Carbon Textiles Modified with Copper-Based Reactive Adsorbents as

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Carbon textiles modified with copper-based reactive adsorbents as efficient media for detoxification of chemical warfare agents Marc Florent, Dimitrios A. Giannakoudakis, Rajiv Wallace, and Teresa J. Bandosz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10682 • Publication Date (Web): 27 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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ACS Applied Materials & Interfaces

Carbon textiles modified with copper-based reactive adsorbents as efficient media for detoxification of chemical warfare agents

Marc Florent,1 Dimitrios A. Giannakoudakis,1,2 Rajiv Wallace,1 Teresa J. Bandosz1,2*

1

Department of Chemistry, The City College of New York, New York, NY 10031 USA

2

Ph.D. Program in Chemistry, The Graduate Center of the City University of New York, New

York, NY 10016

*Whom correspondence should be addressed to. Tel.: (212)650-6017; Fax: (212)650-6107); Email: [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT Carbon textile swatch was oxidized and impregnated with copper hydroxynitrate. A subsample was then further heated at 280 oC to form copper oxide. The swatches preserved their integrity through the treatments. As final products, they exhibited remarkable detoxification properties for the nerve agent surrogate dimethyl chlorophosphate (DMCP). Based on the amount of reactive copper phases deposited on the fibers, their adsorption capacities were higher than those of the bulk powders. After a one day exposure to DMCP (1:1 weight ratio adsorbent/DMCP), 99 % of the initial amount of DMCP was eliminated. A synergistic effect of the composite components was clearly seen. GC-MS studies showed that the main surface reaction product was chloromethane. Its formation indicated hydrolysis as a detoxification path. Surface analyses showed phosphate bonding to the fibers and formation of copper chloride. The appearance of the latter species results in a clear textile color change, which suggests the application of these fabrics not only as catalytic protection agents but also as sensors of nerve agents.

Keywords: carbon fibers, copper, textile, nerve agent, degradation, adsorption, detoxification, sensor

1. Introduction Since their massive use during World War I, chemical warfare agents (CWA) have been a worldwide concern resulting in various international treaties to control their production and use.1-2 The last multilateral convention was signed in 1993 and implemented in 1997.3 In spite of


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this, they are still developed and consider as weapon of mass destruction.4 The usage of Sarin in a terrorist attack in Japan (1995) or very recently in Syria are reminders of the threat they pose not only to military personnel in a war zone, but also to civilians.5 This highlights the need to develop new materials that could adsorb, destroy and sense such deadly chemicals. Sarin belongs to a family of organophosphates known as nerve agents for their ability to attack the nervous system resulting in death from respiratory failure.6 These molecules are highly reactive and rapidly hydrolyze in a basic environment. Thus, treatment with an alkaline solution is one of the main methods to destroy stockpile of nerve agents or decontaminate polluted surfaces.7-9 Yet, people who can potentially be in contact with such chemical warfare agents, such as soldiers, need wearable personal protective equipment that could sense the presence of nerve agents in their environment and detoxify it. Impregnated activated carbons are the main protective materials used nowadays in gas mask canisters .10 Many new materials, mostly metal-based (hydr)oxides such as zinc,11 copper,12 zirconium,13 iron,14 manganese,15-16 or metal-organic frameworks17-18 have been investigated to replace carbons or, as a new active phase, to deposit on an activated carbon support. Among those new reactive adsorbents, copper-based materials could be of particular interest due to the broad range of color they may exhibit while undergoing various reactions. Thus, a previous study on copper hydroxynitrate indicated a visible color change from blue-green to yellow-orange upon reaction with dimethyl chlorophosphate (DMCP), a nerve agent surrogate.12 Such change of color might be a direct and facile way of detecting odorless nerve agents. Besides being deployed in air and thus targeting respiratory system, many chemical weapons also act through a skin contact. Thus wearing protective garments is a must.9 Moreover, the professionals who can be subjected to CWAs at any moment also need equipment such as



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tents, backpack, etc., that would not only work as a simple barrier but can also destroy the threat and self-decontaminate. Thus, many research efforts have been recently focused on the development of the next generation of protective textiles either by synthesizing new functionalized synthetic fibers,19-20 or by immobilizing a reactive phase on fabrics.21 Carbon fibers are synthetic fibers that present various interesting properties. In particular, they have high strength-to-weight ratio, are chemically and thermally stable, and lightweight so they could be easily added to military garments without adding to a soldier’s burden.22-26 The objective of this study is to investigate the modified carbon fibers as a new class of protective fabrics against CWAs. Copper hydroxynitrate has been chosen as an active phase being able to catalytically destroy toxic species at ambient conditions. It has been shown previously to have high detoxification capabilities toward, DMCP, especially when in composite with graphite oxide.12 DMCP is a known surrogate of nerve agents such as sarin.12,

18, 27-28

Indeed, due to the high toxicity and restricted access to CWAs, much studies are done on simulants which present close physico-chemical properties to the real CWAs, but are less toxic.29 The copper-modified textile was also subjected to a thermal treatment in order to transform the active phase on the carbon fabrics surface to CuO. Both swatches were exposed to DMCP. Analyses of the surfaces, headspace and extracts were carried out to assess the degradation mechanisms.

Moreover, the sensing capability based on the color changes of the copper

salts/oxides was evaluated.

2. Experimental 2.1. Materials Carbon fiber (CFinitial) textile swatches were obtained from Fibre Glast and washed 1 day with acetone to remove any post-synthesis surface treatments (sizings). After drying, they were then 4 ACS Paragon Plus Environment

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oxidized with a mixture of sulfuric and nitric acid 75/25 v/v for 1 day at a room temperature, washed in a Soxhlet apparatus, and dried. The carbon fibers after this treatment are referred to as CF. Copper hydroxy nitrate was prepared by precipitation from a solution of Cu(NO3)2 with NaOH. The precipitating agent was added at a rate of 40 ml/min. The molar ratio of Cu(NO3)2:NaOH in the reaction mixture was 1:2. The precipitate was then filtered, washed with water and dried at 60 oC. Deposition of the active phase on CF was done by a successive immersion of the swatches in an ethanolic dispersion of copper hydroxynitrate. The modified textile is referred to as CF-CuON. The recovered unattached powder is referred to as CuON. CF-CuON was than heated at 280 oC for 2 hours in air. This sample is referred to as to CF-CuO. CuON powder heated at 280 oC for 2 hours in air is referred to as CuO. Deposition of copper hydroxynitrate on the initial carbon fiber, before oxidation, was carried out in the same conditions as those used in the case of CF samples. The material obtained is referred to as CFinitial-CuON. 2.2. Methods 2.2.1. Potentiometric titration: Potentiometric titration of the carbon textiles, initial and oxidized, was performed using an 888 Titrando automatic titrator (Metrohm). The samples were suspended in NaNO3 (0.1M) solution and titrated with NaOH (0.1 M) starting from pH ~ 3.2 (pH was adjusted with HCl 0.1M if necessary) up to pH ~ 10. The proton binding curves, Q, were obtained from the titration data,30 and the pKa distributions f(pKa) of the groups on the surface of the samples were calculated by finding stable solutions of the Fredholm integral, relating Q to f(pKa), using the numerical procedure SAIEUS.31-32



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2.2.2. Nitrogen adsorption: N2 adsorption isotherms were measured at −196 °C using an ASAP 2020 (Micromeritics). Prior to the analysis the samples were outgassed at 90 °C to a constant vacuum (10-4 Torr). The BET surface area, SBET was calculated from the isotherms. 2.2.3. XRD: X-Ray- diffractograms of the materials were measured on a PANalytical X'Pert Pro Powder Diffraction diffractometer equipped with a PIXcel1D detector, using a CuKα radiation (λ = 0.154 nm) over the range of 10–60° 2θ with steps of 0.026° 2θ. 2.2.4. SEM: Scanning electron micrographs were obtained using a Zeiss Supra 55 VP instrument with electron dispersive X-ray spectroscopy (EDX) at an acceleration voltage of 5 keV. 2.2.5. Fourier transform infrared spectroscopy (FTIR): FTIR spectra were collected on a Nicolet 380 spectrometer using attenuated total reflectance (ATR) with a diamond crystal. 32 scans were averaged and background corrected. 2.2.6. TA-MS: TG curves were collected with a SDT Q600 (TA instruments). The samples were heated up to 1000 °C at a rate of 10 °C/min in an air or helium flow (100 mL/min). Decomposition products in helium were analyzed by mass spectrometry using a gas analysis system Omnistar GSD 320 (Pfeiffer Vacuum). 2.2.7. Reactive adsorption of CWA surrogate: The samples were exposed to dimethyl chlorophosphate (DMCP 96 %) vapors in hermetically closed glass vessels containing two open vials. One vial contained 20 mg of an adsorbent and the other one 20 µl of surrogate. After 1 and 7 days, the headspace was sampled with a syringe through a septum and analyzed by GCMS. The internal container containing the adsorbent was weighted before and after the exposure to the surrogate agent to calculate the mass gain as a result of the adsorption. 1 day and 7 days were arbitrarily chosen to attain a sufficient weight gain and thus minimize the error when weighting. A drawing of the setup is shown in Figure S1A of the Supporting Information.


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2.2.8. Degradation of CWA surrogate: 10 mg of samples were introduced in one end of a horizontal vial. The same mass (10 mg) of DMCP (96 % purity) was spread in the other end and the vial was hermetically closed. Spreading the few microliters of surrogate on the wall of the container allows a better and faster evaporation. At specific times, 1 ml of acetonitrile was introduced through a septum and the vial was shaken to extract the remaining and weakly bonded DMCP. The extract was analyzed by GC-MS. A drawing of the setup is shown in Figure S1B of the Supporting Information. The decrease of DMCP as a function of time was fitted to a first order kinetic equation: ሾ‫ܲܥܯܦ‬ሿ ቇ = −݇‫ݐ‬ ln ቆ ሾ‫ܲܥܯܦ‬ሿ଴ Where [DMCP]0 and [DMCP] are respectively the amounts of DMCP in the acetonitrile extract represented by their peaks area in the chromatograms measured by GCMS initially and at the time t. k is the rate constant in min-1 and t the time in min. 2.2.9. GC-MS: The reactor headspace and acetonitrile extracts were analyzed by GC coupled with a MS detector (Shimadzu Q5000). The column (Restek XTI-5 capillary column) was heated from 50 to 340 oC at a rate of 40 oC/min. Helium was used a carrier gas. The eluting analytes were detected and characterized by mass spectrometry with an electron ionization detector.

3. Results and Discussion Since carbon fibers are rather stable in standard conditions of temperature and pressure, they needed to be oxidized to promote the deposition of the active phase. Such process is expected to form functional groups, which will act as seeds for the polar active phase deposition. A mixture of concentrated sulfuric and nitric acids at room temperature was found mild enough not to break the fibers and to retain the bulk properties of the swatches and yet it was strong



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enough to oxidize the fiber surfaces. The pKa distributions of the different groups present on the surface of the carbon were derived from the titration curve of the carbon fibers dispersed in NaNO3.30-32 Proton binding curves and pKa distributions of the surface acidic functional groups are presented in Figure 1. The initial fibers were rather bare of surface groups (0.05 mmol/g), with only functionalities of pKa =3 and pKa = 7, both in similarly very small quantities were detected. After oxidation, the surface became acidic.

A variety of functional groups (2.2

mmol/g) of pKa ranging from 4 to 10 was formed on their surface.

Figure 1. (A) Proton binding curves and (B) pKa distributions of the groups present on the surface of the carbon textile swatches, initial (CFinitial) and oxidized (CF). The carbon fibers, initial and oxidized, were impregnated with copper hydroxynitrate. Without oxidation, the copper phase powder did not attach to the fiber surface, and was easily removed by a mechanical treatment. On the other hand, the impregnation of the oxidized carbon fiber with copper hydroxyl nitrate resulted in a homogeneous bluish coloration of the swatch (Figure 2A). After heating of CF-CuON at 280 oC, the swatch gets a remarkable copper-like


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metallic tone. Interestingly, the color was not black, as it would be expected for CuO. This may indicate a reduction of the copper(II) phase into copper(0) at 280 oC in the presence of carbon.

Figure 2. (A) Pictures of the carbon fiber swatches, CF-CuON (i and iii) and CF-CuO (ii and iv) before (i and ii) and after (iii and iv) exposure to DMCP vapors. (B) TG and DTG curves in air for CF-CuON, CF-CuO and for the non-oxidized carbon textile (CFinitial) before and after impregnation with CuON. (C) X-ray diffractograms of CF-CuON and CF-CuO textiles and of CuON and CuO powders. Diffraction peaks corresponding to Cu2O () and Cu () are indicated on the diffraction patterns, along with the Miller indices of CuON (rouaite) and CuO (tenorite) phases.

In order to characterize the copper phases immobilized on the surfaces of the carbon fibers, CF-CuON and CF-CuO were analyzed by XRD. The obtained diffractograms are shown 9 ACS Paragon Plus Environment


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in Figure 2B. They are compared to that of CuON, the unattached powder recovered from the impregnation process, and to that of CuO (the CuON powder heated at 280 oC for 2 hours). On the diffractograms of the modified textiles a broad peak, typical of amorphous carbon, is revealed with a maximum at 24.5 o 2θ. The results also show that the active phase deposited on CF-CuON is the monoclinic rouaite, a copper hydroxynitrate Cu2(OH)3NO3. CuON heated at 280 oC converts into copper oxide CuO (tenorite), and it is the phase found on CF-CuO. On the latter sample, cuprite (Cu2O) and copper (Cu) are also present. This indicates that the presence of carbon fiber indeed leads to some reduction of the Cu(II) phase. This explains the metallic copper color of the swatch. Carbothermal reduction of copper sulfate on activated carbon has already been reported, although at high temperature (750 oC), and the same three copper phases (Cu, Cu2O and CuO) were formed.33 In our case, the high dispersion of the copper phase is likely responsible for its reduction taking place at a low temperature. The thermal analysis (TA) of CFinitial (non-oxidized carbon fiber), CFinitial-CuON (impregnated non-oxidized carbon fiber), CF-CuON (impregnated oxidized carbon fibers) and CF-CuO (CF-CuON heated at 280 oC) was carried out to estimate the amount of active phase deposited on the swatches. The TG and DTG curves measured in air are collected in Figure 2C. CFinitial completely burns and CFinitial-CuON almost completely burns, leaving an ash content of only 0.5 wt%. This confirms that without oxidation the copper phase hardly attaches to the carbon fibers. However, a shift to a lower ignition temperature for the modified swatch was found. This indicates that even though only small amount of copper was attached to the surface, it was able to catalytically promote the carbon fiber oxidation process. CF-CuON shows a small weight loss around 230 oC, which corresponds to the conversion of Cu2(OH)3NO3 to CuO.12


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The carbon phase burns between 400 and 550 oC. Above that temperature the masses of both CF-CuON and CF-CuO remain constant and the ashes consist of CuO since Cu and Cu2O that formed as a result of the presence of carbon, should be oxidized in air at a high temperature.34 The ash content is 8.3 wt% for CF-CuO and 6.4 wt% for CF-CuON, which correspond to a copper content of 6.6 and 5.1 wt% for CF-CuO and CF-CuON, respectively. The lower content of copper in CF-CuON as compared to CF-CuO, means that CF-CuON contains more carbon than CF-CuO. This implies that during the heating process to 280 oC that transforms CF-CuON into CF-CuO some carbon is consumed, resulting in a copper enrichment of CF-CuO as compared to CF-CuON. This is an indication of the carbothermal reduction process. Indeed, the reduction of Cu2(OH)3NO3 into Cu2O or Cu is possible due to the oxidation of some of the carbon matrix, in which carbon is released in the form of CO or CO2. Because of this CF-CuO is expected to have less carbon per copper element than CF-CuON The porosity of the materials was analyzed using nitrogen adsorption at −196 °C. The adsorption isotherms are shown in Figure S2 of the Supporting Information. The isotherms of the CuON and CuO powders are characteristic of mesoporous materials with slit-shape pores. Heating CuON to form CuO results in the loss of porosity. Thus, the specific surface area of CuO (32 m2/g) is 7 times smaller as compared to that of CuON (220 m2/g). The carbon textile swatch is non porous (SBET = 5 m2/g). Despite the high porosity of CuON, its deposition on CF does not significantly increase the surface area of this material (SBET = 8 m2/g). This low porosity remains upon the heating of CF-CuON to form CF-CuO (SBET = 7 m2/g). It is interesting to note that a hypothetical physical mixture of CF and CuO is expected to have SBET of 7.2 m2/g (based on the SBET measured for the single components and their content in the composites measured by TA), which is similar to the one actually measured for CF-CuO. However, a hypothetical



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mixture of CF and CuON is expected to have SBET of 25.6 m2/g, which is higher than the one measured for CF-CuON. This difference may indicate some aggregation of CuON/change in its morphology during the deposition on the carbon fibers. The SEM images of CF-CuON and CF-CuO are shown in Figure 3 along with the maps of elements. The shapes of the carbon fibers are well preserved after oxidation, impregnation and heat treatments. After impregnation with the CuON active phase, small particles can be seen on the fibers. This CuON phase is very well-dispersed, which is in agreement with the observed homogeneous coloration of the carbon swatches. The thermal transformation of CF-CuON into CF-CuO results in the aggregation of these particles. Thus, large copper-containing aggregates can be seen on the surface of the fibers together with well-dispersed small particles. This disparity reflects the different phases present on the surface, namely CuO, Cu2O and nanoparticles of Cu0.

Figure 3. SEM images and maps (C, O and Cu) of (A) CF-CuON and (B) CF-CuO.

The chemistry of two copper-based active phases deposited on the carbon textile swatches was also analyzed by FTIR spectroscopy. The spectra of CF-CuO and CF-CuON are 12 ACS Paragon Plus Environment

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shown in Figure S3 of the Supporting Information. The spectra reveal very similar features and the main difference is a small peak at 1295 cm-1, which appears only on the spectrum of CFCuON. This peak represents the symmetric NO2 stretching vibration in copper nitrate. The samples were exposed to DMCP vapors in a vials-in-vial closed adsorption system. As seen in Figure 2A, both swatches change colors to greenish hue as a result of DMCP vapors exposure. This specific feature of these materials might make them the potential indicators of the exposure level to chemical warfare agents. The adsorption capacities of the samples were estimated from their weight gain after 1 and 7 days of exposure to DMCP. The results are seen in Figure 4A. After 1 day, the weight gains are high and similar for both materials. These weight gains, 129 and 120 mg/g for CFCuON and CF-CuO, respectively, are comparable to the values reported on other powder materials (see Table S1 of the Supporting Information). For instance, it was reported that a copper hydroxynitrate composite containing 20 wt% graphite oxide showed weight gains of 98 mg/g after 24 hours and 181 mg/g after 100 hours exposure to DMCP.12 However, with an increase in the exposure time, the weight gain on CF-CuON decreases, while the one on CF-CuO increases. This indicates that during the reactive adsorption of DMCP on the latter sample, some volatile compounds are formed and released from the surface. The initial carbon swatch, before any oxidation treatment do not show any weight gain after 1 day of exposure. However, after 1 week a small weight uptake can be measured (25 mg/g). The adsorption on the oxidized CF alone, without copper phase, shows a loss of weight instead of weight uptake. This indicates that the oxidized carbon phase reacts with DMCP, which results in a loss of carbon, likely as CO2 and/or water from functional groups. However, after one week the weight loss is smaller than



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after one day, indicating that after a first phase of reactions that damage/affect the carbon phase, DMCP and its reaction products adsorb on the carbon fibers. The decrease of the weight uptake in the case of CF-CuON shows that the reactions on the carbon matrix become significant during a long exposure time. On the other hand, it does not show such a significant effect on the weight gain of CF-CuO. This is probably due to the fact that CuO is still very active after 1 week. Indeed, after 1day exposure to DMCP vapors the weight uptakes measured on CuON and CuO bulk powders were found to be 150 and 160 mg/g, respectively,. However, after 1 week exposure the weight uptake of CuON powder slightly increased to 180 mg/g, while the one on CuO considerably increases to 504 mg/g. The hypothetical weight gains assuming physical mixtures of the fibers and active phases were calculated and they are compared to the measured ones in Figure 4A. In all cases, the measured weight gains are larger than the hypothetical ones, showing a clear synergy between the carbon phase and the copper phases that enhances their reactive adsorption capability. To broad the spectrum of catalytic applications of our modified textiles we have also tested their performance against 2-chloroethyl ethyl sulfide (CEES) as a surrogate of mustard gas. Although the weight uptakes shown in Figure S4 confirm the lower adsorption capability of the copper phases toward CEES as compared to DMCP, they still adsorb some CEES up to 50 and 74 mg/g for CF-CuON and CF-CuO, respectively, after 7 days of exposure. This is a better protection than the carbon fiber alone, which does not adsorb any CEES. Interestingly, no change of color could be observed following the adsorption of CEES.


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A 160

B

140

− 0.4 ⋅10 −3 t

0.0

120 -0.5

100

ln( [DMCP]/[DMCP]0 )

DMCP weight gain / mg/g

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80 60 40 20 0

− 2.0 ⋅10 −3 t

-1.0

-1.5

− 6.6 ⋅ 10 −3 t -2.0

CFinitial

-2.5

-20 -40

1 day measured 1 day hypothetical 7 days measured 7 days hypothetical

-60 -80 CFinitial CF

CF CF-CuON CF-CuO

-3.0 0

50

100

− 8.8 ⋅ 10 −3 t 150

200

250

300

350

Time / min

CF-CuON CF-CuO

Figure 4. (A) Weight uptakes of CFinitial, CF, CF-CuON and CF-CuO after 1 and 7 days of DMCP exposure. (B) Evolution of the DMCP concentration in the acetonitrile extracts as a function of the exposure time. Lines are linear fits of the data points.

The increase in the weight of CF-CuON followed by its decrease after a long exposure to DMCP combined with a change of color, are clear indications of reactions occurring on the surface of the modified fibers. In order to investigate these reactions, the headspace of the reaction vessel was analyzed by GC-MS. The measured chromatograms are collected in Figure S5. They show that DMCP, which is represented by a peak at retention time 3.1 min, was almost completely removed after 1 week, and the only reaction product detected in the headspace was chloromethane, indicating a degradation of DMCP through a hydrolysis path. In order to evaluate the extent of DMCP degradation on the surface of the modified fibers and the effect of the active copper phase addition, an extraction with acetonitrile was carried out after different exposure times of the adsorbing materials to DMCP (same weight as the weight of



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adsorbent). Figure 4B compares peak areas of DMCP from chromatograms of the extracts. It shows that the non-oxidized initial carbon swatch does not react with DMCP. After oxidation, and without the copper phase, the degradation of DMCP occurs readily on the oxidized carbon fibers, as expected from the weight loss of carbon mentioned above. The addition of the copper phases increases the degradation rate of DMCP. The kinetics of degradation are higher on CuON than those on CuO. Thus, the rate constants, derived from the linear fittings of the DMCP contents in the acetonitrile extracts as a function of time, are found to be 2·10-3 min-1 for CF, 6.6·10-3 min-1 for CF-CuO and 8.8·10-3 min-1 for CF-CuON. After 1 day exposure to DMCP (1:1 DMCP/adsorbent) 99 % of DMCP was removed. Thus, only 0.1, 0.2 and 1.2 % of the initial amount of DMCP injected could be detected in the extracts of CF-CuO, CF-CuON and CF, respectively. Interestingly, no degradation products were detected in the extracts, indicating that those products are strongly adsorbed on the fiber surfaces, or hidden by the strong peak of the acetonitrile solvent. This would be expected for products such as methanol or chloromethane. SEM images together with elemental maps of the exposed samples (Figure 5) show dramatic transformations of the copper phase deposited on the carbon fibers. The fibers structure of CF-CuON after contact with DMCP remained intact, however the copper phase has been modified. The well-dispersed small particles of copper hydroxynitrate have converted into big aggregates. The element maps indicate that these aggregates contain mainly copper, chloride and phosphorus.

In the case of CF-CuO, the fibers are also intact and copper, chloride and

phosphorus are found on their surface. In addition, filaments consisting mainly of copper and chloride are detected. Phosphorous is not visible in these structures.


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Figure 5. SEM images and C, O, Cu Cl and P element maps of (A) CF-CuON and (B) CF-CuO after DMCP exposure. The atomic percentage of each element is shown in the corresponding panel. Inset: magnification of one of the copper filament formed on CF-CuO

FTIR spectra of CF-CuO and CF-CuON spectra after DMCP exposure are shown in Figure S3. For comparison, the gas phase DMCP spectrum is also included. The spectra of both 17 ACS Paragon Plus Environment


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materials show new features due to the reaction of DMCP with the active phases. Thus, two bands representing C-H vibration in CH3O appear at 2905 and 2980 cm-1.35 In addition, a band at 1055 cm-1 appears that corresponds to CH3-O or P-OCH3 stretching vibrations. It indicates the presence of phosphorous ester.35-36 The P-OCH3 vibration is expected to give a very strong signal, as seen in pure DMCP spectrum.36 However, the ratio of this band intensity to the one of the C-H vibration (2905 cm-1) is much smaller in the case of exposed materials than for pure DMCP. This suggests that the phosphorous ester groups reacted with the surface, resulting in a cleavage of the P-O bond and leaving CH3O groups on the surface of the fibers. Interestingly, an intense P=O band visible at 1308 cm-1 on the spectrum of DMCP, does not appear on those of the exhausted materials. Yet, the small doublet around 1250 cm-1 can be attributed to P=O, suggesting a weakening of the P=O bond due to its interaction with the surface.36 In addition, the NO band at 1295 cm-1 on CF-CuON disappeared, indicating a release of NO or NO2 during DMCP reactive adsorption, as reported previously.12 In order to identify the species formed on the surfaces, the samples were gradually heated in an inert atmosphere and the released gases were analyzed by mass spectroscopy. The m/z thermal profiles are shown in Figure 6. Before exposure to DMCP, CF-CuON shows a remarkable release of NO (m/z = 30) originating from the conversion of the copper hydroxynitrate to copper oxide. As expected CF-CuO, on the other hand, does not show such a release owing to the absence of nitrate in this material. After CF-CuON exposure to DMCP vapors, the peak of nitrate decomposition disappears. This shows the involvement of nitrate in the detoxification process and corroborate to previous reports of a NO/NO2 release during the reaction of DMCP with the copper hydroxynitrate surface.12


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As seen based on the analyzing of m/z thermal profiles, below 100 oC, only water is released from the surface of CF-CuON and CF-CuO. Since the boiling point of DMCP is at 80 o

C, these results show that DMCP and its degradation products are strongly bonded to the

surfaces. The species bonded to the surface of CF-CuON are decomposed/vaporized at 200 and 240 oC. This suggests two binding sites of different energies. Similarly, DMCP and its degradation products are released from the surface of CF-CuO at 190 and 250 oC. Both materials show a release of a phosphoryl fragment P=O (m/z = 47) indicating the presence of phosphate on the surface. The strongest signal measured is from m/z = 50, which confirmed that the main released product is chloromethane. Interestingly, the m/z thermal profiles for CF-CuON show also a release of methanol (m/z =31 and m/z = 29), which is not detected in the case of CF-CuO. The m/z thermal profiles of the oxidized carbon fiber before impregnation (CF) are shown in Figure S6. It shows the release of CO2 (m/z = 44) at a relatively low temperature (peak at 250 oC), but also some release of NO (m/z = 30) and a small amount of SO2 (m/z = 64). These are attributed to the oxidation of the carbon swatch by the sulfuric-nitric acid mixture. After exposure to DMCP, interestingly these peak are disappearing, showing the involvement of these species in the DMCP removal. However, almost no fragment due to DMCP or its degradation products can be seen. Only a very small amount of m/z = 50 (CH3Cl) and m/z =15 (CH3) are detected. These indicate that the surface functional groups of CF reacted with DMCP, but almost no product retained on the surface. This is in agreement with the loss of weight observed for this sample.



1.5

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CF-CuON

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Ion current / nA

Ion current / nA

A

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18 0.5

CF-CuO 18

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1.0

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Temperature / °C

200

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Temperature / °C

Figure 6. m/z thermal profiles of the fragments related to DMCP and its derivatives released from the surface of CF-CuON and CF-CuO, (A) before and (B) after exposure to DMCP. m/z values are indicated next to the corresponding lines.

The detection of chloromethane in the headspace indicates that hydrolysis of DMCP is the main degradation mechanism on both materials, CF-CuON and CF-CuO. DMCP bonds through its phosphoryl group to the surface, which weakens the P=O bond. The chlorophosphate


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is then hydrolyzed on the surface and hydrochloric acid, is formed. The hydrochloric acid reacts then with the methyl ester to release chloromethane (Figure 7). B

A

Hydrolysis

Figure 7. (A) Reaction mechanisms on the surfaces of CF-CuON and CF-CuO, and (B) the degradation products bonded to the surface The surface analyses also indicated the formation of copper chloride suggesting that the released HCl also reacts with the copper species leading to the replacement of OH-, NO3- or O2-, by a Cl- anion. This would release H2O or HNO3, which can in turn hydrolyze DMCP:

This release of NO2 has been reported in previous studies,12 and is supported by the absence of NO in the decomposition products detected from the thermal analysis of CF-CuON exposed to DMCP. In addition to the reaction with copper, DMCP also reacts with CF. Indeed, the acidic surface functional groups introduced by the oxidation treatment can also degrade the nerve agent. However there is no adsorption of the reaction product and it results in a loss of mass. The formation of green copper chloride is responsible for the observed color change that can be used to sense the presence of the organochlorophosphate. Phosphate are strongly bonded 21 ACS Paragon Plus Environment


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to the surface through the P=O bonds, and so are the methoxy groups, which react with the copper chloride during the thermal analysis to form chloromethane. All these hydrolyzed products of DMCP, methanol, chloromethane or dimethyl phosphate are much less toxic than is DMCP. Interestingly, CEES, which present a reactive C-Cl bond did not led to the color change, which can be linked to the small amount adsorbed. In order to verify if other chloroalkane or organophosphate without chlorine could give a false positive, dichloromethane and triisopropyl phosphate (TIPP) were also tested, and no change of color was observed. Direct contact of these compounds with CuON and CuO powder was also carried out. The images shown in Figure 8, clearly demonstrate that only the compound with chlorophosphate result in a change of color indicating a higher reactivity with the surface to form CuCl2. Indeed, even though alkyl chloride such as CH2Cl2 or CEES also have chloride as a leaving group, their carbon in the C-Cl bond is expected to have much less affinity to nucleophile than the phosphorus of a chlorophosphate, which is surrounded by electron-withdrawing groups. Thus, DMCP is more susceptible to bond to nucleophilic groups on the material surface (O, OH), replacing Cl- that can then take the place of the nucleophile that left the surface.


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Figure 8: CuON (A) and CuO (B) powders before and after contact with DMCP, TIPP, CH2Cl2 and CEES.

It has to be noted that in general, real nerve agents do not possess chlorine but a fluorine. In the absence of the possibility to work with real warfare agent, it is assumed that real CWA, such as sarin would react in a similar way as DMCP. Indeed, fluorine is also a halogen, smaller with higher electronegativity, which actually may increase the reactivity of the P-F bond as compared to that of P-Cl. Thus, similarly to the chloride leaving DMCP when reacting with CFCuO or CF-CuON, it is known that F- is also the leaving group when the nerve agent react with the acetylcholinesterase enzyme.37-38 And since copper fluoride is white/ light blue, it is expected that its contact with CF-CuON or CF-CuO it will trigger a change of color, similarly to DMCP. Another important issue is the life-span and reusability of the protective material. Since copper hydroxynitrate may slowly transform to CuO, CF-CuO should be more thermodynamic stable material than is CF-CuON. In addition the thermal treatment at 280 oC, should assure thermal stability up to that temperature. Thus, the weight uptake on a one-year old material CFCuO was measured after one day exposure to DMCP. Regeneration of the material was attempted in the same condition as the preparation (280 oC for 2 hours in air). The results shown 23 ACS Paragon Plus Environment


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in Figure S7, indicate a smaller intake after one year, at 92 mg/g. Yet a change of color was still visible. However after the first regeneration, no change of color could be seen when exposed again to DMCP. In addition, the weight uptake decreases with each regeneration. After the third one, the material was not protecting anymore. This is likely related to the exhaustion of its active centers upon reaction with the decomposition products and inability of their thermal regeneration (salt decomposition) at the relatively low temperature of 280 oC.

4. Conclusions A mild oxidation of carbon textiles functionalized their surfaces. This allowed the high dispersion and the strongly bonded copper hydroxynitrate on the fibers. The swatches containing CuO, Cu and Cu2O could be easily prepared by heating the hydroxynitrate-modified textiles to 280 oC. Both materials tested showed remarkable detoxification abilities against the nerve agent surrogate DMCP, which is almost completely decomposed during 24 hours of direct contact with the samples. While the oxidized carbon swatches are good decontaminants, the kinetics of degradation are highly improved by the addition of the copper phase. On the modified fibers with CuO or CuON, the DMCP degradation rate constants are two and three times higher, respectively, than that on CF alone. In addition to their significant high detoxification capabilities, the modified fabrics can be used to sense the presence of DMCP which changes its colors through the formation of green copper chloride. The detoxification occurs through hydrolysis, producing mostly the much less toxic chloromethane. The nitrates present in CFCuON enhance the hydrolysis of DMCP releasing NO or NO2. This release of oxidative species causes a loss of adsorbent mass by oxidizing the carbon support. Deposition of the copper phase results in a clear synergy of the composite components enhancing the detoxification performance. In terms of detoxification, both CF-CuON and CF-CuO show similar 24 ACS Paragon Plus Environment

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detoxification abilities. CF-CuON presents the advantage of a faster kinetics and a better sensing due to a higher contrast in the change of color. However CF-CuO, due to the thermal treatment presents the advantage of a better anchoring to the carbon support, a higher stability and the possibility to partially regenerate its activity.

Acknowledgment This work was supported by Army Research Office (grant No. W911-13-0225)

SUPPORTING INFORMATION Supporting Information includes: comparison of the weight gains reported for various adsorbents after 1 day exposure to DMCP, schematic representation of the setup used for adsorption and degradation of the CWA surrogate, nitrogen adsorption isotherms, FTIR spectra of CF-CuON and CF-CuO before and after exposure to DMCP, weight uptakes of CF, CF-CuON and CF-CuO after 1 and 7 days of CEES exposure, chromatograms of the headspace of CF-CuON and CFCuO after contact with DMCP vapors, and weight uptake of a one-year old CF-CuO, after several thermal regeneration processes.



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Carbon Textiles Modified with Copper-Based Reactive Adsorbents as

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