the literature cannot be left unremarked and suggested to revisit the radiation properties of these hydroxides and oxyhydroxide. In this work, we focused on both hydrogen production and defect formation in order to provide a global view of radiolytic phenomena inside irradiated nanomaterials. Actually little information exists relating particle size to radiolytic phenomena, On the one hand, one can assume that, once produced, H2 would be released easier from smaller particles; therefore working with nanoparticles would maximize the yields that can be achieved in aluminum and other hydroxides.

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On the other hand, nanomaterials have a very high amount of defect sites that can quench radiolytic events, starting with high specific surfaces.12-13 With such nanomaterials, we were very careful to prepare dry samples in order to understand the phenomena occurring without the contribution of adsorbed water. In this study, the quantification of molecular hydrogen was not only done at room temperature as it is commonly the case but also after annealing at high temperature (up to 250°C). Annealing was also used to study the impact of the nanostructuration on the thermal stability of the defects induced by radiation.

2. Experimental Methods Sample preparation and characterization The main investigation was led on powders of oxyhydroxides AlOOH (Boehmite) and hydroxides Al(OH)3 (Bayerite). In order to ensure reproducibility all samples were taken from the same batch procured from Sasol®, Germany in the highest purity available. AlOOH was studied in two different crystallite sizes (respectively named AlOOH S and AlOOH L for small and large particles), while one particle size was investigated in the case of Al(OH)3. In this paper we were interested by the radiolysis of structural water, therefore only dry samples were prepared and irradiated. Samples were prepared by heating the powder using a Carbolite tube furnace under high vacuum conditions (10-4 mbar). Thermogravimetric measurements were performed with a Mettler-Toledo TGA/DSC. The optimum heating conditions to dry 500 mg of material adopted were 170°C during 5 hours for AlOOH and 130°C during 4 hours for Al(OH)3. Samples dedicated to H2 quantification were irradiated in Pyrex ampules evacuated at 10-4 mbar, filled with 0.8 mbar of ultrahigh purity helium gas and then sealed. EPR sample cells were standard low paramagnetic impurities glass tubes evacuated and flame-sealed. X-Ray Diffraction (XRD) was performed at the LAPA Laboratory in CEA Saclay. X-Ray diffraction (XRD) was used for three main purposes: phase identification, crystallite size estimation and comparison before and after irradiation. Material structures were investigated using a photon microprobe, built on a Rigaku RU200 rotating anode X-ray generator running at 55 kV

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and 21 mA. The beam delivered by a molybdenum anode (Kα ~ 17.5 keV – 0.70932Å) was monochromatized by a toroidal multilayer mirror. Diffraction patterns were collected in transmission mode with an image plate (GE Healthcare). After scanning (Storm820 – GE Healthcare), the acquired image was circularly integrated with FIT2D (ESRF) to obtain a typical I=f(2) diagram. Data processing was carried out with the EVA software (Bruker AXS) and the ICDD-JCPDS database. Crystallite size was determined using Scherrer equation14: 2

(1)

K is the Scherrer constant, lambda is the wavelength of X-radiation, 2θ the diffraction angle, B is the peak width and L is the crystallite size. Electron paramagnetic resonance (EPR) was conducted at Laboratoire National Henri Becquerel (LNHB), CEA-Saclay, France and Laboratoire des solides irradies (LSI), Ecole Polytechnique, France. Spectra were acquired on X-Band EMX Bruker spectrometers (X-Band) with a 100 Hz field modulation. In most cases microwave power and amplitude modulation were 10 mW, 0.2 mT, respectively. The microwave frequency was measured with a frequency counter. Quantification was estimated using a hydroxyl-TEMPO sample ((2,2,6,6-TetramethylpipeRIDin1-yl)oxyl or (2,2,6,6-tetramethylpipeRIDin-1-yl)oxidanyl) as a standard. The error on the conversion factor is estimated at 25%. Fourier transform infrared spectroscopy (FTIR) was performed on a Bruker Tensor 27 FTIR spectrophotometer using the ATR (attenuated total reflectance) technique with a Golden Gate accessory. Spectra were collected over the range of 4000-500 cm−1 at a 4 cm−1 resolution from 100 scans. Data were analyzed using OPUS software. The specific surface areas, noted BET-SSA, were calculated from nitrogen sorption isotherms using BrunauerEmmeteTeller (BET) method and a Micromeritics system. Adsorption-desorption isotherms with nitrogen were collected on a (ASAP 2010 Instrument) apparatus at 77 K at the Laboratoire de Mesures et Modélisation de la Migration des Radionucléides (L3MR), CEASaclay, France. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Perkin Elmer Optima 2000 DV) was used for the detection of trace metals (see supporting information) at Laboratoire d'Intégration de Systèmes et des technologies (LISL), CEA-Saclay, France.

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Irradiation Experiments Electron beam irradiations were conducted using a linear accelerator located in NIMBE, Saclay, France. The pulse-width was 10 ns and the electron energy was 10 MeV and the repetition rate was 5 Hz. No sample heating was detected. The typical dose delivered per pulse determined using the Fricke dosimeter was: 28 Gy/pulse. 15-16 Materials were also irradiated using heavy ions of GANIL (Grand Accélérateur National d’Ions Lourds) in order to detect if any Linear Energy Transfer (LET) exists. The ion used in these experiments was 36Ar18+ at 95 MeV/nucleon for a LET value ranging between 709 and 762 eV/nm for Al(OH)3 and 837 and 894 eV/nm for AlOOH. Calculations with SRIM program were achieved to estimate the energy loss deposited in the irradiation window and the glass walls and to determine the deposited energy in the samples. 17

H2 determination An Agilent 6890 Trace-Gas Chromatograph (Agilent Technologies, Santa Clara, CA) combined with a pulsed-discharge He photoionization detector from VICI Instrument Co, Inc. (PPD model D-3, Houston, TX) has been employed for the determination of molecular hydrogen released at room temperature from electron irradiated samples. The estimated error in the gas measurement is less than 10%. The carrier gas used was ultra-high-purity helium (Helium 6.0). This technique provides the precision and detection limit (10 ppb) suitable for our dry samples. As for hydrogen released after annealing above room temperature, µ-GC (μGC-R3000 SRA instrument) (NIMBE) and Agilent 450 (LRMO) using ultrahigh purity argon (argon 6.0) as the carrier gas were used. Gas chromatographs were equipped with a Pomp turbo HiCube 80Eco (Pfeiffer) that provides a secondary vacuum.

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Results Characterization of the materials The characteristics of the various material used are summarized in Table 1. Both XRD and specific surface analysis confirm the nanoparticle form of the material. Moreover, XRD shows that we have almost pure phases (Figures S1, S2 and S3). Structure characterization was also done using FTIR (Figures S4 and S5). IR spectrum related to AlOOH L shows sharp bands at 3282 cm-1 and 3084 cm-1attributed to OH stretching (Al-OH asymmetric and symmetric respectively). Similar bands are seen in the case of AlOOH S though they are significantly larger, indicating a more disordered structure in this latter material. 7, 18 Al(OH)3 spectra show sharp bands at 3546, 3655 and 3460 cm-1. The last two bands are also attributed to Al-OH symmetric and asymmetric stretching respectively.19-20 Drying conditions were optimized considering the weight loss between 20˚C and the temperature of the first derivative thermogravimetric curve (DTG) peak due to loss of water that is not chemically bound (Figure S6). Care was taken not to exceed the temperatures higher than that of the first DTG peak, where water loss is due to structural water. Trace-elements were determined using ICP-AES (Figure S7). Fe and Chromium impurities that may have an impact on the radiolytic phenomena are present only in minor amounts: 85, 65, and 50 μg/g for AlOOH L, Al(OH)3 and AlOOH S. XRD and FTIR were used to check sample integrity before and after thermal treatment and irradiation. The same characterization methods done on pristine powders described above were repeated after heat treatment and irradiation (Figures S2 to S5). No differences were seen between pristine, treated and irradiated powders.

Hydrogen production For all three materials, the H2 production was analyzed up to 120 kGy. It was proportional to the dose. H2 radiolytic yields were deduced from the slopes of the different curves (Figure 1). These experiments had to be reproduced many times in order to determine the experimental uncertainty of the measured yields. This uncertainty varied from one type of material to another and was the

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highest in the case of AlOOH L. The conditioning process was absolutely more difficult in this case It is worth noting that up to 120 kGy, AlOOH S did not produce quantifiable H2 and irradiations were pursued in this case until 390 kGy to measure a yield H2 produced from irradiated AlOOH depends on crystallite sizes: AlOOH L has the greater radiolytic yield (0.05 ± 0.02) x 10-7 mol/J) while AlOOH S produces only a minimal amount of H2 (a maximum of (0.004 ± 0.002) x 10-7 mol/J). Contrary to the results obtained by Westbrook et al, 11

studied nanoparticles of Al(OH)3 definitely produces H2 under irradiation (0.021 ±

0.005) x 10 7 mol/J) (The corresponding radiolytic yields of H2 are presented in Table 2). However, these yields may be only the tip of the iceberg. Irradiated materials are known to be able to trap radiolytic species and release them upon activation by dissolution

21

or heating. 22-23

Therefore, in a second phase, we analyzed whether the hydroxide and oxohydroxide we studied could store some H2 upon irradiation. In our case, dissolution experiments conducted on irradiated Al(OH)3 at 170 kGy, did not allow to measure hydrogen released, the concentration of trapped hydrogen was lower than to 1.1 x 10-11 mol/kg. We suggested starting with Al(OH)3 since it is supposed to produce the highest H2 quantities (it has more OH groups than AlOOH), due to the low concentrations obtained from this Bayerite, the dissolution was not conducted on AlOOH S and AlOOH L. However post irradiation annealing experiments up to 250˚C show that significant amounts of hydrogen or hydrogen precursors were trapped in AlOOH L and Al(OH)3 (Figure 2). Higher temperatures lead to a progressive transformation of the various materials into Al2O3. Radiolytic yield of hydrogen are presented in Table 2.

Radiation Induced Defects (RID) EPR spectra of paramagnetic centers originating from e-beam irradiation of Al(OH)3 and AlOOH S and L have been measured and will be discussed in this section. In order to give a general idea of the trapped centers formed at room temperature, EPR spectra of irradiated hydrates, are compared at 120 kGy in Figure 3 and represented as a function of the dose in Figure 4. The spectra were normalized by the sample weight.

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Two narrow signals separated by 50 mT are identified in AlOOH L and Al(OH)3 EPR spectra. This doublet was assigned to H radicals and was observed in alkaline earth hydroxides irradiated with rays at 77 K (described width 0.32-0.40 mT) 9, 24 and at room temperature in AlOOH and fluoride compounds of aluminum.24 The central part of the spectrum is more complex and has to be deconvoluted in order to assign the various signals. The simulations of the EPR spectra corresponding to various systems are represented in Figure 4 and a typical deconvolution is shown in Figure S8, using the spectra of AlOOH S at 6 kGy as an example. AlOOH S and AlOOH L present two similar components: 2.024,

A broad component RID I, with 3.09

∆

2.0034 and ∆

∆

1.7

,

. It is the major component formed at low doses. 2.010

A narrower singlet RID II, with

0.001 and ∆

1.7

(no anisotropy could be

determined). It superposes progressively to RID I and become the major component at high doses. A third, minor component RID III is seen specifically in AlOOH L at high field 1.998, ∆

(

0.2

(Figure 4).

Annealing reveals the presence of a preexisting anisotropic signal RID IV with

2.063 (Figure

6, 7 and S10). This signal is observable only when the intensity of the other signals has decreased but does not appear to be created upon annealing. Unlike other centers, this defect is stable up to 300°C (Figure 7). In Al(OH)3, the shape of the EPR signal remains unchanged whatever the dose is. It is dominated by a wide slightly asymmetric singlet. The related spin parameters are: 2.0026 and ∆

∆

0.84

,∆

3.88

2.030,

. The spin parameters of RID I’ are close

to those found for RID I. The anisotropy is slightly more important for RID I’. We suppose that this defect is similar to that of RID I so it will be called RID I’ thereafter. Some very small signals are superimposed on this signal (Figure 4 and RT data in Figure 8): -

A narrow peak at 7 kGy (called RID III’) which characteristics are close to those of RID III: a

-

factor of 1.998 and a width of 0.2 mT,

A signal with a hyperfine interaction above 30 kGy (called RID VI’).

In order to determine if defects are located on the surface or not, EPR spectra were recorded before and after exposing the irradiated samples to air. Only a small difference was observed: RID I and

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I’ signal decreases and RID III/RID III’ disappeares. The concentration of H radical remains unchanged when sample was exposed to air. This result leads us to suppose that most reactions are not happening on the surface and that H radicals and the major induced defects reside in the bulk and are trapped inside our powder structure. Signal attribution The spin parameters of RID I and RID I’ are in the range found for oxygen centers and Al(OH)3.6, 25-26

in Al2O3

corresponds to a hole trapped in filled 2p orbitals of oxygen, it has a spin .27 We checked also that similar centers were

½ and an axial anisotropy with

formed in Al2O3 nanoparticles formed by dehydration of the various materials. It is tempting to assign RID II to an

center in another site of stabilization. Nevertheless a quasicenter in such a

isotropic signal seems incompatible with an close to

range (in this case

should be

). Steinike and al. reported the formation of ozonide in mechanical treated Hydrargillite,

a polymorph of the Gibbsite. 6 Despite the poor quality of the Figures reported in the literature, some similarities can be found between RID II signal and the signal reported by Steinike et al. The authors assigned this center to ozonide radical

formed by reaction (2) of O2 with

→

:8

(2)

However, our samples have been extensively degassed, and thus we can wonder if radiolysis can produce dissolved oxygen in the material, as already observed in glasses.28 Ozonide radical was 2.012 in Barium hydroxides8 and in different oxides such as

observed as a singlet with quartz. 29

The presence of a third type of oxygen centered defect must be taken into account: RID IV. The anisotropy of the RID IV signal characterized by to

permits to assign this signal

centers. 30 It is generally associated with the recombination of

centers followed by hole

trapping (Reaction 3 and 4): ⋯ ⋯

⋯

→

⋯

⋯ →

⋯ ⋯

(3) ⋯

(4)

Subsequent trapping of holes can lead to the observed ozonide by, for example, reaction 5 and 6: ⋯

9

⋯

→

⋯

⋯


(5)


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⋯

⋯

⋯

→

⋯

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⋯

(6)

RID III cannot be attributed to an oxygen centered defect. Classically, g factors smaller than that of the free electron are attributed to electrons centers. 31-32,33 Indeed, the adsorption of O2 from air leads to the decrease of RID III. These observations enable us to attribute the spectrum to electrons captured by surface anionic vacancies (usually called FS centers). The last type of RT defect is RID VI’. It is characterized by 11 lines resulting from a hyperfine structure from two Al3+ nuclei and is supposed to be

…

…

described by Kuruc et al in

aluminum hydroxide samples. 26

Defect yields and stability Defect yields and stability are tightly connected, the concentration of a defect is representative of the competition between its formation and the various annealing processes. The behavior of defect (in concentration) as a function of the dose is represented in Figure 5 and Figure S9 while their thermal annealing behavior is represented in Figure 6 to 8. H atom maximum concentration is achieved at very low doses in both AlOOH L and Al(OH)3, and no yield can be derived from the data (Figure S9). When considering temperature effect, the stability of H radicals varies markedly from one system to another. They are not stabilized in AlOOH S, disappear immediately above room temperature in Al(OH)3, and anneal in AlOOH L only above 80°C. The difference of stability between AlOOH S and AlOOH L can be attributed to a higher disorder in AlOOH S as H atoms require highly symmetric cages to be trapped. 34 We can notice that H atoms are less stable in the studied nanoparticles than in the work of Scholz et al. 34 In this latter study, it was possible to trap H radicals in thermally untreated AlOOH powder up to 150ºC. 34 The behavior of oxygen centered defects is more complex (Figure 5). In Bayerite, RID I’ increases almost linearly with the dose. In Boehmite RID I saturates slowly with the dose while RID II accumulates. This confirms the possible involvement of RID I as an intermediate in the formation of RID II. The initial yields of formation of RID defects are estimated using the slope at the origin (see Table 2). They all have values in the 10-7 mol.J-1 range.

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Oxygen centered defects disappeared at higher temperatures than H radicals except in AlOOH L where H radicals and RID I anneal synchronously. There may be a limited effect of particle size on RID stability, as RID II anneal more efficiently in AlOOH S than in AlOOH L, but this difference in stabilization is less pronounced than in the case of H atom. We can also notice that in Al(OH)3 RID I’ evolves above 120°C in ozonide like species RID II’ with a gz=2.009± 0.005 and in peroxy like species RID V’ with a g factor of 2.016 ± 0.005. (Although the anisotropy is different from those of RID IV it remains compatible with an O2- center (Figure 8) The formation of these species can follow reaction (3) to (6) with an additional O- center serving as a hole source. From these data, we must distinguish three types of behavior: -

Some defects do not accumulate because they are intrinsically unstable. This concerns especially the reductive defects (H atoms and F centers, Table 2) and confirms that hydrogen production is probably the preferred pathway of evolution of the reductive equivalents produced by irradiation,

-

Some defects, like RID I in AlOOH, are not intrinsically unstable, but do not accumulate. An explanation of this saturation would be that they undergo further reactions under irradiation. The particle size has little effect on these processes.

-

Some defects accumulate with the dose, probably because they cannot evolve further at room temperature, like RID I’ in Al(OH)3 or they are already very oxidized, like RID II in AlOOH. They are the final state of evolution of the oxidant equivalent produced by irradiation.

Discussion : H2 yields measured in this study are compared to the literature in Table 3. We can notice that values are among the lowest for hydroxides. Of course, our results were obtained from electron irradiation, and most literature data focuses on gamma rays. Therefore dose rate effect can be suspected, but this discrepancy can also arise from sample preparation (our drying conditions were very carefully optimized), and particle size used. We must also keep in mind, that even if G(H2) values were in the range of 10-9 mol/J, these materials produce significant amounts of molecular hydrogen. The yields become really very high when we take into account the H2 trapped inside the materials. For example, the total yield of H2 in AlOOH reaches 0.12×10-7 mol/J, when normalized to the amount of water contained in AlOOH

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(15% in weight), it corresponds to a yield of 0.8×10-7 mol/J, to be compared to a primary H2 yield in bulk water of 0.45×10-7 mol.J-1. Besides providing H2 yield measurement, our study points out the existence of two sources of H2 in irradiated hydroxides. The first one corresponds to an immediately released H2 (H2 released), which would be a classical radiolytic H2. The second one corresponds to a thermally activated H2 detrapping (H2 detrapped). This pathway may be independent from the immediate released one as there is a 200°C gap without almost any H2 production before H2 detrapped onset (Figure 2). For the H2 released, we can derive general mechanistic rules for the hydroxide and oxohydroxide studied here. The formation of both H radicals and

centers point towards the existence of homolytic

dissociation process ⋯

→

⋯

(7)

With H radicals recombination being the natural source of immediately released hydrogen (H2 released) →

(8)

As a competing reaction, the as formed H atoms can also be trapped, but the trapping sites are rapidly saturated with the dose: →

(9)

We probably have ionic processes in parallel, as trapped electron has been observed as RID III. ⋯

→

⋯

(10)

The produced protons will naturally react with the hydroxyl of the material to yield water. Within this reaction scheme, H atoms would have a high probability to be produced in separate crystallites in AlOOH S, owing to their size, and would have difficulties to encounter. This nanoconfinement effect would limit H2 production by reaction (8). To test this hypothesis, we conducted high LET radiations with Argon ions. This type of irradiation produces mainly electronic excitations and is well known to favor radical radical reactions due to the high concentration of reactive species in the tracks. As a matter of fact, high LET irradiations restored H2 released production

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in AlOOH S (G (H2 released) =3×10-9 mol/J), and increased it in AlOOH L (G (H2 released)) = 3×10-9 mol/J) and Al(OH)3 (G (H2 released)= 2×10-9 mol/J). However, owing to the activation of the material, defect production and thermal H2 detrapping could not be reliably analyzed. Considering the production pathway of molecular hydrogen upon heating, the most straightforward mechanism would be the thermally activated H radicals detrapping 2

(11)

However, we have many indications that trapped H radical is probably not the H2 detrapped precursor. First the temperature of disappearance of H radicals does not correspond to the H2 detrapping in any of the tested solids. Secondly the concentration of H trapped is much lower than that of H2 detrapped (H concentrations are reported in Figure S9). An alternative mechanism would be a pathway leading to H2 detrapped in crystallite trapping of H2 formed by H atom recombination →

(12)

In fact, if reaction (8) was occurring on the crystallite surface and reaction (12) inside, very different diffusion/thermal requirements would be expected for the two types of H2. However, owing to its very high specific surface/low crystallite size, AlOOH S would be then expected to favor strongly H2 released compared to AlOOH L. This is obviously not the case. In the absence of any direct proof of H2 trapped presence inside the material (a proof that would be difficult to obtain owing to the low concentration involved), we cannot also exclude alternative EPR silent precursors for thermally activated H2 production. For example, H2 production have been described to occur at cationic vacancies in Mg(OH)2 35 but the relativistic electron we used are not expected to produce efficiently atom displacement. Reduced iron sites could also seem as a good precursor.36 Indeed the concentration in iron (1.2 x 10-3 mol/kg) can withstand the H2 detrapped yields we measured but we did observe only a partial reduction of iron in these systems (30% at 140 kGy in AlOOH L). The mechanism of

production may even be a complex combination of these different

contributions and deserves a dedicated investigation. However, all these models suggest the same

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explanation why AlOOH S presents few or no H2 detrapping at 250°C: the oxygen centered defects react with the H2 detrapped precursor more easily than in other materials because of the small particle size and of its greater disorder. Besides mechanisms, we must comment on the efficiency of the radiolytic processes. When looking at radiolytic efficiencies, we must keep in mind that we look only at final yields that encompass complex recombination processes occurring at very different time scales. The RID, owing to their stability and their early production, give good indications of the initial processes occurring within the material. When the various RID yields can be compared at low doses, where the defect production has not yet saturated in AlOOH (Table 2), the dissociation efficiencies are similar for AlOOH L and Al(OH)3 and slightly lower for AlOOH S. As stated earlier, RID yields are very high, and are comparable to the OH° primary yields in water. Reaction 7 is obviously very efficient independently of the material nanostructure and would deserve further investigations by time resolved techniques. In Al(OH)3 the difference between RID production and H2 production shows the efficiency of the various recombination processes occurring on long time scales. There is here more than one order of magnitude between RID production and the final (detrapped or released) H2 production. This indicates that the detrapped H2 is only a portion of the trapped one. The most obvious reaction in competition with the detrapping would be the following ⋯

→

⋯

(13)

This hypothesis could explain why H2 detrapping upon heating occurs efficiently only once most RID defects have been annealed. These observations must be modulated for AlOOH L and AlOOH S. In these two materials in the high dose range, the RID I production saturates, even if H2 production carries on, as in AlOOH L. Considering the comparatively much lower yield in RID II, this saturation cannot correspond solely to

⋯

⋯

recombination. It is indeed the signature of an additional energy

dissipation process occurring when the mass concentration in probable phenomenon is the non-geminate H and

⋯

⋯

is sufficient. The most

recombination, but we cannot exclude

that the high concentration of defect decreases the efficiency of reaction (7). If the initial energy deposition process seems very efficient in aluminum hydroxides and oxyhydroxides, diverse

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processes (confinement, recombination, H2 or H2-precursor back reaction) can explain the limited H2 production at RT compared to the initially high defect yields and the sensitivity of the H2 production to the history/nanostructure of the material.

Conclusion Aluminium hydroxides and oxohydroxides present H2 and oxygen centered defect production under electron irradiation. The material nanostructure has an impact on H2 production even more important than its atomic composition. This impact does not seem connected to the initial steps of the radiolytic processes that are very efficient in all the tested materials, but rather to nanoconfinement effects during the reactive species diffusion. A significant part of the absorbed energy remains stored into the materials, and induces a secondary H2 production upon heating. The effect of adsorbed water on these same particles will be elucidated in further studies. Supporting Information. DRX and FTIR of the samples before and after thermal treatment and irradiation. ATG/DTG and ICP-AES analysis of the samples. Additional EPR spectra and defect quantifications. This material is available free of charge via the Internet at http://pubs.acs.org.” Corresponding Author *[email protected] ACKNOWLEDGMENT This work has been supported by CEA, AREVA NC and AREVA TN. We thank Romain Dagnelie for providing us the access to the BET measurements. We thank Michel Tabarant for the ICP-AES analysis and Marie-Noëlle Amiot for EPR access as well as Bruno Boizot for EPR experimental support. We thank Vincent Dauvois for his interesting advices and help. We thank Jean-Jacques Pielawski for his implication in this subject.

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16. Fricke, H. H., J. E, Attix, F. H., Roesch, W. C., Eds.; , In Radation Dosimetry, 2nd Ed. Academic Press: New York and London 1966, 2, 167. 17. Ziegler, J. F.; Ziegler, M. D.; Biersack, J. P., Srim – the Stopping and Range of Ions in Matter (2010). Nucl. Instrum. Methods 010, 268, 1818-1823. 18. Alex, T. C.; Kumar, R.; Roy, S. K.; Mehrotra, S. P., Anomalous Reduction in Surface Area During Mechanical Activation of Boehmite Synthesized by Thermal Decomposition of Gibbsite. Powder. Technol 2011, 208, 128-136. 19. Gallas, J. P., Goupil, J. M., Vimont, A., Lavalley, J. C., Gil, B., Gilson, J. P., & Miserque, O. Quantification of Water and Silanol Species on Various Silicas by Coupling IR Spectroscopy and in-situ Thermogravimetry. Langmuir 2009, 25, 5825-5834. 20. Xu, B.; Smith, P., Dehydration Kinetics of Boehmite in the Temperature Range 723–873k. Thermochim. Acta. 2012, 531, 46-53. 21. De Las Cuevas, C.; Miralles, L.; Pueyo, J. J., The Effect of Geological Parameters on Radiation Damage in Rock Salt: Application to Rock Salt Repositories. Nucl. technol. 1996, 114, 325-336. 22. Johnson, R. E.; Quickenden, T. I., Photolysis and Radiolysis of Water Ice on Outer Solar System Bodies. J. Geophys. Res. E: Planets 1997, 102, 10985-10996. 23. Teolis, B. D.; Shi, J.; Baragiola, R. A., Formation, Trapping, and Ejection of Radiolytic O2 from Ion-Irradiated Water Ice Studied by Sputter Depth Profiling. J. Chem. Phys. 2009, 130, 134704. 24. Vedrine, J. C.; Imelik, B.; Derouane, E. G., Influence of the Physical State of Three Different Aluminum Hydroxides on the Interaction between Trapped H Atom and Lattice Protons: An Epr and Endor Study. J. Magn. Reson (1969) 1974, 16, 95-109. 25. Shiyan, I. N.; Serikov, L. V.; Smekalina, T. V.; Vasiliev, A. A., Epr Studies of Aluminium Oxide Phase Compositions. React. Kinet. Catal. Lett. 1990, 41, 291-294. 26. Kuruc, J., Paramagnetic Centers by X-Ray-Irradiation of Aluminium Hydroxide. J. Radioanal. Nucl. Chem. 1991, 154, 61-72. 27. Henderson, B.; Wertz, J., Defects in the Alkaline Earth Oxides. Adv. Phys. 1968, 17, 749855. 28. Ollier, N.; Rizza, G.; Boizot, B.; Petite, G., Effects of Temperature and Flux on Oxygen Bubble Formation in Li Borosilicate Glass under Electron Beam Irradiation. J. Appl. Phys. 2006, 99, 073511. 29. Griscom, D. L., Diffusion of Radiolytic Molecular Hydrogen as a Mechanism for the Post Irradiation Buildup of Interface States in Sio2 on Si Structures. J. Appl. Phys. 1985, 58, 25242533. 30. Griscom, D. L.; Merzbacher, C. I.; Weeks, R. A.; Zuhr, R. A., Electron Spin Resonance Studies of Defect Centers Induced in a High-Level Nuclear Waste Glass Simulant by GammaIrradiation and Ion-Implantation. J. Non-Cryst. Solids 1999, 258, 34-47. 31. Chen, W., Optically Detected Magnetic Resonance of Defects in Semiconductors. In Epr of Free Radicals in Solids, Lund, A.; Shiotani, M., Eds. Springer US: 2003; Vol. 10, pp 601-625. 32. Janak, J. F., G Factor of the Two-Dimensional Interacting Electron Gas. Phys. Rev. 1969, 178, 1416-1418. 33. Chen, W. M., Epr of Free Radicals in Solids Ii. Trends in Methods and Applications. Chapter: Optically Detected Magnetic Resonance of Defects in Semiconductors., 2nd ed.; Springer, 2012.

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34. Scholz, G., & Stösser, R. , Atomic Hydrogen as Spin Probe in Thermally and Mechanically Activated Materials. Phys. Chem. Chem. Phys. 2002, 4, 5448-5457. 35. Scoville, J.; Sornette, J.; Freund, F. T., Paradox of Peroxy Defects and Positive Holes in Rocks Part Ii: Outflow of Electric Currents from Stressed Rocks. J. Asian Earth Sci. 2015, 114, 338-351. 36. Shipko, F.; Douglas, D. L., Stability of Ferrous Hydroxide Precipitates. J. Phys Chem. 1956, 60, 1519-1523. 37. Ghormley, J.; Stewart, A., Effects of γ-Radiation on Ice1. J. Am. Chem. Soc. 1956, 78, 2934-2939. 38. J.A. Ghormley, a. A. O. A., A. E. C, In 115th Am. Chem. Soc. Meeting San Franciscom California, 1949. 39. Lefort, M. C. a. M., Sur Le Mécanisme Chimique Primaire De Radiolyse De L'eau. J. Chim. Phys 1955, 52, 545-555. 40. Scheuer, W. D. a. O., Décomposition De L’eau Par Les Rayons. Compt. Rend 1913, 156, 466-467. 41. P. Gunther, a. L. H., Quantum Yields of Gas Reactions Induced by Shortwave Ultra-Violet Light. Z. Physik. Chem 1939, 374. 42. L.I.Barsova, T. K. Y., S. L. Orlov, and M. B. Zubareva, Radiolytic Conversions of Strontium and Barium Hydroxides and Their Crystal Hydrates. Khimiya Vysokikh Energii 1990, 24, 329-334.

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Table 1. Physicochemical parameters of the materials.

AlOOH L

AlOOH S

Al(OH)3

Boehmite 94%

Boehmite 95%

Bayerite 90%

Crystallite size (nm)

18

5

20

Density

3

3

2.4

BET-specific surface (m2/g)

41

270

110

Phase composition

Table 2. H2 radiolytic yields at room temperature and detrapped at high temperature from the different hydroxides. Defect yields are defined in the dose region where their production increases linearly with the dose. When yields cannot be determined, concentrations in defects are given.

Molecular hydrogen yield

Radicals yield at RT

Concentration at RT

(mol/J)

(mol/J)

(µmol/kg)

Gaseous RT

Detrapped

G (RID I)

G (RID II )

x 10-7

x 10-7

x 10-7

x 10-9

AlOOH L

0.05 ± 0.02

0.02 ± 0.005

2.3

AlOOH S

(0-0.004)

(0-0.008)

Al(OH)3

0.021 ±

0.03 ± 0.01

H

RID III

7.4

330

100-170

1.3

6.3

0

0-2

3.1

-

4-30

0-3

0.005

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Table 3. H2 radiolytic yields from different hydroxides found in the literature and compared with our results.

Molecular hydrogen yield

Reference

(mol/J) x10-7

20

H2 (Ice)

0-0.7

37-41

Ca(OH)2

0.21

10

Sr(OH)2

0.13

42

Ba(OH)2.H2O

0.093

42

Ba(OH)2

0.073

42

AlOOH

(0.057-0.13)

11

Mg(OH)2

0.053

42

Ba(OH)27.4H2O

0.052

42

AlOOH L

0.05 ± 0.02

Our results

Sr(OH)2.H2O

0.042

42

Sr(OH)27.4H2O

0.031

42

Al(OH)3

0.021 ± 0.005

Our results

AlOOH S

(0-0.004) ± 0.002

Our results

Al(OH)3

0

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Figure 1. Typical molecular hydrogen production curve (µmol/kg) from e-beam irradiated aluminum hydroxides and oxyhydroxides as a function of the dose: (a) AlOOH Large-particle size and (c): AlOOH Small-particle size (b): Al(OH)3.

Figure 2. Effect of temperature on hydrogen detrapping from e-beam irradiated samples at 120 kGy. Cumulative H2 radiolytic yields are plotted with respect to temperature (40˚C/hour), the highest temperature

21



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was chosen to repeat the experiment and define extreme values for the detrapping and their associated experimental error (open square).

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Figure 3. Comparison of EPR spectra of e-beam irradiated AlOOH L, S and Al(OH)3 at 120 kGy.

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Figure 4. EPR spectra for dry samples after e-beam irradiation at 7, 30 and 120 kGy at RT. Black solid lines represent experimental data, colorful solid lines represent the sum of theoretical fits.

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Figure 5. Evolution of the concentration of RID I and RID II and RID I’ in e-beam irradiated AlOOH L, AlOOH S and Al(OH)3. The error bars refer to the reproduction of the same quantification at a single dose.

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Figure 6. AlOOH L annealing showing radiation induced defects shapes and H radicals with respect to temperature (top right and left respectively) and the disappearance in intensity (bottom). Temperatures (top) are represented as follows: Violet: RT, pink: 40°C,

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Figure 7. AlOOH S annealing showing radiation induced defects shapes with respect to temperature (top right and left) and the disappearance in intensity (bottom). Temperatures (top) are represented as follows: Olive: RT, black: 40°C, orange: 80°C, purple: 120°C,

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Figure 8. EPR spectra (right) and relative intensity (left) of e-beam irradiated Al(OH)3 at 120 kGy after annealing at different temperatures. EPR spectra are represented as follows: Pink: RT, wine: 40°C, purple: 80°C, orange: 120°C, black: 160°C and olive 200°C

Figure 9. EPR spectra of electron irradiated Al(OH)3 at 120 kGy and annealed one hour at 225°C. Straight lines represent the experimental data and dotted ones the simulations. RID I’ (black), RID II’ (green), and RID V’ (violet).

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Radiolytic Events in Nanostructured Aluminum Hydroxides - The

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