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A Combined DRIFTS and DR-UV-Vis Spectroscopic in Situ Study on the Trapping of CHI by Silver-Exchanged Faujasite Zeolite 3

Mouheb Chebbi, Bruno Azambre, Laurent Cantrel, and Alain KOCH J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07112 • Publication Date (Web): 03 Aug 2016 Downloaded from http://pubs.acs.org on August 8, 2016

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The Journal of Physical Chemistry

A Combined DRIFTS and DR-UV-Vis SpectroscopicIn situStudy on the Trapping of CH3I by Silver-Exchanged Faujasite Zeolite M. Chebbia, B. Azambrea*, L. Cantrelb, A. Kocha a

Université de Lorraine, Laboratoire de Chimie et Physique-Approche Multi-Echelle des

Milieux Complexes (LCP-A2MC- EA n°4362), Institut Jean-Barriol FR2843 CNRS, Rue Victor Demange, 57500 Saint-Avold, France b

Institut de Radioprotection et de Sûreté Nucléaire (IRSN), PSN-RES, SAG, 13115 Saint-Paul

Lez Durance, France. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ; Telephone number: +33(0)387939106.

ABSTRACT

The mechanism of CH3I adsorption and thermal decomposition by a Ag/Y sorbent with 23 wt% of silver and Si/Al ratio of 2.5 was investigated using two different spectroscopic techniques. On the one hand, DR-UV-Vis spectroscopy was used in order to monitor the evolution of silver species and their transformation to silver iodide AgI clusters during increasing exposure to gaseous methyl iodide and upon heating under dry and wet atmospheres. On the other hand, the evolution of adsorbed organic species was investigated

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under similar conditions by DRIFTS. The latter data were correlated with the evolution of gaseous products in order to elucidate the mechanistic pathways involved in the trapping and catalytic decomposition of CH3I.

1. INTRODUCTION

During the irradiation of the fuel matrix within the nuclear power plant, different radioactive fission products (FPs) are generated. A severe nuclear accident can lead to significant radioactive release into the environment. Indeed, a Loss Of Coolant Accident (LOCA) for instance can lead to the core overheat when safety systems are not operating. FPs released from the degraded fuel are next transported through the Reactor Coolant System (RCS) and a part of them, the most volatile ones, can reach the nuclear containment building. To avoid over-pressure of the containment, most of the nuclear power plants are equipped with containment venting systems (CVS) and for a part of them some filtering devices are added. After opening of the CVS if needed, some FPs can be released in the environment due to loss of the confinement. The amount of activity release strongly depends on the filtering efficiency of the CVS. Very recently in link with Fukushima accident, a complete review of filtering CVS concerning technologies, performances, implementations as well as containment venting strategies was undertaken.1,2 Volatile FPs are the most difficult to trap. In that respect, iodine is of particular interest because gaseous CH3I and I2 are some high contributors to the dose at short term because of the 131I isotope. 3-5Therefore, iodine capture in the gaseous effluents is a critical issue for public safety in case of nuclear accident. In order to mitigate the release of radioactive iodine, an improvement may consist in combining current filtration devices (such as metallic filters1, aqueous scrubbers6 or sand bed filters7) with an additional porous sorbent. Among the possible adsorbing materials, zeolites


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are obvious candidates,thanks to their ion-exchange abilities, high specific surface areas, tunable pore sizes and structures, and chemical, thermal and mechanical stability. 8 Indeed, since the 70s, many academic and industrial research works have shown that Ag/zeolites with faujasite or mordenite structures are among the best zeolitic materials to ensure retention and immobilization of volatile iodine species such as I2 and CH3I. 9-13In that respect, most past and recent studies aimed to assess the retention ability and trapping stability of I-containing molecules on conditions more or less representative of severe nuclear accidents. However, only a very few of them have been devoted to the investigation of the trapping mechanism, and this stands mainly for commercial Ag/MOR zeolites (Ionex-400 and Ionex-900). Chapman and co-workers14 have reported that on a H2-prereducedAg/MOR zeolite, two different AgI phases coexisted after I2 adsorption: an α-AgI phase present within the pores and larger γ-AgI nanoparticles present on the outer zeolite surface. However, the direct trapping of I2 without H2 pretreatment, leads to the formation of α-AgI phase which is confined inside the internal zeolitic pores. It was therefore concluded that the latter configuration was a more secure route for radioactive iodine capture. Some attempts to elucidate the mechanism of CH3I trapping were also conducted by combining different ex-situ characterization tools. 15 It was suggested thatthe CH3I molecule is cleaved at low temperatures on the zeolitic surface to form a I- fragment and surface methoxy species. The iodide anion then reacts with silver to form occluded AgI precipitates in the main channels of the mordenite structure. On the other hand, surface methoxy species may undergo secondary reactions with adsorbed water to form methanol, which further reacts to yield dimethylether. On the other hand, catalysis studies with other halomethane molecules (such as CH3Cl) reported the formation of higher alkanes and alkenes at mediumhightemperatures. 16


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Nevertheless and despite the advances made, some aspects relevant to the mechanism of CH3I trapping and catalytic decomposition by silver zeolites remain widely unexplored. In the nuclear context, a good comprehension of these processes is of vital interest, since this can provide a strong basis to design more efficient sorbents, for instance by better understanding the effects of acidity, pore topology or silver electronic state. Molecular approaches are also a prerequisite in order to be able to predict the sorbent behavior in severe accident conditions, i. e. in presence of other gases (H2O, H2, CO2, NOx, Cl2…), with possible inhibitors for iodine sorption, at low iodine concentration or at quite high temperatures (~100°C) for middle time scale (2-3 weeks). On the other hand, CH3I being also a good methylating reagent17, this molecule can be used also as a “model” to investigate the reactions of hydrocarbon fragments which have been reported to play an important role in Fisher-Tropsch synthesis18. Here also, the optimization of the associated catalytic processes requires a better assessment of the adsorption and reactivity mechanisms. Understanding the CH3I adsorption/desorption mechanism by silver zeolites at a molecular level requires both the monitoring of AgI formation processes inside the porous framework, but also to gain insights of the catalytic transformations undergone by carbonaceous fragments in function of the temperature. Such information can be obtained using in situ spectroscopic approaches. The present paper attempts to elucidate the trapping mechanism and decomposition of CH3I by silver zeolites usingin situ Diffuse Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS) andDiffuse Reflectance UV-Vis Spectroscopy(DRS-UV-Vis) combined with gasphase reactor measurements. This study is focused on faujasite Ag/Y zeolite, because the Ytype may be preferred over the X-type thanks to higher Si/Al ratio (>2) and better chemical stability towards humidity and NOx, which are formed by radiolytic processes in nuclear context. Moreover, faujasite-type zeolites are fascinating host media in order to study clusters and contaminants stabilization, due to the existence of large supercages. Diffusional


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constraints of iodine species through such a large pore system are also expected to be greatly reduced in comparison with small- or medium-pore zeolites. In the first part of this study, an in-depth characterization of the Ag/Y zeolite is provided. Time- or temperature-resolved spectroscopies in the infrared and UV-Vis ranges are then used in order to monitor all types of surface processes. Spectral data obtained in adsorbed phase are then correlated to those from quantitative gas-phase FTIR spectroscopy in order to give an accurate picture of all the mechanistic pathways involved in the trapping and catalytic decomposition.

2.EXPERIMENTAL SECTION

2.1.Materials The parent NH4/Y zeolite, with Si/Al =2.5 was supplied (under ammonium form) by Zeolyst International (CBV 500). The zeolite was converted to its protonic form (H/Y (2.5)), by calcination in a muffle furnace at 500 °C for 2 hours (v= 5°C/min, dwell at 200°C for 1h). Silver containing zeolite was prepared by ion-exchanging three times 2g of parent NH4/Y (2.5) zeolite with 100mL of 0.01 M silver nitrate (Sigma-Aldrich, purity > 99.9 %) solution. Each ion exchange was carried out at room temperature for 24h in the dark to avoid any photo-reduction of silver species. Between each ion-exchange, the catalyst was washed with distilled water and dried at 80°C overnight. Prior to adsorption and characterization studies, the Ag/Y zeolite was calcined using the same procedure as the one reported above for the parent zeolite. The obtained zeolite is denoted as xAg/Y (Si/Al ratio), where x is the percent by weight of silver. Elemental ICP analyses (Na, Ag, Al and Si) were performed at the Central Analysis Service of the CNRS (France) for zeolites before and after ion-exchange with silver.


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ATR infrared spectroscopy was used in order to measure the percentage of exchanged ammonium ions with silver, before zeolite calcination. The different measurements were carried out using Bruker Alpha spectrometer equipped with Alpha P ATR accessory and diamond crystal. ATR-IR spectra were collected between 400 and 4000 cm-1 with a resolution of 4 cm-1 and 32 scans. It was found that about 73% of NH4+ cations present initially was substituted by Ag+ cations before calcination 2.2.Physico-chemical characterizations before/afterCH3I adsorption test Textural, structural and electronic characterizations of the silver zeolite before and after an adsorption test with methyl iodide were carried out usingN2 porosimetry at 77K, TEM (Transmission Electron Microscopy), PXRD (Power X-ray Diffraction), DRS-UV-Vis, and CO adsorption followed by DRIFTS. Porosimetric properties were obtained from N2 adsorption isotherms recorded at 77 K 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 adsorbed impurities. Specific surface areas (SBET) were determined using the BET equation between 0.05 > P/P0> 0.3. The micropore volume and external surface area (Sext) were calculated according to the t-plot method. 19The total pore volume (Vpore) was measured from N2 adsorption isotherms at P/P0 = 0.97. PXRD patterns were collected using a Rigaku-Miniflex II (Japan) with Cu Kα radiation (λ= 1.5405Å). Powder diffraction patterns were recorded between 5 and 70° (2θ) using increments of 0.02°and a counting time of 2 s. DRS UV-Vis spectra were collected on a Cary 4000 UV-Vis spectrometer (Agilent Technologies) equipped with a double monochromator and DRA 900integrating sphere coated


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with polytetrafluoroethylene (PTFE).Spectra were registered at ambient conditions between 200 and 800 nm, with a resolution of 2 nm and a scan rate of 600 nm/min. A non-absorbing standard (Spectralon) was used as reference. DRIFTS experiments relevant to the dynamic adsorption of CO were carried out on a Varian Excalibur 4100 FTIR spectrophotometer equipped with a diffuse reflectance Graseby Specac optical accessory and a SpectraTech environmental chamber. Before each run, the sample was pre-treated under flowing helium (Air Liquide, 99.99 %, flow rate of 60 ml/min) from room temperature to 450°C (5°C/min) and then cooled down to 35°C. Background spectrum was recorded and then the zeolite powder was exposed to 1000 ppm CO/He (30 ml/min) at 35°C.Timd-resolved DRIFTS spectra were collected (co-addition of 100 scans with a resolution of 4 cm-1) until the zeolite surface sites were saturated with CO. 2.3 Insitu DRIFTS experiments of CH3I adsorption/decomposition The equipment used for DRIFTS experiments was the same as the one described in the last section for the adsorption of CO. First, the silver zeolite was pre-treated in situ inside the environmental cell under Argon (Air Liquide, 99.99 %, flow rate of 60 ml/min) from 30 °C to 200°C (30 min dwell) at 5°C/min, followed by another heating from 200 °C to 500°C (5°C/min, 60 min dwell at 500°C). Then, the sample was cooled down to 100°C in order to reach the temperature used in adsorption experiments. During the cooling, background spectra were collected at decreasing temperatures from 500°C to 100°C every 50°C. Then, a 1333 ppm/Ar concentration of methyl iodide was introduced to the sample. Appropriate dilution was achieved through the use of a certified CH3I bottle (Air Products, 2000 ppm ± 0.5% Argon) and Brooks 5850s mass-flow controllers (total flow rate of 60 ml/min). Time-resolved spectra were recorded (100 scans, Resolution of 4 cm-1) while increasingly saturation of the sample with the incoming CH3I. Once surface saturation was reached, a degassing step was performed under argon at 100°C in order to eliminate any physisorbed species. Then,


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Temperature Programmed Desorption (TPD) experiments were performed by ramping the temperature from 100 °C to 500°C at 10°C/min under argon (flow rate of 60 ml/min). 2.4. In situ DRS-UV-Vis experiments Time- and/or temperature-evolution of silver species during exposure to gaseous methyl iodide were recorded using a Harrick Praying Mantis diffuse reflectance accessory coupled to a heated reaction chamber, both attached to a Cary 4000 UV-Vis spectrophotometer. The temperature program and other methodologies were similar to those used for in situ DRIFTS experiments except that theCH3I concentration and total flow rate were set to 667 ppm and 30 ml/min, respectively. 2.5. Reactor experiments The adsorption capacity of the Ag23/Y (2.5) zeolite in CH3I, as well as the temperature profiles of decomposition products, were determined using dedicated breakthrough and TPD experiments, respectively. Experiments were performed in fixed-bed reactor (Carbolite tubular furnace with Eurotherm controller), using a sample mass of 200 mg, and a total flow rate of 150 mL/min (which corresponds to a GHSV of 20000 h-1).The adsorption/desorption sequence was almost similar to the one described for in situ spectroscopic experiments: (i) insitu pretreatment (ii) adsorption at 100°C until saturation of the zeolite bed (iii) evacuation under Ar at 100 °C (1 hour) then heating from 100 to 600 °C (v = 10°C/min). During the adsorption phase, the CH3I concentration at the reactor inlet was set to 1333 ppm. The composition of the gas mixture at the reactor outlet was continuously monitored every 2 min using a heated FTIR gas cell (Cyclone Series – Specac, optical path length = 2 m, V = 0.19 L) coupled to the Varian Excalibur 4100 FTIR spectrometer and DTGS detector. The temperature of the gas cell was maintained at 120 °C to avoid any condensation. FTIR spectra, referenced to Ar background, were recorded using a 2 cm-1 resolution and co-addition of 50 scans. Quantitative determination of the temperature profiles of CH3I and its


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decomposition products were obtained after calibration of spectral data (either in absorbance or area) with appropriate standards. When necessary, specific subtraction procedures were implemented in order avoid any spectral interference between products before their quantification.

3. RESULTS AND DISCUSSION

3.1. Textural, structural and electronic characterization of the Ag/Y zeolite The main textural and chemical characteristics of the zeolitic sorbent before and after the ionic exchange procedure with silver are listed in Table 1.

Na Structure

Label

NIE*

Si/Al

Ag/Al

Faujasite (3D)

SBET

Vmicro

Vpore

(m2/g)

(cm3/g)

(cm3/g)

Ag (wt. %) (wt.%)

Na/Al

H/Y (2.5)

0

2.50

0

0

3.30

0.43

756

0.256

0.352

23Ag/Y (2.5)

3

2.50

22.8

0.74

0.62

0.09

433

0.146

0.217

* Number of ionic exchanges

Table 1 : Textural and chemical characteristics of the parent and silver zeolites. The three successive exchanges used to synthesize the 23Ag/Y(2.5) zeolite leads to a significant decrease (by ~ 43%) of the micropore volume and the specific surface area. This decrease was of the same order magnitude when considering the total pore volume (by ~ 38%). This can be explained by the high content (~23 wt%) of incorporated silver species, which partially block either the accessibility to the network of supercages or reduces the free space within the supercages.


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Figure 1 : X-Ray diffractograms of parent and silver zeolites (before and after CH3I adsorption test). The diffraction peaks of faujasite are indexed and those typical of AgI (βphase) are shown (stars).

The phase purity of the parent H/Y (2.5) zeolite was examined by PXRD. Its diffraction pattern ( Figure 1) was found to be consistent with that reported in the literature for the faujasite structure. 20XRD characterization was also carried out in order to evaluate the zeolite structure after silver introduction and also to assess the presence of Ag crystalline phases (AgO, Ag2O and Ag° particles). The silver-exchanged zeolite shows peaks at the same position, but with very different relative intensities (Figure 1).Indeed, it’s known that the incorporation of silver cations in zeolites induces a significant change of the relative electronic densities of hkl


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planes. This phenomenon is related to a redistribution of intra-zeolite charge-compensating cations as stated in many studies in the literature. 21 22

Such behavior is more pronounced for heavy elements such as silver because scatter X-rays

more strongly than the other elements of the zeolite (H, Na, Si, Al). 23In our study, this fact can be also related to the insertion of a high silver content (≈ 23 wt%). On the other hand, no diffraction peaks attributed to metallic silver phases were observed. Overall, it seems likely that most of introduced silver species are well-dispersed within the internal zeolitic structure and more specifically at cation-exchange positions. More information was obtained on silver speciation by DR-UV-Vis spectroscopy. Some literature studies have shown that the coexistence of different kinds of silver species can be discriminated using this technique.24-28The attribution of the absorptions corresponding to exchanged Ag+ species (208-238 nm) and Ag° nanoparticles (above 350 nm) is relatively consensual. However, the distinction between Agnδ+ and Ag°m species, whose maxima were reported to be in the range 240-350 nm25, is more difficult due to the dependence of absorption characteristics towards the cluster size. On Figure 2are displayed the DR-UV-Vis spectra of 23Ag/Y (2.5) and those of H/Y (2.5). The spectrum of 23Ag/Y (2.5) zeolite after subtraction of H/Y contribution is also considered. It can be seen that the absorption of the parent zeolite is not significant compared to silver-exchanged one. For the silver-exchanged zeolite, the strongest band observed at around 214nm and the shoulder at 226 nm are attributed to the 4d10 → 4d95s1 electron transition of isolated silver cations present at two different exchange sites.24More tentatively, the absorptionsat240 and 255 nm are attributed to small silver cationic clusters Agnδ+ of different sizes within the faujasite supercages.27,28In addition, a weak band was also observed at 305 nm, which can be assigned to neutral Ag°mclusters.27,28


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In line with the white color of the material, all these data seem to indicate that silver species in 23Ag/Y(2.5) zeolite are mainly present as isolated cations or very small charged clusters on exchange positions. Nevertheless, the presence of minor amount of small silver metallic aggregates on the internal or external surface also coexist with the former species.

Figure 2 : DR-UV-Vis spectrum of (a) calcined 23Ag/Y (2.5) zeolite before CH3I adsorption test, (b) 23Ag/Y (2.5) after subtraction of the parent zeolite and (c) parent zeolite H/Y (2.5). DRIFTS of adsorbed CO at 35°Cwas also used to probe the electronic state of supported silver species as well as to gain insight on the location of silver cations within the porous network. The more electron-deficient are the exchanged Ag+ species, the higher will be the frequency of adsorbed CO. 29The usual absorption range for these species is between 2200 and 2151 cm-1. 30It was also reported that Ag° can also adsorb CO, but only at low temperature (with an IR frequency at about 2060 cm-1).On Figure 3, is plotted the time-evolution of DRIFTS spectra during increasing exposure to diluted CO (1000 ppm). All CO-related bands increased with time up to surface saturation (t = 30 min) and no apparent reduction of the


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sample was noticed. The two major bands at 2178 cm-1 and 2195 cm-1are assigned to Ag+(CO) monocarbonyl species. Their IR frequencies are similar to those reported in the literature for Ag/Y zeolites. 31The presence of two bands indicates the presence of two types of Ag+ sites with different electron acceptor properties. According to 31, the Ag+(CO) monocarbonyl band centered at 2178 cm-1has to be assigned to Ag+ cations in SII positions. The weaker band at 2195 cm-1is attributed to CO interaction with Ag+ cations having lower O coordination, i.e. possibly those siting on SIII sites. In addition, weaker bands were observed at lower wavenumbers, which can be tentatively assigned to interaction of CO with non-exchanged protons or other silver clusters such as Agnδ+ or Ag°m, whose presence was confirmed by DRUV-Vis spectroscopy. To sum up, all the above characterization techniques have revealed that silver species are well dispersed within the faujasite structure and present mainly as silver Ag+ cations located in SII and SIII exchange sites. Other charged or metallic silver (Ag°m, Agnδ+) aggregates were also observed, but in lower amounts. The adsorption and decomposition of methyl iodide on these silver species is considered now.


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Figure 3 :Time-resolved DRIFTS spectra of adsorbed CO at 35°C on 23Ag/Y (2.5). 3.3. Insights on CH3I adsorption and catalytic decomposition from reactor studies The breakthrough curves obtained following the in situ pre-treatment of 23Ag/Y(2.5) and H/Y zeolites at 500 °C and a dynamic adsorption test with [CH3I]0=1333 ppm and T=100°Care displayed in Figure 4 ((A) and (B)). Under the employed conditions, saturation of the zeolite bed was reached within 66 min. This corresponds to an adsorption capacity of 209 mg/g (by integration the area under breakthrough curve, Figure 4 (A)). It is worth noting that the adsorption capacity of the parent H/Y(2.5) zeolite was only equal to 7 mg/g(Figure 4 (B)). This large increase of adsorption capacity highlights that specificCH3I adsorption only occurs on silver sites.


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Figure 4 : Breakthrough curves obtained during a CH3I adsorption experiment at 100 °C in reactor with 23Ag/Y (2.5)(A)and with the parent H/Y (2.5) zeolite (B). In the case of experiment performed with silver zeolite, a small amount of dimethylether was observed (Figure 4 (A)), as well as traces of methanol at the outlet of fixed-bed reactor. In order to explain the formation of oxygenates observed at low temperatures during the adsorption phase or the TPD (Figure 4 (A) and 5), the CH3I breakthrough experiment shown in Figure 4 (A) was repeated in presence of 1% water in the feed. This experiment (not shown here) showed the formation of significant amounts of DME and methanol together with H2O adsorption. Considering the very different amounts of oxygenates produced with or without water, it seems clear that the methanol desorbed during the TPD (Figure 5) is due to the presence of water traces in gas cylinders, which could adsorb simultaneously to CH3I during the adsorption phase.


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After saturation of the sorbent bed with CH3I at 100°C and exposure to ambient air, the color of the 23Ag/Y(2.5) zeolite changed from white to pale yellow. This color is characteristic of a silver iodide AgI (β) phase, whose existence was confirmed by XRD( Figure 1,green curve).The formation of AgI was reported to occur through a precipitation reaction between silver species and the iodide ions I- after a first step involving the dissociation of CH3I. 14,15 This experiment was repeated twice in order to study the catalytic decomposition of adsorbed CH3I species. Once the CH3I adsorption capacity of the silver zeolite was reached, the weakly-held species were removed (by Ar purge) and the sample was heated from 100 °C to 600 °C. OnFigure 5, is displayed the temperature evolution profiles of the desorbed products. Upon increasing the temperature, dimethylether (DME), methanol (MeOH) and iodomethane were observed to desorb from 120 °C. The CH3I peak at 250 °C is consistent with the existence of chemisorbed iodomethane species. Iodine balance between adsorbed and desorbed CH3I amounts indicated that the relative proportion of chemisorbed methyl iodide represents about 14% of the CH3I adsorbed initially at 100 °C, 7% being physisorbed and 79% transformed to by-products, such as AgI precipitates. On the other hand, the formation of DME and MeOH was also observed by Nenoff et al. 15ona Ag/MOR sorbent at 150°C. Their production involves first the dissociation of CH3I catalyzed by Brönsted acid sites and the formation of surface methoxy intermediates. The latter species react with traces of water to form methanol while Brönsted acid sites are regenerated. Formation of dimethylether was explained by a secondary reaction between methanol and surface methoxy species. 15


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Figure 5 : Temperature profiles of the by-products generated from the thermaldecomposition of adsorbed CH3I at 100°C on 23Ag/Y (2.5).

At medium T° (250-350 °C), higher alkanes (CH4, C3H8) and alkenes (C2H4, C3H6) were produced from the decomposition of chemisorbed CH3I species. In the literature, these species were also observed from the catalyzed decomposition of halomethane molecules both on Ag/13X zeolites and acidic aluminophosphates (HSAPO-34)[16,32]. C2 and C3 hydrocarbons arise from coupling reactions catalyzed by protonic and silver sites between dissociated carbonaceous fragments. At temperatures above 420 °C, small amounts of CO are formed and molecular iodine (I2not shown on Figure 5) is also observed above 500 °C. At the end of the TPD experiment, the darkening of the zeolite sample suggests the formation of coke and/or the existence of silver reduction processes. More details on the formation of all these products will be given in the next sections. 3.4. In situ DRIFTS study– reaction of adsorbed organic species


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The H/Y zeolite prepared by thermal treatment of the parent zeolite (in ammonium form) displayed very poor retention for CH3I (7 mg/g at 100°C). Hence, a peculiar attention will be devoted to the behavior of 23Ag/Y (2.5) in order to better elucidate the reactions pathways involved during CH3I capture. Reactor experiments described in the previous section were repeated using DRIFTS in order to obtain information on the in situ evolution of CH3I ad-species during the adsorption at 100 °C and subsequent TPD.

A

B

Figure 6 : Time-resolved DRIFTS difference spectra corresponding to the interaction at 100 °C of CH3I (1333 ppm/Ar) with the surface of the 23Ag/Y (2.5): (A) Hydroxyls region; (B) CHx stretching region.

On Figure 6 ((A) and (B))are displayed the difference spectra representing the interactions of the activated silver zeolite with adsorbed iodomethane at different times of exposure in the hydroxyl (-OH) and methyl (-CH3) stretching regions, respectively in the 3700-3500 cm-1 and 3000-2800 cm-1 ranges. The perturbation of hydroxyl groups by adsorbed CH3I molecules induces important modifications in the hydroxyl region (Figure 6 (A)).The main evolution is


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related to the decrease of the 3635 cm-1 band assigned to Brönsted acid groups (Si-OH-Al). 33

The broad and positive absorption, which grows symmetrically around 3528 cm-1, denotes

the presence of H-bonded complexes between Si-OH-Al groups and adsorbed CH3I molecules. These observations can be compared to those previously reported for CH3Cl adsorption by H/Y zeolites. 33About the latter, it was found that the interaction of CH3Cl with bridged Si-OH-Al groups leads to the decrease of the peak intensity at 3640 cm-1, and the apparition of a new broad band in the range 3400-3100 cm-1. By increasing the amount of introduced CH3Cl, this broad band became more intense and the peak at 3640 cm-1 decreased progressively until its complete disappearance at high chloromethane pressures. 33-36In addition, it was observed that CH3Cl molecules were never accessible to OH groups belonging to the sodalite cages and hexagonal prisms 33. In accordance with the known kinetic diameter of CH3I (about 6 Å37), it can be also concluded in the present study that CH3I molecules are only accessible to the bridged Si-OH-Al located in the large cages (supercages).It is worth noting that the adsorption of CH3I also leads to the decrease of the intensities of the bands located at 3687 and 3740 cm-1. These bands are assigned to extraframework Al-OH and Si-OH species, respectively. 33,38This result indicates that CH3I molecules can also be in interaction with these hydroxyl groups. These observed changes following the adsorption of CH3I on 23Ag/Y(2.5) were also accompanied by the increase of positive absorptions at 2855, 2877, 2963, 2977 and 3020 cm1

(Figure 6 (B)), all related to CHx surface species. According to the presence of numerous

bands and the fact that all these bands do not increase at the same rate, the corresponding vibrations cannot be assigned to a single surface species. This observation can be related for instance to the existence of both molecular CH3I and oxygenates (such as MeOH and DME)within the same temperature range(Figure 4 (A) and Figure 5). More specifically, we also showed that some CH3I molecules interact with different kinds of OH groups (Figure 6


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Page 20 of 40

(A)).These interactions are probably related to the existence of the CH3I peak centered at 250 °Con the TPD profile displayed on Figure 5. Overall, it can be concluded that the absorptions visible on Figure 6 (B) belong probably both to strongly adsorbed methoxy species (and/or dissociated CH3) as well as adsorbed CH3I in molecular form. In order to further discriminate between these two species according to their relative stability and IR frequencies, TPD experiments on the CH3I-saturated sample were carried out by ramping the temperature from 100 °C to 500°C under Ar. From Figure 7, it can be inferred that the set of bands at 2855, 2963 and 3020 cm-1(in green) decreases more rapidly with the temperature than the bands centered at 2870 and 2977 cm-1. Hence, the former bands are assigned to molecular CH3I adsorbed on Ag/Y zeolite. By contrast, the decrease in intensity of the bands at 2870 and 2977 cm-1occurs mostly at temperatures higher than 200°C. In line with their higher thermal stability, these bands are assigned to dissociated CH3 fragments on silver metallic sites and/or methoxy (SiOCH3Al)species. Thus, methoxy species may be formed from MeOH adsorption or by reaction between the positively-charged carbon atoms of the H-bonded complex with the oxygen atoms of zeolite framework, this process generating HI as a by-product. 33,39,40Bushell et al. 41have also reported that alkyl iodide can dissociate below room temperature at clean metallic surfaces yielding chemisorbed iodine and an adsorbed alkyl group. Since metallic silver was also detected in the present study by XRD and DRS-UV-Vis, some methyl groups are likely to be present over the catalyst surface after dissociation of CH3I on silver metallic sites.


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Figure 7 : DRIFTS difference spectra of 23 Ag /Y (2.5) surface in the CHx stretching region: during TPD from 100 to 500 °C. Prior to TPD, the zeolite was saturated with CH3I adsorbed species. The interpretation of DRIFTS spectra corresponding namely to CH3 deformation modes is also addressed here. However, the strong absorption of the zeolite framework below 1700 cm1

precluded the accurate monitoring on these modes at temperatures higher than 100°C.

Nevertheless, a DRIFTS spectrum of the CH3I-saturated surface at 100°C in the lowwavenumbers region is given on Figure 8. The two bands at 1242 and 1455 cm-1correspond to the asymmetric and symmetric bendings of methyl groups for molecular CH3I adsorbed on the zeolite. Two other bands at 1065 and 1120 cm-1 are characteristic for methoxy species. 42

Finally, the small contributions observed at 963 cm-1 and 1660 cm-1canbe attributed to some

C-H and C=C vibration modes of adsorbed alkenes.16,39 The generation of alkenes is in agreement with FTIR data from gas-phase reactor studies (Figure 5), where the formation of ethene and propene was observed at medium temperatures.


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The complete IR attribution of these surface species, likewise the IR frequencies of CH3I in different physical states, are gathered in Table 2.

Figure 8 : DRIFTS difference spectrum of 23Ag/Y (2.5) after saturation with CH3I (1333 ppm/Ar) at 100°C in the CH3 deformation region.

Vibrational modes

CH3I gas

CH3I liquid

[18,

[this work]

this work] υ2, CH3 symmetric deformation υ5, CH3 asymmetric deformation

CH3I adsorbed

Methoxy

Alkenes

[16, 43,

[42,43,

[39]

this work]

this work]

1251 cm-1

1237 cm-1

1242 cm-1

-

-

-1

-1

-1

-

-

1445 cm

-1

1424 cm

-1

1455 cm

-1

2 υ5, overtone

2850 cm

2823 cm

2855 cm

-

-

υ1, CH3 symmetric stretching

2969 cm-1

2948 cm-1

2963 cm-1

2870 cm-1

-

υ4, CH3 asymmetric stretching

3065 cm-1

-

3020 cm-1

2977 cm-1

-

-1

C-O stretching

-

-

-

1065 cm

-

CH3 rocking

-

-

-

1120 cm-1

-

C-H alkene deformation

-

-

-

-

963 cm-1

C=C alkene

-

-

-

-

1660 cm-1

Table 2 : Wavenumbers and assignments of IR bands resulting from iodomethane adsorption on Ag/Y zeolite. Putting all the above data together, it appears that some CH3I molecules remained intact on the silver zeolite surface at temperatures up to 350°C (end of the TPD peak on Figure 5). A


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fraction however dissociates to yield either methoxy species or adsorbed methyl groups on metallic silver sites while also releasing an adsorbed I-fragment or gaseous HI. In the present study, the presence of HI could not be ascertained. However, it is possible that the generated hydroiodic acid instantaneously reacts with silver cations to give molecular AgI, this process regenerating a Brönsted acid group (eq. (1) and (2)): CH3I + SiOHAl → SiOCH3Al + HI (1) Ag+ + HI →AgI + H+ (2) In presence of a high silver content, the heterolytic dissociation of methyl iodide over silver cationic species (i.e. without involvement of Brönsted acid sites) could not be neglected, since it may directly produce silver iodide and surface methoxy species without formation of HI (eq. (3)): CH3I + Ag+ + [SiOAl]-→ AgI + SiOCH3Al(3) In that respect, reactions corresponding to eq.(2) and (3) are expected to be strongly promoted by the very high thermodynamic stability of silver iodide precipitate (pKps = 16.08 [44]). Once generated, methoxy species and adsorbed methyl fragments on metallic sites can also participate to a variety of reactions. According to Figure 5, the catalyzed thermal decomposition of methyl iodide involves three main stages: (i) the formation of oxygenates(MeOH, DME) at low temperatures (between 100 and 350 °C), (ii) the formation of methane as well as olefins and other alkanes (between 250 and 400°C), (iii) the existence of partial oxidation reactions occurring at temperatures above 500°C.Hence, the reactions involved in stage (ii) have some common points with those involved in the well-documented methanol to hydrocarbon (MTHC) processes. 45Due to the importance of MTHC catalysis in the industry, relevant mechanistic studies performed on acidic and metal-loaded zeolites are


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already available in the literature.45-47Hence, the elemental processes associated with those mechanisms are beyond the scope of the present study. Nevertheless, and because some byproducts generated by the reaction of CH3Iwith Ag/Y zeolite bearing protonic sites are similar to those involved in MHTC technology, some important aspects are also recalled below. Overall, the main reaction pathways leading to the different products observed during CH3ITPD experiments are summarized on Figure 9. At low temperatures, the production of methanol has been related to the reaction of water with surface methoxy species (eq. (4), 45): SiOCH3Al + H2O → SiOHAl + CH3OH (4) Dehydration of methanol over the acidic sites of the zeolite leads to the formation of dimethyl ether (eq. (5)): SiOCH3Al + CH3OH → CH3OCH3 + SiOHAl (5) The equilibrium mixture composed by MeOH, DME and water is converted on Ag/Y zeolite at higher temperatures to C2 and C3 olefins, and also alkanes (Figure 5).These by-products are produced by consecutive methylation and hydrogenation/dehydrogenation reactions involving different possible intermediates, such as carbenes or methyl oxonium ylides. This type of reaction between reactive intermediates and trapped HC in the zeolite structure is generally related to a “hydrocarbon pool” (HP) mechanism.45In addition to acidic sites, the dissociation of CH3I on metallic silver has been demonstrated to produce methyl species41, which may undergo rapid fragmentation to CH2-, CH, C and H species. Hence, it seems that both acidic and silver sites contribute to the HP mechanism for the studied Ag/Y zeolite. In the literature, it has also been reported that higher olefins, paraffins, aromatics and naphthenes can also be formed under MTHC conditions, mainly via hydrogen transfer,


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alkylation, isomerization, polycondensationand other secondary reactionscatalyzed by zeolitic materials. 45Moreover, it was also found that HC conversions strongly depend on the the zeolite pore topology, the acidity, the presence of metal promoters as well as important reaction parameters such as the temperature and residence time. Such an example of zeoliteinduced selectivity was evidenced for H-ZSM-5 and H-Y. 47H-ZSM-5 promotes the production of mono-ring aromatics from methanol and is only little subjected to coking. By contrast, on a large pore zeolite such as H-Y, the reactions leading to heavier HC products predomine. Over a certain temperature, these products further transform to H-rich products (first paraffins and then methane), and a H-deficient coke. 47 On CH3I-TPD data displayed on Figure 5, the production of CH4 up to 570°C seems to corroborate these results, because a darkening of the sample was simultaneously observed at the end of the TPD experiment. Finally, at higher temperatures redox processes can be proposed to explain the formation of CO (from 420 °C) on the one hand and I2 and HI on the other hand (from 500 °C). CO formation can result from partial oxidation of methane with water. On the other hand, molecular iodine can be formed by the redox thermal decomposition of silver iodide, which also produces metallic silver as by-product:48 2AgI → 2Ag° + I2 (6) The reaction network leading to the possible decomposition pathways of CH3I on Ag/Y zeolite is depicted on Figure 9.


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Figure 9 : mechanistic scheme representing the reactions pathways involved during CH3I adsorption and catalytic decomposition by 23Ag/Y (2.5) zeolite. 3.4. In situDR-UV-Vis study – reaction of iodine species with silver Relevant information on the processes leading to AgI precipitates was obtained using in situ DR-UV-Vis spectroscopy. On Figure 10 (A), are represented the temporal evolution of optical spectra in function of increasing exposure to methyl iodide (up to saturation coverage) at 100°C. The DR-UV-Vis spectrum of the silver zeolite recorded at 100°C after an in situ pretreatment in the cell at 200 °C under Ar (black line, bottom) is roughly similar to the one displayed on Figure 2and recorded under ex situ conditions. According to the spectral assignments made previously, it indicates the presence of dispersed silver species, namely Ag+ cations on exchange positions, but also the presence of charged (Agnδ+) or metallic


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(Ag°m) silver clusters. By exposing the zeolite to increasing levels of CH3I, new and strong absorptions develop immediately and a significant redshift is also observed (Figure 10 (A)).

(B)

(A)

Figure 10 : (A) Time-resolved DR-UV-Vis spectra of 23Ag/Y (2.5) exposed to CH3I until saturation; (B) CH3I-TPD on 23Ag/Y(2.5):spectral evolution in function of temperature (100 °C → 500 °C) and comparison with spectra of bulk AgI and the sample exposed to ambient air after CH3I adsorption test. Spectralon was used as reference. Moreover, TPD measurements achieved just after surface saturation with methyl iodide have shown that the absorption edge slightly shifts to higher wavelengths as the temperature is increased from 100 to 500°C (Figure 10 (B)). It is worth noting that even at 500 °C, the most important part of the absorptions related to these trapped iodine species remain confined in the UV range. Comparison between this set of in situ spectra and the optical spectrum of a commercial bulk AgI reference (in green, Figure 10 (B)) reveals that both spectra do not possess similar absorption features. In line with its yellowish color, the AgI reference has an intense absorption band in the visible region. Interestingly, the latter spectrum much more resembles to that of a spent (i.e. after CH3I adsorption test) silver zeolite sample (violet,


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Figure 10 (B)) after a prolonged exposure to ambient atmosphere. In order to better understand these evolutions, these temperature-programmed experiments were repeated but 1% H2O was now added to the feedgas (spectra not shown here). This procedure was used in order to extrapolate from in situ to ex situ conditions and to assess the effect of humidity. Under wet conditions, it was observed that the redshift of the absorption edge was more significant than under dry conditions. Thus, it can be concluded that the time of exposure to CH3I, the temperature and the handling conditions (such as the presence of moisture or oxygen) influence the absorption characteristics (and color) of iodine precipitates inside the zeolite. Interpretation of all the above observations is considered now. In presence of CH3I, dispersed silver species, such as the Ag+ cations in exchange positions present in the supercages of the faujasite structure, react with CH3I to form a single molecule of silver iodide. The AgI formation involves a precipitation reaction between the Ag+ species and iodide ions I- after a first step involving the dissociation of CH3I. This CH3I dissociation may occur both on silver or acid sites, depending on the temperature. On the other hand, the contribution from molecularly adsorbed CH3I species to UV-Vis spectra has also to be considered since it was observed both from TPD (Figure 5) and DRIFTS (Figure 6 (A) and (B)) studies. According to its absorption characteristics in liquid state (λmax = 260 nm), it is proposed that adsorbed CH3I species may slightly contribute to the absorptionsobserved in the 200-300 nm range (Figure 10 (A) and (B)) likewise some of the trapped olefins and oligomers produced from CH3I decomposition. Nevertheless, AgI clusters are known to also present strong absorptions in this range, which probably predomine over those related to organic species. In the literature, the incorporation of AgI clusters in vapor phase inside different zeolitic structureshas been investigated by UV-Vis spectroscopy. 49,50The redshift observed by increasing the loading in AgI was explained by the growth of confined (AgI)n clusters inside the pores and


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semiconductor quantum size effects. By increasing the size of semiconductor particles from the sub-nanometric to the nanometric range, a decrease of the spacing between the HOMO and LUMO orbitals is expected. 51-54Hence, the related absorptions become progressively displaced from the UV to the visible region above a certain cluster size. By confronting the spectral evolution of Figure 10 (A)with the model spectra from these literature studies49,50, it can be established that an aggregation of molecular AgI species occurred to form higher clusters of AgI in the course of reaction with CH3I. Furthermore, the size increase of silver iodide clusters ((AgI)n) within supercages continues progressively until reaching the storage capacity of supercages. For the faujasite structure, we can estimate that the higher clusters present in the supercages are of (AgI)4 type. 50 After exposure to ambient atmosphere, water vapor or high temperatures, AgI-related absorptions become more redshifted (Figure 10 (B)). This seems in line with the existence of a coalescence/sintering process. Indeed, a part of AgI clusters seems to move from the internal porosity to the external surface while forming cristalline precipitates, which can easily be observed by XRD and DRS-UV-Vis (

Figure 1 and Figure 10 (B)). It seems likely that humid conditions promote the formation of a micro-solution within the pores, which favors silver mobility and promotes the coalescence/sintering of AgI species. In agreement with this discussion, a schematic representation of silver iodide formation within the supercages as well their sintering outside of the zeolite pores is provided on Figure 11.


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Figure 11 : Schematic representation of silver iodide formation inside the supercages of 23Ag/Y(2.5) zeolite after being exposed to CH3I. For the sake of clarity, the faujasite structure was described by a 2D network of four connected supercages, each one being simply represented by an octagon, with four windows.

CONCLUSION

The aim of this study was to elucidate the mechanism of methyl iodide trapping and decomposition by a silver faujasite zeolite, in the context of severe nuclear accident. The 23wt%Ag/Y sorbent with Si/Al ratio of 2.5 and prepared using a 3-fold ionic exchange with silver nitrate was characterized by N2 porosimetry at 77K, XRD, DR-UV-Vis and DRIFTS of adsorbed CO. Correlations between the data obtained using all these techniques showed a great dispersion of silver species within the faujasite framework, with most part of


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silver being present as Ag+ isolated cations on SII and SIII exchange sites, the other part existing as charged Agnδ+ or metallic Ag°m clusters. Due to its high silver content, the 23wt%Ag/Y sorbent presented a high adsorption capacity of 209 mg/g towards CH3I and a good trapping stability, with about 79% of iodine being trapped as stable AgI precipitate. For the first time, such as approach using gas-phase FTIR, DRIFTS and DRS-UV-Vis was used to unravel the CH3I trapping mechanism. Gaseous and adsorbed intermediates involved in the catalytic decomposition of CH3I to hydrocarbons in the range 100-500 °C were investigated by combining TPD-reactor studies and in situ DRIFTS. At low-medium temperatures, part of the CH3I is adsorbed molecularly, forming H-bonded complexes with hydroxyl groups, namely the bridged Si-OH-Al located in the supercages and bearing Brönsted acidity. A part of adsorbed CH3I dissociates to form surface methoxy species and methyl groups. Methoxy species were found to be involved in the formation of methanol and dimethylether, which are produced above 100 °C. In addition, methane, higher alkanes (propane) and alkenes (such as ethene and propene) are formed from methanol and dimethyl ether above 250 °C, by a complex network of reactions similar to those encountered in MTHC processes. The processes leading to the growth of AgI precipitates in the internal and external surface of zeolite cristallites were directly observed by in situ DR-UV-Vis spectroscopy. It was found that AgI clusters of increasing size are first formed exclusively within the supercages. Quantum confinement concepts were used in order to explain the differences in the absorption bands observed for trapped AgI clusters with those corresponding to AgI in bulk state. Two different processes may be responsible for the formation of yellowish AgI precipitates: (i) the presence of silver metallic clusters or nanoparticles on the zeolite external surface before test;


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(ii) the effect of temperature and humidity, which promotes the growth of AgI clusters outside of the pores. Further works have to be performed in order to assess the CH3I trapping mechanism of silver zeolite in presence of gaseous “contaminants” present under severe accidental conditions, such as H2O, COx or NOx. ACKNOWLEDGMENTS 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" managed by the National Research Agency (ANR) under grant agreement n° ANR-11-RSNR-0013-01. REFERENCES (1) Jacquemain, D.; Guentay , S.; Basu, S.; Sonnenkalb, M.; Lebel, L.; Allelein, H. J.; Liebana, B.; Eckardt, B.; Ammirabile, L. Status Report on Filtered Containment Venting; OECD/NEA/CSNI, Report NEA/CSNI/R (2014) 7, 2014. (2) Jacquemain, D.; Guentay, S.; Basu, S.; Sonnenkalb, M.; Lebel, L.; Allelein, H. J.; Liebana, B.; Eckardt, B.; Ammirabile, L. In CSNI post-Fukushima activity on Filtered Containment Venting Systems: Status in OECD countries and guidance for improvements and future designs, International Congress on Advances in Nuclear Power Plants (ICAPP 2015), Nice, France, May 3-6, 2015. (3) Clément, B.; Cantrel, L.; Ducros, G.; Funke, F.; Herranz, L.; Rydl, A.; Weber, G.; Wren, C. State of the Art Report on Iodine Chemistry; OCDE report, NEA/CSNI/R (2007)/1, 2007.


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(4) Jubin, R. T. In Organic Iodine Removal from Simulated Dissolver Off-Gas Systems Using Partially Exchanged Silver Mordenite, Proceedings of the 17th Air Cleaning Conference, Colorado, U.S.A., Aug 2-5, 1982. (5) Haefner, D.R.; Tranter, T.J. Methods of Gas Phase Capture of Iodine from Fuel Reprocessing off-gas: A literature Survey; INL/EXT-07-12299, Idaho National Laboratory, Idaho Falls, U.S.A., 2007. (6) Grob, D. In Filtered Containment Venting System, NRC Public Meeting, Rockville, U.S.A., 2012. (7) Guieu S. M. In French NPPs Filtered Containment Venting Design, International Society for Nuclear Air Treatment Technologies, 33rd Nuclear Air Cleaning Conference, St Louis, U.S.A., June 22-24, 2014. (8) 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. (9) 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, U.S.A., 1969. (10) Pence, D.T.; Maeck, W.J. Silver Zeolite: Iodine Adsorption Studies; Idaho Nuclear Corporation, Idaho Falls, U.S.A., 1969. (11) Pence, D.T.; Duce, F.A.; Maeck, W.J. In Developments in the Removal of Airborne Iodine Species with Metal Substituted Zeolites, Proceedings of the 12th AEC Air Cleaning Conference, Tennessee, U.S.A., Aug 28-31, 1972.


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(12) Staples, B.A.; Murphy, L.P., Thomas, T.R. In Airborne Elemental Iodine Loading Capacities of Metal Zeolites and a Dry Method for Recycling Silver Zeolites, Proceedings of the 14th ERDA Air Cleaning Conference 1, Sun Valley, Idaho, U.S.A., Aug 2-4, 1976. (13) Thomas, T. R.; Staples, B. A.; Murphy, L. P.; Nichols, J. T. Airborne Elemental Iodine Loading Capacities of Metal Exchanged Zeolites and a Method for Recycling Silver Zeolites; Idaho National Engineering Laboratory, Idaho Falls, U.S.A., 1977. (14) Chapman, K. W.; Chupas, P. J.; Nenoff, T. M. Radioactive Iodine Capture in SilverContaining Mordenites through Nanoscale Silver Iodide Formation. J. Am. Chem. Soc. 2010, 132, 8897-8899. (15) 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. (16) Wei, Y.; Zhang, D.; Liu, Z.; Su, B. L. Highly Efficient Catalytic Conversion of Chloromethane to Light Olefins over HSAPO-34 as Studied by Catalytic Testing and In Situ FTIR. J. Catal. 2006, 238, 46-57. (17) Avila-Zarraga, J. G.; Martinez, R. Efficient Methylation of Carboxylic Acids with Potassium Hydroxide/Methyl Sulfoxide and Iodomethane. Synth. Commun. 2001, 31, 21772183. (18) Su. C.; Liao, C-H.; Wang, J.-D.; Chiu, C.-M.; Chen, B.-J. The Adsorption and Reactions of Methyl Iodide on Powdered Ag/TiO2. Catal. Today 2004, 97, 71-79. (19) Lippens, C. B.; 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.


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(20) Treacy, M. M. J.; Higgins, J. B. Collection of Simulated XRD Powder Patterns for Zeolites Fifth (5th) Revised Edition; Elsevier Sciences, 2007. (21) Fonseca, A. M.; Neves, I. C. Study of Silver Species Stabilized in Different Microporous Zeolites. Microporous Mesoporous Mater. 2013, 181, 83-87. (22) Zhang, A.; Shi, T. B.; Jia, C. Z.; Chen, Y.; He, M. Y. Adsorptive Removal of Aromatic Organosulfur Compounds over the Modified Na-Y Zeolites. Appl. Catal. B: environ. 2008, 82, 1-10. (23) Chester, A. W; Derouane, E. G. Zeolite Characterization and Catalysis, A Tutorial; Springer, 2001. (24) Aspromonte, S. G.; Mizrahi, M. D.; Schneeberger, F. A.; Ramallo Lopez, J. M.; 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, 25433-25442. (25) Bartolomeu, R.; Azambre, B.; Westermann, A.; Fernandes, A. ; Bertolo, R.; Issa Hamoud, H.; Henriques, C. ; Da Costa, P. 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. (26) Bartolomeu, R.; Bertolo, R.; Casale, S.; Fernandes, A.; Henriques, C.; Da Costa, P.; Ribeiro, F. Particular Characteristics of Silver Species on Ag-Exchanged LTL Zeolite in K and H Form. Microporous Mesoporous Mater. 2013, 169, 137-147. (27) Shibata, J.; Shimizu, K.-i.; Takada, Y.; Shichi, A.; Yoshida, H.; Satokawa S.; Satsuma, A.; Hattori, T. Structure of Active Ag Clusters in Ag Zeolites for SCR of NO by Propane in the Presence of Hydrogen. J. Catal. 2004, 227, 367-374.


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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 36 of 40

(28) Sayah, E.; Brouri, D.; Wu, Y.; Musi, A.; Da Costa, P.; Massiani, P. A TEM and UV– visible Study of Silver Reduction by Ethanol in Ag–Alumina Catalysts. Appl. Catal. A: Gen. 2011, 406, 94-101. (29) Lamberti, C.; Zecchina, A.; Groppo, E.; Bordiga, S. Probing the Surfaces of Heterogeneous Catalysts by In Situ IR Spectroscopy. Chem. Soc. Rev. 2010, 39, 4951-5001. (30) Hadjiivanov, K. I.; Vayssilov, G. N. Characterization of Oxide Surfaces and Zeolites by Carbon Monoxide as an IR Probe Molecule. Adv. Catal.2002, 47, 307-511. (31) Tarach, K.; Marek, K. G.; Chrzan, M.; Walas, S. Quantification of Silver Sites in Zeolites: Carbon Monoxide Sorption Monitored by IR Spectroscopy. J. Phys. Chem. C .2014, 118, 23751-23760. (32) Belapurkar, A. D.; Rao,K. A.; Gupta, N. M.; Iyer, R. M. On the Interaction of CH3I over Silver-Exchanged Molecular Sieves. Surf. Technol. 1984, 21, 263-272. (33) Jaumain, D.; Su, B.-L. Monitoring the Brönsted Acidity of Zeolites by Means of In Situ FT-IR and Catalytic Testing using Chloromethane as Probe Molecule. Catal. Today 2002, 73, 187-196. (34) Su, B.-L.; Barthomeuf, D. Quantitative Measurement by Infrared Spectroscopy of the Protonic Acidity of H-SAPO-37 and HY Using Benzene as a Probe. J. Catal. 1993, 139, 8192. (35) Su, B.-L.; Norberg, V. Characterization of the Brønsted Acid Properties of H(Na)-Beta Zeolite by Infrared Spectroscopy and Thermal Analysis. Zeolites 1997, 19, 65-74.


36

Page 37 of 40

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


(36) Su, B.-L.; Manoli, J. M.; Potvin, C.; Barthomeuf, D. Acidity and Location of Cations (H+, Na+) in HEMT and NaEMT Determined Using Benzene Adsorption. J. Chem Soc., Faraday Trans. 1993, 89, 857-862. (37) Choi, B. S.; Park, G. I.; Kim, J. H.; Lee, J. W.; Ryu, S. K. Adsorption Equilibrium and Dynamics of Methyl Iodide in a Silver Ion-Exchanged Zeolite Column at High Temperatures. Adsorption 2001, 7, 91-103. (38) Kubelkova, L.; Novakova, J.; Nedomova, K. Reactivity of Surface Species on Zeolites in Methanol Conversion. J. Catal. 1990, 124, 441-450. (39) Jaumain, D.; Su, B.-L. Direct Catalytic Conversion of Chloromethane to Higher Hydrocarbons over a Series of ZSM-5 Zeolites Exchanged with Alkali Cations. J. Mol. Catal. A: Chem. 2003, 197, 263-273. (40) Murray, D. K.; Chang, J.-W.; Haw, J. F. Conversion of Methyl Halides to Hydrocarbons on Basic Zeolites: a Discovery by In Situ NMR. J. Am. Chem. Soc. 1993, 115, 4732-4741. (41) 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, 9556-9566. (42) Natal-Santiago, M. A.; Dumesic,J. A. Microcalorimetric, FTIR, and DFT Studies of the Adsorption of Methanol, Ethanol, and 2,2,2-Trifluoroethanol on Silica. J. Catal. 1998, 175, 252-268. (43) Konya, Z.; Hannus, I.; Kiricsi, I. Infrared Spectroscopic Study of Adsorption and Reactions of Methyl chloride on Acidic, Neutral and Basic Zeolites. Appl. Catal. B: Environ. 1996, 8, 391-404.


37


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

Page 38 of 40

(44) Olmsted, J.; Williams, G. M. Chemistry: "The Molecular Science", second ed.; Wm. C. Brown Publishers, Iowa, 1997. (45) Khadzhiev, S. N.; Magomedova, M. V.; Peresypkina, E. G. Mechanism of Olefin Synthesis from Methanol and Dimethyl Ether over Zeolite Catalysts: A Review. Petroleum Chem. 2014, 54, 243-269. (46) Huela, V.; Huguet, E.; Cammarano, C.; Lacarriere, A.; Durand, R.; Leroi, C.; Cadours, R.; Coq, B. Conversion of Methyl Mercaptan and Methanol to Hydrocarbons over Solid Acid Catalysts – A Comparative Study. Appl. Catal. B: Environ. 2014, 144, 547-553. (47) Schulz, H. “Coking” of Zeolites During Methanol Conversion: Basic Reactions of the MTO-, MTP- and MTG Processes. Catal. Today 2010, 154, 183-194. (48) 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, 656-664. (49) Kodaira, T.; Ikeda, T.; Takeo, H. Optical and X-ray Diffraction Study of AgI Clusters Incorporated into Zeolite LTA. Eur. Phys. J. D. 1999, 9, 601-604. (50) Kodaira, T.; Ikeda, T.; Takeo, H. Incorporation of AgI Clusters into the Cages of Zeolites LTA and FAU Observed by Optical Spectra and X-ray Diffraction Patterns. Chem. Phys. Letters 1999, 300, 499-503. (51) Efros, A. L. Interband Absorption of Light in a Semiconductor Sphere. Sov. Phys. Semicond. 1982, 16, 772-775. (52) Kayanuma,Y. Quantum-Size Effects of Interacting Electrons and Holes in Semiconductor Microcrystals with Spherical Shape. Phys. Rev. B. 1988, 38, 9797-9805.


38

Page 39 of 40

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


(53) Tang, Z. K.; Nozue, Y.; Goto, T. Quantum Size Effect on the Excitation Energy and the Oscillator Strength of PbI2 Clusters in Zeolite. J. Phys. Soc. Jpn. 1991, 60, 2090-2094. (54) Brus, L. E. Electron–Electron and Electron‐Hole Interactions in Small Semiconductor Crystallites: The Size Dependence of the Lowest Excited Electronic State. J. Chem. Phys. 1984, 80, 4403-4409.


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