Evaluation of Silver Zeolites Sorbents Toward Their Ability to Promote

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Evaluation of Silver Zeolites Sorbents Towards their Ability to Promote Stable CH3I Storage as AgI Precipitates Bruno Azambre, and Mouheb Chebbi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02366 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 12, 2017

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ACS Applied Materials & Interfaces

Evaluation of Silver Zeolites Sorbents Towards their Ability to Promote Stable CH3I Storage as AgI Precipitates Bruno Azambrea*, Mouheb Chebbia† a

Laboratoire de Chimie et Physique-Approche Multi-Echelle des Milieux Complexes (LCP-

A2MC- EA n°4362), Institut Jean-Barriol FR2843 CNRS, Université de Lorraine, Rue Victor Demange, 57500 Saint-Avold, France KEYWORDS: Iodomethane, TPD, adsorbent, nuclear power plant, XRD, N2 sorptiometry at 77K.

ABSTRACT

In this study, up to 13 different silver zeolites sorbents were prepared by repeated ion-exchange from their parent structures (FAU X and Y, MOR, *BEA, MFI, FER), characterized and evaluated for their ability to capture methyl iodide in the context of nuclear severe accident. A novel methodology was implemented to establish structure-activity relationships between sorbent properties and iodine trapping stability. After saturation of the zeolite bed with CH3I during a dynamic breakthrough experiment at 100°C, a two-steps quantitative desorption method was elaborated with the aim to quantify separately the CH3I fractions trapped by physisorption,

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chemisorption or reacted as AgI precipitates. Besides, an analysis of the mechanisms involved in CH3I sorption and decomposition processes was also carried out. Overall, Ag/Y zeolites displayed the highest fractions trapped as stable AgI precipitates, thanks to the presence of high amounts of dispersed silver species at accessible locations in the large supercages, and their low sodium content.

1. INTRODUCTION Since the Fukushima accident in 2011, some interest has been renewed worldwide for R&D works on atmospheric source-term mitigation technologies.1 During a sequence of nuclear severe accident (SA), the threat existing on the integrity of the containment building, due to an uncontrolled rise of pressure, can be prevented by venting. Nevertheless, it is essential that the venting line should be equipped by one or several mitigation technologies to capture radiotoxic releases before their dissemination in environment.2 Nowadays, many countries involved in the production of nuclear energy are currently implementing or improving Filtered Containment Venting Systems (FCVS) in order to improve the safety of their nuclear power plants.2-4 Existing FCVS technologies, which could be classified as “dry” and “wet”, are known to be efficient for aerosol retention, but much less for volatile iodine (I2 and organic iodides).5-8 Among “dry” systems, sorbents based on molecular sieves and activated carbons present a great interest because of their extended trapping surfaces.1 In that respect, inorganic sorbents such as silver zeolites were identified from pioneered works in the 1970’s for their ability to promote stable iodine trapping by formation of AgI precipitates.9,10-12 Moreover, silver zeolites are expected to be robust and to withstand elevated temperatures, humidity, irradiation and oxidizing conditions. Under SA conditions, the efficiency of iodine trapping may be impacted by the


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nature of the sorbent itself, but also by temperature, bed dimensioning/flowing conditions, irradiation as well as potential inhibitors such as steam and other gases.1,5,6 Usually, retention performances are determined by a Decontamination Factor (DF), which represents the ratio of iodine concentrations measured at the inlet and outlet of a sorbent bed. DF measurements are useful to compare the filtration efficiency of several sorbents for a given set of operating conditions (provided the sensitivity of the detection technique is sufficiently high1) but fail in providing crucial information on the trapping stability or on trapping mechanisms. Although many zeolite structural types exist, most of investigations have been limited to Ag/X and Ag°/MOR zeolites.9,10,13 These sorbents showed good filtration behavior under normal operating conditions (with DF about 103-104 11-13), but numerous uncertainties remain under SA conditions, especially regarding the stability of iodine trapping. Hence, a reminding gap for the application of silver zeolites in nuclear power plants is the lack of information available on the impact of zeolite structure and silver content on retention properties. Insufficient knowledge on the molecular mechanisms involved in iodine trapping, especially under SA conditions, is also an obstacle towards a rational design of these sorbents. The present study will be specifically focused on CH3I retention, because it is the most widespread compound in the class of organic iodides.14,15. The in-depth characterization of selected silver zeolite sorbents was already presented likewise the interpretation of adsorption capacities and diffusional constraints determined from dynamic breakthrough experiments.16 Here, we propose for the first time to evaluate the stability of volatile iodine species trapped over a wide range of silver zeolites with different structural types (FAU X and Y, MOR, *BEA, FER and MFI) and silver contents (0-35 wt%). Fractions trapped by physisorption, chemisorption and AgI precipitates will be determined by establishing material balances between data obtained


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during the adsorption phase with those collected in the course of two-steps subsequent Temperature-Programmed Desorption (TPD) experiments. Finally, a discussion on the relative efficiencies of silver use by the different zeolites will be given.

2. EXPERIMENTAL 2.1.Materials Commercial zeolites of structural types FAU (Y, Si/Al = 2.5, CBV 300), MOR (Si/Al = 10, CBV21A), FER (Si/Al = 10.5, CP914C), *BEA (Beta, Si/Al = 10.1, CP814E*), MFI (ZSM-5, 11.5, CBV 2314) were all provided by Zeolyst in ammonium form. A 13X (NaX) zeolite (Si/Al=1.2, 60-80 mesh) was supplied by Sigma Aldrich. The diffraction patterns of the parent structures were found to be consistent with those of reference materials,17 with no phase impurities detected. Silver incorporation by ion-exchange into the different zeolitic frameworks consisted in stirring 2 g of the parent zeolites for 24h with an aqueous solution of silver nitrate (0.01 M) at room temperature. Then, the exchanged-zeolites were vacuum-filtered and dried at 80°C overnight. Whenever necessary, this ion-exchange procedure was repeated up to 2 (MOR) or 3 times (Y, MFI, and 13 X) in order to achieve higher silver contents. Prior to adsorption and characterization studies, all zeolitic materials were calcined under air in a muffle furnace with a heating rate of 5°C.min-1 from room temperature to 200°C (plateau of 1 hour) and then to 500°C (plateau of 2 hours). In order to avoid any photo-reduction of silver species, all preparation steps were done in the dark. In the following, the obtained zeolitic sorbents are denoted as xAg/Structure (Si/Al) where x is the mass percentage in silver.


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A “benchmark” Ag/13X zeolite (Si/Al=1.2, 382280, Ag84Na2[(AlO2)86(SiO2)106].xH2O) with 35 % wt silver was purchased from Sigma-Aldrich for comparison purposes. This sorbent was denoted in the present study as 35Ag/13Xcomm (1.2). 2.2.Physicochemical characterizations Elemental ICP analyses (Ag, Na, Al and Si) were performed at the Service Central d’Analyses of the CNRS and at the Service d’Analyses des Roches et des Minéraux (France). Measurements of the content of each element were performed using two-times ICP analyses. The uncertainty in the determination of each element was certified to be about 2 % of its value (in wt %), except for elements whose concentration is close to the detection limit (< 0.05 %). Powder X-ray diffraction (PXRD) measurements before and after test were carried out using a Rigaku-Miniflex II (Japan) with the Cu Kα radiation (λ= 0.15418 nm). PXRD patterns were recorded between 5° and 70° (2θ) using increments of 0.02° and a counting time of 2 s. Porosimetric properties were obtained from N2 adsorption isotherms recorded at -196°C on an automated Autosorb IQ sorptiometer supplied by Quantachrome. Prior to each adsorption measurement, samples were outgassed in situ in vacuum at 80°C for 1h and then at 350°C for 6h to remove most of adsorbed impurities. Specific surface areas (SBET) were determined using the BET equation (0.05 < P/P0 < 0.35). Microporous volume (Vmicro) was calculated according to the t-plot method.18,19 The total pore volume (Vpore) was measured from N2 adsorption isotherms at P/P0 = 0.97, whereas the mesopore volume (Vmeso) was deduced by the difference: Vmeso = Vpore – Vmicro. UV-Vis spectra in Diffuse Reflectance (DR) mode were collected on a Varian Cary 4000 UVVis spectrometer equipped with a double monochromator and DRA900 integrating sphere.


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Spectra were recorded between 200 and 800 nm, with a resolution of 2 nm and a scan rate of 600 nm.min-1. A Spectralon standard was taken as reference. 2.3.Description of gas-phase dynamic tests CH3I adsorption/desorption properties were studied in a fixed-bed reactor (tubular Carbolite MTF furnace with Eurotherm controller), using a sample mass of 200 mg, and a total flow rate of 150 mL.min-1 (GHSV of 20000 h-1). The experimental setup16 devoted to CH3I dynamic sorption tests can be divided in three main parts: (i) the generation of the CH3I inlet concentration (1333 ppm/ argon), (ii) a fixed-bed reactor with a quartz tube where the zeolite bed (200 mg, grain size between 200 and 630 µm) is maintained between two pieces of quartz wool; (iii) the detection/quantification system composed by a Varian 4100 Excalibur FTIR spectrometer equipped with a heated gas cell (C2 Cyclone Series – Specac, optical path length = 2 m, V = 0.19 L). An in situ pre-treatment up to 500°C (heating rate 5°C/min with a 30 min dwell at 200°C and 1h at 500°C) was done prior to each retention tests in order to remove most of adsorbed impurities, namely moisture. Then, the retention test was started at 100°C up to saturation of the zeolite bed by adsorbed CH3I species. After completion of the adsorption phase, a degassing step was performed under argon at 100°C in order to eliminate the weakly adsorbed (physisorbed) species. Finally, TPD experiments were performed in order to obtain complementary information on the trapping stability. These tests consisted in ramping the temperature from 100 to 500°C at 10°C.min-1 under argon. During this phase, chemisorbed CH3I was desorbed (orange zone, Fig. 1), likewise the emissions due to carbonaceous by-products generated through catalytic decomposition processes, as stated in our previous study.

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I2 formation in the course of TPD

was also assessed qualitatively (appearance of pink coloration) by using a cyclohexane liquid trap placed downstream the FTIR spectrometer.


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A typical CH3I adsorption/desorption experiment is shown in the case of the 22.8Ag/Y (2.5) zeolite on Figure 1. For each zeolite, all the CH3I species adsorbed or desorbed during these experiments (corresponding to the blue, violet and orange zones on Fig. 1) were quantified by integration of breakthrough curve or TPD signals. Detailed procedures about experimental conditions and quantitative exploitation of gas-phase IR spectra were already given in our previous study.16

Figure 1: Typical CH3I adsorption/desorption profile (example of quantification of the different trapped CH3I species for 22.8Ag/Y (2.5) zeolite).

3. RESULTS


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3.1.Characterization of silver-exchanged zeolites In this study, up to 11 different silver zeolites were evaluated for their ability to trap methyl iodide. Their most important chemical characteristics are displayed in Table S1 (ESI) together with those of their parent materials (in protonated or sodium forms). Except faujasite X zeolites, all zeolitic sorbents have low sodium contents. As expected, the amounts of silver incorporated depend on the Si/Al ratio, the zeolitic structure and the number of successive exchanges. Owing to their small Si/Al ratio and higher Cationic Exchange Capacities (CEC),21 the highest silver contents were logically reached for faujasite X and Y zeolites, with about 23 wt% silver incorporated after three successive exchanges. A maximum of 35 wt% silver was even found for the commercial 35Ag/13Xcomm (1.2) sorbent. For the other structures (MOR, MFI, FER, *BEA), which were all characterized by Si/Al ratio of the order of 10, the silver content remained in the range 3.4-9.0 wt%. Assuming that each Ag+ exchanges one NH4+ (or one Na+ for faujasite X zeolites), the theoretical exchange degree (corresponding to Ag/Al values in Table S1) was found to be always below 100% for all silver zeolites. Hence, over-exchange leading to silver aggregation on the external surface, probably did not occur in our synthesis conditions. This was further confirmed by XRD analyses (not shown here), which did not reveal the existence of a crystallized Ag2O phase or large silver metallic nanoparticles for all studied zeolites.16 Rather, preliminary experiments by DR-UV-Vis spectroscopy by us and others have shown that for all silver zeolites, Ag+ ions located at exchange positions coexisted with charged Agδ+n (and sometimes metallic Ag°m clusters) confined in the internal porosity.16,20,22-25 The existence of the latter species is due to auto-reduction processes occurring in the course of the thermal treatment of the sorbents. Those phenomena were reported to be promoted in the presence of zeolite water as depicted by Eq. (1):26,27


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nAg+ + ZOn- + (n-δ)/2 H2O → Agnδ+ + (n-δ) H+ + ZOn- + (n-δ)/4 O2 Eq. (1) with ZOn- = zeolitic framework and n > δ. Hence, the Ag/Al ratio provided in Table S1 cannot be used directly to estimate the real exchange degree. Textural characteristics of these sorbents were also examined by N2 porosimetry at 77K (Table S1). Generally, significantly higher micropore volumes (Vmicro) were measured for the large-pore faujasite (X and Y) zeolites, even after exchange with silver, and also to a lesser extent for mordenite sorbents. By contrast, the decrease of porosimetric characteristics was comparatively more important for other zeolitic sorbents with smaller pore size, such as Ag/MFI, Ag/BETA and Ag/FER zeolites. This can be reasonably understood, considering that the presence of silver species as Ag+, Agδ+n, Ag°m entities in the cavities of such zeolites induces more restrictions to the N2 probe than similar species located in larger pores.

3.2.CH3I retention performances at 100°C Adsorption capacities at breakthrough and saturation, as well as diffusional constraints, were determined previously for all the investigated zeolites.16 The purpose of the current study being to focus on the trapping stability of adsorbed CH3I species, only a brief summary of former experiments is provided here for a better clarity. As stated in our recent study,16 breakthrough curves obtained during the adsorption phase at 100°C differed by the time required for breakthrough (t5% defined by C/C0 = 0.05) and the steepness of C/C0 profiles after breakthrough. All along retention experiments, CH3I was the only I-containing product, HI and I2 being not detected.


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Sample

Qsat (mmol/g)

Physisorbed CH3I Qphys I (mmol/g) (%)

0.116 ± 0.029

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Chemisorbed CH3I Qchem I (mmol/g) (%)

11± 3

Iodine as precipitate* QAgI +NaI (mmol/g) (%)

0.067 ± 0.020

70 ± 4

0.429 ± 0.004

9.1Ag/Y (2.5)

0.613 ± 0.037

19 ± 4

22.8Ag/Y (2.5)

1.571 ± 0.094

7±2

0.116 ± 0.029

17 ± 4

0.262 ± 0.078

76 ± 4

1.168 ± 0.012

13X (1.2) (Na-X)

0.648 ± 0.039

39 ± 8

0.253 ± 0.063

35 ± 9

0.227 ± 0.068

26 ± 1

0.169 ± 0.002

7.3Ag/13X (1.2)

1.050 ± 0.063

16 ± 3

0.168 ± 0.042

27 ± 7

0.283 ± 0.085

57 ± 3

0.598 ± 0.006

23.4Ag/13X (1.2)

1.649 ± 0.099

9±2

0.148 ± 0.037

22 ± 6

0.363 ± 0.109

69 ± 3

1.138 ± 0.011

35Ag/13Xcomm (1.2)

1.881 ± 0.113

7±2

0.132 ± 0.033

17 ± 4

0.320 ± 0.096

76 ± 4

1.430 ± 0.014

5.9Ag/MOR (10)

0.585 ± 0.035

28 ± 6

0.164 ± 0.041

23 ± 6

0.134 ± 0.040

49 ± 2

0.287 ± 0.003

7.3Ag/MOR (10)

0.845 ± 0.051

34 ± 7

0.284 ± 0.071

23 ± 6

0.194 ± 0.058

9Ag/MFI (11.5)

0.599 ± 0.036

32 ± 7

0.198 ± 0.050

4±1

4.2Ag/FER (10.4)

0.451 ± 0.027

76 ±16

0.343 ± 0.086

3.4Ag/BETA (10.1)

0.324 ± 0.019

33 ± 7

0.107 ± 0.027

43 ± 2

0.364 ± 0.004

0.025 ± 0.008

64 ± 3

0.397 ± 0.004

18 ± 5

0.081 ± 0.024

6

0.027

27 ± 7

0.088 ± 0.026

40 ± 2

0.130 ± 0.001

* obtained from molar balance using adsorption capacities at saturation (expressed in mmol/g).

Table 1: Molar amounts and fractions of iodine (in %) stored as physisorbed and chemisorbed CH3I species, as well as AgI precipitates. Overall, adsorption capacities at saturation (Table 1) were found to increase with silver content, almost regardless of the zeolite structure. Indeed, an almost linear relationship is obtained (Fig. 2) between these two parameters. Hence, our data indicate that the amount of dispersed silver species that could be introduced by ion exchange is a more influent parameter on CH3I adsorption than the pore size and pore topology typical of each zeolitic structure. On the other hand, a small deviation towards the linear trend is observed on Fig. 2 at the highest silver contents (35 wt% corresponding to 35Ag/13Xcomm (1.2) sorbent). For exchange degrees close to 100%, larger silver-containing aggregates or silver species in less accessible locations (for


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instance Ag+ cations or small clusters located in sodalite cages of the faujasite structure) may exist in larger proportions. From our adsorption data, it seems likely that these species are less efficient to promote CH3I retention, resulting in a non-optimal use of silver. On the other hand, it was also established in our former study that adsorption capacities at breakthrough, which take into account diffusional limitations, were comparatively much more influenced by the framework type (pore size and connectivity) and other chemical parameters (presence of sodium, Si/Al ratio) than simply the silver content. More particularly, it was found that large-pore Ag/Y zeolite with silver content above 15 wt% and to lesser extent Ag/MOR zeolites are the most efficient sorbents in terms of CH3I filtering properties and are therefore good candidates for FCVS application. By contrast, diffusional constraints were more pronounced for small-to-medium pore zeolites, resulting in shortened breakthrough times. We also showed previously by XRD and DR-UV-Vis spectroscopy that the color change (from white to yellowish) underwent by silver zeolites after test is due to the formation of AgI precipitates.16 In another previous study,20 the mechanism of CH3I retention/decomposition on silver zeolites was addressed in detail for 22.8Ag/Y (2.5) sorbent using in situ spectroscopic techniques. It was found that some adsorbed methyl iodide molecules first undergo dissociation on silver species, leaving an adsorbed methyl fragment or a methoxy species simultaneously to the formation of molecular AgI such as depicted by Eq. (2): Agnδ+ + (n-δ) H+ + ZOn- + nCH3I → nAgI + CxHyn+ + ZOn-

Eq. (2)

Where ZOn- = zeolitic framework, CxHy = C-by products and n > δ.


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Figure 2 : Evolution of CH3I adsorption capacities at saturation (Qsat expressed in mmol/g) in function of silver content (mmol/g) for all silver-exchanged zeolites. Some Ag/X zeolites (in red or violet) were not considered for linear correlation due to their high sodium contents (see the green arrows). In a second step, molecular AgI species further coalesce to give (AgI)n clusters (n < 4-5) within the supercages of the faujasite structure, as witnessed by the progressive growth and blueshift of absorptions in UV-region (colorless). However, as the spent sorbent was exposed to ambient conditions, a rapid growth of AgI species occurred as they diffuse from the internal towards the external surface of zeolite cristallites, characterized by the presence of an absorption edge in the visible region (close to bulk AgI at 432 nm).20 3.3.Desorption of ad-CH3I species


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Upon completion of the adsorption phase at 100°C, methyl iodide was substituted by pure argon (Fig. 1 and section 2.3.). Under these conditions, a displacement of the adsorption equilibrium occurred and weakly adsorbed species were evacuated. A period of 1 hour was sufficient to reach the steady state for all tested zeolites. Those weakly-held species can be considered as responsible of the reversible part of the CH3I storage. They also represent the fraction that could easily escape from the sorbent filter under severe nuclear accident conditions. Hence, an adequate sorbent should in principle promote as much as possible the irreversible storage of iodine. In the next section, they will be quantified and referred as physisorbed species. Hence, the part of adsorbed iodine strongly bound to the sorbents (i.e. remaining after the evacuation step at 100°C) consists either in chemisorbed CH3I (defined in our case as desorbed at temperatures higher than the adsorption temperature of 100°C) or AgI precipitates. For Ag°/MOR zeolites, Nenoff et al.29 surprisingly reported that no CH3I was present in the pores of Ag°/MOR after adsorption, the only iodine species being thought to be occluded AgI entities. In our case, TPD experiments were performed under argon from 100 to 600°C after completion of the adsorption phase. Single or multiple CH3I desorption peaks being observed above 100°C for all investigated zeolites (Fig. 3), this confirms that AgI is not the sole product formed during retention and that chemisorbed CH3I species co-exist with AgI entities. Overall, the profiles of CH3I emissions recorded during TPD experiments (Fig. 3) seem to rely on many factors, including the pore structure of the parent zeolite, the silver content and speciation as Ag+ cations or charged and neutral clusters, and the presence of secondary sorption sites (such as Na+ cations). Moreover, some limitations exist regarding the interpretation of CH3I profiles alone. In addition to gaseous methyl iodide, desorption of adsorbed CH3I led also to the formation of catalytic decomposition by-products, such as oxygenates (methanol (MeOH),


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dimethylether (DME), formaldehyde) at low-medium temperatures, and higher alkanes and alkenes, methane, carbon monoxide and even coke at higher temperatures. Their emission profiles are displayed for the investigated zeolites in Fig. S1 (in ESI). Hence, a catalytically “active” zeolite would release comparatively a lesser proportion of CH3I during TPD because it will decompose to a greater extent in the course of the TPD. The second limitation relies more on diffusional aspects. Owing to the presence of precipitated AgI entities and catalytic by-products in partially occluded pores, it is also possible that some CH3I molecules may encounter some difficulties to diffuse from the pores towards the external surface. This should result in a “delayed” desorption, with some CH3I peaks displaced towards higher temperatures.

Figure 3: CH3I emission profiles during TPD experiments for silver faujasite (X&Y; (A)) and for zeolites belonging to other structural types (*BEA, MOR, MFI; (B)).


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CH3I-TPD profiles are described now. Starting with the faujasite X series, the parent NaX (13X) zeolite displayed a complex pattern with at least 4 CH3I peaks, located at 200, 260, 320 and 400°C. All these peaks are assigned to the desorption of CH3I interacting with Na+ sites present in different locations within the faujasite structure. After single or repeated exchanges with Ag+ (corresponding to Ag(Na)/X zeolites on Fig. 3 (A)), an intense peak is observed at 240250°C. This feature, also observed on the TPD pattern of the 22.8Ag/Y (2.5) zeolite is attributed to CH3I adsorbed on Ag+ cations located in the supercages of the faujasite framework. In our previous study,20 the existence of strongly bound CH3I was also observed on 22.8Ag/Y (2.5) by in situ DRIFTS in the CHx stretching region (IR frequencies of 3020, 2963 and 2855 cm-1). Other features above 300°C on the TPD patterns of Ag(Na)/X zeolites (Fig. 3 (A)) are ascribed to CH3I adsorbed on silver clusters or on residual Na+ sites. For these faujasite zeolites, the amounts of oxygenates (Fig. S1 in ESI, MeOH, DME) released during TPD experiments are found to increase with the silver content and are noteworthy higher for X zeolites compared with Y zeolites. The formation of such oxygenates involves the decomposition of surface methoxy species, produced upon CH3I adsorption and is potentially catalyzed by the presence of residual humidity (more important for the low Si/Al ratio).20,29,30 Above 250-300°C, catalytic decomposition to methane, formaldehyde and CO is also promoted for Ag/X zeolites whereas C3 (propene, propane) and C2 (ethene) hydrocarbons (from coupling reactions between dissociated methyl fragments and oxygenates30,31) are rather formed on Ag/Y. Silver zeolites with other types of frameworks (MFI, MOR, FER, *BEA) exhibited rather different desorption patterns than Ag/FAU (X and Y). Due to higher Si/Al ratio (of the order of 10) and lower silver contents (3-9 wt%), the amounts of gases evolved during TPD experiments are much less intense (Fig. 3 (B)). Depending on the exchange level, two or three peaks (at 180,


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320 and 400°C, respectively) are observed in the case of Ag/MOR zeolites. By contrast, Ag/MFI and Ag/FER zeolites displayed a unique and weak signal at 180-200 °C, whereas the CH3I desorption on Ag/BETA corresponds to a broad peak centered at 320°C. The formation of Ccontaining by-products was also observed for all these zeolites but a detailed interpretation of catalytic decomposition processes for the different structures is beyond the scope of this study. 3.4. Quantitative evaluation of trapping stability So as to clarify the discussion about the relative stability of CH3I trapping by the different zeolites, the amounts adsorbed in reversible or irreversible way were quantified separately. These determinations could be made rather easily because, as already mentioned, no other I-containing products (I2 and HI) were produced from CH3I during the adsorption and desorption phases. Reversibly-adsorbed species, referred as Qphys in Table 1, were determined from the area under the curve relative to the decay of CH3I in the gas phase (Fig. 1 near 100 min). The irreversible part of trapping is represented both by chemisorbed CH3I species and AgI precipitates formed from CH3I dissociation. Chemisorbed amounts (Qchem in Table 1) were calculated from the area of the CH3I desorption peaks (Fig. 1 and 3), whereas the amount of iodine stored as AgI (QAgI, Table 1) was deduced by molar balance using the following equation (Eq. 3): QAgI = Qsat – (Qphys + Qchem) Eq. (3) The different types of fractions adsorbed were then expressed as percentages of the corresponding adsorption capacities (listed in Table 1 in mmol/g in order to take into account the iodine trapped as AgI precipitate) in order to provide a comparison between the different zeolites. From the data gathered in Table 1 and the ranking of silver zeolites shown on Fig. 4, it can be deduced that the reversible/irreversible character of CH3I trapping greatly differ among the silver zeolites investigated. On the left side of the diagram (Fig. 4), faujasite X and Y zeolites with


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silver content higher than 20 wt% display the best properties in terms of trapping stability, with more than 90% of the adsorbed CH3I being trapped irreversibly (with about 70-80 % existing as AgI precipitated entities). Those zeolites have both the highest silver contents and also superior micropore volumes by comparison with other structures (Table S1). By contrast, the parent NaX zeolite displays a significant tendency towards physisorption (39 ± 8 %). Accordingly, the percentage trapped as NaI (26 ± 1 %) is rather low compared with AgI (57-76% for Ag/X zeolites). This result can be paralleled with the works of Thomas et al.12 devoted to the trapping of I2 at 150°C by different kinds of cation-exchanged 13X molecular sieves (Na+, Ag+, Pb2+ and Cd2+). Although in their study all adsorbents displayed high adsorption capacities for iodine, only silver was able to promote irreversible trapping (61% of I2 chemisorbed against 8 -15% for the other cations). This can be easily explained considering the higher stability of silver iodide precipitate over NaI from a thermodynamic viewpoint, and also the tendency of the latter to be easily solubilized by water (AgI is insoluble32). By contrast with silver faujasite zeolites (Fig. 4), the 4.2Ag/FER (10.4) material with low silver content and small pores enhances the reversible part of trapping at the expense of the irreversible one. Nevertheless, this behavior cannot be solely related to the low silver content of this zeolite because 3.4Ag/BETA (10.1) with less silver is more efficient in promoting AgI formation. For the ferrierite structure, the CH3I kinetic diameter (about 5-6 Å10) is above the theoretical pore size (3.5 x 4.8 Å; 4.2 x 5.4 Å33) and physisorption is promoted at the expense of AgI formation. Therefore, such small-pore zeolites are not adequate for long-term CH3I capture. Other silver zeolites such as Ag/MFI and Ag/MOR display an intermediate behavior (Fig. 4).


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Figure 4: Ranking corresponding to the ability of the different sorbents to promote irreversible trapping as AgI. Best sorbents are on the left side of the diagram. According to some of our past studies,34 the pseudo-unidimensional character of the pores in the mordenite structure makes them more prone to pore blockage of diffusional limitations in comparison with 3D zeolites (such as faujasite). This may impact negatively their retention behavior. To our knowledge Ag/MFI zeolites were never investigated for iodine capture. The MFI structure of the 9Ag/MFI (11.5) consists in straight and zig-zag channels wide of 5 and 5.5 Å,33 respectively, and is very efficient in catalyzing many hydrocarbons reactions thanks to a high acidity and specific metallic sites. Interestingly, rather good trapping properties (64 ± 3 % as AgI) were achieved with 9Ag/MFI (11.5) despite the sterical limitations existing in this


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structure. According to TPD results (Fig. 3 and Fig. S1 in ESI), it seems likely that CH3I could be more easily dissociated in this zeolite, which is a prerequisite for AgI formation. To sum up, it can be concluded that zeolites of faujasite Y structure with high silver and low sodium contents particularly promote irreversible CH3I trapping, namely due to the reaction of methyl iodide with accessible silver cations to yield AgI precipitates. Ag/MFI and Ag/MOR zeolites have rather satisfying behavior although not as good as Ag/Y and even when similar silver contents are compared. Small-pore sorbents such as Ag/FER zeolites disadvantages strong retention at the expense of weakly-held species and are not a good choice for nuclear applications. 3.5.Efficiency of silver use by the different sorbents In previous sections, it was shown that the proportions stored as AgI differ among the sorbents. Calculations of I/Ag ratio should allow in principle to identify the best formulations in terms of silver use, which is important from an economical viewpoint. For instance, I/Ag ratio < 1 could mean that some silver atoms remains inaccessible to iodomethane molecules to form AgI precipitates whereas ratio > 1 could indicate the presence of molecularly adsorbed CH3I. In literature,35 the silver use through I/Ag ratio was estimated using simply the adsorption capacity at saturation of the sorbent, which could be misleading. Indeed, such a calculation could not discriminate between different forms of trapping. In the present study, we judge pertinent to calculate I/Ag ratio using three different approaches: (i) a global I/Ag ratio taking classically into account all trapped fractions (as in the literature); (ii) a second I/Ag ratio whose the contribution from physisorbed species has been eliminated; hence, only the irreversible part of trapping is considered; (iii) a third I/Ag ratio in which only the fraction trapped as AgI precipitates is taken


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into account (physisorbed and chemisorbed fractions have been removed from the adsorption capacity). These ratios are gathered in Table 2 for the different sorbents. Except perhaps for Ag/Y zeolites, these three types of I/Ag ratio were found to differ significantly between each other when the “molecular” contributions (reversible or irreversible) to the trapping were important. A straightforward example can be found for instance in Table 2 for the 4.2Ag/FER (10.4) sorbent. The use of global I/Ag ratio could indicate a very good use of silver (I/Ag = 1.17), but re-calculation of this ratio by eliminating physisorbed or all molecularly adsorbed components would show a whole different picture (I/Ag = 0.28 and 0.07 respectively). Hence, only a very small proportion of silver atoms is involved in AgI production for the silver ferrierite sorbent. In that respect, the formation of AgI in the course of CH3I retention is very important because it is the most stable form of trapping. Bulk AgI precipitates were found by us to be thermally stable at least up to 500-600°C and more likely up to 700-800°C, as deduced from preliminary thermogravimetric (TG) experiments (see Fig. S2 in ESI). AgI precipitates are also known to be highly insoluble in water.32 This is important in nuclear context where relative humidity could be very high.1


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Sample

Global

I/Ag w/o

I/Ag (as AgI

I/Ag

physisorption

precipitate only)

9.1Ag/Y (2.5)

0.73

0.59

0.51

16.8Ag/Y (2.5)

0.79

N. D*

N. D

22.8Ag/Y (2.5)

0.74

0.69

0.57

7.3Ag/13X (1.2)

1.58

1.33

0.90

23.4Ag/13X (1.2)

0.76

0.69

0.52

35Ag/13Xcomm (1.2)

0.58

0.54

0.44

5.9Ag/MOR (10)

1.08

0.77

0.53

7.3Ag/MOR (10)

1.24

0.82

0.54

9Ag/MFI (11.5)

0.74

0.50

0.47

4.2Ag/FER (10.4)

1.17

0.28

0.07

3.4Ag/BETA (10.1)

1.03

0.69

0.41

* N.D: Not determined.

Table 2 : I/Ag ratio obtained from different calculations considering raw Qsat values (global I/Ag) or values corrected from the contributions of physisorption (3rd column) or physisorption + chemisorption (as AgI only, 4th column). In order to compare more accurately the ability of each sorbent to use silver efficiently, we judge more useful to consider the last I/Ag ratio (precipitate, 4th column in Table 2), because it directly reflects the part of silver that could be used to store iodine under the most stable form. Overall, I/Ag ratio in the range 0.41-0.54 were obtained for most of silver-exchanged zeolites. This indicates that about one silver atom over two is used to form AgI. The significance of such values is discussed now. In comparison with I2 (the other main iodine volatile species), the


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irreversible trapping of CH3I (as main representative for organic iodides) is generally perceived as more difficult.15 Considering that a cleavage of bonds in these molecules is mandatory for the formation of stable AgI precipitates,20 this is in agreement with the higher dissociation energy of the C-I bond (213 kJ.mol-1) compared with the I-I bond (148 kJ.mol-1).36 In other words, these values indicate an easier adsorption/dissociation mechanism for I2 as compared with CH3I. In our case, significantly higher I/Ag ratio (of the order of 1) were calculated from I2 retention tests (results not shown here) than for CH3I retention tests. This confirmed the higher reactivity of I2 compared with CH3I in presence of silver zeolites. The lower energetic cost for the dissociation of the I-I bond does not probably explain alone the more efficient use of silver when comparing the effects of I2 and CH3I exposure. Other parameters, such as the silver electronic state and the presence of by-products from catalytic decomposition reactions can also play a role on iodine trapping. As shown in Fig. S1 (in ESI), the formation of C-by products from CH3I dissociation (such as oxygenates, CO or other kinds of saturated/unsaturated hydrocarbons) is observed in rather large amounts for most of the investigated silver zeolites. The presence of such molecules inside the pores could slow down the diffusion of methyl iodide and/or partially inhibit its reaction with silver sites due to adsorption competition.16 By contrast, no by-product was observed during I2 retention tests (not shown here). In conclusion, this may justify that all silver atoms in silver-exchanged zeolites could not be used (actually about one silver atom over two) at 100°C for the production of AgI precipitates from CH3I. On the other hand, a detailed characterization of silver zeolites by DRS-UV-Vis, XRD, HRTEM etc…was reported in our recent study.16 It was shown that all the investigated silverexchanged zeolites presented a significant amount of charged silver clusters in addition to silver cations at exchange positions. The presence of Agnδ+ species is due to auto-reduction processes


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occurring during calcination/pre-treatment, as also reported elsewhere.27 On the Figure 5 below is depicted a possible mechanism involving the reaction of a binuclear silver species with methyl iodide. It is seen that, while the Iδ- moiety interacts with Ag2+, the CH3δ+ moiety reacts with a OH group (for instance a Brönsted Si(OH)Al acid) to finally yield a methoxy species. Reaction with CH3I with different types of surface OH species was observed by DRIFTS in our former study focused on Ag/Y. 20 Besides molecularly adsorbed CH3I (responsible of CH3I emissions during TPD, as shown in Fig. 3), surface methoxy species were also clearly identified by DRIFTS. 20 In Figure 5, the material balance of the reaction between CH3I and a Ag2+ cluster indicates that only one half of silver atoms is used to produce a molecular AgI entity, leading to a I/Ag ratio = 0.5. Even if the existence of charged silver clusters was observed for all investigated zeolites, it is important to remember that the amount of binuclear species and their accessibility within the different host frameworks may vary a lot depending on the type of silver zeolite. Even if other mechanisms are probably operative and could involve other forms of silver as well (e.g. isolated cations, higher clusters or nanoparticles), this molecular view is rather convenient to explain the observed results. For instance, it can be used to account for the higher formation of AgI in the large supercages of faujasite X and Y than in the smaller pores of other structures (Fig. 4).


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Figure 5 : Mechanistic scheme showing the reaction between a silver binuclear cluster and CH3I inside a zeolite pore. The I/Ag ratio of 0.90 found for 7.3Ag/13X (1.2) could be considered as surprising. Nevertheless, it can be easily explained by the high sodium content of this material (5.86 wt%, Table S1), which is responsible of the formation of less stable NaI (40-50 %) precipitates in addition to AgI (50-60%). Hence, 7.3Ag/13X (1.2) has not to be considered as better than 22.8Ag/Y (2.5). Finally, it is also worth noting that Ag/MOR zeolites behave also well in terms of silver use, promoting irreversible trapping (however both as chemisorbed CH3I and AgI). Finally, the amounts of iodine (expressed in mmol/g) stored as AgI precipitates during CH3I retention tests were plotted in function of silver content (mmol/g) on Fig. 6. NaX (1.2) and 7.3Ag/13X (1.2) zeolites were removed from the linear correlation (y = 0.488x, R2 =0.970) because part of iodine is also present as NaI (or NaI + AgI in the case of 7.3Ag/13X (1.2)) and our methodology cannot strictly distinguish between these two kinds of precipitates. Because of its too small pore size, 4.2Ag/FER (10.4) was also excluded. Overall, it is interesting to note that the amount of iodine stored by silver-exchanged zeolites as AgI precipitates almost strictly depends on the silver content. Hence, it could be deduced that a high amount of dispersed silver species (as Ag+ cations on exchange positions and/or small silver clusters) promotes the


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most irreversible form of trapping. The very high affinity of silver for iodine32,37 is the driving force for the formation of AgI precipitates and the silver speciation/location seems to play a minor role in that respect, provided that silver aggregates are small enough to make easier iodine diffusion. Nevertheless, it has also to be remembered that, besides storage as AgI, other less stable (molecular) forms of trapping co-exist (Fig. 4, Table 1). These forms are in general more preponderant at low silver contents and for other structures than faujasite.

Figure 6: Iodine amounts stored as AgI (expressed in mmol/g) in function of the silver content (mmol/g) for some silver zeolites.


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4. CONCLUSION

In this study, we aimed to explore, for different series of silver-exchanged zeolites, the relationships existing between their chemical, structural and textural parameters and their ability to capture CH3I irreversibly. Following CH3I breakthrough experiments carried out at 100°C, it was found that the adsorption capacities at saturation were mainly function of the total amount of exchanged silver species, almost regardless of the zeolite structure. However, the working capacities of the sorbents depended on many chemical and/or physical factors, including the silver and sodium contents, and the diffusional limitations imposed by the zeolite crystallite size, the pore structure and the presence of occluded silver species or carbonaceous by-products inside the pores (from CH3I dissociation). The formation of AgI was observed for all silver zeolites following CH3I retention tests. The amount of AgI precipitates was found to be correlated with the amount of silver existing in the exchanged zeolites. Nevertheless, silver iodide precipitates were found to co-exist with other less stable forms of trapping (physisorbed and chemisorbed CH3I ad-species). A specific methodology was employed to rank the different silver-exchanged zeolites according to the distribution of the physisorbed, chemisorbed and AgI fractions. Overall, Ag/Y zeolites of faujasite structure with high silver content (> 15 wt%) were found to be almost optimal sorbents for CH3I retention, with more than 90 % of irreversible trapping (76 % as AgI). By contrast, other kinds of faujasite sorbents such as Ag/X zeolites, are less interesting due to their high sodium content and greater sensitivity to humidity. Large- or medium-pore zeolites with lesser silver contents, such as Ag/MOR and Ag/MFI, behaved also


26

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well for CH3I retention and can also be considered for nuclear application, if the amount of silver has to be limited for economic reasons.

ASSOCIATED CONTENT Supporting Information Chemical composition and textural properties of silver zeolites. Temperature profiles of the C-by products generated from the thermal decomposition of adsorbed CH3I at 100°C on the different investigated zeolites. TG curve of bulk AgI under inert atmosphere (Alfa Aesar, 11419) (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Tel.: +33372749856. Present Addresses -

† IMT Atlantique (Mines Nantes), Energy and Environment Systems Department, Laboratoire GEPEA-UMR CNRS 6144, 4 Rue Alfred Kastler 44307 Nantes Cedex 3.

Author Contributions -

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources -

European Atomic Energy Community’s (Euratom) Seventh Framework Programme FP7/2007-2013 (grant agreement n° 323217) ;


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-

Page 28 of 34

National Research Agency ANR-MIRE (grant agreement n° ANR-11- RSNR-0013-01).

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The research leading to these results is partly funded by the European Atomic Energy Community’s (Euratom) Seventh Framework Programme FP7/2007-2013 under grant agreement n° 323217. This work has been also supported by the French State under the program "Investissements d'Avenir" called MIRE, managed by the National Research Agency (ANR) under grant agreement n° ANR-11-RSNR-0013-01.

REFERENCES (1)

Herranz, L. E.; Lind, T.; Dieschbourg, K.; Riera, E.; Morandi, S.; Rantanen, P.; Chebbi, M.; Losch, N. State of the Art Report : Technical Bases for Experimentation on Source Term Mitigation Systems; Passam-Theor-T04 [D2.1], 2013.

(2)

Cantrel, L.; Herranz, L.; Guieu, S.; Albiol, T.; Collet, R.; Lind, T.; Kärkelä, T.; Mun, C.; Jacquemain, D.; Chebbi, M. Overview of Ongoing and Planned R&D Works on Delayed Radioactive Releases and Filtrered Containment Ventig Systems Efficiencies in a Severe Accident, Proceeding of ICAPP 2015, Nice, May 03-06, 2015.

(3)

Proceedings of the Specialists' Meeting on Filtered Containment Venting Systems; OECD/NEA/CSNI, Report NEA/CSNI/R (1988) 148, 1988.

(4)

Note on the Outcome of the May 1988 Specialists' Meeting on Filtered Containment


28

Page 29 of 34

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


Venting Systems; OECD/NEA/CSNI, Report NEA/CSNI/R (1988) 156, 1988. (5)

Jubin, R. T. A Literature Survey of Methods to Remove Iodine from Off-Gas Streams Using Solid Sorbents; ORNL/TM-6607, Oak Ridge National Laboratory: Tennessee, 1979.

(6)

Haefner, D.; Tranter, T. Methods of Gas Phase Capture of Iodine from Fuel Reprocessing off-Gas: A Literature Survey; INL/EXT-07-12299, Idaho National Laboratory: Idaho Falls, ID, 2007.

(7)

Jacquemain, D.; Guentay, S.; Basu, S.; Sonnenkalb, M.; Lebel, L.;Allelein, H. J.; Liebana, B.; Eckardt, B.; Ammirabile, L. Status Reporton Filtered Containment Venting; OECD/NEA/CSNI, Report NEA/CSNI/R (2014) 7, 2014.

(8)

Riley, B. J.; Vienna, J. D.; Strachan, D. M.; McCloy, J. S.; Jerden, J. L. Materials and Processes for the Effective Capture and Immobilization of Radioiodine: A Review. J. Nucl. Mater. 2016, 470, 307–326.

(9)

Burger, L. L; Scheele, R. D; Wiemers, K. D. Selection of a Form for Fixation of Iodine129; PNL-4045/UC-70, Pacific Northwest Laboratory: Washignton, 1981.

(10) M Scheele, R. D ; Burger, L. L; Matsuzaki, C. L. Methyl Iodide Sorption by Reduced Silver Mordenite, PNL-4489/UC-70, Pacific Northwest Laboratory: Washignton, 1983. (11) Maeck, W. J.; Pence, D. T.; Keller, J. H. A Highly Efficient Inorganic Adsorber for Airborne Iodine Species (Silver Zeolites Development Studies); Idaho Nuclear Corporation, Idaho Falls, ID, 1969. (12) Thomas, T. R.; Staples, B. A.; Murphy, L. P.; Nichols, J. T. Airborne Elemental Iodine


29


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 34

Loading Capacities of Metal Exchanged Zeolites and a Method for Recycling Silver Zeolites; Idaho National Engineering Laboratory: Idaho Falls, ID, 1977. (13) Pence, D. T.; Duce, F. A.; Maeck, W. J.; First, M. W. Developments in the Removal of Airborne Iodine Species with Metal Substituted Zeolites, Proceeding of the 12th AEC Air Cleaning Conference, Tennessee, Aug 28-31, 1972. (14)

Campbell, D. O.; Malinauskas, A. P.; Stratton, W. R. Chemical Behavior of Fission Product Iodine in Light Water Reactor Accidents. Nucl. Technol. 1981, 53 (2), 111–119.

(15) Clément, B.; Cantrel, L.; Ducros, G.; Funke, F.; Herranz, L.; Rydl, A.; Weber, G.; Wren, C. State of the Art Report on Iodine Chemistry; OECD/NEA/CSNI, Report, NEA/CSNI/R (2007) 1, 2007. (16) Chebbi, M.; Azambre, B.; Cantrel, L.; Huvé, M.; Albiol, T. Influence of Structural, Textural and Chemical Parameters of Silver Zeolites on the Retention of Methyl Iodide. Microporous Mesoporous Mater. 2017, 244, 137–150. (17)

Treacy, M. M. J.; Higgins, J. B. Collection of Simulated XRD Powder Patterns for Zeolites, 5th ed.; Elsevier Sciences, 2007.

(18) Lippens, B.C.; Linsen, B.G; De Boer, J.H. Studies on Pore Systems in Catalysts I. The Adsorption of Nitrogen; Apparatus and Calculation. J. Catal. 1964, 3, 32-37. (19) Roth, W. J.; Gil, B.; Makowski, W.; Marszalek, B.; Eliasova, P. Layer like Porous Materials with Hierarchical Structure. Chem. Soc. Rev. 2015, 3400–3438. (20) Chebbi, M.; Azambre, B.; Cantrel, L.; Koch, A. A Combined DRIFTS and DR-UV-Vis Spectroscopic in Situ Study on the Trapping of CH3I by Silver-Exchanged Faujasite


30

Page 31 of 34

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


Zeolite. J. Phys. Chem. C 2016, 120 (33), 18694–18706. (21)

Jacobs, P. A; Flanigen, E. M; Jansen, J. C; Bekkum, H. van. Introduction to Zeolite Science and Practice, 2th ed.; Elsevier sciences, 2001.

(22) Chebbi, M.; Azambre, B.; Cantrel, L. Trapping of CH3I by Silver-Exchanged Zeolites : Influence of Structural and Chemical Parameters on Adsorption Performances and Trapping Stability, 18th International Zeolite Conference (18th IZC), Rio de Janeiro, June 19-24, 2016. (23)

Bartolomeu, R.; Azambre, B.; Westermann, A.; Fernandes, A.; Bértolo, R.; Hamoud, H. I.; Henriques, C.; Da Costa, P.; Ribeiro, F. Investigation of the Nature of Silver Species on Different Ag-Containing NOx Reduction Catalysts: On the Effect of the Support. Appl. Catal. B Environ. 2014, 150–151, 204–217.

(24) Aspromonte, S. G.; Mizrahi, M. D.; Schneeberger, F. A.; López, J. M. R.; Boix, A. V. Study of the Nature and Location of Silver in Ag-Exchanged Mordenite Catalysts. Characterization by Spectroscopic Techniques. J. Phys. Chem. C 2013, 117 (48), 25433– 25442. (25) Shibata, J.; Takada, Y.; Shichi, A.; Satokawa, S.; Satsuma, A.; Hattori, T. Influence of Zeolite Support on Activity Enhancement by Addition of Hydrogen for SCR of NO by Propane over Ag-Zeolites. Appl. Catal. B Environ. 2004, 54 (3), 137–144. (26) Sun, T; Seff, K. Silver Clusters and Chemistry in Zeolites. Chem. Rev. 1994, 94 (4), 857870. (27)

Baker, M. D.; Ozin, G. A.; Godber, J. Far-Infrared Studies of Silver Atoms, Silver Ions,


31


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 32 of 34

and Silver Clusters in Zeolites A and Y. J. Phys. Chem. 1985, 89 (2), 305–311. (28)

Stöber, D.; Hildmann, B. O.; Böttner, H.; Schelb, S.; Bachem, K.-H.; Binnewies, M. Chemical Transport Reactions during Crystal Growth of PbTe and PbSe via Vapour Phase Influenced by AgI. J. Cryst. Growth 1992, 121 (4), 656–664.

(29) Nenoff, T. M.; Rodriguez, M. A.; Soelberg, N. R.; Chapman, K. W. Silver-Mordenite for Radiologic Gas Capture from Complex Streams: Dual Catalytic CH3I Decomposition and I Confinement. Microporous Mesoporous Mater. 2014, 200, 297–303. (30) Khadzhiev, S. N.; Magomedova, M. V.; Peresypkina, E. G. Mechanism of Olefin Synthesis from Methanol and Dimethyl Ether over Zeolite Catalysts: A Review. Pet. Chem. 2014, 54 (4), 245–269. (31) Bushell, J.; Carley, A. F.; Coughlin, M.; Davies, P. R.; Edwards, D.; Morgan, D. J.; Parsons, M. The Reactive Chemisorption of Alkyl Iodides at Cu(110) and Ag(111) Surfaces: A Combined STM and XPS Study. J. Phys. Chem. B 2005, 109 (19), 9556– 9566. (32) Brown, L. S. Chemistry: The Molecular Science, 2nd Edition (Olmsted, J. III; Williams, G.M.); Wm. C. Brown Publishers: Iowa, 1997. (33) Baerlocher, C.; McCusker, L.; Olson, D. Atlas of Zeolite Framework Types; 6th Revised Edition, Elseiver Sciences, 2007. (34) Westermann, A.; Azambre, B. Impact of the Zeolite Structure and Acidity on the Adsorption of Unburnt Hydrocarbons Relevant to Cold Start Conditions. J. Phys. Chem. C 2016, 120 (45), 25903–25914.


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(35)

Wu, L.; Sawada, J. a.; Kuznicki, D. B.; Kuznicki, T.; Kuznicki, S. M. Iodine Adsorption on Silver-Exchanged Titania-Derived Adsorbents. J. Radioanal. Nucl. Chem. 2014, 302 (1), 527–532.

(36) L Lippert, E. The Strengths of Chemical Bonds, von T. L. Cottrell. Butterworths Publications Ltd., London 1958. Angew. Chemie 1960, 72 (16), 602–602. (37) Pearson, R. G. Absolute Electronegativity and Hardness: Application to Inorganic Chemistry. Inorg. Chem. 1988, 27 (4), 734–740.


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Physisorbed CH3I Physisorbed CH3I ChemisorbedCH3I CH3I Chemisorbed

Ag-exchanged zeolites

CH3I under precipitates CH3Itrapped stored as AgI AgI precipitates 120 100 80

FAU (3D)

Ag+ Agnδ+

MOR (1D)

Ag°m

Ag+ Ag+

Ag°

m

Ag+

FER (2D)

Ag+ Ag δ+ Agnδ+ n

CH3I Breakthrough curves at T = 100°C + Ar-TPD

60 40 20 0

22.8Ag/Y (2.5)


5.9Ag/MOR (10)

4.2Ag/FER (10.4)

Evaluation of Silver Zeolites Sorbents Toward Their Ability to Promote

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