Adsorption Of Methyl Iodide Research Articles

Hydrophobic nanosheet silicalite-1 zeolite for iodine and methyl iodide capture

Effective capture of radioactive iodine from nuclear fuel reprocessing is of great importance for public safety as well as the secure utility of nuclear energy. In this work, a hydrophobic nanosheet silicalite-1 (NSL-1) zeolite with an adjustable size was developed for efficient iodine (I2) and methyl iodide (CH3I) adsorption. The optimized all-silica zeolite NSL-1 exhibits an excellent I2 uptake capacity of 553 mg/g within 45 min and a CH3I uptake capacity of 262 mg/g within 1 h. Benefiting from the reduced thickness and enhanced porosity, microporous NSL-1 possesses enhanced iodine adsorption capacity and fast adsorption kinetics, which is a considerable high value among inorganic materials. Unexpectedly, the remarkable characters of high hydrophobicity, acid-resistance and anti-oxidation endow it a higher iodine uptake capacity than traditional aluminosilicate zeolites. More importantly, the high uptake selectivity toward I2 possessed by NSL-1 owing to its hydrophobic skeleton under simulated dynamic conditions. The low cost, facile and scalable synthesis of NSL-1 further highlights great prospects for applications in the nuclear industry. This work provides useful insights for designing efficient adsorbents for iodine capture.

Effects of moisture and aging upon decomposition of methyl iodide by reduced silver mordenite

Reduced silver mordenite has been considered as a sorbent for the capture of organic iodides, especially methyl iodide, from off-gases produced by aqueous used nuclear fuel reprocessing operations. The adsorption capacity of this material has been unpredictable especially when NOx and water are present. Previous work has found that a catalytic decomposition reaction is occurring on the surface but few determinations have been made of the kinetics of this reaction. The work presented tested the adsorption behavior and apparent catalytic reaction rate in humid conditions and compared those to dry conditions testing. Both experiments observed a first order reaction with rate constants of 0.0847 L/g sorbent/s and 0.1202 L/g sorbent/s respectively. Such a reduction in apparent rate constant is possibly due to either water obstructing methyl iodide adsorption or product desorption limitation. Changes in the adsorption profile were also apparent between these two, with the humid conditions experiment reaching saturation sooner than the dry conditions experiment. Additionally, an experiment into the effects of sorbent storage in a controlled laboratory environment was performed. The performance of the sorbent materials that were stored with silver in the zerovalent state was slightly inferior to those materials that were stored in ionic form (Ag+) and reduced to zerovalent silver immediately prior to subjecting them to sorption test. The materials stored with silver in the ionic form (and reduced just prior to application) behaved essentially similarly to the freshly synthesized (and reduced) sorbents in the sorption tests. This suggests that zerovalent silver experiences some oxidation resulting in deactivation of some sites.

Study on iodine adsorption capacity and adsorption mechanism of AgY

ABSTRACT To systematically study the methyl iodide (CH3I) adsorption performance and adsorption mechanism of Ag ion exchange NaY zeolite (AgY), AgY adsorbents with different Ag content were selected. 16.6% AgY was capable of storing 150 mg/g and 275 mg/g of CH3I when the penetrated CH3I reached 1‰ and 100% inlet concentration. TEM and EDS results presented a high relationship between Ag dispersion and CH3I adsorption capability. The formation of a stable AgI complex rationalized the CH3I adsorption mechanism on AgY. Compared to AgX, AgY was verified to possess a strong acid resistance by FT-IR, SEM and XRD. The effect of three key parameters (temperature, humidity and gas velocity) on CH3I adsorption was carefully investigated by orthogonal experiment design. The experimental results showed that AgY had better iodine removal performance at a high temperature, low humidity, and low flow rate. This work provides technical reference for further engineering applications of 16.6%AgY in spent fuel reprocessing plants.

Adsorption of toxic methyl iodide over Beta, ZSM-5 and SSZ-13 supported silver adsorbents

The efficient capture of radioactive isotopes of iodine uranium fission reactions is a major issue for safe nuclear energy. To investigate the toxic methyl iodide (CH3I) retention performance of Ag/Beta, Ag/ZSM-5 and Ag/SSZ-13, these Ag zeolites with identical Ag loadings and close Si/Al were prepared. The limited pore volume of ZSM-5 makes a larger size of Ag particles (∼7 nm) than Ag/Beta and Ag/SSZ-13. Silver, in the form of Ag+ ions, enters the cages of the zeolites as Lewis acid sites. In terms of the CH3I retention performance, Ag/Beta rivals the currently reported Ag zeolites applied to capture of radioactive iodine compounds. The CH3I storage capacity of Ag/Beta reaches 1.76 g/gAg. The strong CH3I affinity on Ag/Beta is confirmed by DFT method. The CH3I adsorption energy reaches 121.24 kJ/mol. Kinetics analysis shows that CH3I adsorption on three Ag zeolites observes pseudo-second order model. Good fitting of intra-particle diffusion model on Ag/SSZ-13 also demonstrates that intragranular diffusion is the rate-determining step of CH3I adsorption. The mechanism of CH3I retention is CH3I diffusion within the zeolites and then the formation of AgI species. This work expands the research field of Ag zeolite and contributes to its promising application in the capture of radioactive iodine.

Nanoporous N-Rich Covalent Organic Frameworks with High Specific Surface Area for Efficient Adsorption of Iodine and Methyl Iodide

Nanoporous N-Rich Covalent Organic Frameworks with High Specific Surface Area for Efficient Adsorption of Iodine and Methyl Iodide

A mixed-ligand Co(II) MOF synthesized from a single organic ligand to capture iodine and methyl iodide vapour.

Mixed-ligand metal-organic frameworks (MOFs) are usually synthesized from two or more organic ligands as initial reactants, and MOFs synthesized from one organic ligand precursor through partial in situ reactions remain very limited. Herein, by introducing an imidazole-tetrazole bifunctional ligand, 5-(4-imidazol-1-yl-phenyl)-2H-tetrazole (HIPT), as a single ligand and performing in situ hydrolysis of the tetrazolium group, a mixed-ligand Co(II)-MOF based on HIPT and 4-imidazol-1-yl-benzoic acid (HIBA), [Co2(μ3-O)(IPT)(IBA)]·x solvent (Co-IPT-IBA), was constructed and applied to capture I2 and methyl iodide vapours. Single crystal structural analyses reveal that Co-IPT-IBA exhibits a 3D porous framework with 1D channels based on the relatively few reported ribbon-like rod SBUs. The nitrogen adsorption-desorption isotherms indicate that the BET surface area of Co-IPT-IBA is 168.5 m2 g-1 and it possesses both micropores and mesopores. Due to its porosity, nitrogen-rich conjugated aromatic rings, and Co(II) ions, Co-IPT-IBA was applied to capture iodine molecules in vapour and exhibited an adsorption capacity of 2.88 g g-1. By combining the IR, Raman, XPS and grand canonical Monte Carlo (GCMC) simulation results, it was deduced that the tetrazole ring, coordination water molecules, and the redox potential of Co3+/Co2+ facilitate iodine capture. The presence of mesopores was also responsible for the high iodine adsorption capacity. In addition, Co-IPT-IBA showed the ability to capture methyl iodide in vapours with a moderate capacity of 625 mg g-1. The transformation of crystalline Co-IPT-IBA to amorphous MOFs may be due to the methylation reaction. This work represents a relatively rare example of methyl iodide adsorption by MOFs.

Tetraethylene glycol-based polymer networks for the efficient removal of radioactive methyl iodide and iodine vapor

A new application of aliphatic polyether derivatives: the highly efficient adsorption of methyl iodide and iodine vapor.

Adsorption of methyl iodide on reduced silver‐functionalized silica aerogel: Kinetics and modeling

AbstractThe low concentration methyl iodide (CH3I) adsorption process on reduced silver‐functionalized silica aerogel (Ag0‐Aerogel) was studied. The kinetic data were acquired using a continuous flow adsorption system. Because the corresponding physical process was observed, the shrinking core model (SCM) was modified and applied. An average CH3I pore diffusivity was calculated, the CH3I‐Ag0‐Aerogel reaction was identified as a 1.40 order reaction instead of first order reaction, and the nth order reaction rate constant was determined. This modified SCM significantly increases the accuracy of adsorption behavior prediction at low adsorbate concentration. Modeling results indicate that the overall adsorption process is controlled by the pore diffusion. However, at low adsorbate concentration (ppbv level), the CH3I adsorption is limited to the surface reaction due to the low uptake rate in a predictable time period.

Design of highly efficient adsorbents for removal of gaseous methyl iodide using tertiary amine-impregnated activated carbon: Integrated experimental and first-principles approach

Activated carbons (ACs) impregnated with triethylenediamine (TEDA) and quinuclidine (QD) are prepared for the removal of methyl iodide (CH3I) gas in the event of nuclear accidents. The effect of the tertiary amine loading (1, 5, and 10 wt.%) on the adsorption capability of the ACs is evaluated under dry and humid (RH 75%) conditions. Experimental evaluation of the 'breakthrough' combined with rigorous first-principles calculations enables us to identify the fundamental mechanism underlying the superior adsorption capability of QD to TEDA for real application, especially under humid conditions. The adsorption of CH3I by the impregnated ACs under dry conditions strongly depends on the physical properties (porosity) of the adsorbent. On the other hand, the chemical properties and the tertiary amine loading play dominant roles in CH3I adsorption at 75% RH. In particular, this study suggests a novel mechanism of the protonation reaction, rather than the already known alkylation of tertiary amines, to capture CH3I chemically under humid conditions. The molecular-level behavior of amines and water is a key factor for the enhanced adsorption of methyl iodide. The adsorbents developed herein can be applied to the efficient removal of radioactive methyl iodide in severe accidents.

Adsorptive removal of gaseous methyl iodide by triethylenediamine (TEDA)-metal impregnated activated carbons under humid conditions.

Removal of gaseous radioactive iodine (131I and 129I) compounds from nuclear facilities is an important issue. Herein we assessed the adsorptive capacity of gaseous non-radioactive methyl iodide (CH3127I) as a simulant on two commercial TEDA-metal impregnated activated carbon(AC)s. The characterizations of the ACs were determined ICP-MS, XPS, and 77 K N2 isotherms. As a result, it was found that one AC has a small amount of TEDA but a well-developed porosity, and the other one was abundant with TEDA, but the porosity was relatively less developed. The methyl iodide removal performances were evaluated under 10 ppm and 400 ppm using breakthrough experiments under various relative humidities (RH). Desorption was also carried out using nitrogen after adsorption to investigate adsorption affinity. Methyl iodide adsorption capacity of TEDA-rich AC decreased significantly as RH increased at 10 ppm. Conversely, performance degradation was clearly observed from less TEDA-impregnated AC with well-developed porosity as RH increased at 400 ppm. It is demonstrated that the amount of physisorbed methyl iodide is decreased as RH increased. Although moisture decreases the adsorption amount, it enhances the adsorption affinity. Also, additional TEDA impregnation to ACs results in improving the performance under severe condition (RH90%, 400 ppm).

Investigation of gaseous wet methyl iodide adsorption on Ag nanoparticles embedded in organic–inorganic hybrid silica gels

X-ray photoelectron spectroscopy (XPS), single-particle photoluminescence, and lifetimes were adopted to investigate the interaction properties between gaseous wet methyl iodide (CH3I) and Ag nanoparticles (Ag NPs), which are embedded into a propylethylenediamine (NH2CH2CH2NHCH2CH2CH2-, PEDA) anchored silica gel. Based on the obtained XPS resolutions and shifts of binding energies, a stronger interaction of CH3I than Ag NPs toward SiO2, methylation of amines and silanols by CH3I, the partial interaction in the Ag NPs and CH3I interface, condensation of SiO2 and PEDA, and stabilization of Ag NPs by PEDA were explained. In addition, an enhanced luminescence of Ag NP-embedded PEDA-anchored SiO2 gel (Ag NP-PEDA gel) by surrounding CH3I, and Ag NP stabilization by PEDA and CH3I were also observed from the single-particle photoluminescence and lifetimes.

Energetics of Adsorbed Methyl and Methyl Iodide on Ni(111) by Calorimetry: Comparison to Pt(111) and Implications for Catalysis

The heat of adsorption and sticking probability of methyl iodide were measured on Ni(111) at 100 and 160 K using single-crystal adsorption calorimetry (SCAC). At 100 K, methyl iodide adsorbs molecularly with a heat of 102 kJ/mol on terrace sites in the low-coverage limit, giving a standard enthalpy of formation (ΔHf°) of CH3I(ad) of −87 kJ/mol. A heat of 122 kJ/mol is also measured on defect sites, probably step edges. Calorimetry of the dissociative adsorption of methyl iodide on Ni(111) at 160 K yielded an integral heat of adsorption of −270 kJ/mol at 0.04 ML, providing the energetics of adsorbed methyl, with ΔHf°[CH3(ad)] = −71 kJ/mol and a CH3–Ni(111) bond enthalpy of 218 kJ/mol. This is 22 kJ/mol stronger than the reported value for H3C–Pt(111) bonds, explaining the greater activity of Ni catalysts for hydrogenolysis in comparison to Pt. The measured energetics for methyl were compared to density functional theory (DFT) calculations from previous literature, showing that these methods systematically ...

Capture of Elemental and Organic Iodine from Dilute Gas Streams by Silver-exchanged Mordenite

The treatment of off-gas streams arising from reprocessing of used nuclear fuel (UNF) is an area of active study by the U.S. Department of Energy. Such off-gas streams contain volatile fission products, including long-lived 129I. Although 129I is released into the off-gas at multiple points within the chemical reprocessing flowsheet, previous research has focused on removal from the dissolver off-gas stream (DOG). The DOG is expected to contain up to 98% of iodine in UNF at ppm levels within the stream. Other off-gas streams will also contain iodine but at substantially lower concentrations. Recent work has shown that compliance with U.S. regulations will likely require capture of iodine from these dilute streams in addition to capture from DOG.In particular, the vessel off-gas (VOG) stream is expected to contain 1-3% of the total iodine inventory at ppb concentrations. A review of literature also indicates that the speciation of iodine in the VOG stream will differ from that of the DOG, with the DOG containing primarily I2 and the VOG containing a mixture of I2 and organic iodine species.Silver-exchanged mordenite (AgZ) has been identified for use in the removal of iodine from off-gas streams. It is an effective capture material for I2 at the concentrations expected in the DOG, but little is known about its performance in gas streams that may contain both I2 and organic iodides at very dilute concentrations.The experiments to be described were designed to separately characterize the adsorption of I2 and methyl iodide on AgZ through extended duration testing. Simulated vessel off-gases containing low levels of either I2 or methyl iodide were contacted with AgZ sorbent beds for up to four months. Through the use of sorbent beds in series and varied sampling times, key parameters such as adsorption rate, decontamination factor, and performance over time could be determined for the capture of each species by AgZ.This paper will discuss the literature relating to the speciation of iodine in the VOG and will discuss the results of adsorption experiments. An examination of the difference between capture of I2 and organic iodides will be conducted, and the impacts of those results on future iodine removal work will be considered.

Method for direct deconvolution of heat signals in transient adsorption calorimetry

Abstract A method of heat signal analysis is presented for transient adsorption calorimetries including single crystal adsorption calorimetry (SCAC) which uses fast Fourier transforms (FFT) to determine the instrument response function and deconvolute the heat-versus-time signals. The method utilizes a heat signal generated by a laser pulse of known power-versus-time to extract the instrument response function for the calorimeter. The instrument response function is then used to extract the heat power signal from a molecular beam heat pulse of unknown intensity. This method allows for the extraction of the total heat deposited by the molecular beam pulse without any kinetic modeling even in the event of complex reaction dynamics. This method is compared to previous methods used to analyze SCAC data using example data from the two-step dissociative adsorption of methyl iodide on Pt(111). It is found to be equally accurate for extracting total heats and simpler to perform than the previous methods.

Surface kinetics and energetics from single crystal adsorption calorimetry lineshape analysis: Methyl from methyl iodide on Pt(1 1 1)

The first use of single crystal adsorption calorimetry (SCAC) to probe the kinetics of surface chemical reactions is presented. It is applied to study a common situation encountered in catalytic mechanisms, wherein a gas quickly populates a molecularly adsorbed state that then converts to more stable products on a slower timescale (10–1000ms). We show that for such a two-step process, detailed analysis of the heat-detector signal’s time-response lineshape to a single pulse of gas provides the rate constant for the second step and the heats of reaction for both of the elementary reactions. We apply this analysis to the dissociative adsorption of methyl iodide (CH3I) on Pt(111) at 270K, to measure the heats of reaction for the elementary steps involved and the rate constant for the slow step, all as a detailed function of coverage. At low coverage, the reaction is CH3Ig→CH3Iad→CH3,ad+Iad, followed at high coverage by CH3,ad→CHad+2Had and CH3,ad+Had→CH4,g (above 0.08ML total CH3,ad plus Iad). These results provide the rate constant for the dissociation of CH3Iad and the heats of formation of both CH3,ad and CHad. These two heats agree with values determined at 320K where the rates are so fast that lineshape analysis is not needed, proving the validity of the lineshape analysis method introduced here for analyzing SCAC data.

Surface free energy analysis of adsorbents used for radioiodine adsorption

In this work, the surface free energy of biomass-based activated carbons, both fresh and impregnated with triethylenediamine, has been evaluated. The contribution of Lifshitz van der Waals components was determined by the model proposed by van Oss et al. The results obtained allowed predicting the most probable configurations of the impregnant onto the carbon surface and its influence on the subsequent adsorption of radioactive methyl iodide.

Fundamental Studies of Methyl Iodide Adsorption in DABCO Impregnated Activated Carbons

Methyl iodide capture from a water vapor stream using 1,4-diazabicyclo[2.2.2]octane (DABCO)-impregnated activated carbons is, for the first time, fundamentally described here on the atomic level by means of both molecular dynamics and grand canonical Monte Carlo simulations. A molecular dynamics annealing strategy was adopted to mimic the DABCO experimental impregnation procedure in a selected slitlike carbon pore. Predictions, restricted to the micropore region, are made about the adsorption isotherms of methyl iodide, water, and nitrogen on both impregnated and bare activated carbon models. Experimental and simulated nitrogen adsorption isotherms are compared for the validation of the impregnation strategy. Selectivity analyses of the preferential adsorption toward methyl iodide over water are also reported. These simulated adsorption isotherms sum up to previous experimental studies to provide an enhanced picture for this adsorption system of widespread use at nuclear plant HVAC facilities for the capture of radioactive iodine compounds.

Energetics of Adsorbed CH3 on Pt(111) by Calorimetry

The enthalpy and sticking probability for the dissociative adsorption of methyl iodide were measured on Pt(111) at 320 K and at low coverages (up to 0.04 ML, where 1 ML is equal to one adsorbate molecule for every surface Pt atom) using single crystal adsorption calorimetry (SCAC). At this temperature and in this coverage range, methyl iodide produces adsorbed methyl (CH(3,ad)) plus an iodine adatom (I(ad)). Combining the heat of this reaction with reported energetics for Iad gives the standard heat of formation of adsorbed methyl, ΔH(f)(0)(CH3,ad), to be −53 kJ/mol and a Pt(111)–CH3 bond energy of 197 kJ/mol. (The error bar of ±20 kJ/mol for both values is limited by the reported heat of formation of I(ad).) This is the first direct measurement of these values for any alkyl fragment on any surface.

Influence of carbon xerogel textural properties on the dynamic adsorption of methyl iodide

X-ray microtomography coupled to image analysis has been used to study the influence of the adsorbent pore texture and the experimental conditions on the dynamic adsorption of methyl iodide in packed filters. By applying this imaging technique the internal axial adsorption profiles for increasing exposure times to the gas stream are analysed. This experimental technique establishes a new technology to study in situ the dynamic adsorption of volatile compounds. Resorcinol–formaldehyde based carbon xerogels have been used as adsorbents, as their pore texture can be tuned by changing the synthesis conditions. The textural characteristics of the adsorbents (surface areas and pore volumes) have been assessed by using nitrogen and carbon dioxide adsorption as well as mercury porosimetry. The methyl iodide dynamic adsorption results show that, for the same gas flow rate and CH 3I inlet concentration, the adsorbed amount is highly dependent on large pore volumes. Thus, samples with almost the same micropore volumes (adsorption volumes) have different methyl iodide adsorption capacities, which are related to, the above mentioned, large pores. The influence of both the gas carrier flow rate and the methyl iodide inlet concentration on the adsorption can be explained using the so-called linear driving force model. This approach takes into account the fact that internal transport limitations are directly related to the pore texture. Moreover, the simulation of the dynamic adsorption process has allowed relating the simulated axial concentration profiles to the experimental X-ray microtomography data.

Density Functional Theory Study of Methyl Iodide Adsorption and Dissociation on Clean and K-Promoted β-Mo2C Surfaces

We have studied the effect of K on the adsorption and dissociation of methyl iodide on the β-Mo2C(001) surface using density functional theory calculations, and the results were compared with experimental data. The most favorable sites for methyl iodide adsorption are 3-fold sites on both clean and K-promoted surface. The changes in the work function fit our model as the molecule withdraws charge from the surface. The C−I bond weakens when the molecule adsorbs on the surface, and this effect is more noticeable on the K-promoted surface. The dissociation to an I adsorbed atom and a methyl group adsorbed is energetically favorable for both surfaces, but there is a lower activation barrier on the K-doped surface.