Decontamination of 2-chloroethyl ethylsulfide using titanate nanoscrolls

Titanate nanoscrolls, a recently discovered variant of TiO2 nanocrystals, are tested as reactive sorbent for chemical warfare agent (CWA) decontamination. The large surface area of the uncapped tubules provides the desired rapid absorption of the contaminant while water molecules, intrinsic constituents of titanate nanoscrolls, provide the necessary chemistry for hydrolytic reaction. In this study the decomposition of 2-chloroethyl ethylsulfide (CEES), a simulant for the CWA mustard, was monitored using C NMR. The NMR spectra reveal reaction products as expected from the hydrolysis of CEES. This demonstrates that titanate nanoscrolls could potentially be employed as a decontaminant for CWAs. 2005 Elsevier B.V. All rights reserved. Hydrolysis reactions have shown promising reactivity and desirable products for the decontamination of a number of CWAs [1]. However, the solubility of some CWAs such as bis(2-chloroethyl) sulfide (HD or mustard) by aqueous solutions is often very limited, especially for thickened agents with added polymeric thickener [1]. This hinders severely the effectiveness of aqueous solutions for CWA decontamination [1]. An alternative approach to liquid decontamination media is the use of solid sorbent systems for the decontamination of CWAs and clean-up of hazardous waste pollutants [1]. Large surface area and sufficient reactivity toward pollutant detoxification are two key properties of the sorbent materials for such applications. Strong absorbance removes the agent rapidly from the affected surface and creates immediate relief. Once trapped with0009-2614/$ see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.100 * Corresponding author. Fax: +1 919 962 0480. E-mail address: [email protected] (A. Kleinhammes). in the porous solid, the adsorbate undergoes chemical reactions which could render them harmless. The reaction chemistry involving detoxification of CWAs is shared by some organic pollutants and pesticides [1]. Recent experiments showed that powders of metal oxide, MgO [2], CaO [3], Al2O3 [4], and TiO2 [5], when penetrated by CWAs, initiate reactions found in liquid decontamination schemes in addition to binding agents to their surfaces. Moreover catalysis was suspected as a means of sustaining some of the reactions [3]. It was expected that powders of nanocrystals would be more effective than powders of conventional microcrystals owing to larger surface areas and larger numbers of available reactive sites due to surface, corner, and edge defects [3,6]. In reality, much of that surface area would not be accessible to adsorbates due to aggregation of nanocrystallites rendering many facets of nanocrystallites inaccessible. In contrast to nanocrystals, nanostructures with large aspect ratios such as nanotubes and nanoscrolls might stack together irregularly without 82 A. Kleinhammes et al. / Chemical Physics Letters 411 (2005) 81–85 loosing surface areas. One of such nanostructures is titanate nanoscrolls discovered in 1998 [7,8]. While the exact structure and composition are still under investigation, the dimension and shape as observed by TEM are clear [7–16]. The tubules have an average outer diameter of 12 nm and an inner diameter of 5 nm [10–12]. TEM characteristics such as a spiral-like cross-section reveal unambiguously the scroll structure (like a paper scroll). The spacing between the spiral walls of the nanoscroll is approximately 0.78 nm [10–12]. The uncapped tubules are on average between 0.3 and 1 lm long giving them large aspect ratio and large surface area. The surface area based on BET measurements is approximately 250 m/g for titanate nanoscrolls as compared to 50 m/g for typical TiO2 nanocrystals [8]. Here, we demonstrate that titanate nanoscrolls have the potential to be an effective decontaminant for CWAs. Titanate nanoscrolls were produced by hydrothermal synthesis following the procedure as originally proposed by Kasuga et al. [7,8] and followed by other authors [10–16]. 2 g of TiO2 anatase nanocrystals (Aldrich 39953, 32 nm average particle size, 45 m/g surface area) was immersed in 200 ml of 10 M NaOH and heated in Teflon lined autoclave at 130 C for 72 h. The produced material was sonicated, filtered, and washed repeatedly with distilled H2O until the solution above the white precipitate becomes neutral (pH 7). Dilute 0.1 M HCl was also used sometimes to speed up the process of reaching neutral pH. The precipitated material was then dried at temperatures around 55 C for 3–5 h. The mass of the dried material exceeds the mass of the starting TiO2 nanocrystals by a factor of approximately 1.6 indicating that the material contains H2O molecules. The estimated ratio of H2O to TiO2 is about 3.5. Na NMR was used to determine the Na content in the sample. The washed and dried nanoscrolls contain approximately 5 · 10 Na/(g nanoscrolls) yielding a Na/Ti ratio of 0.07. This ratio is about 5 times higher in the sample immediately following the hydrothermal synthesis but without washing. Pure 2-chloroethyl ethylsulfide (CEES) liquid, a simulant of HD, was used to evaluate the decontamination effect of titanate nanoscrolls. In the experiment, 3 ll of Fig. 1. SEM and TEM micrographs of titanate nanoscrolls. The images de microto nanometers: (a) single particle, (b) surface of particle resembling nanoscroll. pure CEES was added by a micro-syringe to 30 mg of titanate nanoscrolls (approximately 10 wt% CEES) packed in a magic-angle-spinning (MAS) rotor (4 mm MAS rotor from Chemagnetics/Varian). No additional water was added to the sample. The reaction path of CEES with titanate nanoscrolls was investigated by C NMR. The CEES sample is not C-enriched. Free induction decays (FIDs) were recorded using a pulsed spectrometer in a magnetic field of 9.4 T under MAS at a spinning rate of 10 kHz. The spectra were collected by signal averaging of 18500 scans with a recycle delay of 5 s. Chemical shifts (CS) are reported with respect to TMS based on adamantane as a secondary reference. Figs. 1a, b show scanning electron microscopy (SEM) pictures of our titanate nanoscrolls where the porosity of titanate nanoscroll aggregates is clearly revealed. Transmission electron microscopy (TEM) resolves the tubular structure with inner diameter of 5 nm and outer diameter of 12 nm (Fig. 1c). The wall separation is about 0.70 nm. Fig. 2 shows the X-ray diffraction (XRD) pattern (Cu Ka line) of titanate nanoscroll powder. The prominent peak at 2h = 10.27 was attributed to the separation of the walls of nanoscrolls and corresponds to a d-spacing of 0.86 nm. Suzuki and Yoshikawa [14] measured the d-spacing associated with this low angle diffraction peak as a function of temperature and found that d decreases from 0.92 nm at room temperature to 0.79 nm at 200 C. They attributed the observed change to water desorption from the space between the walls of nanoscrolls which deflate when water molecules are removed. Based on thermogravimetric measurements they estimated that nanoscrolls at room temperature contain 3H2O per formula unit which they postulate to be H2Ti3O7 Æ nH2O with n < 3. Our nanoscrolls show a dspacing of 0.86 nm as determined by XRD whereas TEM shows a d-spacing of 0.70 nm. Since TEM images were taken in ultrahigh vacuum and thus devoid of H2O, the larger d-spacing determined by XRD is consistent with water trapping between the walls under ambient conditions. The relative change in the d-spacing is similar to that reported previously [14]. CEES (CH3CH2SCH2CH2Cl) and HD (ClCH2CH2SCH2CH2Cl) can be detoxified by stripping the chlorine monstrate the porosity of the material on length scales ranging from a spider web network of scrolls, (c) TEM image resolving individual 20 40 60 80 100 2 theta Nano-Scrolls